An Innovative Method to Generate Iodine(V and III

An Innovative Method to Generate Iodine(V and III)-Fluorine Bonds and Contributions to the Reactivity of Fluoroorganoiodine(III) Fluorides and Related Compounds

Vom Fachbereich Chemie der Universität Duisburg-Essen

zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation

von Anwar Abo-Amer aus Irbid / Jordanien

Referent: Korreferent: Tag der mündlichen Prüfung:

Prof. Dr. H.-J. Frohn Prof. Dr. G. Geismar 26.01.2005

Die experimentellen Arbeiten wurden in der Zeit von Juli 2001 bis April 2004 unter Anleitung von Herrn Prof. Dr. H.-J. Frohn im Fach Anorganische Chemie des Fachbereiches Chemie am Campus Duisburg der Universität Duisburg-Essen durchgeführt.

ACKNOWLEDGMENTS I would like to thank my supervisor Prof. Dr. Hermann-Josef Frohn (Distinguished Professor Inorganic Chemistry), for his guidance, encouragement, support throughout my graduate study, his willingness to share his technical knowledge and for having patience with me. He acted as the driving force behind this research. He provided his knowledge and expertise. He spent many time for constructive discussion, which enriched my knowledge, skill and my experience. I sincerely thank Prof. Dr. G. Geismar, the Korreferent, for his encouragement, support and constructive discussion. Also, I’m very grateful to Prof. Dr. Vadim Bardin for many fruitful discussions concerning topics in fluorine and boron chemistry. I have to thank my colleague Dr. Nicolay Adonin for helpful discussions. He provided not only scientific, but also moral support, and most of all friendship, throughout my study and research. I am also grateful to many other persons and I would like to acknowledge their significant contributions to my study: - Karsten Koppe, who has provided me with constant support, kind guidance and significant contribution, not only on my academic life but also on my personal life. - Wassef Al Sekhaneh, who inspired my research with his incredible knowledge. - Dietmar Jansen, Petra Fritzen, Christoph Steinberg, Andre´ Wenda, and Oliver Brehm, which all inspired my research with their incredible knowledge and helped for a warm and supportive environment. Special thanks are given to many faculty and staff members of the chemistry department (Duisburg-Essen Universität) for their assistance during my graduate study. In particular, thanks are pressed to Dr. Ulrich Flörke for the X-Ray crystallographic work. Special thanks to Mrs. Beate Römer and Mr. Manfred Zähres for NMR spectrometric measurements. My utmost appreciation and thanks are given to my wife, Eman Abu-Jadoua, for her love and support throughout my graduate career. I also thank my daughter, Mimas, and my son, Yamen, for bringing so much joy the moment they joined into my life in Germany. I warmly thank my parents, brothers and sisters for continuous inspiration and encouragement. The support of many friends through out my research (Prof. Dr. Alaa Hassan, Prof. Dr. Mohammad Shabat) has also been much appreciated.

“After great pain, a formal feeling comes” Emily Dickinson

Dedicated to…

My Daughter Mimas, My Son Yamen, My Wife Eman, My Mother and Father

Table of Contents

I

Table of Contents 1

Introduction

1

1.1

Bonding and Structure in Polyvalent Iodine Compounds

1

1.2

(Difluoroiodo)arenes

4

1.3

(Tetrafluoroiodo)arenes and (Difluorooxoiodo)arenes

6

1.3.1

(Tetrafluoroiodo)arenes

6

1.3.2

(Difluorooxoiodo)arenes

7

1.4

Iodine Pentafluoride

7

1.5

Iodonium Salts

9

1.5.1

Diaryliodonium Salts

9

1.5.2

Alkenyl(aryl)iodonium Salts

12

2

Objectives

14

2.1

Preparative Aspects

14

2.1.1


14

2.1.2


15

2.1.3


15

2.1.4


15

2.1.5

Iodonium Salts

16

2.2

Reactivity, Structure, and Spectroscopy

17

3

Results and Discussion

19

3.1

Preparation of Iodine Pentafluoride (IF5) by a New Methodological Approach

19

3.1.1

Introduction

19

3.1.2

Relevant Reactivities of I(V)-F and I(V)-O Bonds

19

3.1.3

The Reaction of I(V)-O Compounds with aHF in a Two Phase System

20

3.1.4

The Important Steps in the Preparation of IF5

20

3.1.5

The Influence of the HF Concentration on the IF5 Formation

21

Table of Contents

3.2

4-Fluoro-1-(tetrafluoroiodo)benzene by Oxygen-Fluorine Substitution

3.3

23

4-Fluoro-1-(difluorooxoiodo)benzene (p-C6H4FIOF2) by Treatment of 4-Fluoro-iodylbenzene with Hydrofluoric Acid

3.4

II

24

(Difluoroiodo)arenes (ArIF2) by Oxygen-Fluorine Substitution on ArIO with Hydrofluoric Acid as Reagent

25

The Influence of the HF Concentration on the Formation of (Difluoroiodo)arenes (ArIF2)

3.5

26

A Convenient Route to (Difluoroiodo)benzenes (ArIF2) Directly from (Diacetoxyiodo)benzenes

28

3.6

Iodonium Salts

30

3.6.1

The Synthesis of Diaryliodonium Salts Starting from (Difluoroiodo)arenes

30

3.6.2

The Synthesis of Alkenyl(aryl)iodonium Salts Starting from (Difluoroiodo)arenes

3.6.2.1

trans-1,2,3,3,3-Pentafluoroprop-1-enyl(fluorophenyl)iodonium Tetrafluoroborates

3.6.2.2

31

31

trans-1,2,3,3,3-Pentafluoroprop-1-enyl(pentafluorophenyl)iodonium Tetrafluoroborate

33

3.6.2.3

Preparation of Trifluorovinyl(fluorophenyl)iodonium Tetrafluoroborates

33

3.6.2.4

Preparation of Trifluorovinyl(pentafluorophenyl)iodonium Tetrafluoroborate

34

3.7

Selected Reactivities of Fluoro(difluoroiodo)benzenes C6H4FIF2 35

3.7.1

Reactivities with Nucleophiles and Lewis Bases

35

3.7.1.1

The Reaction of p-C6H4FIF2 with Trimethylsilylacetate

35

3.7.1.2

The Interaction of ArIF2 with 2,2´-Bipyridine

36

3.7.1.3

The Interaction of ArIF2 with (C6H5)3PO

37

3.7.1.4

The Reaction of ArIF2 with [NMe4]F

37

Table of Contents 3.7.1.4.1

III

The Reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : 1) in Dichloromethane

38

3.7.1.4.2

The 1 : 2 Reaction of p-C6H4FIF2 with [N(CH3)4]F in Dichloromethane

40

3.7.1.4.3

The 1 : 0.5 Reaction of p-C6H4FIF2 with [N(CH3)4]F in Dichloromethane

41

3.7.1.4.4

The Reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : 1) in Acetonitrile

41

3.7.1.4.5


42

3.7.1.4.6

The 1 : 2 Reaction of o-C6H4FIF2 with [N(CH3)4]F in Dichloromethane

42

3.7.1.4.7

The 1 : 2 Reaction of m-C6H4FIF2 with [N(CH3)4]F in Dichloromethane

43

3.7.1.5

The Reaction of p-C6H4FIF2 with CsF

44

3.7.1.5.1

The Reaction of p-C6H4FIF2 with CsF (1 : 1) in Acetonitrile

44

3.7.1.5.2


45

3.7.2

Reactions of C6H4FIF2 with Lewis and Brønsted Acids

46

3.7.2.1

The Reaction of p-C6H4FIF2 with C6H5PF4

46

3.7.2.2

The Reactions of p-C6H4FIF2 with Alcohols (MeOH, EtOH, CF3CH2OH)

47

3.7.2.3

The Reaction of p-C6H4FIF2 with CF3CO2H

48

3.7.2.4

The Reaction of p-C6H4FIF2 with aHF

49

3.8

Selected Reactivities of Iodonium Salts

52

3.8.1

Reactions with Lewis Bases

52

3.8.1.1

The Reaction of [p-C6H4F(CF2=CF)I][BF4] with Naked Fluoride

52

3.8.1.2

The Reaction of [p-C6H4F(C6H5)I][BF4] with Naked Fluoride

54

3.8.1.3

The 1 : 1 Reaction of [p-C6H4F(C6H5)I]F with Naked Fluoride in Dichloromethane

55

3.8.2

Reactions with Nucleophiles

56

3.8.2.1

The Reaction of [p-C6H4F(trans-CF3CF=CF)I][BF4] with (p-C6H4F)3As in CH2Cl2

3.8.2.2

The Reaction of [p-C6H4F(trans-CF3CF=CF)I][BF4] with (p-C6H4F)3P in CH2Cl2

3.8.2.3

56

The Reaction of [p-C6H4F(trans-CF3CF=CF)I][BF4] with 2,2´-Bipyridine in CH2Cl2

3.8.2.4

56

57

The Attempted Reaction of [p-C6H4F(CF2=CF)I][BF4] with (p-C6H4F)3P in aHF

58

Table of Contents

IV

3.9

The Results of 1H, 13C, and 19F NMR Spectroscopic Studies

59

3.9.1

19

59

3.9.2

The NMR Spectroscopic Studies of 4-Fluoro-1-(tetrafluoroiodo)benzene

F NMR Spectroscopic Studies of IF5

(p-C6H4FIF4) 3.9.3

The NMR Spectroscopic Studies of 4-Fluoro-1-(difluorooxoiodo)benzene (p-C6H4FIOF2)

3.9.4

60

The NMR Spectroscopic Comparison of C6H4XI, C6H4XI(OAc)2, and C6H4XIF2 (X = o-, m-, and p-F)

3.9.5

59

62

The Temperature Dependence of 19F NMR Chemical Shifts in Monofluoro(difluoroiodo)benzenes

67

3.9.6

NMR Spectroscopic Studies on Iodonium Salts

70

3.9.6.1

Asymmetric Diaryliodonium Tetrafluoroborates

70

3.9.6.2


72

3.9.6.3

Trifluorovinyl(fluorophenyl)iodonium Tetrafluoroborates

76

3.9.6.4

Alkenyl(pentafluorophenyl)iodonium Tetrafluoroborates

79

3.10

Thermal Stabilities of Selected (Difluoroiodo)benzenes and Aryl-Containing Iodonium Salts

81

3.11

X-Ray Crystal Structure Analysis

83

3.11.1

The Crystal Structures of p-C6H4FIF2 and o-C6H4FIF2

83

3.11.2

The Crystal Structure of [m-C6H4F(C6H5)I][BF4]

90

3.11.3

The Crystal Structure of [p-C6H4F(trans-CF3CF=CF)I][BF4]

94

3.11.4

The Crystal Structure of p-C6H4FIOF2

98

3.12

The Inductive and Resonance Parameters of Selected I(III)Substituents in Iodonium Salts Using Taft`s Method

101

4

Experimental Section

104

4.1

Materials, Apparatus, and Methods

104

4.1.1

General Methods

104

4.1.2

Spectroscopic, Physical, and Analytical Measurements

105

Table of Contents

V

4.1.2.1

NMR Spectroscopy

105

4.1.2.1.1

1

105

4.1.2.1.2

11

B NMR Spectroscopy

105

4.1.2.1.3

19

F NMR Spectroscopy

105

4.1.2.1.4

13

C NMR Spectroscopy

105

4.1.2.2

Differential Scanning Calorimetry (DSC) Measurements

107

4.1.2.3

Melting Point Measurements

107

4.1.2.4

X-Ray Single Crystal Measurements

107

4.1.2.5

Weighing of Electrostatic Materials

107

4.1.3

Solvents, Chemicals, and Starting Compounds

108

4.1.3.1

Solvents

108

4.1.3.2

Chemicals

109

4.1.3.2.1

Available in the Laboratory

109

4.1.3.2.2

Commercially Available Chemicals

109

4.1.3.3

Starting Compounds

111

4.1.3.3.1

The Preparation of (Diacetoxyiodo)arenes ArI(O2CCH3)2

111

4.1.3.3.2

The Preparation of Iodosylbenzenes ArIO

114

4.1.3.3.3

The Preparation of p-Fluoroiodylbenzene p-C6H4FIO2

115

4.1.3.3.4

The Preparation of Phenyldifluoroborane

116

4.1.3.3.5

The Preparation of Perfluorovinyldifluoroborane

117

4.1.3.3.5.1 The Preparation of Potassium Perfluorovinyltrifluoroborate

117

4.1.3.3.5.2 The Preparation of Lithium Perfluorovinyltrimethoxyborate

119

4.1.3.3.6

H NMR Spectroscopy

The Preparation of trans-1,2,3,3,3-Pentafluoroprop-1-enyldifluoroborane

120

4.1.3.3.6.1 The Preparation of Potassium trans-1,2,3,3,3-Pentafluoroprop-1enyltrifluoroborate

121

4.1.3.3.6.2 The Preparation of Lithium trans-1,2,3,3,3-Pentafluoroprop-1enyltrimethoxyborate

122

4.1.3.3.6.3 The Preparation of trans-1,2,3,3,3-Pentafluoropropene

123

4.2

Synthetic Procedures and Spectroscopic Data

124

4.2.1

An Innovative Preparation of Iodine Pentafluoride

124

4.2.1.1

Starting from Iodine(V) Oxide

124

4.2.1.2

Starting from Sodium Iodate

124

Table of Contents 4.2.1.3

VI

The Influence of the HF Concentration on the IF5 Formation: Reaction of NaIO3 with aHF

125

4.2.2

The Preparation of 4-Fluoro-1-(tetrafluoroiodo)benzene

126

4.2.3

The Preparation of 4-Fluoro-1-(difluorooxoiodo)benzene

127

4.2.4

The Preparation of (Difluoroiodo)benzenes from Iodosylbenzenes

128

The Influence of the HF Concentration on the Formation of (Difluoroiodo)arenes (ArIF2) 4.2.5

A Convenient Route to (Difluoroiodo)benzenes ArIF2 Directly from (Diacetoxyiodo)benzenes

4.2.6

142

The Preparation of Trifluorovinyl(pentafluorophenyl)iodonium Tetrafluoroborate

4.3

141

The Preparation of Trifluorovinyl(monofluorophenyl)iodonium Tetrafluoroborates

4.2.10

137

The Preparation of trans-1,2,3,3,3-Pentafluoroprop-1-enyl(pentafluorophenyl)iodonium Tetrafluoroborate

4.2.9

133

The Preparation of trans-1,2,3,3,3-Pentafluoroprop-1-enyl(monofluorophenyl)iodonium Tetrafluoroborates

4.2.8

132

The Preparation of Monofluorophenyl(phenyl)iodonium Tetrafluoroborates

4.2.7

130

145

Selected Reactivities of Fluoro(difluoroiodo)benzenes C6H4FIF2

147

4.3.1


147

4.3.1.1


147

4.3.1.2


147

4.3.1.3


148

4.3.1.4

The Reaction of p-C6H4FIF2 with [NMe4]F

149

4.3.1.4.1


149

4.3.1.4.2


150

4.3.1.4.3

The Reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : 0.5) in Dichloromethane

151

4.3.1.4.4

The Reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : 1) in Acetonitrile

152

4.3.1.4.5

The Reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : 3) in CH2Cl2

153

4.3.1.4.6

The Reaction of m-C6H4FIF2 with [N(CH3)4]F (1 : 2) in Dichloromethane

154

Table of Contents

VII

4.3.1.4.7

The Reaction of o-C6H4FIF2 with [N(CH3)4]F (1 : 2) in Dichloromethane

154

4.3.1.5


155

4.3.1.5.1


155

4.3.1.5.2


155

4.3.2


157

4.3.2.1


157

4.3.2.2

The Reactions of ArIF2 with Alcohols (MeOH, EtOH, CF3CH2OH)

157

4.3.2.3


158

4.3.2.4


159

4.4


161

4.4.1


161

4.4.1.1

The Reaction of [p-C6H4F(CF2=CF)I][BF4] with Naked Fluoride in CH2Cl2

161

4.4.1.2

The Reaction of [p-C6H4F(C6H5)I][BF4] with Naked Fluoride in CH2Cl2

162

4.4.1.3

The 1 : 1 Reaction of [p-C6H4F(C6H5)I]F with Naked Fluoride in CH2Cl2

163

4.4.2

Reactions with Nucleophiles

164

4.4.2.1


4.4.2.2


4.4.2.3

165


4.5

165


4.4.2.4

164

166

The Determination of the Inductive and Resonance Parameters of Selected I(III)-Substituents in Iodonium Salts Using Taft`s Method

167

5

Summary

170

5.1

Generation of Iodofluorides and Organoiodofluorides

170

5.1.1


170

5.1.2


171

Table of Contents

VIII

5.1.3


172

5.1.4


172

5.2

The First Synthesis of Perfluoroalkenyl(aryl)iodonium Tetrafluoroborate Salts

5.3

Reactivity, Structure, and Spectroscopy of Monofluoro(difluoroiodo)benzenes

5.4

173

174

General Reactivities of Perfluoroalkenyl(aryl)iodonium Tetrafluoroborate Salts

175

6

References

176

7

Appendix

183

7.1

NMR Spectroscopic Data of I-F and Related Compounds

183

7.2

Solubility of ArIF2 in Different Solvents

184

7.3

Solubility of HF in Methylene Chloride

185

7.4

The Interatomic Distances and Angles of p-C6H4FIF2, o-C6H4FIF2, [m-C6H4F(C6H5)I][BF4], p-C6H4FIOF2 [p-C6H4F(trans-CF3CF=CF)I][BF4]

185

7.5

List of Figures

194

7.6

List of Schemes

195

7.7

List of Tables

197

7.8

List of Symbols and Abbreviations

199

7.9

List of Publications, Presentations and Conferences

201

Curriculum Vitae

Introduction

1

1

Introduction

1.1

Bonding and Structure in Polyvalent Iodine Compounds

The concept of hypervalency was introduced by Musher[1] in 1969. By definition in hypervalent molecules the octet rule is not obeyed, that means that there are more than four pairs of electrons around the central atom in the conventional Lewis formula. More simply, hypervalent molecules or ions are containing central atoms of group 15 – 18, non-metals of groups V – VIII of the main groups, in a higher valency than the stable one given by the valency rule 8 – group number. In such compounds the central atom uses a p-orbital to form a linear bond to two ligands. Such bonds, termed "hypervalent", are longer and weaker than (normal) two-centre two-electron covalent bonds.[2, 3, 4, 5] The description of such bonding systems by molecular orbital theory led to the concept of 3center-4-electron or similar poly-centre bonds (hypervalent bonds).[6,

7]

Supported by

computational work this concept is now accepted.[8, 9] The most common hypervalent iodine compounds are aryl–λ3–iodanes (ArIL2) with a decet structure and pseudotrigonal bipyramidal geometries (T-shaped molecules) and aryl-λ5– iodanes (ArIL4) with a dodecet structure and square pyramidal geometries. Bonding in ArIL2 compounds uses essentially a pure 5p orbital in the linear L-I-L bond, the hypervalent three-centre-four-electron bond (3c-4e bond), with two electrons from the doubly occupied 5p orbital of iodine and one electron from each p-orbital of the ligands L. The least electronegative ligand in ArIL2, the aryl group, is bound by a normal two-centre-two-electron covalent bond with C(sp2) hybridization in the CAr–I σ-bond.[10, 11] In the MO-scheme of the IL2 subunit with three molecular orbitals the two molecular orbitals of lower energy, bonding and nonbonding orbitals, are filled (Fig. 1). Partial positive charge has to be assigned to the central iodine atom (ca. +1.0 a.u.),[8] while partial negative charge on both apical heteroatom ligands (L = F: ca. –0.5 a.u.).[8] The filled nonbonding molecular orbital has a node at the central iodine atom. The partial positive charge on iodine in the highly polarised 3c–4e bond makes the aryl-λ3-iodane an electrophilic agent. The inherent nature of the 3c–4e bond explains the preferred orientation of more electronegative ligands in the apical positions. For non-metals of the same group more electropositive central atoms are energetically favoured for hypervalent species: thus in general, λ3-iodanes are more stable than analogous λ3bromanes and λ3-chloranes.[10, 11]

Introduction

2 antibonding

L Ar

I

:

nonbonding

:

bonding

L L

L

I

Figure 1: Molecular orbital scheme for the three centre-four electron bond in the IL2 group. For the designation of hypervalent compounds the Martin Arduengo [N-X-L] notation is usually used[12], in which N is the number of valence electrons surrounding the central atom X and L is the number of ligands bonded to the X-atom. According to this designation, six structural types of polyvalent iodine species (1 – 6) are the most common. The first two species, 8-I-2 (1) and 10-I-3 (2), called λ3-iodanes, are conventionally considered as derivatives of iodine(III), whereas the next two, 10-I-4 (3) and 12-I-5 (4) λ5-iodanes, represent the most typical structural types of pentavalent iodine.[13] .. I L

.. L

8-I-2 1

L

L R

I

:

:

I

: L 10-I-3 2

L 10-I-4 3

O

L L

L

L

L

I .. 12-I-5 4

L

L

L

L

I

L L

L L

L 14-I-6 5

L I

L

L L

L 14-I-7 6

Species 1 – 4 are common in organic chemistry. The 10-I-3 species have an approximately Tshaped structure with a collinear arrangement of the most electronegative ligands. Including the free electron pairs, the ψ-geometry of iodine is a distorted trigonal bipyramide. 8-I-2 species (iodonium cations) (1) are usually considered as cationic part of salts with pseudotetrahedral geometry of the central I-atom. Caused by the positive partial charge on iodine and the open moiety of iodine, additional contacts to basic sites of the anion are observed.[14, 15] The I-C distances in both species 1 and 2 are approximately equal to the sum of the covalent radii of iodine and carbon, ranging generally from 2.00 to 2.10 Å. Compounds of iodine(III) with one carbon ligand are represented by organic iodosyl compounds (RIO, where R is usually aryl) and their derivatives (RIX2, where X represents an electronegative ligand). The second iodine(III) class with two carbon ligands on iodine includes various iodonium salts (R2I+ X–). The overwhelming majority of known, stable organic compounds of polyvalent iodine belong to these two classes. The two heteroatom ligands X attached to iodine in RIX2 are commonly represented by fluorine, chlorine, O-, N-, and strongly electronegative C-substituents. In general, only RIX2 derivatives bearing the

Introduction

3

most electronegative substituents X are sufficiently stable.[13, 14] The bonding in iodine(V) compounds containing divalent ligands such as oxygen may also be described in terms of hypervalency. Two singly occupied atomic orbitals of oxygen interact with a doubly occupied 5p orbital of iodine forming three molecular orbitals: one bonding (doubly occupied), one nonbonding localised on oxygen (doubly occupied), and one antibonding (unoccupied). The result is a highly polarised I–O bond with considerable positive partial charge on iodine and negative partial charge on oxygen. Such hypervalent bonds are designated as 2c–4e bonds (fig. 2).[11] On the other hand, compounds of the IOL3 type are constructed from three different bonds. In PhIOF2 there is one 2c–2e I–C bond, one 3c–4e IF2 bond, and one 2c–4e I–O bond.[16]

:

.

O

.

:

. .

C

I

C

O . .

. . I

ψ3

antibonding

ψ

nonbonding

ψ1

bonding

2

O

I

Figure 2: The molecular orbital scheme for the hypervalent 2c-4e I-O bond. The bonding in iodine(V) compounds, IL5, with a square pyramidal structure may be described in terms of one 2c–2e bond between iodine and the ligand in the apical position, trans to the lone pair, and two orthogonal, hypervalent 3c–4e bonds, accommodating four ligands.[17, 18a] Aryl-λ5-iodanes ArIL4 have a square pyramidal structure with the aryl group in the apical position and four ligands in basal positions.

Ar L L

I ..:

L L

A very high fugalibility (leaving group ability) of iodanyl groups (λ1) is among the most

Introduction

4

important features of iodonium salts, often describes as λ3-iodanes[18b], which makes it possible to generate highly reactive species such as carbenes, nitrenes, cations, and arynes under mild conditions. Furthermore λ3-iodanes, RIX2, are suitable oxidizing agents and allow the transformation of a wide range of functionalities such as alcohols, amines, sulfides, alkenes, alkynes, and carbonyl groups.[10]

1.2


Actually (difluoroiodo)arenes have received a widespread practical application in organic synthesis as versatile fluorination reagents. Generally, they are more reactive than the analogous bromides and chlorides.[19] There is a considerable number of different methods of synthesis for this widely applied class.[20] (Difluoroiodo)arenes were synthesised for the first time by Dimroth and Bockemüller from iodosylbenzenes and 40 % aqueous hydrogen fluoride as impure products in 1931:[21] ArIO

+

2 HF

K[HF2] CHCl3

ArIF2

+

H2O

(1)

Garvey, Halley, and Allen used a mixture of 46 % aqueous HF and glacial acetic acid:[22]

ArIO

+

2 HF / CH3CO2H

ArIF2

+

H2O

(2)

In 1966, Carpenter reported a method, which can be described as chlorine-fluorine substitution on (dichloroiodo)arene using HF in the presence of mercury(II) oxide:[23]

ArICl2

+

2 HF / HgO

ArIF2

+ HgCl2 + H2O

(3)

The isolation of readily hydrolysible (difluoroiodo)arenes is the mean problem in all above mentioned methods owing to the fact that the reaction mixture contains water. To overcome this disadvantage, Schmidt and Meinert proposed the electrochemical oxidation of iodoarenes in acetonitrile solution in the presence of silver fluoride as supporting electrolyte and fluoride source giving the pure (difluoroiodo)arenes.[24] For a high yield the electrochemical preparation of para-substituted (difluoroiodo)arenes Et3N ⋅ n HF was recently used as reagent.[25, 26] Moreover, (difluoroiodo)arenes are formed readily when the corresponding iodosyl or bis(trifluoracetoxy)iodoarenes are treated with sulfur tetrafluorid at –20 °C.[27] All the by-

Introduction products

5

in

this

reaction

are

volatile

and

can

be

removed

by

evaporation.

(Difluoroiodo)arenes are afforded in high purity:

ArIO

+

SF4

ArI(O2CCF3)2

+

2 SF4

-20 °C -20 °C

ArIF2

+ SOF2

ArIF2

+ 2 SOF2 + 2 CF3COF (5)

(4)

Schmeißer reported for the first time the oxidative addition of fluorine to C6F5I. C6F5IF2 was obtained by using elemental fluorine at low temperature:[28, 29]

ArfI

+

F2

-100 °C CCl3F

ArfIF2

(6)

Xenon difluoride was also used to obtain (difluoroiodo)arenes:[30]

ArI

+

XeF2

20 - 40 °C

ArIF2

+

Xe

(7)

The fluorination of various iodoarenes with elemental fluorine, diluted with nitrogen to avoid the fluorination of the aromatic ring which contained donating substituents, have been published:[31, 32]

ArI

+

F2

-100 °C CCl3F

ArIF2

(8)

A modified three step method for preparing (difluoroiodo)arenes from iodoarenes in a pure form was reported parallel to this work. (Dichloroiodo)arenes were prepared by the reaction of iodoarenes with chlorine gas (eq. 9). The products were hydrolysed to form the corresponding iodosylarenes (eq. 10), which were treated after purification with 46 % aqueous HF to produce (difluoroiodo)arenes (eq. 11):[33]

ArI

+

Cl2

ArICl2

ArICl2

+

2 NaOH

ArIO

+

H 2O

ArIO

+

2 HF

ArIF2

+

H2O

(9) + 2 NaCl

(10) (11)

Introduction

1.3

6

(Tetrafluoroiodo)arenes and (Difluorooxoiodo)arenes

1.3.1 (Tetrafluoroiodo)arenes The chemistry of iodine(V) compounds or λ5-iodanes is substantially less developed in comparison with the chemistry of I(III). Recently there has been an increasing interest in I(V) especially in their fluorinated compounds.[34] Iodine(V) compounds may have the general formula IL5, IZL3, and IZ2L where L is a monovalent and Z a divalent ligand. The bonding system of IL5 can be described in terms of one 2c–2e bond I–Lapical and two orthogonal 3c–4e bonds, accommodating the basal IF2 subunits. In the case of RIF4, the R-ligand is placed in the apical position.[19, 35] The oxidative fluorination of organoiodides can be used to prepare (tetrafluoroiodo)arenes (RIF4). This method produces very often RIF4 in mixtures with (difluoroiodo)arenes, and their separation is difficult. The first reported method for the preparation of ArIF4 used the fluorination of ArI by nitrogen-diluted F2 in CCl3F. In the first step ArI reacts with F2 at –100 °C giving slightly soluble ArIF2 in CCl3F, which can - as far as dissolved - further interact with F2 at –40 °C and form ArIF4.[36-38] Fluorination of iodoarenes with an excess of one of the following fluorinating agents XeF2, ClF3, BrF3, BrF5, C6F5BrF2 and C6F5BrF4 led to the corresponding (tetrafluoroiodo)arene compounds:[27, 30, 37, 39 – 41] 3 ArI

+

-78 - 20 °C

4 ClF3

3 ArIF4

+

2 Cl2

(12)

Another approach to ArIF4 preferentially developed for aryl groups with electronwithdrawing substituents is the nucleophilic substitution on IF5. Arylsilanes and arylmetal compounds of thalium, lead, bismuth, and cadmium have been used:[19a, 42 – 46]

PhSiF3

+

IF5

+

2 Py

Si(C6F5)4

+

4 IF5 + 2 Py

Cd(C6F5)2

+ 2 IF5

PhIF4

+

SiF4 2 Py

(13)

4 C6F5IF4

+

SiF4 2 Py

(14)

2 C6F5IF4

+

CdF2

(15)

ArfIF4 can be produced by electrophilic substitution using the highly electrophilic [IF4]+ cation[47a]. No ArfIF4 was formed by oxidative fluorination between iodoarenes ArfI and IF5 under non-acidic conditions:[47b]

Introduction ArfH

7 [IF4]+

+

ArfIF4

+

H+

(16)

(Tetrafluoroiodo)arenes were obtained in quantitive yield also by heating iodylarenes with sulphur tetrafluoride:[48, 49] ArIO2

+

2 SF4

ArIF4

+

2 SOF2

(17)

(Difluorooxoiodo)arenes react in the same manner with SF4, moreover their use is safer because they are less explosive than iodylarenes:[27, 37, 45, 50] ArIOF2

+

SF4

ArIF4

+

SOF2

(18)

1.3.2 (Difluorooxoiodo)arenes (Difluorooxoiodo)arenes were obtained by dissolving iodylarenes in hot 40 % aqueous hydrofluoric acid:[51 – 53]

ArIO2

+

2 HF

ArIOF2

+

H2O

(19)

Alternative procedures are the reaction of (tetrafluoroiodo)arenes with equivalent amounts of hexamethylsiloxane (eq. 20) or simply with water (eq. 21) or iodylarenes (eq. 22):[49] ArIF4

+

( (CH3)3Si)2O

ArIOF2

+

2 (CH3)3SiF

(20)

ArIF4

+

H2 O

ArIOF2

+

2 HF

(21)

ArIF4

+

ArIO2

2 ArIOF2

1.4

(22)


Iodine pentafluoride, IF5, is the only known binary interhalogen compound of iodine(V). Iodine pentafluoride is a colourless liquid with a melting point of 9.6 °C and a boiling point of 98 °C.

Introduction

8

Iodine pentafluoride is a versatile and well-known fluorinating agent. It can be used, for example, to prepare fluorohydrocarbons and fluoroalkyl sulfides, to form adducts with oxides of nitrogen and to convert metals to fluorides.[56] IF5 was first prepared in 1862 by heating of iodine with silver fluoride:[54] 3 I2

+

IF5

5 AgF

+

5 AgI

(23)

Thirty years later, Moissan reported the direct synthesis using iodine and elemental fluorine.[55] It has been found that iodine(V) fluoride can be prepared by reacting iodine oxygen compounds with sulfur tetrafluoride. Such I-O starting materials are iodine oxides (I2O5), alkali metal iodates (NaIO3, KIO3) and alkaline earth metal iodates (Mg(IO3)2, Ca(IO3)2, Ba(IO3)2). The reactants must be used in anhydrous form, because water reacts as well with sulfur tetrafluoride as with iodine pentafluoride:[56] I2O5

+

2 IF5

5 SF4

+

5 SOF2

(24)

In 1963, Fawcett reported a new method of preparing iodine pentafluoride by fluorinating anhydrous iodine pentaoxide (I2O5) with pure carbonyl fluoride at high temperature:[57] I2O5

+

2 IF5

5 COF2

+

5 CO2

(25)

The reaction between iodine and fluorine is primarily a heterogeneous solid-gas reaction. Because of the high reaction enthalpy iodine sublimates and reacts instantaneously with fluorine in the gas phase. At a temperature above 250 °C IF7 becomes the favoured product. Therefore it is useful in the direct synthesis of IF5 to look for homogeneous and moderate temperature conditions. Principally the presence of an inert solvent may be useful. In the technical process IF5 itself is used as slightly dissolving medium for I2:[58]

I2

+

5 F2

IF5 as solvent 200 - 300 °C

2 IF5

(26)

In a modified method molten iodine was reacted with gaseous fluorine at 114 - 280 °C (eq. 27):[59, 60]

Introduction

I2 (l)

+

9

5 F2 (g)

114 - 280 °C

2 IF5

(27)

Recently, a new industrial process for producing a mixture of perfluoroalkanes and iodine pentafluoride was reported. The reaction of perfluoroalkyl iodide with gaseous fluorine in an inert liquid solvent gives the corresponding perfluoroalkane and iodine pentafluoride. The liquid solvent must be inert toward both fluorine gas and iodine pentafluoride (eq. 28):[61] 2 Rf-I

1.5

+

6 F2

2 IF5

+

2 Rf-F

(28)

Iodonium Salts

According to the conventional classification, iodonium salts contain the positively charged 8–I–2 cation with two carbon ligands bonded to iodine(III) and a negatively charged counter ion, of the general formula R2I+ X–. 1.5.1 Diaryliodonium Salts Diaryliodonium salts belong to the most common, stable and well-investigated class of polyvalent iodine compounds. The methods for synthesising symmetrical and unsymmetrical diaryliodonium salts have been well developed and their properties have been well investigated. The iodosyl compounds and their diacetates are active electrophilic reagents, which readily react with aromatic hydrocarbons to form iodonium salts. This reaction is the most widespread method for the synthesis of unsymmetrical diaryliodonium salts:[62, 63, 64] ArIO

+

Ar'H

+ H2SO4

[ArAr'I][HSO4]

+

H2O

(29)

In fact, this reaction is a two step process. In the first step the aryliodoso compound ArIO reacts with H2SO4 forming the iodonium salt [ArIOH][HSO4] as the reactive electrophile: ArIO

+

H2SO4

[ArIOH][HSO4]

(30)

In the following step the iodonium cation [ArIOH]+ attacks Ar'H and an electrophilic substitution takes place:

Introduction

10

[ArIOH][HSO4]

+

[ArAr'I][HSO4]

Ar'H

+

H2O

(31)

In another approach, potassium iodate was used as precursor of the iodine electrophile for preparing symmetrical iodonium salt:[65]

2 ArH

+ KIO3

+

H2SO4

-0.5 O2

[Ar2I][HSO4] + H2O + KOH

(32)

The reaction mechanism likely involves the formation of iodyl sulfate, which following reacted with ArH, initially to iodylarene (ArIO2). After protonation this reacts with ArH: KIO3 + 2 H2SO4 ArH

+ [IO2][HSO4]

ArH + [ArIOH][HSO4]

[IO2][HSO4] + K[HSO4] + H2O

(33)

[ArIOH][HSO4]

(34)

[Ar2I][HSO4]

+ 0.5 O2 +

H 2O

(35)

Also iodine triacetates react with aromatic hydrocarbons under proton assistance:[65, 66] 2 ArH + I(O2CCF3)3 + H+

[Ar2I]+

+ 3 CF3CO2H

(36)

In a two step reaction, also iodine trichloride can be used as starting material for iodonium salts:[67, 68]

ICl3

+

MR

RICl2

+

MCl

(37)

RICl2

+

MR

[R2I]Cl +

MCl

(38)

A mixture of iodosyl and iodyl compounds undergoes a base-catalyzed transformation forming iodonium iodates:[63, 64, 69]

ArIO2

+

[OH]-

[ArIO3H]-

Ar'IO

+

[ArIO3H]-

[ArAr'I][IO3]

(39) +

[OH]-

(40)

Introduction

11

Other approaches involve the nucleophilic substitution at I(III) using organometallic compounds. By this method bis(pentafluorophenyl)iodonium chloride was prepared from either C6F5Li or (C6F5)2Cd. This approach is also useful for the preparation of unsymmetrical diaryliodonium salt:[16, 19, 70] (C6F5)2Cd

+

2 C6F5ICl2

2 [(C6F5)2I]Cl

+

CdCl2

(41)

C6F5Li

+

C6F5ICl2

[(C6F5)2I]Cl

+

LiCl

(42)

Arylsilanes can be used for introducing aryl groups into IF3. In acidic medium the iodonium cation is favoured over ArIF2:[16, 19, 70] [Ar2I][BF4]

2 ArSiF3 + IF3 + BF3

+

2 SiF4

(43)

Several new approaches to prepare aryliodonium salts were developed in the past few years. Ochiai and co-workers reported a new efficient regioselective synthesis of diaryliodonium tetraarylborates by the reaction of (diacetoxyiodo)arenes with sodium or potassium tetraarylborates in acetic acid:[71]

ArI(OAc)2

+

2 M[BAr'4]

HOAc

[ArAr'I][BAr'4]

+ 2 MOAc

+ BAr´3

(44)

In a regioselective manner, a variety of diaryliodonium and heteroaryliodonium sulfonates were prepared from (diacetoxyiodo)benzene and readily available aryl boronic acids:[72]

ArI(OAc)2

+

Ar'B(OH)2

HX

[ArAr'I][X] + HOAc + B(OH)2(OAc)

(45)

Also ligand-transfer reactions are reported in literature:[73] [trans-TfO(Pr)CC(Pr)(Ph)I][OTf] + ArLi

[Ar(ph)I][OTf] + Li+ + Pr C C Pr (46)

Introduction

12

1.5.2 Alkenyl(aryl)iodonium Salts Alkenyliodonium salts are highly reactive compounds. Only a few of these salts have been known. In the last two decades such compounds have become available and their chemistry was developed. Meanwhile alkenyliodonium salts were used for synthetic applications. Alkenyliodonium salts can be obtained either by reaction of iodine(III) species with activated alkenes or by nucleophilic addition to the triple bond. The first general method for preparing alkenyliodonium compounds was reported by Ochiai and coworkers:[74, 75] R1 R2

SiMe3 R3

+

PhIO

[Et3O][BF4]

R1

IPh

- Me3SiOEt

R2

R3

BF4

(47)

In eq. 48, starting materials of the iodonium type were used. This procedure allowed the stereospecific synthesis of alkenyliodonium salts with retention of the configuration in the olefinic part:[76]

R1

SnBu3

R2

H

+ [Ph(CN)I][OTf]

R1

IPh

R2

H

OTf + Bu3SnCN (48)

Alkenyliodonium salts can be prepared also from the reaction of alkenyl boronic acids or esters with (diacetoxyiodo)arenes, again with retention of the configuration:[77]

R1

H

R2

B(OH)2

+ PhI(OAc)2

BF3 Et2O, Na[BF4], H2O

- NaOAc, - B(OH)2(OAc)

R1

H

R2

IPh

BF4

(49)

Alkenyl(aryl)iodonium salts can be obtained also by treating alkenylzirconium derivatives with (diacetoxyiodo)arenes:[78]

R

H

H

ZrCp2Cl

+ PhI(OAc)2

THF, Na[BF4], H2O

R

H

-NaOAc, -ZrCp2Cl(OAc)

H

IPh

BF4

(50)

The electrophilic addition of aryliodosyl compounds to alkynes leads to alkenyliodonium salts:[79, 80, 81]

Introduction

RC CH

13

+

PhIO

CF3SO3H

R

-H2O

IPh

TfO

H

OTf

(51)

The nucleophilic addition of halides X- to alkynyliodonium salts in acidic medium resulted in Z-ß-X-substituted alkenyliodonium salts:[82, 83]

RC CIPh BF4

+

LiX

AcOH

X = F, Cl, Br

X

IPh

R

H

BF4 + LiOAc

(52)

Recently, one example of Z and E-ß-fluorine substituted alkenyliodonium salts was reported a) by the addition of aqueous hydrofluoric acid to alkynyliodonium salts (eq. 53)[84] and b) by mean of the reactive (difluoroiodo)toluene to mono-substituted alkynyl compounds (eq. 54)[85], respectively.

RC CIPh BF4

RC CH

+ HFaq

+

p-TolIF2

Et3N 5 HF

F

IPh

R

H

R

I(F)Tol-p

F

H

BF4

(53)

(54)

Objectives

2

Objectives

2.1

Preparative Aspects

14

Although some of the iodine fluorides like IF5 and its organo derivatives like ArIF4, ArIOF2, and ArIF2 are known over a long time, their preparation and practical application are still under investigation.[21, 35, 45, 50, 54]

2.1.1 Iodine Pentafluoride Iodine pentafluoride, IF5, is the only known binary interhalogen compound of iodine(V). It is industrially used in the telomerisation of CF2=CF2 and in synthetic chemistry as oxidising and fluorinating agent. The direct reaction of the elements - the only practically applied version - is accompanied by a lot of problems, mainly resulting from the fact that fluorine is a very reactive gas. Resulting IF5 dissolves I2 whereas F2 is not good soluble in IF5. Therefore it is useful to blow F2 through the liquid IF5 phase still containing I2 in the final stage and feed the gas of the outlet back into a reactor operating in a medium or initial reaction stage. An important further demand is to avoid local excess of F2 combined with temperatures above ca. 300 °C, because under such conditions IF7 is formed.[59] Such a processes require specific experience and precaution because of the highly corrosive nature of the reactants and the product, and the vigorous mode of the strong exothermic reaction. The disproportionation of I2 in the presence of AgF is too expensive and the oxygen-fluorine substitution reaction using an excess of sulfur tetrafluoride or carbonyl fluoride at high temperature under pressure demands autoclave techniques for corrosive materials and highest safety standards for the gaseous, highly poisoning and expensive starting materials SF4 and COF2. To overcome the above mentioned disadvantages it is necessary to develop a real alternative process based on a new methodology. This means concretely for the preparation of iodine pentafluoride to avoid expensive reagents like fluorine and utilise a minimum of corrosionresistant equipment. More common, the fundamental basis for an industrially applicable new method for preparing iodine pentafluoride should be investigated and developed in the present work.

Objectives

15

2.1.2 (Tetrafluoroiodo)arenes Until now (tetrafluoroiodo)arenes were used as versatile fluorination reagents only for academic purposes. The methods for preparing ArIF4 are limited by the nucleophilic substitution at IF5

[19, 42 – 45]

or by the fluorination of iodoarenes with an excess of one of the

following fluorinating agents XeF2, ClF3, BrF3, BrF5, C6F5BrF2 and C6F5BrF4

[30, 37, 39 – 41]

or [47,

by reacting iodylarenes with sulphur tetrafluoride under pressure and elevated temperatures. 48]

A disadvantage of the above mentioned fluorinating methods are the hard to prepare and

difficult to handle reagents. One of the tasks of this work was the adaption of oxygen-fluorine substitution to the preparation of (tetrafluoroiodo)arenes with the clear aim to isolate pure products.

2.1.3 (Difluorooxoiodo)arenes (Difluorooxoiodo)arenes are a class of compounds intermediate between the class of (iodyl)and (tetrafluoroiodo)arenes. They have no known practical application. The chemistry of (difluorooxoiodo)arenes is substantially less developed in comparison to ArIO2 and ArIF4. The known methods of preparation are either the oxygen-fluorine substitution on iodylarenes, ArIO2,[51

–

53]

or

the

partial

hydrolysis

or

fluorine-oxygen

substitution

on

(tetrafluoroiodo)arenes, ArIF4.[49] In the first process, the product could not be isolated and was used for further reactions without purification. The main disadvantage of the second procedure is the use of non easy available ArIF4 as starting material. In this work the preparation and isolation of a pure (difluorooxoiodo)arene will be investigated by the aimed oxygen-fluorine substitution.

2.1.4 (Difluoroiodo)arenes (Difluoroiodo)arenes are potential fluorinating reagents for alkenes, alkynes, carbonyl compounds, organo sulfur compounds, and iodoalkanes. But they are not yet practically applied, due to the lack of convenient and cheap methods of preparation. Recently, there has been

an

increasing

interest

in

the

preparation,

reactivity,

and

application

of

(difluoroiodo)arenes. Generally, eight methods for the preparation of (difluoroiodo)arenes are known from literature. The oldest method of Dimroth and Bockemüller[21] or the modified one of Garvey[22] is characterised by the treatment of the corresponding aryliodoso compound with 46 % HF in acetic acid. The product was used without separation or isolation. The method of

Objectives

16

Carpenter[23] involves a one-step reaction of (dichloroiodo)arenes with aqueous hydrofluoric acid and mercuric oxide in methylene chloride. The product was used directly without isolation or purification. The fluorination of iodoarenes with XeF2 was described by Zupan[30] or with F2 by Ruppert[32] and Naumann.[31] Beside these general methods, electrochemical oxidation reactions of para-substituted iodoarenes were investigated in the presence of Et3N ⋅ n HF,[25, 26] and of iodobenzene in acetonitrile solution in the presence of AgF.[24] In 2002, Hara and coworkers[33] reported a modified method based on the Carpenter method, in which para-iodobenzene derivatives were chlorinated, subsequently hydrolysed with aqueous NaOH, and treated with aqueous HF. The main disadvantage of all prior mentioned preparative methods is the difficulty of isolating a pure product or the use of expensive or uncommon fluorinating agents like F2, SF4, ClF3, BrF5, C6F5BrF2, or XeF2. Additionally, in the oxidative fluorinating processes an excess or even a local excess of the fluorinating agent produces by-products such as (tetrafluoroiodo)arenes or oxidative addition products of the aryl fragment. Although several methods have been reported for the preparation of (difluoroiodo)arenes, there is still a need of a convenient and cheap preparative method. Therefore the preparation and isolation of pure (difluoroiodo)arenes with good available reagents and a convenient and easy method to scale up will be investigated and developed in this work.

2.1.5 Iodonium Salts Polyfluoroorganodifluoroboranes are unique reagents for the introduction of polyfluoroorgano groups (alkynyl, alkenyl, and aryl) into XeF2.[89 corresponding

polyfluoroorganoxenonium

– 91]

Under these acidic conditions the

tetrafluoroborates

and

in

few

cases

polyfluoroorganoxenonium polyfluoroorganotrifluoroborates were obtained. trans-2-XCF=CFBF2 showed a differentiated reactivity, depending on the nature of X. X = H, F, and Cl underwent xenodeboration whereas X = CF3, C4F9, C4H9, and Et3Si formed no Xe-C compounds. It should be mentioned that cis-X-CF=CFBF2 (X = CF3 and C2F5) underwent xenodeboration. The before summarised results can not be rationalised by electronic effects. Instead of this the steric aspects in the transition state was discussed.[91d,e] We were interested to find out if the non-reactivity of trans-RfCF=CFBF2 towards XeF2 could be generalised for other hypervalent F-E-F triads. Therefore we decided to investigate the reactivity of the related hypervalent IF2 group in ArIF2 with trans-CF3CF=CFBF2.

Objectives

17

Pentafluoro(difluoroiodo)benzene is considered as a stronger Lewis acid than the corresponding non-fluorinated and monofluorinated (difluoroiodo)arenes, therefore we were interested to investigate the reactivity of the hypervalent IF2 group under the specific electronic influence of the C6F5 and C6H4F group with perfluoroalkenylboranes, especially trans-CF3CF=CFBF2. Additionally, in this work unsymmetric diaryliodonium and perfluoroalkenyl(aryl)iodonium salts are objects of investigation. The chemistry of alkenyldifluoroboranes is generally well established. Surprisingly, there are only few communications concerning the preparation and reactivity of perfluoroalkenyl difluoroboranes, which were recently reported by Frohn et al.[87] Generally, perfluoroalkenyl(phenyl)iodonium salts are promising electrophilic perfluoroalkenylating agents. They should be able to react with nucleophiles under transfer of the perfluoroalkenyl group. Thus they provide a useful route for the synthesis of substituted alkenes. The general methods for the preparation of alkenyl(phenyl)iodonium salts are the reaction of λ3-iodanes with alkenylmetal compound.[74 – 78] For this investigation, the polyfluorinated trans-alkenyldifluoroborane and corresponding borate salts (K[trans-CF3CF=CFBF3], Li[trans-CF3CF=CFB(OMe)3]) will be prepared for the first time in this work, while the perfluorovinylborane (CF2=CFBF2) was recently reported by this group.[92, 93]

2.2

Reactivity, Structure, and Spectroscopy

The high polarisation of the I-F bond in the IF2 group in (difluoroiodo)arenes encourages to investigate their reactivity with Lewis acids as well as with Lewis bases. The knowledge of influences which govern the intermolecular interactions in the crystalline state of (difluoroiodo)arenes is still limited. Most systematic information derived from C6F5IF2 with a very strong electron-withdrawing organo group. Here the structure revealed strong intermolecular contacts between the positively charged iodine centre and negatively charged fluorine atoms of the IF2 group of neighbour molecules. This contacts lead to a chain arrangement in the crystal. The dependence of chemical shift values in NMR spectroscopic

Objectives

18

studies on pentafluoro(difluoroiodo)benzene solutions from the basicity of the solvent stressed the coordination ability at the partial positive charged iodine centre.[47b] It will be desirable to obtain structural as well as spectroscopic information of (difluoroiodo)arenes with less electron-withdrawing aryl groups to prove the general feature of the intermolecular interactions. Very recently, an experimental study has shown that C6F5IF2 with its strong electrophilic centre is able to coordinate suitable nucleophiles.[87] It is expected that (difluoroiodo)arenes with less electron-withdrawing aryl groups will show a similar behaviour. Therefore the coordination of basic reactants on (difluoroiodo)arenes will be examined. The fluoride acceptor ability of (difluoroiodo)arenes marks a special case of the above mentioned reactivity. Schrinner has reported the existence of an unstable fluoride adduct of C6F5IF2 and/or a fluoride-bridged adduct of C6F5IF2.[88] The lower fugalibility of less electron-withdrawing aryl groups compared with the C6F5 group prompted us to examine the reaction of (difluoroiodo)benzenes with fluoride sources. Until now the class of perfluoroalkenyl(aryl)iodonium salts is still unknown. For this class, a high effective positive charge on the iodine centre is expected, which will influence the reactivity and stability of this class. Beside the preparation of prototypes of perfluoroalkenyl(aryl)iodonium salts, it is aim of this work to characterise their spectroscopic and structural features. Furthermore the electronic influence (σI- and σR-values) of perfluoroalkenyliodine(+) substituents will be determined using perfluoroalkenyl(aryl)iodonium salts.


19

3


3.1

Preparation of Iodine Pentafluoride (IF5) by a New Methodological Approach

3.1.1

Introduction

Several methods for preparing iodine pentafluoride have been reported up to day (Scheme 1).[54-61] The transformation of iodine-oxygen bonds to iodine-fluorine bonds, however, has not yet been satisfactory investigated, despite of the industrial application potential of IF5 and its use as reagent in organic synthesis. Therefore its preparation under an optimised new approach is an important goal. I2

+

I-source

+

I2O5

+

2 IF5

5 F2

IF5

fluorooxidisers SF4

2 IF5

or COF2

fluorooxidisers = XeF2, ClF3, BrF3, BrF5, ...

Scheme 1: Known approaches to IF5.

3.1.2

Relevant Reactivities of I(V)-F and I(V)-O Bond

The Lewis-acidic nature of IF5, due to the comparatively high positive charge on iodine(V), enables the easy contact to bases, even to oxygen of the water molecule. The interaction with water results in the substitution of two fluorine atoms of a ≡IF2 fragment and formation of one ≡I=O bond (highly exothermic reaction). If enough water is offered, IF5 is converted to iodic acid via the intermediates IOF3 and IO2F (Scheme 2).

IF2 IF5

+

H2O IOF3

Scheme 2: The reactivity of I-F bonds towards water.

I IO2F

O

+ 2 HF HIO3


20

The hydrolysis of iodine pentafluoride is principally an equilibrium reaction. We were interested to use this reaction in the opposite direction. We have found that 48 % aqueous HF is not a suitable reagent to obtain IF5 from I2O5. Even anhydrous hydrogen fluoride does not allow the separating of IF5 from the reaction mixture by distillation or crystallisation (Scheme 3). I2O5

+

10 HF (48 %)

2 IF5

+

5 H2O

I2O5

+

10 HF (aHF)

2 IF5

+

5 H2O

Scheme 3: The transformation of I–O bonds to I–F bonds.

3.1.3

The Reaction of I(V)-O Compounds with aHF in a Two Phase System

We have developed a solution for the previously mentioned problem of generating I–F bonds from the corresponding I–O bonds.[94] Our methodical approach to prepare IF5 starts from I(V)-O compounds with aHF in a two phase system (Scheme 4). I2O5 or NaIO3 were used as I(V)-O starting materials, suspended in a non-basic, polar and with HF / H2O immiscible solvent such as dichloromethane. In a fast reaction the I(V)-O starting material formed IF5 at –30 °C in a good yield (72 %). The IF5-CH2Cl2 phase was collected as upper colourless phase.

MxIyOz

aHF, solv.a

y IF5

+

z H2O

+

x MF

x = 1, y = 1, z = 3, M = Na, x = 0, y = 2, z = 5 a immiscible with HF / H2O like CH2Cl2 Scheme 4: The general route to IF5 starting from I–O compounds.

3.1.4

The Important Steps in the Preparation of IF5

It is assumed that the formation of iodine pentafluoride proceeds via the two known intermediates. The first intermediate is IO2F which is insoluble in organic solvents because it is strongly associated. By the same reason the intermediate IOF3 is also not transferred into


21

the organic phase. IOF3 is allowed to react with further HF to form the less associated IF5 molecule which distributes preferentially into the organic solvent phase. After separation of the organic phase, IF5 can be isolated from the CH2Cl2 solution (Scheme 5). IO3-

+

2 HF

IO2F

+

H2O

+

F-

(1)

IO2F : insoluble in polar organic solvents; strongly associated: (IO2F)n IO2F

+

2 HF

IOF3

+

H2O

(2)

IOF3 : insoluble in nonbasic organic solvents; strongly associated: (IOF3)n IOF3

+ 2 HF

IF5

+

H2 O

(3)

IF5 : weakly associated in polar organic solvents; soluble in polar solvents

Scheme 5: The main steps of the preparation of IF5.

3.1.5

The Influence of the HF Concentration on the IF5 Formation

IF5 was obtained in good yield when I2O5 or iodates M[IO3] reacted with aHF in the two phase system HF / CH2Cl2 at low temperature of –30 °C (sec. 3.1). To get more insight into this reaction the content of water in HF was systematically varied. Figure 3 compiles the result for HF with starting concentrations from 53 % to 100 % and shows that the yield of iodine pentafluoride determined in the CH2Cl2 phase strongly decreases with the increase of the water content in HF. Additionally to the HF concentration the reaction temperature was varied in the range from –70 to –1 °C. Generally, a slight increase in the yield of IF5 was found with increasing temperature. The deviation obtained with 82 % HFaq at –1 °C may be an experimental error. The before mentioned tendencies to form IF5 cannot simply be explained by the law of Le Chatelier. Due to the higher exothermic character of the hydrolysis of IF5 the formation of IF5 should be favoured at lower temperatures. Our experiment finding in producing IF5 is in agreement with this prognosis.


22

But there are also other factors which should be considered, e.g. the solubility of HF or HFaq in CH2Cl2 (table 28, appendix). The solubility of HF and HFaq in CH2Cl2 increases with temperature. Hydrolysis of IF5 in CH2Cl2 is “protected” by this fact. It should be mentioned that the amount of IF5 in the CH2Cl2 phase was determined by

19

F

NMR using the internal standard C6F6. For the standard it was assumed that the distribution of the standard in HF or HFaq was negligible and that the temperature had no considerable influence on the distribution of the standard.

100

100 % HF

Yield of IF5 (%)

90 80

95 % HF

70

90 % HF

60 50 82 % HF

40 30 70 % HF

20 10

65 % HF 53 % HF

0 -70

-60

-50

-40

-30

-20

-10

0

Temp. (°C)

Figure 3: The influence of the HF concentration (concentration of HF introduced in the experiment) on the yield of IF5 in the CH2Cl2 phase at different reaction temperatures.


3.2

23

4-Fluoro-1-(tetrafluoroiodo)benzene by Oxygen-Fluorine Substitution

The preparation of (tetrafluoroiodo)arenes by fluorine addition to the corresponding iodoarenes is a not easy process. Fluorinating agents used for this purpose are xenon difluoride, elemental fluorine or chlorine trifluoride. The use of this fluorooxidisers under mild conditions (low temperature) ends preferentially with (difluoroiodo)arenes and under harder conditions the cleavage of the C-I bond and the fluorination of the aryl group is a competing reaction channel. When iodylarenes were treated with 48 % HF or 70 % HF the desired product was not obtained, but with an excess of aHF in the presence of dichloromethane the product was produced at < 20 °C in good yield (63 %).[94] In the two phase reaction system, the target product was distributed preferentially into the organic solvent phase (eq. 55). After separation of the organic phase, and evaporation of the solvent p-C6H4FIF4 could be isolated as a white crystalline solid melting at 89 °C without decomposition.

F

IO2

+

4 HF

CH2Cl2 -30 °C

F

IF4

+

2 H 2O

(55)

For the reaction of ArIO2 with HF the pathway is offered in Scheme 6. The formation of the target compound is described via four intermediates. In the first step HF is added to the polar I=O bond forming ArIO(OH)F which has best preconditions to associate via hydrogen bridges and I=O⋅⋅⋅I contacts. This intermediate can in a subsequent step add a second HF molecule. ArI(OH)2F2 may dehydrate to ArIOF2 or undergo hydroxy-fluorine substitution. The path to ArI(OH)F3 is not unambiguous clear as the OH–F substitution in ArIOF2, because the addition of one HF molecule on ArIOF2 leads to the same intermediate. The final step, the hydroxy-fluorine substitution produces ArIF4. It should be noted that under this strong protic conditions no cleavage of the C-I bond was observed despite of the high polarity and the negative partial charge on C(1).


ArIO2

+

H+

24

+

F-

OH Ar

F

I O F Ar I

F O

+

+

H+

H+

+ F-

+

F-

Ar F I F + F OH

H+

+ F-

CH2Cl2 HF / H2O CH2Cl2 HF / H2O CH2Cl2 HF / H2O CH2Cl2 HF / H2O

OH Ar

(1)

I O F Ar

Ar

F I OH F OH

F

I

F O

(2)

Ar F I F OH F

+

H2 O

(3)

+

H2O

(4)

Ar F I F

F F

Scheme 6: The main steps from ArIO2 to ArIF4.

3.3

4-Fluoro-1-(difluorooxoiodo)benzene (p-C6H4FIOF2) by Treatment of 4-Fluoro-iodylbenzene (p-C6H4FIO2) with Hydrofluoric Acid

The synthesis of (difluorooxoiodo)arenes has been reported as an intermediate compound and the product was used directly without separation. The preparation proceeded from iodylarenes (ArIO2), but no reaction details or yield are given.[51 – 53] 4-Fluoro(difluorooxoiodo)benzene was easily prepared from the corresponding iodylarene by treatment with aqueous 48 % HF at 70 °C similar to.[94] When the reaction was carried out at 20 °C, no reaction could be observed. p-Fluoro-(difluorooxoiodo)benzene was obtained in quantitative yield as a white crystalline solid and decomposed at 192 °C. The reaction of ArIO2 with aqueous 48 % HF could proceed analogue to the above mentioned pathway (Scheme 6).


3.4

25

(Difluoroiodo)arenes (ArIF2) by Oxygen-Fluorine Substitution on ArIO with Hydrofluoric Acid as Reagent

In contrast to the intensively investigated (dichloroiodo)arenes only a smaller number of papers deals with (difluoroiodo)arenes. The preparation of pure (difluoroiodo)arenes and the isolation from mixtures in glass equipment involved the main problem. Therefore it is still an attractive task to develop a convenient process for preparing (difluoroiodo)arenes. Our concept

started

with

a

literature

procedure

for

forming

the

corresponding

(diacetoxyiodo)arenes (ArI(OAc)2) and subsequent treatment with aqueous NaOH provided iodosylarenes (ArIO) in high yield (70 - 92 %). Oxygen-fluorine substitution on iodosylarenes, suspended in methylene chloride, with aqueous 48 % HF at 20 °C ended with (difluoroiodo)arenes free of impurities in good yield (81 - 91 %).[94] In the two-phase reaction the product (difluoroiodo)arene was distributed preferentially in the organic solvent phase, which could be easily separated and after evaporation of the solvent the white crystalline solid could be isolated (Scheme 7). (Difluoroiodo)arenes dissolve in CH2Cl2 and CH3CN without reaction but in protic solvent like MeOH and AcOH a nucleophilic substitution proceeded which ended finally with ArIY2 (Y = OMe, OAc).

ArI

CH3CO3H

ArI(OAc)2

NaOH aq

ArIO

HF aq / CH2Cl2 - H2 O

ArIF2

Ar = o, m, p-C6H4F

Scheme 7: A convenient and general approach to (difluoroiodo)arenes. Parallel to our work, Hara offered an alternative method for the preparation of (difluoroiodo)arenes. In the first step he oxidised aryliodides by chlorination. This step was followed by hydrolysis of (dichloroiodo)arenes and finally by treatment of the resulting iodosylarenes with aqueous hydrogen fluoride in dichloromethane.[33] It is assumed that in the oxygen-fluorine substitution process the formation of (difluoroiodo)arenes proceeds via the intermediate ArI(OH)F, which is insoluble in solvents like CH2Cl2 because being probable associated. The intermediate ArI(OH)F is able to react with further HF to form fast the weakly associated (difluoroiodo)arenes which distribute preferentially into the organic solvent phase (Scheme 8).

Results and Discussion O Ar

I

+

+

OH Ar I F

H

+

+

H

26

+

+

F

F

OH Ar I F

-

-

Ar

F I F

+

H2O

Scheme 8: The main steps in the formation of (difluoroiodo)arenes.

The Influence of the HF Concentration on the Formation of (Difluoroiodo)arenes (ArIF2) When p-C6H4FIO in the presence of CH2Cl2 was treated with HF of different concentrations (aHF to 24 % HFaq) two liquid phases were formed and the yield of p-C6H4IF2 in the CH2Cl2 phase was determined by

19

F NMR using C6F6 as internal standard. The temperature was

varied between –70 °C and 35 °C. With aHF as starting reagent from –70 °C to –1 °C a low yield of p-C6H4FIF2 of only ca. 40 % was found. The yield decreased significantly when the temperature was raised to 35 °C (13.5 %). Increasing amounts of water in the reagent HFaq (24 % – 80 % HFaq) increased the yield of the product up to 91.5 %. In case of HF-H2O mixtures no significant influence of the temperature was found in contrast to aHF. At present time we can only formulate assumptions to explain this unexpected reactivity. In comparison to IF5 the I-F bond in p-C6H4FIF2 is less polar and more easy polarisable. Higher proton activity in the reagent aHF may polarise and ionise ArIF2 according to eqs. 56 and 57.

p-C6H4FIF2

+

HF

p-C6H4FIF+

+ [FHF]-

(56a)

p-C6H4FIF2

+ 2 HF

p-C6H4FI2+

+ 2 [FHF]-

(56b)

p-C6H4FIF+

+ x HF

[p-C6H4FIF(FH)x]+

(57a)

p-C6H4FI2+

+ n HF

[p-C6H4FI(FH)n]2+

(57b)

Eqs. 56 and 57 show a borderline description with a ionic product which should be less soluble in CH2Cl2.


27

100

Yield of p -C6 H4 FIF2 (%)

90 80 70 60

70 % HF

50

80 % HF

40 30

aHF

20 10 0 -70

-60

-50

-40

-30

-20

-10

0

10

20

30

Temp. (°C)

Figure 4: The influence of temperature and HF concentration (aHF - 70 %) on the yield of p-C6H4FIF2 in the reaction of ArIO with HF.

100

24 % HF 95

32 % HF

Yield of p -C6 H4 FIF2 (%)

90 85

48 % HF

80 75 70 65 60

-40

-30

-20

-10

0

10

20

30

Temp. (°C)

Figure 5:

The effect of HF concentration (48 % - 24 %) on the yield of p-C6H4FIF2 at different temperatures.


3.5

28

A Convenient Route to (Difluoroiodo)benzenes (ArIF2) Directly from (Diacetoxyiodo)benzenes

After the successful conversion of ArIO (Ar = o-, m-, p-C6H4F) with only one portion of aqueous HF (48 %) in dichloromethane at 20 °C, it was interesting to apply the same method directly on ArI(OAc)2. Treatment of ArI(OAc)2 with ca. 10 time excess of aqueous HF (48 %) in dichloromethane at 20 °C gave two solution phases. The

19

F NMR spectra of the

dichloromethane phase showed no complete conversion of ArI(OAc)2 to ArIF2 (table 23, sec. 4.2.5). Repeated separation and treatment of the CH2Cl2 phase with fresh portions of aqueous HF (48 %) each in a stoichmetric excess gave the pure product ArIF2 with a yield of 54 – 77 % (Scheme 9).

ArI(OAc)2

+

HF aq

CH2Cl2

Ar = o-, m-, p-C6H4F

ArIF2

+

2 HOAc

Scheme 9: The overall reaction of ArI(OAc)2 with HFaq. The formation of ArIF2 proceeds via the known intermediate ArI(F)OAc (sec. 3.7.1.1). ArI(F)OAc is soluble in polar organic solvents. ArI(F)OAc reacted with further HF to form the ArIF2 molecule with the symmetric hypervalent IF2 unit which is distributed preferentially into the polar organic solvent phase. Repeated treatment of the organic phase with HF led to the complete conversion of ArI(OAc)2. After removal of the polar organic solvent ArIF2 could be isolated (Scheme 10). Ac Ar

I O

Ac Ar

Ac

O +

H+

+

+

+

F-

+

-

Ar

O I F

+

HOAc

Ac

O I F

CH2Cl2

H

F

CH2Cl2

F Ar

I F

Scheme 10: The main steps in the preparation of ArIF2 from ArI(OAc)2.

+

HOAc


29

Acyloxy-fluorine substitution on (diacetoxyiodo)arenes is less favoured than oxygen-fluorine substitution on ArIO because the starting material as well as the intermediate ArI(F)OAc and the co-product HOAc are soluble in the product phase CH2Cl2. HOAc in the CH2Cl2 phase may attack the product ArIF2 and initiated the back reaction. In the case of ArIO as starting material no problem resulted from water, which is practically immiscible with dichloromethane (sec. 3.4).


30

3.6

Iodonium Salts

3.6.1

The Synthesis of Diaryliodonium Salts Starting from (Difluoroiodo)arenes

Aryl(phenyl)iodonium tetrafluoroborates [Ar(C6H5)I][BF4] were prepared quantitatively by the reaction of (difluoroiodo)arenes (Ar = o-, m-, and p-C6H4F) with phenyldifluoroborane at –50 °C in CH2Cl2 as white solids (eq. 58). The resulting aryl(phenyl)iodonium tetrafluoroborates are soluble in polar, non-coordinating solvents like CH2Cl2. Generally, diaryliodonium tetrafluoroborates are stable at room temperature for a long time when stored as solids under dry argon.

ArIF2

+

CH2Cl2 -50 °C

C6H5BF2

[Ar(C6H5)I][BF4]

(58)


The nucleophilic fluoro-aryl substitution is associated with the cleavage of the carbon(sp2)boron bond on one side and the iodine-fluorine bond on the other side and will be explained as follows: the hypervalent (F-I-F, 3c-4e) bond is considered to have a large electrostatic component. Thus the terminal fluorine atoms possess a high basicity and thereby a preferred reactivity towards Lewis acids. In the Lewis acid-base reaction between ArIF2 and C6H5BF2 the iodine(III) centre becomes more electrophilic and parallel the nucleophilicity of the phenyl group (pyramidalisation of boron) arises (Scheme 11). Such a four-centre intermediate state assists the migration of the phenyl group from boron to iodine(III). Subsequently the effective Lewis acid BF3 abstracts the remaining fluorine atom at iodine in the intermediate [Ar(C6H5)I]F and the iodonium cation is formed beside the [BF4]– anion. The last step is favoured mainly by the win of lattice energy. F I F

F F

B F

Scheme 11: The interaction of ArIF2 with C6H5BF2 under fluorine-aryl substitution at ArIF2.

Results and Discussion 3.6.2

31

The Synthesis of Alkenyl(aryl)iodonium Salts Starting from (Difluoroiodo)arenes

In the course of this study we were interested to apply the before discussed method for the preparation of perfluoroalkenyl(aryl)iodonium tetrafluoroborate salts. Our preparative approach

was

based

on

the

interaction

of

perfluoroalkenyldifluoroboranes

with

(difluoroiodo)arenes. Our target products, perfluoroalkenyl(aryl)iodonium tetrafluoroborate salts, with a highly electrophilic iodine(III) centre, offer the perspective to investigate interactions with nucleophiles. The long-term goal of this investigation is the stereospecific electrophilic

transfer

of

perfluoroalkenyl

groups

(trans/cis-RfCF=CF)

to

selected

nucleophiles.

3.6.2.1


The colourless starting materials formed a yellow suspension when molar equivalents of (difluoroiodo)arenes reacted with the strong Lewis acid trans-1,2,3,3,3-pentafluoroprop-1enyldifluoroborane in methylene chloride at –60 °C to give trans-1,2,3,3,3-pentafluoroprop-1enyl(aryl)iodonium tetrafluoroborates in high yield (75 - 86 %) (Scheme 12). It is worth to note that the product of the reaction of trans-1,2,3,3,3-pentafluoroprop-1-enyldifluoroborane with (difluoroiodo)arenes affords the alkenyl(aryl)iodonium salt stereospecifically with retention of the configuration of the alkenyl group. Iodonium salts with the trans-CF3CF=CFBF3 anion are unstable when stored for a longer time, both in solution or even as solids, even at low temperature (–70 °C). This property may be explained by a nucleophilic interaction of the trans-CF3CF=CF group of the fluoroborate anion (not proved by an independent experiment). Additionally, it is advisable to run the reaction below –60 °C because trans-1,2,3,3,3-pentafluoroprop-1-enyldifluoroborane has a high vapour pressure and furthermore it is not recommended to store the iodonium salts as solution more than one day at ambient temperature. All trans-1,2,3,3,3-pentafluoroprop-1enyl(aryl)iodonium tetrafluoroborates were slightly soluble in polar non-basic solvents like CH2Cl2.


ArIF2

+

32

trans-CF3CF=CFBF2

CH2Cl2

[Ar(trans-CF3CF=CF)I][BF4]

-60 °C


Scheme 12: The preparation of trans-1,2,3,3,3-pentafluoroprop-1-enyl(aryl)iodonium tetrafluoroborates. The formation of the target compounds proceeded via an acid-base interaction between basic fluorine atoms of the IF2 group and the acidic borane. The intermediate formed alkenyl(aryl)iodinefluoride [Ar(trans-CF3CF=CF)I]F reacts with BF3 rapidly under abstraction of fluoride and formation of trans-1,2,3,3,3-pentafluoroprop-1-enyl(aryl)iodonium tetrafluoroborate (Scheme 13).

F F 1. I F

F

B 2. F

F

B

F

F

F

I

CF3

F

F

F

F

CF3

F


F B F I

F F

F

F F

CF3

Scheme 13: The interaction of ArIF2 and trans-CF3CF=CFBF2 under fluorine-alkenyl substitution of ArIF2.

Results and Discussion 3.6.2.2

33

trans-1,2,3,3,3-Pentafluoroprop-1-enyl(pentafluorophenyl)iodonium Tetrafluoroborate

The

formation

of

trans-1,2,3,3,3-pentafluoroprop-1-enyl(pentafluorophenyl)iodonium

tetrafluoroborate started at above –60 °C. The resulting insoluble white solid product was isolated easily from the mother liquor at 20 °C in a high yield (92 %). Predictably, the above result demonstrates that the abstraction of a fluorine atom from the IF2 group in C6F5IF2 is more difficult compared with C6H4FIF2 due to higher positive partial charge on iodine. On the other hand, the salt [C6F5(trans-CF3CF=CF)I][BF4] is insoluble in the polar non-basic solvent CH2Cl2 but good soluble in the basic solvents CH3CN and CH3NO2. The insolubility in CH2Cl2 may reflect a strong contact between the cation and anion. Additionally, this iodonium salt is stable at 20 °C under a dry argon atmosphere for more than two months. trans-1,2,3,3,3-Pentafluoroprop-1-enyl(pentafluorophenyl)iodonium tetrafluoroborate melts at 160 – 162 °C. Tonset in DSC was determined to 161.2 °C (endothermic).

3.6.2.3

Preparation of Trifluorovinyl(fluorophenyl)iodonium Tetrafluoroborates

Trifluorovinyl(monofluorophenyl)iodonium tetrafluoroborate salts were formed in high yields (78 - 94 %) by reaction of equimolar amounts of (difluoroiodo)arenes with perfluorovinyldifluoroborane CF2=CFBF2 at –60 °C (Scheme 14). The isomeric trifluorovinyl(monofluorophenyl)iodonium salts were soluble in the polar non-basic organic solvent CH2Cl2 but they were insoluble in non-polar organic solvents like n-pentane. The trifluorovinyl(monofluorophenyl)iodonium tetrafluoroborate salts were unstable when stored for a longer time, both in solution and in the solid-state, even at low temperature (–70 °C). It is not recommended to store the iodonium salt in solution more than one day. The instability may be caused by the fact that the CF2=CF group will be eliminated as electrophile. It is advisable to run the reaction below –60 °C because trifluorovinyldifluoroborane has a high vapour pressure. The constitution of the alkenyl(aryl)iodonium salts was proved by 1H, 13C, 11B and 19

F NMR spectroscopy.

ArIF2

+

CF2=CFBF2

CH2Cl2 -60 °C

[Ar(CF2=CF)I][BF4]

Ar = o-, m-, p-C6H4F Scheme 14: The preparation of trifluorovinyl(fluorophenyl)iodonium tetrafluoroborates.


34

Preparation of Trifluorovinyl(pentafluorophenyl)iodonium Tetrafluoroborate

In a similar manner as described before, the trifluorovinyl(pentafluorophenyl)iodonium tetrafluoroborate salt was formed in high yield (91 %) by the reaction of equimolar amounts of pentafluoro(difluoroiodo)benzene with perfluorovinyldifluoroborane CF2=CFBF2 at –40 °C (Scheme 14). It should be mentioned that no reaction proceeded when the temperature was below –40 °C. The higher temperature in relation to trans-CF3CF=CFBF2 was necessary for the first step of abstracting of fluoride. It demonstrates the lower acidity of CF2=CFBF2 compared with trans-CF3CF=CFBF2. It is advisable to run the reaction at –40 °C to –30 °C. The solid product of the reaction can be easily separated from methylene chloride by decantation of the mother liquor. The low solubility of this salt in CH2Cl2 indicates a weaker contact between the cation and anion relative to the trans-1,2,3,3,3-pentafluoroprop-1enyl(pentafluorophenyl)iodonium

case.

The

trifluorovinyl(pentafluorophenyl)iodonium

tetrafluoroborate salt is stable at 20 °C under argon atmosphere for at least two months and shows a melting point of 108 – 110 °C without decomposition. Tonset was found in DSC to be 109.7 °C (endothermic).


35

3.7

Selected Reactivities of Fluoro(difluoroiodo)benzenes C6H4FIF2

3.7.1


3.7.1.1


The substitution of only one F-atom of the IF2 group by a OAc group in the 1 : 1 reaction of ArIF2 with Me3SiOAc was complicated by the formation of a mixture of products: ArI(F)OAc, ArI(OAc)2, beside unreacted ArIF2. The isolation of the desired product ArI(F)OAc (Ar = p-C6H4F) could not be achieved by crystallisation because all by-products showed a similar solubility. ArI(F)OAc was independently produced by the reaction of ArI(OAc)2 with aqueous hydrogen fluoride with ca. 10 % yield. It is worth emphasising that the intermediate ArI(F)OAc is a particular case of the class of compounds with the general formula [Ar-I-X][Z], where X is a heteroatomic substituent and Z is a nucleofugality group. There are some well-known derivatives of these compounds with X = OH, Z = OTf;[81] X = OH, Z = FSO3;[95] X = OH, Z = HSO4;[11] X = OH, Z = BF4;[96] X = F, Z = BF4.[97] In most cases the structures of these compounds were only postulated, but not substantiated. There is no structural proof for products of the type [Ar-I-X][Z]. The nature of this compound was only based subsequent chemical transformations.[98 – 99] After establishing a convient method for the preparation of ArIF2 it was interesting to convert ArIF2 into the corresponding derivative ArI(F)OAc.[94] As shown by Brel et al., (difluoroiodo)arenes (ArIF2) was reacted with trimethylsilyltriflate in CH2Cl2 solution to yield [ArIF][OTf] which was without separation introduced directly into the reaction with acetylene.[100a] The addition of trimethylsilylacetate to the pre-cooled solution of p-fluoro(difluoroiodo)benzene (–78 °C) in dichloromethane (1 : 1 ratio) led to a mixture of p-C6H4FI(F)OAc and p-C6H4FI(OAc)2. The desired compound p-C6H4FI(F)OAc was present in 47 % yield and the by-product p-C6H4FI(OAc)2 in 14 % beside 39 % of unreacted starting material p-C6H4FIF2 (Scheme 15).

p-C6H4FIF2

Me3SiOAc

p-C6H4FI(F)OAc + p-C6H4FI(OAc)2 + Me3SiF

Scheme 15: Preparation of p-C6H4FI(F)OAc. The distribution of products after the consumption of the equimolar amount of Me3SiOAc


36

showed that p-C6H4FI(F)OAc was not desactivated enough towards a further F-OAc substitution. In the present case a nucleophilic substitution mechanism, under electrophilic assistance describes the pathway for the above conversion of p-C6H4FIF2 into p-C6H4FI(F)OAc. Due to the partial charges on I–F and Si–OAc basic fluorine interacts with acidic silicon and OAc is transferred to iodine (Scheme 16). For p-C6H4FI(F)OAc we can write a borderline formula [p-C6H4FIOAc]F which explains the further substitution of the negatively charged fluorine by an additional OAc group. Driving force in the F–OAc substitution is the thermodynamically favoured SiF bond of Me3SiF. Subsequent nucleophilic attack of acetate anions on the iodonium cation results in the addition of an acetate anion and elimination of fluoride under formation of Me3SiF.[101] (Scheme 16) CH3 CH3 δ δ Si F

δ

I

F

F

δ

δ OAc

CH3

- FSiMe3

OAc I

F

F + Me3SiOAc

- FSiMe3

F

OAc I OAc

Scheme 16:

Proposed reaction path for the preparation of p-C6H4FI(F)OAc and p-C6H4FI(OAc)2.

3.7.1.2


The interaction of ArIF2 (Ar = o-, p-C6H4F) with 2,2´-bipyridine in CH2Cl2 at 20 °C was accompanied by the formation of deep yellow coloured solutions. Deshielding of the IF2 group (∆δ = 1.8 ppm) and shielding of the o-F atom (∆δ = –0.5 ppm) were observed. No such significant shift was found in case of the para-isomer. But the isolation of the adducts ArIF2 ⋅ bipy (Ar = o-, p-C6H4F) as pure solids failed. In contrast to Ar = o-, p-C6H4F, the adduct of


37

C6F5IF2 with 2,2´-bipyridine was obtained and characterised by its crystal structure.[100b] Owing to the higher positive charge on iodine in the latter case, the formation of the adduct was more probable. The interaction between ArIF2 (Ar = o-, p-C6H4F) and 2,2´-bipyridine was too weak. Attempts to separate the adduct by crystallisation ended with the isolation of the individual starting compounds.

3.7.1.3


Treatment of ArIF2 (Ar = o-, p-C6H4F) with (C6H5)3PO in CH2Cl2 resulted in a deep yellow solution. The 19F NMR signal of the IF2 group was slightly deshielded (∆δ = 0.6 ppm) and of the o-F atom slightly shielded (∆δ = –0.5 ppm). After cooling the solution (–40 °C) white crystals were isolated, which represented the adduct (C6H5)3PO ⋅ HF as proved by its X-ray structure. The desired product could not be isolated from the solution by succeeding low temperature crystallisation. Relating to the positive charge on iodine and the hard Lewis base character of (C6H5)3PO, the formation of an adduct was expected. The change of the chemical shifts of the IF2 group and the o-F signal and the change of the solution colour (deep yellow) indicates that an interaction between o-C6H4FIF2 and (C6H5)3PO had taken place. But crystallisation did not allow the isolation of a defined adduct. This result has a similarity with the result of the reaction of ArIF2 (Ar = o-, p-C6H4F) with 2,2´-bipyridine.

3.7.1.4

The Reaction of ArIF2 with [N(CH3)4]F

Due to the fluoride donor and fluoride acceptor characteristic behaviour of C6F5IF2 which was recently reported by Frohn et al.,[47b] the addition of fluoride as a strong nucleophile to ArIF2 (Ar = o-, m-, p-C6H4F) should be possible. Tetramethylammonium fluoride was used as a source of fluoride, where fluoride is only weakly coordinated by the electrophilic tetramethylammonium cation. Therefore [NMe4]F is known as naked fluoride.[88] Frohn and co-workers have reported an unstable adduct in case of the reaction of C6F5IF2 with [NMe4]F. A low frequency chemical shift of the C6F5 group in the 19F NMR spectra up to 4 ppm and a small shift of the IF2 group in the same direction beside the presence of a new I-F signal (2c-2e) at high frequency (–5 ppm) was observed. The adduct was detected in a suspension in the presence of other unspecified by-products.[88]


38

3.7.1.4.1 The Reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : 1) in Dichloromethane The treatment of equimolar amounts of p-C6H4FIF2 and [N(CH3)4]F in dichloromethane at –60 °C under intensive stirring resulted in the immediate formation of a white precipitate. In the mother liquor and in the solid product an unexpected compound with two signals of equal integral at –109.2 ppm (s) and –110.9 ppm (m) was observed beside [N(CH3)4][pC6H4FIF3]. In the CH2Cl2 mother liquor only two signals of p-C6H4FIF2 at –109.4 ppm and –174.6 ppm were present. After washing the solid with cold CH2Cl2 several times the same two components were observed in the mother liquor. When the dry solid was suspended in CH3CN (–40 °C) additional to the IF2 and C6H4F group a further signal for I-F appeared at –25.0 ppm ([ArIF3]–). The 19F NMR signals of the unexpected compound shifted in CH3CN to –99 ppm (s) and –111 ppm (m). The signal of the carbon-bonded fluorine of [p-C6H4FIF3]– had been shifted (–110.7 ppm) by 6 ppm compared with p-C6H4FIF2 in CH3CN. In contrast to this significant shift, the IF2 group value was nearly unchanged. Additionally to the above mentioned compounds the signal of the by-product [HF2]– appeared. When warmed to 0 °C the I-F signal became sharper and the relative integral increased from 0.5F (–40 °C) to 0.7F and finally at 24 °C to 0.81F. Parallel to the increasing integral their proceeded a shift from –25.0 ppm at –40 °C to –23 ppm at 24 °C. The ratio of [p-C6H4FIF3]– and the unexpected compound changed in the MeCN mother liquor significantly from –40 °C to 24 °C namely from 2 : 1 to 5 : 1, respectively. What is the nature of the unexpected compound with 2 signals of equal integral? p-C6H4FIF2 possesses two electrophilic centres: I(III) and the o- and p- positions relative to the IF2 group. The carbon electrophilic sites are generated by polarisation of the π-electrons of the aryl group (Scheme 17). Thus, the π-electron density on C-2, C-6, and C-4 is decreased. At these positions a fluoride anion can principally attack the aryl group. Additionally the fluorine atom bonded at C-4 increases the electrophilicity of C-4 and favours the addition of fluoride there.

Scheme 17: The formation of the Meisenheimer complex.


39

The resulting adduct of the Meisenheimer type can eliminate fluoride in a reversible reaction. This elimination is favoured with raising temperature and explains the growing of the ratio [p-C6H4FIF3]– to [4,4-C6H4F2IF2]– from –40 to +24 °C. Further proof for the Meisenheimer complex will be given later. It is the first time that such a complex was reported for polyvalent halogen compounds. For the desired compound [NMe4][p-C6H4FIF3] the following signals were assigned: –25.0 ppm (b), which belongs to the I-F bond (3c-4e), –110.7 ppm, which is characteristic for the aryl-bonded fluorine, and –169.0 ppm, which is associated with the IF2 group.

MF + p-C6H4FIF2

CH2Cl2

M[p-C6H4FIF3] + M[4,4-C6H4F2IF2]

-60 °C

M = [NMe4] Scheme 18: The reaction of [NMe4]F with p-C6H4FIF2.

By VSEPR theory a square planar environment around iodine is expected for the [p-C6H4FIF3]– anion, which is related to the [IF4]– anion with D4h symmetry.[102] In [ArIF3]– the three iodine-fluorine bonds are not chemically equivalent. For both fluorine atoms of the symmetrical hypervalent triad F–I–F (3c-4e) a similar shift value can be expected as for the IF2 group in neutral p-C6H4FIF2. The observed low frequent shift of the IF2 group (up to 6 ppm) is in agreement with an increasing partial negative charge on fluorine (IF2 triad) in the anion. The signal of the fluorine atom of the asymmetrical hypervalent C-I-F triad (–25.0 ppm) which is comparable to the bond in C6F5XeF with a

19

F resonance of ca.

[86a]

–3.5 ppm in dichloromethane at –78 °C.

[4,4-C6H4F2IF2]– was present in an amount of 29.7 % in the MeCN mother liquor beside the main product [NMe4][ArIF3]. In earlier work[87], the addition of F– to IF2 in C6F5IF2 was observed but followed by a fast decomposition. Principally, we cannot exclude the addition of a second fluoride anion to p-C6H4FIF2 under formation of the [p-C6H4FIF4]2– but the

19

F NMR contradicts such a constitution. The


40

situation for the dianion can be compared with the formation of [IF5]2– from IF3 with F–

[86b]

and with the formation of [XeF5]– from XeF4 and [N(CH3)4]F.[103] However, the predicted insolubility of a dianionic iodate salt in dichloromethane contradicts to the observation in the mother liquor and washing solutions. The formation of the [(p-C6H4F)2IF2]– anion as an alternative proposal for the unexpected compound cannot be excluded a priori. Eqs. 59 – 61 formulate a potential approach to this product. +

p-C6H4FIF2

[N(CH3)4][ p-C6H4FIF3]

(59)

[p-C6H4FIF3]- +

p-C6H4FIF2

(p-C6H4F)2IF + [IF4]-

(60)

(p-C6H4F)2IF +

[N(CH3)4] F

[N(CH3)4] [(p-C6H4F)2IF2]

(61)

[N(CH3)4]F

However in the 19F NMR spectra neither the tetrafluoroiodate(III) anion ([IF4]– δ = 100.7 ppm at –35 °C in CH3CN) nor the iodine-bonded fluorine atoms of [(p-C6H4F)2IF2]– could be observed.[102]

3.7.1.4.2 The 1 : 2 Reaction of p-C6H4FIF2 with [N(CH3)4]F in Dichloromethane A suspension resulted in the 1 : 2 reaction of p-C6H4FIF2 with [N(CH3)4]F at –60 °C in CH2Cl2. In the CH2Cl2 mother liquor the 19F NMR indicated four compounds: [p-C6H4FIF2], [4,4-C6H4F2IF2]–, [HF2]–, and F–. The first three components in the molar ratio of 33 : 1.3 : 1. Due to the broad shape of the F– signal the integral gave no reliable value. The –40 °C MeCN solution of the solid showed - directly measured – the presence of the two anions [p-C6H4FIF3]– and [HF2]– in the molar ratio of 9.3 : 1. Ten minutes later only the decomposition products could be detected: p-C6H4FIF2 and [HF2]– in the ratio 1 : 10.8. Two facts concerning the solid product should be stressed: 1. no Meisenheimer complex [4,4-C6H4F2IF2]– was found and 2. the relative integral of the third iodine-bonded fluorine of [p-C6H4FIF3]– was 1 and showed resonance at the highest frequency (–14.9 ppm). In conclusion, [p-C6H4FIF3]– can be observed when F– is used in excess, but dissolved in MeCN at –40 °C the iodate anion has a high fluoride donor ability and decomposes.


41

3.7.1.4.3 The 1 : 0.5 Reaction of p-C6H4FIF2 with [N(CH3)4]F in Dichloromethane In the fast reaction of one equivalent of p-C6H4FIF2 with a half equivalent of [N(CH3)4]F in dichloromethane at –60 °C a suspension resulted. In the mother liquor and in the CH2Cl2 washings of the solid product two compounds were observed in the 19F NMR spectra. The first contained a set of signals at –26.7 ppm (b), –109.7 ppm (m) and –174.6 ppm (s) and the second set of two signals at –109.0 ppm (m) and –111.3 ppm (s). When the solid was treated with BF3 ⋅ Et2O in CH2Cl2 at –40 °C the [4,4-C6H4F2IF2]– anion and the I-F signal at –26.7 ppm disappeared and a new signal at –148 ppm (s) developed. The disappearance of [4,4-C6H4F2IF2]– is in agreement with the high fluoride donor ability of the Meisenheimer complex. Original purpose of the 1 : 0.5 experiment was the preparation of the dinuclear [(p-C6H4FIF2)2F]– anion. From the

19

F NMR we can not decide unambiguously about the

–

number of n in [(p-C6H4FIF2)nF] . The broad IF signal at –26.7 ppm showed a resonance between [ArIF3]– and F– and a relative integral of only 0.2 to 0.3 instead of 0.5 (Scheme 19). 0.5 [N(CH3)4] F

X

CH2Cl2 / -60 °C

[N(CH3)4][(p-C6H4FIF2)2F]

p-C6H4FIF2 0.5 [N(CH3)4] F CH2Cl2 / -60 °C

[N(CH3)4][(p-C6H4FIF2)nF] + [NMe4][4,4-C6H4F2IF2]

Scheme 19: The 1 : 0.5 reaction of [NMe4]F with p-C6H4FIF2.

3.7.1.4.4 The Reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : 1) in Acetonitrile In this reaction an alternative experimental strategy was used to avoid the attack of naked fluoride on the solvent. Acetonitrile was condensed to the cooled 1 : >1 solid mixture (–192 °C) of p-C6H4FIF2 and [N(CH3)4]F. After melting a suspension resulted. The

19

F NMR

spectrum of the acetonitrile mother liquor at –30 °C showed two sets of signals the first: –14.2 (b), –110.2 (m), –169.5 (s), the second –100.7 (s), and –111.0 (m) ppm additionally –142.5 (d) ppm. At 24 °C the broad signal at –14.2 ppm shifted to –18.5 ppm and became sharp.


p-C6H4FIF2 + MF

42 CH3CN -196 °C to -35 °C

M = [NMe4]

M[p-C6H4FIF3] + M[4,4-C6H4F2IF2] by-product : M[HF2]

Scheme 20: The reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : >1) in acetonitrile. The 19F NMR data in table 24 (sec. 4.3.1.4.4) show that the desired product [p-C6H4FIF3]– has been formed beside [4,4-C6H4F2IF2]–. Until now, however, all attempts to isolate one of the products had failed due to a similar solubility of both components in organic solvents and their thermal lability.

3.7.1.4.5 The Reaction of p-C6H4FIF2 with [N(CH3)4]F (1 : 3) in Dichloromethane To the solid and cooled mixture (–192 °C) of p-C6H4FIF2 and [N(CH3)4]F in a 1 : 3 ratio dichloromethane was condensed. To check the reactivity the suspension was monitored by 19F NMR spectroscopy at –60 °C. In the CH2Cl2 mother liquor 19F NMR showed four compounds and/or anions: [p-C6H4FIF2], [4,4-C6H4F2IF2]–, [HF2]–, and F– for the first three in the molar ratio of 2 : 1 : 1, whereas the broad signal of F– could not be reliably integrated. In the CH2Cl2 washing of the solid a mixture of three compounds was observed: [p-C6H4FIF2], [4,4-C6H4F2IF2]– and [HF2]– in the relative molar ratio of 10 : 1 : 1, respectively. Despite of the high ratio of fluoride not all Lewis acid ArIF2 had reacted. It is notable that the 19

F NMR spectra does not show a significant shift of the signals of ArIF2 after the addition of

all three equivalents of [N(CH3)4]F. It became apparent that, in general, an increase in the stoichoimetric amounts of naked fluoride (more than 2 equivalents) resulted in an increase of [4,4-C6H4F2IF2]– in the product.

3.7.1.4.6 The 1 : 2 Reaction of o-C6H4FIF2 with [N(CH3)4]F in Dichloromethane The 1 : 2 reaction of o-C6H4FIF2 and [N(CH3)4]F in dichloromethane gave a solution at –60 °C. The

19

F NMR spectrum of the solution showed a mixture of three compounds and/or

anions: o-C6H4FIF2, [2,2-C6H4F2IF2]– and F– the first two in the relative molar ratio 1 : 71, respectively. Because of the broadening of the F– signal no reliable integral could be determined. Previously in case of the p-isomer (3.7.1.4.1-4), the formation of [4,4-C6H4F2IF2]– was observed beside [p-C6H4FIF3]–. The addition of the fluoride anion to the ortho derivative gave


43

only one I(III)-product [2,2-C6H4F2IF2]– due to the higher positive partial charge on C-2 in the o-isomer with respect to C-4 in the p-isomer (Scheme 21).

Scheme 21: The addition of the fluoride anion to o-C6H4FIF2. The addition of the fluoride anion to the aromatic ring seems to be the kinetically favoured reaction path. The preferred formation of [2,2-C6H4F2IF2]– over [o-C6H4FIF3]– seems to be not based on the strength of the dipole-ion interaction. It may be deduced on the higher lability of the I(III)⋅⋅⋅F– interaction in comparison to the Cδ+⋅⋅⋅F– interaction.

3.7.1.4.7 The 1 : 2 Reaction of m-C6H4FIF2 with [N(CH3)4]F in Dichloromethane The 1 : 2 reaction of m-C6H4FIF2 and [N(CH3)4]F in dichloromethane at –60 °C under intensive stirring ended with a solution. The

19

F NMR indicated three signals at –56.6 ppm

(b), –111.7 ppm, and –174.8 ppm additionally to the signal of the by-product [HF2]– at –147.9 ppm. The addition of fluoride proceeded only at the iodine centre. No Meisenheimer complex was observed. C-3 is not activated by the IF2 group for F– addition. Thus no Meisenheimer complex of the type [3,3-C6H4F2IF2]– was found (see Scheme 21). This result shows clearly the contrast to the ortho- and para-isomers, which possess in addition to I(III) an electrophilic carbon centre. The interaction of m-C6H4FIF2 with naked fluoride does not result in the formation of the [m-C6H4FIF3]– anion because the 19F shift values of the fluoride in the asymmetric Ar-I-F unit at ca. –20 ppm was absent. The observed resonance of –56.6 ppm is characteristic for F– which interacts with acidic centre, here the I(III) centre. In agreement with this interpretation is the high frequent shift of m-F (∆δ = 2.8 ppm) relative to m-C6H4FIF2 itself in CH2Cl2 at –40 °C (–108.9 ppm). Even the excess of fluoride did not help to realise the [m-C6H4FIF3]– anion.


44


As mentioned previously (sec. 3.7.1.2 and 3.7.1.4) ArIF2 (Ar = o-, m-, p-C6H4F) compounds have the ability to coordinate the base fluoride. In the salt CsF the base fluoride is combined with the large alkaline metal cation Cs+ which is equal to a relatively small lattice energy. The treatment of ArIF2 (Ar = p-C6H4F) in CH3CN solution with the insoluble solid CsF showed no formation soluble Cs[ArIF3]. The

19

F NMR spectra of the mother liquor showed

beside the unshifted signals of ArIF2 the two signals of the Meisenheimer complex.

3.7.1.5.1 The Reaction of p-C6H4FIF2 with CsF (1 : 1) in Acetonitrile The heterogeneous reaction of p-C6H4FIF2 with CsF in CH3CN at 20 °C in the ratio of 1 : 1 ended with a suspension. After 3 h only 60 % of p-C6H4FIF2 had reacted. This conversion increased slightly to 62 % after 9 h. In the mother liquor of the reaction two new signals of equal integrals developed at δ = –108.7 ppm (m) and –110.5 ppm (s) besides the signals of p-C6H4FIF2. The signal of the C6H4F and the IF2 group of p-C6H4FIF2 are shifted to low frequency –107.9 ppm and –171.7 ppm, respectively. Washing of the white solid several times with CH3CN showed always the same mixture. The 19F NMR multiplet at –108.7 ppm is assigned to two fluorine atoms at C-4 and the singlet at –110.5 ppm assigned to both fluorine atoms bonded to iodine in agreement with the anion [4,4-C6H4F2IF2]–. Due to the absence of the signal of a third fluorine bonded to iodine at high frequency, the formation of [p-C6H4FIF3]– can be excluded. (Scheme 22) CsF

X

CH3CN / 20 °C

Cs[p-C6H4FIF3]

p-C6H4FIF2 CsF CH3CN / 20 °C

Cs[4,4-C6H4F2IF2]

Scheme 22: Reaction of p-C6H4FIF2 with CsF (1 : 1) in CH3CN. In conclusion, no fluoride addition to iodine was detectable in the MeCN solution when CsF was used as fluoride source. Only the Meisenheimer adduct Cs[4,4-C6H4F2IF2] was formed.


45

3.7.1.5.2 The Reaction of p-C6H4FIF2 with CsF (1 : 2) in Acetonitrile The heterogeneous reaction of p-C6H4FIF2 with CsF in CH3CN at 20 °C in the ratio 1 : 2 ended with a white suspension. After 2 days of stirring at 20 °C only 57 % of p-C6H4FIF2 had reacted. In the mother liquor p-C6H4FIF2 was the main compound (95 rel. mole %) beside [4,4-C6H4F2IF2]– with two signals in the integral ratio 1 : 1 at δ = –108.7 ppm (m) and –110.6 ppm (s). In the first washing of the white solid with CH3CN, [4,4-C6H4F2IF2]– was found as major product (68 rel. mole %) beside p-C6H4FIF2 (32 rel. mole %). Continued washing of the solid resulted in solutions with traces of p-C6H4FIF2 (5 rel. mole %) beside [4,4-C6H4F2IF2]– (95 rel. mole %).(Scheme 23) 2 CsF

X

CH3CN / 20 °C

Cs2[p-C6H4FIF4]

p-C6H4FIF2 2 CsF CH3CN / 20 °C

Cs[4,4-C6H4F2IF2]

Scheme 23: Reaction of p-C6H4FIF2 with CsF in CH3CN in the ratio 1 : 2.

In light of the reported observations, there was no influence of the excess of CsF on the kind of product. Whether Cs[p-C6H4FIF3] nor Cs2[p-C6H4FIF4] were observed as soluble products.


46

3.7.2


3.7.2.1


p-Fluorophenyl(phenyl)iodonium

hexafluorophosphate

([p-C6H4F(C6H5)I][PF6])

was

prepared as white solid by the reaction of p-fluoro(difluoroiodo)benzene (p-C6H4FIF2) with phenyltetrafluorophosphorane (C6H5PF4)[104] at –60 °C in CH2Cl2 in a good yield (93 %) (Scheme 24). An intensive blue colour resulted when the phosphorane came into contact with p-C6H4FIF2. The colour disappeared after intensive stirring. The resulting salt was colourless and like the before described asymmetrical diaryliodonium tetrafluoroborates soluble in the polar “non-basic” CH2Cl2, while it was insoluble in non-polar organic solvents like n-pentane. The p-fluorophenyl(phenyl)iodonium hexafluorophosphate salt was stable at room temperature for at least 3 months, both in solution (CH2Cl2) and as solid under an argon atmosphere. The p-fluorophenyl(phenyl)iodonium hexafluorophosphate salt was molten at 126 °C without decomposition like the diaryliodonium salts [Ar(C6H5)I][BF4] (Ar = o-, m-, pC6H4F).

p-C6H4FIF2

+

C6H5PF4

CH2Cl2 -60 °C

[p-C6H4F(C6H5)I][PF6]

Scheme 24: Preparation of the p-fluorophenyl(phenyl)iodonium hexafluorophosphate salt. The nucleophilic fluoro-aryl substitution at I(III) is associated with the cleavage of the carbon -phosphorane bond on one side and of the iodine-fluorine bond on the other side. The hypervalent (F-I-F, 3c-4e) bond has a large electrostatic component. Thus the terminal negatively charged fluorine atoms possess a high basicity and thereby a defined reactivity towards Lewis acids. In the Lewis acid-base reaction between C6H5PF4 and p-C6H4FIF2, (Scheme 25), the iodine(III) centre becomes more electrophilic and parallel the nucleophilicity of the phenyl group arises. The intermediate product of interaction assists the migration of the phenyl group from P(V) to iodine(III). Subsequently PF5 as effective Lewis acid abstracts the remaining fluorine atom at iodine in the intermediate [p-C6H4F(C6H5)I]F and the iodonium cation is formed beside the [PF6]– anion. This step is favoured mainly by the win of lattice energy.


47 F F

2

I

F F

1

F

F

Ar

P F

Scheme 25: The aryl-fluorine substitution resulting from the interaction of p-C6H4FIF2 and C6H5PF4.

3.7.2.2

The Reactions of p-C6H4FIF2 with Alcohols (MeOH, EtOH, CF3CH2OH)

p-C6H4FIF2 reacted readily with ROH (R = Me, Et, CF3CH2) to yield a clear solution at 20 °C. When ROH was added to p-C6H4FIF2 the signal of the IF2 group disappeared and the signal of the aryl-bonded fluorine atom shifted to low frequency (1 ppm). After treatment of the solution with dry Na2CO3 and evaporation of MeOH the solid product dissolved only partially in CH2Cl2. The mother liquor represents a mixture (1 : 8.4) of ArIF2 and ArI(F)OMe (Ar = p-C6H4F). The new compound p-C6H4FI(F)OMe was characterised by its aryl-F resonance at –109.5 ppm and the I-F resonance at –118.6 ppm. The shift of the iodine-bonded fluorine from –174.4 ppm (IF2) to –118.6 ppm can be best interpretated by the borderline description as [ArIF][OMe] of the asymmetric surrounding of I(III). In the case of EtOH and CF3CH2OH the reaction took ca. 0.5 h, while MeOH reacted directly. For removing HF, Na2CO3 was used (Scheme 26).

p-C6H4FIF2

ROH 20 °C

p-C6H4FI(OR)2 + 2 HF

Na2CO3 20 °C

p-C6H4FI(F)OR + p-C6H4FIF2

Scheme 26: Reaction of ROH with p-C6H4FIF2.

The 19F NMR spectra of the interaction of ArIF2 (Ar = p-C6H4F) and MeOH or EtOH showed the presence of only one signal for aryl-bonded fluorine but shifted to smaller frequency and no IF2 signal. This observation is no unambiguous proof for the formation of p-C6H4FI(OR)2. The fast exchange of both axial ligands at the IX2 group with ROH cannot be excluded (Scheme 27). The fact that no signal of HF was observed before the treatment with Na2CO3 can be explained by a rapid exchange between HF and ArIF2.


48

H F

O

R

I

F

F

Scheme 27: The proposed interaction of ROH with p-C6H4FIF2.

3.7.2.3


Treatment of p-C6H4FIF2 in CH2Cl2 solution with two equivalents of CF3CO2H gave a mixture of substitution products additionally to the starting compound p-C6H4FIF2. 28.5 % of mono-substituted p-C6H4FI(F)OAcf and 6 % of di-substituted p-C6H4FI(OAcf)2 resulted after 3 h of stirring at 20 °C. After addition of NaF to the reaction mixture the yield of p-C6H4FI(F)OAcf and p-C6H4FI(OAcf)2 increased to 39 % and 17 %, respectively. The amount of both reactants p-C6H4FIF2 and CF3CO2H decreased parallel. The above results show that fluoride-trifluoroacetate substitution did not take place quantitively. This can be explained by the weak nucleophilic nature of the trifluoroacetate anion and only partial removal of HF. ArIF2 + H +

+

[OAcf]-

[ArIF]+ + [OAcf]- +

F- + H+

(62a)

The fluoride anion is a stronger nucleophile than the trifluoroacetate anion which favoured the back-reaction. Varvoglis[16] had shown that ArI(OAc)2 dissociated in strong acids to yield [ArIOAc][OAc] which is in equilibrium with the starting material:

ArI(O2CCH3)2

H+

[ArI(O2CCH3)]+ + CH3CO2H

(62b)

In the related example, discussed before, the reaction of ArIF2 (Ar = p-C6H4F) with Me3SiOAc (sec. 3.7.1.1), it was shown that ArIF2 reacted only partially to yield ArI(F)OAc (47 %) and ArI(OAc)2 (14 %). The differences between both experiments are focused in two main aspects: the OAc anion is more basic and nucleophilic than OAcf, and the second concerns the acidity of the medium. In case of AcfOH we have a protic acid whereas in the


49

reaction with Me3SiOAc we have a weak Lewis acid. The influence of NaF can be explained by the basicity of F– which allows to fix the byproduct HF in eq. 62a and thus to shift the equilibrium to the side of products. At that point it is appropriate to comment why the reaction rate for the substitution of the second iodine-bonded fluorine is lower than for the first. The first substitution ends with ArI(F)OAcf (Ar = p-C6H4F). From the high frequent resonance of iodine-bonded fluorine we can deduce a high participation an iodonium salt resonance form [ArIF][OAcf]. In the corresponding cation the I-F bond (2c-2e) is stronger than in the IF2 (3c-4e) group. Despite the positive partial charge on iodine in [ArIF]+ the substitution of I-F by I-OAcf is unfavoured. The mechanism of the p-C6H4FI(F)OAcf formation contains two sequential steps: weaken of the I-F bond by interaction with H+ and attack of the nucleophilic [OAcf]– anion at I(III) (Scheme 28). O

F

F

H

I

O

F

CF3

O -HF CF3

F

O

I F

Scheme 28: Formation of p-C6H4FI(F)OAcf.

3.7.2.4


The low temperature treatment of ArIF2 (Ar = p-C6H4F) with aHF resulted immediately in the formation of a deep blue solution. Only the signal of aryl-bonded fluorine in the

19

F NMR

spectra was observed at around –97.7 ppm at –80 °C. In the 1H NMR spectra two sets of signals at 8.8 ppm and 7.8 ppm for H-2,6 and H-3,5 were present. When the reaction temperature was increased to –40 °C the colour of the solution became green and in addition another signal of aryl-bonded fluorine of low integral appeared at –101.5 ppm which represents [(p-C6H4F)2I][F(HF)n]. At 0 °C the colour of the solution became yellow, δ = –97.7 ppm had disappeared, and the signal of aryl-bonded fluorine was the only signal in 19F NMR for [(p-C6H4F)2I][F(HF)n] (–101.2 ppm). Beside the signals of IF5 were observed. (Scheme 29) 19

F NMR spectroscopy showed no IF2 resonance whether at –80 °C nor at 0 °C and the aryl-F

resonance at the unusual position of ca. –98 ppm proposed a very strong electron-withdrawing


50

second substituent at the aryl group. We assume IF+ / I(FH)n2+ to be such substituents. Eqs. 63a and 63b describe the fluoride abstraction under the action of the super acid aHF. At moment we cannot decide between both products [ArIF(FH)n]+ and [ArI(FH)m]2+. The initially blue intensive colour proposed a charge-transfer from the aryl group to iodine(III) and is comparable with the observation in the reaction of p-C6H4FIF2 with C6H5PF4. p-C6H4FIF2

+

n+1 HF

p-C6H4FIF2

+

m+2 HF

-80 °C

[p-C6H4FIF(FH)n]+

+

[HF2]-

(63a)

[p-C6H4FI(FH)m]++

+

2 [HF2]-

(63b)

When the temperature was raised to –40 °C the colour changed from deep blue to green and beside [ArIF]+ the signal of [Ar2I]+ appeared at lower frequency. At 0 °C the colour changed to yellow and [ArIF]+ had disappeared. Beside [Ar2I]+ both signals of IF5 were detected. The molar ratio of [Ar2I]+ to IF5 was 1 : 0.54, theoretically 1 : 0.6 corresponding to eq. 64. 2 ArIF2

+

n HF

[Ar2I][F(HF)n]

0.6 IF5

+

3 I-

1.8 I2

+

0.6 IF5

+ 0.2 I2

(64) (65)

The yellow colour derived from dissolved I2. 1/3 of the reaction volume was used to determine the amount of I2 and IF5 by iodometry. Experimentally 1.89 I2 were found instead of 2 I2 (eqs. 64 and 65). Principally two reaction paths can be suggested to explain the products of ArIF2 in aHF: [Ar2I]+, IF5 and I2. The first path is characterised by the attack of H+ to the π-electron system on C-1. The π-electron density is enriched by the polarisation of the aryl group by I(III) or better the IF+ substituents. Subsequent heterolysis of the C-I bond and addition of F– to I(III) results in IF3 and C6H5F. The latter was not detected by

19

F NMR. IF3 is known to

disproportionate spontaneously to IF5 and I2. (Schemes 29 and 30)

p-C6H4FIF2 + aHF

-80 - 0 °C

5 IF3

Scheme 29: Solvolysis of ArIF2 in aHF.

C6H5F

+

3 IF5

+

< IF3 >

I2


51

-40 - 0 °C

p-C6H4FIF2 + aHF

C6H5F

+

IF3 5x

p-C6H4FIF+

3 IF5 +

C6H5F (p-C6H4F)2IF + H

+

n HF

[(p-C6H4F)2I][F(HF)n]

BF3 . Et2O

I2

[(p-C6H4F)2I][BF4]

Scheme 30: The proposed mechanism of the reaction of p-C6H4FIF2 with aHF. A second possible path starts again with [ArIF]+ which interacts with the basic F-site of ArIF2. The F-bridge enables R to migrate under formation of [Ar2IF] and IF2+. The latter adds F– from HF and disproportionate. IF5 and I2 are the products. In the presence of HF, [Ar2IF] loses F– and forms [Ar2I]+ which was observed and isolated after treatment with BF3 ⋅ Me2O as [BF4]– salt. (Scheme 31) R

F

R I

I F

F

R

R I

F

R2I+ +

I F

F

Scheme 31: The interaction of [RIF]+ with RIF2. A proof of the key step in path 1 could be achieved in the experiment of a mixture of p-C6H4FIF2

and

C6H5F

with

aHF

which

showed

the

exclusive

formation

of

[(p-C6H4F)2I][F(HF)n] as a blue suspension at –80 °C. But the stability of the colour was displayed even at 0 °C. (Scheme 32)

p-C6H4FIF2

+

C6H5F

aHF -80 °C

[(p-C6H4F)2I][F(HF)n] BF3 . Et2O [(p-C6H4F)2I][BF4]

Scheme 32: The reaction of p-C6H4FIF2 with C6H5F in aHF.


52

3.8


3.8.1


3.8.1.1

The Reaction of [p-C6H4F(CF2=CF)I][BF4] with Naked Fluoride

Alkenyl(aryl)iodonium fluoride salts were reported by the reaction of ArIF2 (Ar = pCH3C6H4) with alk-1-ynes in the presence of Et3N ⋅ 5 HF in CH2Cl2 at 0 °C. The crude products were used for the further transformations without isolation. The presence of a doublet signal in 1H NMR with a coupling constant 3J(H,F) of 15 Hz for the olefinic hydrogen (δ = 6.7 ppm) was considered as a proof for alkenyl(aryl)iodonium salt with a cisconfiguration in the alkenyl group. But no I-F signal was reported for the proposed compounds alkenyl(aryl)iodonium fluoride.[85, 100] The reaction of [p-C6H4F(CF2=CF)I][BF4] with a slight excess of [NMe4]F in CH2Cl2 solution at –60 °C ended with a white suspension. The solid product was identified as [NMe4][BF4]. The mother liquor contained a mixture of products: [p-C6H4F(CF2=CF)I]F (49.4 %), p-C6H4FI (32.7 %), 1,3-C6H4F2 (4.7 %), p-C6H4F(CF=CF2) (4.5 %), CF3CF2H (3.1 %), and CF2=CHF (< 1 %). The products were identified by 19F NMR. The desired product [p-C6H4F(CF2=CF)I]F was the main product formed (ca. 50 % yield) of the direct addition of the fluoride anion to the positively charged iodine centre. This addition was accompanied by the formation of the insoluble co-product [NMe4][BF4]. Additionally to the main product by-products deriving from consecutive reactions were observed. The mixture of by-products prevents the isolation of [p-C6H4F(CF2=CF)I]F by crystallisation. Additionally the slow decomposition of the main product was hindering the isolation. The formation of the by-products can be deduced from a favoured intermediate species, which derived formally from the initial heterolytic cleavage of the alkenyl carbon-iodine bond affording aryl iodide and primary the vinyl cation [CF2=CF]+ (Scheme 33). This cation may be trapped by another intermediate species, benzyn, which results from the base attack of F– on the m-CH bond of the iodonium compound. Generally, because of the fugality of the aryliodonio moiety, alkenyl(aryl)iodonium salts display

a

significant

reactivity

as

electrophiles

in

copper-mediated

nucleophilic

substitutions,[74] palladium-catalyzed coupling reactions,[142] and nucleophilic substitution by


53

enolates.[143] Ochiai, Okuyama, and co-workers recently quantified the nucleofugality of the phenyliodonio moiety as 106 more reactive than the triflate group. They also reported some elimination

products

from

the

solvolysis

of

trans-1-decen-1-yl(phenyl)iodonium

tetrafluoroborate.[144] In addition to pathways via the vinyl cation, other paths are possible. The initial homolytic cleavage of the vinylic C-I bond creates the iodoarene cation radical which may be followed by a single electron transfer (SET). Alkylidene-carbene and migration-elimination-addition modes are also possible. The observation of CF2=CHF is consistent with this mechanism as there is no change in charge at the transition state for a homolytic cleavage and developing of the iodoarenes radical cation should be destabilised by electron-withdrawing substituents. The mechanism for the 1,3-C6H4F2 formation will be discussed in the following section.

[p-C6H4F(CF2=CF)I][BF4]

CF2=CFH

F-

< H- >

[p-C6H4F(CF2=CF)I]F

F C

C

F

+ p-C6H4FI + F-

F

2F F CF3CF2H

p-C6H4F(CF=CF2)

Scheme 33: Reaction of [p-C6H4F(CF2=CF)I][BF4] with naked fluoride.


54

The Reaction of [p-C6H4F(C6H5)I][BF4] with Naked Fluoride

Diaryliodonium fluoride salts were reported by the reaction of diaryliodonium hydrogen sulphate with anhydrous hydrogen fluoride or gaseous hydrogen fluoride in the presence of basic barium salt (e.g. Ba(OH)2),[145] or by the reaction of diaryliodonium salts with silver oxide and reacting the product with hydrogen fluoride.[146] p-Fluorophenyl(phenyl)iodonium fluoride could conveniently be prepared by the interaction of a dichloromethane solution of p-fluorophenyl(phenyl)iodonium tetrafluoroborate and tetramethylammonium fluoride in a 1 : 1 stoichiometry at –60 °C (eq. 66). The exact stoichiometry was necessary to avoid the formation of [HF2]– as by-product.

[p-C6H4F(C6H5)I][BF4] + [NMe4]F

CH2Cl2

[p-C6H4F(C6H5)I]F + [NMe4][BF4]

-60 °C

(66)

The reaction of [p-C6H4F(C6H5)I][BF4] with [NMe4]F in dichloromethane at –60 °C proceeded smoothly. [NMe4][BF4] precipitated and the aimed product [p-C6H4F(C6H5)I]F was formed in quantitative yield, dissolved in CH2Cl2. This result is different to that of the reaction of [p-C6H4F(CF2=CF)I][BF4] with naked fluoride under similar conditions which gave [p-C6H4F(CF2=CF)I]F in only ca. 50 % yield accompanied by a mixture of by-products. Increasing the temperature of the [ArArÍ]F-CH2Cl2 solution up to 24 °C was accompanied by mainly decomposition of the product under formation of a 1,3-difluorobenzene (79 rel. mole %) and iodobenzene via benzyne formation (Scheme 34). In addition there is a trace amount (2.6 rel. mole %) of a monofluorobenzene compound C6H4FX, where X is not o-F or p-F atom. In contrast to perfluoroalkenyl(aryl)iodonium fluoride which underwent destruction with time even at low temperature (–60 °C) [ArArÍ]F is stable until 0 °C. The formation of benzyne has been reported as an intermediate by the reaction of diphenyliodonium salts with a base such as ″[Bu4N]F″.[141] [p-C6H4F(C6H5)I]F

CH2Cl2 0 °C

I+

F-

- PhI

+ HF

H F F

Scheme 34: The interaction of the [p-C6H4F(C6H5)I]+ cation with the base F–.

Results and Discussion The

19

55

F NMR spectra of the compound [p-C6H4F(C6H5)I]F contained the characteristic

signals for I-F and aryl-bonded fluorine with equal integrals. The I-F signal was observed as a very broad signal at low temperature (–60 °C) at –18.2 ppm which shifted significantly to lower frequency (–27.7 ppm) at 0 °C with a less broad signal (τ1/2 = 56 Hz). In contrast to the I-F signal the aryl-F signal remained practically unchanged at ca. –110 ppm.

3.8.1.3

The 1 : 1 Reaction of [p-C6H4F(C6H5)I]F with Naked Fluoride in Dichloromethane

Treatment of [p-C6H4F(C6H5)I]F with an additional equivalent of [NMe4]F in CH2Cl2 proceeded very fast at low temperature (–60 °C). The

19

F NMR spectrum showed that the

aryl-F signal at –111.7 ppm had been shifted to lower frequency by 1.1 ppm compared with the initial situation and that [HF2]– at –148.2 ppm had been formed. Additionally a very broad singlet developed at –59.8 ppm when the temperature was raised to 0 °C (τ1/2 = 130 Hz). The different interaction of ArArÍF and [ArArÍ]+ with F– should be compared and interpreted. Common in both cases is the addition of fluoride to the partially positive charged iodine centre at –60 °C under formation of [ArArÍF2]– and ArArÍF, respectively. In case of ArArÍF the addition is concurring with the H+-abstraction from the solvent under [HF2]– formation. The less expressed tendency to add F– can be deduced to the lower positive partial charge on I(III) in the neutral ArArÍF compound compared with the iodonium cation. A second difference in reactivity becomes obvious when warmed to 0 °C. The reaction of ArArÍF with F– shows no products deriving from arene intermediates as there are: 1,3C6H4F2 or 1,3-C6H4FI. This means that the acidification of the four o-H atoms in ArArÍF is to low that abstraction by fluoride can proceed. Furthermore it is an important hint for the interpretation of the latter reaction of [ArArÍ]+ with F– where at –60 °C first ArArÍF was formed and after warming to 20 °C the arene formation proceeded. Due to the asymmetric hypervalent bond C-I-F in ArArÍF the highly ionic I-F part will be cleaved heterolytically at higher temperature (destruction of the ionic adduct) and the base F– can attack the o-H positions in [ArArÍ]+, which are activated by the positive charge on iodine. The positive charge on iodine reached its maximum in the cationic species.


56

3.8.2 Reactions with Nucleophiles The perfluoroalkenyl(monofluorophenyl)iodonium cation contains two organo groups both bonded via a sp2-hybridised carbon to iodine(III). But both groups differ in their electronwithdrawing character. Therefore, it was of basic interest which of both is preferentially transferred to a nucleophile. Three n-nucleophiles of the pniktides were chosen as examples: 2,2´-bipyridine, tris(p-fluorophenyl)phosphane, and tris(p-fluorophenyl)arsane. Principally two different kind of products can be discussed. The addition product of the nucleophile to the electrophilic iodine(III) centre and alternatively the addition product of one of the electrophilic organo groups to the lone pair of the nucleophile. Additionally, we cannot exclude the electrophilic substitution of one C-H bond in the aryl part of the nucleophile.

3.8.2.1


In order to enlighten the reactivity of perfluoroalkenyl(fluorophenyl)iodonium tetrafluoroborate

salts

with

n-nucleophiles

(Lewis

bases),

a

reaction

of

[p-C6H4F(trans-

CF3CF=CF)I][BF4] with (p-C6H4F)3As was carried out in CH2Cl2 in a 1 : 1 ratio at 20 °C. Surprisingly, after stirring for 5 days [p-C6H4F(trans-CF3CF=CF)IAs(p-C6H4F)3][BF4] was detected in the reaction mixture in relative molar ratio 1 : 9 to the expected main product [(p-C6H4F)3(trans-CF3CF=CF)As][BF4]. The rate of the reaction shows that even at higher temperature the reaction proceeds very slowly due to the low basicity of the Lewis base. Overall, the cleavage of the I–C(alkenyl) bond was preferred. Additionally to [(p-C6H4F)3(trans-CF3CF=CF)As][BF4] and [p-C6H4F(trans-CF3CF=CF)IAs(p-C6H4F)3][BF4] traces of an unknown cis-CF3-CF=CF compound was found. The formation of [(p-C6H4F)3(trans-CF3CF=CF)As][BF4] proceeded in a satisfactory yield (ca. 70 %), regio- and stereospecific.

3.8.2.2


The reaction of [p-C6H4F(trans-CF3CF=CF)I][BF4] with (p-C6H4F)3P in CH2Cl2 at 20 °C in a ratio 1 : 1 gave a mixture of products. After 55 h of stirring 66.7 % of the iodonium salt had been converted. The expected product [(p-C6H4F)3(trans-CF3CF=CF)P][BF4] was found in a relative molar ratio 2.5 : 2.3 to the unexpected by-product (p-C6H4F)3PF2 (see eqs. 67a and


57

67b below). [Ar(trans-CF3CF=CF)I][BF4] + Ar3P

[Ar3(trans-CF3CF=CF)P][BF4] + ArI (67a)

[Ar(trans-CF3CF=CF)I][BF4] + Ar3P

Ar3PF2

+

ArI

(67b)

Ar = p-C6H4F

The main reaction 67a can formally be described as nucleophilic substitution at C-1 of the alkenyl

group.

The

regio-

and

stereospecific

formation

of

[(p-C6H4F)3(trans-

CF3CF=CF)P][BF4] is in agreement with the description in Scheme 35. F3C

F

C6H4F-p F I

P

C6H4F-p C6H4F-p

F

Scheme 35: The nucleophilic substitution of p-C6H4FI in [p-C6H4F(trans-CF3CF=CF)I][BF4] by (p-C6H4F)3P. Albeit the formation of (p-C6H4F)3PF2 was unexpected in a non trace amount. Formally oxidation of phosphor(III) was accompanied by the reduction of iodine(III) to iodine (I). This reactivity is in contrast to the previous case of (p-C6H4F)3As, where no oxidation of As(III) to Ar3AsF2 took place. The result does not contradict to a general rule because (p-C6H4F)3As is a less strong reducing agent than (p-C6H4F)3P.

3.8.2.3


Alkenyl(aryl)iodonium salts coordinated by neutral nitrogen ligands are not known in the case of perfluoroalkenyl(aryl)iodonium cations, probably due to the facile oxidation of the nitrogen atom by iodine(III).[11, 19, 105] Recently, Ochiai reported the adduct of the alkynyl(phenyl)iodonium cation with 1,10-phenanthroline.[106] Equimolar amounts of [p-C6H4F(trans-CF3CF=CF)I][BF4] and 2,2´-bipyridine reacted quickly in methylene chloride at –20 °C to give the corresponding adduct [p-C6H4F(trans-


58

CF3CF=CF)I ⋅ 2,2´-bipyridine][BF4]. The reaction was accompanied by a change of colour from colourless to black green. The

19

F NMR analysis showed that all fluorine signals were strongly shifted by the adduct

formation except the CF3 group. For the p-fluorine atom (aryl group) a shift to lower frequency (∆δ = 1.4 ppm) was observed parallel to a larger shift of the F-1 atom (low frequent shift up to 5.5 ppm) whereas the F-2 atom shifted only 2 ppm (low frequent). We can assume that both nitrogen atoms of 2,2´-bipyridine (bidentate ligand) interacted with iodine(III). For the [BF4]– anion we noticed a low frequent shift up to 5 ppm. Consequently, the [BF4]– anion interaction via the fluorine atoms became weaker or zero. All C-bonded F atoms shifted to lower frequency. The higher partial charge on fluorine is responsible for this shielding. Unfortunately, the X-ray diffraction data were not good enough to solve the structure of the adduct.

3.8.2.4


The preparation and isolation of [(p-C6H4F)3(CF2=CF)P][BF4] in CH2Cl2 was complicated by the formation of a mixture of products. The isolation of the desired product [(p-C6H4F)3(CF2=CF)P][BF4] was not achieved because all other products were soluble in the polar organic solvent. Obviously, the reaction of the alkenyl(aryl)iodonium salt with (p-C6H4F)3P made additional experiments necessary. Using aHF as solvent should make the iodonium cation more naked for reactions. But we could not estimate the decrease of nucleophilicity of the phosphane by protonation. When (p-C6H4F)3P was treated in aHF a blue suspension resulted. Addition of the cold and colourless solution of [p-C6H4F(CF2=CF)I][BF4] in aHF at low temperature (–78 °C) changed the colour. A greenish suspension resulted without full dissolution of the phosphane even at higher temperature (0 °C). The 19F NMR result presented a significant low frequent shift for all fluorine atoms up to 3.5 ppm. This observation means that probably the direct addition of (p-C6H4F)3P to the iodonium centre took place without elimination of iodobenzene to form [p-C6H4F(CF2=CF)I ⋅ {(pC6H4F)3P}][BF4]. The low frequent shift of the tetrafluoroborate anion indicated that the anion is not or less interacting with the I(III) centre compared to the starting material.


3.9

The Results of 1H, 13C, and 19F NMR Spectroscopic Studies

3.9.1

19

59

F NMR Spectroscopic Studies of IF5

Iodine pentafluoride is soluble as well in polar, “non-coordinating” solvents like CH2Cl2 as in basic coordinating solvents like MeCN. Due to the positive partial charge on iodine in the IF5 molecule there is a strong interaction of I(V) and basic nitrogen in MeCN. The lone pair in IF5 (square pyramidal structure) does not hinder the base to interact below the equatorial IF4 plane (for comparison: adduct formation of IF5 and F–).[107] As a result of the MeCN coordination the positive charge on iodine is diminished and the equatorial and axial fluorine atoms show resonance at lower frequency (CH2Cl2: F(ax) = 59.2, F(eq) = 12.0 ppm; CH3CN: F(ax) = 53.0, F(eq) = 5.4 ppm).[87] The 19F NMR spectrum of iodine pentafluoride in CH2Cl2 contains two well-resolved signals. One quintet for the F(ax) at δ = 59.2 ppm with 2J(F(ax),F(eq)) = 89.4 Hz and one doublet for F(eq) at δ = 12.0 ppm with 2J(F(eq),F(ax)) = 89.4 Hz. The 19F NMR data are consistent with a square pyramidal geometry in which one fluorine atom occupies the axial position and four fluorine atoms the equatorial positions. From literature it is known that IF5 is a non-rigid molecule. At higher temperature coalescence of both signals takes place.[107 - 109]

3.9.2

The NMR Spectroscopic Studies of 4-Fluoro-1-(tetrafluoroiodo)benzene (p-C6H4FIF4)

This compound is a white, moisture sensitive solid and thermally stable above ambient temperature. The NMR spectroscopic properties of p-C6H4FIF4 are related to those of RfIF4 (Rf = CF3, C3F7, C4F9)[38] and C6F5IF4, in particular its 19F NMR spectrum in CH2Cl2 indicates that the aryl group occupies the apical position in the square pyramidal molecule.[37] The

19

F NMR spectrum of p-C6H4FIF4 contains two well-resolved signals one for the IF4

group at –23.7 ppm and another for the 4-F-atom at –102.9 ppm. In comparison to p-C6H4FIF2, the fluorine atoms in p-C6H4FIF4 bonded to iodine are deshielded. In comparison to 4-C6H4FI the F-4 resonance in 4-C6H4FIF4 shifted 12.4 ppm to higher frequency explainable by a stronger p-p π-backbond F-4-C-4 caused by the polarisation of the aryl π-system by the IF4 group. Both 1H resonances in 4-C6H4FIF4 appeared deshielded compared to the parent compound 4-C6H4FI (0.6 to 0.7 ppm).


60

A significant high frequent shift of 67.7 ppm was measured for C-1. Only the shift value of C-3,5 remained nearly unaffected by the change of the oxidation state of iodine. The resonance of C-2,6 in p-C6H4FIF4 shifted to lower frequency relative to ArI (9.0 ppm). A multiplet signal for C-1 due to coupling with –IF4 group was observed. Selected chemical shift values and coupling constants of p-C6H4FIF4 are summarised in table 1.

3.9.3

The NMR Spectroscopic Studies of 4-Fluoro-1-(difluorooxoiodo)benzene (p-C6H4FIOF2)

The

19

F NMR spectra of 4-fluoro-1-(difluorooxoiodo)benzene in methylene chloride or

acetonitrile solutions consist of two signals. The IOF2 group gave rise to a singlet at –26.9 (MeCN) or –24.5 ppm (CH2Cl2) in basic MeCN or non-basic polar CH2Cl2. This resonance is close to that of the IF4 group (p-C6H4FIF4: –23.7 ppm in CH2Cl2 and –24.3 ppm in MeCN). The F-4 resonance is shifted 13.1 ppm to higher frequency when the oxidation number on iodine is increased from +I to +V. Both 1H resonances in p-C6H4FIOF2 are shifted to higher frequency by 0.6 or 0.7 ppm compared with p-C6H4FI.

CH2Cl2

CH3CN –102.8

p-C6H4FIOF2

p-C6H4FIOF2

4

–26.9

–24.5

–24.3

–23.7

6.9 7.6b 7.5d 7.5f 7.5h

7.7 8.3a 8.2c 8.2e 8.1g

H3,5

H δ/ppm

H2,6

1

-

-

-

154.8i

87.1

C1

-

-

-

130.2

139.2

-

-

-

117.6

117.9

C3,5

C δ/ppm

C2,6

13

-

-

-

165.6

162.9

C4

4

-

-

-

-

3.4

J(C1,F4)

3

-

-

-

9.6

7.8

J(C2,6,F4)

2

-

-

-

23.7

22.1

J(C3,5,F4)

Coupling constants J/Hz 1

-

-

-

257.1

246.8

J(C4,F4)

61

J(H2,6,H3,5) = 9.2 Hz, 4J(H2,6,F) = 4.7 Hz;

f 3

J(H3,5,H2,6) = 8.7 Hz, 3J(H3,5,F) = 8.5 Hz;

J(H2,6,F) = 4.9 Hz; h 3J(H3,5,H2,6) = 8.9 Hz, 3J(H3,5,F) = 8.9 Hz; i 2J(C1,IF4) = 9.0 Hz.

e 3

g 3

J(H2,6,H3,5) = 9.2 Hz,

J(H2,6,H3,5) = 8.8 Hz, 4J(H2,6,F) = 4.4 Hz; b 3J(H3,5,H2,6) = 8.5 Hz, 3J(H3,5,F) = 8.5 Hz; c 3J(H2,6,H3,5) = 9.4 Hz, 4J(H2,6,F) = 4.6 Hz; d 3J(H3,5,H2,6) =

–102.2

9.0 Hz, 3J(H3,5,F) = 9.0 Hz;

a3

CH3CN –102.8

p-C6H4FIF4

–102.9

CH2Cl2

p-C6H4FIF4

-

IF4/IOF2

F δ/ppm

–115.3

p-F

19

CH2Cl2

Solvent

p-C6H4FI

Compound

Table 1: Selected NMR chemical shifts and J(19F-13C) of p-C6H4FI, p-C6H4FIOF2, and p-C6H4FIF4 at 24 °C



62

The NMR Spectroscopic Comparison of C6H4XI, C6H4XI(OAc)2, and C6H4XIF2 (X = o-, m-, and p-F)

A comparison of the NMR chemical shifts of C6H4XI, C6H4XI(OAc)2 and C6H4XIF2 (where X = o-, m-, and p-F) (tables 3 and 4) exhibits characteristic tendencies depending on the oxidation state of iodine, here I(I) and I(III). The

19

F NMR spectra of all three

monofluoro(difluoroiodo)benzenes, dissolved in methylene chloride or acetonitrile at different temperatures, gave singlets for the IF2 group at –165.5, –176.3, and –174.6 ppm (CH2Cl2) or –161.7, –170.3, and –168.2 ppm (CH3CN) for the ortho-, meta-, and paraisomers, respectively. This chemical shift values are close to that of known ArIF2 compounds (table 2).[38, 111] In all monofluoro(difluoroiodo)benzenes the IF2 group appeared as a singlet, no coupling between aryl-bonded protons and/or fluorine and the IF2 group was observed for the IF2 signal, even in the case of 2-fluoro-1-(difluoroiodo)benzene where a 4JF,F was potentially expected. Missing JF,F and JF,H couplings of the IF2 group can be deduced on non free rotation of the IF2 group relative to the aryl plane. A windscreen wiper like movement of the IF2 group is associated by a variation of 4JF,F and 5JF,H which gives rise to broad lines. At comparable conditions, the fluorine resonances (m-, p-F) of the aryl C-F bonds in ArIF2 are shifted to high frequency approximately 3.1 and 6.4 ppm for m-F and p-F, respectively, when compared with C6H4FI, whereas a low frequency shift of ca. 3 ppm was observed for the ortho derivative. The differences in the observed shift tendency of C6H4F when comparing m- and p-F-C6H4IF2 with C6H4FI can be explained by a high p-p-π-back bond of the F-atom initiated by the polarisation of the aryl π-system by the IF2 group. The opposite behaviour of o-C6H4FIF2 is in agreement with an agostic interaction of the o-F atom with the IF2 group. Under the influence of the field of the positively charged I(III)-centre the F-C p-p-π-back bond is low and instead of that a significant dipole-dipole interaction F⋅⋅⋅IF2 is possible. In the case of C6H4FI(OAc)2 derivatives the aryl-F chemical shift showed a higher frequency shift of 0.5 ppm and 1.9 ppm, for ortho-, and para-isomers, respectively, compared with the difluoride analogous. In the 1H spectra of ArIX2 (Ar = o-, m-, and p-C6H4F), high frequency shifts of ca. 0.5 to 1.0 ppm are found for all proton resonances in relation to C6H4FI due to both the change of oxidation state and nature of X (F, OAc).


63

In all ArIX2 compounds, a significant high frequency shift was observed for C-1 (up to 35 ppm). It is worth to notice that C-2,6 showed in the para-, and meta-isomers a significant low frequency shift up to 8 ppm, but in the ortho-isomer only a 3.5 ppm low frequency shift. Also, the carbon atoms bonded to fluorine remain nearly unaffected by the change of the oxidation state or by comparing X = F and OAc except in the case of the ortho-isomer. The other carbon atoms remained nearly unaffected (high frequency up to 1 ppm) except C-4 in the ortho-, and meta-isomers which showed high frequency shifts of 5.6 and 3.7 ppm, respectively.


64

Table 2: 19F NMR spectra of the IF2 group for some (difluoroiodo)arenes Compound

Solvent

Temperature

δ/ppm

Ref.

C6H5IF2

THF

–40 °C to –50 °C

–174.0

31

CDCl3

20 °C

–177.8

33

CDCl3

20 °C

–175.9

32

THF

–40 °C to –50 °C

–174.3

31

CDCl3

20 °C

–177.3

33

CDCl3

20 °C

–176.6

32

p-C6H4BrIF2

THF

–40 °C to –50 °C

–173.5

31

p-C6H4ClIF2

THF

–40 °C to –50 °C

–173.4

31

CDCl3

20 °C

–177.0

33

CDCl3

20 °C

–178.2

32

THF

–40 °C to –50 °C

–172.8

31

CDCl3

20 °C

–178.1

33

p-C6H4(IF2)2

THF

–40 °C to –50 °C

–173.6

31

o-C6H4(IF2)2

THF

–40 °C to –50 °C

–158.8

31

p-C6H4FIF2

THF

–40 °C to –50 °C

–172.3

31

CD2Cl2

20 °C

–174.4

110

THF

–40 °C to –50 °C

–172.7

31

CD2Cl2

20 °C

–176.2

110

THF

–40 °C to –50 °C

–162.4

31

CDCl3

20 °C

–165.0

32

CH3CN

35 °C

–160.5

87

CH2Cl2

35 °C

–158.6

87

p-CH3C6H4IF2

p-NO2C6H4IF2

m-C6H4FIF2

o-C6H4FIF2

C6F5IF2

–176.3 –174.4 -

–111.5

–108.4j

–108.6o

–115.3

–108.9r

–107.0s

3-C6H4FI

3-C6H4FIF2

3-C6H4FI(OAc)2

4-C6H4FI

4-C6H4FIF2

4-C6H4FI(OAc)2

8.3

8.0

7.7

8.0

7.9k

7.5

-

-

-

H2

7.4

7.4

6.9

-

-

-

7.5f

7.4b

6.9

H3

1

-

-

-

7.5p

7.4l

7.5

7.6g

7.8c

7.3

H4

H δ/ppm

7.4

7.4

6.9

7.7q

7.8m

7.2 - 7.0

7.8h

7.5d

7.1

H5

8.3

8.0

7.7

8.0

7.9n

7.2 - 7.0

8.3i

8.3

7.8

H6

65

J(F,H3) = 8.4 Hz, 4J(F,H4,6) = 5.6 Hz, 4J(F,H6) = 2.8 Hz; b 3J(H3,F) = 7.6 Hz, 3J(H3,H4) = 7.6 Hz; c 3J(H4,H3) = 7.6 Hz, 3J(H4,H5) = 6.8 Hz, 4 J(H4,F) = 6.8 Hz; d 3J(H5,H6) = 8.4 Hz, 3J(H5,H4) = 8.4 Hz; e 3J(F,H3) = 8.2 Hz, 4J(F,H4) = 5.5 Hz, 4J(F,H6) = 5.5 Hz; f 3J(H3,F) = 7.7 Hz, 3J(H3,H4) = 7.7 Hz, 4J(H3,H5) = 1.2 Hz; g 3J(H4,H5) = 7.8 Hz, 3J(H4,H3) = 7.9 Hz, 4J(H4,F) = 5.6 Hz, 4J(H4,H6) = 1.6 Hz; h 3J(H5,H6) = 8.3 Hz, 3J(H5,H4) = 8.3 Hz, 4J(H5,H3) = 1.2 Hz; i 3J(H6,H5) = 7.8 Hz, 4J(H6,F) = 5.9 Hz, 4J(H6,H4) = 1.7 Hz; j 3J(F,H2) = 8.1 Hz, 3J(F,H4) = 8.1 Hz, 4J(F,H5) = 6.0 Hz; k 3 J(H2,F) = 8.6 Hz; l 3J(H4,F) = 8.4 Hz, 3J(H4,H5) = 8.2 Hz, 4J(H4,H6) = 2.1 Hz; m 3J(H5,H6) = 8.0 Hz, 3J(H5,H4) = 7.7 Hz, 4J(H5,F) = 5.5 Hz; n 3 J(H6,H5) = 8.5 Hz, 4J(H6,H4) = 2.0 Hz, 4J(H6,H2) = 2.0 Hz; o 3J(F,H2) = 7.8 Hz, 3J(F,H4) = 7.7 Hz, 4J(F,H5) = 6.3 Hz; p 3J(H4,H5) = 8.5 Hz, 3J(H4,F) = 8.3 Hz, 4J(H4,H6) = 2.4 Hz, 4J(H4,H2) = 0.6 Hz; q 3J(H5,H6) = 8.3 Hz, 3J(H5,H4) = 8.1 Hz, 4J(H5,F) = 5.9 Hz; r 3J(F,H3,5) = 8.1 Hz, 4J(F,H2,6) = 4.9 Hz, 6J(F,IF2) = 1.5 Hz; s 3J(F,H3,5) = 8.3 Hz, 4J(F,H2,6) = 4.9 Hz.

-

–96.8e

2-C6H4FI(OAc)2

a 3

–166.1

–97.3a

2-C6H4FIF2

IF2 -

F δ/ppm

–94.5

C6H4F

19

2-C6H4FI

Compound

Table 3: 1H and 19F NMR chemical shifts of C6H4XI, C6H4XIF2, and C6H4XI(OAc)2 (X = o-, m- and p-F) in CH2Cl2 at 24 °C


139.2

132.6f

123.0b

120.3

87.1

117.9e

115.3

3-C6H4FIF2

3-C6H4FI(OAc)2i

4-C6H4FI

4-C6H4FIF2

4-C6H4FI(OAc)2j

g3

118.5

118.7

117.9

162.5

J(C1,IF2) = 10.6 Hz;

137.8

122.6

163.9

162.5

116.5

116.8

c 3

118.5

118.7

117.9

132.3

132.4

131.5

126.6

126.8

126.0

C5

J(C2,IF2) = 5.0 Hz;

164.5

164.5

162.9

119.2

118.7

115.0

135.1

136.0

130.4

C4

C δ/ppm

115.8

C3

13

3.5

3.1

3.4

8.0

8.6

7.9

23.4

22.3

25.1

C1,F

e 2

8.8

8.5

7.8

25.0

26.8

23.7

252.7

254.5

244.6

C2,F

J(C6,IF2) = 4.6 Hz;

d 3

137.8

132.6g

139.2

131.0

125.5d

133.6

137.3

135.7

139.6

C6

253.6

252.5

246.8

21.0

21.0

20.9

7.9

7.9

7.3

C4,F

J(C1,IF2) = 11.5 Hz;

22.7

23.1

22.1

253.6

253.4

250.8

22.2

22.0

23.6

C3,F

f 3

Coupling constants J/Hz

22.7

23.1

22.1

7.9

7.9

8.1

3.5

3.2

3.6

C5,F

8.8

8.5

7.8

3.5

3.8

3.3

-

-

1.4

C6,F

66

J(C2,IF2) = 4.2 Hz;

J(C6,IF2) = 4.2 Hz; h δ = 177.1 (s, -CO2), 20.1 (s, CH3); i δ = 176.8 (s, -CO2), 20.2 (s, CH3); j δ = 176.7 (s, -CO2), 20.2 (s, CH3).

b 2

117.4c

93.6

3-C6H4FI

J(C1,IF2) = 14.0 Hz;

124.9

109.0

2-C6H4FI(OAc)2h

a 2

158.4

116.1a

2-C6H4FIF2

159.6

161.9

C2

81.2

C1

2-C6H4FI

Compound

Table 4: 13C NMR chemical shifts and J(13C-19F) of C6H4XI, C6H4XIF2, and C6H4XI(OAc)2 (X = o-, m-, and p-F) in CH2Cl2 at 24 °C



The

Temperature

67 Dependence

19

of

F

NMR

Chemical

Shifts

in

Monofluoro(difluoroiodo)benzenes The

19

F NMR spectra of monofluoro(difluoroiodo)benzenes in different solvents and at

different temperatures gives insight into the nature of the inter- or intramolecular interaction between iodine and fluorine. The IF2 group of o-fluoro(difluoroiodo)benzene showed resonance high frequent with respect to p- and m-fluoro(difluoroiodo)benzene 9.1 or 11.2 ppm in CH2Cl2 caused by the agostic effect of the o-F-atom (fig. 6). Likewise, in all monofluoro(difluoroiodo)benzenes the IF2 resonance is shifted to high frequency when basic solvents like CH3CN are used due to coordination of the basic solvent at the positive iodine centre. Thus, the signal of IF2 group is shifted by 6 ppm for the p- and m-derivatives but in case of o-fluoro(difluoroiodo)benzene only less than 4 ppm due to the still present agostic effect. In case of F-2 MeCN has to compete with this fluorine atom (fig. 6). Table 5 shows that the shift of the IF2 group signal is most pronounced to low frequency at low temperature due to the strong interaction between the basic solvent and the positive iodine centre, which can be considered as a type of an adduct formation. δ F δ F δ I F

δ

Figure 6: The intramolecular interaction (agostic effect) of the o-fluorine atom with I(III) in o-fluoro(difluoroiodo)benzene.

The F-4 fluorine atom shows a negligible temperature dependence in both solvents, in contrast to the F-2- and F-3-fluorine atoms. At lower temperatures the coordination of the basic solvent is favoured. Additionally to the coordination of the basic solvent molecule at I(III) we have to assume an intermolecular I⋅⋅⋅F interaction (fig. 7), especially in CH2Cl2. In the weak coordinating solvent CH2Cl2 this effect is more pronounced: for (p-C6H4FIF2) ∆ [δ(IF2 / –80 °C) – δ(IF2 / 24 °C)] = –0.96 ppm (low frequency), for (m-C6H4FIF2) ∆ [δ(IF2 / –80 °C) – δ(IF2 / 24 °C)] = –1.87 ppm, for (o-C6H4FIF2) ∆ [δ(IF2 / –80 °C) – δ(IF2 / 24 °C)] =


68

–1.91 ppm. Iodine(III) when interacting with basic solvents or basic sites of the neighbour molecule diminishes its positive partial charge. These interactions are favoured at lower temperature.

F

δ

δ

Fδ

δI

I F

δ

δ

F

F

Figure 7: The intermolecular interaction of ArIF2 molecules.

F

Results and Discussion Table 5:

69

The temperature dependence of the

19

F NMR resonances of the monofluoro-

(difluoroiodo)benzene Compound

Solvent

C6H4F

IF2

24

–97.7

–165.5

0

–98.0

–165.9

–30

–98.2

–166.5

–80

–98.7

–167.4

24

–94.4

–161.7

0

–94.5

–162.1

–30

–94.7

–162.4

24

–108.4

–176.3

0

–108.6

–176.8

–30

–108.9

–177.3

–80

–109.2

–178.1

24

–104.9

–170.3

0

–105.0

–170.6

–30

–105.3

–171.0

24

–109.0

–174.6

0

–109.1

–174.8

–30

–109.2

–175.2

–80

–109.2

–175.5

24

–104.5

–168.2

0

–104.5

–168.3

–30

–104.4

–168.4

(°C) o-C6H4FIF2

CH2Cl2

CH3CN

m-C6H4FIF2

CH2Cl2

CH3CN

p-C6H4FIF2

CH2Cl2

CH3CN

δ/ppm

Temperature


70

3.9.6

NMR Spectroscopic Studies on Iodonium Salts

3.9.6.1

Asymmetric Diaryliodonium Tetrafluoroborates

Monofluorophenyl(phenyl)iodonium tetrafluoroborates, dissolved in methylene chloride at 24 °C, showed a singlet at ca. δ = –146 ppm for the [BF4]– anion in their 19F NMR spectra and a multiplet for the fluorine atom in the phenyl group. The high frequent shift value of the [BF4]– anion is indicative for a significant interaction with an electrophilic cation (table 6). The fluorine resonance of the FC6H4 group in all three diaryliodonium tetrafluoroborates [C6H4X(C6H5)I][BF4] (X = o-, m-, and p-F) is shifted to high frequency by 1.8 ppm, 3.2 ppm, and 4.4 ppm for the ortho-, meta-, and para-F isomer, respectively, compared with the C6H4FIF2 analogous. The 1H NMR spectra reveal that the resonances for [C6H4X(C6H5)I][BF4] (X = o-, m-, and p-F) protons appear at approximately the same values, compared with the starting materials C6H4FIF2 and C6H5BF2. In the

13

C NMR spectra, significant low frequent shifts are measured for C-1 (C6H4F) in all

three iodonium tetrafluoroborates [C6H4F(C6H5)I][BF4] (F = o-, m-, and p-) compared with chemical shift of the starting material C6H4FIF2 (17.6 ppm, 11.8 ppm, 12.1 ppm, for ortho-, meta-, and para-F, respectively). In case of [o-C6H4F(C6H5)I][BF4], a low frequent shift was measured for C-5 (11.1 ppm) and C-6 (7.9 ppm) and small high frequency shifts were observed for C-2 (2.1 ppm), C-3 (0.4 ppm) and C-4 (0.4 ppm) in o-C6H4F-group. In contrast to the ortho-derivative, in the meta-derivative high frequency shifts for C-2 (5.7 ppm), C-4 (2.3 ppm), C-5 (1.8 ppm), and C-6 (6.3 ppm) were observed. The shift of C-3 remained nearly unaffected

by

the

formation

of

iodonium

salt

from

ArIF2.

Similarly,

the

[p-C6H4F(C6H5)I][BF4] exhibit a small high frequency shift for C-2,6 (5.5 ppm), C-3,5 (1.2 ppm) and C-4 (0.6 ppm) (table 7). The 11B NMR spectra reveal that the signal for all three diaryliodonium tetrafluoroborates in CH2Cl2 appear at approximately –2.0 ppm, which is typical for [BF4]– anions.

–146.0

–145.8

–105.2b

–104.6c

[3-C6H4F(C6H5)I][BF4]

[4-C6H4F(C6H5)I][BF4]

H δ/ppm

8.3 – 8.1 (4H), 7.8 (1H), 7.7 (2H), 7.4 (2H)

8.2 (1H), 8.2 (1H), 8.0 (1H), 7.9 (1H), 7.9 (1H), 7.7 (3H), 7.5 (1H)

8.3 (1H), 8.2 (2H), 7.9 (2H), 7.7 (2H), 6.6 (2H)

FC6H4(C6H5)

1

–2.0

–2.1

–2.1

BF4

B δ/ppm

11

C1

C2,6

C3,5

C4

105.8 138.1 119.9 165.1 119.9 138.1 113.2 135.9 133.1 133.7

C6

[4-C6H4F(C6H5)I][BF4]

C5

111.3 123.1 163.5 121.0 134.2 131.8 112.8 136.3 133.2 133.9

C4

[3-C6H4F(C6H5)I][BF4]

C3

C6H5 C1,F

C2,F

-

8.3

9.3

25.6

23.3

256.5

21.6

C3,F

256.2

20.8

8.0

C4,F

FC6H4

23.3

7.8

-

C5,F


9.3

3.2

3.3

C6,F

J(F,H3,5) = 8.3 Hz, 4J(F,H2,6) = 4.6 Hz

c 3

J(F,H2) = 7.6 Hz, 3J(F,H4) = 7.6 Hz, 4J(F,H5) = 6.0 Hz

b 3

160.5 117.3 136.4 137.9 127.8 112.7 132.9 135.9 133.5 22.6 253.7

C2

C δ/ppm

98.5

C1

C6H4F

13

[2-C6H4F(C6H5)I][BF4]

Compound

71

J(F,H3) = 8.2 Hz, 4J(F,H4) = 5.5 Hz, 4J(F,H6) = 5.5 Hz

a 3


Table 7: 13C NMR chemical shifts and J(19F-13C) of monofluorophenyl(phenyl)iodonium tetrafluoroborates in CH2Cl2 at 24 °C

–146.6

–95.5a

[2-C6H4F(C6H5)I][BF4]

F δ/ppm BF4

19

C6H4F

Compound

Table 6: 1H, 19F, and 11B NMR chemical shifts of monofluorophenyl(phenyl)iodonium tetrafluoroborates in CH2Cl2 at 24 °C



72

trans-1,2,3,3,3-Pentafluoroprop-1-enyl(fluorophenyl)iodonium

Tetrafluoro-

borates trans-1,2,3,3,3-Pentafluoroprop-1-enyl(monofluorophenyl)iodonium tetrafluoroborates are soluble in methylene chloride and this solution can be used for spectroscopic measurements at 24 °C. Their

19

F NMR spectra show sharp, well-resolved signals for each kind of fluorine

(table 8). The 19F NMR signals of the trans-CF3CF=CF group in the iodonium salt are strongly shifted compared with the trans-CF3CF=CFI (appendix 7.1). The 19F NMR signal of the F-1 atom is shifted low frequent (ca. 32 ppm), whereas the F-2 atom is shifted 31 ppm to high frequency. The CF3 group is unaffected (high frequent shift of max. 0.4 ppm). It is worth note that all 19F NMR signals of the trans-CF3CF=CF group are systematically affected by the position of the fluorine in the aryl group. From o-F to p-F the

19

F resonances of the trans-CF3CF=CF group are shifted to lower

frequency with a maximum effect on F-1 of 2.0 ppm. This shift tendency can be interpreted by a decreasing inductive influence from o- to p-C6H4F on I(III). Another argument is based on the fact that with a larger distance of F from C-1 (aryl group) the aryl group can take over more of the positive charge from I(III). Furthermore, a coupling constant between ortho-fluorine and F-1 and F-2(cis) of the alkenyl group was found, which was determined to 5.7 and 4.3 Hz, respectively. The fluorine signal for the C6H4F group was strongly shifted to high frequency compared with the parent compounds C6H4FIF2 of 4.2 ppm, 5.0 ppm, and 7.5 ppm for ortho-, meta-, and para-F, respectively. This observation is in agreement with the above mentioned property of the C6H4F group to take over positive charge increasingly from o- to p-F. A sharp singlet for the BF4 group was observed at –141.8 - –142.5 ppm. The

11

B

NMR

spectra

showed

for

all

three

trans-1,2,3,3,3-pentafluoroprop-1-

enyl(monofluorophenyl)iodonium tetrafluoroborates quintets and appeared at approximately the same value (–2.0 ppm). The 1H NMR spectrum of [o-C6H4F(trans-CF3CF=CF)I][BF4] revealed that most protons


73

appeared at approximately the same values as in the starting compound o-C6H4FIF2, only H-3 was shifted to high frequency (0.6 ppm). In case of [m-C6H4F(trans-CF3CF=CF)I][BF4] only H-2,6 were shifted to high frequency (0.3 ppm). In [p-C6H4F(trans-CF3CF=CF)I][BF4] only H-2,6 was shifted to high frequency (0.4 ppm). Similar to the diaryliodonium salts, significant low frequent shifts are measured for C-1 of the aryl group in alkenyl(aryl)iodonium tetrafluoroborates with 12.9 ppm, 13.5 ppm, and 18.0 ppm, for meta-, para- and ortho-F, respectively, when compared with the chemical shifts of the starting materials monofluoro(difluoroiodo)benzene. In case of [m-C6H4F(transCF3CF=CF)I][BF4], high frequent shifts for C-2,3,4,5 and C-6 (up to 6.7 ppm) were observed. In case of [p-C6H4F(trans-CF3CF=CF)I][BF4], a larger high frequent shift of C-2,6 (up to 8 ppm) was measured. The shift values of C-3,5 and C-4 remained nearly unaffected when going

from

ArIF2

to

alkenyl(aryl)iodonium

salt.

Similarly

for

[o-C6H4F(trans-

CF3CF=CF)I][BF4], a slight high frequent shift for C-2,3,4,5 and C-6 (up to 3 ppm) was observed. (table 9)

74

–140.2j

–120.8k

–120.1g

–119.4c

F2

F δ/ppm

–68.6l

–68.4h

–67.7d

CF3

–142.5

–142.5

–141.8

BF4

8.4

8.2

-

H2

7.5

-

8.0

H3

1

-

7.9

7.6

H4

H δ/ppm

7.5

7.7

7.7

H5

8.4

8.2

8.4

H6

–2.2

–2.1

–1.3

BF4

B δ/ppm

11

J(F2,F1) = 141.2 Hz, 3J(F2,F3) = 19.4 Hz, 6J(F2,o-C6H4F) = 4.3 Hz;

d 3

J(F3,F2) = 19.4 Hz, 4J(F3,F1) = 10.6 Hz;

e 3

J(F,H2) = 7.5 Hz, 3J(F,H4) = 7.5

J(F,H3) = 9.8 Hz, 4J(F,H4) = 8.5 Hz, 5J(F,F1) = 5.3 Hz, 6J(F,F2) = 5.3 Hz; b 3J(F1,F2) = 141.2 Hz, 4J(F1,F3) = 10.5 Hz, 6J(F1,o-C6H4F) = 5.7 Hz; c

–101.4i

–139.1f

–138.2b

–93.1a

–103.5e

F1

C6H4F

19

i 3

J(F,H3,5) = 8.3 Hz, 4J(F,H2,6) = 4.1 Hz;

19.4 Hz, 4J(F3,F1) = 10.6 Hz.

Hz;

j 3

J(F1,F2) = 141.8 Hz, 4J(F1,F3) = 10.6 Hz;

k 3

J(F2,F1) = 141.8 Hz, 3J(F2,F3) = 19.4 Hz;

l 3

J(F3,F2) =

Hz, 4J(F,H5) = 6.5 Hz; f 3J(F1,F2) = 142.0 Hz, 4J(F1,F3) = 10.5 Hz; g 3J(F2,F1) = 142.0 Hz, 3J(F2,F3) = 19.3 Hz; h 3J(F3,F2) = 19.3 Hz, 4J(F3,F1) = 10.5

3

a 3

CF3CF=CF)I][BF4]

[4-C6H4F(trans-

CF3CF=CF)I][BF4]

[3-C6H4F(trans-

CF3CF=CF)I][BF4]

[2-C6H4F(trans-

Compound

at 24 °C

Table 8: 1H, 19F, and 11B NMR chemical shifts of trans-1,2,3,3,3-pentafluoroprop-1-enyl(monofluorophenyl)iodonium tetrafluoroborates in CH2Cl2


110.2

104.4

[3-C6H4F(transCF3CF=CF)I][BF4]

[4-C6H4F(transCF3CF=CFI][BF4]

139.9

124.7

160.8

C2

121.1

163.6

118.7

C3

166.3

122.8

138.8

C4

C6H4F

13

121.1

134.9

129.3

C5 22.9

8.7

-

133.3 125.5d 144.9e 116.4f

139.9 125.0g 144.2h 116.0i

C2

138.4 125.4a 145.2b 116.5c

C1

C1,F

C6

Alkenyl CF3

C δ/ppm

9.5

25.9

256.6

C2,F

23.4

258.6

21.2

C3,F

259

20.9

8.2

C4,F

C6H4F

23.4

8

3.2

C5,F


9.5

3.5

-

C6,F

J(C2,F2) = 266.1 Hz, 2J(C2,F3) = 43.2 Hz, 2J(C2,F1) = 30.8 Hz; i 1J(C3,F3) = 276.8 Hz, 2J(C3,F2) = 36 Hz, 3J(C3,F1) = 5 Hz.

J(C2,F1) = 30.0 Hz; f 1J(C3,F3) = 277.0 Hz, 2J(C3,F2) = 35.9 Hz, 3J(C3,F1) = 4.6 Hz; g 1J(C1,F1) = 349.8 Hz, 2J(C1,F2) = 62.6 Hz, 3J(C1,F3) = 3.0 Hz;

h1

2

Hz, 2J(C3,F2) = 36 Hz, 3J(C3,F1) = 5 Hz; d 1J(C1,F1) = 352.0 Hz, 2J(C1,F2) = 61.8 Hz, 3J(C1,F3) = 3.0 Hz; e 1J(C2,F2) = 266.5 Hz, 2J(C2,F3) = 43.6 Hz,

J(C1,F1) = 351.2 Hz, 2J(C1,F2) = 62.4 Hz, 3J(C1,F3) = 2.8 Hz; b 1J(C2,F2) = 267.3 Hz, 2J(C2,F1) = 43.5 Hz, 2J(C2,F3) = 30.5 Hz; c 1J(C3,F3) = 276.8

98.1

C1

[2-C6H4F(transCF3CF=CF)I][BF4]

a 1

75

C NMR chemical shifts and J(13C-19F) of trans-1,2,3,3,3-pentafluoroprop-1-enyl(monofluorophenyl)iodonium tetrafluoroborates in

CH2Cl2 at 24 °C

13

Compound

Table 9:



76

Trifluorovinyl(fluorophenyl)iodonium Tetrafluoroborates

Trifluorovinyl(monofluorophenyl)iodonium tetrafluoroborates are soluble in methylene chloride at 20 °C and display three doublets of doublets corresponding to the trifluorovinyl group in the

19

F NMR spectra. It is useful to compare the CF2=CF group in iodonium salts

with CF2=CFI (appendix 7.1). The 19F NMR spectra showed that the F-1 signal was shifted to low frequency (7.6 ppm), while the F-2(cis) signal was very strongly shifted (15.3 ppm) to high frequency. Similarly, a significant high frequency shift for F-2(trans) was observed (8.8 ppm). The 19F NMR spectra show that all 19F NMR signals of CF2=CF group are not affected by the position of fluorine in the aryl group. Compared with ArIF2 the signal of fluorine in the aryl group was strongly shifted to high frequency by 2.6 ppm, 4.1 ppm, 7.2 ppm for ortho-, meta-, and para-F, respectively. The 19F chemical

shift

of

the

[BF4]–

anion

in

trifluorovinyl(monofluorophenyl)iodonium

tetrafluoroborates appeared at –143.2 to –143.9 ppm. The

11

B NMR signal of the three trifluorovinyl(monofluorophenyl)iodonium tetrafluoro-

borates was located at approximately (–2.1 ± 0.1 ppm). The 1H NMR spectra of [o-C6H4F(CF2=CF)I][BF4] exhibit that the protons were unaffected except H-3, which was shifted to high frequency (0.5 ppm) when compared with o-C6H4FIF2. In case of [m-C6H4F(CF2=CF)I][BF4], all protons were affected (up to 0.2 ppm) except H-5 which remained unaffected. H-2,6 in [p-C6H4F(CF2=CF)I][BF4] were shifted to high frequency (0.3 ppm). Similar to the trans-1,2,3,3,3-pentafluoroprop-1-enyl(monofluorophenyl)iodonium tetrafluoroborates, significant low frequent shifts are measured for the C-1 atoms (aryl group) of trifluorovinyl(monofluorophenyl)iodonium tetrafluoroborates (12 ppm up to 17.4 ppm, for para-, meta-, and ortho-F, respectively, compared with the chemical shifts of monofluoro(difluoroiodo)benzenes). In case of [m-C6H4F(CF2=CF)I][BF4], the shift of C-3,4 and C-5 was small (high frequent up to 3 ppm); but a significant high frequency shift was observed for C-2 and C-6 (up to 7 ppm). The same trend was found in case of [p-C6H4F(CF2=CF)I][BF4]: a significant high frequent shift for C-2 and C-6 (up to 6.5 ppm), while the position of C-3,4 and C-5 was nearly constant. For [o-C6H4F(CF2=CF)I][BF4], a small high frequency shift of C-2,3,4,5 and C-6 (up to 2.2 ppm) was observed.

–157.9i

–101.7h

[4-C6H4F(CF2=CF)I][BF4]

c 3

–79.0k

–79.1g –143.9

–143.2

–143.6

BF4

d 3

8.3

8.1

-

H2

7.5

-

7.9

H3

-

7.6

7.5

H4

H δ/ppm

J(F,H3,5) = 8.2 Hz, 4J(F,H2,6) = 4.3 Hz;

i 3

7.5

7.8

7.6

H5

8.3

8.1

8.3

H6

–2.1

–2.2

–2.3

BF4

B δ/ppm

11

e 3

J(F1,F2(cis)) = 127.1 Hz,

j 3

J(F2(cis),F1) = 127.4 Hz, 2J(F2(cis),F2(trans)) =

J(F2(trans),F1) = 59.8 Hz, 2J(F2(trans),F2(cis)) = 26.0 Hz;

J(F1,F2(cis)) = 127.3 Hz, 3J(F1,F2(trans)) = 60.3 Hz;

g 2

J(F,H2) = 7.6 Hz, 3J(F,H4) = 6.6 Hz;

J(F2(cis),F1) = 127 Hz, 2J(F2(cis),F2(trans)) = 26.2 Hz;

27.3 Hz; k 3J(F2(trans),F1) = 60.2 Hz, 2J(F2(trans),F2(cis)) = 27.3 Hz.

h 3

f 3

J(F2(trans),F1) = 61.0 Hz, 2J(F2(trans),F2(cis)) = 25.3 Hz;

J(F1,F2(trans)) = 60.1 Hz;

C6H4F) = 6.4 Hz;

3

–98.9j

–98.6f

–79.0c

F2(trans)

1

77

J(F1,F2(cis)) = 126.9 Hz, 3J(F1,F2(trans)) = 61.0 Hz, 5J(F1,o-C6H4F) = 3.9 Hz; b 3J(F2(cis),F1) = 126.8 Hz, 2J(F2(cis),F2(trans)) = 25.3 Hz, 6J(F,o-

–158.1e

–104.3d


a 3

–157.8a

–98.0b

F δ/ppm

F2(cis)

19

F1

–94.8

C6H4F


Compound

Table 10: 1H, 19F, and 11B NMR chemical shifts of trifluorovinyl(monofluorophenyl)iodonium tetrafluoroborates in CH2Cl2 at 24 °C


105.8


139.1

123.4

160.1

C2

120.7

163.0

117.9

C3

166.1

121.8 120.7

134.2

128.4

C5

C δ/ppm

137.8

C4

C6H4F

13

139.1

132.2

137.9

C6

C2 155.3b 155.3d 155.1f

C1 100.5a 100.5c 100.6e

Alkenyl

-

8.2

23.4

C1,F

9.5

26.2

255.5

C2,F

23.4

257.6

21.3

C3,F

258

20.7

8.2

C4,F

C6H4F

23.4

8

3.2

C5,F


9.5

3.5

-

C6,F

78

Hz.

Hz; e 1J(C1,F1) = 324 Hz, 2J(C1,F2(trans)) = 63.1 Hz, 2J(C1,F2(cis)) = 29.1 Hz; f 1J(C2,F2(cis)) = 312.2 Hz, 1J(C2,F2(trans)) = 288.8 Hz, 2J(C2,F1) = 32

Hz; c 1J(C1,F1) = 324 Hz, 2J(C1,F2(trans)) = 63 Hz, 2J(C1,F2(cis)) = 29.5 Hz; d 1J(C2,F2(cis)) = 312.4 Hz, 1J(C2,F2(trans)) = 289.3 Hz, 2J(C2,F1) = 32

J(C1,F1) = 324.9 Hz, 2J(C1,F2(trans)) = 63.3 Hz, 2J(C1,F2(cis)) = 29.7 Hz; b 1J(C2,F2(cis)) = 312.4 Hz, 1J(C2,F2(trans)) = 289.7 Hz, 2J(C2,F1) = 31.6

111.2


a1

98.7

C1


Compound

Table 11: 13C NMR chemical shifts and J(13C-19F) of trifluorovinyl(monofluorophenyl)iodonium tetrafluoroborates in CH2Cl2 at 24 °C


Results and Discussion 3.9.6.4 The

19

79

Alkenyl(pentafluorophenyl)iodonium Tetrafluoroborates F NMR spectra of the alkenyl(pentafluorophenyl)iodonium tetrafluoroborate salts

[C6F5(trans-CF3CF=CF)I][BF4] and [C6F5(CF2=CF)I][BF4] were measured in MeCN. Wellresolved signals for each fluorine were obtained. Only [C6F5(CF2=CF)I][BF4] was slightly soluble in CH2Cl2. Thus a direct comparison with the salts discussed before suffers from the different nature (solvates) of the dissolved species. The

13

C NMR spectra reveal that the signals of the aryl carbons for both [C6F5(trans-

CF3CF=CF)I][BF4] and [C6F5(CF2=CF)I][BF4] carbons appear at approximately the same values. The signal of C-1 atom is strongly shifted to low frequency (up to 20 ppm) comparing with C-1 in C6F5IF2 spectra.[47b] The other atoms (C-2,6, C-3,5, and C-4) were shifted slightly to higher frequency (up to 2.5 ppm). The signals for C-1 and C-2 in the CF2=CF group resonate at higher frequency when compared with the monofluorophenyl salts (up to 3 ppm). C-1 in the trans-CF3CF=CF group is shifted to high frequency (up to 2.5 ppm) when compared with [C6H4F(CF2=CF)I][BF4] while the other two carbons remain unchanged. The

chemical

shifts

and

coupling

constants

of

trans-1,2,3,3,3-pentafluoroprop-1-

enyl(pentafluorophenyl)iodonium tetrafluoroborate and trifluorovinyl(pentafluorophenyl)iodonium tetrafluoroborate are summarised in tables 12 and 13.

–155.3

–154.9

F3,5

C6F5 F1 –138.2b –157.0h

–140.0a –140.9e

F δ/ppm

F4

19

–95.7g

–117.8c

F2

XCF=CF

–77.2f

–67.4d

trans-X

–148.1

–152.0

BF4

B

–1.43

–1.35

BF4

δ/ppm

11

80

C6F5

147.3

147.6

C

2,6

138.5

138.6

C

3,5

147.4

147.8

C

4

145.1b 156.3e

103.2d

C

2

XCF=CF

127.5a

C

1

–

116.2c

trans-X

26

26

C ,F

1

C6F5

258

256.1

C ,F

2

257

257.3

C3,F

263

263.7

C4,F

J(C3,F1) = 5 Hz;

J(C2,F1) = 31 Hz.

3

2

J(C1,F1) = 327 Hz, 2J(C1,F2(trans)) = 64 Hz, 2J(C1,F2(cis)) = 31 Hz;

d 1

e 1

J(C2,F2(cis)) = 313 Hz, 1J(C2,F2(trans)) = 290 Hz,

J(C1,F1) = 354 Hz, 2J(C1,F2) = 63 Hz; b 1J(C2,F2) = 268 Hz, 2J(C2,F3) = 43.6 Hz, 2J(C2,F1) = 30.1 Hz; c 1J(C3,F3) = 276.5 Hz, 2J(C3,F2) = 36 Hz,

85.8

[C6F5(CF2=CF)I][BF4]

a 1

84.6

[C6F5(trans-CF3CF=CF)I][BF4]

C

1

Table 13: 13C NMR chemical shifts and J(13C-19F) of alkenyl(pentafluorophenyl)iodonium tetrafluoroborates in CD3NO2 at 24 °C 13 Compound C δ/ppm Coupling constants J/Hz

J(F4,F3,5) = 19.9 Hz, 4J(F4,F2,6) = 7.0 Hz; b 3J(F1,F2) = 138.8 Hz, 4J(F1,F3) = 10.4 Hz, 5J(F1,F2,6(C6F5)) = 5.2 Hz; c 3J(F2,F1) = 138.8 Hz, 3J(F2,F3) = 19.4 Hz, 6J(F2,F2,6(C6F5)) = 4.0 Hz; d 3J(F3,F2) = 19.4 Hz, 4J(F3,F1) = 10.3 Hz; e 3J(F4,F3,5(C6F5)) = 19.9 Hz, 4J(F4,F2,6(C6F5)) = 6.7 Hz; f3 J(F2(trans),F1) = 60.7 Hz, 2J(F2(trans),F2(cis)) = 27 Hz; g 3J(F2(cis),F1) = 125.2 Hz, 2J(F2(cis),F2(trans)) = 27 Hz, 6J(F2(cis),F2,6(C6F5)) = 5.2 Hz; h3 J(F1,F2(cis)) = 125.2 Hz, 3J(F1,F2(trans)) = 60.7 Hz, 5J(F1,F2,6(C6F5)) = 3.3 Hz.

–120.5


a3

–119.6

F2,6


Compound

Table 12: 19F and 11B chemical shifts of alkenyl(pentafluorophenyl)iodonium tetrafluoroborates in CH3CN at 24 °C



3.10

81

Thermal Stabilities of Selected (Difluoroiodo)benzenes and ArylContaining Iodonium Salts

The thermal behaviour and stability of XC6H4IF2, [XC6H4(trans-CF3CF=CF)I][BF4], [XC6H4(CF2=CF)I][BF4], [XC6H4(C6H5)I][BF4] (X = o-, m-, and p-F), [C6F5(transCF3CF=CF)I][BF4], and [C6F5(CF2=CF)I][BF4] have been investigated by differential scanning calorimetry (DSC) and the results are presented in table 14. Table 14: Data of thermal properties of XC6H4IF2, [XC6H4(trans-CF3CF=CF)I][BF4], [XC6H4(CF2=CF)I][BF4] (X = o-, m-, and p-F), [p-C6H4F(C6H5)I][PF6], [C6F5(trans-CF3CF=CF)I][BF4],

[C6F5(CF2=CF)I][BF4],

and

p-C6H4FIF4,

determined by DSC measurements and by visual melting point ___________________________________________________________________________ decomposition visual mp Tmpa Tdeca ∆Hb ___________________________________________________________________________ 61 78.0 237.9 59.48 o-C6H4FIF2 m-C6H4FIF2

76

86.4

202.1

11.62

p-C6H4FIF2

101

107.9

204.5

5.41

p-C6H4FIF4

89

92.4

241.2

12.8

[o-C6H4F(trans-CF3CF=CF)I][BF4]

129 - 131

134.2

c

c

[m-C6H4F(trans-CF3CF=CF)I][BF4]

134 - 136

141.5

c

c

[p-C6H4F(trans-CF3CF=CF)I][BF4]

90 - 91

92.2

c

c

[o-C6H4F(CF2=CF)I][BF4]

57 - 58

61.7

185.0

98.21

[m-C6H4F(CF2=CF)I][BF4]

71 - 73

72.6

181.4

92.8

[p-C6H4F(CF2=CF)I][BF4]

102 - 103

108.5

183.0

90.57


160 - 162

161.2

199.6

1.50


108 - 110

109.7

141.5

35.49

[o-C6H4F(C6H5)I][BF4]

140 - 142

145.6

c

c

[m-C6H4F(C6H5)I][BF4]

125 - 127

131.6

c

c

[p-C6H4F(C6H5)I][BF4]

134 - 136

141.6

277.6

32.24


125 - 126

129.6

c

c

___________________________________________________________________________ Tmp: melting point (endothermic); Tdec : decomposition point (exothermic) a

onset temperature; b enthalpy (Jg–1); c no sharp exothermic signal followed the melting point

signal.


82

The results of the thermal analysis indicate that C6H4FIF2 (o-, m-, and p-F) have a suitable thermal stability and thereby this property is similar to that reported recently by Frohn et al. for C6F5IF2.[47b, 87] All three C6H4FIF2 compounds (o-, m-, and p-F) were thermally stable up to 200 °C. Table 14 shows that the stability of o-C6H4FIF2 was higher than that of meta- and para-F. The higher thermal stability of the ortho-isomer may be caused by the intramolecular interaction (agostic effect). This effect should be more expressed in the case of C6F5IF2 which is really more stable than the nonfluorinated ArIF2 analogues compounds.[87] Even p-C6H4FIF4 showed high thermal stability higher than that of p-C6H4FIF2. For the salts [C6H4X(trans-CF3CF=CF)I][BF4] (X = o-, m-, and p-F), no real point of decomposition was found by DSC till 500 °C. In the other iodonium salts [C6H4X(CF2=CF)I][BF4] (X = o-, m-, and p-F), the decomposition took place at 185.0, 181.4 and 183.0 °C for ortho-, meta-, and para-F, respectively. Different to the C6H4F compounds, the salts [C6F5(alkenyl)I][BF4] (alkenyl = CF2=CF, transCF3CF=CF) decomposed shortly after the meting point, what indicates that the stability of the electrophilic cation in combination with the [BF4]– anion is strongly associated with the missing mobility in the solid state. In the three diaryliodonium tetrafluoroborates [C6H4F(C6H5)I][BF4] (X = o-, m-, and p-F) only [p-C6H4F(C6H5)I][BF4] displays unambigously an exothermic effect for the decomposition process in DSC measurements. In case of [o-C6H4F(C6H5)I][BF4], [m-C6H4F(C6H5)I][BF4] and [p-C6H4F(C6H5)I][PF6] no clear exothermic effect for decomposition was observed but in a second scan of the same sample no melting point (endothermic effect) was measured again, which confirms the decomposition in the first scan. Previously, it has been reported that the anion has an important effect on the stability and reactivity of iodonium salts. It was suggested that the distance between the centres of the cation and the anion is the key factor in this effect.[112] Recently Naumann has found that the stability of diaryliodonium salts increases from tetrafluoroborates to higher stability of trifluoromethanesulfonate

via

intermediate

trifluoroacetate.[113]


showed only an endothermic consumption (306.9 °C) of energy after melting which may be caused by the sublimation.


3.11

83

X-Ray Crystal Structure Analysis

Crystallographic data for p-C6H4FIF2, o-C6H4FIF2, [m-C6H4F(C6H5)I][BF4], p-C6H4FIOF2 and [p-C6H4F(trans-CF3CF=CF)I][BF4] were obtained from single crystals and are listed in tables 15 - 18. The molecular structures of p-C6H4FIF2, o-C6H4FIF2, [m-C6H4F(C6H5)I][BF4], [p-C6H4F(trans-CF3CF=CF)I][BF4] and p-C6H4FIOF2 are presented in figures 8, 11, 15, 18, and 21, respectively. The interatomic distances, angles and torsion angles of p-C6H4FIF2, o-C6H4FIF2, [m-C6H4F(C6H5)I][BF4], [p-C6H4F(trans-CF3CF=CF)I][BF4] and p-C6H4FIOF2 are presented in the appendix (tables 29 – 34).

3.11.1

The Crystal Structures of p-C6H4FIF2 and o-C6H4FIF2

p-C6H4FIF2 was crystallised from methylene chloride at –25 °C as white needles. The crystals belonged to the orthorhombic space group Cmca with a = 6.289(3) Å, b = 7.070(4) Å, c = 30.415(14) Å, α = β = γ = 90 °, V = 1352.3(12) Å3 and Z = 8 (calculated density = 2.554 mg / m3). Single crystals of o-C6H4FIF2 containing solvent molecules were received from CH2Cl2 at –20 °C. o-C6H4FIF2 ⋅ 0.45 CH2Cl2 crystallised in the monoclinic space group C2/c with a = 18.851(6), b = 12.742(6), c = 7.028(3) Å, α = 90 °, β = 106.927(9) °, γ = 90 °, V = 1614.9(11) Å3 and Z = 8 (calculated density = 2.322 mg / m3). Figures 8 and 14 show both molecules. Relevant interatomic distances and angles are given in the caption of the figures. In the molecular structures of p-C6H4FIF2 and o-C6H4FIF2, the IF2 group is not exactly linear (∠ F-I-F = 168.5(2) ° and 169.08(19) °). All angles C-I-F were determined to ca. 86 °. In both structures, a T-shaped environment was observed at iodine, whereby the two F-atoms were in the axial positions and the ipso C-atom of the C6H4F group took one of the three equatorial sites in the Ψ-trigonal bipyramide. The hypervalent bond IF2 (3c–4e) in both compounds was slightly unsymmetric.[1, 13] The IF2 triad in p-C6H4FIF2 for example showed I–F distances which differ by 0.4 %. All I–F distances in p-C6H4FIF2 (2.016(6) and 2.024(5) Å) and o-C6H4FIF2 (2.014(4) and 2.015(5) Å) are equal in the limit of 3σ. Comparing the I–F distances in p-C6H4FIF2 and o-C6H4FIF2 with those in C6F5IF2 (2.025(2) and 1.959(2) Å)[47b] or in C6F5IF4 (1.910(4) and 1.929(4) Å)[114] a trend due to the different


84

effective charges on iodine is detectable. The only additionally reported I–F distances in p-MeC6H4IF2 (2.14 and 1.56 Å) should be incorrect,[115] because the sum of covalent radii of I and F is 1.97 Å[116] and the value of 1.56 Å is smaller within 21 %. (From literature it is known that the I-F distance in C6F5IFn decreases with increasing n)[47b, 114] The I–C distance in p-C6H4FIF2 (2.084(9) Å) is longer than in o-C6H4FIF2 (2.049(9) Å). The course of the distance I-Cipso in C6F5IFn molecules (n = 0, 2, 4) is known in literature and increasing from C6F5IF2 (2.068(4) Å), over C6F5I (2.077(4) Å) to C6F5IF4 (2.081(7) and/or 2.088(5) Å). On the first sight, it is unexpected that the I-Cipso distance in o-C6H4FIF2 is shorter than in comparable compounds containing more fluorine atoms in the aryl group (e.g. C6F5). It was supposed that the high effective positive charge on iodine in C6F5IF4 causes a positive partial charge on C(1). Consequently C-I becomes longer because of electrostatic repulsion. A further argument derived from the repulsion of both o-F atoms and one IF2 triad in the IF4 group. The perpendicular orientation of one IF2 group guarantees the minimum repulsion. But in such a case the second IF2 group shows the maximum repulsion. In the staggered conformation of the IF4 group, which is realised in the molecule structure, all four F atoms bonded to iodine underlay repulsion from both o-F atoms. In o-C6H4FIF2 we can discuss an intramolecular interaction (agostic effect) (I⋅⋅⋅Fortho = 3.12(5) Å). Caused by this interaction the I-C distance (2.049(9) Å) is short (1.5 % shorter than in C6F5IF4). F(1), I, and F(2) define a plane which forms an angle with the aryl plane of 69.41(69) ° (F(1)I-C(1)-C(2)) in p-C6H4FIF2 and 68.7(8) ° in o-C6H4FIF2 (F(2)-I-C(1)-C(6)). o-C6H4FIF2 and p-C6H4FIF2 show significant intermolecular interactions. I(III) of each ArIF2 forms two contacts to two fluorine atoms of two ArIF2 molecules in neighbourhood. Thus, rhombohedral I2F2 units are established. Overall a zigzag chain of iodine edge-chaired I2F2 units with I-F contacts of 2.920(4) and 2.978(5) Å in the ortho-isomer and 2.966(6) and 3.031(5) Å in the para-isomer are not the shortest I-F contacts known in literature (fig. 10 and 12). In C6F5IF2 with stronger electrostatic I-F interactions the shortest I⋅⋅⋅F contact was determined to 2.742(2) Å.[47b, 87]


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The packing of the o-C6H4FIF2 molecules and CH2Cl2 in the crystal shows a layer structure along [001] with alternating orientation of the phenyl groups above and below the layer. The space between the layers which is not filled by the IF2 group is taken over by CH2Cl2.

Figure 8:

The molecular structure of p-C6H4FIF2. Selected distances [Å], angles [°], and torsion angles [°] of the p-C6H4FIF2 molecule: I(1)-F(1) 2.016(6), I(1)-F(2) 2.024(5), I(1)-C(1) 2.084(9), C(4)-F(3) 1.358(12); F(1)-I(1)-F(2) 168.5(2), F(1)I(1)-C(1) 83.9(3), F(2)-I(1)-C(1) 84.6(3); (F(1)-I-C(1)-C(2) 69.41(69).


Figure 9:

The structure of p-C6H4FIF2 shows two orientations for the phenyl group around the IF2 axis.

Figure 10:

86

Intermolecular contacts in p-C6H4FIF2: the zigzag chain of I2F2 rhomboids.


Figure 11:

87

The molecular structure of o-C6H4FIF2. Selected distances [Å], angles [°], and torsion angles [°] of the o-C6H4FIF2 molecule: I(1)-F(11) 2.015(5), I(1)-F(12) 2.014(4),

I(1)-C(1)

2.049(9),

C(6)-F(13)

1.357(10);

F(11)-I(1)-F(12)

169.08(19), F(12)-I(1)-C(1) 84.2(2), F(11)-I(1)-C(1) 84.9(2), F(2)-I-C(1)-C(6) 68.7(8).

Figure 12: The packing of the o-C6H4FIF2 molecules, viewed down the [100] axis, showing only the intermolecular I⋅⋅⋅F contacts and carbon C(1) atom of the aryl rest.


Figure 13: The packing of the o-C6H4FIF2 molecules, viewed down the [001] axis.

Figure 14: The packing of the o-C6H4FIF2 molecules, viewed down the [010] axis.

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Table 15: Crystal data and structure refinement of p-C6H4FIF2 and o-C6H4FIF2 p-C6H4FIF2

o-C6H4FIF2

Chemical formula

C6H4F3I

C6.45H4.90Cl0.45F3I

Formula weight

259.99

282.26

Temperature

150 K

153(2) K

Wavelength

0.71073 Å

0.71073 Å

Crystal system

Orthorhombic

Monoclinic

Space group

Cmca

C2/c

Unit cell dimensions

a = 6.289(3) Å

a = 18.851(6) Å

b = 7.070(4) Å

b = 12.742(6) Å

c = 30.415(14) Å

c = 7.028(3) Å

α

90 °

90 °

β

90 °

106.927(9) °

γ

90 °

90 °

Volume

1352.3(12) Å3

1614.9 Å3

Z

8

8

Calculated density

2.554 mg / m3

2.322 mg / m3

Absorption coefficient

4.706

4.095

F(000)

960

1050 3

Crystal size

0.60 x 0.35 x 0.12 mm

0.20 x 0.10 x 0.10 mm3

Theta range

2.68 to 27.07 °

1.96 to 28.31 °

Limiting indices

–8