Remarkable two-step, four-electron oxidative addition reactions at ...

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Remarkable two-step, four-electron oxidative addition reactions at phosphorus [P(I)–P(V)] in terminal electrophilic phosphinidene complexes1 Todd W. Graham, Konstantin A. Udachin, Marek Z. Zgierski, and Arthur J. Carty

Abstract: The reactivity of the terminal phosphinidene complex [Re(CO)5{η1-P(NiPr2)}]AlCl4 towards the oxidants S8 and Me3SnN3 has been investigated. These result in unprecedented four-electron oxidative addition reactions, affording complexes containing the P(V) ligands η1-P(=S)(Cl)(NiPr2) and η1-P(=NR)(Cl)(NiPr2), respectively. DFT calculations support the proposed mechanism. Key words: electrophilic phosphinidene, low coordinate phosphorus, rhenium, oxidative addition, P(I) ligand. Résumé : On a étudié la réactivité de phosphinidènes terminaux [Re(CO)5{01-P(NiPr2}]AlCl4 vis-à-vis des oxydants S8 et Me3SnN3. Ces interactions donnent lieu à des réactions d’addition oxydantes à quatre électrons inconnues jusqu’à maintenant qui conduisent à la formation de complexes comportant respectivement les ligands P(V) 01P(=S)(Cl)(NiPr2} et 01-P(=NR)(Cl)(NiPr2}. Des calculs selon la théorie de la fonctionnelle de la densité sont en accord avec le mécanisme proposé. Mots-clés : phosphinidène électrophile, phosphore à faible coordination, rhénium, addition oxydante, ligand P(I). [Traduit par la Rédaction]

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Introduction Over the years since the isolation and structural characterization of the first terminal, cationic, electrophilic phosphinidene complex of molybdenum in 2001 (1a), it has become clear that an extensive range of these phosphinidene-based analogues of Fischer carbenes can be synthesized. Examples of [LnM(=PNR2)] complexes with metals from groups VI (1), VII (2), VIII (1b, 3), IX (4), and X (5) have been described, and a comprehensive picture of bonding and reactivity in these molecules is beginning to emerge, which complements an extensive reaction manifold already established for transient (6a, 6b) and nucleophilic (6c, 7) phosphinidenes. The bonding in terminal, electrophilic phosphinidene complexes consists primarily of a σ donor interaction from one phosphorus lone pair to the metal center with weak π back donation from the metal into the vacant p orbital on phosphorus. The electrophilicity of the phosphorus is partially alleviated by π donation from the NR2 group. Structurally, these bonding interactions are typically manifested in slightly shortened M–P bonds, planar geometries at the nitrogen substituents, and short P–N bonds

(1b, 2–4). Accordingly, aminophosphinidene ligands (P– NR2) are regarded as isoelectronic to CO, with phosphorus in the +1 oxidation state. Chemically, as expected from the bonding model, such complexes undergo facile nucleophilic addition at the P(I) center forming adducts with Lewis bases (2, 8), in contrast to nucleophilic phosphinidene complexes such as [Cp*Ir(PPh3)(=PMes*)] (9), which react with electrophilic reagents. The oxidative addition reaction is a key step in many important organometallic transformations (10). It typically proceeds via an addition of a reagent X–Y to a metal complex [M], resulting in the formation of a new complex of the form [M](X)(Y) and in a change of the oxidation state of the metal from (n)+ to (n + 2)+. Many of the reactions with terminal aminophosphinidene complexes we have previously described have resulted in conversion of the P(I) ligand to P(III); for example, conversion to a coordinated phosphaalkene (R2NP=CR2) or phosphaimine (R2NP=N-N=CPh2) ligand (3b). We have also observed reactions in which activated X–H bonds (X = SiR3, OR, or C6H4N2Ph) oxidatively add to the P(I) ligand, affording coordinated functionalized phosphines (P(H)(X)NR2) (2, 11).

Received 7 August 2007. Accepted 23 August 2007. Published on the NRC Research Press Web site at canjchem.nrc.ca on 28 September 2007. This article is dedicated to Dr. George Michael Bancroft for his pioneering application of Mössbauer and electron spectroscopy to fundamental and applied science and for the leadership he has provided to the Canadian Synchrotron community. T.W. Graham, K.A. Udachin, M.Z. Zgierski, and A.J. Carty.2 Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, ON K1A 0R6, Canada; The Ottawa–Carleton Chemistry Institute, University of Ottawa, Ottawa, ON K1N 6N5, Canada. 1 2

This article is part of a Special Issue dedicated to Professor G. Michael Bancroft. Corresponding author (e-mail: [email protected]).

Can. J. Chem. 85: 885–888 (2007)

doi:10.1139/V07-119

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Scheme 1. Cl i

N Pr2 P Cl OC CO Re CO OC S

C 2a O

S

- AlCl3

Re

OC

CO CO

C O

A

i

N Pr2 P Cl OC O Re C OC CO C 2b O

R N

P OC

BuS OC OC

P Re

1/8 S8

Ni Pr2

OC t

CO AlCl4 CO

BuSH

C O

5

We now wish to report a new and unprecedented type of oxidative addition reaction, which results in the conversion of a P(I) center in cationic [Re(CO)5{η1-P(NiPr2)}]AlCl4 (1) to a P(V) ligand in neutral [Re(CO)5{η1-P(=S)(Cl)(NiPr2)}] (2) via reaction with S8. A similar four-electron oxidative addition occurs on reaction of 1 with Me3SnN3 affording the novel neutral dimer [Re(CO)5{P(Cl)(NiPr2)=N(AlCl2)}]2 (3).

Results and discussion The complex [Re(CO)5{η1-P(NiPr2)}]AlCl4 (1) (2), prepared by halide abstraction from [Re(CO)5{η1-P(Cl)NiPr2}] (4) by AlCl3, reacts smoothly with S8 in dichloromethane, precipitating AlCl3 and affording a quantitative yield of the new compound [Re(CO)5{η1-P(=S)(Cl)(NiPr2)}] (2) (Scheme 1), which displays a 31P NMR resonance at δ 100 (see Experimental Section), sharply upfield of that in 1 (δ 956). The 1H NMR spectrum of 2 shows only resonances due to the iPr groups. A single crystal X-ray analysis3 (Fig. 1) revealed that a new one-electron donor P(V) ligand P(=S)(Cl)NiPr2 is coordinated to a typical square-pyramidal Re(CO)5 fragment. The Re(1)—P(1) bond length of 2.4972(8) Å is significantly longer than the corresponding Re—P distance in the precursor phosphinidene complex 1 (2) (2.446(3) Å). The P(1)—S(1) separation of 2.181(1) Å is considerably longer than the short P=S separation of Ph3P=S (1.950(3) Å) (12), which may be attributed to competitive π bonding to phosphorus from the sulfur atom and the nitrogen lone pair of the NiPr2 substituent. In agreement with this, the P(1)—N(1) bond length (1.661(3) Å) indicates substantial residual π character (4). While oxidation of R3P to form R3P=S is a well-known reaction (13), the direct conversion of P(I) to P(V) is a new reaction pathway for coordinated phosphorus ligands. Complex 2 is likely formed from the intermediate complex A (Scheme 1) via oxidative sulfuration; the highly electrophilic phosphorus center in A then 3

Ni Pr2

R N

. . Ni Pr2 - N2 P CO Re CO C O

AlCl4

i

P

Ni Pr2 Cl

Re(CO)5 C - RCl +Cldimerization

B

RN3

OC t

P

Re(CO)5

S

H

AlCl2

AlCl4

Cl AlCl3 Ni Pr2

Re(CO)5

Cl

Pr2N

1 0.5

P

N

AlCl2 N

Cl2Al

3

Ni Pr2 P Cl Re(CO)5

Fig. 1. ORTEP diagrams of complexes 2 and 3. Hydrogen atoms have been omitted for clarity, and thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (°): 2: Re(1)—P(1), 2.4972(8); P(1)—S(1), 2.181(1); P(1)— N(1), 1.661(3); Re(1)–P(1)–S(1), 102.16(4); Re(1)–P(1)–Cl(2), 112.31(4); Re(1)–P(1)–N(1), 118.1(1); S(1)–P(1)–Cl(2), 105.28(5); S(1)–P(1)–N(1), 103.6(1); Cl(2)–P(1)–N(1), 113.3(1). 3: Re(1)—P(1), 2.5249(7); P(1)—Cl(1), 2.1298(9); P(1)—N(1), 1.660(2); P(1)—N(2), 1.600(2); N(2)—Al(1), 1.861(2); Al(1)— Cl(2), 2.143(1); Al(1)—Cl(3), 2.152(1); Re(1)–P(1)–Cl(1), 104.03(3); Re(1)–P(1)–N(1), 118.48(8); Re(1)–P(1)–N(2), 117.11(8); P(1)–N(2)–Al(1), 133.3(1); N(2)–Al(1)–Cl(3), 111.85(8); Cl(2)–Al(1)–Cl(3), 104.80(4).

reacts with chloride ion to form the product. Although we have not been able to observe intermediate A, DFT analyses (14) of the cations 1, A, and the neutral product 2 support the proposed mechanism. The energy of intermediate A formed by adding sulfur to the phosphinidene cation 1 lies 5.45 eV below the sum of the energies of 1 and atomic S, thus A is an energetically stable species. However, addition of chloride ion to A provides an additional stabilization energy of 5.82 eV. There is therefore a strong driving force for

X-ray data for 2: FW 524.91; orthorhombic; Pna2(1); a = 24.342(1) Å; b = 7.6634(7) Å; c = 9.2297(8) Å; T = 125(2) K; V = 1721.7(3) Å3; Z = 4; Dcalcd = 2.025 Mg/m3; 4709 independent reflections (R(int) = 0.0234); GOF = 1.150; R1 = 0.0184; wR2 = 0.0458. © 2007 NRC Canada

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the oxidation from P(I) in 1 to P(V) in 2. The minimized geometry of 2 matches the major features of the X-ray data with the HOMO being a localized 3p orbital on the sulfur atom. The LUMO of A is centered on phosphorus, which also bears a positive charge (+0.52 e (Mulliken)), consistent with nucleophilic attack by Cl– at this atom. Experimental and calculated (DFT) P—S and P—N bond lengths in 2 confirm that the ground state has substantial contributions from the mesomeric form 2b (Scheme 1). Synthetically, we have found that 2 can also be generated directly from the neutral terminal chlorophosphido complex [Re(CO)5{η1-P(Cl)NiPr2}] (4) and S8, although the reaction is qualitatively much slower than with complex 1. 31P NMR spectroscopy of solutions of 1 showed no indication of the presence of any of the phosphide complex 4, and exhaustive pentane extractions of solid complex 1 afforded no trace of highly soluble 4. While we cannot completely eliminate the possibility of trace amounts of 4 in equilibrium with 1, we believe the experimental evidence coupled with the DFT calculations favor the intermediacy of A in the oxidative conversion of 1 to 2. By contrast, the reaction of 1 with tBuSH affords the new P(III) complex [Re(CO)5{P(NiPr2)(H)(StBu)}]AlCl4 (5), which results from oxidative addition of the S–H bond to the P(I) ligand. This compound has been fully characterized (see Experimental Section) and displays a 31P NMR resonance at δ –11. The 1H NMR spectrum of 5 shows the expected tBu and iPr resonances, in addition to a characteristic P–H signal at δ 8.62 with a large 1JPH of 424 Hz. The reaction of 1 with Me3SnN3 readily affords the new complex [Re(CO)5{P(Cl)(NiPr2)=N(AlCl2)}]2 (3) (Scheme 1). The 31P NMR spectrum of the product shows a high-field resonance at δ 81 (cf. δ 100 in 2), and the 1H NMR spectrum shows only the presence of iPr signals (see Experimental Section). The X-ray analysis4 confirms that in the centrosymmetric structure, the rhenium and phosphorus ligand environments in 3 are closely related to those in 2, with a one electron P(V) fragment coordinated to each octahedral rhenium center in the dimeric molecule. The P–N(2) bonds of the two equivalent P(V) ligands are bound to two AlCl2 units forming an N2Al2Cl4 bridge between [Re(CO)5{P(Cl)(NiPr2)}] entities. The P(1)—Cl(1) (2.1298(9) Å) and P(1)—N(1) (1.660(2) Å) distances are typical, and the P=N separation (1.600(2) Å) is as expected for an R3P=N ligand (15). The µ-AlCl2 fragments have an average Al—Cl separation of 2.147 Å and average N—Al distances of 1.855 Å. By analogy with compound 2, 3 likely forms by initial conversion to the extremely electrophilic intermediate B. Nucleophilic attack of the RN nitrogen in B on AlCl4– forms C; subsequent chloride attack on phosphorus with concomitant elimination of RCl and dimerization forms compound 3. These results, coupled with previous studies describing facile P=C, P=N, and P=S bond-forming reactions at both

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terminal (3b) and doubly bridging (16) phosphinidene complexes, illustrate the synthetic potential of electrophilic P(I) centres for the generation of new types of coordinated P(III) and P(V) ligands.

Conclusion In conclusion, we have demonstrated for the first time that terminal electrophilic phosphinidene complexes can undergo two step, four-electron oxidative addition reactions leading to new types of coordinated P(V) ligand systems. We attribute the remarkable and unprecedented reactivity of the phosphinidene complexes to the low oxidation state (i.e., P(I)) and coordination number of the phosphinidene ligand, both of which favor oxidative addition. We note that maingroup compounds, which contain phosphorus in low coordination environments (17), are known, and we anticipate exciting and complementary reactivity patterns to emerge.

Experimental5 General comments All procedures were carried out using standard Schlenk techniques or in a glovebox under a nitrogen atmosphere. THF was distilled from Na/benzophenone. Dichloromethane and pentane were purified using solvent purification columns containing alumina (dichloromethane) or alumina and copper catalyst (pentane). CDCl3 and CD2Cl2 were vacuumdistilled from AlCl3 and then CaH2. C6D6 was vacuumdistilled from Na/benzophenone. NMR spectra were recorded at 400.13 MHz (1H), 161.98 MHz (31P{1H}), or 100.61 MHz (13C{1H}). Cl2PNiPr2 (18) was prepared according to the literature procedures. Re2(CO)10 and AlCl3 were obtained from Strem Chemicals. All other reagents were obtained from Aldrich and were used as received. [Re(CO)5{␩1-P(=S)(Cl)(NiPr2)}] (2) 13 mg (0.0507 mmol) of S8 was added to 250 mg (0.399 mmol) of [Re(CO)5{P(NiPr2)}]AlCl4, the mixture was stirred for 1 h, and then the solvent was removed in vacuo. The solid was dissolved in ~3 mL of THF and filtered, and then pentane was added dropwise until the solution became cloudy. Cooling to − 45 °C overnight resulted in the formation of large crystals of AlCl3(THF)2, which were filtered off. This was repeated twice until all AlCl3(THF)2 was removed from the solution. On standing, compound 2 crystallized in 50% yield. Analytically pure compound 2 may also be prepared by adding 1/8 equiv. of S8 to [Re(CO)5(P(Cl)NiPr2)] at − 45 °C in THF and stirring for 2 h, followed by recrystallization from THF–pentane at − 45 °C. Yield: 67%. 1H NMR (δ, CDCl3, 25 °C): 3.96 (sept., 3JHH = 6.7 Hz, 2H, CH(CH3)2); 1.61 (d, 3JHH =

4

X-ray data for 3: FW 2418.97; monoclinic; P21/c; a = 9.176(1) Å; b = 18.493(2) Å; c = 13.725(1) Å; β = 123.120(6)°; V = 1950.7(4) Å3; Z = 1; Dcalcd. = 2.059 Mg/m3; 5442 independent reflections (R(int) = 0.0240); GOF = 0.945; R1 = 0.0201; wR2 = 0.0477.

5

Supplementary data for this article are available on the journal Web site (canjchem.nrc.ca) or may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 5219. For more information on obtaining material, refer to cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 661898, and 661599 contain the crystallographic data for this manuscript. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or [email protected]). © 2007 NRC Canada

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6.7 Hz, 6H, CH(CH3)2); 1.50 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2). 31P NMR (δ, CDCl3, 25 °C): 100. IR (νCO, cm–1, CH2Cl2): 2150 (m, sh); 2087 (m, sh); 2045 (s); 2024 (s). Anal. calcd. for C11H14ClNO5PSRe: C, 25.2; H, 2.7; N, 2.7. Found: C, 25.5; H, 2.5; N, 2.4. [Re(CO)5{␩1-P(NiPr2)(Cl)(=NAlCl2)}]2 (3) 10 mL of CH2Cl2 was added to 55 mg (0.412 mmol) of AlCl3 and 200 mg (0.406 mmol) of [Re(CO)5(P(Cl)NiPr2)], and the mixture was stirred for 5 min. After cooling the suspension to − 45 °C, 84 mg (0.404 mmol) of Me3SnN3 was added. The mixture was stirred for 4.5 h and was then filtered through Celite. The solution was concentrated in vacuo to ~1 mL and was then cooled to − 45 °C overnight resulting in the formation of a white crystalline product, which was collected and dried in vacuo. Yield: 145 mg, 60%. 1H NMR (δ, CD2Cl2, − 45 °C): 4.53 (m, 4H, CH(CH3)2); 1.48 (d, 3 JHH = 6.8 Hz, 12H, CH(CH3)2); 1.45 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2); 1.44 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2). 31P NMR (δ, CDCl3, 25 °C): 80.5 (br, s). IR (νCO, cm–1, CH2Cl2): 2154 (w); 2102 (w); 2093 (w); 2052 (s); 2045 (s); 2030 (m, sh). Anal. calcd. for C22H28Al2Cl6N4O10P2Re2: C, 21.9; H, 2.3; N, 4.6. Found: C, 21.6; H, 2.3; N, 4.6. [Re(CO)5{␩1-P(StBu)(H)(NiPr2)}]AlCl4 (5) [Re(CO)5{η1-P(NiPr2)}]AlCl4 in 10 mL of dichloromethane was prepared as described earlier from 200 mg (0.406 mmol) of [Re(CO)5(P(Cl)NiPr2)]. The suspension was cooled to − 80 °C, and then 46 µL (0.408 mmol) of t BuSH was added. The mixture was warmed to room temperature, resulting in dissolution of the solid, and was stirred for 20 min. The solvent was removed in vacuo, and the residue was recrystallized from dichloromethane–ether at − 45 °C. Yield: 60%. 1H NMR (δ, CDCl3, 25 °C): 8.62 (d, 1 JPH = 424 Hz, 1H, P-H); 3.70 (sept., 3JHH = 6.8 Hz, 2H, CH(CH3)2); 1.63 (s, 9H, C(CH3)3); 1.42 (d, 3JHH = 6.7 Hz, 6H, CH(CH3)2); 1.38 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2). 31P NMR (δ, CDCl3, 25 °C): –11. IR (νCO, cm–1, CH2Cl2): 2156 (w); 2053 (s); 2035 (m, sh). Anal. calcd. for C15H24AlCl4NO5PSRe: C, 25.1; H, 3.4; N, 2.0. Found: C, 24.9; H, 3.0; N, 2.0.

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