Synthesis and physical-chemical properties of ionic

J. Chim. Phys. (1998) 95, 1626-1639 Q EDP Sciences. Les Ulis

Synthesis and physical-chemical properties of ionic liquids based on l-n-butyl-3-methylimidazolium cation P.A.Z. ~ u a r e z ' ,S. in loft^, J.E.L. ~ullius', R.F. de ~ o u z aand ' J. ~ u ~ o n t ' ~ '

' Instituto de Quimica, UFRGS, AV. Bento Gonpalves, 9500, CEP 91501-970, Porto Alegre, RS, Brazil Present address: lnstituto de Quimica, PUCRS, Porto Alegre, RS, Brazil (Received 8 January 1998; accepted 76 March 7998)

Correspondence and reprints.

RESUME La reaction du chlonue de I-n-butyl-3-mCthylimidazolium (BMI.CI) avec le tetrafluoroborate ou l'hexafluoropliosphate de sodium produit les sels fondus B M . X (1, X = BF4 et 2, X = PFb). Les composes 1 et 2 sont des liquides visqueux dans une grande gamme de temperature (jusqu'a 192 K). Les analyses par XR et RMN ('H et '"C),densite, viscosite et conductivite suggerent que le compose 2 presente un comportement quasi-moleculaire. Le colnpose l est quasi-molecdaire en dessous de 279 K, mais a plus hautes temperat~uesil est probableinent sous une forme ionique qui resulte d'interactions entre les ions i~nidazoliumet tetrafluoroborate a travers des liasons du type pont d'hydrogene. mots clCs : l -b~1tyl-3-rnethyli1nidazolium, sel fondu, paire ionique, propri6tis physicochiiniques.

ABSTRACT The reaction of l-n-butyl-3-~netl~ylimidazoliurn chloride (BMI.CI) with sodium tetrafluoroborate or sodium hexafluorophosphate affords the molten salts Bh4I.X (1, X= BF4 and 2, X= PFs). Compounds l and 2 are viscous liquids within a wide range of temperature (down to 192 K). IR, NMR, density, viscosity and conductivity measurements suggest that compound 2 behaves quasi-molecular. Compound 1 is quasi-molecular below 279 K, but at higher temperatures is probably composed of irnidazolium and tetrafluoroborate ions in an extended hydrogen-bonded network.

key words : I-butyl-3-methylimidazolium, molten salt, ion pair, physical-chemical properties.

Synthesis and properties of imidazolium ionic liquids

1627

INTRODUCTION Among the various known room temperature molten salts, those obtained fiom the mixture of 1,3-dialkylimidazolium chloride with AlCl, have been receiving special attention due to their large applications, for example, in electrochemistry [l -41 and as solvents in two-phase catalytic oligomerization reactions [5-81. The applications of al~uninumbased ionic liquids as solvents for transition metal complexes and in electrochemistry are limited because these ionic liquids are moisture sensitive and, moreover, most of transition metal complexes and organic substrates are not inert to the organo-aluminate compounds. Air and water stable molten can be obtained by the substitution of the chloro-aluminate anion by weakly complexing anions such as BF4 [9], PF6 [l01 and CF3S03 [ l ] ] in the 1-ethyl-3-methylimidazoliumcompound.

However, the relative high melting point of these compounds limits their application as solvent for two-phase catalysis and electrocl~emistry. Nevertheless, air and water stable ionic liquids with a wide range of liquid phase have been recently prepared fiom I -n-butyl-3-metl~ylimidazolium associated with non-coordinating anions such as BF4 1

and PFa 2 [12]. These ionic liquids are fluid within a wide range of temperature (down to 192 K), possess an increased electroche~nicalwindow (up to 7.0 V) [l31 and are excellent solvents for organoinetallic catalyst precursors [12,14,15]. In particular, the large electrochemical window offers the use of these ionic liquids as unique solvents for electrochemical and spectroscopic investigations. However, the applications of these ionic liquids presupposes the knowledge of their physical-chemical properties. We report herein several physical-chemical properties of the ionic liquids 1 and 2. Moreover, we present some evidences suggesting that these liquids possess a quasimolecular structure. Some aspects of the present work have been communicated previously [l 31. J. Chim. Phys.

P.A.Z. Suarez et al.

EXPERIMENTAL

General. All preparative procedures were carried out under dry argon using standard Scldenk tube techniques. The solvents were distilled from appropriate drying agents under argon. The NaX salts were available from Aldrich and have been used as purchased.

Spectroscopic Data. 'H NMR and "C{'H)NMR were recorded on a Varian Gemini 200 MHz spectrometer. The chemical shifts were measured in ppm relative to TMS as external standard. IR spectra were recorded on a Mattson Galaxy 3000 FTIR spectrophotometer, the wavenumbers were calibrated against standard polystyrene film.

IlYeltirtg and Freezing measurements. The calorimetric experiments were carried out on a 12000 PL-DSC.

Deterntination of Solubility of l in water. All solubilities were measured in a Schlenk tube under argon. The Schlenk tube containing a known solution of 1 in water was kept in a constant-temperature bath. Then, the system was slowly cooled, under efficient stirring, until the solution becomes clouded and the temperature was measured.

Deizsity hfeasureinettfs. All densities were measured in a Pyrex dilatometric tube under argon. The dilatorneter was colnposed of a bulb (5 cm" with a capillary miliinetricaly graduated column (0.1 cm internal diameter; 20 cm long) and was calibrated with distilled water. The dilatoineter was placed in a constant-temperature bath and the vol~uneof a known mass of ionic liquid was determined.

Condiictivity measurements. All the conductivity measurements were camed out under argon in a Schlenk tube as cell, using a commercial platinum electrode calibrated at 298 K, by using 1 delnal aqueous KC1 solution [16]. Resistance measurements were performed with a HP Model 4265B Wheatstone bridge.

Synthesis and properties of imidazoliurn ionic liquids

1629

Viicosity measurements. A Ostwald viscosimeter adapted for use under argon was used. The viscosimeter was calibrated with glycerol [17]. At least three runs were made for each sample at each temperature, and the mean flow time was used in the calculations of the absolute viscosity. The viscosimeter was placed in a constanttemperature bath. Syntltesk. The l-n-butyl-3-methylimidazolium chloride salt 3 was prepared as

described by Wilkes and co-workers [18]. Compounds 1 and 2 have been prepared according to literature procedure [l 31. Compound 1 was further purified by dissolving in water (1 :l compound llwater) at room temperature followed by decantation at ca. 279 K and dried over anhydrous magnesium sulfate. Compound 2 was washed several

times with water and dried over anhydrous magnesium sulfate.

RESULTS AND DISCUSSION

The reaction of NaBF4 or NaPF6 with I-n-butyl-3-metl~ylimidazoliurnchloride 3 in acetone at room temperature affords after 24 hours compounds 1 and 2 in high yields (Scheme 1). nBu'

@Me

Cl-

+ NaX

- NaCI

,TB"- a-Ms v

1, X= BF4 2. X=PFr

X

Scheme 1. Synthesis of compounds 1 and 2. Compounds 1 and 2 are viscous liquids imiscible with non-polar solvents, like hexane and toluene. Compound 2 is imiscible with water and can be purified by washing with water. The miscibility of I with water depends on the composition (molten saltlwater ratio) and on the temperature (Figure 1). For example, a homogeneous 111 (wlw, molten salt/water) mixture is fonned at temperatures above

5°C and a two-phase mixture at temperatures below 5°C.


o -"o.,

-

0' V

-

4

-

E

-

1 -12

-

-

I

20

'

I

'

I

40

'

I

'

I

'

I

60

'

I

'

,

-

-

80

Water content (%,w/w) F i g ~ r eI . Partirrl nliscibility of BMl. BF.

ill

water (whv).

Spectroscopic Data. The I R spectra of compounds 1 and 2 (see Table I) show peaks between 3100

-

3200 cm-' that can be attributed to the aromatic C-H stretching, whereas those below 3000 cm-' can be attributed to diphatic C-H stretching [19,20]. The observed aromatic C-H stretching are characteristic for hydrogen-bonds (C-H.--F interactions) 1211. This

fact is in sharp contrast to those observed for the related l-ethyl-3-methylimidazolium tetrafluoroborate [g] or hexafluorophospl~ate[l01 melts where no hydrogen-bonds have been detected in the IR spectra. Only small difference in the C-H stretching frequency in the IR of 1 and 2 have been observed and, thus, it is difficult to correlate between these Frequency cl~angesand the precise energy of the hydrogen bond [21]. However, the strength of the C-H.-F interactions increases in the order 1 < 2. This fact was fitrther corroborated by NMR, viscosity, density and conductivity data (see below).


1631

TABLE I :Infrared spectral data (3200-2800 cm-', film) of compounds 1 and 2.

The 'H and

'

3

Assignment Aromatic C-Hstr.

1

2

3 165 s

3168 S

Aromatic C-H str.

3 122 m

3 125 m

Aliphatic C-Hstr.

2965 s

2965

Aliphatic C-H str.

2940 m

2940 m

Aliphatic C-Hstr.

2880 W

2880 W

S

chemical ~ shifts observed in the NMR spectra of compounds 1 and 2

are summarized in Tables I1 and 111, respectively. For atom position number used see

scheme 2.

Scheme 2. lmidazolium atom numbering scheme.

It is interesting to note the downfieId shift of H-C2 in compound 2 compared to compound 1. This is probably due to the hydrogen-bond with the PF6 anion being stronger than that formed with the BF4 a n i ~ n . ~

TABLE I1 : Proton chemical shifts (ppm) of compounds 1 , 2 and 3.

" neat. in CDC13. J Chim. Phys

P.A.Z.Suarez et al.

1632

TABLE III :Carbon chemical shifts (ppm) of compounds 1 , 2 and 3. Carbon C2

C4 CS C6 C7 C8 C9 C10

2"

1"

136.4 136.5 123.3 123.5 122.0 122.2 49.3 48.9 3 1.6 3 1.5 18.8 19.0 12.8 12.6 35.4 35.8 " neat. h in CDC13.

3b

132.0 123.8 121.1 50.4 32.7 20.0 14.0 37.2

Glass Transition, Viscosities and Densities. The glass transition temperatures, viscosities (q3o) and densities (p) are summarized in Table IV. The experimental densities of 1 and 2 could be least-square fitted to

equations of the form: p = a + b (t-60)

[l]

where t is the temperature in "C.1231 The values of tlie fit parameters for compounds 1 and 2 are a= 1.15, 1.35 g.cm-3and b= -6.51 .104,- 8.24.1o4 g . ~ m - ~ . " respectively. ~-', TABLE IV: Glass transition temperrtures (Tg), viscosities and densities at 30°C for compounds 1 and 2.

Compound

1 2

Tg (K) 192 212

1130 (Poises)

2.33 3.12

P30

(&m3) 1.17 1.37

Compounds 1 and 2 show a glass transition at 192 K and 212 K, respectively. These temperatures are far below the melting point founded for the analogous l-ethyl-3methylimidazolium tetrafluoroborate and hexafluoropl~osphatecompounds (288 K and


1633

333 K, respectively) [9-10]. This result shows that the presence of a long alkyl chain (butyl) at the N | of the imidazolium cation inhibits the crystallization of these melts.[ll] It is interesting to note the high value of the viscosity displayed by compound 2 compared to 1. This fact is probably related to stronger H F interactions present in 2 resulting in more compact structures. The strong hydrogen bonds in 2 may also explain its high density since a linear dependence of the density is expected with the size of the anion. [23]

Conductivity.

The specific conductivities, k, of 1 and 2 were least-square fitted to equations of the form 2

k = k + £ , 0 6 0 ) + Âr (t-60) , a

[2]

2

where t is the temperature in °C [23]. The equivalent conductivities, A, were calculated by A = kUlp

[3]

where k is the specific conductivity, calculated from equation (2), M is the molecular weight of the ionic liquid and p its density, obtained from equation (1). As observed for other molten salts based on 1,3-dialkylimidazolium chloride-AJCU melts,[23] compounds 1 and 2 follow the Vogel-Tammann-Fulcher equation for conductivity ,/2

A = Ar exp[-MT-T )]

[4]

0

where T is the 'ideal' glass transition temperature and A 0

A

and k are substance A

specific constants. The fited parameters for specific conductivities (equation 2) and equivalent conductivities (equation 4) are given in Table V. J. Chim. Phys.


1634

TABLE V : Least-square-fit parameters for equation (2) specific conductivities and for equation (4) equivalent conductivities of compounds 1 and 2.

Comp. k,. 10'

k l s 1 o4

k2* 106

h(AA)

To

kA 1 0 . ~

T range

It is interesting to note that the conductivities of the molten salts 1 and 2 are significantly lower than those observed for the l-n-butyl-3-methylimidazolium tetrachloroaluminate analogue (see Figure 2) [23]. This indicates that the interaction between the anion and cation in the organo-aluminate molten salt is essentially electrostatic with a small contribution of the hydrogen bonds. Contrarily, a strong interaction between anions and cations in compounds 1 and 2 possesses an additional contribution fiom the hydrogen bonds.

The higher conductivities values observed for compound l compared with 2 at temperatures above 279 K is an indication of stronger cation-anion interaction in compound 2 than in l . However, at temperatures below 279 K, the conductivity as well as the other physical-chemical properties of these two molten salts are almost identical. This strongly suggest that at temperatures below 279K compounds 1 and 2 possess similar structures in the liquid phase (see below). The differences in the physical-chemical properties of the molten salts 1 and 2 can be explained in terms of cation-anion affinity via hydrogen bonds. In this respect, it was recently demonstrated that the structure of basic ionic liquids derived fiom l-ethyl-3methylirnidazoliwn halides should not be considered as statistical aggregates of anions and cations, but preferentially as three dimensional networks of anions and cations, linked together by hydrogen bonds. [22] Moreover, each imidazoliurn cation is

Synthesis and properties of imidazoliurn ionic liquids

1635

hydrogen bonded to three anions, via the three protons of the aromatic ring. Depending on the cation-anion affinity cations and anions build a quasi-molecular state.[22]

Figure 2. Dependence ofthe spec1f7ccondrtctivi~of compounds I-n-butyl-3-mcthylimidazolium tetrachlorml~~minate (a) (231, 1 (h) and 2 (c) on the tempcratiire.

It is reasonable to assiune from IR, NMR, density, viscosity and conductivity data that compound 2 exists in a quasi-molecular state between 273 and 353 K. On the other hand, compound 1 builds this quasi-molecular structure at temperatures below 279 K. At higher temperatures this co~npoundis composed of imidazoliurn and tetrafluoroborate ions in an extended hydrogen-bonded network. This is to our knowledge the first example of a dual (ionic polymer

-

ionic pair) behaviour of a

molten salt. We propose that the term quasi-molecular expresses bonding between cations and anions that energetically might be compared with covalent bonds. In the case of ionic liquids, this quasi-molecular bond results from strong electrostatic interactions with additional hydrogen bonds between the anions and cations. This particular behaviour of the ionic liquid offers the application of 1 as an unique solvent for both, polar and non-polar two-phase catalysis.[24] J. Chim. PhyS.

1636


ACKNOWLEDGMENTS We thank the PADCT and FAPERGS for partial financial support, the CNPq for a fellowship (to S. E.) and CENPES for scholarships (to P. Z. S. and J. E. Dullius).

SUPPLEMENTARY MATERIALS AVAILABLE Tables of densities and specific conductivity of BMI.BF4 and BM.PF6 (1 page).

REFERENCES 1 Hussey C.L. (1983) in Adv. in Molten Salt Chem. G . Mamantov and C. Marnantov Eds., Elsevier, New York, 5, 185 p.

2 Pagni R.M. (1987) in Adv. in Molten Salt Chem. G. Marnantov and C. Mamantov Eds., Elsevier, New York, 6,211. 3 Hussey C.L. (1 988) Pure Appl. CeVhcnt. 60, 1763.

4 Osteryoung R.A. (1987) in Adv. in Molten Salt Chem. G . Marnantov and C. Marnantov Eds., Elsevier, New York, 6 , 3 2 9 ~ . 5 Chauvin Y., Gilbert B., Guibard I(1990) J. Chem. Soc., Chem. Commun. 1715. 6 Chauvin Y., Einlofi S., Olivier H. (1 995) Ind. Eng. C,'hcnt.Res. 34, 1149.

7 Einloft S., Dietrich F., Souza R.F., Dupont J. (1996) Polyhedron. 15,3257. 8 Chauvin Y., Olivier H., Wyrvalski C. N., Simorl L. and Souza R. F. (1996) J. Catal. 165,275. 9 Wilkes J.S., Zaworotko M.J. (1992) J. Chcnt. Soc., Chem. Commun. 965. 10 Fuller J., Carlin R.T., De Long H.C.,Haworth D. (1994) J. Chem. Soc., Chem.

Comntun. 299. 1 1 Cooper E.I., O'Sdlivan M.E.J. (1990) in Proceedings ofthe 7th Int. Symposium qf Molten Salts, Physical Electrochemistry and High Temperature Materials Division

Proceedings Hussey C. L., Wilkes J. S., Flengas S. N. and Ito Y., Eds., 90, 17,386.


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12 Suarez P.A.Z., Dullius J.E.L., Einlofi S., Souza R.F., Dupont J. (1996) Polyhedron. 15, 1217.

13 Suarez P.A.Z., Selbach V.M., Dullius J.E.L., Einloft S., Piatnicki C.M.S., Azambuja D.S., Souza R.F., Dupont J. (1997) Elcctrochim. Acta 42,2533. 14 Monteiro A.L., Zinn F.K., Souza R.F., Dupont J. (1997) Tetrahedron Assym. 8, 177. 15 Suarez P.A.Z., Dullius J.E.L., Einlofi S., Souza R.F., Dupont J. (1997) Inorg.

Chirn. Acta 255,207. 16 Jones G., Bradshaw B.C. (1933) J. Am. Chem. Soc. 55,1780. 17 S e y r J.B., Oberstar H.E. (195 1) Indust. (e Eng. Chem. 43,2117. 18 Wilkes J.S., Levisky J.A., Wilson R.A., Hussey C.L. (1982) Inorg. Chem. 21, 1263. 19 Tait S., Osteryoung R.A. (1984) Inorg. C:hem. 23,4352. 20 Dieter M.K., Dymek Jr. C.J., Heilner N.E., Rovang J.W., Wilkes J.S. (1988) J.

Am. Chcm. Soc. 110,2722. 21 Elaiwi A, Hitchoock P.B., Seddon K.R., Srinivasan N., Tan Y., Welton T., Zora J. A. (1995) J. C.'hem. Soc., Dalron Trans. 3467. 22 Avent A.G., Chaloner P.A., Day M.P., Seddon K.R., Welton T. (1994) J. Chem.

Soc., Dalton Trans. 3405. 23 Fannin Jr. A.A., Floreani D.A., King L.A., Landers J.S., Piersma B.J., Stech D.J., Vaughn R.L., Wilkes J.S., Williams J.L. (1984) J. Phys. Chem. 88,2614. 24 a) Dullius J.E.L., Suarez P.A.Z., Einlofi S., Souza R.F., Dupont J., Fischer J., De Cian A. (1 998) Organometallics in press. b ) Dullius J.E.L., Suarez P.A.Z., Einloft S., Souza R.F., Dupont J. (1996) Brazilian Patent PI 9605670-3. c) DulIius J.E.L., Suarez P.A.Z., Einlofi S., Souza R.F., Dupont J. (1996), Brazilian Patent PI 9605493-0.

J. Chim. Phys.


1638

ANNEX 1. Supplementary materials Table S1. Experimental and adjusted densities data of BMI.PF6 and BMI.BF4 at different temperatures (in

Temperature

(K) 278.5

BMI.PF6 Experimental Adjusted 1,391

279.0

Experimental

1,390 1,390

284.0

1,386

1,386

294.0

1,377

1,377

303.0

1,369

1,370

304.0

BMI.BF4

1,369

Adjusted

1,185 1,185

1,185 1,181

1,l 75

1,175 1,169

1,169

1,168


1639

Table S2- Experimental and adjusted Specific Conductivities data (.lo4) of BMI.PF6 and BMI.BF4

at different temperatures in s.cm".

Temperature

BMI.BF4 Experimental

Adjusted

BMI.PF6 Experimental

Adjusted 4.22 4.20 4.19 4.23 4.36 4.42 5.22 5.51 7.49 10.7 1 11.41 14.57 19.01 20.93 24.98 30.97 38.18 46.43 55.18 66.65 74.27 105.06 107.77

J. Chim. Phys.