Complexing and exchange of boron, aluminum, and gallium chlorides with some Lewis bases1

BORIS GLAVINCEVSKI A N D SYDNEY K. BROWNSTEIN Dicision of Chemistry, Nutional Reseurch Council, Otta~c.a, Ont., Catlada K I A OR9

Received March 31, 1981

BORIS GLAVINCEVSKI and SYDNEY K. BROWNSTEIN. Can. J . Chem. 59, 3012 (1981). Boron trichloride and dimethyl sulfide form a 1:1 complex. I t exchanges with excess boron trichloride by a dibplacement

mechanism with an activation energy of 5.0 f 0.5 kcallmol and with excess dimethyl sulfide by a dissociation mechanism which has an activation energy of 20 f 1 kcallmol. The 1 : 1 complex of AICI, and dimethyl sulfide exchanges very rapidly with excess dimethyl sulfide. Equilibria and exchange reactions of AlCl,[(CH,),O], AICI,[(CH,),O],, and (CH,),O are described. Rapid exchange is always found in the analogous gallium systems. BC1, reacts with chloride ion to give BC1,- and B,C17-, which exchange rapidly with excess BCI,. AICI, reacts with C1- and BCI,- to give AlCI,- which does not exchange with AICI,.

BORIS GLAVINCEVSKI et SYDNEY K. BROWNSTEIN. Can. J. Chem. 59. 3012 (1981). Le trichlorure de bore donne un complexe 1 : 1 avec le sulfure de methyle. En presence d'un exces de trichlorure de bore, il y a un

echange suivant un mecanisme de deplacement avec une inergie d'activation de 5,0 + 0.5 kcal/mol et en presence d'un exces de sulfure de methyle suivant un mecanisme de dissociation avec une Cnergie d'activation de 20 f 1 kcal/mol. Le complexe 1: 1 forme entre AICI, et le sulfure de methyle echange tres rapidement avec le sulfure de methyle en exces. Des equilibres et des reactions d'echange de AICI, [(CH,),O] et AlCI,[(CH,),O], avec (CH,),O sont decrits. On a toujours trouve un echange rapide dans les systemes semblables du gallium. BCI, reagit avec I'ion chlorure pour donner BCI,- et B2C1,-, complexes qui echangent rapidement avec le BCI, en exces. AICI, reagit avec C 1 et BC14- pour donner AICI, qui n'echange pas avec AICI,.

Introduction well established (16). With aluminum as central ~~~i~ acid-base interactions involving boron atom equilibria have been reported between AICI,,

trihalides have been widely studied and a variety of C1-. AlC1,-3 and AIzCl7- (17, 18). The positions of

exchange rates and mechanisms have been repor- the depend upon the

tcd (1-6). Exchange of the acid-base complex with Experimental excess of Lewis acid has only been reported for boron trifluoride and occurs by a displacement Boron trichloride and aluminum trichloride were purified by

fractional distillation and stored in containers on the vacuum mechanism' Exchange of the with an system used for sample preparation. Sulfur dioxide and tetra- excess of Lewis base has been found to occur via a methylammonium chloride were purified and manipulated as dissociation mechanism with acetonitrile and tri- previously reported (19).

methylamine as bases and BC13 and BBr, as Lewis - Proton, "B, '7A1, and 7LGa resonance spectra were obtained

acids (4, 5). ~h~~ far, studies of aluminum halide on a Varian Associates XLlOO spectrometer equipped with Nicolet Fourier Transform and multinuclear attachments. All

have been limited lo the state and spectra are referenced with respect to internal tetramethylsilane to solutions of the halide in acetonitrile, a strongly (20). For cross-comparison with existing compilations of boron coordinating solvent (7-14). In acetonitrile both resonance spectra (21), we find E BCI, of 32,085,482 (lit.

cationic and anionic aluminum species have been 32,085,48612 and - B(OCH,), of 32.084,591 (lit. 32,084.5651,' well within the variation which may be caused by solvent identified. It is possible that different results might The most common reference for 2 7 A has been AI(H20)63+,

be obtained if the aluminum halide and Lewis base !-,In order to compare our results with previous literature values 5 are allowed to react in an inert solvent. sulfur <AI(H,o),~+ was measured and found to be 26,056,780(1it26.057 x dioxide, a good solvent for ~ 1 ~ 1 , and its :f 1nf;ared spectra of potassmm bromtde pellets of some prod-

&ucts were obtatned on Beckman Acculab 4 and Perkln-Elmer plexes' chosen for this study' This "lvent is "'-283 spectrometers Ramdn spectra of sollds and sulfur dtoxlde not truly inert since it forms a molecular complex solut~ons were obtalned by excttatlon wtth the 514 5 nm line of a with aluminum trichloride (15). The structure of the Coherent Rddtatlon argon Ion laser and recorded wlth a Jarrell- complex has not been determined although a dimer Ash 25-100 double monochromator equipped with photon

with SQ, coordinated to at each end ofthe counting detection. Lifetimes in the exchanging systems were

Al,Cl, chain has been suggested. determined by comparison of experimental spectra with those calculated by a stochastic method (22).

The tetrachl~IT)borate anion is well known but Tetramethylammonium (TMA) chloroborate: 227.2 mg (2.073 thus far no evidence has been reported for B,C17- mmol) of well dried TMACl were dissolved in sulfur dioxide and although the existence of the analogous B2F7- is

2These values are calculated from the various types of results 'NRCC No. 19604. reported in ref. 21.

0008-404218 1/203012-04$01.00/0 01981 National Research Council of CanadalConseil national de recherches du Canada

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GLAVINCEVSKI A N D BROWNSTEIN 3013

2.073 mmol of BCI, were added to the solution. All volatile materials were removed under vacuum to give 458.5 mg (2.073 mmol) of product. Anal. calcd: B 4.77. C162.5;found: B 4.58. C1 61.296.

Results and Discussion Sulfur dioxide and methylene chloride solutions

with a ratio of boron trichloride to dimethyl sulfide from 3 : 1 to 1 : 2 and a mole fraction of complex from 0.022 to 0.149 were studied by proton. l1B, and 13C resonance spectroscopy over a temperature range from 245 to 299K. The rate constants as a function of temperature are shown in Fig. 1. The relation- ships between reaction rates, rate constants, and lifetimes via nuclear magnetic resonance studies have been known for many years (1,2). Exchange between a free and complexed species, as in eq. [I] , can occur either by dissociation of the complex or by displacement of one species in the complex by excess of the free species. In the former case the activation enthalpy for exchange equals the heat of dissociation and the lifetime of the complex is independent of the concentration of the free spe- cies. For exchange by displacement the activation enthalpy will be less than the heat of dissociation and the lifetime of the complex will be inversely proportional to the concentration of the free spe- cies.

In the present study, we have determined the energetics and exchange mechanisms for exchange of both excess Lewis acid and base upon the same complex, boron trichloride - dimethyl sulfide. From "B resonance spectroscopy it was possible to obtain the lifetimes of free and complexed boron trichloride as a function of temperature and con- centration. At a given temperature the lifetime of complexed BCl, varied with concentration of free BCl, and a constant value for the rate constant could only be obtained for the kinetics of a dis- placement reaction. Results for three samples are shown in Fig. 1. The Arrhenius activation energy for BCl, exchange was calculated to be 5.0 + 0.5 kcallmol.

The second exchange reaction for this system, exchange of free and complexed dimethyl sulfide, was studied by 'H and 13C spectroscopy. It was found that the lifetime of complexed dimethyl sulfide was independent of the concentration of free dimethyl sulfide and a constant value of the rate constant for exchange was obtained by as- sumption of a dissociation mechanism. These kin- etic results, also shown in Fig. 1, yield an Arrhenius activation energy for dimethyl sulfide exchange of

FIG. 1. Temperature dependence for exchange rates with the complex (CH,),S.BCl,.

20 + 1 kcal/mol. The order of activation energies also agrees with the proposed mechanisms.

It therefore appears that exchange of excess Lewis acid by a displacement mechanism and excess Lewis base by a dissociation mechanism may be the usual mechanisms for complexes of boron trihalides. A possible route for exchange with excess boron trichloride is via tetra-coordinate sulfur with the incoming BCl, coordinating to tQe remaining lone pair of electrons on sulfur followed by departure of BCl,. This possibility would allow a low energy pathway for exchange via displace- ment. It is difficult to visualize two molecules of dimethyl sulfide coordinated to a single boron trichloride. The higher energy dissociation mech- anism of exchange with excess dimethyl sulfide indicates that a lower energy pathway via a dis- placement reaction is not available. Thus the mechanistic results for dimethyl sulfide exchange are also consistent with chemical intuition.

It would be useful to determine if the mechan- isms for exchange with BCl, as Lewis acid were

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CAN. J . CHEM. VOL. 59, 1981

TABLE 1. Chemical shifts of species in the exchange studies

Compound 6'H" - 613Cc - 1 lBb - - -7Ald 7 - " G a

BC1, TMA BCI, TMA B ~ C ~ , ~ 1 ~ 1 , TMAAICI, GaC1, TMAGaCI,

16.62 32.091.085 20.30

19.63 18.22 58.20 64.31 65.35

2.79 44.9 (CN)

3.2 50 (CN)

3.2 50 (CN)

32,085,482 32,084.185 32,084,373

" I n ppm to low field of TMS. C for (CH,O),B is 32,084,591 c In ppm to low iield of TMS. "E for (CH,CH,)AI IS 26.060.416 in CH,C12

also followed by AlCl, and GaCl,. For the alumi- num chloride - dimethyl sulfide system in sul- fur dioxide solution and for the systems GaC1,- (CH,),S and GaC1,-(CH,),O exchange was too fast to be measured by the nmr technique. The shielding parameters are listed in Table 1. It is to be noted that a complex 2(CH3),S.A1C1, has been reported (21), suggesting that a low energy dis- placement pathway for exchange of dimethyl sulfide may be available in this system.

Although exchange in the system AlC1,-(CH3),S was too fast to study, in the system AlC1,-(CH,),O the rates were in a convenient range. The geometry of 2: 1 complexes of an ether with AlCl, was found to be a trigonal bipyramid with the ethers at the axial sites (23). Only a 1: l complex between diethyl ether and AlC1, was reported for a mix- ture of the reactants (13). We found both 2: 1 and 1: l complexes of dimethyl ether with AlCl, in sulfur dioxide solution. Their nuclear magnetic resonance parameters are listed in Table 1.

Samples were prepared with from 4.0 to 1.5 equivalents of (CH,),O for each AICI,. In those samples containing more than two equivalents of dimethyl ether, signals are observed only for the 2 : l complex and for free ether. There is no evidence for the 1 : 1 complex. Therefore the kinet- ics of the exchange between at least one of the dimethyl ether species in the 2: 1 complex and free dimethyl ether can be measured directly. The exchange rates for (CH,),O in the 2: 1 complex are listed in Table 2. Since they are independent of the

TABLE 2. Exchange rates for dimethyl ether in the 2 : l complex with free dimethyl ether and that in the 1 : 1 complex

Mole fraction T ("C) k s-'

concentration of free dimethyl ether this implies that exchange occurs via a dissociation of at least one bound ether from the 2: l complex. From an Arrhenius plot of the average rate for these samples an activation energy of 29.3 +_ 0.5 is obtained. The limited temperature range and number of data points precludes an accurate measure but an acti- vation energy of about 30 kcallmol is reasonable for a dissociation process. It compares reasonably well with the value of 20 kcallmol for the dissociation mechanism found for BCl, .(CH,),S.

In those samples containing less than two equiva- lents of ether per AICI, only signals of the 2 : 1 and 1 : I complexes are observed. The exchange be- tween these species can then be determined with- out interference. The exchange rates for (CH3)20 in the 2: 1 complex with that in the 1 : 1 complex are

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GLAVINCEVSKI A N D BROWNSTEIN 3015

also listed in Table 2. Within the rather large limits of error the rates are the same for exchange of the 2: 1 complex with either free ether or the 1: l complex. This is also strong confirmation that the exchange occurs via dissociation of one dimethyl ether from the 2 : 1 complex.

Disproportionation and ionization to yield AlCl,- and AICIZ+, both solvated, have been reported when AICl, is dissolved in acetonitrile (7). Very recent results present evidence for some CH,CN. AlCl, and various solvated CH3CN species (24). Since the behaviour of neat acetonitrile is rather different from that found for dimethyl ether and dimethyl sulfide in sulfur dioxide as solvent, the system AlC1,-acetonitrile was investigated in SO, solvent. With an excess of aluminum trichloride only the complex CH3CN.A1Cl3 and uncomplexed AlCl, were observed via 'H, 13C, and 27Al reson- ance spectroscopy. The shielding parameters are listed in Table 1. With approximately equal con- centrations of AlCl, and CH,CN both the 1:l complex and the ionized species previously repor- ted were observed via their 27Al resonances. The shielding parameters for AlC1,- and A1C12+ are also listed in Table 1 and agree with the previously reported values (7). With ratios of two or three CH3CN for each AlCl, only the ionized species were observed. At -60°C in the proton resonance spectra of these solutions considerable fine struc- ture was observed for the coordinated acetonitrile, suggesting different environments for these spec- ies. These results show that CH,CN, in an inert solvent, with an excess of AlCl,, behaves similarly to (CH,),S and (CH,),O as a Lewi5 base. The rather different behaviour when acetonitrile is in excess is presumably due to its strong coordinating tendency for the aluminum ion.

It is possible to study exchange and competitive complexing of the Group I11 chlorides BCl,, AlCl,, and GaC1, with the common Lewis base, chloride ion. The reaction of chloride ion and BCl, to give BCl,- is long established but further reaction to give B2C17- has not been previously reported. There is rapid boron exchange on the nmr time scale between BCl,- and BCl,. The chemical shift for the boron resonance in a solution of composi- tion B,CL7- is not the weighted average of those for BCI, and BC1,-. With an excess of BCI, the chemical shift of the single line observed is that of a weighted average for B2C17- and BCl,. The shield- ing parameters are listed in Table 1. These results suggest, but do not prove, the existence of BZC17- in solution. It was not possible to isolate TMAB2C17 since a reaction with the solvent occurred upon

concentration, or standing at room temperature, to yield an insoluble product with a very different elemental composition.

In sulfur dioxide as solvent AlCl, and AlCl, are observed as separate 27Al resonances with no evidence of reaction or exchange. Perhaps the complex of AICl, with the SO, solvent is stronger than that which might be formed with A1C1,-. In a competition experiment for chloride ion it was found that for an initially equimolar mixture of C1-, BCI,, and AlCl, only BCI, and AlC1,-- were ob- served at equilibrium. There was no evidence for BCl,- nor BAlC1,-. Shielding parameters for these and the corresponding gallium species are given in Table 1.

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