Solutions of Gallium Trichloride in Ethers: A 71 Ga NMR Study and the X-Ray Structure of GaCl3 • Monoglyme Stefan Böck, Heinrich Nöth*, and Astrid Wietelmann Institute oflnorganic Chemistry, University of Munich, Meiserstraße 1, D-8000 München 2 Dedicated to Prof. Dr. G. Fritz on the occasion o f his 70 th birthday Z. Naturforsch. 45b, 979-984 (1990); received January 22, 1990 71Ga NMR Spectra, Gallium Trichloride-Ether, Gallium Trichloride-Tetrahydrofuran, rä-Dichloro-bis(dimethylglycolether)gallium-tetrachlorogallate Solutions of GaCl 3 in various ethers have been studied by 7lGa NMR spectroscopy. <S7lGa data indicate that the predominant species in diethylether and tetrahydrofuran solutions are G aC l3 0 (C 2H 5) 2 and G aC l3 -2 0 C 4Hg, respectively. However, in monoglyme solution disso­ ciation occurs and the product crystallizing from the solution is [c 7's-GaCl2(monoglyme)2]- G aCl4 as demonstrated by an X-ray structure determination of the solvate GaCl 3 • monoglyme. Introduction deduced from spectroscopic studies (IR, Raman, Aluminium trichloride, which crystallizes in an N M R ) [8]. Dissociation of A1C1 3 in diethylether ionic lattice, dissolves in many polar solvents and has been observed not only by the electrical con­ forms a large number of coordination compounds. ductance of its solution but also by : 7A1 N M R spectroscopy [1,9, 10], Dissociation processes are [Al(OH2)6]Cl3 crystallizes from acid aqueous solu­ increasingly favoured in the series diethylether, tions of A1C1 3 [1], The compound A1C1 3-2C H 3CN THF, dimethylglycolether (monoglyme), and di- can be obtained from solutions of A1C1 3 in acetoni- trile, and this compound was shown to be methyldiglycolether [10], Moreover, crown ethers L support ionization, as shown by the complex of [A1C1(NCCH3)5][A1C14]2 • CHjCN by X-ray struc­ 2 A1C1 -4-crown-12 [11] producing the salt ture determination [2]. In addition, a second 3 acetonitrile adduct has been found to be [C12A1L]A1C14. Many additional cations have been detected in [A1(NCCH3)6](A1C14)3 [3]. Pyridine (py) yields sev­ solutions o f A1C1 in highly polar solvents by A1 eral compounds with A1C1 3 depending on the 3 27 mode of preparation, and the structures of NMR spectroscopy. However, the question whether the solid in equilibrium with the solution AlCl3-3py and AlCl 3-2py have been determined reflects the solution state needs still further explo­ to be mer-A1C13 • 3 py and [/ra^ 9-Cl2Al(py) 4][AlCl4], respectively [4], From diethylether the compound ration. From this point of view it was of considera­ ble interest to compare the behaviour of GaCl 3 to ­ A1C13-0 (C 2H5)2, containing a tetracoordinated aluminium atom, has been isolated [5]. Moreover, wards ethers as solvents with that of A1C13. Al­ though 71Ga NMR spectroscopy is not as versatile two different kinds of A1C1 3-2THF compounds (THF = tetrahydrofuran) have been character­ as 27A1 NMR spectroscopy, the method can be ized. The first one results from the action of THF used to get information on solution species. on the adduct (Me 2N)3SiCl- A1C13. It is the molec­ Results ular compound A1C1 3-2THF with a pentacoordi- nated Al-atom in a trigonal-bipyramidal environ­ Gallium trichloride in diethylether ment and the THF molecules in apical positions The low melting adducts GaX 3 O E t2 (X = Cl, [6]. In contrast, if toluene is added to a solution of Br, I) have been isolated and characterized [12], A1C1 in tetrahydrofuran the solid A1C1 -2T H F , 3 3 and it is well known that G aC l 3 dissolves much which separates, is the ionic compound [cis- more readily in organic solvents than does A1C13. C1 A1(THF) ]A1C1 [7] whose existence was first 2 4 4 In contrast to A1C1 3 solutions in diethylether those of G aC l 3 show only very weak electrical conduc­ * Reprint requests to Prof. Dr. H. Nöth. tivity. 0.204 and 0.123 M solutions of GaCl 3 in di­ Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen ethylether are almost non-conducting, while the 0932 - 0776/90/0700 - 0979/$ 01.00/0 specific electrical conductivity of comparable 980 St. Böck et al. ■ Solutions of Gallium Trichloride in Ethers AICI3 solutions in diethylether is 1.2-10 ~2 cm2 ß“ 1 exhibits two 71Ga NM R signals, a broad and rela­ [13]. Thus, G aC l3 in diethylether appears to form tively weak signal at Ö = 329 ppm and a strong and almost no ionic products. fairly sharp one at 3 = 261 ppm. On dilution, the In consonance with these findings is the broad broad resonance vanishes while the other becomes 7lGa NMR signal at S = 260 ppm which we ob­ increasingly sharper (260 versus 160 Hz at half serve at much lower field than reported by Akitt height). These results clearly indicate dissociation et al. (<S71Ga = 137 ppm) [14]. This signal corre­ of GaCl3, presumably according to sponds to the formation of Cl3Ga • OEt2. Exchange 2G aC l3 L G aC l2L2+ + G aC l4“ (1) of this etherate with solvent ether is slow as two sets of 13C NM R signals are observed. or 2 G aC l 3 • L G aC l2L+ + G a C lf + L (2) The 71Ga NMR chemical shift (S = -329 ppm) Gallium trichloride in tetrahydrofuran and dioxane corresponds best with tetracoordination: both a pentacoordinated as well as hexacoordinated gal­ Tetrahydrofuran dissolves GaCl 3 quite readily. lium center should show a resonance at much Solutions in the concentration range 0.12-0.97 higher field [17]. We therefore assign this signal to m ol /1 show negligible electrical conductivity in a tetracoordinated GaCl3 L adduct. Whether dis­ contrast to A1C1 3 solutions in this solvent [15]. sociation occurs according to eq. ( 1 ) or ( 2) cannot These solutions produced a detectable 7,Ga NMR be decided on the basis of the 71Ga NMR spectra signal neither at ambient temperature nor at lower alone. temperature (-20 °C). Also, the isolated G aC l3 • THF, dissolved in C 6D6, gave no 7lGa reso­ The structure of [GaCl2(monoglyme)2]GaCl4 nance. Obviously, quadrupolar relaxation is very The adduct GaCl3 • monoglyme separating from fast for this compound. As expected, two l3C monoglyme solutions forms well shaped crystals NMR signals are observed for GaCl 3 TH F. The from dichloromethane solutions suitable for an carbon atoms bonded to the oxygen atom are X-ray crystallographic study. The crystals are shifted to lower field compared with the free ligand orthorhombic, and 8 GaCl3- monoglyme units are while the carbon atoms of the other CH 2 groups found in the unit cell. However, as depicted in are slightly better shielded. Broad resonances are Fig. 1 the compound is not a molecular adduct but observed for the two sets of CH 2 groups. rather the salt [Cl2Ga(monoglyme) 2]GaCl4. Bond­ A 71Ga NM R spectrum may allow to distinguish ing parameters are summarized in Table I. between tetra- or pentacoordination in solution. The tetrachlorogallate anion deviates only No 7,Ga NMR signal could be recorded for satu­ slightly from a regular tetrahedron, the C l-G a- rated solutions of GaCl 3 in dioxane. This definitely also excludes the formation of the readily detecta­ ble GaCl4_ anion. This is ascertained by the very low electrical conductivity of the GaCl 3 solution in dioxane. In this respect, GaCl 3 behaves like A1C13 in dioxane, where a 27Al NMR signal at ö = 66 ppm points to the presence of pentacoordinated Al [17]. N o signal at Ö = 105 ppm, typical for the presence of A1C14~ [17] was found. This excludes dissociation of A1C1 3 in dioxane into solvated A1C12+ and A1C14“. Gallium trichloride in monoglyme and diglvme Compared to solutions of GaCl 3 in diethylether or tetrahydrofuran the solubility of this halide in monoglyme is low (< 0.1 M). This is one of the rea­ ^>C I3 sons why no 71Ga NMR signal was recorded. In Fig. 1. ORTEP plot of [Cl 2Ga(monoglyme):]GaCl4. contrast, a 0.34 M solution of GaCl 3 in diglyme Thermal ellipsoids represent a 30% probability. St. Böck et al. ■ Solutions of Gallium Trichloride in Ethers 981 Table I. Selected bond distances (in A) and bond angles neat liquid fits, with tetracoordination [14], The (in degrees) of [GaCl 2(monoglyme) 2]GaCl4. Estimated 71Ga NMR signal for solutions of GaCl 3 in di- standard deviations in parenthesis. ethylether is found at lower field (Ö = 260 ppm); Bond distances this chemical shift value would correspond with Ga 1 - Cl 2 2.187(2) Ga2-08 2.064(8) the form ation of G aC l4~ [17]. The line width G a l- C l 2 2.177(2) G a 2 - 0 11 2.081(6) (—7500 Hz), however, excludes such an assign­ G a l- C l 3 2.171(2) 02-C 1 1.365(18) ment, as does the low electrical conductivity. G a 1-C 14 2.170(2) 02-C3 1.540(9) G a2 -C 1 5 2.204(5) 05-C6 1.435(12) Therefore, we have to conclude that the principal G a2 -C 1 6 2.223(4) 0 5 - C 4 1.437(15) species in solution is GaCl 3 • OE t2. G a 2 - 0 5 2.039(9) 08-C7 1.502(12) In contrast to A1C13, gallium trichloride seems Ga2-02 2.093(5) OH-C12 1.496(17) not to form a stable GaCl 3-2THF adduct. Bond angles GaClj-THF was isolated from THF solutions, Cl 1- G a l -C12 110.4(1) C 1 5 - G a 2 - 0 5 96.4(3) and neither a solution of this compound in ben­ Cl 1- G a l - C l 3 1 1 0 .