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. 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. ( 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 . 1( 1) C16-Ga2-05 94.1(3) zene nor in tetrahydrofuran allowed the detection C12-Gal-C13 109.0(1) 02-Ga2-05 96.4(3) of a 71Ga NMR signal. This observation falls in C13-Gal-C14 109.0(2) C15-Ga2-08 97.0(3) line with the higher stability of AlCl 3-2N M e3 as C 1 2 -G a l -C 1 4 1 1 0 .2 (2 ) C 1 6 - G a 2 - 0 8 96.4(3) C13-Gal-C14 109.0(2) 02-Ga2-08 84.0(3) compared to GaCl 3-2N M e3 [18], However, one C 1 5 -G a2 -C 16 1 0 1 . 1( 1) 0 5 - G a 2 - 0 8 161.0(3) might expect the formation of GaCl 3-2THF since C 1 5 -G a2 - 0 2 168.0(2) C15-Ga2-0 11 91.9(2) G aC l3 reacts with dioxane, to produce a 1:1 adduct C 1 6 -G a2 - 0 2 90.7(2) C16-Ga2-0 11 166.7(2) containing pentacoordinated gallium [16]. The asymmetric electron distribution around gallium in such complexes may generate a strong field gra Cl bond angles ranging from 108.1(1) to 110.4(1) dient allowing rapid relaxation and preventing the and the Ga-Cl bond lengths from 2.170(2) to observation of a 71Ga NMR signal. Moreover, 2.187(2) A. A1C13 is a stronger Lewis acid for hard bases [19], The cationic part of the structure is more inter and this may be an additional factor causing A1C1 3 esting, the most notable feature being the c/s-ori- to dissociate appreciably in THF solution, from entation of the chlorine atoms at the hexacoordi- which [/ra«.9-Cl2A l(TH F)4]AlCl4 can be isolated nated gallium center. Consequently, the average [7], Dissociation of this type occurs with GaCl 3 GaCl distance (2.213 A) is larger than in the tetra- only in polyether solutions as shown here. coordinated GaCl4~ anion (2.176 A), in spite of However, in the salt [Cl 2Ga(monoglyme) 2]G aCl4 the positive charge in the cation. the chlorine atoms are a in cw-position in contrast Due to the small OGaO bite angles of the mono- to [/ra«.9-Cl2Al(THF)4]+ [7], Reasons advanced for glyme ligand (80.1(3) and 78.9(3)° respectively), a this different arrangement remain tentative and strong distortion from regular octahedral symme any further discussion should be postponed until try is noted, the cations local symmetry approach the structure of [Cl 2Al(monoglyme)2]+ has been es ing the point group symmetry C with the two 2 tablished. fold axis bisecting the ClGaCl angle (101.1(1)°). The cw-arrangement is also found in The transoid bond angles also deviate strongly [Cl2Ga(dipy)2]+, a cation possessing two chelating from the ideal 180°. Respective values are ligands [19], while the chlorine atoms in C15-Ga2-02 168.0(2)°, C16-Ga2-OH [Cl2G a(py)4]+ are in rrans-position [20]. The struc 166.7(2)°, and 0 5 -G a 2 -0 8 161.0(3)°. tural difference between [Cl2Ga(dipy)2]+ and [Cl2Ga(py)4]+ has been explained by steric interac Discussion tions of the hydrogen atoms next to the nitrogen The formation of coordination complexes of atom. In [ds-Cl 2Ga(dipy)2]+ this interaction is gallium trihalides, and GaCl 3 in particular, is well avoided. Similarly, the [?ra« 5-Cl2Ga(py)4]+ geome known [12], G aC l 3 ■ OE t2 has been isolated as a low try is possible due to rotation of the pyridine mole melting solid (m.p. 16 °C) and its molecular consti cules around the GaN bonds in order to minimize tution established by vibrational spectroscopy. H -H interactions of the hydrogen atoms in ortho- The value J71Ga = 137 ppm, as observed for the position. Arguments of this kind are, however, not 982 St. Böck et al. - Solutions of Gallium Trichloride in Ethers fully valid for monoglyme as a ligand due to the The signals at 66.6 and 15.9 result from flexibility of its 0 2C2 skeleton. G aC l3 O E t2 the others from the solvent, <571Ga The GaCl bond lengths in czs-Cl 2G aL 4 cations (ppm): 260, h(l/2) = 2700 Hz. Equivalent conduc are shorter than those in trans-C\2Gah4+ species tivity at 18 °C: <1.2-10 1 Q~x cm -1 for 0.204 and (2.213 A in [Cl2Ga(monoglyme)2]+, 2.265 Ä in 0.123 M solutions. [Cl2G a(dipy)2]+ and 2.31 Ä in [/ra«s-Cl 2Ga(py)4]+). Galliwntrichloride-tetrahydrofuran A comparison of the GaO bond distances for Cl2Ga(monoglyme)2+ (average 2.069 Ä) with those a) 3.0 ml of tetrahydrofuran (42 mmol) were in pentacoordinated GaCl3- 1,4-dioxane (2.21 Ä) cooled to 0 °C. 520 mg of GaCl 3 (2.9 mmol) were then added with stirring. A clear solution resulted [16] and G a2Cl4- 1,4-dioxane (2.03 Ä) [21], where which was immediately used. Equivalent conduc the dioxane acts as a monodentate ligand, shows a tivity of the solution (0.97 M, 0.137 M) at 20 C: violation of the general rule that the respective < 1 .7 -1 0-7 Q~x cm-1. No 71Ga NMR signal was bond lengths should increase as the coordination observed. [1] M. J. Taylor, in G. Wilkinson (ed.): Comprehensive 11 ] J. J. Atwood, H. Elgamel, G. H. Robinson, S. G. Coordination Chemistry, Vol. 3, p. 105, Pergam on Bott, J. A. Weeks, and W. E. Hunter, Inclus. Pheno Press, New York, London (1987). mena 2,367(1984). [2] I. R. Beattie, P. J. Jones, J. A. K. Howard, L. E. 12] H. Ishihara and H. Negita, Bull. Chem. Soc. Jpn. Smart, J. J. Gilmore, and J. W. Akitt, J. Chem. Soc. 58, 2731 (1985). Dalton Trans. 1979, 528; M. Dalibart, J. Deroualt, 13] Yu. M. Kessler, N. M. Alpastova, and O. R. M. T. Forel, and P. Caillet, J. Mol. Struct. 63, 233 Osipov, Russ. Chem. 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