Organometallic complexes of group 13 metals (Al - Tl)

This specific class of compounds has a long-known history in main group element chemistry. Many compounds have been prepared in the past and their structures and reactivities have been studied in detail. However, novel class of substituents such as sterically demanding terphenyl groups have been developed in the last decade, yielding novel group 13 organometallic compounds. These are valuable starting reagents for so far unknown complexes.

Me₃M and Mes₃M (M = B - Tl) are the only group 13 organometallics, which have been completely structurally characterized.[1] In contrast to the trimesityls, the solid-state structures of the trimethyls differ significantly. BMe₃ is monomeric and adopts the expected D_3h symmetric geometry,[2] however, the molecules pack in layers through weak van der Waals or electrostatic interactions to a closed packed array. AlMe₃ forms a well-known dimer with bridging Me groups (also in the gas phase), whereas GaMe₃, InMe₃ and TlMe₃ form two dimensional networks build by tetrameric units of MMe₃.[3] The latter are formed through weak intermolecular metal×××Me interactions with long metal×××C bond distances. The tetramers are connected to each other by even longer metal×××Me bridges yielding two-dimensional networks. Interestingly, among the tetragonal phase a second polymorph of GaMe₃, a monoclinic phase, in which the tetramers are replaced by a polymeric chain, was described. The solid-state structures of Et₃Ga and t-Bu₃Gawere also determined. Et₃Ga consists of layers of wavelike networks build by four independent molecules in the unit cell. The Ga atoms interact to neighboring methylene and methyl carbon atoms. The intermolecular Ga×××C distances range from 308.7 to 359.3 pm. The wide Ga-C-C bond angle (113.3(2) - 118.8(2)°), which is comparable to that observed in solid BEt₃ (118.7 - 119.3°), was explained by hyperconjugation of the gallium p-orbital into C-H orbitals. These intermolecular interactions are typically weaker or even suppressed by using sterically demanding organic substituents monomeric (kinetic stabilization). For instance, t-Bu₃Ga crystallizes with six independent molecules in the asymmetric unit.[4] Five of the six independent t-Bu₃Ga molecules show weak contacts to adjacent t-Bu₃Ga molecules with Ga–C bond distances ranging from 3.2 and 3.6 Å, whereas only one t-Bu₃Ga molecule remains uncoordinated. The intermolecular interactions result in molecules which slightly deviate from exact trigonal planar environment (357.5 - 359.5°). Compared to the gas phase structure of t-Bu₃Ga (Ga–C 203.4(2) pm; C–Ga–C 120°), the average Ga–C bond length observed in the solid state is slightly shorter.

References

[1] a) J. F. Blount, P. Finocchiaro, D. Gust, K. Mislow, J. Am. Chem. Soc. 1973, 95, 7019; b) , M. M. Olmstad, P. P. PowerJ. Am. Chem. Soc. 1986, 108, 4235; c) J. J. Jerius, J. M. Hahn, A. F. M. M. Rahman, O. Mols, W. H. Ilsley, J. P. Oliver, Organometallics 1986, 5, 1812; d) O. T. Beachley, Jr., M. R. Churchill, J. C. Pazik, J. W. Ziller Organometallics 1986, 5, 1814; e) J. T. Leman, A. R. Barron, Organometallics 1989, 8, 2214; f) J. Blumel, B.; Werner, T. Krauter, B. Neumüller, Z. Anorg. Allg. Chem. 1997, 623, 309.

[2] R.; Boese, A. J. Downs, T. M. Greene, A. W. Hall, C. A. Morrison, S. Parsons, Organometallics 2003, 22, 2450.

[3] a) McGrady, G. S.; Turner, J. F. C.; Ibberson, R. M.; Prager, M. Organometallics 2000, 19, 4398; b) N. W. Mitzel, C. Lustig, R. J. F. Berger, N. Runeberg, Angew. Chem., Int. Ed. Engl. 2002; 41, 2519; c) Blake, A. J.; Cradock, S. J. Chem. Soc. Dalton Trans. 1990, 2393.

[4] A. Kuczkowski, S. Schulz, M. Nieger, Appl. Organomet. Chem. 2004, 18, 244.