Structure of triamidoaluminum complexes: a theoretical ab initio/IMOMM study.

Abstract

The electronic structure of three-coordinated complexes of aluminum Al(NR2)3 has been studied through theoretical calculations. In the unsubstituted system Al(NH2)3, ab initio calculations (MP2/6-31G(d,p) level) show that the optimal geometry results from a conrotatory motion of the three amido substituents starting from the fully conjugated planar species. The energy difference between these two structures is found to be small (less than 0.5 kcal/mol). In methylated species Al(NMe2)3, steric effects become important and the planar geometry is destabilized. The conrotatory geometry corresponds to the absolute minimum and is located 14.2 kcal/mol below the planar structure (MP2/6-31G(d,p) level). Several coupled motions of the amido substituents have been computed and have been found to stabilize the system with respect to the fully conjugated structure. A rough estimate of the steric repulsion is calculated by comparison between the unsubstituted and methylated species. Finally, the real molecules Al[N(SiMe3)2]3 and Al(NiPr2)3 have been studied through a coupled quantum mechanics/molecular mechanics method. In accordance with the experimental data, it is found that the conrotatory minimum is the absolute minimum in the R = SiMe3 case whereas a less symmetrical minimum is found in the R = iPr case. In this last minimum, an amido group is almost deconjugated and the two other groups move in a conrotatory manner. The different behavior of these two systems may originate from the quasi-spherical shape of the SiMe3 group, which leads to unavoidable steric repulsion.