Nonagostic M···H–C Interactions. Synthesis, Characterization, and DFT Study of the Titanium Amide Ti2Cl6[N(t-Bu)2]2

Abstract

The compound Ti2Cl6[N(t-Bu)2]2 (1) has been synthesized by treating TiCl4 with di(tert-butyl)amine, HN(t-Bu)2. Compound 1 crystallizes in two different polymorphs from pentane, both conforming to the space group P21/n. In both polymorphs, 1 exhibits a close Ti···C contact of 2.634(3) Å between titanium and a γ-methyl group in one of the two tert-butyl groups of the bound amido ligand. Interestingly, the γ-methyl group adopts a rotational conformation that maximizes the Ti···H distances, the shortest of which are 2.36(2) and 2.62(2) Å. Even though the former distance is within the range characteristic of agostic interactions, the rotational orientation of the methyl group suggests that the Ti···H interactions are repulsive rather than attractive. DFT and NBO analysis confirms this supposition: there is no evidence of weakening of the C–H bond closest to the titanium and no evidence of significant overlap of titanium orbitals with the C–H bonding orbitals of the γ-methyl group involved in the close contact. Further evidence that the close contact is repulsive was obtained from a DFT study of a series of related complexes in which the N(t-Bu)2 ligand is replaced with a NR(t-Bu) ligand, where the substituent R not involved in the close contact is Et, Me, or SiMe3. All of these latter substituents, which are sterically smaller than a t-Bu group, enable the amide group to pivot in such a way as to move the tert-butyl group farther from the metal center. The results suggest that the short Ti···C and Ti···H distances seen crystallographically for 1 are actually the result of intraligand and interligand steric repulsions involving the amide substituent not involved in the close contact. The lack of an agostic interaction despite the close contact (and the low electron count of the Ti center) is ascribed to the strong σ- and π-donor properties of the amide and chloride ligands, which raise the energies of the empty orbitals on Ti.