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

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• 作者：Charles W. Spicer ; Charity Flener Lovitt ; Gregory S. Girolami
• 刊名：Organometallics
• 出版年：2012
• 出版时间：July 9, 2012
• 年：2012
• 卷：31
• 期：13
• 页码：4894-4903
• 全文大小：377K
• 年卷期：v.31,no.13(July 9, 2012)
• ISSN：1520-6041

文摘

The compound Ti<sub>2sub>Cl<sub>6sub>[N(t-Bu)<sub>2sub>]<sub>2sub> (1) has been synthesized by treating TiCl<sub>4sub> with di(tert-butyl)amine, HN(t-Bu)<sub>2sub>. Compound 1 crystallizes in two different polymorphs from pentane, both conforming to the space group P2<sub>1sub>/n. In both polymorphs, 1 exhibits a close Ti路路路C contact of 2.634(3) 脜 between titanium and a 纬-methyl group in one of the two tert-butyl groups of the bound amido ligand. Interestingly, the 纬-methyl group adopts a rotational conformation that maximizes the Ti路路路H distances, the shortest of which are 2.36(2) and 2.62(2) 脜. Even though the former distance is within the range characteristic of agostic interactions, the rotational orientation of the methyl group suggests that the Ti路路路H interactions are repulsive rather than attractive. DFT and NBO analysis confirms this supposition: there is no evidence of weakening of the C鈥揌 bond closest to the titanium and no evidence of significant overlap of titanium orbitals with the C鈥揌 bonding orbitals of the 纬-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)<sub>2sub> ligand is replaced with a NR(t-Bu) ligand, where the substituent R not involved in the close contact is Et, Me, or SiMe<sub>3sub>. 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路路路C and Ti路路路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 蟽- and 蟺-donor properties of the amide and chloride ligands, which raise the energies of the empty orbitals on Ti.