Nonagostic M···H–C
Interactions.
Synthesis, Characterization, and DFT Study of the Titanium Amide Ti<sub>2</sub>Cl<sub>6</sub>[N(<i>t</i>-Bu)<sub>2</sub>]<sub>2</sub>
- Publication date
- Publisher
Abstract
The compound Ti<sub>2</sub>Cl<sub>6</sub>[N(<i>t</i>-Bu)<sub>2</sub>]<sub>2</sub> (<b>1</b>) has been synthesized
by treating
TiCl<sub>4</sub> with di(<i>tert</i>-butyl)amine, HN(<i>t</i>-Bu)<sub>2</sub>. Compound <b>1</b> crystallizes
in two different polymorphs from pentane, both conforming to the space
group <i>P</i>2<sub>1</sub>/<i>n</i>. In both
polymorphs, <b>1</b> exhibits a close Ti···C
contact of 2.634(3) Å between titanium and a γ-methyl group
in one of the two <i>tert</i>-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(<i>t</i>-Bu)<sub>2</sub> ligand is replaced with a NR(<i>t</i>-Bu)
ligand, where the substituent R not involved in the close contact
is Et, Me, or SiMe<sub>3</sub>. All of these latter substituents,
which are sterically smaller than a <i>t</i>-Bu group, enable
the amide group to pivot in such a way as to move the <i>tert</i>-butyl group farther from the metal center. The results suggest that
the short Ti···C and Ti···H distances
seen crystallographically for <b>1</b> 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