Synthesis and structural characterisation of [TpMe2Nb(CH3)2(CH3CCCH3)] and [TpMe2NbCl(CH3)(CH3CCCH3)]: is there an intrinsic α-agostic interaction in alkyl complexes of the [TpMe2Nb(alkyne)] moiety?

Abstract

The methyl complexes [Tp^Me2Nb(CH₃)₂(CH₃CCCH₃)] (2) and [Tp^Me2NbCl(CH₃)(CH₃CCCH₃)] (3) have been prepared from the corresponding dichloride, and their structures investigated by X-ray diffraction. In each case, the Nb–CH₃ moiety displays a normal geometry in comparison with other methylniobium derivatives, with no evidence for the structural distortions which attend an α-agostic interaction. The NMR spectra of 2 and 3, including those of the CH₂D isotopomers, also point to a conventional methyl–metal complex, with no isotopic perturbation of resonance which can signal agostic behaviour. Likewise, the IR spectra of the CH₃, CH₂D and CD₃ isotopomers of 2 and 3 indicate a conventional bonding situation, although the asymmetric Nb–CH₃ potential leads to two distinct ν^is_CD modes in the CH₂D spectrum. Taken together, these results lead us to conclude that there is no intrinsic α-agostic interaction in the methyl complexes 2 and 3, and that the α-agostic structures deduced for their longer chain alkyl congeners are predominantly the result of steric interactions between the alkyl moiety and the bulky Tp^Me2 ligand.