C–H and C–C agostic interactions in cycloalkyl tris(pyrazolyl)boratoniobium complexes

Cycloalkyl niobium complexes, TpMe2NbX(R)(MeCCMe) (TpMe2 = hydrotris(3,5-dimethylpyrazolyl)borate; X = Cl, R = c-C3H5 (2a); X = Br, R = c-C3H5 (2b), X = Cl, R = c-C5H9 (3), X = Cl, R = c-C6H11 (4),) have been prepared from TpMe2NbCl2(MeCCMe). The cyclopropyl complex 2a shows no sign of C–H agostic interactions either in the solid state (X-ray) or in solution. In contrast, the NMR spectra of 3 and 4 are temperature dependent as a consequence of an equilibrium between a major α-agostic species and a minor non-agostic one. Hybrid QM/MM calculations are used to rationalise the behaviour of these cycloalkyl species, and illustrate the subtle interplay of steric and electronic effects in these systems.