Journal of the American Chemical Society, Vol.118, No.24, 5665-5671, 1996
A Solid-State NMR-Study of Tungsten Methyl-Group Dynamics in (W(Eta(5)-C(5)Me(5))Me(4))(PF6)
We report the results of Magic Angle Spinning (MAS) C-13 and static H-2 NMR studies of the dynamics of the methyl groups coordinated to tungsten in [WCp*Me(4)][PF6] (Cp* = eta(5)-C(5)Me(5)). The temperature-dependent broadening of the axial methyl C-13 line can be ascribed to interference between H-1 decoupling and methyl motion when the motional rate and decoupling nutation frequency are comparable. This proposal is consistent with the absence of broadening in the H-2 labeled compound and the motional rate constants inferred from H-2 NMR lineshape and T-1 studies, which range from 10(7) s(-1) at 25 degrees C to 10(3) s(-1) at -125 degrees C. The measured barrier to axial methyl hopping (26 kJ/mol) is among the highest reported to date. An X-ray crystal study of [W(eta(5)-C(5)Me(4)Et)Me(4)][PF6] reveals no evidence (in terms of the core geometry) for an agostic interaction in either the axial methyl group or the equatorial methyl groups.