C-13 CP MAS NMR-STUDIES OF THE STEREOCHEMICAL NONRIGIDITY OF TITANIUM TETRACYCLOPENTADIENYL IN THE SOLID-STATE

Variable-temperature C-13 CP/MAS NMR spectroscopy of Ti(eta-C5H5)2(sigma-C5H5)2 has shown that sigmatropic rearrangement of the sigma-cyclopentadienyl rings occurs for the molecule in the crystalline state. Magnetization-transfer experiments at 165 K are consistent with [1,2] shifts being the main pathway for the rearrangement, but there is evidence for the possibility of [1,3] shifts as a minor pathway. These measurements, in conjunction with spectral line-shape analysis in the exchange broadening regime, have been used to estimate Arrhenius activation parameters for [1,2] shifts of E(a) = 33.2 +/- 1.0 kJ mol-1 and A = 2.9 X 10(10) s-1. The activation barrier is similar to that suggested by solution NMR studies, implying that control of the rearrangement is principally by electronic factors. In contrast to the results of solution NMR studies, only at the highest temperature (369 K) accessible before thermal decomposition of the solid intervenes is there evidence that sigma-/eta-cyclopentadienyl ligand functionality interchange may be significant on the NMR time scale.