Density functional study of hydrogen exchange and methane elimination from bis(cyclopentadienyl)tungsten methyl hydrides and their ansa-bridged analogues

Abstract

Theoretical calculations have been carried out using density functional theory on the hydrogen exchange and methane elimination reactions of [W(η-C5H5)2(Me)H] and [W{(η-C5H4)2CH2}(Me)H]. In both cases the hydrogen exchange proceeds along a similar reaction path on which lies a η1-H σ complex. The midpoint of the exchange reaction is a η2-H,H σ complex. For [W(η-C5H5)2(Me)H] a transition state is proposed which is calculated to lie 81 kJ mol–1 above the ground state. This compares with an experimental free energy of activation of 106 kJ mol–1 for [W(η-C5H5)2(Me)D]. Elimination of methane is calculated to have an overall reaction energy of 19.6 kJ mol–1 for [W(η-C5H5)2(Me)H] and 78.3 kJ mol–1 for [W{(η-C5H4)2CH2}(Me)H]. The activation energy for elimination from the two compounds is estimated as around 80 and 85 kJ mol–1 respectively. The principal reason for the relative stability of the [W(η-C5H5)2] product is its relaxation to a parallel ring triplet state which is calculated to lie 67.9 kJ mol–1 below the lowest excited singlet state. For [W{(η-C5H4)2CH2}] the singlet and triplet states are separated by 13.3 kJ mol–1 with the rings inclined at an angle constrained by the ansa bridge.