Is the bipyridyl thorium metallocene a low-valent thorium complex? A combined experimental and computational study

Abstract

Bipyridyl thorium metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th(bipy) (1) and [η⁵-1,3-(Me₃C)₂C₅H₃]₂Th(bipy) (2) have been investigated by magnetic susceptibility and computational studies. The magnetic susceptibility data reveal that 1 and 2 are not diamagnetic, but they behave as temperature independent paramagnets (TIPs). To rationalize this observation, density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations have been undertaken, which indicated that Cp′₂Th(bipy) has indeed a Th(IV)(bipy²⁻) ground state (f⁰d⁰π*², S = 0), but the open-shell singlet (f⁰d¹π*¹, S = 0) (almost degenerate with its triplet congener) is only 9.2 kcal mol⁻¹ higher in energy. Complexes 1 and 2 react cleanly with Ph₂CS to give [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th[(bipy)(SCPh₂)] (3) and [η⁵-1,3-(Me₃C)₂C₅H₃]₂Th[(bipy)(SCPh₂)] (4), respectively, in quantitative conversions. Since no intermediates were observed experimentally, this reaction was also studied computationally. Whereas coordination of Ph₂CS to 2 in its S = 0 ground state is not possible, Ph₂CS can coordinate to 2 in its triplet state (S = 1) upon which a single electron transfer (SET) from the (bipy²⁻) fragment to Ph₂CS followed by C–C coupling takes place.