The rising interest in the development of homogenous sulfur atom transfer (SAT) catalysts is attributable to their applications to both the desulfurisation of crude oil, and the utilisation of abundant elemental sulfur in generating valuable organosulfur feedstock. Although SAT catalysts containing group 6 elements, such as molybdenum and tungsten have shown great promise, these systems often suffer from aerobic instability. Our research has focused on the development of highly efficient novel molybdenum SAT catalysts with greater oxygen and water tolerance than those currently known. We demonstrate that μ-oxo-bridged binuclear Mo(V) species, [Mo₂O₃(L)₂(THF)₂], where L = 2-hydroxybenzaldehyde thiosemicarbazone, are effective catalysts for SAT. Our system allows for fine-tuning of the electron density by alteration of ligand substituents, which has a marked impact on the catalytic efficiency. We have also screened a large range of potential sulfur donor and acceptor substrates. Herein, we report our attempts to isolate intermediates in the catalytic mechanism by cycle decoupling. Through the use of strongly coordinating ligands bipyridine (bpy) and dimethyl aminopyridine (DMAP), a Mo(IV) mono-oxo complex, [MoO(L)(bpy)], and Mo(VI) mono-oxo mono-sulfido species, [MoO(S)(L)(DMAP)], have been identified through spectroscopic or crystallographic characterisations. We also discuss our investigation into an analogous tetradentate thiosemicarbazone system as a control to gain a better understanding of the role of the vacant coordination site in the catalytic cycle.

General structure of the investigated μ-oxo-bridged binuclear Mo(V) SAT catalysts, confirmed by x-ray crystallography.

Stephen Marriott

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