DFT insights into the bifunctional nature of a bioinspired tungsten complex

Abstract

Tungsten model complexes that replicate aspects of enzymatic activity are scarce due to challenges in reducing the readily prepared WO₂²⁺ moiety. As a result, studies of bioinspired catalytic cycles remain limited, since tungsten in nature primarily facilitates redox processes such as Oxygen Atom Transfer (OAT). In our group, we recently observed that pyridine-2-thiolate (PyS)-supported tungsten dioxido complexes can be reduced to phosphine-stabilized W(IV) compounds, opening up opportunities for investigating rarely observed reduced participants in the OAT catalytic cycles. Reactivity studies revealed the activation of DMSO – an expected oxotransferase substrate, and O₂ – a substrate typically associated with oxygenase enzymes. The activation of the latter encouraged a detailed computational DFT study (ORCA/PBE0-D4). Although heavy elements like tungsten usually remain in diamagnetic spin states, our findings indicate that O₂ is activated by [WO(PyS)₂(PMe₃)] in two successive steps, binding as η¹-(O₂)⁻ in a paramagnetic W(V) intermediate, which converts to a much more stable diamagnetic W(V) species after spin crossover. The oxidation to W(VI) is triggered by the η¹- to η²-haptotropic shift of O₂. This complex converts to the W(VI) dioxide with the highest energy barrier of the O₂ activation reaction (ΔG^‡ 93 kJ mol⁻¹), but the regeneration of the catalytic W(IV) species is even more difficult (ΔG^‡ 118 kJ mol⁻¹). The high coordination flexibility of the ancillary PyS ligands is key to reaching different spin states, which are essential for dioxygen activation. Interestingly, the observed reactivity may resemble the inactivation pathway of tungsten enzymes under aerobic conditions.