Ligand-to-metal charge transfer facilitates photocatalytic oxygen atom transfer (OAT) with cis-dioxo molybdenum(vi)-Schiff base complexes

Abstract

Systems incorporating the cis-Mo(O)₂ motif catalyse a range of important thermal homogeneous and heterogeneous oxygen atom transfer (OAT) reactions spanning biological oxidations to platform chemical synthesis. Analogous light-driven processes could offer a more sustainable approach. The cis-Mo(O)₂ complexes reported here photocatalyse OAT under visible light irradiation, and operate via a non-emissive excited state with substantial ligand-to-metal charge-transfer (LMCT) character, in which a MoO π*-orbital is populated via transfer of electron density from a chromophoric salicylidene-aminophenol (SAP) ligand. SAP ligands can be prepared from affordable commercially-available precursors. The respective cis-Mo(O)₂-SAP catalysts are air stable, function in the presence of water, and do not require additional photosensitisers or redox mediators. Benchmark OAT between phosphines and sulfoxides shows that electron withdrawing groups (e.g. C(O)OMe, CF₃) are necessary for photocatalytic activity. The photocatalytic system described here is mechanistically distinct from both thermally catalysed OAT by the cis-Mo(O)₂ motif, as well as typical photoredox systems that operate by outer sphere electron transfer mediated by long-lived emissive states. Both photoactivated and thermally activated OAT steps are coupled to establish a catalytic cycle, offering new opportunities for the development of photocatalytic atom transfer based on readily-available, high-valent metals, such as molybdenum.