Photoactivated cis-dioxo molybdenum(VI)-Schiff base complexes for oxygen atom transfer reactions

To mimic the single-electron transfer (SET) mechanisms prevalent in molybdoenzymes, a number of recently reported molybdenum-oxo catalytic systems use light activation to achieve effective oxygen atom transfer (OAT). The wider applicability of such systems is hindered by complicated ligand synthesis, limited reaction scope, and the need for additional mediators to achieve catalytic turnover. This study focuses on the development of relatively simple molybdenum complexes that are capable of independent photocatalytic OAT reactions.
Thirty-one [cis-Mo(VI)O2]-based complexes with a range of salicylidene aminophenol (SAP) ligands were synthesised and characterised using NMR, FTIR, UV-visible spectroscopies and mass spectrometry. Structural diversity was achieved by introducing functional groups with different electronic properties into the ligand scaffold. The substituents did not affect the spectroscopic profiles of the complexes, but strongly influenced their OAT reactivity. Only the complexes bearing strong electron-withdrawing groups (EWGs) achieved high catalytic turnovers in the benchmark photocatalytic OAT reaction between PPh3 and DMSO under irradiation. This allowed the rational design of the most active OAT photocatalyst to date, [cis-Mo(VI)O2(SAP-(CN)2)(MeOH)], which exhibited quantitative substrate conversion after 70 minutes of irradiation.
The catalytic mechanism associated with the photoactive [cis-Mo(VI)O2(SAP-(R)2)(MeOH)] complexes was investigated using in situ FTIR spectroscopy, 1D and 2D NMR spectroscopy, as well as TD-DFT calculations and electrochemical analysis. Studies strongly suggested the formation of a reduced monomeric [Mo(IV)O]-based catalyst intermediate during photocatalytic OAT. Furthermore, the activity of the photocatalysts was observed to be governed by the strength of EWGs present in the catalyst structure, and not the generation of the LMCT-excited states that initiate the catalytic cycle. The obtained results led to the proposal of three potential reaction mechanisms that could operate under the employed conditions. Further studies are required to elucidate which pathway dominates the OAT reaction catalysed by the novel photoactive [cis-Mo(VI)O2]-based complexes.