Inner-Sphere Electron Transfer is Rate Limiting in the Photoreduction of N‑Oxides by Re-complexes

In this study, we investigate the key mechanistic steps of the photocatalytic deoxygenation of nitrous oxide (N2O), a significant greenhouse gas, and pyridine N-oxide (PNO), an important intermediate in pharmaceutical synthesis, using a rhenium(I) tricarbonyl bipyridine complex, [ReI(bpy)(CO)3Cl] (Re–Cl), in the presence of a sacrificial electron donor (DIPEA). Optimization of the reaction conditions revealed that the addition of water (3 m) enhances catalytic efficiency, particularly for PNO reduction, suggesting a crucial role of proton availability in facilitating N–O bond cleavage. Through a combination of techniques such as nanosecond transient absorption spectroscopy, infrared spectroscopy, and density functional theory (DFT) calculations, we elucidate the mechanism of this photocatalytic transformation. The reaction is initiated by photoexcitation of the Re–Cl complex, followed by reductive quenching by DIPEA, generating the singly reduced species (1ERS), which serves as the catalytically active species. The deoxygenation mechanism proceeds via proton-assisted N–O bond cleavage, forming a metal-hydroxo intermediate. The rate-limiting step is identified as the electron transfer to the substrate. The presence of water facilitates proton transfers and ligand exchanges during PNO photodeoxygenation but has no significant impact on N2O reduction.