Photocatalytic CO₂ Reduction with Manganese Complexes Bearing a κ²‑PN Ligand: Breaking the α‑Diimine Hold on Group 7 Catalysts and Switching Selectivity

The fundamental challenge of reducing CO2 into more valuable energy-containing compounds depends on revealing new catalysts for this process. By removal of the long-standing limitation of α-diimine ligation, which is dominant in photocatalytic complexes in this area, new visible-light, CO2-reducing photocatalysts based on Mn and Re supported by κ2-PN phosphinoaminopyridine ligands were identified. These catalysts, [M­{κ2-(Ph2P)­NH­(NC5H4)}­(CO)3Br], displayed excellent product selectivity and, by a change of only the metal center, gave a dramatic product switch from CO with M = Mn to HCO2H with M = Re. The catalyst systems were explored with variation of the ligand, electron donor, solvent, and photosensitizer. The products were definitively traced using 13CO2 as a substrate. Both complexes quenched the excited-state photosensitizer Ru­(bpy)32+*, suggesting oxidative quenching as a potential entry into the catalytic cycle