Photocatalytic CO Reduction with Manganese Complexes Bearing a κ-PN Ligand: Breaking the α-Diimine Hold on Group 7 Catalysts and Switching Selectivity.

The fundamental challenge of reducing CO 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, CO-reducing photocatalysts based on Mn and Re supported by κ-PN phosphinoaminopyridine ligands were identified. These catalysts, [M{κ-(PhP)NH(NCH)}(CO)Br], displayed excellent product selectivity and, by a change of only the metal center, gave a dramatic product switch from CO with M = Mn to HCOH with M = Re. The catalyst systems were explored with variation of the ligand, electron donor, solvent, and photosensitizer. The products were definitively traced using CO as a substrate. Both complexes quenched the excited-state photosensitizer Ru(bpy)*, suggesting oxidative quenching as a potential entry into the catalytic cycle.