Highly Efficient and Robust Photocatalytic Systems for CO₂ Reduction Consisting of a Cu(I) Photosensitizer and Mn(I) Catalysts

The development of highly efficient, selective, and durable photocatalytic CO₂ reduction systems that only use earth-abundant elements is key for both solving global warming and tackling the shortage of energy and carbon resources. Here, we successfully developed CO₂ reduction photocatalysts using [Cu₂(P₂bph)₂]²⁺ (CuPS) (P₂bph = 4,7-diphenyl-2,9-di­(diphenylphosphinotetramethylene)-1,10-phenanthroline) as a redox photosensitizer and fac-Mn­(X₂bpy)­(CO)₃Br (Mn­(4X)) (X₂bpy = 4,4′-X₂-2,2′-bipyridine (X = −H and −OMe) or Mn­(6mes) (6mes = 6,6′-(mesityl)₂-2,2′-bipyridne)) as the catalyst. The most efficient photocatalysis was achieved by Mn­(4OMe): The total quantum yield of CO₂ reduction products was 57%, the turnover number based on the Mn catalyst was over 1300, and the selectivity of CO₂ reduction was 95%. Electronic and steric effects of the substituents (X) in the Mn complexes largely affected both the photocatalytic efficiency and the product selectivity. For example, the highest selectivity of CO formation was achieved by using Mn­(6mes) (selectivity S_CO = 96.6%), whereas the photocatalytic system using Mn­(4H) yielded HCOOH as the main product (S_HCOOH = 74.6%) with CO and H₂ as minor products (S_CO = 23.7%, S_H2 = 1.7%). In these photocatalytic reactions, CuPS played its role as an efficient and very durable redox photosensitizer, while remaining stable in the reaction solution even after a turnover number of 200 had been reached (the catalyst used had a turnover number of over 1000).