Redox-Inactive Cation Assisted Direct Productions of Oxalic Acid and Dimethyl Oxalate via Oxidative CO Coupling Catalyzed by Coplanar Dicobalt Complexes

Metal–metal cooperativity is one of the most striking features of dinuclear complexes and provides the cornerstone of many of their unique reactivities. In this research, an open-shell planar dinuclear hydroxycarbonylcobalt complex (2) bearing a Robson-type macrocyclic ligand was synthesized. With the assistance of redox-inactive cations, this dicobalt complex was found to catalyze the direct production of oxalic acid via oxidative CO coupling from CO, O₂, and H₂O in good efficiency (turnover number (TON) up to 55.6) and selectivity (up to 94%) at ambient conditions (room temperature and a total gas pressure of 2 atm) under visible light irradiation. By utilizing the same protocol, the catalytic direct production of dimethyl oxalate from CO, O₂, and methanol was also achieved using a dinuclear methoxycarbonylcobalt complex (6), which was synthesized by the esterification of 2 with methanol, as the catalyst, in a TON up to 24.7 with high selectivity (94%). Moreover, the addition of triflic acid further improved the yield of dimethyl oxalate to a TON of 43.8 but with a relatively lower selectivity (83%). The presence of redox-inactive cations was crucial for the selective production of dimethyl oxalate; otherwise, the methyl formate would form as the major product.