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.