A Polymeric Unimolecular CO Dehydrogenase Mimic with both Inner and Outer Spheres for Enhanced Photocatalytic CO2 Reduction in Aqueous Solution

Inspired by carbon monoxide dehydrogenase (CODH), mimicking its inner and outer spheres is a promising strategy in CO2 reduction catalyst design. However, artificial CODH-like catalysts are generally limited to the inner sphere effect and only applicable in organic solvents or for electrocatalysis. Herein, we report an aqueous CODH mimic with both inner and outer spheres for photocatalysis. In this polymeric unimolecular catalyst, the inner sphere is composed of cobalt porphyrin with four appended amido groups and the outer sphere consists of four poly(2(dimethylamino)ethyl methacrylate) (PDMAEMA) arms. Upon visible light irradiation (λ > 420 nm), the as-prepared photocatalyst exhibits a high turnover number (TONCO = 1731.2) in the reduction of CO2 into CO, which is superior to most reported molecular photocatalysts in aqueous solution. The mechanism studies indicate that, in this water-dispersible and structurally well-defined CODH mimic, the cobalt porphyrin core serves as the catalysis center, amido groups function as hydrogen-bonding pillars helping to stabilize the CO2 adduct intermediate, while PDMAEMA shell renders both the water solubility and a CO2 reservoir through reversibly capturing of CO2. The present work has clarified the significance of inner and outer coordination sphere effects for improving the aqueous photocatalytic CO2 reduction performance of CODH mimics.