CO2 Reduction by Transition-Metal Complex Systems: Effect of Hydrogen Bonding on the Second Coordination Sphere

Homogeneous electrocatalysts typified by transition-metal complex show transcendent potency in efficient energy catalysis through molecular design. For example, metal complexes with elaborate design performed wonderful activity and selectivity for electrocatalytic CO₂ reduction. Primary coordination sphere of metal complexes plays a key role in regulating its intrinsic redox properties and catalytic activity. However, the overall reduction efficiency of CO₂ is also bound up with the substrate activation process. Transition-metal complexes are hoped to exhibit reasonable redox potential, reactive activity, and stability, while binding and activating CO₂ molecules to achieve efficient CO₂ reduction. Construction of second coordination sphere, especially hydrogen-bonding network of transition-metal complexes, is reported to be the “kill two birds with one stone” strategy to realize efficient CO₂ reduction catalysis via systematic catalyst properties modulation and substrate activation. Herein, we present recent progress on the construction of hydrogen-bonding network in the second coordination sphere of metal complexes by ligand modification or the introduction of exogenous organic ligand, and the resulted productive enhancement of the catalytic performance of metal complexes by the improvement of adsorption capacity and activation of CO₂, proton transfer rate, and stability of reaction intermediates, and so forth.