For CO2 Reduction, Hydrogen-Bond Donors Do the Trick

Carbon dioxide is the major pollutant responsible for climate change, an unfortunate byproduct of powering our society with fossil fuels. There is currently great interest in applying renewable energy to capture and reduce CO2 to provide carbon-neutral fuels. This approach mimics natural photosynthesis, which utilizes CO2 for energy storage and as a structural building block. Achieving this goal requires catalysts that can reduce CO2 to higher energy products or fuel precursors. A popular target reaction is the two-electron reduction of CO2 to CO. Along with H2, CO can be used in Fischer–Tropsch processes to generate liquid fuels. In this issue of ACS Central Science, Chapovetsky, Welborn, and co-workers describe an integrated experimental and computational investigation into mechanistic pathways for a series of cobalt complexes that catalyze this reaction. The most active catalyst contains architectural features that are known to facilitate CO2 reduction in other molecular catalysts. However, their detailed analysis contained some surprises and illuminates new ways in which the secondary structure can be harnessed to promote high catalytic activity. The initial cobalt catalyst reported by Chapovetsky and co-workers contains four secondary amines along the ligand backbone (1 in Figure 1). The pendant N−H groups appear to be poised to assist CO2 binding through hydrogenbonding interactions; this binding motif has been observed in a structurally similar Ni(cyclam) catalyst. The N−H assist hypothesis was buoyed by the strong positive dependence on the number of groups and catalytic rate. However, the calculated energies for CO2 binding in this fashion contained an unexpected result: the ring flip required to position the N−H for CO2 binding came at a prohibitively high energetic cost, and is inaccessible under catalytic conditions. Instead, the reason analogues with sequentially methylated amines (pendant R2N−H groups replaced by R2N−CH3) have lower catalytic activity is their increased steric profile, which inhibits CO2 binding.