Oxo-functionalised mesoionic NHC nickel complexes for selective electrocatalytic reduction of CO₂ to formate

Strategies for the conversion of CO₂ to valuable products are paramount for reducing the environmental risks associated with high levels of this greenhouse gas and offer unique opportunities for transforming waste into useful products. While catalysts based on nickel as an Earth-abundant metal for the sustainable reduction of CO₂ are known, the vast majority produce predominantly CO as a product. Here, efficient and selective CO₂ reduction to formate as a synthetically valuable product has been accomplished with novel nickel complexes containing a tailored C,O-bidentate chelating mesoionic carbene ligand. These nickel(ii) complexes are easily accessible and show excellent catalytic activity for electrochemical H⁺ reduction to H₂ (from HOAc in MeCN), and CO₂ reduction (from CO₂-saturated MeOH/MeCN solution) with high faradaic efficiency to yield formate exclusively as an industrially and synthetically valuable product from CO₂. The most active catalyst precursor features the 4,6-di-tert-butyl substituted phenolate triazolylidene ligand, tolerates different proton donors including water, and reaches an unprecedented faradaic efficiency of 83% for formate production, constituting the most active and selective Ni-based system known to date for converting CO₂ into formate as an important commodity chemical.