Second Coordination Sphere Effects in an Earth-Abundant Monometallic Complex as Catalyst Dictate Highly Selective Photochemical Conversion of CO2 to HCOOH

Photochemical reduction of CO2 to formic acid (HCOOH) using molecular transition metal complexes as catalysts has been previously documented to proceed via either of the two mechanisms: CO2 insertion into metal hydride bonds, or hydrogenation of CO2. We present herein photoreduction of CO2 to HCOOH that adopts both these pathways by a single monometallic molecular catalyst featuring earth-abundant cobalt - a characteristic not observed in other molecular catalysts for CO2 photoreduction to date. This dual functionality stems from distinctly different, active forms of the catalyst, at different temporal stages of irradiation. We employ in situ NMR spectroscopy to monitor potential catalytic intermediates at various time points of irradiation by visible light. These observed states align with the kinetic profiles of product evolution, proving instrumental in revealing the catalyst's unique behavior. We advocate for the integration of such in situ NMR monitoring in future studies as it provides a valuable tool for an informed design of photochemical CO2 reduction by molecular catalysts.