Highly efficient electrocatalytic CO₂ reduction by a Cr^III quaterpyridine complex

Design tactics and mechanistic studies both remain as fundamental challenges during the exploitations of earth-abundant molecular electrocatalysts for CO₂ reduction, especially for the rarely studied Cr-based ones. Herein, a quaterpyridyl Cr^III catalyst is found to be highly active for CO₂ electroreduction to CO with 99.8% Faradaic efficiency in DMF/phenol medium. A nearly one order of magnitude higher turnover frequency (86.6 s^-1) over the documented Cr-based catalysts (<10 s^-1) can be achieved at an applied overpotential of only 190 mV which is generally 300 mV lower than these precedents. Such a high performance at this low driving force originates from the metal-ligand cooperativity that stabilizes the low-valent intermediates and serves as an efficient electron reservoir. Moreover, a synergy of electrochemistry, spectroelectrochemistry, electron paramagnetic resonance, and quantum chemical calculations allows to characterize the key Cr^II, Cr^I, Cr⁰, and CO-bound Cr⁰ intermediates as well as to verify the catalytic mechanism.