Selective Catalytic Conversion of Nitrite to Ammonium by an Oxygen-Tolerant Molecular Cobalt Complex

Molecular Co complexes of a bis-pyridine-monooxime ligand (HBPML), [CoIII(BPML)2]+ (1) and [CoII(HBPML)Br2] (2), have been synthesized and thoroughly characterized. Electrocatalytic nitrite (NO2–) reduction catalyzed by 1 was investigated in a 0.1 M sodium phosphate buffer solution (PBS) at pH 7, which revealed the selective conversion of NO2– to ammonium (NH4+) with 99% Faradaic efficiency and a turnover frequency of ∼65 h–1. Experimental investigations revealed that the initiation of the catalytic reaction begins through the coordination of NO2– to the CoI site via the dissociation of one of the pyridine arms of the ligand, which makes the catalyst highly selective for the NO2– reduction reaction (NO2– RR). In fact, 1 was found inactive for the oxygen reduction reaction in PBS at pH 7, thus efficiently functioning NO2– RR under an oxygen atmosphere. Complex 2 is converted to 1 through a disproportionation reaction in the buffer solution and catalyzes the NO2– RR, implying that 2 is not capable of assisting the reduction of NO2– to NH4+. Further, theoretical investigations have been performed to understand the reaction mechanism. The detailed reaction mechanism of the NO2– RR has been demonstrated by combining experimental observations and in-silico studies. Overall, the study underscores the significance of ligand design aspects on the electrocatalytic reduction of NO2– to NH4+ by a molecular Co complex.