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

Molecular Co complexes of a bis-pyridine-monooxime ligand (HBPML), [Co^III(BPML)₂]⁺ (1) and [Co^II(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 Co^I 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.