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.