Bioinspired Nitrite Reduction to Ammonia by Molecular Nickel Polypyridine–Quinoline Complex

Abstract

In nature, the enzyme cytochrome-c-nitrite reductase (CcNiR) selectively reduces nitrite to ammonia, involving four nitrite-reduced intermediates, e.g., nitric oxide (NO), nitroxyl species (NO– and its conjugate acid HNO), and hydroxylamine (NH2OH). Similar to enzymatic nitrite reduction, the molecular nickel nitrito complex containing a polypyridine–quinoline ligand, [NiII(2PyN2Q)(k-ONO)](BF4)] (1-ONO) exhibits nitrite reduction to ammonia, involving all four intermediates under controlled electrochemical and stoichiometric chemical conditions. All four nitrite reduced intermediates, along with ammonia (NH3) have been detected and characterized by 1H and 31P{1H} NMR, EPR, and spectrophotometric studies. Moreover, the 1-ONO complex shows electrocatalytic nitrite reduction to ammonia in nonaqueous and aqueous media with excellent Faradaic efficiencies of 86% and 98%, using tetra-butyl ammonium nitrite (TBAN) and sodium nitrite (NaNO2) respectively. The 15N-labeled ammonia was obtained using Na15NO2 as the only source of nitrogen. Further, the in situ-generated nickel–nitrosyl complex {Ni-NO}10 (3) was characterized by 15N NMR , UV–vis, FT-IR (with 15NO labeling) studies. Complex 3 provides other nitrite-reduced species upon stepwise reduction with CoCp*2 and PTBP, supporting its intermediacy during the catalytic cycle. Unlike the 1-ONO complex, the all-four-pyridine-containing nickel nitro complex [NiII(N4Py)(k-NO2)](BF4)] (2-NO2) yields NH2OH as the exclusive product rather than NH3. The presence of a redox-active, sterically demanding quinoline moiety in the primary coordination sphere of the 1-ONO complex facilitates stepwise nitrite reduction, involving the key intermediates NO, NO–, HNO, and NH2OH, to ammonia (NH3) through proton-coupled electron transfer pathways. A comprehensive computational study outlines the reaction energy profile and elucidates the ligand-controlled product selectivity.