Selective Nitrite Reduction at Heterobimetallic CoMg Complexes

Abstract

Heme-containing nitrite reductases bind and activate nitrite by a mechanism that is proposed to involve interactions with Brønsted acidic residues in the secondary coordination sphere. To model this functionality using synthetic platforms that incorporate a Lewis acidic site, heterobimetallic CoMg complexes supported by diimine–dioxime ligands are described. The neutral (μ-NO_2)CoMg species 3 is synthesized from the [(μ-OAc)(Br)CoMg]+ complex 1 by a sequence of one-electron reduction and ligand substitution reactions. Data are presented for a redox series of nitrite adducts, featuring a conserved μ-(η^1-N:η^1-O)-NO_2 motif, derived from this synthon. Conditions are identified for the proton-induced N–O bond heterolysis of bound NO_2– in the most reduced member of this series, affording the [(NO)(Cl)CoMg(H_2O)]+ complex 6. Reduction of this complex followed by protonation leads to the evolution of free N_2O. On the basis of these stoichiometric reactivity studies, the competence of complex 1 as a NO_2– reduction catalyst is evaluated using electrochemical methods. In bulk electrolysis experiments, conducted at −1.2 V vs SCE using Et_3NHCl as a proton source, N_2O is produced selectively without the competing formation of NH_3, NH_2OH, or H_2.

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