Selective Nitrite Reduction at Heterobimetallic CoMg Complexes

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-NO2)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)-NO2 motif, derived from this synthon. Conditions are identified for the proton-induced N-O bond heterolysis of bound NO2(-) in the most reduced member of this series, affording the [(NO)(Cl)CoMg(H2O)](+) complex 6. Reduction of this complex followed by protonation leads to the evolution of free N2O. On the basis of these stoichiometric reactivity studies, the competence of complex 1 as a NO2(-) reduction catalyst is evaluated using electrochemical methods. In bulk electrolysis experiments, conducted at -1.2 V vs SCE using Et3NHCl as a proton source, N2O is produced selectively without the competing formation of NH3, NH2OH, or H2.