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-(η1-N:η1-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.