Molecular Structure of Nitro- and Nitrito-Copper Complexes as Reaction Intermediates in Electrochemical Reduction of Nitrite to Dinitrogen Oxide

Abstract

Abstract Copper-nitrito and -nitro isomers, [Cu(ONO)(tpa)]PF6 and [Cu(NO2)(tpa)]PF6 (tpa = tris[(2-pyridyl)methyl]amine) were isolated and the molecular structures were determined by X-ray analysis. [Cu(ONO)(tpa)]PF6 (C18H18N5O2PF6Cu) crystallizes in the monoclinic space group P21/a with a = 13.374(2), b = 14.033(2), c = 13.455(2) Å, β = 119.10(1)°, V = 2206.5(6) Å3, and Z = 4. [Cu(NO2)(tpa)]PF6 (C18H18N5O2PF6Cu) crystallizes in the orthorhombic space group I/ba2 with a = 16.718(3), b = 17.554(3), c = 14.785(3) Å, V = 4338(1) Å3, and Z = 8. Those nitrito- and nitro-complexes exist as an equilibrium mixture in solutions. Electrochemical reduction of NO2− in the presence of [Cu(H2O)(tpa)](ClO4)2 at −0.4 V in H2O (pH 7.0) catalytically produced N2O with concomitant evolution of a small amount of NO via the nitro and nitrito adducts.