Further evidence supporting an inner sphere mechanism in the NO reduction of the copper(II) complex Cu(dmp)22+ (dmp = 2,9-dimethyl-1,10-phenanthroline)

Described are further studies directed towards elucidating the mechanism of the nitric oxide reduction of the copper(II) model system, Cu(dmp)22+ (I, dmp = 2,9-dimethyl-1,10-phenanthroline). The reaction of I with NO in methanol results in the formation of Cu(dmp)2+ (II) and methyl nitrite (CH3ONO), with a second order rate constant kNO = 38.1 M−1 s−1 (298 K). The activation parameters for this reaction in buffered aqueous medium were measured to be ΔH‡ = 41.6 kJ/mol and ΔS‡ = −82.7 kJ/mol deg. The addition of azide ion (N3−) as a competing nucleophile results in a marked acceleration in the rate of the copper(II) reduction. Analysis of the kinetics for the NO reduction of the bulkier Cu(dpp)22+ (IV, dpp = 2,9-diphenyl-1,10-phenanthroline) and the stronger oxidant, Cu (NO2-dmp)22+ (V, NO2-dmp = 5-nitro-2,9-dimethyl-1,10-phenanthroline), gave the second order rate constants kNO = 21.2 and 29.3 M−1 s−1, respectively. These results argue against an outer sphere electron transfer pathway and support a mechanism where the first step involves the formation of a copper-nitrosyl (Cu(II)-NO or Cu(I)-NO+) adduct. This would be followed by the nucleophilic attack on the bound NO and the labilization of RONO to form the nitrite products and the cuprous complex.