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)(2)(2+) (I, dmp = 2,9-dimetliyl-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 k(NO) = 38.1 M-1 s(-1) (298 K). The activation parameters for this reaction in buffered aqueous medium were measured to be Delta H-double dagger = 41.6 kJ/mol and AS = -82.7 kJ/mol deg. The addition of azide ion (N-3(-)) 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)(2)(2+) (IV, dpp = 2,9-diphenyl-1,10-phenanthroline) and the stronger oxidant, Cu(NO2-dmp)(2)(2+) (V, NO2-dmp = 5-nitro-2,9-dimethyl-1,10-phenanthroline), gave the second order rate constants k(NO) = 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. (c) 2005 Elsevier Inc. All rights reserved.