Further evidence supporting an inner sphere mechanism in the NO reduction of the copper(II) complex Cu(dmp)2(2+) (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-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 (298K). The activation parameters for this reaction in buffered aqueous medium were measured to be DeltaH(double dagger)=41.6 kJ/mol and DeltaS(double dagger)=-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)(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 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.
Lim
Mark D
MD
Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA.
Capps
Kenneth B
KB
Karpishin
Timothy B
TB
Ford
Peter C
PC
D001386:Azides / Q000737:chemistry; D002413:Cations, Divalent / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D015394:Molecular Structure; D009569:Nitric Oxide / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D013057:Spectrum Analysis
D001386:Azides; D002413:Cations, Divalent; D008024:Ligands; D009942:Organometallic Compounds; C505200:bis(2,9-dimethyl-1,10-phenanthroline)copper(II); D009569:Nitric Oxide; D003300:Copper