Investigation of the nitric oxide reduction of the bis(2,9-dimethyl-1,10-phenanthroline) complex of copper(II) and the structure of [Cu(dmp)2(H2O)](CF3SO3)2

The reaction between nitric oxide (NO) and the Cu(II) complex Cu(dmp)(2)(H2O)(2+) (1, dmp=2,9-dimethyl-1,10-phenanthroline) has been quantitatively examined in solution. In neutral or alkaline aqueous solutions or in methanol, 1 (E-1/2=0.58 V vs NHE in water) is reduced by NO to give the corresponding cuprous species Cu(dmp)(2)(+) plus RONO (R = H, CH3). The analogous reaction for Cu(phen)(2)(H2O)(2+) (2, phen = 1,10-phenanthroline) (E-1/2=0.18 V) is much slower and does not go to completion. In aqueous solutions, the conjugate base of 1 reacts with NO at a slower rate. At low pH, the cuprous complex Cu(phen)(2)(+) reacts quantitatively with NO2- in the reverse reaction to give 2 plus NO, detected with a NO-sensitive electrode. The crystal structure of the triflate salt of 1 was determined, and the cupric ion was found to be 5-coordinate with two bidentate dmp ligands and one H2O ligand. The triflate salt of 1, [Cu(dmp)(2)(H2O)](CF3SO3)(2), crystallizes in the orthorhombic system, space group Pbca, with a=20.531(4) Angstrom, b=21.342(8) Angstrom, c=15.345(5) Angstrom, V=6723(4) Angstrom(3), Z=8, and R=0.051 (R-W=0.053). Kinetics studies for the NO reduction of 1 show the reaction to be first order in [NO], to be slower in basic media, and to be inhibited by higher concentrations of different buffers. This behavior is rationalized in terms of a mechanism where NO must form an inner sphere complex with Cu(II), but such binding at the fifth coordination site is inhibited by prior binding to OH-or to the conjugate base of the buffer.