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

Abstract

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 (E1/2 = 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) (E1/2 = 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) Å, b = 21.342(8) Å, c = 15.345(5) Å, V = 6723(4) Å3, Z = 8, and R = 0.051 (Rw = 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.