Binding of nitrite and its reductive activation to nitric oxide at biomimetic: copper centers

The reactivity of nitrite towards the copper(II) and copper(I) centers of a series of complexes with tridentate nitrogen donor ligands has been invest igated. The ligands are bis[(1-methylbenzimidazol-2-yl)methyl]amine (l-bb), bis[2-(1-methylbenzimidazol-2-yl)ethyl] amine (2-bb), and bis[2-(3,5-dimet hyl-1-pyrazolyl)ethyl]amine (ddah) and carry two terminal benzimidazole (l- bb, 2-bb) or pyrazole (ddah) rings and a central amine donor residue. While 2-bb and ddah form two adjacent six-membered chelate rings on metal coordi nation, l-bb forms two smaller rings of five members. The binding affinity of nitrite and azide to the Cu(II) complexes (ClO4- as counterion) has been determined in solution. The association constants for the two ligands are similar, but nitrite is a slightly stronger ligand than azide when it binds as a bidentate donor. The X-ray crystal structure of the nitrite complex [ Cu(ddah)(NO2)]ClO4 (final R=0.056) has been determined: triclinic P (1) ove r bar space group, a=8.200(2) Angstrom, b=9.582(3) Angstrom, c=15.541(4) An gstrom. It may be described as a perchlorate salt of a "supramolecular" spe cies resulting from the assembly of two complex cations and one sodium perc hlorate unit. The copper stereochemistry in the complex is intermediate bet ween SPY and TBP, and nitrite binds to Cu(II) asymmetrically, with Cu-O dis tances of 2.037(2) and 2.390(3) Angstrom and a nearly planar CuO2N cycle. O n standing, solutions of [Cu(ddah)(NO2)]ClO4 in methanol produce the dinucl ear complex [Cu(ddah)(OMe)](2)(ClO4)(2), containing dibridging methoxy grou ps. In fact the crystal structure analysis (final R=0.083) showed that the crystals are built up by dinuclear cations, arranged on a crystallographic symmetry center, and perchlorate anions. Electrochemical analysis shows tha t binding of nitrite to the Cu(II) complexes of 2-bb and ddah shifts the re duction potential of the Cu(II)/Cu(I) couple towards negative values by abo ut 0.3 V. The thermodynamic parameters of the Cu(II)/Cu(I) electron transfe r have also been analyzed. The mechanism of reductive activation of nitrite to nitric oxide by the Cu(I) complexes of l-bb, 2-bb, and ddah has been st udied. The reaction requires two protons per molecule of nitrite and Cu(I). Kinetic experiments show that the reaction is first order in [Cu(I)] and [ H+] and exhibits saturation behavior with respect to nitrite concentration. The kinetic data show that [Cu(2-bb)](+) is more efficient than [Cu(1-bb)] (+) and [Cu(ddah)](+) in reducing nitrite.