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
investigated. The ligands are bis[(1-methylbenzimidazol-
2-yl)methyl]amine (1-bb), bis[2-(1-methylbenzimidazol-
2-yl)ethyl]amine (2-bb), and bis[2-(3,5-dimethyl-
1-pyrazolyl)ethyl]amine (ddah) and carry two terminal
benzimidazole (1-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 coordination, 1-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 P1Å space group, a=8.200(2) Š,
b=9.582(3) Š, c=15.541(4) Š. It may be described as a
perchlorate salt of a ªsupramolecularº species resulting
from the assembly of two complex cations and one
sodium perchlorate unit. The copper stereochemistry
in the complex is intermediate between SPY and
TBP, and nitrite binds to Cu(II) asymmetrically, with
Cu-O distances of 2.037(2) and 2.390(3) Š and a
nearly planar CuO2N cycle. On standing, solutions of
[Cu(ddah)(NO2)]ClO4 in methanol produce the dinuclear
complex [Cu(ddah)(OMe)]2(ClO4)2, containing
dibridging methoxy groups. 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 that binding of nitrite
to the Cu(II) complexes of 2-bb and ddah shifts the
reduction potential of the Cu(II)/Cu(I) couple towards
negative values by about 0.3 V. The thermodynamic
parameters of the Cu(II)/Cu(I) electron transfer have
also been analyzed. The mechanism of reductive activation
of nitrite to nitric oxide by the Cu(I) complexes
of 1-bb, 2-bb, and ddah has been studied. 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.