A structural model for oxidized type II copper nitrite reductase with a polyimidazole tripodal ligand

Abstract

The new copper(II)–nitrite complex [Cu II(L)(ONO)]ClO 4 ( 2), L=[(bis(1-methylimidazol-2-yl)methyl)(2-(pyridyl-2-yl)ethyl)amine] was synthesized and characterized by cyclic voltammetry, UV–Vis spectroscopy, X-band EPR and X-ray crystallography. Complex 2 crystallizes in the monoclinic system, space group P2 1/ n and has the metal center in a somewhat distorted square pyramidal geometry, with three nitrogen atoms from the chelate ligand and one nitrite ion outlining the basal plane. One imidazole nitrogen from the tripodal molecule is at the apical position at 2.158(2) Å from the metal. Spectrophotometric and electrochemical experiments in water solution reveal that complex 2 readily releases the nitrite ligand in this medium ( t 1/2<10 s at 25 °C) yielding the precursor complex 1, [Cu II(L)(S)] 2+. The affinity of complex 1 for nitrite was evaluated by spectrophotometric titration, following the appearance of the Cu(II)–nitrite band at 376 nm, and by the cyclic voltammetric peak potential data. Both experiments were carried out in methanolic solution ( μ=0.1 mol L −1, TBAPF 6, T=25±1 °C). The formation constant value obtained from the voltammetric experiments (1.2 ± 0.1 × 10 3 M −1) agrees quite well with that from the spectrophotometric titration (1.4 ± 0.2 × 10 3 M −1) and is within the range of values observed for other Cu(II)–nitrite complexes. The geometrical arrangements of the tripodal ligand and the nitrite ion coordination allows considering complex 2 a potential biomimetic structural model for type II copper in nitrite reductases.