Structural, electronic and electron paramagnetic resonance study of (pyridine-2,6-dicarboxylato)copper(II) complexes with substituted imidazoles

Abstract

The molecular and electronic structures of a series of copper(II) complexes [Cu(pydca)L][pydca = pyridine-2,6-dicarboxylate; L = imidazole, 1-, 2- or 5-methylimidazole, 2-ethylimidazole or histamine (4-aminoethylimidazole)] have been studied. The structures where L is 1- (1-mim), 2- (2-mim) or 5-methylimidazole (5-mim) have been determined by single-crystal X-ray diffraction techniques: [Cu(pydca)(1-mim)(H2O)]·2H2O, triclinic, space group P1, a = 8.908(2), b = 15.729(3), c = 5.320(4) Å, α = 97.04(2), β = 91.83(2), γ = 101.64(2)°, Z = 2; [Cu(pydca)(2-mim)(H2O)]·H2O 2, monoclinic, space group P21/c, a = 12.105(3), b = 13.421(3), c = 8.277(3) Å, β = 96.9(0)°, Z = 4; [Cu(pydca)-(5-mim)(H2O)]·0.5MeOH 3, triclinic, space group P1, a = 7.575(2), b = 11.766(2), c = 16.203(3) Å, α = 69.39(3), β = 86.41(3), γ = 82.00(3)°, Z = 4, with two independent molecules in the unit cell. In each case the complex consists of discrete, monomeric units. The copper atom is co-ordinated to two carboxylate oxygen atoms [Cu-O(average) 2.0205(4), 2.0410(6) and 2.037(6) Å], to the pyridiyl nitrogen [Cu-N 1.895(4), 1.918(7) and 1.903(5) Å] and the imidazolyl nitrogen [Cu-N 1.925(4), 1.929(8) and 1.925(5) Å in 1-3]. The distorted square-pyramidal geometry is completed by a longer axial bond [Cu-O 2.390(4), 2.295(5) and 2.287(6) Å] to the oxygen atom of a water molecule. The effects of the 2-methyl substituent in 2 lead to a substantially longer Cu-N(pyridyl) bond. The solid-state EPR spectra of the complexes revealed strong dipole-dipole coupling except for that with L = imidazole where antiferromagnetic exchange was involved. The frozen-glass spectra had an unusual lineshape, atypical of tetragonally or tetrahedrally distorted mononuclear copper(II) complexes. The anisotropic EPR spectra of the five-co-ordinate copper(II) complexes were fitted by an orthorhombic spin Hamiltonian from which the g and 63Cu hyperfine matrices and linewidth parameters were determined by computer simulations. The methyl and ethyl derivatives of the [Cu(pydca)L(H2O)] complexes display similarities with superoxide dismutase with respect to the EPR characteristics, viz. the hyperfine and linewidth anisotropy.