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 P, 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 P, 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.