Synthesis, characterization, and X-ray crystal structures of mononuclear copper(II) complexes with unsymmetrical quadridentate ligands involving an imidazole moiety and discrete imidazolate-bridged binuclear copper(II) complexes with non-identical co-ordination geometries in the binuclear unit

Abstract

Mononuclear copper(II) complexes with unsymmetrical quadridentate Schiff bases involving an imidazole moiety [Cu(HL¹)][ClO₄](1) and [Cu(HL²)][ClO₄](2) have been prepared and characterized, where H₂L¹ denotes the 1:1:1 condensation product of acetylacetone, ethylenediamine, and 4-formylimidazole and H₂L² denotes that of acetylacetone, propane-1,3-diamine, and 4-formylimidazole. The structure of the unsymmetrical quadridentate ligand has been established by the ¹H n.m.r. spectrum of [Ni(HL¹)][ClO₄]. The deprotonated complex [Cu(L¹)], which can function as ligand at the imidazolate nitrogen, has been isolated and characterized. X-Ray crystal structures of complexes (1) and (2) have been determined. Complex (1) crystallizes in the monoclinic space group P2₁/n with a= 22.146(2), b= 9.659(1), c= 7.126(1)Å, β= 98.35(1)°, and Z= 4. Complex (2) crystallizes in the triclinic space group P with a= 10.551(4), b= 11.602(3), c= 7.307(1)Å, α= 98.95(1)°, β= 96.99(2), γ= 113.04(2)°, and Z= 2. In both complexes (1) and (2), the copper(II) atom assumes a square-planar co-ordination environment with N₃O donor atoms of unsymmetrical quadridentate ligands. Two imidazolate-bridged binuclear copper(II) complexes [Cu(L¹)Cu(tfacac)₂](3)(tfacac = trifluoroacetylacetonate) and [Cu(L¹)Cu(tebima)][PF₆][ClO₄]·Me₂CO (4){tebima = tris[2-(N-ethylbenzimidazolyl)methyl]amine} have been prepared by the reaction of (1)(as a-site component complex) and [Cu(tfacac)₂] or [Cu(tebima)][ClO₄]₂·2H₂O, exhibiting an unsaturated co-ordination site (b-site component complex), in the presence of one equivalent of triethylamine. Complexes (3) and (4) have been characterized by elemental analyses, melting points, molar electrical conductivities, i.r., electronic, and e.s.r. spectra, magnetic susceptibilities, and single-crystal X-ray diffraction. Complex (3) crystallizes in the triclinic space group P with a= 11.462(4), b= 16.708(8), c= 7.692(2)Å, α= 93.96(3), β= 109.98(2), γ= 71.74(3)°, and Z= 2. Complex (4) crystallizes in the triclinic space group P with a= 13.131(5), b= 19.285(6), c= 10.878(4)Å, α= 93.89(2), β= 92.83(2), γ= 114.64(2)°, and Z= 2. The X-ray structural analyses of (3) and (4) verify the discrete imidazolatebridged binuclear structures in which the two co-ordination geometries in the binuclear unit are non-identical. In the complex (3), one copper atom has a square-planar co-ordination geometry and the other has a square-based pyramidal co-ordination geometry. In (4), one copper atom has a square-planar co-ordination geometry and the other a geometry intermediate between square-based pyramidal and trigonal bipyramidal. The Cu–N(imidazolate) bond distances are 1.966(3)Å for (3) and 1.957(8)Å for (4). The magnetic susceptibility data gave the antiferromagnetic interaction parameters 2J=–112 cm^–1 for (3) and –140 cm^–1 for (4). The e.s.r. spectra of complexes (3) and (4) measured in frozen solutions (77 K) at X-band frequency showed broad ΔM= 1 and weak ΔM= 2 absorptions.