Self-assembly of copper(II) complexes with a dibasic tridentate ligand and monodentate N-heterocycles: structural, magnetic and EPR studies

Three ternary copper(II) complexes of general formula [Cu(bhac)(hc)], with a tridentate Schiff base, acetylacetone benzoylhydrazone (H2bhac) and monodentate N-heterocycles (hc=pyrazole, imidazole and pyridine), have been synthesized. Elemental analysis, various forms of spectroscopy (infrared, electronic absorption and EPR), cyclic voltammetry, cryomagnetic measurements and X-ray crystallography were used for the characterization of the complexes. Analytical data, infrared and electronic spectral features and molar conductivity values are consistent with the proposed molecular formulae and the +2 oxidation state of the metal ion in these complexes. In each complex, the enolate-O, the imine-N and the deprotonated amide-O donor tridentate ligand (bhac2−) and the sp2 N donor heterocycle form an O2N2 square-plane around the metal ion. The whole molecule of none of the complexes is perfectly planar because of different orientation of the phenyl ring plane of the tridentate ligand and that of the heterocycle ring plane with respect to the plane containing rest of the molecule. In the solid state, the complexes having the pyrazole and the pyridine as the heterocyclic ligand, exist as centrosymmetric dimeric species due to very weak apical coordination of the metal bound enolate-O. The complex of imidazole has no such apical coordination and exists as a monomer. In the crystal lattice, the molecules of these complexes are involved in a variety of intermolecular non-covalent interactions such as O-H⋯O, N-H⋯O, N-H⋯N, C-H⋯π and π⋯π. Self-assembly via these interactions lead to a one-dimensional arrangement of [Cu(bhac)(Hpyrz)]·C2H5OH, a two-dimensional layered structure of [Cu(bhac)(Himdz)] and a three-dimensional network of [Cu(bhac)(py)]. Cryomagnetic and EPR spectral measurements indicates weak antiferromagnetic spin-exchange in all the three complexes.