Substituent and solvent effects in the spectra and structure of some mixed-ligand copper(II) chelates containing β-ketoenols

Abstract

The infrared and the ligand-field excitation spectra of a series of new mixed-ligand copper(II) chelates that encompass N,N-dimethyl- N′-ethyl-ethylenediamine (dmeen) and the anion of a substituted β-ketoenol (1,3-dione) were obtained in the solid state and in solution. Information related to the electronic excitation spectra, the IR spectra, the molar conductivity and the magnetic properties of the newly obtained and characterised chelates are presented and discussed. The molar conductivity in nitromethane reveals a predominance of electrostatic interactions between the [Cu(β-dione)dmeen] + entity and bulky polyatomic anions that counterbalance the positive charge while the IR spectra disclose that the bidentate ligands form chelate rings with copper as the common vertex. The resulting CuN 2O 2 chromophore attains a square-coplanar structure and exhibits a tendency for axial ligation which is enhanced when electron-attracting substituents are attached to the β-dionato moiety. The tendency for axial ligation is partially fulfilled when suitable nucleophiles are present. Covalent interactions prevail when chloride is present and upon coordination it presumably occupies the apex of a square-pyramidal structure. Chain-like bidentate anions enable copper(II) to achieve coordination saturation and when configuration requirements demand it, they distort the square-planar arrangement of the initial CuN 2O 2 chromophore forming distorted octahedral structures. Covalent solute-solvent interactions are revealed by shifts in the ligand-field excitation spectra that are enhanced as the nucleophilicity of the solvent increases. Linear dependence of the ligand-field excitation maximum on solvent parameters related to donor properties is generally observed.