Synthesis, structure, spectra and redox interconversions in copper(II) complexes of 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine

Abstract

Copper(II) complexes of 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (dppt) with ClO₄^-, BF₄^-, NO₃^-, Cl^- and Br^- as counter ions have been prepared. The crystal structure of the complex [Cu(dppt)₂(H₂O)₂][ClO₄]₂ has been determined. The co-ordination geometry of the copper ion is elongated octahedral, the equatorial plane being formed by the N(2) nitrogens of both the triazine units and two pyridyl nitrogens and the axial positions being occupied by water molecules. The copper ion can be best described as centrosymmetric with the two dppt ligands being strictly coplanar to each other. The electronic and EPR spectral properties illustrate the strong axial interaction by solvent molecules with the weakly ligated CuN₄ plane. The markedly positive redox potential (0.57-0.34 V vs. NHE) of these complexes is ascribed to the π -acceptor properties of the ligand rather than to steric effects of the phenyl substituents. The E_½ and ΔE_p values are significantly dependent on the solvent and the anions. The spectral and electrochemical behaviour of the copper(II) complex of 3-(2-pyridyl)-5,6-bis(p-sulfonatophenyl)-1,2,4-triazine, the sulfonato derivative of dppt, in aqueous solution at various pH revealed that it could not act as an effcient catalytically active redox agent.