Electrochemical synthesis and structural characterization of homoleptic and heteroleptic cobalt, nickel, copper, zinc and cadmium compounds with the 2-hydroxy-1,4-naphthoquinone ligand

Abstract

The electrochemical oxidation of anodic metals (cobalt, nickel, copper, zinc or cadmium) in an acetone solution of the ligand 2-hydroxy-1,4-naphthoquinone (HL) afforded homoleptic [ML2] compounds. The addition to the cell of co-ligands (L′) such as 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen) or 1-methylimidazole (N-MeIm) allowed the one-step synthesis of heteroleptic [ML2Ln′] compounds. The crystal structures of [CoL2(H2O)2]·MeOH (1), [CoL2(H2O)(MeOH)] (2), [CuL2(N-MeIm)2] (3), [CoL2(bipy)] (4), [ZnL2(bipy)] (5) and [CdL2(bipy)] (6) have been determined by X-ray diffraction techniques. In compounds 1 and 2 the metal is in a distorted octahedral [CoO6] geometry with the quinone acting as a monoanionic bidentate ligand. In both complexes, classical O–H⋯O-type intermolecular interactions dominate the crystal packing behavior. In 1 this gives rise to tetrameric supramolecular units that are connected through coordinated water molecules. In 2 dimers are formed and these are grouped in layers, which are held together by coordinated water molecules. In 3 the copper atom is in a regular square planar [CuO2N2] environment with the ligand acting in a monodentate manner. In this case the crystal packing involves non-classical C–H⋯O and π–π-stacking interactions. Complexes 4, 5 and 6 are six coordinated [MO4N2] systems with a distorted octahedral geometry for 4 and 5 and a trigonal prismatic geometry for 6. This difference in molecular geometry is reflected in the crystal packing of the compounds. Thus, in 4 and 5 the non-classical C–H⋯O intermolecular interactions responsible for the crystal packing involve the C–H groups in positions 6 and 6′ of the 2,2′-bipyridine and the oxygens of the two phenolate groups, which give rise to ribbons of molecules. In 6 these interactions involve the C–H groups in positions 5 and 5′ of the 2,2′-bipyridine, which give rise to molecular layers. All compounds were characterized by microanalysis, IR, UV-visible (Co, Ni and Cu complexes) and 1H NMR spectroscopy (Zn and Cd complexes) and FAB mass spectrometry.