Synthesis and characterization of new Co(III) mixed-ligand complexes, containing 2-hydroxy-aryloximes and α-diimines. Crystal and molecular structure of [Co(saox)(bipy)2]Br

A series of 12 cobalt (III) complexes of 2-hydroxy-aryloximes (H2oxime) with an α-diimine (enR), under the general formula [Co(oxime)(enR)2]Br · 2H2O were synthesized and characterized. The IR and H NMR spectra indicate the bidentate coordination mode of the ligands and the dianionic character of the oxime ligand in the complexes, while the electronic excitation spectra are indicative of an octahedral geometry around cobalt(III). The octahedral environment with CoN5O chromophore was confirmed by X-ray structure analysis of the solvated [bis(2,2′-bipyridine)-(2-hydroxy-benzaldoximato)cobalt(III)]bromide, [Co(saox)(bipy)2]Br · 0.166bipy · 0.15CH3OH · 1.75H2O. The phenolic oxygen as well as the oximic nitrogen plus two nitrogen atoms, each one from a different bipy molecule, build the equatorial plane. The oximic chelate ring can be described as an extentend delocalized π system. The crystal structure of one of the investigated oxime ligands, the 2-hydroxy-benzophenonoxime (H2bpox) was also determined by X-ray analysis, verifying the strong intra-and intermolecular hydrogen bonds.Theoretical calculations (DFT, ZINDO) with gaussian-98 for the prediction of the electronic spectrum gave good correlation with the experimental one of the titled compound in the solid state. The MLCT transition was found as a shoulder at 340 nm, while the d–d absorptions at 408 and ∼600 nm are characteristics of octahedral Co(III) symmetry. The absorption spectra at different solvents showed a solvatochromic effect. The cyclic voltammetry study in CH3CN showed the reversible redox couples Co(I)/Co(II) and Co(II)/Co(III), as well as the reversible oxidation of bipy or phen and the irreversible oxidation of the oxime ligands.

Graphical abstractIn the 12 novel complexes [Co(oxime)(enR)2]Br · 2H2O, with octahedral arrangement of the chromophore CoN5O, the rare for mononuclear Co(III) complexes, bidentate dianion character of the oxime ligand is established. The theoretical investigation with gaussian-03 for the prediction of the electronic spectrum and especially the ZINDO calculations, gave good correlation with the experimental spectrum in the solid state of the complex [Co(saox)(bipy)2]Br. The cyclic voltammogram study gave all the expected waves for the redox processes of the metal Co(III) and the oxime ligands, as well as for the α-diimines bipy and phen.Figure optionsDownload full-size imageDownload as PowerPoint slide