Synthesis, electrochemical and in situ spectroelectrochemical studies of new transition metal complexes with two new Schiff-bases containing N 2O 2/N 2O 4 donor groups

Abstract

New Schiff-base copper and cobalt complexes, [Cu(L 1)], [Cu(L 2)] and [Co(L 1)], [Co(L 2)] (where L 1 = N- N′-bis(3,5-di- tert-butylsalicylaldimine)-1,4-cyclohexane bis(methylamine) and L 2 = N- N′-bis(3,5-di- tert-butylsalicylaldimine)-1,8-diamino-3,6-dioxaoctane), were synthesized and characterized using elemental analysis, IR spectra, UV–Vis spectra, magnetic susceptibility measurements, 1H and 13C NMR spectroscopy, thermal analysis and molar conductance ( Λ M). Their electro-spectrochemical properties were investigated using cyclic voltammetric (CV) and thin-layer spectroelectrochemical techniques in a dichloromethane solution (CH 2Cl 2). The CV of [Cu(L 2)] showed a lower oxidation potential than that of [Cu(L 1)] under the same experimental conditions. The oxidation wave (II) of [Cu(L 2)] was accompanied by an EC process (II′), which was not observed for [Cu(L 1)]. Also, [Cu(L 2)] exhibited a reduction process, but [Cu(L 1)] did not. These results indicate that the Cu(II) ion in [Cu(L 2)] is coordinated by N 2O 4 donor sites while [Cu(L 1)] presents a square-planar structure with N 2O 2 donor sites. Both oxidation processes for [Co(L 1)] and [Co(L 2)] are based on the cobalt center, and they are assigned to Co(II)/Co(III) couples. The spectroelectrochemical results indicate that the oxidized species of [Cu(L 2)] is similar to that of [Cu(L 1)], the only difference being that the absorption bands of the oxidized species for [Cu(L 2)] shift to lower energy compared with those of [Cu(L 1)] because of their different coordination environment. The geometry of [Cu(L 2)] changed into square-planar after the complex was totally oxidized and the neutral complex was only recovered following the EC process, as observed from the CV of [Cu(L 2)]. For the two cobalt complexes, the bands corresponding to the π → π ∗transitions disappeared and new bands with small red shifts and of lower intensity were observed during the oxidation process. These new bands are attributed to the LMCT transition as observed in the case of the oxidation processes of the cobalt complexes.