Transition metal complexes with thiosemicarbazide-based ligands. Part 63. Syntheses, structures and physicochemical characterization of the first chromium(III) complexes with pyridoxal semi- and thiosemicarbazones

Abstract

With pyridoxal semi-/thiosemicarbazones (PLSC/PLTSC) ligands for the first time chromium complexes were obtained. In the reaction of ethanolic solution of Cr(NO3)3 and K3[Cr(NCS)6] and the ligands in mole ratio 1:1 or 1:2, the following complexes were formed: [Cr(PLSC)(PLSC–H)](NO3)2·H2O (1), K[Cr(PLSC–H)(NCS)3]·EtOH (2), [Cr(PLTSC)(PLTSC–H)](NO3)2·2H2O (3), [Cr(PLTSC)2](NO3)3 (4) and [Cr(PLTSC)(NCS)3]·2H2O (5). All the complexes have mer-octahedral structure which in the cases of the complexes 2, 4 and 5 was proved by single-crystal X-ray diffraction analysis. The Schiff bases coordinate in the usual tridentate ONX manner (X=O/S; PLSC/PLTSC). The pyridoxalic fragment is zwitter ion regardless of the form of the coordinated ligands: neutral (keto/thion) and monoanionic (enolic/thiolic). In addition to the above complexes, X-ray crystallography was used to characterize neutral and protonated forms of PLSC, i.e., PLSC·2H2O and PLSC·HNCS, the latter one being obtained as a by-product of the reaction of formation of the complex 2 using K3[Cr(NCS)6] and PLSC in the mole ratio 1:2. The X-ray analyses of these ligand forms have shown that in the case of PLSC·HNCS the ligators Ophenolic, Nazomethine, Oketo due to strong hydrogen O2–H⋯N3 bond are placed in cis-position to each other (pro-binding conformation) which is not the case with PLSC·2H2O. The compounds are characterized thoroughly by also IR/UV–Vis spectral analyses, electrochemical and thermal methods.