Syntheses, structural diversity and thermal behavior of first row transition metal complexes containing potential multidentate ligands based on 2,6-diacetylpyridine and benzyl carbazate

Abstract

The Schiff base ligand, N′-(1-{6-[1-(Benzyloxycarbonyl-hydrazono)-ethyl]-pyridin-2-yl}-ethylidene)-hydrazinecarboxylic acid benzyl ester (bc2-dap), was synthesized by the reaction of 2,6-diacetylpyridine (dap) with benzyl carbazate (bc) in methanol. Also first row transition metal complexes of the general formula [M(NCS)2(bc2-dap)]·nH2O; M = Mn (1), Fe (2), Co (3), Zn (4) or Co (5); n = 0 (1–4) and n = 1 (5), [Fe(bc2-dap)(H2O)2]·2Cl·4H2O (6), [Ni(NCS)2(bc2-dap)]·CH3OH (7) and [Co(NCS)2(bc-dap)] (8) have been synthesized using a template method and characterized by spectroscopic techniques and single crystal X-ray diffraction. Complexes 1–4 are isotypic, while compounds 1–6 comprises of discrete mononuclear units with pentagonal bipyramidal geometry. In contrast compounds 7 and 8 adopt octahedral and tetrahedral geometries, respectively. In all of the complexes 1–6, the Schiff base acts as a pentadentate ligand that binds via the pyridine nitrogen, azomethine nitrogen and carbonyl oxygen atoms. The Schiff base ligand occupies the equatorial plane and isothiocyanate anions for 1–5 and water molecules for 6 occupy the axial positions. Compound 7 has octahedral geometry in which the Schiff base ligand acts as a tetradentate ligand and in this case one of the carbonyl oxygen atoms is not involved in coordination. Furthermore, the N-bound isothiocyanate anions are coordinated as terminal ligands. In compound 8, the Schiff base behaves as a bidentate ligand through the azomethine nitrogen and pyridine nitrogen atoms leading to a tetrahedral geometry with the remaining two sites are occupied by isothiocyanate ions. Thermal reactivity of the ligand and complexes (1–8) was studied by TG-DTA and the complexes all undergo endo- followed by exothermic decomposition to give metal oxides, whereas the ligand decomposes completely to give gaseous products.