Effect of molar ratios of reagents and solvent on the complexation process of nickel(II) ions by the N2O3-donor Schiff base

Abstract

The new N2O3-donor pentadentate Schiff base (C29H26N2O3) was synthesized and characterized by different techniques. The single crystal X-ray analysis confirmed that in the solid state one half of the molecule contains the phenol-imine form while in the other one the proton of phenol group is moved towards the azomethine group creating the zwitterionic form. In solution, the studied Schiff base exists mainly in the phenol-imine tautomer but in more polar solvents (MeOH or EtOH) stabilization of the more polar keto-amine form is also observed. The effect of polar protic solvents (methanol and ethanol) on synthesis of NiII complexes with the Schiff base was studied. The synthesis of compounds was performed at the stoichiometric metal:ligand 1:1 and 2:1 ratio. This allowed to obtain four different nickel(II) complexes. The products were characterized using FITR spectroscopy, X-ray diffraction analysis and thermogravimetric methods. When equimolar amounts of reagents were used in both solvents, the mononuclear complexes were formed, [Ni(C29H24N2O3)]·(CH3OH)(H2O)0.25 (1a) and [Ni(C29H24N2O3)]·H2O (2a). In both compounds NiII ions have distorted square planar geometry formed by N2O2-core of Schiff base ligand. Increasing the concentration of metal ions in the synthesis, it was possible to obtain tetranuclear ([Ni4(C29H24N2O3)2(CH3COO)2(CH3OH)3.65(H2O)0.35]·(CH3OH)7 (1b)) and hexanuclear ([Ni6(C29H24N2O3)2(C16H17N2O2)2(CH3COO)2]·(C2H5OH) (2b)) complexes. In crystal 1b the NiII ions have octahedral geometry whereas in complex 2b the metal centres are four- and six-coordinate.