Hydrazone Schiff base-manganese(II) complexes: Synthesis, crystal structure and catalytic reactivity

Abstract

Five dissymmetric tridentate Schiff base ligands, containing a mixed donor set of ONN and ONO were prepared by the reaction of benzhydrazide with the appropriate salicylaldehyde and pyridine-2-carbaldehyde and characterized by FT-IR, 1H and 13C NMR. The complexes of these ligands were synthesized by treating an ethanolic solution of the appropriate ligand and one equivalent Et 3N with an equimolar amount of MnCl 2 · 4H 2O or alternatively by a more direct route in which an ethanolic solution of benzhydrazide was added to ethanolic solution of appropriate salicylaldehyde and MnCl 2 · 4H 2O solution to yield [MnCl(L 1)(H 2O) 2], [Mn(L 2) 2(H 2O) 2], [MnCl(L 3)], [MnCl(L 4)] and [MnCl 2(H 2O)(L 5)]. The hydrazone Schiff base ligands and their manganese complexes including HL 1–4 and L 5 (HL 1 = benzoic acid (2-hydroxy-3-methoxy-benzylidene)-hydrazide, HL 2 = benzoic acid (2,3-dihydroxy-benzylidene)-hydrazide, HL 3 = benzoic acid (2-hydroxy-benzylidene)-hydrazide, HL 4 = benzoic acid (5-bromo-2-hydroxy-benzylidene)-hydrazide, L 5 = benzoic acid pyridine-2-yl methylene-hydrazide) were characterized on the basis of their FT-IR, 1H and 13C NMR, and molar conductivity. The crystal structures of HL 1 and [MnCl 2(H 2O)L 5] have been determined. The results suggest that the Schiff bases HL 1, HL 2, HL 3, and HL 4 coordinate as univalent anions with their tridentate O,N,O donors derived from the carbonyl and phenolic oxygen and azomethine nitrogen. L 5 is a neutral tridentate Schiff base with N,N,O donors. ESI-MS for the complexes Mn–L 2,3,5 provided evidence for the presence of multinuclear complexes in solution. Catalytic ability of Mn–L 1–5 complexes were examined and found that highly selective epoxidation (>95%) of cyclohexene was performed by iodosylbenzene in the presence of these complexes and imidazole in acetonitrile.