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

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 Et3N with an equimolar amount of MnCl2 · 4H2O or alternatively by a more direct route in which an ethanolic solution of benzhydrazide was added to ethanolic solution of appropriate salicylaldehyde and MnCl2 · 4H2O solution to yield [MnCl(L1)(H2O)2], [Mn(L2)2(H2O)2], [MnCl(L3)], [MnCl(L4)] and [MnCl2(H2O)(L5)]. The hydrazone Schiff base ligands and their manganese complexes including HL1–4 and L5 (HL1 = benzoic acid (2-hydroxy-3-methoxy-benzylidene)-hydrazide, HL2 = benzoic acid (2,3-dihydroxy-benzylidene)-hydrazide, HL3 = benzoic acid (2-hydroxy-benzylidene)-hydrazide, HL4 = benzoic acid (5-bromo-2-hydroxy-benzylidene)-hydrazide, L5 = 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 HL1 and [MnCl2(H2O)L5] have been determined. The results suggest that the Schiff bases HL1, HL2, HL3, and HL4 coordinate as univalent anions with their tridentate O,N,O donors derived from the carbonyl and phenolic oxygen and azomethine nitrogen. L5 is a neutral tridentate Schiff base with N,N,O donors. ESI-MS for the complexes Mn–L2,3,5 provided evidence for the presence of multinuclear complexes in solution. Catalytic ability of Mn–L1–5 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.

Preparation and characterization of dissymetric tridendate Schiff base ligands and their Mn(II) complexes are reported. In this study crystal structures of one of the ligands and one of Mn-complexes were determined. It was demonstrated that these dissymetric hydrazone Schiff base manganese(II) complexes (Mn–L1–5) are highly selective catalysts for oxidation of cyclohexene by PhIO under mild conditions in the presence of imidazole.Figure optionsDownload as PowerPoint slide