Catalysis of alkene epoxidation by manganese(II) and (III) complexes of both Schiff base and reduced Schiff base ligands utilizing environmentally benign H2O2

Abstract

Mono and disubstituted tridentate Schiff base ligands were synthesized from the reaction of 2(2-aminoethyl)pyridine with 5-bromosalicylaldehyde (H-L1) and 3,5-dibromosalicylaldehyde (H-L2) respectively, which were subsequently reacted with NaBH4 to yield the corresponding reduced Schiff base ligands H-rL1 and H-rL2. The complexes; MnIII-(L1)2 (1), MnIII-(rL1)2 (3), and MnII-(L2)2 (2), MnII-(rL2)2 (4) were obtained by the reaction of the ligands with {Mn(ClO4)2·6H2O}. The ligands and complexes were characterized by UV–Vis, ESI-MS and FT-IR spectroscopy. The 1H NMR spectra of the ligands as well as the crystal structures, and electrochemical properties of the complexes were obtained. The crystal structures of MnIII-(L1)2 and MnII-(L2)2 show coordination of the ligands in tridentate mode in octahedral geometry. Cyclic voltammetric studies of Mn-(L1)2 and Mn-(L2)2 in acetonitrile solution show two prominent reversible peaks attributed to the redox processes: Mn(II)/Mn(III), (E1/2 = 0.203–(−0.204 V) and Mn(III)/Mn(IV), (E1/2 = 1.103–0.195 V) versus Ag/AgCl while that of Mn-(rL4)2 showed three quasi reversible peaks suggesting three redox couples: Mn(II)/Mn(III) (E1/2 = −0.109); Mn(III)/Mn(IV), (E1/2 = 0.125 and (Mn(IV)/Mn(V), (E1/2 = 0.649) vs Ag/AgCl. Epoxidation of cyclohexene and 1-hexene by hydrogen peroxide catalyzed by the manganese complexes gave epoxide yield of 42–53% with a turnover of 10.50–13.25 after 20 h of reaction for the reduced Schiff base complexes {MnIII-(rL1)2 and MnII-(rL2)2} at 0 °C.