Synthesis, characterization, catalytic oxidation and biological activity of ruthenium(III) Schiff base complexes derived from 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione

Abstract

A series of air stable low spin Ru(III) complexes of the type [RuX 2(EPh 3) 2(L)] (where X = Cl or Br; E = P or As; L = monobasic bidentate Schiff base ligand) have been synthesized from the reaction of ruthenium(III) precursors, viz. [RuX 3(EPh 3) 3] (where X = Cl, E = P; X = Cl or Br, E = As) and [RuBr 3(PPh 3) 2(CH 3OH)] and Schiff bases derived from the condensation of DHA (3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione) with methylamine (HL1), cyclohexylamine (HL2) and 2-aminopyridine (HL3) in benzene under reflux. In all these reactions, the Schiff base ligand replaces one triphenylphosphine or triphenylarsine molecule, one chloride or bromide and one methanol from the precursors. These complexes have been characterized by elemental analyses, FT-IR, UV–Vis and EPR spectroscopy, together with magnetic susceptibility measurements. Elemental analyses and IR studies shows that the Schiff base ligands behave as monobasic bidentate ligands coordinating through the oxygen atom of the deprotonated phenolic group and the nitrogen atom of the azomethine group. The redox behavior of the complexes has been investigated by the cyclic voltammetric technique. All the complexes display two quasireversible oxidations, (Ru IV/Ru III) in the range 0.67–0.82 V and (Ru V/Ru IV) in the range 1.00–1.17 V, and an irreversible reduction in the range −0.79 to −0.92 V. Further, the catalytic efficiency of one of the ruthenium complexes ( 10) was determined for the oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones in the presence of N-methylmorpholine- N-oxide(NMO) as co-oxidant. The formation of high valent Ru V O species as a catalytic intermediate is proposed for the catalytic process. Furthermore, the in vitro toxicity of these complexes was tested against the growth of bacterial species viz., Staphylococcus aureus (209p) and Escherichia coli ESS (2231).