Iron(iii) and cobalt(iii) complexes with both tautomeric (keto and enol) forms of aroylhydrazone ligands: catalysts for the microwave assisted oxidation of alcohols

Abstract

Two Schiff bases derived from the condensation of 2-hydroxybenzohydrazide with 3,5-di-tert-butyl-2-hydroxybenzaldehyde (H₂L¹) or with 2,3-dihydroxy benzaldehyde (H₂L²) were used to synthesize the Fe(III) and Co(III) complexes [Fe(L¹)(HL¹)] (1) and [Co(L²)(HL²)] (2), respectively. The compounds were characterized by elemental analysis, IR, ESI-MS and single crystal X-ray analysis. Structural studies indicated the presence of both keto and enol tautomeric forms of the ligand in 1 and 2. The complexes (mainly 1) act as catalysts in the microwave-assisted solvent-free peroxidative oxidation (by tert-butylhydroperoxide, TBHP) of primary and secondary alcohols. A facile, efficient and selective synthesis of ketones was achieved with a yield up to 96% and a TON up to 500, after 30 min under low power (15 W) microwave irradiation (complex 1 as catalyst). 2-Pyrazinecarboxylic acid (Hpca) shows a promoting effect.