Electronic and steric effects in cobalt Schiff bases complexes: Synthesis, characterization and catalytic activity of some cobalt(II) tetra-halogens-dimethyl salen complexes

The synthesis, characterization and catalytic activity of a series of tetra-halogeno-dimethyl salen cobalt (II) complexes are reported in this paper. The investigated complexes of cobalt (II) with Schiff bases are: ocu'-di-methyl Salen cobalt (II) [Co(dMeSalen)], 3,3',5,5'-tetra chloro alpha,alpha'-di-methyl Salen cobalt (II), [Co(tCldMeSalen)], 3,3'-di-bromo 5,5'-di-chloro alpha,alpha'-di-methyl Salen cobalt (II), [Co(tBrdMeSalen)], 3,3',5,5'-tetra bromo alpha,alpha'-di-methyl Salen cobalt (II), [Co(tBrdMeSalen)] and 3,3',5,5'-tetra iodo alpha,alpha'-di-methyl Salen cobalt (II), [Co(tIdMeSalen)] (where Salen is bis(salicylaldehyde)ethylenediamine). The characterization of the complexes was performed by elemental analysis, cyclic voltammetry, UV-Vis, IR and EPR spectroscopies. The study was made in DMF, and pyridine was used for coordination as axial base. The redox potential is influenced by the substituent grafted on aromatic ring and in the azomethynic position and also by the molecules coordinating in axial position (solvent, DMF, or pyridine). The catalytic oxygenation of 2,6-di-tert-butylphenol by these complexes leads to the obtention of benzoquinone and diphenoquinone products. The cobalt (II) complexes form reversible adducts with molecular oxygen. (c) 2006 Elsevier B.V. All rights reserved.