Reductive coupling of benzyl bromide catalyzed by a novel dicobalt complex having two salen units

Abstract

A novel dicobalt complex [Co^II₂L] has been synthesized, where (L)⁴⁻ is a dinucleating ligand derived by the 2 ∶ 2 ∶ 1 condensation of ethylenediamine, salicylaldehyde, and 5,5′-methylenebis(salicylaldehyde), and it has two N₂O₂ metal-binding sites which are linked to each other with a methylene spacer. This complex was characterized by UV-VIS, IR, and NMR as well as mass spectroscopy. Its redox behavior was investigated in dmf using cyclic voltammetry in comparison with that for the corresponding mononuclear complex [Co(salen)]. The redox couples to Co^III–Co^II and Co^II–Co^I for [Co^II₂L] were observed at +0.09 and −1.20 V vs. Ag–AgCl, respectively. These potentials are quite similar to those for [Co(salen)]. The electro-generated [Co^I₂L]²⁻ reacts with alkyl halide at each metal center to give an organocobalt complex. A further one-electron reduction of the compound yields an unstable intermediate that undergoes rapid decomposition by cleavage of the cobalt–carbon bond. The electrolysis of benzyl bromide at −1.40 V vs. Ag–AgCl in the presence of the dicobalt complex yields bibenzyl as the major product. On the other hand, when [Co(salen)] was used as a catalyst, toluene was obtained as the major product. The difference in the product distribution is due to the structural properties of the catalysts.