A comparative study of the speciation in methanol solution and activity in the oxidative coupling of 2,6-di-tert-butylphenol presented by the complexes: Copper(II)-N,N′-di-tert-butylethylenediamine-halogen and some copper(II)-diamine-halogen complexes

Abstract

The oxidative coupling of 2,6-di-methylphenol is an industrial reaction catalyzed by the copper(II)-di-tert-butylethylenediamine-dibromo complex. In order to get more insight about the behavior of the N,N′-dialkyl-diamine cupric complexes and the halide effect on the catalyic activity, a spectrophotometric study of the speciation in methanol of copper(II) coordinated with N,N′-di-tert-butylethylenediamine or 1,5-pentanediamine and their respective halides chloride and bromide, was carried out at 303K. Formation constants and individual calculated electronic spectra of all species, were obtained. With the purpose to compare the catalytic activity of these complexes with other diamine complex systems in the oxidative coupling of 2,6-di-methylphenol, the oxidative coupling of 2,6-di-tert-butylphenol was used as a catalytic model system.Solutions with the most abundant complexes were tested and correlated to their catalytic activity. These data are compared with several studies of the copper(II) diamine complexes, with or without chloride or bromide ions. The catalytic activity in the oxidation of 2,6-di-tert-butylphenol was monitored in methanol solution, following the corresponding quinone formation, at 418nm (ε=3.95x104Lmol−1cm−1 at 303K). The obtained formation constants of the copper(II)-di-tert-butylethylenediamine system are: logβ110=4.13±0.12, logβ120=6.56±0.12. For the ternary copper(II)-di-tert-butylethylenediamine chloro or bromo systems are: logβ111=6.10±0.09 and logβ111=6.69±0.04, respectively. While of the copper(II)-1,5-pentanediamine system are logβ110=3.32±0.11, logβ210=5.73±0.10.Kinetic studies of the oxidative coupling of 2,6-di-tert-butylphenol catalyzed by the copper(II)-diamine systems indicate that the copper(II)-1,5-pentanediamine presents low activity, while the copper(II)-N,N′-di-tert-butylethylenediamine shows a very similar activity to the copper(II)-1,4-butanediamine system reported earlier. Further studies are needed in order to explain the reasons why the complexes generated with this two different diamines display such similar activity.