Spectrophotometric determination of formation constants for the Cu–ethylenediamine–halogen (chloride and bromide) system and their catalytic effect on the oxidative coupling of 2,6-di-tert-butyl-phenol

In order to explain the mechanism of the dimerization of 2,6-di-tert-butyl-phenol when catalyzed by the copper–ethylenediamine complexes, a spectrophotometric study of the speciation of copper(II) complexes in methanol of Cu(II), ethylendiamine and Cl− or Br− was carried out at 303 K. The formation constants obtained for the copper chloride system are: log β101 = 2.90 ± 0.03, log β102 = 6.39 ± 0.03 and log β103 = 8.62 ± 0.04, for the copper bromide system are log β101 = 3.01 ± 0.10, log β102 = 5.50 ± 0.08, for the copper–ethylendiamine complexes are log β110 = 6.13 ± 0.05 and log β120 = 10.54 ± 0.08, and for the ternary copper–ethylenediamine chloride or bromide systems are log β111 = 10.21 ± 0.03 and log β111 = 10.07 ± 0.03, respectively. Knowing the speciation of the copper–ethylenediamine–halide systems, the kinetic studies can be correlated with the species in solution. Comparative studies of the oxidation reaction of 2,6-di-tert-butyl-phenol using different copper(II) complexes with chloride or bromide and ethylenediamine as catalyst are reported. Their catalytic activity in the oxidation of 2,6-di-tert-butyl-phenol was monitored in methanol solution, following the corresponding quinone formation, at 418 nm (ε = 3.95 × 104 mol−1 L cm−1 at 303 K). The results indicate that the most active species are [Cu(en)X]+, where X is bromide or chloride, Both complexes have similar activity.