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.

Graphical abstractThe oxidative coupling of 2,6-di-tert-butyl-phenol catalyzed using different copper(II) complexes with chloride or bromide and ethylenediamine are reported. Their speciation in methanol of these complexes are correlated to their catalytic activity finding the most active species are the complexes [Cu(en)X]+.Figure optionsDownload full-size imageDownload as PowerPoint slide