Currently, catalysts with oxidative activity are required to create valuable chemical, agrochemical, and pharmaceutical products. The catechol oxidase activity is a model reaction that can reveal new oxidative catalysts. The use of complexes as catalysts using iron (III) and structurally simple ligands such as pyrazine (pz), quinoxaline (qx), and phenazine (fz) has not been fully explored. To characterize the composition of the solution and identify the abundant species which were used to catalyze the catechol oxidation, the distribution diagrams of these species were obtained by an equilibrium study using a modified Job method in the HypSpec software. This allows to obtain also the UV-vis spectra calculated and the formation constants for the mononuclear and binuclear complexes with Fe3+ including: [Fe(pz)]3+, [Fe2(pz)]6+, [Fe(qx)]3+, [Fe2(qx)]6+, [Fe(fz)]3+, and [Fe2(fz)]6+. The formation constants obtained were log β110 = 3.2 ± 0.1, log β210 = 6.9 ± 0.1, log β110 = 4.4 ± 0.1, log β210 = 8.3 ± 0.1, log β110 = 6.4 ± 0.2, and log β210 = 9.9 ± 0.2, respectively. The determination of the catechol oxidase activity for these complexes did not follow a traditional Michaelis-Menten behavior.
Keywords: catecholase activity; formation constants; iron (III) complexes.