Formation of reactive oxygen species by vanadium complexes

Abstract

The role of the ligand in vanadium IV and V complexes to produce reactive oxygen species (OH and O2-) by Fenton and Haber–Weiss reactions is analyzed theoretically, in gas phase and considering solvated ions. By using reactivity indexes, we found that the presence of the oxo group induces the electron donating process over complexes that in first instance does not present it. Also, we show that the vanadyl ion promotes the production of OH radicals, which is a contrary behavior presented by others vanadium complexes that may act as O2- producers or scavengers. For Haber–Weiss reactions, the analysis of the Gibbs free energy predict the following ordering: H2O>NH3>Cl->OH->C2O42-, Cl->C2O42-, NH3>oxo, H2O>oxo, NH3>oxo, Cl-, for ligands with vanadium V and IV in the formation of reactive oxygen species (ROS); which allow for the first time a criteria to predict ROS activity in vanadium complexes. All the results were obtained by using the Density Functional Theory, in its Kohn–Sham version, with four different exchange–correlation functionals (PBE, PBE0, B3LYP and M06-2X) coupled with the aug-cc-pVDZ basis set. It is well-known that transition metals sometimes exhibit a multireference character, for that reason several tests were considered over the vanadium complexes reported here. In particular, T1 test, multiconfiguration self-consistent field and coupled-clusters calculations were performed in order to test the feasibility of our results. In general, we can say that the M06-2X exchange–correlation functional exhibits the best performance to study theoretically vanadium complexes.