Theoretical Study of Aromatic Hydroxylation in [Cu2(H-XYL)O2]2+ Complex Mediated by a Side-on Peroxo Dicopper Core and the Cu-ligands Effects

In this work, the aromatic hydroxylation mechanism in [Cu2(H-XYL)O2]2+ complex mediated by a peroxo dicopper core and the Cu-ligands effects are investigated by using hybrid density functional theory (DFT) the broken symmetry B3LYP method. Based on the calculated free-energy profiles, we proposed two available mechanisms. The first reaction steps of both mechanisms involve O-O bond cleavage concerted with C-O bond formation, and the second step involves substrate Wagner-Meerwein rearrangements by a [1,2] H shift (HA shift from CA to CC) or (HA shift from CA to OA) across the phenyl ring to form the stable dienone intermediates, and this is followed by the protonation of bridging oxygen atoms to produce the final hydroxylated dicopper(II) product. The HA shift from CA to CC mechanism is the energetically most favorable, in which the first reaction step is rate-limiting reaction, with a calculated free-energy barrier of 19.0 kcal mol-1 and deuterium kinetic isotope effect of 1.0, in agreement with the experimental observations. The calculation also shows that the reaction started from the P-type species of [Cu2(H-XYL)O2]2+ which is capable of direct hydroxylation of aromatic substrates without the intermediacy of an O-type species. Finally, we designed some new complexes with different Cu-ligands and found one which computationally possesses a higher activity in mediating hydroxylation of the ligand based aromatic substrate; here, Cu loses a pyridyl ligand donor by dissociation, compared [Cu2(H-XYL)O2]2+ complex.