Generation of HO• Radical from Hydrogen Peroxide Catalyzed by Aqua Complexes of the Group III Metals [M(H2O)n]3+ (M = Ga, In, Sc, Y, or La): A Theoretical Study

The mechanism of the rate-limiting stage of alkanes oxidation with hydrogen peroxide, that is, formation of the hydroxyl radicals, catalyzed by aqua complexes, [M(H2O)](3+) (1), of the group III metals exhibiting a unique stable non-zero oxidation state (M = Ga, In, Sc, Y, or La) was theoretically studied in detail at the DFT level. The mechanism involves the substitution of a H2O ligand for H2O2, protolysis of the coordinated H2O2, substitution of another H2O ligand for H2O2, generation of the hydroxyl radical upon the homolytic O-O bond cleavage in the key complex [M(H2O)((n-2))(H2O2)(OOH)](2+), and closure of the catalytic cycle. The substitution steps proceed via the dissociative mechanism D (M = Ga and Y) or the associative mechanisms A [M = Sc, La, or In (second substitution)] or I-a [M = In (first substitution)]. The general catalytic activity of 1 is determined by three main factors, that is, (i) lability of the complexes, (ii) acidity of the metal-bound ligands, and (iii) the H2O2 activation toward the homolytic O-O bond cleavage. The H2O2 activation, in turn, depends on the strength of the M-O2H2 bond, delocalization of the spin density within a coligand (OOH-), and ability of the coligand to be easily oxidized. The calculations predict that the catalytic activity of 1 increases along the row of the metals Al approximate to La < Y approximate to In < Sc < Ga.