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)n]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 Ia [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 ≈ La < Y ≈ In < Sc < Ga.