A Double Arene Hydroxylation Mediated by Dicopper(II)−Hydroperoxide Species

The dicopper(II) complex [Cu2(L)]4+ (L = α,α‘-bis{bis[2-(1‘-methyl-2‘-benzimidazolyl)ethyl]amino}-m-xylene) reacts with hydrogen peroxide to give the dicopper(II)−hydroquinone complex in which the xylyl
ring of the ligand has undergone a double hydroxylation reaction at ring positions 2 and 5. The dihydroxylated
ligand 2,6-bis({bis[2-(3-methyl-1H-benzimidazol-2-yl)ethyl]amino}methyl)benzene-1,4-diol was isolated by
decomposition of the product complex. The incorporation of two oxygen atoms from H2O2 into the ligand
was confirmed by isotope labeling studies using H218O2. The pathway of the unusual double hydroxylation
was investigated by preparing the two isomeric phenolic derivatives of L, namely 3,5-bis({bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino}methyl)phenol (6) and 2,6-bis({bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino}methyl)phenol (7), carrying the hydroxyl group in one of the two positions where L is
hydroxylated. The dicopper(II) complexes prepared with the new ligands 6 and 7 and containing bridging
μ-phenoxo moieties are inactive in the hydroxylation. Though, the dicopper(II) complex 3 derived from 6
and containing a protonated phenol is rapidly hydroxylated by H2O2 and represents the first product formed
in the hydroxylation of [Cu2(L)]4+. Kinetic studies performed on the reactions of [Cu2(L)]4+ and 3 with H2O2
show that the second hydroxylation is faster than the first one at room temperature (0.13 ± 0.05 s-1 vs
5.0(±0.1) × 10-3 s-1) and both are intramolecular processes. However, the two reactions exhibit different
activation parameters (ΔH⧧ = 39.1 ± 0.9 kJ mol-1 and ΔS⧧ = −115.7 ± 2.4 J K-1 mol-1 for the first
hydroxylation; ΔH⧧ = 77.8 ± 1.6 kJ mol-1 and ΔS⧧ = −14.0 ± 0.4 J K-1 mol-1 for the second hydroxylation).
By studying the reaction between [Cu2(L)]4+ and H2O2 at low temperature, we were able to characterize
the intermediate η1:η1-hydroperoxodicopper(II) adduct active in the first hydroxylation step, [Cu2(L)(OOH)]3+
[λmax = 342 (ε 12 000), 444 (ε 1200), and 610 nm (ε 800 M-1cm-1); broad EPR signal in frozen solution
indicative of magnetically coupled Cu(II) centers].