A double arene hydroxylation mediated by dicopper(II)-hydroperoxide species

The dicopper(II) complex [Cu-2(L)](4+) (L = alpha,alpha'-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-H-1-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 (H2O2)-O-18. 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-H-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(ll) complexes prepared with the new ligands 6 and 7 and containing bridging mu-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 [Cu-2(L)](4+). Kinetic studies performed on the reactions of [Cu-2(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) x 10(-1) s(-1)) and both are intramolecular processes. However, the two reactions exhibit different activation parameters (DeltaH(double dagger) = 39.1 +/- 0.9 U mol(-1) and DeltaS(double dagger) = -115.7 +/- 2.4 J K-1 mol(-1) for the first hydroxylation; DeltaH(double dagger) = 77.8 +/- 1.6 kJ mol(-1) and DeltaS(double dagger) = -14.0 +/- 0.4 J K-1 mol(-1) for the second hydroxylation). By studying the reaction between [Cu-2(L)](4+) and H2O2 at low temperature, we were able to characterize the intermediate eta(1):eta(1)-hydroperoxodicopper(II) adduct active in the first hydroxylation step, [Cu-2(L)(OOH)](3+) [lambda(max) = 342 (epsilon 12 000), 444 (epsilon 1200), and 610 nm (epsilon 800 M-1 cm(-1)); broad EPR signal in frozen solution indicative of magnetically coupled Cu(II) centers].