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

The dicopper(II) complex [Cu2(L)]4+ (L)R,R¢-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 H2 18O2. 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 í-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 (¢Hq ) 39.1 ( 0.9 kJ mol-1 and ¢Sq ) -115.7 ( 2.4 J K-1 mol-1 for the first hydroxylation; ¢Hq ) 77.8 ( 1.6 kJ mol-1 and ¢Sq)-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 è1:è1-hydroperoxodicopper(II) adduct active in the first hydroxylation step, [Cu2(L)(OOH)]3+ [ìmax ) 342 ( 12 000), 444 ( 1200), and 610 nm ( 800 M-1cm-1); broad EPR signal in frozen solution indicative of magnetically coupled Cu(II) centers].