Reactivity study of a hydroperoxodicopper(II) complex: Hydroxylation, dehydrogenation, and ligand cross-link reactions

Employing a binucleating phenol-containing ligand PD'OH, a mu-phenoxo-mu-hydroperoxo dicopper(II) complex [Cu-2(II)(PD'O-)(-OOH)(RCN)(2)](ClO4)(2) (1, R = CH3, CH3CH2 or C6H5CH2; lambda(max) = 407nm; v((O-O)) = 870 cm(-1) ; J. Am. Chem. Soc. 2005, 127, 15360) is generated by reacting a precursor dicopper(I) complex [Cu-2(I)(PD'OH)(CH3CN)(2)]( ClO4)(2) (2) with O-2 in nitrile solvents at -80 degrees C. Species 1 is unable to oxidize externally added substrates, for instance, PPh3, 2,4-tert-butylphenol, or 9,10-dihydroanthracene. However, upon thermal decay, it hydroxylates copper-bound organocyanides (e.g., benzylcyanide), leading to the corresponding aldehyde while releasing cyanide. This chemistry mimics that known for the copper enzyme dopamine-beta-monooxygenase. The thermal decay of 1 also leads to a product [Cu-3(II)(L'')(2)(Cl-)(2)](PF6)(2) (6); its X-ray structure reveals that L'' is a Schiff base-containing ligand which apparently derives from both oxidative N-dealkylation and then oxidative dehydrogenation of PD'OH; the chloride presumably derives from the CH2Cl2 solvent. With an excess of PPh3 added to 1, a binuclear Cu(I) complex [Cu-2(I)(L')(PPh3)(2)](ClO4)(2) (5) with a cross-linked PD'OH ligand L' has also been identified and crystallographically and chemically characterized. The newly formed C-O bond and an apparent k(H)/k(D) = 2.9 +/- 0.2 isotope effect in the benzylcyanide oxidation reaction suggest a common ligand-based radical forms during compound 1 thermal decay reactions. A di-mu-hydroxide-bridged tetranuclear copper(II) cluster compound [{Cu-2(II)(PD'O-)(OH-)}(2)](ClO4)(4) (8) has also been isolated following warming of 1. Its formation is consistent with the generation of [Cu-2(II)(PD'O-)(OH-)](2+), with dimerization a reflection of the large Cu center dot center dot center dot Cu distance and thus the preference for not having a second bridging ligand atom ( in addition to the phenolate O) for dicopper(II) ligation within the PD'O- ligand framework.