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Reactivity Study of a Hydroperoxodicopper(II) Complex:  Hydroxylation, Dehydrogenation, and Ligand Cross-Link Reactions

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posted on 2006-09-04, 00:00 authored by Lei Li, Amy A. Narducci Sarjeant, Kenneth D. Karlin
Employing a binucleating phenol-containing ligand PD‘OH, a μ-phenoxo-μ-hydroperoxo dicopper(II) complex [CuII2(PD‘O)(OOH)(RCN)2](ClO4)2 (1, R = CH3, CH3CH2 or C6H5CH2; λmax = 407 nm; ν(O-O) = 870 cm-1; J. Am. Chem. Soc. 2005, 127, 15360) is generated by reacting a precursor dicopper(I) complex [CuI2(PD‘OH)(CH3CN)2](ClO4)2 (2) with O2 in nitrile solvents at −80 °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-β-monooxygenase. The thermal decay of 1 also leads to a product [CuII3(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 [CuI2(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 kH/kD = 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-μ-hydroxide-bridged tetranuclear copper(II) cluster compound [{CuII2(PD‘O-)(OH-)}2](ClO4)4 (8) has also been isolated following warming of 1. Its formation is consistent with the generation of [CuII2(PD‘O-)(OH-)]2+, with dimerization a reflection of the large Cu···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.