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

Employing a binucleating phenol-containing ligand PD‘OH, a μ-phenoxo-μ-hydroperoxo dicopper(II) complex [Cu^II₂(PD‘O⁻)(⁻OOH)(RCN)₂](ClO₄)₂ (1, R = CH₃, CH₃CH₂ or C₆H₅CH₂; λ_max = 407 nm; ν_(O-O) = 870 cm^-1; J. Am. Chem. Soc. 2005, 127, 15360) is generated by reacting a precursor dicopper(I) complex [Cu^I₂(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 [Cu^II₃(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^I₂(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-μ-hydroxide-bridged tetranuclear copper(II) cluster compound [{Cu^II₂(PD‘O^-)(OH^-)}₂](ClO₄)₄ (8) has also been isolated following warming of 1. Its formation is consistent with the generation of [Cu^II₂(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.