Synthesis and reactivity of (μ-η2:η2-peroxo)dicopper(II) complexes with dinucleating ligands: Hydroxylation of xylyl linker with a NIH shift

New hexadentate dinucleating ligands having a xylyl linker, X–L–R, were synthesized, where X–L–R = 1,3-bis[bis(6-methyl-2-pyridylmethyl)aminomethyl]-2,4,6-trimethybenzene (Me₂–L–Me) and 1,3-bis[bis(6-methyl-2-pyridylmethyl)aminomethyl]-2-fluorobenzene (H–L–F). They form dinuclear copper(I) complexes, [Cu₂(X–L–R)]²⁺ (Me₂–L–Me (1) and H–L–F (2)). The copper(I) complexes in acetone at −78 °C react with O₂ to produce intra- and intermolecular (μ-η²:η²-peroxo)dicopper(II) species depending on the concentrations of the complexes:  both complexes generate intramolecular (μ-η²:η²-peroxo)dicopper(II) species [Cu₂(O₂)(X–L–R)]²⁺ (1-O₂ and 2-O₂) at the concentrations below ∼5 mM, whereas at ∼60 mM, both complexes produce intermolecular (μ-η²:η²-peroxo)dicopper(II) species, which were confirmed by the electronic and resonance Raman spectroscopies.  The electronic spectrum of 1-O₂ in acetone at concentrations below ∼5 mM showed an absorption band at (λ_max = 442 nm, ε = 5600 M⁻¹ cm⁻¹) assignable to the ${\mathrm{\pi }}_{\sigma }^1$(O–O)-to-Cu(II) ($({\text{d}}_{x^2-y^2}+{\text{d}}_{x^2-y^2})$ component) LMCT transition in addition to an intense band attributable to the ${\mathrm{\pi }}_{\sigma }^1$(O–O)-to-Cu(II) ($({\text{d}}_{x^2-y^2}-{\text{d}}_{x^2-y^2})$ component) LMCT transition (λ_max = 359 nm, ε = 21000 M⁻¹ cm⁻¹), indicating that the (μ-η²:η²-peroxo)Cu(II)₂ core of 1-O₂ takes a butterfly structure. Decomposition of 1-O₂ resulted in hydroxylation of the 2-position of the xylyl linker with 1,2-methyl migration (NIH shift), suggesting that the hydroxylation reaction proceeds via a cationic intermediate as proposed for closely related (μ-η²:η²-peroxo)Cu(II)₂ complexes having a xylyl linker. Kinetic study of the decomposition (hydroxylation of the xylyl linker) of 1-O₂ suggests that a stereochemical effect of the methyl group in the 2-position of the xylyl linker has a significant influence on a transition state for decomposition (hydroxylation of the xylyl linker).