Characterization of a 1:1 Cu-O₂ adduct supported by an anilido imine ligand

Copper(I) complexes of sterically hindered anilido imine ligands o-C ₆H₄{N(C₆H₃ⁱPr ₂)}{C(R)=NC₆H₃ⁱPr₂} ^- (L¹, R = H; L², R = CH₃) have been prepared and characterized by spectroscopic and X-ray crystallographic methods. These complexes are highly reactive with O₂, and in the case of L² the product of low-temperature oxygenation was fully characterized by spectroscopic, X-ray crystallographic, and computational methods. The resonance Raman spectrum features an isotope-sensitive vibration at 974 cm ^-1 (Δ(¹⁸O) = 66 cm^-1), consistent with assignment as an O-O stretch. Despite the asymmetric coordination environment provided by the supporting anilido imine ligand, the X-ray crystal structure confirms rather symmetric side-on binding of the O₂ moiety to the copper center, and the O-O bond length of 1.392(2) Å indicates that this intermediate has significant Cu(III)-peroxo character. Theoretical calculations support this interpretation and predict that while a fully optimized end-on singlet geometry can be obtained, it is higher in energy than the side-on isomer by 3.5 kcal mol^-1 at the CASPT2/TZP level.