Characterization of a 1:1 Cu−O₂ Adduct Supported by an Anilido Imine Ligand

Copper(I) complexes of sterically hindered anilido imine ligands o-C6H4{N(C6H3iPr2)}{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.