Electronic tuning of β-diketiminate ligands with fluorinated substituents: Effects on the O₂-reactivity of mononuclear Cu(I) complexes

Copper(i) complexes with the β-diketiminate ligands HC{C(R)N(Dipp)}{C(R′)N(Dipp)}^- (Dipp = C₆H ₃ⁱPr_2-2,6; L¹, R = CF₃, R′ = CH₃; L², R = R′ = CF₃) have been isolated and fully characterized. On the basis of X-ray structural comparisons with the previously reported complex LCu(CH₃CN) (L = HC{C(CH₃)N(Dipp)}₂^-), the ligand environments at the copper centers in the analogous nitrile adducts with L¹ and L² impose similar steric demands. L¹Cu(CH₃CN) reacts instantaneously at low temperature with O₂ to form a thermally-unstable intermediate with an isotope-sensitive vibration at 977 cm^-1 (928 cm^-1 with ¹⁸O₂), in accord with the peroxo O-O stretch associated with side-on coordination for LCu(O ₂). However, L²Cu(CH₃CN) is unreactive toward O₂ even at room temperature. Evaluation of the redox potentials of the nitrile adducts and the CO stretching frequencies of the carbon monoxide adducts revealed an incremental adjustment of the electronic environment at the copper center that correlated with the extent of ligand fluorination. Furthermore, theoretical calculations (DFT, CASPT2) predicted that an increasing extent of Cu(ii)-superoxo character and end-on coordination of the O ₂ moiety in the Cu/O₂ product (L² > L ¹ > L) are accompanied by increases in the free energy for the oxygenation reaction, with L² unable to support a Cu/O₂ intermediate. Calculations also predict the 1:1 Cu/O₂ adducts to be unreactive with respect to hydrogen atom abstraction from hydrocarbon substrates on the basis of their stability towards both reduction and protonation.