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

Copper(I) complexes with the beta-diketiminate ligands HC{C(R) N(Dipp)}{C(R') N(Dipp)}(-) (Dipp = C6H3 Pr-i(2-) 2,6; L-1, R = CF3, R' = CH3; L-2, 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 L-1 and L-2 impose similar steric demands. (LCu)-Cu-1(CH3CN) reacts instantaneously at low temperature with O-2 to form a thermally-unstable intermediate with an isotope-sensitive vibration at 977 cm(-1) (928 cm(-1) with O-18(2)), in accord with the peroxo O-O stretch associated with side-on coordination for LCu(O-2). However, (LCu)-Cu-2(CH3CN) is unreactive toward O-2 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/O-2 product (L-2 > L-1 > L) are accompanied by increases in the free energy for the oxygenation reaction, with L-2 unable to support a Cu/O-2 intermediate. Calculations also predict the 1 : 1 Cu/O-2 adducts to be unreactive with respect to hydrogen atom abstraction from hydrocarbon substrates on the basis of their stability towards both reduction and protonation.