Influence of Electron-Withdrawing Substituents on the Electronic Structure of Oxidized Ni and Cu Salen Complexes

Nickel (Ni(Sal(CF3))) and copper (Cu(Sal(CF3))) complexes of an electron-poor salen ligand were prepared, and their one-electron oxidized counterparts were studied using an array of spectroscopic and theoretical methods. The electrochemistry of both complexes exhibited quasi-reversible redox processes at higher potentials in comparison to the M(Sal(R)) (R = Bu-t, OMe, NMe2) analogues, in line with the electron-withdrawing nature of the para-CF3 substituent. Chemical oxidation, monitored by ultraviolet visible near-infrared (UV-vis-NIR) sioectroscopy, afforded their corresponding one-electron oxidized products. Ligand-based oxidation was observed for [Ni(Sal(CF3))](+center dot), as evidenced by sharp NIR transitions in the UV-vis NIR spectrum and a broad isotropic signal at g = 2.067 by solution electron paramagnetic resonance (EPR) spectroscopy. Such sharp NIR transitions observed for [Ni(Sal(CF3))](+center dot) are indicative of a delocalized electronic structure, which is in good agreement with electrochemical measurements and density functional theory (DFT) calculations. In addition, the increased Lewis acidity of [Ni(Sal(CF3))](+center dot), evident from the EPR g-value and DFT calculations, was further quantified by the binding affinity of axial ligands to [Ni(Sal(CF3))](+center dot). For [Cu(Sal(CF3))](+), an intense ligand-to-metal charge transfer band at 18 700 cm(-1) in the UV-vis NIR spectrum was observed, which is diagnostic for the formation of a Cu-III species [J. Am. Chem. Soc., 2008, 130, 15448-15459]. The Cu-III character for [Cu(Sal(CF3))](+) is further confirmed by F-19 NMR analysis. Taken together, these results show that the electron-deficient salen ligand H(2)Sal(CF3) increases the Lewis acidity of the coordinating metal center.