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

Nickel (Ni(SalCF3)) and copper (Cu(SalCF3)) 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(SalR) (R = tBu, OMe, NMe2) analogues, in line with the electron-withdrawing nature of the para-CF3 substitu-ent. Chemical oxidation, monitored by UV-Vis-NIR spectroscopy, afforded their corresponding one-electron oxidized products. Lig-and-based oxidation was observed for [Ni(SalCF3)]+•, as evidenced by sharp NIR transitions in the UV-Vis-NIR spectrum and a broad isotropic signal at g = 2.067 by solution EPR spectroscopy. Such sharp NIR transitions observed for [Ni(SalCF3)]+• are indicative of a delocalized electronic structure, which is in good agreement with electrochemical measurements and DFT calculations. In addition, the increased Lewis acidity of [Ni(SalCF3)]+•, evident from the EPR g-value and DFT calculations, was further quantified by the binding affinity of axial ligands to [Ni(SalCF3)]+•. For [Cu(SalCF3)]+, an intense LMCT band at 18700 cm-1 in the UV-Vis-NIR spectrum was observed, which is diagnostic for the formation of a CuIII spe-cies [J. Am. Chem. Soc., 2008, 130, 15448 – 15459]. The CuIII character for [Cu(SalCF3)]+ is further confirmed by 19F NMR anal-ysis. Taken together, these results show that the electron-deficient salen ligand H2SalCF3 increases the Lewis acidity of the coordinat-ing metal center.