文摘
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 substituent. Chemical oxidation, monitored by ultraviolet鈥搗isible鈥搉ear-infrared (UV鈥搗is鈥揘IR) spectroscopy, afforded their corresponding one-electron oxidized products. Ligand-based oxidation was observed for [Ni(SalCF3)]+鈥?/b>, as evidenced by sharp NIR transitions in the UV鈥搗is鈥揘IR spectrum and a broad isotropic signal at g = 2.067 by solution electron paramagnetic resonance (EPR) spectroscopy. Such sharp NIR transitions observed for [Ni(SalCF3)]+鈥?/b> 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(SalCF3)]+鈥?/b>, evident from the EPR g-value and DFT calculations, was further quantified by the binding affinity of axial ligands to [Ni(SalCF3)]+鈥?/b>. For [Cu(SalCF3)]+, an intense ligand-to-metal charge transfer band at 18鈥?00 cm鈥? in the UV鈥搗is鈥揘IR spectrum was observed, which is diagnostic for the formation of a CuIII species [J. Am. Chem. Soc., 2008, 130, 15448鈥?5459]. The CuIII character for [Cu(SalCF3)]+ is further confirmed by 19F NMR analysis. Taken together, these results show that the electron-deficient salen ligand H2SalCF3 increases the Lewis acidity of the coordinating metal center.