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

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• 作者：Linus Chiang ; Khrystyna Herasymchuk ; Fabrice Thomas ; Tim Storr
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：June 15, 2015
• 年：2015
• 卷：54
• 期：12
• 页码：5970-5980
• 全文大小：653K
• ISSN：1520-510X

文摘

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 = ^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(Sal^CF3)]^+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(Sal^CF3)]^+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(Sal^CF3)]^+鈥?/b>, evident from the EPR g-value and DFT calculations, was further quantified by the binding affinity of axial ligands to [Ni(Sal^CF3)]^+鈥?/b>. For [Cu(Sal^CF3)]⁺, 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 Cu^III species [J. Am. Chem. Soc., 2008, 130, 15448鈥?5459]. The Cu^III character for [Cu(Sal^CF3)]⁺ is further confirmed by ¹⁹F NMR analysis. Taken together, these results show that the electron-deficient salen ligand H₂Sal^CF3 increases the Lewis acidity of the coordinating metal center.