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

Linus Chiang †, Khrystyna Herasymchuk †, Fabrice Thomas §, and Tim Storr *†

† Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada

§ Département de Chimie Moléculaire, Chimie Inorganique Redox (CIRE), UMR-5250, Université Grenoble Alpes, BP 53, 38041 Grenoble Cedex 9, France

Inorg. Chem., Article ASAP

DOI: 10.1021/acs.inorgchem.5b00783

Publication Date (Web): May 27, 2015

*E-mail: tim_storr@sfu.ca.

Synopsis

Nickel (Ni(SalCF3)) and copper (Cu(SalCF3)) complexes of an electron-deficient salen ligand were prepared, and their one-electron oxidized counterparts were studied using an array of experimental and theoretical methods. Ni(SalCF3) undergoes ligand oxidation to afford a delocalized ligand radical, while Cu(SalCF3)exhibits metal-based oxidation to afford a CuIII species.

電子求引基をサレンにつけることで、配位子が電子不足になり酸化されにくくなる。これにより、錯体の一電子過程において配位子ではなく金属が酸化され、Cu(II)→Cu(III)となる。

Abstract

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-CF3substituent. Chemical oxidation, monitored by ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy, afforded their corresponding one-electron oxidized products. Ligand-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 electron paramagnetic resonance (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 density functional theory (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 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 CuIII species [J. Am. Chem. Soc., 2008,130, 15448–15459]. The CuIII character for [Cu(SalCF3)]+ is further confirmed by 19F NMR analysis. Taken together, these results show that the electron-deficient salen ligand H2SalCF3increases the Lewis acidity of the coordinating metal center.

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