title: Influence of Electron-Withdrawing Substituents on the Electronic Structure of Oxidized Ni and Cu Salen Complexes.
abstract: 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 = (t)Bu, 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) spectroscopy, afforded their corresponding one-electron oxidized products. Ligand-based oxidation was observed for [Ni(Sal(CF3))](+•), 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))](+•) 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))](+•), evident from the EPR g-value and DFT calculations, was further quantified by the binding affinity of axial ligands to [Ni(Sal(CF3))](+•). 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 (19)F NMR analysis. Taken together, these results show that the electron-deficient salen ligand H2Sal(CF3) increases the Lewis acidity of the coordinating metal center.
authors.affiliation: †Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
authors.forename: Linus
authors.initials: L
authors.lastname: Chiang
authors.affiliation: †Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
authors.forename: Khrystyna
authors.initials: K
authors.lastname: Herasymchuk
authors.affiliation: §Département de Chimie Moléculaire, Chimie Inorganique Redox (CIRE), UMR-5250, Université Grenoble Alpes, BP 53, 38041 Grenoble Cedex 9, France.
authors.forename: Fabrice
authors.initials: F
authors.lastname: Thomas
authors.affiliation: †Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
authors.forename: Tim
authors.initials: T
authors.lastname: Storr
mesh_terms: D060326:Chemistry Techniques, Synthetic; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005029:Ethylenediamines / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D056951:Photoelectron Spectroscopy; D011725:Pyridines / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
chemical_list: D005029:Ethylenediamines; D009942:Organometallic Compounds; D011725:Pyridines; D003300:Copper; D009532:Nickel; C011452:disalicylaldehyde ethylenediamine