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 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(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 H2SalCF3 increases
the Lewis acidity of the coordinating metal center.