New Insights into the Electronic Structure and Reactivity of One-Electron Oxidized Copper(II)-(Disalicylidene)diamine Complexes

The neutral and one-electron oxidized Cu­(II) six-membered
chelate 1,3-Salcn (1,3-Salcn = N,N′-bis­(3,5-di-tert-butylsalicylidene)-1,3-cyclohexanediamine)
complexes
have been investigated and compared with the five-membered chelate 1,2-Salcn (N,N′-bis­(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) complexes. Cyclic voltammetry of Cu(1,3-Salcn) showed two reversible redox waves at 0.48 and
0.68 V, which are only 0.03 V higher than those of Cu(1,2-Salcn). Reaction of Cu(1,3-Salcn) with 1 equiv of AgSbF6 afforded the oxidized complex which exists as a ligand-based
radical species in solution and in the solid state. The X-ray crystal
structure of the oxidized complex, [Cu(1,3-Salcn)]SbF6, exhibited an asymmetric metal
binding environment with a longer Cu–O bond and quinoid distortion
in the phenolate moiety on one side, demonstrating at least partial
ligand radical localization in the solid state. The ligand oxidation
is also supported by XPS and temperature dependent magnetic susceptibility.
The electronic structure of the [Cu(1,3-Salcn)]+ complex was further probed by UV–vis–NIR,
resonance Raman, and electron paramagnetic resonance (EPR) measurements,
and by theoretical calculations, indicating that the phenoxyl radical
electron is relatively localized on one phenolate moiety in the molecule.
The reactivity of [Cu(1,3-Salcn)]+ with benzyl alcohol was also studied. Quantitative conversion of
benzyl alcohol to benzaldehyde was observed, with a faster reaction
rate in comparison with [Cu(1,2-Salcn)]+. The kinetic isotope effect (KIE = k(H)/k(D)) of benzyl alcohol oxidation by [Cu(1,3-Salcn)]+ was estimated to be 13, which is smaller
than the value reported for [Cu(1,2-Salcn)]+. The activation energy difference between [Cu(1,2-Salcn)]+ and [Cu(1,3-Salcn)]+ was in good agreement with the energy
calculated from KIE. This correlation suggests that the Cu­(II)-phenoxyl
radical species, characterized for [Cu­(1,2-salcn)]+ is more reactive for hydrogen abstraction from benzyl alcohol
in comparison to the 1:1 mixture of Cu­(III)-phenolate and Cu­(II)-phenoxyl
radical species, [Cu(1,2-Salcn)]+. Thus, the Cu­(II)-phenoxyl radical species accelerates benzyl alcohol
oxidation in comparison with the Cu­(III)-phenolate ground state complex,
in spite of the similar activated intermediate and oxidation pathway.