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

Abstract

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.

Properties of the indole ring in metal complexes. A comparison with the phenol ring

“…Complexes 1,2-salcn + , 1,3-salcn + , and saloph + showed similar reaction kinetics, suggesting that the reaction mechanisms are similar [87,92]. The kinetic isotope effect (KIE) with the use of α-d 2 -benzyl alcohol was estimated to be very large (k D /k H N 10), which indicates that hydrogen abstraction from α-position of the benzyl alcohol is involved in the rate determining step of the oxidation (Scheme 3) [95,96].…”

“…The resonance Raman spectrum of 1,3-salcn + exhibited a characteristic phenoxyl radical ν 7a Raman band at 1494 cm − 1 in CH 2 Cl 2 solution. From the results, 1,3-salcn + can be assigned to the Cu(II)-phenoxyl radical species with the radical electron localized on one of the two phenolate moieties both in the solid state and in solution [87]. On the other hand, the crystal structure of [Cu(4,5-diMeOsalophen)]SbF 6 (4,5-diMeO-saloph + ) (4,5-diMeOsalophen = N,N′-bis(3,5-di(tert-butyl)salicylidene)-1,2-diamino-4,5-dimethoxybenzene) showed that the counter anion is located at the proximal position of the 1,2-phenylenediamine moiety (Fig.…”

“…8(B)) [87]. The C-O bond length of the phenolate moiety interacting with the counter anion is shorter than that of the other phenolate moiety (1.266(5) vs. 1.304(5) Å), while the Cu-O bond length of the phenolate moiety interacting with the counter anion is longer than that of the other Cu-O bond (1.996(3) vs. 1.912(3) Å).…”

“…The highest reactivity was observed for 1,3-salcn + , and this may be taken as due to the fact that this complex exists as the Cu(II)-phenoxyl radical which is the active species of the oxidation; complexes 1,2-salcn + and saloph + were less reactive probably because of the mentioned equilibrium. The fact suggests that the hydrogen abstraction in the rate-determining step can be accelerated only by the localized phenoxyl radical owing to its higher hydrogen affinity than Cu(III) ion and the delocalized ligand radical [85,87,93]. On the other hand, 4,5-diMeO-saloph + , which is assigned to the o-diiminobenzene radical complex, was found to form a substrate saturation system following a different reaction kinetics from the other three complexes and less reactive than 1,3-salcn + .…”

“…Hydrogen abstraction mechanisms of oxidation of benzyl alcohol: (A) by the Cu(II)-localized phenoxyl radical of 1,3-salcn + and (B) by 1,3-salcn + and saloph + in the state of equilibrium between the Cu(II)-localized phenoxyl radical and the other different oxidized species[87,92].…”