Proton-Coupled Oxidation of Aldimines and Stabilization of H-Bonded Phenoxyl Radical-Phenol Skeletons.

Abstract

Stable phenoxyl radicals H-bonded to phenols were successfully isolated. The effect of the intermolecular H-bonding to the concerted proton coupled electron transfer (CPET) reactions of the aldimines and the stability and spin distribution of the H-bonded phenoxyls are reported. Salts of iminium-phenol derivatives as cations and the corresponding imine-phenolato derivatives coordinated to zinc(II) as anions, [ZnII(ArRO)Cl2]-[ArROH2]+, were isolated, where ArOH are aldimine derivatives. Notably, [ZnII(ArRO)Cl2]-[ArROH2]+ salts undergo CPET reactions in air affording phenoxyl analogues, [ZnII(ArOH)Cl2].ArO●·CH3CN. [ZnII(ArRO)Cl2]-[ArROH2]+ salts incorporate intermolecular iminium-phenolato, [(Zn)Ar-O--+HN═CH-] H-bonds, while [ZnII(ArOH)Cl2].ArO●·CH3CN moieties contain intermolecular phenoxyl-phenol, [Ar-O-HO-Ar]●, H-bonds. The phenoxyls are presented in two forms, [(Zn)Ar-O●---HO-Ar (zinc phenoxyl) ↔ (Zn)Ar-OH---●O-Ar (free phenoxyl)]. In crystals, the spin density scatters on both phenolic fragments corresponding to a delocalized state, while in solution the latter form has been calculated as a ground electronic state. The X-band EPR spectra of crystals, solutions and frozen glasses were analyzed. The powder spectra at g = 2.0030 ± 0.0005 and the frozen glass spectra at g = 2.0075 ± 0.0003 follow the hyperfine patterns due to 14N (I = 1) nuclei. In fluid solutions, the g values of the hyperfine signals due to 14N and 1H nuclei are 2.0078 ± 0.0001. 1-3 exhibit absorption bands at 350-390 nm due to π → π* intraligand charge transfer (ILCT) transitions, while the radical species, in addition to π → π transitions at 405-440 nm, display phenol to phenoxyl intervalence charge transfer (IVCT) transitions at 600-650 nm. The cyclic voltammograms (CVs) of 1-3 depend on the scan rates; at lower scan rates (100-400 mV/s) the CPET reactions occur at -0.92 to -0.96 V versus Fc+/Fc couple, whereas at higher scan rates (1000-2400 mV/s), the oxidation occurs by the electron transfer (ET) path at 0.05-0.12 V. Thus, a potential shift of ∼1.0 V is recorded due to CPET reactions facilitated by H-bonding.