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

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, [Zn II (Ar R O)Cl 2 ] − [Ar R OH 2 ] +, were isolated, where ArOH are aldimine derivatives. Notably, [Zn II (Ar R O)Cl 2 ] − [Ar R OH 2 ] + salts undergo CPET reactions in air affording phenoxyl analogues, [Zn II (ArOH)Cl 2 ].ArO ● ·CH 3 CN. [Zn II (Ar R O)Cl 2 ] − [Ar R OH 2 ] + salts incorporate intermolecular iminium–phenolato, [(Zn)Ar–O – – + HN═CH−] H-bonds, while [Zn II (ArOH)Cl 2 ].ArO ● ·CH 3 CN 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 14 N ( I = 1) nuclei. In fluid solutions, the g values of the hyperfine signals due to 14 N and 1 H 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.