화학공학소재연구정보센터
Inorganic Chemistry, Vol.58, No.22, 15060-15077, 2019
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 Hbonding 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((ArO)-O-R)Cl-2](-)[(ArOH2)-O-R](+), were isolated, where ArOH are aldimine derivatives. Notably, [Zn-II((ArO)-O-R)Cl-2](-)[(ArOH2)-O-R](+) salts undergo CPET reactions in air affording phenoxyl analogues, [Zn-II(ArOH)Cl-2]center dot ArO center dot center dot CH3CN. [Zn-II((ArO)-O-R)Cl-2](-)[(ArOH2)-O-R](+) salts incorporate intermolecular iminium-phenolato, [(Zn)Ar-O--+HN=CH-] H-bonds, while [Zn-II(ArOH)Cl-2]center dot ArO center dot center dot CH3CN moieties contain intermolecular phenoxyl-phenol, [Ar-O-HO-Ar](center dot), H-bonds. The phenoxyls are presented in two forms, [(Zn)Ar-O-center dot---HO-Ar (zinc phenoxyl) <-> (Zn)Ar-OH---O-center dot-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 N-14 (I = 1) nuclei. In fluid solutions, the g values of the hyperfine signals due to (14) N and H-1 nuclei are 2.0078 +/- 0.0001. 1-3 exhibit absorption bands at 350-390 nm due to pi ->pi* intraligand charge transfer (ILCT) transitions, while the radical species, in addition to pi ->pi transitions at 405-440 nm, display phenol to phenoxyl intervalence charge transfer (WCT) 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 similar to 1.0 V is recorded due to CPET reactions facilitated by H-bonding.