Spectroscopic and electrochemical characterization of di-tert-butylated sterically hindered Schiff bases and their phenoxyl radicals.

A series of sterically hindered N-arylsalicylaldimines (SAs) previously prepared from substituted salicylaldehydes (X-Sal, where X = H, Cl, Br, NO2, OH, OCH3) and 2,6-di-t-butyl-1-hydroxyaniline (LxH) and 2,5-di-t-butylaniline (Lx'H) were characterized by 1H and 13C NMR, UV-Vis and electrochemical methods. The electronic spectra (ES) (X = OH, OCH3) in alcoholic solvents (MeOH, EtOH, PrOH, iso-PrOH) unlike other solvents exhibit a new absorption band in the region 630-675 nm (epsilon = 19-242 M(-1) cm(-1)), which are not characteristic for other SA known in literature. The ESR studies of primary phenoxyl radicals generated from LxH by their oxidation with PbO2 reveal that some of them with the time are converted to more stable secondary Coppinger's radical. The cyclic voltammograms of LxH and Lx'H except NO2-substituted ones in CH3CN are similar and along with two or three irreversible anodic waves at the potentials ranging from 0.0 to +1.9 V versus Ag/AgCl, also display one or two irreversible reduction waves at potentials -0.6 to +0.5 V. A series of a new SA prepared from 3,5-di-t-butylsalicylaldehyde and mono-substituted anilines (X = H, o-, p-F, Cl, Br, OCH3, p-t-butyl, 5,6-benzo) were characterized by analytical, spectroscopic (IR, UV-Vis, 1H and 13C NMR), and electrochemical techniques. The ES spectra of o-, p-Cl, p-Br, o-CH3 and 5,6-benzo-substituted SA did not exhibit expected absorptions at 400-500 nm in alcoholic solutions.