Pincer-type bisnitroxide radicals involving tetramethylenedioxy and o-xylylenedioxy bridges

Abstract

Two new pincer-type bisnitroxides have been synthesized, where tetramethylenedioxy and o-xylylenedioxy are chosen as a bridge between the p-position of two tert-butyl phenyl nitroxide moieties. The X-ray crystal structure analysis clarified the linear and U-shaped molecular structures for the former and latter compounds, respectively. In the toluene solution ESR of the tetramethylenedioxy derivative, five hyperfine splitting lines were recorded, but a center signal was somewhat lowered. The ESR spectra of the o-xylylenedioxy derivative displayed a typical 1/2/3/2/1 five-line pattern at 370 K, and an “alternating line-width” effect appeared in 210–230 K. These spectra were reproduced by simulation, based on the proposed model where a few conformers have differing exchange couplings. The contribution showing strong exchange is reasonably ascribable to the U-shaped conformer. Thus, the solid and solution experimental results are compatible with each other.Keywards: biradical, nitroxide, aminoxyl, ESR, exchange interaction.