Biradical pincers involving two phenylene, biphenylene or terphenylene arms: Evidence for approach of two terminal nitroxides

Abstract

Four new pincer-type bisnitroxides, o-C6H4{CH2O-(p-C6H4)n-N(O•)-But}2 (abbreviated as pBi and pTer for n = 2 and 3, respectively; pPh for n = 1 (known)), and o-C6H4{CH2O-(p-C6H4)n-1-(m-C6H4)–N(O•)-But}2 (mBi and mTer for n = 2 and 3, respectively) were prepared, where o-xylylenedioxy was chosen as a hinge and phenylene, biphenylene, and terphenylene were selected as an arm. The crystallographic analysis clarified the intramolecular proximity of the two radical groups was observed for mBi, whereas pBi and mTer possess an open V-shape. In toluene solution ESR, five hyperfine splitting lines were recorded for every compound at room temperature or higher. The antiferromagnetic exchange interaction is estimated as 2J/kB = −0.05 to −0.0002 K from ESR simulation. The orders of the exchange interactions are as follows: Ph > Bi > Ter regarding the arm lengths and m > p regarding the positional isomers. The temperature at which alternating linewidths appeared was found to be a convenient temperature scale in this study.