Exchange Coupling Mediated Through-Bonds and Through-Space in Conformationally Constrained Polyradical Scaffolds: Calix[4]arene Nitroxide Tetraradicals and Diradical

Calix[4]arenes constrained to the 1,3-alternate conformation and functionalized at the upper rim
with four and two tert-butylnitroxides have been synthesized and characterized by X-ray crystallography,
magnetic resonance (EPR and 1H NMR) spectroscopy, and magnetic studies. The 1,3-alternate nitroxide
tetraradical and diradical provide unique polyradical scaffolds for dissection of the through-bond and through-space intramolecular exchange couplings. In addition, detailed magnetic studies of the previously reported
calix[4]arene nitroxide tetraradical, which possesses cone conformation in solution, reveal conformational
dependence of exchange coupling. Through-bond coupling between the adjacent nitroxide radicals is
mediated by the nitroxide−m-phenylene−CH2−m-phenylene−nitroxide coupling pathway, and through-space coupling is found between the diagonal nitroxide radicals at the conformationally constrained N···N
distance of 5−6 Å. Magnetic studies of the calix[4]arene polyradical scaffolds in frozen solutions show that
the through-bond exchange coupling in the 1,3-alternate calix[4]arene tetraradical is antiferromagnetic,
while that in cone calix[4]arene tetraradical is ferromagnetic. The through-space exchange couplings are
antiferromagnetic in both cone and 1,3-alternate calix[4]arene tetraradical, as well as in the 1,3-alternate
calix[4]arene diradical. The exchange coupling constants (|J/k|) are of the order of 1 K.