Calixarenes 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 calixarene 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 calixarene polyradical scaffolds in frozen solutions show that the through-bond exchange coupling in the 1,3-alternate calixarene tetraradical is antiferromagnetic, while that in cone calixarene tetraradical is ferromagnetic. The through-space exchange couplings are antiferromagnetic in both cone and 1,3-alternate calixarene tetraradical, as well as in the 1,3-alternate calixarene diradical. The exchange coupling constants (|J/k|) are of the order of 1 K.
ASJC Scopus subject areas
- Colloid and Surface Chemistry