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

Authors

• Andrzej Rajca, University of Nebraska - LincolnFollow
• Sumit Mukherjee, University of Nebraska-Lincoln
• Maren Pink, Indiana University
• Suchada Rajca, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

2006

Citation

J Am Chem Soc. 2006 October 18; 128(41): 13497–13507.

Comments

Copyright 2006 Rajca et al.

Abstract

Calix[4]arenes constrained to 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 throughbond

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-phenylenenitroxide 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 Kelvin.