High-spin polyarylmethyl polyradical: Fragment of a macrocyclic 2-strand based upon calix[4]arene rings

Annelated triple-calix[4]arene-based tetradecaether 1-(OMe) ₁₄ is synthesized. Its three isomers, 1X-(OMe) ₁₄, 1Y-(OMe) ₁₄, 1Z-(OMe) ₁₄, are isolated and characterized. The corresponding triarylmethyl carbopolyanion is prepared and oxidized to polyradical 1. SQUID magnetometry for 1 in THF-d ₈/2-MeTHF gives S = 6.2, below the theoretical value of 7 expected for strong ferromagnetic coupling between ₁₄ 'unpaired' electrons. Chemical quenching studies suggest that approximately one deuterium from the deuterated solvent is incorporated in 1, thus accounting in part for S < 7. Study of the temperature dependence of magnetization indicates that ferromagnetic coupling between 'unpaired' electrons is not very strong in 1; i.e., the thermal population of low-spin excited states is detected in the cryogenic temperature range. Upon exposure of 1 in THF-d ₈/2-MeTHF to ambient temperature for 30 min, a narrow distribution of spin systems with an 'average' S = 2.5 is obtained; upon a longer exposure time of 24 h, an 'average' S = 1 is produced.