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.
Synthesis, crystallography, and magnetic characterization of a stable macrocyclic tetranitroxide 1, a calix[4]arene, which is functionalized with four tert-butylnitroxides at the upper rim, is described. In solution, 1 has a 4-fold symmetric fixed cone conformation on the NMR time scale and small, but nonnegligible, exchange interactions between the radicals (30 K > /J/k/ >>1.8 mK). In the solid state, dimerization of one diagonal pair of nitroxides leads to a pinched cone conformation for 1 with strong intradimer antiferromagnetic coupling with /J/k/ = 200-300 K (singlet-triplet energy gap, DeltaE(ST) approximately 1 kcal/mol).
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