observed for macrocycle 6. Lower conversion to the triplet
dication was observed upon addition of a second equivalent
of PIFA to macrocycle 4, although the stability of the dication
was high. After 2 h at room temperature, the same signal
for 42+ with similar intensity was observed, indicating that
the diradicals are stable at room temperature. Integration of
the EPR signal ascribed to the triplet state of 62+ versus a
standard sample of Fremy’s salt18 showed that roughly 90%
of triplet 62+ was produced upon oxidation of 6 by 2 and 4
equiv of PIFA, followed by addition of TFA (Figure 2).
not absence, of TFA3,23 led us to use these conditions in our
EPR study.
The role of the acid in creating a triplet ground state
remains unclear. However, we have shown that the TFA does
not react irreversibly with the neutral macrocycles. 1H NMR
signals of neutral 4 are broad and then shifted in the presence
of incrementally increasing amounts of added TFA, but the
original spectrum for 4 is regenerated after addition of
bicarbonate base.
Previous reports of diradicals generated from open chain
analogues of 4-6 have included a distance between diradi-
cals derived from EPR zero field splitting parameters.3,24 This
distance is difficult to interpret because a variety of
conformations of the open chain materials are possible. In
contrast, the cores of structures 4-6 are well defined. The
dominant resonance contributors to the dication diradical of
these materials are structures with the radical cations located
at alternating nitrogens to maximize the distance between
charges. Thus, the diagonal N-N distance of 7.45 Å found
in the X-ray structure of 4 might correspond to the distance
between the two radical sites in 42+. Indeed, a zero field
splitting of 103 G for 4 provides a distance of 8.1 ( 0.3 Å
between the two spins that create the triplet state. A splitting
of 111 ( 10 G provides a distance of 7.9 ( 0.3 Å for that
of 6. The splitting was determined for 6 at different
temperatures, and the value was temperature independent.
The core structure is unlikely to be altered substantially upon
oxidation, but the N-aryl groups are likely to carry some spin
density, creating an effective distance between spins that is
larger than simply the diagonal N-N distance. A more
detailed analysis of the role of these groups on the electronic
and magnetic properties of tetraazacyclophanes will be the
subject of future studies, along with the use of our facile
synthesis to prepare variants of these materials that adopt
organized solution or solid phase structures.
Figure 2. EPR spectrum of 6 at 20 K in CH2Cl2/1% TFA after
addition of 2 equiv of PIFA.
Temperature-dependent EPR studies and magnetic sus-
ceptibility measurements were conducted on 62+ to evaluate
further the multiplicity of the ground state. The intensity of
the g ) 4 signal displayed simple Curie behavior between
10 and 45 K, indicating a ground state triplet or nearly
degenerate singlet and triplet states. Magnetic susceptibility
was measured at 25 °C using the Evans method.19,20 In
methylene chloride without added TFA, the dication dis-
played a magnetic moment of 1.12 µB, suggesting that the
singlet is the major species under these conditions, as was
determined by EPR spectroscopy. However, the magnetic
susceptibility was dependent on the amount of TFA added.
The different concentrations of 62+ in the magnetic suscep-
tibility and EPR experiments, the dependence of the triplet
population on medium, and the potential dependence of
solvent magnetic susceptibility upon addition of TFA make
it difficult to correlate the two experiments. However, the
Acknowledgment. We are grateful to Professor Albert
J. Fry for help with cyclic voltammetry and for the use of
his equipment. We thank Professor Gary W. Brudvig for
assistance with EPR analysis and Susan De Gala for X-ray
analysis. We also appreciate an exchange of manuscripts and
helpful comments from Prof. S. Blackstock, whose related
work is reported in the previous paper of this issue. We are
grateful to DOE for support of this work.
µ
eff of a solution of 6 and 2 equiv of oxidant in a methylene
chloride solution containing 10% TFA was 1.9 ( 0.2 µB,
indicating that roughly 45% of 6 was converted to triplet
62+ and that the singlet and triplet states lie close to each
other in energy. The ground state of dication diradicals is
well known to depend on medium,21,22 and previous observa-
tions of triplet triarylamine diradicals in the presence, but
Supporting Information Available: Experimental pro-
cedures and spectroscopic and analytical data for 1-23.
X-ray diffraction data for 4. Cyclic voltammetry and EPR
data. This material is available free of charge via the Internet
(18) Weil, J. A.; Bolton, J. R.; Wertz, J. E. In Electron Paramagnetic
Resonance; John Wiley and Sons: New York, 1994; p 499.
(19) Evans, D. F. J. Chem. Soc. 1959, 2003-5.
(20) Live, D. H.; Chan, S. I. Anal. Chem. 1970, 42, 791-2.
(21) Thomaides, J.; Maslak, P.; Breslow, R. J. Am. Chem. Soc. 1988,
110, 3970-3979.
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(23) Wienk, M. M.; Janssen, R. A. J. Am. Chem. Soc. 1996, 118, 10626-
10628.
(24) Yoshizawa, K.; Chano, A.; Ito, A.; Tanaka, K.; Yamabe, T.; Fujita,
H.; Yamachui, J. Chem. Lett. 1992, 369-372.
(22) Miller, J. S.; Dixon, D. A.; Calabrese, J. C. Science 1988, 240,
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Org. Lett., Vol. 1, No. 13, 1999