are dependent upon mass of the diradical. Values of both Msat
and wT would be lower if nitroxide diradical 2, which is an
electrostatic amorphous powder, is incompletely transferred to
the reaction vessel. Most importantly, the measured values of
This research is supported by the National Science Foundation
(CHE-0414936, CHE-0718117, and DMR-0216788 for the
purchase of EPR spectrometer), and by the Air Force Office
of Scientific Research (FA9550-04-1-0056).
ꢀ
1
wT (in units of emu K mol ) are nearly identical to the
Notes and references
numerical values of Msat (in the unit of m ). Therefore,
B
wT E 1 is found for 2, after values of wT are corrected by
spin concentration (Msat). Although the wT vs. T plots are
nearly flat (Fig. 4, inset plot), small errors in correction for
diamagnetism may have significant impact on such plots,
especially at higher temperatures. Thus, only a lower limit of
1 A. Rajca, J. Wongsriratanakul and S. Rajca, Science, 2001, 294,
1
503–1505.
A. Rajca, J. Wongsriratanakul and S. Rajca, J. Am. Chem. Soc.,
004, 126, 6608–6626.
E. Fukuzaki and H. Nishide, J. Am. Chem. Soc., 2006, 128,
996–1001.
2
3
2
ꢀ
1
4 Magnetic Properties of Organic Materials, ed. P. M. Lahti, Marcel
Dekker, New York, 1999, pp. 1–713; Molecular Magnetism, ed.
K. Itoh and M. Kinoshita, Gordon and Breach, Kodansha, 2000,
pp. 1–337; J. A. Crayston, J. N. Devine and J. C. Walton, Tetra-
hedron, 2000, 56, 7829–7857; A. Rajca, Chem.–Eur. J., 2002, 8,
more than 100–200 K (0.2–0.4 kcal mol ) for the singlet–
triplet energy gap (2J/k) may be estimated. A more accurate
value of 2J/k may be obtained from the wT vs. T plots in
the solid state, using rigorous point-by-point correction for
diamagnetism.
4
834–4841; A. Rajca, Adv. Phys. Org. Chem., 2005, 40, 153–199;
N. M. Shishlov, Russ. Chem. Rev., 2006, 75, 863–884.
Diarylnitroxide radicals are defined as nitroxides connected to two
aryl groups: A. R. Forrester, J. M. Hay and R. H. Thomson,
Organic Chemistry of Stable Free Radicals, Academic Press,
London, 1968, ch. 5, pp. 180–246.
For solid diradical 2, the wT vs. T plot shows a
downward turn at lower temperatures from the value of
5
ꢀ
1
wT E 0.93 emu K mol at 290 K (Fig. 5). These data indicate
that diradical 2 possesses a triplet ground state (S = 1)
with the singlet–triplet energy gap, 2J/k, that is comparable
or greater than the thermal energy at room temperature
6
H. Oka, H. Kouno and H. Tanaka, J. Mater. Chem., 2007, 17,
1
7 Z. Delen and P. M. Lahti, J. Org. Chem., 2006, 71, 9341–9347.
209–1215.
8
9
M. Tanaka, K. Matsuda, T. Itoh and H. Iwamura, J. Am. Chem.
Soc., 1998, 120, 7168–7173.
A. Rajca, M. Vale and S. Rajca, J. Am. Chem. Soc., 2008, 130,
9099–9105.
0 The analogous triarylmethyl diradical is air sensitive: A. Rajca and
(
2J/k Z 300 K), and the presence of the antiferromagnetic
intermolecular interactions between the S = 1 diradical
molecules.
1
1
1
1
S. Utamapanya, J. Org. Chem., 1992, 57, 1760–1767.
1 K. Shiraishi, A. Rajca, M. Pink and S. Rajca, J. Am. Chem. Soc.,
2
005, 127, 9312–9313.
2 The actual yield may be lower due to diamagnetic by-products that
are EPR-silent.
3 Values of wT E 0.4 emu K mol that are significantly below wT =
ꢀ1
ꢀ1
1
.0 emu K mol expected for an S = 1 diradical are obtained.
4 C. Berti and L. Greci, J. Org. Chem., 1981, 46, 3060–3063.
5 C. P. Sar, T. Kalai, N. M. Baracz, G. Jerkovich and K. Hideg,
1
1
´
´
´
´
Synth. Commun., 1995, 25, 2929–2940.
6 M. J. Frisch et al., GAUSSIAN 03, Revision E.01, Gaussian,
Wallingford, CT, 2004.
7 Singlet–triplet energy gaps are calculated using broken-symmetry
singlets with S(S + 1) E 1.0 (Table S3, ESIw). This level of theory
usually overestimates the singlet–triplet gaps for triplet ground
state organic diradicals, and underestimates such gaps for the
singlet ground state diradicals: D. Y. Zhang, D. A. Hrovat,
M. Abe and W. T. Borden, J. Am. Chem. Soc., 2003, 125,
1
1
Fig. 5 SQUID magnetometry for solid diradical 2.
1
1
2823–12828; C. J. Cramer and B. A. Smith, J. Phys. Chem.,
996, 100, 9664–9670.
In the w vs. T plot, a broad maximum is observed at 37 K,
indicating that these intermolecular antiferromagnetic inter-
actions are stronger by about an order of magnitude compared
1
1
2
8 A. Rajca, Chem. Rev., 1994, 94, 871–893; R. J. Bushby, N. Taylor
and R. A. Williams, J. Mater. Chem., 2007, 17, 955–964.
9 A. Rajca, S. Mukherjee, M. Pink and S. Rajca, J. Am. Chem. Soc.,
2006, 128, 13497–13507.
0 Planar alkylarylnitroxide diradical: A. Rajca, M. Takahashi,
M. Pink, G. Spagnol and S. Rajca, J. Am. Chem. Soc., 2007,
2
0
to those in planar alkylarylnitroxide diradicals. The broad
maximum at 37 K is more pronounced in samples with lesser
content of monoradical (Fig. S8, ESIw). Unlike for the planar
1
29, 10159–10170.
21 (a) F. A. Neugebauer and S. Bamberger, Chem. Ber., 1974,
07, 2362–2382; (b) For diradical 2 at 140 K, isotropic g =
+ g + g )/3 = 2.0048, Fig. S4 and Table S1, ESIw].
2
0
alkylarylnitroxide diradicals,
neither the dimer model
1
(
Heisenberg pairs of S = 1 diradicals) or chain model
one-dimensional Heisenberg chains of S = 1 diradicals)
(
g
x
y
z
(
14
2
2 All simulated components of g-tensor, D-tensor, and N-hyperfine
tensor for 2 have slightly lower values compared to those for a
planar alkylarylnitroxide diradical (ref. 20) (Table S1, ESIw). This
is consistent with greater delocalization of spin density away from
nitroxide into relatively extended p-system in diarylnitroxide
provided good numerical fits, even if a monoradical was
included in the fit. These relatively strong intermolecular
antiferromagnetic interactions are expected to lower the value
ꢀ1
of wT, below 1.00 emu K mol expected for an S = 1 state,
even at room temperature.
diradical 2.
2
4
1
23 EPR and H NMR spectra for the identical sample of 2, as well as
EPR spectra for this SQUID sample, are shown in Fig. S6, S7 and
In summary, nitroxide diradical 2 is the first isolated
diarylnitroxide diradical that is stable in the solid state at
room temperature and it possesses the triplet ground state with
strong ferromagnetic coupling.
S11 (ESIw).
ꢀ1
2
4 The value of wT E 0.93 emu K mol at 290 K may also be affected
by the correction for diamagnetism, for which the accuracy may be
decreased accuracy because of small weights of samples.
4
374 | Chem. Commun., 2009, 4372–4374
This journal is ꢁc The Royal Society of Chemistry 2009