EPR studies of the bistability in CF3CNSSN

Article abstract of DOI:10.1080/10426500490427637

Condensation of CF₃C(=NH)NH₂ with SCl₂ yielded the dithiadiazolylium salt, [CF₃CNSSN]Cl which could be reduced to [CF₃CNSSN]₂ with Zn/Cu couple in SO ₂. [CF₃CNSSN]₂ is a diamagnetic dimer in the solid state but melts at 308 K to give a paramagnetic liquid. On cooling the radical enters a metastable state before immediately resolidifying below 295 K. The thermal hysteresis was probed by EPR spectroscopy. Metastable samples spontaneously solidified after an induction period, typically between ca. 5 and 30 min, consistent with the variable time necessary to form a seed crystal from which crystallization can occur.

Full text of DOI:10.1080/10426500490427637

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Phosphorus, Sulfur, and Silicon and the Related
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EPR STUDIES OF THE BISTABILITY IN CF3CNSSN
Delia A. Haynes a , Eric J. L. McInnes b , Jack Passmore c & Jeremy M. Rawson a
a University of Cambridge , Cambridge, United Kingdom
b University of Manchester , Manchester, United Kingdom
c The University of New Brunswick , Fredericton, New Brunswick, Canada
Published online: 12 Aug 2010.
To cite this article: Delia A. Haynes , Eric J. L. McInnes , Jack Passmore & Jeremy M. Rawson (2004) EPR STUDIES
OF THE BISTABILITY IN CF3CNSSN, Phosphorus, Sulfur, and Silicon and the Related Elements, 179:4-5, 869-873, DOI:
10.1080/10426500490427637
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Phosphorus, Sulfur, and Silicon, 179:869–873, 2004
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490427637
EPR STUDIES OF THE BISTABILITY IN CF3CNSSN
Delia A. Haynes,a Eric J. L. McInnes,b Jack Passmore,c
and Jeremy M. Rawsona
University of Cambridge, Cambridge, United Kingdom;a
University of Manchester, Manchester, United Kingdom;b
and The University of New Brunswick, Fredericton,
New Brunswick, Canadac
(Received August 17, 2003; accepted October 3, 2003)
Condensation of CF3C( NH)NH2 with SCl2 yielded the dithiadia-
zolylium salt, [CF3CNSSN]Cl which could be reduced to [CF3CNSSN]2
with Zn/Cu couple in SO2. [CF3CNSSN]2 is a diamagnetic dimer in
the solid state but melts at 308 K to give a paramagnetic liquid. On
cooling the radical enters a metastable state before immediately re-
solidifying below 295 K. The thermal hysteresis was probed by EPR
spectroscopy. Metastable samples spontaneously solidified after an in-
duction period, typically between ca. 5 and 30 min, consistent with the
variable time necessary to form a seed crystal from which crystallization
can occur.
Keywords: Bistability; dithiadiazolyl; metastable
For some time we have been interested in the use of thiazyl
radicals as novel magnetic materials, e.g., the dithiadiazolyl radi-
cal p-O2NC6F4CNSSN orders as a ferromagnet at 1.3 K1 and p-
NCC6F4CNSSN orders as a canted antiferromagnet below 36 K.2 More
recently we and others have observed bistability in a number of dithi-
azolyl radicals; the benzo-derivative BDTA was found3 to form a spin-
paired diamagnetic structure on sublimation. More recent studies have
shown that BDTA solidifies to form a metastable paramagnetic phase
on cooling from the liquid state.4 This phase orders as an antiferromag-
net at 11 K. Other dithiazolyl radicals have been shown by Oakley to
undergo solid-state transitions between diamagnetic (or antiferromag-
netic) and paramagnetic phases.5 Perhaps the most striking behavior
Address correspondence to Jeremy M. Rawson, Department of Chemistry, Univer-
sity of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom. E-mail:
jmr31@cos.cam.ac.uk
869

870
D. A. Haynes et al.
is that of trithiatriazapentalenyl, C2S3N3, which exhibits bistability
between 230 K and 320 K.6 This type of behavior provides the opportu-
nity to use these derivatives in room temperature data storage devices.
Phase switching of this type requires the crossing of two free energy
surfaces with a small activation energy barrier to interconversion, e.g.,
minor perturbation of molecular coordinates in order to transform be-
tween solid state structures.5,6
We were interested in preparing new materials which exhibit nar-
rower regions of bistability around room temperature and recently re-
ported the magnetic behavior of CF3CNSSN (1).7 The structure of 1 in
the solid state is a spin-paired dimer8 but converts to a paramagnetic
liquid on melting. The detailed structure of 1 in the liquid phase is
hard to probe but the large increase in volume coupled with its param-
agnetism indicate uncoupling of the spin-paired dimers. We speculated
that electrostatic association of radicals may be favored with the energy
barrier to breaking down the π∗-π∗ dimerization and electrostatic inter-
actions providing an activation barrier to interconversion. Here we re-
port a novel synthesis of 1 and probe its stability in the metastable state.
RESULTS AND DISCUSSION
The synthesis of CF3CNSSN was first reported by Mews and coworkers
in 1985 from the reaction of CF3CN with (NSCl)3, followed by reduction
of the resultant salt, [CF3CNSSN]Cl with Zn.8 It also has been prepared
by the rearrangement of the isomeric radical CF3CNSNS.9 We sought a
more direct route to this material (Scheme 1) and found that condensa-
tion of trifluoroacetamide, CF3C( NH)·NH2, with SCl2 yielded the salt
[CF3CNSSN]Cl in good yield (45%). Subsequent reduction with Zn/Cu
couple in SO2 yielded the desired radical which was readily purified by
vacuum sublimation (10−2 Torr, 35–20◦C).
SCHEME 1 Synthesis of 1.
X-band EPR studies on pure 1 clearly revealed the bistability be-
tween 295 and 308 K evident in the magnetic measurements7 and
showed that there were no marked differences in the width of the hys-
teresis loop as a function of heating/cooling rate (Figure 1a). In order

EPR Studies of the Bistability in CF CNSSN
871
3
FIGURE 1 (a) Intensity of the EPR spectrum of 1 with dwell times of 2 min
and 20 min between measurements. (b) Intensity of the EPR spectrum of 1 as
a function of time at fixed temperatures in the metastable region.

872
D. A. Haynes et al.
to probe the stability of the metastable state the sample was heated
into the liquid phase, cooled into the metastable region and the EPR
signal measured as a function of time at fixed temperatures [294(2),
296(2), and 298(2) K]. In all cases an abrupt decrease in EPR inten-
sity was observed [5 min at 298 K, 30 min at 296 K, and 15 min at
294 K] (Figure 1b). The abrupt decrease in EPR signal is indicative of
a complete and rapid transformation from the metastable liquid phase
to the solid state. No clear time/temperature correlation appeared evi-
dent, consistent with the variable time to form a nucleation site for the
crystallization process.
CONCLUSION
The current studies show that the super-cooled liquid state of
CF3CNSSN is not indefinitely stable. Stabilization of this state might
be achieved through the generation of a liquid crystal phase between
the solid and liquid phases. Approaches to this end are underway.
EXPERIMENTAL
Radical 1 is extremely air-sensitive and its low melting point made han-
dling difficult. For EPR studies we found that sublimation directly into
a quartz EPR tube joined with a quartz-glass graded seal minimized
sample degradation.
Preparation of [CF3CNSSN]Cl
Trifluoroacetamide (2.0 g, 17.85 mmol) was stirred in CH2Cl2 (50 ml)
and SCl2 (2.8 ml, 35.3 mmol) was added slowly. The mixture was re-
fluxed at ca. 43◦C for 14 h under N2 until no HCl evolution could be
detected. The orange precipitate was filtered, washed (2 × 10 ml of
CH2Cl2), and dried in vacuo. Yield 1.7 g, 45%. +EI-MS 172.9 (M+).
Preparation of [CF3CNSSN]
[CF3CNSSN]Cl (1.00 g, 4.8 mmol) and Zn/Cu couple (0.16 g, 2.45 mmol)
were stirred in SO2 in one limb of a three-limbed reaction vessel for 18 h.
The resultant deep red solution was filtered into the middle-limb and
the SO2 removed to yield a dark solid which was then vacuum trans-
ferred into the third limb (10−2 Torr, −196◦C). It was then sublimed to
yield 1 as a metallic green-black crystalline product. Recovered yield

EPR Studies of the Bistability in CF CNSSN
873
3
0.32 g, 39%. +EI-MS 172.9 (M+); Found C: 13.5%, H: 0.0%, N: 15.3%;
Calc. for 1. C: 13.9%, H: 0.0%, N: 16.2%.
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