Thermal hysteresis in dithiadiazolyl and dithiazolyl radicals induced by
supercooling of paramagnetic liquids close to room temperature: a study
of F3CCNSSN and an interpretation of the behaviour of F3CCSNSCCF3†
Hongbin Du,a Robert C. Haddon,b Ingo Krossing,a Jack Passmore,*a Jeremy M. Rawson*c and
Melbourne J. Schrivera
a Department of Chemistry, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6E2.
E-mail: passmore@unb.ca
b Department of Chemistry and Physics, and, Advanced Carbon Materials Center, University of Kentucky,
Lexington, KY 40506, USA
c
Department of Chemistry, The University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW.
E-mail: jmr31@cus.am.ac.uk
Received (in Cambridge, UK) 14th March 2002, Accepted 20th June 2002
First published as an Advance Article on the web 22nd July 2002
The trifluoromethyl-substituted dithiadiazolyl and dithiazo-
but predict that the planar electrostatic dimer 2g cf. 1g (below)
is thermodynamically favoured. The p*–p* interaction ob-
served in the crystal structure of 2 arises through the additional
electrostatic stabilisation (2 dimers ? tetramer+DH ≈ 250 kJ
mol21). These energies are in qualitative agreement with a
simple electrostatic model using point charges (ESI SUP-
03†).
The melting of 1 (35 °C) is associated with an abrupt increase
in paramagnetism (Fig. 2a) and an extraordinarily large increase
in volume. The volume of 1 is increased by ca. 30% on melting
[r(liquid, 60 °C) = 1.41 g cm23; r(solid, 283–303 K6) = 2.00
g cm23] cf. hydrocarbons 5–20%.7 Larger values are often
associated with dramatic changes of structure on melting e.g.
AlCl3 (83%).7 The melting of 2 is also accompanied by a large
volume increase of 22%, attributed4 to the breakdown of the
p*–p* bonding interaction and an increase in intermolecular
contact to the sum of the van der Waals radii (the expected
increases are ca. 36% for 1 and 28% for 2).
lyl radicals, F3CCNSSN (1) and F3CCSNSCCF3 (2) asso-
ciate through p*–p* covalent and electrostatic Sd+…Nd2
interactions in the solid state, but melt with a dramatic
volume increase to generate paramagnetic liquids; these
radicals exhibit thermal hysteresis, which arises through a
meta-stable super-cooled liquid state, close to room tem-
perature.
The dithiazolyl radical TTTA has recently been shown1 to
exhibit room temperature bistability with thermal hysteresis
between 234 and 317 K. The behaviour arises through a solid–
solid transformation between a low temperature diamagnetic
phase and a high temperature paramagnetic phase.1 Other
examples of this behaviour have been reported in related thiazyl
radicals.2 In this paper we report the observation of thermal
hysteresis in the magnetic properties of the dithiadiazolyl
radical (1) and the dithiazolyl radical (2). In both cases the meta-
stable state arises through super-cooling of the liquid phase.
Samples of 1 and 2 were prepared according to literature
methods (ESI SUP-01†).3,4 The structures of 1 and 2 have been
reported previously,4,5 but are worthy of some comment. The
structure of 1 comprises twisted cofacial dimers (Fig. 1a) with
intradimer S…S contacts of 2.997(2) and 2.978(2) Å. This p*–
p* association leads to spin-pairing, generating a closed shell,
diamagnetic, ground state. The electrostatic Sd+...Nd2 inter-
dimer interactions (Fig. 1a) propagate through the crystal
structure (ESI SUP-02†) and give rise to an extended net-
work.
The abrupt change in susceptibility at the phase transition can
be modelled using the domain model of Sorai.8 Attempts to
1
model the liquid phase behaviour of 1 as either an S = ⁄2 Curie–
Weiss paramagnet or as a pure open-shell dimer, (e.g. 1g)
proved unsuccessful (ESI SUP-05†).
The magnetic data clearly indicate some degree of associa-
tion in the melt either as a p*–p* covalently-bonded dimer such
as 1a (as found in the solid state, see Fig. 1a) or as an
electrostatic dimer e.g. 1g. The large increase in volume implies
that the dimer is the electrostatically bound, planar, 1, rather
than the p*–p* bonded 1a or 1b. However, B86P86/SVP
calculations give favourable dimerisation enthalpies for all
three isomers, but with stability of the dimers increasing in the
order 1g, 1a ≈ 1b (ESI SUP-02†). Other isomers are also
possible.9
The simplest, adequate model, involves an equilibrium
between paramagnetic monomers and an open-shell exchange-
coupled dimer (J = 2260 K) in the liquid phase. The curve-fit
is a little insensitive to the values of DHeqm and DSeqm (required
The isoelectronic radical 2 exists as a diamagnetic tetramer in
the solid state linked via electrostatic contacts and p*–p*
interactions4 [3.097(50), 3.239(36) Å] (Fig. 1b). Theoretical
calculations (ESI SUP-03†) on 2 show that the enthalpy of
dimer formation via either of the closed-shell interactions 2a or
2b, is not favourable, in agreement with solution EPR studies,4
† Electronic supplementary information (ESI) available: Synthesis of 1;
packing diagrams of 1 and 2; ab initio calculations on the association modes
of 1 and 2; estimates of the energy of association of 2 based on a simple
electrostatic model; molecular electrostatic potential maps of 1 and 2 and a
comparison of the charge distribution in 1 and 2 from semi-empirical and ab
initio calculations; curve-fits of the magnetic data of 1 utilising a Curie–
Weiss, exchange-coupled dimer and monomer–dimer equilibrium models
Fig. 1 (a) Asymmetric unit of 1 illustrating the tetrameric motif with dimers
linked via electrostatic S…N interactions: (b) structure of 2 illustrating the
tetrameric motif with dimers linked via electrostatic S…N interactions.
1836
CHEM. COMMUN., 2002, 1836–1837
This journal is © The Royal Society of Chemistry 2002
Du, Hongbin
