Crystal structures, EPR and magnetic properties of 2-ClC6H 4CNSSN and 2,5-Cl2C6H3CNSSN

Article abstract of DOI:10.1039/c0cc04296j

The β-sheet structure associated with chlorinated aromatics (d _Cl...Cl ≈ 4.0 ) has been implemented to drive formation of π-stacked structures of dithiadiazolyl radicals. Both title compounds exhibit an increase in paramagnetism above 150 K but

Full text of DOI:10.1039/c0cc04296j

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COMMUNICATION
ꢀ
Crystal structures, EPR and magnetic properties of 2-ClC H CNSSN
4
6
ꢀ
and 2,5-Cl C H CNSSN w
2
6 3
a
b
c
c
Antonio Alberola, Emma Carter, Christos P. Constantinides, Dana J. Eisler,
cd
b
Damien M. Murphy and Jeremy M. Rawson*
Received 8th October 2010, Accepted 14th December 2010
DOI: 10.1039/c0cc04296j
The b-sheet structure associated with chlorinated aromatics
˚
(
d
Clꢁ ꢁ ꢁCl E 4.0 A) has been implemented to drive formation of
p-stacked structures of dithiadiazolyl radicals. Both title
compounds exhibit an increase in paramagnetism above 150 K
but solid-state EPR studies indicate that the origin of the
paramagnetism in these two systems is different.
There has been considerable interest in thiazyl radicals and their
selenium analogues as building blocks for the construction of
1
magnetic materials. One family of radicals whose magnetic
properties have attracted some attention are the 1,2,3,5-dithia-
2,3
diazolyl (DTDA) radicals (1). Previous solution EPR and more
recent UV/vis studies on derivatives of 1 have revealed a very
Fig. 1 p*–p* modes of association in 1,2,3,5-dithiadiazolyl radicals.
ꢂ1
4
favourable dimerisation enthalpy (ca. 35 kJ mol ). As a
consequence the majority of derivatives have been found to adopt
one of the p*–p* dimer motifs (Fig. 1, A–D) which effectively
quenches their paramagnetism in the solid state.
crystal-engineering principles to weaken the dimerisation
process in the solid state and to probe the resultant effects on
the electronic structure. Chlorinated aromatics have a tendency
to adopt a p-stacked b-sheet structure in which the short-
5
˚
axis (corresponding to the p-stacking direction) is ca. 4 A,
substantially longer than the typical intra-dimer contacts in
6
˚
derivatives of 1 (2.9–3.1 A). If the strength of the ClꢁꢁꢁCl
interaction is of comparable magnitude to the dimerisation energy
then weakening or cleavage of the p*–p* dimer is anticipated. In
this communication we report the synthesis, structures and
magnetic properties of two chlorinated derivatives, 2 and 3, both
of which adopt p-stacked chloro-aryl rings. In both cases SQUID
magnetometry reveals the onset of paramagnetism above 150 K
and EPR studies reveal two differing microscopic mechanisms for
the increase in bulk paramagnetism; thermal population of an
excited triplet state associated with the p*–p* dimer (in 3) and
breakdown of the p*–p* dimer (in 2) to generate increasing
In a small number of instances dimerisation has been
suppressed and can lead to long range magnetic order in some
2
derivatives. We have been particularly interested in utilising
a
Dep. De Quimica Fisica I Analitica-Quimica Fisica,
Universitat Jaume I, Av. De Vicent Sos Baynat s/n,
E-12071 Castello de la Plana, Spain
The School of Chemistry, Cardiff University, Main Building,
b
numbers of S = 1 radical centres.
Park Place, Cardiff, UK CF10 3AT
Department of Chemistry, The University of Cambridge,
2
c
Radicals 2 and 3 were prepared from the corresponding
2
Lensfield Road, Cambridge, UK CB2 1EW
Dept of Chemistry and Biochemistry, The University of Windsor,
d
aromatic nitriles according to standard experimental methods
ꢂ1
and were purified by vacuum sublimation (100–70 1C, 10 Torr)
401 Sunset Av., Windsor, ON, Canada N9B 3P4.
E-mail: jmrawson@uwindsor.ca
to yield 2 and 3 as black lustrous blocks and needles
respectively (see SUP-01).z Both 2 and 3 adopt structures in
which the chloro-aromatics adopt the anticipated p-stack
motif (see SUP-02) with Clꢁ ꢁ ꢁCl contacts in the range
w Electronic supplementary information (ESI) available: Preparation
and crystallographic details for 2 and 3; discussion of the crystallo-
graphic studies on 3; figures of the p-stacked structures of 2 and 3;
analysis of geometries of cisoid p*–p* dimers; reported Clꢁ ꢁ ꢁCl con-
tacts in chlorinated p-conjugated planar organics; representative low T
˚
.693(3)–4.100(3) A. However the packing of the heterocyclic
3
EPR spectrum of 2 illustrating S = 1 defect sites; temperature-
2
rings is significantly different.
dependence of the EPR intensity of 3. CCDC 796396–796397. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0cc04296j
Radical 2 crystallised in the monoclinic space group Pc with
four molecules in the asymmetric unit, each of which have
2
532 Chem. Commun., 2011, 47, 2532–2534
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5
–300 K in both warming and cooling modes. Data were
corrected for diamagnetism. No significant differences in
sample susceptibility were observed between heating or
cooling modes. The temperature dependence of wT vs. T for
2
and 3 is presented in Fig. 4. Both materials are essentially
diamagnetic below 150 K although a small Curie-tail was
apparent upon cooling to low temperature. This was assigned
to trace defects (0.7% for 2 and 0.22% for 3) of DTDA
radicals in the lattice and confirmed by low temperature
EPR studies. On warming from 150 K up to room temperature
a steady increase in wT was observed. At 300 K the para-
magnetism corresponds to 13% S = 1 Curie spins for 2 and
Fig. 2 The two crystallographically independent molecules in the
structure of 2; (a) conventional cisoid p*–p* dimer; (b) orthogonal
p*–p* dimer; and (c) the SOMO–SOMO bonding interaction between
radicals in the orthogonal dimer pictured with a contour value of
2
1
5% S = 1 Curie spins for 3.
2
Variable temperature X-band EPR studies (5–300 K) on
polycrystalline solid samples of 2 and 3 revealed a small
number of S = 1 radical defect centres in the lattice at low
0.035 au.
2
temperatures which were also apparent in the SQUID data.
These were readily modelled as isolated DTDA radicals by
conventional intramolecular dimensions. These four molecules
associate in the solid state to generate two dimers (Fig. 2a and b);
one of type A and one containing the unprecedented
orthogonal dimerisation mode, E (Fig. 1). At 180 K, the
4
b,8
comparison with previous studies
and analogous spin
= 2.002, g = 2.008,
z x y z
g = 2.021, A = 14, A 4 1, A 4 1 G; see SUP-06).
Hamiltonian parameters were observed (g
x
y
N
N
N
˚
Sꢁ ꢁ ꢁS distances [3.160(3) A] in dimer A fall at the longer end
However the temperature evolution of their EPR spectra
above 150 K is significantly different and reveals alternative
pathways for the increase in paramagnetism in these two
materials.
˚
of the conventional range for type A dimers [3.09(6) A,
SUP-03]. The orthogonal dimer E has no precedent in DTDA
chemistry but still offers the potential for a net p*–p* bonding
interaction between singly-occupied MO’s (Fig. 2c) with
˚
intradimer Sꢁ ꢁ ꢁS distances of 3.288(3) and 3.240(3) A, and
˚
Sꢁ ꢁ ꢁN distances of 3.274(6) and 3.474(7) A.
A similar perpendicular p*–p* interaction has been observed
in the isoelectronic diselenadiazolyl radicals, albeit with the
two heterocycles oriented somewhat differently, i.e. with the
Se–Se bond bridging the two Se–N bonds rather than Se–Se
7
and Nꢁ ꢁ ꢁN edges.
The structure of 3 proved difficult to determine satisfactorily
and an ordered structure could only be determined when using
a super-cell (see SUP-04). Radical 3 crystallises in the triclinic
ꢀ
space group P1 with two cisoid dimers of type A (Fig. 3) in the
asymmetric unit. The heterocyclic ring geometry is unexceptional
˚
but the intra-dimer Sꢁ ꢁ ꢁS distances [3.156(6)–3.268(5) A]
are notably longer than those observed in other cisoid dimers
˚
of this type [3.09(6) A, SUP-03] indicative of some weakening
of the p*–p* dimer interaction. The inter-dimer Sꢁ ꢁ ꢁS
˚
contacts are somewhat longer [4.075(5)–4.185(6) A] though
notably the dichlorophenyl rings are almost evenly spaced
˚
[
3.619(4)–3.710(4) A] consistent with the preferred b-sheet
˚
structure (3.77–4.02 A, see SUP-05).
Magnetic studies on both 2 and 3 were undertaken on a
Quantum Design SQUID magnetometer in an applied field of
1
000 G for radical 2 and 5000 G for radical 3 in the range
Fig. 4 Temperature dependence of wT for 2 (top) and 3 (bottom)
in the range 5–300 K. The solid line (bottom) corresponds to a
Bleaney–Bowers model (see text).
Fig. 3 One of the two crystallographically independent cisoid dimers
in the asymmetric unit of (3)
2
.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2532–2534 2533

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Fig. 6 (left) Closed-shell singlet ground state afforded for strongly
interacting p*–p* dimers; (right) thermally accessible triplet excited
states for weakly bonded p*–p* dimers.
Our EPR studies reveal different microscopic origins for the
observed paramagnetism. A detailed study of these and other
chlorophenyl derivatives will be the subject of a full paper.
Notes and references
Fig. 5 Solid state X-band EPR spectra of (a) 2 and (b) 3 at 220 K.
Inset shows the half-field resonance associated with the formally
spin-forbidden S = 1 term (relative EPR intensities are ꢄ600 for
z Crystal data for 2: C
7 4 2 2
H ClN S , M = 215.69, monoclinic, Pc,
a = 9.5948(19), b = 14.020(3), c = 12.332(3) A, b = 91.24(3)1,
˚
3
˚
V = 1658.6(6) A , m(Mo-Ka) = 0.899, T = 180(2) K, Z = 8,
ꢂ3
D
R
c
= 1.728 mg m , F(000) = 872, independent reflections 4388
int = 0.056). The structure was solved by direct methods and refined
2
and ꢄ1 for 3).
(
by full-matrix least-squares on F using the SHELXTL program
2
In 2 there is a steady increase in intensity of the spectrum in
10
package.
(I 4 2s(I)) = 0.046, wR
ꢀ
2 2 2
Crystal data for 3: C Cl N S , M = 250.13, triclinic, P1, a =
Non-hydrogen atoms were anisotropically refined.
the g = 2 region above 150 K (a double integral of the EPR
signal follows the behaviour of the w vs. T data from SQUID
magnetometry; see SUP-07). Whilst there is some dipolar
broadening associated with the increase in the number of
paramagnetic centres generated, no additional features are
observed (Fig. 5a). Conversely the EPR spectrum of 3
above 150 K shows clear evidence for the presence of a triplet
state reflected in (i) additional features attributable to zero-
field splitting and (ii) the observation of the forbidden
R
1
2
(all data) = 0.133, S = 1.164 (all data).
7
H
3
˚
7.3271(2), b = 10.3563(3), c = 24.6666(7) A, a = 88.096(2), b =
3
˚
8
T = 180(2) K, Z = 8, D
1.458(2), g = 77.0090(10)1, V = 1803.60(9) A , m(Mo-Ka) = 1.127,
ꢂ3
c
= 1.842 mg m , F(000) = 1000,
independent reflections 5799 (Rint = 0.0395). The structure was solved
2
by direct methods and refined by full-matrix least-squares on F using
10
the SHELXTL program package.
anisotropically refined. R (I 4 2s(I)) = 0.049, wR
S = 1.050 (all data).
Non-hydrogen atoms were
1
2
(all data) = 0.112,
1
2
A. Alberola, J. M. Rawson and A. L. Whalley, J. Mater. Chem.,
006, 16, 2560; K. Preuss, Dalton Trans., 2007, 2357.
DM
spin Hamiltonian parameters for this S = 1 species were
= 2.002, g = 2.008, g = 2.021; |D| = 0.0183,
E| = 0.0008 cm . Fig 5b also includes a contribution from
s
= ꢃ2 transition in the half-field region (Fig 5b). The
2
A. J. Banister, N. Bricklebank, I. Lavender, J. M. Rawson,
C. I. Gregory, B. K. Tanner, W. Clegg, M. R. J. Elsegood and
F. Palacio, Angew. Chem., Int. Ed. Engl., 1996, 35, 2533;
A. Alberola, R. J. Less, C. M. Pask, J. M. Rawson, F. Palacio,
P. Oliete, C. Paulsen, A. Yamaguchi, R. D. Farley and
D. M. Murphy, Angew. Chem., Int. Ed., 2003, 42, 4782.
g
x
y
z
ꢂ1
|
a rhombic S = 1 EPR spectrum associated with isolated
2
DTDA radicals (see above). The behaviour of 3 is consistent
with an S = 0 ground state with thermally accessible triplet
9
3 G. Antorrena, J. E. Davies, M. Hartley, F. Palacio, J. M. Rawson,
J. N. B. Smith and A. Steiner, Chem. Commun., 1999, 1393;
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S. I. Pascu and J. M. Rawson, Chem. Commun., 2005, 4726.
4 (a) J. Britten, N. G. R. Hearns, K. E. Preuss, J. F. Richardson and
S. Bin-Salomon, Inorg. Chem., 2007, 46, 3934; (b) S. A. Fairhurst,
K. M. Johnson, L. H. Sutcliffe, K. F. Preston, A. J. Banister,
Z. V. Hauptman and J. Passmore, J. Chem. Soc., Dalton Trans.,
1986, 1465; P. S. White, A. J. Banister, H. Oberhammer and
L. H. Sutcliffe, Chem. Commun., 1987, 66; A. J. Banister and
J. Passmore, et al., Chem. Commun., 1987, 66; W. V. F. Brooks,
N. Burford, J. Passmore, M. J. Schriver and L. H. Sutcliffe, Chem.
Commun., 1987, 69.
configuration (Fig. 6). A fit of both the w vs. T data and EPR
signal intensity to the Bleaney–Bowers equation up to
2
2
30 K provides an estimate of the singlet–triplet separation
J/k E ꢂ1300 K (comparable with previously reported
9
dithiadiazolyl radicals with 2J/k E ꢂ2400 K). Above 230 K,
both w and the EPR signal intensity rise more steeply than the
model predicts, consistent with small thermal expansion of
the intradimer Sꢁ ꢁ ꢁS distance leading to a weakening of the
exchange coupling. An improved fit (up to 280 K) can
be achieved using
a small temperature-dependence of
2
J [J = ꢂ880 + 0.005T (Fig. 4)].
5 Crystal Engineering: The Design of Organic Solids, G. R. Desiraju,
Materials Science Monographs, 54, Elsevier Press, 1989.
The paramagnetism in 2 may be due to the breakdown of
1
6
J. M. Rawson, A. J. Banister and I. Lavender, Adv. Heterocycl.
Chem., 1995, 62, 137.
7 N. Feeder, R. J. Less, J. M. Rawson, P. Oliete and F. Palacio,
the dimer E, generating S = radicals. Conversely, whilst
2
both 2 and 3 possess dimers of type A, the shorter mean
intradimer SꢁꢁꢁS contact in 2 (cf. the dichlorophenyl derivative 3)
may lead to stronger bonding and a less accessible triplet
configuration. Thus the paramagnetism in 2 appears to arise
Chem. Commun., 2000, 2449.
8
S. A. Fairhurst, L. H. Sutcliffe, K. F. Preston, A. J. Banister,
A. S. Partington, J. M. Rawson, J. Passmore and M. J. Schriver,
Magn. Reson. Chem., 1993, 31, 1027.
1
2
from formation of S = states whereas the paramagnetism in
9 A. Decken, T. S. Cameron, J. Passmore, J. M. Rautiainen,
R. W. Reed, K. V. Shuvaev and L. K. Thompson, Inorg. Chem.,
3
appears due to thermal population of a triplet configuration.
The current studies reveal that partial chlorination of the
2007, 46, 7436; K. V. Shuvaev, A. Decken, F. Grein, T. S. M. Abedin,
L. K. Thompson and J. Passmore, Dalton Trans., 2008, 4029.
0 G. M. Sheldrick, SHELXTL Crystallographic suite for PC, Siemens
Analytical Instrument Division, Madison, WI, USA, 1997.
aromatic substituent is sufficient to weaken the conventional
1
p*–p* dimerisation process associated with DTDA radicals.
2
534 Chem. Commun., 2011, 47, 2532–2534
This journal is c The Royal Society of Chemistry 2011