Structure and magnetic properties of a sulfur-nitrogen radical, methylbenzodithiazolyl

Article abstract of DOI:10.1039/b004992l

An X-ray crystal structure of the methylbenzodithiazolyl radical MeC₆H₃S₂N, MBDTA, reveals that it forms a regularly spaced π-stack motif in the solid state; variable temperature magnetic studies show that the susceptibili

Full text of DOI:10.1039/b004992l

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J O U R N A L O F
Structure and magnetic properties of a sulfur±nitrogen radical,
methylbenzodithiazolyl{
a
Gordon D. McManus, Jeremy M. Rawson,* Neil Feeder, Fernando Palacio* and
a
a
b
b
Patricia Oliete
C H E M I S T R Y
a
Department of Chemistry, The University of Cambridge, Lens®eld Road, Cambridge, UK
CB2 1EW. E-mail: jmr31@cam.ac.uk
Instituto de Ciencia de Materiales de Aragon, CSIC-Universidad de Zaragoza, Zaragoza,
Spain E-50009
b
Received 22nd June 2000, Accepted 12th July 2000
Published on the Web 4th August 2000
An X-ray crystal structure of the methylbenzodithiazolyl
radical MeC N, MBDTA, reveals that it forms a
regularly spaced p-stack motif in the solid state; variable
temperature magnetic studies show that the susceptibility
passes through a broad maximum at 140 K indicative of
exceptionally strong antiferromagnetic coupling for an
Crystals of MBDTA suitable for single crystal§ X-ray studies
were formed as black rhombs by vacuum sublimation
6
3 2
H S
22
(10 Torr, 50±55 ³C), just below the decomposition temperature
60 ³C). The asymmetric unit of MBDTA contains a single
molecule of unexceptional geometry (Fig. 1) with the fused ring
(
4
essentially planar (maximum deviation is for the heterocyclic N
organic
radical
possessing
extended
magnetic
Ê
which lies 0.035 A above the mean plane). Molecules of MBDTA
form a herringbone array with p-stacking along the crystal-
interactions. Its behaviour can be modelled as a two-
dimensional Heisenberg square lattice of S~1/2 ions with
J~272 K.
5
lographic c-axis in a similar manner to NDTA rather than the
6
slipped p-stacks observed for BBDTA, QDTA and TDP-DTA.
The distance between equivalent atoms in neighbouring rings
1
W oÈ lmershauser ®rst described the syntheses and characterisa-
Ê
in the stack coincides with the length of the c-axis [5.851(3) A],
but the molecular plane is inclined at 60.7³ to the ab plane and
tion of the benzo-fused dithiazolyl radicals BDTA and
MBDTA in 1984, and reported that both BDTA and
MBDTA showed no tendency to dimerise in the solid state.
the closest contact (d
3
in Fig. 2) along the stacking direction is
.742 A. A view of MBDTA in the ab plane is shown in Fig. 3.
In addition to the intra-stack contacts, there are a series of
Ê
3
2
Whilst solution EPR studies by Passmore et al. on BDTA
2
1
determined the dimerisation energy to be y0 kJ mol , their
crystallographic study showed that BDTA crystallised as a
¼
Ê
inter-stack S S contacts in the range 3.708±3.819 A (Fig. 2).
2
¼
All of these intermolecular S S contacts fall outside the sum
7
of the minor van der Waals radii for S (y3.2 A), but within
the sum of the major van der Waals radii (y4.0A). These
intermolecular contacts give rise to a two-dimensional sheet of
interactions in the crystallographic bc plane.
¼
centrosymmetric dimer in the solid state with an S
S
Ê
Ê
separation of 3.175(1) A. The two unpaired electrons form a
multi-centre bonding interaction via a p*±p* interaction
between singly occupied molecular orbitals (SOMOs), thereby
Ê
2
rendering the compound diamagnetic. Nevertheless, the small
dimerisation energy indicates that a ®ne balance is to be
expected between monomeric and dimeric structures. We now
report the structure and magnetic properties of MBDTA.
1
MBDTA was prepared according to the literature method
Scheme 1), although the ®nal reduction step proved proble-
(
matic and was dependent on reducing agent and solvent. The
optimum recovered yield (11% based on 3,4-dimercapto-
toluene) used Ag powder in MeCN.{ Similar problems have
been encountered by Oakley et al. during the synthesis of the
3
benzobis(dithiazolyl), BBDTA.
Variable temperature dc magnetic susceptibility studies (1.8±
3
00 K) were carried out on a polycrystalline sample of
MBDTA on a SQUID magnetometer in an applied ®eld of
T. Additional isothermal magnetisation measurements were
1
made at 1.8 and 3 K. Diamagnetic corrections were made for
the sample holder and sample (Pascal's constants). A plot of
{
Analytical data for MBDTA: Calcd for C
N, 8.3%; Found C, 49.2%; H, 3.6%; N, 8.3%; EPR (CH
g~2.003, a ~11.4 G.
Crystal data for MBDTA: C
space group Pbca, a~16.778(6), b~14.757(4), c~5.851(3) A, Z~8,
7
H
6
NS
2
: C, 50.0%; H 3.6%;
2
2
Cl , 298 K),
N
Scheme 1 Reagents and conditions: (i) SO
2 2 2 2
Cl , re¯ux CH Cl ; (ii)
§
7 6 2
H NS , M~168.25, orthorhombic,
Me SiN , CH Cl , 0 ³C; (iii) Ag powder, MeCN.
3
3
2
2
Ê
2
1
T~180(2) K, m(Mo-Ka)~0.645 mm . Of 2065 re¯ections measured,
268 were unique (Rint~0.0479) and were used in all calculations. The
®nal wR ~0.1315 (all data), R
1
{A 3D representation of MBDTA as a PDB ®le is available as
supplementary data. For direct electronic access see http://
www.rsc.org/suppdata/jm/b0/b004992l/
2
1
[Fw2s(F)]~0.0523. CCDC 1145/235.
See http://www.rsc.org/suppdata/jm/b0/b004992l/ for crystallographic
®les in .cif format.
DOI: 10.1039/b004992l
J. Mater. Chem., 2000, 10, 2001±2003
This journal is # The Royal Society of Chemistry 2000
2001

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Ê
Fig. 1 Selected intramolecular bond lengths (A) and angles (³) for
MBDTA are: S(1)±N(1) 1.653(3), S(1)±C(1) 1.743(4), S(2)±N(1)
1
9
.656(4), S(2)±C(6) 1.740(4), C(1)±C(6) 1.394(5), N(1)±S(1)±C(1)
9.7(2), N(1)±S(2)±C(6) 99.9(2), S(1)±N(1)±S(2) 113.8(6), C(6)±C(1)±
S(1) 113.43(3), C(1)±C(6)±S(2) 113.2(2).
the molar susceptibility vs. temperature (Fig. 4) exhibits a
broad maximum at 140 K. The absence of any ®eld dependent
magnetisation below 140 K indicates that the maximum in x is
associated with the onset of low dimensional antiferromagnetic
order. This exceptionally high temperature for xmax is
indicative of very strong antiferromagnetic exchange interac-
tions between spins and this is re¯ected in the room
temperature effective magnetic moment [ca. 1.3mB at 300 K]
which is considerably less than that expected for an S~1/2
Fig. 3 Molecular packing of MBDTA viewed in the ab plane,
illustrating the strongly slipped p-stack along the crystallographic c-
axis and the large separation of dithiazolyl-rich planes along the
crystallographic a-axis.
tion and the (positive) contribution from the antiferromagnetic
correlations.
B
paramagnet [ca. 1.7m ]. Below ca. 25 K the susceptibility curve
The low dimensionality of the magnetic structure is
consistent with the two-dimensional nature of the dithiazolyl
interactions in the crystal structure and the compound was
analysed as a two dimensional Heisenberg system, using high
temperature series expansions for the square planar Heisenberg
increases rather sharply most likely due to the contribution
from a small mole fraction of non correlated paramagnetic
centres in the sample. The paramagnetic origin of such an
increase is con®rmed by the isothermal magnetisation mea-
surements at 1.8 and 3.0 K which can be ®tted to the Brillouin
function corresponding to 0.8¡0.1% of S~1/2 paramagnetic
centers plus a term which depends linearly on the magnetic
8
model. A good agreement between experimental and calcu-
lated data was observed with an exchange term, J~272 K. The
adequacy of the ®t was re¯ected in the excellent comparison
between theoretical and experimental values of both the
®eld. This term combines the (negative) diamagnetic contribu-
m B m
position, t ~k Tmax/|J|S(Sz1), and height, x ~xmax|J|/
2
N g m , of the maximum of the susceptibility in reduced
2
B
units (Fig. 5). In the square planar Heisenberg model the
theoretical values for these constants are, respectively, 2.495
8
and 0.0469 for S~1/2. These values are in good agreement
m m
with the calculated values t ~2.578 and x ~0.0466, using |J|/
B
k ~72 K and g~2.
Fig. 4 Variation of molar susceptibility of MBDTA ($) as a function
of temperature. The solid line in the high temperature region represents
the best ®t to the two-dimensional square-planar Heisenberg model,
with J~272(¡1) K; in the low temperature region the solid line
represents the best ®t to the presence of non-correlated paramagnetic
centres in the 2D system (see text). The inset shows the Curie behaviour
of the paramagnetic centres.
Fig. 2 Molecular packing diagram of MBDTA viewed perpendicular
¼
to the crystallographic a axis. Selected intermolecular S S contacts
Ê
~3.719, d ~3.742, d ~3.768, d ~3.819 A.
are: d
1
~3.708, d
2 3 4 5
2002
J. Mater. Chem., 2000, 10, 2001±2003

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anisotropy antiferromagnet will normally show a transition
from the antiferromagnetic to the spin±¯op state in the
isothermal magnetisation curve.
Whilst the fused nature of the BDTA and MBDTA ring
systems provides an opportunity for p-delocalisation of the
spin density away from the heterocyclic ring, theoretical
2
calculations have shown that the majority of the spin density
is still localised on the S and N atoms. Consequently the main
pathway for magnetic exchange can be considered to be via the
¼
¼
intermolecular S N and S S interactions already described.
This gives rise to a two-dimensional sheet structure in the bc
plane; propagation along the crystallographic a direction relies
¼
Ê
on N H interactions [at 3.7 A to the H atom attached to C(4)]
and the degree of spin delocalisation onto the C ring (Fig. 3).
6
It is, presumably, the weakness of this third interaction which
precludes the onset of long range magnetic order. We are
presently examining new derivatives in which more extensive p-
delocalisation onto the substituents is anticipated, thereby
paving the way towards long range magnetic order at high
temperature.
Fig. 5 Experimental temperature dependence of the susceptibility, in
reduced units, of MBDTA ($) compared to representative examples of
the S~1/2 square planar Heisenberg antiferromagnetic model;
[
C
(
Cu(C
5
H
5
NO)
CuBr
)][PF ] (r), [Cu(pz)
6
(BF
(#), (2-NH
][ClO
4
)
2
]
(©), CuF
2
?2H
2
O
(%), (2-NH
2
-5-Cl-
2
CuBr (), [Cu(pz) -
5
H
5
N)
2
4
2
-5-Me-C
]
5
H
5
N)
2
4
NO
3
6
2
4
2
(,). Data are scaled to the high
temperature series prediction (solid line).
Acknowledgements
The two-dimensional magnetic behaviour of MBDTA
compares very well with other representative examples of the
S~1/2 square planar Heisenberg model. In Fig. 5 the magnetic
susceptibility of MBDTA, corrected for diamagnetic and
We would like to thank the Royal Society for an equipment
grant (J.M.R.), the EPSRC for a studentship (G.D.M.) and the
CICYT (Grant. No. MAT97-0951) for ®nancial support.
uncorrelated paramagnetic contributions, is represented in
8
reduced units together with that of Cu(C
5
H
5
NO)
(2-NH
4 2
and Cu(pz) (ClO )
6 4 2
(BF ) ,
8
O, (2-NH
9,10
CuF
C
2
?2H
N) CuBr
2
2
-5-Cl-C
[Cu(pz) (NO
5
H
3
N)
)]PF
2
CuBr
4
,
2
-5-Me-
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1
antiferromagnet will order at T~0 K.
However, small
deviations from this model can trigger magnetic ordering.
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dimensionality to the 2D Ising model which can undergo
magnetic ordering at a ®nite temperature. Magnetic interac-
tions between layers would cause a crossover in the lattice
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3
4
5
(
susceptibility at Tmax~1.9 K and it orders at T ~0.62 K
T
max). Thus, Cu(C
5 5 6 4 2
H NO) (BF ) shows the maximum of the
6
1
2
N
with a ratio T
(
Cl-C
N
/Tmax~0.33. Other examples are CuF
2
?2H
2
O
1
Tmax~26 K, T ~10.9 K and T /T ~0.42), (2-NH -5-
3
N
N
max
2
9
H
5 3
N)
2
CuBr
4
(Tmax~1.06 K,
and (2-NH -5-Me-C H N) CuBr
~0.44 K and T
T
N
~0.74 K and
T
N
/
7
8
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Tmax~0.70)
max~0.82 K, T
2
5
3
2
4
9
(
T
N
N
/Tmax~0.54). In the case
of MBDTA, despite the very large exchange interaction
between radicals, long range order does not seem to become
apparent down to 1.8 K, the lowest temperature measured
during these experiments. The magnetic ordering transition in
x(T) measurements is normally manifested as a kink in the x(T)
curve, but the contribution from uncorrelated paramagnetic
centres renders its observation dif®cult in this instance. In
addition, M(H) measurements provide no indication of the
presence of antiferromagnetic ordering at 1.8 K either. A low
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J. Mater. Chem., 2000, 10, 2001±2003
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