Supramolecular architecture, crystal structure and transport properties of the prototypal oxobenzene-bridged bisdithiazolyl radical conductor

Article abstract of DOI:10.1039/c3cc46686h

Supramolecular CH...π interactions cause a ruffling of the otherwise coplanar ribbon-like arrays of radicals in the structure of the oxobenzene-bridged bisdithiazolyl 1a. The material displays a conductivity σ(300 K) = 6 × 10^-3 S cm^-1 (E_act = 0.16 eV) and orders antiferromagnetically below 4 K. At applied fields above 1 kOe the material displays metamagnetic behavior. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc46686h

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Supramolecular architecture, crystal structure and
transport properties of the prototypal oxobenzene-
bridged bisdithiazolyl radical conductor†
Cite this: Chem. Commun., 2014,
50, 785
Received 2nd September 2013,
Accepted 25th September 2013
Joanne W. L. Wong,a Aaron Mailman,a Stephen M. Winter,a Craig M. Robertson,b
Rebecca J. Holmberg,c Muralee Murugesu,c Paul A. Dubed and Richard T. Oakley*a
DOI: 10.1039/c3cc46686h
www.rsc.org/chemcomm
Supramolecular CHꢀ ꢀ ꢀp interactions cause a ruffling of the other-
The overlay of these molecular ribbons gives rise to a variety of
wise coplanar ribbon-like arrays of radicals in the structure of the patterns, including head-over-tail p-stacks A (1d,e),2c,d slipped p-stacks
oxobenzene-bridged bisdithiazolyl 1a. The material displays a con- B (1c),2b and brick wall motifs C (1b).2a While the bandwidth W
ductivity r(300 K) = 6 ꢁ 10ꢂ3 S cmꢂ1 (Eact = 0.16 eV) and orders generated by these packing patterns is insufficient to overcome U,
antiferromagnetically below 4 K. At applied fields above 1 kOe the these materials nonetheless display high conductivity, with s(300 K)
material displays metamagnetic behavior.
in the range of 10ꢂ4 to 10ꢂ2 S cmꢂ1, and with thermal activation
energies Eact = 0.20 to 0.10 eV. In the case of 1b, metallization has
Heterocyclic thiazyl radicals have been widely studied as building been achieved by compression to just 3 GPa.2a Here we report the
blocks for magnetic and conductive materials, in part because the preparation and structural characterization of the prototypal deriva-
presence of sulfur (a heavy heteroatom) enhances intermolecular tive 1a. Despite its small size, the basal proton plays a major role in
magnetic and electronic interactions.1 Within this context determining solid state structure and transport properties.
oxobenzene-bridged bisdithiazolyls 1 represent a new and potentially
powerful building block for the design of molecule-based conductors.2
Their highly delocalized spin distributions help suppress dimerization
and, at the same time, decrease the Mott-Hubbard onsite Coulomb
barrier to charge transfer U.3 From a structural perspective, the crystal
packing of these radicals is strongly influenced by intermolecular
SꢀꢀꢀO0 and SꢀꢀꢀN0 contacts, which serve as supramolecular synthons4
to generate ribbon-like arrays, as in Chart 1.
Scheme 1
To prepare the desired framework, while at the same time avoiding
chlorination of the basal carbon,2b we started with 2,6-diamino-anisole
2,5 which was converted sequentially into the 3,5-bisthiocyanate 3 and
Chart 1
a Department of Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1,
Canada. E-mail: oakley@uwaterloo.ca
b Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK
c Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada
d Brockhouse Institute for Materials Research, McMaster University, Hamilton,
Ontario L8S 4M1, Canada
† Electronic supplementary information (ESI) available: Details of synthetic,
Fig. 1 (a) X-band EPR spectrum of 1a in CS2. (b) CV scan of [1a][ONf] with
Pt electrodes, 0.1 M n-Bu4NPF6 electrolyte, scan rate 100 mV sꢂ1
spectroscopic and magnetic work. CCDC 957990. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc46686h
.
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3,5-dithiol 4 (Scheme 1). Subsequent cyclocondensation of 4 with
thionyl chloride yielded the desired salt [1a][Cl] without any contamina-
tion with [1c][Cl]. Metathesis with silver nonaflate AgONf in MeCN
afforded, after solvent removal, metallic green crystals of [1a][ONf].
Reduction of the latter with octamethylferrocene (OMFc) in MeCN
yielded metallic bronze microcrystals of 1a.
The electronic structure of the radical in solution has been
probed by EPR spectroscopy and cyclic voltammetry (Fig. 1). The
X-band EPR spectrum of 1a in CS2 is consistent with a highly
delocalized spin distribution, consisting of a doublet of pentets
arising from hyperfine coupling to two equivalent 14N nuclei with
aN = 0.354 mT, and to the basal proton with aH = 0.203 mT. Cyclic
voltammetry on a solution of [1a][ONf] in MeCN (Fig. 1b) indicates a
reversible +1/0 couple with E1/2 = +0.061 V vs. SCE and a second
irreversible reduction, corresponding to the ꢂ1/0 couple, with a
cathodic peak potential Epc = ꢂ0.525 V vs. SCE. From the difference
Fig. 3 (a) Tilted-T CHꢀ ꢀ ꢀC0 contacts along the 21 axis. End-on views of
molecular ribbons running perpendicular to the (b) x- and (c) y-directions,
with dihedral angles showing extent of ruffling.
differences, the most notable arising from the presence of edge-to-
face or tilted-T CHꢀꢀꢀp interactions8 along the 21 axes (Fig. 3). This
local herringbone arrangement forces radicals out of coplanarity,
and causes a ruffling of the otherwise planar ribbon-like arrays along
x and y, as indicated by the dihedral angles of 142.41 and 134.81. The
resulting slipped p-stacks, with a plate-to-plate separation of 3.47 Å,
are inclined at an angle of 29.81 to the c-axis.
DC magnetic susceptibility (w) measurements on 1a have been
performed over the temperature range T = 2–300 K. Initial field-cooled
experiments (Fig. 4a) at a field of H = 1 kOe suggest Curie–Weiss
behavior with strong local ferromagnetic interactions; a fit to the data
from 30–300 K affords a C-constant of 0.364 emu molꢂ1 and a large y
value of +15.6 K. Below 10 K, there is a rapid increase in both w and wT,
the latter reaching a maximum value near 9.0 emu K molꢂ1 near 5.0 K.
in the Epc for the two couples we estimate a cell potential Ecell
=
0.56 V, to date the lowest observed for a radical of this type, which
augurs well for a low onsite Coulomb potential U.6
Fig. 2 Unit cell of 1a, with ribbon-like arrays of radicals following the
d-glide planes. Intra-ribbon Sꢀ ꢀ ꢀO0 and Sꢀ ꢀ ꢀN0 connectors are in blue,
CHꢀ ꢀ ꢀC0 contacts along the 21 axes in green, and inter-ribbon Sꢀ ꢀ ꢀS0
contacts in red. The ORTEP drawing and table below specify distances.
Fig. 4 (a) Field-cooled DC susceptibility w of 1a from T = 2–100 K with H = 1 kOe;
insert shows wT vs. T (from 2–30 K). (b) Magnetization (M) vs. T plots at different fields
(H). (c) Overlays of AC susceptibility w0 and w00 (ꢁ10) vs. T plots at 10, 100 and
1000 Hz. (d) Magnetization (M) vs. field (H) plots at different temperatures.
X-ray diffraction analysis of microcrystals of 1a required synchro-
tron radiation methods. The crystal structure‡ belongs to the polar
orthorhombic space group Fdd2 and consists of slipped p-stacks of
At magnetic fields lower than 1 kOe, the susceptibility no longer
evenly spaced radicals running parallel to the c-axis. The unit cell increases continuously with decreasing temperature. Instead, a
drawing in Fig. 2 provides a view of the 16 symmetry related radicals maximum emerges in the w(T) plot near 4 K, indicative of the onset
(Z = 16), and illustrates the ribbon-like motifs generated by the d-glide of antiferromagnetic interactions below this temperature. The results
planes that run perpendicular to the x and y directions. Also shown is of subsequent M(T) measurements (Fig. 4b) over a range of magnetic
the network of intermolecular SꢀꢀꢀO0, SꢀꢀꢀN0, SꢀꢀꢀS0 and CHꢀꢀꢀC0 fields H are consistent with metamagnetic behavior,9 that is, a
contacts (all inside the respective van der Waals separation).7
system which, at low field, possesses an AFM ordered ground state,
While the projection shown in Fig. 2 suggests architectural but which experiences field induced re-alignment at higher applied
similarities with other radicals of this type, there are some important fields, so as to produce an FM ordered state. From the position of the
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maximum in the low field M(T) plots, the ordering temperature TN architecture (Chart 1C) with a well developed 2D electronic
for the AFM state can be estimated near 4 K. The critical field Hc structure.2a By contrast, the role of the CHꢀꢀꢀp supramolecular
required to invert the ordering can be approximated in terms of the synthons in 1a is to disrupt the coplanarity of radicals, to ruffle
turnover point in the M(T) plots, where the AFM maximum at 4 K in the molecular ribbon motif and produce a slipped p-stack architec-
the M(T) curves disappears; on this basis we estimate Hc B1 kOe.
ture with a more 1D electronic structure. As a result the performance
AC susceptibility measurements (Fig. 4c) support this interpretation of 1a, in terms of s(300 K) and Eact, is marginally lower than that of
and provide a more precise measure of the ordering temperature. At all 1b. The material is nonetheless very close to the Mott insulator/metal
cycling frequencies from 0.1 Hz to 1 kHz, the in-phase component w0 transition, and its ruffled ribbon structure is likely more compres-
shows a clear, frequency independent maximum at 4.5 K, which sible12 than the layered sheets of 1b, in which case its pressure
corresponds closely to the value obtained from DC cooling curve induced metallization may be a relatively facile process.
00
experiments. The response of the out-of-phase component w is much
weaker, as would be expected for a low dimensional system.
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support.
The field dependence of the phase transition can be seen more
clearly in isothermal magnetization measurements. As shown in
Fig. 4d, typical paramagnetic behavior is observed above 15 K, but
below this temperature the M(H) curves rise rapidly, indeed far more
quickly than related radicals that undergo field induced FM ordering.10
Also in contrast to other radical-based metamagnets, and despite the
low field required to induce magnetization reversal, the magnetization
is extremely slow to reach the full FM saturation value Msat = 1 mB
expected for a S = 12 system with a nominal g = 2. At T = 2 K, the value of
M(H) is still rising at H = 50 kOe, the limit of our experiments, but has
only reached 0.8 mB, or 80% of the full saturation value. This result is
perhaps not too surprising, when considered in conjunction with the
low value of TN/y = 0.3 and a positive Weiss constant in a material that
displays AFM ordering. Given the complexity of the crystal structure,
and the consequently large number of magnetic exchange interactions,
low dimensional and/or frustrated character to the magnetic response
is likely, and in such circumstances magnetization plateaus are
commonly observed.11
We reported earlier the results of ambient pressure, variable
temperature conductivity (s) measurements on the derivatives
1b–1e.2 Similar measurements, using the four-probe method on
cold-pressed pellets, have now been performed on 1a. Results for
the entire set are illustrated in Fig. 5, in the form of log plots of s
against 1/T. As may be seen, the conductivity of 1a s(300 K) =
6 ꢁ 10ꢂ3 S cmꢂ1, places it near the top of the pack, but its
activation energy, Eact = 0.16 eV, is a little higher than the average.
Notes and references
‡ Crystal data at 296.15 K for 1a: C6H1N2S4 M = 245.33, orthorhombic, a =
27.876(3) Å, b = 31.252(4) Å, c = 4.0004(5) Å, V = 3485.2(7) Å3, space group
Fdd2 (#42), Z = 16, Dcalcd = 1.870 g cmꢂ3, m = 1.042 mmꢂ1, flack = 0.06(8);
122 parameters were refined using 1873 unique reflections (Rint = 0.0235)
to give R = 0.0235 and Rw = 0.524 (observed data).
1 (a) J. M. Rawson, A. Alberola and A. Whalley, J. Mater. Chem., 2006,
16, 2560; (b) R. G. Hicks, in Stable Radicals: Fundamentals and Applied
Aspects of Odd-Electron Compounds, ed. R. G. Hicks, John Wiley & Sons,
Ltd., Wiltshire, 2010, p. 317; (c) I. Ratera and J. Veciana, Chem. Soc. Rev.,
2012, 41, 303; (d) R. C. Haddon, ChemPhysChem, 2012, 13, 3581;
´
(e) R. T. Boere and T. L. Roemmele, Comp. Inorg. Chem. II, 2013, vol. 1,
p. 375; ( f ) J. M. Rawson, J. J. Hayward, Handbook of Chalcogen Chemistry:
New Perspectives in Sulfur, Selenium and Tellurium, 2013, vol. 2, p. 69;
(g) A. W. Cordes, R. C. Haddon and R. T. Oakley, Phosphorus, Sulfur Silicon
Relat. Elem., 2004, 179, 673; (h) K. V. Shuvaev and J. Passmore, Coord.
Chem. Rev., 2013, 257, 1067; (i) K. E. Preuss, Dalton Trans., 2007, 2357.
2 (a) A. Mailman, S. M. Winter, X. Yu, C. M. Robertson, W. Yong, J. S. Tse,
R. A. Secco, Z. Liu, P. A. Dube, J. A. K. Howard and R. T. Oakley, J. Am.
Chem. Soc., 2012, 134, 9886; (b) X. Yu, A. Mailman, K. Lekin, A. Assoud,
C. M. Robertson, B. C. Noll, C. F. Campana, J. A. K. Howard, P. A. Dube
and R. T. Oakley, J. Am. Chem. Soc., 2012, 134, 2264; (c) X. Yu, A. Mailman,
K. Lekin, A. Assoud, P. A. Dube and R. T. Oakley, Cryst. Growth Des., 2012,
12, 2485; (d) X. Yu, A. Mailman, P. A. Dube, A. Assoud and R. T. Oakley,
Chem. Commun., 2011, 47, 4655.
3 (a) N. F. Mott, Proc. Phys. Soc., London, Sect. A, 1949, 62, 416;
(b) J. Hubbard, Proc. R. Soc. London Ser. A, 1963, 276, 238.
4 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311.
5 (a) N. J. Clecak, S. Jose, R. J. Cox, L. Gatos, S. L. Solar, S. Jose and
H. K. Wurster, US Pat., 3489558, 1970; (b) A. Banerjee and
I. N. Ngwendson, US Pat., 20070179311 A1, 2007.
´
6 R. T. Boere and T. L. Roemmele, Coord. Chem. Rev., 2000, 210, 369.
7 (a) A. J. Bondi, J. Phys. Chem., 1964, 68, 441; (b) I. Dance, New
J. Chem., 2003, 27, 22.
8 (a)S.Paliwal,S.GeibandC.S.Wilcox,J. Am. Chem. Soc., 1994, 116, 4491; (b)P.
Hobza, H. L. Selzle and E. W. Schlag, J. Am. Chem. Soc., 1994, 116, 3500;
(c) A. Gavezzotti and G. R. Desiraju, Acta Crystallogr., 1988, B44, 427;
(d) J. Bernstein, J. A. R. P. Sarma and A. Gavezzotti, Chem. Phys. Lett., 1990,
174, 361; (e) S. E. Wheeler and K. N. Houk, Mol. Phys., 2009, 107, 749.
9 J. M. D. Coey, Magnetism and Magnetic Materials, Cambridge
University Press, 2010.
10 (a) S. M. Winter, K. Cvrkalj, P. A. Dube, C. M. Robertson,
M. R. Probert, J. A. K. Howard and R. T. Oakley, Chem. Commun.,
2009, 7003; (b) W. Fujita, K. Takahashi and H. Kobayashi, Cryst.
Growth Des., 2011, 11, 575; (c) H. Nagashima, S. Fujita, H. Inoue and
N. Yoshioka, Cryst. Growth Des., 2004, 4, 19; (d) T. Ishida,
K. Tomioka, T. Nogami, H. Yoshikawa, M. Yasui, F. Iwasaki,
N. Takeda and M. Ishikawa, Chem. Phys. Lett., 1995, 247, 7.
11 (a) G. Catilla, S. Chakravarty and V. J. Emery, Phys. Rev. Lett., 1995,
75, 1823; (b) M. Hase, H. Kuroe, K. Ozawa, O. Suzuki, H. Kitazawa,
G. Kido and T. Sekine, Phys. Rev. B: Condens. Matter Mater. Phys.,
2004, 70, 104426; (c) M. Kumar, S. E. Dutton, R. J. Cava and
Z. G. Soos, J. Phys.: Condens. Matter, 2013, 25, 136004.
Fig. 5 Plots of log s versus 1/T for radicals 1a–e; data for 1c correspond
to the MeCN adduct. Data for 1b–1e are from ref. 2.
Strong lateral FꢀꢀꢀS0 contacts in 1b, in combination with SꢀꢀꢀO0
and SꢀꢀꢀN0 interactions, link ribbons of radicals into rigorously
coplanar sheets of radicals, which gives rise to a brick wall
12 K. Lekin, A. A. Leitch, J. S. Tse, X. Bao, R. A. Secco, S. Desgreniers,
Y. Ohishi and R. T. Oakley, Cryst. Growth Des., 2012, 12, 4676.
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