The first semiquinone-bridged bisdithiazolyl radical conductor: A canted antiferromagnet displaying a spin-flop transition

Article abstract of DOI:10.1039/c1cc10598a

The alternating ABABAB π-stacked bis-1,2,3-dithiazolyl radical 2a (2, R₂ = Ph) has a conductivity σ of 3 × 10^-5 S cm^-1 at 300 K, and orders as a spin-canted antiferromagnet (T _N = 4.5 K) which undergoes a spin-f

Full text of DOI:10.1039/c1cc10598a

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The first semiquinone-bridged bisdithiazolyl radical conductor: a canted
antiferromagnet displaying a spin-flop transitionw
Xin Yu,a Aaron Mailman,a Paul A. Dube,b Abdeljalil Assouda and Richard T. Oakley*a
Received 31st January 2011, Accepted 1st March 2011
DOI: 10.1039/c1cc10598a
The alternating ABABAB p-stacked bis-1,2,3-dithiazolyl radical
2a (2, R2 = Ph) has a conductivity r of 3 ꢀ 10ꢁ5 S cmꢁ1 at
300 K, and orders as a spin-canted antiferromagnet (TN = 4.5 K)
which undergoes a spin-flop transition to a field-induced ferro-
magnetic state saturating (at 2 K) at H B 20 kOe.
this material with silver triflate (AgOTf) in MeCN yields a
deep purple solution which, upon solvent removal, leaves
metallic green crystals of the triflate salt [2a][OTf]. This
material may be recrystallized from MeCN, and its structure
has been confirmed by X-ray diffraction.z Reduction of
[2a][OTf] with octamethylferrocence (OMFc) yields radical
2a as lustrous black needles.
Interest in the use of thiazyl radicals as building blocks for
magnetic and conductive materials is growing rapidly, in part
because the heavy heteroatom (sulfur) enhances intermolecular
magnetic and electronic interactions.1 Within this context
radicals based on the resonance stabilized, pyridine-bridged
bisdithiazolyl framework 1 (Chart 1) are appealing targets, as
their solid state structures and transport properties can be fine-
tuned by variations in the size of the exocyclic ligands R1/R2,2
and/or by replacement of sulfur by its heavier congener
selenium.3,4 The steric bulk of the ligands also helps suppress
dimerization, but generally gives rise to herringbone arrays of
slipped p-stacks in which orbital overlap along the stacking
direction is diminished. In order to avoid the herring-bone
mold found for 1 we are now exploring radicals based on
the isoelectronic semiquinone-bridged skeleton 2. Herein we
report the preparation, structure and transport properties of 2a
(2, R2 = Ph), the first example of this new class of radical.
Scheme 1
Cyclic voltammetry on a solution of [2a][OTf] in MeCN
1
2
(Fig. 1) indicates a reversible +1/0 couple with E = 0.108 V
vs. SCE. The second reduction, corresponding to the ꢁ1/0 couple,
1
2
1
2
is not reversible, but the cell potential Ecell = E (ox) ꢁ E (red) for
2a may nonetheless be estimated in terms of the difference in
the two cathodic peak potentials Epc. The Ecell value (0.60 V) so
obtained is substantially smaller than that typically found for 1,2
which suggests that, in the solid state, semiquinone-bridged
radicals 2 will enjoy a lower onsite Mott-Hubbard Coulomb
repulsion energy U.6 The spin distributions of the two systems
are, however, quite similar, as indicated by the EPR spectrum of
2a (Fig. 1), which consists of a five-line pattern (g = 2.00093)
arising from hyperfine coupling to the two equivalent nitrogens
(I(14N) = 1) with aN = 0.352 mT.
Chart 1
The synthetic sequence to 2a (Scheme 1) begins with a
double Herz cyclization of 4-phenyl-2,6-diamino-phenol dihydro-
chloride 35 with sulfur monochloride in MeCN at reflux, a
process which affords the semiquinone-bridged bis-dithiazolylium
chloride [2a][Cl] as a blue-black insoluble solid. Metathesis of
a Department of Chemistry, University of Waterloo, Waterloo,
Ontario, N2L 3G1, Canada. E-mail: oakley@uwaterloo.ca
b Brockhouse Institute for Materials Research, McMaster University,
Hamilton, Ontario L8S 4M1, Canada
w Electronic supplementary information (ESI) available: Details of
synthetic, spectroscopic and magnetic work. CCDC 810524–810525.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc10598a
Fig. 1 (a) CV scan of [2a][OTf] with Pt electrodes, 0.1 M n-Bu4NPF6
supporting electrolyte, scan rate 100 mV sꢁ1. (b) X-band EPR
spectrum of 2a in toluene at 20 1C.
c
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Fig. 4 Plot of wT vs. T for 2a at H = 1 kOe. Inset shows ZFC-FC
plots of w vs. T at H = 100 Oe.
H = 1 kOe (Fig. 4) indicates paramagnetic behavior, with
strong local ferromagnetic interactions; a Curie–Weiss fit
to the data above 100 K affords C = 0.349 emu K molꢁ1
and y = +32.8 K. Application of the Baker model10 for a
Fig. 2 Unit cell drawing of 2a, viewed perpendicular to the yz plane,
with intermolecular S3ꢂ ꢂ ꢂO0 contacts shown with dashed lines.
1
Heisenberg 1D FM-coupled chain (p-stack) of S = centers
2
to the data over the range T = 6–30 K yields exchange
coupling constants J = 29.5 ꢁc1mꢁ1 for interactions along the
p-stacks, and zJ0 = ꢁ2.5 cm for the cumulative interstack
(mean field) interactions.
Additional measurements at lower fields revealed the onset
of a slight low temperature discontinuity in w(T) for H o 200 Oe,
and a subsequent ZFC-FC experiments at H = 100 Oe
(Fig. 4, inset) established a phase transition at 4.5 K. The
resulting state may be viewed in terms of spin-canted AFM-
ordering based on the antiparallel alignment of the FM-coupled
chains (p-stacks). The degree of canting is, however, very
small, as no spontaneous magnetization could be detected.
The value of TN = 4.5 K obtained from the ZFC-FC
measurements nonetheless compares well with that (6.2 K)
Fig. 3 Two views of the alternating ABABAB p-stacks in 2a, with
intrastack S1ꢂ ꢂ ꢂS30 contacts d1 and d2.
1
11
estimated using the expression kTc E 2S2 (|zJ |ꢂJ) , which
0
2
may be derived from mean field theory, with S = 12 and using
values of J and zJ0 cmꢁ1 obtained from the Baker fit to the 1D
FM chain model.
The crystal structure of 2a belongs to the non-centric space
group P212121, and consists (Fig. 2) of sheets of approximately
coplanar radicals laced together by a series of intermolecular
S3ꢂ ꢂ ꢂO0 contacts (2.858 A). These sheets are packed in layers
to produce alternating ABABAB p-stacks in which adjacent
radicals are related by the 2-fold screw axis along x (Fig. 3).
Nearest neighbors along the p-stacks are separated by S1ꢂ ꢂ ꢂS30
contacts d1 (3.680 A) and d2 (3.806 A), both of which are
nominally outside the Sꢂ ꢂ ꢂS0 van der Waals separation.7 At a
molecular level there are small increases in the S–N and S–S
distances in the radical relative to those in the cation that can
be related to the occupation of an antibonding SOMO.2 The
CQO distance in the radical (1.225(4) A) is also slightly longer
than in the cation (1.204(2) A).
The alternating p-stack motif found for 2a, coupled with
the paucity of close intercolumnar Sꢂ ꢂ ꢂS0 contacts, suggests a
relatively one-dimensional electronic structure which might be
considered susceptible to a symmetry-lowering, spin-pairing
instability, as observed in related radicals with ABABAB
p-stacked structures.8 However, variable temperature (from
T = 2–300 K) magnetic susceptibility w measurements (corrected
for diamagnetic contributions)9 establish that this is not the
case. For example, a plot of wT (field cooled) vs. T at a field
Fig. 5 Plots of M vs. H (left) and dM/dH vs. H (right) for 2a at T = 2,
5 and 10 K.
Magnetization (M) vs. field (H) measurements (Fig. 5, left)
on 2a at different temperatures have provided additional
insight into its magnetic structure. At T = 2 K, for example,
c
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the value of M rises sharply with H, reaching a plateau at
1.00 Nb, the saturation value expected for a S = 12 system with
a nominal g E 2, just above H = 20 kOe. There is no evidence
of hysteresis. At temperatures above TN, M rises less steeply,
and with decreasing dM/dH, so that saturation does not occur
out to H = 50 kOe, the limit of the experiment. These results
are broadly consistent with metamagnetic behavior,12 in which
the antiparallel alignment of the FM ordered radical p-stacks
is reversed by the field, so that all the chains become aligned in
parallel, producing a field-induced FM ordered material.
However, a characteristic feature for such systems is an
inflexion in the M vs. H plot, with dM/dH reaching a maximum
at the field where magnetic realignment occurs. Several non-
metallic radicals and radical ions displaying this behavior have
been reported.13,14 In this case, however, no inflexion is
observed in the M vs. H plot, nor is any maximum apparent
in the dM/dH vs. H plot (Fig. 5, right) prior to saturation, even
at T = 2 K. This result, plus the fact that ZFC-FC measure-
ments establish that AFM ordering collapses above H = 200 Oe,
is more consistent with a spin-flop transition.15–17 The profiles
of the M vs. H curves at T > TN are also in keeping with such
a process.15
Notes and references
z Crystal data at 296(2) K for [2a][OTf]: C13H5F3N2O4S5 M = 470.49,
monoclinic, a = 8.4534(4), b = 12.9782(6), c = 16.2121(7) A, b =
102.762(1), V = 1734.69(14) A3, space group P21/c (#14), Z = 4, Dc =
1.802 g cmꢁ3, m = 0.723 mmꢁ1; 300 parameters were refined using 4183
unique reflections (Rint = 0.0289) to give R = 0.0366 and Rw = 0.0721
(observed data). Crystal data at 296(2) K for 2a: C12H5N2OS4 M =
321.42, orthorhombic, a = 6.8011(7), b = 11.3785(12), c = 15.6252(16) A,
V = 1209.2(2) A3, space group P212121 (#19), Z = 4, Dc = 1.766 g cmꢁ3
,
m = 0.774 mmꢁ1; 174 parameters were refined using 3431 unique reflec-
tions (Rint = 0.0444) to give R = 0.0547 and Rw = 0.0739 (observed data).
1 (a) A. W. Cordes, R. C. Haddon and R. T. Oakley, Adv. Mater., 1994,
6, 798; (b) J. M. Rawson, A. Alberola and A. Whalley, J. Mater.
Chem., 2006, 16, 2560; (c) R. G. Hicks, Org. Biomol. Chem., 2007, 5,
1321; (d) R. G. Hicks, in Stable Radicals Fundamentals and Applied
Aspects of Odd-Electron Compounds, ed. R. G. Hicks, John Wiley &
Sons, Ltd, Wiltshire, 2010, pp. 317.
2 (a) L. Beer, J. L. Brusso, A. W. Cordes, R. C. Haddon, M. E. Itkis,
K. Kirschbaum, D. S. MacGregor, R. T. Oakley, A. A. Pinkerton
and R. W. Reed, J. Am. Chem. Soc., 2002, 124, 9498;
(b) A. W. Cordes, R. C. Haddon and R. T. Oakley, Phosphorus,
Sulfur Silicon Relat. Elem., 2004, 179, 673.
3 (a) J. L. Brusso, K. Cvrkalj, A. A. Leitch, R. T. Oakley,
R. W. Reed and C. M. Robertson, J. Am. Chem. Soc., 2006,
128, 15080; (b) A. A. Leitch, X. Yu, S. M. Winter, R. A. Secco,
P. A. Dube and R. T. Oakley, J. Am. Chem. Soc., 2009, 131, 7112.
4 (a) C. M. Robertson, A. A. Leitch, K. Cvrkalj, R. W. Reed, D. J. T.
Myles, P. A. Dube and R. T. Oakley, J. Am. Chem. Soc., 2008, 130,
8414; (b) C. M. Robertson, A. A. Leitch, K. Cvrkalj, D. J. T. Myles,
R. W. Reed, P. A. Dube and R. T. Oakley, J. Am. Chem. Soc., 2008,
130, 14791.
5 B. K. Chen, Y. J. Tsai and S. Y. Tsay, Polym. Int., 2006, 55, 930.
6 (a) N. F. Mott, Proc. Phys. Soc., London, Sect. A, 1949, 62, 416;
(b) J. Hubbard, Proc. Roy. Soc. (London), 1963, A276, 238.
7 (a) A. J. Bondi, A. J. Phys. Chem., 1964, 68, 441; (b) I. Dance,
New J. Chem., 2003, 27, 22.
8 A. A. Leitch, R. W. Reed, C. M. Robertson, J. F. Britten, X. Yu,
R. A. Secco and R. T. Oakley, J. Am. Chem. Soc., 2007, 129, 7903.
9 R. L. Carlin, Magnetochemistry, Springer-Verlag, New York, 1986.
10 G. A. Baker, G. S. Rushbrooke and H. E. Gilbert, Phys. Rev.,
1964, 135, A1272.
11 H. Murata, Y. Miyazaki, A. Inaba, A. Paduan-Filho, V. Bindilatti,
N. F. Oliveira, Z. Delen and P. M. Lahti, J. Am. Chem. Soc., 2008,
130, 186.
Fig. 6 Log plot of conductivity of 2a vs. inverse temperature.
Variable temperature 4-probe conductivity (s) measure-
12 (a) O. Kahn, Molecular Magnetism, VCH, New York, 1993; (b) A. Das,
G. M. Rosair, M. S. El Fallah, J. Ribas and S. Mitra, Inorg. Chem.,
2006, 45, 3301; (c) J. P. Sutter, A. Lang, O. Kahn, C. Paulsen,
L. Ouahab and Y. Pei, J. Magn. Magn. Mater., 1997, 171, 147.
13 (a) G. Chouteau and C. Veyret-Jeandey, J. Phys. (Paris), 1981, 42,
1441; (b) T. Kobayashi, T. Takiguchi, K. Amaya, H. Sugimoto,
A. Kajiwara, A. Harada and M. Kamachi, J. Phys. Soc. Jpn., 1993,
62, 3239; (c) T. Ishida, K. Tomioka, T. Nogami, H. Yoshikawa,
M. Yasui, F. Iwasaki, N. Takeda and M. Ishikawa, Chem. Phys.
Lett., 1995, 247, 7; (d) H. Nagashima, S. Fujita, H. Inoue and
N. Yoshioka, Cryst. Growth Des., 2004, 4, 19.
14 (a) W. Fujita, K. Takahashi and H. Kobayashi, Cryst. Growth
Des., 2011, 11, 575; (b) 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.
15 (a) A. N. Bogdanov, A. V. Zhuravlev and U. K. Roßler, Phys. Rev.
B: Condens. Matter Mater. Phys., 2007, 75, 094425; (b) J. M. D.
Coey, Magnetism and Magnetic Materials, Cambridge University
Press, Cambridge, 2010, p. 198.
ments (Fig. 6) on 2a indicate that s(300 K) (3 ꢀ 10ꢁ5 S cmꢁ1
)
is superior to that typically found for the slipped p-stack
arrangements of 1.2 Moreover, the thermal activation energy
E
act = 0.20 eV, derived from data collected over T = 160–300 K,
is substantially lower than that (0.4–0.5 eV) typically found for
1.2 This improved performance, which probably derives from a
combination of increased intermolecular overlap afforded by the
almost direct superposition of the radicals, and a decreased onsite
Coulomb potential U, augurs well for the use of semiquinone-
bridged radicals 2 in the design of new single-component,
molecular electronic and magnetic materials.
The present results also reveal the importance of the lateral
Sꢂ ꢂ ꢂO0 contacts18 as supramolecular synthons in the develop-
ment of sheet-like networks of radicals which are resistive to
Peierls-type distortions. The incorporation of the semiquinone
unit, as found in 2, therefore represents a significant structural
as well electronic advance over pyridine-bridged radicals 1.
The availability of the carbonyl oxygen for coordination to
metals19 is also a potentially attractive feature.
16 T. Sugano, T. Goto and M. Kinoshita, Solid State Commun., 1991,
80, 1021.
17 (a) Y. Teki, K. Itoh, A. Okada, H. Yamakage, T. Kobayashi,
K. Amaya, S. Kurokawa, S. Ueno and Y. Miura, Chem. Phys.
Lett., 1997, 270, 573; (b) T. Kobayashi, M. Takiguchi, K. Amaya,
H. Sugimoto, A. Kajiwara, A. Harada and M. Kamachi, J. Phys.
Soc. Jpn., 1993, 62, 3239.
18 A. Decken, A. Mailman, J. Passmore, J. M. Rautiainen,
W. Scherer and E.-W. Scheidt, Dalton Trans., 2011, 40, 868.
19 K. E. Preuss, Dalton Trans., 2007, 2357.
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4655–4657 4657