An alternating π-stacked bisdithiazolyl radical conductor

Article abstract of DOI:10.1021/ja071218p

A general synthetic route to the resonance-stabilized pyrazine-bridged bisdithiazolyl framework, involving the reductive deprotection of 2,6-diaminopyrazine-bisthiocyanate and cyclization with thionyl chloride, has been developed. An N-methyl bisdithiazolyl radical, 4-methyl-4H-bis[1,2,3] dithiazolo[4,5-b: 5′,4′-e]pyrazin-3-yl, has been prepared and characterized in solution by electron paramagnetic resonance spectroscopy and cyclic voltammetry. Its crystal structure has been determined at several temperatures. At 295 K, the structure belongs to the space group Cmca and consists of evenly spaced radicals π-stacked in an alternating ABABAB fashion along the x-direction. At 123 K, the space group symmetry is lowered by loss of C-centering to Pccn, so that the radicals are no longer evenly spaced along the π-stack. At 88 K, a further lowering of space group symmetry to P2 ₁/c is observed. Extended Huckel Theory band structure calculations indicate a progressive opening of a band gap at the Fermi level in the low-temperature structures. Magnetic susceptibility measurements over the range 4-300 K reveal essentially diamagnetic behavior below 120 K. Variable-temperature single-crystal conductivity (σ) measurements indicate that the conductivity is activated, even at room temperature, with a room-temperature value σ_RT = 0.001 S cm^-1 and a thermal activation energy E_act = 0.19 eV. Under an applied pressure of 5 GPa, σ_RT is increased by 3 orders of magnitude, but the conductivity remains activated, with E_act being lowered to 0.11 eV at 5.5 GPa.

Full text of DOI:10.1021/ja071218p

Published on Web 06/02/2007
An Alternating π-Stacked Bisdithiazolyl Radical Conductor
Alicea A. Leitch,1a Robert W. Reed,1a Craig M. Robertson,1a James F. Britten,1b
Xueyang Yu,1c Richard A. Secco,1c and Richard T. Oakley*,1a
Contribution from the Department of Chemistry, UniVersity of Waterloo, Waterloo, Ontario N2L
3G1, Canada, Department of Chemistry, McMaster UniVersity, Hamilton, Ontario L8S 4L8,
Canada, and Department of Earth Sciences, UniVersity of Western Ontario,
London, Ontario N6A 5B7, Canada
Received February 20, 2007; E-mail: oakley@uwaterloo.ca
Abstract: A general synthetic route to the resonance-stabilized pyrazine-bridged bisdithiazolyl framework,
involving the reductive deprotection of 2,6-diaminopyrazine-bisthiocyanate and cyclization with thionyl
chloride, has been developed. An N-methyl bisdithiazolyl radical, 4-methyl-4H-bis[1,2,3]dithiazolo[4,5-b:
5′,4′-e]pyrazin-3-yl, has been prepared and characterized in solution by electron paramagnetic resonance
spectroscopy and cyclic voltammetry. Its crystal structure has been determined at several temperatures.
At 295 K, the structure belongs to the space group Cmca and consists of evenly spaced radicals π-stacked
in an alternating ABABAB fashion along the x-direction. At 123 K, the space group symmetry is lowered by
loss of C-centering to Pccn, so that the radicals are no longer evenly spaced along the π-stack. At 88 K,
a further lowering of space group symmetry to P21/c is observed. Extended Hu¨ckel Theory band structure
calculations indicate a progressive opening of a band gap at the Fermi level in the low-temperature structures.
Magnetic susceptibility measurements over the range 4-300 K reveal essentially diamagnetic behavior
below 120 K. Variable-temperature single-crystal conductivity (σ) measurements indicate that the conductivity
is activated, even at room temperature, with a room-temperature value σRT ) 0.001 S cm-1 and a thermal
activation energy Eact ) 0.19 eV. Under an applied pressure of 5 GPa, σRT is increased by 3 orders of
magnitude, but the conductivity remains activated, with Eact being lowered to 0.11 eV at 5.5 GPa.
tors.7,8 Incorporation of nitrogen atoms into spin-bearing sites
helps suppress dimerization, and intermolecular interactions
Introduction
Virtually all electrically conductive molecular materials rely
on charge transfer (CT) as the means of generating charge
carriers.2 Two components, a donor and an acceptor, are
required, although single-component organic metals can be made
by incorporating the two moieties into the same molecule.3 An
alternative to the CT paradigm is to construct conductors from
molecules that already possess potential charge carriers, that
is, to use neutral radicals as the building blocks. As originally
proposed,4 this approach focused on the use of highly delocal-
ized odd-alternant hydrocarbon radicals, notably phenalenyls,
derivatives of which have received considerable attention.5
Spiro-conjugated biphenalenyls, for example, have been shown
to display room-temperature conductivities σRT as high as 10-1
between neighboring sulfurs provide a potential pathway for
charge migration. However, early attempts to generate super-
(5) (a) Goto, K.; Kubo, T.; Yamamoto, K.; Nakasuji, K.; Sato, K.; Shiomi,
D.; Takui, T.; Kubota, M.; Kobayashi, T.; Yakusi, K.; Ouyang, J. J. Am.
Chem. Soc. 1999, 121, 1619. (b) Morita, Y.; Aoki, T.; Fukui, K.; Nakazawa,
S.; Tamaki, K.; Suzuki, S.; Fuyuhiro, A.; Yamamoto, K.; Sato, K.; Shiomi,
D.; Naito, A.; Takui, T.; Nakasuji, K. Angew. Chem., Int. Ed. 2002, 41,
1793. (c) Koutentis, P. A.; Chen, Y.; Cao, Y.; Best, T. P.; Itkis, M. E.;
Beer, L.; Oakley, R. T.; Brock, C. P.; Haddon, R. C. J. Am. Chem. Soc.
2001, 123, 3864. (d) Takano, Y.; Taniguchi, T.; Isobe, H.; Kubo, T.; Morita,
Y.; Yamamoto, K.; Nakasuji, K.; Takui, T.; Yamaguchi, K. J. Am. Chem.
Soc. 2002, 124, 11122. (e) Small, D.; Zaitsev, V.; Jung, Y.; Rosokha, S.
V.; Head-Gordon, M.; Kochi, J. K. J. Am. Chem. Soc. 2004, 126, 13850.
(f) Huang, J.; Kertesz, M. J. Am. Chem. Soc. 2003, 125, 13334. (g) Huang,
J.; Kertesz, M. J. Am. Chem. Soc. 2006, 128, 1418.
(6) (a) Chi, X.; Itkis, M. E.; Patrick, B. O.; Barclay, T. M.; Reed, R. W.; Oakley,
R. T.; Cordes, A. W.; Haddon, R. C. J. Am. Chem. Soc. 1999, 121, 10395.
(b) Itkis, M. E.; Chi, X.; Cordes, A. W.; Haddon, R. C. Science 2002, 296,
1443. (c) Chi, X.; Itkis, M. E.; Kirschbaum, K.; Pinkerton, A. A.; Oakley,
R. T.; Cordes, A. W.; Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 4041.
(d) Chi, X.; Itkis, M. E.; Reed, R. W.; Oakley, R. T.; Cordes, A. W.;
Haddon, R. C. J. Phys. Chem. B 2002, 106, 8278. (e) Mandal, S. K.; Itkis,
M. E.; Chi, X.; Samanta, S.; Lidsky, D.; Reed, R. W.; Oakley, R. T.; Tham,
F. S.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127, 8185. (f) Pal, S. K.;
Itkis, M. E.; Tham, F. S.; Reed, R. W.; Oakley, R. T.; Haddon, R. C. Science
2005, 309, 281. (g) Mandal, S. K.; Samanta, S.; Itkis, M. E.; Jensen, D.
W.; Reed, R. W.; Oakley, R. T.; Tham, F. S.; Donnadieu, B.; Haddon, R.
C. J. Am. Chem. Soc. 2006, 128, 1982.
S cm-1 6
, although a metallic ground state has yet to be
demonstrated.
Heteroatom radicals, especially heterocyclic thiazyls, have
also been explored as building blocks for molecular conduc-
(1) (a) University of Waterloo. (b) McMaster University. (c) University of
Western Ontario.
(2) For recent reviews on CT conductors, see: (a) Bendikov, M.; Wudl, F.;
Perepichka, D. F. Chem. ReV. 2004, 104, 4891. (b) Je´rome, D. Chem. ReV.
2004, 104, 5565. (c) Geiser, U.; Schlueter, J. A. Chem. ReV. 2004, 104,
5203. (d) Yamada, J.; Akutsu, H.; Nishikawa, H.; Kikuchi, K. Chem. ReV.
2004, 104, 5057.
(7) (a) Rawson, J. M.; Alberola, A.; Whalley, A. J. Mater. Chem. 2006, 16,
2560. (b) Hicks, R. G. Org. Biomol. Chem. 2007, 5, 1321.
(3) (a) Tanaka, H.; Okano, Y.; Kobayashi, H.; Suzuki, W.; Kobayashi, A.
Science 2001, 291, 285. (b) Kobayashi, A.; Fujiwara, E.; Kobayashi, H.
Chem. ReV. 2004, 104, 5243.
(4) (a) Haddon, R. C. Nature 1975, 256, 394. (b) Haddon, R. C. Aust. J. Chem.
1975, 28, 2333. (c) Haddon, R. C. Aust. J. Chem. 1975, 28, 2343.
(8) (a) Cordes, A. W.; Haddon, R. C.; Oakley, R. T. AdV. Mater. 1994, 6,
798. (b) Cordes, A. W.; Haddon, R. C.; Oakley, R. T. In The Chemistry of
Inorganic Ring Systems; Steudel, R., Ed.; Elsevier: Amsterdam, 1992; p
295. (c) Beer, L.; Cordes, A. W.; Myles, D. J. T.; Oakley, R. T.; Taylor,
N. J. CrystEngChem 2000, 2, 109.
9
10.1021/ja071218p CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 7903-7914
7903

A R T I C L E S
Chart 1
Leitch et al.
Chart 2
imposed radical π-stacks invariably afforded S-S linked
π-dimer stacks, that is, charge density wave (CDW) or Peierls9
distorted structures which were insulating or weakly semicon-
ducting.8 A potentially more serious problem with early thiazyl
radical materials was their high onsite Coulomb repulsion energy
U.10 Thus, even when radical dimerization could be avoided,
intermolecular orbital overlap and the associated solid-state
bandwidth W were insufficient to overcome Coulomb repulsion,
and the materials were trapped in Mott insulating states.11
Attempts to improve conductivity have therefore focused on
the development of more delocalized (low U) radicals which
display a strong network of intermolecular interactions in the
solid state, and hence a large bandwidth W, the criterion for a
metallic ground state being that W > U.
We are approaching this design challenge in two ways. The
first involves the replacement of sulfur by selenium, a tactic
employed early on in the design of CT salts,18 and initial results
are encouraging.19 The second approach, the one addressed
herein, involves the replacement of the central pyridine ring of
1 by a pyrazine bridge, as in 2. While the values of U for both
1 and 2 are expected to be comparable, the more exposed
periphery of 2, with its less heavily substituted framework,
should allow for stronger intermolecular interactions, and hence
an improved bandwidth W. We also hoped to be able to move
away from the slipped π-stack arrays found for 1 to alternating
but vertically aligned ABABAB π-stacks (Chart 1), an adjust-
ment expected to afford greater overlap and hence larger
bandwidth.15
With these energetic and bandwidth criteria in mind, we
prepared a variety of resonance-stabilized radicals,12 including
those based on the bisdithiazolyl framework 1 (Chart 1).13
A
We recently reported the first example of 2, with R ) Et. To
our surprise, this compound crystallized as closed-shell dimers
instead of π-stacked radicals. Moreover, two very different
modes of association were observed (Chart 2), one involving a
localized C-C σ-bond (R-[2]2, R ) Et) and the other a lateral
S-S σ-bond (â-[2]2, R ) Et).20 We have now prepared and
structurally characterized the corresponding methyl derivative
2 (R ) Me). In contrast to the ethyl derivative, this compound
adopts a crystal structure in which the molecules are π-stacked
in alternating ABABAB arrays, as shown in Chart 1. This
arrangement affords a material with a room-temperature con-
ductivity that is markedly improved over those seen for all
derivatives of 1; indeed, the value of σRT ≈ 10-3 S cm-1 is the
highest ever observed for a thiazyl radical conductor. The
material is not, however, a metal. Low-temperature crystal-
lographic studies reveal a series of changes in space group
associated with the opening of a band gap at the Fermi level
and the formation of a semiconducting ground state. In this
paper, we describe the preparation and structural characterization
of 2 (R ) Me). Variable-temperature magnetic susceptibility
and conductivity measurements, the latter at ambient and
elevated (<5.5 GPa) pressures, are also presented, and the results
are interpreted in the light of the EHT band structure calcula-
tions.
notable feature of these materials is that they do not form
π-dimers in the solid state, in part because of the buffering effect
of the beltline R1 and R2 substituents, but also because of the
extent of spin delocalization in the radicals.14-16 However, while
the estimated gas-phase disproportionation enthalpies ∆Hdisp
found for these materials suggest a reduced value for U in the
solid state,17 their bulk conductivities remain activated, with
thermal activation energies Eact ) 0.4-0.5 eV and room-
temperature conductivities σRT ≈ 10-6 S cm-1. Extended Hu¨ckel
Theory (EHT) band structure calculations have indicated
bandwidths W of less than 0.5 eV, values which are comparable
to those found in many conductive CT salts, but clearly still
too small to offset U, which is a maximum for a half-filled
1
band, that is, an f ) /2 system. This conclusion prompts the
question as to how to modify the bisdithiazolyl framework 1 in
order to increase the solid-state bandwidth W sufficiently to
overcome the onsite Coulomb repulsion U.
(9) Peierls, R. C. Quantum Theory of Solids; Oxford University Press: London,
1955; p 108.
(10) (a) Barclay, T. M.; Cordes, A. W.; George, N. A.; Haddon, R. C.; Itkis,
M. E.; Mashuta, M. S.; Oakley, R. T.; Patenaude, G. W.; Reed, R. W.;
Richardson, J. F.; Zhang, H. J. Am. Chem. Soc. 1998, 120, 352. (b) Boere´,
R. T.; Roemmele, T. L. Coord. Chem. ReV. 2000, 210, 369. (c) Kaszynski,
P. J. Phys. Chem. A 2001, 105, 7626. (d) Kaszynski, P. J. Phys. Chem. A
2001, 105, 7615.
(11) Mott, N. F. Metal-Insulator Transitions; Taylor and Francis: London,
1990.
Results and Discussion
(12) Beer, L.; Haddon, R. C.; Itkis, M. E.; Leitch, A. A.; Oakley, R. T.; Reed,
R. W.; Richardson, J. F.; VanderVeer, D. G. Chem. Commun. 2005, 1218.
(13) Cordes, A. W.; Haddon, R. C.; Oakley, R. T. Phosphorus, Sulfur, Silicon
Relat. Elem. 2004, 179, 673.
Synthesis. Construction of the pyrazine-bridged bisdithiazolyl
framework 2 required a lengthy synthetic sequence (Scheme
(14) Beer, L.; Brusso, J. L.; Cordes, A. W.; Haddon, R. C.; Itkis, M. E.;
Kirschbaum, K.; MacGregor, D. S.; Oakley, R. T.; Pinkerton, A. A.; Reed,
R. W. J. Am. Chem. Soc. 2002, 124, 9498.
(15) (a) Beer, L.; Brusso, J. L.; Cordes, A. W.; Haddon, R. C.; Godde, E.; Itkis,
M. E.; Oakley, R. T.; Reed, R. W. Chem. Commun. 2002, 2562. (b) Beer,
L.; Britten, J. F.; Brusso, J. L.; Cordes, A. W.; Haddon, R. C.; Itkis, M. E.;
MacGregor, D. S.; Oakley, R. T.; Reed, R. W.; Robertson, C. M. J. Am.
Chem. Soc. 2003, 125, 14394.
(16) Beer, L.; Britten, J. F.; Clements, O. P.; Haddon, R. C.; Itkis, M. E.;
Matkovich, K. M.; Oakley, R. T.; Reed, R. W. Chem. Mater. 2004, 16,
1564.
(17) ∆Hdisp is the enthalpy change for the conversion of two gas-phase radicals
R into a cation/anion pair, i.e., 2 R a R+ + R-, and is equal to the
difference between the ionization potential (IP) and electron affinity (EA).
The cell potential, Ecell ) E1/2(ox) - E1/2(red), is the difference between
the half-wave potentials for the oxidation and reduction processes.
(18) (a) Beechgaard, K.; Cowan, D. O.; Bloch, A. N. J. Chem. Soc., Chem.
Commun. 1974, 937. (b) Engler, E. M.; Patel, V. V. J. Am. Chem. Soc.
1974, 96, 7376.
(19) (a) Beer, L.; Brusso, J. L.; Haddon, R. C.; Itkis, M. E.; Leitch, A. A.;
Oakley, R. T.; Reed, R. W.; Richardson, J. F. Chem. Commun. 2005, 1543.
(b) Beer, L.; Brusso, J. L.; Haddon, R. C.; Itkis, M. E.; Kleinke, H.; Leitch,
A. A.; Oakley, R. T.; Reed, R. W.; Richardson, J. F.; Secco, R. A.; Yu, X.
J. Am. Chem. Soc. 2005, 127, 1815. (c) Brusso, J. L.; Cvrkalj, K.; Leitch,
A. A.; Oakley, R. T.; Reed, R. W.; Robertson, C. M. J. Am. Chem. Soc.
2006, 128, 15080. (d) Brusso, J. L.; Derakhshan, S.; Itkis, M. E.; Kleinke,
H.; Haddon, R. C.; Oakley, R. T.; Reed, R. W.; Richardson, J. F.; Robertson,
C. M.; Thompson, L. K. Inorg. Chem. 2006, 45, 10958.
(20) Leitch, A. A.; McKenzie, C. E.; Oakley, R. T.; Reed, R. W.; Richardson
J. F.; Sawyer, L. D. Chem. Commun. 2006, 1088.
9
7904 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

An Alternating
π
-Stacked Bisdithiazolyl Radical Conductor
A R T I C L E S
Scheme 1
Figure 1. EPR spectrum of 2 (R ) Me) in CH2Cl2, SW ) 3 mT; for the
simulation, L/G ) 0.01 mT and LW ) 0.024 mT.
anates.24 Accordingly, the reaction of 6 with Bu3P in anhydrous
MeCN afforded a deep orange solution, and addition of small
amounts of water to this mixture led to the precipitation of the
monosubstituted aminophosphiniminodithiol 8 as an orange
microcrystalline solid; this material served as an effective
alternative to 7.25 Reaction of 8 with thionyl chloride in the
presence of pyridine afforded the desired bisdithiazolylium
framework in the form of the protonated salt [2][Cl] (R ) H).
The crude insoluble chloride was purified by conversion to a
tetrachlorogallate, which could be deprotonated with Proton-
Sponge to afford the ternary zwitterion 9. Alkylation of this
free base with Meerwein salts R3OBF4 (R ) Me, Et) caused
some initial difficulties, as the product was invariably contami-
nated by protonated material. However, when the alkylation was
performed in the presence of Proton-Sponge, protonation was
effectively suppressed, and the desired N-alkylated salts [2][BF4]
(R ) Me, Et) were obtained in good yield.
Chemical reduction of [2][BF4] (R ) Me, Et) was achieved
using decamethylferrocene (DMFc) in MeCN. In the case of R
) Et, the reaction afforded a light brown crystalline solid which
was sublimed in vacuo to yield a mixture of amber blocks of
R-[2]2 (R ) Et) and black needles of â-[2]2 (R ) Et). The
structures, relative energetics, and thermal interconversion of
these two dimers were described earlier.20 The methyl compound
2 (R ) Me) crystallizes directly from the reaction mixture as
copper-colored needles; its crystal structure and transport
properties are described below.
1), the first step of which was the preparation of 2,6-
diaminopyrazine 5. This latter compound can be generated in
small quantities by the hydrogenation of the corresponding
diazide 4, which is itself readily prepared from dichloropyrazine
3.21 With more convenient reagents, for example, sodium
borohydride, only one azide is fully reduced, while the other is
trapped as a tetrazole.21 We have found, however, that complete
reduction of both azides can be achieved using iron in aqueous
acetic acid;22 after an alkaline workup, the diamino compound
5 can be extracted in 70% yield (from 3). In previous studies,
we showed that the pyridine-bridged bisdithiazolyl framework
1 can be made, in a single step, via a double Herz cyclization
of diaminopyridine with sulfur monochloride.14-16 Unfortu-
nately, the 3,5-positions of diaminopyrazine are less susceptible
to electrophilic substitution than are the corresponding sites in
diaminopyridine, as a result of which double Herz cyclization
of 5 with sulfur monochloride fails. Assembly of the desired
tricyclic skeleton of 2 therefore required, as a first step, the
conversion of the diaminopyrazine 5 to its bisthiocyanate
derivative 6. Reductive deprotection of thiocyanates has typically
been achieved with aqueous sodium sulfide, followed by
acidification to afford the diamino-dithiol 7.4,23 However, the
use of this approach on 6 afforded, after an acidic workup, an
intractable brown solid that did not undergo a cyclization
reaction with thionyl chloride or sulfur monochloride.
Electron Paramagnetic Resonance Spectroscopy and Cy-
clic Voltammetry. The extent of spin delocalization in the
pyrazine-bridged radicals 2 (R ) Me, Et) has been probed by
electron paramagnetic resonance (EPR) spectroscopy and cyclic
voltammetry. The X-band EPR spectrum of 2 (R ) Me),
recorded on a sample dissolved in CH2Cl2 at room temperature,
is shown in Figure 1, along with a simulation from which the
1
14N and H hyperfine coupling constants were extracted. The
coupling constants aN and aH for both radicals 2 (R ) Me, Et)
are listed in Table 1; for purposes of comparison, those of the
related pyridine-bridged radicals 1 (R1 ) Me, Et; R2 ) H) are
also provided. In both pairs of radicals, the hyperfine patterns
We therefore turned to the use of tributylphosphine, a reagent
commonly employed in the reductive deprotection of thiocy-
(21) Shaw, J. T.; Brotherton, C. E.; Moon, R. W.; Winland, M. D.; Anderson,
M. D.; Kyler, K. S. J. Heterocycl. Chem. 1980, 11, 17.
(22) Zienkiewicz, J.; Kaszynski, P.; Young, V. G., Jr. J. Org. Chem. 2004, 69,
2551.
(23) (a) Baker, A. J.; Hill, S. A. J. Chem. Soc. 1962, 3464. (b) Lochon, P.;
Me´heux, P.; Ne´el, J. Bull. Soc. Chim. Fr. 1967, 11, 4387. (c) Okada, M.;
Marvel, C. S. J. Polym. Sci., Part A-1 1968, 6, 1259.
(24) Flowers, W. T.; Holt, G.; Omogbal, F.; Poulos, C. P. J. Chem. Soc., Perkin
Trans. 1 1976, 2394.
(25) The structural constitution of this compound has been confirmed by single-
crystal X-ray diffraction. The crystals belong to the trigonal space group
P3121, with a ) b ) 12.0557(4) Å, c ) 55.325(4) Å, Z ) 12 (at T ) 200
K). There are two hydrogen-bonded molecules in the asymmetric unit, each
with the molecular formula shown in Scheme 1.
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7905

A R T I C L E S
Leitch et al.
Table 1. Hyperfine Coupling Constants (mT), g-Values, and Half-Wave Potentialsa
1 (R1
)
Me, R2
)
H)
1 (R1
)
Et, R2
)
H)
2 (R
)
Me)
2 (R ) Et)
aN
0.313 (2N)
0.062 (1N)
0.318 (2N)
0.061 (1N)
0.309 (2N)
0.073 (1N)
0.266 (1N)
0.313 (2N)
0.073 (1N)
0.266 (1N)
a
a
H (CH)
H (NR)
0.230 (1H)
0.228 (1H)
0.034 (NCH3)
2.0082
<0.02 (NCH2CH3)
0.053 (NCH3)
2.0087
0.025 (NCH2CH3)
2.0087
g-value
E1/2
E1/2
E1/2
2.0082
-1/0
-0.95c
-0.130
1.294
-0.95c
-0.870c
-0.0104
1.610
-0.853c
-0.0291
1.585
0/+1
-0.146
+1/+2
1.272
b
Ecell
0.77d
0.76d
0.82d
0.85d
b
a E1/2-values (volts) in MeCN, referenced to saturated calomel electrode (SCE). Ecell ) E1/2(0/+1) - E1/2(-1/0). c Irreversible behavior; Epc-value quoted.
d Ecell estimated as Epc(0/+1) - Epc(-1/0).
Table 2. Crystallographic Data for 2 (R ) Me)
are dominated by coupling to two equivalent dithiazolyl
nitrogens, the value of aN being approximately one-half of that
T
observed in monofunctional 1,2,3-dithiazolyl radicals.26 There
is also a weaker coupling to the pyrazine nitrogen and to the
N-methyl (or N-ethyl) protons, as well as a relatively large
hyperfine interaction with the basal nitrogen of the pyrazine
ring. In the pyridine-bridged systems, this is replaced by
coupling to the basal CH proton.
88(2) K
123(2) K
295(2) K
formula
fw
a, Å
b, Å
c, Å
C5H3N4S4
247.35
C5H3N4S4
247.35
C5H3N4S4
247.35
9.244(2)
6.5344(17)
27.035(7)
90.004(4)
1633.0(7)
2.012
6.5456(6)
9.2558(8)
27.040(2)
-
1638.2(2)
2.006
6.6488(2)
27.2177(5)
9.2797(2)
-
1679.30(7)
1.957
â, deg
V, Å3
The results of cyclic voltammetric measurements on the [2]-
[BF4] salts (R ) Me, Et), presented in the form of half-wave
potentials E1/2, are summarized in Table 1. Both radicals show
a reversible 0/+1 wave in the range -0.01 to -0.03 V vs SCE,
individual values being shifted anodically by some 120 mV with
respect to the analogous couples of 1 (R1 ) Me, Et; R2 ) H),
as expected from the more electronegative pyrazine core. A more
anodic reversible +1/+2 wave is also present. As in 1 (R1 )
Me, Et; R2 ) H), reduction of the radical to the anion (the -1/0
couple) is irreversible, a feature which we have previously
attributed to the cleavage of one of the S-S27 (or S-N)28 bonds
upon reduction of the radical. The cell potential values Ecell cited
in Table 1 are thus only estimates based on the difference
between the cathodic peak potentials (Epc) of the oxidation and
reduction processes. Nonetheless, the trends in both E1/2 and
Ecell map well onto those observed for the pyridine-based
radicals 1, and are broadly consistent with the computed gas-
phase IP, EA, and disproportionation enthalpy ∆Hdisp data
previously estimated for resonance-stabilized bisdithiazolyls.14,15
Crystal Structure. Copper-colored needles of 2 (R ) Me)
suitable for X-ray and single-crystal conductivity measurements
were grown by slow, diffusion-controlled mixing of solutions
of [2][BF4] and DMFc. In order to interpret and understand the
transport properties (vide infra) of the material, its crystal
structure was determined at 295, 123, and 88 K. Crystal data at
these temperatures are provided in Table 2, and pertinent
intermolecular S-S′ and S-N′ contacts are listed in Table 3.
Figure 2 shows ORTEP drawings of the asymmetric units at
the three temperatures.
F(calcd), g cm-3
space group
Z
P21/c
Pccn
Cmca
8
1.11
8
8
µ, mm-1
λ, Å
10.266
10.015
1.54178
829/0/
0.71073
4040/182/
236
direct methods
0.0720, 0.1398
1.54178
1399/0/
130
direct methods
0.0570, 0.1617
data/restraints/
parameters
solution method
R, Rw (on F2)a
79
direct methods
0.0387, 0.1105
a R ) [∑||Fo| - |Fc||]/[∑|Fo|] for I > 2σ(I); Rw ) {[∑w||Fo| - |Fc| | ]/
2
2 2
[∑(w|Fo| )]}1/2
.
4
Table 3. Summary of Contacts and Angles in 2 (R ) Me)
88 K
123 K
Pccn
295 K
Cmca
Contactsa
P21/c
Lateral (σ)
3.281(2)
S1-S2′
S2-S4′
S1-N4′
S2-N4′
S3-N3′
3.270(7), 3.294(7)
3.418(8), 3.449(7)
3.179(14), 3.155(13)
3.099(12), 3.193(11)
3.023(13), 2.939(14)
3.305(1)
3.466(1)
3.192(3)
3.179(3)
3.059(3)
3.435(2)
3.166(5)
3.152(5)
2.989(4)
Intrastack (π)
S1-S4′
S4-S1′
S2-S2′
3.727(6), 3.928(6)
3.750(6), 3.841(6)
3.627(4), 4.001(4)
3.760(2)
3.874(2)
3.662(3), 3.966(3)
3.865(1)
3.865(1)
3.851(1)
Interstack (π)
S1-S2′
S2-S1′
S3-S3′
S4-S4′
3.667(7), 3.844(6)
3.828(7), 3.684(6)
4.120(3), 3.948(3)
4.079(4), 3.690(4)
3.697(2)
3.831(2)
3.964(3), 4.121(3)
3.723(3), 4.056(3)
3.831(1)
3.831(1)
4.093(1)
3.936(1)
δb
0.026(8), 0.024(8)
2.6(3), 2.8(3)
0.018(3)
2.17(10)
0
0
φc
a Distances in angstroms. b Maximum deviation from the mean molecular
plane. c Tilt angle of the perpendicular to mean plane from the stacking
axis.
At 295 K, the crystal structure of 2 (R ) Me) belongs to the
orthorhombic space group Cmca and is based on arrays of eVenly
spaced radicals π-stacked along the x-direction. Within the
molecular units, bond distances and angles are typical of a
bisdithiazolyl radical.14,15 Two views of the crystal structure,
showing the unit cell packing and the lateral S-S′ and S-N′
interactions (Table 3) between neighboring molecules, are
provided in Figure 3. There are eight radicals in the unit cell,
all of which lie on crystallographic mirror planes at x ) 0 and
(26) (a) Cordes, A. W.; Mingie, J. R.; Oakley, R. T.; Reed, R. W.; Zhang, H.
Can. J. Chem. 2001, 79, 1352. (b) Barclay, T. M.; Beer, L.; Cordes, A.
W.; Oakley, R. T.; Preuss, K. E.; Taylor, N. J.; Reed, R. W. Chem. Commun.
1999, 531. (c) Preston, K. F.; Sutcliffe, L. H. Magn. Reson. Chem. 1990,
28, 189.
(27) Antonello, S.; Benassi, R.; Gavioli, G.; Taddei, F.; Maran, F. J. Am. Chem.
Soc. 2002, 124, 7529.
(28) Alternatively, N-S bond cleavage could occur. See, for example: Barclay,
T. M.; Cordes, A. W.; Goddard, J. D.; Mawhinney, R. C.; Oakley, R. T.;
Preuss, K. E.; Reed, R. W. J. Am. Chem. Soc. 1997, 119, 12136.
9
7906 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

An Alternating
π
-Stacked Bisdithiazolyl Radical Conductor
A R T I C L E S
Figure 4. Alternating ABABAB π-stacks of 2 (R ) Me) at 295 K.
Intrastack S-S′ contacts (in green) and interstack S-S′ contacts (in red)
are defined in Table 3.
Figure 2. ORTEP drawings of the asymmetric units of 2 (R ) Me) at
295, 123, and 88 K, showing atom numbering schemes.
Figure 5. Cross-linking of rows of π-stacks in 2 (R ) Me) in the Cmca
space group at 295 K.
Figure 6. Appearance of the (4,1,1h) reflection below 143 K, heralding the
loss of C-centering. The strong (3,1,1h) reflection is allowed in both P- and
C-lattices.
intermolecular S-S′ contacts, electronic communication in the
y-direction is minimal.
Figure 3. Two views of the Cmca unit cell of 2 (R ) Me) at 295 K.
Lateral intermolecular S-S′ contacts (in red) and S-N′ contacts (in green)
are defined in Table 3.
Cooling crystals of 2 (R ) Me) leads to small but expected
contractions in unit cell dimensions and intramolecular bond
lengths. A second data set, collected at 123 K, could be solved
and refined in the original space group (Cmca), but the
asymmetry of the thermal ellipsoids (elongation along the
stacking direction) apparent in the 295 K structure (Figure 2)
was more serious and obliged us to consider a symmetry-
lowering distortion. A full search over all observed reflections
at 123 K revealed several violations of C-centering and a change
in Bravais lattice from C to P. In order to establish the
temperature range for this transition, we monitored the evolution
of the (4, 1, 1h) reflection as a function of temperature (Figure
6). At 153 K, this peak was absent, as expected ((h+k) odd)
for a C-lattice. However, near 143 K, the reflection emerged
from the background, thereby signaling the transition to a
P-lattice. Further cooling to 123 K led to an increase in intensity
of this peak. Subsequent evaluation of the systematic absences
of the 123 K data in a primitive orthorhombic setting allowed
us to conclude that the correct space group is Pccn, and the
structure reported here is based on that assignment.
1/2, with consecutive plates along the x-direction related by the
1
3
a-glide at z )
/ and / so as to produce superimposed
4 4
ABABAB arrays with an interplanar separation of a/2, or 3.347-
(2) Å. This arrangement (Figure 4) affords a series of intraco-
lumnar S-S′ contacts (Table 3) up and down the π-stack which,
although outside of the normal van der Waals range,29 are well
1
oriented for strong π-overlap. The c-glide operation at y ) /4
and 3/4 generates pairs of dovetailed arrays of these stacks, the
two halves of which are laced together by a network
(Figure 4 and Table 3) of ladder-like intercolumnar π-type S-S′
contacts (S1-S2′, S3-S3′, and S4-S4′) that extend lattice-
wide along both the x- and z-directions. The packing of the
radicals into double rows of interconnected π-stacks running
parallel to the z-direction gives rise to the dovetailed motif
shown in Figure 5. These double rows are then locked together
by the four-center σ-type centrosymmetric S3-N3′ interactions
noted earlier (Figure 3). As a result of the absence of
(29) Bondi, A. J. Phys. Chem. 1964, 68, 441.
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7907

A R T I C L E S
Leitch et al.
Figure 7. Transformation of evenly spaced radical π-stacks to π-dimer
stacks as a function of temperature. Intermolecular S1-S4′ contacts (Table
3) are shown in green.
Figure 8. Plots of øP and øPT (inset) versus T for 2 (R ) Me).
Magnetic Susceptibility Measurements. Magnetic suscep-
tibility measurements on 2 (R ) Me), recorded on a Faraday
balance over the temperature range 9-300 K, are shown in
Figure 8. The results are presented in the form of a plot of øP,
the paramagnetic susceptibility after correction for diamagnetic
contributions (øD ) 117.29 × 10-6 emu mol-1).32 At ambient
temperature, the material is weakly paramagnetic, with øP )
277 × 10-6 emu mol-1 at 300 K. With decreasing temperatures,
the value of øP decreases slightly, reaching a minimum value
of 101 × 10-6 emu mol-1 near 100 K. Further cooling leads to
a rapid increase in øP, which we interpret as heralding the
presence of paramagnetic defects in the lattice. Such features
are commonly observed in crystalline radicals, and their
susceptibility should follow Curie-Weiss behavior. Consistently,
a Curie-Weiss fit (shown in Figure 8) to the data between T )
12 and 81 K provides values of C ) 0.0091 emu K mol-1 and
Θ ) -5.2 K. Also illustrated in Figure 8 is a plot of øPT versus
temperature. Below 100 K, øPT remains approximately constant,
with a value of 2% of that expected (0.375) for a Curie
paramagnet with S ) 1/2. This residual paramagnetism amounts
to the contribution of the defect electrons to the total moment.
Above 120 K, the value of øPT begins to increase steadily from
its threshold value, reaching a value of 0.09 emu K mol-1 at
300 K; this corresponds to a spin defect concentration of
approximately 25%. Similar paramagnetic enhancements of
otherwise diamagnetic π-dimer stacks have been observed
elsewhere,33,34 and these have been attributed to the uncoupling
of the weak intradimer bonds and the generation of additional
spin defects.
Conductivity Measurements. We have carried out four-
probe conductivity (σ) measurements on both pressed pellet and
single-crystal samples of 2 (R ) Me). The results are self-
consistent: the conductivity at 295 K from a pressed pellet
sample is 6.4 × 10-4 S cm-1, while for a single crystal σ (295
K) increases to 9.7 × 10-4 S cm-1. These values constitute the
highest conductivities ever observed for a thiazyl radical. The
temperature dependence of the single-crystal conductivity over
the range 73-295 K, shown in Figure 9, confirms that the
conductivity is activated. It is also apparent that the thermal
activation energy Eact is itself temperature dependent, with two
distinct and approximately linear log σ vs 1/T regimes, one
spanning the range 295-120 K, the other 120-80 K. In the
high-temperature range, the derived Eact is 0.19 eV, while in
Concomitant with loss of C-centering is a crossover in the
definition of the b- and c-axes, as may be seen in Table 2. More
importantly, the molecular plates along the ABABAB π-stacks
are free to move away from their otherwise fixed positions at
x ) 0 and 1/2. The resulting deviations (Table 3) are, however,
small. The molecules remain essentially planar (δ ) 0.018 Å)
and tilt very slightly (φ ) 2.2°) away from perpendicularity
with respect to the stacking direction (Figure 7). The network
of intermolecular interactions also changes.30 Consecutive
molecules along the π-stack in the new unit cell are related by
symmetry (a 2-fold rotation about c) but are not equally spaced
up and down the π-stacks. As a result, the four previously
equivalent S1-S4′ contacts associated with a given molecule
separate into two pairs, at 3.760(2) and 3. 874(2) Å, above and
below the molecule, respectively, to produce a small but
significant alternation in the plate-to-plate separation. The four
ladder-like S1-S2′ contacts which were identical in Cmca
evolve into equivalent pairs above and below the molecule.
Finally, the S3-S3′ and S4-S4′ interactions separate into four
distinct contacts (two long and two short).
A third data set was collected at 88 K, and inspection of the
systematic absences indicated a loss of two of the three glide
planes of Pccn and a further reduction in space group symmetry
to primitive monoclinic. The presence of the remaining glide
plane established the setting to be P21/c, and the structure was
solved and refined in this space group.31 The slight increase in
the R-value (Table 2) can be ascribed to the need to refine a
greater number of weak reflections and a merohedral twinning
arising from the pseudo-orthorhombic nature of the lattice. There
is a switchover in unit cell vectors (a T b) from the Pccn
settings (Table 2), but the cell constants remain close to those
seen at 123 K. The symmetry lowering leads to two crystallo-
graphically independent molecules per asymmetric unit, so that
consecutive molecules along the ABABAB π-stacks are no
longer equivalent. The two molecules are nonetheless closely
related, being nearly planar (δ ) 0.026 and 0.24 Å), and the
tilt angle φ for both increases only slightly (to 2.6° and 2.8°)
relative to that seen at 123 K. The four possible S1-S4′ contacts
(Table 3 and Figure 7) are now completely independent but
fall into two groups (long and short), as seen at 123 K, indicative
of a slight but continued migration toward a bond-alternating
π-dimer stack.
(30) Detailed illustrations of these changes are provided in the Supporting
Information.
(32) Estimated from Pascal’s constants. Carlin, R. L. Magnetochemistry;
Springer-Verlag: New York 1986.
(31) The 88 K data set could be solved in Pccn, if over 40 systematic absence
violations were ignored, but attempts to refine the solution gave rise to
unreasonably large thermal ellipsoids and a high R-factor. Lower symmetry
orthorhombic space groups were considered, but none was satisfactory.
Only when the space group symmetry was lowered to monoclinic P21/c
were all systematic absence requirements met and a satisfactory refinement
achieved. This change allowed atoms to move away from positions
otherwise defined by the extra glide planes of Pccn.
(33) (a) Beekman, R. A.; Boere´, R. T.; Moock, K. H.; Parvez, M. Can. J. Chem.
1998, 76, 85. (b) Andrews, M. P.; et al. J. Am. Chem. Soc. 1991, 113,
3559. (c) Cordes, A. W.; Haddon, R. C.; Hicks, R. G.; Kennepohl, D. K.;
Oakley, R. T.; Palstra, T. T. M.; Schneemeyer, L. F.; Scott, S. R.; Waszczak,
J. V. Chem. Mater. 1993, 5, 820.
(34) Barclay, T. M.; Cordes, A. W.; Haddon, R. C.; Itkis, M. E.; Oakley, R. T.;
Reed, R. W.; Zhang, H. J. Am. Chem. Soc. 1999, 121, 969.
9
7908 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

An Alternating
π
-Stacked Bisdithiazolyl Radical Conductor
A R T I C L E S
Figure 11. Single-crystal IR transmission spectrum of 2 (R ) Me).
Figure 9. Single-crystal conductivity of 2 (R ) Me) at ambient pressure,
with derived Eact-values.
Figure 12. EHT dispersion curves for the three crystal structures of 2 (R
) Me), showing the emergence of a band gap at lower temperatures. The
solid blue lines are the COs arising from the radical SOMOs. Higher-lying
orbitals (green dashed lines) overlap in the P2/1c structure.
crystal sample of 2 (R ) Me) at ambient temperature. Figure
11 shows the IR transmittance over the range 650-11000 cm-1
.
Figure 10. Pressure dependence of σ (294 K) (top) and Eact (bottom) of 2
(R ) Me).
In the mid-IR region, between 650 and 3100 cm-1, there is a
series of absorptions arising from molecular vibrations of the
molecule. Underlying these modes, however, is a strong
background absorption which, we believe, arises from solid-
state processes (vide infra). In addition, there is a strong, well-
developed, low-lying absorption cutoff beyond 3000 cm-1
which, as will be described below, corresponds to the valence-
to-conduction band excitation of a semiconductor. The optical
energy gap Eg has a threshold value near 0.48 eV, increasing
to near 0.75 eV at the transmission cutoff.
the low-temperature region it is 0.12 eV. Below 80 K, the
activation energy appears to drop even further, but there are
insufficient data to warrant a numerical analysis. An interpreta-
tion of these changes is provided below.
We have also probed the pressure dependence of the
conductivity of 2 (R ) Me) using a cubic anvil press. As
illustrated in Figure 10, the conductivity σ (at 294 K) increases
steadily with applied pressure, reaching a value near 0.2 S cm-1
at 5.0 GPa. Analysis of the temperature dependence (from 20
to 90 °C) of the conductivity indicates that it remains activated
over the entire pressure range studied. From a series of plots of
log σ vs 1/T, we have derived the activation energies Eact as a
function of pressure, and these results are also plotted in Figure
10. Initially the value of Eact decreases steadily to a plateau value
near 0.13 eV between 3 and 4 GPa. It then begins to decrease
more rapidly around 5 GPa, reaching a value of 0.11 eV at 5.5
GPa, the limiting pressure of the apparatus. Comparison of these
results with those obtained from the slipped π-stack bisdithia-
zolyl 1 (R1 ) Me, R2 ) Ph)35 reveals that the pressure responses
of the conductivity for the two compounds are similar; that is,
both show an increase in conductivity of about 3 orders of
magnitude over a 5 GPa range. However, for the present
compound the decrease in the activation energy with increasing
pressure is somewhat smaller.
Band Structures. In order to develop an understanding of
the electronic structure of 2 (R ) Me), and to rationalize the
results of the crystallographic, magnetic, conductivity, and
optical measurements, we have carried out EHT band structure
calculations on the three crystal structures of 2 (R ) Me)
reported above. The results are shown in Figure 12, in the form
of dispersion curves for the crystal orbitals (COs) arising from
the eight radical singly occupied molecular orbitals (SOMOs)
in the unit cell, as tracked along the reciprocal space direction
that corresponds to the π-stacking direction of each cell.36,37 In
previous studies of the electronic structures of slipped π-stack
bisdithiazolyls 1, we concluded that their relatively low
bandwidths (W ) 0.4-0.5 eV) were associated with a loss in
intermolecular overlap occasioned by the slippage of the
radicals.14,15 Calculations on a model π-stack consisting of
(36) In the two orthorhombic structures (Cmca and Pccn), there is an absolute
correspondence between the directions of the real and reciprocal unit cell
vectors, and in the 88 K (P21/c) structure the â angle is such that the
correspondence is very nearly exact.
Near-Infrared Spectrum. As a complement to the conduc-
tivity measurements, and the activation energies derived there-
from, we have examined the near-infrared spectrum of a single-
(37) The COs are plotted from Γ (0,0,0) to X (1/2,0,0) for the two orthorhombic
structures, and from Γ (0,0,0) to Y (0,1/2,0) for the monoclinic cell. Note
that, for the calculations on the 295 K structure, we used the special position
of the primitive cell rather than that of the full C-centered version so as to
allow for a more direct comparison of the three band structures.
(35) Beer, L.; Brusso, J. L.; Haddon, R. C.; Itkis, M. E.; Oakley, R. T.; Reed,
R. W.; Richardson, J. F.; Secco, R. A.; Yu, X. Chem. Commun. 2005,
5745.
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7909

A R T I C L E S
Leitch et al.
perfectly superimposed radicals suggested that bandwidths
approaching 2 eV could be achieved. At 295 K, in the Cmca
structure, the alternating orientation of radicals along the π-stack
produced by the a-glide precludes perfect superposition and
reduces orbital interactions along the stack, that is, those
associated with the S1-S4′ and S2-S2′ contacts. However, the
same feature also allows for increased interactions between the
stacks, that is, through the S1-S2′, S3-S3′, and S4-S4′
contacts, so that significant bandwidth can still be attained.
Based on the energetic spread of the eight COs, the estimated
bandwidth W is close to 1.5 eV, approaching that predicted for
a perfectly superimposed π-stacked structure.38 On the basis of
this description, such a bandwidth might well be expected to
of a heterocyclic thiazyl radical dimerized through a localized
C-C σ-bond.20 Moreover, its conversion to the â-isomer
provides a sobering caveat that, in the absence of lattice effects,
ring-opening S-S σ-dimerization of bisdithiazolyl radicals is
the thermodynamically preferred outcome.
In the light of these results, the crystal structure of 2 (R )
Me) at 295 K (Cmca), consisting of bisdithiazolyl radicals
locked by glide planes into alternating but apparently evenly
spaced π-stacked arrays, was a welcome and exciting finding.
This is a hitherto unobserved packing pattern for a neutral radical
(f ) 1/2) conductor, although similar symmetric ABABAB
π-stacking arrangements defined by glide planes have been
observed in CT salts.41 The strongly interconnected π-stacks in
2 (R ) Me) suggest a well-developed two-dimensional elec-
tronic structure, a view confirmed by the band structure
calculations. Given the apparent convergence of the necessary
energetic and structural features, it was at first surprising that
its conductivity is activated, not metallic.
So why is 2 (R ) Me) not a metal at room temperature?
One possible reason is that the material is a Mott insulator.41
The weak but increasing paramagnetism above 120 K could be
interpreted in terms of evenly spaced (undimerized) π-stacks
consisting of strongly antiferromagnetically coupled radicals.
The activated conductivity would then be a consequence of
strong electron correlation; that is, the bandwidth W is insuf-
ficient to overcome the Coulomb repulsion U. Although the band
calculations indicate a remarkably high bandwidth (W ≈ 1.5
eV) for a molecular material, even this may not be sufficient to
offset short-range correlation.42 The other possible explanation
is that the material is a semiconductor. Indeed, such a description
would be the natural default if the diamagnetic π-dimer stacked
structure identified at 88 K were also observed at room
temperature. The band gap value, Eg ) 0.42 eV, estimated from
the EHT calculations would then be satisfyingly close to the
threshold optical excitation found from near-IR measurements.
It would also be about twice the value of the thermal activation
energy, Eact ) 0.19 eV, obtained from the variable-temperature
conductivity measurements, a result expected for an intrinsic
semiconductor. The moderate response of the conductivity to
pressure would also be in keeping with stacked π-dimer
structures.43 However, this simple explanation is, by itself,
insufficient, as it ignores the magnetic and structural changes
that occur upon heating the material above 120 K.
1
give rise to an f ) /2 metal.
The loss of C-centering occasioned by cooling crystals of 2
(R ) Me) to 123 K, and the associated onset of an alternation
in the plate-to-plate separation along the π-stack, should lead
to the opening of an energy gap at the Fermi level and the
creation of a semiconducting state. The band structure calcula-
tions on the Pccn cell support this conclusion, although the
observed band gap Eg is only 0.09 eV. However, upon further
cooling and transformation to the monoclinic P21/c cell, the
magnitude of the calculated band gap increases quite dramati-
cally to 0.42 eV, in spite of the fact that the structural changes
between the Pccn and P21/c structures appear to be extremely
small.
Discussion
The design of single-component conductive materials com-
posed of neutral radicals requires the development of molecular
building blocks which, in the solid state, give rise to a low on-
site Coulomb repulsion U and a large bandwidth W. In principle,
resonance-stabilized bisdithiazolyls 1 respond to both these
challenges. Their delocalized spin distributions lead to reduced
disproportionation energies and cell potentials relative to those
seen in earlier generations of thiazyl radicals, while their exposed
sulfur-rich peripheries should allow for strong intermolecular
interactions. Moreover, the structural evidence to date indicates
that these radicals resist carrier-quenching dimerization. How-
ever, the slipped π-stack packing motifs (Chart 1) that these
systems invariably adopt15,16 lead to a severe decrease in
intrastack overlap and lowered bandwidths, as a result of which
a metallic ground state has not been achieved.
As noted earlier, previous work on diamagnetic π-stacked
thiazyl dimers has shown that, with increasing temperature, an
increase in the magnetic response is often observed.33,34 This
“spin breakout” can be rationalized in terms of a random
uncoupling of dimers and the generation of radical defects.
Figure 13 illustrates this process as it applies to the slightly
tilted π-dimer stacks of the P21/c structure of 2 (R ) Me). In
the absence of any residual defects (which are, in reality,
present), the structure would be as shown in Figure 13A, with
all radicals paired to afford a perfectly diamagnetic ground state.
Heating the structure causes an opening of some of the dimers,
to produce geminal (Figure 13B) and/or non-geminal (Figure
Replacement of the pyridine ring of 1 with a pyrazine ring
in 2 was therefore undertaken in the belief that removal of the
R2 group of 1 would favor more nearly superimposed π-stacking
and lead to an improved bandwidth (Chart 1).39 However, as
the structures (Chart 2) of the two phases of 2 (R ) Et)
illustrated, removal of the steric protection afforded by the basal
ligand also renders the radical vulnerable to cofacial σ-dimer-
ization.40 The R-dimer (R ) Et) thus represents the first example
(38) In perfectly superimposed π-stacked CT salts of dithiadiazolyl and
diselenadiazoyl radicals, EHT bandwidths approaching 3.0 eV have been
reported. For example, see: Bryan, C. D.; Cordes, A. W.; George, N. A.;
Haddon, R. C.; MacKinnon, C. D.; Oakley, R. T.; Palstra, T. T. M.; Perel,
A. S. Chem. Mater. 1996, 8, 762.
(39) The removal of buffering R-groups to enhance intermolecular interactions
was an approach used early on in the design of dithiadiazolyl radicals. For
example, see: Cordes, A. W.; Haddon, R. C.; Hicks, R. G.; Kennepohl,
D. K.; Oakley, R. T.; Schneemeyer, L. F.; Waszczak, J. V. Inorg. Chem.
1993, 32, 1554.
(40) This effect is well recognized for purely organic radicals. See, for
example: Zaitsev, V.; Rosokha, S. V.; Head-Gordon, M.; Kochi, J. K. J.
Org. Chem. 2006, 71, 520.
(41) Heuze´, K.; Fourmigue´, M. A.; Batail, P.; Coulon, C.; Cle´rac, R. B.; Canadell,
E. C.; Auban-Senzier, P. A.; Ravy, S. D.; Je´rome, D. AdV. Mater. 2003,
15, 1251.
(42) Ohno, K.; Noguchi, N.; Yokoi, T.; Ishii, S.; Takeda, J.; Furuya, M.
ChemPhysChem 2006, 7, 1820.
(43) Britten, J. F.; Clements, O. P.; Cordes, A. W.; Haddon, R. C.; Oakley, R.
T.; Richardson, J. F. Inorg. Chem. 2001, 40, 6820.
9
7910 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

An Alternating
π
-Stacked Bisdithiazolyl Radical Conductor
A R T I C L E S
Figure 15. Schematic density of states (DOS) diagram showing donor (D0)
and acceptor (D-) Hubbard defect bands in 2 (R ) Me), with low-
temperature excitations between these states (A) and high-temperature
excitations from/to the valence/conduction bands (B).
Figure 13. Radical defects (B, C) generated upon heating perfectly ordered
π-dimer stacks (A). A fully symmetric π-radical stack (D) is also shown.
however, be split by Coulomb correlations into donor (D0) and
acceptor (D-) Hubbard sub-bands.44 The low-temperature
conductivity (<120 K) may thus be viewed as arising from
variable-range hopping between these weakly metallic midgap
states (Figure 15A). These processes can be collectively
associated with a thermal activation energy Eact ) 0.12 eV
(Figure 9). At elevated temperatures (>120 K), the conductivity
stems from hopping that also involves excitations between the
midgap states and the band edges of either the valence or
conduction bands (Figure 15B), processes for which Eact ) 0.19
eV, a value in good agreement with the band gap estimated
from the optical measurements and EHT calculations. The
infrared spectrum can also be explained within this model. The
poorly resolved background below 3000 cm-1 (Figure 11) can
be ascribed to electronic excitations between the midgap states
and the valence/conduction bands, while the strong, broad band
above 3000 cm-1 corresponds to valence-to-conduction band
excitations.
Figure 14. Head-over-tail π-dimer stacking in dithiazolyl radical 10.
13C) radical defects. At the same time, the previously regular
tilt angle (φ) of the dimers is replaced by a random clockwise
or counterclockwise rocking. Any of these structural modifica-
tions, or a combination thereof, will give rise to a gradual
increase in magnetization and, at the same time, lead to a loss
of crystallographic correlation in the orientation of the molecules
along the stacking direction. This loss of correlation creates the
illusion of higher symmetry. The observation of C-centering
and assignment of the Cmca space group is thus not a result of
perfect ordering of uncoupled radicals, as depicted in Figure
13D, but rather a consequence of thermally induced disorder
along the π-stacks of the P21/c cell, with the Pccn cell
representing an intermediate phase in this process. The elongated
thermal ellipsoids for the sulfur atoms in the Cmca structure
(Figure 2) attest to the fact that the high-symmetry space group
is an artifact of the diffraction experiment and that, over the
entire temperature range studied, the transport properties of the
material are best rationalized in terms of a structure consisting
of defect-contaminated π-dimer stacks.
Given the above structural description, we propose a hopping
model44 to account for the conductivity of 2 (R ) Me) as a
function of temperature. We have used this approach before to
rationalize the transport properties of the dithiazolyl radical 10
(Figure 14),34 the solid-state structure of which showed a head-
over-tail π-dimer stacking motif similar to the ABABAB
arrangement described here. In the present case, topological
defects and lattice disorder give rise to a finite density of states
(Figure 15) within the band gap of the ideal P21/c structure
(Figure 12), even at low temperatures (<120 K). With increasing
temperature, the radical spins generated by uncoupling of the
dimers increase the density of these defect states. These states
are not localized and contribute to the conductivity. They may,
Summary and Conclusions. We have developed a synthetic
route to a new class of resonance-stabilized radical, one based
on the use of a pyrazine bridging unit between two 1,2,3-
dithiazole rings. Two examples of these highly delocalized
radicals 2 (R ) Me, Et) have been characterized in solution by
EPR spectroscopy and cyclic voltammetry. From these studies,
we conclude that these materials should enjoy a relatively low
onsite Coulomb repulsion U in the solid state. Structural studies
on the ethyl derivative 2 (R ) Et) reveal that the radicals
crystallize in two different dimeric modifications; in both cases,
the radical spins are paired and not available to serve as charge
carriers.20 By contrast, the corresponding methyl derivative 2
(R ) Me) crystallizes in alternating ABABAB π-stacked arrays,
with rows of π-stacks dovetailed together by π-type interannular
S-S′ interactions. The conductivity of this material is the highest
ever seen for a neutral thiazyl radical, with a room-temperature
value σRT ≈ 0.001 S cm-1. The material is not, however, a
metal, as the conductivity remains activated, even under an
applied pressure of 5.5 GPa. The high-symmetry space group
Cmca observed crystallographically at ambient temperatures is
1
nonetheless consistent with an f ) /2 metallic state. Variable-
temperature magnetic measurements confirm diamagnetic be-
havior below 120 K, and low-temperature crystallographic work
establishes a sequential lowering in space group symmetry from
Cmca (295 K) to Pccn (123 K) to P21/c (88 K), changes which
give rise to an opening of a band gap at the Fermi level and the
formation of a semiconducting ground state. While the activated
(44) Mott, N. F.; Davis, E. A. Electronic Processes in Non-Crystalline Materials,
2nd ed.; Oxford University Press: Oxford, 1993.
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7911

A R T I C L E S
Leitch et al.
10 min, a solution of 5 (10.0 g, 0.0908 mol) in 100 mL of MeOH was
added dropwise to the reaction mixture over 1 h, and the mixture was
stirred for an additional hour at -78 °C. The mixture was warmed to
room temperature and stirred for 16 h before being added to 2 L of
ice/water. After 1 h, the yellow product 6 was filtered off and washed
with water; yield 15.0 g (0.0669 mol, 74%). IR: 3442 (w), 3339 (w),
3183 (w), 2168 (s), 2157 (s), 1626 (s), 1513 (s), 1430 (m), 1309 (m),
1254 (m), 1151 (w), 1036 (w), 764 (s), 676 (m), 480 (s), 454 (m), 409
(w) cm-1. Anal. Calcd for C6H4N6S2: C, 32.13; H, 1.80; N, 37.47.
Found: C, 32.17; H, 1.59; N, 37.20.
conductivity at room temperature could be ascribed to Mott
insulator behavior, we believe that the high-symmetry Cmca
space group is an artifact arising from the loss of long-range
correlation in the slightly tilted π-stacks of the P21/c structure.
As a result, at ambient temperatures, the material is still best
described as a semiconductor, with conductivity arising from
hopping within and to/from midgap states produced by radical
defects present in the lattice. It remains to be seen if modifica-
tions to the framework of 2, that is, changes in the R group or
the replacement of sulfur by selenium, as has recently been
achieved for 1,19 will afford more conductive materials, and
perhaps a metallic ground state.
Preparation of 2-Tributylphosphinimino-6-aminopyrazine-3,5-
dithiol, 8. Tributylphosphine (27.0 mL, 0.108 mol) was added to a
thick slurry of 6 (10.4 g, 0.0463 mol) in 250 mL of MeCN to give a
red solution. Upon brief stirring, a fine orange precipitate was formed.
After 30 min, water (1.70 mL, 0.0943 mol) was added, and the reaction
mixture was stirred for an additional 30 min before filtration to afford
an orange microcrystalline powder. The product, 8, which crystallizes
as a hydrate, was washed with 2 × 150 mL of MeCN and 1 × 150 mL
of diethyl ether; yield 12.8 g (0.0326 mol, 70%); mp 115 °C dec. IR:
3422 (w), 3276 (w), 3084 (w), 2252 (w), 1582 (m), 1553 (m), 1521
(m), 1421 (m), 1358 (m), 1292 (m), 1225 (m), 1209 (m), 1144 (m),
1113 (w), 1096 (w), 1044 (m), 937 (m), 906 (m), 891 (w), 833 (w),
Experimental Section
General Procedures and Starting Materials. The reagents pyra-
zine, sodium azide, ammonium thiocyanate, tri-n-butylphosphine,
thionyl chloride, pyridine, iron powder, gallium trichloride, Proton-
Sponge, decamethylferrocene (DMFc), trimethyloxonium, and triethy-
loxonium tetrafluoroborates were obtained commercially. All were used
as received, save for DMFc, which was sublimed and recrystallized
from acetonitrile before use. All solvents were of at least reagent grade;
acetonitrile (MeCN) and dichloroethane (DCE) were dried by distillation
from P2O5. All reactions were performed under an atmosphere of dry
nitrogen. 2,6-Dichloropyrazine 345 and the corresponding diazide 421
were prepared according to literature methods. Melting points are
uncorrected. Fractional sublimations were performed in an ATS series
3210 three-zone tube furnace, mounted horizontally, and linked to a
series 1400 temperature control system. Infrared spectra (Nujol mulls,
KBr optics) were recorded on a Nicolet Avatar FTIR spectrometer (at
2 cm-1 resolution), and visible spectra were collected using a Beckman
780 (w), 459 (w) cm-1 31P NMR (δ, CDCl3): 54.6 (s, 1P). Anal. Calcd
.
for C16H33N4OPS2: C, 48.95; H, 8.47; N, 14.27. Found: C, 48.22; H,
7.93; N, 13.91. The crude material was desolvated by heating it
at 80 °C/10-3 Torr.
Preparation of 4H-Bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]pyrazin-2-ium
Tetrachlorogallate, [2][GaCl4] (R ) H). Pyridine (6.40 mL, 0.0791
mol) was added to a slurry of 8 (14.9 g, 0.0398 mol) in 400 mL of
MeCN. A solution of SOCl2 (31.0 mL, 0.425 mol) in 30 mL of MeCN
was added dropwise to the slurry over 30 min to give a brown mixture
(slight exotherm), which was set to reflux for 1 h. After the mixture
cooled to room temperature, the brown solid [2][Cl] (R ) H) was
filtered off and washed with 200 mL of MeCN and 200 mL of DCE;
yield 9.92 g (0.0369 mol, 93%). IR: 1634 (w), 1562 (w), 1548 (w),
1535 (w), 1425 (s), 1319 (s), 1083 (m), 885 (w), 868 (w), 802 (w),
1
DU 640 spectrophotometer. H and 31P NMR spectra were run on a
Bruker Avance 300 MHz NMR spectrometer. Low-resolution mass
spectra (70 eV, EI, DEI and CI, DCI) were run on a Micromass Q-TOF
Ultima Global LC/MS/MS system or a JEOL HX110 double-focusing
mass spectrometer. Elemental analyses were performed by MHW
Laboratories (Phoenix, AZ).
751 (w), 718 (m), 683 (w), 618 (m), 506 (m), 481 (m), 447 (m) cm-1
.
Gallium trichloride (8.00 g, 0.0454 mol) was added to a slurry of crude
[2][Cl] (R ) H) (10.1 g, 0.0375 mol) in 150 mL of MeCN to afford a
dark violet-blue solution, which was stirred for 15 min before being
filtered to remove any undissolved material. The solvent was removed
from the filtrate by flash distillation to leave a purple-bronze residue
that was triturated in 30 mL of a 1:1 HOAc/DCE solution. The gallate
[2][GaCl4] (R ) H) was filtered and washed several times with HOAc/
DCE mixtures and then neat HOAc, followed by diethyl ether; yield
10.3 g (0.0231 mol, 62%). IR: 3194 (w), 1615 (w), 1559 (w), 1544
(w), 1427 (m), 1415 (m), 1330 (m), 1311 (m), 1088 (w), 880 (w), 847
Preparation of 2,6-Diaminopyrazine, 5. Sodium azide (5.50 g, 84.6
mmol) was slowly added as a powdered solid to a stirred solution of
dichloropyrazine 3 (6.00 g, 40.3 mmol) in 75 mL of dimethylsulfoxide,
and the mixture was heated to 70 °C in an oil bath for 1 h. The turbid
orange solution was poured onto 1 L of ice/water, and the resulting
white fibrous precipitate of the diazide 4 was filtered off on a Bu¨chner
funnel and washed with water (caution! 46). The crude (and still wet)
diazide was transferred into a 500 mL flask, to which was added 200
mL of water, 100 mL of EtOH, 3 mL of HOAc, and iron filings (10.0
g). The mixture was heated to a gentle boil for 30 min, cooled, and
filtered. The filtrate was made alkaline (to pH > 12) with NaOH, and
the resulting gelatinous precipitate was filtered off. The aqueous filtrate
was then flash evaporated and the residual solid extracted with 3 ×
300 mL of boiling DCE. The combined extracts were evaporated to
afford crude 2,6-diaminopyrazine 5 as an off-white solid. Recrystalli-
zation from DCE afforded fibrous needles; yield 3.10 g (28.2 mmol,
70% from 3); mp 135-137 °C (lit.21 mp 136-137 °C). IR: 3407 (w),
3312 (s), 3167 (s, br), 1632 (s), 1587 (m), 1537 (vs), 1293 (s), 1249
(m), 1137 (m), 1000 (m), 821 (m), 753 (w), 620 (w), 572 (w), 473 (w)
(w), 723 (m), 635 (m), 505 (s), 479 (s) cm-1
.
Preparation of Bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]pyrazine, 9. A
sample of [2][GaCl4] (R ) H) (8.19 g, 0.0184 mol) was dissolved in
150 mL of MeCN to afford a blue solution that was filtered to remove
any undissolved solid. A solution of Proton-Sponge (4.76 g, 0.0222
mol) in 150 mL of MeCN was added dropwise to the filtrate over 15
min. After 20 min, the blue-gray solid 9 was collected by filtration
and washed with 3 × 100 mL of MeCN; yield 4.30 g (0.0180 mol,
100%). IR: 1605 (w), 1412 (s), 1264 (s), 1124 (w), 1086 (m) cm-1
.
cm-1
.
Preparation of 4-Methyl-4H-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]-
pyrazin-2-ium Tetrafluoroborate, [2][BF4] (R ) Me). Proton-Sponge
(2.03 g, 0.00945 mol) was added to a slurry of Me3OBF4 (5.58 g, 0.0377
mol) in 50 mL of DCE to afford a yellow slurry. Zwitterion 9 (4.39 g,
0.0189 mol) was then added, and the blue-green reaction mixture was
stirred for 16 h at room temperature. The brown solid was filtered off
and washed with 2 × 40 mL of DCE. In order to remove protonated
impurities, the crude product was rapidly stirred in 200 mL of warm
(70 °C) HOAc for 40 min. The material so obtained was further purified
by double recrystallization from MeCN to afford [2][BF4] (R ) Me)
Preparation of 2,6-Diaminopyrazine-3,5-bisthiocyanate, 6. A
solution of bromine (29.0 g, 0.181 mol) in 100 mL of cold MeOH was
added dropwise to a cold (-78 °C) solution of NH4SCN (27.6 g, 0.363
mol) in 500 mL of MeOH over 1 h to afford a colorless slurry. After
(45) (a) Klein, B.; Hetman, N. E.; O’Donnell, M. E. J. Org. Chem. 1963, 28,
1682. (b) Klein, B.; Berkowitz, J. J. Am. Chem. Soc. 1959, 81, 5160.
(46) Azides can be explosive, although there is no evidence that this one is.
Avoid spatulas and glass frits, and minimize exposure to light (the dry
solid turns blue on exposure to air).
9
7912 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

An Alternating
π
-Stacked Bisdithiazolyl Radical Conductor
A R T I C L E S
as lustrous red crystals; yield 1.64 g (0.00491 mol, 26%); mp 273 °C
dec. IR: 1614 (w), 1541 (m), 1503 (s), 1337 (s), 1198 (m), 1120 (m),
1085 (s), 1051 (s), 1019 (s), 951 (w), 891 (m), 877 (w), 720 (m), 663
respect to the quasi-reference electrode in a single-compartment cell
fitted with Pt electrodes and referenced to the Fc/Fc+ couple of ferrocene
at 0.38 V vs SCE.47 The Epa-Epc separation of the reversible couple
was within 10% of that of the Fc/Fc+ couple.
1
(s), 526 (m), 513 (w), 488 (m), 477 (m) cm-1. H NMR (δ, CD3CN):
3.49 (s, 3H, CH3). UV-vis (MeCN): λmax 623 nm (log ꢀ ) 4.6). Anal.
Calcd for C5H3BF4N4S4: C, 17.97; H, 0.90; N, 16.77. Found: C, 18.08;
H, 0.73; N, 17.00.
EPR Spectra. The X-band EPR spectra were recorded at ambient
temperature using a Bruker EMX-200 spectrometer on samples of 2
(R ) Me, Et) dissolved in degassed dichloromethane. Hyperfine
coupling constants were obtained by spectral simulation using Simfo-
nia.48
Preparation of 4-Ethyl-4H-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]-
pyrazin-2-ium Tetrafluoroborate, [2][BF4] (R ) Et). Proton-Sponge
(1.76 g, 0.00819 mol) was added to a colorless solution of Et3OBF4
(6.18 g, 0.0325mol) in 60 mL of DCE to give a clear yellow solution.
Zwitterion 9 (3.77 g, 0.0162 mol) was then added, and the blue-green
reaction mixture was stirred for 16 h at room temperature. The red-
brown crude product was collected by filtration and washed with 2 ×
45 mL of DCE. To remove protonated impurities, the crude product
was warmed at 70 °C in 200 mL of HOAc for 40 min. This material
was further purified by double recrystallization from MeCN to give
[2][BF4] (R ) Et) as lustrous red crystals; yield 2.70 g (0.00775 mol,
48%); mp 276 °C dec. IR: 1538 (m), 1503 (s), 1335 (s), 1198 (m),
1181 (m), 1069 (s), 1032 (s), 887 (m), 789 (m), 722 (m), 657 (m), 549
(w), 519 (m), 512 (m), 483 (m) cm-1. 1H NMR (δ, CD3CN): 4.02 (q,
2H, NCH2CH3, J ) 7.2 Hz), 1.27 (t, 3H, NCH2CH3, J ) 7.2 Hz). UV-
vis (MeCN): λmax 623 nm (log ꢀ ) 4.5). Anal. Calcd for C6H5-
BF4N4S4: C, 20.70; H, 1.45; N, 16.09. Found: C, 20.86; H, 1.25; N,
16.31.
X-ray Measurements. The needle of 2 (R ) Me) used for the 295
and 123 K data sets was glued to a glass fiber with epoxy, and the
crystal for the 88 K data set was mounted on a loop with paratone.
The 295 and 123 K data sets were collected using ω-scans with a Bruker
SMART6000 CCD detector on a D8 three-circle goniometer and
parallel-focused Cu KR radiation from a Rigaku RU-200 fine-focus
rotating anode generator at 5 kW. The 88 K data set was collected
using φ- and ω-scans on a Bruker SMART APEX II diffractometer
with an APEX II CCD area detector on a D8 three-circle goniometer
and Mo KR (λ ) 0.71073 Å) radiation. The data were scanned using
Bruker’s APEX 2 program and integrated using Bruker’s SAINT
software.49 Processing of all the data was completed using the APEX
2 software by direct methods using SHELXS-9050 and refined by least-
squares methods on F2 using SHELXL-97,51 incorporated in the
SHELXTL52 suite of programs. A merohedral twin law (180° rotation
about the c-axis) was applied in order to refine the 88 K data. All non-
hydrogen atoms were refined anisotropically; hydrogen atoms were
located on difference maps and refined isotropically. Details of data
collection and refinement are presented in Table 2.
Band Structure Calculations. Band electronic structure calculations
were performed with the EHMACC suite of programs53 using the
Coulomb parameters of Baasch, Viste, and Gray54 and a quasi-split-
valence basis set adapted from Clementi and Roetti;55 numerical values
are tabulated elsewhere.56 The off-diagonal elements of the Hamiltonian
matrix were calculated with the standard weighting formula.57 Atomic
positions were taken from the crystallographic data.
Magnetic Susceptibility Measurements. Magnetic susceptibility
measurements on 2 (R ) Me) were performed over the temperature
range 9-310 K on a George Associates Faraday balance operating at
0.5 T.
Preparation of 4-Methyl-4H-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]-
pyrazin-3-yl, 2 (R ) Me). A carefully degassed (four freeze-pump-
thaw cycles) solution of [2][BF4] (R ) Me) (0.250 g, 0.748 mmol) in
75 mL of MeCN was added to an equally degassed solution of DMFc
(0.256 g, 0.784 mmol) in 130 mL of MeCN. After 30 min, fine gold
crystals of analytically pure 2 (R ) Me) were collected by filtration
and washed with 3 × 15 mL of MeCN; yield 0.152 g (0.614 mmol,
84%). IR: 1308 (w), 1212 (w), 1030 (m), 842 (w), 697 (w), 650 (w),
516 (m), 483 (w), 433 (w) cm-1. Crystals suitable for crystallographic
work and single-crystal conductivity measurements were obtained by
the slow diffusion of a degassed solution of [2][BF4] (R ) Me) (0.030
g, 0.0898 mmol) in 10 mL of MeCN into a similarly degassed solution
of DMFc (0.030 g, 0.0919 mmol) in 15 mL of MeCN through a fine-
porosity frit. The fine copper needles were harvested after 16 h. Anal.
Calcd for C5H3N4S4: C, 24.28; H, 1.22; N, 22.65. Found: C, 24.40;
H, 1.20; N, 22.81.
Ambient-Pressure Conductivity Measurements. Ambient-pressure
single-crystal conductivity measurements on 2 (R ) Me) were made
using a four-probe configuration along the needle axis (which corre-
sponds to the crystallographic x-direction), with in-line contacts made
using silver paint. The measurements were performed on a custom-
made helium variable-temperature probe using a Lake Shore 340
temperature controller. A Keithley 236 unit was used as a voltage source
and current meter, and two 6517A Keithley electrometers were used
to assess the voltage drop between the potential leads in the four-probe
configuration.
Preparation of 4-Ethyl-4H-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]-
pyrazin-3-yl, 2 (R ) Et). A sample of [2][BF4] (R ) Et) (0.700 g,
0.00201 mol) and DMFc (0.689 g, 0.00211 mol) were combined in 30
mL of degassed MeCN to give a fuschia-colored solution. After 2 h,
the light brown solid was filtered off and washed with 3 × 10 mL of
MeCN; yield 0.393 g (0.00150 mol, 75%). Vacuum sublimation of the
isolated material at 10-4 Torr in a three-zone furnace along a
temperature gradient from 100 °C to 50 °C gave a mixture of amber
blocks of R-[2]2 and black needles of â-[2]2, which could be manually
separated. Resublimation of the mixed material at 10-4 Torr along a
temperature gradient from 110 °C to 50 °C afforded 100% black needles
of â-[2]2. IR of R-[2]2: 1606 (w), 1582 (w), 1562 (m), 1526 (w), 1496
(w), 1343 (w), 1262 (w), 1227 (w), 1179 (w), 1123 (w), 1067 (w),
1002 (w), 864 (w), 836 (w), 796 (w), 774 (w), 694 (m), 662 (w), 641
(m), 550 (w), 511 (w), 483 (w), 467 (w), 429 (w) cm-1. IR of â-[2]2:
1534 (m), 1434 (s), 1403 (w), 1353 (w), 1334 (s), 1262 (m), 1224 (s),
1183 (m), 1065 (m), 880 (s), 788 (w), 727 (m), 668 (w), 648 (w), 648
(w), 546 (w), 485 (w), 464 (w), 420 (w) cm-1. Anal. Calcd for â-phase
C6H5N4S4: C, 27.57; H, 1.93; N, 21.43. Found: C, 27.61; H, 2.00; N,
21.26.
High-Pressure Conductivity Measurements. High-pressure tem-
perature conductivity experiments on 2 (R ) Me) were carried out in
(47) Boere´, R. T.; Moock, K. H.; Parvez, M. Z. Anorg. Allg. Chem. 1994, 620,
1589.
(48) WinEPR Simfonia, version 1.25; Bruker Instruments, Inc.: Billerica, MA,
1996.
(49) SAINT, version 6.22; Bruker Advanced X-ray Solutions, Inc.: Madison,
WI, 2001.
(50) Sheldrick, G. M. SHELXS-90. Acta Crystallogr. A 1990, 46, 467.
(51) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(52) SHELXTL, Version 6.12, Program Library for Structure Solution and
Molecular Graphics; Bruker Advanced X-ray Solutions, Inc.: Madison,
WI, 2001.
(53) EHMACC, Quantum Chemistry Program Exchange, program no. 571.
(54) Baasch, H.; Viste, A.; Gray, H. B. Theor. Chim. Acta 1965, 3, 458.
(55) Clementi, E.; Roetti, C. At. Data Nucl. Data Tables 1974, 14, 177.
(56) Cordes, A. W.; Haddon, R. C.; Oakley, R. T.; Schneemeyer, L. F.;
Waszczak, J. V.; Young, K. M.; Zimmerman, N. M. J. Am. Chem. Soc.
1991, 113, 582.
Cyclic Voltammetry. Cyclic voltammetry was performed using a
PINE Bipotentiostat, model AFCClBP1, with scan rates of 50-100
mV s-1 on solutions (<10-3 M) of [2][BF4] (R ) Me, Et) in oxygen-
free MeCN (dried by distillation from CaH2) containing 0.1 M tetra-
n-butylammonium hexafluorophosphate. Potentials were scanned with
(57) Ammeter, J. H.; Bu¨rgi, H. B.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem.
Soc. 1978, 100, 3686.
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7913

A R T I C L E S
Leitch et al.
a cubic anvil press58 using pyrophyllite (Al4Si8O20(OH)4) as the
pressure transmitting medium. Sample pressure was determined from
previous calibrations of the applied hydraulic load against pressures
of structure transformations in standards at room temperature (Hg L
T I at 0.75 GPa, Bi I T II at 2.46 GPa, Tl 1 T III at 3.70 GPa, and
Ba 1 T II at 5.5 GPa).59 Temperature was applied by Joulean heating
of a cylindrical Nb foil (0.127 mm thick) furnace and monitored with
a Pt/(Pt + 10% Rh) thermocouple, using a pressure-corrected electro-
motive force.59,60 Two Pt electrodes contacted the disk-shaped (∼6.1
mm diameter and ∼0.35 mm thick), pre-compacted powder sample
which was contained in a boron nitride (σBN ≈ 10-11 S cm-1) cup.
Four-wire alternating current (Solartron 1260 impedance analyzer)
resistance measurements were made at a frequency of 1 kHz. Separate
experiments were performed at room temperature vs pressure (0.3-
5.5 GPa) and at fixed pressures vs temperature (22-90 °C). In the
constant room-temperature experiments, resistance was measured on
increasing/decreasing pressure over 5-6 h. In the variable-temperature
experiments, resistance was measured at fixed temperature intervals
of 5-8 °C on heating/cooling at constant pressure. Pressure was
increased and the temperature was cycled again. The contiguous disk-
shaped sample was extracted from the recovered pressure cell, and the
sample geometry was measured to convert resistance to conductivity.
The IR spectrum of the recovered sample showed no change upon
comparison with the pre-compression spectrum.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada (NSERCC) for
financial support. We also acknowledge the NSERCC for a
Canada Graduate Scholarship to A.A.L. and the Canada Council
for a Killam Research Fellowship to R.T.O. We are grateful to
Dr. Mikhail Itkis and Prof. Robert Haddon of the University of
California at Riverside for performing single-crystal conductiv-
ity, near-IR, and magnetic measurements on 2 (R ) Me).
Supporting Information Available: Complete ref 33b, pack-
ing diagrams, and intermolecular contacts (PDF); details of
X-ray crystallographic data collection and structure refinement,
tables of atomic coordinates, bond distances and angles,
anisotropic thermal parameters, and hydrogen atom positions
(CIF). This information is available free of charge via the
Internet at http://pubs.acs.org.
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