A bimodal oxobenzene-bridged bisdithiazolyl radical conductor

Article abstract of DOI:10.1021/cg300107t

The preparation and structural characterization of the methyl-substituted oxobenzene-bridged bisdithiazolyl radical 3b is described. Crystals of 3b belong to the monoclinic space group C2/c and contain two distinct radical environments, A and B. There are eight A radicals in the unit cell, which occupy general positions and form alternating twisted π-stacks running parallel to the c-axis. The four B radicals also adopt an alternating π-stack pattern, but each molecule lies on a crystallographic 2-fold rotation axis, and the overlay of neighboring radicals is centrosymmetric. Stacks of A radicals are linked by close intermolecular S???O′ and S???N′ contacts into ribbon-like arrays that weave along the y-direction, and the B radical stacks are located in columnar cavities generated by the out-of-register alignment of the ribbons of A radicals. Variable temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, a result in accord with DFT estimated exchange energies for intrastack radical-radical interactions. Four-probe conductivity measurements indicate a conductivity σ(300 K) = 9.0 × 10^-4 S cm^-1, with a thermal activation energy E _act = 0.13 eV.

Full text of DOI:10.1021/cg300107t

Article
pubs.acs.org/crystal
A Bimodal Oxobenzene-bridged Bisdithiazolyl Radical Conductor
Xin Yu,† Aaron Mailman,† Kristina Lekin,† Abdeljalil Assoud,† Paul A. Dube,§ and Richard T. Oakley*,†
†Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
§Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario L8S 4M1, Canada
S
* Supporting Information
ABSTRACT: The preparation and structural characterization of the
methyl-substituted oxobenzene-bridged bisdithiazolyl radical 3b is
described. Crystals of 3b belong to the monoclinic space group C2/c
and contain two distinct radical environments, A and B. There are eight
A radicals in the unit cell, which occupy general positions and form
alternating twisted π-stacks running parallel to the c-axis. The four B
radicals also adopt an alternating π-stack pattern, but each molecule lies
on a crystallographic 2-fold rotation axis, and the overlay of neighboring
radicals is centrosymmetric. Stacks of A radicals are linked by close
intermolecular S···O′ and S···N′ contacts into ribbon-like arrays that
weave along the y-direction, and the B radical stacks are located in columnar cavities generated by the out-of-register alignment of
the ribbons of A radicals. Variable temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically
coupled system, a result in accord with DFT estimated exchange energies for intrastack radical−radical interactions. Four-probe
conductivity measurements indicate a conductivity σ(300 K) = 9.0 × 10−4 S cm−1, with a thermal activation energy Eact = 0.13
eV.
The development of N-alkylated pyridine-bridged bisdithia-
zolyl radicals 1 (Chart 1) represented a major step forward, as
INTRODUCTION
■
The idea that the unpaired electron in a neutral organic radical
might serve as a carrier of charge, thereby allowing the
development of single component molecular metals with a half-
filled (f = 1/2) energy band, was first proposed by Haddon in
1975.1 In practice, however, the realization of conductivity in
carbon-based radicals has not been easy, although considerable
progress has been made in recent years by using phenalenyls
and spirophenalenyls.2 Other potential building blocks include
highly stable, light heteroatom (N, O) radicals, such as aminyls,
nitroxyls, and verdazyls,3 but in these systems spin density is
too localized, and intermolecular hopping of the unpaired
electrons is suppressed by the large onsite Coulomb repulsion
associated with the f = 1/2 band structure.4
Heavy atom radicals, notably thiazyls and selenazyls, have
also been pursued as building blocks for conductive materials,5
in the belief that the expected strong overlap between orbitals
on neighboring sulfur (or selenium) atoms will increase solid
state bandwidth W and hence facilitate charge migration.
However, when coupled with the relatively confined spin
distribution inherent in early heterocyclic thiazyls, this
increased orbital overlap led to a pronounced tendency for
radical association. More importantly spin localization caused a
high onsite Coulomb repulsion energy U,6 so that even when
dimerization could be avoided, intermolecular overlap and the
resulting bandwidth were still insufficient to overcome charge
repulsion, and conductivity was low.7 While dramatic improve-
ments in conductivity could be induced by p-type doping,8 the
restricted spin distribution in the neutral (f = 1/2) materials
remained a serious impediment to their use in the design of
purely neutral, single component systems.
Chart 1
resonance stabilization between the two dithiazolyl “wings”
effectively doubled spin delocalization.9 Consistently, the gas
phase disproportionation energies ΔHdisp and solution-based
10
electrochemical cell potentials Ecell of these radicals, which
provide indirect measures of U, were significantly reduced
relative to those of earlier monocyclic derivatives. However,
most examples of 1 crystallized as slipped π-stack arrays locked
into herringbone packing patterns (Figure 1a), a motif which
compromised bandwidth W and hence charge transport,
although replacement of sulfur by its more spatially extensive
congener selenium afforded significant improvements in both
conductivity11 and magnetic properties.12 Isoelectronic replace-
ment of the basal CR2 unit by nitrogen, as in the N-alkylated
Received: January 24, 2012
Revised: March 5, 2012
Published: March 29, 2012
© 2012 American Chemical Society
2485
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
Scheme 1
Figure 1. Packing motifs found in 1, 2, and 3: (a) herringbone π-
stacks, (b) alternating head-over-tail π-stacks, and (c) slipped ribbon
π-stacks.
pyrazine-bridged radicals 2, afforded new solid state structure
types. Thus, while removal of the steric protection afforded by
the basal ligand rendered the ethyl derivative 2b susceptible to
dimerization,13 the corresponding methyl compound 2a
displayed an alternating ABABAB π-stack structure (Figure
1b),14 the bandwidth and conductivity of which were
dramatically improved relative to those seen in the herringbone
structures adopted by 1. However, at low temperatures (<120
K) the evenly spaced π-stacks of 2a collapsed into dimers.
To break the herringbone mold found in 1, while at the same
time avoiding the spin-quenching dimerization found for 2, we
are currently exploring the structural and transport properties
of radicals based on the resonance stabilized, oxobenzene-
bridged bisdithiazolyl framework 3, in which the NR1 moiety of
1 is replaced by a carbonyl group. While the molecular
electronic structure of these new radicals is similar to that of 1,
the polarity of the carbonyl CO bond leads to the development
of strong intermolecular S···O′ interactions15 in the solid state.
These supramolecular synthons16 are capable of generating
rigid, ribbon-like arrays which pack as slipped π-stacks (Figure
1c) and afford improved charge transport.
Here we expand on our recent work on the chloro- and
phenyl-substituted derivatives 3a17 and 3c,18 and describe the
preparation and structural characterization of the methyl-
substituted compound 3b (R = Me).19 From a structural
perspective this latter material has proved particularly
interesting, as it contains two crystallographically distinct
radical environments, one packed within the superstructure
created by the other. Variable temperature magnetic suscept-
ibility and conductivity measurements on 3b are reported, and
the results interpreted in the light of Density Functional
a
Table 1. Electrochemical Potentials for 1 and 3
b
b
c
E1/2 (−1/0)
−0.835
−0.940
E1/2 (0/+1)
Ecell
d
1 (R2 = Cl)
0.005
−0.136
−0.104
0.195 (0.158)
0.073 (0.042)
0.108 (0.071)
0.84
e
f
h
1 (R2 = Me)
0.80
e
1 (R2 = Ph)
3a (R = Cl)
3b (R = Me)
3c (R = Ph)
−0.956
−0.481
−0.557
−0.529
0.85
f
g
g
g
h
0.64
f
h
0.59
f
h
0.60
a
b
c
Volts, in MeCN. Reference to SCE. Ecell = |E1/2 (0/+1) − E1/2
d
e
f
(−1/0)|. R1 = Me, see ref 9b. R1 = Me, see ref 9d. Irreversible
g
reduction, only the cathodic peak potential Epc is cited. E1/2 and Epc
h
(in parentheses). Ecell estimated as |Epc (0/+1) − Epc (−1/0)|.
couple of 3a, 3b, and 3c which reflect the electron-withdrawing
power of the axial ligand R; similar trends are observed for
derivatives of [1].9b,d Also provided in Table 1 is a comparison
of the cell potentials Ecell for 1 and 3. In those cases where the
−1/0 couple is irreversible the magnitude of Ecell has been
estimated in terms of the difference in the Epc values for the
−1/0 and 0/+1 couples. Collectively, the low Ecell values
observed for the oxobenzene-bridged radicals augur well for
improved charge transport in these materials.
Based on the electrochemical data we selected octamethyl-
ferrocene OMFc (E1/2(ox) = −0.038 V vs SCE)12b,20 as a
suitable reagent to convert [3b][OTf] to the corresponding
radical 3b, and subsequent bulk reduction of [3b][OTf] with
OMFc was certainly effective in producing quantities of
microcrystalline radical sufficient for magnetic and conductivity
measurements. While slow codiffusion of MeCN solutions of
OMFc and [3b][OTf] in a glass H-cell also produced
predominantly microcrystalline material, a few crystalline
shards of 3b suitable for single crystal X-ray diffraction work
were isolated. Powder diffraction analysis of the bulk
microcrystalline material confirmed a structural match and
phase uniformity.
Crystallography. The structural information for 3b
originates from a single crystal gleaned from a codiffusion
(H-cell) reduction. Table 2 lists crystal data, while Figure 2
illustrates ORTEP drawings of the two radicals in the
asymmetric unit, and provides the atom numbering scheme.
Intramolecular distances and angles are all normal for radicals
of this type; intermolecular S···O′, S···N′, and S···S′ contacts are
listed in Table 3. As noted above, the structure and phase purity
of the bulk material was confirmed by powder diffraction
methods; the results of a Rietveld refinement performed in
GSAS are illustrated in Figure 3.21
Theory (DFT) and Extended Huckel Theory (EHT) band
̈
electronic structure calculations.
RESULTS
■
Synthesis and Electrochemistry. The preparative se-
quence to the oxobenzene-bridged bis-1,2,3-dithiazolyl 3b
(Scheme 1) starts from 2,6-dinitro-p-cresol 4 which, upon
reduction with Sn/HCl yields the bishydrochloride of 2,6-
diamino-4-methylphenol 5. This latter compound undergoes a
double Herz cyclization with sulfur monochloride at reflux in
acetonitrile to afford the chloride salt [3b][Cl] as an insoluble
purple-brown powder. This material can be converted into the
more soluble triflate (OTf−) salt by metathesis with silver
triflate. The choice of reducing agent for the conversion of the
salt [3b][OTf] to the neutral radical 3b was made on the basis
of cyclic voltammetric (CV) measurements on solutions of
[3b][OTf] in MeCN, with 0.1 M n-Bu4NPF6 as supporting
electrolyte and Pt wire electrodes. As may be seen from the
compilation of half-wave potentials listed in Table 1, there are
some differences in the potentials for the reversible 0/+1
Crystals of 3b belong to the monoclinic space group C2/c,
with Z = 8. There are 1 1/2 molecules in the asymmetric unit
(Figure 2), with the B radicals occupying special positions,
2486
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
Table 2. Crystal Data for 3b
formula
M
C10.5H4.5N3O1.5S6
389.03
a, Å
25.828(3)
14.5799(14)
7.3586(7)
95.687(2)
2757.4(5)
1.871
b, Å
c, Å
β, deg
V, Å3
ρcalcd (g cm−1
space group
Z
)
C2/c
8
temp (K)
296(2)
μ (mm−1
)
0.993
λ (Å)
0.71073
data/restraints/params
solution method
R, Rw (on F2)
4020/0/192
direct methods
0.0582, 0.0774
Figure 3. Observed and calculated powder X-ray diffraction patterns
for 3b (λ = 1.5406 Å).
Figure 2. ORTEP drawing (50% probability ellipsoids) of the two
molecules (A and B) associated with the asymmetric unit of 3b,
showing atom numbering schemes. Molecule B is bisected by a 2-fold
rotation axis.
a
Table 3. Intermolecular Contacts
d1
d2
d3
d4
d5
d6
d7
d8
d9
d10
d11
S3a···O1b′
O1a···S1b′
S3a···N1b′
N1a···S1b′
S2a···S1b′
S2b···S1b′
S3a···S2a′
N2a···S1a′
O1a···S1a′
O1a···S2a′
S1a···S2a′
2.974
3.131
3.244
3.101
3.584
3.710
3.761
3.165
2.975
2.968
3.722
a
Distances in Å; see Figures 4 and 5 for definitions of d1-d11.
bisected by the 2-fold rotation axes parallel to the b-axis at x =
0, 1/2. The A radicals are located in general positions. The unit
cell drawing shown in Figure 4 illustrates the packing of the
radicals viewed parallel to the c-axis, and highlights the zigzag
arrays of A radicals generated by the 2-fold screw axes at x = 1/
4 and 3/4. The B radicals at x = 0, 1/2 are nested in the
pockets formed by the out-of-register ribbons of A, and locked
in place by a complex network of intermolecular S···S′, S···O′,
and S···N′ contacts d1-d5, all of which are within the respective
van der Waals separation.22 There is also a set of short S···O′,
and S···N′ interactions (d8-d10) which serve as structure-
making links along the zigzag, ribbon-like arrays of A-radicals
(Figure 5).
Figure 4. Packing of radicals in the unit cell of 3b, viewed parallel to
the c axis (above). Alternation of radicals along the A and B π-stacks is
shown below. The B radicals lie on a 2-fold rotation axis, and the
methyl protons are disordered. Intermolecular S···S′ (blue),
S···O′(red), and S···N′ (green) contacts are shown with dashed lines.
centrosymmetric, an arrangement which gives rise to a pair of
intermolecular S···S′ contacts d6. By contrast, the overlay of
radicals along the π-stacks of A is less symmetric, and the
orientation of consecutive radicals is rotated by an angle of
89.8°. As a result there are no short S···S′ interactions save for
that afforded by the rather oblique contact d7. The interplanar
separation δ (3.413 Å) is nonetheless quite close; by contrast,
in the B radical π-stacks, δ = 3.627 Å. Overall, the perfect head-
Both the A and B molecules form bases for alternating, but
evenly spaced π-stacks, as illustrated in Figures 4 and 5. The
head-over-tail overlay of the B radicals along the π-stacks is
2487
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
An additional feature associated with the packing of the A
radicals in 3b is the trans-antarafacial, four-center interaction
generated by the centrosymmetric approach of radicals in
adjacent π-stacks. While the associated S···S′ intermolecular
distance (d11 = 3.722 Å) is nominally outside the van der
Waals separation,22 the magnetic and electronic consequences
of these interactions are of considerable importance; these
effects are discussed below.
Magnetic Measurements. Variable temperature DC
magnetic susceptibility (χ) measurements on 3b have been
carried out over the temperature range 2−300 K. The results
are illustrated in Figure 7 in the form of cooling curve plots of χ
Figure 7. Field-cooled χ versus T (left) and χT versus T (right) plots
for 3b at H = 1 kOe.
Figure 5. Ribbons of A radicals running parallel to b (above) and
skewed π-stacking of A radicals (below); alternate layers are
interconverted by a 2-fold screw axis. Intermolecular S···S′ (blue),
S···O′ (red), and S···N′ (green) contacts are shown with dashed lines.
and χT versus T, measured using an external field of H = 1 kOe.
Inspection of the χT versus T plot suggests a strongly
antiferromagnetic (AFM) response, since for a paramagnetic
S = 1/2 system with a nominal value of g ≈ 2 the high
temperature limit of χT should fall near 0.375 emu K mol−1. In
this case, χT at 300 K is near 0.20 emu K mol−1. Not
surprisingly, attempts to perform even a Curie−Weiss fit to the
high temperature data were unsuccessful, a finding that we
attribute to the fact that the crystal structure contains two
different S = 1/2 sublattices, one based on A radicals, with eight
spins per cell, and the other composed of B radicals, with four
spins per cell. In addition, the large number of close
intermolecular contacts between pairs of A radicals, pairs of
B radicals, and hybrid A/B pairs suggests a complex array of
magnetic interactions, and an overall magnetic structure which
is unlikely to conform to a simple description based on a 1D- or
2D-magnetic (chain or ladder) model, let alone follow Curie−
Weiss behavior.
over-tail stacking adopted by the B radicals of 3b is reminiscent
of that found in the phenyl-substituted oxobenzene-bridged
derivative 3c (Figure 6),18 while the twisted packing of π-stacks
That being said, a qualitative understanding of the dominant
magnetic interactions in 3b can be reached by using DFT
calculations to estimate pairwise exchange energies J between
radicals in the A and B π-stacks, that is, the values of J1, J2, and
J3 shown in Figure 8. These three dimers are defined
respectively by the close contacts d11, d7 and d6 in Figures
4 and 5. On the basis of previous experience,11c it is these π-
stack interactions, rather than those involving lateral interstack
approaches, that are most likely to produce a strong
antiferromagnetic response. The computational approach,
which has been successfully applied to a variety of nitrogen-
centered radicals,23 heterocyclic thiazyls24,25 and phenalenyls,26
employs exchange energies estimated using the broken
symmetry DFT methods developed by Noodleman and
Yamaguchi.27 Accordingly, the exchange energy J for any pair
of interacting radicals, defined with reference to the Heisenberg
Hamiltonian Hex = −2J{S1·S2}, can be derived from the total
Figure 6. Alternating π-stacked architectures in 3c (ref 18) and 2a (at
295 K, ref 14), with intermolecular S···S′ (blue) contacts shown with
dashed lines.
found for the A radicals is more reminiscent of the architecture
found in the N-methyl pyrazine-bridged radical 2a.14 It should
be noted, however, that in 2a the mean planes of the radicals
are (at 295 K) rigorously perpendicular to the stacking
direction, whereas the A radicals in 3b are inclined by 68.1°
to the c-axis.
2488
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
associated exchange energy (J = −108.4 cm−1)29 is even more
negative, a finding which may provide an explanation for the
strong AFM response observed for 2a.30 However, the
magnitude of these exchange interactions falls far short of
those seen in more localized radical dimers, such as the
centrosymmetrically bridged m-cyanophenyl-dithiadiazolyl31
+
and many salts of the binary cation S3N2 , including
[S3N2][AsF6].32 In these latter systems (Figures 9b and 9c)
the S···S′ bridges are much shorter, so that the electronic
structures of the dimers should no longer be considered as
open shell singlets,33 but rather as a closed shell singlets best
described in terms of a classical 4-center 2-electron bond
model.34
Conductivity. Variable temperature conductivity (σ)
measurements on the oxobenzene-bridged radicals 3a, its
acetonitrile adduct 3a·MeCN, and 3c were reported in earlier
work.17,18 Similar experiments on 3b, using the four-probe
method on cold-pressed pellets, have now been performed. The
results for all four compounds are illustrated in Figure 10, in the
Figure 8. Pairwise magnetic exchange interactions within the π-stacks
of the A and B radicals in 3b, with corresponding DFT estimated
exchange energies J1, J2, and J3.
energies of the triplet (ETS) and broken symmetry singlet
(EBSS) states and their respective expectation values ⟨S2⟩,
according to eq 1. Single point total energies ETS and EBSS were
calculated using the hybrid exchange correlation functional
UB3LYP and polarized, split-valence basis sets with double-ζ
(6-31G(d,p)) functions. Atomic coordinates were taken from
the crystallographic data.
−(E − E
⟨S ⟩ −⟨S ⟩
TS
)
TS BSS
J =
2
2
(1)
BSS
The numerical results given in Figure 8 indicate a large
negative (AFM) J1 value, suggesting an isolated open-shell
singlet state for these dimers. By contrast the small, albeit
positive value for J2 indicates a 1D ferromagnetic interaction
along the π-stacks of the A radicals. Finally, while the value of J3
found for the π-stacks of the B radicals is smaller than that of J1,
it is clearly large enough to suggest a well-developed 1D AFM
chain interaction.28 Of course, these conclusions exclude the
cumulative effects of lateral interstack interactions, which are
likely to further enrich the magnetic description. The present
results nonetheless establish the presence of strong AFM
magnetic responses within both the A and B π-stacks, the
largest arising from the trans-antarafacial approach character-
ized by J1.
Figure 10. Plots of log σ versus 1/T for oxobenzene-bridged radicals.
Derived thermal activation energies are listed in Table 4. Data for 3a,
3a·MeCN, and 3c are from refs 17 and 18.
form of log plots of σ against 1/T; values of σ(300 K) and the
Arrhenius activation energy Eact are summarized in Table 4.
The N-methyl pyrazine-bridged radical 2a,14 which displays a
similar alternating π-stack architecture (Figure 6), also
possesses four-center, trans-antarafacial interactions (Figure
9a) akin to that found in 3b. The S···S′ separations (3.831 Å)
are comparable to those found in 3b (d11 = 3.722 Å), and the
Table 4. Conductivity and Activation Energy Data
a
a
b
3a·MeCN
3a
3b
3c
σ(300 K)
3.0 × 10−3
4.0 × 10−3
9.0 × 10−4
3.0 × 10−5
(S cm−1
)
Eact (eV)
0.11
0.16
0.13
0.20
a
b
Ref 17. Ref 18.
While there are variations within the group 3a, 3a·MeCN, 3b,
and 3c, their collective performance is markedly and uniformly
superior to that of related pyridine-bridged materials, both in
terms of enhanced conductivity and lowered activation energy.
Indeed the values of σ(300 K) are comparable to those found
in selenium-based variants of 1.11 Moreover, while the
conductivity of 3b is activated, indicative of a Mott insulating
ground state, the value of Eact = 0.13 eV is second only to that
found for 3a·MeCN, a material which, to our knowledge,
displays the lowest thermal activation energy ever reported for a
neutral f = 1/2 radical.17
Figure 9. Centrosymmetric four-center S···S′ contacts in (a) the
pyrazine-bridged bisdithiazolyl 2a (ref 14), (b) m-cyanophenyl-
dithiadiazolyl (ref 31), and (c) [S3N2][AsF6] (ref 32). For 2a the
estimated exchange energy J is also provided.
Band Structure Calculations. To explore the electronic
structure of 3b we have carried out a series of EHT band
structure calculations based on the crystal structure geometry.
2489
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
As with the results obtained for other oxobenzene-bridged
materials17 the numerical data must be viewed with caution, as
for strongly correlated Mott insulating systems such as these
the tight-binding approximation fails to provide a proper
description of the ground state. The method nonetheless
provides qualitative insight into the direction and extent of
intermolecular orbital interactions within and between the
radical π-stacks. As in the case of 3a and 3c, we focus on the
dispersion of the crystal orbitals (COs) arising from the
interactions of the singly occupied molecular orbitals (SOMOs)
on neighboring molecules along and between the radical π-
stacks. Idealized views of this antibonding A2 symmetry orbital
are illustrated in Figure 11.
Figure 13. EHT band structure of different models of 3b, showing (i)
CO dispersion of a 2D array (along y and z) of A radicals, (ii) a full 3D
set of A radicals, and (iii) the full 3D structure with A and B radicals.
(d11, Figure 5), which gives rise to the strong AFM exchange
interactions J1. Nominally, and again speaking within the limits
of the EHT method, this weak dimerization generates a very
small band gap. On moving from the 2D to 3D lattice of A
radicals, as in Figure 13(ii), the band gap almost closes, as
might be expected from the increase in dimensionality. Finally,
when the cavities in the superstructure of A radicals are filled
with π-stacks of B radicals (Figure 13(iii)), a continuum of COs
is generated, giving rise to an overall bandwidth W = 0.48 eV.
That being said, the orbital interactions between the A and B
radical π-stacks do not appear to be extensive, so that to a first
approximation it is tempting to describe the overall structure in
terms of two separate sublattices (A and B), with the former
possessing stronger electronic interactions.
Figure 11. Idealized views of the A2 π-SOMO of 3b.
Given the unusual nature of the crystal structure, that is, the
presence of two distinct sublattices, we have performed
calculations on several idealized structures, the first based on
a single ribbon of A radicals, π-stacked along the z direction and
running parallel to y, as shown in Figure 5. The second model
addresses the full 3D sublattice of A radicals, the honeycomb
nature of which is illustrated by the space-filling superstructure
in Figure 12. The effects of the inclusion of the sublattice of B
DISCUSSION AND CONCLUSION
■
The use of secondary bonding interactions36 or supramolecular
synthons16 to influence if not control the solid state structure of
thiazyl radicals has a long history. For example, the role of
nitrile CN···S′31,37and covalent F···S′38contacts to control
supramolecular architecture, and four-center (S···N′)2 inter-
actions to generate centrosymmetric pairings,39 has been well
recognized. More recently, the importance of intermolecular
carbonyl CO···S contacts, both in thiazyl chemistry17,18,40 and
elsewhere,15 has also been noted. In this light the concerted
effect of chelating S···O′ and S···N′ interactions to lock adjacent
radicals into 1D ribbon-like arrays (Figure 14), is not surprising.
In the case of 3a·MeCN, these ribbons are packed into slipped
π-stacks (Figure 1c) which afford a large electronic bandwidth
and correspondingly high conductivity.17 The crystal structure
of 3b described here is more complex, in that it is bimodal, with
two completely different radical environments. One of these
Figure 12. Two sublattices of 3b, viewed parallel to the stacking
direction, with B radicals occupying cavities generated between out-of-
register ribbons of A radicals.
radicals is then considered as a perturbation of the electronic
structure of the A sublattice.
The results of the band structure calculations are summarized
in Figure 13 in the form of dispersion curves along the direction
Γ → Z for the SOMO-based COs,35 which collectively would
constitute the f = 1/2 band if the ground state were metallic.
Inasmuch as the number of bands reflects the number of
molecules in the unit cell, there are (i) four bands in the 2D
lattice based on A radicals, (ii) eight bands for the full 3D lattice
of A radicals, and (iii) twelve bands (8A + 4B) for the full 3D-
lattice based on both radicals A and B. As may be seen in Figure
13(i) the four COs are split into two pairs, an effect which can
be traced back to the influence of the trans-antarafacial overlap
Figure 14. Ribbon-like arrays of radicals created by intermolecular
S···O′ and S···N′ supramolecular synthons.
2490
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
radicals (A) adopts a ribbon-like structure akin to that found in
3a·MeCN, but consecutive ribbons along the stacking direction
are now related by a 2-fold screw axis rather than a translation,
an effect which leads to a doubling of the cell dimension. The
out-of-register packing of these 1D arrays of A radicals affords
an open 2D framework, a pseudoporous superstructure, within
which π-stacked columns of B radicals are sequestered (Figure
12).
In the present case the degree of doping, charge transfer
between the A and B sites, is likely to be very small, but may be
sufficient to improve charge transport. Testing these ideas, by
making materials with crystallographically distinct radical
centers, is a nontrivial task. It would, however, be relatively
easy to generate bimodal materials with two chemically distinct f
= 1/2 building blocks, either by cocrystallization techniques48
or by using mixed biradicals.49 Charge polarization in such
systems could lead to a smaller, possibly vanishing, Mott−
Hubbard gap.
Bimodal radicals are rare. Indeed to our knowledge 3a, with
three molecules in the asymmetric unit, is the only other radical-
based material known to exist with more than one crystallo-
graphically distinct site. That being said there are a number of
reports of dithiadiazolyl and dithiazolyl radicals with two
distinct radical environments, but in these systems at least one
of the two radicals is dimerized, as a result of which there is no
question of magnetic communication between the two sites.41
Rawson has reported a dimer formed by association of two
chemically different dithiadiazolyls,42 but again in the absence
of unpaired spins there is no magnetic manifestation of the
presence of different building blocks. Likewise examples of
radicals doped into host structures are rare. The incorporation
of thiazyl radicals into a variety of diamagnetic hosts, both
organic and inorganic, has been described, in all cases affording
materials in which the paramagnetic centers are essentially
isolated.43 There is also an example of a verdazyl radical
clathrate in which the radicals are locked into columnar arrays,
an arrangement which gives rise to strong AFM coupling along
EXPERIMENTAL SECTION
■
General Methods and Procedures. The reagents sulfur
monochloride, octamethylferrocene (OMFc), silver triflate (trifluor-
omethanesulfonate) and 2,6-dinitro-p-cresol 4 were obtained
commercially. All were used as received save for OMFc, which was
sublimed in vacuo and recrystallized from acetonitrile before use. The
solvents acetonitrile (MeCN), dichloroethane (DCE), dichloro-
methane (DCM) and carbon disulfide were of at least reagent
grade. MeCN was dried by distillation from P2O5 and CaH2, and both
DCE and DCM by distillation from P2O5. All reactions were
performed under an atmosphere of dry nitrogen. Melting points are
uncorrected. Infrared spectra (Nujol mulls, KBr optics) were recorded
on a Nicolet Avatar FTIR spectrometer at 2 cm−1 resolution, and
visible spectra were collected on samples dissolved in MeCN using a
Perkin-Elmer Lambda 35 UV−vis spectrophotometer. 1H NMR
spectra were run on a Bruker Avance 300 MHz NMR spectrometer
and low resolution Electro-Spray Ionization (ESI) mass spectra were
recorded on a Micromass Q-TOF Ultima Global LC/MS/MS system.
Elemental analyses were performed by MHW Laboratories, Phoenix,
AZ 85018.
the π-stacks (as observed here).44 More recently Poppl has
̈
shown that nitronyl nitroxides incorporated into a copper
framework Cu3(btc)2 (btc = benzenetricarboxylic acid) modify
the magnetic properties of the host lattice.45 In the case of 3b
the extent of magnetic communication between the two
sublattices is difficult to assess, as strong AFM coupling is
anticipated in both.
Preparation of 2,6-Diamino-4-methylphenol Bishydrochlor-
ide 5. Following the procedure described for 2,6-diaminophenol,50 tin
powder (12.7 g, 0.107 mol) was added in three portions to a
suspension of 2,6-dinitro-p-cresol 4 (5.5 g, 27.7 mmol) in 50 mL of
concentrated HCl at 0 °C. When effervescence had ceased the mixture
was allowed to warm to room temperature, then filtered through glass
wool and the solvent removed from the filtrate by flash evaporation.
The remaining solid was washed with 10−15 mL of concentrated HCl,
then redissolved in 10−15 mL of deionized water to give a pale yellow
solution. HCl gas was slowly bubbled through the solution for ∼15
min until the white crystalline product 5 formed. This material was
collected by filtration and air-dried. A second, smaller portion of 5
crystallized from the filtrate and was also collected by filtration. Total
In regard to the charge transport properties of 3b, the
question arises as to the origin of its high conductivity and low
thermal activation energy. The electrochemical evidence
suggests an intrinsically low U for oxobenzene-bridged radicals
in general, and the π-stacked slipped ribbon architecture
previously described for 3a·MeCN clearly provided a significant
enhancement in bandwidth (W = 1.02 eV). In that light, the
performance of the unsolvated chloro radical 3a and the methyl
radical 3b is perplexing, as the results of the EHT calculations
for both suggest somewhat weaker electronic interactions than
those seen in 3a·MeCN.17 Indeed the values of W estimated for
3a (0.52 eV) and 3b (0.48 eV) are comparable to that found for
the highly one-dimensional, and less conductive, phenyl-
substituted material 3c (W = 0.49 eV).46
This seemingly anomalous performance of 3a and 3b, both
of which display high conductivity and low excitation energies
in spite of a relatively low bandwidth, may be related to the fact
that they are bi- (or tri-) modal. The different electronic
environments (crystal fields) experienced by the crystallo-
graphically distinct radicals may give rise to subtle differences in
their individual electronic properties, which would be manifest
in their ion energetics (IP and EA values), effects that are not
registered in the EHT calculations. In essence charge transport
in 3b (and 3a) may be aided by a form of self-doping, a
phenomenon which has been observed for heterostructures
formed at the interfaces between inorganic oxide Mott
insulators.47 For these latter systems it has been shown that
charge transfer at the surface can lead to significant improve-
ments in conductivity, even to the generation of metallic states.
1
yield 5.00 g (23.7 mmol, 85%); mp >250 °C. H NMR (DMSO) δ:
2.11 (s, 3H), 6.05 (s, 2H).51Anal. Calcd. for C7H12N2OCl2: C, 39.83;
H, 5.73; N, 13.27. Found: C, 40.01; H, 5.61; N, 13.41.
Preparation of [3b][Cl]. Sulfur monochloride (16 mL, 27.0 g, 200
mmol) was added dropwise to a solution of 4-methyl-2,6-
diaminophenol bishydrochloride 5 (4.33 g, 19.9 mmol) in 250 mL
anhydrous MeCN, and the mixture was heated under gentle reflux
overnight. The purple-brown precipitate of [3b][Cl] was filtered off,
washed thoroughly with MeCN and dried in vacuo. Yield 5.22 g (17.1
mmol, 89%); mp >250 °C. IR (cm−1): 1669 (s), 1406 (s), 1583 (s),
1090 (w), 1002 (m), 950 (w), 913 (w), 846 (m), 783 (w), 756 (s),
656 (w), 489 (m), 471 (w).
Preparation of [3b][OTf]. Silver triflate (2.89 g, 11.2 mmol) was
added to a slurry of crude [3b][Cl] (2.96 g, 10.0 mmol) in 150 mL
anhydrous MeCN, to afford a deep purple solution, which was gently
heated at reflux for 1 h, then filtered to remove a gray precipitate of
AgCl. The solvent was flash distilled from the filtrate to leave crude
product as a dark red solid (3.86 g), which was recrystallized from hot
MeCN to give metallic green shards of [3b][OTf] (2.51 g, 6.15 mmol,
61%); mp >250 °C. Anal. Calcd. for C8H3F3N2O4S5: C, 23.52; H,
0.74; N, 6.86. Found: C, 23.36; H, 0.91; N, 7.00. IR (cm−1): 1683 (s),
1409 (s), 1288 (s), 1270 (s), 1238 (s), 1159 (s), 1030 (s), 1006 (m),
853 (w), 762 (s), 660 (m), 638 (s), 660 (m), 938 (s), 573(w), 515
2491
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
(m), 485 (m), 468 (m). UV−vis: λmax = 568 nm, ε = 1.52 × 104 L
ASSOCIATED CONTENT
* Supporting Information
■
mol−1 cm−1.
S
Preparation of 3b. Method 1. Bulk Material for Conductivity
and Magnetic Measurements. A solution of [3b][OTf] (250 mg,
0.613 mmol) in 75 mL of degassed MeCN (3 freeze−pump−
thaw cycles) was filtered into a solution of OMFc (210 mg,
0.944 mmol) in 175 mL of similarly degassed MeCN to yield a
blue solution and a blue-black precipitate. After 1 h of stirring at
room temperature, the purple microcrystalline product was
filtered off, washed with MeCN and dried under vacuum, (69
mg, 0.266 mmol, 43% yield); mp >250 °C. Anal. Calcd. for
C7H3N2OS4: C, 32.41; H, 1.17; N, 10.80. Found: C, 32.46; H,
1.17; N, 10.96. IR (cm−1): 1575 (s, br), 1456 (s), 1377 (s),
1285 (s, br), 1134 (w), 1029 (w), 1066 (w), 825 (w), 768 (w),
727 (m), 681 (m), 634 (m), 503 (w), 471 (w), 438 (w).
Method 2. Slow Diffusion for Single Crystals. A solution of
[3b][OTf] (75 mg, 0.184 mmol) in 15 mL degassed (4 freeze−
pump−thaw cycles) MeCN was allowed to diffuse slowly into a
similarly degassed solution of OMFc (64 mg, 0.221 mmol) over a 6 h
period, affording 3b as microcrystalline solid with a few single crystals
which were manually separated for single crystal work.
Cyclic Voltammetry. Cyclic voltammetry was performed using a
PINE Bipotentiostat, Model AFCClBP1, with scan rates of 50−250
mV s−1 on solutions of [3b][OTf] in MeCN (dried by distillation
from P2O5 and CaH2) containing 0.1 M tetra-n-butyl-ammonium
hexafluorophosphate. Potentials were scanned with 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. The Epa−Epc separation of the reversible couples were within
10% of that of the Fc/Fc+ couple.52
Crystallography. Crystals were glued to glass fibers with epoxy. X-
ray data for 3b were collected using omega scans with a Bruker APEX I
CCD detector on a D8 3-circle goniometer and Mo Kα (λ = 0.71073
A) radiation. The data were scanned using Bruker’s SMART program
and integrated using Bruker’s SAINT software.53 The structures were
solved by direct methods using SHELXS-9054 and refined by least-
squares methods on F2 using SHELXL-9755 incorporated in the
SHELXTL56 suite of programs. Powder X-ray diffraction data on bulk
3b were collected at ambient temperature on a powder diffractometer
equipped with a position sensitive detector (INEL) using Cu Kα1
radiation (λ = 1.5406A°). The total 2θ range was 0−112°, measured in
steps of 0.029°. Starting with the space group, unit cell and the
molecular coordinates available from the single crystal data set, the
unit cell dimensions were refined by Rietveld methods57 using the
GSAS program package.58
Magnetic Susceptibility Measurements. DC magnetic suscept-
ibility measurements on 3b were performed over the temperature
range 2−300 K on a Quantum Design MPMS SQUID magnetometer.
Diamagnetic corrections were made using Pascal’s constants.59
Conductivity Measurements. Four-probe temperature depend-
ent conductivity measurements on cold pressed pellet (1 × 1 × 5 mm)
samples of 3b were performed over the range 140−300 K using home-
built equipment. Silver paint (Leitsilber 200) was used to apply the
electrical contacts.
Details of single crystal X-ray crystallographic data collection
and structure refinement, tables of atomic coordinates, bond
distances and angles, anisotropic thermal parameters, and
hydrogen atom positions in CIF format. Archival files for DFT
calculations of magnetic exchange energies. This information is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: oakley@uwaterloo.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERCC) for financial support and for a
Vanier Graduate Scholarship to K.L.
REFERENCES
■
(1) (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.
(2) (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) 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. (c) Haddon, R. C.;
Sarkar, A.; Pal, S. K.; Chi, X.; Itkis, M. E.; Tham, F. S. J. Am. Chem. Soc.
2008, 130, 13683. (d) Pal, S. K.; Itkis, M. E.; Tham, F. S.; Reed, R. W.;
Oakley, R. T.; Haddon, R. C. J. Am. Chem. Soc. 2008, 130, 3942.
(e) Bag, P.; Itkis, M. E.; Pal, S. K.; Donnadieu, B.; Tham, F. S.; Park,
H.; Schlueter, J. A.; Siegrist, T.; Haddon, R. C. J. Am. Chem. Soc. 2010,
132, 2684. (f) Kubo, T.; Katada, Y.; Shimizu, A.; Hirao, Y.; Sato, K.;
Takui, T.; Uruichi, M.; Yakushi, K.; Haddon, R. C. J. Am. Chem. Soc.
2011, 133, 14240.
(3) (a) Lahti, P. Adv. Phys. Org. Chem. 2011, 45, 93. (b) Ratera, I.;
Veciana, J. Chem. Soc. Rev. 2012, 41, 303. (c) Hicks, R. G. In Stable
Radicals: Fundamentals and Applied Aspects of Odd-Electron Com-
pounds; Hicks, R.G., Ed.; John Wiley & Sons, Ltd.: Wiltshire, U.K.,
2010; pp 248−280. (d) Saito, G.; Yoshida, Y. Bull. Chem. Soc. Jpn.
2007, 80, 1.
(4) The importance of avoiding f = 1/2 states was recognized early
on in the development of charge transfer salts. Complete charge
transfer, to generate f = 1/2 states, afforded insulating materials. See:
(a) Torrance, J. B. Acc. Chem. Res. 1979, 12, 79. (b) Garito, A. F.;
Heeger, A. J. Acc. Chem. Res. 1974, 7, 232.
(5) (a) Rawson, J. M.; Alberola, A.; Whalley, A. J. Mater. Chem. 2006,
16, 2560. (b) Hicks, R. G. In Stable Radicals: Fundamentals and Applied
Aspects of Odd-Electron Compounds; Hicks, R.G., ed., John Wiley &
Sons, Ltd.: Wiltshire, U.K., 2010; pp 317−380.
(6) (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.
Band Structure Calculations. EHT Band electronic structure
calculations were performed with the Caesar 2.0 suite of programs60
using the Coulomb parameters of Baasch, Viste and Gray61 and a
quasi-split valence basis set adapted from Clementi and Roetti.62 The
off-diagonal elements of the Hamiltonian matrix were calculated with
the standard weighting formula.63 Atomic positions were taken from
crystallographic data.
Exchange Energy Calculations. All calculations were performed
using the UB3LYP functional and the split-valence double-ζ basis set
6-31G(d,p), as contained in the Gaussian 09W suite of programs.64
Exchange energies J1, J2, and J3 for interacting pairs of radicals in 3b
were computed from eq 1, using single point energies of the triplet and
broken symmetry singlet states and their respective ⟨S2⟩ expectation
values. Tight convergence criteria were employed, and atomic
coordinates were taken from crystallographic data.
́
(b) Boere, R. T.; Roemmele, T. L. Coord. Chem. Rev. 2000, 210, 369.
(c) Kaszynski, P. J. Phys. Chem. A 2001, 105, 7626. (d) Decken, A.;
Mailman, A.; Passmore, J.; Rautiainen, J. M.; Scherer, W.; Scheidt, E.-
W. Dalton Trans. 2011, 40, 868.
(7) (a) Cordes, A. W.; Haddon, R. C.; Oakley, R. T. Adv. Mater.
1994, 6, 798. (b) Oakley, R. T. Can. J. Chem. 1993, 71, 1775.
(8) (a) Bryan, C. D.; Cordes, A. W.; Haddon, R. C.; Glarum, S. H.;
Hicks, R. G.; Kennepohl, D. K.; MacKinnon, C. D.; Oakley, R. T.;
Palstra, T. T. M.; Perel, A. S.; Schneemeyer, L. F.; Scott, S. R.;
Waszczak, J. V. J. Am. Chem. Soc. 1994, 116, 1205. (b) Bryan, C. D.;
Cordes, A. W.; Fleming, R. M.; George, N. A.; Glarum, S. H.; Haddon,
2492
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
R. C.; MacKinnon, C. D.; Oakley, R. T.; Palstra, T. T. M.; Perel, A. S. J.
Am. Chem. Soc. 1995, 117, 6880. (c) Bryan, C. D.; Cordes, A. W.;
Goddard, J. D.; Haddon, R. C.; Hicks, R. G.; MacKinnon, C. D.;
Mawhinney, R. C.; Oakley, R. T.; Palstra, T. T. M.; Perel, A. S. J. Am.
Chem. Soc. 1996, 118, 330.
(9) (a) Cordes, A. W.; Haddon, R. C.; Oakley, R. T. Phosphorus,
Sulfur, Silicon Relat. Elem. 2004, 179, 673. (b) 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. (c) 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. (d) 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.
Rawson, J. M. Polyhedron 2005, 24, 2579. (c) Deumal, M.; LeRoux, S.;
Rawson, J. M.; Robb, M. A.; Novoa, J. J. Polyhedron 2007, 26, 1949.
(25) (a) Decken, A.; Mattar, S. M.; Passmore, J.; Shuvaev, K. V.;
Thompson, L. K. Inorg. Chem. 2006, 45, 3878. (b) Leitch, A. A.;
Oakley, R. T.; Reed, R. W.; Thompson, L. K. Inorg. Chem. 2007, 46,
6261. (c) Jose, D.; Datta, A. Cryst. Growth Des. 2011, 11, 3137.
(26) 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.
(27) For details of the theoretical procedure, see: (a) Noodleman,
L.; Norman, J. G. J. Chem. Phys. 1979, 70, 4903. (b) Noodleman, L. J.
Chem. Phys. 1981, 74, 5737. (c) Takano, Y.; Taniguchi, T.; Isobe, H.;
Kubo, T.; Morita, Y.; Yamamoto, K.; Nakasuji, K.; Takui, T.;
Yamaguchi, K. Chem. Phys. Lett. 2002, 358, 17. (d) Yamaguchi, K.
In Self-Consistent Field Theory and Applications; Carbo, R.,
Klobukowski, M., Eds.; Elsevier: Amsterdam, 1990; p 727. (e) Deumal,
M.; Robb, M. A.; Novoa, J. J. Prog. Theor. Chem. Phys. 2007, 16, 271.
(28) Despite the apparent similarity between the head-over-tail π-
stacking observed for the B radicals of 3b and the single π-stacks of 3c
(Figure 5), the magnetic interactions are quite different. While the
calculated values of J3 in 3b point to an AFM interaction between
adjacent radicals, the magnetic data for 3c are well described by a 1D
FM chain model.
(29) This value was calculated using equation 1, at the same level of
theory as for J1,2,3, and with atomic coordinates taken from the
reported crystal structure.
(30) There is also a noncentrosymmetric four-center interaction in
2a. At 295 K, the two S···S′distances are longer (3.936 and 4.093 Å),
but the calculated exchange energy (J = −61.1 cm−1) indicates an even
stronger AFM response.
(31) Cordes, A. W.; Haddon, R. C.; Hicks, R. G.; Oakley, R. T.;
Palstra, T. T. M. Inorg. Chem. 1992, 31, 1802.
(32) Gillespie, R. J.; Ireland, R.; Vekris., J. E. Can. J. Chem. 1975, 53,
(10) The disproportionation energy ΔHdisp is the enthalpy change for
the conversion of two gas phase radicals R into a cation/anion pair,
that is, 2R ⇄ R+ + R−, and is equal to the difference between the
ionization potential (IP) and electron affinity (EA). The cell potential
is defined here as the numerical difference between the half-wave
potentials for the oxidation and reduction processes, that is, Ecell
E1/2(ox) − E1/2(red).
=
(11) (a) 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.
(b) Brusso, J. L.; Cvrkalj, K.; Leitch, A. A.; Oakley, R. T.; Reed, R. W.;
Robertson, C. M. J. Am. Chem. Soc. 2006, 128, 15080. (c) Leitch, A. A.;
Yu, X.; Winter, S. M.; Secco, R. A.; Dube, P. A.; Oakley, R. T. J. Am.
Chem. Soc. 2009, 131, 7112.
(12) (a) Leitch, A. A.; Brusso, J. L.; Cvrkalj, K.; Reed, R. W.;
Robertson, C. M.; Dube, P. A.; Oakley, R. T. Chem. Commun. 2007,
3368. (b) Robertson, C. M.; Leitch, A. A.; Cvrkalj, K.; Reed, R. W.;
Myles, D. J. T.; Dube, P. A.; Oakley, R. T. J. Am. Chem. Soc. 2008, 130,
8414.
(13) Leitch, A. A.; McKenzie, C. E.; Oakley, R. T.; Reed, R. W.;
Richardson, J. F.; Sawyer, L. D. Chem. Commun. 2006, 1088.
(14) Leitch, A. A.; Reed, R. W.; Robertson, C. M.; Britten, J. F.; Yu,
X.; Secco, R. A.; Oakley, R. T. J. Am. Chem. Soc. 2007, 129, 7903.
(15) (a) Iwaoka, M.; Takemoto, S.; Tomoda, S. J. Am. Chem. Soc.
2002, 124, 10613. (b) Junming, L.; Yunxiang, L.; Subin, Y.; Weiliang,
Z. Struc. Chem. 2011, 22, 757. (c) Navarrete-Vazquez, G.; Alaniz-
́
Palacios, A.; Tlahuextl, M.; Bernal-Uruchurtu, M.; Tlahuext, H.
CrystEngComm 2012, 14, 1256.
(16) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311.
(17) Yu, X.; Mailman, A.; Lekin, K.; Assoud, A.; Robertson, C. M.;
Noll, B. C.; Campana, C. F.; Howard, J. A. K.; Dube, P. A.; Oakley, R.
T. J. Am. Chem. Soc. 2012, 134, 2264.
3147.
(33) For a more detailed discussion of open- versus closed-shell
singlet thiazyl radical dimers, see: Clarke, C. S.; Jornet-Somoza, J.;
Mota, F.; Novoa, J. J.; Deumal, M. J. Am. Chem. Soc. 2010, 132, 17817.
(34) (a) Small, R. W. H.; Banister, A. J.; Hauptman, Z. V. J. Chem.
Soc., Dalton Trans. 1984, 1377. (b) Gimarc, B. M.; Trinajstic, N. Pure
́
Appl. Chem. 1980, 52, 1443. (c) Gimarc, B. M. Pure Appl. Chem. 1990,
62, 423.
(35) Given that the crystal structure of 3b belongs to the monoclinic
space group C2/c, the correspondence between the directions of the
real and reciprocal unit cell vectors is only approximate.
(36) Alcock, N. W. Bonding and Structure, Structural Principles in
Inorganic and Organic Chemistry; Ellis Horwood: Cornwall, U.K., 1990;
p 321.
(37) (a) Banister, A. J.; Bricklebank, N.; Clegg, W.; Elsegood, M. R.
J.; Gregory, C. I.; Lavender, I.; Rawson, J. M.; Tanner, B. K. Chem.
Commun. 1995, 679. (b) Banister, A. J.; Bricklebank, N.; Lavender, I.;
Rawson, J. M.; Gregory, C. I.; Tanner, B. K.; Clegg, W.; Elsegood, M.
R. J.; Palacio, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 2533.
(c) Alberola, A.; Less, R. J.; Palacio, F.; Pask, C. M.; Rawson, J. M.
Molecules 2004, 9, 771. (d) Alberola, A.; Burley, J.; Collis, R. J.; Less, R.
J.; Rawson, J. M. J. Organomet. Chem. 2007, 692, 2750. (f) Haynes, D.
A. CrystEngComm 2011, 13, 4793.
(18) Yu, X.; Mailman, A.; Dube, P. A.; Assoud, A.; Oakley, R. T.
Chem. Commun. 2011, 47, 4655.
(19) In previous papers (ref 17 and 18) we referred to 3a and 3c as
semiquinone-bridged bisdithiazolyls. Based on the suggestions of a
referee we employ here the name oxobenzene-bridged bisdithiazolyl,
which conforms more closely to the formal nomenclature. The full
IUPAC name for 3b is 4-oxo-8-methyl-3H,4H-benzo[1,2-d:5,4-d′]
bis[1,2,3]dithiazol-3-yl.
(20) Reported as −0.02 V versus SCE by Hobi, M.; Ruppert, O.;
Gramlich, V.; Togni, A. Organometallics 1997, 16, 1384.
(21) GSAS refinement of the powder diffraction data on 3b, collected
at 295(2) K, afforded a = 25.6415(13) Å, b = 14.4848(14) Å, c =
7.29933(34) Å, β = 95.597(5)°, V = 2698.14(27) Å3, with Rp = 0.0406
and Rwp = 0.0745.
(22) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441. (b) Dance, I. New J.
Chem. 2003, 27, 22.
(23) (a) Novoa, J. J.; Deumal, M. Struct. Bonding (Berlin) 2001, 100,
(38) (a) Beer, L.; Cordes, A. W.; Myles, D. J. T.; Oakley, R. T.;
Taylor, N. J. CrystEngComm 2000, 2, 109. (b) Banister, A. J.; Batsanov,
A. S.; Dawe, O. G.; Howard, J. A. K.; Davies, J. E.; Rawson, J. M.;
Smith, J. N. B. Phosphorus, Sulfur, Silicon Relat. Elem. 1997, 124 & 125,
553. (c) Rawson, J. M.; Banister, A. J.; Lavender, I. Adv. Heterocycl.
Chem. 1995, 62, 137.
(39) (a) Cozzolino, A. F.; Vargas-Baca, I.; Mansour, S.;
Mahmoudkhani, A. H. J. Am. Chem. Soc. 2005, 127, 3184.
(b) Cozzolino, F.; Vargas-Baca, I. Cryst. Growth Des. 2011, 11, 668.
(40) Decken, A.; Jenkins, H. D. B.; Mailman, A.; Passmore, J.;
Shuvaev, K. V. Inorg. Chim. Acta 2008, 361, 521.
33. (b) Jornet, J.; Deumal, M.; Ribas-Arino, J.; Bearpark, M. J.; Robb,
̃
M. A.; Hicks, R. G.; Novoa, J. J. Chem. Eur. J. 2006, 12, 3995.
(24) (a) Rawson, J. M.; Luzon, J.; Palacio, F. Coord. Chem. Rev. 2005,
249, 2631. (b) Luzon, J.; Campo, J.; Palacio, F.; McIntyre, G. J.;
(41) (a) Barclay, T. M.; Cordes, A. W.; George, N. A.; Haddon, R.
C.; Itkis, M. E.; Oakley, R. T. Chem. Commun. 1999, 2269. (b) Brusso,
2493
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494

Crystal Growth & Design
Article
J. L.; Clements, O. P.; Haddon, R. C.; Itkis, M. E.; Leitch, A. A.;
Oakley, R. T.; Reed, R. W.; Richardson, J. F. J. Am. Chem. Soc. 2004,
126, 8256.
(42) Allen, C.; Haynes, D. A.; Pask, C. M.; Rawson, J. M.
CrystEngComm 2009, 11, 2048.
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. W.; Morokuma, K.; Zakrzewski, V. G.; Voth, G., A.;
Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
revision A.02; Gaussian, Inc.: Wallingford CT, 2009.
(43) (a) Langley, P. J.; Rawson, J. M.; Smith, J. N. B.; Schuler, M.;
Bachmann, R.; Schweiger, A.; Palacio, F.; Antorrena, G.; Gescheidt, G.;
Quintel, A.; Rechsteiner, P.; Hulliger, J. J. Mater. Chem. 1999, 9, 1431.
(b) Suess, H. I.; Wuest, T.; Sieber, A.; Althaus, R.; Budde, F.; Luethi,
H. P.; McManus, G. D.; Rawson, J. M.; Hulliger, J. CrystEngComm
2002, 432. (c) Potts, S. V.; Barbour, L. J.; Haynes, D. A.; Rawson, J. M;
Lloyd, G. O. J. Am. Chem. Soc. 2011, 133, 12948.
(44) Gilroy, J. B.; Lemaire, M. T.; Patrick, B. O.; Hicks, R. G.
CrystEngComm 2009, 11, 2180.
(45) Jee, B.; Koch, K.; Moschkowitz, L.; Himsl, D.; Hartman, M;
Poppl, A. J. Phys. Chem. Lett. 2011, 2, 357.
̈
(46) From an analogous EHT band electronic structure calculation
on 3c, using atomic coordinates taken from the reported crystal
structure.
́
(47) (a) Gonzalez, I.; Okamoto, S.; Yunoki, S.; Moreo, A.; Dagotto,
E. J. Phys.: Condens. Mater. 2008, 20, 264002. (b) Yunoki, S.; Moreo,
A.; Dagotto, E.; Okamoto, S.; Kancharla, S. S.; Fujimori, A. Phys. Rev. B
2007, 76, 064532.
(48) (a) Murata, H.; Mague, J. T.; Aboaku, S.; Yoshioka, N.; Lahti, P.
M. Chem. Mater. 2007, 19, 4111. (b) Seber, G.; Freitas, R. S.; Mague, J.
T.; Paduan-Filho, A.; Gratens, X.; Bindilatti, V.; Oliveira, N. F Jr.;
Yoshioka, N.; Lahti, P. M. J. Am. Chem. Soc. 2012, 134, 3825.
(49) (a) Cameron, T. S.; Lemaire, M. T.; Passmore, J.; Rawson, J. M.;
Shuvaev, K. V.; Thompson, L. K. Inorg. Chem. 2005, 44, 2576.
(b) Decken, A.; Cameron, T. S.; Passmore, K.; Rautiainen, J. M.; Reed,
R. W.; Shuvaev, K. V.; Thompson, L. K. Inorg. Chem. 2007, 46, 7436.
(50) Kanamori, D.; Yamada, Y.; Onoda, A.; Okamura, T.; Adachi, S.;
Yamamoto, H.; Ueyama, N. Inorg. Chim. Acta 2005, 358, 331.
(51) Cai, L.; Xie, W.; Mahmoud, H.; Han, Y.; Wink, D. J.; Li, S.;
O’Connor, C. J. Inorg. Chim. Acta 1997, 263, 231.
(52) Boere, R. T.; Moock, K. H.; Parvez, M. Z. Anorg. Allg. Chem.
́
1994, 620, 1589.
(53) SAINT, version 6.22; Bruker Advanced X-ray Solutions, Inc.:
Madison, WI, 2001.
(54) Sheldrick, G. M. SHELXS-90. Acta Crystallogr. A 1990, 46, 467.
(55) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures; University of Gottingen: Gottingen, Germany, 1997.
̈
̈
(56) SHELXTL, Program Library for Structure Solution and Molecular
Graphics, version 6.12; Bruker Advanced X-ray Solutions, Inc.:
Madison, WI, 2001.
(57) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65.
(58) Larson, A. C.; Von Dreele, R. B. Report No. LA-UR-86-748;
General Structure Analysis System (GSAS), Los Alamos National
Laboratory: Los Alamos, NM, 1987.
(59) Carlin, R. L. Magnetochemistry; Springer-Verlag: New York,
1986.
(60) CAESAR, Program for Crystal and Electronic Structure Analysis,
version 2.0; PrimeColor Software, Inc.: Cary, NC, 1998.
(61) Basch, H.; Viste, A.; Gray, H. B. Theor. Chim. Acta 1965, 3, 458.
(62) Clementi, E.; Roetti, C. At. Data. Nucl. Data Tables 1974, 14,
177.
(63) Ammeter, J. H.; Burgi, H. B.; Thibeault, J. C.; Hoffmann, R. J.
̈
Am. Chem. Soc. 1978, 100, 3686.
(64) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J.
A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;
Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C. Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
2494
dx.doi.org/10.1021/cg300107t | Cryst. Growth Des. 2012, 12, 2485−2494