Semiquinone-bridged bisdithiazolyl radicals as neutral radical conductors

Article abstract of DOI:10.1021/ja209841z

Semiquinone-bridged bisdithiazolyls 3 represent a new class of resonance-stabilized neutral radical for use in the design of single-component conductive materials. As such, they display electrochemical cell potentials lower than those of related pyridine-bridged bisdithiazolyls, a finding which heralds a reduced on-site Coulomb repulsion U. Crystallographic characterization of the chloro-substituted derivative 3a and its acetonitrile solvate 3a·MeCN, both of which crystallize in the polar orthorhombic space group Pna2₁, revealed the importance of intermolecular oxygen-to-sulfur (CO...SN) interactions in generating rigid, tightly packed radical π-stacks, including the structural motif found for 3a·MeCN in which radicals in neighboring π-stacks are locked into slipped-ribbon-like arrays. This architecture gives rise to strong intra- and interstack overlap and hence a large electronic bandwidth W. Variable-temperature conductivity measurements on 3a and 3a·MeCN indicated high values of θ(300 K) (>10 ^-3 S cm^-1) with correspondingly low thermal activation energies E_act, reaching 0.11 eV in the case of 3a·MeCN. Overall, the strong performance of these materials as f = ¹/ ₂ conductors is attributed to a combination of low U and large W. Variable-temperature magnetic susceptibility measurements were performed on both 3a and 3a·MeCN. The unsolvated material 3a orders as a spin-canted antiferromagnet at 8 K, with a canting angle φ = 0.14° and a coercive field H_c = 80 Oe at 2 K.

Full text of DOI:10.1021/ja209841z

Article
pubs.acs.org/JACS
Semiquinone-Bridged Bisdithiazolyl Radicals as Neutral Radical
Conductors
Xin Yu,† Aaron Mailman,† Kristina Lekin,† Abdeljalil Assoud,† Craig M. Robertson,‡ Bruce C. Noll,§
Charles F. Campana,§ Judith A. K. Howard,‡ Paul A. Dube,∥ and Richard T. Oakley*,†
†Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
‡Department of Chemistry, University of Durham, Durham DH1 3LE, U.K.
§Bruker AXS, Inc., Madison, Wisconsin 53711, United States
∥Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1
S
* Supporting Information
ABSTRACT: Semiquinone-bridged bisdithiazolyls 3 represent a
new class of resonance-stabilized neutral radical for use in the design
of single-component conductive materials. As such, they display
electrochemical cell potentials lower than those of related pyridine-
bridged bisdithiazolyls, a finding which heralds a reduced on-site
Coulomb repulsion U. Crystallographic characterization of the
chloro-substituted derivative 3a and its acetonitrile solvate
3a·MeCN, both of which crystallize in the polar orthorhombic
space group Pna21, revealed the importance of intermolecular
oxygen-to-sulfur (CO···SN) interactions in generating rigid, tightly
packed radical π-stacks, including the structural motif found for 3a·MeCN in which radicals in neighboring π-stacks are locked
into slipped-ribbon-like arrays. This architecture gives rise to strong intra- and interstack overlap and hence a large electronic
bandwidth W. Variable-temperature conductivity measurements on 3a and 3a·MeCN indicated high values of σ(300 K) (>10−3
S
cm−1) with correspondingly low thermal activation energies Eact, reaching 0.11 eV in the case of 3a·MeCN. Overall, the strong
performance of these materials as f = 1/2 conductors is attributed to a combination of low U and large W. Variable-temperature
magnetic susceptibility measurements were performed on both 3a and 3a·MeCN. The unsolvated material 3a orders as a spin-
canted antiferromagnet at 8 K, with a canting angle ϕ = 0.14° and a coercive field Hc = 80 Oe at 2 K.
more than 35 years ago.9 In practice, however, the realization of
INTRODUCTION
■
conductivity in radical-based materials is not easy. For example,
in light heteroatom (N, O) radicals such as aminyls, nitroxyls,
and verdazyls, the spin density is too localized, and
intermolecular hopping of the unpaired electrons is suppressed
by a large on-site charge repulsion, causing such materials to be
insulators.10 Overcoming the charge repulsion problem, which
can be understood within the language of Mott−Hubbard
theory,11,12 requires that nearest-neighbor interactions (ex-
pressed in terms of the intermolecular resonance integral β) be
maximized and that the barrier to charge transport, the Mott−
Hubbard on-site Coulomb repulsion parameter U, be
minimized. When the electronic bandwidth W (= 4β)13 is
sufficient to offset charge repulsion, that is, when W > U, a
metallic state should prevail. Toward this end, a wide range of
delocalized polycyclic organic radicals, notably phenalenyls,
The conventional design strategy for inducing electrical
conductivity in organic materials is based upon the use of
charge transfer (CT) to generate charge carriers in otherwise
closed-shell molecules.1 As a result, conductive systems
generally require two components, a donor and an acceptor,
although it is possible to incorporate both partners into a single
molecule.2 The accumulated arsenal of semiconductors, metals
and superconductors based on donors such as tetrathiafulvalene
(TTF) and acceptors such as tetracyanoquinodimethane
(TCNQ) and C60 attest to the success of the CT paradigm.3,4
In addition, if one of the CT partners contains a transition
metal5 or possesses an attached “outrigger” radical,6 materials
can be made that display both conductive and magnetic
signatures, a combination of potential value in the emerging
field of molecular spintronics.7
Within the latter context, that is, the growing interest in spin-
correlated conductivity, the pursuit of single-component
molecular materials that possess both carriers of charge and
spin holds much appeal.8 In principle, molecular radicals
represent ideal building blocks for such systems, and indeed,
the possibility that the unpaired electron supplied by a neutral
radical might serve as a charge carrier was proposed by Haddon
spirophenalenyls,14,15 and betainic CT radicals3 16 have been
e,
explored, but while high conductivity has been observed for
some spirophenylenyls, a metallic ground state has yet to be
achieved.
Received: October 19, 2011
Published: January 24, 2012
© 2012 American Chemical Society
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While light-heteratom radicals are poor conductors, the
localization of spin density that suppresses charge transport
serves to enhance magnetic properties by focusing the weak
through-space intermolecular magnetic exchange interactions.17
Indeed, the first nonmetal-based molecular ferromagnets were
constructed from such building blocks.18 However, the Curie
temperatures of these systems are low (TC < 2 K), and in the
absence of spin−orbit coupling, magnetic anisotropy and the
concomitant coercive fields Hc are essentially nonexistent. In an
attempt to balance the potentially conflicting electronic
requirements for charge transport and magnetic performance,
that is, spin delocalization versus spin localization, strong versus
weak (orthogonal) orbital overlap, we have pursued the
development of heavy-heteroatom radicals, notably thiazyls
and selenazyls, in the belief that the presence of nitrogen atoms
in spin-bearing sites will suppress dimerization while orbital
interactions between neighboring sulfur and selenium atoms,
respectively, will generate pathways for charge migration and/
or magnetic coupling. However, initial attempts to generate
superimposed radical π-stacks from simple monocyclic thiazyl
radicals afforded diamagnetic, Peierls-distorted19 structures that
were insulating or weakly semiconducting.20
3p orbitals on sulfur, resulting in improved conductivity.
Moreover, the introduction of spin−orbit coupling effects27
occasioned by the presence of the heavy heteroatom selenium
gives rise to some remarkable magnetic effects, including
Dzyaloshinsky−Moriya spin canting28 and hard ferromagnet-
ism,29 with TC values as high as 17.5 K at ambient pressure and
rising to 24 K at 2 GPa.30
In an attempt to break away from the herringbone mold
found for 1, we explored the structures of N-alkylated pyrazine-
bridged radicals 2. While removal of the steric protection
afforded by the basal ligand makes these radicals more
susceptible to dimerization, as in 2b (R = Et),31 we were
eventually able to prepare radical 2a (R = Me), which displays a
superimposed but alternating ABABAB π-stack structure32
whose electronic bandwidth and hence conductivity are both
dramatically improved relative to those seen in the herringbone
structures found for 1. However, at low temperatures (<120 K),
the evenly spaced π-stacks of 2a collapse into dimers, thereby
producing a diamagnetic ground state. As a continuation of
these efforts to move away from the herringbone packing of the
π-stacks found in 1 while at the same time avoiding the spin-
quenching dimerization found for 2, we have explored the
effect on structure and function of the isoelectronic
replacement of the NR1 moiety in 1 with a carbonyl group to
produce the resonance-stabilized semiquinone-bridged bisdi-
thiazolyl system 3. While the molecular electronic structure of
these new radicals is similar to that of both 1 and 2, the polarity
of the carbonyl CO bond in 3 leads to the development of
strong intermolecular S···O′ interactions in the solid state, and
these supramolecular synthons33 give rise to more rigid, more
tightly bound frameworks that display improved charge
transport properties. Following the synthetic procedure
developed for the recently reported phenyl-substituted
derivative 3b (R = Ph),34 we have now prepared the
corresponding chloro-substituted compound 3a (R = Cl). In
the present paper, we describe the spectroscopic, electro-
chemical, and structural characterization of 3a and its
acetonitrile solvate 3a·MeCN. Variable-temperature magnetic
susceptibility and conductivity measurements are also reported,
A more serious problem with early, relatively localized thiazyl
radicals was their high on-site Coulomb repulsion energy U.21
Thus, even when dimerization could be avoided, intermolecular
overlap between neighboring singly occupied molecular orbitals
(SOMOs) and the resulting solid-state bandwidth W were
insufficient to overcome charge repulsion, as a result of which
the materials were trapped in Mott-insulating states. Improve-
ments in conductivity could be induced by p-type doping,22
which lowered U by changing the degree of band filling, but
1
within the confines of the half-filled band (f = /2) paradigm,
the primary design challenge continues to be the development
of new radical systems with more delocalized spin distributions
and inherently low U values.
Chart 1
and the results are interpreted in the light of Extended Huckel
̈
Theory (EHT) electronic band structure calculations.
RESULTS
■
Building Block Synthesis. The general preparative route
to semiquinone-bridged bisdithiazolyls 3 starts from the
appropriate 4-substituted phenol 4 (Scheme 1), which can be
readily oxidized with nitric acid to the corresponding 2,6-
dinitrophenol 5. Reduction of 5 with Sn/HCl then yields the
N-Alkylated pyridine-bridged bisdithiazolyl radicals 1 (Chart
1) were developed in response to this need for greater spin
delocalization. As a result of the resonance stabilization
effect,15f their gas-phase disproportionation energies ΔHdisp
23
and electrochemical cell potentials Ecell
,
which provide
Scheme 1
indirect measures of U, are lower than those of simple
monocyclic derivatives.24 In addition, variations in the exocyclic
groups R1 and R2 allow for fine-tuning of solid-state structures
and hence intermolecular magnetic and electronic interac-
tions.25 However, most derivatives of this type crystallize as
slipped π-stack arrays of radicals locked into herringbone
packing patterns, a motif that compromises bandwidth and
hence charge transport. As a result, their room-temperature
conductivities σRT are on the order of 10−6−10−5 S cm−1.
However, replacement of sulfur by its heavier congener
selenium26 increases the bandwidth by virtue of the greater
spatial extension of the 4p orbitals on selenium relative to the
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bishydrochloride of 2,6-diaminophenol 6, which undergoes a
double Herz cyclization with sulfur monochloride in MeCN at
reflux to afford the chloride salt [3][Cl] as an insoluble blue-
black solid.35 This crude material may be converted into more
soluble triflate (CF3SO3− or OTf−) and/or hexafluoroantimo-
nate (SbF6−) salts by metathesis with AgOTf or AgSbF6.
Solutions of salts of 3+ in MeCN are deep-blue in color,
although the associated absorption maxima (see the Exper-
imental Section) are somewhat blue-shifted relative to those
observed for salts of 1+.
with (excess) bulk cation, a phenomenon that has been
observed in the electrochemistry of simple monocyclic
dithiazolylium cations37 and related materials.21b,38 In view of
the irreversibility of the −1/0 couple, it is more difficult to
establish the corresponding cell potentials Ecell for 3a and 3b.
We can nonetheless make an upper-limit estimate of the
magnitude of Ecell by taking the difference in the Epc values for
the −1/0 and 0/+1 couples, and by so doing Ecell was found to
be near 0.60 V for both radicals, which is substantially lower
than that observed (near 0.80 V) for 1 (Table 1).
Electrochemistry. To determine suitable chemical reduc-
ing agents for the generation of 3, we explored its electro-
chemical behavior by cyclic voltammetry (CV), starting from
solutions of [3][OTf] in MeCN with 0.1 M n-Bu4NPF6 as a
supporting electrolyte and Pt wire electrodes. As may be seen in
Table 1, there are some differences in the half-wave potentials
Isolation of Radicals. On the basis of the half-wave
potentials observed for the 0/+1 couples for 3a and 3b and our
experience with the chemical reduction of salts of 1+ to the
respective radicals 1, we initially selected octamethylferrocene
(OMFc) [E1/2(ox) = −0.038 V vs SCE]29,39 as a suitable
reagent to convert [3a][OTf] and [3b][OTf] to the
corresponding radicals. This proved to be an ideal choice for
3b, which was readily obtained as black needles by codiffusion
of MeCN solutions of OMFc and [3b][OTf] in a glass H-cell.
Crystallization of radical 3a proved to be a more challenging
exercise but eventually a more productive one. Initial attempts
to reduce [3a][OTf] with OMFc using H-cell techniques and
MeCN as the solvent afforded a microcrystalline precipitate
displaying a weak IR band near 2200 cm−1, the presence of
which led us to suspect (correctly) the formation of an
acetonitrile solvate. To produce larger crystals of this material,
we explored the use of milder reducing agents, including
dimethylferrocene (DiMFc), whose potential (0.263 V vs
SCE)29,40 is by itself insufficient to reduce [3a][OTf].
However, in practice, and aided by lattice energy (solvation)
effects, DiMFc functioned perfectly, affording fine black needles
of the acetonitrile solvate 3a·MeCN when added slowly to
solutions of [3a][OTf] in acetonitrile. The structure of this
material is described below.
With the isolation of 3a·MeCN successfully completed, the
question then arose as to whether it might be possible to
generate crystals of the unsolvated radical 3a. To this end, we
considered alternative solvents, eventually settling on propioni-
trile (EtCN). However, this choice required that we switch
counteranions, as [3a][OTf] displayed insufficient solubility in
EtCN. Success was finally achieved using [3a][SbF6] as the
precursor salt and OMFc as the reducing agent. This
combination afforded 3a as very small splintered needles that
were shown by IR and elemental analysis to be free of solvent.
Single-crystal and powder X-ray diffraction studies and
transport property measurements were performed on this
material.
a
Table 1. Electrochemical Potentials for 1 and 3
b
b
c
E1/2(−1/0)
E1/2(0/+1)
Ecell
d
e
1 (R2 = Cl)
−0.835
−0.956
−0.481
0.005
0.84
0.85
0.64
0.60
1 (R2 = Ph)
3a (R = Cl)
3b (R = Ph)
−0.104
0.195 (0.158)
f
g
g
h
h
i
f
−0.529
0.108 (0.071)
a
b
c
In volts, measured in MeCN. Referenced to SCE. Ecell = E1/2(0/+1)
− E1/2(−1/0). R1 = Me; see ref 25b. R1 = Me; see ref 36.
Irreversible reduction; only the cathodic peak potential Epc is cited.
E1/2; Epc is given in parentheses. Ecell estimated as Epc(0/+1) −
Epc(−1/0). From ref 34.
d
e
f
g
h
i
for the reversible 0/+1 couples of 3a and 3b that can be related
to changes in the electron-withdrawing power of the axial
ligand R; similar trends have been observed for derivatives of
1.36 More importantly, the results establish that both
semiquinone-bridged salts [3][OTf] are more easily reduced
(by about 200 mV) than the corresponding pyridine-bridged
materials [1][OTf]. For the second reduction process, which
corresponds to the −1/0 couple, the difference between the
two systems is more profound. Not only is there a large anodic
shift in the half-wave potential (nearly 400 mV) upon moving
from 1 to 3, but in the latter case, the reduction wave is strongly
irreversible (Figure 1). This irreversibility, which is independ-
ent of scan rate and sample concentration, may stem from
comproportionation of the electrochemically produced anion
EPR Spectra. The spin distribution in radicals of type 3 has
been explored by EPR spectroscopy. In our earlier
communication, we reported the X-band EPR spectrum of
3b,34 which consists of a broad five-line pattern arising from
hyperfine coupling to two equivalent dithiazolyl nitrogens
[I(14N) = 1], the value of aN (0.352 mT) being similar to,
although slightly larger than, that observed for 1b (0.320
mT).36 Likewise, solutions of 3a, obtained by dissolving crystals
of either unsolvated 3a or its MeCN adduct in toluene or
dichloromethane, display a strong five-line EPR signal (Figure
2) reminiscent of that observed for the related pyridine-bridged
radical 1a.25b For both systems, the aN values (Table 2) are
approximately one-half of those found in monofunctional 1,2,3-
dithiazolyls, as would be expected given the fact that the spin
density is “shared” between two dithiazolyl rings.41
Figure 1. CV scans of (left) [3a][OTf] and (right) [1a][OTf] in
MeCN with 0.1 M n-Bu4NPF6 supporting electrolyte. Data for
[1a][OTf] were taken from ref 25b.
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withdrawing effect of the carbonyl group. Somewhat surpris-
ingly, however, the gas-phase disproportionation energies
(ΔHdisp) of 3a (4.62 eV) and 1a (4.53 eV) are almost
identical. This finding, which is inconsistent with the
experimental cell potentials Ecell (Table 1), may simply reflect
the difficulty of the hybrid B3LYP functional in handling spin
correlation in the semiquinone system, a problem suggested by
the higher level of spin contamination (⟨S2⟩) found in 3a.
Crystallography. The crystal structures of the semi-
quinone-bridged radical 3a and its acetonitrile solvate
3a·MeCN were determined by single-crystal X-ray diffraction.
Given the small size of the crystals of the unsolvated radical and
the consequent paucity of reflections and relatively high R
values, we cross-checked the single-crystal results by powder
diffraction analysis (Figure 3).42 This experiment also
confirmed the phase uniformity of the bulk material. Table 3
lists crystal data, while Figure 4 illustrates ORTEP drawings of
Figure 2. (a) X-band EPR spectra (spectral width = 3.0 mT) of (left)
3a and (right) 1a in toluene; hyperfine coupling constants a in mT are
shown. (b) Kohn−Sham spin density isosurfaces and (c) atomic spin
densities ρ and aN values (in mT) derived from single-point UB3LYP/
EPR-III/6-31G(d,p) calculations at the UB3LYP/6-311G(d,p)-opti-
mized geometries.
a
Table 2. Computed Ion Energetics
Figure 3. Observed and calculated powder X-ray diffraction patterns
for unsolvated 3a (λ = 1.5406 Å).
3a
1a
IP
6.66
6.31
EA
2.04
1.78
⟨S2⟩
0.8141
4.62
0.7797
4.53
b
ΔHdisp
EN
Table 3. Crystal Data
c
2.16
2.03
a
compound
formula
[3a][SbF6]·MeCN
3a·MeCN
3a
Gas-phase ΔSCF values in eV from (U)B3LYP/6-311G(d,p)
b
c
calculations. ΔHdisp= IP − EA. Mulliken electronegativity EN =
(IP + EA)/2.
C8H3ClF6N3OS4Sb
556.57
C8H3ClN3OS4
320.82
C18Cl3N6O3S12
839.43
M
a (Å)
b (Å)
c (Å)
V (Å3)
14.3729(7)
14.3342(7)
16.5005(8)
3399.5(3)
2.179
9.4913(3)
23.5986(8)
5.3124(2)
1189.88(7)
1.791
48.779(3)
3.8228(2)
14.2036(9)
2648.6(3)
2.105
We also compared the spin distributions in the semiquinone-
and pyridine-bridged radicals obtained using density functional
theory (DFT) calculations at the UB3LYP/EPR-III/6-31G-
(d,p) level. Illustrations of the Kohn−Sham spin density
isosurfaces of 3a and 1a are shown in Figure 2, along with
numerical details for each. The results support the qualitative
similarity of the two radicals but also reveal some potentially
important differences. For example, not only is the spin density
on nitrogen (ρN = 0.172) in 3a greater than that in 1a (ρN =
0.149), in accord with the observed (and calculated) aN values,
but so too is ρS for the thiazyl sulfur (Figure 2). Both of these
features are consistent with the possibility of stronger
intermolecular magnetic and electronic interactions for the
semiquinone-bridged radical. We also used DFT methods to
compare the gas-phase ion energetics of the two systems. As
shown in Table 2, the calculated ionization potential (IP),
electron affinity (EA), and Mulliken electronegativity (EN) of
3a are greater than those of 1a; these results are consistent with
the electrochemical potentials and indicative of the electron-
−1
ρcalcd (g cm )
space group
Z
Pbca
Pna21
Pna21
8
4
4
T (K)
296(2)
300(2)
100(2)
−1
μ (mm )
2.335
9.300
12.373
λ (Å)
0.71073
1.54178
1770/1/155
1.54178
2785/400/423
data/restr./
parameters
4096/30/267
solution
method
R, Rw (on F2)
direct
direct
direct
0.0458, 0.0803
0.0216, 0.0561
0.0808, 0.2027
the asymmetric units of 3a and 3a·MeCN and provides the
atom-numbering schemes; selected intramolecular metrics are
provided in Table 4. To check the effect of redox changes on
the internal structural parameters, we also structurally
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Figure 4. ORTEP drawings (50% probability ellipsoids) of the
asymmetric units of unsolvated 3a and 3a·MeCN, showing the atom-
numbering schemes. The three crystallographically distinct radicals in
3a are labeled A, B, and C.
characterized the salt [3a][SbF6]·MeCN. This revealed small
contractions in the S−S, S−N, and S−C distances in the cation
a
Table 4. Internal Metrics
b
compound
[3a][SbF6]·MeCN
3a·MeCN
3a
Figure 5. Unit cell of 3a·MeCN viewed parallel to (a) the c axis and
(b) the b axis. In (b), the layer with y < 0.5 is shaded. Intermolecular
S···O′ (red) and S···N′ (green) contacts are shown with dashed lines.
S−S
2.062(2)
1.593(4)
1.316(4)
1.703(4)
1.217(4)
2.093(4)
1.625(2)
1.312(8)
1.732(12)
1.216(3)
2.10(4)
1.59(6)
1.32(4)
1.73(8)
1.21(3)
S−N
N−C
S−C
C−O
contacts inside the van der Waals separation43 between radicals
in neighboring π-stacks along y, although there is one
interaction (S1···S2′ = 3.750 Å) that may provide some
interstack electronic communication.
a
Distances in Å; for averaged values, the numbers in parentheses are
the greater of the difference and the standard deviation. Averaged
over three molecules.
b
Inspection of the ribbonlike layers within the π-stacks of
3a·MeCN reveals some interesting features that may be
compared to the herringbone structures observed for radicals
of type 1. For example, as illustrated in Figure 6, there is
considerable π-type overlap between radicals in neighboring
stacks of 3a·MeCN. This feature is not possible in a
herringbone packing pattern. The mean planes of radicals
along the π-stacks of 3a are separated by a distance δ = 3.497 Å,
which is similar to the interplanar separations seen in radical π-
stacks of 1, although there is no single interatomic separation
that lies within the standard van der Waals separation.43 The
electronic consequences of this arrangement are developed
below.
Like the acetonitrile adduct 3a·MeCN, crystals of the
unsolvated radical 3a, as grown from propionitrile, belong to
the polar orthorhombic space group Pna21. The crystal
architecture of the two variants is, however, profoundly
different. There are three independent molecules (A, B, and
C) in the asymmetric unit of 3a (Figure 4), and with Z = 4, this
gives rise to a total of 12 molecules in the unit cell. Each of the
three radicals A, B, and C forms the basis for an evenly spaced,
slipped π-stack running parallel to the b axis. The unit cell
drawing provided in Figure 7 illustrates the packing of the
radicals in the xz plane. Also indicated in this drawing is the
labyrinth of lateral intermolecular S···S′, S···O′, and S···N′
contacts that stitch the radicals together. All of these contacts,
which are defined numerically in the Supporting Information,
relative to those seen in the radical. Similar variations between
the cation and radical oxidation states of 1 were observed and
can be related to the removal of an electron from a
predominantly antibonding SOMO.25
Crystals of the acetonitrile solvate 3a·MeCN possess an
orthorhombic structure with Z = 4 and display a π-stacked
architecture in which the molecules are linked laterally into
coplanar arrays by intermolecular S···O′ and S···N′ contacts. As
shown in Figure 5, the cell dimensions and symmetry of the
Pna21 space group dictate a packing pattern in which the
radicals in the coplanar arrays are aligned in regular (not
alternating) slipped π-stacks running along the z direction.
Nearest neighbors along x and y are related by n-glide planes,
producing a cross-braced appearance when the π-stacks are
viewed along the y direction. Each of the 2D arrays of π-stacks
comprises ribbons of radicals inclined in the same direction
(not in a herringbone pattern) at an angle θ = 49.6° with
respect to the c axis. Within each ribbon, neighboring radicals
are linked by short S···O′ and S···N′ contacts that are defined
numerically in the Supporting Information. The interlocking of
the radicals by these supramolecular synthons33 produces a
zigzag or wavelike array within the folds of which the
acetonitrile solvate molecules are perfectly sequestered; the
radicals and solvent molecules are locked together by short
polar MeCN···S′ interactions. There are no intermolecular
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Figure 6. Two views of slipped π-stacks in 3a·MeCN: (a) view
perpendicular to the molecular planes; (b) side view illustrating
interlayer interactions along and between the π-stacks. Intermolecular
S···O′ (red) and S···N′ (green) contacts are shown with dashed lines.
Figure 8. Slipped π-stacking of the three symmetry-independent
radicals in unsolvated 3a. Staggering of the radicals affords close
intracolumnar S1···S2′ and S3···S4′ interactions. The views at the
bottom illustrate the slippage of adjacent radicals along the local
directions x and y.
a
Table 5. Structural Parameters of the π-Stacks in 3a
compound
radical A
radical B
radical C
S1···S2′
S3···S4′
3.633
3.574
3.374
0.300
1.771
3.453
3.625
3.359
0.249
1.808
3.545
3.575
3.368
0.231
b
δ
c
dx
d
dy
1.792
a
b
c
All distances in Å. Interplanar separation along π-stacks. Lateral
Figure 7. Packing of A, B, and C radicals in the xz plane of the unit
cell of unsolvated 3a, showing lateral intermolecular S···S′ (blue),
S···O′(red), and S···N′ (green) contacts.
slippage in the x direction of neighbors along π-stacks (see Figure 8) .
Longitudinal slippage in the y direction of neighbors along π-stacks
(see Figure 8) .
d
are within the respective van der Waals separations.43 The
overall structural motif can be described in terms of wavelike
arrays of C radicals that weave along the z direction,
neighboring molecules being related by 21 axes at x = 0 and
1/2 and linked by four-center (S···N′)2 secondary-bonding44
bridges. The B radicals also form wavelike arrays along the z
direction, but nearest neighbors are now related by the n-glides
Band Structures. To explore and compare the electronic
structures of the radicals of 3a and 3a·MeCN, we carried out a
series of EHT band structure calculations on the crystal
structure geometries. The results must be viewed with caution,
as the tight-binding approximation fails to provide a proper
description of the ground state of strongly correlated systems
such as these. The method nonetheless provides qualitative
insight into the direction and extent of intermolecular orbital
interactions within and between the radical π-stacks. As in
previous work, we focused on the dispersion of the crystal
orbitals (COs) arising from the interactions of the SOMOs
along the π-stacking direction.46 Idealized views of this
antibonding A2 symmetry orbital are illustrated in Figure 9.
The results are summarized in Figure 10 in the form of
dispersion curves for the SOMO-based COs, which collectively
1
3
at x = /4 and /4 and are linked by S···O′ and S···N′ contacts.
Finally, the A radicals serve as bridges between these two
chains, interacting strongly with each.
Despite the rather oblique alignment of the three
independent π-stacks, when they are viewed parallel to the z
direction (Figure 8), close examination reveals that the
individual columns share common features. First, in all three
π-stacks, the interplanar separation δ is closer than that seen in
3a·MeCN and nearer to the value observed in graphite.45
Second, slippage of the radicals occurs almost exclusively by a
longitudinal movement of neighboring radicals, as indicated by
the value of dy in Table 5; the small value of dx coupled with
the almost equal intermolecular S1···S2′ and S3···S4′
interactions indicates virtually no lateral slippage. Staggering
of this type and to this degree has been seen before in π-stacked
radicals of type 125b and in those systems was associated with
relatively weak (orthogonal) intrastack overlap.
1
would constitute the f = /2 band if the ground state were
metallic. Inasmuch as the number of bands reflects the number
Figure 9. Idealized views of the A2 π-SOMO of 3.
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of molecules in the unit cell, there are four bands for 3a·MeCN
but a total of 12 for unsolvated 3a. In each case, the spread of
Table 6. Conductivity and Magnetic Parameters
a
3a·MeCN
3a
3b
σ(300 K) (S cm−1)
Eact (eV)
3.0 × 10−3
0.11
4.0 × 10−3
3.0 × 10−5
0.16
0.20
b
b
C (emu K mol−1)
0.389
0.351
0.349
b
b
θ (K)
−61.7
−27.0
32.8
c
TN (K)
−
8.0
4.5
a
b
c
See ref 34. From a Curie−Weiss fit to the 100−300 K data. Does
not order magnetically above 2 K.
Figure 10. EHT dispersion curves for (left) 3a·MeCN and unsolvated
(right) 3a.
the set of COs across the Brillouin zone gives a qualitative
estimate, within the tight-binding approximation, of the solid-
state bandwidth. On this basis, it is immediately apparent that
the bandwidth W = 1.02 eV found for the acetonitrile adduct
3a·MeCN is exceptionally large. Moreover, the strong
dispersion observed along a* (Γ → X) and c* (Γ → Z) also
indicates a well-developed two-dimensional (2D) electronic
structure, in accord with the slipped-ribbon architecture
illustrated in Figure 5b. In contrast, the unsolvated material
3a shows only limited interactions along the π-stacks (Γ → Y)
and even weaker dispersion along the remaining two principal
directions of reciprocal space. Overall, the bandwidth W is
estimated to be 0.52 eV. The EHT electronic bandwidths for
the radicals thus clearly indicate the ribbonlike π-stacked
architecture of the solvated material as having the most well-
developed electronic structure, the one most likely to confer
high conductivity. In contrast, in the unsolvated structure, the
interactions are weaker, in terms of both magnitude and
directionality, and indeed are reminiscent of those seen in the
herringbone structures of pyridine-bridged radicals 1.
Figure 11. Field-cooled (left) χ versus T and (right) χT versus T plots
for 3a·MeCN at H = 1 kOe.
sample to 2 K. The high-temperature cooling curve data (with
H = 1 kOe) for the unsolvated material 3a (Figure 12) also
suggested an antiferromagnetically coupled Curie−Weiss
paramagnet with θ = −27.0 K, but upon cooling below 10 K,
this material displayed a rapid increase in both χ and χT. The
surge was more pronounced at lower field (H = 100 Oe), and a
subsequent zero-field-cooled/field-cooled (ZFC/FC) experi-
ment revealed a sharp bifurcation in χ(T) at 8 K for the ZFC
and FC sweeps, indicative of a phase transition to a spin-canted
antiferromagnetically ordered state with TN = 8 K. Field-
independent magnetization experiments confirmed the order-
ing temperature, and back-extrapolation of M to T = 0 K
afforded a value of the spontaneous magnetization Mspont = 2.5
mNβ, which was used to estimate the spin-canting angle ϕ =
0.14°.48 Measurements of M as a function of field were also
performed, and these indicated a weak, quasi-linear M versus H
dependence out to 5 T. Cycling of the field revealed a subtle
but distinct hysteretic response in M(H), giving rise to a
coercive field Hc = 80 Oe at 2 K.
Conductivity Measurements. We reported previously the
results of variable-temperature conductivity (σ) measurements
on 3b.34 Similar measurements using the four-probe method on
cold-pressed pellets were performed here on 3a·MeCN and
unsolvated 3a. The results are persented in Figure 13 in the
form of plots of log σ against 1/T. Values of σ(300 K) and the
Arrhenius activation energy Eact are summarized in Table 6
along with the results of measurements previously reported for
3b and 1a. Comparison of the performance of 3a, 3a·MeCN,
and 3b with that of related pyridine-bridged materials, of which
1a is representative,25b indicates that the semiquinone-bridged
materials are uniformly superior in terms of both enhanced
conductivity and lowered activation energy. Indeed, the values
of σ(300 K) for 3a and 3a·MeCN are among the highest ever
observed for a neutral f = 1/2 radical. Not only are they several
orders of magnitude higher than those for radicals of type 1, for
which σ(300 K) is generally in the range 10−6−10−5 S cm−1, but
they also rival that of the very best selenium-based variant of 1,
namely, 5 × 10−3 S cm−1 when R1 = Me, R2 = Cl.49 Moreover,
Magnetic Properties. Previously reported variable-temper-
ature direct current (DC) magnetic susceptibility (χ) measure-
ments34 on 3b taken at low field (H = 100 Oe) over the range
2−300 K indicated strong local ferromagnetic (FM) coupling
along the π-stacks. Analysis of the data over the range T = 6−30
K in terms of the Baker model47 for a Heisenberg 1D FM-
1
coupled chain (π-stack) of S = /2 centers yielded exchange
coupling constants J = +29.5 cm−1 for interactions along the π-
stacks and zJ′ = −2.5 cm−1 for the cumulative interstack (mean-
field) interactions. Upon further cooling, the FM chains
become aligned in an antiparallel fashion to form a spin-canted
́
antiferromagnetic (AFM) state with a Neel temperature (TN)
of 4.5 K. With increasing field, the AFM chains undergo a spin-
flop transition to a field-induced FM state.
Here we carried out similar magnetic measurements on
3a·MeCN and unsolvated 3a (Table 6). The data for
3a·MeCN, illustrated in Figure 11 in the form of cooling
curve plots of χ and χT versus T measured using an external
field of H = 1 kOe, indicated strongly antiferromagnetically
coupled paramagnetic behavior. Consistently, the results of a
Curie−Weiss fit over the range 100−300 K afforded a large
negative θ value (−61.7 K). There was no evidence, however,
for a structural or magnetic phase transition upon cooling the
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pyridine-bridged systems 1, where the N-alkyl group creates an
insulating buffer between neighboring molecules, the carbonyl
oxygen atom in 3 plays an active structure-making role, giving
rise to tightly packed structures linked by short intermolecular
S···O′ contacts. These supramolecular synthons afford a rich
variety of novel packing motifs, including the head-over-tail
motif found for 3b and the stacked-ribbon architecture found in
3a·MeCN (Figure 14). Only in the case of the unsolvated
radical 3a does the packing of the π-stacks resemble the
herringbone pattern typically observed for pyridine-bridged
radicals 1.
Figure 14. Packing patterns found in 1, 2, and 3: (a) slipped-ribbon π-
stacks; (b) herringbone π-stacks; (c) alternating head-over-tail π-
stacks.
Figure 12. (a) Field-cooled χ versus T plot for unsolvated 3a at H = 1
kOe. (b) Field-cooled χT versus T plot for unsolvated 3a at H = 1
kOe. The inset shows a ZFC/FC plot of χ versus T at H = 100 Oe. (c)
Decay of the spontaneous magnetization M with increasing temper-
ature for unsolvated 3a. (d) Hysteresis in cycling of M versus H
measurements at T = 2 K.
These new structural motifs give rise to interesting magnetic
properties. The head-over-tail π-stacking previously found for
3b affords remarkably strong FM interactions along the π-
stacks, and upon cooling to 4.5 K, the FM chains undergo
antiparallel AFM ordering.34 In the case of 3a (unsolvated),
AFM ordering is also observed, but the unusual crystal
structure, with three independent molecules in the asymmetric
unit, precludes a detailed interpretation of its magnetic
structure, although the orthogonal overlap condition of the π-
stacks suggests an FM interaction. Attempts to model the χ(T)
1
data above 100 K using either a 1D FM or AFM chain S = /2
model were unsuccessful. This is perhaps not unexpected in
view of the fact that there are three distinct π-stacks (A, B, and
C), but given this structural complexity, it is surprising that the
bulk material is capable of ordering magnetically. The origin of
the strong local AFM interactions in the solvated material
3a·MeCN is a little clearer. The χ(T) data from 150 to 300 K
could be fitted using a molecular-field-modified Bonner−Fisher
1D S = 1/2 antiferromagnetically coupled chain model50 which,
when referenced to the Hamiltonian Hex = −2JS1·S2, afforded J
and zJ′ values of −32.4 and 12.8 cm−1 respectively. However,
modeling the 2−100 K data produced an equally acceptable fit
but with J = −38.1 cm−1 and zJ′ = 45.2 cm−1. The variation in
and magnitude of the mean-field zJ′ term perhaps indicates the
inadequacy of the 1D model (the zJ′ values are too large) or
may simply reflect changes in exchange energies occasioned by
contraction of the unit cell upon cooling.
However, the charge transport properties of these semi-
quinone-bridged materials constitute their most remarkable and
appealing features. As a set, structures 3a, 3a·MeCN, and 3b
display room-temperature conductivities several orders of
magnitude higher than those of their pyridine-based counter-
parts 1a and 1b. The thermal activation energies are also much
lower, indeed lower than those observed for selenium-based
variants of 1, where the heavy-atom effect affords improved
orbital overlap (a larger W) and a softer core (a lower U). The
question therefore arises as to the origin of this improved
performance in the semiquinone materials. Is it a result of an
intrinsically lower U or an improved bandwidth W, or are there
̂
̂
̂
Figure 13. Plots of log σ versus 1/T for representative radicals. The
derived thermal activation energies are provided in Table 6. Data for
1a and 3b were taken from refs 25b and 34, respectively.
while the conductivity of these new radicals remains activated,
indicative of a Mott-insulating ground state, the value of Eact
=
0.11 eV derived for 3a·MeCN constitutes, to our knowledge,
the lowest thermal activation energy ever reported for a neutral
1
f = /2 radical. In contrast, radicals 1 show Eact values in the
range 0.40−0.50 eV,25b while that of the best of their selenium
analogues is lowered to 0.17 eV.49
DISCUSSION
■
The semiquinone-bridged bisdithiazolyls 3a and 3b constitute
the first members of a new class of resonance-stabilized π-
radical for use in the design of f = /2 conductors. Their
1
electrochemical cell potentials Ecell, which are lower than those
observed for the pyridine-bridged radicals 1a and 1b, augur well
for improved performance by virtue of a reduced on-site
Coulomb repulsion term U. Moreover, in contrast to the
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of anhydrous MeCN, and the mixture was heated under gentle reflux
overnight. The blue-black solid was filtered off and washed with
MeCN, hot DCE, and CS2 to remove all of the residual sulfur halides.
The resulting solid, crude [3a][Cl] (R = Cl) was dried in vacuo. Yield
4.22 g (13.4 mmol, 77%); mp >250 °C. IR (cm−1): 1671 (s), 1279 (s),
1097 (m), 1019 (m), 834 (m), 819 (m), 777 (m), 763 (m), 733 (m),
621 (w), 607 (w), 487 (m), 469 (m).
additional features that need to be considered? As noted above,
the electrochemical evidence suggests an intrinsically lower U,
and the π-stacked slipped-ribbon architecture for 3a·MeCN
clearly provides a significant enhancement in W. In essence, the
operational guidelines provided by the Mott−Hubbard model
seem to work, at least qualitatively.
Preparation of [3a][SbF6]. Solid NOSbF6 (3.92 g, 14.8 mmol)
was added to a slurry of crude [3a][Cl] (4.22 g, 13.4 mmol) in 100
mL of anhydrous MeCN, affording a deep-purple solution that was
gently heated at reflux for 1 h and then filtered. The solvent was flash-
distilled from the filtrate, leaving [3a][SbF6] as a golden solid (4.62 g,
8.96 mmol, 67.0% yield), which was crystallized from acetic acid or
EtCN; mp >250 °C. Anal. Calcd for C6N2S4OClSbF6: C, 13.98; N,
5.43. Found: C, 13.71; N, 5.10. IR (cm−1): 1671 (s), 1281 (s), 1270
(s), 1107 (m), 1025 (m), 857 (w), 841 (m), 874 (m), 654 (s), 559
(w), 490 (w). UV−vis: λmax = 573 nm, ε = 1.6 × 104 L mol−1 cm−1.
Crystals of the solvate [3a][SbF6]·MeCN suitable for X-ray diffraction
work were grown from MeCN.
Preparation of [3a][OTf]. AgOTf (3.42 g, 13.3 mmol) was added
to a slurry of crude [3a][Cl] (3.82 g, 12.1 mmol) in 100 mL of
anhydrous MeCN, affording a deep-purple solution that was gently
heated at reflux for 1 h and then filtered to remove a gray precipitate of
AgCl. The solvent was flash-distilled from the filtrate, leaving
[3a][OTf] as a golden solid, which was crystallized from hot MeCN
or EtCN as metallic green shards. Yield 3.07 g (7.16 mmol, 59.1%);
mp >250 °C. Anal. Calcd for C7N2S5O4F3: C, 19.60; N, 6.53. Found:
C, 19.86; N, 6.61. IR (cm−1): 1683 (s), 4180 (s), 1442 (s), 1412 (m),
1283 (m), 1267 (s), 1264 (s), 1248 (s), 1224 (s), 1175 (s), 1167 (s),
1102 (m), 1028 (s), 857 (w), 839 (m), 785 (m), 758 (w), 640 (m),
635 (m), 576 (w), 517 (m), 486 (w), 472 (w).
Preparation of 3a·MeCN. Method 1. Bulk Material for
Conductivity and Magnetic Measurements. A solution of
[3a][OTf] (214 mg, 0.50 mmol) in 30 mL of degassed MeCN
(three freeze−pump−thaw cycles) was added to a solution of excess
DiMFc (326 mg, 1.00 mmol) in 100 mL of similarly degassed MeCN.
After 30 min, the fine microcrystalline precipitate of 3a·MeCN was
filtered off, washed with 5 × 30 mL of MeCN, and dried in vacuo.
Yield 140 mg (0.44 mmol, 88%); mp >250 °C. Anal. Calcd for
C8H3ClN3OS4: C, 29.95; H, 0.94; N, 13.10. Found: C, 29.98; H, 1.02;
N, 13.30. IR (cm−1): 2246 (w, ν(CN)), 1603 (s, br, ν(CO)), 1366
(w), 1311 (m, br), 1117 (m), 997 (m), 952 (m, br), 902 (w), 807 (w),
710 (s), 589 (w), 502 (m), 467 (w), 452 (m).
Method 2. Slow Diffusion for Single Crystals. A solution of
[3a][SbF6] (150 mg, 0.291 mmol) in 15 mL of degassed MeCN (four
freeze−pump−thaw cycles) was allowed to diffuse slowly into a
similarly degassed solution of DiMFc (300 mg, 1.40 mmol) in 15 mL
of MeCN. After 15 h, purple needles of 3a·MeCN (31 mg, 0.11 mmol,
38%) suitable for crystallographic work were collected, washed with
MeCN, and dried under a stream of nitrogen.
SUMMARY AND CONCLUSION
■
The preparation and characterization of the chloro-substituted
semiquinone-bridged bisdithiazolyl radical 3a takes the neutral
radical conductor concept a major step forward. While a
metallic state has not been achieved, the two crystalline
modifications 3a and 3a·MeCN are both Mott insulators, and
the acetonitrile adduct nonetheless displays, to our knowledge,
the lowest activation energy ever observed for an f = 1/2 single-
component system and therefore the smallest Mott−Hubbard
gap. It remains to be seen whether the performance of the
materials reported here is open to improvement by chemical
modification (further variations in R) or the application of
physical pressure. In the latter context, we recently described
the pressure-induced conversion of selenium-based variants of
1 into weakly metallic states,30 and the present compounds may
respond even more dramatically.
EXPERIMENTAL SECTION
■
General Methods and Procedures. The reagents sulfur
monochloride, OMFc, DiMFc, AgOTf, AgSbF6, and 4-chlorophenol
(4a) were obtained commercially. All were used as received save for
OMFc and DiMFc, which were sublimed in vacuo and recrystallized
from MeCN before use. The solvents MeCN, EtCN, dichloroethane
(DCE), dichloromethane (DCM), carbon disulfide, hexane, and
diethyl ether were of at least reagent grade. MeCN and EtCN51
were dried by distillation from P2O5 and CaH2 and DCM by
distillation from P2O5. All reactions were performed under an
atmosphere of dry nitrogen. Melting points are uncorrected. IR
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 PerkinElmer Lambda
35 UV−vis spectrophotometer. 1H NMR spectra were run on a Bruker
Avance 300 MHz NMR spectrometer, and low-resolution electrospray
ionization mass spectra were recorded on a Micromass Q-TOF Ultima
Global LC/MS/MS system. Elemental analyses were performed by
MHW Laboratories (Phoenix, AZ).
Preparation of 4-Chloro-2,6-dinitrophenol (5a). Nitric acid
(93.7 g, 1.49 mol) was added dropwise to a solution of 4a (45.0 g,
0.350 mol) in 300 mL of glacial acetic acid at room temperature. The
dark-orange mixture was stirred at 75 °C for 30 min and then poured
onto 400 mL of crushed ice. The light-yellow solid was collected by
filtration, washed with H2O, and dried in air to give 5a with a yield of
66.9 g (0.306 mol, 87%). Recrystallization from 1:1 (v:v) DCM/
Preparation of 3a. Method 1. Bulk Material for Conductivity
and Magnetic Measurements. A solution of [3a][OTf] (313 mg,
0.730 mmol) in 30 mL of degassed EtCN (three freeze−pump−thaw
cycles) was added to a stirred solution of OMFc (316 mg, 1.09 mmol)
in 100 mL of similarly degassed EtCN, yielding a green solution with a
crystalline precipitate. After 2 h at room temperature, the purple
microcrystalline product was filtered off, washed with EtCN, and dried
under vacuum. Yield 174 mg (0.621 mmol, 85% yield); mp >250 °C.
Anal. Calcd for C6ClN2OS4: C, 25.76; N, 10.01. Found: C, 26.00; N,
9.81. IR (cm−1): 1615 (s), 1115 (m), 994 (s), 905 (m), 831 (m), 811
(m), 734 (s), 604 (w), 584 (w), 499 (m), 465 (m), 445 (m).
Method 2. Slow Diffusion for Single Crystals. A solution of
[3a][SbF6] (75 mg, 0.145 mmol) in 15 mL of degassed MeCN (four
freeze−pump−thaw cycles) was allowed to diffuse slowly into a
similarly degassed solution of OMFc (100 mg, 0.346 mmol) over a 6 h
period, affording 3a (20 mg, 0.071 mmol, 49% yield) as very fine,
hairlike needles.
hexane afforded yellow needles, mp 79−80 °C (lit.52 80−81 °C). H
1
NMR (CDCl3): δ 8.29 (s, 2H, Ph), 11.40 (s, 1H, OH).
Preparation of 2,6-Diamino-4-chlorophenol Bishydrochlor-
ide (6a). Following the procedure described for 2,6-diaminophenol,53
tin powder (31.4 g, 0.264 mol) was added in portions to a mixture of
5a (25.1 g, 0.115 mol) and SnCl2·2H2O (26.1 g, 0.116 mol) suspended
in 100 mL of concentrated HCl at 0 °C and left to stir at 0 °C for 1 h.
HCl gas was then passed through the reaction mixture for 2 min, and
the resulting precipitate was collected by filtration and washed with 20
mL of concentrated HCl. The product was recrystallized twice from
1:1 (v:v) HCl/H2O and dried at 110 °C under vacuum to give 6a as
white needles in a yield of 15.7 g (0.0679 mol, 59% yield); dec >230
1
°C. H NMR (DMSO): δ 6.72 (s, 2H, Ph), 7.61 (s, 7H, NH3, OH).
Anal. Calcd for C6H9Cl3N2O: C, 31.13; H, 3.29; N, 12.10. Found: C,
31.38; H, 3.71; N, 12.26.
Cyclic Voltammetry. CV was performed using a PINE
Bipotentiostat, Model AFCClBP1, with scan rates of 50−250 mV s−1
on solutions of [3a][OTf] in MeCN (dried by distillation from P2O5
Preparation of [3a][Cl]. Sulfur monochloride (11.7 g, 86.6 mmol)
was added dropwise to a solution of 6a (4.01 g, 17.3 mmol) in 100 mL
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and CaH2) containing 0.1 M tetra-n-butyl-ammonium hexafluor-
ophosphate. 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.54
EPR Spectroscopy. The X-band EPR spectrum of 3a was recorded
at ambient temperature on a Bruker EMX-200 spectrometer using a
sample of MeCN-solvated or unsolvated radical dissolved in degassed
toluene. Hyperfine coupling constants were obtained by spectral
simulation using Simfonia55 and WinSim.
standard weighting formula.67 Atomic positions were taken from
crystallographic data. In the case of 3a·MeCN, the MeCN molecules
were not included in the calculations.
ASSOCIATED CONTENT
■
S
* Supporting Information
Complete refs 4a and 63; details of single-crystal X-ray
crystallographic data collection and structure refinement and
tables of atomic coordinates, bond distances and angles,
anisotropic thermal parameters, and hydrogen atom positions
(CIF); intermolecular contacts in 3a and 3a·MeCN; fitting of
magnetic data for 3a·MeCN; and archival files from G09
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
Crystallography. Crystals were glued to glass fibers with epoxy. X-
ray data for [3a][SbF6] were collected using ω scans with a Bruker
APEX I CCD detector on a D8 three-circle goniometer and Mo Kα (λ
= 0.71073 Å) radiation. The data were scanned using Bruker’s SMART
program and integrated using Bruker’s SAINT software.56 The
structure was solved by direct methods using SHELXS-90 and refined
by least-squares methods on F2 using SHELXL-97 incorporated in the
SHELXTL suite of programs.57 X-ray data for the radical 3a·MeCN
were collected using ω scans with a Bruker Microstar Cu rotating-
anode (λ = 1.54178 Å) diffractometer equipped with a Proteum 135
CCD detector. The data were integrated with SAINT and scaled with
X-SCALE, which is part of the Bruker Proteum software package.58
Structural solutions56 and least-squares refinements were conducted
using the Olex2 interface59 to the SHELX suite. The slightly low
coverage (93%) reflects the fact that the uncollected high-angle
reflections would have required a lot more Cu rotating-anode
instrument time, which was not available. The refinement showed
no ill effects because of this minor omission. X-ray data for the
unsolvated radical 3a were also collected on a Bruker Proteum-135
CCD system equipped with a Cu MicroSTAR rotating anode (λ =
1.54178 Å). The frames were integrated with the Bruker SAINT
software package and corrected for absorption effects using the
multiscan method SADABS. The structure was solved and refined
using the Bruker SHELXTL software package. In view of the small size
of the crystal and the resulting paucity of data, the final R (0.0808) and
Rw (0.2027) were not unexpected. Powder X-ray diffraction data on
bulk 3a were collected at ambient temperature on a powder
diffractometer equipped with a position-sensitive detector (INEL)
using Cu Kα1 radiation (λ = 1.54056 Å). The total 2θ range was 0−
112°, measured in steps of 0.029°. Starting with the space group, unit
cell, and molecular coordinates available from the single-crystal data,
the unit cell dimensions were refined by Rietveld methods60 using the
GSAS program package.61
Magnetic Susceptibility Measurements. DC magnetic suscept-
ibility measurements on 3a·MeCN and 3a were performed over the
temperature range 2−300 K on a Quantum Design MPMS SQUID
magnetometer. Diamagnetic corrections were made using Pascal’s
constants.62
Conductivity Measurements. Temperature-dependent four-
probe conductivity measurements on cold pressed pellet samples (1
mm × 1 mm × 5 mm) of 3a·MeCN and 3a were performed over the
range 140−300 K using an Oxford Instruments MagLab EXA system
and home-built equipment. Silver paint (Leitsilber 200) was used to
apply the electrical contacts.
Molecular Electronic Structure Calculations. All of the DFT
calculations were performed with the Gaussian 09W suite of
programs63 using the (U)B3LYP hybrid functional and polarized
split-valence basis sets with triple-ζ [6-311G(d,p)] functions. Full
geometry optimization was invoked for the calculation of the total SCF
energies of cation, anion, and radical states of 1a and 3a, from which
the gas-phase IP and EA values were derived. Atomic spin densities ρ
and hyperfine coupling constants aN (in mT) were taken from single-
point (U)B3LYP/EPR-III/6-31G(d) calculations at the 6-311G(d,p)-
optimized geometries.
AUTHOR INFORMATION
■
Corresponding Author
oakley@uwaterloo.ca
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERCC) and the Engineering and
Physical Sciences Research Council (U.K.) for financial
support. We also acknowledge the NSERCC for a Vanier
Graduate Scholarship to K.L.
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