Single-Component Conductors: A Sturdy Electronic Structure Generated by Bulky Substituents

Article abstract of DOI:10.1021/acs.inorgchem.6b00556

While the introduction of large, bulky substituents such as tert-butyl, a'SiMe₃, or a'Si(isopropyl)₃ has been used recently to control the solid state structures and charge mobility of organic semiconductors, this crystal engineering strategy is usually avoided in molecular metals where a maximized overlap is sought. In order to investigate such steric effects in single component conductors, the ethyl group of the known [Au(Et-Thiazdt)₂] radical complex has been replaced by an isopropyl one to give a novel single component molecular conductor denoted [Au(iPr-Thiazdt)₂] (iPr-Thiazdt: N-isopropyl-1,3-Thiazoline-2-Thione-4,5-dithiolate). It exhibits a very original stacked structure of crisscross molecules interacting laterally to give a truly three-dimensional network. This system is weakly conducting at ambient pressure (5 S·cm^-1), and both transport and optical measurements evidence a slowly decreasing energy gap under applied pressure with a regime change around 1.5 GPa. In contrast with other conducting systems amenable to a metallic state under physical or chemical pressure, the Mott insulating state is stable here up to 4 GPa, a consequence of its peculiar electronic structure.

Full text of DOI:10.1021/acs.inorgchem.6b00556

Article
pubs.acs.org/IC
Single-Component Conductors: A Sturdy Electronic Structure
Generated by Bulky Substituents
Agathe Filatre-Furcate,† Nathalie Bellec,† Olivier Jeannin,† Pascale Auban-Senzier,‡ Marc Fourmigue,†
́
§,∥
Jorge Iniguez, Enric Canadell,*,∥ Benjamin Briere,⊥ Vinh Ta Phuoc,⊥ and Dominique Lorcy*,†
́
̀
̃
†Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
(MaCSE), Campus de Beaulieu, Batiment 10A, 35042 Rennes cedex, France
‡Laboratoire de Physique des Solides UMR 8502 CNRS-Universite
Paris-Sud, Universite
́
de Rennes 1, Matier
̀
e Condensee
́ ̀
et Systemes Electroactifs
̂
́
́
Paris-Saclay, F.91405 Orsay cedex, France
§Materials Research and Technology Department, Luxembourg Institute of Science and Technology (LIST), 5 avenue des
Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg
∥Institut de Cien
̀
cia de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, E-08193 Bellaterra, Spain
⊥GREMAN UMR 7347 CNRS, Universite
́
F. Rabelais, Parc de Grandmont, 37200 Tours, France
S
* Supporting Information
ABSTRACT: While the introduction of large, bulky sub-
stituents such as tert-butyl, −SiMe3, or −Si(isopropyl)3 has
been used recently to control the solid state structures and
charge mobility of organic semiconductors, this crystal
engineering strategy is usually avoided in molecular metals
where a maximized overlap is sought. In order to investigate
such steric effects in single component conductors, the ethyl
group of the known [Au(Et-thiazdt)2] radical complex has
been replaced by an isopropyl one to give a novel single
component molecular conductor denoted [Au(iPr-thiazdt)2]
(iPr-thiazdt: N-isopropyl-1,3-thiazoline-2-thione-4,5-dithiolate). It exhibits a very original stacked structure of crisscross
molecules interacting laterally to give a truly three-dimensional network. This system is weakly conducting at ambient pressure (5
S·cm−1), and both transport and optical measurements evidence a slowly decreasing energy gap under applied pressure with a
regime change around 1.5 GPa. In contrast with other conducting systems amenable to a metallic state under physical or
chemical pressure, the Mott insulating state is stable here up to 4 GPa, a consequence of its peculiar electronic structure.
use as multicomponent molecular conductors,5 metal bis-
INTRODUCTION
■
(dithiolene) complexes, when neutral and radical, have also
been studied, but to a lesser extent, as precursors of single
component molecular conductors.6,7 The substitution scheme
and the shape of the dithiolene ligands within such neutral
metal bis(dithiolene) complexes can also hinder short sulfur−
sulfur contacts (S···S) leading to strong intermolecular
interactions. This is true indeed with bulky ligands such as in
[M(DL-bordt)2],8 [M(S2C2Ph2)2],9 and [M(S2C2(OCn)2)2]10
(M = Au, Ni) (Chart 1). Another restriction to favorable
overlap can be also a nonplanar structure as found in
[Cu(dmdt)2] where a dihedral angle of 80.29° is observed
between the planes of the two TTF dithiolate ligands
coordinated to the Cu ion (Chart 1).11 Nevertheless, in this
case, the ligands take an arrangement similar to the “κ-type”
arrangement found in organic superconductors12 with a close
packing of the dmdt ligands allowing for a very high room
temperature conductivity for this complex (110 S.cm−1).13
The development of molecular semiconductors with bulky
groups to specifically control the overlap interactions and
associated charge mobilities has proved to be an efficient
approach in organic field effect transistors. For instance, devices
built with pentacenes substituted at the 6,13 positions by −C
C−SiR3 groups (as in TIPS when R = iPr)1 exhibit improved
performances. Similarly, tert-butyl groups introduced on a
tetrathiafulvalene core provide high mobility materials (>1 cm2
V−1 s−1).2 Such a strategy has not been investigated purposely
in organic conductors and metals, because the steric hindrance
generated by these groups can easily disrupt strong
intermolecular interactions needed for large band dispersion
and, therefore, decreases the conductivity even if close packing
is generally thermodynamically favored.3
For example, in mixed valence salts of metal bis(dithiolene)
complexes such as C[M(dmit)2] (M = Ni, Pd and dmit = 1,3-
dithiole-2-thione-4,5-dithiolate), the size and the shape of the
counterion (C+ = NMe4 , NMe3Et+, NMe2Et2 , ...), even if it
does not participate directly in the electronic delocalization,
strongly affect the organization and interactions of the main
actor, M(dmit)2, and thus the physical properties.4 Besides their
+
+
Received: March 5, 2016
Published: June 6, 2016
© 2016 American Chemical Society
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Chart 1
Among the various metal bis(dithiolene) complexes
investigated as precursors of conducting materials, neutral
gold complexes are valuable and original candidates with one
radical per site, at variance with organic conductors with mixed
valence character.14,15Along these lines, we have recently
reported a single component conductor based on a neutral
radical gold dithiolene complex, [Au(Et-thiazdt)2] (Et-thiazdt =
N-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate), which behaves
as a Mott insulator, with a two-dimensional layered structure
(Chart 2, R = Et).16
the additional methyl should prevent the same type of
organization as the one observed for the gold complexes with
an ethyl substituent or a hydroxyethyl one.
Herein we present the influence of the bulkier isopropyl
group (R = iPr) on the structural and electrochemical
properties of the complexes as well as on the conducting
properties of the corresponding materials. A very original
crisscross solid state organization is found, associated with a
peculiar electronic structure, which makes this compound
highly resistant to physical pressure effects, at variance with
earlier examples.
Chart 2
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of the thiazoline-2-thione ring 120
substituted by an isopropyl (iPr) group was realized according
the chemical strategy developed for other thiazoline-2-thiones
starting from primary amine as outlined in Scheme 1.21
Scheme 1
The quest for stabilizing the metallic state in this system
prompted us to perform different chemical modifications of the
ligand skeleton. Therefore, we have successfully developed a
series of neutral radical gold dithiolene complexes by replacing
the outer sulfur atoms of the thiazoline-2-thione rings by
oxygen atoms and/or by replacing the coordinating sulfur atom
by selenium atoms.17 Actually, these substitutions preserve the
geometry and the solid state organization of the prototypical
[Au(Et-thiazdt)2] complex, as these compounds were all
isostructural, but induce highly anisotropic chemical pressure
effects, as demonstrated by the analysis of unit cell evolutions,
with associated changes in conductivity. We also performed
another type of structural modification of the ligand skeleton
that consists of replacing the ethyl group of the thiazole ring by
a hydroxyethyl one.18 Even if the hydroxyethyl group is slightly
bulkier than the ethyl one, the neutral complex [Au(EtOH-
thiazdt)2] proved isostructural with the previously reported
examples of neutral gold dithiolene complexes belonging to the
same family (Chart 2, R = (CH2)2OH).
In order to modify the solid state organization of such radical
gold complexes to a larger extend, we focused our attention on
a bulkier nitrogen substituent such as an isopropyl group (R =
iPr). The choice of the isopropyl group is not a random
selection. Indeed, it has been demonstrated that an isopropyl
group connected to a thiazole core can adopt two types of
conformations with, in both cases, one methyl above and one
below the plane of the thiazole core.19 Thus, the disposition of
The dithiocarbamate salt was prepared by adding carbon
disulfide and triethylamine to a solution of the 2-propanamine
in diethyl ether. The reaction of chloroacetaldehyde with the
dithiocarbamate salt followed by cyclization and dehydration in
the presence of sulfuric acid led to thiazoline-2-thione 1.
Formation of the protected dithiolate ligand 2 was realized
through the bismetalation of 1 with LDA followed by the
1
addition of S8 and bromopropionitrile (Scheme 1). The H
NMR spectrum of 2 shows some peculiarities. Indeed at room
temperature it exhibits three broad signals for the isopropyl
group, one at 1.76 ppm for the two CH3 and two at 5.24 and
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5.96 ppm for the C−H with an intensity ratio 6:0.5:0.5.
Variable temperature 1H NMR experiments performed in
CDCl3 were performed by decreasing the temperature by steps
of 10 °C. Upon lowering the temperature the methyl signal at
1.76 ppm decoalesces into two sharp doublets at 1.83 and 1.69
ppm while the two signals for the C−H give two well resolved
septuplets at 5.3 and 5.96 ppm with the intensity ratio 48:52
(Figure 1).
dependence of the 1H NMR spectra carried out on this
complex 3 shows that, upon lowering the temperature, the
broad signals for the C−H at 5.74 ppm and at 4.62 ppm change
into two sharp multiplets, due to the superposition of two
septuplets of almost equal intensity, with a ratio of 92:8. Thus,
among those four distinguishable signals, two representing 92%
are attributed to one rotamer syn or anti and the two others for
8% for the other rotamer in either cis or trans configuration. For
the broad signal assigned to the CH3 at 1.58 ppm, it
decoalesces into two doublets at 1.60 and 1.56 ppm with a
ratio of 55:45 (Figure S1) and certainly due to the low amount
of one of the rotamers (8%) or the superposition of the signals
the four sets of signals are not noted. Thus, contrary to what
was observed for the precursor 2, in the case of the dithiolene
complex 3 one of the rotamers, syn or anti, exists predominantly
in solution.
Structural and Electrochemical Properties. The mono-
anionic gold complex 3 recrystallized in CH3CN was isolated as
dark green crystals, and the molecular structure has been
elucidated by X-ray diffraction. It crystallizes in the monoclinic
system, space group C2/c. Selected bond lengths are collected
in Table 1. The molecular structure of complex 3 (Figure 2) is
characterized by a trans configuration and the syn/syn
conformation with the two C−H of the isopropyl groups
pointing toward the metallacycles. The metallacycle of this
monoanionic complex is slightly distorted from planarity along
the S···S axis (5.13(7)°).
Determination of the redox potentials was carried out on
[PPh4][Au(iPr-thiazdt)2] 3 by cyclic voltammetry (Figure 3)
performed in CH2Cl2 using NBu4PF6 as supporting electrolyte.
This dithiolene complex exhibits an irreversible reduction
process at −0.95 V vs SCE and two reversible monoelectronic
oxidation waves at E1 = +0.52 V and E2 = +0.63 V vs SCE. The
two oxidation processes correspond to the formation of the
neutral and to the monocationic species, respectively. Note that
these values are very close to those found for the ethyl-
substituted analogue, [Au(Et-thiazdt)2]− (E1 = +0.52 V and E2
= +0.65 V vs SCE).
1
Figure 1. H NMR spectra of the isopropyl group, C−H and CH3
protons, of 2 at various temperature.
This observation is reminiscent of those performed by
Roussel and co-workers on thiazoline analogues with an N-
isopropyl group.19 This phenomenon indicates the existence of
two possible rotamers due to hindered rotation of the isopropyl
group, because of the presence of the cyanoethylthio groups,
and thus to different orientations of the C−H of the isopropyl
group relative to the thiazoline-2-thione plane. The C−H is
oriented either toward the CS bond leading to the anti
rotamer or toward the cyanoethylthio groups to give the syn
rotamer (Chart 3). Contrariwise, for the unsubstituted 1, the
rotation about the N−C bond is relatively unhindered for the
interconversion to occur.
Oxidation of this anionic complex has been realized by
electrocrystallization performed in acetonitrile in the presence
of NBu4PF6 as the supporting electrolyte upon application of a
current intensity of 0.3 μA. After a couple of days, crystals of
the neutral complex [Au(iPr-thiazdt)2] 4 suitable for an X-ray
diffraction study were collected on the anode. It crystallizes in
the monoclinic system, P21/c space group with two crystallo-
graphic independent Au complexes, each of them located on an
inversion center. The two neutral complexes, molecules A and
B, are planar with very small S···S folding angles of 2.15(12)°
for Au1 and 1.07(13)° for Au2 compared to the monoanionic
species (Table 1). As already observed for the monoanionic
species, the neutral complex derived from the coordination of
two dissymmetrical dithiolene ligands adopts a trans config-
uration. Concerning the disposition of the isopropyl group with
respect to the plane of the complex, the anti/anti conformation,
with the two C−H of the isopropyl groups pointing toward the
CS bond of the thiazole ring, is obtained. In the solid state,
the neutral complex crystallizes in a different conformation
(anti/anti) than the one obtained for the monoanionic species
(syn/syn).
Chart 3
The synthesis of the monoanionic gold complex 3 was
realized by adding sodium methanolate to a solution of 2, in
order to deprotect the dithiolate ligand, followed by the
addition of KAuCl4 and PPh4Cl. As previously observed for the
ligand 2, the 1H NMR spectrum of 3 displays at room
temperature broad signals due to the existence of rotamers.
However, for this gold complex only three broad signals were
observed, two for the C−H at 5.74 ppm and at 4.62 ppm and
one for the CH3 at 1.58 ppm. Therefore, here again the rotation
of each isopropyl group can be hindered leading to the
existence of different conformers: anti/anti, syn/anti, and syn/
syn. Furthermore, due to the presence of dissymmetrical
ligands, this complex 3 is susceptible to adopt a trans and cis
configuration, leading to six plausible possibilities, three
conformations for each configuration (Chart 4). Temperature
Comparison of the bond length between the monoanionic
and the neutral species, [Au(iPr-thiazdt)2]−1,0, shows that
oxidation induces pronounced modifications on the metalla-
cycle, with shortening of the C−S bonds and lengthening of the
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Chart 4
Table 1. Intramolecular Bond Lengths (in Å) in
Monoanionic and Neutral Dithiolene Complexes trans-
[Au(iPr-thiazdt)2]−1,0, Together with the Folding Angle θS···S
(in deg) of the Metallacycle along the S···S Axis
[Au(iPr-thiazdt)2]•
[Au(iPr-thiazdt)2]−
molecule A
molecule B
Au−S1
Au−S2
S1−C1
S2−C2
C1−C2
C1−N1
C2−S3
N1−C3
S3−C3
C3−S4
θS···S
2.3275(5)
2.3123(5)
1.754(2)
1.739(2)
1.346(3)
1.412(2)
1.741(2)
1.360(3)
1.736(2)
1.677(2)
5.16(5)
2.3109(13)
2.3196(11)
1.736(5)
1.716(5)
1.369(7)
1.385(6)
1.740(5)
1.372(6)
1.740(5)
1.658(5)
2.15(12)
2.3079(12)
2.3188(12)
1.732(5)
1.714(5)
1.366(7)
1.399(6)
1.744(5)
1.389(6)
1.733(5)
1.658(5)
1.07(13)
Figure 3. Cyclic voltamogram of [Au(iPr-thiazdt)2]− in CH2Cl2 with
[NBu4][PF6] 0.1 M as electrolyte, E in V vs SCE, scan rate 100 mV·
s−1.
an angle of ≈70° (Figure 4). Within the stacks, the shortest S···
S contacts between neighboring complexes amount to 3.693(5)
and 3.750(5) Å and involve the sulfur atoms of the
metallacycles (Figure 5, top). Actually these distances are in
the range of twice the van der Waals radius of sulfur (3.70 Å).
Figure 2. Molecular view (top) and side view (bottom) of the
monoanionic species of the [PPh4][Au(iPr-thiazdt)2] 3.
CC bond, and to a lesser extent within the thiazole rings.
This bond length evolution upon oxidation indicates the
extensive delocalization of the spin density on the dithiolene
moieties as already observed for other gold dithiolene
complexes.16−18
The two crystallographically independent molecules A and B
alternate and form uniform stacks −Au1−Au2−Au1−Au2−
along a with a distance Au1···Au2 of 3.483(1) Å. Due to the
bulky isopropyl groups on the thiazole rings, there is now a
large rotation of the long axis of adjacent molecules along a by
Figure 4. Projection view along a of the unit cell of [Au(iPr-thiazdt)2]
4.
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other perpendicular directions, completely differs from that
observed in the other reported complexes belonging to the
same family with R = Et or R = (CH2)2OH.16−18 In the latter
indeed, regular stacks of neutral radicals interact only laterally
to form two-dimensionnal layered structures. In other words,
the steric constraints brought by the isopropyl groups in 4 limit
the stacking of the radical complexes to a very specific crisscross
overlap but allow for sizable S···S contacts between these stacks
in the two perpendicular directions.
Magnetic Properties. The temperature dependence of the
paramagnetic susceptibility was determined in the 2−300 K
temperature range. The paramagnetic susceptibility, corrected
for the Pascal diamagnetic term (3 × 10−4 cm3·mol−1 at room
temperature), is almost temperature independent in agreement
with a Pauli-type susceptibility of a good conductor and in the
same range as the one observed for [Au(Et-thiazdt)2] (1.1 ×
10−4 cm3·mol−1 at room temperature) (Figure S2). At low
temperature, a Curie tail corresponds to less than 0.2% of S =
1/2 species attributable to paramagnetic defaults.
Transport Properties. The temperature dependence of the
resistivity was measured on single crystals along the a stacking
direction (axis of the needle) at ambient pressure and under
hydrostatic pressure. The room temperature conductivity at
ambient pressure, averaged on different crystals, amounts to
σ(RT, 1 bar) = 5 S·cm−1. The temperature dependence of the
resistivity at ambient pressure shows an insulating behavior
with an activation energy Ea = 0.058 eV as deduced from the fit
of the data with a law of the type ρ = ρ0 exp(Ea/kT) between 70
and 200 K (Figures 6 and S3 top panel).
Figure 6. Temperature dependence of the resistivity of [Au(iPr-
thiazdt)2] for sample #1 at ambient pressure and sample #2 under
pressure (0.4, 0.63, and 1.2 GPa). The red lines are the Arrhenius fits
to the data giving the activation energies.
The pressure dependence of the conductivity at room
temperature, σRT, follows an exponential law up to 1.5 GPa
(Figure 7), a further indication of an activated behavior with an
activation energy which varies linearly with pressure. Above 1.5
GPa, however, σRT adopts a linear pressure dependence (inset
of Figure 7) indicating the proximity of a metallic state. By
comparison, the prototypical [Au(Et-thiazdt)2] complex
exhibits a much higher amplitude of variation in the exponential
regime σRT (1.5 GPa)/σRT (0) ∼ 1000 (instead of 40 for 4),16
and reaches a metallic state with σRT = 1000 S·cm−1 at 2 GPa,
while for [Au(iPr-thiazdt)2] 4, σRT only reaches 300 S·cm−1at
2.6 GPa.
Figure 5. View of the short S···S contacts within the stacks along the
stacking axis a (top) and between the stacks along the bc plane with
molecules A (Au1) (middle) and molecules B (Au2) (bottom).
As illustrated in Figure 5, much shorter intermolecular S···S
contacts (3.36−3.37 Å) can be also observed in the bc plane.
These S···S contacts involve the exocyclic sulfur atom of the
thiazole core and the sulfur atom of the metallacycle of two
molecules A or two molecules B leading to potentially strong
interactions in the bc plane and giving the whole structure a
three-dimensional character (Figure 5).
It should be stressed here that this original crisscross
structure within 1D stacks, interacting sideways in the two
This very limited effect of pressure is also evidenced in the
activation energies determined from the Arrhenius fit of
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lower energy by approx 0.1 eV, up to 4.0 GPa, but remains
roughly the same in both shape and width. In addition, the low
energy optical conductivity increases, mainly above 1.3 GPa,
but no signature of a Drude peak is observed. Note that these
results are consistent with transport measurements since DC
conductivity increases with pressure and a metallic regime is
not reached. As shown in Figure 9, both pressure dependence
Figure 7. Pressure dependence of the room temperature conductivity
of [Au(iPr-thiazdt)2] with a log scale in the main plot and a linear scale
in the inset. The red line is an exponential fit to the data between 0
and 1.5 GPa.
resistivity data. Indeed, Ea decreases from 0.058 eV at ambient
pressure (or 0.065 eV at 0.4 GPa) to still 0.021 eV at 2.6 GPa
(Figure S3). Note that the lower value for Ea obtained at
ambient pressure compared to 0.4 GPa could be due to thermal
contraction effects and that Ea is determined in a smaller
temperature range (50−125 K) at high pressure (see
Supporting Information for more details).
Figure 9. Pressure dependence of the gap, Δ, obtained from optical
spectroscopy (blue symbols) and from transport measurements, Δ =
2Ea (red symbols).
Optical Measurements. In order to complement the
transport results and get a better insight on the electronic
properties of 4, optical measurements were also performed at
room temperature under pressure, allowing also higher
pressures to be reached, up to 4.0 GPa. Optical conductivity
spectra as a function of pressure are shown in Figure 8.
and values of the gap obtained by optical measurements are in
good agreement with the activation energies extracted from
transport measurements. Thus, both transport and optical data
exhibit a change of behavior around 1.5 GPa, indicating the
vicinity of a metallic state.
In addition, phonon modes exhibit large changes with
increasing pressure (Figure S4). Particularly, the spectral weight
of the mode at 1130 cm−1 strongly decreases with pressure and
seems to disappear above 1.3 GPa. Moreover, the vibration
modes around 1300 cm−1 are continuously screened as pressure
is increased, indicating the growth of a far-infrared electronic
background. The optical spectral weight is roughly conserved at
6000 cm−1 (Figure S5), and this implies that most changes in
electrodynamics properties with pressure involve an energy
scale of the order of the SOMO bandwidth, as predicted by
DFT calculations (see below).
In conclusion, both transport and optical measurements for 4
indicate that its Mott insulating behavior is rather robust and
withstands applied external pressure much better than the
prototypical ethyl-substituted complex [Au(Et-thiazdt)2]. The
regime change observed around 1.5 GPa stabilizes a more
conductive but not fully metallic state.
Figure 8. Optical conductivity spectra of [Au(iPr-thiazdt)2] at 0, 0.5,
1.3, 2.2, and 4.0 GPa. Solid lines are the slope of the edge of the mid-
infrared band at 0 and 4.0 GPa (see text). Shaded area corresponds to
the diamond absorption of the anvil cell. Inset: optical conductivity at
0 GPa. The solid line is a fit of experimental data using Lorentz
oscillators.
Electronic Structure. First-principles density functional
theory (DFT) calculations were carried out to understand the
transport and optical properties of this new gold dithiolene
complex and the differences from those of related systems
sharing the solid state organization of [Au(Et-thiazdt)2].16,17
Since [Au(iPr-thiazdt)2] 4 is a radical, let us first consider the
different ways in which the unpaired electron of each molecule
can interact in the crystal structure. As in all gold dithiolene
complexes with two ligands, identical or exhibiting very small
structural differences (see Table 1), the singly occupied
molecular orbital (SOMO) is a π type orbital delocalized
over the two ligands and with a weak participation of the gold
orbitals.16 Thus, when looking for the different ways to interact
of the individual molecules, we must focus on the short
At ambient pressure, the optical conductivity mainly consists
of narrow peaks in the far-infrared due to phonons, and a broad
mid-infrared excitation. Apart phonon modes, the optical
conductivity can roughly be fitted with a single Lorentz
oscillator of width approximately 3450 cm−1 (0.43 eV),
centered at 3760 cm−1 (0.46 eV) (inset of Figure 8). This
latter excitation is assigned to electronic transition within
SOMO bands across the Fermi level. These results are in good
agreement with DFT calculations (see below). The optical gap,
determined as the intersection of the slope of the edge of the
mid-infrared band and the x-axis, is estimated to 0.17 eV. As
pressure is increased, the mid-infrared band clearly shifts to
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contacts between the ligands. A schematic representation of the
structural plot in Figure 4 is shown in Figure 10 where the
nonmetallic state, and a metallic one, which differ by a tiny
energy difference of 2.7 meV per unit cell. Similar energy
differences were previously calculated for the [Au(Et-thiazdt)2]
based systems.16,17 Note that, because of the usual over-
estimation of the stability of the metallic state in DFT, these
energy differences should be taken as lower limits. The
localized state is the ground state and is made of stacks with
molecules with an unpaired spin up and down alternating along
a (i.e., the blue coupling is antiferromagnetic (AFM)). The
chains are coupled tridimensionally in such a way that the red
couplings are ferromagnetic (FM). Localized states where the
intrachain coupling is FM are higher in energy. Thus, [Au(iPr-
thiazdt)2] is predicted to be nonmetallic, in agreement with the
resistivity measurements. The calculated band structure for this
state is reported in Figure 11a (every band in this figure is
Figure 10. Schematic representation of the crystal structure of
[Au(iPr-thiazdt)2] along a (see Figure 4) where the molecules at the
same height are plotted with the same color. The different types of
arrows are used to indicate the different directions in which the
unpaired electron of the molecule can interact (see main text).
molecules lying at the same height along the a direction are
plotted with the same color. Because of the crisscross
organization of successive molecules along a, the interactions
along this direction within an individual chain (those
symbolized by a blue arrow in Figure 10) may seem weaker
than in the prototypical [Au(Et-thiazdt)2]. However, this is not
so. In the present case there are four intrachain S···S contacts of
3.693, 3.750, 3.752, and 3.802 Å whereas in [Au(Et-thiazdt)2]
there are only two longer S···S contacts of 3.841 Å because of
the slipping of two adjacent molecules. Thus, the band
dispersion along the chain direction is expected to be larger
in the present case. These chains are not as isolated as a quick
look at the structure could suggest. As mentioned above, within
the same plane, there are two S···S contacts per molecule of
3.362 or 3.372 Å depending on the plane (see Figure 5).
However, these contacts are of the lateral π type, which is
usually weak. DFT calculations for an isolated plane of
molecules show that indeed they are very weak and can be
disregarded in the following discussion. However, as a result of
the crisscross structure, the outer CS bonds of a molecule at
a given height fall practically on top of the outer CS bond of
a molecule of the adjacent stack lying half a unit cell below. In
that way, a chain of CS bonds almost on top of each other is
generated along the a direction such that every carbon atom is
on top of a sulfur atom and vice versa. These C···S contacts are
relatively short (∼3.4−3.6 Å), and since the C and S pz orbitals
point almost directly toward each other, the interactions are of
the strong σ type. Even if only two of the atoms of the molecule
participate in one of these interactions, the excellent σ type
overlap makes them very significant. These interactions are
symbolized with a red arrow in Figure 10. We can thus describe
the situation saying that the unpaired electrons couple
tridimensionally because the different stacks interact through
these chains of interactions which also run along a but couple
neighboring chains. Equivalently, since the unpaired electron is
delocalized all over the molecule, we could describe the
situation saying that the chains couple because the unpaired
electrons can undergo extended interactions along the crossed
step chains indicated with dashed green lines in Figure 10.
Using the experimental crystal structure, we carried out
calculations for different ways to couple the electrons in the
SOMOs. We could locate two different states, a localized,
Figure 11. Band structure for the nonmetallic (a) and metallic (b)
states of [Au(iPr-thiazdt)2]. Bands marked with an asterisk are the
SOMO bands. The energy zero refers to the highest occupied level of
the system. Γ, X, Y, Z, and R refer to the (0, 0, 0), (1/2, 0, 0), (0, 1/2,
0), (0, 0, 1/2), and (1/2, 0, 1/2) points of the monoclinic Brillouin
zone.
actually the superposition of two bands, one associated with
spin up and more localized in one of every two molecules along
the stacks, and the other with spin down and more localized in
the other set of molecules along the stack).
As noted for other gold bis(dithiolene) complexes, the single
occupied molecular orbital (SOMO) and the orbital immedi-
ately below (SOMO−1) differ by an energy which is not very
large and must be considered in understanding the electronic
structure of these molecular solids. From the eight bands in the
central part of Figure 11a, the upper four (i.e., those marked
with an asterisk) are mostly built from the SOMO whereas the
lower four are mostly built from the SOMO−1. In contrast
with the case of the [Au(Et-thiazdt)2] based systems, the
overlap of the two sets of bands is weak and does not play any
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Article
role in determining the transport properties. In addition, the
dispersion of the SOMO bands is larger in the present case, a
feature which agrees with the relatively large conductivity. In
contrast, the width of the SOMO−1 bands is considerably
smaller. The twist of successive molecules along the stack due
to the steric requirements of the bulky isopropyl substituents
leads thus to very different band structures for the present
system and the [Au(Et-thiazdt)2] based ones.
The four SOMO bands in Figure 11a can be described as two
pairs of practically folded bands along the Γ−X direction
because there are two practically equivalent molecules along
this direction and two equivalent stacks in the unit cell. The
two upper bands have considerably different energies at Γ, and
the two bands meet along the Γ−Z and Γ−Y directions,
conferring an important dispersion to these bands along the
directions perpendicular to the chain direction. In other words,
although the coupling along the chain direction is the largest,
the different chains are quite strongly coupled through the
outer CS bonds. The important feature of this band structure
is that there is an avoided crossing between the second and
third SOMO bands. Since only half of the SOMO bands can be
filled, there is a gap separating the highest occupied and lowest
unoccupied band levels and the conductivity must be activated,
in agreement with the transport measurements. The calculated
band structure for the metallic state is shown in Figure 11b. It is
very similar to that of the nonmetallic state except for the
absence of the avoided crossing between the second and third
SOMO bands. The origin of the avoided crossing in the
nonmetallic state lies in the fact that every molecule in the
antiferromagnetically coupled chain is also part of two chains of
weaker but ferromagnetic interactions along the same direction.
Under such conditions the crossing of the second and third
SOMO bands is not allowed. In contrast, the two sets of
interactions are AFM in the metallic state and the crossing is
allowed. It is therefore clear that the gap resulting from this
avoided crossing stabilizes the system and makes the non-
metallic state more stable.
Although both [Au(iPr-thiazdt)2] 4 and [Au(Et-thiazdt)2]
have a localized nonmetallic state as the ground state, the origin
of this feature is completely different. In [Au(Et-thiazdt)2], the
band gap is an indirect gap which can disappear simply by
increasing the band dispersion, something which can be easily
brought about by applying pressure.16 In contrast, the band gap
for [Au(iPr-thiazdt)2] 4 is the consequence of an avoided
crossing between two bands. As discussed above, this is a
consequence of the topological properties of the lattice and the
fact that the SOMO is delocalized over the dithiolene ligands.
Thus, using pressure to modify the band dispersion cannot
remove this gap unless a very anisotropic compression of the
crystal structure leads to a structural change such that the
nature of the intermolecular interactions is strongly modified.
In principle, both an increase (as it presumably occurs for
[Au(Et-thiazdt)2]) or decrease of the intermolecular inter-
actions may result from the application of a moderate pressure.
However, the avoided crossing must persist in both cases. As an
illustration of this situation, the calculated band structure when
the intermolecular distance was shortened by 0.1 Å along the
chain axis (i.e., a 2.9% contraction along a) is show in Figure
12. As reasoned above, the band gap arising from the avoided
crossing is kept without much change. Consequently, an
activated conductivity is expected if under pressure the essential
topology of the lattice is kept or if the structure evolves to a
different topology in such a way that the intermolecular
Figure 12. Band structure for the nonmetallic state of [Au(iPr-
thiazdt)2] when the molecules were rigidly brought together by 0.1 Å
along the chain axis. The energy zero refers to the highest occupied
level of the system. Γ, X, Y, Z, and R refer to the (0, 0, 0), (1/2, 0, 0),
(0, 1/2, 0), (0, 0, 1/2), and (1/2, 0, 1/2) points of the monoclinic
Brillouin zone.
interactions are not strong. However, the results in Figure 6
clearly argue in favor of the first possibility. Thus, in clear
contrast with other single component molecular conductors, be
they organic molecules (bisdithiazolyls)22 or metal complexes
23
(Ni(dmit)2, Au(Et-thiazdt)2,16) which can be made metallic
under physical or chemical pressure,17 [Au(iPr-thiazdt)2] is
expected to keep the nonmetallic behavior under pressure, in
agreement with our transport measurements.
CONCLUDING REMARKS
■
We have reported here the influence of steric effect on both the
organization and the properties of a single component
molecular conductor, namely, [Au(iPr-thiazdt)2]. Despite the
presence of two bulky isopropyl groups on the planar radical
gold dithiolene complex, the system has found a way to favor
strong intermolecular overlap interactions by adopting an
original crisscross overlap, at variance with the one observed for
the prototypical [Au(Et-thiazdt)2]. First-principles calculations
show that the ground state of the system is nonmetallic with
antiferromagnetic coupling of the SOMO electrons along the
stacks but ferromagnetic coupling between the stacks. At
variance with [Au(Et-thiazdt)2], which exhibits an indirect band
gap amenable to closing under pressure leading to a metallic
state,16,17 the isopropyl complex [Au(iPr-thiazdt)2] possesses a
direct band gap resulting from an avoided band crossing. This
symmetry constraint is maintained under applied pressure and
strongly stabilizes the nonmetallic behavior, as experimentally
observed from transport and optical measurements.
EXPERIMENTAL SECTION
General. NMR spectra were recorded at room temperature using
■
CDCl3 unless otherwise noted. Chemical shifts are reported in ppm
1
referenced to TMS for H NMR. High resolution mass spectrometry
(HRMS) was performed using the fast bombardment (FAB)
ionization mode and the (ESI) electrospray ionization mode by the
́
Centre Regional de Mesures Physiques de l’Ouest, Rennes. CVs were
carried out on a 10−3 M solution of complex in CH2Cl2−[NBu4][PF6]
0.1 M. Potentials were measured versus saturated calomel electrode
(SCE). Chemicals and materials from commercial sources were used
without further purification.
1,3-Thiazoline-2-thiones 1. At 0 °C, to a solution of isopropyl-
amine (2.1 mL, 25 mmol) and triethylamine (3.8 mL, 27.5 mmol) in
200 mL of diethyl ether, CS2 (1.7 mL, 27.5 mmol) was slowly added.
The reaction mixture was stirred for 30 min at 0 °C. The white solid
was filtered, washed with diethyl ether, and dried under vacuum. The
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Inorganic Chemistry
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dithiocarbamate salts are used in the next step without further
found, 1317.1187. Anal. Calcd for C36H34N2PS8: C, 44.16; H, 3.50; N,
2.86. Found: C, 43.98; H, 3.52; N, 2.80.
1
purification. White powder: yield = 51% (3 g); mp 132 °C; H NMR
3
Electrocrystallization. Crystals of [Au(iPr-thiazdt)2] were pre-
pared electrochemically using a two compartment cell (12 mL) with Pt
electrodes. An acetonitrile solution of [PPh4][Au(iPr-thiazdt)2] (8
mg) and nBu4NPF6 (100 mg) was placed in the anodic compartment,
and an acetonitrile solution of nBu4NPF6 (100 mg) in the cathodic
compartment. Black needles of [Au(iPr-thiazdt)2], were collected on
the anode upon application of a constant current of 0.4 μA for 10 days.
Crystallography. Single-crystal diffraction data were collected on
APEXII, Bruker-AXS diffractometer, Mo Kα radiation (λ = 0.71073
Å), for all compounds. The structures were solved by direct methods
using the SIR97 program,24 and then refined with full-matrix least-
squares methods based on F2 (SHELXL-97)25 with the aid of the
WINGX26 program. All non-hydrogen atoms were refined with
anisotropic atomic displacement parameters. H atoms were finally
included in their calculated positions. Details of the final refinements
are given In Table 2. The crystal structure of 3 was refined with one
extra water molecule, located close to the inversion center, with a 0.5
occupation parameter.
(D2O, 300 MHz) δ 1.15 (t, 9H, CH3, JHH = 7.3 Hz), 1.17 (d, 6H,
CH3, 3JHH = 6.6 Hz), 3.09 (q, 6H, CH2, 3JHH = 7.3 Hz), 4.25 (sept, 1H,
CH, 3JHH = 6.6 Hz); 13C NMR (D2O, 75 MHz) δ 8.3 (CH3),19.8 (N-
CH(CH3)2), 46.6 (CH2), 49.9 (N-CH(CH3)2), 207.9 (CS).
To a solution of the dithiocarbamate salt (12.8 g, 54 mmol) in
acetonitrile (100 mL), chloroacetaldehyde (7.2 mL, 56.7 mmol, 50%
in water) was slowly added. The pale yellow solution was stirred at
room temperature overnight. The volume was reduced to about 10 mL
by rotary evaporation, and the brown residue was slowly added to 25
mL of concentrated sulfuric acid cooled at 0 °C. The reaction mixture
was stirred during 20 min, hydrolyzed with ice water, and extracted
with dichloromethane. The organic phase was washed with water and
dried over MgSO4, and the solvent was removed under reduced
pressure to give thiazoline-2-thione 1, which was purified by column
chromatography using dichloromethane as eluent. Beige powder (Rf =
0.55): yield = 91% (4.9 g); mp 56 °C; 1H NMR (300 MHz) δ 1.38 (d,
6H, CH3, 3JHH = 7.4 Hz), 5.37 (sept, 1H, CH, 3JHH = 7.4 Hz), 6.64 (d,
3
3
1H, CH, JHH = 4.6 Hz), 7.12 (d, 1H, CH, JHH = 4.6 Hz); 13C
NMR (75 MHz) δ 21.4 (CH3), 50.3 (CH), 111.4 (CH), 127.2 (
CH), 186.0 (CS); HRMS calcd for [M + H]+ (ASAP) C6H10NS2,
160.02492; found, 160.0249. Anal. Calcd for C6H9NS2: C, 45.25; H,
5.70; N, 8.79; S, 40.26. Found: C, 45.14; H, 5.69; N, 8.80; S, 40.77.
4,5-Bis(cyanoethylthio)-1,3-thiazoline-2-thiones 2. Under an inert
atmosphere, at −10 °C, freshly prepared LDA (7.5 mL, 12.0 mmol)
was added to a solution of 1,3-thiazoline-2-thione 1 (1.27 g, 8.0 mmol)
in 40 mL of anhydrous THF. After stirring for 30 min at −10 °C,
sulfur S8 (0.38 g, 12.0 mmol) was added and the solution was stirred
for an additional 30 min. A solution of 16.0 mmol of LDA was added
to the reaction medium. Stirring was continued for 3 h, at −10 °C, and
0.51 g (16.0 mmol) of sulfur was added. After 30 min 6.6 mL (80.0
mmol) of 3-bromopropionitrile was added, and the reaction mixture
was stirred overnight. The solvent was removed in vacuo, and the
residue was dissolved in dichloromethane. The organic phase was
washed with water and then dried over MgSO4. The resulting oil was
purified by column chromatography using diethyl ether/dichloro-
methane as eluent (0.1/9.9). 2 was isolated as an orange oil: (Rf =
Table 2. Crystallographic Data
3
4
formulas
FW (g mol−1
system
space group
a (Å)
C36H36AuN2OPS8
997.08
C12H14AuN2S8
639.70
)
monoclinic
C2/c
monoclinic
P21/c
29.0982(7)
7.3725(2)
22.3171(5)
90
6.9668(6)
12.8949(10)
21.2391(13)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
T (K)
125.1210(10)
90
92.757(4)
90
3915.97(17)
150(2)
1905.8(2)
150(2)
Z
4
4
1
Dcalc (g·cm−1
)
1.688
2.229
0.57); yield = 40%. For the main rotamer (52.4%): H NMR (300
MHz) δ 1.69 (d, 6H, CH3,3JHH = 7.1 Hz), 2.82 (t, 4H, CH2,3JHH = 7.1
Hz), 2.91 (t, 4H, CH2,3JHH = 7.1 Hz), 5.96 (sept, 1H, CH, 3JHH = 7.1
Hz); 13C NMR (75 MHz) δ 18.2 (N−CH(CH3)2), 18.5 (S-CH2−
CH2-CN), 31.6 (S-CH2-CH2-CN), 53.4 (CH), 118.3 (CN), 126.2 (
C), 135.6 (C), 186.9 (CS). For the other rotamer: 1H NMR (300
MHz) δ 1.83 (d, 6H, CH3,3JHH = 7.0 Hz), 3.07 (t, 4H, CH2,3JHH = 7.0
Hz), 3.23 (t, 4H, CH2,3JHH = 7.0 Hz), 5.30 (sept, 1H, CH, 3JHH = 7.0
Hz); 13C NMR (75 MHz) δ 20.8 (N−CH(CH3)2), 18.9 (S-CH2-CH2-
CN), 33.3 (S-CH2-CH2-CN), 56.0 (CH), 118.3 (CN), 129.0 (C),
136.4 (C), 187.6 (CS); HRMS calcd for [M]+(ASAP)
C12H15N3S4, 329.01433; found, 329.0142. Anal. Calcd for
C12H13N3S4: C, 44.01; H, 4.00; N, 12.83. Found: C, 43.80; H, 3.80;
N, 12.59.
μ (mm−1
)
4.257
8.593
total refls
29144
4337
abs corr
multiscan
4485 (0.0227)
4087
multiscan
4337 (0.0409)
3133
uniq refls (Rint)
uniq refls (I > 2σ(I))
R1, wR2
0.0169, 0.039
0.020, 0.0406
1.05
0.0315, 0.0596
0.0532, 0.0645
1.067
R1, wR2 (all data)
GOF
Resistivity Measurements. The resistivity measurements were
performed along the long axis of the needles (a crystallographic axis)
of [Au(iPr-thiazdt)2]. Gold pads were evaporated on the crystals in
order to improve the quality of the contacts and gold wires were glued
with silver paste on those contacts. Then a standard four point
technique was used with a low frequency lock-in detection (Iac = 1 μA)
for measured resistances below 10 kΩ and dc measurement for higher
resistances (Idc = 0.1−1 μA).
Resistivity measurements were also performed under high hydro-
static pressure in a CuBe clamped cell up to 1.2 GPa (#2) and a
NiCrAl clamped cell up to 2.6 GPa (#3) with silicone oil (Daphne
7373) as the pressure transmitting medium. The pressure at room
temperature was extracted from the resistance of a Manganin gauge in
the pressure cell, and it is this value which is indicated in the figures.
However, the loss of pressure during cooling is estimated to 0.2 GPa,
slightly decreasing with pressure. For experiments in the CuBe
pressure cell, low temperatures have been provided by a cryocooler
equipment down to 25 K with a copper−constantan thermocouple
inside the pressure cell as the thermometer. For experiments in the
NiCrAl pressure cell, a homemade cryostat equipped with a 4 K pulse
[PPh4][Au(iPr-thiazdt)2] 3. Under inert atmosphere and at room
temperature, 7 mL (7.0 mmol) of a 1 M solution of sodium
methanolate was added to 0.27 g (0.82 mmol) of 1,3-thiazoline-2-
thione 2. After complete dissolution of the compound, 0.19 g (0.5
mmol) of potassium tetrachloroaurate(III) hydrate (KAuCl4.H2O)
dissolved in 5 mL of dry methanol was added to the reaction medium.
Stirring was continued for 5 h, and 0.19 g (0.5 mmol) of
tetraphenylphosphonium chloride (PPh4Cl) dissolved in 3 mL of
anhydrous methanol was added to the reaction medium. Then the
reaction was stirred overnight at room temperature under argon
atmosphere. The opaque dark green solution was filtered. The
resulting solid was washed with methanol and recrystallized in
acetonitrile. Dark green crystals: yield = 38% (150 mg); mp 226 °C;
1H NMR at room temperature (300 MHz) δ 1.58 (broad signal, 6H,
CH3), 4.62 (broad signal, 2H, CH, 8%), 5.74 (broad signal, 2H, CH,
92%), 7.61 (m, 8H, CHAr), 7.76 (m, 8H, CHAr), 7.89 (m, 4H, CHAr);
HRMS calcd for [2C+, A−] (ESI) for C60H54N2P2S8Au, 1317.1188;
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Inorganic Chemistry
Article
tube has been used with a Cernox thermometer located outside the
pressure cell. Cooling and warming curves (performed at about 0.4 K/
min down to 50 K) have been averaged in order to annihilate the
temperature gradient (0 to 10 K) between the sample and the
thermometer.
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* Supporting Information
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dominique.lorcy@univ-rennes1.fr.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the CDIFX (Rennes) for access to the X-ray
diffraction facilities and the ANR (France) for financial support
through project No. 12-BS07-0032. Work in Bellaterra was
supported by MINECO (Spain) through Grant FIS2012-
37549-C05-05 and Generalitat de Catalunya (2014SGR301).
J.I. was financially supported by the Fond National de
Recherche Luxembourg through a PEARL grant (FNR/P12/
4853155/Kreisel) and by MINECO-Spain (Grant No.
MAT2013-40581-P). E.C. acknowledges support of the Spanish
MINECO through the Severo Ochoa Centers of Excellence
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