Anisotropic chemical pressure effects in single-component molecular metals based on radical dithiolene and diselenolene gold complexes

Article abstract of DOI:10.1021/ja3065649

On the basis of the reported radical neutral complex [Au(Et-thiazdt) ₂] (Et-thiazdt = N-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate), a series of single-component conductors derived from [Au(Et-thiazdt)₂], also noted as [AuS₄(=S)₂], has been developed, by replacing the outer sulfur atoms of the thiazoline-2-thione rings by oxygen atoms and/or by replacing the coordinating sulfur atoms by selenium atoms toward the corresponding diselenolene complexes. Comparison of the X-ray crystal structures and transport properties of the four isostructural complexes, noted as [AuS₄(=S)₂], [AuS₄(=O)₂], [AuSe₄(=S)₂], and [AuSe₄(=O)₂], shows that the oxygen substitution on the outer thiazoline ring actually decreases the conductivity by a factor of 100, despite a contracted unit cell volume reflecting a positive chemical pressure effect. On the other hand, the S/Se substitution increases the conductivity by a factor of 100, and the pressure needed to transform these semiconductors into the metallic state is shifted from 13 kbar in [AuS₄(=S)₂] to only ?6 kbar in [AuSe₄(=S)₂]. Analysis of unit cell evolutions and ab initio band structure calculations demonstrates the strongly anisotropic nature of this chemical pressure effect and provides an explanation for the observed changes in conductivity. The greater sensitivity of these neutral single-component conductors to external pressure, as compared with classical radical salts, is also highlighted.

Full text of DOI:10.1021/ja3065649

Article
pubs.acs.org/JACS
Anisotropic Chemical Pressure Effects in Single-Component
Molecular Metals Based on Radical Dithiolene and Diselenolene Gold
Complexes
Gilles Yzambart,† Nathalie Bellec,† Ghassan Nasser,† Olivier Jeannin,† Thierry Roisnel,†
,†
‡
§
,§
,†
́
Marc Fourmigue,* Pascale Auban-Senzier, Jorge Iniguez, Enric Canadell,* and Dominique Lorcy*
́
̃
†Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
(MaCSE), Campus de Beaulieu, Bat 10A, 35042 Rennes cedex, France
‡Laboratoire de Physique des Solides, UMR 8502 CNRS-Universite
de Paris-Sud, Bat 510, F-91405 Orsay cedex, France
§Institut de Cien
cia de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, E-08193 Bellaterra, Spain
́
de Rennes 1, Matier
̀
e Condensee
́
et System
̀
es Electroactifs
̂
́
̀
S
* Supporting Information
ABSTRACT: On the basis of the reported radical neutral
complex [Au(Et-thiazdt)2] (Et-thiazdt = N-ethyl-1,3-thiazo-
line-2-thione-4,5-dithiolate), a series of single-component
conductors derived from [Au(Et-thiazdt)2], also noted as
[AuS4(S)2], has been developed, by replacing the outer
sulfur atoms of the thiazoline-2-thione rings by oxygen atoms
and/or by replacing the coordinating sulfur atoms by selenium
atoms toward the corresponding diselenolene complexes.
Comparison of the X-ray crystal structures and transport
properties of the four isostructural complexes, noted as
[AuS4(S)2], [AuS4(O)2], [AuSe4(S)2], and [AuSe4(O)2], shows that the oxygen substitution on the outer thiazoline
ring actually decreases the conductivity by a factor of 100, despite a contracted unit cell volume reflecting a positive chemical
pressure effect. On the other hand, the S/Se substitution increases the conductivity by a factor of 100, and the pressure needed to
transform these semiconductors into the metallic state is shifted from 13 kbar in [AuS4(S)2] to only ≈6 kbar in [AuSe4(
S)2]. Analysis of unit cell evolutions and ab initio band structure calculations demonstrates the strongly anisotropic nature of this
chemical pressure effect and provides an explanation for the observed changes in conductivity. The greater sensitivity of these
neutral single-component conductors to external pressure, as compared with “classical” radical salts, is also highlighted.
Chart 1
INTRODUCTION
■
Beyond the well-developed use of metal bis(dithiolene)
complexes as precursors of multicomponent molecular
conductors in mixed-valence salts such as C[Ni(dmit)2]2 (C+
1
= NMe4 , TTF+, ...), an emerging strategy is to use these
complexes as precursors of single-component molecular
conductors, that is, neutral complexes lacking any C+
counterion.2 Along these lines, several neutral metal bis-
(dithiolene) complexes bearing a noninnocent tetrathiafulva-
lene backbone were successfully developed by Kobayashi et al.,
such as [Ni(tmdt)2] (Chart 1).3 Another route toward neutral
single-component dithiolene complexes is based on neutral
gold complexes lacking this TTF backbone.4 Such radical
species can be easily obtained by oxidation of the AuIII
monoanionic dithiolene complexes, particularly with electron
rich ligands like [Au(bdt)2],5 [Au(α-tpdt)2],6 and [Au-
(F2pdt)2]7 (Chart 1). The regular stacking of these radical
complexes in the solid state5,7 and the presence of interstack
interactions allow for high room temperature conductivity
(10−1−10−3 S cm−1), although the temperature dependence is
characteristic of semiconducting behavior.
+
Received: July 10, 2012
Published: September 25, 2012
© 2012 American Chemical Society
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In this context, we recently reported the synthesis and
properties of a very promising system based on the dithiolene
ligand N-alkyl-1,3-thiazoline-2-thione-4,5-dithiolate (R-
thiazdt).8 Indeed, electrochemical oxidation of the anionic
[Au(Et-thiazdt)2]− afforded the neutral radical complex [Au-
(Et-thiazdt)2].9 In the solid state, the radical complex organizes
into uniform, nondimerized stacks, with sizable interstack
interactions. Its room temperature conductivity is strongly
pressure dependent, from 0.33 S cm−1 at ambient pressure up
to 1000 S cm−1 at 21 kbar. Furthermore, from the room
temperature semiconducting behavior, a metallic state can be
stabilized above 13 kbar. Ab initio band structure calculations
demonstrated unambiguously that the indirect gap observed at
ambient pressure could be indeed suppressed by contracting
the conducting slabs along the b stacking direction and/or the a
interstack direction. Such theoretical approaches to pressure
effects are particularly useful, as experimental structural
determinations under pressure are still not a routine work.10
Because of the soft nature of molecular conductors, pressure
has always been a favorite tool in the toolbox of solid-state
physicists to explore the phase diagram of mixed valence
multicomponent conducting salts, from hydrostatic11 to
uniaxial pressure effects.12 Chemical substitution strategies
offer another possibility to explore their phase diagram, as
illustrated in the use of counterions of different volume in
that the introduction of smaller oxygen atoms could bring the
molecules closer to each other and exert a positive chemical
pressure on the system. Accordingly, we have prepared the N-
ethyl-1,3-thiazoline-2-one-4,5-dithiolate ligand and its corre-
sponding neutral gold bis(dithiolene) complex, noted as
[AuS4(O)2]. On the other hand, we also investigated the
introduction of heavier selenium atoms in the metallacycles, as
the S/Se substitution usually increases intermolecular inter-
actions and consequently band dispersion in the solid state.
Accordingly, we have prepared the two novel diselenolate
ligands, i.e., N-ethyl-1,3-thiazoline-2-one-4,5-diselenolate and
N-ethyl-1,3-thiazoline-2-thione-4,5-diselenolate and their corre-
sponding Au bis(diselenolene) complexes, noted as [AuSe4(
O)2] and [AuSe4(S)2], respectively (Chart 2).
Chart 2
TMTTF13,14 or TMTSF salts, for example, in the PF6 /AsF6 /
−
−
SbF6 or ClO4 /ReO4 series.15,16 One can also mention the
halide-containing salts such as κ-[BEDT-TTF]2[Cu(N(CN)2)-
X]17 or (o-Me2TTF)2X,18 with X = Cl, Br, I, or the (EDT-
TTF-CONMe2)X salts with X = Br or AsF6.19 Note also
ternary compounds where the crystal structure includes solvent
molecules of different size.20 The effects of such substitutions
have been described as chemical pressure effects. Positive
chemical pressure effects are associated with smaller atoms or
molecules, leading to a cell contraction with electronic
consequences similar to those of an applied hydrostatic
pressure, and negative pressure effects are associated with the
more counterintuitive situation where larger unit cells lead to
electronic behaviors observed only under higher pressures.21
Deuteration has been also investigated in cation radical salts for
that purpose as the volume of the CD3 group has been
estimated to be 1.0−1.5% less than that of the CH3 group in
the region of 50−100 K,22 with different effects on metal−
insulator or metal−superconductor transitions in TMTSF,23
BEDT-TTF,24 or DCNQI salts.22
−
−
−
In this paper, we describe the structural and electronic
properties of the monoanionic species based on the three
analogues of the all-sulfur [AuS4(S)2]−1 complex, that is,
[AuS4(O)2]−1, [AuSe4(O)2]−1, and the [AuSe4(S)2]−1,
and their oxidation to the corresponding neutral radical
complexes. The observed striking evolutions of their transport
properties within the whole series are rationalized on the basis
of a combined experimental and theoretical approach to their
crystallographic and electronic structures, demonstrating that
atom substitutions can afford highly anisotropic chemical
pressure effects, an original approach besides uniaxial physical
pressure effects to explore the phase diagram of these single-
component molecular conductors.
RESULTS AND DISCUSSION
■
Synthesis and Crystal Structures. The synthesis of the
monoanionic gold dithiolene and diselenolene complexes was
carried out starting from the N-ethyl-1,3-thiazoline 2-thione 1,
according to the synthetic strategy described in Scheme 1. In
Single-component conductors such as [Au(tmdt)2]25 are also
pressure sensitive,26 as well as organic radicals27 behaving as
Mott insulators,28 which exhibit an insulator to metal transition
under high pressure.29 The striking physical pressure effects
unraveled in the single-component [Au(Et-thiazdt)2] con-
ductor9 raise questions about the possible effects that chemical
pressure could play within these single-component conductors.
In the absence of any counterion, the only possibility left for
investigating chemical pressure effects is based on atom
substitution on the thiazdt backbone of the organic ligand.
We have thus followed two different strategies to test the
chemical pressure influence. In the one hand, we focused our
attention on the exocyclic sulfur atoms of the reference
complex [Au(Et-thiazdt)2], noted as [AuS4(S)2] in the
following to highlight the modified atoms on the Et-thiazdt
ligand. Indeed, these two exocyclic sulfur atoms do not
participate notably in the interactions between neighboring
molecules in the conducting slabs. It was therefore anticipated
Scheme 1. Synthetic Procedures toward Anionic Gold
Complexes
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order to form the N-ethyl-1,3-thiazoline-2-one 2, we first
transformed 1 into the corresponding thiomethyl-1,3-thiazo-
lium salt, thanks to the alkylation of the exocyclic sulfur atom
with MeI. The subsequent reaction of this salt with MeONa
allowed us to isolate 2 in 82% yields. Then, we synthesized the
organic hidden form of the dithiolate and diselenolate ligands,
where these ligands are protected by cyanoethyl groups. The
protected dithiolate and diselenolate ligands were formed by
reacting the thiazole rings 1 and 2 at −10 °C in THF with
successively (i) lithium diisopropylamide (LDA), (ii) sulfur or
selenium, and (iii) 3-bromopropionitrile (Scheme 1). These
organic hidden forms of the ligands are easily purified by
column chromatography. Deprotection of the dithiolate and
diselenolate ligands was realized in basic medium using sodium
methanolate. Then KAuCl4 was added to the solution, followed
by the addition of Et4NBr or PPh4Cl to the reaction mixture.
The corresponding dithiolene complexes [PPh4][AuS4(O)2]
and the diselenolene monoanionic gold complexes [PPh4]-
[AuSe4(O)2] and [NEt4][AuSe4(S)2] were isolated in 30−
50% yields. Black crystals, suitable for X-ray diffraction studies
were obtained for every complex after recrystallization in
acetonitrile.
Table 1. Intramolecular Bond Lengths (in Å) in
Monoanionic and Neutral Dithiolene and Diselenolene
Complexes [AuS4(S)2]−,•, [AuS4(O)2]−,•, [AuSe4(
S)2]−,•, and [AuSe4(O)2]−,•, Together with the Folding
Angle of the Metallacycle along the Chalcogen···Chalcogen
Axis, θS···S and θSe···Se
9
9
[AuS4(S)2]−
[AuS4(S)2]•
[AuS4(O)2]− [AuS4(O)2]•
Au−S1
Au−S2
S1−C1
S2−C2
C1−C2
C2−N1
C1−S3
N1−C3
S3−C3
C3−X4
θS···S
2.3321(16)
2.3287(15)
1.744(6)
1.760(5)
1.332(8)
1.416(7)
1.731(6)
1.356(7)
1.743(6)
1.658(6)
10.9(1)
2.3196(13)
2.3186(14)
1.713(5)
1.720(6)
1.372(8)
1.390(7)
1.738(5)
1.370(7)
1.749(6)
1.641(6)
1.2(1)
2.317(1)
2.326(1)
1.740(3)
1.750(2)
1.338(4)
1.402(3)
1.751(3)
1.371(3)
1.787(3)
1.215(3)
8.42(5)
2.309(3)
2.322(3)
1.708(12)
1.738(11)
1.379(17)
1.389(14)
1.736(12)
1.370(15)
1.799(13)
1.206(15)
0.6(2)
[AuSe4(O)2]− [AuSe4(O)2]• [AuSe4(S)2]− [AuSe4(S)2]•
Au−S1
Au−S2
Se1−C1
Se2−C2
C1−C2
C2−N1
C1−S3
N1−C3
S3−C3
C3−X4
θSe···Se
2.433(2)
2.445(2)
1.879(7)
1.902(6)
1.339(10)
1.395(8)
1.761(7)
1.36(1)
2.4432(17)
2.4164(17)
1.833(17)
1.903(18)
1.42(3)
2.4405(6)
2.4456(6)
1.874(6)
1.910(6)
1.342(8)
1.398(7)
1.739(6)
1.369(7)
1.749(6)
1.659(6)
16.8(1)
2.4368(3)
2.4410(4)
1.862(4)
1.875(4)
1.374(5)
1.390(4)
1.740(3)
1.373(5)
1.755(4)
1.649(4)
2.82(5)
The molecular structures of the anionic moieties of the three
complexes depicted in Figure 1 show the same trends: (i) a
1.38(2)
1.729(17)
1.36(2)
1.792(8)
1.218(10)
5.7(2)
1.83(2)
1.18(2)
0.4(4)
those observed for other monoanionic homoleptic gold
dithiolene or diselenolene complexes, with Au−Se bond
distances around 2.44 Å and Au−S bond distances around
2.32 Å. The main difference induced by the outer chalcogen
atom is observed in the folding angle along the X···X axis,
which is significantly smaller in the thiazoline-2-one when
compared with the thiazoline-2-thione. This can be ascribed to
the difference of electronegativity/polarizability of the outer
chalcogene atom.
Electrochemical Studies. Cyclic voltammograms (CV) of
the three novel anionic complexes have been recorded in
CH2Cl2 using [NBu4][PF6] as supporting electrolyte. The CV
of [PPh4][AuS4(O)2] is shown in Figure 2 as an example.
[PPh4][AuS4(O)2] and [PPh4][AuSe4(O)2] exhibit upon
anodic investigation two reversible monoelectronic processes
corresponding to the oxidation of the monoanionic species into
the neutral radical and then to the oxidation into the
monocationic species. Upon cathodic investigation, the
reduction wave is irreversible and corresponds to the reduction
Figure 1. Molecular structure of the monoanionic species in
[PPh4][AuS4(O)2] (a) and side views of the monoanionic species
in [PPh4][AuS4(O)2] (b), [PPh4][AuSe4(O)2] (c), and [NEt4]-
[AuSe4(S)2] (d).
square-planar geometry around the Au atom, (ii) the obtention
of the trans isomer only, even if the cis and trans isomers can be
formed due to the dissymmetry of the dithiolene ligands, and
(iii) the metallacycles are nonplanar and are folded along the
X···X axis (X = S, Se) with an angle of about 5°−17° while the
outer thiazole cores are planar. These geometrical features are
similar to those observed on the analogous monoanionic gold
dithiolene complex, [NEt4][AuS4(S)2].9 Intramolecular bond
lengths (Table 1) within the metallacycles are comparable to
Figure 2. Cyclic voltammogram of [PPh4][AuS4(O)2] in CH2Cl2
with 0.1 M [NBu4][PF6] as electrolyte (E in V vs SCE, scan rate 100
mV s−1).
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into the dianionic species. As already observed for Ni
complexes with similar ligands,8a the first oxidation process of
the dithiolene [AuS4(O)2]− occurs at a less anodic potential
than for the corresponding diselenolene [AuSe4(O)2]−
(Table 2). Note also that the substitution of sulfur for oxygen
bonds in the outer thiazole ring are not significantly modified.
These structural modifications upon oxidation demonstrate the
strong delocalization of the spin density on the metallacycles, as
already observed with the few examples of gold dithiolene
complexes reported so far where both monoanionic and neutral
forms were characterized, that is, [Au(α-tpdt)2],6 [Au-
(dddt)2],30,31 [Au(F2pdt)2],7 and [AuL2] with L = 1,2-di(4-
tert-butylphenyl)ethylene-1,2-dithiolate.32 In the solid state
(Figure 4), the neutral radical complexes adopt the same
Table 2. Redox Potentials for the Various Gold Dithiolene
and Diselenolene Complexes in CH2Cl2 with 0.1 M
[NBu4][PF6] (E in V vs SCE, ν = 100 mV s−1)
complex
Ered
E1
E2
ref
ox
ox
a
a
a
a
[AuS4(O)2]
[AuSe4(O)2]
[AuSe4(S)2]
[AuS4(S)2]
−1.05
−1.07
−0.89
−0.90
0.39
0.44
0.52
0.52
0.86
0.88
−
this work
this work
this work
9
a
0.65
a
Irreversible process.
increases the overall electron-donating ability of the complexes,
as illustrated by the comparison of the first redox potential of
[AuS4(O)2]− (+0.38 V) with that of [AuS4(S)2]− (+0.52
V). The diselenolene complex [AuSe4(S)2]− shows a
different behavior, as an irreversible oxidation process is
observed at 0.52 V on the first anodic scan, presumably due
to adsorption phenomena.
The Neutral Radical Complexes. Oxidation of the
monoanionic species has been realized electrochemically
upon application of a current intensity of 0.2−0.5 μA in the
presence of NBu4PF6 as supporting electrolyte. Crystals of the
neutral species were collected on the anode and investigated by
X-ray diffraction. The neutral complexes [AuS4(O)2],
[AuSe4(O)2], and [AuSe4(S)2] crystallize in the mono-
clinic system, space group P21/a. They are isostructural with
the all-sulfur analog [AuS4(S)2].9 The molecular structures of
the three complexes show that all these neutral species are
planar, as depicted in Figure 3 for [AuSe4(O)2], with a
chalcogen···chalcogen folding angle (θS···S and θSe···Se in Table
1) strongly decreased, when compared with the monoanionic
complexes.
Figure 4. Projection view along b of the unit cell of [AuSe4(O)2].
structure as [AuS4(S)2], characterized by a layered structure,
with uniform stacks running along b and interacting laterally
along a. Within the stacks, a lateral slip is observed between
neighboring complexes.
We have therefore now at hand a series of four isostructural
radical complexes, with either S or O atoms on the outer
thiazoline ring, with either S or Se atoms to coordinate the gold
atom. The all-sulfur compound, [AuS4(S)2] was reported to
behave as a semiconductor at ambient pressure, with σRT = 0.33
S cm−1 and Eact = 0.12 eV [ρ = 1/σ = ρ0 exp(Eact/kT)]. It
becomes metallic at pressures above 13 kbar.9
The resistivity measurements (Figure 5) were carried out on
single crystals of the three novel complexes along the long axis
of the needles (b crystallographic axis). The room temperature
Figure 3. Molecular structure of the neutral species [AuSe4(O)2].
Comparison of the geometrical characteristics between the
monoanionic and neutral states of the complexes allowed us to
analyze the modifications occurring upon oxidation (Table 1).
These modifications occur essentially on the bond lengths of
the metallacycles and follow the same trends for the dithiolene
and the diselenolene derivatives. For instance, upon oxidation,
the X−C (X = S, Se) bonds are shortened while the CC
bond is lengthened. Apart from this CC bond, the other
Figure 5. Temperature dependence of the resistivity of the four
complexes at ambient pressure.
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conductivity at ambient pressure, σRT (1 bar), is 0.33 × 10−2
S
cm−1 for [AuS4(O)2], 0.18 S cm−1 for [AuSe4(O)2], and
40 S cm−1 for [AuSe4(S)2].
Upon cooling, a semiconducting behavior is observed for the
three complexes with activation energies (determined between
200 and 300 K) of 0.18, 0.06, and 0.05 eV for [AuS4(O)2],
[AuSe4(O)2], and [AuSe4(S)2], respectively. It follows
that the replacement of the outer sulfur atom by oxygen
actually decreases σRT (1 bar) by 2 orders of magnitude, while
the replacement of the coordinating sulfur atoms with selenium
atoms increases σRT (1 bar) by 2 orders of magnitude. The two
combined substitutions cancel each other in [AuSe4(O)2]
compared to the original [AuS4(S)2], but the activation
energy is smaller in the diselenolene compound. These trends
are also confirmed by the temperature dependence of the
paramagnetic susceptibility, determined for the three com-
pounds in the 2−300 K temperature range (Figure 6). Two
Figure 7. Evolution of the unit cell parameters within the series. The
negative pressure effects are highlighted in red.
in the (a, b) conducting planes, with important consequences
on the intermolecular interactions.
Indeed, as illustrated below in Figure 8 for the two selenolate
complexes [AuSe4(S)2] and [AuSe4(O)2], the shortest
Figure 6. Temperature dependence of the paramagnetic susceptibility
of [AuS4(O)2], [AuSe4(O)2], and [AuSe4(S)2]. The red line is
a fit to the Bonner−Fischer model (see text) for the less conducting
[AuS4(O)2] compound.
Figure 8. View of the short Se···Se and S···Se contacts between
neighboring [AuSe4(O)2] (left) and [AuSe4(S)2] (right) (a)
within the stacks and (b) between two neighboring stacks.
different behaviors can be observed. Indeed, for [AuS4(O)2],
the magnetic susceptibility is weakly temperature dependent
(10−3 cm3 mol−1) and well fitted with a Bonner−Fisher model
for a uniform spin chain with J/k = −211(4) K (J = −144
cm−1), together with a Curie tail amounting to 1.4% magnetic
defaults, indicating a stronger spin localization associated with a
less conducting behavior. Contrariwise, the paramagnetic
susceptibility for the more conducting [AuSe4(O)2] and
[AuSe4(S)2] diselenolene compounds, corrected for the
Pascal diamagnetic term, is almost temperature independent.
The smaller value is observed with the most conducting
[AuSe4(S)2] compound. A Curie tail at the lowest
intrastack (Figure 8a) and interstack (Figure 8b) intermolecular
S···S, Se···Se, or S···Se contacts are actually longer when we
replace the outer sulfur atom in [AuSe4(S)2] by an oxygen
atom in [AuSe4(O)2]. Similar trends are found when going
from [AuS4(S)2] to [AuS4(O)2].
If we now concentrate on the influence of the nature of the
chalcogen atom within the metallacycles, for example by
comparing the unit cell dimensions of [AuS4(S)2] and
[AuSe4(S)2], or [AuS4(O)2] and [AuSe4(O)2], we can
observe that the effect of the S/Se substitution is also highly
anisotropic (Figure 7), as it mainly affects the a axis associated
with lateral interactions between stacks within the conducting
slabs. Besides, the increase along the b stacking axis is
considerably smaller than what would be expected from the
difference of S and Se van der Waals radii (evolution of the van
der Waals radii when going from one S atom to one Se is
+5.26%). Therefore, stronger interactions are observed within
the diselenolene series along both the a and b axes. These
interactions are even more pronounced with the diselenolene
bearing outer sulfur atoms, [AuSe4(S)2].
1
temperatures corresponds to only 0.47 and 1.2% of S = /2
species attributable to paramagnetic defaults in [AuSe4(O)2]
and [AuSe4(S)2], respectively.
Anisotropic Chemical Pressure Effect. While the
increase of conductivity and decrease of activation energy
upon selenium substitution could be anticipated, the opposite
behavior upon oxygen introduction appears very surprising, also
considering that the unit cell volume does indeed decrease
notably when replacing the exocyclic sulfur atoms by oxygen
atoms. However, as shown in Figure 7, this unit cell contraction
is far from isotropic. Indeed, the unit cell volume contraction is
actually accompanied by a notable expansion along the a and b
axes of the unit cell (in red in Figure 7), demonstrating that the
anticipated positive chemical pressure effect is actually reversed
These combined results corroborate the information
obtained from transport and magnetic measurements. The
presence of the smaller oxygen atoms in [AuS4(O)2], when
compared with the original all sulfur [AuS4(S)2] complex,
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Article
induces a compression along the c axis, making the slabs closer
to each other but creating a concomitant dilatation along the a
and b axes, within the conducting slabs. As a consequence, the
conductivity of [AuS4(O)2] is 100 times lower than that of
the all sulfur analog and the activation energy is higher.
Concerning the diselonolate complex [AuSe4(S)2], a 100
times increase of the conductivity is observed as it reaches 40 S
cm−1 at room temperature. Upon cooling, a semiconducting
behavior is observed with much smaller activation energy than
for the dithiolene analog. In the [AuSe4(O)2] complex,
replacement of the sulfur atoms by selenium ones counter-
balances the effect of the exocyclic oxygen atoms. Actually, the
conductivity measured for [AuSe4(O)2] is approximately the
same as the one determined for [AuS4(S)2] (Figure 5).
Physical Pressure Effects, Compressibility, and Mode-
lization. The evolution of the conductivity of [AuS4(S)2]
under pressure has been already reported.9 The all-sulfur
complex becomes metallic above 13 kbar, and the metal−
insulator transition that appeared below 70 K is totally
suppressed above 17 kbar. It is therefore interesting at this
stage to compare this behavior with that of the most
conducting member of the series, that is, [AuSe4(S)2].
As mentioned above, [AuSe4(S)2] shows an 100-fold
increase in conductivity at ambient temperature and pressure.
Under pressure, its room temperature conductivity increases
strongly, from 40 to 2600 S cm−1 at 12.5 kbar, and exhibits two
different regimes (Figure 9). An exponential pressure depend-
different applied pressures on different compounds is not quite
reliable because of the volume changes during cooling. Finally,
the change of regime for the conductivity observed above 6
kbar is confirmed by the metallic temperature dependencies of
the resistivity for [AuSe4(S)2] at 7 and 12 kbar (inset of
Figure 9).
The 9 kbar chemical pressure value associated with the S4/
Se4 substitution can be compared with similar pressure effects
reported in an isostructural series of organic conductors such as
the TMTTF−TMTSF salts. In the generic phase diagram of
́
these salts, Jerome reported indeed a 26 kbar shift between the
sulfur salt, (TMTTF)2PF6, and the selenium salt,
(TMTSF)2PF6.33 The much smaller 9 kbar chemical pressure
effect observed here tends to indicate a higher sensitivity of the
neutral single component structures investigated here, by
contrast with ionic multicomponent radical salts.
In our previous paper on the all sulfur complex [AuS4(
S)2], first principles band structure calculations have shown the
presence of a small indirect gap, while simulations for slightly
compressed cells along the a (transverse) and b (stacking)
directions were able to show that the gap closes and the
metallic state is stabilized. Analogous calculations were
performed here for the whole series. Using the experimental
crystal structures, we have found that the delocalized (metallic)
and localized (antiferromagnetic, AFM) states differ by a small
energy difference and compete for the ground state in all these
solids. The localized AFM state is the ground state for
[AuS4(S)2], [AuS4(O)2], and[AuSe4(O)2], whereas the
metallic state is favored for [AuSe4(S)2]. The AFM state is
made of stacks with molecules with an unpaired spin up and
down alternating along b. The calculated band structures for
the ground state of the four systems are reported in Figure 10
(in those for the AFM systems every band is the superposition
of two bands, one associated with spin up and localized in one
of every two molecules along the stacks and the other with spin
down is localized in the other set of molecules along the stack,
whereas in that of the metallic systems both bands are equally
delocalized). As noted for other gold bis(dithiolene) com-
plexes,7 the singly occupied molecular orbital (SOMO) and the
orbital immediately below (SOMO−1) differ by an energy that
is on the same order of magnitude as the intermolecular
transfer integrals. This leads to the overlap of the SOMO and
SOMO−1 bands with the consequence that, in contrast with
most molecular conductors, two bands per molecule must be
explicitly considered. Although the crystallographic unit cell
contains only two molecules, we had to double it in order to be
able to describe the antiferromagnetic state. Consequently, the
band structures of Figure 10 contain eight bands, four
originating from the SOMO and four from the SOMO−1.
Since every molecule contributes with three electrons to the
bands, six pairs of bands should be filled. Note the small
indirect gap separating the sixth and seventh pairs of bands in
the band structures of the [AuS4(S)2] and [AuSe4(O)2]
(Figure 10, parts b and c, respectively) and the noticeably larger
gap in that of [AuS4(O)2] (Figure 10d). Two pairs of bands
overlap at the Fermi level in the band structure of [AuSe4(
S)2] (Figure 10a) which thus corresponds to a metallic state.
Thus, taking into account the usual overestimation of the
stability of the metallic state in DFT, the band structures of
Figure 10 suggest that [AuS4(S)2] and [AuSe4(O)2]
should have similar conductivities, whereas [AuS4(O)2] and
[AuSe4(S)2] should exhibit lower and higher conductivities,
respectively, in nice agreement with the above experimental
Figure 9. Pressure dependence of the room temperature conductivity
of [AuSe4(S)2]. The black lines are guides to the eye. Inset:
temperature dependence of the resistivity of [AuSe4(S)2] at ambient
pressure (green), 7 kbar (purple), and 12 kbar (red) and for
comparison [AuS4(S)2] at 9 kbar (black).
ence is first observed up to 4 kbar followed by a linear pressure
dependence above 6 kbar. These two different regimes can be
associated with a semiconducting behavior below 4 kbar (with a
linear variation of the activation energy with pressure) and a
metallic behavior above 6 kbar. The same feature was already
observed for [AuS4(S)2] but with a regime change at higher
pressure (around 13 kbar),9 i.e., shifted by 9 kbar.
This 9 kbar shift also appears when comparing the
temperature dependence of the resistivity for [AuSe4(S)2]
at ambient pressure (insert of Figure 9) with the data obtained
for [AuS4(S)2] under different pressures.9 We note a
similarity between the behavior of [AuSe4(S)2] at ambient
pressure and [AuS4(S)2] at 9 kbar (also plotted in the inset
of Figure 9), with an even smaller activation energy. However,
the comparison between activation energies determined at
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energy), the magnetic susceptibility shows a Pauli-type
paramagnetic behavior, typical of metallic systems.
The nature of the different bands was previously discussed in
detail for [AuS4(S)2].9 In the context of the present work, it
is important to emphasize that, as noted in Figure 10b, the top
two bands at Γ originate from the SOMO and the third band
from the top originates from the SOMO−1. The width of the
SOMO and SOMO−1 bands strongly depend on the strength
of the interactions along the stack direction (i.e., the b-
direction). The SOMO and SOMO−1 are very directional π-
type orbitals making strong interactions along the stacks. Thus,
since replacement of Se for S in the central backbone of the
molecules leads to a reinforcement of these interactions, the
indirect gap decreases (and eventually disappears), leading to
an increase in the conductivity.
To analyze the effect of the replacement of O for S in the
outer part of the molecule, it is interesting to run simulations
for slightly expanded cells along the a- and b-directions. As it is
clear from Figure 10, the conductivity of these materials
increases with the width of the upper SOMO and SOMO−1
bands, which leads to the decrease of the indirect band gap and
eventually to the semimetallic overlap. In the simulations that
we have carried out, the molecules were moved rigidly along
the chosen directions. The calculated band structures when the
molecules move away by 0.2 Å are shown in Figure 11. Note
that the band structure for [AuSe4(S)2] with expansion along
both the a- and b-directions (Figure 11c) is almost identical to
that of [AuS4(S)2] (Figure 10b). It turns out that the band
structure is especially sensitive to the expansion along the
stacking b-direction. Since as examined above the volume
decrease resulting from the O for S replacement is associated
with an increase in the a and b cell constants (Figure 7), the
simulations of Figure 11 provide a simple rationale for the
observed conductivity decrease.
CONCLUSIONS
■
Chemical pressure effects by atom substitution are known to
play an important role in molecular conductors. We have
shown here within an original series of single-component
conductors derived from [Au(Et-thiazdt)2] that such effects are
actually strongly anisotropic. The replacement of sulfur by
oxygen atoms when going from [AuS4(S)2] to [AuS4(
O)2] induces indeed a reduced cell volume but a concomitant
dilatation of the conducting slabs with an associated 100-fold
reduced conductivity. Similar trends are found in the
diselenolene series. Such effects should be carefully considered
in the future when discussing positive or negative chemical
pressure effects. Of particular note is also the high sensitivity of
such neutral single-component conductorsby comparison
with multicomponent saltsto external pressure, a sensitivity
attributable to their neutral, more compressible character.
Analysis of unit cell evolutions and ab initio band structure
calculations demonstrate the strongly anisotropic nature of this
chemical pressure effect and provide an explanation for the
observed changes in conductivity.
Figure 10. Band structure for the metallic ground state of [AuSe4(
S)2] (a) and the antiferromagnetic ground state of [AuS4(S)2] (b),
[AuSe4(O)2] (c), and [AuS4(O)2] (d). The dotted line refers to
the Fermi level in part a and the highest occupied level of the system
in parts b−d. Γ, X, Y, Z, and M refer to the (0, 0, 0), (1./2, 0, 0), (0, 1/2,
0), (0, 0, 1/2), and (1/2, 1/2, 0) points of the monoclinic Brillouin zone.
results. Except for [AuS4(O)2], there is a very small indirect
band gap around the Fermi level (and this must also apply for
[AuSe4(S)2] because of the above-mentioned overstabiliza-
tion of the metallic state in DFT), and consequently, the
conduction should be activated. However, because of the very
weak gap the materials may be considered in practice as
semimetals, thus explaining the fact that whereas the
conductivity is activated (although with a small activation
EXPERIMENTAL SECTION
■
General. All commercial reagents were used as purchased. The
thiazoline-2-thione 1 was synthesized according to a literature
procedure.9 All air-sensitive reactions were carried out under an
argon atmosphere. Melting points were measured on a Kofler hot-
stage apparatus and are uncorrected. H NMR and 13C NMR spectra
1
were recorded on a Bruker AV300III spectrometer. Chemical shifts are
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Hz). 13C NMR (DMSO-d6, 75 MHz): δ 13.5, 18.6, 48.1, 122.5, 137.1,
174.5. HRMS: calcd for C6H10NS2 [C+] 160.025 47, found 160.0254.
Anal. Calcd for C6H10NIS2: C, 25.09; H, 3.51; N, 4.88; S, 22.33.
Found: C, 25.25; H, 3.77; N, 4.90; S, 22.42. Sodium methanolate
solution (1 M in MeOH, 9.4 mL, 9.4 mmol) was added under an inert
atmosphere to a solution of the thiazolium salt (1.8 g, 6.3 mmol) in
dry methanol (20 mL). The reaction was stirred at room temperature
for 5 h and then extracted with dichloromethane and washed with
saturated NaCl solution. The concentrated solution was purified by
chromatography on silica gel using CH2Cl2/Et2O (9/1) as eluant to
afford thiazoline 2 as a colorless oil in 84% yield (0.68 g). Rf = 0.57
1
(SiO2, CH2Cl2/Et2O, 9/1). H NMR (CDCl3, 300 MHz): δ 1.31 (t,
3
3
3H, CH3, J = 7.3 Hz), 3.76 (q, 2H, CH2, J = 7.3 Hz), 6.12 (d, 1H,
HC, J = 5.4 Hz), 6.56 (d, 1H, HC, J = 5.4 Hz). 13C NMR
(CDCl3, 75 MHz): δ = 14.5, 40.2, 101.1, 124.2, 171.7. HRMS: calcd
for C5H7NONaS [M + Na]+ 152.014 61, found 152.0145.
3
3
General Procedure for the Synthesis of the Cyanoethyl
Protected Ligands 3 and 4. To a −10 °C cooled solution of
thiazoline-2-thione 2 (3.9 mmol, 0.5 g) in dry THF (40 mL) was
added a solution of LDA, freshly prepared from n-BuLi (5.8 mmol, 3.6
mL, 1.6 M in hexane) and diisopropylamine (5.8 mmol, 0.82 mL) in
dry THF (10 mL). After stirring for 30 min at −10 °C, sulfur (5.8
mmol, 0.19 g) or selenium (5.8 mmol, 0.46 g) was added and the
solution was stirred for an additional 30 min. To the medium, a
solution of LDA (7.8 mmol, 4.9 mL of n-BuLi in hexane added to 7.8
mmol, 1.1 mL of diisopropylamine in 10 mL of dry THF) was added.
The reaction mixture was stirred at −10 °C for 3 h, and sulfur (7.8
mmol, 0.25 g) or selenium (7.8 mmol, 0.62 g) was added. After 30
min, 3-bromopropionitrile (1.66 mL, 20 mmol) was added and the
reaction mixture was stirred overnight. The solvent was evaporated in
vacuo, and the residue was extracted with CH2Cl2. The concentrated
solution was purified by chromatography on silica gel using CH2Cl2/
Et2O (95/5) as eluant to afford the protected ligand.
3,3′-[(3-Ethyl-2-oxo-2,3-dihydro-1,3-thiazole-4,5-diyl)]bis-
(thio)]dipropanenitrile 3. Light brown solid in 48% yield (0.56 g).
Rf = 0.28 (SiO2, CH2Cl2/Et2O, 95/5). Mp: 66−67 °C. 1H NMR
3
(CDCl3, 300 MHz): δ 1.30 (t, 3H, CH3, J = 7.1 Hz), 2.71 (t, 2H,
CH2, 3J = 7.0 Hz), 2.75 (t, 2H, CH2, 3J = 7.0 Hz), 3.06 (t, 2H, CH2, 3J
3
3
= 7.0 Hz), 3.13 (t, 2H, CH2, J = 7.0 Hz), 4.01 (q, 2H, CH2, J = 7.1
Hz). 13C NMR (CDCl3, 75 MHz): δ 18.5, 18.6, 31.9, 32.3, 40.9, 116.1,
117.5, 117.7, 131.7, 169.7. HRMS: calcd for C11H14N3OS3 [M + H]+
300.0299, found 300.0308. Anal. Calcd for C11H13N3OS3: C, 44.12; H,
4.38; N, 14.03; S, 32.12. Found: C, 44.04; H, 4.42; N, 13.71; S, 32.67.
3,3′-[(3-Ethyl-2-oxo-2,3-dihydro-1,3-thiazole-4,5-diyl)bis-
(seleno)]dipropanenitrile 4. Yellow powder in 43% yield (0.66 g).
Rf = 0.30 (SiO2, CH2Cl2/Et2O, 95/5). Mp: 111 °C. 1H NMR
Figure 11. Band structure calculated for [AuSe4(S)2] when the
molecules are rigidly separated by 0.2 Å along (a) the stacking b-
direction, (b) the interstack a-direction, and (c) along both the
stacking b- and interstack a-directions. The dotted line refers to the
Fermi level in parts a and b, and the highest occupied level of the
system in part c.
3
(DMSO-d6, 300 MHz): δ 1.15 (t, 3H, CH3, J = 7.1 Hz), 2.90−2.96
3
(m, 4H, CH2), 3.03−3.12 (m, 4H, CH2), 3.90 (q, 2H, CH2, J = 7.1
Hz). 13C NMR (DMSO-d6, 75 MHz): δ 14.1, 18.4, 18.5, 23.7, 24.8,
41.9, 107.1, 119.4, 119.5, 127.6, 170.9. HRMS: calcd for
C11H13N3ONaS80Se2 [M + Na]+ 417.900 19, found 417.9003. Anal.
Calcd for C11H13N3OSSe2: C, 33.59; H, 3.33; N, 10.68; S, 8.15.
Found: C, 33.29; H, 3.42; N, 10.45; S, 8.16.
quoted in parts per million (ppm) referenced to tetramethylsilane.
Mass spectra were recorded with Varian MAT 311 instrument by the
Centre Reg
́
ional de Mesures Physiques de l’Ouest, Rennes, France.
Elemental analyses were performed at the Centre Reg
́
ional de Mesures
Physiques de l’Ouest, Rennes, France. Tetrahydrofuran was distilled
from sodium−benzophenone. Column chromatography was per-
formed using silica gel Merck 60 (70−260 mesh). Cyclic voltammetry
was carried out on a 10−3 M solution of the complex in CH3CN or in
CH2Cl2, containing 0.1 M n-Bu4NPF6 as supporting electrolyte.
Voltammograms were recorded at 0.1 V s−1 on a platinum disk
electrode (A = 1 mm2). The potentials were measured versus a
saturated calomel electrode (SCE).
3,3′-[(3-Ethyl-2-thioxo-2,3-dihydro-1,3-thiazole-4,5-diyl)bis-
(seleno)]dipropanenitrile 5. A similar procedure as the one
described above using 1,3-thiazoline-2-thione 1 (3.45 mmol, 0.5 g)
as starting material gave 5 as a yellow solid in 46% yield (0.65 g). Rf =
1
0.47 (SiO2, CH2Cl2/Et2O, 19/1). Mp: 134−135 °C. H NMR (300
3
MHz, CDCl3) δ 1.35 (t, 3H, CH3, J = 7.1 Hz), 2.83−2.89 (m, 4H,
3
CH2), 3.08−3.17 (m, 4H, CH2), 4.54 (q, 2H, CH2, J = 7.1 Hz). 13C
NMR (75 MHz, CDCl3) δ 13.3, 19.3, 19.4, 23.6, 25.2, 46.5, 117.7,
117.8, 118.1, 130.8, 188.7. HRMS: calcd for C11H13N3S280Se2 [M+•]
410.8881, found 410.8886. Anal. Calcd for C11H13N3S2Se2: C, 32.28;
H, 3.20; N, 10.26; S, 15.67. Found: C, 32.23; H, 3.20; N, 10.05; S,
15.88.
[PPh4][AuS4(O)2]. To a dry two-necked flask containing
thiazoline-2-thione 2 (0.25 g, 0.84 mmol) was added under nitrogen
at room temperature 8 mL of 1 M NaOMe in MeOH (8 mmol). The
mixture was stirred for 30 min and a solution of KAuCl4 (166 mg, 0.44
mmol) in dry MeOH (12 mL) was slowly added to the medium. The
1,3-Thiazoline-2-one 2. To a solution of 1,3-thiazoline-2-thione 1
(4 g, 27.6 mmol) in dry acetone (30 mL) was slowly added
iodomethane (2.1 mL, 33.1 mmol). The reaction was stirred at room
temperature for 2 h under an inert atmosphere. The medium was
cooled to 0 °C and the precipitate was filtrated and washed with a
minimum amount of cold acetone. The 2-thiomethyl-1,3-thiazolium
salt was dried under vacuum to afford a white powder in 95% yield
(7.5 g). Mp: 146−147 °C. 1H NMR (DMSO-d6, 300 MHz): δ 1.43 (t,
3
3H, CH3, J = 7.3 Hz), 3.01 (s, 3H, CH3), 4.35 (q, 2H, CH2, 3J = 7.3
3
3
Hz), 8.14 (d, 1H, HC, J = 4.0 Hz), 8.45 (d, 1H, HC, J = 4.0
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Table 3. Crystallographic Data of Monoanionic Species
compound
formula
FW (g·mol−1
(PPh4)[AuS4(O)2]
C34H30AuN2O2PS6
918.90
(NEt4)[AuSe4(O)2]
C18H30AuN3O2S2Se4
897.38
(NEt4)[AuSe4(S)2]
C18H30AuN3S4Se4
929.54
)
crystal system
space group
a (Å)
monoclinic
C2/c
monoclinic
C2/c
monoclinic
P21/c
18.3110(6)
8.0874(3)
25.3857(8)
90
17.9569(15)
10.7960(9)
14.2220(12)
90
9.5164(7)
12.9011(9)
10.9576(7)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
T (K)
104.610(1)
90
102.209(4)
90
96.033(3)
90
3637.8(2)
150(2)
2694.8(4)
150(2)
1337.81(16)
150(2)
Z
4
4
2
Dcalc (g cm−3
)
1.678
2.212
2.307
μ (mm−1
)
4.466
11.037
11.265
total refls
29344
3721
3048
abs corr
multiscan
4170 (0.0377)
3861
multiscan
3721
multiscan
3048
uniq refls (Rint)
uniq refls (I > 2σ(I))
R1, wR2
3226
2757
0.0161, 0.0532
0.0219, 0.0782
1.418
0.0426, 0.1246
0.0506, 0.13
1.137
0.0298, 0.0826
0.0356, 0.0852
1.178
R1, wR2 (all data)
GOF
Table 4. Crystallographic Data of Neutral Species
compound
formula
FW (g·mol−1
[AuS4(O)2]
C10H10AuN2O2S6
579.53
[AuSe4(O)2]
C10H10AuN2O2S2Se4
767.13
[AuSe4(S)2]
C10H10AuN2S4Se4
799.25
)
crystal system
space group
a (Å)
monoclinic
P21/a
monoclinic
P21/a
monoclinic
P21/a
14.135(3)
4.1370(11)
13.460(3)
90
14.7952(3)
4.1628(1)
13.4493(3)
90
14.5116(5)
4.0984(3)
14.3554(5)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
T (K)
97.14(3)
90
95.717(8)
90
92.613(4)
90
781.0(3)
150(2)
824.21(3)
150(2)
852.89(8)
100(2)
Z
2
2
2
Dcalc (g cm−3
)
2.464
3.091
3.112
μ (mm−1
)
10.222
18.012
17.641
total refls
2024
1857
16837
abs corr
multiscan
2024
multiscan
1857
multiscan
1968 (0.0493)
1774
uniq refls (Rint)
uniq refls (I > 2σ(I))
R1, wR2
1439
1374
0.0401, 0.0986
0.0648, 0.121
0.961
0.0548, 0.1283
0.0929, 0.1554
1.074
0.0206, 0.0393
0.0272, 0.0410
1.109
R1, wR2 (all data)
GOF
reaction mixture was stirred for 4 h at room temperature, and a
solution of PPh4Cl (0.21 g, 0.55 mmol) was added. After an additional
stirring overnight, the precipitate was filtered, washed with MeOH,
and recrystallized in CH3CN to afford the title complex in 30% yield
(115 mg) as black crystals. Mp: 206 °C (dec). 1H NMR (CD3CN, 300
MHz): δ 1.19 (t, 6H, CH3, J = 7.2 Hz), 3.68 (q, 4H, CH2, J = 7.2
Hz), 7.64−7.78 (m, 16H, HAr), 7.89−7.95 (m, 4H, HAr). HRMS: calcd
for C58H50N2O2P2S6Au [2C+,A−]+ 1257.133 75, found 1257.1338.
Anal. Calcd for C34H30N2O2PS6Au: C, 44.44; H, 3.29; N, 3.05; S,
20.94. Found: C, 44.33; H, 3.24; N, 3.13; S, 21.02.
was stirred for 30 min and a solution of KAuCl4 (132 mg, 0.35 mmol)
in 10 mL of dry MeOH was slowly added to the medium. The reaction
mixture was stirred for 4 h at room temperature, and a solution of
PPh4Cl (0.19 g, 0.5 mmol) was added. After additional stirring
overnight, the precipitate was filtered, washed with MeOH, and
recrystallized in CH3CN to afford the title complex in 27% yield (95
3
3
1
mg) as black crystals. Mp: 213 °C (dec). H NMR (CD3CN, 300
3
3
MHz): δ 1.20 (t, 6H, CH3, J = 7.1 Hz), 3.72 (q, 4H, CH2, J = 7.1
Hz), 7.64−7.78 (m, 16H, HAr), 7.89−7.95 (m, 4H, HAr). HRMS: calcd
for C58H50N2O2P2S280Se4Au [2C+,A−]+ 1448.911 54, found 1448.9139.
Anal. Calcd for C34H30N2O2PS2Se4Au: C, 36.90; H, 2.73; N, 2.53; S,
5.79. Found: C, 36.68; H, 2.66; N, 2.66; S, 5.76.
[PPh4][AuSe4(O)2]. To a two necked flask containing thiazoline-
2-thione 2 (0.25 g, 0.64 mmol) was added under nitrogen at room
temperature 6 mL of 1 M NaOMe in MeOH (6 mmol). The mixture
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Article
[NEt4][AuSe4(S)2]. To a two-necked flask containing 1,3-
thiazoline-2-thione 2 (0.25 g, 0.61 mmol) was added 6 mL of 1 M
NaOMe in MeOH (6 mmol) under nitrogen at room temperature.
The mixture was slowly stirred for 30 min and a solution of KAuCl4
(125 mg, 0.33 mmol) in 10 mL of dry MeOH was slowly added to the
medium. The reaction mixture was stirred for 4 h at room
temperature, and a solution of NEt4Br (105 mg, 0.5 mmol) was
added. After additional stirring overnight, the precipitate was filtered,
washed with MeOH, and recrystallized in CH3CN to afford the title
complex in 50% yield (0.14 g) as black crystals. Mp: 238 °C (dec). 1H
NMR (CD3CN, 200 MHz): δ 1.21 (m, 12H, CH3); 1.27 (t, 6H, CH3,
room temperature values, and the loss of pressure occurring during
cooling is estimated to 2 kbar. Low temperatures have been provided
by cryocooler equipment down to 25 K except for experiments under
pressure on [AuSe4(S)2], which were performed in a variable-
temperature insert in an helium cryostat down to 1.5 K.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic files in CIF format and cyclic voltamo-
gram of [AuSe4(S)2]. This material is available free of charge
via the Internet at http://pubs.acs.org.
3
3
3J = 7.1 Hz); 3.16 (q, 8H, CH2, J = 7.1 Hz); 4.18 (q, 4H, CH2, J =
7.1 Hz). Anal. Calcd for C18H30N3S4Se4Au: C, 23.26; H, 3.25; N, 4.52;
S, 13.80. Found: C, 23.30; H, 3.25; N, 4.59; S, 14.15.
AUTHOR INFORMATION
Corresponding Author
dominique.lorcy@univ-rennes1.fr; marc.fourmigue@univ-
rennes1.fr; canadell@icmab.es
■
Electrocrystallizations. Crystals of [AuS4(O)2], [AuSe4(
O)2], or [AuSe4(S)2] were prepared electrochemically using a
standard H-shaped cell (12 mL) with Pt electrodes. An acetonitrile
solution of [NEt4][AuS4(O)2] (10 mg), [PPh4][AuSe4(O)2] (10
mg), or [PPh4][AuSe4(S)2] (10 mg) was placed in the anodic
compartment, and nBu4NPF6 (100 mg) was placed in both
compartments. Black needle crystals of [AuS4(O)2], [AuSe4(
O)2], or [AuSe4(S)2] suitable for X-ray diffraction studies were
obtained on the anode upon application of a constant current of 0.4
μA for 10 days.
Crystallography. Crystals were picked up with a cryoloop and
then frozen at 150 or 100 K under a stream of dry N2. Data were
collected on an APEX II Bruker AXS diffractometer with graphite-
monochromated Mo KR radiation (λ = 0.710 73 Å). Structures were
solved by direct methods (SIR97)34 and refined (SHELXL-97)35 by
full-matrix least-squares methods, as implemented in the WinGX
software package.36 Absorption corrections were applied. Hydrogen
atoms were introduced at calculated positions (riding model, included
in structure factor calculations but not refined). Twin refinement was
used for (NEt4)[AuSe4(O)2], (NEt4)[AuSe4(S)2], [AuS4(
O)2], and [AuSe4(O)2]. The two twin domains were determined
with the Cell_now program (Bruker AXS), a twin data reduction was
done with the SAINT program (Bruker AXS), and a multiscan
absorption correction was applied using the Twinabs program (Bruker
AXS), generating a HKLF 5 data set constructed from all observations.
The elevated GOF value of 1.418 for (PPh4)[AuS4(O)2] is the
consequence of a refinement on an uncut hkl data set. The Gof value
(1.13) obtained after refinement of a data set cut between 5.5 and 0.77
Å (that remove not well integrated low θ reflections) is closer to the
ideal value. Details of the final refinements are given in Table 3 for
anionic gold complexes and Table 4 for neutral compounds.
Computational Details. We have used the spin-polarized
generalized gradient approximation37 (σ-GGA) approach to density
functional theory38 (DFT) as implemented in the Vienna Ab Initio
Simulation Package (VASP).39 We used the projector augmented
wave40 method to represent the ionic cores, solving explicitly for the
following electrons: 5d and 6s of Au; 3s and 3p of S; 4s and 4p of Se;
2s and 2p of O, N, and C; and 1s of H. Electronic wave functions were
represented with a plane wave basis truncated at 400 eV. A careful
convergence test for Brillouin zone integrations was necessary.
Accurate energy differences between various (e.g., antiferromagnetic
vs metallic) electronic solutions were obtained using Γ-centered 4 × 8
× 4 k-point grids for the simulation cell with four molecules in it,
which results from the crystalline cell (two molecules) and an
antiferromagnetic ordering along b.
Resistivity Measurements. The resistivity has been measured
along the long axis of the needles (b crystallographic axis). Gold pads
were evaporated on the surface of the crystals in order to improve the
quality of the contacts. Then a standard four-points technique was
used with a low frequency lock-in detection (Iac = 0.1−1 μA) for
measured resistances below 50 kΩ and dc measurement for higher
resistances (Idc = 0.1−0.01 μA). Hydrostatic pressures were applied at
room temperature in a NiCrAl clamp cell using Daphne 7373 silicone
oil as the pressure transmitting medium. The pressure was determined,
at room temperature, using a manganin resistance gauge located in the
pressure cell close to the sample. The pressures indicated here are
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from University Rennes 1 for a Ph.D. grant
(to G.Y.) is gratefully acknowledged. We thank the CDIFX
(Rennes, France) for access to X-ray data collection facilities.
This work was also supported by MINECO-Spain (Projects
FIS2009-12721-C04-03, MAT2010-18113, MAT2010-10093
and CSD2007-00041).
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