Synthesis and physical properties of a new single-component molecular conductor [Au(dhdt)2]

Article abstract of DOI:10.1016/j.poly.2008.10.012

We have succeeded in synthesizing a new Au(III) complex coordinated by a dihydro-TTF-ditiolato ligand, dhdt^2- (4,5-dihydrotetrathiafulvalene-4,5-dithiolato) as a tetrabutlylammoium or tetraphenylphosphonium salt. Crystal structural analysis of (Ph₄P)[Au(dhdt)₂] revealed that the Au(III) ion adopts a square planar coordination and the mean Au-S bond length suggests the Au ions are in the trivalent Au(III) state. A neutral complex, [Au(dhdt)₂], was obtained by the electrochemical oxidation of (n-Bu₄N) [Au(dhdt)₂]. Conducting behavior of [Au(dhdt)₂] was semiconductive with conductivity at room temperature of 8.3 Scm^-1 and activation energy of 23 meV. Magnetic susceptibility of [Au(dhdt)₂] was well explained by the sum of the Curie and temperature independent components.

Full text of DOI:10.1016/j.poly.2008.10.012

Polyhedron 28 (2009) 1634–1637
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and physical properties of a new single-component molecular
conductor [Au(dhdt)2]
*
Hiroyuki Nishikawa , Wataru Yasuoka, Kumiko Sakairi, Hiroki Oshio
Department of Chemistry, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 25 November 2008
We have succeeded in synthesizing a new Au(III) complex coordinated by a dihydro-TTF-ditiolato ligand,
dhdt2À (4,5-dihydrotetrathiafulvalene-4,5-dithiolato) as a tetrabutlylammoium or tetraphenylphospho-
nium salt. Crystal structural analysis of (Ph4P)[Au(dhdt)2] revealed that the Au(III) ion adopts a square
planar coordination and the mean Au–S bond length suggests the Au ions are in the trivalent Au(III) state.
A neutral complex, [Au(dhdt)2], was obtained by the electrochemical oxidation of (n-Bu4N) [Au(dhdt)2].
Conducting behavior of [Au(dhdt)2] was semiconductive with conductivity at room temperature of
8.3 ScmÀ1 and activation energy of 23 meV. Magnetic susceptibility of [Au(dhdt)2] was well explained
by the sum of the Curie and temperature independent components.
Keywords:
Molecular conductor
Single-component
Au complex
Dihydro-TTF
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
already reported the Ni complex with a TTF-type ligand which has
the reduced p-electron system compared with those of TTF-dithio-
Most of electrical conducting molecular materials have gener-
ally been composed of charge transfer complexes, or radical salts
based on organic donors or acceptors; more than two constituent
molecules were required for the molecular conductors. Although
considerable efforts have been devoted to preparation of single-
component molecular conductors, the conductivity was not more
than the semiconductor [1]. In 2001, however, Kobayashi and co-
workers succeeded in developing the first single-component
molecular metal, [Ni(tmdt)2] (tmdt2À = trimethylenetetrathiaful-
valenedithiolato), which retained the metallic state down to 0.6 K
[2]. The metallic state of this Ni complex originates in a small
HOMO–LUMO energy gap leading to crossing the valence and con-
duction bands and then the formation of semi-metallic Fermi sur-
face [3]. From this point of view, metal complexes with extended
lato ligands, [Ni(dhdt)2], and it showed semiconducting behavior
[5]. We disclose here the synthesis and physical properties of a
new Au complex with p-reduced TTF-ditholato type ligands.
2. Experimental
2.1. Materials
All solvents and chemicals were reagent-grade, purchased com-
mercially, and used without further purification unless otherwise
noted. Tetrahydrofuran (THF) was distilled from sodium benzo-
phenone ketyl under a nitrogen atmosphere prior to use, and
methanol, diethyl ether and acetonitrile were distilled from CaH2.
Tetrabutylammonium hexafluorophosphate, which was used for
electrochemical measurements, was recrystallized from ethanol
(EtOH) before use.
p-electron system are one of the promising molecules for the sin-
gle-component metals.
On the other hand, we are currently investigating to develop or-
2.2. Syntheses of (n-Bu4N)[AuIII(dhdt)2] and (Ph4P)[AuIII(dhdt)2]
ganic
p-donors, which can destabilize the metallic states and en-
hance the electron correlation in the molecular conductors in
Synthesis of (n-Bu4N)[AuIII(dhdt)2] was performed according to
the method for the Ni complex reported previously as shown in
Scheme 1. To a THF solution of the cyanoethyl-protected dihydro-
tetrathiafulvalenedithiolate 1 (30 mg, 0.08 mmol) was added a
methanol solution of tetrabutylammonium hydroxide at À78 °C
and the mixture was allowed to warmed to room temperature.
To the mixture was added dropwise a methanol solution of hydro-
gen tetrachloroaurate(III) tetrahydrate (16 mg, 0.039 mmol) at
À78 °C. After the reaction mixture was allowed to warm to room
temperature, the resulting precipitate was filtered off. Addition of
order to realize superconducting transition. We prepared organic
donors with the reduced
p-electron systems compared with the
conventional TTF donors, leading to the enhancement of the on-site
Coulomb repulsion, and succeeded in developing several pressure-
induced superconductors from such a
p-reduced donor [4]. Reduc-
tion of the -electron system of the single-component conducting
p
metal complexes with extended TTF ligands is interesting. We have
* Corresponding author. Tel./fax: +81 29 853 4426.
E-mail address: nishikaw@chem.tsukuba.ac.jp (H. Nishikawa).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.10.012

H. Nishikawa et al. / Polyhedron 28 (2009) 1634–1637
1635
Scheme 1.
diethyl ether to the filtrate afforded (n-Bu4N)[AuIII(dhdt)2] as
n-Bu4NPF6 as a supporting electrolyte at a scan rate of 50 mV sÀ1
,
brown powder in 54% yield.
using glassy-carbon working and platinum counter electrodes
and a saturated calomel electrode (SCE) as the reference electrode.
The redox potentials of Au complex were summarized in Table 1
together with those of Ni complex, (Ph4P)[NiIII(dhdt)2]. While the
irreversible step corresponding to the oxidation from monovalent
[Au(dhdt)2]À to neutral Au complex was observed at À0.06 V, the
reduction step from [Au(dhdt)2]À to divalent [Au(dhdt)2]2À at
À0.85 V. Compared with the corresponding potentials of the Ni
complex, oxidation to the neutral complexes occurs at almost same
potentials, but the reduction potential to the divalent species of the
Au complex is more negative than that of the Ni complex.
Tetraphenylphosphonium salt, (Ph4P)[AuIII(dhdt)2],
was
obtained by slow diffusion of THF–methanol solution of tetraphen-
ylphosphonium bromide to THF–methanol solution of (n-Bu4-
N)[AuIII(dhdt)2] to give black needle crystals suitable for X-ray
structural analysis.
2.3. Preparation of neutral complex [Au(dhdt)2]
Neutral complex, [Au(dhdt)2], was prepared by electrochemical
oxidation of (n-Bu4N)[AuIII(dhdt)2] at a constant current of 0.2–
0.5 lA in benzonitrile or benzonitrile with 5–20% methanol or eth-
anol containing n-Bu4NBr as a supporting electrolyte at 25 °C. All
the solvents were purified before use. Concentration of (n-Bu4-
N)[AuIII(dhdt)2] and n-Bu4NBr were 0.3–3.0 and 25 mM, respec-
tively. For all the reaction condition, only powder samples were
obtained. Anal. Calc. for [Au(dhdt)2] (C12H8AuS12): C, 19.64; H,
1.10. Found: C, 19.77; H, 1.22%.
3.2. Crystal structure of (Ph4P)[AuIII(dhdt)2]
X-ray structural analysis for (Ph4P)[AuIII(dhdt)2] revealed that
the complex crystallized in the monoclinic space group P21/c con-
sisting of two [AuIII(dhdt)2]À anions and one tetraphenylphospho-
nium cation. The Au(III) ions are located on the center of inversion,
so that the stoichiometry of tetraphenylphosphonium cation and
[AuIII(dhdt)2]À anion is 1: 1. Fig. 1a shows the molecular structure
of the [AuIII(dhdt)2]À anion. The Au(III) ions of both the crystallo-
graphically independent [AuIII(dhdt)2]À anions adopt a square
planar coordination surrounded by four sulfur atoms from dhdt2À
ligands. The Au–S bond lengths are 2.288(3)–2.308(3) Å, which
are comparable to the mean Au–S bond length (2.296 Å) observed
in [AuIII(tmdt)2] [6], suggesting the Au ions in [Au(dhdt)2]À anions
are trivalent Au(III). The [Au(dhdt)2]À anions are arranged in a
windmill manner [7] in the bc plane and they stacked along the
a-axis as shown in Fig. 1b. The cations are located in the hollow
surrounded by four [Au(dhdt)2]À anions. There is no sulfur–sulfur
contacts shorter than the sum of van der Waals radii between
the [Au(dhdt)2]À anions.
2.4. X-ray crystallography
Diffraction data were collected at 293 K on a Bruker SMART
APEX diffractometer fitted with a CCD type area detector, and a full
sphere of data were collected using graphite-monochromated Mo
Ka radiation (k = 0.71073 Å). The data frames were integrated
using SAINT and merged to give a unique data set for structure deter-
mination. Total reflections collected were 18646 of which inde-
pendent reflections were 5686 (Rint = 0.0531). The structure was
solved by direct methods and refined by the full-matrix least-
squares method on all F2 data using the SHELEX 5.1 package (Bruker
Analytical X-ray Systems). Non-hydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms were included in
calculated positions and refined with isotropic thermal parameters
riding on those the parent atoms.
3.3. Electrical conductivity and magnetic susceptibility of [Au(dhdt)2]
Crystal data for (Ph4P)2[AuIII(dhdt)2]2: C72H56Au2P2S24, F.W. =
2146.48; black needle, monoclinic, space group P21/c, a =
8.304(1), b = 22.249(3), c = 21.571(3) Å, b = 95.738(2)°, V = 3965.5
(8) Å3, Z = 2, R = 0.0521, Rw = 0.1339.
Fig. 2 shows the temperature dependence of resistivity of the
neutral complex [Au(dhdt)2] measured on a compress pellet. The
electrical conductivity at room temperature was
which is higher than that of the corresponding Ni complex,
[Ni(dhdt)2], measured for a single crystal (
rt = 0.084 S cmÀ1) [5].
r ,
rt = 8.3 S cmÀ1
2.5. Electrical conductivity and magnetic susceptibility
r
As seen in Fig. 1, the conducting behavior was the activation-type,
Temperature dependence of resistivity was measured on a com-
pressed pellet of a powder sample of [Au(dhdt)2] by the four-probe
dc method using gold wire contacted to the pellet by carbon paste.
Magnetic susceptibility was measured for a powder sample in the
temperature range of 2–300 K using a superconducting quantum
interference device magnetometer with 20000 Oe field application.
The diamagnetic contribution of [Au(dhdt)2] was calculated to be
À305.46 Â 10À6 emu molÀ1 using Pascal’s law. Magnetization data
were collected as a function of the applied field up to 5 T at 1.8 K.
and the activation energy was calculated to be Ea = 23 meV from
the Arrhenius plot,
ture-independent constant, kB, the Boltzmann constant, and
q
(T) = Aexp(
D
E/kBT), where A is a tempera-
E,
D
the activation energy. The activation energy of the Au complex is
smaller than that of the Ni complex (Ea = 40 meV).
Since the Au(III) ion has d8 electronic configuration, the neutral
Au dithiolato complex has one unpaired electron. Thus magnetic
Table 1
Redox potentials of (Ph4P)[MIII(dhdt)2] (M = Au and Ni) in acetonitrile (V vs. SCE,
3. Results and discussion
25 °C).
3.1. Electrochemical property of Au complex
Compound
E1
E2
(Ph4P)[AuIII(dhdt)2]
(Ph4P)[NiIII(dhdt)2]
À0.06
À0.12
À0.85
À0.52
The redox potentials of (Ph4P)[AuIII(dhdt)2] were investi-
gated by cyclic voltammetry at 25 °C in acetonitrile containing

1636
H. Nishikawa et al. / Polyhedron 28 (2009) 1634–1637
Fig. 1. Crystal structure of (Ph4P)[Au(dhdt)2]. (a) Molecular structure of [Au(dhdt)2]À anion. (b) Crystal structure viewed along a-axis.
0.4
0.3
0.2
0.1
14
xm
12
calc.
x
m T
10
8
6
4
2
0
0.0
50
100
150
200
250
300
T/ K
Fig. 2. Temperature dependence of resistivity of [Au(dhdt)2].
Fig. 3. Temperature dependence of magnetic susceptibility and
vmT of [Au(dhdt)2].
The solid line is fit by the sum of the Curie–Weiss law and temperature
a
properties of the neutral Au complex should be very attractive [6].
In order to investigate magnetism of the neutral complex
[Au(dhdt)2], temperature dependence of the magnetic susceptibil-
ity was measured by using a SQUID magnetometer at 20000 Oe
from 300 to 2 K (Fig. 3). The susceptibility at 300 K was
independent component.
7.94 Â 10À4 emu molÀ1. The
v
mT value was 0.238 emu molÀ1 K at
300 K, which is smaller than the expected value of
0.375 emu molÀ1 K for S = 1/2, and decreased gradually with

H. Nishikawa et al. / Polyhedron 28 (2009) 1634–1637
1637
obtained by the electrochemical oxidation of (n-Bu4N)[Au(dhdt)2]
to give a dark brown powder sample. Conducting behavior of the
neutral [Au(dhdt)2] complex was semiconductive with room tem-
0.15
0.10
0.05
0.00
perature conductivity of
Ea = 23 meV, indicating higher conductivity than those of the neu-
tral Ni complex [Ni(dhdt)2] (
rt = 0.084 S cmÀ1, Ea = 40 meV). Mag-
r
rt = 8.3 S cmÀ1 and activation energy of
r
netic susceptibility of [Au(dhdt)2] was well explained by the sum
of the Curie component and the constant value attributable to
the conduction
p-electron. The Curie term is about 20% of the
S = 1/2 paramagnetic system. Details on the electronic and mag-
netic structure of this complex is continuously investigated as well
as preparation of a single crystal of the neutral [Au(dhdt)2] com-
plex for X-ray structural analysis and further investigation on the
physical properties.
5. Supplementary data
0
5
10
15
20
25
30
H T-1/ kG K-1
CCDC 702730 contains the supplementary crystallographic data
for (Ph4P)2[AuIII(dhdt)2]2. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Fig. 4. Magnetization (M) vs. magnetic field strength (H/T) plot for [Au(dhdt)2].
decrease of temperature, and then sudden drop was observed at
20 K. The magnetic susceptibility was well reproduced by the
sum of the Curie–Weiss law and the temperature independent
Acknowledgement
component attributable to the conduction
whole temperature range as shown by the solid line in Fig. 3. The
least-squares fit is found for the Curie constant
p-electron over the
This work was partly supported by a Grant-in-Aid for Scientific
Research (C) (20550118) from JSPS.
C = 7.98 Â 10À2 emu molÀ1 K, Weiss temperature h = À3.46 K, and
vp = 5.75 Â 10À4 emu molÀ1, respectively. The value of Curie term
is about 20% of the S = 1/2 paramagnetic system, which is fairly
large for impurity. Fig. 4 shows the magnetic field dependence of
magnetization measured at 1.8 K up to 5 T with a SQUID magne-
tometer. The M/Nb values increased with increasing magnetic filed
strength, and did not show saturation up to 5 T. Although the sat-
uration was not complete, the magnetization under the applied
field of 5 T was ca. 0.15 Nb which is much smaller than the value
for S = 1/2 and comparable to the result obtained by the magnetic
susceptibility measurement.
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4. Conclusion
A gold dithiolato complex coordinated by the dihydro-TTF
ligand of which
p-electron system is reduced compared with
those of TTF-dithiolato metal complexes was newly synthesized
and isolated as tetra-n-butylammoium salt of [Au(dhdt)]À
monoanion. Crystal structure of tetraphenylphosphonium salt,
(Ph4P)[Au(dhdt)], was revealed by X-ray crystallography. The
Au(III) ions in the [Au(dhdt)]À anions have a square planar coordi-
nation and the mean Au–S bond length suggests the Au ions are
in the trivalent Au(III) state. A neutral complex [Au(dhdt)2] was