New unsymmetrical donor dimethyl(ethylenedioxy)tetraselenafulvalene (DMEDO-TSeF): Structures and properties of its cation radical salts

Article abstract of DOI:10.1039/b511820d

A novel unsymmetrical tetraselenafulvalene (TSeF)-type donor dimethyl(ethylenedioxy)tetraselenafulvalene (DMEDO-TSeF) has been synthesized without the use of the highly toxic reagent CSe₂ and the structures and properties of its cation radical salts with octahedral anions are described. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b511820d

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New unsymmetrical donor dimethyl(ethylenedioxy)tetraselenafulvalene
(DMEDO-TSeF): Structures and properties of its cation radical salts{
Takashi Shirahata,* Megumi Kibune and Tatsuro Imakubo*
Received 19th August 2005, Accepted 13th September 2005
First published as an Advance Article on the web 26th September 2005
DOI: 10.1039/b511820d
TMTSF at a moderate yield (14%). We applied these mild
conditions to the synthesis of DMEDO-TSeF (Scheme 1) and the
following best result was obtained; to a mixture of selone 1
(215 mg, 0.71 mmol) and selone 2 (226 mg, 0.68 mmol) in benzene
(90 mL) was added HMPT (0.74 mL, 4.08 mmol), and the solution
was stirred for 2 h at room temperature. After the removal of the
solvent under reduced pressure, the crude products were separated
by column chromatography (SiO2/CS2–CH2Cl2) and then purified
by preparative gel permeation chromatography (GPC/CS2). The
target DMEDO-TSeF was isolated as purple-red needles (31 mg,
0.065 mmol, 10%) together with the self-coupling products
TMTSF (8%) and BEDO-TSeF (11%). The molecular structure
of the DMEDO-TSeF was characterized by NMR, MS, IR,
elemental analysis and X-ray structure analysis.{§ Table 1
summarizes the half-wave potentials of DMEDO-TSeF and the
related symmetrical donors measured under the same conditions.
DMEDO-TSeF showed two reversible redox waves, and the first
half-wave potential and DE value of DMEDO-TSeF lies midway
between those of TMTSF and BEDO-TSeF.
A novel unsymmetrical tetraselenafulvalene (TSeF)-type donor
dimethyl(ethylenedioxy)tetraselenafulvalene (DMEDO-TSeF)
has been synthesized without the use of the highly toxic reagent
CSe2 and the structures and properties of its cation radical salts
with octahedral anions are described.
Donor molecules containing the ethylenedioxy group show
excellent features such as high solubility towards most organic
…
solvents and the ability to form CH O hydrogen bonds.
However, fusion of the [1,4]dioxene ring has been limited to the
[1,3]dithiole and [1,3]thiaselenole units due to synthetic difficulties.1
Among them, bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF or
BO)2 and bis(ethylenedioxy)diselenadithiafulvalene (BEDO-STF)3
containing two ethylenedioxy groups on the edges of the molecular
framework have been recognized as providing stable organic
metals, and two superconductors have been discovered from BO
salts.4 Recently, we have achieved the first synthesis of bis(ethyl-
enedioxy)tetraselenafulvalene (BEDO-TSeF) without the use of
highly hazardous CSe2. It is the most promising ethylenedioxy
group-containing donor for the development of organic metals
and superconductors, and we showed that its crystal structure and
electrochemical properties are successfully inherited from the
parent molecules of bis(ethylenedithio)tetraselenafulvalene (BETS)5
and BO.6 On the other hand, several unsymmetrical donors with
one ethylenedioxy group have also been synthesized. However,
almost all their cation radical salts have possessed an unstable
metallic nature except for a few examples.7 In order to stabilize the
metallic nature, we have designed a new unsymmetrical TSeF-type
donor DMEDO-TSeF, which is a hybrid molecule between
tetramethyltetraselenafulvalene (TMTSF)8 and BEDO-TSeF. In
this communication, we report the synthesis of DMEDO-TSeF
together with the structures and physical properties of its cation
radical salts of octahedral anions (PF6, AsF6 and SbF6).
The black needle crystals of (DMEDO-TSeF)2X (X = PF6,
AsF6 and SbF6) were prepared by the galvanostatic oxidation
(0.5 mA) of DMEDO-TSeF (ca. 5 mg) in the presence of the
The coupling reaction of selone 2 should be conducted under
mild conditions, i.e. hexamethylphosphorous triamide (HMPT)–
benzene at room temperature, since the C–Se bonds are weakened
by the electron-donating ability of the dioxene ring.6 In the course
of our study to investigate the application of the above mild
conditions for the formation of the central double bond, we found
that it is applicable to the coupling reaction of various kinds of
selone. In fact, the coupling reaction of 1 with HMPT afforded
Scheme 1 Synthesis of DMEDO-TSeF.
Table 1 Half-wave potentials of DMEDO-TSeF and related
analoguesa
Donor
E11=2/V
E21=2/V
DE = E21=2 2 E11=2
TMTSF
20.01
+0.04
+0.08
+0.35
+0.35
+0.34
0.36
0.31
0.26
Imakubo Initiative Research Unit, RIKEN, 2-1 Hirosawa, Wako,
Saitama 351-0198, Japan. E-mail: tsirahat@riken.jp; imakubo@riken.jp
{ Electronic supplementary information (ESI) available: experimental
details of the synthesis of DMEDO-TSeF, crystal structure analysis for
neutral DMEDO-TSeF and band calculation for (DMEDO-TSeF)2X
(X = PF6, AsF6 and SbF6). See DOI: 10.1039/b511820d
DMEDO-TSeF
BEDO-TSeF
a
vs. Cp2Fe–Cp2Fe+ couple, in PhCN with 0.1 M n-Bu4NBF4, glassy
carbon working electrode, 100 mV s21, room temperature.
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with the transition temperature observed in the resistivity
measurements. The Pauli-like behavior remained below the
transition temperature and huge hysteresis was observed in the
range of 215–270 K, indicating that this phase transition is a first-
order metal–metal transition.
We have performed X-ray crystal structure analyses of these
three salts at room temperature (293 K). The unit cell contains
two donor molecules and one counter anion, giving donor to
anion ratio of 2 : 1. As shown in Fig. 3, the donor molecules of
these salts stack in a head-to-tail manner along the a-axis and are
…
connected by short Se Se contacts shorter than the sum of the
van der Waals radii10 along the b-axis. This packing diagram
resembles that of the (TMTSF)2X system11 and affords quasi one-
dimensional Fermi surfaces (refer to the electronic supplementary
information). The donor arrangement for these three salts depends
on the anion size, and there is a distinct difference in inter-column
interaction. In the PF6 salt, the donor molecules are aligned along
Fig. 1 Temperature dependence of the resistivity for (DMEDO-
TSeF)2X; X = PF6, AsF6 and SbF6.
corresponding tetra-n-butylammonium salts (ca. 40 mg) as a
supporting electrolyte in chlorobenzene (20 mL). All these salts are
crystallized in a triclinic b-type structure;§ however, their conduct-
ing properties and donor arrangements are influenced by the size
of the counter anion. Fig. 1 shows the temperature dependence
of the resistivity for these salts. The PF6 salt is metallic down to
215 K, and then a distinct phase transition occurred following the
resistivity jump. Below the transition temperature, the metallic
behavior remained down to ca. 10 K; however, the single crystals
of the PF6 salt were extremely fragile after the phase transition.
Owing to the fragility of this type of single crystal, it is always
difficult to measure resistivity at low temperature.7d,9 Therefore, it
has not been possible to measure the warming process of the
temperature dependence of the resistivity in spite of many efforts,
e.g. a very slow cooling/warming rate (0.1 K min21). On the other
hand, the AsF6 and SbF6 salts showed metal–semiconductor
transition around 40 K.
˚
the b-axis with a slipping distance of 4.7 A. On the other hand, the
˚
slipping distances of the AsF6 and SbF6 salts are 3.2 A in either
salt. It is considered that this structural difference affects their
conducting properties and that the large slips of donor molecules
Fig. 2 shows the temperature dependence of the static magnetic
susceptibility for the PF6 salt. The room temperature value of x
is 2.9 6 1024 emu mol21, which is appropriate for the Pauli
paramagnetism of common organic metals. With the lowering of
the temperature, the static magnetic susceptibility of the PF6
salt decreased gradually and showed an upturn at 235 K. The
temperature showing the maximum susceptibility (215 K) agrees
Fig. 3 Crystal structure for (DMEDO-TSeF)2X at 293 K; (a), (b) X =
PF6, (c), (d) X = AsF6 and (e), (f) X = SbF6 viewed along the crystal-
lographic a-axis for (a), (c) and (e), and projection on to the ac-plane for
…
(b), (d) and (f). The dotted lines indicate Se Se contacts shorter than
˚
sum of the van der Waals radii [PF6: d1 = 3.680(1) A; AsF6: d1 = 3.680(2),
˚
˚
d2 = 3.645(2) A; SbF6: d1 = 3.629(2), d2 = 3.582(2), d3 = 3.726(1) A]. In (a),
(c) and (e), the minor orientations of anions are omitted for clarity. In (b),
(d) and (f), the ordered and disordered fluorine atoms are represented
with axes and open ellipsoids, respectively, and the major and minor
orientations are showed by the black and gray lines, respectively [PF6:
0.55(5)/0.45(5); AsF6: 0.51(5)/0.49(5); SbF6: 0.67(4)/0.33(4)].
Fig. 2 Temperature dependence of the static magnetic susceptibility for
(DMEDO-TSeF)2PF6 with an applied field of 10 kOe. The inset shows
cooling and warming data around the transition temperature.
4400 | J. Mater. Chem., 2005, 15, 4399–4402
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˚
donor molecules diminished to 3.2 A from the room temperature
˚
value of 4.7 A. Therefore, the structural change is not simple
˚
thermal contraction but the molecules glide by 1.5 A along the
molecular long axis. As a result of this immense structural change,
it is quite reasonable that the single crystals are extremely fragile
during phase transition.
In summary, we have successfully synthesized a new unsymme-
trical donor DMEDO-TSeF without the use of highly toxic
reagents. Novel organic metals with octahedral anions have been
prepared and their crystal structure and physical properties have
been investigated. Among them, the first-order phase transition
observed in the PF6 salt at 215 K will attract much interest not
only as a phenomenon of basic science but also as a building
block for a molecular device because of the huge molecular
˚
motion (1.5 A per molecule) during phase transition and the wide
temperature range of hysteresis.12,13 Further studies on the physical
properties of (DMEDO-TSeF)2X are now in progress together
with the exploration of related materials.
Fig. 4 Temperature dependence of the cell parameters for (DMEDO-
TSeF)2PF6. The values of cell parameters are normalized to their
respective values at 293 K. The solid and broken lines indicate the cooling
and warming process, respectively.
Notes and references
observed in the PF6 salt play an important role in first-order
metal–metal transition (vide infra). The anions of these three salts
were taken in a cavity formed by the zigzag-type donor stacks
and showed disorder based on uni-axial rotation. In the SbF6
salt, the motion of the octahedral anion is a jiggle rather than a
rotation because large SbF6 anions are installed in the cavity more
precisely.
{ Selected data of new compound DMEDO-TSeF: purple-red needles, mp
199 uC (decomp.); m/z (EI, 70 eV): 480 (M+ for C10H10O278Se80Se3); dH
(270 MHz, CD2Cl2) 4.27 (4H, s, OCH2CH2O), 2.00 (6H, s, CH3); IR (neat,
n/cm21) 1624 (s), 1443 (m), 1368 (m), 1268 (m), 1240 (w), 1134 (s);
Elemental analysis: Calc. for C10H10O2Se4: C, 25.13; H, 2.11. Found: C,
25.14; H, 2.11%.
§ Crystal data for DMEDO-TSeF: C10H10O2Se4, M = 478.02, purple-red
needles (0.50 6 0.06 6 0.05 mm), monoclinic, P21/c (#14), a = 13.958(4),
3
˚
˚
b = 8.285(2), c = 11.867(3) A, b = 109.478(6)u, V = 1293.8(6) A ,
m = 11.323 mm21, Z = 4, 9395 reflections measured, 3223 unique
(Rint = 0.0571). Final R indices [I . 2s(I)]: R1 = 0.0501, wR2 = 0.1262.
GOF = 1.076. Crystal data for (DMEDO-TSeF)2PF6: C20H20O4Se8PF6,
The unit cell parameters of the PF6 salt have been measured at
various temperatures from 293 to 220 K. As shown in Fig. 4, the
values of cell parameters a, b and c regularly decrease with
temperature down to 220 K due to thermal contraction. We tried
to perform structure analysis at 200 K (below the transition
temperature of 215 K) in order to investigate the low-temperature
metallic phase. However, the refinement of the cell parameters
was unsuccessful due to the appearance of many incommensurate
reflections. In the warming process, coherence of the crystals was
recovered above 215 K, and the cell parameters showed drastic
change due to the generation of a new phase. The cell parameters
of the PF6 salt at 220 K are summarized in Table 2 compared with
the room temperature values of the PF6, AsF6 and SbF6 salts. It is
interesting that the new phase observed in the warming process is
isostructural with the AsF6 and SbF6 salts and it should be also
noted that the drastic changes of the unit cell angles are
characteristic in this phase transition. According to preliminary
structure refinement, the slipping distance between neighboring
¯
M = 1101.01, black needles (0.20 6 0.05 6 0.02 mm), triclinic, P1 (#2), a =
˚
7.302(3), b = 7.977(3), c = 13.500(6) A, a = 73.739(10), b = 79.794(8), c =
79.329(10)u, V = 735.2(5) A , m = 10.062 mm21, Z = 1, 5474 reflections
3
˚
measured, 3607 unique (Rint = 0.0935). Final R indices [I . 2s(I)]: R1 =
0.0504, wR2 = 0.1176. GOF = 0.847. Crystal data for (DMEDO-
TSeF)2AsF6: C20H20O4Se8AsF6, M = 1144.96, black needles (0.50 6
¯
0.03 6 0.02 mm), triclinic, P1 (#2), a = 7.269(3), b = 7.632(3), c =
3
˚
˚
14.345(6) A, a = 78.664(8), b = 86.234(8), c = 69.542(7)u, V = 731.1(5) A ,
m = 11.187 mm21, Z = 1, 4486 reflections measured, 3177 unique
(Rint = 0.0351). Final R indices [I . 2s(I)]: R1 = 0.0718, wR2 = 0.1872.
GOF = 0.942. Crystal data for (DMEDO-TSeF)2SbF6: C20H20O4Se8SbF6,
¯
M = 1191.79, black needle (0.40 6 0.03 6 0.02 mm), triclinic, P1 (#2),
˚
a = 7.281(3), b = 7.930(3), c = 14.519(5) A, a = 79.806(8), b = 89.564(9),
c = 65.582(7)u, V = 749.2(4) A , m = 10.702 mm21, Z = 1, 5603 reflections
3
˚
measured, 3688 unique (Rint = 0.0586). Final R indices [I . 2s(I)]:
R1 = 0.0652, wR2 = 0.1442. GOF = 0.862. CCDC reference numbers
281658–281661. For crystallographic data in CIF format see DOI: 10.1039/
b511820d
1 H. Yamochi, in TTF Chemistry—Fundamentals and Applications of
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low-temperature phases of PF6 salt and the room-temperature phase of
AsF6 and SbF6 salts
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PF6
PF6
AsF6
SbF6
T/K
˚
293
220a
293
293
a/A
˚
7.302(3)
7.977(3)
13.500(6)
73.739(10)
79.794(8)
79.329(10)
735.2(5)
7.178(4)
7.623(5)
14.174(8)
79.375(12)
87.655(14)
68.949(13)
711.2(7)
7.269(3)
7.632(3)
14.345(6)
78.664(8)
86.234(8)
69.542(7)
731.1(5)
7.281(3)
7.930(3)
14.519(5)
79.806(8)
89.564(9)
65.582(7)
749.2(4)
b/A
˚
c/A
a/deg
b/deg
c/deg
6 T. Imakubo, T. Shirahata and M. Kibune, Chem. Commun., 2004,
1590–1591.
3
˚
V/A
a
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