A purely organic paramagnetic metal, κ-β″-(BEDT-TTF) 2(PO-CONHC2H4SO3), where PO = 2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl free radical

Article abstract of DOI:10.1021/cm100984z

A novel bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)-based purely organic paramagnetic conducting salt that contains the aminoxyl radical was prepared. Five BEDT-TTF salts containing the TEMPO radical were prepared. An anion with the even smaller PO (2, 2, 5, 5-tetramethyl-3-pyrrolin-1-oxyl) radical was also prepared. The sulfonic acid H2 was prepared by reacting 3-carboxy-2, 2, 5, 5-tetramethyl pyrrolin-1-oxyl (PO-COOH) with 2-aminoethanesulfonic acid mmol) in the presence of DCC and DMAP in 30 mL of CH₂Cl₂. Single-crystal X-ray diffraction data for PPh ₄2 were collected using a Quantum CCD area detector and a Rigaku AFC-7R diffractometer. Black needlelike crystals of k-β''-(BEDT-TTF) ₂(POCONHC₂H₄SO₃), were obtained by the conventional constant-current electrocrystallization method in a mixed solvent of PhCl and CH₃CN with 15 mg of BEDT-TTF and 70 mg of PPh₄2. The salt is metallic down to 1.7K, indicating a stable metal.

Full text of DOI:10.1021/cm100984z

762 Chem. Mater. 2011, 23, 762–764
DOI:10.1021/cm100984z
A Purely Organic Paramagnetic Metal, K-β00-(BEDT-
TTF)2(PO-CONHC2H4SO3), Where PO = 2,2,5,5-
Tetramethyl-3-pyrrolin-1-oxyl Free Radical†
ducing aminoxyl radicals (R2NO ), but as of yet, no other
3
group has prepared a metallic salt.6 Similarly, we pre-
pared five BEDT-TTF salts containing the TEMPO
radical (see scheme) and all are semiconductors.7a-e
We then reported the first metallic salt (albeit only down
to 210 K) containing the smaller PROXYL radical, β00-
(BEDT-TTF)2(PROXYL-NHCOCH2SO3).7f We then pre-
pared an anion with the even smaller PO (2,2,5,5-tetra-
methyl-3-pyrrolin-1-oxyl) radical, PO-NHCOCH2SO3- (1),
but the resultant BEDT-TTF salt had an R-type packing
motif and was a semiconductor.7g In this communication, we
report the larger anion PO-NHCOCH2CH2SO3- (2) which
includes the same PO radical but with the longer ethylene
group instead of the methylene group of anion 1. This new
anion 2 has provided a BEDT-TTF salt, the structure and
properties of which are reported.
Hiroki Akutsu,*,‡ Shinji Yamashita,‡ Jun-ichi Yamada,‡
Shin’ichi Nakatsuji,‡ Yuko Hosokoshi,§ and
Scott S. Turner#
‡Graduate School of Material Science, University of Hyogo,
3-2-1, Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297,
Japan, §Department of Physical Science, Graduate School
of Science, Osaka Prefecture University, 1-1 Gakuen-cho,
Naka-ku, Sakai, Osaka 599-8531, Japan, and
#Chemical Sciences, University of Surrey, Guildford,
Surry GU2 7XH, United Kingdom
Received April 9, 2010
Revised Manuscript Received May 10, 2010
Organic conducting salts, based on bis(ethylenedithio)-
tetrathiafulvalene (BEDT-TTF), have been prepared with
a wide variety of counteranions.1 The transport pro-
perties range from insulating through semiconducting to
metallic and superconducting and are dependent in part on
the packing of the BEDT-TTF molecules. The packing
motifs have been classified into several types designated by
the labels R, β, β00, κ, λ, θ, etc.1,2 Evidently, the donor
arrangements play a crucial role in determining the trans-
port properties, because κ- and β-type salts usually have
metallic properties, R-type salts are usually semiconduc-
tors, β00-typesaltsareweakmetalsorsemimetals,and soon.
After the discovery of the BEDT-TTF-based paramag-
The sulfonic acid H2 was prepared by reacting 3-carboxy-
2,2,5,5-tetramethylpyrrolin-1-oxyl (PO-COOH, 100 mg,
0.54 mmol) with 2-aminoethanesulfonic acid (H2NCH2-
CH2SO3H, 70 mg, 0.65 mmol) in the presence of DCC
(H11C6-NdCdN-C6H11, 150 mg, 0.65 mmol) and DMAP
(4-(CH3)2N-pyridine, 180 mg, 1.30 mmol) in 30 mL of CH2Cl2
at room temperature with stirring over three days. Metathesis
with PPh4Br gave PPh42 as yellow crystals, which were
recrystallized from acetone (yield 45%). Single-crystal X-ray
diffraction data for PPh42 were collected using a Quan-
tum CCD area detector and a Rigaku AFC-7R diffracto-
meter.8 The asymmetric unit contains one PPh4 cation
netic superconductor, β00-(BEDT-TTF)4[(H3O)Fe(C2O4)3]
3
PhCN,3 charge-transfer (CT) salts with transition metal
anions have attracted great interest.4 In particular, the
λ-(BETS)2FeCl4 family have unique electromagnetic pro-
perties that emerge from an interaction between localized
and itinerant electrons.5 However, the interaction is small
(<10 K), probably because there is no direct overlap
between the pπ orbitals of the donor molecule and the d
orbitals of the transition metal. To make CT salts that
have larger interactions, one strategy has been to use a
different source of unpaired electron density, organic free
radicals, which can directly overlap with the donor pπ
orbitals. To this end, many researchers have tried intro-
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Tsujii, M.; Nakatsuji, H.; Hosoito, H. Chem. Lett. 1993, 1361.
Hutchison, K. A.; Srdanov, G.; Menon, R.; Gabriel, J. -C. P.; Knight,
B.; Wudl, F. J. Am. Chem. Soc. 1996, 118, 13081. Nakatsuji, S.; Anzai,
H. J. Mater. Chem. 1997, 7, 2161. Nakamura, Y.; Koga, N.; Iwamura,
H. Chem. Lett. 1991, 69. Mukai, K.; Senba, N.; Hatanaka, T.;
Minakuchi, H.; Ohara, K.; Taniguchi, M.; Misaki, Y.; Hosokoshi, Y.;
Inoue, K.; Azuma, N. Inorg. Chem. 2004, 43, 566. Aonuma, S.;
Casellas, H.; Faulmann, C.; Garreau de Bonneval, B.; Malfant, I.;
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2001, 11, 337.
(7) (a) Akutsu, H.; Yamada, J.; Nakatsuji, S. Chem. Lett. 2003, 32,
1118. (b) Akutsu, H.; Yamada, J.; Nakatsuji, S. Synth. Met. 2005, 152,
377. (c) Akutsu, H.; Masaki, K.; Mori, K.; Yamada, J.; Nakatsuji, S.
Polyhedron 2005, 24, 2126. (d) Yamashita, A.; Akutsu, H.; Yamada, J.;
Nakatsuji, S. Polyhedron 2005, 24, 2796. (e) Akutsu, H.; Yamada, J.;
Nakatsuji, S.; Turner, S. S. Solid State Commun. 2006, 140, 256.
(f) Akutsu, H.; Sato, K.; Yamashita, S.; Yamada, J.; Nakatsuji, S.;
Turner, S. S. J. Mater. Chem. 2008, 18, 3313. (g) Akutsu, H.;
Yamashita, S.; Yamada, J.; Nakatsuji, S.; Turner, S. S. Chem. Lett.
2008, 37, 882.
† Accepted as part of the “Special Issue on π-Functional Materials”.
*Corresponding author.
(1) Williams, J. M.; Ferraro, J. R.; Thorn, R. J.; Carlson, K. D. ;
Geiser, U.; Wang, H. H.; Kini, A. M.; Whangbo, M.-H. Organic
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r
pubs.acs.org/cm
Published on Web 06/02/2010
2010 American Chemical Society

Communication
Chem. Mater., Vol. 23, No. 3, 2011 763
0.75. The structure consists of alternating donor and
anion layers propagated along the crystallographic c axis.
There are two crystallographically independent donor
layers with κ- and β00-type arrangements shown in panels
b and c in Figure 1, respectively.10 For each donor A-D
(Figure 1b,c) we estimate the charge, using bond lengths12
of þ0.17, þ0.83, þ0.85, and þ0.60, respectively. The
charges, normalized by the total number of holes in the
asymmetric unit, are þ0.14, þ0.67, þ0.70, and þ0.49 for
A-D, respectively. The band filling for the κ- and β00-
layers is different at 0.80 and 0.70, respectively.13 The
result suggests that the κ-layer has more electrons than the
β00-layer, in order to equalize their Fermi level. The band
fillings are slightly different from the formula band filling
of 0.75, which may make both the β00- and κ-layer metallic
although the κ-layer has considerble charge dispropor-
tionation.14c In the anionic layers (Figure 1d), the anions
are all oriented in the same direction and no short con-
tacts were observed between the spin centers of the PO
˚
radicals (the shortest O O distance is 5.880(19) A).
3 3 3
Figure 1a shows that the sulfonate groups (-SO3-) are
oriented to face the β00-layers and the N-O atoms, which
carry the unpaired spin, are oriented to face the κ-layers.
The anisotropy of the anionic layers along the c-axis
appears to lead to the structure having the two crystal-
lographically independent donor layers. In fact, a mag-
netically important short contact between the anionic
spin center O atom and a sulfur atom of the BEDT-TTF
Figure 1. (a) Crystal structure of 3. (b) Packing arrangements of donors
in the κ-layer and (c) the β00-layer. Dashed lines indicate S S contacts
3 3 3
˚
shorter than the sum of the van der Waals radii (<3.70 A). (d) Packing
arrangement of the anion layer. (e) Molecular structure of the κ-layer
donors and the anions showing the short S O contact.
3 3 3
and one anion. The magnetic susceptibility of a poly-
crystalline sample of PPh42 was measured using a
Quantum Design MPMS-5S SQUID magnetometer.
The data can be modeled by a Curie-Weiss law with
C = 0.370 emu K/mol and θ = -0.39 K.
˚
molecule in the κ-layer (O5 S1) of 3.638(10) A is observed
3 3 3
(Figure 1e),15 although it is slightly longer than the sum of
˚
the van der Waals radii (3.37 A). In addition, there are
two independent N-O distances for the PO moieties of
1.252(13) A for N2-O5 and 1.286(14) A for N4-O10
(Figure 1e). Only the latter is close to the N-O distance in
Black needlelike crystals of κ-β00-(BEDT-TTF)2(PO-
CONHC2H4SO3), 3, were obtained by the conventional
constant-current electrocrystallization method in a mixed
solvent of PhCl (18 mL) and CH3CN (2 mL) with 15 mg of
BEDT-TTF and 70 mg of PPh42. X-ray diffraction data
were recorded at 293 K with the same CCD system as used
for PPh42.9 The crystal structure of 3 is shown in Figure
1a. In the asymmetric unit there are four BEDT-TTF
molecules and two anions. Therefore, the formula charge
of BEDT-TTF is þ0.5 and the formula band filling is
˚
˚
˚
PPh42(1.281(4) A) and is in the range found for the TEMPO
radicals (1.27-1.30 A).16 As mentioned, the O5 atom has a
˚
short contact with the S1 atom of the donor, suggesting some
electron donation from the PO radical to the κ-layer.17
The electrical resistivity of the single crystals were mea-
sured by a conventional four probe method. At 300 K a
resistivity of 1.6 ꢀ 10-2 Ω cm was observed. The tem-
perature dependence of the electrical resistivity is shown
in Figure 2a. From 300 K the resistivity decreases gradu-
ally with decreasing temperature to 70 K, below which it
decreases more rapidly down to 1.7 K. Thus, this is the
(8) Crystal data for PPh42: C35H38N2O5PS, M=629.73, triclinic P1,
˚
˚
˚
a = 11.6417(6) A, b = 11.7774(7) A, c = 13.7069(13) A, R =
3
˚
112.458(3)°, β=99.9654(9°), γ=99.1630(10)°, V=1657.4(2) A ,
Z = 2, Dc = 1.262 g cm-3, MoKR, λ = 0.71073 A, θmax = 27.5°,
˚
T=295 K, total data 14488, unique data 6879, μ=0.1893 mm-1
435 parameters, R=0.070, Rw=0.057 on |F|, and S=1.094 (see the
,
Supporting Information for further details).
(9) Crystal data for 3: C31H34N2O5S17, M=1059.74, triclinic P1, a=
˚
˚
˚
8.711(2) A, b=11.8136(4) A, c=40.528(7) A, R=89.448(2)°, β=
3
˚
87.3821(12)°, γ=83.2487(9)°, V=4137.3(12) A , Z=4, Dc=1.70
(12) Guionneau, P.; Kepert, C. J.; Bravic, G.; Chasseau, D.; Truter,
M. R.; Kurmoo, M.; Day, P. Synth. Met. 1997, 86, 1973.
(13) Only a small number of charge-transfer salts have been reported
with two crystallographically independent conducting layers, each
of which have different band fillings (see refs 14a-c).
gcm-3, MoKR, λ=0.71073 A, θmax =27.5°, T=293 K, total data
˚
27895, unique data 6689, μ = 0.930 mm-1, 989 parameters,
R = 0.080, Rw = 0.075 on |F|, and S = 1.151 (see the Supporting
Information for further details).
(10) Several BEDT-TTF salts have been reported that have two differ-
ent donor layers with entirely different structures (see ref 11).
(11) Schlueter, J. A.; Geiser, U.; Wang, H. H.; Kini, A. M.; Ward, B. H.;
Parakka, J. P.; Daugherty, R. G.; Kelly, M. E.; Nixon, P. G.;
Winter, R. W.; Gard, G. L.; Montgomery, L. K.; Koo, H. -J.;
Whangbo, M. -H. J. Solid State Chem. 2002, 168, 524. Akutsu, H.;
Akutsu, A.; Turner, S. S.; Day, P.; Canadell, E.; Firth, S.; Clark, R. J. H.;
Yamada, J.; Nakatsuji, S. Chem. Commun. 2004, 18. Akutsu, H.;
Yamada, J.; Nakatsuji, S.; Turner, S. S. Solid State Commun. 2007,
144, 144. Martin, L.; Day, P.; Akutsu, H.; Yamada, J.; Nakatsuji, S.;
Clegg, W.; Harrington, R. W.; Horton, P. N.; Hursthouse, M. B.;
McMillan, P.; Firth, S. CrystEngComm 2007, 9, 865.
(14) (a) Ishikawa, Y.; Saito, K.; Kikuchi, K.; Kobayashi, K.; Ikemoto,
I. Bull. Chem. Soc. Jpn. 1991, 64, 212. (b) Kato, R.; Fujiwara, M.;
Kashimura, Y.; Yamaura, J. Synth. Met. 2001, 120, 675. (c) Kawamoto,
T.; Mori, T.; Kakiuchi, T.; Sawa, H.; Shirahata, T.; Kibune, M.; Yoshino,
H.; Imakubo, T. Phys. Rev. B 2007, 76, 134517.
(15) The shortest S O contact between the other independent anion 2
3 3 3
˚
and a BEDT-TTF molecule (S15-O10, Figure 1e) is 3.989(11) A.
(16) Shibaeva, R. N. Zh. Strukt. Khim. 1975, 16, 330.
(17) An oxidized TEMPO has the shorter N-O distance [e.g., 1.195(5)
A in (TEMPO)(TCNQF4)] (see ref 18).
˚
(18) Nakatsuji, S.; Takai, A.; Nishikawa, K.; Morimoto, Y.; Yasuoka,
N.; Suzuki, K.; Enoki, T.; Anzai, H. Chem. Commun. 1997, 275.

764 Chem. Mater., Vol. 23, No. 3, 2011
Communication
Figure 2. (a) Temperature-dependent electrical resistivity of 3. (b) Cal-
culated Fermi surfaces of(I, II) κ- and (III, IV) β00-layers atthe bandfilling
of (I) 0.80, (II) 0.75, (III) 0.70, and (IV) 0.75, respectively.
first metal containing an aminoxyl radical that is stable
to such low temperatures.
The electronic structure has been characterized using
€
extended Huckel tight-binding band structure calcula-
Figure 3. (a) Temperature-dependent magnetic susceptibility (χ-T plot)
of the powder sample of 3. The dashed line is calculated on the basis of an
alternate Heisenberg 1D chain model. Inset shows a 1D chain of the anions.
(b) χ-T plot after subtracting the anionic Heisenberg term from the total
magnetic susceptibility (2-300 K). The solid line is a guide to the eyes.
tions19 for each of the different two-dimensional donor
sheets. Figure 2b shows Fermi surfaces of the κ- (I, II) and
β00-layers (III, IV) at the band filling of 0.80 (I), 0.75 (II),
0.70 (III), and 0.75 (IV), respectively. Despite of the
difference in the band filling both Fermi surfaces for the
κ- and β00-layers are similar to each other. The band
dispersion (Figure S1 in the Supporting Information) of
the β00-layer has no significant midgap but that of κ-layer
has a mid-gap-like split, suggesting that the κ-layer can
become a Mott insulator if the band fillng is 0.75. In
addition, the calculation including the charge-ordering
effect was also calculated for κ-layer. Fermi surface area
decreses with increarsing the charge-ordering effect
(Figure S1e in the Supporting Information).
range expected for Pauli paramagnetic susceptibility of
organic metals (2-6 ꢀ 10-4 emu/mol),1 suggesting no
charge localization on the donor layers and normal metal-
lic behavior. The small negative J values indicate that
there is a short-range weak antiferromagnetic PO PO
interaction. As shown in the inset of Figure 3a, the two
independent anions form 1D alternating chains with
3 3 3
˚
short N-O O-N contacts of just less than 6 A. It is
through this 1D network that the PO radicals appear to
3 3 3
interact with each other although the intermolecular
distances are relatively long. Alternatively the S
O
The temperature-dependent magnetic susceptibility of
3 was measured using a Quantum Design SQUID mag-
netometer from 2 to 300 K (MPMS-5SH) and from 0.5 to
3 3 3
short contact as shown in Figure 1e suggests that the
weak antiferromagnetic interaction is mediated by the
κ-type donor layers. Because the salt is a metal, it is
postulated that the itinerant electrons mediate the antiferro-
magnetic interaction. However, if this is the case, 2D-like
magnetic behavior should be observed, which contradicts
with the observed 1D Heisenberg-like result. The confirma-
tion of which route is significant is now in progress.
3
2 K (MPMS-XL5 with an IQUANTUM iHelium3 He
refrigerator). The χ-T curve (Figure 3a) has a broad
maximum around 1 K, below which χ decreases with
decreasing temperature. We also measured the magnetic
susceptibility of a bundle of the needle crystals oriented in
four glass capillaries (1 mmφ) parallel and perpendicular
to the crystal long axis (//b), but no anisotropy was
observed (Figure S2 in the Supporting Information).
The result suggests that the χ drop is not caused by an
antiferromagnetic transition. In fact, the powder data
(Figure 3a) can be modeled by a 1D Heisenberg S = 1/2
alternating chain expression20 with p (spin concentration)=
0.975, J1=-0.80, J2=-0.69 K. The p value is close to 1.0,
as predicted for the PO radical, suggesting that this moiety
dominates the Heisenberg term. To obtain the Pauli para-
magnetic contribution from the BETD-TTF layers (χET),
we subtracted the anionic Heisenberg term from the total
data. The resultant curve is shown in Figure 3b (from 2 to
300 K).21 The value of 3.5-4.2 ꢀ 10-4 emu/mol is in the
In conclusion, we have made a novel BEDT-TTF-based
purely organic paramagnetic conducting salt that contains
the aminoxyl radical. X-ray analysis indicates that there are
two crystallographically independent donor sheets with
κ- and β00-type structural motifs. The salt is metallic down
to 1.7 K, indicating a stable metal. A weak antiferromagnetic
interaction between the PO radicals was also observed.
Supporting Information Available: Additional information
and figures (PDF); crystallographic information (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
(21) Note that there is a deviation of the observed data from the best
fitting curve in the low-temperature region (<50 K, see Figure 3a).
Therefore we omitted the χET values below 2 K in Figure 3a. Such
uncertainty in the estimation of χET at low temperature may
indicate that the data does not exactly follow the 1D alternating
chain model.
(19) Mori, T.; Kobayashi, A.; Sasaki, Y.; Kobayashi, H.; Saito, G.;
Inokuchi, H. Bull. Chem. Soc. Jpn. 1984, 57, 627.
(20) Hall, J. W.; Marsh, W. E.; Weller, R. R.; Hatfield, W. E. Inorg.
Chem. 1981, 20, 1033.