The first organic paramagnetic metal containing the aminoxyl radical

Article abstract of DOI:10.1039/b806694a

We have prepared an organic magnetic anion, PROXYL-4-CONHCH ₂SO₃^- (1). The electrocrystallisation of BEDT-TTF with PPh₄1 gave the first purely organic paramagnetic metal, β″-(BEDT-TTF)₂(1). The salt shows a broad metal-insulator transition at approximately 210 K. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b806694a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
The first organic paramagnetic metal containing the aminoxyl radical†
Hiroki Akutsu, a Keiko Sato,a Shinji Yamashita,a Jun-ichi Yamada,a Shin’ichi Nakatsujia and Scott S. Turnerb
*
Received 21st April 2008, Accepted 10th June 2008
First published as an Advance Article on the web 18th June 2008
DOI: 10.1039/b806694a
We have prepared an organic magnetic anion, PROXYL-4-CON-
ꢀ
HCH2SO3 (1). The electrocrystallisation of BEDT-TTF with
PPh41 gave the first purely organic paramagnetic metal, b00-
(BEDT-TTF)2(1). The salt shows a broad metal–insulator transition
at approximately 210 K.
Since the first synthesis of the donor molecule BEDT-TTF (bis
(ethylenedithio)tetrathiafulvalene, ET) three decades ago,1 many
The acid 1 was prepared by reacting 3-carboxy-PROXYL (0.40 g,
2.1 mmol) with H2NCH2SO3H (0.29 g, 2.6 mmol) in the presence of
N,N0-dicyclohexylcarbodiimide (DCC, 0.53 g, 2.6 mmol) and
ET-based conducting salts with a wide variety of anions have been
prepared.2 In the salts, the donor molecules usually form quasi-2-D
conducting layers, which are separated by the counteranions.3
dichloromethane at ambient temperature with stirring over four days.
Usually the anions play no significant role in the transport properties
4-dimethylaminopyridine (DMAP, 0.63 g, 5.2 mmol) in 30 mL of
Metathesis of H1 with PPh4Br yielded PPh41 (2) as yellow crystals,
and prevent the donors from approaching one another in the crystals.
which were then recrystallised from acetonitrile (yield 27%). X-Ray
In fact, a salt with smaller anions is usually a more stable metal than
diffraction data of 2 were collected on a Rigaku AFC-5R 4-circle
an isomorphous salt with larger anions.
diffractometer at room temperature.‡ The asymmetric unit contains
Since the discovery of the paramagnetic organic superconductor,
one PPh4 cation and one anion. The N–O distance in the PROXYL
˚
part is 1.272(3) A, which is similar to the corresponding distance in
b00-(ET)4[(H3O)Fe(C2O4)3]$PhCN,4 organic magnetic conductors
have attracted great interest.5,6 In particular, l-(BETS)2FeCl4 and its
derivatives show unique physical properties.5 Their interesting
properties emerge from interactions between magnetic moments
incorporated into a conducting crystal. However, magnetic interac-
tions in these salts are small, probably because the magnetic centres
(here transition metals) are surrounded by ligands such as Clꢀ, Brꢀ,
C2O42ꢀ, etc., through which the centres must interact indirectly.
Alternatively organic free radicals can be used as the source of
unpaired electron density. The spin is delocalised over a few atoms on
a ‘bare’ orbital. Therefore, the orbital containing the unpaired
electron can potentially overlap directly with orbitals occupied by
conduction electrons. For this reason many researchers have intro-
duced organic radicals into organic conducting salts but all reported
salts have only shown semiconducting behaviour until now.7
Our recent focus is on preparing new organic magnetic anions that
combine the organic free radical TEMPO with the sulfonate group
(–SO3ꢀ).8 However, all of the ET-based salts of these anions show
only semiconducting behaviour. As mentioned above, smaller anions
usually provide more conductive and/or metallic salts. Therefore, we
have chosen the smaller PROXYL radical. We have now prepared an
organic magnetic anion, PROXYL-CONHCH2SO3ꢀ (1) which is the
smallest sulfonate that we have thus far prepared. We have been able
to synthesise the ET salt of 1, the structure and properties of which
are reported in this communication.
9
˚
TEMPO radicals (1.27–1.30 A). The temperature-dependent
magnetic susceptibility of a polycrystalline sample from 2–300 K
using a Quantum Design MPMS-5S SQUID magnetometer obeys
the Curie–Weiss law with C ¼ 0.379 emu K molꢀ1 and q ¼ ꢀ0.36 K.
The conventional constant-current electrocrystallisation in a mixed
solvent of m-dichlorobenzene and acetonitrile with 15 mg of ET and
70 mg of 2 gave black block-like crystals. The X-ray diffraction
measurement was performed on a Rigaku AFC-5R 4-circle diffrac-
tometer at ambient temperature. The resulting data were solved as
b00-(ET)21 (3).x A donor to anion ratio of 2 : 1 indicates that each ET
molecules has a formula charge of +1/2. The crystal structure of
3 shown in Fig. 1 has an asymmetric unit with two ET molecules and
aGraduate School of Material Science, University of Hyogo, 3-2-1, Kouto,
Kamigori-cho, Ako-gun, Hyogo, 678-1297, Japan. E-mail: akutsu@sci.
u-hyogo.ac.jp; Fax: +81 791 58 0164; Tel: +81 791 58 0164
bDepartment of Chemistry, Warwick University, Gibbet Hill Road,
Coventry, UK CV4 7AL
Fig. 1 An ORTEP view (50% probability level) of b00-(ET)21 (3) along
the c-axis.
† CCDC reference numbers 685677 and 685678. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b806694a
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 3313–3315 | 3313

View Article Online
Fig. 4 Temperature dependence of electrical resistivity for 3. Inset is the
expansion from 160–290 K.
Fig. 2 Packing arrangement of the donor layer in 3 (ORTEP, 50%
probability ellipsoids). Dashed lines indicates S/S contacts shorter than
˚
the van der Waals distance (<3.7 A).
and Fermi surfaces (Fig. 3, right) are shown. Only small hole and
electron pockets are observed, which is common for b00-type salts.12
b00-Type ET salts are usually semimetals or ‘weak’ metals.
The temperature-dependent electrical resistivity of 3 measured by
the four-probe AC method is shown in Fig. 4. The resistivity grad-
ually falls with decreasing temperature, indicating metallic conduc-
tivity (see inset in Fig. 4). Thus, this is the first aminoxyl-radical
containing organic conductor that shows metallic transport. The
resistivity reaches a minimum at approximately 210 K, increasing
gradually down to 150 K, and then increasing rapidly as for a semi-
conductor. Fig. 5a shows the cT–T plot of 3. If the PROXYL
radicals had non-interacting spin moments and the ET molecules
were magnetically silent, the cT–T plot would be a horizontal line,
with cT ¼ 0.352 emu K molꢀ1 (¼94% of s ¼ 1/2 spin (0.375 emu
K molꢀ1)) as shown in Fig. 5a. However, the actual magnetic curve
deviates from the straight line in two regions, (i) low temperature
region (<20 K) and (ii) high temperature region (>120 K), as shown
in Fig. 5a. The data in the range of 2–20 K (i) can be modelled by the
Curie–Weiss law with C ¼ 0.352 emu K molꢀ1 and q ¼ ꢀ1.05 K.
The C value is 94% of that of the s ¼ 1/2 spin (0.375 emu K molꢀ1) on
one anion. The N–O distance of the PROXYL moiety in 3 is
˚
1.274(10) A, which is the same as that in 2. The unit cell consists of
alternating donor/anion layers along the b-axis. In the anion layer,
there is a PROXYL spin dimer about the centre of symmetry with
˚
a short O/O contact of 4.649(10) A. In addition, the observed
shortest contact between the oxygen atom of the PROXYL moiety
˚
and sulfur atoms of ET molecules is 3.570(8) A, which is 0.2 A longer
˚
˚
than sum of the van der Waals radii (3.37 A). The structure suggests
that it is unlikely that the salt shows significant interactions between
conducting and magnetic electrons. The donor layers are in a b00-type
packing motif as shown in Fig. 2. The central C]C distances of the
two crystallographically independent donor molecules (A and B
˚
shown in Fig. 2) are almost the same, 1.367(10) and 1.368(10) A,
respectively. The estimated charges on both ET molecules calculated
using bond lengths10 are also almost the same, +0.50 and +0.49,
respectively. In other words the positive charge is equally distributed
over the donor layer and not localised as in charge disproportion-
ation. The electronic structure has been characterised using extended
Huckel tight-binding band structure calculations on the 2-dimen-
¨
sional ET sheet.11 The calculated values of the intermolecular overlap
integrals (ꢁ10ꢀ3) p1, p2, p3, a1, a2, s1, s2 and s3 (see Fig. 2) are ꢀ6.34,
ꢀ4.73, ꢀ1.62, ꢀ8.21, ꢀ9.18, ꢀ10.23, ꢀ16.45 and ꢀ10.25, respec-
tively. The values of a1, a2, s1, s2 and s3 are larger than those of p1,
p2 and p3, indicating that the side-by-side interactions are stronger
than the face-to-face interactions. The dispersion relation (Fig. 3, left)
Fig. 5 (a) cT–T plot for salt 3 where c is the magnetic susceptibility per
half of the unit cell [(ET)21]. The solid horizontal line is drawn at cT ¼
0.352. (b) c–T plot after subtracting the Curie term [cCurie-Weiss ¼ 0.352/
(T ꢀ 1.05)] from the total susceptibility.
Fig. 3 Electronic band structure calculated for 3.
3314 | J. Mater. Chem., 2008, 18, 3313–3315
This journal is ª The Royal Society of Chemistry 2008

View Article Online
Notes and references
ꢀ
‡ Crystal data for 2: C34H38N2O5P1S1, M ¼ 617.72, triclinic P1, a ¼
˚
12.042(3), b ¼ 12.306(3), c ¼ 11.691(4) A, a ¼ 95.75(3), b ¼ 111.52(2),
ꢂ
3
g ¼ 83.20(2) , V ¼ 1597.4(8) A , Z ¼ 2, Dc ¼ 1.284 g cmꢀ3, MoKa, l ¼
˚
0.71073 A, qmax ¼ 27.5ꢂ, T ¼ 294 K, total data 7690, unique data 7342,
˚
m ¼ 0.195 mmꢀ1, 426 parameters, R ¼ 0.052, Rw ¼ 0.061 on |F| and S ¼
1.025. The structure was solved by direct methods and all non-H atoms
were subjected to anisotropic refinement by full-matrix least-squares on F
using CrystalStructure Ver. 3.8.2. CCDC 685677.†
ꢀ
x Crystal data for 3: C30H34N2O5S17, M ¼ 1047.63, triclinic P1,
˚
a ¼ 12.436(4), b ¼ 19.528(6), c ¼ 9.326(3) A, a ¼ 97.06(3), b ¼ 110.22(3),
ꢂ
3
g ¼ 83.44(3) , V ¼ 2102.9(12) A , Z ¼ 2, Dc ¼ 1.654 g cmꢀ3, MoKa, l ¼
˚
0.71073 A, qmax ¼ 27.5ꢂ, T ¼ 295 K, total data 10383, unique data 9644,
˚
m ¼ 0.9138 mmꢀ1, 521 parameters, R ¼ 0.064 Rw ¼ 0.068 on |F| and S ¼
1.134. The structure was solved by direct methods and all non-H atoms
were subjected to anisotropic refinement by full-matrix least-squares on F
using CrystalStructure Ver. 3.8.2. CCDC 685678.†
1 M. Mizuno, A. F. Garito and M. P. Cava, J. Chem. Soc., Chem.
Commun., 1978, 18.
2 H. Mori, J. Phys. Soc. Jpn., 2006, 75, 051003.
3 J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser,
H. H. Wang, A. M. Kini and M.-H. Whangbo, Organic
Superconductors (Including Fullerenes): Synthesis, Structure,
Properties and Theory, Prentice Hall, Englewood Cliffs, NJ, 1992.
4 M. Kurmoo, A. W. Graham, P. Day, S. J. Coles, M. B. Hursthouse,
J. L. Caulfield, J. Singleton, F. L. Pratt, W. Hayes, L. Ducasse and
P. Guionneau, J. Am. Chem. Soc., 1995, 117, 12209.
Fig. 6 Temperature dependent electrical resistivities of 3 under pressure.
Inset shows pressure dependence of the slopes of the Arrhenius plots
(250–300 K) which are equal to the activation energy for semiconductors.
5 H. Kobayashi, H. Cui and A. Kobayashi, Chem. Rev., 2004, 104,
5265.
6 E. Coronado and P. Day, Chem. Rev., 2004, 104, 5419; T. Enoki and
A. Miyazaki, Chem. Rev., 2004, 104, 5449.
the PROXYL moiety, suggesting that this part of the salt dominates
the Curie–Weiss term. It is also likely that the Weiss constant of
ꢀ1.05 K is caused by some small antiferromagnetic interaction
between the PROXYL moieties. Actually, the structure shows that
the anions form spin dimers with a short >N–O/O–N< contact.
The deviation in the high temperature region (ii) may be a contribu-
tion from the ET cations. To obtain the temperature dependence of
the contribution, we subtracted the Curie–Weiss term from the total
data. The resultant magnetic susceptibility curve is shown in Fig. 5b.
The magnetic susceptibility at 300 K is 3 ꢁ 10ꢀ4 emu molꢀ1 which
compares well to the Pauli paramagnetic susceptibility of usual
organic metallic conductors (2–6 ꢁ 10ꢀ4 emu molꢀ1).3 This is
consistent with the metallic transport property of the salt at room
temperature. The magnetic susceptibility gradually decreases with
decreasing temperature to 0–1 ꢁ 10ꢀ4 emu molꢀ1. Since the absolute
susceptibility is proportional to the density of states (DOS), the
gradual decrease corresponds to a decrease in DOS, which is
consistent with the broad MI transition. The upturn below 120 K
suggests the transition is not caused by a density wave but is due to
charge ordering, which is also common to b00-type salts.12,13 We have
therefore not found any evidence of an interaction between the
localised spins and the conduction electrons.
7 T. Sugano, T. Fukasaka and M. Kinoshita, Synth. Met., 1991, 41–43,
3281; E. Fujiwara, S. Aonuma, H. Fujiwara, T. Sugimoto and
Y. Misaki, Chem. Lett., 2008, 37, 84; S. Otsubo, H. Cui, H.-J. Lee,
H. Fujiwara, K. Takahashi, Y. Okano and H. Kobayashi, Chem.
Lett., 2006, 35, 130; M. M. Matsushita, H. Kawakami, Y. Kawada
and T. Sugawara, Chem. Lett., 2007, 36, 110; T. Sugimoto,
S. Yamaga, M. Nakai, K. Ohmori, M. Tsujii, H. Nakatsuji and
H. Hosoito, Chem. Lett., 1993, 1361; K. A. Hutchison, G. Srdanov,
R. Menon, J.-C. P. Gabriel, B. Knight and F. Wudl, J. Am. Chem.
Soc., 1996, 118, 13081; S. Nakatsuji and H. Anzai, J. Mater. Chem.,
1997, 7, 2161; Y. Nakamura, N. Koga and H. Iwamura, Chem. Lett.,
1991, 69; K. Mukai, N. Senba, T. Hatanaka, H. Minakuchi, K. Ohara,
M. Taniguchi, Y. Misaki, Y. Hosokoshi, K. Inoue and N. Azuma,
Inorg. Chem., 2004, 43, 566; S. Aonuma, H. Casellas, C. Faulmann,
B. Garreau de Bonneval, I. Malfant, P. Cassoux, P. G. Lacroix,
Y. Hosokoshi and K. Inoue, J. Mater. Chem., 2001, 11, 337.
8 H. Akutsu, J. Yamada and S. Nakatsuji, Chem. Lett., 2001, 208;
H. Akutsu, J. Yamada and S. Nakatsuji, Synth. Met., 2001, 120,
871; H. Akutsu, J. Yamada and S. Nakatsuji, Acta Crystallogr.,
Sect. C, 2003, 59, o433; H. Akutsu, J. Yamada and S. Nakatsuji,
Acta Crystallogr., Sect. E, 2003, 59, o1141; H. Akutsu, J. Yamada
and S. Nakatsuji, Chem. Lett., 2003, 32, 1118; H. Akutsu,
J. Yamada and S. Nakatsuji, Synth. Met., 2005, 152, 377;
H. Akutsu, K. Masaki, K. Mori, J. Yamada and S. Nakatsuji,
Polyhedron, 2005, 24, 2126; A. Yamashita, H. Akutsu, J. Yamada
and S. Nakatsuji, Polyhedron, 2005, 24, 2796; H. Akutsu,
J. Yamada, S. Nakatsuji and S. S. Turner, Solid State Commun.,
2006, 140, 256.
The electrical resistivity under pressure up to 15 kbar was
measured using a clamp-type pressure cell (Fig. 6), in order to expand
the metallic region. However, the resistivity under static pressure has
no metallic regions and the activation energies increase with
increasing pressure as shown in the inset of Fig. 6. Uniaxial stress may
expand the metallic region.
9 R. N. Shibaeva, Zh. Strukt. Khim., 1975, 16, 330.
10 P. Guionneau, C. J. Kepert, G. Bravic, D. Chasseau, M. R. Truter,
M. Kurmoo and P. Day, Synth. Met., 1997, 86, 1973.
11 T. Mori, Energy band calculation software package. See http://
www.op.titech.ac.jp/lab/mori/lib/program.html.
12 T. Mori, Bull. Chem. Soc. Jpn., 1998, 71, 2509.
13 T. Yamamoto, H. M. Yamamoto, R. Kato, M. Uruichi, K. Yakushi,
H. Akutsu, A. Sato-Akutsu, A. Kawamoto, S. S. Turner and P. Day,
Phys. Rev. B, 2008, 77, 205120.
In conclusion, the smallest purely organic paramagnetic anion
provides the first organic paramagnetic metal containing the
aminoxyl radical. The salt shows metallic behaviour down to 210 K,
followed by a broad metal–insulator transition.
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 3313–3315 | 3315