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Synthetic Metals 161 (2011) 365–368

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Synthetic Metals

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ynthesis, structure and conductivity of molecular conductor-(BEDT-TTF)(HSO4)

ong-Yu Chena,∗, Qi Fangb, Ping Lia, Wen-Tao Yub, Cui-Ying Xuc, Jin-Biao Zhangc, De-Qing Zhangc

School of Chemistry and Chemical Engineering, TaiShan Medical University, 271016, ChinaState Key Laboratory of Crystal Materials, Shandong University, 250100, ChinaInstitute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China

r t i c l e i n f o

rticle history:eceived 10 August 2010eceived in revised form0 November 2010ccepted 24 November 2010

a b s t r a c t

A new BEDT-TTF-based radical salt, ˇ-(BEDT-TTF)(HSO4) [BEDT-TTF = bis(ethylenedithio)tetrathiaful-valene], has been synthesized by oxidative electrocrystallization. The crystal structure was determinedby four-circle X-ray diffraction. The crystal belongs to monoclinic system, C2/c space group withthe unit cell parameters of: a = 1.5996(2) nm, b = 1.06173(10) nm, c = 1.15083(11) nm, ˇ = 120.534(8)◦,V = 1.6834(3) nm3, and R = 0.0611. In the title crystal, the BEDT-TTF radicals are stacked to form columns

vailable online 31 January 2011

eywords:olecular conductor

EDT-TTF

along the [1 0 1] axis and the molecular planes of BEDT-TTFs in adjacent columns are parallel to eachother. The inter-stack side-by-side S. . .S short contacts along the b-axis help to form a kind of cationicconducting sheet parallel to (1 0 1) plane. The room temperature conductivity at certain direction on(1 0 1) plane of ˇ-(BEDT-TTF)(HSO4) single crystal was measured to be 11.7 S cm−1. From 100 K to 290 K,the resistivity–temperature curve demonstrates its typical semiconductivity with an excitation energy

onductivity

tructureharge-transfer salts

of 0.26 eV.

. Introduction

BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene, also abbrevi-ted as ET] has been intensively studied as a most promisinglectro-donor in molecular conductors [1]. More attention haseen paid to it owing to the structural and conductivity diver-ity of charger-transfer complexes. Through electrocrystallization,EDT-TTF can form various structural types of charge-transfer com-lexes with different anions. For its structural diversity, theseharge transfer complexes electrical properties span a wide rangerom insulator to semiconductor to superconductor. About half of

olecular superconductors are based on BEDT-TTF and the highestuperconducting temperature (Tc) to date of these salts are 11.6 Kat ambient temperature) in �-(ET)2Cu[N(CN)2]Br [2] and 14.2 K (at2 Kbar applied pressure) in ˇ′-(ET)2ICl2 [3], which make it the bestystem to probe the relation between substance electrical conduc-ivity and its structure. So the interest in this donor is still rising,nd works on creating new organic conductors are being activelytudied. The synthesis of new conductors based on ET, as a rule, is

ither through searching of new counterions to donors, or throughhanging synthetic condition with same counterion to obtain dif-erent structure. The different structures constructed by the sameonors and counterions are more meaningful for the investiga-

∗ Corresponding author.E-mail address: binboll@126.com (H.-Y. Chen).

379-6779/$ – see front matter © 2011 Published by Elsevier B.V.oi:10.1016/j.synthmet.2010.11.035

© 2011 Published by Elsevier B.V.

tion of the relationship between the structure and conductivity orsuperconductivity.

Oxygen-containing tetrahedral anions such as ClO4−, BrO4

−,ReO4

− are among the earliest species used, owing to the realizationof ambient pressure superconductivity in (TMTSF)2ClO4. Molecularconductor based on ET with HSO4

− anions, �-(BEDT-TTF)3(HSO4)2,was described earlier in [4,5]. This paper reports a new molecularconductor with HSO4

− anions, ˇ-(BEDT-TTF)(HSO4), investigationof its crystal structure and conducting properties.

2. Experimental

2.1. Synthesis and crystal growth

BEDT-TTF was prepared according to the previous report [6] andrecrystallized twice from chloroform prior to use. MoS4(NEt4)2 wassynthesized according to the literature [7]. The black plate crys-tals were obtained by electrochemical oxidation of BEDT-TTF inthe presence of MoS4(NEt4)2 in 1,1,2-trichloroethane and a littleof water after 40 days. The anion HSO4

1− instead of MoS42− was

derived from the solvent of 1,1,2-trichloroethane.

2.2. Structure characterization

The crystal structure was determined by Brucker P4 four-circle diffractometer with Mo K� radiation (� = 0.071073 nm) thatwas diffracted by a black plate crystal with the dimension of

366 H.-Y. Chen et al. / Synthetic Metals 161 (2011) 365–368

Fig. 1. Molecular structure of title compound. Symmetry code: a (−x + 1, −y + 1, −z); b (−x, y, 1/2 − z).

0 1] di

0ωhtdwHHv

Fd

Fig. 2. A packing diagram of the title crystal viewed along [1

.38 mm × 0.20mm × 0.10 mm. Intensities were measured by the− 2� scan technique, and in the range of 2.42◦ ≤ � ≤ 28.00◦,= −1 → 21, k = −1 → 14, l = −15 → 13, 2490 independent reflec-

ion with I > 3� were collected. The structure was resolved by

irect method with SHELEX-97 programs, and 123 parametersere refined by full-matrix least-squares method, of which non-atoms were refined with anisotropic temperature factors and-atoms with isotropic temperature factors. The refinement con-erged to R = 0.0611. CCDC 787883 contains the supplementary

ig. 3. The b-axis view of the packing diagram of the title crystal (the BEDT-TTF molecisorderd half occupied tetrahedral hydrosulfate anions have been shown).

rections (corresponding HSO4− has been omitted for clarity).

crystallographic data for this paper. These data can be obtainedfree of charge from the Cambridge Crystallographic Data Center viawww.ccdc.cam.ac.uk/data request/cif.

2.3. Conductivity characterization

Variable temperature electrical conductivity was measured overthe range of 120–290 K by the standard four-probe method, wherefour electrodes on crystal were made directly by gold wires with

ules located on the y = 0.5 layer have been deleted for clarity and only one set of

H.-Y. Chen et al. / Synthetic Metals 161 (2011) 365–368 367

F t contb 2 − x,

g0taaic

3

tMiaao

teˇ

Ffi

ig. 4. Connection modes of ET cation with four neighbor molecules by S. . .S shoreen shown). Symmetry code: a: (1 − x, 1 − y, −z); b: (1/2 − x, 1/2 + y, 1/2 − z); c: (1/

old paint. The spacing between neighboring wires were 0.15 mm,.25 mm, 0.15 mm respectively (see insert figure in Fig. 5). Thewo wires in the middle (wire 2 and wire 3) were connected toKEITHLEY-2001 digital voltmeter and the other two wires (wire 1nd wire 4) to a KEITHLEY-220 programmable current source. Dur-ng the measurement (fixed or varying temperature), the electricalurrent was kept constant 1 �A.

. Result and discussion

The BEDT-TTF hydrosulfate salt were obtained during attemptso synthesize molecular conductor based on the BEDT-TTF and the

oS42− anion, strategies to promote S. . .S interactions include the

ncorporation of sulfur atoms in the periphery of the compensatingnions or the use of cluster sulfide complexes derived from MoS4

2−

nion. The hydrosulfate anions is believed to form after oxidation

f S2− cleavage of MoS4

2− anion.The structure of the title crystal is shown in Fig. 1. It belongs

o monoclinic system, C2/c space group with the unit cell param-ters of: a = 1.5996(2) nm, b = 1.06173(10) nm, c = 1.15083(11) nm,= 120.534(8)◦, V = 1.6834(3) nm3. The asymmetric unit contains

ig. 5. The resistance (�)–temperature (T) curve (warming process) of single crystal ˇ-(xed.

acts (only one set of disorderd half occupied tetrahedral hydrosulfate anions have−1/2 + y, 1/2 − z); d: (1/2 + x, 1/2 − y, −1/2 + z).

half an independent BEDT-TTF cation and half a HSO4− anion. The

BEDT-TTF cations are arranged face to face in crystallographicallyequivalent columns along the [1 0 1] axis [Fig. 2]. In the column,the donor take up ROB (ring over bond) [8] packing motif withthe distance between up–down the overlap BEDT-TTF least-squareplanes being 0.38060 nm. The molecular planes of BEDT-TTFs inadjacent columns are parallel to each other [Fig. 3], where this pack-ing motif is generally referred as ˇ-type [7]. Inside the columnsof BEDT-TTF, there are no S. . .S contacts smaller than the sum ofthe van der Waals radii, while there is a considerable amount ofshort interstack side-by-side S. . .S contacts smaller than the sumof the van der Waals radii between the outermost two pairs Satoms of ET molecules in adjacent stacks. Every BEDT-TTF cationis netted by four surrounding BEDT-TTF cations through S. . .S con-tacts (3.484(2) A) [Fig. 4]. This kind of contacts is known to bethe most characteristic feature of the structure of BEDT-TTF-based

quasi-2D metals. The HSO4

− anions are located disorderly on a two-fold axis (Fig. 1). The atom O1, O2, O1′b, O2′b (symmetry code b:−x, y, −1/2−z) rotate around S5 with O1′, O2′, O1b, O2b showingtetrahedral configurations (shown in Figs. 3 and 4), where theiroccupancies are equal to be 0.50.

BEDT-TTF)(HSO4). The insert figure is for the crystal sample with four gold wires

368 H.-Y. Chen et al. / Synthetic Me

Table 1Selected bonds lengths( ´A) for the complexes [symmetry code: a (−x + 1, −y + 1, −z)].

S(1)–C(3) 1.716(4) S(1)–C(4) 1.738(4)S(2)–C(3) 1.728(4) S(2)–C(5) 1.734(4)

Bifitoctab

dr2wllcsietaooeCedlC

S(3)–C(5) 1.738(4) S(3)–C(2) 1.814(6)S(4)–C(4) 1.728(4) S(4)–C(1) 1.791(6)C(5)–C(4) 1.368(7) C(3)–C(3)a 1.383(9)

The distribution of the bond lengths (shown in Table 1) in theEDT-TTF radical cations suggests that the donor in the title crystal

s a radical cation with a charge of +1 [9,10]. It is primarily con-rmed by the elongation of the central C C bond in the cation upo 0.1383(9) nm, which is consistent with a donor oxidation statef +1 in tetrathiafulvalene-based molecules. So, according to theharge distribution on the BEDT-TTF molecules, we can confirmhe counter anion is HSO4

− anion instead of SO42− anion, where H

tom is difficult to locate by X-ray diffraction and also is difficult toe confirmed by elemental analysis for its small quantity.

The measured room temperature conductivity along the longestimension of the crystal is �0 = 11.7 S cm−1 (corresponding to aesistance of 8.5 × 10−2 cm). The crystal sample was cooled from90 K to 100 K and then naturally warmed back to 290 K [Fig. 5]. Thearming process was slower and closer to thermodynamic equi-

ibrium in comparison with the cooling process. An approximatelyinear dependence between ln (R/R0) and T−1 (its linearly fittingoefficient is 0.9641) demonstrate that the dependence of � on T isubject with Arrhenius law, thus we can conclude the title crystals a typical semiconductor from 100 K to 290 K with an excitationnergy of 0.26 eV. There was no evidence for any discontinuities inhe conductivity or changes of activation energy over this temper-ture range. According to structure analysis of the title crystal, thebserved bond values indicate that the donor molecule has a degreef ionicity � = +1. Extended-Hückel calculations [11] show the high-st occupied molecular orbital of ET as bonding with respect to the

C bonds and antibonding with respect to the C–S bonds. So, theffect of the positive electrical charge residing on the ET moleculeiminishes the bonding character of the C C bond (resulting in a

engthening of the C C bond) and the antibonding character of the–S bond(resulting in a shortening of the C–S bonds). It is likely

[

[

tals 161 (2011) 365–368

that the absence of close stacks of cations in the structure is aresult of relatively large coulumbic repulsion between the highlycharged cations. In contrast to �-(BEDT-TTF)3(HSO4)2 [5], whichshows metallic conductivity with a metal–insulator transition at127 K, the more highly charged +1 cations account for no evidencefor metallic behavior in the title crystal.

4. Conclusion

We here reported a new BEDT-TTF based charge transfer salt ˇ-(BEDT-TTF)(HSO4). Its single crystal structure was determined byfour-circle diffractometer. Variable temperature electrical conduc-tivity curve shows it is a semiconductor with an excitation energyof 0.26 eV from 100 K to 290 K. Structure analysis shows that theET molecules in the crystal are fully ionized, which it is expected toaccount for no evidence for metallic behavior in the title salt.

Acknowledgements

We gratefully acknowledge the financial support of the NationalNatural Science Foundation of China (No. 50673054) and the Foun-dation of Key Laboratory of Organic Solide Institute of ChineseAcademy of Science.

References

[1] T. Ishiguro, K. Yamaji, Organic Conductors, 2nd edition, Springer, Berlin, 1998.[2] A.M. Kini, U. Geiser, H.H. Wang, K.D. Carlson, J.M. Williams, W.K. Kwok, K.G.

Vandervoort, J.E. Thompson, D.L. Stupka, D. Jung, M.-H. Whangbo, Inorg. Chem.29 (1990) 2555–2557.

[3] H. Mori, J. Phys. Soc. Jpn. 75 (2006) 51003 (1–15).[4] L.C. Porter, H.H. Wang, M.M. Miller, J.M. Williams, Acta Cryst. C43 (1987) 2201.[5] A. Miyazaki, T. Enoki, G. Saito, Synth. Met. 70 (1995) 793–794.[6] K.S. Varma, A. Bury, N.J. Harris, A.E. Underhill, Synthesis (1987) 837–838.[7] O.W. Mcdonald, G.D. Friesen, L.D. Rosenhein, W.E. Netton, Inorg. Chim. Acta 72

(1983) 205–210.[8] T. Mori, Bull. Chem. Soc. Jpn. 71 (1998) 2509–2526.

[9] P. Guionneau, C.J. Kepert, G. Bravic, D. Chasseau, M.R. Truter, M. Kurmoo, P. Day,

Synth. Met. 86 (1997) 1973–1974.10] L.-K. Chou, M.A. Quijada, M.B. Clevenger, G.F. de Oliveira, K.A. Abboud, D.B.

Tanner, D.R. Talham, Chem. Mater. 7 (3) (1995) 530–534.11] T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito, H. Inokuchi, Bull. Chem.

Soc. Jpn. 57 (1984) 627–633.