New organic metal κ-(BETS)4Hg3Cl8

Synthetic Metals 139 (2003) 535–538

New organic metal ␬-(BETS)4 Hg3Cl8 E.I. Zhilyaeva a,∗ , O.A. Bogdanova a , V.V. Gritsenko a , O.A. Dyachenko a , R.B. Lyubovskii a , K.V. Van b , A. Kobayashi c , H. Kobayashi d , R.N. Lyubovskaya a a

Institute of Problems of Chemical Physics, Russian Academy of Sciences at Chernogolovka RAS, Chernogolovka, Moscow Region, MD 142432, Russia b Institute of Mineralogy, Russian Academy of Sciences, Chernogolovka, MD 142432, Russia c Spectrochemical Centre, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan d Institute for Molecular Science, Okazaki 444, Japan Received 6 April 2003; accepted 16 May 2003

Abstract New bis(ethylenedithio)tetraselenafulvalene (BETS) based radical cation salts with chloromercurate anions have been prepared by electrocrystallization. The salt (BETS)4 Hg3 Cl8 is metallic down to 32 K, at lower temperatures the resistance grows smoothly being lower than the starting value. The salt (BETS)4 Hg2 Cl6 (C2 H5 Cl)x , x ∼ 1, undergoes a metal-to-insulator transition near 100 K. The crystal structure of (BETS)4 Hg3 Cl8 is formed by the layers of the BETS radical cations of the ␬-type packing and inorganic layers composed of the [Hg2 Cl6 ]2− anions and the HgCl2 molecules. © 2003 Elsevier B.V. All rights reserved. Keywords: Organic conductors based on radical cation salts; Electrical conductivity; X-ray diffraction

1. Introduction Mercury halides have been found to form anions of various compositions and structures in radical cation salts of bis(ethylenedithio)tetrathiafulvalene (ET) (see [1] and references therein), which, in turn, affect the structure of conducting layers formed by the ET molecules and, therefore, define electroconducting properties of the compounds. In particular, the radical cation salts (ET)4 Hg2.89 Br8 and (ET)4 Hg2.78 Cl8 have polymeric chains of the anions and the ␬-type conducting layers, and undergo a superconducting transition at Tc = 4.3 K [2,3] and 1.8 K/12 kbar [3,4], respectively. Therefore, it was interesting to synthesize and study radical cation salts with halogenomercurate anions based on the selenium containing ET analogue, bis(ethylenedithio)tetraselenafulvalene (BETS). S

S

S

S

S

Se

Se

S

Se

S

S

Se

S

S

S

S

S

Se

Se

S

Se

S

S

Se

ET

BETS

BEDSe

Earlier several BETS bromomercurate phases were found [5,6], Shubnikov-de-Haas oscillations being ob∗ Corresponding author. Fax: +7-096-515-35-88. E-mail address: [email protected] (E.I. Zhilyaeva).

0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00355-2

served in ␪-(BETS)4 HgBr4 (C6 H5 Cl) [6,7], and a superconducting transition at Tc ∼ 2 K being revealed in ␬-(BETS)4 Hg2.84 Br8 [6]. The BETS salt with the iodomercurate anion, ␪-(BETS)4 Hg3 I8 , is composed of the ␪-type conducting organic layers, the [Hg2 I6 ]2− anions, and the HgI2 molecules, and shows the M–I transition at 95 K [8]. In this paper, we report on the synthesis of new BETS radical cation salts with chloromercurate anions, ␬-(BETS)4 Hg3 Cl8 , (BETS)4 Hg2 Cl6 (C6 H5 Cl)x , x ∼ 1, and (BETS)2 Hg3 Cl7 , their electrical conductivities and the crystal structure of (BETS)4 Hg3 Cl8 .

2. Experimental BETS was synthesized according to the procedure described in [9]. (n-Bu4 N)HgCl3 was prepared as described in [10]. (BETS)4 Hg3 Cl8 was isolated as an individual compound by electrochemical oxidation of BETS (2.7 mM) in a tetrahydrofuran (THF) solution of 2.3 mM (n-Bu4 N)HgCl3 and 23 mM HgCl2 at 40 ◦ C and a current of 0.2 ␮A for 2 weeks. Chem. Anal. Found: C, 15.86; H, 0.97. Calc. for C40 H32 S16 Hg3 Cl8 : C, 15.13; H, 1.02%. The crystal grown in this manner were unusable for the X-ray analysis. Good quality, black shiny rhombs of ␬-(BETS)4 Hg3 Cl8 together with elongated plates of the other phase were obtained by

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E.I. Zhilyaeva et al. / Synthetic Metals 139 (2003) 535–538

electrocrystallization at 50 ◦ C, using a chlorobenzene solution of 3 mM BETS, 2 mM (n-Bu4 N)HgCl3 , 0.22 mM HgCl2 and a constant current of 0.07 ␮A applied for 45 days, HgCl2 being added to the cathode compartment. The stoichiometry of different phases of the crystals was determined by the electron probe microanalysis (EPMA). The crystals of (BEDSe)2 Hg2 Br6 and (BEDSe)3 HgCl4 prepared according to [11,12], respectively, and comprising the BEDSe molecule, an isomer of BETS, were used as reference compounds. EPMA showed S:Hg:Cl = 16:2.9:8.1 for rhombs that afforded the (BETS)4 Hg3 Cl8 composition. For elongated plates EPMA revealed S:Hg:Cl = 16:2:7 for freshly prepared samples and 16:2:(6–7) for the stored ones. Chem. Anal. Found for stored samples: C, 17.06; H, 2.83. Calc. for C20 H16 S8 Se8 HgCl3 ((BETS)2 HgCl3 ): C, 16.55; H, 1.11%. These results allowed us to assume the (BETS)4 Hg2 Cl6 (C6 H5 Cl)x composition, x ∼ 0–1. (BETS)2 Hg3 Cl7 was prepared from a solution containing 3 mM of BETS, 2 mM of (n-Bu4 N)HgCl3 and 4 mM of HgCl2 in chlorobenzene at 49 ◦ C and a current of 0.1 ␮A. EPMA showed S:Hg:Cl = 8:3:6.8. Chem. Anal. Found: C, 12.47; H, 1.16. Calc. for C20 H16 S8 Se8 Hg3 Cl7 : C, 12.03; H, 0.80%. Conductivity of single crystals was measured by a standard dc four-probe technique. The main crystallographic data for ␬-(BETS)4 Hg3 Cl8 are: C40 H32 Cl8 Hg3 S16 Se16 , M = 3173.18, monoclinic, a = 38.23(4) Å, b = 8.694(6) Å, c = 11.37(1) Å, β = 106.55(9)◦ , V = 3623.8(6) Å3 (the crystallographic parameters were refined by the least-squares method using 25 strong automatically centered reflections at 8◦ < 2θ < 32◦ , λ = 0.70926 Å), space group C2, Z = 2, dCalc. = 2.907 g/cm3 , F(000) = 2896; µ(Mo K␣) = 10.895 mm−1 . The maximum and minimum transmission factors were 0.32 and 0.12, respectively. The quality of the experiments was controlled using two standard reflections, whose intensities were measured in every 50 reflections. These two check reflections remained constant within accuracy during the whole experiment. X-ray experimental data for the single crystal (0.31 mm × 0.28 mm × 0.11 mm) were collected at room temperature on an automatic four-circle KM-4 diffractometer (KUMA, Poland), ω/2θ scanning, scan width ω = 0.64 + 0.35 tan q, scan rate ω = 2.4–6.6◦ min−1 , Mo K␣ radiation with a graphite monochromator; 5579 reflections was measured among which 1783 with the intensities I > 2σ(I) were used to find a model and refine the structure, (2θ)max = 60.16◦ ; the interval of h, k, l variation was: −49 ≤ h ≤ 49, 0 ≤ k ≤ 12, 0 ≤ l ≤ 15. X-ray absorption in the crystal was taken into account on the final stage of refinement using the SHELX-76 considering crystal faceting. The crystal structure of (BETS)4 Hg3 Cl8 was solved by direct methods followed by the Fourier syntheses using the SHELX-86 [13] and SHELXL-93 [14] programs. The Hg, Cl, and Se atoms were refined by the least-squares method in an anisotropic approximation. The S and C atoms were

refined in an isotropic approximation using F. The absolute structure parameter was 0.03(6). Hydrogen atoms were not localized. The final value of the R-factor was 0.088 for 1091 F > 4s(F). The maximal shift/ESD was 0.055 in the last cycle of the refinement. Residual electron density in the final difference synthesis δ = 2.106 and −1.356 e Å−3 .

3. Results and discussion To synthesize BETS chloromercurates we used (n-Bu4 N) HgCl3 as a supporting electrolyte. Electrochemical oxidation of BETS in chlorobenzene at room temperature yields (BETS)4 Hg2 Cl6 (C6 H5 Cl)x , x ∼ 1. The temperature of 50 ◦ C provides the mixture of the crystals of two compounds, namely, (BETS)4 Hg2 Cl6 (C6 H5 Cl)x (elongated thin plates of 5–7 mm length) and ␬-(BETS)4 Hg3 Cl8 (rhombic plates 0.4 mm × 0.4 mm × 0.1 mm in size). The addition of small amounts of HgCl2 to the electrolyte results in the increased share of ␬-(BETS)4 Hg3 Cl8 in the electrocrystallization product. Larger quantities of HgCl2 provide the formation of a mixture of the (BETS)4 Hg2 Cl6 (C6 H5 Cl)x salt and a new phase, namely, (BETS)2 Hg3 Cl7 , the latter being produced in an individual state at the HgCl2 /(n-Bu4 N)HgCl3 ratio equal to 2:1. ␬-(BETS)4 Hg3 Cl8 can be isolated as the only phase by using THF, the crystals being of unsatisfactory quality. Finally, the better quality of the ␬-(BETS)4 Hg3 Cl8 crystals has been attained in chlorobenzene at 50 ◦ C and low current density. To determine the composition of chloromercurates and identify the phases we used EPMA, which allows a precise estimation of the S:Hg:Cl ratio for every crystal with respect to a reference compound. The (BETS)4 Hg3 Cl8 composition found from EPMA is the same as that derived from the X-ray analysis. The comparison of the content of chlorine in the freshly prepared samples and those stored for a long time revealed its decreasing under storage and allowed a conclusion on the presence of the solvent in (BETS)4 Hg2 Cl6 (C6 H5 Cl)x . The crystal structure of ␬-(BETS)4 Hg3 Cl8 is presented in Fig. 1. It is characterized by an alternation of the BETS radical cation layers parallel to the bc plane and the chloromercurate layers along the a axis. The structure of the radical cation layer is shown in Fig. 2. The radical cations form nearly orthogonal dimers of the ␬-type. Each dimeric unit of the donor molecule has a “double bond over-ring” arrangement similarly to that in the other BETS based salts of the ␬-type packing [15,16]. A number of Se · · · Se contacts inside the dimers shorter than the sum of van der Waals radii has been found in the structure of ␬-(BETS)4 Hg3 Cl8 . The shortened intermolecular Se · · · Se, Se · · · S, and S · · · S contacts have also been found between the radical cations from the neighboring dimers. In the chloromercurate layers, the [Hg2 Cl6 ]2− anions alternate with neutral HgCl2 molecules along the b axis of the crystal. Mercury atoms in the [Hg2 Cl6 ]2− dimer have a distorted tetrahedral configuration of the Hg–Cl bonds. Abnor-

E.I. Zhilyaeva et al. / Synthetic Metals 139 (2003) 535–538

537

Fig. 1. Crystal structure of ␬-(BETS)4 Hg3 Cl8 .

1.0

R(T)/Ro

0.8

3

1

0.6

2

0.4 0.2 0

50

100

150

200

250

300

T, K

mally great thermal vibrations of the Hg atoms (Fig. 3) indicate their statistical disordering. A similar structure of the anion sheets in which the HgX2 molecules alternate with the [Hg2 X6 ]2− anions was found earlier in ␬-(d8 -ET)4 Hg3 Br8 [17] and ␪-(BETS)4 Hg3 I8 [8]. It is of interest to compare the structures of ␬-(BETS)4 Hg3 Cl8 and ␬-(ET)4 Hg2.78 Cl8 [3]. The features of the ET chloromercurate are a complex structure involving incommensurate Hg sublattice and a nonstoichiometric amount of mercury [1,3]. The volume of the unit cell of ␬-(BETS)4 Hg3 Cl8 is somewhat greater than that of ␬-(ET)4 Hg2.78 Cl8 [3,18]. This is associated with a larger size of the BETS donor molecule. Probably the expansion of the cation layers provided a stoichiometric amount of mercury to be involved in the unit cell. Both salts have a ␬-type packing motif for donor molecules. The mode of intermolecular overlap in dimeric units is the same, of a “double bond over-ring” type. There are shortened chalcogen · · · chalcogen contacts between the neighboring dimers in the both salts. The main distinction consists in the

fact that the ET-based salt has no shortened S · · · S contacts inside the dimer [3] in contrast to ␬-(BETS)4 Hg3 Cl8 , where a number of shortened intradimer Se · · · Se contacts have been found. Figs. 4 and 5 show the temperature dependencies of relative electrical resistance of single crystals for ␬-(BETS)4 Hg3 Cl8 and (BETS)4 Hg2 Cl6 (C6 H5 Cl)x from 298 K down to liquid helium temperatures. The temperaturedependent resistance curves for the sulfur-containing analogues, ␬-(ET)4 Hg2.78 Cl8 and ␤

-(ET)4 Hg2 Cl6 (C6 H5 Cl) prepared from the authors original data, cited in [1] are also presented for comparison.

2.0 1.6

R(T)/Ro

Fig. 2. Structure of the cation layer of ␬-(BETS)4 Hg3 Cl8 .

Fig. 4. Temperature dependency of relative electrical resistance: (1) for ␬-(BETS)4 Hg3 Cl8 ; (2) for ␬-(ET)4 Hg2.78 Cl8 at ambient pressure; (3) for ␬-(ET)4 Hg2.78 Cl8 at p = 9 kbar.

1.2

1 0.8

2 0.4 0.0 0

50

100

150

200

250

T, K Fig. 3. Structure of the anion chain in ␬-(BETS)4 Hg3 Cl8 .

Fig. 5. Temperature dependency of relative electrical resistance: (1) for (BETS)4 Hg2 Cl6 (C6 H5 Cl)x ; (2) for ␤

-(ET)4 Hg2 Cl6 (C6 H5 Cl).

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The resistance of ␬-(BETS)4 Hg3 Cl8 decreases with the temperature decrease down to ∼32 K. Several small jumps in the resistance appear probably as a result of cracks in the crystal. At lower temperatures the resistance grows smoothly being less than the room temperature value. Unlike ␬-(BETS)4 Hg3 Cl8 , ␬-(ET)4 Hg2.78 Cl8 reveals the stepless resistance decrease down to ∼5 K and an anomaly of the electrical conductivity below 80 K, which may be suppressed at high pressure [1,3,4]. At p = 9 kbar the resistance curve for ␬-(ET)4 Hg2.78 Cl8 shows no anomaly. The resistance minimum near 5 K at ambient pressure is shifted to ∼ 25 K with the pressure increase up to 9 kbar [1,3,4]. In fact, the temperature dependence of resistance of ␬-(BETS)4 Hg3 Cl8 is similar to that of ␬-(ET)4 Hg2.78 Cl8 at applied pressure of 9 kbar. The similarity of the resistance curves for ␬-(BETS)4 Hg3 Cl8 and ␬-(ET)4 Hg2.78 Cl8 at p = 9 kbar together with the shortened intradimer chalcogen–chalcogen contacts in ␬-(BETS)4 Hg3 Cl8 compared to its absence in ␬-(ET)4 Hg2.78 Cl8 could indicate that the crossover from ET to BETS in the ␬-chloromercurate phase is equivalent to the application of approximately 9 kbar pressure to ␬-(ET)4 Hg2.78 Cl8 . Taking into account that pressure of 12 kbar resulted in the appearance of superconductivity in ␬-(ET)4 Hg2.78 Cl8 [1,3,4], one may conclude that ␬-(BETS)4 Hg3 Cl8 is a promising object for further investigation at applied pressure. For the second chloromercurate phase, the resistance of (BETS)4 Hg2 Cl6 (C6 H5 Cl)x slightly decreases down to 100 K. Below this temperature the resistance sharply increases with the temperature decrease undergoing a metal-insulator transition. On the contrary, for the ET based salt of similar composition, ␤

-(ET)4 Hg2 Cl6 (C6 H5 Cl), the resistance decreases linearly down to 1.3 K. Thus, in this case the crossover from ET to BETS affects the transport properties destabilizing the metallic state in (BETS)4 Hg2 Cl6 (C6 H5 Cl)x as compared to ␤

-(ET)4 Hg2 Cl6 (C6 H5 Cl). This can be associated with a possible difference in crystal structures. We failed to measure conductivity of the third chloromercurate BETS-phase, (BETS)2 Hg3 Cl7 , because of poor quality of the crystals. In a conclusion, the substitution of the sulfur atoms for selenium ones in the five-membered rings of the ET molecules in the ␬-chloromercurate salt resulted in both the expansion of the unit cell and the formation of shortened chalcogen · · · chalcogen contacts inside the dimers composed of the radical cations. The former resulted in the stoichiometric composition and the latter caused the same changes in the character of the temperature dependency of the resistance, which had earlier been caused by 9 kbar pressure. The studies of conduc-

tivity of (BETS)4 Hg3 Cl8 under pressure will be carried out.

Acknowledgements This work was supported by Russian Foundation for Basic Research, Projects nos. 01-03-33009 and 00-03-32809.

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