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The low-temperature crystal structure of bis­(ethyl­enedi­thio)­tetra­thia­fulvalene (BEDT-TTF), C10H8S8, is similar to its high-temperature structure. The room-temperature central bond lengths of this mol­ecule are often used as reference in empirical methods to estimate the charge carried by the (BEDT-TTF)xn+ cations in the BEDT-TTF mol­ecule-based organic conductors. We show that the method we previously reported can still be used with low-temperature BEDT-TTF salts data. Moreover, we confirm the purely thermal origin of the ordering of the ethyl­ene group.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270199016753/sk1337sup1.cif
Contains datablocks global, BEDT-TTF

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270199016753/sk1337BEDT-TTFsup2.hkl
Contains datablock BEDT-TTF

CCDC reference: 144631

Comment top

The BEDT-TTF molecule, (I), is used to build the very well known (BEDT-TTF)xn+Xn- salts which exhibit such a diversity of physical and structural properties (Williams et al., 1992). In our aim to correlate structural and conducting properties in this series we assume that knowledge of the low-temperature structural properties of the neutral molecule will be important. Indeed, one uses the room temperature intramolecular structural parameters of this molecule as a reference to analyse the intramolecular structural parameters of the corresponding cation in the corresponding family salts. \sch

In particular we developed a statistical method to give an estimation of the charge carried by the BEDT-TTF cation in such salts (Guionneau et al., 1997) from a combination of the averaged values of the four central bond lengths. This method shows the linear variation of the charge with the distance δ = (b+c)-(a+d) where b and c are the averaged values of the central S—S bond lengths while a and d are the averaged values of the central C—C bond lengths. The estimation of the charge carried, Q, in such a way was obtained with an accuracy of 10%, the equation of linear variation was found to be Q=6.347–7.463δ. The central bond lengths of the neutral BEDT-TTF molecule were used as a reference point to obtain this equation. In order to know if we could use this method with low-temperature data of BEDT-TTF salts we needed to compare the values of δ for the room temperature and a temperature of around 100 K for which a lot of structural data are available in the studied series. The averaged central bond length values are close at room temperature and 100 K: a = 1.343 (4), b = 1.756 (3), c = 1.760 (3), d = 1.333 (4) Å (Guionneau et al., 1997) and a = 1.352 (4), b = 1.763 (3), c = 1.771 (3) and d = 1.349 (4) Å, respectively. The associated δ values are so very close at room temperature [0.840 (15) Å] and 100 K [0.834 (15) Å]. The small difference between these two values (-0.006 Å) corresponds to a difference in the charge estimation (+0.04 e-) which appears lower than the accuracy of the method (0.10 e-). It also shows that the charge calculated with this method from low-temperature data could be slightly overestimated as suspected in Gaultier et al. (1999). Thus, these results seem to confirm the validity of the δ method as a rough but quick and easy tool to estimate the charge partition in BEDT-TTF salts. Such a partition has been calculated using this method and successfully explains the conducting properties of some organic conductors (Martin et al., 1999; Coronado et al., 1998). It has also been used to show the change in the degree of ionicity that can occur when cooling (Gaultier et al., 1999; Guionneau et al., 1998).

In general, one of the features of the BEDT-TTF entities is the disorder that can affect the ethylene extremities. At room temperature such a disorder often appears in BEDT-TTF molecule based salts. It was also found in neutral BEDT-TTF (Kobayashi et al., 1986; Guionneau et al., 1997). One of the commonly used estimates of the disordering is the deviation from the normal corresponding C—C bond lengths (around 1.53 Å). In the title molecule, ethylene groups (C13—C14 and C18—C19) bond lengths are much closer from the normal values at 100 K [1.518 (3) Å and 1.517 (3) Å, respectively] than at room temperature [1.467 (3) Å and 1.421 Å]. Moreover, at 100 K, the displacement parameters of the ethylene carbon atoms appear to be of the same order of magnitude as those of the other carbon atoms of the molecule at room temperature while they are three times higher at room temperature. Such a thermal ordering of the neutral BEDT-TTF should be kept in mind when trying to understand the BEDT-TTF entities intramolecular changes that may occur by cooling in the corresponding salts.

The molecular packing of BEDT-TTF at room temperature, described as based on dimers (Kobayashi et al., 1986), remains identical at 100 K. It is worth noting that this behaviour is different from that of the TTF crystals where TTF or tetrathiofulvalene corresponds to the core of the BEDT-TTF molecule, which undergo a structural transition at low temperature (Batsanov, 1998).

Experimental top

Crystals were prepared according to Mizuno et al. (1978).

Computing details top

Data collection: SMART (Siemens, 1996); cell refinement: SMART; data reduction: SAINT (Siemens, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1999).

Figures top
[Figure 1] Fig. 1. Displacement ellipsoid view (90% probability) of the BEDT-TTF molecule at 110 K. The distances discussed in the text are as follows: a is C10—C15, b is the average value of C10—S4, C10—S3, C15—S2 and C15—S1, c is the average value of C12—S4, C11—S3, C16—S1 and C17—S2, d is the average value of C11—C12 and C16—C17.
bis(ethylenedithio)tetrathiafulvalene top
Crystal data top
C10H8S8F(000) = 784
Mr = 384.64Dx = 1.794 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
a = 6.6583 (1) ÅCell parameters from 425 reflections
b = 13.733 (2) Åθ = 20.5–34.1°
c = 17.414 (1) ŵ = 1.23 mm1
β = 116.57 (1)°T = 100 K
V = 1424.1 (1) Å3Needles, orange
Z = 40.20 × 0.15 × 0.10 mm
Data collection top
SMART Siemens CCD
diffractometer
3251 independent reflections
Radiation source: fine-focus sealed tube2714 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.037
ω scansθmax = 27.4°, θmin = 2.0°
Absorption correction: empirical (using intensity measurements)
SADABS (Sheldrick, 1996)
h = 88
Tmin = 0.768, Tmax = 0.915k = 1717
12398 measured reflectionsl = 2222
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.027Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.061H-atom parameters constrained
S = 1.09 w = 1/[σ2(Fo2) + (0.0191P)2 + 1.0583P]
where P = (Fo2 + 2Fc2)/3
3251 reflections(Δ/σ)max = 0.001
163 parametersΔρmax = 0.35 e Å3
0 restraintsΔρmin = 0.36 e Å3
Crystal data top
C10H8S8V = 1424.1 (1) Å3
Mr = 384.64Z = 4
Monoclinic, P21/cMo Kα radiation
a = 6.6583 (1) ŵ = 1.23 mm1
b = 13.733 (2) ÅT = 100 K
c = 17.414 (1) Å0.20 × 0.15 × 0.10 mm
β = 116.57 (1)°
Data collection top
SMART Siemens CCD
diffractometer
3251 independent reflections
Absorption correction: empirical (using intensity measurements)
SADABS (Sheldrick, 1996)
2714 reflections with I > 2σ(I)
Tmin = 0.768, Tmax = 0.915Rint = 0.037
12398 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0270 restraints
wR(F2) = 0.061H-atom parameters constrained
S = 1.09Δρmax = 0.35 e Å3
3251 reflectionsΔρmin = 0.36 e Å3
163 parameters
Special details top

Experimental. The sample was cooled at a rate of 2 K min-1. A half sphere was collected based on ω scans at values of Φ = 0, 88, 180°. Frames were collected at 0.3° intervals and at 20 s per frame. At the end of these runs, the first 100 frames were repeated in order to check that no alteration had occurred.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
S20.21293 (8)0.85826 (4)0.04260 (3)0.01247 (12)
S60.14782 (9)0.69753 (4)0.08621 (3)0.01417 (12)
S40.30107 (8)0.98716 (4)0.21250 (3)0.01312 (11)
S50.39553 (9)0.72435 (4)0.15221 (3)0.01568 (12)
S10.27815 (8)0.88339 (4)0.02020 (3)0.01156 (11)
S30.18790 (8)1.00502 (4)0.15326 (3)0.01180 (11)
S80.42843 (9)1.05090 (4)0.39193 (3)0.01408 (12)
S70.16013 (9)1.06906 (4)0.31922 (4)0.01596 (12)
C170.0339 (3)0.77851 (14)0.03902 (13)0.0111 (4)
C100.0304 (3)0.95583 (15)0.13432 (13)0.0109 (4)
C160.1892 (3)0.79056 (14)0.06840 (13)0.0108 (4)
C110.0116 (3)1.02797 (14)0.26384 (13)0.0114 (4)
C150.0068 (3)0.90362 (14)0.06337 (13)0.0111 (4)
C130.0651 (3)1.07854 (16)0.42725 (13)0.0144 (4)
H1320.01051.11550.46300.017*
H1310.10601.01240.45190.017*
C120.2119 (3)1.02056 (14)0.29065 (13)0.0111 (4)
C140.2736 (4)1.12825 (15)0.43150 (14)0.0148 (4)
H1420.37281.14660.49170.018*
H1410.22891.18870.39700.018*
C190.0312 (4)0.73258 (16)0.19720 (13)0.0147 (4)
H1920.01400.80340.20300.018*
H1910.02250.69870.23470.018*
C180.2788 (4)0.71025 (18)0.22912 (14)0.0199 (5)
H1820.30510.64230.25030.024*
H1810.36490.75300.27890.024*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
S20.0094 (2)0.0162 (2)0.0109 (3)0.00037 (19)0.0037 (2)0.00320 (19)
S60.0153 (3)0.0143 (2)0.0134 (3)0.00399 (19)0.0069 (2)0.00112 (19)
S40.0093 (2)0.0206 (3)0.0093 (2)0.00016 (19)0.0040 (2)0.0039 (2)
S50.0146 (3)0.0202 (3)0.0132 (3)0.0070 (2)0.0070 (2)0.0061 (2)
S10.0092 (2)0.0139 (2)0.0100 (2)0.00088 (18)0.0029 (2)0.00227 (19)
S30.0089 (2)0.0164 (2)0.0088 (2)0.00107 (19)0.00284 (19)0.0018 (2)
S80.0103 (2)0.0221 (3)0.0083 (3)0.0002 (2)0.0028 (2)0.0036 (2)
S70.0115 (2)0.0251 (3)0.0131 (3)0.0016 (2)0.0071 (2)0.0061 (2)
C170.0145 (10)0.0103 (9)0.0091 (10)0.0006 (8)0.0059 (8)0.0011 (7)
C100.0075 (9)0.0130 (9)0.0103 (10)0.0003 (7)0.0023 (8)0.0008 (8)
C160.0146 (10)0.0099 (9)0.0084 (10)0.0004 (8)0.0054 (8)0.0018 (8)
C110.0139 (10)0.0115 (9)0.0087 (10)0.0008 (8)0.0051 (8)0.0017 (8)
C150.0095 (9)0.0117 (9)0.0118 (10)0.0012 (7)0.0046 (8)0.0006 (8)
C130.0156 (10)0.0185 (10)0.0117 (10)0.0004 (8)0.0085 (9)0.0034 (8)
C120.0143 (10)0.0119 (9)0.0077 (9)0.0007 (8)0.0055 (8)0.0008 (7)
C140.0136 (10)0.0171 (10)0.0130 (11)0.0008 (8)0.0053 (9)0.0041 (8)
C190.0202 (11)0.0162 (10)0.0099 (10)0.0022 (8)0.0085 (9)0.0001 (8)
C180.0193 (12)0.0287 (12)0.0126 (11)0.0031 (9)0.0080 (9)0.0046 (9)
Geometric parameters (Å, º) top
S2—C171.768 (2)S3—C111.778 (2)
S2—C151.770 (2)S8—C121.757 (2)
S6—C171.745 (2)S8—C141.817 (2)
S6—C191.824 (2)S7—C111.755 (2)
S4—C101.758 (2)S7—C131.811 (2)
S4—C121.771 (2)C17—C161.347 (3)
S5—C161.746 (2)C10—C151.352 (3)
S5—C181.835 (2)C11—C121.350 (3)
S1—C151.762 (2)C13—C141.518 (3)
S1—C161.768 (2)C19—C181.517 (3)
S3—C101.763 (2)
C17—S2—C1593.47 (10)C17—C16—S1117.06 (15)
C17—S6—C1996.61 (10)S5—C16—S1117.83 (12)
C10—S4—C1294.51 (10)C12—C11—S7129.74 (16)
C16—S5—C18103.49 (10)C12—C11—S3116.91 (16)
C15—S1—C1693.92 (10)S7—C11—S3113.01 (11)
C10—S3—C1194.05 (10)C10—C15—S1122.65 (16)
C12—S8—C1499.75 (10)C10—C15—S2122.88 (16)
C11—S7—C13100.70 (10)S1—C15—S2114.29 (11)
C16—C17—S6122.49 (16)C14—C13—S7113.38 (15)
C16—C17—S2117.49 (15)C11—C12—S8127.73 (16)
S6—C17—S2119.64 (12)C11—C12—S4116.77 (16)
C15—C10—S4122.88 (16)S8—C12—S4115.22 (12)
C15—C10—S3123.01 (16)C13—C14—S8112.34 (15)
S4—C10—S3113.99 (12)C18—C19—S6115.01 (15)
C17—C16—S5125.08 (16)C19—C18—S5117.19 (16)

Experimental details

Crystal data
Chemical formulaC10H8S8
Mr384.64
Crystal system, space groupMonoclinic, P21/c
Temperature (K)100
a, b, c (Å)6.6583 (1), 13.733 (2), 17.414 (1)
β (°) 116.57 (1)
V3)1424.1 (1)
Z4
Radiation typeMo Kα
µ (mm1)1.23
Crystal size (mm)0.20 × 0.15 × 0.10
Data collection
DiffractometerSMART Siemens CCD
diffractometer
Absorption correctionEmpirical (using intensity measurements)
SADABS (Sheldrick, 1996)
Tmin, Tmax0.768, 0.915
No. of measured, independent and
observed [I > 2σ(I)] reflections
12398, 3251, 2714
Rint0.037
(sin θ/λ)max1)0.648
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.027, 0.061, 1.09
No. of reflections3251
No. of parameters163
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.35, 0.36

Computer programs: SMART (Siemens, 1996), SMART, SAINT (Siemens, 1996), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), PLATON (Spek, 1999).

 

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