Synthesis and Properties of 2-alkylidene-1,3-dithiolo[4,5-d]-4,5- Ethylenediselenotetrathiafulvalene Derivatives and Crystal Structures of Their Cation Radical Salts

Tetrathiafulvalene derivatives condensed with 2-alkylidene-1,3-dithiole moiety, MeDTES


Synthesis and Property of MeDTES, EtDTES, and CPDTES
The synthesis of MeDTES, EtDTES, and CPDTES is outlined in Figure 2. The triethyl phosphite-mediated cross-coupling reaction of 4,5-ethylenediseleno-1,3-dithiole-2-thione (1) [17] with the ketone 2 [18] afforded phosphonate ester 3 in a 62% yield [19]. MeDTES, EtDTES and CPDTES were synthesized in a 68−86% yield by the Wittig−Horner reaction of 3 with corresponding ketone in the presence of lithium diisopropylamide (LDA) in THF at −78 °C.Molecular structures of all the new compounds were determined by NMR, MS, and elemental analyses.Redox potentials were measured by cyclic voltammetry (CV) in benzonitrile.The cyclic voltammograms of MeDTES, EtDTES, and CPDTES showed two pairs of reversible redox waves and one pair of irreversible waves.The redox potentials are summarized in Table 1.The first oxidation potentials of these donors are lower by 0.01−0.02V than those of MeDTET and BEDT-TTF [20], which could be due to the smaller electronegativity of the selenium atom compared with that of the sulfur atom.There is not, however, much difference in the electron-donating ability between these new donors and MeDTET.The theoretical calculation of MeDTES was carried out using a hybrid method of Hartree-Fock and density functional theory (DFT) B3LYP methods using the 6−31G(d) basis set. Figure 3 shows an optimized structure and the HOMO of MeDTES.The TTF moiety adopts a chair conformation and the dihedral angles are 17.0 and 20.7°.The 2-isopropyridene-1,3-dithiole unit also bends with the dihedral angle of 21.7°.The terminal six-membered ring of MeDTES has an eclipsed conformation [15] and the selenium atoms of the ethylenediseleno group protrude from the DT-TTF skeleton, that is, the ethylenediseleno group is so bulky that is a crucial factor to form a dimer structure.The HOMO of MeDTES distributes mainly to the DT-TTF moiety and hardly distributes to the ethylenediseleno group.

Preparation of Cation Radical Salts
Single crystals of the cation radical salts of MeDTES and CPDTES were prepared by electrocrystallization in tetrahydrofuran, 1,2-dichloroethane (containing 6% ethanol, v/v), and chlorobenzene (containing 6% ethanol, v/v) in the presence of corresponding tetra-n-butylammonium salts as the supporting electrolyte (Table 2).The salts of MeDTES with Au(CN) 4 − (4), ReO 4 − (5), and 6) and the CPDTES salt with I 3 − (7) were obtained as platelet crystals, although EtDTES did not afford single crystalline salts.Crystal data of 5−7 are summarized in Table 3.     Intermolecular overlap integrals are calculated by the extended Hückel method [23] and are listed in Figure 6.Intrastack overlap integral c (−26.80  10 −3 ) is much larger than integrals b1, b2, and p.The calculated band dispersion and the Fermi surfaces for 4 based on the tight-binding approximation are shown in Figure 7 [23].The energy band shows a half-filled band because of the 1:1 donor-anion ratio and uniform donor column.The flat Fermi surfaces suggest that this salt is a one-dimensional metal.This salt shows, however, low conductivity (σ rt = 4.4  10 −3 S cm −1 ) at room temperature and semiconducting behavior with an activation energy of E a = 0.15 eV.The redox potential difference, (∆E = E 2 − E 1 ), corresponds to the on-site Coulomb interaction U.The ∆E value of MeDTES (0.30 V) is almost the same as that of BEDT-TTF (0.32 V [20]).These facts suggest that this salt is a Mott insulator derived from the relatively large on-site Coulomb interaction U. − ion are crystallographically independent and they are located in the general position, giving the 1:1 donor−anion ratio.An oxygen atom is found around the center of inversion with a half occupancy.Since the origin of the oxygen is not clear, we assumed it as a part of the solvent, tetrahydrofuran (THF).However, the refinement did not converge to give the THF molecule.Therefore we conclude that the oxygen atom is either water (H 2 O) or of oxonium ion (H 3 O + ) origin, which might explain the conducting property of the salt.The calculated overlap integral a1 (−2.04  10 −3 ) is much smaller than the integral a2 (21.47  10 −3 ), indicating strong dimerization along the stacking direction (Figure 10).Interestingly, there is a large overlap integral q2 because of the existence of short Se•••S contacts.The energy band structure and Fermi surface were calculated by the tight-binding approximation [23].There are two energy branches and there is no energy gap between upper and lower bands (Figure 11).If the origin of oxygen atom is an oxonium ion, the Fermi level (E F ) crosses the upper branch, which results in the quasi-one-dimensional metal (Figure 12a).However, the σ rt of the salt is 1.3  10 −2 S cm −1 and this salt shows semiconducting behavior with small activation energy of 0.058 eV.When the composition of 5 is (MeDTES) + (ReO 4

−
)(H 2 O) 0.5 , the E F crosses just through the degeneracy point, which suggest that the salt possesses Fermi points at the B and X points (Figure 12b).The conducting property supports the idea that the origin of oxygen is water and the composition of the salt 5 is (MeDTES)(ReO 4 )(H 2 O) 0.5 .As shown in Figure 15, Molecules A and B form the head-to-head dimer with an interplanar distance of 3.40 Å and a slipping distance of 0.76 Å along the molecular long axis [5].The intradimer overlap integral a (−41.80  10 −3 ) is larger than those of the interdimer p (−14.10  10 −3 ) and q (−11.27 10 −3 ).The calculated energy band structure is shown in Figure 16.There are two branches, which are separated by a gap because of the dimerized structure.When the infinite iodide chain consists of polyiodide, such as pentaiodide I 5

−
, the Fermi level (E F ) crosses the upper branch, which makes the salt metallic.However, this salt behaves as a semiconductor with an activation energy of 0.051 eV and shows low conductivity σ rt = 5.3  10 −2 S cm −1 at room temperature.When the composition of 6 is (MeDTES) + (I 3 − )(DCE) 0.25 , the E F lies at the gap (Figure 17), which suggests that the salt 6 is a band insulator.Conductivity measurement supports the idea that the composition of the salt 6 is (MeDTES)(I 3 )(DCE) 0.25 .(CPDTES)(I 3 ) (7) crystallizes in triclinic space group P 1.The ORTEP drawings of the CPDTES molecule are shown in Figure 18.The molecule takes a flat chair-like structure and the dihedral angles of the TTP moiety composed of the S3−S6 and C6−C9 atoms are 5.7(1) and 6.9(2)°.One donor molecule is crystallographically independent.There are two types of I 3 − ion in the crystal and two halves of the ions are also crystallographically independent (Figure 19).  ) and q (7.08  10 −3 ) are much smaller than the intradimer integral a (38.11  10 −3 ).Calculated energy band structure reveals that this salt is a band insulator (Figure 21).This result is consistent with the low conductivity (σ rt = 6.1  10 −4 S cm −1 ) and large activation energy (E a = 0.17 eV) of the salt.

General
All chemicals and solvents are of reagent grade.All reactions were conducted under an argon atmosphere.Dehydrated THF was purchased from Wako Pure Chemical Industries, Ltd., and used without further purification.A THF solution of LDA was prepared from diisopropylamine with an n-hexane solution of n-BuLi (1.6 M, Wako Pure Chemical Industries) prior to use.Column chromatography was carried out with silica gel (Kanto Chemical, 100−210 μm and Wakosil ® , 64−210 μm). 1 H NMR spectra were recorded on a JEOL JNM-EX270 spectrometer.The chemical shifts are given in δ (ppm), downfield from internal tetramethylsilane.Mass spectra were measured on Applied Biosystem MALDI-TOF-MS Voyager-DE™ PRO.The melting points were determined with a Yanaco MP-J3.IR spectra were recorded on a JASCO FT/IR-460 plus spectrometer.Elemental analyses were performed at the Integrated Center for Science, Ehime University.Cyclic voltammetry (CV) measurements were performed using a BAS ALS/chi 617B electrochemical analyzer.The cell for CV consisted of a Pt disk working electrode, a Pt wire counter electrode, and an Ag/AgNO 3 reference electrode.The measurements were carried out in benzonitrile containing 0.1 M tetra-n-butylammonium hexafluorophosphate as a supporting electrolyte.All redox potentials were converted relative to a. ferrocene/ferrocenium (Fc/Fc + ) couple.All computations were performed with the Gaussian 09 program package [26] using the 6-31G(d) basis set [27].Density functional theory (DFT) calculations were carried out using a hybrid method of Hartree-Fock and B3LYP method [28−30].

Preparation of Cation Radical Salts
Single crystals of the cation radical salts of MeDTES and CPDTES were prepared by the galvanostatic oxidation under the conditions listed in Table 2. Platinum wire electrodes (2.0 mm φ) and standard H-shaped cells were employed.

Band Calculations
From the results of the X-ray crystal structure analysis, intermolecular overlap integrals were calculated using highest occupied molecular orbitals (HOMOs) of the donor molecules obtained by the extended Hückel MO calculations.The electronic band dispersions and Fermi surfaces were calculated using the intermolecular transfer integrals under the tight-binding approximation [23].

Electrical Resistivity Measurement
Electrical resistivities were measured by the four-probe method using YOKOGAWA 7651 programmable direct current source and KEITHLEY 2001 digital multimeter unit.Gold wires (10 μm φ diameter) were attached to a single crystal using carbon paste.The sample was cooled using an Iwatani CryoMini model CRT-HE05-RE cooling system and the temperature was controlled using Lakeshore S331 digital program temperature controller.Electrical properties of 4−7 are summarized in Table 3.

Conclusions
A series of DT-TTF derivatives containing ethylenediseleno group have been synthesized, and crystal structures, band structures, and electrical properties of their cation radical salts have been investigated.There is no distinct difference in the electrochemical properties between the ethylenediseleno-substituted donors and the ethyledithio-substituted donors.In contrast, crystal structures of the cation radical salts are different.It is known that the ethylendithio-substituted DT-TTFs tend to afford the -type molecular conductors with non-half-filled bands [10,11], and the self-aggregating property is the key to form the κ-type molecular arrangement [35].The self-aggregating property is associated with the moderate steric hindrance of the ethylenedithio group.The theoretical calculation of MeDTES reveals that the replacement of sulfur atoms in the ethylenedithio group by larger selenium atoms leads to an enhancement of steric hindrance.As a result, the self-aggregating property has disappeared in the present ethylenediseleno-containing donor system, and MeDTES and CPDTES yielded cation radical salts with various crystal structures.The dimerized structure is observed in the salts 5−7 and the donor-anion ratio of the salts is 1:1.Calculated energy bands of these salts consist of two branches.Their conductivity results support the idea that the E F lies in the gap and crosses just through the degeneracy point.If cation radical salts with a 2:1 donor-anion ratio could be obtained, the E F might cross the upper branch, which is in favor of formation of effective half-filling conductors.At the moment, we have obtained a small size Au(CN) 2 − salt of MeDTES with poor quality and preliminary structure analysis suggests the β-type donor arrangement with the 2:1 donor-anion ratio.This salt could be a candidate for the new organic metals including organic superconductors.Preparation of high quality single crystals of such cation radical salts is underway.

Figure 3 .
Figure 3. (a) Top view and side view of optimized structure of MeDTES are represented by a ball and stick model (left) and a space-filling model (right); (b) The HOMO of MeDTES, of which the energy level is −4.72 eV.(b)

2. 2 . 2 .
Structures and Electrical Properties of (MeDTES)[Au(CN) 4 ] (4) (MeDTES)[Au(CN) 4 ] (4) crystallizes in orthorhombic space group Pnma.The ORTEP drawings of the MeDTES molecule are shown in Figure 4.A half of the donor molecule is crystallographically independent, in which the carbon atoms C3, C4, C6, and C7 are on the mirror plane at b = 0.25.The terminal ethylene bridge of the MeDTES molecule shows a flipping disorder.The gold atom of the Au(CN) 4 − ion also exists on the mirror plane at b = 0.75 and half of the Au(CN) 4 − ion is crystallographically independent.These facts indicate a 1:1 donor−anion ratio.Donor molecules stack uniformly along the c axis with the interplanar distance z = 3.49 Å and the slipping distance x = 2.47 Å in a head-to-head manner.The geometrical parameters x, y, and z are estimated by the literature method[5], and the definition of these parameters is shown in Supplementary Materials.There are four, crystallographically equivalent donor columns in the unit cell: I, II, III, and IV (Figure5).The Se•••Se contact (3.720(1) Å) is shorter than the sum of the van der Waals radii[21], which are represented by blue broken lines in Figure5.The adjacent donor columns I and II (III and IV) are orthogonally linked by the Se•••Se contacts along the b axis.The square-planar Au(CN) 4 − ions also stack along the c axis with the interplanar distance of 3.24 Å.In consequence, this salt shows a segregate columnar structure like (TTF)(TCNQ)[22].

Figure 4 . 4 .Figure 5 .
Figure 4. Top view (left) and side view (right) of ORTEP drawings of the MeDTES molecule in 4. Displacement ellipsoids are drawn at the 50% probability level.Symmetry operator * is x, 0.5 − y, z.

Figure 8 .
Figure 8. Top view (left) and side view (right) of ORTEP drawings of the MeDTES molecule in 5. Displacement ellipsoids are drawn at the 50% probability level.

Figure 9 .
Figure 9. (a) Crystal structure of 5 viewed along the a axis; (b) Columnar structure of MeDTES in 5; (c) Top view and side view of molecular overlaps of a1 and a2.The atoms in the front molecule are represented by solid circles and those in the rear are represented by open circles.

Figure 11 .
Figure 11.Calculated band dispersion of the salt 5.The E F depends on the composition.

Figure 13 .
Figure 13.Top view and side view of ORTEP drawings of (a) Molecule A and (b) Molecule B of MeDTES in 6. Displacement ellipsoids are drawn at the 50% probability level.

Figure 15 .
Figure 15.(a) Top view and side view of molecular overlap of a.The atoms in the front molecule are represented by solid circles and those in the rear are represented by open circles; (b) Molecular arrangement viewed along the molecular long axis.The calculated overlap integrals (×10 −3 ) are a = −41.80,p = −14.10,and q = −11.27.The infinite iodine chains are depicted by pink circles.The hydrogen atoms are omitted for clarity.

Figure 18 . 7 .Figure 19 .
Figure 18.Top view (left) and side view (right) of ORTEP drawings for the CPDTES molecule in 7. Displacement ellipsoids are drawn at the 50% probability level.Se1 As shown in Figure20, donor molecules form a head-to-tail dimer with an interplanar distance of 3.36 Å and a slipping distance of 0.98 Å along the molecular long axis[5].The dimers are connected by short S•••S contacts (3.555(3) and 3.583(4) Å) that are shorter than the sum of the van der Waals radii and form a two-dimensional S•••S network on the ab plane.One of the I 3 − ions is situated in a space surrounded by the donor dimers in the S•••S network on the ab plane.The other I 3 − ion is situated between the dimers along the c axis.Although the S•••S contacts are formed between the dimers, the interdimer overlap integrals p (3.98  10 −3

Figure 20 . 3 −
Figure 20.(a) Top and side views of molecular overlap of a.The atoms in the front molecule are represented by solid circles and those in the rear are represented by open circles; (b) Molecular arrangement viewed along the molecular long axis.The calculated overlaps integrals (×10 −3 ) are a = 38.11,p = 3.98, and q = 7.08.Orange broken lines and pink circles indicate S•••S contacts shorter than the sum of the van der Waals radii and I 3 −

Table 2 .
Conditions of the preparation of 4

Table 3 .
Crystal data, structure refinement details and electrical properties for 4