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), ReO₄⁻ (5), and I₃⁻ (6) and the CPDTES salt with I₃⁻ (7) were obtained as platelet crystals, although EtDTES did not afford single crystalline salts. Crystal data of 5−7 are summarized in Table 3.

(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 (Figure 5). 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 Figure 5. 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].

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 = E₂ − E₁), 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.

(MeDTES)(ReO₄)(H₂O)_0.5 (5) crystallizes in triclinic space group P. The molecular structure of the donor molecule is shown in Figure 8. The MeDTES molecule in this salt shows high planarity except for the terminal ethylene bridge. One donor molecule and one ReO₄⁻ 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 (H₂O) or of oxonium ion (H₃O⁺) origin, which might explain the conducting property of the salt.

As shown in Figure 9, donor molecules form a columnar structure along the a axis and they are linked by Se···S contact (3.682(5) Å) which is shorter than the sum of the van der Waals radii (Se···S = 3.70 Å) [21]. The donor molecules stack in a two-fold periodicity with the interplanar distance of 3.65 Å for a1 and 3.33 Å for a2. The slipping distance x along the molecular long axis is 9.01 Å for a1 and 4.60 Å for a2 [5]. The donor molecules are also slipping by 1.97 Å for a1 and 0.20 Å for a2 along the molecular short axis. The large slipping distance for a1 results in the formation of a large cavity where the ReO₄⁻ ions are situated.

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)⁺(ReO₄⁻)(H₂O)_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)(ReO₄)(H₂O)_0.5.

The MeDTES salt with I₃⁻ (6) crystallizes in triclinic space group P. There are crystallographically independent two MeDTES molecules. The ORTEP drawings of Molecules A and B of MeDTES are shown in Figure 13. TTF moiety of Molecule A shows high planarity, and the 2-isopropylidene-1,3-dithiole unit slightly bends with dihedral angle of 6.3(3)°. Molecule B also has flat chair-like structure with dihedral angles 5.9(1) and 7.3(3)° as shown in Figure 13b.

As shown in Figure 14, there is 1,2-dichloroethane (DCE) solvent molecule at around the center of inversion with a half occupancy. Six iodine atoms I1−I6 are crystallographically independent and they form infinite zig-zag chains. The interatomic distances d1 (I1−I2), d4 (I4−I4), and d7 (I6−I6) are 2.900(2), 2.950(2), and 2.935(2) Å, respectively, which are consistent with the average distance of the I−I bond (2.92 Å) in triiodide I₃⁻ [24,25]. In contrast, the distances d2 (I2−I3), d3 (I3−I4), d5 (I5−I1), and d6 (I6−I5) are 3.271(2)−3.471(3) Å which is close to the mean value (3.44 Å) of the I−I bond length of triiodide (2.92 Å) and the sum of the van der Waals radii of iodine atoms (3.96 Å) [21]. The triiodide often forms an infinite chain and the typical I···I distance is 3.60 Å [24], which is slightly longer than d2, d3, d5, and d6 of the salt 6. There might be two patterns of configuration model of triiodide ions which are depicted in Figure 14b as light blue and pink lines. The infinite zig-zag iodine chains observed in the salt 6 are disordered.

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 I₅⁻, 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)⁺(I₃⁻)(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)(I₃)(DCE)_0.25.

(CPDTES)(I₃) (7) crystallizes in triclinic space group P. 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 I₃⁻ ion in the crystal and two halves of the ions are also crystallographically independent (Figure 19).

As shown in Figure 20, 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 I₃⁻ ions is situated in a space surrounded by the donor dimers in the S···S network on the ab plane. The other I₃⁻ 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) 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.

Triethyl phosphite (26 mL) was added to a mixture of 4,5-ethylenediseleno-1,3-dithiole-2-thione (1) [17] (1.00 g, 3.14 mmol) and diethyl (5-oxo-1,3-dithiolo[4,5-d]-1,3-dithiol-2-yl)phosphonate (2) [18] (737 mg, 2.23 mmol) in toluene (26 mL), and the solution was stirred at 110 °C for 2 h under argon atmosphere. After cooling to room temperature, the solution was evaporated under reduced pressure to eliminate the solvent and excess of triethyl phosphite. The residue was subjected to silica-gel column chromatography (SiO₂/dichloromethane:ethyl acetate = 10:1). The target compound 3 was isolated as an orange solid (828 mg, 1.38 mmol, 62%). mp 132−136 °C; ¹H NMR (CDCl₃, 270 MHz) δ_H 1.37 (t, J = 6.8 Hz, 6H), 3.35 (m, 4H), 4.26 (m, 4H), 5.35 (m, 1H); IR (KBr) ν 2979, 2895, 1256, 1162, 1044, 1016, 954, 794, 764, 543, 532 cm⁻¹; LDI-TOF-MS Calcd. for C₁₃H₁₅O₃PS₆Se₂: 601.74 (M⁺). Found: 602.01 with an isotropic pattern of these selenium atoms; Anal. Calcd. for C₁₃H₁₅O₃PS₆Se₂: C, 26.00; H, 2.52%. Found: C, 25.77; H, 2.60%.

A THF solution of LDA (0.5 M, 0.30 mL, 0.15 mmol) was added to a mixture of 3 (65 mg, 0.11 mmol) and acetone (0.10 mL, 1.4 mmol) in dehydrated THF (5 mL) at −78 °C. The reaction mixture was stirred for 2 h at −78 °C, then quenched by addition of methanol (10 mL) and warmed to room temperature. The resulting precipitate was collected by filtration and washed with methanol. The crude product of MeDTES was purified by silica gel column chromatography (SiO₂/carbon disulfide). Analytically pure MeDTES was isolated as orange plates (48 mg, 0.095 mmol, 86%). mp 222−225 °C (decomp.); ¹H NMR (CS₂-C₆D₆, 270 MHz) δ_H 1.60 (s, 6H), 3.11 (s, 4H); IR (KBr) ν 2925, 2900, 1433, 1399, 1365, 1266, 963, 766 cm⁻¹; LDI-TOF-MS Calcd. for C₁₂H₁₀S₆Se₂: 505.74 (M⁺). Found: 506.24 with an isotropic pattern of these selenium atoms; Anal. Calcd. for C₁₂H₁₀S₆Se₂: C, 28.57; H, 2.00%. Found: C, 28.34; H, 2.26%.

A THF solution of LDA (0.5 M, 1.05 mL, 0.53 mmol) was added to a mixture of 3 (247 mg, 0.41 mmol) and 3-pentanone (0.10 mL, 0.94 mmol) in dehydrated THF (15 mL) at −78 °C. The reaction mixture was stirred for 2 h at −78 °C, then quenched by addition of methanol (20 mL) and warmed to room temperature. The same procedure as for MeDTES was adopted for the separation of the products. EtDTES was isolated as orange solid (159 mg, 0.30 mmol, 73%). mp 208−210 °C (decomp.); ¹H NMR (CS₂-C₆D₆, 270 MHz) δ_H 0.97 (t, J = 7.8 Hz, 6H), 2.00 (q, J = 7.8 Hz, 4H), 3.10 (s, 4H); IR (KBr) ν 2963, 2925, 1265, 765 cm⁻¹; LDI-TOF-MS Calcd. for C₁₂H₁₀S₆Se₂: 533.78 (M⁺). Found: 534.23 with an isotropic pattern of these selenium atoms; Anal. Calcd. for C₁₄H₁₄S₆Se₂: C, 31.57; H, 2.65%. Found: C, 31.43; H, 2.70%.

A THF solution of LDA (0.5 M, 0.30 mL, 0.15 mmol) was added to a mixture of 3 (67 mg, 0.11 mmol) and cyclopentanone (0.10 mL, 1.12 mmol) in dehydrated THF (5 mL) at −78 °C. The reaction mixture was stirred for 2 h at −78 °C, then quenched by addition of methanol (20 mL) and warmed to room temperature. The same procedure as for MeDTES was adopted for the separation of the products. EtDTES was isolated as orange solid (40 mg, 0.075 mmol, 68%). mp 198−201 °C (decomp.); ¹H NMR (CS₂-C₆D₆, 270 MHz) δ_H 1.25 (m, 4H), 1.75 (m, 4H), 3.22 (s, 4H); IR (KBr) ν 2938, 2906, 2869, 1620, 1516, 1425, 1266, 766 cm⁻¹; LDI-TOF-MS Calcd. for C₁₄H₁₂S₆Se₂: 531.76 (M⁺). Found: 532.23 with an isotropic pattern of these selenium atoms; Anal. Calcd. for C₁₂H₁₀S₆Se₂: C, 28.57; H, 2.00%. Found: C, 28.34; H, 2.26%.