A phenol-fused tetrathiafulvalene: modulation of hydrogen-bond patterns and electrical conductivity in the charge-transfer salt

Abstract

A tetrathiafulvalene (TTF) derivative fused with a phenol moiety, PhOH-EDT-TTF, was designed and synthesized as a new electron-donor molecule with hydrogen-bonding (H-bonding) abilities. Electrochemical oxidation of this donor molecule in the presence of Bu₄NClO₄ gave a charge-transfer salt, β-(PhOH-EDT-TTF)₂ClO₄, that contains PhOH-EDT-TTF^+0.5 and ClO₄⁻ in a 2 : 1 ratio. In this salt, the donor and anion molecules are connected with three-centered H-bonds through the phenol hydroxy group, which is in contrast to the linear H-bonds observed in an analogue salt based on the catechol-fused TTF. This variation in the donor-anion H-bonds modulated the arrangement of the donor molecules in the conducting layer, leading to better conducting properties than those of the analogue salt.

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Introduction

Hydrogen-bonded (H-bonded) molecular systems have continuously attracted much attention, because of their unique structural and functional features attributable to the electrostatic and dynamic properties of the H-bond interactions.^1–3 For example, in the field of molecular conductors,³ Murata et al. have realized a purely organic molecular metal based on a H-bonded charge-transfer (CT) complex, where the H-bonds controlled the redox activity and ratio of the component electron donor/acceptor molecules.^3a,c Our group has also found several kinds of interesting phenomena and properties based on H-bonds coupled to the π-electrons in molecular conductors.^4–8 In particular, H-bond dynamics-based unique electronic properties were observed in the catechol-fused ethylenedithiotetrathiafulvalene (H₂Cat-EDT-TTF; Fig. 1 left)-based conductors.^5–7 What is important here is that these features are fundamentally based on the interplay of the electron donating and H-bonding abilities of the component molecule, H₂Cat-EDT-TTF.

Fig. 1 Molecular design of PhOH-EDT-TTF (right) on the basis of the catechol analogue, H₂Cat-EDT-TTF^4a (left).

These results have encouraged us to further explore such TTF-based electron donor molecules with H-bonding abilities. Thus, in this study, we have designed phenol-fused ethylenedithiotetrathiafulvalene, PhOH-EDT-TTF (Fig. 1 right), by eliminating one OH group from H₂Cat-EDT-TTF. This type of symmetry-lowered, mono-substituted TTF derivatives has also extensively been investigated as an interesting component of molecular conductors.^{3a,c–i,8a,b,d,9,10} Here, we report the synthesis, structures, and properties of PhOH-EDT-TTF and its CT salt, β-(PhOH-EDT-TTF)₂ClO₄. Interestingly, this chemical modification modulated not only the donor-anion H-bond pattern but also the donor arrangement in the conducting layer, which resulted in significant improvement of the conducting properties.

Results and discussion

Synthesis, structure, and properties of phenol-fused EDT-TTF electron donor

The synthetic scheme for PhOH-EDT-TTF is depicted in Scheme 1 (for details, see the Experimental section). The Stille coupling reaction of 1,2-dibromo-4-methoxybenzene 1¹¹ with nBu₃SnS(CH₂)₂CN¹² provided the bis(cyanoethylthio) derivative 2, which was then transformed into the 1,3-dithiole-2-thione derivative 3 through decyanoethylation and subsequent cyclization. Demethylation of 3 followed by tert-butyldimethylsilylation gave compound 4, which was then subjected to the Wittig-type reaction with 4,5-ethylenedithio-1,3-dithiole-2-one, to afford the compound 5. Finally, removal of the silyl group from 5 provided the desired compound PhOH-EDT-TTF as an orange powder.

Scheme 1 Synthesis of PhOH-EDT-TTF. Reagents and conditions: (i) nBu₃SnS(CH₂)₂CN, Pd(PPh₃)₄, toluene, 120 °C; (ii) NaOMe, ZnCl₂, nBu₄Br, CH₂Cl₂/MeOH, room temperature; (iii) 1,1′-thiocarbonyldiimidazole, AcOH, THF, −78 °C–room temperature; (iv) BBr₃, CH₂Cl₂, −5 °C to 30 °C; (v) tert-butyldimethylsilyl chloride, Et₃N, N,N′-dimethylamino-4-pyridine, CH₂Cl₂, room temperature; (vi) P(OEt)₃, 115 °C; (vii) nBu₄NF, AcOH, THF, room temperature.

Single crystals of PhOH-EDT-TTF suitable for X-ray structure analysis (Table 1) were obtained by recrystallization from a methanol solution. In the crystal, the TTF skeleton of PhOH-EDT-TTF is slightly distorted in a boat-like fashion (Fig. 2a), which is typical for neutral TTF derivatives. The phenol hydroxy group is disordered in two positions (O1, H1A, and H4 vs. O2, H2A, and H3) with site occupancy factors of 0.74(1) and 0.26(1), respectively. The dominant hydroxy group (O1–H1A) participates in a C–H⋯O-type hydrogen-bond with a methylene group in the neighboring molecule (dashed blue lines in Fig. 2b; d_H⋯O ∼ 2.5 Å), as the hydrogen-bond acceptor, resulting in the formation of one-dimensional chain structures along the a + c direction. Also, the OH proton (H1A) is directed to sulfur atoms on the EDT-TTF skeleton in the neighboring chain with d_H⋯S ∼ 3.0 Å (dashed purple lines in Fig. 2b). Furthermore, S⋯S short contacts are found between the chains (dashed brown lines in Fig. 2b; d_S⋯S ∼ 3.5 Å). As shown in Fig. 2c, the donor molecules form dimeric pairs in a head-to-tail manner (∼3.4 Å), constructing a two-dimensional (2D) sheet-like arrangement.

Table 1 Crystallographic data for PhOH-EDT-TTF and β-(PhOH-EDT-TTF)₂ClO₄

Compound	PhOH-EDT-TTF	β-(PhOH-EDT-TTF)2ClO4
Formula	C12H8O1S6	C24H16Cl1O6S12
FW	360.55	820.56
Crystal system	Monoclinic	Monoclinic
Space group	P21/c (#14)	C2/c (#15)
a (Å)	12.369(5)	33.29(4)
b (Å)	11.196(4)	7.227(7)
c (Å)	10.011(4)	13.232(14)
α (°)	90	90
β (°)	92.537(5)	105.227(18)
γ (°)	90	90
V (Å^3)	1385.1(9)	3072(6)
Z value	4	4
T (K)	273	293
d calc (g cm^−3)	1.729	1.774
λ (Å)	0.71073	0.71073
Goodness of fit (GOF)	1.010	1.134
# of observations	3146	3476
# of variables	184	196
R 1 (I > 2.0σ(I))	0.0819	0.0754
wR2 (all data)	0.2098	0.2367
CCDC number	1590325	1590324

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Fig. 2 Crystal structure of the neutral donor PhOH-EDT-TTF at 273 K. (a) ORTEP drawings of the molecular structure (top: top view, bottom: side view; minor disordered components (O2, H2A, and H3) are shown in green), (b) intermolecular short contacts, and (c) two-dimensional sheet arrangement. In (a), the thermal ellipsoids are scaled to the 50% probability level.

The cyclic voltammetry of PhOH-EDT-TTF in CH₃CN showed two reversible oxidation waves (E¹_1/2 = +0.48 V, E²_1/2 = +0.77 V; vs. SCE), whose values are higher than those of the catechol analogue H₂Cat-EDT-TTF (E¹_1/2 = +0.40 V, E²_1/2 = +0.65 V; vs. SCE)^4a (Fig. 3). This result indicates that the electron-donating ability is significantly decreased by the elimination of one hydroxy group. In fact, a similar trend is predicted by DFT calculations (Fig. 3, right column):¹³ the calculated HOMO energy level of PhOH-EDT-TTF (−4.77 eV) is lower than that of H₂Cat-EDT-TTF (−4.72 eV).^4a Also, the ΔE value (= E²_1/2 − E¹_1/2) is increased from 0.25 V in H₂Cat-EDT-TTF^4a to 0.29 V in PhOH-EDT-TTF, which corresponds to the fact that the degree of HOMO delocalization, especially onto the benzene ring, is decreased with this chemical modification (Fig. 3). Nevertheless, PhOH-EDT-TTF has a sufficiently high electron-donating ability comparable to BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) (E¹_1/2 = +0.49 V, E²_1/2 = +0.72 V; vs. SCE), probably thanks to the residual hydroxy group.

Fig. 3 Comparison of electronic properties of the donor molecules, PhOH-EDT-TTF and H₂Cat-EDT-TTF.^4a The oxidation potentials, E¹_1/2 and E²_1/2, were estimated from CV experiments (in CH₃CN with 0.1 M Bu₄NClO₄, vs. SCE). The energy levels and distributions of HOMO were obtained from DFT calculations (B3LYP/6-31G(d)).¹³

Synthesis, structure, and properties of the CT salt, β-(PhOH-EDT-TTF)₂ClO₄

Electrochemical oxidation of PhOH-EDT-TTF in the presence of nBu₄N·ClO₄ in PhCl–CH₃CN gave brown plate crystals of the titled CT salt. The X-ray crystal structure analysis at room temperature (Table 1) revealed that the space group is C2/c and the asymmetric unit consists of one donor molecule and a half of the ClO₄ anion. Therefore, the chemical formula of this salt is given as (PhOH-EDT-TTF)₂ClO₄, where the donor molecule should be in a +0.5 state, PhOH-EDT-TTF^+0.5. In fact, the TTF skeleton of the donor molecule in this salt (Fig. 4a) is highly planar and has a longer central C=C bond (1.366(8) Å) compared to that in the neutral crystal (1.315(9) Å, Fig. 2a), which are characteristic of oxidized TTF derivatives. The unit cell contains eight donor molecules and four anions (Fig. 4b). These two species separately form a layer structure, as represented by pink and blue colors in Fig. 4b, and these layers are alternately arranged along the a axis. Such a layered crystal structure is frequently seen in TTF-based CT salts and actually the previous H₂Cat-EDT-TTF salts also have a similar structure.^4a

Fig. 4 Crystal structure of the CT salt, β-(PhOH-EDT-TTF)₂ClO₄. (a) Molecular structure of the donor molecule PhOH-EDT-TTF^+0.5 (left: top view, right: side view) and (b) unit cell. In (a), the thermal ellipsoids are scaled to the 50% probability level.

Also, the present PhOH-EDT-TTF^+0.5 molecules form two O–H⋯O-type H-bonds with ClO₄ anions through the phenol hydroxy group (Fig. 5a; d_O1⋯O2 = 2.700(11) Å (blue dashed lines) and d_O1⋯O3 = 3.023(10) Å (green dashed lines)). Although the position of the OH proton is not completely determined (owing to the quality of the diffraction data), this type of H-bonds can be categorized as a three-centered (or bifurcated) H-bond, where one proton donor is connected to two proton acceptors.² Thanks to these donor-anion H-bonds, the donor molecules are arranged along the c axis, accompanied by lateral S⋯S interactions between the EDT-TTF moieties (Fig. 5a). The shortest S⋯S distance in this H-bonded array is 3.661(4) Å (black dashed lines), close to the sum of the van der Waals radii (3.60 Å).¹⁴ Such a H-bonded array is also seen in CT salts of the catechol analogue, β-(H₂Cat-EDT-TTF)₂Br and β′′-(H₂Cat-EDT-TTF)₂Cl; however, the H-bonds are not three-centered but linear (Fig. 5b, O–H⋯Br).^4a Therefore, the difference in the number of hydroxy groups in the donor molecules might make the H-bond pattern different. In addition, the difference in the counter anion in these salts (i.e., ClO₄vs. Br) might also contribute to the difference in the H-bond patterns.¹⁵ The shortest S⋯S distance in the catechol system, β-(H₂Cat-EDT-TTF)₂Br (3.512(2) Å, Fig. 5b), is shorter than that in the present salt (3.661(4) Å, Fig. 5a), which is probably related to the H-bond structure differences mentioned above.

Fig. 5 Donor-anion H-bonded arrays in the crystal of (a) β-(PhOH-EDT-TTF)₂ClO₄ and (b) β-(H₂Cat-EDT-TTF)₂Br.^4a

In the donor layer (Fig. 4b, pink areas), PhOH-EDT-TTF^+0.5 molecules construct a two-dimensional (2D) sheet structure (Fig. 6a), categorized as the β-type arrangement in the BEDT-TTF-based CT salts.¹⁶ Thus, this salt is named β-(PhOH-EDT-TTF)₂ClO₄. The donor molecules are uniformly stacked along the b axis in a head-to-tail manner, where the interplanar separation is about 3.6 Å. These donor stacks are arranged along the c axis through the above-mentioned H-bonds (Fig. 5a). Similar to this compound, the catechol analogue, β-(H₂Cat-EDT-TTF)₂Br,^4a also form a β-type donor arrangement (Fig. 6b) by utilizing the H-bonds shown in Fig. 5b. However, as shown in gray in Fig. 6a and b, the arrangement manners of the H-bonded molecules in these two systems are significantly different from each other: the H-bonded molecules in β-(PhOH-EDT-TTF)₂ClO₄ are arranged horizontally in the 2D layer (Fig. 6a), whereas those in β-(H₂Cat-EDT-TTF)₂Br are arranged diagonally (Fig. 6b). This difference is clearly due to the difference in their donor-anion H-bond patterns (i.e., three-centered or linear; Fig. 5a and b).

Fig. 6 Donor arrangement in (a) β-(PhOH-EDT-TTF)₂ClO₄ and (b) β-(H₂Cat-EDT-TTF)₂Br.^4a The symbols, b, b1, b2, p, q, and r, represent the intermolecular transfer integrals: b = 117, p = −30, q = −31, and r = −19 meV for (a) β-(PhOH-EDT-TTF)₂ClO₄ and b1 = −229, b2 = −60, p = −23, q = −11, and r = 78 meV for (b) β-(H₂Cat-EDT-TTF)₂Br.^4a Gray-colored areas represent the H-bonded molecular arrays in Fig. 5a and b.

Intermolecular transfer integrals in the donor layer of β-(PhOH-EDT-TTF)₂ClO₄ are calculated¹⁷ to be b = 117, p = −30, q = −31, and r = −19 meV (Fig. 6a), indicating the occurrence of a strong interaction in the stack direction (b). In addition, the interstack directions (p, q, and r) also have substantial interactions (up to a quarter of the stack direction), which should be due to the lateral S⋯S interactions resulted from the H-bond formation (Fig. 5a). Reflecting these intermolecular interactions, a quasi-two-dimensional (Q2D) Fermi surface was obtained (Fig. 7a, right) by tight-binding band calculations.¹⁷ Despite having such a 3/4-filled band structure, the single crystal of this salt showed not metallic but semiconducting behavior (Fig. 7b).¹⁸ The room-temperature electrical conductivity and activation energy are 6 S cm⁻¹ and 0.08 eV, respectively. These values indicate that the conducting properties are significantly enhanced compared to those of the catechol analogue, β-(H₂Cat-EDT-TTF)₂Br (0.02 S cm⁻¹ and 0.10 eV, semiconducting).^4a This is because the uniform stacks of donor molecules are realized in the present system β-(PhOH-EDT-TTF)₂ClO₄, in contrast to the strongly dimerized stacks in β-(H₂Cat-EDT-TTF)₂Br (i.e., b1/b2 = 3.8 (≫1), Fig. 6 caption). Thus, one can imagine that the variation in the arrangement manners of the H-bonded donor molecules (Fig. 6a and b) allows this salt to have the uniform stacks, which results in the improvement of the conducting properties. Namely, the H-bond interactions have played an important role in controlling not only the donor-anion contacts, but also the donor arrangement and electronic structure in the conducting layer.

Fig. 7 (a) Electronic band structure and (b) temperature dependence of electrical resistivity (single crystal, four-probe) of β-(PhOH-EDT-TTF)₂ClO₄.

Conclusions

Phenol-fused ethylenedithiotetrathiafulvalene, PhOH-EDT-TTF, was designed and synthesized as a new TTF derivative with H-bonding abilities. Cyclic voltammetry measurements and DFT calculations indicate that this molecule has a sufficiently high electron-donating ability comparable to BEDT-TTF, enabling us to obtain the charge-transfer salt, β-(PhOH-EDT-TTF)₂ClO₄, composed of PhOH-EDT-TTF^+0.5 and ClO₄⁻ in a 2 : 1 ratio. In this salt, the donor molecule forms three-centered H-bonds with two neighboring ClO₄⁻ anions through the phenol hydroxy group, which is in contrast to the catechol-fused analogue that constructs the linear H-bonds. Interestingly, this difference in the H-bond patterns modulated the arrangement of the donor molecules in the conducting layer, leading to uniform stacks of the donors and better conducting properties in the present system, β-(PhOH-EDT-TTF)₂ClO₄. These results indicate that PhOH-EDT-TTF is a promising component molecule in electronic functional materials with H-bonds. Further exploration of PhOH-EDT-TTF-based H-bonded molecular conductors and also the investigation of dynamic properties based on the H-bonds^5–7 are currently underway.

Experimental

Materials and methods

Triethyl phosphite was distilled over sodium before use. Other commercially available materials were used without further purification. All the reactions were carried out under argon atmosphere. ¹H NMR (300 MHz) spectra were measured with a JEOL JNM-AL300 spectrometer with CDCl₃ or acetone-d₆ as a solvent. The residual solvent was used as the internal standard. APCI-MS spectra were recorded on a Bruker micrOTOF II spectrometer. Melting points were measured with a hot-stage apparatus and are uncorrected. Elemental analyses were performed at the Department of Chemistry, Graduate School of Science, The University of Tokyo. Cyclic voltammetry (CV) measurements were performed with an ALS Electrochemical Analyzer Model 610DB in a CH₃CN solution containing 0.1 M nBu₄NClO₄ as a supporting electrolyte at room temperature (working electrode: glassy carbon, counter electrode: Pt wire, reference electrode: saturated calomel electrode (SCE), scan rate: 0.1 V s⁻¹).

Synthesis of 1,2-bis(2′-cyanoethylthio)-4-methoxybenzene (2). 1,2-Dibromo-4-methoxybenzene 1¹¹ (6.4 g, 24 mmol) and 3-[(tributylstannyl)thio]propanenitrile¹² (33 g, 89 mmol) were placed in a 300 mL two-necked round-bottomed flask and dissolved in toluene (200 mL, dehydrated). To the solution was added Pd(PPh₃)₄ (4.2 g, 3.6 mmol) at room temperature. The resulting mixture was stirred at 120 °C for 18 h and then cooled down to 50–60 °C. The insoluble material was removed by filtration and the filtrate was concentrated under reduced pressure. The residue was dissolved in CH₂Cl₂ and filtered through silica gel. The filtrate was concentrated under reduced pressure, and the residue was washed with hexane and methanol, to give 2 (4.7 g, 70%) as a light brown powder. This material was used for the next reaction without further purification. [Analytically pure sample was obtained as a beige powder by silica gel column chromatography using hexane : ethyl acetate = 1 : 1 as eluent.] R_f 0.61 (hexane : ethyl acetate = 1 : 1); mp 66–68 °C; ¹H NMR (300 MHz, CDCl₃) δ 2.56 (t, J = 7.2 Hz, 2H), 2.71 (t, J = 7.2 Hz, 2H), 3.06 (t, J = 7.2 Hz, 2H), 3.20 (t, J = 7.2 Hz, 2H), 3.83 (s, 3H), 6.75 (dd, J = 8.4, 2.4 Hz, 1H), 6.83 (d, J = 2.4 Hz, 1H), 7.77 (d, J = 8.4 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 17.70, 18.14, 28.38, 30.53, 55.55, 111.88, 115.05, 117.73, 118.04, 122.98, 137.17, 140.65, 160.72; anal. calcd for C₁₃H₁₄N₂O₁S₂(H₂O)_0.3: C, 55.02; H, 5.19; N, 9.87. Found C, 54.96; H, 5.03; N, 9.77.

Synthesis of 5-methoxybenzo[d][1,3]dithiole-2-thione (3). The bis(cyanoethylthio) derivative 2 (4.0 g, 14 mmol) was placed in a 500 mL round-bottomed flask and dissolved in CH₂Cl₂ (80 mL, dehydrated). To this solution was added sodium methoxide (10 mL, 50 mmol, 5 M methanol solution) at room temperature, and the resulting mixture was stirred for several minutes at the same temperature. To the reaction mixture was then added a mixture of ZnCl₂ (2.7 g, 20 mmol) and Bu₄NBr (5.5 g, 17 mmol) in methanol (20 mL, dehydrated), and the resulting mixture was stirred for 1 h at room temperature. The reaction mixture was concentrated under reduced pressure and dried in vacuo at room temperature for 1 h. The residue was then suspended in THF (150 mL, stabilizer free) and acetic acid (50 mL). To this suspension, 1,1′-thiocarbonyldiimidazole (3.9 g, 22 mmol) in THF (50 mL, stabilizer free) was added dropwise at −78 °C for 30 min. The resulting mixture was stirred overnight (16 h) with gradual warming to room temperature. To the reaction mixture were added 28% NH₃ aqueous solution and satd NaCl aqueous solution. The organic layer was separated, dried over Na₂SO₄, filtered through silica gel, and concentrated under reduced pressure. The residue was washed with methanol, collected by filtration and then dried in vacuo at room temperature, to give 3 (1.4 g, 47%) as a deep yellow powder. This material was used for the next reaction without further purification. [Analytically pure sample was obtained as a light yellow powder by silica gel column chromatography using hexane : ethyl acetate = 1 : 1 as eluent.] R_f 0.83 (hexane : ethyl acetate = 1 : 1); mp 142.0–142.5 °C; ¹H NMR (300 MHz, CDCl₃) δ 3.85 (s, 3H), 6.96–7.01 (m, 2H), 7.36 (d, J = 8.4 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 55.77, 106.14, 115.28, 122.35, 132.14, 142.40, 159.48, 192.27; anal. calcd for C₈H₆O₁S₃: C, 44.83; H, 2.82; N, 0.00. Found C, 44.87; H, 2.94; N, 0.00.

Synthesis of 5-((tert-butyldimethylsilyl)oxy)benzo[d][1,3]dithiole-2-thione (4). The methoxy derivative 3 (1.4 g, 6.5 mmol) was placed in a 500 mL two-necked round-bottomed flask and dissolved in CH₂Cl₂ (200 mL, dehydrated). To the solution was added BBr₃ (12 mL, 12 mmol, 1 M CH₂Cl₂ solution) at −5 °C. The resulting mixture was stirred at the same temperature for 15 min and then at 30 °C for 13 h. To the mixture were further added BBr₃ several times (74 mL (in total), 74 mmol, 1 M CH₂Cl₂ solution) with stirring at 30 °C. After the methoxy derivative 3 completely disappeared (on TLC), methanol and satd NaCl aqueous solution were added to the reaction mixture. The organic layer was separated, dried over Na₂SO₄, filtered through silica gel, and concentrated under reduced pressure. After drying in vacuo at room temperature for 15 min, the residue was suspended in CH₂Cl₂ (200 mL, dehydrated). To this suspension were successively added Et₃N (1.7 mL, 12 mmol), N,N′-dimethylamino-4-pyridine (0.32 g, 2.6 mmol), and tert-butyldimethylsilyl chloride (1.8 g, 12 mmol), and the resulting mixture was stirred at room temperature for 1.5 h. To the reaction mixture was added water, and the resulting organic layer was separated, dried over Na₂SO₄, filtered through silica gel, concentrated under reduced pressure, and dried in vacuo at room temperature for 1 h, to give 4 (1.85 g, 90%) as a deep yellow solid. R_f 0.80 (hexane : ethyl acetate = 1 : 1); mp 32–34 °C; ¹H NMR (300 MHz, acetone-d₆) δ 0.27 (s, 6H), 1.01 (s, 9H), 7.08 (dd, J = 8.7, 2.4 Hz, 1H), 7.36 (d, J = 2.4 Hz, 1H), 7.64 (d, J = 8.7 Hz, 1H); ¹³C NMR (75 MHz, acetone-d₆) δ −4.51, 18.74, 25.88, 114.35, 121.46, 123.91, 133.71, 143.02, 156.69, 191.39.

Synthesis of TBDMS-protected PhOH-EDT-TTF5. A mixture of thione 4 (629 mg, 2.0 mmol), 4,5-(ethylenedithio)-1,3-dithiole-2-one (625 mg, 3.0 mmol), and triethyl phosphite (8 mL) was stirred at 115 °C for 3 h. The reaction mixture was concentrated in vacuo, and to the residue was then added hexane. The resulting insoluble solid was removed by filtration and the filtrate was concentrated under reduced pressure. The resulting residue was subjected to silica gel column chromatography with a 3 : 1 mixture of hexane and ethyl acetate as eluent, to give 5 (396 mg, 42%) as an orange powder. R_f 0.64 (hexane : ethyl acetate = 3 : 1); mp 114–115 °C; ¹H NMR (300 MHz, CDCl₃) δ 0.18 (s, 6H), 0.97 (s, 9H), 3.30 (s, 4H), 6.61 (dd, J = 8.4, 2.4 Hz, 1H), 6.76 (d, J = 2.4 Hz, 1H), 7.07 (d, J = 8.4 Hz, 1H); HRMS (APCI), calcd for C₁₈H₂₂O₁S₆Si₁⁺: 473.9759 [M]⁺; found: 473.9750; anal. calcd for C₁₈H₂₂O₁S₆Si₁: C, 45.53; H, 4.67; N, 0.00. Found C, 45.82; H, 4.97; N, 0.00.

Synthesis of PhOH-EDT-TTF. TBDMS-protected PhOH-EDT-TTF2 (150 mg, 0.32 mmol) was placed in a 50 mL round-bottomed flask and dissolved in THF (6 mL, stabilizer free). To this solution were added acetic acid (2 drops) and nBu₄NF (0.48 mL, 0.48 mmol, 1 M THF solution) at room temperature. After stirring for 20 minutes at the same temperature, the reaction mixture was directly subjected to silica gel column chromatography with ethyl acetate as eluent. The obtained crude material was washed with hexane and dried in vacuo at room temperature, to give PhOH-EDT-TTF (74 mg, 64%) as a reddish orange powder. A single crystal suitable for X-ray analysis was obtained as an orange plate by slow evaporation of a methanol solution at 4 °C. R_f 0.70 (ethyl acetate); mp 187 °C (dec.); ¹H NMR (300 MHz, acetone-d₆) δ 3.44 (s, 4H), 6.75 (dd, J = 8.4, 2.4 Hz, 1H), 6.96 (d, J = 2.4 Hz, 1H), 7.25 (d, J = 8.4 Hz, 1H) [the OH proton was not detectable by ¹H NMR. A clear ¹³C NMR spectrum was not obtained because of low solubility of PhOH-EDT-TTF in common organic solvents.]; HRMS (APCI), calcd for C₁₂H₈O₁S₆⁺: 359.8894 [M]⁺; found: 359.8886; anal. calcd for C₁₂H₈O₁S₆(C₆H₁₄)_0.05: C, 40.49; H, 2.40; N, 0.00. Found C, 40.51; H, 2.66; N, 0.00.

Electrochemical crystallization of β-(PhOH-EDT-TTF)₂ClO₄

PhOH-EDT-TTF (8.0 mg) was placed in one side of an H-shaped cell equipped with a glass filter. Then, nBu₄NClO₄ (20 mg) was placed in each side of the cell, and the cell was purged with argon gas. These materials were dissolved in a mixed solvent of CH₃CN (2 mL) and degassed PhCl (17 mL). The resulting solution was kept at 20 °C for 2 h, and then an annealed Pt electrode was inserted into each side of the cell. A constant current of 1.0 μA was applied to the solution at 20 °C for 1 week, to afford brown plate crystals of β-(PhOH-EDT-TTF)₂ClO₄ (typical size: 0.30 × 0.05 × 0.01 mm³) suitable for the X-ray crystal analysis and physical property measurements.

Single crystal X-ray structure analysis

X-ray crystallographic measurements were performed on a Rigaku MercuryII CCD X-ray diffractometer (MoKα, λ = 0.71073 Å). The structures were solved by direct methods and refined with a full-matrix least squares technique using CrystalStructure 4.2.4 (Rigaku Corporation (2000–2016)). Anisotropic thermal parameters were applied to all non-hydrogen atoms. The hydrogen atoms were generated geometrically.

CCDC 1590325 (PhOH-EDT-TTF) and 1590324 (β-(PhOH-EDT-TTF)₂ClO₄).†

Electrical resistivity

Electrical resistivity measurements of a single crystal of β-(PhOH-EDT-TTF)₂ClO₄ were performed on a Physical Property Measurement System (Quantum Design) by the conventional four-probe method using carbon paste and gold wires.

Theoretical calculations

Density functional theory (DFT) calculations were performed with Gaussian03 program (revision E.01) at the B3LYP/6-31G(d) level of theory.¹³ Intermolecular transfer integrals were calculated by the extended Hückel method.¹⁷ Electronic band structures were calculated by the tight-binding approximation method.¹⁷

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by a Grant-in-Aid for Scientific Research (No. 15H00988, 16K05744, 17H05143, 24340074, 16H04010) from Japan Society for the Promotion of Science (JSPS), Japan, and the Canon Foundation. The authors also thank Prof. S. Suzuki (Osaka University) for measurements of the APCI-MS spectra and Prof. H. Yoshizawa (ISSP, The Univ. of Tokyo) for assistance in using his Physical Property Measurement System (Quantum Design).

Notes and references

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18. This might be due to strong electron correlations resulting from a relatively narrow bandwidth (∼0.6 eV; Fig. 7a, left). Further details of the electronic structure are currently being investigated by magnetic measurements.

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Footnote

† CCDC 1590324 and 1590325. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qm00574a

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