**Synthesis, Structure and Physical Properties of (trans-TTF-py2)1.5(PF6)**·**EtOH: A Molecular Conductor with Weak CH**···**N Hydrogen Bondings**

**Shohei Koyama 1, Morio Kawai 1, Shinya Takaishi 1, Masahiro Yamashita 1,2, Norihisa Hoshino 3, Tomoyuki Akutagawa 3, Manabu Kanno <sup>1</sup> and Hiroaki Iguchi 1,\***


Received: 31 October 2020; Accepted: 24 November 2020; Published: 26 November 2020

**Abstract:** The studies of crystal structures with hydrogen bonds have been actively pursued because of their moderate stabilization energy for constructing unique structures. In this study, we synthesized a molecular conductor based on 2,6-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene (trans-TTF-py2). Two pyridyl groups were introduced into the TTF skeleton toward the structural exploration in TTF-based molecular conductors involved by hydrogen bonds. In the obtained molecular conductor, (trans-TTF-py2)1.5(PF6)·EtOH, short contacts between the pyridyl group and the hydrogen atom of the TTF skeleton were observed, indicating that hydrogen bonding interactions were introduced in the crystal structure. Spectroscopic measurements and conductivity measurement revealed semiconducting behavior derived from π-stacked trans-TTF-py2 radical in the crystal structure. Finally, these results are discussed with the quantified hydrogen bonding stabilization energy, and the band calculation of the crystal obtained from density functional theory calculation.

**Keywords:** tetrathiafulvalene; molecular conductor; hydrogen bonding

### **1. Introduction**

Since the first metallic molecular conductor, TTF-TCNQ was reported [1], many researchers have been eagerly exploring TTF-based charge-transfer complexes [2,3], and exotic physical properties have been reported in well-designed molecular crystals in recent years [4,5]. In the progress of molecular conductors, modulating crystal structures has been one of the biggest challenges because the physical properties of molecular conductors heavily depend on their structures. Up to now, there have been several attempts to manipulate crystal structures by introducing supramolecular interactions into the TTF skeleton, such as intermolecular hydrogen bond [6–12] and halogen bond [6,13–16]. Among them, utilizing the hydrogen bond has been gathering attention because of its ability to form various unique structures owing to the moderate bond-dissociation energy (~40 kcal/mol) [17]. Although developments of TTF-based molecular crystals with hydroxy, amide, and cyano groups have been reported so far [6–12], TTF-based molecular conductors with pyridyl groups have been scarcely reported [18,19]. Furthermore, the pyridyl group in the TTF skeleton has often been used for metal coordination [20,21] but not for hydrogen bonding. The hydrogen bonding via pyridyl groups has many advantages in the arrangement of crystal structures because of its well-directed interaction derived from the rigid

structure of the pyridyl group. In addition, the pyridyl group is easy to be introduced into π-conjugated molecular backbones by applying a hetero coupling reaction [22]. To investigate a novel crystal structure of TTF-based molecular conductors, we chose 2,6-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene (trans-TTF-py2) as a starting material. This molecule has two pyridyl groups on the TTF skeleton (Scheme 1), and a unique structure based on a hydrogen bonding via the pyridyl groups can be expected. Although TTF-py2 has been reported as a neutral molecular crystal [23] and as a ligand in coordination polymers [24–28], there is no report of trans-TTF-py2-based conductive crystal. Even in trans-bis-substituted TTF molecules, the crystal structure of the molecular conductor was reported in only one paper to the best of our knowledge [29]. In our paper, a molecular conductor, (trans-TTF-py2)1.5(PF6)·EtOH (**TTF-py2\_PF6**), was prepared through electrochemical crystallization. Single crystal X-ray diffraction (SXRD) revealed a one-dimensional π-stacking structure of oxidized trans-TTF-py2, where the nitrogen atoms of the pyridyl groups have short contacts with hydrogen atoms of the adjacent TTF-py2 molecules, suggesting the existence of significant hydrogen bondings. Spectroscopic analyses and the measurement of electrical conductivity of **TTF-py2\_PF6** were carried out. Its band structure and the stabilization energy of hydrogen bondings calculated by density functional theory (DFT) are also discussed.

**Scheme 1.** Synthesis of TTF-py2.

#### **2. Materials and Methods**

#### *2.1. Methods*

All solvents and reagents used in the syntheses were obtained from commercial sources without further purification. IR spectrum was recorded as KBr pellets on a FT/IR-4200 spectrometer of JASCO, Tokyo, Japan at room temperature (RT). UV-Vis-NIR absorption spectrum was measured as KBr pellets on a V-670 spectrophotometer of JASCO, Tokyo, Japan at RT. Both spectra were connected to represent the solid-state absorption spectrum in a wide range (Figure 2b). The ESR spectra were acquired by using a JES-FA100 of JEOL, Tokyo, Japan. Microscopic Raman Spectrum was measured by using a LabRAM HR-800 of HORIBA, Kyoto, Japan at RT.

1H NMR measurements were performed on Bruker AV500 of Bruker Japan, Kanagawa, Japan at RT. The temperature dependence of the electrical conductivity was measured in a liquid He cryostat of a Quantum Design Physical Property Measuring System (PPMS) MODEL 6000 of Quantum Design Japan, Tokyo, Japan by using the two-probe method in direct current (DC) mode with Keithley sourcemeter model 2611 of Tektronix, Beaverton, Oregon, United States. The cooling rate was 2 K/min. The electrical leads (15 μmϕ gold wires) were attached to a single crystal with carbon paste (Dotite XC-12 in diethyl succinate).

#### 2.1.1. Single X-ray Diffraction

The diffraction data for **TTF-py2\_PF6** were collected on a XtaLAB AFC10 diffractometer with a HyPix-6000HE hybrid pixel array detector, graphite monochromated Mo Kα radiation (λ = 0.7107 Å) and a cryogenic equipment GN-2D/S of Rigaku, Tokyo, Japan. The crystal structure was solved using direct methods (SHELXT) followed by Fourier synthesis. Structure refinement was performed using full matrix least-squares procedures with SHELXL [30,31] on *F* [2], where *F* is the crystal structure factor, in the Olex2 software. [32]

#### 2.1.2. Computational Methods

DFT calculations were performed using the gaussian 16 package [33] for estimation of hydrogen bonding with the counterpoise method [34]. The B3LYP functional [35–37] and the cc-pVTZ basis sets [38] were used for the calculation of hydrogen bonding, because the B3LYP functional has been validated in previous studies for the calculation of hydrogen bonds in several models such as clathrate, radical, and cation-anion systems [39–42]. The atomic coordination of trans-TTF-py2 ligands was extracted from the cif files of **TTF-py2\_PF6** reported herein and used for the calculation without any optimizations. The molecular orbital energies were presented against the vacuum level standard.

Amsterdam Modeling Suite (AMS) packages were applied for the calculations of charge transfer integrals [43,44] and band structure [45] with tight-binding approximation. Charge transfer integrals between adjacent trans-TTF-py2 molecules and band structure of **TTF-py2\_PF6** were investigated by the B3LYP/TZP method, and third-order density-functional-based tight binding (DFTB3) model, respectively, without structural optimization.

#### *2.2. Syntheses*

TTF-py2 was synthesized by following already described procedures [46] (Scheme 1) with a little modification (e.g., reaction temperature and the solvent used for washing in the synthesis of **1**, reaction temperature and the addition of the extraction process in the synthesis of **2,**reaction temperature in the reaction in the synthesis of **3**, and recrystallization process in the synthesis of **4**).

#### 2.2.1. Synthesis of 4-(2-bromoacetyl)pyridine Hydrobromide (**1**)

Bromine (20.00 g, 0.13 mol) was added dropwise to 4-acetylpyridine (15.36 g, 0.13 mol) dissolved in 40 mL of HBr aq.(47–49% wt.) at 70 ◦C. Transparent crystalline solid was immediately precipitated in the solution after the addition of bromine. The precipitate was filtered, washed with acetonitrile, and dried in a desiccator to obtain 4-(2-bromoacetyl)pyridine hydrobromide. Yield: 74.8% 1H NMR (500 MHz, D2O, 298 K, ppm) δ 8.78 (d, 2H), 8.19 (d, 2H), 3.72 (s, 2H).

#### 2.2.2. Synthesis of Potassium Isopropylxanthate (**2**)

Potassium hydroxide (14.03 g, 0.25 mol) were dissolved in 100 mL isopropanol at 80 ◦C and then cooled to 40 ◦C. Pale pinkish suspension was obtained after carbon disulfide (19.04 g, 0.25 mol) were slowly added dropwise to the solution. The precipitate was filtered and extracted with acetone to obtain a bright yellow crystalline solid. Yield: 76.3%. 1H NMR (500 MHz, CDCl3, 298 K, ppm) δ 5.33 (sep, 1H), 1.35 (d, 6H).

#### 2.2.3. Synthesis of O-(1-methylethyl)S-[2-oxo-2-(4-pyridinyl)ethyl]carbonodithioate (**3**)

**1** (7.02 g, 25 mmol) dissolved in 75 mL H2O and **2** (1.30 g, 38 mmol) dissolved in 50 mL H2O were mixed with stirring. Formed transparent crystals were filtered, washed with H2O, and dried in a desiccator overnight to give **3**. Yield: 77.8% 1H NMR (500 MHz, CDCl3, 298 K, ppm) δ 8.86 (d, 2H), 7.80 (d, 2H), 5.70( sep, 1H), 4.59 (s, 2H), 1.38 (d, 6H).
