*3.2. Crystal Structure of Neutral Gold Complex* **2**

Although the single crystal size is very small and thin, the crystal structure of neutral complex **2** has been successfully determined by single crystal X-ray structure analysis. The neutral complex **2** crystallizes into the triclinic system with space group of *P*-1. One neutral [Au(etdt)2] molecule and one unexpected THF molecule are crystallographically independent in the unit cell. In traditional donor-acceptor molecular conductor systems, there are several examples reported which contain solvent molecules, such as β-(BEDT-TTF)4[(H3O)Fe(C2O4)3]·PhCN—the first paramagnetic molecular superconductor [38], (Me4N)[Ni(ptdt)2]·Me2CO—a monoanion dithiolate nickel complex [19]. However, to the best of our knowledge, **2** is a rare case of solvent-containing neutral dithiolate complexes with extended-TTF ligands. Since **2** exhibits a distance of 3.240 Å between the oxygen atom of THF and ethylene groups of the etdt ligand (O···H-C), the THF molecules may be stabilized by the weak intramolecular hydrogen bonding. In general, solvent molecules do not contribute to the electronic structure, hence the neutral gold complex **2** is still single-component molecular conductor.

The molecular structure and the packing diagram viewed along the different axes of **2** are shown in Figure 2. The Au(III) atom in the neutral [Au(etdt)2] molecule also show a square-planar coordination geometry, with Au-S distance of 2.318–2.336 Å, and an average S-Au-S angle of 91.10◦, which is similar to that of **1**. The oxidized extended-TTF ligands in single-component molecular conductors usually became planar. However, as shown in Figure 2b, one of the etdt ligands in the neutral [Au(etdt)2] molecule is still bent, which might be due to the space steric hindrance effect caused by the presence of THF molecules. On the other hand, the C=C distances in the TTF unit of the planar ligand are 1.340–1.360 Å, which is longer than that of **1**. Consequently, similar to that of reported single-component molecular conductors, the electrochemical oxidization was mainly carried out at the extended-TTF ligands.

**Figure 2.** (**a**) Top view of the neutral [Au(etdt)2] molecule without THF molecule. (**b**) Side view of the neutral [Au(etdt)2] molecule with THF molecule. (**c**) Crystal structure of **2** viewed along the long axes of molecules. The short S···S contacts (<3.7 Å) are shown as dotted line. THF molecules are omitted for clarity. (**d**) Crystal structure of **2** viewed along the *a*-axis.

As shown in Figure 2c, the neutral [Au(etdt)2] molecules are stacked face-to-face to form a dimeric column along the *b*-axis, with interplanar distances of 3.472 and 3.906 Å, respectively. The dimeric columns are arranged side-by-side along the *a*-axis to form a conduction layer parallel to the *ab* plane. There are many intermolecular S···S short contacts are observed in the molecular layer. Especially along the *a*-axis, a shortest intermolecular S···S distance of 3.394 Å suggests that **2** would be a satisfactory single-component molecular conductor with the relatively high conductivity. As shown in Figure 2d, the [Au(etdt)2] conducting layers are strongly separated by the terminal ethylene groups and THF molecules along the *c*–axis, forming a 2D electronic structure. As mentioned before, such similar 2D electronic structure has also been observed in single-component molecular conductors with bulky ligands, [M(ptdt)2] and [M(hfdt)2].

#### *3.3. Electrical Properties of Neutral Gold Complex* **2**

Since the single crystal size of **2** was very small, resistivity measurements were performed by the standard four-probe method using compressed pellets of polycrystalline samples down to 40 K cooled by the liquid helium. The room-temperature conductivity (σRT) of **2** is about 0.2 S/cm, which is somewhat high for compressed pellet sample of 2D molecular conductor. As shown in Figure 3, the resistivity increases with decreasing temperature, and exhibits a semiconducting behavior with an activation energy (*E*a) of about 0.1 eV in the temperature range of 200–300 K. Considering that the measurements were carried out on compressed pellets, **2** should be a fairly good conductor in the single crystalline state.

**Figure 3.** Temperature dependence of electrical resistivity of **2** measured by using compressed pellets.

#### *3.4. Magnetic Susceptibility of Neutral Gold Complex* **2**

The static magnetic susceptibilities of **2** were measured using a SQUID magnetometer at 5000 Oe in the temperature range of 2–300 K (Figure 4). After correction for the diamagnetic contribution of <sup>−</sup>4.0 <sup>×</sup> <sup>10</sup>−<sup>4</sup> emu/mol, the room-temperature susceptibility of **<sup>2</sup>** was almost zero (small than <sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>5</sup> emu/mol). The susceptibility values can be fitted well by the Curie–Weiss law over the entire temperature range, with a Curie constant of 2.2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>K</sup>·emu/mol and a very small Weiss temperature of −0.15 K, which usually correspond to paramagnetic (*S*1/2) impurities of 0.6%. Consequently, the magnetic susceptibility measurements suggest that **2** is essentially non-magnetic, which is consistent with the dimeric structure, and its semiconducting nature.

**Figure 4.** Temperature dependence of magnetic susceptibility of **2** at the field of 5000 Oe. The blue line is the Curie–Weiss fitting curve as described in the text.

#### *3.5. Electronic Structures and Band Structure Calculations of Neutral Gold Complex* **2**

The MO, band structure, and DOS calculations were performed by the DFT method. The spin polarized molecular orbitals and the energy levels of the neutral [Au(etdt)2] molecule are shown in Figure 5a. The frontier orbitals near the Fermi level are very similar to those of reported gold dithiolate complexes with extended-TTF ligands, such as [Au(tmdt)2]. The singly occupied molecular orbital (SOMO) of [Au(etdt)2] is composed of an anti-symmetric combination of the left- and right-ligand π orbitals and a small contribution of the *d* orbital of the Au(III) atom. As a result, the spin density distribution of the [Au(etdt)2] molecule shown in Figure 5b is mainly distributed on the ligand. As compared to the neutral [Au(tmdt)2], which becomes a magnetic metal exhibiting antiferromagnetic transition at 110 K, the neutral [Au(etdt)2] becomes a non-magnetic semiconductor owing to the dimeric structure.

**Figure 5.** (**a**) Spin polarized molecular orbitals and the energy levels of the neutral [Au(etdt)2] molecule. (**b**) Spin density distributions of the neutral [Au(etdt)2] molecule.

*Crystals* **2020**, *10*, 1001

The band energy dispersion curve and DOS of **2** are shown in Figure 6. The energy dispersion is very small along the *c*\* direction, but exhibits a considerable energy dispersion along the *a*\* and *b*\* directions, indicating the 2D nature of the system. The calculated DOS also give a band gap (Δ*E*) of about 0.20 eV, which is consistent with the semiconducting behavior and *E*<sup>a</sup> (Δ*E* ≈ 2*E*a) for the resistivity measurements.

**Figure 6.** (**a**) The band energy dispersion curve of **2**. The symbols Γ, *X*, *Y*, *Z*, and *Q* represent the following positions in the reciprocal space: Γ (0,0,0), *X* (1/2,0,0), *Y* (0,1/2,0), *Z* (0,0,1/2), and *Q* (0,1/2,1/2). (**b**) The density of states (DOS) of **2**.

#### **4. Conclusions**

In conclusion, a new neutral gold dithiolate complex with an extended-TTF ligand, [Au(etdt)2](THF) (**2**), was prepared. Unlike the reported single-component molecular metals, **2** is a rare case of a solvent-containing single-component molecular conductor. The crystals of **2** are composed of 2D conducting layers of [Au(etdt)2] molecules, which are strongly separated by the terminal ethylene groups and THF molecules. The resistivity measurements performed on the compressed pellets of

samples of **2** exhibit fairly high room-temperature conductivity of 0.2 S/cm and a low activation energy of 0.1 eV, which are consistent with the result of the DFT band structure calculations. The observed non-magnetic behavior of **2** is consistent with the dimeric structure of [Au(etdt)2] molecules, and its semiconducting nature. Such results confirm that the crystal structures and electronic structures of the single-component molecular conductor [M(L)2] system can be tuned by adopting various combinations of M and L.

**Author Contributions:** Conceptualization, B.Z. and A.K.; investigation, K.S., Y.I., B.Z. and A.K.; data curation, K.S., Y.I. and B.Z.; writing—original draft preparation, B.Z., H.K. and A.K.; writing—review and editing, B.Z., H.K. and A.K.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by "JSPS KAKENHI, grant number 17K05846".

**Acknowledgments:** The authors would like to thank Nanotechnology Platform Program (Molecule and Material Synthesis) of MEXT, Japan.

**Conflicts of Interest:** The authors declare no conflict of interest.
