**1. Introduction**

Since the discovery of the first molecular metal, TTF-TCNQ (TTF = tetrathiafulvalene, TCNQ = tetracyanoquinodimethane) charge transfer complex in 1973 [1], many various types of molecular conductors have been reported, including famous examples such as (TMTSF)2PF6 (TMTSF = tetramethyltetraselenafulvalene)—the first molecular organic superconductor [2], and λ- (BETS)2FeCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene)—a field induced superconductor [3,4]. These traditional charge transfer-based molecular conductors inherently consist of more than one kind of molecule, a donor as well as an acceptor. However, the first single-component molecular metal [Ni(tmdt)2] (tmdt = trimethylenetetrathiafulvalenedithiolate) developed in 2001 has opened a new field of conducting materials [5,6]. [Ni(tmdt)2] consists of only one kind of neutral molecule, and exhibits metallic behavior down to 0.6 K. The observation of de Haas-van Alphen oscillations, at very high magnetic fields and low temperatures, showed the existence of the three-dimensional Fermi surfaces [7], which was also proved by ab initio band structure calculations [8]. After that, a number of single-component molecular conductors have been reported, such as [Au(Me-thiazdt)2] (Me-thiazdt = *N*-methyl-1,3-thiazoline-2-thione-4,5-dithiolate)—a single-component molecular metal without any TTF unit [9], and TED (= tetrathiafulvalene-extended dicarboxylate radical)—a single-component pure organic metal [10].

For single-component molecular conductor [M(L)2] systems, an important characteristic is that its electronic structure can be widely tuned by exchanging the central transition metal atom (M) for another transition metal atom, even among isostructural systems. The series of isostructural systems, [M(tmdt)2] (M = Ni, Cu, Pd, Pt, Au) is a typical case. [Ni(tmdt)2] and [Pt(tmdt)2] exhibit very high conductivity and metallic behavior down to extremely low temperatures [11,12]. [Au(tmdt)2] is a

hybrid antiferromagnetic metal, and undertakes an antiferromagnetic transition at around 110 K as well as retaining its metallic state [13,14]. [Cu(tmdt)2] is a hybrid Mott insulator, and shows a one-dimensional antiferromagnetic Heisenberg behavior with magnetic ordering at 13 K [15,16]. However, [Pd(tmdt)2] with an even number of total electrons, is an antiferromagnetic semiconductor, and takes out a magnetic ordering onset exceeding 100 K due to strong electron correlation [17,18].

On the other hand, the difference of the ligands has an important effect or influence on the electronic structures and band structures of single-component molecular conductors. Actually, a variety of electronic structures have been realized by using similar extended-TTF dithiolate ligands with different terminal groups (as shown in Scheme 1). For example, unlike the isostructural [M(tmdt)2] systems with a tight three-dimensional molecular packing, [M(ptdt)2] (M = Ni, Au; ptdt = propylenedithiotetrathiafulvalenedithiolate) [19,20] and [M(hfdt)2] (M = Ni, Au, Pd; hfdt = bis(trifluoromethyl)tetrathiafulvalenedithiolate)[21,22] crystallize in a layered two-dimensional (2D) molecular packing. Especially, [Ni(hfdt)2] is a single-component molecular superconductor with transition temperatures at 5.5 K under high pressures around 8 GPa [23]. Recently, [Pt(dmdt)2] has been found to host strongly correlated massless Dirac electrons with nodal lines at ambient pressure [24].

**Scheme 1.** Chemical structure of single-component molecular conductor [M(L)2] systems mentioned in this paper.

It is well known that, for BEDT-TTF (= bis(ethylenedithio)tetrathiafulvalene) charge transfer complexes, the conformational flexibility of the terminal ethylene groups yields a variety of the crystal structures and the resultant electronic structures. For example, α-(BEDT-TTF)2I3 is a molecular Dirac electron system under high pressure [25], as well as β-, κ-, and θ-(BEDT-TTF)2I3 are ambient pressure molecular superconductors [26,27]. To develop new types of single-component molecular conductors with novel electronic structures and physical properties, we have tried to prepare a series of dithiolate complexes, with etdt (= ethylenedithiotetrathiafulvalenedithiolate) ligand, which contain the same terminal ethylene group to that of BEDT-TTF molecule. Although, some similar dithiolate complexes had been reported by G. Matsubayashi et al. [28,29], their oxidized neutral species have not been studied. We report here, syntheses, crystal structures, and physicals properties of new neutral gold dithiolate complexes, [Au(etdt)2]·THF.

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

#### *2.1. General Methods*

All the syntheses were carried out under argon atmosphere using the Schlenk technique because the anionic state of metal complexes with extended-TTF dithiolate ligands are quite sensitive to oxygen. The etdt ligand [30] and the gold source, tetra-n-butylammonium tetrachloroaurate(III) ( nBu4N·[AuCl4]) [31], were prepared according to procedures reported in literature. Tetrahydrofuran (THF) was distilled over Na metal and benzophenone. Methanol (MeOH) was distilled over Mg metal activated with I2. The supporting electrolytes tetra-n-butylammonium hexafluorophosphate ( nBu4N·PF6) used in the electrocrystallization was recrystallized three times with ethylacetate and dried in vacuo. All other reagents were used as purchased without any further purification.
