© 2003 The Chemical Society of Japan Bull. Chem. Soc. Jpn., 76, 97–102 (2003) 97 c. Jpn. e Chemical ciety of Ja- n e Chemical ciety of Ja- n M 03 2 SJA8 09-2673 178 02 02 .4 .7 Structural and Electrical Properties of Novel Molecular Conductors Based on Extended-TTF Donors BDT-TTP and I - Anions Conducting BDT-TTP Salts with I − Anions H. B. Cui et al. HengBo Cui, Takeo Otsuka, Akiko Kobayashi,* Yohji Misaki, † and Hayao Kobayashi †† Research Centre for Spectrochemistry, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033 †Department of Molecular Engineering, Kyoto University, Yoshida, Kyoto 606-8501 ††Institute for Molecular Science, Okazaki 444-8585 (Received June 26, 2002) With the aim of developing organic conductors with new types of molecular arrangements, some organic conduc- tors composed of elongated π donors BDT-TTP (2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) and simple I − anions were prepared by electrocystallization. The crystal of (BDT-TTP) 3 I has a very unique three-dimensional donor arrangement, where six BDT-TTP molecules are crystallographically independent. Despite the three-dimensional inter- molecular short S≥S network, the examination of CwC bond lengths of six BDT-TTP molecules indicates an inhomoge- neous charge distribution, which is unfavorable for the formation of metallic bands. The resistivity measurements showed the system to be a semiconductor with a very small activation energy. On the other hand, the needle-shaped crystal has β-type donor arrangement and 2:1 stoichiometry. The crystal of β-(BDT-TTP) 2 I retains its metallic state down to 2 K. Though an open Fermi surface was obtained by tight-binding band calculation, the fairly strong two- dimensionality of the electronic structure was suggested. The first organic superconductor, Bechgaard salt was report- ed in 1980; 1 it has exerted an extremely large influence on the development of organic conductors in the last two decades. Though some molecular metals consisting of single-compo- nent molecules were recently discovered, 2,3 almost all the mo- lecular conductors currently studied are composed of more than two chemical species because the charge transfer between the molecules forming conduction band and the other chemical species had long been believed to be indispensable for the gen- eration of the charge carriers. In the case of Bechgaard salt with the chemical composition of D 2 X, the crystal consists of π donors (D) forming a conduction band and inorganic anions (X − ) generating hole carriers in the conduction band by ex- tracting one electron from two D molecules. Consequently, the various types of molecular conductors can be developed by the suitable choice of π donors and inorganic anions. Since the donor molecules are responsible for the transport properties, the discovery of new types of donor arrangements such as β- and κ-type arrangements was of key importance to develop the field of organic conductors. 4–6 In this sense, the trials to obtain new types of molecular arrangements by the rare combination of the donor molecules such as BDT-TTP (2,5-bis(1,3-dithiol- 2-ylidene)-1,3,4,6-tetrathiapentalene) with elongated π system and small size anions such as halogen ions (X − ) might be in- teresting because BDT-TTP is a very good π donor to construct the organic conductors with stable metallic states. 7,8 Due to the elongated π donors and small anions, the number of the strong intermolecular contacts can be expected to increase be- tween donor molecules. In order to reduce the Coulomb repul- sion between the positively charged donor molecules, the multi-dimensional molecular conductors with new packing motif might be expected. Here we present the crystal struc- tures and electrical properties of two kinds of BDT-TTP salts with I − anions. Though a number of studies have been made on the organic conductors with I 3 − anions, the reports on the systems with I − anions are still very rare. This seems to be mainly due to the difficulty in the preparation of good crystals. The I − ions tend to be easily oxidized into the I x − polyiodide (I − ) ion during electrochemical oxidation and to give the crystals with disor- dered anion sites, which prevents any further studies. One of the old famous examples of TTF complexes with I − ions is TTF 7 (I) n (n ≈ 5) reported 25 years ago. 9–11 Since then, several compounds composed of TTF-like donors and I − anions have been reported. 12–18 Experimental Synthesis. All the chemical procedures were performed un- der an inert atmosphere. The solvents were reagent grade and were freshly distilled. Black plate crystals of (BDT-TTP) 3 I and black needle crystals of (BDT-TTP) 2 I were grown by electro- chemical oxidation from the mixed solution of 90% chloroben- zene and 10% ethanol (10 mL) containing BDT-TTP (1.5–2 mg) and tetrabutylammonium salt of I − (30–40 mg) under an argon atmosphere at room temperature. Special H-shaped glass cells with electrodes of 1 mm diameter platinum wires were used. A constant current of 0.1 μA was supplied for 15–20 days. Crystal Structure Determination. The crystal structures of (BDT-TTP) 3 I and (BDT-TTP) 2 I were determined on a RIGAKU MERCURY CCD X-ray system at room temperature. The crystal of (BDT-TTP) 3 I with the size of 0.30 × 0.10 × 0.05 mm 3 and that of (BDT-TTP) 2 I with the size of 0.20 × 0.05 × 0.05 mm 3 were