Synthesis, Structure and Properties of an Unsymmetrical Tetraselenafulvalene Donor Fused with a Pyrazino-Ring (PEDTTSeF) and its Cation Radical Salt

Abstract

Since the discovery of metallic and superconducting states in several cation radical salts derived from tetrachalcogenafulvalenes, the realization of two-dimensional (2D) electron states has been crucial to the development of new organic conducting systems. Among them, bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) has given rise to more superconducting salts than any other tetrathiafulvalene (TTF) derivatives, where transverse intermolecular S±S networks play an essential role in the formation of 2D Fermi surfaces. [1] Furthermore, the selenium substitution of the inner sulfur atoms of BEDT-TTF is expected to enhance the transverse chalcogen±chalcogen interaction and to stabilize the metallic state at low temperature. Such a strong stabilization has been observed in the salts of bis (ethylenedithio)tetraselenafulvalene (BETS), [2] from which new organic superconductors have been discovered. [3] Hence the development of the selenium analog of TTF-type donor is of interest; however unsymmetrical tetraselenafulvalene (TSeF) derivatives are not as well studied because they are difficult to synthesize. On the other hand, recently the pyrazino-condensed TTF-type donors have received attention as materials for realizing the 2D conducting property through the N±S and S±S intermolecular contacts. [4] Such a 2D network has been observed, for example, in the salts of pyrazinoethylenedithio-TTF (PEDTTTF), synthesized by G. C. Papavassiliou et al. [5,6] So far, however, no metallic salts based on PEDTTTF have been found to be stable at low temperatures. With this in mind, we focused on introducing the TSeF framework to PEDTTTF in order to realize the intermolecular network through the N±S and N±Se contacts and to stabilize the metallic state. In this communication, we report the synthesis, crystal structure and electro-chemical properties of a novel unsymmetrical TSeF donor, pyrazinoethylenedithio-TSeF (PEDTTSeF) 1 (Scheme 1), along with the crystal and band structures, and the electrical and magnetic properties of its cation radical salts.PEDTTSeF was synthesized by a cross-coupling reaction of pyrazino-1,3-diselenol-2-one 2 [7] and 4,5-ethylenedithio-1,3-diselenol-2-one 3 [2b] with trimethylphosphite in refluxing toluene. The reaction mixture was reduced in vacuo and the resultant red oil was purified by column chromatography (silica gel, dichloromethane). The crude product obtained was recrystallized from dichloromethane/n-hexane to afford air-stable red crystals in 32 % yield. [8] Scheme 1. X-ray crystallographic analysis was performed on a neutral molecule recrystallized from carbon disulfide/n-heptane. [9] As shown in Figure 1, the TTF moiety has a bent structure with a dihedral angle of 16.9, like the structure of neutral BEDT-TTF, [10] while the pyrazino-ring moiety has a planar structure. The ethylene bridge displayed conformational disorder. Fig. 1. Molecular structure of neutral PEDTTSeF. a) Top view. b) Side view.The electrochemical properties of PEDTTSeF were investigated by cyclic voltammetry in PhCN co...