New Organic Superconductors β-(BDA-TTP)₂X [BDA-TTP = 2,5-Bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene; X^- = SbF₆^-, AsF₆^-, and PF₆^-]

Abstract

The synthesis, electrochemical properties, and molecular structure of a new pi-electron donor, 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP), is described. In contrast to the hitherto-known tetrachalcogenafulvalene pi-donors providing organic superconductors, this donor contains only the bis-fused 1,3-dithiole-2-ylidene unit as a pi-electron system, yet produces a series of ambient-pressure superconductors beta-(BDA-TTP)2X [X = SbF6 (magnetic T(c) = 6.9 K, resistive T(c) = 7.5 K), AsF6 (magnetic T(c) = 5.9 K, resistive T(c) = 5.8 K), and PF6 (magnetic T(c) = 5.9 K)], which are isostructural. The values of the intermolecular overlap integrals calculated on the donor layers of these superconductors suggest a two-dimensional (2D) electronic structure with loose donor packing. Tight-binding band calculations also indicate that these superconductors have the 2D band dispersion relations and closed Fermi surfaces.

Tetrachloroferrate (III) Salts of BDH-TTP [2,5-Bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene] and BDA-TTP [2,5-Bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene]: Crystal Structures and Physical Properties

“…Thus one dithiane ring of each independent BDA-TTP molecule is much flatter than that of each BDA-TTP molecule in the superconducting BDA-TTP salts (6). Additionally, the two trimethylene end groups of each independent BDA-TTP molecule are found on the same side of the molecular plane, which is in contrast to the fact that those of the BDA-TTP molecules in both the neutral state and the superconducting slats appear with opposite orientation with respect to the molecular plane (6). The BDA-TTP molecules form slipped A), so as to avoid the steric hindrance of the less flat dithiane ring.…”

“…1), a sulfur-based non-TTF donor, provides many metallic CT salts stable down to low temperatures regardless of the counteranions used (5) and (ii) new superconducting salts can be obtained from a dithiane analog of BDH-TTP, 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP, Fig. 1), in combination with relatively large anions such as SbF (6). Our attention was thus focused on the Fecontaining BDH-TTP and BDA-TTP salts.…”

Tetrathiapentalene-based organic conductors

“…Yamada et al have developed molecular conductors based on BDH-TTP and 2,5-bis(1,3-dithian-2-ylidene)-1,3, 4,6-tetrathiapentalene (BDA-TTP) [97,98] 1 whose π-electron systems have been reduced by the exchange of double bonds in the outer 1,3-dithiole rings of BDT-TTP with a saturated ethylene or propylene bridge. Cyclic voltammetry revealed that the E value becomes larger than that in BDT-TTP, suggesting an increase of the on-site Coulomb repulsion by reduction of the π-electron system.…”

“…On the other hand, BDA-TTP conductors have a strong tendency to take β-type molecular packing. In these materials, intrastack interaction is smaller compared with that of typical β-type conductors and the degree of dimerization is very large, [98,[129][130][131].…”

Dimensionality and Electrical Properties in Organic Synthetic Metals—Current Results through Selected Recent Examples

“…6. Crystal structure of salt 6 Á Br (30) Recently, Yamada and co-workers presented a crucial result (57) by demonstrating that the TTF core is not essential to the production of organic superconductors. These authors described the first superconducting salts obtained from donors containing the TTP unit as the only pÀelectron system.…”