Structural diversity in conducting bilayer salts (CNB-EDT-TTF)4A

Abstract

The family of recently described salts based on the electron donor CNB-EDT-TTF (5-cyanobenzene-ethylenedithio-tetrathiafulvalene) and different anions A, with general formula (CNB-EDT-TTF)₄A, constitutes an unprecedented type of molecular conductor based on a bilayer structure of the donors. The crystal structures of the bilayer conductors (CNB-EDT-TTF)₄A with small discrete anions are reviewed and analyzed considering previously reported compounds with different anions (A = I₃⁻, ClO₄⁻, BF₄⁻, ReO₄⁻, PF₆⁻ and SbF₆⁻), as well as three new members of this family described here with AsF₆⁻, AuI₂⁻and I₂Br⁻. A common structural feature of this family of compounds is the head-to-head arrangement of the donors, induced by a network of weak CN⋯H–C interactions, which can be described as an effective combination of R²₂(10) and R²₄(10) synthons, forming donor bilayers alternating with anionic layers. The magnitude of the interactions associated with R²₂(10) and R²₄(10) synthons is estimated to be 4.7 and 10.0 kcal mol⁻¹, respectively. In spite of the common donor bilayer arrangement, a rich diversity of structural variations and polymorphs is observed. These structures are related to different types of donor or anion arrangements, which are analyzed. For all anions a β′′-type packing pattern of the donor molecules arranged in virtually identical bilayers is observed. However, for (CNB-EDT-TTF)₄I₃, so far a unique exception in this family of compounds, a distinct κ-type packing of the donors, is also obtained as a metastable phase as shown also by DFT calculations that predict the κ-phase above the β′′-phase. Another type of polymorphism is observed in β′′-(CNB-EDT-TTF)₄A, compounds with tetrahedral anions, related to the tilting of the donor molecules (either alternating or uniform) in successive bilayers, leading to monoclinic or triclinic unit cells, with doubling of one cell parameter corresponding to the thickness of one bilayer (∼25 Å). Finally, the anions, which often appear with fractional site occupation factors, can be ordered in the anionic plane, with different degrees of correlation between successive anion planes as shown in the AsF₆.compound. The implications of these structural features in the electronic properties of these compounds are also discussed.