Synthesis, structures and physical properties of bis(ethylenedithio)tetrathiafulvalenium salts of paramagnetic metallacarborane anions

Abstract

Single phases of the bis(ethylenedithio)tetrathiafulvalenium (et˙⁺) salts [et]₂[Cr(1,2-C₂B₉H₁₁)₂]1 and [et]₂[Fe{1-SC₄H₃-1,2-C₂B₉H₁₀}₂]2(SC₄H₃= thiophen-2-yl) were synthesised via chemical and electrochemical routes, respectively. The crystal structures of both salts feature alternating layers of et units and metallacarborane anions. The packing motif of 1 is very similar to that of the β′-[et]₂X salts {X =[ICl₂]^– or [BrlCl]^–}, with the rod-shaped [Cr(C₂B₉H₁₁)₂]^– anions functioning as paramagnetic analogues of the trihalide anions and the et units being linked by short intermolecular S ⋯ S interactions (3.4–3.6 Å) to form one-dimensional ‘tapes’ extending in the (1 0 0) direction. The et units in 2 are packed more evenly, resulting in a two-dimensional honeycomb network of short intermolecular S ⋯ S interactions (3.4–3.6 Å) within each et layer. The [Fe(C₂B₉H₁₀C₄H₃S)₂]^– anions in 2 pack to form unusual double-layers, in which the thiophene groups are oriented towards each other and away from the et layers, indicating that inter-et S ⋯ S interactions are favoured over et–thiophene S ⋯ S interactions. Both compounds 1 and 2 are semiconductors with complex temperature-dependent conductivities between ca. 75 and 280 K. Their conductivities at room temperature are ca. 2 × 10^–3 S cm^–1(1, compressed pellet) and 5 × 10^–1 S cm^–1(2, single crystals). The absence of metallic behaviour in 1 and 2 may be attributed in part to the loose packing of the et molecules, due to the large size of the anions, in these compounds. The magnetic susceptibilities of 1 and 2 were fitted by the Curie–Weiss law between 25 and 300 K to give Weiss constants of –1.1 and +1.6 K respectively.