Preparation and properties of tetrathia- and tetramethyltetraselena-fulvalene salts of [M6O19]2–(M = Mo or W)

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Abstract
The preparation, X-ray crystal structures, conductivity and optical properties of the salts, [ttf]3[W6O19]1, [ttf]3[Mo6O19]2(ttf = tetrathiafulvalene) and [tmtsf]3[W6O19]·2dmf 3(tmtsf = tetramethyltetraselenafulvalene) are reported. Crystal data: 1, triclinic, space group P, a= 9.965(3), b= 10.503(3), c= 10.634(3)Å, α= 71.93(2), β= 78.63(2), γ= 63.38(3), Z= 1, R= 0.020; 2, triclinic, space group P, a= 9.942(3), b= 10.417(3), c= 10.601(3), α= 72.33(2), β= 78.77(2), γ= 63.52(3)°, Z= 1, R= 0.030; 3, monoclinic, space group P21/c, a= 11.589(4), b= 19.385(5), c= 13.681(3), β= 99.53(2), Z= 2 and R= 0.049. In all the salts the organic molecules form trimerized stacks. A classical ring-over-double bond overlap is observed in 3, in contrast with 1 and 2 which present an unusual intra-trimer criss-cross overlap. In compound 3 the intra-trimer Se ⋯ Se (3.73–3.81 Å) contacts are shorter than those observed in [tmtsf]2X (X = PF6 or BF4) series. The inter-trimer Se ⋯ Se contacts (3.94–4.12 Å) are in the range of the van der Waals separation (4 Å). Short O ⋯ Se (3.17–3.30 Å) contacts are observed. Both electrical and optical measurements and also electronic band-structure calculations reveal that the three salts are semiconductors.