Synthesis, crystal structure and properties of the semiconducting salts (TTF)2[Ni(dtcr)2] and (ET)2[Ni(dtcr)2] based on [Ni(dtcr)2] dianions (dtcr = dithiocroconate)

Abstract

The first examples of CT salts based on [Ni(dtcr)₂] dianions (1) (dtcr = dithiocroconate = 4,5-disulfanylcyclopent-4-ene-1,2,3-trionate), (TTF)₂[Ni(dtcr)₂] (TTF = tetrathiafulvalene) (2) and (ET)₂[Ni(dtcr)₂] [ET = bis(ethylenedithio)tetrathiafulvalene] (3) are reported. The redox-active dianion 1, containing oxo-groups in the periphery of the molecule, has been selected to investigate the role of the oxo-groups in promoting intermolecular interactions and hopefully their conducting properties. The salts 2 and 3 have been prepared by electrocrystallisation methods and 3 shows a semi-metallic behaviour: σ = 1 × 10⁻³ Ω⁻¹ cm⁻¹ at room temperature, with a low activation energy 60 meV, while crystals of 2 were unsuitable for conductivity measurements. The X-ray structural characterisation shows an alternate dianion–(cation)₂ stacking and the capability of the oxo-groups to promote interstack contacts. In 2, the TTF donors are present as face-to-face dimers of monocations (D₂)²⁺. The stacking arrangement is different in 3, where ET monocations stack along two directions ([110] and [10]) in the same manner, with the repeating sequence …(ET)–(ET)–[Ni(dtcr)₂]–(ET)–(ET)… and are almost parallel to each other, with interplanar distances of 3.575 Å. Both structures are built on a dianion and two donor molecules, each one with a charge of +1. Diffuse reflectance combined with vibrational spectra complement structural results well. Crystal data: both 2 and 3 crystallise in the monoclinic space group P2₁/c with a = 8.6340(8) Å, b = 21.586(2) Å, c = 7.5960(8) Å, β = 95.625(11)° and V = 1408.9(2) ų for 2 and with a = 9.3700(7), b = 7.4410(6), c = 28.278(2) Å, β = 99.039(6)°, V = 1947.1(3) ų for 3.