Molecular structure and solid-state properties of the two-dimensional conducting mixed-valence complex [NBu4]0.29[Ni(dmit)2] and the neutral [Ni(dmit)2](H2dmit = 4,5-dimercapto-1,3-dithiole-2-thione); members of an electron-transfer series

Abstract

Single crystals of the mixed-valence radical salt [NBu₄]₂[Ni(dmit)₂]₇·2CH₃CN {ignoring the solvent molecules, the formula may be written as [NBu₄]_0.29[Ni(dmit)₂]; H₂dmit = 4,5-dimercapto-1,3-dithiole-2-thione} were obtained by galvanostatid oxidation of [NBu₄][Ni(dmit)₂] in acetonitrile. The crystal structure of this compound has been determined at 118 K by X-ray diffraction studies: crystals are triclinic, space group P1 or P(assumed), with Z= 2 and unit-cell dimensions a= 13.425(2), b= 22.791(3), c= 24.183(4)Å, α= 108.49(1), β= 103.02(1), and γ= 89.82(1)°. The structure was solved by direct methods and refined by least squares to R= 0.038 for 7 170 unique, observed, diffractometer data. It consists of thick layers of stacked Ni(dmit)₂ entities parallel to (001), and separated by sheets of [NBu₄]⁺ cations and CH₃CN molecules. Strong π interactions within a stack are evidenced by the short stacking distances (3.48–3.57 Å). There is also an extensive interleaving of the Ni(dmit)₂ which involves close S ⋯ S interstack contacts. As a result, this structural arrangement is nearly two-dimensional. Conductivity measurements, carried out using a four-probe technique, show a high (σ= 1–10 Ω^–1 cm^–1 at 300 K) thermally activated (E_a= 0.1–0.02 eV) conductivity. Measurements of the conductivity along the b and a axes, using the Montgomery method, show a low anisotropy (σ_b/σ_aca. 1), consistent with the two-dimensional nature of this compound. The related neutral member of the [Ni(dmit)₂]^n– electron-transfer series (n= 0) has been isolated for the first time and its crystal structure determined: crystals are monoclinic, space group P2₁/a, with Z= 2, a= 17.108(9), b= 5.302(4), c= 7.720(4)Å, and β= 77.09(4)°. The structure was solved by direct methods and refined by least squares to R= 0.041 for 772 unique, observed, diffractometer data. The structure of this semi-conducting compound consists of regular stacks of Ni(dmit)₂ along the [010] direction with short interstack S ⋯ S distances.