Crystal structures, electrical conductivity and band-structure calculations of three new [cation][Ni(C3S5)2]2compounds

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Abstract
The mixed-valence radical salts [smdt][Ni(C3S5)2]21, [dmp][Ni(C3S5)2]22 and [dmm][Ni(C3S5)2]23(C3S5 2–= 1,3-dithiole-2-thione-4,5-dithiolate, smdt =S-methyl-1,3-dithianium, dmp =N,N-dimethylpyrrolidinium and dmm =N,N-dimethylmorpholinium) have been obtained by electrochemical oxidation of the corresponding [cation][Ni(C3S5)2] complexes in acetonitrile. Single-crystal X-ray studies revealed that all three crystallise in essentially the same packing mode. Their structure consists of anionic diads stacked face-to-face along one direction. The stacks form layers of Ni(C3S5)2 units which are separated by the cations. In the structure of 3 two types of Ni(C3S5)2 layers exist, exhibiting different structural and electrical properties. The acceptor molecules neighbouring along the longest axis make an angle of 135 (1), 139 (2) and 152°(3) with each other, thereby forming a herringbone motif. In all three structures, short intermolecular S ⋯ S contacts can be found predominantly between Ni(C3S5)2 units lying side-by-side to each other, but also within the stacks in 3. At ambient pressure, all three Ni(C3S5)2 compounds have room-temperature conductivities ranging between 1 and 70 S cm–1. Their thermal behaviour is consistent with semiconductors having activation energies of about 0.14 eV. Band-structure calculations were carried out and found to be in agreement with the observed conductivities when some degree of electron correlation is taken into account. Calculation of molecular orbital overlaps indicated dimerisation of the anions, with a weak, essentially two-dimensional conduction pathway for salts 1 and 2. In the crystal lattice of 3 one Ni(C3S5)2 layer can be considered as conducting along the stacking direction, while in the second type of layer a two-dimensional conduction pathway appears to be present.