New Radical-Cation Salts Based on the TMTTF and TMTSF Donors with Iron and Chromium Bis(Dicarbollide) Complexes: Synthesis, Structure, Properties

Radical-Cation Salts Based on the TMTTF and TMTSF Donors with Iron and Bis(Dicarbollide) Complexes: recognition of his outstanding contribution to the chemistry of carboranes. Abstract: New radical-cation salts based on tetramethyltetrathiafulvalene (TMTTF) and tetram-ethyltetraselenefulvalene (TMsTSF) with metallacarborane anions (TMTTF)[3,3 (cid:48) -Cr(1,2-C 2 B 9 H 11 ) 2 ], (TMTTF)[3,3 (cid:48) -Fe(1,2-C 2 B 9 H 11 ) 2 ], and (TMTSF) 2 [3,3 (cid:48) -Cr(1,2-C 2 B 9 H 11 ) 2 ] were synthesized by electro-crystallization. Their crystal structures were determined by single crystal X-ray diffraction, and their electrophysical properties in a wide temperature range were studied. The ﬁrst two salts are dielectrics, while the third one is a narrow-gap semiconductor: σ RT = 5 × 10 − 3 Ohm − 1 cm − 1 ; E a ≈ 0.04 eV (aprox. 320 cm − 1 ).


Introduction
Radical-cation salts and charge transfer complexes based on derivatives of tetrathiafulvalene (TTF) constitute a wide class of organic materials with transport properties ranging from insulating to superconducting [1][2][3][4]. This work is part of the systematic study of radical-cation salts of tetrathiafulvalene and its derivatives with metallacarborane anions, of which earlier results were summarized in works [5][6][7].

Results and Discussion
Single crystals of compounds 1-3 suitable for X-ray diffraction studies in the form of thin plates were obtained by electrochemical crystallization (See Supplementary Materials and Table 1). The crystal structure of 1 is formed by the TMTTF radical-cations and the [3,3 -Cr(1,2-C 2 B 9 H 11 ) 2 ] − anions occupying general positions in the unit cell ( Figure 1). (TMTTF)[3,3 -Cr(1,2-C 2 B 9 H 11 ) 2 ] has a pseudo-layered structure, in which anionic layers alternate along the ac diagonal with layers formed by radical-cation dimers ( Figure 2). The dimer formation corresponds to the stoichiometry of the salt: in this case due to the Peierls instability a phase transition should occur with doubling of the stacks period [7]. The distances between the averaged planes of the TMTTF donors in the dimers are 3.38 Å (the planes are drawn through all S atoms), and the dihedral angle between the planes is 0 • by symmetry conditions. There are short intermolecular S . . . S interactions (3.426(1)-3.432(1) Å) of the "face-to-face" type between the TMTTF donors in the dimers.        The TMTTF + radical-cations are non-planar and have a "boat" conformation: the maximum deviations of terminal C(9), C(10), C(11), and C(12) atoms from the plane of the averaged molecule drawn through all sulfur atoms are 0.30-0.36 Å.
The Cr-C and Cr-B bond lengths are 2.173(2)-2.180(2) and 2.232(2)-2.279(2) Å, correspondingly. The distances from the chromium atom to the C 2 B 3 faces of the dicarbollide ligands are equal to 1.68 Å, which is close to the corresponding distances found in the structures of Cs The electrical conductivity measurements have shown that 1 is an insulator with σ 293~1 0 −11 Ohm −1 cm −1 . The low value of electrical conductivity is apparently connected with the absence of conducting layers and dimerization of the radical-cations stacks.
The crystal structure of (TMTTF)[3,3 -Fe(1,2-C 2 B 9 H 11 ) 2 ] (2) is formed by a quarter of the TMTTF radical-cation in a special position placed on the m plane and a quarter of the [3,3 -Fe(1,2-C 2 B 9 H 11 ) 2 ] − anion in the 2/m special position of the unit cell ( Figure 3). The compound 2 is characterized by a structure where the TMTTF cations and the metallacarborane anions form staggered stacks (Figures 4 and 5). The distances between the averaged planes of the TMTTF donors in the dimers are 3.38 Å, and the dihedral angle between the planes is 0 • by symmetry conditions.         The Fe-C and Fe-B bond lengths are 2.0790(9)-2.1001(8) and 2.1001(8)-2.1494(8) Å, correspondingly, and the overlapping values are due to the statistical disordering of carbon and boron atoms in the dicarbollide ligands. The distances from the iron atom to the C 2 B 3 faces of the dicarbollide ligands are equal to 1.53 Å, which is close to the distances in analogous salts of the iron bis(dicarbollide) anion [19,24,25]. The dicarbollide ligands are turned relative to each by 180 • , forming the transoid conformation. The C 2 B 3 faces are parallel by symmetry conditions.
According to the electric conductivity measurements, compound 2 is an insulator with conductivity~10 −10 Ohm −1 cm −1 . The low value of electroconductivity is in an agreement with the 1:1 stoichiometry and non-layered structure of the salt, as well as with the inclination angle of the radical-cations in the stack, at which there is only slight overlap between neighboring radical-cations.