Radical salts of TTF derivatives with the metal–metal bonded [Re2Cl8]

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$X-ray powder diffraction characterization of the elusive ...$

86 E.W. Reinheimer et al. / Journal of Molecular Structure 890 (2008) 81–89 conducted at 110 ± 2 K with graphite monochromated radiation and were corrected for Lorentz and polarization effects. The Siemens SAINT software package was used to integrate the frames. The program SADABS was used to correct for absorption effects [28,29]. The structures were solved by direct methods by the use of the SHELXS-97 program in the Bruker SHELXTL v5.1 interface in the XSEED software package [30–32]. The SHELXL-97 program was used to refine all non-hydrogen atoms with anisotropic thermal parameters by full matrix least squares calculations on F 2 [33]. Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. The solid-state structures for (1 2CH 3CN), (2 6CH 2Cl 2), 3 and 4 are shown in Figs. 1–4. Crystallographic information is summarized in Tables 1 and 2. Relevant bond distances are presented in Tables 3–6. The central Fig. 9. Projection of the crystal structure of 3 in the bc plane. Fig. 10. Projection of the crystal structure of 3 in the ac plane. C@C bond and C–S bonds are the most susceptible to the oxidation state of the donor and have been used by Coppens and coworkers to develop an empirical relationship that can be used to calculate the overall oxidation state of the donor molecule [34]. Calculated average charges for the chalcofulvalenium cations in the compounds, determined via this method, are listed in Table 7. 2.3. Transport properties Electrical conductivity was measured with the four probe technique using a Quantum Design PPMS equipment in the temperature range 300–220 K on single crystals of (1 2CH 3CN) and 3 (below ca. 220 K the resistance of the samples was higher than 10 MX, the limit of our measurement). The contacts between
the platinum wires (25 lm diameter) and the crystals were secured using graphite paste. The samples were measured with cooling and warming rates of 0.5 K min 1 and with a DC intensity current of 0.1 lA. 3. Results and discussion 3.1. Synthesis and structure The radical salts were grown by electrochemical oxidation of the neutral donors ET, TMTTF, TMTSF and o-Me2TTF in the presence of the anion [Re 2Cl 8] 2 . Although the donors are chemically quite similar, different solvents were required in order to grow suitable crystals. (TMTTF) 3[Re 2Cl 8] 2CH 3CN(1 2CH 3CN) and (o-Me 2TTF) 2[Re 2Cl 8] (4) were grown from acetonitrile, (TMTSF)5[Re2Cl8]2 6CH2Cl2 (2 6CH 2Cl 2) from dichloromethane and (BEDT-TTF) 2[Re 2Cl 8] (3) from benzonitrile. It has been well established in the literature that, in the absence of any intermolecular contacts such as p–p or S S interactions, neutral chalcofulvalene donors are non-planar, often exhibiting significant bends of up to 30° along intra-molecular dithiole or diseleno bridges [35]. Upon oxidation of the donor, structural alterations accompanying the formation of the radical cation occur including the adoption of a planar conformation and a lengthening of the central C@C bond with concomitant shortening of the C–S bonds in the central TTF core [36]. The salt (TMTTF) 3[Re 2Cl 8] 2CH 3CN (1 2CH 3CN) crystallizes in the triclinic space group P1 and is characterized by segregated 1D stacks of cations with the anions and solvent molecules occupying the 1D channels present in the structure (Fig. 5). The TMTTF chains are composed of three crystallographically different molecules: A, B and C. The latter two exhibit very short intermolecular contacts (S7–S11 = 3.383(3) Å), less than the sum of the van der Waals radii (3.6 Å), which is the approximate distance that molecule A exhibits with its nearest neighbors. Thus, the chain consists of alternating monomers (A) and dimers (BC) along the a axis. The anions in the interstices are closer to the BC dimers, but no significant directional interactions are evident. According to the central C@C bond and C–S bond distances, molecules B and C bear a charge close to +1, while A is essentially neutral. These assignments are in accord with the strong dimerization observed for the [BC] 2+ dimers, typical of such radical salts, and isolated TMTTF (A) molecules which are close to being neutral. The compound (2 6CH 2Cl 2) is also clearly one-dimensional and crystallizes in the triclinic space group P1. The cations and anions form segregated stacks parallel to the [111] direction (Fig. 6). The TMTSF chains consist of three crystallographically independent TMTSF molecules, arranged in a pattern of the type –BABCC– (Fig. 6). With respect to the intermolecular distances, short Se–Se contacts are evident between A and B (Se2–Se6 = 3.586(4) Å), and between C and C (Se7–Se10 = 3.510(3) Å); in both cases these interactions are much shorter than the sum of the Van der Waals radii ( 4.0 Å) for Se–Se contacts. The shortest B–C distances are over 3.9 Å, thus this chain is actually built from centrosymmetric dimers (CC) and trimers (BAB). The remainder of the cell contents consist of [Re 2Cl 8] 2 anions and dichloromethane molecules. The anions are oriented with their principal axes along the chain and alternate along the [111] axis their position with the solvent molecules in a zig-zag arrangement (Fig. 7). According to the stoichiometry, four positive charges are required to be distributed over five TMTSF molecules, but the estimated charges found for all molecules are well over +1, taking into account the bonding distances of each molecule (+1.4 for A, +1.1 for B and +1.1 for C) [34]. Accurate determination of each donor’s charge is not always possible and is dependent on crystal quality. Based on the Coppens’ formula and the crystallographically derived bond distances, the (CC) di- E.W. Reinheimer et al. / Journal of Molecular Structure 890 (2008) 81–89 87 mers and (BAB) trimers are estimated to possess an overall +2 charge. The crystal structure of 3 is quite different from the previous two cases. In this structure, the anions and cations form mixed layers in the bc plane with an alternating tetragonal arrangement of dimerized ET molecules and anions (Fig. 8). There are two crystallographically independent types of centrosymmetric ET dimers with similar intermolecular distances and orientations. On the basis of the stoichiometry of the compound, all ET molecules should be in the +1 oxidation state. This conclusion was confirmed by the estimation of the charges from the bonding distances (+0.7 and +1.2 for each ET in each dimer) [34]. The intra-dimer contacts between the ET molecules (S S distances range between 3.4 and 3.55 Å) are shorter than the sum of the van der Waals radii (3.6 Å), an indication of the presence of strong electronic interactions. There are no short contacts between dimers, as they appear to be well isolated in the layer by the anions with interlayer distances longer than 4 Å. There are also two crystallographically independent [Re2Cl8] 2 anions with different orientations with respect to the layers, being either perpendicular or parallel to the mean plane of the layers. Consequently, each ET dimer is surrounded by four [Re2Cl8] 2 units, two of each kind. Weak hydrogen bonding is evident between the anions and the ethylene groups, with shortest C–Cl contacts of 3.46(2) Å (C10–Cl1), to produce alternating anion–cation chains in the layer. The solid-state structure of 4 crystallizes in the triclinic space group P1 and is dominated by short S S contacts at distances
Page 1 and 2: Radical salts of TTF derivatives wi
Page 3 and 4: Table 2 X-ray crystallographic and
Page 5: Fig. 7. Projection of the crystal s
Page 9: [9] (a) A. Ota, H. Yamochi, G. Sait

Radical salts of TTF derivatives with the metal–metal bonded [Re2Cl8]

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