Crystal structure of bis(ethylenedithio)tetrathiafulvalenium μ2-acetato-bis[tribromidorhenate(III)] 1,1,2-trichloroethane hemisolvate

The crystal structure of a binuclear monocarboxylato dirhenium(III) complex with a fulvalene derivative is reported. This compound represents a radical cation salt containing a cluster unit with rhenium–rhenium quadruple bond.


Chemical context
In the past few decades, molecular low-dimensional conducting materials have attracted much interest owing to their physical properties, in particular their electrical, magnetic and spectroscopic properties. The packing of radical cations in the crystal and the properties of radical cation salts depend substantially on the type of anions involved (Mori et al., 1999;Mori, 1999). Labile equatorial chloride or bromide groups around the Re 2 6+ cluster unit are the reactive centres in interactions with other chemical compounds and biological macromolecules (Shtemenko et al., 2013(Shtemenko et al., , 2015. Only one radical cation salt containing a rhenium-rhenium quadruple bond has been described so far {(ET) 2 [Re 2 Cl 8 ] [ET = bis-(ethylenedithio)tetrathiafulvalene]; Reinheimer et al., 2008}. In this context, we present the synthesis and crystal structure of a new radical cation salt of ET with the dirhenium(III) anion [Re 2 Br 6 (CH 3 COO)] À . Neither acetic acid nor acetate was used in the synthesis of this radical cation salt. Evidently, the acetate ligand arose by hydrolysis of CH 3 CN (Cotton et al., 1991). Complex compounds of dirhenium(III) with one equatorial carboxylato ligand are not well studied, the struc- ISSN 2056-9890 ture of only three such rhenium compounds having been reported to date (Lau et al., 2000;Vega et al., 2002;Beck & Zink, 2011).

Structural commentary
The title compound ( Fig. 1) consists of bis(ethylenedithio)tetrathiafulvalene (ET) radical cations, 2 -acetato-bis[tribromidorhenate(III)] anions and 1,1,2-trichloroethane molecules in the stoichiometric molar ratio of 1:1:0.5. The solvent molecule is disordered over two orientations of equal occupancy about a twofold rotation axis intersecting the midpoint of the C-C ethane bond. The tetrathiafulvalene fragment adopts an almost planar configuration (r.m.s. deviation = 0.033 Å ) that is typical for ET radical cations. The dihedral angle between the five-membered rings is 0.3 (6) . The carbon atoms of both ethylenedithio fragments (C4/C5 and C9/C10) are disordered over two sets of sites with occupancy ratios of 0.65:0.35 and 0.77:0.23, respectively.
In the anion, each Re III atom is coordinated by three Br atoms forming ReBr 3 units which are linked by a Re-Re multiple bond [2.2174 (10) Å ] and a bridging 2 -acetate ligand, forming a strongly distorted cubic O 2 Br 6 coordination polyhedron around the Re 2 core. The length of the Re-Re bond is very close to the mean value of 2.222 Å for quadruple bonds (Groom et al., 2016), and the six bromine ligands are arranged into an eclipsed conformation. It is also known that the presence of O,O-bridging ligands in such structures has a negligible effect on the Re-Re bond length [it varies in the range 2.2067 (7)-2.2731 (9) Å for compounds with no bridging ligands and in the range 2.2168 (8)-2.2532 (2) Å for compounds with O,O-bridging ligands (Poineau et al., 2015)]. Thus, the structure of the Re 2 Br 6 CH 3 COO À anion corresponds to the typical structure of compounds with quadruple Re-Re bonds in an Re 2 6+ core (Cotton et al., 2005). The Re-Br and Re-O bonds vary in the ranges 2.435 (3)-2.451 (3) Å and 2.009 (15)-2.040 (16) Å , respectively. The distortion from an ideal cubic geometry is mainly due to the short distance between the O atoms of the acetate group [2.24 (2) Å ], while the BrÁ Á ÁBr separations between adjacent Br atoms vary in the range 3.411 (3)-3.553 (4) Å .

Database survey
compounds of bis(ethylenedithio)tetrathiafulvalene with simple Re-containing anions resulted in eight hits, amongst which one closely related structure containing the ET cation and Re 2 Cl 8 anion (Reinheimer et al., 2008). A search for Re 2 Hal x L y anionic moieties, where Hal is a halogen atom and L is the 2 -carboxylic group, resulted in nine hits. Some closely related patterns were found, e.g. one containing the ( 2 -acetato)-hexachloridodirhenate anion exhibiting the same structure of the title compound (Vega et al., 2002), and one containing the di-2 -acetato-bis(dibromidorhenate) anion (Koz'min et al., 1981).

Synthesis and crystallization
The synthesis of the radical cation title salt was performed by galvanostatic anodic oxidation of ET (0.002 mol l À1 ) in a twoelectrode U-shaped glass cell with platinum electrodes. The initial current intensity of 0.1 mA was increased by 0.05 mA per day to a final value of 0.45 mA. A mixture of 1,1,2-trichloroethane/acetonitrile (12:1 v/v) was used as solvent.
After a period of 6-7 weeks, black shiny plate-shaped crystals of the title salt suitable for X-ray analysis were formed.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were placed in idealized positions and refined using a riding-model approximation, with C-H = 0.96-0.97 Å , and with U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. The 1,1,2-trichloroethane molecule is disordered over two sets of sites about a twofold rotation axis with equal occupancy. The C4-C5 and C9-C10 groups of the ET cations are disordered over two orientations with occupancy factors of 0.65/0.35 and 0.77/0.23, respectively. These occupancies were initially obtained as free variables by the full-matrix refinement, and were then fixed in the final refinement cycles. The C-C and C-Cl bond lengths in the solvent molecule were constrained to be 1.52 (1)  to have the same lengths to within 0.005 Å . The atoms of each disordered fragment, including the solvent molecule, were restrained to have approximately the same displacement parameters to within 0.02-0.04 Å 2 . DELU restraints to within 0.01 Å 2 were applied to atoms C4B, C5B, C9B, C10B, C1S and Cl2S. In addition, all non-hydrogen atoms of the solvent molecule were restrained to be approximately isotropic to within 0.03-0.06 Å 2 . Several outlier reflections (67) that were believed to be affected by the contribution of several unresolved minor twin domains were omitted from the final cycles of refinement, reducing the R factor from 0.061 to 0.052. Attempts to refine the structure using a two-component twin model were unsuccessful. Moreover, the crystals of the title compound are stable but show a strong tendency to splicing. The poor quality of the available crystal may account for the rather low bond precision of the C-C bonds and the presence of several large residual density peaks.  (Agilent, 2014); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Special details
Experimental. Absorption correction: CrysAlisPro (Agilent, 2014) Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.