Bis[bis(pentamethylcyclopentadienyl)cobalt(III)] tetrachloridocobaltate(II) dichloromethane disolvate

The title compound, [Co(C10H15)2]2[CoCl4]·2CH2Cl2, was isolated as a dichloromethane solvate and was formed in the reaction between lithium pentamethylcyclopentadienide and anyhydrous cobalt(II) chloride in tetrahydrofuran. There are two decamethylcobaltocenium cations, one tetrachloridocobaltate(II) anion and two dichloromethane solvent molecules in the formula unit. There is a slight disorder of the dichloromethane solvent which was treated with a two-site model [occupancy rates = 0.765 (4) and 0.235 (4)]. The dichloromethane molecules display significant C—H⋯Cl interactions with the tetrachloridocobaltate(II) dianion. The cobalt atom of the decamethylcobaltocenium cation sits on a twofold rotation axis, with only one pentamethylcyclopentadiene ligand being unique and the second generated by symmetry. The cobalt atom of the [CoCl4]−2 ion sits on a special site with -4 symmetry, with one unique chloride ligand and the others generated by the fourfold inversion axis.

The title compound, [Co(C 10 H 15 ) 2 ] 2 [CoCl 4 ]Á2CH 2 Cl 2 , was isolated as a dichloromethane solvate and was formed in the reaction between lithium pentamethylcyclopentadienide and anyhydrous cobalt(II) chloride in tetrahydrofuran. There are two decamethylcobaltocenium cations, one tetrachloridocobaltate(II) anion and two dichloromethane solvent molecules in the formula unit. There is a slight disorder of the dichloromethane solvent which was treated with a two-site model [occupancy rates = 0.765 (4) and 0.235 (4)]. The dichloromethane molecules display significant C-HÁ Á ÁCl interactions with the tetrachloridocobaltate(II) dianion. The cobalt atom of the decamethylcobaltocenium cation sits on a twofold rotation axis, with only one pentamethylcyclopentadiene ligand being unique and the second generated by symmetry. The cobalt atom of the [CoCl 4 ] À2 ion sits on a special site with 4 symmetry, with one unique chloride ligand and the others generated by the fourfold inversion axis.

Related literature
For a related structure with a (THF) 2 LiCl 2 CoCl 2 monoanion and the decamethylcobaltocenium cation, see: Dehnen & Zimmermann (2000) (CCDC 135478). The structure of a related dimer synthesized by Koelle et al. (1986) Table 1 Hydrogen-bond geometry (Å , ). of the dimer. However, in our laboratories, regardless of the stoichiometry used in attempts to make the chloro bridged dimer described by Koelle et al. (1986), the title compound was the only material isolated. The composition of the title compound consists of two decamethylcobaltocenium cations, one tetrachlorocobalt(II) dianion, and two molecules of dichloromethane. The dichloromethane molecules were slightly disordered and the disorder was treated successfully by a two site model with occupancies of 76.5 (4) and 23.5 (4)%. Dehnen & Zimmerman (2000) noted a similar product in attempts to make selenium-bridged compounds and noted the decamethylcoblatocenium ion was formed depending on the temperature of the reaction. In their case, the CoCl 4 −2 ion was linked via chloride bridges to a bis-THF Li + cation.
In the structure reported here, the CoCl 4 −2 ion shows significant interaction with the dichloromethane of solvation.
Recently, Allen et al. (2013) examined the Cambridge Crystallographic Data Base and analyzed crystallographic evidence of C-H···Cl hydrogen bonding for both CH 2 Cl 2 and CHCl 3 . In that paper, for the specific case of CH 2 Cl 2 interacting with Cl-, they note C-H···Cl interactions with H···Cl distances ranging from 2.33 to 2.95 Å and C-H···Cl angles ranging from 120° to 170° for a set of 63 structures. In the title structure, the H···Cl distance is 2.72 Å with a C-H···Cl angle of 145.0°. These parameters would place the C-H···Cl interaction for the title structure very nearly at the median of the structures analyzed by Allen et al. (2013).
In the structure reported here, the CoCl 4 2− ion is also distorted from perfect tetrahedral geometry with the Cl-Co-Cl angles involved in the hydrogen bonding compressed to 100.04 (2)° and the remaining Cl-Co-Cl angles are 114.385 (12)°. While it is important not to make too much of a qualitative observation, the strength of the C-H···Cl interaction may also be responsible for the relative stability of these crystals in open air. Our experience is that, in the abence of a significant attractive interaction, dichlromethane molecules quite easily evaporate from crystals with loss of crystallinity at room temperature.

Experimental
The procedure described by Koelle et al.(1986) was followed using lithium pentamethylcyclopentadienide (LiCp*) and anhydrous cobalt(II) chloride in tetrahydrofuran. Instead of obtaining the hexane soluble brown dimer as described, the reaction produced a green solid. Dissolution of the solid in dichloromethane followed by slow diffusion of diethyl ether produced well formed green prisms of the title compound that are very air stable and retain the dichloromethane of solvation even after several weeks exposure to the open atmosphere at room temperature.

Figure 1
Plot of the title compound displaying the complete molecular fragments with thermal ellipsoids shown at 50% probability level. Only the major component (76.3 (4)%) of the disordered dichloromethane solvate is shown. Symmetry codes: (1) 3/2 − x, 1/2 − y, z; (2) 3/2 − x, 3/2 − y, z; (3) y, 3/2 − x, 3/2 − z; (4) 3/2 − y, x, 3/2 − z   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.21 e Å −3 Δρ min = −1.07 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (