Definitive crystal structure of 1,1′-bis[1,2-dicarba-closo-dodecaborane(11)]

In the title compound, the two {1,2-closo-C2B10H11} cages are linked across a centre of inversion with a C—C distance of 1.5339 (11) Å. By careful analysis of the structure, it is established that the non-linking cage C atom is equally disordered over cage vertices 2 and 3.

The two structural studies of 1,1 0 -bis[1,2-dicarba-closododecaborane(11)] so far reported for which atomic coordinates are available (Hall et al., 1965;Ren & Xie, 2008) agree that the overall molecular structure is that of two 1,2dicarba-closo-dodecaborane(11) units linked via a C1-C1A bond across a centre of inversion. However they differ in their interpretation of the position of the non-linking carbon atom, C2 (and, by symmetry, C2A). In the earlier study, Hall et al. considered two models, one (Case I) in which C2 was disordered over two adjacent cage vertices and another (Case II) in which it was disordered over all five vertices to which C1 is connected, expressing a slight preference for the former model based on R factors, with supplementary evidence coming from inspection of temperature factors and the lengths of cage connectivities. In their later study, Ren & Xie considered only an ordered model, with C2 occupying one of the two C/B disordered sites in Case I of Hall et al., but no justification for this model was given. The two crystals used by Hall et al. and by Ren & Xie are isomorphous, and both data sets were collected at room temperature.
We have recently described two new methods, which distinguish CH from BH vertices in carboranes and heterocarboranes, the Vertex-to-Centroid Distance (VCD) method (McAnaw et al., 2013) and the Boron-Hydrogen Distance (BHD) method (McAnaw et al., 2014). In the present communication, we apply these methods to a precise, lowtemperature data set to unambiguously describe the crystal structure of the title compound, 1,1 0 -bis[1,2-dicarba-closododecaborane(11)].
The crystals used in this determination are also isomorphous with those studied by Hall et al. (1965) and by Ren & Xie (2008), so comment on the positioning of the non-linking cage C atom in all three determinations is warranted. Using the Vertex-to-Centroid Distance (VCD) method (McAnaw et al., 2013) to analyse our Prostructure (only the linking atom C1 identified as carbon with all other cage atoms described as boron and with H atoms allowed positional refinement), we conclude that the second cage C atom is statistically disordered over vertices 2 and 3 (Table 1). On assigning these positions as (essentially) 0.5C+0.5B and completing the refinement we note that all vertex-centroid distances barely change, confirming our contention (McAnaw et al., 2013) that the conclusions from the VCD method are essentially independent of whether vertices have been refined as C or B and thus allowing the method to be applied to literature structures even if an incorrect C/B assignment has been made. Application of the VCD method to the structure of Hall et al. confirms that their partially disordered Case I model was correct, whilst application to the structure of Ren & Xie (which had the second C atom exclusively at vertex 3) shows that their model is incorrect. Boron-Hydrogen Distance (BHD) analysis (McAnaw et al., 2014) of our structure (Table 2) also supports the conclusion that the non-linking C is disordered over vertices 2 and 3. The two shortest vertex-H distances in the Prostructure involve vertices 2 and 3, and when these vertices are assigned as (essentially) 0.5C+0.5B, the refined distances to H increase to values between those expected for 100% B and 100% C.
The final structure determined for 1,1 0 -bis[1,2-dicarbacloso-dodecaborane(11)] is the most precise to date. The e.s.d.'s on comparable molecular parameters are ca half the magnitude of those of Hall et al. (which is nevertheless a remarkably good determination given the hardware used to collect data and the limited number of reflections measured) Figure 1 Perspective view of the title compound, with displacement ellipsoids drawn at the 50% probability level. The label suffix 'A' refers to the symmetry operation (Àx, Ày + 2, Àz + 2). Table 1 Vertex-to-centroid distances (Å ) in studies of 1,1 0 -bis[1,2-dicarba-closo-dodecaborane (11)  1.043 (12) 1.080 (9) 10 1.164 (12) 1.108 (9) 11 1.118 (11) 1.096 (9)  12 1.086 (11) 1.108 (9) and ca a quarter of the magnitude of those of Ren & Xie. The present determination is the only one to have been carried out at low temperature (100 K). The three C1-B distances span the range 1.7308 (9)-1.7427 (9) Å whilst the two C1-C/B connectivities are 1.6950 (8) and 1.6991 (8) Å . Of the remaining connectivities, C/B-C/B is shortest, 1.7215 (9) Å , C/B-B is intermediate, lying in the range 1.7353 (10)-1.7603 (9) Å , and B-B distances are the longest, spanning from 1.7775 (10) to 1.8015 (11) Å . The relative lengths of all of these connectivities are fully consistent with the fact that C has a smaller radius than B, which is the essential basis for the VCD method.

Supramolecular features
The only HÁ Á ÁH contact less than 2.40 Å is H3Á Á ÁH12B at 2.342 (13) Å [symmetry code: (B) Àx + 1 2 , y + 1 2 , Àz + 3 2 ]. Given that vertex B is 50% C and that CH units and BH units in carboranes are protonic and hydridic respectively, with the degree of hydridic character increasing with increasing distance from the C atoms, this might represent a weak dihydrogen bond. The angles at H3 and H12B are 151.1 (7) and 123.2 (6) , respectively.
Of the remaining nine hits revealed by Conquest, one (FASQAR; Herzog et al., 1999) relates to an octamethyl derivative of 1,1 0 -bis[1,2-dicarba-closo-dodecaborane (11)] and eight are concerned with species in which the molecule has been deprotonated at the C2 and C2 0 positions, with the resulting dianion complexing either a transition metal or a main-group element.

Synthesis and crystallization
The compound was prepared by the Cu I -mediated coupling of lithiated ortho-carborane, a method first reported by Yang et al. (1995) for para-carborane and later used by Ren & Xie (2008) for the coupling of ortho-carborane. Purity was confirmed by elemental microanalysis, mass spectrometry and NMR spectroscopy, the last by comparison with the data of Yang et al. (1995). Single crystals for this study were afforded by cooling a solution of the compound in hexane to 243 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The molecule sits on a crystallographic centre of symmetry at the mid-point of the C1-C1A bond. Initially only the linking atom C1 was identified as carbon, with all other cage atoms described as boron and with H atoms allowed positional refinement. This model (the Prostructure) was refined and then analysed by both the VCD (McAnaw et al., 2013) and the BHD (McAnaw et al., 2014) methods. Both methods led to the same conclusion regarding the location of the second C atom, which was found to be disordered between positions 2 and 3. These vertices were assigned boron and carbon occupancies of 0.5, treated as tied variables. Refinement was completed with H atoms continuing to be refined positionally and with U iso (H) = 1.2U eq (C,B). At convergence, cage position 2 is [0.503 (9) C + 0.497 (9)   Data collection: SAINT (Bruker, 2009); cell refinement: APEX2 (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2008); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

(I)
Crystal data

Special details
Experimental. Absorption correction: SADABS-2008/1 (Bruker, 2008) was used for absorption correction. wR2(int) was 0.0508 before and 0.0428 after correction. The Ratio of minimum to maximum transmission is 0.9456. The λ/2 correction factor is 0.0015. 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.

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