Bis(μ4-adamantane-1,3-dicarboxylato-1κO 1:2κO 1′:3κO 3:4κO 3′)octacarbonyl-1κ2 C,2κ2 C,3κ2 C,4κ2 C-tetrakis[tris(4-methylphenyl)phosphane]-1κP,2κP,3κP,4κP-tetraosmium(I)(2 Os–Os)

[Os2(CO)4(tri-p-tolylphosphine)2]2(μ4-adamantane-1,3-dicarboxylate)2 is a molecular loop consisting of two parallel diosmium(I) sawhorse units linked together by two bridging dicarboxylato ligands. The title compound is only the second example of a Group 8 dinuclear sawhorse complex with an adamantane-based dicarboxylato ligand.

data reports 2003,2010), and one is a molecular square consisting of four sawhorse units (Shiu et al., 2002). In all of these, the Ru-Ru axes are parallel rather than perpendicular to one another. The CSD also contains six Os 2 carboxylato sawhorse assemblies: five are molecular loops of two sawhorse units and one is a molecular triangle consisting of three sawhorse units (Fikes et al., 2014;Gwini et al., 2017). In all but one of these assemblies, the Os-Os axes within a molecule are parallel to one another. Only the molecular loop [Os 2 (CO) 6 ] 2 ( 4 -adamantane-1,3-diacetate) 2 has Os-Os axes that are oriented perpendicular to one another (Fikes et al., 2014). No Ru 2 sawhorse assemblies containing adamantane-based dicarboxylato linkers have been reported. Our goal was to investigate the orientation of Os 2 units that would result when using adamantane-1,3-dicarboxylic acid rather than adamantane-1,3-diacetic acid as a starting material.
The structure of the cluster molecule in the title compound is illustrated in Fig. 1. The cluster entity resides on an inversion center and consists of a molecular loop in which two Os 2 (CO) 4 (phosphine) 2 sawhorse units are bridged by two adamantane-1,3-dicarboxylato ligands. The four tri-p-tolylphosphine ligands occupy axial coordination sites with Os-Os-P angles of 170.20 (2) and 170.60 (2) , which are typical for diosmium sawhorse complexes. Like Ru 2 sawhorse carboxylato macrocycles in which the Ru-Ru axes are parallel to one another, the two Os-Os axes in this structure are also parallel. This is in contrast to the related molecular loop [Os 2 (CO) 6 ] 2 ( 4 -adamantane-1,3-diacetate) 2 in which the metal-metal axes within each molecule are oriented perpendicular to one another (Fikes et al., 2014). In the title compound, the Os-Os bond length is 2.7398 (2) Å . In [Os 2 (CO) 6 ] 2 ( 4 -adamantane-1,3-diacetate) 2 , where the axial sites are occupied by carbonyl ligands instead of phosphine ligands, the metal-metal bond lengths are somewhat longer at 2.7433 (3) and 2.7561 (3) Å .
The cluster molecules of the title compound stack so that the Os-Os vectors are nearly parallel to the b axis and nearly perpendicular to the a axis. When viewed down the b axis, the central cavities of the molecular loops align to form narrow channels, as shown in Fig. 2. Because sawhorse clusters with dicarboxylato ligands have sometimes crystallized with solvent molecules trapped in the center of the macrocycle, it is common to list the dimensions of the central cavity (Therrien & Sü ss-Fink, 2009). The cavity in the center of the title compound is a distorted rhombus with unique edge lengths of 4.684 (1) and 4.976 (1) Å as measured from the Os-Os midpoints to the central adamantane carbon atom C58. This cavity is smaller than that in [Os 2 (CO) 6 ] 2 ( 4 -adamantane-1,3diacetate) 2 in which these distances average 5.2 Å (Fikes et al., 2014). The size difference was expected since there are two fewer carbon atoms per linker ligand in the title compound. Packing of the title molecules viewed approximately along the b axis.

Figure 3
Views of the central shapes and core dimensions for (a) the title compound and (b) [Os 2 (CO) 6 ] 2 ( 4 -adamantane-1,3-diacetate) 2 . On the left are perspective views in which atoms toward the front appear larger and atoms toward the back appear smaller. On the right are illustrations of the underlying core shapes in which blue dots represent the centroids of the two Os-Os units and gray dots represent the centroids of the two adamantane moieties.

Figure 1
View of the title molecule showing the atom-labeling scheme. Displacement ellipsoids are scaled to the 35% probability level. For the sake of clarity, all H atoms are omitted. [Symmetry code: (1) 1 À x, 1 À y, 1 -z].
As a result of their small sizes, the centers of the molecular loops cannot serve as a trap for solvent molecules in either one of these complexes. As shown in Fig. 3, the central portions of these two molecular loops also display different shapes. Connecting the centroids of the two Os-Os vectors and the centroids of the two adamantane groups leads to a butterfly shape in the case of [Os 2 (CO) 6 ] 2 ( 4 -adamantane-1,3-diacetate) 2 and an approximate square in the case of the title compound. The butterfly wings of the adamantane diacetato complex are joined at an angle of 126 , while all four centroids are coplanar in the square of the title compound. The distances between adamantane centroids for the title cluster and for [Os 2 (CO) 6 ] 2 ( 4 -adamantane-1,3-diacetate) 2 are 7.087 (2) and 7.598 (2) Å , respectively. Despite the differences in dimensions and spacing for the adamantane-based ligands in these two complexes, the distances between Os-Os centroids are remarkably similar at 8.983 (2) and 8.964 (2) Å , respectively.

Synthesis and crystallization
Os 3 (CO) 12 (73.9 mg, 0.0815 mmol) and adamantane-1,3-dicarboxylic acid (29.2 mg, 0.130 mmol) were added to 7 ml of 1,2-dichlorobenzene in a 35 ml microwave vessel. This solution was stirred and heated in the microwave reactor at 478 K for 13 minutes. The resulting solution had a pale-yellow color. The solvent was removed, then the residue was mixed with 25 ml of 1,2-dichloroethane and 5 ml of acetonitrile and added to a 100 ml round-bottom flask equipped with a magnetic stir bar. Tri(p-tolyl)phosphine (64.0 mg, 0.210 mmol) was added and the mixture was refluxed for 60 min. The solution was cooled to 277 K, 4 ml of n-hexane were added, and the products were isolated by fractional crystallization. The first fraction to precipitate was the desired product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Interstitial solvent molecules could not be modeled in a satisfactory manner, so a solvent mask was generated revealing voids at (1/2, 0, 1/2) and (1/2, 1/2, 0), each with a volume of 394.4 Å 3 and containing about 110 electrons. The contribution of the disordered solvent molecules to the scattering was removed using the SQUEEZE (Spek, 2015) routine in PLATON (Spek, 2020). These solvent molecules are not considered in the given chemical formula and other crystal data.   Special details 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.