Crystal structure of the inverse crown ether tetrakis[μ2-bis(trimethylsilyl)amido]-μ4-oxido-dicobalt(II)disodium, [Co2Na2{μ2-N(SiMe3)2}4](μ4-O)

The first cobalt-containing inverse crown ether, [Co2Na2{μ2-N(SiMe3)2}4](μ4-O), features a central μ4-oxido ligand. Weak intermolecular Na⋯H3C—Si interactions form an infinite chain extending along [010] in the crystal.


Structural commentary
Crystals of (I) suitable for X-ray diffraction were obtained as reaction by-products via crystallization from toluene at 238 K. Attempts at a rational synthesis were not successful. The molecular structure of compound (I) is shown in Fig. 2a and relevant bond lengths and angles are presented in Table 1. The asymmetric unit contains half of a unique molecule comprised of an oxygen atom located on an inversion center, one cobalt atom, one sodium atom, and two -N(SiMe 3 ) 2 ligands with the remainder of the molecule being completed by application of inversion symmetry. Consequently, all opposing M-O-M angles (M = Co, Na) are crystallographically imposed to 180 . The four bridging nitrogen atoms lie slightly out of plane from the four metal atoms, exhibiting a dihedral angle of 8.1 (2) between their respective planes as shown in Fig. 2b.
The majority of cobalt-bridging oxido compounds possess bent angles, so the 4 -oxido ligand in (I) is unusual in that it coordinates linearly to the opposing metal atoms. With a central oxido ligand, by charge balance each cobalt atom has formally an oxidation state of +II. While the paramagnetic nature of (I) prevents confirmation by NMR studies, it is unlikely that the central O atom is actually a hydroxido ligand. The structurally related anionic compound [Na 4 ( 2 -N(SiMe 3 ) 2 ) 4 ( 4 -OH)] À , which bears a central 4 -OH ligand, is noticeably pyramidalized, possessing Na-O-Na angles of 140.1 (2) and 142.4 (2) (Clark et al., 2009). Additionally, the Co1-O1 bond length of 1.8398 (9) Å in (I) is significantly shorter than those of other structurally characterized complexes of Co II bearing approximately linear (a) The molecular structure of (I), showing displacement ellipsoids at the 50% probability level. (b) An alternate view of (I) down the Na-O-Na axis displaying ring offsets. H and C atoms were truncated for clarity. [Symmetry code: (i) Àx + 1, Ày + 1, Àz + 1.]

Supramolecular features
In the solid state, the steric bulk of the trimethylsilylamide ligands prevents further intermolecular interactions of either the cobalt atoms or the oxido ligand, as can be observed in the space filling model of (I) presented in Fig. 3a. Some weak interactions can be noted for sodium, however, which is consistent with the open site around sodium visible in Fig Packing diagram of (I), showing NaÁ Á ÁH contacts forming an infinite chain that extends along [010]. (Symmetry code: Àx + 1, Ày + 1, Àz + 1.)  The sodium atoms and one -Si-CH 3 group from each molecule coordinate to a neighboring -Si-CH 3 group and sodium atom, respectively, forming an infinite chain extending along [010], as illustrated in Fig. 4. The two close NaÁ Á ÁH contact distances of 2.961 and 2.886 Å fall within the range of previously structurally characterized literature examples of various molecules containing sodium bis(trimethylsilyl)amide moieties (2.55-3.0 Å ). For selected examples, see: Driess et al. (1997); Sarazin et al. (2006); Kennedy et al. (2008). This type of intermolecular interaction has been previously noted in the solid state for related potassium-based inverse crown ethers bearing bridging peroxido ligands (Kennedy et al., 1999), and in related sodium-containing precursors (Kennedy et al., 2008).

Synthesis and crystallization
Compound (I) was obtained as single crystals on multiple occasions as a side product of two different reactions; however, attempts at a rational synthesis were not successful. These reactions used conditions and reagents that were nominally free of oxygen and water. Nonetheless, trace oxygen or water are the likely sources of the bridging oxido ligand. Adventitious water (Lu et al., 2010) and oxygen (Kennedy et al., 2008) have both been shown to be potential oxygen-atom sources, and have been previously utilized to generate this type of structure. Additionally, fragmentation of tetrahydrofuran has also been identified as a potential oxygen-atom source in one case (Mulvey et al., 2010).
Method 1: In a glovebox [(IPr)CoCl 2 ] 2 (Matsubara et al., 2012;Przyojski et al., 2013) [IPr = 1,3-di(2,6-diisopropylphenyl)imidazolin-2-ylidene] (50 mg, 0.048 mmol, 1 equiv.) was dissolved in 3 ml toluene and cooled to 238 K. A 238 K solution of NaN(SiMe 3 ) 2 (Sigma-Aldrich, titrated to 0.844M in THF) (22.9 mL, 0.193 mmol, 4 equiv.) was added dropwise to the solution of [(IPr)CoCl 2 ] 2 with stirring. The reaction mixture rapidly changed color from blue to turquoise to green and became turbid. The solution was allowed to warm to ambient temperature and stirred for 1 h. The reaction was filtered through Celite and the filtrate reduced to dryness under vacuum. The resulting green solid was dissolved in a minimal volume of toluene, passed through a Pasteur pipette filter, and stored at 238 K for several days. The resulting precipitate primarily consisted of thin green plates of (IPr)CoCl(N(SiMe 3 ) 2 ) (Hansen et al., 2015), occasionally accompanied by a small number of dark green-blue blocks of (I).
Method 2: While attempting to prepare a compound of the type Na[Co(N(SiMe 3 ) 2 ) 3 ], (I) was occasionally observed as a minor by-product during recrystallization attempts. In a typical reaction anhydrous CoCl 2 (100 mg, 0.77 mmol, 1 equiv.) was suspended in 2 ml THF and cooled to 238 K. NaN(SiMe 3 ) 2 (423.6 mg, 2.31 mmol, 3 equiv.) was dissolved in 10 ml THF, cooled to 238 K, then added to the stirred slurry of CoCl 2 . The reaction mixture was allowed to warm to ambient temperature and stir overnight, over which time it slowly turned green and turbid. The reaction mixture was filtered through Celite and rinsed with additional THF until washings were colorless, leaving a white solid remaining on the Celite pad. The combined THF fractions were combined and concentrated under vacuum to a yield a waxy green solid. The resulting solid was recrystallized from a solution in a minimal volume of toluene cooled to 238 K. The title compound (I) was occasionally observed as blue-green blocks.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed at idealized positions with C-H = 0.98 Å , U iso (H) set to 1.5U eq (C). The initial structure solution and refinements had a

Special details
Experimental. Absorption correction: TWINABS2012/1 (Bruker, 2012) was used for absorption correction. For component 1: wR2(int) was 0.0813 before and 0.0454 after correction. The Ratio of minimum to maximum transmission is 0.77. Final HKLF 4 output contains 11962 reflections, Rint = 0.0892 (2973 with I > 3sig(I), Rint = 0.0335) 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. Refined as a 2-component twin.