Crystal structure of bis(η2-ethylene)(η5-pentamethylcyclopentadienyl)cobalt

In the title compound, the Co—C(olefin) bonds have an average length of 2.022 (2) Å, while the Co—C(pentadienyl) bonds average 2.103 (19) Å. The olefin C=C bonds are 1.410 (1) Å. In the crystal, molecules are linked into chains by weak C—H⋯π interactions.


Chemical context
The title compound, Cp*Co(CH 2 CH 2 ) 2 (Cp* = pentamethylcyclopentadienyl), was first reported in 1981 by Spencer and coworkers (Beevor et al., 1981) in their quest to find a more thermally labile analogue of the related Cp*Co(dicarbonyl) complex. Since this first report, it and other olefin complexes of cobalt with Cp* or Cp (Cp = cyclopentadienyl) have become important precursors for the generation of Cp 0 CoL (L = olefin, pyridine, etc) and Cp 0 Co fragments used as active species in C-H bond activation (Lenges et al., 1997(Lenges et al., , 1998Broere & Ruijter, 2012), cyclotrimerization of alkynes (Dosa et al., 2002;Holmes et al., 2015) and C-S bond activation (Jones & Chin, 1994;Chan et al., 2015). The utility of the Cp*Co(CH 2 CH 2 ) 2 complex in organometallic synthesis has been explored extensively. Examples include the preparation of high-oxidation state Co V complexes  and the preparation of Cp*Co( 5 -pentadienyl) + complexes (Witherell et al., 2008;Ylijoki et al., 2009Ylijoki et al., , 2015.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. Although the cobalt atom is located on a general position, the molecule is essentially C 2v symmetric, which agrees with the symmetry of the 1 H NMR data (Beevor et al.,

Supramolecular features
In the crystal, a weak C-HÁ Á Á interaction is observed between one of the methyl groups and the Cp* ring edge of the adjacent molecule related by a 2 1 screw axis. The shortest contact occurs between the C6-H6C of the methyl group and the C1 atom of the Cp* ring [H6CÁ Á ÁC1 i 2.79, C6Á Á ÁC1 i 3.734 (3) Å , C6-H6CÁ Á ÁC1 i 162 ; symmetry code (i): Àx, y À 1 2 , Àz + 1 2 ] , while the H6CÁ Á ÁCp* ring centroid distance is 3.00 Å . The molecules are linked through the C-HÁ Á Á interactions, forming a helical chain parallel to the b axis (Fig. 2).

Figure 1
The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level for non-H atoms.

Figure 2
Packing diagram of the title compound, viewed down the c axis, showing a chain formed by C-HÁ Á Á interactions. Dotted lines show the shortest C-HÁ Á ÁC contact involved in the interaction. Ellipsoids are drawn at the 50% probability level. The 2 1 screw axes (green) are also shown. tallizes in the monoclinic space group P2 1 /c with unit-cell dimensions of a = 12.5561 (5), b = 7.3323 (3), c = 14.7157 (6) Å and = 108.3520 (10) at 100 K.

Synthesis and crystallization
The title compound was prepared by reduction of [Co 2 (C 10 H 15 ) 2 (-Cl) 2 ] (Koelle et al., 1986) under ethylene. This procedure is an adaptation of that reported by Nicholls & Spencer (1990). All solvents were degassed by purging with nitrogen and dried by passing through activated Al 2 O 3 . A 1% Na amalgam was prepared by addition of Na (305 mg, 13.3 mmol) in small portions to mercury (30.5 g) in a Schlenk flask equipped with a stir bar and rubber septum under a nitrogen atmosphere. The sodium was allowed to disperse completely between additions. Gentle heating with a heat gun may be required to initiate the process after the first addition. The Na amalgam was cooled to room temperature. THF (100 ml) was added to the Schlenk flask, followed by gently bubbling ethylene through the system via a needle for 20 min to ensure saturation. Previously prepared [Co 2 (C 10 H 15 )(-Cl) 2 ] (2.77 g, 6.0 mmol) was removed from the glovebox and rapidly added to the Schlenk flask under a nitrogen purge. The ethylene was bubbled through the THF for an additional 10 min, then the needle was moved to a position ca 1 cm above the solution surface to prevent clogging. The reaction was stirred under ethylene for a total of 1.5 h. Over this timespan, the colour evolved from dark brown to a red/orange colour. At this point, the septum was replaced with a glass stopper and the solvent removed completely under vacuum. The evacuated flask was transferred to the glovebox where the product was taken up in pentane and filtered through Celite, taking care to separate the mercury. The solution was concentrated under vacuum in a Schlenk tube and then sealed with a greased glass stopper. The tube was removed from the glovebox and placed in a 193 K freezer overnight. The next day, the tube was removed from the freezer and immediately immersed in a dry ice/acetone bath and placed under inert atmosphere on the Schlenk line. The solvent was removed by canula transfer at low temperature to isolate the title compound (1.8 g, 60%) as dark-red rectangular crystals. The product was dried under vacuum and transferred to the glovebox where it was stored at 233 K. The NMR spectroscopic data is identical to that previously reported (Beevor et al., 1981;Nicholls & Spencer, 1990 Computer programs: APEX2 and SAINT (Bruker, 2008), SHELXT (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b) and Mercury (Macrae et al., 2006).

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 1. The H atoms of the methyl groups were included at geometrically idealized positions (C-H = 0.98 Å ) and were treated as riding, with U iso (H) = 1.5U eq (C). The H atoms of the ethylene groups were located in a difference-Fourier map and their positions were freely refined, while their U iso (H) values were set to be equal to 1.2U eq of the parent carbon atom.

Bis(η 2 -ethylene)(η 5 -pentamethylcyclopentadienyl)cobalt
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.