Crystal structures of (μ2-η2,η2-4-hydroxybut-2-yn-1-yl 2-bromo-2-methylpropanoate-κ4 C 2,C 3:C 2,C 3)bis[tricarbonylcobalt(II)](Co—Co) and [μ2-η2,η2-but-2-yne-1,4-diyl bis(2-bromo-2-methylpropanoate)-κ4 C 2,C 3:C 2,C 3]bis[tricarbonylcobalt(II)](Co—Co)

The structures of two alkyne-hexacarbonyl-dicobalt complexes are reported, each with potential ATRP initiator substrates as substituents on the alkynes. The complexes each form tetrahedral C2Co2 cluster cores with classical sawhorse conformations, while a feature of the crystal packing is the formation of inversion dimers for both molecules.


Chemical context
In 1954 alkynes were found to act as ligands and displace two carbonyl groups from dicobalt octacarbonyl to form alkynehexacarbonyl-dicobalt complexes (Sternberg et al., 1954). The novelty of these compounds, together with their close isolobal relationship to other members of the 'tetrahedrane series' (Hoffmann, 1982), spawned enormous interest in both the hexacarbonyls and their substituted derivatives. Applications include use in organic synthesis (Melikyan et al., 2012), as biological probes (Salmain & Jaouen, 1993) and in the stabilization of high-performance energetic materials (Windler et al., 2012). Their diverse redox properties (Robinson & Simpson, 1989) have also been exploited in the development of molecular wires (McAdam et al., 1996;Hore et al., 2000;Xie et al., 2012) where alkyne-hexacarbonyl-dicobalt cores are separated by electronically conducting spacers or connecting groups. Our recent interest in incorporating redox-active organometallic species into polymer materials (Dana et al., 2007;McAdam et al., 2008) prompted us to investigate the synthesis of alkyne-hexacarbonyl-dicobalt complexes with potential ATRP initiator functionality by the incorporation of one or more known initiator substrates, such as 2-halo-2methyl propanoyl esters (Wang & Matyjaszewski, 1995;Laurent & Grayson, 2006), into the alkyne system. The structures of two such molecules with 2-bromo-2-methylpropanoate substituents are reported here.

Figure 3
Inversion dimers in the crystal structure of (1). Hydrogen bonds are drawn as dashed lines and symmetry operations are those detailed in Table 2.

Figure 5
C-HÁ Á ÁO hydrogen bonds in the crystal structure of (2). Hydrogen bonds are drawn as dashed lines and symmetry operations are those detailed in Table 4.

Figure 6
Chains of molecules of (2) formed by C-HÁ Á ÁBr hydrogen bonds drawn as dashed lines. Symmetry operations are those detailed in Table 4.

Figure 4
Overall packing for (1) viewed along the b axis. Hydrogen bonds and other interatomic contacts are drawn as dashed lines.

Database survey
The first structure, of dicobalt hexacarbonyl diphenylacetylene, was reported using film data (Sly, 1959
The complexes were then purified by recrystallization from hexane at 273 K. Yields were in the range 70-80%. Complexation was confirmed by the absence of a band at 1860 cm À1 in the infrared spectrum, attributable to the 2 (bridging) carbonyl groups of the dicobalt octacarbonyl starting material. In addition, a hypsochromic shift of approximately 30 cm À1 of the remaining carbonyl stretching frequencies is seen, due to the decrease in electron density at the metal atoms upon coordination of these alkynes. Characteristic IR spectra were recorded for both products as follows: IR (, cm À1 ): (1)

Refinement
All H atoms bound to carbon were refined using a riding model with d(C-H) = 0.99 Å , U iso = 1.2U eq (C) for CH 2 , 0.98 Å , U iso = 1.5U eq (C) for CH 3 atoms. In the final refinement, two reflections from the data for (2) with F o << F c were omitted from the refinement.

Co)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.83 e Å −3 Δρ min = −0.73 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.