Crystal structure of di-μ-hydroxido-κ4 O:O-bis[bis(acetylacetonato-κ2 O,O′)cobalt(III)]

[Co(acac)2(μ-OH)]2 (acac = acetylacetonate) is the first crystal structure reported of a dimeric transition metal bis-acac complex with OH− as the bridging group. The centrosymmetric molecular structure is a [Co2(μ2-OH)2] dimer with each metal coordinated by two acac ligands in a κ2-O,O′ mode.


Chemical context
Well-defined cobalt(III) hydroxide complexes are relatively rare, especially in the absence of amine ligands (Bryndza & Tam, 1988). One of the earliest examples is [Co(acac) 2 (-OH)] 2 (acac is acetylacetonate, C 5 H 7 O 2 ), (I), which was prepared by oxidation of Co(II)(acac) 2 with hydrogen peroxide. The complex reacts with 2,4-pentanedione to form Co III (acac) 3 and may serve as a useful model for hydration and oxidation catalysts (Masłowska & Baranovski, 1978;Bergquist et al., 2003;Zinn et al., 2007;Wang et al., 2009) Boucher and Herrington characterized the complex according to IR and 1 H NMR spectra (Boucher & Herrington, 1971). These data indicated a single diastereoisomer, the identity of which was not clear from the spectra. We now report its crystal structure, confirming that it is centrosymmetric.
The molecular structure of (I) contains a [Co 2 ( 2 -OH) 2 ] motif with each metal coordinated by two acac ligands in a 2 -O,O 0 mode (Fig. 1). The two halves of the dimer are related via inversion symmetry. The Co1Á Á ÁCo1 i distance is 2.8829 (7) Å . This distance falls within the range (2.696-3.355 Å ) of all CoÁ Á ÁCo distances reported in the Cambridge Crystallographic Database (Groom & Allen, 2014) for OH À -bridged Co complexes in which the metals are coordinated by six oxygen atoms. It is well below the average CoÁ Á ÁCo distance of 3.108 Å .

Supramolecular features
There are no significant supramolecular features to discuss with the extended structure of (I). There are weak C-HÁ Á ÁO intermolecular interactions (Table 1) between one methyl group of an acac ligand and the hydroxide oxygen atom. These interactions result in the formation of chains normal to the ac plane (Fig. 2). It should be noted that the hydroxyl H atom does not participate in hydrogen bonding. Examination of the packing diagram shows that the bulky acac ligands prevent any hydrogen-bonding interactions with neighboring molecules.

Database survey
One closely related crystal structure, [Co(L) 2 (-OH)] 2 ; L = 1-(dibenzylamino)-5,5-dimethyl-1,4-dioxohex-2-en-2-olate, has been reported previously (Wang et al., 2009). The ligand in this complex is a modified acac with a tert-butyl group in place of one methyl and a {CON(CH 2 Ph) 2 } group in place of the other methyl group. The coordination environment of the Co III atoms is the same as in (I). The average Co-O L distance of 1.890 Å is similar to the average Co-O acac distance in (I) of 1.881 Å . The average Co-OH distance of 1.907 Å is also comparable to that of (I) (1.911 Å ).
A search of the Cambridge Crystallographic Database (Groom & Allen, 2014) returned 13 dimeric complexes with the general formula [TM(acac) 2 (-X)] 2 ; TM = transition metal, and X = O, OR, NO, or S (Bottomley et al., 1982;Nakahanada et al., 1992;Smith et al., 1972;Sokolov et al., 1999). Complex (I) is the first crystal structure reported that fits this general formula in which the bridging group is OH À . The molecular structure of (I), showing displacement ellipsoids at the 35% probability for non-H atoms and spheres of arbitrary size for H atoms. The unlabeled atoms are related by the symmetry operator (Àx, Ày + 2, Àz + 1). Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view along the a axis of the crystal structure of (I), showing extended chains normal to the ac plane. The weak C-HÁ Á ÁO interactions are shown as red dashed lines. All H atoms except the hydroxide H atom (H1) and the interacting H atoms (H1B) have been omitted for clarity. Color code: blue = Co, red = O, gray = C, green = H.

Synthesis and crystallization
The title complex was synthesized according to the procedures reported by Boucher & Herrington (1971). To a mixture of Co(acac) 2 Á2H 2 O (2 g, 7.27Â10 À3 mol, 1 equiv) and KOAc (3.2 g, 3.26Â10 À2 mol, 4.5 equiv) in methanol (125 ml) was added a solution of H 2 O 2 in water (30% wt , 2 ml). The resulting solution changed color from pink to green. The reaction was stirred at room temperature for 1 h under an ambient atmosphere. The reaction was then concentrated to dryness on a rotary evaporator. The residual green solid was washed with water (3 Â 20 ml) and then acetone (3 Â 20 ml), and then dried in air, leaving the product (0.85 g, 1.55Â10 À3 mol, 43% yield). Crystals, suitable for X-ray diffraction, were grown by slow diffusion of pentane into chloroform solutions of the green product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydroxyl H atom was located in a difference map and its position was allowed to refine freely. Methyl H atom positions, R-CH 3 , were optimized by rotation about R-C bonds with idealized C-H, R-H and HÁ Á ÁH distances. Remaining H atoms were included as riding idealized contributors. Methyl and hydroxide H atom U iso 's were assigned as 1.5U eq of the carrier atom; remaining H atom U iso 's were assigned as 1.2U eq of the carrier atom.