Orientational disorder in the one-dimensional coordination polymer catena-poly[[bis(acetylacetonato-κ2 O,O′)cobalt(II)]-μ-1,4-diazabicyclo[2.2.2]octane-κ2 N 1:N 4]

The one-dimensional coordination polymer, self-assembled from bis(acetylacetonato)cobalt(II) units as metal–complex connectors and 1,4-diazabicyclo[2.2.2]octane (DABCO) as linkers, can serve for a comparative investigation of the magnetic behaviour of analogous compounds. Space filling more symmetric than atom positions leads to pronounced orientational disorder for the DABCO ligand.

The title compound, [Co(C 5 H 7 O 2 ) 2 (C 6 H 12 N 2 )] n , was obtained as a onedimensional coordination polymer from bis(acetylacetonato)diaquacobalt(II), [Co(acac) 2 (OH 2 ) 2 ], and 1,4-diazabicyclo[2.2.2]octane (DABCO), a diamine with good bridging ability and rod-like spacer function. In the chain complex that extends along the c axis, the Co II atom is six-coordinated, the O-donor atoms of the chelating acac ligands occupying the equatorial positions and the bridging DABCO ligands being in trans-axial positions. In the crystal structure, the DABCO ligand is conformationally disordered in a 50:50 manner as a result of its location across a crystallographic mirror plane. The metal-metal distance is very close to that in a related compound exhibiting weak antiferromagnetic exchange between the Co II ions, and the title compound can thus be useful for obtaining more information about the contribution of different bridges to the magnetic coupling between paramagnetic ions.

Chemical context
Self-assembly of coordination polymers from simple building blocks is of considerable interest due to their diverse architectures and potential applications in catalysis and advanced materials, such as magnetic, optic and electronic materials.
In this paper, two simple building blocks, namely 1,4-diazabicyclo[2.2.2]octane (DABCO), a diamine with good bridging ability and rod-like spacer function, and the unsaturated square-planar metal complex bis(acetylacetonato-2 O,O 0 )cobalt(II), [Co(acac) 2 ], have been chosen to design a one-dimensional coordination polymer in which the paramagnetic Co II ions are separated by a distance of 7.2328 (7) Å . This metal-metal distance is very close to the distance of 7.267 (3) Å reported by Ma et al. (2001)
The acetylacetonate (acac) ligand, which is the deprotonated form of acetylacetone (pentane-2,4-dione, acacH), is a well-known mononegative O,O 0 -chelating donor agent and its metal coordination chemistry is well documented [for reviews on the coordination chemistry of acac ligands, see: Aromí et al. (2008);Bray et al. (2007);Vigato et al. (2009)]. For DABCO, the bridging coordination behaviour is most exploited for the generation of coordination polymers and metal-organic frameworks (MOFs), with Zn 2+ being the most common metal ion used in these structures [for representative examples, see: Furukawa et al. (2009);Uemura et al. (2007)].
The complex crystallizes in the orthorhombic Pnnm space group with the metal atom on a special position with site symmetry ..2/m. The Co II atom shows an octahedral environment defined by four equatorial acac O atoms on a mirror plane, with bond lengths ranging from 2.0299 (10) to 2.0411 (10) Å , and with two N atoms of bridging DABCO groups on a twofold rotation axis in the axial positions at distances of 2.3071 (12) Å (Fig. 1).

Supramolecular features
The centrosymmetric DABCO ligand is bonded to two [Co(acac) 2 ] units, which gives rise to the formation of chains extending along the c axis (Fig. 2). The individual chains run parallel in the crystal and do not interact with each other. This polymer is essentially a one-dimensional coordination polymer, the only structural motif that is present being based on the Co II coordination requirements.

Figure 2
The molecular packing of the coordination polymer chains.
Elemental analyses were carried out on a Heraeus CHNO Rapid apparatus (Institute of Inorganic Chemistry, RWTH Aachen University).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Space filling more symmetric than atom positions leads to pronounced orientational disorder (Herberich et al., 1993) for the DABCO ligand over two positions due to mirror symmetry. As a result, the site occupancies of the C atoms are constrained to 0.5. In principle, the same should be true for the associated H atoms, their alternative positions for the different C positions overlap very closely, thus forming the hexagon of local residual electrondensity maxima about the C-atom scaffold shown in Fig. 3. These maxima can be freely refined as H atoms with reasonable C-H geometry and displacement parameters.
H atoms attached to C atoms were calculated, introduced in their idealized positions and treated as riding, with C-H = 0.95 Å and U iso (H) = 1.5U eq (C) for methyl H atoms and U iso (H) = 1.2U eq (C) otherwise. For consistency, we opted to calculate the positions of the DABCO H atoms and fix them in their idealized positions. Due to the fact that the acac ligand lies on a mirror plane, the acac methyl groups are therefore equally disordered over two orientations.