1,4-Diazabicyclo[2.2.2]octane–trans,trans-hexa-2,4-dienedioic acid (1/1)

The title 1:1 co-crystal, C6H12N2·C6H6O4, the dicarboxylic acid molecule is close to planar [r.m.s. deviation from the mean plane = 0.07 (1) Å]. In the crystal, the two molecules are arranged alternately and are linked by O—H⋯N hydrogen bonds, leading to the formation of a chain along the [20-1] direction. The chains are assembled into a two-dimensional framework parallel to the (102) plane through weak C—H⋯O hydrogen bonds between the two types of molecules.


Suk-Hee Moon and Ki-Min Park Comment
Co-crystals made up of two or more components have attracted much attention in recent years owing to their contributions to supramolecular chemistry (Bhogala & Nangia, 2003;Gao et al., 2004), materials chemistry (Hori et al., 2009) and pharmaceutical chemistry (Weyna et al., 2009). As a part of our recent efforts to construct supramolecular architectures using the co-crystal strategy, the crystal structure of a co-crystal consisting of trans,trans-hexa-2,4dienedioic acid and 4,4′-bipyridine molecules has been reported by us (Moon & Park, 2012). In this paper we present a co-crystal structure of trans,trans-hexa-2,4-dienedioic acid with 1,4-diazabicyclo[2.2.2]octane.
The title compound is shown in Fig. 1. The asymmetric unit contains one 1,4-diazabicyclo[2.2.2]octane molecule and one trans,trans-hexa-2,4-dienedioic acid molecule. The dicarboxylic acid molecule is essentially planar, with an r.m.s. deviation from the mean plane of 0.07 Å.
In the crystal structure, both components are arranged alternately, and linked by intermolecular O-H···N hydrogen bonds, leading to the formation of a one-dimensional chain. Additionally, the chains are assembled into a twodimensional framework through weak intermolecular C-H···O hydrogen bonds between 1,4-diazabicyclo[2.2.2]octane and dicarboxylic acid molecules (Fig. 2, Table 1).

Experimental
A mixture of stoichiometric amounts of trans,trans-hexa-2,4-dienedioic acid and 1,4-Diazabicyclo[2.2.2]octane in DMF (in a 1:1 volume ratio) was heated until the two components dissolved and was then kept at room temperature. Upon slow evaporation of the solvent, X-ray quality single crystals were obtained.

Refinement
All H-atoms were positioned geometrically and refined using a riding model. C-H = 0.95 Å for Csp 2 , C-H = 0.99 Å for methylene C and O-H = 0.84 Å for the hydroxyl groups; U iso (H) = 1.2U eq (parent atom).  The molecular structure of the title compound, showing displacement ellipsoids drawn at the 50% probability level.

Computing details
Hydrogen atoms are shown as spheres of arbitrary radius. The dashed line indicates a hydrogen bond.

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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.