Cholic acid–quinoxaline (2/1)

In the title inclusion compound, 2C24H40O5·C8H6N2, the unit cell contains two molecules of cholic acid (3α,7α,12α-trihydroxy-5β-cholan-24-oic acid) and one molecule of quinoxaline which implies disorder of the quinoxaline in the space group P21. The amphiphilic molecules of cholic acid assemble, in an antiparallel arrangement, via O—H⋯O hydrogen bonds, into typical corrugated host bilayers which are lipophilic on the outside and lipophobic on the inside. The host framework belongs to the so called α-trans subtype. The quinoxaline molecules are accommodated in lipophilic channels formed between neighboring bilayers with only van der Waals interactions between host and guest. There is a crystallographic twofold screw axis directed along an empty channel in the host framework; however, neighboring guests in any one channel are related by a unit-cell translation along the b axis. Thus, the overall structure is a 1:1 superposition of two such channels related by the crystallographic twofold screw axis.

In the title inclusion compound, 2C 24 H 40 O 5 ÁC 8 H 6 N 2 , the unit cell contains two molecules of cholic acid (3,7,12-trihydroxy-5-cholan-24-oic acid) and one molecule of quinoxaline which implies disorder of the quinoxaline in the space group P2 1 . The amphiphilic molecules of cholic acid assemble, in an antiparallel arrangement, via O-HÁ Á ÁO hydrogen bonds, into typical corrugated host bilayers which are lipophilic on the outside and lipophobic on the inside. The host framework belongs to the so called -trans subtype. The quinoxaline molecules are accommodated in lipophilic channels formed between neighboring bilayers with only van der Waals interactions between host and guest. There is a crystallographic twofold screw axis directed along an empty channel in the host framework; however, neighboring guests in any one channel are related by a unit-cell translation along the b axis. Thus, the overall structure is a 1:1 superposition of two such channels related by the crystallographic twofold screw axis.
Data collection: CrysAlis CCD (Oxford Diffraction, 2007); cell refinement: CrysAlis RED (Oxford Diffraction, 2007); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and Mercury (Macrae et al., 2006); software used to mode of the bilayers (α/β). Among numerous guest molecules that have cocrystallized with cholic acid no larger arenes or aromatic azaheterocycles have been reported. This is probably due to the problems with accommodating large molecules of fixed geometry within corrugated host channels. Quinoxaline easily cocrystallized with cholic acid, because as a low melting solid it could be used for cocrystallization without the need for any additional solvent.
In (I) ( Fig. 1), the host molecules are arranged in typical antiparallel bilayers and the framework can be classified as α-trans (Fig. 2). Four molecules of the host generate a cyclic motif of O-H···O hydrogen bonds (Fig. 3, Table 1) that assembles molecules into a two-dimensional polymeric structure (host bilayer). The hydrogen bonds are not completely buried on the inside of the bilayer as they partially line the grooves on the corrugated bilayer surface. The quinoxaline molecules are accommodated in lipophilic channels formed between neighboring bilayers and there are only van der Waals interactions between host and guest. The unit cell contains two molecules of the bile acid and one molecule of quinoxaline.
In P2 1 this implies disorder of the guest and this is the case for (I): the crystallographic symmetry of the empty channel is higher than the symmetry of the guest arrangement within the channel. Neighbouring guests are related by translation along b [7.8968 (3) Å] and not by the crystallographic 2 1 axis operating along the channel (Fig. 4). There is no long-distance order in the channels because no reflections in addition to the Bragg reflections were detected. Thus, the model of the crystal structure of the title compound reveals superposition of two channels related by the crystallographic twofold screw axis ( Fig. 4).

Experimental
The title compound was obtained by dissolving cholic acid (0.1 g, 0.24 mmol) in melted quinoxaline (0.7 g, 5.38 mmol) and evaporation of the excess of quinoxaline at 60°C for two days. The resulting colorless plates were stable in air.

Refinement
In the absence of significant anomalous scattering effects, Friedel pairs were averaged. The absolute configuration of cholic acid was assigned from the known configuration of the starting material. All H atoms were located in electron-density difference maps. For refinement all H atoms were placed at calculated positions, with C-H = 0.96-0.98 Å and O-H = 0.82 Å, and were refined as riding on their carrier atoms with U ĩso (H) = 1.2U eq (C, O). No restraints were imposed on geometry of the disordered quinoxaline molecules (occupancy factor 1/2). Fig. 1. : The molecular structure of the title compound with displacement ellipsoids shown at the 50% probability level. H atoms bound to C atoms are omitted for clarity.   3α,7α,12α-trihydroxy-5β-cholan-24-oic acid-quinoxaline (2/1)

Crystal data
where P = ( 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 Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.