Theobrominium perchlorate dibenzo-18-crown-6 3.25-hydrate

The co-crystal, C7H9N4O2 +·ClO4 −·C20H24O6·3.25H2O, consists of theobrominium (3,7-dimethyl-2,6-dioxo-1H-purin-9-ium) cations, perchlorate anions and dibenzo-18-crown-6 and water molecules. The crown ether is in a bent conformation, in which the planes of the aromatic rings subtend an angle of 63.7 (1)°. Intermolecular O—H⋯O hydrogen bonding between the water molecules and the O atoms of the cyclic ether delimit an empty space reminiscent of a hollow cage. The water molecules are additionally linked to the cations by N—H⋯O hydrogen bonding. One of the positions of the water molecules is occupied only fractionally (25%) and is located outside this framework.

The co-crystal, C 7 H 9 N 4 O 2 + ÁClO 4 À ÁC 20 H 24 O 6 Á3.25H 2 O, consists of theobrominium (3,7-dimethyl-2,6-dioxo-1H-purin-9-ium) cations, perchlorate anions and dibenzo-18-crown-6 and water molecules. The crown ether is in a bent conformation, in which the planes of the aromatic rings subtend an angle of 63.7 (1) . Intermolecular O-HÁ Á ÁO hydrogen bonding between the water molecules and the O atoms of the cyclic ether delimit an empty space reminiscent of a hollow cage. The water molecules are additionally linked to the cations by N-HÁ Á ÁO hydrogen bonding. One of the positions of the water molecules is occupied only fractionally (25%) and is located outside this framework.
VK is grateful to the Studienstiftung des Deutschen Volkes for a PhD scholarship.
that are likely to interact with bioreceptors which show some similarities with crown ethers.
In the crystal structure of the title compound the dibenzo-18-crown-6 molecule is in the usual bent conformation (Fig. 1) The angle between the planes of the aromatic rings is 63.7 (1)°, which is slightly lower than the one reported for the crystal structure of the neat molecule (Lima et al., 2008). The oxygen atoms of the ether build hydrogen bonds with two water molecules above (O1W and O3W) and one below (O2W) the central part of the ring (Fig. 2). The resulting geometric arrangement is reminiscent of a hollow cage with O-atoms on the vertices and H-bonds defining the sides. The "cages" are interlinked with one another via H-bonds between water molecules. The theobrominium ions are connected to the H-bonding framework via intermolecular N-H···O hydrogen bonding between N9 and O3W ( Table 1).
The pyrimidine ring of the theobromine molecule appears to be superimposed over one of the aromatic rings of dibenzo-18-crown-6. The angle enclosed by the planes of the purine and benzene ring is 9.18 (8)°. Due to this relatively large value of the interplanar angle, π-π stacking interactions between both aromatic moieties appear to be unlikely.

Experimental
Theobromine (18 mg, 0.1 mmol) was dissolved in aqueous HClO 4 solution (5.6 ml, 6.4%) and added to the suspension of AgClO 4 (20 mg, 0.1 mmol) and dibenzo-18-crown-6 (36 mg, 0.1 mmol) in a mixture of toluene (6.3 ml) and dichloromethane (0.3 ml). The biphasic suspension was stirred vigorously for 1.5 hrs. and filtered. After 6 weeks of slow solvent evaporation at room temperature and several cycles of filtration the mother liquor was cooled for 10 days at 4°C. One colourless pentagonal prismatic crystal could be isolated among thin colourless intergrown needles.

Refinement
All C-H and N-H H atoms were positioned with idealized geometry and were refined isotropic with U iso (H) = 1.2 U eq (C, N) (1.5 for methyl H atoms) using a riding model with C -H = 0.970 Å for methylene, 0.930 Å for aromatic, 0.97 Å for methyl and 0.86 Å for N-H H atoms. The O-H H atoms of the water molecules at O1W, O2W and O3W were located in difference map, their bond lengths were set to 0.82 Å and afterwards they were refined isotropic with U iso (H) = 1.5 U eq (O) using a riding model. The position of the water molecule O4W is occupied to only 25% and its H atoms were not located.   View of the dibenzo-18-crown-6 molecule and the co-crystallizing water molecules with intermolecular O-H···O hydrogen bonding shwon as dashed lines.   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.