Hydronium perchlorate–dibenzo-18-crown-6 (1/1): monoclinic polymorph

The asymmetric unit of the title compound, H3O+·ClO4 −·C20H24O6, contains two molecules/ions of each species. Both dibenzo-18-crown-6 molecules have a complexed hydronium ion inside their cavity with O—H⋯O and O—H⋯(O,O) links between the two species. The associated perchlorate anions also accept O—H⋯O hydrogen bonds from the hydronium ion. Both crown ether molecules are present in a butterfly conformation with approximate C 2v symmetry and their cavities are closed by the benzene ring of a neighbouring molecule. The packing is consolidated by C—H⋯O and C—H⋯π interactions.

The asymmetric unit of the title compound, H 3 O + ÁClO 4 À Á-C 20 H 24 O 6 , contains two molecules/ions of each species. Both dibenzo-18-crown-6 molecules have a complexed hydronium ion inside their cavity with O-HÁ Á ÁO and O-HÁ Á Á (O,O) links between the two species. The associated perchlorate anions also accept O-HÁ Á ÁO hydrogen bonds from the hydronium ion. Both crown ether molecules are present in a butterfly conformation with approximate C 2v symmetry and their cavities are closed by the benzene ring of a neighbouring molecule. The packing is consolidated by C-HÁ Á ÁO and C-HÁ Á Á interactions.
The published structure determined at room temperature is triclinic(space group P-1, a = 8.582 Å, b = 10.486 Å, c = 26.293 Å, α = 79.45°,β = 82.00° and γ = 79.36°, V =2272.5 Å) with asymmetric unit consisting of two independent molecules of macrocycle with complexed hydronium ions. The neutrality of the compound is ensured by two perchlorate anions. The data of crystal structure, presented in this paper, were collected at room temperature (testing stage) and at 120 K (final data collection). We found the complex monoclinic, P2 1 /c space group, with unit-cell parameters a = 8.6535 Å, b = 26.7823 Å, c = 19.1707 Å, β = 99.9987° and doubled unit cell volume V = 4372.05 Å 3 . The difference between both structures is in their system of hydrogen bonds. In Chekhlov's structure, the hydronium ion is held by three hydrogen bonds inside the crown cavity. In presented structure, hydronium ion and crown-ether form only two hydrogen bonds. The third hydrogen atom of hydronium ion is shared with perchlorate anion which makes it to point out of the cavity. This hydrogen bond causes that the perchlorate anions are not disordered as it was observed in Chekhlov's structure. Consequently, sharp maxima in difference Fourier map could be used for localizing hydrogen positions in both oxonia cations (Fig 3) and the found hydrogen positions could be refined without restraints. The distance between hydronium and oxygen atoms in macrocycles are 1.637 Å (O21-H1···O3) and 1.864 Å (O21-H9···O5) for one crownether molecule and 1.895 Å (O22-H10···O11) and 1.661 Å (O22-H8···O9) for the other one. The length of hydrogen bond between hydronium ion and perchlorate is 1.732 Å (O21-H7···O17) and 1.687 Å (O22-H6···O14). The distances between hydrogen atoms and oxygen atoms in hydronium correspond to the extent of their participation in hydrogen bonding system: O-H distance close (but still longer) to the standard value 0.983 Å has been only found for the weakest hydrogen bond O22-H10···O11. For stronger hydrogen bonds O-H distance becomes significantly longer, taking the maximum value 1.29 (4) for O22-H6···O14. The O-H and corresponding H···O distances for oxonia are summarized in Table 2. The hydronium ions are enclosed in the crown-ether cavities by phenyl ring of neighbouring molecules. This arrangement is stabilized due to CH-π interactions between phenyl rings and CH 2 groups of crownether (the distance between centroid of phenyl ring C11→C16 and H37b in ethylen group is 2.989 Å and between centriod of phenyl ring C31→C36 and H17a in ethylen group is 2.870 Å) and due to the face-to-edge orientation of phenyl rings (distance between the centriod of phenyl ring C21→C26 and H13 of phenyl ring C11→C16 is 3.207 Å and between the centriod of phenyl ring C1→C6 and H33 in phenyl ring C31→C36 is 3.004 Å).

Experimental
Dibenzo-18-crown-6, perchloric acid and acetonitrile were purchased by Fluka. Crystals were prepared by slow evaporation of equimolar mixture of dibenzo-18-crown-6 (0.05M) and perchloric acid (0.05M) in acetonitrile to yield colourless prisms of the title compound.

Special details
Refinement. The refinement was carried out against all reflections. The conventional R-factor is always based on F. The goodness of fit as well as the weighted R-factor are based on F and F 2 for refinement carried out on F and F 2 , respectively. The threshold expression is used only for calculating R-factors etc. and it is not relevant to the choice of reflections for refinement.
All the H atoms were discernible in difference Fourier maps and could be refined to reasonable geometry. Despite of it the H atoms bonded to carbon atoms were constrained to ideal positions. The O-H distances and angles in hydronium ions were not restrained.
The isotropic temperature parameters of hydrogen atoms were calculated as 1.2*U eq of the parent atom.
The program used for refinement, Jana2006, uses the weighting scheme based on the experimental expectations, see _refine_ls_weighting_details.