meso-4,4′-Difluoro-2,2′-{[(3aR,7aS)-2,3,3a,4,5,6,7,7a-octahydro-1H-1,3-benzimidazole-1,3-diyl]bis(methylene)}diphenol

In the crystal structure of the title compound, C21H24F2N2O2, there are two intramolecular O—H⋯N hydrogen bonds involving the N atoms of the imidazolidine ring and the hydroxy groups. The crystal studied was a meso compound obtained by the reaction of the aminal (2S,7R,11S,16R)-1,8,10,17-tetraazapentacyclo[8.8.1.18,17.02,7.011,16]cosane with 4-fluorophenol. The imidazolidine ring has a twisted conformation with a CH—CH—N—CH2 torsion angle of 44.99 (14)° and, surprisingly, the lone pairs of the N atoms are disposed in a syn isomerism, making the title compound an exception to the typical ‘rabbit-ear effect’ in 1,2-diamines. In the crystal, molecules are linked via C—H⋯F hydrogen bonds, forming chains along the c-axis direction. These chains are linked via another C—H⋯F hydrogen bond, forming a three-dimensional network.

In the crystal structure of the title compound, C 21 H 24 F 2 N 2 O 2 , there are two intramolecular O-HÁ Á ÁN hydrogen bonds involving the N atoms of the imidazolidine ring and the hydroxy groups. The crystal studied was a meso compound obtained by the reaction of the aminal (2S,7R,11S,16R)-1, 8,10,17-tetraazapentacyclo[8.8.1.1 8,17 .0 2,7 .0 11,16 ]cosane with 4-fluorophenol. The imidazolidine ring has a twisted conformation with a CH-CH-N-CH 2 torsion angle of 44.99 (14) and, surprisingly, the lone pairs of the N atoms are disposed in a syn isomerism, making the title compound an exception to the typical 'rabbit-ear effect' in 1,2-diamines. In the crystal, molecules are linked via C-HÁ Á ÁF hydrogen bonds, forming chains along the c-axis direction. These chains are linked via another C-HÁ Á ÁF hydrogen bond, forming a three-dimensional network.

Related literature
For a related structure, see: Rivera et al. (2011). For a discussion of the 'rabbit-ear effect' in 1,2-diamines, see: Hutchins et al.(1968 Table 1 Hydrogen-bond geometry (Å , ). (17) 148 (2)   Moreover, the benzyl groups are located in an unexpected 1,3-diequatorial syn arrangement in the heterocyclic ring with dihedral angle between the planes containing the aromatic rings of 53.80 (30)°. The nonbonding pairs of amino groups involved in the intermolecular hydrogen bonding interactions do not suffer the `rabbit-ear effect′ having a syn arrangement demonstrating that the title compound is an exception of this effect.
The stability of the crystal lattice of the title compound is related with non classical intermolecular interactions C-H···F that hold molecules linked in extended chains along the c axis. There are O-H···C and C-H···F weak interactions (table 1), the latter involve halogen group in molecular contact with an electron-deficient C-H bond of the aromatic ring of a second molecule.

Refinement
All H atoms were discernible in difference Fourier maps and could be refined to reasonable geometry. According to common practice H atoms bonded C atoms were kept in ideal positions with C-H distance 0.96 Å during the refinement. The hydroxy H atoms were found in difference Fourier maps and their coordinates were refined freely. All H atoms were refined with thermal displacement coefficients U iso (H) set to 1.5U eq (C, O) for hydroxy groups and to 1.2U eq (C) for the CH-and CH 2 -groups.

Computing details
Data    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. The program used for refinement, Jana2006, uses the weighting scheme based on the experimental expectations, see _refine_ls_weighting_details, that does not force S to be one. Therefore the values of S are usually larger than the ones from the SHELX program.