Crystal structure of the di-Mannich base 4,4′-dichloro-3,3′,5,5′-tetramethyl-2,2′-[imidazolidine-1,3-diylbis(methylene)]diphenol

In the title compound, the imidizadoline ring adopts an envelope conformation and the nitrogen lone pairs are oriented in a syn disposition. The crystal packing is stabilized by C—H⋯O hydrogen-bonding interactions.

The title compound, C 21 H 26 Cl 2 N 2 O 2 , was prepared in a solvent-free microwaveassisted synthesis, and crystallizes in the orthorhombic space group Pna2 1 . The imidazolidine ring adopts an envelope conformation and its mean plane is almost perpendicular to the two pendant aromatic rings [dihedral angles = 84.61 (9) and 86.54 (9) ]. The molecular structure shows the presence of two intramolecular O-HÁ Á ÁN hydrogen bonds between the phenolic hydroxy groups and imidazolidine N atoms. The two 3-chloro-6-hydroxy-2,4-dimethylbenzyl groups are located in a cis orientation with respect to the imidazolidine fragment. As a result, the lone pairs of electrons on the N atoms are presumed to be disposed in a syn conformation. This is therefore the first example of an exception to the 'rabbit-ears' effect in such 2,2 0 -[imidazolidine-1,3diylbis(methylene)]diphenol derivatives.
In a previous report (Rivera & Quevedo, 2013), the title compound (I) was obtained under solvent-free conditions by ISSN 2056-9890 heating a 1:4 mixture of TATD and 4-chloro-3,5-dimethylphenol in an oil bath with stirring at 423 K for 20 min. Drawbacks of this synthesis include the long reaction time and a requirement of considerable effort to optimize the reaction conditions and temperature control. We therefore subsequently explored this reaction under solvent-free, microwaveassisted conditions. The reaction was found to proceed smoothly under microwave irradiation in only 3 min at 403 K, in modest yield.

Supramolecular features
With both hydroxy groups of (I) involved in intramolecular hydrogen bonds, the only directional interaction in the crystal is a C13-H13Á Á ÁO2 i bond (Table 1 and Fig. 2), which links adjacent molecules in a head-to-tail fashion into zigzag chains, extending along the c-axis direction (Fig. 2).

Database survey
A search in the Cambridge Structural Database (Groom & Allen 2014) revealed previous reports of six structures of related 2,2 0 -[imidazolidine-1,3-diylbis(methylene)]diphenol compounds (Rivera et al., 2011(Rivera et al., , 2012a(Rivera et al., ,b,c, 2013(Rivera et al., , 2014. Each of these also shows intramolecular O-HÁ Á ÁN hydrogen bonds between the two imidazolidine N atoms and the hydroxy groups. In addition, the DÁ Á ÁA distances in these compounds compare well with those observed in the title compound. As with (I), the imidazolidine ring in the p-tert-butylphenol derivative , adopts an envelope conformation whereas, in the other five the ring adopts a twist conformation. Furthermore, unlike the title compound, the nitrogen lone pairs in all six of the related derivatives are oriented in an anti disposition. The title molecule, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines. Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) Àx þ 1; Ày þ 1; z þ 1 2 .

Figure 2
A perspective view along the a axis of the crystal packing of the title compound,. The C-HÁ Á ÁO hydrogen bonds are shown as dashed lines.

Synthesis and crystallization
A mixture of 1,3,6,8-tetrazatricyclo[4.4.1.1 3,8 ]dodecane (0.100 g, 0.6 mmol) and 4-chloro-3,5-dimethylphenol (0.375 g, 2.4 mmol) without any solvent was exposed to microwave irradiation in a CEM Discover reactor (with 250 W as the maximum power) for 3 min at a temperature of 403 K. Once cooled to room temperature, the reaction mixture was dissolved with CHCl 3 which was removed under reduced pressure to yield the crude product. This was further purified by column chromatography on silica gel using a mixture of benzene:ethyl acetate (80:20) as eluent (yield 21%, m.p. = 421-422 K). Single crystals in the form of needles shorter than 1 mm were obtained from a chloroform:ethanol (50:50) solution by slow evaporation of the solvent at room temperature over a period of one week.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All the H atoms were located in difference electron density maps. The hydroxy H atoms were freely refined. C-bound H atoms were fixed geometrically (C-H = 0.95 to 0.99 Å ) and refined using a riding model, with U iso (H) set to 1.2U eq (1.5U eq for methyl groups) of the parent atoms. The methyl groups were allowed to rotate but not to tip.