Racemic 1,2,3,4,7,8,9,10-octafluoro-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine: an octafluorinated analogue of Tröger’s base

The title compound, C15H6F8N2, possesses a non-crystallographic twofold axis. The dihedral angle between the two benzene rings is 98.4 (2)°. The crystal structure involves intermolecular C—H⋯F hydrogen bonds.

Recently, a considerable interest has developed in relatively unfunctionalized receptors with concave aromatic surfaces (termed molecular clefts or tweezers). Wilcox et al. (1987) pioneered the incorporation of the Tröger's base framework in chiral molecular clefts by fusing the methanodiazocine core of Tröger's base with two bicyclic aromatic building blocks.
Later, molecular clefts comprising two or three fused methanodiazocine cores have been reported (Artacho et al., 2006, and references cited therein). Our interest in the title compound was raised due to the prospect of using highly fluorinated aromatic systems in the design of molecular clefts, thus providing a possibility to explore different supramolecular interactions.
Synthesis of halo-derivatives of Tröger's base was pioneered by Wärnmark (Jensen & Wärnmark, 2001). Later, a number of fluoro-, chloro-, bromo-, and iodo-derivatives of Tröger's base, with the halogen atoms in different positions on the aromatic rings were reported (Sergeyev &Diederich, 2004 andHansson et al., 2003). However, they typically contain only one halogen atom on each aromatic ring of the methanodiazocine skeleton. Exceptions are the recently reported 2,4,8,10tetrafluoro- (Li et al., 2005) and tetrabromo-analogs (Faroughi et al., 2006) of Tröger's base. However, polyhalo-analogs of Tröger's base such as the octafluoro analog presented here are unprecedented. To the best of our knowledge, no X-ray crystal structure of a Tröger's base analog with fluorine in the aromatic ring has been reported.
The racemic octafluoro analog of Tröger's base crystallizes in the centrosymmetric space group P2 1 /c with one molecule in the asymmetric unit. The molecule has a non-crystallographic twofold symmetry axis through the bridging carbon C13.
The CSM (Continuous Symmetry Measure) is 0.0183 (Zabrodsky et al., 1993). Bond lengths and angles are within expectations. TLS analysis returns quasi-isotropic values for the librational amplitudes, and the values for the resulting corrections of the bond lengths are all below the 2σ level. The dihedral angle between the two benzene rings is 98.4 (2)°, which lies within the normal range for analogs of Tröger's base (Dolensky et al., 2007). Cohesion in the structure appears to be mainly provided by aromatic π-π interactions between the fluorinated benzene rings, leading to pairwise ordering of enantiomers around the centers of inversion, with an interplanar distance of under 4 Å. The most clear examples of this in the structure , with a slippage of 3.251 (3) Å. Cg(x) indicates the centroid of benzene ring x, perp. indicates the perpendicular distance between the ring planes. Due to the lack of suitable hydrogen bond donors, the N-atoms display no close contacts whatsoever. There are a number of F-π contacts in the structure, e.g. F(3)···Cg(2) ii 3.672 (3) Å, C3-F3···Cg(2) ii 161.7 (2)°. Also, H-F contacts occur that are substantially shorter than the van der Waals radii, but these are not usually understood as hydrogen bonds. They are given in Table 1. supplementary materials sup-2 Experimental 2,3,4,5-Tetrafluoroaniline (330 mg, 2 mmol) and paraformaldehyde (120 mg, 4 mmol) were added under vigorous stirring to CF 3 COOH (4 ml) at −15°C. The resulting mixture was allowed to reach room temperature and stirred for 14 days, then slowly added to a stirred mixture of ice and 30% aqueous NH 3 (7 ml). Extraction with CH 2 Cl 2 (2 x 20 ml), drying of the organic layer over MgSO 4 , and removal of the solvent in vacuo gave a crude product which was purified by flash chromatography (SiO 2 /CH 2 Cl 2 ). Yield of the title compound: 135 mg (37%). Crystals suitable for X-ray diffraction were grown by slow evaporation from CHCl 3 solution.    ,2,3,4,7,8,9,10-octafluoro-5,6,11,12-tetrahydro-5,11-\ methanodibenzo[b,f]