Isotypic crystal structures of 1-benzyl-4-(4-bromophenyl)-2-imino-1,2,5,6,7,8,9,10-octahydrocycloocta[b]pyridine-3-carbonitrile and 1-benzyl-4-(4-fluorophenyl)-2-imino-1,2,5,6,7,8,9,10-octahydrocycloocta[b]pyridine-3-carbonitrile

Two isotypic title compounds comprise a 2-iminopyridine ring fused with a cyclooctane ring. In one compound, the cyclooctane ring adopts a twisted chair–chair conformation, while in the second, this ring adopts a twisted boat–chair conformation.


Chemical context
The pyridine skeleton is of great importance to chemists as well as to biologists as it is found in a large variety of naturally occurring compounds and also in clinically useful molecules having diverse biological activities. Its derivatives are known to possess antimicrobial (Jo et al., 2004) and antiviral (Mavel et al., 2002) activities. The heterocyclic 1,4-dihydropyridine ring is a common feature in compounds with various pharmacological activities such as antimicrobial (Hooper et al., 1982) and antithrombotic (Sunkel et al., 1990) activities. The chemistry of imines in particular is of special interest in the literature due to their numerous practical applications (Echevarria et al., 1999). Imines have attracted much attention because of their wide variety of applications in the electronics and photonics fields (Wang et al., 2001). Imines and their complexes have a variety of applications in the biological, clinical and analytical fields (Singh et al., 1975;Patel et al., 1999). Our interest in the preparation of pharmacologically active 2-imino pyridines led us to synthesise the title compounds and we have undertaken the X-ray crystal structure determination of these compounds in order to establish their conformations.

Structural commentary
The structures of compounds (I) and (II) are shown in Figs. 1 and 2, respectively. The cyclooctane ring adopts a twisted ISSN 1600-5368 chair-chair conformation in compound (I) and twisted boatchair conformation (Wiberg, 2003) in compound (II).

Figure 4
Partial packing diagram of the title compound (II). Dashed lines represent intermolecular hydrogen bonds and C-HÁ Á Á contacts. For clarity, H atoms not involved in hydrogen bonding have been omitted.
involving the phenyl ring as acceptor (Tables 1 and 2 ,Figs. 3,4). In each case, the resulting supramolecular structure is a layer propagating parallel to the (110) plane.

Synthesis and crystallization
The two compounds were prepared in a similar manner using 4-fluoro aldehyde (1 mmol) for compound (I) and 4-bromo aldehyde (1 mmol) for compound (II). A mixture of cyclooctanone (1mmol), respective aldehyde (1 mmol) and malononitrile (1 mmol) were taken in ethanol (10 mL) to which p-toluenesulfonic acid (pTSA) (0.5 mmol) was added. The reaction mixture was heated under reflux for 2-3 h. After completion of the reaction (TLC), the reaction mixture was poured into crushed ice and extracted with ethyl acetate. The