2.1. Synthesis

Nitrobenzylidene-iodoanilines were prepared as previously described (Glidewell, Howie et al., 2002) and subsequently reduced using a fivefold molar excess of sodium borohydride in refluxing methanol. After work-up, crystals of (1), (2), (6) and (7) suitable for single-crystal X-ray diffraction were grown by slow evaporation of solutions in ethanol. Compound (8) is an oil at ambient temperature and we have not been able to induce crystallization, even at reduced temperatures: (3) and (4) are solids at ambient temperature but, despite repeated efforts, we have so far been unable to obtain any crystals of either compound which are suitable for single-crystal X-ray diffraction.

2.2. Data collection, structure solution and refinement

Diffraction data for (1), (2), (6) and (7) were collected at 120 (2) K using a Nonius Kappa-CCD diffractometer with graphite-monochromated Mo K radiation ( = 0.71073 Å). Other details of the cell data, data collection and refinement are summarized in Table 1, together with details of the software employed (Ferguson, 1999; Nonius, 1997; Otwinowski & Minor, 1997; Sheldrick, 1997a,b; McArdle, 2003).¹

For (1) and (6) the space group P2₁/c was uniquely assigned from the systematic absences: likewise, the space group P2₁2₁2₁ was uniquely assigned for (2). Compound (7) is triclinic, and the space group was selected and confirmed by the subsequent analysis. The structures were solved by direct methods and refined with all data on F². A weighting scheme based upon P = [F_o² + 2F_c²]/3 was employed in order to reduce statistical bias (Wilson, 1976). All H atoms were located from difference maps and they were included in the refinements as riding atoms with the bond distances C-H 0.95 (aromatic) or 0.99 Å (CH₂) and N-H 0.88 Å. The absolute configuration of (2) in the crystal selected for study was established by the use of the Flack parameter (Flack, 1983), whose final value was -0.01 (2) with 1191 Friedel pairs present.

Supramolecular analyses were made and the diagrams were prepared with the aid of PLATON (Spek, 2003). Details of molecular conformations are given in Table 2. Details of hydrogen-bond dimensions and short intramolecular contacts, and iodonitro interactions are given in Tables 3 and 4. Figs. 1-13 show the molecular components, with the atom-labelling schemes, and aspects of the supramolecular structures.

3.1. Molecular conformations

The molecular conformations of iodo-(N-benzyl)anilines can be defined in terms of just four torsional angles and these are given in Table 2 for (1), (2) and (4)-(7). For the central spacer unit, the torsional angle C11-C17-N1-C21 shows that for (2) this unit is essentially trans planar, whereas for all other examples discussed here this torsional angle is close to ±90°, indicative of a markedly non-planar unit. The iodinated ring C21-C26 is generally nearly co-planar with the fragment C21-N1-C17. Since the labelling of the iodinated ring is determined by the location of the I substituent, the values show, for example, that in (1) and (2) the I substituent lies off different edges of the molecule: thus in (1) the I is on the same edge as the N-H bond, while in (2) it is on the opposite edge (Figs. 1 and 3). It seems possible that the short intramolecular N-HI contacts in (1) (Table 3) play a significant role in controlling the overall conformation of (I). Likewise in the 2-iodo derivative (7), there is a short intramolecular N-HI contact which may be significant in controlling the orientation of the iodinated ring. A similar difference occurs between (4) and (5) (Glidewell, Low et al., 2002). In (2) and (7), as well as in (4), the nitrated ring is nearly co-planar with the adjacent C11-C17-N1 fragment, but this is far from the case in (1), (5) and (6). In all cases there are some fairly short intramolecular C-HO contacts (Table 3). The biggest deviation of the nitro group from co-planarity with the adjacent aryl ring occurs in (7), where moreover the two molecules show a marked difference: apart from this, the molecules in the selected asymmetric unit are close to being mirror images (see Fig. 11). It will be noted (see §3.2) that the structures of all these compounds exhibit direction-specific intermolecular interactions and it is thus probable that the energies associated with these intermolecular interactions are of similar magnitudes to the potential-energy barriers associated with rotation about the single bonds specified in Table 2.

One consequence of the conformational behaviour revealed by the torsional angles is that each molecule is chiral in the solid state, having no internal symmetry whatsoever. Compounds (1), (6) and (7) all crystallize in centrosymmetric space groups containing equal numbers of the two enantiomeric forms, as do (4) and (5) [P2₁/n and Pbca, respectively (Glidewell, Low et al., 2002)]: however, (2) crystallizes in the chiral space group P2₁2₁2₁ where each crystal can contain only one enantiomer. Since (2) is almost certainly racemic in solution, its crystallization is as a conglomerate: while the absolute configuration of the molecules in the crystal selected for study was readily determined, this configuration has no chemical significance.

The bond lengths and inter-bond angles show no unusual values.

3.2. Supramolecular structures

3.3. General comments on the supramolecular structures

The molecular constitutions of (1)-(9) [see (II)] allow the possibility of six different types of direction-specific intermolecular interaction. These are N-HO and C-HO, hydrogen bonds, iodonitro interactions, and aromatic stacking interactions. In the event, no single compound in this series whose structure has been determined to date exhibits more than four of these types, while (1) exhibits only one type, namely C-HO hydrogen bonds. Compounds (4)-(6) all exhibit three types of direction-specific intermolecular interaction, but the selection is, in fact, different for each: while the structures of all three of these compounds contain C-HO hydrogen bonds, only that of (5) contains N-HO and C-H(arene) hydrogen bonds, only that of (6) contains N-H(arene) hydrogen bonds, and only that of (4) contains iodonitro interactions. In addition, while the structures of both (4) and (6) contain aromatic stacking interactions, these are absent from the structure of (5).

Not only do the number and identity of the types of intermolecular interactions vary, but the structural consequences of these interactions also show a wide range of behaviour. Thus, while (2) and (7) both contain the same four types of intermolecular interactions, namely N-HO and C-HO hydrogen bonds, and iodonitro and stacking interactions, the consequences of these are entirely different in that the overall supramolecular structures of (2) and (7) are three- and one-dimensional, respectively. Compounds (1) and (7), exhibiting one and four types, respectively, in the intermolecular interactions, both have one-dimensional supramolecular structures; (5) and (6) both have two-dimensional supramolecular structures; (2) and (4), exhibiting four and three types, respectively, of intermolecular interaction, both have three-dimensional supramolecular structures.