Received 12 October 2004
Isonicotinamide has been shown to form many 1:1 co-crystals with monofunctional carboxylic acids, but with propionic acid it forms a co-crystal containing two acid molecules and one isonicotinamide molecule per formula unit, C6H6N2O·2C3H6O2. The crystal structure consists of `supermolecules' made up of of one isonicotinamide molecule and two acid molecules, and the asymmetric unit contains two of these supermolecules. One of the acid molecules is hydrogen bonded to the pyridine function, and the other to the amide function of the isonicotinamide. Further N-HO hydrogen bonds connect these supermolecules into chains which run along the  direction. The chains are linked into layers perpendicular to (010) by C-HO and -stacking interactions. The layers are then linked together by further C-HO interactions.
Isonicotinamide has been shown to crystallize with carboxylic acids in a 1:1 stoichiometry to form a robust building block or `supermolecule' consisting of two amide and two acid molecules, (I) (Aakeröy et al., 2002). When a saturated solution of isonicotinamide in warm propionic acid was allowed to cool, colourless crystalline laths were obtained. Single-crystal X-ray diffraction revealed these to be a co-crystal consisting of isonicotinamide and propionic acid in a 1:2 ratio, viz. (II).
Similar preparative routes with formic and acetic acids both yielded 1:1 co-crystals (Oswald, 2004). Attempts to prepare a 1:1 co-crystal with propionic acid failed. For example, a 1:1 mixture of propionic acid and isonicotinamide in ethanol yielded only crystals of (II); even in the presence of excess isonicotinamide, the only crystals obtained were isonicotinamide itself and (II).
The crystal structure of (II) consists of supermolecules comprising two acid and one isonicotinamide molecule. One acid forms an R22(8) motif with the amide moiety (Bernstein et al., 1995). Another acid molecule forms a hydrogen bond to the pyridine N atom, supported by a weaker C-HO hydrogen bond (Fig. 1 and Table 1). There are two supermolecules in the asymmetric unit and, in the terminology of Aakeröy et al. (2002), both are in the trans-trans conformation.
The independent supermolecules hydrogen-bond together using the second amide donor and the carbonyl group from the propionic acid molecules located at the pyridine end of the supermolecules. This builds up a helical chain in which successive supermolecules are aligned approximately perpendicular to one another (Figs. 2-4; hydrogen-bond dimensions are listed in Table 1). The chains run along the a direction, and they comprise all the conventional N-HO and O-HO hydrogen bonds in the crystal structure (see Table 1); additional C-HO interactions (C5A-H5AO2U and C5B-H5BO2V) are also formed within the chains (Desiraju & Steiner, 1999).
Successive helical chains are distributed along the c direction at z = ¼, ¾, ..., etc. (Fig. 5). Though there are no direct hydrogen-bonding interactions between neighbouring chains, weak C-HO interactions are formed between chains located one lattice-repeat away from each other (e.g. the red and blue chains in Fig. 5; see also Fig. 6). These interactions involve C2A-H2AO3T and C2B-H2BO3S. Supermolecules in neighbouring chains are interleaved to produce stacks of supermolecules along a (Fig. 5). Stacks containing only the supermolecules based on isonicotinamide molecule A occur at z = ½, while stacks containing only those based on molecule B occur at z = 0, 1, ... etc. Within the stacks, pairs of pyridine moieties are -stacked across inversion centres (Fig. 7). The stacking distances are 3.34 and 3.33 Å for the A and B pyridine rings, respectively.
Thus, layers are formed in the ac-plane by chains of hydrogen-bonded supermolecules linked by weak C-HO and -stacking interactions. The layers are connected via C-HO hydrogen bonds involving pairs of C4T-H4T1O2T and C4V-H4V1O2S interactions disposed about inversion centres (Fig. 8).
| || Figure 1 |
The two crystallographically independent supermolecules, with the atomic numbering. Displacement ellipsoids are drawn at the 30% probability level. Conventional hydrogen bonds are shown in heavy dashes and the HO distances span 1.78 (4)-1.96 (4) Å (see Table 1). The C-HO hydrogen bonds (shown as open dashes) are quite weak for this type of interaction (2.73 and 2.72 Å).
| || Figure 2 |
Hydrogen-bonded chains in the crystal structure of (II). Hydrogen bonds link supermolecules into chains. This view is approximately along the direct lattice direction . Hydrogen bonds are shown as dashed lines, weak C-HO hydrogen bonds are shown in turquoise.
| || Figure 3 |
Hydrogen-bonded chains in the crystal structure of (II). Successive supermolecules are approximately perpendicular to each other; this view is perpendicular to (001).
| || Figure 4 |
Hydrogen-bonded chains in the crystal structure of (II). View of the chain, showing the atom numbering; view approximately along .
| || Figure 5 |
Packing of hydrogen-bonded chains in the crystal structure of (II), forming a layer perpendicular to b*. Neighbouring chains are distributed along the c axis. Different chains (as shown in Figs. 2-4) are shown in different colours. This view is along a, cf. Fig. 2.
| || Figure 6 |
Packing of hydrogen-bonded chains in the crystal structure of (II), forming a layer perpendicular to b*. As Fig. 5, but with the green molecule deleted to reveal C-HO hydrogen bonds formed between the blue and red chains shown in Fig. 5. C-HO hydrogen bonds within chains are shown in turquoise, those between chains are shown in magenta.
| || Figure 7 |
Packing of hydrogen-bonded chains in the crystal structure of (II), forming a layer perpendicular to . Neighbouring chains are connected by -stacking interactions. This figure shows two chains viewed along . One chain is shown in ball-and-stick representation, the other as wireframe.
| || Figure 8 |
Full packing diagram of the crystal structure of (II), viewed along . Different layers (as shown in Figs. 5-7) are shown in the top and bottom halves of the figure. C-HO hydrogen bonds connect the layers.
All materials were obtained from Aldrich and used as received. Isonicotinamide (0.50 g, 4.10 mmol) was dissolved in an excess of propionic acid (2.40 g, 32.43 mmol) and warmed until all the solid dissolved. The solution was cooled to room temperature, producing colourless laths.
H atoms were placed on C atoms in calculated positions [Uiso(H) = 1.2Ueq(C)] and allowed to ride on their parent atoms [C(phenyl)-H = 0.95, C(methylene)-H = 0.99 and C(methyl)-H = 0.98 Å]. Amide and hydroxyl H atoms were located in difference maps and refined freely, the former subject to the restraint N-H = 0.95 (3) Å. The ranges of N-H and O-H bond lengths were 0.91 (2)-0.96 (1) and 0.75 (5)-0.87 (4) Å, respectively.
Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT ; program(s) used to solve structure: SHELXTL (Sheldrick, 2001); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL, MERCURY (Taylor & Macrae, 2001) and DIAMOND (Crystal Impact, 2004); software used to prepare material for publication: SHELXTL, EnCIFer (Allen et al., 2004) and PLATON (Spek, 2003), as incorporated in WinGX (Farrugia, 1999)..
We thank the EPSRC, the University of Edinburgh and the Cambridge Crystallographic Data Centre for funding.
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