Isonicotinamide–formamide (1/1)

The 1:1 co-crystal of isonicotinamide and formamide, C6H6N2O·CH3NO, consists of hydrogen-bonded dimers, each comprising two isonicotinamide or two formamide mol­ecules. These dimers are connected further by hydrogen bonds into sheets, which are parallel to the (\overline{2}11) plane.

The 1:1 co-crystal of isonicotinamide and formamide, C 6 H 6 N 2 OÁCH 3 NO, consists of hydrogen-bonded dimers, each comprising two isonicotinamide or two formamide molecules. These dimers are connected further by hydrogen bonds into sheets, which are parallel to the (211) plane.

Comment
Isonicotinamide has been shown to crystallize with carboxylic acids in a 1:1 stoichiometry to form a robust building block or 'supermolecule', (I), consisting of two amide and two acid molecules (Aakerö y et al., 2002;Oswald et al., 2004). Amides contain C O and C-NH 2 groups that could act in an analogous way to the C O and C-OH groups of carboxylic acids. The aim of the present investigation was to assess the validity of this analogy in the case of the simplest amide, formamide.
The title co-crystal, (II), crystallizes in the monoclinic space group P2 1 /c with one molecule of each component in the asymmetric unit (Fig. 1). The bond distances and angles are unremarkable.
Amides characteristically form R 2 2 (8) (Bernstein et al. 1995) centrosymmetric dimers through hydrogen bonding between the NH 2 and C O groups. This behaviour is observed in (II), where homomeric dimers are formed (i.e. formamide forms a dimer with another formamide etc.), the two components in each case being related by crystallographic inversion centres. The NÁ Á ÁO distances in the R 2 2 (8) dimers are 2.9239 (16) Å in the case of isonicotinamide and 2.9696 (16) Å for formamide.
In co-crystals of carboxylic acids with isonicotinamide, homomeric R 2 2 (8) dimers are often formed between the amide groups of the isonicotinamide molecules (Aakerö y et al., 2002). The two pyridyl functions at either end of the nicotinamide dimer so formed hydrogen bond to two carboxylic acid molecules in R 2 2 (7) motifs comprising C-OHÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds. Of these interactions, only the R 2 2 (8) dimer formation is observed in (II).
The second donor function of the isonicotinamide forms a hydrogen bond to the carbonyl O atom of the formamide; these interactions build up chains. The chains are linked together through a hydrogen bond between a symmetryequivalent formamide dimer and the pyridine N atom of the isonicotinamide forming an open grid-like layer parallel to the (211) plane (Fig. 2). The second donor function of the formamide molecules serves to link this layer with a symmetry equivalent parallel to (211) filling in the structure.

D-HÁ
H atoms attached to C atoms were placed in idealized positions (C-H = 0.95 Å ) and allowed to ride on their parent atoms with U iso (H) = 1.2U eq (C). H atoms attached to N atoms were located in a difference map and refined freely.
We thank the CCDC, the EPSRC and The University of Edinburgh for funding.   Formation of hydrogen-bonded layers in (II); hydrogen bonds are shown as broken lines. This view is approximately along the (211) reciprocal lattice direction.

Figure 1
The asymmetric unit of (II). Displacement ellipsoids are shown as 30% probability surfaces and H atoms are drawn as circles of arbitrary radii. Isonicotinamide has been shown to crystallize with carboxylic acids in a 1:1 stoichiometry to form a robust building block or `supermolecule′, (I), consisting of two amide and two acid molecules (Aakeröy et al., 2002;Oswald et al., 2004). Amides contain C═O and C-NH 2 groups that could act in an analogous way to the C═O and C-OH groups of carboxylic acids. The aim of the present investigation was to assess the validity of this analogy in the case of the simplest amide, formamide.
The title co-crystal, (II), crystallizes in the monoclinic space group P2 1 /c with one molecule of each component in the asymmetric unit (Fig. 1). The primary bond distances and angles are unremarkable. In co-crystals of carboxylic acids with isonicotinamide, homomeric R 2 2 (8) dimers are often formed between the amide groups of the isonicotinamide molecules (Aakeröy et al., 2002). The two pyridyl functions at either end of the nicotinamide dimer so formed hydrogen bond to two carboxylic acid molecules in R 2 2 (7) motifs comprising C-OH···N and C-H···O hydrogen bonds. Of these interactions, only the R 2 2 (8) dimer formation is observed in (II).
The second donor function of the isonicotinamide forms a hydrogen bond to the carbonyl O atom of the formamide; these interactions build-up chains. The chains are linked together through a hydrogen bond between a symmetryequivalent formamide dimer and the pyridine N atom of the isonicotinamide forming an open grid-like layer parallel to the (−211) planes (Fig. 2). The second donor function of the formamide molecules serves to link this layer with a symmetry equivalent parallel to (−2-11) filling-in the structure.

S2. Experimental
Isonicotinamide (0.49 g, 4.02 mmol) was dissolved in an excess of formamide (1.48 g, 32.10 mmol) and warmed until all the solid dissolved. On cooling, long colourless needles were produced, which fractured into thinner shards, degrading the crystal quality, when attempts were made to cut them to a more suitable length.

S3. Refinement
H atoms attached to C atoms were placed in idealized positions (C-H = 0.95 Å) and allowed to ride on their parent atoms with U iso (H) = 1.2U eq (C). H atoms attached to N atoms were located in a difference map and refined freely. The asymmetric unit of (II). Displacement ellipsoids are shown as 30% probability surfaces and H atoms are drawn as circles of arbitrary radii.

Special details
Experimental. The temperature of the sample was controlled using an Oxford Cryosystems low-temperature device (Cosier & Glazer, 1986). Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.