Pyridine-3-carboxamide–telluric acid (1/1)

The title molecules, pyridine-3-carboxamide and telluric acid, are interconnected by conventional O—H⋯N, N—H⋯O and O—H⋯O hydrogen bonds of moderate strength as well as by π–π interactions between the pyridine rings.


Chemical context
The motivation for the title structure determination follows from the fact that there are relatively a few structure determinations of molecular crystals containing the telluric acid molecule H 6 TeO 6 (Groom et al., 2016). These structure determinations are summarized in Table 1.
H 6 TeO 6 is a weak acid with pK a = 7.68 (1st degree; CRC Handbook, 2017) at room temperature. At the same time, the pK a value for pyridine-3-carboxamide is 3.3 (CRC Handbook, 2009). ÁpK a = pK a (base) À pK a (acid) À 4.4, which indicates that the crystalline product would rather be a co-crystal (Childs et al., 2007). In all the cases listed in Table 1, the H 6 TeO 6 molecules are fully protonated. All of these known structures are co-crystals except for ZARGII where H 6 TeO 6 is an additive molecule in the salt structure.
In most of the listed structures, the molecules of H 6 TeO 6 form columns which are interconnected by O-HÁ Á ÁO hydrogen bonds. Such a situation takes place in KUTBUW (the columns are parallel to the a axis), UREATE, UREATE01, UREATE02 (parallel to the c axis) and VALTUX (parallel to the c axis). Analogous columns parallel to the a axis are present in GUNQUB; however, these columns are formed together with water molecules. In the other two structures, the constituent molecules are surrounded by each other. None of the structures in Table 1 contains a hydrogen bond with disordered hydrogen atoms in which the hydroxyl groups of the telluric acid are involved. Interestingly, neutron diffraction experiments revealed that the cubic form of H 6 TeO 6 (Cohen-Addad, 1971) possesses disordered hydrogen atoms, in contrast to the monoclinic form (Lindqvist & Lehmann, 1973). The H 6 TeO 6 molecule can be considered as an interesting building block for crystal engineering because it can offer each of its six hydroxyl groups for the formation of hydrogen bonds with neighbouring molecules.
3-Pyridinecarboxamide (nicotinamide) is a biologically important molecule which is an active part of the vitamin B3 and nicotinamide adenine dinucleotide (NAD) [see for example Wald (1991) and Williamson et al. (1967)]. The interplanar angles ANG between the pyridine and the amide groups in the 3-pyridinecarboxamide or 3-carbamoylpyridin-1-ium molecules span a large angle because these two moieties are connected by a single C-C bond (bond D, Fig. 1). (This single bond corresponds to the bond C1-C6 in the title structure.) Thus 3-pyridinecarboxamide as well as 3-carbamoylpyridin-1-ium molecules can easily accommodate to the environment for optimization of the amide interactions. It seems that the bond length D tends to be longer in the 3-carbamoylpyridin-1-ium molecules than in 3-pyridinecarboxamide molecules. This phenomenon can easily be explained by the elongation of the C-NH + bonds in comparison to the the C-N bonds in the conjugated bonds system present in the pyridine rings, and thus by a tendency to a slight elongation of bond D.

Structural commentary
The title molecules are shown in Fig. 2. The interplanar angle between the pyridine non-hydrogen atoms and the nonhydrogen amide atoms is 15.25 (8) . Table 2 lists the hydrogen bonds present in the title structure. The parameters of these hydrogen bonds place them in the category of moderate hydrogen bonds (Gilli & Gilli, 2009). The sheets composed of the telluric acid molecules only are held together by O-HÁ Á ÁO hydrogen bonds (Fig. 3). These sheets alternate with the 3-pyridinecarboxamide molecules, which are interconnected by hydrogen bonds as well as by -electronÁ Á Á-electron ring interactions . These sheets are parallel to (001). The presence of these sheets is so far unique among the known structures of molecular crystals with H 6 TeO 6 (see also the Chemical context section).
The secondary amine nitrogen is the acceptor of the strongest hydrogen bond present in the structure (O7-H1O7Á Á ÁN1; Table 2). The primary amine hydrogen H1N2 is donated to one of the hydroxyl oxygen atoms of the telluric acid while H2N2 is donated to the oxygen atom of the amide group (O4).
The most important piece of knowledge derived from the study of the title structure is the functionality of the telluric acid molecule, which can become a constitutional part of the hydrogen-bonding pattern. This property of the telluric acid molecule has not been so far studied in depth in molecular crystals because of scarcity of relevant structural data.
1522 Jan Fábry C 6 H 6 N 2 OÁH 6 O 6 Te Acta Cryst. Dependence of the C-C bond distance D, which interconnects the pyridine and the amide groups, on the interplanar angle (ANG) between these groups in 3-pyridinecarboxamide molecules (black squares) or 3carbamoylpyridin-1-ium molecules (red circles). The interplanar angle has been calculated from the non-hydrogen atoms of these groups. The title structure, which belongs among 3-pyridinecarboxamide molecules, is depicted by a green triangle. Table 1 Overview of the known structure determinations of molecular crystals containing the H 6 TeO 6 molecule.

Figure 2
The title molecule with anisotropic atomic displacements shown at the 50% probability level (PLATON; Spek, 2009).

Supramolecular features
The telluric acid molecules H 6 TeO 6 form sheets (Fig. 3) parallel to (001). Each telluric acid molecule donates four hydrogen atoms to four symmetry-equivalent telluric acid molecules and accepts four hydrogen atoms from these molecules. These hydrogen bonds are arranged in centrosymmetric graph-set motifs R 2 2 (8) (Etter et al., 1990): (symmetry codes as in Table 2).
Another hydrogen atom of the telluric acid is donated to atom N1, thus forming a chain with graph-set motif C(3). The chain is composed of the atoms O7-H1O7Á Á ÁN1 (Figs. 4 and 5) and this hydrogen bond is the strongest of all the hydrogen bonds present in the title structure ( Table 2).

Synthesis and crystallization
Equimolar amounts of 3-pyridinecarboxamide (0.40 g) and telluric acid (0.75 g) were dissolved in water (10 ml). Colourless crystals of the title compound were obtained by slow evaporation over the course of three weeks.