Bis(3-carbamoylpyridin-1-ium) phosphite monohydrate

Two of the constituent molecules in bis(3-carbamoylpyridin-1-ium) phosphite monohydrate, i.e. the phosphite anion and the water molecule, are situated on the symmetry plane. The molecules are held together by moderate N—H⋯O and O—H⋯N, and weak O—H⋯O and C—H⋯Ocarbonyl hydrogen bonds in which the primary and secondary amine and water H atoms are involved. The H atom directly bonded to the P atom avoids hydrogen bonding, as usual.

However, interest in the preparation of the title hydrated salt was called for with respect to an investigation of the configuration of the -NH 2 group and its dependence on its environment.
It was hoped that 3-carbamoylpyridine (nicotinamide) would make a salt or a co-crystal with phosphorous acid, H 3 PO 3 . It is difficult to predict which of these two forms would be prefererred, because of a small difference of ÁpK a = pK a (base) À pK a (acid) (Childs et al., 2007). [The pK a values for 3-carbamoylpyridine and H 3 PO 3 are 3.3 and 1.3 (first degree), respectively (CRC Handbook, 2009).]

Structural commentary
The title molecules are shown in Fig. 1. The resulting structure turned out to be a monohydrated salt. Table 1 lists the hydrogen bonds, which are shown in Fig. 2. The secondary amine hydrogen H1n1 is involved in the strongest hydrogen bond present in the structure (N1-H1n1Á Á ÁO3 i ). Its parameters indicate that this hydrogen bond is situated on the boundary between strong and moderate hydrogen bonds (Gilli & Gilli, 2009). The amide hydrogen H1n2 is donated to the water oxygen, while H2n2 is donated to atom O3 of the phosphite anion. Atom O2 is an acceptor of water hydrogen H1ow. Water hydrogen H2ow is donated to a pair of O3 ISSN 2056-9890 atoms. The carbonyl oxygen O1 is an acceptor of two weak C-HÁ Á ÁO hydrogen bonds, namely C3-H1c3Á Á ÁO1 ii and C4-H1c4Á Á ÁO1 iii . The water oxygen atom is also an acceptor of hydrogen H1c2 (see Table 1).
Phosphite and fluorophosphonate, as well hydrogen phosphite and hydrogen fluorophosphonate, are similar molecules. Either molecule can be involved, not only in isostructural compounds, but even in mixed crystals (Fá bry et al., 2012). Similarity regarding not only the shape of the molecules but also the avoidance both of P-bonded fluorines and hydrogens of involvement in strong or moderate hydrogen bonds (Matulková et al., 2017). The latter article shows a plot of the dependence of P-F distance on the longest P-O distance in flourophosphonate and hydrogen fluorophosphonate molecules. The P-F distance tends to be longer in [FPO 3 ] 2À than in [HFPO 3 ] À . Fig. 3 shows a similar plot for the phosphites and hydrogen phosphites between both molecules despite the larger spread of P-H distances in phosphite molecules because of the lower accuracy of the H-atom determinations by X-ray diffraction experiments. The reason why the P-H bond tends to be longer follows from the conservation of the overall bond valence sum of the central P 5+ or P 3+ atom. It is worth pointing out that the tabulated value of the bond valence parameter for the P-H bond seems to yield too high values. For example, for the important values of the P-H distances, i.e. 1.28, 1.33 and 1.37 Å (cf. Fig. 3), the bond valences (Brese & O'Keeffe, 1991)   The title molecule, with anisotropic atomic displacement ellipsoids shown at the 50% probability level (PLATON; Spek, 2009 (2) 3.263 (3) 146.8 (9) Symmetry codes: (i) Àx þ 3 2 ; Ày þ 2; z þ 1 2 ; (ii) x; y þ 1; z À 1; (iii) Àx þ 3 2 ; Ày þ 1; z À 1 2 ; (iv) Àx þ 3 2 ; Ày þ 2; z À 1 2 ; (v) Àx þ 3 2 ; Ày þ 1; z þ 1 2 ; (vi) x; y; z À 1; (vii) Àx þ 1; y; z.

Figure 2
View of the title structure. C, H, N, O and P atoms are represented by gray, small gray, blue, red and violet circles, respectively. [Symmetry codes: The C-NH 2 group tends to be fairly planar for short C-N bonds (Fá bry et al., 2014). In agreement with a short C-N bond length [C6-N2 = 1.3232 (18) Å ] in the title structure, the best plane through C6/N2/H1n2/H2n2 reveals a maximum deviation of about 0.05 (2) Å for each hydrogen, while 2 = 12.6.

Database survey
The applied crystallographic databases were the Cambridge Crystallographic Database (Version 5.39, with updates to May 2018; Groom et al., 2016) and the Inorganic Crystal Structure Database (June 2018; ICSD, 2018). The search was carried out for all phosphites or hydrogen phosphites with a cation of one kind.

Synthesis and crystallization
The title structure was prepared by slow evaporation of a water solution (18 ml) of equimolar amounts of nicotinamide (1.49 g) and phosphorous acid (1 g). Colourless crystals were isolated after two months.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All the H atoms were discernible in the difference electron-density map. The aryl H atoms were constrained by the constraints C-H = 0.95 Å and U iso (H) = 1.2U eq (C). Water hydrogen H2ow was refined freely, while H1ow was restrained with a distance restraint of 0.84 Å with elasticity 0.02 Å (Mü ller, 2009), and with U iso (H) = 1.5U eq (O). The hydrogens of the primary amine N2 group and the secondary amine N1 group were constrained by U iso (H) = 1.2U eq (N). The P-H hydrogen was refined isotropically. Three reflections, i.e. 952, 10,5,2 and 11,5,2, were discarded from the refinement because |I(obs) À I(calc)|/(I) > 20.
Since the phosphite oxygens revealed large displacement ellipsoids, the anharmonic displacement parameters upto the fourth grade were included for atoms P1, O2 and O3. The dependence of the longest P-O distance (Å ) on the P-H distance (Å ) in hydrogen phosphites (red circles); phosphites are represented by black squares and the title phosphite structure by a green triangle.

Special details
Refinement. This part differs from the original article by Thanigaimani et al. (2006). It also differs from the refinement by Thanigaimani et al. (2006) by a different threshold for the consideration of the observed diffractions: F 2 > 3sigma(F 2 ) has been used as criterion for observed diffractions by JANA2006 which was used for the calculation of the corrected structural model. Three diffractions 9 5 -2, 10 5 -2, 11 5 -2 were discarded from the refinement because |I(obs)-I(calc)|/σ(I) > 20. Since the Flack parameter turned out to equal to 0.012 (13) in the final stage of refinement it was set to 0. 726 Friedel pairs used in the refinement.  (3)