Comment

Nitrogen-containing heterocyclic compounds are used extensively as structural components of pharmaceuticals and agrochemicals (Ahangar et al., 2011; Gobis et al., 2012), due to their high biological activity and generally low toxicity. They also play a vital role in organometallic catalysts (Bianchini et al., 2003; Li & Hor, 2008) and dye-sensitized solar cells (Moorcraft et al., 2008; Wu et al., 2010). Within this class, compounds containing pyridine rings have attracted particular attention. 2-Aminopyridine and its derivatives are used as dyes (Patel & Patel, 2009) and pyridinium cation derivatives often possess antibacterial and antifungal activities (Sepcic, 2000; Sliwa, 1989). Some aminopyridines are found to demonstrate pharmacological activity as K⁺-channel inhibitors. By investigating three-dimensional iso-Laplacian diagrams, Nino & Munoz-Caro (2001) found a common reactivity pattern in the charged forms.

Crystal engineering can provide a key to answering why and how molecules pack in particular ways, and provides a systematic approach to the design of new crystal structures with desirable physical and chemical properties (Lam & Mak, 2000). Hydrogen bonding has emerged as one of the most powerful forces in crystal engineering due to its selectivity, strength and directional properties (Aakeroy & Seddon, 1993). Along with other delicate noncovalent interactions like - stacking and electrostatic interactions, hydrogen bonding has the potential to assemble supramolecular architectures. In the title salt, 2-amino-4-methylpyridinium nitrate, (1), and in the analogous salt 2-amino-5-methylpyridinium nitrate, (2) (Yan et al., 2012), the endocyclic N atoms are protonated to form cations and the nitrates serve as the counter-anions. In the presence of anions, a hydrogen bond is strengthened by two to three times (40-190 kJ mol^-1) compared with a hydrogen bond involving uncharged molecular species (10-65 kJ mol^-1) (Lam & Mak, 2000).

Tautomerism plays an important role in biological systems and the different tautomers that can potentially exist in each DNA base may play a role in DNA mutation (Akai et al., 2005). Amino-imino tautomerism has been widely investigated by many researchers and it is firmly established that neutral 2-aminopyridines exist predominantly in the aminopyridine form and not in the pyridone imine form. Akai et al. (2005) found that amino-imino tautomerism of neutral 2-aminopyridines could be induced by photoexcitation. Protonation of 2-aminopyridines causes significant electron redistribution in the conjugate acid form, as determined by the relative contributions of the `amino' and `imino' resonance forms (Chapkanov, 2010), and they may even exist predominantly in the `imino' form (Spinner, 1962).

Protonation, hydrogen bonding and electron distribution will have a significant impact on the structures of (1) and (2) and their properties and bioactivities. In order to understand their structures and properties better, we report here the supramolecular architectures and the results of resonance studies of (1) and (2), which were the products obtained in the attempted preparation of 2-amino-4-methylpyridine and 2-amino-5-methylpyridine Schiff base complexes. The synthetic details for (1) are essentially identical to those we reported recently for (2) (Yan et al., 2012). Hydrogen-bonding geometries for (1) are given in Table 1. Comparisons of important structural parameters for 2-amino-4-methylpyridine with (1) and 2-amino-5-methylpyridine with (2) are made in Table 2.

The crystal structures of the free bases 2-amino-4-methylpyridine (Kvick & Noordik, 1977) and 2-amino-5-methylpyridine (Nahringbauer & Kvick, 1977) have already been reported, and their structures are similar. Two planar molecules form hydrogen-bonded dimers, in which the two molecules are related by a centre of inversion. For 2-amino-4-methylpyridine, the dimeric units are linked in a cyclic manner through intermolecular C-H interactions, packing perpendicular to the a axis to form layers. These layers are held together by van der Waals interactions between the methyl groups along the a axis. For 2-amino-5-methylpyridine, the dimers are packed in a herringbone fashion, with an angle of 52.5° between the planes of the two different sets of dimers.

The structure of (1) is composed of discrete 2-amino-4-methylpyridinium cations and nitrate anions; the asymmetric unit is shown in Fig. 1. Since no acid was added to the reaction, it is presumed that the hydroxy O atom of the Schiff base that was formed by the reaction of 2-amino-4-methylpyridine and 1,3-dihydroxyacetone coordinated with the metal ion and released H⁺, which then protonated the pyridine N atom of 2-amino-4-methylpyridine to form the pyridinium ion. Previously reported theoretical calculations by Nino & Munoz-Caro (2001) show that protonation of aminopyridine leads to a change of hybridization of the amine group from pyramidal to planar. They propose that this is due to an increase in conjugation between the amino group and the charged pyridinium cation, but this is not in accordance with the experimental result that the dihedral angle between the planes of the pyridine ring and the amine group in (1) is 24 (4)°. This may be because the theoretical calculations did not take into account the impact of the NO₃^- groups, which can change the direction of the hydrogen bonding.

Comparing (1) with neutral 2-amino-4-methylpyridine, the C1-N2 bond length has decreased by approximately 0.03 Å, indicating an increase in the bond order. In addition, the C2-C3 and C4-C5 bond lengths have decreased by approximately 0.02 and 0.04 Å, respectively, while the N1-C1, C1-C2, C3-C4 and C5-N1 bond lengths are not significantly different. Similarly, comparing (2) with neutral 2-amino-5-methylpyridine, the C1-N2 and C4-C5 bond lengths have decreased by approximately 0.04 and 0.02 Å respectively, while the N1-C1, C1-C2, C3-C4 and C5-N1 bond lengths are not significantly different, indicating the increase in the bond orders of C1-N2 and C4-C5. In theory, the C2-C3 bond length should decrease. However, as the difference of 0.011 (5) Å is less than 3, it is not statistically distinguishable from the neutral molecule due to the data quality. Apart from this bond length, other data is quite regular. These structural effects are consistent with a significant redistribution of electron density within the pyridinium cations of (1) and (2). In both (1) and (2), the C1-N2, C2-C3 and C4-C5 bond lengths reflect more of the double-bond character in resonance forms (II) and (IV) compared with the free-base forms in which the `amino' resonance forms contribute more. This indicates the contributions from resonance forms (I) and (II), and (III) and (IV), respectively (see Scheme).

In the crystal structure of (1), the 2-amino-4-methylpyridinium cations and nitrate anions are linked in a cyclic manner through N-HO hydrogen bonds with an R₄³(12) graph-set motif (Etter et al., 1990). These units are extended into a one-dimensional zigzag chain structure (Fig. 2) lying parallel to the a axis, through a cyclic R₂²(8) association involving amine N-HO and aromatic C-HO hydrogen bonds to nitrate O-atom acceptors, and a cyclic R₁²(4) association involving bifurcated pyridinium N-HO hydrogen bonds to two nitrate O-atom acceptors. As viewed along the b axis, these infinite one-dimensional chains are combined to generate an infinite two-dimensional layer (Fig. 3) via weak intermolecular C-HC interactions between discrete chains. Two adjacent two-dimensional layers, as viewed along the c axis, are connected through offset face-to-face - stacking interactions, forming a two-layer unit. This is consistent with the experimental observation that the compound crystallizes as plates.

The Mulliken charge distributions of (1) and (2) are shown in Table 3, and the Wiberg bond orders of (1) and (2) are shown in Table 4. According to Table 3, the net charges distributed on the three atoms of the amine groups are -0.012802 and 0.081896, respectively. Considering the relative electronegativity of N and C atoms, there should be more negative charge distributed on the amine groups. Thus, we can conclude that part of the positive charge has transferred from the newly added proton to the amine group, as shown in resonance forms (II) and (IV). Using this analysis method, we can also conclude that there is partial charge transfer to the pyridine rings. As shown in Table 4, the bond order of C1-N2 is greater than that of N1-C1 for both (1) and (2), indicating that the amine groups conjugate with the pyridine rings. In addition, the bond orders of C2-C3 and C4-C5 are greater than others. Thus, we can conclude that both (1) and (2) are represented essentially or largely by resonance form (II) for (1) and (IV) for (2) (see Scheme).

The comparison and assignment of the IR spectra of 2-amino-4-methylpyridine with (1) and 2-amino-5-methylpyridine with (2) are given in Tables 5 and 6, respectively. Since the IR bands of the two groups are very similar, we can take 2-amino-4-methylpyridine and (1) as a representative to discuss. The spectroscopic changes in (1) compared with 2-amino-4-methylpyridine point to a structural change that is far more drastic than the mere addition of a proton to give an ion represented mainly by resonance form (I), and they show more of the character of resonance form (II). The absorption maximum at 3432 cm^-1 in 2-amino-4-methylpyridine is assigned to an NH₂ antisymmetric stretch, _asNH₂. Two bands are found at 3303 and 3133 cm^-1, belonging to the Fermi doublet caused by a resonance between the symmetric NH₂ frequency (_sNH₂ of the dimeric 2-amino-4-methylpyridine) and its 2NH₂ overtone, due to N-HN hydrogen-bond formation with the participation of the NH₂ group (Arnaudov & Dinkov, 1998). In contrast, in (1), the band at 3398 cm^-1 is assigned to NH. A series of bands have emerged between 3000 and 2763 cm^-1, owing to Fermi resonance of combination bands due to the formation of a secondary amine salt. Within the range 1700-1300 cm^-1, the band of 2-amino-4-methylpyridine at 1647 cm^-1 can be assigned to an NH₂ scissor vibration NH₂ according to Spinner (1962). The bands at 1615, 1556, 1490 and 1448 cm^-1 are assigned to the pyridine-ring skeleton stretching bands, and Katritzky & Hands (1958) believe that it is the strong electron-donating ability of NH₂ that leads to the drastic shift of the band to 1615 cm^-1. The bands at 1466 and 1372 cm^-1 are assigned to the CH₃ bending vibrations _asCH₃ and _sCH₃. In contrast, for (1), the band at 1669 cm^-1 is assigned to an NH₂⁺ scissor vibration (Akai et al., 2005), NH₂⁺, with a higher frequency shift of 22 cm^-1 because of the partial charge transfer from N_pyH to NH₂. The band at 1627 cm^-1 is assigned to a C=N⁺ stretching vibration (Akai et al., 2005), C=N⁺, in resonance form (II), and the intense absorption maximum at 1384 cm^-1 is assigned to an NO₃^- antisymmetric stretching vibration, _asNO₃^-. The band at 1487 cm^-1 is assigned to _asCH₃, and the _sCH₃ band has been swamped by the _asNO₃^- vibration. It is obvious that the pyridine-ring skeleton stretching bands have disappeared, which is further evidence of the resonance form (II) character. As all these band assignments are also applicable to (2) and 2-amino-5-methylpyridine, this strengthens our conclusion above that these compounds are represented essentially or largely by resonance form (II) for (1) and resonance form (IV) for (2).

From the evidence presented above, we can conclude that the protonation of 2-amino-4-methylpyridine and 2-amino-5-methylpyridine and the introduction of NO₃^- leads to drastic changes in their crystal structures. Compound (1) can resonate between forms (I) and (II), and (2) between forms (III) and (IV), with partial charge redistribution and aromatic character distortion. They are represented essentially or largely by resonance form (II) for (1) and resonance form (IV) for (2). Zeng & Ren (2007) studied the tautomerism of 2-aminothiazole in solution, noting that solvation will play an important role in affecting the tautomeric equilibrium and that increasing the polarity of the medium causes a shift in the tautomeric equilibrium toward the imino form. The addition of variable amounts of a salt to the solutions has been shown to increase the population of imino species (Annese et al., 1994). Likewise, we can presume that the negative charge of NO₃^- groups adjacent to the NH₂ groups in (1) and (2) can impose an inductive effect, promoting the partial transfer of the positive charge from N_pyH to NH₂. This might be the reason why (1) and (2) exist predominately in resonance form (II) for (1) and resonance form (IV) for (2) compared with 2-amino-4-methylpyridine and 2-amino-5-methylpyridine.

Experimental

2-Amino-4-methylpyridine (0.216 g, 2.0 mmol) and the 1,3-dihydroxyacetone dimer (0.180 g, 1.0 mmol) were dissolved in methanol (20 ml) and the solution was stirred for 6 h at 333 K. Sm(NO₃)₃·6H₂O (0.444 g, 1.0 mmol) was then added and the solution was stirred for a further 4 h. The resulting solution was filtered and the filtrate was left for slow evaporation at room temperature in air. Colourless plate-shaped crystals of (1) formed when the solvent had almost completely evaporated. The crystals were then preserved in toluene for further analysis. For the synthesis, structure determination and crystal structure of (2), see Yan et al. (2012). The natural bond orbital (NBO) analyses of (1) and (2) were carried out by the density functional theory (DFT) method at the B3LYP/6-31 level. The atomic coordinates used in the calculations are from crystallographic data. All calculations were conducted on a Pentium IV computer using the GAUSSIAN03 program (Frisch et al., 2003). The four compounds, 2-amino-4-methylpyridine, (1), 2-amino-5-methylpyridine and (2), were characterized by IR spectra recorded in KBr pellets using a Nicolet 170SX spectrophotometer in the 4000-400 cm^-1 region.

All H atoms except for those of the methyl group were found from difference Fourier maps and refined without constraints. The methyl H atoms were positioned geometrically and refined using a riding model, with C-H = 0.96 Å and U_iso(H) = 1.5U_eq(C).

Data collection: SMART (Bruker, 2000); cell refinement: SMART; data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: WQ3020 ). Services for accessing these data are described at the back of the journal.

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