organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

Journal logoSTRUCTURAL
CHEMISTRY
ISSN: 2053-2296

An amino–imino resonance study of 2-amino-4-methyl­pyridinium nitrate and 2-amino-5-methyl­pyridinium nitrate

aKey Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, Shandong 266100, People's Republic of China
*Correspondence e-mail: fanyuhua301@163.com

(Received 1 September 2012; accepted 21 November 2012; online 13 December 2012)

The contributions of the amino and imino resonance forms to the ground-state structures of 2-amino-4-methyl­pyridinium nitrate, C6H9N2+·NO3, and the previously reported 2-amino-5-methyl­pyridinium nitrate [Yan, Fan, Bi, Zuo & Zhang (2012[Yan, X., Fan, Y., Bi, C., Zuo, J. & Zhang, Z. (2012). Acta Cryst. E68, o2084.]). Acta Cryst. E68, o2084], were studied using a combination of IR spectroscopy, X-ray crystallography and density functional theory (DFT). The results show that the structures of 2-amino-4-methyl­pyridine and 2-amino-5-methyl­pyridine obtained upon protonation are best described as existing largely in the imino resonance forms.

Comment

Nitro­gen-containing heterocyclic compounds are used extensively as structural components of pharmaceuticals and agrochemicals (Ahangar et al., 2011[Ahangar, N., Ayati, A., Alipour, E., Pashapour, A., Foroumadi, A. & Emami, S. (2011). Chem. Biol. Drug Des. 78, 844-852.]; Gobis et al., 2012[Gobis, K., Foks, H., Bojanowski, K., Augustynowicz-Kopec, E. & Napiorkowska, A. (2012). Bioorg. Med. Chem. 20, 137-144.]), due to their high biological activity and generally low toxicity. They also play a vital role in organometallic catalysts (Bianchini et al., 2003[Bianchini, C., Mantovani, G., Meli, A., Migliacci, F., Zanobini, F., Laschi, F. & Sommazzi, A. (2003). Eur. J. Inorg. Chem. pp. 1620-1631.]; Li & Hor, 2008[Li, F. & Hor, T. S. (2008). Adv. Synth. Catal. 350, 2391-2400.]) and dye-sensitized solar cells (Moorcraft et al., 2008[Moorcraft, L. P., Morandeira, A., Durrant, J. R., Jennings, J. R., Peter, L. M., Parsons, S., Turner, A., Yellowlees, L. J. & Robertson, N. (2008). Dalton Trans. pp. 6940-6947.]; Wu et al., 2010[Wu, K. L., Hsu, H. C., Chen, K., Chi, Y., Chung, M. W., Liu, W. H. & Chou, P. T. (2010). J. Chem. Soc. Chem. Commun. 46, 5124-5126.]). Within this class, compounds containing pyridine rings have attracted particular attention. 2-Amino­pyridine and its derivatives are used as dyes (Patel & Patel, 2009[Patel, N. B. & Patel, A. L. (2009). Indian J. Chem. Sect. B, 48, 705-711.]) and pyridinium cation derivatives often possess anti­bacterial and anti­fungal activities (Sepcic, 2000[Sepcic, K. (2000). J. Toxicol. Toxin Rev. 19, 139-160.]; Sliwa, 1989[Sliwa, W. (1989). Heterocycles, 29, 557-595.]). Some amino­pyridines are found to demonstrate pharmacological activity as K+-channel inhibitors. By investigating three-dimensional iso-Laplacian diagrams, Nino & Munoz-Caro (2001[Nino, A. & Munoz-Caro, C. (2001). Biophys. Chem. 91, 49-60.]) 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[Lam, C. K. & Mak, T. C. W. (2000). Tetrahedron, 56, 6657-6665.]). 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[Aakeroy, C. B. & Seddon, K. R. (1993). Chem. Soc. Rev. 22, 397-407.]). Along with other delicate noncovalent inter­actions like ππ stacking and electrostatic inter­actions, hydrogen bonding has the potential to assemble supra­molecular architectures. In the title salt, 2-amino-4-methyl­pyridinium nitrate, (1)[link], and in the analogous salt 2-amino-5-methyl­pyridinium nitrate, (2)[link] (Yan et al., 2012[Yan, X., Fan, Y., Bi, C., Zuo, J. & Zhang, Z. (2012). Acta Cryst. E68, o2084.]), 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[Lam, C. K. & Mak, T. C. W. (2000). Tetrahedron, 56, 6657-6665.]).

[Scheme 1]

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[Akai, N., Ohno, K. & Aida, M. (2005). Chem. Phys. Lett. 413, 306-310.]). 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[Akai, N., Ohno, K. & Aida, M. (2005). Chem. Phys. Lett. 413, 306-310.]) 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[Chapkanov, A. G. (2010). Struct. Chem. 21, 29-35.]), and they may even exist predominantly in the `imino' form (Spinner, 1962[Spinner, E. (1962). J. Chem. Soc. pp. 3119-3126.]).

Protonation, hydrogen bonding and electron distribution will have a significant impact on the structures of (1)[link] and (2)[link] 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)[link] and (2)[link], 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)[link] are essentially identical to those we reported recently for (2)[link] (Yan et al., 2012[Yan, X., Fan, Y., Bi, C., Zuo, J. & Zhang, Z. (2012). Acta Cryst. E68, o2084.]). Hydrogen-bonding geometries for (1) are given in Table 1[link]. Comparisons of important structural parameters for 2-amino-4-methyl­pyridine with (1)[link] and 2-amino-5-methyl­pyridine with (2)[link] are made in Table 2[link].

The crystal structures of the free bases 2-amino-4-methyl­pyridine (Kvick & Noordik, 1977[Kvick, Å. & Noordik, J. (1977). Acta Cryst. B33, 2862-2866.]) and 2-amino-5-methyl­pyridine (Nahringbauer & Kvick, 1977[Nahringbauer, I. & Kvick, Å. (1977). Acta Cryst. B33, 2902-2905.]) 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⋯π 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)[link] is composed of discrete 2-amino-4-methyl­pyridinium cations and nitrate anions; the asymmetric unit is shown in Fig. 1[link]. 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[Nino, A. & Munoz-Caro, C. (2001). Biophys. Chem. 91, 49-60.]) 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)[link] 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)[link] 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)[link] 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 3σ, 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)[link] and (2)[link]. In both (1)[link] and (2)[link], the C1—N2, C2—C3 and C4—C5 bond lengths reflect more of the double-bond character in resonance forms (II)[link] and (IV)[link] compared with the free-base forms in which the `amino' resonance forms contribute more. This indicates the contributions from resonance forms (I)[link] and (II), and (III) and (IV), respectively (see Scheme[link]).

In the crystal structure of (1)[link], the 2-amino-4-methyl­pyridinium cations and nitrate anions are linked in a cyclic manner through N—H⋯O hydrogen bonds with an R43(12) graph-set motif (Etter et al., 1990[Etter, M. C., MacDonald, J. C. & Bernstein, J. (1990). Acta Cryst. B46, 256-262.]). These units are extended into a one-dimensional zigzag chain structure (Fig. 2[link]) lying parallel to the a axis, through a cyclic R22(8) association involving amine N—H⋯O and aromatic C—H⋯O hydrogen bonds to nitrate O-atom acceptors, and a cyclic R12(4) association involving bifurcated pyridinium N—H⋯O 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[link]) via weak inter­molecular C—H⋯C inter­actions between discrete chains. Two adjacent two-dimensional layers, as viewed along the c axis, are connected through offset face-to-face ππ 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)[link] and (2)[link] are shown in Table 3[link], and the Wiberg bond orders of (1)[link] and (2)[link] are shown in Table 4[link]. According to Table 3[link], 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)[link] and (IV)[link]. Using this analysis method, we can also conclude that there is partial charge transfer to the pyridine rings. As shown in Table 4[link], the bond order of C1—N2 is greater than that of N1—C1 for both (1)[link] and (2)[link], 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)[link] and (2)[link] are represented essentially or largely by resonance form (II)[link] for (1)[link] and (IV)[link] for (2)[link] (see Scheme[link]).

The comparison and assignment of the IR spectra of 2-amino-4-methyl­pyridine with (1)[link] and 2-amino-5-methyl­pyridine with (2)[link] are given in Tables 5[link] and 6[link], respectively. Since the IR bands of the two groups are very similar, we can take 2-amino-4-methyl­pyridine and (1)[link] as a representative to discuss. The spectroscopic changes in (1)[link] 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, νasNH2. Two bands are found at 3303 and 3133 cm−1, belonging to the Fermi doublet caused by a resonance between the symmetric NH2 frequency (νsNH2 of the dimeric 2-amino-4-methyl­pyridine) and its 2δNH2 overtone, due to N—H⋯N hydrogen-bond formation with the participation of the NH2 group (Arnaudov & Dinkov, 1998[Arnaudov, M. & Dinkov, Sh. (1998). Spectrosc. Lett. 31, 1687-1703.]). In contrast, in (1)[link], the band at 3398 cm−1 is assigned to νNH. 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 δNH2 according to Spinner (1962[Spinner, E. (1962). J. Chem. Soc. pp. 3119-3126.]). The bands at 1615, 1556, 1490 and 1448 cm−1 are assigned to the pyridine-ring skeleton stretching bands, and Katritzky & Hands (1958[Katritzky, A. R. & Hands, A. R. (1958). J. Chem. Soc. pp. 2202-2204.]) 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 δasCH3 and δsCH3. In contrast, for (1)[link], the band at 1669 cm−1 is assigned to an NH2δ+ scissor vibration (Akai et al., 2005[Akai, N., Ohno, K. & Aida, M. (2005). Chem. Phys. Lett. 413, 306-310.]), δNH2δ+, with a higher frequency shift of 22 cm−1 because of the partial charge transfer from NpyH to NH2. The band at 1627 cm−1 is assigned to a C=Nδ+ stretching vibration (Akai et al., 2005[Akai, N., Ohno, K. & Aida, M. (2005). Chem. Phys. Lett. 413, 306-310.]), νC=Nδ+, in resonance form (II), and the intense absorption maximum at 1384 cm−1 is assigned to an NO3 anti­symmetric stretching vibration, νasNO3. The band at 1487 cm−1 is assigned to δasCH3, and the δsCH3 band has been swamped by the νasNO3 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)[link] and 2-amino-5-methyl­pyridine, this strengthens our conclusion above that these compounds are represented essentially or largely by resonance form (II) for (1)[link] and resonance form (IV) for (2)[link].

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)[link] can resonate between forms (I) and (II), and (2)[link] 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)[link] and resonance form (IV) for (2)[link]. Zeng & Ren (2007[Zeng, Y. & Ren, Y. (2007). Int. J. Quantum Chem. 107, 247-258.]) 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[Annese, M., Corradi, A. B., Forlani, L., Rizzoli, C. & Sgarabotto, P. (1994). J. Chem. Soc. Perkin Trans. 2, pp. 615-621.]). Likewise, we can presume that the negative charge of NO3 groups adjacent to the NH2 groups in (1)[link] and (2)[link] can impose an inductive effect, promoting the partial transfer of the positive charge from NpyH to NH2. This might be the reason why (1)[link] and (2)[link] exist predominately in resonance form (II) for (1)[link] and resonance form (IV) for (2)[link] compared with 2-amino-4-methyl­pyridine and 2-amino-5-methyl­pyridine.

[Figure 1]
Figure 1
The structure of (1)[link], showing 30% probability displacement ellipsoids and the atom-numbering scheme.
[Figure 2]
Figure 2
A view of the packing of (1)[link], showing the zigzag chain parallel to the a axis. Hydrogen bonds are shown as dashed lines.
[Figure 3]
Figure 3
A view of the packing of (1)[link], showing the two-dimensional network perpendicular to the b axis. Hydrogen bonds are shown as dashed lines.

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)[link] 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)[link], see Yan et al. (2012[Yan, X., Fan, Y., Bi, C., Zuo, J. & Zhang, Z. (2012). Acta Cryst. E68, o2084.]). The natural bond orbital (NBO) analyses of (1)[link] and (2)[link] 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[Frisch, M. J., et al. (2003). GAUSSIAN03. Gaussian Inc., Pittsburgh, Pennsylvania, USA.]). The four compounds, 2-amino-4-methyl­pyridine, (1)[link], 2-amino-5-methyl­pyridine and (2)[link], were characterized by IR spectra recorded in KBr pellets using a Nicolet 170SX spectrophotometer in the 4000–400 cm−1 region.

Crystal data
  • C6H9N2+·NO3

  • Mr = 171.16

  • Orthorhombic, P b c a

  • a = 8.4150 (7) Å

  • b = 12.8669 (11) Å

  • c = 15.2441 (14) Å

  • V = 1650.6 (2) Å3

  • Z = 8

  • Mo Kα radiation

  • μ = 0.11 mm−1

  • T = 298 K

  • 0.45 × 0.43 × 0.22 mm

Data collection
  • Bruker SMART CCD area-detector diffractometer

  • Absorption correction: multi-scan (SADABS; Sheldrick, 1996[Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.]) Tmin = 0.951, Tmax = 0.976

  • 7806 measured reflections

  • 1453 independent reflections

  • 865 reflections with I > 2σ(I)

  • Rint = 0.067

Refinement
  • R[F2 > 2σ(F2)] = 0.049

  • wR(F2) = 0.152

  • S = 1.11

  • 1453 reflections

  • 129 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • Δρmax = 0.20 e Å−3

  • Δρmin = −0.14 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯AD—HH⋯ADAD—H⋯A
N1—H1⋯O1i0.90 (3)1.92 (3)2.819 (3)171 (3)
N1—H1⋯O3i0.90 (3)2.45 (3)3.120 (3)131 (2)
N2—H2A⋯O2ii0.90 (4)2.16 (4)2.986 (4)153 (3)
N2—H2B⋯O2iii0.90 (4)2.11 (4)2.998 (4)170 (3)
C2—H2C⋯O3iii0.93 (3)2.36 (3)3.283 (4)168 (2)
C4—H4⋯C6iv0.91 (3)2.92 (4)3.760 (5)156 (3)
Symmetry codes: (i) [x-{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+1]; (ii) [x, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]; (iii) [x+{\script{1\over 2}}, -y+{\script{1\over 2}}], -z+1; (iv) [x-{\script{1\over 2}}, y, -z+{\script{3\over 2}}].

Table 2
Comparison of important bond lengths (Å) for 2-amino-4-methyl­pyridine with (1)[link], and 2-amino-5-methyl­pyridine with (2)[link]

Bond2-Amino-4-methyl­pyridine(1)[link]2-Amino-5-methyl­pyridine(2)[link]
C1—N21.363 (2)1.331 (4)1.364 (2)1.329 (4)
N1—C11.347 (2)1.338 (4)1.338 (2)1.344 (4)
C2—C31.383 (2)1.359 (4)1.365 (2)1.354 (4)
C4—C51.383 (2)1.342 (5)1.375 (2)1.348 (4)
C1—C21.410 (2)1.395 (4)1.400 (2)1.410 (4)
C3—C41.397 (2)1.391 (5)1.393 (2)1.399 (5)
C5—N11.348 (2)1.347 (4)1.347 (2)1.363 (4)

Table 3
Mulliken charge distribution of (1)[link] and (2)[link]

For the method of Mulliken charge calculation, see: Guerra et al. (2003[Guerra, C. F., Handgraaf, J. W., Baerends, E. J. & Bickelhaupt, F. M. (2003). J. Comput. Chem. 2, 189-210.]).

Atom(1)[link](2)[link]Atom(1)[link](2)[link]
N1−0.537839−0.595197H3 0.163031
H10.2595240.258697C4−0.0409940.028576
N2−0.554437−0.517579H40.077307 
H2A0.2629660.379254C50.1775130.146575
H2B0.2786690.220221H50.1552190.188731
C10.5697720.506648C6−0.333533−0.414497
C2−0.0471070.010988H6A0.1203890.156658
H2C0.0757230.070531H6B0.1226090.175007
C30.137316−0.079661H6C0.2051590.179073

Table 4
Wiberg bond orders of (1)[link] and (2)[link]

For the method of Wiberg bond-order calculation, see: Mayer (1985[Mayer, I. (1985). Theor. Chim. Acta, 4, 315-322.]).

Bond(1)[link](2)[link]
C1—N21.22851.3490
N1—C11.20621.1956
C2—C31.47091.5728
C4—C51.57921.5677
C1—C21.31221.2291
C3—C41.27381.2589
C5—N11.17481.1696

Table 5
Comparison of IR spectra of 2-amino-4-methyl­pyridine and (1)[link] in the range 3500–1300 cm−1

2-Amino-4-methyl­pyridineBand(1)[link]Band
νasNH23432Fermi resonance3000–2763
νsNH23303νNH3398
νsNH23133  
δNH2 scissor1647δNH2 scissor1669
Skeleton vibration1615νC=Nδ+1627
Skeleton vibration1556  
Skeleton vibration1490  
δasCH31466δasCH31487
Skeleton vibration1446νasNO31384
δsCH31372  

Table 6
Comparison of IR spectra of 2-amino-5-methyl­pyridine and (2)[link] in the range 3500–1300 cm−1

2-Amino-5-methyl­pyridineBand(2)[link]Band
νasNH23454Fermi resonance3000–2765
νsNH23306  
νsNH23168  
δNH2 scissor1635δNH2 scissor1669
Skeleton vibration1608νC=Nδ+1627
Skeleton vibration1564  
Skeleton vibration1502  
δasCH31457δasCH31473
Skeleton vibration1393νasNO31384
δsCH31377  

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 Uiso(H) = 1.5Ueq(C).

Data collection: SMART (Bruker, 2000[Bruker (2000). SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SMART; data reduction: SAINT (Bruker, 2000[Bruker (2000). SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); software used to prepare material for publication: SHELXTL.

Supporting information


Comment top

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 answer 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 non-covalent interactions like ππ stacking and electrostatic interactions, hydrogen bonding has the potential to assemble supramolecular architectures. In the title compound, 2-amino-4-methylpyridinium nitrate, (1), and in the analogous compound 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 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 are essentially identical to those we reported recently with the crystal structure of (2) (Yan et al., 2012). Selected bond lengths and angles for (1) are given in Table 1, and hydrogen-bonding geometries are given in Table 2. Comparisons of important structural parameters for 2-amino-4-methylpyridine with (1), and 2-amino-5-methylpyridine with (2), are made in Table 3.

The crystal structures of the free bases 2-amino-4-methylpyridine (Kvick & Noordik, 1977) and 2-amino-5-methylpyridine (Nahringbauer & Kvick, 1977) have been already reported, and their structures are similar. Two planar molecules form hydrogen-bonded dimers, in which the two molecules are related by a centre of symmetry. For 2-amino-4-methylpyridine, the dimer units are linked cyclically through intermolecular C—H···π 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 herring-bone 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 H+ 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 hybridisation 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 pyridine ring and the amine group in (1) is 23.90°. 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) is less than 3σ, 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 cyclically through N—H···O hydrogen bonds of R43(12) graph set (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—H···O and aromatic C—H···O hydrogen bonds to nitrate O-atom acceptors, and a cyclic R12(4) association involving bifurcated pyridinium N—H···O 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—H···C interactions between discrete chains. Two adjacent two-dimensional layers, as viewed along the c axis, are connected through offset face-to-face ππ stacking interactions, forming a two-layer unit. This is consistent with the experimental result 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 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 amine groups. Thus, we can conclude that part of the positive charges have transferred from the newly added protons to the amine groups, 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, (1), 2-amino-5-methylpyridine and (2) are given in Tables 6 and 7, 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, νasNH2. Two bands are found at 3303 and 3133 cm-1, belonging to the Fermi doublet caused by a resonance between the symmetric NH2 frequency (νsNH2 of the dimeric 2-amino-4-methylpyridine) and its 2δNH2 overtone, respectively, due to N—H···N 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 νNH. 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 δNH2 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 δasCH3 and δsCH3. In contrast, for (1), the band at 1669 cm-1 is assigned to an NH2δ+ scissor vibration (Akai et al., 2005), δNH2δ+, with a higher frequency shift of 22 cm-1 because of the partial charge transfer from NpyH to NH2. The band at 1627 cm-1 is assigned to a CNδ+ stretching vibration (Akai et al., 2005), νCNδ+, in resonance form (II) and the intense absorption maximum at 1384 cm-1 is assigned to an NO3- antisymmetric stretching vibration, νasNO3-. The band at 1487 cm-1 is assigned to δasCH3, and the δsCH3 band has been swamped by the νasNO3- 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, we can conclude 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 NpyH 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.

Related literature top

For related literature, see: Aakeroy & Seddon (1993); Ahangar et al. (2011); Akai et al. (2005); Annese et al. (1994); Arnaudov & Dinkov (1998); Bianchini et al. (2003); Chapkanov (2010); Etter et al. (1990); Frisch (2003); Gobis et al. (2012); Katritzky & Hands (1958); Kvick & Noordik (1977); Lam & Mak (2000); Li & Hor (2008); Moorcraft et al. (2008); Nahringbauer & Kvick (1977); Nino & Munoz-Caro (2001); Patel & Patel (2009); Sepcic (2000); Sliwa (1989); Spinner (1962); Wu et al. (2010); Yan et al. (2012); Zeng & Ren (2007).

Experimental top

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 NBO analyses of (1) and (2) were carried out by the density functional theory (DFT) method at the B3LYP/6-31 level. 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.

Refinement top

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 Uiso(H) = 1.5Ueq(C).

Computing details top

Data collection: SMART (Bruker, 2000); cell refinement: SMART (Bruker, 2000); 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 (Sheldrick, 2008).

Figures top
Fig. 1. The structure of (1), showing 30% probability displacement ellipsoids and the atom-numbering scheme.

Fig. 2. A view of the packing of (1), showing the zigzag chain parallel to the a axis. Hydrogen bonds are shown as dashed lines.

Fig. 3. A view of the packing of (1), showing the two-dimensional network perpendicular to the b axis. Hydrogen bonds are shown as dashed lines.
2-amino-4-methylpyridinium nitrate top
Crystal data top
C6H9N2+·NO3F(000) = 720
Mr = 171.16Dx = 1.378 Mg m3
Orthorhombic, PbcaMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2abCell parameters from 1536 reflections
a = 8.4150 (7) Åθ = 2.7–21.8°
b = 12.8669 (11) ŵ = 0.11 mm1
c = 15.2441 (14) ÅT = 298 K
V = 1650.6 (2) Å3Plate, colourless
Z = 80.45 × 0.43 × 0.22 mm
Data collection top
Bruker SMART CCD area-detector
diffractometer
1453 independent reflections
Radiation source: fine-focus sealed tube865 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.067
ϕ and ω scansθmax = 25.0°, θmin = 2.7°
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
h = 109
Tmin = 0.951, Tmax = 0.976k = 1513
7806 measured reflectionsl = 1418
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.049H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.152 w = 1/[σ2(Fo2) + (0.0379P)2 + 0.9001P]
where P = (Fo2 + 2Fc2)/3
S = 1.11(Δ/σ)max < 0.001
1453 reflectionsΔρmax = 0.20 e Å3
129 parametersΔρmin = 0.14 e Å3
0 restraintsExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.0052 (17)
Crystal data top
C6H9N2+·NO3V = 1650.6 (2) Å3
Mr = 171.16Z = 8
Orthorhombic, PbcaMo Kα radiation
a = 8.4150 (7) ŵ = 0.11 mm1
b = 12.8669 (11) ÅT = 298 K
c = 15.2441 (14) Å0.45 × 0.43 × 0.22 mm
Data collection top
Bruker SMART CCD area-detector
diffractometer
1453 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
865 reflections with I > 2σ(I)
Tmin = 0.951, Tmax = 0.976Rint = 0.067
7806 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0490 restraints
wR(F2) = 0.152H atoms treated by a mixture of independent and constrained refinement
S = 1.11Δρmax = 0.20 e Å3
1453 reflectionsΔρmin = 0.14 e Å3
129 parameters
Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2sigma(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
N10.3102 (3)0.12928 (18)0.46825 (16)0.0534 (7)
H10.233 (4)0.132 (2)0.427 (2)0.064*
N20.5005 (4)0.1613 (2)0.36306 (18)0.0682 (8)
H2A0.425 (4)0.157 (3)0.322 (2)0.082*
H2B0.599 (5)0.149 (3)0.343 (2)0.082*
N30.4582 (3)0.3718 (2)0.64011 (17)0.0578 (7)
O10.5940 (3)0.3700 (2)0.67303 (14)0.0837 (9)
O20.3422 (2)0.3620 (2)0.68888 (14)0.0809 (8)
O30.4434 (3)0.3804 (2)0.56049 (15)0.0829 (8)
C10.4630 (3)0.1419 (2)0.44629 (18)0.0471 (7)
C20.5770 (3)0.1344 (2)0.51237 (19)0.0523 (8)
H2C0.682 (4)0.1403 (19)0.4941 (18)0.063*
C30.5334 (4)0.1157 (2)0.59676 (19)0.0591 (8)
C40.3729 (5)0.1030 (3)0.6157 (2)0.0719 (10)
H40.345 (4)0.089 (3)0.672 (2)0.086*
C50.2646 (4)0.1098 (3)0.5513 (2)0.0675 (9)
H50.148 (5)0.104 (2)0.562 (2)0.081*
C60.6562 (5)0.1077 (3)0.6686 (2)0.0929 (13)
H6A0.76050.11320.64360.139*
H6B0.64560.04200.69780.139*
H6C0.64040.16290.71010.139*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
N10.0386 (13)0.0688 (16)0.0529 (15)0.0035 (12)0.0031 (11)0.0000 (12)
N20.0655 (19)0.095 (2)0.0442 (16)0.0039 (17)0.0015 (13)0.0054 (15)
N30.0416 (14)0.0797 (19)0.0522 (16)0.0026 (12)0.0002 (12)0.0010 (13)
O10.0417 (13)0.154 (3)0.0554 (14)0.0082 (13)0.0054 (11)0.0007 (14)
O20.0403 (12)0.144 (2)0.0589 (14)0.0010 (13)0.0076 (11)0.0066 (14)
O30.0608 (15)0.135 (2)0.0527 (14)0.0099 (13)0.0041 (11)0.0174 (14)
C10.0435 (16)0.0543 (17)0.0435 (15)0.0017 (13)0.0014 (12)0.0011 (13)
C20.0387 (16)0.0600 (19)0.0582 (18)0.0001 (13)0.0058 (14)0.0011 (15)
C30.071 (2)0.060 (2)0.0462 (17)0.0051 (16)0.0099 (16)0.0053 (14)
C40.088 (3)0.083 (2)0.0444 (18)0.010 (2)0.0144 (18)0.0014 (17)
C50.0501 (19)0.085 (2)0.067 (2)0.0061 (17)0.0169 (17)0.0022 (18)
C60.116 (3)0.089 (3)0.073 (3)0.007 (2)0.047 (2)0.0022 (19)
Geometric parameters (Å, º) top
N1—C11.338 (4)C2—C31.359 (4)
N1—C51.347 (4)C2—H2C0.93 (3)
N1—H10.90 (3)C3—C41.391 (5)
N2—C11.331 (4)C3—C61.509 (4)
N2—H2A0.90 (4)C4—C51.342 (5)
N2—H2B0.90 (4)C4—H40.91 (3)
N3—O31.225 (3)C5—H51.00 (4)
N3—O21.233 (3)C6—H6A0.9600
N3—O11.248 (3)C6—H6B0.9600
C1—C21.395 (4)C6—H6C0.9600
C1—N1—C5122.1 (3)C2—C3—C4118.7 (3)
C1—N1—H1121 (2)C2—C3—C6120.9 (3)
C5—N1—H1117 (2)C4—C3—C6120.4 (3)
C1—N2—H2A119 (2)C5—C4—C3120.0 (3)
C1—N2—H2B121 (2)C5—C4—H4122 (2)
H2A—N2—H2B114 (3)C3—C4—H4118 (2)
O3—N3—O2121.8 (3)C4—C5—N1120.4 (3)
O3—N3—O1119.5 (2)C4—C5—H5123 (2)
O2—N3—O1118.7 (3)N1—C5—H5117 (2)
N2—C1—N1119.3 (3)C3—C6—H6A109.5
N2—C1—C2122.6 (3)C3—C6—H6B109.5
N1—C1—C2118.2 (3)H6A—C6—H6B109.5
C3—C2—C1120.7 (3)C3—C6—H6C109.5
C3—C2—H2C123.7 (18)H6A—C6—H6C109.5
C1—C2—H2C115.6 (18)H6B—C6—H6C109.5
C5—N1—C1—N2179.8 (3)C1—C2—C3—C6179.9 (3)
C5—N1—C1—C20.1 (4)C2—C3—C4—C50.2 (5)
N2—C1—C2—C3179.2 (3)C6—C3—C4—C5179.7 (3)
N1—C1—C2—C30.5 (4)C3—C4—C5—N10.3 (5)
C1—C2—C3—C40.6 (5)C1—N1—C5—C40.5 (5)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O1i0.90 (3)1.92 (3)2.819 (3)171 (3)
N1—H1···O3i0.90 (3)2.45 (3)3.120 (3)131 (2)
N2—H2A···O2ii0.90 (4)2.16 (4)2.986 (4)153 (3)
N2—H2B···O2iii0.90 (4)2.11 (4)2.998 (4)170 (3)
C2—H2C···O3iii0.93 (3)2.36 (3)3.283 (4)168 (2)
C4—H4···C6iv0.91 (3)2.92 (4)3.760 (5)156 (3)
Symmetry codes: (i) x1/2, y+1/2, z+1; (ii) x, y+1/2, z1/2; (iii) x+1/2, y+1/2, z+1; (iv) x1/2, y, z+3/2.

Experimental details

Crystal data
Chemical formulaC6H9N2+·NO3
Mr171.16
Crystal system, space groupOrthorhombic, Pbca
Temperature (K)298
a, b, c (Å)8.4150 (7), 12.8669 (11), 15.2441 (14)
V3)1650.6 (2)
Z8
Radiation typeMo Kα
µ (mm1)0.11
Crystal size (mm)0.45 × 0.43 × 0.22
Data collection
DiffractometerBruker SMART CCD area-detector
diffractometer
Absorption correctionMulti-scan
(SADABS; Sheldrick, 1996)
Tmin, Tmax0.951, 0.976
No. of measured, independent and
observed [I > 2σ(I)] reflections
7806, 1453, 865
Rint0.067
(sin θ/λ)max1)0.595
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.049, 0.152, 1.11
No. of reflections1453
No. of parameters129
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.20, 0.14

Computer programs: SMART (Bruker, 2000), SAINT (Bruker, 2000), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Selected geometric parameters (Å, º) top
N1—C11.338 (4)C1—C21.395 (4)
N1—C51.347 (4)C2—C31.359 (4)
N1—H10.90 (3)C3—C41.391 (5)
N2—C11.331 (4)C3—C61.509 (4)
N2—H2A0.90 (4)C4—C51.342 (5)
N2—H2B0.90 (4)
C1—N1—C5122.1 (3)N1—C1—C2118.2 (3)
C1—N1—H1121 (2)C3—C2—C1120.7 (3)
C5—N1—H1117 (2)C2—C3—C4118.7 (3)
C1—N2—H2A119 (2)C2—C3—C6120.9 (3)
C1—N2—H2B121 (2)C4—C3—C6120.4 (3)
H2A—N2—H2B114 (3)C5—C4—C3120.0 (3)
N2—C1—N1119.3 (3)C4—C5—N1120.4 (3)
N2—C1—C2122.6 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O1i0.90 (3)1.92 (3)2.819 (3)171 (3)
N1—H1···O3i0.90 (3)2.45 (3)3.120 (3)131 (2)
N2—H2A···O2ii0.90 (4)2.16 (4)2.986 (4)153 (3)
N2—H2B···O2iii0.90 (4)2.11 (4)2.998 (4)170 (3)
C2—H2C···O3iii0.93 (3)2.36 (3)3.283 (4)168 (2)
C4—H4···C6iv0.91 (3)2.92 (4)3.760 (5)156 (3)
Symmetry codes: (i) x1/2, y+1/2, z+1; (ii) x, y+1/2, z1/2; (iii) x+1/2, y+1/2, z+1; (iv) x1/2, y, z+3/2.
Comparison of important bond lengths (Å) for 2-amino-4-methylpyridine with (1), and 2-amino-5-methylpyridine with (2) top
Bond2-Amino-4-methylpyridine(1)2-Amino-5-methylpyridine(2)
C1—N21.363 (2)1.331 (4)1.364 (2)1.329 (4)
N1—C11.347 (2)1.338 (4)1.338 (2)1.344 (4)
C2—C31.383 (2)1.359 (4)1.365 (2)1.354 (4)
C4—C51.383 (2)1.342 (5)1.375 (2)1.348 (4)
C1—C21.410 (2)1.395 (4)1.400 (2)1.410 (4)
C3—C41.397 (2)1.391 (5)1.393 (2)1.399 (5)
C5—N11.348 (2)1.347 (4)1.347 (2)1.363 (4)
Mulliken charge distribution of (1) and (2) top
For the method of Mulliken charge calculation, see: Guerra et al. (2003).
Atom(1)(2)Atom(1)(2)
N1-0.537839-0.595197H30.163031
H10.2595240.258697C4-0.0409940.028576
N2-0.554437-0.517579H40.077307
H2A0.2629660.379254C50.1775130.146575
H2B0.2786690.220221H50.1552190.188731
C10.5697720.506648C6-0.333533-0.414497
C2-0.0471070.010988H6A0.1203890.156658
H2C0.0757230.070531H6B0.1226090.175007
C30.137316-0.079661H6C0.2051590.179073
Wiberg bond orders of (1) and (2) top
For the method of Wiberg bond-order calculation, see: Mayer (1985).
Bond(1)(2)
C1—N21.22851.3490
N1—C11.20621.1956
C2—C31.47091.5728
C4—C51.57921.5677
C1—C21.31221.2291
C3—C41.27381.2589
C5—N11.17481.1696
Comparison of IR spectra of 2-amino-4-methylpyridine and (1) in the range 3500–1300 cm-1 top
2-Amino-4-methylpyridineBand(1)Band
νasNH23432Fermi resonance3000–2763
νsNH23303νNH3398
νsNH23133
δNH2 scissor1647δNH2 scissor1669
Skeletal vibration1615νCNδ+1627
Skeletal vibration1556
Skeletal vibration1490
δasCH31466δasCH31487
Skeletal vibration1446νasNO3-1384
δsCH31372
Comparison of IR spectra of 2-amino-5-methylpyridine and (2) in the range 3500–1300 cm-1 top
2-Amino-5-methylpyridineBand(2)Band
νasNH23454Fermi resonance3000-2765
νsNH23306
νsNH23168
δNH2 scissor1635δNH2 scissor1669
Skeletal vibration1608νCNδ+1627
Skeletal vibration1564
Skeletal vibration1502
δasCH31457δasCH31473
Skeletal vibration1393νasNO3-1384
δsCH31377
 

Acknowledgements

This research was supported by the National Natural Science Foundation of China (grant Nos. 20971115 and 21071134), the Special Foundation for Young Teachers of Ocean University of China (grant No. 201113025) and the Natural Science Foundation of Shandong Province (grant No. ZR2012BQ026).

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