Crystal structure of 2-aminobenzothiazolinium nitrate and theoretical study of the amino-imino tautomerism of 2-aminobenzothiazole

Description of the crystal structure

The crystal structure of compound 2 has been reported [32], and here we report the crystal structure of compound 1. Selected bond lengths and angles of compound 1 are listed in Table 1, and hydrogen bonding geometry is given in Table 2. As shown in Figure 1A, the crystal structure of compound 1 is totally different from that of compound 2 due to protonation of the ring nitrogen atom and the presence of the NO₃^- anion. In compound 2, the molecules related by an inversion center are linked via N–H···N hydrogen bonds involving the amino groups, forming dimers. In turn, these dimers are linked via a second N–H···N hydrogen bond, forming an infinite two-dimensional network parallel to the b axis [32]. In contrast, for compound 1, there are two 2-aminobenzothiazolinium cations and two nitrate anions in the asymmetric unit. To differentiate the similar two 2-aminobenzothiazolinium cations, the cation containing N1 and N2 is designated as A, whereas the cation containing N3 and N4 is designated as B. Cations A and B are in different layers. The layer containing O4, O5, O6, N6, and A is described as layer A, whereas the layer containing O1, O2, O3, N5, and B is described as layer B. As shown in Figure 1B and 1C, for both layer A and layer B, the 2-aminobenzothiazolinium cations and nitrate anions are linked in a cyclic manner through the amine and aromatic N–H···O hydrogen bonds with an R²₂(8) graph-set motif [33].

Figure 1

(A) The atomic labeling scheme for an asymmetric unit of compound 1. (B) A view of the packing showing layer A of 2-aminobenzothiazolinium nitrate perpendicular to the c axis, hydrogen bonds are shown as dashed lines. (C) A view of the packing showing layer B of 2-aminobenzothiazolinium nitrate perpendicular to the c axis, hydrogen bonds are shown as dashed lines.

These units are extended into parallel one-dimensional zigzag chains lying parallel to the a axis, through a cyclic R²₁(4) association involving the amine N–H···O bifurcated hydrogen bonds to two nitrate O acceptors. However, compared with layer A, the units in layer B are also linked through a cyclic R²₃(10) association involving the additional aromatic C–H···S hydrogen bonds to an aromatic S acceptor, and the two N–H···O hydrogen bonds mentioned above. In addition, as viewed along the c axis, such 1-D infinite chains in layer B are combined to generate an infinite 2-D network (Figure 1C), via an aromatic C–H···O hydrogen bond to a nitrate O acceptor.

Quantum chemistry calculations

All the atom labels in compounds 1–3 are consistent with cation A. Comparison of the corresponding Wiberg bond orders for compounds 1–3 is made in Table 3. Comparison of corresponding NPA atomic charges for compounds 1–3 is presented in Table 4. MEP is related to the electronic density and is a very useful descriptor in understanding sites for electrophilic attack and nucleophilic reactions, as well as hydrogen bonding interactions [34]. The electrostatic potential V(r) is also well suited for analyzing processes based on the ‘recognition’ of one molecule by another, as in drug-receptor, and enzyme-substrate interactions, because it is through their potentials that the two species first ‘see’ each other [35]. To predict reactive sites for the electrophilic and nucleophilic process for the studied molecules, MEP surface was obtained based on the optimized geometry. The negative (red and yellow) regions of MEP are related to electrophilic reactivity (Figure 2B, C) and the positive (blue) ones to nucleophilic reactivity (Figure 2A).

Figure 2

(A) The total electron density mapped with electrostatic potential surface of compound 1. (B) The total electron density mapped with electrostatic potential surface of compound 2. (C) The total electron density mapped with electrostatic potential surface of compound 3.

For compound 2, the Wiberg bond orders of N1–C1 and N2–C1 are higher than that for a single bond and lower than that for a double bond, and the Wiberg bond order of N1–C1 is much higher than that for N2–C1 (Table 3). This suggests that the NH₂ group is slightly conjugated with the benzothiazole system and partial negative charges are shifted from NH₂ to N1 (Table 4). This suggestion is supported by the fact that the NPA atomic charge distributed on NH₂ is positive (0.054). As the conjugation extent is very slight, H2A and H2B moieties are not coplanar with the benzothiazole system and NH₂ maintains the pyramidal configuration. As shown in Figure 2B, the negative region is found around N1 with the V(r) value being -0.048 a.u. Thus, the proton is inclined to attack N1 to form the 2-aminobenzothiazolinium cation. When Fe(NO₃)₃×9H₂ O was added, the 2-aminobenzothiazolinium cations can co-crystallize with NO₃^- to form the supermolecular structure of compound 1. This analysis can explain why compound 1 is formed even though no acid is present.

For compound 3, the Wiberg bond orders of N1–C1 and N2–C1 are also higher than that for a single bond and lower than that for a double bond, and the Wiberg bond order of N2–C1 is much higher than that for N1–C1 (Table 3). This suggests that N1 is slightly conjugated with N2=C1 and partial negative charges have transferred from N1H1 to N2H2B. This suggestion is supported by the fact that N2H2B has much more NPA atomic charges than N1H1 (Table 4). As shown in Figure 2C, the negative region is found around N2 with the V(r) value being -0.055 a.u. Thus, the proton is also inclined to attack N2 to form the 2-aminobenzothiazolinium cation.

For compound 1, the Wiberg bond orders of N1–C1 and N2–C1 are also higher than that for a single bond and lower than that for a double bond, but the bond order of N2–C1 is slightly higher than that for N1–C1 (Table 3). This suggests that after protonation of N1 or N2, the electron configuration experiences a dramatic change compared with compounds 2 or 3. Meanwhile, H2A and H2B are coplanar with the benzothiazole system, indicating that NH₂ is completely conjugated with the benzothiazole. However, H2A and H2B are not coplanar with the benzothiazole system in the crystal structure, which can be attributed to the different environment between the solution and solid state and by the impact of NO₃^-. The proton has the charge of 1, but the NPA atomic charges distributed on N1H1 in compound 1 are increased by approximately 0.544 compared with compound 2 (Table 4). The rest of the charges brought by the proton are largely dispersed to S1 and NH₂. The NPA atomic charges distributed on NH₂ in compound 1 is increased by approximately 0.588 compared with compound 3 (Table 4). The rest of the charges brought by the proton are largely dispersed to S1 and N1H1. This also means that compound 1 has a larger extent of electron delocalization than compounds 2 and 3. As shown in Figure 2A, there is no negative region in the 2-aminobenzothiazolinium cation. The positive regions are found around H1 and H2A with the V(r) values being 0.182 a.u. and 0.152 a.u., respectively. The most positive region is found between H1 and H2A with the V(r) values being 0.240 a.u. Thus, the solvent molecule is inclined to attack H1 or H2A, making the 2-aminobenzothiazolinium cation to be deprotonated to form compounds 2 or 3. In addition, the molecular total energies of compounds 1, 2, and 3 are -778.65 a.u., -778.20 a.u., and -778.10 a.u., respectively. The negative values indicate that compounds 1, 2, and 3 are very stable in the methanol solution, and compound 1 is more stable than compounds 2 and 3, which is consistent with the fact that compound 1 can exist in methanol solution. Compounds 2 and 3 may have approximately equal concentrations in that they have similar molecular total energies. As a result, the 2-aminobenzothiazolinium cation can serve as an intermediate between the amino form and imino form of the tautomeric reaction for 2-aminobenzothiazole. Through protonation and deprotonation among compounds 1, 2, and 3, tautomerism can proceed in two steps (Scheme 2).

Materials and physical measurements

2-Aminobenzothiazole was purchased from Sigma (USA) and used without further purification. The elemental analysis was carried out with a model 2400 Perkin-Elmer analyzer. The infrared spectrum of the compound was recorded in a KBr pellet using a Nicolet 170SX spectrophotometer in the 4000–400 cm^-1 region. The ¹H NMR spectrum was obtained on a Bruker DRX-600 spectrometer. The X-ray diffraction data were collected on a Bruker Smart CCD X-ray single crystal diffractometer.

Synthesis of 2-aminobenzothiazolinium nitrate (1)

2-Aminobenzothiazole (0.30 g, 2.0 mmol) and Fe(NO₃)₃×9H₂ O (0.40 g, 1.0 mmol) were dissolved in methanol (20 mL) and this solution was stirred for 4 h at 60°C, then filtered and the filtrate was left for slow evaporation at room temperature in the air. The colorless needle crystals were obtained in 23% yield after 20 days. ¹H NMR (600 MHz, DMSO-d₆): δ 9.04 (s, 3H), 7.80 (d, 1H, J = 7.8 Hz), 7.42 (d, 1H, J = 7.8 Hz), 7.37 (t, 1H, J = 7.8 Hz), 7.21 (t, 1H, J = 7.8 Hz). A nitrate salt: mp 202–206°C (dec); IR for the nitrate salt: 3104, 1649, 1621, 1578, 1468, 1384, 1321, 1257, 754, 643 cm^-1. Anal. Calcd for C₇ H₇ N₃ O₃ S: C, 39.43; H, 3.31; N, 19.71; S, 15.04. Found: C, 39.14; H, 3.22; N, 20.01; S, 14.88.

Crystallographic data collection and structure determination

Diffraction intensity data of the single crystal of compounds 1 were collected on a Bruker Smart CCD X-ray single crystal diffractometer equipped with a graphite monochromated MoKα radiation (λ=0.71073 Å) by using a φ and ω scan mode at 298 (2) K. The programs used for data collection and cell refinement are the SMART and SAINT programs [36]. Empirical absorption correction was applied using the SADABS programs [37]. All structure solutions were performed with direct methods using SHELXS-97, and the structure refinement was done against F² using SHELXL-97 [38]. All non-hydrogen atoms were found in the final difference Fourier map. Hydrogen atoms were fixed geometrically at calculated distances and allowed to ride on the parent non-hydrogen atoms. Positional and thermal parameters were refined by the full-matrix least-squares method to convergence. The molecular graphics were generated using Diamond 3.1d [39, 40]. The crystallographic data of compound 1 is summarized in Table 5. For the crystallographic data collection and structure determination of compound 2, see [32].

Computational details

Optimizations of geometrical structures and natural bond orbital analyses of the compounds were carried out by using the DFT B3LYP method with 6-311+G* basis set combined with the polarizable continuum model in methanol. The harmonic vibrational frequencies were calculated at the same level of theory for the optimized structure. The vibrational frequency calculations revealed no imaginary frequencies, indicating that an optimal geometry at this level of approximation was found for the compounds. For compound 1, only cation A was selected as the initial model, and the optimized structure was also designated as compound 1. All calculations were conducted on a Pentium IV computer using the Gaussian 03 program [41]. Comparison of the optimized bond lengths and angles with the experimental values of compounds 1 and 2 is shown in Table S1 and Table S2, respectively, and the optimized bond lengths and angles of compounds 3 are shown in Table S3 (in the supplementary material online). The good agreement between the experimental values and the calculated values indicates that the selected method is suitable to calculate these molecules. To investigate the reactive sites of the three compounds, the molecular electrostatic potential was evaluated. The molecular electrostatic potential, V(r), at a given point r(x, y, z) in the vicinity of a molecule, is defined in terms of the interaction energy between the electrical charge generated by the electrons and nuclei of the molecule and a positive test charge (a proton) located at r. The graphics of the MEP map were generated using GaussView 5.0.9 [42].