Characterization of compounds 1 and 2
The compounds were prepared using the crystallization method of slow evaporation of the solvent, which is one of the simplest methods. Compounds 1 and 2 are light-yellow crystalline solids, air stable and soluble in water and organic solvents. Two new crystalline forms were obtained and characterized. Melting point of both compounds increased when compared with the melting point of the picric acid (123ºC) and free base, indicating that a new compound was obtained.
The UV-vis absorption spectra were performed in methanolic solutions (10− 5 molL− 1) after solubilization of the crystals for each compound (See supplementary information figure S1). This technique was employed here to confirm the presence multicomponent system and molar ratio between the PA and bases. Compound 1 has maximum absorption at 243 and 354 nm and compound 2 has absorptions at 286 and 354 nm. Given that PA free presents an intense absorption band at 354 nm, the experiment confirms the presence of the picrate in the new material. All bands observed in the UV-Vis spectra are to the transition \(\pi \to {\pi }^{*}\) for PA [16], DPK [17], and BBBPY [18].
In addition, the solid-state behavior of compounds 1 and 2 were also investigated by infrared (IR) spectroscopy measures. As can be seen (figure S2), in the region close to 3100 cm− 1 a characteristic band assigned to the νN-H stretching vibration was observed for 1 and 2, presenting values of 3099 and 3089 cm− 1, respectively. This can be good evidence of salt formation due to the protonation of the nitrogen atom of the bipyridine derivative. Normally, the NH stretching vibration is observed at around 3500 − 3400 cm− 1. However, for solids 1 and 2 it occurs a red shift due to hydrogen intermolecular interactions between donor and acceptor, contributing to the increase in the bond length -NH, shifting it to lower frequencies vibrations [19]. The νC-O stretching of picrate ion was assigned to the bands in the region 1261 cm− 1 for 1 and 1269 cm− 1 for 2. The bands in the region 1628 cm− 1 in the spectra of DPK-PA were assigned to the νC = O stretching of the ketone. For BBBPY-PA, the bands in the regions 1362, 1306, 1238, and 1165cm− 1 were assigned to the tert-butyl groups. The spectra and detailed IR bands assignments are in the supplementary information Figure S2.
Molecular Structure and Hirshfeld surface analysis of compounds 1 and 2
For 1 and 2, detailed structural information were obtained by single crystal X-ray diffraction, consisting in an equimolar mixture of the bipyridine derivative and the picric acid. Compound 1 (Fig. 1a) crystallizes in the triclinic system and space group P\(\stackrel{-}{1}\), while compound 2 (Fig. 1b) is a non-centrosymmetric structure that crystallizes in the orthorhombic system and space group P212121. Crystallographic data are summarized in Table 1.
Table 1
Crystal data and structure refinement for compounds 1 and 2.
Compound | 1 | 2 |
Empirical formula | (C11H9N2O)(C6H2N3O7) | (C18H25N2)(C6H2N3O7) |
Formula weight | 413.31 | 497.50 |
Temperature (K) | 293(2) | 293(2) |
µ (mm− 1) | 0.128 | 0.099 |
F (000) | 424 | 1048 |
Crystal system | Triclinic | Orthorhombic |
Space group | P\(\stackrel{-}{1}\) | P 212121 |
a (Å) b (Å) c (Å) | 9.378(17) 9.790(15) 10.141(16) | 6.3902(6) 23.4160(3) 16.7320(18) |
α (°) β (°) γ (°) | 99.63 99.09 102.55 | |
V(ų) | 877.5 | 2503.6(5) |
Z | 2 | 4 |
Density (calculated) (mg/m3) | 1.564 | 1.320 |
Crystal size (mm³) | 0.28 x 0.25 x 0.18 | 0.34 x 0.30 x 0.24 |
Reflections collected | 3633 | 17638 |
Observed reflections | 2317 | 6052 |
Parameters | 301 | 481 |
R1 (Fo) | 0.0686 | 0.0619 |
WR2 (Fo2) | 0.1887 | 0.1531 |
S | 1.076 | 1.088 |
CCDC | 2202398 | 2202399 |
Comparing 1 and 2 with others bipyridines picrate structures published elsewhere [7], the [(dpaH)(pic)] differs from structure 1 reported here, due to presence of a secondary amine instead of a ketone. In the compound [(dpaH)(pic)], there is an intramolecular H-bond forming a bridge between the pyridine rings, similar to compound 1. The [(dpaH)(pic)] crystallizes in the monoclinic system and space group P21/c. Another crystal structure, [(bypH)(pic)] [20], crystallizes in the triclinic system (space group P\(\stackrel{-}{1}\)) and presents an intramolecular hydrogen bond. On the other hand, the structure of 2 does not exhibit this intramolecular H-bond probably due to the steric hindrance of the tertbutyl groups. The asymmetric unit of 1 and 2 solids present just one ion-pair with one picrate (monoanionic) and one bipyridine derivative protonated (see Fig. 1). As can be seen for the picrate anion, the nitro group at position 4 is coplanar with the ring for structure 1, with a dihedral angle of 1.42°. On the other hand, for structure 2 the nitro group in position 4 is slightly outside the plane of the ring, with a dihedral angle of 5.49°. For the -NO2 groups at position 6 the dihedral angle was of 3.43° for 1 and in the case of 2 it was 23°. The biggest deviations were encountered for the nitro groups in position 2, with dihedral angles of 29.94° for 1 and 75.59° for 2, in both structures the nitro groups in these positions were identified with a disorder. This occurs probably due to steric effects caused by the phenolic oxygen, for the PA molecule there is an intramolecular hydrogen bond between the phenolic hydrogen and the oxygen of the nitro group in position 2, when the phenol is deprotonated, the negative charge in the phenolic oxygen causes this steric effect.
For compound 1, the DPK molecule has two basic nitrogen atoms able to accept hydrogen, however, only one was protonated. As a result, there is a formation of an N-H\(\cdots\)N intramolecular hydrogen bond, stabilizing the molecular system and the planar conformation of the two pyridine rings (Fig. 2a). The dihedral angle between the two pyridine rings is only 3.82°. For compound 2, also only one nitrogen was protonated, however, the formation of an intramolecular hydrogen bond was not favorable due to steric hindrance. The two pyridine rings are almost coplanar with a dihedral angle of 10.22°, in which the nitrogen atoms are located at opposite way with torsion angle between [N1-C22-C23-N7] of -169.4(3)°. The distances of the bond C1-O1 of the picrate in both 1 (1.233(8) Å) and 2 (1.242(5) Å) are shorter compared with free picric acid (1.351 Å) [21], indicating the deprotonation of PA. The main bond distances and angles are listed in Table S1. In the structure of 1 (Fig. 2b) is observed the formation of a centrosymmetric dimer between the molecules of DPK through the oxygen of the ketone part. It was observed a bifurcated hydrogen interaction in which the major part is the interaction N1-H1\(\cdots\)N7 and the minor part is N1-H1\(\cdots\)O7. For 2 (Fig. 3) it was identified a strong hydrogen bond N1-H1\(\cdots\)O1, similar to many picrate salts, in which this interaction usually occurs [2, 12]. The distances observed here were within the range typically reported for this type of interaction and are summarized in Table 2.
Table 2
H-bond parameters for compounds 1 and 2.
| Interaction | d(D-H) (Å) | d(H···A) (Å) | d(D···A) (Å) | D-H···A (°) | Symmetry |
Compound 1 | N1-H1···O7 | 0.860(2) | 2.351(2) | 2.972(3) | 129.5(2) | -x.-y.-z + 1 |
| N1-H1···N7 | 0.860(2) | 1.932(2) | 2.627(3) | 136.9(2) | x. y. z |
| C29-H29···O1 | 0.930(3) | 2.229(3) | 3.155(4) | 174.2(2) | -x.-y.-z + 1 |
| C19-H19···O6 | 0.930(4) | 2.600(3) | 2.486(4) | 159.3(2) | |
| C20-H20···O6 | 0.930(4) | 2.477(2) | 3.396(4) | 196.7(2) | x.+y-1.+z |
| C21-H21···O5 | 0.930(3) | 2.593(2) | 3.522(4) | 177.1(2) | |
| C26-H26···O5 | 0.930(3) | 2.601(2) | 3.504(4) | 163.8(2) | -x + 1.-y.-z |
| C27-H27···O3 | 0.930(3) | 2.509(5) | 3.373(6) | 154.7(2) | x + 1. + y + 1.+z |
Compound 2 | N1-H1···O1 | 0.86 | 1.803 | 2.626(4) | 159.5 | x + 1.+y.+z |
| C21-H21···O1 | 0.93 | 2.450 | 3.313 | 154.4 | |
| C18-H18···O5 | 0.93 | 2.395 | 3.112 | 133.7 | |
In the crystal packing of 1 and 2 are represented in Fig. 4. In the structure of 1, viewed along b axis, it is observed the formation of layers presenting molecules of picrate (green) intercalated with layers formed for molecules of DPK (blue) (see Fig. 4a). On the other hand, in the packing of compound 2, the molecules of picrate are caged by molecules of BBBPY (Fig. 4b). Thus, there are no similar patterns concerning the supramolecular array of 1 and 2.
To better explore the intermolecular contacts occurring in 1 and 2, we use the Hirshfeld surface (HS) (Figure S3-S4), an important tool to visualize the main intermolecular contacts, including conventional and weak H-bond [22]. Through the fingerprint plots, which are generated from the HS, we can access all non-covalent interactions that contribute to crystal packing stabilization. Considering what is most seen in previous reported structures of picrate salts, the interaction O\(\cdots\)H is almost always there [19, 23], in our case the contributions from this type of interaction is 27.3% for 1 and 19.3% for 2, in the fingerprint plots (Figs. 5 and 6) this interactions can be identified as sharp spikes in the bottom-left of the graphic, the upper spike corresponds to the donor part and the lower spike to the acceptor, the fact that this patterns are long and thin means that it corresponds to an interaction of the type N-H\(\cdots\)O [24]. For compound 1 (Fig. 5) it was identified a layered packing alternating in a pattern ABABA, explaining the highest contributions from the interaction C\(\cdots\)O/O\(\cdots\)C (13.8%) when compared to compound 2 (5.2%). On the other hand, compound 2 (Fig. 6) presents the higher contribution from the interaction H\(\cdots\)H (34.3%), probably due to the presence of the t-butyl groups (for 1 the contribution for this interaction is of 11.9%). This feature also increases the contributions from C\(\cdots\)H/ H\(\cdots\)C interactions for compound 2 (11.5%) when compared with 1 (10.5%). The lower contributions from the contacts C\(\cdots\)C for both structures (0.9% for 1 and 2.0% for 2), along with the absence of an intense color in the region of 1.8Å in the fingerprints, indicates that there’s probably no π\(\cdots\)π, or C-H\(\cdots\)π type of interactions, this interactions are actually seen for the similar structures discussed before [7, 20], for this structures the interactions reported as the important ones for crystal packing are the interactions N-H\(\cdots\)O, along with π\(\cdots\)π and nitro\(\cdots\)π. As for the structures reported in this study the important interactions for the crystal packing were seen as being N-H\(\cdots\)O and C-H\(\cdots\)O. Recent studies highlight the importance of both N-H\(\cdots\)O and C-H\(\cdots\)O H-bonds to stabilize the supramolecular frameworks of picrate salts [25]. A graphic with the percentage of the contribution for both compounds is presented in Figure S5.