Introduction

Crystallization is frequently considered as an art. Notwithstanding, even a highly skilled artist in this area often finds it difficult, sometimes even impossible to get a diffraction-quality single crystal. The major obstacle could be attributed to the fact that although the process of crystallization has been studied historically, the inner theory of the crystallization process/crystal growth, at the molecular level, is still a murkier subject [1, 2]. Even with the advent of highly sophisticated instruments and popular theory, there still exists a long way off from predicting the outcome of a crystallization process accurately at the molecular level. Instead, the popular crystallization theories are usually based on the crystal engineering theory with the background of chemical and physical knowledge to create the supramolecular structures from molecules that are programmed to participate in the multiple interactions of electrostatic forces, H-bonding, cation···π interaction, CH-π, and π···π interactions [3,4,5,6,7,8]. H-bonding is one of the most critical non-covalent associates that determine and control the assembly of the molecules and ions [9,10,11,12]. In recent years, efforts have been made to explore the construction of supermolecules or supramolecular arrays utilizing non-covalent bonding. Thus, the supramolecular construction successfully exploits the hydrogen bonding and other types of non-covalent interactions in creating the supramolecular systems [13]. In this regard, there are many interesting topological structures such as one-dimensional (1-D) tapes, two-dimensional (2-D) sheets, and three-dimensional (3-D) networks which have been built by the H-bonding associates [14, 15].

In zwitterionic compounds, oppositely charged quaternary ammonium groups are connected with the necked carboxylate group via the methylene groups [16,17,18,19,20], and they are also known as betaines. Another group of betaines is the heterocyclic betaines, and the most common heterocyclic unit is the pyridine ring. Although some betaines with pyridine units have been documented [21,22,23,24,25,26], there were very few inter salts with the quinoline and nicotinamide moiety, especially concerning the (carboxymethyl)propanoate simultaneously.

Therefore, it is essential to study the supramolecular interactions of the inter salts derived from quinoline and nicotinamide; herein, we unveil the structures of the two inner salts derived from quinoline and nicotinamide (Scheme 1), respectively. The two organic inner salts are 3-carboxy-2-(quinolinium-1-ylmethyl)propanoate dihydrate (1) and 3-(3-carbamoylpyridinium-1-yl)-2-(carboxymethyl)propanoate (2), and the synthons in them were compiled in Scheme 2 (Scheme 2). The characterization of the inner salts was performed using the following techniques: single-crystal X-ray diffraction (SCXRD), elemental analysis (EA), and melting point measurement.

Scheme 1
scheme 1

The inner salts discussed in this paper

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Scheme 2
scheme 2scheme 2

The synthons in 1-2

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Experimental section

Materials and physical measurements

The chemicals and solvents used in this job are of analytical grade and available commercially and were utilized without further purification. The FT-IR spectra were recorded from the KBr pellets in the range 4000–400 cm−1 on a Mattson Alpha-Centauri device. Microanalytical (C, H, and N) data were registered with a Perkin-Elmer Model 2400II elemental analyzer. Melting points of the inner salts were recorded on an XT-4 thermal apparatus without correction.

Crystal growth of the inner salts 1–2

3-carboxy-2-(quinolinium-1-ylmethyl)propanoate dihydrate (1)

3-Carboxy-2-(quinolinium-1-ylmethyl)propanoate (25.9 mg and 0.1 mmol) was dissolved in methanol (10 mL). The solution was stirred for a few minutes; then, the solution was filtered into a test tube. The solution was left standing at room temperature for several days, and colorless block crystals were isolated after slow evaporation of the methanol solution in the air. The crystals were collected and dried in air to give the title compound 3-carboxy-2-(quinolinium-1-ylmethyl)propanoate dihydrate (1). (yield 22.4 mg, 75.86%). mp 159–161 °C. Elemental analysis: Calc. for C14H17NO6 (295.28): C, 56.89; H, 5.76; N, 4.74. Found: C, 56.85; H, 5.72; N, 4.71. Infrared spectrum (cm−1): 3721 s(br, ν(OH), 3268 s(νas(NH)), 3152 s(νs(NH)), 3066 m, 3023 m, 2971 m, 2932 m, 1657 s(νas(C = O)), 1616 s(νas(COO−)), 1570 m, 1528 m, 1480 m, 1430 m, 1386 s(νs(COO−)), 1344 m, 1304 m, 1286 s(νs(C-O)), 1242 m, 1200 m, 1159 m, 1115 m, 1072 m, 1030 w, 988 w, 946 m, 905 m, 861 m, 818 m, 777 w, 732 m, 690 m, 651 w, 634 m, and 604 m.

3-(3-carbamoylpyridinium-1-yl)-2-(carboxymethyl)propanoate (2)

3-(3-carbamoylpyridinium-1-yl)-2-(carboxymethyl)propanoate (25.2 mg and 0.1 mmol) was dissolved in methanol (12 mL). The solution was stirred for a few minutes; then, the solution was filtered into a test tube. The solution was left standing at room temperature for several days, and colorless block crystals were isolated after slow evaporation of the methanol solution in air. The crystals were collected and dried in air to give the title compound 3-(3-carbamoylpyridinium-1-yl)-2-(carboxymethyl)propanoate (2). (yield: 20 mg, 79.29%). mp 187–189 °C. Elemental analysis: Calc. for C11H12N2O5 (252.23): C, 52.33; H, 4.76; N, 11.10. Found: C, 52.28; H, 4.72; N, 11.06. Infrared spectrum (cm−1): 3648 s(ν(OH)), 3230 s(νas(NH)), 3166 s(νs(NH)), 3074 m, 2982 m, 2936 m, 2855 m, 1708 s(ν(C = O)), 1644 s, 1599 s(νas(COO−)), 1556 m, 1519 m, 1472 m, 1428 m, 1384 s(νs(COO−)), 1324 s(νs(NO2)), 1288 s(ν(C-O)), 1245 m, 1201 m, 1158 m, 1115 m, 1071 m, 1028 m, 987 m, 945 m, 903 m, 858 m, 812 m, 768 m, 726 m, 684 m, 649 m, and 614 m.

X-ray crystallography and data collection

Suitable crystals were mounted on a glass fiber by a Bruker SMART 1000 CCD diffractometer operating at 50 kV and 40 mA power using the Mo Kα radiation (0.71073 Å) with the CCD area detector. Data were collected using the φ and ω scans at 0.9° and 15 s exposures per frame. Data collection and reduction were performed by the utilization of the SMART and SAINT software [27]. The structures were solved by the direct methods, and the non-hydrogen atoms were subjected to anisotropic refinement by the full-matrix least squares against F2 employing the SHELXTL package [28]. The hydrogen atoms anchored to the carbon atoms were ridden at the geometrically calculated sites with C-H = 0.93 Å (aryl), and C-H = 0.97–0.98 Å (for the sp3 C atoms), the hydrogen atoms tied to the nitrogen and oxygen atoms were located on a Fourier difference map and refined without AFIX command, with Uiso(H) = 1.2Ueq(C, N) and Uiso(H) = 1.5 Ueq(O). Further details of the structural analysis are summarized in Table 1. Selected bond lengths and angles for the salts 1–2 are listed in Table 2, and the relevant hydrogen bond parameters are provided in Table 3.

Table 1 Summary of X-ray crystallographic data for compounds 1–2
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Table 2 Selected bond lengths [Å] and angles [°] for 1–2
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Table 3 Hydrogen bond distances and angles in studied structures 1–3
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Results and discussion

Preparation and general characterization

The crystals 1–2 were grown by evaporating the methanol solution at ambient conditions. Both structures are inner salts showing no hygroscopic. The molecular structures and their atom labeling schemes for the both structures are demonstrated in Figs. 1 and 2, respectively. The elemental analysis data for the both crystalline inner salts are in good alignment with their compositions. The infrared spectra of both compounds are consistent with their chemical formulas measured by the elemental analysis and further confirmed by the X-ray diffraction analysis.

Fig. 1
figure 1

Molecular structure of 1 showing the atomic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level

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Fig. 2
figure 2

Molecular structure of 2 showing the atomic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level

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The very strong and broad features at approximately 3152–3721 cm−1 in the IR spectra of both inner salts arise from the O-H or N-H stretching frequencies. Aryl ring (including the pyridine and the quinoline) stretching and bending are connected with the medium intensity bands in the regions of 1500–1630 and 600–750 cm−1, respectively. The inner salts 1–2 exhibit the characteristic bands for the CO2H groups. IR spectroscopy has also been proven to be useful for the detection of the CO2− groups [29, 30].

Structural descriptions

X-ray structure of 3-carboxy-2-(quinolinium-1-ylmethyl)propanoate dihydrate (1)

The crystal structure of the inner salt 1 consists of one 3-carboxy-2-(quinolinium-1-ylmethyl)propanoate and two water molecules in the asymmetric unit (ASU) (Fig. 1). The inner salt 1 crystallizes in the triclinic space group P-1 with one CO2H group ionized, and the other CO2H group remains protonized.

The C-O bond lengths 1.240(3) Å (O(2)-C(1)) and 1.249(3) Å (O(1)-C(1)) concerning the carboxylate are within the range of the deprotonated CO2H unit [31]. In the CO2H group, the two C-O bond lengths are obviously different between O(3)-C(4) (1.299(4) Å) and O(4)-C(4) (1.204(4) Å) with the large Δ value of 0.095 Å. The relatively large Δ value which is expected for the neutral C = O and C-O bonds [32] are also corroborating the reliability of adding the H atoms experimentally by the different electron densities onto the O atoms (Table 3). Quaternization of the quinoline at the cyclic N1 site is reflected in a change in the bond angle. The angle at the unquaternizated ring atom is ca. 118 ° [33], while around the quaternizated N, the angle (C(6)-N(1)-C(10)) is 120.2(3)°. The rms deviation of the quinoline plane is 0.0108 Å, the C1-O1-O2/C4-O3-O4 inclined at 86.8/24.2° from the quinoline plane. Both planes defined by the COO−/COOH groups in the same inner salt were almost perpendicular to each other with a dihedral angle of 90.6°.

Two water molecules produced a water dimer via the O-H···O hydrogen bond with the O···O = 2.825(5) Å. At the 3-carboxy-2-(quinolinium-1-ylmethyl)propanoate, there adhered a water dimer by the CH-O associate present between the 4-CH(C8H8) of the quinoline and the O(O6) atom of one water molecule at the water dimer with C···O = 3.408(2) Å/< CHO = 166(4)° to attain a tri-component aggregate. Both tri-component aggregates were held together by the O(5)-H(5F)···O(2)#1 hydrogen bond donated by the water molecule to one O atom of the carboxylate with O···O = 2.775(4) Å, and CH-O association operated between the 5-CH(C14H14) of the quinoline and the same O(O2) atom of the carboxylate that was involved in the O-H···O H-bond with C···O = 3.355(4) Å< CHO = 128(3)° to create a six-component assembly. The geometries of the CH-O associates were in the limit of the report [34]. Both tri-component aggregates at the six-component assembly were inversionally related. The six-component assemblies were interlinked by the O(3)-H(3)···O(1)#2 H-bond generated between the CO2H and the CO2− with O···O separation = 2.530(3) Å to get the 1D chain extending along the crystallographic a axis direction; here, the O···O separation was ca. 0.1 Å shorter than the document with the CO2H···CO2− associate [35]. The 1D chains were fused together by the O(5)-H(5E)···O(2)#2 hydrogen bond operated between the water molecule and the carboxylate with O···O = 2.818(3) Å to reach 2D sheet extending parallel to the crystallographic ac plane (Fig. 3). The 2D sheets were further piled at the crystallographic b axis direction by the CH2-O associate present between the CH2(C5H5A and C3H3A) spacer of the 3-carboxy-2-(quinolinium-1-ylmethyl)propanoate and the oxygen atom (O1) in the carboxylate with C···O amounting to 3.437(4)–3.595(4) Å and the < CHO = 148(4)/153(4)° to create the 3D network structure. The geometries of the CH2-O associates were close to the document [36]. The I R21(6), II R22(10), III R22(22), IV R42(8), V R43(10), and VI R55(17) graph sets were established in the net.

Fig. 3
figure 3

2D sheet structure of 1 extending parallel to the ac crystallographic plane

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X-ray structure of 3-(3-carbamoylpyridinium-1-yl)-2-(carboxymethyl)propanoate (2)

As 1, the crystal of 2 was also grown by slow evaporation of its CH3OH solution, comprising only one 3-(3-carbamoylpyridinium-1-yl)-2-(carboxymethyl)propanoate in its ASU (Fig. 2). The compound 2 is also an inner salt, in which only one proton of the two carboxyl groups was ionized. It adopts the monoclinic space group P2(1)/n. The C-O bond length of the O(5)–C(6) (1.228(5) Å) is of the common C = O double bond. Quaternization of the nicotinamide on the N1 site is reflected in a change in the bond angle. The angle at the unquaternizated ring N atom is 118.56(13)° [37], while for the quaternizated N, the angle (C(11)–N(1)-C(7)) is 120.5(4)°. And this angle also resembled that found in 1. The nicotinamide moiety adopted the E conformation according to Fausto [38], in which the O(5)-C(6)-C(8)-C(9) torsion angle is −134.7(5)°. The rms deviation of the pyridine ring N1-C7–C11 was 0.0101 Å, the C1-O1-O2/C4-O3-O4 inclined at 94.7(2)/37.6(2)° from the pyridine plane. Both planes defined by the COO−/COOH groups in the same inner salt were almost perpendicular to each other with a dihedral angle of 83.6(2)°. The CONH2 deviated by 137.7(2)° from the pyridine plane.

The 3-(3-carbamoylpyridinium-1-yl)-2-(carboxymethyl)propanoates were linked together by the O(3)-H(3)···O(1)#3 hydrogen bond of the COOH···COO− mode with O···O distance of 2.481(4) Å, and NH-π associate operated between the NH2 of the nicotinamide fragment and the pyridine kernel with N···Cg = 3.352(2) Å to get the 1D chain stretching along the crystallographic b axis direction. The COOH···COO− hydrogen bond parameters were close to that at 1. In this case, the N···Cg distance is in the limit of the reported values [39]. The 1D chains were tethered together by the CH-O association stemmed from the 4-CH(C9H9) of the nicotinamide residue and the OH(O3) at the CO2H with C···O = 3.439(3) Å and < CHO = 143(4)° and CH-O associate between the 5-CH(C10H10) of the nicotinamide residue and the O(O2) atom at the CO2− with C···O = 3.245(2) Å and < CHO = 123(2)º to get 2D sheet spreading parallel to the crystallographic ab plane (Fig. 4). The 2D sheets were further packed along the crystallographic c axis direction through the N(2)-H(2B)···O(2)#2 hydrogen bond established between the NH2 of the nicotinamide residue and the CO2− with N···O = 2.842(5) Å, CH-O association present between the 5-CH(C10H10) of the nicotinamide and the OH(O3) at the carboxyl with C···O = 3.335(2) Å and < CHO = 125(2)°, and CH-O association operated between the 6-CH(C11H11) of the nicotinamide and the O(O2) at the CO2− with C···O = 3.442(2) Å and < CHO = 145(2)° to make the 3D layer network structure. The N···O separation at the N-H···O hydrogen bond is considerably less than the sum of the van der Waals radii for the N and O (3.07 Å) atoms [40]. The geometries of the CH-O associations resembled those at 1 too. There established the VII R22(10), VIII R22(12), IX R22(13), X R22(14), XI R22(16), and XII R33(9) synthons.

Fig. 4
figure 4

2D sheet structure of 2 extending parallel to the ab crystallographic plane

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The Hirshfeld surface analysis

The Hirshfeld surfaces and fingerprint plots were generated from the CIF files of the respective inner salts utilizing the Crystal Explorer (Version 21.5) suite. Hirshfeld surfaces [41] and related fingerprint plots [42, 43] were calculated for each compound (Figs. 5 and 6), and the percentage contribution of the significant non-covalent contacts at each inner salt is compiled in Table 4. The H···H interactions in 1 and 2 occupy 50.8% and 30.0% of the total Hirshfeld surfaces at 1 and 2. The The H···O interactions occupy 34.9% and 47.4% of all Hirshfeld surfaces for 1–2, respectively. 1 has the biggest percent of the H···H interaction for it bears more H atoms than 2. The H···O interaction at 2 has the largest percent because there existed more CH-O contacts, which is also supported by the relative high C…O contact at 2. The H···C interaction composes 5.7% and 8.8% of the total Hirshfeld surfaces at 1–2, respectively.

Fig. 5
figure 5

Fingerprint plots computed from Hirshfeld surfaces of 1. H···H contacts are highlighted in red circle, and H…O contacts are highlighted in green circle

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Fig. 6
figure 6

Fingerprint plots computed from Hirshfeld surfaces of 2. H···H contacts are highlighted in red circle, and H…O contacts are highlighted in green circle

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Table 4 Summary of the various contact contributions to the Hirshfeld surface area in 1–2
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Conclusions

In summary, 2 quinolinium/pyridinium (carboxymethyl)propanoate inner salts have been structurally featured and the underlying supramolecular chemistry has been elucidated in this article, which make new knowledge into the inner salt motifs. In both the inner salts, the cyclic ring N is quaternized. In this report, a complex sets of synthons with small and large circles were made as R21(6), R22(10), R22(12), R22(13), R22(14), R22(16), R22(22), R33(9), R42(8), R43(10), and R55(17). Major of the synthons were not found repeatedly, yet the R22(10) appeared at both 1 and 2.

Though prepared via the same natural solvent evaporation technique, they both showed the 3D net motifs. All bear the 2D sheet substructure, and there also built the subchain at both; it seems that the intra- and interchain non-covalent bonds showed equal merit in the structure propagation. Despite variations in the aryl cores, Both established the strong intermolecular O-H···O H-bonds. Hirshfeld surface analysis discloses that the H···H/H···O intermolecular interactions act as the influential parts to ascertain the crystal packing arrangements. Apart from the robust classical H-bonds, other different sets of the supramolecular associates of CH-O, CH2-O, and NH-π are also crucial in structure expansion. The CH-O contacts were created in both the salts. The various non-covalent bonds coupled with the different functional groups at the quinoline/pyridine resulted in the construction of the higher-dimensional assemblies from the discrete blocks. Based on the inner salts discussed here as well as those from the literatures, the results clearly tell that the quinoline/pyridine is a good segment to be built into the inner salts so as to get the diversiform and stable H-bonded systems.