SulfonateÂ·Â·Â·Pyridinium Supramolecular Synthon: A Robust Interaction

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Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

Sulfonate···Pyridinium Supramolecular Synthon: A Robust Interaction Utilized to Design Molecular Assemblies Arshid A. Ganie, Aadil A. Ahangar, and Aijaz A. Dar* Department of Chemistry, Inorganic Section, University of Kashmir, Hazratbal, Srinagar-190006, India

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S Supporting Information *

ABSTRACT: An approach for supramolecular design through a sulfonate··· pyridinium synthon has been adopted to achieve the structural modulations in molecular complexes of metanilic acid and various pyridyl derivatives. Layered ionic solids [(3-ABSA)2−(4,4′-BPY-H)2+(4,4′-BPY)] (1) and [(3ABSA)−(4,4′-BPP-H)+] (2) have been obtained by reacting metanilic acid (3-ABSA-H) with linear bis-4-pyridines: 4,4′bipyridine (4,4′-BPY) and 4,4′-trimethylenedipyridine (4,4′BPP). 1a, a polymorph of 1, obtained with a slight excess of metanilic acid, also exhibits the layered ionic structure. The tendency of bis-4-pyridines to form infinite N+−H···N assisted chains and hence layered structures is also supported by the literature. Attempts to break down the layered structures by employing nonlinear bis-pyridines, 2,2′-bipyridine (2,2′-BPY) and 1,7-phenanthroline (1,7-phen), lead to the isolation of [(3ABSA)−(2,2′-BPY-H)+·H2O]2 (3) and [(3-ABSA)−(1,7-Phen-H)+(1,7-Phen)·H2O] (4). Compound 3 displays twodimensional (2-D) framework structure with bis-pyridinium cations stacked in rectangular cavities, while in 4 one-dimensional (1-D) trilayer chains associate to form 2-D sheets, which in turn undergo π-interlocking to form a 3-D supramolecule. Monopyridyl derivative 4-phenylpyridine (4-PhPy) yields a pseudo-honeycomb framework [(3-ABSA)−(4-PhPy-H)+·H2O] (5), with pyridinium cations occupying the hives. [(1,5-DNSA)2−(4,4′-BPP-2H)2+·2H2O] (6), obtained with bis-sulfonic acid (1,5DNSA-2H) and linear bis-pyridine, is a 2-D porous solid with water-associated bis-pyridinium chains bridged by bis-sulfonates through a masked sulfonate···pyridinium synthon. Interestingly, the supramolecular synthon manifests itself variedly: ionic interaction in 1, 1a, and 2; charge assisted hydrogen bonding interaction in 4 and 5; and water-masked interaction in 3 and 6. Enhanced thermal and chemical stability in molecular salts 1−6 may be attributed to proton transfer. Incipience of mild to intense color in 1−5 is attributed to extended π-stacking of pyridyl ligands. The products are also characterized by spectroscopic and thermogravimetric methods.

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INTRODUCTION Molecular solids and complexes represent an emergent class of applied materials across a broad spectrum, viz., pharmacy,1,2 agriculture,3,4 industry,5 optoelectronics,6 and so on.7−12 The material properties of these supramolecules depend on packing of the constituents, i.e., crystal packing. Achieving the design or prediction of the crystal packing in the molecular solids continues to be an enigma of crystal engineering. Of the empirical criteria guiding crystal packing, viz., evasion of a vacuum, reduction of repulsive forces, and boosting of attractive forces, the latter involves predictable molecular interactions and hence is a vital option in designing the crystal structures. The vigorous surge in supramolecular chemistry after the early 1990s can be attributed to the recognition of well-defined and predictable structural fragments, i.e., supramolecular synthons, by Etter13 and Desiraju.14 Since then, crystal engineering has come a long way in exploring strong, weak, and very weak noncovalent interactions in supramolecular systems. However, very limited success has been achieved in the rational design of properties based on supramolecular interactions.15 Isoenergetic or nearly isoenergetic supramolecular arrangements, poly© XXXX American Chemical Society

morphs, solvatomorphs, pseudopolymorphs, etc., are a serious challenge before crystal engineers in addition to understanding the very nature of crystal packing.16−22 Robust supramolecular synthons have been identified and documented.23,24 Besides strength and directionality, an important criterion for a robust synthon is the frequency of its occurrence. Modular buildup and design of the cocrystal is possible only with robust and insulated synthons, and synthon− tecton theory has been successful in structural prediction albeit with limitations.25−27 Robust synthons once formed tend not to dissolve and may lead to kinetic aggregation over a thermodynamic one, but with sufficient stability, as has been observed with porous framework organic supramolecules. The difficulty in predicting crystal structures has prompted a surge in studies on synthon hierarchies, and discovery of new robust synthons can lead to better understanding and approaches to address the problem. Received: April 26, 2019 Revised: June 17, 2019 Published: June 26, 2019 A

DOI: 10.1021/acs.cgd.9b00555 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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method. The formation of 1−6 is supported by various spectroscopic, analytical, and thermal methods of analyses. Fourier transform infrared (FT-IR) spectroscopic studies of 1−6 support their formation. The absorption bands at around 1189−1103 and 1350−1300 cm−1 correspond to SO stretching of the sulfonic group, while the absorption bands at 1605−1660 cm−1 and 1550−1600 cm−1 correspond to CN and CC stretching, respectively, Figures ESI-1−6. Compounds 1−6 exhibit stability toward nonpolar and less polar solvents and are soluble in solvents of high polarity only, Table ESI-1. The 1H NMR spectra of 1−6, reported in DMSO-d6, are consistent with their formulation in terms of both the observed chemical shift and peak intensity ratio. 1H NMR spectra with peak assignments are provided in the Supporting Information, Figures ESI-7−12. The formation of 1−6 is also supported by 13 C NMR spectroscopy, Figures ESI-13−18. Products 1, 2, 5, and 6 exhibit high thermal stability as confirmed by their thermal analysis and melting points, 252 (1), 230 (2), 218 (5), and 321 °C (6), when compared to pristine compounds, Table ESI-2. All the products have been further characterized by thermogravimetry, and their definitive solid state structures and crystal packing have been established by the single crystal-X-ray diffraction (SC-XRD) method. Structure Description of 1 and 2. The reaction of metanilic acid with one equivalent of 4,4′-BPY and 4,4′-BPP in aqueous solution yielded 1 and 2 as intense brown solids via slow evaporation, respectively. Compound 1a was obtained by reacting a slight excess of aminobenzenesulfonic acid with 4,4′-BPY. Compounds 1 and 1a crystallize in a triclinic unit cell with centrosymmtric space group P1̅, while 2 crystallizes in a monoclinic unit cell with the P21/n space group. The asymmetric unit of 1 contains two molecules each of metanilate (3-ABSA)− and bis-4-pyridinium (4,4′-BPY-H)+ ions in addition to one molecule of neutral bis-4-pyridine imparting the [(3-ABSA)2−(4,4′-BPY-H)2+(4,4′-BPY)] formula to it. Attempts to obtain a 1:1 molecular salt by reacting bis-4pyridine with a slight excess of metanilic acid under similar conditions yielded polymorph 1a, possessing half the cell contents and nearly half the cell volume of 1. The sulfur centers S1 and S2 of metanilic acid in 1 are in distorted tetrahedral geometry with average bond angle values of 112.82 and 106.21 for O−S−O and 112.62 and 105.82° for C−S−O bonds, respectively. The linear N···N distances in base coformers are 7.044(1) (N3···N4), 7.082(1) (N5···N6), and 7.058(2) (N7··· N8) Å, while the torsion angle values for pyridyl rings are 23.51(1), 16.27(1), and 9.27(1)°, respectively. The protonation at nitrogen centers N4 and N7 is indicated by relaxed C−N−C bond angle values, 120.71 and 121.04° at N4 and N7, compared to the bond angle values of 116.34, 117.41, 117.54, and 116.17 at neutral nitrogen centers N3, N5, N6, and N8, respectively. Molecular structure diagrams of 1 are depicted in Figure 1a, while those of 1a are depicted in Figure ESI-20. Comparative packing diagrams of 1 and 1a are also provided in the Supporting Information, Figure ESI-21. The asymmetric unit of 2 accommodates one molecule each of metanilate (3-ABSA)− and bis-4-pyridinium (4,4′-BPP-H)+ ion. The sulfur center in metanilate has a distorted tetrahedral geometry with average O−S−O and C−S−O bond angle values of 112.49 and 105.81°, respectively. The linear N···N distance in the flexible bis-4-pyridinium linker is 9.395 Å, with a dihedral angle value of 9.12(1)° between pyridyl rings. The C−N−C bond angle value at the protonated nitrogen center N3 is 121.23(2)° compared to the bond angle value of 117.57° at the

Supramolecular salts of 5-sulfosalicylic acid with various bis-4pyridines reported by us recently exhibited preference for formation of the sulfonate···pyridinium synthon over carboxylate···pyridinium, carboxylic···pyridine, and/or hydroxy···pyridine synthons.28 It was also observed that in addition to thermal and chemical stability, proton transfer leads to incipience of color in the products of colorless pristine formers. These results prompted us to further investigate the sparsely reported sulfonate···pyrdinium synthon.29−45 In addition to fewer hits than for the corresponding carboxylic acid···pyridine synthon, our CCDC search indicated a large tendency for proton transfer of the sulfonic proton to pyridyl nitrogen, i.e., salt formation in the sulfonic acid pyridine-based synthon, Chart 1. Here, we Chart 1. CCDC Hits for Aromatic Carboxylic and Sulfonic Acids with Pyridyl Bases

a

The co-crystal form exists because of the unavailability of an acceptor for the proton or in the structure with at least one salt interaction by the same synthon.

report a series of supramolecular aggregates utilizing arylsulfonic acid and various pyridyl derivatives (Chart 2), as a case Chart 2. Starting Materials Used for Synthesis of 1−6

for consideration of sulfonate···pyridinium interaction as a robust supramolecular synthon. Interesting structural modulations have been achieved by varying the pyridyl derivatives as well as the acid formers. As anticipated, the engineered structures of the products exhibit enhanced thermal and chemical stability as well as incipience of color.

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RESULTS Room temperature reaction of metanilic acid (3-ABSA-H) with various pyridyl derivatives, 4,4′-bipyridine (4,4′-BPY), 4,4′trimethylenedipyridine (4,4′-BPP), 2,2′-bipyridine (2,2′-BPY), 1,7-phenanthroline (1,7-Phen), and 4-phenyl pyridine (4PhPy), in aqueous solutions yielded [(3-ABSA)2−(4,4′-BPYH)2+(4,4′-BPY)] (1) and [(3-ABSA)−(4,4′-BPP-H)+] (2), [(3ABSA)−(2,2′-BPY-H)+·H2O]2 (3), [(3-ABSA)−(1,7-PhenH)+(1,7-Phen)·H2O] (4), and [(3-ABSA)−(4-PhPy-H)·H2O] (5). A polymorph of 1, 1a, has been obtained under similar conditions but with an excess of metanilic acid. The reaction of 1,5-napthelenedisulfonic acid (1,5-NDSA-2H) with 4,4′-BPP under similar conditions yielded [(1,5-DNSA)2−(4,4′-BPY2H) 2+ ·2H2 O] (6). Compounds 1−6 were obtained as crystalline solids in quantitative yield via the slow evaporation B



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Infinite hydrogen bonded bis-4-pyridinium chains exist in 2, while the pyridinium formers display a rare N+−H···N synthon breakdown in 1, i.e., form hydrogen bonded trimers (BPY−H··· BPY···H−BPY)2+, which then associate via C−H···N interactions to form infinite chains.47 The cationic chains are further associated into 2-D layers by π-stacking in 1 and by π-stacking as well as C−H···π interactions in 2. The bond parameters of these π interactions are highlighted in Figure 3. The charged metanilate and pyridinium layers associate via electrostatic forces with some assistance from weak secondary interactions: C−H···O (C33−H33···O1, 3.088(1) Å, 167.79(2)°) in 1 and C−H···O (C7−H7···O1 3.302(1) Å, 149.78(1)°; C18−H19···O1, 3.278(2) Å, 131.47(1)°) and C− H···N (C17−H17···N1, 3.501(1) Å, 165.22(1)°) in 2. In light of the apparent tendency of linear bis-4-pyridines to form infinite N+-H···N assisted hydrogen bonded chains and hence layered solids with aminoaryl sulfonates, it will be intriguing to investigate the affect of nonlinear bis-pyridines on the solid state aggregation behavior of these sulfonatepyridinium based supramolecular assemblies. In this direction nonlinear bis-pyridines: 2,2′-BPY and 1,7-Phen were reacted with metanilic acid in an aqueous medium to yield 3 and 4. Structure Description of 3. Compound 3 crystallizes as dark brown blocks in a monoclinic crystal system with the P21/n space group. Two molecules each of metanilate, bis-pyridinium, and lattice water constitute the asymmetric unit. The pyridinium coformers are nearly planar with N1−C5−C6−N2 and N3− C15−C16−N4 torsion angles of 0.11 and 5.42°, respectively. The bond angle values at protonated nitrogen centers N1 and N3 are relaxed to 122.69 and 123.17°, as compared to bond angle values of 116.75 and 117.06° at neutral N2 and N4 centers, respectively. The sulfur centers S1 and S2 in metanilate ions are in distorted tetrahedral geometry with average bond angle values of 108.48 and 108.71°, respectively. The molecular structure diagram of 3 is depicted in Figure 4. The sulfonate···pyridinium

Figure 1. Molecular structure diagram of [(3-ABSA)2−(4,4′-BPYH)2+(4,4′-BPY)] (1) and [(3-ABSA)−(4,4′-BPP-H)+] (2) depicted in a and b, respectively.

neutral nitrogen (N2) end. The molecular structure diagram of 2 is depicted in Figure 1b. Two metanilate ions associate to form a centro-symmetric hydrogen bonded dimer through interactions: N1−H1N···O2 (D···A, 2.933(1) Å; D−H···A, 167.94(1)°) in 1 and N1−H1N··· O4 (2.992(1) Å, 158.39(2)°) and H2−H2N···O2 (3.117(1) Å, 165.48(2)°) in 2. The dimers further aggregate via hydrogen bonding interactions with adjacent aminoaryl sulfonates to form similar one-dimensional (1-D) hydrogen bonded tapes in both 1 and 2, Figures ESI-19 and ESI-22. The 1-D hydrogen bonding tapes form two-dimensional (2-D) sheets via weak slipped π−π interactions in 1, while very weak hydrogen−hydrogen (H···H) dispersive forces bind 1-D hydrogen bonded tapes into 2-D anionic sheets in 2, Figure 2. Both 1 and 2 with linear bis-4-pyridine coformers exist as layered ionic solids. The few and far between structures of bis-4pyridines reported with aminoaryl sulfonic acids also exhibit a layered ionic structure,35,46 similar to 1 and 2, indicating a tendency of linear bis-4-pyridines to form layered ionic solids which consist of infinite N−H+···N assisted bis-pyridinium chains. Unlike anionic metanilate layers, the cationic bis-4pyridinium layers in 1 and 2 exhibit significant packing variation.

Figure 2. Aggregation of metanilate ions into 2-D anionic layers in 1 and 2 depicted in parts a and b, respectively. C



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Figure 3. Packing diagrams of 1 (top) and 2 (bottom).

and N1−H1···N5 (2.821(1) Å, 151.99(1)°) interactions. Four 2,2′-bispyridinium ions stacked by π interactions in each void are placed alternately with an average π−π separation of around 3.632 Å. The packing diagram of 3 is depicted in Figure 5. Structure Description of 4. Compound 4 crystallizes as yellow blocks in the triclinic crystal system with a centrosymmetric space group, P1̅. The asymmetric unit consists of one metanilate ion and two molecules of 1,7-Phen, one protonated and one neutral, in addition to a molecule of lattice water. The sulfur center in the metanilate is in distorted tetrahedral geometry with average O−S−O and C−S−O bond angle values of 112.3 and 106.4°, respectively. Protonation at the N3 center of phenanthrolium is indicated by a relaxed C13−N3−C18 bond angle value of 122.23°, compared to bond angle values of 116.58, 116.95, and 116.64° at neutral nitrogen centers N2, N4, and N5 of phenanthrolium and phenanthroline moieties. The molecular structure diagram of 4 is depicted in Figure 6. Two metanilate units form a hydrogen bonded centrosymmetric dimer via N1−H1N···O3 (3.054(2) Å, 168.31(1)°) interactions between amine and sulfonic groups. Further hydrogen bonding interactions by the residual sulfonate oxygen atoms O1 and O2 form [(3-ABSA)−(1,7-Phen)+(1,7-Phen)· H 2 O] 2 dimers via direct O2···H3N−N3 (2.680(1)Å, 172.04(2)°) and water molecule assisted hydrogen bond interactions O1···H4B−O4 (2.901(2) Å, 159.88(1)° and O4− H4C···N4 (2.833(1) Å, 171.92(1)°). These dimers are associated into trilayer 1-D hydrogen bonded tape through interactions between amine hydrogen and a lattice water molecule: N1−H2N···O4 (3.230(2) Å, 152.53(1)°). The 1-D hydrogen bonded tapes have fascinated structural chemists.48 The 1-D tapes further associate via C17−H17···O4 (3.294(2) Å, 153.18(1)°) weak interactions into 2-D sheets (Figure ESI-25), which interlock through π−π stacking between phenanthroline rings to form a 3-D network. The packing diagram of 4 is depicted in Figure 7. π-Stacking of phenanthroline rings is depicted in Figure ESI-26.

synthons in 3 are masked by lattice water and an amine group, Figure ESI-23.

Figure 4. Molecular structure diagram of [(3-ABSA)−(2,2′-BPY-H)+· H2O]2 (3).

The metanilate ions associate via amino···sulfonate interaction, N5−H5B···O4 (2.974(2) Å, 149.67(1)°), and waterdimer masked sulfonate···sulfonate interaction, O6···H8A−H8 (2.747(4) Å, 172.28(2)°), O8−H8B···O7 (2.732(9) Å, 173.52(2)°), and O7−H7B···O3 (2.732(2) Å, 173.52(1)°), forming the [3-ABSA:H2O]22− dimer with the R44(4) secondary synthon. The residual active hydrogen bonding functionalities further associate the hydrated-metanilate dimer via O1···H5A− N5 (3.010(4) Å, 173.33(2)°), O7−H7A···O4 (2.790(5) Å, 171.87(3)°), and O5−H6A···N5 (3.052(2) Å, 172.69(2)°) interactions into a 2-D framework. In addition to R44(4), the growth unit of 3 consists of R43(4) and R22(2) secondary synthons. The rectangular cavities formed by water associated metanilate ions in a 2-D framework of 3 are occupied by bispyridinium ions held by N3−H3···O8 (2.696(2) Å, 156.62(1)°) D



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Figure 5. Packing diagram of 3.

into a 3-D pseudo-honeycomb structure, with a skelton of water associated metanilate and phenylpyridinium ions occupying the hives. The molecular structure and packing diagram of 5 are depicted in Figure 8 and Figure 9, respectively. In light of the structural modulations achieved by varying the pyridyl bases in 1−5, it will be enticing to probe the effect of altering the functionalities of sulfonic acid on supramolecular aggregation behavior. To investigate if linear bis-4-pyridines can retain the layered ionic structures with further functionalized sulfonic acids, 4,4′-BPP was reacted with bis-sulfonate 1,5DNSA to yield 6. Structure Description of 6. Pallid needles of 6 crystallizes in the monoclinic I2/a space group with half a molecule of each former and a lattice water molecule in the asymmetric unit. The bis-4-pyrdine is protonated at both ends with a C6−N1−C10 bond angle value of 121.84(1)° and a linear N···N distance of 9.281(2) Å between nitrogen centers. The sulfur centers in dianionic 1,5-NDSA are in distorted tetrahedral geometry with average bond angle values of 108.7 and 108.2°, respectively. As in 3, the sulfonate···pyridinium synthon is masked by water which bridges the formers via N1−H1···O4 (2.653(1) Å, 171.14°) and O4−H4C···O3 (2.758(2) Å, 162.06°). Pendant hydrogen H4B furthers hydrogen bonds to the sulfonate oxygen (O4−H4B···O3:2.813(2) Å, 174.82(1)°) of different disulfonate formers forming a centro-symmetric R42(4) synthon, which repeats over the lattice at every water assisted joint of formers. The [1,5- NDSA]2− anions apparently bridge the water assisted bis-4-pyridyl chains into a 2-D supramolecular framework with reactangular voids. Interestingly, 6 represents a subtle example of limited porous organic supramolecules. The molecular structure and packing diagram of 6 are depicted in Figures 10 and 11, repectively. The porous 2-D sheets of 6 are associated into a 3-D aggregate by π-stacking (3.431−3.469 Å) between pyridyl rings of 4,4′BPP and very weak C10−H10···O1 (3.061(1), 123.16(2)) interactions. Unlike 1 and 2, which bear stacked linear bispyridinium chains held strong by π-stacking of both pyridyl

Figure 6. Molecular structure diagram of [(3-ABSA)−(1,7-PhenH)+(1,7-Phen)·H2O] (4).

Intriguing structural modulation of layered ionic solids 1, 1a, and 2 was achieved using nonlinear bis-pyridines in 3 and 4. Only one nitrogen center of 2,2′-BPY and 1,7-Phen is protonated and participates in the hydrogen bonding to form 3 and 4 with diverse structural scaffolds. It will be interesting to investigate the structural modulations that may be achieved by linear-monopyridyl bases. Reaction of 4-Phpy with 3-ABSA-2H in an aqueous medium yielded another modulated supramolecule 5. Structure Description of 5. Light brown crystals of 5 pack in monoclinic space group P21/c. The asymmetric unit is constituted of one molecule each of metanilate, phenylpyridinium, and water. 4-PhPy is nearly planner with a torsion angle of 9.59° between aryl rings. Protonation at the nitrogen center of 4-PhPy by a sulfonic proton is also indicated by a relaxed C7−N2−C11 bond angle value of 121.15°. The formers are associated via N2−H2N···O2 (2.774(2) Å, 174.69(1)°) and water assisted hydrogen bonding interactions, O4−H4A···O3 (2.867(2) Å, 156.54(2)°) and O4−H4B···O1 (2.861(1) Å, 164.48(3)°), into centro-symmetric dimeric species [{(3ABSA)+(4-PhPy-H)−·H2O}2]2. These dimers are further associated via hydrogen bonding interactions of the amine hydrogen of metanilate N1−H1A···O2 (3.099(1) Å, 172.84(3)°) and N1−H1B···O1 (3.231(2) Å, 153.84(1)°) E



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Figure 7. Packing diagram of 4. Different 2-D layers are depicted in different colors.

rings, bis-4-pyridinium in 6 undergoes partial and feeble πstacking via one pyridyl ring only.

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DISCUSSION Besides strength and directionality, the defining criterion for a robust supramolecular synthon is its frequency of occurrence. Although reported few and far between,29−45 sulfonate··· pyridinium interactions are not categorized as robust supramolecular synthons.23,24 Our recent account of supramolecular assemblies based on this synthon indicated its facile manifestation.28 Molecular salts 1−6 further indicate persistent sulfonate···pyridinium synthon formation, even in the presence

Figure 8. Molecular structure diagram of [(3-ABSA)−(4-PhPy-H)+· H2O] (5).

Figure 9. Packing diagram of 5. F



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pyridinium and sulfonate moieties assemble independently. In 3 and 6, the synthon is masked by solvent molecules, while in 4 and 5 sulfonate and pyridinium centers are in direct contact. Synthon masking has been reported previously for sulfonate··· pyridinium as well as other supramolecular synthons.32,44 The articulation of the sulfonate ···pyridnium synthon in 1, 1a, and 2 is debatable. In addition to being a structural unit and assembled through a known synthetic operation, a supramolecular synthon should be clung to by intermolecular interaction.24 Like in 3−6, the supramolecular design in 1, 1a, and 2 is achieved by a preconceived rational approach, and the supramolecules possess alike building units but with ionic interaction between the sulfonate and pyridinium centers. Hence, in the broader context of the definition, the ionic sulfonate−pyridinium interaction in 1, 1a, and 2 should qualify as a supramolecular synthon. The transfer of a sulfonic proton to a pyridine nitrogen center engenders various discernible properties in the resultant supramolecular assemblies. The molecular salts 1−6 as well as others reported earlier exhibit exceptional thermal stability as indicated by higher melting points than those of the corresponding formers, ESI Table-2. Thermogravimetric analyses also indicate enhanced thermal stability for 1−6 and is discussed ahead. The reported molecular salts also exhibit no or very low solubility in solvents of low polarity, ESI Table-2. Incipience of color in the supramolecular salts obtained from pallid formers is reported. Thermal Properties. The sharp endothermic peaks observed in DTA curves at 252.2, 231.1, 178.4, 156.5, 218.3, and 321.1 °C for 1−6, respectively, indicate their melting points. Remarkable phase changes in 3 and 4 observed during melting point measurements are also supported by DTA curves. The melting points of 1−6 correlated with their formers are tabulated in ESI Table-1. TGA curves of nonsolvated salts 1 and 2 are stable until 210 °C, while 3−6 show an early weight loss of 5−6%, which corresponds to a loss of lattice water. Other than a loss of lattice solvent, the reported molecular salts 5 and 6 exhibit augmented thermal stabilities until around 200 and 330 °C, respectively, while 3 and 4 show early weight loss. The normalized TGA curves of 1−6 are depicted in Figure 12, while the individual TGA-DSC curves are given in Figures ESI-23 and 33.

Figure 10. Molecular structure diagram of [(1,5-DNSA)2−(4,4′-BPY2H)2+·2H2O] (6).

Figure 11. Packing diagram of 6. Top: space-fill depiction along b axis. bottom: ball−stick depiction along a axis.

of other possible competing interactions. Moreover, the structural modulations achieved in 1−6 (as well as 1−4 reported earlier28) by varying flexibility, stoichiometry, isomerism, or shape in pyridyl derivatives and/or functional groups in a sulfonic acid former indicate an opportunity toward achieving rationally engineered supramolecular assemblies. There is an apparent tendency of linear bis-4-pyridines to form nonsolvated ionic layered solids as indicated by 1, 1a, and 2 and the literature reports.35,46 The anticipated breakdown of the layers in these ionic solids is achieved by employing nonlinear bis-pyridines: 2,2′-BPY and 1,7-Phen as well as 4-PhPy in 3−5, respectively. The molecular complexes 3−5 represent a variety of framework structures: 3 is 2-D frameworks with nearly rectangular cavities occupied by bis-pyridinium cations, while 5 is a 3-D pseudo-honeycomb framework in which pyridinium cations occupy the hives. Interestingly, 4 packs into a rare 3-D πinterlocked network. The structural alteration of layered ionic solids has also been achieved by using bis-aryl sulfonic acid 1,5NDSA-2H with linear bis-4-pyridine in 6. In nonsolvated complexes 1 and 2, the ionic layers are primarily packed by electrostatic forces, while in unpiled ionic solids 3−6 lattice water molecules act as supramolecular cement. Compared to the sparsely reported sulfonic acid−pyridine interaction, with ubiquitous existence in the salt form, analogous carboxylic acid−pyridine interaction is a popular robust synthon with nearly equal preference for both the cocrystal as well as salt form, Chart 1. The sulfonate···pyridinium synthon manifests itself diversely in 1−6. In 1, 1a, and 2 through ionic interaction,

Figure 12. TGA curves of 1−6.

Absorption Properties. In order to understand the effect of proton transfer and molecular stacking interactions, diffuse reflectance (DR) UV−visible absorptions of molecular complexes 1−6 as well as the pristine components were recorded. The comparative absorption spectra of 1−6 are given in Figure 13, while the individual spectra compared with starting materials G



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EXPERIMENTAL SECTION

Methods and Materials. 3-Aminobenzenesulfonic acid (99%, Sigma-Aldrich), 4,4′-bipyridyl (>99.9%, Sigma-Aldrich), 4,4′-trimethylenedipyridine (99%, Sigma-Aldrich), 2,2′-bipyridyl (99%, SigmaAldrich), 1,7-phenanthroline (99%, Sigma-Aldrich), 4-phenylpyridine (99%, Sigma-Aldrich), and 1,5-napthelenedisulfonic acid (99%, SigmaAldrich) were used as procured. Single distilled H2O and methanol (SD fine) were used for the cocrystallization. Melting points were determined on the MP70 Melting Point System capillary apparatus (Mettler Toledo) in closed end capillaries. The pH was determined on a Labtronics Micro Processor pH meter (LT-49). Infrared spectroscopic data for molecular salts were obtained using an Agilent Technologies Cary 630 FT-IR (4000−650 cm−1) in ATR mode. Ground crystals of salts were placed on the crystal plate of the infrared instrument, and the spectrum was recorded. Samples for 1H NMR were recorded on a Bruker 400 MHz spectrometer in DMSO-d6. Thermal gravimetric analyses of these samples were performed on a Simultaneous Thermal Analyzer-STA (LINSEIS, USA 6807/8835/ 16) using an alumina crucible at a heating rate of 10 °C. DR−UV−vis studies were carried out on a Schimadzu 2600 spectrometer in a BaSO4 medium. SC-XRD Studies. Single-crystal data for 1−3 and 6 were collected on a Rigaku Saturn CCD diffractrometer using a graphite monochromator (Mo Kα, λ = 0.71073 Å), while the data collection for 4 and 5 was carried out on an XTALAB Mini diffractometer. The selected crystals were mounted on the tip of a glass pin using mineral oil or feviquick and placed in a cold flow produced with an Oxford Cryocooling device. Complete hemispheres of data were collected using ω and φ scans (0.3°, 16 s per frame). Integrated intensities were obtained with Rigaku Crystal Clear-SM Expert 2.1 software, and they were corrected for absorption correction. Structure solution and refinement were performed with the SHELX package. The structures were solved by direct methods and completed by iterative cycles of ΔF syntheses and full-matrix least-squares refinement against F (Table 1). Synthesis of [(3-ABSA)2−(4,4′-BPY-H)2+(4,4′-BPY)] (1 and 1a). 3ABSA-H (86 mg, 0.5 mmol for 1; 129 mg, 0.75 mmol for 1a) and 4,4′BPY (78 mg, 0.5 mmol) were dissolved separately in 10 mL of hot distilled water and then mixed together. The resultant solution was filtered while hot. Intense orange blocks of 1 and 1a were obtained in

Figure 13. DR−UV−visible spectra of 1−6.

are given in Figures ESI-34−39. The starting materials, i.e., 3ABSA-H, 4,4′-BPY, 4,4′-BPP, 2,2′-BPY, and 4-PhPy absorb in the UV region due to π−π* electronic transitions, while the transition is red-shifted in 1,5-NDSA-2H and 1,7-Phen due to a fused aryl skelton. Interestingly, the molecular salts 1−6 exhibit significantly different absorption properties due to π-stacking and strong charge-assisted hydrogen bond interactions (N+− H···O) between the crystal formers. The red shift of absorption bands to the visible region in 1−5 leads to the incipience of yellow to pale orange color in π-stacked supramolecules. The shift of absorption spectra of aryl sulfonate on deprotonation and bis-pyrdines on protonation has been observed previously.28,49,50 Water masked molecular salt 6 is an off-white solid and undergoes an odd blue shift of the π−π* transition in 1,5NDSA-2H to 310 nm. The apparent difference in optical behavior of supramolecule 6 may be possibly attributed to a lack of significant π-stacking in the essentially 2-D framework of 6. Furthermore, the significant difference in the absorption properties of molecular complexes 1−5, compared to that of the pristine components, can be attributed to proton transfer as well as interactive conjugation in the supramolecular networks. Table 1. Crystal Data Details of Compound 1−6 compound

1

1a

2

3

4

5

6

CCDC no. sample ID empirical formula fw temp [K] crystal system space group a [Å] b [Å] c [Å] α [deg] b [deg] γ [deg] V [Å3] Z D(calcd) [Mg/cm3] μ [mm−1] Θ range [deg] reflns collected indep. reflns GOF R1 (I0 > 2s(I0)) wR2 (all data)

1911508 AAG-147 C42H38N8O6S2 814.92 293 triclinic P1̅ 10.0972(2) 11.6893(2) 16.5679(3) 86.748(2) 75.999(2) 78.640(2) 1860.14(6) 2 1.455 0.207 25.000 6532 5834 1.117 0.0378 0.1282

1911509 DA-13 C21H19N4O3S 407.46 293 triclinic P1̅ 8.6057(11) 10.1239(13) 11.9259(15) 77.620(5) 86.919(6) 70.125(5) 954.2(2) 2 1.418 0.201 25.000 3347 2621 1.026 0.0576 0.1570

1911510 AAG-197 C19H21N3O3S 371.45 293 monoclinic P21/n 9.9677(6) 8.5694(5) 20.3249(11) 90 92.630(6) 90 1734.27(17) 4 1.423 0.212 25.000 3052 2279 1.060 0.0498 0.1199

1911511 MAA-01 C16H17N3O4S 347.39 150 monoclinic P21/n 13.484(3) 17.139(2) 14.206(4) 90 103.59(3) 90 3191.1(13) 8 1.446 0.229 25.000 5553 4072 1.040 0.0527 0.1299

1911512 DA-73 C30H25N5O4S 551.61 293 triclinic P1̅ 8.656(17) 11.664(19) 13.98(2) 67.76(4) 82.67(5) 86.52(6) 1296.11(4) 2 1.414 0.173 25.000 4562 2941 1.101 0.0778 0.2401

1911513 DA-42 C17H18N2O4S 346.39 293 monoclinic P21/n 8.086(7) 18.878(14) 11.252(9) 90 103.454(11) 90 1671(2) 4 1.377 0.217 25.00 2936 2354 1.049 0.0606 0.1866

1911514 AAG-316 C23H26N2 O8S2 522.58 150 monoclinic I2/a 17.9281(9) 7.9611(4) 17.2609(9) 90 113.401(6) 90 2261.0(2) 4 1.535 0.291 25.000 1993 1731 1.046 0.0376 0.0983

H



Article

1−2 weeks by slow evaporation at room temperature. Yield: 120 mg (73.0%). MP: 230 °C. pH 4.96 (for aqueous mixture). IR ν (neat): 3399(w), 3332(w), 3045(vw), 1632(w), 1591(vs), 1479(s), 1401(s), 1189(vs), 1103(s), 779(vs). 1H NMR δ, 400 MHz, DMSO-d6, ppm: 8.80 (d, 4H, 3JH−H = 4.1 Hz), 7.95 (d, 4H, 3JH−H = 4.2 Hz), 7.39 (t,1H, 3 JH−H = 8.1 Hz), 7.35 (d,1H), 7.30 (s, 1H), 7.03 (d, 1H, 3JH−H = 4.1 Hz). 13C NMR δ, 400 MHz, DMSO-d6, ppm: 155.28, 149.62, 147.06, 143.39, 138.00, 128.80, 125.55, 117.22, 116.93, 114.48, 34.30, 29.98. Anal. Calcd for C42H38N8O6S2 (814.24): C, 61.90; H, 4.70; N, 13.75. Found: C, 61.45; H, 4.91; N, 13.16. Synthesis of [(3-ABSA)−(4,4′-BPP-H)+] (2). 3-ABSA-H (86 mg, 0.5 mmol) and 4,4′-BPP (99 mg, 0.5 mmol) were dissolved separately in 10 mL of hot distilled water and then mixed together. The resultant solution was further heated for 2 h and filtered while hot. Brown block crystals were obtained in 2−3 weeks by slow evaporation at room temperature. Yield: 141 mg (76.0%). MP: 219 °C. pH: 6.38 (for aqueous mixture). IR ν (neat): 3421 (w), 3324(w), 2087(br), 1595(s), 1476(s), 1177(vs), 1095(s), 1029(vs), 700(s). 1H NMR δ, 400 MHz, DMSO-d6, ppm: 8.5 (d, 4H, 3JH−H = 8.1 Hz), 7.5 (d, 4H, 3JH−H = 8.1 Hz), 7.10 (t, 1H, 3JH−H = 4.1 Hz), 7.01 (d, 1H, 3JH−H = 8.2 Hz), 6.99 (s, 1H), 2.02 (t, 4H, 3JH−H = 8.1 Hz), 1.97 (m, 2H, 3JH−H = 4.1 Hz). 13C NMR δ, 400 MHz, DMSO-d6, ppm: 155.28, 149.62, 147.06, 143.39, 138.00, 128.80, 125.55, 117.22, 116.93, 114.48, 34.30, 29.98. Anal. Calcd for C19H21N3O3S (371.13): C, 61.44; H, 5.70; N, 11.31. Found: C, 61.33; H, 5.45; N, 11.98. Synthesis of [(3-ABSA)−(2,2′-BPY-H)+·H2O] (3). 3-ABSA (86 mg, 0.5 mmol) and 2,2′-BPY (78 mg, 0.5 mmol) were dissolved separately in 10 mL of hot distilled water and then mixed together. The resultant solution was heated with stirring and filtered hot. The filtrate yielded light yellow block crystals by slow evaporation in a week. Yield: 121 mg (67%). MP: 207 °C. pH 5.38. IR ν (neat): 3406(w), 3347(w), 2120(br), 1636(s), 1601(s), 1159(vs), 1132(s). 1H NMR δ, 400 MHz, DMSO-d6, ppm: 8.71 (d, 2H, 3JH−H = 4.1 Hz), 8.42 (d, 2H, 3JH−H = 8.2 Hz), 8.01 (t, 1H, 3JH−H = 4.1 Hz), 7.53 (t, 1H, 3JH−H = 4.1 Hz), 7.51 (d, 1H, 3JH−H = 4.2 Hz), 7.37 (m, 1H, 3JH−H = 4.1 Hz), 7.17 (d, 2H, 3JH−H = 4.1 Hz). 13C NMR δ, 400 MHz, DMSO-d6, ppm: 154.42, 149.90, 149.30, 138.70, 134.24, 129.77, 125.15, 123.91, 122.37, 121.45, 119.57. Anal. Calcd for C16H17N3O4S (347.39): C, 55.32; H, 4.93; N, 12.10. Found: C, 55.03; H, 4.91; N, 12.26. Synthesis of [(3-ABSA)−(1,7-Phen-H)+(1,7-Phen)·H2O] (4). 3-ABSA (86 mg, 0.5 mmol) and 1,7-Phen (90 mg, 0.5 mmol) were dissolved separately in 10 mL of hot distilled water and then mixed together. Solution was heated and stirred for 1 h and filtered hot. Filtrate yielded brown colored crystals by slow evaporation after a week. Yield: 125 mg (70%). MP: 156 °C. pH 4.48. IR ν (neat): 3446(w), 3197(w), 2071(br), 1622(s), 1595(s), 1229(vs), 1028(vs), 838(vs). 1H NMR δ, 400 MHz, DMSO-d6, ppm: 9.51 (d, 2H, 3JH−H = 4.1 Hz), 8.07 (d, 2H, 3 JH−H = 4.2 Hz), 8.52 (d, 1H, 3JH−H = 4.1 Hz), 8.18 (d, 1H, 3JH−H = 4.1 Hz), 8.03 (d, 1H, 3JH−H = 4.2 Hz), 7.80 (m, 1H, 3JH−H = 4.1 Hz), 7.46 (t, 1H, 3JH−H = 4.1 Hz), 7.12 (d, 1H, 3JH−H = 4.2 Hz). 13C NMR δ, 400 MHz, DMSO-d6, ppm: 151.09, 150.61, 150.15, 148.26, 145.29, 137.01, 134.08, 133.65, 130.70, 129.72, 127.79, 126.76, 126.38, 123.99, 123.70, 122.99, 122.29, 119.64. Anal. Calcd for C30H25N5O4S (551.16): C, 65.32; H, 4.57; N, 12.70. Found: C, 65.13; H, 4.91; N, 13.06. Synthesis of [(3-ABSA)−(4-PhPy-H)+·H2O] (5). 3-ABSA-H (86 mg, 0.5 mmol) and 4-PhPy (78 mg, 0.5 mmol) were dissolved separately in 10 mL of hot distilled water and then mixed together. The resultant solution was heated with stirring and filtered hot. Filtrate yielded brown crystals by slow evaporation in a week. Yield: 121 mg (73%). MP: 207 °C. pH 5.58. IR ν (neat): 3432(w), 3343(w), 2085(br), 1631(s), 1595(s), 1481(vs), 1153(s), 1028(vs). 1H NMR δ, 400 MHz, DMSOd6, ppm: 8.7 (d, 2H, 3JH−H = 4.1 Hz), 8.0 (d,2H, 3JH−H = 4.1 Hz), 7.9 (d, 2H, 3JH−H = 4.1 Hz), 7.5 (d, 2H, 3JH−H = 4.1 Hz), 7.3 (s,1H), 7.2 (m, 1H, 3JH−H = 4.1 Hz), 6.9 (m, 1H, 3JH−H = 8.2 Hz). 13C NMR δ, 400 MHz, DMSO-d6, ppm: 151.37, 149.76, 147.20, 138.69, 136.32, 130.87, 129.89, 129.31, 127.83, 122.89, 120.70, 119.83, 117.19. Anal. Calcd for C17H18N2O4S (346.10): C, 58.95; H, 5.24; N, 8.09. Found: C, 58.53; H, 5.63; N, 8.26. Synthesis of [(1,5-NDSA)2−(4,4′-BPP-2H)2+·2H2O] (6). 1,5-NDSA2H (180 mg, 0.5 mmol) and 4,4′-BPP (78 mg, 0.5 mmol) were

dissolved separately in 10 mL of methanol and then mixed together. The resultant solution was stirred and then filtered. Off-white crystals were obtained from filtrate by slow evaporation immediately in a few hours. Yield: 221 mg (82%). MP: 207 °C. pH 3.38. IR ν (neat): 3395(w), 3078(w), 2128(br), 1632(s), 1502(s), 1241(vs), 1159(s), 1028(s). 1H NMR δ, 400 MHz, DMSO-d6, ppm: 8.20 (d, 1H, 3JH−H = 8.1 Hz), 8.13 (d, 4H, 3JH−H = 4.1 Hz), 7.45(d, 1H, 3JH−H = 8.1 Hz),7.40 (d, 1H, 3JH−H = 4.1 Hz), 7.02 (t, 4H, 3JH−H = 4.2 Hz), 3.41(t, 4H, 3JH−H = 4.1 Hz), 2.72 (m, 2H, 3JH−H = 4.1 Hz). 13C NMR δ, 400 MHz, DMSO-d6, ppm: 162.73, 144.19, 141.90, 129.98, 129.55, 127.35, 124.58, 124.48, 34.70, 29.13. Anal. Calcd for C23H26N2O8S2 (522.11): C, 52.86; H, 5.02; N, 5.36. Found: C, 52.49; H, 5.33; N, 5.83.

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CONCLUSION Control over structural modulations in supramolecular assemblies based on the less explored sulfonate···pyridinium synthon has been achieved through synthesis of 1−6. 1, 1a, and 2 indicate a facile tendency of bis-4-pyridines to form nonsolvated layered ionic solids, and the desired structural modulations of these layered solids have been achieved by alteration of the bis-pyridine coformers: 2,2′-BPY forms a pyridinium encapsulated 2-D framework (3), 1,7-Phen forms a π-interlocked supramolecule (4), and 4-PhPy yields a pseudohoneycomb-type supramolecular aggregate (5). The structural modulations in ionic layered solids have also been achieved by utilizing bis-sulfonate 1,5-NDSA to yield a subtle and limited 2D porous organic supramolecule (6). Molecular salts 1−6 indicate persistent sulfonate···pyridinium synthon formation, even in the presence of other potential competing interactions. Together with our recently reported supramolecules based on this synthon, 1−6 represent a case for inclusion of sulfonate···pyridinium interaction as a robust supramolecular synthon, as it is strong, directional, and can be reproduced at will. Moreover, the structural modulations reported here by varying coformers indicate an opportunity toward achieving rationally engineered supramolecular assemblies based on this synthon. Ubiquitous existence of this synthon in the salt form provides thermal and chemical stability to the resultant supramolecular assemblies. Incipience of color in the resultant molecular complexes has been confirmed by a red shift in π−π* absorption bands primarily due to extended πinteractions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.9b00555. FT-IR of 1−6, 1H NMR spectra of 1−6, 13C NMR spectra of 1−6, molecular structure and packing diagrams of 1−6, TGA-DTA thermo-grams of 1−6, diffuse reflectance UV−vis spectra of pristine components and 1−6, MP and solubility tables of 1−6, photo of crystals and grinded powders of 1−6, and HR-MS spectra of 1−6 (PDF) Accession Codes

CCDC 1911508−1911514 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. I



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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +91-9596034885. E-mail: [email protected]/ [email protected] ORCID

Aijaz A. Dar: 0000-0002-6180-274X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to UGC-New Delhi and SERB-DSTNew Delhi for BSR (No. F. 30-357/2017) and ECR (ECR/ 2017/000058) projects, respectively. D.A.A. also acknowledges host Prof. G. R. Desiraju and funding agency INSA-New Delhi for the Visiting Scientist Award. The authors also acknowledge Darshan Mhatre for his assistance in solving the data collected for 4 and 5.

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ABBREVIATIONS 4,4′-BPY, 4,4′-bipyridine; 4,4′-BPP, 4,4′-trimethylenedipyridine; 2,2′-BPY, 2,2′-bipyridine; 1,7-Phen, 1,7-phenanthroline; 4-PhPy, 4-phenylpyridine; 1,5-NDSA-2H, 1,5-napthelenedisulfonic acid

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SulfonateÂ·Â·Â·Pyridinium Supramolecular Synthon: A Robust Interaction

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