Anion assisted supramolecular self-assemblies of succinate and malate adducts: crystal structures and theoretical modelling

Abstract

A series of five dicarboxylic acid complexes of succinic acid and malic acid, hexamethylenetetramine succinic acid (HMTSA) (1), bis(3-methylanilinium) succinate succinic acid (3MASSA) (2), 4-methoxyanilinium hydrogen succinate (4MAHS) (3), 2-aminoanilinium hydrogen malate (2AAHM) (4) and 4-ethoxyanilinium hydrogen malate (4EAHM) (5), have been synthesized and structurally analyzed; also, the supramolecular organizations within their crystalline solids have been studied. Strong hydrogen bonds such as O–H⋯O, N–H⋯O and O–H⋯N are dominant in the formation of one- and two-dimensional supramolecular frameworks. In addition, C–H⋯O and C–H⋯N weak interactions also play important roles in the framework of supramolecular self-assembly. The geometries of the crystal structures have been optimized by the DFT/B3LYP method using the 6-31g(d,p) basis set, and the results have been found to be comparable with experimental results. Computational analysis of the crystal packing by the PIXEL method provided deeper insight into the nature and domination of intermolecular interactions in the overall stabilization. Hirshfeld surfaces and their associated 2D fingerprint plot analyses revealed that closer molecular contacts may aid the crystal packing. The reported compounds were also characterized with FT IR spectroscopy and thermogravimetric analysis.

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1. Introduction

Crystal engineering is a tool for the design and supramolecular synthesis of desirable new organic and inorganic compounds with specific physicochemical properties, such as non-linear optics, electrical conductivity, magnetism, and catalysis.¹ Crystal engineering has received increasing acknowledgment in industrial and medical fields because of its unique property of not altering the inherent chemical properties of the molecule. A comprehensive understanding of the interactions between the molecules is required in the prediction of supramolecular patterns that exist in crystal structures.² The forces responsible for the self-assembly of molecular species vary from strong covalent bonds to weak non-covalent bonds (hydrogen bonding and intermolecular forces). The study of non-covalent interactions is crucial, as it governs molecular recognition, host–guest chemistry, catalysis, molecular folding and mechanically interlocked molecular architectures.³ The self-assembly of molecular species through non-covalent interactions is one of the fundamental techniques that have been used for the construction of supramolecular aggregates with specific properties.⁴ Supramolecular chemistry focuses on non-covalent interactions to disclose the fundamental role of intra- and intermolecular forces in the construction of larger structures.⁵ Generally, the rationalization of supramolecular synthons is the key issue in identifying the distinct sets of interactions that occur between molecules.⁶ It is well known that frameworks depend on the shape, size and availability of the functional groups of the interacting molecules.⁷ Carboxylic acids, both aliphatic and aromatic, form organic and metal–organic frameworks because of their excellent bridging capabilities with various complementary functional groups, such as amides, nitriles, and alcohols.⁸ The basic motifs which carboxylic groups adopt with complementary functional groups are linear dimers (D), catemers of linear chains (C) and rings (R).⁹ Reports on CSD analysis depict that the interaction of dicarboxylic acids with amines (complementary functional groups) forms more supramolecular heterosynthons than supramolecular homosynthons.¹⁰ Aliphatic dicarboxylic acids prefer to crystallize in the extended trans conformation, thereby forming infinite linear or nonlinear tapes or chains through head to tail hydrogen bonding between adjacent molecules.¹¹ Also, dicarboxylic acids exist in different forms with bases, such as neutral, singly ionized and doubly ionized. We report here five crystal structures of dicarboxylic acids: three of succinic acid and two of malic acid (Scheme 1). Interestingly, the complexes of succinic acid reported here exist in all three forms as (i) acid–base co-crystals, (ii) succinates with unionized succinic acid and (iii) singly ionized hydrogen succinates. We focused our attention on the study of the stabilization of the solid networks, specifically with two dicarboxylic acids, succinic acid (saturated dicarboxylic acid) and malic acid (hydroxyl substituted dicarboxylic acid), with different substituted bases. The structural properties of hydrogen bonding were analyzed with a combination of experimental and computational techniques. This observation lends substantial support to delineate the participation of hydrogen bonds in the construction of multidimensional architectures.

Scheme 1 (a) Succinic acid, (b) malic acid, (c) hexamethylenetetramine, (d) 3-methylaniline, (e) 4-methoxylaniline, (f) 2-aminoaniline and (g) 4-ethoxyaniline.

2. Experimental section

2.1. Hexamethylenetetramine succinic acid (1)

Compound 1 was prepared by mixing a hot methanolic solution of succinic acid (0.118 g; 1 mM) with a hot methanolic solution of hexamethylenetetramine (0.140 g; 1 mM) in a 1 : 1 ratio. The mixture was heated to 40 °C in a water bath for about 20 minutes with subsequent stirring. The mixture was then allowed to cool to room temperature and was left undisturbed for slow evaporation. Colourless crystals of 1 were obtained after a few weeks.

2.2. Bis(3-methylanilinium) succinate succinic acid (2)

Compound 2 was prepared by mixing a hot methanolic solution of succinic acid (0.118 g; 1 mM) with a hot methanolic solution of 3-methylaniline (0.098 g; 1 mM) in a 1 : 1 M ratio. The mixture was warmed in a water bath for 25 minutes with continuous stirring. The solution was then allowed to cool to room temperature. Colourless crystals of 2 were obtained after a few weeks.

2.3. Bis(4-methoxyanilinium) hydrogen succinate (3)

Compound 3 was prepared by mixing a hot methanolic solution of succinic acid with a hot methanolic solution of 4-methoxyaniline in a 1 : 1 M ratio. The mixture was warmed in a water bath for 25 minutes with continuous stirring. The mixture was left undisturbed to evaporate slowly under ambient conditions. Colourless crystals were obtained after a few days.

2.4. 2-Aminoanilinium hydrogen malate (4)

Compound 4 was prepared by mixing a hot methanolic solution of malic acid with a hot methanolic solution of 2-aminoaniline in a 1 : 1 M ratio. The mixture was warmed in a water bath for 25 minutes with continuous stirring. The mixture was then allowed to cool to room temperature and was left undisturbed for slow evaporation. Colourless crystals were obtained after a few days.

2.5. 4-Ethoxyanilinium hydrogen malate (5)

Compound 5 was prepared by mixing a hot ethanolic solution of malic acid with a hot ethanolic solution of 4-ethoxyaniline in a 1 : 1 M ratio. The mixture was warmed in a water bath for 25 minutes with continuous stirring for about 15 minutes. The mixture was then allowed to cool to room temperature and was left undisturbed for slow evaporation. Colourless crystals were obtained after a few days.

2.6. Physical measurements

The FT-IR spectra of compounds 1 to 5 were recorded with a Perkin Elmer Spectrum One FT-IR spectrophotometer in the spectral region of 4000 to 450 cm⁻¹ using KBr pellets. Thermogravimetric experiments were carried out on a Sii Nano instrument (TGDTA 6300) from 20 °C to 600 °C under nitrogen atmosphere at a heating rate of 20 °C min⁻¹.

2.7. X-ray crystallography

The X-ray intensity data of all the compounds were collected using a Bruker AXS Kappa APEXII CCD diffractometer. APEX2 software was used for data collection and also for indexing the reflections and determining the unit cell parameters. Unit cell determination and data reduction were performed using XPREP and SAINT software.¹² All structures were solved by direct methods using the SHELXS-97 program and refined with SHELXL-97 (ref. 13) using the full-matrix least-squares procedure. The final refinement was performed using the full matrix least-squares method with anisotropic thermal parameters for non-H atoms on F². The displacement ellipsoid plots of the compounds are shown in the ESI.† The molecular graphics were generated using ORTEP3 (ref. 14) and Mercury 2.4.¹⁵

2.8. Theoretical calculations

Density functional theory (DFT) calculations were carried out using the Gaussian 09W program¹⁶ at an HP Z400 high performance workstation. The atomic coordinates from the final X-ray refinement method were taken as the starting coordinates for the quantum mechanical calculations. The molecular geometries of the structures were optimized using Becke's three parameter exact exchange functional (B3)¹⁷ combined with a gradient-corrected correlation functional of Lee, Yang and Parr (LYP)¹⁸ by the 6-31G(d,p) basis set at the global minimum energy level. No constraints on bond length, angle or dihedral angle were applied in the calculations, and all the atoms were free to optimize.

2.9. Hirshfeld surface analyses

Hirshfeld surfaces and their associated two dimensional (2D) fingerprint plots were calculated using Crystal Explorer software.¹⁹ Hirshfeld surface analysis is a graphical tool for the picturization of overall molecular contacts. The molecular Hirshfeld surface of a crystal structure is constructed based on the electron distribution calculated from the sum of spherical atom electron densities.²⁰ The mapping of a crystal structure with normalized contact distance (d_norm) relies on two distances, normalized by the van der Waals distance: d_e, the distance from the point to the nearest nucleus external to the surface, and d_i, representing the distance to the nearest nucleus internal to the surface. The bright red spots on the surface indicate stronger hydrogen bond interactions, while the change in colours represents weaker and longer contacts. The surface analysis and their corresponding 2D fingerprint plot clearly exhibit the quantitative information of all interactions present between the molecules under investigation. It is, therefore, of interest to investigate how hydrogen bonds and also closer contacts impact the stability of the overall crystal structure.

2.10. PIXEL energy calculations

The PIXEL method is a semi-empirical technique for the evaluation of lattice and intermolecular energies of organic crystals. This method takes the entire electron density of a molecule into account to provide comprehensive information on interactions. The breakdown of the total intermolecular energy into coulombic, polarisation, dispersion and repulsion energies (Coulomb–London–Pauli model) divulges the relationship between molecular constituents. In the present study, PIXELC calculation was carried out using the CLP computer program.²¹ The electron density of individual molecules was calculated using GAUSSIAN09W quantum chemistry software.

3. Results and discussion

All the compounds were characterized by X-ray crystallography, FT-IR spectroscopy and thermal analysis to determine their molecular structures and supramolecular organizations as well as the nature of their crystal forms and their stabilities. Theoretical DFT calculations, Hirshfeld analysis and PIXEL energy calculations were performed to support the experimental output.

3.1. X-ray crystallography

The structure refinement parameters of compounds 1 to 5 are given in Table 1, and the geometrical characteristics of the hydrogen bonds are given in Table 2. In compound 1, the succinic acid molecule is disordered over two positions with refined site occupancies of 0.868(4) and 0.132(4), respectively. Suitable similarity restraints were used for the bond lengths and bond angles of the disordered compounds. All the hydrogen atoms associated with the carboxylic oxygen of the compounds were located from the difference electron density map and allowed to ride on the parent atom, whereas the hydrogen atoms bound to the N-atom identified from the difference map were restrained to a suitable distance with a s.u. of 0.01 Å. The hydrogen atoms associated with all the carbon atoms were placed in the calculated positions and refined using the riding model. Also in compound 5, during the early stages of refinement, the ethoxy phenyl moiety of the cation exhibits positioned disorder with respect to the hydrogen DL-malate, which exhibits configurational disorder. The disorder was modeled using two sets of atomic sites whose arbitrary positions were initially identified from the difference-electron density map. The C–N bonds of the cations share the same sites of the positionally disordered major and minor components, which are 0.70 and 0.30, respectively. The major and minor components of the disordered DL-hydrogen malate anion within the selected asymmetric unit show S and R configurations at the stereogenic carbon atoms C3 and C3′, respectively. The bond lengths of the anions of both the components were made equal using suitable similarity restraints with a s.u. of 0.02 Å, while in the cations, the aromatic C–C bond distances were 1.39 (0.02 Å) and C–C = 1.52(2) Å and C–O = 1.45(2) Å. In addition, the anisotropic displacement parameters of both the components corresponding to partial occupancy atoms occupying essentially the same region of space were made equal using suitable similarity restraints with an effective s.u. of 0.02 Ų. Rigid bond restraints were applied for the disordered components to make the adjacent atom move in the connecting bond direction, subject to the condition that the refined occupancies of both components are 0.70 and 0.30, respectively. All hydrogen atoms were located from electron density map regions apart from those in the minor occupancy fragments, which were included in the calculated positions. All the hydrogen atoms bonded to the carbon atoms were then treated as riding atoms in the geometrically idealized positions, with C–H distances of 0.093 (aromatic), 0.96 (CH₃) and 0.97 (CH₂). For all the other compounds, the refinements of the exact positions of the hydrogen atoms were judged from the appearance of strong difference-electron density peaks near the O atoms and the carbonyl C–O distances with the residual index values. The ORTEP diagrams of compounds 1 to 5 are given in ESI Fig. 1–5.†

Table 1 Crystal structures and refinement parameters of compounds 1 to 5

Molecular formula	C10H18N4O4	C22H30N2O8	C22H30N2O10	C10H14N2O5	C12H17NO6
Mr	258.28	450.48	482.48	242.23	271.27
Crystal size (mm^3)	0.30 × 0.20 × 0.20	0.30 × 0.20 × 0.20	0.25 × 0.25 × 0.30	0.35 × 0.30 × 0.25	0.25 × 0.25 × 0.20
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Triclinic
Space group	P21/c	P21/c	P2(1)/n	P2(1)/c	P1̄
A (Å)	6.0011(3)	9.3856(4)	14.320(2)	10.5893(8)	5.9736(3)
B (Å)	18.2772(10)	11.0691(6)	9.6613(13)	14.2294(11)	7.5564(4)
C (Å)	11.7435(7)	21.5537(10)	17.751(3)	7.4272(4)	16.1174(7)
α (°)	90	90	90	90	81.600(2)
β (°)	99.398(2)	92.617(3)	96.215(5)	94.524(3)	81.012(2)
γ (°)	90	90	90	90	70.7390(10)
Volume (Å^3)	1270.78(12)	2236.88(19)	2441.4(7)	1115.64(13)	674.87(6)
Z	4	4	4	4	2
ρcalcd (mg m^−3)	1.350	1.338	1.313	1.442	1.335
μ (MoKα) (mm^−1)	0.105	0.102	0.104	0.117	0.108
F (000)	552	960	1024	512	288
T (K)	296(2)	296(2)	293(2)	293(2)	293(2)
Range of indices	−7,4; −21,21; −13,13	−11,11; −13,13; −26,26	−19,19; −12,12; −23,23	−12,12; −16,16; −8,8	−7,7; −9,9; −20,20
Reflections collected	12 113	20 104	35 202	10 306	14 420
Unique	2235	4164	6191	1862	3087
θ range for data collection (°)	2.08 to 24.99	2.07 to 25.50		2.40 to 24.49
R (int)	0.0249	0.0297	0.0422	0.0337	0.0226
Goodness of fit	1.068	1.094	1.071	1.075	1.070
Final R indices [I > 2sigma(I)]	0.0554	0.0499	0.0582	0.0516	0.0351
wR2 [I > 2sigma(I)]	0.1447	0.1401	0.1433	0.1402	0.0911
R indices (all data)	0.0672	0.0625	0.1090	0.0633	0.0450
wR2 (all data)	0.1568	0.1485	0.1832	0.1488	0.0964

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Table 2 Geometrical parameters for hydrogen bonds in compounds 1 to 5^a

Compound	Bond (symmetry)	dD–H (Å)	dH⋯A (Å)	dD⋯A (Å)	<D–H⋯A (°)
a Symmetry codes: (i) −x + 2, y − 1/2, −z + 3/2 (ii) −x + 1, −y + 1, −z + 1 (iii) x + 1, y, z (iv) x, −y + 3/2, z + 1/2 (v) −x + 1, y + 1/2, −z + 3/2;^1 (i) x + 1, y − 1, z (ii) x + 1, y, z (iii) −x + 1, y + 1/2, −z + 1/2 (iv) −x + 1, y − 1/2, −z + 1/2 (v) x, −y + 1/2, z − 1/2 (vi) −x + 2, −y, −z + 1;^2 (i) x − 1/2, −y + 3/2, z − 1/2 (ii) −x + 1/2, y − 1/2, −z + 1/2 (iii) x, −y + 1, −z (iv) x, y − 1, z;^3 (i) x, y + 1, z (ii) x − 1, y, z (iii) x + 1, y, z (iv) x + 1, y − 1, z;^4 (i) −x + 2, −y + 1, −z + 1 (ii) −x + 2, y − 1/2, −z + 3/2 (iii) x, y, z + 1.^5
Compound 1	O(4)–H(4)⋯N(1)^i	0.82	1.85	2.659(4)	169.1
	C(8)–H(8A)⋯O(3)^ii	0.97	2.55	3.465(5)	158.3
	C(10)–H(10B)⋯N(2)^iii	0.97	2.65	3.570(3)	159.3
	C(6)–H(6B)⋯N(4)^iv	0.97	2.60	3.505(3)	155.5
	C(7)–H(7A)⋯O(3)^v	0.97	2.66	3.502(7)	145.2
	O(1)–H(1)⋯N(3)	0.82	1.88	2.695(3)	178.7
Compound 2	O(1A)–H(1A)⋯O(4B)^i	0.82	1.67	2.480(2)	171.3
	N(1B)–H(1B3)⋯O(3B)^ii	0.89	1.82	2.703(2)	174.8
	N(1A)–H(1A1)⋯O(1A)^iii	0.89	2.13	2.940(2)	151.1
	N(1A)–H(1A2)⋯O(2B)^iv	0.89	1.91	2.793(3)	173.9
	N(1A)–H(1A3)⋯O(4A)^v	0.89	1.88	2.754(2)	166.6
	N(1B)–H(1B1)⋯O(2A)^vi	0.89	2.00	2.863(3)	163.6
	O(3A)–H(3A)⋯O(1B)	0.82	1.67	2.474(2)	168.5
	N(1B)–H(1B2)⋯O(3A)	0.89	2.26	3.001(2)	140.3
	N(1B)–H(1B2)⋯O(2B)	0.89	2.40	3.084(2)	133.4
Compound 3	O(1)–H(1)⋯O(8)^i	0.82	1.66	2.475(2)	176.4
	O(5)–H(5)⋯O(3)	0.82	1.68	2.487(2)	166.5
	N(1)–H(1A)⋯O(6)	0.89	2.27	2.742(3)	112.7
	N(1)–H(1B)⋯O(8)^ii	0.89	2.26	2.824(3)	121.4
	N(1)–H(1C)⋯O(2)^iii	0.89	2.58	3.323(3)	140.9
	N(2)–H(2C)⋯O(2)	0.89	2.08	2.727(3)	128.5
	N(2)–H(2C)⋯O(4)^iv	0.89	2.37	2.820(3)	111.4
	N(2)–H(2D)⋯O(5)^iv	0.89	2.21	3.091(3)	173.2
	N(2)–H(2E)⋯O(7)^ii	0.89	2.07	2.759(3)	133.6
Compound 4	N(1)–H(1A)⋯O(2)^i	0.922(18)	1.815(19)	2.722(3)	168(3)
	N(1)–H(1B)⋯O(3)	0.904(18)	1.986(19)	2.877(3)	168(3)
	N(1)–H(1B)⋯O(5)	0.904(18)	2.41(3)	2.955(3)	119(2)
	N(1)–H(1C)⋯O(2)^ii	0.906(18)	2.16(2)	2.999(3)	155(3)
	N(2)–H(2C)⋯O(2)^ii	0.916(19)	2.12(2)	3.029(3)	170(3)
	O(4)–H(4)⋯O(1)^iii	0.82	1.71	2.528(2)	171.6
	O(5)–H(5)⋯O(1)	0.82	2.02	2.688(3)	138.2
Compound 5	O(3)–H(3A)⋯O(2)^i	0.82	1.76	2.577(8)	176.2
	O(5)–H(5A)⋯O(1)	0.82	2.18	2.654(10)	116.9
	O(5)–H(5A)⋯O(4)^ii	0.82	2.41	2.977(9)	126.7
	N(1)–H(1A)⋯O(2)	0.932(13)	1.881(14)	2.807(4)	171.9(13)
	N(1)–H(1B)⋯O(1)^iii	0.950(12)	1.781(15)	2.705(8)	163.5(13)
	N(1)–H(1C)⋯O(5)^iv	0.896(13)	1.936(14)	2.787(5)	157.8(13)

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3.2. Hexamethylenetetramine succinic acid

The co-crystal of succinic acid with hexamethylenetetramine crystallizes in the monoclinic system with the P2₁/c space group. Succinic acid interacts with only two nitrogen atoms of hexamethylenetetramine on either side through O–H⋯N hydrogen bonds. The O1–H1⋯N3 and O4–H4⋯N1 hydrogen bonds between the acid and base form a C₂²(12) motif which repeats to form an infinite acid–base chain constrained to the 2₁ screw axis along the [010] direction at (0 y 1/4), as shown in Fig. 1. In the chain, the neighboring succinic acid molecules lie parallel to each other, whereas the pendant hexamethylenetetramine moieties form an infinite one dimensional zig–zag chain that runs along the b axis. Further, the chains stack on top of each other, forming a layer through C8–H8A⋯O3 (C⋯O = 3.465 Å) interactions and generating a two dimensional supramolecular sheet along the (001) plane, as shown in Fig. 2. Of the four nitrogens in hexamethylenetetramine, two are involved in strong hydrogen bonding linkages with succinic acid on both sides, and the other nitrogen atoms participate in weak C–H⋯N interactions, forming molecular sheets. The sheets are stacked parallel to the (−1 0 2) plane via intermolecular C7–H7A⋯O3 and C6–H6B⋯N4 interactions with distances of C⋯O = 3.502 Å and C⋯N = 3.505 Å, respectively. The carboxylate oxygen O3 of succinic acid undergoes bifurcated C7–H7A⋯O3 hydrogen bonds that interlink the adjacent sheets, forming a 3D supramolecular network, as shown in Fig. 3. Meanwhile, C10–H10B⋯N2 joins the acid and base components within the sheet; although the sheets stack parallel to each other, slippage exists between the sheets.

Fig. 1 Part of the crystal structure of 1, showing the formation of acid–base chains through O–H⋯N hydrogen bonds constrained along the 2₁ screw axis.

Fig. 2 Representation of two dimensional supramolecular sheet formations through O–H⋯N and C–H⋯O hydrogen bonds in compound 1 parallel to the (001) plane. The hexamethylenetetramine molecules are shown as space filled representations.

Fig. 3 Three dimensional network formation through O–H⋯N, C–H⋯O and C–H⋯N hydrogen bonds in compound 1.

3.3. Bis(3-methylanilinium) succinate succinic acid

The asymmetric unit of the crystal contains two molecules of protonated 3-methylaniline, one molecule of succinate dianion and an unprotonated succinic acid molecule. In this compound, the succinic acid–succinate anions are arranged alternatively along the chain in a DAAD pattern with a graph set notation of C₂²(14) through intermolecular O3A–H3A⋯O1B and O1A–H1A⋯O4B hydrogen bonds (O⋯O = 2.4734 (18) and 2.4795 (18) Å; O–H⋯O = 168.6° and 171.3°) forming syn–syn contacts which extend as an anionic chain along the b-axis (Fig. 4). In the chain, both the unionised and the diionic acids are arranged perpendicularly with a bond angle of (85.93 (6)°), forming a twisted chain, probably due to steric repulsion of the carboxylate-carboxyl oxygen atoms. From the torsion angle measurement, it is found that both the succinic acid and succinate molecules exist in the anti conformation. The carboxyl O–H of the acid molecule and the N–H of the 3-methylanilinium cation act as donors, while the succinate molecule carbonyl and carboxylate oxygens act as acceptors, thus forming the supramolecular motif R₂²(6). Each cation forms four N–H⋯O hydrogen bonds, including N1B⋯H1B2⋯O2B, N1B⋯H1B3⋯O3Bⁱⁱ [symmetry code: (ii) x + 1, y, z], N1Aⁱⁱ–H1A2⋯O2B [symmetry code: (ii) –x + 1, –y + 1, –z + 1] and N1Aⁱⁱ–H1A1⋯O1Aⁱⁱⁱ [symmetry codes: (ii) –x + 1, –y + 1, –z + 1; (iii) –x + 1, y + 1/2, –z + 1/2] hydrogen bonds with adjacent carboxylate and carboxylic molecules. Two 3-methylanilinium cations form an amide⋯amide homodimer synthon, generating a rectangular grid, by interacting with carboxylate oxygen atoms through an R₂⁴(8) ring motif. Each diionic succinate molecule in the chain interacts with four 3-methylanilinium cations, whereas each succinic acid molecule interacts with two 3-methylanilinium cations through two N–H⋯O interactions via N1A–H1A3⋯O4A^v and N1B–H1B1⋯O2A^vi [symmetry codes: (v) x, −y + 1/2, z − 1/2; (vi) −x + 2, −y, −z + 1]. In this structure, the carbonyl oxygens of succinate dianions O2B and O3B form bifurcated hydrogen bonds with 3-methylanilinium cations through N–H⋯O interactions; thus, the 3-methylanilinium cation interlinks three anionic chains. The adjacent chains are interconnected with each other via N–H⋯O interactions with bridged anilinium cations (Fig. 5), forming an extended molecular sheet along the (001) plane. The molecular sheet is interconnected centrosymmetrically by an R₄⁴(18) motif through N–H⋯O hydrogen bonds, as depicted in Fig. 6a and b.

Fig. 4 The space filled representation of the succinate–succinic acid one dimensional network extending along the [001] direction formed through O–H⋯O hydrogen bonds in compound 2.

Fig. 5 Part of the structure of 2, showing the formation of O–H⋯O and N–H⋯O hydrogen bonds between the 3-methylanilinium succinate ions and succinic acid (the hydrogen atoms not involved in the hydrogen bonding were removed for clarity).

Fig. 6 (a) The inversion related (001) hydrogen bonded sheets, depicted as yellow and violet, in compound 2. The N–H⋯O hydrogen bonds connecting the sheets are shown in red (the hydrogen atoms not involved in bonding were removed for clarity). (b) The space filled representation of the inversion related ionic sheets in compound 2, viewed down the a-axis.

3.4. Bis(4-methoxyanilinium hydrogen succinate)

The asymmetric unit of this title compound contains protonated 4-methoxy anilinium cations and monoionised succinate molecules in a ratio of 2 : 2. From the torsion angles, the carbon backbones of the succinic acids confirm that one of the hydrogen succinate molecules is almost planar (C1–C2–C3–C4 = −164.4(2)°), whereas the other is completely bent or twisted (C5–C6–C7–C8 = 61.0(3)°). The hydrogen succinate monoanions are connected by two strong O–H⋯O hydrogen bonds, forming a zigzag infinite chain along the (101) direction, as shown in Fig. 7. The two succinate monoanions are arranged alternatively along the chain in an ADAD pattern with a graph set notation of C₂²(14) through syn–syn contacts. The O1 atoms of the linear succinate anions undergo bifurcated hydrogen bonding with the bent hydrogen succinate anions, thereby interlinking antiparallel chains through C6–H6A⋯O1 hydrogen bonds, generating 2D anionic sheets. The 4-methoxyanilinium cations generate a rectangular grid by interacting with the carboxylate oxygen atoms of parallel anion chains through four N–H⋯O hydrogen bonds (N1–H1A⋯O2, N1–H1B⋯O4, N2–H2A⋯O5 and N2–H2C⋯O2), forming a R₃⁵(12) ring motif. In addition, two N–H⋯O (N1–H1C–O6 and N2–H2B⋯O7) hydrogen bonds connect the antiparallel chains to the parallel chains. Thus, the 4-methoxyanilinium cations play a major role in holding the chains together with six N–H⋯O hydrogen bonds, generating a two dimensional molecular network. It should be noted that the inversion related cations are pendant on both faces of the molecular network, as shown in Fig. 8. The O1 and O2 atoms of the linear succinate anion and the O5 oxygen atom of the bent or twisted succinate anion are involved in bifurcated interactions with neighbouring molecules. Hence, the linear succinate monoanion acts as a single hydrogen bond donor and a fivefold hydrogen bond acceptor.

Fig. 7 Part of the crystal structure of 3, showing the formation of an anion–anion chain through O–H⋯O hydrogen bonds constrained along the 2₁ screw axis.

Fig. 8 Two dimensional network formation through O–H⋯O, N–H⋯O and C–H⋯O hydrogen bonds with cations pendant on both sides in compound 3.

3.5. 2-Aminoanilinium hydrogen malate

In compound 4, the asymmetric unit consists of one molecule of hydrogen malate anion and one molecule of 2-aminoanilinium cation. The amino group (NH₂) at the ortho position acts as a hydrogen bond donor to the respective acceptor oxygen atom. The hydrogen malate anions form a one dimensional head to tail chain linked through O4–H4⋯O1 hydrogen bonds extending along the [001] direction, forming a C(7) motif. The adjacent anionic chains are bridged by the 2-aminoanilinium cations through three N–H⋯O hydrogen bonds, such as N1–H1B⋯O3, N1–H1B⋯O5 and N1–H1C⋯O2. The nitrogen atom (N1) of the NH₃⁺ donates the H1B hydrogen, forming a bifurcated hydrogen bond with the acceptor oxygen atoms O3 and O5, whereas the oxygen atoms O2 at (−x + 2, y − 1/2, −z + 3/2) act as bifurcated acceptors with the hydrogen atom donors H2C (of the NH₂ group) and H1C (of the NH₃⁺ group) from the 2-aminoanilinium cations, forming R₂¹(5), R₁²(7) and R₆⁶(25) motifs. This self assembly of anions and cations forms two dimensional ripple-like sheets extending parallel to the (001) plane (Fig. 9). Adjacent inversion related (001) supramolecular sheets are connected through the N–H⋯O hydrogen bond N1–H1A⋯O2, which resembles an intertwined two dimensional supramolecular framework extending parallel to the (001) plane; Fig. 10 illustrates the molecular self-assembly, which resembles an interwoven architecture.

Fig. 9 Part of the crystal structure of 4, showing the formation of anion–cation two dimensional ripple sheets formed through N–H⋯O hydrogen bonds along the (001) plane.

Fig. 10 Part of the crystal structure of 2-AAHM, showing an intertwined two dimensional supramolecular framework extending parallel to the (001) plane. The two different two dimensional sheets are represented in different colours.

3.6. 4-Ethoxyanilinium hydrogen malate

Here, the hydrogen bonds generated from the minor components are excluded from the discussion. The asymmetric unit consists of one molecule of hydrogen malate anion and one molecule of 4-ethoxyanilinium cation. The carboxyl (COOH) and carboxylate groups are linked head to tail through O3–H3A⋯O2 hydrogen bonds to form a C(7) chain motif which extends along the [010] direction, generating a one dimensional chain, and the hydroxyl groups that form O5–H5A⋯O4 hydrogen bonds with the carboxyl oxygen atom O4 form a C(6) chain motif which further extends infinitely along the [100] direction orthogonal to the propagation of the C(7) chain motif, as shown in Fig. 11. The [010] and [100] chains form a two dimensional anionic supramolecular grid built by a R₄⁴(22) ring motif parallel to the (001) plane, as shown in Figure12a. The 4-ethoxyanilinium cations are anchored with the anionic grid through three N–H⋯O hydrogen bonds N1–H1A⋯O2, N1–H1B⋯O1 and N1–H1C⋯O5, as shown in Figure12b, and the self-assembly of anions and cations through N–H⋯O and O–H⋯O hydrogen bonds result in a two dimensional supramolecular sheet parallel to the (001) plane. The adjacent anionic-cationic sheets are separated by a distance equal to the length of the c-axis, whose stacking down the [001] direction constitutes the packing of the crystalline solid.

Fig. 11 Part of the crystal structure of 5, showing the formation of an anion–anion chain through two different types O–H⋯O hydrogen bonds.

Fig. 12 (a) Part of the crystal structure of 4EAHM, showing the formation of a two dimensional anionic sheet built from a R₄⁴(22) ring motif extending along the (001) plane. (b) Part of the crystal structure of 4EAHM, showing the formation of a two dimensional molecular sheet with cations trapped between the anions.

3.7. CSD analysis

Analysis of reported succinate and succinic acid structures from CSD shows the salient features of the salt and co-crystal formation. Separate searches were performed for co-crystals and salts (includes the addition of neutral acid molecule) using CSD Version 5.31.²² 64 structures were found for succinic acid co-crystals. Viewing the C–C–C–C skeleton of the succinic acid moiety, most of the structures exhibit planar conformations, while 6 structures have twist conformations. It is of interest to note that the succinic acids with planar structures are constrained to a special crystallographic position where the central C–C bond lies in the centre of inversion. As observed in 1, the formation of O–H⋯N hydrogen bonds is a common feature, with the acid moiety interlinking the base (Ref codes: GAWLOG, RESGAY, RESGIG, RESGOM, RESHAS, RESHIH, RETZEW, SOSBAD and VIGDEV), forming zero to one dimensional networks. The structures with head to tail ring nitrogens act as O–H⋯N hydrogen bond acceptors and form acid–base one dimensional networks, as observed in compound 1 (Ref codes: CUJMIE, GALBIF, JAZBES, JEDLAG, LATLEY, TACCIL, UMINOT, UNEFEY, VOQBOT and XUBVEW). Searches for succinate salts gave a list of 122 structures, which include metal complexes (36) and solvates (42). The search results included 7 structures of succinate (di-ion), 24 structures of hydrogen succinate (mono-ion) and 8 structures of succinate with a neutral succinic acid. In the reported structures of phenylethylanilinium succinate (Refcode: COCPIU)²³ and ethyl ammonium hydrogen succinate (Refcode: KIXJIK),²⁴ the adjacent ionic chains are interlinked by cations through strong N–H⋯O hydrogen bonds which form two dimensional sheets of ionic molecules. As observed in 2 and 3, the adjacent sheets are inversely related to each other, with the cations pendant on both faces. In all the structures with succinate/succinic acid molecules, HOLNIG, HOLNIG01, IHESOD, ISUTEV, KOHPOM, MOSMIR and PUTFAL, the acid-ion moieties are connected through syn–anti contacts, except for KIXJIK, which shows syn–syn contact. Interestingly, from the reported structures, the cations with more than one hydrogen bond at the NH⁺ donor sites exhibit steric effects with the acid-ion contact, leading to a syn–anti contact, whereas in the structures with lone NH₃⁺ donor sites, the acid ion linkages show syn–syn contacts, as formed in 2 and 3.

Systematic examination of 45 malate crystals on CSD reveals that the hydrogen malate structures AMHMAM, CECPOP02, CECPOP03, EMIMAO, EYOBAV, NOJWEO, NOJWIS, NOJWOY, RUPVER, VECJOD and YETMAL are reported to have NH₃⁺ substituted cations. As a consequence of the presence of NH₃⁺ substitution in the cations, these structures exhibit similar supramolecular networks to those observed in (4) and (5). In the crystal structure of L-histidinium hydrogen L-malate (CSD ref code: REJZUC²⁵), it is interesting that even in the presence of NH₃⁺ substitution in the cations, the anions form a one dimensional substructure via hydrogen bonds between the hydroxyl hydrogens and the carboxylate oxygens. Also, the presence of additional hydrogen bonding sites in the cation interrupts the formation of the preferred two dimensional anionic networks, as in (4). It is understood from these observations that the anions form acid–acid and acid-ion networks of their own, depending on the type of interaction they undergo with the counter ions.²⁶

3.8. Optimized geometry

The molecular geometry of all the compounds was optimized by DFT/B3LYP using 6-31g(d,p) calculations to confirm the molecular structure in the ground state. The compounds were optimized to the global energy minimum level, which is listed in the ESI.† Table 1a and b† confirm that the XRD molecular conformations are very close to the calculated molecular structures. Small disagreements in the geometrical values were noticed between two calculations. The maximum bond lengths and angles differ by 0.05 Å and 5°, respectively, compared to the XRD values. DFT calculations were performed in the gaseous state and the geometry is also optimized freely without any restriction, whereas the experimental method involves the crystalline state, in which the crystal packing field and other non-covalent interactions play dominant roles.

3.9. Thermal analysis

The thermal behavior of the compounds was studied through thermogravimetric analysis, which provides information regarding the phase change and decomposition of the crystal system. The DTA and TGA were carried out using a Sii Nano instrument (TG/DTA 6300) at 20 °C min⁻¹ in nitrogen atmosphere. It can be observed from the DTA curves that all the compounds undergo melting with complete decomposition. The salts and the co-crystals of succinic acid undergo endothermic decomposition. In all the compounds, there is no loss below 100 °C; hence, the crystal reflects solvent molecules during crystallization; also, this indicates that the compounds are moisture free. The DTA curve of compound 1 (Fig. 13a) shows two endothermic peaks at 159 °C and 213 °C, which are attributed to the melting of the sample. The endothermic peak at 159 °C represents the melting of the compound. The compound undergoes complete decomposition above 350 °C. The TGA of 3-methylanilinium succinate succinic acid(II) showed three steps of decomposition (Fig. 13b). From the DTA curve, the endothermic peaks at 134 °C, 178 °C and 260 °C represent the melting of the compounds. The major peak at 134 °C is the melting point of the crystal, as the major weight loss occurs after that point. Compound 3 showed two steps of weight loss decomposition; the endothermic peaks at 119 °C and 241 °C represent the melting of the sample (Fig. 13c). Compound 4 showed two steps of weight loss decomposition; the endothermic peak at 145 °C represents the melting of the compound, and the peak at 256 °C represents the complete decomposition of the compound (Fig. 13d). Compound 5 also showed two steps of decomposition; from the DTA curve, the endothermic peak at 161 °C is attributed to the melting of the compound, and it undergoes decomposition at around 300 °C (Fig. 13e). It is noted that compound 1 undergoes complete decomposition up to 1043 °C, which may be due to the presence of hexamethylenetetramine; compounds 2 and 5 undergo decomposition up to 600 °C, while compounds 3 and 4 undergo complete decomposition up to 250 °C. Comparing the thermal stabilities, compounds 2 to 5 sustain stability up to 100 °C, while compound 1 showed gradual weight loss from the beginning, possibly due to the presence of fewer strong and weak hydrogen bonds compared to the other compounds.

Fig. 13 TG and DTA curves of compounds 1 to 5.

3.10. FT-IR spectroscopy results

FT IR spectra were recorded at ambient temperature using a Perkin Elmer Spectrum One FT IR spectrophotometer; the spectra were obtained over the range of 4000 to 450 cm⁻¹ using KBr pellets. For compound 1 (Fig. 14a) the broad band observed at 3418 cm⁻¹ is due to the presence of free O–H groups. The overlapping peaks between 2964 and 2383 cm⁻¹ are due to CH and NH stretching. The peak at 1643 cm⁻¹ is due to the C=O group of the acid. The C–O stretch peak at 1308 cm⁻¹ and bending at 1440 cm⁻¹ confirm the presence of the –COOH group. This confirms the formation of co-crystals between hexamethylenetetramine and succinic acid. In the spectrum of compound 2 (Fig. 14b), the peak at 2927 cm⁻¹ corresponds to NH₃⁺ in-plane symmetric stretching, and the peak at 2622 cm⁻¹ corresponds to NH₃⁺ symmetric stretch out of plane vibrations. The vibration peak at 1539 cm⁻¹ is due to NH₃⁺ asymmetric bending, the peak at 1711 cm⁻¹ represents the presence of the C=O group of the saturated free acid, and the peaks at 1613 and 1386 cm⁻¹ correspond to COO– stretching. The multiple bands between 1120 and 946 cm⁻¹ are due to the in-plane aromatic C–H bending vibrations. Thus, the spectrum establishes the presence of NH₃⁺ groups due to the protonation of amine groups and the presence of free acid. In compound 3 (Fig. 14c), the broad peak over the range of 3500 cm⁻¹ to 2500 cm⁻¹ with multiple peaks corresponds to the O–H and N–H stretching vibrations. The peaks of C=O and C–O stretching shifted lower to 1628 cm⁻¹ and 1252 cm⁻¹, which is attributed to the presence of the carboxylate ion; this confirms the protonation of the compound. The peak at 1503 cm⁻¹ corresponds to the N–H bending of the cation. The peak observed at 1454 cm⁻¹ represents the C–O–H plane bending vibration. In compound 4 (Fig. 14d), the peaks at 3425 cm⁻¹ and 3221 cm⁻¹ represent the stretching vibrations of the O–H groups of malic acid. The peaks at 3010 cm⁻¹ and 1572 cm⁻¹ are due to the N–H stretching and bending of the of NH₂ and NH₃⁺ cations. The peaks at 1695 cm⁻¹ and 1283 cm⁻¹ correspond to the C=O and C–O stretching vibrations. In compound 5 (Fig. 14e), the sharp peak at 3447 cm⁻¹ corresponds to the presence of the alcoholic O–H of malic acid. The broad peak over the range of 3100 cm⁻¹ to 2500 cm⁻¹ corresponds to the O–H and N–H stretching vibrations. The peak at 1997 is attributed to the presence of aromatic C–H bending vibrations. The peaks at 1710 cm⁻¹ and 1307 cm⁻¹ correspond to the C=O and C–O stretching vibrations of the acid. The peak at 1574 is attributed to the N–H bending of the cation. The shifts in the frequencies of the functional groups in all the compounds manifest their involvement in hydrogen bonding.

Fig. 14 FTIR spectra of compounds 1 to 5.

4. Hirshfeld surface analysis

Hirshfeld surface (HS) analysis is an approach for the graphical visualization of molecular contacts in colored contouring with 2D fingerprints (FP) to simultaneously divulge the full distribution of the involved interactions.

The intermolecular interactions of all the molecular structures (1 to 5) were quantified using Hirshfeld surface analysis and were mapped over d_norm values (Table 3), as shown in Fig. 15. The relative contributions of various interactions from the fingerprint plots of compounds 1 to 5 are presented in Fig. 17, which clearly exhibits the modes of interaction that exist in the crystal structures. Comparison of the atom⋯atom contacts showed that the value of O⋯H is considerably greater for malate crystals than for other contacts. The fingerprint plot of 1 shows two characteristic spikes; the sharper one provides evidence for C–H⋯N hydrogen bonds, while the weaker spike represents O–H⋯O hydrogen bonds.

Table 3 Range of d_norm values mapped over the Hirshfeld surfaces

Compound	de	di
HMTSA	−0.736	1.505
3-MASSA	−0.861	1.368
4-MAHS	−0.870	1.456
2-AAHM	−0.821	1.180
2-EAHM	−0.790	1.460

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Fig. 15 Hirshfeld surfaces of compounds 1 to 5, showing the possible interactions with neighbouring molecules.

Fig. 16 Fingerprint plots of compounds 1 to 5, showing full and resolved contacts.

Fig. 17 Bar chart showing the contributions of the percentages of contacts in the Hirshfeld surfaces of compounds 1 to 5.

From Table 4 and Fig. 16, it is clear that the maximum contribution is derived from O⋯H contacts (40%) due to O–H⋯O and N–H⋯O interactions. The next highest contribution is from H⋯H contacts (30%) due to interaction between substituent groups. Reasonable contribution from H⋯O contacts (15%) accounts for the presence of C–H⋯O interactions. The remarkable contribution of the H⋯N contact of compound 1 clearly depicts the presence of C–H⋯N interactions. The favourable contributions from C⋯H, H⋯C, O⋯C, O⋯O, C⋯C, and C⋯O contacts correspond to the existence of relatively weak interactions.

Table 4 Comparison of various atom⋯atom contacts predicted from FP plots

Contacts	HMTSA	3-MASSA	4-MAHS	2-AAHM	2-EAHM
O⋯H	39.8	39	45.6	49.4	48.9
H⋯H	41.1	38.4	31.5	30.4	30.4
H⋯O	8.6	16.2	15.2	13.1	14.8
C⋯H	2.3	2.5	2	2.1	2.5
H⋯C	0.1	1.6	1.6	1.2	0.6
O⋯C	0.2	0.4	1.1	1.2	0.3
O⋯O	1.2	0.5	0.8	1.1	2
C⋯C	0	0.6	0.7	0.2	0.3
C⋯O	0.2	1.5	1.6	1.2	0.2
H⋯N	6.4	0	0	0	0

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5. Pixel energy calculations

Calculations of the total energy of organic crystals using the pixel method have been reported by several research groups. Most of these studies showed that acid base salts exhibit repulsive destabilizing interactions due to the presence of polar groups. The intermolecular energies of molecular pairs (from the .mlc file) involved in the crystal packing, along with the symmetries of all the compounds, are listed in Table 5. In this study, the obtained values agree well with the reported results. The centroid–centroid distance of the interacting molecules of all the compounds ranges from 5 to 7 Å. It is noted that the repulsion energy is almost less than the sum of the other energies. The attractive energies are larger in magnitude and contribute to the stability of the crystals. It has been observed that the energy values of the O–H⋯O interactions are greater than all the other hydrogen bonds in all the structures, which signifies the major contribution of this interaction to the stability of the crystal packing.

Table 5 Total intermolecular interaction energies calculated from PIXEL energy calculations in kJ mol⁻¹

	Cd⋯Cd distance	Ecoul	Epol	Edisp	Erep	Etot	Symmetry
Compound 1
C(10)–H(10B)⋯N(2)	6.808	−3.6	−1.4	−5.1	4.3	−5.8	x + 1, y, z
C(8)–H(8A)⋯O(3)	5.581	−7.8	−2.9	−13.1	7.8	−16.0	−x + 1, −y + 1, −z + 1
C(6)–H(6B)⋯N(4)	6.107	−3.1	−0.4	−6.8	1.2	−9.0	x, −y + 3/2, z + 1/2
C(7)–H(7A)⋯O(3)	6.345	−4.1	−1.0	−6.7	2.5	−9.1	x + 1, y + 1/2, −z + 3/2
O(4)–H(4)⋯N(1)	6.790	−69.9	−33.4	−22.0	83.2	−42.1	−x + 2, y + 1/2, −z + 3/2
Compound 2
O(1A)–H(1A)⋯O(4B)	7.572	−94.3	−47.8	−14.9	101.8	−55.2	x + 1, y − 1, z
N(1B)–H(1B3)⋯O(3B)	5.444	−99.2	−36.7	−19.7	238.4	82.8	x + 1, y, z
N(1A)–H(1A3)⋯O(4A)	7.112	−2.9	−0.5	−5.9	2.5	−6.7	x, −y + 1/2, z − 1/2
N(1B)–H(1B1)⋯O(2A)	5.860	−58.7	−23.3	−17.9	138.9	39.1	−x + 2, −y, −z + 1
O(3A)–H(3A)⋯O(1B)	7.305	−90.6	−47.8	−15.3	99.6	−54.1
N(1B)–H(1B2)⋯O(3A)	5.284	−19.6	−6.7	−15.4	53.0	11.4
N(1B)–H(1B2)⋯O(2B)	7.308	−14.4	−6.7	−9.8	41.2	10.2
Compound 3
O5–H5⋯O3^i	6.848	−110.4	−45.2	−14.9	103.1	−67.4	x − 1/2, −y + 3/2, z − 1/2
O1–H1⋯O8	6.458	−100.7	−39.7	−14.0	89.3	−65.1
N1–H1A⋯O6	5.568	−77.2	−43.4	−19.6	276.4	136.1
N1–H1B⋯O8^ii	5.806	−50.6	−10.2	−14.6	41.7	−33.6	−x + 1/2, y − 1/2, −z + 1/2
N1–H1C⋯O2^iii	6.237	−13.6	−2.0	−7.4	4.3	−18.7	x, −y + 1, −z
N2–H2C⋯O2	6.744	−45.7	−17.4	−14.3	95.0	17.6
N2–H2E⋯O7^ii	7.268	−65.8	−22.6	−13.7	113.3	11.2	−x + 1/2, y − 1/2, −z + 1/2
N2–H2C⋯O4^iv	6.886	−41.6	−45.0	−14.7	258.0	156.6	x, y − 1, z
N2–H2D⋯O5^iv	6.535	−5.5	−4.0	−11.1	14.7	−5.9	x, y − 1, z
Compound 4
O(3)–H(3A)⋯O(2)^i	7.556	−30.7	−15.9	−13.2	45.9	−13.8	x, y + 1, z
O(5)–H(5A)⋯O(4)^ii	5.974	−25.4	−5.3	−8.1	3.7	−35.2	x − 1, y, z
N(1)–H(1B)⋯O(1)^iii	6.976	−70.0	−23.7	−14.3	107.3	−0.8	x + 1, y, z
N(1)–H(1C)⋯O(5)^iv	5.837	−12.6	−20.9	−23.5	37.7	−19.3	x + 1, y − 1, z
Compound 5
N(1)–H(1A)⋯O(2)^i	6.719	−60.1	−30.8	−13.9	147.2	42.4	−x + 2, −y + 1, −z + 1
N(1)–H(1C)⋯O(2)^ii	6.005	−47.8	−25.2	−21.4	127.2	32.8	−x + 2, y − 1/2, −z + 3/2
O(4)–H(4)⋯O(1)^iii	7.427	−37.5	−22.9	−12.8	80.3	7.0	x, y, z + 1

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The lattice energy calculations of all the compounds are given in Table 6. As compound 2 and 3 consist of 4 molecules in the asymmetric unit (two anions and two cations), the PIXEL module calculations are not applicable in these cases; hence, five different molecular pairs were created, as shown in Fig. 12. Pair 1 consists of one A anion and one A cation, Pair 2 consists of one A anion and one B cation, Pair 3 consists of one succinic acid and one succinate anion, Pair 4 consists of one B anion and one B cation, and Pair 5 consists of one B anion and one A cation molecule. Compound 1 involves a single O–H⋯N interaction and has a greater stabilizing energy of −42.1 kJ mol⁻¹, whereas the C–H⋯O and C–H⋯N interactions are stabilized by the dispersion energy, contributing more than 75% to the total interaction energy, with energies ranging from −5.8 to −16.0 kJ mol⁻¹. It is noted that the C–H⋯O hydrogen bond dominated the C–H⋯N hydrogen bond and was found to contribute significantly to the crystal packing. In compound 2, all the interactions show greater repulsion, and their energy values were found to be less than 200 kJ mol⁻¹, which is quite common in molecular crystals. This may be due to the strong pull between the atoms involved in ring motif interactions. Despite the destabilization, the summation of other energies according to the “Madulong mode” contributes to the stability of the crystal. In compound 3, most of the hydrogen bonds were found to be stabilized by coulombic interactions, while a few are nearly repulsive, which indicates their interactions with twisted succinic acid. In compounds 4 and 5, although the repulsive energy is little high as compared to that of other compounds, coulombic interactions provided significant contributions to the stabilization, followed by polarization and dispersion energies. On the whole, the total interaction energies of the few N–H⋯O interactions are destabilizing in nature, which may be due to the deviation of the optimum N–H⋯O bond angles or their modes of interaction in the motifs. However, the crystals are greatly stabilized by attracting forces contributed by other N–H⋯O and O–H⋯O contacts.

Table 6 Total lattice energies calculated from PIXEL energy calculations in kJ mol⁻¹

	Ecoul	Epol	Edisp	Erep	Etot
Compound 1
	−88.0	−40.7	−74.7	−103.7	−99.6
Compound 2
Pair I	−23.9	−22.4	−35.4	76.6	−5.2
Pair II	−43.3	−17.0	−34.3	105.7	11.1
Pair III	−108.3	−48.2	−38.3	110.7	−84.0
Pair IV	−68.1	−25.5	−38.2	161.2	29.4
Pair V	−32.0	−19.2	−33.5	40.6	−44.1
Compound 3
Pair I	−117.0	53.5	−35.1	108.0	−97.8
Pair II	−61.4	−36.5	−30.5	188.5	60.1
Pair III	−53.9	−22.1	−32.1	127.2	19.1
Pair IV	−76.5	−30.9	−29.8	165.1	27.8
Pair V	−77.1	−36.2	−31.0	181.2	36.9
Compound 4
	−72.3	−62.0	−84.7	125.3	−93.8
Compound 5
	−127.4	−64.6	−88.8	289.6	8.7

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6. Conclusion

Succinic acid, being versatile in nature, demonstrated its ability to form co-crystals and salts with its complementary functional group. The interplay of strong hydrogen bonds, such as O–H⋯O, N–H⋯O and O–H⋯N hydrogen bonds, is dominant in the formation of one- and two-dimensional supramolecular frameworks in these solid-state structures. It is noted that weak hydrogen bonds of the type C–H⋯N and C–H⋯O in 1 were involved in the construction of supramolecular sheets, even though hexamethylenetetramine is the source of four hydrogen bond acceptors and three weak donors. In compounds 2 and 3, the protonated NH₃⁺ group participates in four N–H⋯O hydrogen bonds which interlink the anionic network to construct a two dimensional supramolecular sheet of anions and cations with the cation pendant on both faces of the sheet. The succinate anions of 2 and 3 form a one dimensional anionic scaffold through O–H⋯O hydrogen bonds; the carboxyl–carboxylate contact is syn–syn. It can clearly be observed that compound 1 does not form any carboxyl–carboxylate contacts. The adjacent anionic chains of 2 and 3 are anti-parallel to each other and are constrained to a crystallographic inversion centre. In contrast to this, in 1, both carbonyl hydrogens find their respective N acceptors, resulting in hydrogen bonds that form chains built of acid–base asymmetric units which are also constrained along the 2₁ axis with direction [010] at (0, y, 1/4). It is understood that in 1, although the number of strong hydrogen bond donor sites is fewer compared to compounds 2 and 3, the presence of weak hydrogen bonds gain importance in the construction of three dimensional networks. It is notable that the contribution of weak hydrogen bonds gains importance in the absence of strong hydrogen bonds. This indicates that malate salts with the presence of additional hydroxyl groups have greater chances of forming linear two dimensional sheets. In 4, the amino group substituted cations adjust themselves to be accommodated between the malate anionic layers, leading to puckered anionic networks, while in compound 5, the neighboring hydrogen malate chains are aligned parallel to each other, resulting in a planar anionic two dimensional layer. It is of interest to note the trigonal pyramidal molecular geometry of the NH₃⁺ group in the 4-ethoxyanilinium cations provides hydrogen bond donor sites to the selective acceptor oxygen atoms in the anionic network, by which the cations mutually adjust themselves to be accommodated in the anionic scaffolding, resulting in a flat anionic network. Hence, the presence of additional hydrogen bonding sites in the cations interrupts the formation of the preferred two dimensional anionic network, as in 5. In conclusion, it is observed that the formation of anionic two dimensional networks in both succinate and hydrogen malate salts are largely determined by the cation substitutions, and the anionic-cationic supramolecular networks can be controlled by the cations.

The occurrence of motifs in the supramolecular organization of these compounds exhibited the predictability of supramolecular architectures to some degree. The elevation of the melting points of the compounds elucidates the thermal stability of the structure due to the presence of strong and weak hydrogen bonds. The Hirshfeld surface analysis and fingerprint plots revealed the prevalence of the intermolecular interactions in the crystal structures. The theoretical PIXEL energy calculations expanded upon the various intermolecular potentials and their relative contributions to the stability of the structures.

Acknowledgements

The authors thank Dr. Babu Varghese and SAIF, IIT Madras for providing the intensity data collection facility and FTIR studies.

References

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Footnote

† Electronic supplementary information (ESI) available: The CCDC deposition numbers are CCDC 908167, 908168, 1485474 for the succinate salts and CCDC-1485475-1485476 for the malate salts. Observed and optimized molecular geometries of compounds 1 to 5 are shown in the supplementary tables. The ORTEP diagrams of compounds 1 to 5 are given in supplementary Fig. 1–5. CCDC 1485474–1485476. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra15683e

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