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. 2023 Aug 10;8(33):30776–30787. doi: 10.1021/acsomega.3c05132

π-Stacking among the Anthracenyl Groups of a Copper Complex Resulted in Doubling of Unit Cell Volume To Provide New Polymorphs

Abhay Pratap Singh 1, Jubaraj B Baruah 1,*
PMCID: PMC10448684  PMID: 37636968

Abstract

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Two polymorphs of the 9-N-(3-imidazolylpropylamino)methylanthracene (Hanthraimmida) containing hydrated copper(II)-2,6-pyridinedicarboxylate complex are reported. The two polymorphs have either lamellar or Herringbone arrangements of π-stacks among the anthracenyl groups of organocation. The difference between the two polymorphs originated from having face-to-face stacking arrangements between the two anthracenyl groups of the symmetry independent cations within the unit cell in one of the polymorphs. The π-stacked anthracenyl groups in consecutive layers of the polymorphs are oriented in one direction in the polymorph designated as P1, whereas the polymorph designated as P2 has such orientations in opposite directions. The unit cell volume of the polymorph P2 (Z = 4) has approximately twice the volume of the polymorph P1 (Z = 2); it happend due to coalescence of two unit cells of P1 in the ab-crystallographic plane. A mixed methanol/water solvate of the copper complex is also reported. It has a channel-like arrangement of the cations; has the anions and the solvents within the cation embraced channel-like enclosures. This complex is unstable, once taken out from the methanol solvent, it transforms in real time to P2 by replacements of the methanol molecules by water molecules.

1. Introduction

The π-stacking among aromatic units have profound interest in fundamental,1 biological,2 and material sciences.3 The magnitude of aromatic π-stacking interactions is very small (∼2 kJ mol–1).4 However, those contribute to design catalysts5 and magnetic,6 optical,7 high energy materials.8 Aromatic π-stacking in a self-assembly influences physical properties like conductance, proton transfer, etc.9 Herringbone arrangement is a term used for organized π-stacks and has broad dimensions for material design.10 The formations of Herringbone structures are guided by solvent,11 host–guest interactions,12 effect of substituents,13 and stimuli.14 Stacking among the phenyl rings occur in several ways such as edge-to-face, face-to-face, or oblique types, as illustrated in Figure 1a.15 It may be seen that the face-to-face parallel molecules either having eclipsed (Figure 1a (iii) or partly eclipsed (Figure 1a (vi)) geometries may be further arranged in different ways at translated positions in the case of eclipsed pairs, or as in a uni-directional manner (Figure 1a (vii)) or in bi-directions (Figure 1a (viii)). Further complicacies may be brought on them by arranging such layers as monolayers or bilayers or more and their combinations. Such self-assemblies may be extended infinitely or truncate as combination of different numbers of units. Two such possibilities by extending chains of parallel stacked anthracene rings in a unidirectional or bidirectional manner are shown in the Figure 1b (ix, x). Among the poly-aromatic compounds, various structural and packing arrangements of anthracene-based compounds are well studied.16 The stacking among anthracene rings largely influence the nature of self-assemblies17 and are reflected in the self-assemblies of metal complexes with anthracene-based ligands.18 The stacking arrangements of molecules differ from system to system and can result in different types of stacking arrangements, among which the Herringbone or lamellar structures have been known to provide independent properties originating from packing patterns.19 The spatial organization of stacks among aromatic planes controlled by a flexible part shows an effect on photo-luminescence.20 We felt that a compound having unsymmetrical aromatic group such as the anthracenyl-group will be suitable to explore stacks among anthracenyl groups which may adopt lamellar or Herringbone arrangements, as shown in ix and x of the Figure 1b. There are three important requirements to observe such stacking patterns; (a) face-to-face stacks among the planar rings, (b) inclined stacks in the same direction with respect to a vertical axis, (c) there must be organized, unidirection linear chains maintaining the slanting orientations to result lamellar structure or in the opposite directions to provide a Herringbone structure. These arrangements could lead to polymorphs but there has been no systematic approach available to obtain them, so we followed a design principle to organize stacks of π-components of a cationic part in layered anionic arrangements. For this purpose, we choose to study metal pyridine carboxylates having organic cation derived from 9-N-(3-imidazolylpropylamino)methyl anthracene (Hanthraimmida). The Hanthraimmida has a π-aromatic anthracenyl group appended though flexible linkers (Figure 1d). It is known to provide cocrystal with pyridine dicarboxylic acid,21 where the organo cations of Hanthraimmida shows symmetry nonequivalent forms. The literature suggests that symmetry-independent molecules provide polymorphs22 or synthon variations.23 It is also known that the π-stacked 2,6-pyridinedicarboxylate of a metal complexes having imidazolium cation form a layered structure.24 Furthermore, the anionic part from metal-2,6-pyridinedicarboxylate complexes assemble as dimer (Figure 1d) involving the stacks of chelated ligands, and they are also known to form anionic 1-D-layer-like arrangements.25 Hence, if the cationic part is derived from Hanthraimmida in such complexes, the anions will provide the required 1D arrangement to a set of anthracenyl group to form different stacking arrangements. The flexible tether to the anthracenyl unit would allow the anthracenyl unit to dictate the spatial orientations through interplay of weak interactions to provide polymorphs. With these backgrounds, we demonstrate here a pair of polymorphs that have lamellar and Herringbone arrangements of the anthracenyl groups and also an intermediate complex that provided the polymorph that has Herringbone arrangements.

Figure 1.

Figure 1

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(a) Different π-interactions caused by the position and orientations of the stacked species shown by taking benzene as an example, (b) two different types of arrangements among the anthracene rings in parallel but in a (ix) unidirectional and a (x) bidirectional inclined manner. (c) Design principle for lamellar and Herringbone arrangements of the cationic part in a layered anionic reference frame. (d) Structure of dication from Hanthraimmida and the chelate-chelate stacks between 2,6-pyrdinedicarboxylate in its metal complex anion.

2. Experimental Section

9-N-(3-Imidazolylpropylamino) methyl anthracene was prepared by the reported procedure.21

2.1. Synthesis, Characterization, and Intermediate Isolation

[(H3anthraimmida)Cu(26pdc)2]2·9H2O (P1 and P2): To a well-stirred solution of 2,6-pyridinedicarboxylic acid (33.42 mg, 0.2 mmol) and copper(II) acetate monohydrate (19.96 mg, 0.1 mmol) in analytical grade methanol (20 mL), Hanthraimmida (31.54 mg, 0.1 mmol) was added. The resulting solution was stirred for about 1 h and kept undisturbed for crystallization in open conditions, which yielded flake-type crystals. The crystals were hand-picked after decanting the supernatant solution. (Isolated yield: 80%). Anal. calcd for C70H76Cu2N10O25: C, 53.01%; H, 4.83%; N, 8.83%. Found: C, 52.70%; H, 5.60%; N, 8.87%. IR (cm–1): 3375 (br, w, νO–H), 1614 (s, νC=O), 1575 (s, νC=N). Decomposition pt. 220.1 °C. UV–vis: 386 and 790 nm.

The same reaction was carried out with higher concentrations of the reactants used in the polymorph P1 by keeping the volume of methanol the same (20 mL of methanol, and four times higher concentration of the reactants as that were used for preparation of P1) after stirring at room temperature for 12 h, provided a green precipitate. The precipitate formed was dissolved in water and the resulting solution was kept undisturbed for crystallization, under open air at 27 °C. After 5 days, the green crystals of P2 appeared, and these were collected by decantation. (Isolated yield: 75%). Decomposition pt. 257 °C. Anal. calcd for C70H76Cu2N10O25: C, 53.01%; H, 4.83%; N, 8.83%. Found: C, 52.80%; H, 4.95%; N, 8.87%. UV–vis: 370 nm and 806 nm. IR spectra of the P2 was having insignificant difference from P1.

The intermediate [(H3anthraimmida)Cu(26pdc)2].2.5H2O.2CH3OH (INT) was prepared by adding Hanthraimmida (63.08 mg, 0.2 mmol) to a well-stirred solution of 2,6-pyridinedicarboxylic acid (66.84 mg, 0.4 mmol) and copper(II) acetate monohydrate (39.92 mg, 0.2 mmol) in methanol (20 mL). The solution was stirred for about 3 h. On standing, bluish green crystals of INT appeared from the solution after 12 h; these crystals once taken out from solution were found to be unstable. The crystals were collected and put in paraffin oil and mounted in a diffractometer. The crystals of the complex once taken out from the solution became opaque and transformed to P2. This conversion was monitored under a microscope with time (please refer to the supplementary video). The IR of the neat crystals were immediately recorded by picking up the crystals from solution. The PXRD spectra of the fresh wet samples were recorded and had a matching pattern with the simulated one from the crystallographic information file (Figure 5S). IR (cm–1): 3314 (br, w, νO–H), 2945 (s, νC–H) 1614 (w, νC=O), 1575 (w, νC=N), 1020 (s, νC–O).

2.2. Crystallographic Study

The X-ray single crystal diffraction data for the salts were collected by a Bruker APEX-II CCD diffractometer at room temperature. Data refinement and cell reductions were carried out by Bruker SAINT Software. Data reduction and cell refinements were performed using SAINT and XPREP software. Structures were solved by direct methods using SHELXS-97 and were refined by full-matrix least-squares on F2 using SHELXL-14 and OLEX2 programs. All non-hydrogen atoms were refined in anisotropic approximation against F2 of all reflections. Hydrogen atoms were placed at their geometric positions by riding and refined in the isotropic approximation. The crystallographic parameters are listed in Table 1. For the structure of the INT solved at room temperature, a solvent mask was calculated by using OLEX2 software after removing all the solvent molecules. This showed that there were 119 electrons in a volume of 462 Å3 in one void per unit cell. This is consistent with the presence of 2[CH3OH], 2.5[H2O] molecules per asymmetric unit to account the theoretical count 122 electrons per unit cell required for this composition. In case of the INT structure solved from data collected at low temperatures, a solvent mask was also calculated, and in this case, 116 electrons were found in a volume of 458 Å3 in one void per unit cell. This is again consistent with the presence of 2[CH3OH], 2.5[H2O] per Asymmetric Unit to account for 122 electrons per unit cell within the constraints of experimental accuracy of X-ray diffraction. The formula presented in the Table 1 for INT reflects two water molecules as there was residual electron density accounting for half molecule of water. This residual density remained unresolved after several sets of data from independent data collection. The solvent percentage was confirmed by thermogravimetric analysis.

Table 1. Crystallographic Parameters of the P1, P2, and INT.

parameters P1 P2 INT
formula Cu2C70H76N10O25 Cu2C70H76N10O25 CuC37H41N5O12
CCDC 2250640 2250641 2250642
mol. wt. 1584.48 1584.48 811.29
crystal system triclinic monoclinic triclinic
space group P1̅ P21/c P1̅
a (Å) 11.380(7) 11.338(4) 9.70(2)
b (Å) 15.195(10) 41.360(12) 13.71(3)
c (Å) 22.011(14) 15.328(5) 14.69(3)
α (°) 79.784(17) 90 78.23(4)
β (°) 75.809(17) 94.020(9) 79.74(5)
γ (°) 88.457(18) 90 83.84(7)
V (Å3) 3631(4) 7170(4) 1877(7)
density, g cm–3 1.449 1.468 1.435
abs. coeff., mm–1 0.673 0.682 0.652
F (000) 1648 3296 846
total no. of reflections 12,769 12,633 6607
reflections, I > 2σ(I) 8107 8946 5464
max. θ/° 24.999 25.000 25.000
ranges (h, k, l) –13 ≤ h ≤ 13 –13 ≤ h ≤ 13 –11 ≤ h ≤ 11
  –18 ≤ k ≤ 18 –49 ≤ k ≤ 49 –16 ≤ k ≤ 16
  –26 ≤ l ≤ 26 –18 ≤ l ≤ 18 –17 ≤ l ≤ 17
complete to 2θ (%) 99.9 99.9 99.6
data/restraints/parameters 12,769/0/983 12,633/1/986 6607/0/500
GooF (F2) 1.109 1.096 1.016
R indices [I > 2σ(I)] 0.0833 0.0523 0.0555
wR2 [I > 2σ(I)] 0.1713 0.1125 0.1488
R indices (all data) 0.1391 0.0881 0.0704
wR2 (all data) 0.2102 0.1395 0.1709
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3. Results and Discussion

Two different polymorphs of the complex [(H3anthraimmida)Cu(26pdc)2]2.9H2O (26pdc = 2,6-pyridinedicarboxylate) were obtained at different time intervals from different concentrations of reactants from the reaction of 2,6-pyridinedicarboxylic acid with Hanthraimmida. A reaction performed at a relatively shorter duration (1 h) and at low concentration yielded the polymorph (P1). Whereas when the reaction with the same reactants continued for longer time (12 h) at relatively higher concentrations of the reactant, it resulted in the formation of another polymorph (P2). The same reaction at an intermittent concentration than the polymorphs but performed for approximately three hours provided another solvate of the complex, which was the intermediate [(H3anthraimmida)Cu(26pdc)2]. The 2.5H2O·2CH3OH complex referred to as INT is shown in Scheme 1. The solid-state structures of all the complexes were characterized by single crystal X-ray diffraction. The intermediate complex INT was unstable, and once crystals of INT were taken out from the supernatant solution, it changed to the polymorph P2. Hence, it could not be isolated in bulk quantity and stored as the solid sample. We have taken out the complex and kept in an argon atmosphere for 30 min; under this condition also, the crystals were decomposed and the powder XRD of those crystal after 2 h resembled close to the polymorph P2. The crystal morphologies and color of the crystals of the solvate INT and the two polymorphs were different, those could be visually distinguished. The transformation of INT to P2 could be also easily seen through the naked eyes, as the crystals of INT picked up from solution turns opaque and greenish yellow in real time. We have monitored the transformation by observing the crystals soaked in mother liquor placing them on a microscopic glass slide, placed under a microscope and recorded a video, which is enclosed as a Supporting Information (Video 1). The transformation in real time was clearly visible and suggested that the crystals comes closer once the solvent was getting evaporated from the glass plate and the crystal of the INT formed aggregated mass which provided the crystals of the P2. The crystals of P1 and P2 are stable and did not interconvert in the methanol solvent. The IR spectra (a and d Figure 2(i)) and the elemental analysis of the P1 and P2 had close resemblances between them. Each polymorph had the characteristic IR peaks from the 26pdc at 1614–1619 cm–1. The O–H stretches in each case was observed as a broad peak in the range of 3315–3375 cm–1. The IR spectra of the solvate INT had differences from the polymorphs as it had showed a sharp peak at 3314 cm–1. This IR-peak was used as to demarcate the instability of the INT crystals once taken out of the reaction medium. The crystals after 5 min loses the strong broad absorption at 3314 cm–1 (b of Figure 2(i)), and the IR spectra (c of Figure 2(i)), showed the peaks of the polymorph P2.

Scheme 1. Preparation of the P1, P2, and INT.

Scheme 1

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Figure 2.

Figure 2

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(i) IR (neat, cm–1) spectra of the P1 (a), INT immediately after preparation (b), INT, 5 min after taking out from the solution (c) and P2 (d). (ii) DSC of the three forms heated up to 200 °C with a heating rate of 10 °C/min. (iii) Overlaid X-band ESR spectra of the solid samples of P1, P2, INT, and INT after 5 min at 297 K (calibrated with dpph g = 2). (iv) PXRD patterns of (a) P1, (b) INT, and (c) INT after 5 min and (d) P2. (v) PXRD patterns of (a) P1, (b) P1 after heating at 150 °C, (c) P2 and (d) P2 after heating at 150 °C.

The thermal stability of two polymorphs were checked by recording their individual thermogram and differential scanning calorimetry (DSC) (Figures 9S–12S). From the TG, the polymorphs and the intermediate had showed weight loss due to the loss of the water of crystallization around, these were also reflected in the corresponding DSCs (Figure 2(ii)) around 81 °C (heating rate 10 °C), whereas, the polymorph P2 had lost the water molecules at 104 °C. The polymorph P1 was decomposed above 242 °C, and the P2 was decomposed above 264 °C. In order to check the transformation between the two forms, we have recorded the DSC up to 120 °C (heating rate 5 °C) (Supporting Figures 13S and 14S). As the polymorphs were decomposed above 242 °C, DSC plots of the polymorphs up from 0° to 120 °C were recorded, followed by cooling in a cyclic manner. The P1 showed a broad endothermic peak at 54–93 °C, whereas the P2 had showed multiple endothermic peaks in the range 60°–111 °C due to loss of water molecules (Supporting Figures 14S and 15S). Both the polymorphs in their respective cooling cycle did not show the original peak observed in the first cycle in this temperature range, showing no absorption of water by the dehydrated sample took place. The DSC curve of INT is also enclosed (Figure 17S), and it is quite similar to that of P2, as it was reported earlier in the text that INT is unstable after removal from the solvent. The thermogram of INT indicates a weight loss of approximately 13.29%, indicating the removal of solvent molecules consisting of two methanol and two and half water molecules. To ensure the stability of molecule during the collection of data, the TG measurement was conducted at a rate of 50 °C/min as INT is unstable at room temperature for a longer period of time. The DTG curve reveals a peak at 65 °C, indicating the loss of methanol molecules and another peak at 138 °C, indicating the loss of water. The IR spectra of the fresh sample of INT has the methanol C–H stretches at 2945 and 2832 cm–1 which disappeared upon standing and shows a broad peak at 3425 cm–1 (νOH) due to replacement of methanol by water.

The X-ray photoelectron spectra of both the polymorphs were not distinguishable, but they confirmed the presence of copper(II) ions (Figures 6S and 7S). The ESR spectra of the P1 had characteristic peaks of perpendicular and parallel components of gav = 2.18 of copper(II) in a distorted octahedral geometry (Figure 2(iii)). The P2 had showed a broad peak for the perpendicular component of gav = 2.21 and the parallel component was not distinct, and the INT showed peak at gav = 2.19. The ESR peak of the INTFigure 2(iii) observed in the fresh sample immediately transformed to show the peak of the polymorph P2. Experimentally determined powder XRD patterns of the P1, P2, and INT are shown in (Figure 2(iv)a,b, and d). The pattern of each form of the complex was compared with the respective powder XRD generated from their crystallographic information file (CIF) by using Mercury software (Supporting Figures 3S–5S). Each experimentally determined PXRD matched well with the respective one generated from CIF, showing the phase purity of the bulk samples. The transformation of INT to P2 was reflected in the instantaneously recorded PXRD of the freshly prepared INT (c of Figure 2(iv)), it had a matching peak with the PXRD generated from its CIF, showing only one form in the crystals. Once the PXRD of the same sample was re-recorded, it was found that the PXRD was matching with the PXRD of the P2 (d of Figure 2(iv)). The textures of the crystals were also changed. This occurred due to the replacements of the methanol molecules from the crystals by incorporation of water molecules. The P1 and P2 were independently desolvated each by heating at 150 °C for 2 h in an oven. The PXRD patterns of both the samples were recorded and both transformed to a common form. It was found that the anhydrous form of the P1 had a similar pattern as that of the hydrate. The X-ray diffraction pattern of the dehydrated form of P2 was similar to the dehydrated form of P1 (Figure 2(v)).

3.1. Crystal Structures

We have determined the X-ray crystal structure of both the polymorphs and also of the intermediate complex INT. For the structure determination of INT, the crystals were placed in paraffin oil and a good diffracting crystal was picked up to collect the diffraction data immediately. The crystal structures of the two polymorphs P1 and P2 had showed that both had two symmetry independent molecules; that is to suggest that they had two symmetry independent cations as well as two anions in their respective asymmetric unit; this was apart from each having nine water molecules. In fact, these should be considered to have 4.5 moles of water per complex from chemical point of view. Each organic cation in the structures of the P1 and P2 had independent orientations of the imidazole unit with respect to the anthracenyl group; and conformation of those organocations were not equivalent. The structure of the two polymorphs are shown in the Figure 3a,b. The coordination environment around copper(II) ion, of the both polymorphs were identical having two 2,6-pyridinedicarboxylates coordinated to copper(II) ion each providing NO2 type of coordination environment. The metal–ligand bond parameters are listed in supporting Table 3S. In the unit cell of each polymorph, there were two symmetry independent hexa-coordinate [Cu(26pdc)2]2– anions; and the two chelated unit of the complex were found as stacked dimers. This is not unusual as there are large numbers of chelate complexes where stacking among the coordinated ligands form assemblies26 and 2,6-pyridinedicarboxylate complexes24,25 are not exception to those. The distance between the two planes of stack dimers of the anions, in P1 was 3.676 Å, and in P2, it was 3.601 Å. There were also two symmetry independent dications in the respective unit cell of each polymorph. These organic dications were formed by protonation of Hanthraimmida at the N–H next to anthracenyl group and at the sp2-N atom of the imidazole. The imidazolium portion of the cations were involved in the formation of charge-assisted hydrogen bonds. Both the +N–H(imidazole) of the symmetry-independent cations of P1 were hydrogen-bonded to the anions through intervening water molecules. It had two types of +N–H(imidazole)···O(water) hydrogen bonds connecting the anions. In one site, there were two intervening hydrogen-bonded water molecules, and in another site, there was one intervening hydrogen-bonded water molecule. In P2, the two imidazolium cations were involved in hydrogen bonds in a different manner, and it had +N–H(imidazole)···O(water) and +N–H(imidazole)···O(26pdc) hydrogen bonds contributing to the assembly. Both the polymorphs had independent types of chain-like arrangements among the cations and anions; these chains were organized in such a way that anthracene rings from two independent chains. These chains were assembled in face-to-face stackings to form layer-like arrangements. The rings were inclined with respect to the anionic chains. Those bilayers of the P1 were oriented in the same direction, providing a lamellar structure, whereas, in the case of P2, the layers were organized in the opposite directions as alternate bilayers to provide the herringbone structure. These arrangements provided differences in the packing of the two polymorphs. In the literature, flexible dyads such as of monoalkoxynaphthalene–naphthalimide undergoes thermochromic transformation due to changes in the π-stacks.23 In such systems, the flexible alkyl chains had a role to guide the stacks. In the present example, we had (CH2)3-tether linking the anthracenyl group with the imidazolium cation. Thus, there were various conformations to guide the orientations of those rings and to bring in varieties in the self-assemblies. It may be noted that the bisphosphoramidate and its methanol solvate were reported to have single and two molecules in the asymmetric units; in that case, the packing patterns were guided by edge to face C–H···π interactions.27 Our examples had two symmetry independent molecules with lamellar and Herringbone arrangements that have provided avenues for a new polymorphic compounds of such a kind.

Figure 3.

Figure 3

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Crystal structure of (a) P1 and (b) P2 (water molecules are omitted for clarity) and the packing patterns of (c) P1 and (d) P2 showing the orientations of π-stacks in the two polymorphs.

In the crystal structure of the INT, the asymmetric unit had a [Cu(26pdc)2]2– anion and a H3anthraimmida cation (Figure 4a). One of the carbonyl oxygen atom of the [Cu(26pdc)2]2– was hydrogen bonded to the N-H of imidazolium unit on the cation. This compound had also anthracene rings in parallel positions, but those were located at translated positions. The packing diagram of INT showing the encapsulated anions in cationic enclosures is shown in Figure 4b,c. There were non-eclipsing parallel rings of the anthracenyl group as pairs along the b-crystallographic direction. The planes of these anthacenyl-groups had a centroid to centroid distance of 4.56 Å. This distance suggested no stacking interactions among the anthracenyl groups. In the self-assembly, four neighboring cations were organized by forming hydrogen bonds through the oxygen atoms of C=O of the anions with the N–H bond of the –+NH2– unit of the cation. They provided laterally embraced enclosures of anthracenyl groups to encapsulate two [Cu(26pdc)2]2– anions (Figure 4b). A view along the b-crystallographic axis showed these enclosures as channel-like arrangements. The anions were encapsulated in these cation embraced channel-like structure.

Figure 4.

Figure 4

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(a) Crystal structure of INT, showing hydrogen bonds of the anion with methanol molecules and cations, (the water molecules are omitted for clarity), (b) enclosures created by the π-embracing among the cations providing the surrounding to accommodate that solvent and anions. (c) The channels including the water and methanol molecules.

The water molecules in the self-assembly contributed to segregate the anions by providing hydrogen-bonded bridges connecting anions in another channel. In the overall assembly, a view from the bc-plane showed that the anions were in pairs formed through the stacks between the two chelated ligands. Those stacked ligands of the [Cu(26pdc)2]2– were separated by a centroid to centroid distance 3.55 Å; on the other hand, the anions within the channels were held apart without stacks among them. Therefore, there was possibly a tendency to break the barricades of the channels to form the stacked arrangements among the anions. This could be a factor for the INT transforming to the P2. The INT is an anion encapsulated assembly in a cationic noncovalent enclosure, and there are examples of such class reported in the literature based on different methodologies.28 The channel-like arrangements of noncovalent structures are useful to assemble and disassemble to facilitate ion transport.26 In our case, we found that this structure was very unstable and turned to π-stacked structure spontaneously upon exposure to moisture or removal of the solvent. The structure of P2 had Herringbone cross-beddings structures, which relates to the structures of sediments formed on the sea shores by tides. Those structural patterns have a look of the structural arrangements of the bones of a fish. Such patterns on the soil in a sea-shore occurs due to periodic flows of water during the forward and backward movement of water in tide and ebb. Those periodic patterns have the fore-sets in successive parallel structured arrangements in opposite directions.29 There are also examples of self-assemblies where changes in the stacking among the molecules caused gate opening and enabled to design in the artificial ion channel by externally adding a π-stacking component.30 In the present case, there was spontaneous formation of polymorph P2 having a Herringbone cross-bed structure, from the intermediate complex INT. The transformation resulted in P2 which had extensive stacking among the rings. The channel-like arrangements of INT broke down upon replacement of methanol molecules by water. In this case, the molecules of solvents once get apart, the molecule reorganizes to form the one form that has stacking among the planar portions of the anions as well as among the cations. Accordingly, the solvent molecules get into the respective positions of the stable form of the polymorph. This process is favorable as methanol is a low boiling solvent, and it escapes easily from the lattice to get replaced by water that has a higher hydrogen bonding ability. Attempted synthesis of only methanol solvate of the complex in dry methanol in inert atmosphere yielded INT, suggesting the propensity of it to crystallize out as solvate having methanol and water together. The INT had two and half water molecules and two methanol molecules.

The polymorph P2 had 4.5 water molecules per complex; although crystallographically, there were 9 molecules per unit cell that has two complex molecules in the unit cell holding those. Theoretically, at ambient conditions, the volume of a molecule of methanol is 6.9 × 10–23 cm3 and the volume of a water molecule is 2.98 × 10–23 cm3. There were 4.5 molecules of water of the polymorph P2 (per complex) that has a volume of 12.06 × 10–23. However, INT has two and half water molecules together with two methanol molecules and has a volume 21.25 × 10–23 cm3. As replacement of methanol by water will require lesser volume, hence replacement of methanol by water was possible. Furthermore, the water molecules have much higher numbers of hydrogen bond sites than that of methanol to provide a stable structure. Accordingly, the incorporation of water molecule to expel out the methanol molecules of the INT was feasible with a minimal volume changes. There are known 1,10-phenanthroline solvates having different π-stacks and certain ones are transient; after removal from the mother liquor, it also forms a hydrate with Z′ = 3.31a The role of the solvent in such a case is to provide pivot to guide the orientations of the ring; in an earlier study, we also had shown the role of DMSO and DMF to provide different sets of hydrogen bonding environment resulting in Z′ = 1 or 2.31b In the present case, there were stacked dimers of anions that provided the pivot in each case to link to the organic cations, by direct hydrogen bonds or through intervening water molecules. The orientations of the anion dimers were different, and the assembling of these guided the orientations of the cations to have different Z′. It was shown that the hydrates having a lower amount of water molecules provide channel-like structures31 and we do find a similar aspect having channel-like structure in the INT. The structure of the INT at low temperatures (120 K, the CCDC No. 2265340) was determined; in this case, it had 2.5 water molecules and two methanol molecules in the complex as the solvent of crystallization. We could not resolve the crystallographic disorder in the water molecules in the structure. Hence, the structure was solved by squeezing the water molecules and the CIF of the structure is enclosed as Supporting Information. There was least change in the unit cell parameters with respect to the unit cell parameters at room temperature, confirming it to be the same structure. These discussions as well as the observations relating to the channel, suggest its propensity to form stacked structures through easy loss of the methanol molecules from the channels. The transformation took place by accepting water molecules from the solvent (99% methanol) to replace methanol; water is a better hydrogen bond forming species than methanol and to utilize higher numbers of hydrogen bonds than methanol to provide stability.

3.2. Structural Comparison Relating Conversion of Polymorphs and Solvate

We had sorted out the dihedral angles that contributed to the orientations to the planar anthracenyl rings by focussing on the flexible portion of the single bonded −(CH2)3– tether. The dihedral angles were found to be different for the symmetry independent cations in both the polymorphs (listed in Table 4S). A better picture was obtained by drawing the overlaid diagram of the cations by keeping the anthracenyl group of each cation fixed as eclipsing each other with respect to the plane of the paper (Figure 5a). In P1 and P2, the symmetry-independent cations had differences in the angular orientations shown by designating them by _a and _b in Figure 5a. The spatial dispositions of the +N–H(imidazole) in P1 were toward same side, whereas in P2, these were projected inward and outward toward the anthracenyl group. The most intriguing feature is that the projection of the +N–H(imidazole) in the two polymorphs are in opposite direction in such an overlaid diagram. This is one of the key issue the hydrogen bond as the +N–H(imidazole) participated in both the polymorphs and contributed to provide the directionality to the respective self-assembly. In the case of the INT, the cation had orientation of the +N–H(imidazole) resembling of one of the cation marked red in Figure 4a. To be more precise, these affected the orientations of the planes drawn including the planar anthracenyl group of individual cation with respect to the plane drawn on the plane of the imidazole ring. The angles bisecting those two planes of the cations of the P1 were 43.03° and 46.23°, respectively (Figure 4c–e), whereas those were 65.72° and 42.82° in P2. The cation of INT had the bisecting angle 85.72°. Hence, the similar orientations of the +N–H(imidazole) of the one cation of P2 with the cation of the INT was not guiding the bisecting angle between the two planes and it was the central tether that made the difference in this orientations of the planes. This analysis had clarified that the only one bisecting angle between the anthracenyl part; and the imidazole part of P2 had a significant difference from the other three orientations that had close similarities of the bisecting angles. This provided different inclinations among the rings as proposed to have provided the Herringbone arrangement in P2.

Figure 5.

Figure 5

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(a) Overlaid diagram of the cations of the P1, P2, and INT showing the differences in the orientations of the imidazolium cation with respect to anthracene ring ( _a and _b represent symmetry independent cations of the unit cell of the respective polymorph). The angles between independent planes constructed over the rings of the anthracene and imidazolium part of each cation of (b) P1, (c) P2, and (d) INT.

The crystals of the P1 belonged to the triclinic P-1 space group, whereas the crystals of the P2 belonged to monoclinic P21/c. There were large differences in the unit cell volumes of the two polymorphs, P1 has a volume 3631(4) Å3, whereas the P2 has a volume about twice of this, precisely 7170(4) Å3. There was a large difference in the length of the b-crystallographic axis, for P1, it was 15.195(10) Å, whereas for P2, it was 41.360(12) Å. Crystal densities of the polymorphs were similar. The intermediate complex INT was crystallized in the triclinic P – 1 space group. It has a unit cell volume 1877(7) Å3 which is about four times smaller than the respective volume in P2 (z = 4), but about half the volume of the P1 (z = 2). On the other hand, the two symmetry-independent molecules in the unit cell of each polymorph were with different torsion angles (Table 4S). Hence, the systems are complicated to understand in a straight forward manner, it requires answers to accommodate the consequences of solvents to to guide the arrangements to have the symmetry independent molecules. This aspect is a push forward to have new insight into the varieties in conformational flexible molecules in solid-state self-assemblies. In this regard, we had an advantage that both P1 and P2 had the same numbers of symmetry independent molecules and there was a doubling in unit cell. The analysis on the unit cell suggested that if two unit cells of P1 along ab-crystallographic faces were overlapped, it would provide similar packing to the P2. Hence, it was possible to generate the coalesced unit cell, that was very similar to the unit cell of P2 (Figure 6). This coalescence had accommodated the required π-stacking among the anthracenyl groups that was present in the P2 (Figure 6b). The doubling of unit cell volumes occurs in inorganic compounds due to electronic effect32a and deformation of geometry.32b Doubling and quadruplet increase in volumes of unit cell among polymorphs without conformational changes of the molecules were reported earlier.32c We also had reported the extended domain as a factor to enhance unit cell volumes in polymorphic solvates.32d The evaporation rates of solvents also provide time-dependent π-stacks among donors and acceptor molecules.33 and our results showed this aspect as a new example. In this study, the additional point is on the role of concentration dependence together with time dependence in the formation of the three forms of the complex. Indeed, we observe a similar propensity to form stacks among the planar units. Switching between π-systems from one form to other;34 their property changes35 and desolvation leading to isostructural forms36 are well known. The hydrolysis of ester containing complex in the solid state converting to crystalline carboxylate compound was reported by us.37 In the present case, the formation of the P2 from the INT involved very small amount of mass loss, but it caused while getting converted to P2 about 4-times increase in the unit cell volume from INT had occurred, and it was not a single crystal to single crystal transformation.

Figure 6.

Figure 6

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Different unit cells to show the doubling of unit cell volume through π-stacking, (a) Unit cell of P1, (b) coalescence of ab-crystallographic faces of two unit cells of P1, and (c) unit cell of P2.

4. Conclusions

Two polymorphs having lamellar or Herringbone arrangements of the anthracenyl groups were formed under different crystallization conditions due to the propensity to maximize π-stacks among planar anthracene rings. In these examples, the copper-pdc anions served as hinges to the cations in different ways to influence the spatial orientations of the anthracenyl groups deciding the extent of face-to-face π-stacking among them. The most important aspect observed in this study was the P2 (z = 4) having double the unit cell volume that of P1 (z = 2) that occurred due to assembling of two unit cells along ab crystallographic faces through π-stacking. The coalescence of two independent unit cells of a polymorph through π-stacking converting to another polymorph is unprecedented and open a new avenue for understanding. This study also have illustrated the fact that assemblies in solution at different concentration could provide crystallization of mixed solvate as intermediate INT, which transformed to P2 by the replacement of methanol from channel-like enclosures of the INT to maximize hydrogen bonds and π-stacking.

Acknowledgments

The authors thank the Ministry of Education, Government of India, New Delhi, for financial support through a departmental grant (F.O. 5-1/2014-TS. VII), the Department of Chemistry, IIT Guwahati, and Department of Science and Technology, New Delhi India, for using the SCXRD facility (Sanction No. SR/FST/CS-II/2017/23C) provided by them for structure elucidation and various instrument facilities, and the Central Instrumentation facility of IIT Guwahati for general facilities. The authors thank Mr. Ravinder Chahal of the Department of Physics, IIT Guwahati, for help in recording the video of the crystal transformation.

Data Availability Statement

The crystallographic information files are submitted to the Cambridge Crystallographic Database center and have the CCDC numbers 2250640–2250642, 2265340.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c05132.

  • PXRD patterns; UV–visible spectra; TG and DSC; PES; hydrogen bond tables; and selected torsion angles of the polymorphs; general aspects of experimental section, crystallographic parameters; and hydrogen bonds (PDF)

  • Video on the crystal of INT transforming to P2 is also included (MP4)

Author Contributions

The manuscript was written through contributions of both authors. Both authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao3c05132_si_001.pdf (1.5MB, pdf)
ao3c05132_si_002.mp4 (117.8MB, mp4)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao3c05132_si_001.pdf (1.5MB, pdf)
ao3c05132_si_002.mp4 (117.8MB, mp4)

Data Availability Statement

The crystallographic information files are submitted to the Cambridge Crystallographic Database center and have the CCDC numbers 2250640–2250642, 2265340.

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