The solubility and stability of heterocyclic chalcones compared with trans-chalcone

Heterocyclic chalcones are a recently explored subgroup of chalcones that have sparked interest due to their significant antibacterial and antifungal capabilities. Herein, the structure and solubility of two such compounds, (E)-1-(1H-pyrrol-2yl)-3-(thiophen-2-yl)prop-2-en-1-one and (E)-3-phenyl-1-(1H-pyrrol-2-yl)prop2-en-1-one, are assessed. Single crystals of (E)-1-(1H-pyrrol-2-yl)-3-(thiophen-2yl)prop-2-en-1-one were grown, allowing structural comparisons between the heterocyclic chalcones and (2E)-1,3-diphenylprop-2-en-1-one, trivially known as trans-chalcone. The two heterocyclic chalcones were found to be less soluble in all solvents tested and to have higher melting points than trans-chalcone, probably due to their stronger intermolecular interactions arising from the functionalized rings. Interestingly, however, it was found that the addition of the thiophene ring in (E)-1-(1H-pyrrol-2-yl)-3-(thiophen-2-yl)prop-2-en-1-one increased both the melting point and solubility of the sample compared with (E)-3-phenyl-1-(1H-pyrrol-2-yl)prop-2-en-1-one. This observation may be key for the future crystal engineering of heterocyclic chalcones for pharmaceutical applications.


Introduction
1,3-Diarylprop-2-en-1-ones, trivially known as chalcones, are organic molecules which feature two aromatic rings linked by an enone backbone. The name 'chalcone' is derived from the Greek word chalkos, which translates as 'copper', since the majority of naturally occurring chalcones have this colour (Sahu et al., 2012). Chalcones have a variety of uses, ranging from being pharmacophores (Onyilagha et al., 1997), acting as nematicides (Simmonds et al., 1990), and in the synthesis of other more complex compounds (Lasri & Ismail, 2018;Zhuang et al., 2017). For example, chalcones are used as reactants in the Robinson annulation for ringed compounds (Safaei-Ghomi & Alishahi, 2006). In addition to their pharmaceutical applications, some chalcones are known to be fluorescent when the aromatic rings are functionalized with electron-donating groups, affording them the potential to be used as probes for mechanistic investigations (Zhuang et al., 2017). Chalcones can be synthesized in a variety of ways, including Friedel-Crafts acylations (Shotter et al., 1978), palladium cross-coupling reactions (Eddarir et al., 2003) and using the Wittig reaction between an appropriate aromatic aldehyde and ylide (Xu et al., 1995).
Chalcones which contain a pyrrole, thiophene or furan aromatic ring instead of the traditional phenyl ring are known as heterocyclic chalcones and are of interest because of their enhanced pharmacophoric properties. For example, when ISSN 2052-5206 # 2020 International Union of Crystallography combined with antibiotics like benzylpenicillin (penicillin G), heterocyclic chalcones featuring the furan functional group instead of the traditional phenyl ring show significantly greater antibacterial activity against Bacillus subtilis, Escherichia coli and Staphylococcus aureus (Sridhar et al., 2011;Tran et al., 2012). Little has been reported on the uses of unsubstituted heterocyclic chalcones, however. From this perspective, fully characterizing the intermolecular interactions involved in these compounds is vital as they relate to emergent properties such as solubility, which are critical in engineering materials with desired properties for pharmaceutical applications.
In this article, the solubility limits of two heterocyclic chalcones ( Fig. 1) and trans-chalcone are deduced for a variety of polar and non-polar solvents. The crystal structure of 1 was solved via single-crystal X-ray diffraction for the first time and is compared with that of 2 (Gong et al., 2008) and transchalcone (Wu et al., 2006). Analysis of these molecules in the solid state highlights the stark differences in their intermolecular interactions, providing an insight into their solubilities. Compound 2 and the polymorphs of trans-chalcone were verified using X-ray powder diffraction and matched with the previously reported structures. The thermal properties of the two heterocyclic chalcones were collected and compared with that of trans-chalcone in order to demonstrate the differences in intermolecular interaction strength.

Synthesis of heterocyclic chalcones and crystal growth
Heterocyclic chalcones 1 and 2 were synthesized using the Claisen-Schmidt condensation (see the supporting information for detailed experimental information; 1 H and 13 C NMR spectra for both 1 and 2 are shown in Figs. S1-S6). Single crystals of 1 and 2 were grown from n-hexane, toluene, tetrahydrofuran (THF), ethyl acetate, chloroform, acetone, ethanol and methanol at a concentration of 0.05 mol dm À3 by slow evaporation (Figs. S7 and S8 in the supporting information). This was achieved by covering the solutions with Parafilm and piercing the film with a small hole, approximately 2 mm in diameter. The solutions were left at room temperature (295 K) for two weeks to allow all the solvent to evaporate. After growth from slow evaporation, larger crystals of both heterocyclic chalcones exhibited a pale-yellow colour similar to that of trans-chalcone, although thinner crystals appeared colourless. Crystals of 1 tended to adopt a block-like morphology (Fig. 2), whereas 2 tended to form both needle and plate-like structures. The single crystal of 1 grown from THF was used for single-crystal X-ray diffraction.

Solubility of the heterocyclic chalcones compared with trans-chalone
The solubility limits of the chalcones being considered here were found by forming saturated solutions using the solvents listed in Table 1, using a method which has been described previously (experimental details can be found in the supporting information; Salman et al., 2015). Powder X-ray diffraction was used initially, to assure that the crystal structures of the samples being tested were consistent. The solubilities of the three chalcones in Table 1 show that all three dissolved well in polar solvents such as THF and acetone, relative to non-polar solvents such as n-hexane and toluene.
Solubility relies on three sets of intermolecular interactions: solute-solute, solvent-solvent and solute-solvent. If the enthalpy associated with the formation of solute-solvent interactions is greater than the sum of the enthalpies for solute-solute and solvent-solvent interactions, then the solute will dissolve into the solvent. Overall, the heterocyclic chalcones are much less soluble than trans-chalcone in all the solvents tested. This will predominantly be caused by the weaker solute-solute interactions between the trans-chalcone molecules compared with the interactions between the  Skeletal formulae for the two heterocyclic chalcones studied here. 1 is  Crystals of (a) 1 and (b) 2, grown using the slow evaporation method from methanol at a concentration of 0.05 mol dm À3 . heterocyclic chalcone molecules. In order to confirm this, the polarities of the three chalcones were calculated. Molecules of each chalcone were optimized using density functional theory (DFT) in GAUSSIAN09 (Frisch et al., 2009) with the B3LYP functional set to the 6-31G level (Xue & Gong, 2009). For 1, there are two different conformers which depend on the location of the sulfur atom, being either syn or anti with respect to the carbonyl group. This is reflected by the disorder within the crystal structure and so a weighted average was used to calculate the polarity of 1, based on the polarity of both conformers. trans-Chalcone was calculated to be the most polar of the three chalcones being considered (3.36 D), followed by 2 (1.98 D) and then 1 as the least polar (1.56 D). The decrease in polarity associated with the heterocyclic chalcones results in a decrease in solubility for the non-polar solvents n-hexane and toluene, as shown in Table 1.
The functionalized rings found in 1 and 2 provide a greater variety of intermolecular interactions between each other, which increases the solute-solute interactions and in turn reduces their solubility compared with trans-chalcone. When comparing 1 and 2, the solubility of 1 is greater in polar aprotic solvents than that of 2. This is probably due to the lone pair featured on the thiophene ring of 1 providing more intermolecular interactions with the aprotic solvent than 2 is able to do.
Chalcone 2 is generally less soluble than 1 in all tested solvents, except toluene and ethanol. The differences in solubility can be elucidated by looking at the molecular electrostatic potentials of the three chalcones ( Fig. 3 and Fig. S9 in the supporting information). The solubility of transchalcone and 2 in toluene is probably due to a favourable interaction involving the non-heterocyclic phenyl ring, which is not present in 1. Solubility in protic solvents such as ethanol and methanol should be similar for both heterocyclic chalcones, as the solute-solvent interactions will be mediated predominantly via the carbonyl group.

Stability of the heterocyclic chalcones compared with trans-chalcone
Thermal analysis using differential scanning calorimetry (DSC) indicates that the melting points of 1 and 2 are 423.42 and 410.95 K, respectively (Figs. S10-S12). On recrystallization from the melt, a different polymorph of trans-chalcone formed, resulting in two separate melting points being recorded (Fig. 4). The first melting occurred at 330.24 K (form I, verified with powder X-ray diffraction in Figs. S13-S16) and the second at 325.95 K (form II, verified with powder X-ray diffraction in Figs. S17-S20).
The use of heterocyclic rings in both 1 and 2 increases the stability of the chalcone, indicated by 1 and 2 having significantly higher melting points than trans-chalcone, implying stronger intermolecular interactions between the chalcone molecules of 1 and 2. The crystal structures of both 1 and 2 have similar hydrogen-bonding motifs ( Fig. 5; see also Figs. S21-S29, and Tables S2 and S3 for single-crystal and powder XRD data relating to 1 and powder XRD data relating to 2). The distance between the donor pyrrole and the acceptor carbonyl functional groups in the crystal structures of both 1 and 2 are 2.832 Å or 2.841 Å , and 2.817 Å , respectively, which are approximately the length of a short hydrogen-bond interaction found in the secondary structure of proteins Molecular electrostatic potentials for trans-chalcone, 1 and 2. The range for all three plots is between À6.5 Â 10 À2 a.u. (red) to 6.5 Â 10 À2 a.u. (blue), with an isovalue of MO = 0.02 e Å À3 and a density of 0.0004.

Figure 4
The DSC heating cycles for all three chalcones being considered, in the temperature range 313 to 453 K. (Langkilde et al., 2008). Form I of trans-chalcone features no hydrogen bonding within its crystal structure; but form II does, between the carbonyl group and the phenyl ring in the third position on the enone backbone (Wu et al., 2006). The hydrogen-bonding interactions in 1 and 2 are stronger than those found in trans-chalcone form II due to the nitrogen atom having a higher electronegativity than carbon. This will result in a higher melting point for the heterocyclic chalcones, which we have observed here via DSC.
Though the hydrogen bonding for 1 and 2 is similar, the melting point is higher for 1 than for 2, suggesting there is another significant intermolecular interaction to consider. Both 1 and 2 feature L-shaped arrangements in the molecular packing (Fig. 6), suggesting that interactions also play a role in stabilizing the structure. In 1, the thiophene and pyrrole rings appear to adopt an L-shaped arrangement with similar rings on adjacent molecules, which relies on a lone-pairinteraction for the thiophene rings [the interaction covers a distance of 3.636 (5) Å ] and an H-interaction for the pyrrole rings [the interaction covers a distance of 2.877 (2) Å ]. In 2, the phenyl and pyrrole rings form an L-shaped arrangement between each other in adjacent molecules, which arise from H-interactions (the interactions cover 3.122 (2) Å for benzene-benzene interactions and 5.327 (3) Å for benzenepyrrole interactions). However, the interaction distances are greater in 2 than 1 and the interactions in 2 do not lie within the van der Waals radii of the atoms, where hydrogen is recorded to have a van der Waals radius of 1.68 Å , carbon 1.30 Å and sulfur 1.83 Å (Mantina et al., 2009).
To estimate the strengths of the L-shaped arrangements in both heterocyclic chalcone crystal structures, the dissociation energies of the t-shaped dimer equivalent between the benzene, pyrrole and thiophene rings were compared using values which are recorded in the literature. Comparing the dissociation energies of the benzene-benzene and thiophenethiophene t-shaped dimers, the thiophene-thiophene dimer has a larger dissociation energy than the benzene analogue, reported to be À2.60 and À2.46 kcal mol À1 (1 kcal mol À1 = 4.184 kJ mol À1 ), respectively (Tsuzuki et al., 2002). This is due to the high polarizability of the sulfur atom in the thiophene ring, increasing the dispersion forces between the two heterocyclic rings which in turn increases the strength of the dimer interaction (Tsuzuki et al., 2002). As the hydrogenbonding interactions are very similar for both 1 and 2, it seems most likely that the differences in stability are primarily due to the differences in the energy of the t-shaped arrangements of the aromatic rings and therefore the interactions with the systems of the aromatic rings.
Form II of trans-chalcone is also reported to feature t-shaped H-interactions within its crystal structure between the phenyl rings (Wu et al., 2006). Even though the t-shaped dimer for the benzene-benzene system is stronger than that for the benzene-pyrrole system found in 2, due to the nitrogen atom being less polarizable than carbon, trans-chalcone form II is much less stable than 2 due to the hydrogen-bonding interactions being weaker. Form I of trans-chalcone features arrangements which are similar to a stackedinteraction between the phenyl rings. Though the displaced stacked dimer Two molecules each of (a) 1 and (b) 2, showing the hydrogen bonding (green lines) within their crystal structures. The visualization software that was used was Mercury 4.1.3 (Macrae et al. 2006(Macrae et al. , 2008.

Figure 6
Molecules of (a) 1 and (b) 2, showing key interactions. Orange spheres represent the centroids of the aromatic rings. Compound 1 shows lonepair-interactions between the thiophene rings and H-interactions between pyrrole rings. Compound 2 shows H-interactions. for benzene has a slightly larger dissociation energy than the tshaped dimer [reported to be À2.48 kcal mol À1 ; (Tsuzuki et al., 2002)], the dissociation energy is still smaller than that for the t-shaped thiophene dimer which has a dissociation energy of À2.60 kcal mol À1 (Tsuzuki et al., 2002). This reduces the stability of the polymorph when compared with the heterocyclic chalcones.

Conclusion
In conclusion, the heterocyclic chalcones investigated here are less soluble than trans-chalcone due to the weak solute-solute interactions present in trans-chalcone's molecular packing, highlighted by large differences in melting points when compared with 1 and 2.
Within the heterocyclic chalcone crystal structures, there are two main bonding motifs, hydrogen bonding and t-stacked dimer arrangements, which involve interactions with the system of the heterocyclic rings. In both 1 and 2, the hydrogen bonding is similar, but the thiophene group leads to a more stable t-shaped arrangement in 1, which correlates with a higher melting temperature. Interestingly, however, 1 is still more soluble than 2 in the majority of polar solvents tested here, which suggests that the increase in solute-solute interactions does not overcome the additional solute-solvent interactions arising from the extra heterocyclic ring.
The results found may apply to the design and synthesis of more pharmaceutically attractive heterocyclic chalcones, tuning both the stability of the compound and its solubility simultaneously. The heterocyclic chalcones may also be suitable for interactions with proteins if they are found to feature bioactivity.