Conformation and Visual Distinction between Urea and Thiourea Derivatives by an Acetate Ion and a Hexafluorosilicate Cocrystal of the Urea Derivative in the Detection of Water in Dimethylsulfoxide.

Structures of different solvates and solute-solvent interactions of 4-(3-(4-nitrophenyl)urido)benzoate (L 1 ) and methyl-4-(3-(4-nitrophenyl)thiourido)benzoate (L 2 ) with different solvents are analyzed. The solution of L 1 with tetrabutylammonium acetate (TBAA) in dimethylsulfoxide (DMSO) is colorless, but a similar solution of L 2 with TBAA is orange. On the other hand, respective solutions of these urea and thiourea derivatives with tetrabutylammonium fluoride (TBAF) in DMSO are orange. Urea derivative L 1 facilitates the reaction of TBAF with glass to form tetrabutylammonium hexafluorosilicate, which on further interaction with L 1 forms cocrystal 2L 1 ·(TBA)2SiF6. Reorganization of hydrogen-bonded self-assembly of 2L 1 ·(TBA)2SiF6 in DMSO caused by water is established by a dynamic light scattering study. With an increase in the amount of water in the solution, visual color changes from orange to colorless, and the color changes are reversed upon the addition of a dehydrating agent such as molecular sieves. Solvates of L 1 with DMSO, dimethylformamide (DMF), and dimethylacetamide are quasi-isostructural. The respective self-assembly of these solvates differs due to orientations of aromatic rings and the carbomethoxy group across the thioamide/amide bond. Significant differences in self-assemblies of the respective DMSO solvate of L 1 and L 2 are observed; self-assembly of the former has dimeric subassemblies as repeat units, whereas the latter has monomeric subassemblies. DMF solvates of L 1 and dimethylacetamide of L 1 are built by dimeric subassemblies to form self-assembled structures, but these subassemblies differ in the orientation of the carbomethoxy group across the urea units.

Introduction

Urea- and n class="Chemical">thiourea-based molecules have a special status to form anion-guided noncovalent assemblies.[1−4] A thiourea derivative noncovalently bound to anions has higher stability than the corresponding urea derivative bound to the same anion.[5] Ability of an anion to deprotonate urea and the solvent-guided deprotonation and conformational changes of a urea derivative are the key factors in anion recognition.[6,7] Certain anion-guiding chemical reactions of thiourea derivatives such as hydrolysis and bromination are useful in anion detection.[8] With the aid of a syn or anti conformer of a thiourea derivative, distinction of acetate and fluoride ions is possible.[9] On the other hand, conformations play an important role in self-assemblies of urea[10−12] and thiourea[13−15] derivatives. The specific ability associated with a particular conformation to bind an organic substrate has impact on organocatalysis.[16−18] Lesser numbers of conformers of symmetric urea and thiourea derivatives in comparison to unsymmetrically substituted ones ease their structural study. In general, conformational changes in solution and the solvent-guided stabilization of conformers are independent issues. The first one is a dynamic process mentioned in literature, which is highly dependent on continuously available energy to cause rotation across a bond by crossing a rotational activation barrier, and such conformational changes can be frozen. However, the latter aspect is due to solvation or self-assembly for stabilizing a conformer in the solid state which may not be the stable one present in solution.[19] Such a conformational adjustment may not necessarily be yielding a thermodynamically stable conformer but may lead to a kinetically stable one that can be obtained by self-assembly or solvation.[20] It was earlier shown that depending on the metal ion different conformations of urea can be stabilized;[21] hence, an integrated approach dealing with the consequent effects of solvents and anions on conformations is required. Solvents generally play an important role in guiding stabilization of a particular conformer of a urea derivative.[22,23] The present literature is not suggestive enough to provide integrated details regarding the differences in structures of solvates and their respective assembly in solution. Furthermore, self-assemblies formed by anions such as hydroxide acetate and fluoride show color changes in specific solvents through deprotonation,[3,4] which leaves scope to study their assemblies under different conditions. Thus, we have taken up an integrated approach to understand the structure and properties of unsymmetrical urea or thiourea derivatives methyl 4-(3-(4-nitrophenyl)urido)benzoate (L) and methyl-4-(3-(4-nitrophenyl)thiourido)benzoate (L); each having identical substituents. The chosen examples have the urea/thiourea part attached to a nitrophenyl group and a carbomethoxy-phenyl group at two different sides (Figure a). Such compounds would show syn–syn or syn–anti conformers as illustrated in Figure b. The stabilization of different conformationally adjusted molecules (Figure c) in the solvate may replicate the solvent–urea/thiourea interactions in solution. In these examples, the carbomethoxy group has also a provision to adopt different orientations, with either of such conformations leading to a conformational polymorph in the solid state. Conformational polymorphs arising from orientations of the ethyl part of the carboethoxy group are available in the literature.[24,25] It is also interesting to note that quasi-structural polymorphs in molecular crystals are obtained through crystallization from different solvents.[26] Thus, we have investigated self-assemblies of L and L in solution and in the solid state and have studied the consequences of their interactions with solvents and anions.

Figure 1

(a) Urea and thiourea derivatives. (b) Syn–syn and syn–anti conformers and (c) different conformers with respect to orientations of the carbomethoxy group.

(a) Urea and n class="Chemical">thiourea derivatives. (b) Syn–syn and syn–anti conformers and (c) different conformers with respect to orientations of the carbomethoxy group.

Results and Discussion

Interaction of Solvents with L and L

UV spectra of the compounds L and L in different solvents show nonoverlapping spectra, reflecting characteristic shifts in the absorption maxima. Compounds L and L dissolved in dimethylsulfoxide (n class="Chemical">DMSO) show sharp absorbance at 347 and 331 nm, respectively. These absorptions are solvent-dependent; compound L dissolved in solvents such as acetonitrile, tetrahydrofuran, acetone, and 1,4-dioxane shows absorptions at 334–337 nm range, whereas the same compound dissolved in dimethylformamide (DMF) or DMSO shows absorption at 345–347 nm (Figure a). These shifts observed in the UV spectra recorded for solutions of L in different solvents (Figure b) do not follow a trend to relate with solvent polarity. As a result of the differences in hydrogen bonding of these compounds L and L with solvents in different ways, the gap between n−π* differs in each case in different solvents, which contributes to the positions at which UV-absorption maximum of these compounds would occur in different solvents.

Figure 2

UV absorption of (a) L and (b) L in different solvents (10–5 M).

1H NMR spectra of L and L in different deuterated solvents as shown in Figure clearly show that solvents (a) affect the chemical shifts of the aromatic protons of the carbomethoxyphenyl ring, (b) have the exchangeable N–H signals, and (c) have the least effect on the chemical shifts of protons on the nitrophenyl ring. Depending on the aprotic solvent, N–H protons appear at different chemical shift positions, whereas solutions of the compounds in methanol-d4 show complete exchange of N–H protons with O–H protons of the solvent. Among the deuterated solvents, 1H NMR spectra recorded in acetone-d6 shows chemical shifts in the 1H NMR of L and L, which are distinctly different from those signals obtianed in other deuterated solvents. In this particular case, the e and f protons of L are shifted downfield (Figure a); in the case of L, the e proton is shifted downfield and overlaps with the signal from d protons as depicted in Figure b. This is attributed to the syn–anti conformer (I and II of Figure b) stabilized in acetone-d6 over the syn–syn conformer observed in other solvents. 2D-HOMOCOSY 1H NMR spectra of L in DMSO-d6 and acetone-d6 establish the assignments of the chemical shift positions of the aromatic protons from their respective coupling schemes (Figure S1 and S2). It may be noted that the syn–anti conformer of the thiourea derivative forms an adduct with acetone.[27] The gas-phase density functional theory B3LYP/6-31++G(d,p) level shows that the syn–anti conformer of L forms a stable adduct with acetone, which has 98.42 kJ mol–1 lower energy as compared to that of the adduct with the syn–syn conformer of L and acetone (Figure S3). The N–H bond present next to the nitrophenyl group of the more stable form projects away from carbonyl of acetone, whereas the N–H next to the carbomethoxyphenyl unit has a suitable projection for the hydrogen bond with acetone. Thus, one possible reason for a nominal change in the chemical shift positions of the 1H NMR signals of the nitrophenyl group in these compounds in acetone is due to the delocalization of electrons over the nitrophenyl group. Due to such delocalizations, this N–H group adopts a partial double bond character on the C–N bond.

Figure 3

Solvent-dependent 1H NMR spectra of (a) L and (b) L in (i) DMSO-d6, (ii) acetone-d6, (iii) acetonitrile-d3, and (iv) methanol-d4.

Solvent-dependent 1H NMR spectra of (a) L and (b) L in (i) n class="Chemical">DMSO-d6, (ii) acetone-d6, (iii) acetonitrile-d3, and (iv) methanol-d4.

A possible conversion of a syn–syn to syn–anti conformer using the acetone solvent is confirmed by recording the n class="Chemical">1H NMR spectra of L proportions of acetone-d6 and DMSO-d6 as a mixture of solvents (Figure ). Spectramarked as (v) in Figure are less symmetric in the acetone-d6 solvent as signals from the protons on the two aromatic rings that are clear A2B2 patterns in other cases are not very clear in this case and suggest them to be more closer to the AA′BB′ pattern. In comparison to signals d and f, signals assigned for c and e are insignificantly affected upon the increase in the ratio of deuterated acetone in the mixed solvent. As the concentration of acetone-d6 increased, the overlap of the signals from d and f took place; thus, the signal f moved drastically downfield. As a result of the change of the conformation, the proton marked as f gets deshielded.

Figure 4

1H NMR spectra of L in (i) DMSO-d6 and (v) acetone-d6 and (ii–iv) different ratios (v/v) of DMSO-d6 and acetone-d6 (ii) 1:1, (iii) 1:2, and (iv) 1:5.

1H NMR spectra of L in (i) n class="Chemical">DMSO-d6 and (v) acetone-d6 and (ii–iv) different ratios (v/v) of DMSO-d6 and acetone-d6 (ii) 1:1, (iii) 1:2, and (iv) 1:5.

Variable temperature 1H NMR in the temperature range 280–292 K (Figure S4) and concentration-dependent n class="Chemical">1H NMR (Figure S5) of L in acetone-d6 did not show notable changes, except for the slight change observed in chemical shifts of N–H protons. This suggests that there is no free rotation for conformational changes but the solvent forms assemblies with spontaneously formed conformation-adjusted molecules.

Crystallization of L and L from solution in different solvents yielded crystals of three solvates of L suitable for X-ray analysis; they are L·DMF, L·n class="Chemical">DMA, and L·DMSO and one for L, namely, L·DMSO. Urea or thiourea derivatives adopt the syn–syn or syn–anti conformer[28−31] across the C=O/C=S bond. Hence, these examples provide a way to make a systematic structural study.

An asymmetric unit of each solvate contains one parent molecule with a solvent molecule except in L·DMF, which has a pair of parent molecules and n class="Chemical">DMF molecules. Thus, this is an example of multiple numbers of symmetry nonequivalent molecules in the unit cell. Symmetry nonequivalent molecules in the asymmetric unit of the solvate of amide-containing compounds are not an exception.[32] In the present examples of solvates, each adopts a syn–syn orientation across the urea portion of the solvate. This is due to the locking of the conformer by the solvent molecule in each case through bifurcated hydrogen bonds. This is also another reason that none of the solvate has conventional urea tape motifs.[33]

Generally, self-assemblies of urea derivatives[34,35] are relatively straightforward to respective assemblies of substituted n class="Chemical">thioureas;the latter are more sensitive to interplay of weak interactions.[36] In the crystal structure of L·DMSO solvate, two urea molecules form a dimeric subassembly by bridging DMSO solvent molecules. Such dimers are formed as a consequence of contribution from C–H···O bonds (Figure a). In such a subassembly, oxygen atoms of urea and nitro functional groups are the hydrogen bond acceptors. Oxygen atoms of DMSO are engaged with two NH of thiourea in bifurcated hydrogen bonds, and also the two oxygen atoms of the nitro group of L form a bifurcated hydrogen bond with a C–H bond of DMSO. These bifurcated hydrogen bonds are responsible for holding two independent neighboring solvates as illustrated in Figure b.

Figure 5

Hydrogen-bonded assemblies of (a) L·DMSO solvate and (b) L·DMSO solvate. (c) Overlaid diagram of DMSO solvates of L and L and (d) bifurcated hydrogen bonds in L·DMSO and L·DMSO.

Hydrogen-bonded assemblies of (a) L·n class="Chemical">DMSO solvate and (b) L·DMSO solvate. (c) Overlaid diagram of DMSO solvates of L and L and (d) bifurcated hydrogen bonds in L·DMSO and L·DMSO.

Noncovalently linked subassemblies within the self-assembly of thiourea solvate L·n class="Chemical">DMSO are different from the one present in the assembly of the corresponding urea solvate L·DMSO. In the former case, the oxygen atom of urea and nitro groups is the hydrogen bond acceptor, but in the latter case, only the oxygen atom of the nitro group is the hydrogen bond acceptor. Hydrogen bonds between the DMSO molecule and L or L that is responsible to hold a particular form of the conformer of urea/thiourea are shown in Figure d. Each has a similar geometry, but the difference in bond parameters arises from the differences in the ionic radii of sulfur and oxygen atoms. This causes distinction between crystal packing of these DMSO solvates (Figure S6). Details of hydrogen bond parameters of the two DMSO solvates are given in Table , and the rest are listed in Table S1. Orientations of the phenyl rings in two solvates are shown in the overlaid diagram Figure c, and the corresponding torsion angles are listed in Table S2.

Table 1

Hydrogen-Bond Parameters of DMSO Solvates

solvates	D–H···A	dD–H (Å)	dH···A (Å)	dD···A (Å)	∠D–H···A (deg)
L1·DMSO	N(1)–H(1)···O(6)	0.86	2.06	2.876	159
	N(2)–H(2)···O(6)	0.86	2.08	2.899	158
L2·DMSO	N(2)–H(2)···O(5)	0.86	2.10	2.925	160
	N(3)–H(3)···O(5)	0.86	2.01	2.849	166

DMSO solvates, L·n class="Chemical">DMSO and L·DMSO, show S=O IR-stretching at 1017 and 1026 cm–1, respectively (Figure S7). IR stretching at 1725 and 1708 cm–1 from the solvate L·DMSO originates from the carbonyl of urea and ester functional groups. Solvate L·DMSO has C=O stretching from the ester group at 1710 cm–1. Solvate L·DMSO has a sharp C=S stretching at 1593 cm–1.

The hydrogen-bonded dimeric subassemblies within self-assemblies of L·n class="Chemical">DMF and L·DMA solvates shown in Figure a,b have clear differences in the orientation of the respective carbomethoxy group. Carbonyl of the carbomethoxy group is involved in bifurcated hydrogen bonds with two C–H bonds in L·DMA, whereas there is no such hydrogen bond in the assembly of L·DMF solvate. Oxygen atoms of solvent molecules are hydrogen bond acceptors forming N–H···O bonds in each case. Packing diagrams of the two solvates differ, which are shown in Figure S8. The carbonyl group of the carbomethoxy part of L·DMA projects in the same direction as that of carbonyl of urea [(i) of Figure c], whereas in L·DMF carbonyl groups project away from each other [(ii) of Figure c]. Gas-phase density functional theory B3LYP/6-31++G(d,p) level calculations show that the two conformers arising from orientations of the carbomethoxy group of L have comparable energy (Figure S9). Thus, differences in the orientations of the ester group in the two solvates are due to the consequence of packing requirements. The weak interactions are analyzed by Hirshfeld surface analysis. Hirshfeld surfaces of different solvates highlighting different weak interactions present in each crystal packing are shown in Figure S10. Fingerprint plots (Figure S11) with O···H interactions and relative contributions of various interactions expressed as percentages are tabulated in Table S3. Contributions from O···H interactions in thiourea solvate L·DMSO are 4.6–5.9% lower as compared to that of urea solvates. The higher contributions of O···H interactions in L·DMSO, L·DMF, and L·DMA solvates are obviously due to the movement of the sulfur atom of thiourea to the oxygen atom of urea derivatives.

Figure 6

Self-assemblies of (a) L·DMF solvate, (b) L·DMA solvate, and (c) two conformers of L from orientations of the carbomethoxy group.

Self-assemblies of (a) L·DMF solvate, (b) L·n class="Chemical">DMA solvate, and (c) two conformers of L from orientations of the carbomethoxy group.

Interactions of TBAA and TBAF with L and L

Interactions of anions with receptors based on urea and n class="Chemical">thiourea are well-studied.[35] A solution of urea derivative L does not show color change (Figure a) on the addition of tetrabutylammonium acetate (TBAA), whereas a solution of compound L in DMSO shows a yellow-orange color upon interaction with TBAA (Figure b). Compound L is selective to show the color change with only tetrabutylammonium fluoride (TBAF) among several other tetrabutylammonium salts (Figure S12a). The inherent basicity associated with certain anions such as hydroxide or acetate causes noticeable spectral changes of solution of L in a similar wavelength as apparent in their respective spectra. However, such changes are insignificant to cause visual color changes or intensities that are comparably low to make a comparison with the drastic changes observed from interactions with a fluoride anion. The binding constant of L with TBAF is 2.49 × 105 M–1, whereas those of chloride, bromide, and acetate salts are 0.12 × 105, 0.15 × 105, and 0.55 × 105 M–1, respectively. L shows color changes identical with TBAF, TBAA, or hydroxide but does not show color change with other salts (Figure S12b). In this case, the binding constants are 4.52 × 105 , 0.25 × 105, 0.16 × 105, and 3.98 × 105 M–1 for tertrabutylammonium fluoride, chloride, bromide, and acetate, respectively. These clearly establish the selectivity of binding of L as fluoride ≫ acetate > chloride ≈ bromide and of L as fluoride ≥ acetate > chloride > bromide. The gradual addition of TBAA to L shows a red-shifted absorption at 416 nm which develops through an isobestic point at 378 nm (Figure S12c). Thus, it is possible to distinguish compounds L and L independently in DMSO solution by adding TBAA. Differentiation of urea and thiourea derivatives[38,39] by an acetate anion is due to the higher acidic character of thiourea in DMSO.

Figure 7

Color changes observed by naked eye in solution (i) without and (ii) with TBAA (in DMSO 10–3 M) of (a) L and (b) L.

Color changes observed by naked eye in solution (i) without and (ii) with TBAA (in n class="Chemical">DMSO 10–3 M) of (a) L and (b) L.

1H NMR titration spectra of L and L (aromatic region is shown for clarity) with n class="Chemical">TBAA in DMSO-d6 are shown in Figure . Clear changes in the chemical shift positions of proton signals appearing in the aromatic region are observed, in addition to the changes imparted on the exchangeable protons. In the case of L, a half equivalent of TBAA shifts the f signal downfield (Figure a) and the signals d and e are slightly shifted, which is due to the hydrogen bonding of L with the acetate anion to form adduct A as illustrated in Figure a. Upon the increase of TBAA concentration to 1 equiv, there are no further changes in the chemical shifts of aromatic protons. Two N–H hydrogen atoms become more acidic upon the formation of A′ (Figure a) as reflected in their chemical shifts, and these signals appear at 13.0 and 13.3 ppm. This indicates that there is no deprotonation to generate anionic species, so the color of this solution is unaffected by TBAA. The situation with L is quite different. In this case, deprotonation takes place with half equivalent of TBAA which bridges two thiourea molecules forming B. Upon formation of anion B, the signals d and f merge as there is an exchange of protons from N–H with an acetate ion (Figure b). Also, on further addition of TBAA to this solution, a stable 1:1 adduct B′ is generated (Figure b). These solution studies suggest that the interactions of the acetate with urea and thiourea derivatives differ. This is attributed to the fact that in the case of urea, there is a competition for retaining tape-like structures by intermolecular hydrogen bonding and urea binding with acetate anion to form cyclic hydrogen-bonded subassemblies, which is illustrated as A′ in the right side of Figure a. Hence, such a competition does not allow a higher intake of acetate to the hydrogen-bonded assembly more than 1 equiv nor deprotonates the urea. By contrast, in the case of the thiourea derivative, the softer sulfur atoms of the thiourea derivative allow deprotonation to form B′ as shown in the right side of Figure b. Because of such effects, color differences arise in the two solutions.

Figure 8

Aromatic region of 1H NMR (DMSO-d6) spectra during titration of (a) L and (b) L with (i) 0.0, (ii) 0.5, and (iii) 1 equiv TBAA.

Aromatic region of 1H NMR (n class="Chemical">DMSO-d6) spectra during titration of (a) L and (b) L with (i) 0.0, (ii) 0.5, and (iii) 1 equiv TBAA.

Because of the presence of such an anion, the color of the solution turns yellow-orange, which is usual due to the interactions of TBAF with n class="Chemical">urea and thiourea derivatives;[37] for which L and L are no exceptions. UV–visible titrations of L as well as that of L with fluoride ions show isobestic points at 370 and 378 nm, respectively (Figure S13). Hence, on interaction of L or L with TBAF, one product is formed. Compound L shows color change with TBAF, acetate, or hydroxide in an identical manner but does not show color change with other salts (b of Figure S12). Similar spectral changes of L or L occur in the presence of tetrabutylammonium hydroxide which is suggestive of deprotonation of urea or thiourea derivatives (Figure S14). The interactions of L or L with TBAF are tested in different solvents such as acetonitrile, tetrahydrofuran, acetone, 1,4-dioxane, DMF, and DMSO. The absorption maxima of L or L showed greater shifts in the DMSO solvent compared with other polar aprotic solvents (Figure S17). This result is reflected in the 1H NMR titration of L with TBAF. Shifts in the positions of signals of aromatic protons close to the urea part are observed, whereas the protons present at the meta-positions are magnetically equivalent, which are less affected (Figure S18). This is due to binding of urea N–H of L and TBAF in the DMSO solvent.

1H NMR spectra of L with half equivalent of n class="Chemical">TBAF shifts the signal e of L, and it overlaps with the signal from d proton, whereas the signals c and d are not significantly shifted and signal f is nominally shifted downfield from 7.71 to 7.73 ppm. These results suggest that half equivalent of TBAF forms an assembly between the monoanion of L and neutral L. TBAF initially forms a dimeric assembly similar to the one with TBAA, but dianion of L is formed upon the addition of more amount of TBAF. For such a dianion, only two aromatic signals in the aromatic region (Figure ) are observed. UV–visible spectra of L with excess amounts of fluoride shift the absorption maximum toward a higher side eventually to 450 nm. Dianions (III) shown in Figure have an amino-quinone backbone with a flanking C=S bond and are self-assembled by an H2F+ cation. No HF2– anions are isolated during interactions with TBAF that was reported with related substrates.[40] This suggestion is based on the observed overlapping of two aromatic signals, which is likely to be from a resonance-stabilized structure suggested in spectra (iii) of Figure . 4-Nitrophenyl containing symmetric thiourea derivatives form dianions by interacting with fluoride ions;[6] in the present example, the ester containing a phenyl ring is similar to the nitrophenyl group as an electron-withdrawing group that adopts a similar resonance structure.

Figure 9

Aromatic region of 1H NMR (DMSO-d6) spectra of L with (i) 0.0, (ii) 0.5, and (iii) 1 equiv of TBAF.

Aromatic region of 1H NMR (n class="Chemical">DMSO-d6) spectra of L with (i) 0.0, (ii) 0.5, and (iii) 1 equiv of TBAF.

Cocrystals of thiourea with n class="Chemical">TBAF are well-known.[41] Attempted crystallization to obtain crystalline adducts of L or L with TBAF yielded crystals only in one case; that is crystals of product of L reacting with TBAF in a glass vessel. In this particular case, the cocrystal of L with tetrabutylammonium hexafluorosilicate having a composition 2L·(TBA)2SiF6 (1) was obtained. The crystal structure of this cocrystal has four L molecules, two SiF62– anions and two TBA counter cations, in an asymmetric unit. One SiF62– anion is held by N–H···F and C–H···F hydrogen bonds (Figure a). Fluorine atoms of the SiF62– anion act as a hydrogen bond acceptor. SiF62– anions are formed by the reaction of liberated hydrofluoric acid from the reaction of urea with TBAF and the silica of the glass vessel. Reactions involving fluoride ions to generate hexafluorosilicate using a neutral receptor in a glass vial is of interest to dissolve silica to an easily transformable substrate. Such a condition was observed earlier with a tripodal ligand which is much complex to be synthesized.[42] Alternatively, hexafluorosilicates generated by fluoride on reaction with the glass vessel are known.[43−46]

Figure 10

(a) Hydrogen bonds between L and SiF62– anion and (b) CPK model showing the surrounding of the SiF62– anion.

(a) Hydrogen bonds between L and n class="Chemical">SiF62– anion and (b) CPK model showing the surrounding of the SiF62– anion.

Two L molecules and two tetrabutylammonium cations encapsulate one n class="Chemical">SiF62– anion through hydrogen bonds as shown in the space-filling model (Figure b). Furthermore, the neutral cocrystal shows color change upon dissolution, which is not the case with other cationic receptors containing hexafluorosilicate salt.[43−46] Thus, we clearly show that neutral hydrogen-bonded species has a role in the activation of urea, leading to deprotonation of urea.

The present cocrystal 1 is interesting: it does not have an intense color in the solid state and appears as a light yellow crystal. Once the cocrystal is dissolved in n class="Chemical">DMSO, it shows absorption at 481 nm, which is the characteristic absorption maxima of the L generated upon interaction with fluoride or hydroxide. Thus, this cocrystal is an intermediate neutral complex, which in solution in DMSO forms deprotonated species of the host to exhibit a color. In fact, the compound L provides colorless solution in the presence of TBAF in 1,4-dioxane, and this particular solvent has established literature to form a stable solvate with urea derivatives.[47]

Furthermore, the assemblies formed between L and hexafluorosilicate anion in n class="Chemical">DMSO solution can be disassembled by adding water reversibly. In UV–visible spectroscopic titration of L with TBAF, the changes in absorptions pass through an isobestic point, showing that only one type of species is formed (Figure S13a). Because we could not isolate the fluoride cocrystal but could isolate hexafluorosilicate cocrystal 2L·(TBA)2SiF6, we studied the UV–visible absorption of 2L·(TBA)2SiF6 in DMSO solution by adding water and removing the added water by a dehydrating agent.

The presence of water in the solution shows a decrease in absorption at 481 nm, a peak characteristic of the deprotonated form of L and an increase in absorption at 347 nm which is due to neutral L. A similar result was found when we recorded the UV–vis absorption of 2L·(n class="Chemical">TBA)2SiF6 at different time intervals on moisture absorptions or addition of water (Figure S20). When the added water was removed by a molecular sieve, a colorless solution is formed. Such a process can be repeatedly done as shown in Figure . This observation suggests that the DMSO solvent facilitating the assembly of deprotonated L with hexafluorosilicate is opposed by water to show deprotonation–protonation equilibrium.

Figure 11

Changes in UV–visible absorption of 2L·(TBA)2SiF6 at 481 nm in the absence and presence of water in DMSO upon alternative addition of water and a molecular sieve.

Changes in UV–visible absorption of 2L·(TBA)2SiF6 at 481 nm in the absence and presence of n class="Chemical">water in DMSO upon alternative addition of water and a molecular sieve.

To differentiate energies associated with an encapsulated structure where (TBA)2n class="Chemical">SiF62– is held between two L or it would remain as a discrete cocrystal with composition 2L·(TBA)2SiF6, optimization of these compositions was carried out by DFT and was found that inclusion is favorable (Figure S19). The dynamic light scattering study has shown that in solution, 2L·(TBA)2SiF6 (Figure S21) remains as an aggregate with an average size of 218.9 nm. Upon addition of water, such an aggregation changes to form a new aggregation with an average size of 279.9 nm. Thus, this shows that water molecules reorganize the self-assembly of the cocrystal by changing the hydrogen-bond schemes to cause the color change by influencing the aggregation-induced absorption property.

Conclusions

Stabilizations of syn–syn and syn–anti forms of L and L in solution are dependent on solvents. DMSO prefers the stabilization of syn–syn geometry, whereas n class="Chemical">acetone favors the syn–anti forms of L or L. As a packing requirement, orientations of carbonyl groups of esters in DMF and DMA solvates of L differ in their respective solvate, but in solution, the study of these solvents could not differentiate such orientations. The specific ability of acetate to deprotonate thiourea but not urea provides means for visual distinctions. The intensification of color upon dissolution of cocrystal of L with a hexafluorosilicate anion provides the evidence about the existing difference between its self-assemblies in solution and in the solid state and the participation of the solvent in changing assemblies. This aspect is further reflected in the difference in the aggregation size in the DLS study of DMSO solution with or without water. In DMSO solution, equilibrium between anion-assisted deprotonation of urea or thiourea derivative can be reversibly tuned by adding water followed by removing water from the solvent. Replica self-assemblies present in solution are reflected in the solid state in certain solvates, but the number of issues related to the interplay of weak interactions makes the necessity to deal each assembly independently. On the basis of low temperature and concentration-dependent study, it is clear that the signatures of the orientations of carbomethoxy groups or the overall conformation stabilized by the solvent in particular solutions do not necessarily get reflected in solid-state structures of solvates. The observed differences in orientations in the solid state relate to the propensity of the crystals to form tightly packed structures. A similar aspect is reflected in the self-assembly of cocrystal 1 and its changes in dissolution as in solution, the participation of the solvent and water molecules matters. Such processes resulting in reversible assembly and disassembly in solution as demonstrated may be tuned to show properties and reactivity related to aggregation that may in turn have practical utilities in delivery, detection, signal transduction, and chemical reactivity.

Experimental Section

Synthesis and Characterization

To a solution of methyl-4-aminobenzoate (0.151 g, 1 mmol) in n class="Chemical">diethylether (30 mL), 4-nitrophenylisocyanate (0.164 g, 1 mmol) or 4-nitrophenylisothiocyanate (0.180 g, 1 mmol) was added and stirred at room temperature overnight. A yellow precipitate was formed; the filtered and dried samples were L or L. Different solvates of L and L were obtained by slow evaporation of the respective solution in the corresponding solvent.

DMSO Solvate of L (L·DMSO)

Isolated yield: 65%. 1H NMR (600 MHz, n class="Chemical">DMSO-d6): 9.53 (s, 1H), 9.32 (s, 1H), 8.21 (d, J = 6 Hz, 2H), 7.92 (d, J = 6 Hz, 2H), 7.71 (d, J = 6 Hz, 2H), 7.62 (d, J = 6 Hz, 2H), 3.84 (s, 3H). 13C NMR (150 MHz, DMSO-d6): 165.8, 151.7, 145.9, 143.6, 141.2, 130.4, 125.1, 123.1, 117.9, 117.7, 51.8. IR (KBr, cm–1): 3356 (w), 3308 (w), 2919 (w), 1725 (s), 1708 (m), 1596 (s), 1573 (s), 1543 (s), 1497 (s), 1433 (s), 1413 (s), 1334 (s), 1302 (m), 1284 (m), 1248 (s), 1194 (s), 1171 (s), 1109 (s), 1017 (s), 954 (m), 854 (s), 825 (m), 769 (s), 751 (s), 732 (s), 692 (w). Elemental Anal. Calcd for C17H19N3O6S is: C, 51.85; H, 4.82. Found: C, 51.74; H, 4.75. ESI mass: calcd 316.0933 [M + H+]; found, 316.0934 [M + H+].

DMF Solvate of L (L·DMF)

Isolated yield: 63%. 1H NMR (400 MHz, n class="Chemical">DMSO-d6): 9.54 (s, 1H), 9.32 (s, 1H), 8.26 (s, 1H), 8.21 (d, J = 8 Hz, 2H), 7.93 (d, J = 8 Hz, 2H), 7.70 (d, J = 8 Hz, 2H), 7.61 (d, J = 8 Hz, 2H), 3.81 (s, 3H), 2.88 (s, 3H), 2.72 (s, 3H). IR (KBr, cm–1): 3363 (s), 3311 (m), 3065 (w), 2932 (w), 1732 (s), 1710 (s), 1656 (s), 1595 (s), 1576 (s), 1540 (s), 1500 (s), 1440 (s), 1415 (s), 1344 (s), 1326 (s), 1306 (s), 1270 (s), 1248 (s), 1196 (s), 1169 (s), 1122 (w), 1122 (s), 1035 (w), 1007 (w), 965 (w), 898 (s), 856 (s), 822 (s), 789 (w), 773 (s), 740 (s).

DMA Solvate of L (L·DMA)

Isolated yield: 68%. 1H NMR (400 MHz, n class="Chemical">DMSO-d6): 9.54 (s, 1H), 9.32 (s, 1H), 8.21 (d, J = 8 Hz, 2H), 7.92 (d, J = 8 Hz, 2H), 7.70 (d, J = 8 Hz, 2H), 7.61 (d, J = 8 Hz, 2H), 3.81 (s, 3H), 2.93 (s, 3H), 2.78 (s, 3H), 1.95 (s, 3H). IR (KBr, cm–1): 3361 (s), 3301 (s), 3083 (w), 2944 (w), 1716 (s), 1625 (s), 1596 (s), 1571 (m), 1545 (m), 1502 (s), 1437 (s), 1413 (s), 1333 (w), 1312 (w), 1298 (m), 1270 (s), 1243 (s), 1197 (s), 1171 (s), 1128 (m), 1108 (s), 1012 (m), 963 (m), 897 (m), 856 (s), 826 (s), 770 (s), 751 (s), 740 (s).

DMSO Solvate of L (L·DMSO)

Isolated yield: 66%. 1H NMR (600 MHz, n class="Chemical">DMSO-d6): 10.60 (s, 1H), 10.56 (s, 1H), 8.23 (d, J = 6 Hz, 2H), 7.95 (d, J = 6 Hz, 2H), 7.84 (d, J = 6 Hz, 2H), 7.71 (d, J = 6 Hz, 2H), 3.80 (s, 3H). 13C NMR (150 MHz, DMSO-d6): 179.8, 166.4, 146.5, 144.3, 143.2, 130.5, 125.7, 125.1, 122.8, 122.5, 52.6. IR (KBr, cm–1): 3439 (br, m), 1710 (s), 1593 (w), 1553 (s), 1503 (s), 1437 (s), 1409 (m), 1328 (s), 1303 (w), 1282 (m), 1251 (s), 1199 (w), 1175 (s), 1108 (s), 1026 (s), 951 (s), 848 (s), 827 (w), 814 (w), 752 (m), 728 (s), 691 (m). Elemental Anal. Calcd for C17H19N3O5S2 is: C, 49.82; H, 4.64. Found: C, 49.76; H, 4.55. ESI mass: calcd 332.0705 [M + H+]; found, 332.0703 [M + H+].

Cocrystal 2L·(TBA)2SiF6

Compound L (0.315 g, 1 mmol) was dissolved in 15 mL DMF. To this solution, n class="Chemical">TBAF (1.5 mmol) was added, and a homogeneous solution was prepared. The resulting solution was kept for crystallization. After 20–25 days, light yellow crystals appeared. Isolated yield: 35%. 1H NMR (600 MHz, DMSO-d6): 10.04 (s, 2H), 8.14 (d, J = 6 Hz, 2H), 7.85 (d, J = 6 Hz, 2H), 7.78 (d, J = 6 Hz, 2H), 7.69 (d, J = 6 Hz, 2H), 3.81 (s, 3H), 3.17 (t, J = 12 Hz, 8H), 1.57 (m, 8H, −CH2−), 1.32 (m, 8H, −CH2−), 0.94 (t, J = 6 Hz, 12H, −CH3). 13C NMR (150 MHz, DMSO-d6): 166.6, 154.0, 148.7, 145.4, 140.5, 130.6, 125.3, 122.4, 118.2, 117.9, 57.9, 532.2, 23.4, 19.5, 13.8. 19F NMR (600 MHz, DMSO-d6): −122.2. IR (KBr, cm–1): 3356 (br, m), 2962 (s), 2875 (s), 1714 (s), 1597 (s), 1567 (m), 1546 (m), 1503 (s), 1461 (w), 1434 (s), 1413 (s), 1382 (s), 1322 (s), 1304 (s), 1285 (s), 1244 (s), 1200 (s), 1173 (s), 1109 (s), 1036 (w), 857 (s), 772 (s), 753 (s).

Crystallographic Study

X-ray single-crystal diffraction data for L and L, solvates, and cocrystals were collected at 298 K with Mo Kα radiation (λ = 0.71073 Å) with the use of a Bruker Nonius SMART APEX CCD diffractometer equipped with a graphite monochromator and an APEX CCD camera. For data collection, indexing, and determination of the unit cell parameters, SMART software was used. SAINT and XPREP software were used in data reduction and cell refinement, and finally, structures were solved by the direct method and full-matrix least-squares on F2 approximation with the aid of SHELXL-14. All nonn class="Chemical">hydrogen atoms were refined in anisotropic approximation against F2 of all reflections. Hydrogen atoms were placed at their geometric positions and either set to be “fixed” or “riding” and refined with isotropic approximation. Crystallographic data are tabulated in Table .

Table 2

Crystallographic Parameters of Solvates and Cocrystals

compound	L1·DMF	L1·DMA	L1·DMSO	L2·DMSO	2L1·(TBA)2SiF6
formula	C18H20N4O6	C19H22N4O6	C17H19N3O6S	C17H19N3O5S2	C62H98N8O10F6Si
CCDC no.	1551200	1551199	1551198	1551201	1551202
mol wt	388.38	402.41	393.41	409.47	1257.59
space group	P1̅	P1̅	P1̅	P1̅	P1̅
a/Å	7.7434(3)	8.8491(7)	8.2476(16)	8.9102(6)	13.6859(6)
b/Å	12.1722(5)	10.9303(15)	9.1570(17)	9.9426(7)	17.7916(8)
c/Å	20.2617(7)	10.9736(10)	13.002(3)	11.7194(8)	30.7988(14)
α/deg	95.227(3)	105.249(10)	80.729(15)	91.392(4)	93.988(4)
β/deg	93.915(2)	93.074(7)	80.544(14)	104.200(4)	96.865(4)
γ/deg	98.882(2)	103.966(9)	80.501(14)	106.984(4)	109.357(4)
V/Å3	1872.41(12)	985.96(19)	946.2(3)	957.44(12)	6976.8(6)
density/g cm–3	1.378	1.355	1.381	1.420	1.197
Abs. coeff/mm–1	0.105	0.103	0.210	0.312	0.107
F(000)	816	424	412	428	2697
total no. of reflections	6504	3567	3191	3215	25235
reflections, I > 2σ(I)	3582	1418	1878	1874	12 117
max θ/deg	25.05	25.24	25.04	25.05	25.25
ranges (h, k, l)	–9 ≤ h ≤ 9	–10 ≤ h ≤ 10	–9 ≤ h ≤ 9	–10 ≤ h ≤ 10	–16 ≤ h ≤ 16
	–14 ≤ k ≤ 14	–12 ≤ k ≤ 13	–10 ≤ k ≤ 10	–10 ≤ k ≤ 11	–21 ≤ k ≤ 21
	–24 ≤ l ≤ 24	–13 ≤ l ≤ 8	–15 ≤ l ≤ 15	–10 ≤ l ≤ 13	–35 ≤ l ≤ 36
completeness to 2θ (%)	98.10	99.80	95.10	94.70	99.88
data/restraints/parameters	6504/0/511	3567/0/266	3191/0/247	3215/0/247	25 235/7/1587
GooF (F2)	1.081	1.078	1.069	1.010	1.058
R indices [I > 2σ(I)]	0.0532	0.0839	0.0790	0.0479	0.0935
wR2 [I > 2σ(I)]	0.0966	0.1332	0.1689	0.1255	0.1533
R indices (all data)	0.1107	0.1879	0.1280	0.0941	0.1709
wR2 (all data)	0.1357	0.1730	0.1945	0.1478	0.1862

UV–Visible Experiments

Stock solutions of L and L (1 × 10–3 M) and different tetrabutylammoinum salts were independently prepared in DMSO. All UV–vis measurements were recorded in a 3 mL quartz cell with a 10 mm path length at room temperature. UV–vis spectroscopic titrations were recorded by adding different amounts of anion solution to L or L n class="Chemical">DMSO solutions. The effect of water on 2L·(TBA)2SiF6 was studied by recording the UV–visible spectra of 2.5 mL (1 × 10–5 M) cocrystal 2L·(TBA)2SiF6 DMSO solution. To this solution, water (1.6 wt %) was added, and UV–visible spectra were recorded. After recording the UV–visible spectra of such a solution after the addition of water, the solution was kept with molecular sieve (200 mg, average size 0.5 nm) for 1 h and decanted. The UV–visible spectrum of the decanted solution was taken again. The process was repeated three times.