Supramolecular Assembly of Tris (4-carboxyphenyl) arenes

Article pubs.acs.org/crystal

Supramolecular Assembly of Tris(4-carboxyphenyl)arenes: Relationship between Molecular Structure and Solid-State Catenation Motifs Holden W. H. Lai,*,†,# Ren A. Wiscons,† Cassandra A. Zentner,† Matthias Zeller,‡ and Jesse L. C. Rowsell†,§ †

Department of Chemistry and Biochemistry, Oberlin College, 119 Woodland Street, Oberlin, Ohio 44074, United States Department of Chemistry, Youngstown State University, 1 University Plaza, Youngstown, Ohio 44555, United States

‡

S Supporting Information *

ABSTRACT: The crystal structures of seven 1,3,5-tris(4carboxyphenyl)arenes with functionalized central arene rings are reported. The formation of (6,3) hcb hexagonal sheets as a result of carboxylic acid dimer formation was observed in most of the crystal structures, with the exception of two compounds with functional groups capable of forming hydrogen bonds, namely, 2,4,6-tris(4-carboxyphenyl)-1,3-diaminobenzene and 2,4,6-tris(4-carboxyphenyl)-3-methylaniline. These structures were found to incorporate THF solvent molecules in their hydrogen-bonding motif, giving rise to distorted pseudohexagonal arrays. Functional groups on the central ring were found to influence stacking distances, stacking offsets, inclination angles, degree of catenation, and dimensions of solvent-occupied channels. To better understand and appreciate these complicated crystal structures, they were categorized into four distinct stacking/catenation families: simple stacking, single-layer offset catenation, double-layer offset catenation, and rotated-layer catenation. The unique structure of the unfunctionalized parent compound 1,3,5-tris(4-carboxyphenyl)benzene is rationalized in light of the structural behavior of its derivatives.

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INTRODUCTION

Individual molecules of 1 have three carboxylic acid functional groups and molecular 3-fold rotational symmetry. Carboxylic acids are known to form R22(8) dimers (Figure 1),4 a robust synthon present in both the solid and liquid states.3 A recent survey of the Cambridge Structural Database (CSD) found that over a third of all carboxylic acids form R22(8) dimers.5 If carboxylic acid dimers were formed with no interruption in the self-assembly of 1, (6,3) hcb hexagonal sheets with >2 nm pore diameter would result. Silly,6 Ruben and co-workers,7 as well as Lackinger and his group8,9 employed scanning tunneling microscopy to study the 2-D self-assembly of 1 at the liquid−solid interface. Consistent with the carboxylic acid dimer synthon, it was determined that (6,3) hexagonal sheets were overall the most prevalent assembly. However, 2-D phases with alternative hydrogen-bonded networks were also observed. The R22(8) carboxylic acid dimer synthon was also found in the solid-state 3D assembly of 1. Hexagonal sheets formed in the crystal structure of 1,10 reported by our group, and that of one of its derivatives, 2,4,6-tris(4-carboxyphenyl)mesitylene (9, Table 1), described by Moorthy and co-workers.11 However, these two crystal structures have distinct packing motifs that

The understanding of extended lattice motifs of crystalline molecular compounds has greatly expanded within the past decades. Despite these advances, it is not yet possible to reliably predict a crystalline phase from solely its molecular constituents, making crystal structure prediction a challenge even for simple systems. Crystal engineering seeks to solve this predicament, aiming to rationalize and reliably predict crystal structures based on understanding and exploitation of intermolecular interactions, thus enabling the design and synthesis of molecular solid-state structures.1,2 Central to crystal engineering is the concept of supramolecular synthons, structural units formed by predictable intermolecular interactions.3 While supramolecular synthons are helpful for predicting local intermolecular interactions, they often provide little insight into the global topology of a crystal structure. Carboxylic acids are a popular choice for supramolecular synthons in the rational design of molecular crystals because of the directionality and predictability of their interactions. In recent years, our group and others have attempted to understand the self-assembly of 1,3,5-tris(4-carboxyphenyl)benzene (tcpb, 1, Table 1) and its derivatives. Among this compound and its chemical derivatives, the carboxylic acids are expected to provide the driving force that guides the overall topology of tcpb derivatives’ structures. © XXXX American Chemical Society

Received: October 3, 2015 Revised: November 26, 2015

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DOI: 10.1021/acs.cgd.5b01416 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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compare the motifs in the solid-state assembly of tcpb derivatives. The present paper follows up on our recent report on the structure and properties of parent compound 1 and systematically explores the effect of central ring chemical functionalization on its supramolecular assembly. The effect of functional groups on the molecular conformation of tcpb derivatives will be discussed, followed by the effect of molecular conformation on the formation, stacking, and catenation of the hexagonal and pseudohexagonal sheets. Finally, the solvent-occupied channels and the overall stacking and catenation motifs of the structures are compared, and the findings are used to rationalize the unique structure of the unfunctionalized parent compound 1.

Table 1. Molecular Structures of 1,3,5-Tris(4carboxyphenyl)benzene Derivatives

source

code

R1

R2

R3

ref 10 this work this work this work this work this work this work this work ref 11

1 2 3 4 5 6 7 8 9

H H H H H H H H CH3

H H H CH3 H CH3 NH2 H CH3

H CH3 OCH3 OCH3 NH2 NH2 NH2 NO2 CH3

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RESULTS AND DISCUSSION Synthesis and Crystallization of 2−8. The synthesis and crystallization of 1 and 9 have been reported elsewhere.10,11 Compounds 2−7 were synthesized by Suzuki-Miyaura coupling of three equivalents of 4-methylcarboxyphenylboronic acid with the corresponding functionalized tribromobenzenes, followed by hydrolysis of the methyl esters to yield carboxylic acids (Table 2). Although the hydrolyzed homocoupling product, 4,4′-biphenyldicarboxylic acid, was observed in the coupling reactions, this side product could be minimized by exclusion of oxygen and water during synthesis. The small traces of the homocoupling products that still formed were readily removed in the crystallization process. Because of deactivation of nitrobenzene toward bromination,14 an alternative approach avoiding the use of tribromonitrobenzene was developed for the preparation of 8. Compound 8 was synthesized by the cyclocondensation of 2 equiv of 4-methylacetophenone and 1 equiv of 4-methylbenzaldehyde to form 2,4,6-tris(4methylphenyl)pyrylium tetrafluoroborate. The pyrylium core was then reacted with nitromethane to yield 2,4,6-tris(4methylphenyl)nitrobenzene, which was oxidized with potassium permanganate to yield the triacid (Scheme 1). Single crystals of 3−8 were grown by vapor diffusion using either THF or 1,4-dioxane as the solvent, and either acetonitrile or propionitrile as the antisolvent. Single crystals of 2 were grown by slow cooling from a heated solvent mixture of 1:1 (v:v) THF/water. Single-Crystal X-ray Diffraction (SCXRD) of 2−8. Structure determinations of the crystals for these compounds were particularly challenging because of their solvent-occupied channels and tendency to quickly desolvate upon removal from the mother liquor. Back Fourier transform methods (SQUEEZE, as implemented in the program Platon15) were applied to account for electron density of unresolved or extensively disordered solvent molecules in the channels of the frameworks. For several tcpb derivatives, disorder of framework and/or solvent molecules was too pronounced, resulting in diffraction to only very low resolution. Compounds for which no data with sufficiently high resolution could be obtained include 7 and 2,4,6-tris(4-carboxyphenyl)-3,5-dimethylaniline, grown by vapor diffusion using the solvent/antisolvent pair 1,4dioxane/acetonitrile. Some of the remaining structures, such as 6 grown by vapor diffusion using the solvent/antisolvent pair 1,4-dioxane/acetonitrile, had sufficient diffraction intensity but suffered from non-Bragg behavior such as streaking of diffracted intensity between Bragg peaks (see Figure S12 for examples of representative diffraction images). Possible causes for this behavior include stacking faults of hexagonal sheets or other dislocations. However, due to the labile nature of the materials

Figure 1. R22(8) carboxylic acid dimer, an eight-membered ring consisting of two hydrogen bond donors and two hydrogen bond acceptors.

cannot be rationalized by the carboxylic acid dimer synthon alone. Crystal structures of 1 and 9 both exhibit inclined polycatenation (ICAT) but differ in their degrees of catenation (DOC),12 whereas the structure of 1 has a DOC of (7/7), the structure of 9 has a DOC of (5/5) (see Note 1 in Supporting Information (SI)). A DOC of (7/7) indicates that there are two motifs, with seven external rings from one motif catenating a single ring of the other motif, and vice versa (Figure 2).13 To better understand these distinct structural features of 1 and its derivatives, a systematic study was conducted to analyze and

Figure 2. DOC of (7/7) found in the structure of 1. Seven layers of hexagonal sheets (three dark green and four blue) catenate one hexagon (shown in light green). B



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Table 2. Synthesis of 2−7a

code

R1

R2

R3

overall yield (%)

2 3 4 5b 6b 7b

H H H H H H

H H CH3 H CH3 NH2

CH3 OCH3 OCH3 NH2 NH2 NH2

93 62 75 94 98 78

The first step was performed under nitrogen atmosphere for all compounds. bThe second step, hydrolysis of the methyl esters, was performed under nitrogen atmosphere.

a

the study. Of the structures included, that of 3 is the only structure that is noncatenated; i.e., layers of hexagonal sheets are not anchored against shifting relative to one another, which would make this material more likely to suffer from dislocations and stacking faults and also explains its particular structural instability toward desolvation. On the basis of this behavior, it is reasonable to speculate that crystals with even more pronounced non-Bragg behavior might also have noncatentated, layered structures. Crystal structure data of compounds 2−8 are summarized in Table 3. Additional structure determination details and ORTEP style plots for all compounds are given in the SI. Powder X-ray Diffraction (PXRD) and Thermal Gravimetric Analysis (TGA) of 2−8. Crystals of 2−8 were filtered, dried over a glass frit, and analyzed by PXRD and TGA. Upon filtration, all crystals, with the exception of 5, slowly became opaque as solvent evaporated from within the crystals; macroscopic fractures and cracks became evident for larger crystals. Crystals of 2, 4, 6, and 7 had PXRD diffractograms that did not match the calculated diffractograms from SCXRD data, likely due to structural transformations as a result of solvent evaporation. Crystals of 3 and 8 lost crystallinity completely upon filtration. Compound 5 retained the core features of its calculated diffractogram from SCXRD data. The peaks are shifted to lower diffraction angles, indicating an expansion of its framework between the single crystal structure measured at 100 K and the experimental powder diffractogram taken at room temperature (see Figures S13−19 for comparisons of the measured PXRD diffractograms and diffractograms calculated from SCXRD data). The observed disagreement between experimental and calculated PXRD diffractograms is consistent with a general trend for porous molecular crystals as a result of framework collapse upon evacuation of solvent molecules in voids or channels.16 Tcpb 1 is a notable exception, which was found to be unusually stable even after complete solvent evacuation.10 Despite the drastic changes in the crystal structures of 2−4 and 6−8 upon filtration and air-drying, TGA, in conjunction with NMR analyses of the residues revealed that solvent molecules remained in the solids of 3−8

Scheme 1. Synthesis of 8

and their rapid loss of crystallinity upon removal from mother liquor, it was not possible to use electron microscopy or diffraction techniques to further investigate the origin and exact nature of the non-Bragg behavior. For some of the materials, the degree of non-Bragg diffraction was too pronounced to allow for a meaningful refinement of the overall structures. These structures were therefore excluded from this report. Crystals falling into this category include 8 and 2,4,6-tris(4carboxyphenyl)-meta-xylene grown by vapor diffusion using the solvent/antisolvent pair tetrahydrofuran/acetonitrile. The reported structures of 6, 7, and 8 were determined from crystals grown from different solvents (see the crystallization conditions in the Experimental Section). Of the reported structures, compounds 8 and especially 3 did show some non-Bragg behavior, resulting in increased R values and low structure quality indicators. The nature of the frameworks and structural motifs obtained for compounds 3 and 8 are, however, unambiguous, and they are thus included in C



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Table 3. Crystal Structure Data of 1−9a 1c

compound molecular formula formula weight crystal system space group a (Å) b (Å) c (Å) β (deg) volume (Å3) % void volumeb Z, Z′ final R indices (I > 2σ(I)) compound

2

C27H18O6 438.44 monoclinic I2 31.419(6) 30.116(6) 45.320(9) 90.412(2) 42880(14) 37.1 56, 14 R1 = 0.060, wR2 = 0.138

molecular formula formula weight crystal system space group a (Å) b (Å) c (Å) volume (Å3) % void volumeb Z, Z′ final R indices (I > 2σ(I))

3

C28H20O6 452.46 orthorhombic Cmca 30.4631(10) 23.4366(8) 8.4589(3) 90 6039.2(4) 31.9 8, 0.5 R1 = 0.0334, wR2 = 0.0957

6-THF C32H29NO7 539.58 orthorhombic Pbca 26.571(3) 7.2638(8) 32.672(4) 6305.9(13) 16.2 8, 1 R1 = 0.0610, wR2 = 0.1664

C28H20O7 468.46 monoclinic C2/c 53.575(3) 31.8450(18) 25.6252(14) 118.323(3) 38485(4) 68.6 24, 3 R1 = 0.1479, wR2 = 0.5277 7-THF

C31H28N2O7 540.57 orthorhombic Pbca 26.523(4) 7.2629(6) 32.349(3) 6231.5(12) 16.8 8, 1 R1 = 0.0658, wR2 = 0.1519

4 C29H22O7 482.49 orthorhombic Pca21 7.2941(5) 33.145(2) 33.380(2) 90 8070.1(9) 44.7 8, 2 R1 = 0.0938, wR2 = 0.2687 8 C27H17NO8 483.42 orthorhombic Iba2 14.692(9) 29.009(18) 31.930(19) 13609(14) 36.7 16, 2 R1 = 0.1095, wR2 = 0.2755

5 C27H19NO6 453.43 orthorhombic Cmca 32.015(3) 24.655(2) 23.540(2) 90 18581(3) 34.1 24, 1.5 R1 = 0.0672, wR2 = 0.2851 9−2DME11 C30H24O6 660.75 orthorhombic Pbcn 27.090(3) 9.0376(9) 31.579(3) 7731.3(14) 41.6 8, 1 R1 = 0.1222, wR2 = 0.3324

a

The SQUEEZE routine was used to correct for the influence of disordered solvent molecules in all structures, with the exception of 9, in which the solvent molecules in the channels were modeled to be dimethoxyethane (DME). bThe void volume was determined after removal of all nonframework molecules using CCDC Mercury computer software, calculated using contact surface, with a probe radius of 1.2 Å and approximate grid spacing of 0.7 Å. cData of the I2 polymorph of 1, from ref 10.

up to temperatures between 100 and 200 °C (see the SI for NMR spectra and TGA traces). Molecular Conformations of 1−9. Functional groups on the central arene ring affect the molecular conformations of tcpb derivatives because of the steric repulsion between the functional groups and the hydrogen atoms at the ortho positions of the outer arene rings. This repulsion leads to increases in torsion angles between the central and outer arene rings of tcpb derivative molecules. To probe this effect, the average torsion angles between the central and outer arene rings were plotted against the average steric A values17 (see Note 2 in SI) of the three substituents on the central arene ring in compounds 1−9 (Figure 3). As expected, torsion angles increase with the steric A values of the substituents on the central arene ring. Compound 9, which has three methyl groups on the central arene ring and the highest average steric A value (1.7 kcal/mol for methyl), has three outer arene rings that are nearly orthogonal to the central ring (average torsion angle = 87.66°). An understanding of the ways in which functional groups on the central arene ring of the tcpb derivatives affect their molecular conformations aids in the rationalization of their solid-state assemblies; molecules become less planar the larger the torsion angles are between the central and substituent arene rings, which affects their ability to π-stack in the solid state. This phenomenon will be discussed in more detail in the following sections. Formation of (6,3) hcb Hexagonal and Pseudohexagonal Sheets. The formation and stacking of (6,3) hcb sheets held together entirely by R22(8) dimers are conserved in

Figure 3. Correlation between the average torsion angle (deg) between central and outer arene rings and the average steric A value of substituents on the central ring for the crystal structures of 1−9. The A value15 (kcal/mol) of hydrogen is zero (see SI for tables of all torsion angles and steric A values).

the crystal structures of 1−5 and 8−9. The hexagonal sheets formed were found to be especially distorted among tcpb derivatives that have large torsion angles between central and outer arene rings, such as 4 and 9 (Figure 4d,h). The structures of 6 and 7 also have large torsion angles and distorted pseudohexagonal sheets, but the distortion is primarily due to interruption of the carboxylic acid dimer pattern by solvent molecules. Two parallel sides of the hexagons are shortened by hydrogen bonding of the carboxylic acid toward a THF molecule and a methyl or amine group (Figure 4f). The hydrogen-bonding motif here forms an eight-membered ring D



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Figure 5. (a) One third of the carboxylic acid groups of 6 form R23(8) connections that consist of COOH and NH2 groups from two molecules of 6 and two THF molecules, forming one side of the hexagon. The gray, white, blue, and red atoms are carbon, hydrogen, nitrogen, and oxygen atoms, respectively; (b) this R23(8) hydrogenbonding motif consists of two donors, three acceptors, and forms an eight-membered ring. Crystal structure refinement determined that in the structure of 6 the methyl group can take the place of the amino group and form the same hydrogen-bonding motif. Red dashed lines indicate hydrogen bonds.

carboxylate, creating coulombic repulsion that would prevent the formation of R22(8) dimers and hydrogen bonding between the carboxylic acid group and THF molecules as observed in 5−7. Stacking of (6,3) Hexagonal and Pseudohexagonal Sheets. Stacking of the (6,3) hexagonal sheets is conserved in the crystal structures of 1−5, 8, and 9. Analogous stacking of pseudohexagonal sheets is observed in 6 and 7. To better understand and quantify the stacking distances between hexagonal sheets, the distance S (Figure 6) was measured

Figure 4. Complete hexagons from the (6,3) hcb sheets in the crystal structure of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, (g) 8, and (h) 9, and pseudohexagonal sheet in the structures of (f) 6 and 7. The hexagonal sheets are held together by R22(8) carboxylic acid hydrogen bonds (for 6 and 7, R22(8) and R23(8)). The gray, white, blue, and red atoms are carbon, hydrogen, nitrogen, and oxygen atoms, respectively. Lengths of the diagonals of the hexagons were determined in units of Å by measuring the interatomic distance between the atoms directly attached to the central arene rings that are located at the opposite vertices of the hexagons, and subtracting the van der Waals radii18 of the atoms. The directions of the black arrows also indicate the direction along which catenating hexagonal or pseudohexagonal sheets stack, except in the case of 3 (c), which does not exhibit polycatenation.

Figure 6. Distance S (in Å) used to quantify the stacking distances between hexagonal sheets. The red spheres are the centroids of the central arene rings of the tcpb derivative molecules.

between the central arene rings of the stacked tcpb derivative molecules. Generally, tcpb derivative molecules with greater torsion angles between the central and outer arene rings were found to have larger stacking distances S (Figure 7). This correlation is expected because orthogonal outer rings push stacked molecules further apart. Inclined Polycatenation. Inclined polycatenation (ICAT) is observed in all of the tcpb derivative crystal structures with the exception of 3. The DOC, however, are different among these structures (Figure 8). DOC and Inclination angles (Figure 9), the acute angles formed by two catenating hcb sheets in the ICAT structures, were determined using TOPOS19 and are tabulated in Table 4. Among the polycatenated crystal structures, functional groups on the central ring affect the degree of catenation by changing two structural features: the dimensions of the hexagonal channels and the stacking distances between hexagonal sheets. As depicted in Figure 4, all of the catenating hexagonal sheets stack along a diagonal of the hexagonal channel, indicated by the black arrows. Crystal structures of 2− 9 all have shorter diagonals relative to 1 because of the functional groups that are present at the vertices of the hexagonal channels. As a result, fewer catenating hexagonal

and consists of three donors and two acceptors (Figure 5). It is therefore categorized as an R23(8) motif.4 While the amino functional group, which is weakly basic and capable of both accepting and donating hydrogen bonds, did form hydrogen bonds with solvent molecules to interrupt the hcb sheet formations in 6 and 7, these interactions were not observed in 5, which has a single amino group. There was no evidence of proton transfer between the amine and carboxylic acid groups in the crystal structures of 5−7. Hydrogen atoms at both carboxylic acid as well as amine groups were resolved in difference density electron maps for fully occupied and major disordered moieties. Any significant proton transfer from carboxylic acid to amine would create a negatively charged E



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Figure 9. Inclination angle, θ, from the crystal structure of 1.

Table 4. Inclination Angles (Figure 8) and DOC of the ICAT Structures of 1, 2, 4−9

Figure 7. Correlation between the average stacking distances S (Figure 6) and average torsion torsion angle (deg) between central and outer arene for the crystal structures of 1−9.

sheets will fit along the diagonal, leading to lower DOC. This explains why crystal structures of tcpb derivatives with one functional group on the central ring (2, 5, 8) have a DOC of (6/6), lower than the DOC of (7/7) in the crystal structure of nonfunctionalized 1. With multiple functional groups on the central ring, as is the case with both 4 and 9, the torsion angles between the central and outer arene rings increases due to steric repulsion. This leads to increased stacking distances (Figure 7). With shorter diagonal lengths in the hexagonal channels and increased stacking distances between hexagonal sheets, both 4 and 9 have a DOC of (5/5), lower than what is observed in the structure of the monofunctionalized tcpb derivatives other than 3. Crystal structures of 6 and 7 have pseudohexagonal channels with the shortest diagonals as a result of the THF-amine/methyl hydrogen-bonding motif (Figure 5). As a result, these structures have the lowest DOC of (4/4) among the polycatenated crystal structures. Solvent-Filled Channels. Polycatenation influences the dimensions of the solvent-occupied channels in the structures of 1−9 (Figure 10). As expected, the noncatenated crystal structure of 3 has the largest % void volume reported here, at 69%, and a channel diameter of greater than 2 nm. The catenated structures with hexagonal sheets connected entirely through carboxylic acid dimers have solvent-occupied volumes

crystal structure

inclination angle (deg)

DOC

1 2 4 5 6 7 8 9

67.2 49.6 84.5 55.3 84.9 84.8 66.7 61.9

(7/7) (6/6) (5/5) (6/6) (4/4) (4/4) (6/6) (5/5)

within a range of 30−50% and channel diameters of less than 1 nm. Structures of 6 and 7 on the other hand, which have smaller and distorted pseudohexagonal sheets due to hydrogen bonding with THF molecules, have reduced solvent-occupied volumes of less than 20%. For the calculation of % void volumes in 6 and 7, the tightly bound THF molecules are considered an intrinsic part of the hydrogen-bonded framework for 6 and 7, not as disordered guest molecules in the voids. Stacking and Catenation Motifs. As discussed in the previous sections, supramolecular synthons and functional groups on the central arene ring can help rationalize structural motifs in the solid-state assembly of tcpb derivatives. While hydrogen bonding and the R22(8) carboxylic acid dimer synthon reliably lead to the formation of (6,3) hcb hexagonal or pseudohexagonal sheets, steric demand of the functional groups on the central arene ring affects the molecular conformations of

Figure 8. DOC in crystal structures of 1, 2, and 4−9. (a) 1 DOC (7/7), (b) 2 DOC (6/6), (c) 8 DOC (6/6), (d) 5 DOC (6/6), (e) 4 DOC (5/5), (f) 9 DOC (5/5), and (g) 6 and 7 DOC (4/4), which are isomorphous. Green molecules form catenating hexagonal or pseudohexagonal sheets that are analogous to the purple ones. The green molecules are shown in alternating light and dark for clarity. The gray, white, and red atoms in (g) are respectively carbon, hydrogen, and oxygen atoms that belong to the ordered THF molecules that are part of the hcb sheets (Figure 5). F



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Figure 10. Depiction of solvent-occupied channels of the 1 × 1 × 1 unit cells of the crystal structures of 1 (a, view along the c-axis), 2 (b, view along the c-axis), 3 (c, view along the c-axis), 4 (d, view along the a-axis), 5 (e, view along the c-axis), 6 and 7 (f, view along the b-axis), 8 (g, view along the a-axis), and 9 (h, view along the b-axis). Channels are displayed in purple using the CCDC Mercury computer software, calculated using contact surface, with a probe radius of 1.2 Å and approximate grid spacing of 0.7 Å.

tcpb derivatives, which has implications for the stacking distances between hexagonal sheets and the DOC. Though these structural motifs are essential for the most basic analysis of tcpb derivatives’ crystal structures, they are inadequate for rationalizing the subtle differences between the structures’ stacking and catenation motifs. To better understand and appreciate the complicated structures of tcpb derivatives, crystal structures of 1−9 were categorized into four distinct stacking/ catenation families: simple stacking, single-layer offset catenation, double-layer offset catenation, and rotated-layer catenation. The hexagonal sheets in the crystal structure of 3 exhibit simple stacking because the layers are tightly stacked with no offsets or gaps of sufficient size to allow interweaving with inclined hexagonal sheets. The structure consists of three crystallographically independent molecules, as depicted in Figure 11. The centroid-to-centroid distance between the central arene rings of molecules depicted in violet and green in Figure 11b is at the upper limit of what is typically considered parallel displaced π−π stacking (3.3−3.8 Å).20 Centroid-tocentroid distances between the green and blue molecules and between the violet and blue molecules are out of that range. However, close packing facilitated through edge-to-face CH-π interactions between the outer arene rings (Figure 11b) leaves little to no space between hexagonal sheets. Polycatenation, which requires space between stacked hcb sheets for catenating layers to interweave, is therefore not possible and not observed in the crystal structure of 3. Molecules are stacked in one direction only, with continuous solvent-occupied channels propagating the entire length of the crystal, but being isolated from each other. This results in a structure with an unusually

Figure 11. (a) Stacking of (6,3) hexagonal sheets in the crystal structure of 3; (b) centroid-to-centroid distances (Å) between the central arene rings of the three stacking crystallographically independent molecules (centroids shown as small orange spheres). Molecules are colored by symmetry equivalence.

large void volume of ca. 69%, double that of its polycatenated analogues. Single-layer offset catenation, the most common catenation among the structures reported here, also features the simplest form of catenation. Crystal structures belonging to this family (2, 4, 6, 7, 9) are made up of a single crystallographically independent layer of a (6,3) hexagonal or pseudohexagonal sheet. Parallel layers are slipped along each other, having just enough offset to create a gap that can accommodate a single layer of another catenating hexagonal or pseudohexagonal sheet (Figure 12). The way this catenation motif is realized differs substantially between structures 2, 4, 6, 7, and 9. All structures belonging to this catenation family, with the exception of the isomorphous structures of 6 and 7, feature different space groups, inclination angles, DOC, as well as distinct π-stacking G



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Figure 12. (a) Stacking of (6,3) hexagonal sheets in the crystal structure of 2. Offset stacking leads to a gap (indicated by black arrows) between the hexagonal sheets; (b) the gap is ideally sized to accommodate a single layer of a catenating hexagonal sheet (shown in blue). A similar topology is also found in the crystal structures of 4, 6, 7, and 9.

Figure 13. (a) Stacking of (6,3) hexagonal sheets in the crystal structure of 8. Offset stacking leads to a gap (indicated by black arrows) between every other hexagonal sheet; (b) the gap is ideally sized to accommodate two layers of catenating hexagonal sheets (shown in dark green and dark blue).

Figure 14. (a−c) Stacking (6,3) hexagonal sheets in the crystal structure of 1. Rotated stacking leads to openings between the hexagonal sheets. (d) The gap indicated by yellow and red arrows are ideally sized to accommodate four and three layers of catenating hexagonal sheets (shown in light blue and light green), respectively. A similar topology is also found in the crystal structure of 5, which has 3 + 3 rotated stacking as opposed to 3 + 4.

H



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of nearly 200 °C, and differentiates both 1 and 5 drastically from compounds 2−4 and 6−8, which all undergo substantial changes in their structures, or lose crystallinity altogether, upon removal from mother liquor. The combination of catenation with extensive closest π-stacking and rotation of stacks seems to give rise to a unique stability to tcpb derivatives not observed for compounds that are π-stacked but not catenated (such as 3), or compounds that are only catenated, but without extensive closest π-stacking. Further experiments, including variable-temperature PXRD and gas adsorption studies, will be necessary to demonstrate that 5 can retain its original crystal structure and porosity after complete solvent evacuation. If 5 proves to be permanently porous and similarly stable as 1, it would be exciting to test for its potential applications in molecular separations, as there has been growing interests in developing hydrogen-bonded organic frameworks (HOF) for separation and catalysis.21−23 These experiments, however, are outside the scope of the present paper, which focuses on supramolecular assembly. The unique rotated-layer catenation observed for 1 and 5 also helps to rationalize the unusually large number of crystallographically independent molecules observed in the two polymorphs of 1, with Z′ numbers of 14 for the I2 polymorph, and 56 for the P1 polymorph, the largest number of crystallographically unique but chemically identical entities within one crystal lattice reported to date.10,24 In the structure of 5, there are stacks of three symmetry-independent molecules, which are π-stacked with another three-molecule thick stack of the same type. The three molecules on top are related to those at the bottom by an inversion center in the middle of the sixmolecule stack. Inversion centers are also located between neighboring stacks, at the center of the carboxylate dimers of the second molecule of the three-molecule stacks. The overall number of crystallographically independent molecules in 5 is thus three, the thickness of half of the six-molecule stack. In the structure of 1, no symmetry operation is conceivable that would be able to convert a three-molecule stack into a four molecule stack, either within one seven-molecule stack or between molecules in parallel stacks. The smallest symmetry independent unit in 1 has to consist of two parallel stacks of each seven molecules as there cannot be any nontranslational symmetry either within one stack, or between directly neighboring stacks. Fourteen independent molecules are indeed observed in the simpler of the two polymorphs, and in the less symmetric case it is four times that value, 56.

distances. Despite these differences, there seems to be a preference among tcpb derivatives to form crystals with this catenation motif, pointing toward kinetic and/or thermodynamic factors favoring this arrangement. Less prevalent is multilayer offset catenation, with only one case of double-layer offset catenation observed among the tcpb derivatives. Multilayer offset catenation is similar to single-layer offset catenation, but with stacking of hexagonal sheets alternating between closest stacking and offset stacking such that after a certain number of tightly stacked layers, there is an offset ideally sized to accommodate the same number of catenating hexagonal sheets (Figure 13). The crystal structure of 8, the only structure belonging to this catenation family, consists of one unique double-layer of (6,3) hexagonal sheets. This motif emulates single-layer polycatenation, with a double layer in 8 taking the role of single layers in 2, 4, 6, 7, and 9. Both single and double layer offset catenation seem to be associated with medium-to-large torsion angles between the central and peripheral arene rings, and all structures with torsion angles above 40° belong to one of these families (Figure 3). An entirely different catenation motif is realized in the crystal structures of 1 and 5, both of which can be categorized as rotated-layer catenated. In the crystal structure of 1, closest stacking occurs within stacks of three or four hexagonal sheets (green and blue molecules, Figure 14a). Within these stacks, 1 stacks the same as 3, leaving no space or gap between layers for other molecules to interweave. Immediately above a closestpacked three-molecule stack follows a four-molecule stack of closest-stacked hexagonal sheets. The four-molecule stack (blue, Figure 14a) is rotated by ca. 60° with respect to the three-molecule stack (green, Figure 14a). These sevenmolecule thick stacks fit perfectly within one opening formed by a catenating hexagonal sheet (Figure 14b). The same kind of rotated 3 + 4 stacking continues above the blue molecules in Figure 13b, but the stack above is shifted by one molecule (dark green molecules, Figure 14c). As a result of this rotated stacking, undulating channels are created through which interweaving hexagonal sheets can fit (Figure 14d,e). An analogous rotated-layer catenation is observed in the structure of 5, which has 3 + 3 rotated stacking as opposed to 3 + 4 in 1. The DOC is (6/6) for the 3 + 3 stacks of 5 as opposed to (7/ 7) for the 3 + 4 arrangement in 1. There is a correlation between the extent of π-stacking and the torsion angles between central and outer arene rings. Among all investigated structures, 1, 3, and 5 have the smallest average torsion angles between the central and outer arene rings, with 33.7°, 36.4°, and 36.4°, respectively (Figure 3). Small torsion angles appear to be a prerequisite for the formation of closest, as opposed to offset, π-stacks made up of three or more molecules as observed in 1, 3, and 5. All other structures investigated have arene−arene torsion angles above 40° and exhibit single or double layer offset catenation. The similarities between the structures of 1 and 5 extend beyond their solid-state arrangements, differentiating themselves from the other examples by an increase in framework stability. Among the examples investigated, 1 and 5 are the only two structures that survive removal from the mother liquor and air-drying, as evidenced by the agreement of the powder diffractogram of 5 measured after filtration and air-drying with that of its SCXRD structure. These data suggest that the structure of 5 may have stability similar to that of 1, which was shown to retain its single crystal structure up to a temperature

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CONCLUSIONS A series of 1,3,5-tris(4-carboxyphenyl)arenes with functionalized central arene rings were synthesized, and their crystal structures were determined. The two important supramolecular synthons found in the nonfunctionalized 1,3,5-tris(4carboxyphenyl)benzenecarboxylic acid dimers leading to (6,3) hcb sheets and π-stacking of the (6,3) sheetsare conserved in all of the structures reported, with the exception of 6 and 7, which incorporate THF molecules in their pseudohexagonal sheets. The different structures demonstrate that functionalization at the central arene ring affects both molecular conformations and solid-state self-assembly. Functionalization at the central ring leads to steric repulsion between the functional groups and the hydrogen atoms at the ortho position of the outer arene rings, leading to larger torsion angles between the central and outer arene rings. Molecules with larger torsion angles were generally found to have larger I



Article

by direct methods using the SHELXTL suite of programs26,27 and refined by full matrix least-squares against F2 with all reflections using Shelxl2013 or Shelxl201428,29 and the graphical interface Shelxle.30 H atoms attached to carbon, nitrogen, and oxygen atoms were positioned geometrically and constrained to ride on their parent atoms, with carbon hydrogen bond distances of 0.95 Å for aromatic C−H, 0.99 and 0.98 Å for aliphatic CH2 and CH3, 0.88 Å for NH2 moieties and 0.84 Å for carboxylic acid moieties, respectively. Methyl and hydroxyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. Uiso(H) values were set to a multiple of Ueq(C/O/N) with 1.5 for CH3 and OH, and 1.2 for C−H, CH2, and N−H units, respectively. SCXRD details are reported in Table 3 (compounds 2− 8) or in Table S1 of the SI (homo coupling product dimethyl biphenyl-4,4′-dicarboxylate and synthetic intermediates 2,4,6-tris(4carboxyphenyl)anisole trimethyl ester, 2,4,6-tris(4-carboxyphenyl)aniline trimethyl ester and 2,4,6-tritolylnitrobenzene). Additional details for each structure are given in the SI, including disorder, pseudosymmetry handling, restraints and constraints used, and details on solvent-filled void space and numbers of electrons corrected for. Complete crystallographic data, in CIF format, have been deposited with the Cambridge Crystallographic Data Centre. CCDC 1422491− 1422501 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Synthesis of 2,4,6-tris(4-methylphenyl)pyrylium Tetrafluoroborate. The procedure described by Kotra and co-workers31 was modified as follows: A 100 mL 3-neck round-bottom flask, equipped with two septa and a condenser topped with a gas inlet adapter, was purged with nitrogen. Twenty milliliters of anhydrous toluene was added using a gastight syringe, followed by the addition of 4methylacetophenone (6.5 mL, 0.049 mol) and 4-methylbenzaldehyde (3.0 mL, 0.025 mol). The reaction mixture was heated to 80 °C. Boron trifluoride diethyl etherate (6.5 mL) was then added via syringe, after which the color of the reaction mixture turned from yellow to deep red. The reaction mixture was then heated to reflux for 2 h under nitrogen. After the mixture was allowed to cool to room temperature, 10 mL of acetone were added, followed by 250 mL of diethyl ether. The resulting precipitate was collected by suction filtration and dried over a glass frit. Yield 3.73 g (8.5 mmol, 35%); yellow solid: 1H NMR (400 MHz, DMSO-d6) δ 9.03 (s, 2H), 8.54 (d, 2H, J = 8.4 Hz), 8.48 (d, 4H, J = 8.4 Hz), 7.62−7.60 (m, 6H), 3.343 (s, 9H); 13C NMR (100 MHz, DMSO-d6) δ 169.88, 164.41, 147.07, 146.49, 130.91, 130.45, 130.07, 129.07, 126.96, 113.88, 21.91, 21.86. APCI-MS+ m/z Calcd for C26H23O+ [M − BF4−] 351.17, found 351.2. Synthesis of 2,4,6-tris(4-methylphenyl)nitrobenzene. The procedure described by Dimroth and co-workers32 for the synthesis of 2,4,6-triphenylnitrobenzene was modified as follows: In a 100 mL round-bottom flask, 2,4,6-tris(4-methylphenyl)pyrylium tetrafluoroborate (3.39 g, 7.7 mmol), nitromethane (0.6 mL, 11.2 mmol), and 34 mL of absolute ethanol were stirred at room temperature and purged with nitrogen. Triethylamine (1.8 mL, 12.9 mmol) was added rapidly using a gastight syringe. The reaction mixture was then heated at reflux for 3 h, during which time the mixture turned from a yellow slurry to a red/brown solution. The reaction mixture was rotary evaporated to reduce the solvent volume by half and the residue was allowed to cool in a refrigerator overnight. The resulting precipitate was collected by suction filtration and recrystallized from acetic acid. Yield 1.35 g (3.4 mmol, 4%): light yellow needles: 1H NMR (400 MHz, DMSO-d6) δ 7.74 (d, 2H, J = 8.0 Hz), 7.71 (s, 2H), 7.35 (d, 4H, J = 8.0 Hz), 7.32− 7.28 (m, 6H); 13C NMR (100 MHz, DMSO-d6) δ 148.24, 142.83, 138.68, 138.64, 135.55, 134.74, 133.65, 130.11, 129.87, 128.34, 128.26, 127.71, 21.19, 21.16. SCXRD data are given in the SI. S y n t h e s i s a n d C r y s t a l G ro w t h of 2, 4 , 6- T r i s (4 carboxypheny)nitrobenzene (8). 2,4,6-Tris(4-methylphenyl)nitrobenzene (1.0 g, 2.5 mmol) was dissolved in 15 mL of pyridine in a 100 mL round-bottom flask, and 15 mL of deionized (DI) water and potassium permanganate (2.04 g, 12.9 mmol) were added. The resulting purple slurry was then heated to reflux. When the color of the reaction mixture turned completely from purple to brown/black, more

stacking distances between hexagonal sheets, leading to lower DOC, and to single or double layer offset catenation. Small torsion angles, below 40°, allow for extensive closest π-stacking. In the case of compound 3 this led to infinite closest stacking with complete elimination of catenation, resulting in open hexagonal channels with 69% of solvent-occupied volume in its structure. The other structure type found for molecules with small torsion angles and extensive π-stacking is rotated-layer catenation, as observed for structures of 1 and 5. This latter solid-state arrangement is the only one among tcpb derivatives yet observed that survives removal from mother liquor and air drying without collapse of the hcb framework, making these two rotated-layer catenation structures uniquely stable among this family of closely related structure types. Although the carboxylic acid dimer synthon did not itself provide rationalization of the global topologies of the crystal structures reported, it reliably predicted the 2-D (6,3) hexagonal sheet assembly, even though pseudohexagonal sheets found in 6 and 7 were interrupted by solvent molecules. This prediction simplified the analysis of the crystal structures and focused the analysis on the stacking and catenation of (6,3) hexagonal or pseudohexagonal sheets. The tcpb derivative crystal structures reported demonstrate the significant crystal structural changes that are possible with subtle molecular modifications, but more examples are necessary to fully understand the relationship between solid-state assembly and molecular structure. As evident from crystal structures of 6 and 7, which incorporated solvent molecules into their frameworks, crystallization conditions can affect the resulting crystal structures. Future work on this system should therefore not only focus on obtaining structures of new tcpb derivatives, but also consider the possibility of polymorphism and/or different solvates for a given molecule.

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EXPERIMENTAL SECTION

All reagents were purchased from Sigma-Aldrich and used without further purification, with the following exceptions: 4-(methylcarboxy)phenylboronic acid (OxChem), 2,4,6-tribromotoluene and tripotassium phosphate (Acros Organics), 2,4,6-tribromoaniline (TCI America), 2,4,6-tribromo-3-methylaniline (Alfa Aesar), concentrated HCl and potassium carbonate (Fisher Scientific), ethyl acetate, ethanol, and methanol (Pharmco-Aaper). A Varian 400 MHz NMR spectrometer was used. All NMR spectra were taken at room temperature with spinning at 20 Hz. Shimming and locking were performed using the Varian VnmrJ software. An Agilent 1100 Series LC/MSD system was used for atmospheric pressure chemical ionization mass spectrometry. Samples were dissolved in HPLC grade methanol (Pharmco-Aaper) and carried into the mass spectrometer by a mobile phase consisting of pure methanol at a flow rate of 0.500 mL/min. High-resolution mass spectrometry for crystals of 2-6 and 8 were performed at the University of Akron Mass Spectrometry Facility. A Rigaku Ultima IV diffractometer (λ = 1.5418 Å) with Bragg−Brentano geometry was used for room temperature PXRD experiments. A scan speed of 4 deg/min was used. Single Crystal X-ray Diffraction. Instruments used include a Bruker SMART APEX CCD diffractometer with Mo K-α radiation, a Bruker AXS Prospector CCD diffractometer with Cu K-α radiation, and a Bruker AXS D8 Quest CMOS diffractometer with Mo K-α radiation. Data for the crystal structure of 5 were collected at the Advanced Photon Source at Argonne National Lab using source Beamline 15-ID B ChemMatCARS and a Bruker AXS APEXII CCD diffractometer as the measurement device. The wavelength of the synchrotron radiation was 0.41328 Å. The Apex225 suite of programs was used for collection of diffraction data, unit cell determinations, data integration and correction for absorption and other systematic errors. The space groups were assigned and the structures were solved J



Article

J = 8.0 Hz), 7.61 (m, 6H), 2.01 (s, 3H). 13C NMR (100 MHz, DMSOd6) 167.58, 167.54, 146.13, 143.85, 142.82, 137.04, 132.89, 130.38, 130.11, 130.05, 129.73, 127.93, 127.33, 18.85. APCI-MS+ m/z Calcd for C28H19O6− [M − H+] 451.1182, found 451.1204. 1H NMR data are consistent with those previously reported in the patent literature.35 Single crystals for SCXRD analysis were grown by slow cooling. Crude product (400 mg) was mixed with 40 mL of 1:1 (v:v) tetrahydrofuran/water and sonicated in a 50 mL media bottle. The resulting slurry was tightly capped and heated at 70 °C in an oven, during which the media bottle was shaken every 15 min. After 2 h of heating, the mixture phase-separated. The media bottle was then removed from the oven and allowed to cool to room temperature overnight. The two phases were then combined by gently swirling the mixture. The resulting solution was refrigerated, and needle-shaped crystals grew after 2 days. 2,4,6-Tris(4-carboxyphenyl)anisole (3). White solid. Yield: 270 mg (0.58 mmol, 62% yield): 1H NMR (400 MHz, DMSO-d6) 12.99 (s, 3H), 8.04 (d, 4H, J = 8.0 Hz), 8.00 (d, 2H, J = 8.0 Hz), 7.92 (d, 2H, J = 8.0 Hz), 7.80 (d, 4H, J = 8.0 Hz), 7.75 (s, 2H), 3.12 (s, 3H). 13C NMR (100 MHz, DMSO-d6) 167.60, 167.56, 155.18, 143.67, 142.55, 135.90, 135.33, 130.34, 130.21, 130.08, 129.83, 129.77, 127.45 61.05. APCI-MS+ m/z Calcd for C28H19O6− [M − H+] 467.1131, found 467.1085. Single crystals for SCXRD analysis were grown by vapor diffusion with the solvent/antisolvent pair 1,4-dioxane/acetonitrile: the crude product (150 mg) was mixed with 2.8 mL of 1,4-dioxane and sonicated. The mixture was then filtered by syringe filtration, separated into 2 aliquots, followed by vapor diffusion with acetonitrile. Blockshaped crystals grew in 2 days. 2,4,6-Tris(4-carboxyphenyl)-3-methylanisole (4). White solid, contains 25 mol % pyridine. Yield: 334 mg (0.70 mmol, 75% yield): 1 H NMR (400 MHz, DMSO-d6) 12.97 (s, 3H), 8.03 (d, 2H, J = 8.0 Hz), 7.99 (m, 4H), 7.73 (d, 2H, J = 8.0 Hz), 7.55 (d, 2H, J = 8.0 Hz), 7.48 (d, 2H, J = 8.0 Hz), 7.31 (s, 1H), 3.02 (s, 3H), 1.91 (s, 3H). 13C NMR (100 MHz, DMSO-d6) 167.64, 167.58, 167.56, 154.55, 145.69, 142.59, 142.44, 137.68, 136.63, 135.00, 131.63, 131.37, 130.51, 130.07, 130.07, 129.96, 129.88, 129.86, 129.70, 129.60, 129.43, 60.76, 19.08. APCI-MS+ m/z Calcd for C29H21O7− [M − H+] 481.1287, found 481.1317. Single crystals for SCXRD analysis were grown by vapor diffusion with the solvent/antisolvent pair 1,4-dioxane/acetonitrile: the crude product (65 mg) was mixed with 3.0 mL of 1,4-dioxane and sonicated. The mixture was then filtered by syringe filtration, separated into three aliquots, followed by vapor diffusion with acetonitrile. Block-shaped crystals grew in 1 week. 2,4,6-Tris(4-carboxyphenyl)aniline (5). Hydrolysis of the methyl esters was performed under nitrogen atmosphere. Green solid, contains 23 mol % ethyl acetate. Yield: 338 mg (0.87 mmol, 94% yield): 1H NMR (400 MHz, DMSO-d6) 12.93 (s, 3H), 8.05 (d, 4H, J = 8.0 Hz), 7.93 (d, 2H, J = 8.0 Hz), 7.79 (d, 2H, J = 8.0 Hz), 7.68 (d, 4H, J = 8.0 Hz), 7.47 (s, 2H), 4.65 (s). 13C NMR (100 MHz, DMSOd6) 167.66, 167.58, 144.38, 144.09, 142.33, 130.46, 130.36, 130.34, 130.03, 129.81, 128.78, 128.44, 127.23, 126.13. APCI-MS+ m/z Calcd for C27H18NO6− [M − H+] 452.1134, found 452.1091. NMR data are consistent with those previously reported.36 Single crystals for SCXRD analysis were grown by vapor diffusion with the solvent/antisolvent pair tetrahydrofuran/acetonitrile: the crude product was mixed with tetrahydrofuran and sonicated. The mixture was then filtered by syringe filtration, followed by vapor diffusion with acetonitrile. Plate-shaped crystals grew in a few weeks. 2,4,6-Tris(4-carboxyphenyl)-3-methylaniline (6). Hydrolysis of the methyl ester was performed under nitrogen atmosphere. Yellow solid, contains 5 mol % biphenyl-4,4′dicarboxylic acid (homocoupling product). Yield: 424 mg (0.90 mmol, 98% yield): 1H NMR (400 MHz, DMSO-d6) 12.94 (s, 3H), 8.08 (d, 2H, J = 8.0 Hz), 7.99 (d, 2H, J = 8.0 Hz), 7.94 (d, 2H, J = 8.0 Hz), 7.62 (d, 2H, J = 8.0 Hz), 7.48 (d, 2H, J = 8.0 Hz), 7.44 (d, 2H, J = 8.0 Hz), 7.00 (s, 1H), 1.83 (s, 3H). 13C NMR (100 MHz, DMSO-d6) 167.65, 167.60, 167.55, 146.56, 144.08, 143.48, 133.42, 131.18, 130.81, 130.77, 130.46, 130.36, 130.29, 130.07, 129.63, 129.59, 129.57, 129.03, 127.87, 127.61, 124.09, 19.20. APCI-

potassium permanganate (2.95 g, 18.7 mmol) was added directly to the reaction mixture. Heating at reflux was then continued until the reaction mixture once again turned from purple to brown/black. The reaction mixture was then filtered to remove manganese(IV) oxide (deep brown solid). More potassium permanganate (3.1 g, 19.6 mmol) was then added to the filtrate, and the resulting purple solution was heated at reflux. When the reaction mixture turned from purple to brown/black, more potassium permanganate was added (2.1 g, 13.2 mmol). The mixture was then heated at reflux until all of the purple color disappeared. Manganese(IV) oxide was removed from the mixture by suction filtration, and the filtrate was washed with ethyl acetate (3 × 20 mL) and acidified with 1 M HCl. The resulting precipitate was collected by suction filtration and dried over a glass frit. Yield 778 mg (1.6 mmol, 64.3%): light yellow powder: 1H NMR (400 MHz, DMSO-d6) 13.14 (s, 3H), 8.04−8.02 (m, 8H), 7.97 (s, 2H), 7.60 (d, 4H, J = 8.4 Hz); 13C NMR (100 MHz, DMSO-d6) 167.39, 167.25, 148.38, 142.21, 142.04 140.41, 134.34, 131.57, 131.29, 130.36, 130.21, 129.77, 128.81, 128.27; APCI-MS − m/z Calcd for C27H16NO8− [M − H+] 482.0876, found 482.0829. Single crystals for SC-XRD analysis were grown by vapor diffusion experiments on the benchtop at 20 °C. The crude product (33.3 mg) was dissolved in 1 mL of 1,4-dioxane. The solution was filtered by syringe filtration, followed by vapor diffusion with propionitrile. Plates suitable for SCXRD developed within 3 days. Synthesis of 1,3-Diamino-2,4,6-tribromobenzene. In a 100 mL round-bottom flask, a mixture of m-phenylenediamine (1.78 g, 16.5 mmol) and 50 mL of acetic acid was stirred at room temperature until all solids were completely dissolved. Stirring was then continued with the round-bottom flask immersed in an ice water bath. Bromine (3 mL, 60 mmol) was added dropwise using a Pasteur pipet. After the bromine was added, the reaction mixture was removed from the ice water bath and allowed to stir at room temperature for 2 h. The black mixture was suction filtered, and the black solid was allowed to air-dry over a glass frit. The black solid was then extracted with chloroform, filtered, and the filtrate was completely evaporated to leave behind a brown solid. Yield: 1.8 g (5.2 mmol, 31.5%): 1H NMR (400 MHz, CDCl3) 7.44 (s, 1H), 4.51 (s, 4H). 13C NMR (100 MHz, CDCl3) 141.65, 133.23, 96.10, 95.28. NMR data are consistent with those previously reported.33 General Procedure for Suzuki Coupling and Hydrolysis. In a nitrogen glovebox, substituted tribromobenzene (0.93 mmol), 4(methylcarboxy)phenylboronic acid (500 mg, 2.8 mmol), tetrakis(triphenylphosphine)palladium(0) (40 mg, 0.043 mmol), and tripotassium phosphate (620 mg, 2.9 mmol) were mixed with 3 mL of commercial anhydrous methanol and 10 mL of commercial anhydrous tetrahydrofuran in a 20 mL scintillation vial charged with a stir bar. The vial was tightly capped, removed from the glovebox, and stirred at 90 °C in an aluminum heating block for 3 days. After the reaction mixture cooled to room temperature, the solvent was completely evaporated under a stream of air. The homocoupling side product, dimethyl biphenyl-4,4′-dicarboxylate, was observed in varying amounts. The identity of this side product was confirmed by SCXRD. Structure details are given in the SI and agree with data reported previously.34 The residue was extracted with dichloromethane and filtered through a glass frit. The solvent was completely evaporated from the filtrate to obtain the trimethyl esters (SCXRD details for the trimethyl esters of 2,4,6-tris(4-carboxyphenyl)anisole and 2,4,6-tris(4-carboxyphenyl)aniline are given in the SI). The crude trimethyl esters were dissolved in 15 mL of pyridine. Fifteen milliliters of 2.5 M aqueous NaOH were added to the pyridine solution, and the resulting biphasic mixture was heating at reflux for 20 min. The mixture was allowed to cool to room temperature, and enough 1 M aqueous HCl was added such that the two phases combined. The solution was then washed twice with 15 mL of ethyl acetate. Concentrated HCl was then added to the aqueous layer. The resulting precipitate was collected by suction filtration and allowed to air-dry over a glass frit. 2,4,6-Tris(4-carboxyphenyl)toluene (2). White solid. Yield: 400 mg (0.87 mmol, 93% yield): 1H NMR (400 MHz, DMSO-d6) 12.99 (s, 3H), 8.03 (d, 4H, J = 8.0 Hz), 7.98 (d, 2H, J = 8.0 Hz), 7.88 (d, 2H, K



Article

MS+ m/z Calcd for C27H19N2O6− [M − H+] 466.1291, found 466.1327. Single crystals for SCXRD analysis were grown by vapor diffusion with the solvent/antisolvent pair tetrahydrofuran/acetonitrile: the crude product was mixed with tetrahydrofuran and sonicated. The mixture was then filtered by syringe filtration, followed by vapor diffusion with acetonitrile. Needle-shaped crystals grew in a few weeks. 2,4,6-Tris(4-carboxyphenyl)-1,3-diaminobenzene (7). Hydrolysis of the methyl ester was performed under nitrogen atmosphere. Brown solid, contains 10 mol % pyridine and 14 mol % biphenyl-4− 4′dicarboxylic acid (homocoupling product). Yield: 338 mg (0.72 mmol, 78% yield): 1H NMR (400 MHz, DMSO-d6) 12.89 (s, 3H), 8.10 (d, 2H, J = 8.0 Hz), 7.97 (d, 4H, J = 8.0 Hz), 7.64 (d, 4H, J = 8.0 Hz), 7.57 (d, 2H, J = 8.0 Hz), 7.01 (s, 1H). 13C NMR (100 MHz, DMSO-d6) 167.61, 167.58, 146.85, 143.66, 143.10, 139.38, 132.39, 131.80, 131.12, 131.09, 130.22, 129.60, 129.45, 127.78. APCI-MS+ m/ z Calcd for C27H19N2O6− [M − H+] 467.12, found 467.2. Single crystals for SCXRD analysis were grown by vapor diffusion with the solvent/antisolvent pair tetrahydrofuran/acetonitrile: the crude product was mixed with tetrahydrofuran and sonicated. The mixture was then filtered by syringe filtration, followed by vapor diffusion with acetonitrile. Needle-shaped crystals grew in a few days.

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Crystallography at the Advanced Photon Source, SCrAPS, program supported by Indiana University. The authors thank Drs. Josh Chen and Maren Pink for data collection. ChemMatCARS Sector 15 is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE1346572. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. The authors thank Professor Davide M. Proserpio for help with using the TOPOS program. The authors also thank Selim Gerislioglu and Chrys Wesdemiotis at the University of Akron Mass Spectrometry Facility for collecting high-resolution mass spectrometry data.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01416. Details of single crystal structure determinations, Ortep style plots for all new structures reported, and experimental details for structures of side products and intermediates. Selected diffraction images, comparisons of measured PXRD diffractograms and diffractograms simulated from SCXRD experiments, TGA/DSC traces, 1 H and 13C NMR spectra, tables of torsion angles, as well as CIFs, are available free of charge (PDF) Accession Codes

CCDC 1422491−1422501 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address #

Department of Chemistry, Stanford University, Stanford, CA 94305, United States. Notes

The authors declare no competing financial interest. § J.L.C.R., who passed away on January 30, 2015, was the principal investigator who designed this research. The authors are grateful for his unsurpassed mentorship and friendship.

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ACKNOWLEDGMENTS The X-ray diffractometers at Youngstown State University were funded by NSF Grants DMR-1337296, CHE-0087210, by Ohio Board of Regents Grant CAP-491, and by Youngstown State University. The powder X-ray diffractometer at Oberlin College was purchased with support of NSF grant DMR-0922588. Data for 5 were collected at Beamline 15-ID B ChemMatCARS at the Advanced Photon Source through the Synchrotron L



Article

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