Can Entropically Unfavorable Topological Hydrogen‐Bonded Networks Form From Tetrakis‐ or Pentakis(4‐Carboxyphenyl)benzene Derivative?

ABSTRACT

Control of network topology is one of the most important issues in reticular chemistry. Herein, we report the network structures of the missing pieces of carboxyphenylbenzene (CPB) derivatives, the simplest series of building blocks for hydrogen‐bonded organic frameworks (HOFs), and elucidate their network topology in a crystalline state. 1,2,3,5‐Tetrakis‐ and 1,2,3,4,5‐pentakisCPBs (1235CPB and 12345CPB) form H‐bonded networks with sql ‐ and cem ‐topologies, respectively. Furthermore, it is remarkable that 1235CPB forms a more complicated cqe ‐topological network structure with three kinds of voids in crystalline states under the same crystallization condition, although the formation occurs infrequently. The network structure with cqe ‐topology is hitherto unknown in molecule‐based framework materials. Other molecules in the series of CPB gave no H‐bonded network with complicated topology, enabling us to estimate the limitation for generating entropically unfavorable networks on the basis of a simple consideration of the number of cases for the network formation. We also report thermal behaviors, porosity, and structural transformation of these H‐bonded network crystals from a materials perspective.

1,2,3,5‐Tetrakis‐ and 1,2,3,4,5‐pentakis(4‐carboxyphenyl)benzene derivatives (1235CPB and 12345CPB) form H‐bonded networks with sql ‐ and cem ‐topologies, respectively. Furthermore,1235CPB forms a more complicated cqe ‐topological network structure with three types of voids in crystalline states under the same crystallization condition, although the formation occurs infrequently. The cqe ‐network is hitherto unknown in molecule‐based framework materials.

1. Introduction

It has been crucial for developing organic functional materials to design molecule‐based network structures precisely because the network topology significantly affects their electronic behaviors [1], the adsorption behavior of chemicals [2], and other chemical properties [3]. Huge numbers of combinations of organic ligands and metal centers, including secondary building units (SBUs) or molecular components with multiple functional groups, have realized versatile network topologies in metal‐organic frameworks (MOFs) [4, 5, 6] or covalent organic frameworks (COFs) [7, 8]. Network topology of frameworks has been categorized using three‐letter codes as demonstrated by O'keffee and Yaghi [9, 10], and can be analyzed using ToposPro software provided by Proserpio and Blatov [11, 12]. Precise control of the network topology of frameworks is particularly important [13], and a handful of excellent examples were reported [14, 15]. Zhao and coworkers demonstrated that the network topology of COFs composed of 4,4',4'',4'''‐(ethene‐1,1,2,2‐tetrayl)tetraaniline and terephthalaldehyde derivatives can be controlled by the substituents of the terephthalaldehyde: the aldehyde with hydroxy groups gave the kgm (Kagome)‐network, while that with ethoxy or butoxy groups gave the sql (square lattice)‐network [14]. These structures are referred to as topological isomers. Sun, Wang, and coworkers reported that a kgm ‐topological 2D COF and dia (diamond)‐topological 3D COF can be selectively synthesized by temperature control during the polymerization reaction of 4,4',4'',4'''‐(1,4‐phenylenebis(azanetriyl))tetrabenzaldehyde PATB and p‐phenylenediamine [15]. These structures are referred to as dimensional isomers. More interestingly, structural transformation from the 2D to 3D COF was accomplished in the latter system.

Molecule‐based network structures constructed through hydrogen bonds (H‐bonds), referred to as hydrogen‐bonded organic frameworks (HOFs) [16, 17, 18, 19, 20, 21, 22, 23], have also been of interest due to their high crystallinity, enabling the revelation of their crystal structures by facile single‐crystal X‐ray diffraction (SCXRD) analysis. However, formation of topological and dimensional isomers of HOFs is much less reported compared with the interpenetration isomers observed in hcm (honeycomb)‐topological [24, 25, 26], and acs ‐topological [27] networks. As exceptional examples, we reported that bowl‐shaped hexacarboxylic acid CPSM yielded both an undulated hxl (hexagonal)‐network sheet and a complex bi‐layered sheet formed via a trefoil‐knot H‐bonded network under the same crystallization conditions [28]. Tohnai and coworkers constructed porous organic salts with various network topologies, such as dia , lon (lonsdaleite), uni, and sod (sodalite), using tetrahedral tetrasulfonic acid and triphenylmethyl amine derivatives [29].

To investigate the relationship between the geometry of tectons and the H‐bonded network structures in HOFs systematically, Wuest and coworkers prepared hexa‐, penta‐, and tetra‐substituted benzene derivatives with diaminotriazine (DAT) groups and revealed their crystal structures [30]. A DAT group, however, can form multiple H‐bonds, making it difficult to identify and categorize the network topologies. On the other hand, it is well‐known that shape‐persistent rigid molecules possessing three or more carboxy groups can form a robust networked structure with clear topology by the formation of H‐bonded dimers of the carboxy groups [31, 32]. In particular, a series of carboxyphenylbenzene derivatives (CPBs) possessing three or more 4‐carboxyphenyl groups at various positions of the benzene core are the simplest and appropriate tectons of HOFs to explore the relationship between geometry and symmetry of tectons and network topology of HOFs. To date, hexa‐substituted CPB [33], tris‐substituted CPBs at 1,3,5‐, 1,2,4‐, and 1,2,3‐positions (135CPB [24], 124CPB [34], and 123CPB [35], respectively), tetra‐substituted CPBs at 1,2,3,4‐ and 1,2,4,5‐positions (1234CPB [36] and 1245CPB [37], respectively) were reported to form HOFs with fully networked structures via forming the self‐complementary H‐bonded dimer of the carboxy groups.

In this study, we were interested in the last remaining tetra‐substituted derivative, 1235CPB, as well as penta‐substituted 12345CPB, to complete a series of CPBs and reveal their H‐bonded network structures formed in crystalline states. Tetrasubstituted derivatives have three isomers: 1245CPB, 1234CPB, and 1235CPB with point groups of D _2v, D _2h, and D _2v, respectively (Figure 1a–c). Assuming that carboxyl groups can invariably form H‐bonded dimers to yield a 2D network structure, 1234CPB can provide a ladder network with triangular void I from two types of tectons oriented by inversion operations (Figure 1e), while 1245CPB can yield an sql ‐topological 2D network with rhombic void II through translational operations of tectons oriented in one type (Figure 1f). These network structures were indeed observed in the crystalline state [36, 37]. Similarly, it is easy to imagine that 1235CPB can form a sql ‐network with rhombic void II by translational tiling of the tecton in two types of orientations related by inversion symmetry operations (Figure 1g). 12345CPB can also form a cem ‐network [38] with both rhombic and trigonal voids I and II (Figure 1h), where the cem ‐network is a 2D network with a vertex surrounded by three triangles and two squares (i.e., 3³.4²).

FIGURE 1.

Schematic representation of the dangling bond‐free 2D hydrogen‐bonded network structures of tetrakis‐, and pentakis‐(4‐carboxyphenyl)benzene derivatives. Molecular structures of (a) 1234CPB, (b) 1245CPB, (c) 1235CPB, and (d) 12345CPB, and the simplest corresponding network structures with sql ‐ or cem ‐topology (e–h). More complicated possible 2D network structures of (i) 1245CPB with kgm ‐topology, (j) 1235CPB with cqe ‐topology, and (k) 12345CPB with fsz ‐topology. Building blocks are colored blue, red, gray, cyan, pink, and green, depending on the molecular orientation with different rotational angles within the network structures. These networks involve three kinds of voids with triangular (I), rhombic (II), and hexagonal (III) geometries.

On the other hand, it is theoretically possible for 1245CPB to form a kgm ‐topological network with voids I and III from the tectons in three kinds of different orientations (Figure 1i). Indeed, pyrene‐based tetracarboxylic acid was reported to form a kgm ‐like topological network [39], in addition to a sql ‐network [40], although a H‐bond between the carboxy groups is not an ideal self‐complementary dimer but a truncated dimer. However, no HOF exhibits a kgm ‐topological network composed of 1245CPB was reported. Similarly, 1235CPB can form a cqe ‐topological network with trigonal, rhombic, and hexagonal voids I, II, and III, from the tectons in four kinds of different orientations (Figure 1j), where the cqe ‐network is a 2D network with two types of vertices: one is surrounded by two triangles, one square, and one hexagon (i.e., 3².4.6) and the other is by triangle, one square, and two hexagons (i.e., 3.4.6²). In the case of 12345CPB, an fsz ‐topological network with trigonal and hexagonal voids I and III, composed of tectons with five kinds of different orientations, is theoretically possible to form (Figure 1k), where the fsz ‐network is a 2D network with a vertex surrounded by four triangles and one hexagon (i.e., 3⁴.6).

2. Results and Discussion

2.1. Synthesis and Crystallization

Tecton 1235CPB was synthesized by Suzuki–Miyaura cross‐coupling reaction of tetrabromobenzene derivative 1, which was prepared according to literature [41], and methoxycarboxyphenylboronic acid (2), followed by hydrolysis of the resultant ester 3 (Scheme 1a). Similarly, 12345CPB was synthesized according to Scheme 1b. 2,3,4,5,6‐Pentabromobenzenaminium chloride (4) was reduced by tert‐butyl nitrite [42], and the resultant pentabromobenzene (5) was reacted with 2 via a Suzuki‐Miyaura cross‐coupling reaction to give penta‐substituted ester 6. Hydrolysis of 6 in the presence of KOH gave 12345CPB. During purification, the product was thoroughly washed with water, CHCl₃, and methanol, resulting in relatively lower isolation yields (i.e., 43%).

SCHEME 1.

Syntheses of (a) 1235CPB and (b) 12345CPB.

Tecton 1235CPB was recrystallized from a mixed solution of o‐dichlorobenzene (DCB) and THF at 40°C, giving platelet thin crystals (Figure S1). During SCXRD analyses, we happened to encounter a very rare topological framework, that is, a layered HOF with a cqe ‐topological network, referred to as cqe‐1235CPB‐L1. Repeated crystallization under the same conditions and SCXRD analyses of 1235CPB, however, revealed that most of the single crystals exhibited another topological network different from cqe‐1235CPB‐L1. The crystals have a sql ‐topological network, which is further interpenetrated to give HOF sql‐1235CPB‐P1. These two kinds of crystals exhibit a closely similar morphology. SCXRD‐suitable single crystals of HOF cqe‐1235CPB‐L1 were merely obtained despite many efforts. These indicate that cqe‐1235CPB‐L1 is a kind of disappearing polymorph, that is, a crystal form that has been prepared and identified experimentally at least once, but does not form in subsequent attempts to prepare the same crystal form by the same procedure [43, 44]. Moreover, 1235CPB was recrystallized from a mixed solution of DMF and aromatic solvent such as DCB or methyl benzoate (MeBz) at 100°C, to give platelet crystals (Figure S2). SCXRD analysis revealed that the crystal also has a sql ‐topological network, which, on the other hand, is stacked without interpenetration to give layered HOF sql‐1235CPB‐L1. Tecton 12345CPB was recrystallized from a mixed solution of THF and MeBz at 40°C to give a layered HOF with a cem ‐topological network, referred to as cem‐12345CPB‐L1 (Figure S3). This form was also obtained at 100°C using a DMF/MeBz solvent system. The crystallization conditions for HOFs and crystal data are summarized in Table 1, Tables S1, and S2.¹ The crystal structures of the above‐mentioned HOFs are discussed in the next section.

TABLE 1.

Crystallization conditions and resultant frameworks.

Host	good solvent	Template solvent	T/°C	Obtained HOF
1235CPB	THF	DCB	40	cqe‐1235CPB‐L1 (minor precipitate) sql‐1235CPB‐P1 (major precipitate)
1235CPB	DMF	DCB	100	sql‐1235CPB‐L1
1235CPB	DMF	MeBz	100	sql‐1235CPB‐L1
12345CPB	THF	MeBz	40	cem‐12345CPB‐L1
12345CPB	DMF	MeBz	100	cem‐12345CPB‐L1

2.2. Crystal Structure

2.2.1. Crystal Structure of HOF cqe‐1235CPB‐L1

Tecton 1235CPB crystallized into space group P‐1. The crystal structure is composed of two symmetrically independent molecules, A and B (colored cyan and pink, respectively), with slightly different molecular conformations (Figure 2a). The dihedral angles of the peripheral phenyl groups (1, 2, 3, and 5) against the central benzene rings in molecule A are 55.2°, 59.4°, 42.4°, and 39.8°, respectively, while those in molecule B are 43.3°, 62.1°, 55.1°, and 14.6°, respectively. The molecules are connected via self‐complementary H‐bonding between the carboxy groups to form a cqe ‐topological network sheet (Figure 2b), where the molecules colored in cyan and pink are related to those colored in dark blue and red, respectively, by inversion symmetry operations, and therefore, the network is composed of four kinds of components with different orientations. There are no dangling bonds of the carboxy groups in the structure. It is noteworthy that a cqe ‐topological molecular network has not been reported for MOFs, COFs, and HOFs, to our best knowledge. The sheet has three kinds of voids (I, II, and III) with trigonal, rhombic, and hexagonal apertures, respectively. The sheet was slip‐stacked with an AA manner to yield layered HOF cqe‐1235CPB‐L1 (Figure 2c). The layer is slightly undulated, probably due to packing forces. Some of the included DCB molecules were refined crystallographically (Figure S4a,b). The molecules of DCB sandwiched the carboxyphenyl dimers from both sides of the plane of the dimer through face‐to‐face intermolecular interactions, which stabilize the crystal structure (Figure S4c). On the other hand, solvent molecules accommodated in the void were not identified due to severe disorder. Since pure bulk crystals of cqe‐1235CPB‐L1 were not obtained, TG or NMR analysis to determine a host‐guest stoichiometry could not be conducted.

FIGURE 2.

Crystal structures of HOFs (a–c) cqe‐1235CPB‐L1, (d–f) sql‐1235CPB‐P1, and (g–i) sql‐1235CPB‐L1. (a,d,g) Symmetrically independent two molecules A (cyan or dark blue) and B (pink or cyan) drawn by anisotropic displacement ellipsoids with 50% probability. (b,e,h) H‐bonded network structures with cqe ‐, sql‐, and sql‐topologies, respectively. (c,f,i) Assembly manners of the 2D layers viewed from the top and side of the layers. Dihedral angles (ω) of the peripheral phenylene rings (1,2,3,5) against the central benzene ring are inserted in (a,d,g). Trigonal, rhombic, and hexagonal voids are referred to as I, II, and III, respectively, in (b,e,h). Phenyl group 5 of molecule B in sql‐1235CPB‐P1 is disordered in two positions, one of which is shown in (e,f).

2.2.2. Crystal Structures of HOFs with sql‐Network

In most cases of crystallization under the condition using the THF/DCB solvent system at 40°C, 1235CPB yields HOF sql‐1235CPB‐P1 belonging to space group P222₁. The crystal structure is composed of two independent C ₂‐symmetric molecules A and B (colored by dark blue and cyan, respectively) with different molecular conformations (Figure 2d): The dihedral angles (ω) of the peripheral phenyl groups (1, 2, 3, and 5) against the central benzene rings in molecule A are 55.2°, 63.4°, 55.2°, and 43.5°, respectively, while those in molecule B are 53.9°, 57.1°, 53.9°, and 56.0°/65.6°, respectively. The molecules are connected via the H‐bonding of the carboxy groups to form a sql ‐topological 2D networked sheet (Figure 2e), where molecules A and B align in an antiparallel manner. The 2D sheet has voids II with a rhombic aperture. The sql ‐topological 2D network is interwoven with a dihedral angle of 40.6°, giving 3D catenated HOF sql‐1235CPB‐P1 (Figure 2f). The interwoven network is supported by multiple CH/O interactions that are formed between the hydrogen atom of the phenylene ring and the oxygen atom of the carboxy group, with CH···O distances of 2.32–2.48 Å (Figure S5). In the void, molecules of DCB are accommodated, although they were not refined crystallographically due to their severe disorder. As described before, a different crystallization condition, i.e., slow evaporation of a mixed solution of DCB and DMF at 100°C, resulted in a change of assembly manner of the sql ‐network sheet to give HOF sql‐1235CPB‐L1. The crystal belonging to space group P2₁/c is composed of two independent molecules A and B (colored by dark blue and cyan, respectively) with slightly different molecular conformations (Figure 2g): The dihedral angles (ω) of the peripheral phenyl groups (1, 2, 3, and 5) against the central benzene rings in molecule A are 53.7°, 46.1°, 55.5°, 20.9°, respectively, while those in molecule B are 53.0°, 43.1°, 58.9°, 19.3° respectively. It is noteworthy that the value of ω5 in sql‐1235CPB‐L1 is smaller than that in sql‐1235CPB‐P1 due to the layered staking structure. The molecules in sql‐1235CPB‐L1 form the same sql ‐topological 2D network as sql‐1235CPB‐P1 (Figure 2h). However, the 2D sheet was slip‐stacked to yield layered HOF sql‐1235CPB‐P1 (Figure 2i). CH/π and π/π interactions are observed between the layers (Figure S6). Inclusion channels originating from void II are formed along the c‐axis. In the void, solvent molecules of DCB are accommodated with a host: guest molar ratio of 1: 5.

To investigate the effect of crystallization temperature on the appearance of cqe‐1235CPB‐L1 and sql‐1235CPB‐P1, powder X‐ray diffraction measurements were performed on precipitates obtained at various crystallization temperatures using a mixed solution of THF and DCB. Solvated crystals of HOFs accommodating halogenated aromatic solvents such as dichlorobenzene are known to exhibit low X‐ray diffraction contrast [45, 46, 47]. Therefore, PXRD measurements were performed using synchrotron radiation X‐rays (λ = 1.00 Å) as shown in Figure 3. Crystallization at low temperatures yields a precipitate with a characteristic peak at 2θ = 4.18° attributed to the (021) plane of cqe‐1235CPB‐L1, although its single crystal was not identified. Conversely, as the crystallization temperature rises from room temperature to 60°C, the corresponding diffraction peak of the resultant precipitate becomes extremely weak and then disappears. Meanwhile, the characteristic peak at 2θ = 3.79° attributed to the (002) plane of sql‐1235CPB‐P1 is observed for all precipitates. These results indicate that cqe‐1235CPB‐L1 can generate in kinetically controlled environments, but thermodynamically more stable sql‐1235CPB‐P1 is always preferentially produced.

FIGURE 3.

PXRD (λ = 1.00 Å) patterns of 1235CPB recrystallized from a THF/DCB solution at various temperatures ranging from −10 to 60°C, together with the simulated patterns of cqe‐1235CPB‐L1, sql‐1235CPB‐P1, and sql‐1235CPB‐L1.

2.2.3. Crystal Structure of HOF cem‐12345CPB‐L1

The penta‐substituted tecton yields HOF cem‐12345CPB‐L1 belonging to space group P‐1. The crystal includes one symmetrically dependent molecule (Figure 4a). The dihedral angles (ω) of the peripheral phenyl groups (1, 2, 3, 4, and 5) against the central benzene rings are 56.3°, 61.6°, 58.7°, 70.6°, and 47.7°, respectively. As observed in other penta‐substituted ring systems [48], all peripheral phenyl groups incline in the same direction due to the steric repulsion. The molecules are connected via the H‐bonding of the carboxy groups to form a cem ‐topological 2D networked sheet (Figure 4b), where molecules with two kinds of orientations related by inversion symmetry operation align in an antiparallel manner. The 2D sheet has voids I and II with triangle and rhombic apertures. The layers are slip‐stacked in an inverted manner to give the HOF (Figure 4c) CH/O and CH/π interactions are observed between the layers (Figure S7). In the void, solvent molecules MeBz are accommodated, although they were not refined crystallographically due to their severe disorder.

FIGURE 4.

Crystal structure of layered HOF cem‐12345CPB‐L1. (a) Symmetrically independent molecule drawn by anisotropic displacement ellipsoids with 50% probability. (b) cem ‐topological H‐bonded network structure. (c) Stacking manner of 2D layers viewed from the top and side of the layers. Dihedral angles (ω) of the peripheral phenylene rings (1,2,3,4,5) against the central benzene ring are inserted in (a). Trigonal and rhombic voids are referred to as I and II, respectively, in (b).

2.2.4. Network Formation Probability

Regarding the 2D networking pattern, tetratopic or pentatopic tectons 1245CPB, 1235CPB, and 12345CPB have formed three simple networks with sql ‐ or cem ‐topology and one complex network with cqe ‐topology, although six kinds of 2D networks are potentially possible to form as shown in Figure 1. Particularly, it is noteworthy that only one complex topology (i.e., cqe ‐topology) was obtained among three (i.e., kgm ‐, cqe ‐, and fsz ‐topologies). To interpret these results, we evaluated the network formation probability P for each network using the following simple model, like a “jigsaw puzzle,” considering only the combination probability (Scheme 2). In the case of 1245CPB and 1235CPB, the central molecule (0) with four binding sites is bonded by four molecules (1) to (4) that have pre‐defined orientations (i.e., 60°, 120°, 180°, 240°, 300°, and/or 360°‐rotated orientations) necessary to form the desired topological networks. Similarly, in the case of 12345CPB, the central molecule (0) is bonded by five molecules (1) to (5). Enthalpic factors, such as intermolecular H‐bonds and π/π interactions, are not considered.

SCHEME 2.

Probability evaluation for forming (a) sql ‐ and kgm ‐toplogical networks from 1245CPB, (b) sql ‐ and cqe ‐topological networks from 1235CPB, and (c) cem ‐ and fsz ‐topological networks from 12345CPB based on simple combination.

When 1245CPB forms an sql ‐topological network, the four peripheral molecules (1) to (4) have the same orientation in a 0° rotation. The probability that four indistinguishable equivalent molecules occupy four equivalent binding sites is 1 [i.e., P( sql ) = 1]. Similarly, the probability of forming the kgm ‐network from two molecules in a 60° rotation and two molecules in a 120° rotation is calculated to be P( kgm ) = 1/6. Therefore, a probability ratio to form a kgm ‐network against an sql ‐network is 1/6. In the case of 1235CPB, the formation probabilities of the sql ‐ and cqe ‐networks are calculated to be P( sql ) = 1/6 and P( cqe ) = 1/12, respectively. The probability ratio to form cqe ‐network against the sql ‐one is, therefore, 1/2. These results indicate that the kgm ‐network of 1245CPB has less chance to form than the cqe ‐network of 1235CPB in the corresponding competitive network formation processes. These results indicate that in each of the two competing network formation processes, the kgm ‐network of 1245CPB has less chance to form than the cqe ‐network of 1235CPB. Indeed, the kgm ‐network of 1245CPB has not been known, while the cqe ‐network of 1235CPB was obtained in this study despite its lower formation frequency than that of the sql ‐network.

In the case of 12345CPB, the formation probabilities of the cem ‐ and fsz ‐networks are calculated to be P( cem ) = 1/10 and P( fsz ) = 1/120, respectively. The probability ratio to form fsz ‐network against the cem ‐one is 1/12, indicating that the fsz ‐topological network may be rarely obtained in a competitive crystallization condition. Indeed, when 12345CPB was recrystallized at various temperatures, the resulting crystalline precipitates had the same diffraction patterns (Figure S8).

2.2.5. Thermal Stability and Porosity

The obtained HOFs are composed of the H‐bonded 2D sheets containing voids I, II, and/or III, but the shapes of inclusion cavities in the HOFs differ from each other, influencing the solvent removal process and gas sorption behaviors. Figure 5 shows visualized surfaces of the inclusion cavities for each HOF. In HOF cqe‐1235CPB‐L1, three types of inclusion channels originating from voids I, II, and III are formed along the a‐axis, and narrow tunnels, formed along the c‐axis, connect the channels, giving two dimensionally extended inclusion spaces (Figure 5a). Ratio of the solvent accessible volume (void ratio) is 61.8% [49]. In sql‐1235CPB‐P1, channels with a parallelogram‐shaped opening are formed along the b‐axis, and the channels are connected by slightly wider tunnels parallel to the a‐axis formed between layers that mutually penetrate with a dihedral angle of 40° (Figure 5b). Consequently, this HOF also possesses a 2D inclusion space. The void ratio is 60.8%. In sql‐1235CPB‐L1, channels similar to sql‐1235CPB‐P1 are formed along the c‐axis (Figure 5c). Although sql‐1235CPB‐L1 possesses a layered structure, thin tunnels form every four layers due to the distortion and non‐uniform conformation of the carboxyphenyl group, connecting the channels via this narrow tunnel, giving two‐dimensionally extended inclusion spaces. The void ratio is 48.2%. In cem‐12345CPB‐L1, 1D channels are formed along the a‐axis, but openings within the channel are obstructed by periodically arranged “beams”, resulting in bottlenecks (Figure 5d). The void ratio is 46.9%.

FIGURE 5.

Visualized solvent‐inclusion spaces in crystals of (a) cqe‐1235CPB‐L1, (b) sql‐1235CPB‐P1, (c) sql‐1235CPB‐L1, and (d) cem‐12345CPB‐L1. Each crystal includes three views from different directions. Inside and outside surfaces of the spaces are colored with cyan and dark blue, respectively.

Thermal analyses were conducted using crystalline powder of the HOFs. Since HOFs cqe‐1235CPB‐L1 and sql‐1235CPB‐P1 were not able to separate, a powder sample obtained by recrystallization at 60°C, which mostly contains sql‐1235CPB‐P1, was used for the analyses. Thermal gravimetric (TG) analysis was conducted on HOFs sql‐1235CPB‐P1, sql‐1235CPB‐L1, and cem‐12345CPB‐L1 (Figure S9). TG curve of sql‐1235CPB‐P1 showed a weight loss of 57% up to 180°C, which corresponds to a host: guest ratio of 1: 5. HOF sql‐1235CPB‐L1 showed stepwise weight loss of 45% in total up to 200°C, indicating a host: guest ratio of 1: 3. The TG curve of cem‐12345CPB‐L1 showed a distinct stepwise weight loss, with a total weight loss of 42% up to 180°C, which corresponds to a host: guest ratio of 1: 3.5, although no plateau region was observed in the TG curve. The estimated host: guest ratios from these TG analyses are in good agreement with those determined by ¹H NMR spectra (Figures S10–S12).

To reveal structural changes upon release of the guest molecules included in the cavity of HOFs, variable‐temperature (VT) PXRD measurements were performed. Crystalline powder of sql‐1235CPB‐P1 obtained by recrystallization at 60°C from a mixed solvent of THF and DCB was used for the experiment. At around room temperature, the diffraction intensity is very weak due to disordered chlorinated solvent molecules, while diffraction peaks at 2θ of 5.67° and 11.50°, corresponding to the (002) and (004) planes, become obvious starting around 100–150°C (Figure S13a). The diffraction pattern remains up to 250°C, indicating that this interpenetrated framework retains its original structure even after solvent molecules are released. This result does not contradict the findings of many previous studies, which indicate that HOFs with interpenetrating structures exhibit greater structural rigidity [46, 50, 51, 52]. Crystalline powder of sql‐1235CPB‐L1 obtained by recrystallization at 100°C from a mixed solvent of DMF and MeBz was used for the experiment, resulting in clear PXRD patterns at around room temperature (Figure S13b). As the temperature increased, irregular peak changes were observed up to 100°C, and then no obvious peaks were observed up to 300°C. This indicates that the structural periodicity of the framework is significantly disrupted during the solvent desorption process. Subsequently, peaks at 16°–17° are observed around 300°C. These peaks might be attributed to a shrunk framework. Crystalline powder of cem‐12345CPB‐L1 obtained by recrystallization at 40°C from a mixed solvent of THF and MeBz was used for the experiment. The original diffraction peak at 2θ of 6.07°, corresponding to the (010) and/or (001) plane, was slightly shifted to 6.99° at 150°C (Figure S13c). The resultant pattern remained up to 250°C, indicating the second form (12345CPB‐2) can be a robust framework.

2.2.6. Crystal Structures of the Shrunk Frameworks

To reveal crystal structures after the structural changes, the removal of the included MeBz molecules was conducted by a solvent exchange method. Crystals of sql ‐1235CPB‐L1 were immersed in hexane for 1 h at room temperature, followed by heating at 80°C. Some of the crystals retained single‐crystallinity, allowing SCXRD analysis of the resultant crystal encapsulating hexane, referred to as 1235CPB‐2(hexane). As shown in Figure 6, two H‐bonded carboxy dimers in the 2D rhombic framework cleave, and with molecular movement, H‐bonding reformation occurs into a branched H‐bonded tetramer. The O···O distance of the complementary dimers in the backbone ranges from 2.53 to 2.80 Å. The branched H‐bond, on the other hand, has an O···O distance ranging from 2.76 to 2.95 Å, which is slightly longer than that of the dimer. To date, we have demonstrated that such tetratopic tectons bearing two carboxyl groups in the ortho‐position of an aryl group readily undergo H‐bond cleavage and reformation [53, 54, 55, 56], and this phenomenon was also observed in the present system. In the channels, hexane molecules are included, while in the discrete voids, no aromatic molecules are accommodated (Figures S14 and S15). The estimated molar ratio of 1235CPB: hexane is 4: 1. Single crystals of cem‐12345CPB‐L1 were heated at 80°C for 5 min under air, and the resulting crystal was subjected to SCXRD. Although data quality is not sufficient, we revealed the crystal structure of transformed HOF 12345CPB‐2 (Figure 7). Again, two H‐bonded dimers cleaved, and the 2D rhombic void collapsed by shrinking. The carboxy groups were trapped by molecules of water that came from the atmosphere. Two molecules of MeBz were encapsulated in isolated spaces between the voids (Figures S16 and S17).

FIGURE 6.

Structural transformation from sql‐1235CPB‐L1 to 1235CPB‐2(hexane). (a) Schematic representation of the framework transformation via cleavage and rearrangement (reformation) of H‐bonding. (b) H‐bonded network with branched H‐bonded tetramer motifs. (c) Stacking manner of the layers. Molecules of hexane included in the void are omitted for clarity.

FIGURE 7.

Structural transformation from cem‐12345CPB‐L1 to 12345CPB‐2. (a) Schematic representation of the framework transformation via cleavage and rearrangement (reformation) of H‐bonding with water insertion. (b) Stacking manner of the water‐trapped layers.

These results indicate that void II with a rhombic aperture in layered HOFs easily collapses through cleavage of H‐bonds during the solvent desorption process, resulting in the shrinkage of the frameworks. Interestingly, when a drop of MeBz was added to crystalline powder of the shrunk frameworks, 12345CPB‐2 was transformed back into cem‐12345CPB‐L1, while no interconversion was observed for 1235CPB (Figure S18).

2.3. Porosity

To evaluate the permanent porosity, the activated HOFs were prepared by immersing in hexane, followed by heating at 80–140°C under vacuum for 4 days. The complete removal of the included solvent molecules was confirmed by ¹H NMR spectroscopy (Figures S19–S21). Structures of the activated HOFs were characterized as follows (Figure 8a). An activated sample of sql‐1235CPB‐P1 (denoted as sql‐1235CPB‐P1a) exhibits a closely similar PXRD pattern with that of the as‐formed crystal sql‐1235CPB‐P1, although the diffraction intensity became lower. This indicates that the activated HOF did not undertake structural changes upon activation. An activated sample of sql‐1235CPB‐L1 (denoted as sql‐1235CPB‐L1a) exhibits the same PXRD pattern as that of sql‐1235CPB‐2(hexane), indicating that structural changes occurred through dissociation and reformation of H‐bonds to give a HOF with narrower pores. An activated sample of cem‐12345CPB‐L1 (denoted as cem‐12345CPB‐L1a) exhibits a similar PXRD pattern to that of 12345CPB‐2, indicating that the structural changes occurred through the H‐bonded dissociation, although it is not clear whether water molecules were inserted or not.

FIGURE 8.

(a) PXRD patterns of activated HOFs (λ = 1.00 Å): (i) observed sql‐1235CPB‐P1a, (ii) simulated sql‐1235CPB‐P1, (iii) observed sql‐1235CPB‐L1a, (iv) simulated 1235CPB‐2(hexane), (v) observed cem‐12345CPB‐P1a, (vi) simulated 12345CPB‐2. Gas sorption isotherms of (b) 1235CPB‐P1a, (c) 1235CPB‐L1a, and (d) 12345CPB‐L1a. N₂ (black circle) and CO₂ (green triangle) sorption isotherms were recorded at 77 K and 195 K, respectively. Solid and open symbols indicate adsorption and desorption processes, respectively.

sql‐1235CPB‐P1a showed no uptake N₂ and a type‐I(b) CO₂ adsorption isotherm with uptake of 83.6 cm⁻³ (STP) g⁻¹ at P/P ₀ of 1.0 (Figure 8b). These results can be explained by a slightly smaller kinetic diameter of CO₂ than N₂ for narrow pores of the HOF and by a favorable quadruple interaction between the CO₂ and framework. Brunauer–Emmett–Teller (BET) specific surface area [S_A(BET)] was calculated to be 266 m² g⁻¹, based on the CO₂ sorption isotherm (Figure S21). Activated HOF sql‐1235CPB‐L1a, on the other hand, showed a gate‐open‐like CO₂ sorption isotherm with hysteresis (Figure 8c), indicating that sql‐1235CPB‐L1a has a narrower channel and/or bottleneck than that of sql‐1235CPB‐P1a. Amount of adsorbed CO₂ is 86.4 cm⁻³ (STP) g⁻¹ at P/P ₀ of 1.0, similar to that of sql‐1235CPB‐P1a. In the case of cem‐12345CPB‐L1a, the CO₂ sorption isotherm has a rapid rise with a small step at P/P ₀ of ca. 0.03, and reaches a plateau at P/P ₀ of 0.2. An uptake amount of CO₂ is 263.8 cm⁻³ (STP) g⁻¹ at P/P ₀ of 0.94. The N₂ sorption isotherm, on the other hand, shows a gate‐open‐like profile at P/P ₀ of 0.10 with obvious hysteresis (Figure 8d). S_A(BET) was calculated to be 1335 m² g⁻¹, based on the CO₂ sorption isotherm (Figure S22). These results show that the interpenetrated HOF is rigid and shows no structural changes during the gas sorption, while the layered HOFs may be flexible and show structural changes upon gas sorption.

3. Conclusions

In this study, we prepared hydrogen‐bonded organic frameworks (HOFs) of the missing pieces of carboxyphenylbenzene (CPB) derivatives, that is, 1,2,3,5‐tetrakis‐ and 1,2,3,4,5‐pentakisCPBs (1235CPB and 12345CPB, respectively). Recrystallization of 1235CPB from a THF/DCB solution under 40°C concomitantly yielded two types of HOFs, sql‐1235CPB‐P1 and cqe‐1235CPB‐L1. The former is a framework based on interpenetrated sql ‐network sheets, while the latter has a layered framework with more complicated cqe ‐network sheets. sql‐1235CPB‐P1 is primarily formed, but lowering the temperature causes a small amount of cqe‐1235CPB‐L1 to form simultaneously. 1235CPB also yielded a layered HOF with sql ‐network sheets, sql‐1235CPB‐L1, by recrystallization from a DMF/DCB solution at 100°C. In the case of 12345CPB, only a layered framework based on cem ‐network sheets, cem‐12345CPB‐L1, was obtained when crystallization was conducted at various temperatures.

The network structure with cqe ‐topology is hitherto unknown in molecule‐based framework materials. Other molecules in the series of CPB gave no H‐bonded network with complicated topology, enabling us to estimate the limitation for generating entropically unfavorable networks on the basis of a simple consideration of the number of cases for network formation. Additionally, it is revealed that the interpenetrated framework retains the structure after activation, while the layered frameworks were shrunk via cleavage and reformation of hydrogen bonds upon activation. The gas sorption behaviors reflect these structural features.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll73250‐sup‐0001‐SuppMat.pdf.

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article.