1,8-Dihydroxy Naphthalene—A New Building Block for the Self-Assembly with Boronic Acids and 4,4′-Bipyridine to Stable Host–Guest Complexes with Aromatic Hydrocarbons

The new Lewis acid–base adducts of general formula X(nad)B←NC5H4-C5H4N→B(nad)X [nad = 1,8-O2C10H6, X = C6H5 (2c), 3,4,5-F3-C6H2 (2d)] were synthesized in high yields via reactions of 1,8-dihydroxy naphthalene [nadH2] and 4,4′-bipyridine with the aryl boronic acids C6H5B(OH)2 and 3,4,5-F3-C6H2B(OH)2, respectively, and structurally characterized by multi-nuclear NMR spectroscopy and SCXRD. Self-assembled H-shaped Lewis acid–base adduct 2d proved to be effective in forming thermally stable host–guest complexes, 2d × solvent, with aromatic hydrocarbon solvents such as benzene, toluene, mesitylene, aniline, and m-, p-, and o-xylene. Crystallographic analysis of these solvent adducts revealed host–guest interactions to primarily occur via π···π contacts between the 4,4′-bipyridyl linker and the aromatic solvents, resulting in the formation of 1:1 and 1:2 host–guest complexes. Thermogravimetric analysis of the isolated complexes 2d × solvent revealed their high thermal stability with peak temperatures associated with the loss of solvent ranging from 122 to 147 °C. 2d, when self-assembled in an equimolar mixture of m-, p-, and o-xylene (1:1:1), preferentially binds to o-xylene. Collectively, these results demonstrate the ability of 1,8-dihydroxy naphthalene to serve as an effective building block in the selective self-assembly to supramolecular aggregates through dative covalent N→B bonds.


Introduction
The last decades have witnessed increased interest in the rational design of supramolecular architectures based on the self-assembly of various suitable building blocks [1][2][3][4][5][6][7][8][9][10][11][12]. Amongst the various strategies, the utilization of intermolecular dative covalent N→B bond interactions in the design and self-assembly of well-defined supramolecular structures has become a promising synthetic approach with potential in host-guest chemistry, gas storage, and crystal engineering [13][14][15][16]. Examples of this recent development are trigonal planar catechol-based aryl boronic esters [17]. These strongly Lewis acidic building blocks, in combination with N-containing strongly Lewis basic ligands, have been demonstrated to facilitate self-assembly to give-through coordinate covalent N→B bonds-boronic ester-based supramolecular structures, such as coordination polymers, cage-and nanostructures, and gels [18][19][20][21][22][23][24][25][26][27]. Recently, the groups of Hőpfl and Morales-Rojas [28] have shown that reactions of catechol-based aryl boronic esters with bipyridine linkers in the presence of various aromatic hydrocarbon solvents readily form self-assembled Lewis-type N→B adducts with aromatic hydrocarbons incorporated. It was hypothesized that upon N→B coordination, the aromatic bipyridine linker becomes increasingly electron-deficient, enabling the recognition and selective isolation of host-guest complexes with aromatic hydrocarbons [29]. However, catecholate-based boronic esters, despite their high Lewis

Results and Discussion
At the outset, we investigated the ability of our recently prepared boronic esters o general formula R-Bnad (1a-e), where nad = 1,8-naphthtalenediolate (a: R = mesityl; b: R = 2,6-dichlorophenyl; c: R = phenyl; d: R = 3,4,5-trifluorophenyl; e: R = pentafluorophenyl to form stable Lewis acid-base adducts with 4,4′-bipyridine (Scheme 2). The reaction were performed in acetone-D6 as the solvent and monitored by 11 B and 1 H NMR spectros copy. However, the 1 H NMR spectra of all five reactions exclusively gave one set of sig nals, even when an excess of 4,4′-bipyridine or one of the respective boronic esters wa used, indicating rapid equilibrium to occur at room temperature. The 11 B NMR spectra on the other hand, showed very different spectral features ( Figure 1). Thus, a 1:2 mixtur of 4,4′-bipyridine and the bulkiest and least Lewis acidic ester 1a (R = mesityl) showed signal at around 29 ppm, which is close to the 11 B NMR chemical shift of 1a (δ = 27 ppm in CDCl3) [30], showing that the equilibrium strongly favors the individual acid and bas components over adduct 2a. For the system 4,4′-bipyridine/1b (1:2), two distinct signals a ca. 27 ppm and 7 ppm were observed; the former can be assigned to ester 1b, while th latter is due to the desired Lewis acid-base adduct 2b. In contrast, mixtures of 4,4′-bipyr idine with 1c, 1d, and 1e, respectively, showed up-field shifted signals at ca. 12 ppm (1c) 6 ppm (1d), and 4 ppm (1e), indicating the formation of the respective adducts 2c, 2d, and 2e. Scheme 2. Equilibrium between 4,4′-bipyridine/1a-e and the Lewis acid-base adducts 2a-e in dilut acetone-d6 at room temperature.

Results and Discussion
At the outset, we investigated the ability of our recently prepared boronic esters of general formula R-Bnad (1a-e), where nad = 1,8-naphthtalenediolate (a: R = mesityl; b: R = 2,6-dichlorophenyl; c: R = phenyl; d: R = 3,4,5-trifluorophenyl; e: R = pentafluorophenyl) to form stable Lewis acid-base adducts with 4,4 -bipyridine (Scheme 2). The reactions were performed in acetone-D 6 as the solvent and monitored by 11 B and 1 H NMR spectroscopy. However, the 1 H NMR spectra of all five reactions exclusively gave one set of signals, even when an excess of 4,4 -bipyridine or one of the respective boronic esters was used, indicating rapid equilibrium to occur at room temperature. The 11 B NMR spectra, on the other hand, showed very different spectral features ( Figure 1). Thus, a 1:2 mixture of 4,4 -bipyridine and the bulkiest and least Lewis acidic ester 1a (R = mesityl) showed a signal at around 29 ppm, which is close to the 11 B NMR chemical shift of 1a (δ = 27 ppm in CDCl 3 ) [30], showing that the equilibrium strongly favors the individual acid and base components over adduct 2a. For the system 4,4 -bipyridine/1b (1:2), two distinct signals at ca. 27 ppm and 7 ppm were observed; the former can be assigned to ester 1b, while the latter is due to the desired Lewis acid-base adduct 2b. In contrast, mixtures of 4,4 -bipyridine with 1c, 1d, and 1e, respectively, showed up-field shifted signals at ca. 12 ppm (1c), 6 ppm (1d), and 4 ppm (1e), indicating the formation of the respective adducts 2c, 2d, and 2e.
Molecules 2023, 28, x FOR PEER REVIEW 2 of 12 their high Lewis acidity, readily hydrolyze, particularly when used in "wet" solvents. Our group has recently shown that 1,8-dihydroxy naphthalene-based aryl boronic esters are as strongly Lewis acidic as their catechol-based counterparts but significantly more hydrolytically stable (Scheme 1) [30]. Based on our recent findings, we envisioned that 1,8dihydroxy naphthalene should be an effective building block in the self-assembly with aryl boronic acids and a suitable bipyridine linker to form thermally stable host-guest complexes with aromatic hydrocarbons.

Results and Discussion
At the outset, we investigated the ability of our recently prepared boronic esters of general formula R-Bnad (1a-e), where nad = 1,8-naphthtalenediolate (a: R = mesityl; b: R = 2,6-dichlorophenyl; c: R = phenyl; d: R = 3,4,5-trifluorophenyl; e: R = pentafluorophenyl) to form stable Lewis acid-base adducts with 4,4′-bipyridine (Scheme 2). The reactions were performed in acetone-D6 as the solvent and monitored by 11 B and 1 H NMR spectroscopy. However, the 1 H NMR spectra of all five reactions exclusively gave one set of signals, even when an excess of 4,4′-bipyridine or one of the respective boronic esters was used, indicating rapid equilibrium to occur at room temperature. The 11 B NMR spectra, on the other hand, showed very different spectral features ( Figure 1). Thus, a 1:2 mixture of 4,4′-bipyridine and the bulkiest and least Lewis acidic ester 1a (R = mesityl) showed a signal at around 29 ppm, which is close to the 11 B NMR chemical shift of 1a (δ = 27 ppm in CDCl3) [30], showing that the equilibrium strongly favors the individual acid and base components over adduct 2a. For the system 4,4′-bipyridine/1b (1:2), two distinct signals at ca. 27 ppm and 7 ppm were observed; the former can be assigned to ester 1b, while the latter is due to the desired Lewis acid-base adduct 2b. In contrast, mixtures of 4,4′-bipyridine with 1c, 1d, and 1e, respectively, showed up-field shifted signals at ca. 12 ppm (1c), 6 ppm (1d), and 4 ppm (1e), indicating the formation of the respective adducts 2c, 2d, and 2e. Scheme 2. Equilibrium between 4,4′-bipyridine/1a-e and the Lewis acid-base adducts 2a-e in dilute acetone-d6 at room temperature. To confirm our structural assignments, efforts were undertaken to grow single cry tals suitable for X-ray analysis from concentrated acetone solutions of 4,4′-bipyridine an 2a-e. The results for 2a, 2b, and 2d are shown in Figure 2 and reveal that two boronic este units bind to a 4,4′-bipyridine moiety linked via dative N→B bonds. Compounds 2a an 2b can structurally be described as double-tweezers. 2a exhibits an almost perfect double tweezer-shaped structure with nearly planar bipyridine units, while 2b is markedl twisted with a twisting angle of the bipyridine moiety of about 35°. Adduct 2d, on th other hand, is composed of a more H-shaped structure with a nearly planar bipyridin unit (twisting angle ~10°). The bond parameters for all three adducts are similar, with B N and B-O distances ranging from 1.61 to 1.67 Å and 1.45-1.47 Å, respectively. The centra boron atoms in 2a, 2b, and 2d have slightly distorted tetrahedral coordination environ ments, with O-B-O and N-B-O angles ranging from 111-116° to 102-105°. Unfortunately it was not possible to obtain suitable single crystals of 2c and 2e from acetone as the so vent. Not only was 2e highly soluble in acetone, but it also slowly degraded to a secon product exhibiting a relatively sharp signal in the 11 B NMR with a chemical shift of ca. ppm (see also Figure 1). This new product crystallized from acetone after a week and wa identified by NMR spectroscopy and SCXRD as a spirocyclic 4,4′-bipyridinium borate (se supporting information). However, switching the solvent from acetone to benzene re sulted in the formation of single crystalline material, which by X-ray analysis was ident fied as the phenyl derivative 2c × benzene ( Figure 2). Similar to 2d, adduct 2c × benzen has an H-shaped structure with a nearly planar bipyridine unit (twisting angle ~ 5°) an average B-O and B-N distances of 1.45 and 1.67 Å, respectively. Notably, 2c × benzene co crystallizes with two molecules of benzene, both being entrapped via π-π stacking wit the 4,4′-bipyridine moiety and C-H … π contacts with both the phenyl and the naphthalen substituents. To confirm our structural assignments, efforts were undertaken to grow single crystals suitable for X-ray analysis from concentrated acetone solutions of 4,4 -bipyridine and 2a-e. The results for 2a, 2b, and 2d are shown in Figure 2 and reveal that two boronic ester units bind to a 4,4 -bipyridine moiety linked via dative N→B bonds. Compounds 2a and 2b can structurally be described as double-tweezers. 2a exhibits an almost perfect double-tweezer-shaped structure with nearly planar bipyridine units, while 2b is markedly twisted with a twisting angle of the bipyridine moiety of about 35 • . Adduct 2d, on the other hand, is composed of a more H-shaped structure with a nearly planar bipyridine unit (twisting angle~10 • ). The bond parameters for all three adducts are similar, with B-N and B-O distances ranging from 1.61 to 1.67 Å and 1.45-1.47 Å, respectively. The central boron atoms in 2a, 2b, and 2d have slightly distorted tetrahedral coordination environments, with O-B-O and N-B-O angles ranging from 111-116 • to 102-105 • . Unfortunately, it was not possible to obtain suitable single crystals of 2c and 2e from acetone as the solvent. Not only was 2e highly soluble in acetone, but it also slowly degraded to a second product exhibiting a relatively sharp signal in the 11 B NMR with a chemical shift of ca. 0 ppm (see also Figure 1). This new product crystallized from acetone after a week and was identified by NMR spectroscopy and SCXRD as a spirocyclic 4,4 -bipyridinium borate (see supporting information). However, switching the solvent from acetone to benzene resulted in the formation of single crystalline material, which by X-ray analysis was identified as the phenyl derivative 2c × benzene ( Figure 2). Similar to 2d, adduct 2c × benzene has an H-shaped structure with a nearly planar bipyridine unit (twisting angle~5 • ) and average B-O and B-N distances of 1.45 and 1.67 Å, respectively. Notably, 2c × benzene co-crystallizes with two molecules of benzene, both being entrapped via π-π stacking with the 4,4 -bipyridine moiety and C-H . . . π contacts with both the phenyl and the naphthalene substituents. It was also possible to synthesize and isolate adducts 2c and 2d via a one-pot procedure in excellent yields starting from the respective boronic acids, without the need of preparing the respective esters 1c and 1d (Scheme 3). For example, when two equivalents of 1,8-dihydroxy naphthalene and one equivalent of 4,4′-bipyridine were reacted with two equivalents of the respective aryl boronic acid in acetone as a solvent, 2c and 2d were isolated in yields of 80% and 90%, respectively, as bright orange crystalline materials. Both compounds were fully characterized by 1 H, 13 C, 19 F, and 11 B NMR spectroscopy as well as by combustion analysis.
Encouraged by these results, the self-assembly with diboronic acids was investigated. Thus, upon reacting 1,8-dihydroxy naphthalene with 4,4′-bipyridine and 1,4-phenylene diboronic acid in a 2:1:1 ratio in acetone as the solvent, an orange crystalline material formed. However, once precipitated, the isolated material proved to be insoluble in common organic solvents, rendering its characterization by NMR spectroscopy impossible. Attempts to grow single crystals suitable for X-ray analysis failed as well. On the other hand, replacing 4,4′-bipyridine with trans-1,2-di(4-pyridyl)ethylene gave-upon reaction with 1,8-dihydroxy naphthalene and 1,4-phenylene diboronic acid in acetone-an orange microcrystalline precipitate, from which single crystals suitable for X-ray analysis could be obtained. The results revealed the compound to be polymeric Lewis acid-base adduct It was also possible to synthesize and isolate adducts 2c and 2d via a one-pot procedure in excellent yields starting from the respective boronic acids, without the need of preparing the respective esters 1c and 1d (Scheme 3). For example, when two equivalents of 1,8-dihydroxy naphthalene and one equivalent of 4,4 -bipyridine were reacted with two equivalents of the respective aryl boronic acid in acetone as a solvent, 2c and 2d were isolated in yields of 80% and 90%, respectively, as bright orange crystalline materials. Both compounds were fully characterized by 1 H, 13 C, 19 F, and 11 B NMR spectroscopy as well as by combustion analysis.
Encouraged by these results, the self-assembly with diboronic acids was investigated. Thus, upon reacting 1,8-dihydroxy naphthalene with 4,4 -bipyridine and 1,4-phenylene diboronic acid in a 2:1:1 ratio in acetone as the solvent, an orange crystalline material formed. However, once precipitated, the isolated material proved to be insoluble in common organic solvents, rendering its characterization by NMR spectroscopy impossible. Attempts to grow single crystals suitable for X-ray analysis failed as well. On the other hand, replacing 4,4bipyridine with trans-1,2-di(4-pyridyl)ethylene gave-upon reaction with 1,8-dihydroxy naphthalene and 1,4-phenylene diboronic acid in acetone-an orange microcrystalline precipitate, from which single crystals suitable for X-ray analysis could be obtained. The results revealed the compound to be polymeric Lewis acid-base adduct 3. Polymer 3 is composed of diboronic ester units coordinated with the 4,4 -bipyridine moieties in a zig-zag fashion (Scheme 3, Figure 3a). Due to poor diffraction and multiple twinning of the single crystals measured, only the connectivity of the polymer could be confirmed.

3.
Polymer 3 is composed of diboronic ester units coordinated with the 4,4′-bipyridine moieties in a zig-zag fashion (Scheme 3, Figure 3a). Due to poor diffraction and multiple twinning of the single crystals measured, only the connectivity of the polymer could be confirmed.
In contrast, the reaction of 1,3-phenylene diboronic acid under otherwise identical conditions did not lead to the formation of a polymeric structure; instead, the rectangular dimer 4 (yield 94%) was formed as confirmed by single-crystal X-ray analysis (Scheme 3, Figure 3b)

3.
Polymer 3 is composed of diboronic ester units coordinated with the 4,4′-bipyridine moieties in a zig-zag fashion (Scheme 3, Figure 3a). Due to poor diffraction and multiple twinning of the single crystals measured, only the connectivity of the polymer could be confirmed.
In contrast, the reaction of 1,3-phenylene diboronic acid under otherwise identical conditions did not lead to the formation of a polymeric structure; instead, the rectangular dimer 4 (yield 94%) was formed as confirmed by single-crystal X-ray analysis (Scheme 3, Figure 3b)   In contrast, the reaction of 1,3-phenylene diboronic acid under otherwise identical conditions did not lead to the formation of a polymeric structure; instead, the rectangular dimer 4 (yield 94%) was formed as confirmed by single-crystal X-ray analysis (Scheme 3, Figure 3b). Both bipyridine units in 4 are markedly twisted, with twisting angles of about Encouraged by the ability of 2c × benzene to form a stable host-guest complex with two molecules of benzene, a detailed study of the inclusion properties of H-shaped 2d was undertaken (Scheme 4). 2d was selected because of its enhanced hydrolytic stability in solution and the ability of the fluorine substituents to significantly decrease the electron density at boron, thereby increasing the stability of the Lewis acid-base adducts to be formed. Suspensions of 1,8-dihydroxy naphthalene, 4,4 -bipyridine, and 3,4,5-trifluorophenyl boronic acid in the respective aromatic hydrocarbon solvent (i.e., benzene, toluene, o-xylene, mxylene, p-xylene, mesitylene, aniline) were heated until clear solutions were obtained. Upon slowly cooling to room temperature, crystalline solids were obtained in isolated yields ranging from 34-81%. In addition to being characterized by 1 H NMR spectroscopy and combustion analysis, all compounds were analyzed by X-ray crystallography.
Molecules 2023, 28, x FOR PEER REVIEW 6 of 12 Encouraged by the ability of 2c × benzene to form a stable host-guest complex with two molecules of benzene, a detailed study of the inclusion properties of H-shaped 2d was undertaken (Scheme 4). 2d was selected because of its enhanced hydrolytic stability in solution and the ability of the fluorine substituents to significantly decrease the electron density at boron, thereby increasing the stability of the Lewis acid-base adducts to be formed. Suspensions of 1,8-dihydroxy naphthalene, 4,4′-bipyridine, and 3,4,5-trifluorophenyl boronic acid in the respective aromatic hydrocarbon solvent (i.e., benzene, toluene, o-xylene, m-xylene, p-xylene, mesitylene, aniline) were heated until clear solutions were obtained. Upon slowly cooling to room temperature, crystalline solids were obtained in isolated yields ranging from 34-81%. In addition to being characterized by 1 H NMR spectroscopy and combustion analysis, all compounds were analyzed by X-ray crystallography. The results of the X-ray analysis are shown in Figure 4 and reveal that 2d is capable of forming various stable host-guest complexes with all aromatic hydrocarbon solvents used in this study via π-π stacking between the 4,4′-bipyridyl unit of 2d and the respective aromatic solvent. Thus, in benzene as a solvent, 2d × benzene is formed (Figure 4a), which is composed of three benzene molecules per two molecules of 2d. Identical results were obtained with aniline as the solvent, generating host-guest complex 2d × aniline ( Figure  4b) with a 2d/aniline ratio of 2:3. In contrast, carrying out the self-assembly reactions in toluene and p-xylene gave the host-guest complexes 2d × toluene and 2d × p-xylene (Figure 4c,d), respectively, which consist of one aromatic guest per one molecule of 2d (1:1 complex). Interestingly, crystallization experiments in meta-xylene gave two crystallographically distinct single crystals of 2d × m-xylene; the results for one revealed a classical 1:1 host-guest complex (Figure 4e), while X-ray analysis of the second one revealed a 1:2 complex (Figure 4f) with two additional unbound m-xylene molecules per three equivalents of the 1:2 complex. On the other hand, X-ray analysis of crystalline 2d × o-xylene obtained from o-xylene as a solvent confirmed a 1:2 complex (Figure 4g). The X-ray analysis for 2d × mesitylene (Figure 4h) also confirms a 1:2 host-guest complex but with two molecules of 2d and two molecules of unbound mesitylene in the unit cell. The results of the X-ray analysis are shown in Figure 4 and reveal that 2d is capable of forming various stable host-guest complexes with all aromatic hydrocarbon solvents used in this study via π-π stacking between the 4,4 -bipyridyl unit of 2d and the respective aromatic solvent. Thus, in benzene as a solvent, 2d × benzene is formed (Figure 4a), which is composed of three benzene molecules per two molecules of 2d. Identical results were obtained with aniline as the solvent, generating host-guest complex 2d × aniline (Figure 4b) with a 2d/aniline ratio of 2:3. In contrast, carrying out the self-assembly reactions in toluene and p-xylene gave the host-guest complexes 2d × toluene and 2d × p-xylene (Figure 4c,d), respectively, which consist of one aromatic guest per one molecule of 2d (1:1 complex). Interestingly, crystallization experiments in meta-xylene gave two crystallographically distinct single crystals of 2d × m-xylene; the results for one revealed a classical 1:1 hostguest complex (Figure 4e), while X-ray analysis of the second one revealed a 1:2 complex (Figure 4f) with two additional unbound m-xylene molecules per three equivalents of the 1:2 complex. On the other hand, X-ray analysis of crystalline 2d × o-xylene obtained from o-xylene as a solvent confirmed a 1:2 complex (Figure 4g). The X-ray analysis for 2d × mesitylene (Figure 4h) also confirms a 1:2 host-guest complex but with two molecules of 2d and two molecules of unbound mesitylene in the unit cell.   Table 1 compares the host-guest ratios of 2d × solvent determined from the X-ray analysis of single crystals with those obtained from the 1 H NMR data of the bulk materials; the latter were dried in air for circa 24 h. It was found that crystals of 2d × o-xylene and 2d × m-xylene, upon drying in air, lose guest molecules to form host-guest complexes with a formal 1:1 host-guest ratio. While in 2d × aniline and 2d × benzene, the 2:3 host-guest ratio  Table 1 compares the host-guest ratios of 2d × solvent determined from the X-ray analysis of single crystals with those obtained from the 1 H NMR data of the bulk materials; the latter were dried in air for circa 24 h. It was found that crystals of 2d × o-xylene and 2d × m-xylene, upon drying in air, lose guest molecules to form host-guest complexes with a formal 1:1 host-guest ratio. While in 2d × aniline and 2d × benzene, the 2:3 host-guest ratio remains unchanged, and 2d × mesitylene contains significantly more mesitylene than what was found via X-ray analysis of single crystalline material. This seems to imply that 2d × mesitylene may crystallize in various forms with different host-guest ratios. To study the thermal behavior of the host-guest complexes 2d × solvent, a thermogravimetric analysis of the bulk materials was performed; the results are shown in Figure 5, Figures S30 and S31, and Table S1. Figure 5 (bottom) shows the TG curve of 2d, revealing complete weight loss to occur at about 260 • C. This indicates that the acid-base adduct 2d either sublimes or that the decomposition and evaporation of its individual acid and base components occur within the same temperature range. The TG graphs of all host-guest compounds exhibit a two-step weight loss. This is exemplarily shown for the solvent adducts 2d × benzene, 2d × toluene, 2d × o-xylene, 2d × m-xylene, and 2d × p-xylene ( Figure 5), for which the observed weight losses are consistent with the stoichiometry of the bulk materials established by 1 H NMR spectroscopy (Table 1). After the loss of the guest, the residual material likely consists of the solid phase established for solvent-free 2d. In this series, for 2d × benzene and 2d × toluene, the peak temperatures (T peak ) associated with the liberation of the guest were found to be 131 • C and 147 • C, respectively, suggesting stronger host-guest interactions in 2d × toluene, and consistent with the results of DSC analysis ( Figures S32 and S33). For the xylene series, the peak temperatures for the loss of solvent are 122 • C (2d × m-xylene), 133 • C (2d × o-xylene), and 136 • C (2d × p-xylene). The slightly higher peak temperatures for 2d × o-xylene and 2d × p-xylene suggest somewhat stronger host-guest interactions compared to 2d × m-xylene, which is supported by DSC measurements (Figures S34-S36).
In a related study, Hőpfl and Morales-Rojas recently disclosed the thermal properties of self-assembled host-guest complexes derived from reactions of 1,2-catechol and 3,4,5-trifluorphenylboronic acid with 4,4 -bipyridine in various aromatic hydrocarbon solvents [28]. Thermogravimetric analysis of these catechol-based host-guest complexes revealed the peak temperatures associated with the loss of solvent to be 65 • C (m-xylene), 71 • C (o-xylene), 76 • C (p-xylene), 121 • C (toluene), 139 • C (benzene), and 100 • C (mesitylene). Except for benzene, these values are significantly lower than what was measured for our 1,8-dihydroxy naphthalene-based host-guest complexes 2d × solvent ( Figure 5). The replacement of catechol with the 1,8-dihydroxy naphthalene ligand at the central boron appears to result in stronger π-type host-guest interactions between the 4,4 -bipyridine unit and the aromatic solvent. Unfortunately, Hőpfl and Morales-Rojas did not disclose thermodynamic data from DSC measurements for comparison [28]. In a related study, Hőpfl and Morales-Rojas recently disclosed the thermal properties of self-assembled host-guest complexes derived from reactions of 1,2-catechol and 3,4,5trifluorphenylboronic acid with 4,4′-bipyridine in various aromatic hydrocarbon solvents [28]. Thermogravimetric analysis of these catechol-based host-guest complexes revealed the peak temperatures associated with the loss of solvent to be 65 °C (m-xylene), 71 °C (oxylene), 76 °C (p-xylene), 121 °C (toluene), 139 °C (benzene), and 100 °C (mesitylene). Except for benzene, these values are significantly lower than what was measured for our 1,8dihydroxy naphthalene-based host-guest complexes 2d × solvent ( Figure 5). The replacement of catechol with the 1,8-dihydroxy naphthalene ligand at the central boron appears to result in stronger π-type host-guest interactions between the 4,4′-bipyridine unit and the aromatic solvent. Unfortunately, Hőpfl and Morales-Rojas did not disclose thermodynamic data from DSC measurements for comparison [28].
Based on the results from the host-guest chemistry with m-, o-, and p-xylene, we wondered whether self-assembled 2d would selectively incorporate one of the three xylene isomers. Thus, 1,8-dihydroxy naphthalene, 4,4′-bipyridine, and 3,4,5-trifluoroboronic acid were reacted in an equimolar solvent mixture of m-, p-, and o-xylene (1:1:1). The obtained crystalline material was collected and analyzed by 1 H NMR spectroscopy using DMSO-D6 as the solvent (see supporting information for more details). The results revealed a higher selectivity of 2d for o-xylene (ca. 57%), with an m-, p-, to o-xylene ratio of about 2:1:4, respectively, which is similar to those of the respective catechol-based host-guest complexes reported by Hőpfl and Morales-Rojas [28]. Considering the results from the TGA and DSC measurements, the fairly low uptake of p-xylene appears to be surprising but perhaps can be attributed to kinetic effects during the crystallization process.
Finally, to further broaden the scope of the host-guest approach, the modified bipyridine linker 1,2-bis(4-pyridyl)ethane was employed in reactions with 1,8-dihydroxy naphthalene and various boronic acids. Unfortunately, in none of the cases could host-guest complexes with aromatic hydrocarbon solvents be crystallized and structurally determined. Nonetheless, single crystals of the solvent-free double-tweezers 5 and 6 could be Based on the results from the host-guest chemistry with m-, o-, and p-xylene, we wondered whether self-assembled 2d would selectively incorporate one of the three xylene isomers. Thus, 1,8-dihydroxy naphthalene, 4,4 -bipyridine, and 3,4,5-trifluoroboronic acid were reacted in an equimolar solvent mixture of m-, p-, and o-xylene (1:1:1). The obtained crystalline material was collected and analyzed by 1 H NMR spectroscopy using DMSO-D 6 as the solvent (see supporting information for more details). The results revealed a higher selectivity of 2d for o-xylene (ca. 57%), with an m-, p-, to o-xylene ratio of about 2:1:4, respectively, which is similar to those of the respective catechol-based host-guest complexes reported by Hőpfl and Morales-Rojas [28]. Considering the results from the TGA and DSC measurements, the fairly low uptake of p-xylene appears to be surprising but perhaps can be attributed to kinetic effects during the crystallization process.
Finally, to further broaden the scope of the host-guest approach, the modified bipyridine linker 1,2-bis(4-pyridyl)ethane was employed in reactions with 1,8-dihydroxy naphthalene and various boronic acids. Unfortunately, in none of the cases could host-guest complexes with aromatic hydrocarbon solvents be crystallized and structurally determined. Nonetheless, single crystals of the solvent-free double-tweezers 5 and 6 could be obtained from acetone solutions and were characterized by single-crystal X-ray crystallography (Figure 6a,b).
Molecules 2023, 28, x FOR PEER REVIEW 10 of 12 obtained from acetone solutions and were characterized by single-crystal X-ray crystallography (Figure 6a,b).

Conclusions
We have demonstrated the ability of 1,8-dihydroxy naphthalene to self-assemble with various aryl boronic acids and 4,4′-bipyridine as a linker to give double-tweezer and H-shaped Lewis acid-base adducts in very good isolated yields by means of N→B bond formation. In addition, a series of host-guest complexes, 2d × solvent, was prepared via reactions of 4,4′-bipyridine, 1,8-dihydroxy naphthalene, and 3,4,5-trifluorophenyl boronic acid with various aromatic hydrocarbons. SCXRD analysis of these solvent adducts revealed host-guest interactions to primarily occur via π···π contacts between the 4,4′-bipyridyl linker and the aromatic solvents to give 1:1 and 1:2 host-guest complexes, which was confirmed by 1 H NMR spectroscopic analysis of the bulk samples. TGA measurements of the bulk samples showed well-defined weight losses at peak temperatures ranging from 122 to 147 °C, which in every case could be attributed to the complete loss of solvent (guest). Self-assembled Lewis acid-base adduct 2d, when recrystallized from an equimolar mixture of m-, p-, and o-xylene (1:1:1), preferentially binds to o-xylene, generating 2d × o-xylene as the major component, similar to what was reported for structurally related catechol-based host-guest complexes [28]. Collectively, the present contribution demonstrates the effectiveness of 1,8-dihydroxy naphthalene in constructing supramolecular structures via dative N-B bond formation.

Conclusions
We have demonstrated the ability of 1,8-dihydroxy naphthalene to self-assemble with various aryl boronic acids and 4,4 -bipyridine as a linker to give double-tweezer and H-shaped Lewis acid-base adducts in very good isolated yields by means of N→B bond formation. In addition, a series of host-guest complexes, 2d × solvent, was prepared via reactions of 4,4 -bipyridine, 1,8-dihydroxy naphthalene, and 3,4,5-trifluorophenyl boronic acid with various aromatic hydrocarbons. SCXRD analysis of these solvent adducts revealed host-guest interactions to primarily occur via π···π contacts between the 4,4bipyridyl linker and the aromatic solvents to give 1:1 and 1:2 host-guest complexes, which was confirmed by 1 H NMR spectroscopic analysis of the bulk samples. TGA measurements of the bulk samples showed well-defined weight losses at peak temperatures ranging from 122 to 147 • C, which in every case could be attributed to the complete loss of solvent (guest). Self-assembled Lewis acid-base adduct 2d, when recrystallized from an equimolar mixture of m-, p-, and o-xylene (1:1:1), preferentially binds to o-xylene, generating 2d × o-xylene as the major component, similar to what was reported for structurally related catechol-based host-guest complexes [28]. Collectively, the present contribution demonstrates the effectiveness of 1,8-dihydroxy naphthalene in constructing supramolecular structures via dative N-B bond formation.