Tribenzotriquinacene Fused Dimers (“Fusodimers”)—Synthesis and Host–Guest Properties

ABSTRACT

The synthesis and structural properties of two stereoisomeric tribenzotriquinacene (TBTQ) derivatives, syn‐4 and anti‐4, in which two rigid TBTQ bowls share a common benzene ring, are described. Owing to the orthogonal orientation of the three indane wings in each of the TBTQ units, four veratrole rings point either at right angles or parallel into the 3‐space. In contrast to the convex‐concave anti‐isomer, the biconcave fusodimer syn‐4 was found to host C₆₀ and C₇₀ fullerenes as 1:1 complexes both in solution and in the solid state, as shown by ¹H NMR spectroscopy and x‐ray crystallography. Complexation‐induced chemical shift (CIS) measurements gave similar binding constants, K _a = 382 (± 4) for syn‐4⊂C₆₀ and K _a = 410 (± 8) M^{− 1} for syn‐4⊂C₇₀. Structural details of the torsion and widening of the formally C _2v‐symmetric host skeleton upon embedding of the fullerenes into the deep biconcave molecular surface of syn‐4 were determined along with intermolecular peculiarities of the solid‐state complexes.

Two tribenzotriquinacene units merged with each other in either syn‐ or anti‐orientation of the bowls have become accessible in a short synthesis. In contrast to the anti‐isomer, the syn‐isomer forms 1:1 host–guest complexes with both C₆₀ and C₇₀ in solution and in the solid state, as shown by ¹H NMR spectroscopy and x‐ray diffraction. The Cartesian orientation of the four electron‐rich wings is maintained in both “fusodimers”.

1. Introduction

Tribenzotriquinacene 1 (TBTQ, Figure 1), a structurally unique molecule bearing three indane units oriented orthogonally to each other in space [1, 2, 3, 4, 5], has been used as a versatile building block towards the construction of many molecular bowls [6, 7, 8, 9, 10, 11, 12, 13] and capsules [14, 15, 16, 17, 18, 19, 20, 21]. As the TBTQ framework possesses only a relatively small molecular cavity, the molecule itself is not an effective host for molecular binding. However, binding strength and profile can be enhanced by the addition of various appendages to the indane wings [6, 7, 8, 11]. For example, triptycene‐functionalized TBTQ 2 [6] and tris(thianthreno)triquinacene 3 [7, 11] were found to form 1:1 and/or 1:2 complexes with C₆₀ and/or C₇₀, with binding constants (K ₁ or K ₂) in the range of 3 × 10² to 6 × 10³ M⁻¹ [22]. Even the extension of the TBTQ core with three TBTQ‐based centrohexaindane units has been achieved with the resulting complex with C₆₀ having a K ₁ = 1.5 × 10⁴ M⁻¹ [23].

FIGURE 1.

Structures of parent TBTQ 1 (side view) and wing‐extended TBTQ hosts 2 and 3.

We envisaged that stitching two or several TBTQ bowls into a single, yet larger molecular entity could be an attractive strategy towards the construction of effective hosts. While the non‐covalent or reversibly covalent assembly approaches have been shown to produce aesthetically pleasing TBTQ‐based molecular capsules [14, 15, 16, 17, 18, 19, 20, 21], the resulting molecular ensembles mostly do not possess robust stability to withstand changes in the molecular environment such as temperature, solvent polarity, and pH values, and their guest binding and capsulation behavior are yet to be unfolded. To our knowledge, there is only one report of a self‐assembled TBTQ‐octamer that acted as a host for tetraalkylammonium ions and the pyrene molecule [17].

2. Results and Discussion

Herein we report the short synthesis of two diastereomeric tribenzotriquinacenes that bear two mutually covalently linked TBTQ units, syn‐4 and anti‐4, from readily available starting materials in four steps, based on the established sequence involving 2‐benzyhydryl‐substituted 1,3‐indanediols (Figure 2) [1, 5, 24]. Actually, these two compounds are not truly dimeric but represent truncated dimers, which we coin “fusodimers” here, with the TBTQ cores sharing one aromatic ring. In fact, the same molecular skeleton, but with different and unlike substitution patterns in the two molecular moieties of syn‐5 and anti‐5, had already been reported by Hopf and coworkers using a different and relatively long synthesis strategy [10]. In their report, apart from the x‐ray crystal data of isomer anti‐5, no additional information with regard to the host guest properties of these dimers were disclosed. Here we provide further structural and host–guest complexation properties of both of the newly prepared fusodimers syn‐4 and anti‐4. As it turns out, due to the larger molecular cavity and the deeper concave shape, isomer syn‐4 is a highly effective host for fullerenes such as C₆₀ and C₇₀. Hence, compounds 4 can serve as alternative building blocks, similar to the parent TBTQ 1 and its derivatives, to be used in the construction of complex molecular systems, and further π‐extended warped polyaromatic compounds [25, 26, 27].

FIGURE 2.

Structures of the diastereomeric pairs of TBTQ fusodimers 4 and 5.

The synthesis of the fusodimers 4 began with 4‐bromoveratrole 6, which was treated with n‐butyllithium, followed by two‐fold reaction with terephthalaldehyde 7 to give diol 8 in 78% yield as a mixture of stereoisomers (Scheme 1). The mixture was subjected to double condensation with indanedione 9 [28, 29] in the presence of para‐toluenesulfonic acid to obtain the tetraketones 10 in 72% yield. Compounds 10 were reduced with lithium aluminum hydride to give the tetraalcohols 11 in 99% yield. Whereas the product 10 was isolated as a mixture of diastereomers, intermediate 11 was a particularly complex mixture and used without purification. The final step of the synthesis was the four‐fold cyclodehydration of tetrol 11 using orthophosphoric acid in xylenes as a solvent. Stereoisomer syn‐4 was obtained in 13% yield by repeated recrystallization of the product mixture from methanol, while isomer anti‐4 was isolated from the combined mother liquors by column chromatography in 10% yield. The summed product yield of 4 (23%) obtained with the bis‐indanediols 11 represents a remarkably good outcome in view of the yields of comparable two‐fold cyclodehydration reactions achieved with simple and similarly electron‐rich 2‐benzhydyryl‐1,3‐indanediols (45%−50%) [28, 29].

SCHEME 1.

Synthesis of the fusodimers syn‐4 and anti‐4.

The structural identities and spectral assignments of all compounds were confirmed by combined ¹H and ¹³C NMR spectroscopy and mass spectrometry. As expected, the intermediates 8, 10 and 11 were mixtures of diastereomers and full NMR signal assignment was not possible, but the relative chemical shift values within specific regions and the relative integrals matched well with the expected structures. The mass spectrometric data obtained by electrospray ionization (ESI) confirmed the presence of the molecular ions [M + Na]⁺ of all new compounds with the expected m/z values (see Supporting Information).

The structures of the title compounds syn‐4 and anti‐4 were ascertained by ¹H and ¹³C NMR and 2D NMR spectroscopy, mass spectrometry and x‐ray crystallography (see Figures S1–S4 and Table S1). For the syn‐4 dimer, the ¹H NMR spectrum shows the presence of three sets of isolated arene protons through the singlets at δ 7.44, 6.89, and 6.79 ppm. Also, two singlets at δ 4.33 and 4.25 ppm reflect the two sets of bridgehead protons, two singlets at δ 3.81 and 3.80 ppm indicate the two different sets of methoxy groups, and a singlet at δ 1.67 ppm is due to the methyl protons (Figure 3, top). For the anti‐4 dimer, the respective groups of protons give rise to slightly different chemical shifts (Figure 3, bottom). While the methoxy resonances of syn‐4 are almost isochronous (Δδ = 0.01 ppm), those of anti‐4 are separated slightly further (Δδ = 0.06 ppm). Notably, the reverse relation holds true for the two sets of bridgehead protons (Δδ = 0.09 vs. 0.92 ppm). In both cases, the ¹H NMR spectra nicely confirm the molecular symmetries of the isomers (syn‐4: C _2v, anti‐4: C _2h).

FIGURE 3.

¹H NMR spectra (500 MHz, CDCl₃) of the TBTQ fusodimers syn‐4 and anti‐4.

Slow evaporation of a toluene solution of syn‐4 afforded single crystals suitable for x‐ray diffraction analysis, which revealed the presence of a solvate bearing one molecule of syn‐4 and one of toluene in the asymmetric unit. The molecular structure consists of two TBTQ units that share a benzene ring (A) within the central s‐indacene unit with the apical methyl groups oriented toward the same hemisphere (Figure 4a) [30]. The width XY and the depth of the concavity of syn‐4 were measured as 7.09 and 3.73 Å, respectively. The ideal C _2v symmetry of the polycyclic framework is reduced to C ₁ as the molecule is slightly twisted. The angle (∠_ind‐ind) between two indane planes shared within the indacene unit was measured as 6.9°. The angle (∠XAY) between two axes of the inner indane moieties was found to be 177.7°, indicating a slight outward bending of the s‐indacene unit. The angles between the axes of the three indane units of one TBTQ moiety, XA, XB, and XC, and those between the corresponding axes YA, YD, and YE of the other one ranged from 84.4° to 89.8°, thus deviating slightly from perfect orthogonality. Moreover, the dihedral angle between two apical methyl groups ∠_Me‐Me was determined as 6.0°, also differing to some extent from the ideal value of 0° (Figure 4b). A molecule of toluene was found to be encapsulated in the cavity with the closest distance of 3.45 Å. In the crystal packing, molecules of syn‐4 and the translation equivalents form a linear chain along the b‐axis through C−H···O interactions with a length of 2.65 Å (Figure 4c). Moreover, the glide reflection equivalent of each syn‐4 molecule formed another linear chain along the b‐axis so that C−H··· π interactions with distances in the range of 2.91 to 2.93 Å were witnessed between neighboring chains.

FIGURE 4.

X‐ray crystal structure of TBTQ fusodimer syn‐4: (a) Top view showing the width the cavity and the angles between the axes of the indane units, the centroids of the benzene rings are indicated in blue; (b) side view showing the depth of the cavity, the twisting of concave skeleton and the encapsulation of a molecule of toluene; (c) linear chain observed in the crystal packing. Hydrogen atoms are omitted for clarity.

Single crystals of anti‐4 suitable for x‐ray structural analysis were obtained from 1,1,2,2‐tetrachloroethane (TCE) solution. X‐ray diffraction analysis revealed that there is only half of the anti‐4 molecule together with four TCE solvates in the asymmetric unit [30]. Since the centroid of the anti‐4 molecule sits on the inversion center, two separated conical cavities by TBTQ cores are inverted to each other with the expected C _2h symmetry. The vertical separation between the concave up‐oriented TBTQ core and the concave down‐oriented counterpart was measured as 5.65 Å. The width and the depth of two identical conical cavities are 8.27 and 2.96 Å, respectively (Figure 5a). The angles between the three axes of indane units XA, XB, and XC and their symmetrical equivalents (xa, xb, and xc) range from 87.9° to 92.3°, indicating slightly less deviation from perfect orthogonality and inward bending within the TBTQ cores as compared to syn‐4. Unlike syn‐4, no molecular twisting was observed as the dihedral angle (∠_Me‐Me) was found to be 180°. Moreover, the two central indane planes are coplanar, as the angles (∠_ind‐ind) and (∠XAx) are 0° and 180°, respectively, because of the inversion symmetry (Figure 5b). Short contacts between the veratrole rings (X···x’ and Y···y’) of two adjacent anti‐4 molecules allow the formation of infinite chains. The stacking is not parallel, and the intermolecular distance of the two closest rings was measured to be 3.49 Å. Two solvated TCE molecules are accommodated in each of the conical cavities of a given anti‐4 molecule with intermolecular distances of 3.43 Å (Figure 5c). Another two TCE molecules are coordinated to the two apical methyl groups along the chain with a distance of 3.49 Å.

FIGURE 5.

X‐ray crystal structure of TBTQ fusodimer anti‐4: (a) Side view showing the width and the depth of the two conical cavities and the angles between the axes of the indane units, the centroids of the benzene rings are indicated in blue; (b) side view showing the lack of twisting of the skeleton; (c) linear chain with the different accommodation of TCE solvate molecules observed in the crystal packing. All the solvates and hydrogen atoms are omitted for clarity.

We expected that, from the sizes of the cavity shown in the x‐ray crystal structures of syn‐4 and anti‐4, these two compounds may be capable of binding fullerenes. Job plot and/or complexation‐induced chemical shift (CIS) measurements were performed to determine the binding stoichiometry and binding constants in the solution state using toluene‐d ₈ as the solvent. The Job plots indicated a 1:1 binding stoichiometry in both cases involving syn‐4 with C₆₀ and C₇₀ (Figures S6 and S10). The most pronounced downfield shifts of ¹H NMR resonances were noted for the two sets of equivalent methoxy protons, and the concentration‐dependent chemical shift values were fitted into a 1:1 binding model [31, 32], giving binding constants K _a of 382 (± 1.1%) and 410 M^{− 1} (± 2.0%) for C₆₀ and C₇₀, respectively (Figures S5, S7, S9, and S11). This indicates that fusodimer syn‐4 has similar binding affinity towards the fullerene guests C₆₀ and C₇₀. On the other hand, no observable trend could be obtained from CIS spectroscopy involving anti‐4 with either C₆₀ or C_{70 ,} suggesting that this stereoisomer is not an effective host for fullerenes (Figure S8). In comparison to the more extended TBTQ‐fullerene complexes mentioned in the Introduction, fusodimer syn‐4 exhibits relatively weak binding due to the less pronounced cavity of its polycyclic framework.

Crystals suitable for x‐ray structural analysis were also obtained from toluene solutions of the host–guest complexes of syn‐4⊂C₆₀ and syn‐4⊂C₇₀ [30]. X‐ray diffraction analysis of syn‐4⊂C₆₀ revealed that the twisting of the skeleton of syn‐4 is reduced upon encapsulation of a C₆₀ molecule, as the dihedral angle ∠_Me‐Me decreases from 6.0° to 4.2° (Figure 6a). After complexation with C₆₀, the angle (∠_ind‐ind) increases significantly from 6.9° to 16.6° while the angle (∠XAY) decreases from 177.7° to 168.1°, which is likely due to the steric congestion of the host by the bulky fullerene guest (Figure 6b). A slight outward bending of the s‐indacene unit was observed as the width of the cavity decreased to 7.02 Å. The angles between the axes of the indane units, XA, XB, and XC, as well as YA, YD, and YE ranged from 88.0° to 91.7°, suggesting that almost ideal orthogonality of the TBTQ cores is attained upon complexation with C₆₀. The distances between the centroid of C₆₀ and the central benzene ring A of syn‐4 were determined as 7.08 Å, and the corresponding distances to the benzene rings B, C, D, and E of the other four TBTQ units were found to be 7.08, 6.67, 6.50, 6.68, and 6.67 Å, respectively. Offset π‐π stackings were observed where the distances between the periphery of C₆₀ and the four dimethoxyindane wings range within 3.17−3.18 Å. Moreover, π–π stacking between C₆₀ and wing B’ of a neighboring molecule of syn‐4 was also observed with a distance of 6.74 Å (Figure 6c). Interestingly, pairs of two sideward adjacent C₆₀ guest molecules exhibit short contacts with centroid‐to‐centroid distances of 9.94 Å. However, only dimeric subunits but not infinite chains of C₆₀ are formed, as both ends of such pairs were solvated by toluene molecules at the periphery of C₆₀.

FIGURE 6.

X‐ray crystal structure of host–guest complex syn‐4⊂C₆₀: (a) Side view showing the slight twisting of the concave skeleton; (b) side view showing the width and depth of the cavity and the angles between the axes of indane units upon the encapsulation of C₆₀; the centroids of the benzene rings are shown in blue; (c) dimeric subunits observed in the crystal packing. All solvate molecules and hydrogen atoms are omitted for clarity.

Encapsulation of a C₇₀ molecule instead of C₆₀ was found to reduce the twisting of the molecular skeleton of host syn‐4 even further. X‐ray diffraction analysis of syn‐4⊂C₇₀ revealed that the dihedral angle (∠_Me‐Me) is only 2.1° in this case (Figure 7a). Outward bending of two central indane planes was also observed, with the angles (∠_ind‐ind and ∠XAY) measured as 13.9° and 170.0°, respectively, while the width XY of the cavity remain at 7.04 Å (Figure 7b). Similar to the complexation of syn‐4 with C₆₀, the angles between three axes of indane units XA, XB, and XC as well as YA, YD, and YE range from 86.4° to 91.6°, again suggesting almost ideal orthogonality within each of the TBTQ cores upon complexation of syn‐4 with C₇₀. A subtle difference in the encapsulation mode of C₇₀ with host syn‐4 was found in comparison with that of C₆₀. Intuitively, the orientation of the ellipsoidal C₇₀ guest within the complex should be co‐axial with the host syn‐4, such that the major axis of the fullerene would be oriented perpendicular to the central benzene ring A. However, a tilt with an angle (∠_tilt) of 15.5° was observed between the major axis of the ellipsoid of C₇₀ and the perpendicular of central benzene ring A of syn‐4. The distances between the centroid of C₇₀ and the five benzene rings A and B−E of host syn‐4 were measured as 7.61, 6.94, 7.04, 6.83, and 6.72 Å, respectively. Offset π‐π stackings were also observed in the crystal of syn‐4⊂C₇₀, where the distances between the periphery of C₇₀ and the four dimethoxyindane wings range from 3.04 to 3.39 Å. Furthermore, π‒π stacking between C₇₀ of a given complex syn‐4⊂C₇₀ and the benzene ring B’ of the host of a neighboring complex was observed with a distance of 7.25 Å, slightly longer as compared to the case of syn‐4⊂C₆₀ (Figure 7c). In further analogy, dimeric subunits were also observed in the crystal packing of syn‐4⊂C₇₀. Two neighboring C₇₀ molecules exhibit short contacts with centroid‐to‐centroid distances of 10.0 Å.

FIGURE 7.

X‐ray crystal structure of host–guest complex syn‐4⊂C₇₀: (a) Side view showing the slight twisting of the concave skeleton; (b) side view showing the width and depth of the cavity and the angles between the axes of the indane units upon the encapsulation of C₇₀; (c) dimeric subunit observed in the crystal packing. All solvate molecules and hydrogen atoms are omitted for clarity.

3. Conclusion

In summary, we have disclosed an efficient, short synthesis of two symmetrically functionalized tribenzotriquinacene dimers, syn‐4 and anti‐4, involving four‐fold cyclodehydration in the last step. The two concave TBTQ moieties share a single central benzene ring, with the two cavities of these “fusodimers” being oriented either to the same or to two different hemispheres. Both title compounds were fully characterized by NMR spectroscopy, mass spectrometry, and x‐ray crystallography. The large biconcave framework of syn‐4, containing five electron‐rich arene rings, provides a large polyaromatic pocket and is well suited for the encapsulation of the fullerene molecules C₆₀ and C₇₀ with similar binding strength. In contrast, stereoisomer anti‐4 was found to be devoid of any binding capabilities with fullerenes, obviously due to the unfavorable mutual orientation of the two TBTQ bowls. The association constants and structural details of the 1 : 1 host–guest complexes syn‐4⊂C₆₀ were elucidated by CIS spectroscopy and x‐ray crystallography, respectively. The presence of the eight methoxy groups at the peripheral veratrole entities in both syn‐4 and anti‐4 can provide handles for further refunctionalization and extension of the rigid polycyclic framework. Even the six unsubstituted benzhydrylic bridgeheads of these particular fusodimers may be utilized for further modulation at later stages. Owing to the intrinsic orthogonality of the TBTQ core, the perpetuated Cartesian geometry of fused TBTQ oligomers may be used for the development of novel unusual 3D‐nonplanar polyaromatic compounds.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Additional supporting information can be found online in the Supporting Information section. The authors have cited an additional reference within the Supporting Information [33].

Supporting File 1: chem71574‐sup‐0001‐SuppMat.docx

Ng C.‐F., He L., Cheung Y.‐Y., Cao X.‐P., Kuck D., and Chow H.‐F., “Tribenzotriquinacene Fused Dimers (“Fusodimers”)—Synthesis and Host–Guest Properties.” Chemistry – A European Journal 32, no. 41 (2026): e71574. 10.1002/chem.71574

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.