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Chiral separation and characterization oftriazatruxene-based face-rotating polyhedra:the role of non-covalent facial interactions†

Pei Zhang, Xinchang Wang, Wei Xuan, Pixian Peng, Zhihao Li, Ruqiang Lu,Shuang Wu, Zhongqun Tian and Xiaoyu Cao *

We constructed a series of novel chiral molecular face-rotating

polyhedra (FRP) from two 10,15-dihydro-5H-diindolo[3,2-a:30,20-c]-

carbazole (triazatruxene) derivatives and trans-1,2-cyclohexane diamine,

and investigated how facial interactions and the positions of sub-

stituents determine the diastereoselectivity and geometry of the

final assemblies.

Discrete molecular polyhedra have attracted broad researchinterest due to their potential applications in catalysis,1 molecularrecognition2 and gas storage.3 Although molecular polyhedraconnected through non-covalent bonds have been extensivelystudied,4 these polyhedra cannot be purified using chromato-graphy due to the high reversibility of non-covalent bonds orhigh polarity of the polyhedra. As a result, thorough investigationon the diastereoisomers of molecular polyhedra is challenging. Byintroducing dynamic covalent chemistry (DCC)5 into the synthesisof molecular polyhedra,6 their chemical stability has been greatlyincreased and thus facilitating their separation.6c On the otherhand, chiral molecular polyhedra are particularly interesting due totheir unique asymmetric nano-sized cavity7 and potential applica-tions in asymmetric catalysis1a,8 and separation of small chiralmolecules.9 Nonetheless, chiral separation of molecular polyhedraand thorough characterization of each diastereoisomer are evenmore challenging and seldom reported.10

Recently, our group constructed a series of molecular face-rotating polyhedra (FRP) using facial building blocks exhibitingtwo-dimensional chirality, i.e., truxene derivatives (TR)11 andtetraphenylethylene (TPE).12 The molecular FRP possess aunique form of supramolecular chirality, exhibiting the largestmolar ellipticity ever reported (4.6 � 106 deg cm2 dmol�1 for(AAAA)-1).11 The molecular FRP systems also provide insights

into the mechanism of racemization processes of chiral polyhedra.13

In our previous work, we have shown that the alkyl chains of TR arenearly perpendicular to the truxene plane, inducing van der Waalsrepulsive forces among the faces of polyhedra (Fig. 1a). According tocomputational results, those facial interactions are essential for thehigh diastereoselectivity of the assembly process of molecular FRP.But no control experiment has been reported. Recently, a rigidC3-symmetric molecule, i.e. 10,15-dihydro-5H-diindolo[3,2-a:30,20-c]-carbazole (triazatruxene or TAT) has been used as an electron-rich material.14 Interestingly, TAT derivatives can also be used asface-rotating building blocks: three nitrogen atoms at 5, 10, and15 positions form either a clockwise (C) or an anti-clockwise (A)directionality (Fig. 2). More importantly, the alkyl chains of TATare nearly parallel to its triazatruxene plane, thus avoiding vander Waals facial interactions once assembled into molecularpolyhedra (Fig. 1b).

In this work, we assemble two C3-symmetric TAT derivativeswith enantiopure trans-1,2-cyclohexane diamine into a series ofmolecular FRP. The two TAT derivatives, i.e., TAT-CHO-1 andTAT-CHO-2, have three aldehyde groups at 2, 7, 12 and 3, 8, 13positions, respectively (Fig. 2). Interestingly, the assembly of an

Fig. 1 (a) Alkyl groups in TR are nearly perpendicular to the truxene planeand fill the cavity of molecular polyhedra. (b) Alkyl groups in TAT-CHO-1are nearly parallel to the triazatruxene plane, and the cavity of molecularpolyhedra is empty.

State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry

and Chemical Engineering, Collaborative Innovation Center of Chemistry for Energy

Materials (iChEM) and Key Laboratory of Chemical Biology of Fujian Province,

Xiamen University, Xiamen 361005, P. R. China. E-mail: xcao@xmu.edu.cn

† Electronic supplementary information (ESI) available: Experimental details such asNMR, mass spectra and crystal data. CCDC 1578604 and 1578652. For ESI andcrystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc02049c

Received 15th March 2018,Accepted 6th April 2018

DOI: 10.1039/c8cc02049c

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4686 | Chem. Commun., 2018, 54, 4685--4688 This journal is©The Royal Society of Chemistry 2018

achiral facial building block (TAT-CHO-1) and a chiral buildingblock (CHDA) produces all five possible diastereoisomers,which are separated using chiral column chromatography.The five diastereoisomers exhibit different NMR patterns andchiral optical properties. Moreover, the subtle structural differencebetween these two building blocks leads to distinct geometry anddiastereoselectivity of the final assemblies.

Facial building blocks TAT-CHO-1 and TAT-CHO-2 weresynthesized following two different synthetic approaches accordingto the literature (for detailed synthetic procedure and characteriza-tions, see the ESI†). The oxidation of 2,7,12-tribromo-5,10,15-tributyl-triaza-truxene using butyl lithium and N,N-dimethylformamide gaveTAT-CHO-1, whereas 5,10,15-tributyl-triazatruxene and dichloro-methyl methyl ether using tin(IV) chloride as a catalyst affordedTAT-CHO-2.

The condensation reaction between TAT-CHO-1 and (1R,2R)-CHDA was carried out in toluene with a trace amount of trifluoro-acetic acid as a catalyst to give FRP-8. FRP-8 was first characterizedusing high-resolution mass spectrometry, which showed one peak atthe mass-to-charge ratio m/z = 1430.89803 (calc. m/z = 1430.89240 for[M + 2H]2+) (Fig. S24, ESI†), corresponding to the molecular [4+6]polyhedra constructed using four equivalents of TAT-CHO-1 and sixequivalents of CHDA. However, the nuclear magnetic resonance(NMR) spectra suggested a mixture of stereoisomers in FRP-8, asindicated by the complex peaks of imine protons (Fig. S5, ESI†).

We used chiral high performance liquid chromatography(HPLC) to separate FRP-8, which showed five peaks eluting at7.8, 11.8, 15.7, 21.2, and 27.4 minutes in the 8 : 23 : 27 : 15 : 18ratio, respectively (Fig. 3b). The five stereoisomers were thencollected and characterized separately. We inferred the molecularsymmetry of each stereoisomer from the numbers of their NMRpeaks. Aromatic protons of the TAT moiety at the 13 positionexhibited one, four, six, four and one sets of peaks, respectively(Fig. 3c). Our previous report demonstrated that for a face-rotating

octahedron, there are five possible stereoisomers, i.e., CCCC (T),CAAA (C3), CCAA (S4), CCCA (C3) and AAAA (T), and these stereo-isomers would exhibit one, four, six, four and one sets of NMRpeaks, respectively. The number of stereoisomers in FRP-8 andtheir molecular symmetries are in accordance with the speculation,thus indicating that all the five possible stereoisomers in theoryhave been found for FRP-8. Although the rotational pattern of eachstereoisomer cannot be fully assigned using NMR, it is clear thatthe first and fifth peaks represent T-symmetric CCCC or AAAA, thesecond and the fourth peaks represent C3-symmetric CCCA orCAAA, and the third peak represents S4-symmetric CCAA.

Finally, we fully assigned the directionality of the fivestereoisomers by comparing their experimental circular dichroism(CD) spectra with calculated spectra. In the experimental CDspectra (Fig. 4b), the fifth peak exhibited the largest CD intensityat 370 nm. We calculated the CD spectra of the five possiblestereoisomers using tight-binding approximations to a time-dependent density functional based tight binding (TD-DFT+TB)approach. The calculated CD spectra of the five stereoisomersshowed that AAAA polyhedra have the largest CD intensity at itshighest CD intensity (Fig. 4c). Hence, the fifth peak was assigned tothe AAAA polyhedra. Similarly, the directionalities of the otherpeaks were also assigned accordingly (Fig. 4b and c).

The assembly of TAT-CHO-1 and chiral 1,2-diamines producedall five possible stereoisomers, exhibiting no apparent diastereos-electivity during the assembly process. By contrast, the assembly ofTR and chiral 1,2-diamines exhibited high diastereoselectivity andproduced only one diastereoisomer. Hence, this work representsthe first control experiment to illustrate how facial interactionsdetermine the diastereoselectivity of molecular FRP. We havepreviously tried to assemble truxene-based molecular FRP withoutfacial interaction by introducing shorter methyl or ethyl groups, butthe poor solubility of the resulting FRP prevented their separationand full characterization.

We then studied the assembly of TAT-CHO-2 and (1R,2R)-CHDA into R-FRP-9, following a similar procedure to that ofFRP-8. The mass spectra of R-FRP-9 showed one major peak at

Fig. 2 (a) Triazatruxene derivatives TAT-CHO-1 as a facial building block.(b) TAT-CHO-2 as a facial building block. (c) The assembly of TAT-CHO-1and enantiopure CHDA gave [4+6] molecular FRP, which contain fivediastereoisomers. (d) The assembly of TAT-CHO-2 and enantiopure CHDAgave a [2+3] molecular prism, which contains only one diastereoisomer.

Fig. 3 (a) Five possible diastereoisomers of a face-rotating octahedronand their molecular symmetry. (b) Chiral HPLC analysis of FRP-8 indicatesfive diastereoisomers. (c) Partial NMR spectra of the five diastereoisomersexhibit one, four, six, four and one sets of doublet, respectively.

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m/z = 1430.90521 (calc. m/z = 1430.89240 for [M + H]+) (Fig. S25,ESI†), corresponding to the [2+3] condensation product of twoequivalents of TAT-CHO-2 and three equivalents of CHDA. Inthe NMR spectra of R-FRP-9, six imine protons exhibited onlyone set of singlet, indicating that all the imine protons areinterchangeable through symmetry operations. Single crystalstructures demonstrated that R-FRP-9 is a molecular prism withtwo parallel faces (Fig. 5a). In theory, a face-rotating [2+3] prismhas three possible stereoisomers, i.e., AA, CA and CC. In thesingle crystal structures of R-FRP-9, only a C3-symmetric AAprism was found, in accordance with the high molecularsymmetry inferred from the NMR spectra.

Thorough investigations on the stereoisomers of R-FPR-9were carried out using chiral HPLC and CD spectroscopy.Chiral separation analysis of R-FRP-9 showed a single peakeluting at 10.3 minutes (Fig. S26a, ESI†). Theoretical CD spectrawere also calculated using the TD-DFT+TB approach. Theexperimental CD spectra of R-FRP-9 are in accordance withthe calculated CD spectra of the AA diastereoisomer (Fig. 5b).These results confirmed that the assembly of TAT-CHO-2 and(1R,2R)-CHDA gave only an AA prism, exhibiting high diastereo-selectivity. Similarly, TAT-CHO-2 and (1S,2S)-CHDA assembledinto S-FRP-9 with only a CC diastereoisomer.

Moreover, in the crystal structures of FRP-9, there are bothintra- and intermolecular p–p interactions between the aromatictriazatruxene faces. The intramolecular distance between the twofaces of one molecular prism (Fig. 5c) was 4.0 Å, and theintermolecular distance between two prisms was 3.8 Å (Fig. 5c),indicating intra- and intermolecular p–p interactions. In addition,the intramolecular p–p interactions within FRP-9 are probablyresponsible for its high diastereoselectivity, similar to the highdiastereoselectivity introduced by the facial interactions throughvan der Waals repulsive forces in the FRP based on TR.15 Inter-molecular interactions between FRP-9 lead to a one-dimensionalcolumn in solid states (Fig. 5c).

In conclusion, we reported a series of chiral molecular FRPbased on TAT derivatives. Two C3 symmetric planar molecules wereused as facial-building blocks and assembled with enantiopureCHDA into different assemblies. TAT-CHO-1 with three aldehydegroups at 2, 7, and 12 positions resulted in [4+6] polyhedra. Due tothe lack of facial interactions, the assembly of TAT-CHO-1 andCHDA exhibited no apparent diastereoselectivity and resulted in allfive possible stereoisomers. We separated the five diastereoisomersand characterized them using NMR, MS, and CD spectroscopy.TAT-CHO-2 with three aldehyde groups at 3, 8, and 13 positionsassembled with CHDA into a [2+3] molecular prism, and gave onlyone diastereoisomer out of the three possible diastereoisomers.The high diastereoselectivity can be attributed to the intra-molecular p–p interactions between two faces of the molecularprism. Our research systematically investigated the influences offacial interactions on the diastereoselectivity of molecular FRP andthe influences of substituents on the geometry and diastereoselec-tivity of the final assemblies.

This work was supported by the 973 Program (No.2015CB856500), the NSFC (No. 21722304, 21573181, and91227111) and Fundamental Research Funds for the CentralUniversities (No. 20720160050) of China. We thank Hang Qufor single crystal measurements and Hongxun Fang for HPLCanalysis.

Conflicts of interest

There are no conflicts to declare.

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Fig. 4 (a) Optimized structure of (AAAA)-FRP-8 using DMol at the TZPlevel. (b) Experimental CD spectra of five diastereoisomers of FRP-8.(c) Calculated CD spectra of five diastereoisomers using the TD-DFT+TBmethod.

Fig. 5 (a) Single crystal structure of R-FRP-9 using (1R,2R)-CHDA asvertices. (b) Experimental (solid line) and calculated (dash line) CD spectraof FRP-9, using (1R,2R)-CHDA (red line) and (1S,2S)-CHDA (blue line) asvertices. (c) Packing motif of R-FRP-9 in crystals.

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15 The assembly of TR and (1R,2R)-CHDA does not result in any [2+3]molecular prism. Because the butyl groups in TR are nearly perpendi-cular to its truxene plane, the formation of the [2+3] prism wouldinduce high steric hindrance between the two faces of the prism.

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