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S41467-021-21889-8.Pdf ARTICLE https://doi.org/10.1038/s41467-021-21889-8 OPEN Stereoselectivity in spontaneous assembly of rolled incommensurate carbon bilayers ✉ ✉ Taisuke Matsuno 1 , Yutaro Ohtomo 1, Maki Someya1 & Hiroyuki Isobe 1 The periodicity of two-dimensional entities can be manipulated by their stacking assembly, and incommensurate stacks of bilayers are attracting considerable interest in materials sci- ence. Stereoisomerism in incommensurate bilayers was first noted with incommensurate 1234567890():,; double-wall carbon nanotubes composed of helical carbon networks, but the lack of structural information hampered the chemical understanding such as the stereoselectivity during bilayer formation. In this study, we construct a finite molecular version of incommensurate carbon bilayers by assembling two helical cylindrical molecules in solution. An outer cylindrical molecule is designed to encapsulate a small-bore helical cylindrical molecule, and the spontaneous assembly of coaxial complexes proceeds in a stereoselective manner in solution with a preference for heterohelical combinations over diastereomeric, homohelical combinations. The rational design of incommensurate bilayers for material applications may be facilitated by the design and development of molecular versions with discrete structures with atomic precision. 1 Department of Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo, Japan. ✉email: [email protected]; [email protected] NATURE COMMUNICATIONS | (2021) 12:1575 | https://doi.org/10.1038/s41467-021-21889-8 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-21889-8 hen two graphitic sheets are stacked to form carbon to encapsulate a small-bore helical CNT molecule, and the coaxial Wbilayers, there emerges an interesting example of ste- i-DWNT structure has been determined via spectroscopic and reoisomerism in the majority of cases called incom- crystallographic analyses. Unexpectedly, the spontaneous assem- mensurate pairs. The stereoisomerism originates from the twisted bly is controlled not only by cylinder diameters but also by gra- orientations of two graphitic layers, and the induced chirality and – phitic helicity, and the helical handedness of the inner CNT periodicity result in intriguing electronic properties1 3 Owing to molecule is discriminated by the outer CNT molecule. For the the dimensionality reduction via facial discrimination, this unique assembly of i-DWNT complexes, heterohelical inner/outer com- type of stereoisomerism is easier to comprehend by rolling the binations of (P)/(M) and (M)/(P) are preferred over homohelical carbon bilayers in the form of incommensurate double-wall – combinations of (P)/(P) and (M)/(M). Interestingly, this is carbon nanotubes (i-DWNTs)4 6. Thus, in the case of i-DWNT, opposite of the suggestions made previously for infinite CNTs. stereoisomerism appears in a pair of helical carbon nanotubes We anticipate that the stereoselectivity observed with the (CNTs) comprising an incommensurate set of (n1,m1)- and (n2, incommensurate pairs of finite CNT molecules may facilitate the ≠ m2)-chiral indices with the relationship of m1/n1 m2/n2. For molecular design of chiral moiré patterns for materials instance, when (20,4)-CNT encapsulates (9,6)-CNT, two dia- applications8,9. stereomers emerge from facial combinations of the bilayer stacks of helical carbon networks having unidentical chiral angles (Fig. 1). Each of the diastereomeric orientations of the carbon Results bilayers gives rise to a set of enantiomer pairs; therefore, the Design and synthesis of outer CNT molecules. The i-DWNT coaxial assembly of incommensurate pairs of CNTs results in four complexes were designed and synthesised by using [n]phenacenes stereoisomers of (P)/(P), (M)/(M), (P)/(M) and (M)/(P) for the as common bases of cylindrical molecules that have different double-wall combination of (20,4)- and (9,6)-CNTs7. Although diameters. With chrysene ([4]phenacene) panels, the first exam- such stereoisomerism in carbon bilayers has long attracted ple of helical CNT segments was previously prepared in the form attention, particularly in the fields of physics and materials sci- of belt-persistent [4]cyclochrysenylenes ([4]CC)10,11, and with a ence, the structures have not been well understood and realised in contorted chrysene panel of dibenzochrysene, a small-bore helical a discrete form for molecular entities. CNT molecule was synthesised in the form of [3]cyclo-3,11- Here we show that incommensurate pairs of finite CNT dibenzochrysenylene ([3]CdbC, Fig. 2)12. We envisioned that an molecules, i.e., i-DWNT complexes, can be assembled in a outer CNT molecule may be designed to encapsulate the small- spontaneous manner through the design of tight-fitting cylind- bore [3]CdbC by using larger [n]phenacene panels and sought rical molecules. An outer helical CNT molecule has been designed possible structures by using a simple yet effective 2D-mapping method13,14. The small-bore helical CNT segments were (P)- and chiral index diameter (dt) chiral angle ( ) (M)-(9,6)-[3]CdbC, which are hereafter denoted as (P)-(9,6) and Outer + Inner = (20,4) + (9,6) = 1.77 nm + 1.04 nm = 8.9° + 23.4° (M)-(9,6) db CNT CNT . The small-bore [3]C C molecule has a geometric diameter (dt) of 1.04 nm, and, considering the interlayer distance diastereomers – 15,16 (P) + (P) or (M) + (M)(P) + (M) or (M) + (P) of carbon bilayers as 0.34 0.38 nm , the ideal geometric dia- meter for outer cylindrical molecules should be in the range of 1.72–1.80 nm. By mapping [n]phenacenes on a graphitic sheet in various orientations and combinations (Supplementary Fig. 1), we were delighted to find that an isomer of [4]cyclo-3,11-fulmi- nenylene ([4]CF) at a chiral index of (20,4) showed a geometric enantiomers enantiomers (P)-(20,4) (M)-(20,4) (P)-(20,4) (M)-(20,4) + + + + (P)-(9,6) (M)-(9,6) (M)-(9,6) (P)-(9,6) Fig. 1 Stereoisomerism of i-DWNT. An example of a combination of two helical CNTs, i.e., (20,4)- and (9,6)-CNTs, is shown. Combinations of two carbon layers in twisted orientations give rise to two diastereomers, each of Fig. 2 Helical CNT molecules. Chemical structures of small-bore [3]CdbC them existing as a pair of enantiomers. molecules and large-bore [4]CF molecules. 2 NATURE COMMUNICATIONS | (2021) 12:1575 | https://doi.org/10.1038/s41467-021-21889-8 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-21889-8 ARTICLE 1) 2-hexyl- simple 1H NMR spectrum of [4]CF with six aromatic resonances benzaldehyde P+Ph •Br– (four doublets and two singlets) showed the presence of a single 3 (2.0 eq.) diastereomer. Among the four possible structures of [4]CF, tBuOK (2.1 eq.) R THF, reflux, 3 h isomers with D4 or D2d symmetry can afford this simple spectrum. Upon further chromatographic separations of [4]CF 2) h , I (20 mol%) 2 R with amylose-loaded silica gels (CHIRALPAK-IA columns), we – + THF (15 eq.) Br•Ph3 P detected an enantiomer pair, which confirmed the selective toluene, rt, 9 h P 1 45% (2 steps) 2 production of D4-symmetric [4]CF. Two enantiomers, ( )-(20,4) R = hexyl and (M)-(20,4), with helical carbon networks were finally separated by preparative HPLC with CHIRALPAK-IA columns (Bpin)2 (6.0 eq.) Bpin (Supplementary Figs. 3 and 4). [Ir(cod)OMe]2 (10 mol%) R dtbpy (20 mol%) mesitylene Stereoselective spontaneous assembly of the i-DWNT complex. 100 °C, 24 h R We thus obtained four helical CNT molecules: a pair of enan- pinB db 12 84% 3 tiomers of small-bore [3]C C molecules , (P)- and (M)-(9,6), and a pair of enantiomers of large-bore [4]CF molecules, (P)- and 1) PtCl2(cod) (1.0 eq.) (M)-(20,4) (see Fig. 2). Upon mixing these four cylindrical CsF (6.0 eq.) molecules in dichloromethane, the i-DWNT complexes were R THF, reflux, 24 h spontaneously assembled unexpectedly in a stereoselective man- 2) oDCB, 100 °C, 3 h 4 ner. As shown in Fig. 4a, each cylindrical molecule shows six 3) PPh3 (10 eq.) R aromatic resonances as a racemate. Heterohelical combinations of oDCB, rt, 30 min (P)-(20,4)⊃(M)-(9,6) and (M)-(20,4)⊃(P)-(9,6) are diastereoi- 100 °C, 24 h somers of homohelical combinations of (P)-(20,4)⊃(P)-(9,6) and 19% (3 steps) (M)-(20,4)⊃(M)-(9,6) (cf. Fig. 1), and these four stereoisomers, [4]CF including two sets of diasteromeric complexes, should afford 24 MALDI-TOF MS [[4]CF = (P)- & (M)-(20,4)] aromatic resonances in the simplest case. However, when we db m/z = 1978 [M]+ mixed an equimolar amount of the [3]C C and [4]CF racemates containing four cylindrical molecules, we obtained a 1H NMR spectrum comprising 12 aromatic resonances (Fig. 4). This spectrum suggested the presence of specific processes during the 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 complexation, and we sought the origin and the structural m/z identity by separately preparing two diastereomeric complexes, 1H NMR [[4]CF = (P)- & (M)-(20,4)] i.e., heterohelical and homohelical i-DWNT complexes. We first mixed heterohelical molecules of (P)-(20,4) and (M)-(9,6) and 1 dd ss obtained a H NMR spectrum comprising 12 aromatic reso- d d nances (8 × d and 4 × s) that perfectly matched the one observed from the racemate mixture. This observation thus showed that the complex formed in the racemate mixture was an enantiomeric 8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 mixture of (P)-(20,4)⊃(M)-(9,6) and (M)-(20,4)⊃(P)-(9,6).We Chemical shift (ppm) next mixed homohelical molecules of (P)-(20,4) and (P)-(9,6) Fig.
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