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An atropisomeric M2L4 cage mixture displaying
guest-induced convergence and strong guest
emission in water†
Takahiro Tsutsui,‡a Lorenzo Catti,‡b Kenji Yozac and Michito Yoshizawa*aIntroduction of atropisomeric axes into a bent bispyridine ligand leads to the quantitative formation of
a complex mixture of atropisomeric M2L4 cages upon treatment with metal ions. Whereas the isomerratio of the obtained cage mixture, consisting of up to 42 isomers, is insensitive to temperature andReceived 10th June 2020
Accepted 17th July 2020solvent, the quantitative convergence from the mixture to a single isomer is accomplished upon
encapsulation of a large spherical guest, namely fullerene C60. The observed isomerization with otherguests depends largely on their size and shape (e.g., <10 and 82% convergence with planar triphenylene
and bowl-shaped corannulene guests, respectively). Besides the unusual guest-induced convergence,

DOI: 10.1039/d0sc03223a

the present cage mixture displays the strongest guest emission (FF ¼ 68%) among previously reported

rsc.li/chemical-science

MnLm cages and capsules, upon encapsulation of a BODIPY dye in water.

Introduction
Biological assemblies such as viral capsids and ferritins display
well-organized, spherical nanoarchitectures yet consist of
unsymmetrical protein subunits bearing exible moieties
before assembly (Fig. 1a).1 On the other hand, synthetic chemists are apt to design highly symmetrical subunits with suﬃcient conformational rigidity to precisely construct sphericalnanostructures through hydrogen bonding2 and coordinationdriven self-assemblies,3 e.g., a spherical M2L4 cage (Fig. 1b).4
The cavity size and properties of such articial assemblies arealso highly controllable by the use of the well-designed
components.5 In contrast, desymmetrized, synthetic subunits
with one or more isomerizable moieties give rise to a complex
mixture of product isomers via self-assembly (e.g., Fig. 1c). So
far, no rational method has been reported to converge suchcomplex, articial mixtures (so-called dynamic combinatorial
libraries)6 into a single, three-dimensional isomer.
Herein we report the design and synthesis of bent bispyridine ligand 1 with two atropisomeric axes (Fig. 2a). Treatment of
the isomerizable and desymmetrized ligands with metal ionsleads to the quantitative formation of a complex mixture ofatropisomeric M2L4 cages 2 (Fig. 2b). Although the isomer ratioof the resultant cages is insensitive to temperature and solvent,
the selective and quantitative convergences from the complex
mixture to a single isomer are demonstrated upon encapsulation of spherical corannulene dimer and fullerene C60, respectively. In addition, the present cage mixture displays thestrongest guest emission (FF ¼ 68%), as compared to previously
reported MnLm cages and capsules,7 upon encapsulation ofa uorescent BODIPY dye.
As the simplest model, we chose a highly symmetrical M2L4
structure to examine its desymmetrization and convergence. It
has been established that bent bispyridine ligands with a virtual
C2v symmetry generate M2L4 cages or capsules as a single
isomer upon complexation with metal ions (Fig. 1b).4 Their hostabilities have been widely investigated by many groups so far.8,9Thus we introduced atropisomeric axes10 into the ligand andaLaboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo
Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: yoshizawa.m.ac@m.titech.ac.jpb

WPI Nano Life Science Institute, Kanazawa University, Kakuma-machi, Kanazawa
920-1192, Japanc

Bruker AXS, 3-9 Moriya-cho, Kanagawa-ku, Yokohama 221-0022, Japan† Electronic supplementary information (ESI) available. CCDC 1907426. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc03223a‡ These authors contributed equally.

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Fig. 1 Schematic representation of the self-assemblies of (a) a biological nanostructure, (b) an M2L4 cage from symmetrical ligands andmetal ions, and (c) a complex mixture of M2L4 cage isomers fromdesymmetrized ligands and metal ions studied herein.

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Schematic representation of (a) bispyridine ligand 1 (R ¼
–OCH2CH2OCH3) and, (b) the optimized structures of the atropisomers in equilibrium (DFT calculation, B3LYP/6-31* level, R ¼–OCH3) and the formation of M2L4 cage 2 as a complex isomericmixture.
Fig. 2

designed 1,4-naphthylene-embedded, bent ligand 1 (Fig. 2a).
The new bispyridine ligand adopts three atropisomers owing tohindered rotation around the phenyl–naphthyl bonds so thatthe corresponding atropisomeric M2L4 cage, possessing
a spherical hydrophobic cavity (1 nm in diameter), is anticipated to be present as an equilibrium mixture of up to 42
isomers (Fig. 3).

1

Formation of a complex mixture of M2L4 cages

H NMR spectra (500 MHz, DMSO-d6, r.t.) of (a) ligand 1 and (b)a complex mixture of cage isomers 2. (c) 1H DOSY NMR (500 MHz,DMSO-d6, 298 K) and (d) ESI-TOF MS (DMSO, r.t.) spectra of isomers 2.
Optimized structures of 2: (e) an all-syn isomer and (f and g) an all-anti
isomer (substituents and counterions are omitted for clarity).

First we observed the quantitative formation of a complex
mixture of M2L4 cage 2 isomers by mixing atropisomeric ligand
1, PdCl2(DMSO)2, and AgNO3 (in a 2 : 1 : 2 ratio) in DMSO-d6 for
1 h at room temperature (Fig. 2).11,12a As expected, the 1H NMR
spectrum of the product mixture showed complicated andbroadened, aromatic signals in the range of 9.7 to 6.6 ppm, incontrast to the simple and sharp signals observed for free ligand
1 (Fig. 4a and b). The 1H DOSY NMR spectrum revealed thatthese proton signals have the same diﬀusion constant (D ¼ 1.2
 1010 m2 s1; Fig. 4c),12b indicating the existence of Pd(II)linked cage 2 as an isomeric mixture in equilibrium. The
exclusive formation of M2L4 assemblies 2 was unambiguouslyconrmed by ESI-TOF MS analysis: prominent peaks wereobserved at m/z 759.9, 1033.9, and 1582.3, assignable to the [2 –n$NO3]n+ species (n ¼ 4, 3, and 2, respectively; Fig. 4d). In
a manner similar to 2, Pt(II)-analogue 20 was formed quantitatively and its M2L4 structure was conrmed by NMR and MS
analyses.11 The existence of various isomers of 20 was alsoestablished by the complicated and broadened 1H NMR signals(Fig. S23†).
The theoretical calculation of cage 2 indicated that diﬀerences in energy are relatively small among representative
isomers (DE < 1.8 kcal mol1; e.g., Fig. 4e, f and S22†),11 supporting the complex signals observed in the 1H NMR spectrum

Results and discussion

Fig. 3Fig. 4

Schematic representation of the possible 42 isomers of M2L4 cage 2. These isomers are in equilibrium in solution.

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(Fig. 4b). The optimized structure of 2 (e.g., an all-anti isomer,
Fig. 4g) claried that the spherical cavity (580 Å3) is surrounded by eight naphthalene panels and the cavity diameter is
1.3 nm. Although the cavity shape and size are almost identicalto those of anthracene-based M2L4 capsule 3 reported previously (Fig. S1†),9,11 the bent polyaromatic ligands of 2 areproximate to each other yet unstacked.

Guest-induced convergence to a single isomer
Whereas the isomer ratio of cage 2 remained intact even uponchanging the solvent (e.g., CD3CN, CD3OD, and D2O) andtemperature (e.g., 10 to 110  C), guest encapsulation allowed
the complex mixture to convert into a single isomer in water.
When a simple planar and hydrophobic guest, triphenylene (Tp;
excess), was mixed with a D2O solution of 2 (0.6 mM) at room
temperature for 1 h (Fig. 5a), the 1H NMR signals of 2 changed
from completely broad13 to slightly sharp (Fig. 5b and c), due tothe quantitative incorporation of the guests (2 equiv.) into thehost cavity through the hydrophobic eﬀect.14 The NMR signals
were unchanged even at elevated temperature (e.g., 80  C). The
ESI-TOF MS analysis of the product clearly supported theChemical Science
formation of 1 : 2 host–guest complex 2$(Tp)2 (Fig. S27c†). This
result indicated the possibility of wide-ranging host abilities of
2 in water and selective isomerization of 2 through an appropriate guest stimulus.15
Next, the treatment of small spherical adamantane (Ad) andbowl-shaped corannulene (Cor) with cage mixture 2 gave rise tohost–guest complexes 2$Ad and 2$(Cor)2 in a quantitative
manner, respectively, under the same aqueous conditions(Fig. 5a).15 Interestingly, the resultant 1H NMR spectra revealed
further conversion from the complex mixtures to a single
isomer upon binding of the sterically demanding guests, whose
shapes are similar to the spherical cavity of 2. The percentagesof the generated, major host isomer accommodating Ad and(Cor)2 guests were estimated to be 75 and 82, respectively(Fig. 5d and e), on the basis of the reference proton signal of themethoxy groups of 2 around 2.96–2.83 ppm. The aliphatic
proton signals of bound Ad appeared around 0.7 ppm, due tothe aromatic shielding eﬀect of 2 (Fig. S28a†). The elevenaromatic signals were clearly observed in the 1H NMR spectrumof 2$(Cor)2 (Fig. 5e). Noteworthy, a similar 1H NMR spectrum
was obtained from a mixture of cage 20 , which is an analogue of
2 with Pt(II) ions, and excess Cor in D2O at room temperature for
7 h (Fig. S30†).11 The nding revealed that the naphthalene
panels embedded in the spherical framework of 2 can ip evenat ambient temperature, because the bonds between the Pt(II)hinges and the pyridyl groups are xed under these conditions.
The quantitative conversion of the mixture into a single
isomer was accomplished upon incorporation of sphericalfullerene C60 (C60) into cage 2. Mixing of cage 2 with excess C60
in D2O at 100  C for 24 h yielded 1 : 1 host–guest complex 2$C60
exclusively. Intense ESI-TOF MS peaks at m/z ¼ 940.2 and 1274.2
were assignable to [2$C60 – n$NO3]n+ species (n ¼ 4 and 3;
Fig. 5g). Notably, the 1H NMR spectrum of 2$C60 exhibited only
eleven signals in the aromatic region, indicating the presence ofa single isomer (100% selectivity; Fig. 5f). In the 13C NMR
spectrum of 2$C60, a single signal for the C60 guest was observed
at 140.1 ppm (Fig. S32†), in a manner similar to that of theprevious C60-incorporated M2L4 capsule (141.2 ppm).9a The
broadened, new UV-visible absorption bands derived from theguest were also found around 400–730 nm (Fig. S36†).16 In
short, these results revealed that the “atropisomeric” dynamic
combinatorial library6 of cage 2 can be strictly controlled by its
size (volume) and shape complementary guest molecules.

Crystallographic analysis of the single isomer(a) Schematic representation of the guest-induced convergence of a complex mixture of 2. 1H NMR spectra (500 MHz, D2O, r.t.)of (b) cage 2, (c) 2$(Tp)2, (d) 2$Ad, (e) 2$(Cor)2, and (f) 2$C60. (g) ESITOF MS spectrum (H2O) of 2$C60 and the expansion and simulation of
the [2$C60 – 4$NO3]4+ signals.
Fig. 5This journal is © The Royal Society of Chemistry 2020

The direction of the eight naphthalene panels on 2$C60 was
eventually determined by X-ray crystallographic and theoreticalanalyses.11,17,18 As we expected, the crystal structure showed that
one molecule of C60 is fully captured by the cage-shaped M2L4
framework of 2 (Fig. 6a). Remarkably, all the bent bispyridine
ligands adopted a syn conformation and aligned roundly in thesame direction (Fig. 6b), most probably to avoid the steric
repulsion between the naphthalene panels of the adjacentligands. The p-stacking interactions between C60 and thenaphthalene panels were observed at eight positions (d ¼ 3.2–
3.4 Å in interplanar distance; Fig. S38†). The theoretical

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Chemical ScienceFig. 6 X-ray crystal structure of 2$C60 (all-syn isomer): (a) space-ﬁlling
representation for C60 and (b) space-ﬁlling representation for thenaphthalene panels (the substituents are replaced by H atoms forclarity). (c) Optimized structure of 2$C60 (all-anti isomer, R ¼ –H).

calculations of 2$C60 (R ¼ –H) on the basis of the present crystal
structure indicated that the energy of the all-syn isomer (Fig. 6a)
is 10.6 kcal mol1 lower than that of an all-anti isomer (Fig. 6c)in the gas phase. The cage framework possesses four small
openings (approximately 4  5 Å2) between the polyaromatic
ligands, in contrast to previous M2L4 capsule 3 with ananthracene-based, closed shell.9 Preliminary host–guest studies
showed the selective binding of one molecule of functionalized
fullerenes (e.g., diethyl malonate-derivatized C60) by the present
cage in water (Fig. S40†).11

Strong emission of an encapsulated BODIPY derivative
Finally, we encountered that naphthalene-based Pt(II)-cage 20
displays very strong guest emission upon encapsulation of
a BODIPY dye in water.19 Aer stirring a mixture of cage 20 andwater-insoluble, pentamethyl boron-dipyrromethene PMB (9equiv.) in D2O at 80  C for 1.5 h (Fig. 7a), the formation of 1 : 1
host–guest complex 20 $PMB was evidenced by NMR, MS, andUV-visible analyses. The obtained, broad 1H NMR signals indicated the existence of a complex isomeric mixture including theguest dye (Fig. S41a†). The 1 : 1 host–guest ratio of the productwas conrmed by the ESI-TOF MS spectrum, which shows clear
MS peaks for [20 $PMB – n$NO3]n+ species (n ¼ 4, 3; Fig. S41b†).
The UV-visible spectrum of the pale yellow solution of 20 $PMB
in water showed new absorption bands in the range of 410–
550 nm, derived from encapsulated PMB (Fig. 7b). The observed
bands were slightly red-shied (Dl ¼ 10 nm) with respect tothose of free PMB in CH3CN and their intensity indicated theencapsulation eﬃciency being 25%. The optimized structureof 20 $PMB (R ¼ –H) suggested that a slightly bent PMB framework is fully accommodated in the polyaromatic shell of 20(Fig. 7c).11
Host–guest complexes featuring high emissivity in water
have attracted considerable attention related to biomolecularsensors. However, the majority of coordination cages and8148 | Chem. Sci., 2020, 11, 8145–8150

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Fig. 7 (a) Schematic representation of the formation of highly emissive host–guest complex 20 $PMB. (b) UV-visible spectra and photograph (H2O, r.t.) of 20 $PMB, 20 , and PMB (in CH3CN). (c) Optimized
structure of 20 $PMB (R ¼ –H, all-syn isomer). (d) Fluorescence spectrum (H2O, r.t., 80 mM based on 20 , lex ¼ 500 nm) of 20 $PMB and its
photograph (lex ¼ 365 nm).

capsules disturb guest emission upon encapsulation, owing to
the heavy metal eﬀect.3–6 Our previous studies exceptionallydemonstrated strong uorescence (lmax ¼ 535 nm, FF ¼ 48%)
from a 1 : 1 host–guest complex, composed of anthracene-based
capsule 3 and PMB in water.7c Notably, the aqueous solution of
20 $PMB emitted much stronger green uorescence witha quantum yield of 68% at lmax ¼ 514 nm, upon irradiation at
500 nm (Fig. 7d). The band shape and maximum wavelength
are similar to those of free PMB in ethanol (lmax ¼ 522 nm,
FF ¼ 78%). To the best of our knowledge, this represents thestrongest guest emission in coordination host compounds reported so far.7 As compared to capsule 3 with a relatively rigid
cavity,9 the exible naphthalene-based cavity of 20 is able toadjust to the shape of the bulky PMB guest (Fig. 7c), which
reduces the steric host–guest repulsion. Once the cage encapsulates the guest, the exibility of the naphthalene panels islargely suppressed due to the eﬃcient host–guest interactions.
These eﬀects might suppress emission quenching processes,
observed in the case of 3$PMB, to a large degree (DFF ¼ +20%).19

Conclusions
We have challenged the perfect conversion of a complex mixtureof atropisomeric M2L4 cages into a single isomer by an external
stimulus. The cage mixture including up to 42 isomers was
generated upon incorporation of two atropisomeric axes into
the bent bispyridine ligands of a common and highly symmetrical M2L4 structure. Unusual convergence of the mixed isomers
was demonstrated upon encapsulation of guest molecules.
Particularly, the encapsulation of a fullerene guest led to thequantitative formation of a single isomer, due to its size andshape complementarity to the spherical host cavity. Thus, thepresent system can be regarded as a novel dynamic combinatorial library capable of incorporating large guests up to 1 nm.
The isomer structure (all-syn) of the M2L4 cage was fully characterized by NMR, MS, UV-visible, X-ray crystallographic, andtheoretical analyses. As an additional host function, the cage

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mixture encapsulated a uorescent BODIPY dye and the resultant 1 : 1 host–guest complex exhibited intense guest emission(FF ¼ 70%) in water. We believe that the present study isa small yet rst “synthetic” step to approach large and wellorganized, biological capsules, composed of multiple unsymmetrical subunits, in vitro.

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Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
This work was supported by JSPS KAKENHI (Grant No.
JP18H01990/JP19H04566) and “Support for Tokyotech
Advanced Researchers (STAR)”. We thank Prof. Munetaka Akita
and Dr Naoki Noto for helpful discussion and L. C. thanks the
JSPS and Humboldt Postdoctoral Fellowship.

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11 See the ESI.† The geometry optimizations of the cages andhost–guest complexes were performed with molecularmechanics (MM) calculations (Forcite module, Materials
Studio, version 5.5.3). Particularly, the optimized

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Chemical Sciencestructures and energies of all-syn/anti 2$C60 isomers were
calculated on the basis of the crystal structure.
12 (a) Ligand 1 was synthesized in three steps using 3pyridineboronicacid
pinacolester,
1,4dibromonaphthalene,
and1,5-dibromo-2,3,4-tri(2methoxyethoxy)-benzene;11 (b) The diﬀusion constantindicates the product size (outer diameter) being
approximately 2 nm.
13 In the 1H NMR spectrum, the signals of a mixture of 2 in D2O(Fig. 5b) are much broader than those in DMSO-d6 (Fig. 4b),
because of strong interactions between the naphthalenebased bispyridine ligands, induced by the hydrophobic
eﬀect.
14 The ESI-TOF MS analysis of the products elucidated thehost–guest ratios (except for the labile 2$Ad complex underMS conditions). In addition, the ratios were also estimatedby the 1H NMR integrations of the products aer
disassembly in DMSO-d6 (at 80  C for 1 h under highdilution conditions).
15 The guest-induced isomerization of coordination cage
frameworks is uncommon, whereas there have been
several reports on guest-induced structural changes ofcoordination cages: (a) M. Scherer, D. L. Caulder,D. W. Johnson and K. N. Raymond, Angew. Chem., Int. Ed.,
1999, 38, 1588–1592; (b) S. Hiraoka and M. Fujita, J. Am.
Chem. Soc., 1999, 121, 10239–10240; (c) K. Umemoto,K. Yamaguchi and M. Fujita, J. Am. Chem. Soc., 2000, 122,
7150–7151; (d) D. M. Wood, W. Meng, T. K. Ronson,

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A. R. Stefankiewicz, J. K. M. Sanders and J. R. Nitschke,
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R. A. Bilbeisi, J. D. Thoburn, J. K. Clegg and J. R. Nitschke,
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16 The absorption bands of the electron-decient C60 guest are
sensitive to the polyaromatic host framework due to theeﬃcient host–guest interactions.7f The electron-donating
properties of the host framework are decreased by the
replacement of the anthracene panels with naphthalene
panels so that large blue shis of the guest absorption
bands were observed in 2$C60 as compared with ananthracene-based host–guest analogue.9a
17 The brown block crystals of 2$C60 were obtained by slow
concentration of a 2 : 1 acetonitrile/water solution of 2$C60
over 3 weeks at room temperature. Whereas there havebeen many reports on the successful X-ray crystallographic
analysis of M2L4 hosts,4,8,9 reports on host–guest complexes
including one C60 guest are still rare.1818 (a) C. Garcı́a-Simón, M. Costas and X. Ribas, Chem. Soc. Rev.,
2016, 45, 40–62; (b) B. Chen, J. J. Holstein, S. Horiuchi,
W. G. Hiller and G. H. Clever, J. Am. Chem. Soc., 2019, 141,
8907–8913.
19 Cage 20 was chosen for this study due to the pronounced
heavy-atom uorescence quenching eﬀect of the Pd(II)analogue. A detailed study of the emission dynamics has
been performed with 3$PMB, using uorescence lifetimeand femtosecond transient absorption measurements.7c

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