Synthesis and Structure of [9]Cycloparaphenylene Catenane: An All-Benzene Catenane Consisting of Small Rings

Article abstract of DOI:10.1021/acs.orglett.9b04599

A catenane consisting of two [9]cycloparaphenylenes ([9]CPPs) has been synthesized. Density functional theory calculations suggested that [n]CPPs (n = 5, 6) are highly strained upon the formation of catenanes compared with the corresponding uncatenated CP

Full text of DOI:10.1021/acs.orglett.9b04599

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Letter
Synthesis and Structure of [9]Cycloparaphenylene Catenane: An All-
Benzene Catenane Consisting of Small Rings
Yasutomo Segawa,* Motonobu Kuwayama, and Kenichiro Itami*
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ABSTRACT: A catenane consisting of two [9]cycloparaphenyl-
enes ([9]CPPs) has been synthesized. Density functional theory
calculations suggested that [n]CPPs (n = 5, 6) are highly strained
upon the formation of catenanes compared with the corresponding
uncatenated CPPs, whereas [n]CPP catenanes (n ≥ 7) are not
strained. The synthesis of ([9]CPP)([9]CPP)catenane was
accomplished via the following route: (i) a spirosilylation, (ii) a
nickel(0)-mediated macrocyclization, (iii) a desilylation, and (iv)
reductive aromatization reactions. An X-ray diffraction analysis
revealed a catenated structure of ([9]CPP)([9]CPP)catenane.
nnovative methods for the synthesis of mechanically
interlocked molecules (MIMs) have unlocked previously
I
inaccessible areas of science and technology.1 Representative
MIMs such as catenanes and molecular knots exhibit distinct
topologies due to their three-dimensionally interlocked
structures.1,2 Since the first synthesis of a catenane by
Wasserman in 1960,3 synthetic methods for MIMs have been
extensively investigated. Prior to the 1980s, methods based on
covalent templates and statistical threading have been
predominantly used for the synthesis of catenanes and
rotaxanes.4 Sauvage and coworkers subsequently developed a
method that is based on the use of a 1,10-phenanthroline−Cu
complex, which improved the product yield and later enabled
the first synthesis of a molecular trefoil knot.5 In addition to
this coordinative template method, several types of reversible
interactions, for example, electrostatic and π−π stacking
interactions as well as hydrogen and halogen bonding, have
been widely used to date.6 However, all of these methods
require polar or coordinative functional groups and thus
impose severe limitations on the synthesis of nonpolar MIMs.
Except for one example of a cycloalkane-based catenane,7
reports on the synthesis of heteroatom-free MIMs that consist
solely of carbon and hydrogen atoms have remained elusive
until very recently.
Figure 1. (a) Previously reported mechanically interlocked cyclo-
paraphenylenes (m = 0, 1). (b) Synthetic strategy for mechanically
interlocked cycloparaphenylenes.
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remain elusive. In 2016, Rader, Mullen, and coworkers
reported a detailed mass analysis of a mixture of CPPs,
which suggested the potential presence of CPP catenanes and
trefoil knots, although results of further spectral or structural
investigations were not reported at that time given that
individual components proved difficult to isolate.9 Moreover,
Cycloparaphenylene (CPP) is a challenging building block
for the construction of topological structures.8 CPPs have
received much attention during the past decade due to their
highly symmetric structure and radial π-conjugation mode.
Since the first synthesis of [n]CPPs (n = 9, 12, 18; n = number
of paraphenylene units) by Bertozzi, Jasti, and coworkers in
2008,8h several CPP derivatives have been synthesized using
strain-releasing strategies. Whereas the generation of CPP
catenanes and knots (Figure 1a) has been actively sought after
in the field of synthetic chemistry, their synthesis and isolation
Received: December 22, 2019
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isolation of these CPP MIMs enabled us to reveal their
characteristic physical properties such as the vortex-like
dynamic motion of the CPP trefoil knot and the rapid energy
transfer between the two rings in ([12]CPP)([9]CPP)-
catenane.
Herein we report the synthesis, structure, and photophysical
properties of ([9]CPP)([9]CPP)catenane (1). The synthesis
of small CPPs has been widely investigated as (i) the
photophysical properties of relatively small CPPs ([5]−
[11]CPPs) differ dramatically from each other depending on
their ring size13 and (ii) the synthesis of smaller CPPs is highly
challenging due to their high intrinsic strain.14 Therefore, CPP
catenanes that consist exclusively of small CPPs are of high
interest with respect to both their photophysical properties and
the synthetic difficulties associated with their generation. Prior
to the experimental efforts, we theoretically investigated the
relationship between the strain energy and the ring size of CPP
catenanes.
Initially, we calculated the strain energies of CPP catenanes
to examine the effect of an interlocked system on the intrinsic
strain. Density functional theory (DFT) calculations at the
B3LYP/6-31G(d) level of theory were used for all of the
structures. The strain energies14 of the catenanes consisting of
two [n]CPPs are plotted in Figure 2. The strain increases for n
= 6 and is very high for n = 5. This is due to a repulsion from
the benzene ring that resides within the CPP ring, suggesting
that the ring size of [5]CPP is too small for the formation of
catenane structures. Because the strain does not significantly
change for n > 6, the effect of the strain on the formation of the
catenane can be considered negligible when the size is larger
than that of [6]CPP. This result represents an important
guideline for the molecular design of CPP catenanes.
We attempted the synthesis of [9]CPP catenane 1 (Figure
3a). The C-shaped unit 2 has already been reported as a
precursor for ([9]CPP)([12]CPP)catenane.11a Compound 2
was subjected to our previously developed spirosilylation
conditions.11a The dilithiation of 2 followed by the addition of
SiHCl3(tmeda)15 afforded spirosilane 3 in 49% yield.
Figure 2. Strain energies of catenated and uncatenated [n]CPPs as a
function of the ring size (n) calculated at the B3LYP/6-31G(d) level
of theory on hypothetical homodesmotic reactions.
because of the synthetic limitations, all previously reported π-
conjugated catenanes contain phenanthroline moieties.10 Thus
efficient synthetic routes to heteroatom-free catenanes and
knots are in high demand.
Very recently, we have established a new strategy to
synthesize heteroatom-free MIMs.11 Because spirobi-
(dibenzosilole) is quantitatively converted into two biphenyls
upon treatment with fluorides in alcohol,12 the spirosilane unit
can be used as a removable template for the construction of
interlocked structures. We have used this “traceless” synthetic
methodology to synthesize and isolate the ([12]CPP)([12]-
CPP)catenane and ([12]CPP)([9]CPP)catenane as well as a
[24]CPP trefoil knot (Figure 1a,b).11a The synthesis and
Figure 3. (a) Synthesis of ([9]CPP)([9]CPP) catenane 1. Reaction conditions: (i) n-BuLi; then TMEDA, SiHCl3, THF/Et2O. (ii) Ni(cod)2, 2,2′-
bipyridyl, DMF. (iii) n-Bu4NF, dioxane/n-BuOH. (iv) Sodium naphthalenide, THF; then I2. Abbreviations: cod = 1,5-cyclooctadiene, DMF = N,N-
dimethylformamide, TMEDA = N,N,N′,N′-tetramethylethylenediamine. (b) Plausible intermediates 4a,b for the generation of 1 and [9]CPP from
3 with the relative total energies (ΔE) calculated by using the B3LYP/6-31G(d) level of theory. (c) ORTEP drawing of 1 with thermal ellipsoids at
50% probability. One of disordered [9]CPPs, solvent molecules, and hydrogen atoms are omitted for clarity.
B
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Spirosilane 3 was also obtained as a side product during the
synthesis of ([9]CPP)([12]CPP)catenane. A homocoupling
reaction of 3 using Ni(cod)2 and 2,2′-bipyridyl furnished
pseudocatenane 4a. Without complete purification, a desilyla-
tion reaction using TBAF and a reductive aromatization
reaction with sodium naphthalenide were carried out
sequentially. Thus [9]CPP catenane was obtained in 1.6%
yield relative to 3 and [9]CPP was obtained as a side product
in 16% yield; other side products might be the polymeric
insoluble compounds obtained from the homocoupling step.
The formation ratio of catenane to CPP is 1:10, which is much
lower than that of [12]CPP catenane and [12]CPP (∼1:1).
This might be due to the fact that the formation ratio of the
pseudocatenane 4a is decreased relative to 4b on account of
the strain of 4a. DFT calculations indicated that 4a is 6.4 kcal·
mol−1 higher in energy than 4b, which may imply the difficulty
of the macrocyclization step for the formation of 4a (Figure
3b). The molecular structure of 1 was confirmed unequivocally
by X-ray crystallography (Figure 3c). A single crystal of 1 was
obtained from the vapor diffusion of hexane into a THF
solution of 1 at room temperature. Compound 1 exhibits an
interlocked catenane structure with disorder in one of the
[9]CPP rings, and hexane molecules are incorporated in the
voids of the crystal.
The photophysical properties of 1 were measured and
compared with those of [9]CPP to investigate the effect of the
catenane structure on the electronic properties. UV−vis
absorption spectra, fluorescence spectra, absolute fluorescence
quantum yields (ΦF), and the fluorescence lifetime (τ) of 1
and [9]CPP are shown in Figure 4a. Both the absorption and
fluorescence spectra of 1 are similar to those of [9]CPP,
although the fluorescence spectrum is slightly bathochromi-
cally shifted, which is similar to the case of [12]CPP and its
catenane. The absolute fluorescence quantum yield of 1 (ΦF =
0.54) is slightly lower than that of [9]CPP (ΦF = 0.73). Time-
resolved fluorescence measurements were conducted, which
revealed fluorescence lifetimes of 11.3 ns. According to the
equations ΦF = kr × τ and kr + knr = τ−1, the radiative (kr) and
nonradiative (knr) decay rate constants from the singlet excited
state were determined (kr = 4.8 × 107 s−1; knr = 4.1 × 107 s−1).
Compared with those of uncatenated [9]CPP (kr = 6.9 × 107
s−1; knr = 2.5 × 107 s−1), both rate constants are affected by the
catenane structure. The frontier molecular orbitals of 1 and
[9]CPP with their respective orbital energy values are shown
in Figure 4b. The HOMO and HOMO−1 are localized on
each [9]CPP ring, whereas the LUMO and LUMO+1 are
delocalized over the two rings.
Figure 4. (a) UV−vis absorption (solid lines) and fluorescence
(dotted lines) spectra as well as absolute fluorescence quantum yields
(ΦF) and fluorescence lifetime (τ) of 1 and [9]CPP in CH2Cl2.
Excitation wavelength: 340 nm; emission wavelength for the lifetime
measurement of 1: 510 nm. (b) Frontier molecular orbitals of 1 and
[9]CPP were calculated at the B3LYP/6-31G(d) level of theory
(isovalue: 0.02).
red-shifted relative to that of [9]CPP, which indicates weak
π−π interactions for the catenated structure. Further
investigations into the synthesis of heteroatom-free catenanes
and knots are currently in progress in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04599.
Experimental details and spectra of all new compounds
(PDF)
In summary, we have synthesized and structurally charac-
terized a catenane that consists of two [9]CPP rings. DFT
calculations indicated that [n]CPPs (n = 5,6) are highly
strained upon the formation of catenanes, whereas [n]CPPs (n
≥ 7) are not. The synthesis of ([9]CPP)([9]CPP)catenane
(1) was accomplished via the following synthetic route: (i) a
spirosilylation of C-shaped units (2), (ii) a macrocyclization
via a nickel(0)-mediated homocoupling, (iii) a fluoride-
mediated desilylation, and (iv) sequential reductive aromatiza-
tion reactions. The three-step yield of 1 relative to spirosilane 3
(1.6%) is lower than that of ([12]CPP)([12]CPP)catenane
(16%) and similar to that of ([12]CPP)([9]CPP)catenane
(1.5%). A single-crystal X-ray diffraction analysis of [9]CPP
catenane revealed a catenated structure in which one of the
two [9]CPP rings is disordered. The optical properties of 1 are
similar to those of [9]CPP, albeit the emission of 1 is slightly
Cartesian coordinates of optimized structures (XYZ)
Accession Codes
CCDC 1974070 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Yasutomo Segawa − Nagoya University, Nagoya, Japan,
and JST, Nagoya, Japan; orcid.org/0000-0001-6439-
8546; Email: ysegawa@nagoya-u.jp
C
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Kenichiro Itami − Nagoya University, Nagoya, Japan,
JST, Nagoya, Japan, and Nagoya University, Nagoya,
Japan; orcid.org/0000-0001-5227-7894;
Email: itami@chem.nagoya-u.ac.jp
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Itami, K. Design and Synthesis of Carbon Nanotube Segments.
Angew. Chem., Int. Ed. 2016, 55, 5136−5158. (h) Jasti, R.;
Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. Synthesis, character-
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Motonobu Kuwayama − Nagoya University, Nagoya,
Japan, JST, Nagoya, Japan, and Nagoya University,
Nagoya, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.9b04599
Notes
The authors declare no competing financial interest.
̈
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(9) Zhang, W.; Abdulkarim, A.; Golling, F. E.; Rader, H. J.; Mullen,
K. Cycloparaphenylenes and Their Catenanes: Complex Macrocycles
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ACKNOWLEDGMENTS
■
This work was supported by the ERATO program from JST
(JPMJER1302 to K.I.), the KAKENHI Funding Program from
MEXT (JP1905463 to K.I.; JP16K05771, JP19H02701, and
JP19K22183 to Y.S.), a Grant-in-aid for Scientific Research on
Innovative Areas “π-Figuration” (17H05149 to Y.S.), and the
Murata Science Foundation (to Y.S.). Calculations were
performed using resources of the Research Center for
Computational Science, Okazaki, Japan. ITbM is supported
by the World Premier International Research Center Initiative
(WPI), Japan.
̈
̈
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