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VOLU M E 9 • NUMBE R 3 • 2009

THE CHEMICAL RECORDAN OFFICIAL PUBLICATION OF THE JAPAN CHEMICAL JOURNAL FORUM

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ISSN: 1527-8999

Personal Account

THE
CHEMICALRECORD

Organoplatinum-Mediated Synthesis of
Cyclic π-Conjugated Molecules: Towards
a New Era of Three-Dimensional
Aromatic Compounds
Shigeru Yamago,*[a,b] Eiichi Kayahara,[a,b] and Takahiro Iwamoto[a]Institute for Chemical Research, Kyoto University, Uji 611-0011 (Japan)
E-mail: yamago@scl.kyoto-u.ac.jp[b]
CREST, Japan Science and Technology Agency, Tokyo 102-0076 (Japan)[a]Received: October 18, 2013
Published online: January 22, 2014
Dedicated to Professor Teruaki Mukaiyama on the occassion of the 40th anniversary of the Mukaiyama
aldol reaction

ABSTRACT: This article describes the most recent developments in the synthesis of threedimensional π-conjugated molecules and the elucidation of their properties made by our researchgroup. Various cycloparaphenylenes (CPPs) of different sizes and a cage-like 3D molecule weresynthesized based on the platinum-mediated assembly of π-units and subsequent reductive elimination of platinum. The assembly of π-units by this method mimics the self-assembly process forthe formation of supramolecular ligand–metal complexes with 3D cages and polyhedral structures.
Furthermore, reductive elimination of platinum successfully took place with high efﬁciency, despite
the high strain energy of the target molecule. Several size-dependent physical properties of CPPs,
namely the photophysical, redox, and host–guest chemistries, were also clariﬁed. These results areof use for a molecular-level understanding of CNT physical properties as well as fullerene peapods.
Theoretical and electrochemical studies suggest that small CPPs and their derivatives should be
excellent lead compounds for molecular electronics. DOI 10.1002/tcr.201300035
Keywords: cycloparaphenylenes, fullerenes, host–guest systems, nanotubes, supramolecularchemistry

1. Introduction
It is obvious that one must think logically to take researchforward. However, the beginning of research is not necessarily
logical, and in many instances, is actually triggered in illogicalways, such as by a serendipitous ﬁnding, an instinct, curiosity, passion, and so on. For organic chemists, molecular structure or shape is undoubtedly one of the “illogical” triggers
that initiates research. Cycloparaphenylenes (CPPs) are suchmolecules.
CPPs are hoop-shaped π-conjugated molecules in which
paraphenylene (PP) units are linked in a cyclic manner(Figure 1). CPPs are the shortest structural unit of armchaircarbon nanotubes (CNTs) and are also structural constituents

84 www.tcr.wiley-vch.deof fullerenes. For example, C60 contains [5]CPP units (thenumber in the brackets indicates the number of PP units
constituting CPP). Because of their unique structure, CPPs
have attracted the attention of theoretical, physical, and synthetic chemists for more than half a century.[1] It is worth
noting that organic chemists had instinctually recognized theimpact of the CPP structure before the discovery of fullerenes
and CNTs. By analogy to the huge potentials of CNTs andfullerenes in materials science and technology, especially forphoto-, electro-, and photoelectronic applications,[2] CPPs also
hold signiﬁcant potential as key molecular units for futurematerials with new and/or improved properties. In addition,

Chem. Rec. 2014, 14, 84–100© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

Platinum-Mediated Synthesis of Cyclic π-Conjugated Compounds

CPP

Armchair CNT

Fullerene (C60)

Fig. 1. Structures of a CPP, armchair CNT, and C60. The CPP unit is highlighted as red in the CNT and C60.

CPPs would be a seed compound for the syntheses of structurally uniform CNTs,[3] as CNTs are currently prepared as
mixtures of varying size, helicity, and length by physicalmethods such as laser vaporization, arc discharge of carbon,
and aromatic hydrocarbon combustion, and separated bytedious multistep procedures. Therefore, the bottom-up synthesis of structurally uniform CNTs (at least in size andhelicity) from CPPs by organic synthesis would be an excellent
alternative to current CNT mass-production methods. In
Shigeru Yamago received his B.S. and Ph.D.
in chemistry from the Tokyo Institute ofTechnology in 1986 and 1991, respectively,
as an organic chemist under the supervisionof Professor Eiichi Nakamura. During thattime, he joined Professor Peter Vollhardt’s
group at UC Berkeley as a summer student(1988). He became an assistant professor at
the same institute in 1991, moved to Kyoto University in 1995 as
an assistant professor, Osaka City University in 2003 as full professor, and to his current position at the Institute for Chemical
Research at Kyoto University in 2006. He was a visiting scientist in
Dr. Chryssostomos Chatgilialoglu’s group at Consiglio Nazionaledelle Ricerche in Bologna (2000). He was a research fellow of thePrecursory Research for Embryonic Science and Technology(PRESTO) program in the Japan Science and Technology Agency(JST) from 2002 to 2006, and has been a principal investigator of
the Core Research for Evolutional Science and Technology(CREST) program in JST since 2010. He was a recipient of the
Incentive Award in Synthetic Organic Chemistry from the Societyof Synthetic Organic Chemistry, Japan (SSOCJ), in 2001, theChemical Society of Japan Award for Creative Work in 2010, theAsian Core Program Lectureship Award from Singapore in 2010,
the DIC Functional Materials Award from SSOCJ in 2012, theIchimura Academic Award in 2012, and the Technical Development Award from the Adhesion Society of Japan in 2013. His
research interests include synthetic organic and polymer chemistry,
radical chemistry, element chemistry, and materials chemistry.

Chem. Rec. 2014, 14, 84–100

addition, the cavity of CPPs offers a unique opportunity insupramolecular chemistry to study concave–convex π–πinteractions[1d] and fabricate hierarchically ordered π-materials,
which mimic fullerene peapods.[4]Despite the potential impact of CPPs in fundamental andapplied chemistry and materials science, a lack of synthetic
methods for their preparation have inhibited progress inunveiling their unique properties until recently.
In 2008, Jasti, Bertozzi, and co-workers reported forthe ﬁrst time the synthesis of [9], [12], and [18]CPPs.[5] A
signiﬁcant feature of their synthesis is the use of 3,6-cisdimethoxycyclohexa-1,4-diene 1 as a masked aromatic ring
(Scheme 1a). The cyclohexadiene unit provided the curvatureand rigidity necessary for macrocyclization leading to cyclic
precursor 2 through the Pd-catalyzed Suzuki–Miyaura coupling
reaction of 1a (X = I) and 1b (X = Bpin, pin refers topinacolate). Subsequent reductive aromatization afforded thecorresponding CPPs. Immediately after this report, Itami andco-workers reported in 2009 the selective synthesis of [12]CPP using 1,4-cis-cyclohexane 3 as a masked aromatic ring

Eiichi Kayahara received his B.Sc. and
Ph.D. from Osaka City University and
Kyoto University in 2006 and 2011. Hestudied the precision control of polymer
structure based on the heteroatommediated living radical polymerization
reaction under the supervision of Professor Shigeru Yamago. During his educational career, he was a visiting student in the Professor
Christopher C. Cummins group at Massachusetts Institute ofTechnology, United States (2008). He was also a recipient of
the JSPS Predoctoral Fellowship from 2008 to 2011. Since
2011, he has been an assistant professor at the Institute for
Chemical Research, Kyoto University (with Prof. Shigeru
Yamago). His research interests include the synthesis of neworganic molecules and their applications to new organic
materials on the basis of new and efﬁcient organic reactions,organometallic reactions, and reactive intermediates.
Takahiro Iwamoto was born in Osaka,
Japan, in 1986. He received his B.Sc. (2009)
from Kyoto University and his M.Sc. (2011)
from the same university under the supervision of Professor Shigeru Yamago. He iscurrently a Ph.D. student in the samegroup, focusing on the synthesis and supra
molecular chemistry of new cyclic π-conjugated molecules. He
has been a Predoctoral Research Fellow of the JSPS since 2011. In
2012, he worked with Professor Lawrence T. Scott’s group at
Boston College as an exchange student.

© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

www.tcr.wiley-vch.de85

THE CHEMICAL RECORD(Scheme 1b).[6] After stepwise macrocyclization with 3a (X = I) and3b (X = Bpin) giving cyclic precursor 4, oxidative dehydration
afforded the CPP. Subsequent extension of this method by Jasti
allowed for the selective synthesis of a series of [6]–[12]CPPs,[7] as wellas the gram-scale syntheses of [8] and [10]CPPs.[8] At present, thesmallest CPP able to be synthesized is [6]CPP. Itami and co-workers
also extended their synthetic method for the size-selective synthesis ofa series of [9]–[16]CPPs.[9] These methods have also been applied tothe synthesis of several derivatives by Jasti,[10] Itami,[11] and Müllen.[12]Our group developed a different synthetic route toward
CPPs and reported the selective synthesis of [8]CPP in 2010
(Scheme 1c).[13] The synthesis relies on the formation of “platinum square” 6, which was prepared by the transmetalation of

4,4′-bisstannylated biphenyl 5c (X = SnMe3) and Pt(cod)Cl2(cod refers to 1,5-cyclooctadiene). Reductive elimination of theplatinum species from 6 afforded [8]CPP, which was the smallest CPP prepared at that time. Application of this synthetic
method permitted the random synthesis of [8]–[13]CPPs,[14]selective synthesis of [6] and [10]CPPs,[15] and various CPP
derivatives.[16] Furthermore, an extension of the concept of
the formation of 6 led us to synthesize a cage-like threedimensional π-conjugated molecule.[17] Isobe and co-workers
applied our method to the synthesis of the simplest structuralunit of chiral CNTs.[18] We further unraveled several uniqueproperties of CPPs including size-dependent photophysicalproperties,[14,19] redox properties,[14,15b] and size-selective host–
guest chemistry.[20] The most recent developments in the synthesis of CPPs and their derivatives and the elucidation of theirproperties made by our group are presented in this article.

2. Molecular Design
2.1. Synthetic Planning
Our initial idea for the synthesis of CPPs came from ourongoing project toward the development of new reagents forliving radical polymerization.[21] We have already reported
that organotellurium compounds are excellent agents for providing control over molecular weight and molecular weight
distribution in radical polymerization.[22] Furthermore, theorganotellurium group at the living polymer end can be transformed into a variety of functional groups under anionic,
cationic, and radical conditions.[22,23] During this study, welearned that the C–Te–C bond angle of organotellurium compounds is nearly 90° (Figure 2a), since 5s and 5p orbitals do
not form hybridized orbitals owing to their signiﬁcant energy
differences. Besides the structural uniqueness, this bond angle
minimizes the activation energy of the homolytic substitution
reaction between polymer-end radicals and organotellurium
compounds, which is the key mechanism in controlling themolecular weight distribution in living radical polymerization.[24] We have also demonstrated that organotellurium
compounds undergo reductive elimination (ligand coupling

Scheme 1. Synthetic routes to CPPs.

a)

b)
Y

Y
n

n

- 4 "Y"94.6 o
n

n

Y

n

Y
n

n

B3LYP/6-31G*+LANL2DZ(Te)n7
g: Y = Te, h: Y = I+, i: Y = Pt(L)

Fig. 2. a) In silico optimized structure of dimethyltelluride and b) a possible synthetic route to CPPs from asquare-shaped precursor.

86 www.tcr.wiley-vch.deChem. Rec. 2014, 14, 84–100© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

Platinum-Mediated Synthesis of Cyclic π-Conjugated Compounds

reaction) under mild conditions.[25] These results, coupled with acontinued interest directed toward curved π-conjugated molecules,[26] ignited a new synthetic route to CPPs utilizing squareshaped organotellurium compound 7g (Y = Te) as an intermediate.
Once 7g forms, subsequent reductive elimination of tellurium from
7g would give [4n]CPP (n = integer) (Figure 2b).
We immediately expected two signiﬁcant challenges toarise in this synthetic route. One is the efﬁcient formation of thedesired cyclic macrocycles; that is, obtaining the desired compound in good yield and high selectivity. The precursors are notnecessarily square-shaped, and any cyclic precursors of this sortwould give CPPs. However, a C–Y–C bond angle of 90° would
be suitable for the predictable and selective synthesis of CPPs of
a certain size. The other challenge is the reductive elimination
to form highly strained CPPs (see below); to the best of ourknowledge, no studies exist showing reductive elimination to be
applicable for the synthesis of highly strained molecules. Despite
these challenges, we were optimistic in our synthetic strategy,
and commenced synthesis of the “tellurium square”.
Despite our optimistic expectations, however, the synthesis of tellurium square 7g was unsuccessful so far. During thisstudy, we also learned that Stang and co-workers had alreadyreported the synthesis of “iodonium square” 7h (Y = I+), in
which a 4,4′-biphenyl unit (n = 2) was connected through thehypervalent iodine species.[27] Thus, we also synthesized 7h asreported, and attempted to convert it to CPPs. However, no
CPPs were detected.
We were also aware that the structure of 6 is a covalently
bonded version of a palladium or platinum square which isformed by the self-assembly of 4,4′-bipyridyl with palladiumand platinum (Scheme 2).[28] Therefore, the use of platinum or
palladium would be a promising strategy for the formation of
7. Since bisaryl palladium(II) species undergo reductive elimination at ambient temperature and are not expected to have a
sufﬁcient lifetime to form a square complex, we selected platinum square 7i as the next target.
Prior to this study, to the best of our knowledge, only two
examples of the synthesis of cyclic multinuclear platinumcomplexes were known in which the platinum atoms are covalently connected with conjugated molecules. Youngs andco-workers reported the preparation of butadiyne-bridged
cyclic tetranuclear platinum complex 8 (Figure 3).[29] However,no attempts were reported for the reductive elimination of
the platinum complex to form a cyclic hexadecayne. Bäuerle
and co-workers reported the reaction of a diyne-substituted
terthiophene with PtCl2(dppp) (dppp refers to 1,3-bis(diphenylphosphino)propane) to give bisplatino-macrocycle 9
in good yield.[30] Subsequent reductive elimination of platinum
from 9 afforded the corresponding cyclodimeric terthiophenediyne in moderate yield. The diyne units were further converted to thiophene by treatment with disodium sulﬁde, giving
cyclo[8]thiophene.
After we started our work, Bäuerle and co-workers also
reported the direct synthesis of cyclic oligothiophenes mediated by cyclic platinum intermediate 10, which was formed by
the transmetalation of a bisstannylated quinquethiophene and
PtCl2(cod).[31] While the structure of 10 was not fully characterized, reductive elimination of platinum afforded a mixtureof cyclic oligothiophenes (m = 1–6). These results suggest thatformation of the cyclic platinum complex having PP units
analogous to the sides of a square is possible; however, itremained unclear whether reductive elimination of platinum

N(L)M' NN M'(L)N

N

N(L)M' NN
N M'(L)(L)M'X2
selfassemblyM' = Pd, Pt
N

Scheme 2. Synthesis of (4,4′-bipyridyl)4M4 (M = Pd, Pt) complex throughthe self-assembly process pioneered by Fujita and co-workers.

Fig. 3. Structures of oligomeric cyclic platinum compounds prepared by Youngs and Bäuerle.

Chem. Rec. 2014, 14, 84–100© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

www.tcr.wiley-vch.de87THE CHEMICAL RECORD

Table 1. Calculated diameters and strain and HOMO/LUMO
energies of [n]CPPs.[a]nDiameter[b](nm)Strain energy[c](kJ/mol) [kcal/mol]

HOMO
energy (eV)

LUMO
energy (eV)

4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

5.70
7.05
8.40
9.77
11.13
12.49
13.87
16.61
16.61
17.99
19.36
20.74
22.11
23.49
24.87
26.23
27.62

602.7 [144.1]
490.5 [117.2]
406.8 [97.23]
356.5 [85.20]
307.1 [73.40]
279.6 [66.82]
246.6 [58.93]
229.4 [54.83]
205.21 [49.05]
194.33 [46.45]
176.14 [42.10]
168.33 [40.23]
153.82 [36.76]
148.27 [35.44]
136.63 [32.65]
132.76 [31.73]
122.96 [29.39]−4.498−4.665−4.915−4.968−5.105−5.125−5.198−5.209−5.263−5.259−5.289−5.289−5.311−5.309−5.326−5.324−5.332−1.990−1.955−1.780−1.802−1.699−1.720−1.666−1.677−1.632−1.651−1.638−1.638−1.622−1.630−1.617−1.622−1.621[a]Data obtained from DFT calculations at the B3LYP/6-31G* level of theory.
Determined from the diameter of an inscribed circle containing the ipso
carbons of each PP unit. [c]A group equivalent reaction [(n + 1) terphenyl →[n]CPP + n biphenyl] used for Itami[33] and Bachrach[34] was employed.
[b]from the complex would take place when considering thesigniﬁcant strain energy of the ﬁnal products.
2.2. Structure, Strain Energies and HOMO/LUMO
Energies of CPPs
The structure and electronic properties of [n]CPPs (n = 4–20)
were estimated using density functional theory (DFT) calculations at the B3LYP/6-31G* level of theory prior to the synthesis.[14] The most stable conformation of even-numbered[n]CPPs is an alternating zigzag orientation of PP units withnearly D(n/2)h point group symmetry, and that of odd-numbered
CPPs has a helical arrangement of PP units. The dihedral
angles between two adjacent PP units of even CPPs are 33–35°
for large CPPs (n > 10), the values of which are similar to thoseof linear oligoparaphenylenes (35.8°).[32] The dihedral angles
become smaller as CPPs are decreased in size, and the calculated dihedral angles of [8] and [6]CPPs are 31° and 27°,
respectively. These results are virtually identical to the reports
from Itami and co-workers[33] and Bachrach and Stück.[34]Diameters, strain energies, and HOMO/LUMO energies of
the CPPs are summarized in Table 1. As there is no ﬁxed
deﬁnition of the CPP diameter, we have deﬁned it here as thediameter of a circle passing through the ipso carbons of each
PP unit.

88 www.tcr.wiley-vch.de

The CPP strain energies increase with a decrease in thenumber of PP units, and a small but distinct odd–even effectoriginating from the conformational differences between odd
and even CPPs could be observed. [20]CPP, which was thelargest and least strained CPP among the calculated CPPs,
possesses 123 kJ mol−1 of strain energy, a value almost the sameas that of cyclopropane (117 kJ mol−1).[35] [12]CPP, which was
prepared for the ﬁrst time by Jasti and Bertozzi, possesses
205 kJ mol−1 of strain energy, which is very close to that of
cyclopropene (230 kJ mol−1). The strain energies of [8]CPP,
our ﬁrst synthetic target, and [5]CPP, the structural unit ofC60, are 307 and 491 kJ mol−1, respectively. These results
clearly indicate that CPPs are highly strained molecules.
However, considering that the highly strained hydrocarbon
cubane (strain energy of 649 kJ mol−1) had already been prepared, we supposed that small CPPs, such as [5]CPP, could
possibly be synthesized by selecting an appropriate syntheticroute and reaction conditions.
HOMO and LUMO energies of the CPPs increase anddecrease, respectively, as the size of the CPP decreases. The size
dependences of the HOMO/LUMO energies of CPPs are in
sharp contrast to those of linear oligoparaphenylenes, for which
the HOMO and LUMO energies increase and decrease,respectively, as the result of an increase in the effective conjugation when the number of PP units is increased. The conﬂicting results for CPPs most likely originate from the size
dependence of the aromaticity of the PP units, which decrease
in benzenoid character and increase in quinoid character as
the number of PP units decreases.[1e,36] Therefore, smaller CPPs
have stronger polyene character than larger analogues. Thissize-dependent decrease in aromaticity accompanied by the
structural transformation to quinoid form was recently experimentally proven by Raman spectroscopy.[37] The HOMO/
LUMO energies of CPPs exhibit odd–even effects due to theexistence of the helical conformation in the odd CPPs.
However, the PP units of CPPs rotate freely at ambient temperature; accordingly, the experimentally observed physicalproperties of CPPs showed less pronounced odd–even effects,
a result reﬂected by the time-averaged structure and orbitalenergies (see below).
The HOMO/LUMO gap for [5]CPP was calculated to be
2.71 eV, a value very close to the experimentally obtained
HOMO/LUMO gap for C60 (2.88 eV). Since many C60
derivatives have been applied to charge-transporting materials
in organic electronics, especially for photovoltaics,[2c,2d] small
CPPs such as [5]CPP and its derivatives would be prominent
lead materials for molecular electronics.
Theoretical calculations suggest that, besides size as discussed above, a variety of factors, such as functionality, length,
and helicity, inﬂuence the electronic structure of CPPs andtheir derivatives (Figure 4). For example, a cyclic tetramerof ﬂuorene, similar in size to [8]CPP but containing aChem. Rec. 2014, 14, 84–100© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

Platinum-Mediated Synthesis of Cyclic π-Conjugated Compounds

LUMO (eV)HOMO (eV)(C6H4)8([8]CPP)[C6H3(C[CH3]2)-C6H3]4(C6F4)8(C5NH3)8(C10H4)8(C14H4)8-1.70 (-1.60)-5.45 (-5.35)

-1.60
-4.90

-2.73
-6.86

-2.54
-5.45

(-2.76)(-4.34)(-2.38)(-5.00)Fig. 4. Structures and HOMO/LUMO energies of [8]CPP and its derivatives obtained by DFT calculations at theB3LYP/6-31G* level of theory. Structures of tubular compounds and energy values in parentheses were obtained at theB3LYP/3-21G* level of theory.

Scheme 3. Selective synthesis of [12]CPP.

dimethylmethylene bridge in the biphenyl unit of the CPP,
possesses a lower LUMO energy, higher HOMO energy, and,thus, narrower HOMO/LUMO gap than that of [8]CPP.
Replacement of the hydrogen atoms of a CPP with ﬂuorine(perﬂuoro[8]CPP) and introduction of an electronegative
nitrogen atom to a CPP (a cyclic tetramer of 2,5-connectedbipyridine) act to decrease both HOMO and LUMO energies.
Furthermore, tubular compounds formed by an increase in thelength of a CPP possess far narrower HOMO/LUMO gaps
compared to the corresponding CPP having the same diameter.
These results clearly suggest the ease of orbital-energy engineering of CPPs compared to fullerenes, given that the availability
and structural ﬂexibility of fullerenes and CNTs are quite
limited. Therefore, CPPs offer unique structural and electronic
diversities for molecular electronics. All compounds shown in
Figure 4 are our synthetic targets.

3. Synthesis of CPPs
3.1. Selective Synthesis of [8] and [12]CPPs
We chose [8]CPP as an initial target (Scheme 1c).[13] 4,4′Bis(trimethylstannyl)biphenyl 5c prepared from commercially

Chem. Rec. 2014, 14, 84–100

available 4,4′-dibromobiphenyl was treated with two equivalents of PtCl2(cod) to give bisplatinated biphenyl 5d. Then,
5d was heated with an equimolar amount of 5c in THF,
giving the square-shaped platinum complex 6e in good yield.
Alternatively, 6e was also obtained in high yield by heating a
1 : 1 mixture of 5c and PtCl2(cod). Precipitation of 6e wasobserved during the reaction, which may shift the equilibrium toward the formation of 6e and also reduce the transformation of 6e into unwanted side products such as linear
oligomeric compounds. The cod ligand in 6e was substituted with dppf to give 6f, and subsequent reductive elimination of platinum by the addition of bromine afforded[8]CPP. The overall yield of [8]CPP was 37% in our ﬁrst
report but has now been increased to 57% by optimization of
the conditions.
[12]CPP was also selectively synthesized starting from
4,4′′-bis(trimethylstannyl)terphenyl in a similar manner
through platinum square 11 in 43% overall yield(Scheme 3).[14] More than 100 mg of [12]CPP was obtained ina large-scale preparation. Several carbon- and heteroatombridged CPP derivatives were also successfully synthesizedin our group, starting from the corresponding carbon- andheteroatom-bridged biphenyl derivatives.[16]© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

www.tcr.wiley-vch.de89

THE CHEMICAL RECORD

SnMe3
X

+

X
5
c: X = SnMe3
d: X = Pt(cod)Cl

Scheme 4. Synthesis of the simplest structural unit of helical CNTs by Isobe
and co-workers.

Isobe and co-workers successfully applied our synthetic
method to the synthesis of the simplest structural unit ofhelical CNTs starting from chrysene derivative 12 bearing
Bpin groups (Scheme 4).[18a] After boron–platinum transmetalation to form a platinum square, reductive elimination ofplatinum by the addition of triphenylphosphine afforded thedesired 13 in good yield as a mixture of rotational isomers. The
helical chirality of the products is preserved at ambient temperature; the products were optically resolved using HPLC
equipped with a chiral stationary phase, permitting the isolation of the optically active, simplest helical CNT structuralunit for the ﬁrst time. Isobe and co-workers also succeeded in
the synthesis and optical resolution of the simplest structuralunit of zigzag single-walled CNTs (SWCNTs)[18b] and πlengthened compounds prepared from Pigment Red 168[18c]using the same synthetic strategy.
3.2. Random Synthesis of [n]CPPs (n = 8–13)[14]The above results clearly demonstrate the efﬁcacy of theplatinum-square route for the synthesis of [4m]CPPs(m = integer). However, the results also suggest that thestructural diversity with respect to ring size is quite limited. To
expand the synthetic versatility, we next attempted to employ
different PP units as the precursors for cyclic platinum complexes. If the C–Pt–C bond-angle preference in the platinumcomplex is 90°, a rectangular platinum complex would selectively form. Once such a complex forms, the reductive elimination of platinum from the complex would give varieties ofeven CPPs.
To examine this hypothesis, a 1 : 1 mixture ofbisplatinated biphenyl 5d and 4,4′′-bis(trimethylstannyl)
terphenyl 14 was heated in 1,2-dichloroethane at 50°C for
32 h, and the resulting mixture was treated with dppf followed
by bromine (Scheme 5). While we anticipated the selective
formation of [10]CPP, a mixture of [8], [9], [10], [11],and [12]CPP was obtained in 2.4%, 3.2%, 5.3%, 4.7%, and90 www.tcr.wiley-vch.de

n
SnMe3
14

[8]-[13]CPPs(n = 1-6)

Scheme 5. Random synthesis of [8]–[13]CPPs.

2.3% overall yields, respectively, as conﬁrmed after separation
by gel-permeation chromatography (GPC) and MALDITOF-MS analysis. The products and their ratios were dependent on the reaction conditions employed for platinumcomplex formation. When 5d and 14 were heated for 20 hunder otherwise identical conditions, [9], [10], [11], [12], and[13]CPP were formed in 5.4%, 9.6%, 7.7%, 3.8%, and 0.8%
yields, respectively; [8]CPP was not observed. A mixture of
CPPs was also obtained by mixing 5c, 14, and Pt(cod)Cl2 in aratio of 1 : 1 : 2 at 70°C in 1,2-dichloroethane for 24 h. After
ligand exchange and bromine-induced reductive elimination,[8]–[12]CPPs were obtained in 0.7%, 3.0%, 7.3%, 9.8%, and3.7% yields, respectively.
[10]CPP is expected to form from rectangular platinumcomplex 16, and [8], [9], [11], and [12]CPPs should form
from tetranuclear platinum complexes 6, 15, 17, and 11,
respectively (Scheme 6). In order for [13]CPP to form, threeterphenyl and two biphenyl units or one terphenyl and ﬁvebiphenyl units must couple, indicating the formation of apenta- or hexanuclear platinum intermediate. It has alreadybeen reported that the transmetalation between arylstannanes
and platinum is reversible, and that the aryl groups on thediarylplatinum complexes undergo scrambling.[38] Therefore,
the formation of unexpected platinum complexes must be
due to equilibrium during the transmetalation and/or ligandexchange reactions. It is unclear at this moment whether theformation of platinum complexes is thermodynamically controlled through the self-assembly process, as platinum deposition was observed under the current conditions. Though
the detailed mechanism is not fully understood, this syntheticroute is attractive because it gives various CPPs, including odd
CPPs, which can be easily separated using GPC depending on
their molecular size.
3.3. Alternative Synthetic RouteWhile the above-mentioned method could produce several
CPPs in reasonable overall yields, selective synthesis could only

Chem. Rec. 2014, 14, 84–100© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

Platinum-Mediated Synthesis of Cyclic π-Conjugated Compounds

Scheme 6. Possible synthetic precursors of [9], [10], and [11]CPPs.

(dppf)PtBrn(dppf)Pt

Br

Ni(cod)2
dppf
THF
heat

(dppf)Pt

Pt(dppf)oxidantnn(dppf)Pt

18
j: n = 3, k: n = 2, l: n = 1

Pt(dppf)

tolueneheat

N[10]CPP (N = 5)[8]CPP (N = 3)[6]CPP (N = 1)

19

Scheme 7. Selective synthesis of [10], [8], and [6]CPPs.

be achieved for [8] and [12]CPPs. Furthermore, the method
could not be applied to the synthesis of CPPs smaller than[8]CPP despite their potential as molecular electronic materials. Jasti and co-workers recently succeeded in the synthesis of[6]CPP, the smallest CPP so far synthesized; however, theirroute requires numerous reaction steps, providing a low overall
yield (0.7% over eight steps).[7b] Therefore, a new and highyielding synthetic route for small CPPs is highly sought after.
We recently developed a new and selective route towardplatinum-square formation that was effective for the sizeselective synthesis of [10], [8], and [6]CPPs (Scheme 7).[15b]The new synthetic route relied on the synthesis of platinum
square 19 from U-shaped p-haloaryl-substituted dinuclear
platinum complex 18. Upon treatment of 18j–18l under
Yamamoto coupling conditions[39] employing two equivalentsof Ni(cod)2 and dppf at 50°C, platinum complexes 19j–19l
were formed in 20–63% yields. [10] and [8]CPPs wereobtained in 75% and 74% yields from 19j and 19k, respectively, using the Br2-induced reductive elimination adopted
from our previous synthesis.[7b]In contrast, treatment of 19l with Br2 did not afford[6]CPP at all. However, the use of XeF2[40] instead of Br2 as anoxidant induced the desired reductive elimination, giving[6]CPP in 47% yield. Fluorine-based oxidants such as AgF,
ﬂuoride anion sources such as tetrabutylammonium ﬂuoride,

Chem. Rec. 2014, 14, 84–100

and several neutral and electron-deﬁcient ligands such as
P[OCH(CF3)2]3,[41] acetonitrile, and cod, were also effective informing [6]CPP. Under optimized conditions, [6]CPP was
obtained in 8.9% overall yield from commercially available
1,4-diiodobenzene. Therefore, this new synthetic route signiﬁcantly increases the availability of [6]CPP.
We also selectively synthesized [10]CPP starting fromL-shaped cis-coordinated p-iodobiphenyl 20m by applying a
protocol developed by Osakada and co-workers.[42] Thus, 20m
was treated with a stoichiometric amount of Pd(dba)2 (dbarefers to dibenzylideneacetone) and 2,2′-bipyridyl (bpy); theresulting intermediate was further treated with a stoichiometricamount of AgBF4. Although we anticipated the formation of[8] or [16]CPP via the selective dimerization or tetramerizationof 20m, respectively, [10]CPP was exclusively formed in 13%
isolated yield (Scheme 8).[15a]A plausible mechanism for the formation of [10]CPP
from 20m involves oxidative addition of 20m to a Pd(dba)2/
bpy species to afford the corresponding aryl–Pd(II) complex
20n (L′ may be cod or bpy). The cationic Pd complex generated from 20n upon treatment with silver salt induces anaryl-coupling reaction to give trimer 21,[42a] from which elimination of 4,4′-bispalladium biphenyl occurs to give triangulartrinuclear platinum complex 22.[42b,43] Finally, reductive elimination of platinum from 22 gives [10]CPP.

© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

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THE CHEMICAL RECORD(L)Pt(cod)Pt

Pt(L)X

1. Pd(dba)2/bpy
2. AgBF4
Pd(L')
X

20
m: X = I
n: X = Pt(L`)(L' = cod or bpy)(L)Pt

Pd(L')
21

Scheme 9. Self-assembly of trispyridyl ligand 23 to form cage molecule 24
reported by Fujita and co-workers.

Pt(L)

(L)Pt

(L')Pd+[10]CPP

Pd+(L')
2

Pt(L)
22

Scheme 8. Selective synthesis of [10]CPP from L-shaped cis-coordinatedplatinum 20m.

suggesting the existence of two competing factors affecting
the chemical shift. The chemical shift of the aromatic hydrogen in [n]paracyclophanes shows a lower magnetic ﬁeld insmaller cyclophanes[44] due to a decrease in aromaticity andthe existence of a deshielding effect caused by an increase inring strain. Therefore, the observed size dependence of the
proton chemical shift of [6] and [7]CPPs is probably derived
from ring strain, but that of larger CPPs cannot be explainedby strain. Further studies are needed to clarify this uniquesize dependence.
3.5. Synthesis of a 3D π-Conjugated Molecule[17]Fig. 5. 1H NMR chemical shifts of [n]CPPs in CDCl3.

3.4. NMR Spectra of CPPs
All CPPs showed a single peak in the 1H NMR spectra in
the aromatic region and two peaks in the 13C NMR spectra
even at low temperatures such as −80°C, indicating that theparaphenylene units freely rotate on the NMR timescale. The
rotational barrier of the paraphenylene unit in [12]CPP was
estimated by theoretical calculations as 16 kJ mol−1 by Itami
and co-workers.[33]The 1H NMR chemical shifts of the CPPs, including
CPPs prepared by Bertozzi, Jasti, and Itami, are summarized in
Figure 5. Among [8]–[19]CPPs, the proton resonances shift
downﬁeld with an increase in CPP size, reaching a maximum at∼7.7 ppm. In contrast, the change in proton resonances from
[6] to [7]CPPs show an upﬁeld shift with the increase in size,

92 www.tcr.wiley-vch.de

Despite the detailed mechanism for the formation of theplatinum square (e.g., 6 from 5) being unclear, the process
by which complex formation occurs can be regarded as acovalent bonding version of the self-assembly of structurally
related coordination complexes (Scheme 1c vs. Scheme 2).
The numerous existing reports on supramolecular transitionmetal–ligand complexes with 3D cages and polyhedral structures[45] may suggest that these complexes would be suitableprecursors for novel hoop-shaped π-conjugated molecules.
For example, Fujita reported that trispyridyl ligand 23 andpalladium(II) or platinum(II) selectively and quantitatively
formed octahedral, hexapalladium or platinum cage 24 by selfassembly involving a ligand–metal interaction (Scheme 9).[45a] If a
structurally related cage consisting solely of covalent bonds was toform, such a molecule would be a possible precursor of a 3D
π-conjugated molecule.
To
examinethispossibility,
1,3,5-tris(ptrimethylstannylphenyl)benzene 25c was ﬁrst treated withthree equivalents of Pt(cod)Cl2 to afford trisplatinated 25d,
which was further treated with an equimolar amount of 25c. A
white precipitate appeared during the reaction; this solid, isolated in 81% yield, was identiﬁed as the desired octahedralshaped, hexanuclear platinum complex 26e (Scheme 10).
Treatment of 26e with dppf, followed by the reductive elimination of platinum from the resulting dppf complex 26f by

Chem. Rec. 2014, 14, 84–100© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

Platinum-Mediated Synthesis of Cyclic π-Conjugated Compounds

Scheme 10. Synthesis of 3D π-conjugated molecule 27.

addition of XeF2, afforded cage-like 3D molecule 27 in 21%
yield. The structure of 27 was initially elucidated from 1H and
13
C NMR spectroscopy and MS, and was ﬁnally and unambiguously determined using single-crystal X-ray crystallographic analysis.
The calculated strain energy of 27 was 501 kJ mol–1,
which is higher than that of [6]CPP (407 kJ mol–1) and even[5]CPP (491 kJ mol–1).[14] In contrast, precursor 26 did notexhibit signiﬁcant strain because the bond angle of cissubstituted platinum would be close to its ideal angle of 90°.
These results strongly suggest that the use of reductive elimination is an excellent strategy for synthesizing highly strained
molecules.
The key steps of the synthesis are the platinum-mediatedassembly of π-units by transmetalation and subsequent reductive elimination of platinum. The assembly process mimics theself-assembly of metal and ligand species that form structurally
related cage-like coordination complexes. As many metal–organic frameworks with 3D structures are already known
and can easily be prepared, the assembly/reductive elimination
strategy described here would provide a variety of new 3D
π-conjugated molecules with different structures and topologies, which are otherwise difﬁcult to obtain using conventional
synthetic methods.

4. Properties of CPPs
4.1. UV and Fluorescence Spectra of CPPs
The UV-vis absorption and ﬂuorescence spectra of the CPPs
are shown in Figure 6.[14,19] All CPPs absorbed UV-vis light ataround λmax ≈ 340 nm regardless of their size. The absorption
coefﬁcient (ε) slightly increased with an increase in the sizeof the CPP. This insensitivity of λmax to size is because the
HOMO/LUMO transition is forbidden with no or very small
oscillator strengths in all cases, as suggested by the timedependent DFT calculations.[14,46] The strong absorption canbe assigned to the sum of nearly degenerate HOMO-2 andChem. Rec. 2014, 14, 84–100

Fig. 6. a) UV-vis (solid lines) and ﬂuorescence (dashed lines) spectra andb) ﬂuorescence emission of [6], [8], [9], [10], [11], [12] and [13]CPPs in
chloroform. Fluorescence spectra are normalized to the UV-vis absorption
intensities.

HOMO-1 to LUMO, and HOMO to nearly degenerate
LUMO+1 and LUMO+2 transitions. In sharp contrast to
the size dependency of the HOMO/LUMO energies of
CPPs, HOMO-1 and HOMO-2 increased and LUMO+1
and LUMO+2 decreased as the number of PP units wasincreased. Consequently, the energy gaps of the allowed transitions are almost the same regardless of CPP size. The small
CPPs exhibited small shoulder peaks at λ ≈ 380–450 nm; theseabsorptions are thought to be related to the HOMO/LUMO
transition.
All CPPs ﬂuoresced with the exception of [6]CPP, and the
Stokes shift became larger and more red-shifted with a decrease© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

www.tcr.wiley-vch.de93THE CHEMICAL RECORD

Fig. 7. Schematic energy diagram for the relaxation process of CPP in the
excited state. Reprinted with permission from reference [19]. Copyright 2012,
Royal Society of Chemistry.

in CPP size. Fluorescence quantum yields were quite high forlarger CPPs (e.g., 0.72 for [13]CPP) but decreased considerably with a decrease in CPP size (e.g., 0.084 for [8]CPP).[19]Furthermore, time-resolved ﬂuorescence spectroscopy showedthat the lifetime of the singlet excited state was dependent on
the size of the CPP, and smaller CPPs had longer lifetimes
compared to their larger counterparts, e.g., the lifetimes of [8]and [13]CPPs were measured as 17.6 and 2.0 ns, respectively.
In addition, the polarity of the solvent, such as toluene, chloroform, and acetonitrile, did not affect the lifetime. All theseresults indicate the larger structural change from the Franck–
Condon state to the relaxed state in smaller CPPs and theabsence of intra- or intermolecular charge transfer involved at
the excited state. The larger Stokes shift of the smaller CPPs isreﬂected by the smaller HOMO/LUMO gap compared to thelarger CPPs, as discussed in Section 2.2.
The structural relaxation in the excited state as detected by
the ﬂuorescence upconversion method showed that the decay
lifetimes of the initial components became shorter for smaller
CPPs, e.g., 0.27 and 4.31 ps for [8] and [13]CPP, respectively.
These results suggest that smaller CPPs undergo a faster but
larger structural relaxation from the Franck–Condon state to
the relaxed state compared to larger CPPs. Theoretical calculations suggest that the structural relaxation accompanies theplanarization of each PP unit in CPP as well as the increase in
the quinoidal contribution of the benzene unit, and that thestructural change is more signiﬁcant in smaller CPPs thanlarger ones (Figure 7).
4.2. Redox Properties of CPPs
All of the CPPs examined showed reversible oxidation waves
as measured using cyclic voltammetry (CV) in 1,1,2,2tetrachloroethane (C2H2Cl4) solutions containing 0.1 mol L–1
Bu4NPF6 at room temperature (Figure 8 and Table 2).[14,15b]
This clear reversibility indicates that the oxidized intermediates

94 www.tcr.wiley-vch.deFig. 8. Cyclic voltammogram of [8]CPP in Bu4NPF6/C2H2Cl4 at room
temperature.

Table 2. Half-wave oxidation potentials of [n]CPPs obtained using
cyclic voltammetry.[a]nE (vs. Fc/Fc+)

6
8
9
10
11
12
13

0.29
0.59
0.70
0.74
0.79
0.83
0.85

[a]
V vs. ferrocene/ferrocenium couple; Bu4NPF6 (0.1 M) in 1,1,2,2tetrachloroethane; scan rate = 20 mV/s; scan range = 0–1.25 V.

are stable under ambient temperatures; indeed, we recently
succeeded in isolating the radical cation and dication of[8]CPP.[47] We initially thought that the oxidation wave
observed in the CV experiments was a one-electron process,but we now believe that the wave consists of two-electron
processes. Detailed studies are now in progress. Reduction
wave(s) could not be observed within the potential window of
this solvent.
The oxidation potential was strongly affected by the sizeof the CPPs, with [6]CPP exhibiting the lowest half-wave
oxidation potential of 0.29 V (vs. the ferrocene/ferrocenium
couple). The oxidation potential was observed to increase withlarger CPP sizes, reaching a plateau around 0.85 V with anincrease in the number of PP units. This trend is in good
agreement with the trend observed for CPP HOMO energies,
and the oxidation potential is likely to reﬂect the time-averaged
HOMO energy of the dynamic structures of the CPPs.

Chem. Rec. 2014, 14, 84–100© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

Platinum-Mediated Synthesis of Cyclic π-Conjugated Compounds

Scheme 11. a) Size-selective encapsulation of C60 by [10]CPP and b) in silico optimized structure of [10]CPP⊃C60
by DFT calculations at the M06-2X/6-31G* level of theory.

4.3. Host–Guest Chemistry
4.3.1. Selective Encapsulation of C60 by [10]CPP
By analogy to the existence of layered carbon networks withcurved surfaces, e.g., multiwalled CNTs,[48] bucky onions,[49]and, especially, fullerene peapods in which SWCNTs encapsulate fullerenes,[4] the concave cavity of the CPPs should act as ahost for π-conjugated molecules with a convex surface, such as
fullerenes. Such a host–guest complex would be a suitable modelfor elucidating convex–concave π–π interactions.[1d,50] Furthermore, the properties of CNTs are modulated by the encapsulation of fullerenes in fullerene peapods, and the preparation of
CPP host–guest complexes encompassing fullerenes would
provide good models to elucidate the peapod properties in thehomogeneous phase. Although Kawase has reported the complexation of cycloparaphenyleneacetylenes with fullerenes,[51] at
the time of starting this work, there were no reports regarding
the formation of host–guest complexes with CPPs so far.
1
H NMR and UV-vis spectroscopic studies on the host–
guest chemistry between CPPs and C60 revealed that C60 was
selectively encapsulated by [10]CPP, forming a 1 : 1 complex,[10]CPP⊃C60 (Scheme 11a).[20a] The size selectivity is worthemphasizing, in that only [10]CPP interacted with C60 among
the [8]–[12]CPPs, as clariﬁed by 1H NMR analysis by observing the downﬁeld shift of the [10]CPP proton signal before
and after C60 addition (Figure 9a). The proton signals of theother CPPs did not change. The same downﬁeld shift wasobserved when C60 was added to a solution of isolated [10]CPP(Figure 9b).
The binding constant (Ka) of complex formation between
C60 and [10]CPP was 2.79×106 L mol–1 in toluene, as measured
by ﬂuorescence quenching experiments. The results revealed
that [10]CPP and C60 are stabilized by about 38 kJ mol–1 as aresult of encapsulation; this value is about 100 times larger thanthat obtained for [6]cycloparaphenyleneacetylene⊃C60 andvery similar to that of [6](1,4)naphthyleneacetylene⊃C60,[1d,51a]
which is the strongest host reported for C60 so far consisting of
simple hydrocarbons until very recently (see below).

Chem. Rec. 2014, 14, 84–100

The structure of the complex optimized by DFT calculations at the M06-2X/6-31G* level of theory[52] revealed that
C60 sits snugly inside the cavity of [10]CPP (Scheme 11b). Theinterlayer distance between C60 and [10]CPP as calculated
from the diameter of C60 and [10]CPP is 0.335 nm. The
observed interlayer distance coincides with the interplanar vander Waals distance between graphite sheets,[53] clearly indicating the importance of attractive van der Waals interactions for
maximizing concave–convex π–π interactions. The structure of
the complex was recently proven by single-crystal X-ray crystallographic analysis by Jasti and co-workers, and the X-ray
structure is essentially identical to the calculated structure.[8]The dihedral angles between two adjacent PP units of[10]CPP⊃C60 are 26–28°, which is signiﬁcantly smaller thanthose of free [10]CPP (32–33°).[14,33,34] However, since a singlet
was observed in the 1H NMR spectrum of [10]CPP⊃C60 evenat low temperatures (−80°C), the PP unit in the complex must
be structurally ﬂexible and be rapidly ﬂuttering at this temperature, and the C60 molecule inside the [10]CPP cavity also
freely rotates.
Very recently, Isobe and co-workers reported that a cyclic
tetramer derivative of chrysene 13 also encapsulates C60 withan extremely high binding constant (log Ka = 11.5 intoluene).[54] Since the binding constant of the complex was sostrong, it can be considered as a bearing surface on the molecular scale; bearing molecule 13 holds the fullerene journal
tightly to prevent its run-out motion, and the fullerene journal(or shaft) rolls anisotropically in the bearing despite the tightholding constraint.
4.3.2. Selective Encapsulation of C70 by [10] and [11]CPPs
SWCNT encapsulation of fullerenes with an anisotropic shapeis of great interest because several geometrically distinct orientations are possible, unlike spheroidal C60. C70 has an ellipsoidal
shape with long (0.796 nm) and short (0.712 nm) axes, and has
the highest aspect ratio (0.89) among fullerenes.[55] On encapsulation by SWCNTs, this molecule adopts one of two different© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

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THE CHEMICAL RECORD

Fig. 9. 1H NMR spectra in CDCl3 at room temperature of a) [8]–[12]CPPs i) before and ii) after C60 addition, andb) isolated [10]CPP i) before and ii) after C60 addition. Reprinted with permission from reference [20a]. Copyright2011, John Wiley and Sons.

Fig. 10. Molecular structure of C70–CNT peapod with a) lying orientation and b) standing orientation.

orientations, namely “lying”, where the long axis of C70 isparallel to the tube axis, and “standing”, where it is perpendicular (Figure 10a and 10b, respectively).[56] Okazaki andco-workers reported that the transition from lying to standing
occurred at a tube diameter of 1.406 nm.[57] However, thefactor(s) controlling the orientation of C70 is (are) unclear, as theinteraction between neighboring fullerenes is also proposed toplay a signiﬁcant role in determining the orientation.[58]In sharp contrast to the selective interaction between C60
and [10]CPP, both [10] and [11]CPPs formed a 1 : 1 complex
with C70, giving the resulting peapods as determined by 1H
NMR and UV-vis spectroscopy.[20b] The binding constants of[10]CPP⊃C70 and [11]CPP⊃C70 in toluene were 8.4×104 L
mol−1 and 1.5×105 L mol−1, respectively, at 25°C,[59] meaningthat complex formation was highly exergonic with ΔG values of−28 kJ mol−1 and −30 kJ mol−1 for [10] and [11]CPP, respectively. While these values are slightly lower than that of [10]CPP
and C60, this was one of the strongest host–guest interactions
between C70 and a simple hydrocarbon observed to date.

96 www.tcr.wiley-vch.deIn silico structural optimization and single-crystal X-ray
crystallographic analysis of the complexes revealed that C70 was
found to exist in two distinct orientations inside the CPP
cavity, namely lying and standing, depending on the speciﬁc
size of the CPP (Figure 11). In [10]CPP⊃C70, C70 adopts thelying orientation inside the [10]CPP cavity, interacting with itat the equatorial region (Figure 11a). Note that X-ray crystal
analysis was performed on the complex of [10]CPP and
cyclopropyl-substituted C70 28 (Figure 12). This mode ofinteraction is virtually identical to that observed between C60
and [10]CPP. C70 and [10]CPP were separated by 0.35 nm,
supporting the importance of the attractive van der Waals
interaction. These results are not surprising, as the short axis of
C70 (0.712 nm) is almost identical to the diameter of C60(0.710 nm). Since the diameter of “free” [10]CPP is 1.380 nm,
the orientation of C70 in [10]CPP⊃C70 is consistent with thatobserved in CNTs.[57,60]
In contrast, C70 adopted the standing conformation in[11]CPP⊃C70, interacting with [11]CPP along the long axis

Chem. Rec. 2014, 14, 84–100© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

Platinum-Mediated Synthesis of Cyclic π-Conjugated Compounds

Fig. 11. ORTEP drawings of a) [10]CPP⊃28, b) [10]CPP⊃28 (top view), c)[11]CPP⊃C70, and d) [11]CPP⊃C70 (top view) with 50% probability thermal
ellipsoids. Solvent molecules and hydrogen atoms are omitted for clarity. O:
red; C: gray. Reprinted with permission from reference [20b]. Copyright 2013,
John Wiley and Sons.

Fig. 12. Structure of functionalized C70 28.

(Figure 11b). Since the diameter of free [11]CPP is 1.520 nm,
the selectivity for the standing orientation is consistent
with that observed for CNTs.[57,60] The interaction between[11]CPP and C70 in its standing orientation involves the radial
deformation of [11]CPP to an ellipsoidal shape. Interaction
between C70 and [11]CPP is less ordered than that observed
between [10]CPP and C70, but the interfacial distances,deﬁned as those between the centroid of a PP unit of [11]CPP
and the nearest centroid of a hexagon or pentagon of C70, cameclose to 0.35 nm (0.361–0.387 nm, except for in the helicalregion of the PP units) upon deformation. Therefore, this“induced-ﬁt” maximizes the van der Waals interactions along
the C70 long axis and the CPP. This is because CPPs are highly
radially elastic molecules, and the deformation occurs readily
with low energy loss.
It is noteworthy that the orientations of C70 units encapsulated within [11]CPP⊃C70 were well ﬁxed with no orientational disorder, while for [10]CPP⊃28 half of the C70 units in

Chem. Rec. 2014, 14, 84–100

the complex showed a directional disorder along the longaxis of C70. The radially elastic character of CPPs should contribute to the tight encapsulation, as orientational disorder has been commonly observed in the crystals of many
fullerene inclusion complexes such as C60 complexes of rigid
cyclic porphyrins.[61]The above results strongly suggest that the orientation of
C70 in a C70 peapod is kinetically controlled by the interaction
between C70 and the CNT depending on its size, even under
the preparation conditions of fullerene peapods (∼400°C undervacuum). The kinetically formed structure would be further
stabilized by the interaction between neighboring C70 molecules in both the lying and standing orientations.
In the crystal packing, [10]CPP⊃28 aligned along the aaxis, as highlighted in blue in Figure 13a, and the other complexes (purple) ﬁlled the space. [11]CPP⊃C70 aligned along theb axis (Figure 13b), and this highly ordered alignment generated a one-dimensional ordering of C70 separated by the vander Waals distance. Therefore, this type of one-dimensional
fullerene array opens up possibilities for the application ofthis crystal to anisotropic electronic devices such as chargetransport materials.[62]5. Conclusions and Future Perspectives
Several cyclic π-conjugated molecules, as exempliﬁed by CPPs
and a cage-like 3D molecule, were synthesized based on theplatinum-mediated assembly of π-units to form closed-cagecompounds, from which reductive elimination of platinum
gave the target molecule. The assembly of π-units by thismethod mimics the self-assembly process for the formation of
supramolecular ligand–metal complexes. Furthermore, despite
the high strain energy of the target molecule, reductive elimination of platinum successfully took place with high efﬁciency.
As there are numerous examples in the synthesis of supramolecular transition metal–ligand complexes with 3D cages andpolyhedral structures through self-assembly of ligands andmetals, the current assembly/reductive elimination strategy
would provide a variety of new cyclic π-conjugated moleculeswith different structures and topologies, which are challenging
to obtain by conventional synthetic methods.
Several size-dependent physical properties of CPPs, suchas photophysical properties, redox properties, and host–guest
chemistry, were also clariﬁed. These results would be useful forthe molecular-level understanding of CNT physical properties
as well as fullerene peapods. Theoretical and electrochemical
studies suggest that small CPPs and their derivatives should be
excellent lead compounds for molecular electronics. Since thestructural ﬂexibilities and the availability of pure samples arepotentially higher for CPPs and their derivatives than thoseof fullerenes and CNTs, CPPs, their derivatives, and 3D© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

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THE CHEMICAL RECORD

Fig. 13. Three-dimensional array of complexes a) [10]CPP⊃28 and b) [11]CPP⊃C70. Solvent molecules andhydrogen atoms are omitted for clarity. Reprinted with permission from reference [20b]. Copyright 2013, John Wileyand Sons.

molecules offer unique structural and electronic diversities fororganic materials used in molecular electronics.

[2]

Acknowledgements
We thank all of our collaborators and co-workers whose names
appear in the references, in particular, Prof. Tetsuro Majima of
Osaka University for photophysical studies, Prof. Toshiyasu
Suzuki of the Institute for Molecular Science for electrochemical studies, Prof. Taheharu Haino of Hiroshima University forthe host–guest chemistry, and Prof. Hikaru Takaya of Kyoto
University and Nobuhiro Yasuda of the Japan Synchrotron
Radiation Research Institute for the X-ray crystallographic
analysis. We are also thankful for the ﬁnancial support provided by the Core Research for Evolutional Science and Technology (CREST) from the Japan Science and Technology
Agency (to S. Y.), a Grant-in-Aid for Scientiﬁc Research fromthe Japan Society for the Promotion of Science (JSPS) (toE. K.), and a JSPS Fellowship for Young Scientists (to T. I.).

[3]

[4]

[5]
[6]
[7]
[8]

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