Synthesis and photophysical studies of chiral helical macrocyclic scaffolds via coordination-driven self-assembly of 1,8,9,16-tetraethynyltetraphenylene. Formation of monometallic platinum(II) and dimetallic platinum(II)-ruthenium(II) complexes

Article abstract of DOI:10.1021/ja106599j

This paper is concerned with the synthesis and reactions of enantiopure 1,8,9,16-tetraethynyltetraphenylene (3). We obtained 3 in 34% yield through four steps starting from 1,8,9,16-tetrahydroxytetraphenylene (2a) via a functional group interconversion st

Full text of DOI:10.1021/ja106599j

Published on Web 10/29/2010
Synthesis and Photophysical Studies of Chiral Helical
Macrocyclic Scaffolds via Coordination-Driven Self-Assembly
of 1,8,9,16-Tetraethynyltetraphenylene. Formation of
Monometallic Platinum(II) and Dimetallic
Platinum(II)-Ruthenium(II) Complexes
Fang Lin,† Hai-Yan Peng,† Jing-Xing Chen,† David T. W. Chik,‡ Zongwei Cai,‡
Keith M. C. Wong,§,| Vivian W. W. Yam,§,| and Henry N. C. Wong*,†,|,⊥
Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic
Chemistry, The Chinese Academy of Sciences, 345 Ling Ling Road, Shanghai 200032, China,
Department of Chemistry, Hong Kong Baptist UniVersity, Waterloo Road, Kowloon, Hong Kong
SAR, China, Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong
Kong SAR, China, and Institute of Molecular Functional Materials, Department of Chemistry,
and Center of NoVel Functional Molecules, The Chinese UniVersity of Hong Kong, Shatin, New
Territories, Hong Kong SAR, China
Received November 20, 2009; E-mail: hncwong@cuhk.edu.hk
Abstract: This paper is concerned with the synthesis and reactions of enantiopure 1,8,9,16-tetraethy-
nyltetraphenylene (3). We obtained 3 in 34% yield through four steps starting from 1,8,9,16-tetrahydroxy-
tetraphenylene (2a) via a functional group interconversion strategy. On the basis of this chiral “helical”
building block, three rigid helical macrocycles 14, 15, and 22 were designed. Complexes 14 and 15 were
constructed via coordination-driven self-assembly with platinum(II) complexes 8 and 9b, while 22 cannot
be obtained successfully. Then macrocycle 28 was designed on the structural basis of 22 to which octyl
chains were introduced, in the hope of improving the solubility of the complex. Macrocycle 28 was finally
formed and was characterized by NMR spectroscopy, elemental analysis, and electrospray mass
spectrometry. For the enantiopure 15 and 28, circular dichroism (CD) spectra also exhibited chiral properties.
Complexes 27 and 28 both exhibited an intense emission band at 621 nm in acetonitrile at 298 K upon
excitation at λ > 420 nm.
Scheme 1
Introduction
Since the modestly efficient methods for the construction of
tetraphenylene (1) and its derivatives were recorded,1-3 con-
siderable attention has been devoted to the design and syntheses
of novel tetraphenylene derivatives. Interest in this domain stems
from the saddle shape geometry of 1 that should facilitate
excellent three-dimensional dispositions crucial for the realiza-
tion of helical frameworks,4 novel molecular clathrates,5 and
asymmetric catalysts.3d Due to the high barrier for inversion of
the central cyclooctatetraene ring of 1,3f it is therefore of interest
to prepare optically active tetraphenylene derivatives which can
be employed as chiral self-assembly building blocks for three-
dimensional scaffolds.3a-e We recently reported the synthesis
of 1,8,9,16-tetrahydroxytetraphenylene (2a), 1,8,9,16-tetrakis-
(diphenylphosphino)tetraphenylene (2b) (Scheme 1), and the
† Shanghai Institute of Organic Chemistry, The Chinese Academy of
Sciences.
‡ Hong Kong Baptist University.
(3) (a) Wen, J.-F.; Hong, W.; Yuan, K.; Mak, T. C. W.; Wong, H. N. C.
J. Org. Chem. 2003, 68, 8918–8931. (b) Lai, C. W.; Lam, C. K.; Lee,
H. K.; Mak, T. C. W.; Wong, H. N. C. Org. Lett. 2003, 5, 823–826.
(c) Hui, C. W.; Mak, T. C. W.; Wong, H. N. C. Tetrahedron 2004,
60, 3523–3532. (d) Peng, H.-Y.; Lam, C.-K.; Mak, T. C. W.; Cai,
Z. W.; Ma, W.-T.; Li, Y.-X.; Wong, H. N. C. J. Am. Chem. Soc. 2005,
127, 9603–9611. (e) Wu, A.-H.; Hau, C.-K.; Wong, H. N. C. AdV.
Synth. Catal. 2007, 349, 601–608. (f) Bachrach, S. M. J. Org. Chem.
2009, 74, 3609–3611. (g) Huang, H.; Stewart, T.; Gutmann, M.;
Ohhara, T.; Niimura, N.; Li, Y.-X.; Wen, J.-F.; Bau, R.; Wong,
H. N. C. J. Org. Chem. 2009, 74, 359–369. (h) Huang, H.; Hau, C.-
K.; Law, C. C. M.; Wong, H. N. C. Org. Biomol. Chem. 2009, 7,
1249–1257.
§ The University of Hong Kong.
| Institute of Molecular Functional Materials.
⊥ Department of Chemistry and Center of Novel Functional Molecules,
The Chinese University of Hong Kong.
(1) Rapson, W. S.; Shuttleworth, R. G.; van Niekerk, J. N. J. Chem. Soc.
1943, 326–327.
(2) For reviews, see: (a) Mak, T. C. W.; Wong, H. N. C. In ComprehensiVe
Supramolecular Chemistry; MacNicol, D. D., Toda, F., Bishop, R.,
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9
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A R T I C L E S
Lin et al.
Scheme 2
chiral rodlike platinum complexes employing 2a and 2b as
building blocks.3d As compared with 2a and 2b, 1,8,9,16-
tetraethynyltetraphenylene (3) is more helical-like because the
introduction of ethynyl groups on C-1, C-8, C-9, and C-16
renders 3 an excellent candidate for constructing rigid helical
supermolecules.
knowledge, there have been no literature reports on macrocycles
that are assembled by enantiopure tetraphenylene derivatives.
Results and Discussion
Synthesis of Enantiopure 1,8,9,16-Tetraethynyltetraphenylene
(3). Appropriate substituents on C-1, C-8, C-9, and C-16 on
tetraphenylene (1) endow 1 superior potential as a rigid building
block in the formation of helical macrocycles. Nonetheless, the
steric hindrance on these positions due to the molecular shape
also leads to the difficulty in realizing 3. Enantiopure 2a was
prepared by resolution through (S)-camphorsulfonylation.3d Due
to the defined absolute configuration of the (S)-camphorsulfonyl
group, enantiomerically pure (S,S)-2a and (R,R)-2a were
obtained after subsequent desulfonylation steps. With enan-
tiopure 2a in hand, enantiopure 3 was primarily obtained by
transition metal coupling reactions. Due to the steric hindrance
of the tetraphenylene scaffold, triflate 5 failed to react with
trimethylsilylacetylene under the conditions of Sonogashira9 and
Kumada reactions.10 In these attempts, starting material 5 was
recovered. We then switched our efforts to a Negishi reaction,11
since zinc reagents should be more reactive. Trimethylsilyl-
ethynyl zinc chloride did react with triflate 5 catalyzed by
Pd(dppp)Cl2 in diglyme at 100 °C. Without further purification,
trimethylsilyl groups were removed by K2CO3 in MeOH
(Scheme 2). After careful chromatography, 3 was separated in
a disappointing yield of only 7%, which was too low to fulfill
the next step for the synthesis of macrocycles. In this procedure,
products with two remaining triflic groups were also detected.
These results indicate that the Negishi reaction was by no means
efficient for the introduction of ethynyl groups to the sterically
compact tetraphenylene molecule.
Coordination-driven self-assembly of supramolecular scaf-
folds such as sticks, squares, helices, cylinders, and cages have
significantly grown over the past decade.6 Compared to co-
valently bonded organic counterparts, the preparation of
metal-organic supermolecules enjoys higher efficiency and ease
after elaborate designs. Herein, we wish to report an efficient
synthesis of 3 as well as the realization of a series of enantiopure
helical macrocycles containing the structural motif of 3. We
believe that such helical macrocycles possess attractive potential
applications as luminescent and nonlinear optical materials,
molecular wires, and molecular electronics, because of the
alkynyl group’s inherent property of linear geometry, structural
rigidity, π-electron delocalization, and ability to interact with
metal via pπ-dπ overlap.7 It is noteworthy that 3 can also
transfer its chirality to the macrocycle cores, which make the
macrocycles possess optical rotation values.8 To the best of our
(4) (a) Rajca, A.; Safronov, A.; Rajca, S.; Shoemaker, R. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 488–491. (b) Marsella, M. J.; Kim, I. T.; Tharn,
F. J. Am. Chem. Soc. 2000, 122, 974–975.
(5) Man, Y.-M.; Mak, T. C. W.; Wong, H. N. C. J. Org, Chem. 1990, 55,
3214–3221.
(6) For reviews, see: (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem.
ReV. 2000, 100, 853–907. (b) Seidel, S. R.; Stang, P. J. Acc. Chem.
Res. 2002, 35, 972–983. (c) Swiegers, G. F.; Malefetse, T. J. Coord.
Chem. ReV. 2002, 225, 91–121. (d) Holliday, B. J.; Mirkin, C. A.
Angew. Chem., Int. Ed. 2001, 40, 2022–2043. (e) Cotton, F. A.; Lin,
C.; Murillo, C. A. Acc. Chem. Res. 2001, 34, 759–771. (f) Fujita, M.
Chem. Soc. ReV. 1998, 27, 417–425. (g) Lee, S. J.; Lin, W. Acc. Chem.
Res. 2008, 41, 521–537. (h) Schwab, P. F. H.; Levin, M. D.; Michl,
J. Chem. ReV. 1999, 99, 1863–1934. (i) Fujita, M.; Tominaga, M.;
Hori, A.; Therrien, B. Acc. Chem. Res. 2005, 38, 369–378. (j) Ward,
M. D. Chem. Commun. 2009, 4487–4499.
We therefore directed our attention to the use of cyano groups
instead of a trimethylsilylethynyl group, as the former has a
(8) (a) Katz, T. J. Angew. Chem., Int. Ed. 2000, 39, 1921–1923. (b) Graule,
S.; Rudolph, M.; Vanthuyne, N.; Autschbach, J.; Roussel, C.; Crassous,
J.; Re´au, R. J. Am. Chem. Soc. 2009, 131, 3183–3185.
(9) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
16, 4467–4470. For review, see: (b) Hegedus, L. S. J. Organomet.
Chem. 1993, 457, 167–272. (c) Alami, M.; Linstrumelle, G. Tetra-
hedron Lett. 1991, 32, 6109–6112.
(7) (a) Ziessel, R.; Hissler, M.; Elghayoury, A.; Harriman, A. Coord.
Chem. ReV. 1998, 178-180, 1251–1298. (b) Khan, M. S.; Kakkar,
A. K.; Long, N. J.; Lewis, J.; Raithby, P. R.; Marder, T. B.; Wittmann,
F.; Friend, R. H. J. Mater. Chem. 1994, 4, 1227–1232. (c) Yam, V. W.-
W.; Yu, K.-L.; Wong, K. M.-C.; Cheung, K.-K. Organometallics 2001,
20, 721–726. (d) Yam, V. W.-W. Acc. Chem. Res. 2002, 35, 555–
563. (e) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Y. J. Am. Chem.
Soc. 2002, 124, 6506–6507. (f) Yam, V. W.-W.; Wong, K. M.-C.;
Zhu, N. Y. Angew. Chem., Int. Ed. 2003, 42, 1400–1403. (g) Yam,
V. W.-W.; Chan, K. H.-Y.; Wong, K. M.-C.; Chu, B. W.-K. Angew.
Chem., Int. Ed. 2006, 45, 6169–6173.
(10) Tamao, K.; Sumitani, K.; Kumada, M. J. Am. Chem. Soc. 1972, 94,
4374–4375.
(11) (a) King, A. O.; Okukado, N.; Negishi, E. J. Chem. Soc., Chem.
Commun. 1977, 683–684. (b) Negishi, E.; Okukado, N.; King, A. O.
J. Am. Chem. Soc. 1978, 100, 2254–2255.
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Figure 2. Linear linker 8 and the angle linker 9a.
Figure 1. ORTEP drawing of 6.
Scheme 4
Scheme 3
EI mass spectrum was observed at m/z 400.1250, which is in
good agreement with the theoretical value of 400.1252 for the
molecular formula C32H16 of 3. By utilizing a sequential
functional group transformation route as shown in Scheme 3,
we then prepared both enantiopure (S,S)-3 and (R,R)-3 in 34%
yield starting from enantiopure (S,S)-2a and (R,R)-2a, respectively.
Synthesis of Platinum(II) Complexes 8 and 9b. In light of
the inherent rigidity and helicity of 3, it was anticipated that
the tectons self-assembled with 3 should adhere strictly to the
directional-bonding methodology,6b,d and also a relatively
flexible structure was necessary. Then the rigidity and chirality
can be transferred through 3 to the whole macrocycles after
self-assembly.14 Herein we designed two types of ditopic metal
containing tectons to link with 3, i.e. the linear linker 8 and the
angle linker 9a that are shown in Figure 2. Platinum(II) alkynyl
complexes were designed taking account their robust abilities
of constructing well-defined finite supramolecular structures and
potential electrochemical, photochemical, and optoelectronic
properties.7
With enantiopure 3 in hand, our target therefore was to
synthesize the platinum complex 8 and 9a and to connect them
with 3 via coordination-driven self-assembly protocol. The
preparation of 8 is outlined in Scheme 4. 11 was prepared
according to the literature.16 The resulting 1,4-bis[2-trans-
Pt(PEt3)2Cl]phenylbutadiyne 8 was obtained by reacting 11 with
2 equiv of trans-Pt(PEt3)2Cl2 in the presence of a CuCl catalyst
in a 1:1 mixture of toluene/piperidine at 100 °C. The structure
of tecton 8 was confirmed by elemental analysis, 1H, 13C{1H},
31P{1H} NMR, IR spectroscopy, mass spectrometry, and X-ray
crystallographic analysis. Four equivalent phosphorus atoms in
8 gave rise to a sharp singlet at 16.2 ppm in the 31P{1H} spectra
(121.5 MHz, CDCl3), with a set of 195Pt satellites (1JPt-P ) 2378
Hz). The geometry around each Pt is close to a square planar
geometry (Figure 3).
much smaller bulk than the latter and, as a result, could probably
reduce the influence of steric hindrance for palladium catalyst
coupling reaction. Nitrile 7 could be converted to 3 by using a
functional group interconversion strategy. In order to introduce
cyano groups to triflate 5, a protocol established by Rice was
employed.12 Under the conditions of Pd(PPh3)4, Zn(CN)2, and
anhydrous DMF, with heating at 150 °C for 2 days, we were
able to achieve the isolation of 1,8,9,16-tetracyanotetraphenylene
(6) in an almost quantitative yield (Figure 1). The crucial key
to this success was to keep the reaction under absolutely
anhydrous conditions. Compound 6 exhibited strong signals at
m/z 404 in its mass spectrum. 1H NMR, elemental analysis, and
X-ray crystallographic results also supported the structure of 6.
Subsequently, 6 was reduced by DIBAL-H at 45 °C and was
followed by acidification with 20% sulfuric acid. In this way,
tetraphenylene-1,8,9,16-tetraal (7) was obtained in a moderate
yield.13 Its H NMR spectra (300 MHz, CDCl3) exhibited a
1
singlet at δ 9.53 ppm that can be assigned to the four aldehyde
protons. Eventually, reaction of 7 with the Bestmann reagent14
allowed its direct conversion to 3 in 63% yield. Compound 3
exhibited a singlet at δ 2.85 ppm that can be assigned to terminal
ethynyl protons. Moreover, the molecular ion peak of 3 in its
(12) Kubota, H.; Rice, K. C. Tetrahedron Lett. 1998, 39, 2907–2910.
(13) Miller, A. E. G.; Biss, J. W.; Schwartzman, L. H. J. Org. Chem. 1959,
24, 627–630.
(14) (a) Mu¨ller, S.; Liepold, B.; Roth, G. J.; Bestmann, H. J. Synlett 1996,
521–522. (b) Roth, G. J.; Bernd, L.; Mu¨ller, S. G.; Bestmann, H. J.
Synthesis 2004, 59–62.
The synthesis of 9a was shown in Scheme 5. In our previous
design, starting from phenanthrene 12, through three steps,17,14
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Lin et al.
Figure 4. ORTEP drawing of 9b.
Figure 3. ORTEP drawing of 8.
semitransparent oil resulted in the reaction flask. After diluting
this oil with CH2Cl2, a semitransparent thin film formed, whose
solubility was too low that it is practically insoluble in almost
all organic solvents, even under heating. However, if CH2Cl2
was utilized to dilute the reaction mixture, and the organic layer
was washed consecutively by water and aqueous NH4Cl to
remove CuCl and the base, macrocycles 14 and 15 with good
solubility were obtained respectively after the removal of
solvents in vacuum.
Scheme 5
The solubility of the aforementioned thin films was so low
that their identification by NMR and mass spectra was impos-
sible. Notwithstanding, from the limited evidence of elemental
analysis and the IR spectrum,20 we anticipate that cross-linking
products were formed, whose specific structures are still
unknown.
NMR analysis of the soluble products indicated formation
of the target macrocyclic species. There are only signals of
protons on the aromatic rings and PEt3 ligands in both 1H NMR
spectra of macrocycles 14 and 15. The 31P{1H} NMR spectrum
of 14 showed two sharp singlets at 12.7 and 15.8 ppm, both
with a set of 195Pt satellites (1JPt-P ) 2389 and 2397 Hz),
respectively, which was similar to the 31P{1H} NMR spectrum
of 15. The two sharp phosphorus singlets suggested that there
were two different phosphorus atoms in 14 or 15, which can
be explained as the transfer of rigidity from 3 to the whole
macrocycles so that the two PEt3 ligands were fixed and
therefore differentiated.
ESI-MS spectra further substantiated the formation of these
two macrocycles. The ESI mass spectrum gave the (M + H)+
peak, with an m/z ratio of 2617.7992 (calcd 2617.8152) for 14.
As for macrocycle 15, the (M + K)+ peak, m/z ) 2559.7539
(calcd 2559.7711) was observed. Elemental analysis results also
confirmed the proposed structures.
The poor yield of 14 can be explained by the following
observation. When the X-ray structure of 9b is compared
with that of 8, there is a 120.83° dihedral angle existing in
the biphenyl backbone. While, in structure 8, the two Pt (II)
moieties are almost opposite to each other and the backbone
is in a plane (Figure 6). That means complex 8 needs more
energy than 9b to distort to a proper dihedral angle in order
to fit the rigid 3.
cis-[Pt2(bipy)(PEt3)4Cl2]2 (9a) was prepared by using 2 equiv
of cis-Pt(PEt3)2Cl2. However, neither the CuCl catalyzed condi-
tions employing toluene/piperidine as solvents with heating18a
nor milder conditions using THF/Et2NH at room temperature18b,c
gave the corresponding cis-complex. It was found that only
trans-[Pt2(bipy)(PEt3)4Cl2]2 9b was obtained, whose structure
was supported by an X-ray diffraction study (Figure 4). The
isolation of the resulting trans-isomer 9b indicates that the cis-
isomer 9a is not kinetically stable and unable to resist a thermal
or a photochemical isomerization. This result was unexpected,
as the trans-isomer 9b does not possess the appropriate
conformation of the designed macrocycle.
Self-Assembly and Characterization of Platinum(II) Complexes
14 and 15. The self-assembly of macrocycle 14 and 15 is outlined
in Scheme 6 using a typical procedure (Figure 5).19 In the
formation of macrocycle 14, complex 8 was unable to react
completely, 60% of which was recovered, while, in the
formation of macrocylce 15, complex 9b was able to react
completely according to a TLC study. In the process of workup,
both reactions demonstrated a similar phenomenon; i.e. when
the reaction solvent was removed directly under vacuum, a
(15) (a) Anderson, S.; Neidlein, U.; Gramlich, F.; Diederich, F. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1596–1600. (b) Ba¨hr, A.; Droz, A. S.;
Pu¨ntener, M.; Neidlein, U.; Anderson, S.; Seiler, P.; Diederich, F. HelV.
Chim. Acta 1998, 81, 1931–1963. (c) Droz, A. S.; Diederich, F.
J. Chem. Soc., Perkin Trans. 1 2000, 4224–4226.
(16) Hay, A. S. J. Org. Chem. 1962, 27, 3320–3321.
(17) Bailey, P. S.; Erickson, R. E.; Caserio, M. C.; Roberts, J. D. Org.
Synth. Coll. Vol. 5 1973, 489–492.
(18) (a) Ohshiro, N.; Takei, F.; Onitsuka, K.; Takahashi, S. J. Organomet.
Chem. 1998, 569, 195–202. (b) Onitsuka, K.; Yamamoto, S.; Taka-
hashi, S. Angew. Chem., Int. Ed. 1999, 38, 174–176. (c) Lee, S. J.;
Hu, A.; Lin, W. J. Am. Chem. Soc. 2002, 124, 12948–12949.
(19) Procedures: a dilute solution of 3 and 2 equiv of either Pt(II) complex
8 or 9b in a 1:1 mixture of THF/Et2NH was stirred for about 48 h in
a catalytic amount of CuCl at room temperature, until 3 was completely
consumed.
For the formation of 15, we supposed that, after a long
period of reaction time, the two components connect to each
other by metal-carbon covalent bonds in a certain style,
which is a dynamic stable state in a dilute solution.21 In such
a self-assembly system, bonding processes are reversible and
the formation of products are in continuous equilibrium, just
like many supramolecular systems with “error-checking” or
“proof-reading” properties,22 that ensures the eradication of
thermodymanically unstable products. However, a change in
(20) The resulting thin film formed from 3 and 9b showed that the element
contents (C, 49.82; H, 6.14) were very close to those values of a
complex formed from 4 mol of complex 9b linking with 1 mol of 3
(C, 49.61; H, 6.16). As expected, the IR spectrum showed the
disappearance of ν (Ct CsH) stretches at ∼3286 cm-1 and the
appearance of strong ν (Ct C) stretches at ∼2090 cm-1
.
(21) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.;
Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 898–952.
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Scheme 6
external factors, such as concentration, catalyst, pressure, and
temperature, etc., can have a dramatic effect upon the
equilibrium and therefore change the product distribution,
even though the initial product has formed. As for our self-
assembly systems, the condensation of solution led to the
collapse of the equilibrium, which resulted in the irreversible
formation of cross-linking products. On the other hand, the
removal of the catalyst (CuCl) also disturbed the balance of
the system, and macrocycles were therefore generated.
Synthesis and Characterization of Platinum(II)-Ruthenium(II)
Dimetallic Complex. The synthesis of 14 and 15 prompted us to
undertake a more challenging synthesis of another rigid helical
macrocycle in which bipyridine moieties were introduced as
second metal binding sites, leading to coordination with a second
Figure 5. Ball-and-stick model of 14 and 15 (Chem3D).
Figure 6. Capped sticks models of 9b and 8.
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Lin et al.
16, which is also supported by elemental analysis, 13C{1H},
31P{1H} NMR, and IR spectroscopy.
With complex 16 and tetraethynyltetraphenylene 3 in hand,
we then explored the potential of the self-assembly to form
macrocycle 22. But unfortunately, the method used for the
synthesis of macrocycles 14 and 15 unexpectedly failed for 22.
A red precipitate was generated whose solubility was too low
1
to be identified. The H NMR spectra (300 MHz, acetone-d6)
of this precipitate indicated that the signal of ethynyl proton
disappeared, while only signals of aromatic rings and PEt3
ligands were found. The 31P{1H} NMR spectra (121.5 MHz,
acetone-d6) showed two sharp singlets which were similar to
those observed for 14 and 15. However, the results of the mass
spectrum and elemental analysis were unable to substantiate the
nature of this macrocycle. When (S,S)-3 and (R,R)-3 respectively
were used in the same method to assemble with 16, the
corresponding products with opposite specific rotation and
reversed circular dichroism (CD) spectra were observed, which
indicated that chiral 3 did connect with 16 in a certain unknown
manner rather than the predesigned one. We supposed that the
bonds connecting bipyridine and the benzene ring could rotate
randomly, which was unfavorable for the self-assembly of 22
(Scheme 8). This unsuccessful attempt inspired us to make a
partial modification on the Ru(II)-Pt(II) bipyridine building
block 16.
Figure 7. Synthesis of a rigid helical macrocycle in which bipyridine
moieties were introduced as second metal binding sites utilizing complex
16.
metal in an intermolecular manner (Figure 7). Dimetallic
complex 16 was accordingly designed for this purpose. The
synthesis of 16 from 4,4′-dibromo-2,2′-bipyridine 17 was
accomplished in five steps (Scheme 7). Complex 20 was allowed
to react with cis-Ru(bpy)2Cl2 21 in EtOH, which was followed
by treatment with aqueous NH4PF6 solution, affording complex
1
Octyl chains were introduced into 16 to form compound 27,
on account of improving the solubility of the self-assembly
products. In this way, the two acetylenic Pt(II) reacting positions
can be fixed to some extent, as the introduction of the long
aliphatic chains can somehow restrict the random bond rotation
between bipyridine and the benzene ring. The synthesis of 27
was accomplished in five steps by the aforementioned methods
with a moderate yield (Scheme 9) and was unequivocally
16 in a moderate yield. In the H NMR spectrum (300 MHz,
CDCl3) of 20, the width between proton H1 and H2 was just
0.03 ppm, while, in that of complex 16 (300 MHz, acetone-
d6),23 the signal of H1 was shifted dramatically to low field and
the corresponding width increased to 0.43 ppm. This phenom-
enon can be explained by the fact that the four aryl rings in 16
experienced a deshielding effect on H1 after coordinating with
Ru(II). These data are consistent with the expected structure of
Scheme 7
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Scheme 8
Scheme 9
characterized by 1H, 13C{1H}, 31P{1H} NMR, IR spectroscopy,
and ESI-MS spectrometry.
which gave (M-3PF6)3+ and (M-4PF6)4+ peaks, with respective
m/z ratios of 1438.4879 and 1042.6286. The (M-PF6)+ peak
of 28 (m/z ) 4605.4) was out of the range of the spectrometer.
With complex 27 and 3 in hand, we then explored the self-
assembly of macrocycle 28 (Figure 8). To a dilute solution of
3 in a 15:1 mixture of THF/piperidine was added a solution of
27 in THF in a dropwise manner (Scheme 10). Then the
resulting red solution was stirred at room temperature for 3 days
until a product with high polarity formed. After the workup
process, a leaf-shaped red solid was obtained with good
solubility in acetone and acetonitrile. The structure of macro-
cycle 28 was therefore substantiated by ESI-MS spectrometry,
1
The H NMR, 31P{1H} NMR, and IR spectra were consistent
with structure 28. Also the elemental analysis results [C, 51.21;
H, 6.00; N, 3.49 (calcd C, 51.58; H, 5.63; N, 3.54)] confirmed
the proposed molecular formula.
(23) Complex 16 is just slightly soluble in CDCl3 but soluble in acetone-
d6. The 1H NMR spectrum of 16 shows that the protons of 16 have a
similar chemical shift in both CDCl3 and acetone-d6.
(24) (a) Masai, H.; Sonogashira, K.; Hagihara, N. Bull. Chem. Soc. Jpn.
1971, 44, 2226–2230. (b) Sacksteder, L.; Baralt, E.; DeGraff, B. A.;
Lukehart, C. M.; Demas, J. N. Inorg. Chem. 1991, 30, 2468–2476.
(c) Choi, C. L.; Cheng, Y. F.; Yip, C.; Phillips, D. L.; Yam, V. W. W.
Organometallics 2000, 19, 3192–3196.
(22) Fang, Y.-Q.; Polson, M. I. J.; Hanan, G. S. Inorg. Chem. 2003, 42,
5–7.
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A R T I C L E S
Lin et al.
Figure 9. CD spectra of (R,R)-3, 15, 28 and (S,S)-3, 15, 28 in acetonitrile
at concentrations of 1.0 × 10-5 M.
Figure 8. Ball-and-stick model of 28 (Chem3D).
(S,S)-3 and (R,R)-3 were used to form the enantiomeric
compounds of 15 and 28 respectively. The CD spectrum of 28,
15, and 3 are shown in Figure 9. Metallocycle 15 exhibited
one major band and two minor bands with a red shift in energy
and higher intensities as compared with compound 3. The CD
spectra of 28 exhibited one major band and one minor band
with much higher intensities as compared with 3 and macrocycle
15, which indicates the occurrence of chiral properties during
the self-assembly processes.
Photophysical Properties. The electronic absorption spectra
of selected compounds are shown in Figure 10, and their
photophysical data are summarized in Table 1. Alkyne 3 showed
a high-energy absorption band at about 246-254 nm, which is
attributed to the π-π* transition. The dinuclear alkynylplati-
num(II) complex 9b exhibited higher-energy absorption bands
at 246-278 nm and lower-energy absorption bands at 300-320
nm. In view of the spectroscopic studies on the alkynylplati-
num(II) phosphine complexes,24 the higher-energy absorption
bands are assigned as a π-π* intraligand (IL) transition of the
alkynyl linker 13, while the lower-energy absorption bands are
ascribed to the admixture of π-π* IL and dπ(Pt) f π*(Ct CR)
Figure 10. Electronic absorption spectra of selected compounds in
acetonitrile at 298 K.
metal-to-ligand charge transfer (MLCT) transitions. The het-
erometallic platinum(II)-ruthenium(II) complexes 27 and 28
displayed similar electronic absorption spectra, in which there
are higher-energy absorption bands at 290-334 nm and lower-
energy absorption bands at 428-462 nm. The higher-energy
absorption bands are attributed to the π-π* intraligand (IL)
Scheme 10
9
16390 J. AM. CHEM. SOC. VOL. 132, NO. 46, 2010

Studies of Chiral Helical Macrocyclic Scaffolds
A R T I C L E S
Table 1. Photophysical Data of Selected Compounds in Acetonitrile at 298 K
Emission
Absorption
max/nm (ꢀmax/dm3 mol-1cm-1
b
Compound
Medium (T/K)
λ
)
λ
max/nm (τo/µs)
Φem
a
3
MeCN (298)
Solid (298)
Solid (77)
246 (42 970), 254 (36 830), 288 (2085)
-
-
520 (<0.1)
488 (36.7)
458 (43.7)
nPrCN (77)
c
9b
27
28
MeCN (298)
Solid (298)
Solid (77)
246 (35 905), 278 (20 440), 300 (21 645), 320 (23 925)
430 (<0.1)
547 (<0.1)
490 (82.1)
455 (62.0)
<10-3
nPrCN (77)
MeCN (298)
Solid (298)
Solid (77)
254 (77 280), 290 (109 770), 326 sh (56 430), 428 (16 850), 462 (21 740)
248 (100 940), 290 (125 580), 334 (83 435), 428 (16 830), 462 (20 600)
621 (1.7)
623 (0.7)
625 (3.2)
594 (5.9)
13.9 × 10-2
8.6 × 10-2
nPrCN (77)
MeCN (298)
Solid (298)
Solid (77)
621 (1.7)
625 (0.2)
623 (4.5)
595 (5.1)
nPrCN (77)
b
a Cannot be detected. Luminescence quantum yield, measured at room temperature using [Ru(bpy)3]2+ in degassed acetonitrile as a standard. c Too
weak to be measured with great accuracy.
suggestive of an efficient energy transfer or internal conversion
from the higher energy mixed π-π* intraligand (IL)/dπ(Pt) f
π*(Ct CR) metal-to-ligand charge transfer (MLCT) excited state
to the lowest-lying triplet dπ(Ru) f π*(bpy) metal-to-ligand
charge transfer (MLCT) excited state. Although the configura-
tion of the platinum(II) metal centers in macrocycle 28 is
preorganized or locked by the TETP bridging ligand in an
inward direction, no observable metal-metal-to-ligand charge
transfer (MMLCT) emission arising from the Pt...Pt interaction
could be found. It may probably be due to the presence of the
sterically bulky triethylphosphine ligands, which prevent the two
platinum(II) metal centers from coming into close proximity.
On the other hand, the MMLCT excited state, even if it does
Figure 11. Emission spectra of 27 and 28 in degassed acetonitrile at
298 K.
exist in the heterometallic platinum(II)-ruthenium(II) com-
plexes, would readily undergo energy transfer to the lowest-
lying dπ(Ru) f π*(bpy) metal-to-ligand charge transfer (MLCT)
excited state, given its anticipated higher energy than the dπ(Ru)
f π*(bpy) MLCT state.
transition of the polypyridyl and alkynyl ligands, with some
mixing of dπ(Pt) f π*(Ct CR) metal-to-ligand charge transfer
(MLCT) transitions.24 On the other hand, the lower-energy
absorption bands at 428-462 nm, which are absent in the
homometallic platinum(II) complexes, are assigned as a dπ(Ru)
f π*(bpy) metal-to-ligand charge transfer (MLCT) transition,
typical of other ruthenium(II) polypyridyl complexes.25 Upon
photoexcitation, some of them showed intense emission for
various media and temperatures. Complex 9b showed emission
at 430 nm in acetonitrile at 298 K, which is assigned to an origin
of the metal-perturbed IL excited state. Upon excitation at λ >
420 nm, the heterometallic platinum(II)-ruthenium(II) com-
plexes 27 and 28 exhibited an intense emission band at 621 nm
in acetonitrile at 298 K (Figure 11), with large Stokes shifts
and lifetimes in the microsecond range, which are indicative of
their triplet parentage. On the basis of the characteristic emission
observed at a similar energy in the well-studied ruthenium(II)
tris(bipyridine) system,25 together with the fact that the alky-
nylplatinum(II) phosphine complexes usually displayed emission
at a relatively higher energy (λem < 600 nm), the origin of such
an emission band at 621 nm is ascribed to the triplet dπ(Ru) f
π*(bpy) metal-to-ligand charge transfer (MLCT) excited state.
Excitation at a higher energy (λ ) 340 nm) yielded the same
low-energy emission band. The excitation spectra of 27 and 28
closely resemble the corresponding electronic absorption spectra
(Figure 10). The presence of the excitation band at 345 nm is
Conclusion
In this article, we report the synthesis of chiral 1,8,9,16-
tetraethynyltetraphenylene (3), which was employed as a chiral
building block. Three types of chiral, helical, and rigid mac-
rocycles were designed, whose chirality would be endowed by
connecting with 3. In this connection, three types of Pt(II) or
Pt(II)-Ru(II) complexes 8, 9b, and 27 were initially prepared,
which led to the formation of three designed helical assemblies
14, 15, and 28 via coordination-driven self-assembly, albeit the
yield of 14 was very low. Their characterization was carried
out by NMR and IR spectroscopy, ESI-MS spectrometry, and
elemental analysis. Moreover, the CD spectra of these macro-
cycles also indicated chiral properties. Some of the platinum(II)
and heterometallic platinum(II)-ruthenium(II) compounds were
selected for photophysical study and were shown to exhibit
interesting luminescence behavior. Emission characteristic of
the lowest-lying triplet dπ(Ru) f π*(bpy) MLCT excited state
was observed in the heterometallic platinum(II)-ruthenium(II)
compounds, while higher energy emission of a metal-perturbed
9
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A R T I C L E S
Lin et al.
ref. No. CUHK403909), the External Cooperation Program of the
Chinese Academy of Sciences (BGrant No GJHZ200816), and the
University Grants Committee Areas of Excellence Scheme (AoE/
P-03/08). F.L. acknowledges with thanks the Croucher Foundation
(Hong Kong) for a Shanghai Studentship. The Science and
Technology Commission of the Shanghai Municipal Government
also partially supported this program. We thank Professors Li-Xin
Dai and Xue-Long Hou for helpful discussions.
IL origin typical of platinum(II) alkynyl phosphines was
observed in the homometallic platinum(II) species.
Acknowledgment. Dedicated to Professor Eiichi Nakamura on
the occasion of his 60th birthday. The Institute of Molecular
Functional Materials is an Area of Excellence under the University
Grants Committee (Hong Kong). This work was supported by a
grant from the Croucher Foundation (Hong Kong), a General
Research Fund from Research Grants Council (Hong Kong) (RGC
Supporting Information Available: Experimental details,
X-ray crystallographic data of 6, 8, and 9b. This material is
available free of charge via the Internet at http://pubs.acs.org.
(25) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Hor-
wood: Chichester, U.K., 1990. (b) Denti, G.; Serroni, S.; Campagna,
S.; Ricevuto, V.; Balzani, V. Coord. Chem. ReV. 1991, 111, 227–
236. (c) Scandola, F.; Indelli, M. T.; Chiorboli, C.; Bignozzi, C. A.
Top. Curr. Chem. 1990, 158, 73–149.
JA106599J
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