Novel platinum-acetylide metallocycles constructed via a stepwise fragment coupling approach and their aggregation behaviour

Article abstract of DOI:10.1039/c3cc42403k

Through a stepwise fragment coupling approach, a family of novel platinum-acetylide metallocycles with well-defined shape and size were successfully constructed. Their aggregation behavior was investigated. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc42403k

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Novel platinum–acetylide metallocycles constructed via
a stepwise fragment coupling approach and their
aggregation behaviour†
Cite this: DOI: 10.1039/c3cc42403k
Received 3rd April 2013,
Accepted 23rd May 2013
Zhong-Yu Li, Lin Xu, Cui-Hong Wang, Xiao-Li Zhao and Hai-Bo Yang*
DOI: 10.1039/c3cc42403k
www.rsc.org/chemcomm
Through a stepwise fragment coupling approach, a family of novel aggregation of platinum–acetylide metallocycles, which might be
platinum–acetylide metallocycles with well-defined shape and size were due to the difficulties in their synthesis.
successfully constructed. Their aggregation behavior was investigated.
It should be noted that some previous methods employed to
construct the covalent macrocycles often suffer from low yields. With
Macrocyclic host molecules have played important roles in the the aim of constructing well-defined platinum–acetylide metallo-
development of supramolecular chemistry.1 The milestone research cycles with high efficiency, in this study, a stepwise fragment
progress in crown ethers,2a cryptands,2b and spherands2c has laid the coupling strategy was explored to build well-defined organometallic
solid foundation of supramolecular chemistry as an independent macrocycles from predesigned platinum acetylide building blocks
discipline. With the development of supramolecular chemistry, a with appropriate geometrical structures. Moreover, by using the
series of novel macrocyclic host molecules such as heteracalixaro- functionalized building blocks, a new metallocycle decorated with
matics,3a,b pillar[n]arenes,3c and [n]cycloparaphenylenes3d have a hydrophobic long chain was obtained. Its aggregation behaviour
been well investigated and new energy has been instilled into has been well investigated.
macrocyclic chemistry. Thus the design and construction of novel
macrocyclic molecules has always been one of the driving forces to fragment coupling approach as shown in Scheme 1. According to the
stimulate the development of supramolecular chemistry. geometry of rhomboidal macrocycles, 601 fragment 1 and 1201 linker
Two novel metallocycles 5a-b were prepared via a stepwise
Recently, the design and self-assembly of nanostructured aggre- 2a-b were employed to build such metallocycles. The [2+1] fragment
gates from macrocyclic molecules via a ‘‘bottom-up’’ strategy have 3a with a strictly geometric angle was prepared via a coupling
attracted considerable attention. A great number of nano/micro-
structured materials that form vesicle, fiber, microtube and micro-
sphere structures through the supramolecular self-assembly of
macrocycles have been reported. Some of these materials have
presented wide applications in diverse fields such as supramolecular
amphiphiles,4a,b supramolecular polymers,4c drug delivery,4d
fluorescent sensors,4e organogels,4f etc. As an important functional
linker, platinum(II) acetylide has been extensively explored in the
areas of organometallic gels,5a solar cells,5b,c functional oligomers,5d,e
and luminescent materials5f because of its unique p-conjugated
character. For instance, we have prepared a series of novel
platinum–acetylide complexes, which exhibited potential applica-
tions as luminescent organometallic gels.6 However, compared to
the considerable work on the supramolecular self-assembly of
platinum–acetylide oligomers, very few studies have explored the
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department
of Chemistry, East China Normal University, 3663 N. Zhongshan Road, Shanghai
200062, P.R. China. E-mail: hbyang@chem.ecnu.edu.cn; Fax: +86021 62235137
† Electronic supplementary information (ESI) available: Details of synthesis and
Scheme 1 Preparation of metallocycle 5a and exo-functional metallocycle 5b.
Reagents and conditions: (i) CuI, THF, Et2NH, rt; (ii) K2CO3, THF, MeOH, rt; and
(iii) CuI, THF, Et2NH, rt.
characterization, crystallographic data, UV/vis and emission spectra, and addi-
tional SEM images. CCDC 931704. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3cc42403k
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reaction7 of 1 and 2a by using CuI as the catalyst. Then the
deprotection reaction7 of 3a with K2CO3 in a mixture of methanol
and THF led to the formation of the key intermediate oligomer 4a
with terminal alkynes. The final novel metallocycle 5a was prepared
through the [3+1] coupling of oligomer 4a with fragment 1, and
isolated as a yellow solid in 55.6% yield. Through a similar method,
another metallocycle 5b decorated with a hydrophobic long chain was
obtained in 63.8% yield.
Fig. 2 (a) UV-vis absorption spectra and (b) emission spectra of metallocycles
The molecular structures of platinum–acetylide metallocycles
5a-b were characterized using multiple nuclear NMR (1H, 31P,
and 13C), infrared spectroscopy, mass spectrometry, and elemental
5a-b in dichloromethane at 298
5b = 5.16 Â 10À6 M).
K
(concentration: 5a
=
5.79
Â
10À6 M;
1
analysis. The H NMR spectrum of 5a-b showed only one set of
the absorption spectrum of 5b showed two intense absorption bands
in the range of 307–325 nm and 407–433 nm. According to the
previous spectroscopic investigation on trans-[Pt(PEt3)2(CRCR)2]
and its derivatives,8a the low energy absorption bands are described
as an admixture of intraligand (IL) [p–p*(CRCR)] and metal-to-
ligand charge transfer (MLCT) [d(Pt) p–p*(CRCR)] transition with
predominant IL character. The higher energy absorption bands are
likely attributed to an admixture of p–p* and n–p* transitions
localized on the alkoxyphenyl fragments with MLCT (dp(Pt) -
p*(CRCR)) transition.8 Upon being excited at 358 nm, macrocycles
5a-b displayed strong emission at ca. 457 nm with a shoulder at
455 nm in a dilute dichloromethane solution (ca. 10À5 M, Fig. 2b).
It is well known that the hydrophobic interaction of alkyl chains and
the hydrogen bonding between amide groups enables the orientation
and ordered aggregation of the molecules. The morphologies of
aggregates oligomer 4b and macrocycles 5a-b were examined using
scanning electron microscopy (SEM). SEM samples were prepared by
depositing the solution of complexes onto a SiO2/Si substrate (1 Â
1 cm2). Interestingly, it was found that the oligomer 4b and macrocycles
5a-b displayed obviously different morphologies even under the same
conditions. The morphologies of these platinum–acetylide complexes
were heavily dependent on the molecular shapes as shown in Fig. 3 and
ESI.† For example, upon exposure to the solvent mixture of dichloro-
methane–methanol (v/v, 1 : 1), the morphologies of oligomer 4b
featured twining agglomerates while ordered fibers with a flake-like
appearance were found in the case of 5b. Notably, 5a could not form
any ordered aggregation under the same conditions (ESI†). This
difference might be caused by the structural difference among oligomer
4b, macrocycle 5a, and 5b. In the case of 5a, due to the lack of alkyl
chains, the solubility of 5a turned out to be very poor, which led to
the absence of ordered aggregation. Macrocycle 5b preferred the
face to face mode because of the existence of the rigid rhomboidal
ring, allowing for the formation of well-organized aggregates.
signals, respectively, which indicated the formation of highly sym-
metrical structures. For instance, the 1H NMR spectrum of metallo-
cycle 5a displayed two singlets at 7.88 and 8.74 ppm, respectively,
which were assigned to the protons on 4-, 5- and 9-, 10-substituted
location of pyrene, respectively. The doublet–doublet at 7.91–7.96 ppm
attributed to the protons on 2-, 3-, 6-, 7-substituted location of pyrene
was found as well. Moreover, the 31P{1H} NMR spectra of 5a-b dis-
played a sharp singlet (ca. 11.97 ppm for 5a and 12.53 ppm for 5b) with
two 195Pt satellites (JPt–P, ca. 2371.8 Hz for 5a and 2358.9 Hz for 5b),
respectively, which supported the existence of symmetrical metallo-
cycles. Additionally, the investigation of MALDI-TOF mass provided
further strong support for the formation of metallocycles 5a-b. For
instance, in the mass spectra, the peaks at m/z = 2469.75 for 5a
(0.03 Da deviation) and m/z = 4013.09 (0.08 Da deviation) for 5b,
attributed to the [M + H]+ ion, were observed, respectively. These
peaks were isotopically resolved and their isotopic resolution is in
excellent agreement with the theoretical distribution (ESI†).
X-ray crystallographic analysis unambiguously established the
structure of metallocycle 5a as a discrete platinum–acetylide rhom-
boid (Fig. 1). Pale-yellow crystals suitable for single-crystal X-ray
analysis were grown by slow diffusion of ethyl acetate into a chloro-
form solution of 5a at room temperature. X-ray crystallographic
analysis showed that the platinum–acetylide macrocycles featured a
rhomboidal ring with a size of 1.40 Â 1.43 nm. The ORTEP
representation of the crystal structure showed that all atoms (except
for the triethylphosphine ligands) lay approximately in the same
plane. The platinum atoms in the complexes were found to adopt a
slightly distorted trans-square-planar geometry with C–Pt–P in the
range of 87.21 and 92.11, which might be caused by the steric
demand of the bulky triethylphosphine ligands (ESI†).
The investigation of UV-visible absorption and emission spectra
of the platinum–acetylide macrocycles 5a-b in diluted dichloro-
methane solutions was carried out at 298 K (Fig. 2). The absorption
spectra of macrocycles 5a-b exhibited similar patterns. For example,
Fig. 3 SEM images of (a) oligomer 4b and (b) macrocycle 5b prepared in
Fig. 1 Ball-and-stick model of the X-ray crystal structure of metallocycle 5a.
dichloromethane–methanol (v/v, 1 : 1).
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By employing a functionalized precursor, a metallocycle decorated with
a hydrophobic long chain was obtained via this strategy as well. The
sharp NMR signals in both 31P {1H} and 1H NMR spectra along with
the solubility of these species ruled out the formation of oligomers.
Moreover, X-ray crystallographic analysis of 5a provided further
evidence for the unambiguous determination of rhomboidal metallo-
cycles. These newly designed platinum–acetylide metallocycles are
air- and humidity-stable at room temperature. The supramolecular
aggregation of metallocycle 5b was studied through concentration- and
temperature-dependent absorption and emission spectra. Furthermore,
the concentration-dependent 1H NMR spectroscopic studies were
performed as well. All results indicated that the hydrophobic interaction
between alkyl chains and the hydrogen-bonding between amides are
the driven forces during the formation of ordered aggregates. Moreover,
the morphologies of these platinum–acetylide complexes were depen-
dent on the molecular shape. These findings obviously enrich the
library of novel macrocyclic host molecules and provide a new platform
to design potential functional nanostructured aggregates.
Fig. 4 Schematic representation of the supposed patterns of the formation of
different supramolecular aggregates.
This work was financially supported by NSFC/China (No. 21132005
and 91027005) and Fok Ying Tung Education Foundation (No. 131014).
Notes and references
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cycle 5b, so these oligomer molecules were ready to intertwine
and form unordered aggregates (Fig. 4).
To obtain insights into the aggregation behaviour of 5b, further
spectroscopic investigation was carried out. Detailed concentration-
dependent and variable-temperature UV-vis and emission spectro-
scopy of 5b were performed in dichloromethane (ESI†). Although no
significant changes were observed in UV-vis absorption spectra,
macrocycle 5b exhibited concentration-dependent emission proper-
ties in dichloromethane solution. As shown in Fig. 5a, it was found
that the increase in the concentration of 5b from 1.03 Â 10À7 to
5.16 Â 10À6 M at 298 K led to a significant increase in the intensity of
the emission band at 402 nm with a slight red-shift (4.0 nm). This
observation is consistent with the formation of the excimer during the
aggregation process as illustrated in previous reports.9 Moreover,
upon increasing the concentration from 5.16 Â 10À6 to 5.16 Â
10À4 M (Fig. 5b), the emission band was found to drop dramatically
in intensity with an obvious red-shift (ca. 28 nm), which indicated that
the main aggregation pattern of 5b was changed into some complicated
manner from the dimer in the higher concentration range.10 Further-
1
more, the concentration-dependent H NMR spectroscopic studies of
5b in CD2Cl2 were performed to probe the driving forces for the
aggregation process (ESI†). It was found that the NH resonance dis-
played an obvious downfield shift upon the increase of concentration
(d = 7.817 ppm at 1.45 mg mLÀ1 and d = 7.856 ppm at 25.02 mg mLÀ1),
which was indicative of the existence of hydrogen-bonding inter-
actions during the formation of supramolecular aggregates.6
In conclusion, through a stepwise fragment coupling approach,
a family of novel platinum–acetylide metallocycles with a well-defined
shape and size were successfully constructed in modest yields.
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.