A divergent approach to the precise synthesis of giant organometallic dendrimers using platinum-acetylides as building blocks

Article abstract of DOI:10.1039/b210830e

Giant platinum-acetylides dendrimers were precisely synthesized by a divergent method; the sixth generation dendrimer, the diameter of which is larger than 10 nm, has 189 Pt atoms per molecule, and its molecular weight is as high as 139750.

Full text of DOI:10.1039/b210830e

A divergent approach to the precise synthesis of giant organometallic
dendrimers using platinum–acetylides as building blocks
Kiyotaka Onitsuka, Atsushi Shimizu and Shigetoshi Takahashi*
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka
567-0047, Japan. E-mail: takahashi@sanken.osaka-u.ac.jp
Received (in Cambridge, UK) 6th November 2002, Accepted 6th December 2002
First published as an Advance Article on the web 20th December 2002
Giant platinum–acetylides dendrimers were precisely syn-
thesized by a divergent method; the sixth generation
dendrimer, the diameter of which is larger than 10 nm, has
189 Pt atoms per molecule, and its molecular weight is as
high as 139750.
amounts of 1 remained after 48 h, and a structural defect in the
product was detected by H NMR (Fig. 1(a)). Two signals
1
assignable to the protons of terminal acetylenes were observed
at d 3.00 and 3.05. The former is attributed to a diethynyl–
monoplatinum group and the latter to a monoethynyl–diplati-
num group. The integral ratio of these signals relative to that of
the methyl groups at the core suggests that 25% of the product
has on average one unreacted terminal acetylene, even though
75% of the product has the expected structure. This problem
was resolved by using a 25 mol% excess of the building block
1 in the reaction. Disappearance of the signal assignable to the
To develop novel functionalized nano-size materials, the
incorporation of metallic species into dendrimers has attracted
much attention, since transition metal complexes have charac-
teristic magnetic, electronic, and photo-optical properties as
well as distinctive reactivity.1 In particular, organometallic
dendrimers offer the advantage of making it possible to tailor
dendritic molecules with desirable functionalities due to not
only the availability of a wide variety of organic compounds
that coordinate to many kinds of metal atoms but also the
flexibility of the coordination modes of organic ligands to the
metal.2 Although many organometallic dendrimers have been
prepared so far, most of them contain metallic species at specific
positions in the molecule; i.e., at the core or at the periphery.
Dendrimers composed of organometallic complexes in each
generation are current challenging targets. Some examples of
such dendrimers have been reported using an appropriate choice
of organometallic complexes for the core and building
blocks.3–7 However, large dendrimers greater than fourth-
generation have not yet been prepared.
1
terminal acetylenes in the H NMR spectrum clearly showed
that the reaction proceeded completely to give the desired
fourth-generation dendrimer G4 (Fig. 1(b)). Although the
reaction mixture contained not only G4 but also considerable
amounts of 1, these were easily separated by reprecipitation.
Platinum–acetylide complexes are thermally robust and
stable, even when exposed to air and moisture, and can be
obtained in high yields by a well-established synthetic method-
ology.8 Since some platinum–acetylide complexes show unique
properties,9 platinum–acetylide dendrimers are of special
interest due to their potential to provide new nano-size
organometallic materials. Recently, we developed a convergent
route to the synthesis of platinum–acetylide dendrimers up to
the third-generation.10 We present here a divergent approach to
platinum–acetylide dendrimers up to the sixth-generation,
which is much larger organometallic dendrimers than those
have been reported previously.
The synthetic route up to the third-generation dendrimer is
shown in Scheme 1. The building block for divergent synthesis
is the mononuclear platinum–acetylide complex (1), which was
prepared from 1-bromo-3,5-diiodobenzene using the trime-
thylsilyl and triisopropylsilyl groups to protect the terminal
acetylene.10,11 We used 2,4,6-triethynylmesitylene (2), the
methyl groups of which are very important for characterization
of the resulting dendrimers, as the core. The treatment of 1 with
2 in a molar ratio of 3+1 using a CuCl catalyst in diethylamine
at room temperature for 24 h resulted in the quantitative
formation of the first-generation dendrimer (G1), which was
fully characterized by spectral analyses. The reaction of the
deprotected first-generation dendrimer (G1A), which was gen-
erated by reacting G1 with Bu4NF, with 6 equiv. of 1 gave the
second-generation dendrimer (G2), which was subsequently
grown to the third-generation dendrimer (G3) in a similar
manner. In these reactions, the building block 1 was completely
consumed and the desired dendrimers were obtained as the sole
product.
On the other hand, when we tried to prepare the fourth-
generation dendrimer by reacting G3A with 24 equiv of 1, small
Scheme 1
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Table 1 GPC data for dendrimers G1–G6
Although molecular ions of these dendrimers, except for G1
and G1A, were not detected even by MALDI-TOF mass
spectrometry, the number of unreacted terminal acetylenes can
be sensitively estimated by 1H NMR in each generation because
all of the dendrimers have three methyl groups at the core
regardless of the generation number. Successive reactions led to
the formation of up to the sixth-generation dendrimer (G6). In
the 1H NMR spectra of G5 and G6, no signal due to the terminal
acetylenes was observed, indicating no significant amounts of
structural defects. The sixth-generation dendrimer G6 has 189
Pt atoms per molecule, and its molecular weight is as high as
139750 (Scheme 2).
a
Dendrimer
Mn
FW
Mn/FW
G1
G2
G3
G4
G5
G6
2800
2869
0.98
0.92
0.74
0.57
0.42
0.29
6700
11900
19100
29100
40200
7285
16116
33778
69102
139750
a Determined by using polystyrene standards.
The relative molecular size of these dendrimers was investi-
gated by gel permeation chromatography (Table 1). The number
averaged molecular weight (Mn) of G1 is coincidentally
consistent with the formula weight (FW). Whereas the FW
grows exponentially with an increase in the number of the
generation, the Mn value increases proportionally, so that G6
has an Mn value of less than one-third of the formula weight.
The molecular size of G6 was measured by small angle neutron
scattering (SANS), which showed that the radius was 56.8 ± 0.2
Å. This result is in very good agreement with the diameter of G6
(12 nm) calculated by molecular modeling.
In summary, we have precisely synthesized a large organo-
metallic dendrimer, the diameter of which is larger than 10 nm,
by using platinum–acetylide as a repeating unit. With the facile
chemistry described here, nanoscale organometallic objects
may be realized. Further studies focused on the properties of
these nano-size organometallic dendrimers are now in pro-
gress.
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports and
Culture, Japan. We thank Dr. Masaaki Sugiyama (Department
of Physics, Kyusyu Univ.) and Prof. Toshiharu Fukunaga
(Research Reactor Institute, Kyoto Univ.) for measuring
SANS.
Notes and references
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Fig. 1
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Scheme 2
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