Synthesis of organometallic dendrimers by ligand exchange reactions: Reversible bonding of dendrons to a core in transition metal acetylide dendrimers

Article abstract of DOI:10.1039/b101583o

Ligand exchange reactions of chloride ligands on a trinuclear palladium acetylide core with platinum acetylide dendrons having a 4-pyridyl group at a focal point result in the quantitative formation of novel organometallic dendrimers, which easily revert to the core and the dendrons by treatment with Bu₄NCl.

Full text of DOI:10.1039/b101583o

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Synthesis of organometallic dendrimers by ligand exchange reactions:
reversible bonding of dendrons to a core in transition metal acetylide
dendrimers
Kiyotaka Onitsuka, Asako Iuchi, Masanori Fujimoto 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) 19th February 2001, Accepted 9th March 2001
First published as an Advance Article on the web 30th March 2001
Ligand exchange reactions of chloride ligands on a trinu-
clear palladium acetylide core with platinum acetylide
dendrons having a 4-pyridyl group at a focal point result in
the quantitative formation of novel organometallic den-
drimers, which easily revert to the core and the dendrons by
treatment with Bu4NCl.
mass spectrum, full characterization was performed by means
of NMR spectral analyses. For example, the 31P NMR spectrum
of GD3 exhibited two singlet signals at d 11.0 and 10.8 in a 4+3
integral ratio. The former was assignable to the phosphine on
the platinum atoms bound to the p-methoxyphenylethynyl
groups on the surface of the dendron, whereas the latter was due
1
to the phosphines on inner platinum atoms. In the H NMR
Dendrimers are three-dimensional macromolecules with reg-
ularly hyperbranched structures, and have a wide range of
potential applicable to the development of new materials.1 In
recent years, attention has shifted towards dendritic molecules
containing metal centers since they exhibit characteristic
properties such as redox behavior and catalysis.2 Previously we
have synthesized novel organometallic dendrimers composed
of platinum acetylide units.3,4 On the other hand, configura-
tional and constitutional changes in a dendritic molecule
responding to simple external stimuli are of interest for the
practical application of dendrimers.5,6 Thus, we engaged in the
construction of organometallic dendrimers containing switch-
able metal components by adopting coordination bonds. Herein
we report the syntheses of novel organometallic dendrimers in
which platinum–acetylide dendrons are reversibly bonded to a
palladium acetylide core.
spectrum, a sharp signal due to methoxy protons appeared at d
3.78 with twelve-fold intensity of those of the signals due to a-
and b-protons of pyridyl groups at the focal point.
We then examined the synthesis of dendrimers by the ligand
exchange reaction of the trinuclear platinum–acetylide complex
It is well known that halide ligands on transition metal atoms
are reversibly substituted for neutral ligands such as phosphine
and pyridine. We have therefore designed platinum–acetylide
dendrons having a 4-pyridyl group at a focal point and have
achieved successful preparation by a convergent method as
shown in Scheme 1. Reaction of the platinum acetylide complex
1 with 1,3,5-triethynylbenzene derivative 2, which has two
terminal acetylenes and one pyridylethynyl group, in the
presence of a Cu( ) catalyst gave the first generation dendron
I
GD1 in 74% yield. Similar reactions of diplatinum 3 and
hexaplatinum complexes 5, which were prepared as reported
previously,4 with 4 gave second and third generation dendrons
GD2 and GD3 in 60 and 44% yield, respectively (Scheme 2).
Trace analysis on these reactions by GPC showed that sharp
peaks due to GD2 with a higher molecular weight relative to
those of 3 and 4 grew with consumption of the substrates.
Purification by column chromatography on alumina followed
by reprecipitation from CH2Cl2–hexane gave a single product
which was confirmed by GPC analysis. Although no molecular
ion peaks of these dendrons except for GD1 were detected in the
Scheme 1 Reagents and conditions: i, Cu(I) cat., Et2NH, room temp., 1 d,
Scheme 2 Reagents and conditions: i, Cu(I) cat., Et2NH, room temp., 3 h,
74%.
60%; ii, Cu( ) cat., Et2NH, room temp., 3 h, 44%.
I
DOI: 10.1039/b101583o
Chem. Commun., 2001, 741–742
This journal is © The Royal Society of Chemistry 2001
741

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was found that the treatment with GD1 in the presence of
NaBArA4 (ArA = 3,5-(CF3)2C6H3) resulted in complete ligand
exchange and produced the first generation dendrimer G1 in a
quantitative yield (Scheme 3). Similarly, the second generation
dendrimer G2 was also prepared by the reaction of 7 with GD2
in quantitative yield. However, no exchange of chloride ligand
with GD3 took place and third generation dendrimer G3 did not
form at all. The difference in reactivity between GD2 and GD3
may arise from the steric effect of GD3.
The absence of structural defects in dendrimers G1 and G2
was confirmed by spectral analysis. A singlet signal assignable
to PEt3 on palladium was observed at d 32.2 in the 31P NMR
spectrum of G2 while the spectrum of 7 showed a signal at d
32.7. The 1H NMR spectrum of G2 exhibited a sharp singlet at
d 2.87 due to the methyl protons of the central mesitylene
moiety while the methyl signal of 7 was observed at d 3.14. In
1
the H NMR spectrum of G2 the integral ratio of the central
methyl signal relative to that of the methoxy protons situated at
the surface was 1+4. These data clearly suggest that all chloride
ligands on palladium were replaced with the pyridyl groups of
GD2, and are consistent with the structure of G2.
Treatment of dendrimer G1 with excess Bu4NCl in benzene
led to the quantitative formation of core 7 and dendron GD1
(Scheme 4). Similar treatment of dendrimer G2 regenerated 7
and GD2 in quantitative yield. These results suggest that the
bonding of dendrons GD1 and GD2 to core 7 can be reversibly
controlled by the ligand exchange reaction. Quantitative
formation of G1 and dissociation into 7 and GD1 were repeated
up to three times by successive treatment of NaBArA4 and
Bu4NCl, respectively. These results show the first examples of
morphology control in dendrimers by chemical stimuli other
than light.6
Scheme 4 Reagents and conditions: i, 9 Bu4NCl, benzene, room temp., 30
min, quantitative yields.
This work was supported by Grant-in-Aid for Scientific
Research on Priority Areas, (No. 11136224 ‘Metal-assembled
Complexes’) from the Ministry of Education, Science, Sports
and Culture. We thank The Material Analysis Center, ISIR,
Osaka University, for support of spectroscopic and elemental
analyses.
Notes and references
1 D. A. Tomalia, A. M. Naylor and W. G. A. Goddard III, Angew. Chem.,
Int. Ed. Engl., 1990, 29, 138; J. Issberner, R. Moors and F. Vögtle,
Angew. Chem., Int. Ed. Engl., 1994, 33, 2413; A. W. Bosman, H. M.
Janssen and E. W. Meijer, Chem. Rev., 1999, 99, 1665; A. Adronov and
J. M. J. Fréchet, Chem. Commun., 2000, 1701.
2 M. A. Hearshaw and J. R. Moss, Chem. Commun., 1999, 1; I. Cuadrado,
M. Morán, C. M. Casado, B. Alonso and J. Losada, Coord. Chem. Rev.,
1999, 193–195, 395; G. R. Newkome, E. He and C. N. Moorefield, Chem.
Rev., 1999, 99, 1689.
3 N. Ohshiro, F. Takei, K. Onitsuka and S. Takahashi, Chem. Lett., 1996,
871; N. Ohshiro, F. Takei, K. Onitsuka and S. Takahashi, J. Organomet.
Chem., 1998, 569, 195.
4 K. Onitsuka, M. Fujimoto, N. Ohshiro and S. Takahashi, Angew. Chem.,
Int. Ed., 1999, 38, 689.
Scheme 3 Reagents and conditions: i, 3.3 NaBArA4, benzene, room temp.,
30 min, quantitative yields.
5 A. Archut, G. C. Azzellini, V. Balzani, L. De Cola and F. Vögtle, J. Am.
Chem. Soc., 1998, 120, 12187.
6 M. Smet, L.-X. Liao, W. Dehaen and D. V. McGrath, Org. Lett., 2000, 2,
511; Y. Takaguchi, S. Suzuki, T. Mori, J. Motoyoshiya and H. Aoyama,
Bull. Chem. Soc. Jpn., 2000, 73, 1857.
6 with dendrons.7 Although chloride ligands on complex 6 are
easily replaced with pyridine in the presence of KPF6, the
reaction of complex 6 with GD1 allowed only partial ligand
exchange to give a mixture of the desired dendrimer and other
complexes having one or two GD1 in the molecule. Therefore,
the core was changed to a palladium–acetylide complex 7 and it
7 K. Onitsuka, H. Ogawa, T. Joh and S. Takahashi, Chem. Lett., 1988,
1855.
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Chem. Commun., 2001, 741–742