Published on Web 09/07/2006
Ionic Core-Shell Dendrimers with an Octacationic Core as
Noncovalent Supports for Homogeneous Catalysts
†
†
‡
Rob van de Coevering, Alfred P. Alfers, Johannes D. Meeldijk,
§,¶
,§
Elo ´ı sa Mart ´ı nez-Viviente, Paul S. Pregosin,*
Robertus J. M. Klein Gebbink,* and Gerard van Koten*,†
,
†
Contribution from the Faculty of Science, Organic Chemistry and Catalysis, Utrecht UniVersity,
Padualaan 8, 3584 CH, Utrecht, The Netherlands, Faculty of Science, Inorganic Chemistry and
Catalysis, Utrecht UniVersity, Sorbonnelaan 16, 3584 CA, Utrecht, The Netherlands, and
Laboratory of Inorganic Chemistry, ETH Z u¨ rich, H o¨ nggerberg, CH-8093, Z u¨ rich, Switzerland
Abstract: Ionic core-shell dendrimers with an octacationic core have been applied as noncovalent supports
for homogeneous catalysts. Catalytically active arylpalladium complexes, which bear a tethered sulfato
group, were noncovalently attached to the ionic core-shell dendritic supports via a straightforward ion-
1
exchange reaction under mild conditions. Diagnostic shifts in H NMR and Overhauser contacts show that
the sulfato groups of the catalysts are located close to the octacationic core of the dendritic support in the
resulting assemblies. The location of the catalytic Pd(II) sites has been varied via two strategies: by
increasing the dendrimer generation and/or by shortening of the sulfato tether. In addition, a metallodendritic
assembly was prepared, which bears an alternative shell of apolar dodecyl groups. Both the dendrimer
size and the nature of the dendritic shell have no influence on the binding properties of the dendritic supports,
i.e., the octacationic dendrimers of generations 1-3 form discrete 1:8 assemblies with the arylpalladium
complexes. The structural aspects and the nature of the metallodendritic assemblies have been studied
by means of pulse gradient spin-echo NMR diffusion methods, Overhauser spectroscopy, and electron
microscopy (TEM). These techniques showed that the dendritic supports and arylpalladium complexes
are strongly associated in solution to give unimolecular assemblies of nanoscopic dimensions. Membrane
dialysis can recover these metallodendritic assemblies due to their nanoscopic size. The catalytic
performances of the metallodendritic assemblies are comparable, but slightly lower than the performance
of the unsupported catalyst.
Introduction
bond between the metal and the dendrimer would be preferable.
Functionalization of a dendritic structure using covalent bonds,
however, often requires harsh reaction conditions and, as a
consequence, calls for dendritic supports with sufficient chemical
and thermal stability.
In the past decade many research groups have been involved
in the synthesis and application of dendrimer-supported homo-
geneous catalysts. The nanoscopic size of the dendrimer
support facilitates recovery of the catalyst, for example, by
1
,2
3
means of nanofiltration. Furthermore, the high solubility and
(
2) Some recent examples of dendritic catalysts include: (a) Fujihara, T.; Obora,
monodisperse structure of the dendritic support allow for the
formation of well-defined supported homogeneous catalysts,
which can be characterized and studied in situ by various
standard analysis techniques. In the reported systems, the
homogeneous catalyst, which is often a transition metal complex,
is attached to a dendritic support either via a covalent or a dative
bond. To reduce the chance of metal leaching, a stable covalent
Y.; Tokunaga, M.; Sato, H.; Tsuji, Y. Chem. Commun. 2005, 4526-4528.
(
b) Helms, B.; Liang, C. O.; Hawker, C. J.; Fr e´ chet, J. M. J. Macromolecules
2
005, 38, 5411-5415. (c) Lemo, J.; Heuze, K.; Astruc, D. Org. Lett. 2005,
7
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Catal. 2004, 346, 1093-1096. (h) Slagt, M. Q.; Stiriba, S.-E.; Kautz, H.;
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†
Utrecht University, Faculty of Science, Organic Chemistry and
Catalysis.
‡
Utrecht University, Faculty of Science, Inorganic Chemistry and
Catalysis.
§
ETH Z u¨ rich.
¶
Current address: Inorganic Chemistry Department, Faculdad de
Qu ´ı mica, Universidad de Murcia, Apdo. 4021, 30071, Murcia, Spain.
(
1) For recent reviews on dendrimer-supported homogeneous catalysts see, for
example: (a) van Heerbeek, R.; Kamer, P. C. J.; van Leeuwen, P. W. M.
N.; Reek, J. N. H. Chem. ReV. 2002, 102, 3717-3756. (b) Kreiter, R.;
Kleij, A. W.; Klein Gebbink, R. J. M.; van Koten, G. Top. Curr. Chem.
(3) For reviews on the use of nanofiltration for catalyst recycling: (a) Dijkstra,
H. P.; van Klink, G. P. M.; van Koten, G. Acc. Chem. Res. 2002, 35, 798-
810. (b) Vankelecom, I. F. J. Chem. ReV. 2002, 102, 3779-3810.
2
001, 217, 163-199. (c) Astruc, D.; Chardac, F. Chem. ReV. 2001, 101,
2
991-3023.
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J. AM. CHEM. SOC. 2006, 128, 12700-12713
10.1021/ja060079t CCC: $33.50 © 2006 American Chemical Society