Tris(2,2′-bipyridyl)ruthenium(II) with branched polyphenylene shells: A family of charged shape-persistent nanoparticles

Article abstract of DOI:10.1002/anie.200704256

(Figure Presented) Cores for thought: Dendrimers based on an octahedral symmetry and with a central positively charged transition-metal complex have been prepared up to the third generation (see schematic representation). The shape-persistent dendrimers with a high density of aromatic rings are accessible by either a partly convergent synthesis or a divergent strategy in which the metal complex proved to be stable to high-temperature Diels-Alder reactions.

Full text of DOI:10.1002/anie.200704256

Communications
DOI: 10.1002/anie.200704256
Dendrimers
Tris(2,2’-bipyridyl)ruthenium(II) with Branched Polyphenylene Shells:
A Family ofCharged Shape-Persistent Nanoparticles**
Monika C. Haberecht, Jan M. Schnorr, Ekaterina V. Andreitchenko, Christopher G. Clark, Jr.,
Manfred Wagner, and Klaus Müllen*
Dedicated to Professor Klaus Hafner on the occasion of his 80th birthday
Polyphenylene dendrimers with poly(pentaphenylben-
zene)[1,2] branches (PPDs) are special within dendrimer
chemistry because of their stiff, mostly radial arms that do
not allow backfolding, thus rendering the molecules shape-
persistent.[3] As a result, their overall shapes are defined by
the geometry of the particular core unit, and different cores
(Figure 1a–c) have been proven to generate structural
diversity.[4]
The highest core symmetry that approaches spherical
PPDs[5] has, however, been limited to a four-armed, tetrahe-
dral tetraphenylmethane core (Figure 1c), since higher sym-
metries, such as octahedral, are challenging to achieve in
organic chemistry. A powerful tool for building structures
with controlled symmetry is instead provided by the use of
organometallic complexes as core units such as the well-
known tris(2,2’-bipyridyl)ruthenium(II) complex ({Ru(bpy)3},
Figure 1d).[6] This complex has already been employed as a
functional core in a variety of non-shape-persistent dendrim-
ers,[7–9] mainly for investigating the effect that site isolation
Figure 1. PPD cores (the arrows indicate the positions for dendrimer
growth): a) biphenyl, b) hexaphenylbenzene, c) tetraphenylmethane
(Td), and d) {Ru(bpy)3}.
has on the properties of the ruthenium-based chromophore[8]
(for example, excited-state lifetimes) or for the design of
light-harvesting systems.[9] Moreover, it possesses an almost
perfect octahedral coordination geometry[10] and is shape-
persistent itself, and thus can serve as the desired PPD core
when dendritic wedges are attached to the six positions para
to its nitrogen atoms (Figure 1d).
In addition, the {Ru(bpy)3} core provides valuable syn-
thetic handles: First, it introduces two positive charges within
the center of the stiff and nonpolar PPD backbone, thus
giving the dendrimer the character of a large, weakly
coordinatingdication. Second, this core is constructed by
metal complexation, which is expected to deliver a facile and
versatile tool for the synthesis of desymmetrized PPDs if the
ligand (dendron) attachment is performed stepwise.
The reaction sequence established for the synthesis of
high-generation, monodisperse polyphenylene dendrimers is
based on a [4+2] Diels–Alder cycloaddition of a triisopro-
pylsilyl (TIPS) protected ethynyl-substituted cyclopentadie-
none branchingunit to an ethynyl-substituted core or
dendrimer, followed by removal of the TIPS groups, which
activates the molecule for further growth.[2] Since 2,2’-
bipyridine has no known reactivity under Diels–Alder con-
ditions, it offers the opportunity to either grow the dendrimer
divergently or to synthesize the polyphenylene dendrons first
and then build the dendrimer by metal complexation in the
final step (“convergent” approach).
[*] Dr. M. C. Haberecht, Dipl.-Chem. J. M. Schnorr,[+]
Dr. E. V. Andreitchenko, Dr. C. G. Clark, Jr., Dr. M. Wagner,
Prof. Dr. K. Müllen
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-350
E-mail: muellen@mpip-mainz.mpg.de
[+] Present address: Department of Chemistry
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
[**] We gratefully acknowledge support of this work by the Deutsche
Forschungsgemeinschaft (DFG)within the frame of the Sonder-
forschungsbereich (SFB)625. C.G.C. is grateful for financial support
from a U.S. National Science Foundation MPS Distinguished
International Research Fellowship (MPS-DRF; award: DMR-
0207086)and from the Max Planck Society. We thank C. Beer for
synthetic support and S. Türk for MALDI-TOF measurements.
The key component in the two synthetic strategies for
{Ru(bpy)3}-cored PPDs is 4,4’-bis(ethynyl)-2,2’-bipyridine (1).
The reported synthesis of 1 consists of six steps and is
complicated by weakly soluble 2,2’-bipyridine (bpy) inter-
mediates.[11] Therefore, a new five-step synthesis was devel-
Supporting information for this article is available on the WWW
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oped for 1, which is more convenient since the bpy moiety is
formed in the penultimate step (Scheme 1).
Startingfrom 2-bromopyridine, 2-bromo-4-iodopyridine
(3) was prepared in two steps.[12] Subsequent selective
Figure 2. Illustrations of a)[G3] Td-cored PPD [4] and b)[G3] {Ru-
(bpy)3}-cored PPD 18.
dynamic radius of 2.5 nm for dendrimer 18, which is slightly
larger than that determined for the Td species (2.2 nm).[4] This
findingindicates a decrease in the lareg voids between
branches, as the physical radii are almost identical. Whereas
the tetrahedral dendrimer distributes 144 phenylene rings
within the hydrodynamic volume, the density of the {Ru-
(bpy)3}-cored dendrimer is increased to 216 aromatic rings.
Therefore, it is likely that the surface crowding, beyond which
defined dendrimer growth no longer occurs, is reached earlier
for the herein presented octahedral PPDs than for the Td-
cored species, which were obtained up to the fourth gener-
ation.
Scheme 1. Synthesis of 2,2’-bipyridine 1: a) ,b)[12]; c)TIPS-acetylene,
[PdCl2(PPh3)2], CuI, NEt3, toluene, 08C, 18 h, 67%; d)[Sn 2(nBu)6],
[Pd(PPh3)4], toluene, 1208C, 7 days, 65%; e)TBAF, THF, RT, 30 min,
89%. TBAF=tetrabutylammonium fluoride.
Sonogashira–Hagihara coupling gave 4, which was converted
into bipyridine 2 by employinga Stille couplingprocedure.
Removal of the TIPS groups of 2 with tetrabutylammonium
fluoride (TBAF) gave 4,4’-bis(ethynyl)-2,2’-bipyridine (1) in
26% overall yield from 2-bromopyridine.
[13]
A series of dendrons (5–8) was prepared for the synthesis
of dendrimers by “convergent” metal complexation. These
dendrons were prepared from 1 by utilizingthe iterative
reaction sequence of Diels–Alder cycloadditions and TIPS
deprotections described above (Scheme 2). The building
blocks 10[2] and 11[14] were prepared by literature methods.
To obtain the desired PPDs, dendrons 5–7 were treated
with [RuCl2(dmso)4][15] in DMF at 1408C for three to four
days, which led to the formation of the first ([G1]) to third
([G3]) generation dendrimers 16–18 (Scheme 3). The syn-
thesis of the corresponding[G4] dendrimer, derived from the
largest bpy derivative 8, was unsuccessful.
Divergent dendrimer synthesis required the use of the
[G1] TIPS-ethynyl-functionalized 2,2’-bipyridine 12. The [G1]
{Ru(bpy)3}-based dendrimer 19 was then formed upon com-
plexation of 12 with [RuCl2(dmso)4] in DMF at 1408C
(Scheme 4). Further dendrimer synthesis was again achieved
by iterative Diels–Alder cycloadditions with cyclopentadie-
none 11 and cleavage of the TIPS groups. The crucial point is
that the {Ru(bpy)3} core survives the reaction cycles, thereby
enabling growth to higher generations. As for the convergent
strategy, synthesis was successful up to the [G3] dendrimer 23,
but the corresponding[G4] species was not obtained.
Interestingly, both strategies are apparently limited to the
synthesis of the third generation and, moreover, both were
found to give comparable overall yields.
The size, shape, and polyphenylene density of the [G3]
{Ru(bpy)3}-cored dendrimer 18 were compared with those of
the related [G3] species with a tetraphenylmethane (Td) core
(Figure 2).[4] Diffusion NMR measurements revealed a hydro-
Figure 3. 1H NMR spectra (700 MHz, CD2Cl2, 303 K)of [G1] dendrim-
ers 16, 24, 25, and 26.
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In addition to their increased
density, the {Ru(bpy)3}-cored den-
drimers differ from previously
reported PPDs since they carry the
positive charges of the metal com-
plex within their center. The TIPS-
protected species 19, 21, and 23 were
found to be completely soluble in
hydrocarbon solvents, which sug-
gests that only the dendrimer surface
defines the solubility properties,
whereas the positive charge together
with the chloride counterions are
shielded within the dendrimer back-
bone. The counterions of the [G1]
dendrimer 16 were efficiently
exchanged upon treatment with the
sodium salt of methyl orange, am-
monium hexafluorophosphate, or
sodium tetraphenylborate to give
the new salts 24, 25, and 26, respec-
tively. As can be seen from the
1H NMR spectra (Figure 3), the bpy
resonances of the dendrimer are
sensitive to the anion exchange,
whereas the polyphenylene regions
remain unaffected. The resonances
that are most influenced are H5 and
H6 of bpy, which suggests their
proximity to the anions in cases
where coordination is strong(for
example, chloride). The tetraphenyl-
borate salt is a different case, and the
complexity of the spectrum suggests
interactions with dendrimer aryl
groups. A future challenge is the
combination of these nanoscale di-
cations with anions of a similar size
to construct large dendrimer salts.
The introduction of charge would
add an additional tool for the assem-
bly of complex supramolecular
architectures.[16]
Two other strategies to influence
the supramolecular behavior of such
charged PPDs involve the modifica-
tion of their shape or the generation
of inhomogeneous dendrimer surfa-
ces. Both goals are reached by a
desymmetrization of the dendrimer
core through the attachment of dif-
ferent dendrons. Since desymme-
trized PPDs have so far only been
accessible
by
statistical
approaches,[17] the applicability of a
stepwise formation of a {Ru(bpy)3}
derivative as a potential controlled
route for desymmetrization was
evaluated. Therefore, the dichloro-
Scheme 2. Syntheses of 2,2’-bipyridylpolyphenylene dendrons 5–8: a) 9, o-xylene, 1408C, 20 h, 97%;
b) 10, o-xylene, 1408C, 2 days, 90%; c) 11, o-xylene, 1408C, 20 h, 48%; d)TBAF, THF, RT, 1 h, 98%;
e) 10, o-dichlorobenzene, 1758C, 3 days, 45%; f) 11, o-xylene, 1408C, 2 days, 90%; g)TBAF, THF,
RT, 1 h, 98%; h) 10, o-dichlorobenzene, 1758C, 3 days, 32%.
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Scheme 3. “Convergent” syntheses of {Ru(bpy)3}-cored PPDs 16–18: a)–c) [RuCl2(dmso)4], DMF, 1408C, 3 days. a)55%, b)23%, c)44%.
ruthenium compound 27 was synthesized and treated with the
[G2] dendron 6, which led to the formation of macromolecule
28 (Scheme 5) in 34% overall yield from [RuCl2(dmso)4].
This result establishes the ease of this desymmetrization
strategy and further work, concentrating on the design of
dumbbell-type and amphiphilic structures,[16] is in progress.
In summary, this is the first description of shape-persistent
dendrimers that are based on octahedral symmetry, and
moreover, possess a positively charged transition-metal com-
plex at their center. The molecules are accessible with
structural perfection up to the third generation by either a
partly convergent synthesis or a divergent strategy in which
the metal complex proved to be stable to high temperature
Diels–Alder conditions. The {Ru(bpy)3}-based dendrimers
were shown to possess a strongly enhanced global density of
aromatic rings compared to the related tetrahedral structure;
nevertheless, efficient counterion exchange was demonstrated
for the first generation dendrimers. The resulting opportunity
to combine the salts with larger anions and the possibility of
the stepwise attachment of dendrons gives access to various
dendrimer salts and charged desymmetrized PPDs, which are
of interest for further investigations of the self-assembly of
shape-persistent polyphenylene structures.
Received: September 14, 2007
Published online: January 23, 2008
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Scheme 4. Divergent syntheses of {Ru(bpy)3}-cored PPDs 19–22 a)DMF, 140 8C, 4 days, 48%; b)TBAF, THF, RT, 1 h, 82%; c) 11, ethylene glycol,
o-xylene, 1408C, 3 days, 55%; d)TBAF, THF, RT, 1 h, 60%; e) 11, ethylene glycol, o-xylene, 1408C, 4 days, 81%.
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Scheme 5. Synthesis and schematic representation of desymmetrized
compound 28: a)DMF, 140 8C, 3 days, 64%; b)EtOH/CHCl 3, 908C,
6 days, 46%.
Keywords: coordination modes · dendrimers · ruthenium ·
.
supramolecular chemistry · synthetic methods
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