Synthesis and Characterization of Dendritic Necklaces: A Class of Outer-Sphere-Outer-Sphere Connected Dendronized Organoplatinum Polymers

Article abstract of DOI:10.1002/anie.200351613

Molecular jewelry! Up to 880 dendrimer beads can be joined together with Pt complexes to make a long necklace of dendrimers (see picture). The polymerization of dendrimers that contain reactive functional subunits on the dendrimer surface opens a new route to a novel class of dendronized polymers of high molecular weights and in good yields.

Full text of DOI:10.1002/anie.200351613

Angewandte
Chemie
A Chain of Dendrimers
Synthesis and Characterization of Dendritic
Necklaces: A Class of Outer-Sphere–Outer-
Sphere Connected Dendronized Organoplatinum
Polymers**
Figure 1. Synthesis of dendritic necklaces by outer-sphere–outer-sphere
connection of bifunctional dendrimers.
Hak-Fun Chow,* Cham-Fai Leung, Wei Li, Kai-
Wai Wong, and Luan Xi
platinum coordination chemistry as a method for the poly-
merization of bis(ethynyl) surface-functionalized G1–G3
dendritic beads 1–3, we demonstrate that higher-order 1D
dendritic necklaces with high degree-of-polymerization
values (DP, from 30–880) could be prepared in high yields.
We also describe a general synthetic protocol for the
construction of dendrimers that contain a limited number of
functional groups on the surface sector.
Because bis(ethynyl) polyether-based dendrimers readily
form trans-dialkynyl organoplatinum(ii) complexes upon
treatment with trans-[(Et3P)2PtCl2], this reaction was chosen
to link the dendrimers 1–3 together.[8] To avoid random
polymerization of the dendritic beads, only two of the surface
moieties were functionalized with “reactive” long chain 10-(4-
ethynylphenoxy)decyloxyl residues while the rest were dec-
orated with “inert” 3-(4-tert-butylphenoxy)propoxy groups.
The synthesis of the target dendritic monomers 1–3
required the availability of three bromo-dendrons 10, 11,
and 12[9] (Scheme 1). Hence, the G1-dendrimer 4 and G2-
Some years ago Newkome and Vögtle proposed the concept
of dendritic networks[1] whereby simple dendrimer/dendron
subunits could be connected together to form macromolec-
ular assemblies with nanoscopic dimensions.[2] At about the
same time, Schlüter and co-workers began to report their
pioneering works on the synthesis and characterization of a
special kind of dendritic networks, also known as cylindrical
dendronized polymers,[3,4] based on an inner-sphere–inner-
sphere connection[2] approach, that is, the resulting dendron-
ized polymers may be viewed as constructed from the
connection of individual dendrons through their internal
focal-point functional groups. A large number of such inner-
sphere–inner-sphere connected dendronized polymers were
reported. Some of them were shown to have cylindrical
geometry[4] while some were known to self-assemble into
spherical nanostructures.[5] To our surprise, examples of
dendritic networks created by the alternative outer-spher-
e–outer-sphere connection strategy[2] are less studied. A few
examples were already reported but invariably they all
involve a random polymerization of dendrimer subunits that
have a large number of reactive functional groups on the
dendrimer surface.[6] We report herein a new variant of the
outer-sphere–outer-sphere connection strategy that allows us
to prepare a novel class of nanoscopic dendritic networks
(hereafter called dendritic necklaces because of their struc-
tural resemblance to untied necklaces) by the controlled
polymerization of dendrimer subunits containing only two
reactive surface functional moieties (Figure 1).[7] We also
show that the resulting dendritic necklaces, in contrast to
randomly connected dendritic networks, can be fully charac-
terized by solution techniques such as NMR, MS, GPC and
laser light scattering (LLS) to enable a thorough assessment
of their structural identity and purity. Employing organo-
[*] Prof. Dr. H.-F. Chow, C.-F. Leung, W. Li
Department of Chemistry
The Chinese University of Hong Kong
Shatin, NT (Hong Kong SAR)
Fax: (+852)26035057
E-mail: hfchow@cuhk.edu.hk
Dr. K.-W. Wong, L. Xi
Department of Physics
The Chinese University of Hong Kong
Shatin, NT (Hong Kong SAR)
[**] We thank the Research Grants Council, HKSAR (CUHK4201/99P)
for the financial support, Prof. D. T. W. Chan for mass spectral and
Prof. C. Wu and Mr. T. Ngai for LLS measurements
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Reagents and conditions: a) hydroquinone (0.45equiv),
Cs2CO3, dibenzo-24-crown-8, DMF, 1108C, 2 h.
Angew. Chem. Int. Ed. 2003, 42, 4919 –4923
DOI: 10.1002/anie.200351613
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4919

Communications
The G1–G3 diiodide surface-functionalized
dendrimers 4–6 were then converted to the
corresponding bis(trimethylsilylethynyl) den-
drimers 7–9, respectively, by the Sonogashira
coupling reaction with 1-trimethylsilylethyne.
The trimethylsilyl groups were then removed
under basic conditions to afford the target
bifunctional dendrimers 1–3, respectively, from
compounds 7–9. The structures of the target
monomers 7–9 were confirmed by 1H and
13C NMR and mass spectroscopic measure-
ments,[9] and their purity and homogeneity
(polydispersity index (PDI) = 1.04) were also
evaluated by GPC analysis (Figure 2).
Copolymerization reactions of a 1:1 molar
ratio (20–30 mm) of the bifunctional dendrim-
ers 1–3 and trans-[(Et3P)2PtCl2] were conducted
in the presence of CuI, diisopropylamine in
CH2Cl2 to afford G1–G3 dendritic necklaces
17–19, respectively (Scheme 3). The light
yellow organoplatinum products were isolated
by successive precipitation from the addition of
a CH2Cl2 solution to acetone and to methanol
1
and their structures were characterized by H,
13C and 31P NMR spectroscopy. Figure 3
1
showed the H NMR spectra of the G2-mono-
mer 2 and the resulting G2-dendritic necklace
18. Prior to Pt complexation, the terminal
acetylenic proton gave a sharp singlet at d =
3.0 ppm and an AB system (d = 7.4 and
6.8 ppm) due to the aromatic protons of the 4-
ethynylphenoxy surface groups. After complex-
ation, the positions of the AB system were
shifted upfield (d = 7.2and 6.7 ppm) while the
signal of the acetylenic proton due to the end
group could hardly be observed because of the
high degree of polymerization (DP ꢀ 90, see
below). The 31P{1H} NMR spectra of the G1-17
and G2- 18 dendritic necklaces consisted of one
31P signal located at about d = 17.5 ppm with
two 195Pt satellite signals [1J(Pt,P) ꢀ 2380 Hz] of
dendrimer 5 containing two aryl iodide surface moieties were
constructed by anchoring the mono-iodo surface-functional-
ized[10] G1-bromide 10 (2.2 equiv) and G2-bromide 11
(2.2 equiv), respectively, to a hydroquinone core in the
presence of Cs2CO3 in reasonable yields. Conversely, a
differentially (methoxymethyl (MOM) and benzyl) protected
hypercore 13[9] was used in the synthesis of the diiodo surface-
functionalized G3-dendrimer 6 (see Scheme 2). Hence, the
two MOM groups were cleaved under acidic conditions to
afford a bis(phenol) intermediate 14, which was then treated
first with 2.2 equivalents of the symmetrical G2-bromide 12 to
give a partially constructed G3-dendrimer 15. The benzyl
groups were then removed and the product 16 was reacted
with 2.2 equivalents of the unsymmetrical G2-bromide 11 to
give the G3-dendrimer 6.
Figure 2. GPC chromatograms of bis(ethynyl) surface-functionalized
dendrimers 1–3. A=UV absorbance, arbitrary units.
4920
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 4919 –4923

Angewandte
Chemie
one sixth intensity (Figure 4). Conversely, the
31P{1H} NMR spectrum of the G3 dendritic
necklace 19 consisted of one major 31P signal at
about the same position (d = 16.3 ppm). How-
ever, two additional 31P signals (d = 20.2 and
14.0 ppm) of lower intensities were also noted.
These signals probably arose from 31P reso-
nances of the end group, which became more
evident due to the lower DP value (ꢀ 30) of the
G3-dendritic necklace 19.
The molecular weights and distributions of
the necklaces 17–19 in THF were determined
by LLS and GPC studies (Table 1) and the
measured values were not influenced by the
monomer concentration used in the copoly-
merization. Furthermore, the molecular-
weight values of the G1-dendritic necklace 17
obtained from GPC studies were also inde-
pendent of the sample concentration used in
the measurements,[11] which suggests the values
in Table 1 reflect the molecular weights of
individual dendritic necklaces and not those of
the aggregates. The PDI values obtained were
rather large (ꢀ 2.0) and were independent of
dendrimer generation. Examination of the
LLS data indicated that this new outer-
sphere–outer-sphere
connection
strategy
could produce large nanoscopic-sized (Rg ꢀ
Scheme 2. Reagents and conditions: a) HCl, EtOH, THF, 08C, 1 h; b) 12 (2.2 equiv),
Cs2CO3, dibenzo-24-crown-8, DMF, 608C, 3 h; c) H2 (1 atm), 10% Pd/C, EtOH, EtOAc
(1:1), 258C, 6 h; d) 11 (2.2 equiv), Cs2CO3, dibenzo-24-crown-8, DMF, 608C, 3 h;
e) TMSC·CH, [Pd(PPh3)2Cl2], CuI, NEt3, PPh3, toluene, 608C, 24 h; f) K2CO3, MeOH,
THF, 258C, 1.5h.
100 nm) dendritic necklaces of high molecular
5
¯
weights (Mw > 1.8 ” 10 ). It was also noted that
the copolymerization was sensitive to steric
inhibition as the DP value dropped signifi-
cantly from 880 to 30 on going from the first to
the third generation. Nonetheless, the DP
values were still comparable to the high-end values of
dendronized polymers obtained from inner-sphere–inner-
sphere connection strategy.[4c] Furthermore, GPC meas-
¯
urements tended to produce much smaller Mw values as
polystyrenes were not good calibration standards for
¯
dendritic polymers. The ratio of the Mw values obtained
from LLS and GPC, values that indicated how much the
GPC results underestimated the real molar mass of the
tested compounds, showed a gradual decrease (from 14.0
to 1.5) from the G1- to the G3-dendritic necklace. The
ratios are much greater than unity as the hyperbranched
dendritic necklaces are expected to have a much larger
mass per unit length than polystyrene. However, as the size
of the dendritic beads was getting larger for the G3-
necklace, the long hydrocarbon linker groups were
Figure 3. 1H NMR (300 MHz, CDCl3) spectra of a) G2-dendritic necklace
18 and b) G2-dendrimer 2.
Scheme 3. Reagents and conditions: HN(iPr)2, CuI, CH2Cl2, 258C, 12 h.
Angew. Chem. Int. Ed. 2003, 42, 4919 –4923
www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4921

Communications
Table 1: LLS and GPC data of G1-G3 dendritic necklaces 17–19.
expected to stiffen and result in an
increase of hydrodynamic volume
and hence a decrease of the ratio.
A scanning tunneling micro-
scopic (STM) study on spin-casted
(3000 rpm) samples of dendritic
necklaces 17–19 from diluted THF
solutions (ꢀ 10 mgmLÀ1) on highly
Dendritic necklaces Yield% [a]
LLS[b]
DP[d]
GPC[c]
Mw (DLLS)/
¯
¯
3
3
¯
¯
Mw ”10
Rg [nm]
Mw ”10 PDI Mw (GPC)
G1 17
G2 18
G3 19
88
78
58
1500 (1.7)[e] 8.8”102 170
290 (3.1)
180 (5.8)
107[f]
82[g]
1.9 14.0
0.9”102 76
0.3”102 124
1.7
2.0
3.5
1.5
117[g]
[a] Yield after precipitation from solvents. [b] Solvent: THF. [c] Solvent: THF. Polystyrenes were used as
¯
calibration standards. [d] Calculated from Mw determined by LLS. [e] Values in brackets are the
calculated molecular weights of the repeating unit. [f] Sample concentration=2.7 mgmLÀ1. [g] Sample
oriented
pyrolytic
graphite
concentration=0.54 mgmLÀ1
.
(HOPG) showed that the necklaces
existed in aggregate form and
adopted
a coiled structure. An
increase in the spin-casting speed
to 5000 rpm facilitated the rate of solvent evaporation and
prevented aggregate formation, and the dendritic necklaces
were found across the steps of the freshly cleaved HOPG
surface as thin threads (Figure 5). The mean cross-section
Figure 5. STM images of dendritic necklaces a) 17 b) 18 and c) 19
spin-casted (5000 rpm) on HOPG.
diameter of the individual threads was found to increase from
1.1 to 1.6 and then to 2.0 nm on going from G1–G3, thus
showing that the size of the dendritic beads also played a role
in controlling the molecular dimensions of these dendritic
necklaces.
In summary, we report here a general method for the
synthesis of functional dendrimers that contain a limited
number of reactive functional groups on the dendritic surface
and also a new strategy for the construction of a novel class of
structurally defined dendritic necklaces by using an outer-
sphere–outer-sphere connection strategy. One notable fea-
ture of this method is that the dendritic necklaces thus
obtained have a very high degree of polymerization. We
believe this new method offers a new entry towards the
construction of nanoscale shape-controlled polymers. Two
important structural elements in this strategy are the length of
the linker groups and the size of the dendrimer beads, which
Figure 4. 31P{1H} NMR (161.9 MHz, CDCl3) spectra of dendritic neck-
laces a) G1 17, b) G2 18, and c) G3 19.
4922
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 4919 –4923

Angewandte
Chemie
are expected to be the key variables that can be used to
control the three dimensional morphology of the dendritic
networks. Hence, dendritic necklaces that have tight packing
dendrimer subunits may be realized by using shorter linker
molecules and larger dendrimer beads. Furthermore, multi-
block dendritic necklaces that have segregated regions can
also be constructed from dendrimer beads and linker units
that are of different physical properties.
Received: April 8, 2003 [Z51613]
Keywords: dendrimers · nanostructures · platinum · polymers
.
[1] The term “networks” coined by Newkome and Vögtle has a
different conception from that used in polymer chemistry. In
polymer chemistry, networks are generally referred to highly
cross-linking systems while dendritic networks do not necessarily
have a cross-linking architecture.
[2] a) G. R. Newkome, C. N. Moorefield, F. Vögtle, Dendritic
Molecules. Concepts. Syntheses. Perspectives, Wiley-VCH, Wein-
heim, 1996, chap. 9; b) G. R. Newkome, C. N. Moorefield, F.
Vögtle, Dendrimers and Dendrons. Concepts, Syntheses, Appli-
cations, Wiley-VCH, Weinheim, 2001, chap. 9.
[3] a) R. Freudenberger, W. Claussen, A.-D. Schlüter, H. Wallmeier,
Polymer 1994, 35, 4496 – 4501; b) W. Claussen, N. Schulte, A.-D.
Schlüter, Macromol. Rapid Commun. 1995, 16, 89 – 94.
[4] For reviews and highlights on cylindrical dendronized polymers,
see a) A. D. Schlüter, Top. Curr. Chem. 1998, 197, 165 – 191;
b) H. Frey, Angew. Chem. 1998, 110, 2313 – 2318; Angew. Chem.
Int. Ed. 1998, 37, 2193 – 2197; c) A. D. Schlüter, J. R. Rabe,
Angew. Chem. 2000, 112, 860 – 880; Angew. Chem. Int. Ed. 2000,
39, 864 – 883.
[5] V. Percec, C.-H. Ahn, G. Ungar, D. J. P. Yeardley, M. Möller, S. S.
Sheiko, Nature 1998, 391, 161 – 164.
[6] a) S. Watanabe, S. L. Regen, J. Am. Chem. Soc. 1994, 116, 8855 –
8856; b) J. Li, D. R. Swanson, D. Qin, H. M. Brothers, L. T.
Piehler, D. Tomalia, D. J. Meier, Langmuir 1999, 15, 7347 – 7350;
c) D. J. Díaz, G. D. Storrier, S. Bernhard, K. Takada, H. D.
Abruꢀa, Langmuir 1999, 15, 7351 – 7354; d) M. T. Reetz, D.
Giebel, Angew. Chem. 2000, 112, 2614 – 2617; Angew. Chem. Int.
Ed. 2000, 39, 2498 – 2501.
[7] A recent paper describing the synthesis of G1- and G2-dendritic
monomers that have bifunctional surface moieties for outer-
sphere–outer-sphere connection had appeared. However, no
polymerization study was disclosed, see R. N. Ganesh, J.
Shraberg, P. G. Sheridan, S. Thayumanavan, Tetrahedron Lett.
2002, 43, 7217 – 7220.
[8] K. Sonogashira, Y. Fujikura, T. Yatake, N. Toyoshima, S.
Takahashi, N. Hagihara, J. Organomet. Chem. 1978, 145, 101 –
108.
[9] See Supporting Information for details of synthetic procedures
and physical data.
[10] The methodology used in the synthesis of the surface-function-
alized G1-bromide 10 and G2-bromide dendrons 11 was similar
to that described in the literature, see K. L. Wooley, C. J.
Hawker, J. M. J. Frꢁchet, J. Chem. Soc. Perkin Trans. 1 1991,
1059 – 1076 for details.
¯
[11] The estimated GPC mass-average molar mass (Mw) of the G1-
dendritic necklace 17 at concentrations 0.11, 0.54, and
2.70 mgmLÀ1 were 1.05 ” 105, 1.04 ” 105 and, 1.07 ” 105 respec-
tively.
Angew. Chem. Int. Ed. 2003, 42, 4919 –4923
www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4923