Synthesis of highly functionalised dendrimers based on polyhedral silsesquioxane cores

Article abstract of DOI:10.1039/a801510d

A number of dendrimers based on polyhedral silsesquioxane cores have been synthesised and characterised. The new molecules are prepared by repetitive hydrosilation/alkenylation reactions, which provide a facile and high yield route to dendrimers with a high density of branch ends per generation. Judicious choice of hydrosilating agent [HSiCl₃, H(CH₃)SiCl₂ or H(CH₃)₂SiCl] produces dendrimers with varying numbers of chain ends, and alkenylating agents of different lengths produce molecules with different physical properties. The 24-vinyl functionalised dendrimer, 5, has been characterised using single-crystal X-ray diffraction techniques. Copyright 1998 by the Royal Society of Chemistry.

Full text of DOI:10.1039/a801510d

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Synthesis of highly functionalised dendrimers based on polyhedral
silsesquioxane cores†
Paul-Alain Jaffrès and Russell E. Morris*
School of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY16 9ST
A number of dendrimers based on polyhedral silsesquioxane cores have been synthesised and characterised.
The new molecules are prepared by repetitive hydrosilation/alkenylation reactions, which provide a facile and
high yield route to dendrimers with a high density of branch ends per generation. Judicious choice of
hydrosilating agent [HSiCl , H(CH )SiCl or H(CH ) SiCl] produces dendrimers with varying numbers of
3
3
2
3 2
chain ends, and alkenylating agents of different lengths produce molecules with different physical properties.
The 24-vinyl functionalised dendrimer, 5, has been characterised using single-crystal X-ray diffraction
techniques.
Dendrimers are molecules with a globular structure in which
well defined branches radiate from a central core, becoming
more branched and crowded as they extend out to the peri-
the exterior of the molecules, generally in high yield with reten-
tion of the silsesquioxane structure intact. Examples of reac-
tions of POSS species can be found in the recent review by
1
10
phery. Since the first successful synthesis of a symmetrical
Lichtenhan and references therein. We report here a facile
2
branched dendrimer, this class of polymers has received con-
divergent synthetic route to dendrimers using these POSS
molecules as the core. These preparative routes produce a high
density of functional groups on the exterior of the dendrimer
without resorting to repetitive synthesis of large numbers of
generations. These molecules have been previously used as core
species to produce what can be described as zeroth generation
dendrimers, for example, the octopus molecules prepared
siderable interest, with possible applications ranging from drug
delivery agents, micelle mimics, nanoscale building blocks, high
3
performance polymers and nanoscale reactors. The rigid
spheroidal architecture of these molecules leads to properties
such as low intrinsic viscosity, high solubility and miscibility,
and high reactivity (from the presence of many chain ends). An
alternative application that has achieved some attention is that
1
1,12
by Bassindale and Gentle
and the ferrocene derivatised
4–6
13
of catalysis, where dendrimers have some striking advantages
over both homogeneous and heterogeneous catalysts includ-
ing the possibility of preparing highly active yet recyclable
catalysts.
octasilsesquioxane species of Moràn et al. More recently,
Hong and co-workers and Feher have reported the use of
polyhedral silsesquioxanes as cores for dendrimers, based on
14
functionalising phosphine derivatised dendrimers, and func-
Dendrimers are generally prepared using two approaches; a
divergent method where successive dendrimer layers (gener-
ations) are added to a core and a convergent method where the
arms of the dendrimer are synthesised first and subsequently
attached together at a focal point to produce the final molecule.
A major drawback with dendrimer synthesis, especially the
divergent method, is that it often requires many repetitive steps
in order to build the dendrimer outwards leading to multi-step
and often low yield preparations. Our goal is to prepare den-
drimers that can be used as supports for catalysts, combining a
high number of active sites on the exterior of the molecule with
the possibility of separating the catalyst from the reaction mix-
ture using ultrafiltration techniques, so taking advantage of the
relatively large size of the dendrimers to produce a molecule
that combines the advantages of homogeneous and hetero-
geneous catalysts while minimising the disadvantages of each.
Recently, a large amount of literature has emerged based on
the use of pentacyclooctasiloxanes (more commonly known
as polyhedral oligomeric silsesquioxanes or POSS, which are
molecules of formula R Si O ; R = H, CH , vinyl, C H , etc.)
tionalising the rather unstable γ-aminopropyl-derivatised
silsesquioxane core respectively.
15
Results
The synthesis of the dendrimer molecules was accomplished
according to Scheme 1, using a similar repetitive hydrosilation/
16–18
vinylation procedure to those published previously.
The 24-
Cl dendrimer, 2, can be prepared in quantitative yield (as mea-
sured by NMR spectroscopy) via the hydrosilation of octavinyl-
silsesquioxane, 1, using trichlorosilane. This compound can
also be synthesised by the hydrosilation of octahydridosilsesqui-
oxane, using vinyltrichlorosilane, although compound 1 is the
preferred starting material as it is extremely simple to syn-
19
thesise (or it can be obtained from Aldrich); the bulky nature
of the silsesquioxane means that the unwanted side reaction,
where addition to the vinyl groups occurs in the β-position, is
suppressed. Hydrosilation of compound 1 can also be com-
pleted with a number of other chlorosilane molecules, including
dichloromethylsilane and chlorodimethylsilane, to produce
dendrimers such as the 16-Cl and 8-Cl derivatised molecules,
3 and 4, shown in Fig. 1. This illustrates a method by which
we have control over the number of reactive chain ends a
dendrimer possesses.
8
8
12
3
6
11
7–9
as building blocks from which polymers can be constructed.
Polyhedral silsesquioxane molecules are easy to prepare in high
yield (some are commercially available through Aldrich) and
standard organic manipulations can be applied to functionalise
The vinylation of 2 using vinylmagnesium bromide produces
the 24-vinyl dendrimer, 5, in 81% yield. Compound 5 can then
be used as a starting material for the subsequent preparation of
the second generation dendrimer by hydrosilation with tri-
chlorosilane (to produce the 72-Cl dendrimer 6) and by a repeat
of the vinylation reaction to produce the 72-vinyl dendrimer 7,
although the yield from this last reaction is reduced (to 21%)
*
†
E-mail: rem1@st-andrews.ac.uk
Supplementary data available: experimental details and characteris-
ation for compounds 2–11. For direct electronic access see http://
www.rsc.org/suppdata/dt/1998/2767/, otherwise available from BLDSC
(
No. SUP 57403, 8 pp.) or the RSC Library. See Instructions for
Authors, 1998, Issue 1 (http://www.rsc.org/dalton).
J. Chem. Soc., Dalton Trans., 1998, Pages 2767–2770
2767

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Cl
Cl
Cl
Cl
Cl
Cl
Si
Si
Cl
Si
Si
O
O
Si
Cl
Cl
Cl
Cl
O
O
O
Si
O
O
Si
O
Cl
Si
(i)
O
O
Cl
Cl
Cl
Cl
Si
Cl
Si
Si
Si
O
Si
O
Si
Si
Cl Cl
Cl
Cl
Si
Cl
Cl
Cl
1
2
(ii)
Si
Si
Cl
Si
Si
Si
(iii)
Si
Cl
Cl
Si8O12
Si
3
Si
8
Si
6
Si
(iv)
5
Si
Si8O12
Si
3
8
7
Scheme 1 Synthetic scheme for the production of 24-Cl dendrimer, 2, 24-vinyl dendrimer, 5, 72-Cl dendrimer, 6 and the 72-vinyl dendrimer 7. (i)
HSiCl , Speier’s catalyst, Et O, reflux, 8 h; (ii) CH CHMgBr, THF, 80 ЊC, 24 h; (iii) HSiCl , Speier’s catalyst, Et O, reflux, 8 h; (iv) CH CHMgBr,
3
2
2
3
2
2
THF, 80 ЊC, 18 h
compared to the lower generations. All molecules have been
characterised spectroscopically and the 24-vinyl dendrimer, 5,
has also been partially characterised using single-crystal
X-ray diffraction, although because of extensive disorder of the
carbon atoms at the periphery of the dendrimer, only a partial
structure refinement was possible. Fig. 2 shows the results of
this diffraction study. Taken in conjugation with the spectro-
scopic and analytical data reported in this paper, the diffraction
experiment confirms the molecular structure of 5 as the defect-
free 24-vinyl functionalised dendrimer.
added to the next generation of dendrimer depends on the
number of reactive sites on the core and how many monomers
can be added to each reactive site (their valency). For previous
carbosilane dendrimers the maximum number of reactive sites
on the core is four (using tetravinylsilane or tetraallylsilane),
and on successive hydrosilation/vinylation reactions involving
16–18
the trivalent SiCl unit as the reactive site a branching progres-
3
sion of 4→12→36→108 occurs for the zeroth, first, second and
third generation dendrimers respectively. Utilising the octafunc-
tionality of a polyhedral oligomeric silsesquioxane core we get
a progression of 8→24→72→216 for the same generations,
providing a route to silane dendrimers with a high density of
exterior functionalised sites with fewer steps than previously
published methods. Other examples of using core and branch-
ing units of high multiplicity to form dendrimers with a large
number of chain ends can be found in the review by Tomalia
The 24-Cl dendrimer, 2, can also be reacted with allyl-
magnesium bromide to produce the 24-allyl dendrimer, 8. This
reaction occurs in high yield and the crude product is almost
1
pure (within the detection limit of H NMR spectroscopy).
All of the chloro-functionalised dendrimers undergo similar
alkenylation reactions, all in relatively high yield, to produce
dendrimers with varying numbers of chain ends. For example,
the 16-Cl dendrimer 3 also undergoes this reaction under
similar conditions to produce molecule 9.
20
and Hurst. Dendrimers with polyhedral silsesquioxane cores
will also approach their De Gennes limit (the size where surface
congestion becomes too great for further defect-free gener-
ations to be added) in fewer generations than those prepared
with other cores. These molecules will also be more spherically
symmetric for the lower generation numbers than other
dendrimers because the spatial constraints imposed by eight
dendrimer arms are greater than those of other core molecules.
The use of octafunctional polyhedral oligomeric silsesqui-
Discussion
Dendrimers synthesised using the divergent method start with a
core, representing the zeroth generation, possessing one or
more reactive sites. The number of monomer units that can be
2
768
J. Chem. Soc., Dalton Trans., 1998, Pages 2767–2770

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Fig. 1 Molecular structures of compounds 3, 4, 8, 9, 10 and 11. The
cubic silsesquioxane core is shown as lines linking the silicon atoms.
The oxygen atoms would be near the centres of these lines
oxane cores provides an easy route to highly functional den-
drimer molecules. They may also be advantageous in tuning the
properties of the dendrimer to suit the environment in which a
particular application will occur. For example, much of the
interest in polyhedral silsesquioxane molecules in polymer
chemistry is in their use as ‘hard blocks’ to modify the thermal
Fig. 2 (a) Ball and stick and (b) thermal ellipsoid representations of
the crystal structure of the 24-vinyl dendrimer, 5. For clarity, the
10
and mechanical stability of materials. The use of POSS
molecules in dendrimers should have a similar effect, leading to
dendrimers with different properties from those based on, for
example, tetravinylsilane cores. The properties of the dendrimer
can also be altered by changing the length of the carbon chain
between branching silicon atoms by using different alkenyl
Grignard reagents in the alkenylation step. This will have a
marked effect on the physical characteristics of the molecules,
especially, of course, their size and the number of generations
that can be synthesised before surface congestion becomes the
size-limiting factor. It will almost certainly play an important
role in any catalytic utility of these molecules, affecting, for
example, the selectivity of a catalyst. The effect of the chain
length on the physical properties of the dendrimers can be seen
clearly by noting the difference in appearance of the 24-vinyl
dendrimer, which is a crystalline solid, and the 24-allyl den-
drimer, which is a viscous liquid. Similar variations in physical
properties have been reported for other carbosilane dendrimers,
where varying the length of alkenylating agents from C to C
disordered vinyl groups are not shown on the thermal ellipsoid plot
SiCl3 groups towards nucleophiles, and vinyl chain ends
towards addition reactions, should allow facile derivatisation of
these dendrimers with useful catalytic functionality.
Experimental
All manipulations were carried out under an atmosphere of
argon. Solvents were dried according to established procedures.
1
6
Compound 1 was synthesised according to reported methods.
13
Proton NMR spectra were recorded at 300.13 MHz, C NMR
spectra at 75.47 MHz and Si NMR spectra at 59.63 MHz on
a Bruker AM300 spectrometer operating in the Fourier-
transform mode with, for C spectra, noise proton decoupling:
reference Si(CH ) . The FTIR spectra were recorded on a
29
13
3
4
Perkin-Elmer 1710 spectrometer. Chemical analysis was accom-
plished by the University of St. Andrews Microanalysis service.
Example synthetic procedures follow. Complete methods can
be found in SUP 57403.
2
10
2
1
produced dendrimers varying markedly in physical properties.
The two types of dendrimer reported here, chloro- and vinyl-
functionalised, are ideal platforms on which to build synthetic
strategies aimed at the preparation of new supported catalysts
and we are continuing our research in order both to optimise
the synthetic conditions for their synthesis and to utilise the
molecules as catalyst supports. The high reactivity of the
Syntheses
1,3,5,7,9,11,13,15-Octakis[2-(trichlorosilyl)ethyl]pentacyclo-
3
,9 5,15 7,13
[9.5.1.1 .1 .1 ]octasiloxane 2. The compounds HSiCl (6
3
Ϫ1
i
ml, 59.3 mmol) and H [PtCl ] (0.1 mol L in Pr OH, 10 drops)
2
6
J. Chem. Soc., Dalton Trans., 1998, Pages 2767–2770
2769

View Article Online
were added to a solution of octavinylpentacyclooctasiloxane
(1.0 g, 1.58 mmol) in diethyl ether (50 ml). The resulting
a partial structure refinement could be accomplished, but
2 2
1
agreement values of R(F ) = 0.17, RЈ(F ) of 0.25 and goodness
of fit of 2.9 were obtained for a full-matrix least-squares refine-
ment against 2309 reflections, of which 1325 were observed
according to the criterion I > 3σ(I). The paucity of data is pre-
sumably due to the intrinsic high levels of thermal motion and/
or disorder at the periphery of the dendrimer. These problems
are highlighted by the large thermal displacement factors
associated with all the atoms. The silicon and oxygen and the
eight methylene carbon atoms were refined with anisotropic
thermal displacement parameters, and the other carbon atoms
isotropically. The carbon atoms of the vinyl groups were
located in Fourier-difference maps, but the quality of the crys-
tal data is such that we were unable to refine their positions
without bond distance and angle restraints. There is evidence in
the Fourier-difference maps for a number of alternative posi-
tions for the terminal vinyl carbon (i.e. there may be some dis-
order of the vinyl groups), but these could not be modelled
satisfactorily. The experiment does, however, confirm the basic
connectivity of compound 5.
mixture was heated at reflux for 8 h and at 20 ЊC for 15 h. The
solvent was removed in vacuo to give 2.65 g (97.8%) of 2 as a
white solid. Compound 2 can also be synthesised by adding
Ϫ1
i
H [PtCl ] (0.1 mol L in Pr OH, 2 drops) to a solution of
2
6
22
octahydridopentacyclooctasiloxane (0.15 g, 0.35 mmol) and
vinyltrichlorosilane (9 ml, 70 mmol). The resulting mixture was
heated at reflux for 22 h. The solvent was removed in vacuo to
1
give 0.77 g (100%) of 2 as a white solid. H NMR (CDCl ): δ
3
1
3
0
3
.95 (m, CH , 16 H), 1.45 (m, CH , 16 H). C NMR (CDCl ): δ
2 2 3
29
.41 (O SiCH ), 16.77 (CH SiCl ). Si NMR (CDCl ): δ 12.82
3
2
2
3
3
(
SiCl ), Ϫ67.48 (O SiC).
3
3
1
,3,5,7,9,11,13,15-Octakis[2-(trivinylsilyl)ethyl]pentacyclo-
3,9 5,15 7,13
[
(
9.5.1.1 .1 .1 ]octasiloxane 5. Vinylmagnesium bromide
1  in THF, 45 ml) was added to a solution of 2 (2.65 g, 1.54
mmol) in THF (40 ml). The resulting solution was stirred at
room temperature for 17 h. Ammonium chloride (1  in H O,
2
5
0 ml) was added and the solution was extracted with diethyl
ether (2 × 80 ml). The combined organic layers were washed
CCDC reference number 186/1059.
with brine, dried over Na SO and concentrated in vacuo. The
residue (2.33 g) was loaded onto a column of silica gel and
See http://www.rsc.org/suppdata/dt/1998/2767/ for crystallo-
graphic files in .cif format.
2
4
eluted with 95:5 hexane–diethyl ether to afford compound 5
(
5
(
1.90 g, 81%) (Found: C, 50.7; H, 6.9. C H O Si requires C,
64
104 12 16
Acknowledgements
1
0.8; H, 7.1%). H NMR (CDCl ): δ 0.62 (m, CH , 16 H), 0.76
3 2
3 3
m, CH , 16 H), 5.78 (dd, J = 7.4, J = 16.8 Hz, SiCH, 24
We thank the Engineering and Physical Sciences Research
Council for support and R. E. M. thanks the Royal Society of
Edinburgh for the provision of a Scottish Office Education and
Industry Department Personal Research Fellowship.
2
HH
HH
13
H), 6.08–6.21 (m, CH᎐CH , 48 H). C NMR (CDCl ): δ 4.13
(
CH ). Si NMR (CDCl ): δ Ϫ18.2 [Si(CH᎐CH ) ], Ϫ67.48
(
᎐
2
3
CH Si). 4.34 (O SiCH ), 134.32 (CH᎐CH ), 134.66 (CH᎐
2 3 2 2
29
2
3
2 3
Ϫ1
O SiC). IR/cm (KBr disc): 3050m, 2945m, 1593m (C᎐C),
3
1
404s, 1263m, 1116vs (SiOSi), 1009s, 955s, 710s, 595s, 470m.
,3,5,7,9,11,13,15-Octakis{2-{tris[2-(trichlorosilyl)ethyl]silyl}-
References
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2 D. Tomalia and R. Dewald, US Pat., 4 507 466, 1985.
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heated at reflux for 27 h. The solvent was removed in vacuo to
Ϫ1
3 R. Dagani, Chem. Eng. News, 1996, June 3, 30, and refs. therein.
3
2
6
4
5
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i
13
6
7
J. Haggin, Chem. Eng. News, 1995, Feb. 6, 26.
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3
P. Harrison and R. Kannengieser, Chem. Commun., 1996, 415.
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2
3
3
2
29
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CH SiCl ). Si NMR (CDCl ): δ 12.31 (SiCl ), 11.02 (CH Si),
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Ϫ67.02 (O Si).
3
(
Am. Chem. Soc. Div. Polym. Chem.), 1996, 37, 765.
1
,3,5,7,9,11,13,15-Octakis{2-{tris[2-(trivinylsilyl)ethyl]silyl}-
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1
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(4.68 g, 1.03 mmol) in THF (50 ml). The resulting solution
was stirred at room temperature for 16 h and refluxed for 2 h.
Ammonium chloride (1  in H O, 50 ml) was added and the
solution was extracted with diethyl ether (2 × 80 ml). The com-
bined organic layers were washed with brine, dried over Na SO4
and concentrated in vacuo. The residue (4.63 g) was loaded onto
a column of silica gel and eluted with 95:5 hexane–diethyl ether
to afford compound 7 (0.89 g, 21%). C NMR (CDCl ): δ 2.55
br, CH CH ), 4.86 (br, CH CH ), 134.55 (br, CH᎐CH ). Si
NMR (CDCl ): δ 10.45 (CH SiCH ), Ϫ18.25 (CH SiCH᎐
CH ), Ϫ66.59 (O Si).
3
91.
6
1
3 M. Moràn, C. M. Casado, I. Cuadrado and J. Losada,
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(
2 2 2 2 2
19 T. N. Martynova and V. P. Korchkov, Zh. Obshch. Khim., 1982, 52,
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2
3
2
2
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1
Crystal structure determination of compound 5
Crystal data. C H O Si , M = 1522.9, monoclinic, a =
3.731(1), b = 13.869(1), c = 23.37(1) Å and β = 94.36(5)Њ, U =
437(4) Å , T = 220 K, space group P2 /c (no. 14), Z = 2, µ(Mo-
Kα) = 2.68 mm , 2309 unique reflections measured. The crys-
2
64
104 12 16
997, 9, 1475.
1
4
3
1
Ϫ1
tal showed essentially no diffraction beyond 2θ 30Њ and so only
Received 23rd February 1998; Paper 8/01510D
©
Copyright 1998 by the Royal Society of Chemistry
2
770
J. Chem. Soc., Dalton Trans., 1998, Pages 2767–2770