Modular construction and hierarchical gelation of organooxotin nanoclusters derived from simple building blocks

Article abstract of DOI:10.1039/b710971g

Mixtures of an appropriate carboxylic acid and n-butylstannoic acid constitute modular gelation systems, in which the formation of a well-defined 'tin-drum' nanocluster subsequently underpins the hierarchical assembly of nanostructured fibres, which form self-supporting gel-phase networks in organic solvents. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710971g

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Modular construction and hierarchical gelation of organooxotin
nanoclusters derived from simple building blocks{{
Uwe Hahn,a Andrew R. Hirst,b Juan Luis Delgado,a Adrien Kaeser,a Be´atrice Delavaux-Nicot,a
Jean-Francois Nierengarten*a and David K. Smith*b
Received (in Cambridge, UK) 17th July 2007, Accepted 24th October 2007
First published as an Advance Article on the web 29th October 2007
DOI: 10.1039/b710971g
Mixtures of an appropriate carboxylic acid and n-butylstannoic
acid constitute modular gelation systems, in which the
formation of a well-defined ‘tin-drum’ nanocluster subsequently
underpins the hierarchical assembly of nanostructured fibres,
which form self-supporting gel-phase networks in organic
solvents.
Understanding the controlled self-assembly of nanoscale architec-
tures from simple well-defined molecular-scale building blocks
provides a major challenge to chemical science and a source of
nanostructured advanced materials.1 Over recent years, a wide
range of structurally diverse, low molecular weight gelators has
been developed.2 These molecules assemble through non-covalent
interactions into nanoscale fibres, which ultimately form sample-
spanning interpenetrated networks. These networks, although only
constituting a small percentage (e.g. 1% wt/vol) of the total sample,
are capable of preventing the flow of bulk solvent, through a
combination of capillary forces and solvent–gelator interactions. In
these materials, molecular scale information programmed by
organic synthesis is expressed on the macroscopic level via
nanoscale self-assembly. This approach generates soft materials
with diverse applications.3 Modular4 and multi-component5
gelation systems have been of specific interest as they have
exquisite tunability and there is great potential to introduce
function. Modular systems rely on the development of molecular
scaffolds which act as generic gelating units. Examples of multi-
component systems include complexes formed through acid–base
interactions6 or metal–ligand bonds7 which subsequently assemble
into fibres via other non-covalent interactions. This paper reports
our preliminary characterisation of a novel and unexpected
modular, multi-component gelation system, in which tin assembles
with carboxylate ligands to yield organooxotin nanocluster
modules, which then hierarchically assemble into gels.
Scheme 1 Gelators investigated in this paper.
this paper were synthesised by heating the appropriate carboxylic
acid and n-butylstannoic acid in benzene under Dean–Stark
conditions (Scheme 1) and fully characterised (see ESI{).
Intriguingly, during the synthesis, gelation of some samples
occurred. Although the supramolecular packing of tin-drums in
crystals has previously been explored,12 there have been no
previous reports of tin-drum gelators, and we therefore became
interested in the behaviour of these nanoclusters.
We investigated the gelation of 1b in benzene, using simple,
reproducible tube-inversion methodology to monitor the tempera-
ture of the gel–sol phase boundary (Tgel) at a range of
concentrations (Fig. 1).13 As commonly observed, increasing the
concentration of the gelator increased the Tgel value until a plateau
value was reached. Gelator 1b forms room temperature gels at
concentrations of ca. 5 mM (2.6 wt%), and the Tgel value then
increases to its plateau value of 57.5 uC at a concentration of
The formation of organooxotin clusters has been of consider-
able interest owing to the structurally diverse products which can
be generated in a simple one-pot assembly process (Scheme 1).8
For example, ferrocenes,9 porphyrins10 and fullerenes11 have all
been assembled around the ‘tin-drum’ nanocore. The clusters in
aLaboratoire de Chimie de Coordination du CNRS, 205 route de
Narbonne, 31077, Toulouse, Cedex 4, France.
E-mail: jfnierengarten@lcc-toulouse.fr; Fax: +33 (0)561 553003
bDepartment of Chemistry, University of York, Heslington, York, UK
YO10 5DD. E-mail: dks3@york.ac.uk; Fax: +44 (0)1904 432516
{ Electronic supplementary information (ESI) available: Synthesis and
characterisation data. See DOI: 10.1039/b710971g
Fig. 1 Tgel data as a function of concentration (in benzene) for gelator
1b.
{ The HTML version of this article has been enhanced with a colour
image.
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with peripheral aliphatic tails, are the most potent gelators,
reaching saturation at low concentrations (ca. 10 mM). Gelators
3b and 4b are less effective, requiring concentrations of 30 mM and
100 mM respectively to achieve saturation. In terms of the number
of molecules required for gelation, nanoclusters 1b and 2b are
therefore most effective – indicating that they have higher self-
assembly association constants. Furthermore, although 1b and 2b
have higher molecular masses (Mr) than 3b and 4b, they are also
more effective in terms of weight % loading. This indicates that
alkyl tails enhance the assembly process, possibly through van der
Waals interactions.
Gelators 1b, 2b and 3b, which contain aromatic rings, are all
significantly more effective than gelator 4b. In this case, the
aromatic rings are replaced with methyl ether groups indicating
that p–p stacking, as previously observed in tin-drum crystals,12
may play an important role in the assembly process.
Fig. 2 Tgel data determined in different solvents plotted as a function of
the normalised Reichardt solvent parameter – open circles indicate that
gelation was not observed.
10 mM (5.2 wt%). The increase in the macroscopic thermal
stability of the system on increasing concentration corresponds to
the evolution of a sample-spanning network. In the ‘plateau
region’, addition of further gelator no longer enhances the
materials properties, suggesting that the network has achieved its
optimal structure in terms of fibre growth and the number of
network contact points.
Interestingly, although nanoclusters 1b and 2b are the most
potent gelators at low concentration, once the plateau region is
attained the macroscopic thermal stability is relatively independent
of the peripheral groups, with the Tgel values of all gelators being
between 50 uC and 65 uC. This indicates that although 1b and 2b
have higher affinities for self association, their ‘plateau-region’
network formation is somehow limited (see below).
We considered the possibility that the gelation process may
represent a chemical change in the tin-drum nanocluster. However,
in all cases, gelation was thermoreversible. Furthermore, there was
no difference in the NMR spectra (1H, 13C and 119Sn) before and
after gelation, demonstrating that the gelation of these nanoclus-
ters is a physical process based on non-covalent interactions
between clusters and does not correspond to a chemical
transformation.
Nanocluster 5b only formed gels in 1-bromonaphthalene – in
other solvents either dissolution or precipitation occurred. In
1-bromonaphthalene, a cloudy gel with a plateau region Tgel value
of 50 uC was observed – effective gelation given that this solvent is
more polar than toluene." Interestingly, 5b is the only nanocluster
in which the aromatic ring is not substituted by ethers. We propose
that the absence of ethers modifies the solubility properties of 5b,
highlighting the fact that gelation is a delicate balance between
solvation, self-assembly and precipitation events.16
We next investigated the influence of solvent on the gelation of
1b in order to gain further insight into the self-assembly
mechanism.14 The thermal properties of 1b were determined at
constant concentration (20 mM) in a range of different solvents.
The Tgel values could be related to the solvent polarity parameter
Field emission gun scanning electron microscopy (FEGSEM) of
the xerogel of 1b (Fig. 3, dried from benzene) indicated the
formation of ‘one-dimensional’ interpenetrated nanofibres ca.
50 nm in width. Surprisingly, however, these fibres were relatively
short, typically having lengths of only ca. 400 nm. Usually, the
fibres underpinning gels are many micrometres long.2 It has
been reported that steric hindrance on the periphery of fibrillar
architectures can limit the lengths to which they grow;17 we propose
that the alkyl chains on the periphery of nanocluster 1b may
limit fibre growth and hence explain why the Tgel of nanocluster 1b
was limited in the plateau regime. Unfortunately all other gels
proved difficult to image reproducibly from benzene.
N
ET (Reichardt’s polarity parameter) (Fig. 2).15 In general terms,
N
Tgel decreased as ET increased,§ and if the solvent became too
polar, then gelation was no longer observed. This indicates that the
non-covalent interactions between the clusters which underpin
gelation have an electrostatic component (e.g. dipole–dipole
interactions between Sn(d+) and O(d2)).
We then carried out a thermal investigation of all of these new
gels (Table 1), determining the concentration and weight %
required for the onset of network ‘saturation’, as well as the Tgel
value in the plateau region. Modifying the peripheral groups of the
organooxotin nanoclusters directly controls the concentration at
which the plateau region is established. Nanoclusters 1b and 2b,
Wide-angle X-ray powder diffraction (XRD) can help elucidate
the molecular structure of organogels.18 We therefore performed a
preliminary investigation of the xerogel of 1b (dried from benzene),
and the XRD pattern (Fig. 4A) revealed a series of sharp peaks,
Table 1 Summary of thermal characterisation of gelators 1–5 in
benzene
Onset of
plateau
region/mM
Onset of
plateau
region/wt%
Plateau
region
Tgel/uC
Code
Mr
1b
2b
3b
4b
5ba
a
5195.3
5700.3
2237.8
1589.2
1877.5
10
10
30
100
50
5.2
5.7
6.7
15.9
9.4
57.5
59.5
62.5
53.5
50
Solvent: 1-bromonaphthalene.
Fig. 3 FEGSEM image of gelator 1b as a benzene xerogel.
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4944 | Chem. Commun., 2007, 4943–4945

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assembly of nanostructures incorporating different carboxylic
acids. This will yield greater control over self-assembly, tune the
materials properties of the gels, and yield highly functional
hierarchical hybrid nanomaterials.
We thank the SCIC and Dr B. Escuder for technical assistance
and general advice regarding XRD. Dr A. Hirst acknowledges the
University Jaume I, Spain for a visiting researcher mobility grant.
The German Academic Exchange Service (DAAD) is acknowl-
edged for a postdoctoral fellowship to Dr U. Hahn. DKS thanks
EPSRC for funding (EP/C520750/1).
Notes and references
§ In previous hydrogen-bond mediated gels we have observed dependence
on the Hildebrand polar solubility parameter;14a however, in this case, no
correlation was observed.
" Although the normalised Reichardt parameter for 1-bromonaphthalene
is not provided in reference 15, those of 1-chloronaphthalene and
1-iodonaphthalene are 0.194 and 0.191 respectively.
1 (a) G. M. Whitesides, Small, 2005, 1, 172–179; (b) I. W. Hamley, Angew.
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R. G. Weiss and P. Terech, Springer, Heidelberg, 2006; (b) D. K. Smith,
Tetrahedron, 2007, 63, 7283–7284.
Fig. 4 XRD patterns of the xerogels (from benzene) formed from (A)
gelator 1b and (B) gelator 3b.
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indicative of ordered structures. The first three peaks could be
˚
˚
˚
indexed as hexagonal packing (23.47 A, 13.83 A, 11.99 A,
representing spacings of 1, 1/!3, 1/!4, respectively). Only the major
peaks could be indexed – the minor peaks may represent different
structuring, or a degree of polymorphism, in the powder. Gelator
3b had broader diffraction peaks, which indicates that in the
absence of alkyl tails the packing is less ordered, reinforcing the
importance of these groups in generating an ordered assembly.
Again, the peaks could be preliminarily indexed to hexagonal
˚
˚
˚
packing (12.60 A, 7.23 A, 6.30 A), the smaller dimensions being
consistent with smaller nanoclusters (i.e. no alkyl tails). More
detailed X-ray studies will be a focus of future work on this system.
Crystallography10,12 indicates that tin-drum nanoclusters are
flattened cylinders with the carboxylic acid ligands arranged
around the equator. Disc-like objects are an appropriate shape for
self-assembly (stacking) into columnar structures.12,19 Indeed, a
single crystal X-ray structure of compound 5b (to be reported
elsewhere), which is similar to 3b, indicated a cluster diameter of
˚
19.7 A and height of 14.5 A. Due to interdigitation, the cluster–
˚
˚
cluster repeat length was ca. 13 A. Our results are consistent with a
model in which the nanoclusters assemble into hexagonally packed
columnar fibrils. We propose that columnar aggregation is driven
by electrostatic interactions, for example between Sn(d+) and
O(d2), combined with p–p stacking interactions for 1–3b.
Furthermore, van der Waals interactions and the packing of alkyl
chains support assembly for 1–2b.
In conclusion, this preliminary communication reports a novel
multi-component gelation system based on the assembly of ‘tin-
drum’ modules which hierarchically assemble into fibrillar
nanomaterials. The clusters form as a consequence of tin–
carboxylate (metal–ligand) interactions, and then hierarchically
assemble into fibres as a consequence of polar interactions (dipole–
dipole interactions and p–p stacking) supported by van der Waals
forces. The nanocluster module is highly tunable – a wide range of
different carboxylic acids may be employed to assemble structures
of this type. Further work is currently in progress to explore the
13 For a typical example see: A. R. Hirst, D. K. Smith, M. C. Feiters and
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´
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