Self-Assembly of organostannoxanes: Formation of gels in aromatic solvents

Article abstract of DOI:10.1021/om900474a

Organostannoxane drums [n-BuSn(O)O₂C-C₆H ₄-4-OR]₆[R = -CH₃ (1); -C₉H ₁₉ (2); -C₁₁H₂₃ (3)] and [n-BuSn(O)O ₂C-C₆H₃-3,5-(OR)<

Full text of DOI:10.1021/om900474a

Organometallics 2009, 28, 4593–4601 4593
DOI: 10.1021/om900474a
Self-Assembly of Organostannoxanes: Formation of Gels
in Aromatic Solvents
Vadapalli Chandrasekhar,*,† Kandasamy Gopal,† Puja Singh,†
Ramakirushnan Suriya Narayanan,† and Andrew Duthie‡
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India, and ‡School of
Life and Environmental Sciences, Deakin University, Geelong Victoria 3217, Australia
Received June 4, 2009
Organostannoxane drums [n-BuSn(O)O2C-C6H4-4-OR]6 [R = -CH3 (1); -C9H19 (2); -C11H23 (3)]
and [n-BuSn(O)O2C-C6H3-3,5-(OR)2]6 [R = -CH3 (4); -C9H19 (5)] were synthesized by the
reaction of n-BuSn(O)(OH) with the corresponding carboxylic acid in a 1:1 stoichiometry. Analo-
gous reactions involving [n-Bu2SnO]n in a 1:1 stoichiometry afforded the diorganostannoxane ladders
{[n-Bu2SnO2C-C6H4-4-OR]2O}2 [R = -CH3 (6); -C9H19 (7); -C11H23 (8)] and {[n-Bu2SnO2C-
C6H3-3,5-(OR)2]2O}2 [R = -CH3 (9) and -C9H19 (10)]. Compounds 1-10 could also be prepared
by a solventless methodology, which involved grinding the reactants together in a mortar and pestle at
room temperature. Compounds 1-10 exhibit gelation behavior in aromatic solvents. In contrast, in
aliphatic solvents gelation behavior was not observed. Among the organostannoxanes reported here, 2,
3, 5, and 8 were found to be extremely efficient gelators based on their critical gelation concentration
values. The microstructure of the organometallic gels, investigated by optical and scanning electron
microscopy, reveals the presence of cross-linked network structures. The gels formed from 2 and 3 can
be converted into xerogels by removal of solvent. The latter can be reconverted into the original gels by
treatment with aromatic solvents.
Introduction
core)-supported multiferrocene, -porphyrin, -fluorene,
and pyrazolyl assemblies.4-9 Similarly, Nierengarten and
co-workers have reported the preparation of hexa- and
dodecafullerene assemblies supported on a organotin drum
core.10
We were interested in examining if the organostannox-
ane synthesis strategy can be extended to the preparation of
organometallic gels. Recently, there has been a growing
interest in the study of low molecular weight organogels
and metallogels, owing to diverse applications in drug
delivery, sensors, templates for synthesis of nanostruc-
tures, biomimetics, etc.11 Although many gelators are
polymers, there has also been a great deal of interest in
Organotin cages and clusters have been of interest be-
cause of the large structural diversity that is present in this
family and also because of their importance in catalysis and
other applications.1-3 Most of these compounds are pre-
pared, in general, by reactions involving organotin oxides,
hydroxides, or oxide-hydroxides with protic acids. For
example, the reaction of n-butylstannonic acid, [n-BuSn-
(O)OH]n, with various carboxylic acids affords dendrimeric
hexanuclear drums, [n-BuSn(O)O2CR]6, containing
a
[Sn6O6] core. Similarly, the reaction of protic acids with
[n-Bu2SnO]n affords tetranuclear ladders, {[n-Bu2SnO2-
CR]2O}2. The reactions of organotin oxides/oxide-hydro-
xides with phosphorus-based acids are more complex and
have been a rich source of structurally complex organotin
cages. In spite of the complexity of the products, the current
state-of-the art in organostannoxane synthesis is reasonably
advanced and allows the assembly of desired organotin
architectures. It is therefore possible to design organotin
compounds and decorate the organostannoxane cores with
suitable functional peripheries. Utilizing this strategy we
have recently reported organooxotin-(drum and ladder
(4) Chandrasekhar, V.; Nagendran, S.; Bansal, S.; Kozee, M. A.;
Powel, D. R. Angew. Chem., Int. Ed. 2000, 19, 1833.
(5) Chandrasekhar, V.; Nagendran, S.; Bansal, S.; Cordes, A. W.; Vij,
A. Organometallics 2002, 21, 3297.
(6) Chandrasekhar, V.; Gopal, K.; Nagendran, S.; Singh, P.; Steiner,
A.; Zacchini, S.; Bickley, J. F. Chem.-Eur. J. 2005, 11, 5437.
(7) Chandrasekhar, V.; Nagendran, S.; Azhakar, R.; Kumar, M. R.;
Srinivasan, A.; Ray, K.; Chandrashekar, T. K.; Madavaiah, C.;
Verma, S.; Priyakumar, U. D.; Sastry, G. N. J. Am. Chem. Soc. 2005,
127, 2410.
(8) Chandrasekhar, V.; Thilagar, P.; Sasikumar, P. J. Organomet.
Chem. 2006, 691, 1681.
*To whom correspondence should be addressed. E-mail: vc@iitk.ac.
in. Phone: (þ91) 512-259-7259. Fax: (þ91) 521-259-0007/7436.
(1) Chandrasekhar, V.; Nagendran, S.; Baskar, V. Coord. Chem. Rev.
2002, 235, 1.
(9) Chandrasekhar, V.; Thilagar, P.; Steiner, A.; Bickley, J. F.
Chem.;Eur. J. 2006, 12, 8847.
(10) Hahn, U.; Gegout, A; Duhayon, C.; Coppel, Y.; Saquet, A.;
Nierengarten, J. F. Chem. Commun. 2007, 516.
(2) Chandrasekhar, V.; Gopal, K.; Thilagar, P. Acc. Chem. Res. 2007,
40, 420.
(3) Holmes, R. R. Acc. Chem. Res. 1989, 22, 190.
(11) (a) Sangeetha, N.; Maitra, U. Chem. Soc. Rev. 2005, 34, 831.
(b) Ajayaghosh, A.; Praveev, V. K.; Vijayakumar, C. Chem. Soc. Rev.
2008, 37, 109.
r
2009 American Chemical Society
Published on Web 06/25/2009
pubs.acs.org/Organometallics

4594 Organometallics, Vol. 28, No. 15, 2009
Chandrasekhar et al.
Scheme 1
recent years in discovering small molecules that can form
gels in organic and aqueous solutions. Examples of non-
polymeric gelators reported in the literature include ster-
oids, alkylamide derivatives, and fatty acids. The gelation
property of these small-molecule gelators arises as a result
of the utilization of noncovalent van der Waals forces, π-π
stacking, and hydrogen-bonding interactions.12,13 We rea-
soned that reacting the organotin oxide-hydroxide [n-BuSn-
(O)(OH)]n or the organotin oxide with substituted ben-
zoic acids 4-RO-C6H4-COOH or 3,5-(OR)2-C6H3-COOH
would afford organostannoxanes of defined drum or ladder
structures. The intermolecular interactions among the
substituents present in the periphery of these stannoxane
cores would allow such molecules to self-assemble in solu-
tion, producing lamellar supramolecular structures con-
taining large free volumes, which can trap solvent
molecules and hence induce gelation. During the course
of this work a preliminary communication was published
by Nierengarten et al. on organotin gelators.14 Herein, we
describe the synthesis and characterization of organoox-
otin drums, [n-BuSn(O)O2C-C6H4-4-OR]6 [R= -CH3 (1);
-C9H19 (2); -C11H23 (3)] and [n-BuSn(O)O2C-C6H3-3,5-
(OR)2]6 [R = -CH3 (4); -C9H19 (5)], and organooxotin
ladders, {[n-Bu2SnO2C-C6H4-4-OR]2O}2 [R = -CH3 (6);
-C9H19 (7); -C11H23 (8)] and {[n-Bu2SnO2C-C6H3-3,5-
(OR)2]2O}2 [R = -CH3 (9) and -C9H19 (10)]. These
compounds induce gel formation in various aromatic
solvents even at room temperature. We also report the
microstructures of some representative examples
of these new organostannoxane gels. These gels can
also be transformed into xerogels by removal of solvent
in vacuo.
Results and Discussion
Synthesis. The reaction of n-butylstannonic acid, [n-BuSn-
(O)OH]n, with various alkoxy-substituted benzoic acids in a
1:1 stoichiometric ratio in refluxing toluene afforded hexa-
nuclear organostannoxane drums 1-5 in quantitative yields
(Schemes 1 and 2). Under similar conditions the reactions
of the benzoic acids with [n-Bu2SnO]n in a 1:1 ratio afforded
the tetranuclear ladder compounds 6-10 (Schemes 3 and 4).
A solvent-free methodology, which we have recently demon-
strated,15 also can be adopted for the preparation of 1-10.
In this latter technique the reactants are ground together
followed by a workup to afford the final products (see
Experimental Section).
The 119Sn NMR spectra of 1-5 are characterized by the
presence of a single resonance in a narrow chemical shift
window (ranging from -484.9 to -486.1 ppm) (Table 1).
These chemical shifts are the signature of the drum structure
of organostannoxanes, which contain six equivalent tin
atoms each with a 1C,5O coordination environment.2,3 On
the other hand, compounds 6-10 show two signals in their
119Sn NMR spectra (ranging from -188 to -203 ppm),
indicating the presence of two types of tin atoms in a tetra-
nuclear ladder structure (Table S1).2,3
Gelation Properties of Drums and Ladders. Compounds 1-
10 are not soluble in solvents such as n-hexane or diethyl
ether. However, they are soluble in solvents such as dichloro-
methane, chloroform, acetone, benzene, toluene, methanol,
and ethanol. We observed a viscous gel formation when the
solutions of these compounds in aromatic solvents were
allowed to age for about 48 h at room temperature. The
solution turned viscous in about 10-12 h before becoming
completely immobile (48 h). In contrast, gel formation is
not observed in aliphatic solvents. This type of solvent
(12) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489.
(13) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201.
(14) Hahn, H.; Hirst, A. R.; Delgado, J. L.; Kaeser, A.;
Delavaux-Nicot, B.; Nierengarten, J. F.; Smith, D. K. Chem. Commun.
2007, 4943.
(15) Chandrasekhar, V.; Baskar, V.; Boomishankar, R.; Gopal, K.;
Zacchini, S.; Bikley, J. F.; Steiner, A. Organometallics 2003, 22, 3710.
(16) Chandrasekhar, V.; Gopal, K.; Nagendran, S.; Steiner, A.;
Zacchini, S. Cryst. Growth Des. 2006, 6, 267.

Article
Organometallics, Vol. 28, No. 15, 2009 4595
Scheme 2
Scheme 3
Scheme 4

4596 Organometallics, Vol. 28, No. 15, 2009
Chandrasekhar et al.
Table 1. Critical Gelation Concentration (CGC, wt %)a for
Various Organostannoxanes in Different Aromatic Solvents
method (Figure 1). Gels obtained from benzene or toluene
show Tgel at around 45-60 °C (Table 2).
Optical micrographs of the drum gels 1-3 formed from
various aromatic solvents are indicative of the lamellar
nature of these gels (Figure 1). The microstructure of the
drum gels was studied by environmental scanning electron
microscopy (ESEM). This technique allows the visualiza-
tion of the morphology under low-vacuum conditions. An
ESEM micrograph of the drum gel 2 (formed in toluene)
reveals the formation of an intertwined fibrous network
(Figure 2). Long chains with diameters of 5-10 μm con-
sisting of intertwined fibers are generated, which aggregate
to form fibrous networks. Such fibrous network formation
is an indication for a typical sol-gel of low molecular mass
gelators.12,13 A similar pattern was also observed for the
drum gels 3 and 5. The microstructure of the drum gel 5
(formed from toluene) reveals that an intertwined tape
network is formed with a diameter of 10-15 μm (Figure
S1, Supporting Information). However, we were unable to
study the microstructure of gels formed by 1 and 4 because
even under ESEM conditions fast evaporation of the
solvent occurred from the gel.
The gels formed with 2 and 3 (in toluene or mesitylene) do
not suffer any shrinkage in volume upon removal of the
solvent (sol) in vacuum (∼3 Torr). We have studied this over
a period of 2 days. The resulting material is a xerogel
(Figures 3a,b).19 These xerogels can be manually crushed
in a mortar to obtain granules and are highly soluble in polar
aliphatic solvents. The organostannoxane structure remains
intact in the xerogels also. Evidence for this comes from
the119Sn NMR of the dissolved granules of the xerogel. A
single resonance at -484 ppm (in CDCl3) was observed
consistent with the drum structure (Table S1).1-3 The CP
MAS 119Sn NMR of xerogels of 2 and 3 showed broad
isotropic signals at -485 and -518 ppm, respectively, indica-
ting that the stannoxane drums retain their structures in the
xerogels. Thermal analysis of the xerogel of 2 (Figure S2a)
revealed a gradual loss in weight percent (5.3%) until the
onset of its decomposition at ∼278 °C, while the thermo-
gravimetric curve of the xerogel of 3 (Figure S2b) showed no
significant weight loss prior to the onset of its decomposition
at ∼270 °C. The surface areas for xerogels of 2 and 3, as
obtained from nitrogen adsoption measurements, were
found to be 0.26 and 1.17 m2/g, respectively, indicating that
the porosity of these xerogels is not significant.
stannoxane benzene toluene p-xylene mesitylene chlorobenzene
drum 1
drum 2
drum 3
drum 4
drum 5
ladder 6
ladder 7
ladder 8
ladder 9
ladder 10
6.8
2.9
2.8
4.9
2.3
7.2
4.5
3.2
6.5
4.1
5.9
1.4
1.6
4.3
1.1
6.8
3.9
2.8
6.6
3.4
6.0
1.3
1.1
4.3
0.8
6.7
4.0
2.1
6.2
3.2
6.1
1.3
0.9
4.1
0.5
6.8
3.9
2.0
5.9
2.0
5.2
0.9
0.8
4.0
0.6
6.8
3.8
2.4
5.9
2.3
a CGC measurements were performed in the concentration range
from 30 to 100 mg/mL.
dependence on gel formation has been reported in some
other systems also.17,18 In the current system aromatic
solvents appear to influence the self-assembly (aggregation)
of drum molecules and the microstructure of the resultant
sol-gels.
The gelation efficiency of the organostannoxane gelators
was studied in terms of their critical gelation concentration
(CGC) measurements (wt %). These data are summarized
in Table 1. It can be seen that in general all the drums are
more efficient than the corresponding ladders in inducing
gelation.
This can be traced to the spherical Sn6O6 core in the former
in comparison to the flat Sn4O2 structural motif present in
the latter. The role of the peripheral substituents is also quite
crucial. While even a simple methoxy substituent on the
aromatic carboxylate ligand is able to induce gelation, the
presence of long-chain alkylene spacers between the alkoxy
and the aromatic groups dramatically improves the effi-
ciency of gelation. For example, compounds 2, 3, and 5 have
a CGC of <1.5 wt % for toluene, p-xylene, mesitylene, and
chlorobenzene. Among these, the CGC of 5 is the lowest
(Table 1). These values suggest that 2, 3, and 5 can be
considered as supergelators,18 which require very low con-
centrations for inducing gelation. Among the ladders, com-
pound 8 is the most efficient gelator followed closely by 10.
In order to understand the nature of the organostannoxanes
(1-10) in solution during the process of gelation, their 119Sn-
{1H} NMR spectra were recorded in C6D6. Even after the
solutions became sufficiently viscous (aging for 14 h) broad
resonances, in the same chemical shift region as was observed
in CDCl3 solutions, could be detected (Table S1). After
about 48 h the gels became completely immobile, and at this
stage 119Sn NMR signals could not be detected presumably
because of large relaxation effects in the immobile gel phase.
The gel-sol transition temperatures (Tgel) of the drum and
ladder gels 1-10 were determined by the tube inversion
It is also of interest to note that the xerogels 2 and 3 can be
readily transformed to gels on the addition of aromatic
solvents. These transformations are independent of the type
of aromatic solvent. In case of compounds 1, 4, and 5
removal of solvent resulted in amorphous solids, which
could be again transformed into gels upon treatment with
the aromatic solvent.
The tetranuclear ladder compounds 6-10 also form gels in
the presence of aromatic solvents (for optical micrograph of
7 and 10 see Figure S3 of the Supporting Information).
Microstructures of ladder gels 7, 8, and 10 could be investi-
gated by ESEM (Figure S4 of the Supporting Information
shows an ESEM micrograph of 8). These gels exist in a plate-
like morphology. Ladder gels form amorphous solids upon
removal of solvent under reduced pressure. Similar to the
drum gel 5, the amorphous solid could be again transformed
into a gel in the presence of aromatic solvent.
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G.; Dordick, J. S. Chem. Mater. 2006, 18, 5988. (c) Jonkheijm, P.;
Schoot, P. V-D.; Schenning, A. P. H. J.; Meijer, E. W. Science 2006, 313,
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J. Org. Chem. 2007, 72, 7270. (e) Jeong, Y.; Hanabusa, K.; Masunaga,
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(18) (a) Luboradzki, R.; Gronwald, O.; Ikeda, A.; Shinkai, S. Chem.
Lett. 2000, 1148. (b) Suzuki, M.; Yumoto, M.; Kimura, K.; Shirai, H.;
Hanabusa, K. Chem. Commun. 2002, 704. (c) Kobayashi, H.; Friggeri,
A.; Koumoto, K.; Amaike, M.; Shinkai, S.; Reinhoudt, D. N. Org. Lett.
2002, 4, 1423. (d) Mukhopadhyay, S.; Maitra, U.; Ira.; Krishnamoorthy,
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Desvergne, J. P.; Brotin, T.; Meerschaut, D.; Clavier, G.; Placin, F.;
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(19) Holthoff, E. L.; Bright, F. V. Acc. Chem. Res. 2007, 40, 756.

Article
Organometallics, Vol. 28, No. 15, 2009 4597
Figure 1. (a) Photograph of the gel formed by 2 in toluene. (b-d) Optical micrograph of drum gels in toluene: (b) 1; (c) 2; (d) 3.
Table 2. Tgel (°C) for Various Organostannoxane Gels
stannoxanes
sional hexagonal disks are proposed to propagate to form
a two-dimensional sheet. Side-wise folding of the sheet can
result in the formation of a fiber where the inner and outer
surface of the fiber contains noninteracting alkyl chains
(n-Bu groups on tin). These can be utilized for hydrophobic
interactions between the fibers.
solvent
1a
2b
3b
4a
50
5b 6a
7b
8b
9a
10b
benzene 42-45 50-52 50
toluene 45
55
52-55 50 50-53 57-60
50 48-50 53-55
48-50 50-53 48-50 55 45 55
a At 8 wt % of stannoxane concentration. b At 5 wt % of stannoxane
concentration.
Conclusion
Although tentative, we propose a gelation mechanism for
the formation of the fibrous network in drum gels 2 and 3
(Scheme 5). Each drum contains six alkoxy groups in the
periphery in an approximate hexagonal arrangement. In-
teraction of each drum with six others (hydrophobic inter-
actions of the alkyl chains) results in a two-dimensional
hexagonal disk. Similar examples of hydrophobic interac-
tions are known in the literature.20 At this point, there are
six trigonal-shaped voids created within the hexagonal
supramolecular architecture around each drum. These
two-dimensional hexagonal disks further interact with each
other to form a three-dimensional network. This process
can occur in two ways (Scheme 5). In model A, the hexago-
nal disks stack one above the other. The driving force for
this is the hydrophobic alkyl chain interactions (between
n-Bu groups on tin). Such stacking leads to the formation
of columns and eventually hexagonal columnar fibers. The
surface of the fibers contains alkoxy groups, hydrophobic
interactions among which can cause cross-linking. In the
second model, model B, the above-described two-dimen-
We have successfully assembled a series of dendritic organo-
stannoxane drums and ladders containing multiple substituents.
The design of these assemblies was carried out in such a manner
that the peripheral substituents present around the organostan-
noxane core would form supramolecular networks in solution
through hydrophobic, weak intermolecular van der Waals, or
π-π interactions. All of these organostannoxanes function as
low molecular weight organometallic gelators in aromatic sol-
vents. A few dendritic organostannoxane drums are supergelators
with critical gelation concentrations of <1.5 wt % for toluene,
p-xylene, mesitylene, and chlorobenzene. The morphology of
these organometallic sol-gels was studied by optical and ESEM
techniques. Also, the xerogels can be readily transformed to gels
by addition of aromatic solvents. We are extending these studies
to investigate sensor applications of organostanoxane gels.
Experimental Section
Reagents and General Procedure. Solvents and other general
reagents used in this work were purified according to standard
procedures.21 The following chemicals were purchased and
(20) (a) Kumar, S. Chem. Soc. Rev. 2006, 35, 83. (b) Ishi-i, T.;
Hirayama, T.; Murakami, K-i.; Tashiro, H.; Thiemann, T.; Kubo, K.;
Mori, A.; Yamasaki, S.; Akao, T.; |Tsuboyama, A.; Mukaide, T.; Ueno,
K.; Mataka, S. Langmuir 2005, 21, 1261.
(21) Vogel’s Text Book of Practical Organic Chemistry, 5th ed.;
Furniss, B. S., Hannaford, A. J., Smith, P. W. G., Tatchell, A. R., Eds.;
ELBS and Longman: London, 1989

4598 Organometallics, Vol. 28, No. 15, 2009
Chandrasekhar et al.
Figure 2. ESEM micrographs of drum gel 2. (a) Close view of 2 showing the cross-linked fibers in toluene. (b) Close view of long fibers
of the gel formed by 2 in chlorobenzene.
Figure 3. ESEM micrograph of the xerogel formed from sol-gel. (a) Xerogel of 2. (b) Xerogel of 3.
used as such without further purification: [n-BuSn(O)OH]n, [n-
Bu2SnO]n, HO2C-C6H4-4-OC9H19, HO2C-C6H4-4-OC11H23,
H3CO2C-C6H3-3,5-(OH)2 (all from Aldrich, USA), HO2C-
C6H4-3,5-(OCH3)2 (Fluka, Switzerland), and HO2C-C6H4-4-
OCH3 (Spectrochem, India).
the nitrogen adsorption measurements. Measurements were
carried out using Smart Sorb 92/93 suface area analyzer, made
by Smart Instruments Co. Pvt. Ltd.
Solid state 119Sn NMR of xerogels of compound 2 and 3 were
measured using a Jeol Eclipse Plus 400 spectrometer (at 149.05 MHz)
equipped with a 6 mm MAS probe and were referenced against
SnMe4. The spectra were obtained using cross-polarization (contact
time 5 ms, recycle delay 10 s). Crystalline c-Hex4Sn (δ -97.35) was
used as a secondary reference.
Instrumentation. Melting points were measured using a JSGW
melting point apparatus and are uncorrected. Elemental ana-
lyses were carried out using a Thermoquest CE instruments
model EA/110 CHNS-O elemental analyzer. 1H and 119Sn
NMR spectra were recorded in CDCl3 solution (unless speci-
fied) on a JEOL JNM Lambda spectrometer operating at 400.0
and 150.0 MHz, respectively. The chemical shifts are referenced
Thermogravimetric analysis was carried out on a Perkin-
Elmer Pyris 6 thermogravimetric analyzer.
General Synthetic Procedure. Synthesis of HO2C-C6H4-3,5-
(OC9H19)2. This compound was prepared by adopting a liter-
ature procedure.22 A mixture of K2CO3 (16.59 g, 120.00 mmol)
and H3C-O2C-C6H3-3,5-(OH)2 (3.36 g, 20.00 mmol) in
1
with respect to tetramethylsilane (for H) and tetramethyltin
(for 119Sn). All the 119Sn NMR spectra were recorded under
broadband decoupled conditions. Optical images were obtained
with a LABOMAD DIGI 3 optical microscope operating from
40ꢀ to 1000ꢀ. ESEM measurements were performed on a FEI
QUANTA 200 microscope equipped with a tungsten filament
gun and operating at 20 keV. The micrographs were recorded at
WD 10.6 mm in varied magnification. TEM images were
recorded on a JEOL JEM 2000 FX-II transmission electron
microscope operating at 200 keV.
MeCN (50 mL) was refluxed for 30 min under
a
N2 atmosphere. To this was added dropwise a solution of
n-C9H19Br (12.43 g, 60.00 mmol) in MeCN (10 mL), and the
mixture was stirred for 20 h. The reaction mixture was then
cooled to room temperature and filtered. Evaporation of the
solvent from the reaction mixture resulted in a residue, which
Nitrogen Sorption Measuremets. The samples of xergels of 2
and 3 were degassed at 100 °C for 4 and 5 h, respectively, prior to
(22) Schultz, A.; Diele, S.; Laschat, S.; Nimtz, M. Adv. Funct. Mater.
2001, 11, 441.

Article
Organometallics, Vol. 28, No. 15, 2009 4599
Scheme 5. Proposed Mechanism for the Gelation Process in Drums 2 and 3
was washed with CH2Cl2 (2ꢀ50 mL). The filtrate was washed
with a 0.5 M NaOH solution (50 mL) and water (2ꢀ50 mL),
dried in MgSO4, filtered, and evaporated, yielding a colorless
oil. The latter was treated with KOH (15.71 g, 280.00 mmol)
in EtOH (75 mL) and heated under reflux for 6 h. An oily
solid was obtained after evaporation of the solvent from the
reaction mixture. This was cooled to 0 °C and treated with
H2O (50 mL) followed by 36.5% HCl (20 mL). A white
precipitate was formed. This was filtered, washed with water,
dried under vacuum, and identified as the title compound. Yield:
Solvent-Free Method. This involved the grinding of a stoi-
chiometric mixture of the organotin precursor and a substituted
benzoic acid at room temperature using a mortar and a pestle.
The progress of the reaction was monitored by noting the
morphological changes of the mixture as well as monitoring
its solubility (in organic solvents) with time. When the mixture
was completely soluble, it was extracted with a minimum
amount of dichloromethane. Evaporation of the solvent aff-
orded the product, whose structural identity was established by
its 119Sn{1H} NMR spectroscopy.
Specific details of each reaction and the characterization data
of the products obtained are given below.
[n-BuSn(O)O2C-C6H4-4-OCH3]6 (1). Conventional method:
[n-BuSn(O)OH]n (0.63 g, 3.00 mmol), HO2C-C6H4-4-OCH3
(0.46 g, 3.00 mmol), yield 0.89 g (85.8%).
Solvent-free method: [n-BuSn(O)OH] (0.11 g, 0.50 mmol),
HO2C-C6H4-4-OCH3 (0.08 g, 0.50 mmol), grinding time 30 min,
yield 0.16 g (88.4%).
Mp: 145 °C (dec). Anal. Calcd (%) for C72H96O24Sn6: C,
42.02; H, 4.70. Found: C, 42.03; H, 4.69. 1H NMR (δ, ppm): 0.86
(t, J=7.32 Hz, 18H, n-butyl CH3), 1.18-1.83 (m, 36H, n-butyl
CH2), 3.75 (s, 18H, -OCH3), 6.79 (d, J=8.39 Hz, 12H, aromatic
1
6.86 g (84.4%). H NMR (δ, ppm): 0.88 (t, J = 6.01 Hz, 6H,
CH3), 1.28-1.81 (m, 28H, CH2), 4.01-4.06 (m, 4H, -OCH2),
7.33 (s, 2H, aromatic CH), 7.71 (s, 1H, aromatic CH).
Synthesis of Organostannoxanes 1-10. Two synthetic proce-
dures were adopted for the preparation of 1-10 as outlined
below.
Conventional Method. A stoichiometric mixture of the orga-
notin precursor and a substituted benzoic acid was taken in
toluene (70 mL) and heated under reflux for 6 h. The water
formed in the reaction was removed by using a Dean-Stark
apparatus. The reaction mixture was filtered and evaporated to
dryness to afford the corresponding products.

4600 Organometallics, Vol. 28, No. 15, 2009
Chandrasekhar et al.
CH), 7.96 (d, J = 8.39 Hz, 12H, aromatic CH). 119Sn NMR
(δ, ppm): -484.9 (s).
[n-BuSn(O)O2C-C6H4-4-OC9H19]6 (2). Conventional meth-
od: [n-BuSn(O)OH]n (0.42 g, 2.00 mmol), HO2C-C6H4-4-
OC9H19 (0.53 g, 2.00 mmol), yield 0.87 g (95.3%).
Solvent-free method: [n-BuSn(O)OH] (0.11 g, 0.50 mmol),
HO2C-C6H4-4-OC9H19 (0.13 g, 0.5 mmol), grinding time
30 min, yield 0.21 g (90.9%).
Mp: 120 °C (dec). Anal. Calcd (%) for C120H192O24Sn6: C,
52.77; H, 7.09. Found: C, 52.70; H, 7.00. 1H NMR (δ, ppm): 0.80
(t, J = 6.83 Hz, 18H, CH3), 0.85 (t, J = 7.31 Hz, 18H, n-butyl
CH3), 1.19-1.41 (m, 96H, CH2), 1.67-1.78 (m, 24H, CH2), 3.89
(t, J = 6.46 Hz, 12H, -OCH2), 6.77 (d, J = 8.79 Hz, 12H,
aromatic CH), 7.94 (d, J = 8.55 Hz, 12H, aromatic CH). 119Sn
NMR (δ, ppm): -485.1 (s).
[n-BuSn(O)O2C-C6H4-4-OC11H23]6 (3). Conventional meth-
od: [n-BuSn(O)OH]n (0.42 g, 2.00 mmol), HO2C-C6H4-4-
OC11H23 (0.59 g, 2.0 mmol), yield 0.93 g (95.3%).
Solvent-free method: [n-BuSn(O)OH] (0.11 g, 0.5 mmol),
HO2C-C6H4-4-OC11H23 (0.15 g, 0.5 mmol), grinding time 30
min, yield 0.22 g (87.7%).
Mp: 110 °C (dec). Anal. Calcd (%) for C132H216O24Sn6: C,
54.68; H, 7.51. Found: C, 54.65; H, 7.55. 1H NMR (δ, ppm): 0.80
(t, J = 6.59 Hz, 18H, CH3), 0.85 (t, J = 7.31 Hz, 18H, n-butyl
CH3), 1.19-1.43 (m, 120H, CH2), 1.66-1.80 (m, 24H, CH2),
3.89 (t, J = 6.59 Hz, 12H, -OCH2), 6.77 (d, J = 8.75 Hz, 12H,
aromatic CH), 7.94 (d, J = 8.79 Hz, 12H, aromatic CH). 119Sn
NMR (δ, ppm): -485.1 (s).
[n-BuSn(O)O2C-C6H3-3,5-(OCH3)2]6 (4). Conventional meth-
od: [n-BuSn(O)OH]n (0.42 g, 2.00 mmol), HO2C-C6H3-3,5-
(OCH3)2 (0.37 g, 2.00 mmol), yield 0.71 g (93.7%).
Solvent-free method: [n-BuSn(O)OH]n (0.11 g, 0.50 mmol),
HO2C-C6H3-3,5-(OCH3)2 (0.09 g, 0.50 mmol), grinding time
30 min, yield 0.18 g (94.8%).
Mp: 155 °C (dec). Anal. Calcd (%) for C78H108O30Sn6: C,
41.86; H, 4.86. Found: C, 41.81; H, 4.91. 1H NMR (δ, ppm): 0.79
(t, J = 7.33 Hz, 18H, n-butyl CH3), 1.04-1.71 (m, 36H, n-butyl
CH2), 3.44 (s, 36H, -OCH3), 6.91 (s, 6H, aromatic CH), 7.78 (s,
12H, aromatic CH). 119Sn NMR (δ, ppm): -486.1 (s).
[n-BuSn(O)O2C-C6H3-3,5-(OC9H19)2]6 (5). Conventional
method: [n-BuSn(O)OH]n (0.21 g, 1.00 mmol), HO2C-C6H3-
3,5-(OC9H19)2 (0.41 g, 1.00 mmol), yield 0.54 g (89.6%).
Solvent-free method: [n-BuSn(O)OH]n (0.11 g, 0.50 mmol),
HO2C-C6H3-3,5-(OC9H19)2 (0.21 g, 0.50 mmol), grinding time
30 min, yield 0.29 g (95.2%).
Mp: 230 °C (dec). Anal. Calcd (%) for C96H164O14Sn4: C,
57.16, H 8.19. Found: C 57.11, H 8.21. 1H NMR (δ, ppm): 0.79
(t, J = 7.33 Hz, 12H, CH3), 0.86 (t, J = 7.33 Hz, 24H, n-butyl
CH3), 1.23-1.39 (m, 80H, CH2), 1.58-1.72 (m, 24H, CH2), 3.84
(t, J = 6.45 Hz, 8H, -OCH2), 6.92 (d, J = 8.44 Hz, 8H,
aromatic CH), 7.88 (d, J = 8.44 Hz, 8H, aromatic CH). 119Sn
NMR (δ, ppm): -188.9 (s), -204.1 (s).
{[n-Bu2SnO2C-C6H4-4-OC11H23]2O}2 (8). Conventional
method: [n-Bu2SnO]n (0.25 g, 1.00 mmol), HO2C-C6H4-4-
OC11H23 (0.29 g, 1.00 mmol), yield 0.49 g (92.8%).
Solvent-free method. [n-Bu2SnO]n (0.10 g, 0.40 mmol), HO2C-
C6H4-4-OC11H23 (0.12 g, 0.40 mmol), grinding time 30 min, yield
0.20 g (93.9%).
Mp: 210 °C (dec). Anal. Calcd (%) for C104H180O14Sn4: C,
58.66, H 8.52. Found: C 58.65, H 8.55. 1H NMR (δ, ppm): 0.82
(t, J = 7.29 Hz, 12H, CH3), 0.96 (t, J = 7.29 Hz, 24H, n-butyl
CH3), 1.12-1.40 (m, 96H, CH2), 1.61-1.84 (m, 24H, CH2), 3.78
(t, J = 6.50 Hz, 8H, -OCH2), 6.88 (d, J = 8.46 Hz, 8H,
aromatic CH), 7.78 (d, J = 8.46 Hz, 8H, aromatic CH). 119Sn
NMR (δ, ppm): -188.2 (s), -205.0 (s).
{[n-Bu2SnO2C-C6H3-3,5-(OCH3)2]2O}2 (9). Conventional
method: [n-Bu2SnO]n (0.50 g, 2.0 mmol), HO2C-C6H3-3,5-
(OCH3)2 (0.37 g, 2.0 mmol), yield 0.78 g (93.5%).
Solvent-free method: [n-Bu2SnO]n (0.10 g, 0.40 mmol), HO2C-
C6H3-3,5-(OCH3)2 (0.07 g, 0.40 mmol), grinding time 30 min,
yield 0.16 g (94.2%).
Mp: 195 °C (dec). Anal. Calcd (%) for C68H108O18Sn4: C,
48.37, H 6.45. Found: C 48.38, H 6.50. 1H NMR (δ, ppm): 0.84
(t, J = 7.32 Hz, 24H, n-butyl CH3), 1.22-1.61 (m, 48H, n-butyl
CH2), 3.74 (s, 24H, -OCH3), 6.71 (s, 4H, aromatic CH), 7.61
(s, 8H, aromatic CH). 119Sn NMR (δ, ppm): -192.72 (s),
-205.7 (s).
{[n-Bu2SnO2C-C6H3-3,5-(OC9H19)2]2O}2 (10). Conventional
method: [n-Bu2SnO]n (0.25 g, 1.00 mmol), HO2C-C6H3-3,5-
(OC9H19)2 (0.41 g, 1.00 mmol), yield 0.60 g (92.3%).
Solvent-free method: [n-Bu2SnO]n (0.10 g, 0.40 mmol),
HO2C-C6H3-3,5-(OC9H19)2 (0.16 g, 0.40 mmol), grinding time
30 min, yield 0.23 g (90.6%).
Mp: 220 °C (dec). Anal. Calcd (%) for C132H228O18Sn4: C,
61.50, H 8.91. Found: C 61.46, H 8.96. 1H NMR (δ, ppm): 0.85
(t, J = 7.31 Hz, 24H, CH3), 0.92 (t, J = 7.31 Hz, 24H, n-butyl
CH3), 1.21-1.54 (m, 120H, CH2), 1.66-1.74 (m, 32H, CH2),
3.90 (t, J = 6.75 Hz, 16H, -OCH2), 6.68 (s, 4H, aromatic CH),
7.45 (s, 8H, aromatic CH). 119Sn NMR (δ, ppm): -195.4 (s),
-206.8 (s).
Gelation Experiments and Determination of Critical Gelation
Concentrations (CGC). A weighed amount of organostannox-
ane along with an appropriate solvent (2 mL) were placed in a
glass vial (2.5 cm length and 1 cm diameter) and heated with a
hot-gun until the solid completely dissolved, affording an
anisotropic solution. This was cooled to room temperature
and left for 4 h. At this stage the state of the solution was
monitored visually by turning the test vial upside-down. The
material was classified as a gel if it did not exhibit gravitational
flow while turning down the vial.
Mp: 110 °C (dec). Anal. Calcd (%) for C174H300O30Sn6: C,
1
58.30; H, 8.44. Found: C, 58.29; H, 8.48. H NMR (δ, ppm):
0.79 (t, J = 7.03 Hz, 36H, CH3), 0.84 (t, J = 7.33 Hz, 18H,
n-butyl CH3), 1.14-1.55 (m, 168H, CH2 CH2), 1.71-1.79 (m,
36H, CH2), 3.64 (t, J = 6.78 Hz, 24H, -OCH2), 6.81 (s, 6H,
aromatic CH), 7.65 (s, 12H, aromatic CH). 119Sn NMR (δ,
ppm): -486.0 (s).
{[n-Bu2SnO2C-C6H4-4-OCH3]2O}2 (6). Conventional meth-
od: [n-Bu2SnO]n (0.50 g, 2.00 mmol), HO2C-C6H4-4-OCH3
(0.30 g, 2.00 mmol), yield 0.75 g (97.0%).
Sample Preparations. Optical Microscopic Experiments. A
small amount of gel was taken on a plain glass slide (gel samples
were fabricated using 8 wt % of the gelators 1, 4, 6, and 9; for
other samples 5 wt % of gelators was used), air-dried for 15 min,
and viewed under an optical microscope.
ESEM Experiments. Samples of the sol-gels were prepared
by the freeze-drying method from their gel phases.23 In a typical
procedure, the gel was prepared in a rectangular thin wall glass
container [1 ꢀ 3 ꢀ 3 cm3]. After gelation, it was cooled by using
liquid nitrogen and sealed. This was used for taking ESEM
pictures. For the xerogel samples, the above-described gel was
placed on a plain glass slide. Over a period of 2 days, the solvent
Solvent-free method: [n-Bu2SnO]n (0.10 g, 0.40 mmol), HO2C-
C6H4-4-OCH3 (0.06 g, 0.40 mmol), grinding time 30 min, yield
0.14 g (91.6%).
Mp: 260 °C (dec). Anal. Calcd (%) for C64H100O14Sn4: C,
49.01, H 6.43. Found: C 49.05, H 6.45. 1H NMR (δ, ppm): 0.84
(t, J = 7.31 Hz, 24H, n-butyl CH3), 1.22-1.81 (m, 48H, n-butyl
CH2), 3.81 (s, 12H, -OCH3), 6.79 (d, J = 8.40 Hz, 8H, aromatic
CH), 7.96 (d, J = 8.40 Hz, 8H, aromatic CH). 119Sn NMR (δ,
ppm): -199.2 (s), -203.5 (s).
{[n-Bu2SnO2C-C6H4-4-OC9H19]2O}2 (7). Conventional meth-
od: [n-Bu2SnO]n (0.50 g, 2.00 mmol), HO2C-C6H4-4-OC9H19
(0.53 g, 2.00 mmol), yield 0.90 g (89.0%).
Solvent-free method: [n-Bu2SnO]n (0.10 g, 0.40 mmol), HO2C-
C6H4-4-OC9H19 (0.11 g, 0.40 mmol), grinding time 30 min, yield
0.18 g (88.8%).
(23) For the preparation of dry samples for SEM observations, see:
Jeong, S. W.; Shinkai, S. Nanotechnology 1997, 8, 179.

Article
Organometallics, Vol. 28, No. 15, 2009 4601
was removed under vacuum (∼3 Torr) to produce a dried
measurements. We thank Prof. Nishith Verma of the
Department of Chemical Engineering, IIT Kanpur,
and his group members for the surface area measure-
ments.
material (xerogel). This was then examined by ESEM.
Acknowledgment. We are thankful to the Department
of Science and Technology, New Delhi, for financial
support. V.C. is thankful to DST for a J. C. Bose National
fellowship. V.C. is a Lalit Kapoor chair professor. We
thank the electron microscopy facility of the Advanced
Centre for Materials Science, IIT Kanpur, for ESEM
Supporting Information Available: Thermogravimetric curves
for xerogels of compound 2 and 3 and additional optical and
SEM images of gels and xerogels. This material is available free
of charge via the Internet at http://pubs.acs.org.