Modular nanometer-scale structuring of gel fibres by sequential self-organization

Article abstract of DOI:10.1039/b511259a

Ag(I) and Cu(II) complexes of a series of simple bis(urea) ligands form soft metallogels. X-ray crystallographic results suggests that the gels' structure is based on hydrogen bonding to counter anions and thus suggests a route to tunable gel rheological

Full text of DOI:10.1039/b511259a

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Modular nanometer-scale structuring of gel fibres by sequential
self-organization
Lucas Applegarth,a Nigel Clark,a A. Christine Richardson,b Andrew D. M. Parker,a
Ivana Radosavljevic-Evans,a Andres E. Goeta,a Judith A. K. Howarda and Jonathan W. Steed*a
Received (in Columbia, MO, USA) 8th August 2005, Accepted 13th September 2005
First published as an Advance Article on the web 4th October 2005
DOI: 10.1039/b511259a
Ag(I) and Cu(II) complexes of a series of simple bis(urea)
ligands form soft metallogels. X-ray crystallographic results
suggests that the gels’ structure is based on hydrogen bonding
to counter anions and thus suggests a route to tunable gel
rheological properties.
Gels comprise a partially ordered liquid trapped by a highly
Unlike more flexible bis(ureas) containing long alkyl chains, 1 and
2 do not form gels alone in a variety of organic solvents such as
tetrahydrofuran, acetonitrile, methanol, chloroform and toluene.
However addition of ethyl groups to give 3 does result in the
formation of an organogel (Fig. 1a) in chlorofom : methanol (50 :
50 v/v) at concentrations of 10 mM, comparable to those for long
chain alkyl systems.15,22,23 The gel was characterized by frequency
sweep rheometry. The resultant storage modulus was greater than
the loss modulus and frequency independent, both aspects are
characteristics of gels.24 The value of the storage modulus was
16 kPa, implying a weak gel, despite its opacity, which suggests
significant structuring and crystallinity.
porous network of fibres resulting in viscoelastic macroscopic
properties. As well as being fascinating because of their
nanometer-scale structure, recent work has shown that the highly
porous, partially ordered network in gels, coupled with their
formation by spontaneous self-organization gives them tremen-
dous technological possibilities, for example as selective sorbents or
uptake agents,1–6 in the controlled formation of highly porous
polymers,7 or in the synthesis and support of catalytic nano-
particles.8–10 The gel host matrix can comprise a wide range of
fibrous materials e.g. based on organic11,12 or metal oxide
polymers,13 or on non-covalent assembly of molecular compounds
such as cholesterol derivatives, surfactants, elaborated porphyrin
and phthalocyanine derivatives, carbohydrates and peptide deriva-
tives, extended bis- and tris(urea) compounds, phenylenevinylene
derivatives and oligoamides.14–19 However, most of the materials
reported to date are based on polymers.11,12 The vast majority of
these systems contain long chain alkyl groups in order to arrest full
crystallization and limit aggregation to nanometer scale gel fibres.
Gels may be considered to be either hard or soft based on their
rheological characteristics. There is significant current technologi-
cal interest in soft gels that can be readily induced to flow by either
a change of temperature or being stressed beyond the yield point.20
We now report a simple, readily prepared series of rigid
bis(urea) ligands in which gelation occurs to give soft gels via a
hierarchical series of self-organization steps templated by metal
The gel was dried gently and imaged using SEM and AFM.
A representative image is shown in Fig. 2a. The dried xerogel
comprises a dense fibrous network with large fibres of up to
500 nm width. On higher magnification the strands are revealed to
be collinear fibre bundles with the narrowest fibres measuring ca.
60 nm. Addition of Ag(I) salts AgBF4 and AgNO3 to 3 again
results in gel formation. In the case of AgBF4 the appearance of
the fibres is markedly different with narrower strands showing
markedly less bunching, Fig. 2b. Addition of AgNO3 also
dramatically affects the morphology of the gel fibres with both
narrow fibres of width ca. 35 nm and broader, well-defined, flat
ribbons observed.
The unalkylated analogue, 2, alone does not form a gel in
thf/water and precipitates as a microcrystalline solid. However,
…
salts. The intrinsic ability of bis(ureas) to aggregate via NH OLC
reversible hydrogen bonded interactions17,18,21 is modulated by,
and can be switched by reversible coordination interactions to
metal cations and by hydrogen bonding to conjugate anions. The
resulting gels and the consequent nanostructuring of metal ions
offers interesting technological possibilities.
The bis(urea) dipyridyl ligands 1 and 2 are readily prepared
in one step by the reaction of commercial diisocyanates with
3-aminopyridine. The strategy is very versatile given the wide
availability of commercial isocyanates as industrial intermediates.
aDepartment of Chemistry, University of Durham, South Road, Durham,
DH1 3LE, UK. E-mail: jon.steed@durham.ac.uk
Fig. 1 Photographs of (a) organogel of 3 from chloroform/methanol, (b)
comparison of solutions of free ligand 2 (left) and 2/AgBF4 in thf/water
(right) (c) metallogel comprising 1/Cu(NO3)2 from aqueous methanol.
bDepartment of Biological and Biomedical Sciences, University of
Durham, South Road, Durham, DH1 3LE, UK
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Fig. 3 (a)
X-ray
structure
of
the
metallomacrocycle
[{Ag(2)(MeCN)}2](NO3)2 in 4 (two repeats). Each chain interacts with
…
its neighbours via NH anion hydrogen bonds. (b) Polar p-stacked chains
of macrocycles. Ag (cyan), O (red), N (blue), C (grey), H (light grey).
discrete nitrate sandwich complex formed by the monodentate,
p-tolyl analogue of 2.26,27 The interior of the macrocycle is
occupied by two acetonitrile molecules interacting weakly with the
˚
Ag(I) centre (Ag–NCMe 5 2.556(5) A) to give a very unusual
T-shape geometry. The acetonitrile molecules are situated slightly
off-centre, apparently maximising van der Waals interactions with
the ligand. In order to aid in understanding gel formation it is
interesting to look at the solid-state packing of the macrocycles. In
addition to being linked by hydrogen bonding to nitrate ions, the
macrocycles also form polar stacks in the solid state (Fig. 3b). It is
possible that the stronger hydrogen bonding to nitrate results in
the crystallization of 4 while in the case of the reaction of 2 with
AgBF4 the complex is more soluble allowing gel formation.
Additional ethyl groups, as in complexes with 3, renders the whole
system more soluble again promoting gel formation and reducing
Fig. 2 Representative SEM images of dried xerogels (a) 3 alone from
chloroform/methanol (50 : 50 v/v) (b) 3 wih AgBF4 and (c) 3 with AgNO3.
reaction of 2 with AgBF4 in the same medium thf/water (2 : 1 v/v)
results in the formation of a semi-transparent, weak gel, thus the
presence of the labile metal salt promotes gel formation, Fig. 1b.
Reaction of 2 with AgNO3 in a variety of solvent mixtures
(2 in either thf/water, chloroform/methanol or toluene/methanol;
AgNO3 in MeCN) again does not result in gel formation but
allows the isolation of
a crystalline product of formula
[{Ag(2)(MeCN)}2](NO3)2?2CHCl3?1.5H2O 4. The complex was
characterized by X-ray crystallography{ (Fig. 3) and suggests some
insight into the aggregation process of 2 and 3 with Ag(I) salts and,
by inference, the structure of the metallogels formed by 2 and 3
with AgBF4, and 3 with both AgBF4 and AgNO3.
Complex 4 comprises a discrete 2 + 2 metallomacrocycle with
approximately linear coordination at the Ag(I) centres (N–Ag–N 5
163.41(14)u) and four externally directed urea groups hydrogen
2
bonding to NO3 anions. The anions bridge between adjacent
macrocycles via unsymmetrical pairwise R22(8) hydrogen bonded25
…
Fig. 4 SEM image of metallogel derived from 1 and copper(II) nitrate.
This journal is ß The Royal Society of Chemistry 2005
rings (Fig. 3a), supported by CH O interactions similar to the
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Fig. 5 X-ray crystal structure of [Co(1)2(H2O)4](NO3)2?H2O 5 showing hydrogen bonding interactions. CH hydrogen atoms omitted for clarity.
indices based on 5923 reflections with I . 2s (I) (refined on F2), 335
parameters. Lp and absorption corrections applied ,m 5 0.479 mm21
CCDC 281274 & 281275. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b511259a.
crystallinity. The structure of 4 also suggests a reason why the gels
should be relatively free-flowing despite their highly structured
nature. If gel aggregation is via hydrogen bonding to anions then
the gel rheological properties will be linked to the strength of the
(relatively weak) hydrogen bonds and to the p-stacking. The van
der Waals dimensions of the metallomacrocycle-nitrate unit are
ca. 2 6 1.5 nm and hence the smallest fibres would comprise
ca. 10–15 molecules in width. This aggregation mode contrasts to
.
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…
the NH OLC hydrogen bonding proposed for other bis(urea)
gels which do not contain metals.23
Analogous gelation behaviour was also observed for ligand 1 in
the presence of copper(II) nitrate, the incorporation of the metal
being readily apparent from the blue coloration of the decanted
material (Fig. 1c). The copper nitrate gel proved to be crystalline
and adopted a structure involving well-defined ribbons of highly
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variation of anion identity and urea hydrogen bond acidity (e.g. via
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Notes and references
{ Crystal data for 4: C28H26Ag1Cl3N8O5.74, M 5 780.66, orthorhombic,
space group Pmn21 (No. 31), a 5 43.826(4), b 5 5.2608(5), c 5
14.0960(14) A, V 5 3250.0(5) A , Z 5 4, Dc 5 1.595 g cm23, F000
5
3
˚
˚
˚
1575.730, Mo Ka radiation, l 5 0.71073 A, T 5 120 K, 2hmax 5 55.6u,
27657 reflections collected, 7287 unique (Rint 5 0.02). Final GooF 5 0.8655,
R1 5 0.0473, wR2 5 0.1110, R indices based on 6271 reflections with I .
3.00s(I) (refinement on F2), 419 parameters, 7 restraints. Lp and
absorption corrections applied, m 5 0.920 mm21. Absolute structure
parameter 5 0.55(3). 5: C36H56CoN14O16, M 5 999.88, triclinic, space
¯
˚
3
group P1 (No. 2), a 5 9.3380(6), b 5 11.3324(7), c 5 11.4099(7) A,
˚
26 D. R. Turner, B. Smith, E. C. Spencer, A. E. Goeta, I. R. Evans,
D. A. Tocher, J. A. K. Howard and J. W. Steed, New J. Chem., 2005,
29, 90.
a 5 74.1670(10), b 5 71.2920(10), c 5 80.3580(10)u, V 5 1095.99(12) A ,
Z 5 1, Dc 5 1.515 g cm23, F000 5 525, MoKa radiation, l 5 0.71073 A,
˚
T 5 120(2) K, 2hmax 5 61.0u, 14659 reflections collected, 6590 unique
(Rint 5 0.0205). Final GooF 5 1.092, R1 5 0.0436, wR2 5 0.1027, R
27 D. R. Turner, E. C. Spencer, J. A. K. Howard, D. A. Tocher and
J. W. Steed, Chem. Commun., 2004, 1352.
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