Gelation of La(III) cations promoted by 5-(2-pyridyl)tetrazolate and water

Article abstract of DOI:10.1039/b605006a

Addition of water to the product formed when LaCl₃ and 1H-5-(2-pyridyl)tetrazole (LH) were treated with an excess of triethylamine in ethanol, resulted in the reversible formation of a hydrogel, which when further treated with EtOH-ⁱPrOH produced crystals of [LaL₃(H ₂O)₃]·(H₂O)₄, as determined by X-ray crystallography. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605006a

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Gelation of La(III) cations promoted by 5-(2-pyridyl)tetrazolate and
water{
Philip C. Andrews,*a Peter C. Junk,a Massimiliano Massia and Morry Silbersteinb
Received (in Cambridge, UK) 6th April 2006, Accepted 31st May 2006
First published as an Advance Article on the web 3rd July 2006
DOI: 10.1039/b605006a
hydrogel is obtained by the addition of water to a La(III) complex
in which the metal cation is coordinated by 5-(2-pyridyl)tetrazolate
anions. In this case, gel formation was completely unexpected since
the tetrazolate ligand was not specifically designed to form a cross-
linked architecture with the ability to swell in contact with water.
We are currently studying the formation of Ln complexes with
N donor chelating heterocycles to assess the possibility of using
them as ‘‘protecting’’ ligands in the formation of Ln clusters.13 In
the course of these studies we reacted LaCl3 with 1H-5-(2-
pyridyl)tetrazole (LH) using a modification of the synthetic
procedure recently described by Roesky et al.14 LH was
synthesized by nucleophilic addition of NaN3 to the cyano group
of 2-pyridinecarbonitrile in water, catalysed by ZnBr2.15 The
capacity of L2 to form mononuclear Ln complexes of the type
LnL3(H2O)3 has been reported for Ln = Gd, Eu, and Dy.16 In
attempting the synthesis of the clusters, an excess of triethylamine
was added to an ethanolic solution containing both LaCl3 and LH
Addition of water to the product formed when LaCl3 and
1H-5-(2-pyridyl)tetrazole (LH) were treated with an excess of
triethylamine in ethanol, resulted in the reversible formation of
a hydrogel, which when further treated with EtOH–iPrOH
produced crystals of [LaL3(H2O)3]?(H2O)4, as determined by
X-ray crystallography.
Due to their unique properties, lanthanides have found applica-
tions in a wide range of fields, e.g. medicinal chemistry,1,2 the
design of novel luminescent3 or magnetic materials,4 anti-corrosion
coatings5 and precursors for the deposition of ultra-pure
lanthanide oxides as high-k gate dielectrics in Si-based field effect
transistors.6 Special attention has recently focussed on forming gel-
like systems containing lanthanide cations or complexes, with the
intent of improving the thermal stability and the mechanical
properties of the material.7 The advantage of such gels is that as
‘‘soft materials’’ they offer facile processability by means of cheap
fabrication techniques such as microfluidics, micromolding in
capillaries (MIMICS), and soft lithography.8 The citrate gel
process has been extensively employed for the preparation of
ceramic materials based on ultrapure lanthanide chromites,
LnCrO3, and for the deposition of luminescent Ln2O3–Eu (Ln =
Gd, Lu) thin films.9 Moreover, nanostructured hydrogels have
proven to be optimal candidates as responsive materials in
biological applications such as biomaterials and biosensors.10
Although a rigorous definition of ‘‘gel’’ is not straightforward,
these systems can be roughly described as either physically (i.e. by
means of hydrophobic interaction or hydrogen bonds) or
covalently cross-linked polymeric architectures which swell in
contact with a solvent.10 Cross-linking is essential in preventing the
complete dissolution of the material, and thus formation of a
‘‘liquid’’ solution.
in a 1
: 3 stoichiometric ratio.{ Because of its relative
thermodynamic acidity (pKa = 4.8), the tetrazole ring is readily
deprotonated by triethylamine. Upon addition of the base a white
precipitate is formed which is insoluble in both apolar and polar
organic solvents, even after prolonged heating and/or sonication.
However, when mixed with hot water (80 uC), gelation occurs
within a few minutes (Fig. 1).
To gain some understanding of the structure of the gel we
analyzed the dry material by means of IR and NMR spectroscopy.
The presence of a strong broad band around 3300 cm21 in the IR
spectrum, even after several days of drying at 50 uC under reduced
pressure, suggested the presence of a hydrated compound. The
absorptions at 1606 and 1571 cm21, which we ascribe to the CLN
and NLN stretches of the ligand, are typical of the tetrazole ring.
1H-NMR on the sample (dispersed in carefully dried DMSO after
several hours of sonication) showed four broad signals at 8.75,
Three main strategies have been followed for the preparation of
lanthanide containing gels: (i) lanthanide salts or complexes are
simply added to a gelating matrix, which can be a polymer or a
silica glass precursor;11 (ii) the ligands of the lanthanide complex
are endowed with chemical groups able to covalently graft to the
framework;7 (iii) the lanthanide cations are anchored to specific
chemical functionalities of the matrix.12
We report here the first example of gelation of a La complex in
water without the need for an additional gelating polymer. The
aSchool of Chemistry, Monash University, Melbourne, Victoria 3800,
Australia. E-mail: phil.andrews@sci.monash.edu.au;
Fax: +61-3-9905 5509; Tel: +61-3-9905 4597
bDepartment of Medical and Radiation Sciences, Monash University,
Melbourne, Victoria 3800, Australia
{ Electronic supplementary information (ESI) available: Crystal structures
of LaL3(H2O)?(H2O)4. See DOI: 10.1039/b605006a
Fig. 1 Synthesis and formation of the La pyridyltetrazolate hydrogel.
Chem. Commun., 2006, 3317–3319 | 3317
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8.08, 7.95 and 7.24 ppm, corresponding to the four hydrogen
atoms of the pyridine ring, confirming the presence of L.15 The
resonance at 3.33 ppm indicated the presence of water, in
accordance with the IR spectrum. The NMR analysis also showed
the presence of a small amount of triethylamine. The two triplets at
1.05 and 0.98 ppm along with the quartet signal at 2.41 ppm
suggest that Et3N is present both as base and conjugate acid. The
second quartet, which should be around 3.2 ppm, is almost
completely masked by the strong absorption. From integration of
the NMR signals we have estimated the dry product to be
composed of tetrazolate anions, amine (as Et3N and [Et3NH]OH)
and water in the ratio of 1 : 0.1 : 3. Moreover, the total amine
content is present as 50% base and 50% conjugate acid. On the
basis of these spectroscopical data, we conclude that the core of the
gel, is mainly composed of hydrated La3+ cations coordinated by L
ligands. The exact role of the base (NEt3) is not yet clear and is
being further investigated. However, we believe that the presence
of a mixture of triethylamine and triethylammonium results from
the reversible protonation of the amine by the acidic water
molecules coordinated to the La centre, with concomitant
Fig. 2 Crystal structure of LaL3(H2O)3. Hydrogen atoms and lattice
˚
water molecules are not shown for clarity. Selected bond distances (A):
La1–O1 2.486(2); La1–O2 2.5252(19); La1–O3 2.539(2); La1–N1 2.675(2);
La1–N5 2.763(2); La1–N6 2.654(2); La1–N10 2.743(2); La1–N11 2.662(2);
La1–N15 2.763(2).
formation of
a hydroxy anionic complex of the type
[LaL3(H2O)x(OH)][HNEt3].§ The addition of an excess of water
thus promotes the formation of a three dimensional hydrogen-
bonded architecture, which also explains the reversible behaviour
of the gel upon mechanical and thermal stimulation, upon which it
becomes temporarily less viscous.
inclusion of four water molecules in the cavities between them. The
extensive hydrogen bonding which then forms between all the
water molecules, and which also involves the nitrogen atoms of
the tetrazole rings, establishes the three dimensional network in the
solid state, and is most likely the key factor in determining the
formation of the hydrogel structure. This conclusion is supported
by the fact that adding EtOH or iPrOH (which do not possess the
same capacity for creating three dimensional hydrogen-bonded
structures as H2O does) to the hydrogel disrupts the hydrogen
bonding and results in the crystallization of LaL3(H2O)3, with only
water incorporated in the lattice. Moreover, no other solvent other
than water is able to provide for gel formation.
In attempting to fully understand the role of L and triethylamine
in the gelation process we repeated the synthetic procedure
replacing LH with picolinic acid. However, we have seen no
evidence of gel formation for this system. In addition, to assess
whether triethylamine is necessary for gel formation, we synthe-
sized the neutral LaL3(H2O)3 complex by metathesis of LaCl3 with
LNa in water. Heating and/or sonicating these crystals in water,
even for several hours, has never led to gel formation. We therefore
conclude that both L, and the presence of a small amount of
triethylamine, are necessary for the formation of the gel.
Dilution of the hydrogel in a 1 : 1 mixture of EtOH–iPrOH
results in the formation, after a few days, of colourless block-
shaped crystals." The structure was analyzed by single crystal
X-ray diffraction and determined to be the neutral
[LaL3(H2O)3]?(H2O)4 complex (Fig. 2),I in which each La3+ is
nonacoordinated by three bidentate ligands L and three molecules
of water. Four more shared lattice water molecules are present in
the second coordination sphere. This supports our predicted
structure of the complex in the gel core, [LaL3(H2O)x
(OH)][HNEt3], in that the IR spectra of the dry material and of
the crystals, [LaL3(H2O)3]?(H2O)4, are very similar. Given this, and
inferring that the crystals result from the breakdown of the gel
structure, we further analysed the intermolecular bonding in the
solid state packing of the crystals. Although pyridyltetrazoles tend
to maximize p-staking interactions between the aromatic systems,17
in this case this is hampered by the three coordinated water
molecules which are bound to each La cation. However, for each
metal, one of the ligands arranges itself in an offset face-to-face
p-stacking configuration with the ligand of a neighbouring
Importantly, and rather surprisingly, gelation is a completely
reversible process, and the dry starting material can be recovered
intact on evaporation of the water from the hydrogel at 70 uC (IR
spectra before and after the gelation are identical). In addition, the
gel structure has proven to be stable over periods of months and
there is no evidence for hydrolysis of the complex with subsequent
release of the tetrazolate ligand from the starting material, even
when treated with hot water over sustained periods of time. This
stability most likely results from the relatively low basicity of the
tetrazolate ligand.
We are currently exploring the possibility of forming pyridylte-
trazolate based hydrogels using other lanthanide cations.
Preliminary results indicate that Eu does not seem to support
gelation, perhaps due to the lower Brønsted acidity of the heavier
elements. However, we have been able to form a stable mixed
La–Eu pyridyltetrazolate based gel containing a metal : metal ratio
of up to 1 : 1 suggesting that the La complex is capable of being
used as a matrix for other Ln cations. This work is still in progress.
In summary, we have described the preparation of a hydrogel
using a La complex and triethylamine. On the basis of the
spectroscopic evidence we have determined that the gel is formed
when the complex, in which La is coordinated to 5-(2-
pyridyl)tetrazolate, undergoes deprotonation caused by the pre-
sence of the base, triethylamine. The resulting complex is then
trapped in a hydrogen-bonded three dimensional network formed
…
complex (C C 3.7 A; N C 3.4 A), forming at the same time a
…
˚
˚
vertex-to-face arrangement between the pyridine ring and another
…
˚
tetrazole (C N 3.4 A). More extensive non-covalent interactions
between the complex molecules is further offset by the competitive
3318 | Chem. Commun., 2006, 3317–3319
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2hmax = 56.6u, 30874 reflections collected, 6653 unique (Rint = 0.0478).
Final GooF = 1.055, R1 = 0.0317, wR2 = 0.0573, R indices based on 5280
reflections with I > 2s(I) (refinement on F2), 467 parameters, 18 restraints.
Lp and absorption corrections applied, m = 1.631 mm21. CCDC 604028.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b605006a
by the addition of water. We have also demonstrated that for
gelation to occur both the ligand LH, and the base are required.
To the best of our knowledge this is the first time in which direct
gelation of a lanthanide complex in water is presented without the
need of a specifically designed self-assembling ligand or a
polymeric gelating matrix.
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Notes and references
{ Synthetic details. All materials were used as received without further
purification. 1H-5-(2-pyridyl)tetrazole was synthesized from the corre-
sponding 2-pyridinecarbonitrile and NaN3 in H2O following the literature
procedure.15 LaCl3 was prepared by dissolution of La2O3 in 33% HCl.
§ Gel formation, ([LaL3(H2O)x(OH)][HNEt3]): LaCl3 (1 mmol) and 5-
(2-pyridyl)tetrazole (3 mmol) were added to 20 ml of ethanol.
Triethylamine (4.5 mmol) was added dropwise to the solution under
vigorous stirring. After 24 h the white precipitate which formed was filtered
off and washed with 30 ml ethanol, 15 ml diethyl ether, and 15 ml hexane,
before being dried for 3 h under reduced pressure. Yield 0.24 g, mp >330 uC
(decomp.). 1H-NMR (300.13 MHz, 30 uC d6-DMSO) d 8.75 (1H, br s, H6);
8.32 (1H, br s, H3); 8.08 (1H, br s, H4); 7.40 (1H, br s, H5); 3.33 (5H, s,
H2O ); 2.41 (0.3H, q, J = 7.2 Hz, N(CH2CH3)3); 1.05 (0.4H, t, J = 7.0 Hz,
HN(CH2CH3)3); 0.98 (0.4H, t, J = 7.2 Hz. N(CH2CH3)3). IR (diamond
ATR) nmax. (cm21): 3310br, 1606s, 1571w, 1462s, 1424s, 1287s, 1159w,
1058w, 1002w, 801w, 733w and 718w. Gelation occurred on heating to
80 uC, the solid (0.2 g) dispersed in 10 ml H2O.
9 P. S. Devi, J. Mater. Chem., 1993, 3, 373; Q. Liu, Q. Liu and L. Luo,
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" Synthesis of [LaL3(H2O)3]?(H2O)4. Addition of a 1 : 1 mixture of
ethanol–isopropanol to the gel (formed as above) resulted in the formation
of large colourless block-shaped crystals which were analysed as
[LaL3(H2O)3]?(H2O)4. Yield (not maximised) 0.046 g, mp > 312 uC
(decomp.). 1H-NMR (300.13 MHz, 30 uC d6-DMSO) d 8.75 (1H, br s, H6);
8.10 (1H, d, J 7.7, H3); 7.94 (1H, t, J 7.7, H4); 7.38 (1H, t, J 5.8 H5); 3.33
(y7H, s, H2O).13C-NMR (75 MHz, 30 uC, d6-DMSO) d 161.25 (CN4);
149.52 (C2); 149.37 (C6); 138.15 (C4); 122.97 (C3); 120.98 (C5). Anal calc.
for [LaL3(H2O)3]?1.5(H2O): C 32.8, H 3.2, N 32.0; found: C 33.0, H 3.7, N
32.3. IR (diamond ATR) nmax. (cm21): 3300br, 1636w, 1605s, 1570w,
1461w, 1427s, 1386s, 1284s, 1248w, 1156w, 1099w, 1059w, 1035w, 1002w,
796w, 750w and 728w.
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I Crystal structure analysis. Crystal data analysed using X-Seed:
C18H24LaN15O6.50, M = 693.43, 0.16 6 0.12 6 0.10 mm3, monoclinic,
˚
space group P21/n (No. 14), a = 9.1993(18), b = 17.748(4), c = 16.947(3) A,
b = 99.54(3)u, V = 2728.7(9) A , Z = 4, Dc = 1.688 g cm23, F000 = 1384,
17 A. T. Rizk, C. A. Kilner and M. A. Halcrow, CrystEngComm, 2005, 7,
359.
3
˚
˚
Nonius Kappa CCD, MoKa radiation, l = 0.71073 A, T = 123(2) K,
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 3317–3319 | 3319