Interpenetrated networks from a novel nanometer-sized pseudopeptidic ligand, bridging water, and transition metal ions with cds topology

Article abstract of DOI:10.1039/b502788h

The combination of a new pseudopeptidic ligand, transition metal ions, and bridging water molecules results in the formation of [M(μ-TBG)(μ-H ₂O)(H₂O)₂] · 2H₂O (M: Cu, Co and H₂TBG: terephthaloylbisglycine); both compounds show rare two-fold interpenetrated three-dimensional cds-nets and reversible loss of coordinated and lattice water molecules. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502788h

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Interpenetrated networks from a novel nanometer-sized pseudopeptidic
ligand, bridging water, and transition metal ions with cds topology{
George E. Kostakis,ab Luigi Casella,a Nick Hadjiliadis,*b Enrico Monzani,a Nikolaos Kourkoumelisc and
John C. Plakatouras*b
Received (in Cambridge, UK) 24th February 2005, Accepted 23rd May 2005
First published as an Advance Article on the web 21st June 2005
DOI: 10.1039/b502788h
proceeds smoothly and produces crystalline solids formulated as
[M(m-TBG)(m-H2O)(H2O)2]?2H2O [M: Cu (1), Co (2)].3b Both
compounds were characterized crystallographically and they were
found to belong to the P2/a space group with one half molecule in
the asymmetric unit.{ A water molecule is included, and it is
connected to the network, eventually formed, with four hydrogen
bonds. Each metal ion is coordinated, in a rather regular
octahedron (with larger Jahn–Teller distortion in the case of Cu,
in 1), to two oxygen atoms belonging to two monodentate
carboxylates of two different TBG ligands, two bridging and two
terminal water molecules, in trans positions (mean M–O distances,
The combination of a new pseudopeptidic ligand, transition
metal ions, and bridging water molecules results in the
formation of [M(m-TBG)(m-H2O)(H2O)2]?2H2O (M: Cu, Co
and H2TBG: terephthaloylbisglycine); both compounds show
rare two-fold interpenetrated three-dimensional cds-nets and
reversible loss of coordinated and lattice water molecules.
In recent years there has been extensive interest in metal–organic
polymer chemistry due to the variety of the composition and
topology of the produced compounds, and also to their interesting
functional properties and potential applications. The development
of rational synthetic routes by self-assembly has afforded an
important number of coordination polymers with specific
topologies.1 Metal–ligand coordination has been well used in the
directed assembly of extended porous metal–organic networks.
The ability to control the design of coordination networks arises
from the management of the coupling of the coordination
properties of individual metal ions and ligand functionality. One
of the key points for such studies is the design or choice of
components that organize themselves into desired patterns with
useful functions. Considerable use has been made of the rigid
linear bridging ligand 1,4-benzenedicarboxylic (terephthalic) acid
and also of related ligands in which various spacer groups connect
carboxylic pairs.2
˚
1: 2.092, 2: 2.102 A). The differences in M–O bonds are also
reflecting the two different kinds of H2O molecules in the
coordination sphere and can preclude the existence of hydroxide
species. The system’s coordination is presented in Fig. 1. The
structure of [M(m-TBG)(m-H2O)(H2O)2] is constructed from one-
dimensional chains interlinked through bridging water and TBG
groups into an open framework, three-dimensional structure. As
shown in Fig. 2, the chains consist of square planar nodes (the
terminal water molecules are not counted) linked through TBG
ligands. There are two different metal–metal distances: (a) a short
˚
one for the water bridge (1: 4.0878(8), 2: 3.986(2) A) and (b) a long
˚
one for the TBG bridge (1: 17.207(4), 2: 17.438(9) A). The
structure propagates in one direction as {M(TBG)} chains through
the bridging terephthaloylbisglycinate ligands. These linear chains
are linked in turn through a {M(H2O)}2+ undulating chain.
Adjacent {M(TBG)} chains cross at an angle of y48u to produce
the grid pattern seen in projection along c. The water bridges
propagate along the a axis, while the TBG bridges propagate
alternately parallel to the two diagonal directions of the bc face
forming this way a cds network.4
Our synthetic strategy is to introduce flexibility on the aromatic
rigid scaffold in addition to groups that would allow extra
stabilizing interactions and eventually to build up a higher
dimensional motif through metal–ligand interactions. Our first
approach is terephthaloylbisglycine,3a which is longer than
terephthalate, the carboxylate groups are reasonably free to rotate
and the amide bond can be the source of hydrogen bonds that
would be able to stabilize a network. Furthermore, bearing in
mind that aminoacids and peptides provide the glycine chelate ring
for coordination to metal ions, we could expect a bis-chelating
bridging behavior that could lead to an unprecedented polymer.
The reaction between equivalent amounts of the corresponding
metal nitrate and terephthaloylbisglycine (H2TBG) in water, at r.t.,
The rhombic channels shown in Fig. 3, exhibit a 68-atom
2
˚
connect to generate a cavity of y50 A cross section area. (Fig. S1)
aDipartimento di Chimica Generale, Universita´ di Pavia, Via Taramelli
12, 27100, Pavia, Italy
bDepartment of Chemistry, University of Ioannina, 451 10, Ioannina,
Greece. E-mail: iplakatu@cc.uoi.gr
Fig. 1 The coordination sphere of the Cu atoms and the ligand bonding
˚
cDepartment of Physics, University of Ioannina, 451 10, Ioannina,
Greece
in 1. 2 has a very similar structure. Selected bond lengths/A and angles/u, 1:
Cu(1)–O(1) 1.987(2), Cu(1)–O(4) 2.291(2), Cu(1)–O(5) 1.998(2); O(1)–
Cu(1)–O(4) 93.18(8), O(1)–Cu(1)–O(5) 88.80(10), O(4)–Cu(1)–O(5)
89.71(10); 2: Co(1)–O(1) 2.053(3), Co(1)–O(4) 2.182(2), Co(1)–O(5)
2.069(4); O(1)–Co(1)–O(4) 92.09(13), O(1)–Co(1)–O(5) 89.89(15), O(4)–
Co(1)–O(5) 90.10(14).
{ Electronic supplementary information (ESI) available: Figures, H-bond
Tables for the structures, vibrational, electronic and EPR spectra, thermal
analyses, XRPD patterns for original, dehydrated and rehydrated samples
and a vrml file with the topological presentation of the structures. See
http://dx.doi.org/10.1039/b502788h
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Fig. 4 A part of the H-bonding network in 1 showing the interaction
between the interpenetrating nets.
monodentate, and according to the IR spectrum is at least
bidentate chelating. The carbonyl oxygen appears to interact with
the metal ions, while the amide bond remains protonated.5
(Table S3) The electronic spectra for the intermediate of 1 suggest
reduction of the coordination number, while for 2, distorted
pseudotetrahedral geometry of the coordination sphere about
Co(II).6 (Fig. S5)
Fig. 2 The infinite 2D-layers based on {Cu3(TBG)(H2O)2}2 rings as part
of a 3D network in the structure of 1.
Although these channels have significant size there is no significant
void space as a consequence of the interpenetration of a second
three-dimensional gridwork, as illustrated in Fig. 3. (See also Fig.
S2) Interpenetration obviously is due to the length of the TBG
ligand. The two networks, dependent crystallographically, are
chemically connected through the lattice water, which forms four
H-bonds, two as a donor to the non-bonded oxygen atom of a
carboxylic group and to the amide oxygen of a different ligand but
of the same grid and two as acceptor from a terminal water
molecule of the previous grid and a peptide nitrogen of the other
grid. (Fig. 4, Tables S1–2)
When a dehydrated sample of either 1 or 2 is exposed in ‘‘wet’’
media,7 it reabsorbs the lost water molecules and reforms its
original structure, as shown from XRPD patterns.
We believe that this is due to the fact that above 140 uC, though
the 3D network is destroyed, the ligand remains intact and
coordinated to the metal ions, forming probably a lower
dimensional motif. There is an interaction which may not be
important for the stabilization of the initial interpenetrated motif
but could assist the stabilization of the dehydrated structure. The
distance of the least square planes of the phenyl rings of the two
Despite the coordination and the extended H-bonding network,
thermogravimetric analyses, under nitrogen, for 1 and 2 indicate
complete and rapid loss of all water molecules. Both ligated and
non-bonded water loss occur simultaneously in the temperature
ranges 80–120 and 75–125 uC for 1 and 2, respectively. The
anhydrous intermediates are stable up to 200 and 310 uC,
respectively, around where the decomposition begins, to give
metal oxides as final residues. (Fig. S3)
˚
different nets is 3.49 A, but we cannot characterize it as stacking
˚
due to the large offset of the ring centroids (2.12 A). Perhaps
dehydration reduces this offset and the interaction becomes
stacking and the metal ions remain close. This way the water
bridge can be easily formed upon rehydration.
The liquid He temperature EPR spectrum for a powder of 1
(Fig. S6) is characterized by a split broad signal centered at g y2.
It is indicative of pairs of interacting CuII paramagnetic centers.
The g 5 4 ‘‘semiforbidden’’ transitions which are predicted in the
case of a pure S 5 1 triplet, are not resolved even at high
microwave power.8 This indicates that the Cu–Cu coupling is
comparable to or smaller than the microwave energy i.e. 0.3 cm21
in our case. The spectrum for 2 is characterized by g values at 5.8,
3.9 and a weak trough at g 5 2. This is typical of high-spin
monomeric Co2+ S 5 3/2 centers with no resolvable magnetic
coupling between them. The g values at 5.8 and 3.9 indicate a
severe deviation from axial symmetry. This is due to the
rhombicity of the ligand field imposed by the three different kinds
of ligands in trans positions, as can be seen in the crystal structure.
To our knowledge complexes 1 and 2 are unique in metal
organic hybrid polymer chemistry with a number of novel features
which can be separated into two different categories. Those of
structural interest: (i) Though the cds topology possesses a
fundamental place in 4-connected networks and has received
considerable attention, our case is the most distorted.9 (ii) They are
the first examples of interpenetrated networks built by metal ions
X-ray powder diffraction on the stable intermediates suggests
that the initial structure collapses on heating. Infrared and
electronic spectroscopies provide useful tools for the elucidation
of the dehydrated structures. The carboxylate group is not
Fig. 3 Schematic presentation of the interpenetrating cds networks as
derived from the crystallographic data of 1. Spheres represent metal
centres while short and long bars represent water and TBG bridges,
respectively. The distortion of the cds network can be seen from the
differences in bridge lengths as well as from the dihedral angles defined by
the metal centres.
3860 | Chem. Commun., 2005, 3859–3861
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0.01 mol) in CHCl3 (20 ml), which was added slowly while maintaining
the temperature at 0 uC. After 2 h at r.t., the solution was evaporated,
yielding a yellow powder that was recrystallized from H2O. The purity
of the intermediate was checked with TLC (CHCl3 : AcOEt, 4 : 1). The
yellow powder was dissolved in MeOH (50 ml) and KOH (1.7 g,
0.03 mol) in MeOH (20 ml) was added slowly. After 48 h at r.t., the
solution was stripped to dryness, and the white residue was dissolved in
H2O and acidified down to pH 5 2 with HCl. After 5 h at 4 uC the
product was filtered and dried in vacuo. Yield 2.7 g, 95%. mp/uC 273(1)
(dec). Elemental analysis of TBGH2 (calc) C: 51.13(51.43) H 4.20(4.32)
N 9.87(10.00)%. IR (KBr), n/cm21 3358, 3307 (N–H) (s), 3086 (w),
2944 (w), 2749 (w) (O–H), 2637 (w), 2563 (w), 1711 (s) (CLO), 1639 (s)
(CLO amide), 1554 (s) (d N–H), 1501(w), 1408 (m), 1355 (m), 1328 (m),
1275 (w), 1228 (s) (C–O), 1002 (w), 936 (m), 866 (w), 832 (w), 730 (m).
1H NMR (250 MHz, DMSO)/ppm 3.94 (d, J 5 5.92 Hz, 4H), 7.96 (s,
4H), 8.97 (t, J 5 5.86 Hz, 2H), 12.62 (bs, 2H). 13C NMR (100 MHz,
DMSO)/ppm 42.1, 128.2, 137.1, 166.7, 172.1. ESI-MS m/z 560.9 (2M +
H)+, 319.0 (M + H + K)+, 281.2 (M + H)+; (b) 1: TBGH2 (0.2 g,
0.71 mmol) was suspended in H2O (10 ml) and to this NaOH 0.5 M
solution was added until the solution became clear (ca. 1.5 ml). To this a
solution of Cu(NO3)2?3H2O (0.35 g, 1.43 mmol) in 5 ml of H2O was
added. Greenish microcrystalline solid was formed after 3 h, which was
filtered, washed with ethanol and diethyl ether, and dried in vacuo.
Yield: 0.22 g, 71% based on TBGH2. For the preparation of crystals
50 mg of TBGH2 were dissolved in a mixture of 10 ml H2O and 3 ml
DMF. To this Cu(NO3)2?3H2O (85 mg in 5 ml of H2O) was added and
the resulting solution was left to evaporate slowly. A few crystals were
isolated after almost 7 weeks, washed with ethanol and dried in air.
Similar procedures were followed for the preparation of 2.
and two different linkers with such an immense difference in size
(one monoatomic and one nanometer-sized bridge). The closest
example we could find is [Cu(bpe)(H2O)(SO4)] (bpe 5 trans-1,2-
bis(4-pyridyl)ethylene),9b,10 and those related with important
advances in metal organic framework chemistry: (iii) They are
the first examples in the literature of 3D metal organic polymers
derived from a pseudopeptidic ligand. (iv) Bridging water
molecules occur in several crystal hydrates11 but, when unsup-
ported by other bridging ligands, are very rare in metal organic
polymers. A characteristic example is [Ni3(CTC)2(m-H2O)(H2O)]n
(CTC32
5 cis,cis-1,3,5-cyclohexanetricarboxylate), where the
water bridge is supported by a carboxylate bridge.12 A search in
the CSD13 reveals only two examples of unsupported water
bridges in 1D polymers. These are the carboxylates
[CoL2(m-H2O)(H2O)2] (L
5
3,4-dimethoxybenzoate14a and
3-hydroxy-4-methoxybenzoate14b). (v) Reversible loss of solvated
molecules is rather common in metal organic hybrid materials but
the reversible loss of bridging ligands is certainly rare.
Encouraged by the structure and properties of complexes 1 and
2, we are currently exploring the reactions of the TBGH2 ligand
with other metals in a variety of conditions, as well as the magnetic
and catalytic properties of the complexes presented here.
We thank Prof. Yiannis Deligiannakis, Department of
Environmental and Natural Resources Management, University
of Ioannina, for the EPR study and Dr Massimo Boiocchi, Centro
Grandi Strumenti, University of Pavia, and the Horizontal
Laboratory Network, University of Ioannina for the X-ray
measurements.
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7 In a typical experiment, a quantity ca. 50 mg of a sample of 1 or 2 is
heated at 140 uC in the oven, for 30 min. After the identity of the
intermediate is checked, the dry sample is placed in a glass desiccator,
beside a water filled beaker or is suspended in a ‘wet’ solvent like diethyl
ether or chloroform. The initial weight of the sample is restored faster in
the solvents, and this procedure can be repeated at least three times
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rehydrated samples.
Notes and references
{ Single crystal X-ray data: 1: C12H20CuN2O11, M 5 431.85, monoclinic,
˚
a 5 8.1755(6), b 5 6.8256(14), c 5 15.795(2) A, b 5 102.996(7)u,
3
˚
V 5 858.8(2) A , T 5 295(2) K, space group P2/a (no. 13), Z 5 2, m(Mo-
Ka) 5 1.334 mm21, 2713 reflections measured, 2504 unique (Rint 5 0.0255),
1541 (I ¢ 2sI) used for refinement. Final R(F2) 5 0.0471. 2:
C12H20CoN2O11, M 5 427.22, monoclinic, a 5 7.971(5), b 5 6.907(5),
8 A. Bencini and D. Gatteschi, EPR of Exchange Coupled Systems,
Springer-Verlag, Berlin, 1990.
3
˚
˚
c 5 16.012(5) A, b 5 103.899(19)u, V 5 855.7(5) A , T 5 295(2) K, space
group P2/a (no. 13), Z 5 2, m(Mo-Ka) 5 1.065 mm21, 2656 reflections
measured, 2484 unique (Rint 5 0.0217), 1632 (I ¢ 2sI) used for refinement.
Final R(F2) 5 0.0660. Despite the room temperature data, all hydrogen
atoms were located from difference Fourier maps. The C–H protons were
placed in idealized positions and treated as riding on the parent carbon
atoms. The amide and the water protons were refined isotropically leading
to small isotropic displacement parameters. For 2, restraints provided by
SHELX (DFIX and EADP (proton pairs)) were applied for the isotropic
refinement. CCDC 265226 and 267859. See http://dx.doi.org/10.1039/
b502788h for crystallographic data in CIF or other electronic format.
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3 (a) TBGH2 was synthesized by the reaction of a solution of glycine
methyl ester hydrochloride (2.51 g, 0.02 mol) and Et3N (5.6 ml, 0.04 mol)
in CHCl3 (30 ml) with a solution of terephthaloyl chloride (2.03 g,
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