Coordination polymers with pyridine-2,4,6-tricarboxylic acid and alkaline-earth/lanthanide/transition metals: Synthesis and X-ray structures

Article abstract of DOI:10.1039/b814066a

Pyridine-2,4,6-tricarboxylic acid (ptcH₃) reacts with Cd(ii), Mn(ii), Ni(ii), Mg(ii), Ca(ii), Sr(ii), Ba(ii), Dy(iii) salts forming different products depending on the reaction conditions. In the presence of pyridine at room temperature the ace

Full text of DOI:10.1039/b814066a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Coordination polymers with pyridine-2,4,6-tricarboxylic acid and
alkaline-earth/lanthanide/transition metals: synthesis and X-ray structures†
Madhab C. Das,a Sujit K. Ghosh,a E. Carolina San˜udob and Parimal K. Bharadwaj*a
Received 14th August 2008, Accepted 24th November 2008
First published as an Advance Article on the web 26th January 2009
DOI: 10.1039/b814066a
Pyridine-2,4,6-tricarboxylic acid (ptcH3) reacts with Cd(II), Mn(II), Ni(II), Mg(II), Ca(II), Sr(II), Ba(II),
Dy(III) salts forming different products depending on the reaction conditions. In the presence of
pyridine at room temperature the acetate, chloride or nitrate salt of Cd(II) breaks the ligand to form an
open framework structure with the empirical formula, {[Cd(Ox)(H2O)2]H2O}n (Ox = oxalate), 1. In the
absence of pyridine, no crystalline compound could be isolated at room temperature (RT). However,
under hydrothermal conditions and in the absence of pyridine, a discrete tetrameric complex with the
formula, {[Cd2(cda)2(H2O)4](H2O)3}2 (cdaH2 = 4-hydroxypyridine-2,6-dicarboxylic acid), 2, is formed
where the carboxylate group at the 4-position of the ligand is reduced to a hydroxyl group. When Ni(II),
Mn(II), Mg(II), Ca(II), Sr(II), Ba(II), Dy(III) salts are used in place of Cd(II), no crystalline product
could be isolated at RT. But under hydrothermal conditions, coordination polymers
({[Ni1.5(ptc)(pip)0.5(H2O)4]·H2O}n, (pip = piperazine), 3; {Mn1.5(ptc)·2H2O}n, 4; {Mg3(ptc)2·8H2O}n, 5;
{[Mg(ptc)(H2O)2]·1/2[Mg(H2O)6]·H2O}n, 6; {Ca1.5(ptc)·2H2O}n, 7; {Sr1.5(ptc)·5H2O}n, 8;
{[Ba(ptc)(H2O)][Ba(ptcH2)H2O]}n, 9; {[Dy(ptc)·3H2O]·H2O}n, 10) are formed. The structures exhibit
different dimensionality depending on the nature of the metal ions. In 1 a discrete acyclic water
hexamer is also identified. All the compounds are characterized in the solid state by X-ray
crystallography, IR and elemental analysis.
coordination polymers with s-block metal ions by K. M. Fromm
Introduction
has improved this literature a lot.4a,4b Transition metals are more
The development and identification of novel solid-state architec-
natural choices owing to the well-defined coordination geome-
tures is a general theme in the pursuit of new functional materials.
tries of d-block cations and cation clusters. Compared to this,
However, at times, finding strategies to prepare a designed solid-
reluctance in using alkaline earth cations as building blocks arises
state architecture can be highly empirical, and obtaining a phase
from their unpredictable coordination numbers and geometries
as no ligand field stabilization effects govern their bonding.5
The main issue for framework assembly with alkaline earth
ions is their high affinity for oxygen donors, particularly water.
Given the less predictable nature of alkaline earth assemblies, it
seems counter-intuitive to use them as building blocks although
less structural predictability does not preclude high degrees of
order or functionality. Also, lack of intrinsically useful properties
(i.e. magnetism or variable oxidation states) might appear to
make alkaline earth metals less interesting. However these metals
do have some advantages for application in materials science;
they are nontoxic, cheap and generally amenable to aqueous
preparation. For these reasons alkaline earth salts are the preferred
formulations for a host of commercial materials, including many
common pharmaceuticals and colorants.
We6 and others7 are exploring the coordination chemistry of
pyridine-2,4,6-tricarboxylic acid (ptcH3) to construct MOFs with
different dimensionality. This ligand has been chosen as aromatic
carboxylates are known to form thermally stable MOFs with
different types of metal ions. Another reason for choosing aro-
matic polycarboxylates is the inherent negative charge associated
with them that helps in the charge compensation of the metal
ion in the framework. We have used transition, alkaline earth and
lanthanide metals to build MOFs with different structures. Taking
into account all the previous studies6–7 along with the present one,
with the desired organization of components becomes a trial
and error process. Metal–organic framework (MOF) structures
can be synthesized by hydro(solvo)thermal or room temperature
reactions between metal ions and multi-dentate ligands. In most
cases, it may not be possible to predict the structural pattern by
looking at the ligand structure while, in a few cases, unexpected
reactions1–3 such as ligand oxidative coupling, hydrolysis or
substitution, can occur making it almost impossible to predict
the final structure. However, such ligand transformations provide
alternate routes for the construction of new MOFs.
The vast majority of work on coordination polymers has used
transition metals. However the chemistry of alkaline earth metals
has been a largely undeveloped area. There have been a few
studies4 on the coordination behavior of these metals in both
aqueous and nonaqueous media. However, the pioneering work on
aDepartment of Chemistry, Indian Institute of Technology, Kanpur, 208016,
India. E-mail: pkb@iitk.ac.in
bDepartament de Qu´ımica Inorga`nica, Universitat de Barcelona, Diagonal
647, 08028, Barcelona, Spain
† Electronic supplementary information (ESI) available: X-Ray crys-
tallographic files in CIF format for the structure determination of
compounds 1–10; additional figures, selected bond lengths and angles
for the compounds 1–10. CCDC reference numbers 697859–697868. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b814066a
1644 | Dalton Trans., 2009, 1644–1658
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Table 1 Symmetric stretching (ns), asymmetric stretching (nas), their
differences (Dn) and the binding mode of the carboxylate groups of
compounds 3–10
of indefinite composition almost instantaneously. The reaction
under hydrothermal condition in the absence of a base, affords
a discrete tetrameric compound (2) in good yield. With a Co(II)
salt and pyridine at room temperature, the ligand forms a 1D
coordination polymer (the same can be prepared7b without using
pyridine as reported by Cheng et al.) without any ligand cleavage.
The same reaction under hydrothermal condition, does not give
any crystalline product. Several Ni(II) salts when allowed to react
with the ligand, with or without a base like pyridine or piperazine,
give a mixture of products but on hydrothermal reaction in the
presence of piperazine afford only the crystalline product 3 in high
yields (40% or more). Thus, the breakage of the ligand to oxalate is
observed only with Cd(II) and Zn(II)6a in the presence of pyridine.
However, neither Cd(II) nor Zn(II) can break the ligand when a
base other than pyridine is used. At this stage, we are unable to
suggest a mechanism for the cleavage of the ligand. Further studies
are underway to access the role of various metal ions and presence
of pyridine on the cleavage of this ligand to form various polymeric
metal oxalate compounds.
Compound nas
ns
Dn
Binding mode
3
1639
1608, 1556 1367
1629, 1563 1365
1630, 1533 1395
1624
1617, 1510 1370
1642, 1583 1440
1608, 1528 1394
1390, 1360 249, 279 Unidentate
4
241, 189 Unidentate
246, 198 Unidentate
235, 138 Unidentate, chelate
5
6
7
1384, 1356 240, 268 Unidentate
8
247, 140 Unidentate, chelate
202, 143 Unidentate, chelate
214, 134 Unidentate, chelate
9
10
we have attempted to rationalize how the dimensionality of the
MOFs formed by the ligand ptcH3 varies with the nature of the
metal ion.
Results and discussion
All the compounds are stable in air, sparingly soluble in water and
insoluble in common organic solvents. High yields of the prod-
ucts indicate that they are thermodynamically stable under the
prevailing reaction conditions. Each exhibits a strong absorption
band in the region, 1350–1550 cm-1 diagnostic8 of coordinated
carboxylates. The uni-dentate and chelate mode of coordination of
carboxylate groups in 2–10 can be distinguished by FTIR spectra.9
A separation (Dn) of 40–120 cm-1 between asymmetric nas(O–C–O)
and symmetric ns(O–C–O) indicates chelating mode, while a
separation of 210–270 cm-1 is indicative of the uni-dentate mode of
binding (Table 1). The absence of characteristic bands at around
1700 cm-1 rules out the presence of protonated carboxylate groups
in the complexes 2–1010 except for complex 9.
Literature survey reveals existence of several cadmium oxalate
structures under different reaction conditions. The compound
[Cd2(m5-C2O4)(m3-OH)2],11 which was synthesized from hydrother-
mal reaction of CdCl2, H2C2O4, imidazole and H2O shows a five-
bridging coordination mode of oxalate unit having inter-crossed
[Cd(OH)]• corrugated sheets and a 3D [Cd2(ox)]• network. Rao
et al. reported alkali halide incorporated cadmium oxalate12–13
synthesized by metathetic reaction employing hydrothermal meth-
ods. The same group also reported14 a 3D cadmium oxalate open
framework constructed from amine and K+ ion. A polymeric
cadmium oxalate species quite similar to 1, was prepared15 by
decomposition of ascorbic acid in the presence of Cd(II) ion
under mildly acidic condition. The structure of 1 consists of
oxalate anions bonded to Cd(II) forming a 2D open framework
structure. Each oxalate anion binds four Cd(II) ions extending
along the crystallographic b axis where two of the O atoms act
as bridging ones. Along the crystallographic a axis, however, each
oxalate shows D2h symmetry binding two Cd(II) ions. The almost
Structures
Structure of {[Cd(Ox)(H2O)2]H2O}n, (1). The Cd(II) ion
breaks the ligand in the presence of pyridine forming a Cd(II)–
oxalate coordination polymer, 1. The counter anion does not play
any role in this transformation as the same product (1) is formed
with different salts of Cd(II) such as acetate, chloride or nitrate.
In the absence of pyridine or in the presence of a different base
such as piperazine or tetraalkylammonium hydroxide, reaction
between a Cd(II) salt and the ligand affords a white precipitate
2
˚
square voids thus formed have the dimension, 5.99 ¥ 6.00 A
(atom to atom distance). Each Cd(II) shows hepta-coordinated
pentagonal bipyramidal geometry with ligation from five O atoms
of three oxalate groups and two water molecules (Fig. 1). All bond
Fig. 1 A perspective view of 2D open framework structure of complex 1.
The Royal Society of Chemistry 2009 Dalton Trans., 2009, 1644–1658 | 1645
This journal is
©

View Article Online
distances and bond angles are within normal ranges. Selected bond
lengths and bond angles involving the metal ion are collected in
the ESI (Table S1).† The present structure appears to be identical
to that of {Cd2(C2O4)·6H2O}n15 reported by Orioli et al. Although
the crystal data parameters of this reference compound is the same
as that of ours, the space group for the triclinic system used by
Structure of {[Cd2(cdaH)2(H2O)4](H2O)3}2, (2). The structure
of 2 consists of a discrete tetra nuclear Cd(II) cluster where
the carboxylic acid group at the 4-position of the ligand is
converted to hydroxyl group and remain uncoordinated (Fig. 3).
This conversion of pyridine-2,4,6-tricarboxylic acid (ptcH3) to
chelidamic acid (cdaH3) is readily achieved under hydrothermal
condition. Each terminal Cd(II) ion in 2 is bonded to the pyridine
N and two carboxylate O at the 2- and 6-positions of one cdaH2-
and two carboxylate O at the 2-positions of another cdaH2-
ligands giving a NO4 coordination in the pentagonal plane. Two
water molecules occupy the axial sites completing pentagonal
bipyramidal coordination geometry. Each of the two Cd(II) ions in
the middle is bonded to one carboxylate O at the 5-position of the
previous cdaH2- and the pyridine N and two carboxylate O at the 2-
and 6-positions of the neighboring cdaH2-, along with one water
molecule equatorially. Two water molecules are bonded axially
giving each metal ion a hepta-coordinated pentagonal bipyramidal
geometry.
¯
them is P1 (No. 1) whereas we have used the higher symmetric P1
(No. 2) to solve the structure.
A striking feature of this complex is identification of a discrete
acyclic water hexamer that was also present in the crystal structure
of the above reference compound15 but not mentioned by the
authors. Among small water clusters, the hexamer is subjected
to a number of experimental and theoretical studies as this cluster
is the building block of ice16 and also exhibits17 some of the
properties of bulk water. Theoretical calculations18 have predicted
five low energy isomers for this cluster that are iso-energetic.
While chair,19 boat,20 and planar cyclic21 forms of hexamers have
been identified in crystal hydrates, the ‘cage’ conformer predicted
to be most stable at very low temperature has been observed22
by vibration–rotation tunneling spectroscopy and to the best of
our knowledge, so far, no organic or metallo-organic compound
is known where an acyclic water hexamer has been detected.
This water hexamer has a center of inversion symmetry and is
made up of one ‘free’ and two metal-bound water molecules with
their symmetry related molecules through H-bonding interactions,
and is depicted in Fig. 2 along with its immediate H-bonding
coordination environment. In the hexameric unit, the average
Structure of {[Ni1.5(ptc)(pip)0.5(H2O)4]·H2O}n, (3). The struc-
ture consists of Ni(II), ptc3-, piperazine and water. It can be
described as a piperazine-bridged dimer where each Ni(II) ion
is equatorially coordinated to the pyridine N, carboxylate O at
2- and 6-positions and a piperazine N and the two axial sites are
occupied by two water molecules showing a distorted octahedral
coordination geometry. Another Ni(II) ion is axially coordinated
to the carboxylate present at the 4-position of the ptc3- in a
monodentate fashion thus joining the dimeric moieties and form a
1D coordination polymeric chain along the crystallographic a axis.
The ligand ptc3- has a m4 coordination mode (Type II, Scheme 1) in
this complex. Free water molecules occupy the voids in between the
parallel chains (Fig. 4). These water molecules are involved in H-
bonding interactions to ‘O’ of carboxylates as well as metal-bound
water molecules of neighboring chains to reinforce the structure.
˚
distance between the oxygen atoms is 2.863 A, which is comparable
˚
with the O ◊ ◊ ◊ O distance in liquid water (2.85 A), while for
16b,17b
˚
the gas phase this value is ca. 0.1 A longer.
The water
molecules present in clefts of biological/abiological molecules are
very different from the gas phase or bulk liquid water. Here, the
principle of maximizing H-bonding interactions is followed where
the environment plays a major role. While Ow1 acts as two H-bond
donors to two different carboxylate O atoms, Ow3 acts as two H-
bond donors to one carboxylate O atom and a water molecule
(Ow1). The H-bonding parameters are listed in Table S2 (ESI†).
Interestingly, the hexameric water clusters act as pillars separating
two layers of coordination polymers with an approximate distance
Structure of {Mn1.5(ptc)·2H2O}n, (4); {Ca1.5(ptc)·2H2O}n, (7).
Compounds 4 and 7 are isomorphous and hence only the
structural description of 4 is presented. The asymmetric unit
consists of one ptc3- unit, two Mn(II) and two metal-bound water
molecules (Fig. 5). Atom Mn(2), exhibits a distorted octahedral
surrounding (Fig. S1, ESI†), in which the basal plane is formed
˚
of 5.488 A between the layers leading to an overall H-bonded
3-D structure.
Fig. 2 A view of the discrete acyclic water hexamer in 1. Symmetry codes: (i) 2 - x, 1 - y, -1 - z; (ii) 2 - x, 1 - y, -z; (iii) 1 + x, y, z.; (iv) 2 - x, 1 - y, 1 - z;
(v) 4 - x, -1 - y, 1 - z (vi) -x, 1 - y, -1 - z; (vii) x, y, 1 + z; (viii) x, y, -1 + z.
1646 | Dalton Trans., 2009, 1644–1658
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Fig. 3 A view of the discrete tetranuclear Cd(II) complex in 2.
Scheme 1 Coordination modes of ligand ptc.
by four carboxylate O atoms from four ptc3- ligands. This plane is
almost perpendicular to the mean plane of the pyridine ring (ca.
86.7◦). Two water molecules occupy the axial positions. On the
other hand, Mn(1) forms two five-membered chelate rings with
one ptc3- ligand (bite angle = 68.99◦, 66.90◦). One carboxylate
O atom and one O atom of a water molecule (Ow1) along with
the chelating atoms form a basal plane of pentagonal bipyramidal
geometry. Axial sites are occupied by two carboxylate O atoms
from two ptc3- units. All M–O and M–N distances are within
normal ranges23 and are listed in Table S1 (ESI†). There is a m7
coordination motif of the ligand ptc3- in this complex (Type VII,
Scheme 1).
Two sidewise pillars constructed from alternate repeating units
of Mn(1) and a carboxylate group [C(7)O(1)O(2)] are bridged
by two symmetry related carboxylate O atoms (O3) forming
four-membered Mn2O2 sets of Mn(1) dimers. In each of the
ladders, the pyridine rings of ptc3- units are involved in two
different p–p stacking interactions (centroid-to-centroid distances
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1644–1658 | 1647
©

View Article Online
Fig. 4 The 1D coordination polymeric structure in complex 3.
Fig. 5 Coordination environment of 4 (ORTEP drawing with 50% probability of ellipsoids). Symmetry codes: (i) 1 + x, y, z; (ii) x, -1 + y, z; (iii) -x, 1 -
y, 1 - z; (iv) -1 - x, 1 - y, -z; (v) x, 1 + y, z; (vi) -1 + x, y, z; (vii) -1 - x, 1 - y, 1 - z; (viii) -x, -y, 1 - z; (ix) 1 + x, 1 + y, z; (x) -x, 1 - y, -z; (xi) -x, 2 - y, 1 - z.
˚
˚
Structure of {Mg3(ptc)2·8H2O}n, (5). The asymmetric unit of 5
consists of three Mg(II), two ptc3- units and eight water molecules.
The Mg(2) is coordinated by six atoms to form a distorted
octahedral geometry where four water molecules are bonded
equatorially. The axial sites are occupied by two carboxylate O
atoms (O3, O5) from two different ptc3- units. Mg(1) also exhibits a
distorted octahedral geometry with equatorial coordination from
ring N and two O atoms from two carboxylate groups at the
2- and 6- positions of a ptc3- ligand (i.e. NO2 donor set). The
remaining equatorial site is occupied by another carboxylate O
atom at the 2-position belonging to a different ptc3- unit and the
axial sites are occupied by two water molecules (Ow5, Ow6). The
same geometry is attained by Mg(3) atom with similar mode of
bonding. There are two different coordination modes for ptc3- unit
in this complex: one with a connectivity number of m6 and the
other m3 (Type I, VI, Scheme 1). Fig. 8 represents the coordination
environment of 5, in which the trinuclear unit is the building
block of the structure. These building units are propagating the
1D coordination polymeric chains along the crystallographic a
axis. The pyridine rings of one chain are involved in p–p stacking
are 3.205(4) and 3.299(4) A with offset distance of 0.51 and 0.69 A
respectively) in opposite faces with two different Mn(1)–Mn(1)
˚
corner distances of 7.819 and 7.385 A respectively (Fig. 6). These
layers of ladder-cum-pillars run along the crystallographic a axis.
Octahedrally coordinated Mn(2) connects these layers through
two different carboxylate O atoms to form a robust 3-D network.
(Fig. 7).
For complex 7, bond lengths and angles at the calcium atoms
are compiled in Table S1 (ESI†). The Ca–O (carboxylate) bond
lengths in 7 lie in the ranges of 2.266(4)–2.551(4) A. A study based
on the data extracted from the Cambridge Structural Database
(CSD) found a range of 2.27–2.49 A for Ca–O bonds involving
monodentate carboxylate.24 So, the values observed in this case
˚
˚
˚
are thus exceptional only for Ca2–O1 = 2.551(4) A. The CSD
˚
˚
study mentioned also gives 2.31–2.49 A and 2.33–2.60 A for
Ca–OH2 bond lengths in seven- and eight-coordinate complexes,
respectively. This is consistent for the Ca–OH2 (terminal) bond
˚
lengths which lie between 2.340(5)–2.382(4) A in complex 7.
Here also, the pyridine rings of ptc3- units are p-stacked in
opposite faces with two different centroid-to-centroid distances
˚
˚
˚
interactions (centroid-to-centroid distance of 3.478(4) A with
of 3.350(4) and 3.455(4) A (offset distances are 0.58 and 1.13 A
respectively).
˚
offset distance of 1.09 A) with the pyridine rings of neighboring
1648 | Dalton Trans., 2009, 1644–1658
This journal is The Royal Society of Chemistry 2009
©

View Article Online
Fig. 6 p–p stacking interactions among ptc units in complex 4.
Fig. 7 View of 3D framework of 4.
chains along the b direction. The metal-bound water molecules
of one chain are involved in an intricate array of H-bonding
interactions to ‘O’ of carboxylates as well as the water molecules of
another chains. These H-bonding interactions among the chains
extend the structure along the crystallographic b axis (Fig. S2,
Table S3, ESI†). The lengths of the Mg–O bonds fall in the narrow
range 2.008(5)–2.200(5) which are within normal limits4d,g,m and
are summarized in Table S1 (ESI†).
molecules and one free water molecule. The Mg(II) ion in the
general position (Mg1) shows a distorted pentagonal bipyramidal
geometry with equatorial coordination from two ptc3- units (NO4
donor set), while the axial sites are occupied by two water
molecules (Type V, Scheme 1). Thus each organic anion binds two
Mg1 ions to form a coordination polymeric chain. These chains
runs parallel to the b axis making void spaces in between that are
occupied by inorganic hexa-aqua magnesium cations (the Mg2
position coinciding with a crystallographic center of symmetry),
and free water molecules (Fig. 9). These water molecules are
involved in an intricate array of H-bonding interactions to un-
bound ‘O’ of carboxylates as well as metal-bound water molecules
of neighboring 1D chains. Calculation of the solvent accessible
Structure of {[Mg(ptc)(H2O)2]·1/2[Mg(H2O)6]·H2O}n (6).
For this compound, the asymmetric unit comprises of one ptc3-
ligand, one Mg(II) ion in a general position, another Mg(II) ion
on a crystallographic center of symmetry, five metal-bound water
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1644–1658 | 1649
©

View Article Online
Fig. 10 View of the coordination environment of the metal center in
complex 8 (ORTEP drawing with 50% probability of ellipsoids). Symmetry
codes: (i) 0.5 - x, 0.5 + y, z; (ii) 1 - x, 1 - y, -z; (iii) 0.5 - x, -0.5 + y, z;
(iv) 1 - x, y, 0.5 - z; (v) -x, y, 0.5 - z.
hand, Sr(1) is also coordinated to nine atoms, including (O1,
N1, O6) the pair of symmetry related NO2 donor set, another
pair of symmetry related oxygen (O3) from a carboxylate group.
The remaining oxygen atom comes from a coordinated water
molecule (Ow5) to give Sr(1) an overall singly capped square
antiprism geometry (Fig. S3, ESI†). The Sr–O and the carboxylate
C–O bond distances in this network span the range 2.542(4)–
Fig. 8 Coordination environment of 5 (ORTEP drawing with 50%
probability of ellipsoids). Symmetry codes: (i) -0.5 + x, -0.5 + y, z;
(ii) 0.5 + x, 0.5 - y, 0.5 + z; (iii) x, -y, -0.5 + z; (iv) -0.5 + x, 0.5 - y,
-0.5 + z; (v) 0.5 + x, 0.5 + y, z; (vi) x, -y, 0.5 + z.
volume25 reveals that 6 contains ~13% void volume. All Mg–O
bond lengths and angles are within limits ranging from 2.039(3) to
˚
˚
˚
2.849(4) A and 1.228(6)–1.269(6) A respectively, consistent with
the values observed in other metal complexes.4p Selected bond
distances and angles for this complex are summarized in Table
S1 (ESI†). The ligand ptc3- has a m8 coordination mode in this
complex (Type X, Scheme 1). It is interesting to note that there
are three m2 carboxylate oxygen atoms (O1, O3, O6) which bridge
2.251(3) A and are listed in Table S1 (ESI†). The presence of a free
water molecule in the asymmetric unit differs this structure slightly
from {[M(H2O)6][M(ptc)(H2O)2]2·2H2O}n (M = Fe/Co).7b
Structure of {Sr1.5(ptc)·5H2O}n, (8). The asymmetric unit in
this structure comprises of one ptc3- ligand, two Sr(II) atoms and
5 metal-bound water molecules. Fig. 10 shows the coordination
environment of two crystallographically independent Sr(II) ions.
Quite differently from the coordination environments in 1–6, the
metal Sr(2) in 8 is nine-coordinated by O atoms, of which five
are from water molecules and remaining are from three different
ptc3- units out of which two are involved in chelation, resulting in
a singly capped square antiprism (Fig. S3, ESI†). On the other
˚
the independent Sr atoms to form a Sr(1)Sr(2) dimer (3.982(3) A).
To the best of our knowledge no such metallo-dimer is yet known
where the dimer is formed by m2 bridging of three different car-
boxylate oxygen atoms. Another dimer, namely Sr(2)Sr(2) dimer,
˚
is formed (4.353(3) A) by the bridging of two symmetry related
water molecules (Ow1) with a tetrahedral bridging angle (Sr2–
Ow1–Sr2 = 108.47(15)◦) suggesting a tetrahedral coordination
Fig. 9 A perspective view of complex 6.
This journal is
1650 | Dalton Trans., 2009, 1644–1658
The Royal Society of Chemistry 2009
©

View Article Online
environment (others angles are: H1W1–Ow1–H2W1 = 106(6)◦,
H1W1–Ow1–Sr2 = 105(5)◦, 116(5)◦; H2W1–Ow1–Sr2 = 103(3)◦,
119(3)◦ around the water molecule. While Sr(1) is shared by the
two Sr(1)Sr(2) dimeric units, Sr(2) is shared by one Sr(1)Sr(2)
and another Sr(2)Sr(2) dimeric units. Thus these dimeric chains
can be best described as [Sr(2)Sr(2)Sr(1)]n which run along the
crystallographic c axis. The other point to note is that these two
types of metals do not lie in a plane but rather form puckered
sheets. These metal oxide layers are interconnected by ptc3- ligands
as pillars to form a 3D network as shown in Fig. 11. These
pillars are approximately perpendicular to the puckered metal
oxide sheets and the pyridine rings of these pillars are parallel
with respect to each other with a centroid-to-centroid distance of
two water molecules coordinated to each of the Ba atoms (Fig. 12).
There is no uncoordinated water molecule in the structure. The
coordination polyhedra around Ba(II) ions can be described as a
trigonal hexadecahedron and trigonal dodecahedron for Ba1 and
Ba2 respectively (Fig. S4, ESI†). The Ba(1) ion is bonded to four
mono-dentate carboxylate O atoms (O3, O4, O8, O11) from four
ptc3- units, two chelate oxygen atoms (O9, O10) from a different
ptc3- unit, one NO2 donor set (O1, N1, O6) from another ptc3- unit,
and a water molecule (Ow2) completing ten coordination. This
is a rare example of six multicarboxylate ligands coordinating
to one metal center. The coordination environment of Ba(2) is
quite different. Here, the metal ion is bound to two mono-dentate
carboxylate oxygen atoms (O9, O10) from two ptc3- ligands, two
chelate oxygen atoms (O3, O4) from another ligand unit, and a
water molecule (Ow1) attaining eight coordination geometry. The
ligand has two coordination modes in this structure, one with
a connectivity number of m9 and the other m7 (Type VIII, IX,
Scheme 1). All bond distances involving the metal ion are within
normal range.26 The N(1)–Ba(1) and N(2)–Ba(2) distances in 9
˚
˚
3.483(4) A (offset distance = 1.21 A) showing p–p interactions.
The faces of the aromatic rings are alternate with respect to the
position of ring N atom as can be seen in 5.
Structure of {[Ba(ptc)(H2O)][Ba(ptcH2)H2O]}n, (9). This com-
pound displays a complicated 3D structure, containing two
crystallographically independent Ba atoms, two ptc3- ligands and
˚
(2.947(10) and 2.991(9) A respectively) are larger than N(1)–Sr(1)
˚
distance (2.708(4) A) found in 8, which is in accordance with
the larger radius of barium than strontium. The Ba ◊ ◊ ◊ O bond
distances are also within normal range.4o,p In contrast to 8, in
which two types of metal dimers form puckered sheets, the metals
in 9 (Ba1, Ba2), form a zigzag type of dimer (namely Ba1Ba2
˚
dimer) with a Ba–Ba separation of 4.523(2) and 4.455(2) A. It
is worth noting that there are two sets of chelate oxygen atoms
(O3, O4 and O9, O10) from two unique ligand moieties which
are basis of these dimeric chains where each type of metal ions
sit in separate rows of the zigzag chain (Fig. 13). These metal
oxide dimeric sheets are running along crystallographic a axis.
Moreover, each of the two unique ligands simultaneously chelates
to two unique cations and bridges to two neighboring metal oxide
chains like a pillar. The p–p interaction is the total interaction
between two aromatic moieties including van der Waals’ and other
electrostatic interactions having energies in the 3–10 kcal mol-1
range that stabilize molecules in many crystals.27 The pillars in 9
are also held together by significant p–p interactions. The pillars
which bridges Ba(2), appears to have significant p–p interactions,
with two different centroid-to-centroid distances of 3.573(4) and
Fig. 11 View of the 3D structure of 8 along the b axis.
Fig. 12 A view of the coordination environment of 9. Symmetry codes for (a): (i) -x, 2- y, -1 - z; (ii) -x, 2 - y, -z; (iii) -1 - x, 1 - y, -z; (iv) 2 - x, 2 - y,
-1 - z; (v) 1 - x, 1 - y, -2 - z; (vi) x, y, 1 + z; (vii) x, -1 + y, z; (viii) 1 - x, 2 - y, -1 - z; (ix) -1 - x, -y, -z. For (b): (i) -x, 2 - y, -1 - z; (ii) -x, 2 - y, -z;
(iii) -1 - x, 1 - y, -z; (iv) 2 - x, 2 - y, -1 - z; (v) 1 - x, 1 - y, -2 - z; (vi) x, y, 1 + z; (vii) x, -1 + y, z; (viii) -x, 1 - y, -z; (ix) -1 + x, y, z.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1644–1658 | 1651
©

View Article Online
Fig. 13 p–p stacking interactions among ptc units in complex 9.
Fig. 14 View of the 3D structure of 9.
˚
˚
A striking feature of this structure is the arrangement of Dy(III)
ions in infinite single helices extending along the crystallographic b
axis with intervening ptc3- ligands. As the compound crystallizes in
the centro-symmetric space group P21/c, there are equal numbers
of left and right-handed chains. A left-handed helix is depicted
in the Fig. 16. The helical structure is a result of metal–ligand
interactions coupled with stereoelectronic characteristics of the
ligand and the conditions prevailing during the synthesis. The
3.909(4) A (offset distances of 1.45 and 1.91 A respectively) among
the participating pyridine rings. On the other hand, the pyridine
rings of the pillars connecting Ba1 atoms appear to have a short
˚
centroid-to-centroid distance of 3.382(4) A (offset distance =
˚
0.96 A) which is clearly attributed to strong p–p interaction.
˚
Another centroid-to-centroid distance of 4.127(4) A which is large
enough, cannot be involved in p–p interaction. Fig. 14 represents
the 3D structure of 9.
˚
pitch of the helix is calculated to be 7.24 A based on the
Structure of {[Dy(ptc)·3H2O]·H2O}n, (10). The single crystal
analysis reveals that complex 10 is a 3D open MOF constructed
with ptc3-, Dy(III) ion and water molecules. The asymmetric unit
consists of one Dy(III) ion, one ptc3-, three metal-bound and one
free water molecules. The Dy(III) ion is nine coordinated (Fig. 15)
and is described as singly-capped square anti-prism with bonding
from five carboxylate O atoms from three ptc3- units, one ring N
atom from one of the ligands and three water molecules (Fig. S5,
ESI†). The Dy–O and Dy–N bond lengths are within the normal
range. Each ptc3- unit links five Dy(III) centers and four O atoms
in addition to the ring N atom (Type XI, Scheme 1).
repeated unit containing two Dy(III) ions and 2 ligands. The
stability of the final helical structure relies on the coordinate
bonds that each metal makes with ptc3- ligand and the medium
value of the pitch is a result of topology of this ligand along with
electrostatic repulsive interactions among the metal centers.
Magnetic measurements
Variable temperature magnetic susceptibility data were collected
for 4 in the 2 to 300 K temperature range with an applied magnetic
field of 0.3 T. The susceptibility variation with temperature is
1652 | Dalton Trans., 2009, 1644–1658
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Fig. 15 View of the coordination environment around Dy(III) in 10.
Symmetry codes: (i) x, -0.5 - y, 0.5 + z; (ii) 2 - x, -y, 2 - z; (iii) x,
1 + y, z.
Fig. 17 (a) Temperature dependence for the susceptibility per Mn(II) ion
in 4 (c is shown in circles, cT is shown as triangles). (b) Magnetization
plot per Mn(II) ion in 4. The solid line is the Brillouin function for Mn(II)
ion with g = 2.0.
Structural trends
The strategies to control network structure are important for
design and synthesis of functional solids whose properties are
inherent in their 1D, 2D or 3D structures. There have been recent
trends in studying the dimensionality of s-block coordination
networks. Fromm has investigated28 the polymeric structures of
hydrated Ca(II) and Ba(II) polyether complexes as their iodide
salts where the M(II) units are cross-linked between metal-bound
water molecules and uncomplexed iodide ions by H-bonding
interactions with concluding remarks by the authors that the
dimensionality of the polymers formed were primarily related to
the number of water molecules per cationic complex, which in
turn, could be controlled by the denticity of the polyether ligands
and the radii of the cations used. In a more specific approach with
sulfate ions, Squattrito and Cai have studied the coordination
behavior of mono-4n and disulfonate4m ligands with alkali and
alkaline earth cations to obtain lamellar structures. Here, the
authors have shown that with the decreasing charge/radius ratio
of the cation there is the increment in coordination ability of the
ligand.
Fig. 16 View of the left-handed single helix in compound 10:(a) ball-and–
stick representation and (b) space-filling representation.
shown in Fig. 17a. As the temperature decreases, the susceptibility
increases, following the Curie–Weiss law, with a negative Weiss
temperature, indicating weak antiferromagnetic coupling between
the Mn(II) ions. The cT vs T plot decreases slowly as temperature
goes down, until at temperatures below 80 K the decrease in the
cT product is more pronounced. The magnetization increases
with increasing field in a nearly linear fashion, and is at all times
well below the calculated Brillouin curve for an isolated Mn(II)
with g = 2.0, which supports the antiferromagnetic coupling in
4 (Fig. 17b). A close look at the structure shows a 3D grid of
Mn(II) ions where each Mn(II) ion is linked to four other Mn(II)
by syn, anti-carboxylato arms of the ptc3- ligands. This bridging
mode will favour weak antiferromagnetic interactions between the
metal centres, as observed in the magnetic data presented here.
The nature of the metal largely governs which class of structure
is formed and this can be understood by considering the interplay
of the basic factors governing s-block metal coordination, that
is, charge, size and electronegativity. As the most electronegative
metal investigated, Mg–ligand bonds have a higher covalent
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1644–1658 | 1653
©

View Article Online
contribution than the others, whilst as the smallest group 2 ion,
the higher charge density on Mg(II) means that it tends to make
stronger interactions with Lewis bases. It is known that Mg–OH2
bonds are considerably more stable than the other M–OH2 bonds
considered here.29 As can be seen, the asymmetric unit of 5 contains
eight water molecules, the highest among the complexes discussed
here. Crystallization from water is in effect a competition reaction
between the carboxylate groups and neutral water molecules; the
higher covalent nature of magnesium thus appears to favor direct
hydration of metal over less interaction with the carboxylate group
as can be seen for 6. In the 3D structure of Ca(II), Sr(II) and
Ba(II) (complex 7, 8, 9 respectively), there is markedly lower level
of hydration. Previous studies on this ligand by us6 and others7
have shown that this ligand readily reacts with transition metals
as well as lanthanides to yield a wide variety of structures. In
this present paper, we have extended our investigation to systems
containing alkaline earth metals, a few of the transition metals and
a lanthanide ion. A schematic representation showing the various
coordination modes of the ligand ptc3- with different connectivity
numbers (m) used in this study as well as in previous studies6,7 are
given (Scheme 1). As a comparison, atomic radii of ions, ligand
connectivity (m), range in coordination number and structural
variation are listed in Table 2, which provides an understanding
of the key factors determining variation in these structures. The
coordination numbers of the transition metals vary from 6 to 7
and for the lanthanides and alkaline earth metals this span is 8–
10 except for Ca(II) ion. So, coordination number increases as a
result of increase in ionic radii. Although the ligand connectivity
numbers down the table is not in gradual increment order, it
can be said that metals with higher radii have generally higher
connectivity numbers. For the case of Fe, the connectivity number
is high which may be considered in a very little difference in radii
first part contains the metal ions from Fe to Zn (ionic radii 0.64
˚
to 0.74 A) and second part contains metal ions from Mn to Ba
˚
(ionic radii 0.80 to 1.35 A). In contrast, the other alkaline earth
metals along with lanthanides have higher 3D networks due to
their high ligand connectivity number and coordination number.
So, in general, structures of higher dimensions are obtained going
down the table.
Conclusion
We have shown the formation of a Cd–oxalate coordination
polymer as a result of breakage of the ligand, pyridine-2,4,6-
tricarboxylic acid (ptcH3) at RT in the presence of pyridine.
The solid-state structure also shows the presence of a discrete
acyclic water hexamer. However, under hydrothermal condition,
ligand ptcH3 is reduced to chelidamic acid that binds Cd(II)
forming a 1D coordination polymer. Prior to this study, struc-
tures of coordination polymers of seven transition and three
lanthanide metals with the ptcH3 ligand were known.6–7 We have
now expanded this series by adding few more transition and
lanthanide metals as well as several alkaline earth metals. With this
structural information now available, it is possible to rationalize
the seemingly random variety of coordination networks formed
with this ligand. Thus, a tentative trend in the coordination ability
of metal ions toward aromatic carboxylates is emerging: alkaline-
earth metals ª trivalent lanthanides > divalent transition metals.
This study presents a strategy to form coordination networks
not only with different levels of aggregation between metal ions
and the ligand, but also with different degrees of dimensionality
difference in the number of metal–carboxylate interactions that
can form, as rationalized by cation radii and coordination number,
manifests into a dimensional evaluation between the networks,
propagating down Table 2. This study also presents a strategy
to form coordination networks with soft closed shell cations of
Group 2. These metals should not be discounted as potential
candidates for building blocks in extended coordination networks
for potential applications.
˚
among the transition metal ions (maximum difference is 0.10 A
between Fe and Zn) except for the case of Mn, which is relatively
larger in size. Either 1D or 2D structures are obtained for the
first row transition metals (exception for Mn) as well as Mg whose
ionic radii, ligand connectivity and coordination numbers are very
similar. Thus, we can divide the whole Table 2 in two parts: the
Table 2 Ionic radii, ligand connectivity (m), metal coordination number (CN), and structure dimensionality (D) of M-ptc compounds
Complex
Metal ion
Ionic radii
m
CN
D
Reference
{[Fe(H2O)6][Fe(ptc)(H2O)2]2·2H2O}n
{Mg3(ptc)2·8H2O}n,
Fe
0.64
0.65
0.65
0.72
0.72
0.72
0.72
0.74
0.74
0.74
0.74
0.80
0.90
0.90
0.91
0.99
1.00
1.13
1.35
5
3/6
5
4
3
5
4
5
3
5
3
7
7
6
6
7
7
6/7
6
7
6
6
6
6
6/7
6
6
6
6
9
10
9
6/7
9
9
1
1
1
1
1
1
1
1
1
2
2
3
3
3
3
3
3
3
3
7b
Mg
Mg
Ni
This work
This work
This work
6d
7b
6c
7b
6e
6a
6a
This work
6b, 7a
7a
This work
This work
6b
{[Mg(ptc).1/2[Mg(H2O)6]·H2O}n
{[Ni1.5(ptc)(pip)0.5(H2O)4]·H2O}n
{[Ni(ptcH)2][Ni(bpy)(H2O)4]·6H2O}
{Cu3(ptc)2(H2O)8]·4H2O}n
Ni
Cu
Cu
Co
Co
Zn
Zn
Mn
Pr
{[Cu (ptc)]·1/2[Cu((H2O)6]·H2O}n
{[Co(H2O)6][Co(ptc)(H2O)2]2·2H2O}n
{[Co2(ptcH)2(4,4¢-bipy)(H2O)4]·2H2O}
{Zn1.5(ptc)(H2O)6}n
{Zn1(ptcH)(H2O)4.33
{Mn1.5(ptc)·2H2O}n
}
3
{[Pr(ptc)·2H2O]·2H2O}
{Na2NiPr(ClO4)(C2H6O2)(ptc)2(H2O)8}·4.5H2O}n
{[Dy(ptc)·3H2O]·H2O}n
{Ca1.5(ptc)·2H2O}n
Pr
Dy
Ca
Nd
Sr
{[Nd(ptc)·2H2O]·2H2O
{Sr1.5(ptc)·5H2O}n
8
7/9
This work
This work
{Ba2(ptcH)2·2H2O}n
Ba
8/10
1654 | Dalton Trans., 2009, 1644–1658
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
=
=
n(C C + C N)]; 1483 w for n(Car–C); 1384 s, 1323 w, 1311 w for
ns(O–C–O); 1161 w, 1070 w, 1052 m for dip(C–H); 937 w for dring
Experimental section
;
Materials
826 m for d(O–C–O); 777 m for dop(C–H); 608 w for p (CO2);
530 w, 485 w for n(M–O).
The metal salts and 2,4,6-trimethylpyridine were acquired from
Aldrich and used as received. All solvents were obtained from
S.D. Fine Chemicals (India) and were purified prior to use
following standard procedures.
{[Cd2(cdaH)2(H2O)4](H2O)3}2, (2). Synthesis of this com-
pound was achieved hydrothermally by reacting 1 mmol amounts
of Cd(NO3)2·4H2O and ptcH3 in 5 ml of water in a 25 ml Teflon-
lined stainless steel autoclave. The autoclave was heated under
autogenous pressure to 180 ◦C for 2 days and then kept at 60 ◦C
for a further 12 h period. Upon cooling to RT, the desired product
appeared as long colorless rectangular parallelopipeds in ~60%
yield. In this reaction condition, the carboxylic acid group at the
4-position of ptcH3 was converted to hydroxyl to afford chelidamic
acid (cdaH3 = chelidamic acid). Anal. Calcd. for C14H20N2O17Cd2
: C, 23.58; H, 2.82; N, 3.92%. Found: C, 23.63; H, 2.85; N, 3.86%.
Main IR features (cm-1, KBr pellet): 3420 vs for n(O–H); 3050 w
Physical measurements
The IR spectra (KBr pellets) were recorded on a Perkin-Elmer
Model 1320 spectrometer in the 4000–400 cm-1 spectral regions.
Elemental analyses (C, H, and N) were performed on a LECO
CHNS-932 microanalytical analyzer. Magnetic measurements
were performed at the Servei de Mesures Magne`tiques (Universitat
de Barcelona) on crushed polycrystalline samples using a Quan-
tum Design SQUID MPMS-XL magnetometer equipped with a
5 T magnet. The diamagnetic corrections were evaluated from
Pascal’s constants.
=
=
for n(C–H); 1606 vs for [nas(O–C–O) + n(C C + C N)]; 1443 m
for n(Car–C); 1363 s, 1272 m for ns(O–C–O); 1105 w, 1027 w,
1015 m for dip(C–H); 939 w for dring; 827 m for d(O–C–O); 783 m
for dop(C–H); 589 w for p (CO2); 470 w, 438 w for n(M–O + M–N).
X-Ray structural studies
{[Ni1.5(ptc)(pip)0.5(H2O)4]·H2O}n, (3). This compound was
synthesized hydrothermally, as above by reacting Ni(NO3)2·6H2O,
piperazine (pip) and ptcH3 in one molar a◦mount each in the
presence of 6 ml water for three days at 170 C and then cooled
for 7 h. The final product obtained as long green needles, were
collected by filtration, washed with water, then MeOH, and finally
with ether. Yield ~40%. Anal. calcd for C10H17N2O11Ni1.5: C, 27.98;
H, 3.99; N, 6.53%. Found: C, 27.93; H, 4.03; N, 6.42%. Main IR
features (cm-1, KBr pellet): 3368 vs for n(O–H); 1639 vs for [nas(O–
Single crystal X-ray data on 1–10 were collected at 100 K on
a Bruker SMART APEX CCD diffractometer using graphite-
˚
monochromated Mo-Ka radiation (l = 0.71069 A). The linear
absorption coefficients, scattering factors for the atoms, and the
anomalous dispersion corrections were taken from International
Tables for X-ray Crystallography. The data integration and
reduction were processed with SAINT30 software. An empirical
absorption correction was applied to the collected reflections with
SADABS31 using XPREP.32 The structure was solved by the direct
method using SHELXTL33 and was refined on F2 by full-matrix
least-squares technique using the SHELXL-9734 program pack-
age. The non-hydrogen atoms were refined anisotropically while
the hydrogen atoms attached to carbon atoms were positioned
geometrically and treated as riding atoms using SHELXL default
parameters. The hydrogen atoms of water molecules were located
from difference Fourier maps and refined freely keeping the O–H
=
=
C–O) + n(C C + C N)]; 1390 s, 1360 s for ns(O–C–O); 921 m for
ring; 758 m for dop(C–H); 595 w for p(CO2); 558 w, 490 w, 467 w
d
for n(M–O + M–N).
{Mn1.5(ptc)·2H2O}n, (4). This compound was synthesized by
mixing 1 mmol of ptcH3 and 1 mmol of MnCl2·4H2O in 4 ml
of water in Teflon-lined autoclave, which was heated under
autogenous pressure to 180 ◦C for 84h. Upon cooling to room
temperature the desired product appeared as pale pink cubic in
ca. 60% yield. The same crystalline product could be obtained
when acetate or sulfate salt of Mn(II) is used in place of chloride
salt. Anal. calcd for C8H6N1O8Mn1.5: C, 29.42; H, 1.85; N, 4.29%.
Found: C, 29.46; H, 1.89; N, 4.25%. Main IR features (cm-1, KBr
pellet): 3381 vs for n(O–H); 3050 w for n(C–H); 1608 vs, 1556 s
˚
bond distances constrained to ~0.82 A with the DFIX command.
In 2, Ow6 is disorder over two positions with occupancy of 0.5 and
both were refined as isotropic. For complex 5, the Flack parameter
did not refine to a sensible value and therefore the polar direction
in the crystal could not be determined. The crystal and refinement
data are collected in Table 3.
=
=
for [nas(O–C–O) + n(C C + C N)]; 1455 m for n(Car–C); 1367
vs, 1278 m for ns(O–C–O); 1226 m, 1105 w, 1025 w for dip(C–H);
944 w for dring; 747 m for dop(C–H); 618 w for p(CO2); 498 w, 467 w
for n(M–O + M–N).
Synthesis
Pyridine-2,4,6-tricarboxylic acid, (ptcH3). This compound
was synthesized in 50% yield by oxidation of 2,4,6-
trimethylpyridine with aqueous alkaline KMnO4 following a
literature method.35
{Mg3(ptc)2·8H2O}n, (5). This was prepared in ca. 54% yield
as colorless needles on hydrothermal reaction of 1 mmol of ptcH3
and 1 mmol of Mg(NO3)2·6H2O (or MgCl2·6H2O) in 5 ml of water
in Teflon-lined autoclave which was heated under autogenous
pressure to 140 ◦C for 72 h. Upon cooling to room temperature the
final product was collected by filtration, washed with water and
then MeOH. Anal. calcd for C16H20N2O20Mg3: C, 30.34; H, 3.18;
N, 4.45%. Found: C, 30.36; H, 3.14; N, 4.48%. Main IR features
(cm-1, KBr pellet): 3469 vs for n(O–H); 3101 m for n(C–H); 1629
{[Cd(Ox)(H2O)2]H2O}n, (1). Reaction of Cd(NO3)2·4H2O
and ptcH3 in 2 : 1 molar ratio at room temperature in the presence
of excess pyridine afforded crystals of 1 in 1 h in ~50% yield. The
same product as crystals could be isolated in comparable yield
when acetate or chloride salt of Cd(II) was used in place of the
nitrate salt. Anal. calcd for C2H6O7Cd: C, 9.44; H, 2.3%. Found:
C, 9.50; H, 2.42%. Main IR features (cm-1, KBr pellet): 3439 vs
for n(O–H); 3095 w for n(C–H); 1615 vs, 1534 s for [nas(O–C–O) +
=
=
vs, 1563 s for [nas(O–C–O) + n(C C + C N)]; 1452 m for n(Car–
C); 1365 s, 1284 m for ns(O–C–O); 1108 m, 1029 m for dip(C–H);
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1644–1658 | 1655
©

View Article Online
1656 | Dalton Trans., 2009, 1644–1658
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
934 w, 919 w for dring; 838 m for d(O–C–O); 733 m for dop(C–H);
534 w, 490 w for n(M–O + M–N).
dip(C–H); 952 w for dring; 792 m for dop(C–H); 535 w, 516 w for
n(M–O + M–N).
{[Mg(ptc)(H2O)2]·1/2[Mg(H2O)6]·H2O}n, (6). The filtrate
from the reaction of Mg(NO3)2·6H2O and ptcH3 on further
evaporation afforded 6 as colorless hexagonal-shaped crystals in
~15% yield. Anal. calcd for C8H14N1O12Mg1.5: C, 27.25; H, 4.00;
N, 3.97%. Found: C, 27.28; H, 4.05; N, 4.02%. Main IR features
(cm-1, KBr pellet): 3366 vs for n(O–H); 3045 w for n(C–H); 1662
vs, 1611 s for [nas(O–C–O) + n(C C + C N)]; 1452 m for n(Car–
C); 1365 s, 1284 m for ns(O–C–O); 1108 m, 1029 m for dip(C–H);
934 w, 919 w for dring; 838 m for d(O–C–O); 733 m for dop(C–H);
534 w, 490 w for n(M–O + M–N).
Acknowledgements
We gratefully acknowledge the financial support received from the
Council of Scientific and Industrial Research, New Delhi, India
(Grant No. 1638/EMR II) and an SRF to MCD. ECS acknowl-
edges the financial support from the Spanish Government, (Grant
CTQ2006/03949BQU and Juan de la Cierva fellowship).
=
=
References
1 X.-M. Chen and M.-L. Tong, Acc. Chem. Res., 2007, 40, 162.
2 L. Han, X. Bu, Q. Zhang and P. Feng, Inorg. Chem., 2006, 45, 5736.
3 X.-M. Zhang, Coord. Chem. Rev., 2005, 249, 1201.
{Ca1.5(ptc)·2H2O}n, (7). Standard hydrothermal conditions
did not afford compound 7. A different approach was adopted for
its synthesis. In a typical experiment, 1 mmol of ptcH3 and 1 mmol
of Ca(NO3)2·4H2O and few drops of triethylamine were taken in a
10 ml of water in Teflon-lined autoclave. The autoclave was heated
under autogenous pressure to 180 ◦C for 96 h and then left to
cool to room temp. Colorless prismatic crystals were obtained in
ca. 58% yield. Anal. calcd for C8H6N1O8Ca1.5: C, 31.58; H, 1.99;
N, 4.60%. Found: C, 31.60; H, 2.04; N, 4.64%. Main IR features
(cm-1, KBr pellet): 3424 vs for n(O–H); 3063 m for n(C–H); 1624 vs
for [nas(O–C–O) + n(C C + C N)]; 1445 m for n(Car–C); 1384 m,
1356 m, 1322 s for ns(O–C–O); 1010 m for dip(C–H); 944 w, 930 w
for dring; 785 m for dop(C–H); 591 w for p(CO2); 514 w, 469 w for
n(M–O + M–N).
4 (a) K. M. Fromm, Coord. Chem. Rev., 2008, 252, 856; (b) W. Maudez,
M. Meuwly and K. M. Fromm, Chem.–Eur. J., 2007, 13, 8302; (c) S.
Fox, I. Busching, W. Barklage and H. Strasdeit, Inorg. Chem., 2007, 46,
818; (d) J. A. Rood, B. C. Noll and K. W. Henderson, Inorg. Chem., 2006,
45, 5521; (e) H. Schmidbaur, O. Kumberger and J. Riede, Inorg. Chem.,
1991, 30, 3101; (f) A. Uchtman and R. J. Jandacek, Inorg. Chem., 1980,
19, 350; (g) L. R. Nassimbeni and H. Su, J. Chem. Soc., Dalton Trans.,
2000, 349; (h) M. J. Plater, A. J. Roberts, J. Marr, E. E. Lachowski and
R. A. Howie, Chem. Commun., 1998, 797; (i) M. Westerhausen, Inorg.
Chem., 1991, 30, 90; (j) P. B. Hitchcock, M. F. Lappert, G. Lawless
and B. J. Royo, J. Chem. Soc., Chem. Commun., 1990, 1141; (k) M. J.
McCormick, K. B. Moon, S. R. Jones and T. P. Hanusa, J. Chem.
Soc., Chem. Commun., 1990, 778; (l) L. B. Cole and E. M. Holt, Inorg.
Chim. Acta, 1989, 195; (m) J. Cai, C.-H. Chen, C.-Z. Liao, X.-L. Feng
and X.-M. Chen, Acta Crystallogr., Sect. B, 2001, 57, 520; (n) A. J.
Shubnell, E. J. Kosnic and P. J. Squattrito, Inorg. Chim. Acta, 1994,
216, 101; (o) R. Murugavel, V. V. Karambelkar, G. Anantharaman
and M. G. Walawalkar, Inorg. Chem., 2000, 39, 1381; (p) Z. Fei, T. J.
Geldbach, R. Scopelliti and P. J. Dyson, Inorg. Chem., 2006, 45, 6331;
(q) S. Chadwick, U. Englich, K. Ruhlandt-Senge, C. Watson, A. E.
Bruce and M. R. M. Bruce, J. Chem. Soc., Dalton Trans., 2000, 2167;
(r) M. A. Guino-o, C. F. Campanab and K. Ruhlandt-Senge, Chem.
Commum., 2008, 1692; (s) K. T. Quisenberry, C. K. Gren, R. E. White,
T. P. Hanusa and W. W. Brennessel, Organometallics, 2007, 26, 4354;
(t) M. J. Harvey, K. T. Quisenberry, T. P. Hanusa and V. G. Young, Jr.,
Eur. J. Inorg. Chem., 2003, 3383.
=
=
{Sr1.5(ptc)·5H2O}n, (8). When Sr(NO3)2 (or SrCl2·6H2O) was
taken in place of Ca(NO3)2·4H2O keeping the reaction condition
the same as in 7, compound 8 could be isolated in ~55%
yield as colorless rectangular parallelopipeds. Anal. calcd for
C8H12N1O10.5Sr1.5: C, 22.79; H, 2.86; N, 3.32%. Found: C, 22.84;
H, 2.83; N, 3.35%. Main IR features (cm-1, KBr pellet): 3475 vs
for n(O–H); 3060 w for n(C–H); 1630 vs, 1533 s for [nas(O–C–
=
=
O) + n(C C + C N)]; 1463 m for n(Car–C); 1395 s, 1348 m for
ns(O–C–O); 1277 w, 1230 m for dip(C–H); 930 w for dring; 784 m for
dop(C–H); 550 w, 491 w, 469 w for n(M–O + M–N).
5 (a) N. S. Poonia and A. V. Bajaj, Chem. Rev., 1979, 79, 389; (b) D. E.
Fenton, in Comprehensive Coordination Chemistry, Eds. S. Wilkinson,
and R. D. Gillard, J. A. McCleverty, Pergamon, Oxford, 1987, Vol. 3,
pp. 1.
{[Ba(ptc)(H2O)][Ba(ptcH2)H2O]}n, (9). When Ba(NO3)2 (or
BaCl2·6H2O) was taken in place of Ca(NO3)2·4H2O keeping the
reaction condition the same as in 7, compound 9 could be isolated
in ~58% yield as colorless rectangular parallelopipeds. Anal. calcd
for C16H10N2O14Ba2: C, 26.36; H, 1.38; N, 3.84%. Found: C, 26.41;
H, 1.35; N, 3.89%. Main IR features (cm-1, KBr pellet): 3423 vs for
n(O–H); 3068 m for n(C–H); 1727 m for protonated carboxylate;
1642 vs, 1582 s for [nas(O–C–O) + n(C C + C N)]; 1548 m for
n(Car–C); 1440 m, 1390 m for ns(O–C–O); 1246 m, 1192 m, 1108 w
for dip(C–H); 1001 w, 942 w, 902 w for dring; 787 m for dop(C–H);
555 w for p(CO2); 520 w, 468 w for n(M–O + M–N).
6 (a) S. K. Ghosh, G. Savitha and P. K. Bharadwaj, Inorg. Chem., 2004,
43, 5495; (b) S. K. Ghosh and P. K. Bharadwaj, Eur. J. Inorg. Chem.,
2005, 43, 4886; (c) S. K. Ghosh, M. S. E. Fallah, J. Ribas and P. K.
Bharadwaj, Inorg. Chim. Acta, 2006, 359, 468; (d) S. K. Ghosh and
P. K. Bharadwaj, Inorg. Chim. Acta, 2006, 359, 1685; (e) S. K. Ghosh
and P. K. Bharadwaj, J. Che. Sci., 2005, 117, 23.
7 (a) H.-L. Gao, Y. Long, B. Ding, H.-S. Wang, P. Cheng, D.-Z. Liao
and S.-P. Yan, Inorg. Chem., 2006, 45, 481; (b) H.-L. Gao, B. Ding, Y.
Long, P. Cheng, D.-Z. Liao, S.-P. Yan and Z.-H. Jiang, Inorg. Chem.
Commun., 2005, 8, 151.
=
=
8 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordina-
tion Compounds, Wiley & Sons, New York, 5th edn., 1997.
9 S. D. Robinson and M. F. Uttley, J. Chem. Soc., Dalton Trans., 1973,
1912.
10 L. J. Bellamy, The Infrared Spectra of Complex Molecules, Wiley, New
{[Dy(ptc)·3H2O]·H2O}n, (10). This was prepared in ~55%
yield as light purple rectangular parallelopipeds crystals on
hydrothermal reaction of 1 mmol of Dy(NO3)3·5H2O with 1 mmol
of the ligand under identical experimental condition as 7. Anal.
calcd for C8H10N1O10Dy1: C, 21.70; H, 2.27; N, 3.16%. Found: C,
21.68; H, 2.30; N, 3.13%. Main IR features (cm-1, KBr pellet):
3383 vs for n(O–H); 3055 w for n(C–H); 1631 m, 1645 vs, 1580 s
York, 1958.
11 J. Lu, Y. Li, K. Zhao, J.-Q. Xu, J.-H. Yu, G.-H. Li, X. Zhang, H.-Y. Bie
and T.-G. Wang, Inorg. Chem. Commun., 2004, 7, 1154.
12 R. Vaidhyanathan, S. Natarajan and C. N. R. Rao, Eur. J. Inorg. Chem.,
2003, 1675.
13 R. Vaidhyanathan, S. Natarajan and C. N. R. Rao, J. Solid State Chem.,
2002, 167, 274.
14 P. A. Prasad, S. Neeraj, S. Natarajan and C. N. R. Rao, Chem.
Commum., 2000, 1251.
=
=
for [nas(O–C–O) + n(C C + C N)]; 1447 m for n(Car–C); 1394 s,
1353 m, 1278 w for ns(O–C–O); 1236 w, 1110 m, 1026 m for
15 P. Orioli, B. Bruni, M. D. Vaira, L. Messori and F. Piccioli, Inorg.
Chem., 2002, 41, 4312.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1644–1658 | 1657
©

View Article Online
16 (a) M. Matsumoto, S. Saito and I. Ohmine, Nature, 2002, 416, 409;
(b) N. H. Fletcher, The Chemical Physics of Ice, Cambridge University
Press, Cambridge, 1970.
17 (a) J. K. Gregory, D. C. Clary, K. Liu, M. G. Brown and R. J. Saykally,
Science, 1997, 275, 814; (b) D. Eisenberg, and W. Kauzmann, The
Structure and Properties of Water, Oxford University Press, Oxford,
1969; (c) G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford
University Press, Oxford, 1997.
18 (a) K. Kim, K. D. Jordan and T. S. Zwier, J. Am. Chem. Soc., 1994,
116, 11568; (b) J. Tsai and K. D. Jordan, Chem. Phys. Lett., 1993, 213,
181.
19 (a) R. Custecean, C. Afloroaei, M. Vlassa and M. Polverejan, Angew.
Chem., Int. Ed., 2000, 39, 3094; (b) S. K. Ghosh and P. K. Bharadwaj,
Inorg. Chem., 2003, 42, 8250.
20 (a) K. M. Park, R. Kuroda and T. Iwamoto, Angew. Chem., Int. Ed.
Engl., 1993, 32, 884; (b) Y.-P. Ren, L.-S. Long, B.-W. Mao, Y.-Z. Yuan,
R.-B. Huang and L.-S. Zheng, Angew. Chem., Int. Ed., 2003, 42, 532.
21 (a) S. K. Ghosh and P. K. Bharadwaj, Angew. Chem., Int. Ed., 2004,
43, 3577; (b) J. N. Moorthy, R. Natarajan and P. Venugopalan, Angew.
Chem., Int. Ed., 2002, 41, 3417.
23 F. H. Allen, Acta Crystallogr., 2002, B58, 380.
24 M. M. Harding, Acta Crystallogr., 1999, D55, 1432.
25 A. L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht
University, Utrecht, The Netherlands, 2003.
26 S. M. F. Lo, S. S. Y. Chui and I. D. Williams, Acta Crystallogr., 1998,
C54, 1846.
27 (a) C. A. Hunter, Chem. Soc. Rev., 1994, 23, 101, and the references
therein; (b) M. Dan, A. K. Cheetham and C. N. R. Rao, Inorg. Chem.,
2006, 45, 8227.
28 K. M. Fromm, Chimia, 2002, 56, 676.
29 A. M. Jones, and M. P. Lord, Macmillan’s Chemical and Physical Data,
Macmillan Press, London, 1992, pp. 486.
30 SAINT+, 6.02ed, Bruker AXS, Madison, WI, 1999.
31 G. M. Sheldrick, SADABS, Empirical Absorption Correction Program,
University of Go¨ttingen, Germany, 1997.
32 XPREP, 5.1 ed. Siemens Industrial Automation Inc., Madison, WI,
1995.
33 G. M. Sheldrick, SHELXTL TM Reference Manual: version 5.1, Bruker
AXS, Madison, WI, 1997.
34 G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refine-
ment, University of Go¨ttingen, Go¨ttingen, Germany, 1997.
35 L. Syper, K. Kloc and J. Mlochowski, Tetrahedron, 1980, 36, 123.
22 K. Liu, M. G. Brown, C. Carter, R. J. Saykally, J. K. Gregory and D. C.
Clary, Nature, 1996, 381, 501.
1658 | Dalton Trans., 2009, 1644–1658
This journal is
The Royal Society of Chemistry 2009
©