Reversible water intercalation accompanied by coordination and color changes in a layered metal-organic framework

Article abstract of DOI:10.1021/ic9003356

A new two-dimensional 3d-4f mixed-metal mixed dicarboxylate (homocyclic and heterocyclic) of the formula [Gd₂(H₂O) ₂Ni(H₂O)₂(1,2-bdc)₂(2,5-pydc) ₂] · 8H₂O (1; 1,2-H₂bdc = 1,2-benzenedicarboxylic acid and 2,5-H₂pydc = 2,5- pyridinedicarboxylic acid) has been prepared by employing the hydrothermal method. The structure has infinite one-dimensional-Gd-O-Gd-chains formed by the edge-shared GdO₉ polyhedral units, resulting exclusively from the connectivity between the Gd³⁺ ions and the 1,2-bdc units. The chains are connected by the [Ni(H₂O)₂(2,5-pydc)₂] ^2-metalloligand, forming the two-dimensional layer arrangements. The stacking of the layers creates hydrophilic and hydrophobic spaces in the interlamellar region. A one-dimensional water ladder structure, formed by the extraframe-work water molecules, occupies the hydrophilic region while the benzene ring of 1,2-bdc occupies the hydrophobic region. To the best of our knowledge, the present compound represents the first example of a 3d-4f mixed-metal carboxylate in which two different aromatic dicarboxylate anions act as the linkers. The stabilization energies of the water clusters have been evaluated using density functional theory calculations. The water molecules in 1 are fully reversible accompanied by a change in color (greenish blue to brown) and coordination around Ni²⁺ ions (octahedral to distorted tetrahedral).

Full text of DOI:10.1021/ic9003356

4
942 Inorg. Chem. 2009, 48, 4942–4951
DOI:10.1021/ic9003356
Reversible Water Intercalation Accompanied by Coordination and Color Changes
in a Layered Metal-Organic Framework
Partha Mahata, K. V. Ramya, and Srinivasan Natarajan*
Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science,
Bangalore 560 012, India
Received February 19, 2009
A new two-dimensional 3d-4f mixed-metal mixed dicarboxylate (homocyclic and heterocyclic) of the formula
[Gd (H O) Ni(H O) (1,2-bdc) (2,5-pydc) ] 8H O (1; 1,2-H bdc = 1,2-benzenedicarboxylic acid and 2,5-H pydc = 2,5-
pyridinedicarboxylic acid) has been prepared by employing the hydrothermal method. The structure has infinite one-
2 2 2 2 2 2 2 2 2 2
3
dimensional -Gd-O-Gd- chains formed by the edge-shared GdO polyhedral units, resulting exclusively from the
9
3
connectivity between the Gd ions and the 1,2-bdc units. The chains are connected by the [Ni(H O) (2,5-pydc) ]
+
2-
2
2
2
metalloligand, forming the two-dimensional layer arrangements. The stacking of the layers creates hydrophilic and
hydrophobic spaces in the interlamellar region. A one-dimensional water ladder structure, formed by the extraframe-
work water molecules, occupies the hydrophilic region while the benzene ring of 1,2-bdc occupies the hydrophobic
region. To the best of our knowledge, the present compound represents the first example of a 3d-4f mixed-metal
carboxylate in which two different aromatic dicarboxylate anions act as the linkers. The stabilization energies of the
water clusters have been evaluated using density functional theory calculations. The water molecules in 1 are fully
2
+
reversible accompanied by a change in color (greenish blue to brown) and coordination around Ni ions (octahedral to
distorted tetrahedral).
Introduction
host-guest chemistries such as photopolymerization reac-
3
tions, catalysis, etc. The guest molecules can play a useful
The research in the area of metal-organic frameworks
MOFs) continues to be interesting for their unique struc-
role in the control of crystal structures and the physical and
(
4
1
chemical behavior of two-dimensional host lattices.
tures and tunable properties. Many of the known MOFs
have three-dimensional structures, and the synthesis of two-
dimensional ones is also important because some of them can
Though the formation of layered MOFs based on single-
metal ions has been established, reports on the use of two
5
2
different metal ions are few. We have been interested in the
replicate the behavior of naturally occurring clays. The
layered compounds can also participate in a variety of
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(g) Yuge, T.; Miyata, M.; Tohnai, N. Cryst. Growth Des. 2006, 6, 1271. (h)
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Matsumoto, A.; Miyata, M. Angew. Chem., Int. Ed. 2005, 44, 7059. (i)
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Verlag GmbH & Co. KGaA: Weinheim, Germany, 2004. (l) Song, J. L.;
Mao, J. G.; Sun, Y. Q.; Zeng, H. Y.; Kremer, R. K.; Clearfield, A. J. Solid State
Chem. 2004, 177, 633.
*
(
Corresponding author. E-mail: snatarajan@sscu.iisc.ernet.in.
1) (a) Morris, R. E.; Wheatley, P. S. Angew. Chem., Int. Ed. 2008, 47,
966. (b) Furukawa, H.; Kim, J.; Ockwig, N. W.; O’Keeffe, M.; Yaghi, O. M.
4
J. Am. Chem. Soc. 2008, 130, 11650. (c) Horike, S.; Dinca, M.; Tamaki, K.;
Long, J. R. J. Am. Chem. Soc. 2008, 130, 5854. (d) Maspoch, D.; Ruiz-
Molina, D.; Veciana, J. Chem. Soc. Rev. 2007, 36, 770. (e) Goto, Y.; Sato, H.;
Shinkai, S.; Sada, K. J. Am. Chem. Soc. 2008, 130, 14354. (f) Rao, C. N. R.;
Natarajan, S.; Vaidhanathan, R. Angew. Chem., Int. Ed. 2004, 43, 1466. (g)
Eddaoudi, M.; Kim, J.; Vodak, D.; Sudik, A.; Wachter, J.; O’Keefe, M.;
Yaghi, O. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4900. (h) Kitawaga, S.;
Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334. (i) Bradshaw,
D.; Warren, J. E.; Rosseinsky, M. J. Science 2007, 315, 977. (j) Ferey, G.;
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Mater. 2007, 6, 142.
(
(
2) Chen, C. L.; Beatty, A. M. J. Am. Chem. Soc. 2008, 130, 17222.
3) (a) Matsumoto, A.; Sada, K.; Tashiro, K.; Miyata, M.; Tsubouchi, T.;
(5) (a) Wang, F. Q.; Zheng, X. J.; Wan, Y. H.; Sun, C. Y.; Wang, Z. M.;
Wang, K. Z.; Jin, L. P. Inorg. Chem. 2007, 46, 2956. (b) Kim, Y. J.; Park, Y.
J.; Jung, D. Y. Dalton Trans. 2005, 2603. (c) He, F.; Tong, M. L.; Yu, X. L.;
Chen, X. M. Inorg. Chem. 2005, 44, 559. (d) Zhang, J. J.; Xia, S. Q.; Sheng, T.
L.; Hu, S. M.; Leibeling, G.; Meyer, F.; Wu, X. T.; Xiang, S. C.; Fu, R. B.
Chem. Commun. 2004, 1186. (e) Yu, Z. T.; Liao, Z. L.; Jiang, Y. S.; Li, G. H.;
Chen, J. S. Chem.;Eur. J. 2005, 11, 2642.
Tanaka, T.; Odani, T.; Nagahama, S.; Tanaka, T.; Inoue, K.; Saragai, S.;
Nakamoto, S. Angew. Chem., Int. Ed. 2002, 41, 2502. (b) Nakano, K.; Sada,
K.; Nakagawa, K.; Aburaya, K.; Yoswathananont, N.; Tohnai, N.; Miyata,
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K. H. Angew. Chem., Int. Ed. 2003, 42, 1360.
pubs.acs.org/IC
Published on Web 4/17/2009
© 2009 American Chemical Society

Article
Inorg. Chem., Vol. 48, No. 11, 2009
4943
study of the formation of MOFs using both transition metals
the sample-to-detector distance fixed at 6.03 cm and the detector
position (2θ) fixed at -25°. The data were reduced using
SAINTPLUS, and an empirical absorption correction was
and lanthanides, which resulted in a layered solid, [Gd
8
6
(
H O) Co{C N H (COO) } ]. Our continued investiga-
2
3
5
1
3
2 3
9
applied using the SADABS program. The structure was solved
tions also revealed the formation of a new pillared layered
structure by use of two different carboxylic acids (homocyclic
and heterocyclic), [La (H O) ][{C H N(COO) } {C H
1
and refined using SHELXL97 present in the WinGx suite of
0
1
1
programs (version 1.63.04a). All of the hydrogen atoms of the
carboxylic acids and the water molecules were initially located in
the difference Fourier maps, and for the final refinement, the
hydrogen atoms were placed in geometrically ideal positions and
held in the riding mode. Final refinement included atomic
positions for all of the atoms, anisotropic thermal parameters
for all of the non-hydrogen atoms, and isotropic thermal para-
meters for all of the hydrogen atoms. Full-matrix least-squares
2
2
4
5
3
2 2
6
4
7
(
COO) }]. Similarly, we wanted to investigate whether it
2
would be possible to prepare a 3d-4f mixed-metal mixed-
dicarboxylate (benzene and pyridine) compound. During the
course of these studies, we have now prepared a new 3d-4f
mixed-metal compound formed with benzene-1,2-dicar-
boxylate and pyridine-2,5-dicarboxylate, [Gd (H O)Ni
2
2
2
refinement against |F | was carried out using the WinGx pack-
age of programs. Details of the structure solution and
(
H O) (1,2-bdc) (2,5-pydc) ].8H O (1). The structure con-
2 2 2 2 2
11
tains cationic one-dimensional gadolinium phthalate chains,
+
final refinements are given in Table 2. CCDC 650346 contains
the crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crys-
tallographic Data Center (CCDC) via www.ccdc.cam.ac.uk/
data_request/cif.
[
(
Gd(1,2-bdc)] , connected by an anionic [Ni(H O)2
2
2-
2,5-pydc)2] metalloligand, forming a two-dimensional
structure with decorated Kagome topology. The structure
contains one-dimensional water ladders in the interlamellar
space, which can be reversibly adsorbed. The removal of
water is also accompanied by a change in color from light
greenish blue to brown. In this paper, we present the synth-
esis, structure, reversible adsorption, and also stability stu-
dies of the water assembly of 1.
Results and Discussion
Structure. The compound has 32 non-hydrogen atoms
3+
in the asymmetric unit, of which one Gd
one Ni ion are crystallography independent. Of these,
ion and
2+
2
+
the Ni ions occupy a special position (2b) with a site
occupancy of 0.5. There are one 1,2-bdc anion and one
2,5-pydc anion present in the asymmetric unit (see the
Supporting Information, Figure S3). The two carboxy-
late groups of 1,2-bdc are connected only with the
Experimental Section
Synthesis and Initial Characterization. The title com-
pound was prepared by employing the hydrothermal method.
Gd
.5 mM) were dispersed in 7 mL of water. 2,5-Pyridinedicar-
boxylic acid (0.1705 g, 1 mM), 1,2-benzenedicarboxylic acid
0.1678 g, 1 mM), and NaOH (0.08 g, 2 mM) were added with
2 3 2 2
O (0.09 g, 0.25 mM) and Ni(OAc) 4H O (0.127 g,
3
0
3+
Gd ions. Two of the carboxylate oxygen atoms [O(1)
and O(5)] of 1,2-bdc have μ connectivity (connects two
3
(
3
+
Gd
ions and a carbon atom). The two carboxylate
continuous stirring, and the mixture was homogenized at room
temperature for 30 min. The final reaction mixture was sealed in
a 23 mL poly(tetrafluoroethylene)-lined stainless steel autoclave
and heated at 180 °C for 72 h. The initial pH value of the reaction
mixture was 3, and no appreciable change in the pH was noted
after the reaction. The final product, containing large quantities
of light-greenish-blue rectangular crystals, was filtered, washed
with deionized water under vacuum, and dried at ambient
conditions (yield 70% based on Gd). Elem anal. Calcd for
groups of 2,5-pydc show differences in connectivity: the
carboxylate group at the 2 position of the pyridine ring
2
+
has simple monodentate connectivity with the Ni ion,
whereas the carboxylate group at the 5 position of
the pyridine ring has bidentate connectivity with two
3+
Gd
ions. The nitrogen atom of the pyridine ring is
2+
bonded with the Ni ion (see the Supporting Informa-
tion, Figure S4).
The Gd ion is coordinated by nine oxygen atoms and
has a distorted tricapped trigonal-prismatic environment
1: C, 28.85; H, 3.05; N, 2.24. Found: C, 28.62; H, 3.11; N,
3
+
2.31. Energy-dispersive analysis of X-ray analysis on many
single crystals revealed a Gd/Ni ratio of 2:1.
Powder X-ray diffraction (PXRD) patterns were recorded on
well-ground samples in the 2θ range of 5-50° using Cu KR
radiation (Philips X’pert; see the Supporting Information,
Figure S1). The X-ray diffraction (XRD) patterns indicated
that the product is a new material, with the pattern being entirely
consistent with the simulated XRD pattern generated based on
the structure determined using the single-crystal XRD. The IR
spectrum was recorded on a KBr pellet (Perkin-Elmer, SPEC-
TRUM 1000; see Supporting Information, Figure S2). The
observed IR frequencies are listed in Table 1.
[GdO , CN = 9]. Of the nine oxygen atoms, one oxygen
9
atom, [O(6)], is a coordinated water molecule and the
remaining ones are from the carboxylate group (1,2-bdc
2
+
and 2,5-pydc). The Ni
ion is coordinated by four
oxygen atoms and two nitrogen atoms from the pyridine
ring, forming a distorted octahedral environment
[
NiO N , CN = 6]. Of the four oxygen atoms, two
4 2
symmetry-generated oxygen atoms [O(11)] are coordi-
nated water molecules and the remaining two oxygen
atoms are from the carboxylate group (2,5-pydc).
Single-Crystal Structure Determination. A suitable single
crystal was carefully selected under a polarizing microscope and
carefully glued to a thin glass fiber. The single-crystal data were
collected on a Bruker AXS SMART Apex CCD diffractometer
˚
The Gd-O bonds have an average distance of 2.45 A,
and the Ni-O and Ni-N bonds have average distances
˚
of 2.08 and 2.05 A, respectively. The O-Gd-O bond
at 293(2) K. The X-ray generator was operated at 50 kV and
˚
angles are in the range of 50.71(7)-167.03(7)°, and the
3
5 mA using Mo KR (λ = 0.710 73 A) radiation. Data were
collected with an ω scan width of 0.3°. A total of 606 frames were
collected in three different settings of j (0, 90, and 180°), keeping
(
8) SMART (version 5.628), SAINT (version 6.45a), XPREP, and
SHELXTL; Bruker AXS Inc.: Madison, WI, 2004.
9) Sheldrick, G. M. Siemens area correction absorption correction pro-
(
::
::
gram; University of Gottingen: Gottingen, Germany, 1994.
(10) Sheldrick, G. M. SHELXL-97 program for crystal structure solution
and refinement; University of Gottingen: Gottingen, Germany, 1997.
(11) Farrugia, J. L., WinGx suite for small-molecule single crystal crystal-
lography. J. Appl. Crystallogr., 1999, 32, 837.
(
6) Mahata, P.; Sankar, G.; Madras, G.; Natarajan, S. Chem. Commun.
005, 5787.
7) Mahata, P.; Ramya, K. V.; Natarajan, S. Chem.;Eur. J. 2008, 14,
839.
::
::
2
(
5

4944 Inorg. Chem., Vol. 48, No. 11, 2009
Mahata et al.
Table 1. Observed IR Bands for 1
ν
as(COO)
single
ν
as(COO)
ν
as(COO)
[triple
Ν
[triple
s
(COO)
ν
[doublec
a
(COO)
ν
a
(COO)
[single
[
[double
δ(CHaromatic)δ(CHaromatic
)
bands
ν(H
2
O) δ(H
2
O) coordination] coordination] coordination] coordination] oordination] coordination]
1592(s) 1554(s) 1542(s) 1415(s) 1392(s) 1366(s)
in-plane
out-of-plane δ(COO)
wavenumber 3000- 1620(s)
1130-
1040(m)
880-
810(m)
760-
650(m)
-1
(
cm
)
3600(s)
a
˚
Table 3. Selected Bond Distances (A) Observed in 1
Table 2. Crystal Data and Structure Refinement Parameters for 1
empirical formula
fw
cryst syst
C
30
H
38
N
2
O
28Gd
2
Ni
bond
amplitude
bond
amplitude
1247.84
monoclinic
P2 /c (No. 14)
12.791(2)
7.2645(13)
23.439(4)
90.0
Gd(1)-O(1)
Gd(1)-O(2)
Gd(1)-O(3)
Gd(1)-O(4)
Gd(1)-O(5)
2.526(2)
2.601(2)
2.358(2)
2.459(2)
2.5789(19)
2.359(2)
Gd(1)-O(7)
Gd(1)-O(1)#1
Gd(1)-O(5)#2
Ni(1)-O(11)
Ni(1)-O(12)
Ni(1)-N(1)
2.349(2)
2.382(2)
2.429(2)
2.121(3)
2.042(2)
2.051(2)
a
a
space group
˚
1
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
Gd(1)-O(6)
98.450(3)
90.0
a
Symmetry transformations used to generate equivalent atoms:
1
1
1
1
#
1, -x, y - /
2
, -z + /
2
; #2, -x, y + /
2
2
, -z + / .
3
˚
V (A )
2154.3(7)
2
293(2)
1.924
3.573
Z
T (K)
-
3
F
calc (g cm
)
-
1
μ (mm
)
θ range (deg)
λ(Mo KR) (A)
R indices [I > 2σ(I)]
R indices (all data)
2.20-28.01
0.710 73
R1 = 0.0278, wR2 = 0.0504
˚
R1 = 0.0368, wR2 = 0.0530
2
P
P
||/ |F
2
P
a
2
2
R1 =
||F
o
| - |F
c
o
|; wR2 = { [w(F
o
-2 F
c
) ]/
P
2
2
1/2
2
2
[
w(F
o
) ]} . w = 1/[σ (F
o
) + (aP) + bP], P = [max(F
o
,0) + 2
2
(
F
c
) ]/3, where a = 0.0184 and b = 0.8239.
O/N-Ni-O/N bond angles are in the range of 80.17
9)-180.0(9)°. The selected bond distances are listed in
Table 3.
The two-dimensional structure of the compound can be
(
simplified by considering the connectivity between the
GdO polyhedra. Thus, each of the GdO polyhedral
9
9
units are connected via the μ carboxylate oxygen atoms
3
[
-
3
O(1) and O(5)] by sharing the edges to give rise to infinite
Gd-O-Gd- chains, with a Gd-Gd distance of
˚
.991 A (see the Supporting Information, Figure S5).
The cationic -Gd-O-Gd- chains are formed exclu-
2
+
sively by the 1,2-bdc units (Figure 1a). The Ni ions are
connected by two 2,5-pydc anions and two water mole-
2
-
cules, forming a typical dianionic, [Ni(H O) (pydc) ]
2
,
2
2
metalloligand (Figure 1b). The metalloligand connects
the gadolinium carboxylate chains, giving rise to a neutral
two-dimensional structure (Figure 1c). The layers are
arranged in such a way that there are exclusive regions
in the interlamellar region that are hydrophilic and
hydrophobic (Figure 2a). The hydrophilic regions are
due to the presence of guest water molecules, coordinated
water molecules [O(6) and O(11)], and carboxylate oxy-
gen atoms (see the Supporting Information, Figure S6).
The projecting aromatic phthalate units form the hydro-
phobic regions.
Figure 1. (a) One-dimensional -Gd-O-Gd- chains formed by the
connectivity between 1,2-bdc and the Gd ions in 1. (c) Structure of the
Layer formation is essentially facilitated by the con-
3+
2
+
2-
nectivity between Ni and 2,5-pydc, which also connects
the gadolinium phthalate chains. The phthalate units do
not contribute to the formation of the network connec-
tivity except providing the necessary oxygen atoms
dianionic, [Ni(H O) (pydc) ] , metalloligand. (d) Layered structure
2
2
2
2-
formed from the connectivity between the [Ni(H
2
O) (pydc)
2
2
]
metallo-
ligand and the -Gd-O-Gd- chains. The 1,2-bdc unit has been shown
partially for clarity.
3
+
3+
2+
ion is connected with two Ni ions and two
ions through the 2,5-pydc ligands (Figure 1c).
needed for the nine-coordinated Gd ions. To analyze
the topology of the layer, we can consider the connectivity
Gd
Gd
3
+
3
+
2+
3+
2+
of 2,5-pydc with the Gd
Ni
and Ni
ion is connected with four Gd
ions. Here, each
ions and each
Thus, both of the Gd and Ni ions act as the four-
connected node, and the connectivity between them gives
2
+
3+

Article
Inorg. Chem., Vol. 48, No. 11, 2009
4945
Water is important for the study of many of biological
1
2
processes. The identification of water clusters in diverse
environments can lead to a better understanding of the
behavior of the water molecule. Up to now, water clus-
ters, (H O) , with n = 2-8, 10, 12, 14, 16, 18, and 45, have
2
n
1
3-23
been obtained in diverse hosts.
Among all of the
water clusters, the one-dimensional water structure ap-
pears to draw considerable interest because of its impor-
2
4
tance in biology. One-dimensional water structures are
generally built from an infinite connection of water
molecules or through the linking of one or more types
2
5
2
6
of smaller cyclic water clusters. In light of this, forma-
tion of the water ladder in the present system by con-
nectivity of the tetramer and dimer units is new.
Stability Studies of the Water Cluster. The observation
of the water ladders in the present structure prompted us
to investigate the stability of such water clusters using
simple theoretical models. We have also performed stu-
dies on similar one-dimensional water clusters that have
been observed in other related systems of compounds in
the literature (Table 4). To understand the stability of the
one-dimensional water ladder in 1, we have carried out an
ab initio calculation using density functional theory. To
aid in our understanding, we have also calculated the
stabilization energies of water clusters having one-dimen-
sional connectivity similar to that found in other MOFs
Figure 2. (a) Arrangement of the layers in 1. Note the presence of
alternate hydrophobic and hydrophilic regions. (b) View of the arrange-
ment of the one-dimensional water cluster units (ladder) in 1.
(
13) For (H : Manikumari, S.; Shivaiah, V.; Das, S. K. Inorg. Chem.
002, 41, 6953.
3
14) For (H O) : (a) Macgillivary, L. R.; Atwood, J. L. J. Am. Chem. Soc.
2
O)
2
2
1
(
2
rise to a network with two triangles and two hexagons
around the node (see the Supporting Information, Figure
S7), which is similar to that found in two-dimensional
Kagome networks.
997, 199, 22592. (b) Keutsch, F. N.; Cruzan, J. D.; Saykally, R. J. Chem.
Rev. 2003, 103, 2533.
(15) For (H O) : (a) Supriya, S.; Manikumari, S.; Raghavaiah, P.; Das, S.
2
4
K. New J. Chem. 2003, 27, 218. (b) Supriya, S.; Das, S. K. New J. Chem. 2003,
27, 1568. (c) Long, L. S.; Wu, Y. S.; Huang, R. B.; Zheng, L. S. Inorg. Chem.
2004, 43, 3798. (d) Tao, J.; Ma, Z. J.; Huang, R. B.; Zheng, L. S. Inorg. Chem.
2004, 43, 6133. (e) Sun, Y. Q.; Zhang, J.; Ju, Z. F.; Yang, G. Y. Aust. J. Chem.
2005, 58, 572. (f) Mahata, P.; Natarajan, S. Inorg. Chem. 2007, 46, 1250.
A careful analysis of the structure reveals that the
extraframework water molecules [O(100), O(200), O
(
300), and O(400)] form cooperative hydrogen-bond
interactions, giving rise to a ladderlike one-dimensional
structure (Figure 2b). Thus, the water molecules O(100)
and O(300) form a tetrameric unit because of the presence
of a center of symmetry, while the water molecules O(200)
and O(400) form a simple dimer. The connectivity
between the two units forms the one-dimensional water
ladder (Figure 2b). The one-dimensional water ladder
also forms extensive hydrogen-bond interactions with the
coordinated water molecules [O(6) and O(11)] (see the
Supporting Information, Figure S8), giving rise to a new
type of water assembly in this structure. Further analysis of
the water cluster also reveals some interesting aspects.
Thus, all of the extraframework water molecules, O(100),
O(200), O(300), and O(400), have a tetrahedral arrange-
ment with other water molecules and the carboxylate
oxygen atoms (see the Supporting Information, Figure
S9). This is similar to the arrangement of the water
molecules in ice with active participation from the
lone pair of the oxygen atoms. The coordinated water
molecules, O(6) and O(11), however, have a trigonal
arrangement with regards to other water molecules and
the carboxylate oxygen atoms. All of the water molecules,
however, appear to position themselves in such a way as
to form a distorted R-Po-like topology (see the Supporting
Information, Figure S10).
2 6
(16) For (H O) : (a) Moorthy, J. N.; Natarajan, R.; Venugopalan, P.
Angew. Chem., Int. Ed. 2002, 41, 3417. (b) Ghosh, S. K.; Bharadwaj, P. K.
Inorg. Chem. 2003, 42, 8250. (c) Ghosh, S. K.; Bharadwaj, P. K. Inorg. Chem.
004, 43, 5180. (d) Ye, B. H.; Ding, B. B.; Weng, Y. Q.; Chen, X. M. Inorg.
Chem. 2004, 43, 6866. (e) Liao, Y. C.; Jiang, Y. C.; Wang, S. L. J. Am. Chem.
Soc. 2005, 127, 12794.
2
2 8
(17) For (H O) : (a) Atwoood, J. L.; Barbour, L. J.; Ness, T. J.; Raston, C.
L.; Raston, P. L. J. Am. Chem. Soc. 2001, 123, 7192. (b) Doedens, R. J.;
Yohannes, E.; Kahn, M. I. Chem. Commun. 2002, 62. (c) Ma, B. Q.; Sun, H.
L.; Gao, S. Chem. Commun. 2005, 2336. (d) Prasad, T. K.; Rajasekharan, M.
V. Cryst. Growth Des. 2006, 6, 488. (e) Ghosh, S. K.; Bharadwaj, P. K. Eur. J.
Inorg. Chem. 2005, 24, 4886.
2
(18) For (H O)10: (a) Barbour, L. J.; Orr, G. W.; Atwood, J. L. Nature
(
London) 1998, 393, 671. (b) Barbour, L. J.; Orr, G. W.; Atwood, J. L. Chem.
Commun. 2000, 859. (c) Michaelides, A.; Skoulika, S.; Bakalbassis, E. G.;
Mrozinski, J. Cryst. Growth Des. 2003, 3, 487. (d) Bergougant, R. D.; Robin,
A. Y.; Fromm, K. M. Cryst. Growth Des. 2005, 5, 1691.
2
(19) For (H O)12: (a) Ghosh, S. K.; Bharadwaj, P. K. Angew. Chem., Int.
Ed. 2004, 43, 3577. (b) Neogi, S.; Savitha, G.; Bharadwaj, P. K. Inorg. Chem.
2004, 43, 3771.
(
20) For (H
K. Inorg. Chem. 2005, 44, 3856.
21) For (H O)16: Ghosh, S. K.; Bharadwaj, P. K. Inorg. Chem. 2004, 43,
6887.
22) For (H
setty, N.; Barnes, C. L.; Katti, K. V. J. Am. Chem. Soc. 2003, 125, 6955.
23) For (H O)45: Lakshminarayanan, P. S.; Suresh, E. I.; Ghosh, P. J.
2
O)14: Ghosh, S. K.; Ribas, J.; Fallah, M. S. E.; Bharadwaj, P.
(
2
(
2
O)18: Raghuraman, K.; Katti, K. K.; Barbour, L. J.; Pillar-
(
2
Am. Chem. Soc. 2005, 127, 13132.
(24) (a) Sainz, G.; Carrell, C. J.; Ponamerev, M. V.; Soriano, G. M.;
Cramer, W. A.; Smith, J. L. Biochemistry 2000, 39, 9164. (b) Jude, K. M.;
Wright, S. K.; Tu, C.; Silverman, D. N.; Viola, R. E.; Christianson, D. W.
Biochemistry 2002, 41, 2485. (c) Tajkhorshid, K.; Nollert, P.; Jensen, M. O.;
e
Miercke, L. J. W.; O’Connell, J.; Stroud, R. M.; Schulten, K. Science 2002,
296, 525.
(
12) Buck, U.; Huisken, F. Chem. Rev. 2000, 100, 3863.

4946 Inorg. Chem., Vol. 48, No. 11, 2009
Mahata et al.
Table 4. One-Dimensional Water Structures Observed in Other Host-Guest Systems
smallest water
cluster, (H O)
total size of repeating
unit, (H O)
serial no.
1
formula
2
n
2
n
ref
[Gd
,2-benzenedicarboxylic acid; 2,5-H
[TACD] 3H O (TACD = 1,4,7,10-tetraazacyclododecane)
[Cd(phen)(Hpppn)].4H O (phen = 1,10-phenanthroline; pppn =
-phosphonopropionate)
2
(H
2
O)
2
Ni(H
2
O)
2
(1,2-bdc)
2
(2,5-pydc)
2
] 8H
2
O (1,2-H
2
bdc =
n = 2, 4
n = 12
present
compound
26a
3
1
2
pydc = 2,5-pyridinedicarboxylic acid)
2
3
2
n = 4
n = 6
n = 10
n = 14
3
2
26b
3
I
II
4
5
[Cu Cu (pydc)
2
2
(bpe) ] 7H O (pydc = pyridine-2,6-dicarboxylate;
bpe = trans-1,2-bis(4-pyridyl)ethylene)
[Cu(mal) (picH) ] 5H O (mal = malonate dianion; picH = protonated
2
2
n = 4, 10
n = 4
n = 24
n = 7
26c
26d
3
2
2
2
3
2
-amino-4-picoline)
6
7
8
[C20
[APDO] 4H O (APDO = trans-4,4 -azopyridine dioxide)
H
25SnN
3
O
2
] 2.5H
2
O
n = 6
n = 5
n = 6
n = 11
n = 8
n = 12
26e
26f
26g
3
0
2
3
[(enH
en = ethylenediamine)
[Cd (bpa) Cl ] 6H
[Fe(bipy)
[Cu(2-mi)
Water molecules are disordered.
2
)
3
{Ni(AEDP)
2
}] 6H
2
O (AEDPH
4
= 1-aminoethylenediphosphonic acid;
3
a
9
0
2
2
4
2
O [bpa = N,N -bis(picolinamide)azine]
n = 4
n = 6
n = 4
n = 7
n = 11
n = 7
26h
26i
26j
3
2 2 2
(CN) ] 2.5H O
a
1
1
0
1
3
b
2
2
(male)] 3H O (2-mi = 2-methylimidazole; male = maleate)
3
a
b
Hydrogen atoms of the water molecules were not assigned.
2
6
27
set of 6-31+G(d,P), using the Gaussian 03 software
and related sytems. For calculations of the one-dimen-
sional water clusters formed by the connectivity between
smaller cyclic water clusters, we first calculated the stabi-
lization energy of the small water cluster (fragment) and
then the total stabilization energy due to connectivity
between the small water cluster units. We have performed
all of the calculations at the B3LYP level with a basis
2
8
package. For 1, on basis of the crystal structure geo-
metry, we have made an evaluation of the stability of the
water tetramers [O(100) ꢀ 2 and O(300) ꢀ 2] using the
single-point energy calculation without any symmetry
constraints. The stabilization energy was found to be -
14.57 kcal/mol for the tetramer. Similarly, for the dimer
[
of calculations, we have considered the (H O) unit
O(200) and O(400)], it was -2.89 kcal/mol. In the next set
2
12
(
25) (a) Cheruzel, L. E.; Pometun, M. S.; Cecil, M. R.; Mashuta, M. S.;
Wittebort, R. J.; Buchanan, R. M. Angew. Chem., Int. Ed. 2003, 42, 5452. (b)
Rather, B.; Zaworotko, M. J. Chem. Commun. 2003, 830. (c) Wakahara, A.;
Ishida, T. Chem. Lett. 2004, 33, 354. (d) Neogi, S.; Bharadwaj, P. K. Inorg.
Chem. 2005, 816. (e) Kim, H. J.; Jo, H. J.; Kim, J.; Kim, S. Y.; Kim, D.; Kim,
K. CrystEngComm 2005, 7, 417. (f) Fei, Z. F.; Zhao, D. B.; Geldbach, T. J.;
Scopelliti, R.; Dyson, P. J.; Antonijevic, S.; Bodenhausen, G. Angew. Chem.,
Int. Ed. 2005, 44, 4720. (g) Zhang, X. L.; Ye, B. H.; Chen, X. M. Cryst.
Growth Des. 2005, 5, 1609. (h) Birkedal, H.; Schwarzenbach, D.; Pattison, P.
Angew. Chem., Int. Ed. 2002, 41, 754. (i) Sreenivasulu, B.; Vittal, J. J. Angew.
Chem., Int. Ed. 2004, 43, 5769. (j) Saha, B. K.; Nangia, A. Chem. Commun.
(dodecameric unit) formed by the connectivity between
the two tetrameric units and the two dimeric units, which
is the basic repeating unit (the total number of hydrogen
bonds in this unit is 10) of the water ladder (Figure 3a).
The calculated stabilization energy for the (H O) unit
2
12
was found to be -47.82 kcal/mol. In Figure 3b, we show
the plots of the electron densities and electrostatic
potentials for the tetrameric and dodecameric units. As
can be seen, there appears to be considerable electronic
delocalization in the water cluster units due to the hydro-
gen-bond interactions.
2
005, 3024. (k) Lau, B. Y.; Jiang, F. L.; Yuan, D. Q.; Wu, B. L.; Hong, M. C.
Eur. J. Inorg. Chem. 2005, 3214.
26) (a) Pal, S.; Sankaran, N. B.; Samanta, A. Angew. Chem., Int. Ed. 2003,
(
4
5
2, 1741. (b) Zhang, X. M.; Fang, R. Q.; Wu, H. S. Cryst. Growth Des. 2005,
, 1335. (c) Hu, N. H.; Li, Z. G.; Xu, J. W.; Jia, H. Q.; Niu, J. J. Cryst. Growth
In Figure 4, we have pictorially represented the various
water clusters considered for comparison with the water
ladders observed in the present compound. One-dimen-
sional water clusters, formed by the connectivity between
a tetramer and a monomer, have been observed in the
organic host system, [TACD] 3H O (TACD = 1,4,7,10-
Des. 2007, 7, 15. (d) Choudhury, S. R.; Jana, A. D.; Colacio, E.; Lee, H. M.;
Mostafa, G.; Mukhopadhyay, S. Cryst. Growth Des. 2007, 7, 212. (e)
Rolando, L. G.; Berenice, M. D. M.; Victor, B.; Herbert, H.; Hiram, I. B.;
Luis, S. Z. R. Chem. Commun. 2005, 44, 5527. (f) Ma, B. Q.; Sun, H. L.; Gao,
S. Chem. Commun. 2004, 2220. (g) Li, M.; Chen, S.; Xiang, J.; He, H.; Yuan,
L.; Sun, J. Cryst. Growth Des. 2006, 6, 1250. (h) Ye, B. H.; Sun, A. P.; Wu, T.
F.; Weng, Y. Q.; Chen, X. M. Eur. J. Inorg. Chem. 2005, 1230. (i) Ma, B. Q.;
Sun, H. L.; Gao, S. Eur. J. Inorg. Chem. 2005, 3902. (j) Jin, Y.; Che, Y.;
Batten, S. R.; Chen, P.; Zheng, J. Eur. J. Inorg. Chem. 2007, 1925.
3
2
2
6a
tetraazacyclododecane; Figure 4a).
Our theoretical
evaluation gave an energy of -15.26 kcal/mol for the
water tetramer and an overall stabilization energy of -
49.36 kcal/mol for the (H O) unit.
(
(
27) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
2
10
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A.
D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.
One-dimensional water tapes formed by the connectiv-
ity between a hexamer and a monomer have been
observed in the channels of a metal phosphonocarbox-
ylate network, [Cd(phen)(Hpppn)] 4H O (phen = 1,10-
3
2
phenanthroline and pppn = 3-phosphonopropionate;
Figure 4b). Here, the calculations reveal a stabilization
26b
energy of -26.06 kcal/mol for the hexamer and an
overall stabilization energy of -140.81 kcal/mol for the
(H O) unit.
2
14
Water chains with tetrameric and decameric clusters
have been observed in [Cu Cu (pydc) (bpe) ] 7H O
I
2
II
2
2
3
2
[pydc = pyridine-2,6-dicarboxylate and bpe = trans-
1,2-bis(4-pyridyl)ethylene; Figure 4c]. Our calculations
2
6c

Article
Inorg. Chem., Vol. 48, No. 11, 2009
4947
energy of -55.43 kcal/mol has been obtained for the
connected tetramers, (H O) . Comparable values have
been reported for this cluster.
2
7
2
6d
Another example of this type of water cluster is the
formation of one-dimensional chains by sharing between
the cyclic hexamer units in [C H SnN O ] 2.5H O
2
0
25
3
2
3
2
2
6e
(
Figure 4e). Here, the stabilization energy was found
to be -20.46 kcal/mol for the hexamer, and the total
stabilization energy for the two-connected hexamer,
(
H O) , was found to be -46.49 kcal/mol.
2
11
One-dimensional water clusters have also been ob-
served by the edge sharing of cyclic water pentamer units
in the organic host, [APDO] 4H O (APDO = trans-4,
2
3
0
26f
4
-azopyridine dioxide; Figure 4f).
We obtained a
stabilization energy of -13.44 kcal/mol for the water
pentamer and a total stabilization energy of -27.66
kcal/mol for the two-connected pentamer, (H O) unit.
2
8
One-dimensional water clusters have also been
observed in the proton-transfer salt host, (enH ) [Ni
2
3
(AEDP) ] 6H O (AEDPH4 = 1-aminoethylenedipho-
sphonic acid and en = ethylenediamine; Figure 4g).
2 2
3
2
6g
Here, the calculations show a stabilization energy
of -36.97 kcal/mol for the hexamer and -81.41 kcal/
mol for the two-connected hexameric unit, (H O) .
2
12
The calculations on the various one-dimensional water
structures gave an opportunity to correlate the stabiliza-
tion energies with the number of water molecules (n) of
various (H O) units observed in different host-guest
2
n
systems. The stabilization energies are plotted against n
see the Supporting Information, Figure S11). As can be
noted, the stabilization energies are not linear with the
(
size of the (H O) units (n = 2-12). Additionally, (H O)
2
n
2
n
clusters with similar n values appear to have different
energies. This behavior suggests that the stabilization
energies of the water clusters depend not only on n but
also on other factors such as distances and angles between
the water molecules and the size and shape of the water
cluster units.
Dynamics of the Water Molecules. Thermogravimetric
analysis (TGA) was performed to investigate the thermal
stability of the water structure as well as the framework of
1 (see the Supporting Information, Figure S12). TGA was
carried out (Mettler-Toledo) in an oxygen atmosphere
Figure 3. (a) One decameric water cluster unit present in the
one-dimensional water ladders in 1, which was used for the calculation
of the stabilization energy (see the text). (b) Plots of the electron densities:
(
(
i) the tetramer [O(100) ꢀ 2 + O(300) ꢀ 2], (ii) the dodecamer,
H
2
O)12. Plots of the electrostatic potentials: (iii) the tetramer, (iv) the
dodecamer.
(flow rate = 20 mL/min) in the temperature range of 30-
850 °C (heating rate = 5 °C/min). The TGA studies show
yielded a stabilization energy of -80.57 kcal/mol for the
decameric cluster and -31.6 kcal/mol for the tetramer.
The overall stabilization energy of -199.09 kcal/mol has
been obtained for the (H O) unit.
In all of these cases, the cyclic water clusters are con-
nected by smaller linkers such as a monomer, a dimer, or a
tetramer of water, giving rise to one-dimensional water
chains. This is similar to the water ladder observed in the
present compound. In the literature, there are reports of
other one-dimensional water clusters formed by sharing
through the corners or edges, and we have performed a
similar study on these water molecules as well.
an initial weight loss of 17% in the temperature range of
50-180 °C, which is consistent with the calculated value
of 17.3% for the loss of the extraframework and coordi-
nated water molecules. The second sharp weight loss at
∼ 400 °C corresponds with the loss of the carboxylate
units. The total observed weight loss of 68% corresponds
well with the loss of the carboxylate and water mole-
cules (calcd 66.7%). The final calcined product was found
to be crystalline by PXRD and corresponds with
a mixture of Gd O (JCPDS: 12-0797) and NiO (JCPDS:
2
24
2
3
47-1049).
Because the initial weight loss was observed below 200
°C and also corresponded with the loss of water mole-
cules, we wanted to examine the possible reversible nature
of the water adsorption in the sample. For this, the sample
was taken in the TGA apparatus (Figure 5a) and heated
from 30 to 180 °C (heating rate = 5 °C/min), keeping the
sample for 30 min at 180 °C. This was followed by cooling
The formation of one-dimensional water chains by
corner sharing of tetrameric units has been observed in
[
picH = protonated 2-amino-4-picoline; Figure 4d).
Cu(mal) (picH) ] 5H O (mal = malonate dianion and
2 2 2
3
2
6d
The stabilization energy of -28.32 kcal/mol has been
obtained for the tetramer, and the overall stabilization

4948 Inorg. Chem., Vol. 48, No. 11, 2009
Mahata et al.
Figure 4. Various one-dimensional water structures from the literature and compared in the present study (only the fragment of the water cluster used
2
6a
in the calculations is shown): (a) water decamer unit in [TACD] 3H O (TACD = 1,4,7,10-tetraazacyclododecane);
2
(b) water (H
2
O)14 unit in
3
26b
O (phen = 1,10-phenanthroline; pppn = 3-phosphonopropionate); (c) water (H
I
II
[
Cd(phen)(Hpppn)] 4H
pydc = pyridine-2,6-dicarboxylate; bpe = trans-1,2-bis(4-pyridyl)ethylene];
2
2
O)24 unit in [Cu
2
Cu (pydc)
O (mal = malonate
(f) water octamer in [APDO] 4H
2
(bpe) ] 7H
2
2
O
3
3
2
6c
[
dianion; picH = protonated 2-amino-4-picoline);
(
en = ethylenediamine).
(d) water heptamer in [Cu(mal) (picH) ] 5H
2
2
2
3
26d
26e
(e) water (H
2
O)11 unit in [C20
H
25SnN
3
O
2
]b2.5H O;
2
2
O
3
0
26f
APDO = trans-4,4 -azopyridine dioxide); (g) water (H
2
O)12 unit in [(enH
2
)
3
{Ni(AEDP) }] 6H
2
2
O (AEDPH = 1-aminoethylenediphosphonic acid;
4
3
2
6g
to 30 °C (cooling rate = 2 °C/min) along with the
introduction of water vapor (75 min) and also cooling
for 1 h at 30 °C. In this rehydation process, the dry sample
returns to 98.9% of the initial weight of the original
sample from 83% at the completely dehydrated state.
The dehydration and rehydration processes were re-
peated four times, and in each case, similar behavior
was observed. The final sample after the four cycles of

Article
Inorg. Chem., Vol. 48, No. 11, 2009
4949
Figure 6. Water vapor adsorption-desorption isotherms of 1.
We have also carried out DSC measurements in
the temperature range of 25-225 °C (heating rate =
5
°C/min) to learn about the energetics of the dehydration
process (Figure 5b). The DSC studies indicate that the
dehydration process is endothermic with three distinct
peaks at 163, 186, and 200 °C, respectively. The three
peaks probably correspond with removal of the water
molecules from different environments in the crystal
structures. The total molar enthalpy calculated from the
DSC measurements is 55.46 kcal, which corresponds with
an energy of 4.62 kcal per water molecule. Similar values
2
5a
have been obtained previously.
Encouraged by the stability of the framework and the
reversibility of the hydration-dehydration cycles, we
sought to investigate the sorption behavior. For this, a
preheated sample (∼180 °C) was used and the adsorption
studies were carried out using nitrogen and water as the
Figure 5. (a) Reversible water uptake study of 1 using TGA. The red
curve represents the heating and cooling cycle. The blue trace shows the
weight loss and weight gain. (b) Differential scanning calorimetry (DSC)
study of 1. Note the three endothermic peaks.
probe molecules. The adsorption isotherms of N at 77 K
2
indicate no significant uptake of nitrogen, and only sur-
face adsorption was observed (see the Supporting Infor-
mation, Figure S16). Water adsorption, measured at
dehydration-rehydration was examined by PXRD,
which indicated the same PXRD pattern as the original
sample (see the Supporting Information, Figure S14).
This indicates that the water molecules are fully reversi-
ble. Our efforts to investigate the dehydrated phase (in
situ) using the single-crystal XRD were not successful
because the sample appears to lose the single crystalline
nature upon removal of the water molecules.
2
98 K, shows a gradual uptake, and a total of 248 mL/g
(
(
11.42 mol of H O/mol of I) was adsorbed at P/P = 1
2
Figure 6). The observed rapid onset of water vapor
0
sorption at very low pressures is characteristic of the
presence of micropores in the solid. Figure 6 also shows
considerable hysteresis in the desorption curve, which
indicates that the dry sample has a high affinity for the
water molecules. Similar behavior has been reported by
Kitagawa et al. for [Gd (imidc) (H O) ](H O), which
2
shows reasonable water vapor sorption but nonporous
behavior toward N in its dehydrated state.
Optical, Magnetic, and Electron Paramagnetic Reso-
nance (EPR) Studies. The diffuse-reflectance UV-vis
spectra at room temperature were recorded for the so-
dium salts of 1,2-bdc and 2,5-pydc and for the present
compound (see the Supporting Information, Figure S17).
In all cases, the main absorption bands are at ∼280 nm,
which may be due to the intraligand absorption (ILA; n f
π* or π f π*). The low intense peak at ∼400 nm for the
sodium salt of 2,5-pydc may be due to the ILA of the
heterocyclic aromatic system, which was not observed in
The dehydration/removal of the water molecules also
brings about a change in the color of the sample. Thus, the
compound changes from light greenish blue to brown
after dehydration. The dehydrated sample, brownish in
color, returns to the original color within 5 min upon
exposure to atmospheric conditions (see the Supporting
Information, Figure S15). This change of color could be
due to the change of the crystal-field-splitting energy of
30
2
2
3
2
2
2
+
2+
the Ni
ions, where the Ni
ions change from the
octahedral state to the four-coordinated state, possibly
a highly distorted tetrahedral coordination, after removal
of the bound water molecules.
2
9
(
29) Gill, N. S.; Taylor, F. B.; Hatfield, W. E.; Parker, W. E.; Fountain, C.
S.; Bunger, F. L. Tetrahalo complexes of Dipositive Metals in the First
Transition Series. Inorganic Syntheses; Tyree, S. Y., Jr., Ed.; McGraw-Hill,
Inc.: New York, 1967; Vol. 9, pp 136-142.
(30) Maji, T. K.; Mostafa, G.; Chang, H. C.; Kitagawa, S. Chem.
Commun. 2005, 2436.

4950 Inorg. Chem., Vol. 48, No. 11, 2009
Mahata et al.
spectrum of the dehydrated compound show differences
from that of the original compound. The ILA band of the
dehydrated phase shows a 10 nm shift and appears at
290 nm. The absorption bands at 355 and 485 nm
probably originate from the d-d transition of the four-
coordinated d (Ni ) ions. The absorption bands can be
8
2+
3
assigned to the T (F) f A (F) and T (F) f T (P)
3
3
3
1
2
1
1
transitions, respectively, based on a distorted tetrahedral
2
+
31
geometry of Ni ions. Thus, the light greenish blue of
the hydrated compound and the brown of the dehydra-
ted sample could be due to the A (F) f T (F) and
3
3
2
g
1g
3
3
T (F) f T (P) transitions in the visible region. The
1
1
relative intensity of the observed d-d transition for
2
+
the octahedral Ni ions appears to be low because this
transition is Laporte-forbidden. The light greenish blue
of the original compound is due to vibronic coupling of
2
+
the octahedral Ni ions, while a comparatively intense
d-d transition (darker brown) of the dehydrated sample
may be due to partially Laporte-allowed transitions
2
+
33
(
d-p mixing) of the tetrahedral Ni ions.
To investigate of the rehydration behavior more
clearly, the UV-vis spectra of the dehydrated samples
were studied by exposing the sample to atmospheric
conditions (Figure 7b). As can be noted, the intensities
3
3
3
3
of the T (F) f A (F) and T (F) f T (P) transitions
1
2
1
1
decrease continuously along with an increase in the
intensity of the A (F) f T (F) transition as a function
3
3
2
g
1g
of time. This observation corresponds well with the
change of color from brown to greenish blue.
Room-temperature solid-state photoluminescence
(PL) studies were carried out on ground samples (see
the Supporting Information, Figure S18). The aromatic
dicarboxylates used in the formation of MOFs absorb
strongly in the UV region and are an attractive ligands for
sensitizing the lanthanide ion (Eu, Dy, Sm, and Tb) via
Figure 7. (a) UV-vis spectra of 1 (blue trace) and the dehydrated phase
redtrace). (b) UV-vis spectra of the dehydratedsample after exposure to
the atmosphere for various times (0-10 min). The inset shows the spectra
in the range of 450-800 nm.
3
4
(
the antenna effect. On the other hand, MOF com-
pounds of Gd do not show this behavior because of the
absence of any ionic resonance levels below the triplet
2
+
35
the present compound because of the presence of Ni
ions. A shoulder at 330 nm and a broad band at ∼635 nm
state of the ligand. The emission spectra of the sodium
salts of 1,2-bdc and 2,5-pydc and 1 exhibit a main emis-
sion band at ∼425 nm, which can be assigned to the
intraligand luminescence (π* f n or π* f π). The
additional peak for the sodium salt of 2,5-pydc and 1
at ∼485 nm may be due to the intraligand transition
arising from the heterocyclic system (pyridine).
Temperature-dependent magnetic susceptibility mea-
surement for 1 has been performed on well-ground sam-
ples using a SQUID magnetometer in the 2-300 K
temperature range, with an applied field of 1000 Oe (see
the Supporting Information, Figure S19). The observed
effective magnetic moment of the present compound
were also observed for I, which may be due to the d-d
transition of the d (Ni ) ion. These absorptions at 330
8
2+
3
and 635 nm can be assigned to the A (F) f T (P) and
3
2
g
1g
3
3
A (F) f T (F) transitions, respectively. The electro-
31
2
g
1g
3
+
7
ions (4f ) could not be
nic transitions from the Gd
observed clearly because the excited-state energy for the
3
Gd ion is high, with the lowest-energy transition ( S f
+
8
6
P7/2) generally appearing at λ < 300 nm, which overlaps
with the strong intraligand bands observed for 1.
3
2
The change of color from greenish blue to brown upon
dehydration suggested the possibility of studying the
changes in the electronic transitions in the dehydrated
sample. For this, the compound was heated under va-
cuum in a glass tube for 2 h at 180 °C and the resulting
brown sample was carefully transferred to a quartz-sealed
cuvette under inert conditions (glovebox). The UV-vis
spectra of the dehydrated sample was recorded at room
temperature (Figure 7a). As can be noted, the UV-vis
at room temperature (300 K) is 18.12 μ , which is very
B
3
close to the value expected for two Gd ions and one
+
2
+
3+
2+
Ni ion ( μspin-only for Gd is 7.93 and for Ni is 2.83).
(33) Huhhey, J. .E; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of structure and reactivity, 4th ed.; Pearson Education: Upper
Saddle River, NJ, 2000.
(34) (a) Serre, C.; Pelle., F.; Gardant, N.; Ferey, G. Chem. Mater. 2004, 16,
1
177. (b) Millange, F.; Serre, C.; Marrot, J.; Gardant, N.; Pelle, F.; Ferey, G.
(
31) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier:
New York, 1968.
32) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer-
Verlag: Heidelberg, Germany, 1994.
J. Mater. Chem. 2004, 14, 642. (c) Serre, C.; Millange, C.; Thouvenot, N.;
Gardant, N.; Pelle, F.; Ferey, G. J. Mater. Chem. 2004, 14, 1530.
(35) Chandler, B. D.; Cramb, D. T.; Shimizu, G. K. H. J. Am. Chem. Soc.
2006, 128, 10403.
(

Article
Inorg. Chem., Vol. 48, No. 11, 2009
4951
3
+
The Gd
because L (orbital quantum number) for f system is 0,
ions do not have any orbital contribution
compoundcouldbe employed as a water sensor. Stabilization
energies of the water molecules, calculated using density
functional theory, suggest that the values are comparable
to other similarly reported water structures. The role
of hydrated transition-metal ions in MOFs is still not
explored fully, and the present study suggests that other
related compounds could be prepared with as yet unex-
plored properties. Research in this direction is presently
underway.
7
2
+
and for the case of Ni , the orbital contributions are
completely suppressed for the octahedral geometry with a
6
2
2
t
e electronic configuration in the ground state. The 1/
g
g
χM vs T curve is shown as an inset of Figure 12. Above 100
K, the magnetic behavior can be fitted to the Curie-
Weiss law, with C = 10.41 emu/mol and θ = -3.18 K.
p
The small negative value of θ indicates weak antiferro-
P
magnetic interactions.
The EPR spectrum at room temperature was recorded
with diphenylpicrylhydrazyl as the standard (see the
Supporting Information, Figure S20). Two bands ob-
served with a superposition at g = 2.34 and 1.83 corre-
Acknowledgment. S.N. thanks the Department of
Science and Technology, Government of India, for the
award of a research grant, and the authors thank the
Council of Scientific and Industrial Research, Govern-
ment of India, for the award of a fellowship (to P.M.) and
a research grant. S.N. also thanks the Department of
Science and Technology, Government of India, for the
award of the RAMANNA fellowship.
3
+
36
sponds with the Gd ions. EPR spectra corresponding
to Ni have not been observed because of the weak EPR
2
+
2
+
signal of the Ni ions.
Conclusions
One-dimensional water ladders, occupying the interlamel-
lar spaces, appear to be reversibly adsorbed along with
changes in the color of the sample. The fast response of the
dehydrated sample to the presence of water suggests that this
Supporting Information Available: X-ray crystallographic
files in CIF format, bond angle table, hydrogen-bond table,
additional structural plots, IR, TGA, PXRD, stabilization
energy plot of water clusters, image of samples in hydrated
and dehydrated states, and N adsorption, UV, PL, magnetic,
2
(
36) Szyczewski, A.; Lis, S.; Kruczynski, Z.; But, S.; Elbanowski, M.;
and EPR plots. This material is available free of charge via the
Internet at http://pubs.acs.org.
Pietrzak, J. J. Alloys Compd. 1998, 275-277, 349.