Different geometrical arrangements in carboxylate coordination polymers of flexible dicarboxylic acid

Article abstract of DOI:10.1016/j.jssc.2011.05.004

Dicarboxylate coordination polymers (15) of Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II), respectively, derived from (7-carboxymethoxy-naphthalen-2-yloxy)- acetic acid (L₁H₂) are synthesized and characterized. Depending on the coordination sites around the metal centers and coordination mode of the ligand, dimensionality of these polymers varies. The dicarboxylates adopt three spatial orientations: in-plane linear coordination, out-of-plane cis coordination and out-of-plane trans coordination mode. Both the cis and trans out-of-plane coordination modes are found to exist only if the ancillary ligand pyridine is coordinated to the metal ion. When the aquoligand coordinates the in-plane linear coordination mode of L₁ predominates. The coordination polymers 4 and 5 show photoluminescence in solution. The dicarboxylate of (5-carboxymethoxy-naphthalen-1-yloxy)-acetic acid (L ₂H₂) does not form coordination polymer under ambient conditions, but prefers to remain as uncoordinated anion providing hydrophobic confinement to hexa-aquometal(II) cation. Compound 3 crystallizes in P2 ₁ space group and it shows broadband ultra-violet fluorescence centered at 352.9 nm on focusing 632.8 nm He:Ne laser.

Full text of DOI:10.1016/j.jssc.2011.05.004

Journal of Solid State Chemistry 184 (2011) 1726–1734
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Different geometrical arrangements in carboxylate coordination polymers of
flexible dicarboxylic acid
n
Himangshu Deka a, Rupam Sarma a, Satchi Kumari b, Alika Khare b, Jubaraj B. Baruah a,
a Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
b Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Dicarboxylate coordination polymers (1–5) of Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II), respectively,
derived from (7-carboxymethoxy-naphthalen-2-yloxy)-acetic acid (L1H2) are synthesized and char-
acterized. Depending on the coordination sites around the metal centers and coordination mode of the
ligand, dimensionality of these polymers varies. The dicarboxylates adopt three spatial orientations:
in-plane linear coordination, out-of-plane cis coordination and out-of-plane trans coordination mode.
Both the cis and trans out-of-plane coordination modes are found to exist only if the ancillary ligand
pyridine is coordinated to the metal ion. When the aquoligand coordinates the in-plane linear
coordination mode of L1 predominates. The coordination polymers 4 and 5 show photoluminescence
in solution. The dicarboxylate of (5-carboxymethoxy-naphthalen-1-yloxy)-acetic acid (L2H2) does not
form coordination polymer under ambient conditions, but prefers to remain as uncoordinated anion
providing hydrophobic confinement to hexa-aquometal(II) cation. Compound 3 crystallizes in P21 space
group and it shows broadband ultra-violet fluorescence centered at 352.9 nm on focusing 632.8 nm
He:Ne laser.
Received 25 November 2010
Received in revised form
19 April 2011
Accepted 1 May 2011
Available online 14 May 2011
Keywords:
Coordination polymers
Flexible carboxylic acids
Structural study
Self-assembly
Fluorescence
Nonlinear optics
& 2011 Elsevier Inc. All rights reserved.
1. Introduction
with different transition metal ions (Fig. 1). These three different
geometrical patterns arise from orientation of the dicarboxylate
Coordination polymers [1–8] are extensively studied in the
recent years for gas storage, [9–12] sensing, [13] molecular recogni-
tion, [14] molecular magnetism, [15–17] catalysis [18–19] and as
nonlinear optical material [20]. Carboxylate coordination polymers,
in this respect, are of special interest for their diverse structural
features [21–30]. Carboxylate ligands possess several binding modes
in different metal carboxylate complexes [31–35]. Metal complex/
coordination polymers with flexible carboxylate arm such as mal-
onate and other carboxylate ligands are also studied with great
interest to understand their structures under variety of conditions
[36–42]. However, there are the scopes for much work in this area to
understand the control over structural topology as the reaction
condition and complexity arising from supramolecular features that
are associated with directional properties can guide such structures.
Much research has been carried out using hydrothermal conditions
[43] and we here demonstrate how room temperature reactions can
lead to coordination polymers and their structure vary with the
metal ions, the coordinating mode of anionic ligand and the ancillary
ligands. We show here three types of geometries adopted by the
di-anion of (7-carboxymethoxy-naphthalen-2-yloxy)-acetic acid (L1)
groups of the flexible arms of the carboxylate ligand (L1). These
orientations lead to cyclic or open chain polymeric structures with
metal ions.
2. Experimental section
The ligands (7-carboxymethoxy-naphthalen-2-yloxy)-acetic acid
L1H2 and (5-carboxymethoxy-naphthalen-1-yloxy)-acetic acid L2H2
were prepared as described earlier [44]. The X-ray structure of each
coordination polymer is determined; corresponding CIF files were
used to simulate the powder X-ray pattern and the experimentally
determined X-ray powder pattern are compared with these (sup-
plementary materials). The fluorescence spectra were measured
from solutions of coordination polymer (10ꢀ4 M in desired solvent)
followed by excitation at 260 nm.
2.1. Synthesis of complex [MnL1(H2O)3]n
1
The ligand (7-carboxymethoxy-naphthalen-2-yloxy)-acetic
acid (0.028 g, 0.1 mmol) along with NaOH (0.008 g, 0.2 mmol)
was dissolved in water (20 mL). To this solution MnCl2 ꢁ 4H2O
(0.0197 g, 0.1 mmol) was added with constant stirring at room
temperature. Precipitation occurred in the reaction mixture.
n Corresponding author.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.05.004

H. Deka et al. / Journal of Solid State Chemistry 184 (2011) 1726–1734
1727
Fig. 1. Structure of L1H2 and possible coordination modes of the ligand L1.
The precipitate was dissolved in pyridine (10 mL). A small amount
2.5. Synthesis of the complex [{Cd L1 (H2O)3} ꢁ H2O]n
5
of dimethylformamide (3 mL) was added. The reaction mixture
was stirred for half an hour. The resulting solution was filtered to
discard any insoluble material and the filtrate was kept for
crystallization. After 6 days diffraction quality crystals were
collected and dried in air. Yield of the pure crystalline complex
60% (with respect to Mn). IR (KBr, cmꢀ1): 3478 (w), 3390 (w),
1608 (w), 1568 (s), 1515 (w) 1474 (w), 1427 (m), 1385 (m), 1331
(w), 1323 (w), 1253 (w), 1212 (s), 1171 (m), 1062 (w), 1052 (w),
844 (m) and 834 (m). Thermogravimetry: weight loss 80–140 1C
13.2% (calcd. 12.5% for loss of three water molecules); residue due
to formation of MnO2 at 575 1C (calcd. 22.7%, experimental
23.63%).
Complex 5 was synthesized in a similar procedure as in 1 but
with CdCl2 ꢁ H2O (0.02 g, 0.1 mmol) as the metal source. Colorless
crystals were collected after 1 week. Yield of the pure crystalline
complex: 55% (with respect to Cd). IR (KBr, cmꢀ1): 3463 (bw),
1614 (s), 1518(w), 1476 (w), 1422 (w), 1391 (w), 1339 (w), 1252
(w), 1213 (s), 1174 (w), 1052 (w), 841 (m), 709(w). 1HNMR
(400 MHz, DMSO-d6, ppm): 4.49 (s, 4H, –CH2ꢀ, 6.94 (m, 4H,
aromatic), 7.64 (d, 2H, aromatic, 3J¼9.2 Hz). Thermogravimetry:
14.58% weight loss 75–110 1C (calcd. 14.07% for loss of four water
molecules); residue at 525 1C for CdCO3 (calcd. 45.83%, experi-
mental 47.92%).
2.6. Synthesis of complex [{Cd (H2O)6} ꢁ L2 ꢁ (H2O)2] 6
2.2. Synthesis of complex [{Ni (L1)(Py)3(H2O)} ꢁ H2O]n
2
Complex 6 was synthesized in a similar procedure as in 5 but
using (5-carboxymethoxy-naphthalen-1-yloxy)-acetic acid as
ligand. Yield of the pure crystalline complex 71% (with respect
to Cd). IR (KBr, cmꢀ1): 3444 (bs), 1596 (s), 1574 (s), 1435 (w),
1412 (s), 1337 (m), 1271 (s), 1088 (w), 1069 (w), 788 (w).
Thermogravimetry: residue 31.25% at 550 1C from continuous
weight loss in the temperature range 225–550 1C (calcd. 31.27%
for formation of CdCO3).
Complex 2 was synthesized in a similar procedure as in 1 but
with NiCl2 ꢁ 6H2O (0.0237 g, 0.1 mmol) as the metal source. Green
colored diffraction quality crystals were collected after a week
and dried in air. Yield of the pure crystalline complex 84% (with
respect to Ni). IR (KBr, cmꢀ1): 3477 (w), 1614 (s), 1514 (w), 1461
(w), 1434 (w), 1381 (w), 1328 (w), 1210 (s), 1177 (m), 1062 (s),
846(s) and 713(m). Thermogravimetry: 55.98% weight loss
90–325 1C (calcd. 58.62% for loss of three pyridines along with
two water molecules); residue due to formation of NiO at 450 1C
(calcd. 12.24%, experimental 11.34%).
3. X-ray crystallography
The X-ray crystallographic data were collected at 296 K with
˚
2.3. Synthesis of complex [CuL1(H2O)(Py)2]n
3
MoK
a radiation (l¼0.71073 A) using a Bruker Nonius SMART
CCD diffractometer equipped with graphite monochromator. The
SMART software was used for data collection and also for
indexing the reflections and determining the unit cell parameters;
the collected data were integrated using SAINT software. The
structures were solved by direct methods and refined by full-
matrix least-squares calculations using SHELXTL software [45].
All the non-H atoms were refined in the anisotropic approx-
imation against F2 of all reflections. The H atoms attached to O
and crystallized water molecule were located from the difference
Fourier synthesis map and refined with isotropic displacement
coefficients It was necessary to apply restrains to optimize the
distances of some hydrogen atoms of water molecules. Crystal
parameters and details of the final refinement parameters are
shown in Table 1.
Complex 3 was synthesized in a similar procedure as in 1 but
with CuCl2 ꢁ 2H2O (0.017 g, 0.1 mmol) at room temperature. After
1 week blue colored needle shaped crystals were collected and
dried in air. Yield of the pure crystalline complex: 67% (with
respect to Cu). IR (KBr, cmꢀ1): 3387 (w), 1629 (s), 1604 (s), 1511
(w). 1447 (w), 1408 (m), 1266 (m), 1203 (s), 1165 (w), 1054 (w),
832 (w), 764 (w) and 699 (m). Thermogravimetry: 30.0% weight
loss 125–190 1C (calcd. 33.65% for loss of two pyridines along
with a water molecule); residue due to formation of CuO at 500 1C
(calcd. 15.5%, experimental 16.0%).
2.4. Synthesis of complex [Zn L1(H2O)2]n
4
Complex 4 was synthesized in a similar procedure as in 1 but
with ZnCl2 (0.0136 g, 0.1 mmol) as the metal source. Colorless
crystals were collected a week after and dried in air. Yield of the
pure crystalline complex 85% (with respect to zinc). IR (KBr, cmꢀ1):
3477(w), 1614(s), 1514(w), 1461(w), 1434(w), 1381(w), 1328(w),
1210(s), 1177(m), 1062(s), 846(s) and 713(m). 1H NMR (400MHz,
DMSO-d6, ppm): 4.539 (s, 4H, –CH2ꢀ), 6.97(d, 2H, aromatic,
3J¼8.8 Hz), 7.06(s, 2H, aromatic), 7.70 (d, 2H, aromatic, 3J¼8.8 Hz).
Thermogravimetry: 8.9% weight loss 35–110 1C (calcd. 8.5% for loss
of two water molecules); residue at 525 1C for Zn(O2CCH3)2 (calcd.
48.45%, experimental 42.85%).
4. Results and discussion
The reactions of (7-carboxymethoxy-naphthalen-2-yloxy)-
acetic acid in the presence of sodium hydroxide with transition
metal salts of manganese(II), nickel(II), copper(II), zinc(II) and
cadmium(II) lead to coordination polymers with varying compo-
sitions as well as dimensionalities. The reactions carried out are
shown in Scheme 1.
Stoichiometric amount of MnCl2 ꢁ 4H2O, L1H2 and NaOH in
water react to form a two dimensional coordination polymer 1

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H. Deka et al. / Journal of Solid State Chemistry 184 (2011) 1726–1734
Table 1
Crystallographic parameters of complexes 1–6.
Compound no.
1
2
3
4
5
6
Formulae
C14 H16 Mn O9
383.21
C29 H27 N3 Ni O8
604.25
C24 H22 Cu N2 O7
513.98
C14 H14 O8 Zn
375.62
C14 H14 Cd O10
454.65
C14 H26 Cd O14
530.75
Formula wt.
Crystal system
Space group
Triclinic
Pꢀ1
Triclinic
Monoclinic
P21
Triclinic
Pꢀ1
Triclinic
Pꢀ1
Triclinic
Pꢀ1
Pꢀ1
4.7577(2)
9.0066(8)
13.3081(14)
7.3001(4)
6.8256(6)
6.3129(4)
˚
a (A)
5.8248(3)
10.7658(10)
15.4196(15)
6.8904(7)
7.6324(4)
8.6984(8)
6.5100(4)
˚
b (A)
26.9737(13)
13.4554(13)
12.8284(7)
14.9548(13)
13.4751(7)
˚
c (A)
a
b
g
(deg)
87.754(3)
89.692(3)
84.740(3)
743.79(6)
90.299(6)
94.313(6)
103.408(5)
1449.9(2)
90.00
91.089(4)
97.389(4)
95.697(4)
704.96(7)
102.797(4)
97.981(4)
92.445(4)
854.97(13)
93.300(3)
100.110(3)
113.129(2)
496.45(5)
110.873(7)
90.00
(deg)
(deg)
3
1152.9(2)
˚
V (A )
Z
2
2
2
2
2
1
Density (mgmꢀ3
)
1.711
0.937
394
1.384
0.723
628
1.481
0.995
530
1.770
1.784
384
1.766
1.327
452
1.775
1.171
270
Abs. coeff. (mmꢀ1
)
F(000)
Total no. of reflections
Reflections, I42 (I)
Max. (deg)
Completeness to 2
7186
8123
9774
5430
11,104
3106
5524
s
2686
4632
3426
2424
1645
25.50
97.2
25.00
90.4
23.98
97.0
25.00
97.5
25.50
97.8
25.00
93.6
y
y
(%)
Data/restraints/parameters
2686/0/235
1.245
0.0433
0.0466
4632/2/378
1.028
0.0826
0.1574
3426/1/312
1.033
0.0427
0.0519
2424/0/210
1.047
0.0448
0.0521
3106/8/231
1.090
0.0881
0.0920
1645/6/141
1.193
0.0505
0.0508
Goof (F2)
R indices [I42
s(I)]
R indices (all data)
Scheme 1. Synthesis of coordination polymers.
having a composition [Mn (L1)(H2O)3]n. The aquo group of the
complex shows a broad IR absorption at 3478 cmꢀ1. The asym-
metric and symmetric stretching of the carboxylate ligand
appears at 1608–1385 cmꢀ1, respectively. The coordination poly-
mer 1 crystallizes in the triclinic space group Pꢀ1. All the
manganese(II) centers in 1 adopt octahedral geometries. How-
ever, there are two different coordination environments for
manganese(II) centers in the structure. Mn1 center is coordinated
to two carboxylate ligands and two aquoligands while Mn2 is
coordinated by two carboxylate oxygens and four aquoligands
there by completing the hexa-coordination. The carboxylate
ligand is coordinated to the metal centers with both bridging
and monodentate binding mode there by linking three mangane-
se(II) centers. This binding mode exhibiting by the ligand L1
carboxylate coordination polymers [46]. The Mn1 centers are
connected to a nearby Mn1 center by the m2 bidentate bridging
mode of the ligand through O4 and O5 coordination. This bridging
elongates the molecule along a crystallographic axis. There is no
bridging between any two Mn2 centers. Mn2 center is coordi-
nated by the ligand in a monodentate fashion and is connected to
the Mn1 center through the coordination by the other end of the
spacer ligand L1. This bridging extends the molecule along the c
crystallographic axis thereby providing it a two dimensional
architecture as shown in Fig. 2a. Further to this, the two dimen-
sional sheets of the coordination polymer 1 are connected to each
other through O3–H3a ꢁ ꢁ ꢁ O5, O3–H3 ꢁ ꢁ ꢁ O6 and O9–H9 ꢁ ꢁ ꢁ O2
interactions. These short range interactions holds the nearby 2D
sheets together and impart the molecule a three dimensional
hydrogen bonded network structure. The weak interactions
in 1 can be assigned as m3 Z2 Z1 and is found to exhibit in
ꢀ :

H. Deka et al. / Journal of Solid State Chemistry 184 (2011) 1726–1734
1729
Fig. 2. (a) Two dimensional sheet of the coordination polymer 1 and (b) hydrogen bonded network formed by the coordination polymer 1.
among the 2D sheets are shown in Fig. 2b and the interactions are
tabulated in supplementary table 1s.
polymeric chains. This channel is shown in Fig. 3(c). Helical
structure is held by a layer of water structures in ordered manner.
The reaction of copper(II) chloride monohydrate and (7-car-
boxymethoxy-naphthalen-2-yloxy)-acetic acid (L1H2) in presence
of sodium hydroxide in water/pyridine leads to the formation of a
one dimensional coordination polymer having the composition
[CuL1(H2O)(Py)2]n (3). For this complex the IR stretching frequen-
cies for the coordinated carboxylate ligands appear at 1629 and
1408 cmꢀ1 and for the coordinated pyridine ligands appear at
832 cmꢀ1. The single crystal X-ray analysis shows that the
coordination polymer 3 crystallizes in the monoclinic P21 space
group. The crystal structure of 3 is shown in Fig. 4 and it is
interesting to observe that the ligand coordinate the metal
centers in cis fashion (Fig. 1). The copper (II) centers adopt a
The reaction between NiCl2 ꢁ 6H2O and L1H2 in presence of
sodium hydroxide in water/pyridine results in the formation of an
one dimensional coordination polymer having composition
[{Ni(L1)(Py)3(H2O)} ꢁ H2O]n (2). The compound exhibits character-
istic IR stretching frequencies for the coordinated carboxylate
ligands at 1614 and 1381 cmꢀ1 as well as for the pyridine ligands
at 846 cmꢀ1. The single crystal X-ray analysis shows that the
coordination polymer 2 crystallizes in the triclinic space group
Pꢀ1. All the nickel(II) centers in the structure adopt a near
octahedral geometry; the hexa-coordination around the metal
center is satisfied by two carboxylate, three pyridine and one
˚
aquo groups. The Ni–Oaquo bond (Ni–O8, 2.07 A) is longer than the
˚
˚
Ni–OL bond (Ni–O1, 2.03 A and Ni–O4, 2.05 A). The Ni–N bonds
slightly distorted (t¼0.18) square pyramidal geometry with both
˚
are even much longer lying in the range of 2.09ꢀ2.12 A. However,
pyridines and the ligand L1 occupying two coordination sites and
these distances are common for octahedral nickel(II) complexes
the fifth coordination site is occupied by a water molecule. This
[47,48]. The carboxylate ligand L1 here connects the nearby metal
index t is zero for a perfectly square-pyramidal and a unity for
centers with a trans m2
ꢀ
Z1
:
Z
1 binding mode. In other words both
perfectly trigonal-bipyramidal geometry (calculated as a differ-
the carboxylate ligating sites are coordinated to the metal center
in a monodentate fashion, there by nullifying the possibility of the
coordination polymer of being a two dimensional like in the case
of 1. The bridging between the nearest nickel(II) centers are
ence between two largest angles divided by 60) [49]. The Cu–OL
˚
˚
bond distances are Cu–O1, 1.97 A and Cu–O5, 1.98 A while the
˚
Cu–Oaquo bond distance is 2.22 A. The nearest Cu1 centers are
connected by the ligand with a m2 Z1 binding mode. This
ꢀ
Z1
:
provided by the trans m2
ꢀ
Z1
:
Z1 binding mode of the ligand L1
bridging, in fact, results in the 1D coordination polymeric struc-
tures of 3 extending along the b crystallographic axis (Fig. 4b).
The ligand L1 reacts with anhydrous zinc(II) chloride to result
in the formation of another one dimensional coordination poly-
mer 4 [ZnL1(H2O)2]n. The compound crystallizes in the triclinic
Pꢀ1 space group. The carboxylate ligand is coordinated to the
metal centers with a similar binding mode as in the coordination
that elongate the polymer along the b crystallographic axis. The
coordinated aquo groups are involved in both intramolecular and
intermolecular hydrogen bond formation viz. O8–H8a ꢁ ꢁ ꢁ O3 and
O8–H8b ꢁ ꢁ ꢁ O3, respectively. These interactions hold one chain to
another and there by forms a two dimensional hydrogen bonded
sheets as shown in Fig. 3b. It is noteworthy that the coordination
polymer holds one water molecule of crystallization per nickel
atom. These water molecules remain hydrogen bonded to the one
dimensional chain forming a water channel in between two
polymer 1. Each of the ligand L1 adopts the m3 Z2 Z1 binding
ꢀ :
mode thereby linking three zinc(II) centers. However the differ-
ence with 1 arises in that all the zinc(II) centers are penta

1730
H. Deka et al. / Journal of Solid State Chemistry 184 (2011) 1726–1734
coordinated with three of the coordination sites occupied by the
carboxylate ligand and other two by aquoligands. These coordi-
nation lead to a slightly distorted (
t
¼0.18) trigonal bipyramidal
geometry around the zinc(II) centers. Equatorial positions in the
trigonal bipyramid are occupied by the three carboxylato oxygens
˚
˚
with bond distances of Zn1–O5, 1.96 A; Zn1–O7, 1.97 A and Zn1–
˚
O1, 2.00 A while the axial sites are occupied by two aquoligands
˚
˚
with bond distances Zn1–O6, 2.08 A and Zn1–O8, 2.14 A. The
bridging binding mode leads to binuclear zinc(II) units that are
connected to each other through the monodentate binding mode
of the spacer ligand L1 at the rear end. This bridging extends the
molecule along the b crystallographic axis resulting in the 1D
coordination polymer. In the crystal lattice the polymeric chains
are held together via inter-molecular hydrogen bonding through
water molecules and formed a layer structure. The aquo groups
between the layers are inter-molecularly hydrogen-bonded with
oxygen atoms of carboxylates via O6–H6 ꢁ ꢁ ꢁ O4, O6–H6b ꢁ ꢁ ꢁ O1,
O6–H6b ꢁ ꢁ ꢁ O2, and O8–H8 ꢁ ꢁ ꢁ O4 interactions. The hydrogen
bond parameters are included in supplementary table 1s. Besides
these, the polymeric chains are also interconnected by weak
˚
˚
C2–H2a ꢁ ꢁ ꢁ O3, 2.73 A and C13–H13 ꢁ ꢁ ꢁ
p, 2.85 A interactions.
We had earlier shown that the zinc complexes derived from
analogous isomeric ligand of naphthalene on coordination gen-
erate template to coordinate sodium ions [42], but such observa-
tions are not observed with the L1. The paddle wheel structures
of carboxylate coordination polymer are generally formed at
room temperature reactions of terephthalate type of ligands
[46] (Fig. 5).
The ligand L1 reacts with cadmium chloride monohydrate to
result in the formation of one dimensional coordination polymer
5 [{CdL1(H2O)3}.H2O]n. The FT-IR spectra show the stretching
frequencies for the carboxylate ligands appearing at 1614 and
1391 cmꢀ1 due to the asymmetric and symmetric stretching
mode. The stretching due to aquoligands as well as the water of
crystallization appears at 3463 cmꢀ1. All the cadmium(II) centers
are hepta coordinated by four oxygen atoms of carboxylates and
three aquo molecules. All the Cd–O bond distances are in the
Fig. 3. (a) Crystal structure of coordination polymer 2 showing the trans m2 Z1 Z1
ꢀ :
binding mode of L1 (the hydrogen atom in oxygen O7 could not be located),
(b) self-assembly of complex 2 through weak interactions and (c) water channels
encapsulated by the 1D chains of 2.
˚
range 2.23–2.46 A. There is one water molecule of crystallization
that remains hydrogen bonded in the crystal lattice. However the
hydrogen atoms attached to this oxygen atom O10 could not be
located due to the poor reflection data. The carboxylate ligand is
ꢀ :Z
coordinated to the metal centers with m2 Z2 2 binding mode as
both the carboxylate groups are in chelating mode. This bridging
extends the coordination polymer along the c crystallographic
axis. The aquoligands in the molecule are involved in hydrogen
bonding through O9–H9a ꢁ ꢁ ꢁ O7, O9–H9b ꢁ ꢁ ꢁ O4, O8–H8a ꢁ ꢁ ꢁ O2,
O8–H8b ꢁ ꢁ ꢁ O9, O7–H7a ꢁ ꢁ ꢁ O10 interactions which lead to a
complicated three dimensional hydrogen bonded network. For
simplicity, only some of these interactions in the coordination
polymer are shown in Fig. 6.
It is interesting to note here that the use of the isomeric ligand
(5-carboxymethoxy-naphthalen-1-yloxy)-acetic acid (L2H2) in
place of the L1H2 in a similar reaction with metal(II) chloride
results in the formation of salts comprising of hexa-aquometal(II)
ion and (5-carboxymethoxy-naphthalen-1-yloxy)-acetate ions
(L2) crystallizing with a water molecule (Scheme 2). This type of
products are obtained in the case of the reaction of (5-carboxy-
methoxy-naphthalen-1-yloxy)-acetic acid with both manganese
chloride tetrahydrate and cadmium chloride hydrate in water in
the presence of sodium hydroxide. Both complexes are character-
ized by conventional spectroscopic methods as well as by single
crystal and X-ray powder diffraction techniques. However the
diffraction data obtained for the salt of hexa-aquomanganese(II)
ion and (5-carboxymethoxy-naphthalen-1-yloxy)-acetate ion
co-crystallized with a water molecule is of poor quality to be
reported, nevertheless it is good enough to make sure about the
Fig. 4. (a) cis m2
(b) formation of one dimensional coordination polymer in complex 3.
ꢀ :
Z1 Z1 binding mode of L1 in coordination polymer 3 and

H. Deka et al. / Journal of Solid State Chemistry 184 (2011) 1726–1734
1731
Fig. 5. Formation of one dimensional coordination polymer 4 through m2 Z2 Z1 coordination mode of L1.
ꢀ :
Fig. 6. Hydrogen bonding in the one dimensional chains of coordination polymer 5.
Scheme 2. Synthesis of cadmium complex of L2.
structural pattern. The crystal structure of the hexa-aquocadmium(II)
ion and (5-carboxymethoxy-naphthalen-1-yloxy)-acetate ion is
shown in Fig. 7a. There is one uncoordinated water molecule in the
lattice. The complex cation, the dicarboxylate anions and the water
molecules are connected by a number of hydrogen bond interactions
such as O4–H4b ꢁ ꢁ ꢁ O1, O5–H5a ꢁ ꢁ ꢁ O7, O7–H7a ꢁ ꢁ ꢁ O1, O7–
H7b ꢁ ꢁ ꢁ O2, etc. as shown in Fig. 7b. The hydrogen bond parameters
are tabulated in supplementary table 1s.
In order to confirm the phase purity of the bulk materials,
powder X-ray diffraction (PXRD) of all the compounds were
determined. The PXRD experimental and simulated patterns of
all of them are shown in the supplementary materials. As shown
in Fig. 8, all major peaks of experimental powder X-ray patterns
(PXRD) of compound 1 matches well that of simulated PXRD,
indicating their reasonable crystalline phase purity. PXRD analy-
sis of the bulk samples of the other coordination polymers 2–5
and the molecular complex 6 are carried out and found to match
well with the simulated patterns (please refer to the supplemen-
tary materials).
The thermal stability of all these complexes was studied by
thermogravimetry. The curves for weight loss vs. temperature at
heating rate 5 1C/min up to 550 1C from room temperature have
shown that each complex except compound 6 loses the solvent
molecules acting either as solvent of crystallization or ligands, at
relatively low temperature (cf. o250 1C). The theoretical and
experimental weight losses are calculated on the basis of the
formula obtained from single crystal X-ray. The calculated and
experimental values tallies well with such loss of weight. The
complex 6 loses weight continuously from 250 to 550 1C to form
cadmium carbonate. In the case of zinc complex the final weight
of the residue is close to zinc acetate; whereas in the case of
manganese, nickel and copper complexes corresponding stable
oxides are observed as residues. Metal carboxylate complexes on
heating generally leads to metal oxides [50–51]. In some of our

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H. Deka et al. / Journal of Solid State Chemistry 184 (2011) 1726–1734
Fig. 7. (a) Crystal structure of molecular complex 6 and (b) hydrogen bonded self-assembly of complex 6 through weak interactions.
Fig. 8. Experimental and simulated PXRD pattern for the coordination polymer 1 (top: experimental; bottom: simulated).
cases such as cadmium we have obtained carbonate derivatives
residues, such observations are not exceptional to carboxylate
complexes of f-block metal but are less frequent [52].
results in precipitation. As this precipitate can be dissolved in
pyridine we have tried to carry out the fluorometric titration in
pyridine as solvent. Interestingly, in pyridine (7-carboxymethoxy-
naphthalen-2-yloxy)-acetic acid is not fluorescence active, in fact,
addition of pyridine solution (10ꢀ2 M, methanol) to a 10ꢀ4 M
solution of L1H2 results in quenching of fluorescence (please refer
to supplementary material). Compounds 4 and 5 are soluble in
dimethylsulfoxide (DMSO); we have recorded the fluorescence
spectrum of these compounds in DMSO. Comparison of the
emission spectra of the free acid with the coordination polymers
shows that the free acid, 4 and 5 emit at the same wavelength
333 nm. As the MLCT or LMCT transitions can be ignored in case
of a d10—electronic configuration the emission observed in
5. Photoluminescence study
Photoluminescence properties of the acid (7-carboxymethoxy-
naphthalen-2-yloxy)-acetic acid as well as the coordination poly-
mers 4 and 5 are studied in the solution state. The UV absorption
spectra show that the methanolic solution of the acid has three
absorption bands at 273, 309 and 323 nm. Among them the band
at 273 nm arises due to
p–p* transition of the ligand. Emission
spectra were recorded for the samples by exciting them at
260 nm. Upon excitation, L1H2 emits at 333 nm. We have tried
to follow a fluorometric titration of the acid with incremental
addition of metal(II) acetate viz. Zn(OAc)2 and Cd(OAc)2.
However, the addition of a metal salt to the solution of L1H2
compounds 4 and 5 are only because of the p–p* transition of
the ligand. The emission spectra for the compounds as well as for
the ligand are shown in Fig. 9. We have not observed any
significant change by adding pyridine to the DMSO solutions of
compounds 4 and 5. However, there is reduction of intensity of

H. Deka et al. / Journal of Solid State Chemistry 184 (2011) 1726–1734
1733
incidence of 451. The emitted signal was collected by a lens on to the
monochromator. Monochormator was scanned in the ultraviolet
range from 300 to 400 nm. The broadband ultra-violet fluorescence
was observed centered at 352.9 nm as shown in Fig. 10. The proposed
mechanism for the observed fluorescence is three-photon absorption
induced fluorescence. This multiphoton absorption confirms that the
coordination polymer 3 may have scope for nonlinear optical
application in particular for frequency up conversion.
7. Conclusion
In conclusion, we have synthesized five coordination polymers of
dicarboxylate of (7-carboxymethoxy-naphthalen-2-yloxy)-acetic
acid with transition metal ions viz. Mn(II), Ni(II), Cu(II), Zn(II) and
Cd(II), respectively, with or without ancillary ligand at ambient
conditions. Three different orientations of the ligand are identified
and observed that the ligand L1 can coordinate to metal with: in
plane-linear coordination mode, out of plane cis coordination mode
and out of plane-trans coordination mode. Coordination polymer 1
has a two dimensional architecture whereas all other polymers
under consideration are one dimensional coordination polymers.
The aquoligands in coordination polymers 2, 4 and 5 are involved in
hydrogen bonding among each other; there by leads to three
dimensional supramolecular architecture. It is observed that in case
of the coordination polymers 1, 4 and 5 where water is the only
ancillary ligand, the ligand L1 adopts the in-plane linear coordination
mode. However, in coordination polymers 2, L1 adopts the out of
plane trans coordination mode and in 3 it adopts the out of plane-cis
coordination mode and in both the cases pyridine remains coordi-
nated to the metal center. Therefore, it can be concluded that the cis
and trans out-of-plane coordination modes are found to exist only if
the ancillary ligand pyridine also remains coordinated to the metal
center along with aquoligands and origin of the different orientation
of the ligand lies at the steric congestion around the metal center.
If the aquoligands happen to be the only ancillary ligand the in-plane
linear coordination mode of L1 predominates. Literatures are there
accounting for such dependence of architecture of coordination
polymers on the coordination environment [18]. The isomeric
(5-carboxymethoxy-naphthalen-1-yloxy)-acetic acid does not lead
to coordination polymer, rather remain as anion for hexa-aquometal
(II) cation. The thermal as well as luminescence properties of the
acid L1H2 and the complexes are studied. (7-Carboxymethoxy-
naphthalen-2-yloxy)-acetic acid is highly fluorescence active and
quenching of fluorescence can be obtained by addition of pyridine to
a solution containing L1H2; however, the zinc (II) and cadmium (II)
complexes are fluorescence active. The coordination polymer 3 may
have scope for nonlinear optical application as confirmed by the
multiphoton absorption process.
Fig. 9. Fluorescence emission spectra for compounds 4, 5 and L1H2.
352.9
300
320
340
360
380
400
Wavelength (nm)
Fig. 10. Multiphoton induced emission in UV range, confirming frequency up
conversion in compound 3.
fluorescence similar but less prominent than the one dissolved in
pyridine. Thus, it suggests that complexes 4 and 5 dissociates in
DMSO solution and fluorescence emission of the ligand as well as
undissociated forms are observed, which shows quenching of
fluorescence of the complexes in comparison to the metal com-
plexes. Naphthalene derivatives are known to form intramolecular
exciplex [53] and nitrogen heteroaromatic molecules such as
pyridine, picoline, etc. are known to be quencher for such exciplex
fluorescence [54]. In our case also fluorescence quenching is
presumably through exciplex quenching upon addition of pyridine.
Supplementary materials
The hydrogen bond parameters, fluorescence emission of
complexes, powder diffractions of the polymers, CIF files for the
coordination polymers 1–6 are given as supplementary materials.
The crystallographic data files of the compounds are submitted to
Cambridge Crystallographic Database and have the CCDC num-
bers 774467, 774468 and 779596–779599.
6. SHG properties
Since coordination polymer 3 crystallizes in a non-centrosym-
metric space group it is expected to show nonlinear optical properties
[55,56]. To explore the nonlinearity in the compound, a film was
prepared by the deposition of drop of a solution of the sample in
pyridine–water mixed solvent (1:2 ratio) over glass plates followed
by room temperature drying. This resulted in the thin film on cover
slit. The 632.8 nm He:Ne laser was focused on to the thin film sample
to a power density of 106 W/m2. The laser was launched at an angle
Acknowledgments
The authors thank Department of Science and Technology,
New Delhi, India for financial support. The author R.S. thanks
CSIR, New Delhi, India for fellowship.

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H. Deka et al. / Journal of Solid State Chemistry 184 (2011) 1726–1734
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