Syntheses, structure and properties of Alkaline-earth metal salts of 4-Nitrophenylacetic acid

Article abstract of DOI:10.1007/s12039-016-1182-1

The synthesis, crystal structure, spectral characteristics and thermal properties of alkaline-earth metal salts of 4-nitrophenylacetic acid (4-npaH) namely, [Mg(H₂O)₆](4-npa)₂?4H ₂O (4-npa = 4-nitrophenyl-acetate) (1), [Ca(H ₂O) ₂(4-npa) ₂] (2) and [Sr(H ₂O) ₃(4-npa) ₂] ?4.5H ₂O(3) are reported. In 1, the 4-npa ion functions as a charge balancing counter anion for the octahedral [Mg(H ₂O) ₆] ²⁺ unit with the Mg(II) ion situated on a centre of inversion. The two unique lattice water molecules link the [Mg(H ₂O) ₆] ²⁺ cations and 4-npa anions with the aid of O-H ?O interactions. Compounds 2 and 3 are one-dimensional (1-D) coordination polymers containing an eight coordinated Ca(II) situated in a general position and a nine coordinated Sr(II) located on a twofold axis. The μ₂-bridging tridentate binding modes of the crystallographically independent 4-npa ligands in 2 and the unique 4-npa ligand in 3 link the bivalent metal ions into an infinite chain with alternating Ca ?Ca separations of 3.989 and 4.009 ?, respectively, and a single Sr ?Sr separation of 4.194 ? in the 1-D chain. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1182-1

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1182-1
Syntheses, structure and properties of Alkaline-earth metal
salts of 4-Nitrophenylacetic acid
BIKSHANDARKOIL R SRINIVASANa,∗, KIRAN T DHAVSKARa and CHRISTIAN NÄTHERb
aDepartment of Chemistry, Goa University, Goa 403 206, India
bInstitut für Anorganische Chemie, Christian-Albreschts Universität Kiel, D-24098 Kiel, Germany
e-mail: srini@unigoa.ac.in
MS received 31 May 2016; revised 28 July 2016; accepted 28 August 2016
Abstract. The synthesis, crystal structure, spectral characteristics and thermal properties of alkaline-earth
metal salts of 4-nitrophenylacetic acid (4-npaH) namely, [Mg(H2O)6](4-npa)2·4H2O (4-npa = 4-nitrophenyl-
acetate) (1), [Ca(H2O)2(4-npa)2] (2) and [Sr(H2O)3(4-npa)2]·4.5H2O(3) are reported. In 1, the 4-npa ion func-
tions as a charge balancing counter anion for the octahedral [Mg(H2O)6]2+ unit with the Mg(II) ion situated on
a centre of inversion. The two unique lattice water molecules link the [Mg(H2O)6]2+ cations and 4-npa anions
with the aid of O-H· · · O interactions. Compounds 2 and 3 are one-dimensional (1-D) coordination polymers
containing an eight coordinated Ca(II) situated in a general position and a nine coordinated Sr(II) located on a
twofold axis. The μ2-bridging tridentate binding modes of the crystallographically independent 4-npa ligands
in 2 and the unique 4-npa ligand in 3 link the bivalent metal ions into an infinite chain with alternating Ca· · · Ca
separations of 3.989 and 4.009 Å, respectively, and a single Sr· · · Sr separation of 4.194 Å in the 1-D chain.
Keywords. Alkaline-earth; 4-nitophenylacetic acid; one-dimensional; μ2-bridging tridentate; coordination
polymer.
1. Introduction
offers the possibility of disposing the –COOH group
differently by placing additional substituents like –NH2,
–OH, –NO2, etc., on the six membered ring to fine
tune the structure with the aid of secondary interac-
tions, resulting in a rich structural chemistry for the
alkaline-earth metal benzene carboxylates. When syn-
thesis is performed in aqueous medium, the small size,
high hydration tendency and a preferred hexa coordi-
nation number favour the formation of an octahedral
[Mg(H2O)6]2+ ion charge balanced by carboxylate
anion leading to monomeric structures. The –NO2
group is isoelectronic with –COOH but is devoid of
metal binding capabilities. Hence, we have investi-
gated the chemistry of nitrobenzoic acids with alkaline-
earths. Although 1-D coordination polymers could
be constructed using 2-nitrobenzoic acid (2-nbaH),8b
2-carbamoyl-4-nitrobenzoic acid,8g 3-nitrophthalic acid,7f
only monomeric structures could be assembled for Ca9b
or Sr9c with 4-nitrobenzoic acid (4-nbaH). The intro-
duction of a –CH2 group trans to the nitro functionality
between the ring and the –COOH in 4-nbaH results in
4-nitrophenylacetic acid (4-npaH). In the pair of link-
ers which differ by a methylene group, the –COOH
moiety (Figure 1) is attached to a rigid six membered
ring in 4-nbaH while it is more flexible in 4-npaH.
In the literature few papers are reported on metal
4-nitrophenylacetate (4-npa) compounds other than
The past two decades have witnessed the develop-
ment of an extensive s-block metal chemistry in terms
of the structural characterization of several alkali
and alkaline-earth metal carboxylates.1 8 These studies
have revealed that the alkaline-earths exhibit different
degrees of hydration. Unlike Mg(II) which forms many
compounds containing the octahedral [Mg(H2O)6]2+
ion,6 the heavier congeners contain less number of coor-
dinated water molecules and adopt an extended struc-
ture viz., one- or two-dimensional due to carboxylate
bridging. The heavier metal ions exhibit a higher coor-
dination number; for example the preferred coordi-
nation number of Ca(II) is eight,8 while Sr(II)3e,f or
Ba(II)5a,b exhibit coordination numbers of nine or more.
The flexibility of Ca, Sr and Ba to adopt variable coor-
dination geometries1a,b and their oxophilic nature are
probably responsible for a higher denticity (viz., tri-
dentate or more) of the carboxylate ligand to satisfy
the higher than six coordination number of these metal
ions, resulting in the formation of extended structures.
This was well demonstrated for alkaline-earths by link-
ing them into a one-dimensional (1-D) chain employing
2-aminobenzoic acid as a linker.7a An aromatic ring
∗For correspondence
Dedicated to Prof. M.S. Wadia on the occasion of his 80th birthday.

Bikshandarkoil R Srinivasan et al.
a water bath. The insoluble starting material slowly
started dissolving with brisk effervescence. The heating
was stopped when there was no more evolution of CO2.
At this stage, the reaction mixture was almost clear, and
pH was close to neutral. The hot reaction mixture was
filtered and kept aside for crystallization at low tem-
perature. Colourless crystals of 1 which separated in a
few days were collected by filtration in ∼60% yield.
The use of calcium carbonate, strontium carbonate
and barium carbonate instead of MgCO3 in the above
reaction afforded compounds 2, 3 and 4, respectively.
(Yield = 65–72%) Anal. Calcd(%) for C16H32MgN2O18
(564.74 g/mol) 1: C, 34.03; H, 5.71;N, 4.96; MgO, 7.14;
Found (%): C, 34.02.17; H, 5.59; N, 4.51; MgO, 7.38;
Anal. Calcd(%) for C16H16CaN2O10 (436.39 g/mol) 2:
C, 44.04; H, 3.70; N, 6.42; CaCO3, 22.93; Found(%): C,
44.99; H, 3.72; N, 6.11; CaCO3, 21.31; Anal.Calcd(%)
for C16H27SrN2O15.5(583.01 g/mol) 3: C, 32.96; H, 4.68;
N, 4.80; SrCO3, 23.10; Found: C, 33.29; H, 4.476; N,
4.79; SrCO3, 24.95; Anal. Calcd(%) for C16H14BaN2O9
(515.62 g/mol) 4: H2O, 3.40; BaCO3, 38.27; Found(%):
H2O, 3.49; BaCO3, 38.13.
Figure 1. Rigid versus flexible –COOH group in 4-nbaH
and 4-npaH.
alkaline earths.10 In this study, we have investigated the
reactions of alkaline-earth carbonates with 4-npaH to
structurally characterize the products. The results are
described in this paper.
2.3 X-ray crystal structure determination
2. Experimental
Single crystal X-ray structure analysis of 1 and 3 was
performed at the Sophisticated Analytical Instrument
Facility (SAIF), Indian Institute of Technology (IIT)
Madras. Intensity data were collected using Bruker
AXS Kappa Apex II CCD Diffractometer for 1 and
3 and an Image Plate Diffraction System (IPDS-1)
from STOE for 2. The structures were solved with
direct methods using SHELXS-97 and refinement was
done against F2 using SHELXL-97.11 All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms
attached to the aromatic ring were introduced in calcu-
lated positions and included in the refinement by rid-
ing on their respective parent C atoms. The O-H and
N-H, hydrogen atoms were located in difference map,
their bond lengths were set to ideal values (for 2) and
afterwards they were refined using a riding model. The
technical details of data acquisition and selected crystal
refinement results are summarised in Table 1.
2.1 Materials and methods
All the chemicals used in this study were of reagent
grade and were used as received without any further
purification. Infrared (IR) spectra of the solid samples
diluted with KBr were recorded on a Shimadzu (IR
Prestige-21) FT-IR spectrometer from 4000–400 cm−1
at a resolution of 4 cm−1. Raman spectra were recorded
using 785 nm laser radiation for excitation on an Agiltron
PeakSeeker Pro Raman instrument. UV-Visible spec-
tra were recorded in water using a Shimadzu UV-2450
double beam spectrophotometer using matched quartz
cells. Isothermal weight loss studies were performed in
a temperature controlled furnace. TG-DTA study was
performed in flowing oxygen in Al2O3 crucibles at heat-
ing rate of 10 K min−1 using a STA-409 PC simultane-
ous thermal analyser from Netzsch. Elemental analyses
(C, H and N) were performed on a Variomicro cube
CHNS analyser.
3. Results and Discussion
2.2 Synthesis of [Mg(H2O)6](4-npa)2·4H2O (1), [Ca
(4-npa)2(H2O)2] (2), [Sr(4-npa)2(H2O)3]·4.5H2O (3),
and [Ba(4-npa)2(H2O)] (4)
3.1 Synthetic aspects, spectral and thermal studies
The reaction of alkaline-earth metal carbonates MCO3
A mixture of 4-npaH (1.812 g, 10 mmol) and magne- (M = Mg or Ca or Sr or Ba) with 4-npaH in 1:2 mole
sium carbonate (5 mmol) was taken in distilled water ratio in a hot aqueous solution resulted in the dissolu-
(∼100 mL) and the reaction mixture was heated on tion of MCO3. Filtration followed by slow evaporation

Alkaline-earth metal salts of 4-nitrophenylacetic acid
Table 1. Crystal data and selected refinement results for (1), (2), and (3).
Empirical formula
Formula weight (g/mol)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C16H32N2MgO18 (1)
564.74
293(2)
0.71073
Monoclinic
P21/c
C16H16N2CaO10 (2) C16H27N2SrO15.5 (3)
436.39
200(2)
0.71073
Triclinic
P¯ı
583.01
293(2)
0.71073
Monoclinic
C2/c
13.5398(9)
8.1318(5)
11.8364(9)
90
102.042(3)
90
1274.54(15)
2
1.472
7.7599(8)
10.9920(12)
12.1866(14)
108.538(13)
105.120(12)
90.597(12)
946.46(18)
2
29.118(4)
11.2307(16)
7.9877(11)
90
100.988(6)
90
b (Å)
c (Å)
α(◦)
β(◦)
γ (◦)
Volume (Å3)
Z
2564.2(6)
4
Dcalc (mg/m3)
Absorption coefficient (mm−1
F(000)
1.531
1.497
)
0.156
596
0.391
452
2.175
1196
Crystal size (mm3)
θ range for data collection (◦)
Index ranges
0.35 × 0.30 × 0.25
2.940 to 24.996
−15 ≤ h ≤ 16
−9 ≤ k ≤ 9
−14 ≤ l ≤ 14
13872 / 2239
[R(int) = 0.0351]
0.18 × 0.12 × 0.07 0.08 × 0.05 × 0.04
2.73 to 27.00
−9 ≤ h ≤ 9
2.804 to 24.996
−34 ≤ h ≤ 27
−13 ≤ k ≤ 13
−9 ≤ l ≤ 9
−14 ≤ k ≤ 14
−15 ≤ l ≤ 15
8221 / 4019
Reflections collected / unique
18321 / 2267
[R(int) = 0.0646]
[R(int) = 0.1290]
Reflections with I>2σ(I)
Completeness to θ
Absorption correction
Max. and min. Transmission
1716
100%
2769
97.2%
1515
99.9%
Semi-empirical from equivalents Numerical
0.975 and 0.937
Semi-empirical from equivalents
0.8818 and 0.9678 0.895 and 0.802
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
2239 / 16 / 209
1.029
R1 = 0.0406,
wR2 = 0.0795
R1 = 0.0610,
wR2 = 0.0875
4019 / 0/ 263
0.957
2266 / 5 / 173
1.003
R1 = 0.0433,
wR2 = 0.1072
R1 = 0.0886,
wR2 = 0.1280
R1 = 0.0405,
wR2 = 0.0887
R1 = 0.0744,
wR2 = 0.1010
R indices (all data)
Largest diff. peak and hole(e Å−3) 0.240 and −0.194
0.347 and −0.369 0.447 and −0.301
in the 3500–3000 cm−1 region. However, it is found
that compounds 1 and 3 tend to lose water gradu-
ally with time. Compound 1 loses its lattice water
once air dried as evidenced by changes in IR spectra
(Figure S2 in Supplementary Information), and TG-
DTA curves (Figure 2) of air dried sample of 1.
of the reaction mixture afforded compounds 1–4 in
good yield. Out of these, 1–3 gave crystals suitable for
X-ray structure determination; only a polycrystalline
material could be obtained for 4. Hence, for com-
pound 4 with a composition [Ba(4-npa)2(H2O)], the
structure could not be determined. An analysis revealed
that all compounds which contain metal:4-npa in 1:2
ratio are differently hydrated with the metal:water ratios
being 1:10, 1:2, 1:7.5 and 1:1 for Mg, Ca, Sr and Ba,
respectively.
A comparison of the IR spectra (Figure S1 in Sup-
plementary Information) of fresh samples of 1–4 with
4-npaH, reveals that the band due to –COOH group
of the free acid occurring at 1710 cm−1 is shifted to
lower energies in 1–4 due to the formation of metal
carboxylate. The presence of water in 1–4 can be
evidenced by the characteristic profile of the spectra
A weight loss of 21.94% above 100◦C accounts for
weight loss of six coordinated water, instead of ten if
lattice water is considered. This suggests that the lattice
waters of the compound 1 are more easily lost when
subjected to dryness. Hence, the first DTA endother-
mic peak is assigned to the removal of coordinated
water. The DTA peak at 557◦C is an exothermic event
and represents the decomposition of complex into its
oxide with the observed residue of 7.38% in agreement
with the calculated value of 7.14%. Similarly, 3 also
loses water over a period of time but slower than 1.

Bikshandarkoil R Srinivasan et al.
rehydrated back upon cooling to room temperature. The
Raman spectra of 1–3 and the free ligand 4-npaH are
very similar (Figure S7 in Supplementary Information).
It is interesting to note that compounds 1-3 exhibit the
symmetric stretching vibration of the −NO2 group at
∼1339 cm−1 as the most intense signal which is also
observed in the free ligand. The asymmetric stretch-
ing vibration of the −NO2 group is also observed as a
strong signal at ∼1597 cm−1. The optical spectra of 1 to
3 in water exhibit absorption maxima (Figure S8) which
are close to 291 nm, the λmax of the free acid. This signal
can thus be assigned for an intra-ligand charge transfer
of 4-npa.
3.2 Description of Crystal structures
Compound 1 crystallises in the centrosymmetric mon-
oclinic P21/c space group with the Mg(II) located on
an inversion centre. The crystal structure consists of
a central Mg(II) ion, three unique terminal water lig-
Figure 2. TG-DTA curves for 1 at 10◦C/min.
This behaviour is clearly evidenced from differences ands, an independent 4-npa anion and two lattice water
observed in TG profile of freshly air dried and aged (or molecules (Figure 3). In view of the special position
old) sample of 3 (Figure S3 in Supplementary Infor- of Mg(II), the asymmetric unit consists of a half of
mation). A fresh sample shows total weight loss of the formula unit. In this compound, 4-npa is not coor-
23. 27% accounting for loss of 7.5 moles of water, dinated to Mg(II) but functions as a charge balanc-
while an aged sample of 3 is comparatively drier as it ing counter anion. The metric parameters of the 4-npa
shows weight loss of only 12.07% at the same temper- anion are in the normal range (Table S1 in Supple-
ature. This change is also observed in the correspond- mentary Information). The central metal is linked to
ing DTA curves of both samples. In fresh sample of 3, six aqua ligands forming the well-known [Mg(H2O)6]2+
two endothermic peaks, at 79◦C and 130◦C respectively unit observed in several Mg(II) carboxylates listed in
are observed, while only a single endothermic peak at the Cambridge Database. The Mg-O bond distances
126◦C is observed in the DTA curve of aged sample. range from 2.0510(15) to 2.0855(16) Å (Table 2). The
The differences appear in the region between 3500– trans O-Mg-O angles exhibit ideal values of 180◦ while
3000 cm−1 of IR spectra of 3 (Figure S4 in Supplemen- the cis O-Mg-O vary between 88.37(7) to 91.63(7)◦
tary Information) which clearly indicate that the water (Table S2 in Supplementary Information) indicating a
content in two samples is no more the same. Above slightly distorted {MgO6} octahedron.
500◦C the compound is decomposed into its carbonate
An analysis of the crystal structure of 1 reveals that
(%SrCO3 = 24.95%). It is interesting to note that 2 the [Mg(H2O)6]2+ cations, 4-npa anion and the lattice
is not only air stable but also doesn’t lose any water water are interlinked with the aid of O-H· · · O interac-
even at 100◦C. The TG-DTA curves exhibit (Figure S5 tions (Table 3). The O4, O5 and O6 atoms of the 4-npa
in Supplementary Information) an endothermic event at anion, and the O8 and O9 of the lattice water function as
131◦C and a corresponding mass loss of 8.45% which H-acceptors while the H-atoms of the coordinated and
indicates loss of two coordinated water molecules. The lattice water molecules function as H-donors. The O9-
stability at 100◦C and dehydration at 131◦C is can be H9B· · · O8 interaction between the lattice waters results
evidenced from IR spectra (Figure S6 in Supplemen- in the formation of a water dimer, which is further H-
tary Information) of compound 2, heated at these tem- bonded to two symmetry related [Mg(H2O)6]2+ cations
peratures. Similarly, compound 4 is thermally stable as and three different 4-npa anions (Figure 4).
weight change is observed only when it is heated to
The O8 of the lattice water forms H-acceptor bonds
150◦C. The barium carbonate residue formed (38.13%) with a [Mg(H2O)6]2+ cation via O2-H2C· · · O8 and
on decomposition of 4, indicates the presence of a sin- a second lattice water via O9-H9B· · · O8 interactions,
gle water molecule. This is confirmed by a mass loss while the H atoms of the lattice water further link it with
of 3.40% (expected for one water=3.49%) observed two 4-npa anions via the H-donor bonds O8-H8A· · · O5
at 150◦C for compound 4. However, 4 is immediately and O8-H8B· · · O6 (Figure S9 in Supplementary

Alkaline-earth metal salts of 4-nitrophenylacetic acid
Figure 3. The crystal structure of 1 showing the atom labelling scheme and the coordination
sphere of Mg(II). Displacement ellipsoids are drawn at the 50% probability level excepting
for the H atoms, which are shown as spheres of arbitrary radii. Intramolecular H-bonding is
shown in broken lines.
Information). The O9 of the second lattice water forms positions. Its structure (Figure 5) consists of an octaco-
H-acceptor bonds with two different [Mg(H2O)6]2+ ordinated central Ca(II), two terminal water molecules,
cations via O1-H1A· · · O9 and O1-H1B· · · O9 interac- and two crystallographically independent 4-npa ligands,
tions, while the H atoms of the lattice water further link both of which behave as μ2-bridging tridentate ligands
4-npa anion and the first lattice water via O9-H9A· · · (Figure S10 in Supplementary Information). The geo-
O4 and O9-H9B· · · O8 interactions respectively lead- metric parameters of the unique 4-npa ligands are in
ing to the formation of alternating layers of cations the normal range (Table S1). Each calcium is linked to
and anions (Figure S9). Thus the unique lattice water two monodentate water, and to six O atoms from four
molecules (O8 and O9) linked into a H-bonded dimer symmetry related 4-npa anions, resulting in a distorted
serve as a link between the alternating layers of cations square antiprismatic {CaO8} polyhedron (Figure S10 in
and anions.
Supplementary Information). The Ca-O bond distances
Compound 2 crystallizes in the centrosymmetric vary between 2.3359(16) and 2.6636(17) Å (Table 2)
triclinic space group P¯ı with all atoms located in general while the O-Ca-O angles scatter in a very broad range
Table 2. Selected bond lengths [Å] for 1, 2, and 3.
[Mg(H2O)6](4-npa)2·4H2O (1)
Mg1-O1
Mg1-O2
Mg1-O3
2.0855(16)
2.0510(15)
2.0607(15)
Mg1-O1i
Mg1-O2i
Mg1-O3i
2.0856(16)
2.0510(15)
2.0607(15)
[Ca(H2O)2(4-npa)2] (2)
Ca1-O11ii
Ca1-O2iii
Ca1-O21
Ca1-O22
Ca1-O12
2.3359(16)
2.3449(16)
2.4209(18)
2.4333(17)
2.4346(16)
Ca1-O1
Ca1-O2
Ca1-O11
Ca1-Ca1iii
Ca1-Ca1ii
2.4355(18)
2.6516(18)
2.6636(17)
3.9889(10)
4.0086(10)
[Sr(H2O)3(4-npa)2]·4.5H2O (3)
Sr1-O1
Sr1-O2
Sr1-O5
Sr1-O6
Sr1-O1iv
2.710(3)
2.623(3)
2.773(12)
2.605(4)
2.495(3)
Sr1-O1v
Sr1-O6vi
Sr1-O2vi
Sr1-O1vi
Sr1-Sr1iv
2.495(3)
2.605(4)
2.623(3)
2.710(3)
4.1940(7)
Symmetry transformations used to generate equivalent atoms: i) −x, −y−1,
−z+1; ii) −x+1, y+1, −z+1; iii) −x+2, −y+1, −z+1, iv) −x+1, −y, −z;
v) x, −y, z+1/2; vi) −x+1, y, −z+1/2.

Bikshandarkoil R Srinivasan et al.
Table 3. Hydrogen bonding geometry [Å and ◦] for 1, 2, and 3.
D-H· · · A
d D-H)
d H· · · A)
d D· · · A)
<DHA
Symmetry code
[Mg(H2O)6] (4-npa)2·4H2O (1)
O3-H3B· · · O5
O1-H1A· · · O9
O1-H1B· · · O9
O9-H9A· · · O4
O3-H3A· · · O4
O8-H8A· · · O5
O8-H8B· · · O6
O2-H2D· · · O4
O2-H2C· · · O8
O9-H9B· · · O8
0.870(17)
0.827(16)
0.837(16)
0.869(17)
0.830(17)
0.861(17)
0.825(17)
0.857(16)
0.838(16)
0.857(17)
1.818(17)
1.975(16)
2.064(16)
1.922(18)
2.27(2)
2.683(2)
2.785(2)
2.890(3)
2.778(2)
2.986(2)
2.735(2)
2.955(18)
2.677(2)
2.707(2)
2.956(3)
173(3)
166(3)
168(3)
168(3)
144(3)
156(3)
155(3)
171(3)
172(3)
165(2)
x, −y−1/2, z+1/2
x, y, z
−x, −y−1/2, −z+1/2
−x, −y−1, −z
−x, y+1/2, −z+1/2
x, y, z
−x+1, −y, −z+1
x, −y−1/3, z+1/2
x, −y−1/2, z+1/2
x, −y−1/2, z+1/2
1.928(19)
2.186(18)
1.828(16)
1.873(17)
2.120(18)
[Ca(H2O)2(4-npa)2] (2)
O21-H10· · · O1
O21-H20· · · O14
O22-H30· · · O12
O22-H40· · · O4
0.840
0.840
0.840
0.840
1.892
2.259
1.895
2.234
2.720
3.096
2.730
3.070
168.60
174.84
172.16
173.48
−x+1, y+1, −z+1
−x+2, y−1/2, −z+1/2
−x+3, −y, −z+1
−x+1, −y, −z
[Sr(H2O)3(4-npa)2]·4.5H2O (3)
C7-H7· · · O3
0.93
2.63
3.359(7)
3.322(11)
2.719(5)
2.854(6)
135.7
−x+1/2, y−1/2, −z+1/2
−x+1, y−1, −z+1/2
x, −y, z−1/2
C5-H5A· · · O8
O6-H6A· · · O2
O6-H6B· · · O7
0.88(2)
0.902(19)
0.909(19)
2.54(8)
1.86(3)
1.97(2)
150(14)
159(7)
163(6)
−x+1/2, y−1/2, −z+1/2
D = Donor and A = Acceptor.
A pair of the first unique ligand (O1, O2) are linked
to a pair of Ca(II) ions with Ca· · · Caii separation of
3.989 Å (Figure S10) which results in the formation
of a tricyclic dicalcium-dicarboxylate unit. This is the
basic building block of the coordination polymer. This
tricyclic unit differs from the well-known cyclic eight
membered dimetallic-dicarboxylate unit in several dinu-
clear carboxylates where the (–COO)− functions as a
symmetrical μ2-bridging bidentate ligand.
A second independent pair of μ2-bridging triden-
tate 4-npa ligands (O11, O12) bind to two symmetry
related Ca(II) in an identical manner with Ca1-O11
and Ca1-O12 distances of 2.6636(17) and 2.6516(18) Å
respectively. The O11 oxygen is further linked to
a symmetry related Ca(II) center at a distance of
2.3359(16) Å, resulting in the formation of tricyclic
dicalcium-dicarboxylate with Ca· · · Cai separation of
4.009 Å (Figure S11 in Supplementary Information).
Thus the alternating pairs of two crystallographically
unique 4-npa ligands connect pairs of {Ca(H2O)2}2+
units into an extended chain with alternating Ca· · · Ca
separation of 3.989 and 4.009 Å. The formation of a 1-D
coordination polymer extending along a-axis can be ex-
plained as a result of the combined effect of the two
unique 4-npa ligands (Figure 4). The binding mode of
the carboxylate ligand, as well the terminal waters is very
similar to that observed in other Ca(II) 1-D polymers23
like [Ca(H2O)2(2-nba)2] (2-nba = 2-nitrobenzoate) and
Figure 4. The water dimer (O8, O9) is H-bonded to two
[Mg(H2O)6]2+ cations and three 4-npa anions (For symmetry
relation see Table 4).
from 50.83(5) to 155.19(6)◦ (Table S2) indicating
the distortion of the {CaO8} polyhedron. The first unique
μ2-bridging tridentate 4-npa ligand (O1, O2) which
exhibits a μ2-η2: η1 coordination mode binds to Ca(II)
in a bidentate fashion with Ca1-O1 and Ca1-O2 dis-
tances of 2.4355(18) and 2.6516(18) Å respectively.
The O2 oxygen is further linked to a symmetry related
Ca(II) center at a distance of 2.3449(16) Å.

Alkaline-earth metal salts of 4-nitrophenylacetic acid
Figure 5. The crystal structure of 2 showing the atom labelling scheme and the eight coordination
around Ca(II). Displacement ellipsoids are drawn at the 50% probability level excepting for the H
atoms, which are shown as spheres of arbitrary radii. Symmetry code: i) −x+1, y+1, −z+1; ii)
−x+2, y+1, −z+1 (top); A portion of the 1-D chain along a-axis due to the μ2-bridging tridentate
coordination modes of both the unique ligands (botttom).
[Ca(H2O)2(4-nba)2]·2dmp (2-nba = 2-nitrobenzoate; ligand are in the normal range (Table S1). The observed
dmp = dimethylpyrazole). A scrutiny of the crystal Sr-O bond distances (Table 2) vary from 2.495(3) to
structure reveals that the H atoms of the coordinated 2.773(12) and the O-Sr-O angles scatter in a wide range
water molecules O21 and O22 function as H-donors from 48.07(9) to 157.18(16) (Table S2). The hydrogen
and are linked to the O1, O4, O12 and O14 atoms of atoms attached to the lattice waters O7, O8 and O9
the unique 4-npa anions via O-H· · · O interactions could not be located. The central Sr(II) exhibits nine
(Table 3).
coordination (Figure 6) and is bonded to six oxygen
Compound 3 crystallizes in the centrosymmetric atoms of four different symmetry related 4-npa anions
monoclinic space group C2/c. Its crystal structure con- (as in 2) and three oxygen atoms of three terminal water
sists of a unique Sr(II) located on a twofold axis, two molecules, resulting in a distorted tricapped trigonal
independent coordinated water molecules (O5 and O6) prismatic {SrO9} coordination polyhedron (Figure S12
of which O5 is situated on a twofold axis, an unique in Supplementary Information).
4-npa anion and three lattice water molecules (O7, O8,
Despite the differing coordination number of the cen-
O9). The geometric parameters of the unique 4-npa tral metal in 2 and 3, it is interesting to note that the

Bikshandarkoil R Srinivasan et al.
Figure 6. The crystal structure of [Sr(H2O)3(4-npa)2]·4.5H2O 3 showing the atom labelling
scheme and the coordination around Sr(II). Displacement ellipsoids are drawn at 50% probability
level excepting for the H atoms, which are shown as circles of arbitrary radius. For clarity the lat-
tice water molecules are not shown. Symmetry code: i) −x+1, −y, −z; ii) x, −y, z+1/2; iii) −x+1,
y, −z+1/2 (top); A portion of the 1-D coordination polymer extending along c axis due to the μ2-
bridging tridentate coordination of the unique 4-npa ligand. For clarity the three coordinated water
molecules are not displayed (bottom).
Sr(II) salt of 4-npa 3 is structurally very similar to that (Figure 5). The presence of three monodentate water
of 2. The Sr(II) ions are linked into an infinite 1-D ligands coordinated to Sr(II) in 3 (instead of two around
chain by the unique μ2-bridging tridentate 4-npa ligand Ca in 2) can explain the nona coordination of Sr(II).
Table 4. Comparative structural features of the alkaline-earth salts of 4-nbaH and 4-npaH.
Compound
S. G.
C.N.
Function of 4-nba or 4-npa
Structure type
Ref
[Mg((H2O)6](4-nba)2·2H2O
[Ca(4-nba)2(H2O)4]
P¯ı
6
7
9
9
6
8
9
–
Free anion
Monomer
Monomer
Monomer
1-D Polymer
Monomer
1-D Polymer
1-D Polymer
–
9a
9b
9c
9d
P21/c
P21/c
P21/c
P21/c
P¯ı
C2/c
–
Monodentate, bidentate
Free anion, bidentate
Bidentate, μ2-bridging bidentate
Free anion
μ2-bridging tridentate
μ2-bridging tridentate
–
[Sr(4-nba)(H2O)7](4-nba)·2H2O
[Ba(4-nba)2(H2O)5]
[Mg(H2O)6](4-npa)2·4H2O
[Ca(4-npa)2(H2O)2]
This work
This work
This work
This work
[Sr(4-npa)2(H2O)3]·4.5H2O
[Ba(4-npa)2(H2O)]
S. G. = space group; C.N. = coordination number.

Alkaline-earth metal salts of 4-nitrophenylacetic acid
As in 2, a tricyclic distrontium-dicarboxylate unit is the Acknowledgements
basic building block of the 1-D coordination polymer.
The authors thank the Sophisticated Analytical Instru-
In view of the special position of Sr(II) in the crystal
structure and an unique 4-npa ligand (unlike two crys-
tallographically independent 4-npa ligands in 2) a sin-
gle Sr· · · Sr separation of 4.194(1) Å is observed in the
1-D chain due to the linking of pairs of {Sr(H2O)3}2+
units by pairs of the unique 4-npa ligand. In view of
the difficulty to locate the H-atoms of lattice water, a
discussion of H-bonding in 3 is not presented.
ment Facility (SAIF), Indian Institute of Technology
(IIT) Madras for single crystal X-ray analysis of 1 and
3 reported in this paper. Financial assistance to the
Department of Chemistry, Goa University at the level of
DSA-I under the Special Assistance Programme (SAP)
by the University Grants Commission, New Delhi is
gratefully acknowledged.
A comparison of the structures of 1 to 3 with the alka-
line-earth metal salts of 4-nitrobenzoic acid (4-nbaH)
reveals a rich structural chemistry in this group of
compounds all of which crystallize in centrosymmetric
space groups (Table 4). The Mg(II) salt of both acids
4-npaH and 4-nbaH contain the octahedral [Mg(H2O)6]2+
cation. The Ba(II) salt of 4-npaH 4 contains a single
water molecule unlike [Ba(4-nba)2(H2O)5]. The Ca(II)
and Sr(II) salts of 4-nbaH contain four and seven coor-
dinated water molecules unlike in 2 and 3, which can
also explain their monomeric structure. The present
work gives two new examples of structurally character-
ized compounds viz., 2 and 3 both of which are based
on a tricyclic dimetal-dicarboxylate unit.
References
1. (a) Banerjee D and Parise J B 2011 Cryst. Growth Des.
11 4704; (b) Fromm K M and Gueneau E D 2004 Poly-
hedron 23 1479; (c) Fromm K M 2008 Coord. Chem.
Rev. 252 856; (d) Batten S R, Neville S M and Turner
D R 2009 In Coordination Polymers: Design, Analysis
and Application (Cambridge UK: RSC Publishing); (e)
Eddaoudi M, Moler D B, Li H, Chen B, Reineke T M,
O’Keeffe M and Yaghi O M 2001 Acc. Chem. Res. 34
319; (f) Burgess K M N, Xu Y, Leclerc M C and Bryce
D L 2014 Inorg. Chem. 53 552
2. (a) Chanthapally A, Quah H S and Vittal J J 2014 Cryst.
Growth Des. 14 2605; (b) Raja D S, Luo J H, Yeh
C T, Jiang Y C, Hsu K F and Lin C H 2014 Crys-
tEngComm 16 1985; (c) Arlin J, Florence A J, Johnston
A, Kennedy A R, Miller G J and Patterson K 2011
Cryst. Growth Des. 11 1318; (d) Dinca˘ M and Long J R
2005 J. Am. Chem. Soc. 127 9376; (e) Mallick A, Saha
S, Pachfule P, Roy S and Banerjee R 2010 J. Mater.
Chem. 20 9073; (f) Murugavel R, Anantharaman G,
Krishnamurthy D, Sathiyendiran M and Walawalkar
M G 2000 J. Chem. Sci. 112 273
3. (a) Kang M, Liu T, Wang X, Luo D, Li R and Lin Z 2014
Inorg. Chem. Commun. 44 155; (b) Yang D-L, Zhang
X, Yang J-X, Yao Y-G and Zhang J 2014 Inorg. Chim.
Acta 423 62; (c) Lian C, Liu L, Guo X, Long Y, Jia S,
Li H and Yang L 2016 J. Solid State Chem. 233 229; (d)
Srinivasan B R, Shetgaonkar S Y, Dhavskar K T, Sundar
J K and Natarajan S 2012 Indian. J. Chem. 51A 564;
(e) Wang J-H, Tang G-M, Qin T-X, Yan S-C, Wang Y-T,
Cui Y-Z and Ng S W 2014 J. Solid State Chem. 219 55;
(f) Srinivasan B R, Shetgaonkar S Y and Raghavaiah P
2009 Polyhedron 28 2879
4. Conclusions
The synthesis, spectral characteristics and thermal
properties of the alkaline-earth metal salts of 4-
nitrophenylacetic acid are reported along with a
description of the structures of three of these salts. The
Mg(II) salt 1 is a monomer and is a molecular container
for a water dimer. In contrast, the Ca(II) and Sr(II) salts
are one-dimensional coordination polymers due to the
μ2-bridging tridentate nature of the 4-npa ligands in 2
and 3.
Supplementary Information (SI)
Crystallographic data (including structure factors) for
the structures of [Mg(H2O)6](4-npa)2·2H2O (1), [Ca
(4-npa)2(H2O)2] (2), and [Sr(4-npa)2(H2O)3]·4.5H2O(3)
reported herein have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary
publication no. CCDC 1482247 (1), CCDC 1482248
(2) and CCDC 1482249 (3). Copies of the data can be
obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1 EZ, UK. (fax: +44-
(0)1223-336033 or email: deposit@ccdc.cam.ac.uk).
Electronic supplementary information (for Figures S1–
S12 and Tables S1–S2) is available at www.ias.ac.in/
chemsci.
4. (a) Guan L and Wang Y 2015 J. Solid State Chem.
230 243; (b) Ghazzali M, Khair M, Al-Farhan K and
Reedijk J 2014 Inorg. Chim. Acta 409 503; (c) Tan Y-H,
Xu Q, Gu Z-F, Gao J-X, Wang B, Liu Y, Yang C-S and
Tang Y-Z 2016 J. Mol. Struc. 1119 346; (d) Tahmasian
A, Safarifard V, Morsali A and Joo S W 2014
Polyhedron 67 81; (e) Zhang D, Zhang R, Li J,
Qiao W and Wang S 2013 Inorg. Chem. Commun.
35 307; (f) Natarajan S, Sundar J K, Athimoolam
S and Srinivasan B R 2011 J. Coord. Chem. 64
2274
5. (a) Srinivasan B R, Shetgaonkar S Y and Raghavaiah P
2012 Indian. J. Chem. 51A 1064; (b) Srinivasan B R,
Shetgaonkar S Y and Raghavaiah P 2008 J. Chem. Sci.
120 249; (c) Srinivasan B R, Sawant J V and Raghavaiah

Bikshandarkoil R Srinivasan et al.
8. (a) Murugavel R and Banerjee S 2003 Inorg. Chem.
P 2007 J. Chem. Sci. 119 11; (d) Srinivasan B R,
Shetgaonkar S Y, Kohli S and Rajnikant 2010 Indian. J.
Chem. 49A 1468
Commun. 6 810; (b) Srinivasan B R, Shetgaonkar S Y
and Raghavaiah P 2009 Polyhedron 28 2879; (c) Srini-
vasan B R, Shetgaonkar S Y, Saxena M and Näther C
2012 Indian J. Chem. 51A 435; (d) Odabas¸og˘lu M and
Büyükgüngör O 2007 Acta Crystallogr. E63 m712; (e)
Yuan R X, Xiong R G, Chen Z F, You X Z, Peng S M
and Lee G H 2001 Inorg. Chem. Commun. 4 430; (f)
Senkovska I and Thewalt U 2005 Acta Crystallogr. C61
m448; (g) Srinivasan B R, Shetgaonkar S Y and Ghosh
N N 2011 J. Coord. Chem. 64 1113; (h) Yano S, Numata
M, Kato M, Motoo S and Nishimura T 2001 Acta Crys-
tallogr. E57 m488; (i) Natarajan S, Srinivasan B R,
Sundar J K, Ravikumar K, Krishnakumar R V and
Suresh J 2012 J. Chem. Sci. 124 781
6. (a) Murugavel R, Karambelkar V V and Anantharaman
G 2000 Indian J. Chem. A39 843; (b) Wiesbrock F,
Schier A and Schmidbaur H 2002 Z. Naturforsch. B57
251; (c) Dhavskar K T, Bhargao P H and Srinivasan B R
2016 J. Chem. Sci. 128 421; (d) Dale S H, Elsegood
M R J and Kainth S 2003 Acta Crystallogr. 59C m505;
(e) Srinivasan B R, Shetgaonkar S Y and Raghavaiah
P 2009 Indian J. Chem. 48A 1662; (f) Morgant G,
Angelo J, Desmaele D, Dichi E, Sghaier M, Fraisse B,
Retailleau P, Brumas V, Fiallo M M L and Tomas A 2008
Polyhedron 27 2097; (g) Indrani M, Ramasubramanian
R, Fronczek F R, Braga D, Vasanthacharya N Y and
Kumaresan S 2009 J. Chem. Sci. 121 413
7. (a) Murugavel R, Karambelkar V V, Anantharaman G
and Walawalkar M G 2000 Inorg. Chem. 39 1381;
(b) Murugavel R and Korah R 2007 Inorg. Chem. 46
11048; (c) Song W D, Guo X X and Zhang C H 2007
Acta Crystallogr. E63 m399; (d) Srinivasan B R and
Shetgaonkar S Y 2010 J. Coord. Chem. 63 3403; (e)
9. (a) Srinivasan B R, Sawant J V, Näther C and Bensch W
2007 J. Chem. Sci. 119 243; (b) Srinivasan B R, Sawant
J V and Raghavaiah P 2006 Indian J. Chem. 45A 2392;
(c) Srinivasan B R, Raghavaiah P and Sawant J V
2007 Acta Crystallogr. E63 m2251; (d) Srinivasan B R,
Sawant J V, Sawant S C and Raghavaiah P 2007 J. Chem.
Sci. 119 593
Srinivasan B R, Shetgaonkar S Y, Sawant J V and 10. (a) Sheng G H, Cheng X S, You Z L and Zhu H L 2015
Raghavaiah P 2008 Polyhedron 27 3299; (f) Srinivasan
B R, Shetgaonkar S Y and Näther C 2011 Z. Anorg.
Allg. Chem. 637 130; (g) Srinivasan B R, Shetgaonkar
S Y, Näther C and Bensch W 2009 Polyhedron 28 534;
(h) Murugavel R, Baheti K and Anantharaman G 2001
Inorg. Chem. 40 6870
Synth. React. Inorg. Metal org and Nano-Metal Chem.
45 1273; (b) Ali N, Tahir M N, Ali S, Iqbal M, Munawar
K S and Perveen S 2014 J. Coord. Chem. 67 1290; (c)
Sharutin V V, Sharutina O K and Senchurin V S 2014
Russian J. Inorg. Chem. 59 326
11. Sheldrick G M 2015 Acta Crystallogr. C71 3