Synthesis and structural characterization of a barium coordination polymer based on a μ2-monoatomic bridging 4-nitrobenzoate

Article abstract of DOI:10.1080/00958972.2019.1666980

The synthesis, spectral characterization, crystal structure and properties of [Ba(H₂O)₂(NMF)₂(4-nba)₂], 1 (NMF = N-methylformamide; 4-nba = 4-nitrobenzoate), are reported. ¹H and ¹³C NMR sp

Full text of DOI:10.1080/00958972.2019.1666980

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Synthesis and structural characterization of a
barium coordination polymer based on a µ2-
monoatomic bridging 4-nitrobenzoate
Pooja H. Bhargao & Bikshandarkoil R. Srinivasan
To cite this article: Pooja H. Bhargao & Bikshandarkoil R. Srinivasan (2019): Synthesis and
structural characterization of a barium coordination polymer based on a µ2-monoatomic bridging 4-
nitrobenzoate, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2019.1666980
To link to this article: https://doi.org/10.1080/00958972.2019.1666980
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1666980
Synthesis and structural characterization of a barium
coordination polymer based on a l2-monoatomic
bridging 4-nitrobenzoate
Pooja H. Bhargao and Bikshandarkoil R. Srinivasan
School of Chemical Sciences, Goa University, Goa, India
ABSTRACT
ARTICLE HISTORY
Received 19 June 2019
Accepted 30 August 2019
The synthesis, spectral characterization, crystal structure and prop-
erties of [Ba(H2O)2(NMF)2(4-nba)2], 1 (NMF ¼ N-methylformamide;
1
4-nba ¼ 4-nitrobenzoate), are reported. H and 13C NMR spectral
KEYWORDS
data reveal the presence of NMF in 1. A strong band at
1660 cmꢀ1 in the infrared spectrum indicates the binding of
amide oxygen to Ba(II) which is confirmed by the single crystal
structure. The unique Ba(II) in 1 situated on a mirror plane exhib-
its nine coordination and is bonded to two symmetry related
monodentate terminal NMF ligands via the amide oxygen, and a
terminal aqua ligand. The l2-monoatomic bridging binding mode
of each of a crystallographically independent 4-nba ligand and a
unique water ligand link the Ba(II) cations into an infinite chain
extending along a, leading to the formation of a 1D coordination
polymer with BaꢁꢁꢁBa separations of 4.2522(3) Å. In the chain, the
fBaO9g polyhedra are linked in a face sharing fashion. Thermal
decomposition of 1 results in the formation of BaCO3 residue. A
comparative study of the structural chemistry of several barium
coordination polymers is described.
Barium;
N-methylformamide;
4-nitrobenzoate;
l2-monoatomic bridging;
coordination polymer
1. Introduction
The past decade has witnessed a rapid growth of publications on the chemistry of
alkaline-earth metal based materials [1–18]. Of the alkaline-earth metals the heavier
CONTACT Bikshandarkoil R. Srinivasan
srini@unigoa.ac.in
Dedicated to Prof. A.V. Salker on the occasion of his 63rd birthday.
Supplemental data for this article can be accessed here.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
P. H. BHARGAO AND B. R. SRINIVASAN
congeners starting from Ca are known to be structurally flexible and exhibit variable
coordination numbers while the lighter elements Be and Mg prefer four- and six-
coordination, respectively [1, 2]. Barium exhibits coordination numbers ranging from 7
to 12 with nine being found in many compounds [14–17]. It is well documented in
the literature that by using carboxylate ligands, the heavier alkaline-earths starting
from Ca can be linked via the bridging binding mode of the carboxylate ligand, into
1D or 2D or 3D coordination polymers [19–28]. The use of multitopic ligands viz. di- or
tricarboxylic acids leads to higher dimensionality (2D and 3D) [23, 30] while many
monocarboxylic acids afford a 1D chain polymer [24, 25]. Several examples of barium
coordination polymers based on carboxylate ligands have appeared in recent literature
[9–15].
A variety of synthetic methodologies [19] ranging from hydrothermal/solvothermal,
mechanochemical, sonochemical, gel method, diffusion method, etc. [30–39] have
been employed by several research groups for compound preparation. However, con-
ventional syntheses performed under ambient reaction conditions in aqueous medium
continue to be an often used method [20–29]. In earlier studies, we demonstrated
that an aqueous reaction of readily available reagents viz. substituted benzene carbox-
ylic acid with alkaline-earth metal carbonate is a convenient method for preparation
of coordination polymers of Ca, Sr, and Ba [22, 23, 40–43]. We have shown that in this
acid–base reaction, the insoluble nature of the starting metal carbonate reagent is
advantageous both to monitor the course of the reaction and also in product isola-
tion. In the case of 4-nitrobenzoic acid (4-nbaH), this reaction readily afforded a 1D
coordination polymer containing five terminal aqua ligands, viz. [Ba(H2O)5(4-nba)2]
[44], 1a, while zero-dimensional solids of formula [Mg(H2O)6](4-nba)2ꢁ2H2O [45],
[Ca(H2O)4(4-nba)2] [46], and [Sr(H2O)7(4-nba)](4-nba)ꢁ2H2O [47] were obtained for Mg,
Ca and Sr, respectively. In recent work we have shown that dehydration of the
zero-dimensional compound [Ca(H2O)4(4-nba)2] followed by reaction with an O-donor
ligand like N-methylformamide (NMF) results in formation of the anhydrous 1D poly-
meric compound [Ca(NMF)2(4-nba)2] [48]. Application of this strategy to the pentaaqua
barium compound 1a in an effort to enhance its dimensionality resulted in the forma-
tion of the water deficient compound [Ba(H2O)2(NMF)2(4-nba)2], 1, containing terminal
NMF ligands which exhibits a triple chain polymeric structure. The results of these
investigations are described in this paper.
2. Experimental
2.1. Materials and methods
All chemicals used in this study were of reagent grade and used as received without
any further purification. The precursor [Ba(H2O)5(4-nba)2] (1a) used for the synthesis of
1 was prepared by a literature method [44]. Infrared (IR) spectra of the solid samples
diluted with KBr were recorded on a Shimadzu (IR Prestige-21) FT-IR spectrometer
from 4000–400 cmꢀ1 at a resolution of 4 cmꢀ1. Raman spectra were recorded using
785 nm laser radiation for excitation on an Agiltron Peak Seeker Pro Raman instru-
ment. UV–Visible spectra of aqueous solutions were recorded using a Shimadzu UV-
2450 double beam spectrophotometer using matched quartz cells. The

JOURNAL OF COORDINATION CHEMISTRY
3
photoluminescence spectra were obtained on a xenon flash lamp technology based
Cary Eclipse Fluorescence Spectrophotometer (G9800A) from Agilent Technologies.
Isothermal weight loss studies were performed in a temperature controlled electric fur-
nace. Elemental analysis (C, H, and N) was performed on an Elemental Vario Micro
cube CHNS analyzer. TG-DTA study was performed in flowing air in Al2O3 crucibles at
a heating rate of 10 K minꢀ1 using a STA-409 PC simultaneous thermal analyzer from
1
Netzsch. H and 13C NMR spectra were recorded using a Bruker 400 MHz (Avance) FT-
NMR spectrometer.
2.2. Synthesis of [Ba(H2O)2(NMF)2(4-nba)2] (1)
A powdered sample of 1a (0.560 g, 1 mmol) was taken in a beaker and heated in a
temperature controlled oven to obtain the anhydrous compound [Ba(4-nba)2], 1 b, in
near quantitative yield. The product was cooled to room temperature and an excess
of NMF (5 mL) was added into this with stirring to obtain a clear solution which was
set aside for crystallization. The crystals of 1 were isolated and washed with ether and
air dried. Alternatively, a powdered sample of 1a (0.560 g, 1 mmol) was stirred well to
dissolve in NMF (5 mL), and the reaction mixture was set aside for crystallization at
room temperature to obtain 1 which was isolated as above.
A mixture of BaCO3 (0.1973 g, 1 mmol) and 4-nbaH (0.334 g, 2 mmol) in water
(25 mL) was heated on a water bath for ꢂ30 min to obtain a clear solution. The reac-
tion mixture was filtered and its pH was found to be neutral. The filtrate was concen-
trated to ꢂ10 mL and NMF (5 mL) was added to it. The reaction mixture was kept
undisturbed for crystallization. The crystalline product was isolated by filtration,
washed with ether and air dried (yield: 75%).
Anal. Calcd for C18H22N4O12Ba (1): C, 34.66; N, 8.98; H, 3.56. Found: C, 34.36; N, 9.17;
H, 3.35%. IR (KBr, cmꢀ1): 3500–3000(br), 3318(s), 3126(w), 3081(w), 3048(w), 2910(m),
1660(s), 1611(w) 1565(s), 1518(m), 1402(s), 1348(s), 1320(m), 1251(m), 1156(m),
1108(m), 1013(m), 985(m), 958(m), 883(m), 835(m), 794(s), 774(m), 719(m), 514(s).
Raman (cmꢀ1): 3924(m), 3084(w), 2903(w), 1592(s), 1522(w), 1403(m), 1341(s),
1250(w), 1102(m), 957(w), 861(m), 623(w). UV-Vis (in H2O): 274 nm.
1H NMR (D2O) d: 2.63 (s, 3 H), 7.86 (d, 2 H; J ¼ 8.8 Hz), 7.90 (s, 1 H), 8.15 (d,
2 H, J ¼ 8.8 Hz).
13C NMR (100 MHz, D2O) d: 24.27(C9), 123.34(C3, C7), 129.45(C4, C6), 142.46(C2),
148.74(C5), 164.74 (C8), 173.51(C1). DTA (ꢃC): 114 (endo), 190 (endo), 404 (exo), 549 (exo).
2.3. X-ray structure determination
Intensity data for 1 were collected at room temperature on a Bruker Axs Kappa Apex3
CMOS diffractometer with graphite monochromated Mo Ka radiation (k ¼ 0.71073 Å) at
the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology
(IIT) Madras. The data integration and reduction were carried out using SAINT-PLUS
software. The structures were solved by direct methods using SHELXT-2014 and refine-
ment was done against F2 using SHELXL 2014 [49]. All nonhydrogen atoms were
refined anisotropically. All hydrogens attached to the carbon atoms of the aromatic

4
P. H. BHARGAO AND B. R. SRINIVASAN
Table 1. Crystal data and selected refinement results for 1.
Empirical formula
C18H22 N4BaO12
Formula weight (g molꢀ1
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
)
623.73
296(2)
0.71073
Orthorhombic
Pnma
7.3486(3)
b (Å)
31.9323(13)
c (Å)
10.1572(4)
90.00
2383.47(17)
4
1.738
1.735
1240
b (ꢃ)
Volume (Å3)
Z
Dcalc (mg/m3)
Absorption coefficient (mm–1
F(000)
)
Crystal size (mm3)
0.15 ꢄ 0.10 ꢄ 0.05
h range for data collection (ꢃ)
Index ranges
3.422–25.00
–8 ꢅ h ꢅ 8 –37 ꢅ k ꢅ 37 –12 ꢅ l ꢅ 12
20066/2130 (R(int) ¼ 0.0547)
99.8%
Reflections collected/unique
Completeness to h
Absorption correction
Max. and min. transmission
Refinement method
Multi scan
0.7454 and 0.6374
Full-matrix least-squares on F2
2130/3/175
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
1.095
R1 ¼ 0.0314, wR2 ¼ 0.0671
R1 ¼ 0.0427, wR2 ¼ 0.0712
1.659 and –0.539
1908263
Largest diff. peak and hole (e Å–3
CCDC No.
)
ring of 4-nba and the methyl and amido carbonyl group of NMF ligands were intro-
duced in calculated positions and included in the refinement riding on their respective
parent C atoms. The technical details of data acquisition and selected crystal refine-
ment results are summarized in Table 1.
3. Results and discussion
3.1. Synthesis, spectral, and thermal investigations
Dehydration of [Ba(H2O)5(4-nba)2], 1a, by heating followed by reaction of the anhyd-
rous compound [Ba(4-nba)2], 1b, with N-methylformamide (NMF) was performed in
order to enhance its dimensionality and obtain crystals suitable for X-ray structure
determination (Scheme 1).
An excess amide was used to function both as solvent and ligand. The dehydration
reaction proceeded as expected, followed by NMF incorporation and crystals of 1 suit-
able for X-ray study were obtained (Figure S1). Our efforts to obtain a barium 4-nitro-
benzoate compound containing NMF by a direct method under nonaqueous
conditions have not been fruitful so far. Hence, it appears that loss or exchange of
two aqua ligands from 1a by NMF probably facilitates its structural reorganization
leading to the formation of 1.
1
An investigation of the crystalline product by H and 13C NMR revealed the pres-
ence of NMF in addition to 4-nitrobenzoate as evidenced by the characteristic 1H

JOURNAL OF COORDINATION CHEMISTRY
5
Scheme 1. Synthesis of 1 from 1a.
chemical shifts for the hydrogen attached to the amide carbon and methyl protons of
NMF (Figure S2). The 13C NMR spectrum (Figure S3) exhibits a total of seven signals,
five of which can be assigned for the distinct carbons of 4-nba. The signals at 164.7
and 24.3 ppm are characteristic of the amide carbon and the methyl carbon attached
to the electronegative N. An intense band at 1660 cmꢀ1 which is not present in the
precursor 1a can be assigned for the vibration the amide carbonyl (Figure 1). In add-
ition, the IR spectrum revealed the presence of water in 1 by a broad signal in the
–OH region, thus indicating that the final product still contains some water. Since the
free NMF ligand exhibits the carbonyl band at ꢂ1690 cmꢀ1 it could be inferred that
the amide oxygen is bonded to the metal as explained in the crystal structure
(vide infra).
Pyrolysis of the product revealed the chemical composition of Ba:4nba:water:NMF
to be 1:2:2:2, indicating that the above synthetic methodology affords a water defi-
cient and not an anhydrous compound. In view of the above, especially the binding
of amide oxygen to Ba, the synthesis was performed by a direct dissolution of 1a in
NMF which also afforded 1. Further, a one-pot synthesis was performed, wherein 1a
was generated in situ by an aqueous reaction of BaCO3 with 4-nbaH and further reac-
tion with NMF (Scheme 1) which also afforded the same product as evidenced by
identical IR spectra and powder pattern. A comparison of the pattern of the NMF con-
taining compound with that of the precursor compound 1a (Figure S4) reveals that a
new phase is formed, which is in good agreement with the pattern calculated from
the single crystal data (vide infra).
The IR spectrum of 1 exhibits several sharp bands in the mid-IR region, indicating
the presence of the organic moieties. The appearance of a feature at ꢂ3318 cmꢀ1 can
be attributed to the –NH stretch of the NMF. A comparison of the IR spectra of the
free ligand 4-nbaH, 1a and 1 reveal the absence of the –COOH signal in 1a and 1
which is observed at 1690 cmꢀ1 in the free acid (Figure S5). The differing nature of 1a
and 1 is revealed by a change in the spectra in the –OH region and the observation
of an intense amide carbonyl vibration at 1660 cmꢀ1 in 1. Both compounds exhibit an
intense signal at 1350 cmꢀ1 which can be assigned for the symmetric stretch of the
nitro group of 4-nba. This signal is the most intense band in the Raman spectra of
both compounds (Figure S6). Compound 1 absorbs in the UV region and the spectrum
is similar to that of 4-nbaH or 1a. The intense band at 274 nm in the UV-Vis spectrum
(Figure S7) can be assigned to the intramolecular charge transfer transition of the

6
P. H. BHARGAO AND B. R. SRINIVASAN
Figure 1. Infrared spectra of 1 and 1a in the region 2000–400cmꢀ1. Signal of amide carbonyl.
ꢆ
4-nitrobenzoate. The photoluminescence of 1 was investigated by exciting a pow-
dered sample of 1 with 280 nm radiation using an excitation slit width of 10 nm and
an emission slit width of 5 nm at a scan rate of 600 nm per minute. For comparison,
the same instrument settings were employed for the study of the free ligand 4-nbaH
and the precursor 1a (Figure S8). The fluorescence emission spectra of all the com-
pounds are similar with emission at a longer wavelength of 379 nm, which can be
ꢆ
assigned to an intraligand p ! p transition of the 4-nitrobenzoate ligand. Compared
to the free ligand, the fluorescent intensity is diminished in 1, which indicates that
binding of 4-nba to the closed shell Ba(II) does not bring about any major changes
except a reduction in the emission intensity. It is interesting to note that the fluores-
cence emission of the pentaaqua Ba(II) compound 1a is almost the same as the free
ligand but slightly more than that of 1 (Figure S8).
Isothermal weight loss analysis of 1 at 100 ꢃC showed a mass loss of 15.24% which
is much more than the value (5.72) expected for the loss of two moles of water and
less than that expected for the loss of 2 NMF (18.9). An IR spectral analysis of 1 heated
at 100 ꢃC reveals the loss of amide in addition to water as evidenced by a diminishing
of the intensity of the amide carbonyl signal (Figure S9). Hence this mass loss can be
assigned for the loss of two H2O molecules and also for a partial loss of NMF ligands.
It is observed that heating at 120 ꢃC results in a total disappearance of the amide car-
bonyl peak (Figure S9). Pyrolysis of the bulk compound performed in a temperature
controlled furnace at 800 ꢃC results in the formation of 31.55% of residual BaCO3,
which is in agreement with the theoretical value of 31.64%. The formation of carbon-
ate residue was inferred by a characteristic spot test for carbonate. The TG-DTA curves
add more credence to the thermal behavior of 1. The TG curve exhibits two endother-
mic events at 114 and 190 ꢃC, respectively (Figure S10), and the total mass loss of
ꢂ25% is in agreement with the loss of aqua and NMF ligands. The DTA curve displays
two exothermic events at 404 and 549 ꢃC, which can be due to the degradation of the
carboxylate moieties leading to decomposition of 1. In many alkaline-earth 4-nitroben-
zoates exothermic events have been observed above 400 ꢃC [44–47]. The observed

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 2. The crystal structure of [Ba(H2O)2(4-nba)2(NMF)2] (1) showing the atom labelling scheme
and the coordination sphere of Ba(II) in 1 (top). Displacement ellipsoids are drawn at the 30%
probability level except for the H atoms, which are shown as circles of arbitrary radii. The distorted
monocapped square antiprism fBaO9g coordination polyhedron in 1 (bottom). Symmetry code: (i)
x, ꢀy þ 1/2, z; (ii) x ꢀ 1/2, -y þ 1/2, ꢀz þ 1/2; (iii) x ꢀ 1/2, y, ꢀz þ 1/2; (iv) x þ 1/2, y,ꢀz ꢀ 1/2.
residual BaCO3 of 29.12% in the TG experiment is slightly less than the
expected value.
3.2. Description of crystal structure
Compound 1 crystallizes in the centrosymmetric orthorhombic space group Pnma
with Ba1, O6 and O7 located on a mirror plane and all other atoms situated in general
positions. In view of the special position of the central metal and the aqua ligands,
the asymmetric unit of 1 having molecular formula [Ba(H2O)2(NMF)2(4-nba)2] consists
of a unique Ba(II), a crystallographically independent 4-nba ligand, a unique terminal
NMF ligand, and two crystallographically unique aqua ligands O6 and O7, respectively
(Figure S11). The geometric parameters of the NMF and 4-nba ligands are in the nor-
mal range (Table S1).
The central barium exhibits nine coordination (Figure 2) and is bonded to two sym-
metry generated terminal NMF ligands via the amide oxygen O5, a terminal aqua lig-
and O6, and four symmetry related 4-nba ligands via O3 and to two symmetry related
water molecules O7 resulting in a distorted monocapped square antiprismatic fBaO9g

8
P. H. BHARGAO AND B. R. SRINIVASAN
Table 2. Selected bond lengths and angles of [Ba(H2O)2(NMF)2(4-nba)2] (1).
Bond lengths (Å)
Ba1-O5
Ba1-O7
Ba1-O3
Ba1-O6
Ba1-O7iv
2.749(3)
2.768(5)
2.803(3)
2.923(5)
2.953(4)
Ba1-O5i
Ba1-O3i
Ba1-O3ii
Ba1-O3iii
BaꢁꢁꢁBai
2.749(3)
2.803(3)
2.782(3)
2.782(3)
4.2522(3)
Bond angles (ꢃ)
O5-Ba1-O5i
O5-Ba1-O7
O5i-Ba1-O7
O5-Ba1-O3ii
O5i-Ba1-O3ii
O7-Ba1-O3ii
O5-Ba1-O3iii
O5i-Ba1-O3iii
O7-Ba1-O3iii
O3ii-Ba1-O3iii
O5-Ba1-O3i
O5i-Ba1-O3i
O7-Ba1-O3i
O3ii-Ba1-O3i
O3iii-Ba1-O3i
O5-Ba1-O3
O5i-Ba1-O3
O7-Ba1-O3
98.39(14)
70.95(10)
70.95(10)
136.67(9)
79.11(9)
67.41(9)
79.11(9)
136.67(9)
67.41(9)
74.50(11)
135.38(9)
78.59(9)
143.06(5)
87.03(6)
132.50(4)
78.59(9)
135.38(9)
143.07(5)
O3ii-Ba1-O3
O3iii-Ba1-O3
O3i-Ba1-O3
O5-Ba1-O6
O5i-Ba1-O6
O7-Ba1-O6
O3ii-Ba1-O6
O3iii-Ba1-O6
O3i-Ba1-O6
O3-Ba1-O6
O5-Ba1-O7iv
O5i-Ba1-O7iv
O7-Ba1-O7iv
O3ii-Ba1-O7iv
O3iii-Ba1-O7iv
O3i-Ba1-O7iv
O3-Ba1-O7iv
O6-Ba1-O7iv
132.50(4)
87.03(6)
73.87(11)
66.55(9)
66.55(9)
112.52(13)
142.29(6)
142.29(6)
71.79(9)
71.79(9)
130.74(7)
130.74(7)
122.89(12)
67.86(9)
67.86(9)
64.65(9)
64.65(9)
124.59(13)
Symmetry transformations used to generate equivalent atoms: (i) x, –y þ 1/2, z; (ii) x–1/2, –y þ 1/2, –z þ 1/2; (iii)
x–1/2, y, –z þ 1/2; (iv) x þ 1/2, y, –z þ 1/2.
polyhedron (Figure 2). The Ba-O bond distances vary between 2.749(3) and 2.953(4) Å
in 1 (Table 2) while the O-Ba-O bond angles range from 64.65(9) to 142.29(6)ꢃ. These
values are in agreement with reported data for other barium coordination polymers
[44, 50].
It is interesting to note that the amide oxygen O5 of NMF makes the shortest bond (Ba-
O5¼ 2.749(3) Å) which is shorter than the Ba-O bond length of the terminal water O6.
Unlike O6, the second unique water O7 functions as a l2-bridging bidentate ligand (Figure
S12) and is bonded to a Ba at 2.768(5) Å; O7 makes another contact with a second Ba(II) at
a longer distance of 2.953(4) Å accompanied by a BaꢁꢁꢁBa separation of 4.2522(3) Å. As a
result of this bridging binding mode of O7, the Ba(II) ions are linked into an infinite chain
extending along the a axis (Figure 3). Each Ba(II) in the chain is bonded to three terminal
ligands, two of which are O5 of NMF and the third is O6 of water (Figure S13). The unique
4-nba ligand functions as a monoatomic bridging m2-g2 ligand (Figure S12) and is bonded
to two symmetry related Ba(II) ions at Ba-O distances of 2.782(3) and 2.803(3) Å, respect-
ively, via the O3 of the carboxylate functionality.
In the crystal structure, a pair of such monoatomic bridging 4-nba ligands link a
pair of Ba(II) ions and extend the structure along a (Figure 4) resulting in a double
chain. The net result of the binding of the terminal NMF ligand (O5), terminal aqua lig-
and (O6) and the bridging 4-nba ligand (O3) and bridging water (O7) is the linking of
Ba(II) ions into a triple chain coordination polymer (Figure S14). Each Ba(II) in the 1-D
chain carries three terminal ligands and exhibits a separation of 4.2522(3) Å with the
adjacent Ba(II) ions on either side.
An analysis of the crystal structure of 1 reveals that H6A and H7A are attached to
the oxygens of the terminal and bridging water, respectively, H1A bonded to N1 of

JOURNAL OF COORDINATION CHEMISTRY
9

10
P. H. BHARGAO AND B. R. SRINIVASAN

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 3. A portion of an infinite chain extending along the a axis due to the l2-bridging bidentate
(O7) aqua ligand in 1. For clarity, only the terminal aqua ligands around each Ba(II) are shown.
NMF, the H4 on C4 of 4-nba and H8 linked to C8 of NMF function as H-donors. The
O1 and O4 of 4-nba and O6 of terminal water are H acceptors resulting in three vari-
eties of H-bonding interactions, O-HꢁꢁꢁO, N-HꢁꢁꢁO and C-HꢁꢁꢁO, respectively (Table S2),
which can be classified as intrachain or interchain interactions. The C4-H4ꢁꢁꢁO1 inter-
action linking the hydrogen of a 4-nba ligand of one chain with the nitro O1 of an
adjacent chain is an interchain interaction and serves to link the parallel polymeric
chains (Figure S15).
A comparison of the crystal structure of 1 with that of the precursor [Ba(H2O)5(4-
nba)2], 1a, reveals similarities and quite a few differences. In both the compounds
which are 1D, Ba(II) is nine coordinate. Unlike the five terminal aqua ligands, a biden-
tate 4-nba and a symmetrically bridging bidentate 4-nba in 1a, the title compound 1
contains two types of terminal ligands, NMF and water. There are two unique bridging
bidentate ligands, water (O7) and 4-nba (O3), both of which extend the structure
along the same direction resulting in a triple chain. The 4-nba ligand exhibits monoa-
tomic bridging in 1 unlike the symmetrical bridging (m2-g1:g1) mode of 4-nba in 1a
which can account for the differing BaꢁꢁꢁBa separations of 4.2522(3) and 6.750(1) Å,
respectively, in these compounds. In the 1-D chain, the fBaO9g polyhedra in 1a are
discretely linked via the C7 of the symmetrical bridging 4-nba ligand which joins the
adjacent polyhedra (Figure 5). In contrast, a face shared linking arrangement of the
fBaO9g polyhedra is observed in 1 due to the presence of three common sites (O3,
O3 and O7) for each Ba in the chain which carries three terminal ligands, two NMF
(O5 and O5i) and a water (O6). Although the Ba ions in both 1 and 1a exhibit a dis-
torted monocapped square antiprismatic geometry, the presence of five terminal aqua
ligands and a bidentate 4-nba in 1a instead of three terminal ligands in 1 can explain
the differing polyhedral linking in the chains of 1a and 1.

12
P. H. BHARGAO AND B. R. SRINIVASAN
Figure 4. A portion of an infinite chain extending along the a axis due to the unique l2-g2-biden-
tate 4-nba ligand in 1. For clarity, only O5 of the terminal NMF ligands around each Ba(II) are
shown. The bridging and the terminal aqua ligands are not displayed (top). A view of the triple
chain polymer showing only the O atoms of the bridging ligands (O3 of 4-nba and O7 of water)
and the terminal NMF ligands O5. O6 is terminal water (bottom).
3.3. Comparative study of barium coordination polymers
The availability of several Ba(II) coordination polymers based on carboxylate linkers
[9–11, 14, 15, 21, 23, 34, 40, 44, 50–81] permits a comaparative study of these com-
pounds, the details of which are given below. For this study a total of fifty eight Ba(II)
carboxylates (Table 3) containing examples of 1-, 2- and 3D coordination polymers of
Ba(II) are examined. A majority of the 2- and 3D compounds except entry nos. 23–27
and 52–58 in Table 3 are prepared by hydrothermal or solvothermal methods. All the
1D compounds are prepared under ambient conditions. All the 1D compounds and
many 2D compounds contain coordinated water molecules whose number varies from
two to five while many three-dimesional compounds have less coordinated water per
Ba or no water as has been observed in sixteen of the thirty one entries. Although
some 2D and 3D compounds have been prepared under ambient conditions, synthe-
ses performed under hydrothermal conditions result in less hydration as compared to

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 5. A portion of the 1D chain showing discrete versus face sharing fBaO9g polyhedra in
[Ba(H2O)5(4-nba)2] 1a (top) and [Ba(H2O)2(NMF)2(4-nba)2] 1 (bottom). In 1a, the O1 and O2 are
attached to C7 of the symmetrical bridging 4-nba. In 1, O5, O5i and O6 are the terminal ligands
and the O3, O3 and O7 face is shared between adjacent Ba(II) ions in the chain. Symmetry code:
(i) x, –y þ 1/2, z.
a non-hydrothermal synthesis as can be evidenced by the Ba(II) compounds of ben-
zene-1,3,5-tricarboxylic acid (entry nos. 36, 41 and 42) which are prepared hydrother-
mally. In contrast the Ba(II) framework (entry no. 58) from the same linker prepared
under ambient conditions contains eight moles of water for three Ba(II) ions. In all
these compounds the oxophilic Ba(II) is bonded to O-donor ligands resulting in
fBaOxg (x ¼ 7 to 12) polyhedra. However, three instances (entry nos. 12, 25 and 45)
are observed where the Ba(II) is bonded to a single N in addition to oxygen and one
example where Ba is bonded to two nitrogens (entry no. 21). In these compounds the
reported Ba-N distances are 2.911(5), 3.047(4), 2.906(7) and 2.9662(16) Å, respectively,
and are longer compared to the Ba-O distances. Based on the reported Ba-O distances
which range from 2.625(16) (entry no. 31) to 3.189(7) Å (entry no. 36) in these com-
pounds, it can be inferred that Ba-O bond lengths scatter in the range 2.6 to 3.2 Å. In
view of a wide range of Ba-O bond lengths and O-Ba-O bond angles, in general the
fBaOxg poyhedra are distorted from regular geometries.
Out of the fifty eight entries in Table 3, twenty compounds contain more than one
unique Ba in the crystal structure and thus a total of ninety six Ba(II) sites are
observed. A survey of the coordination numbers of barium reveals the structural flexi-
bility of Ba(II) to exhibit coordination numbers ranging from seven to twelve. The pre-
ferred coordination number is nine which is observed in forty seven Ba(II) sites
including the title compound. The coordination number eight is observed in twenty
nine Ba(II) centers while more than a dozen examples of fBaO10g polyhedra are

14
P. H. BHARGAO AND B. R. SRINIVASAN
observed. A majority (forty-six out of fifty-eight) of the compounds in Table 3 crystal-
lize in centrosymmetric space groups.
The BaꢁꢁꢁBa separations in these compounds are determined by a combination of
many factors, which include the electronic and steric requirements of the central
metal, the denticity, flexibility, bridging behavior and H-bonding characteristics of the
ligand. Although the 2D and 3D compounds can in principle have more than one sep-
aration, only the shortest BaꢁꢁꢁBa separation is listed in Table 3. The BaꢁꢁꢁBa separations
vary from 3.8798(8) Å in the 2D coordination polymer (entry no. 11) derived from 4,40-
sulfobis(benzoic acid) to 7.626(19) Å in the 2D Ba(II) polymer (entry no. 22) derived
from 3-[(3-carboxy-phenoxy)phthalic acid]. The BaꢁꢁꢁBa separation of 3.8798(8) Å (entry
no. 11) is shorter than the sum of the van der Waals radii (4.28 Å), indicating weak
metal–metal interactions. Several more, including the title compound, belong to this
category. The BaꢁꢁꢁBa separations range between 4.0877(7) to 4.937(7) Å in all the 3D
polymers while these values span a broader range between 3.8798(8) to 7.626(19) Å in
both 1D and 2D compounds.
A comparison of the BaꢁꢁꢁBa distances in 1 and 1a where the same 4-nba ligand
bridges the metal centers illustrates that when one carboxylate oxygen binds to a
minimum of two Ba(II) ions (monoatomic bridging) as in 1, the BaꢁꢁꢁBa distances are
shorter, which can be explained due to the geometrical constraints. When the carb-
oxylate ligand exhibits symmetrical bridging (m2-g1:g1) binding mode, the separations
tend to be longer. In the 3D polymers and also in many other compounds it is noted
that the carboxylate group exhibits multidentate (tri or tetradentate) binding wherein
at least one carboxylate oxygen is linked to a minimum of two metal ions, which can
explain the shorter separations. This is demonstrated by the nonanuclear compound
[Ba9(CH3COO)14(ClO4)4] wherein the acetates exhibit three different bridging binding
modes viz. m5-g3:g3, m4-g2:g2 and m3-g2:g2, resulting in the binding of a minimum of
two metals to each carboxylate oxygen. The observed BaꢁꢁꢁBa separation is 4.27 Å.
Longer separations between central metals can occur in a multicarboxylate ligand
wherein each carboxylate exhibits monodentate binding as observed in the 2D com-
pound [Ba(H2O)5(H2PMA)] (H4PMA is pyromellitic acid). The dianion of pyromellitic acid
functions as a tetradentate bridging ligand (m4-g1:g1:g1:g1) binding to four Ba(II) ions
(Figure S16); each Ba(II) is bonded to five terminal aqua ligands. This results in the lon-
ger BaꢁꢁꢁBa separation of 6.65 Å and each (H2PMA) ligand is linked to four fBaO9g poly-
hedra in this compound leading to a discrete linking of the fBaO9g polyhedra. A
similar argument holds for the long BaꢁꢁꢁBa separation of 6.765(3) Å in the 2D com-
pound [Ba(H2IDC)2(H2O)4] ꢁ2 H2O (entry no. 21) where each –COO moiety binds to a
single Ba ion. In the 3D barium terephthalate compound each carboxylate oxygen
functions as a monoatomic bridge and thus exhibits m7-octadentate binding mode
(Figure S17). In this anhydrous compound, the BaꢁꢁꢁBa separation is very short
at 4.123 Å.
4. Conclusion
In this article, we have shown that coordination polymers can be synthesized under
ambient conditions by a proper choice of reagents. The 1D barium coordination

JOURNAL OF COORDINATION CHEMISTRY
15
polymer described in this report is a new addition to the growing list of structurally
characterized alkaline-earth metal coordination polymers based on 4-nitrobenzoic acid
linker, containing terminal amide ligands. The shorter metal-amide oxygen distances in
1 indicate that amides can be considered as useful ancillary ligands for the synthesis
of new alkaline-earth coordination polymers. A comparative study of 58 barium coord-
ination polymers reveals a rich and variable structural chemistry of Ba(II) based on
bridging carboxylate ligands.
Acknowledgments
The authors thank the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of
Technology (IIT) Madras for single crystal X-ray analysis of 1 reported in this paper. The authors
thank one of the reviewers for useful suggestions.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
BRS thanks the University Grants Commission (UGC), New Delhi for financial support to the
School of Chemical Sciences (formerly Department of Chemistry), at the level of DSA-I under the
Special Assistance Program (SAP) vide letter F.No. 540/14/DSA-I/2015/(SAP-I). PHB is a recipient
of the BSR Fellowship (Supernumerary fellowship for “Single Girl Child”) by UGC, New Delhi vide
letter F.No.25-1/2014-15(BSR)/7-69/2007(BSR).
ORCID
Bikshandarkoil R. Srinivasan
http://orcid.org/0000-0002-6598-6726
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