Synthesis, crystal structure and Thermogravimetry of ortho-phthalic acid bridged coordination polymer of Copper(II)

Article abstract of DOI:10.1007/s12039-016-1089-x

Coordination polymer of Cu(II) bridged by o-phthalic acid alone is not known. The reaction of CuCl₂.2H₂O with (2-butoxycarbonyl)benzoic acid yielded three dimensional coordination polymer bridged by o-phthalic acid. X-ray crystal structure shows structure with monoclinic P2₁/c space group. o-Phthalic acid molecules act as bridge between two Cu(II), one carboxylate binds to one Cu(II) as bidentate while the other carboxylate binds to another Cu(II) as monodentate. The four planar co-ordination positions of Cu(II) are satisfied by two chelated carboxylates while fifth and sixth co-ordination positions are satisfied by monodentate carboxylates. EPR and TGA of the coordination polymer are also reported. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1089-x

J. Chem. Sci. ꢀc Indian Academy of Sciences.
DOI 10.1007/s12039-016-1089-x
Synthesis, crystal structure and Thermogravimetry of ortho-phthalic
acid bridged coordination polymer of Copper(II)
∗
BABITA SARMA, SAURAV BHARALI and DIGANTA KUMAR DAS
Department of Chemistry, Gauhati University, Guwahati 781 014, Assam, India
e-mail: digantakdas@gmail.com
MS received 21 April 2015; revised 31 March 2016; accepted 1 April 2016
Abstract. Coordination polymer of Cu(II) bridged by o-phthalic acid alone is not known. The reaction of
CuCl2.2H2O with (2-butoxycarbonyl)benzoic acid yielded three dimensional coordination polymer bridged by
o-phthalic acid. X-ray crystal structure shows structure with monoclinic P21/c space group. o-Phthalic acid
molecules act as bridge between two Cu(II), one carboxylate binds to one Cu(II) as bidentate while the other
carboxylate binds to another Cu(II) as monodentate. The four planar co-ordination positions of Cu(II) are
satisfied by two chelated carboxylates while fifth and sixth co-ordination positions are satisfied by monodentate
carboxylates. EPR and TGA of the coordination polymer are also reported.
Keywords. o-Phthalic acid; coordination polymer; X-ray crystal structure; Copper(II); EPR; TGA.
1
. Introduction
2. Experimental
Coordination polymers have multifunctional properties 2.1 Materials and methods
with tremendous potential for applications in many
1
–3
4
5,6
All the chemicals and solvents were purchased from
Merck. The organic solvents were distilled CuCl .2H O
fields like catalysis, separation, hydrogen storage,
7
8
magnetic devices, non-linear optics, etc. By selecting
appropriate metal ions and organic linkers, coordination
polymers of different structures such as 1D chains, lad-
2
2
was recrystallised from double distilled water before
use. FTIR and UV-Visible spectra were recorded on
Perkin Elmer RX1 FT-IR spectrophotometer and Shi-
madzu UV-1800 (with a 1.0 cm path length quartz cell)
9
–13
ders, 2D grids and 3D networks could be designed.
Polycarboxylic acids have been widely used as link-
ers, as they can coordinate by a number of different
modes, to synthesise coordination polymers of different
1
spectrophotometer, respectively. H NMR spectra were
recorded in Bruker ULTRASHILD 300 MHz instru-
ment. The EPR spectra were recorded in JEOL JES-
FA200. Electrochemical analysis was performed in CHI
1
4,15
structures.
o-Phthalic acid has been widely explored
16–33
as linker in synthesising coordination polymers.
600B Electrochemical Analyzer (USA) comprising of
This immense popularity of o-phthalic acid as linker in
coordination polymers is because of its ability to pro-
vide 26 different modes of coordination.34 In all these
reported coordination polymers, at least one other lig-
and is found to be present together with o-phthalic acid.
o-phthalic acid in the absence of other ligands resulted
in monomeric complexes when allowed to react directly
with Cu(II) ion at room temperature.35 There is no
report of coordination polymer based on o-phthalic acid
alone without incorporating other ligands.
We report herein for the first time, room temperature
synthesis of three dimensional coordination polymer
of Cu(II) with o-phthalic acid without incorporating
any other ligand. To prevent formation of mononuclear
complex, o-phthalic acid was not provided directly but
generated in situ from (2-butoxycarbonyl)benzoic acid.
three electrode cell assembly. Platinum disc was used
as working electrode, Ag-AgCl (3 M NaCl) as refer-
ence while a platinum wire as the counter electrode. The
electrodes were cleaned as per reported procedure.36
Single crystal X-ray diffraction data were recorded
employing Bruker Smart APEX II (3 circle X-ray
diffractometer). The intensity data for the complex were
collected using Mo-Kα radiation (0.71073 Å) at 293 K
on a Bruker SMART APEX and Bruker KAPPA APEX
37
II diffractometers, respectively. The SMART program
was used for collecting frames of data, indexing the
reflections, and determination of lattice parameters;
38
SAINT program for integration of the intensity of
39
reflections and scaling; SADABS program for empir-
ical absorption corrections. The structures were solved
by direct methods (SHELXS-97) and standard Fourier
∗
2
For correspondence
techniques, and refined on F using full matrix least

Babita Sarma et al.
squares procedures (SHELXL-97) using the SHELX-97 The peaks appearing in the region 1595–1611 cm 1
−
4
0
41
package incorporated in Win GX. All non-hydrogen can be attributed to νasym(COO) vibrations while the
−
1
atoms were refined anisotropically except for the two peaks in the range 1382–1396 cm can be attributed to
lattice water molecules which are in a highly disordered νsym(COO) vibrations.
state. Hydrogen atoms were assigned idealized posi-
3
[
.2 EPR spectrum of
tions and given thermal parameters equivalent to either
.5 (methyl hydrogen atoms) or 1.2 (all other hydro-
{Cu(O C-μ -C H -μ -CO ) }.2H O]
1
2
2
6
5
1
2 2
2
n
gen atoms) times the thermal parameter of the carbon
atoms to which they are attached. Hydrogen atoms of
the lattice water molecules were not found.
The EPR spectrum of the complex was recorded as
polycrystalline solid and has been shown in figure 1.
The giso value and geometric parameter G, i.e., the
measurement of exchange interaction between the cop-
per centers were evaluated by using the following
2.2 Synthesis of (2-butoxycarbonyl)benzoic acid (L)
1.48 g (10 mmol) phthalic anhydride and 0.91 mL
42,43
expression.
(
10 mmol) of 1-butanol were taken in 20 mL methanol
ꢀ
ꢁ
◦
giso = g + 2g /3
(1)
(2)
and stirred at 60 C for 10 min; 2–3 drops of pyri-
dine were added and stirring continued for another 2 h.
An off-white product was obtained which was recrys-
ꢁ
⊥
ꢀ
ꢁ ꢀ
ꢁ
G = g − 2.0023 / g − 2.0023
ꢁ
⊥
tallised from methanol. Yield: 0.304 g (85%); M.p. The calculated value of g tensor parameter were g =
ll
◦
1
1
1
78 C. FTIR (KBr, cm ): 3475 (ν ), 2962 (ν ), 2.63 and g = 2.47. Hence g > g > 2.0023 which
O−H
C−H
⊥
ll
⊥
44
1
725 (ν ), 1417(ν ), 1125 and 1293 (ν ). H reveals that d
is the ground state. The value of G
C=O
C=C
C−O
x2−y2
NMR (CDCl , 300 MHz, δ in ppm, TMS): 7.99-7.47 was calculated to be 1.34 which is less than 4 indicating
3
(
2
m, 4H), 4.17 (m, 2H), 1.67-1.58 (m, 2H), 1.42-1.30 (m, effective interaction between the copper centers.45
H), 7.2 (t, 3H). Anal.Calcd. for C H O : C 64.86%;
12
14
4
H 6.35%. Found: C 64.76%; H 6.48%.
5000
The synthetic strategy for the preparation of the
ligand (L) is summarized in scheme 1 below:
2.3 Synthesis of
[
{Cu(O C-μ -C H -μ -CO ) }.2H O]
2
2
6
5
1
2 2
2
n
CuCl .2H O (0.170 g, 1 mmol) and L (0.358 g, 1 mmol)
2
2
were stirred in 20 mL CH OH at room temperature for
3
0
2
h. Blue crystals were precipitated out from the reac-
tion mixture on standing for seven days. Recrystallisa-
tion was done from 1:10 (v/v) mixture of CH OH: H O.
3
2
Yield: 70%. MW 429.72.
3
. Results and Discussion
-
5000
3.1 FTIR spectrum of
2
000
3000
[
{Cu(O C-μ -C H -μ -CO ) }.2H O]
2 2 6 5 1 2 2 2
n
Magnetic Field (G)
The FT-IR spectrum of the complex showed char-
Figure 1. EPR spectrum of the coordination polymer in
acteristic peaks due to the ligation of carboxylate. polycrystalline state.
O
O
O
°
6
0 C
O
O
+
HO
OH
py, MeOH
O
(
L)
Scheme 1. Synthetic path for L.

o-phthalic acid bridged Cu(II) polymer
3
.3 Thermogravimetric analysis (TGA) of
of the coordination polymer and formation of CuO as
the final residue (calculated value of weight loss is
[
{Cu(O C-μ -C H -μ -CO ) }.2H O]
2
2
6
5
1
2 2
2
n
8
1.49%).
Figure 2 shows the TGA curve of the coordination
◦
polymer. The weight loss was obseved from 180 C
3.4 Magnetic moment measurement of
◦
to 400 C. The first weight loss is a well-defined step
[
{Cu(O C-μ -C H -μ -CO ) }.2H O]
◦
2 2 6 5 1 2 2 2
n
with initiation temperature 180 C due to loss of guest
water molecules while the second and the third weight
losses are combined ones with initiation temperatures
ca. 270 C and 340 C, respectively. The total weight
loss in the three step process is 81.95% which accounts
for the loss of phthalic acids from one monomeric unit
2+
To investigate the spin state of Cu in the complex,
the magnetic susceptibility measurement was done at
2
◦
◦
98 K from mass susceptibility measurement. The mass
susceptibility, χ , is calculated using the equation:
g
χg = [CBal × l × (R − R )] /109 × m
(3)
o
Where l = the sample length (cm), m = the sample
mass (g), R = the reading for the tube plus sample,
R = the empty tube reading, C = the balance cali-
0
.6
.5
.4
.3
0
0
Bal
bration constant. Then molar susceptibility, χ = χ ×
m
g
0
molecular mass of the complex. The effective magnetic
0
moment, μ , is then calculated using the expression:
eff
√
0
.2
.1
μeff = 2.83 (χ × 298). The experimental value of
m
magnetic moment was calculated to be 1.78 BM which
is almost equal to the spin only magnetic moment value
due to Cu ion, which is 1.732 BM. Hence, the Cu(II)
ions in the co-ordination polymer are magnetically
independent and should be well separated.
0
0
100
200
300
400
500
600
700
2+
o
Temperature ( C)
Figure 2. TGA curve of the coordination polymer.
Table 1. Crystal data and structure refinement for [{Cu(O2C-μ2-C6H5-μ1-CO2)2}.2H2O]n.
CCDC Number 903426
Empirical formula
Formula weight
:C16 H10 Cu O8.2H2O
:489.79
Temperature
:296(2) K
Wavelength
Crystal system, space group
Unit cell dimensions
:0.71073 Å
:Monoclinic, P21/c
:a = 11.0782(11) Å alpha = 90 deg.
b = 6.6521(7) Å beta = 101.798(4) deg.
c = 12.1645(12) Å gamma = 90 deg.
3
Volume
:877.50(15) Å
3
Z, Calculated density
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Limiting indices
Reflections collected / unique
Completeness to theta = 27.49
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
:2, 1.854 Mg/m
−
1
:1.328 mm
:494.0
:0.38 × 0.32 × 0.24 mm
:3.42 to 27.49 deg.
:−14 <= h <= 14, −7 <= k <= 8, −15 <= l <= 15
:8917 / 2004 [R(int) = 0.0275]
:94.7%
2
Full-matrix least-squares on F
:1899 / 0 / 147
:1.065
:R1 = 0.0545, wR2 = 0.1878
:R1 = 0.0634, wR2 = 0.2144
:0.023(6)
:0.000
:0.901 and −1.389 e.A
Extinction coefficient
Shift/esd max
Largest diff. peak and hole
−
3
2
o
2 2
c
2 2 1/2
2
2
o
2
2
c
wR2 = {ꢀ[w(F −F ) ]/ꢀ[w(F ) ]} where w = 1/[σ (F )+(aP) +bP] with P is [2F +
o
2
Max(F ,0)]/3
o

Babita Sarma et al.
other carboxylates (of these two phthalates) binds to
3.5 X-ray crystallography of
[
{Cu(O C-μ -C H -μ -CO ) }.2H O]
another Cu(II) through only one O atom while the other
O dangles as keto group. The other two trans coordi-
nation sites of the Cu(II) are fulfilled by carboxylate
O atoms of phthalate of which the remaining carboxy-
lates chelate to two Cu(II). In this way, a three dimen-
sional network is generated where each phthalate acts
as a bridge between two Cu(II), and coordinating to one
Cu(II) by the monodentate carboxylate oxygen and to
another Cu(II) by chelating carboxylate.
2
2
6
5
1
2 2
2
n
Discrete single dark blue crystals of the compound suit-
able for X-ray studies were obtained from the con-
centrated reaction mixture by allowing the resulting
solution to stand for seven days at room temperature.
Recrystallization of the complex was done from 1:10
(v/v) mixture of CH OH: H O. The complex crystal-
3 2
lizes in the monoclinic crystal system with P 2 /c space
1
group. The crystallographic data has been shown in
table 1. Selected bond lengths [Å] and bond angles [deg.]
for the compound have been shown in tables 1 and 2.
The unit cell structure of the complex is shown in
figure 3. The structure shows that the butoxy group from
the ligand is lost during the formation of the complex
and phthalic acid acts as a bridging ligand in the com-
plex. Each Cu(II) ion is coordinated with four phtha-
late ligands completing an octahedral structure with six
O atoms. Out of the four phthalate two coordinates
to the Cu(II) through both O atoms of one carboxy-
late as chelate. These two chelating carboxylates are
found to be in trans position to each other. Each of the
In the compound the two trans O-Cu(II)-O angles
created each by the two O atoms of a chelating carboxy-
late and the Cu(II) ion were found to be equal which is
◦
5
1
3.19 . The other two O-Cu(II)-O angles were of value
◦
26.81 . Interestingly the axial Cu(II)-O bonds were not
perfectly perpendicular to the plane of four equatorial
O atoms from two chelating carboxylates. Each of the
axial Cu(II)-O bonds were found to be tilted slightly
towards one of the two equatorial chelating carboxy-
late pairs. The O-Cu(II)-O angles involving one axial O
atom and the O atoms from one pair of chelating car-
◦
◦
boxylate are 92.67 and 90.99 while these angles with
◦
◦
the other pair of carboxylate are 89.01 and 92.67 .
The distance between the two Cu(II) bound O atoms
belonging to the same carboxylate is 2.216 Å while
the distance between the two O atoms (on the same
Table 2. Selected bond lengths [Å] & bond angles [deg.]
for [{Cu(O2C-μ2-C6H5-μ1-CO2)2}.2H2O]n.
Cu(1)-O(3)
Cu(1)-O(4)
O(3)-Cu(1)-O(4)
1.943(10) side of the coordination core) belonging to two differ-
1.973(11)
ent carboxylates is 4.22 Å. Hence, the equatorial O4
91.0(5)
core around the Cu(II) ion in the complex is rhombohe-
dral. The asymmetric unit of the complex is shown in
figure 4.
Symmetry transformations used to generate equivalent
atoms
Figure 3. Packing of Cu complex showing the coordina- Figure 4. Phthalic acid coordinating with Cu1 through
tion environment around Cu along (101) plane. carboxylate oxygen in monodentate fashion.

o-phthalic acid bridged Cu(II) polymer
The coordination of two carboxylic groups of a
phthalate to two different Cu(II) finally gives rise to
a polymeric structure. Figure 5 shows the two dimen-
sional structure of the polymeric system. In the three
dimensional structure shown in figure 6 specific holes
are clearly visualised. It also shows van der Waals inter-
action (π − π) between the phthalate aromatic rings.
The crystal packing arrangement of the compound is
stabilized by the intermolecular O-H· · · O interactions
(
∼2.158 Å) between solvent water hydrogen and car-
boxylate O atom.
Figure 7. Observed (above) and calculated (below) pow-
der X-ray diffraction of {-Cu(μ2-O2C-C6H5-CO2-μ1-)2-}.
2H2O.
3.6 Powder X-ray diffraction study
To prove the bulk purity of the complex synthesized
by us, powder X-ray diffraction data was also collected
for the complex. A close similarity observed between
observed and calculated X-ray diffraction patterns
prove the bulk purity of the complex (figure 7).
Figure 5. Two dimensional structure of the polymeric
Cu(II) complex.
4
. Conclusions
A novel three dimensional co-ordination polymer of
Cu(II) with o-phthalic acid was obtained on reaction
of CuCl and 4-(2-butoxycarbonyl)benzoic acid. X-ray
2
crystallographic studies have shown a 3D coordination
polymer with specific holes. TGA curve showed three
weight losses with final composition as CuO.
Supplementary Information (SI)
All additional information pertaining to characterization
of the cocrystals, namely, FT-IR spectrum (figure S1),
1
H NMR spectrum (figure S2) and CIF (pdf file) are
available at www.ias.ac.in/chemsci.
Acknowledgements
DST, New Delhi and UGC, New Delhi are thanked
for FIST and SAP to the Department, respectively.
B.S. thanks UGC for fellowship under RFSMS. SAIF,
G.U. and CIF, IIT Guwahati are thanked for X-ray
crystallographic data and EPR spectrum, respectively.
Figure 6. Three dimensional polymeric structure of the
Cu(II) complex.

Babita Sarma et al.
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