A new set of Cd(II)-coordination polymers with mixed ligands of dicarboxylate and pyridyl substituted diaminotriazine: Selective sorption towards CO2 and cationic dyes

Article abstract of DOI:10.1039/c7dt01657c

On the basis of a mixed ligand system of L(NH₂)₂ (6-(pyridin-4-yl)-1,3,5-triazine-2,4-diamine) and dicarboxylic acids, three new Cd(ii) coordination polymers viz. {[Cd_0.5(tdc)_0.5(L(NH₂)₂)_0.5(H₂O)]·DMF·H₂O}_n (1), {[Cd_0.5(bdc)_0.5(L(NH₂)₂)(H₂O)]·DMF·H₂O}_n (2), and {[Cd(ipa)(L(NH₂)₂)(DMF)]·H₂O}_n (3) (tdcH₂ = thiophene-2,5-dicarboxylic acid, bdcH₂ = benzene-1,4-dicarboxylic acid, ipaH₂ = benzene-1,3-dicarboxylic acid) were synthesized under diverse reaction conditions and characterized by single crystal X-ray diffraction, PXRD, elemental analysis, IR spectroscopy and TGA. While 1 and 2 revealed 1D chain structures, 3 acquired a 2D square net structural arrangement. Gas adsorption measurements of the desolvated framework 3 showed a moderate uptake of CO₂ under ambient conditions with good selectivity over N₂ and CH₄. The solid state luminescence properties were studied for all three coordination polymers. Moreover, a dye adsorption study on 3 exhibited selective adsorption towards a cationic dye.

Full text of DOI:10.1039/c7dt01657c

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DOI: 10.1039/C7DT01657C
A new set of Cd(II)˗coordination polymers with mixed ligand of dicarboxylate and
pyridyl substituted diaminotriazine: selective sorption towards CO2 and cationic dye†
Santanu Chand,‡ Syed Meheboob Elahi,‡ Arun Pal, Madhab C. Das*
Department of Chemistry, Indian Institute of Technology Kharagpur, 721302, India,
ABSTRACT: On the basis of the mixed ligand system of L(NH2)2 (6-(pyridin-4-yl)-1,3,5-
triazine-2,4-diamine) and dicarboxylic acids, three new Cd(II) coordination polymers viz.
{[Cd0.5(tdc)0.5(L(NH2)2)0.5(H2O)]·DMF·H2O}n(1),{[Cd0.5(bdc)0.5(L(NH2)2)(H2O)]·DMF·H2O
}n,(2) and {[Cd(ipa)(L(NH2)2)(DMF)]·H2O}n, (3) (tdcH2 = thiophene-2,5-dicarboxylic acid,
bdcH2 = benzene-1,4-dicarboxylic acid, ipaH2 = benzene-1,3-dicarboxylic acid) were
synthesized under diverse reaction conditions and characterized by single crystal x-ray
diffraction, PXRD, elemental analysis, IR spectroscopy and TGA. While 1 and 2 revealed 1D
chain structure, 3 acquired a 2D square net structural arrangement. Gas adsorption
measurements of the desolvated framework 3 showed moderate uptake of CO2 at ambient
conditions with good selectivity over N2 and CH4. Solid state luminescence property was
studied for all three coordination polymers. Moreover, dye adsorption study on 3 exhibited
selective adsorption towards a cationic dye.

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DOI: 10.1039/C7DT01657C
Introduction
Coordination polymers which consists of metal ions linked by organic bridging
ligands, have attracted great attention due to not only their topological features but also many
potential applications such as adsorption,1 magnetic,2 electrical,3 catalyst,4
photoluminescence5 and sensing6 and so on. The roles of the organic linkers and the spacers
are crucial to engineer the pore surfaces7 by varying in lengths which can also possibly be
considered as kinetic gate of sorption phenomena. The rational select and design of the
framework components is expected to allow the affinity of the internal pore surface toward
CO2 which can facilitate its selective capture over the other gases like CH4 and N2.8-9 The
potential towards selective CO2 adsorption as well as storage is exemplified by the use of
suitably designed organic ligand and/or spacer with Lewis basic sites for the construction of
porous complexes. It was established that upon introduction of amide10 and amine11
functionality the performance of the coordination polymers toward selective CO2 and
hydrocarbon absorption can also be improved.
Many strategies has been developed for the establishment of the synthesis of mixed
ligand complexes using various polycarboxylate and N,N′-donor ligand to manipulate the
overall structures with required functionality.12 An enormous number of complexes from 1D
to 3D frameworks has already been synthesized by using linear rod-like N,N′-donor ligands
as spacers such as, 4,4′-bipyridine; pyrazine; 4,4′-azobis(pyridine) in combination with
various polycarboxylate ligands as linkers. Here, we have chosen a pyridyl substituted
diaminotriazine denoted as L(NH2)2, containing two free –NH2 groups in the triazine core to
construct both 1D and 2D networks by varying the reaction conditions. The spacer L(NH2)2
has flattened topology that can engage in multiple strong directional interactions in a single
plane thereby co-crystallization with other organic ligands predictably favors the formation of
chains, tapes, and layers structures.13 Individual molecular topology and interactions play a
direction role to the formation of networks of different dimensions, though in practice,
accurate predictions of the dimensionality of structure remain difficult.14 Thus, we anticipated
that the spacer L(NH2)2 while immobilized into the framework of a coordination polymer
may have the ability to participate in hydrogen-bonding, π···π interactions and other non-
covalent interactions with the guest molecules for specific applications such as selective CO2
sorption and selective dye uptake property. Previous reports15 of metal-L(NH2)2 complexes
suggested that only pyridyl nitrogen atoms of L(NH2)2 are preferable sites for coordination to
form discrete complexes. To the best of our knowledge, there is no report on coordination

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polymer constructed by L(NH2)2 as a spacer with metal ions or clusters. Moreover, it can act
DOI: 10.1039/C7DT01657C
as a rigid organic linker by using both pyridyl and the terminal triazine nitrogen atoms for
coordination comparable to complexes made from 4,4ꞌ-bipyridine and its extended analogues.
To this end, we report the synthesis and structures of three Cd(II) coordination
polymers (1-3) under solvothermal conditions as well as room temperature crystallization
forming different 1D or 2D networks with multiple H-bonding and or non-covalent
interactions among the constituents. Gas adsorption measurements of the desolvated complex
3 showed promising selectivity of CO2 over N2 and CH4, at 273 K and 298 K. Dye adsorption
study for 3 showed the selectivity of a cationic dye over neutral and anionic dye. Solid state
photoluminescence properties of all the complexes were also investigated.
Experimental Section
Materials. All reagents and solvents were used as received from commercial
suppliers without further purification. The ligand 6-(pyridin-4-yl)-1,3,5-triazine-2,4-diamine
L(NH2)2 was prepared according to a modified literature procedure.15a
Scheme 1. Spacer and linkers used to construct coordination polymers in this study.
Physical Measurements. The FT-IR spectra were recorded from KBr pellets in the
range of 400–4000 cm−1 on a Perkin-Elmer RX1 spectrophotometer. Elemental analyses (C,
H, and N) were carried outusing a Perkin-Elmer instrument, series II Model 2400
elementalanalyzer. PXRD patterns were recorded with Cu Kα radiation (1.5418 Å) on a
Bruker D8 Advance diffractometer. Thermal analyses were performed using a Netzsch
TG209 F3 Tarsus thermal analyzer from room temperature at a heating rate of 5 °C min−1

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under nitrogen atmosphere. The luminescence spectra for the powdered solid samples were
DOI: 10.1039/C7DT01657C
measured at room temperature using a Horiba Fluorolog spectrophotometer. The UV−Vis
spectra for the dye solutions were measured at room temperature using a Shimadzu UV-2600
1
UV-Vis spectrophotometer. The excitation slit as well as the emission slit were 2.5nm. H
and 13C-NMR spectra were recorded using a Bruker Avance II 400 spectrometer. Mass
(MALDI-TOF) spectrum was recorded using a Bruker MALDI-TOF/TOF mass spectrometer.
Low pressure gas adsorption measurements were performed using a static volumetric system
(Micromeritics 3 Flex) at different temperature.
X-ray Crystallography. The crystal and refinement data for 1-3 are collected in
Table 1. In each case, a crystal of suitable size was selected from the mother liquor and
immersed in paratone oil, and then it was mounted on the tip of a glass fiber and cemented
using epoxy resin. Single crystal X-ray data were collected on a Bruker SMART APEX II
CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71069 Å). The
linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion
corrections were taken from International Tables for X-ray Crystallography. The data
integration and reduction were processed with SAINT16 software. The structure was solved
by the direct method using SHELXTL17 and was refined on F2 by full-matrix least-squares
technique using the SHELXL-9718 program package. For all the cases non-hydrogen atoms
were refined anisotropically. The hydrogen atoms of water molecules and amine groups were
located from difference Fourier maps. All other H atoms were placed in calculated positions
using idealized geometries (riding model) and assigned fixed isotropic displacement
parameters. Attempts to identify the solvent molecules failed in complex 1. Instead, a new set
of F2 (hkl) values with the contribution from the solvent molecules withdrawn was obtained
by the SQUEEZE procedure implemented in PLATON.19 All the bond lengths (Å) and bond
angles (°) are listed in supporting information (Table S1, ESI).
Preparation of {[Cd0.5(tdc)0.5(L(NH2)2)0.5(H2O)]·DMF·H2O}n(1). A mixture of
Cd(NO3)2.4H2O (0.1 mmol, 0.031gm), H2tdc (0.1 mmol, 0.017 g), and L(NH2)2 (0.1mmol,
0.019 g) was dissolved in DMF (5 ml) and stirred for 1h. The solution was left to stand at
room temperature. Colorless crystals were obtained after about one month in 61% (0.031 g)
yield. Elemental analysis for C14H14CdN6O6S, calcd: C, 33.15%; H, 2.76%; N, 16.57%.
Found: C, 32.89%; H, 2.92%; N, 16.51%. FT-IR (KBr pellet, cm−1): 3441(b), 3061.3(w),
2920.9(w), 1664.5(m), 1584.4(s), 1527.6(m), 1411.2(s), 1304.6(w), 1213.9(s), 1155.4(s),
1087.3(m), 873.6(s), 691.4(s), 657.6(m), 522.6(w).

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DOI: 10.1039/C7DT01657C
Table 1. Crystal Data and structure refinements for 1-3
Compounds
Empirical formula
Formula weight
Temperature (K)
Radiation
1
2
3
C14 H14CdN6O6S
506.77
C30H42CdN14O10
871.17
C38 H38 Cd2 N14 O11
1091.62
293(2)
298(2)
298(2)
Mo-Kα
Mo-Kα
Mo-Kα
Wavelength(λ)
Crystal system
Space group
a[Å]
0.71069 Å
Monoclinic
P2/c
0.71069 Å
Monoclinic
P21/c
0.71069 Å
Triclinic
Pī
9.900(5)
15.969(5)
7.269(5)
11.2961(7)
23.8537(15)
7.1245(5)
10.2875(18)
11.4771(18)
11.7608(17)
b[Å]
c[Å]
α[°]
β[°]
90.00
106.527(5)
90.00
104.469(2)
110.226(9)
94.949(9)
γ[°]
Volume[Å3]
90.00
1101.7(10)
2
90.00
1858.8(2)
2
109.795(9)
1193.0(3)
1
Z
Density (calculated)
[Mg/m3]
1.528
1.556
1.519
Absorption
1.125
504
0.662
0.960
coefficient [mm-1]
F(000)
896
548
Refl. used [I > 2σ(I)] 2731
2808
3207
4171
5827
Independent
reflections
Rint
3269
0.0678
0.0385
0.0765
Refinement method
full-matrix least
full-matrix
squares on F2
1.062
least full-matrix least
squares on F2
squares on F2
GOF
1.062
1.083
Final
R1=0.0294
R1=0.0294
R1=0.0630
Rindices[I>2σ(I)]
wR2=0.0749
wR2=0.0347
wR2=0.1737
Preparation of {[Cd0.5(bdc)0.5(L(NH2)2)(H2O)]·DMF·H2O}n(2). It was prepared as
described for 1, except that H2tdc was replaced with H2bdc (0.1 mmol, 0.017 g). Colorless
crystals of 2 were obtained after 25 days yield: 59% (0.052 g) based on metal. Elemental
analysis for C30 H42CdN14O10, calcd: C, 41.32%; H, 4.82%; N, 22.49%. Found: C, 41.18%;

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H, 4.97%; N, 22.25%. FT-IR (KBr pellet, cm−1): 3493.3(b), 2930.7(w), D1O6I: 6105.1.013(9m/C7)D,T01657C
1569.2(m), 1502.7(s), 1392(s), 1107.9(s), 1008.4(m), 843.6(s), 746.5(s), 662.4(m), 524.8(s).
Preparation of {[Cd(ipa)(L(NH2)2)(DMF)]·H2O}n(3). A mixture of Cd(NO3)2·4H2O
(0.1 mmol, 0.031gm), H2ipa (0.017 g, 0.10 mmol), and L(NH2)2 (0.019 g, 0.10 mmol) in 5
mL of DMF was sealed in a 10 mL teflon-lined stainless steel autoclave, which was then
heated under autogenous pressure to 110 °C for 2 days. Slow cooling of the reaction system
afforded colorless crystals suitable for single crystal X-ray diffraction. Yield: 0.029 g (53%).
Elemental analysis for C38 H38 Cd2 N14 O11, calcd: C, 41.69%; H, 3.65%; N, 17.9%. Found:
C, 41.51%; H, 3.69%; N, 17.75%. IR (cm−1): 3413.3(b), 3059.2(w), 2928.3(w), 1668.6(m),
1629.8(s), 1599(s), 1533(s), 1391(s), 1229.7(m), 1146.9(m), 1094.2(w), 1007(m), 859.3(s),
783.5(s), 692.2(s), 636(m).
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Results and discussion
All the complexes were synthesized either by solvothermal condition or by room temperature
slow evaporation process. The formula of all the complexes is confirmed by single-crystal X-
ray diffraction, elemental microanalysis studies and thermal gravimetric analysis (TGA). The
phase purity of the bulk sample was confirmed by powder X-ray diffraction (PXRD). All the
complexes are stable in air and once formed is insoluble in most of the common organic
solvents. High yields of the products indicate that they are thermodynamically stable under
the established reaction conditions. Each exhibits multiple strong absorption bands in the
region 1350–1550 cm-1 corresponding to asymmetric and symmetric stretching vibrations of
coordinated carboxylate groups. The absence of characteristic bands at around 1700 cm-1
rules out the presence of protonated carboxylate groups in these complexes, all of which are
consistent with the result based on single-crystal X-ray diffraction analysis.
Crystal structure of {[Cd0.5(tdc)0.5(L(NH2)2)0.5(H2O)]·DMF·H2O}n(1). Complex 1
crystallizes in the monoclinic crystal system with the space group P2/c. Single crystal X-ray
structure determination reveals that Cd(II) ions are linked together by the tdc2- ligands that
adopt the μ2-κ2,κ2 coordination mode20 to form 1D chains (Scheme S2a, ESI). The
asymmetric unit of 1 consists of a half of Cd(II) ion, half of tdc2-ligand and half of L(NH2)2
spacer. The coordination environment around the Cd(II) ion is shown in Figure 1a depicting
distorted pentagonal bipyramidal geometry. The terminal N atom of the triazine core of
spacer L(NH2)2 remains free. The neighboring pair of 1D chains hold the L(NH2)2 spacers in

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opposite directions which are in π···π stacking [3.641(1) Å] interactions with other spacers
DOI: 10.1039/C7DT01657C
from another pair of 1D chain as shown in Figure 1b. The coordinated water molecule is
simultaneously H-bonded with a metal bound carboxylate O atom of one chain and the
terminal N atom of the of the triazine core of spacer L(NH2)2 from another chain (Figure 1c).
The free ˗NH2 group interacting with the remaining metal bound carboxylate O atom (Figure
1c) along with one C–H···O interaction stabilize the overall structure. All the interactions are
listed in Table S2, ESI.
(a)
(b)
(c)
Figure 1. (a) Coordination environment around Cd(II) in (a) complex 1 (b) π···π stacking of
L(NH2)2 spacers along the 1D chain (symmetry operation (a): -x, y, 1.5-z). (c) Representation
of H-bonding interactions in complex 1 (symmetry operation (a): -x, -y, 0.5+z). H atoms are
omitted for clarity.

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Crystal structure of {[Cd0.5(bdc)0.5(L(NH2)2)(H2O)]·DMF·H2O}n (2). DCOoI: m10.p10l3Ve9iex/wC7A2DrtiTcl0e1O6n5l7inCe
crystallizes in the monoclinic crystal system with space group P21/c. Single crystal X-ray
structure determination reveals that Cd(II) ions are linked together by the btc2- ligands that
adopt the μ2-κ1,κ1 coordination mode20 forming 1D chain (Scheme S2b, ESI) structure. The
asymmetric unit of 2 consists of a half of Cd(II) ion, half of bdc2- ligand and one full L(NH2)2
with one lattice DMF and one H2O molecule. Figure 2a represents the coordination
environment of Cd(II) center adopting a distorted octahedral geometry. Unlike 1, the Cd-bdc
chains in 2 are flanked with L(NH2)2 ligands in both directions leaving the terminal N atom
of the triazine core unbound to the metal center. The flanked ligands from two nearby Cd-bdc
chains are in weak π···π stacking interactions (centroid-to-centroid distance of 3.779(1) Å
and 3.445 (1) Å; (Figure 2b). The 1D framework of 2 is also supported by several other O–
H···O and C–H···O weak interactions (Table S3, ESI). Interestingly, a planar tetrameric
hydrogen bonded loop is formed through the H-bonding interactions among the coordinated
H2O, lattice H2O, one free –NH2 group and one uncoordinated carboxylate O atom as
depicted in Figure 2c, (Ow1···O2 =2.663(1) Å, <Ow1−H2w1···O2 = 170.08(1)°; Ow2···O2
=2.937(1) Å, <Ow2−H2w2···O2 = 158.97(2)°; N5···Ow2 = 2.943(2) Å, <N5−H5B···Ow2 =
154.53(1)°; N5···Ow1 = 2.924(1) Å, <N5−H5B···Ow1 = 154.52(1)°.) The coordinated H2O
and the lattice H2O molecules are further hydrogen bonded with terminal N atom of the
triazine unit and O atom of the lattice DMF molecule respectively forming two dangling H-
bonds from the opposite corner of the loop. This O atom of the DMF molecules makes
another H-bonding interaction with the remaining free –NH2 group of the triazine core, thus
completing a discrete H-bonded network among the guest molecules with the host framework
(Figure 2c). All non-bonding and H-bonding interactions in complex 2 are listed in Table S3,
ESI.

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DOI: 10.1039/C7DT01657C
(a)
(b)
(c)
Figure 2. (a) Coordination environment around Cd(II) in (a) complex 2 (b) π···π stacking of
L(NH2)2 spacers along the 1D chain (symmetry operation (a): 1-x, -y, 1-z). (c) Representation
of H-bonding interactions in complex 2. H atoms are omitted for clarity.

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DOI: 10.1039/C7DT01657C
Crystal structure of {[Cd(ipa)(L(NH2)2)(DMF)]·H2O}n (3). Complex 3 crystallizes in the
triclinic crystal system with the space group Pī. The coordination environments of Cd(II)
ions is shown in Figure 3a. In this structure, Cd(II) center is surrounded by two ipa2- ligands,
two L(NH2)2 units (one through pyridyl nitrogen and another with terminal triazine nitrogen)
and one solvent DMF molecule coordinated through O atom, thus depicting a distorted
octahedral geometry (Figure 3a). In contrast to 1 and 2, both the pyridyl and the terminal N
atom of triazine core are bound to the metal center in 3. Single crystal X-ray structure
determination reveals that Cd(II) ions are linked together by the ipa2- ligands that adopt the
μ2-κ1,κ2 coordination mode20 to form 1D chains (Scheme S2c, ESI). Furthermore, these
dicarboxylic linked 1D chains are bridged by L(NH2)2 units from both the ends to form a 2D
network (with 4,4 net) structure (Figure 3b). The 2D nets are stacked over each other in ABC
fashion21 (Figure 3c) leading to decrease in pore size to one-sixth of (cross section 2.1×3.1Å2)
the expected pore size (with cross sectional value 10.28×11.76 Å2) as depicted in Figure 3d.
The packing index of 51.5% (percent filled space) for 3 as calculated by PLATON19 also
supports the above fact whereas the potential solvent accessible volume is calculated to be
34.5% of the unit cell volume (411.3Å3 per unit cell volume of 1193.0 Å3).
Notably, among the two free –NH2 groups of the spacer L(NH2)2, one is H-bonded
with chelating carboxylate O atom of ipa2- ligand and the metal bound carboxylate O atom of
another ipa2- ligand. The remaining free –NH2 group is H-bonded to the O atom of the DMF
molecule and one of the N atom of the triazine unit. All these H-bonding interactions along
with three other C−H···O/N interactions stabilizes the overall network (Figure 3e). The H-
bonding as well as the non-bonding interactions are listed in ESI (Table S4).
It is worth mentioning that in contrast to the first two structures, where the spacer
L(NH2)2 adopts almost planar orientation with minimum dihedral angle between the planes of
pyridyl ring and the triazine ring (3.55° for 1, 3.32° for 2), these planes are severely tilted
with a dihedral angle of 33.34° in 3. This difference in orientation of the L(NH2)2 spacer
might help the coordination of both the N ends of L(NH2)2 to the Cd(II) center giving rise to
such a net structure.

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DOI: 10.1039/C7DT01657C
(a)
(b)
(c)
(d)
(e)
Figure 3. (a) Coordination environment around Cd(II) in complex 3, (b) One 2D net of 3
where L(NH2)2 acts as a spacer bridging the metal-carboxylate chains through coordination of
both the pyridyl nitrogen and terminal triazine nitrogen atom, (c) ABC stacking of 4,4
network, different color represents different 2D nets, (d) A representation showing the
available space in complex 3. (e) Representation of H-bonding interaction in complex 3.
(symmetry operation (a): 2-x, 1-y, -z). H atoms are omitted for clarity.

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Gas sorption properties. The 2D structural framework with potenDtiOaIl: 10s.1o03lv9/eCn7DtT01657C
accessible volume of 34.5% of 3 prompted us to check its gas sorption property. The guest
solvent molecules were exchanged with methanol for 2 days. The solvent free activated
framework 3a was obtained by evacuating at ambient temperature for 12h, followed by
evacuation at 100 °C for 24h under ultra high vacuum. N2 adsorption at 77K revealed only
surface adsorption which is a common phenomenon for many dynamic 2D and 3D MOFs.22
However, CO2 sorption study on 3a showed a type I adsorption behavior with moderate
uptake capacity (Figure 4) at 273 and 298 K, confirming its microporous nature. It is well-
known that CO2 has the ability to act as a Lewis acid and prefers in weak interactions
between the electron-deficient carbons and electron-rich sites of O or N from the
functionalized frameworks.10-11 Thus, the adsorption towards CO2 might be attributed due to
the presence of electron rich free –NH2 functionality within the framework. This uptake
capacity value is comparable to a diaminotriazine functionalized 3D porous DAT-MOF-1
with high density of –NH2 sites into the host framework23a and other porous MOFs.23b-c,24
In order to explore the potential on gas separation properties under ambient
conditions, the sorption isotherms of CH4 and N2 were also measured at 273 K and 298 K. As
depicted in Figure 4, 3a takes up systematically more CO2 than N2 and CH4. At 273 K and 1
atm pressure, complex 3 uptakes 24.3 cm3g−1 CO2, 11.4 cm3g−1 CH4 and 1.3 cm3g−1 N2, as
shown in Figure 4, whereas at 298 K and 1 atm pressure, these values are 19.8 cm3g−1, 4.5
cm3g−1and 1.1 cm3g−1 respectively. Although the CO2 uptake amount is low compared to the
many well known porous MOFs, we were interested to check its adsorption selectivity over
other gases which were calculated from the single-component isotherm data.25 For CO2
adsorption, this value reports the ratio of the adsorbed amount of CO2 at 0.15 bar to the
adsorbed amount of other gases at 0.75 bar (conditions relevant to CO2 capture from flue
gas).25 While the selectivity towards CO2 uptake over N2 are found to be 41.3 and 27.8; the
selectivity over CH4 are 6.2 and 9.3 at 273 K and 298 K respectively which are comparable to
some of the reported MOFs24 and SOFs.26 Especially, the selectivity value of CO2/CH4
separation (6.2) at 273 K is comparable to the high values reported for other microporous
MOF materials including Tb(BCB)23b (ca.4.1), La(BTB)27a (ca. 6.4), and InOF-1427b (ca. 3.1).
The CO2 sorption selectivity may be attributed to the smaller kinetic diameters of CO2
compared to those of N2 and CH4 (CO2, 3.30 ̊A; N2, 3.64 ̊A; CH4, 3.80 Å); thus enabling CO2
molecules to diffuse easily into the pores. Besides, the selective sorption of CO2 rather than
N2 can also be attributed to the significant quadrupole–quadrupole interactions.10-11 The
several O-donors, pyridyl, triazine core and free amine groups of L(NH2)2 on the pore walls

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can interact effectively with quadrupole molecules, which may induce strong DiOnIt:e1r0a.1c03t9io/Cn7sDT01657C
between the framework and CO2 molecules because of its higher quadrupole moment
compared to N2.28
Figure 4. The CO2, CH4 and N2 gas sorption isotherms on 3a at 273 K (left) and 298 K
(right). Filled and open shapes indicate adsorption and desorption data points respectively.
Dye adsorption studies. Removal of dyes from industrial waste water has received
much attention from an environmental point of view.29 Complexes with nano-sized pores
have been applied in the field of selective adsorption and separation of dyes.30 Compared
with other traditional techniques, such as chemical degradation and membrane filtration
technologies,31 the method based on ion exchange adopting by ionic complexes possesses
incomparable advantages in user-friendly control and low energy-cost.
To this end, the dye adsorption performance of 3 has been studied using one
representatives of each of the three types of dye: cationic (crystal violet, CV; Figure 5),
anionic (methyl orange, MO; Figure S4, ESI) and neutral (Methyl red, MR; Figure S4, ESI).
A freshly prepared 20 mg of 3 was immersed in methanol solutions of each dye at room
temperature with known concentrations (4.1g/L) in a vial. CV solution started a significant
decrease in the color intensity of the solution in addition of 3, indicating a quick adsorption of
CV by 3. This observation was exclusive for CV, as no obvious color change was observed
for other dyes such as Methyl Orange (MO), Methyl Red (MR) (Chart S1, ESI). Figure 5a
shows photographic images of the solution of CV (4.1g/L solution) in the presence of 3 at
various time intervals. Figure S6 (ESI) shows optical microscopy images of 3 and dye-loaded
3. The microcrystals of 3 appeared transparent and block shaped within 200 µm in length as
prepared and turned to deep violet after adsorbing CV keeping the morphology intact during

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the dye uptake process. Dye adsorption ability of 3 was determined by UV-Vis sDpOeIc: t1r0o.10s3c9o/Cp7yDT01657C
at different time intervals as shown in Figure 5b. UV-Vis spectra suggest that the
concentration of CV gradually decreased with time and it was observed that within first 30
min, 3 adsorbed 46% of CV with an adsorption capacity of 221 mg/g (241 g/mole of the
monomer of 3). To check the dye capture behavior, kinetic study was performed with sample
concentration of 4.1g/L at 298K indicating fast adsorption kinetics of CV on surface of 3
(Figure S7, ESI). On the other hand, concentrations of MR and MO remained almost same
even after prolonged exposure.
(a)
(b)
Figure 5. (a) Photographic images of a solution of CV in the presence of complex 3. (b)
UV−Vis spectra of dye adsorption study of CV in methanol solution of 3 at various time
intervals. The inset shows the color change of bulk sample 3 before and after dye adsorption
study.

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Therefore, coordination polymer 3 has an ability to adsorb the cationic dye CV over anionic
DOI: 10.1039/C7DT01657C
and neutral dye. It should be noted that pore size of 3 is not sufficient enough to
accommodate bulky dye molecules like CV in the inner surface of its pores. However, we
presume that this may be the case of surface adsorption of CV onto 3.32 The possible reason
for the selective adsorption of cationic dye is still unclear, however it might be attributed to
several chemical interactions such as hydrogen bonding interaction, π···π stacking interaction
and interactions with the free basic amine sites within the host framework.33 Moreover, 3
retained its crystallinity during the dye uptake process as confirmed by the PXRD analysis
(Figure S8, ESI). It is already established that cationic framework prefers to adsorb anionic
dye and the vice versa. However, reports on preferential adsorption of cationic dye on the
neutral frameworks remain scarce.34
Photoluminescence properties. Luminescent compounds are currently of great
interest because of their various applications in chemical sensors, photochemistry and
electroluminescence display. Many studies have shown that coordination polymers with zinc,
cadmium ions exhibit photoluminescence properties.5,35 To this end, luminescence properties
of complexes 1-3 as well as free L(NH2)2, H2tdc, H2ipa and H2bdc were studied in the solid
state at room temperature under the same experimental conditions. The emission peaks of
L(NH2)2, H2tdc, H2ipa and H2bdc were observed at 464 nm, 460 nm, 458 nm and 496 nm
upon excitation at 280 nm for all the ligands (Figure S10, ESI). These emissions are due to
intra-ligand n→ π* or π → π* charge transitions of L(NH2)2, H2tdc, H2bdc and H2ipa.35a
complex 1 exhibits emission at 454 nm upon excitation at 285 nm which might be caused by
the mixture characteristic of intra-ligand charge transitions of L(NH2)2 and H2tdc.35c The
emission band of complex 2 appears in 457 nm upon excitation at 280 nm which is similar to
that of free L(NH2)2, which can be assigned to the intra-ligand charge transition of L(NH2)2.
Complex 3 displays emission peak at 446 nm upon excitation at 280 nm which may be
assigned to the mixture characteristic of intra-ligand charge transitions of L(NH2)2 and H2ipa.
Compared to the free L(NH2)2 spacer, the emission band of 1, 2 and 3 is 10 nm, 7 nm and 18
nm blue-shifted respectively as shown in Figure 6.35b

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DOI: 10.1039/C7DT01657C
Figure 6. Solid-state photoluminescent spectra of coordination polymers 1- 3.
Thermal stability and powder X-ray diffraction. To estimate the thermal stability
of the complexes, TGA were performed for all three complexes in N2 atmosphere with
heating rate 5°C/min. For 1, the weight loss 21.5% occurs at 260°C which corresponds to the
loss of uncoordinated DMF, H2O and coordinated H2O molecules (calculated value 22.0%)
beyond which the framework starts to degrade (Figure S11, ESI). For 2, the weight loss
12.5% occurs at 260°C, which corresponds to the loss of uncoordinated DMF and H2O
molecules (calculated value 12.9%) followed by degradation of the framework (Figure S11,
ESI). Similarly, the TGA curve of 3 shows a 19.8% weight loss at 32−170°C which can be
attributed to the loss of both lattice water molecule and the coordinated DMF solvent
molecule as well (calculated 20.3%, Figure S11, ESI) and is stable until 250oC followed by
degradation of the framework.
In order to confirm the phase purity of these coordination polymers, the PXRD
patterns were recorded for 1-3, and they were in good agreement with the corresponding
simulated patterns generated from single crystal diffraction data, indicating pure phase of
each bulk sample (Figure S12 for 1, Figure S13 for 2, Figure S14 for 3, ESI). The shifting of
few peaks in 1 and 2 might be attributed to the flexibility of the framework which is heavily
dependent on the content of guest molecules. The different intensities between the simulated
and experimental patterns may be caused by the favored orientation of the powder samples.
Moreover, the PXRD pattern of the guest free activated sample 3 was compared with the
experimental pattern which revealed perfect agreement between the two (Figure S14, ESI)
indicating the retention of crystalline phase after the gas sorption study.

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DOI: 10.1039/C7DT01657C
Conclusions
In summary, a successful attempt was made to construct coordination polymers of
different dimensionality self assembled by suitable coordination of Cd(II) ion with pyridyl
substituted diaminotriazine spacer and three different dicarboxylate organic linkers. All the
complexes are synthesized in pure phase during their bulk production and are moderately
thermally stable. The activated sample of 3 showed moderate selectivity for CO2/N2 and
CO2/CH4 separation. The luminescent nature of the complexes is possibly caused by the
mixture characteristic of intra-ligand charge transitions of mixed ligands. Moreover, this
complex can efficiently and selectively adsorb a cationic dye. Given the presence of two free
basic –NH2 groups of this spacer, the present work will be helpful for providing valuable
information for the rational design and synthesis of diaminotriazine-based 3D porous
complexes for enhanced CO2 capture, separation and molecular recognition through
directional H-bonding and non-covalent interactions.
Acknowledgments
S.C. thanks IIT Kharagpur for research fellowship. S.M.E. thanks the DST-SERB, New
Delhi, India for national post doctoral fellowship. M.C.D. gratefully acknowledges the
financial support received from SERB, New Delhi as Early Carrier Research Award
(ECR/2015/000041).
1
†Electronic supplementary information (ESI) available: H and 13C NMR spectra of the
spacer, UV-Vis spectra, PXRD patterns, solid-state photo luminescent spectra of the used
ligands, TGA plot, selected bond lengths and bond angles, non-covalent interaction,
hydrogen bonding parameter table and X-ray crystallographic files (CIF) for 1–3. CCDC
1540574–1540576. For ESI and crystallographic data in CIF or other electronic format see
DOI:
‡S.C. and S.M.E. contributed equally to this work.

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DOI: 10.1039/C7DT01657C
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DOI: 10.1039/C7DT01657C
Graphical Abstract
A new set of Cd(II)˗coordination polymers with mixed ligand of dicarboxylate and
pyridyl substituted diaminotriazine: selective sorption towards CO2 and cationic dye†
Santanu Chand,‡ Syed Meheboob Elahi,‡ Arun Pal, Madhab C. Das*
Department of Chemistry, Indian Institute of Technology Kharagpur, 721302, India.
Three Cd(II)-CPs have been synthesized with three organic dicarboxylates as linkers and a
pyridyl substituted diaminotriazine as a spacer. CP3 showed its potential for CO2/N2 and
CO2/CH4 separation selectivity and its ability for selective adsorption to a cationic dye.
………………………………………………………………………………………………..