Structural variation of transition metal coordination polymers based on bent carboxylate and flexible spacer ligand: Polymorphism, gas adsorption and SC-SC transmetallation

Article abstract of DOI:10.1039/c6ce00151c

Reaction of the bent dicarboxylate ligand H₂OBA (H₂OBA = 4,4′-oxybisbenzoic acid) and the flexible linker 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (L₁), under diverse reaction conditions, forms two polymorphic Co(ii) coordination polymers (CPs): {[Co(OBA)(L₁)]·DMF}_n (1), as a three dimensional (3D) framework with a pcu alpha-Po primitive cubic topology, and {[Co(OBA)(L₁)]·DMF}_n, (2), as a two dimensional (2D) structure with a 6-c uninodal net topology. Gas adsorption measurements of the desolvated Co(ii) CPs show negligible uptake of all gases in 1, while 2 exhibits moderate uptake of CO₂, with good selectivity over N₂ and CH₄. With Zn(ii), reaction of H₂OBA and L₁ produces a different 2D CP, {[Zn_0.5(OBA)_0.5(L₁)_0.5]}_n (3). Finally, three isostructural Cd(ii) CPs, {[Cd(OBA)(L₁)]·DMF}_n (4), {[Cd(OBA)(L₁)]·DEF}_n (5), and {[Cd(OBA)(L₁)]·DMA}_n (6) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, DMA = N,N-dimethylacetamide), that differ only in the lattice solvent molecules and show 2D structural arrangements are prepared. Interestingly, CP 4 undergoes single-crystal to single-crystal (SC-SC) transmetallation reaction at room temperature, yielding isostructural {[Cu(OBA)(L₁)]·DMF}_n (7) that cannot be synthesized independently. Moreover, the luminescence properties of compounds 1, 2, 3, and 4 have been studied in the solid state at room temperature. All the complexes are characterized by elemental analysis, IR, TGA, PXRD and single crystal X-ray diffraction.

Full text of DOI:10.1039/c6ce00151c

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Revised manuscript
(CE‐ART‐01‐2016‐000151)
Structural variation of transition metal coordination polymers based on
bent carboxylate and flexible spacer ligand: polymorphism, gas
adsorption and SC-SC transmetallation†‡
Arun Pal,a Santanu Chand,a S. Senthilkumar,b Subhadip Neogi*b and Madhab C.
Das*a
aDepartment of Chemistry, Indian Institute of Technology Kharagpur 721302, India, E-
mail: mcdas@chem.iitkgp.ernet.in
bInorganic Materials & Catalysis Division, Central Salt & Marine Chemicals Research
Institute (CSIR), Bhavnagar-364002, Gujarat, India. E-mail: sneogi@csmcri.org
________________________________________________________________________
Abstract. Reaction of the bent dicarboxylate ligand H2OBA (H2OBA = 4,4'-
oxybisbenzoic acid) and flexible linker 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (L1),
under diverse reaction conditions, forms two polymorphic Co(II) coordination polymers
(CPs): {[Co(OBA)(L1)].DMF}n (1), as a three dimensional (3D) framework with pcu
alpha-Po primitive cubic topology, and {[Co(OBA)(L1)].DMF}n, (2), as two dimensional
(2D) structure with 6-c uninodal net topology. Gas adsorption measurements of the
desolvated Co(II) CPs show negligible uptake for all gases in 1, while 2 exhibits
moderate uptake of CO2, with good selectivity over N2 and CH4. With Zn(II), reaction of
H2OBA and L1 produces a different 2D CP {[Zn0.5(OBA)0.5(L1)0.5]}n (3). Finally, three
isostructural Cd(II) CPs: {[Cd(OBA)(L1)].DMF}n (4), {[Cd(OBA)(L1)].DEF}n (5), and
{[Cd(OBA)(L1)].DMA}n (6) (DMF = N, N-dimethylformamide, DEF = N, N-
diethylformamide, DMA = N, N-dimethylacetamide) are prepared that differ only in the
lattice solvent molecules and show 2D structural arrangements. Interestingly, CP 4
undergoes single-crystal to single crystal (SC-SC) transmetallation reaction at room
temperature, yielding isostructural {[Cu(OBA)(L1)].DMF}n, (7) that cannot be
synthesized independently. Moreover, the luminescent properties of compounds 1, 2, 3, 4
have been studied in the solid state at room temperature. All the complexes are
characterized by elemental analysis, IR, TGA, PXRD and single crystal X- ray
diffraction.

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Introduction.
Coordination polymers (CPs), consisting of metal ions linked together by organic
bridging ligands,1-2 are new class of polymeric materials that dwell between the interface
of molecular coordination chemistry and material science.3 Over the past few years
porous coordination polymers have drawn the attention due to their synthetic control that
allows the understanding of high surface area and adjustable pore dimensions as well as
promising applications in different fields such as gas storage, gas separations, magnetism,
catalysis, optics, sensing, photoluminescence and many others.4-7 The very high
adsorption behaviors, and excellent reversibility kinetics is a result of their many
favorable attributes, such as reproducible and facile syntheses, high porosity, chemical
modification and amenability to scale-up.8-10
Since the process of self-assembly can frequently be modulated by various
external factors, such as metal source, ligands, solvents, templates, pH value,
temperature, and so on, the same reactants can usually result in completely different
structures because of the extreme sensitivity of self-assembly to the reaction conditions.11
Among all other factors, the judicious choice of appropriate organic ligands has proven to
be one of the most effective ways to manipulate versatile structures, where the length,
rigidity, coordination modes, functional groups, or substituents in the organic ligands can
bring about consequential effects on the final structures.12
The synthesis of mixed ligand CPs using polycarboxylates and N,N′-donor
spacers has been established as a significant strategy to manipulate the overall structures
with required functionality.13 Much effort has been devoted to using linear rod-like
ligands (linkers) such as, 4,4′-bipyridine; pyrazine; 4,4′-azobis(pyridine); 1,2-di(4-
pyridyl) ethane; 1,2-di(4-pyridyl)ethylene; 1,3-bis(4-pyridyl)propane in combination with
various polycarboxylate ligands to synthesize numerous CPs ranging from 1D to 3D
networks.14 Since altering of the length and functionalization of a spacer is synthetically
feasible, structural diversity and variation of porosity can easily be attained by the use of
flexible carboxylates with neutral linkers.15 Here, we have chosen the V-shaped 4,4′-
oxybisbenzoate (OBA) as the bent carboxylate and L1(1,4-bis(3-pyridyl)-2,3-diaza-1,3-
butadiene) as the linear spacer to construct both 2D and 3D structures by varying the
reaction conditions. The pyridyl based linker can pillar the metal-carboxylate motifs into

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different dimensionality to alter the structural frameworks. To this end, we report the
synthesis and structures of seven CPs (1-7) under solvothermal conditions as well as
room temperature crystallization forming different 2D layers or 3D networks. Solid state
photoluminescence properties of 1, 2, 3, 4 were also investigated.
In the current work, the role of the modulators has been extended further to
produce two new polymorphs (1 and 2) with different topology and diverse pore systems.
From a fundamental point of view, the study of polymorphism in coordination chemistry
is important as it allows to develop a basic understanding of the factors influencing
crystal growth, such as reaction temperature, solvent system, pressure etc.16 Additionally,
producing new materials through polymorphism leads to exceptional structural diversity
and improved storage capacities or shape/size selective catalysis. In the case of CPs,
however, there are not too many examples of polymorphism.16 Gas adsorption
measurements of the desolvated Co(II) polymorphs show negligible uptake of all gases
for 1, while 2 shows moderate uptake of CO2, with good selectivity over N2 and CH4, at
273K.
Interestingly, CP 4 undergoes complete transmetallation reaction to produce the
Cu(II)-analogue through a single-crystal-to-single-crystal (SC-SC) fashion. The
conversions of CPs to other isostructural CPs via the replacement of a framework metal
ion (i.e., transmetallation) is very fascinating and has been documented very recently.17
However, SC-SC transmetallation is a challenging task since cleavage/ formation of coordinate
bonds often leads to complete loss of structural integrity and particularly, single crystallinity. To
better elucidate the exchange mechanism, investigations of the metal replacement behavior
through SC-SC fashion are therefore indispensable.18
Experimental section
Materials.
Co(NO3)2.6H2O (Merck), Zn(NO3)2.6H2O (Merck), Cd(NO3)2.4H2O
(Merck), Cu(NO3)2.3H2O (Merck), 3-pyridinecarboxaldehyde (Sigma Aldrich),
Hydrazine hydrate (Merck), 4,4'-oxybisbenzoic acid (H2OBA) (Alfa Aesar), organic
solvents (DMF, DMA, DEF) were used as obtained, without further purification.

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Synthesis.
Synthesis of 1, 4-bis (3-pyridyl)-2, 3-diaza-1, 3-butadiene (L1).
Ligand 1, 4-bis(3-pyridyl)-2, 3-diaza-1, 3-butadiene (L1) was synthesized by adopting the
procedure with slight modifications.19 3-pyridinecarboxaldehyde (3.75 ml, 40 mmol) was
dissolved in 30 ml dry ethanol. Then hydrazine hydrate (0.97 ml, 20 mmol) was added
drop wise to this solution and the mixture was allowed to stir at room temperature for 24
h under the nitrogen atmosphere. The yellow precipitate thus obtained was collected and
washed with cold ethanol/hexane mixture (1:1, 20 ml) and dried in high vacuum. Yield:
1
90% (4.75 gm). H NMR (400 MHz, CDCl3, ppm): 8.96 (s, 2 H, pyridyl), 8.69 (d, 2 H,
13
pyridyl), 8.65 (s, 2 H, H-C=N), 8.20 (t, 2 H, pyridyl), 7.39 (q, 2 H, pyridyl). C NMR
(100 MHz, CDCl3, ppm): δ 123.9, 129.8, 135, 150.6, 15202, 159.9 (Fig. S1, ESI). Mass
(MALDI-TOF): m/z 211.457 (M+1). FT-IR (KBr pellet, cm-1): 1629(s), 1588.8(s),
1480.3(s), 1417.8(s), 1305.9(s), 1227(s), 1115.1(s), 1088.8(s). Anal. Calcd for C12H10N4:
C, 68.57%; H, 4.76%; N, 26.67%. Found: C, 68.46%; H, 4.59%; N, 26.52%. M.P: 1440C.
{[Co(OBA)(L1)].DMF}n, 1. A mixture of Co(NO3)2.6H2O (0.1 mmol, 0.0291 gm),
H2OBA (0.1 mmol, 0.026 gm), L1 (0.1 mmol, 0.021 gm) was dissolved in DMF (5 ml)
and stirred for 1 h. Then the solution was left to stand at room temperature. Red crystals
were obtained after about one month in 60% (35.9 mg) yield based on metal. Elemental
analysis, Calcd: C, 58.14%; H, 4.18%; N, 11.70%. Found: C, 58.21%; H, 4.19%; N,
11.55%. FT-IR (KBr pellet, cm−1): 3441(b), 3063.3(w), 2921.21(w), 2846.5(w),
1664.5(m), 1585(s), 1528(m), 1410(s), 1306.4(w), 1218.4(s), 1157.1(s), 1089.5(m),
1001.6(w), 873.6(s), 691.4(s), 657.6(m), 522.6(w).
{[Co(OBA)(L1)].DMF}n, 2. A mixture of Co(NO3)2.6H2O (0.1 mmol, 0.0291 gm),
H2OBA (0.1 mmol, 0.026 gm) and L1 (0.1 mmol, 0.021 gm) and DMF (5 ml) was placed
in a 15 ml Teflon-lined stainless steel vessel, heated to 110 0C for 48 h and then cooled to
room temperature over 10 h. Red block crystals of 2 were obtained in 50% (29.93 mg)
yield based on metal. Elemental analysis, Calcd: C, 58.14%; H, 4.18%; N, 11.70%.
Found: C, 58.21%; H, 4.20%; N, 11.57%. FT-IR (KBr pellet, cm−1): 3421.1(b),
3055.9(m), 2937.5(w), 2852(w), 1680.9(s), 1628(m), 1590(s), 1555(m), 1546.1(m),

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1503.3( m), 1470.4(m), 1412(s), 1309.2(m), 1243.4(s), 1164.5(m), 1088.8(m), 878.29(s),
845(m), 782.89(s), 700.66(s), 654.61(s), 628.3(m), 513.2(m), 509.87(m).
{[Zn0.5(OBA)0.5(L1)0.5]}n, 3. The complex 3 was synthesized similarly as 1, except that
Co(NO3)2.6H2O was replaced with Zn(NO3)2.6H2O (0.1 mmol, 0.030 gm). Yellow
crystals of 3 were obtained after 15 days in 60% (31.91 mg) yield based on metal.
Elemental analysis, Calcd: C, 58.67%; H, 3.38%; N, 10.53%. Found: C, 58.55%; H,
3.25%; N, 10.45%. FT-IR (KBr pellet, cm−1): 3035.8(m), 2799.4(w), 2482.1(m),
1927.6(m), 1677.8(s), 1580(s), 1488.5(s), 1430(m), 1366.8(s), 1319.7(m), 1231.7(s),
1150.8(s), 1089.5(m), 1049.5(m), 1001.5(m), 873.6(s), 778.5(s), 684.3(s), 643.4(s),
495(m).
{[Cd(OBA)(L1)].DMF}n, 4. The complex 4 was synthesized similarly as 1, except that
Co(NO3)2.6H2O was replaced with Cd(NO3)2.4H2O (0.1 mmol, 0.031 gm). Yellow
crystals of 4 were obtained after 15 days in 70% (45.63 mg) yield based on metal. 4 could
also be synthesized under solvothermal condition (5 ml DMF) with similar molar ratio at
110oC for 2 days. Elemental analysis, Calcd: C, 53.38%; H, 3.83%; N, 10.74%. Found:
C, 53.22%; H, 3.77%; N, 10.88%. FT-IR (KBr pellet, cm−1): 3447.4(b), 3065.8(m),
2930.9(w), 2852(w), 1680.9(s), 1628(m), 1595.4(s), 1540(s), 1500(m), 1402(s),
1309.2(m), 1240.1(s), 1164.5(s), 1088.8(m), 881.58(s), 842.11(m), 782.89(s), 697.37(s),
651.32(m), 519.74(m).
{[Cd(OBA)(L1)].DEF}n, 5. The complex 5 was synthesized similarly as 2, except that
Co(NO3)2.6H2O and DMF were replaced with Cd(NO3)2.4H2O (0.1 mmol, 0.031 gm),
and DEF (5 ml). Yellow crystals of 5 were obtained in 50% (34 mg) yield based on
metal. Elemental analysis, Calcd: C, 54.71%; H, 4.26%; N, 10.29%. Found: C, 54.80%;
H, 4.15%; N, 10.11%. FT-IR (KBr pellet, cm−1): 3410(b), 3068.3(w), 2975.3(w),
2927.4(w), 1670.7(s), 1597(s), 1542 (s), 1399(s), 1299.3(w), 1245.1(s), 1164.2(m),
1102.9(w), 880.7(s), 772.3(s), 697.6(s), 650.5(m), 522.4(w).

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{[Cd(OBA)(L1)].DMA}n, 6. The complex 6 was synthesized similarly as 1, except that
Co(NO3)2.6H2O and DMF were replaced with Cd(NO3)2.4H2O (0.1 mmol, 0.031 gm) and
DMA (5 ml). Yellow crystals of 6 were obtained after 15 days in 30% (20 mg) yield
based on metal. Elemental analysis, Calcd: C, 54.06%; H, 4.05%; N, 10.51%. Found: C,
53.93%; H, 3.91%; N, 10.44%. FT-IR (KBr pellet, cm−1): 3414.3(b), 3056.2(w),
2927.3(w), 1711.6(m), 1630.7(s), 1595(s), 1539(s), 1394(s), 1231.7(m), 1150.85(m),
1096.7(w), 1008.7(m), 860.3(s), 778.5(s), 691.4(s), 637.2(m).
{[Cu(OBA)(L1)].DMF}n, 7. This complex is a transmetallated complex from complex 4.
Yellow crystals of 4 gradually turned into intense blue crystals of 7 when dipped in a
solution of Cu(NO3)2·3H2O (0.05 M) in DMF at RT for 5 days. Elemental analysis,
Calcd: C, 57.70%; H, 4.15%; N, 11.61%. Found: C, 57.56%; H, 4.09%; N, 11.53%. FT-
IR (KBr pellet, cm−1): 3401(b), 3049.1(w), 2921.1(w), 1670.7(m), 1597(s), 1556.1(s),
1502(w), 1387(s), 1306.4(m), 1258.4(s), 1164.2(m), 1102.9(m), 1055.8(w), 873.6(m),
772.3(s), 691.4(s), 663.9(m), 495(m).
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 out on a Perkin-Elmer instrument, series II Model 2400
elemental analyzer. PXRD patterns were recorded using Cu Kα radiation (1.5418 Å) on a
Bruker D8 Advance diffactometer. Thermal analyses were performed on a Netzsch STA
449C thermal analyzer from room temperature at a heating rate of 10oC min-1 in nitrogen
atmosphere. The luminescence spectra for the powdered solid samples were measured at
room temperature on a Horiba Fluorolog Spectrophotometer. The excitation slit, as well
1
as the emission slit were 2.5 nm. H, 13C-NMR spectrum was recorded using a Bruker
Avance II 400 spectrometer. Mass (MALDI-TOF) spectrum was recorded using a Bruker
MALDI-TOF/TOF mass spectrometer. Energy-dispersive X-ray spectroscopic data
(EDX) was performed with a supra 40, Carl Zeiss Pvt. Ltd instrument. Low pressure gas
adsorption measurements were performed using a static volumetric system
(Micromeritics ASAP 2020) at different temperatures.

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X-ray Crystallography. The crystal and refinement data for 1-7 are collected in Table 1
and Table 2. 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 at 100 K on a
Bruker SMART APEX II CCD diffractometer using graphite-monochromated Mo-K
radiation ( = 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 SAINT20
software. An empirical absorption correction was applied to the collected reflections with
SADABS using XPREP.21 The structure was solved by the direct method using
SHELXTL22 and was refined on F2 by full-matrix least-squares technique using the
SHELXL-9723 program package. For all the cases non-hydrogen atoms were refined
anisotropically. Attempts to identify the highly disordered solvent molecules in
compound 1 were failed. 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.24
Table 1. Crystal data and structure refinements for complexes 1–3.
1
2
C29H25CoN5O6
598.47
100(1)
Mo-Kα
0.71069 Å
Monoclinic
P21/c
14.556(5)
12.648(5)
17.312(5)
90.00
3
C26H18ZnN4O5
531.81
100(1)
Mo-Kα
0.71069 Å
Monoclinic
C2/c
10.284(5)
19.302(5)
11.701(5)
90.00
Empirical formula C26H18CoN4O5
Formula weight
Temperature (K)
Radiation
Wavelength(λ)
Crystal system
Space group
a[Å]
b[Å]
c[Å]
α[°]
525.37
100(1)
Mo-Kα
0.71069Å
Orthorhombic
Pnna
21.638(5)
20.983(5)
12.364(5)
90.00
β[°]
γ[°]
90.00
90.00
5614(3)
8
1.243
110.840(5)
90.00
2978.7(18)
4
1.335
0.625
102.472(5)
90.00
2267.9(16)
4
1.558
1.131
Volume[Å3]
Z
Density [Mg/m3]
Absorption
0.650

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coefficient [mm-1]
F(000)
Refl. used [I >
2σ(I)]
2152
4524
1236
4897
1088
2152
Independent
reflections
7003
6188
2357
R int
0.0696
0.0500
0.0364
Refinement
method
GOF
full-matrix least full-matrix least full-matrix least
squares on F2
1.019
squares on F2
1.039
squares on F2
1.098
Final R
indices[I>2σ(I)]
R1= 0.0569
wR2= 0.1500
R1=0.0728
wR2=0.2205
R1=0.0328
wR2=0.0897
R indices (all
R1= 0.0843
R1=0.0884
R1=0.0366
data)
wR2= 0.1727
wR2=0.2355
wR2=0.0922
Table 2. Crystal data and structure refinements for complexes 4–7.
4
C29H25CdN5O6
651.94
100(1)
Mo-Kα
0.71069 Å
Monoclinic
P21/c
14.298(5)
12.999(5)
16.979(5)
90.00
5
6
7
Empirical formula
Formula weight
Temperature (K)
Radiation
Wavelength(λ)
Crystal system
Space group
a[Å]
C31H29CdN5O6 C30H27CdN5O6 C29H25CuN5O6
679.98
100(1)
665.97
100(1)
603.08
100(1)
Mo-Kα
0.71069 Å
Monoclinic
P21/c
14.261(5)
13.024(5)
17.038(5)
90.00
Mo-Kα
0.71069 Å
Monoclinic
P21/c
14.499(5)
12.841(5)
16.771(5)
90.00
Mo-Kα
0.71069 Å
Monoclinic
P21/c
14.268(5)
12.993(5)
16.902(5)
90.00
b[Å]
c[Å]
α[°]
β[°]
γ[°]
108.280(5)
90.00
2996.5(18)
4
108.197(5)
90.00
3006.3(18)
4
108.244(5)
90.00
2965.5(18)
4
107.945(5)
90.00
2980.9(18)
4
Volume[Å3]
Z
Density [Mg/m3]
1.445
1.500
1.492
1.344
Absorption coefficient 0.777
0.778
0.787
0.781
[mm-1]
F(000)
Refl. used [I > 2σ(I)]
1320
5347
1380
3890
1352
3735
1244
4441
Independent reflections 6216
4873
0.0678
5192
5866
0.0740
R int
0.0363
0.1040
full-matrix
least squares
on F2
Refinement method
full-matrix least full-matrix
full-matrix least
squares on F2
least squares
squares on F2
on F2

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GOF
1.040
1.036
1.073
1.050
Final R
indices[I>2σ(I)]
R indices (all data)
R1=0.0556
wR2=0.1573
R1= 0.0638
wR2=0.1641
R1=0.0357
wR2=0.0847
R1=0.0508
wR2=0.0929
R1=0.0561
wR2=0.1355
R1=0.0818
wR2=0.1537
R1=0.0781
wR2=0.1844
R1=0.1009
wR2=0.2284
Results and Discussion
Crystal structure of {[Co(OBA)(L1)].DMF}n, (1). Single crystal X-ray study reveals
that compound 1 crystallizes in the orthorhombic system with Pnna space group.
Structural investigation shows that asymmetric unit of 1 contains one Co(II) ion, one
OBA2- ligand and one L1 linker where two symmetry related Co(II) centers (Co1, Co1A)
are bridged by two carboxylate groups of two OBA2- ligands to form a Co2(COO)2
dimeric unit with a Co(II)…Co(II) separation of 4.129(9) Å (Fig. 1a). As shown in
Figure 1b, each Co(II) ion adopts a distorted octahedral geometry via coordinating with
four oxygen atoms (O2, O3, O4, O5) from three OBA2- anions in the equatorial plane and
two nitrogen atom (N3, N4) from two L1 ligand at the axial sites. The Co
̶ N bond lengths
vary from 2.156(2) to 2.157(2) Å, while the Co O bond distances lie within 2.0026(18)–
̶
2.223(2) Å, which are in the normal range (Table S1, ESI). Interestingly, two carboxylate
groups of OBA2- ligand adopt different coordination modes in 1. While one carboxylate
group adopts bidentate chelating mode, the other adopts μ2-η1: η1 bidentate bridging
mode forming the dimeric Co(II) unit (Fig. 1a) which is also established by careful
examination of asymmetric and symmetric stretching frequency of carboxylate groups in
FTIR spectra (Table S2, ESI).25 The bent OBA2- ligand, acting as µ3 - bridging ligands,
connect the dimeric units to form corrugated 2D layers. The axially coordinated L1
spacers connect these layers resulting in the formation of a 3D framework. A close
inspection of the structure of 1 divulges that terminal pyridyl groups of L1 are engaged in
π···π stacking interactions (centroid-to-centroid distance of 3.916(1) Å). Moreover, two
intermolecular C—H···O interactions exist between the spacer and the OBA2- ligand.
These secondary interactions are responsible for stabilizing the overall 3D structure
(Table S3, ESI). Figure 2 represents the 3-D packing diagram of 1, showing the 1D
channels of dimension 10.31 Å X 12.46 Å (considering atom to atom distance), running

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along the c axis. The solvent composition in 1 was established from a combination of
elemental analysis and TGA, which is in good agreement with the PLATON24 calculated
solvent-accessible volume (1294.5 Å3, 23.1 %) from the single crystal structure analysis.
Furthermore, topological analysis was carried out to get more insight into the structure,26
which showed that the 3D framework can be best described as a pcu alpha-Po primitive
cubic topology with point symbol: {4^12.6^3} (Fig. S2, ESI), using L1 and OBA2-
ligands as linkers and dimeric Co(II) units as connectors.
Fig. 1: (a) Coordination environment around Co metal of complex 1, (b) Core view of
complex 1 (symmetry operation (A): 0.5-x, 0.5+y, 1.5-z). H atoms are omitted for clarity.

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Fig. 2: View of 2D corrugated layers, joined by L1 spacer and 3-D packing diagram of 1,
showing 1D channels running along the c axis.
Crystal structure of {[Co(OBA)(L1)].DMF}n, (2).
Interestingly, solvothermal reaction of H2OBA, L1 and Co(NO3)2.6H2O in similar
stoichiometric ratio, as used for 1, yielded crystals with new morphology. Single crystal
X-ray study reveals the formation of a different CP {[Co(OBA)(L1)].DMF}n, (2), which
crystallizes in the monoclinic crystal system with P21/c space group. Further structural
investigations indicate that although, asymmetric unit of 2 exactly resembles to that of 1,
orientations of OBA2- ligand changes (vide infra). A comparison of structure between 1
and 2 revels that the coordination modes of OBA2- or L1 ligand, octahedral coordination
environment around Co(II) center, as well as dimeric Co2(COO)2 units are common.
However, the distance between two Co(II) centers in 2 (4.0430(11) Å) is found to be
shorter than that in 1 (Fig. S3, ESI). Clearly, 2 exhibit polymorphism phenomena with 1.
It should be mentioned here that the significance of polymorphism in coordination
polymers (framework isomerism) has recognized immense attraction,16 where this
phenomenon, to our anticipation, originates due to different synthetic approach applied in
this study (room temperature for 1 vs solvothermal in 2 while keeping the molar ratio of
reactants and solvent system same). Most strikingly, orientation of the bent OBA2- ligand
with respect to planar spacer in 2 is quite distinct than that of 1. As depicted in Figure 3,
the shortest and the longest distances among the centers of aromatic rings of the spacer
ligand to the centers of the aromatic moieties of OBA2- ligand vary significantly in both
the complexes (6.81 in 1 vs 6.94 Å in 2 and 12.28 in 1 vs 15.52 Å in 2). Also, the

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dihedral angle between the mean planes of the spacer and phenyl carboxylate at the far
end of the OBA2- ligand is 44.24o in 1, which is extended to almost twice (86.0o) in 2.
Because of the different orientation of the OBA2- ligand, the dimeric Co(II) units bridged
by these ligands are extended in two directions giving rise to a 2D layer structure (Figure
4), rather than 3D. It should be noted that similar to 1, weak π···π stacking interactions
(centroid-to-centroid distance of 3.8036(8) Å compared to 3.916(1) Å in 1) also exists
among adjacent aromatic moieties of L1 ligands in 2. Moreover, lattice DMF molecules
are engaged in C—H···O interactions with the OBA2- ligand and pyridyl moiety of the
spacer, where these weak intermolecular interactions render stability to the overall 2D
structure (Table S3, ESI). Total potential solvent accessible volume is 910.7Å3 per unit
cell volume of 2978.7 Å3 (30.6%) as estimated by PLATON.24 The topological analysis
revels a 6-c uninodal net topology having point symbol: {3^6.4^6.5^3} (Fig. S4, ESI).26
Fig. 3: Different orientations of OBA2- ligand with respect to planar spacer in the
asymmetric units of CP 1 (a) and CP 2 (b). The dihedral angles between the planes as
shown in the figure are 44.24o in (a, for CP 1) and 86.0o in (b, for CP 2). H atoms are
omitted for clarity.

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Fig. 4: Representation of 2-D packing diagram of complex 2 viewed along
crystallographic c axis.
{[Zn0.5(OBA)0.5(L1)0.5]}n, (3). Compound 3 crystallizes in the monoclinic crystal system
with the space group C2/c. Single crystal X-ray structure determination reveals a 2D
zigzag structure. The asymmetric unit of 3 consists of a half of Zn(II) ion, half of OBA2-
ligand and half of L1 spacer without any lattice solvent molecule. Figure 5 (and Fig. S5,
ESI) depicts the coordination environment around Zn(II) center, adopting a highly
distorted octahedral geometry surrounded by two symmetry-related nitrogen atoms from
two L1 ligands and two sets of symmetry related oxygen atoms from two OBA2- ligands.
The two carboxylate groups of the OBA2- ligand show the bidentate chelating
coordination mode (Table S2, ESI).25 The Zn
2.326(4) Å, while the Zn O bond lengths are from 2.308(3) to 2.620(3) Å (Table S1,
ESI). The Zn O bond length of 2.620(3) Å is longer than usual Zn
O bond lengths.27 The
̶ N bond lengths vary from 2.288(4) to
̶
̶
̶
Zn···Zn distances separated by L1 ligand is 11.701(5) Å, while the same created by
OBA2- ligand approximates to 13.809(5) Å. It is worth mentioning that in contrast to the
rest of the structures, where spacer L1 adopts almost planar orientation with minimum
dihedral angle between the terminal pyridyl rings (e.g. 6.15o for 1, 6.27 o for 2, 3.04 o for
4), the pyridyl rings of spacer in 3 is severely tilted with dihedral angle between the two
planes of pyridyl rings being 77.35o. Figure 6 represents the 2-D packing diagram which
shows zigzag structure along the crystallographic a axis. This 2-D structure is further

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stabilized by several intermolecular non-covalent interactions among the constituent
molecules as listed in Table S3, ESI.
Fig. 5: Coordination environment around Zn metal of complex 3 (symmetry operation
(a): -x, y, 0.5-z).
Fig. 6: Representation of 2-D packing diagram of complex 3.
Crystal structures of {[Cd(OBA)(L1)].DMF}n, (4), (5), (6). Single-crystal structure
analyses reveal that 4−6 are isostructural and crystallize in the monoclinic crystal system.
Hence, only the structure of 4 is described in detail. The coordination environments along

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with packing diagrams of other complexes and the selected bond lengths, angles and
nonbonding interactions of all the complexes are listed in the Supporting Information
(Fig. S8-S13, Table S1, S3, ESI). Single crystal X-ray structure determination reveals 4
as a 2D layer structure. The asymmetric unit of compound 4 consists of a one cadmium
(II) ion, one OBA2- ligand, one L1 and a lattice DMF molecule. As shown in Figure 7, the
Cd(II) center adopts a distorted pentagonal bipyramidal geometry surrounded by five
oxygen atoms (O2, O3, O4, O4a, O5) from three different OBA2- ligands in the equatorial
plane and two nitrogen atoms (N1, N2) from two different L1 ligands at the axial
positions. The Cd ̶ N bond lengths vary from 2.288(4) to 2.326(4) Å, while the Cd ̶ O
bond lengths are from 2.308(3) to 2.620(3) Å, which are in the normal range (Table S1,
ESI). In complex 4, two carboxylate groups of OBA2- ligand adopt two different
coordination modes: one carboxylate group adopts bidentate chelating mode and the
other adopts μ2-η1: η2-bridging mode which are also established by careful examination
of asymmetric and symmetric stretching frequency in FTIR spectra (Table S2, ESI).25
Two symmetry related Cd(II) centers (Cd1, Cd1a) are bridged by two carboxylate group
of OBA2- ligand to form a dimeric Cd2(COO)2 core structure (Fig. S6, ESI). The distance
between two symmetry related Cd(II) centers in this dimeric unit is 3.857(1) Å. These
Cd2(COO)2 units are further capped with N atoms of the spacers at both ends and
extended in two directions to form a layer structure (Fig. S7, ESI). Weak π···π stacking
interactions (centroid-to-centroid distance of 3.6044(8) Å) among adjacent aromatic
moieties of L1 ligands, several other C—H···O and C—H···N non-covalent interactions
(Table S3, ESI) among the DMF, spacer L1 and OBA2- ligand further stabilize the 2D
layers.
Fig. 7: Core view of complex 4 (symmetry operation (a): 1-x, -y, 2-z).

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Transmetallation reaction.
Direct synthesis of the Cu(II) analogue of 4 by replacing Cd(NO3)2·6H2O with
Cu(NO3)2·3H2O, as described above, was not successful. To this end, we attempted for
transmetallation reaction. When a crystal of complex 4 was dipped into the solution of
Cu(NO3)2.3H2O in DMF (0.05 M) at room temperature, the pale yellow crystal gradually
turned into greenish blue within 5 days (Fig. S34, ESI). Crystal size, shape and
morphology of the mother crystal remains unaltered throughout the exchange process, as
observed from optical microscopy and diffractometer mounted single crystal
photographs. This observation excludes any possibility of dissolution of the mother
crystal followed by re-crystallization at the surface and growth of a new phase (Fig. S35,
ESI). Moreover, the PXRD pattern of the metal exchanged material does not show any
formation of a new phase and is comparable to that of 4 (Fig. S29, ESI), demonstrating
that structural integrity is preserved during the metal exchange process. Complete metal-
exchange was further supported by EDX of the transmetallated crystal (Fig. S36, ESI).
In spite of the fact that blue crystals are poorly diffracting, we were fortunate to
obtain the single crystal X-ray structure, which showed that the transmetallated crystal
has identical crystal system and structure to that of 4, with the formula
{[Cu(OBA)(L1)].DMF}n (7). As shown in Figure S14 (ESI), the Cu(II) center adopts a
similar distorted pentagonal bipyramidal geometry surrounded by five oxygen atoms
from three different OBA2- ligands in the equatorial plane and two nitrogen atoms from
two different L1 ligands at the axial positions. The heptacoordination of Cu(II) is not so
common in literature (CSD search for heptacoordination of Cu(II) gave only 87 hits
(version 5.36, Nov 2014)). The Cu
̶
N and Cu
N and Cd
̶
O bond lengths are in the normal range28
and almost identical to the corresponding Cd
̶
̶
O lengths of the mother crystal 4
(Table S1, ESI). As depicted in Figure 8, the distance between two symmetry related
Cu(II) centers in the dimeric unit is 3.860(11) Å, which is slightly larger than the
separation of Cd(II) dimer (3.857(1) Å) in 4. However, the centroid-to-centroid distance
among adjacent aromatic moieties of L1 ligands of 3.621(8) Å in 7 is slightly larger than
3.604(8) Å in 4. The 2D packing diagram is depicted in Fig.S15 (ESI), which unveils
several non-covalent interactions (Table S3, ESI) that further stabilize the 2D layers.
Since all our attempts for the direct synthesis of 7 were unsuccessful, we suggest that SC-

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SC metal exchange process is the only route to obtain 7. To our understanding, the
cooperative process of simultaneous breaking of metal-ligand bonds and formation of
new bonds around the incoming metal center may facilitate the formation of 7, retaining
the pristine coordination sphere, which is difficult to synthesize otherwise. However, the
reverse transmetallation does not ensue for the Cu(II) exchanged product even in a
concentrated DMF solution of Cd(NO3)2·6H2O (1 M) under long exchange time (1
month), suggesting that the Cu(II) structure is thermodynamically more stable than the
corresponding Cd(II) structure. Also, exchange behavior with Ni(II) or Co(II) metal ions
for the pristine 4 exhibits no replacement under ambient condition with long exposure
time (one month) or under warm condition. Thus, this metal-exchange phenomena may
be correlated to the relative stabilities of the metal ligand complexes that lie in the order
CoII<NiII<ZnII<<CuII in the Irving–Williams series.17d
Fig. 8: Coordination environment around Cu metal of complex 7. H atoms are omitted
for clarity.
PXRD Patterns and Thermal Analyses:
In order to confirm the phase purity of these coordination polymers, the PXRD
patterns were recorded for CPs 1-7, and they were comparable to the corresponding

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simulated ones (Fig. S23-S29, ESI), indicating pure phase of each bulk sample. To check
the thermal stabilities of compounds 1–7, their TGA curves were investigated under
nitrogen atmosphere with a heating rate of 10oC per min (Fig. S30-S33, ESI). For
compound 1, the weight loss 11.7% occurs at 270oC, which can be attributed to the loss
of lattice DMF molecule (calculated value 12.21%) and then decomposition of the
structure occurs. For 2, the weight loss of 12.40% in the temperature range 100-279oC
corroborates to the loss of the lattice DMF molecule (calculated value 12.21%), followed
by degradation of the framework. Compound 3 is stable up to 270oC and after that, the
decomposition occurs gradually. For 4, the weight loss 11.47% occurs at 260oC, which
corresponds to the loss of uncoordinated DMF molecule (calculated value 11.21%)
followed by degradation of the framework. For 5, the weight loss 15% occurs at 280oC,
which corresponds to the loss of uncoordinated DEF molecule (calculated value 14.85%)
beyond which the framework starts to degrade. For 6, weight loss 13.30% occurs at
173oC which can be attributed to the loss of lattice DMA molecule (calculated value
13.08%) and then decomposition of the framework occurs. For 7, weight loss
corresponding to the release of uncoordinated DMF molecule is 12.44% at 273oC
(calculated value 12.12%) followed by decomposition of the framework. All the other
o
weight losses beyond the temperature region of 270- 280 C could not be elucidated in
certainty. Given that the as synthesized CPs are not stable beyond this temperature
region, it is expected that the strong metal-carboxylate interactions that tighten the
backbone of the ligand to enhance the resistance to pyrolysis is absent in these CPs.29
The frameworks stability of the parent complex (4) and the transmetallated complex (7)
are pretty much similar to each other.
Gas Adsorption Studies:
The porous nature of 1 and 2 satisfy the essential prerequisites for gas sorption
measurements. To this end, gas adsorption studies were conducted up to the relative
pressure (p/p0) of 1.0 on the activated compounds. Prior to gas adsorption measurements,
CP 1 and 2 were immersed in chloroform for 5 days and heated thereafter at 100 °C for
12 h under vacuum to produce the guest-free frameworks, 1′ and 2′, respectively. The
stability of the guest free frameworks, 1′ and 2′ were confirmed by comparing the

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individual PXRD patterns with that of the respective as-synthesized frameworks, which
revealed perfect agreement in both the cases (Fig S39, ESI). However, compound 1′ does
not adsorb any gases. Even N2 adsorption at 77 K reveals only surface adsorption (Fig.
S37, ESI). In contrast, the CO2 adsorption experiment of 2′ showed a typical type I
adsorption behavior (Figure 9a) at 273 and 298 K, confirming its microporous character.
The desolvated compound starts to take up CO2 gradually from the low-pressure region.
The maximum CO2 uptakes (1 atm) at 273 and 298 K are 57.6 and 17.2 cm3 g-1,
respectively. It is noteworthy that the present CO2 sorption results are comparable with
the values observed for well-known MOFs like ZIF-9, ZIF- 100, other porous materials
like zeolites1a and activated carbon.30a Furthermore, it is comparable to MOFs affording
pore wall functionalized by acylamide or amide groups, and MOFs decorated by
coordination-unsaturated metal sites.4c-4d
(a)
(b)
Fig. 9: (a) The CO2 gas sorption isotherm of 2′ at 273 K (blue) and 298 K (red). Filled
and open shapes indicate adsorption and desorption data points, respectively. (b) Isosteric
heat of CO2 adsorption (Qst) of 2′.
To better understand the CO2 sorption properties, we calculated the isosteric heat
of CO2 adsorption (Qst) of 2′ by the Clausius−Clayperon equation2c from the isotherms
obtained at 273 and 298 K. As shown in Figure 9b, the Qst value for CO2 reaches 54.0 kJ
mol−1 at high coverage. It should be noted that this value is significantly higher than
known MOFs having –NH2 group such as, bio-MOF-11 (45 kJ/mol), CAU-1 (48 kJ/mol),
NH2-MIL-53(Al), USO-1-Al-A (50 kJ/mol), or other MOFs like CuTATB-60, PCN-6 (35

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kJ/mol), HKUST-1 (35 kJ/mol), IRMOF-1, MOF-5 (34 kJ/mol), MIL-53(Cr) (32 kJ/mol),
NOTT-140 (25 kJ/mol) and UMCM-1 (12 kJ/mol). Such a high value implies that the azo
moieties of pyridine linkers inside the framework are involved in strong intermolecular
interactions with the adsorbed CO2 molecules. Presumably, the slight hysteresis observed
for CO2 adsorption isotherm can be correlated with this large Qst value. To further
explore the potential properties of 2′ on gas separation under ambient conditions, the
adsorption isotherms of CH4 and N2 were measured at 273 K. As shown in Figure 10, N2
molecules did not diffuse into the pores. However, CH4 gas molecules, having a
reasonable polarizability (25.93 × 10-25 cm3), enter only moderately (6.8 cc/g at 273 K
and up to 1 bar) into the pore. In sharp contrast, the high quadrupole moment and
polarizability of CO2 (13.4 × 10–40 C m2 and 26.3 × 10-25 cm3, respectively)2d induce
better interaction with the channels and interior wall of the CP, composed of basic azo
functionalities. These interactions should be the crucial factors for superior CO2
adsorption over other gases.
The significant challenge regarding gas separation under ambient conditions, plus
the favorable distinguishing adsorption behavior of the material prompted us to examine
the selective CO2 capture ability over N2 and CH4 gases at 273 K. The selectivity (S) for
adsorption of CO2 over other gases was calculated from the single-component isotherm
data. For CO2 capture, this value typically reports the ratio of the adsorbed amount of
CO2 at 0.15 bar to the adsorbed amount of other gases at 0.75 bar; the value is normalized
for the pressures chosen, according to equation: S = (qCO2/qo)/(pCO2/po), where q is the
amount adsorbed and p is the relative pressure (o = Other gases).2e

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Fig. 10: The CO2 (blue), CH4 (pink) and N2 (green) adsorption of 2′ at 273 K. Filled and
open shapes indicate adsorption and desorption data points respectively.
The CO2 selectivity over N2 and CH4 are found to be 207.2 and 17.5 respectively.
Though, numerous MOFs with significant CO2 uptake have been reported, only limited
examples of porous MOFs are known that exhibit CO2/N2 selectivity over 200.30b The
high selectivity value suggest that 2′ may find potential applications for selective CO2
capture from flue gas. Although, the CO2/CH4 selectivity for 2′ at 273 K, is lower than
CO2/N2 selectivity, the value is comparable to some polyimide networks or ZIFs.4b, 30c
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 cobalt, zinc, cadmium ions exhibit
photoluminescence properties.31 To this end, luminescence properties of compound 1-4
as well as free ligands L1 and H2OBA were studied in the solid state at room temperature
under the same experimental conditions. The emission peaks of L1 and H2OBA were
observed at 476 nm and 463 nm upon excitation at 260 nm and 320 nm respectively (Fig.
S38, ESI). These emissions are due to intra-ligand π* → n or π* → π charge transitions
of L1 and H2OBA.31c The CPs 1 and 2 display broad band with the main peak at 450 nm,
445 nm and the shoulder peak at 370 nm, 363 nm upon excitation at 330 nm and 320 nm
respectively which might be caused by the mixture characteristic of intra-ligand charge
transitions of L1 and H2OBA (Fig. 11). The emission results of 1 and 2 are the indication
of close relationship of photoluminescent behavior with the coordination environment
around the metal ions.31d The complex 3 exhibits emission at 362 nm as shoulder peak
and at 466 nm as main peak upon excitation at 330 nm which may be assigned to the
mixture characteristic of intra-ligand charge transitions of L1 and H2OBA (Fig. 11). The
emission band of compound 4 appears in 477 nm upon excitation at 320 nm which is
similar to that of free H2OBA, which can be assigned to the intra-ligand charge transition
of H2OBA (Fig. 11). The compound 4 exhibits a small red-shift (14 nm) with respect to

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free H2OBA ligand, which may be attributed to the metal-ligand coordination
interactions.
Fig. 11: Solid-state photoluminescent spectra of coordination polymers 1 - 4.
Summary:
In summary, self-assembly of a V-shaped organic linker and a flexible N, N'-
spacer with different transitions metal ions leads to the formation of seven new
complexes as two or three dimensional coordination polymers. Interestingly, using
similar stoichiometric ratio of the reactants, variation of the synthetic conditions (room
temperature vs. solvothermal reaction) leads to two different coordination polymers with
varying dimensionalities (two and three dimension) that exhibit polymorphism
phenomenon. Furthermore, the yellow crystal of Cd(II) CP undergoes single-crystal to
single-crystal transmetallation reaction to produce isostructural Cu(II) CP under ambient
conditions. Although, SC-SC replacement of solvent molecules and organic linkers have
much been studied in CPs, only a handful example exists on metal exchange process.
Metal exchange in coordination polymers leads to improved catalytic and gas storage
properties and allow obtaining those structures that cannot be synthesized by de novo
route. The CPs 4-6 are isostructural and only differ in lattice solvent molecules. All the
CPs are produced in pure phase during their bulk synthesis and are moderately thermally
stable. Gas adsorption measurements of the desolvated Co(II) CP-2 shows moderate
uptake of CO2, with good selectivity over N2 and CH4, at 273K. CPs 1-4 exhibit normal
luminescence properties as expected.

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† CCDC reference numbers 1446366-1446372. For crystallographic data in CIF or other
electronic format see DOI:
‡Electronic supplementary information (ESI) available: X-ray crystallographic files
in CIF format for the structure determination of CPs 1-7, additional figures, tables for
selected bond length and bond angle, non-bonding interactions. See DOI:
Acknowledgements.
We gratefully acknowledge the financial support received from ISIRD-SRIC, IIT
Kharagpur and a JRF (UGC) to A. P. CSIR-CSMCRI Communication No. 011/2016. We
thank Dr. Susan Sen, Kyoto University for helpful discussion.
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DOI: 10.1039/C6CE00151C
Graphical Abstract
Structural variation of transition metal coordination polymers based on
bent carboxylate and flexible spacer ligand: polymorphism, gas
adsorption and SC-SC transmetallation
Arun Pal, Santanu Chand, S. Senthilkumar, Subhadip Neogi and Madhab C. Das
Seven new coordination polymers have been synthesized and characterized by EA, IR,
TGA, PXRD and SC-XRD. Polymorphism, SC-SC transmetallation and gas sorption
properties have been studied.