Structural library of coordination polymers based on flexible linkers exploiting the role of linker coordination angle: Synthesis, structural characterization and magnetic properties

Article abstract of DOI:10.1039/c3ce42637h

Herein we report six new coordination polymers [Co(1,2-pda)(1,2-bix)] _n (1a), [Co(hfipbb)(1,2-bix)]_n·nH₂O (2a), [Co(ADA)(1,2-bix)]_n (3a), [Co(ADA)(1,3-bix)] _n·nH₂O (3b), [Co(1,4-pda)₂(2-pztz)] ₂[Co(H₂O)₆]·2nH₂O (4a), [Co₂(μ-OH)(1,3-pda)(4-ptz)]_n (4b) based on the flexible carboxylate ligands 1,2-phenylenediaceticacid (1,2-H₂pda), 1,3-phenylenediaceticacid (1,3-H₂pda), 1,3-adamantanediacetic acid (H₂ADA), 4,4′-(hexa-fluoroisopropylidene)bis(benzoicacid) (H₂hfipbb) and secondary N-donor ligands 1,3-bis(imidazole-1- ylmethyl)-benzene (1,3-bix), 1,2-bis(imidazole-1-ylmethyl)-benzene (1,2-bix), 5-(4-pyridyl) tetrazole (4-ptz) and 2-(2H-tetrazol-5-yl)pyrazine (2-pztz). All the compounds were characterized by single crystal X-ray diffraction analysis, IR spectroscopy, elemental analysis and bulk homogeneity by powder X-ray diffraction. A subtle relation between the position of the coordinating groups in the linker (known as linker coordination angle, LCA) and the dimensionality and topology of the final architectures of the title compounds has been discussed systematically by comparing with the structurally related reported compounds [Co(1,4-pda)(1,4-bix)]_n (1b), [Co(hfipbb)(1,4-bix) _0.5]_n (2b) and [Co₂ (μ-OH)(1,4-pda)(4-ptz)] _n·nH₂O (4c). The compounds studied in the present work are classified into four different mixed linker classes, based on the skeleton geometry of the linker. Finally, temperature dependent magnetic susceptibility measurements have been studied and the relevant zero-field splitting parameters have been determined.

Full text of DOI:10.1039/c3ce42637h

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PAPER
Structural library of coordination polymers based
on flexible linkers exploiting the role of linker
coordination angle: synthesis, structural
Cite this: CrystEngComm, 2014, 16,
4816
characterization and magnetic properties†
*
Bharat Kumar Tripuramallu, Paulami Manna and Samar K. Das
Herein we report six new coordination polymers [Co(1,2-pda)(1,2-bix)]n (1a), [Co(hfipbb)(1,2-bix)]n·nH2O
(2a), [Co(ADA)(1,2-bix)]n (3a), [Co(ADA)(1,3-bix)]n·nH2O (3b), [Co(1,4-pda)2(2-pztz)]2[Co(H2O)6]·2nH2O (4a),
[Co2(μ-OH)(1,3-pda)(4-ptz)]n (4b) based on the flexible carboxylate ligands 1,2-phenylenediaceticacid
(1,2-H2pda), 1,3-phenylenediaceticacid (1,3-H2pda), 1,3-adamantanediacetic acid (H2ADA), 4,4′-(hexa-
fluoroisopropylidene)bis(benzoicacid) (H2hfipbb) and secondary N-donor ligands 1,3-bis(imidazole-1-
ylmethyl)-benzene (1,3-bix), 1,2-bis(imidazole-1-ylmethyl)-benzene (1,2-bix), 5-(4-pyridyl) tetrazole (4-ptz)
and 2-(2H-tetrazol-5-yl)pyrazine (2-pztz). All the compounds were characterized by single crystal X-ray
diffraction analysis, IR spectroscopy, elemental analysis and bulk homogeneity by powder X-ray
diffraction. A subtle relation between the position of the coordinating groups in the linker (known as linker
coordination angle, LCA) and the dimensionality and topology of the final architectures of the title com-
pounds has been discussed systematically by comparing with the structurally related reported compounds
Received 27th December 2013,
Accepted 5th February 2014
[Co(1,4-pda)(1,4-bix)]n (1b), [Co(hfipbb)(1,4-bix)0.5]n (2b) and [Co2 (μ-OH)(1,4-pda)(4-ptz) ]n·nH2O (4c). The
compounds studied in the present work are classified into four different mixed linker classes, based on the
skeleton geometry of the linker. Finally, temperature dependent magnetic susceptibility measurements have
been studied and the relevant zero-field splitting parameters have been determined.
DOI: 10.1039/c3ce42637h
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of the coordinating groups in a simple phenyl linker are
ortho-, meta-, and para-bidentate modes. The angle between
Introduction
Because of the enormous varieties of fascinating structural
topologies and great potential applications as solid func-
tional materials, metal organic frameworks (MOFs) and coor-
dination polymers (CPs) have attracted considerable
attention in recent years.1 One of the most successful design
strategies for constructing CPs is to take advantage of the ver-
satility of several bridging ligands. Organic ligands with well-
defined geometries may greatly influence the final structures
of coordination polymers (CPs).2 In the case of designing
coordination polymers by using mixed linkers, the position
of the coordinating groups on the linker often plays a major
role in directing the dimensionality.3 The general positions
the positions of the coordinating groups in the linker, which
is known as the linker coordination angle (LCA), varies
according to the geometry of the spacer; as a result, the over-
all coordination geometry of the concerned linker in the
resulting CP alters, for example, bent, linear, etc. Zhou and
co-workers reported the application of a mixture of bridging
linkers with different LCAs in a coordination-driven self
assembly process.4 Coordination networks, based on rigid
ligands exploiting LCAs, have been studied extensively due to
their regular and well defined coordination modes.5 In con-
trast, coordination polymers constructed using flexible
ligands are still limited, because of the unpredictable nature
of such systems arising from the flexible nature of spacers
that allows more of degrees of freedom.6
Currently considerable efforts have been devoted to
enriching the structural library of coordination networks
based on flexible ligands. The major parameters to be consid-
ered in designing ditopic flexible ligand based coordination
networks are the nature of the secondary ligand, coordina-
tion availability at the metal coordination sphere, length of
the spacer and position of the coordinating groups. All these
School of Chemistry, University of Hyderabad, P.O. Central University,
Hyderabad- 500046, India. E-mail: skdsc@uohyd.ernet.in, samar439@gmail.com;
Fax: +91 40 2301 2460; Tel: +91 40 2301 1007
† Electronic supplementary information (ESI) available: Complete list of bond
lengths and bond angles, 1/χM vs. T plots, powder X-ray diffraction patterns,
TGA curves and diffuse reflectance spectra and X-ray crystallographic data for
compounds 1a, 2a, 3a, 3b, 4a and 4b; CCDC 978685–978690. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/
c3ce42637h
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factors have a tendency to modulate the conformations of the
flexible ligands which is primarily responsible for the forma-
tion of higher dimensional structures. In ditopic flexible
linkers, the geometrical orientation of two coordinating
groups, associated with the secondary linker, results in the
construction of coordination networks of varying topologies
that can exhibit unique functional properties. Cao and co-
workers reported a series of coordination polymers based on
flexible ligands and used these compounds as functional
materials for potential applications.7 In our previous reports,
we have discussed the factors affecting the conformational
modulation of flexible ligands by exploring the role of the
secondary linker and metal ion.8 Cao's group and we have
demonstrated conformational control of a flexible ligand,
H2pda (trans to cis conformation) by introducing a rigid aux-
iliary ligand, e.g., 4,4′-bipyridine/rigid tetrazole linker.8,9 Shi
and coworkers reported a series of Zn(II) coordination poly-
mer containing compounds from isomeric phenylenediacetic
acid (1,4-, 1,3-, 1,2-H2pda) and dipyridyl ligands.10 The litera-
ture reports of mixed linker coordination networks, based on
flexible linkers exploiting the position of the coordinating
groups in the linker, are very rare. We have been primarily
working on coordination networks based on flexible linkers
and reported our thoughts (in a conceptual manner) during
analyzing the self-assembly processes of flexible linkers and
transition metal ions that we have used in our laboratory.11
This concept prompted us to study the influence of the
linker coordination angles (LCAs) of various flexible ligands
in designing coordination networks of versatile topologies.
We have chosen diverse linkers, e.g., flexible phenylene
diacetates,12 bent flexible adamantane diacetic acid,13
bent 4,4′-(hexa-fluoroisopropylidene)bis(benzoicacid),14 flexi-
ble bisimidazolyl nitrogen donars,15 and rigid tetrazoles16 in
exploring the library of coordination networks with possible
varied linker coordination angles. We have described the
design and synthesis of a new series of coordination polymer
Experimental section
Materials and methods
All the chemicals were received as reagent grade and used
without any further purification. The ligands 1,4-bix, 1,3-bix,
1,2-bix, 4-ptz, 2-tzpz were prepared according to the literature
procedures.17 Elemental analyses were determined by a FLASH
EA series 1112 CHNS analyzer. Infrared spectra of solid sam-
ples were obtained as KBr pellets on a JASCO – 5300 FTIR
spectrophotometer. Powder X-ray diffraction patterns were
recorded on a Bruker D8-Advance diffractometer using graphite
monochromated CuKα1 (1.5406 Å) and Kα2 (1.54439 Å) radia-
tion. Magnetic susceptibilities were measured in the tempera-
ture range 2–300 K on a Quantum Design VSM-SQUID. All
compounds were synthesized in 23 mL Teflon-lined stainless
vessels (Thermocon, India).
Synthesis of [Co(1,2-pda)(1,2-bix)]n (1a)
A mixture of CoCl2·6H2O (0.25 mmol, 59.5 mg), 1,2-pda
(0.25 mmol, 48.5 mg) and 1,2-bix (0.25 mmol, 60.0 mg) was
dissolved in 10.0 mL of distilled water and the pH of the
reaction mixture was adjusted to 6.35 by 0.5 M NaOH solu-
tion. Then the resulting mixture was stirred for 30 min and
transferred to a 23 mL Teflon-lined stainless vessel, sealed,
and heated at 160 °C for 72 h and then cooled to room tem-
perature over 48 h to obtain red block crystals. Yield: 60.3%
(based on Co). Anal. calcd for C24H22CoN4O4 (Mr = 489.39): C,
58.90%; H, 4.53%; N, 11.44%. Found: C, 58.45%; H, 4.35%;
N, 11.01%. IR (KBr pellet, cm−1): 3140, 2997, 2916, 1695, 1572,
1520, 1369, 1265, 1226, 1024, 949, 852, 810, 748, 721, 657.
Synthesis of [Co(hfipbb)(1,2-bix)]n·nH2O (2a)
A mixture of CoCl2·6H2O (0.25 mmol, 59.5 mg), H2hfipbb
(0.25 mmol, 98.0 mg) and 1,2-bix (0.25 mmol, 60.0 mg) was
dissolved in a solvent mixture of H2O (10.0 mL) + DMF
(1.0 mL) and stirred for 30 min. The pH of the reaction
mixture was adjusted to 6.10 by adding 5 M NaOH solution
and the resulting reaction mixture was placed in a 23 mL
Teflon-lined stainless steel autoclave, which was sealed and
heated at 160 °C for 72 h. The autoclave was allowed to
cool to room temperature for 48 h. Deep blue colored
block-shaped crystals of compound 2a were obtained in a
65.5% yield (based on Co). Anal. calcd for C31H22N4F6O5Co
(Mr = 703.46): C, 52.93%; H, 3.15%; N, 7.96%. Found: C,
52.65%; H, 2.98%; N, 7.48%. IR (KBr pellet, cm−1): 3516,
3138, 3015, 1699, 1612, 1556, 1518, 1462, 1244, 1170, 1091,
968, 949, 777, 717, 653.
containing
[Co(hfipbb)(1,2-bix)]n·nH2O (2a), [Co(ADA)(1,2-bix)]n (3a),
[Co(ADA)(1,3-bix)]n·nH2O (3b), [Co(2-pztz)(1,4-
compounds:
[Co(1,2-pda)(1,2-bix)]n
(1a),
pda)2]2[Co(H2O)6]·2nH2O (4a) and [Co2(μ-OH)(1,3-pda)(4-ptz)]n
(4b). In order to understand the affect of linker coordination
angle (LCA), the above-mentioned compounds (present work)
are compared with three other compounds reported earlier
by us,8 namely [Co(1,4-pda)(1,4-bix)]n (1b) [Co(hfipbb)(1,4-
bix)0.5]n (2b) and [Co2 (μ-OH)(1,4-pda)(4-ptz)]n·nH2O (4c). In
the present study, the mixed coordination networks are
broadly classified into four different classes based on the
geometry of their skeleton, as (flexible, flexible), (bent, flexi-
ble), (bent flexible, flexible) and (rigid, flexible); the first
term in the bracket corresponds to the carboxylate linker
and the second term to the secondary N-donor linker. Addi-
tionally, the magnetic exchange interactions between the
metal centers and single ion anisotropy of the compounds
are characterized through variable temperature magnetic
susceptibility measurements and the relevant results have
been described.
Synthesis of {Co(ADA)( 1,2-bix)}n (3a)
A mixture of CoCl2·6H2O (0.25 mmol, 59.5 mg), H2ADA
(0.25 mmol, 63.3 mg) and 1,2-bix (0.25 mmol, 60.0 mg) in
H2O : MeOH (10 : 1) was stirred for 30 min and sealed in a
25 mL Teflon-lined stainless steel autoclave (pH = 7.00). The
resulting reaction mixture was heated at 130 °C for 4 days,
and then slowly cooled to room temperature. Blue crystals of
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3a were obtained in a 60.3% yield (based on Co). Anal. calcd
for C28H30CoN4O4 (Mr = 545.49): C, 61.65%; H, 5.54%; N,
10.27%. Found: C, 61.49%; H, 5.35%; N, 10.42%. IR (KBr
pellet, cm−1): 3408, 3128, 3101, 2962, 2904, 2838, 1672, 1572,
1528, 1446, 1402, 1254, 1101, 953, 750, 723, 673.
graphite monochromated X-ray beam with a crystal to detector
distance of 60 mm, and a collimator of 0.5 mm. The scans were
recorded with an ω scan width of 0.3°. Data reduction was
performed by SAINTPLUS.18b Intensities were corrected for
absorption using SADABS.18c The structures were solved using
SHELXS-9718d and full-matrix least-squares refinements were
performed using SHELXL-97.18e All the non-hydrogen atoms
were refined anisotropically. Hydrogen atoms on the C atoms
were introduced in calculated positions and were included in
the refinement riding on their respective parent atoms. Crystal
data and structure refinement parameters for all the com-
pounds are summarized in Table 1. A complete list of bond
lengths and bond angles are given in the section of ESI.†
Synthesis of {Co(ada)( 1,3-bix)}n (3b)
The ligand 1,2-bix was replaced by ligand 1,3-bix (0.25 mmol,
60.0 mg) and allowed to react with CoCl2·6H2O (0.25 mmol,
59.5 mg) and H2ADA (0.25 mmol, 63.3 mg) to obtain the com-
pound 3b and blue crystals were filtered off in a 40% yield
(based on Co). Anal. calcd for C28H34CoN4O5 (Mr = 565.52): C,
59.47%; H, 6.060%; N, 9.907%. Found: C, 59.23%; H, 6.11%;
N, 9.76%. IR (KBr pellet, cm−1): 3452, 3397, 3150, 3095, 2838,
1649, 1545, 1523, 1446, 1419, 1358, 1232, 1150, 1112, 942,
816, 739, 657.
Results and discussion
Synthesis
In order to exploit the role of the linker coordination angles
(LCAs) in the coordination polymers, based on flexible
linkers, we chose the following linkers: phenylenediacetates,
isomeric bis(imidazole-1-ylmethyl)-benzene linkers, bent flex-
ible carboxylate linker 1,3-adamantane diacetic acid, bent
carboxylate linker H2hfipbb and rigid tetrazoles. The title
compounds were synthesized under hydrothermal conditions
and the reaction procedures were optimized to obtain the
best yields. CoCl2·6H2O was taken as the metal source in the
synthesis of the compounds due to its versatility in forming
octahedral, square pyramidal and tetrahedral coordination
geometries. The syntheses of the compounds were performed
at 160 °C for 1a and 2a, and 130 °C for 3a, 3b, 4a and 4b in
an aqueous medium.
Synthesis of [Co(2-pztz)(1,4-pda)2]2[Co(H2O)6]·2nH2O (4a)
A mixture of CoCl2·6H2O (0.50 mmol, 119.0 mg), 1,4-pda
(0.25 mmol, 48.5 mg) and 2-pztz (0.25 mmol, 37 mg),
dissolved in H2O (10.0 mL), was stirred for 30 min and the
pH of the reaction mixture was adjusted to 5.4 by adding 5 M
NaOH solution. The reaction mixture was placed in a 23 mL
Teflon-lined stainless steel autoclave, sealed and heated at
130 °C for 96 h. The autoclave was allowed to cool to 30 °C
for 48 h. Deep red block-shaped crystals of compound 4a
were obtained in a 65.5% yield (based on Co). Anal. calcd for
C30H38Co3N12O16 (Mr = 999.51): C, 36.05%; H, 3.83%; N, 16.81%.
Found: C, 35.15%; H, 3.24%; N, 16.69%. IR (KBr pellet, cm−1):
3362, 3026, 2926, 1684, 1616, 1574, 1523, 1410, 1253, 1170,
972, 935, 842, 746.
Linker coordination angle (LCA)
In ditopic linkers, the coordination angle between the posi-
tions of two coordinating groups in a linker is called the
linker coordination angle. For a simple rigid phenyl linker,
three different linker coordination angles (i.e. 60°, 120°, and
180°) are possible between the two coordinating groups. Dif-
ferent terminologies are used to represent the position of the
coordinating groups in the linker such as, ortho, meta, para
and (1,4)-, (1,3)-, and (1,2)- etc. In the case of designing coor-
dination architectures with mixed linkers, the LCAs of both
the linkers should be considered in obtaining the desired
topologies. The task becomes more difficult in the case of
flexible ditopic linkers, as they have more degrees of freedom.
To reduce the complexity in measuring the linker coordi-
nation angle for flexible linkers, we have considered the
angle between the two atoms of the linker where the coordi-
nating groups are connected, for example, the LCA of 1,3-pda
is measured as the angle between two CH2 groups of the
acetate side chains with respect to the benzene ring,
which is 120°. An important point to remember in quanti-
fying the LCA is that it is different from the dihedral angle
between the two coordinating groups and the angle
between two coordinating atoms. The LCAs of the ligands
phenylenediacetates, bis(imidazole-1-ylmethyl)-benzene, bent
Synthesis of [Co2(μ-OH)(1,3-pda)(4-ptz)]n (4b)
A mixture of CoCl2·6H2O (0.50 mmol, 119 mg), 1,3-pda
(0.25 mmol, 48.5 mg) and 4-ptz (0.25 mmol, 36.65 mg) was
dissolved in 10 mL distilled H2O which was adjusted to pH = 4.20
with 5 M NaOH solution. The resulting final reaction mixture
was sealed in a 23 mL Teflon-lined stainless steel autoclave
and heated at 130 °C for 3 days. Dark red block-shaped crystals
were obtained, filtered off and collected in a 55.2% yield. Anal.
calcd for C16H12Co2N5O5 (Mr = 472.17): C, 40.70%; H, 2.56%; N,
14.83%. Found: C, 40.15%; H, 2.02%; N, 14.09%. IR (KBr pellet,
cm−1): 3574, 1614, 1601, 1442, 1419, 1394, 1290, 1221, 974, 842,
760, 715, 630.
Single crystal X-ray structure determination of the
compounds
Crystal data of the compounds 1a, 3a, 3b, and 4b were col-
lected on an Oxford Xcalibur Gemini Eos CCD diffractometer
at 298 K using Mo-Kα radiation (λ = 0.7107 Å). Data reduction
was performed using CrysAlisPro (version 1.171.33.55)18a and
SHELXL-97 was used to refine the structures. The compounds
2a and 4a were mounted on a three circle Bruker SMART-APEX
CCD area detector system under a Mo-Kα (λ = 0.71073 Å)
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Table 1 Crystal data and structural refinement parameters for compounds
1a
2a
3a
Empirical formula
Formula weight
T (K)/λ (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C24H22CoN4O4
489.39
298(2), 0.71073
Monoclinic
P21/c
8.5118(7)
22.703(2)
11.3906(8)
90.00
C31H22N4F6O5Co
703.46
298(2), 0.71073
Orthorhombic
Ccca
13.177(8)
15.668(8)
18.950 (16)
90.00
C28H30CoN4O4
545.49
298(2), 0.71073
Monoclinic
P2(1)/c
11.5867(3)
12.6968(4)
17.5141(5)
90.00
β (°)
105.401(7)
90.00
2122.1(3)
4, 1.532
0.850/1012
1.034
0.0535/0.0934
0.0875/0.1061
0.401/−0.313
90.00
90.00
5977(5)
8, 1.563
0.660/2856
1.248
0.0687/0.1377
0.0867/0.1440
0.618/−0.307
96.216(3)
90.00
2561.42(13)
4, 1.415
0.712/1140
1.056
0.0398/0.1043
0.0529/0.1123
0.627/−0.227
γ (°)
Volume (Å3)
Z, ρcalcd (g cm−3
)
μ (mm−1), F(000)
Goodness-of-fit on F2
R1/ wR2 [I > 2σ(I)]
R1/wR2 (all data)
Largest diff peak/hole (e Å−3
)
3b
4a
4b
Empirical formula
Formula weight
T (K)/λ (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C28H34CoN4O5
565.52
298(2), 0.71073
Monoclinic
P2(1)/n
13.1976(9)
10.4914(7)
20.3760(15)
90.00
108.808(8)
90.00
C30H38Co3N12O16
999.51
C16H13N5O5Co2
473.17
298(2), 0.71073
Monoclinic
P21/n
8.9732(4)
20.4712(9)
10.4084(5)
90.00
102.998(5)
90.00
1862.95(15)
4, 1.687
1.819, 948
1.109
0.0448/0.1063
0.0565/0.1120
0.879/−0.302
298(2), 0.71073
Triclinic
¯
P1
9.3717(8)
10.1843(9)
11.7377(11)
112.9560(10)
106.240(2)
90.6960(10)
967.32(15)
1, 1.716
1.360, 511
1.027
0.0322/0.0767
0.0385/0.0802
0.357/−0.229
γ (°)
Volume (Å3)
2670.6(3)
4, 1.407
Z, ρcalcd (g cm−3
)
μ (mm−1), F(000)
0.688, 1188
1.024
0.0440/0.1027
0.0653/0.1166
0.422/−0.376
Goodness-of-fit on F2
R1/wR2 [I > 2σ(I)]
R1/wR2 (all data)
Largest diff peak/hole (e Å−3
)
carboxylate ligands H2hfipbb and H2ADA and rigid tetrazoles
employed in this work are shown in the following Scheme 1
and Table 2
Description of crystal structures
(Flexible, flexible)
In the case of tetrazole ligands, as shown in Scheme 1,
the LCA is measured between the tetrazole core ring and
the position of the nitrogen atom in the pyridine ring. The
LCAs mentioned in the above linkers are considered in
their ideal geometry but in actual case, these angles will
[Co(1,2-pda)(1,2-bix)]n (1a) and [Co(1,4-pda)(1,4-bix)]n (1b).
Compound 1b is
structural details have been previously reported,8 whereas,
compound 1a is 2D coordination polymer containing
a 3D coordination polymer and the
a
compound, that crystallizes in the monoclinic space group P21/c.
The asymmetric unit consists of one crystallographically
independent Co(II) atom in a distorted tetrahedral geometry,
one 1,2-pda2− ligand and one 1,2-bix ligand. The distortion
parameter τ4 (= 0.87), calculated by the four-coordinate geome-
try index (τ4 = {360° − (α + β)}/141, where α and β are the two
largest angles of the tetrahedra), indicates slight distortion in
its tetrahedral geometry.18f The tetrahedral geometry around
the Co(II) atom is comprised of two oxygen atoms from two dif-
ferent 1,2-pda2− ligands and two nitrogen atoms from two dif-
ferent 1,2-bix ligands (Fig. 1a). The carboxylate linker 1,2-pda2−
coordinates to the Co(II) center in μ1-η1,η0 coordination mode
on either side and connects the adjacent Co(II) tetrahedra in a
typical trans conformation with an anticlinal torsion angle of
vary to
10° based on the coordination consequences
adopted by the linkers, e.g., hfipbb2− will have LCAs of
111°, 108° etc., ADA2− has LCAs of 118°, 122° etc. In our
previous article, we reported the factors affecting the con-
formational modulation of flexible linkers in the self-
assembly of coordination networks,8 in which only a few
flexible linkers were used to explain these factors, but
in order to rationalize this concept of flexibility in terms
of coordination and dimensionality, an elaborated study
is required. In view of this, in this article, we have dem-
onstrated
a library of coordination networks exploring
the possible flexible linkers with rigid and bent secondary
ligands.
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adopt a cis conformation to form a molecular box, but in this
case, 1,2-bix adopts a trans conformation, thereby forming 2D
layers. The presence of flexibility in the ligands employed,
results in the formation of a wave like layer with crests and
troughs at the metal acid chains. These layers are stacked
along the c axis in an ABAB fashion and the adjacent layers are
tilted to ~7 Å and the alternate layers are arranged in a back-to-
back fashion as shown in Fig. 1d.
(Bent, flexible)
[Co(hfipbb)(1,2-bix)]n·nH2O (2a) and [Co(hfipbb)(1,4-bix)0.5 n
]
(2b). Compound 2a is a 2D extended coordination polymer
crystallizing in the orthorhombic space group ccca. Compound
2b is a 3D layered pillared framework, as described in our
previous report,8 in which the 2D metal–hfipbb layers are
pillared by the 1,4-bix linkers. The structural details and
crystallographic details of 2b have been reported; herein we
have discussed only compound 2a. The molecular diagram of
2b consists of the Co(II) atom in a slightly distorted tetrahedral
{CoN2O4} geometry with a τ4 of 0.93, constituted by the oxygen
atoms from two different hfipbb2− units,19 and nitrogen atoms
from two different 1,2-bix20 ligands and one lattice water
molecule as shown in the Fig. 2a. The bond distances of Co–O
and Co–N are in the range 1.957(3)–2.023(3) Å and the bond
angles around the Co(II) centre are in the range 100.17°–
112.32°. The carboxylate groups in H2hfipbb are completely
deprotonated and each carboxylate group in hfipbb2−
coordinates to the metal center in μ1-η1,η0 coordination mode
by creating a separation of 13.17 Å between the two metal
centers. The dihedral angle between the two benzene rings in
the hfipbb2− anion is 74.77°, which is higher than that of
68.57° in the neutral H2hfipbb ligand. The connectivity of
hfipbb2− with the metal centers results in the formation of 1D
metal acid zig-zag chains along the crystallographic b axis as
shown in Fig. 2b. The metal–hfipbb systems are very well
known in the literature for the formation of paddle-wheel
structures having open coordination sites at the metal apical
position, which is used to extend the dimensionality by the N-
donor ligands.14 On the other hand, in the crystal structure of
compound 1, metal–hfipbb forms 1D chains, that are extended
to form 2D (4,4) connected sheets with the aid of 1,2-bix
ligands.15 The 1,2-bix ligand connects the Co(II) atoms of the
adjacent 1D chains in a typical trans conformation with an
antiperiplanar torsion angle of 151.84° (viewed through N2–
C13–C13–N2) and creates a separation of 10.236 Å between the
Scheme 1 Schematic representation of linker coordination angles of
the flexible linkers employed in this study.
132.64° (viewed through N4–C24–C14–N2). The acetate side
chains twist with respect to the phenyl ring through various
extents i.e. 101.46° (through C4–C3–C2–C1) and 96.92°
(through C7–C8–C9–C10). 1,2-pda2− separates the two Co(II)
centers with a distance of 8.512 Å to form a 1D metal–acid
chain along the crystallographic a axis (Fig. 1b). These adjacent
chains are connected to each other with the aid of 1,2-bix
ligands along the b axis to form a 2D layer in the ab plane.
1,2-Bix connects the Co(II) atoms of the adjacent chains in a
typical trans conformation with an antiperiplanar torsion angle
of 151.22° (viewed through N4–C24–C14–N2) and creates sepa-
ration of 12.114 Å between the two metal centers in the adja-
cent chains. The dihedral angles between the imidazole ring
planes and the least-squares plane of the phenyl group are
74.01° and 77.50°, respectively. The connectivity of the 1,2-bix
to the metal acid chains results in the formation of 2D (4,4)
connected sheets with dimensions of 12.114 × 8.512 Å2
(Fig. 1c). The linker coordination angle between the two imid-
azole rings in 1,2-bix is 60°, and due to the shorter distance
between the two coordinating N atoms it has a tendency to
Table 2 Linker coordination angles of various linkers employed in this work (see Scheme 1)
Linker name
1,4-
1,3-
1,2-
Flexible
pda
bix
H2ADA
Rigid
ptz
1,4-pda = 180° (linear)
1,4-bix = 180° (linear)
120° (bent flexible)
1,3-pda = 120° (bent)
1,3-bix = 120° (bent)
1,2-pda = 60° (chelated)
1,2-bix = 60° (chelated)
4-ptz = 180° (linear)
120° (bent)
3-ptz = 120° (bent)
2-ptz = 60° (chelated) 2-pztz = 60°,120° (chelated, bent)
H2hfipbb
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Fig. 1 (a) Coordination environment around the CoII ion in 1a with hydrogen atoms omitted for clarity. (b) 1D zig-zag chains formed due to
Co-1,2-pda connectivity. (c) The overall 2D (4,4) connected sheets. (d) 3D stacking of 2D layers showing the tilting between two layers and its
topological representation.
two adjacent chains. The overall connectivity of the 1,2-bix
ligands with the metal acid chains results in the formation of
(4,4) corrugated sheets with dimensions of 13.177 × 10.236 Å2
(Fig. 2c). The CF3 groups, present on the hfipbb2−, decorate the
surface of the 2D layers. The corrugated nature of the 2D layers
allows the two layers to stack over each other to form a double
layer with CF3 groups above and below the double layer
(Fig. 2d). These double layers are again stacked over each other
resulting in the formation of two different types of cavities i.e.,
fluorinated cavities, that are formed due to stacking of the dou-
ble layers and non fluorinated cavities, formed due to the
stacking of the monolayers as shown in Fig. 2e. Two solvent
molecules per cavity are present in the crystal structure and
located exactly on the plane of the monolayer; as a result they
are present in the nonflourinated cavity as shown in Fig. 2e.
3a. Although compound 3a forms a 2D grid-like layer struc-
ture, in this compound, Co(II) has a slightly distorted tetrahe-
dral geometry instead of octahedral geometry with
a
distortion parameter τ4 of 0.78. Each Co(II) ion coordinates
with two oxygen atoms from two different ADA2− ligands and
two N atoms from two 1,2-bix ligands (Fig. 3a). The bond dis-
tances of Co–O and Co–N are in the range 1.964(2)–2.057(6) Å
and the bond angles around the Co(II) centre are in the range
95.75°–106.30°. Both carboxylate groups of the ADA2− ligand
adopt the μ1-η1:η0 coordination mode and each carboxylate
group connects one Co(II) atom in a mono-dentate fashion.
The adjacent Co(II) atoms are connected by the ADA2− ligand
in cis conformation with a torsion angle of 10.55° (viewed
through C1–C2–C14–C13) creating a Co⋯Co separation of
6.925 Å affording a 1D chain through the b axis. Interestingly,
these chains have alternating right and left turns along the
pitch of the ADA2− resulting in the formation of a 1D zig-zag
structure (Fig. 3b). The Co(II) centres in these chains are
again connected by 1,2-bix along the c axis creating a separa-
tion of 13.648 Å. The 1,2-bix linker connects the Co(II) centers
(Bent flexible, flexible)
{Co(ADA)(1,2-bix)}n (3a). The bent flexible linker H2ADA
reacts with the Co(II)-1,2-bix system, resulting in compound
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Fig. 2 (a) Molecular diagram of compound 2a with atom labeling at the coordination sphere and (b) 1D zig-zag chains formed due to Co–hfipbb
connectivity. (c) 2D (4,4) connected topology viewing the square grid. (d) Stacking of two monolayers to form a double layer. (e) 3D stacking of
double layers showing the two different types of cavities.
in a typical cis conformation in which the dihedral angle
between the two imidazole rings is 30.39° and the dihedral
angles between the imidazole rings and benzene rings are
75.31° and 75.21°, respectively. This associated connection of
ADA2− and 1,2-bix with the Co(II) centre results in a 2D grid
like layer structure along the crystallographic bc plane. Due
to the 1D zig-zag pattern of the Co(II)–ADA2− chains, the par-
allelograms formed by the connectivity of 1,2-bix ligands
result in two different dimensions (to be named A and B) as
shown in Fig. 3c. Parallelogram A has a larger area with
dimensions of 18.265 × 11.612 Å2 and B has a smaller area
with dimensions of 20.016 × 8.234 Å. These parallelograms
are arranged in an alternate ABAB fashion in both the
directions along the Co(II)–ADA2− zig-zag chains and Co(II)-
1,2-bix chains. These 2D layers perfectly stack on each other
in an eclipsed manner to arrange the parallelograms to form
1D channels (Fig. 3d). In addition, intermolecular hydrogen
bonding was observed between the C–H groups of the ADA2−
linkers of one layer and carboxylate O atoms of the ADA2−
linkers of another layer through C8–H8⋯O1 (with symmetry
code 2 − x, 0.5 + y, 0.5 − z) with a bond distance of 3.622 Å,
along with some weak C–H⋯N interactions between the C–H
groups of the ADA2− ligands and N atoms of the imidazole
rings. Hence, the two dimensional layers are connected to
each other through C–H⋯O and C–H⋯N interactions to
form a 3D supramolecular framework.
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Fig. 3 (a) The immediate coordination environment of the Co(II) metal center in compound 3a. (b) 1D zig-zag chains formed due to Co–ADA
connectivity. (c) 2D (4,4) connected topology viewing the square grid with two different types of the parallelograms. (d) 3D eclipsed stacking of layers.
{Co(ADA)(1,3-bix)}n·nH2O (3b). Compound 3b is obtained
by replacing the ligand 1,2-bix (LCA 60.0°) with 1,3-bix (LCA
120.0°). The molecular diagram consists of the Co(II) ion in
a distorted tetrahedral geometry with two O atoms from
two different ADA2− ligands, two N atoms from two different
1,3-bix ligands and a lattice water molecule (Fig. 4a). The
distortion in the tetrahedral geometry around the Co(II)
centre, τ4, is 0.73 which is considerably larger compared to
the τ4 values determined for compounds 1a, 2a and 3a. The
Co(II)–O bond lengths are in the range 2.052–2.158 Å, and
Co(II)–N bond lengths are in the range 2.062–2.082 Å, which
are in good accordance with the literature. The bond angles
around the Co(II) centre are in the range 91.01–153.1°
indicating severe distortion in the tetrahedral geometry
around the metal centre. In this compound, both carboxylate
groups of the acetate side chains in ADA2− adopt the μ1-η1:η0
coordination mode and coordinate to the Co(II) centers by
creating a separation of 10.19 Å. The acetate side chains in
the ADA2− linker adopt a trans conformation with a torsion
angle of 122.02° (viewed through C15–C16–C27–C28). The
connectivity of ADA2− with the Co(II) centers results in the
formation of 1D chains. In these Co(II)–ADA2− chains, all the
adamantane rings are located on the same side of the axis
unlike the zig-zag pattern observed in 3a (Fig. 4b). These 1D
chains are connected by the 1,3-bix linker by coordination of
the imidazole nitrogen atoms to the Co(II) centers to form 2D
sheets. The ligand 1,3-bix is a 1,3-positioned ligand with an
LCA of 120.0°, which exists in a trans conformation with a
torsion angle of 117.38 Å and creates a separation of 11.14 Å
between the two Co(II) centers of two adjacent Co(II)–ADA2−
chains. The overall connectivity of the Co(II) centers with the
ADA2− and 1,3-bix ligands results in the formation of 2D (4,4)
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Fig. 4 (a) Molecular diagram of compound 3b describing the coordination sphere. (b) 1D zig-zag chains formed due to Co–ADA connectivity.
(c) 2D (4,4) connected topology viewing the square grid. (d) Figure viewing the eclipsed stacking of monolayers containing R44(10) hydrogen
bonding rings with the lattice water molecules in the channels and the empty channels.
connected corrugated sheets rather than planar sheets
(Fig. 4c). The flexibility, present in the ligand 1,3-bix, imparts
diagonal connectivity between the two Co(II) centres of adja-
cent chains instead of linear connectivity; as a result, the 1D
Co(II)–ADA2− chains are arranged in a wavy ABAB fashion
rather than a planar AAAA fashion. One lattice water molecule
per molecular formula unit is present in the crystal structure.
These lattice water molecules are present exactly on the plane
of the sheets and located in the void spaces created by the
(4,4) connectivity of the metal centers. Interestingly, two lat-
tice water molecules per parallelogram are arranged in alter-
nate parallelograms as shown in Fig. 4d. These lattice water
molecules form strong hydrogen bonding interactions
between the carboxylate oxygens of the same sheets forming
ten membered R44(10) rings inside the parallelograms alterna-
tively. However no hydrogen bonding is observed between the
adjacent sheets through lattice water molecules.
(Flexible, rigid)
[Co(1,4-pda)2(2-pztz)]2[Co(H2O)6]·2nH2O (4a). Compound 4a
is a 1D ion pair compound that crystallizes in the triclinic space
¯
group P1. The molecular diagram consists of a 1D anionic
chain [Co(2-pztz)(1,4-pda)2]2− and lattice cationic [Co(H2O)6]2+
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units. The repeating unit in the 1D chain consists of two
crystallographically independent Co(II) atoms in tbp geometry
bridged by two 2-pztz1− ligands21 and connected to four 1,4-pda2
center in a chelating mode and the resulting Co(2-pztz)2+ com-
plex cations are in turn connected by the nitrogen atom N4
(adjacent to nitrogen atom N3) of the tetrazole ring to form a
six membered dimer ring with a Co–Co separation of 4.179 Å.
The overall connectivity of the 2-pztz1− ligands to the metal cen-
ters results in the formation of Co-dimer {Co2(2-pztz)2} rings
as shown in Fig. 5b. The 2-pztz1− in the crystal structure
exhibits a μ3 coordination mode (μ2 from tetrazole ring and μ1
−
ligands (Fig. 5a). The tbp geometry of each Co(II) ion in the
anionic chain is furnished by three nitrogen atoms from two dif-
ferent 2-pztz1− ligands in the basal plane and two 1,4-pda2−
ligands in the apical positions. The pyrazine nitrogen atom N1
and tetrazole nitrogen atom N3 of 2-pztz coordinate to the Co(II)
Fig. 5 (a) ORTEP view of the basic unit of 4a. Hydrogen atoms of carbon atoms have been removed for clarity. Thermal ellipsoids are at the
30% probability level. (b) Co-dimer ring formed due to chelated connectivity of 2-pztz1−. (c) Molecular loop formed due to cis connectivity of the
1,4-pda2− ligand. (d) 1D extended chain formed due to connectivity of 2-pztz and 1,4-pda ligands (e) 2D packing diagram illustrating the position
of cationic species in the cavities.
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from pyrazine ring) and the other nitrogen atom N2 in the pyr-
azine ring remains uncoordinated. The {Co2(2-pztz)2} dimer
connects to other dimers through two pairs of 1,4-pda2− ligands
in a typical cis conformation. Each 1,4-pda2− coordinates to the
metal center through the μ1 coordination mode and the acetate
side chains in the skeleton are twisted with respect to each
other through a synclinal torsion angle of 49.06 Å (viewed
through C11–C12–C17–C18) indicating a typical cis conforma-
tion. The presence of 1,4-pda2− in the cis conformation results
in the formation of {Co2(1,4-pda)2} molecular loops as shown in
Fig. 5c. Overall, the connectivity of these loops with the dimers
results in the formation of a 1D extended chain along the crys-
tallographic c axis as shown in Fig. 5d. The hexa aqua cobalt(II)
cations compensate the charge of the anionic chains by staying
as a lattice component. From the crystal packing diagram,
[Co(H2O)6]2+ cations are located between the two adjacent
chains along the plane and nearly sandwiched between the two
{Co2(1,4-pda)2} loops along with the lattice water molecules
(Fig. 5e). As anticipated, due to the presence of lattice water mol-
ecules and coordinated water molecules in the hexa aqua cat-
ionic complex, strong hydrogen bonding was expected. The four
coordinated water molecules in each [Co(H2O)6]2+ cation are
hydrogen bonded to carboxylate oxygen atoms of 1,4-pda2− moi-
eties of four anionic 1D chains and two lattice water molecules
through O–H⋯O interactions.
([Co2(μ-OH)(1,3-pda)(4-ptz)]n (4b) and [Co2(μ-OH)(1,4-pda)
(4-ptz)]n·nH2O (4c). Both compounds crystallize in the monoclinic
space group P21/n. The compounds 4b and 4c are isostructural
except the carboxylate ligand 1,3-pda in 4b is replaced by 1,4-pda
in 4c and a lattice water molecule is present in compound 4c,
which is absent in compound 4b. The structural description of
compound 4c was elucidated in our previous report,8 herein,
we describe the structural description of 4b and the structural
consequences obtained by replacing 1,4-H2pda with 1,3-H2pda
in terms of torsion angles and length of the ligand, etc. The
asymmetric unit of compound 4b consists of two Co(II) atoms
in tbp geometry connected by the μ2-hydroxy group and one
1,3-pda2− and 4-ptz1− anion (Fig. 6a). Both the acetate groups in
the 1,3-pda2− anion adopt μ2-η1,η1 coordination modes in a
typical cis conformation with a synclinal torsion angle of
29.84° (viewed through C8–C7–C9–C10), to form a metallo
macrocycle, whereas the torsion angle exhibited by the acetate
side chains of 1,4-pda2− in compound 4b is perfectly cis with a
synclinal torsion angle of 2.30 Å. The major structural changes
observed in the metallocycles are formed due to the cis connec-
tivity of the carboxylate ligands, i.e. a 22-membered macrocycle
with dimensions of 7.531 × 8.109 Å2 was observed in the case
of 1,4-pda2− whereas a 20-membered macrocycle with dimen-
sions of 8.322 × 7.946 Å2 was formed in the case of the 1,3-pda2−
associated compound. Also, the benzene rings of pda2− in the
metallocycle formed in compound 4c are faced parallel to each
other due to 1,4-connectivity in the pda2− ligand, but the ben-
zene rings are not faced parallel to each other in compound 4b
due to 1,3-connectivity in the pda2− ligand (Fig. 6b). The 20
membered macrocycle, formed in 4b extends its dimensional-
ity with four other macrocycles through the bridging hydroxyl
group (μ2-OH) to form a 2D network (Fig. 6c). The co-ligand ptz
in μ4 coordination mode (μ3 from the tetrazole ring and μ1
from the pyridine ring of the ptz) again connects this 2D net-
work. The connectivity of the tetrazole to the Co(II) centres
results in the formation of a 1D chain in which the adjacent
Co(II) ions are bridged by the nitrogen atoms of the tetrazole
ring and μ2-OH groups. The connectivity of pda2− in a cis con-
formation results in the formation of macrocycles which
extend their dimensionality by bridging with μ2-OH groups and
the tetrazole ring to form 2D layers as shown in Fig. 6d.
Role of linker coordination angle
Scheme 1 displays the possible LCAs of the phenylenediacetates,
isomeric bis(imidazole-1-ylmethyl)-benzene linkers, bent carbox-
ylate ligand hfipbb2−, bent flexible carboxylate linker ADA2− and
rigid tetrazoles. The combination of these linkers in construct-
ing mixed linker coordination networks can often lead to
intriguing dimensionalities. The compounds discussed in this
study are present under four different classes to demonstrate
the rational affect of the linker coordination angle.
(Flexible, flexible)
In this section, the two compounds 1a and 1b were synthe-
sized with a mixed (flexible, flexible) linkers system. As
shown in Scheme 2, the connectivity of 1,2-pda and 1,2-bix
results in the formation of 2D layers in compound 1a and the
connectivity of 1,4-pda and 1,4-bix results in the formation of
a 3D framework in compound 1b. In both compounds, the
Co(II) ion is present in the same {CoN2O4} tetrahedral geome-
try, but the linker coordination angle (LCA) changes the
dimensionality of the compounds. In compound 1a, the LCAs
of both the ligands are 60° whereas, the LCAs of the ligands
in compound 1b are 180°. The difference in these LCAs
changes the length of the linker, resulting in changes in the
annex of the linker. The linker with the lowest LCA generally
creates the minimum separation between the metal polyhe-
dra. The separation created by the 1,2-pda and 1,2-bix ligands
in compound 1a is 8.512 Å and 12.114 Å, respectively, while
the separation created by the 1,4-pda and 1,4-bix ligands in
compound 1b is 12.80 Å and 15.09 Å. Due to the longer
length, the ligands protrude outside the layer to form a 3D
framework, whereas, the short linkers connect to the metal
centres in the same layer.
In both compounds, the carboxylate ligands (1,2-pda and
1,4-pda) and N-donor secondary ligands (1,2-bix and 1,4-bix)
adopt a trans conformation. The flexibility of the linkers is
one of the reasons of why the longer ones protrude outside
the 2D plane to form a 3D structure (1b) and the shorter ones
remain in the same plane (1a). Compounds 1a and 1b are
mixed linker coordination networks in which both the linkers
exhibit the same LCA values, i.e. either 60° and 60° in 1a or
180° and 180° in 1b. Perhaps in this system, the lower LCAs
form 2D structures and the higher LCAs form 3D structures
and the linkers with mixed LCAs form either 2D or 3D only.
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Fig. 6 (a) ORTEP view of the basic unit of 4b. Hydrogen atoms of carbon atoms have been removed for clarity. Thermal ellipsoids are at the 40% probability
level. (b) Molecular box formed due to the cis conformation of the pda2− ligand (left for 1,3-pda2− in 4b and right for 1,4-pda2− in 4c). (c) Four connectivity
extension of molecular boxes through the μ2-OH group. (d) Overall 2D network of compound 4b (phenylene rings of 1,3-pda2− are omitted for clarity).
(Bent, flexible)
in Scheme 3, the compounds 2a and 2b are constituted by a
Co–hfipbb-(1,n)-bix (n = 2,4) composition matrix. Compound 2a
is a 2D structure and 2b is a 3D layered-pillared framework.
Both the compounds contain metal–hfipbb architectures which
In this section, the two compounds 2a and 2b with a
(bent, flexible) mixed linker system are discussed. As shown
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(Bent flexible, flexible)
In this section, we tried to rationalize the effect of LCA
on the (bent flexible, flexible) mixed linker system. We chose
the 1,3-positioned adamantane diacetate linker with a LCA
of 120° which also includes flexibility in its geometry. The
associated connectivity of ADA2− with flexible bix linkers
results in interesting topologies which differ from the
expected ones. Both the compounds are 4,4 connected 2D
layers but the topology in both cases are different from each
other. In compound 3a, the 1,2-bix linker with a LCA of 60°
exists in the cis conformation and creates a large separation
of 13.648 Å between the two metal centers, but interestingly
the 1,3-bix linker with a LCA of 120° exists in the trans con-
formation and creates a separation of 11.146 Å between the
two Co(II) centers in 3b. Usually 1,3-bix in a trans conforma-
tion creates a larger separation than 1,2-bix in the cis con-
formation, but the opposite is observed in the case of
compounds 3a and 3b. The main advantage of flexible
linkers is that they can modulate their length according to
the available coordination configurations by modulating
their conformations. In this case, the coordination restric-
tions, imposed by the ADA2− in compounds 3a and 3b, dic-
tates the geometrical parameters of the bix linkers. In the
case of compound 3a, ADA2− forms a 1D zig-zag chain with
left and right turns, whereas ADA2− forms a 1D linear chain
in 3b. The connectivity of these 1D chains with the bix linker
forms different topologies; in the case of 1,2-bix, two differ-
ent parallelograms are formed in the 2D layer and in the
case of 1,3-bix, parallelograms with unique dimensions are
formed in the 2D layer. This difference in the skeleton of the
2D sheets can be correlated with the geometrical parameters
exposed by the bix linkers (Scheme 4).
Scheme 2 Affect of dimensionality by changing the linker coordination
angle of the isomeric bis(imidazole-1-ylmethyl)-benzene (bix) ligands
and phenylene diacetate (pda) ligands.
Scheme 3 Affect of dimensionality by changing the linker coordination
angle of the isomeric bis(imidazole-1-ylmethyl)-benzene (bix) ligands
with the metal–hfipbb system.
are extended by bix linkers. In compound 2a, metal–hfipbb
forms 1D chains, which are connected by 1,2-bix ligands to
form 2D layers whereas in 2b, metal–hfipbb forms 2D helical
double layers which are connected by 1,4-bix to form a 3D
layered-pillared framework. The linker coordination angles of
1,2-bix and 1,4-bix are 60° and 180°, respectively; as a result,
the length of the ligands in the crystal structures are 7.75 Å
and 11.21 Å, respectively. In both compounds, the bix linker
adopts a trans conformation with variable torsion angles
(151.84° for 2a, and 180° for 2b). The longer 1,4-bix favors
the formation of metal–carboxylate paddlewheel layers in com-
pound 2b, since its length can separate these layers with a
definite distance to reduce the intermolecular repulsions.
However the shorter 1,2-bix does not favor the formation of
paddlewheels, as a result the metal–hfipbb forms 1D chains.
In our previous report,11a the steric hindrance created by a
secondary linker at the coordination sphere for the formation
of paddlewheels was studied. From these two examples, it can
be correlated that the formation of paddlewheels would be
more favorable with longer secondary linkers. In fact, this
observation is in accordance with other systems with longer
linkers, e.g., 1,2-dpe, bpy as found in the literature.19
(Flexible, rigid)
In this section, compounds 4a, 4b and 4c in a (flexible, rigid)
mixed linker system are considered. The important parame-
ter in this discussion (for this type of mixed linker system) is
the rigid secondary linker. The flexible primary linker always
Scheme 4 Influence of LCA of secondary bix linker on modulating the
topology of 2D layers in isomeric bis(imidazole-1-ylmethyl)-benzene
(bix) ligands in the metal–ADA system.
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Table 3 The structural outcomes of all the compounds presented in
this study
adopts a cis conformation. As shown in Scheme 5, both com-
pounds 4b and 4c are isostructural, even though the LCAs of
the carboxylate linkers are 180° in 4b and 120° in 4c. In both
these compounds, the flexible carboxylate ligands adopt a cis
conformation which primarily decreases the dimensionality.
Usually cis conformations form low dimensional structures
whereas trans conformations form high dimensional struc-
tures. In this scenario, the other factor, i.e. the LCA of the
secondary linker, plays an important role in tuning the
dimensionality. In compounds 4a and 4b the secondary
linker is 4-ptz with a LCA of 180°; as a result, both these com-
pounds are 2D iso-structural compounds. Whereas in 4a,
when 1,4-pda adopts a cis conformation, a 1D chain is
formed, this is because the secondary ligand 2-tzpz has an
LCA of 60°. In the case of tetrazole linkers, the LCA was
measured between the tetrazole ring and the position of the
nitrogen atom in the pyridine ring. When the LCA is 60°,
the skeleton of the tetrazole resembles 2,2′-bipyridine and
adopts a chelated coordination mode thereby blocking the
two coordination sites of Co(II) centers, which results in the
lower dimensional compound 4a; the cis conformation of
1,4-pda also favors the formation of a 1D chain, as observed
in the case of 4a. We tried to rationalize this effect with
2-ptz with a LCA of 60°, but we were unable to synthesize a
2-ptz-1,4-pda assembled compound, so we chose 2-tzpz
which has two LCAs: 60° and 120°, and fortunately in the
crystal structure of 4a, 2-tzpz utilizes only the LCA of 60°
which helps us to elucidate the affect of the LCA. Overall, in
the mixed linker (flexible, rigid) system, where the flexible
linker adopts a cis conformation, the LCA of the secondary
linker decides the dimensionality of the final architectures.
Also from these three examples, the (flexible, rigid) system
always forms a 2D or 1D dimensional structure instead of
higher dimensionality 3D systems. The overall structural
outcomes are tabulated in Table 3.
LCA (acid,
N-donor)
Mixed linker
typea
Compound
Dimensionality
1a
1b
2a
2b
3a
3b
4a
4b
4c
60,60
F,F
F,F
2D
3D
2D
3D
2D
2D
1D
2D
2D
180,180
120,60
120,180
120,60
120,120
180,60
120,180
180,180
B,F
B,F
BF,R
BF,R
F,R
F,R
F,R
a
F = Flexible, B = Bent, BF = Bent flexible, R = Rigid.
PXRD and thermogravimetric analysis
To ensure the phase purity of the compounds, presented in
this study, powder X-ray diffraction (PXRD) patterns of the
powder samples were recorded. The diffraction patterns for
the simulated data (calculated from single crystal data)
matched well with the observed data which proves the bulk
homogeneity of the crystalline solids (see section 3 of ESI†for
the PXRD patterns). The experimental patterns have a few
un-indexed diffraction peaks and some are slightly broad-
ened and shifted in comparison to the simulated patterns
but still it can be regarded that the bulk as-synthesized mate-
rials represent the compounds.
TGA plots were recorded under flowing N2 for crystalline
samples of the title compounds in the temperature range
40–1000 °C (see relevant plots in section-4, ESI†). Compound
1a is stable up to 296 °C and undergoes a steep weight loss up
to 402 °C followed by a continuous weight loss up to 1000 °C
with a residual mass of 29.3%. Compound 2a loses one crys-
talline water with a weight loss of 2.63% (theoretical weight
loss 2.5%) in the temperature region of 40 °C to 115 °C and
the network remains stable up to 295 °C. The dehydrated net-
work undergoes a sharp weight loss up to 455 °C and then
undergoes continuous weight loss up to 1000 °C. The
thermogravimetric curves of compounds 3a and 3b follow the
same pathway, as both compounds are stable up to 350 °C
followed by decomposition of the associated organic moieties
in two steps at 475 °C and 535 °C with gradual weight loss up
to 1000 °C. Compound 4a remains stable up to 100 °C and
loses six coordinated water molecules of [Co(H2O)6]2+ and two
solvent water molecules in the temperature region of 100 °C
to 225 °C with a weight loss of 14.9% (theoretical 14.4%); the
dehydrated compound remains stable up to 306 °C and
undergoes continuous weight loss up to 1000 °C. Compound
4b is thermally stable up to 350 °C and undergoes a weight
loss continuously in two steps at higher temperatures.
Electronic absorption properties
Solid state diffuse reflectance spectra for all compounds were
measured in the region 200–800 nm (the relevant spectra are
presented in section 5 of the ESI†). The absorption maxima
Scheme 5 Affect of dimensionality by changing the linker coordination
angle of the isomeric phenylenediacetate ligands with different rigid
tetrazole ligands.
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at higher wavelengths 547 nm, 574 nm (for 1a), 529 nm,
581 nm (for 2a), 529 nm, 590 nm (for 3a) 523 nm, 585 nm
(for 3b), 527 nm for 4a and 522 nm for 4b are tentatively
assigned to d–d transitions in the d7 Co(II) ion. The energy
bands, observed at lower wavelengths in their electronic
absorption spectra, are due to π–π* transitions from the phe-
nyl rings which are comparable with the transitions observed
in the free ligands.
The χM data in the temperature range 2–300 K was fitted by
the following expressions for S = 3/2 systems with dominant
zero field splitting effects, D,22 (eqn (1)–(4)):
χ = (Ng2β2/KT)[A/B]
(1)
∥
where A = [1 + 9exp(−2D/KT)] and B = [4(1 + exp(−2D/KT)]
χ = (Ng2β2/KT)[C/D]
(2)
⊥
where C = [4 + (3KT/D) (1 − exp(−2D/KT)] and D = [4(1 + exp(−2D/KT)].
Magnetic properties
Compound 2a
χM′ = [(χ + χ )/3]
(3)
(4)
∥
⊥
The magnetic properties of compound 2a, as plots of χMT
vs. T and χM vs. T are shown in Fig. 7a. The χMT value at
room temperature (300 K) is 2.40 cm3 K mol−1, which is
much higher than the expected value for an isolated CoII ion
(χMT = 1.875 cm3 K mol−1 for a S = 3/2 ion). The χMT value
decreases gradually to 1.31 cm3 K mol−1 at 2 K. 1/χM vs. T fol-
lows the Curie Weiss law in the temperature range 2–300 K
with a negative Weiss constant θ = −5.23 K and C = 2.403 cm3
K mol−1. The higher value of χMT indicates spin-orbit cou-
pling of the tetrahedral Co(II) ion, which can be estimated by
the zero-field splitting (ZFS) effects of a single-ion of Co(II).
χM = χM′/{1–χM′(2zJ′/Ng2β2)}
The parameters N, β and K have their normal meanings.
The best fit was obtained from 2–300 K with g = 2.27 (2)
D = −6.07 (1) cm−1 and zJ′ = 2.49 (6) with a agreement
factor of 1.82 × 10−4 . The value of D, calculated from the
above expressions is in the range expected for a tetrahedral
metal center (i.e. D = −36 to + 13 cm−1).22b D = −6.07 cm−1
indicates a 2D = 12.14 cm−1 splitting between the ground
MS = 3/2 levels and the excited MS = 1/2 levels.
Fig. 7 Plots of χMT vs. T and χM vs. T (inset) for the compounds 2a, 3b, 4a and 4b in the temperature range of 2–300 K. The red line indicates
fitting using the theoretical model (see text).
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Compound 3b
the lack of a proper model for such a system makes it diffi-
cult to estimate the exchange parameters for this compound.
The magnetic properties of compound 3b are roughly the
same magnetic properties as compound 2a. The plots of χMT
vs. T and χM vs. T for compound 3b in the temperature range
2–300 K are shown in Fig. 7b. The χMT value at room tempera-
ture is 2.54 cm3 K mol−1 which is higher than that of compound
3b and it decreases slowly down to 2 K (1.39 cm3 K mol−1).
The Curie Weiss fitting reveals a value of θ = −7.4 K and
C = 2.5 cm3 K mol−1, indicating the antiferromagnetic nature of
the compound. The observed spin-orbit coupling was estimated
by the zero-field splitting (ZFS) effects using the same equation
as mentioned above. The best fitting of χM vs. T data in the tem-
perature range 2–300 K gave g = 2.27 (2) D= −8.45 (1) cm−1 and
zJ′ = 3.62 (4). The higher D value indicates the larger splitting
between the MS = 3/2 levels and MS = 1/2 levels compared to
compound 2a.
Conclusion
In designing coordination polymers based on flexible linkers,
several factors influence the self-assembly process; among
those, the angle between the position of the coordinating
groups, i.e. the linker coordination angle (LCA), is an impor-
tant factor that alters the dimensionality and topology of the
coordination networks. This concept has been discussed
elaborately in this article taking a series of compounds,
based on flexible linkers having intriguing skeletons. The
self-assembly of a (flexible, flexible) mixed linker system
reveals the amendment of dimensionality according to the
length of the linker, which is dependent on LCA. The forma-
tion of the Co–hfipbb matrix as a 1D chain in 2a and a 2D
interpenetrated double layer in 2b, assisted by the secondary
N-donor bix linkers having LCAs of 60° (for 2a), and 180°
(for 2b) reveals the essential role of the position of the coor-
dinating groups on the linker. The associated connectivity of
the Co–ADA matrix with secondary N-donor bix linkers hav-
ing LCAs of 60° (for 3a), and 120° (for 3b) results in the for-
mation of 2D layers with different topologies. Compounds
4a, 4b and 4c are coordination networks based on {M-pda-tz}
constituents in which the flexible carboxylates 1,4-pda in 4a
and 4c and 1,3-pda in 4b adopt a cis conformation which
have the same structural consequences on the dimensional-
ity; in this situation, the LCA of the tetrazole directs the
dimensionality in obtaining 2D structures in 4b and 4c with
tetrazole LCAs of 180° and a 1D structure for 4a with a
tetrazole LCA of 60°. All the compounds discussed in the
study are a series of examples to enrich the structural library
of coordination networks based on flexible linkers exploiting
the role of linker coordination angle. Finally the temperature
dependent magnetic susceptibility measurements divulge the
zero-field splitting parameters of compounds 2a and 3b and
the antiferromagnetic exchange interactions between the
metal centers in 4a and 4b.
Compound 4a
Both χM vs. T and χMT vs. T plots of compound 4a are
presented in Fig. 7c. The room temperature χMT value of
compound 4a is 8.76 cm3 K mol−1 which is much higher than
the spin only value of 5.625 cm3 K mol−1 for the three Co(II)
ions (S = 3/2 and g = 2) indicating an unquenched orbital
contribution of the Co(II) ion. By lowering the temperature,
the χMT value continuously decreases to 2.41 cm3 K mol−1 at
2 K. The data follow the Curie Weiss law with the Weiss con-
stant θ = −20 K agreeing with the antiferromagnetic behavior
of the compound, reflecting a plausible antiferromagnetic
interaction between the two Co(II) ions in the dimer through
N atoms of the tetrazole ring. In compound 4a the two Co(II)
ions are bridged by the nitrogen atoms of the tetrazole ring
and the remaining Co(II) atom is in the lattice position of a
perfect octahedron. This factor makes it difficult to analyze
the exchange phenomenon through the bridge. Because of
the lack of a suitable model for such a system, fitting of the
magnetic susceptibility and the relevant exchange parameters
could not be estimated.
Compound 4b
Acknowledgements
The investigation of the magnetic study of compound 4b in
the temperature range 2–300 K reveals that the χMT value at
room temperature is 8.37 cm3 K mol−1 which is much more
higher than the spin only value (3.75 cm3 K mol−1 ) for two
Co(II) ions (S = 3/2 and g = 2). As the temperature is lowered,
there is a continuous sharp decrease in the χMT value which
reaches a minimum value of 0.35 cm3 K mol−1 at 2 K. The
data of the χM vs. T and χMT vs. T plots (Fig. 7d) indicates the
strong antiferromagnetic exchange between neighboring
Co(II) metal ions. The strong antiferromagnetic exchange is
also supported by the high Weiss constant θ = −160 K. As
described in the crystal structure of the compound 4b, a 1D
chain is formed by the connectivity of the Co(II) ions bridged
by double carboxylate bridges, a hydroxo bridge, tetrazolate
N2 and tetrazolate N3 bridge. The magnetostructure of the
compound contains many pathways for magnetic interaction;
The authors thank the Science & Engineering Research Board
(a statutory body under the Department of Science and Technol-
ogy, Government of India) for financial support (project num-
ber SB/S1/IC/034/2013). The National X-ray Diffractometer
facility at the University of Hyderabad by the Department of
Science and Technology, Government of India, is gratefully
acknowledged. We are grateful to UGC, New Delhi, for provid-
ing infrastructure facility at University of Hyderabad under UPE
grant. BKT and PM thank CSIR, India for their fellowships.
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