Structural and functional studies on ternary coordination polymers from 5-bromoisophthalate and imidazole based flexible linker

Article abstract of DOI:10.1039/c3ra45411h

Three ternary coordination polymers (CPs), namely, {[Cd₂(BrIP) ₂(BITMB)(H₂O)₂]·(THF) ₂·H₂O}_n (CP1), [Cu(BrIP)(BITMB)(H ₂O)]_n (CP2), {[Ni(BrIP)(BITMB)(H₂O)] ·(THF)(H₂O)₂}_n (CP3) were synthesized by solvothermal reactions between H₂BrIP (5-bromoisophthalate), BITMB (1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene) and respective metal nitrates. Single crystal X-ray diffraction studies reveal a variety of supramolecular interactions such as inter/intra molecular hydrogen bonding, C-H...π, and π...π stacking in the 1D interwoven metal organic triple helical motifs of CP1 and sql networks of CP2 and CP3. Solvents employed for the synthesis of these CPs may be acting as structure directing agents. The photocatalytic properties of all three CPs, for the decomposition of Metanil Yellow by dilute hydrogen peroxide in the presence of visible light, have been evaluated and up to 89% dye removal from aqueous solution was achieved in the case of CP2. Solid state fluorescence studies disclose the promising luminescence properties of synthesized CPs. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3ra45411h

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Structural and functional studies on ternary
coordination polymers from 5-bromoisophthalate
Cite this: RSC Adv., 2014, 4, 7352
and imidazole based flexible linker†
a
ab
Kamal Kumar Bisht,ab Yadagiri Rachuri,ab Bhavesh Parmar and Eringathodi Suresh
*
Three ternary coordination polymers (CPs), namely, {[Cd2(BrIP)2(BITMB)(H2O)2]$(THF)2$H2O}n (CP1),
[Cu(BrIP)(BITMB)(H2O)]n (CP2), {[Ni(BrIP)(BITMB)(H2O)]$(THF)(H2O)2}n (CP3) were synthesized by
solvothermal reactions between H2BrIP (5-bromoisophthalate), BITMB (1,3-bis(imidazol-1-ylmethyl)-
2,4,6-trimethylbenzene) and respective metal nitrates. Single crystal X-ray diffraction studies reveal a
variety of supramolecular interactions such as inter/intra molecular hydrogen bonding, C–H/p, and p/
p stacking in the 1D interwoven metal organic triple helical motifs of CP1 and sql networks of CP2 and
CP3. Solvents employed for the synthesis of these CPs may be acting as structure directing agents. The
photocatalytic properties of all three CPs, for the decomposition of Metanil Yellow by dilute hydrogen
peroxide in the presence of visible light, have been evaluated and up to 89% dye removal from aqueous
solution was achieved in the case of CP2. Solid state fluorescence studies disclose the promising
luminescence properties of synthesized CPs.
Received 26th September 2013
Accepted 20th November 2013
DOI: 10.1039/c3ra45411h
www.rsc.org/advances
species, represent
materials.
a signicant subset of metal organic
Introduction
Application of CPs towards removal of dye contaminants
from water has recently attained much attention.4–7 The strate-
gies for this purpose may be classied in two groups; the rst
being the use of CPs as photocatalysts for the degradation of
dyes and another being the use of CPs as dye adsorbents.5–7
Photocatalytic degradation of dyes is generally accomplished
with or without an oxidizing agent such as dilute hydrogen
peroxide.5,6 Interesting studies on the photocatalytic degrada-
tion of several organic dyes have recently been reported. Etaiw
and others have disclosed the photocatalytic properties of a
number of CPs for dye degradation with hydrogen peroxide.6
The mentioned reports suggests that the open-shell metal ions
such as Cu(II) are an ideal node for fabricating photocatalyst
CPs. These reports further imply that the choice of organic
linkers also play a vital role in the construction of photo-
catalytically active CPs. For instance, aromatic linkers owing to
their capability for electron transfer result in good photo-
catalytic activity of CPs.
The past two decades have witnessed an exponential growth in
the eld of design and construction of new coordination poly-
mers (CPs) or metal organic frameworks (MOFs). Huge research
interest in metal organic materials has grown due to their
application in the various elds such as adsorption, separation,
magnetism, enantioselective catalysis, photocatalysis, molec-
ular recognition, optical as well as luminescent properties.1–4
The aesthetic appeal of supramolecular networks has been
another acclaimed reason for the research interest in this eld.2
The MOFs/CPs have frequently been categorized on the basis of
their dimensionality, porosity or framework interpenetration,
etc. The number of employed organic linkers or metal nodes is
another important criterion to classify the metal organic
species, as the higher the number of components in a designed
supramolecular assembly, the larger the leap towards antici-
pated tailoring of bio-inspired functional materials.1 Thus the
CPs comprising of mixed ligands; polycarboxylate and N-donor
Angular dicarboxylates have frequently been chosen as
robust linkers for the construction of CPs among which iso-
phthalate linkers have emerged as one of the most celebrated,
because of their versatile metal binding modes and ease of
functionalization at C5 which extends the scope for additional
coordination or hydrogen bonding sites.8,9 5-Bromoisoph-
thalate (BrIP) linker has been exploited to assemble a variety of
CPs including ternary CPs in which 5-bromo functionality of
BrIP linker assists the extension of the network assembly
via hydrogen and/or halogen bonds.8 Additionally exible
aAnalytical Discipline and Centralized Instrument Facility, CSIR – Central Salt and
Marine Chemicals Research Institute, Council of Scientic and Industrial Research,
G. B. Marg, Bhavnagar – 364 002, Gujarat, India. E-mail: esuresh@csmcri.org;
sureshe123@rediffmail.com
bAcademy of Scientic and Innovative Research (AcSIR), CSIR – Central Salt and
Marine Chemicals Research Institute, Council of Scientic and Industrial Research,
G. B. Marg, Bhavnagar – 364 002, Gujarat, India
† Electronic supplementary information (ESI) available: FTIR, TGA, PXRD, time
dependent UV-Abs data and selected bond lengths for CP1–CP3. CCDC 939331,
939333 and 939334. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ra45411h
7352 | RSC Adv., 2014, 4, 7352–7360
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imidazole based exo-bidentate ligands are excellent tethers in Fluorolog Horiba Jobin Yvon spectrophotometer respectively. X-
bridging the metal–organic motifs, enhancing the framework Ray powder diffraction data were collected using a Philips X-
dimensionality.10,11 Bis(imidazol-yl-methyl)benzene type ditopic Pert MPD system with Cu Ka radiation. Single crystal structures
linkers are well explored, however, reports on CPs comprising of were determined using BRUKER SMART APEX (CCD)
BITMB are surprisingly intermittent (Scheme 1).11
diffractometer.
In continuation of our efforts to design and synthesize novel
CPs, herein we present three ternary CPs involving angular BrIP
and exible N-donor BITMB with different metal nodes Cd(II),
Cu(II) and Ni(II). Solvents, tetrahydrofuran (THF) and water
employed for solvothermal syntheses also played an important
structure directing role in the assembly of reported CPs.
Attempted solvothermal reactions yielded ternary CPs
{[Cd2(BrIP)2(BITMB)(H2O)2]$(THF)2$H2O}n (CP1), [Cu(BrIP)
(BITMB)(H2O)]n (CP2) and {[Ni(BrIP)(BITMB)(H2O)]$(THF)-
(H2O)2}n (CP3). Crystallographic studies revealed interesting
inter/intra molecular hydrogen bonding, C–H/p, and p/p
interactions among metal organic species and templating
solvents. The structure of CP1 comprises 1D metal–organic
triple helices; however, CP2 and CP3 possess isostructural sql
networks. Interestingly, these materials exhibit promising
luminescence and photocatalytic activity.
Syntheses of the compounds
{[Cd2(BrIP)2(BITMB)(H2O)2]$(THF)2$H2O}n (CP1). H2BrIP
(70 mg, 0.28 mmol), BITMB (75 mg, 0.26 mmol) and
Cd(NO3)2$4H2O (100 mg, 0.32 mmol) were dispersed in 7 mL
H2O–THF (1 : 1) and then sealed in a 14 mL Teon-lined auto-
clave, which was heated at 398 K for 50 h. Aer slow cooling to
room temperature, white crystals suitable for single crystal X-ray
analysis were obtained. Yield ¼ 58%. Elemental analysis (%)
calc.: C, 41.40; H, 4.07; N, 4.71; found: C, 41.48; H, 4.65; N, 4.62;
IR cmꢀ1 (KBr): 3417 (br), 2919 (w), 2367 (w), 1596 (s), 1434 (s),
1547 (s), 1370 (s), 1118 (m), 1089 (m), 1023 (w), 870 (w), 778 (m),
721 (m), 656 (w).
[Cu(BrIP)(BITMB)(H2O)]n (CP2). The synthetic procedure
was same as for CP1 except for a slight variation in ligand
stoichiometry, metal salt and solvents used in the reaction.
(H2BrIP (70 mg, 0.28 mmol), BITMB (75 mg, 0.26 mmol) and
Cu(NO3)2$6H2O (100 mg, 0.41 mmol), 2 mL water, 2 mL
methanol and 3 mL 1,4-dioxane were used). Yield ¼ 55%.
Elemental analysis (%) calc.: C, 49.64; H, 4.17; N, 9.26; found:
C, 49.48; H, 4.35; N, 9.12; IR cmꢀ1 (KBr): 3421 (br), 3110 (w),
2920 (w), 2357 (w), 1618 (s), 1551 (m), 1347 (s), 1302 (s),
1228 (w), 1112 (m), 1024 (w), 954 (w), 838 (w), 778 (m), 708 (m),
653 (w).
{[Ni(BrIP)(BITMB)(H2O)]$(THF)(H2O)2}n (CP3). H2BrIP (70
mg, 0.28 mmol), BITMB (75 mg, 0.26 mmol) and Ni(NO3)2$6H2O
(100 mg, 0.41 mmol) were dispersed in 10 mL H2O–THF (1 : 1)
and then sealed in a 23 mL Teon-lined autoclave, which was
heated at 408 K for 50 h. Yield ¼ 72%. Elemental analysis (%)
calc.: C, 49.18; H, 5.27; N, 7.91; found: C, 48.98; H, 5.65; N, 7.62;
IR cmꢀ1 (KBr): 3439 (br), 3137 (w), 2919 (w), 2361 (s), 1598 (m),
1537 (m), 1436 (m), 1359 (m), 1232 (w), 1120 (w), 1083 (w), 1022
(w), 832 (w), 781 (w), 665 (m).
Experimental
Materials and methods
All reagents and solvents were purchased from commercial
sources and were used without further purication. Distilled
water was used for synthetic manipulations. Ligands were
synthesized by adopting the reported procedures.12 CHNS
analyses were done using a Perkin-Elmer 2400 CHNS/O
analyzer. IR spectra were recorded using KBr pellets on a Perkin-
Elmer GX FTIR spectrometer. For each IR spectra 10 scans were
recorded at 4 cmꢀ1 resolution. TGA was carried out using
Mettler Toledo Star SW 8.10. Absorption and emission spectra
were recorded using Shimadzu UV-3101PC spectrometer and
Typical procedure for investigation of the photocatalytic
activity
The catalytic activity of CP1–CP3 was evaluated for the hetero-
geneous photocatalytic decolourization of Metanil Yellow (MY)
dye by dilute H2O2 solution at ambient temperature. In a typical
experiment, 100 mL glass reactor was charged with 50 mL
aqueous MY solution (1 ꢁ 10ꢀ5 M) and the 15 mg photocatalyst
(CP1–CP3) was dispersed in it for 30 min. in the absence of light
to attain adsorption equilibrium. Subsequently, 2 mL of 0.5 M
H2O2 solution was added and the reaction was illuminated by a
tungsten lament lamp. Stirring was maintained throughout
the reaction and 1 mL aliquot was withdrawn every 30 min
which was then ltered and analyzed on an absorption spec-
trophotometer. A control reaction was also performed where the
dye and H2O2 solutions were treated as per the above protocol in
the absence of solid catalysts. UV-Vis absorption of the with-
drawn aliquots was monitored at 435 nm to estimate the
Scheme 1 anti- and syn-conformers of flexible ligand BITMB (a and b)
and binding modes of angular dicarboxylate BrIP observed in the
present study (c–f).
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concentration of MY solution. Thus, the initial MY concentra- hydrogen atoms were rened anisotropically until convergence
tion (C0) and MY concentration (C) aer a certain photo-irra- was reached. The hydrogen atoms attached to the organic
diation time (t) were determined and the C/C0 ratio was moieties were either located from the difference Fourier map or
computed to evaluate the photocatalytic performance of the stereochemically xed in all the CPs. In the case of CP1 the
synthesized CPs.
hydrogen atom of the lattice water molecules could not be
located from the difference Fourier map.
X-Ray crystallography
Results and discussion
Summary of the crystallographic data, details of hydrogen
bonding interactions and selected bond lengths & bond angles
for CP1–CP3 are presented in Tables 1, 2 and S1,† respectively. A
suitable single crystal of each CP was transferred from the
mother liquor to paratone oil and mounted on the tip of a glass
ber for data collection. Intensity data for all three crystals were
Despite the same organic linkers, structural diversity was
observed among these CPs. The CP1 comprise interwoven triple
helical metal organic motifs glued via weak inter- and intra-
molecular interactions. CP2 and CP3 revealed isostructural sql
networks stacked over one another. The structural diversity of
the studied CPs is attributable to the coordination preferences
of the employed metal nodes, the exibility of both ligands in
adopting different conformations, the versatile coordination
mode of the carboxylate group of BrIP, lattice solvents and
various non-covalant interactions in the supramolecular
assembly. The solvent molecules employed in the reaction also
have a profound effect on the structural motifs in all CPs. The
lattice guests (THF and water) located in CP1 are involved in
non covalent interactions with the ribbon type metal–organic
motif.
˚
collected using Mo Ka (a ¼ 0.71073 A) radiation on a Bruker
SMART APEX diffractometer equipped with CCD area detector
at 150 K. The data integration and reduction were processed
with SAINT soware.13 An empirical absorption correction was
applied to the collected reections with SADABS.14 The struc-
tures were solved by direct methods using SHELXTL and were
rened on F2 by the full-matrix least-squares technique using
the program SHELXL-97.15,16 The contribution of highly disor-
dered solvent molecules (THF–H2O) present in the lattice of CP2
and CP3, to the diffraction data has been removed using the
SQUEEZE subroutine as implemented in PLATON.17 However,
in the case of CP1, a water molecule and two THF molecules
present in the lattice were located from the difference Fourier
Crystal and molecular structure of CP1
map. Renement of the compounds using the modied reec- The asymmetric unit of CP1 contains two independent Cd(II)
tion data set apparently improved the R-factor and GOF. All non- centers (Cd1 and Cd2), two BrIP, one BITMB, two coordinated
Table 1 Crystal data and refinement parameters for CP1–CP3a
Identication code
CP1
CP2
CP3
Chemical formula
Formula weight
Crystal colour
Cd2C41H45N4O13Br2
1186.43
Colourless
0.23 ꢁ 0.11 ꢁ 0.06
150(2)
Monoclinic
P21/c
11.285(2)
11.947(2)
32.654(7)
90
92.262(4)
90
4
4399.1(15)
1.791
2.852
CuC25H25N4O5Br
604.94
Blue
NiC25H25N4O5Br
600.11
Green
Crystal size (mm)
Temperature (K)
Crystal system
Space group
0.09 ꢁ 0.05 ꢁ 0.02
0.05 ꢁ 0.05 ꢁ 0.02
150(2)
150(2)
Triclinic
ꢀ
P1
Triclinic
ꢀ
P1
˚
a (A)
9.298(3)
10.903(3)
13.375(4)
75.732(5)
76.533(5)
70.130(5)
2
1219.2(6)
1.648
2.579
9.320(3)
10.402(3)
13.855(5)
76.135(6)
75.941(6)
71.167(6)
2
1214.1(7)
1.642
2.490
˚
b (A)
˚
c (A)
a (ꢂ)
b (ꢂ)
g (ꢂ)
Z
3
˚
V (A )
Density (Mg mꢀ3
)
m (mmꢀ1
F (000)
)
2356
614
612
Reections collected
Independent reections
Rint
30 525
7756
0.0721
10 411
5387
0.0502
10 166
5211
0.0710
Number of parameters
569
1.239
336
1.018
334
1.091
GOF on F2
Final R1/wR2 (I $ 2s(I)
Weighted R1/wR2 (all data)
CCDC number
0.0863/0.1724
0.1036/0.1788
939331
P
0.0560/0.1172
0.0776/0.1269
939333
0.0859/0.1358
0.1318/0.1524
939334
P
P
P
a
2
R ¼ ||Fo| ꢀ |Fc||/ |Fo|; wR ¼ [ w(Fo ꢀ Fc2)2/ w(Fo2)2]1/2
.
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Table 2 Hydrogen bonding parameters for CP1–CP3
:D–H/A (ꢂ)
Symmetry code
˚
˚
CP
D–H/A
d(H/A) (A)
d(D/A) (A)
CP1
O(9)–H(9C)/O(11)
O(9)–H(9D)/O(4)1
O(10)–H(10C)/O(8)1
C(19)–H(19)/O(8)2
C(27)–H(27B)/O(12)
O(5)–H(5C)/O(4)1
C(22)–H(22)/O(1)2
O(5)–H(5D)/O(2)3
C(10)–H(10)/O(2)4
O(5)–H(5C)/O(3)1
O(5)–H(5D)/O(1)2
1.80(9)
1.78(9)
1.97(7)
2.44
2.56
1.83(5)
2.43
2.06(6)
2.34
1.76(8)
2.05(9)
2.702(12)
2.708(10)
2.743(9)
3.021(12)
3.414(16)
2.709(4)
2.880(6)
2.771(5)
3.239(6)
2.607(6)
2.817(7)
162(10)
169(8)
137(8)
121
147
172(4)
109
154(5)
163
173(10)
172(7)
1 ¼ x, 1 + y; 2 ¼ ꢀx, 1/2 + y, 1/2 ꢀ z
CP2
CP3
1 ¼ x, ꢀ1 + y, z; 2 ¼ 1 + x, ꢀ1 + y, z; 3 ¼ 1 ꢀ
x, 2 ꢀ y, ꢀ z; 4 ¼ 2 ꢀ x, 2 ꢀ y, ꢀz
1 ¼ x, 1 + y, z;2 ¼ 1 ꢀ x, 2 ꢀ y, 1 ꢀ z
water molecules along with one water and two THF molecules manner decorating the ribbon motif. Cd/O distance ranges
˚
˚
as solvent of crystallization (Fig. 1a). CP1 crystallized in a from 2.304(8) to 2.486(6) A/2.299(7) to 2.470(7) A, Cd1/N1
˚
˚
monoclinic system with P21/c space group and can be described distance is 2.247(9) A/2.272(9) A for the coordination around
as one-dimensional CP. Both independent metal centers Cd1 and Cd2 respectively (Table S1†). Dihedral angles between
possess distorted pentagonal bipyramidal geometry with iden- the terminal imidazole rings with the central phenyl ring in the
tical N1O6 hepta-coordination. Independent Cd1 and Cd2 metal syn coordinated BITMB ligand bridging the adjacent metal pairs
centers are bridged via oxygen of two different BrIP ligands, are almost orthogonal (83.6ꢂ and 83.53ꢂ respectively for
resulting in edge sharing between distorted pentagonal bipyr- N1N2C17–C19 and N3N4C28–C30). As depicted in Fig. 2a, a
amids (Fig. 1b). One of the BrIP ligand is involved in m4- close examination of the ribbon motif revealed the existence of
h2h2h1h1 mode of coordination in which each carboxylate the interwoven triple helical strands relating the pairs of screw
group bridges Cd1 and Cd2 pairs. The second BrIP ligand is related Cd centers bridged by the BrIP ligand. The Cd1/Cd2
making m2-h1h1h1h1 chelated coordination to link screw related distance within the BrIP bridged pair through carboxylate
˚
Cd1Cd2 pairs generating a one-dimensional ribbon like pattern oxygen atoms O5 and O7 is 3.78 A. The Cd/Cd distance
as depicted in Fig. 2a. Thus, the pentagonal base of hepta- between the symmetrically translated screw pairs bridged by
˚
coordinated Cd(II) centers (Cd1/Cd2) is provided by the chelated BrIP ligand is 9.87 A.
oxygen atoms O1, O2/O3, O4 from the rst BrIP; O7, O8/O5, O6
The packing diagram viewed down the a-axis shows the
from the second BrIP and O9/O10 water molecules, respectively. lattice THF and water molecules are positioned between the
The bridging coordination of O7 and O5 to Cd2 and Cd1 adjacent alternate layers of the one dimensional motif (Fig. 2b).
generates the Cd1Cd2 dimeric pair which is translated along As depicted in Fig. 3a, water hydrogen atoms H9D and H10C are
the b-axis with 21 symmetry in the ribbon like motif. The
seventh coordination is provided by the terminal nitrogen
atoms N1 and N4 of the BITMB ligand in syn conformation
linking the adjacent screw related Cd pairs in up and down
Fig. 2 One dimensional interwoven metal organic triple helix running
Fig. 1 ORTEP depiction of asymmetric unit of CP1 (lattice solvents are along the b-axis; three individual strands are interconnected through
omitted) (a); coordination environment around metal centre in CP1 (b); carboxylate oxygen atoms from BrIP and terminal nitrogen of syn-
ORTEP depiction of coordination geometry in CP2 (c) and CP3 (d); BITMB tethers; (b) packing diagram viewed down the b-axis depicts
ORTEP diagrams are drawn with 40% probability factor for thermal the sequential arrangement of the helical chains (capped stick model)
ellipsoids and some of the hydrogen atoms are omitted for clarity.
and solvent molecules (CPK model) in the crystal lattice.
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Fig. 3 1D metal organic triple helices in CP1 are held together by C–H/O, p/p stacking and C–H/p interactions along the a-axis (a); C–H/O
and O–H/O hydrogen bonds along the c-axis in CP1 (b).
involved in intramolecular O–H/O interactions with the
carboxylate oxygen atoms O4 and O8 and imidazole hydrogen
H19 with carboxylate oxygen O8 respectively. 1D strands are
stacked down the a-axis and involved in intermolecular C–H/O
contacts engaging the imidazole hydrogen H18 and the
carboxylate oxygen O6.
Crystal and molecular structure of CP2 and CP3
ꢀ
CP2 crystallized in P1 space group of the triclinic system. The
asymmetric unit of CP1 includes one Cu(II), one full molecule
of BITMB, one unit of BrIP ligand and one coordinated water
molecule (Fig. 1c). Cu(II) possesses a distorted square pyra-
midal geometry with N2O3 coordination in which terminal
nitrogen atoms (N1 & N4) from two different BITMB, carbox-
ylate oxygen atoms (O1 & O3) from two different BrIP ligands
(involved in m2-h1h1h0h0 bridging mode), and one water
molecule (O5) are involved. The square base is constituted by
N1, N4 from BITMB, O1, O3 of the BrIP ligand and the cis
angles subtended by the coordinated atoms in the square base
Fig. 3a further depicts the intermolecular C–H/p interac-
tion involving imidazole ring screw pairs bridged by BrIP ligand
˚
is 9.87 A and p/p interaction between the phenyl rings of BrIP
and BITMB between the metal organic strands stacked down
˚
a-axis with Cg3/Cg4 distance 3.621(6) A. Consequently, methyl
hydrogen H31B of the central phenyl ring and imidazole
hydrogen H18 of BITMB are involved in C–H/p contacts with
the centroid of the imidazole ring C1g and C2g, with perpen-
dicular distances from the imidazole ring plane to H31B and
˚
range from 88.51(12) to 91.09(12). Cu(II) is up by 0.34 A from
the square base towards the axially coordinated water mole-
cule O5 (Cu1/O5 ¼ 2.221(3)).
˚
H18 2.98 and 2.70 A respectively (H31B/C1g ¼ 3.00, C31/
The topology of CP2 can be described as a neutral sql
network in which the metal centers are bridged via m2-mono-
dantate mode of the carboxylate oxygen generating [Cu(BrIP)2]n
strands along the a-axis, further tethered by the terminal
nitrogen atoms of BITMB ligand along the b-axis (Fig. 4). The
Cu/Cu distance in the sql nets, bridged by BrIP and pillared by
˚
C1g ¼ 3.877 A symmetry code: 1 ꢀ x, ꢀ1/2 + y, 1/2 ꢀ z; H18/
˚
˚
C2g ¼ 2.80 A, C18/C2g ¼ 3.342 A, symmetry code: 1 ꢀ x, 1/2 + y,
1/2 ꢀ z; C1g and C2g are centroids of the imidazole ring
N1N2C17–C19 and N3N4C28–C30). The observed H/p
distance involving C1g and H31B is slightly longer but resides
within the acceptable range of CH/p interactions, indicating a
weak C–H/p contact.18 The THF and water molecules
residing between the consecutive coordination networks are
involved in hydrogen bonding interactions amongst them and
with the coordination network (Fig. 3b) which anchor these
guest molecules between 1D coordination polymeric strands.
Even though hydrogen atoms of the lattice water molecule
O11 could not be located from the X-ray data they act as
donors, making good short contact with the carboxylate
˚
BITMB is 10.097 and 11.68 A, respectively. Interestingly, BITMB
ligand has taken up anti conformation and the twist angles
between the terminal imidazole rings are 78.43ꢂ to make
effective coordination with the metal. As depicted in Fig. 4a
strong intramolecular O–H/O interaction between the coor-
dinated water hydrogen H5C and uncoordinated carboxylate
oxygen O4, C–H/O interaction between the imidazole
hydrogen H22 and coordinated carboxylate oxygen O1 and C–
H/p interaction between the methylene hydrogen H19B with
the phenyl ring of the BrIP ligand are observed within the 2D
sheets.
The packing diagram of CP2 viewed down the a-axis clearly
exposed a displacement of the adjacent 2D nets alternatively
along the b-axis as depicted in Fig. 4b. Adjacent 2D sheets are
drawn in by strong intermolecular O–H/O and C–H/O
interactions generating supramolecular bilayer assembly of
2D sheets. Consequently, uncoordinated carboxylate oxygen
˚
oxygen O1 (O11/O1 ¼ 2.76 A), O13 of the THF molecule
˚
(O11/O13 ¼ 2.78 A) and as acceptors involving intermolec-
ular O–H/O interactions with H7C of coordinated water and
C–H/O contact with the lattice THF. Another THF molecule
positioned with the opposite orientation is making C–H/O
contact with the methylene hydrogen H27B of BITMB to
stabilize the lattice solvent between the 1D coordination
polymeric layers.
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Fig. 4 sql network in CP2 and intramolecular interactions within the
2D sheet (a); packing of CP2 viewed down the a-axis depicting the
bilayer arrangement of sheets via H-bonding interactions (b).
O2 from BrIP is involved as a bifurcated acceptor with the
water hydrogen H5D and imidazole hydrogen H10 of the
BITMB from the neighbouring (4,4) grid in the bilayer
assembly. The lattice solvent molecules squeezed out from the
structural data may be present in the void between adjacent
bilayer 2D-sheets.
Fig. 5 sql network of CP3 and intramolecular interaction present
within the 2D sheets (a); packing of CP3 viewed down the
a-axis depicting the bilayer arrangement of sheets via H-bonding
interaction (b).
Even though CP3 shows the same crystallographic parame-
ters and 2D sheet-type topology as observed in the case of CP2,
variations in the coordination geometry and weak molecular
interactions are noticed in CP3 because of the different metal
nodes. In CP3 Ni(II) possesses a distorted octahedral coordina-
tion in which the carboxylate groups of BrIP are drawn in m2-
h1h1h1h0 bridging mode. The square base in CP3 is provided by
the chelated/monodentate carboxylate oxygen atoms (O1, O2
and O4), from two different BrIP ligands and the O5 water.
[Ni(BrIP)]n strands running along the b-axis are cross-linked by
the terminal nitrogen atoms N1 and N4 from two BITMB
ligands axially in the N2O4 octahedral coordination. Interest-
ingly, the same intramolecular O–H/O and C–H/p contacts
are retained as observed in CP2, but an additional weak offset
Cg/Cg contact between the phenyl ring of BITMB and BrIP is
observed in CP3 as depicted in Fig. 5a. As depicted in Fig. 5b,
the same bilayer arrangement of sql nets via C–H/O contacts
observed in CP2 is maintained in CP3 also and perhaps the
lattice solvents removed from the crystal data may be positioned
between the bilayer sheets.
(Dn ¼ nasym ꢀ nsym) was used to fetch information on the metal–
carboxylate binding modes. CP1 showed two pairs of nasym and
nsym frequencies at 1596, 1434 (Dn ¼ 162) and 1547, 1370 (Dn ¼
177) cmꢀ1 for the carbonyl functionality indicating the m4- and
m2-modes as observed in the crystal structure. CP2 showed two
pairs of nasym and nsym frequencies at 1618, 1347 (Dn ¼ 271) and
1551, 1302 (Dn ¼ 249) cmꢀ1 corresponding to the carbonyl
functionality of dicarboxylate ligand indicating an unsymmetric
monodentate coordination mode. The FTIR spectrum of CP3
displays nasym and nsym frequencies for symmetric chelating and
unsymmetric monodentate modes at 1598, 1436 (Dn ¼ 136)
cmꢀ1 and at 1537, 1359 (Dn ¼ 178) cmꢀ1, respectively. OH
stretching broad bands in the range 3450–3400 cmꢀ1 for all
three compounds are attributable to the coordinated and/or
lattice water. The thermal stability of the synthesized
compounds was ascertained by TGA (Fig. S2†). CP1 with robust
triple helical motifs showed stability up to 325 ꢂC. However,
approximately 6% of its initial weight was lost in the tempera-
ꢂ
ture range 125–220 C which may be ascribed to the one unit
lattice THF molecule. CP2 have shown thermal stability up to ca.
270 ꢂC, whereas CP3 lost two lattice water molecules (calc. 5.1%,
obs. 5.6%) in the temperature range 100–150 ꢂC. Further lossꢂof
FTIR, TGA and PXRD analyses
The lattice guests and composition of CP1–CP3 were ascer- lattice THF was observed in the temperature range 150–325 C
tained by TGA, elemental and single crystal analyses. FTIR (calc. 10.18%, obs. 10.0%). Experimental PXRD patterns of bulk
spectra (Fig. S1†) revealed valuable information about the samples CP1–CP3 concur with those simulated from respective
coordination modes of BrIP moiety. The difference between single crystal data, conrming the phase purity of bulk samples
asymmetric and symmetric carbonyl stretching frequencies (Fig. S3†).
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Table 3 Spectral details of CP1 and the constituent ligands
Photoluminescence study
Solid state photoluminescence behaviour of CP1–CP3 at
ambient temperature was examined (Fig. 6 and S6†). Absorption
maxima for ligands BITMB and H2BrIP were observed at 273
and 300 nm respectively, however CP1–CP3 showed major
absorption bands at ca. 297, 290, 379 nm, respectively. Upon
excitation with 297 nm, strong emission at 454 nm with an
overall luminescence quantum yield (Foverall) of 5.49% for CP1
was recorded at room temperature.
The redshi in the emission bands (Table 3) of CP1 could be
ascribed to the metal–perturbed intraligand emission and
ligand eld transitions. The good photoluminescence of CP1
may also be attributed to the closed shell metal conguration of
Cd(II). Furthermore, the higher quantum yield of CP1 may also
be attributed to the triple helical motif as such rigid structural
features substantially cease the radiationless vibrational energy
loss.11a,19
lex
lem
Dl
(¼lem ꢀ lex)
CP/ligand
lmax (nm)
(nm)
(nm)
Foverall
CP1
BITMB
H2BrIP
241, 297, 341
236, 273
300
297
273
300
454
305
500
157
32
200
5.49
—
—
visible light a slight decrease in the concentration of MY solu-
tion was observed. To evaluate the effect of H2O2 and visible
light on the MY solution, a blank experiment was performed
which resulted in ca. 10% dye decomposition within 180 min.
Identical experiments in the presence of CP1–CP3 were per-
formed and the time dependent change in the MY absorbance
was monitored using UV-Vis spectroscopy. C/C0 versus irradia-
tion time plot (Fig. 7a) suggests that the copper coordination
polymer CP2 exhibit the best photocatalytic activity among
CP1–CP3. Approximately 89% of the initial amount of MY
degrades within 180 min. in the presence of CP2. Under iden-
tical parameters CP1 catalyzed batch showed 17.6% dye
degradation. Time dependent studies were not performed for
Photocatalytic activity
Recent reports on the application of CPs as heterogeneous
photocatalyst for waste water treatment have encouraged us to
study the photocatalytic activity of synthesized CPs. Most of the
reported coordination polymeric photocatalysts require UV
irradiation for fruitful results and only few of them show
signicant activity under visible light. Thus, we tested CP1–CP3
for the bleaching of Metanil Yellow dye coloration in water
under visible light. Analogous experiments to the existing
reports were performed in the presence of dilute H2O2 and the
general procedure is described in the Experimental section.
Dye–H2O2 mixture was stable in the dark for several hours
without any change in absorbance. However, in the presence of
Fig. 7 Photocatalytic decomposition of MY solution with visible light
irradiation and H2O2 in the presence of CP1, CP2 and without catalyst
under the same experimental conditions; digital images of dye solu-
tions at beginning (t ¼ 0 min) and on completion (t ¼ 180 min) of the
attempted reactions (a); time dependent absorption spectra for CP2
Fig. 6 Solid state absorption spectra for CP1–CP3 (a); emission catalyzed decomposition of MY by visible irradiation and dilute H2O2; a
spectra of CP1 (inset comprises digital photographs captured in kinetic study revealing the pseudo-first order reaction pathway is
daylight and at UV irradiation for CP1) (b).
presented in the inset (b).
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CP3 as this CP signicantly disintegrated during the course of showed versatile binding modes of carboxylate groups from the
the experiment. The differences in the photocatalytic activity of angular BrIP linkers with the metal ions. Moreover, the tem-
CP1 and CP2 may be attributed to the open shell conguration plating effect of solvent molecules which reside in the voids of
of Cu(II) center in CP2 and the dense triple helical framework of the crystal structure also plays a signicant role in dening the
CP1. UV-Vis absorption changes observed with time during MY nal structure and topology of these CPs. A variety of weak
photodegradation catalyzed by CP2 in the presence of H2O2 are molecular interactions such as hydrogen bonding, C–H/p and
depicted in Fig. 7b. Time dependent UV-Vis absorption spectra p/p stacking were observed in these supramolecular assem-
for the blank batch and CP1 catalyzed batch are presented in blies which stabilize the molecule in the crystal lattice. Despite
Fig. S4.† The kinetic data for this batch revealed that the CP2 the interesting structural features, CP1–CP3 also exhibit
catalyzed photodecomposition of MY follows a pseudo-rst signicant photoluminescence and photocatalytic activity. CP1
order reaction pathway. The kinetic results corroborate with showed strong emission with 5.49% overall quantum yield
previous reports which also establish the pseudo-rst order (Foverall) at room temperature. Among the three, CP2 exhibits
nature of heterogeneously catalyzed photodegradation of dyes good photocatalytic activity, which displayed nearly 90%
with dilute H2O2.3
degradation of Metanil Yellow under the standard protocol.
The relevant report by Du et al., on the photocatalytic activity Thus, the present study enriches the research insight for the
of MOFs/CPs explains dye decolouration on the basis of the design and synthesis of CPs possessing photoluminescent
semiconductor theory, which is appropriate for the present case properties and photocatalytic activity.
too.5c The theory suggests that illumination of a photocatalyst
by visible radiation of suitable energy causes excitation of
electrons from the valence band to the conduction band. This
Acknowledgements
electronic transition causes formation of holes in the valence The authors acknowledge CSIR India (HYDEN Project, Grant
band. These holes can accept an electron from the organic no. CSC 0122) for nancial support, Mr Satyaveer Gothwal for
molecules like dyes or water (used as solvent), resulting in the TGA data, Mr V. K. Agrawal for IR data, Mr Viral Vakani for CHN
dye oxidation/degradation or formation of hydroxyl radicals, analysis and Dr P. Paul for all-round analytical support. K.K.B
respectively. Hydroxyl radicals being strong oxidizing species, and B.P acknowledge CSIR (India) and Y.R. acknowledges UGC
further contribute to the dye degradation. However, the (India) for research fellowships.
recombination of excited electrons with holes in the valence
band may drastically suppress the oxidative degradation
processes. In order to hold back the recombination process of
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