Intrinsic structural features of coordination polymers make an impact on dye selectivity

Article abstract of DOI:10.1039/d1ce00481f

Five dicarboxylate coordination polymers having 1-phenyl-3-((pyridin-4-yl)methyl)urea (L) as co-ligands with general formula [M₂L₂(dicarb)(H₂O)₂]_n(dicarb = succinate or fumarate for M = Co and Zn; ter

Full text of DOI:10.1039/d1ce00481f

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Intrinsic structural features of coordination
polymers make an impact on dye selectivity†
Cite this: DOI: 10.1039/d1ce00481f
*
Rinki Brahma and Jubaraj B. Baruah
Five dicarboxylate coordination polymers having 1-phenyl-3-((pyridin-4-yl)methyl)urea (L) as co-ligands
with general formula [M2L2(dicarb)(H2O)2]n (dicarb = succinate or fumarate for M = Co and Zn;
terephthalate for M = Co) were synthesised and characterised. Analysis of the supramolecular features of
self-assemblies revealed that the hydrogen bonds of coordinated water and urea significantly contributed
to the self-assemblies. The C–H⋯π interactions in certain self-assemblies locked the phenyl ring of the
urea moiety. These interactions influenced intermolecular hydrogen bonds in those self-assemblies to
distinguish them through characteristic hydrogen bonds. The free N–H bonds from the urea part exposed
outward from the chain-like arrangements of the cobalt terephthalate coordination polymer enabled it to
selectively adsorb methyl orange dye over phenol red dye. The zinc succinate as well as fumarate
coordination polymers were fluorescent and showed dual emissions.
Received 12th April 2021,
Accepted 17th April 2021
DOI: 10.1039/d1ce00481f
rsc.li/crystengcomm
hydrogen bonds.27 In general, two N–H bonds of a urea
moiety get involved in intermolecular hydrogen bonds in self-
Introduction
Urea is an extensively studied compound and continues to
attract interest from chemists as a sustainable key functional
unit and finds applications as fertiliser1–4 and medicine.5 The
supramolecular chemistry associated with urea-based
compounds has contributed to catalysis,6–10 co-crystals,11,12
and molecular recognition.13–16 Certain assemblies of urea
assembled structures of various urea derivatives forming urea
tapes.33–38 To utilise these hydrogen bond donors and
acceptors to form a co-crystal or to bind to another ion, the
segregation of tape-like arrangements to form new hetero-
synthons would be required. For example, the fundamental
requirement for a 1 : 1 co-crystal formation is the need to
choose components that would prefer heteromeric assembly
to overcome the homomeric assemblies of parent
components.57
Competition to form complementary hydrogen bonds
between the two independent components is the essence in a
hydrogen bonded system. This study stems from the interest
in utilising the second coordination sphere58–60 of a metal
complex for substrate binding. For such a purpose, we have
chosen to study coordination polymers of a pyridine-based
urea, namely 1-phenyl-3-((pyridin-4-yl)methyl)urea (L), to
study its metal dicarboxylate coordination polymers. The
pyridine substituent connected through a flexible methylene
tether to urea moiety provides different provisions for
bifurcated hydrogen bonds illustrated in Fig. 1A and B. Such
assemblies depicted in Fig. 1C would be formed by a hydrate
or solvate of such a compound. Having L as an ancillary
ligand of dicarboxylate coordination polymer will have
structural diversities. First of all, diversities in such
coordination polymers may arise from the different binding
modes of carboxylates. Two such possibilities are shown in
Fig. 1D–F. By changing the coordination modes of
carboxylates, several other structures of dicarboxylate
coordination polymers may be imagined.61,62 Furthermore,
accommodating
guest
molecules
show
guest–guest
interactions17 and certain ones have stimuli-guided tuneable
properties.18,19 The self-assemblies of metal complexes of
urea derivatives20–27 or anion-assisted assemblies28–32 or
guest-assisted assemblies of urea derivatives33–38 exhibit
special structural features. The ionic co-crystals of urea
derivatives are of interest to generate new properties.39–41
Urea-based
complexes
show
interesting
conducting
properties42–44 as well as optical sensing properties towards
different ions.45–49 Urea-based inorganic cages,50,51 metal
organic frameworks,52,53 and coordination polymers54–56 are
of interest in catalysis, ion detection etc. Various
symmetrically or non-symmetrically substituted urea
derivatives adopt different conformations and some of such
conformers are stabilised by metal complexation or through
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati –
781 039, Assam, India. E-mail: juba@iitg.ac.in; Tel: +91 361 2582311
† Electronic supplementary information (ESI) available. The crystallographic
information files are deposited at the Cambridge Crystallographic Database
Centre with CCDC no. 2063021–2063026. The spectroscopic details, and results
of thermogravimetry and powder XRDs studies are available. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1ce00481f
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Fig. 1 (A–C) Some selected ways of self-assembling and (D–F) metal dicarboxylate complexes of L.
the presence of coordinated water molecules or appropriate
co-ligands would guide several other structural scaffolds to
form new self-assemblies. The coordination polymers
(abbreviated as CPs) will provide scope for selective substrate
binding. Varieties in structures of CPs will be possible by
modulating different weak interaction schemes. The changes
in structural features will be caused by (a) the orientation of
the urea moiety, (b) exposure of the urea moiety for direct
approach of a partner molecule or ion, (c) metal coordination
guiding directional aspects of the self-assemblies, (d) spaces
in the packing pattern for guest recognition, (e) gaining
higher stability through host–guest complex formation etc.
Each of these effects would influence the substrate binding
of a constituent co-ligand. With those anticipations, we
prepared and characterised five CPs of cobalt and zinc
succinate, fumarate and terephthalate having L as co-ligands.
The molecular recognition in terms of selectivity in
adsorptions of two dyes is depicted to suggest the role of
intrinsic weak interactions contributing to dye recognition.
fumarate for M = Co and Zn; terephthalate for M = Co) (CP1–
CP5) were prepared at ambient conditions by mixing the
appropriate amounts of the reactants in methanol
(Scheme 1). A reaction of L with zinc(II) acetate in the
presence of maleic acid which is the cis-regiomer yielded the
CP of the trans counterpart, namely the fumarate
coordination polymer. Hence in these cases a cis to trans
isomerisation took place during the course of reaction under
ambient condition to form CP of thermodynamically stable
isomer. The cis–trans isomerisation of maleate to fumarate in
zinc complexes was also reported earlier.63,64 Besides these,
an attempt to prepare analogous terephthalate CP of zinc
with L did not result in the incorporation of L to the metal
site but yielded a zinc-terephthalate metal–organic framework
(MOF). The zinc MOF was characterised by determining the
crystal structure and it was found to be [Zn4(Ter)(H2O)
(DMF)]n (CP6) which was a MOF reported in the literature.64
The CPs were insoluble in organic solvents. All these CPs had
two L units, a dicarboxylate and two water molecules of
crystallisation per metal ion. The IR spectra of the CPs
consistently showed the signals for O–H, N–H and CO
stretching. The ligand L has CO stretch of urea part at
1675 cm−1; upon complexation it gets shifted to lower
wavelength, and in the succinate and fumarate this appeared
in the range 1645–1635 cm−1. The CN of pyridine rings
consistently appeared at 1554–1555 cm−1 in the spectrum of
the ligand as well as in the spectra of the metal complexes.
Results and discussion
1-Phenyl-3-((pyridin-4-yl)methyl)urea (L) was prepared by
conventional condensation reaction of 4-picolylamine with
phenyl isocyanate. The succinate, fumarate and terephthalate
CPs of cobalt and zinc having
L ligands and with
compositions [M2L2(dicarb)(H2O)2]n (dicarb = succinate or
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Scheme 1 Synthesis of different coordination polymers.
Succinic acid has CO stretching at 1720 cm−1, whereas
fumaric acid and terephthalic acid have bands at 1680 cm−1;
the respective bands of O–H bending mode coupled with
CO stretching occur at 1432 cm−1 and 1420 cm−1.
formed by the intermolecular hydrogen bonds between
neighbouring urea N–H bonds and OC bonds. The
hydrogen bond formed by the nitrogen atom of pyridine is
important for the adoption of the observed puckered
geometry of the compound. Theoretical studies on pyridine–
water interactions suggested that an appropriate orientation
of pyridine is guided by local environments from
contributions of hydrogen bonding with water.66,67 In the
present case, the pyridine is tethered to the urea through a
flexible methylene group, which provided flexibility to the
molecule to adopt a puckered structure thereby maximising
hydrogen bonding with water molecules.
The coordination polymers CP1–CP5 have analogous
compositions and also have similar coordination
environments around respective metal ions. The identical
ligands, namely two ligands L, have trans positions, as do the
water molecules and also the dicarboxylate. The dicarboxylate
bridges to the metal ions generate the one-dimensional
chains of each 1-D CP.
As the coordination environments of the CPs are similar,
for a comparison, the metal–ligand bond parameters are
listed in Table 1. The M–O bond distances of water molecules
coordinating to water and the M–O bond of dicarboxylates
are similar and are in the range of 2.06–2.11 Å, which are
representative of Co–O and Zn–O dative bonds found in
conventional coordination complexes.68–72 The M–N bond
lengths are 2.14–2.20 Å which are in the range of typical
Zn–N or Co–N bonds of pyridine–metal complexes. As
anticipated, the Zn–L bonds are slightly longer than the Co–L
bonds due to the difference in the size of the two metal ions.
Solid samples of the three cobalt CPs had visible
absorption bands at 494 nm (CP1), 498 nm (CP2) and 498
4
4
nm (CP3) due to the T2 ← T1 transition characteristic of d7
system in a distorted octahedral environment.
Structural aspects
The crystal structures of the 1-phenyl-3-((pyridin-4-yl)methyl)
urea monohydrate as well as the five CPs listed in Scheme 1
were determined. The water molecule of crystallisation of
1-phenyl-3-((pyridin-4-yl)methyl)urea acts as a hydrogen bond
donor as well as a hydrogen bond acceptor (Fig. 2a). This
provided a four-coordinate hydrogen bonding environment to
each water molecule. In an earlier example we observed four-
coordinated water in co-crystals of aminoisophthalic acid
with quinoline where water molecules anchored four
molecules of zwitterionic aminoisophthalic acid molecules.65
In the present case, the two urea N–H of L hydrogen bond to
an oxygen atom of a water molecule [N1–H⋯O2, dd⋯A
=
2.924(5) Å and N2–H⋯O2, dd⋯A = 2.981(5) Å]. The dissimilar
hydrogen bond parameters of these N–H⋯O bonds showed
an unsymmetrical bifurcated hydrogen bonded geometry.
The two O–H of the same water molecule independently form
hydrogen bonds with the oxygen atom of the carbonyl and
the nitrogen atom of the pyridine. Besides the bifurcated N–
H⋯O bonds, two other hydrogen bonds, namely O2–H⋯O1
[dd⋯A = 2.819(5) Å] and O2–H⋯N3 [dd⋯A = 2.876(5) Å],
contributed to the bridges of the self-assembly. Accordingly,
water acts as a bridging molecule between two Ls and
prevented the conventional hydrogen bonded urea tapes
The metal sites in each coordination polymer have
a
distorted octahedral geometry. There are large numbers of
examples of metal-dicarboxylates of similar flexible carboxylic
acid, and those polymers suggest that the dimensionalities
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Table 1 Metal–ligand bond parameters in the coordination polymers
CP1–CP5
Bond distance CP1
CP2
CP3
CP4
CP5
(Å)/angle (°)
(M = Co) (M = Co) (M = Co) (M = Zn) (M = Zn)
M1–O3
M1–O2
M1–N3
O3–M1–O2
O3–M1–O2
O3–M1–N3
O3–M1–N3
O2–M1–N3
O2–M1–N3
2.066(2) 2.061(2) 2.083(1) 2.071(2) 2.068(2)
2.099(1) 2.091(1) 2.105(2) 2.097(1) 2.093(1)
2.197(4) 2.188(3) 2.149(3) 2.214(3) 2.211(3)
91.28(1) 91.22(1) 89.38(1) 88.43(1) 88.34(5)
88.72(8) 88.78(8) 90.62(3) 91.57(8) 91.66(1)
88.28(6) 87.04(6) 88.92(2) 88.21(6) 87.33(6)
91.72(7) 92.96(7) 91.08(4) 91.79(7) 92.67(8)
87.86(2) 87.72(2) 87.98(8) 87.62(3) 87.86(2)
92.14(3) 92.28(3) 92.02(6) 91.38(2) 92.14(4)
CP1, CP2, CP4 and CP5 are isostructural and they crystallise
in the Pccn space group. The common structural feature of the
CPs is that in each case the M1 atom (Zn1 in the case of the
zinc CPs, Co1 in the case of the Co CPs) lies in an inversion
centre; as does the dicarboxylate. The CPs have prominently
intramolecular hydrogen bonds, interchain hydrogen bonds
and C–H⋯π interactions. As the origins of the CPs other than
CP3 are from two dicarboxylates, namely fumarate and
succinate, with two methane or methylene units as
intervening moieties between two carboxylates (Fig. 2d), CP1,
CP2, CP4, and CP5 have similar hydrogen bonds (Fig. 2b).
CP4 has a difference in the hydrogen bonds with respect to
the other three CPs: it does not have the intermolecular
CO⋯H hydrogen bonds connecting the chains. In all the
cases there are intramolecular hydrogen bonds between an
O–H bond and oxygen atom of CO of the nearest mono-
dentate carboxylate providing a cyclic motif with R11(6) graph
set notation.68–72 The C–H⋯π interactions in CP1, CP2 and
CP3 packing contributing to construction of the two-
dimensional architectures are shown in Fig. 3a–c. Whereas
their packing patterns viewed along the a-crystallographic axis
in the case of CP1 and viewed along the b-crystallographic axis
of CP2 and CP3 are shown in Fig. 3d–f.
These chains are arranged such that the urea parts occupy
the spaces between the two chains of the CP. This self-
encapsulation is facilitated by the formation of urea tapes as
illustrated in Fig. 3b. The distance between the two nearest
neighbouring metal centres in CP1 is 15.87 Å, in CP2 is 15.85
Å, in CP4 is 18.87 Å and in CP5 is 15.88 Å; hence a suitable
orientation of the phenyl ring with respect to a neighbouring
pyridine ring of two independent chains makes it possible to
have uniform C3–H⋯π (C3–H⋯πcentroid
=
3.238 Å)
interactions in the CPs (Fig. 2c) other than CP4. CP4 has a
different orientation of the phenyl group and thus this C3–
H⋯π interaction is lacking. Among the weak interactions,
C3–H⋯π interactions have contributions of 1.5–2.5 kcal
mol−1, and these are very important in conformational
flexible systems.73 Thus, in the present series of CPs, they
contributed to the orientation of phenyl groups thereby
making the differences in the hydrogen bonding scheme in
CP4 (Table 2). Inter-chain CO⋯H–O interactions were also
observed in CP4. To indicate the different motifs present in
Fig. 2 (a) Prominent hydrogen bonds in the assembly of L·H2O. (b)
The urea tapes and (c) C–H⋯π interactions in the self-assembly of the
coordination polymer CP1. (d) Weak interactions in the self-assembly
of CP3 (thermal ellipsoids are drawn with 40% probability).
are influenced by the ancillary ligands. These CPs are not
exceptional in that regard and have very close analogies, yet
possess distinctive structural features due to the
supramolecular features and flexible part of the urea moiety.
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Fig. 3 (a)–(c) C–H⋯π interactions in CP1, CP2 and CP3 respectively, and two-dimensional structures of (d) CP1 viewed along a-crystallographic
axis and (e) CP2 and (f) CP3 viewed along b-crystallographic axis.
Table 2 Comparison of hydrogen bond parameters of the coordination polymers
Distance (Å)/angle (°)
CP1
CP2
CP3
CP4
CP5
N1–H⋯O1 (dD⋯A
N2–H⋯O1 (dD⋯A
N2–H⋯O4 (dD⋯A
O2–H⋯O1 (dD⋯A
O2–H⋯O4 (dD⋯A
O2–H⋯O4 (dD⋯A
∠N1–H⋯O1
)
)
)
)
)
)
2.835(4)
2.876(3)
No interaction
No interaction
2.674(3)
2.675(3)
160(3)
149(4)
No interaction
166
2.829(3)
2.870(3)
No interaction
No interaction
2.688(3)
2.662(3)
158(4)
151(4)
No interaction
152
No interaction
No interaction
2.922(2)
2.753(2)
No interaction
2.669(2)
No interaction
No interaction
167(2)
No interaction
146
2.830(3)
2.874(3)
No interaction
No interaction
2.676(3)
2.672(3)
149
2.834(3)
2.870(3)
No interaction
No interaction
2.685(3)
2.668(2)
157(3)
149(3)
No interaction
152
∠N2–H⋯O1
143
∠N2–H⋯O4
No interaction
150
No interaction
177
∠O2–H⋯O4
∠O2–H⋯O1
No interaction
156(3)
No interaction
178(5)
No interaction
176(4)
∠O2–H⋯O4
166(2)
CP3 has no bifurcated hydrogen bonds among urea moieties. Atom labels are as in Fig. 4.
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Fig. 4 Schematic representations of different motifs in the CPs: (a) and (b) in the self-assemblies of CP1, CP2, CP4, CP5; (c) in CP3.
the CPs, simplified schematics for the two different types of
packing observed are shown in Fig. 4. The presence of
CO⋯H–O interactions in the other CPs provided a robust
hydrogen bonded motif which has R23(13) graph set
carboxylate complexes.75 CP3 is a CP having terephthalate
bridges; the orientations of the urea with respect to
neighbouring urea units were different from those of the
other CPs. In this case the urea tapes were not observed. The
CP has R11(6) intramolecular hydrogen bonded motifs similar
to the other CPs (Fig. 4a). The inter-chain interactions were
through the urea CO on one side of a ligand from a chain
notation74 (Fig. 4a). These interactions provided
a 2D
supramolecular CP in each case. The C–H⋯π interactions
control self-assemblies such as multi-component inorganic
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forming hydrogen bonding with an N–H bond of coordinated
ligand associated with another chain. One NH located next to
a phenyl group forms a hydrogen bond to an oxygen atom of
a CO of a coordinating carboxylate group from a third
chain (Fig. 4c) providing a three-dimensional network. The
supramolecular features and associated properties of first
loss of two water molecules of the lattice (calculated 5.4%). A
second weight loss of 17.12% occurred from 168 °C to 232 °C
due to the loss of one fumarate (calculated 17.03%). Weight
loss was complete at 312 °C to give ZnO (experimental
11.61%, calculated 12.11%). The loss of water molecules at
two stages was also observed in CP2 (121–184 °C). This was
in a similar way to that of the water loss from the hydrated
ligand. Whereas, for the other four coordination polymers
CP1 (at 150–175 °C), CP3 (at 86–111 °C), CP4 (at 126–148 °C),
and CP5 (at 117–168 °C) the two water molecules were lost in
a single step (Fig. S8†). Two points clarified from the
thermogravimetry results were that, water molecule loss
thermally from the coordination polymers occurred at higher
temperatures than the normal boiling point of water. It was
also revealed that loss of water molecules in two steps
occurred due to a change in the coordination sphere to
transform to another form of CPs. Having retained one
coordinated water molecule per metal site, this particular
water molecule probably served as bridge to crosslink the
chains of CP2 by dehydration. The literature suggests that
aqua-bridged cobalt carboxylate complexes were easily
formed, whereas zinc complexes often formed carboxylate
complexes with or without coordinated water.77,78 Water of
crystallisation and coordinated water were observed to
dehydrate at distinguishable temperatures in CPs.79 The
stepwise loss of aqua ligand in different hydrogen bonding
environments upon heating from carboxylate complexes were
reported in specific examples.80,81 Thus, the metal ions and
the intrinsic hydrogen bonding caused differences in the
number of steps in which water was lost in the case of the
zinc CPs compared to the cobalt CPs.
row
transition
metal
dicarboxylate
CPs
having
naphthalimide-functionalised pyridine as co-ligands were
found to be guided by dipolar π-stackings among
naphthalimide rings.76 In the present case, the
supramolecular features mainly stem from the urea part. The
hydrogen bonding of urea and carboxylates caused large
differences in packing patterns among the CPs. The powder
XRD patterns of the CPs showed their phase purity. In each
case the calculated peaks for different {hkl} were reflected in
the experimental powder XRD patterns. The overlaid
experimental powder XRD patterns and the one generated
from respective crystallographic information files are given in
the ESI† (Fig. S7†).
The different types of hydrogen bond environments
around water molecules of the hydrated ligand and the CPs
were reflected in their respective thermograms. The hydrated
ligand showed 7.88% weight loss from 63 °C to 121 °C [Fig.
S8(a)†]. This corresponded to the loss of one molecule of
water of crystallisation (calculated 7.34%). For CP1, a weight
loss of 5.45% in the temperature range 150 °C to 175 °C was
observed. This occurred due to the loss of two molecules of
water of crystallisation (calculated 5.41%). The second weight
loss of 17.72% from CP1 occurred from 175 °C to 233 °C due
to the loss of one succinate (theoretical value 17.44%).
Further weight loss was complete at 342 °C, which yields
CoO (experimental 11.76%, calculated 11.25%). In the cobalt
coordination polymer CP2, dehydration took place in two
steps. The first weight loss of 2.7% occurred from 121 to 156
°C, which corresponded to the loss of one molecule of water
of crystallisation (calculated 2.72%). The second weight loss
amounting 3.82% occurred from 156 to 184 °C due to the
loss of the second molecule of water of crystallisation
(calculated 2.72%). A third weight loss of 17.29% occurred
from 182 °C to 234 °C due to the loss of one fumarate
(calculated 17.19%). Further weight loss was complete at 385
°C, which yields CoO (experimental 11.41%, calculated
11.30%). In CP3, a weight loss of 4.37% was observed from
86 °C to 111 °C due to the loss of two molecules of water of
Photoluminescence study
There are many examples of zinc complexes where the
conformational aspects of ligands as well as the metal to
ligand charge transfer play roles in deciding emission
properties.82–85 There are many fluorescent compounds
which show aggregation-induced emission in solid or in
solution, being referred to as AIEgens. The urea derivatives
have an interest as AIEgens86,87 as their deprotonating ability
and ability to form gels make them advantageous.88,89 A solid
powdered sample of the ligand has an absorption band at
291 nm and it showed a broad emission with closely spaced
fluorescence emission maxima at 473 nm, 492 nm, and 530
nm upon excitation at 330 nm. The large shifts of the
emission peak towards higher wavelength are attributed to
the aggregation-induced emission. The compound L·H2O was
weakly emissive, showing emission peaks at 373 nm and 396
nm, and it also showed aggregation-induced quenching
followed by enhancement in DMSO upon addition of different
fractions of water [Fig. 5(i)]. It was found that with an
increasing concentration of water to 60% water, aggregation-
induced quenching took place at 373 nm and 396 nm,
whereas an increase in the concentration above 60% water
resulted in the enhancement of emission at 373 nm and 400
crystallisation (calculated 5.05%).
A second weight loss
amounting to 22.6% occurred from 111 °C to 238 °C; this
was due to the loss of one terephthalate (calculated 23%).
Further weight loss upon heating until 468 °C yielded CoO
(experimental 9.4%, calculated 10.5%). In CP4, a 5.13%
weight loss took place between 126 °C and 148 °C due to the
loss of two molecules of water of crystallisation (calculated
5.36%). Between 148 °C and 236 °C, a weight loss of 17.36%
was observed due to the loss of one succinate (calculated
17.3%). Further weight loss took place at 265 °C to give ZnO
(experimental 11.38%, calculated 12.11%). In CP5, a 5.7%
weight loss took place between 117 °C and 168 °C due to the
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Fig. 5 (i) Fluorescence emission spectra of L (10−4 M) in DMSO with concentration of water varying from 10% to 90% (
373 nm,
396 nm,
377 nm, * 399 nm). (ii) Solid-state photoluminescence spectra of (a) L·H2O (λem = 473 nm, 492 nm, 530 nm), (b) CP1 (λem = 482 nm), (c) CP2 (λem
409 nm), (d) CP3 (λem = 408 nm), (e) CP4 (λem = 381 nm, 506 nm), (f) CP5 (λem = 394 nm, 467 nm). In all cases, the excitation was carried out λex
330 nm.
=
=
nm. An 1HNMR titration carried out by adding D2O to a
solution of L showed that only the chemical shifts of the N–H
were affected with increasing concentration of D2O. This
suggested that the compound did not change its ground state;
there were no structural changes observed from the titration
(Fig. S10†). UV-visible spectra of the zinc CPs CP4 and CP5
showed a ligand-based π–π* transition; the cobalt CPs had
such a ligand-based absorption as well as the d–d transition.
Namely, CP1–CP3 had additional visible absorption peaks due
found in the present examples of zinc CPs. Furthermore, in a
recent study, aggregates of zinc carboxylate complexes were
found to show emission with low PLQY in a similar region.82
The emission of zinc CPs occurring around 400–510 nm
originated from metal to ligand charge transfer transition,
where aggregation affected the emission positions, whereas
the broad emission of the ligand in the region of 473–530 nm
was due to aggregation-induced emission.
to d–d transition at 494 nm, 498 nm and 498 nm respectively. Dye adsorptions by the coordination
Due to the paramagnetic effect of cobalt(II), CP1–CP3 are non-
emissive [Fig. 5(ii)]. On the other hand, CP4 and CP5 showed
polymers
dual emission: namely, the zinc-based CP4 emitted at 381 nm
and 506 nm whereas CP5 emitted at 394 nm and 467 nm
upon excitation at 330 nm. The compound CP5 has olefin
double bond giving it higher rigidity than CP4, which has a
flexible ethylene bridging unit between the carboxylates.
Hence, orientation of the urea is slightly different between the
CPs; this caused the CPs to have a large packing difference
between them. Thus, changes in the hydrogen bonding
schemes caused differences in the aggregation leading to
changes in the emission positions. Dual emissions were
observed in zinc complexes due to emission through twisted
intramolecular charge-transfer of metal charge transfer
transitions90–92 and in the present case also such a situation
occured with emission at a shorter and a longer wavelength
due to such paths being operative. The solid-state
photoluminescence quantum yields (PLQY) of L·H2O, CP4 and
CP5 were determined and were found to be 0.0899, 0.0474
and 0.0039. In an earlier systematic study on solid-state
photoluminescence of a series of Schiff base-derived zinc
complexes, it was found that the emission positions due to
metal to ligand charge transfer were dependent on the
substituent on the ligand.93 Such emissions occurred in the
range of 375–600 nm and had consistently low PLQYs; very
limited exceptions with high PLQY were observed. The
observed PLQY values were of similar magnitude to those
The present set of CPs are water-insoluble; hence we have
explored their utility in adsorbing dyes through hydrogen
bonding. Dye adsorption as a result of hydrogen bonding
abilities of various metal frameworks has been reported in
the literature.94–96 Two different dyes, namely methyl orange
(MO) and phenol red (PR) (Fig. 6A and B), were chosen with
the expectation that the anionic dye MO would bind to the
available urea N–H sites easily by hydrogen bonds (Fig. 6C)
and PR would have lower binding ability unless it undergoes
a ring opening reaction to bind through sulphonate group as
illustrated in Fig. 6D and E.
A hydrolytic ring opening of PR assisted by a CP would
yield ring opened quinone methide form from bis-phenol
form of PR (Fig. 6B). Indeed, the CPs could be used for
selective adsorption of dye which causes decolourisation of
the dye. All the CPs showed superior decolourising abilities
for MO. The changes in the adsorption profiles of the two
dyes, namely MO and PR, were monitored at 464 nm and 432
nm respectively. The spectral changes are shown in
Fig. 7(a) and (b). The spectra were from the samples after
treating independently for three hours with respective CP as
suspension. From the plots in Fig. 7(a) it was found that CP3
recognised MO selectively as compared with the other CPs. It
was found that 1.986 mg g−1 MO per gram of CP3 was
decolourised; in comparison to this, 1.320 mg g−1
,
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Fig. 6 Structures of methyl orange (A) and two forms of phenol red (B). (C)–(E) Possible hydrogen bonding through sulphonate group of the two
dyes.
Fig. 7 UV-visible spectral changes of (a) methyl orange (MO) and (b) phenol red (PR) after treating with CP1, CP2, CP3, CP4, CP5 and CP6. (c and
d) The relative removal efficiencies (the spectra were taken after treatment of CPs with the dye for 3 h).
1.369 mg g−1, 0.867 mg g−1 and 0.731 mg g−1 of MO
respectively were decolourised by CP1, CP2, CP4 and CP5. In
contrast to these CP1, CP2, CP4 and CP5 could not cause
decolourisation of MO. CP2 caused a fraction of ring opening
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reaction of PR as reflected in development of a new peak at
560 nm. CP2 however decolourised 0.714 mg g−1 of PR. In
this case a small fraction of the PR underwent a ring opening
reaction. CP1 did not show selectivity in adsorption and had
comparable adsorptions of both the dyes in a similar manner
attention. The adsorption of MO with MOF CP6 was
ascertained by us and it was found to be 1.309 mg g−1. Hence,
the study of the adsorption ability for the two dyes of the
present set of CPs has helped to understand the observed
recognition between the two dyes by the CPs.
as CP2.
A
set of zinc CPs derived from 1,3,5-
benzenetricarboxylic acid and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-
butadiene were utilised for cationic dyes,96 but were
ineffective towards adsorption of MO which is an anionic
dye. The relative removal efficiencies are shown in
Fig. 7(c) and (d). Since the amounts adsorbed with respect to
weight percent of CP is very small, we would suggest the CP
decolourising ability to be of a higher priority than dye removal
process to find an application in effective dye removal.
Conclusions
In conclusion we have shown that a change in metal ion and
carboxylate moieties brings about subtle changes in the weak
hydrogen bonding scheme in a series of dicarboxylate CPs.
The metal ions influenced the fluorescence spectra; in case
of zinc complexes dual emissions were observed from solid
powdered samples. The cobalt complexes were non-
fluorescent due to the paramagnetic effect. Having
terephthalate bridges rather than succinate or fumarate
These results indicated that MO adsorption by CP3 was
selective and the same CP was not able to decolourise PR. This
had happened because of the higher hydrogen bond formation
ability of the oxygen atom bearing negative charge of
sulphonate group of MO with urea moiety of the CPs. Thus, it
provided a means to recognise MO through decolourisation
that was affected by formation of hydrogen bonds with the
CPs. We attempted to study a competitive adsorption from a
mixture of MO and PR, but found that an equimolar mixture of
MO and PR showed a common UV-visible peak at 443 nm (Fig.
S12†). This deterred us from studying the competitive
adsorption aspect further. Among different dyes, MO has been
widely studied with different adsorbents, and above 100%
efficiency in its adsorption was suggested with MIL-100-Fe.98
The amounts of adsorption of MO by some adsorbents
reported in the literature are compared in Table 3. In practice
MOFs are good adsorbents of dyes. From the comparison in
Table 3 it is clear that the adsorption observed with the CPs in
this study had low intake capability and will not have practical
application. Furthermore, the analyses of MOFs studied in the
literature were carried out through extrapolation of the data
collected from studies performed in solutions by using them in
suspension or in heterogeneous conditions. In those
experiments proportions of dye used with respect to the
amount of adsorbent were also very low; hence practical
utilities of those claims on the adsorptions also require
provided
a
different self-assembly having urea easily
accessible to bind to MO better than to PR. The presence of
C–H⋯π interactions involving phenyl ring next to urea
caused conformational locking to decide the course of self-
assembled structures of the CPs.
Experimental
Synthesis and characterisation
1-Phenyl-3-((pyridin-4-yl)methyl)urea (L). To a solution of
phenyl isocyanate (0.216 mL, 2 mmol) in diethyl ether (30
mL), 4-aminomethylpyridine (0.203 mL, 2 mmol) was added
dropwise and stirred at room temperature overnight. A white
precipitate was formed; the filtered and dried sample was L.
The precipitate was dissolved in methanol–water solution (10
mL, 1 : 1 v/v) and the solvent allowed to evaporate slowly at
1
25 °C which yielded colourless crystal of L·H2O. Yield 90%. -
HNMR (DMSO-d6, 600 MHz): 8.69 (s, 1H), 8.50 (d, J = 6 Hz,
1H), 7.41 (d, J = 6 Hz, 1H), 7.29 (d, J = 6 Hz, 1H), 7.23 (t, J = 6
Hz, 1H), 6.91 (m, J = 6 Hz, 1H), 6.73 (t, J = 6 Hz, 1H), 4.33 (d,
J = 6 Hz, 2H). 13CNMR (DMSO-d6, 150 MHz): 155.36, 149.67,
149.51, 140.34, 128.70, 122.01, 121.31, 117.87, 41.85 ppm.
HRMS found (m/z) 228.1194, calculated (m + 1/z) 228.11. IR
(cm−1): 1675 (s), 1599 (s), 1555 (s), 1497 (s), 1440 (s), 1415 (s),
Table 3 Amounts of methyl orange adsorbed by various adsorbents
Amount
Adsorbent
(mg g−1
)
Ref.
Chromium terephthalate MOFs (MIL-101) (MIL = Matérial Institut Lavoisier)
Iron 1,3,5-benzenetricarboxylate MOF (MIL-100-Fe)
Chromium 1,3,5-benzenetricarboxylate MOF (MIL-100-Cr)
Chromium amino-functionalised carboxylate MOF (NH2-MIL-101-Al)
{[Zn2(HL1)2(L2)(H2O)2](C2H5OH)3}n (H3L1 = 1,3,5-benzenetricarboxylic acid, L3 =
1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene)
114–194 97
1045
736
188
0.4
98
98
99
96
{[Ag(L3)2](BF4)}n (L3 = N1,N4-bis((pyridin-3-yl)methyl)fumaramide)
Melaminium adipate
37.88
2.35
100
101
{[Ni2(L4)3]·Cl·solvent}n where HL4 = 3,5-di(1H-imidazol-1-yl-phenyl)-2H-tetrazole
[Co2L2(Suc)(H2O)2]n
[Co2L2(Fum)(H2O)2]n
[Zn2L2(Suc)(H2O)2]n
[Zn2L2(Fum)(H2O)2]n
285
102
1.320
1.369
0.867
0.731
Present
work
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Table 4 Crystallographic parameters of the ligand L and the coordination polymers
Parameters
L
CP1
CP2
CP3
CP4
CP5
Formula
CCDC
Mol. Wt.
Space group
a (Å)
b (Å)
c (Å)
α (°)
C13H15N3O2
2063021
245.28
C30H34CoN6O8
2063022
665.56
Pccn
11.653(7)
30.448(14)
8.960(5)
90
C30H32CoN6O8
2063023
663.54
C34H34CoN6O8
2063024
713.60
C30H34N6O8Zn
2063025
672.00
Pccn
11.626(7)
30.46(2)
8.952(4)
90
C30H32N6O8Zn
2063026
669.98
Pna21
Pccn
P21/n
Pccn
12.3714(12)
18.5178(17)
5.6651(5)
90
30.4091(14)
11.6865(5)
8.9577(3)
90
11.4051(14)
8.0096(10)
19.454(2)
90
11.700(4)
30.466(10)
8.959(3)
90
β (°)
90
90
1297.8(2)
1.255
0.087
90
90
3179(3)
1.391
0.598
90
90
3183.4(2)
1.385
0.597
106.613(3)
90
1702.9(4)
1.392
90
90
3170(3)
1.408
0.833
90
90
γ (°)
V (Å3)
3193.4(19)
1.394
0.827
Density (g cm−3
)
Abs. coeff. (mm−1
)
0.564
F (000)
520
1388
1380
742
1400
1392
Total no. of reflections
Reflections, I > 2σ(I)
Max. θ (°)
3226
2393
28.285
−16 ≤ h ≤ 16
−24 ≤ k ≤ 24
−7 ≤ l ≤ 7
100
3226/1/179
1.093
4003
3219
2822
2259
25.043
−25 ≤ h ≤ 36
−6 ≤ k ≤ 13
−9 ≤ l ≤ 10
99.9
2822/0/218
1.000
4351
3639
28.595
−15 ≤ h ≤ 15
−10 ≤ k ≤ 10
−26 ≤ l ≤ 26
100
4351/0/236
1.078
2770
2218
3979
3324
28.343
−15 ≤ h ≤ 15
−40 ≤ k ≤ 40
−11 ≤ l ≤ 11
100
3979/0/218
1.083
28.663
−15 ≤ h ≤ 15
−40 ≤ k ≤ 40
−12 ≤ l ≤ 11
99.5
25.048
13 ≤ h ≤ 13
−36 ≤ k ≤ 36
−10 ≤ l ≤ 10
98.4
Ranges (h, k, l)
Complete to 2θ (%)
Data/restraints/parameters
GooF (F2)
4003/3/218
1.021
2770/6/206
1.045
R indices [I > 2σ(I)]
wR2 [I > 2σ(I)]
R indices (all data)
wR2 (all data)
0.0641
0.1804
0.0928
0.2067
0.0502
0.2083
0.0678
0.2259
0.0444
0.1370
0.0570
0.1477
0.0310
0.0805
0.0403
0.0858
0.0383
0.1159
0.0556
0.1243
0.0420
0.0873
0.0511
0.0909
1316 (s), 1236 (m), 808 (s), 744 (m), 688 (s), 631 (w), 579 (s),
471 (s), 454 (s). UV, solid (λmax): 291 nm.
12.99. IR (cm−1): 1634 (s), 1556 (m), 1490 (w), 1444 (s), 1422
(s), 1396 (m), 1313 (s), 1295 (s), 807 (s), 771 (s), 734 (w), 693
(w), 660 (s), 589 (s), 502 (s), 484 (s). UV-visible, solid (λmax):
282 nm, 494 nm.
General procedure for synthesis of the coordination polymers
[Co2L2(Fum)(H2O)2]n (CP2). Pink needle-like crystals,
isolated yield 64% based on Co. Elemental analysis (%) calcd:
C, 54.25; H, 4.82; N, 12.66; found: C, 53.77; H, 5.07; N, 12.69.
IR (cm−1): 1635 (s), 1554 (m), 1443 (s), 1422 (s), 1384 (s), 1312
(s), 1294 (s), 800 (s), 770 (s), 734 (w), 711 (w), 672 (s), 590 (s),
500 (s), 485 (s). UV-visible, solid (λmax): 282 nm, 498 nm.
[Co2L2(Ter)(H2O)2]n (CP3). Pink block crystals, isolated
yield 63% based on Co. Elemental analysis (%) calcd: C,
57.17; H, 4.76; N, 11.77; found: C, 57.25; H, 5.18; N, 11.72. IR
(cm−1): 1655 (s), 1574 (w), 1550 (w), 1524 (w), 1436 (s), 1424
(s), 1376 (s), 1311 (s), 1271 (m), 1231 (s), 752 (s), 692 (s), 641
(w), 692 (s), 513 (w), 481 (s), 461 (s). UV-visible, solid (λmax):
324 nm, 498 nm.
[Zn2L2(Suc)(H2O)2]n (CP4). White plate-like crystals, isolated
yield 59% based on zinc. Elemental analysis (%) calcd: C, 53.57;
H, 5.06; N, 12.50; found: C, 53.72; H, 5.37; N, 12.78. IR (cm−1):
1634 (s), 1554 (m), 1490 (w), 1444 (s), 1422 (s), 1395 (m), 1313
(s), 1294 (s), 807 (s), 771 (s), 735 (w), 661 (s), 588 (s), 502 (s), 483
(s). UV-visible, solid (λmax)L 262 nm, 368 nm.
A solution of ligand L (0.454 g, 2 mmol) in methanol (10 mL)
was prepared and to this solution dicarboxylic acid (1 mmol)
was added and was stirred for 30 minutes at room
temperature. To this solution the corresponding metal
acetate, namely cobalt acetate tetrahydrate (0.249 g, 1 mmol)
or zinc acetate dihydrate (0.220 g, 1 mmol), was added and
the reaction mixture was refluxed for one hour. A pink
precipitate was formed in the case of cobalt complexes,
whereas a dirty white precipitate was formed in the case of
zinc complexes. The precipitate obtained was redissolved in
DMF–water mixture (1 : 1 v/v, 5 mL). The resulting solution
was filtered and kept undisturbed for crystallisation, which
yielded crystals of the corresponding CP. The CPs were
characterised
by
determining
crystal
structure,
thermogravimetry, IR spectroscopy and powder XRD.
[Co2L2(Suc)(H2O)2]n (CP1). Pink needle-like crystals,
isolated yield 62.5% based on Co. Elemental analysis (%)
calcd: C, 54.09; H, 5.10; N, 12.62; found: C, 53.97; H, 5.27; N,
[Zn2L2(Fum)(H2O)2]n (CP5). White block crystals, isolated
yield 60% based on Zn. Elemental analysis (%) calcd: C,
53.73; H, 4.77; N, 12.54; found: C, 52.51; H, 5.09; N, 12.93. IR
(cm−1): 1634 (s), 1554 (m), 1489 (w), 1443 (s), 1422 (s), 1382
(s), 1312 (s), 1293 (s), 1223 (w), 802 (s), 770 (s), 734 (s), 673
(s), 588 (s), 500 (s), 483 (s). UV-visible, solid (λmax): 281 nm,
384 nm.
Table 5 Photoluminescence quantum yield of solid samples
Compound
Emission wavelength (λex = 330 nm)
PLQY (ΦF)
L·H2O
CP4
473, 492, 530
381, 506
0.0899
0.0474
0.0039
CP5
394, 467
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Crystallographic data. Single-crystal X-ray diffraction data
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Physical measurements
of CP2 were collected with an Oxford SuperNova
diffractometer. The data for CP3 and CP5 were collected with
a Bruker Nonius SMART APEX CCD diffractometer. The data
for CP1 and CP4 were collected with a Bruker D8 Venture
single-crystal X-ray diffractometer. CrysAlisPro software was
used for analysis of data obtained by the Oxford SuperNova
diffractometer, whereas SAINT and XPREP were used for data
obtained with the Bruker Nonius SMART APEX CCD
diffractometer. Direct method was used for structure solution
using SHELXL-2014 software. All non-hydrogen atoms were
refined in anisotropic approximation against F2 of all
reflections. Hydrogen atoms were placed at their geometric
positions by riding and refined in the isotropic
approximation. However, in CP4, the H2C atom could not be
located by riding and hence was fixed at calculated position
and refined. Crystallographic parameters are listed in
Table 4.
Dye adsorption study. The coordination polymers CP1–
CP5 (60 mg each) were each added to separate aqueous
solutions of MO or PR (10 mL, 5 × 10−5 M) and stirred for 3
hours. Then the solutions were centrifuged for 10 min at
7000 rpm (25 °C) to isolate the solid CPs from the liquid. The
visible spectrum of supernatant solution of each was
recorded. The visible spectra of the original aqueous
solutions of MO or PR (10 mL, 5 × 10−5 M) were also
recorded. The amount of dye adsorbed (mg g−1) was obtained
by the following equation:96
1
1HNMR, 13CNMR and 2D HOMO-COSY HNMR spectra were
recorded with a Bruker Ascend-600 MHz using TMS as
internal standard. ESI-mass spectra were recorded with a
high-resolution mass spectrometer (Agilent QTOF 6520).
Thermogravimetric analyses were performed using a thermal
analyzer (SDTQ600) with a simultaneous DTA/TGA system,
under nitrogen with a heating rate of 10 °C min−1. Powder
X-ray diffraction patterns were recorded using a Bruker
powder X-ray diffractometer D2 phaser. Infrared spectroscopy
of the solid samples were recorded with a PerkinElmer
Spectrum Two FT-IR spectrometer in the region 4000–400
cm−1 by using the ATR method. Fluorescence spectra of all
solid samples and aggregation-induced emission were
measured
with
a
Horiba
Scientific
FluoroMax-4
spectrofluorometer. UV-visible spectra of the solid samples
were measured with a PerkinElmer Lambda 750 UV/vis
spectrometer. Dye adsorption by CPs was recorded with a
Cary 100 UV-visible spectrophotometer. Elemental analyses of
CP1–CP5 were recorded with a CHNS elemental analyser
(model: Eurovector EA3000). Quantum yields of solid samples
were measured by taking a definite amount of the solid and
exciting at a required wavelength with a Horiba Fluoromax
Plus spectrofluorometer using the Petite Integrating Sphere
method. The fluorescence emission (Ec) and the scatter (Lc)
for sample and blank (La and Ea) were recorded. From these
spectral measurements (sample and blank), the quantum
yields were calculated by using the equation ϕ = [(Ec − Ea)/(La
− Lc)].
MW
m
Amount of dye adsorbed ¼ ðCi − CeÞV ×
Conflicts of interest
where Ci and Ce are the concentrations of each dye (mol L−1)
There are no conflicts to declare.
before and after treatment with CPs, V is the volume of the
solution (in L), MW is the molecular weight of dye and m is
the mass of the CP used (in g).
Acknowledgements
The authors thank Ministry of Human Resources and
Development, Government of India, New Delhi, for financial
support through departmental grant (No. F. No. 5-1/2014-TS.
VII), Guwahati Biotech Park for elemental analysis of the CPs
and North East Centre for Biological Sciences and Healthcare
Engineering, IIT Guwahati Indian Institute of Technology
Guwahati for use of SCXRD facility (project no. BT/COE/34/
SP28408/2018) for two crystal structures reported here.
The removal percentage of dye was calculated by the
following equation:
ðCi − CeÞ
Percentage of dye removed from solution ¼
× 100%
Ci
The observed values for amount of dye adsorbed by each CP
and the percentage of dye removed are listed in ESI† Table
S1.
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