A mechanistic insight into the rapid and selective removal of Congo Red by an amide functionalised Zn(ii) coordination polymer

Article abstract of DOI:10.1039/d0dt02172e

New CPs [Zn(μ-1κOO′:2κN-L)(H2O)(BDC)0.5]n·n(DMF) (1), [Cd(μ-1κO:2κN-L)2(H2O)2]n (2), and [Pb(μ-1κOO′:2κO′-L)(μ-1κO:2κO′:3κN-L)]n (3) [L = 4-(pyridin-3-ylcarbamoyl)benzoate; BDC = benzene-1,4-dicarboxylate] were synthesized and characterized by elemental, FT-IR, powder, and single-crystal X-ray diffraction analyses. Single crystal X-ray diffraction analysis discloses 1D polymeric architectures for 1 and 2 and a 2D one for 3. The topological analysis exemplifies a 2,2,3-connected 3-nodal net with the point symbol {82·12}2{8}3 for 1, a 2,4-connected bimodal net for 2, and a 3,4,7-connected trinodal net for 3. CP 1 shows a selective removal of the Congo Red (CR) dye amongst various dyes. It can be recycled and reused without any significant loss of its dye removal efficiency. An insight into the selective removal of the Congo dye is provided by in silico studies, being accounted for by anion-π, cation-π, and π-π stacking interactions, involving the Zn(ii) ion, phenyl rings, and Ocarboxylate of L, and the phenyl rings, naphthalene rings, and Osulfonate of CR.

Full text of DOI:10.1039/d0dt02172e

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A mechanistic insight into the rapid and selective removal of Congo Red by
an amide functionalised Zn(II) Metal Organic Framework
Anup Paul,*,† Kabita Das,‡ Anirban Karmakar,† M. Fátima C. Guedes da Silva,† Armando J. L.
Pombeiro*,†
†Centro de Química Estrutura, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco
Pais, 1049-001 Lisboa. Portugal.
‡Department of Environmental Studies, North-Eastern Hill University, Meghalaya, Shillong
793022, India
Abstract:
The new CPs [Zn(-1OO’:2N-L)(H2O)(BDC)0.5]n.n(DMF) (1), [Cd(-1O:2N-L)2(H2O)2]n
(2) and [Pb(-1OO’:2O’-L)(-1O:2O’:3N-L)]n (3)
[L
=
4-(pyridin-3-
ylcarbamoyl)benzoate; BDC = benzene-1,4-dicarboxylate] were synthesized and characterized
by elemental, FT-IR, powder and single-crystal X-ray diffraction analyses. Single crystal X-ray
diffraction analysis discloses 1D polymeric architectures for 1 and 2 and a 2D one for 3. The
topological analysis exemplifies a 2,2,3-connected 3-nodal net with the point symbol
{82.12}2{8}3 for 1, a 2,4-connected bimodal net for 2 and a 3,4,7-connected trinodal net for 3.
The CP 1 exhibits a selective removal of Congo Red (CR) dye amongst various dyes. It can be
recycled and reused without any significant loss of its dye removal efficiency. An insight in the
selective removal of the Congo dye is provided by in silico studies, being accounted for by anion-
π, cation-π and π-π stacking interactions, involving the Zn(II) ion, phenyl rings and Ocarboxylate
of L, and phenyl, naphthalene rings and Osulfonate of CR.
*Corresponding authors, E-mail: kanupual@gmail.com, pombeiro@tecnico.ulisboa.pt.
1

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Introduction
Organic dyes, as important industrial components in paper, textiles, plastics, rubber and coatings
etc. are extensively used.1 Under an environmental perspective, the removal of these harmful
dyes before being released into the natural system is an important mattter.1 A lot of methods for
the elimination of dyes from an aqueous environment have been developed, namely by using
activated carbon, filtration, photocatalysis, adsorption, and so on.2–9 However, some of the
methods, such as using activated carbon and filtration, are merely effective for removal of low
concentrations of pollutants from water. The adsorption method in particular with no chemical
deterioration is considered to be highly advantageous due to its effectiveness, efficiency and
economy. However, a major setback associated with these adsorbents concerns their reusability.
It is thus necessary to develop a kind of material to overcome these difficulties, which will not
only reduce the pollution caused by the dyes but will also achieve a high selectivity, being able
to be recycled.
Over the years, a great attention has been paid on the ever-increasing research area of novel
functional metal-organic frameworks (MOFs) or coordination polymers (CPs) due to their
attractive features, such as luminescence, magnetism, applications in catalysis, etc.10–16 A key
to building an ideal MOF is usually lying in the design of appropriate ligands and generally a
simple adjustment in the ligand framework can result in novel structures.10,11,16 Apart from a
wide range of applications of MOFs in various fields, they have also been utilized as an excellent
material for adsorption of dyes.6–9 However, the majority of MOFs that possess this feature has
high dimensional structures.17–20 On the contrary, MOFs with low-dimensional structures
exhibiting a high dye adsorption are rare, but this ability can be fostered by promoting specific
interactions, such as electrostatic connections, hydrogen-bonding and π-π interaction.17 These
2

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specific interactions between MOFs or CPs and dye endow the former the ability to discriminate
different types of dyes towards a selective separation. The design and synthesis of new low
dimensional MOFs that can lead to the separation of dyes effectively is thus an important issue.
Pyridyl-amide functionalized ligands are known to behave as good linkers for generating
multi-dimensional MOFs that have been used in a number of applications, such as, in gas storage
and separation, optical sensing, catalysis, as well as for adsorption of dyes.13,15,16,21–25 We have
already reported some CPs, based on new amide functionalized ligands, which were found to be
effective heterogeneous catalysts in various organic transformations.13,16,26 Such a kind of
ligands has two advantages: (i) the ligand comprises multiple coordinating groups suitable for
building extended frameworks with a large surface area, and (b) the amide groups are both
hydrogen-bonding donors and acceptors, having the possibility to establish interactions with the
targeted dyes.
In this context, we synthesized a new ligand source (Fig. 1), by incorporating the pyridyl N-
nitrogen at the meta position instead of at para position which was the case in the previously
reported analogous one.13 Although a search in the CSD database found a report on this ligand
by Feng et al.27 generated in situ for the construction of MOFs and its direct synthesis and
isolation has been recently reported by us but not the detailed synthesis protocol.28 Herein, we
describe the detailed syntheses and characterization of 4-(pyridin-3-ylcarbamoyl)benzoic acid
(HL) and of its derived CPs [Zn(-1OO’:2N-L)(H2O)(BDC)0.5]n.n(DMF) (1), [Cd(-
1O:2N-L)2(H2O)2]n (2) and [Pb(-1OO’:2O’-L)(-1O:2O’:3N-L)]n (3). CP 1 was
found to possess an excellent removal ability of Congo Red dye over other dyes. A theoretical
study is also presented in order to get a mechanistic insight of this adsorption process.
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O
O
NH
OH
N
Fig. 1. 4-(pyridin-3-ylcarbamoyl)benzoic acid (HL).
Experimental Section
Materials and methods
Solvents used in the reactions were dried and distilled prior to their use.29 Benzene-1,4-
dicarboxylic acid (BDCA), 3-aminopyridine, thionyl chloride (SOCl2) and the metal nitrates
Zn(NO)3.6H2O (98.0% purity) were purchased from Sigma Aldrich Chemical Co. and used as
1
such. FT-IR spectra were recorded on a Bruker Vertex 70 instrument in KBr pellets. H (300
MHz) and 13C (75.45 MHz) NMR spectra were obtained at room temperature (RT) on a Bruker
Avance II + 300 (UltraShieldTMMagnet) spectrometer using tetramethylsilane [Si(CH3)4] as an
internal reference. Carbon, hydrogen and nitrogen elemental analyses were carried out by the
Microanalytical Service of the Instituto Superior Técnico. Nitrogen adsorption isotherm was
collected on a Micromeritics ASAP 2060 gas sorption instrument at 77 K; the sample was pre-
treated in a vacuum oven at 150 °C for 12 h before analysis. Single crystal X-ray diffraction data
were collected using a Bruker APEX-II PHOTON 100 diffractometer with graphite
monochromated Mo-Kα (λ = 0.71069) radiation. Powder X-ray diffraction (PXRD) was
conducted in a D8 Advance Bruker AXS (Bragg Brentano geometry) θ-2θ diffractometer, with
copper radiation (Cu Kα, λ = 1.5406 Å) and a secondary monochromator, operated at 40 kV and
40 mA. Flat plate configuration was used, and the typical data collection range was between 5˚
and 40˚. Thermal properties were analyzed with a Perkin-Elmer Instrument system (STA6000)
4

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at a heating rate of 10 ˚C min-1 under a dinitrogen atmosphere. Electronic absorption and solid-
state emission spectra at room temperature were recorded on Perkin-Elmer LAMBDA 750 UV-
visible spectrophotometer and Perkin Elmer Fluorescence Spectrometer (LS-55), respectively.
Electrospray ionization mass spectrometric data (ESI-MS) were acquired in DMSO on a Bruker
Micro TOF QII, and THERMO Finnigan LCQ Advantage Max ion trap Mass Spectrometer.
Synthesis and characterization
Synthesis of 4-(pyridin-3-ylcarbamoyl)benzoic acid (HL)
The pro-ligand HL was synthesized in two steps. In the first step its ester form MeL was
synthesized according to a modified procedure reported earlier13, where terephthalic acid
monomethyl ester chloride (1.98 g, 10.0 mmol) in 20 mL THF was added dropwise to a mixture
of 3-aminopyridine (0.94 g, 10.0 mmol) in 20 mL THF and 3 mL triethylamine at 0˚C, and was
stirred for 24 h. The white suspension thus formed was filtered off and washed with 2 mL of
water and dried. The crude product was recrystallized using CH2Cl2 to obtain the pure product
1
(MeL). Yield: 2.10 g (72%). H NMR (DMSO-d6, δ ppm) 10.97 (1H, s, NH), 8.42 (1H, d, H-
py), 8.19 (1H, d, H-py), 8.10 (4H, dd, H-bz), 7.89 (1H, t, H-py), 7.15 (1H, t, H-py), 3.93 (3H, s,
OCH3). Anal. Calcd. for C14H12O3N2: C, 65.62; H, 4.72; N, 10.93%. Found: C, 65.66; H, 4.80;
N, 10.89%. FT-IR (cm-1): 3236(m), 3190(m), 3111(m), 3050(m), 1718(s), 1671(s), 1593(m),
1579(s), 1541(s), 1507(m), 440(s), 1310(s), 1287(s), 1240(m), 1111(s), 784(m), 714(m).
In the second step, MeL was hydrolyzed according to a reported procedure34 to obtain
the pro-ligand HL. Yield: 1.6g (76%). The relevant 1H and 13C-NMR spectra (1H and 13C-NMR)
are given in Figs. S1-S2, ESI. Anal. Calcd for C13H10N2O3: C, 64.46; H, 4.16; N, 11.56. Found:
C, 64.63; H, 4.23; N, 11.61. FT-IR (KBr, cm-1): 3279 (bs),1703 (w), 1643 (s), 1522 (s), 1406(w),
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1274 (br), 1181 (w) 1103 (w), 1010 (w), 865 (w), 790 (s), 700 (br), 641 (w). 1H-NMR (300 MHz,
DMSO-d6, δ ppm): δ 13.21 (s, 1H, COOH), 10.66 (s, 1H, NH), 8.94 (s, 1H, Hpy), 8.33 (d, 1H, J
= 6.0 Hz, Hpy), 8.20 (d, 1H, J = 6.0 Hz, Hpy), 8.06 (s, 4H, Ar-H), 7.44 (t, 1H, Hpy). 13C-NMR (77
MHz, DMSO-d6, δ ppm): 167.17, 165.71, 145.26, 142.47, 138.52, 136.08, 134.02, 129.82,
128.46, 127.88, 124.04. ESI-MS : m/z [(M + H)]+, Calcd. 243.07, found 243.11, [(M + Na)]+,
Calcd. 265.07, found 264.91.
Synthesis of [Zn(-1OO’:2N-L)(H2O)(1,4-BDC)0.5]n.n(DMF) (1)
A mixture of HL (15 mg, 0.067 mmol), Zn(NO3)2.6H2O (10 mg, 0.060 mmol) and
benzene-1,4-dicarboxylic acid (BDCA) (10 mg, 0.060 mmol) in DMF: H2O (1:1, v/v) in a sealed
8 mL glass vessel was heated at 75 ˚C for 28 h. White crystals of 1 deposited at the glass vial,
were separated by filtration and washed with deionized water, DMF and then dried in air. Yield:
60%. Anal. Calcd for C20H20N3O7Zn: C, 50.07; H, 4.20; N, 8.76. Found: C, 49.98; H,
4.23; N, 8.74. IR (KBr/pellet, cm-1): 3322 (br), 1648 (s), 1593 (m), 1531 (s), 1482 (m), 1330
(w), 1295 (w), 1142 (w), 1019 (w), 799 (m), 736 (m), 642 (m).
Synthesis of [Cd(-1O:2N-L)2(H2O)2]n (2)
A similar synthetic procedure as that used for 1 was followed, but using HL (15 mg, 0.067 mmol)
and Cd(NO3)2.4H2O (10 mg, 0.060 mmol) in DMF:H2O (1 mL:1 mL). White crystals of 2
suitable for X-ray diffraction analysis obtained were obtained, removed by filtration and
subsequently washed with water, DMF and dried in air. Yield: 68%. Anal. Calcd for
C26H22CdN4O8: C, 49.50; H, 3.51; N, 8.88. Found: C, 49.55; H, 3.53; N, 8.90. IR (KBr/pellet,
cm-1): 3272 (br), 1669 (s), 1538 (s), 1482 (s), 1427 (w), 1385 (s), 1312 (s), 1286 (m), 1193 (w),
1109 (m), 1020 (w), 945 (w), 872 (w), 805 (s), 741 (s), 644 (s), 626 (m).
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Synthesis of [Pb(-1OO’:2O’-L)(-1O:2O’:3N-L)]n (3)
A similar synthetic procedure as that used for 1 was followed, but using HL (15 mg, 0.067 mmol)
and Pb(NO3)2 (10 mg, 0.060 mmol) in DMF:H2O (1 mL:1 mL). The white crystals of 3 suitable
for X-ray diffraction analysis were filtered off and subsequently washed with water, DMF and
dried in air. Yield: 70%. Anal. Calcd for C26H18N4O6Pb: C, 45.28; H, 2.63; N, 8.12. Found: C,
45.25; H, 2.66; N, 8.10. IR (KBr/pellet, cm-1): 3301 (br), 1648 (m), 1586 (m), 1538 (s), 1481
(w), 1330 (m), 1296 (w), 1184 (w), 1048 (w), 1018 (w), 795 (w), 715 (w), 700 (w).
Crystal structure determination
X-ray quality single crystals of the compounds 1-3 were immersed in cryo-oil, mounted in a
nylon loop and measured at room temperature. Intensity data were collected using a Bruker
APEX-II PHOTON 100 diffractometer with graphite monochromated Mo-Kα (λ 0.71069)
radiation. Data were collected using phi and omega scans of 0.5° per frame and a full sphere of
data was obtained. Cell parameters were retrieved using Bruker SMART30 software and refined
using Bruker SAINT30 on all the observed reflections. Absorption corrections were applied using
SADABS.31 Structures were solved by direct methods by using the SHELXS-2014 package32
and refined with SHELXL-2014/6.32 Calculations were performed using the WinGX System-
Version 2014.1.33 The hydrogen atoms attached to carbon and nitrogen atoms were inserted at
geometrically calculated positions and included in the refinement using the riding-model
approximation; Uiso(H) were defined as 1.2Ueq of the parent atoms for phenyl and 1.5Ueq of the
parent atoms for the methyl groups and nitrogen atoms. Least square refinements with
anisotropic thermal motion parameters for all the non-hydrogen atoms and isotropic ones for the
remaining atoms were employed. Crystallographic data are summarized in Table 1 and selected
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bond distances and angles are presented in Table S1-S2 (Supplementary Information file).
CCDC 2010625-2010627 contains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
In silico Studies
Molecular docking was done using HEX 8.0.0 software and visualization was performed using
Discovery Studio 3.5 software. The parameters used for docking include correlation type shape
only, FFT mode 3D, grid dimension 0.6, receptor range 180, ligand range 180, twist range 360,
distance range 40.
Determination of point of zero charge (pHpzc)
For zero point charge determination (pHzpc) the pH drift method was used.34,35 20 mL
solutions were prepared at different pH (4-12) in distilled water using HCl and NaOH,
whereafter 30 mg of 1 were then added to each solution. The flasks were then kept for 24 h and
the final pH of the solutions were measured. The pHpzc of 1 was determined by plotting the
final pH versus the initial one. The pH measurement was carried out using an HANNA HI
2211 pH meter.
Results and Discussion
Syntheses and Characterisation
The pro-ligand 4-(pyridin-3-ylcarbamoyl)benzoic acid (HL) was synthesized by reacting 3-
aminopyridine with methyl 4-(chlorocarbonyl)benzoate in anhydrous THF in the presence of
triethylamine, followed by hydrolysis according to a procedure reported earlier.28 The
coordination polymers 1-3 were synthesized by the hydrothermal reaction of HL with
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Zn(NO3)2.6H2O [in the presence of benzene-1,4-dicarboxylic acid (BDCA)], Cd(NO3)2.4H2O or
Pb(NO3)2, respectively (Scheme 1).
The pro-ligand HL was characterized by elemental analyses, IR and multinuclear (1H
13
and C) NMR techniques, whereas the CPs 1-3 were characterized by elemental analysis, IR
and thermogravimetric analyses. Furthermore, the structural features of the CPs 1-3 were
unequivocally obtained by single-crystal X-ray diffraction analysis (discussed below). In the 1H
NMR spectrum, HL displays characteristic resonances due to the –COOH and –NH protons at δ
13.30 and 10.63, respectively (Fig. S1). The phenyl ring protons resonate in the range of δ 8.94-
7.44. In its 13C NMR spectrum, the signal due to -COOH appears at δ 166.25, along with other
resonances at their usual positions (Fig. S2). The FT-IR spectrum of the pro-ligand HL displays
a sharp and strong band at 1705 cm-1 attributable to [ν(OCO)] stretching vibrations which upon
coordination to the metal centre shifts to a lower frequency at ca. 1648 cm-1, suggesting, in 1-3,
the coordination of the deprotonated carboxylic acid group (Fig. S3). To corroborate that the
crystal structures of 1-3 are representative of the synthesized bulk materials, powder-XRD was
also carried out. The experimental and simulated powder-XRD patterns of 1-3 clearly
demonstrate that the synthesized bulk materials are of a similar nature (Figs. S4-S6).
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O
HN
N
HO
O
HL
Zn(NO3)2.6H2O
BDCA
Pb(NO3)2
DMF:H2O
75 oC, 24 h
Cd(NO3)2.4H2O
DMF:H2O
75 oC, 24 h
DMF:H2O
75 oC, 28 h
[Zn(L)(H2O)(BDC)0.5]n.n(DMF)
(1)
[Cd(L)2(H2O)2]n
(2)
[Pb(L)2]n
(3)
BDCA = benzene-1,4-dicarboxylic acid
BDC = benzene-1,4-dicarboxylate
Scheme 1. Syntheses of CPs 1-3.
Crystal Structure Analysis of 1-3
Single-crystal X-ray diffraction studies of [Zn(-1OO’:2N-L)(H2O)(BDC)0.5]n.n(DMF) (1)
reveal that it crystallizes in the triclinic P-1 space group, and that the asymmetric unit contains
one Zn2+ ion, one L- ligand, half benzenedicarboxylate, one water ligand and one non-
coordinated DMF molecule (Figs. 2a-b). A rhombic grid type one-dimensional double chain
framework having approximately 8 x 14 Å2 channels is observed along the crystallographic c-
axis (Fig. 2c; potential accessible void of 45.7 Å3 per unit cell). Since the Zn-O bonds are well
below the sum of the Van der Waals radii (1.39 and 1.52 Å, respectively; see below and Table
S1 for selected bond distances and angles), the Zn(II) centre shows a distorted octahedral
geometry with three coordination sites being occupied by the chelating Ocarboxylate atoms and the
Npyridyne from two L- ligands, other two filled by the chelating Ocarboxylate atoms of BDC and the
remaining position being occupied by a water molecule. In this way each metal centre is linked
to two different L- ligands whereas each L- ligand connects two different metal ions.
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The L ligand is almost planar. The dihedral angle between the carboxylate containing phenyl
ring and the pyridyl part is 7.03° and the C5-C8-N1-C9 torsional angle is 178.17°.
(a)
(b)
(c)
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Fig. 2. (a) The molecular structure of framework 1 with partial atom labelling scheme. (b) One
dimensional double chain structure of framework 1 (DMF molecules are represented as space
filling model). (c) Hydrogen bonded network of 1.
The Zn‒O bond lengths are in the range of 2.006(3)-2.521(3)Å and the Zn‒N length is 2.064(3)
Å. The Zn···Zn distance between the two-symmetry related metal centres crosswise the benzoate
or the L moieties are of 10.853(4) or 14.018(4) Å, respectively. The DMF molecules are
encapsulated into the lattice via hydrogen bonding interactions involving the water ligands as
donors [O5-H5B···O7, D···A 2.646(5) Å, <DHA 172°]. Additional N-H····O and O-H····O
interactions are also significant which help to expand and stabilize the framework to the third
dimension (Table S2, supporting information). Intense  interaction between vicinal phenyl
and (slightly offset) pyridine rings were detected (centroidcentroid distances of 3.699 Å).
We have obtained another interesting one-dimensional double chain framework, [Cd(-
1O:2N-L)2(H2O)2]n (2) upon performing the reaction of 4-(pyridin-3-ylcarbamoyl)benzoic
acid (HL) with cadmium(II) nitrate in a dimethylformamide-water solvent mixture. The
asymmetric unit of 2 contains one deprotonated L- ligand, one Cd2+ ion and one water molecule.
Upon symmetry expansion a polymeric structure is revealed (Fig. 3a) where the Cd centre
presents an octahedral coordination environment made of two Ocarboxylate atoms from two L- units
[Cd1-O1, 2.2900(12) Å], two Npyridyl atoms from other two [Cd1-N2, 2.3139(13) Å] and two
water molecules [Cd1-O4, 2.3425(13) Å] (Fig. 3b). In this framework, the organic ligand is
twisted as shown by the dihedral angle of 58.06° between the phenyl and the pyridyl rings, and
the C5-C8-N1-C9 torsional angle of 178.71°. A rhombic grid type one-dimensional double chain
framework is observed having approximately 4 x 8 Å2 channels along the crystallographic a-
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axis (Fig. 3b). Strong  interactions could also be detected in 2 with almost perfect dipole-
dipole stacks of vicinal pyridine moieties (centroidcentroid distances of 3.456 Å).
(a)
(b)
Fig. 3. (a) 1D double chain structure of framework 2. (b) Packing view of compound 2.
The water ligands in 2 simultaneously donate to the non-coordinated Ocarboxylate of L- and to the
Oketo atom in a vicinal chain [O4-H4A···O2: D···A 2.6919(19) Å, <DHA 164°; O4-H4B···O3:
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D···A 2.7546(19) Å, <DHA 171°] therefore expanding the structure to the second and then to
the third dimension.
The single-crystal X-ray diffraction analysis disclosed that asymmetric unit of 3 contains one
Pb(II) cation and two deprotonated L‒ ligands expanding, by symmetry, to the 2D CP [[Pb(-
1OO’:2O’-L)(-1O:2O’:3N-L)]n (3). The Pb(II) centre presents an hexa-coordinate
environment constructed with the Ocarboxylate-atoms from four L- ligands, one of those acting as a
bridging chelate donor, the remaining binding position being engaged with the Npyridyl atom from
a fifth L- moiety (Figs. 4a-c). The Pb‒O bond lengths assume values between 2.3817(17) and
2.925(2) Å (Table S3) and the Pb‒N4 distance is 2.534(2) Å. The distance between the Pb(II)
cations in the {PbOCO}2 bimetallic ring is 5.1144(3) Å, and of 13.360(1) Å when going through
the L skeletons, in {Pb(L)}2. There are two different types of L- ligands in the structure of 3: one
in which this anion coordinates to three different Pb(II) cations through a bridging Ocarboxylate
-
and the Npyridyl- atoms, hence occupying internal positions in the structure, and another where it
is coordinated to two different metal centres by means of bridge-chelating Ocarboxylate groups. The
organic moieties are not planar, the dihedral angles between the phenyl and the pyridyl rings
assuming values of 8.91° and 13.51° depending on the ligand being coordinated to two or to
three metal cations; additionally, and in the same order, the CphenylCNCpyridyl torsion angles are
of 179.14º and 168.48. Framework 3 is stabilized via hydrogen bonding interactions between
the Namide groups which strongly donate to the Oamide atoms [N1-H1N·····O3, D···A 2.117(19)
Å, <DHA 162°; N2-H2N·····O6, D···A 2.6919(19) Å, <DHA 163°].
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(a)
(b)
(c)
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(d)
Fig. 3. (a) The molecular structure of 3 with partial atom labelling scheme, a fragment of its 2D
network viewed down the crystallographic b (b) or c (c) axis, and (d) a chain of Pb(II)-bridging
carboxylate groups which runs along the crystallographic b axis.
Topological Analysis of 1-3
The topological analysis on CPs 1-3 was performed using TOPOS 4.0.1736–38 in order to
comprehend the description of their crystal structures. CP 1 displays a 2,2,3-connected 3-nodal
net with the point symbol {82.12}2{8}3 (Fig. 5a). The framework 2 has a 2,4-connected binodal
net (Fig. 5b), but in the case of 3, each Pb(II) is connected with seven different L- ligands, one
ligand connected to three Pb(II) centres and another ligand to four different metal centres. Thus,
framework 3 represents a 3,4,7-connected trinodal net structure with topological type 3,4,7 (Fig.
5c).
(a)
(b)
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(c)
Fig. 5. One dimensional double chain framework 1 (a) and 2 (b). The zinc and cadmium nodes
are represented in cyan and green color, respectively. The linker is represented in pink colour
and BDC is presented in blue. (c) Topological representation of framework 3 (Pb centres are in
green colour and ligand in pink colour).
Thermogravimetric Analysis of 1-3
Thermogravimetric analyses (TGA) under dinitrogen in the range of 30-700 °C at a heating rate
of 10 ˚C min-1 were carried on CPs 1-3 in order to investigate their thermal stability. The TGA
of 1 demonstrates a weight loss of 3.51 % (calculated = 3.75 %) at ca. 130-160 ºC, accredited to
the release of one coordinated water molecule followed by a weight loss of 13.93 % (calculated
= 15.21 %) at 180-250 ºC ascribed to the liberation of one non-coordinated DMF molecule. At
ca. 250-380 ºC the derived form was thermally stable, but above 380 ºC the polymeric structure
disintegrates leading to the formation of ZnO (Fig. 6). The TGA of 2 displays at 110-130 ºC a
weight loss of 5.91 % (calculated = 5.71 %) due to the release of one coordinated water molecule.
The derived form was thermally stable up to 320 ºC beyond which the polymeric structure
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collapses and forms CdO (Fig. 6). On the other hand, CP 3 exhibits excellent thermal stability
of up to ca. 400 ºC, beyond which it disintegrates to form PbO (Fig. 6).
Figure 6: Thermogravimetric analysis of 1-3.
Photoluminescent Properties of 1-3
The luminescent properties of d10 metal complexes are well known and have marked their
importance in numerous applications, namely in electroluminescence, chemical sensing, etc.39,40
Thus, in order to investigate the luminescent behaviour, the solid-state luminescent properties of
the CPs 1-3 were studied at room temperature. Furthermore, the solid-state luminescent
behaviour of the free pro-ligand HL was also inspected for comparison (Fig. 7). Upon excitation
at λex = 290 nm, HL exhibits an emission band at λem = 415 nm, which could be attributed to
intra-ligand charge transfer transition.41,42 A higher emission peak with ca. 7 nm blue shift is
observed for 1 at λem = 408 nm (λex = 320 nm). The higher emission intensity observed for 1
could be due to intra-ligand transition of the coordinated L-, its coordination to the metal
increasing its rigidity.41,42
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On the contrary, 2 exhibits photoluminescence of a similar nature to that of HL upon excitation
at λex = 320 nm, but with a decrease in emission intensity due to metal−ligand charge transfer
transition.43 A weaker photoluminescence was observed for 3 upon excitation at λex = 320 nm.43
Fig. 7. Solid state emission spectra of HL and CPs 1-3 at room temperature.
Dye Adsorption Studies
Preliminary experimental results (Figs. S7 and S8 for 2 and 3, respectively) demonstrated that 1
is the best compound for selective removal of an organic dye (discussed below). Thus, to appraise
the ability of 1 for the removal of dyes from aqueous solution, four different kinds of dyes were
chosen as model adsorbates, viz. Congo red (CR), rhodamine 6g (Rh-6g), rhodamine B (Rh-b)
and fluorescein natrium (Fl-Na), having different sizes and charges (Fig. 8). Experiments were
typically carried out by immersing 10 mg of 1 in 3 mL of aqueous dye solution (3 × 10 −5 M) in
the dark at room temperature. The mixture was constantly stirred in the dark and the abilities of
the sample to adsorb dyes from aqueous solution were monitored through UV-vis spectroscopy.
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NH2
O
S
Cl-
N
N
+Na-O
O
HN
O
NH+
+Na-O
O
N
N
S
O
O
H2N
O
Congo Red
Rhodamine 6g
Cl-
N
O
N+
+Na-O
O
O
COOH
COO-Na+
Rhodamine B
Fluorescein-natrium
Fig. 8. Structure of Congo red (CR), rhodamine 6g (Rh-6g), rhodamine B (Rh-b) and fluorescein
natrium (Fl-Na) used for dye separation studies.
The adsorption results of 1 towards the four organic dyes are depicted in Fig. 9. The naked eye
visualization demonstrated a substantial adsorption of Congo Red (CR) dye in less than ca. 2
minutes, reaching completeness in only ca. 5 minutes. In contrast, no adsorption of Rh-6g, Rh-
b or Fl-Na dyes were perceived even after more than 60 minutes of stirring at room temperature
in the dark, what is also evidenced by UV-Vis spectroscopy (Fig. 10).
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Fig. 9. Naked eye visualization on the removal of dyes by 1 at room temperature.
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Fig. 10. UV-vis spectra of dye solutions upon adsorption on 1 at room temperature: (A) Congo
red, (B) rhodamine 6g, (C) rhodamine B, and (D) fluorescein natrium.
The removal capacity of 1 (Removal%) was calculated using the following equation:
where Ao and At represent the absorbance of the CR solution before and after the adsorption.
From the above equation, the CR removal capacity of 1 was found to be 99.54%, which is better
than some of the reported CPs used for adsorption or removal of this dye.18,19,44,45 The adsorption
property of 1 was also observed by its color change from transparent white (before adsorption)
to reddish orange (after adsorption) (inset, Fig. S9, ESI). The structural integrity of 1 is
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maintained throughout this adsorption process as supported by the scanning electron microscopy
(SEM) images (Fig. S9, ESI) and EDX (Fig. S10, ESI) recorded before and after the adsorption
of CR.
Furthermore, the point of zero charge (pHpzc)34,35 of the adsorbent 1 was determined to get an
insight into its electronic and adsorption properties. The pHpzc is the pH where an adsorbent has
zero total charge on its surface; above this point (higher pH) the surface is negatively charged,
whereas below it (lower pH) the surface of the adsorbent is positively charged. The pHpzc of 1
was found to be 9.8 (Fig. S11, ESI) indicating that the surface has a net positive charge below
pH 9.8 and a net negative charge above pH 9.8. CR is an anionic dye with a pKa of 5.5; when in
solution, at pH lower than 5.5 it exists in cationic form presenting a blue color, while at higher
pH it exists in anionic form with a red color.46,47 Thus, from our adsorption studies (see Fig. 9)
at pH ca. 6.4, the effective removal of CR by 1 is consistent with the electrostatic attraction
between the positively charged surface of 1 (pHpzc = 9.8) and the anionic CR in aqueous
solution.
To understand if the selective adsorption of CR dye is determined by the presence of the azo (-
N=N-) moiety in the framework and by the charge of the dye, the adsorption properties of 1 were
investigated with related dyes bearing such a group, namely naphthol blue black (NBB) and
orange II sodium salt (OR) (Fig. 11) by following a protocol similar to that mentioned above.
O
ONa
S
O
O2N
NH2 OH
N
O
N
O
N
N
N
N
OH
NaO S
O
S ONa
O
Naphthol blue black (NBB)
Orange II sodium salt (OR)
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Fig. 11. Structure of Naphthol blue black (NBB) and Orange II sodium salt (OR).
The adsorption results of 1 towards these organic dyes are shown in Figs. 12 and 13, respectively.
The naked eye visualization indicated that no or negligible adsorption of NBB dye in the first 5
minutes has occurred, although some adsorption was observed after ca. 60 minutes of stirring at
room temperature in the dark as demonstrated by UV-Vis spectroscopy (Fig. 12). On the other
hand, no adsorption of OR dye was seen even after ca. 60 minutes of stirring, what is also proved
by UV-Vis spectroscopy (Fig. 13). These results clearly demonstrate the favorable selective
adsorption of CR over other dyes even with a common azo moiety and also with a negative
charge. Hence, such a selectivity is not primarily due to electrostatic attraction between the
positively charged surface of 1 and the anionic CR, but is triggered by various noncovalent
interactions including π−π stacking interactions with the framework of 1, as shown by docking
studies (see below, in silico docking studies).
Fig. 12. Naked eye visualization on the removal of NBB dye by 1 at room temperature and its
corresponding UV-vis spectrum.
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Fig. 13. Naked eye visualization on the removal of OR dye by 1 at room temperature and its
corresponding UV-vis spectrum.
Selective Separation of CR dye
Compared to dye adsorption, dye selectivity is more attractive and challenging. This prompted
us to investigate the selectivity of 1 towards the CR dye in mixed-dye solutions. The CR + Rh-
6g, CR + Rh-b and CR + Fl-Na dye mixtures were used. We have also tested the selectivity of 1
towards the CR dye in mixed-dye solutions containing CR + Rh-6g + Rh-b + Fl-Na. As shown
in Figs. 14 and 15, in each case only the CR could be selectively and efficiently adsorbed over a
period of just 5 minutes. This fast and selective adsorption process of 1 is very important for
adsorbents in practical wastewater treatment. As shown in the photographs (see inserted pictures
in Figs. 14 and 15), after soaking 1 in the mixed-dye aqueous solutions, the solution color
changed and finally retained the feature color of Rh-6g, Rh-b, Fl-Na and Rh-6g + Rh-b +Fl-Na,
respectively. These results indicate that 1 can effectively and selectively adsorb CR molecules
into the framework, from a solution containing a mixture of dyes.
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Fig. 14. Naked eye visualization of the adsorption of mixtures of dyes by 1 at room temperature
and corresponding UV-vis spectra of the solutions.
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Fig. 15. Naked eye visualization of the adsorption of mixtures of dyes by 1 at room temperature
and corresponding UV-vis spectra of the solutions.
Reusability of Adsorbent
To assess the reusability of the adsorbent, which contributes to reducing the cost of a practical
application process, methanol, ethanol and acetonitrile were used in desorption and
regeneration experiments, and methanol was found to have the best desorption effect. As soon
as methanol was added to 1 with adsorbed CR (1-CR) the solution gained the CR colour almost
instantaneously (Inset of Fig. 16). 1-CR can expediently release the CR by simply washing the
sample with methanol (for a 10 mg sample, ca. 1 mL methanol solvent was used). In addition,
the dye removal capacity of 1 is fully maintained with recycling, its stability being evidenced by
the IR spectrum of 1 (Fig. S12, ESI) recorded after each cycle. As depicted in Fig. 16, after
recycling three times, 1 showed almost identically rapid and quantitative adsorption of CR.
Accordingly, 1 is an excellent candidate as a selective adsorption material, which eventually can
be applied practically to adsorb CR in mixed-dye containing wastewater. This strong and
selective adsorption of CR by 1 can be accounted for by different types of interactions as
discussed below.
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Fig. 16. Effect of CP 1 recycling on the adsorption of CR.
In Silico Studies
We carried out docking studies in order to get a visual representation and insight into the
selective adsorption of CR on 1 (Fig. 17). The full geometry optimization of Congo Red (CR),
rhodamine 6g (Rh-6g), rhodamine B (Rh-b) and fluorescein natrium (Fl-Na) has been carried out
using ChemDraw 3d software. The type of interaction taking place between 1 and CR was studied
by adopting molecular docking software using HEX 8.0.0 software and visualization was
performed using Discovery Studio 3.5 software.
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Fig. 17. Visualization of the docked CR in 1.
The docking study reveals that the dyes stick to the 1D polymer of 1, with a relative
binding energy of -443.75 (CR), -301.45 (Rh-6g), -304.83 (Rh-b) and -231.70 (Fl-Na) eV. The
results unambiguously support the stronger binding affinity of CR, which is in harmony with the
spectroscopic studies. The stacking of CR in 1 is depicted in Fig. 18 and shows the interactions
that are responsible for the adsorption process, specifically anion-π, cation-π and π−π stacking
interactions (Fig. 18). The phenyl rings of L and the phenyl and naphthalene rings of CR are
involved in the π−π stacking interactions, which are in the range of 3.84-4.13 Å. The anion-π
interactions connect not only the CR Osulfonate with the phenyl ring of L, but also the Ocarboxylate
of L and the CR phenyl ring with distances of 4.28 Å and 4.77 Å, respectively. A strong cation-π
interaction with a distance of 3.58 Å is disclosed between Zn(II) ion and a naphthalene ring.
These interactions are thought to be the main driving force for the selective adsorption or removal
ability of CR by 1, that are favored by the elongated shape of the CR dye and by the structural
widening of 1 due to the BDC spacer which can facilitate e.g. a strong π−π staking with the
ligand framework of 1 leading to an effective adsorption.
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