Heteroleptic 1D coordination polymers: 5-Coordinated zinc(II) polymer as an efficient transesterification catalyst

Article abstract of DOI:10.1016/j.poly.2017.01.027

The synthesis of three 1D coordination polymers (CPs), [Zn(TPPZ)(HCCB)]_n·DMF·H₂O (1) [Cd(TPPZ)(HCCB)]_n(2) and [Co(TPPZ)(HCCB)]_n(3), based on the N,N,N-chelating κ³-2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) and polymerizing carboxylate (–COO^?) 3-(carboxy-methylamino)-4-chlorobenzoic acid (H₃CCB) ligands, using a solvothermal method, have been described. The CPs have been adequately characterized by satisfactory elemental analyses, FT-IR, UV–vis, emission, TGA and PXRD analysis. The structure of 1 has been verified by single crystal X-ray diffraction analysis. The structural data revealed the 5-coordinated heteroleptic polymeric framework is exclusively extended through the H₃CCB carboxylate group and strong N,N,N-chelating TPPZ ligand acts as a branch terminating site. Notably, the structure of 1 is comprised of one unit of each H₂O and DMF solvent in its crystal lattice. In contrast, 2 and 3 exclude lattice solvents, as suggested by PXRD studies, after thermal desolvation of 1. Furthermore, 1 with 5-coordinated Zn(II) centers exhibited an efficient catalytic performance for transesterification reactions, with remarkable stability and good reusability.

Full text of DOI:10.1016/j.poly.2017.01.027

Accepted Manuscript
Heteroleptic 1D Coordination Polymers: 5-Coordinated Zinc(II) Polymer as an
Efficient Transesterification Catalyst
Manzoor A. Wani, Ashish Kumar, Mrituanjay D. Pandey, Rampal Pandey
PII:
S0277-5387(17)30064-5
DOI:
http://dx.doi.org/10.1016/j.poly.2017.01.027
POLY 12431
Reference:
Polyhedron
To appear in:
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Revised Date:
Accepted Date:
12 October 2016
29 December 2016
7 January 2017
Please cite this article as: M.A. Wani, A. Kumar, M.D. Pandey, R. Pandey, Heteroleptic 1D Coordination Polymers:
5-Coordinated Zinc(II) Polymer as an Efficient Transesterification Catalyst, Polyhedron (2017), doi: http://
dx.doi.org/10.1016/j.poly.2017.01.027
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Heteroleptic 1D Coordination Polymers: 5-Coordinated Zinc(II) Polymer
as an Efficient Transesterification Catalyst
Manzoor A. Wania, Ashish Kumarb, Mrituanjay D. Pandeya* and Rampal Pandeya*
aDepartment of Chemistry, Dr. Harisingh Gour Central University, Sagar 470003, MP, India
bDepartment of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005,
UP, India
Abstract:
The synthesis of three 1D coordination polymers (CPs), [Zn(TPPZ)(HCCB)]n·DMF·H2O (1)
[Cd(TPPZ)(HCCB)]n (2) and [Co(TPPZ)(HCCB)]n (3), based on the N,N,N-chelating κ3-
2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) and polymerizing carboxylate (−COO−) 3-
(carboxy-methylamino)-4-chlorobenzoic acid (H3CCB) ligands, using a solvothermal
method, have been described. The CPs have been adequately characterized by satisfactory
elemental analyses, FT-IR, UV/vis, emission, TGA and PXRD analysis. The structure of 1
has been verified by single crystal X-ray diffraction analysis. The structural data revealed the
5-coordinated heteroleptic polymeric framework is exclusively extended through the H3CCB
carboxylate group and strong N,N,N-chelating TPPZ ligand acts as a branch terminating site.
Notably, the structure of 1 is comprised of one unit of each H2O and DMF solvent in its
crystal lattice. In contrast, 2 and 3 exclude lattice solvents, as suggested by PXRD studies,
after thermal desolvation of 1. Furthermore, 1 with 5-coordinated Zn(II) centers exhibited an
efficient catalytic performance for transesterification reactions, with remarkable stability and
good reusability.
Keywords: 1D Coordination Polymers; Heteroleptic; Structural studies; 5-Coordination;
Transesterification catalysis.
Corresponding Author Email: rppandeysu@gmail.com; mrituanjaypandey@gmail.com
1

1. Introduction
The design and synthesis of coordination polymers (CPs) based on mixed ligand systems
have drawn a great deal of attention owing to their interdisciplinary nature, intriguing
topologies and potential applications in diverse areas, including non-linear optics, magnetism,
porosity, chemical sensing and catalysis [1-6]. CPs are inorganic/organometallic polymeric
architectures containing metal centers linked by ligands [7]. These systems extend in one
(1D), two (2D) or three dimensions (3D) through repeating units of a typical coordination
complex [8,9]. Variation of the ligand plays a crucial role in the self–assembly processes and
affords CPs with different structural topologies [10,11]. An extensive range of materials
exhibiting a variety of architectures and functions have been prepared by adjusting the shape,
flexibility, length and symmetry of the spacers [12,13]. CPs capable of displaying interesting
architectures and properties can be synthesized by cautious selection of the ligand and metal
ion. In this context, poly-carboxylate ligands as well as N,N-donor linkers have been
extensively utilized and established their significant candidature [12,13]. Usually, neutral N-
donor ligands readily coordinate to the metal centers to afford specific assemblies, however
the positive charge on the metal centers is not balanced, which requires variation in the
architectures. In contrast, carboxylates bring excellent diversity in the CPs as they can adopt
monodentate, chelating–bidentate, bridging–bidentate, bridging–multidentate and various
other binding modes with the metal centers to provide rich structural motifs, with a charge
balancing capacity as an advantage over N−donor ligands [14-18]. With an objective of
studying the relative coordination abilities of such systems in the same reaction environment,
we attempted a solvothermal synthesis using two types of ligand systems, a bis-
N,N,N−chelating ligand (TPPZ) and a bi-carboxylate ligand (H3CCB), both possessing
multiple potential donor sites in distal orientations. The simultaneous presence of these ligand
systems create a competitive coordinating environment for the metal center under neutral
conditions. It is well documented that designing CPs by incorporating terminal ligands can
regulate the connectivity and geometry by blocking some coordination sites of the metal ions,
often leading to lower dimensionality [19].
On the other hand, CPs have demonstrated fascinating catalytic properties (both
homogeneous and heterogeneous) [20-24], after inspiring performances of the catalytically
active metal-organic framework zeolite materials [25-28]. The most tedious job in preparative
organic chemistry is the practical synthesis of complex molecules, despite applying advanced
2

techniques which necessitates the development of catalysts being easy to prepare and atom-
economical [29]. In this context, due to the vital role of esters in synthetic organic chemistry,
ester bond formation is an elementary and vastly studied methodology [30-37]. Pertaining to
it, the synthesis of esters using the transesterification reaction is recognized as a more
competent method relative to that of the dehydrative condensation of alcohols with
carboxylic acids [34,36]. Herein, we report the rational coordination of the carboxylate group
(in the absence of base), despite the presence of two identical bis-N,N,N-chelating donor sites
in the strong TPPZ ligand, leading to a 1D architecture. An interesting example is the
coordination of a bis–tridentate functional linker to the metal center in such a fashion that one
of its N,N,N-tridentate faces coordinates in a κ3-manner, whereas another equally potential
N,N,N–tridentate site remains uncoordinated (Scheme 1, Fig. 1). In addition, H3CCB bridges
the 1D chain via two remote carboxyl groups. It depicts an unusual result in terms of
coordination pattern governed by ligand competition, probably through the charge neutrality
approach.
2. Experimental section
2.1. Materials and methods
The common solvents and reagents like dimethylformamide (DMF) and potassium
hydroxide (KOH) were procured from SD Fine Chemicals Pvt. Ltd. and used as received.
Solvents were dried and distilled following standard literature procedures [38]. The ligand
2,3,5,6–tetrakis(2–pyridyl)pyrazine (TPPZ) was procured from Sigma Aldrich Chemical Co.
Pvt. Ltd. and 3–(carboxymethyl–amino)–4–chlorobenzoic acid (H3CCB) was prepared and
purified following the procedure given below [39]. Elemental analyses were performed on an
Exeter Analytical Inc. (model CE–440) CHN analyzer. IR and UV/vis spectra were acquired
1
on FT–IR Perkin Elmer-577 and Shimadzu UV-1601 spectrometers, respectively. H NMR
(DMSO–d6) spectra were recorded on a JEOL AL 300 FT–NMR spectrometer at an operating
frequency of 300 MHz. Chemical shifts (δ) are given in parts per million (ppm) relative to
tetramethylsilane (TMS, δ 0.00 ppm). Electrospray Ionization mass spectra (ESI-MS) were
obtained on a Waters Q-Tof Premier Micromass HAB 213 mass spectrometer using a
capillary voltage of 2.6-3.2 kV. Thermogravimetric data were obtained on a Perkin Elmer
Thermal Analyzer 660 at a heating rate of 10 °C/min under a nitrogen atmosphere.
3

2.2. Synthetic procedures
2.2.1. Preparation of 3-(carboxymethylamino)-4-chlorobenzoic acid (H3CCB)
An aqueous solution (10 mL) of 3-amino-4-chlorobenzoic acid (1.720 g, 10 mmol) was
added dropwise to a solution of chloroacetic acid (1.880 g, 20.0 mmol) and KOH (1.120 g,
20.0 mmol) in water (15 mL) over half an hour and heated under reflux for 12 h. After
cooling to room temperature, this gave a white precipitate which was filtered, washed with
water (20 mL) and air dried. Yield: 1.605 g, 69.8%. Anal. Calc. for C9H8NO4Cl: C, 47.08; H,
1
3.51; N, 6.10%. Found: C, 46.91; H, 3.49; N, 6.00%. H NMR (D2O, δ ppm): 7.40 (d, 1H),
7.21 (d, 1H), 6.99 (s, 1H), 3.66 (s, 1H), 3.67 (s, 2H). IR data (KBr pellet, cm-1): 3404 s, 1689
s, 1597 s, 1511 m, 1452 m, 1352 m, 1302 s, 1269 s, 1038 m, 758 s. UV/vis. (solid state, λmax
nm): 360, 303.
2.2.2. Preparation of [{µ2-3-1κO-((carboxylatomethyl)amino)-4-chlorobenzoato}(N,N,N-κ3-
2,3,5,6-tetrakis(2-pyridyl)pyrazine)zinc(II)]n(dimethylformamide)(water)
.
[Zn(TPPZ)(HCCB)]n DMF.H2O (1)
An aqueous solution (3 mL) of Zn(NO3)2·6H2O (0.298 g, 1.0 mmol) was added to a
homogenized mixture of 3-(carboxy-methylamino)-4-chlorobenzoic acid (H3CCB) (0.229 g,
1.0 mmol) and N,N,N-chelating κ3-2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) (0.389 g, 1.0
mmol) in DMF (7 mL) and the ensuing reaction mixture was sonicated for 5 min. The sealed
tube was immersed in an oil bath and the temperature of the bath was maintained at 120°C
for 48 h. It was slowly cooled to room temperature to afford creamy white color block shaped
crystals suitable for single crystal X–ray analyses. The crystals were separated by filtration,
washed several times with water and air dried. Yield: 0.334 g, 49.10%. Anal. Calc. for
C36H32ClN8O6Zn: Calcd: C, 55.90; H, 4.17; N, 14.49%. Found: C, 55.82; H, 3.89; N,
14.28%. IR (KBr pellet, cm–1): 3447 m, 3409 s, 1705 s, 1626 m, 1600 s, 1509 s, 1384 m,
1245 w, 1034 m, 758 s. ESI-MS (DMSO): m/z 680.0676 [M+H]+ (desolvated). UV/vis (solid
state, λmax nm): 399, 388, 380.
2.2.3. Preparation of [{µ2-3-1κO-((carboxylatomethyl)amino)-4-chlorobenzoato}(N,N,N-κ3-
2,3,5,6-tetrakis(2-pyridyl)pyrazine)cadmium(II)]n [Cd(TPPZ)(HCCB)]n (2)
This was prepared following the above procedure for 1, except that Cd(NO3)2·4H2O
(0.308 g, 1 mmol) was used in place of Zn(NO3)2.6H2O. Compound 2 was isolated as a
creamy white polycrystalline solid after cooling the reaction mixture to room temperature.
4

Yield: 0.339 g, 46.4%. Anal. Calc. for C33H23CdClN7O4: C, 54.34; H, 3.18; N, 13.44%.
Found: C, 53.92; H, 2.73; N, 12.98%. IR (KBr pellet, cm–1): 3407 m, 1706 s, 1598 s, 1395 m,
1299 s, 1035 m, 758 s. ESI-MS (DMSO): m/z 729.1423 [M]+ UV/vis (solid state, λmax nm):
398, 383.
2.2.4. Preparation of [{µ2-3-1κO-((carboxylatomethyl)amino)-4-chlorobenzoato}(N,N,N-κ3-
2,3,5,6-tetrakis(2-pyridyl)pyrazine)cobalt(II)]n [Co(TPPZ)(HCCB)]n (3)
This was synthesized following the above procedure for 1, except that Co(NO3)2·6H2O
(0.291 g, 1.0 mmol) was used in place of Zn(NO3)2·6H2O. Compound 3 was isolated as a
pink polycrystalline solid from the reaction mixture after cooling to room temperature. Yield:
0.284 g, 42.1%. Anal. Calc. for C33H23CoClN7O4: C, 58.64; H, 3.43; N, 14.50 %. Found: C,
58.43; H, 3.21; N, 14.01 %. IR (KBr pellet, cm–1): 3404 w, 1701 s, 1597 s, 1392 m, 1298 s,
1037 m, 787 s, 750 s. ESI-MS (DMSO): m/z 675.0661 [M+H]+ UV/vis (solid state, λmax nm):
399, 386, 374.
2.3. X–ray structure determination
Single crystal X–ray data on 1 were collected on a Rigaku R–AXIS RAPID II
diffractometer at room temperature using graphite–monochromated Mo-Kα radiation (λ =
0.71073 Å) [40,41]. The crystallographic and refinement data for 1 are summarized in Table
1. The structure was solved by direct methods (SHELXL 97, 2008) [42-44] and refined by
full–matrix least squares on F2 (SHELXL 97, 2012) [44,45]. Non-hydrogen atoms were
refined with anisotropic thermal parameters. The hydrogen atoms were given a mixed
treatment wherein most of the hydrogen atoms were geometrically fixed using a riding
model, apart from those involved in hydrogen bonding, whose partial coordinates and
isotropic atomic displacement parameters were freely refined. The computer program
‘Mercury 2.3’ was used for analyzing the bond lengths, weak interactions and stacking
distances. The powder X–ray diffraction data was collected using an 18 kW Cu–rotating
anode based Rigaku Tokyo, Japan powder diffractometer at RT and a 2θ range of 10–70° at
a scan step of 0.02°. The XRD data were analyzed by Rietveld and Le Bail refinement
techniques using the FULLPROF package.
3. Results and discussion
3.1. Syntheses and characterization
5

The ligand H3CCB was prepared by the reaction of 3-amino-4-chlorobenzoic acid with
chloroacetic acid under alkaline aqueous conditions following an earlier reported procedure
[39]. The complexes 1–3 were synthesized by the reaction of metal(II) nitrates
[Zn(NO3)2·6H2O, Cd(NO3)2·4H2O and Co(NO3)·6H2O] with H3CCB and TPPZ under
controlled solvothermal conditions [M(II):H3CCB:TPPZ; 1:1:1, 120 °C] (Scheme 1).
H3CCB possesses an amino-carboxylate (–NH–CH2–COOH) and a carboxylate (−COOH)
group and displays various versatile coordination modes [39]. Further, we have also
attempted the synthesis using other first row transition metal ions, namely Mn2+, Ni2+ and
Cu2+, under analogous conditions but remained empty handed of worthy results.
Characterization of 1-3 has been achieved by elemental analyses, spectral (FT−IR, ESI-MS,
electronic absorption, emission), TGA and PXRD studies. The representative 1D CP 1 has
been structurally authenticated by X-ray single crystal analysis. In order to have structural
information on 2-3, their powder X–ray diffraction (PXRD) patterns have been recorded and
compared with the PXRD patterns of 1 (vide-supra).
N
N
N
O
OH
N
N
N
N
N
O
N
N
DMF:H2O (7:3)
M
+
O
M(NO3)2. xH2O
120° C
N
H
O
O
Cl
N
N
Cl
OH
H
N
O
H3CCB
TPPZ
N
H
O
Cl
O
O
M
O
M
1
2
3
M= Zn(II), x=6, ; Cd(II), x=4, ; Co(II), x=6,
Scheme 1. Preparation of heteroleptic complexes 1-3.
As shown in Scheme 1, H3CCB preferably coordinates through only the κ1-COO− mode
along with concomitant coordination of TPPZ, resulting in infinite 1D CPs. This selective
mode of bonding might be attained due to the presence of the bulky bis-N,N,N−chelating
ligand (TPPZ) in close proximity to H3CCB, leading to steric hindrance in the heteroleptic
system and restricting the coordination number to five. Moreover, the charge neutrality factor
of the resulting complex might also play a significant role in controlling the 5-coordination.
6

However, the importance of the solvent nature and ratio (DMF:H2O; 7:3) cannot be neglected
in this regard [46,47].
3.2. Crystal structure of [Zn(TPPZ)(HCCB)]n.DMF.H2O (1)
Coordination polymer 1 crystallized in the monoclinic crystal system with the P21/c
space group (Fig. 1 and Fig. S2 in the SI). An ORTEP view along with the atom numbering
scheme is shown in Fig. 1a and important crystallographic data and refinement parameters
are summarized in Table 1. The crystal structure reveals that the asymmetric unit of 1
consists of one unit each of the Zn(II) ion, bisdeprotonated ligand (HCCB2−) and TPPZ ligand.
However, its crystal lattice also involves one molecule each of DMF and H2O, used as
solvents in the synthesis. The immediate coordination geometry about the metal center is best
described as distorted square pyramidal, comprising three N atoms from κ3-TPPZ and two O
atoms (κ1-O) from two units of HCCB2− (Fig. S3 in SI). The basal plane is formed by three N
atoms of κ3-TPPZ and an O atom of the amino carboxylate κ1-COO− group of the
HCCB2− ligand. The apical position is occupied by the O atom of the aromatic κ1-COO−
group of another HCCB2− unit at a distance of 1.997(2) Ǻ. One should ignore the relatively
short distance of the apical position since the two largest β and α angles of the basal plane are
created by other four atoms, thus they are essentially involved in forming the basal plane of
the distorted square pyramidal structure [48]. The Zn−O and Zn−N bond lengths vary in the
range 1.997(2) to 2.169(2) Ǻ (Table S1), which lie within the range of such reported systems
[39-49]. The coordination depicts that the bond distance Zn01−O1 (1.997(2) Ǻ) is smaller
than Zn01−O4 (2.037(2) Ǻ).
7

Fig. 1. (a) Structure of the asymmetric unit of 1 with the atom numbering scheme (the
hydrogen atoms have been omitted for clarity). (b) 1D extended chain of 1 having the
HCCB2− ligand bridged between the Zn(II) centers and the TPPZ moiety remains at one side
of the chain in a syn–syn fashion.
The mutual difference in bond lengths is explained by tetragonality, which is measured
in terms of the in-plane angular distortion described by the ratio τ [given by (β−α)/60] [48].
This ratio virtually represents the percentage of distortion in a square-pyramidal geometry.
The τ value calculated here is 0.2543, which indicates the presence of a substantial distortion
in the square pyramidal structure, which is further supported by significant deviations of the
O–Zn–N bond angles from 90° [96.11(8)-132.94(8)°]. Two adjacent Zn(II) ions are linked by
the HCCB2− ligand as a bridging ‘spacer’ to form an infinite 1D chain along the [010]
direction. The TPPZ units are regularly present at one side of the chain, connected by three
one sided N atoms (Fig. 1b). The TPPZ ligand is substantially twisted, the dihedral angles
between the pyrazine rings and each of the pyridyl rings are found to be 24.87, 23.61, 34.52
and 33.35°. The first two values are concerned with the coordinated pyridyl rings and the
latter two with non-coordinated ones. The pyrazine ring itself is puckered with the maximum
atomic deviation from the best mean plane being 0.124 Ǻ at C7. The torsion angles
C6−N2−C9−C8 and N5−C7−C6 −N2 are −10.174(350) and 22.891(340)° respectively, and
the two C−N−C planes of the pyrazine ring exhibit a dihedral angle of 11.91°. The dihedral
angle between the mean plane of the pyrazine ring and the equatorial plane defined by N3,
N2, N1 and O4 atoms equals 28.57°. It is not exceptional for the pyrazine ring to be non-
planar since it has been reported in the literature for related structures [50]. The responsible
factors for destabilizing the planar geometry may have electronic as well as steric reasons
[51].
The lattice water molecule (H5a-O5-H5b) is involved in an intermolecular hydrogen
bond, O5–H5a···O3 (2.137 Ǻ), with the oxygen atom of an adjacent aminocarboxylate
moiety and an O5–H5b···N4 (2.149 Ǻ) interaction with the nitrogen atom of a non-
coordinated TPPZ pyridyl ring. Further, the lattice water molecule is also involved in an
O5···H4−C4 (2.589 Ǻ) interaction with the carbon atom of one of the coordinated pyridyl
rings of TPPZ (Fig. S4; Table S2 in SI). Likewise, the lattice DMF molecule exhibits two
C−H···O interactions with coordinated and non-coordinated pyridyl rings of different TPPZ
units with distances of 2.323 and 2.594 Ǻ, respectively (Fig. S4). Additionally, some
8

important C-H···Z (Z = N, Cl) interactions exist [C−H···N (2.720 Ǻ) C−H···Cl (2.779 Ǻ)]
which results in a 2D architecture, wherein the H2O and DMF solvent molecules are
alternatively stabilized (Fig. S4 in SI). An intermolecular π···π aromatic stacking interaction
is exhibited between the subsequently spaced aromatic rings of HCCB2− in a parallel
displaced manner. The interlayer distance is found to be 3.507 Å and the distance between
the centroids of the concerned rings is found to be 3.783 Å (Fig. S5). This type of interaction
may contribute towards the stability of the polymeric architecture. The bis-N,N,N−donor
groups of TPPZ usually adopt a bridging coordination mode, which leads to a number of
transition metal coordination polymers [52]. Conversely, heteroleptic infinite CPs containing
the TPPZ ligand showing coordination to metal centers through one end is quite scarce.
3.3. X–ray powder diffraction studies
To obtain structural information on 2 and 3, their PXRD analyzes were performed in the
limited PXRD data set from 10-70º (2θ) and at a scan rate of 2º.
9

Fig. 2 Simulated PXRD patterns of 1 obtained from single crystal data (top) compared with
the PXRD patterns of the desolvated form of 1 and PXRD patterns of 2 and 3.
Initially, the PXRD patterns of the synthesized complexes 2 and 3 were compared with
simulated PXRD pattern of 1 obtained from X-ray single crystal data (Fig. 2). Notably, most
of the PXRD profiles of 2 and 3 matched with the simulated PXRD pattern of 1. The
differences noted, especially at low 2θ values, are supposed to be attributed to the absence of
lattice H2O and DMF solvents in 2 and 3. To get a deep insight into this, the single crystals of
1 were subjected to dynamic vacuum oven conditions by heating at 160 °C for 2 h, that in
turn resulted in the loss of the transparency of the single crystals without any significant color
change, which is ascribed to the desolvation of the crystal sample. To our delight, the PXRD
pattern of the desolvated form of 1 matched well with those of 2 and 3 (Fig. 2). It strongly
supported similar structures and coordination environments in 1-3. In addition, the lack of
lattice solvents (DMF and water) might be the responsible for not be able to obtain single
crystals of 2 and 3, despite several attempts under analogous reaction conditions.
3.4. Infrared and ESI-MS spectroscopic studies
The IR spectra for 1-3 indicated the presence of both TPPZ and H3CCB ligands (Fig.
S6a-c). In 1, aromatic C–H stretching vibrations occur in the range 3049-2855 cm–1,
associated with the TPPZ ligand [53]. In addition, a stretching vibration at 3409 cm–1 due to
the ν(N−H) group and two bands at 1705 and 1600 cm–1, which may be attributed to the
respective vibrations of νasym(C–O) and νsym(C–O) of the carboxylate groups of H3CCB, are
observed. A broad peak at ~3447 cm–1 indicates the presence of a water molecule in the
lattice. In addition, a vibration at 1626 cm–1 is assignable to the –C=O group of
dimethylformamide present in the crystal lattice. Conversely, 2 and 3 do not show any
vibrations at 1626 cm–1, indicating the absence of a lattice DMF molecule in their structures.
Further, 2 and 3 also exhibit bands around 1705 and 1598 cm–1 analogous to those of 1.
To get the better information about the stability of the CPs in the solution and the
presence of lattice solvent, the ESI-MS spectra of 1-3 were acquired in DMSO (Fig. S7a-c).
In its mass spectrum 1 displayed a molecular ion peak at m/z 680.0676 assignable to (Zn +
TPPZ + HCCB + H)+. It is noteworthy to mention here that a peak corresponding to [Zn +
TPPZ + HCCB + DMF + H2O]+ (calc. m/z 781.2011) is not observed, which suggests the
removal of lattice solvents from 1 in solution. Likewise, 2 and 3 also displayed molecular ion
10

peaks at m/z 729.1423 (Zn + TPPZ + HCCB)+ and m/z 675.0661 (Zn + TPPZ + HCCB +
H)+ in their mass spectra.
3.5. UV/vis and luminescence studies
The solid state electronic absorption spectrum of TPPZ displayed two bands at 301 and
359 nm assignable to the π–π* and n–π* transitions, respectively and H3CCB displayed
multiple distinct peaks in the range 230-327 nm (w) (Fig. S8 in SI). The electronic absorption
spectrum of 1 exhibited several low intensity bands in the region 362-399 nm (Fig. S9a).
These bands are assignable to ligand based transitions pertinent to both H3CCB and TPPZ
ligands. The absorption spectrum of 2 also followed a similar trend to that of 1. Since both 1
and 2 are based on d10 systems, they do not exhibit d-d transition bands above 400 nm
[54,55]. In the case of 3, the spectrum showed several intense peaks towards high energy
(357–374 nm), closely situated, and a peak at 386 nm which may be attributed to some intra-
ligand π–π* transitions of the TPPZ ligand. The d–d transition bands responsible for the pink
color of complex 3 are forbidden and thus are not observed here. Considerable red shifts are
exhibited by the CPs, as well as the presence of several intra-ligand charge transfer bands
pertaining to both ligand species, well supportive to the formation of the CPs.
The ligands H3CCB and TPPZ displayed luminescence bands at 425 and 480, 510 and
528 nm, respectively (Fig. S10 in SI). The luminescence properties of the Zn(II) and Cd(II)
complexes and their potential applications as new luminescent materials have drawn
significant current interest [56]. The photoluminescence properties of 1-3 have been
investigated in the solid state at room temperature. Upon excitation (λex) at 400 nm, 1 and 2
exhibited strong emissions at 605, 539, 484 nm (1) and 575 (broad hump), 542, 486 nm (2)
(Fig. S9b). In contrast, 3 displayed only two weak emissions at 597 and 530 nm (Fig. S9b).
The bands at 539, 484 and 542, 486 nm in 1 and 2 are attributed to LMCT (ligand to metal
charge transfer) transitions [57]. Notably, the emission intensities of 1-3 are quite a bit lower
to those for H3CCB and TPPZ, which may be attributed to the existence of weak interactions
and promoted LMCT transitions. Moreover, significant luminescence quenching for 3 is
ascribed to paramagnetic quenching, due to the presence of the Co(II) ion. The bands around
~540 nm correspond to a TPPZ based emission, whereas the band at ~480 nm may be
attributed to the combined effect of both ligands. The absorption spectra of the heteroleptic
CPs display bands centered around 360 nm that remarkably supports the coordination of the
TPPZ group with the metal ions. It facilitates the occurrence of a ligand–metal energy
11

transfer process and thereby reduces the overall emission intensity of the systems [58]. CPs
1-3 are heteroleptic thus their luminescence behavior is a combined contribution of both the
ligands.
3.6. Thermal behavior
The thermal stabilities of 1-3 were investigated through thermogravimetric analyses in
the temperature range 30–1000 ºC at a heating rate of 10 ºC/min under atmospheric
conditions. TGA thermograms for all the complexes exhibit thermal stability of the
framework (excluding lattice solvents) up to ~250 ºC, whereupon decomposition of the
framework starts (Fig. S9 in SI). For the TGA curve of 1, the initial weight (wt) loss between
40 and 120 ºC (3.268 wt %) is assignable to the expulsion of the lattice water molecule (Fig.
S11a in SI). A subsequent two step weight loss of 8.272 and 3.278 wt % in the temperature
range 120-280 ºC indicates the gradual removal of the DMF molecule (9.480 wt %) present
in the lattice structure and involving various weak intermolecular interactions (Fig. S4). The
TGA curve shows the decomposition of the core framework starts after ~300 ºC and ends at
600 ºC, which is attributed to the removal of the organic linker HCCB2– (30.488 wt %) and is
consistent with the theoretically calculated mass decomposition of HCCB2– (29.480 wt %).
Further weight loss is attributed to the decomposition of the organic chelator TPPZ (51.077
wt %) between 600 and 860 ºC, which is in good agreement with the theoretical value
(50.519). The TGA curve of desolvated 1 does not show the decomposition of any lattice
DMF and H2O molecules (Fig. S11b in SI), like its single crystal form (Fig. S11a in SI).
Thermal stability of the desolvated framework is found up to ~250 ºC and it starts
decomposing with an initial weight loss of ~24.853 wt %, ascribed to the partial removal of
the HCCB2– linker. It is consistent with the decomposition of the whole HCCB2– except for
the coordinated –COO moiety (theoretical calc. wt 26.911 %), which is not decomposed
below 450 ºC. The subsequent weight loss between 440 and 1000 ºC corresponds to the
removal of the chelating TPPZ ligand (55.396 wt %).
Notably, the TGA curves of 2 and 3 do not show any initial weight loss corresponding to
lattice solvent molecules and were found to be stable up to ~250 ºC, similar to the TGA curve
pattern of the desolvated form of 1 (Fig. S11c-d in SI). Initial weight losses between 250 and
400 ºC are assignable to the partial removal of the linker HCCB2– (27.258 wt %, 2; 25.498 wt
%, 3). Consequently, the organic chelator TPPZ is lost between 400 and 1000 ºC (52.392 wt
%, 2; 53.467 wt %, 3), which closely resembles the TGA pattern for desolvated 1. These
12

observations suggest that thermal stabilities of CPs 1-3 are similar, however for the single
crystal form of 1 the thermal weight loss starts at a lower temperature owing to the presence
of lattice solvent molecules. Overall, the TGA results suggest an alike thermal decomposition
pattern for complexes 1-3 in the temperature range 250-995 ºC and thus support the
analogous coordination environment.
3.7. Catalytic studies on 1-3.
The catalytic performance of the commercially available 4- and 5-coordinated zinc
cluster Zn4(OCOCF3)6O·CF3CO2H [59,60] in transesterification reactions motivated us to
examine our 5-coordinated CPs 1-3 as catalysts toward transesterification reactions. The
aqueous stability of 1 was probed from room temperature to 100 °C and it was observed that
1 is very poorly soluble (~10 % only) in aqueous medium. Notably, 1 dissolves in
dimethylsulfoxide (DMSO), which has been found to be a good choice as a solvent for
transesterification reactions [61]. The identity of catalyst 1 is retained in DMSO, as is evident
from its mass spectrum in the same solvent (Fig. S7a in SI). To investigate the catalytic
ability of 1, we employed transesterification reactions by varying both the ester (4a-f) and
alcohol (5a-f) reactants. However, formation of 6a has been chosen as representative reaction
for a catalytic activity discussion (Scheme 2).
Initially, 1 was dissolved in DMSO, and methyl 2-phenylacetate (4a) and benzyl alcohol
(5a) were added in a 1:1 stoichiometry to the solution of 1 (as a homogeneous catalyst). The
formation of the transesterification product benzyl 2-phenylacetate (6a) was significantly
apparent over the course of ~ 4 h (Scheme 2). All the products (6a-f) were formed between 4-
8 h, however in some reactions, complete consumption of the reactants required ~16 h
(Scheme 3, Table 2). We have examined the best suited catalyst loading by varying the molar
percentages and found an efficient catalytic activity of 1 at low catalyst loading (2 mol %).
We investigated from high catalyst loading (10 mol %) to low catalyst loading (0.1 mol %)
13

and found only a subtle change in the catalytic activity between 5 mol % (87%) and 2 mol %
(86 %) of 1 (Table 2). Therefore, we standardized a catalytic loading of 2 mol % in all the
transesterification reactions for 6a-f. Further, we successfully synthesized 6b-6f using 1 as a
catalyst with reasonably good yields (Scheme 3, Table 2). The ensuing transesterification
1
products 6a-6f were characterized by the NMR (1H and 13C) spectral technique and the H
NMR spectrum of the representative compound 6a is given in Fig. S12. Moreover, to have a
clear-cut comparison of the product formation rates, the reaction time was fixed to 4 h for
every reaction and the product yields have been compared, as listed in Table S3. Notably, the
maximum conversion is observed in the case of 6c (89 %) and the minimum for 6f (63 %)
after a reaction time of 4 h.
14

n(product)
n(reactant)
*The yield was calculated using the equation: Yield (%) = 100 ×
mole of the products and reactants.
where n is the
Scheme 3. Formation of transesterification products 6a-6f using Zn-CP 1 as a catalyst.
The recovery and reuse possibility of catalyst 1 was examined and notably the catalyst
was recovered by extracting the organic products 6a-f using CH2Cl2/CHCl3/C6H6, leaving 1
in the reaction vessel. The catalytic efficiency of 1 was observed up to six (6) cycles and a
slight decrease in the catalytic performance was found after four (4) cycles (Table 2).
However, for to four cycles the catalytic efficiency was almost same (Fig. 3). The framework
stability of 1 was found to be excellent, which is evident from the PXRD pattern acquired
after six catalytic runs, being virtually the same as that of fresh CP 1 (Fig. S13 in SI).
Fig. 3. Catalytic performance plot of Zn-CP 1 in the first cycle and after the sixth cycle for
the trans-esterification reaction.
The catalytic performance of 1 at the Zn2+ center can be considered efficient if it
performs better than using the free Zn2+ ion as a catalyst in same solvent and under analogous
conditions. With this objective, we dissolved 2 mol % of the Zn2+ ion [using Zn(ClO4)2·6H2O
as the Zn2+ source] in DMSO and examined its catalytic performance for the
transesterification reaction to yield 6a (Scheme 4). Notably, the Zn2+ ion performed very
poorly as compared to 1, yielding only 52% in the first cycle, which dramatically decreased
to 38% in the second cycle. It indicated very poor catalytic consistency of the free Zn2+ ion
catalyst relative to that of 1.
15

The catalytic activities of CPs 2 and 3 were also investigated under analogous conditions,
wherein 3 exhibited insignificant catalytic efficiency toward the transesterification reactions.
However, CP 2 showed catalytic activity toward the transesterification reactions, with quite a
poor performance for the formation of 6a (~3 times less than 1), which suggests it acts as an
ordinary catalyst for transesterification reactions. With this point of view, detailed catalytic
transesterification reactions using 2 as a catalyst have been omitted. The poor catalytic
performance of Cd(II) based CP 2 relative to that of 1 may be attributed to the poor labile
character of the Cd(II) metal (a second-row transition metal) toward substitution reactions
[62].
Fig. 4. Catalytic transesterification mechanism mediated by 1.
A plausible route for the interaction of the substrates with the catalyst is shown in Fig. 4.
The remarkable catalytic performance of CP 1 may be attributed to the coordinatively
unsaturated labile Zn(II) center (5-coordination) which offers an open site for substrate
binding (6th coordination about the Zn2+ center), resulting in the transesterification product
[63,64].
16

4. Conclusion
In summary, a series of heteroleptic CPs, 1, 2 and 3, have been synthesized and characterized
by X–ray crystallographic, spectroscopic and thermal techniques. The crystal structure of 1
reveals a distorted SPy geometry around the Zn(II) center and extends in a 1D array. The 1D
−
architecture is achieved by the bridging HCCB2 ligand, which exhibited a tendency to
rationally coordinate through its carboxylate goups, along with the chelating κ3-TPPZ ligand
which binds through only one of the two identical tridentate N,N,N–donor sites. Two
adjacent chains are extended parallel to each other and stabilized by π···π stacking
interactions, H–bonding and C–H···π types of weak interactions. The Cd(II) and Co(II) CPs
2 and 3 are found to be structurally similar to 1, as suggested by FT–IR, TGA and PXRD
patterns. Thermal studies also indicated similar stabilities of 1-3. The ligands and 1-3 exhibit
significant fluorescence behavior in the solid state at room temperature, where the intensity of
the H3CCB2− ligand is relatively stronger than the concerned polymeric complexes. Amongst
1-3, CP 1 acts as a stable and reusable efficient catalyst for transesterification reactions by
forming products with high time dependent yields upon low catalyst loading (2 mol %).
Acknowledgements
MAW acknowledges the UGC for financial assistance and the Department of Chemistry,
Dr. Harisingh Gour Central University Sagar, (M.P) India, for providing the research
facilities. MDP acknowledges the University Grant Commission for providing financial
assistance through the UGC one time Start up Grant through scheme [F.No.30-
56/2014(BSR)]. RP acknowledges the Department of Science and Technology for financial
assistance through the DST-Inspire Faculty scheme (IFA12-CH-66). The authors are grateful
to Prof. D. S. Pandey, Department of Chemistry, Banaras Hindu University, Varanasi for
helping at all the critical stages.
17

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20

Table 1. Crystal data and structure refinements for 1.
1
Identification code
Empirical formula
Formula weight
Temperature (K)
Wavelength (Ǻ)
Crystal system
Space group
SHELXL–97
C36H31ClN8O6Zn
772.51
293
0.710
Monoclinic
P21/c
a (Ǻ)
14.64
b (Ǻ)
20.80
c (Ǻ)
11.53
α (deg)
90
β (deg)
94.94
γ (deg)
90
Volume
3499.9
Z
4
Density (calculated)(g/cm3)
Absorption coefficient (mm-1)
F(000)
1.466
0.837
1594.48
Crystal size (mm3)
Theta range for data collection
Reflections collected
Independent reflections
Absorption correction
Refinement method
Goodness–of–fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
1 × 0.8 × 0.6
3.06 to 27.43
7945
4986
Tmin = 0.59, Tmax = 0.71
Full–matrix least–squares on F2
0.984
R1 = 0.042, wR2 = 0.085
R1 = 0.085, wR2 = 0.103
21

Table 2. 1) Performance of 1 at different catalytic loading (Mole %); 2) Catalytic
performances of recovered and reused 1 after several runs.
Yield (%)
Yield (%)
Catalytic
Catalytic runs
of recovered 1
(2 mol %)
loading of 1
10 mole %
5 mole %
2 mole %
1 mole %
0.5 mole %
0.1 mole %
89
87
86
80
73
67
1
2
3
4
5
6
86
86
86
86
85
81
22

Graphical Abstract: Pictogram
Heteroleptic 1D Coordination Polymers: 5-Coordinated Zinc(II) Polymer
as an Efficient Transesterification Catalyst
Manzoor A. Wania, Ashish Kumarb, Mrituanjay D. Pandeya* and Rampal Pandeya*
aDepartment of Chemistry, Dr. Harisingh Gour Central University, Sagar 470003, MP, India
bDepartment of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005,
UP, India
___________________________________________________________________________
__
23

Graphical Abstract : Synopsis
Heteroleptic 1D Coordination Polymers: 5-Coordinated Zinc(II) Polymer
as an Efficient Transesterification Catalyst
Manzoor A. Wania, Ashish Kumarb, Mrituanjay D. Pandeya* and Rampal Pandeya*
aDepartment of Chemistry, Dr. Harisingh Gour Central University, Sagar 470003, MP, India
bDepartment of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005,
UP, India
Three heteroleptic 1D MOFs, [Zn(TPPZ)(HCCB)]n·DMF·H2O (1) [Cd(TPPZ)(HCCB)]n (2)
and [Co(TPPZ)(HCCB)]n (3), based on N,N,N-chelating (TPPZ) and bis-carboxylate
(H3CCB) ligands have been reported. The structures of 1-3 comprise of a polymeric
framework extended through the H3CCB carboxylate group, despite presence of strong
chelating TPPZ ligand. The 5-coordinated Zn(II) centres in the infinite 1D chain of complex
1, exhibited efficient catalytic performance for transesterification reactions with remarkable
stability and good reusability.
24