Transition Metal Ions Regulated Structural and Catalytic Behaviors of Coordination Polymers

Article abstract of DOI:10.1021/acs.cgd.0c00465

Three new coordination polymers (CPs), I [Cu(L)(H2O)]·2H2O, II [Zn(L)], and III [Ag(HL)(CH3CN)], were synthesized by employing a novel pyrazole-based dicarboxylate ligand (H2L) through a solvothermal strategy. Single-crystal structural analysis established that compounds I and II exhibit two-dimensional (2D) sheet structures, while compound III possesses a one-dimensional (1D) ladder structure due to the connectivity of terminal solvent moiety. The geometries of metal ions vary from square planar (Cu2+ ion) to distorted tetrahedral (Zn2+ and Ag+ ion). Topological study showed that compound I has made a (4·62)2(42·62·82) net, while compounds II and III have exposed common (44·62) SQL net and (42·6) net, respectively. The Lewis acidic nature of compounds I, II, and III has been confirmed with various heterogeneous catalytic reactions. For the first time, one-pot multicomponent synthesis of α-amino amidine has been performed by the three CPs and the reaction kinetics has been altered by the variation of Lewis acidic metal centers. The roles of metal ions for structural variations and the catalytic performances are investigated in a series of CPs.

Full text of DOI:10.1021/acs.cgd.0c00465

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Article
Transition Metal-ions Regulated Structural and
Catalytic Behaviors of Coordination Polymers
Nikhil Kumar, Tanmay Rom, Virender Singh, and Avijit Kumar Paul
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.0c00465 • Publication Date (Web): 23 Jun 2020
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Transition Metal-ions Regulated Structural and Catalytic Behaviors of
Coordination Polymers
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Nikhil Kumara, Tanmay Roma, Virender Singhb and Avijit Kumar Paula*
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aDepartment of Chemistry, National Institute of Technology Kurukshetra, Kurukshetra-136119, India
Email id: apaul@nitkkr.ac.in
bDepartment of Chemisrty, Central University of Punjab, Bathinda, 151001, Punjab, India.
Abstract
Three new coordination polymers (CPs), I [Cu(L)(H2O)]·2H2O, II [Zn(L)] and III [Ag(HL)(CH3CN)],
were synthesized by employing a novel pyrazole based dicarboxylate ligand (H2L) through solvothermal strategy.
Single crystal structural analysis established that compound I and II exhibit two-dimensional (2D) sheet structure
while compound III possesses one-dimensional (1D) ladder structure due to the connectivity of terminal solvent
moiety. The geometries of metal-ions vary from square planar (Cu2+-ion) to distorted tetrahedral (Zn2+ and Ag+-ion).
Topological study showed that compound I has made (4.62)2(42.62.82) net while compound II and III have exposed
common (44.62) sql net and (42.6) net, respectively. The Lewis acidic nature of compounds I, II and III has been
confirmed with various heterogeneous catalytic reactions. For the first time, one-pot multicomponent synthesis of α-
amino amidine was performed by the three CPs and the reaction kinetics was altered by the variation of Lewis acidic
metal centers. The roles of metal-ions for structural variations and the catalytic performances are investigated in a
series of CPs.
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Introduction
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Considerable efforts have been devoted to prepare an organic ligand with versatile coordination sites to
promote the construction of higher-dimensional, robust and emerging multifunctional framework materials. In this
perspective, coordination polymers (CPs) and metal-organic frameworks (MOFs) are the most prominent outcome
with porous crystalline nature, lowest framework density and inorganic nodes constituted by metal ions/clusters
coordinated with organic moieties containing at least two requisite binding sites.1-3 Such polymeric frameworks
become center of attraction for researchers to carry out diverse applications in selective gas storage and gas
adsorption,4-7 ion-exchange,8-10 separation,11-13 conductivity,14-16 drug delivery,17-19 sensor devices20-22 and various
catalysis like autocatalysis, bio-catalysis, photocatalysis, organo-catalysis, along with electrocatalysis.23-27 At
present, a large number of metal centered catalysts are available for Lewis acid based catalytic transformations in
well-known reactions like cyanosilylation,28-33 Diels-Alder,34 Ene reaction,35 aldol reaction,36-37 ketalization, etc.38-41
In several catalytic reactions, CPs/MOFs works effectively as pre-catalysts, co-catalysts, cooperative catalysts and
promoters to accelerate the formation of valuable products for large scale applications. The catalytic activities of
CPs/MOFs alter due to the number of donor sites in organic ligands, available functional groups in ligands such as
basic (NH2) and acidic groups (OH and SO3H), oxidation states of metals or metal-clusters, ionic-sizes, shapes and
geometries of complexes.1 Furthermore, the correlation between catalyst structure and performance overcomes all
the above issues up to a greater extent as soon as the CPs are modified with post-synthetic modification,
encapsulated with nanoparticles/clusters or solvent variation during catalytic reaction.
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One-pot multicomponent reactions (MCRs) are enormously applicable as they follow clean and simple
methodology, high yields, low environmental impact, mild reaction conditions, high atom economy and wide
substrate scope. Recently, MCRs have appeared as valuable tool for the synthesis of heterocyclic molecules and
drug intermediates in academia and industry.42 The syntheses of amino amidine and its derivatives are one of the
important and promising class of MCRs due to their potential biological applications such as biosynthesis of purines,
nonvaccine derivatives, antimuscarinic agents, iminopeptides, synthesis of antibiotics and modulation of their
thermodynamic & pharmacokinetic profile.43,44 Very few reports are available for the multicomponent amino
amidine formation reaction catalyzed through the homogeneous or heterogeneous catalysts but CPs/MOFs catalyzed
MCR for amino amidine formation are not investigated so far.45 Therefore, we are keen to focus on coordination
environment and crystal structure-dependent Lewis acid mediated MCR employing newly designed CPs.
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In general, the reaction kinetics of any heterogeneously catalytic reaction depend on reactants, products and
catalyst shape and selectivity which may offer substantial transport resistance to block the entry of certain reactants
into CPs surface boundary. Moreover, the qualitative and quantitative effectiveness of a reaction may vary with the
structure modification of the catalyst.2,3 Hence, we have designed a flexible pyrazole based dicarboxylate with O-
and N- donor ligand [(H2L); 5-(3, 5-dimethyl-1H-pyrazol-1-yl) 1,3-benzenedicarboxylic acid] to synthesize three
coordination polymers with the association of different transition metals (Cu, Zn and Ag) under solvothermal
reaction condition as presented in Scheme 1. The detailed investigations of coordination environment dependent
structures and functionality have been described thoroughly. The Lewis acidic natures of developed CPs have been
verified by cyanosilylation of aldehydes and imines along with their substrate scopes. Additionally, one
multicomponent reaction for the formation of amino amidine has also been investigated with the CPs for the first
time. The combined roles of size, charge and geometry/ coordination environment of the metal-ions have been
established through the Lewis acidic nature of various metal ions.
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Materials syntheses and physical characterizations
We have used all the chemicals as they were received from various suppliers. Copper nitrate
[Cu(NO3)2.3H2O, CDH], zinc nitrate [Zn(NO3)2.6H2O HPLC], silver nitrate [AgNO3, Avra], benzaldehyde (TCI), 4-
chloroaniline (TCI), N-benzylideneaniline (Sigma Aldrich), trimethyl silyl cyanide (Loba Chemie), tert-
butylisocyanide (Sigma Aldrich), ethylacetate (CDH), hexane (CDH), dichloromethane (Avra), and acetonitrile
(Avra) were used for the experiments. The detail synthesis procedure and crystal structure of the pyrazole based
dicarboxylate ligand (H2L) can be found in our earlier report.28 All the three CPs (I, II and III) were synthesized
solvothermally and their crystal structures were analyzed by single crystal X-ray diffractometer (Bruker D8 Quest
PHOTON II). The purity and characterizations of the products were measured by powder X-ray diffraction (PXRD),
UV-Visible, FT-IR and TGA analysis. BRUKER-AXS-D8-ADVANCE diffractometer was utilized to collect all the
PXRD patterns in the 2θ range of 5-50o with Cu-Kα radiation (20 mA, 40 kV; λ = 1.54184 Å). KBr pellets were
used to measure the FT-IR spectra of all the samples (4000 to 400cm-1 region; in Shimadzu FT-IR spectrometer).
UV-Visible spectra were collected on Shimadzu UV-2600 240V spectrophotometer. Atomic absorption
spectroscopy was utilized for elemental analysis of the compounds (in Thermo Finnigan FLASH EA 1112 CHN
analyzer). We collected all the TGA data in N2 atmosphere with the flow rate of 30 mL/min in 30-900°C
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temperature region by 10°C/min heating rate (in HITACHI TGA/DTA 7200 instrument). All the room temperature
NMR spectra (1H and 13C-NMR) were measured on Bruker AVANCE-III spectrometer operating at 400 MHz
Evolution 201.
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Synthesis of I [Cu(L)(H2O)]·2H2O
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Cu(NO3)2·3H2O (0.0241 g, 0.1 mmol) was dissolved into the mixture of H2O and acetonitrile solvent (3 mL
H2O:3 mL CH3CN) in 15 mL glass vial. Then, H2L ligand (0.0260 g, 0.1 mmol) was added to it and stirred for next
2 h. Finally, the reaction mixture was poured into a 13 mL Teflon ligand H2L (0.0260 g, 0.1 mmol) -lined stainless-
steel autoclave and heated at 90oC for 3 days. After being cooled to room temperature, needle-shaped blue crystals
were separated out by filtration. The product was also washed with acetonitrile-water mixture. Here, the solid yield
was ~68% with respect to the Cu-metal. The purity of the product was checked from PXRD pattern. Calculated
elemental (%): C 41.51, H 4.29, N 7.45; found (%): C 41.43, H 4.34, N 7.52. The following characteristic peaks
were found in FT-IR (KBr, cm-1) spectrum; 3334 (s), 3107 (s), 2341(m), 1632 (s), 1575 (s), 1451 (m), 1368 (s),
1047 (m), 913 (m), 783 (s), 726 (s), 643 (m).
Synthesis of II [Zn(L)]
Zn(NO3)2·6H2O (0.0297 g, 0.1 mmol) was taken into a 15 mL glass vial containing a mixture of H2O and
acetonitrile solvent (3 mL H2O:3 mL CH3CN). Then, the ligand H2L (0.0260 g, 0.1 mmol) was added to it and the
mixture was stirred continuously for next 2 h. Finally, the reaction mixture was heated at 125 oC for 4 days in a 13
mL Teflon-lined stainless-steel autoclave. Colorless block-shaped crystals were found after filtration and the product
was washed with respective solvent mixture (Yield ~80% based on Zn metal). The PXRD pattern was measured to
confirm the purity of the compound. Calculated elemental (%): C 48.20, H 3.11, N 8.65; found (%): C 48.36, H
3.16, N 8.58. The following characteristic peaks were found in IR spectrum (KBr, cm-1); 2977 (s), 2920 (s), 2646
(s), 1937 (m), 1705 (s), 1585 (m), 1409 (m), 1254 (s), 902 (m), 757 (s), 680 (s), 561 (m).
Synthesis of III [Ag(HL)(CH3CN)]
AgNO3 (0.1 mmol, 0.0169 g), H2L (0.1 mmol, 0.0260 g) and 6 mL of solvent mixture (3 mL H2O:3 mL
CH3CN) were kept into a 15 mL glass vial and stirred for next 2 h. The reaction mixture was then transferred into a
o
13 mL autoclave and heated at 125 C for 3 days. After cooling down to room temperature, colorless block-shaped
crystals were filtered with the yield of ~85% based on Ag metal. The purity of the product was checked by PXRD
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pattern. Calculated elemental (%): C 44.10, H 3.45, N 10.29; found (%): C 44.24, H 3.39, N 10.21. The following
characteristic peaks were found in IR spectrum (KBr, cm-1); 3119 (s), 3062 (s), 2958 (s), 2591 (m), 2363 (m), 1696
(s), 1556 (s), 1365 (s), 1251 (s), 915 (m), 816 (m), 764 (s), 666 (s).
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Single Crystal Data Collection
The single-crystal X-ray diffraction measurements were performed at Bruker D8 Quest PHOTON II
diffractometer with monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K operating at 20 mA and 50 kV by
using ω and ϕ scan. The final data sets were reduced by APEX3 program while SAINTPLUS46 program was utilized
for integration of diffraction profiles. The absorption correction (multiscan) was carried out by SADABS program.47
We initially solved the structure by SIR 9248 and the full matrix least square method (SHELXL-201649) was used
further which is present in the WinGx suit of programs (Version 1.63.04a).50,51 We successfully located all the non-
hydrogen atoms from Fourier maps and refined with anisotropic displacement parameters at the final cycles. Finally,
we fixed all the hydrogen atoms at calculated positions and included in the refinement process using riding model
associated with isotropic thermal parameters. Four hydrogen atoms of the two lattice water molecules in compound I
were not located. We have summarized all the refinement parameters of the crystal structures I, II & III in Table 1.
Selected bond distances and angles are also given in Table S1, S2 & S3 (see ESI). All the observed hydrogen bonds
are supplied in Table S4, S5 and S6 (see ESI). One can find the crystallographic files with the deposited CCDC
numbers 1994388-1994390 for I, II & III, respectively via www.ccdc.cam.ac.uk/data_request/cif.
General method for catalysis
(a) Cyanosilylation of benzaldehyde
A round bottom (RB) flask with two neck was deaerated with N2 gas purging from one neck and the second
neck was closed tightly with a septum. Then, 37.8 mg of catalyst I (0.1 mmol of I) and 5 mL dry DCM were placed
into the RB flask followed by the addition of 53.06 mg aldehyde (0.5 mmol of benzaldehyde) and 74.4 mg trimethyl
silyl cyanide (TMSCN; 0.75 mmol). The whole mixture was stirred continuously for 4 h. The progress of the
reaction was checked by TLC in ethyl acetate/ n-hexane (1:9) solvent mixture. Upon completion of the reaction, the
catalyst was separated out by filtration and solvent was evaporated under vacuum. Then, ethyl acetate was added to
the reaction mixture and targeted yellow colored product was purified by column chromatography over silica-gel
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(100-200 mesh) using ethyl acetate/hexane (1:5 v/v) mixture as an eluent and further analyzed by H-NMR. The
similar procedure was followed for the catalytic reactions in presence of compounds II and III.
(b) Cyanosilylation of imine
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Under the nitrogen atmosphere, 37.8 mg of compound I (0.1 mmol) was kept into a RB flask. Then, 5 mL
dry dichloromethane (DCM) and 90.61 mg aldimine (N-benzylideneaniline; 0.5 mmol) were added to it. Hereafter,
74.4 mg trimethyl silyl cyanide (TMSCN; 0.75 mmol) was added into RB flask with continuous stirring for next 5 h
at room temperature. The reaction progress was monitored by TLC in ethyl acetate: n-hexane (1:9) solvent mixture.
After completion of reaction, the catalyst was separated out from the resulting reaction mixture by filtration and
solvent was evaporated under vacuum. Thereafter, NaHCO3 solution (1%, 5 mL) was added to it and the organic
compound was extracted into ethyl acetate layer and dried over anhydrous Na2SO4 to get crude product. The
resulted yellow colored aminonitrile was further purified with silica gel (100-200 mesh) column chromatography
using ethyl acetate : n-hexane: (1:12 v/v) as eluent and the structure was analyzed by NMR spectra. The remaining
two catalysts II (32.3 mg; 0.1 mmol of II) and III (40.8 mg; 0.1 mmol of III) were also used for cyanosilylation of
imines.
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(c) One-pot multicomponent amino amidine formation
28 mg catalyst I (0.075 mmol) was procured in a 25 mL round bottom (RB) flask charged with 80 mg of
benzaldehyde (0.754 mmol) and 116 mg of 4-chloroaniline (1.5 mmol) under nitrogen atmosphere. Then, 0.085 mL
tert-butyl isocyanide (6.12 mmol) and 2 mL dichloromethane (dry DCM) were added into RB flask. The reaction
mixture was gently stirred at room temperature for next 5 hours and the progress of reaction was monitored by thin
layer chromatography (TLC) in a solvent mixture of n-hexane: ethyl acetate (9:1, v/v). After completion of reaction,
the reaction content was filtered on the celite bed in order to recover the catalyst. The product was further purify by
silica gel column chromatography (60-120 mesh) using ethyl acetate/ n-hexane (3: 97 v/v) as an eluent to obtain the
pure product as a white solid and the structure was analyzed by NMR spectra. The solid catalyst, which was
separated out from the reaction mixture of all the three given reactions, washed well several times with dry DCM,
dried for 2-3 h to reuse for 5 consecutive cycles. We followed the same procedure for one-pot multicomponent
amino amidine formation with other remaining two catalysts II (24.2 mg; 0.075 mmol of II) and III (30.6 mg;
0.075 mmol of III).
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Results and Discussion
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Crystal Structure of [Cu(L)(H2O)]·2H2O
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Single crystal X-ray diffraction reveals that complex I crystallizes in triclinic crystal system with the space
group P-1 (Table 1). The asymmetric unit encompasses of two independent Cu2+ metal ions with site multiplicity of
0.5 (special position of 1a and 1b), one ligand unit (L), one coordinated and two lattice water molecules as displayed
in Figure 1a. The organic ligand (L) has two carboxylate units and one aromatic pyrazolate ring. The Cu(1) ion is
connected to two oxygen atoms of two different carboxylate groups and two distinct pyrazolate nitrogen atoms of
ligands forming CuO2N2 distorted square planar geometry whereas the Cu(2) ion is associated with two oxygen
atoms of different carboxylate groups and two coordinated water molecules forming CuO4 square planar geometry
(see ESI, Figure S1). All the Cu−O bonds are found in the typical range of 1.922(3) −1.964(5) Å. A significant
elongation has been observed for Cu−N bond length [bond distance of 1.996(4) Å] as compared to Cu−O bond
distance. The various <O/N−Cu−O/N vary in the range of 88.95(16) − 91.00(16)° confirming the square planer
geometry of two metal centers. The selected bond lengths and bond angles are given in Table S1 (see ESI).
Two-dimensional (2D) layer is constructed by the arrangement of copper metals and ligands along the ac
plane as displayed in Figure 1b. Here, the organic ligand (L) is coordinated to three different Cu2+ ions via one
nitrogen atom of the pyrazolate moiety and two oxygen atoms of two carboxylate units (see ESI, Figure S2). Out of
two copper ions, Cu(1) is connected with four ligand units while Cu(2) is coordinated with two ligand units in
opposite corner. The pyrazolate rings are perpendicularly aligned to the phenyl ring of the ligand and existing as free
substituent in the layer structure (see ESI, Figure S3). The two-dimensional structure can be well described by the
topology and net-connectivity. Ligand unit is considered as 3-connected node and Cu(1) has formed 4-connected
node while Cu(2) is considered as linker in topology analysis (see ESI, Figure S4). The present structure contains 4-,
6- and 8-membered rings with common point symbol (4.62)2(42.62.82). Though the hydrogen atoms of two lattice
water molecules are not located in the structure, but the C–H···O and O–H···O hydrogen bonds between pyrazolate
moiety and water molecules support to form the overall close-packed structure (see ESI, Figure S5).
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Crystal Structure of [Zn(L)]
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The compound II also crystallizes in the triclinic P-1 space group (Table 1). Here, the asymmetric unit
consists of 20 non-hydrogen atoms, of which one Zn2+ ion and one ligand (L) are crystallographically independent
(Figure 2a). The zinc-ion has distorted tetrahedral environment (ZnO3N) due to the coordination with three oxygens
and one nitrogen atom of the ligand (see ESI, Figure S6). All the three oxygen atoms and one nitrogen atom belong
to four distinct L unit. The Zn–O bond lengths are observed in the typical range of 1.934(2)−1.981(2) Å whereas
Zn–N bond length is 2.053(3) Å. The various bond angles of O/N−Zn−O/N differ in the range of
99.40(10)−129.03(11)° with the average of 109.65o. The important bond lengths and bond angles are summarized in
Table S2 (see ESI). Overall two-dimensional sheet is generated by the association of zinc metale and ligands
connectivity along the ac plane as displayed in Figure 2b. Each ligand (L) is coordinated to four different Zn2+ ions
through three oxygen atoms of two carboxylate units and one nitrogen of the pyrazolate ring. The rest one oxygen
atom (O3) of the ligand is free from the coordination (see ESI, Figure S7). Here, Zn2 dimeric unit is also present in
the 2D structure, formed by the connectivity of two zinc ions and carboxylate oxygen of L (see ESI, Figure S8). The
dimeric units propagate in the a-axis with the connection of ligands and the pyrazolate rings of the ligands are
hanging towards outside in the 2D layer structure (see ESI, Figure S9). Considering both the zinc ions and ligands as
4-connected node, the topological study reveals that the compound has formed sql net with the point symbol of 44.62
(see ESI, Figure S10). The 2D sheets can form week hydrogen bonding interaction between carboxylate unit and
pyrazolate ring to obtain three-dimensionally (3D) closed packed structure (see ESI, Figure S11).
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Crystal Structure of [Ag(HL)(CH3CN)]
Single crystal structural analysis shows that complex III also crystallizes in the triclinic P-1 space group.
Here, the asymmetric unit contains one crystallographically independent Ag+ metal ion, one L unit and one
acetonitrile unit (solvent). The Ag+ ion has distorted tetrahedral geometry (AgO2N2) connecting two oxygen atoms
carboxylate group, one nitrogen atom of pyrazolate ligand and one nitrogen atom of solvent moiety (Figure 1a and
Figure S12 in ESI). The respective bond lengths of Ag−O fall in the range of 2.337(3) −2.481(3) Å and Ag−N bond
distances are in the range of 2.237(5) −2.302(4) Å closely matching with earlier reported silver(I)-based
coordination compounds.44,52-54 The various <O/N−Ag−O/N angles differ in the range of 84.66(11)− 133.69(17)°
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with an average of 109.03o confirming distorted tetrahedral geometry of the Ag-ion. The important bond lengths and
angles are supplied in Table S3 (see ESI).
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Interestingly, one-dimensional (1D) ladder structure is observed in III by the coordination of silver metals
and ligands (L) along the ac plane as given in Figure 3b. The ligand L is linked with three different Ag+ ions through
one oxygen atom (O3) of carboxylate unit via µ2-connectivity and one pyrazolate nitrogen, keeping other three
oxygen atoms free from the coordination (see ESI, Figure S13). A simple Ag2 dimeric unit is present in the 1D
structure, formed due to the coordination of two silver ions and two µ2-oxygen of L (see ESI, Figure S14). The
dimeric units propagate along the a-axis and the non-bonded pyrazolate moieties are hanged in the ladder structure
(see ESI, Figure S15). Here, the solvent moiety (CH3CN) is coordinated with the Ag+-ion and protects from the
formation of layer structure. Hence, the solvent moiety plays a crucial role for the formation of 1D ladder structure
compared to compounds I and II. The topological study reveals that the compound has formed a 3-connected net
with 42.6 point symbol (see ESI, Figure S16). The inter- and intra-molecular hydrogen bonding interactions support
to extend the present structure into three-dimensional (3D) packing as displayed in Figure S17 (see ESI).
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Characterizations and thermal stability:
The PXRD patterns were measured at room temperature for all the three compounds I, II and III. All
observed peaks are matched well with the simulated pattern created from the single-crystal data confirming the
phase purity of all the compounds as shown in Figures S18-S20 (see ESI). The FT-IR spectra evidently provided
clear information about functional groups and water molecules present in all the CPs I, II and III. The spectra of all
the three compounds are almost similar as displayed in Figures S21-S23 (see ESI). All the characteristic peaks are
marked with the corresponding vibration modes. The very sharp peak is appearing at 3334-3160 cm−1 which is
ascribed to νO-H group present in water molecule for I whereas no sharp peaks have been observed in the same
region for II & III. A broad peak in the region of 2500-3100 cm−1 is assigned to ν O-H of COOH moiety present in
III. The remaining peaks in the region of 2900-3070 cm−1 is observed for ν=C-H (of aromatic ring) and ν-C-H (of
CH3 group) for I, II and III. Again, sharp peaks have been observed in the region of 1556-1705 cm-1 is due to the
νC=O and the peaks below 1451 cm−1 can be attributed to stretching vibration of νC=C. The peaks in the region of
1251 to 561 cm−1 are ascribed as the vibration peak of δ=C-H. The UV-vis spectra display two transition bands at
232 and 282 nm for II whereas the bands are at 253 and 301 nm for III demonstrating the occurrence of ligand to
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ligand charge transfer (LLCT) as well as π-π* transition. In case of CP I, two transition bands appear at 567 and 705
nm indicating d-d transition and ligand to metal charge transfer (LMCT). The UV-vis spectra for all the three CPs
are represented as Figure S24 (see ESI). The thermal stabilities of I, II and III have been checked by performing
thermogravimetric analysis (Figure S25, see ESI). As I contains water molecules, so initial weight loss of 9% at 150
oC implies to the loss of three molecules of water (calculated 14%). There are two lattice water molecules and one
coordinated water molecule which are left at the initial stage. Second weight loss starts from 338°C ascribed to
decomposition of framework. The TGA curve for I shows total weight loss of 77% (calculated 78.2%) till 900°C
and the remaining black colour compound could be CuO.55 The TGA curve for II exhibits a total weight loss of 79%
(calculated 76.8%) till 900°C with yellowish white residue (ZnO).56 In case of III, the initial weight loss of 8%
below 275 oC corresponds to the loss of acetonitrile solvent (calculated 10%) from the compound. Then, the similar
trend was observed in CP III with a total weight loss of 71% (calculated 71.6%) till 900°C and the remaining black
colour compound could be Ag2O.30 The final products of I–III are assumed to be stable metal oxides (CuO, ZnO
and Ag2O) as the similar oxides are found earlier.30,55-56 The calculated weight losses are matched well with the
above mentioned final products (oxides). The final residuals for the three compounds were not characterized by
PXRD as they got stuck with the TGA crucible made of Al2O3.
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Catalytic studies
Metal ions as active centers in CPs and MOFs strongly influence various catalytic transformations by
varying their ionic size, coordination number, oxidation state and connectivity with flexible organic ligands. At
present, CPs emerged as an active catalyst for numerous reactions such as hydrogenation, dehydrogenation,
epoxidation, oligomerization, dehydration, polymerization, condensation and so on. Moreover, a plenty of reactions
such as cyanosilylation, esterification, hydrolysis involve water and catalyzed by acidic sites achieving many
industrial applications. In general, metal ions with fully filled d10 electronic configuration act as strong Lewis acid to
carry out the heterogeneous catalytic reactions.28-32,57 In continuation of our earlier efforts28-32 towards single
component cyanosilylation of imines, now we proceed for the first time one-pot multicomponent amino amidine
formation reaction catalyzed by CPs. The present study illustrates the comparison between three Lewis acidic metal
centers Cu, Zn and Ag for three kinds of catalytic reactions such as cyanosilylation of benzaldehyde and imines
along with the multicomponent amino amidine formation reaction.
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Initially, two well-known reactions namely cyanosilylation of benzaldehyde and imines with their
substrates scope are performed as presented in Table 2. The optimizations of both elementary reactions have also
been carried out by using catalyst I, II and III as summarized in Table S7 and S8 (see ESI). The NMR spectra (1H
and 13C) of products for model reaction along their substrates scope are attached as Figures S26-S37 (see ESI). All
the spectra are harmonized well with earlier communicated literatures.43,44 The corresponding yields of the products
under all the varied conditions have been represented in Table 2. All the present catalysts can actively catalyze both
the elementary reactions with excellent yields (above 90%). Hence, electron withdrawing and electron releasing
group derivatized reactants have also been chosen to perform the cyanosilylation reactions of substituted-
benzaldehyde and imines (yields: 92–98%). Even though, the yields are superior in both the cases, but electron
withdrawing groups favor the reaction by generating more electrophilic centers at the reactants. We can conclude
that catalytic activities of Cu+2, Zn+2 and Ag+-ions are comparable for cyanosilylation reactions.
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At last, a multicomponent one pot amino amidine formation reaction has been performed with
benzaldehyde, 4-chloroaniline and tert-butyl isocyanide in dry DCM solvent by using all the three catalysts I, II &
III (0.075 mmol). From Table 3, it is clear that the catalyst II forms 83% product followed by the catalyst I into
81% and the catalyst III into 58% product. The optimization of the catalytic reaction was also done by varying
reaction time, temperature and catalyst amount (Table 3). Here, first we decrease the reaction time to 4 h which
resulted into lesser yield of product. Then, the increment in the reaction time to 8 h does not affect much product
yield. One set of experiment was also performed at 0-5oC but the product formation was very less as compared to
the room temperature reaction. By increasing the temperature from RT to 70 oC also could not enhance much yield
of product. Afterwards, the catalyst amount was decrease to 0.05 mmol, which turned reduction in the product yield.
One more attempt was done by increasing catalyst amount to 0.1 mmol, though the yield was remain almost similar
as in case of 0.075 mmol. The catalytic activities of each starting materials were carried out by employing
Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, AgNO3 and L, which resulted into very less yields of 15%, 19%, 12% and 9%,
respectively. A blank reaction was executed without any catalyst which converted into 4% product formation. So, it
is confirmed that the catalyst I and II are more active than III for the formation of amino amidine.
In general, all the three Lewis acid-based catalysts have different metal centers like Cu, Zn and Ag but their
catalytic activities differ on the basis of their crystal structures. In catalyst I, Cu2+ (d9) exists in square planar
geometry whereas Zn2+ (d10) and Ag+ (d10) have distorted tetrahedral environment in II and III, respectively. Here,
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the Lewis acidic strength is highest for Zn2+ ion due to the higher charge with respect to the ionic size. Hence, Ag+
ion shows the lowest activity compared to other two metal ions. The heterogeneous natures of the present catalysts
encourage us to check its recyclability. We have performed the experiments by using same catalysts I, II and III up
to next five cycles as shown in Figure 4 & S38-S39(see ESI). No such change have been observed in the resultant
yield after five cycles even all the catalysts were stable and worked effectively for next 5 cycles. There is no
structural dissociation as proved by PXRD data collected after reactions (Figures S18-S20, see ESI).
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A comprehensive literature servey has been done for the synthesis of amino amidine in multicomponent
reaction and cyanosilylation reactions by using different catalysts as given in Table S9-S11 (see, ESI). So far, no
particular report is available for amino amidine formation catalyzed by CPs/MOFs.58-60 The present investigation
is the first example of such reaction at room temperature with excellent yield. Based on the earlier reports,43,44
a
probable mechanistic scheme has been drawn for conversion of benzaldehyde, aniline and isocyanide into amino
amidine (Figure 5). Initially, the aromatic aldehyde reacts with aniline in presence of Lewis acid to form an imine
with the removal of water. Then, imine was further protonated in presence of Lewis acid facilitating the nucleophilic
reaction of isocyanide to form the intermediate. Second molecule of aniline attacks to the intermediate and results
into the formation of amidine, which further shows the [1,3] hydrogen transfer to form the desired product.61 The
proposed mechanistic pathways for the two elementary reactions i.e. cyanosilylation of benzaldehyde and imines,
have also been drawn in Figures S40 & S41 (see, ESI). Both the cyanosilylation reactions follow similar pathway
for the formation of product and the similar kinetics have also been reported earlier.28 At last, we can conclude that
all the catalysts are effective for cyanosilylation reactions but the compound III (Ag+-ion) is least active for
multicomponent reaction.
Conclusions
In summary, we have successfully isolated three new coordination polymers by adopting solvothermal
method with pyrazolyl-carboxylate ligand and three different metal precursors. Single crystal structural studies
showed that Cu2+ and Zn2+-ions form two-dimensional layer structure whereas Ag+-ion exhibits one dimensional
ladder structure with the solvent coordination. The coordination environments are also altered from square planar
geometry to distorted tetrahedral geometry with the variation from d9 transition metal ion to d10 transition metal ion.
Lewis acidic natures of all the three CPs have been confirmed by performing three catalytic transformations namely,
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cyanosilylation of benzaldehyde and imines, and one-pot multicomponent α-amino amidine formation reaction. We
have demonstrated a first example of CP catalyzed α-amino amidine formation reaction using a one-pot
multicomponent reaction strategy. The functional group tolerances for the efficient conversions of various
substituted substrates have also been substantiated through their derivatized products. The three heterogeneous
catalysts are very effective as they show exceptional stability and recyclability. The plausible mechanistic pathway
supports the role of ionic potential as driving force for catalytic conversions followed the trend I ≈ II > III. We
believe that the present work will stimulate further to design new framework materials with multidentate ligands,
and to explore novel functional applications like α-amino amidine formation for the production of biologically
active derivatives and synthetic intermediates for new chemical transformations.
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ASSOCIATED CONTENT
Supporting Information
Tables for bond distances, angles and hydrogen bonds, catalytic studies, PXRD, TGA, FT-IR, NMR, and few figures
of crystal structures have been given. This material is available via the Internet at http://pubs.acs.org.
Corresponding Author
*Email: apaul@nitkkr.ac.in
Notes
The authors declare no competing financial interest
ACKNOWLEDGMENT
AKP and TR gratefully acknowledge to Department of Science and Technology (INSPIRE Award No. IFA14-CH-
144) & Science and Engineering Research Board, India (No. SB/FT/CS-098/2014) for the financial support. VS is
thankful to SERB, India for research grant, EMR/2017/000155. NK acknowledges to UGC-DAE-Consortium for
Scientific Research (No. CRS-M-262/2017) for the support of scholarship.
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41. Maity, K.; Karan, C. K.; Biradha, K. Porous Metal-Organic Polyhedral Framework containing Cuboctahedron
Cages as SBUs with High Affinity for H2 and CO2 Sorption: A Heterogeneous Catalyst for Chemical Fixation
of CO2. Chem.: Eur. J 2018, 24, 10988-10993.
42. Yuan, S.; Qin, J.-S.; Li, J.; Huang, L.; Feng, L.; Fang, Y.; Lollar, C.; Pang, J.; Zhang, L.; Sun, D.; Alsalme, A.;
Cagin, T.; Zhou, H.-C. Retrosynthesis of Multi-Component Metal−Organic Frameworks. Nat. Commun. 2018,
9, 808-818.
43. Kumar, A.; Saxena, D.; Gupta, M. K. Nanoparticle Catalyzed Reaction (NPCR): ZnO-NP Catalyzed Ugi-
reaction in Aqueous Medium. Green Chem. 2013, 15, 2699-2703.
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44. Adiyala, P. R.; Chandrasekhar, D.; Kapure, J. S.; Reddy, C. N.; Maurya, R. A. Synthesis of α-Amino Amidines
through Molecular Iodine-Catalyzed Three-Component Coupling of Isocyanides, Aldehydes and Amines.
Beilstein J. Org. Chem. 2014, 10, 2065–2070.
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45. Aguirre-Díaz, L. M.; Gándara, F.; Iglesias, I.; Snejko, N.; Gutiérrez-Puebla, E.; Monge, M. A. Tunable
Catalytic Activity of Solid Solution Metal–Organic Frameworks in One-Pot Multicomponent Reactions. J. Am.
Chem. Soc. 2015, 137, 6132-6135.
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46. Apex3 v2017.3-0, Saint V8.38A, SAINT V8.38A, Bruker AXS Inc.: Madison (WI), USA, 2018.
47. Krause, L.; Herbst-Irmer, R., Sheldrick, G. M.; Stalke, D. J. Appl. Cryst. 2015, 48, 3-10.
48. Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. Completion and Refinement of Crystal Structures
with SIR92. J. Appl. Crystallogr. 1993, 26, 343−350.
49. Sheldrick, G.M. Crystal Structure Refinement with SHELXL. Acta Crystallogr. Sect. C Struct. Chem. 2015, 71,
3–8.
50. Farrugia, J. L. WinGX suite for Small-Molecule Single-Crystal Crystallography, J. Appl. Crystallogr. 1999, 32,
837-838.
51. Spek, A. L. Single-Crystal Structure Validation with the Program PLATON. J. Appl. Crystallogr. 2003, 36,
7−13.
52. He, M.-Q.; Xu, Y.; Li, M.-X.; Shao, M.; Wang, Z.-X. Various of Silver Phosphinate Inorganic Architectures in
Three-Dimensional Frameworks with Argentophilic Interactions. Cryst. Growth Des. 2019, 19, 2892-2898.
53. Yan, Z.-H.; Li, X.-Y.; Liu, L.-W.; Yu, S.-Q.; Wang, X.-P.; Sun, D. Single-Crystal to Single-Crystal Phase
Transition and Segmented Thermochromic Luminescence in a Dynamic 3D Interpenetrated AgI Coordination
Network. Inorg. Chem. 2016, 55, 1096−1101.
54. Rueff, J.-M.; Perez, O.; Caignaert, V.; Hix, G.; Berchel, M.; Quentel, F.; Jaffrès, P.-A. Silver-Based Hybrid
Materials from meta- or para-Phosphonobenzoic Acid: Influence of the Topology on Silver Release in Water.
Inorg. Chem. 2015, 54, 2152−2159.
55. McHugh, L. N.; Perez, L. J. O.; Wheatley, P. S.; Cordes, D. B.; Slawin, A. M. Z..; Morris, R. E. Synthesis and
Structural Characterisation of the Copper MOF: STAM-NMe2, CrystEngComm. 2019, 21, 5387−5391.
56. Mahata, P; Natarajan, S. Pyridine‐and Imidazoledicarboxylates of Zinc: Hydrothermal Synthesis, Structure, and
Properties. Eur. J. Inorg. Chem. 2005, 2005, 2156−2163.
57. Agarwal, R. A.; Gupta, A. K.; De, D. Flexible Zn-MOF Exhibiting Selective CO2 Adsorption and Efficient
Lewis Acidic Catalytic Activity. Cryst. Growth Des. 2019, 19, 2010-2018.
58. Keung, W.; Bakir, F.; Patron, A. P.; Rogers, D.; Priest, C. D.; Darmohusodo, V. Novel  -Amino Amidine
Synthesis via Scandium (III) Triflate Mediated 3CC Ugi Condensation Reaction. Tetrahedron Lett. 2004, 45,
733–737.
59. Khan, A. T.; Bashar, S.; Lal, M.; Mir, M. H. Formation of Unexpected -Amino Amidine Through Three-
Component ‘UGI Condensation Reaction’. RSC Adv. 2012, 2, 5506–5509.
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60. Saha, B.; Frett, B.; Wang, Y.; Li, H.-Y. A p-Toluenesulfinic Acid-Catalyzed Three-Component Ugi-type
Reaction and its Application for the Synthesis of -Amino Amides and Amidines. Tetrahedron Lett. 2013, 54,
2340–2343.
61. Sagar, A.; Babu, V. N.; Sharada, D. S. Silica Gel Promoted Environment-Friendly Synthesis of -Amino
Amidines and Regioselective Transformation of -Amino Amidines into Amidino Substituted Indazoles. RSC
Adv. 2015, 5, 29066–29071.
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Table 1: Description of crystal data and structure refinement parameters for I, II and III.
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Parameters
Empirical formula
Formula mass
Crystal system
Space group
a (Å)
I, [Cu(L)(H2O)]·2H2O
C13H16CuN2O7
375.79
II, [Zn(L)]
C13H10ZnN2O4
323.60
III, [Ag(HL)(CH3CN)]
C15H14AgN3O4
408.16
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60
Triclinic
P-1
Triclinic
P-1
Triclinic
P-1
8.0988(5)
8.6711(6)
12.4926(8)
108.785(3)
93.346(3)
102.531(3)
802.94(9)
2
8.1006(5)
8.8657(5)
9.6604(6)
84.886(2)
86.986(2)
64.489(2)
623.57(6)
2
8.5690(15)
8.6780(15)
10.9458(19)
102.240(5)
100.389(5)
95.864(6)
774.0(2)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
T/K
296(2)
296(2)
296(2)
ρ (g cm-3)
µ(mm-1)
λ (Mo Kα) (Å)
θ min (°)
θ max (°)
Total data
Unique data
Rint
1.538
1.723
1.751
1.396
1.984
1.326
0.71073
2.92
0.71073
2.55
0.71073
1.95
24.99
25.00
25.00
10200
17943
11999
2833
2196
2709
0.0652
0.0264
0.0666
R indexes [I>2σ(I)]
R1 = 0.0474
wR2 = 0.1323
R1 = 0.0259
wR2 = 0.0857
R1 = 0.0370
wR2 = 0.1076
R indexes (all data)
R1 = 0.0834
wR2 = 0.1743
R1 = 0.0360
wR2 = 0.1264
R1 = 0.0492
wR2 = 0.1236
CCDC numbers
F(000)
1994388
378
1994389
328
1994390
408
GOF on F2
1.149
1.301
1.136
aR1 = ∑║Fo│-│Fc║/∑│Fo│; bwR2 = [∑w(Fo -Fc2)2/∑w(Fo ) ] ,
2
2 2 1/2
w = 1/[σ2(Fo)2 + (aP)2 + bP]. P = [max (Fo , 0) + 2 (Fc2)/3
2
a= 0.0990, b= 0.3796 for I; a= 0.0848, b= 0.2107 for II; a= 0.0694, b= 0.0000 for III.
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Table 2: Substrate scope for the cyanosilylation of benzaldehyde and imines catalyzed by CPs I, II and IIIa.
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O
Ph
Ph
OTMS
5 h, RT, dry DCM
Catalyst I, II & III
N
HN
Ph
Me3SiCN
+
H
4 h, RT, dry DCM
Catalyst I, II & III
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CN
H
Me3SiCN
+
Ph
H
CN
Benzaldehyde (1)
N-benzilidine aniline (3)
4 (97 % )
2 (98 %)
Substrate Variations
Substrate
Product (Yield %)
Substrate
Product (Yield %)
CHO
OTMS
H
N
N
CN
H
H3CO
H
CN
1a
H3CO
2a (94 %)
H3CO
H3C
F
H3CO
3a
N
4a (93 %)
CHO
CHO
CHO
OTMS
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CN
H
Cl
H
CN
1b
1c
Cl
2b (98 %)
H3C
3b
4b (92 %)
OTMS
H
N
N
CN
H
Br
H
CN
Br
F
2c (97 %)
3c
4c (96 %)
F
F
OTMS
H
N
N
CN
H
F
CN
4d (98 %)
H
1d
F
Cl
Cl
2d (98 %)
3d
aAll the reported yields in the present table are average isolated yields.
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Table 3: Optimized conditions for the amino amidine formation reaction with compounds I, II & IIIa.
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Cl
C
N
CHO
+
Cl
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5 h, RT, dry DCM
Catalyst I, II & III)
N
+
N
H
H2N
NH
1 (1eq)
2 (2eq)
Cl
1eq
3
Catalyst
Amount
(mmol)
Temperature (°C)
Time (h)
5
Isolated yield
(%)
Turnover
Number (TON)
I
81
83
58
8.14
8.34
5.83
0.075
RT
II
III
I
II
III
I
II
III
I
II
III
I
II
III
I
74
75
57
84
85
70
71
72
59
84
85
72
72
73
60
84
85
73
15
19
12
9
7.43
7.54
5.73
8.44
8.54
7.03
7.13
7.23
5.93
8.44
8.54
7.23
10.85
11.00
9.04
6.33
6.40
5.50
1.50
1.91
1.20
0.90
0.40
0.075
0.075
0.075
0.075
0.05
RT
RT
0-5
70
4
8
5
5
5
5
RT
RT
II
III
I
II
III
0.1
Cu(NO3)2·3H2O
Zn(NO3)2·6H2O
AgNO3
0.075
0.075
0.075
0.075
-----
RT
RT
RT
RT
RT
5
5
5
5
5
L
No catalyst
4
aReaction conditions: Benzaldehyde (0.754 mmol), 4-chloroaniline (1.50 mmol), tert-butyl isocyanide (6.12
mmol), dry DCM (1 mL), catalyst (0.075 mmol), RT & 5 h.
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Scheme 1: Representation of three CPs with their synthetic conditions, formula and yields.
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(a)
(b)
Figure 1: (a) Asymmetric unit along with the coordination environment of copper (II) ions in compound I. The
solvent molecules are omitted for clarity. (b) Two-dimensional layer arrangement of I along the ac plane. H-atoms
and solvent molecules are omitted for clarity.
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(a
(b)
Figure 2: (a) Asymmetric unit along with the coordination environment of zinc (II) ions in compound II. (b) Two-
dimensional layer view of II along the ac plane. H-atoms are omitted for clarity.
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(a)
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Figure 3: (a) Asymmetric unit along with the coordination environment of silver (I) ions in compound III. (b)
Orientation of two 1D ladders along the ac plane. Note the growth of each ladder along the a-axis. H-atoms are
omitted for clarity.
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Figure 4: Recyclability tests of three catalysts (I, II & III) during amino amidine formation reaction up to 5
consecutive cycles.
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Figure 5: Proposed mechanism for the Lewis acid catalyzed multicomponent amino amidine formation.
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“For Table of Contents Use Only”
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Transition Metal-ions Regulated Structural and Catalytic Behaviors of
Coordination Polymers
Nikhil Kumara, Tanmay Roma, Virender Singhb and Avijit Kumar Paula*
Three new coordination polymers are synthesized by varying transition metal-ions with recently
designed pyrazolyl-benzenedicarboxylate ligand. The roles of metal ions for structural variations
and Lewis acidic behaviors are investigated thoroughly.
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