Pyrene carboxylate ligand based coordination polymers for microwave-assisted solvent-free cyanosilylation of aldehydes

Article abstract of DOI:10.3390/molecules26041101

The new coordination polymers (CPs) [Zn(μ-1κO¹:1κO²-L)(H₂O)₂]_n·n(H₂O) (1) and [Cd(μ₄-1κO¹O²:2κN:3,4κO³-L)(H₂O)]_n·n(H_2<

Full text of DOI:10.3390/molecules26041101

molecules
Article
Pyrene Carboxylate Ligand Based Coordination Polymers for
Microwave-Assisted Solvent-Free Cyanosilylation
of Aldehydes
Anirban Karmakar 1,
*
, Anup Paul 1 , Elia Pantanetti Sabatini 1, M. Fátima C. Guedes da Silva 1 and
Armando J. L. Pombeiro 1,2,
*
1
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049-001 Lisbon, Portugal; kanupual@gmail.com (A.P.); e.pantanettisabatini@studenti.unicam.it (E.P.S.);
fatima.guedes@tecnico.ulisboa.pt (M.F.C.G.d.S.)
Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya Street, 117198 Moscow,
Russia
2
*
Correspondence: anirbanchem@gmail.com (A.K.); pombeiro@tecnico.ulisboa.pt (A.J.L.P.)
Abstract: The new coordination polymers (CPs) [Zn(
µ
-1
κ
O1:1
κ
O2-L)(H2O)2]n
·
n(H2O) (1) and [Cd(µ4-
1
κ
O1O2:2 O3-L)(H2O)]n
κ
N:3,4 n(H2O) ( ) are reported, being prepared by the solvothermal reactions
κ
·
2
of 5-{(pyren-4-ylmethyl)amino}isophthalic acid (H2L) with Zn(NO3)2.6H2O or Cd(NO3)2.4H2O, re-
spectively. They were synthesized in a basic ethanolic medium or a DMF:H2O mixture, respectively.
These compounds were characterized by single-crystal X-ray diffraction, FTIR spectroscopy, ther-
mogravimetric and elemental analysis. The single-crystal X-ray diffraction analysis revealed that
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
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Citation: Karmakar, A.; Paul, A.;
Sabatini, E.P.; Guedes da Silva, M.F.C.;
Pombeiro, A.J.L. Pyrene Carboxylate
Ligand Based Coordination Polymers
for Microwave-Assisted Solvent-Free
Cyanosilylation of Aldehydes.
Molecules 2021, 26, 1101.
compound
sional network. In both compounds, the coordinating ligand (L2−) is twisted due to the rotation of
the pyrene ring around the CH2-NH bond. In compound , the Zn(II) metal ion has a tetrahedral
geometry, whereas, in , the dinuclear [Cd2(COO)2] moiety acts as a secondary building unit and the
Cd(II) ion possesses a distorted octahedral geometry. Recently, several CPs have been explored for
the cyanosilylation reaction under conventional conditions, but microwave-assisted cyanosilylation
of aldehydes catalyzed by CPs has not yet been well studied. Thus, we have tested the solvent-free
microwave-assisted cyanosilylation reactions of different aldehydes, with trimethylsilyl cyanide,
using our synthesized compounds, which behave as highly active heterogeneous catalysts. The coor-
1 is a one dimensional linear coordination polymer, whereas 2 presents a two dimen-
1
2
https://doi.org/10.3390/
molecules26041101
Academic Editors:
dination polymer
1 is more effective than 2, conceivably due to the higher Lewis acidity of the Zn(II)
Catherine Housecroft and
Nikolay Gerasimchuk
than the Cd(II) center and to a higher accessibility of the metal centers in the former framework. We
have also checked the heterogeneity and recyclability of these coordination polymers, showing that
they remain active at least after four recyclings.
Received: 20 January 2021
Accepted: 17 February 2021
Published: 19 February 2021
Keywords: coordination polymer; crystal structure analysis; heterogeneous catalysis; microwave;
solvent-free; cyanosilylation reaction
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iations.
1. Introduction
Coordination polymers are commonly viewed as a type of exceedingly popular func-
tional materials because of their charming 1D, 2D and 3D architectures, in addition to their
substantial utility in numerous applications, which include molecular sensing, gas storage
Copyright:
© 2021 by the authors.
and separation, etc. [1,2]. These materials can be deliberately designed to produce explicit
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
structures, topologies and intriguing properties, which are of high demand [3].
Among various applications of the synthesized coordination polymers (CPs), catalysis
is one of the most important and explored areas of research where they have been employed,
often as heterogeneous catalysts [4–6]. This is largely due to their robustness and insoluble
nature in most of the common organic solvents. The important organic transformations
for which CPs have been developed and employed as heterogeneous catalysts include
Molecules 2021, 26, 1101. https://doi.org/10.3390/molecules26041101
https://www.mdpi.com/journal/molecules

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cyanosilylation of aldehydes or ketones [
of alkanes, alcohols or olefins [ ], Henry reaction [10
transesterification [13], cascade [14] reactions, etc.
7
], Knoevenagel condensation [
8
], oxidation
9
,11], ring-opening of epoxides [12],
Furthermore, in the domain of catalysis, cyanosilylation constitutes an important
C-C bond formation reaction, leading to cyanohydrins, which are regarded as a class of
compounds with interest in chemistry and biology, being broadly applied for the synthesis
of plenty of vital compounds, such as α-hydroxy acids and aldehydes and β-amino alco-
hols [15]. Trimethylsilyl cyanide (TMSCN) is one of the mostly used reagents employed for
the synthesis of cyanohydrins promoted by the presence of a Lewis acid or base catalyst,
due to its facile use which proceeds via nucleophilic addition to carbonyl compounds
to generate cyanohydrin trimethylsilyl ethers. Along these lines, the development of a
powerful catalyst, which can effectively catalyze such a reaction is an important area of
present-day research [16]. Although an array of complexes involving metal ions from
d-, f- and p-blocks have been employed as catalysts for such a transformation [17–19],
they mostly act in a homogeneous manner, and just a modest number of heterogeneous
catalysts have been documented in this direction [20,21].
Other relevant features of a catalytic reaction concern the employed conditions. Al-
though many of the catalytic reactions are being performed under conventional heating,
there is an ever-increasing demand for an alternate energy source which will not only be
cost-effective but also highly efficient and environmentally friendly. Microwave irradiation
(MW) provides a notable technique which can take lesser time to perform a reaction, gen-
erate a better yield and selectivity, and is an ideal choice to look for. For a long time, our
group has been utilizing this technique for carrying out many catalytic transformations [22].
Recently, various CPs have been explored for cyanosilylation of aldehydes under conven-
tional conditions [23]; however, microwave-assisted cyanosilylation of aldehydes catalyzed
by CPs has not yet been significantly studied. Thus, the exploration of appropriate CP
based catalysts for the microwave-assisted solvent-free cyanosilylation of aldehydes is a
topic of great interest.
In the current study, we synthesized the pro-ligand 5-{(pyren-4-ylmethyl)amino}isophthalic
acid (H2L) and constructed the one-dimensional [Zn(
µ
-1
κ
O1:1
κ
O2-L)(H2O)2]n
·
n(H2O) (
1) and
the two-dimensional [Cd(µ4-1 N:3,4
κ
O1O2:2 O3-L)(H2O)]n
κ
κ
·n(H2O) (2) coordination polymers.
They have been characterized by FTIR, thermogravimetric, elemental, and single-crystal and
powder X-ray diffraction analyses. The insolubility and existence of both Lewis acid and
basic (amine) centers in these CPs makes them favorable materials for bifunctional heteroge-
neous catalysis. Thus, we tested their heterogeneous catalytic activity towards the solvent-free
microwave-assisted cyanosilylation reaction of different aldehydes under mild conditions.
2. Results and Discussion
2.1. Synthesis and Characterization
The solvothermal reaction of H2L with Zn(NO3)2.6H2O in ethanol and NH4OH
leads to the formation of the coordination polymer [Zn(
) [L = 5-{(pyren-4-ylmethyl)amino}isophthalate]. Upon performing the solvothermal
reaction using Cd(NO3)2.4H2O in the presence of DMF and water mixture, we obtained the
coordination polymer [Cd(µ4-1 N:3,4 n(H2O) ( ) (Figure 1). In the
O1O2:2 O3-L)(H2O)]n
FTIR spectra, the characteristic strong bands for the asymmetric stretching of coordinated
µ-1
κ
O1:1
κ
O2-L)(H2O)2]n
n(H2O)
·
(
1
κ
κ
κ
·
2
carboxylate groups of 1 and 2
appear at 1640–1641 cm−1 and the symmetric stretching is
observed at 1350–1370 cm−1. The C=C stretching frequencies of the aromatic rings are
present in the 1561–1572 cm−1 and 1411–1421 cm−1 regions. The band attributed to the
secondary amine group can be found in the 2914–2934 cm−1 region.

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Figure 1. Synthesis of coordination polymers
Cd(NO3)2·4H2O, under solvothermal conditions.
1
and
2
by reactions of H2L and Zn(NO3)2
·
6H2O or
2.2. Thermogravimetric Analyses
Thermogravimetric analyses were p◦erformed under dinitrogen from room tempera-
◦
ture to ca. 800 C, with a heating rate of 5 C min−1. Features of the thermal stability of the
coordination polymers 1 and 2 are presented in Figure 2.
(A)
(B)
Figure 2. Thermogravimetric curves for 1 (A) and 2 (B).
◦The coordination polymer
1 shows a total weight decrease of 10.1% between 59 and
263 C, corresponding to the loss of the non-coordinated and coordinated water molecules
◦
(calcd: 10.5%). Afterwards it remains stable up to 388 C and then starts to decompose until
◦
◦
739 C (Figure 2A). Compound
2
shows a weight loss of 3.1% in the 65–135 C temperature
range, which is due to the removal ◦of the non-coordinated water molecule (calcd: 3.5%).
In the temperature range of 136–210 C, it loses the coordinated water molecule, equivalent
to a weight loss of 3.2% (calcd: 3.5%). However, above this temperature, the framework
remains stable up to 364 ◦C and then starts to decompose until 745 ◦C (Figure 2B).
2.3. Crystal Structure Analysis
The asymmetric unit of the coordination polymer
ligand, two coordinated and one crystallization water molecules (Figure 3A,B). The Zn(II)
center presents a distorted tetrahedral environment (
4 = 0.91) [24], and via the deproto-
1
contains one zinc(II) ion, one L2−
τ
nated L2− ligand, it generates a 1D polymeric chain (Figure 3C). The metal ion binds two
carboxylate oxygen atoms from two L2− units [Zn1-O2 2.007(17) Å and Zn1-O3 1.970(16)
Å] and two water molecules [Zn1-O5 2.020(2) Å and Zn1-O6 2.025(2) Å]; the O–Zn–O
bond angles fall within the range of 103.0(7)◦ to 119.7(9)◦. The pyrene and isophthalate
ring of the ligand are almost perpendicular with a dihedral angle of 73.34◦. The observed
non-planarity of the ligand is due to the relative twisting of the CH2-NH group, and the

Molecules 2021, 26, 1101
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C4-N1-C9-C10 torsional angle is 163.21◦. The carboxylate groups, working in a monoden-
tate fashion, are nearly in the plane of the isophthalate ring (dihedral angles of 8.53◦ and
12.92◦). The distance between two symmetry related Zn(II) ions in a chain is 10.239 Å,
which is considerably larger than the shortest intermolecular distance of 5.412 Å between
two metal ions in vicinal chains.
(A)
(B)
(C)
Figure 3.
(
A) Molecular structure of framework
1. (
B
) Schematic representation of framework 1.
(C) Hydrogen bonded 1D chain of framework 1.
The 1D chains of coordination polymer
1 are hydrogen bonded through several O-
H
····O interactions between the coordinated water molecules and the carboxylate-O atoms,
forming a two-dimensional hydrogen bonded network. Moreover, these 2D hydrogen
bonded networks are further connected via π····π interactions (between pyrene rings)
with a distance of 3.489 Å (Supplementary Materials Figure S1A), N-H····π interactions
(between amine NH and pyrene ring, dD-π 3.644 Å, Supplementary Materials Figure S1B)
and C-H····π interactions (between pyrene CH and pyrene ring, dD-π 3.451 Å), which
expand the structure to the third dimension.
The asymmetric unit of [Cd(µ4-1
κ
O1O2:2
κ
N:3,4
κ
O3-L)(H2O)]n
n(H2O) (2) contains
·
one Cd(II) ion, one L2− ligand and one of each coordinated and non-coordinated water
molecules (Figure 4A,B). The distorted octahedral coordination environment of Cd(II) ion is
occupied by four carboxylate-O from three adjacent L2− ligands [Cd1-O1 2.228(2), Cd1-O1
0
2.451(3), Cd1-O3 2.343(2), Cd1-O4 2.401(3)] and one amine NH from a L2− ligand [Cd1-N1
2.470(3)], and the remaining site is engaged with the water molecule [Cd1-O5 2.190(3)].
◦
◦
The O–Cd–O bond angles are within the range of 54.45(9) to 173.60(9) . In this coordination
polymer, the amine NH participates in the metal coordination, which we have not observed
in other coordination polymers. The deprotonated L2− ligand coordinates simultaneously
to four metal ions, with two of them linked by a bridging-chelate carboxylate group,
another one where carboxylate is in a chelate-monodentate mode, and the remaining one
by the amine NH group.

Molecules 2021, 26, 1101
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(B)
(A)
(C)
Figure 4.
(
A) Molecular structure of framework 2. (B) Schematic representation of framework 2.
(C) Two-dimensional structure of framework 2 (the hydrogen-bonded water molecules are indicated
as red balls).
As in
1, the organic ligand in 2 is highly twisted. The least-squares plane of the
isophthalate ring makes a◦dihedral angle of 84.51◦ with the pyrene ring and one of the
carboxylate groups is 40.17 twisted relatively to the plane of the aromatic ring it is attached
to. The crystallographically related Cd-ions are arranged in a dinuclear [Cd2(COO)2] clus-
ter forming a secondary building block unit. The Cd····Cd distance within the dinuclear
clusters is of 3.695 Å. Assembly of six L2− ligands and the dinuclear [Cd2(COO)2] clus-
ter results the construction of a two dimensional framework (Figure 4C). Moreover, the
coordinated and non-coordinated water molecules are hydrogen bonded with carboxyl
groups of the L2− ligand through O-H····O interactions. The amine NH is also engaged in
a hydrogen-bonding interaction with carboxylate-O via N-H····O interaction.
2.4. Catalytic Activity Studies
Taking advantage of the high thermal stability and insolubility in most of the common
organic solvents of the coordination polymers 1 and 2, we tested them as heterogeneous
catalysts for the cyanosilylation reaction of different aldehydes under microwave-assisted
solvent-free conditions (Scheme 1). To begin with, benzaldehyde was taken as a model
substrate, trimethylsilyl cyanide (TMSCN) as the silylating agent and the CP
1 or 2 as
catalyst, and various reaction conditions like solvent, temperature and catalyst amount,
were optimized.

Molecules 2021, 26, 1101
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Scheme 1. Aldehyde cyanosilylation reaction with trimethylsilyl cyanide (TMSCN).
Under optimized conditions, a combination of benzaldehyde (0.105 g, 1.0 mmol),
trimethylsilyl cyanide (250
µ
L, 2 mmol) and catalyst (10.2 mg for
1
and 10.8 mg of◦ 2
;
2.0 mol%) was placed in a Pyrex tube covered with a Teflon cap and stirred at 50 C,
under microwave irradiation (5 W), for 1.5 h, under solvent-free conditions. The desired
product was identified by 1H NMR spectroscopy (Supplementary Materials Figure S4), and
the product yield was calculated, using the same procedure as reported in the literature [21].
We carried out the reactions in the presence of different solvents, such as MeOH,
CHCl3, CH3CN or THF, and in the absence of any solvent. Solvent-free conditions show
the maximum catalytic activity, leading to a product yield of 97% or 85% for coordination
polymer
1
or
2
, respectively (entries 1 and 2, Table 1). The use of other solvents leads to
(entries 3–6, Table 1 and Figure 5B) and 41–76% for
(entries 7–10, Table 1 and Figure 5B). Performing the reaction in CH3CN gives the lowest
lower yields in the range of 57–81% for
2
1
yield of 57% or 41% for compound 1 or 2, respectively. These observations are consistent
with the competition of the solvent with the aldehyde substrate for the coordination to
the metal.
Table 1. Cyanosilylation of benzaldehyde with trimethylsilyl cyanide (TMSCN) with coordination
polymer 1 or 2 as catalyst a.
Catalyst
Amount
(mol%)
Temperature
Yield b (%)
Entry
Catalyst
Solvent
(◦C)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
1
2
1
1
1
1
2
2
2
2
1
1
2
2
1
1
50
50
50
50
50
50
50
50
50
50
50
50
50
50
25
40
60
25
40
60
50
50
50
50
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
5.0
1.0
5.0
2.0
2.0
2.0
2.0
2.0
2.0
-
Solvent free
Solvent free
MeOH
CHCl3
CH3CN
THF
MeOH
CHCl3
CH3CN
97
85
78
67
57
81
55
43
41
76
52
98
30
85
32
81
96
25
80
84
10
18
28
23
THF
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
1
2
2
2
Blank
H2L
Zn(NO3)3
Cd(NO3)3
2.0
2.0
2.0
·
6H2O
·
4H2O
a
Reaction conditions (unless stated otherwise): benzaldehyde (1 mmol), 2.0 mol% (relative to the aldehyde) of
catalyst, trimethylsilyl cyanide (250
number of moles of final product 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile per mole of aldehyde X 100.
µ
L, 2 mmol), 1.5 h reaction time, at 50 ◦C. b Calculated by 1H-NMR as the

Molecules 2021, 26, 1101
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(B)
(A)
(C)
Figure 5.
of benzaldehyde, catalyzed by
solvents catalyzed by compounds
(
A
) Plot of yield vs. time for the microwave-assisted solvent-free cyanosilylation reaction
or . ( ) Yield of cyanosilylation of benzaldehyde in different
(red pillar) and (green pillar). ( ) Plots of yield vs. time for the
1
2
B
1
2
C
microwave-assisted (red line) and normal-heating (blue line) solvent-free cyanosilylation reaction of
benzaldehyde, catalyzed by 1.
In order to optimize the catalyst loading, we changed the catalyst amount from 1.0
to 2.0 mol% and observed a high enhancement of the product yield from 52 to 97% for
1
(entries 11 and 1, Table 1) or from 30 to 85% for
additional increase of that amount (up to 5%) resulted only in an almost undetected yield
improvement for (entry 12, Table 1) and in no yield change for (entry 14, Table 1).
2 (entries 13 and 2, Table 1), but an
1
2
Thus, the optimal catalyst amount was considered as 2 mol% (relative to the aldehyde).
We also carried out the cyanosilylation reaction at different temperatures, to examine
the effect of temperature. We obtained only 32% or 25% of product yield upon performing
the reaction at room temperature (25 ◦C) (entry 15 or 18, Table 1), using
respectively. Increasing the reaction temperature from 25 to 50 C results in an yield
1
or
2
as catalyst,
◦
increase to 97% or 85% for
1 or 2 (entry 1 or 2), respectively. Further increase in the
temperature to 60 ◦C did not improve the reaction yield (entries 17 and 20, Table 1).
A blank test was performed with benzaldehyde and TMSCN without any catalyst
and resulted in only 10% yield of the final product after 1.5 h (entry 21, Table 1). We also
checked the cyanosilylation reaction with zinc nitrate, cadmium nitrate and the free ligand
(H2L) and the obtained reaction yields are within 18–23% (entries 22–24, Table 1). Thus, the
optimized reaction conditions for the cyanosilylation of the aldehyde with TMSCN concern
◦
2 mol% of the coordination polymer catalyst at 50 C without any solvent. Under the
optimized reaction conditions our coordination polymers
1 and 2 produce 97% and 85% of
final product yields after 1.5 h under microwave irradiation, respectively. They correspond
to a turnover number (TON) of 48.5 and 42.5, and a turnover frequency (TOF) of 32.3 and
28.3 h−1
.
After further spreading the reaction time to 2.5 h, the final product yield increased
up to 99% in the case of or 87% for compound , but the TOF value decreases markedly
(from 32.3 h−1 to 19.4 h−1 for and from 28.3 h−1 to 17.0 h−1 for
). The yield vs. time plots
1
2
1
2

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for the solvent-free cyanosilylation reaction of benzaldehyde and TMSCN catalyzed by
1
or
2
is presented in Figure 5A. We also performed the same reaction under normal heating
conditions, instead of microwave, and the product yield was only 87% for compound
1
after 1.5 h, and it took 3 h to reach 99% (Figure 5B). Thus, the use of microwave irradiation
accelerated the reaction quite significantly.
We also tested the activity of our catalysts
1 and 2 towards different substituted
aromatic aldehydes. The aromatic aldehydes containing strong electron-withdrawing
groups, such as nitro, bromo and chloro, exhibit the highest yields in the range of 95–99%
for
1 and 89–96% for 2 (entries 1–4, Table 2). In contrast, the aldehydes containing electron-
donating substituent, such as methyl or methoxy, display lower yields 58–75% (entries 7
and 8, Table 2). The meta- and para-hydroxy substituted aldehydes show moderate reactivity
and produce the corresponding cyanohydrin derivatives within the yield range of 67–91%
(entries 5 and 6, Table 2), being also above the corresponding yields for the aldehydes with
eletron-donor substituents. These behaviors are in accord with the expected effect of the
substituent on the electrophilic character of the aldehyde CHO carbon to undergo attack
by the cyano group of TMSCN.
Table 2. Cyanosilylation of different substituted aldehydes with trimethylsilyl cyanide (TMSCN)
with catalyst 1 or 2 a.
Yield b (%) with 1
Yield b (%) with 2
Entry
Reactant
1
2
3
4
5
6
7
8
4-Nitrobenzaldehyde
3-Nitrobenzaldehyde
>99
98
>99
95
91
88
96
95
94
89
75
67
64
58
4-Bromobenzaldehyde
4-Chlorobenzaldehyde
4-Hydroxybenzaldehyde
3-Hydroxybenzaldehyde
4-Methoxybenzaldehyde
4-Methylbenzaldehyde
75
71
a
Reaction conditions: aldehyde (1 mmol), trimethylsilyl cyanide (250 µL, 2 mmol), 2.0 mol% of catalyst 1 or 2 for
1.5 h at 50 ◦C. b Calculated by 1H NMR.
The recyclability of the catalysts
1 and 2 was also tested. At the end of a reaction
cycle, the catalyst was separated by centrifugation, washed with CH2Cl2 and dried before
recycling it. It could be successfully recycled at least for four consecutive cycles, without
considerably losing its activity, as shown in Figure 6B.
To check the leaching and heterogeneity of our catalysts, we separated the catalyst
by centrifugation after 0.5 h and kept the catalyst-free reaction mixture under the same
environment for 1 h more. After elimination of the solid catalyst 1 or 2 from the reaction
mixture, no perceptible rise in the yield was observed (Figure 6A, blue and green dotted
lines), which substantiates the absence of leaching and the heterogeneous nature of the
catalyst. FTIR and powder X-ray diffraction analyses of the catalyst before and after the
catalysis reaction were performed, to check the structural integrity of our catalysts
1 and 2,
and no significant changes in their patterns were observed, as shown in Supplementary
Materials Figure S2 and S3. These results support that the structures of the catalysts
1 and
2 remain intact after the catalytic reaction.

Molecules 2021, 26, 1101
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(A)
(B)
(A) Plots of product yield vs. time for the solvent-free cyanosilylation reaction, catalyzed
Figure 6.
by
1
(purple line) and (red line) (blue dotted line indicates the catalyst removal after 0.5 h and the
2
1
green dotted line concerns the catalyst removal after 0.5 h). ( ) Effect of the catalyst recycling on
2
B
the final product yield (blue pillars for compound 1 and brown pillars for compound 2).
A comparison of the catalytic activity, towards the cyanosilylation of aldehydes with
TMSCN, of our catalyst 1 with other reported coordination polymers is shown in Table
3. Our catalyst appears to be more efficient for such a reaction. For example, the cyanosi-
◦
lylation reaction of benzaldehyde and TMSCN at 50 C, under solvent-free conditions
catalyzed by [Pr(3,5-DSB)(Phen)]n produce a yield of 78% after 3 h reaction time (entry
2, Table 3) [25]. The heterometallic CP [{Co0.6Ni1.4(H2O)2(Bpe)2}(V4O12)]
·
4H2O Bpe gives
·
◦
a yield of 77%, at 50 C, for 16 h, under solvent-free conditions (entry 3, Table 3) [26].
Another heterometallic coordination polymer, [NaCu(2,4-HPdc)(2,4-Pdc)], catalyzes the
same reaction also under solvent-free conditions, for 24 h reaction time, producing an yield
of 85% (entry 4, Table 3) [27]. Moreover, the mononuclear Sn(II) complex of 3-amino-2-
pyrazinecarboxylate gives a reaction yield of 75%, at 50 ◦C, after 4 h (entry 5, Table 3) [28].
In this context, our catalyst 1 also leads to a 97% yield under solvent-free condition, at 50
◦C, after 1.5 h reaction time (entry 1, Table 3). Therefore, our catalyst produces a higher
product yield, in a shorter time, under solvent-free conditions.

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Table 3. A comparison of catalytic activity of different reported coordination polymers in the cyanosilylation of aldehydes with trimethylsilyl cyanide (TMSCN).
Entry
Catalyst
Solvent/Temp/Time
Aldehyde
Yield (%)
Reference
◦
1
2
3
4
5
6
7
8
9
1
Solvent free/50 C/1.5 h
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
4-Nitrobenzaldehyde
Benzaldehyde
97
78
77
85
75
81
97
57
49
14
This work
[23]
◦
[Pr(3,5-DSB)(Phen)]n
[{Co0.6Ni1.4(H2O)2(Bpe)2}(V4O12)]
Solvent free/50 C/3 h
◦
·
4H2O
·
Bpe
Solvent free/50 C/16 h
[24]
[25]
[26]
◦
[NaCu(2,4-HPdc)(2,4-Pdc)]
[Sn(L1)2]
Solvent free/50 C/24 h
◦
Solvent free/50 C/4 h
◦
1
THF/50 C/1.5 h
DCM/50 C/6 h
pentane/40 C/72 h
Tolene/50 C/72 h
This work
[27]
◦
[Pb(L1)2]2
◦
◦
[Cu3(benzenetricarboxylate)2]n
[Cd2(1,4-NDC)2(DMF)2]n
{[Zn(dpe)(µ-OOCCH3)2](H2O)}n
[28]
[29]
[30]
10
DCM/RT/24 h
0
{[Zn3(4,4 -bpy)4(µ-
11
DCM/RT/24 h
Benzaldehyde
22
[31]
0
O2CCH2CH3)4](ClO4)2(4,4 -bpy)2(H2O)4}n
3,5-DSB = 3,5-disulfobenzoic acid; Phen = 1,10-phenathroline; L1 = 3-amino-2-pyrazinecarboxylate; Bpe = 1,4-NDC = 1,4-naphthalenedicarboxylate; 2,4-H2Pdc = pyridine-2,4-dicarboxylic acid; dpe =
1,2-bis(4-pyridyl)ethene; 4,40-bpy = 4,40-bipyridine.

Molecules 2021, 26, 1101
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Moreover, our catalyst
1
also shows a high efficiency in comparison to other reported
coordination polymers, upon performing the cyanosilylation of benzaldehyde with TMSCN
◦
in the presence of a solvent (yield of 81%, in THF at 50 C after 1.5 h, entry 6, Table 3).
For example, the dinuclear Pb(II)-3-amino-2-pyrazinecarboxylate complex produces the
reaction yield of 97%, at 50 ◦C, in DCM, after 6 h, a higher reaction time than our catalyst
(entry 7, Table 3) [29]. An overall reaction yield of 57% after 48 h at 40 ◦C was obtained
by using the 3D [Cu3(benzenetricarboxylate)2]n MOF as catalyst (entry 8, Table 3) [30].
The 3D coordination polymer [Cd2(1,4-NDC)2(DMF)2]n catalyzes the cyanosilylation of
◦
4-nitrobenzaldehyde with an overall yield of 49% after 72 h at 50 C (entry 9, Table
3) [31]. Moreover, the Zn(II) coordination polymers {[Zn(dpe)(
µ
-OOCCH3)2](H2O)}n and
0
0
{[Zn3(4,4 -bpy)4(
µ-O2CCH2CH3)4](ClO4)2(4,4 -bpy)2(H2O)4}n produce an yield of 14% and
22% upon performing the reaction in DCM, at room temperature, after 24 h, respectively
(entries 10 and 11, Table 3) [32,33].
Based on our previous reports, a similar plausible catalytic reaction mechanism is
proposed, wherein a Zn(II) center activates the aldehyde carbonyl group and trimethylsilyl
cyanide with promotion of the nucleophilic attack of the CN group to the carbonyl carbon,
leading to the construction of a C-C bond and thus to the cyanohydrin.
3. Materials and Methods
The synthesis of the ligand and coordination polymers were performed at a relatively
high temperature. The chemicals were used as received from commercial sources. Bruker
Vertex 70 instrument was used to record the FTIR spectra (4000–400 cm−1) in KBr pellets.
The 1H NMR spectra for the ligand were recorded at room temperature, on a Bruker Avance
II + 300 (UltraShieldTMMagnet) spectrometer, operating at 300.130 MHz, and the chemical
shifts are reported in ppm, using tetramethylsilane as the internal reference. TGA was
carried out with a Perkin-Elmer Instrument system (STA6000), at a heating rate of 5 ◦C
min−1, under a dinitrogen atmosphere. C, H and N elemental analyses for the coordination
polymers were carried out by the Microanalytical Service of the Instituto Superior Técnico.
A D8 Advance Bruker AXS (Bragg Brentano geometry) diffractometer, with Cu-radiation
(Cu Kα, λ = 1.5406 Å), was used to collect powder X-ray data (PXRD), operated at 40 kV
and 40 mA. The typical data-collection range was between 5◦ and 40◦, and a flat plate
configuration was used.
3.1. Synthesis of 5-{(pyren-4-ylmethyl)amino}isophthalic Acid (H2L)
The pro-ligand H2L was synthesized, using a two-step reaction. In the first step,
1-pyrenecarboxaldehyde (0.576 g, 2.5 mmol) and 5-aminoisophtalic acid (0.453 g, 2.5 mmol)
were placed in a round bottom flask, and then dry methanol (25 mL) was added. After stir-
ring overnight at room temperature, a yellow suspension was obtained. The solution was
filtered, and the yellow solid was collected on filter paper, which was further washed
several times, using fresh methanol. The obtained compound was dried in air and used in
next step, without purification.
In the second step, the isolated compound (1.58 g, 4 mmol) was dissolved in 25 mL of
methanol, and NaBH4 was gradually added to the suspension, until a colorless solution
was obtained. Afterwards, the reaction mixture was left, stirring overnight, at room
temperature. Upon completion, the methanol was evaporated, and 10 mL water was
added into it, followed by the acidification until pH = 2, with a diluted solution of HCl.
The obtained white solid product H2L was isolated by filtration and thoroughly washed
with water. Yield: 89%. FTIR (KBr, cm−1): 3843 (wbr), 3381 (s), 3043 (m), 2873 (m), 2590
(m), 2509 (m), 2360 (w), 1932 (w), 1681 (s,
1504 (s, C=C), 1425 (s, C=O), 1330 (s), 1271 (s), 1244 (s), 1137 (m), 1083 (m), 997 (w),
964 (w), 950 (w), 889 (w), 844 (s), 759 (s), 711 (s), 694 (s); 1H-NMR (DMSO-d6):
ν
C=O), 1600 (s, ν C=C), 1542 (s, ν C=C),
asym
ν
ν
sym
δ
8.48–8.45
(1H, m, Ar-H), 8.31–8.26 (4H, m, Ar-H), 8.25–8.15 (4H, m, Ar-H), 7.74 (1H, s, Ar-H), 7.43 (2H,
s, Ar-H), 6.78 (1H, s, N-H), 5.05 (2H, s, -CH2). Anal. Calcd. for C25H19NO4 (M = 397.42): C,
75.55; H, 4.82; N, 3.52. Found: C, 75.52; H, 4.65; N, 3.61.

Molecules 2021, 26, 1101
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3.2. Synthesis of the Coordination Polymer [Zn(L)(H2O)2]n·n(H2O) (1)
A mixture of H2L (10 mg, 0.025 mmol) and Zn(NO3)2.6H2O (7.4 mg, 0.025 mmol)
in 2 mL EtOH was prepared, and afterwards, 0.5 mL of NH4OH (30% aqueous solution)
was added. This mixture was then put into an 8 mL glass vessel, sealed and heated at 75
◦C, for 48 h. Then the reaction mixture was cooled to room temperature, giving colorless
crystals of
1504 (w), 1421 (s,
1
. FTIR (KBr, cm−1) 3381 (mbr), 2914 (w), 1640 (s,
C=C), 1352 (s,
ν
asym C=O), 1561 (s,
ν C=C),
ν
νsym C=O), 1322 (m), 1274 (m), 1199 (s), 1109 (w), 1076 (w),
979 (w), 846 (s), 828 (s), 781 (m), 712 (w). Anal. Calcd. for C25H21NO7Zn (M = 512.80): C,
58.55; H, 4.13; N, 2.73. Found: C, 58.23; H, 3.86; N, 2.42.
3.3. Synthesis of the Coordination Polymer [Cd(L)(H2O)]n·n(H2O) (2)
Cd(NO3)2 6H2O (8 mg, 0.025 mmol) and H2L (10 mg, 0.025 mmol) were dissolved in 2
·
mL of DMF:H2O (2:1, v/v) mixture, and the obtained system was sealed in a capped glass
vessel and heated to 75 ◦C, for 48 h. The mixture was then cooled gradually, and colorless
crystals of the coordination polymer
2934 (w), 1641 (s, C=O), 1572 (s,
2
were obtained. FTIR (KBr, cm−1) 3320 (mbr),
ν
ν
C=C), 1531(s), 1411 (s,
ν
C=C), 1370 (s,
ν
C=O),
asym
sym
1284 (s), 1148 (w), 1080 (w), 1106 (m), 906 (w), 840 (s), 802 (s), 779 (s), 673 (m). Anal. Calcd.
for C25H17CdNO6 (M = 539.82): C, 55.62; H, 3.17; N, 2.59. Found: C, 55.33; H, 3.15; N, 2.78.
3.4. Procedure for the Cyanosilylation Reaction of Aldehydes
A mixture of an aromatic aldehyde (1 mmol), trimethylsilyl cyanide (250
µL, 2 mmol)
and 2.0 mol% catalyst (10.2 mg of or 10.8 mg of ; relative to the aldehyde) was placed in
1
2
a Pyrex tube. The tube was then put in the microwave reactor, and the reaction mixture
was left, stirring under MW irradiation (5 W), at 50 ◦C, in solvent-free conditions, for the
desired time. The catalyst was then separated by centrifugation, and the reaction mixture
was evaporated, using a rotary evaporator. Then the final product was dissolved in CDCl3
and analyzed by 1H NMR (Supplementary Materials Figure S4).
3.5. Crystal Structure Determinations
X-ray quality single crystals of the coordination polymers
1 and 2 were placed in
cryo-oil and mounted in a nylon loop, and the data collection was done by using a Bruker
APEX-II PHOTON 100 diffractometer with graphite monochromated Mo-K
α
(
λ
0.71069)
◦
radiation, at room temperature. Phi and omega scans (0.5 per frame) were used for the data
collection, and a full sphere of data was obtained. Bruker SMART [34] software was used to
obtain cell parameters and refined the data by using Bruker SAINT [34] on all the observed
reflections. By using the SADABS [35] program, the absorption corrections were performed.
The crystal structures were solved by using the SIR 97 program and refined with SHELXL-
2014/6 [36]. Calculations were performed, using the WinGX System-Version 2014.1 [37].
The H-atoms attached to C- and N-atoms were introduced at geometrically calculated
positions and encompassed 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 O- and N-atoms. Least-square refinements with anisotropic thermal motion
parameters for all the non-hydrogen atoms and isotropic ones for the residual atoms were
employed. In Supplementary Materials Table S1, the crystallographic data are summarized;
in Supplementary Materials Table S2, selected bond distances and angles are presented;
and the hydrogen bonding interactions are presented in Supplementary Materials Table S3.
The Cambridge Crystallographic Data Centre (CCDC) codes 2057313 and 2057314 contain
the supplementary crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
4. Conclusions
We synthesized and characterized two new coordination polymers, namely [Zn(
O1:1 O2-L)(H2O)2]n O1O2:2 O3-L)(H2O)]n
n(H2O) ( ) and [Cd(µ4-1 N:3,4 n(H2O) (
constructed from 5-{(pyren-4-ylmethyl)amino}isophthalic acid (H2L) with Zn(NO3)2.6H2O
µ-
1κ
κ
·
1
κ
κ
κ
·
2),

Molecules 2021, 26, 1101
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and Cd(NO3)2.4H2O, respectively, under solvothermal reaction conditions. Single-crystal X-
ray diffraction analyses revealed that the coordination polymer
1 features a one-dimensional
linear structure, whereas presents a two-dimensional network. The presence of Lewis
2
acid (metal ion) and basic (amine group) centers, as well as the insolubility in organic
solvents, makes these CPs suitable for heterogeneous catalysis.
Thus, we tested the catalytic activities of these synthesized coordination polymers
towards the cyanosilylation reaction of various aldehydes with TMSCN, under microwave
irradiation. They show a high activity for such a reaction under solvent-free conditions,
mainly the Zn(II)-coordination polymer (
what may reflect the higher Lewis acidity of the Zn(II) center in the former than that of the
Cd(II) site in the latter. However, the simpler 1D nature of with more accessible metal
centers, in comparison with the two-dimensional coordination polymer , can be another
reason behind the higher catalytic efficiency of . We also demonstrated the heterogeneous
nature, the stability and recyclability of the catalysts and . They can be recycled at
1), which is more active than the Cd(II) one (2),
1
2
1
1
2
least four times, without losing activity appreciably. Moreover, the use of microwave
irradiation in such a reaction reduces the reaction time significantly. This work provides
further indication that coordination polymers constructed from simple ligands, bearing
both Lewis acid and basic centers, can be applied as efficient heterogeneous catalysts in
important types of reactions, such as that investigated herein.
Supplementary Materials: The following are available online, Figure S1: (A) π····π interactions
(between pyrene rings) in coordination polymer 1, Figure S2: FT-IR spectra of catalysts 1 (A) and 2
(B) before (black) and after (red) the cyanosilylation reaction, Figure S3: PXRD spectra of catalysts
1 (A) and 2 (B) simulate (black), before (red) and after (blue) the cyanosilylation reaction, Figure
1
S4: H-NMR spectra of solvent-free cyanosilylation of benzaldehyde with catalysts 1 (A) and 2 (B)
in CDCl3 (entries 1 and 2, Table 1) (The protons are considered in the integrations are indicated in
red colour), Table S1: Crystal◦data and structure refinement details for compounds 1-2, Table S2:
Hydrogen bond geometry (Å, ) in compounds 1-2, Table S3: Selected bond distances (Å) and angles
◦
( ) for compounds 1-2.
Author Contributions: A.K. designed the research strategy, ligand synthesis, catalytic studies, X-ray
analysis and manuscript writing; E.P.S. synthesized compounds
1 and 2; A.P. performed NMR,
catalytic studies and manuscript writing; A.J.L.P. and M.F.C.G.d.S. performed manuscript reading
and correction. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the Fundação para a Ciência e Tecnologia (FCT), Portugal,
project UIDB/00100/2020 of Centro de Química Estrutural, and by the RUDN University Strategic
Academic Leadership Program.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: A. Karmakar and A. Paul express their gratitude to Instituto Superior Técnico
and FCT for Scientific Employment contracts (Contrato Nos: IST-ID/107/2018 and IST-ID/197/2019),
under Decree-Law no. 57/2016, of August 29. The authors also acknowledge the Portuguese NMR
Network (IST-UL Centre) for access to the NMR facility. This work was also supported by the RUDN
University Strategic Academic Leadership Program.
Conflicts of Interest: The authors declare no conflict of interest.
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