Zirconium coordination polymers based on tartaric and malic acids as catalysts for cyanosilylation reactions

Article abstract of DOI:10.1016/j.apcata.2019.117190

The synthesis of chiral materials is a great challenge. This work describes the synthesis of Zr-based coordination polymers composed of chiral ligands derived from natural products. L-tartaric acid, dibenzoyl-L-tartaric acid and L-malic acid were selected

Full text of DOI:10.1016/j.apcata.2019.117190

Journal Pre-proof
Zirconium coordination polymers based on tartaric and malic acids as
catalysts for cyanosilylation reactions
´
´
Juan Amaro-Gahete, Dolores Esquivel, Jose R. Ruiz, Cesar
´
´
Jimenez-Sanchidrian, Francisco J. Romero-Salguero
PII:
S0926-860X(19)30345-X
https://doi.org/10.1016/j.apcata.2019.117190
117190
DOI:
Article Number:
Reference:
APCATA 117190
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Applied Catalysis A: General
Received Date:
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Accepted Date:
7 May 2019
17 July 2019
4 August 2019
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Please cite this article as: Amaro-Gahete J, Esquivel D, Ruiz JR, Jimenez-Sanchidrian C,
Romero-Salguero FJ, Zirconium coordination polymers based on tartaric and malic acids as
catalysts for cyanosilylation reactions, Applied Catalysis A, General (2019),
doi: https://doi.org/10.1016/j.apcata.2019.117190
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Zirconium coordination polymers based on tartaric and malic
acids as catalysts for cyanosilylation reactions
Juan Amaro-Gahete, Dolores Esquivel,* José R. Ruiz, César Jiménez-Sanchidrián and
Francisco J. Romero-Salguero *
Departamento de Química Orgánica, Instituto Universitario de Investigación en
Química Fina y Nanoquímica IUNAN, Facultad de Ciencias, Universidad de Córdoba,
Campus de Rabanales, Edificio Marie Curie, E-14071 Córdoba, España
* Corresponding authors. Tel.: +34957218638; fax: +34957212066. E-mail address:
qo2rosaf@uco.es (F.J.R.-S.); Tel.: +34957211050; fax: +34957212066. E-mail address:
q12esmem@uco.es (D.E.)
Graphical abstract
Highlights
 Several coordination polymers containing Zr and L-tartaric acid, dibenzoyl-L-
tartaric acid or L-malic acid were active in the cyanosilylation of benzaldehyde
with TMSCN.
 Those polymers containing malic acid as ligand and a high Zr to malic acid ratio
showed the highest catalytic activity.
1

 Enantiomeric excess of up to 12.6 % were obtained in Et2O and low temperature
(-10 ºC).
 Electron withdrawing groups in p-substituted benzaldehydes increased the
conversion in the cyanosilylation reaction.
Abstract
The synthesis of chiral materials is a great challenge. This work describes the
synthesis of Zr-based coordination polymers composed of chiral ligands derived from
natural products. L-tartaric acid, dibenzoyl-L-tartaric acid and L-malic acid were
selected as ligands from the chiral pool. Their reaction with zirconium oxychloride gave
rise to several materials that were characterized by different techniques, such as XRD,
Raman spectroscopy, 13C MAS NMR, TGA and N2 adsorption/desorption isotherms.
Significant structural and textural properties have been observed depending on the
ligand, the synthesis procedure and the metal to ligand ratio. These polymers were used
for the first time as catalysts in the cyanosilylation of benzaldehyde with trimethylsilyl
cyanide and their activity was compared with that of UiO-66. Different reaction
parameters and reusability were studied for this asymmetric reaction. Some of the
reported materials were very active in this reaction and provided a remarkable
enantioselectivity depending on the reaction conditions.
KEYWORDS: Coordination polymers, Zr(IV) complexes, chiral linkers,
cyanosilylation reaction, enantioselectivity, heterogenous catalysis.
1. Introduction
Coordination polymers are hybrid materials composed of organic linkers and
metal nodes. [1, 2] Among different organic ligands used for the preparation of
coordination polymers, malic and tartaric acids are promising linkers because they are
readily abundant natural products and, besides two carboxylate groups, contains
hydroxyl groups which can provide multiple coordination modes. In addition, both acids
and their derivatives are simple and inexpensive chiral sources [3, 4].
2

Numerous coordination polymers and metal organic frameworks composed of
malic and tartaric acids have been synthesized in the last years. Thus, for example,
Williams et al. [5] reported two new indium tartrates, one of them with a microporous
3D framework with a channel size of ca. 6 Ǻ and Del Castillo et al. [6] described
cadmium tartrates with open frameworks. Metal tartrates exhibit interesting applications
such as antiferromagnetic coupling by manganese coordination polymers [3],
photoluminescence by some rare-earth MOFs (La, Sm, Eu, Tb and Dy) [4] and
ferroelectricity by cerium coordination polymers [7], among others. Metal malates of a
variety of metal ions have also been synthesized [8]. For instance, the crystal structure
of several chiral and achiral malates of Sr(II), Cu(II), Pb(II), Co(II), Ni(II), Mg(II),
Zn(II) and Cd(II) have been reported in the last years [9-12].
Zirconium coordination polymers are particularly attractive because they
generally exhibit a high stability due to the significant strength of Zr-O bonds. An
example is UiO-66, which is a Zr-terephthalate based MOF, a versatile material for
catalytic applications [13]. Both Lewis and Brönsted sites present in this material can
act complementary to each other, making it very attractive for catalysis [14]. Even, the
strength of the Lewis acid sites can be modulated changing the electronic character of
groups bound to the linker [15]. Also, the important role of the accessibility of
substrates to internal active sites was revealed by comparison of the catalytic activity of
UiO-66 and UiO-67, the latter containing biphenyl-4,4’-dicarboxylate ligands [16].
Other Zr-based MOFs have been used as catalysts, i.e. a bimetallic Zr(Ti)-
naphthalendicarboxylate MOFs [17], which was active in Lewis acid catalyzed
reactions, such as Meerwein-Ponndorf-Verley and isomerization of α-pinene oxide,
among others.
The first example of chiral catalysis with a MOF was reported by Kim et al. [18]
This material was synthesized from a pyridyl derivative of tartaric acid and Zn(II). The
enantiomeric excess in the transesterification of 2,4-dinitrophenyl acetate with 1-
phenyl-2-propanol was 8%. Later, the coordination polymer Ni(L-asp)bipy0.5, with L-
asp = L-aspartate and bipy = 4,4’-dipyridyl, provided enantiomeric excess between
+17% and -6% in the methanolysis of cis-2,3-epoxybutane [19]. A chiral Cu(II)
coordination polymer based on binaphthyl units was essayed as catalyst in the
cyanosilylation of benzaldehyde but it exhibited very low activity [20]. No results on
the enantioselectivity of this process were provided.
3

The cyanosilylation of carbonyl compounds with TMSCN is an interesting
organic reaction that gives rise to cyanohydrins, which are useful intermediates for the
synthesis of a wide variety of compounds, such as α-hydroxyacids, α-hydroxyketones,
β-hydroxyamines, α-aminonitriles and β-aminoalcohols, among others. A great number
of different catalysts are active, including Lewis acids. In addition, the sp2 carbon of the
carbonyl group is transformed into a sp3 stereogenic center and so the asymmetric
cyanohydrins synthesis has been intensively pursued. Nevertheless, conversion and
enantioselectivity are very sensible to the substrate, the catalyst and the reaction
conditions [21]. Most homogeneous catalysts are based on privileged ligands as chiral
building blocks, which usually work with coordinated expensive metals. Although most
of these ligands are commercially available, their high cost prevents their general use in
industry.
Even though several coordination polymers based on tartaric acid and malic acid
has been reported (vide supra), to the best of our knowledge, materials containing them
or very simple readily available derivatives have not been ever used as heterogeneous
catalysts. Furthermore, the use of zirconium as metal node should provide Lewis acid
sites and remarkable stability under reaction conditions. In addition, cyanohydrins have
an asymmetric carbon atom and so the enantioselectivity in this process, which has
hardly been studied for heterogeneous catalysts, can be assessed under different
conditions.
Herein, we report the synthesis of zirconium coordination polymers with three
different chiral organic linkers, i.e., tartaric acid, dibenzoyl tartaric acid and malic acid.
After being characterized by different techniques, they have been tested as catalysts for
the first time in cyanosilylation reactions. The activity and selectivity of these catalysts
under different reaction conditions is reported. The influence of the ligand structure and
the zirconium to ligand ratio has been described. Moreover, the stereoselectivity
towards the chiral cyanohydrins has been determined.
2. Experimental
2.1. Reagents and materials
4

L-(+)-tartaric acid (≥ 99.5%), L-(-)-malic acid (≥ 99.5%), dibenzoyl-L-tartaric
acid (98%), zirconium (IV) oxychloride octahydrate (99.5%) and N,N-
dimethylformamide anhydrous (99.8%) were obtained from Sigma Aldrich. Formic acid
pure (98%) was provided by PanReac AppliChem. For the catalytic activity tests,
trimethylsilyl cyanide (98%) and benzaldehyde (99.5%) were supplied by Aldrich,
whereas dodecane (≥ 99.5%) was provided by Fischer Scientific. All reactants and
solvents were used as received, without further purification.
2.2. Synthesis of the catalysts
The synthesis of all materials was carried out either in an autoclave under
autogenous pressure (A) or in a batch reactor under atmospheric pressure. The molar
ratios of the Zr salt (Z) and linker precursor (tartaric acid (T), malic acid (M) or
dibenzoyl-tartaric acid (DBT)) were 1:1 and 1:3, respectively. A summary of the
nomenclature and reaction conditions are given in Table 1. Thus, Z1T1-110A was
prepared by solvothermal synthesis using an autoclave under autogenous pressure (A)
according to the following procedure. A mixture of 1 mmol of ZrOCl2 • 9H2O (0.322 g)
(Z1) and 1 mmol of tartaric acid (0.150 g) (T1) in 10 mL of N,N-dimethylformamide
(DMF) was heated in a teflon vessel at 110 ºC for 68 h. Later, the solid product was
collected by membrane filtration and washed several times with DMF. To assure the
complete removal of unreacted linker, the material was stirred in 25 mL of DMF for 24
h and then in ethanol for 24 h. Afterwards, the resulting material was dried under
vacuum at 100 ºC for 24 h.
Alternatively, Z1T1-120 was synthesized in a batch reactor. For that, a solution
containing 1 mmol of ZrOCl2 • 9H2O (0.322 g) (Z1), 1 mmol of tartaric acid (0.150 g)
(T1) and 20 mL of DMF was transferred to a batch reactor at 120 ºC for 68 h. Then, the
washing procedure was like the solvothermal synthesis. To increase the crystallinity of
the materials synthesized in batch reactor, in some cases, formic acid (5 eq, 97.5μL) was
added to the reaction mixture as a modulating agent (m).
Two batches of some materials were prepared to confirm the reproducibility in
their synthesis and catalytic properties (see Scheme S1 in supplementary information).
5

Table 1 Nomenclature and reaction conditions for the synthesis of Zr-based
coordination polymers.
Zr:L
Name
Ligand (L)
Solvent
T (oC)
Reactor
ratioa
1:1
Z1T1-110A
Z1T1-120
Tartaric acid
Tartaric acid
Tartaric acid
Tartaric acid
Tartaric acid
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
H2O
110
120
120
120
120
110
120
120
110
150
180
120
120
120
120
autoclave
batch
1:1
Z1T1-120m
Z1T3-120
1:1
batch
1:3
batch
Z1T3-120m
Z1DBT1-110A
Z1DBT3-120
Z1DBT3-120m
Z1M1-110A
Z1M1-150A
Z1M1-180A
Z1M1-120
1:3
batch
Dibenzoyl tartaric acid 1:1
Dibenzoyl tartaric acid 1:3
Dibenzoyl tartaric acid 1:3
autoclave
batch
batch
Malic acid
Malic acid
Malic acid
Malic acid
Malic acid
Malic acid
Malic acid
1:1
1:1
1:1
1:1
1:1
1:3
1:3
autoclave
autoclave
autoclave
batch
H2O
H2O
H2O
Z1M1-120m
Z1M3-120
H2O
batch
H2O
batch
Z1M3-120m
H2O
batch
a Zr:L ratio in the synthesis medium; m: modulating agent.
2.3. Characterization of the catalysts
Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8
DISCOVER A25 with Cu-Kα radiation of 0.15418 nm wavelength and a solid-state
detector. The speed of the goniometer was 0.016 º / min and the sweep between 2º <2θ
<70º. Nitrogen adsorption-desorption experiments were carried out using an Autosorb
iQ/ASiQwin analyzer from Quantachrome Instruments at 77 K. Prior to measurements,
the samples were outgassed at 90 ºC for 24 h. The specific surface area of each material
was determined using the Brunauer-Emmett-Teller (BET) method over a relative
pressure (P/P0) range of 0.025 - 0.996. The thermogravimetric analysis was carried out
to establish the stability of the coordination polymers as a function of temperature. The
TGA records were obtained using a Mettler Toledo TGA/DSC instrument. The
measurements were carried out in an O2 atmosphere with a gas flow of 100 mL/min
with a temperature range between 30 and 900 ºC and a heating ramp of 10 ºC/min.
Raman spectra were acquired with a Renishaw Raman instrument (inVia Raman
Microscope) by excitation with green laser light (532 nm) and a grating of 1800
lines/mm in a range 100-4000 cm-1. A total of 15 scans per spectrum were performed to
improve the signal-to-noise ratio. The 13C CP/MAS NMR spectra were recorded at
100.61 MHz on a Bruker AVANCE 400 WB spectrometer at room temperature. An
6

overall of 1000 free induction decays were accumulated. The excitation pulse and
recycle time were 6 ms and 2 s, respectively. Chemical shifts were measured relative to
a tetramethylsilane standard.
2.4. Catalytic activity
2.4.1. General procedure
The catalytic activity of the materials was studied in the cyanosilylation reaction
[22]. The catalyst (12 mg) was pretreated at 120 ºC overnight under vacuum before the
catalytic reaction and then added to a mixture of benzaldehyde (0.87 mmol),
trimethylsilyl cyanide (2.6 mmol) and dodecane (0.39 mmol) as internal standard. The
reaction was kept under nitrogen at different temperatures and reaction times. Samples
collected at different times were analyzed by gas chromatography using a FID detector
and VF-1MS capillary column (15 m x 0.25 mm ID). Benzaldehyde cyanohydrin was
the only product identified. The enantioselectivity (% e.e.) was calculated by gas
chromatography (VARIAN 450-GC) using a chiral capillary column (Restek Rtr-
bDEXm).
For the cyanosilylation reaction of benzaldehyde and derivatives with TMSCN
in presence of solvent, the molar ratios of reactants were kept, and 1 mL of solvent was
added to the reaction mixture.
2.4.2. Hot filtration tests
The reactions were carried out as described previously, but alternatively after 1
or 3 h of reaction, the mixture was filtered off at room temperature using a syringe fitted
with a 0.20 μm nylon membrane filter, to remove all catalyst particles. In each case, the
clear solution obtained was transferred to a round-bottomed flask at room temperature
under nitrogen atmosphere and stirred for further 17 h.
7

3. Results and discussion
3.1. Characterization of the catalysts
Different reaction conditions for the synthesis of Zr-coordination polymers
based on chiral ligands were employed (see Experimental Section, Table 1). Zirconium
malates were obtained in water at different temperatures. Unlike, zirconium tartrates
could not be formed in water under the conditions studied and therefore DMF was used
as solvent. This solvent was also chosen in the synthesis of zirconium dibenzoyltartrates
due to the high insolubility of dibenzoyl tartaric acid in water. In some cases, the metal
to ligand ratio was a determining parameter in the synthesis. For example, no precipitate
was obtained in batch reactor with a Zr:DBT ratio of 1:1, being required a molar ratio of
1:3. In addition, in order to increase the crystallinity, a modulating agent (formic acid)
was added in those synthesis performed in batch reactor.
After the synthesis, the stability of all coordination polymers was confirmed by
solubility tests in different solvents such as water, ethanol, hexane and ethyl acetate.
The X-ray diffraction (XRD) patterns of some coordination polymers are
depicted in Fig. 1. Under similar reaction conditions, only the material with malate as
linkers exhibited crystallinity. All polymers based on tartaric acid, regardless of the
metal to ligand ratio, showed a broad band at the interval 2θ = 4 - 15º, which indicates
poorly ordered materials (Fig. S1). Similar patterns were observed for those consisting
of dibenzoyl tartaric acid (Fig. S2). Other Zr(IV) coordination polymers such as those
based on 1,3,5-benzenetrisphosphonic acid and carboxylate-substituted derivatives have
also been found to be poorly crystalline [23]. Unlike, malic acid led to amorphous
materials when the metal:ligand ratio was 1:1 whereas crystalline materials were
obtained with a metal:ligand ratio of 1:3 (Fig. S3). This difference of crystallinity for
Zr-based coordination polymers could be related with the nature of the organic linkers.
On one hand, the steric hindrance of the dibenzoyl groups could led to an amorphous
structure and, on the other hand, the presence of two hydroxyl groups in the tartaric
linker provides additional coordination modes that could affect the metal coordination
and, therefore, their crystalline arrangement.
8

c
b
a
10
20
30
40
50
60
70
2(degrees)
Fig. 1. X-ray diffraction patterns of Z1T3-120m (a), Z1DBT3-120m (b) and Z1M3-120m
(c).
The adsorption-desorption isotherms for all samples, regardless of the synthesis
conditions, were essentially type-IV with wide pore size distributions particularly in the
mesopore range. For instance, adsorption-desorption isotherms and pore size
distribution of Z1M1-110A, which has been used as catalyst (vide infra), are depicted in
Fig. S4. Surface areas, pore volumes and pore radii are given for all coordination
polymers in Table 2. All Zr-tartrate materials displayed a low specific surface area
(below 20 m2 g-1), which slightly increased for Zr-dibenzoyl tartrates. Nevertheless, Zr-
malates showed remarkable areas with up to 192 m2 g-1. For these coordination
polymers, a slight decrease of the surface area was observed with the temperature of
synthesis. However, the area dramatically decreased to ca. 30 m2g-1 when the zirconium
to malic acid molar ratio in the synthesis medium was 1:3. In addition, pore radii
increased because the contribution of small pores in a more cross-linked structure is
reduced.
Table 2 Physico-chemical properties of Zr-based coordination polymers
Pore
Pore
radius (Å)
Zr:L
Zr/L
Weight loss
(%)
SBET
Name
volume
(cm3/g)
0.016
0.033
0.019
0.020
ratioa ratiob
(m2/g)
Z1T1-110A
Z1T1-120
1:1
1:1
1:1
1:3
0.8
1.1
1.0
0.8
64.7
57.4
59.9
65.7
7
43
38
49
76
19
8
Z1T1-120m
Z1T3-120
5
9

Z1T3-120m
Z1DBT1-110A
Z1DBT3-120
Z1DBT3-120m
Z1M1-110A
Z1M1-150A
Z1M1-180A
Z1M1-120
1:3
1:1
1:3
1:3
1:1
1:1
1:1
1:1
1:1
1:3
1:3
1.0
1.9
2.2
2.0
2.0
2.4
2.5
1.4
1.4
0.8
0.8
61.9
63.9
61.4
63.2
40.0
41.9
37.4
51.3
50.8
59.9
58.4
8
0.016
0.018
0.108
0.049
0.257
0.207
0.193
0.160
0.146
0.120
0.138
41
19
36
38
27
32
32
37
15
87
94
19
60
26
192
121
117
86
Z1M1-120m
Z1M3-120
190
28
Z1M3-120m
29
a Zr:L ratio in the synthesis medium; b Experimental Zr/L ratio calculated from TGA.
Thermogravimetric analysis was carried out to ascertain the stability and
composition of the different materials (Figs. 2, S5, S6 and S7). Tartaric acid-based
polymers displayed three weight losses. The first one up to 150 ºC is attributed to the
removal of solvent molecules. The second loss between 250 and 350 ºC would
correspond to the decomposition of the ligand molecules, i.e., tartaric acid. The
decomposition of some fragments of the organic ligands in the form of carbonate ions
bound to zirconium cations could be responsible for the third weight loss at ca. 550-650
ºC. Similar losses were observed for polymers with dibenzoyl tartaric acid, although the
third loss occurred at somewhat lower temperature (ca. 450-500 ºC). In the case of
malic acid-based polymers, some differences were observed depending on the metal to
ligand ratio. Those polymers synthesized with a metal to ligand molar ratio of 1:1
exhibited three weight losses: up to 150 ºC, between 300 and 400 ºC and between 550
and 650 ºC. However, the second loss in these materials occurred in two stages.
Nevertheless, more differences were observed for those solids obtained with a metal to
ligand ratio of 1:3. The first loss was centered at ca. 200 ºC, clearly revealing a stronger
interaction with solvent molecules in these materials. In addition, following the second
loss, which was very steep, the weight remained constant.
10

0
20
40
60
80
100
0
20
40
60
80
100
100
200
300
400
500
600
700
800
900
Temperature (ºC)
Fig. 2. Thermogravimetric curves for coordination polymers. Bottom: Z1T1-120 (solid
line); Z1T3-120 (dotted-dashed line); Z1DBT3-120 (dashed line). Top: Z1M1-120 (solid
line); Z1M3-120 (dashed line).
A rough estimate of the metal to ligand ratio can be obtained analyzing the TG
curves of these materials, i.e. losses of organic ligands in relation to the remaining metal
oxides [24]. The calculated values are given in Table 2. As can be seen, the metal to
ligand ratios depended on several factors. In the case of zirconium tartrates, those values
are close to 1, regardless of the relative amounts of Zr and tartaric acid present in the
starting reaction mixture. This is the ratio for the most known zirconium metal organic
framework, i.e. UiO-66 [24]. The metal to ligand ratio increased to ca. 2 for zirconium
dibenzoyl tartrates. Interestingly, when malic acid was used as ligand, the metal to
ligand ratio ranged from 0.8 to 2.5, depending on the relative amounts of the reactants,
i.e. Zr and malic acid, and the reaction conditions.
When the metal to ligand ratio is compared for the three types of ligands under
the same reaction conditions, an increase of the ratio is observed with the decrease in
the number of coordinating functional groups of the ligands (-COOH and –OH).
Consequently, the structure of the coordination polymers is more connected in the
following order: Zr-tartrate ≥ Zr-malate ≥ Zr-dibenzoyl tartrate.
Raman spectra of the materials confirmed the presence of the corresponding
linkers and so the formation of hybrid coordination polymers (Figs. 3, S8, S9 and S10).
11

All materials showed several bands in the range 2800 – 2975 cm-1 attributed to the C-H
stretching of the chiral ligands. A signal centered at 1670 cm-1 can be assigned to the
C=O stretching vibrations. Aromatic C-H stretching bands at 3072 and 3015 cm-1 were
present in materials containing dibenzoyltartaric acid. Additionally, polymers based on
tartaric acid showed a band at 3100 cm-1 indicating hydrogen bonding interaction
between water molecules and carboxyl groups in the material [25]. Zr-O coordination
was also confirmed in the polymers by the vibration modes at 663, 619 and 408 cm-1
ascribable to Zr-O stretching [26].
619
408
663
c
c
b
a
b
a
200 250 300 350 400 450 500 550 600 650 700 750 800
Raman shift (cm-1)
600
1200
1800
2400
3000
3600
Raman shift (cm-1)
Fig. 3. Raman spectra of samples Z1T1-110A (a), Z1DBT1-110A (b) and Z1M1-110A
(c).
In some cases, the linkers in coordination polymers can undergo chemical
transformations. For example, upon heating, lithium L-malate undergoes topotactic
dehydration to form a phase containing the unsaturated fumarate ligand, in which the
original 3-D framework remains intact [27]. To assure that the ligands stay intact during
the synthesis, 13C NMR experiments were accomplished. The 13C CP/MAS NMR
spectra of Z1M1-110A, Z1T3-120m and Z1DBT1-110A without previous activation are
shown in Fig. 4. Z1M1-110A and Z1T3-120m showed similar spectra with two sets of
resonance peaks. A first set located in the range of 160-200 ppm is attributed to the
carboxyl carbon atoms of the chiral ligands, which can be in different chemical
environments. The second set at high field (30-80 ppm) is assigned to the Csp3 of the
organic linkers, being more deshielded the carbon atoms bearing hydroxyl groups. For
Z1DBT1-110A, the 13C NMR spectrum showed additional signals in the 120-140 ppm
range which correspond to the aromatic rings of the benzoyl groups. The presence of
12

signals associated to DMF molecules was present in all materials. These signals were
absent after activation of the catalysts at 120 ºC overnight under vacuum (see
experimental section and Fig. S11 in supplementary information).
DMF
4
O
1
O
O
3
5
O
1
2
O
O
O
O
3,4,5
DMF
2
c
O
OH
1
1
O
O
OH
O
DMF
b
1
2
O
OH
2
O
O
1
O
DMF
a
200
150
100
ppm
50
0
Fig. 4. Solid-state 13C CP/MAS NMR spectra of Z1M1-110A (a), Z1T3-120m (b) and
Z1DBT1-110A (c).
In order to prove that ligands do not undergo racemization under the synthesis
conditions, vibrational circular dichroism measurements were undertaken.
Unfortunately, they were not conclusive. Instead, degradation of the polymer followed
by derivatization of the ligand and chiral GC analysis (Fig. S12 in supplementary
information) demonstrated that the optical purity of the ligands remained intact during
the synthesis procedure.
13

3.2. Catalytic activity
Solvent-free cyanosilylation of benzaldehyde with trimethylsilyl cyanide (TMSCN)
All coordination polymers were tested as catalysts for the cyanosilylation
reaction of benzaldehyde with trimethylsilyl cyanide (TMSCN) (Scheme 1) under
solvent-free conditions. In absence of any catalyst, the blank reaction gave a conversion
of 18 % after 14h at room temperature (Fig. S13).
Scheme 1. Cyanosilylation of benzaldehyde with TMSCN.
Zr-tartrates exhibited relatively low activity as catalysts (Fig. 5). The maximum
conversion obtained for such materials was 47%. Most of dibenzoyl tartrate polymers
were inactive except for Z1DBT1-110A, which gave rise to a 67% conversion.
Fig. 5. Conversion (%) and ee (%) in the cyanosilylation of benzaldehyde with TMSCN
at RT for 14 h.
14

However, the most active coordination polymers were those synthesized from
malic acid. Some of them provided a conversion around 95%. For example, polymer
Z1M1-110A gave a conversion and TON (turnover number) comparable to other
heterogeneous catalysts (Table 3). Under the same conditions, the metal-organic
framework UiO-66 was essayed, resulting in a conversion of 95%. In the case of Zr-
malates, the metal to ligand ratio plays an important role in the catalytic activity. In fact,
those materials with the highest activities have ratios about 2. The rest of Zr-malates
with metal to ligand ratios between 1.4 and 0.8 gave conversions below 45%. In
addition, the conversion decreased as the Zr/L ratio decreased. To compensate the lower
content of Zr in sample Z1M3-120m compared to polymer Z1M1-110A, the catalyst
weight for the latter was increased by 2.5 times, while maintaining the rest of
conditions, and its catalytic activity remained very low, ca. 23%. This fact revealed the
importance of the connectivity of the coordination polymer. Indeed, increasing the
number of ligand molecules around Zr decreases the catalytic activity, thus
corroborating the role of Zr cations as Lewis acid sites. If they are buried by ligands,
they are no longer accessible to the reactants. Similarly, it has been reported that the
catalytic activity of UiO-66 in the Fischer esterification increased when defects
consisting of coordinatively unsaturated Zr ions with Lewis acid properties were present
in its structure [14]. In particular, the activation of TMSCN by adsorption on Lewis acid
sites (Zr ions) has been suggested for the cyanosilylation of benzaldehyde in sulfated-
zirconia catalysts [28]. The metal-organic framework compound
Cu3(BTC)2(H2O)3·xH2O (BTC= benzene 1,3,5-tricarboxylate) was tested as catalyst in
the cyanosilylation of benzaldehyde [29]. Prior to its application it had to be dehydrated
in high vacuum at 100 ºC in order to remove physically and chemically bound water
molecules and so give Lewis acid Cu(II) sites. However, maximum yield was not higher
than 57% even after 72 h. In addition, it was very sensitive to the reaction conditions.
15

Table 3 Activity of some heterogeneous catalysts reported in the literature.
Entry
Catalyst
Solvent
Yield
(%)
96
TON
Ref.
1
2
Z1M1-110A
Cu(II)-
none
16.5
16
This work
[30]
MeOH
79.9
Arylhydrazone
Coordination
Polymers
3
Cu(II) Mixed- none
ligand
95.2
47.6
[12]
Coordination
Polymers
4
5
Bu2SnCl2
primary
none
none
97
99
9.7
[31]
[32]
49.5
amide-based
cationic metal
complexes
Cp–Zr–
6
toluene
100
66.7
[33]
Binol@SBA-
15
7
Eu3+-MCM-41 none
90
57
137
10.9
9.6
17
[34]
[29]
[35]
[36]
8
Cu3(BTC)2
UiO-66-A
UiO-68-V
pentane
9
dichloromethane 96
10
1,2-
85
dichloroethane
11
MIL-101 (Cr) none
96
320
[37]
The three ligands used for the synthesis of the coordination polymers are chiral.
Accordingly, the enantiomeric excess (e.e.) was determined in every reaction. These
results are shown in Figure 5. In general, all materials provided a clear
enantioselectivity but with e.e. values below 3%. Even so, it should be noted that, in the
catalyst free reaction, enantioselectivity was not achieved, just obtaining the racemic
mixture of the corresponding cyanohydrin. Those polymers composed of malic acid
synthesized in autoclaves at different temperatures gave the highest conversion and
16

enantioselectivity. Concerning these materials, a decrease in the e.e. value was observed
as the synthesis temperature increased, which also corresponded with an increase in the
metal to ligand ratio. Recent studies on rare-earth metal complexed to chiral pyrrolyl-
substituted 1,2-diimino ligands in cyanosilylation reactions have reported similar yields
but the products were racemic.[38]
To further investigate this reaction, Z1M1-110A, the most active polymer with
higher enantiomeric excess (e.e.) among catalysts tested, was selected to perform
additional studies on cyanosilylation reactions.
Effect of reaction temperature
In order to ascertain the effect of reaction temperature, particularly on
enantioselectiviy, the cyanosilylation reaction was also carried out at 10 and -10 ºC. As
observed in Table 4, the temperature clearly affected the enantioselectivity. The lower
the temperature, the higher the e.e. value. Thus, at -10 ºC, the e.e. was ca. 6%, even
though the yield was greatly reduced. Although this e.e. is low, it is significant because
the active sites, i.e., Zr cations, are not directly attached to chiral centers but they are
located in a chiral environment provided by chiral ligands. Analogous results were
reported by Lin et al. [39] with coordination polymers consisting of 2,2'-
bis(diphenylphosphino)-1,1'-binaphthyl ligands (BINAP) and different lanthanides,
which gave rise to e.e. below 5% in the cyanosilylation of aldehydes. Obviously, higher
enantiomeric excess have been reported on catalysts based on transition metal
complexes with chiral privileged ligands, such as those metal-organic frameworks [22]
and periodic mesoporous organosilicas [40] containing homochiral vanadium-salen
complexes, which yielded e.e. around 70% and 30%, respectively.
Table 4 Conversion and enantiomeric excess in the cyanosilylation reaction at different
temperatures using Z1M1-110A as catalyst.
Temperature (ºC)
Time (h) Conversion (%)
e.e. (%)
2.9
rt
14
20
20
96
95
55
10
-10
5.1
5.9
17

Solvent and promoter effects
Although previous studies have revealed that this type of reaction progresses
very well under solvent-free conditions [41], additional experiments were performed in
presence of different solvents to examine their role in the cyanosilylation reaction. As
seen in Table 5, the presence of solvent in the reaction media produced a decrease in the
conversion, from 96 % under solvent-free conditions to 18% when DMF was added.
The polarity of the solvent clearly influenced on the cyanosilylation reaction. [42] Non-
polar solvents, as hexane, did not affect the reaction rate. Unlike, the use of polar
solvents such as DMF, CH3CN and THF reduced the yield, which was lower than 25%.
Unlike, the reactions in moderately polar solvents like CH2Cl2 and Et2O gave higher
conversions. This decrease in polar solvents can be probably due to the coordination
ability of the solvent molecules to unsaturated Zr-sites.[37] Although the conversions in
presence of solvents were greatly reduced, the ee values were higher in all cases, with
10.6 and 11.4 % ee in toluene and Et2O, respectively. These results showed the
important role of the solvent on the enantioselectivity of this type of reaction.
The enantioselective cyanohydrin formation has been a subject of study by
numerous researchers. Not only the solvent has shown to have an important impact on
asymmetric catalytic systems, but also the addition of “promoters”, such as
CH3P(O)Ph2, Bu3PO and Ph3PO have been beneficial for the yields and
enantioselectivies obtained.[43] Ph3PO has been found to be a superior cocatalyst [44]
and so, in order to evaluate this effect, additional catalytic experiments on the reaction
of benzaldehyde with TMSCN under different solvents in the presence of Ph3PO were
carried out. The results obtained are listed in Table 4. In most cases, the reaction rates
were enhanced by the addition of the promoter. In contrast, the enantioselectivities
values did not show an increasing tendency in all solvents when the promotor was
added. Only those reactions in CH3CN and DMF increased twice the ee values.
In addition, those conditions providing higher conversions and enantiomeric
excess were selected to perform the experiments at low temperature (-10 ºC) The
cyanosilylation reaction of benzaldehyde in the presence of toluene and Ph3PO at -10ºC
gave a conversion of 15% and ee of 11.1%. Moreover, in Et2O and without any added
Ph3PO, the reaction reached the highest ee, 12.6%. These results confirmed that the
18

yield and enantiomeric excess on asymmetric catalytic systems are very sensitive to
small changes in the reaction conditions.
Table 5 Z1M1-110A catalyzed cyanosilylation reaction of benzaldehyde under different
conditions (solvents and promoter).
Solvent
CH2Cl2
Promoter
-
Conv. (%)
36
ee (%)
9.0
Ph3PO
84
7.1
THF
-
25
9.6
Ph3PO
71
6.3
CH3CN
Hexane
DMF
-
24
74
94
89
4.1
9.2
7.9
8.4
Ph3PO
-
Ph3PO
-
18
3.9
Ph3PO
75
5.3
Toluene
-
44
74
10.6
9.7
Ph3PO
Ph3PO
-
Ph3PO
15(*)
78
70
11.1(*)
11.4
9.3
Et2O
-
19(*)
12.6(*)
Solvent free
-
96
2.9
*The reaction was performed at -10ºC; Promoter, Ph3PO, 0.2 equiv.
Scope of the reaction
In order to assess the scope of this catalytic system, tests were carried out using
the catalyst Z1M1-110A to demonstrate its effectiveness in reactions with different p-
substituted benzaldehydes. The experimental conditions were established considering
the solubility of the different reactants. dichloromethane was selected because it
dissolved all of the tested p-substituted benzaldehydes. In addition, the Ph3PO promoter
was added to the reaction mixture because it improved the catalytic results when using
dichloromethane as solvent (vide supra).
The conversion and enantioselectivity results for the different substrates are
shown in Table 6. It can be observed that the p-substituted benzaldehydes bearing
electron withdrawing substituents such as nitro, cyano or bromo give riseto high
conversion and ee. On the other hand, the p-substituted benzaldehydes with electro
donating groups produced poorer yields in the catalytic process.[37]
19

Table 6 Catalytic cyanosilylation of p-substituted benzaldehydes after 14 h at room
temperature in CH2Cl2 using 0.2 eq of Ph3PO as promoter.
p-substituted benzaldehydes
Conversion (%) ee (%)
p-bromobenzaldehyde
88.7
8.4
O
H
Br
p-nitrobenzaldehyde
54.6
11.3
O
H
O
N
O
p-cyanobenzaldehyde
>99
31.1
50.1
5.2
O
H
N
p-propoxybenzaldehyde
n.a.1
n.a.1
O
H
H3C
O
p-ethylbenzaldehyde
O
H
H3C
1 not available; the enantiomeric reaction products could not be separated by chiral GC.
Heterogeneity tests
In order to check the heterogeneity of the cyanosilylation reaction, two hot
filtration tests were undertaken with Z1M1-110A catalyst at different conversions (Fig.
6). After the removal of the catalyst, the reaction did not progress, thus revealing that
the reaction occurred under heterogeneous conditions.[45] That was also indicative of
the stability of these coordination polymers under the reaction conditions.
20

100
80
60
40
20
0
0
2
4
6
8
10
12
14
time (h)
Fig. 6. Hot filtration test in the solvent-free cyanosilylation of benzaldehyde with
TMSCN. Conversion vs time for the catalyzed reaction (*) and for the reaction after
filtration of the catalyst at 1h (■) and 3 h (●).
4. Conclusions
Zirconium-based coordination polymers with different chiral ligands (L-tartaric
acid, dibenzoyl-L-tartaric acid and L-malic acid) were synthesized under solvothermal
conditions in batch reactor or autoclave. Textural and structural properties of the
resultant polymers were directly related to the chiral ligand, the synthesis conditions and
the metal:ligand ratio. Most of the resulting materials were amorphous except for some
malic acid coordination polymers which showed high cristallinity. Zr-malates exhibited
higher specific surface areas than Zr-tartrates and Zr-dibenzoyltartrates, although it
strongly depended on the Zr to ligand ratio. Several characterization techniques showed
that the organic linkers remained unaltered under the synthesis conditions. Several
materials were active in the cyanosilylation reaction of benzaldehyde with TMSCN,
particularly some coordination polymers containing malic acid as ligand, giving rise to
conversions comparable to UiO-66. In general, a high Zr to malic acid ratio resulted in a
higher catalytic activity. Furthermore, the use of Ph3PO as promoter tended to increase
the conversion whereas the activity in polar organic solvents decreased. Interestingly,
enantiomeric excess of up to 12.6%, depending on the reaction conditions, has been
observed. Different p-substituted benzaldehydes have also been used as substrates
exhibiting moderate to good conversion toward the corresponding cyanosilylated
21

product and variable enantioselectivity. Thus, these materials can be considered as
promising green catalysts for Lewis acid-catalyzed organic processes.
Acknowledgments
The authors wish to acknowledge the financial support from Ramon Areces
Foundation (Spain), Andalusian Regional Government (FQM-346 group), Spanish
Ministry of Science, Innovation and Universities for an FPU teaching and research
fellowship (FPU17/03981) and a project MAT2013-44463-R, Feder Funds, the
technical staff from the Instituto Universitario de Investigación en Química Fina y
Nanoquímica (IUNAN) and Servicio Central de Apoyo a la Investigación (SCAI).
22

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