Aliphatic amine mediated assembly of [M6(mna)6] (M = Cu/Ag) into extended two-dimensional structures: synthesis, structure and Lewis acid catalytic studies

Article abstract of DOI:10.1039/d1nj00544h

Two new two-dimensional layered compounds, [{Ag₆(2-mna)₆Cd(Hen)₂}{Cd(en)₂2H₂O}(H₂O)₈],I, and [{M₆(2-mna)₆Cd(Hen)₂}(H₂en)(H₂

Full text of DOI:10.1039/d1nj00544h

NJC
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Aliphatic amine mediated assembly of [M (mna) ]
6
6
(M = Cu/Ag) into extended two-dimensional
Cite this: New J. Chem., 2021,
4
5, 6503
structures: synthesis, structure and Lewis acid
catalytic studies†
Anupam Sarkar, Ajay Kumar Jana and Srinivasan Natarajan
*
Two new two-dimensional layered compounds, [{Ag
6
(2-mna)
6
Cd(Hen)
2
}{Cd(en) 2H O}(H O)
2 2 2 8
], I, and
[
{M (2-mna) Cd(Hen) }(H en)(H O)10], II, (M = Cu and Ag) were prepared employing a [M -(Hmna) ]-
6
6
2
2
2
6
6
cluster as the starting metalloligand. The metalloligand was prepared and characterized before use.
Ethylenediamine (en) was also employed as a possible ligand/structure directing agent. The prepared
6
compounds have the metalloligand as part of the extended structure. The compounds containing the Ag -
Received 4th February 2021,
Accepted 10th March 2021
cluster (I and II) were isolated from the same reaction mixture but at different time intervals. The structures
of the prepared compounds were determined by single crystal X-ray diffraction studies. The compounds
were investigated for possible Lewis acid catalytic behavior by carrying out cyanation of imine, which
indicated good yield and selectivity. The compounds were also found to be a good proton conductor at
room temperature (98% RH).
DOI: 10.1039/d1nj00544h
rsc.li/njc
combine that with the HSAB principle in the design and
synthesis of new framework compounds. One such organic
1
. Introduction
Inorganic–organic hybrid compounds or metal–organic frame- ligand is 2-mercapto nicotinic acid (2-Hmna). 2-mna possesses
work (MOF) compounds have attracted intense research interest three possible binding sites (S, N, O) with different basicities.
2
6,32
over the past two decades due to their many interesting properties According to HSAB theory,
the S and N centers are softer
in the areas of sorption, separation, catalysis, proton conduction, bases compared to oxygen. One can react the soft bases with
+
+
magnetism, etc. Various developments in the family of compounds soft acids such as Cu or Ag ions. Such attempts have been
+
+
have been reviewed and summarized in many articles over the made and [M -(Hmna) ] (M = Cu /Ag ) octahedral clusters have
6
6
1
–12
33,34
years.
been isolated and characterized.
The octahedral clusters
+
+
From the structural point of view, MOFs exhibit a great are built up from the connectivity between Cu /Ag ions and S
variety as well as diversity as it has been shown that their and N centers of 2-mna and are stabilized by extensive hydrogen
structures can be stabilized either from a single metal node or bonding interactions involving the carboxylic acid group. It has
1
3–17
through metal-oxo clusters as building units.
The MOFs been shown that the carboxylic acid groups can also be sub-
based on metal-oxo clusters have the well-established Keggin sequently reacted with harder acidic metal ions, which gives
1
8,19
20–22
23
35–37
ions,
Anderson ions,
Dawson ions, etc., and also rise to interesting framework structures.
2
4,25
newer ones as nodes.
There have been attempts to incor-
During the course of this study, it occurred to us to explore
2
6
porate the Hard-Soft Acid-Base (HSAB) principle of Pearson in this system of compounds by using additional secondary
the preparation of MOFs. organic ligands such as aliphatic diamines. Aliphatic diamines
Over the years, we have been successful in preparing new have been employed as coordination blocking entities in the
inorganic coordination polymers employing metal-oxo clusters/ synthesis of copper azides with considerable success. In
metal clusters as nodes.
3
8
2
7–31
It is interesting to explore organic addition, aliphatic diamines have been employed as structure
ligands that possess more than one possible linking center and directing agents in the synthesis of many inorganic open-
3
9–43
framework compounds.
In the present study, we attempted
to use ethylenediamine (en) as a possible ligand/structure-
directing agent in the synthesis of new inorganic–organic hybrid
Framework solids Laboratory, Solid State and Structural Chemistry Unit, Indian
Institute of Science, Bangalore-560012, India. E-mail: snatarajan@iisc.ac.in
compounds employing the octahedral cluster, [M
which were successful. We have now prepared two new layered
compounds [{Ag (2-mna) Cd(Hen) }{Cd(en) 2H O}(H O) ], I, and
6 6
-(Hmna) ],
†
Electronic supplementary information (ESI) available. CCDC 2058012–2058014
1
[
CIF, PXRD pattern, IR, UV-Vis, PL, TGA, H NMR]. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/d1nj00544h
6
6
2
2
2
2
8
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{M (2-mna)
NJC
[
6
6
Cd(Hen)
2
}(H
2
en)(H
2
O)10], II, (M = Cu and Ag) under II is the only product even after 30 days of reaction time. (yield:
mild reaction conditions. The prepared compounds were char- 56% based on Cd) (Table 1).
acterized by single crystal X-ray diffraction studies to determine
2
.5. Synthesis of [{Cu
6
(2-mna)
6
Cd(Hen)
2
}Á(H
2
2
en)Á(H O)10] {II
their structure and also for possible Lewis-acid heterogeneous
catalytic reactions by performing cyanation of imine employing a
M
6
(M = Cu)}
number of substrates. In this paper, we present the synthesis, We have attempted a similar reaction employing a [Cu
6
(2-Hmna)
6
]-
]-
structure and heterogeneous catalytic studies of all the compounds. metalloligand. Typically, 0.1 mmol of the [Cu (2-Hmna)
6
6
metalloligand in 2 ml of water was solubilized in ethylenediamine
and taken in a 10 ml reaction tube (solution1). 2 ml of 1 : 1 water/
ethanol mixture was carefully layered on top of solution1 to act as
a buffer layer. Finally, a solution of cadmium nitrate (0.1 mmol) in
2
. Experimental section
2
.1. Materials
The starting compounds needed for the synthesis, CuI, AgNO3,
Cd(NO O, ethylenediamine (en), and EtOH were purchased
2
ml ethanol was slowly added on top of the buffer layer. The
layered solutions were kept at 70 1C for 5 days, which resulted in
good quality single crystals (yield: 46% based on Cd) (Table 1 and
Table S1, ESI†).
3
)
2
Á6H
2
from SD Fine (India). 2-Mercaptonicotinic acid was obtained from
Sigma-Aldrich. The starting materials were used without any
further purifications.
2.6. Initial characterization
The three synthesized compounds were characterized employing
powder X-ray diffraction (PXRD; PANalytical, Empyrean), IR
2
.2. Synthesis of the metalloligands, [Cu (2-Hmna) ] and
(2-Hmna) ]
6 6
6
6
[Ag
(PerkinElmer Spectrum 1000), UV/Vis (PerkinElmer, Lambda-35
The metalloligands [Cu
prepared following the method described elsewhere.
6
(2-Hmna)
6
] and [Ag
6
6
(2-Hmna) ], were
spectrophotometer) and photoluminescence (PerkinElmer LS-55)
spectroscopic methods.
3
3,34,44
a
The PXRD data (CuK ) were recorded in the 2y range of 5–501.
2
.3. Synthesis of [{Ag
O) ] (I)
In a typical reaction, in a 10 ml test-tube, 0.1 mmol of the
Ag (2-Hmna) ]-metalloligand in 2 ml water was solubilized
6
(2-mna)
6
Cd(Hen)
2
}{Cd(en)
2
2H
2
O}Á
The observed PXRD patterns of the synthesized compounds were
compared with the PXRD patterns simulated using the single-
crystal data, to establish phase purity of the samples (Fig. S1, ESI†).
The IR spectra of the synthesized compound exhibited
broad bands. The important observed IR bands are as follows:
(
H
2
8
[
6
6
using ethylenediamine (solution1). 2 ml of 1 : 1 water/ethanol
mixture was layered on top of solution1, to act as a buffer layer.
A solution of cadmium nitrate (0.1 mmol) in 2 ml ethanol was
very carefully layered on top of the buffer layer. The layered
solution was kept at 70 1C for 60 h, which resulted in block
shaped crystals at the interphase. (yield: 25% based on Cd)
À1
the IR bands in the range of 3200–3500 cm are due to the
À1
presence of water molecules. The bands at 1600 and 1580 cm
can be assigned to the asymmetric stretching of coordinated
À
–
COO and C–C stretching of the aromatic rings. The band at
À1
around 1445–1450 cm
could be due to the asymmetric
(Table 1).
stretching of the protonated amine group of the ethylenedi-
4
5,46
amine moiety (Fig. S2, ESI†).
The solid state UV/Vis absorption spectra of the samples
2.4. Synthesis of [{Ag
6
(2-mna)
6
Cd(Hen)
2
}Á(H
2
en)Á(H
2
O)10] (II)
The synthetic strategy for compound II is similar to that of I, were recorded at room temperature. For compounds I and II, a
the only difference is the reaction time. The reaction mixture maximum centered at B355 nm (for Ag) and one maximum at
was kept at 70 1C for 120 h. Compound I starts to convert into B400 nm (for Cu) were observed. This peak could be assigned
compound II after 3 days and appears to be completed after to the p–p* transition of the ligand (2-mna) (Fig. S3, ESI†). The
5
days. We have also extended the reaction time further (30 days) solid state photoluminescence of the compounds was recorded
to check for any further transformation to other unknown/new at room temperature. The PL spectra exhibited maxima at
phases. Powder X-ray diffraction studies indicated that compound around 500–590 nm (lex = 372 nm), which could be assigned
Table 1 Synthesis composition employed for the preparation of compounds, I and II
Product
Composition taken during reaction
BL: metalloligand (0.1 mmol) + H O (2 ml) + en (15 mL)
Duration
2 days
Yield (%)
25
(
(
I)
2
ML: H O + EtOH (1 : 1, 2 ml)
TL: Cd(NO O (0.1 mmol) + EtOH (2 ml)
Á4H
2
3
)
2
2
II) Ag
6
/Cu
6
BL: metalloligand (0.1 mmol) + H
ML: H O + EtOH (1 : 1, 2 ml)
TL: Cd(NO O (0.1 mmol) + EtOH (2 ml)
Á4H
2
O (2 ml) + en (15 mL)
7 days
56/46
2
3
)
2
2
2+
The calculated yields are based on Cd . Compositions given are the molar composition (BL = Bottom layer, ML = Middle layer, TL = Top layer).
Elemental analysis: Obsd (Calcd, %) for I: C 23.21 (23.94), H 2.60 (3.01), N 8.03 (8.88), S 8.14 (8.72); II Ag : C 24.08 (24.72), H 2.55 (3.16), N 7.76
8.24), S 8.89 (9.43); II Cu : C 23.98 (24.72), H 2.46 (3.16), N 7.65 (8.24), S 8.76 (9.43).
6
(
6
6
504
|
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to the p*–p transition of the ligand (for Ag). In the case of the taken in a 25 ml round bottomed flask containing 3 ml of CH
2 2
Cl
Cu -cluster compound, two emissions were observed; one at under a nitrogen atmosphere at 0 1C along with 0.05 mmol of
6
4
40–500 nm and the second at 560–600 nm, when excited at compound II (Ag ). After the catalytic studies, the excess solvent
6
4
00 nm (l ). These emissions are due to the p*–p transition of was removed using a rotary evaporator. The products were char-
ex
4
7–50
1
the ligand (Fig. S4, ESI†).
acterized by employing H NMR (Fig. S20, ESI†).
2.7. Single-crystal structure determination
Suitable single crystals were chosen carefully under a polarizing
microscope and mounted on a glass fiber. The X-ray data were
collected on an Oxford Xcalibur (Mova) diffractometer
equipped with an EOS CCD detector. The single-crystal X-ray
3
. Results and discussion
3.1. Structure of [{Ag
6
(2-
mna)
6
Cd(Hen)
2
}{Cd(en)
2
2H
2 2 8
O}(H O) ], I.
data of Mo K (l = 0.71073) were collected. The cell refinement, The structure of I comprises three silver, two cadmium, two
a
data integration and data reduction were accomplished using ethylenediamine, three mercaptonicotinic acid (mna) and five
5
1
the Crystal Pro program. Direct methods were employed for water molecules (Fig. S5, ESI†). Both the Cd atoms have
solving the structure and the structures were refined using octahedral coordination and occupy special positions (1a for
5
2
SHELX-2014 present in the WinGX suit of programs. The Cd1 and 1c for Cd2), with a site multiplicity of 0.5. The Cd(1) is
hydrogen positions were initially found from the difference coordinated by four carboxylate oxygens and two nitrogens
Fourier map and for the final refinement, the hydrogen posi- from two ethylenediamine molecules. Cd(2), on the other hand,
tions were fixed in a geometrically ideal position and refined is coordinated by four nitrogens from two ethylenediamine
employing a riding model. The full-matrix least-squares refine- molecules in a bis-bidendate mode and has two water mole-
2
ment against F was accomplished using the WinGX package cules. Thus, Cd(2) is a non-coordinated metalloligand. The Ag
5
3
program. The structural parameters and refinement details centers are bonded to two sulfur and one nitrogen from three
are listed in Table 2. The details of the bond lengths and bond different 2-mercaptonicotinic acid (mna) molecules forming a
angles are given in Table S2 (ESI†). The final refinements trigonal coordination. Of the six mna ligands available for
included all the atomic positions, anisotropic thermal para- bonding, four carboxylate oxygens bind with the Cd(1) centers
1
1 1 1
meters for all the non-hydrogen atoms, and the isotropic through the Z Z Z Z binding mode (monodendate) and the
thermal parameters for all the hydrogen atoms. CCDC Nos: remaining two carboxylate units are free (Fig. S6, ESI†). The
2
058012 (for I Ag ), 2058013 (for II Ag ) and 2058014 (for II Cu ) connectivity between the mna and Ag units gives rise to an
6 6 6
6
contains the supplementary crystallographic data. This data octahedral cluster arrangement (Fig. 1a). Within the Ag -clusters,
can be obtained free of charge from the Cambridge Crystallo- the AgÁÁÁAg centers are separated by 2.992 Å, which is less than the
+
graphic Data Centre.†
sum of van der Waals radius of Ag ions and suggests possible
argentophilic interactions. It may be noted that supramolecular
argentophilic interactions have been employed successfully for the
2.8. Catalytic studies
54
2
.8.1. Cyanation of imine. Typically 0.5 mmol of N-benzylidine stabilization of lower-dimensional structures, but observing such
aniline and 0.75 mmol of trimethylsilyl cyanide (TMS-CN) were interactions in higher dimensional structures is limited.
33
Table 2 Crystal data and structure refinement parameters for all the synthesized compounds
Structural parameter
I M
6
(M = Ag)
46 Ag Cd
II M
6
(M = Ag)
44 Ag Cd N12
II M
6
(M = Cu)
22 6
44 Cu Cd N12 O S
Empirical formula
C
44
H
6
2
N
14
O
22
S
6
C
42
H
6
O
22
S
6
C
42
H
6
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (1)
b (1)
Triclinic
Triclinic
Triclinic
P1%
P1%
P1%
12.5557(5)
12.7795(7)
12.8750(7)
61.256(6)
72.894(4)
84.428(4)
11.8171(11)
12.2377(10)
12.5376(11)
65.730(4)
81.992(5)
76.182(4)
1603.3(2)
1
11.6027(5)
12.0784(4)
12.2986(5)
64.612(4)
82.183(4)
75.630(4)
g (1)
V (Å )
3
1728.75(18)
1
1507.52(12)
1
Z
T/K
r (calc/g cm
m (mm
120(2)
2.107
2.524
293(2)
2.095
2.395
120(2)
1.935
2.714
À3
)
À1
)
l (Mo K
y range (1)
a
/Å)
0.71073
3.296–24.996
0.0272
0.71073
1.777–24.999
0.0338
0.71073
3.368–30.526
0.0287
R
int
R indexes [I 4 2s (I)]
R indexes (all data)
R
R
1
= 0.034; wR
= 0.037; wR
2
= 0.082
= 0.084
R
R
1
= 0.038; wR
= 0.030; wR
2
= 0.089
= 0.081
R
R
1
= 0.045; wR
= 0.051; wR
2
= 0.107
= 0.113
1
2
1
2
1
2
P
P
P
P
2
2
2 2 1/2
2
2
2
À2
R
1
=
|F
o
| À | F
c
||/ |F
6
o
|; wR
2
= { [w(F
o
À F
c
)]/ [w(F
o
) ]} . w = 1/[r (F
o
) + (aP) + bP]. P = [max (F
o
, O) + 2(F
c
)
]/3 where a = 0.0342 and
b = 7.5092 for I M
(M = Ag), a = 0.0445 and b = 3.6108 for II M
6
(M = Ag) and a = 0.0447 and b = 5.6620 for II M
6
(M = Cu).
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6
À
6À
2+
Fig. 1 (a) The octahedral Ag
6
-cluster with [Ag
6
(mna)
6
]
. (b) The connectivity between [Ag
(2-mna) Cd(Hen)
6
(mna)
6
]
– clusters and Cd ions. Note the connectivity
Á2H O}].
forms a two-dimensional layer. (c) View of the layer arrangement in [{Ag
6
6
2
}{Cd(en)
2
2
The structure of I can be understood from simpler building 3.2. Structure of [{M
units. Thus, the 2-mna ligand binds through the sulfur and nitrogen (M = Cu/Ag), II
6 6 2 2 2
(2-mna) Cd(Hen) }(H en)(H O)10],
6À
atoms with silver centers forming [Ag
Fig. 1b). The free carboxylate units of the cluster units link with
the Cd(1) center forming a two-dimensional layered arrangement
Fig. 1c). As can be noted, two carboxylate units do not participate in
bonding and two of the coordinations of the Cd(1) centers are taken
up by the two terminal en molecules. The connectivity between
6 6
(mna) ] -cluster units
The asymmetric unit of II (Cu) comprises of three Cu, one Cd, three
mna, one and a half amine and five water molecules (Fig. S10, ESI†).
The asymmetric unit of II (Ag), on the other hand, has three Ag, one
Cd, three mna, one and a half amine and five water molecules
(
(
(Fig. S11, ESI†). Both the compounds have octahedral coordination
with Cd, which occupies a special position [1e for II (Cu); and 1a for
II (Ag)] with a site multiplicity of 0.5. In both the compounds, the Cd
centers are coordinated by four carboxylate oxygens and two nitro-
gens from two different en molecules completing the octahedral
arrangement around the Cd. The Cu/Ag centers are connected
trigonally (three-coordinated) by two sulfur and one nitrogen from
three different mna ligands. Of the six mna ligands originating from
6
À
2+
[
Ag
6
(mna)
6
]
-clusters and Cd ions, thus results in a simple 4 Â 4
connectivity, leading to a square lattice. The formation of a square
lattice is not uncommon in inorganic–organic hybrid compounds
and many examples are known in the literature.
55–61
The layers are
arranged in an ABABÁÁÁfashion along the ‘c’ direction (Fig. S9, ESI†).
The bound en molecule hanging from the Cd(1) center and also the
[
Cd(2)(en) (H O) ]-metalloligand are located in the inter-lamellar
2 2 2
the Cu
center in a mono-dentate fashion and two carboxylate remains free
and Ag
-clusters, the CuÁÁÁCu distances
6 6
/Ag octahedral cluster, four are bonded with the Cd(1)
space. The presence of a free carboxylate group, the hanging en
and the Cd-metalloligand along with free water molecules in the
inter-layer spaces generate extensive hydrogen bonding interac- (unbonded). Within the Cu
6
6
tions. The important hydrogen bonding interactions observed are 2.787 Å and the AgÁÁÁAg distances are 3.128 Å, which is less than
+
+
are listed in Table S3 (ESI†).
A topological analysis with the Cd center and [M (6-mna) ]
cluster as two distinct nodes results in a sql-topology.
the sum of the van der Waals radii of Cu and Ag ions
respectively, suggesting considerable cuprophilic/argentophilic
6À
-
6
6
6
2,63
The interactions within the clusters. As mentioned earlier, the argen-
tophilic/cuprophilic interactions are common in supramolecular
4
2
Schl ¨a fli symbol is {4 Á6 }.
6
506
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Fig. 2 (a) View of the single layer. Note that the en molecules are present within the layer. (b) The layer arrangement in II. Note that the layers are
arranged in an AAAAÁ Á Áfashion.
5
4,59–61
structures
bound extended structures is still limited.
The overall crystal structure of II (Cu) and II (Ag) appears to
the number of such examples in covalently
3
3
6
À
be similar. Thus, the [Ag (mna) ] -clusters are connected to
6
6
2+
2
+
four Cd ions and each Cd ions are connected to four
6
À
[Ag
6
(mna)
6
]
-clusters forming an infinite two dimensional Scheme 1 Scheme for the cyanation of imine under a N2 atmosphere.
layered structure. The ethylenediamine molecules have two
different roles. Thus, one en molecule is bound through one
of the nitrogen centers and hangs freely in the interlamellar employing both Cu6 and Ag -clusters. The structure of I was
6
space (Fig. 2). The other en molecule, fully protonated, occupies isolated with Ag -metalloligand clusters only. In spite of our best
6
the pore opening within the layer. Thus, the en molecules efforts, we have not been able to isolate the Cu -compound with
6
perform two different roles: (a) acting as a ligand by binding the same crystal structure as I. It may be noted that both I and II
to the Cd centers and (b) as a charge compensating space filler were prepared from the same reaction mixture and under the
within the layers. Use of aliphatic amine molecules in the same experimental conditions, but by changing the reaction time
synthesis of framework compounds is well established in the (Fig. 3a). The reaction also appears to be temperature dependent
6
4,65
literature.
The amine molecules in such compounds mainly (Fig. 3b). On the other hand, compound II M (M = Cu) was
6
act as a structure-directing agent, however, dual roles of the isolated from the Cu -cluster (Fig. 4).
6
6
6
amine have also been observed.
Preparation of more than one compound from the same
In the present case, the en molecules perform the role of a reaction mixture is known.67,68 Generally, under such condi-
ligand as well as a space-filling and charge-compensating tions, either the temperature of the reaction (thermodynamic)
agent, which appears to be new in this class of compounds. or the time of the reaction (kinetic) is altered to realize
The layers are stacked one over the other in an AAAAÁ Á Áfashion individual phases (single phase compounds). It may be noted
(
Fig. S13, ESI†).
that the roles of thermodynamic and kinetic factors in the
Similar to compound I, compound II also has a sql net with synthesis of compounds under different reaction conditions
4
2
62,63
69–73
the same Schl ¨a fli symbol {4 Á6 }.
.3. Discussion
The heterometallic compounds I and II were prepared employing time appears to have an effect on the preparation of compounds
have been discussed much in the literature,
including
7
4
Ostwald ripening.
The present study indicates that the reaction temperature and
3
6
6
a three-layered approach at varying temperatures and times. The involving Ag -clusters. Similar effects are not observed with Cu -
en molecules actively participated in the reaction by bonding with clusters, as compound II is only phase isolated irrespective of the
the metal ion, Cd, in all the cases. Thus, en was useful not only to time and temperature of the reaction. The formation of I was
solubilize the [M S ]-clusters but also for the deprotonation of the facilitated by the formation of the [Cd(en) Á2H O]-metalloligand,
6
6
2
2
–
COOH group. The deprotonation would prompt the bonding which on prolonged heating decomposed releasing the en molecule.
À
2+
between the –COO and Cd ions. This synthetic approach The free en molecule was located within the apertures in II. It is
resulted in two layered compounds, which appear to be closely likely that the differences in the reactivity between Ag [4d] and
related. In addition, the structure of compound II was realized Cu [3d] -clusters would have resulted in an isolable intermediate
6
6
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Fig. 3 PXRD patterns of the products of the reaction (a) as a function of reaction time and (b) as a function of reaction temperature. Note the formation
of II between 60 and 72 h of the reaction.
Fig. 4 PXRD patterns of the product of the reaction (a) as a function of reaction time and (b) as a function of reaction temperature for the Cu
Note that irrespective of the reaction time, II is the only product. *-Copper oxide (ICSD No.: 89236).
6
-cluster.
with the former. It is difficult to fully rationalize the formation of to the loss of the two coordinated water molecules, four ethyle-
every compound in a multi-component reaction mixture, as nediamine along with the decomposition of the mna ligand. For
many such reactions have not been fully understood.
compound II (M = Ag), the initial weight loss of around 6% in the
temperature range of 45–90 1C corresponds to the loss of ten
water molecules (calcd 8.8%). A continuous weight loss of
3.4. Thermogravimetric analysis
Thermogravimetric analysis (TGA) of the synthesized compounds was B58.9% in the temperature range of 180–850 1C could be due
À1
carried out in the range of 30–1000 1C (heating rate = 5 1C min
)
to the loss of all ethylenediamine along with the decomposition of
À1
under an oxygen atmosphere (flow rate = 50 ml min ) (Fig. 5). the mna ligand. For compound II (M = Cu), the initial weight loss
For compound I, an initial weight loss of around 5.7% in the of around 7% was observed in the temperature range of 45–115 1C
temperature range of 40–85 1C could be due to the loss of eight possibly due to the loss of ten water molecules (calcd 10%). The
water molecules (calcd 6.6%). A continuous weight loss of second and continuous weight loss in the range of 190–900 1C
B49.2% in the temperature range of 160–920 1C corresponds (57.5%) may correspond to the loss of all the en molecules along
6
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6 6
Fig. 5 TGA plot of I (black), II M (M = Ag) (red) and II M (M = Cu) (blue).
Fig. 6 Plausible mechanism for the cyanation of imine.
with the decomposition of the mna ligand (Table S5, ESI†). The
final calcined products in the case of Ag-containing compounds
have been tabulated in Table 3. The yields of 50% (for I Ag
6
), 90%
(
for II Ag ) and 95% (for II Cu ) have been obtained. When the
6
6
(I and II) were found to be Ag and CdO (ICSD – 44387 and 24802).
catalytic studies were carried out without the catalyst and also
with CdSO /Cd(NO , the observed yields were found to be
The calcined product for the Cu-containing one was found to be
a mixture of CuO and CdO (ICSD – 16025 and 24802, Fig. S14,
ESI†).
From the TGA studies, it appears that the extra-framework
water molecules are removed under 100 1C. We wanted to
explore the stability as well as the crystallinity of the com-
pounds after the lattice water molecules are removed. To this
end, we have heated the Ag-containing compound at up to
4
3 2
)
significantly less (10% and 26%/28%, respectively). We also
carried out the hot filtration study (removing the catalyst from
the reaction mixture) which indicated that there is no significant
increase in the yield of the product (Fig. S17, ESI†). This suggests
that the reaction proceeds catalytically. The recyclability of the
catalysts was attempted for up to three cycles, which indicated
comparable activity (Fig. S18, ESI†). The PXRD pattern after the
catalytic studies reveals that the compounds appear to lose some
crystallinity (Fig. S19, ESI†). From the catalytic studies it is clear
that the overall activity of I is lower compared to II. It is likely
that the aliphatic amino molecules are more accessible in II,
which enhances the catalytic yield (Fig. 6).
353 K temperature in an oven. The PXRD pattern recorded for
the dehydrated sample indicated that both the Ag-containing
compounds, I and II, retained the crystallinity as well as the
structure (Fig. S15, ESI†). We have also carried out in situ IR
studies to follow the dehydration, which also indicated the loss
À1
of water bonds in the range 3500–3200 cm . (Fig. S16, ESI†).
These studies confirm that the loss of lattice water molecules
does not affect the overall crystal structure of the compounds.
3.6. Proton conductivity
The prepared compounds contain extra-framework water molecules
in the inter-lamellar space and participate in H-bonding interactions
3.5. Catalysis
with the H en (Table S3, ESI†). We explored the possible proton
2
3
.5.1. Cyanation of imine. The prepared compounds possess conduction in these compounds. A suitable pellet was prepared and
+
3
octahedrally coordinated Cd and ammonium ions (–NH ) as impedance measurements were carried out at 98% humidity at
Lewis acidic centres, which opens up the possibility to employ room temperature. The measurements were carried out in the
6
these compounds as a catalyst in Lewis acid catalyzed reactions. frequency range of 0.1–10 Hz with a signal amplitude of 0.12 V
Thus, we have carried out the cyanation of imine to explore the
Lewis acidic nature of the prepared compounds. Similar reactions
Table 3 The optimized reaction time and yields from the cyanation of
have been carried out to establish the Lewis acidic characters in
imine employing the prepared compounds
75–78
MOFs.
In this study, trimethylsilyl cyanide (TMS-CN) was
employed as a source of cyanide group for the cyanation of imine Entry
Catalyst
Reaction time (h)
Yield (%)
which leads to the formation of cyanohydrin which is an inter-
mediate in the synthesis of many organic compounds (Scheme 1).
To carry out the catalytic study of cyanation of imine
employing the prepared compounds, we have followed the
1
2
3
4
5
6
Compound I Ag
6
20
16
8
16
16
16
50
90
95
26
28
Trace
Compound II Ag
Compound II Cu
Cd(NO
Á4H
CdSO
Á8H
No catalyst
6
6
2
)
3
2
O
4
2
O
7
9
protocols employed previously. The reaction time and yields
were optimized (Table S6, ESI†). The observed yields of the
Reaction conditions: N-benzylideneaniline (0.5 mmol), TMS-CN (0.75 mmol),
Cl at 0 1C under a N atmosphere.
6 6 6
reaction for I (Ag ) and II (Ag /Cu ) under the reaction conditions and catalyst (0.05 mmol) in CH
2
2
2
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À5
(
O
Alpha, Novocontrol). A proton conductivity value of 3.7 Â 10
8 L. Liu, Z. P. Tao, H. R. Chi, B. Wang, S. M. Wang and
Z. B. Han, Dalton Trans., 2021, 50, 39–58.
À1
À1
À4
À1
À1
cm and 1.1 Â 10
O
cm was observed from the
Nyquist plots of compound I and compound II, respectively (Fig.
S21, ESI†). These observed proton conductivity values are com-
9 W. L. Xue, W. H. Deng, H. Chen, R. H. Liu, J. M. Taylor, Y. Li,
L. Wang, Y. H. Deng, W. H. Li, Y. Y. Wen, G. E. Wang, C. Q. Wan
and G. Xu, Angew. Chem., Int. Ed., 2021, 60, 1290–1297.
0 D. W. Lim and H. Kitagawa, Chem. Rev., 2020, 120,
8
0–85
parable to those reported earlier.
1
1
1
1
1
1
1
1
1
8
416–8467.
1 A. E. Thorarinsdottir and T. D. Harris, Chem. Rev., 2020, 120,
716–8789.
2 V. R. Gimenez, S. Tatay and C. M. Gastaldo, Chem. Soc. Rev.,
020, 49, 5601–5638.
Conclusions
8
The synthesis, structure and characterization of three new layered
inorganic–organic hybrid compounds, [{Ag (2-mna) Cd(Hen) }-
Cd(en) 2H O}(H O) ], I, and [{M (2-mna) Cd(Hen) }(H en)(H O)10],
6
6
2
2
{
2
2
2
8
6
6
2
2
2
3 G. Ren, Z. Li, M. Li, Z. Liang, W. Yang, P. Qiu, Q. Pan and
G. Zhu, Inorg. Chem. Commun., 2018, 91, 108–111.
II, (M = Cu and Ag) have been accomplished. The use of ethylene-
diamine in the reaction mixture gave rise to new structures with
hanging amine groups. It is likely that the use of secondary amine
molecules, not only helps in deprotonation of the carboxylic acid
group of the [{M (2-Hmna) }]-clusters, but also gives rise to inter-
esting new structures. The accessibility of protonated amine mole-
cules in the structure of II promotes the facile Lewis acid catalysis
of cyanation of imine molecules under mild conditions with a high
yield. The present study establishes that the aliphatic amine
molecules may be useful not only for stabilizing new structures,
but also for Lewis acid catalysis. This promising variation is being
pursued vigorously now.
4 Y. Zhang, Y. Wang, L. Liu, N. Wei, M.-L. Gao, D. Zhao and
Z.-B. Han, Inorg. Chem., 2018, 57, 2193–2198.
5 D. Li, X. Yang and D. Yan, ACS Appl. Mater. Interfaces, 2018,
6
6
10, 34377–34384.
6 D. Wang, J. Zhang, G. Li, J. Yuan, J. Li, Q. Huo and Y. Liu,
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8 L. M. R. Albelo, A. R. R. Salvador, D. W. Lewis, A. Gomez,
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1
9 L. H. Wee, C. Wiktor, S. Turner, W. W. Vanderlinden,
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Conflicts of interest
There are no conflicts to declare.
2
2
2
2
2
2
0 X. Wang, Z. Chang, H. Liu, A. Tian, G. Liu and J. Zhang,
Dalton Trans., 2014, 43, 12272–12278.
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Acknowledgements
S. N. thanks the Science and Engineering Research Board (SERB),
Government of India, for the award of a J. C. Bose National
Fellowship. The authors also thank the Council of Scientific and
Industrial Research (CSIR), Government of India, for the award of
a research grant (SN) and a research fellowship (AS and AKJ).
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