A metalloligand appended with benzimidazole rings: Tetranuclear [CoZn3] and [CoCd3] complexes and their catalytic applications

Article abstract of DOI:10.1039/c8nj00734a

A novel Co³⁺-based metalloligand 1 offering appended benzimidazole rings has been prepared and utilized for the synthesis of tetranuclear [CoZn₃] (2 and 2-Cl) and [CoCd₃] (3) heterometallic coordination complexes (HCCs). C

Full text of DOI:10.1039/c8nj00734a

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE
A metalloligand appended with benzimidazole rings:
Tetranuclear [CoZn ] and [CoCd ] complexes and their
3
3
catalytic applications
a
a
a,
Saurabh Pandey, Deepak Bansal and Rajeev Gupta *
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
3+
A novel Co ꢀbased metalloligand
1
offering appended benzimidazole rings has been prepared
and 2ꢀCl) and [CoCd ] (3)
and utilized for the synthesis of tetranuclear [CoZn ] (
2
3
3
DOI: 10.1039/x0xx00000x
heterometallic coordination complexes (HCCs). Crystallographic studies of 2ꢀCl and 3
2
+
2+
illustrate coordination of three secondary metal ions (Zn /Cd ) to the appended
benzimidazole rings of thus producing tetranuclear HCCs. Both HCCs were utilized as the
www.rsc.org/
1
heterogeneous catalysts for Knoevenagel condensation and Henry reactions. The stability and
recyclability experiments illustrate stable nature of both HCCs during the catalysis.
system; size, charge, and coordination environment of a catalytic
Introduction
2
4,25
metal are critical parameters in dictating the catalysis outcome.
On the other hand, both stability and robustness of a catalyst remain
Heterometallic coordination complexes (HCCs) including
heterometallic coordination polymers (HCPs) have always attracted
scientific community due to their intriguing architectures and the
presence of at least two different metals in close proximity. The
persistent development in this field is due to their potential
24,25
3+
important criteria.
In this work, we report a new Co ꢀbased
benzimidazoleꢀappended metalloligand with its unique ability to
coordinate three secondary metals ion to afford tetranuclear HCCs.
The resultant tetranuclear [CoZn ] and [CoCd ] HCCs have been
1ꢀ3
3
3
4
5
6
applications as/in molecular magnetism, luminescence, sensing,
utilized as the heterogeneous catalysts for Knoevenagel
condensation and nitroꢀAldol (Henry) reactions.
7
8
9
optics, redoxꢀproperties and catalysis. HCCs have been mostly
developed using compartmental ligands offering selective binding
1
0
groups for coordinating two different metal ions. In such an
approach, it is important to recognize both electronic and
geometrical requirements of two different metals which make the
Results and Discussion
1
1
Synthesis and characterisation of metalloligand 1
synthesis of HCCs challenging and thus restricting their growth.
The alternate route is to utilize a metalloligand which already
contains a primary metal ion whereas its appended functional
groups could be employed to coordinate a secondary metal ion.
Synthesis of metalloligand 1 involves reaction of ligand H L in its
2
2ꢀ
deprotonated form, L , with Co(II) salt followed by aerial oxidation
12ꢀ14
23a,23b,25b
(Scheme 1).
In FTIR spectrum, both absence of amidic –
The metalloligand approach provides several advantages over the
conventional use of compartmental ligands. The prominent ones are
NH stretches and bathochromically shifted νC=O stretches confirm
coordination via deprotonated Namidate groups (Figure S1,
1
5
23a,23b,25b
ꢀ1
the ease of selection of secondary metal ions and a precise control
ESI).
The stretches at ca. 3315 cm correspond to –NH
1
6
26 1
over the dimensionality of the resultant materials. Therefore,
extensive research has been devoted to develop assorted
groups of the appended benzimidazole rings. H NMR spectrum
of 1 shows a doublet and a triplet at 8.05 and 8.46 ppm
corresponding to pyridyl protons whereas benzimidazole ring
protons were observed between 6.77 – 7.14 ppm (Figure S2, ESI).
The benzimidazole –NH protons were observed at 11.67 ppm. UVꢀ
vis spectrum of 1 in N,Nꢀdimethylformamide (DMF) exhibits a
broad band at ca. 680 nm corresponding to dꢀd transition (Figure
1
7ꢀ20
metalloligands for the synthesis of versatile HCCs.
However, a
critical challenge is to stop at the HCCs level and not allow a
metalloligand to go all the way to produce HCPs. In this context,
our research group has developed a variety of metalloligands for the
2
1ꢀ25
subsequent synthesis of both HCCs and HCPs.
Over the course
2
3a,23b,25b,25c
of metalloligand design, we realized that both HCCs and HCPs with
accessible metal sites are excellent candidates for various catalytic
S3, ESI).
Conductivity measurement confirms a 1:1
27
electrolytic nature of this complex.
24,25
application.
It is wellꢀestablished that for an efficient catalytic
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(a)
Scheme 1. Preparation route for the synthesis of metalloligand 1.
Metalloligand 1 was crystallographically characterised and
displays a Co(III) ion coordinated by four elongated Namidate and two
compressed Npyridine bonds (Figure 1a). The Co(III) ion displays an
octahedral environment in a bisꢀchelated ligand environment. Such
a coordination mode has been observed in our earlier metalloligands
appended with pyridyl, arylcarboxylate, thiazole and thiazoline
22,25
groups.
In 1, four Namidate groups constitute a basal plane while
two Npyridyl atoms occupy the axial positions. The CoꢀNamidate bond
(b)
21–
distances were longer than that of CoꢀN
The unit cell of 1 displays the presence of two metalloligands
connected together via two sodium ions (Figure 1b). Every Na ion
is coordinated to benzimidazoleꢀN11 and amideꢀO4 atoms from one
metalloligand and benzimidazoleꢀN12 of a second one while the
fifth coordination site is occupied by a methanol molecule. The
as observed earlier.
pyridyl
2
5
+
+
involvement of benzimidazoleꢀN coordination to Na ions suggests
the potential of metalloligand 1 in coordinating other secondary
metal ions. Thus, reaction of 1 was performed with suitable
secondary metal salts.
Heterometallic coordination complexes 2 and 3
Figure 1. (a) Thermal ellipsoidal representation of complex 1 where
ellipsoids are drawn at 50% probability level. Only selected
hydrogen atoms are shown whereas sodium ion and lattice solvent
molecules are omitted for clarity. (b) Capped and stick model
2
+
2+
Metalloligand 1 on reaction with acetate salts of Zn and Cd in
MeOH produced tetranuclear HCCs [CoZn ] (2) and [CoCd ] (3),
3
3
respectively (Scheme 2). FTIR spectra of 2 and 3 exhibited ν
C=Oamide
+
ꢀ
1
showing dimeric structure of 1 after association involving Na ions.
stretches at ca. 1560 and 1546 cm , respectively (Figure S4, ESI)
ꢀ
1
that were blueꢀshifted (25–35 cm ) when compared to 1. Superꢀ
imposable FTIR spectra of 2 and 3 suggest their isostructural nature.
The diffusedꢀreflectance absorption spectra of both HCCs exhibited
λmax at ca. 660 nm accompanied with a shoulder at ca. 500 nm
(Figures S5 and S6, ESI). Notably, absorption spectra of 2 and 3
closely match to that of 1 thus suggesting that coordination of
secondary metal ions has not dramatically influenced the electronic
3
+
environment of the Co ion. Xꢀray powder diffraction patterns of
both HCCs 2 and 3 exhibited close match with the simulated
patterns obtained from the single crystal diffraction data of 3
Scheme 2. Preparative route for the synthesis of heterometallic
coordination complexes (HCCs) 2, 2ꢀCl and 3.
Crystal structures of HCCs 2ꢀCl and 3
(Figures S7 and S8, ESI). Such a fact infers the structural
similarities of both HCCs 2 and 3, particularly in absence of the
crystal structure of 2. Both 2 and 3 were further investigated by
thermogravimetric analysis (TGA) to understand the nature of
coordinated and/or lattice solvent molecules (Figures S9, ESI).
TGA for 2 and 3, within the temperature range of 50 ꢀ 375 °C,
exhibited weight changes of 19.28% (calc. 19.91%) and 25.85%
We next attempted to crystallize HCCs 2 and 3 to understand the
2
+
coordination behaviour of metalloligand 1 towards secondary Zn
2+
and Cd ions. Importantly, while 3 was successfully crystallized
from DMSOꢀEtOAc, single crystals of 2 suitable for Xꢀray
diffraction could not be obtained. However, addition of a few drops
of 2N HCl to a solution of 2 in DMF resulted in the crystallization
of a new compound, 2ꢀCl. Gratifyingly, crystal structures of both
HCCs, [CoZn ] (2ꢀCl) and [CoCd ] (3), only differed by the type of
(calc. 26.29%), respectively; corresponding to the loss of two
DMSO, three acetate and two water molecules. A close similarity
between the two samples further establishes a nearly identical
structure and chemical composition of both HCCs.
3
3
anion and solvent molecules coordinated to the secondary metal
ions. Notably, metalloligand 1 presents four benzimidazole rings to
coordinate secondary metal ions. The crystal structures of both
2
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…
HCCs (2ꢀCl and 3) exhibited the involvement of all four appended
benzimidazole rings show N N separation of 3.407–3.469 Å
whereas uncoordinated nitrogen atoms were separated by 5.779 Å
(Figure S10, SI). Nevertheless, both HCCs present an unusual
example of incorporating three secondary metal ions to a single
metalloligand.
benzimidazole rings in coordination to the secondary metal ions. As
3+
observed in the structure of 1, both 2ꢀCl and 3 displayed Co ion in
3
+
a N coordination environment. Notably, Co –N core in HCCs 2ꢀ
6
6
Cl and 3 largely remained unaffected by the coordination of
secondary metal ions as was observed with our earlier
2
1,22
metalloligands (Table 1).
(a)
Both HCCs, 2ꢀCl and 3, were isostructural except coordinated
anions and solvent molecules. The crystal structure of 2ꢀCl exhibits
2
+
the presence of one metalloligand, three secondary Zn ions, three
coordinated chloride ions, one coordinated water and two
coordinated DMF molecules. It is important to mention that the
positive charge from three divalent Zn(II) ions is balanced by the
presence of three chloride ions, one monoꢀanionic metalloligand
and two deprotonated appended benzimidazole rings. Importantly,
2+
all appended benzimidazole rings are coordinated to three Zn ions.
For example, each Zn(II) ion is connected by two converging
benzimidazole rings in addition to a chloride anion and a solvent
molecule. The τ values for three Zn(II) ions were 0.88 (Zn1), 0.90
4
(
Zn2) and 0.93 (Zn3) thus suggesting a predominantly tetrahedral
28
geometry. The remaining two benzimidazoleꢀN atoms (N6 and
N14) remain uncoordinated and are present in protonated form.
(b)
The crystal structure of 3 reveals the presence of one
2
+
metalloligand, three Cd ions, three acetate ions, two coordinated
DMSO and two coordinated H O molecules. The charge due to
2
2+
three Cd ions is balanced by three acetate ions, a monoꢀanionic
metalloligand and two deprotonated benzimidazole rings. Four
appended benzimidazole rings provide eight nitrogen atoms, out of
which six nitrogen atoms (N4, N5, N7, N11, N12 and N13)
coordinate with three Cd(II) ion(s) whereas the remaining two
uncoordinated benzimidazole rings are present in the protonated
2
+
form as was also noted in 2ꢀCl. The acetate ions coordinate to Cd
2
9
ions in a bidentate manner. Among three cadmium ions, Cd1 and
Cd3 exhibit fiveꢀcoordinated distorted squareꢀpyramidal geometry
with two Nbenzimidazole and two Oacetate atoms creating a basal plane
and an axial DMSO molecule. The τ values for Cd1 and Cd3 atoms
5
Figure 2. Thermal ellipsoidal representations of HCCs (a) 2ꢀCl and
b) 3. Thermal ellipsoids are drawn at 50% probability level
were found to be 0.04 and 0.13, respectively therefore suggesting a
(
3
0
predominantly squareꢀpyramidal geometry. The geometry around
the third cadmium ion (Cd2) can be best described as the distorted
octahedral where two Nbenzimidazole and two Oacetate atoms comprise
the basal plane whereas two water molecules are axially
coordinated.
whereas only selected hydrogen atoms are shown and lattice solvent
molecules are omitted for clarity.
Cd3
A closer investigation of both crystal structures revealed as to
why only three and not four secondary metals are coordinated to
metalloligand 1. Interestingly, in both cases, coordination of three
secondary metals resulted in a constrained geometrical alignment of
the appended benzimidazole rings. As shown for HCC 3 in Figure
N4
N11
3.614
N5
Cd2
Cd1
3
, benzimidazole rings coordinated to a secondary metal exhibited
3.682
N13
Co1
…
N N separation in the range of 3.660–3.682 Å. In contrast,
uncoordinated benzimidazole rings display a much larger separation
of their nitrogen atoms (N6…N14: 5.324 Å). Consequently, such an
N7
N14
…
enhanced N N separation did not result in enough convergence of
N6
two benzimidazole rings and as a result did not support coordination
to a fourth secondary metal ion. Similarly, in 2ꢀCl, Znꢀcoordinated
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Figure 3. A truncated structure of HCC 3 showing various bond
distances between Cd(II) ions and separations between two
neighbouring nitrogen atoms. The organic framework around the
Cd(II) ions and coordinated solvents/ anions are removed for
clarity.
Both 2 and 3 further supported the reaction of malononitrile
with different substituted aldehydes to afford the respective
products. Interestingly, electronic and steric properties of the
substituted aldehydes influenced the product yield to some
25b,25c
extent.
For example, while electronꢀwithdrawing substituents
on benzaldehyde (entry 2) increased the product yield; electronꢀ
donating ones (entries 3 and 4) resulted in lower product formation.
Moreover, reaction with bulkier substrates, such as 1ꢀ
naphthaldehyde, 2ꢀnapthyldehyde and 9ꢀanthraldehyde, resulted in
reduced yield as compared to benzaldehyde (entries 5ꢀ7). Such a
fact suggests that bulkier substrates have restricted access towards
Table 1. Selected bond distances (Å) and angles (˚) for complexes
1
, 2ꢀCl and 3.
Bond
1
1.953
1.859
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
2ꢀCl
3
Avg. CoꢀNamide
1.953
1.867
1.960
1.872
Avg. CoꢀNpyridyl
M(1)ꢀN(5)
M(1)ꢀN(13)
M(2)ꢀN(7)
M(2)ꢀN(11)
M(3)ꢀN(4)
M(3)ꢀN(12)
1.999(4)
2.010(4)
2.010(4)
1.977(4)
2.002(4)
2.002(4)
2.244(5)
2.213(5)
2.377(4)
2.262(5)
2.181(5)
2.252(5)
the catalytic secondary metal sites decorated on
a
25b,25c
metalloligand.
Importantly, as shown in Table 2, product
yields were better for HCC 3 when compared to 2. We tentatively
propose that such an observation is primarily related to the better
accessibility of a substrate to the catalytic Cd(II) ion due to its
2+
2+
Where M = Zn (for 2ꢀCl) and Cd (for 3).
25b,25c,36
larger size.
Heterogeneous catalysis
Lewis acidic metals have been subjected to detailed investigations
Table 2. Knoevenagel condensation reactions (KCRs) of assorted
aldehydes with malononitrile using HCCs 2 and 3 as the catalysts.
31
due to their role in promoting numerous organic transformations.
Conventional Lewis acid catalysts are often based on main group as
3
1
well as early transition metals; however, late dꢀblock metals such
9,32
as zinc and cadmium have been particularly noteworthy.
b
%
Yield
Although various Znꢀ and Cdꢀbased homogeneous catalysts have
been developed but the quest for the recyclable and reusable
a
Entry
Ar
2
3
9,32
heterogeneous catalysts has continued. Crystal structures of both
HCCs illustrate the presence of three exposed Lewis acidic metals
c
d
c
d
1
2
3
4
5
6
7
C H
79, 77 , 76
>99, 98 , 96
6
5
2
+
2+
p
ꢀNO
ꢀOCH
ꢀClC H5
2
C
6
H
5
87
74
82
73
70
65
97
94
88
85
80
(
Zn and Cd ) and weakly coordinated solvent molecules. These
structural features prompted us to utilize both HCCs as the catalysts
p
C H
3 6 5
9
for the Lewis acid assisted catalytic reactions. In Lewis acid
p
6
catalyzed reactions, typically activation of a substrate is a
prerequisite and plays a critical role before its transformation into a
1ꢀNaphthyl
2ꢀNaphthyl
9ꢀAnthryl
21a,21b
product.
The presence of weakly coordinated solvents to the
Lewis acidic metals suggests that they could be easily removed
72
2
a,25b
and/or replaced to facilitate the access of a substrate.
In order to
a
b
Conditions: 0.5 mol% catalyst; 75 min, 25
±
1 °C. Yield was
avoid competition from the solvent molecule(s), we planned to
utilize both HCCs as the heterogeneous catalysts. To investigate
potential catalytic applications of both HCCs, Knoevenagel
c
determined by GC. Isolated yield for the second run with reused
catalyst. Isolated yield for the third run with reused catalyst.
d
33,34
33
condensation reactions (KCRs)
and Henry reactions (HRs)
Motivated with successful KCRs, we next targeted nitro–aldol
Henry reactions (HRs). HR is a simple CꢀC bond forming reaction
were selected as both these reactions are known to be catalyzed by
35
2
5b–25d,34,35
between aldehydes or ketones with niroalkanes It is important to
Lewis acidic metals.
Notably, both these reactions are
37
note that there are limited reports on solventꢀfree HRs. Majority of
the reports require presence of a protic solvent such as MeOH or
EtOH. This suggests possible involvement of a protic solvent during
good yet facile synthetic tools for the construction of CꢀC
34,35
bonds.
38
the reaction. However, with the intention to exclude any solvent
participation and potential advantage of heterogeneous catalysis; all
reactions were performed in absence of any solvent. Interestingly,
best conversion was obtained when 4ꢀnitrobenzaldehyde was treated
with nitromethane in presence of only 1 mol% of HCC 3 that
afforded the respective βꢀnitroalkanol in 92 % yield (entry 3, Table
3). Therefore, optimization experiments were performed by the
reaction of 4ꢀnitrobenzaldehyde with nitromethane varying catalyst,
temperature, solvent and time (Table 3). The catalytic results
substantiate the importance of HCCs in HRs as no product was
formed when metalloligand 1 was used as a catalyst. Similarly, no
product formation was noticed with either nitrate or chloride salts of
In a typical KCR, benzaldehyde reacted smoothly with
malononitrile in presence of only 0.5 mol% of catalyst under the
solventꢀfree conditions to give 2ꢀbenzylideneꢀmalononitrile as the
25b–25d
only product.
The completion of the reaction at room
temperature (25±1 °C) that too within 75 min illustrates the
noteworthy catalytic performance of both HCCs (Table 2). In order
to confirm that HCCs indeed drive KCRs, catalytic reactions were
also performed using metalloligand 1 as a catalyst. As expected,
metalloligand 1 produced negligible product (<5%) for a reaction
between benzaldehyde and malononitrile under the otherwise
identical reaction conditions.
2+
2+
Zn or Cd as the catalysts (entries 5–8). However, utilization of
4
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Zn(OAc) or Cd(OAc) as the catalysts resulted in 8ꢀ10 % product
2
2
formation (entries 9 and 10). Solvent screening experiments
exhibited inferior catalytic results when compared to that of solventꢀ
free reactions although use of MeOH still produced the product in
%
Yield
9
0% yield while other solvents were mostly moderately effective
a
b
b
Entry
R
1
2
3
(
entries 11 ꢀ 14). These finding suggest that the presence of a protic
3
8
solvent indeed plays an important role in HRs whereas solventꢀ
free conditions mimic the significant role played by a protic solvent.
We propose that the Lewis acidic metals in 2 and 3 better activate
both substrate and nitromethane in absence of a solvent (vide infra)
and help in driving the reactions.
1
47
86
51
2
3
92
87
Table 3. Control and optimization experiments for Henry reactions.
78
4
5
70
40
79
48
a
b
Entry
Catalyst
Solvent
ꢀꢀꢀ
% Yield
1
2
3
4
5
6
7
8
9
1
2
0
74
92
68
0
ꢀꢀꢀ
3
ꢀꢀꢀ
6
7
56
45
61
47
2ꢀCl
ꢀꢀꢀ
ZnCl
CdCl
Zn(NO
Cd(NO
Zn(OAc)
2
.2H
.2H
.6H
.6H
.2H
.2H
2
O
ꢀꢀꢀ
2
2
O
ꢀꢀꢀ
0
3
)
2
2
O
ꢀꢀꢀ
0
b
Conditions: 1.0 mol% catalyst; 36 h, 60°C. Isolated yield.
3
)
2
2
O
ꢀꢀꢀ
0
2
2
O
ꢀꢀꢀ
8
As mentioned earlier, activation of a substrate is an
important step during the catalysis. We believe that in both
KCRs and HRs, activation of an aldehydic substrate takes
1
1
0
1
Cd(OAc)
2
2
O
ꢀꢀꢀ
10
90
3
CH
tert
BuOH
3
OH
2
5b,25c
place prior to the nucleophilic attack.
postulate, both HCCs and were suspended in CH Cl
containing benzaldehyde. Such impregnated samples exhibited
bathochromically shifted C=O stretches at 1693 ( ) and 1694
) cm , respectively (Figures S11 and S12, ESI) and were
To prove such a
ꢀ
1
2
3
70
2
3
2
2
1
1
3
4
3
3
2
H O
42
30
2
ꢀ
1
3
CH CN
(3
ꢀ
1
shifted by ca. 15 cm when compared to neat C H CHO (1708
6
5
a
b
Conditions: 1.0 mol% catalyst; 36 h, 60 °C. Isolated yield.
ꢀ1 23c
cm ). Such an observation suggests potential coordination
2
5b–25d
of benzaldehyde and therefore its activation.
tested activation of nitromethane used in HRs with both HCCs.
Similar impregnation experiments performed using
nitromethane exhibited the appearance of νN=O band at 1540
We also
After optimization of reaction conditions, we investigated the
influence of electronic substituents on an aldehyde on the product
yield (Table 4). For example, benzaldehydes substituted with
electronꢀwithdrawing nitro group (entries 2ꢀ4) exhibited better
product yield as compared to benzaldehyde (entry 1). However,
product yield was reduced when benzaldehyde was substituted with
electronꢀdonating groups (entries 5 and 6). Finally, use of sterically
bulkier naphthaldehyde considerably reduced the product yield
ꢀ
1
ꢀ1
cm in
2 and 1524 cm in 3 (Figures S13 and S14, ESI). Such
ν
N=O bands were also redꢀshifted when compared to only
ꢀ
1 39
nitromethane (1555 cm ). Both these experiments suggest
that benzaldehyde as well as nitromethane have potentially
coordinated the secondary metals after replacing the loosely
bound solvents and/or anion.
(
entry 7). Such an observation supports KCRs where product yield
depended on the steric bulkiness of a substrate that has most
probably restricted the approach of a substrate towards the Lewis
acidic secondary metals.
Reusability and recyclability experiments
To establish the true nature of heterogeneous catalysis,
reactions between benzaldehyde and malononitrile was
Table 4. Henry reactions of various aldehydes with nitromethane
using HCCs 2 and 3 as the catalysts.
monitored using HCC
3 as a model catalyst. As can be seen
from Figure 4, product formation ceased on removal of the
catalyst
3
(red spheres) and resumes after its addition. This
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simple filtration experiment suggests that HCC
3 acts as a
heterogeneous catalyst and other catalytically active species
2
5
have not been leached out. A simple filtration allowed easy
recovery of both HCCs from the reaction mixture and their
consecutive use for five cycles for a reaction between
benzaldehyde and malononitrile (inset, Figure 4). During these
cycles, both HCCs retained their catalytic ability without
considerable decrease in the product yield. It is noteworthy to
mention that no activation or purification was performed
before reusing both HCCs.
2θ
Figure 5. SEM micrographs (left panels) and XRPD patterns (right
panels) of HCC 3; (bottom) as synthesised, (middle) before
catalysis and (top) after catalysis for the reaction between
benzaldehyde and malononitrile.
Experimental Section
General
All reagents were purchased from the commercial sources and used
without further purification. The solvents were dried and purified
40
using standard procedures. The ligand H L was synthesized as per
our earlier report.
2
Time (min)
Figure 4. Filtration test during KCR between benzaldehyde
26a
and malononitrile in presence of HCC
3 as a catalyst displays
Na[Co(L )CH OH]·2CH OH (1). Ligand H L (0.5 g, 1.258 mmol)
2
3
3
2
that removal of after 30 min lead to the cessation of catalysis
3
was dissolved in dinitrogenꢀflushed DMF (20 mL) and to it was
added solid NaH (0.060 g, 2.516 mmol), resulting in a pale yellow
solution. To the aforementioned mixture, solid [Co(H O) ](ClO )
4 2
whereas reꢀaddition after ca. 60 min results in the resumption
of the catalytic reaction. The inset displays recyclability of
2
6
HCC
3 for four consecutive cycles.
(0.230 g, 0.629 mmol) was added under dinitrogen atmosphere. The
resulting solution was stirred for 2 h in open air during which colour
changed to brownishꢀgreen. Removal of the solvent was followed
In order to prove structural integrity and crystallinity of
both HCCs during the Henry reaction of benzaldehyde with by the addition of 25 mL of MeOH and filtration. Diethyl ether
nitromethane; FTIR, XRPD and SEM studies were performed. vapours were diffused through the methanol filtrate that afforded
crystalline compound. The solid was collected by filtration and
dried under vacuum. Yield: 0.8 g (66 %). C H CoN NaO
7
FTIR spectra of recovered HCCs
nicely overlapped to that of pristine sample thus attesting to
their stability during the catalysis (Figures S15 and S16, ESI).
Similarly, XRPD patterns both for
match to that of asꢀsynthesized sample suggesting that their
crystallinity is maintained during the catalysis (Figures S17
2 and 3 after the catalysis
45
38
14
(
968.79): calcd. C 55.79; H 3.95; N 20.24; found: C 56.05; H 4.12;
−
1
N 19.77. FTIR spectrum (Zn–Se ATR, cm ): 3309 (NꢀH), 1624
(
2
and
3
exhibited a close
1
C=O) and 1522 (C=N) H NMR spectrum (400 MHz, DMSOꢀd6):
δ = 11.86 (s, 2H), 8.46 (t, J = 7.4 Hz, 1H), 8.05 (d, J = 7.5 Hz, 2H),
13
7
.14 (s, 2H), 6.80 (d, J = 21.1 Hz, 6H). C NMR spectrum (100
and S18, ESI). Although SEM images of HCC
3 do suggest MHz, DMSOꢀd ): δ = 170.75 (s), 157.83 (s), 152.46 (s), 141.71 (s),
6
moderate decrease in crystal size after the catalysis; bulk 140.18 (s), 132.81 (s), 124.57 (s), 120.58 (s), 120.40 (s), 117.27 (s),
1
10.49 (s). Diffuse reflectance UV/Vis spectrum; 645 nm. UV/Vis
material illustrated enough crystallinity required for the
desirable recyclability (Figure 5). Collectively, these results
ꢀ
1
ꢀ1
spectrum (DMF): nm (ε M cm ), 680 (175).
confirm robust and stable nature of both HCCs
the catalysis.
2 and 3 during
[
(1)Zn (OAc) (H O) (DMSO) ] (2). Complex 2 was synthesized
3 3 2 2 2
by treating a solution of complex 1 (0.1 g, 0.103 mmol) in methanol
10 ml) with Zn(OAc) (0.067 g, 0.309 mmol). The reaction mixture
(
2
was stirred at room temperature for 2 h. The solid thus obtained was
filtered and dried under vacuum. Yield 112 mg (77 %).
C H Zn CoN O .S (1413.23): calcd. C, 44.19; H, 3.49; N,
52
49
3
14 14
2
1
3.88; found C, 43.92; H, 3.58; N, 14.06. FTIR spectrum (Zn–Se
−
1
ATR, cm ): 3184 (NꢀH), 1631 (C=O) and 1560 (C=N). Diffuse
reflectance UV/Vis spectrum: λmax 660 nm.
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 7 of 11
Journal Name
New Journal of Chemistry
View Article Online
ART34ICLE
DOI: 10.1039/C8NJ007 A
4
7
[
(1)Zn (Cl) (H O)(DMF) ]·3DMF (2ꢀCl). Complex 2 (100 mg)
‘Squeeze’ command in PLATON. The recovered electron count of
274 corresponded to nearly seven DMF molecules. In complex 3,
one coordinated and one lattice DMSO molecules were disordered
and were fixed at two positions using part command. Moreover, all
four carbon atoms (C49, C50, C53 and C54) of both these DMSO
molecules were refined isotropically. Details of the crystallographic
data collection and structure solution parameters are given in table
3
3
2
2
was dissolved in DMF (10 ml) followed by the addition of a few
drops of 2N HCl solution. The resulting solution was diffused by
the vapours of diethyl ether at room temperature. After one week,
red colored crystalline material was obtained
obtained was filtered and dried under vacuum. Yield: 72 mg (66.3
). C H Cl Zn CoN O ·3C H NO (1533.65): calcd. C, 45.67; H,
.
The solid thus
%
48
40
3
3
16
7
3
7
5
.26; N, 17.75; found C, 45.82; H, 4.88; N, 18.12. FTIR spectrum
5.
CCDCꢀ1823596ꢀ1823598
contain
the
supplementary
−
1
(Zn–Se ATR, cm ): 3190 (NꢀH), 1638 (C=O) and 1530 (C=N).
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
Diffuse reflectance UV/Vis spectrum: λmax 655 nm.
[
{1(Cd) (OAc) (DMSO) (H O) }] (3). To a solution of 1 (0.100 g,
3 3 2 2 2
0
.103 mmol) in methanol was added a solution of Cd(OAc) .2H O
Table 5. Crystallographic data collection and structure refinement
parameters for complexes 1, 2ꢀCl and 3.
2
2
(0.067 g, 0.309 mmol) in MeOH (5 ml). A red colored compound
Complex
1
2ꢀCl
3
was precipitated that was filtered, washed with methanol and dried
under vacuum. Yield: 130 mg (81 %). C H Cd CoN O S
14 14 2
Formula
C
Na
90
H
70Co
2
N
28
C
111
H
115Cl
6
Co
2
C
O
60
3
H73Cd CoN14
5
2
49
3
2
O
14
N
37
O
19Zn
6
18
S
6
(
3
1
1554.33): calcd. C, 40.18; H, 3.18; N, 12.62; found: C, 39.73; H,
.40; N, 13.01. FTIR spectrum (Zn–Se ATR, cm ): 3217 (NꢀH),
622 (C=O) and 1546 (C=N). Diffuse reflectance UV/Vis spectrum:
−
1
Formula Wt.
T (K)
System
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
1931.58
293(2)
Monoclinic
P2 /c
14.7170(8)
16.7634(11)
20.2352(11)
90
119.615(4)
90
4340.0(5)
2
1.478
0.476
1988
2994.17
273(2)
Triclinic
Pꢀ1
15.1412(13)
17.3711(15)
25.697(2)
79.384(2)
84.626(3)
86.773(3)
6608.8(10)
2
1866.81
253(2)
Triclinic
Pꢀ1
13.876(5)
14.859(5)
20.392(5)
74.361(5)
71.679(5)
68.406(5)
3654(2)
2
λmax 658 nm.
1
Physical measurements. Conductivity measurements were carried
out in organic solvents with a digital conductivity bridge from
Popular Traders, India (model number: PT 825). Elemental analysis
data were obtained with an Elementar Analysen Systeme GmbH
Vario ELꢀIII instrument. The NMR spectroscopic measurements
were carried out with a Jeol 400 MHz instrument. The FTIR spectra
were recorded with a Perkin–Elmer FTIR 2000 or spectrum–two
spectrometer. The absorption spectra were recorded with a Perkin–
Elmer Lambda 25 spectrophotometer. Xꢀray powder diffraction
γ (°)
V (Å )
3
Z
3
ρ
ꢀ
calc(mg/m )
(mm )
1.503
1.511
3048
1.027
1.697
1.332
1884
1.026
ꢀ
1
F (000)
Goodnessꢀofꢀfit
(XRPD) studies were performed either with an X’Pert Pro from
0.987
Panalytical or a Bruker AXS D8 Discover instrument (CuꢀKα
radiation, λ = 1.54184 Å). The samples were ground and subjected
to the range of θ = 2 –50° with a scan rate of 1° per minute at room
temperature. Thermogravimetric analysis (TGA) was performed
2
(GOF) on F
Final R indices
R
wR
1
= 0.0705,
= 0.0877
R
1
=
0.0670,
R
1
= 0.0675, wR
2
2
wR = 0.1561
2
= 0.1162
[
I > 2σ(I)]
−1
using a DTG 60 Shimadzu at 5 °C min heating rate under the
nitrogen atmosphere.
R
1
indices (all R1 = 0.1430, R1 = 0.1247, R = 0.1213, wR2
data)
CCDC No.
wR
1823596
2
= 0.1097
wR2 = 0.1835
1823598
= 0.1375
1823597
Crystallography. The intensity data for complexes 1 and 3 were
4
1
collected on an Oxford XCalibur CCD whereas for complex 2ꢀCl
General procedure for the catalytic reactions. In a typical KCR,
1 equiv. of aldehyde was treated with malononitrile (2.5 equiv.) in
presence of 0.5 mol% catalyst. In HR, aldehyde (1 equiv.) was
reacted with either nitromethane or nitroethane (5 equiv.) in
presence of 1.0 mol % catalysts. Both catalytic reactions were
performed under solventꢀfree condition. For both KCR and HR, the
reaction mixture was stirred for 75 min and 36 h, respectively.
Reactions were monitored by using thin layered chromatography
(TLC) and/or gas chromatography (GC). After mentioned time, the
reaction was quenched by the addition of ethyl acetate and the
catalyst was filtered off. The filtrate was concentrated under the
reduced pressure. The crude compound was purified by using flash
column chromatography on silica gel using hexanes–ethyl acetate
mixture (5:1) as the eluent. The resultant product(s) was analyzed
4
2
on a Bruker SMART APEX CCD respectively, by using graphiteꢀ
monochromated MoꢀKα radiation (λ = 0.71073 Å). Intensity data
were corrected for Lorentz polarization effects and an empirical
absorption correction (SADABS) was applied.
were solved by direct methods and refined by fullꢀmatrix leastꢀ
4
3,44
The structures
2
45
squares refinement techniques on F by using SHELXLꢀ2016 in
4
6
the WinGX module. All hydrogen atoms were fixed at the
calculated positions with isotropic thermal parameters whereas all
nonꢀhydrogen atoms were refined anisotropically. For complex 2ꢀ
Cl, one of the coordinated DMF molecule (O15, C100, N100, C101,
C102) was found disordered and was refined isotropically.
Moreover, oxygen atom O15 was fixed at two positions using part
command. In addition, one of the –CH hydrogen atoms of the
disordered DMF molecule could not be assigned however, is
included in the molecular formula resulting in difference in the
calculated and reported number of hydrogen atoms in the formulae.
Furthermore, some disordered electron density could not be
resolved and was therefore masked using the solvent masking
1
by GC and NMR spectral techniques. Representative H NMR
spectra of some of the isolated organic products are displayed in
Figures S19 – S25, ESI.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

New Journal of Chemistry
Page 8 of 11
View Article Online
ARTICLE
DOI: 10.1039/C8NJ0073
Journal Na4Ame
Characterization data of few representative products:
Acknowledgements
1
2
ꢀnitroꢀ1ꢀphenylethanol. Yield: 80 mg (51%). H NMR spectrum
(
500 MHz, CDCl ): δ 7.40 – 7.33 (m, 5H), 5.44 (d, J = 9.6 Hz, 1H),
RG acknowledges Science and Engineering Research Board,
New Delhi (EMR/2016/000888) and the University of Delhi
for the financial support. SP and DB respectively thank CSIR,
New Delhi and UGC, New Delhi for their fellowships. Authors
thank CIFꢀUSIC at this university for the instrumental facilities
including crystallographic data collection.
3
4
.59 (dd, J = 13.3, 9.7 Hz, 1H), 4.49 (dd, J = 13.3, 3.1 Hz, 1H), 3.03
s, 1H).
ꢀnitroꢀ1ꢀ(4ꢀnitrophenyl)ethanol. Yield: 129 mg (92%). H NMR
spectrum (400 MHz, CDCl ) δ 8.25 (d, J = 8.6 Hz, 2H), 7.62 (d, J =
(
2
1
3
8
.8 Hz, 2H), 5.60 (dd, J = 8.1, 4.0 Hz, 1H), 4.59 (dd, J = 10.1, 3.6
Hz, 2H), 3.29 (s, 1H).
ꢀnitroꢀ1ꢀ(4ꢀnitrophenyl)propanꢀ1ꢀol. Yield: 118 mg (79%). H
Notes and references
a
1
Department of Chemistry, University of Delhi, Delhiꢀ110007. Eꢀmail:
2
rgupta@chemistry.du.ac.in; Web: http://people.du.ac.in/~rgupta/; Fax:
NMR spectrum (400 MHz, CDCl ) δ 8.31 – 8.17 (m, 4H), 7.58 (dd,
J = 8.4, 5.6 Hz, 4H), 5.55 (d, J = 2.6 Hz, 1H), 5.18 (d, J = 8.4 Hz,
3
+91ꢀ11ꢀ2766ꢀ6605; Tel: +91ꢀ11ꢀ2766ꢀ6646 Ext. 172.
Electronic Supplementary Information (ESI) available: Figures for FTIR,
1
H), 4.84 – 4.63 (m, 2H), 3.05 (d, J = 19.2 Hz, 2H), 1.48 (d, J = 6.8
Hz, 3H), 1.38 (d, J = 6.9 Hz, 3H).
ꢀnitroꢀ1ꢀ(2ꢀnitrophenyl)ethanol. Yield: 122 mg (87%). H NMR
spectrum (500 MHz, CDCl ) δ 8.08 (dd, J = 8.3, 1.1 Hz, 1H), 7.95
1
13
H and C NMR, and UVꢀVis spectra; TGA plots; powder XRD
patterns; and tables for detailed bonding parameters. CCDC Nos:
1
2
1
823596ꢀ1823598. See DOI: 10.1039/b000000x/
3
(
(
1
2
dd, J = 8.0, 0.7 Hz, 1H), 7.74 (td, J = 7.6, 0.9 Hz, 1H), 7.57 – 7.53
m, 1H), 6.05 (dd, J = 9.1, 1.9 Hz, 1H), 4.87 (dd, J = 13.9, 2.4 Hz,
1
(a) T. R. Cook and P. J. Stang, Chem. Rev., 2015, 115, 7001–7045; (b)
R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem. Rev., 2011,
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H), 4.55 (dd, J = 13.8, 9.1 Hz, 1H), 3.22 (s, 1H).
ꢀnitroꢀ1ꢀ(3ꢀnitrophenyl)ethanol. Yield: 111 mg (79%). H NMR
1
spectrum (500 MHz, CDCl ) δ 8.31 (d, J = 1.5 Hz, 1H), 8.23 – 8.19
(
J = 8.7, 3.6 Hz, 1H), 4.63 – 4.57 (m, 2H), 3.34 (dd, J = 224.8, 135.2
Hz, 1H).
3
2
3
(a) S. Srivastava, M. S. Dagur, A. Ali and R. Gupta, Dalton Trans.,
m, 1H), 7.76 (d, J = 8.0 Hz, 1H), 7.60 (t, J = 8.0 Hz, 1H), 5.60 (dd,
2
015, 44, 17453–17461; (b) X.ꢀZ. Li, P. ꢀP. Hao, D. Wang, W. ꢀQ.
Zhang and L. ꢀN. Zhu, CrystEngComm., 2012, 14, 366–369.
1
(a) R. M. Haas, Z. Hern, S. Sproules and C. R. Hess, Inorg. Chem.,
1
ꢀ(4ꢀbromophenyl)ꢀ2ꢀnitroethanol. Yield: 63 mg (48%). H NMR
2
017, 56, 14738−14742; (b) A. Mishra, A. Ali, S. Upreti, M. S.
spectrum (400 MHz, CDCl ) δ 7.53 (d, J = 8.4 Hz, 2H), 7.28 (d, J =
3
Whittingham and R. Gupta, Inorg. Chem., 2009, 48, 5234–5243; (c)
M. U. Anwar, L. N. Dawe, S. S. Tandon, S. D. Bungec and L. K.
Thompson, Dalton Trans., 2013, 42, 7781–7794.
8
3
1
.5 Hz, 2H), 5.43 (dd, J = 9.3, 2.8 Hz, 1H), 4.59 – 4.45 (m, 2H),
.01 (s, 1H).
1
ꢀ(3ꢀmethoxyphenyl)ꢀ2ꢀnitroethanol. Yield: 88 mg (61%).
H
4
(a) P. Mahapatra, S. Ghosh, N. Koizumi, T. Kanetomo, T. Ishida, M.
G. B. Drewd and A. Ghosh, Dalton Trans., 2017, 46, 12095–12105;
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3
1
5
1
1
3.2, 6.4 Hz, 1H), 6.95 (d, 1H), 6.86 (dd, J = 8.7, 2.1 Hz, 1H), 5.44–
.40 (m, J = 9.5 Hz, 1H), 4.61 – 4.46 (m, 2H), 3.80 (s, 3H), 3.66 (s,
(
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1
Waldmann, L. Ungur, L. F. Chibotaru, C. E. Anson and A. K.
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ꢀ(naphthaleneꢀ1ꢀyl)ꢀ2ꢀnitroethanol. Yield: 65 mg (47%).
H
NMR spectrum (400 MHz, CDCl ) δ 8.12 (d, J = 8.5 Hz, 1H), 7.92
3
(d, J = 8.0 Hz, 1H), 7.86 (d, J = 8.2 Hz, 1H), 7.66 (t, J = 7.7 Hz,
1
7
1
H), 7.57 (t, J = 7.4 Hz, 1H), 7.46 (t, J = 7.8 Hz, 1H), 7.33 (d, J =
.1 Hz, 1H), 5.38 – 5.16 (m, 1H), 4.93 (s, 2H), 4.93 (d, J = 1.7 Hz,
H).
5
Conclusions
This work illustrated the synthesis and characterization of a
unique metalloligand
rings. Metalloligand
1
1
offering four appended benzimidazole
was utilized for the synthesis of
7
509–7518; (c) X. Zhang, B. Li, Z.ꢀH. Chen and Z. –N. Chen, J.
tetranuclear [Zn Co]
(
2
and 2ꢀCl
heterometallic coordination complexes (HCCs). The
crystallographic studies of 2ꢀCl and exhibited unusual
)
and [Cd Co]
(3)
3
3
Mater. Chem., 2012, 22, 11427–11441; (d) A. M. Nonat, C. Allain,
S. Faulkner, and T. Gunnlaugsson, Inorg. Chem., 2010, 49, 8449–
3
8
456
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840; (b) V. Stavila, A. A. Talin and M. D. Allendorf, Chem. Soc.
2
+
2+
coordination of three secondary Zn /Cd
appended benzimidazole rings being offered by
ions to the
thus
6
7
,
1
7
producing tetranuclear HCCs. Both HCCs were utilized as the
heterogeneous catalysts for Knoevenagel condensation and
Henry reactions. The stability and recyclability experiments
asserted stable nature of HCCs in catalysis. These HCCs not
only demonstrated the importance of metalloligands in material
design but also in developing stable functional materials.
,
3
5
Rev., 2014, 43, 5994−6010; (c) C. Wang, T. Zhang and W. Lin,
8
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 9 of 11
Journal Name
New Journal of Chemistry
View Article Online
ART34ICLE
DOI: 10.1039/C8NJ007 A
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8
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,
and R. Gupta, Inorg. Chem., 2009, 48, 5234–5243; (c) A. P. Singh
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1
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2
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,
1
1
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DOI: 10.1039/C8NJ00734A
A metalloligand appended with benzimidazole rings:
Tetranuclear [CoZn ] and [CoCd ] complexes and their
3
3
catalytic applications
Saurabh Pandey, Deepak Bansal and Rajeev Gupta
Department of Chemistry, University of Delhi, Delhi – 110 007 (India)
Graphical Abstract
3+
A novel Co -based metalloligand, offering appended benzimidazole rings, has been utilized for the
synthesis of catalytically active tetranuclear [CoZn ] and [CoCd ] heterometallic coordination
3
3
complexes.