Heterometallic coordination polymers: syntheses, structures and heterogeneous catalytic applications

Article abstract of DOI:10.1039/c5nj01223f

A metalloligand Na[Co(L¹)₂] (1) of the ligand H₂L¹ (N²,N⁶-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide) offering appended thiazoline rings has been used for the synthesis of one-dimensional {Co³⁺-Zn²⁺} (2), {Co³⁺-Cd²⁺} (3) and {Co³⁺-Hg²⁺} (4) coordination polymers. 1 offers appended thiazoline rings having both soft sulphur and hard nitrogen donors. The crystal structures of 2 and 3 display the coordination of hard thiazoline-N donors to Zn²⁺ and Cd²⁺ ions. Interestingly, 4 illustrates the bonding through both hard thiazoline-N and soft thiazoline-S donors. The three heterometallic coordination polymers have been used as reusable heterogeneous catalysts for the ring-opening reactions of oxiranes and thiiranes; Knoevenagel condensation of benzaldehydes and benzothialdehydes; and cyanation reactions of aldehydes and carbothialdehydes. Our results demonstrate that the relative size and Lewis acidity of secondary metals potentially control the catalytic outcome via preferential interaction with the substrates.

Full text of DOI:10.1039/c5nj01223f

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Heterometallic coordination polymers: syntheses,
structures and heterogeneous catalytic
applications†
Cite this:DOI: 10.1039/c5nj01223f
Deepak Bansal,a Saurabh Pandey,a Geeta Hundalb and Rajeev Gupta*a
A metalloligand Na[Co(L1)2] (1) of the ligand H2L1 (N2,N6-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide)
offering appended thiazoline rings has been used for the synthesis of one-dimensional {Co3+–Zn2+} (2),
{Co3+–Cd2+} (3) and {Co3+–Hg2+} (4) coordination polymers. 1 offers appended thiazoline rings having
both soft sulphur and hard nitrogen donors. The crystal structures of 2 and 3 display the coordination of
hard thiazoline-N donors to Zn2+ and Cd2+ ions. Interestingly, 4 illustrates the bonding through both hard
thiazoline-N and soft thiazoline-S donors. The three heterometallic coordination polymers have been used
as reusable heterogeneous catalysts for the ring-opening reactions of oxiranes and thiiranes; Knoevenagel
condensation of benzaldehydes and benzothialdehydes; and cyanation reactions of aldehydes and
carbothialdehydes. Our results demonstrate that the relative size and Lewis acidity of secondary metals
potentially control the catalytic outcome via preferential interaction with the substrates.
Received (in Montpellier, France)
3rd June 2015,
Accepted 2nd October 2015
DOI: 10.1039/c5nj01223f
www.rsc.org/njc
close proximity to the secondary metals in the resultant coordi-
nation polymers.22
Introduction
In recent years, rational design and construction of new
The preferential interaction of a substrate with a catalytic
inorganic–organic hybrid materials has been an active research metal center is a key-step in the metal-mediated organic trans-
field.1–4 In this context, utilization of coordination complexes as formations.23a,b Despite the availability of a significant amount
metalloligands offers several advantages of producing materials of spectroscopic, kinetic, and theoretical data to understand
including a fair amount of predictability and therefore tailor- such a vital step, detailed structural and mechanistic informa-
made architectures.1 Importantly, such materials offer not tion is still lacking.23c–f Although extensive attempts have been
only fascinating structural topologies5–8 but also important made to comprehend the activation of a substrate during cata-
applications in sorption,9 separation,10 ion-exchange,11 proton lysis, the molecular-level details are still in the early stage.23c
conduction,12 optics,13 magnetism,14 luminescence15 and cata- For example, Lewis acidity of a metal ion depends on several
lysis.16 Our group has developed several metalloligands offer- factors including the size and charge of the cation.24 Ideally,
ing assorted appended functional groups.17–22 Such appended decent control on two such essential components, charge and
functional groups have been shown to coordinate a variety of size, provides an option to control the Lewis acidity of a metal
secondary metal ions producing assorted materials. While ion and therefore its preference to interact and/or bind a
metalloligands offering appended pyridyl groups have afforded substrate. In the past, we have shown a set of hetero-trimetallic
hetero-trimetallic complexes18 as well as two- (2D) and three- complexes having Zn(II), Cd(II), and Hg(II) ions as the secondary
dimensional (3D) networks;19 thiazole-appended metalloligands catalytic sites where a difference in Lewis acidity of the respective
have been able to introduce hydrogen bonds (H-bonds) in cation was found to control the substrate activation step and
subsequently the reactivity.18b Herein, we show the utilization
of a thiazoline-appended metalloligand of ligand H2L1 (N2,N6-
bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide) in the
a Department of Chemistry, University of Delhi, Delhi, 110 007, India.
E-mail: rgupta@chemistry.du.ac.in; Fax: +91-11-2766 6605;
synthesis of one-dimensional (1D) coordination polymers. These
coordination polymers, containing Zn(II), Cd(II), and Hg(II) ions
as the catalytic sites, act as the heterogeneous catalysts for the
ring-opening reactions of oxiranes and thiiranes; Knoevenagel
condensation reactions of benzaldehydes and benzothialde-
hydes; and cyanation reactions of aldehydes and carbothialde-
hydes. Our results illustrate that the difference in the size
Tel: +91-11-2766 6646 ext. 172
b Department of Chemistry, Guru Nanak Dev University, Amritsar–143 005, Punjab,
India
† Electronic supplementary information (ESI) available: Figures for NMR and
absorption spectra, TGA plots, powder XRD patterns, weak interactions, inclusion
studies, hot-filtration test; crystallographic data and tables for bonding details
and data collection. CCDC 894199, 1059686–1059688. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c5nj01223f
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and Lewis acidity of secondary metals potentially controls whereas two Npyridine rings occupy the axial positions. The
the catalytic outcome via the preferential interaction with the geometry around the Co3+ ion is compressed octahedrally
substrates.
wherein the Co–Namide distances (avg. 1.954 Å) are longer than
Co–Npyridine distances (avg. 1.880 Å) although within the range
as observed before.18,21,26 The solid-state packing of 1 shows
that the complex anions are connected via appended thiazoline
rings to the Na+ cations (Na–N4: 2.531(5) Å; Na–N5: 2.430(5) Å)
and such a bonding generates a chain running in the a-direction.
Such chains are further connected to each other via H-bonds
involving Oamide and pyridyl-H atoms resulting in a 2D network.
The involvement of appended thiazoline rings in coordination to
Na+ ions immediately suggests the potential of 1 as a metallo-
ligand for the synthesis of extended architectures.
Results and discussion
Synthesis and characterization of metalloligand 1
The metalloligand Na[Co(L1)2] (1) was synthesized by the reac-
tion of a deprotonated ligand [L1]2À (where H2L1 = N2,N6-bis(4,5-
dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide) with Co(II) salt
followed by oxidation using O2 (Scheme 1). Complex 1 shows
red-shifted namidate stretching bands at 1618 cmÀ1 suggesting
the bonding with the deprotonated Namidate group which is
further corroborated by the absence of ligand’s nN–H stretching
bands.25a The conductivity data confirm the 1 : 1 electrolytic
Synthesis and characterization of HCPs 2–4
The reaction of metalloligand 1 with Zn(II), Cd(II) and Hg(II)
salts resulted in the isolation of heterometallic coordination
polymers (HCPs) 2, 3 and 4, respectively, (Scheme 2). The FTIR
spectra of HCPs 2–4 exhibit namidate stretching bands between
1622 and 1630 cmÀ1, which are red-shifted (5–15 cmÀ1) when
compared to metalloligand 1.18,21,25a The diffuse-reflectance
absorption spectra show lmax at ca. 670 nm for all three HCPs
(Fig. S4, ESI†). The X-ray powder diffraction (XRPD) patterns of
the three HCPs are in close match to those simulated from the
single-crystal structures (Fig. S5–S7, ESI†). This fact asserts the
phase purity of bulk samples, although minor differences were
observed in diffraction line intensities that can be attributed to
the variation of the preferred orientation of crystallites in the
powdered samples.20b,c All three HCPs were investigated by the
thermogravimetric analysis (TGA) to shed light on the fate of
coordinated and/or lattice solvent molecules and their thermal
stability (Fig. S8, ESI†). HCP 2 exhibits weight loss for two
coordinated and two lattice water molecules (obs. 7.4%; calc. 7.5%)
Similarly, HCPs 3 (obs. 5.8%; calc. 5.4%) and 4 (obs. 4.0%;
calc. 3.6%) display a weight loss of three and two lattice water
molecules, respectively. In all three cases, the presence of coordi-
nated and/or lattice solvent molecules is well-supported by the
microanalysis data.
nature.25b The H NMR spectrum of 1 exhibits thiazoline ring
1
protons as triplets at 2.7 and 3.0 ppm whereas pyridine ring
protons were observed as doublets and triplets at ca. 8.0 and
ca. 8.4 ppm, respectively, (Fig. S1 and S2, ESI†). The DMF solution
of 1 displays lmax at 670 nm in addition to a shoulder at 472 nm
(Fig. S3, ESI†). Metalloligand 1 was crystallographically character-
ized and its molecular structure is shown in Fig. 1. The structure
shows that two tridentate ligands coordinate the Co3+ ion mer-
idionally. Such a coordination mode leaves the appended thiazo-
line rings uncoordinated as noted with our earlier metalloligands
thereby offering appended pyridine or thiazole rings.18,22 The
Co3+ ion is coordinated to four Namidate donors in the basal plane
Scheme 1 Synthetic route for metalloligand 1. Conditions: (i) 2 equiv.
NaH, DMF, N2; (ii) 0.5 equiv. [Co(H2O)6](ClO4)2; and (iii) excess O2.
Crystal structures of HCPs 2–4
All three HCPs were crystallographically characterized to under-
stand the coordination ability of metalloligand 1 towards the
secondary metal ions. Metalloligand 1 offers four appended
thiazoline rings available to further coordinate the secondary
metal ions, and the crystal structures of all three HCPs display
Fig. 1 Thermal ellipsoidal representation of metalloligand 1 (left panel);
thermal ellipsoids are drawn at 30% probability level whereas the Na+ ion
and hydrogen atoms have been omitted for clarity. A packing view along
the b axis (right panel); complex anions are shown in green and orange
whereas the Na+ ions are shown as magenta spheres. See text for details.
Scheme 2 Synthetic routes showing the reaction of metalloligand 1 with
Zn2+, Cd2+ and Hg2+ salts that produce HCPs 2–4.
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Fig. 3 (a) Thermal ellipsoidal representation of HCP 3; thermal ellipsoids
are drawn at 30% probability level whereas the anion, solvent molecule
and hydrogen atoms have been omitted for clarity. (b) Coordination sphere
of the Cd(II) ion with bond distances.
Fig. 2 (a) Thermal ellipsoidal representation of HCP 2; thermal ellipsoids
are drawn at 30% level whereas hydrogen atoms involved in hydrogen
bonding are only shown for clarity. (b) Coordination sphere of the Zn(II) ion
with bond distances.
chain to that of thiazoline-H atoms (H12a and H22a) of the
second chain thus generating 2D sheets of parallel running
chains. A further inspection displays that such 2D sheets are
interconnected to each other via ClO4À ions and lattice methanol
molecules producing a 3D architecture (Fig. S10–S11, ESI†).
their involvement in bonding. The central Co3+ ion displays an
octahedral geometry within a N6 coordination environment in
all HCPs, as also noted in the crystal structure of metalloligand
1 (Tables S1–S3, ESI†). In a HCP, the Co3+ core remains largely
unaffected geometrically as observed for our pyridine and
thiazole appended metalloligands.18,21,22 Such a fact suggests
the modular nature of the appended thiazoline rings in 1 that
adjust to the steric and electronic requirements of the assorted
secondary metal.
The crystal structure of HCP 2 exhibits the generation of a
one-dimensional (1D) chain wherein metalloligands coordinate
to the Zn2+ ions via Nthiazoline and Oamide atoms (Fig. 2). The
Zn(II) ion exhibits octahedral geometry in which two Nthiazoline
(N9 and N9#) and two Oamide (O3 and O3#) atoms constitute the
N2O2 basal plane whereas the axial coordination sites are
occupied by the water molecules (O1w and O2w). Importantly,
both uncoordinated thiazoline rings are nicely arranged to
form intramolecular H-bonds with the coordinated water mole-
cules with OÁ Á ÁN separations close to ca. 2.80 Å. The solid-state
packing displays that the individual 1D chains are connected to
each other via H-bonds between Oamide (O4) of one chain and
that of the thiazoline-H atom (H9a) of the second chain. Such
packÀing is further strengthened due to H-bonding involving the
ClO4 ion (Fig. S9, ESI†).
%
HCP 4 crystallizes in a trigonal cell with the R3 space group.
The crystal structure, similar to 2 and 3, exhibits the generation
of a 1D chain in which metalloligands are connected through
the Hg(II) ions (Fig. 4a). In HCP 4, the Hg(II) ion exhibits a
severely distorted five-coordinate geometry (Fig. 4b) where four
coordination sites are occupied by the thiazoline groups while
the fifth one comes from a ClÀ ion (2.345 Å). Importantly, the
crystal structure displays uniqueness of the thiazoline rings.
%
HCP 3 crystallizes in a triclinic cell with a P1 space group.
The crystal structure, similar to 2, shows the generation of a 1D
chain where metalloligands are connected through Cd(II) ions
(Fig. 3). Every Cd(II) ion displays a distorted octahedral geometry
coordinated by four Nthiazoline atoms (2.260–2.395 Å) and two
Oamide atoms (2.467 and 2.568 Å). The tentative basal plane
around the Cd ion comprises N3O atoms with axial sites occupied
by Nthiazoline and Oamide atoms. In an individual 1D chain,
Oamide atoms (O1 and O3) and thiazoline-H atoms from two
metalloligands (H8a and H25b) show intra-chain H-bonding
interactions (Fig. S10, ESI†). Such individual chains are further
Fig. 4 (a) Thermal ellipsoidal representation of HCP 4; thermal ellipsoids
are drawn at 30% probability level whereas hydrogen atoms have been
omitted for clarity. (b) Coordination sphere of the Hg(II) ion with bond
distances including rotationally disordered thiazoline ring with both N and
S atoms. (c) Honeycomb structure generated due to several weak inter-
connected via inter-chain H-bonding involving O2amide of one actions between 1D chains; see text for details.
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It was noted that the Hg(II) ion displays two short (avg. 2.278 Å) similar reactions involving thiiranes as the substrates have been
and two very long (avg. 2.691 Å) bonds from the thiazoline rings. scarcely developed.18b,30 Such a stark difference is probably due to
A closer look establishes that two out of the four appended the tendency of thiiranes to undergo polymerization.30a Hence,
thiazoline rings per metalloligand were actually flipped to coordi- there is a requirement of a common catalyst that can effectively
nation with the Hg2+ ion through thiazoline-S atoms. Thus, perform RORs of both oxiranes and thiiranes. In addition,
effectively, every Hg(II) ion receives two shorter coordinations performing such reactions heterogeneously remains a challenge
from the thiazoline-N donors (avg. 2.278 Å) and two longer ones despite practical significance.
from thiazoline-S atoms (avg. 2.691 Å). Such bonding behavior
adequately illustrates the tendency of a soft metal ion, Hg(II), to and thiiranes using substituted anilines as the nucleophiles. To
make a bond with a soft S donor. minimize the competition from the solvent molecules and the
All three HCPs were tested for the RORs of a few oxiranes
HCP 4 displays an interesting solid-state packing in the form facile isolation of products and separation of catalysts, solvent-
of a hexagonal honeycomb architecture (Fig. 4c and Fig. S12, free reaction conditions were employed. Under these reaction
ESI†). When viewed from the top, six 1D {Co3+–Hg2+} chains conditions, all three HCPs functioned as heterogeneous cata-
(shown in different colours in Fig. 4c) arrange to constitute a lysts. Thus, when cyclohexene oxide was treated with aniline in
hexagonal ring. In a hexagonal ring, such 1D chains interact the presence of only 2 mol% of any of the HCPs, a smooth
with each other via intermolecular H-bonding involving Oamide reaction took place and the corresponding b-amino alcohol was
atoms (O2 and O3) and hydrogen atoms of thiazoline (H12b) and obtained in good yield (Table 1). Reactions involving aniline as
pyridine (H16) rings. Hexagonal rings are further connected well as assorted para-substituted anilines containing both
to each other through H-bonds involving O4amide atoms and electron-donating and electron-withdrawing groups afforded
pyridine-H3 synthons generating a honeycomb structure.
the corresponding b-amino alcohols in good to high yield.
Interestingly, HCP 3 was found to be the best catalyst whereas
4 was the least effective. We tentatively suggest that a subtle
Catalytic potentials of HCPs
Structural investigations of the three HCPs reveal the presence balance between ionic radius and Lewis acidity of the Cd2+ ion,
of Lewis acidic metal ion strongly coordinated by the thiazoline compared to Zn2+ (small size) and Hg2+ ions (lower Lewis acidity),
rings, while weakly coordinated either by water molecule(s), allows cadmium metal to not only activate the epoxide ring but
Oamide atoms, or the chloride ion. Such a bonding pattern also orient the nucleophile before its attack, which is the major
suggests that the weakly coordinated atom/group could be easily reason for this difference.
replaced by a suitable substrate.19,22 This situation may therefore
To further analyse the catalytic abilities of the three HCPs, the
aid the favourable interaction of the substrate with the secondary ROR of cyclohexene sulfide was carried out using substituted
metal and assist in its activation and thus catalysis.22 In addition, anilines under identical reaction conditions as mentioned for
the infinite dimension of HCPs results in their insolubility in cyclohexene oxide (Table 2). Satisfyingly, cyclohexene sulfide also
common organic solvents, therefore, providing an option to reacted smoothly with aniline or substituted anilines producing
carry out the catalytic reactions heterogeneously. Importantly, the respective trans-2-phenylaminocyclohexanethiol. Interestingly,
the present HCPs offer an excellent opportunity to carry out Hg-based HCP 4 was found to be the best catalyst out of three
comparative catalytic reactions and evaluate the suitability of HCPs. We propose that the high affinity of the Hg(II) ion to
various secondary metal ions in such reactions. We anticipated interact with the soft sulphur atom is the primary reason for the
that the relative size and Lewis acidity of a secondary metal ion
may display preferential interaction towards substrates and such
a difference may control the product outcome.18b,27 To illustrate
Table 1 Ring-opening reactions of cyclohexene oxide with aniline and
para-substituted anilines using HCPs 2, 3 and 4 as catalystsa
such a feat, the focus of catalysis is not towards high yield and/or
high turnover number but to understand the reactivity difference
between the three HCPs and more importantly, the difference
induced by the unique secondary metal ions on catalysis.27
Therefore, comparative catalysis experiments were carried out
with the three HCPs. We selected ring-opening reactions (RORs),
Knoevenagel condensation reactions (KCRs), and cyanation reac-
Yieldb, %
tions (CRs) as these reactions are not only important for the
production of fine chemicals but also are known to be carried
out effectively using a suitable Lewis acidic metal.18a,b
S. no.
R
2
3
4
1
2
3
4
5
–H
83, 82c, 80d
96, 93c, 90d
52
72
65
60
62
Ring-opening reactions. RORs of oxiranes and thiiranes with
various nucleophiles are of immense interest due to the impor-
tance of such products in chemical and pharmaceutical research.28
Activation of an oxirane/thiirane ring is a prerequisite condition
for such a reaction and assorted reagents including Lewis acidic
metal salts have been frequently used for such a purpose.18a,b,29
–C2H5
–OCH3
–F
88
85
88
82
97
92
95
94
–Cl
a
Reaction conditions: Catalyst – 2 mol%; solvent-free reaction; 25 1C;
b
c
4 h stirring. Yield was calculated from the GC. Isolated yield for the
third run with the reused catalyst. Isolated yield for the fourth run
d
Interestingly, compared to the well-established RORs of oxiranes, with the reused catalyst.
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molecular ion peak at m/z [M+ À 31] due to the loss of the
CH2OH fragment.20,31a This reaction indicates that the epoxide
ring interacted with the catalytic secondary metal ion through
the less-hindered side probably due to steric reasons. The results
exhibit the better catalytic performance of HCP 3 when com-
pared to the other two HCPs as also noted in the ROR of
cyclohexene oxide.
Table 2 Ring-opening reactions of cyclohexene sulfide with aniline and
para-substituted anilines using HCPs 2, 3 and 4 as catalystsa
Knoevenagel condensation reactions. Reactions involving
carbon–carbon bond formation are notable in the synthesis
of fine chemicals. Knoevenagel condensation reaction (KCR)
is one of such reactions which is known to produce several
significant synthons and intermediates.32–34 We investigated
the potential application of the three HCPs as heterogeneous
catalysts in the KCRs. In a typical reaction, benzaldehyde reacted
cleanly with malononitrile in the presence of only 2 mol% of
HCPs (2, 3 and 4) to produce 2-benzylidene-malononitrile as the
only product. All three HCPs resulted in smooth reactions of
several substituted benzaldehydes with malononitrile to afford
the desired product in high yield (Table 4). The molecular size
and/or the presence of electronic substituents does not seem to
have considerable effect on the yield of the product.22,35 For
example, sterically demanding 1- as well as 2-naphthaldehyde
and 9-anthraldehyde produced the corresponding products in
yield as good as one noted for benzaldehyde suggesting the
similar accessibility of the substrate to the secondary metal
ion.22 We then attempted similar reactions using substituted
benzothialdehydes as the substrates to confirm if such sub-
strates are activated better by any of the three HCPs.18b Although
the three HCPs produced the respective products in good yield,
Hg-based HCP 4 was indeed more effective (compare entries 1, 3,
and 6), thereby supporting our hypothesis.
Yieldb, %
S. no.
R
2
3
4
1
2
3
4
5
–H
62, 60c, 58d
72, 70c, 68d
92, 90c, 88d
–C2H5
–OCH3
–F
71
75
70
78
70
72
70
75
94
90
85
90
–Cl
a
Reaction conditions: Catalyst – 2 mol%; solvent-free reaction; 25 1C;
4 h stirring. Yield was calculated from the GC. Isolated yield for the
third run with the reused catalyst. Isolated yield for the fourth run
b
c
d
with the reused catalyst.
better performance of HCP 4. Such an HgÁ Á ÁS interaction
activates the thiirane ring effectively and enables the ROR in
an efficient manner. The support for this hypothesis comes from
the crystal structure of HCP 4 wherein the Hg(II) ion displays
coordination from Sthiazole in addition to Nthiazole groups. Such a
fact adequately confirms the tendency of a soft metal ion, Hg(II),
to interact with a soft donor, S.
To illustrate the effectiveness of the present HCPs to control
the regioselectivity, styrene oxide was used as a representative
unsymmetrical substrate (Table 3).18a,b,19,20 Interestingly, a single
product was obtained in all cases, therefore, suggesting good
control over the regioselectivity.31 Despite the fact that a nucleo-
phile (i.e. aniline) could attack either at the benzylic carbon atom
or at the less-hindered carbon atom of the substrate, in every
case, nucleophile attack took place at the benzylic carbon atom
affording a single product.31 A compelling proof came from the
mass spectrum of the reaction which showed the characteristic
Cyanation reactions. With the promising results obtained
for RORs and KCRs, HCPs 2–4 were further tested for CRs.
CRs of carbonyl-based substrates provide a convenient route to
cyanohydrins which are important intermediates for numerous
chemicals.35 Considerable efforts have been invested to design
Table 4 Knoevenagel condensation reactions of assorted aldehydes with
(CN)2CH2 using HCPs 2, 3 and 4 as catalystsa
Table 3 Ring-opening reactions of styrene oxide with aniline and para-
substituted anilines using HCPs 2, 3 and 4 as catalystsa
Yieldb, %
S. no.
R/substrate
2
3
4
Yieldb, %
1
2
3
4
5
6
7
8
–H
96(90)c
94
92(90)c
93
94(98)c
85
S. no.
R
2
3
4
m-OCH3
p-OCH3
m-NO2
89(88)c
82
85(90)c
88
87(94)c
92
1
2
3
4
5
–H
75
77
76
81
78
90, 87c, 86d
65
55
68
52
70
–C2H5
–OCH3
–F
92
86
91
84
p-NO2
92
95
96
1-Naphthyl
2-Naphthyl
9-Anthryl
88(85)c
81
85(88)c
79
89(92)c
85
–Cl
75
72
81
a
Reaction conditions: Catalyst – 2 mol%; solvent-free reaction; 25 1C;
a
b
c
Reaction conditions: Catalyst – 2 mol%; solvent-free reaction; 25 1C;
4 h stirring. Yield was calculated from the GC. Isolated yield for the
d
b
c
4 h stirring. Yield was calculated from GC. Numbers in parenthesis
correspond to the respective sulphur analogue.
third run with the reused catalyst. Isolated yield for the fourth run
with the reused catalyst.
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Lewis acid based catalysts, particularly heterogeneous catalysts having Zn-bound labile water molecules for CRs and KCs. Using
incorporating Lewis acidic sites, for CRs.19,20,36 In this context, 5 mol% catalyst in CH2Cl2, 92% and 85% transformations were
the present HCPs appeared to be good candidates. As illustrated noted for CRs and KCs, respectively. These examples clearly
in Table 5, all three HCPs were able to catalyze CRs of assorted suggest the superior nature of the present HCPs in similar
aldehydes using (CH3)3SiCN as the cyanide source. Benzalde- organic transformation reactions.
hyde as well as assorted substituted analogues provided the
Recyclability and control experiments. KCRs were employed
corresponding cyanohydrins in good to excellent yield. Notably, for recovering the HCPs and subsequently testing their stability
even sterically demanding 1-naphthaldehyde was smoothly con- and recyclability. All three HCPs could be easily recovered by
verted to its respective cyanohydrin which suggests that size- simple filtration and were reused for carrying out KCRs of
selectivity is not a criterion.22 We tentatively suggest that cata- benzaldehyde with malononitrile for three consecutive catalytic
lysis occurs via activation of aldehyde before its reaction with cycles. It is important to mention that no purification or regenera-
cyanide. In order to prove such a postulate, HCP 2 was used as a tion was performed during the recyclability experiments. Gratify-
representative example and impregnated with benzaldehyde and ingly, all three HCPs retained their catalytic efficiency throughout
1-naphthaldehyde by suspending crystals in a CH2Cl2 solution of all such cycles. Such a fact is well-supported by the observation
such substrates. Such impregnated samples displayed a consid- of FTIR spectra (Fig. S15–S17, ESI†) and powder XRD patterns
erable red-shift of CQO stretching bands for both aldehydes. (Fig. S18–S20, ESI†) of the recovered HCPs that are super-
For example, such stretching bands were shifted from 1705 to imposable with those of as-synthesized samples. Both these
1690 cmÀ1 for benzaldehyde and from 1702 to 1690 cmÀ1 for studies assert that the crystallinity and structural integrity of
1-naphthaldehyde (Fig. S13 and S14, ESI†). Such an impregna- the recovered material are preserved during catalysis.
tion experiment adequately suggests the activation of aldehydic
Control experiments further established that HCPs were
substrate(s).20,22 While comparing the product yields for CRs, an essential for catalysis as their removal during the reaction
important difference is quite evident; the yields are always on the ceases the product formation. Fig. S21 (ESI†) illustrates such an
higher side with the Hg-based HCP whenever substrates are experiment using HCP 2 where the formation of 2-benzylidene-
S-based. Such results, comparable to those of RORs of thiiaranes, malononitrile was monitored using GC. As evident, there was
again strenthen our hypothesis that the Hg-based HCP has no product formation after the removal of HCP 2. However,
better ability to activate the S-based substrates.
re-addition of 2 immediately causes the KCR to resume. This
Several groups have utilized assorted coordination polymers experiment therefore infers the true heterogeneous catalytic
for various organic transformations, including ring-opening, nature of HCP 2. Finally, the use of metalloligand 1 or a few
cyanation and Knoevenagel condensation reactions.16 For example, M(II) salts as catalysts (e.g., MCl2 and M(OAc)2) resulted in
Tanaka and co-workers37 have used a Cu-based coordination negligible product formation (o5%) thus again justifying the
polymer for the asymmetric RORs under solvent-free conditions. catalytic impact of HCPs.
In one such experiment, the ROR of cyclohexene oxide with
We also attempted to understand the possible morphological
aniline showed 51% transformation with 51% enantiomeric changes to the crystalline HCPs during the course of catalysis.
excess. Garcia and coworkers38 have tested an iron-based coordi- For such a study, HCP 2 was selected to carry out the KCR
nation polymer for RORs of assorted epoxides with different of benzaldehyde with malononitrile. In a typical experiment,
nucleophiles (alcohols and amines). For example, styrene oxide
on reaction with aniline at room temperature produces the
corresponding ring-opened product in 52% yield. Bharadwaj
and coworkers39 have used a Zn-based coordination polymer
Table 5 Cyanation reactions of assorted aldehydes (entries 1–3) and
carbothialdehyde (entries 4–6) with (CH3)3SiCN using HCPs 2, 3 and 4
as catalystsa
Yieldb, %
S. no.
R
2
3
4
1
2
3
4
5
6
Benzaldehyde
4-Methoxybenzaldehyde
1-Naphthaldehyde
Benzothialdehyde
4-Methoxybenzothialdehyde
Naphthalene-1-carbothialdehyde
72
86
84
75
83
62
96
96
92
84
85
75
80
85
80
90
95
85
Fig. 5 SEM micrographs (left panels) and XRPD patterns (right panels) of
2 (A) before and (B) after the KCRs. The bottom panel displays simulated
XRPD patterns along with the optical photograph of the pristine sample.
a
Reaction conditions: Catalyst–1 mol%; solvent-free reaction; 25 1C;
b
4 h stirring. Yield was calculated from GC.
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crystals of 2 were suspended in a mixture of benzaldehyde and 13C NMR spectrum (100 MHz, DMSO-d6): d = 169.33 (C4),
malononitrile and were analysed by powder XRD and scanning 161.60 (C5), 156.30 (C3), 139.86 (C1), 123.90 (C2), 58.03 (C7),
electron microscopy (SEM) (Fig. 5). It is evident that the powder 32.89 (C6).
XRD patterns remain largely unchanged during catalysis; however,
[{Co(L1)2}Zn(H2O)2]nÁClO4 (2). Complex 1 (0.05 g, 0.066 mmol)
SEM micrographs indicate minor changes which are noticeable was dissolved in MeOH (10 mL) followed by the addition of solid
from the cracks on the crystal surface. We therefore conclude that Zn(NO3)2ÁxH2O (0.04 g, 0.133 mmol). This caused precipitation
the present HCPs function as efficient reusable heterogeneous of a product which was dissolved by the addition of a saturated
catalysts for several Lewis acid assisted organic transformation aqueous solution of NaClO4 (2 mL). The resulting solution was
reactions.
finally stirred for 2 h and passed through a pad of Celite in a
medium porosity frit. To the filtrate, vapors of diethyl ether were
diffused that produced a red crystalline product within 3–4 d.
Yield: 0.045 g (74%). Anal. calc. for C26H26ZnCoN10O10S4Cl.2H2O:
C, 32.44; H, 3.14; S, 13.32; N, 14.55. Found: C, 32.70; H, 2.83; S,
13.84; N, 15.12. FTIR spectrum (Zn-Se ATR, selected peaks): 1611,
1595 (CQO) and 1549 (CQN) cmÀ1. Diffuse reflectance absorp-
tion spectrum (lmax, nm): 670.
Conclusions
The present work has shown the synthesis of Zn2+, Cd2+ and
Hg2+ based heterometallic coordination polymers using metallo-
ligand 1 offering appended thiazoline rings. The crystallographic
studies substantiated the 1D infinite chain structures for all
these coordination polymers. All three coordination polymers
functioned as heterogeneous and reusable catalysts for the ring-
opening reactions of oxiranes and thiiranes; Knoevenagel con-
densation reactions of benzaldehydes and benzothialdehydes;
and cyanation reactions of aldehydes and carbothialdehydes.
The catalytic results illustrated that the structurally distinct
secondary metal ions having different Lewis acidities potentially
control the catalytic outcome via preferential interaction with the
substrates.27
[{Co(L1)2}Cd]nÁClO4 (3). This compound was synthesized in a
similar manner as that of HCP 2 with the following reagents:
complex 1 (0.05 g, 0.066 mmol), Cd(NO3)2ÁxH2O (0.041 g,
0.133 mmol), and a saturated aqueous solution of NaClO4.
Diffusion of diethyl ether vapor into the filtrate produced orange
crystalline blocks within a day. Anal. calc. for C26H22CoN10O8S4ClCdÁ
3H2O: C, 31.49; H, 2.89; N, 13.40; S, 12.27. Found: C, 30.91; H,
2.45; N, 13.70; S, 11.39. FTIR spectrum (KBr, selected peaks)
1622 and 1594 (CQO), 1549 (CQN) cmÀ1. Diffuse reflectance
absorption spectrum (lmax, nm): 670.
[Co(L1)2Hg(Cl)]n (4). Complex 1 (0.05 g, 0.066 mmol) was
dissolved in 5 mL of DMF followed by the layering of a MeOH
solution (3 mL) of HgCl2 (0.036 g, 0.133 mmol). The yellow-
orange crystals that appeared at the interface within a day were
Experimental
General
The reagents and chemicals were procured from the commercial
sources and were used without further purification. The solvents
were dried and/or purified using the standard literature methods.40
The ligand H2L1 (N2,N6-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-
dicarboxamide) was synthesized as per our earlier report.22a
separated and dried in vacuum. Anal. calc. for C26H22CoHgN10
-
O4S4Cl.2H2O: C, 31.30; H, 2.63; N, 14.04; S, 12.85. Found: C,
31.54; H, 2.74; N, 14.18; S, 12.42. FTIR spectrum (Zn-Se,
selected peaks): 1630 (CQO) and 1544 (CQN) cmÀ1. Diffuse
reflectance absorption spectrum (lmax, nm): 670.
Synthesis of carbothialdehydes41. The respective aldehydes
(1.0 eq.) were treated with Lawesson’s reagent (1.5 eq.) in
Syntheses
Na[Co(L1)2] (1). The ligand H2L1 (0.50 g, 1.49 mmol) was toluene and refluxed at 70 1C for 8 h. The resulting pink colored
dissolved in N2 flushed DMF (10 mL) and treated with solid solution was filtered and the solvent was removed under reduced
NaH (0.075 g, 3.129 mmol). The resulting solution was stirred pressure. The oily remains were dissolved in diethyl ether and
for 30 min at room temperature. Solid [Co(H2O)6](ClO4)2 (0.272 g, washed with water several times. Diethyl ether was removed to
0.745 mmol) was added to the aforementioned solution followed afford a semi-solid compound. The isolated compounds displayed
by the bubbling of O2 for 2 min. The solution was finally stirred identical spectral features as reported in the literature.41
for 1 h. The solution was filtered and the DMF was removed
Benzothialdehyde. Yield: 98%. FTIR spectrum (Zn-Se ATR,
under reduced pressure. The crude product was isolated after cmÀ1): 916 (–CQS). 4-Methoxybenzothialdehyde. Yield: 96%.
washing with diethyl ether. This product was re-dissolved in DMF FTIR spectrum (Zn-Se ATR, cmÀ1): 910 (–CQS). MS: (ESI+,
and subjected to vapor diffusion of diethyl ether that produced a MeOH, m/z): calc. 153.0374 for 4-methoxybenzothialdehyde;
dark green crystalline product within two days. Yield 0.8 g (72%). found 153.0372 for M + H+. Naphthalene-1-carbothialdehyde.
Anal. calc. for C26H22N10O4S4NaCoÁ2H2O: C, 39.79; H, 3.34; Yield: 95%. FTIR spectrum (Zn-Se ATR, cmÀ1): 910 (–CQS).
N, 17.85; S, 16.34. Found: C, 39.72; H, 3.71; N, 17.61; S, 16.32.
Catalytic reactions
FTIR spectrum (KBr, selected peaks): 1618 (CQO) and 1559
(CQN) cmÀ1. Molar conductivity (DMF ~ 1 mM solution 298 K):
Cyanation reaction. In a typical cyanation reaction, 1 mol%
LM = 70 OÀ1 cm2 molÀ1. Absorption spectrum (lmax, nm, DMF catalyst (2, 3 or 4) was added to a mixture of carbaldehyde or
(e, MÀ1 cmÀ1)): 670 (140), 470 (sh, 1210). 1H NMR spectrum carbothialdehyde (1 equiv.) and trimethylsilylcyanide (3 equiv.),
(400 MHz, DMSO-d6): d = 8.39 (t, J = 8.1 Hz, 2H), 7.97 (d, J = and the reaction mixture was stirred at room temperature
8.0 Hz, 4H), 3.01 (t, J = 8.1 Hz, 8H), 2.67 (t, J = 8.1 Hz, 8H). for 4 h.
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Knoevenagel condensation reaction. In this reaction, 1 mol% with some of them lying at the two-fold axis of rotation. These
catalyst (2, 3 or 4) was added to a mixture of carbaldehyde or could not be modeled accurately, even by considering them as
carbothialdehyde (1 equiv.) and malononitrile (1.5 equiv.), and disordered water molecules. So these were removed by using the
the reaction mixture was stirred at room temperature for 4 h.
SQUEEZE routine of PLATON.48 The resultant model refined well
Ring-opening reaction. 2 mol% catalyst was added to a and also improved on its R index. The SQUEEZE results show the
mixture of oxirane or thiirane (1.0 equiv.) and aromatic amines recovery of electrons which match well with one water molecule
(1.0 equiv.), and the reaction mixture was stirred at room and 1–1.5 molecules of highly disordered DMF which was used as
temperature for 4 h.
the solvent during crystallization and/or synthesis. The final
General procedure for catalytic reaction. In all cases, reac- model does not show any significant residual electron density
tions were monitored either by thin-layer chromatography (TLC) peak. The model contains SAV of volume 266 Å3 because of the
and/or gas chromatography (GC). After the mentioned time, the removed electron density.
À
catalyst was filtered off and the filtrate was concentrated under
For HCP 3, disorder in the ClO4 ion and a thiazoline ring
À
reduced pressure. The crude product thus obtained was purified was observed. Atoms Cl, O1, O2, O3 and O4 of the ClO4 ion
by flash column chromatography on silica gel using the hexanes– and C25 and C26 of thiazoline ring were occupying two crystallo-
ethyl acetate mixture (5 : 1) as the eluent. The product(s) were graphic independent positions which could be located from the
separated, isolated and analysed by GC, GC-MS, and NMR tech- additional residual electron density. These atoms were modeled
niques. The recovered catalysts were washed with diethyl ether, by splitting them at two positions while also applying restraints.
dried and reused without further purification.
This resulted in improved data convergence and meaningful
bond distances. However, some disordered electron density
could not be resolved and thus corrected by using the SQUEEZE
Physical measurements
The FTIR spectra were recorded using either a Perkin-Elmer command of PLATON.48 The electrons recovered correspond to
FTIR-2000 or a Spectrum-Two spectrometer having Zn-Se ATR. 1–1.5 disordered DMF molecules. The model contains an SAV
The NMR spectra were recorded on a Jeol 400 MHz spectrometer. of volume 154 Å3 because of the removed electron density.
The absorption spectra were recorded using either a Perkin-Elmer
In the case of HCP 4, the refinement showed disorder in some
Lambda-25 or a Perkin-Elmer Lambda-35 spectrophotometer. The atoms (C12, C13, C25, C26, C21, C22, S1, S4 and N3) of the
conductivity measurements were carried out using a digital con- thiazoline rings. The thiazoline ring containing atoms S1 and N3
ductivity bridge from Popular Traders, India (model number: PT showed rotational disorder with both atoms sharing their position.
825). The elemental analysis data were obtained using an Elementar These atoms were observed to occupy two different positions
Analysen Systeme GmbH Vario EL-III instrument. Thermo- which were identified from the Fourier map. The disordered atoms
gravimetric analysis (TGA) was performed using a DTG 60 Shimadzu were successfully modelled by fixing distances between the atoms
at 5 1C minÀ1 heating rate under the nitrogen atmosphere.
by applying restraints. Furthermore, atoms C13, C25, C26, C21,
C22, and S4 were refined isotropically. For these atoms, site
occupancy factors (SOFs) were refined with the help of free variable
X-ray crystallography
Single crystal diffraction data for complexes 1 and 2 were part instruction. Refinement of the SOFs for disordered atoms
collected on an Oxford XCalibur CCD42 whereas for 3 and 4 gave the value 0.68. The structure shows large SAVs as a con-
on a Bruker Kappa Apex-CCD diffractometer,43,44 respectively, sequence of the crystal packing in the absence of any other signi-
that are equipped with graphite monochromatic Mo-Ka radia- ficant electron density. The crystallographic data collection and
tion (l = 0.71073 Å). The frames were collected at room structure refinement parameters are presented in Table S4 (ESI†).
temperature for 1, 2, and 4 and at 100 K for 3. The structures
were solved by the direct methods using SIR-9245 or SHELXS-
9746 and refined by the full-matrix least-squares refinement
techniques on F2 using the program SHELXL-9746 incorporated
in WINGX 1.8.05 crystallographic collective package.47 Hydrogen
atoms were fixed at the calculated position with isotropic thermal
parameters whereas all non-hydrogen atoms were refined aniso-
tropically. For 1, high Ueq was observed for atom C13 which was
successfully resolved by splitting it using the PART command due
to which the checkcif shows an A-level alert.
Acknowledgements
RG gratefully acknowledges the financial support from the
Science and Engineering Research Board (SERB), Govt. of India,
New Delhi and the University of Delhi. Crystallographic data
and instrumental facilities were provided by the CIF-USIC of this
university. DB and SP, respectively, thank UGC and CSIR for their
fellowships. The authors sincerely thank an anonymous reviewer
for helping with the crystallographic work.
For 2, disorder in the perchlorate ion could not be resolved
and the refinement is done with restraints over Cl–O distances.
Some disorder in the complex molecule was also seen which
could only be resolved for one amide oxygen atom O2 and C9 of
one of the thiazoline rings. These two atoms and C26 have been
refined with restraints on their bond distances. The difference
Fourier at this stage showed lots of scattered and diffuse electron
density peaks having heights ranging between 4.50 and 1.08 eÅ3
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