A carboxylate-rich metalloligand and its heterometallic coordination polymers: Syntheses, structures, topologies, and heterogeneous catalysis

Article abstract of DOI:10.1021/acs.cgd.6b00176

This work reports three heterometallic coordination polymers (HCPs), namely, [{(1′)₂Zn₈Na₂(H₂O)₂₁}·20H₂O]_n (2), [{(1′)(1')Cd₈(H₂O)₂₁}·32H₂

Full text of DOI:10.1021/acs.cgd.6b00176

Article
pubs.acs.org/crystal
A Carboxylate-Rich Metalloligand and Its Heterometallic
Coordination Polymers: Syntheses, Structures, Topologies, and
Heterogeneous Catalysis
Sumit Srivastava, Vijay Kumar, and Rajeev Gupta*
Department of Chemistry, University of Delhi, Delhi 110 007, India
S
* Supporting Information
ABSTRACT: This work reports three heterometallic coordi-
nation polymers (HCPs), namely, [{(1′)2Zn8Na2(H2O)21}·
20H2O]n (2), [{(1′)(1″)Cd8(H2O)21}·32H2O]n (3), and
[{(1′)2Mn8Na2(H2O)27}·3H2O]n (4), originated from a
common Co3+ based metalloligand 1 offering eight arylcarbox-
ylic acid groups where 1′ and 1″ respectively contribute eight
and six anionic carboxylate groups. The crystal structure
analyses display three-dimensional nature of all three HCPs
wherein metalloligands are connected through secondary
metals. Detailed topological analyses illustrate that the
metalloligands function as the nodes that are connected to
secondary building units (SBUs) composed of Zn2+, Cd2+, and Mn2+ ions coordinated by the arylcarboxylate groups. All three
HCPs effectively function as the heterogeneous catalysts for the Lewis acid assisted Knoevenagel condensation reactions of
assorted aldehydes with three different active methylene compounds.
coordination-bonding sensitive groups.57−72 We have illustrated
the effect of number, position, and orientation of such
appended functional groups, being offered from a metal-
INTRODUCTION
■
Crystalline coordination polymers constitute an important class
of materials due to their interesting architectures1−10 and
significant structure-based applications in sorption,11,12 sepra-
loligand, on the material design. Using diversified metal-
tion,13,14 sensing,15−17 ion-exchange and transport,18,19 proton
loligands, we have been able to showcase examples of hydrogen
bonded assemblies,54−56 discrete trimetallic complexes,57−61
and two- (2D) and three-dimensional (3D) heterobimetallic
coordination polymers.62−72 These examples adequately
illustrate the significance of the metalloligand concept for the
construction of ordered architectures.
conduction,20−22 optics,23−27 magnetic materials,28,29 devi-
ces,30,31 and catalysis.32−36 In particular, their utilization as
the heterogeneous catalysts in assorted organic transformation
reactions has often resulted due to their crystalline nature and
therefore orderly arrangement of the catalytic centers and their
unique ability to support the facile diffusion of substrates and
reagents, a feature reminiscent of zeolites.32−40 While the
literature presents numerous homometallic coordination
polymers, heterometallic coordination polymers incorporating
two different metals are comparatively fewer.41−44 Such a stark
difference between two categories is due to the synthetic
difficulty in placing two different metals in close proximity.
Although judiciously selected ligands, offering two different
coordination environments suitable to satisfy the geometrical
and/or electronic requirement of two different metals, have
been successfully used for the construction of heterometallic
coordination polymers (HCPs),45−53 such examples are limited
due to cumbersome syntheses.49,50,53 In this context, a well-
defined metalloligand,1 already having a primary metal ion but
offering additional coordination-sensitive functional groups to
potentially ligate a secondary metal ion, offers much better
prospects for the construction of HCPs.
Out of assorted metalloligands, the ones offering appended
arylcarboxylic acid groups (ML1 and ML2) were successfully
utilized for the synthesis of moderately porous crystalline HCPs
with noteworthy heterogeneous catalytic applications (Scheme
1).69−72 As a logical extension, we became interested to
incorporate aryldicarboxylic acid functional groups and evaluate
Scheme 1. Our Earlier Metalloligands (ML1 and ML2) and
the Carboxylate-Rich Metalloligand 1 Used in the Present
Work
Our research group has reported various metalloligands
offering assorted appended functional groups ranging from
hydrogen bonding (H-bonding) sensitive groups54−56 to
Received: February 2, 2016
Revised: March 28, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.6b00176
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
1568 (CO) cm−1. Absorption spectra (λ, nm, DMF (ε, M−1 cm−1):
650 (100), 475 (sh), 300 (sh). Conductivity (∼1 mM, DMF, 298 K):
ΛM = 80 Ω−1 cm2 mol−1 (the range for 1:1 electrolyte in CH3OH is
the role of such a carboxylate-rich metalloligand (1) in the
construction of various HCPs. Herein, we present the
syntheses, structures, topological analyses, and heterogeneous
catalytic properties of {Co3+-Zn2+}, {Co3+-Cd2+}, and {Co3+-
Mn2+} HCPs using a common Co3+-based metalloligand (1)
offering eight appended carboxylic acid groups. The resultant
HCPs display interesting structures and unprecedented top-
ologies and function as the heterogeneous catalysts for the
Knoevenagel condensation reactions.
1
65−160). H NMR spectrum (DMSO-d6, 400 MHz): 7.93 (t, 2H,
H10), 7.90 (s, 4H, H4), 7.57 (d, 4H, H9), 7.43 (s, 8H, H5). 13C NMR
spectrum (DMSO-d6, 100 MHz): δ = 167.07 (C2, COOH), 166.61
(C7, amide), 155.92 (C8), 146.84 (C6), 131.70 (C3), 131.43 (C10/
C9), 125.68 (C5), 124.79(C4).
Synthesis of HCPs. [{(1′)2Zn8Na2(H2O)21}·20H2O]n (2). HCP 2
was synthesized by treating a solution of Zn(OAc)2·2H2O (0.75 g,
0.34 mmol) in CH3OH (3 mL) with a solution of metalloligand 1 (0.1
g, 0.08 mmol) in CH3OH (10 mL). The reaction mixture was stirred
at room temperature for 2 h. Subsequently, a yellowish-green product
was collected, washed several times with CH3OH, and dried under
vacuum at ambient temperature. The crude product thus obtained was
redissolved in water and layered with isopropyl alcohol at room
temperature. After a period of three to four days, green colored crystals
suitable for diffraction analysis were collected. Yield: 0.11 g (84%).
Anal. Calcd for C92H78Co2N12Na2O61Zn8·20H2O: C, 32.74%; H,
3.52%; N, 4.98%. Found: C, 32.60%; H, 3.80%; N, 4.90% . FTIR
spectrum (Zn-Se attenuated total reflectance (ATR), selected peaks):
3347 (OH), 1565 (COO, CO) cm−1. Diffused reflectance
absorption spectra (λmax, nm): 616, 450.
[{(1′)(1″)Cd8(H2O)21}·32H2O]n (3). HCP 3 was prepared in a similar
fashion with an identical scale as that of HCP 2; however, Cd(OAc)2·
2H2O was used instead of Zn(OAc)2·2H2O. Green colored crystals of
3 were obtained after a period of three to four days. Yield: 0.15 g
(90%). Anal. Calcd for C92H80Co2N12O61Cd8·32H2O: C, 28.16%; H,
3.70%; N, 4.28%. Found: C, 28.66%; H, 3.39%; N, 4.16%. FTIR
spectrum (Zn-Se ATR, selected peaks): 3337 (OH), 1569 (COO, C
O) cm−1. Diffused reflectance absorption spectra (λmax, nm): 620, 470.
[{(1′)2Mn8(Na)2(H2O)27}·3H2O]n (4). HCP 4 was also synthesized in
a similar manner with an identical scale as mentioned for HCP 2,
however, using Mn(OAc)2·2H2O instead of Zn(OAc)2·2H2O. Green
colored crystals of HCP 4 were obtained after a period of four to five
days. Yield: 0.12 g (86%). Anal. Calcd for C92H90Co2N12Na2Mn8O67·
3H2O: C, 35.72%; H, 3.13%; N, 5.43%. Found: C, 35.96%; H, 3.39%;
N, 5.16%. FTIR spectrum (Zn-Se ATR, selected peaks): 3345 (OH),
1562 (COO, CO) cm−1. Diffused reflectance absorption spectra
(λmax, nm): 630, 470.
EXPERIMENTAL SECTION
■
Materials and Methods. Reagents of analytical grade were
procured from Sigma-Aldrich, Alfa-Aesar, and Spectrochem and were
used without further purification. The solvents were purified using
standard literature methods.73,74
Synthesis of 2,6-(3,5-Methylbenzoatecarbamoyl)pyridine
(H2L3,5‑COOMe). Pyridine-2,6-dicarboxylic acid (1.0 g, 6.0 mmol) was
added as a solid in one portion to a suspension of dimethyl 5-
aminoisophthalate (2.5 g, 12.00 mmol) in pyridine (10 mL), and the
mixture was stirred at 40 °C for 40 min. Triphenyl phosphite (4.0 g,
13.2 mmol) was added dropwise over 10 min to the aforementioned
mixture, and the temperature was increased to 90 °C. The reaction
mixture was stirred for 4 h at 90 °C. On cooling to room temperature,
a colorless precipitate was obtained, which was filtered, washed with
water, and dried. The crude product was purified by recrystallization
from aqueous CH3OH. Yield: 3.2 g, 97%. Anal. Calcd for
C27H23N3O10·4H2O: C, 52.17%; H, 5.03%; N, 6.76%. Found: C,
52.02%; H, 4.78%; N, 6.34%. FTIR spectrum (Zn-Se (ATR), selected
1
peaks): 3247 (N−H), 1726 (COO), 1665 (CO) cm−1. H NMR
spectrum (CDCl3, 400 MHz): δ = 3.9 (s, 6H, H6), 7.24−7.26 (m, 3H,
H1/H5), 8.56−8.54 (d, 2H, H2), 9.07 (s, 4H, H4), 11.05 (s, 2H, H3).
13C NMR spectrum (CDCl3, 100 MHz): δ = 52.23 (C10), 125.2 (C2/
C6), 125.59 (C8), 126.3(C7), 131.28 (C5), 138.27 (C1), 148.35
(C3), 161.3 (C4), 166.3 (C9).
Synthesis of 1P. The ligand H2L3,5‑COOMe (0.50 g, 0.909 mmol) was
dissolved in dinitrogen-flushed DMF and treated with solid NaH
(0.048 g, 1.99 mmol). The resulting mixture was then stirred for 30
min at room temperature, and solid [Co(H2O)6](ClO4)2] (0.166 g,
0.45 mmol) was added to the aforementioned mixture. After another
30 min of stirring, dry O2 was purged to the solution for 2 min. The
solution was finally stirred for 1 h at room temperature. The reaction
mixture was filtered, followed by the removal of the solvent under
reduced pressure. The product thus obtained was isolated after
washing with diethyl ether. The crude product was dissolved in DMF
and subjected to vapor diffusion of diethyl ether, which afforded a
crystalline material within 2 days. Yield: 0.58 g, 88%. Anal. Calcd for
C54H42N6O20CoNa·4C3H7NO: C, 53.95%; H, 4.80%; N, 9.53%.
Found: C, 53.46%; H, 4.46%; N, 9.55%. FTIR spectrum (Zn-Se
(ATR), selected peaks): 3424 (OH), 1712 (COO), 1608, 1581 (C
O) cm−1. Absorption spectra (λmax, nm, DMF (ε, M−1 cm−1): 650
(95), 475 (sh). Conductivity (∼1 mM, DMF, 298 K): ΛM = 85 Ω−1
cm2 mol−1 (the range for 1:1 electrolyte in DMF is 65−160). 1H NMR
spectrum (DMSO-d6,400 MHz): 3.78 (s, 24H, H1/H1′), 7.95 (s, 4H,
H2/H2′), 7.48 (s, 8H, H3/H3′), 7.69 (d, 4H, H4/H4′), 8.00 (t, 2H,
H5/H5′). 13C NMR spectrum (DMSO-d6, 100 MHz): δ = 51.91
(OMe), 166.24 (C1), 162.48 (C6), 155.58 (C7), 146.33 (C5), 139.74
(C2), 131.13 (C9/C8), 125.23 (C4), 124.31 (C3).
General Procedure for the Knoevenagel Condensation
Reaction. The respective aldehyde and active methylene substrates
were dissolved in ethanol. The solid catalyst (1 mol %, 2, 3, and 4;
based on the repeating unit of the polymer) was then added, and the
reaction mixture was stirred at 50 °C for 1−3 h. The progress of the
reaction was monitored by TLC (5% EtOAc/hexane) and/or GC. The
solvent was removed under the reduced pressure, and the residue was
triturated with ethyl acetate. The catalyst thus separated was collected
by filtration, and the filtrate was concentrated to afford the crude
organic product. This product was purified by column chromatog-
raphy with 100−200 mesh silica using 1:1 EtOAc/hexanes as the
1
eluent. The products were identified by GC, GC−MS, H NMR, and
13C NMR spectroscopy when required. The isolated yield as well as
characterization data for a few representative products is provided
below whereas their 1H and 13C NMR spectra are included in Figures
S14−S25.
Characterization Data for a Few Representative Products.
1
(E)-Ethyl 2-Cyano-3-(4-nitrophenyl)acrylate. Yield: 95%. H NMR
Synthesis of 1. The compound 1P (1 g, 1 mmol) was dissolved in a
mixture of THF/H2O (3:1, v/v; 25 mL) and treated with 8 equiv of
NaOH. The reaction mixture was stirred for 12 h at room temperature.
The resulting solution was neutralized using 3 N HCl. Removal of
THF under reduced pressure resulted in the precipitation of the
product, which was isolated and dried under vacuum. The crude
product was recrystallized from DMF after diffusing vapors of diethyl
ether, which produced a highly crystalline product within 2−3 days.
Yield: 0.35 g, 78%. Anal. Calcd for C110H94Co2N18Na2O46·8C3H7NO:
C, 51.05%; H, 4.80%; N, 11.55%. Found: C, 51.49%; H, 4.33%; N,
11.73%. FTIR spectrum (Zn-Se (ATR), selected peaks): 1714 (COO),
spectrum (400 MHz, CDCl3): δ = 8.35−8.33 (d, J = 8.8 Hz, 2H), 8.29
(s, 1H), 8.11−8.14 (d, J = 8.8 Hz, 2H), 4.44−4.39 (q, J1 = 7.2 Hz, J2 =
14.4, 2H), 1.39−1.43 (t, J = 6.8 Hz, 3H). 13C NMR spectrum (100
MHz, CDCl3): δ = 161.451 151.86, 149.79, 136.97, 131.61, 124.43,
114.64, 107.43, 63.47, 14.20. ESI-MS spectrum (EtOAc): m/z = 246.
(E)-Ethyl 2-Cyano-3-(thiophen-2-yl)acrylate. Yield: 80%. 1H NMR
spectrum (400 MHz, CDCl3): δ = 8.34 (s, 1H), 7.82−7.80 (d, J = 7.8,
1H), 7.79−7.77 (d, J = 7.8, 1H), 7.25−7.23 (m, J = 7.23, 1H), 4.38−
4.33 (q, J = 0.36, 2H), 1.39−1.36 (t, 3H). 13C NMR spectrum (100
MHz, CDCl3): δ = 162.80, 148.83, 137.30, 136.09, 135.28, 128.71,
115.84, 99.36, 62.67, 14.30. ESI-MS spectrum (EtOAc): m/z = 207.
B
DOI: 10.1021/acs.cgd.6b00176
Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Article
Table 1. Crystallographic Data Collection and Structure Refinement Parameters for HCPs 2−4
HCP 2
HCP 3
HCP 4
mol formula
fw
C92H78Co2N12O61Na2Zn8·20H2O
3164.29
293(2)
C92H80Co2N12O61Cd8·32H2O
C92H90Co2N12O67Na2Mn8·3H2O
3736.40
293(2)
triclinic
2984.69
293(2)
triclinic
T (K)
cryst syst
space group
a (Å)
monoclinic
C2/m
P1
P1
̅
̅
19.108(5)
28.704(5)
13.646(5)
90
16.413(5)
18.430(5)
24.700(5)
87.900(5)
85.395(5)
75.058(5)
7195(3)
2
16.280(5)
18.430(5)
23.550(4)
89.061(5)
89.688(5)
77.706(5)
6903(3)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
99.582(5)
90
1380(4)
2
ρ (g cm−3
)
1.424
1.725
1.436
F (000)
3144.0
3632
2968
μ(Mo Kα) (mm−1
collected reflns
unique reflns
no. of obs
)
1.594
1.491
1.040
48728
98875
79829
6652(0.0653)
5423
25307(0.0894)
17973
37922(0.0497)
21111
goodness of fit (F2)
1.148
1.012
1.117
a
b
R1 , wR2 [I > 2σ(I)]
0.0796, 0.2697
0.0944, 0.2843
0.0901, 0.2232
0.1212, 0.2416
0.0775, 0.2130
0.1055, 0.2292
a
b
R1 , wR2 [all data]
a
b
4
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑[w(|Fo|2 − |Fc|2)2]/∑[wFo ]}1/2
.
2-(Phenylmethylene)malononitrile. Yield: 95%. 1H NMR spec-
°/min at room temperature. SEM measurements were performed with
a Jeol SM 6610 LV instrument.
trum (400 MHz, CDCl3): δ = 7.91−7.88 (d, J = 7.9 Hz, 2H), 7.77 (s,
1H), 7.64−7.60 (t, J = 7.3 Hz, 1H), 7.55−7.51 (t, J = 7.9 Hz, 2H). 13C
NMR spectrum (100 MHz, CDCl3): δ = 160.02, 134.74, 130.83,
130.28, 129.87, 114.12, 112.65, 82.92. ESI-MS spectrum (EtOAc): m/
z = 154.
Single-Crystal Structure Determination. The intensity data for
HCPs 2−4 were collected at 298 K with an Oxford XCalibur CCD
diffractometer equipped with graphite monochromated Mo Kα
radiation (λ = 0.71073 Å).75 An empirical absorption correction was
applied using spherical harmonics implemented in the SCALE3
ABSPACK scaling algorithm.75 The structures were solved by direct
methods using SIR-9276 and refined by full-matrix least-squares
refinement techniques on F2 using the program SHELXL-97 for HCP
2 and SHELXH-97 for HCPs 3 and 4 in the WinGX module.77,78 The
hydrogen atoms were fixed at the calculated positions with isotropic
thermal parameters whereas all non-hydrogen atoms were refined
anisotropically. In all cases, hydrogen atoms of the coordinated and
lattice water molecules could not be located from the difference
Fourier map; however, their numbers have been included in the
empirical formulas. In addition, a few highly disordered water
molecules were removed by using the SQUEEZE procedure
implemented in PLATON,79 which considerably improved the
structure convergence; however, their numbers have been added to
the empirical formulas of all HCPs. For HCP 4, some unassigned
electron density was noted very close to Mn atoms (<1 Å) due to
absorption artifacts. Details of the crystallographic data collection and
structural solution parameters are provided in Table 1.
1
2-((Thiophen-2-yl)methylene)malononitrile. Yield: 90%. H NMR
spectrum (400 MHz, CDCl3): δ = 7.87 (s, 2H), 7.80−7.79 (d, J = 3.6
Hz, 1H), 7.27−7.25 (dd, J = 4.8 Hz, 1H). 13C NMR spectrum (100
MHz, CDCl3): δ = 151.30, 138.41, 137.09, 135.48, 129.15, 113.92,
113.08, 78.31. ESI-MS spectrum (EtOAc): m/z = 160.
2-((Furan-2-yl)methylene)malononitrile. Yield: 80%. 1H NMR
spectrum (400 MHz, CDCl3): δ = 7.78−7.7 (d, J = 1.83 Hz, 1H), 7.51
(s, 1H), 7.32−7.24 (d, J = 3.6 Hz, 1H), 6.69−6.68 (dd, J = 1.8 Hz,
1H). 13C NMR spectrum (100 MHz, CDCl3): δ = 149.81, 148.51,
143.29, 123.91, 114.59, 113.99, 112.77. ESI-MS spectrum (EtOAc):
m/z = 144.
2-(3-Methoxybenzylidine)malononitrile. Yield: 95%. 1H NMR
spectrum (400 MHz, CDCl3): δ = 7.73 (s, 1H), 7.48 (s, 1H),
7.45−7.39 (m, 2H), 7.17−7.15 (m, 1H), 3.85 (s, 3H). 13C NMR
spectrum (100 MHz, CDCl3): δ = 162.21, 160.03, 132.12, 130.69,
124.04, 121.48, 114.13, 113.76, 112.71, 83.03, 55.63. ESI-MS spectrum
(EtOAc): m/z = 184.
Physical Methods. The FTIR spectra (Zn-Se ATR, 4000−600
cm−1) were recorded with a PerkinElmer Spectrum-Two spectrometer.
The NMR spectroscopic measurements were carried out with a Jeol
400 MHz spectrometer. The absorption spectra were recorded with
either PerkinElmer Lambda 25 or Lambda 35 spectrophotometers.
The elemental analysis data were obtained with an Elementar Analysen
Systeme GmbH Vario EL-III instrument. Gas chromatographic
analyses were done with a PerkinElmer Clarus-580 instrument having
an Elite-5 column. GC−MS studies were performed with a Shimadzu
instrument (QP 2010) with an RTX-5SIL-MS column. Thermal
gravimetric analysis (TGA) was performed with a DTG 60 Shimadzu
at 5 °C min−1 heating rate under a nitrogen atmosphere. The X-ray
powder diffraction (XRPD) studies were performed with either an
X’Pert Pro from Panalytical or a Bruker AXS D8 Discover instrument
(Cu Kα radiation, λ = 1.54184 Å). The samples were ground and
subjected to the range of θ = 5−30 °C with a scan rate of 1−0.72
RESULT AND DISCUSSION
■
Synthesis and Characterization. The metalloligand
Na[Co(L3,5‑COOH)2] (1) offers eight uncoordinated arylcarbox-
ylic acid groups. This metalloligand on reaction with M(OAc)2·
2H2O salts produced the desired HCPs 2−4 (Scheme 2). It is
important to note that the metalloligand 1 was found to exist in
either its 9-form (1′) or its 7-form (1″) in the resultant HCPs
(Scheme 2). Reaction of 1 with secondary metal salt resulted in
a distinct color change from deep green to pale green followed
by the precipitation of compound. All three HCPs were
recrystallized from H2O. The three HCPs exhibit strong bands
between 1555 and 1570 cm−1 due to νCOO as well as νCO
stretches in their FTIR spectra. In addition, broad features
C
DOI: 10.1021/acs.cgd.6b00176
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
parameters. Metalloligand 1 has four aryldicarboxylic acid
functional groups and as a consequence offers eight free
carboxylic acid groups. HCPs 2−4 are constituted from
metalloligand 1 offering arylcarboxylic acid groups that
coordinate to Zn2+, Cd2+, and Mn2+ ions, respectively. Notably,
metalloligand 1 was found to coordinate the secondary metals
either through all eight carboxylate groups (1′ form) or via six
−COO− and two −COOH groups (1″ form; cf. Scheme 2).
The molecular structures of HCPs 2, 3, and 4 are quite
different; however, the central metalloligand core is nearly
identical, in which two tridentate ligands create an N6
coordination environment around the Co3+ ion. Two ligands
coordinate the Co3+ ion via five-membered chelate rings
between Namide and Npyridine atoms. The Co−Npyridine bond
distances (1.853−1.867 Å) are much shorter than Co−Namide
bond distances (1.938−1.974 Å).57,58 Further, all diagonal
angles are close to linear, ranging from 174.8° to 179.6°,
suggesting a symmetrical arrangement of donors around the
cobalt ion. Notably, two tridentate ligands enforce a com-
pressed octahedral geometry around the Co3+ ion.57,58
HCP 2 was crystallized in the monoclinic cell with C2/m
space group. The unit cell of HCP 2 contains 2 metalloligands,
8 Zn2+ ions, 2 Na+ ions, and 21 coordinated and 20
uncoordinated water molecules. A thermal ellipsoidal repre-
sentation of asymmetric unit without lattice water molecules is
shown in Figure 1a. The crystal structure of HCP 2 shows that
the Co3+-based metalloligands are connected through the
secondary Zn2+ ions. Every metalloligand offers four appended
aryldicarboxylic acid groups, and as a result, a total eight
carboxylate groups are available. Out of eight Ocarboxylate atoms,
O3 and O4 coordinate to Zn1 atom, Ocarboxylate atoms O5 and
O6 bridge Zn2 and Zn3 atoms, O7 ligates to Zn1 while O8
remains free, and the remaining two, O9 and O10, coordinate
to Na+ and Zn2 ions, respectively (Figure 1b). The six-
coordinated Zn1 is ligated by three Ocarboxylate atoms (O3 and
O4 from one metalloligand and O7 from another metal-
loligand) while the remaining three sites are occupied by the
water molecules (O1W, O2W, and O3W) (Figure 1c). The six-
coordinated Zn2 ion is coordinated by four Ocarboxylate atoms
(O5, O5#, O9, and O9#) from different metalloligands and two
axial water molecules (O5W and O4W) (Figure 1c). The
seven-coordinated Zn3 is ligated by two Ocarboxylate atoms (O6
and O6#) from two different metalloligands and five water
molecules (O5W, O6W, O8W, O6W, and O8W) (Figure 1c).
The four-coordinated Na+ ion is ligated by two Ocarboxylate (O10
and O10#) from two different metalloligands and two water
molecules (O5W and O7W). The secondary Zn2+ ions
coordinated by carboxylate groups constitute the secondary
building units (SBUs) that are connected to different
Scheme 2. Metalloligand 1 and Its Two Observed Anionic
Forms 1′ and 1″ and the Resultant Heterometallic
Coordination Polymers 2−4
between 3300 and 3400 cm−1 are indicative of the presence of
coordinated as well as lattice water molecules (Figure S1).
HCPs 2−4 displayed the absorption maxima in the range of
615−630 nm in their diffused reflectance absorption spectra
(Figure S2). Such a band is tentatively assigned to the d−d
transition in the Co3+ ion of the metalloligand core. The three
HCPs were also characterized by measuring their XRPD
patterns. The XRPD patterns of the as-synthesized HCPs 2−4
were found to be in great agreement to those simulated from
the single-crystal structural data. Such a fact indicates phase
purity of the bulk crystalline sample (Figures S3−S5).
Thermogravimetric analysis (TGA) and differential scanning
colorimetry (DSC) studies of HCPs 2−4 reveal a weight loss
between 25 and 180 °C assigned to the loss of water molecules
(Figures S6 and S7). For HCP 2, observed and calculated
weight losses were 7.56 and 7.38% for the loss of 41 (21
coordinated and 20 uncoordinated) water molecules. Similarly,
for HCP 3, observed and calculated weight changes were 10.37
and 9.54% for the loss of 21 coordinated and 32 uncoordinated
water molecules. For HCP 4, thermal weight loss corresponds
to the loss of 9 coordinated and 28 uncoordinated water
molecules (obsd/calcd, 5.44/5.40). Importantly, TGA and
DSC studies were adequately supported by the microanalysis
results and crystallographic data for all three HCPs 2−4.
Thermal studies further suggested that all three HCPs are
thermally stable close to 400 °C.
Crystal Structures and Topological Analysis. All three
HCPs 2−4 were characterized by single crystal X-ray diffraction
analysis and then simplified by the topological analysis to
understand the network generation via the mediation of
metalloligands. For all three HCPs, Table 2 provides selected
bond distances whereas Tables S1−S3 contain detailed bonding
Table 2. Selected Bond Distances and Ranges (Å) for HCPs 2−4
bond
HCP 2
HCP 3
1.946(8)
HCP 4
1.952(4)
Co1−N1
Co1−N2
Co1−N3
Co1−N4
Co1−N5
Co1−N6
1.962
1.859
1.958
1.853(8)
1.856(3)
1.974(8)
1.948(5)
1.960(7)
1.955(4)
1.867(8)
1.858(4)
1.963(7)
1.938(4)
a
M −Ocarboxylate
M −Owater
1.949(6)−2.361(6)
2.052(8)−2.290
2.173(8)−2.581(9)
2.24 (1)−2.515(9)
2.099(5)−2.379(9)
2.01(2)−2.441(7)
a
a
M = Zn2+ (for HCP 2), Cd2+ (for HCP 3), Mn2+ (for HCP 4).
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Figure 1. (a) Asymmetric unit of HCP 2; thermal ellipsoids are drawn at the 30% probability level, whereas hydrogen atoms and lattice water
molecules are omitted for clarity. Color code: light green, Co; yellow, Zn; blue, N; green, Ocarboxylate; purple, Owater; red, Oamide; gray, C; pink, Na. (b)
Wire and stick representation of a selected part of HCP 2 which shows the coordination of metalloligand 1 with secondary Zn2+ and Na+ ions. (c)
Polyhedron representations of the coordination spheres of Zn(II) ions (Zn1−Zn3). (d) Space-filling mode of the extended structure of HCP 2 along
the a-axis. Color code: selected part of HCP in red; yellow, Zn; blue, Na. (e) Topological representation of HCP 2. Color code: green, dimer of
metalloligand 1; yellow, Zn.
metalloligands generating a 3D architecture (Figure 1d). HCP
2 offers solvent-accessible voids (SAVs) of 1935.5 Å3,
corresponding to 26.2% of the unit cell volume (7380.0 Å3),
which are filled by the lattice water molecules.
Zn(II) ions (8Zn1, 6Zn2 and 6Zn3 ions). Herein, Zn1, Zn2,
and Zn3 atoms were found to respectively connect to 2, 3, and
3 ML nodes. Such an arrangement generates a network with an
overall point symbol of {4∧3}4{4∧42.6∧24.8∧108.12∧16}
{4}4 where the individual contribution of Zn1, Zn2, and Zn3
was {4}, {4∧3}, and {4∧3}, respectively, whereas metalloligand
contributed {4∧42.6∧24.8∧108.12∧16} (Figure 1e).
HCP 2 was simplified by the ToposPro software80−83 using
the standard simplification method. HCP 2 contains different
types of SBUs69−72,84,85 that are composed of Zn atom
coordinated by a certain number of arylcarboxylate groups,
such as Zn1(COO)2, Zn2(COO)4, and Zn3(COO)2. Such
SBUs function as the SBU nodes in the resultant network.86−91
Further, Co3+-based metalloligands also act as the multitopic
nodes (defined as the ML nodes) that are connected with
different SBUs to generate a network with a unique
topology.69−72 In the present case, a dimer of metalloligands
acting as a ML node was connected to a total of 20 different
HCP 3 was crystallized in triclinic cell with P1 space group.
̅
The unit cell of HCP 3 contains 2 metalloligands, 8 Cd2+ ions,
and 21 coordinated and 32 lattice water molecules. Figure 2a
displays the asymmetric unit of HCP 3, however, without the
lattice water molecules. In this case, two metalloligands are
connected with 8 symmetrically unique Cd2+ ions (Figure 2b).
Two metalloligands offer 16 carboxylic acid groups; however,
only 14 exist in the anionic carboxylate form and coordinate to
E
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Figure 2. (a) Asymmetric unit of HCP 3; thermal ellipsoids are drawn at the 30% probability level, whereas hydrogen atoms and lattice water
molecules are omitted for clarity. Color code: light green, Co; yellow, Cd; blue, N; green, Ocarboxylate; purple, Owater; red, Oamide; gray, C. (b) Wire and
stick representation of a selected part of HCP 3 which shows the coordination of metalloligand 1 with secondary Cd2+ ions. (c) Wire and stick
representations of the appended aryldicarboxylic acid groups and their coordination with Cd2+ ions (Cd1−Cd8).
Cd2+ ions whereas two groups remain as the carboxylic acid and
participate in intermolecular H-bonding with the lattice water
molecules. Figure 2c displays different coordination modes of
the carboxylate groups as well as carboxylic acid synthons. HCP
3 displayed SAVs of volume 595.5 Å3 that corresponded to
8.3% (7195.0 Å3) of its cell volume.
types of multitopic metalloligand-based nodes, ML-1 and ML-2
(shown in red and blue, respectively), which were connected to
12 and 6 Cd ions, respectively. The network topological
analysis revealed an overall point symbol of {4.6.8}
{4∧15.6∧19.8∧27.10∧5}{4∧3.6∧3.8∧9}{4∧3}{4∧4.6∧2}
{4}2{8}2 (Figure 3c) where metalloligand-based nodes, ML-1
and ML-2, contributed {4∧15.6∧19.8∧27.10∧5} and
{4∧3.6∧3.8∧9}, respectively.84,85 On the other hand, SBUs
based on Cd1, Cd2, Cd3, Cd4, Cd5, Cd6, and Cd8 respectively
provided {4∧4.6∧2}, {4.6.8}, {4}, {8}, {8}, {4∧3}, and {4}
point symbols.
The coordination environments of eight different Cd2+ ions,
described by polyhedra, are shown in Figure 3a. Importantly,
out of eight Cd2+ ions, three are six-coordinated (Cd1, Cd3,
and Cd7) whereas the remaining five are seven-coordinated
(Cd2, Cd4, Cd5, Cd6, and Cd8). The coordination environ-
ment around various Cd2+ ions changes drastically. For
example, six-coordinated Cd1 ion is ligated only to Ocarboxylate
atoms (O3, O4, O8, O13, O23, and O32) whereas six-
coordinated Cd7 atom is ligated with one O27carboxylate atom
and five water molecules (O17W, O18W, O19W, O20W, and
O21W). The Cd(COO)n based SBUs [Cd1(COO)4, Cd2-
(COO)3, Cd3(COO)3, Cd4(COO)2, Cd5(COO)2, Cd6-
(COO)3, Cd7(COO), and Cd8(COO)2] are connected with
different metalloligands to generate a 3D structure (Figure 3b).
Topological analysis of HCP 3 shows that different SBU nodes
are connected to ML nodes;69−72,84,85 however, topological
contribution of SBU Cd7(COO) was not considered as it was
not involved in extending the network. Notably, there were two
HCP 4 was crystallized in triclinic cell with P1 space group.
̅
The unit cell of HCP 4 contains 2 metalloligands, 8 Mn2+ ions,
2 Na+ ions, and 27 coordinated and 3 uncoordinated water
molecules. The asymmetric unit of HCP 4 is presented in
Figure 4a, however, without lattice water molecules. In this case
also, a dimer composed of two metalloligands is involved in
generating the network via Mn(COO)n-based SBUs (Figure
4b). Out of 16 carboxylate groups from a dimer of two
metalloligands, 15 −COO− groups coordinate to 8 Mn2+ and 2
Na+ ions whereas one carboxylate group (with O7 and O8
atoms) does not participate in bonding (Figure 4c).
The local coordination environment of all SBUs, described
by the polyhedron representation, is shown in Figure 5a.
F
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Figure 3. (a) Polyhedron representations of the coordination spheres of Cd2+ ions (Cd1−Cd8) in HCP 3. Color code: yellow, Mn; green, Ocarboxylate
;
purple, Owater; gray, C. (b) Extended structure of HCP 3 along the a-axis. Color code: selected part of HCP in red; light green, Co; yellow, Cd. (c)
Topological representation of HCP 3. Color code: red and blue, two different types of metalloligands Co1 and Co2; yellow, Cd.
Interestingly, out of eight Mn2+ ions, four are six-coordinated
(Mn4, Mn6, Mn7, and Mn8) whereas the remaining four are
seven-coordinated (Mn1, Mn2, Mn3, and Mn5). Various Mn2+
ions receive coordination from a combination of Ocarboxylate and
Owater ranging from all six Ocarboxylate ligation in Mn4 to one
Ocarboxylate and five Owater in Mn8. A combination of bonding
between metalloligands and secondary Mn2+ ions generates a
3D network (Figure 5b). HCP 4 offered large SAVs of 1476.7
Å3 correlating to 21.4% of cell volume (6903.0 Å3). Such voids
were noted to be filled by the lattice water molecules.
utilized as the heterogeneous catalysts for various organic
transformations.32−36 The possibilities to carry out the catalysis
heterogeneously and perform structure-specific reactions have
been the major advantages of coordination polymers.92,93 The
crystal structures of three HCPs show several important
features that suggested their potential applications in catalysis.
For example, (i) all three HCPs contain Lewis acidic secondary
metal ions, Zn(II), Cd(II), and Mn(II), arranged in an orderly
manner; (ii) such secondary metals are coordinated by the
labile water molecules that are likely to be replaced by the
substrates and/or reagents; (iii) all three HCPs are thermally
stable close to 400 °C; and (iv) all three HCPs are insoluble in
common organic solvents therefore suggesting their probable
roles as the heterogeneous catalysts. We selected Knoevenagel
condensation reaction of aromatic aldehydes with substrates
having active methylene groups to provide benzylidene
derivatives.94,95 This reaction has been a favorite test reaction
to assess the strength of solid and heterogeneous catalysts.
Knoevenagel Condensation Reactions. Knoevenagel
condensations (KCs) are significant for their role in C−C
double bond formation and are frequently used in organic
synthesis.94,95 KCs can be catalyzed by various reagents, acids,
and bases, as well as assorted Lewis acids.94−104 In this context,
In topological simplification,80−85 SBUs Mn1(COO)2, Mn2-
(COO)2, Mn3(COO)2, Mn4(COO)4, Mn5(COO)2, Mn6-
(COO)5, and Mn7(COO)3 were found to contribute point
symbols {4}, {4}, {4∧3}, {4∧3}, {4∧3}, {4}, and {4},
respectively. The SBU based on Mn8(COO) did not
contribute to the point symbol as it was not involved in
extending the network. Similarly, the point symbol of
metalloligand-based node was {4∧30.6∧27.8∧68.10∧11}
whereas the overall point symbol of HCP 4 was found to be
{4∧30.6∧27.8∧68.10∧11}{4∧3}3{4}4 (Figure 5c).84,85
Heterogeneous Catalysis. As a new class of crystalline
porous materials, coordination polymers have attracted a great
deal of interest; and a number of such architectures have been
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Figure 4. (a) Asymmetric unit of HCP 4; thermal ellipsoids are drawn at the 30% probability level, whereas hydrogen atoms and lattice water
molecules are omitted for clarity. Color code: light green, Co; yellow, Mn; blue, N; green, Ocarboxylate; purple, Owater; red, Oamide; gray, C. (b) Wire and
stick representation of a selected part of network which shows the coordination of metalloligand 1 with secondary Mn2+ and Na+ ions. (c) Wire and
stick representations of the appended aryldicarboxylic acid groups and their coordination to Mn2+ (Mn1−Mn8) and Na+ ions (Na1 and Na2). Color
code: yellow, Mn; blue, N; green, Ocarboxylate; gray, C.
Lewis acid based heterogeneous catalysts, particularly reusable
ones, are important due to their high efficiency and ease in
product separation.94,95 HCPs 2−4 were tested as the potential
heterogeneous catalysts for KCs using assorted aromatic and
heterocyclic aldehydes with three active methylene compounds,
ethyl cyanoacetate, 1,3-diketocyclohexane, and malononitrile.
To optimize the best reaction conditions, several control
experiments were performed using HCP 2 as a model catalyst
in a reaction of benzaldehyde with ethyl cyanoacetate (Table
3). It was noted that the catalyst was essential, as no product
was obtained without it (entry 1). Entry 2 shows that only 1
mol % HCP 2 was able to catalyze the reaction and produces
the product in 40% at 25 °C; however, a mild elevated
temperature of 50 °C led to nearly quantitative reaction (entry
3). The presence of solvent plays an important role which
probably allows better mixing and thus a facile reaction
between the reagents (entry 4). The quantitative product yield
was noted at 4 h, although 98% transformation had already
taken place within 3 h (entry 5). Ethanol was adjudged to be
the best solvent for the condensation reaction, as other solvents
such as MeOH, MeCN, EtOAc, and toluene resulted in
considerably lower conversion (entry 6). Notably, use of water
as the solvent led to nearly 50% yield (data not shown);
however, it also caused partial dissolution of HCPs and
therefore was not used.105 Entry 7 displays that only 1 mol %
catalyst loading was sufficient for the complete reaction. Finally,
use of M(OAc)2·2H2O salts (M = Zn, Cd, and Mn) as the
catalysts resulted in negligible product formation (entry 8).
Such a fact suggests that catalytically active M(II) ions have not
leached out from the HCPs.
These optimized reaction conditions were employed to
explore the scope of catalysis for all three HCPs, and the results
are summarized in Table 4. Three different types of reagents
having active methylene groups, ethyl cyanoacetate, 1,3-
cyclohexanedione, and malononitrile, were used with assorted
aldehydes (Table 4). Thus, when benzaldehyde was treated
with ethyl cyanoacetate in the presence of only 1 mol % of any
of the HCPs, a smooth reaction took place that produced ethyl
2-cyano-3-phenylacrylate in high yield (entry 1).106 Subse-
quently, a variety of substituted aldehydes were treated with
ethyl cyanoacetate (entries 1−6), 1,3-cyclohexanedione (entries
7−12), and malononitrile (entries 13−18) in the presence of
catalysts, and such reactions afforded the desirable products in
high yield (Table 4).107 Importantly, assorted electronic
substituents present on a substrate did not result in
considerable effect on the yield. For example, substituted
benzaldehydes having p-NO2, p-Cl, or m-OMe substituents
produced the corresponding products in yield as good as noted
for benzaldehyde. Such an observation suggests that the
catalytic reactions are not driven by the electronic factors.
Further, heterocyclic aldehydes such as furan-2-carbaldehyde as
well as thiophene-2-carbaldehyde all provided high yield of the
respective products. Importantly, malononitrile resulted in
nearly quantitative transformation with most of the substrates
H
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Figure 5. (a) Polyhedron representations of the coordination sphere of Mn2+ (Mn1−Mn8). Color code: yellow, Mn; green, Ocarboxylate; purple, Owater
;
gray, C. (b) Extended structure of HCP 4 along the a-axis. Color code: selected part of HCP in red; green, Co; yellow, Mn; blue, Na. (b)
Topological representation of HCP 4. Color code: green; dimer of metalloligands Co1 and Co2; yellow, Mn.
Table 3. Control Experiments for the Knoevenagel Condensation of Benzaldehyde with Ethyl Cyanoacetate Using HCP 2 as a
Model Catalyst
a
entry
catalyst [mol %]
solvent
time (h)
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
EtOH
EtOH
EtOH
4
25
0
2 [1]
4
25
40
2 [1]
4
50
>99
2 [1]
4
25, 50
50
22, 55
2 [1]
EtOH
1, 2, 3, 4
40, 72, 98, >99
45, 18, 22, 35
70, 99, 99
2, 2, 4
2 [1]
MeOH, CH3CN, EtOAc, Toluene
4
4
4
50
2 [0.5, 1, 2]
EtOH
EtOH
50
Zn(OAc)2·2H2O, Cd(OAc)2·2H2O, Mn(OAc)2·2H2O
50
a
Yield was calculated using gas chromatography.
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and crystallinity to delineate possible structural change(s)
during the catalysis. In addition, such studies would also allow
exploring their recyclability. For such studies, HCP 2 was
selected as a model catalyst in the reaction of benzaldehyde
with ethyl cyanoacetate. HCP 2 could be successfully used for
five consecutive cycles without appreciable change in the
catalytic performance (Figure S8). The HCP 2 was recovered
after every catalytic cycle and characterized by the FTIR spectra
and powder XRD studies. In addition, scanning electron
micrograph (SEM) was recorded and compared to that of
pristine sample to evaluate the morphological changes to the
crystalline sample. The powder XRD pattern of the recovered
HCP 2 shows that it maintained its crystallinity as well as
structural integrity in the form of nearly superimposable peaks
as compared to pristine sample (Figure S9). A similar
conclusion can be drawn from the superimposable FTIR
spectrum of the recovered catalysts to that of as-synthesized
sample (Figure S10). These studies together with the
reusability of HCP 2 (cf. Figure S8) adequately support the
stable nature of HCPs in KCs. The SEM image of a single
crystal of recovered HCP 2 illustrates only minor changes to
the crystal surface, therefore further suggesting stable nature of
HCPs (Figure S11).
Mechanistic Insights. The present catalysis results rely on
two important assumptions: First, that the coordinated water
molecules present on the Lewis acidic metals are labile and
therefore are easily replaceable. Such a critical step allows a
substrate to interact with the Lewis acidic metal before its
activation followed by reaction with the active methylene
compound. Second, that a substrate is freely able to diffuse
through the pores and channels of HCPs. In order to support
both suggested points, additional experiments were performed
using HCP 2 as a representative case. To investigate that the
coordinated water molecules are indeed labile in nature and a
substrate is able to replace them, D2O exchange was performed.
For such an experiment, powdered sample of HCP 2 was first
heated at 150 °C under vacuum to remove lattice as well as
coordinated water molecules followed by allowing the sample
to equilibrate in a sealed environment of D2O vapors. Such an
experiment resulted in nearly complete exchange of H2O by
D2O, and the νO−D stretches were observed at ca. 2500 cm−1
with a shift of ca. 950 cm−1 from νO−H stretches (Figure S12).
To illustrate possible inclusion of a substrate within the pores
and channels of HCPs, FTIR spectra were recorded. Thus,
when a powdered sample of HCP 2 was impregnated with a
CH2Cl2 solution containing benzaldehyde, the FTIR spectrum
displayed red-shifted νCO stretches for the impregnated
benzaldehyde by ca. 15 cm−1 when compared to neat
benzaldehyde (Figure S13). Further, inclusion of benzaldehyde
resulted in considerable perturbation to the νO−H stretches of
the coordinated/lattice water molecules. Both these features
point toward inclusion of benzaldehyde within the pores of
HCP. More importantly, red-shifted νCO stretches of the
impregnated benzaldehyde strongly infer its activation by
coordination to the Lewis acidic metal ion. We therefore
conclude that an aldehydic substrate first replaces the
coordinated water molecule(s) followed by its activation by
the Lewis acidic metal before its reaction with the active
methylene compound to produce the desired benzylidene
derivatives.
Table 4. Knoevenagel Condensations of Assorted Aldehydes
with Ethyl Cyanoacetate (A), 1,3-Cyclohexanedione (B), and
Malononitrile (C) Using HCPs 2−4
a
b
yield (%)
entry
R
reagents time (h)
HCP 2
HCP 3 HCP 4
c
1
C6H5
A
A
A
A
A
A
B
B
B
B
B
B
C
C
C
C
C
C
3
3
3
3
3
3
3
3
3
3
3
3
1
1
1
1
1
1
98 (10)
99
92
94
83
87
90
91
75
83
82
85
80
82
96
99
>99
>99
89
90
95
96
86
80
83
77
78
82
80
80
75
65
95
99
>99
>99
80
90
2
4-NO2C6H4
4-ClC6H4
3
86
4
3-MeOC6H4
furan-2-yl
83
5
83
6
thiophen-2-yl
C6H5
86
c
7
80 (8)
8
4-NO2C6H4
4-ClC6H4
85
9
80
10
11
12
13
14
15
16
17
18
3-MeOC6H4
furan-2-yl
81
75
thiophen-2-yl
C6H5
79
c
98 (28)
99
4-NO2C6H4
4-ClC6H4
99
3-MeOC6H4
furan-2-yl
99
85
thiophen-2-yl
95
a
Conditions: catalyst, 1 mol %; solvent, EtOH; temperature, 50 °C.
Yield was calculated using gas chromatography. The values in
b
c
parentheses are the GC yield of the product without any catalyst under
the otherwise identical conditions.
within 1 h (entries 13−18). Such a fact suggests the importance
of an activated substrate in catalysis.
We then focused on plausible size-selective KC reactions108
and selected sterically taxing substrates such as 1-naphthalde-
hyde and 2-naphthaldehyde as well as 9-anthraldehyde (Table
5). For these sterically hindered substrates, the results of KCs
are compared to that of benzaldehyde. In general, product
yields for the bulkier substrates were quite on the lower side
when compared with benzaldehyde. We believe that, while
benzaldehyde (molecular size: 4.2 × 6.03 × 1.4 Å3) could freely
diffuse through the pores and channels offered by all three
HCPs, 1-naphthaldehyde (molecular size: 6.03 × 6.64 × 1.4
Å3), 2-naphthaldehyde (molecular size: 5.66 × 7.21 × 1.4 Å3),
and particularly 9-anthraldehyde (molecular size: 6.22 × 9.28 ×
1.4 Å3) were too much bulkier to have favorable accessibility.
Therefore, for these bulkier substrates, catalytic reaction
primarily occurred on the surface rather than within the
interior of the three HCPs. It is worth to note that yields were
always on the higher side whenever malononitrile was used.
Such a point suggests that an activated substrate could still
result in higher product yield despite catalysis primarily
occurring on the surface. Nevertheless, KC results depict that
substrate size accessibility plays an important role in
heterogeneous catalysis.
Recyclability Experiments. The heterogeneous nature of
catalysis allowed us to recover HCPs and evaluate their stability
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Table 5. Knoevenagel Condensations of a Few Bulkier Aldehydes with Ethyl Cyanoacetate (A), 1,3-Cyclohexanedione (B), and
a
Malononitrile (C) Using HCPs 2−4
a
b
Conditions: catalyst, 1 mol %; solvent, EtOH; temperature, 50 °C. Yield was calculated using gas chromatography.
CONCLUSION
AUTHOR INFORMATION
Corresponding Author
*Fax: +91-11-27666605. E-mail: rgupta@chemistry.du.ac.in.
Notes
■
■
We have utilized a carboxylate-rich Co3+-based metalloligand
for the construction of {Co3+-M2+} (M = Zn, Cd, and Mn)
heterometallic coordination polymers. The structure analyses
exhibited 3D nature of three HCPs wherein metalloligands are
connected through the secondary metals. The topological
analyses illustrated that the metalloligands function as the
nodes that are connected to SBUs composed of Zn2+, Cd2+, and
Mn2+ ions coordinated by the arylcarboxylate groups. All three
HCPs acted as the heterogeneous catalysts for the Knoevenagel
condensation reactions of assorted aldehydes. While the
recyclability experiments substantiated the reusable nature of
HCPs, the mechanistic studies suggested the potential
activation of a substrate by the secondary Lewis acidic metal
ion.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.G. gratefully acknowledges the financial support from the
Science and Engineering Research Board (SERB), New Delhi.
The authors thank the CIF-USIC of this university for the
instrumental facilities including single crystal X-ray diffraction
data collection. S.S. thanks UGC, New Delhi for the Senior
Research Fellowship.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.6b00176.
FTIR spectra, diffuse reflectance absorption spectra,
TGA and DSC plots, powder XRD patterns, SEM
1
images, H and 13C NMR spectra, and tables of bond
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Accession Codes
CCDC 1451491−1451493 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
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DOI: 10.1021/acs.cgd.6b00176
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