Synthesis of Metallomacrocycle and Coordination Polymers with Pyridine-Based Amidocarboxylate Ligands and Their Catalytic Activities towards the Henry and Knoevenagel Reactions

Article abstract of DOI:10.1002/open.201800170

The reactions of 3,3′-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}dibenzoic acid (H₂L) with zinc(II), cadmium(II), and samarium(III) nitrates were studied, and the obtained compounds, [Zn(1κO:2κO′-L)(H₂O)₂]_n (1), [

Full text of DOI:10.1002/open.201800170

DOI: 10.1002/open.201800170
Synthesis of Metallomacrocycle and Coordination
Polymers with Pyridine-Based Amidocarboxylate Ligands
and Their Catalytic Activities towards the Henry and
Knoevenagel Reactions
Anirban Karmakar,* Guilherme M. D. M. Rfflbio, M. Fµtima C. Guedes da Silva, and
[a]
Armando J. L. Pombeiro*
2À
The reactions of 3,3’-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}di-
(L ) orients in different conformations, that is, syn-syn for 1
and anti-anti for 2 and 3. Compound 1 is the first example in
which the syn-syn conformation for this ligand has been ob-
served. These compounds act as heterogeneous catalysts for
the nitroaldol (Henry; in water medium) and Knoevenagel con-
densation reactions of different aldehydes, and the most effec-
tive is zinc coordination polymer 1. Recyclability, heterogeneity,
and size-selectivity tests were performed, which showed that
the catalyst was highly active over at least four recycling runs.
benzoic acid (H L) with zinc(II), cadmium(II), and samarium(III)
2
nitrates were studied, and the obtained compounds,
2
2
[
Zn(1kO:2kO’-L)(H O) ] (1), [Cd(1kO :2kO -L)(H O) ] ·6nH O·
2 2 n 2 2 2 2
nC H O ·1.5nDMF
(2),
and
[Sm(1kO:2kO’O’’:3kO’’’-L)-
4
8
2
(
NO )(H O)(dmf)] ·nDMF (3), were characterized by elemental
3 2 n
analysis, FTIR spectroscopy, thermogravimetric analysis, and X-
ray single-crystal diffraction. Compounds 1 and 3 have 1D
zigzag- and double-chain-type structures, respectively, whereas
2
features a dinuclear metallomacrocyclic complex. The ligand
1. Introduction
[
5]
Coordination polymers are normally described as a group of
highly promising functional materials due to their fascinating
ate ligands has been accomplished. However, it is still a chal-
lenge for chemists to develop efficient synthetic strategies to
targeted structures and expected properties.
1
D, 2D, and 3D architectures (not necessarily porous) as well
as their widespread application in various areas such as gas
Recently, various coordination polymers were investigated as
adaptable supramolecular platforms to develop heterogeneous
[
1]
storage, molecular sensing, catalysis, and separations. Their
synthesis can be carefully designed towards specific structures,
[
6]
catalysts for diverse organic transformations, especially for
[2]
[7a–c]
topologies, and interesting properties. On the other hand,
the synthesis of metallomacrocyclic molecules by using metal-
liquid-phase reactions such as Knoevenagel condensation;
[
7d–g]
[8]
cyanosilylation of aldehydes and ketones;
Henry reaction;
[
3]
[9]
directed self-assembly is a current area of research activity.
oxidation of alkanes, alcohols, and olefins; Mukaiyama aldol
[
10]
[11]
The design and selection of suitable multidentate ligands and
metal ions are quite critical in the construction of self-assem-
reaction; ring opening of epoxides; and transesterificatio-
[
8a,12]
n.
This is mainly due to their unique physical and chemical
[4]
bled polymetallic structures. In recent years, marked improve-
ment in the synthesis of coordination polymers and metallo-
macrocyclic compounds with multidentate aromatic carboxyl-
properties, such as high porosity, catalytic activity, and specific
[
13]
structures with particular functionalities.
The Henry or nitroaldol reaction is one of the most powerful
and atom-economic reactions for CÀC bond formation from an
[
14]
[
a] Dr. A. Karmakar, G. M. D. M. Rfflbio, Prof. M. F. C. Guedes da Silva,
Prof. A. J. L. Pombeiro
Centro de Química Estrutural
Instituto Superior TØcnico
Universidade de Lisboa
aldehyde and a nitroalkane. It is widely used for the synthe-
sis of organic compounds of pharmaceutical significance.
Often, this reaction is performed in the presence of strong
bases, such as alkali metal hydroxides, alkoxides, or amines,
Av. Rovisco Pais, 1049-001, Lisbon (Portugal)
E-mail: anirbanchem@gmail.com
pombeiro@tecnico.ulisboa.pt
which leads to the dehydration of b-nitro alcohols with con-
[15]
comitant formation of a nitroolefin.
The development of
new catalysts and procedures for the Henry reaction is a
matter of current interest, namely, towards the reduction of
toxic byproducts and an increase in yield and diastereoselectiv-
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/open.201800170.
ꢀ
2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
ity. Although many metal complexes that can homogeneously
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
[8b,16]
catalyze this reaction are reported in the literature,
such
homogeneous systems suffer from the limitations of
a
common high catalyst loading and the inability to recycle the
catalyst.
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On the other hand, Knoevenagel condensation is another
Thus, in the present work, we synthesized such a proligand
and constructed two new 1D coordination polymers, namely,
[Zn(1kO:2kO’-L)(H O) ] (1) and [Sm(1kO:2kO’O’’:3kO’’’-L)-
useful CÀC bond-forming reaction that is widely used for the
[17]
synthesis of fine chemicals. It is generally catalyzed by Lewis
acids or bases and is extensively studied in homogeneous sys-
2
2 n
(NO )(H O)(dmf)] ·nDMF (3), and the dinuclear metallomacro-
3
2
n
[
18]
2
2
tems.
Although some coordination polymers also catalyze
cyclic
complex
[Cd(1kO :2kO -L)(H O) ] ·6nH O·nC H O ·
2 2 2 2 4 8 2
this reaction effectively, most of them require high catalyst
1.5nDMF (2). The compounds were characterized by elemental
analysis, FTIR spectroscopy, thermogravimetric analysis, and
single-crystal and powder X-ray diffraction analyses. Moreover,
these coordination polymers contain both Lewis acid and basic
centers (pyridyl and amido groups) and are insoluble in
common organic solvents, which make them promising candi-
dates for bifunctional heterogeneous catalysis. Thus, we tested
their heterogeneous catalytic activity towards the Henry and
Knoevenagel reactions of aldehydes under mild conditions.
[19]
loadings, high temperatures, and long reaction times.
Moreover, both the Henry and Knoevenagel condensation
reactions can be catalyzed by Lewis acids or bases, but a litera-
ture survey showed that most metal organic compounds cata-
lyze such reactions by Lewis acidic metal centers, whereas cat-
alytic activity by both Lewis acid–base containing coordination
[7d,17g–i]
polymers remains scant.
Difficulties in developing such
catalysts result from the fact that these groups can easily neu-
tralize each other. Therefore, it remains a challenge to develop
heterogeneous catalysts that can promote one-pot Lewis acid–
base reactions, for example, the Henry or Knoevenagel con-
densation. To overcome these challenges, our group recently
developed several amide-functionalized coordination polymers,
in which the amide group served as a Lewis basic center and
the metal served as a Lewis acid; these coordination polymers
were shown to be quite effective for catalyzing such reac-
2. Results and Discussion
2.1. Synthesis and Characterization
In general, we used solvothermal reaction procedures for the
syntheses of compounds 1–3. The solvothermal reaction of H2L
with zinc(II) nitrate hexahydrate in the presence of dimethylfor-
[
8]
tions. In the present work, we added more Lewis basic sites
amide and pyridine groups) in our ligand system and con-
mamide, methanol, and NH OH led to the formation of the 1D
4
(
coordination polymer [Zn(1kO:2kO’-L)(H O) ] (1), whereas
2
2 n
structed various coordination compounds with metals having
different Lewis acidities in extreme transition-metal groups,
upon performing a similar reaction with Sm(NO ) ·6H O in a
3
3
2
mixture of dimethylformamide (DMF) and water, the one-di-
3
+
2+
2+
that is, Sm , Zn , and Cd , which are hard, border-line, and
soft Lewis acids, respectively. We aimed to afford bifunctional
coordination polymers that are capable of serving as heteroge-
neous catalysts for the above reactions in a more efficient
manner and to compare the effects of different Lewis acid
metal characters.
mensional framework [Sm(1kO:2kO’O’’:3kO’’’-L)(NO )(H O)-
3
2
(
dmf)] ·nDMF (3) was formed. However, the solvothermal reac-
n
tion of Cd(NO ) ·4H O with H L in a DMF, 1,4-dioxane, and
3
2
2
2
NH OH mixture produced the dinuclear metallomacrocyclic-
4
2
2
type
.5nDMF (2) (Scheme 1).
In the IR spectra of 1–3, the characteristic strong bands of
complex
[Cd(1kO :2kO -L)(H O) ] ·6nH O·nC H O ·
2 2 2 2 4 8 2
1
In this context, we chose 3,3’-{(pyridine-2,6-dicarbonyl)bis(a-
zanediyl)}dibenzoic acid (H L) as the source of an organic linker
2
coordinated carboxylate groups appear at n˜ =1579–1550 and
due to the fact that the degree of deprotonation of the car-
boxylic groups could be easily tuned by changing the reaction
conditions, with expected formation of coordination polymers.
À1
1
385-1372 cm for the asymmetric and symmetric stretching
[
21a,b]
vibrations, respectively.
The bands in the n˜ =1680–
À1
1
670 cm regions are attributed to the C=C stretching fre-
Moreover, the amide functionality in H L could offer additional
2
quencies of the aromatic rings, and those in the n˜ =1623–
hydrogen-bonding sites as well as a basic center to the
frameworks. Various mono- and multinuclear complexes
with interesting architectures and properties and similar types
À1
1
612 cm range are attributed to the stretching frequency of
the amide C=O group. To prove the presence of Lewis acidic
sties in compound 1, we performed pyridine adsorption analy-
[20]
of ligands were recently reported. Moreover, not long ago a
few amide-based coordination polymers were shown to act
as heterogeneous catalysts for various organic transforma-
[
6d,e,8d,e,12a]
tions.
Hence, the two main objectives of the current
work were as follows: 1) to synthesize coordination polymers
by using a pyridine amidocarboxylate-based proligand, namely,
3
,3’-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}dibenzoic acid (H L),
2
as the linker source and different early and late transition-
metal ions under various hydrothermal conditions; 2) to apply
the synthesized coordination polymers as heterogeneous cata-
lysts for the Henry and Knoevenagel condensation reactions of
different aldehydes; 3) to compare the coordination and cata-
lytic behaviors of metals in extreme transition-metal periodic
II
II
10
groups, that is, Zn and Cd (with a closed d shell) in
III
group 12 and the 4f lanthanide Sm in group 3.
2
Scheme 1. Reactions of ligand H L with Zn, Cd and Sm nitrate salts.
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À1
sis and observed a small band at about n˜ =1443 cm , which is
1.959(9) Š]; each binds the metal in a monodentate fashion.
attributed to pyridine adsorption on such sites (Figure S4b,
The OÀZnÀO bond angles, within the range of 98.8(5) to
[21c,d]
[8a,d,23]
Supporting Information).
Due to insolubility in solvents
117.3(5)8, are similar to those found in the literature.
The
mostly commonly used for NMR spectroscopy, these frame-
works were only characterized by infrared spectroscopy, micro-
analysis, thermogravimetric analysis (Figure S4a), and single-
crystal X-ray diffraction.
observed nonplanarity of the organic ligand in 1 may result
from the relative twisting of the two CNO amide groups at-
tached to the pyridine ring (dihedral angles of 16.69 and
21.008), whereas the carboxylate sets are virtually in the plane
of the respective phenyl rings (dihedral angles of 3.35 and
II
9
.188). Due to the conformation of the organic ligand, the Zn
2
.2. Crystal Structure Analysis
cations are 19.505 Š apart in a chain, a distance that is consid-
erably larger than the shortest intermolecular distance of
6.041 Š between two metal ions in vicinal chains.
The molecular structures of compounds 1–3 were determined
by single-crystal X-ray diffraction analysis and are shown in Fig-
ures 1–3. Crystallographic data, selected bond lengths and
angles, as well as relevant hydrogen-bond contacts are pre-
sented in Tables S1–S3, respectively.
In compound 1, the hydrogen-bond interactions include
every amide NH atom, which donates to an amide O atom of a
vicinal molecule, and the water molecules, which contact with
the noncoordinated carboxylate O atoms. The intermolecular
organization in 1 is also characterized by several CÀH···O inter-
actions, which helps to expand the structure to the third
dimension.
Compound 1 crystallizes in the monoclinic C2/c space
group, and the asymmetric unit contains one zinc(II) ion, one
2
À
L
ligand, and two coordinated water molecules (Figure 1a,b);
it features a zigzag-type 1D polymeric chain but expands to
D by means of H-bond interactions (Figure 1c,d). The zinc
center presents a slightly distorted tetrahedral environment
II
3
Complex 2 is a dimeric Cd -based metallomacrocyclic com-
plex (Figure 2a,b) that crystallizes in the triclinic P1
¯
space
[
22]
2+
(
t4 =0.92)
made of two water molecules [Zn1ÀO7,
group. Its asymmetric unit contains one Cd ion, one doubly
2À
1
.985(14) Š; Zn1ÀO8, 2.043(15) Š] and two carboxylate oxygen
deprotonated L ligand, and two coordinated and three non-
2À
II
atoms from two L
units [Zn1ÀO1, 1.974(8) Š; Zn1ÀO5,
coordinated H O molecules. The Cd center presents a six-coor-
2
Figure 1. a) Schematic representation, b) molecular structure with partial atom labeling scheme, c) 1D zigzag structure, and d) 3D hydrogen bonded arrange-
ment of compound 1 [Hydrogen bonded pore sizes (7.5 Š X 8.7 Š)].
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Figure 2. a) Schematic representation, b) crystal structure with partial atom-labeling scheme, and c) hydrogen-bonded packing diagram (the water molecules
are presented as a space-filled model) for complex 2.
dinate environment, in which the four equatorial sites are oc-
cupied by two chelating bidentate carboxylate groups from
154.18], which, in turn, is also mutually hydrogen bonded with
the carboxylates and the coordinated water molecules, there-
fore expanding the structure to the second dimension (Fig-
ure 2c).
2
À
two L ligands [Cd1ÀO1, 2.384(10) Š; Cd1ÀO2, 2.381(9) Š;
Cd1ÀO5, 2.478(10) Š; Cd1ÀO6, 2.345(10) Š] and the axial posi-
tions engaged with two O atoms from water molecules [Cd1À
O7, 2.308(15) Š; Cd1ÀO8, 2.391(15) Š]. The OÀCdÀO bond
angles can be as short as 55.3(3)8 but reach 171.0(5)8 for the
almost colinear apical ligands. In view of the binding mode of
3
+
The asymmetric unit of framework 3 contains one Sm ion
2À
attached to one doubly deprotonated L ligand, one biden-
tate nitrate anion, one DMF molecule, and one water mole-
cule; one noncoordinated DMF molecule is also present in this
unit (Figure 3). The organic ligand acts as a bridging tetraden-
tate chelator by means of the carboxylate groups, and each
one binds two metals in a bridging bidentate syn-syn-type
2À
the carboxylate groups, the deprotonated organic ligand (L )
is almost planar, as attested by the angle between the least-
square planes of the phenyl groups relative to the pyridine
ring (2.11 and 7.028). Regardless of the deviations of the CNO
amide groups from the level of the pyridine ring (dihedral
angles of 6.82 and 15.308), the carboxylate sets deviate margin-
ally from the plane of the respective phenyl rings (dihedral
angles of 3.41 and 7.288); 5.469 Š is the intramolecular dis-
III
mode. Thus, the Sm cations participate in a bimetallic 8-mem-
bered (C O Sm ) metallacycle and a monometallic 18-mem-
2
4
2
bered (C N O Sm) metallacycle that share a CÀOÀSm boun-
12
3
2
dary. The metal has a coordination number of eight (higher
than those of 1 and 2, as expected), and four of the coordina-
tion sites are occupied by the oxygen atoms of the carboxylate
II
tance between the Cd cations, which is very close to the inter-
2À
molecular one (5.473 Š).
groups from four L ligands [Sm1ÀO1, 2.290(4) Š; Sm1ÀO2,
2.381(4) Š; Sm1ÀO5, 2.333(4) Š; Sm1ÀO6, 2.371(4) Š] and two
of the coordination sites are occupied by the chelating nitrate
anion [Sm1ÀO7, 2.530(5) Š; Sm1ÀO8, 2.535(4) Š]; the remain-
ing positions are engaged with a DMF molecule [Sm1ÀO11,
Despite their position towards the inside of the metallacycle
ring, the amide NH groups in 2 work as donors to the non-
coordinated water molecule [N1ÀH1···O10 dD-A =3.027(14) Š,
aDÀH···A 146.48; N3ÀH3···O10 dD-A =2.920(14) Š, aDÀH···A
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Figure 3. a) Schematic representation, b) molecular structure with partial atom-labeling scheme, and c) 1D structure for framework 3.
2
.385(5) Š] and a water molecule [Sm1ÀO10, 2.474(4) Š]. The
H10B···O8 dD-A =3.055(6) Š, aDÀH···A 1558]. Several CÀH···O
interactions also help to extend the structure into the third
dimension.
2À
III
way L coordinates to the Sm ions leads to a double-chain-
type 1D coordination polymer. The metal–oxygen bond
lengths are within the range of those usually encountered for
Additional to the aforementioned intermolecular interac-
tions, the three assemblies are also stabilized by short p···p in-
teractions of 3.779 (in 1), 3.565 (in 2), and 3.611 Š (in 3), always
involving the pyridine and one of the phenyl groups. In such
contacts, the rings are mutually parallel but are displaced by
angles of 18.5, 21.1, and 19.58, respectively.
[8e]
lanthanide–oxygen assemblies,
and the OÀSmÀO bond
2À
angles vary from 50.38(17) to 160.40(17)8. Twisting of the L
ligand in 3 is due not only to the dihedral angles between the
amide groups and the pyridine ring (7.98 and 13.498) but also
to those between the carboxylate sets and the related phenyl
groups (8.49 and 17.918). The shortest distance between two
As shown in Scheme 2, three different conformations can
III
2À
Sm ions in a chain is 5.105 Š, and the shortest distance be-
exist for the deprotonated ligand L , namely, anti-anti (in
tween vicinal chains is 9.443 Š.
which the carboxylate group and amide O atom orient in an
anti fashion), syn-syn (in which the carboxylate group and
amide O atom orient in a syn fashion), and anti-syn. Recently,
The coordinated water molecules are hydrogen bonded to
the O atoms of noncoordinated DMF and to the nitrate group
[
O10ÀH10A···O12 dD-A =2.681(8) Š, aDÀH···A 1508; O10À Zhou et al. prepared a 3D Cu metal–organic framework (MOF)
2À
Scheme 2. Three different conformations of L
.
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by using 3,3’-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}dibenzoate
the reaction conditions (temperature, reaction time, amount of
catalyst, solvent) was performed in a model nitroethane–ben-
zaldehyde system with 1 as the catalyst.
Blank reactions were tested with benzaldehyde in the ab-
sence of any metal catalyst at 708C in water, the reaction yield
of nitroalkanol was only 7% after 48 h (Table S4, entry 19).
2À
[20e]
(
L ) and observed anti-anti and anti-syn conformations,
but
not the syn-syn conformation. However, in our case, we ob-
tained the syn-syn conformation in the 3,3’-{(pyridine-2,6-dicar-
2À
bonyl)bis(azanediyl)}dibenzoate ligand (L ) for coordination
polymer 1 and the anti-anti conformation for compounds 2
and 3.
However, the use of proligand H L led to an overall yield of
2
2À
Moreover, the carboxylate groups in our L ligand can coor-
dinate to the metal centers in various fashions. For example, in
the case of compound 1 the carboxylate groups coordinate to
10% after 48 h (Table S4, entry 23). We also tested the activities
of the nitrate salts of zinc(II), cadmium(II), and samarium(III)
and obtained reaction yields in the range of 9 to 11%
II
the Zn ions in a monodentate fashion, whereas in 2 they coor-
(Table S4, entries 20–22). The use of 1:1 mixtures of H L and
2
II
II
II
III
dinate to the Cd centers in a chelating mode. For compound
the Zn , Cd , and Sm salts led to yields between 12 and 17%
(Table S4, entries 26–28).
3
, a bridging bidentate fashion is observed.
Upon using 3 mol% of 1 as the catalyst, a yield of 72% (syn/
anti=62:38) of the b-nitroalkanol from benzaldehyde was
reached (Table S4, entry 6). With 2 and 3, yields of 66 (syn/
anti=67:33) and 61% (syn/anti=60:40) were obtained, respec-
tively (Table S4, entries 17 and 18). Extending the reaction time
to 72 h did not increase the yield of the reaction. The plot of
yield versus time for the Henry reaction of benzaldehyde and
nitroethane with coordination polymer 1 is presented in Fig-
ure 4a.
3
. Catalytic Activity
A few amide-based MOFs and coordination polymers have
been reported to act as catalysts for the Henry and Knoevena-
[6d,e,19a,f]
gel condensation reactions.
The compounds prepared
in this study, particularly zinc(II) coordination polymer 1, have
both Lewis acid (Zn ) and basic centers (amide group). More-
over, all of their metal centers show at least two labile ligands
2
+
We also tested the effect of solvents, amount of catalyst,
and temperature in the Henry reaction. An increase in the
amount of catalyst 1 from 1.0 to 3.0 mol% enhanced the prod-
uct yield from 52 to 72% (Table S4, entries 6 and 7), but any
further increase in the amount of catalyst did not improve the
reaction yield (Table S4, entries 8 and 9).
(
H O and in the case of 3 also DMF and nitrate). Therefore,
2
they present promising features to act as bifunctional catalysts
for these types of reactions. In addition, on account of their in-
solubility in such solvents, their use as heterogeneous catalysts
[8]
should be particularly promising.
We performed experiments with various solvents (e.g.,
CH CN, THF, MeOH, EtOH, and H O) with catalyst 1 (Table S4,
3.1. Catalytic Activity in the Henry (Nitroaldol) Reaction
3
2
entries 6 and 10–13) to select the most suitable solvent for this
reaction. The results indicated that water (72% yield) was the
We tested the activities of 1–3 as heterogeneous catalysts for
the Henry (nitroaldol) reaction of various aldehydes with nitro-
ethane. In a typical reaction, a mixture of benzaldehyde, nitro-
ethane, and 3 mol% catalyst was placed in a glass vessel, and
best solvent, whereas the worst was CH CN (41% yield) for this
3
catalytic reaction. In THF, methanol, and ethanol, yields of 60,
6
1
8, and 63%, respectively, were obtained (Table S4, entries 10–
then H O was added (Scheme 3). The mixture was capped and
2
2).
heated at 708C for 48 h, after which it was cooled to room
temperature; the solid catalyst was then removed by centrifu-
gation. The products were extracted by using CH Cl , which
Varying the temperature from 25 to 708C improved the
yield of the b-nitroalkanol from 10 to 72% (Table S4, entries 6
and 14–16), but any further increase in the reaction tempera-
ture had a negative effect (Table S4, entry 16). The syn isomer
was the major one, and the system typically led to syn/anti
ratios in the range of 80:20 to 60:40 by using nitroethane as
the substrate. The size of the nitroalkane chain also affected
the yield, and with nitromethane and nitropropane, yields of
2
2
was evaporated under vacuum to give the crude product as a
mixture of b-nitroalkanol diastereomers (syn and anti forms,
with predominance of the former; Scheme 3). The diastereo-
1
mers were analyzed by H NMR spectroscopy (Figure S2), and
all the obtained results are presented in Table S4.
By using benzaldehyde as a test compound, we found that
7
9 and 61%, respectively, were obtained (Table S4, entries 24
and 25).
We also compared the activities of catalyst 1 in the reactions
1
gave a higher product yield than either 2 or 3 after the same
reaction time and temperature. Consequently, optimization of
of a variety of substituted aromatic and aliphatic aldehydes
with nitroethane to produce the corresponding b-nitroalkanols
with yields ranging from 19 to 97% (Table 1). Aryl aldehydes
bearing electron-withdrawing groups exhibited higher reactivi-
ties (Table 1, entries 1 and 2) than those bearing electron-do-
nating moieties; this may be related to an increase in the elec-
trophilicity of the substrate in the former case.
II
The efficiency of 1 relative to other previously reported Cu -,
II
III
Zn -, and Sm -based coordination polymers that have been
used as catalysts in the Henry reaction is shown in Table S6.
Scheme 3. Henry (nitroaldol) reaction of benzaldehyde with nitroethane and
typical conditions.
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Figure 4. a) Plot of yield versus time for the reaction of benzaldehyde and nitroethane with water as solvent at T=708C in the presence of catalyst 1.
b) Effect of catalyst recycling on the yield of the b-nitroalkanol resulting from the Henry reaction of benzaldehyde catalyzed by 1.
has many advantages owing to the unique properties of water,
Table 1. Henry reaction of various aldehydes and nitroethane with cata-
lyst 1.
[a]
which include nontoxicity, safety, and environmental benignity.
In that context, our complexes are new, effective, recyclable
[
b]
[c]
[d]
Entry Aldehyde
Yield [%] Selectivity (syn/anti) TON
(
see below), and environmentally “green” catalysts for the
1
2
3
4
5
6
7
p-nitrobenzaldehyde
p-chlorobenzaldehyde
p-methoxybenzaldehyde 19
p-methylbenzaldehyde 52
p-hydroxybenzaldehyde 60
93
67
65:35
61:39
67:33
62:38
66:34
58:42
85:15
31
22
7
17
20
19
32
Henry reaction.
The reaction mechanism is expected to be identical to that
proposed for related catalytic systems reported by our
[
8d,f,9d]
2+
group.
The Lewis acid center (Zn ) activates both nitro-
cinnamaldehyde
acetaldehyde
59
97
ethane (increasing its acidity) and the aldehyde (increasing its
electrophilic character). The amide group and free pyridyl
group of the ligand function as a Lewis base, and they assist in
deprotonation of acidic nitroethane with the formation of the
reactive nitronate species, which adds to the ligated aldehyde
through nucleophilic intramolecular attack with formation of a
CÀC bond, leading to the b-nitroalkanol. Proton abstraction
from the nitroalkane and protonation of the CÀC coupled spe-
cies is assisted by the ligand (with carboxylate and amide
groups) and also by water, which thus possibly accounts for
the good activity of our catalyst in the presence of water.
[
a] Reaction conditions: catalyst (3.0 mol%), aldehyde (0.5 mmol), nitro-
ethane (0.2 mL, 2.6 mmol), and water (1.0 mL) at 708C. [b] Number of
moles of b-nitroalkanol per 100 moles of aldehyde. [c] Calculated by
1
H NMR spectroscopy. [d] Number of moles of b-nitroalkanol per mole of
catalyst.
The reported yields are either lower or comparable to those
achieved with 1. For example, the reaction of 4-nitrobenzalde-
hyde and nitroethane in the presence of a 3D zinc(II) frame-
work based on 1,3,5-tri(4-carboxyphenoxy)benzene led to an
[
24a]
overall yield of only 15% after 72 h (Table S6, entry 4),
3.2. Catalytic Activity in the Knoevenagel Condensation
whereas a 3D Zn–DABCO (1,4-diazobicyclo[2.2.2]octane) frame-
work produced a higher yield of about 34% after 120 h
Reaction
[
24b]
II
(
Table S6, entry 3);
furthermore, a 2D Zn coordination poly-
We tested the catalytic activities of 1–3 as heterogeneous cata-
lysts for the Knoevenagel condensation of malononitrile with
various aldehydes. In a typical reaction, a mixture of benzalde-
hyde, malononitrile, and catalyst was placed in a glass vessel,
and then THF was added. The mixture was capped and heated
at 508C for 1.5 h and was subsequently quenched by centrifu-
gation and filtration at room temperature. The filtrate was
mer of 5-(benzylamino)isophthalate led to a yield of 80% after
4
8 h at 708C by using 10 mol% of the catalyst (Table S6,
entry 6).
[
24c]
II
The same reaction catalyzed by a Cu –pyridine–
2
3
,3,5,6-tetracarboxylate framework led to a yield of 78% after
[24f]
II
6 h (Table S6, entry 8).
However, a 2D Zn framework of 4-
(
pyridin-4-ylcarbamoyl)benzoate heterogeneously catalyzed
the reaction with an overall yield of 93% at 708C after 48 h
evaporated under vacuum to give the crude product. The resi-
[
8f]
1
(
Table S6, entry 2), a yield that is identical to that of catalyst
due was dissolved in CDCl and analyzed by H NMR spectros-
3
1
1
(Table S6, entry 1).
copy. The H NMR spectra and calculation of the yield for the
For the reaction of unsubstituted benzaldehyde with nitro-
Knoevenagel reaction are presented in Figure S3, and all the
obtained results are presented in Table S5.
ethane, catalyst 1 gave a product yield (72%) that was similar
II
to that obtained with a Zn MOF based on 5-{(pyridin-4-ylme-
By using benzaldehyde as a test compound (Scheme 4), we
found that compound 1 gave a higher product yield than the
other catalysts after the same reaction time and at the same
temperature. Consequently, optimization of the reaction condi-
tions (temperature, reaction time, amount of catalyst, solvent)
was performed in the model malononitrile–benzaldehyde
system with 1 as the catalyst.
[9d]
thyl)amino}isophthalate and similar to that obtained with a
III
[8d]
Sm MOF based on 2-acetamidoterephthalic acid
0%; Table S6, entries 10 and 11).
In our system, interestingly, the reaction yield and the selec-
(71 and
7
tivity were higher in an aqueous medium than in an organic
solvent, which is not common. The use of an aqueous medium
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in THF, and the obtained reaction yields were much lower (in
the range of 29 to 31%; Table S5, entries 16–18) than those
achieved in the presence of catalysts 1–3. Using 1:1 mixtures
of H L and zinc nitrate, cadmium nitrate, and samarium nitrate
2
led to yields between 31 and 37% (Table S5, entries 20–22).
We also investigated the catalytic activity of 1 with different
types of substituted aromatic and aliphatic aldehydes in the re-
action with malononitrile. The results are summarized in
Table 2. p-Nitro- and p-chlorobenzaldehyde produced maxi-
mum yields (100–98%), whereas the lowest one (32%) was ob-
tained with p-methoxybenzaldehyde, which indicates that an
electron-withdrawing substituent promotes the reactivity, in
contrast to an electron-donating moiety; this may be related
to an increase in the electrophilicity of the substrate in the
former case.
Scheme 4. Knoevenagel condensation reaction of benzaldehyde with malo-
nonitrile.
Under the typical conditions of 2 mol% of compound 1 at
5
08C in THF, a yield of 91% of 2-benzylidenemalononitrile was
reached (Table S5, entry 4) after 1.5 h. With catalysts 2 and 3,
yields of 84 and 73%, respectively, were obtained (Table S5, en-
tries 13 and 14). Extending the reaction time increased the re-
action yield very slowly, and complete conversion was ob-
tained after 6 h. The plot of yield versus time for the Knoeve-
nagel condensation reaction of benzaldehyde and malononi-
trile with 1 as the catalyst is presented in Figure 5a.
Table 2. Knoevenagel condensation reaction of various aldehydes with
[
a]
malononitrile with catalyst 1.
The effects of temperature, amount of catalyst, and solvents
were also tested. An increase in the amount of catalyst 1 from
[b]
[c]
Entry
Compound
Yield [%]
TON
1
2
3
4
5
6
7
p-nitrobenzaldehyde
p-chlorobenzaldehyde
p-methoxybenzaldehyde
p-methylbenzaldehyde
p-hydroxybenzaldehyde
cinnamaldehyde
100
98
32
51
48
50
92
33
33
11
17
16
17
31
1
.0 to 2.0 mol% enhanced the product yield from 68 to 91%,
but any further increase did not lead to any significant increase
in catalytic activity (Table S5, entries 4–6).
To select the most suitable solvent, experiments with various
solvents (CH CN, CH Cl , THF, MeOH, and EtOH) were per-
3
2
2
acetaldehyde
formed with coordination polymer 1. The results (Table S5, en-
tries 7–10) indicated that THF (yield of 91%) was the best sol-
[
(
a] Reaction conditions: catalyst 1 (2.0 mol%), THF (1 mL), benzaldehyde
52 mL, 0.5 mmol), and malononitrile (66 mg, 1.0 mmol). [b] Calculated by
vent, whereas the worst one was CH CN (72% yield) for the
1
3
H NMR spectroscopy. [c] Number of moles of product per mole of cata-
lyst.
same reaction time (1.5 h) (Table S5, entries 4 and 7). With
methanol, ethanol, and dichloromethane, yields of 89, 83, and
8
5%, respectively, were obtained (Table S5, entries 8–10) after
the same period of time. Increasing the temperature from 25
to 408C improved the yield of 2-benzylidenemalononitrile
from 43 to 72% (Table S5, entries 11 and 12). A further increase
in the temperature to 508C enhanced the yield up to 91%
A few coordination polymers that are catalytically active for
the Knoevenagel condensation reaction of benzaldehyde with
malononitrile were reported, but the obtained yields were typi-
cally lower or comparable to those obtained with
1
[
25]
II
(
Table S5, entry 4).
(Table S7). For example, a Zn coordination polymer based
on 2,5-dioxidoterephthalate catalyzed the reaction in toluene
and produced an overall yield of 77% after 24 h at 708C
Blank reactions were tested with benzaldehyde in the ab-
sence of the catalyst at 508C in THF and led to a yield of only
4% after 1.5 h (Table S5, entry 15). We also checked the reac-
tivities of Zn(NO ) ·6H O, Cd(NO ) ·6H O, and Sm(NO ) ·6H O
[
25a]
2
(Table S7, entry 3).
Similarly, a 2D zinc(II) coordination poly-
mer built from the 5-acetamidoisophthalate ligand produced a
3
2
2
3 2
2
3 3
2
Figure 5. a) Plot of yield versus time for the Knoevenagel condensation reaction of benzaldehyde and malononitrile catalyzed by 1. b) Effect of catalyst recy-
cling on the yield of 2-benzylidenemalononitrile obtained from the Knoevenagel condensation of benzaldehyde and malononitrile catalyzed by 1.
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[
6e]
6
1% yield after 1.5 h at 408C (Table S7, entry 2). Moreover, a
ment affect the catalytic activity. For both reactions, compound
1 afforded the highest yield, and compound 3 afforded the
lowest one. The former case is consistent with the high Lewis
II
Ni MOF based on methanetetrabenzoate led to an overall
[25c]
II
yield of 78% after 6 h at 1308C (Table S7, entry 6).
MOF constructed from tri(pyridin-4-yl)cyclohexane-1,3,5-tricar-
boxamide led to yield of 80% after 12 h (Table S7,
The reaction catalyzed by [Tb(BTATB)(dmf) (H O)]
n
A Cd
II
acidic character of Zn and with its highest coordinative unsa-
a
turation and least crowded coordination environment (tetrahe-
dral). In contrast, the lowest yield for compound 3 corresponds
to the highest crowded octacoordinated environment around
[
17c]
entry 8).
BTATB =4,4’,4’’-[benzene-1,3,5-triyltris(azanediyl)]tribenzoate}
2
2
{
III
produced 99% yield by using 4 mol% of the catalyst at 608C
for 24 h, that is, a longer time, a higher temperature, and a
higher amount of catalyst than in our case (Table S7,
Sm . Moreover, conceivably due to the presence of both free
basic sites (amide N atom and pyridyl N atom) and to its
simple 1D structure, coordination polymer 1 led to reaction
yields that were higher or similar to the reported ones.
[
17e]
entry 7).
With catalyst 1 at 508C, a 91% yield was reached
in only 1.5 h, and complete conversion was achieved in 3 h.
However, a heterometallic Co/Zn MOF based on 5-(picolinami-
3.3. Size-Selectivity Studies
[
17h]
do)isophthalate
led to complete conversion at a lower tem-
perature and a shorter time (Table S7, entry 10). Thus, and ac-
cording to the above comparisons, catalyst 1 is usually more
active than other reported catalysts. In addition, our catalyst is
cheap, easy to prepare, highly active at low temperatures, and
recyclable without any appreciable loss in activity.
To test the size selectivity of 1 for the Henry and Knoevenagel
reactions, we varied the size and shape of the aldehydes, nitro-
alkanes, and active methylene compounds (Table 3). The alde-
hydes used were as follows: benzaldehyde (4.8 Š”5.9 Š),
naphthaldehyde (5.9 Š”7.0 Š), and 9-anthraldehyde (6.0 Š”
[
9d]
The catalytic process is assumed to follow a mechanism sim-
ilar to that of the Henry reaction (see above), by which the
zinc Lewis acid center interacts with the carbonyl group of
benzaldehyde, increasing the electrophilic character of the car-
bonyl carbon atom. The interaction of a cyano group of malo-
nonitrile with the Lewis acid metal site increases the acidity of
its methylene moiety. The basic sites (carboxylate O atom,
amide N atom, or pyridyl N atom) can abstract a proton from
the methylene group to generate the corresponding nucleo-
philic species, which attacks the carbonyl group of coordinated
benzaldehyde with CÀC bond formation and dehydration.
For both catalytic reactions, the relationship between struc-
ture and catalytic activity in the present study is not clearly un-
derstood, but both the metal ion and the structural arrange-
9.3 Š). The nitroalkanes were nitromethane (2.0 Š”3.3 Š), ni-
[
9d]
troethane (2.2 Š”3.9 Š), and nitropropane (2.2 Š”5.6 Š),
whereas the active methylene compounds were malononitrile
(4.5 Š”6.9 Š), ethyl cyanoacetate (4.5 Š”10.3 Š), and tert-butyl
[
9d]
cyanoacetate (5.8 Š”10.3 Š). We observed that the reaction
yield systematically decreased with an increase in the molecu-
lar size of the substrates. For example, the reaction of benzal-
dehyde with nitropropane led to a yield of 61%, which is
lower than the yield of 79% for nitromethane (Table 3, en-
tries 3 and 1). In the case of nitroethane, the yield was 72%
(Table 3, entry 2). The yields of 1-naphthaldehyde and 9-antral-
dehyde (with molecular sizes that hamper their fitting into the
catalyst cavities) were reduced to 53 and 37%, respectively,
under the same reaction conditions (Table 3, entries 4 and 5).
[a]
[b]
Table 3. Henry reaction and Knoevenagel condensation of substrates of different sizes catalyzed by 1.
Entry
Aldehyde
Nitroalkane
Yield [%]
Entry
Active methylene compound
malononitrile
Yield [%]
91
1
nitromethane
79
6
2
3
nitroethane
72
61
7
8
ethyl cyanoacetate
73
45
nitropropane
tert-butyl cyanoacetate
4
5
nitroethane
nitroethane
53
37
9
malononitrile
malononitrile
82
67
10
[
a] Reaction conditions: catalyst 1 (3.0 mol%), benzaldehyde (52 mL, 0.5 mmol), nitroethane (0.2 mL, 2.6 mmol), and water (1.0 mL) for 48 h at 708C. [b] Re-
action conditions: catalyst (2.0 mol%), THF (1 mL), malononitrile (66 mg, 1.0 mmol), and benzaldehyde (52 mL, 0.5 mmol) for 1.5 h at 508C.
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We also observed a similar phenomenon in the case of the
Knoevenagel reaction: upon increasing the size of the active
methylene compound (malononitrile<ethyl cyanoacetate<
tert-butyl cyanoacetate) or the aldehyde (benzaldehyde<1-
naphthaldehyde<9-antraldehyde), the yield decreased from
type of ligand is very useful to construct coordination poly-
3
+
mers having various architectures with either early (Sm ) or
2
+
2+
late (Zn , Cd ) transition metals. Moreover, for the first time,
we observed the syn-syn (for compound 1) conformation of
such a ligand.
9
1 to 45% (Table 3, entries 6 and 8) or from 91 to 67%
We tested the heterogeneous catalytic activity of the com-
pounds towards the Henry CÀC coupling and Knoevenagel
condensation reactions of various aldehydes and found that, in
(
Table 3, entries 6, 9 and 10), respectively.
These results suggest that both reactions catalyzed by 1 are
II
dependent on its hydrogen-bonded pore sizes (7.5 Š”8.7 Š).
Larger substrates are less reactive owing to difficulties associat-
ed with diffusion into the channels of 1. These size-selective
behaviors support the assumption that catalysis also occurs at
the interior catalytic sites and not only at the exterior ones.
both cases, Zn coordination polymer 1 was the most effective
catalyst, conceivably on account of the higher Lewis acid char-
II
acter and lower coordination number of the metal ion. Zn is a
border-line hard/soft Lewis acid, whereas the hard and soft
III
II
characters of Sm and Cd , respectively, do not appear to con-
stitute more favorable features in our case, although any at-
tempt for generalization should not be proposed. Size-selectiv-
ity studies indicated that the reaction yield decreased with an
increase in the molecular size of the substrates, which supports
the assumption that catalysis also occurs at the interior pores
of the hydrogen-bonded network. In addition, the stability of
catalyst 1 in both reactions was well established, proving that
such a coordination polymer could be used as an effective and
recyclable heterogeneous catalyst in significant types of reac-
tions, which deserve to be further explored.
3
.4. Recyclability and Heterogeneity Tests
We performed recycling experiments of 1 in both the Henry
and Knoevenagel reactions. The catalyst, separated by centrifu-
gation of the supernatant solution, was washed with methanol
or THF and dried in air. It was then recycled in five consecutive
experiments, and only a considerable decrease in yield was ob-
served over the fourth to fifth cycles in either case. The FTIR
spectra of catalyst 1 taken before and after the reaction sug-
gested that the structure of the solid was retained (Fig-
ure S1b). This was confirmed by powder XRD, also performed
before and after the Henry and Knoevenagel reactions (Fig-
ure S1a). Additionally, the filtrate solution, obtained after sepa-
ration of the catalyst, was evaporated to dryness, and the
amount of zinc was determined to be only between 0.015 and
Experimental Section
General Methods
The synthetic work was performed in air and at relatively high tem-
peratures. All chemicals were obtained from commercial sources
0
.018% of the amount used in the reaction, which thus ruled
À1
and were used as received. The infrared spectra (4000–400 cm )
out any significant leaching of the catalyst.
were recorded with a Bruker Vertex 70 instrument in KBr pellets
To verify further the heterogeneity of the system, a proce-
dure similar to that described by Lempers and Sheldon was
(
s=strong, m=medium, w=weak, bs=broad and strong, mb=
medium and broad). Carbon, hydrogen, and nitrogen elemental
analyses were performed by the Microanalytical Service of the Insti-
tuto Superior TØcnico. Thermal properties were analyzed with a
[
26]
followed. The catalytic reaction was started, and at a time
when catalyst 1 was most active, it was removed by centrifu-
gation; the catalyst-free solution was kept under the same
conditions and was monitored with time. An increase in the
yield would indicate continuation of the catalytic reaction and,
thus, dissolution of the catalyst. In this experiment, catalyst 1
was removed by centrifugation once the conversion reached
about 48% (after a 6 h reaction time) for the Henry reaction
and roughly 53% (after a 0.3 h reaction time) for the Knoeve-
nagel reaction, whereupon the supernatant fluid was stirred
for an additional time under the same reaction conditions. As
no further conversion into the product was observed in either
case, the tests demonstrated that 1 was heterogeneous in
nature.
PerkinElmer Instrument system (STA6000) at a heating rate of
À1
5
8Cmin under a dinitrogen atmosphere. Powder X-ray diffraction
(
PXRD) was conducted with a D8 Advance Bruker AXS (Bragg Bren-
tano geometry) theta-2-theta diffractometer, with copper radiation
CuKa, l=1.5406 Š) and a secondary monochromator, operated at
0 kV and 40 mA. The flat-plate configuration was used, and the
(
4
typical data collection range was between 5 and 408. For pyridine
adsorption studies, the FTIR spectrum of the solid was recorded
À1
with an Agilent Cary 630 FTIR spectrometer in the 4000–400 cm
wavenumber range by using the diffuse reflectance infrared Fouri-
er transform (DRIFT) technique. For acidity determinations by FTIR
spectroscopy, the sample was heated to 1008C under vacuum at
À5
around 10 torr for 12 h. Pyridine adsorption was performed at
room temperature for 4 h. The sample was then evacuated for 1 h
at 1008C and was cooled to room temperature before recording
the spectrum.
4
. Conclusions
We successfully synthesized and characterized three coordina-
tion compounds of zinc (1), cadmium (2), and samarium (3) de-
rived from 3,3’-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}dibenzo-
Syntheses
Compound 1: A solution of H L (10.1 mg, 0.025 mmol) and zinc(II)
nitrate hexahydrate (14.9 mg, 0.050 mmol) in DMF/MeOH (1:2,
2
ic acid (H L). Single-crystal X-ray diffraction analysis revealed
2
that 1 and 3 are coordination polymers having zigzag- and
double-chain-type one-dimensional structures, respectively,
whereas 2 features a dinuclear metallacyclic complex. This
2
mL) and containing 30% ammonium hydroxide (NH OH) solution
4
(0.5 mL) was prepared and then transferred to a 8 mL glass vessel,
which was sealed and heated at 708C for 48 h (solvothermal reac-
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tor). Cooling of the solution to room temperature afforded small
colorless crystals of 1. FTIR (KBr): n˜ =3372 (s), 3298 (bs), 3188 (s),
To perform the catalyst recycling experiments, the used catalyst
(separated by centrifugation of the supernatant solution) was
washed with THF and dried in air. It was then reused for the Knoe-
venagel condensation reaction as described above.
2
1
8
362 (w), 1680 (s), 1665 (s), 1612 (s), 1579 (s), 1476 (m), 1440 (s),
372 (s), 1321 (m), 1261 (s), 1145 (m), 1080 (m), 1001 (m), 944 (m),
22 (s), 769 (s), 748 (s), 675 (s), 595 (w), 538 (w), 426 cm (w); ele-
À1
mental analysis calcd (%) for C H N O Zn (504.74): C 49.97, H
2
1
17
3
8
Crystal Structure Determination
3
.39, N 8.32; found: C 49.53, H 3.45, N 8.65.
X-ray-quality single crystals of compounds 1–3 were immersed in
cryo-oil, mounted in a nylon loop, and measured at room tempera-
ture. Intensity data were collected by using a Bruker APEX-II
PHOTON 100 diffractometer with graphite monochromated MoKa
Compound 2: A solution of H L (10.1 mg, 0.025 mmol) and cad-
mium(II) nitrate hexahydrate (15.4 mg, 0.050 mmol) in DMF/diox-
2
ane (1:2, 2 mL) and containing 30% NH OH solution (0.5 mL) was
4
prepared and then transferred to a 8 mL glass vessel, which was
sealed and heated at 708C for 48 h (solvothermal reactor). Cooling
of the solution to room temperature afforded light-yellow crystals
of 2. FTIR (KBr): n˜ =3446 (b), 3153 (s), 1665 (m), 1619 (s), 1561 (s),
(l=0.71069) radiation. Data were collected by using phi and
omega scans of 0.58 per frame, and a full sphere of data was ob-
[
28a]
tained. Cell parameters were retrieved by using Bruker SMART
software and were refined by using Bruker SAINT
[28a]
on all the ob-
1
6
409 (s), 1380 (s), 1170 (w), 1083 (m), 1014 (w), 894 (w), 776 (m),
81 (m), 572 (w), 437 cm (w); elemental analysis calcd (%) for
À1
served reflections. Absorption corrections were applied by using
[
28b]
SADABS.
SHELXS-2014 package
Calculations were performed by using the WinGX System-Version
Structures were solved by direct methods by using the
C50.5H64.5Cd N O
(1409.41): C 43.04, H 4.61, N 7.45; found: C
2
7.5 25.5
[28c]
[28c]
and were refined with SHELXL-2014/6.
4
3.13, H 4.45, N 7.32.
[
28d]
Compound 3: A solution of H L (10.1 mg, 0.025 mmol) and samar-
2
2014.1.
The hydrogen atoms attached to carbon and nitrogen
ium(III) nitrate hexahydrate (22.2 mg, 0.050 mmol) in DMF (2 mL)
and water (0.5 mL) was prepared and then transferred to a 8 mL
glass vessel, which was sealed and heated at 708C for 48 h (solvo-
thermal reactor). Cooling of the solution to room temperature af-
forded colorless crystals of 3. FTIR (KBr): n˜ =3853 (w), 3347 (mb),
atoms were inserted at geometrically calculated positions and
were included in the refinement by using the riding-model approx-
imation; U (H) was defined as 1.2U of the parent atoms for the
phenyl groups and 1.5Ueq of the parent atoms for the methyl
groups and nitrogen atoms. Least-square refinements with aniso-
tropic thermal motion parameters for all the non-hydrogen atoms
and isotropic ones for the remaining atoms were employed. Com-
pound 2 contained disordered and 1,4-dioxane (1,4-diox) mole-
iso
eq
1
1
670 (m), 1623 (s), 1550 (s), 1407 (s), 1385 (s), 1160 (w), 1079 (m),
003 (w), 890 (w), 773 (m), 754 (m), 673 (m), 574 (w), 435 cm (w);
À1
elemental analysis calcd (%) for C H N O Sm (779.91): C 41.58, H
2
7
29
6
12
[
28e]
3
.75, N 10.78; found: C 41.43, H 3.20, N 10.34.
cules that could not be modeled reliably. PLATON/SQUEEZE
was
3
used to correct the data, and a potential volume of 217 Š was
found with 108 electrons per unit cell worth of scattering. The
electron count suggest the presence of one 1,4-dioxane molecule
Procedure for the nitroaldol (Henry) reaction: In a typical reaction,
a mixture of aldehyde (1 mmol), nitroethane (0.3 mL), and complex
1
(5.6 mg, 3 mol%) was placed in a capped glass vessel; then,
(
48 electrons) per asymmetric unit. Elemental and thermogravimet-
water (2 mL) was added. The mixture was heated at 708C for 48 h,
and the reaction was subsequently quenched by centrifugation
and filtration. The filtrate was extracted with dichloromethane. The
organic extracts were collected over anhydrous sodium sulfate;
ric analysis data also support this result. These were removed from
the model and included in the empirical formula. Crystallographic
data are summarized in Table S1, and selected bond lengths and
[29]
angles are presented in Table S2.
subsequent evaporation of the solvent gave the crude product.
1
The residue was dissolved in CDCl and analyzed by H NMR spec-
3
troscopy. The yield of the b-nitroalkanol product (relative to the al-
dehyde) was established typically by taking into consideration the Acknowledgements
1
relative amounts of these compounds, as given by H NMR spec-
[
8]
troscopy and previously reported. The syn/anti selectivity was cal-
This work was supported by the Foundation for Science and
Technology (FCT), Portugal (projects UID/QUI/00100/2013 and
PTDC/QEQ-QIN/3967/2014). A.K. expresses his gratitude to the
FCT for a postdoctoral fellowship (Ref. No. SFRH/BPD/76192/
2011), and G.M.D.M.R. acknowledges the CQE (Group 1) for a
Bolsa de Iniciażo à Investigażo Científica (RD0436-CC930204)
fellowship.
1
culated on the basis of the H NMR spectra (Figure S7). In the
H NMR spectra, the values of the vicinal coupling constants (for
1
the b-nitroalkanol products) between the a-NÀCÀH and the a-OÀ
CÀH protons identified the isomers, that is, J=7–9 and 3.2–4 Hz
[27]
for the syn and anti isomers, respectively.
To perform the recycling experiment, the catalyst isolated by filtra-
tion was first washed and dried. It was then used in the nitroaldol
reaction as described above.
Conflict of Interest
Procedure for the Knoevenagel condensation reaction: A mixture
of benzaldehyde (51 mL, 0.50 mmol), malononitrile (66 mg,
1
2
.0 mmol), and catalyst (5.0 mg of 1, 12.1 mg of 2, 7.8 mg of 3;
mol%) was placed in a capped glass vessel, and then THF (1 mL)
The authors declare no conflict of interest.
was added. The mixture was heated at 508C for 1.5 h, and the re-
action was subsequently quenched by centrifugation and filtration
at room temperature. The filtrate was evaporated under vacuum
Keywords: condensation reactions · coordination polymers ·
heterogeneous catalysis · metallomacrocycles · O ligands
to give the crude product [2-(phenylmethylene)malononitrile]. The
1
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