Zinc metal-organic frameworks: Efficient catalysts for the diastereoselective Henry reaction and transesterification

Article abstract of DOI:10.1039/c4dt00219a

Three new compounds bearing different flexible side functional groups, viz. 2-acetamidoterephthalic acid (H₂L1), 2-propionamidoterephthalic acid (H₂L2) and 2-benzamidoterephthalic acid (H₂L3), were synthesized and their coordination reactions with zinc(ii) were studied. X-ray crystallography showed the formation of novel metal organic frameworks with different dimensionalities, where the side functional groups of amidoterephthalic acid and/or auxiliary ligands were found to play significant roles. These frameworks [Zn₂(L1)₂(4,4′-bipyridine) ₂(H₂O)(DMF)]_n (1), [Zn₄(L2) ₃(OH)₂(DMF)₂(H₂O)₂] _n (2) and [Zn(L3)(H₂O)₂]_n·n/ 2(1,4-dioxane) (3) act as heterogeneous polymeric solid catalysts not only for the diastereoselective nitroaldol (Henry) reaction of different aldehydes with nitroalkanes but also for transesterification reactions. These MOF-based heterogeneous catalysts can be recycled without losing their activity. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00219a

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Zinc metal–organic frameworks: efficient catalysts
for the diastereoselective Henry reaction and
transesterification†
Cite this: Dalton Trans., 2014, 43,
7795
Anirban Karmakar, M. Fátima C. Guedes da Silva and Armando J. L. Pombeiro*
Three new compounds bearing different flexible side functional groups, viz. 2-acetamidoterephthalic acid
(H2L1), 2-propionamidoterephthalic acid (H2L2) and 2-benzamidoterephthalic acid (H2L3), were syn-
thesized and their coordination reactions with zinc(II) were studied. X-ray crystallography showed the for-
mation of novel metal organic frameworks with different dimensionalities, where the side functional
groups of amidoterephthalic acid and/or auxiliary ligands were found to play significant roles. These
frameworks [Zn2(L1)2(4,4’-bipyridine)2(H2O)(DMF)]n (1), [Zn4(L2)3(OH)2(DMF)2(H2O)2]n (2) and [Zn(L3)
(H2O)2]n·n/2(1,4-dioxane) (3) act as heterogeneous polymeric solid catalysts not only for the diastereo-
selective nitroaldol (Henry) reaction of different aldehydes with nitroalkanes but also for transesterification
reactions. These MOF-based heterogeneous catalysts can be recycled without losing their activity.
Received 21st January 2014,
Accepted 26th February 2014
DOI: 10.1039/c4dt00219a
www.rsc.org/dalton
components23 viz. the metal ions, which either provide the
coordinatively unsaturated nodes and/or form the active metal
Introduction
Porous metal–organic frameworks (MOFs), a combination of sites integrated into the linker ligand, and the coordinated
inorganic metal species and organic linkers, have attracted ligands. Hence, depending on the nature of the ligands and
considerable attention in recent years.1 MOFs can be readily binding metal ions MOFs can act as Lewis bases or acids in
prepared from the simple combination and self-assembly of the catalytic medium.
metal ions with organic bridging ligands containing divergent
The Henry or nitroaldol reaction is known as one of the
donor atoms. This metal–ligand directed assembly approach most powerful and atom-economic reactions for C–C bond for-
can yield a new generation of multi-dimensional networks, mation with various functionalized structural motifs.24,25
which contain channels or cavities of various sizes and Usually this reaction is performed with homogeneous basic
shapes.2 Due to the porous features of such MOFs a wide catalysts, such as alkali metal hydroxides, alkoxides or amines,
range of applications can be envisioned namely in nonlinear with
a
rather good efficiency.26 Many homogeneous
optics,3 gas storage,4 catalysis5 and host–guest induced separ- catalysts,27–34 including some copper34j,l and zinc34b,l contain-
ation.6 Catalytic applications of such materials were among ing complexes obtained by our group, have already been
the earliest proposed ones,7 and nowadays many organic reac- reported. However, heterogeneous catalysts for such a trans-
tions can be efficiently catalyzed by MOFs.8
formation are rare.35
A remarkable advancement has been achieved in recent
Transesterifications are important transformations in
years in the development of porous MOFs using multidentate organic synthesis, in industrial and in academic laboratories,36
aromatic carboxylate ligands, due to their robustness and and have important applications in polyester synthesis and
thermal stability.9,10 Some earlier reports relate to the use of biodiesel production.37 Homogeneous catalysts have been
dicarboxylate,11,12 tricarboxylate13–15 and tetracarboxylate16–21 reported to catalyze these reactions38 but high reaction temp-
linkers that are inter-bridged by mono- or multi-nuclear metal erature and acidic conditions are usually required. Some other
nodes, leading to stable MOFs with permanent porosity.22 discrete complexes and coordination polymers can also cata-
MOFs can work as catalysts through two different lyze such a type of reaction.39 However, there is still a demand
to develop new types of catalysts based on cheap and environ-
mentally tolerable metal complexes that could be easily recycl-
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade
able (hence forming an heterogeneous system) and show a
high efficiency under mild conditions.
de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal.
E-mail: pombeiro@tecnico.ulisboa.pt
†Electronic supplementary information (ESI) available. CCDC 968209–968211.
Hence, we report herein the synthesis and characterization
of new ligands bearing different amide side functional groups,
2-acetamidoterephthalic acid (H2L1), 2-propionamidoterephthalic
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt00219a
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symmetric stretching, respectively. The bands in the regions
1660–1610 cm−1 and 1417–1429 cm−1 are attributed to the
CvC stretching frequency of the aromatic rings.40 Due to their
insolubility in common NMR solvents, these frameworks were
only characterized by single crystal and powder X-ray diffrac-
tion, elemental and TG analyses.
Scheme 1 Schematic representation of the H2L1, H2L2 and H2L3
ligands.
Crystal structure analysis
In all the coordination polymers, the zinc metal ions are in the
2+ oxidation state and to fulfill the charge balance require-
ments the ligands are fully deprotonated.
The hydrothermal reaction of 2-acetamidoterephthalic acid
(H2L1) with zinc(II) nitrate hexahydrate in the presence of 4,4′-
acid (H2L2) and 2-benzamidoterephthalic acid (H2L3)
(Scheme 1), which were then applied to the synthesis, under
hydrothermal conditions, of Zn(II)-frameworks. The structural
features of the obtained Zn MOFs [Zn2(L1)2(4,4′-bipyridine)2-
(H2O)(DMF)]n (1), [Zn4(L2)3(OH)2(DMF)2(H2O)2]n (2) and [Zn
(L3)(H2O)2]n·n/2(1,4-dioxane) (3) could be established by single
crystal X-ray diffraction analysis and were subjected to a topo-
logical study.
The obtained frameworks act as heterogeneous catalysts in
the nitroaldol combination of nitroethane with various alde-
hydes as well as in the transesterification reactions of various
aromatic esters.
bipyridine leads to the formation of
a 2D network
[Zn2(L1)2(4,4′-bipyridine)2(H2O)(DMF)]n (1) extended with zinc
(II), L12− ions and bipyridine molecules. Single-crystal X-ray
diffraction studies reveal that 1 crystallizes in the monoclinic
P21/c space group and that the asymmetric unit contains two
zinc(II) ions whose coordination spheres are organized with
two L12− ligands, two 4,4′-bipyridine molecules, one water
molecule and one DMF molecule (Fig. 1). This framework con-
tains two identical networks which are interpenetrated
forming a two dimensional structure with 2-fold interpenetrat-
ing networks (Fig. 2).
The zinc(II) ions in 1 have two distinct coordination
environments. Zn1 is in an octahedral coordination environ-
ment whose distortion can be assessed by the octahedral angle
Results and discussion
Syntheses and characterization
The reaction of H2L1, H2L2 or H2L3 with zinc(II) nitrate hexa- variance (OAV)41a and quadratic elongation (OQE) which
hydrate under hydrothermal reaction conditions leads to the assume values of 143 and 1.045, respectively. One of the car-
formation of [Zn2(L1)2(4,4′-bipyridine)2(H2O)(DMF)]n (1), boxylate arms of two μ2-L12− ligands bridges two Zn1 cations
[Zn4(L2)3(OH)2(DMF)2(H2O)2]n (2) or [Zn(L3)(H2O)2]n·n/2(1,4- [Zn1–O8 and Zn1–O9], the coordination sphere of each metal
dioxane) (3), respectively [L1 = 2-acetamidoterephthalate, L2 = being then fulfilled by ligating to every N-atom of one of the
2-propionamidoterephthalate, L3 = 2-benzamidoterephthalate, bridging bipyridine molecules [Zn1–N5 and Zn1–N6] and to
DMF = dimethyl formamide].
both oxygen atoms of a carboxylate group of the remaining L12
ligand [Zn1–O3 and Zn1–O4]. The Zn1 coordination mode
−
In the IR spectra, the characteristic strong bands of co-
ordinated carboxylate groups of 1,
2 and 3 appear at constitutes a dinuclear core acting as a secondary building
1564–1569 cm−1 or 1337–1372 cm−1 for the asymmetric or the block unit in the construction of the two-dimensional
Fig. 1 (A) Coordination scheme in 1 with partial atom labeling scheme. Hydrogen atoms were omitted for clarity. Symmetry operations to generate
equivalent atoms: (i) 1 − x, 1 − y, −z; (ii) 1 − x, 1 − y, 1 − z; (iii) 1 − x, 2 − y, −z; (iv) x, −1 + y, z. (B) Schematic representation of an asymmetric unit of 1.
7796 | Dalton Trans., 2014, 43, 7795–7810
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Ocarboxylate as acceptors, but also include the intramolecular
DMF contact between one of the methyl hydrogen atoms and
the carbonyl moiety (Table S8†). Expectedly, several π⋯π stack-
ing interactions are also present in these structures, the most
relevant ones concerning contacts involving the Zn1–O3–C6–
O4 metallacycle where the centroid⋯centroid distances can
reach values as short as 3.714 Å. Additionally, C–H⋯π contacts
involve the C53 methyl of DMF as the donor and the N6-con-
taining pyridine groups of vicinal bridging bpy groups, and
the C36 of these bpy groups as donors and the Zn1–O3–C6–
O4 metallacycle as the acceptor. These interactions present a
minimum value of 2.700 Å, are considered as strong,41c and
may have influenced the larger twisted nature of these brid-
ging ligands. These interactions help to stabilize the hydrogen
bonded 3D structures of 1.
Fig. 2 A representation of the 2-fold interpenetrated networks of 1
(one framework is represented in red and the other in green).
The hydrothermal reaction of 2-propionamidoterephthalic
acid (H2L2) with zinc(II) nitrate hexahydrate led to the for-
mation of the 3D network [Zn4(L2)3(OH)2(DMF)2(H2O)2]n (2)
(Fig. 3A). Single-crystal X-ray diffraction studies reveal that 2
crystallizes in the monoclinic P21/c space group and that the
asymmetric unit contains two zinc(II) ions, one-and-a-half L22−
ligand, one bridging hydroxyl anion, one coordinated water
molecule and one coordinated DMF molecule. The framework
is built on tetranuclear µ3-hydroxy-type clusters of [Zn4(OH)2]6+
with a center of inversion in the middle of the (Zn1)2(OH)2
core (Fig. 3B). Every tetranuclear connector is associated with
six L22− ligands, two molecules of water, two molecules of
DMF and two µ3-hydroxyl anions. While Zn1 has lightly dis-
torted octahedral coordination geometry (OAV and OQE of 17.8
and 1.005, respectively)41a the Zn2 cation has a marginally dis-
torted tetrahedral environment (τ4 = 0.91).41d The equatorial
plane of Zn1 includes one of the O-atoms of a L22− chelating
carboxylate group, two bridging hydroxyl anions and the
O-atom of the DMF molecule; the axial sites are occupied by a
water molecule and the O-atom of another L22− chelating car-
boxylate group. Consequently, the L22− linked to the
Zn1 metal cations can be considered to be in the relative cis-
position. The coordination sphere of Zn2 is fulfilled by the
O-atom of a monodentate μ1-L22−, other two O-atoms from two
bridging μ2-L22− ligands and a water molecule. The Zn–O
bond distances range from 1.928(3) to 2.113(2) Å, but those
involving Zn2 do not exceed 1.967(3) Å, and concerning Zn1
do not go below 2.070(3) Å; in any case, the longer bonds cor-
responds to the bridging hydroxyl moiety. The shortest Zn⋯Zn
distance in the structure is 3.1250(7) Å which involves Zn1 and
the peripheral Zn2 ions; the symmetry related Zn ions, in turn,
are slightly further apart [3.1759(6) Å].
polymeric assembly. The Zn2 cation is coordinated to one of
the O-atoms of two distinct L12− ligands [Zn2–O1 and Zn2–O6]
and further coordinated to the N-atom of a non-bridging
bipyridine moiety [Zn2–N3], to a water molecule [Zn2–O12]
and to a DMF residue [Zn2–O11], the latter two occupying the
axial positions of a distorted trigonal bipyramidal coordination
environment around the metal center (τ5 = 0.82).41b Thus, in 1
the coordination behaviour of the two 4,4′-bipyridine mole-
cules is different since one of them bridges between two Zn1
centers and the other is monodentate to Zn2; concerning the
dicarboxylate ligands L12−, one occurs in non-bridging mono-
dentate (µ1-O, to Zn2) and syn–anti-type bridging bidentate
(µ2-O,O′ to two Zn1) binding modes, while the other shows the
µ1-O type (to Zn2) as well as the chelating bidentate µ1-O,O(to
Zn1) ligating method.
The Zn–O bond dimensions in 1 differ depending on
whether the oxygen atoms belong to bridging (the L12− ligand
moieties) or non-bridging ligands (water and DMF). The
longest one pertains to the DMF moiety [Zn2–O11] and the
dimensions that follow concern those of the chelating biden-
tate µ1-O,O carboxylate group, with one of those bonds being
considerably longer than the other [Zn1–O3 vs. Zn1–O4], fol-
lowed by those involving the bridging bidentate µ2-O,O′ car-
boxylate [Zn1–O9 vs. Zn1–O8]. The Zn–O bonds involving the
monodentate carboxylate moieties are the shortest ones found
in this MOF [Zn2–O1 and Zn2–O6]. Expectedly, the Zn–N dis-
tances involving the bridging bipyridine are longer than those
in the terminal one [2.154(3) and 2.048(4) Å, respectively].
Non-covalent interactions are present in 1 (Table S2†). Con-
cerning the N–H⋯O contacts, the amide groups of DMF intra-
molecularly donate to the non-coordinated O-carboxylate
atoms [d(D⋯A) distances of 2.611(5) and 2.616(4) Å], while the
water molecule simultaneously donates to Oamide and to one of
the chelating bidentate Ocarboxylate atoms [d(D⋯A) distances of
2.971(4) and 2.810(5) Å, respectively]. C–H⋯O contacts are also
relevant; the strongest ones [d(D⋯A) distances in the 2.730(5)–
2.902(5) Å range] comprise the phenyl groups as donors and
The packing view of 2 is characterized by open channels
along the crystallographic a axis (Fig. 4) with approximate
dimensions of 10.45 × 7.67 Å2, measured as the distance
between the H10C-methyl and the H29C-methyl groups of the
L22− ligands, respectively. The structure of 2 is also stabilized
by hydrogen bonding interactions (Table S2†), the most signifi-
cant one involving the coordinated water molecule (O31) as
the donor to the carboxylate oxygen atom O21. The amide
H-atoms also interact with carboxylate moieties. Moreover,
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Fig. 3 (A) Coordination scheme in 2 with partial atom labeling scheme. Hydrogen atoms are omitted for clarity. (B) Basic structure of the dihy-
droxo-bridged Zn-tetramer [Zn4(OH)2]3+ unit showing the metal coordination spheres. Symmetry operations to generate equivalent atoms: (i) 2 − x,
2 − y, 1 − z; (ii) 2 − x, −1/2 + y, 1.5 − z; (iii) 1 − x, 2 − y, 1 − z; (iv) −1 + x, y, z; (v) −1 + x, 2.5 − y, −1/2 + z; (vi) x, 1.5 − y, −1/2 + z. (C) Schematic rep-
resentation of an asymmetric unit of 2.
and a free 1,4-dioxane fragment. Compound 3 features a
zigzag type one-dimensional coordination polymeric chain,
but expands to 3D by means of H-bond interactions. The Zn1
center presents a distorted tetrahedral environment (τ4
=
0.87)41d and binds to two carboxylate oxygen atoms of two
neighboring L32− units in a monodentate fashion [Zn1–O1
and Zn1–O3] and to two water molecules [Zn1–O10 and Zn1–
O20]. In this framework the organic ligand is almost planar,
the maximum deviations from its mean least square plane per-
taining to the O4 and O3 atoms (0.181 and 0.172 Å, respect-
ively), the metal atom being shifted by 0.143 Å from it. The
presence of a phenyl ring in the side functionality of the L32−
ligand prevented the formation of a structure with a higher
dimensionality, and a 1D zigzag chain is formed instead
(Fig. 5). However, the intermolecular organization in the
crystal is characterized by hydrogen bonding interactions
involving the carboxylate groups as acceptors and the co-
ordinated water molecules as well as the dioxane molecule as
donors (Table S2†), expanding the structure to the third
dimension (Table S2†) (Fig. 6).
Fig. 4 A representation of the 3D network of 2.
intermolecular C–H⋯O contacts are relevant and help to
stabilize the structure.
Single-crystal X-ray diffraction studies reveal that [Zn(L3)-
(H2O)2]n·n/2(1,4-dioxane) (3) crystallizes in the monoclinic P21/c
space group, and that the asymmetric unit contains one zinc(II)
ion, one L32− ligand, two coordinated water molecules (Fig. 5)
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Fig. 5 One dimensional zigzag structure of 3 with partial atom labeling scheme. Symmetry operations to generate equivalent atoms: (i) −1 + x, 1/2
− y, −1/2 + z; (ii) 1 + x, 1/2 − y, 1/2 + z; (iii) −2 + x, y, −1 + z; (iv) 2 + x, y, 1 + z.
One may conclude from the above observations that the from the frameworks. In the particular case of 1 where Zn2 is
three dicarboxylic ligands H2L1, H2L2 and H2L3 reveal mono-coordinated via carboxylate ligands and Zn1 forms a
different coordination features in the coordination polymers binuclear cluster via bridging carboxylates, we considered the
with zinc(II) metal ions, and the amide substituents play a latter as a single node subsequently coordinated via 8 different
significant role. The more bulky amide side functionality carboxylates and a bipyridine linker; the Zn2 ion, being con-
produces
a low dimensional framework. Three different nected to two ligand moieties, represents a 2-connected node.
ligands produce three different structures which not only Relative to the L12− ligand and the 4,4′-bipyridine, both con-
depend upon the reaction conditions but also on the auxiliary nected to Zn1 and Zn2, they act as 2-connected nodes. The
ligands.
polymeric chain structure thus generated is a 4-nodal (2,2,2,8)-
connected net with the Schläfli symbol {62·82·1416·204·224}-
{6}4{8}6. Moreover, this network also has two different inter-
penetrated nets (Fig. 7A).
MOF 2 can be represented as a complex (2,6)-connected
binodal net, with the Schläfli symbol {812·123}{8}3 and the
2,6T1 type topology (binary.ttd). In this framework the tetra-
nuclear zinc cluster serves as a single node coordinated via six
L22− ligands which, in turn, is connected to two Zn centers
(Fig. 7B). The 1D of 3 represents binodal (2)-connected nets
Topological analysis
To improve the description of the crystal structures of 1, 2 and
3 we performed their topological analysis by reducing their
multidimensional structures to simple node-and-linker nets
where the metallic nodes and the organic linkers represent
secondary building units (SBUs).42 We have carried out these
analyses using TOPOS 4.0.42c In order to do so we have
removed the entire lattice and coordinated solvent molecules
Fig. 6 (A) Schematic representation of an asymmetric unit of 3. (B) Hydrogen bonded networks of 3 with hydrogen bonding interaction drawn in
cyan dotted lines.
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Fig. 7 Node-and-linker-type descriptions of the 2D coordination frameworks in 1 (A), of the 3D coordination frameworks in 2 (B), and of the 1D
coordination frameworks in 3 (C). The metal nodes are represented in green and the linkers in pink color.
which result from the metal coordination to two L32− ligands temperature range of 342–517 °C it shows a weight loss of
in line connected with two Zn centers (Fig. 7C).
61.1%, which corresponds to two molecules of 4,4′-bipyridine
and one ligand (L1) (calcd: 60.6%).
Thermogravimetric analyses
2 exhibits a weight loss of 15.0% in the 37–294 °C tempera-
ture range, which accounts for the total removal of one water
molecule and one DMF molecule from the coordination
sphere of the metal (calcd: 15.4%). The remaining material
then decomposes gradually until 550 °C.
Thermogravimetric analyses were carried out under dinitrogen
from room temperature to ca. 650 °C at a heating rate of 10 °C
min−1. Features of the thermal stability of the frameworks are
illustrated in Fig. 8.
3 loses 10.7% of its weight between 117 and 175 °C, most
likely due to desorption of 1,4-dioxane (calcd: 10.3%). The
residue remains stable up to about 247 °C and then releases
two coordinated water molecules until 351 °C, which corres-
ponds to a weight loss of 8.9% (calcd weight loss 9.3%) and
then gradually decomposes until 550 °C.
1 shows a weight loss of 9.2% between 180 and 250 °C,
corresponding to the loss of one molecule of water and one
molecule of DMF (calcd: 9.3%). Upon further heating, the
anhydrous compound is stable up to 342 °C but in the
Catalytic activities towards the Henry reaction
We have tested the potential catalytic activity of frameworks 1,
2 and 3 as solid heterogeneous catalysts in the nitroaldol (or
Henry) reaction of nitroethane with various aldehydes. In a
typical reaction, a mixture of aldehyde (0.5 mmol), nitroethane
(0.3 mL) and Zn catalyst (3 mol%) in 2 mL MeOH, contained
in a capped glass vessel, was stirred at 70 °C for 48 h, where-
upon the solution was filtered to remove the catalyst. The
solvent was evaporated in a vacuum, giving the crude product
as a mixture of the β-nitroalkanol diastereoisomers (syn and
anti forms, with the predominance of the former) which were
analyzed by 1H NMR (see ESI†).
By using benzaldehyde as a test compound, we found that 1
showed a higher conversion as compared to 2 and 3 after the
same reaction time and at the same temperature. Conse-
quently, the optimization of the reaction conditions
Fig. 8 Thermogravimetric curves for 1, 2 and 3.
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We have also tested the effect of temperature, catalyst
amount and solvents in the Henry reaction. An increase of the
catalyst amount from 1.0 and 3.0 mol% enhances the product
yield from 48 to 95%, respectively (entries 8 and 9, Table 1),
but further rise decreases the reaction yield to 70% (entries 10
and 11, Table 1). We have also examined the effect of different
solvents in this reaction (entries 12–14, Table 1). The reaction
did not proceed in acetonitrile, and in THF only 40% total
yield was achieved. If nitroethane was used as a solvent (added
solvent-free conditions, entry 26), 64% yield was attained. By
using a polar solvent (MeOH or H2O) the yields increased up
to 95 and 78%, respectively (entries 9 and 13, Table 1). These
results point out the possible role of protic and polar solvents
in the proton transfer process of the nitroaldol reaction.34j–l
Ranging the temperature from 20 to 70 °C improved the yield
of β-nitroalkanol from 20 to 95% but further increase in the
reaction temperature had a negative effect (entries 9 and
15–18, Table 1). The systems exhibit diastereoselectivity
towards the syn isomer, typically leading to syn : anti molar
ratios in the range of 80 : 20 to 68 : 32 using nitroethane as the
substrate. The size of the nitroalkane chain also affects the
yields; while with nitropropane the conversion was only 51%
(entry 25, Table 1), with nitroethane or nitromethane values of
95% or 84% were obtained (entries 9 or 24, Table 1).
Scheme 2 Nitroaldol (Henry) reaction.
(temperature, reaction time, amount of catalyst and solvent)
was carried out in a model nitroethane–benzaldehyde system
with 1 as the catalyst precursor (Scheme 2 and Table 1).
Blank reactions were also performed (absence of any metal
source; Table 1, entry 21) using benzaldehyde as the substrate,
at 70 °C and in methanol. No β-nitroalkanol was detected after
a reaction time of 48 h. The nitroaldol reaction also did not
take place when using Zn(NO3)2 or compound H2L1 instead of
catalyst 1 (Table 1, entries 22 and 23, respectively). However,
when 3 mol% of 1 is used as a catalyst, a conversion of 95%
(anti : syn = 27 : 73) of benzaldehyde into β-nitroalkanol is
reached (entry 9, Table 1). With 2 and 3 a conversion of 83%
(anti : syn = 31 : 69) and 91% (anti : syn = 32 : 68) were obtained,
respectively (entries 19 and 20, Table 1). Extending the reaction
time to 72 h did not increase the yield of the reaction. The
plots of yield versus time for the Henry reaction of benz-
aldehyde and nitroethane catalysed by 1, 2 and 3 are presented
in Fig. 9A. The observed high conversion rate in the case of 1
directs it as an efficient catalyst for this reaction.
Although there are some reports on coordination poly-
mers35 which are catalytically active for this kind of reaction,
the yields and selectivity are usually higher for our compounds
as compared to other metal–organic frameworks (Table 2). The
Table 1 Optimization of the parameters of the Henry nitroaldol reaction between benzaldehyde and nitroethane with 1 as the catalyst
Amount of
catalyst (mol%)
Selectivity
(anti/syn)
Entry
Catalyst
Time (h)
T (°C)
Solvent
Yield (%)
TOF (h−1
)
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0.5
1.5
2.5
5
7
9
3.0
3.0
3.0
3.0
3.0
3.0
3.0
1.0
3.0
5.0
7.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
—
70
70
70
70
70
70
70
70
70
70
70
70
70
70
RT
30
50
100
70
70
70
70
70
70
70
70
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
THF
40
68
70
74
76
81
91
48
95
70
71
40
78
—
20
46
70
45
83
91
—
—
—
84
51
64
27 : 73
28 : 72
28 : 72
27 : 73
26 : 74
26 : 74
27 : 73
24 : 76
27 : 73
22 : 78
25 : 75
25 : 75
20 : 80
—
26 : 74
25 : 75
26 : 74
31 : 69
31 : 69
32 : 68
—
26.7
15.1
9.3
4.9
3.6
3.0
1.3
1.0
0.7
0.3
0.2
0.3
0.5
—
0.14
0.3
0.5
0.3
0.6
0.64
—
24
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24a
25b
26c
H2O
CH3CN
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
Solvent free
1
1
2
3
Blank
Zn(NO3)2·6H2O
H2L1
1
1
1
3.0
3.0
3.0
3.0
3.0
—
—
29 : 71
16 : 74
24 : 76
—
—
0.6
0.4
0.4
a Nitromethane was used as the substrate. b Nitropropane was used as the substrate. c Methanol (solvent)-free conditions, using nitroethane as
the solvent (2 mL).
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■
●
◆
Fig. 9 (A) Plots of β-nitroalkanol yield vs. time for the Henry reaction of benzaldehyde and nitroethane with 1 ( ), 2 ( ) and 3 ( ). (B) Plots of
β-nitroalkanol yield vs. time for the Henry reaction of 4-nitro benzaldehyde and nitroethane with 1.
Table 2 Comparison of activities of MOFs as catalysts for the Henry reaction with an aldehyde and nitroethane
Selectivity
Yield (%) syn/anti
Catalyst
Solvent/temp/time
MeOH/70 °C/48 h
Aldehyde
Reference
This work
1
4-Nitrobenzaldehyde 98
4-Nitrobenzaldehyde 15
78 : 22
Not determined 35a
Not determined 35b
Zn(II) MOF with 1,3,5-tri(4-carboxyphenoxy)benzene Solvent free/70 °C/72 h
Zn(II) MOF with terphenyl-3,3,-dicarboxylate and
1,4-diazabicyclo[2.2.2]octane (DABCO)
Cu(II) MOF with pyridine carboxylates
Solvent free/60 °C/120 h 4-Nitrobenzaldehyde 34
Solvent free/70 °C/36 h 4-Nitrobenzaldehyde 78
Not determined 35c
3D zinc(II) framework with 1,3,5-tri(4-carboxyphenoxy)-
benzene, in the reaction of 4-nitrobenzaldehyde and
nitroethane, leads to an overall yield of only 15% after 72 h
reaction time (Table 2);35a with a 1,4-diazabicyclo[2.2.2]octane
(DABCO) functionalized 3D-Zn MOF an yield of 34% after
120 h was obtained (Table 2).35b Moreover, our catalyst 1 exhi-
bits a marked selectivity towards the syn diastereoisomer
(Table 2, syn : anti = 78 : 22) which was not reported in other
cases.
We have also compared the activities of the three Zn(II)
frameworks in the reactions of a variety of para- or ortho-substi-
tuted aromatic and aliphatic aldehydes with nitroethane, pro-
ducing the corresponding β-nitroalkanols with yields ranging
from 21 to 98% (Table 3). The reactivity of 1 is generally higher
than those of 2 and 3 and this may result from the greater
Lewis acid nature of the former, a factor that should promote
the H+-nitroalkane bond cleavage and the electrophilicity of
the aldehyde, thus favoring the global reaction (Scheme 3).
The nature of the substrates is an important factor. Indeed,
aryl aldehydes bearing electron-withdrawing groups exhibit
higher reactivities (Table 3, entries 2 and 4) as compared to
those having electron-donating moieties, which may be related
to an increase of the electrophilicity of the substrate in the
former case.
Table 3 Henry reaction of various aldehydes and nitroethane with
catalysts 1, 2 and 3
Yield (%)
by using 1
Yield (%)
by using 2
Yield (%)
by using 3
Entry
1
Compounds
96
79
69
2
3
98
39
96
21
98
26
4
5
6
93
62
83
84
31
47
90
51
71
7
52
21
40
8
9
CH3CHO
98
91
84
75
97
92
CH3CH2CHO
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processes that appear to be favored by the electrophilicity of
the aldehyde and by the protic and polar characters of the
solvent.
Catalytic activity in the transesterification reaction
In a typical reaction, a mixture of methyl-3-nitrobenzoate
(0.5 mmol) and catalyst 2 (3 mol%) in 2 mL EtOH was added
into a capped glass vessel, and the resulting mixture was
stirred at 100 °C for 24 h. The solution was filtered to remove
the catalyst and the solvent was evaporated in a vacuum,
leading to a crude mixture of products which was analyzed by
1H NMR (see ESI†). Since our frameworks have very low solubi-
lity in alcohols, they are potentially good candidates for hetero-
geneous catalytic transesterification of nitrobenzoates.
When 3 mol% of solid 2 is used as a catalyst, a conversion
of 97% of methyl-3-nitrobenzoate into ethyl-3-nitrobenzoate is
reached after 24 h reaction time (entry 6, Table 4). The yield vs.
time plot for the transesterification reaction of methyl-3-nitro-
benzoate and ethanol using 2 as a catalyst is shown in Fig. 11.
In a similar reaction using 1 and 3, conversions of 37% and
34% (entries 18 and 19, Table 4) were obtained, respectively,
after 24 h. Extending the reaction time to 48 h did not increase
the yield. The high conversion rate suggests that 2 acts as a
more efficient catalyst as compared with 1 and 3 for the trans-
esterification reaction. Thus, optimization of the reaction con-
ditions (temperature, reaction time, amount of catalyst and
solvent) was performed using 2 as the catalyst (Scheme 4).
The increase of the catalyst amount slightly enhances the
product yield from 94 to 97% for the corresponding amounts
of 1.0 and 3.0 mol% of catalyst 2, but a further increase in the
catalyst amount to 7 mol% did not improve the yield (entries
9–11 and 6, Table 4). When 1-propanol was used as a solvent
instead of ethanol the reaction yield decreased from 97 to 64%
(compare entries 6 and 12, Table 4). The reaction conversion
further decreased to 44% with the use of a secondary alcohol
(2-propanol, entry 13, Table 4). Increasing the reaction temp-
erature from room temperature to 100 °C increased the yield of
the reaction from 2 to 97% but a further rise in the tempera-
ture led to a decrease in the overall yield (compare entries 11,
14–17, Table 4). No product was detected when the reaction
was performed in the absence of the catalyst and retaining the
same experimental conditions (entry 20, Table 4). Control reac-
tions were also carried out using Zn(NO3)2 as a catalyst and a
10% conversion was achieved after 24 h (entry 21, Table 4). No
transesterification occurred when H2L2 was used instead of
the catalyst (entry 22, Table 4).
Scheme 3 Proposed catalytic cycle for the formation of the β-nitro-
alkanol in the Henry reaction catalyzed by 1.
In order to examine the lifetime and stability of 1 in the
Henry reaction, the catalyst was recycled in two consecutive
experiments. The observed activity remained over the two
cycles of reaction with just a slight decrease in the second run
(Fig. 10). The FT-IR spectra of catalyst 1 taken before and after
the reaction did not indicate any important changes (ESI,
Fig. S2A†), suggesting the integrity of the polymeric structure
of the solid. This fact is also sustained by PXRD also per-
formed before and after the Henry reaction (ESI, Fig. S3A†).
Recycling experiments for 2 and 3 were also undertaken con-
firming that these frameworks also remain active after the
reactions (ESI, Fig. S1†). The FT-IR spectra of catalysts 2 and 3
before and after the reaction also did not indicate any impor-
tant changes (ESI, Fig. S4†)
A proposed catalytic cycle for the Henry reaction catalyzed
by 1 is presented in Scheme 3. The activation of both the alde-
hyde and the nitroethane by the metal centre is followed by
the formation of the C–C bond upon electrophilic addition
leading to the formation of β-nitroalkanol,34b,43 following
Once having established that compound 2 represents an
excellent catalyst for the transesterification transformation, we
investigated the reaction of various substituted methyl benzo-
ates in ethanol. The substrate with an electron-withdrawing
substituent underwent fast transesterification (Table 5, entry
2) whereas those with electron-donating groups experienced a
slower conversion (Table 5, entries 3–5). Methyl benzoate also
converted to ethyl benzoate with a yield of 73%.
In order to examine the recyclability of catalyst 2, we
removed it by filtration after the transesterification reaction of
Fig. 10 Kinetic profiles in two consecutive reaction cycles employing 1
as a catalyst.
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Table 4 Optimization of the parameters of the transesterification reaction of methyl-3-nitrobenzoate in ethanol with 2 as a catalyst
Amount of
catalyst (mol%)
Entry
Catalyst
Time (h)
T (°C)
Solvent
Yield (%)
TON
TOF (h−1
)
1
2
3
4
5
6
7
9
10
11
12
13
14
15
16
17
18
19
20
21
22
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
2
3.5
6
3.0
3.0
3.0
3.0
3.0
3.0
3.0
1.0
5.0
7.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
—
100
100
100
100
100
100
100
100
100
100
100
100
RT
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
1-PrOH
2-PrOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
5.5
9
1.8
3.0
1.8
1.5
3.8
3.1
2.5
1.3
0.7
1.3
1.3
1.3
0.9
0.6
0.03
0.15
0.7
1.4
0.5
0.47
—
40
56
76
97
97
94
97.1
96
64
44
2.0
11
51
96
37
34
—
13.3
18.6
25.3
32.3
32.3
31.5
32.5
32.1
21.4
14.7
0.7
10
24
48
24
24
24
24
24
24
24
24
24
24
24
24
24
24
50
75
3.7
17.1
32.4
12.3
11.3
—
120
100
100
100
100
100
1
3
Blank
Zn(NO3)2·6H2O
Ligand H2L2
3.0
3.0
10
—
3.3
—
0.14
—
the amount of zinc determined, being only 0.09% of the
amount used in the reaction, thus ruling out any significant
leaching of the catalyst.
The mechanism of metal catalyzed transesterification prob-
ably involves an electrophilic activation of the carbonyl moiety
of the substrate upon binding to the metal centre of the cata-
lyst.44 Accordingly the Lewis acidity of this centre may be rele-
vant in this catalytic reaction. A possible transesterification
mechanism is shown in Fig. 12.
Concluding remarks
The synthesis and characterization of three novel amido tere-
phthalic acids having different hanging side functionalities
are reported. These compounds led successfully to one, two
and three dimensional coordination polymers upon reaction
with zinc(II) metal ions. The structures of the new frameworks
have been determined by single crystal X-ray diffraction analy-
sis. The construction of the multidimensional structures was
facilitated by the hanging side functionality of the ligands,
where the amide substituents appeared to play a significant
role as the bulkier moiety produced a lower dimensional
framework; a dependence on the reaction conditions and on
the auxiliary ligand could also be recognized. Accordingly (i) a
1D coordination polymer (3) of the zinc ions could be obtained
with the L32− ligand, the one with the largest amide group; (ii)
direct binary reactions of L12− with zinc(II) did not lead to a
crystalline polymeric aggregate, but a 2D framework (1) could
be obtained with bipyridine as an auxiliary ligand; (iii) the
combination of L22− with zinc ions produced a 3D framework
(2) with channels along the crystallographic a-axis.
Fig. 11 Plots of the product yield vs. time (two consecutive reaction
cycles) for the transesterification reaction of methyl-3-nitrobenzoate
and ethanol with 2.
Scheme 4 Transesterification reaction.
methyl-3-nitrobenzoate, dried it and reused it as a catalyst in a
subsequent transesterification process observing just a slight
decrease in reactivity (Fig. 11). FT-IR spectra of 2 taken before
and after the reaction (ESI, Fig. S2B†) did not indicate any
important changes. This fact is also sustained by PXRD per-
formed before and after the transesterification reaction (ESI,
Fig. S3B†). This suggests that the integrity of catalyst 2 was
retained after reaction. Additionally, the filtrate solution, after
the separation of the catalyst, was evaporated to dryness and
Taking advantage of the lack of solubility of the synthesized
zinc frameworks in alcohols and other common solvents they
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Table 5 Transesterification reaction of various methyl esters with 2 as a catalyst
Entry
1
Substrate
Time (h)
24
Amount (mol%)
3%
Solvent
EtOH
T (°C)
Yield (%)
73
TON
24.3
TOF (h−1
)
100
1.0
2
3
24
24
3%
3%
EtOH
EtOH
100
100
96
51
32.4
17.0
1.4
0.7
4
5
24
24
3%
3%
EtOH
EtOH
100
100
76
48
24.4
16.0
1.0
0.6
sources and used as received. The infrared spectra
(4000–400 cm−1) were recorded on a BIO-RAD FTS 3000 MX
instrument in KBr pellets; abbreviations: s = strong, m =
medium, w = weak, bs = broad and strong, mb = medium and
1
broad. The H and 13C NMR spectra were recorded at ambient
temperature on a Bruker Avance II + 300 (UltraShield™Magnet)
spectrometer operating at 300.130 and 75.468 MHz for proton
and carbon-13, respectively. The chemical shifts are reported
in ppm using tetramethylsilane as the internal reference;
abbreviations: s = singlet, d = doublet, t = triplet, q = quartet.
Carbon, hydrogen and nitrogen elemental analyses were
carried out by the Microanalytical Service of the Instituto
Superior Técnico. Electrospray mass spectra (ESI-MS) were run
with an ion-trap instrument (Varian 500-MS LC Ion Trap Mass
Spectrometer) equipped with an electrospray ion source. For
electrospray ionization, the drying gas and flow rate were opti-
mized according to the particular sample with 35 p.s.i. nebuli-
zer pressure. Scanning was performed from m/z 100 to 1200 in
methanol solution. The compounds were observed in the posi-
tive mode (capillary voltage = 80–105 V). Thermal properties
Fig. 12 Proposed catalytic cycle for the transesterification reaction
catalyzed by 2.
were tested as heterogeneous catalysts for nitroaldol (Henry)
and transesterification reactions. Framework 1 was the most
effective catalyst for the Henry reaction of nitroethane with
various aldehydes producing the corresponding β-nitroalka-
nols in high yields and with a significant stereoselectivity
towards the syn diastereomer. The yields of the nitroaldol reac-
tion were found to increase with the electrophilicity of the sub-
strates, the protic nature and polar character of the solvent
also playing an important role. Framework 2 was the most
effective catalyst for the transesterification of a variety of esters
with different alcohols.
The above observations provide evidence that MOFs can be
utilized as effective heterogeneous catalysts in important types
of reactions. Further explorations into the use of this type of
MOF as a catalyst in other organic transformations, as well as
mechanistic investigations, are ongoing.
were analyzed with
a Perkin-Elmer Instrument system
(STA6000) at a heating rate of 10 °C min−1 under a dinitrogen
atmosphere. Powder X-ray diffraction (PXRD) was conducted
on a D8 Advance Bruker AXS (Bragg Brentano geometry) theta–
2theta diffractometer, with copper radiation (Cu Kα, λ =
1.5406 Å) and a secondary monochromator, operated at 40 kV
and 40 mA. The flat plate configuration was used and the
typical data collection range was between 5° and 40°.
Synthesis of 2-acetamidoterephthalic acid (H2L1)
A 1.81 g (10 mmol) portion of 2-aminoterephthalic acid was
dissolved in 10 mL of acetic anhydride and the reaction
mixture was refluxed for 4 h at 80 °C, after which 20 mL of
water were added and the solution was further heated until
boiling. The obtained white solid product of 2-acetamidotere-
phthalic acid (H2L1) was filtered off and washed with water
Experimental
The synthetic work was performed in air and at room tempera- until total removal of acetic acid. Yield: 82% (1.83 g). Anal.
ture. All the chemicals were obtained from commercial Calcd for C10H9NO5 (M = 223.18): C, 53.82; H, 4.06; N, 6.28.
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Found: C, 53.15; H, 4.10; N, 6.50. FT-IR (KBr, cm−1): 3138 (bs),
In the second step, the isolated ester (3.13 g, 10 mmol) and
2579 (mb), 1693 (s), 1581 (s), 1535 (s), 1466 (m), 1424 (s), 1386 NaOH (0.8 g, 20 mmol) were dissolved in 20 mL of MeOH :
(m), 1285 (s), 1196 (s), 1150 (w), 1019 (w), 971 (s), 936 (w), 803 water (4 : 1). The reaction mixture was refluxed for 4 h at 80 °C,
(w), 759 (s), 668 (w), 554 (w), 504 (w); 1H-NMR (DMSO-d6): after which the solvent was removed under reduced pressure;
10.98 (1H, s, –NH), 8.98 (1H, s, Ar–H), 8.03 (2H, d, 8.4 Hz, Ar– 10 mL of water were added and the solution was acidified (pH
H), 7.64 (2H, d, 8.4 Hz, Ar–H), 2.14 (3H, s, –CH3); 13C-NMR = 2) with dilute HCl solution. The obtained white solid
(DMSO-d6): 172.5, 168.7, 166.5, 140.5, 135.2, 131.2, 123.0, product H2L3 was filtered off and washed with water until total
120.9, 120.4, 24.9. MS (ESI): m/z: 246.0 [M + Na]+.
removal of the acid. Yield: 81% (2.31 g). Anal. Calcd for
C15H11NO5 (M = 285.25): C, 63.16; H, 3.89; N, 4.91. Found: C,
63.11; H, 3.50; N, 4.72. FT-IR (KBr, cm−1): 3140 (bs), 2582
(mb), 1694 (s), 1618 (m), 1582 (s), 1537 (s), 1465 (m), 1427 (s),
Synthesis of 2-propionamidoterephthalic acid (H2L2)
This compound was synthesized by a two-step procedure.
In the first step, dimethyl-2-aminoterephthalate (2.09 g, 1389 (m), 1294 (s), 1248 (m), 1192 (s), 1152 (w), 1072 (w), 904
1
10 mmol) and NEt3 (1.51 g, 15 mmol) were placed in a round (s), 759 (s), 691 (s), 588 (w), 518(m); H-NMR (DMSO-d6): 12.14
bottom flask and then dry dichloromethane (20 mL) was (1H, s, –NH), 9.28 (1H, s, Ar–H), 8.13 (1H, d, 8.1 Hz, Ar–H),
added. After cooling in an ice bath followed by dropwise 7.97 (2H, d, 6.6 Hz, Ar–H), 7.59–7.75 (4H, m, Ar–H); 13C-NMR
addition of propionyl chloride (1.10 g, 12 mmol) the reaction (DMSO-d6): 169.8, 166.9, 165.3, 141.3, 135.9, 134.7, 132.8,
mixture was stirred overnight at room temperature. Upon 131.9, 129.5, 127.5, 123.8, 121.3, 120.7. MS (ESI): m/z: 308.0
removal of the solvent under reduced pressure a yellow solid [M + Na]+.
was obtained. 20 mL of water were added to the yellow solid
which was then extracted with dichloromethane. The organic
Synthesis of [Zn2(L1)2(4,4′-bipyridine)2(H2O)(DMF)]n (1)
extracts were collected over anhydrous sodium sulfate; sub- A mixture of Zn(NO3)2·6H2O (13 mg, 0.044 mmol), H2L1
sequent removal of the solvent gave the methyl ester of com- (10 mg, 0.044 mmol), and 4,4′-bipyridine (7 mg, 0.44 mmol)
pound H2L2. Yield: 73% (1.93 g).
was dissolved in 5 mL of DMF and water (1 : 1). A white precipi-
In the second step, the isolated ester (2.65 g, 10 mmol) and tate was obtained when the mixture was stirred at room temp-
NaOH (0.8 g, 20 mmol) were dissolved in 20 mL of MeOH : erature for 1 h. The precipitate was dissolved in 0.5 mL of 28%
water (4 : 1). The reaction mixture was refluxed for 4 h at 80 °C, aqueous ammonia solution, the resulting mixture was sealed
after which the solvent was removed under reduced pressure, in an 8 mL glass vessel and heated at 80 °C for 48 h. Sub-
10 mL of water were added and the solution was acidified (pH sequent gradual cooling to room temperature (0.2 °C min−1
)
= 2) with dilute HCl solution. The obtained yellow solid afforded needle-like colorless crystals. Yield: 61% (based on
product H2L2 was removed by filtration and washed with water Zn). Anal. Calcd for C43H39N7O12Zn2 (M = 976.55): C, 52.89; H,
until total removal of the acid. Yield: 62% (1.47 g). Anal. Calcd 4.03; N, 10.04; Found: C, 52.63; H, 4.00; N, 10.21. FT-IR (KBr,
for C11H11NO5 (M = 237.21): C, 55.70; H, 4.67; N, 5.90. Found: cm−1): 3329 (bs), 1668 (s), 1610 (s), 1564 (s), 1514 (s), 1451 (w),
C, 55.35; H, 4.50; N, 5.62. FT-IR (KBr, cm−1): 3345 (bs), 2978 1422 (s), 1363 (s), 1303 (s), 1265 (m), 1245 (m), 1220 (s),
(mb), 2560 (mb), 1683 (s), 1581 (s), 1536 (s), 1471 (m), 1414 (s), 1143 (w), 1069 (s), 1046 (m), 1010 (s), 996 (m), 943 (w),
1305 (m), 1258 (s), 1187 (m), 1084 (w), 1015 (w), 919 (s), 912 (m), 815 (s), 769 (s), 633 (s), 533 (w), 503 (w).
759 (s), 695 (s), 526 (w); 1H-NMR (DMSO-d6): 11.06 (1H, s, –NH),
Synthesis of [Zn4(L2)3(OH)2(DMF)2(H2O)2]n (2)
9.05 (1H, s, Ar–H), 8.02 (2H, d, 8.4 Hz, Ar–H), 7.63 (2H, d, 8.4
Hz, Ar–H), 2.41 (2H, q, 7.5 Hz, –CH2), 1.12 (3H, t, 7.5 Hz, –CH3); Zn(NO3)2·6H2O (6.6 mg, 0.022 mmol) and H2L2 (5 mg,
13C-NMR (DMSO-d6): 172.5, 169.4, 166.9, 141.1, 135.7, 131.6, 0.022 mmol) were dissolved in 4 mL of DMF : 1,4-dioxane : H2O
123.3, 121.2, 120.3, 31.0, 9.7. MS (ESI): m/z: 260.0 [M + Na]+.
(2 : 1 : 1 by volume), sealed in a capped glass vessel and heated
to 80 °C for 48 h. Subsequent gradual cooling to room temp-
erature (0.2 °C min−1) afforded colorless crystals obtained in
Synthesis of 2-benzamidoterephthalic acid (H2L3)
This compound was synthesized by a similar pathway as that ca. 77% yield (based on Zn). Anal. Calcd for C39H46N5O21Zn4
described for H2L2. (M = 1182.29): C, 39.62; H, 3.92; N, 5.92; Found: C, 39.71; H,
In the first stage, dimethyl-2-aminoterephthalate (2.09 g, 4.01; N, 5.76. FT-IR (KBr, cm−1) 3423 (bs), 3282 (bs), 2992 (w),
10 mmol) and NEt3 (1.51 g, 15 mmol) were placed in a round 1660 (s), 1568 (s), 1514 (m), 1417 (s), 1372 (s), 1297 (w), 1255
bottom flask and then dry dichloromethane (20 mL) was (w), 1105 (w), 1067 (w), 921 (w), 813 (w), 775 (s), 550 (mb).
added. After cooling in an ice bath followed by dropwise
Synthesis of [Zn(L3)(H2O)2]n·n/2(1,4-dioxane) (3)
addition of benzoyl chloride (1.68 g, 12 mmol) the reaction
mixture was stirred overnight at room temperature. Upon A mixture of Zn(NO3)2·6H2O (26 mg, 0.088 mmol) and H2L3
removal of the solvent under reduced pressure a yellow solid (10 mg, 0.035 mmol) was dissolved in 5 mL of DMF and 1,4-
was obtained. 20 mL of water were added to the yellow solid dioxane (1 : 1). A white precipitate was obtained when 0.2 mL
which was then extracted with dichloromethane. The organic of 28% aqueous ammonia solution was added to this reaction
extracts were collected over anhydrous sodium sulfate; sub- mixture. The precipitate was dissolved by the addition of an
sequent removal of the solvent gave the methyl ester of com- additional 0.5 mL of 28% aqueous ammonia solution. Then,
pound H2L3. Yield: 87% (2.72 g).
the resulting mixture was sealed in an 8 mL glass vessel and
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heated at 80 °C for 48 h. It was subsequently cooled to room Mo-Kα (λ 0.71069) radiation. Data were collected using phi and
temperature (0.2 °C min−1), affording plate-like colorless crys- omega scans of 0.5° per frame and a full sphere of data was
tals. Yield: 68% (based on Zn). Anal. Calcd for C17H17NO8Zn obtained. Cell parameters were retrieved using Bruker
(M = 428.69): C, 47.63; H, 4.00; N, 3.27. Found: C, 47.53; H, SMART46 software and refined using the Bruker SAINT46a on
3.92; N, 3.10. FT-IR (KBr, cm−1): 3319(s), 3242(s), 3179(s), all the observed reflections. Absorption corrections were
1636(s), 1569(s), 1511(s), 1429(s), 1337(s), 1288(s), 1109(m), applied using SADABS.46a Structures were solved by direct
1032(w), 960(w), 877(m), 819(m), 771(s), 699(s), 611(m), 558(m).
methods using SHELXS-97 package46b and refined using
the SHELXL-97.46b Calculations were performed using the
WinGX System–Version 1.80.03.46c The hydrogen atoms
attached to the carbon atoms and nitrogen atoms of 2 and 3
Procedure for the nitroaldol (Henry) reaction catalyzed by
Zn-MOFs
In a typical reaction, a mixture of aldehyde (1 mmol), were inserted at geometrically calculated positions and
nitroethane (0.3 mL) and Zn-catalyst (29 mg for 1, 35 mg of 2 included in the refinement using the riding-model approxi-
and 13 mg of 2, 3 mol%) was placed in a capped glass vessel mation; Uiso(H) were defined as 1.2Ueq of the parent nitrogen
and then 2 mL MeOH were added into it. The mixture was atoms or the carbon atoms for phenyl and methylene residues
heated at 70 °C for 48 h, and subsequently quenched by cen- and 1.5Ueq of the parent carbon atoms for the methyl groups.
trifugation and filtration at room temperature. The filtrate was The hydrogen atoms of the bridging hydroxide (in 2) and those
evaporated in a vacuum to give the crude product. The residue of coordinated water molecules were located from the final
1
was dissolved in DMSO-d6 and analyzed by H NMR. The yield difference Fourier map and the isotropic thermal parameters
of the β-nitroalkanol product (relatively to the aldehyde) was were set at 1.5 times the average thermal parameters of the
established typically by taking into consideration the relative belonging oxygen atoms; the coordinates of the H-water mole-
amounts of these compounds, as given by 1H NMR and pre- cules in 1 were blocked during the refinement process. There
viously reported results.45 The syn/anti selectivity was calcu- were disordered molecules present in the structure of 2. Since
1
lated on the basis of H-NMR spectra which are presented in no obvious major site occupations were found for those mole-
Fig. S7 (ESI†). In the 1H NMR spectra, the values of vicinal cules, it was not possible to model them. PLATON/SQUEEZE46d
coupling constants (for the β-nitroalkanol products) between was used to correct the data and a potential volume of
the α-N–C–H and the α-O–C–H protons identify the isomers, 2637.7 Å3 was found with 772 electrons per unit cell worth of
being J = 7–9 or 3.2–4 Hz for the syn or anti isomers, scattering. These were removed from the model and not
respectively.45
included in the empirical formula. The modified dataset
In order to perform the recycling experiment, first we improved the R1 value by ca. 67%. Moreover, one of the 2-pro-
washed the used catalyst with methanol and dried at room pionamidoterephthalate ligands in 2 is located close to an
temperature. It was then used for the nitroaldol reaction as inversion center and only three ring carbon atoms and the pro-
described above.
pionamido substituent could be located. To avoid the dupli-
cation of this substituent upon growing the fragment, the
symmetry related carbon ring atoms were generated, renamed
and their s.o.f. changed to 0.5 affording, as a result of this
Procedure for the transesterification reaction catalyzed by
Zn-MOFs
In a typical reaction, a mixture of methyl-3-nitrobenzoate strategy, the whole molecule of the ligand with an occupancy
(180 mg, 1 mmol) and Zn-catalyst (29 mg for 1, 35 mg of 2 and of 0.5 as defined by the multiplicity of the special position.
13 mg of 2, 3 mol%) in 2 mL EtOH was added into a capped The atoms were then flanked by PART 1 and PART 0 and the
glass vessel. The mixture was heated at 100 °C for 24 h. The structure finalized normally, though with application of geo-
solution was then centrifuged to remove the catalyst. The metric restraints. Least square refinements with anisotropic
solvent was evaporated in a vacuum, leading to a crude thermal motion parameters for all the non-hydrogen atoms
product. The product mixture was analyzed by 1H NMR in and isotropic ones for the remaining atoms were employed.
CDCl3. The yield of the ethyl ester product (relatively to the Crystallographic data are summarized in Table S1 (ESI†) and
methyl ester) was established typically by taking into consider- selected bond distances and angles are presented in Tables S3
ation the relative amounts of these compounds, as given by 1H and S4.† CCDC 968209–968211 for 1–3, respectively, contain
NMR. The 1H-NMR spectra are presented in Fig. S8 (ESI†). The the supplementary crystallographic data for this paper.
centrifuged catalyst was washed several times with methanol
and dried at room temperature. After that the recycling exper-
iment was performed under the conditions mentioned above.
Acknowledgements
Crystal structure determination
This work was supported by the Foundation for Science and
X-ray quality single crystals of the compounds were immersed Technology (FCT), Portugal (projects PTDC/QUI-QUI/102150/
in cryo-oil, mounted on a nylon loop and measured at 150 K 2008 and PEst-OE/QUI/UI0100/2013). A. Karmakar expresses
(2) or at room temperature (1 and 3). Intensity data were col- his gratitude to the FCT for his post-doctoral fellowships
lected using a Bruker AXS-KAPPA APEX II or a Bruker APEX-II (SFRH/BPD/76192/2011). The authors acknowledge the Portu-
PHOTON 100 diffractometer with graphite monochromated guese NMR Network (IST-UL Centre) for access to the NMR
This journal is © The Royal Society of Chemistry 2014
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facility, and IST Node of the Portuguese Network of mass-spec-
trometry (Dr Conceição Oliveira) for the ESI-MS measure-
ments. We also thank one of the referees for valuable
suggestions concerning the structure of compounds 2.
1450–1459; (b) A. Dhakshinamoorthy, M. Opanasenko,
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