Ionothermal synthesis, crystal structure, topology and catalytic properties of heterometallic coordination polymers constructed from N-(phosphonomethyl) iminodiacetic acid

Article abstract of DOI:10.1039/c5dt02176f

Reactions of rare earth chlorides, N-(phosphonomethyl)iminodiacetic acid (H₄pmida) and ferrous oxalate dihydrate under ionothermal conditions result in four new isostructural 3d-4f heterometal coordination polymers, [LnFe^IIIFeII6(Hpmida)₆]·2H₂O {Ln = Eu (1), Dy (2), Ho (3) and Y (4)}. All compounds were characterized by infrared spectroscopy, elemental analysis, thermogravimetric analysis, and X-ray diffraction. The compounds feature a very interesting three dimensional (3-D) structure built up from a secondary building unit, [Fe₂(Hpmida)₂]^2- and possess a new topology type and complicated unique tilings. The catalytic properties of compounds 1-4 were investigated showing that these types of compounds are heterogeneous catalysts in the Kn?evenagel condensation with high selectivity.

Full text of DOI:10.1039/c5dt02176f

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Ionothermal synthesis, crystal structure, topology
and catalytic properties of heterometallic
coordination polymers constructed from
N-(phosphonomethyl) iminodiacetic acid†
Cite this: DOI: 10.1039/c5dt02176f
Ting-Hai Yang,*a,b Ana Rosa Silva,b Lianshe Fuc and Fa-Nian Shi*b,d
Reactions of rare earth chlorides, N-(phosphonomethyl)iminodiacetic acid (H4pmida) and ferrous oxalate
dihydrate under ionothermal conditions result in four new isostructural 3d–4f heterometal coordination
polymers, [LnFeIIIFe6II(Hpmida)6]·2H2O {Ln = Eu (1), Dy (2), Ho (3) and Y (4)}. All compounds were charac-
terized by infrared spectroscopy, elemental analysis, thermogravimetric analysis, and X-ray diffraction. The
compounds feature a very interesting three dimensional (3-D) structure built up from a secondary building
unit, [Fe2(Hpmida)2]2− and possess a new topology type and complicated unique tilings. The catalytic pro-
perties of compounds 1–4 were investigated showing that these types of compounds are heterogeneous
catalysts in the Knöevenagel condensation with high selectivity.
Received 9th June 2015,
Accepted 24th June 2015
DOI: 10.1039/c5dt02176f
www.rsc.org/dalton
most attractive areas of materials research.23–26 The great inter-
est in this area is driven by two major reasons: first, the intro-
Introduction
Ionothermal synthesis based on the use of ionic liquids (ILs) duction of a second metal centre may allow the construction
as solvents has gained increasing attention due to the out- of appealing novel topologies and beautiful architectures, and
standing properties of ILs: nonvolatility, wide reaction temp- second, the incorporation of unusual metal coordination
erature range, high polarity, and so forth.1–6 Recently, it has environments may influence the physical properties of the
been explored and utilized in the fields of materials science materials, especially their photoluminescent, magnetic and
and coordination polymers (CPs) because the reaction mecha- catalytic properties.27–29 However, novel TMRECPs are difficult
nisms between the metal and organic components may differ to obtain due to the fact that the competing coordination of
from those of molecular environments under ionic transition metals (TM) and rare earth (RE) occurs to the same
environments.7–10 Usually, ILs behave as solvents, structure- ligand. These complicated competing reactions very often lead
directing templates, or charge-compensating groups in the to the generation of preferably homometallic complexes
reaction systems, thus the influence of both the cationic instead of the heterometallic ones.
and anionic parts of ILs on the CP structures have been
An effective strategy to obtain TMRECPs is the use of
studied.11–15 Accordingly, it is feasible to synthesize CPs in ILs multiple coordination sites bifunctional or multifunctional
which have new structures and novel properties that are in- ligands, such as amino acids,30–32 iminodiacetic acid,33–35 imid-
accessible by traditional methods.16–20 We have reported several azoledicarboxylic acid36–38 and pyridinecarboxylate,39–41 that
series of CPs based on the ionothermal synthesis method.21,22
can hold TM and RE at the same time. While much attention
The design and synthesis of transition metal–rare earth hetero- has been focused on linkers with nitrogen-containing carboxy-
metallic coordination polymers (TMRECPs) are some of the lates, only limited reports have documented TMRECPs with
phosphonate ligands.21,42–44
At the same time, functionalized phosphonates such as
nitrogen-containing phosphonates and carboxyphosphonates
could be the best choice for the construction of TMRECPs
because of their diverse coordinating sites and modes with
different affinities towards different metal centres. Phosphonic
acid, N-(phosphonomethyl) iminodiacetic acid (H4pmida) with
two additional carboxylic groups and one amine group, has
been shown to be an appropriate chelating ligand in a rela-
tively acidic solution.
aSchool of Chemistry & Environmental Engineering, Jiangsu University of Technology,
Changzhou 23001, P R China. E-mail: tinghai_yang@hotmail.com,
fshi96@foxmail.com; Fax: +86-519-86953269; Tel: +86-519-86953269
bDepartment of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
cDepartment of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
dSchool of Science, Shenyang University of Technology, 110870 Shenyang, P R China
†Electronic supplementary information (ESI) available: IR data, TG catalytic data
and table of elected bond lengths and angle of 1–4. CCDC 1055583–1055586. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt02176f
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With the above considerations in mind, we explored hetero- stirring until a homogeneous transparent colourless liquid was
metallic coordination polymers with three different functional formed.
groups that can potentially coordinate two metals through the
To a mixture containing H4pmida (3 mmol, 0.6813 g),
ionothermal synthesis method. Recently, we have reported that FeC2O4·2H2O (3 mmol, 0.540 g) and EuCl3·6H2O (0.5 mmol,
a series of novel TMRECPs based on pyrazole-3,5-dicarboxylic 0.1805 g) [or DyCl3·6H2O (0.1855 g) or HoCl3·6H2O (0.1856 g)
acid were an active and recyclable heterogeneous catalyst in or YCl3·6H2O (0.1517 g)] were added CM (8.0 g) and an
the cyclopropanation of styrene.29 Here we report the syn- aqueous solution of NaOH (0.5 M, 1.0 mL). The suspension
thesis, structural analysis and catalysis of four TMRECPs with was magnetically stirred for 1 h at ambient temperature in
H4pmida.
order to prepare a homogeneous sol, which was transferred to
a Teflon-lined autoclave and placed inside a preheated oven at
150 °C for 72 h. After the autoclave is cooled to room tempera-
ture, gold colored block crystals of compound 1 suitable for
single-crystal X-ray diffraction analyses were isolated from the
final reaction products by filtration after washing several times
Experimental
Materials and methods
All the other starting materials were of reagent quality and with ethanol and distilled water, and dried in air at ambient
were obtained from commercial sources without further purifi- temperature. The purities of the samples were confirmed by
cation. Elemental analyses for C, N and H were performed on powder X-ray diffraction studies and elemental analyses. For 1,
a TruSpec 630-200-200 elemental analyser with a combustion Yield: 0.5214 g, 63%, based on FeC2O4·2H2O. Anal. calcd for
furnace temperature of 1075 °C and an afterburner tempera- C30H46O44N6P6Fe7Eu: C, 18.73; H, 2.41; N, 4.40%. Found: C,
ture of 850 °C. FT-IR spectra were recorded from KBr pellets 18.82; H, 2.48; N, 4.51%. IR (KBr, cm−1): 3456(br), 2977(w),
(Aldrich 99%+, FT-IR grade) on a Mattson 7000 FT-IR spectro- 2913(w), 1690(m), 1608(s), 1459(m), 1431(s), 1410(s), 1379(w),
meter in a range of 4000–400 cm−1 at a resolution of 2 cm−1
.
1340(w), 1281(w), 1260(m), 1218(w), 1148(s), 1107(s), 1057(s),
Thermogravimetric analyses (TGA) were carried out using 993(m), 960(m), 927(m), 911(m), 868(m), 784(m), 718(w), 689(w),
a Shimadzu TGA 50 under air, from room temperature to 630(w), 566(s), 529(m), 516(m), 458(w), 384(w), 311(w).
ca. 700 °C, with a heating rate of 10 °C min−1. Powder X-ray
For 2, Yield: 0.5132 g, 62%, based on FeC2O4·2H2O. Anal.
diffraction (PXRD) patterns were recorded at ambient temperature Found (calcd) for C30H46O44N6P6Fe7Dy: C, 18.63; H, 2.40%; N,
using an Empyrean diffractometer with Cu-Kα radiation (λ = 4.35. Found: C, 18.56; H, 2.36; N, 4.39%. IR (KBr, cm−1): 3450
1.54178 Å) in the 2θ range of 5 to 50° in the reflection mode, (br), 2974(w), 2915(w), 1697(m), 1607(s), 1459(m), 1431(s),
which is equipped with a X’Celerator detector, a curved graph- 1411(s), 1379(w), 1340(w), 1280(w), 1260(m), 1217(w), 1147(s),
ite-monochromated radiation and a flat-plate sample holder, 1106(s), 1057(s), 993(m), 960(m), 927(m), 912(m), 868(m),
in a Bragg–Brentano para-focusing optics configuration (45 kV, 784(m), 718(w), 689(w), 630(w), 563(s), 529(m), 515(m), 458(w),
40 mA).
In the catalytic experiments, the Knöevenagel condensation
385(w), 310(w).
For 3, Yield: 0.4691 g, 56%, based on FeC2O4·2H2O. Anal.
was performed at 60 °C in batch reactors using 0.36 mmol Found (calcd) for C30H46O44N6P6Fe7Ho: C, 18.61; H, 2.39; N,
benzaldehyde, 0.75 mmol malononitrile, 0.18 mmol of n-unde- 4.34%. Found: C, 18.76; H, 2.46; N, 4.39%. IR (KBr, cm−1):
cane (internal standard) and 0.1515 g of compound 1, pre- 3460(br), 2975(w), 2915(w), 1696(m), 1606(s), 1459(m), 1432(s),
viously ground, in 1.00 ml of toluene. The kinetic profiles of 1412(s), 1379(w), 1340(w), 1281(w), 1261(m), 1218(w), 1147(s),
the reactions of compounds 1–4 were performed using the 1108(s), 1057(s), 994(m), 962(m), 927(m), 912(m), 870(m),
same experimental conditions, but using 4.0 mL of toluene. 784(m), 720(w), 690(w), 630(w), 564(s), 531(m), 513(m), 459(w),
The reaction was followed by GC, by withdrawing periodically 388(w), 309(w).
aliquots from the reaction mixture, using a Bruker 450 GC gas
For 4, Yield: 0.3441 g, 42%, based on FeC2O4·2H2O. Anal.
chromatograph equipped with a fused silica Varian Chrom- Found (calcd) for C30H46O44N6P6Fe7Y: C, 19.36; H, 2.49; N,
pack capillary column VF-1 ms (15 m × 0.25 mm id; 0.25 µm 4.52. Found: C, 19.46; H, 2.56; N, 4.59%. IR (KBr, cm−1): 3450
film thickness) and using helium as carrier gas. The obtained (br), 2975(w), 2914(w), 1697(m), 1607(s), 1462(m), 1431(s),
products were confirmed using a Finnigan Trace GC-MS. After 1411(s), 1379(w), 1340(w), 1278(w), 1261(m), 1217(w), 1149(s),
27 hours of reaction the solution was decanted, filtered 1105(s), 1057(s), 993(m), 959(m), 927(m), 913(m), 866(m),
through 0.2 µm PTFE syringe filters to a new reactor and 786(m), 719(w), 688(w), 630(w), 563(s), 530(m), 515(m), 458(w),
stirred at 60 °C over the weekend. A control experiment was 385(w), 309(w).
also performed using the same experimental procedure, but
without the addition of the CP 1.
X-ray crystallographic analysis
Synthesis of compounds [LnFeIIIFe6II(Hpmida)6]·2H2O {Ln = Single crystals with dimensions 0.08 × 0.06 × 0.04 mm for 1,
Eu (1), Dy (2), Ho (3) and Y (4)}. Compounds 1–4 were 0.16 × 0.16 × 0.14 mm for 2, 0.10 × 0.10 × 0.08 mm for 3, and
obtained under the same experimental conditions. In a 0.16 × 0.12 × 0.12 mm for 4 were selected for indexing and
general synthesis, a eutectic mixture (ionic liquid) was pre- intensity data collection at 298 K on a Bruker SMART APEX-II
pared by heating a mixture of choline chloride and malonic CCD diffractometer equipped with graphite-monochromatized
acid (CM) in a 1 : 1 mol ratio to 80 °C with constant magnetic Mo Kα (λ = 0.71073 Å) radiation. A hemisphere of data was
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Table 1 Crystallographic data for compounds 1–4
Compound
1 (Eu)
2 (Dy)
3 (Ho)
4 (Y)
Empirical formula
F. w.
Crystal system
Space group
a = b (Å)
c (Å)
C
30H46O44N6P6Fe7Eu
C30H46O44N6P6Fe7Dy
1934.00
C30H46O44N6P6Fe7Ho
1936.43
C30H46O44N6P6Fe7Y
1912.41
1923.46
Trigonal
ˉ
R3
15.3720(4)
22.8499(2)
90
120
4676.0(4)
3
2.049
2.836
2865
1.029
0.0566, 0.1079
0.0962, 0.1222
1.189, −1.001
Trigonal
Trigonal
Trigonal
ˉ
ˉ
ˉ
R3
R3
R3
15.2875(5)
22.7526(2)
90
120
4661.6(2)
3
2.092
3.076
2874
1.064
15.2935(4)
22.7749(1)
90
120
4613.2(2)
3
2.091
3.142
2877
1.047
15.3080(3)
22.7261(1)
90
120
4612.0(2)
3
2.009
2.804
2793
1.008
α = β (°)
γ (°)
V(Å3)
Z
Dc (g cm−3
)
μ (mm−1
F(000)
)
GOF on F2
R1, wR2 a [I > 2σ(I)]
R1, wR2 a (all data)
(Δρ)max, (Δρ)min (e Å−3
0.0487, 0.1491
0.0600, 0.1551
1.711, −1.204
0.0470, 0.1361
0.0554, 0.1398
2.481, −1.425
0.0448, 0.1380
0.0682, 0.1465
1.052, −0.825
)
a R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2
.
collected in the θ range 3.77–28.29° for 1, 1.78–25.99° for 2,
2.36–25.99° for 3, and 2.36–25.99° for 4 using a narrow-frame
method with scan widths of 0.30° in ω and an exposure time
of 10 s per frame. The measured and observed reflections [I >
2σ(I)] are 9435 and 2577 (Rint = 0.0715) for 1, 7855, 2019 (Rint
=
0.0527) for 2, and 8237 and 2021 (Rint = 0.0393) for 3, and 8116
and 2016 (Rint = 0.0550) for 4 respectively. The data were inte-
grated using the Siemens SAINT program,45 with the intensi-
ties corrected for Lorentz factor, polarization, air absorption,
and absorption due to variation in the path length through the
detector faceplate.
The structures were solved by direct methods using
SHELXS-9746 and refined on F2 by full matrix least-squares
using SHELXL-97.47 All the non-hydrogen atoms were refined
anisotropically. All H atoms were refined isotropically. Crystallo-
graphic and refinement details are listed in Table 1. Selected
bond lengths and angles are given in Tables S1 and S2.† CCDC
reference numbers: 1055583–1055586 correspond to com-
pounds 1–4, respectively.
Fig. 1 Building unit of 1 with the atomic labeling scheme (30% prob-
ability). All H atoms except that attached to O5 were omitted for clarity.
Symmetry codes are the same as those given in Table S1.†
carboxylate oxygen atoms [O(4), O(6), and O(7F)] from two
equivalent Hpmida3− ligands and one nitrogen atom [N(1)].
The Fe3+(2) ion has a highly distorted octahedral environment
Results and discussion
Description of structures
Compounds 1–4 are isostructural, and crystallize in trigonal with Fe(2) being bonded by three phosphonate oxygen atoms
lattices with the R3 space group. Hence only the structure of 1 [O(3), O(3C), and O(3D)] from three equivalent Hpmida3−
ˉ
will be discussed in detail as representative. The asymmetric ligands and three carboxylate oxygen atoms [O(6A), O(6B), and
unit of 1 consists of one Fe2+ ion, one-sixth Fe3+ ion, one-sixth O(6G)] from three equivalent Hpmida3− ligands. The Fe–O(N)
Eu3+ ion, one Hpmida3− ligand and one-third lattice water bond lengths range from 2.102(5) to 2.479(5) Å with slightly
molecule. As shown in Fig. 1, the three metal ions (Eu3+, Fe3+, longer Fe(2)–O6 bond [2.479(5) Å] and the Eu–O bond lengths
and Fe2+) are all six-coordinated with a distorted octahedral are all 2.292(4) Å (Table S1†) which are comparable to those
geometry. Around the Eu3+(1) ion, six positions are occupied 3d–4f heterometal coordination polymers.48,49
by six phosphonate oxygen atoms [O(2), O(2A), O(2B), O(2C),
The carboxylate oxygen atom O(5) of the Hpmida3− ligand
O(2D), and O(2E)] from six equivalent Hpmida3− ligands. The is protonated. The Hpmida3− anion serves as a nine-dentate
Fe2+(1) ion is surrounded by two phosphonate oxygen atoms ligand, chelating one Fe2+ ion, bridging two Fe2+ ions, two Fe3+
[O(1) and O(3G)] from two equivalent Hpmida3− ligands, three ions and one Eu3+ ion, respectively, in a µ9–η4N,O,O,O, η1O,
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Fig. 2 Coordination mode of the Hpmida3− ligand.
Fig. 4 The packing structure of 1 viewed along the b-axis. All H atoms
are omitted for clarity. Cyan polyhedron, coordinated Eu ion; green
polyhedron, coordinated Fe2+ ion; brown polyhedron, coordinated Fe3+
ion; pink polyhedron, CPO3 group.
To further understand the topological structure of these
metal phosphonates, we examined the connection mode of
the metal centers and the organic ligands. All topological
studies were performed with the software package TOPOS50
and only the covalent bonds in the structure were considered.
In compound 1, each crystallographically independent metal
center is taken as a node, with intermetal bridges ensured by
the bridging ligand. Due to its crucial structural importance,
i.e., connectivity to more than two metallic centers (µn), the
center of gravity of the ligand (Hpmida3−) was also con-
sidered as a network node. Each Eu3+ ion is connected to six
Hpmida3− ligands, and is viewed as a 6-connected node. Each
Fe3+ and Fe2+ ion, in turn, is connected to six and three
Hpmida3− ligands, and is considered a 6-connected node
and a 3-connected node, respectively. The Hpmida3− ligand
connects one Eu3+, two Fe3+ and three Fe2+ ions and is viewed
as a 6-connected node. The final network is, thus, tetra-
nodal, 3,6,6,6-connected, with an overall point symbol of
{412·63}2{43}6{46·86·103}-{48·66·8}6 (Fig. 5). The latest TTD data-
base indicates that this nodal connectivity is unprecedented
among CP structures, thus we deposit it as fnc3 in the TTD
database.
Fig. 3 (a) The dimeric unit structure [Fe2(Hpmida)2]2−in compound 1;
(b) layer structure constructed from the [Fe2(Hpmida)2]2−unit in 1; (c)
layer topological structure of 1. green ball, [Fe2(Hpmida)2]2− unit; (d) the
topological structure of connections between layers in 1. Cyan ball, Eu
atom; green ball, [Fe2(Hpmida)2]2− unit.
η1O′, η1O′, η1O″, η1O′′′ fashion (Fig. 2). Thus, the ligand traps
effectively the Fe2+ center [Fe(1)] inside three five-membered
chelating rings. Two such chelates are linked to form the
dimeric unit [Fe2(Hpmida)2]2− (Fig. 3a), in which the distances
between the symmetric Fe(1) is 5.0081(13) Å. The dimer and
the symmetric dimer are alternatively bridged by the carboxy-
late oxygen (O7) of Hpmida3− to form an infinite layer in the
ab plane (Fig. 3b). If the dimer was taken as a node, each
dimeric unit is connected to another four dimers, forming a
Kagome structure plane net whose tiling is composed of tri-
angles and hexagons (Fig. 3c).
Remarkably, connections between these adjacent layers
formed by the anionic [Fe2(Hpmida)2]2− dimeric units are
reinforced by the octahedral Eu atom (Fig. 3d), which connect
to the external phosphonate oxygen (O2) (Fig. S1†). Conse-
quently, a 3-D heterometallic framework (Fig. 4), thus, is con-
structed by linking these 2-D layers via six coordinated Eu
atoms. The overall framework is characterized by the presence
of large empty voids which can contain lattice water molecules
and charge-balance coordinated Fe3+ cations.
Using 3DT and Topos 4 programs50 to study the tilings of
net 1 show that there are two primitive proper tilings (PPT1
and PPT2 Fig. 6) in the unit cell and a natural tiling cannot be
constructed. There are four kinds of tiles in PPT1 and PPT2
(Fig. 6) corresponding to 2[43], 6[43], 6[43] and [424·86] (rep-
resented in four different colours). TOPOS analysis also
revealed the presence of 7 essential rings (4a, 4b, 4c, 4d, 4f, 4g,
and 8b for PPT1 and 4a, 4b, 4d, 4e, 4f, 4 g, and 8b for PPT2)
for each tiling in this 3-D net, which indicate that PPT1 and
PPT2 are quite similar with the same transitivity of [4674].
These 7 essential rings contain six 4-membered rings and one
8-membered ring that are organized together to build this
complicated architecture.
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as sharp, weak bands close to 2900 cm−1. The absorption
bands at 1697 cm−1 are assigned to the vibration of the proto-
nated carboxylate group, which agrees with the single X-ray
crystal structure analyses. The strong peaks at 1700–1300 cm−1
are assigned to the asymmetric and symmetric vibrations of
the coordinated carboxylate groups. The set of bands between
1200 and 900 cm−1 are assigned to stretching vibrations of the
tetrahedral CPO3 groups. Additional weak bands at low energy
for these compounds are found. These bands are probably due
to bending vibrations of the tetrahedral CPO3 groups.
PXRD and thermal stability
The experimental and simulated powder X-ray diffraction
(PXRD) patterns of complexes 1–4 are shown in Fig. 7. The
experimental PXRD patterns at room temperature are in good
agreement with the simulated one based on single-crystal
X-ray solution, indicating the phase purity of the bulk pro-
ducts. The difference in reflection intensities is probably due
to the preferred orientation effects.
Fig. 5 The topology structure of 1. pink ball, ligand; cyan ball, Eu3+
;
green ball, Fe2+; brown ball, Fe3+
.
Thermal analyses were carried out for 1–4 to examine the
thermal stabilities of these complexes (Fig. S5†). The thermo-
gravimetry curves of 1–4 display similar thermal behaviours
and good thermal stability (up to 250 °C) in air, presumably
due to the framework stabilization of the 3-D network. The
weight losses observed up to 250 °C are attributed to the
gradual release of the confined water molecules from the struc-
ture cavities. The weight loss above 250 °C is due to the
decomposition of the compounds and the collapse of the
lattice. The final residue was not characterized.
Catalytic properties
Some CPs have been studied as heterogeneous catalysts in the
Knöevenagel condensation (Fig. 8).51 This is an important C–C
coupling reaction of a carbonyl group with a compound con-
taining an activated methylene group (Fig. 8) and is widely
used for the synthesis of fine chemicals and pharmaceuti-
cals.52 Thus compound 1 was tested as the heterogeneous cata-
Fig. 6 Two typical tiling nets of compound 1 along the x-axis showing
very similar features and having the same transitivity of [4674].
The structures of compounds 2–4 are analogous to 1 except
that the Eu3+ ion in 1 is replaced by Dy3+ in 2, Ho3+ in 3 and
Y3+ in 4. The cell volumes decrease in turn from 1 to 4, in
accordance with the decreasing sequence of the ionic radii
of the corresponding metal ions due to the lanthanide
contraction.
IR spectra
The IR spectra for compounds 1–4 were recorded in the region
from 4000 to 400 cm−1 (Fig. S2–S4†). The presence of lattice
and coordination water molecules is manifested by a broad IR
band of medium intensity in the range of about 3500 cm−1
,
assigned to ν(OH). The C–H stretching vibrations are observed Fig. 7 Experimental and simulated PXRD patterns of 1–4.
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can be seen for 1 that the reaction progressed slower than in
1.0 mL of toluene and in a smaller reactor. Nevertheless, the
most active are 1 and 2, whereas 3 and 4 show similar catalytic
activity. This also shows that the presence of lanthanides is
important for the course of the reaction.
The MOFs only contain Lewis acid sites and Knöevenagel
condensation has been reported as traditionally being cata-
lysed by base catalysts.53 However, copper(II) benzene-1,3,5-tri-
carboxylate (CuBTC) metal–organic framework was found be
an effective heterogeneous catalyst for the Knöevenagel con-
Fig. 8 Knöevenagel condensation of benzaldehyde with malononitrile.
lyst in the Knöevenagel condensation of benzaldehyde with densation despite containing Lewis acid sites.53 A mechanism
malononitrile in 1.0 mL toluene at 60 °C. The material showed
catalytic activity in this reaction when compared to the control
was proposed where the CuBTC works as a Brønsted base in
the reaction and deprotonates the active methylene group of
experiment performed using exactly the same experimental malononitrile. A Brønsted acid defect site is thus formed in
conditions, but without the addition of the catalyst. After
27 hours of reaction the benzaldehyde conversion was 27%,
the MOF and together with the adjacent Lewis acid (iron and
rare earth ions) facilitate the activation of both reactants in the
whereas for the control experiment 7% was obtained. The Knöevenagel condensation.53 The deprotonated active methyl-
turnover number (TON) was 119 for the reaction with com-
ene group of malononitrile attacks the aldehyde and then after
re-protonation and dehydration of the adduct the Knöevenagel
pound 1 and the corresponding turnover frequency (TOF) was
4.4 h−1. In both reactions the major product observed was the condensation product is formed.53
expected Knöevenagel condensation product (Fig. 8)51 with 98
and 95% selectivity, respectively. Benzoic acid was observed as
the minor product. After 27 hours of reaction, the solution was Conclusions
separated from the heterogeneous catalyst by decantation and
In summary, four new 3d–4f hetero-metal organic phospho-
it was filtered, using syringe filters, into a new reactor. After a
weekend stirring (68 hours) at 60 °C no further significant
catalytic activity was observed (%C = 29), proving that the
observed 27% benzaldehyde conversion was from compound 1
and that the reaction was truly heterogeneous. After con-
venient washing with toluene, the filtered material was used in
a second catalytic cycle, using the same experimental con-
ditions. After 27 hours benzaldehyde conversion of 32% was
obtained with a TON of 165 and TOF of 6.1 h−1. In the third
catalytic cycle of reuse and after 27 hours the benzaldehyde
conversion was 28%, with a TON of 161 and TOF of 6.0 h−1. In
the fourth catalytic cycle of reuse and after 27 hours benz-
aldehyde conversion was 8%, with a TON of 86 and TOF of
3.2 h−1. After the second cycle, the PXRD pattern of 1 is
similar to the as-synthesized one (Fig. S7†). Thus the material
could be recycled and reused at least for two more catalytic
cycles without the loss of catalytic activity. The substrate scope
was also extended to o-tolualdehyde and heptaldehyde. The
reaction with malononitrile of o-tolualdehyde gave an aldehyde
conversion of 20% in 4.0 mL of toluene at 60 °C, with a TON
of 94 and TOF of 3.2 h−1 (27 hours of reaction). No by-products
were detected indicating that the reaction was selective for the
Knöevenagel condensation product. The reaction of malono-
nitrile with heptaldehyde in 1.0 mL of toluene at 60 °C gave
57% conversion of aldehyde, with a TON of 236 and TOF of
8.7 h−1 (27 hours of reaction). However the reaction with hept-
aldehyde was not selective as heptanoic acid was also formed
as a by-product.
nate frameworks, [LnFeIIIFe6II(Hpmida)6]·2H2O {Ln = Eu (1), Dy
(2), Ho (3) and Y (4), H4pmida: N-(phosphonomethyl)iminodi-
acetic acid} were synthesized via ionothermal methodology. The
coordination polymers feature a very interesting 3-D frame-
work built from a secondary building unit, [Fe2(Hpmida)2]2−
,
further connecting six coordinated Eu3+ and Fe3+ cations and
possess a new topology type with the point symbol of
{412·63}2{43}6{46·86·103}{48·66·8}6 and complicated unique
tilings. The catalytic properties of 1–4 indicate that these types
of compounds are truly heterogeneous catalysts in the Knöeve-
nagel condensation with high selectivity.
Acknowledgements
We thank Fundação para a Ciência e a Tecnologia (FCT),
FEDER, QREN, COMPETE (PEst-C/CTM/LA0011/2013) and
(FCT: SFRH/BPD/74582/2010) for financial support. F.-N. SHI
acknowledges FCT for the project of PTDC/CTM-NAN/119994/
2010. T.-H. Yang acknowledges the NSF of Jiangsu Province
(BK20140244), The Project Sponsored by the Scientific
Research Foundation for Returned Overseas Chinese Scholars
of State Education Ministry, the NSF for Universities in Jiangsu
Province (13KJB150013), the Science and Technology Bureau of
Changzhou City (no. CJ20140031) and Jiangsu University of
Technology (KYY13035).
The other compounds were also tested in the Knöevenagel
condensation of benzaldehyde with malononitrile in 4.0 mL of
toluene at 60 °C at the same time using a Radleys Carousel.
The kinetic profile of the reactions is presented in Fig. S8.† It
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