Mixed lanthanide succinate-sulfate 3D MOFs: Catalysts in nitroaromatic reduction reactions and emitting materials

Article abstract of DOI:10.1039/c1jm14677g

The first series of mixed succinate/sulfate/Ln MOFs, [Ln₂(C ₂H₄C₂O₄)₂(SO ₄)(H₂O)₂] (RPF-16), where Ln = La, Pr, Nd, and Sm, were hydrothermally obtained and their structures determined by X-ray single crystal diffraction. The crystalline products are a series of isostructural 3D polymeric compounds that crystallise in the monoclinic space group P2(1)/n. Their framework comprises infinite crossing chains of LnO₉ sharing edges polyhedra, kept together by succinate and sulfate anions. Topological simplification gives rise to a 3D uninodal six connected net of type pcu alpha-Po primitive cubic. These well-defined compounds show high chemoselectivity towards reduction of the nitro group and present bifunctional activity for the one-pot reductive amination of heptanal at near-complete conversion of the substrates. A general overview of the room-temperature luminescence behavior in the new RPF-16 Ln materials is also reported. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1jm14677g

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PAPER
Mixed lanthanide succinate–sulfate 3D MOFs: catalysts in nitroaromatic
reduction reactions and emitting materials†
a
a
a
ab
a
Richard F. D’Vries, Marta Iglesias, Natalia Snejko, Susana Alvarez-Garcia, Enrique Guti ꢀe rrez-Puebla
and M. Angeles Monge*a
Received 20th September 2011, Accepted 26th October 2011
DOI: 10.1039/c1jm14677g
2 2 4 2 4 2 4 2 2
The first series of mixed succinate/sulfate/Ln MOFs, [Ln (C H C O ) (SO )(H O) ] (RPF-16), where
Ln ¼ La, Pr, Nd, and Sm, were hydrothermally obtained and their structures determined by X-ray
single crystal diffraction. The crystalline products are a series of isostructural 3D polymeric compounds
that crystallise in the monoclinic space group P2(1)/n. Their framework comprises infinite crossing
chains of LnO sharing edges polyhedra, kept together by succinate and sulfate anions. Topological
9
simplification gives rise to a 3D uninodal six connected net of type pcu alpha-Po primitive cubic. These
well-defined compounds show high chemoselectivity towards reduction of the nitro group and present
bifunctional activity for the one-pot reductive amination of heptanal at near-complete conversion of
the substrates. A general overview of the room-temperature luminescence behavior in the new RPF-16
Ln materials is also reported.
Introduction
and nitrogen-free aliphatic linker remains still a challenge. Only
18–20
a few lanthanideoxalate–sulfates have been reported,
and most
The metal–organic frameworks that contain trivalent lanthanide
+
of them own anionic frames with NH4 cations in their tunnels.
In the search for new architectures of lanthanide MOFs, and
after a long time studying lanthanide sulfate, sulfonate and
succinate compounds, as well as their material properties, in this
work we present the first series of mixed succinate/sulfate/Ln
1,2
cations are widely known for their uses as optical and sensor
3
4
5,6
materials, in gases absorption, as magnetic materials, cata-
7–9
10
lysts and drug carriers. The growing number of reports of
applications of this kind of compounds in different fields depicts
1
1
the importance of the lanthanide MOFs in the development of
new technologies. In the catalysis field, the Ln-MOFs are poorly
used and few examples have been reported in epoxidation of
MOFs, [Ln
2
(C
2
4
H C
O
2 4
)
2
(SO
4
)(H
2
O)
2
] (RPF-16), where Ln ¼
La, Pr, Nd, and Sm. These compounds were hydrothermally
obtained and their structures determined by X-ray single crystal
diffraction. Characterization of the bulk samples by powder
X-ray diffraction, IR and thermogravimetric analysis is also
reported. RPF-16 series has been explored as potential chemo-
selective heterogeneous catalysts for the reduction of nitroarenes
in the presence of other reducible functional groups. The mate-
rials were also tested as bifunctional catalysts for the one-pot
reductive amination of heptanal. Additionally, a general over-
view of the room-temperature luminescence behaviour of the
new RPF-16 materials is reported.
12
13
olefins, oxidation of organic sulfurs, acetalization of alde-
14
2
hydes, and some other reactions. On the other hand, the
sulfate anion is an effective component to built structures with
15
new topologies, and materials with different properties. Most
of the reported works on lanthanide sulfates focus generally on
inorganic structures containing alkali metal or ammonium
16,17
ions.
Concerning organic multidentate ligands (spacers) linking
lanthanide ions (connectors), there is a good number of new
compounds with very different and, in some cases, new topologies.
4
However, the obtaining of new compounds combining SO anions
Experimental section
a
Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC),
Cantoblanco, 28049 Madrid, Spain. E-mail: amonge@icmm.csic.es; Fax:
General information
+
34 913720623
All reagents and solvents employed were commercially available
and used as supplied without further purification: succinic acid
(98% Merck); disodium sulfate (98% Merck); lanthanum nitrate
hexahydrate (99.9% Alfa Aesar); praseodymium nitrate hexa-
hydrate (99.9% Strem Chemicals); neodymium nitrate hexahy-
drate (99.9% Alfa Aesar); samarium nitrate hexahydrate (99.9%
Strem Chemicals).
b
Instituto Qu ꢀı mica-F ꢀı sica Rocasolano, (CSIC), Madrid, E-28006, Spain
†
Electronic supplementary information (ESI) available: Experimental
and simulated powder X-ray diffraction patterns, selected bond
lengths, intramolecular hydrogen bonds, X-ray powder patterns after
TG analysis and X-ray powder patterns after catalytic studies for
compounds 1–4. CCDC reference numbers 828893, 828894, 828895 and
828896. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1jm14677g
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J. Mater. Chem., 2012, 22, 1191–1198 | 1191

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The IR spectra were recorded from KBr pellets in the range
X-Ray powder diffraction
ꢀ
1
4
000–250 cm on a Bruker IFS 66V/S. The thermogravimetric
The X-ray powder diffraction measurements were used to check
the purity of the obtained microcrystalline products by
comparison of the experimental results with the simulated
patterns obtained from single crystal X-ray diffraction data. The
residues for the compounds after TG analyses were analysed by
X-ray powder diffraction and compared with ICSD patterns
reported.
and differential thermal analyses (TGA-DTA) were performed
using Seiko TG/DTA 320U equipment in the temperature range
ꢀ1
ꢁ
between 25 and 1000 C in air (100 mL min flow) atmosphere
ꢁ
ꢀ1
and a heating rate of 10 C min . A Perkin-Elmer CHNS
Analyzer 2400 was employed for the elemental analysis. Powder
X-ray diffraction (PXRD) patterns were measured with a Bruker
ꢁ
D8 diffractometer, with a step size of 0.02 and exposure time of
0
.5 s per step.
Optical measurements
Photoluminescence spectra of individual crystals were obtained
+
at room temperature exciting with one line (488 nm) of an Ar
Synthesis
laser, a Jobin-Yvon HR 460 monochromator, coupled to a N2
The compounds were synthesized under hydrothermal condi-
tions. The molar composition of the initial reaction mixture was
cooled CCD. The excitation light was focused on the sample with
2
ꢀ
3+
2ꢀ
an Olympus microscope which is also used to collect the scattered
light with 100ꢂ objectives. The crystals were larger than the spot
size (around 1 micron). Notch filters of the corresponding
wavelengths were used to eliminate the elastic component of the
collected light. The spectra were corrected by the instrumental
0
.5 SO4 : Ln : Succ : 1500 H O.
2
[
La (C H C O ) (SO )(H O) ] (1) was synthesized by addi-
2 2 4 2 4 2 4 2 2
tion of 0.027 g (0.231 mmol) of succinic acid and 0.016 g
0.115 mmol) of disodium sulfate to a dissolution of lanthanum
(
nitrate hexahydrate (0.1 g, 0.231 mmol) in 6 mL of water with
constant stirring for 15 minutes. The pH of the mixture was
function recorded with a calibrated white source and a CaF
pellet.
2
ꢀ1
fixed at 5.0 (NaOH, 1 mol L ). The resultant reaction mixture
was transferred to a Teflon-lined stainless steel autoclave and
ꢁ
heated at 160 C for 18 hours. Colourless crystals suitable for
Catalytic reduction of nitroaromatic compounds
X-ray diffraction analysis were obtained by quenching the
vessel, after being filtered and washed with water and
acetone (yield 70%).
The catalytic properties in hydrogenation of nitroaromatic
reactions of the RPF-16 compounds were examined under
conventional conditions for batch reactions in a reactor (Auto-
Compounds [Ln (C H C O ) (SO )(H O) ] [Ln ¼ Pr (2), Nd
2
2
4
2
4 2
4
2
2
clave Engineers) of 100 mL capacity in toluene and 1/200 metal/
5
(
3) and Sm (4)] were synthesized using a similar procedure;
substrate molar ratio, 5 ꢂ 10 Pa of H
2
pressure and at 363 K
crystalline products 2 (light green, yield 75%), 3 (violet, yield
8%) and 4 (light yellow, yield 82%) were obtained by quenching
the vessel, after being filtered and washed with water and
acetone. Elemental analysis, calculated for 1 (C 14SLa ): C,
4.95; H, 1.87; S, 4.98; found: C, 14.64; H, 1.75; S, 5.18%;
calculated for 2 (C 14SPr ): C, 14.86; H, 1.86; S, 4.95;
found: C, 14.50; H, 1.76; S, 5.17%; calculated for
temperature. The nitro compound was dissolved in toluene and
the catalyst (0.5 mol%) was added. The mixture was placed in an
6
autoclave. After purging with H the reaction mixture was heated
2
8
H
12
O
2
to the desired temperature and stirred (1500 rpm). The products
composition was determined by means of gas chromatography;
the reaction mixture was centrifuged for removing the catalyst.
The products were identified by gas chromatography/mass
spectrometry (GC-MS). Only experiments with mass balances
1
8
H
12
O
2
3
(
C H O SNd ): C, 14.71; H, 1.84; S, 4.90; found: C, 14.37; H,
8 12 14 2
1
1
.72; S, 4.30%; calculated for 4 (C H O SSm ): C, 14.44; H,
8 12 14 2
>
95% were considered.
.80; S, 4.81; found: C, 14.37; H, 1.77; S, 4.89%.
Results and discussion
Single-crystal structure determination
Details of crystallographic data, data collection and refinement
are summarized in Table 1. An ORTEP representation of the
compounds is shown in Fig. 1.
Single-crystal X-ray diffraction data for the compounds were
obtained in a Bruker–Siemens Smart CCD diffractometer
equipped with a normal focus, 2.4 kW sealed tube X-ray source
ꢁ
Upon determining the crystal structure, the formula of these
isostructural series of compounds turned out to be
(
Mo Ka radiation ¼ 0.71073 A) operating at 50 kV and
2
0 mA. Data were collected over a hemisphere of the reciprocal
[Ln
2
(C
2
H
4
C
2
O
4
)
2
(SO
4
)(H
2
O)
2
], (RPF-16), where Ln ¼ La, Pr,
space by a combination of three sets of exposure. Each expo-
ꢁ
Nd, and Sm. This structural type crystallizes in the monoclinic
space group P2(1)/n. The lanthanide ion is coordinated to nine
oxygen atoms: six of succinate ligands, two of different sulfate
sure of 20 s covered 0.3 in u. The unit cell dimensions were
determined for Least-Square fit of reflections with I > 2s. The
structures were resolved by direct methods. The final cycles of
refinement were carried out by full-matrix least-square analyses
with anisotropic thermal parameters of all non-hydrogen
atoms. The hydrogen atoms were fixed at their calculated
positions using distances and angle constrains. All calculations
9
groups, and one of a water molecule in LnO tricapped trigonal
prisms (Fig. 1 and S2, ESI†). The two crystallographically
independent succinate anions (A and B) present in the structure
have trans conformation. Averages of the torsion angle values
for the four isostructural new compounds were made with the
2
1
ꢁ
were performed using SMART software for data collection,
following results: (C5C4C3C8) ¼ 176.5 and (C1C2C6C7) ¼
22
ꢁ
SAINT for data reduction and SHELXTL to resolve and
163.6 for (A) and (B) ligands, respectively. Concerning the
23
carboxylate groups: the values are (O8C5C4C3) ¼ 71.1ꢁ and
refine the structure.
1
192 | J. Mater. Chem., 2012, 22, 1191–1198
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Table 1 Crystallographic data and refinement details for RPF-16
Compound
1
2
3
4
Empirical formula
FW
Temperature/K
C
8
H
12
O
14SLa
2
C
8
H
12
O
14SPr
2
C
8
H
12
O
14SNd
2
C
8 12 2
H O14SSm
642.07
298(2)
0.71073
646.07
298(2)
0.71073
652.73
298(2)
0.71073
664.94
298(2)
0.71073
ꢁ
Wavelength/A
Crystal system
Space group
Unit cell dimensions
Monoclinic
P2 /n
Monoclinic
P2 /n
Monoclinic
P2 /n
Monoclinic
P2 /n
1
1
1
1
ꢁ
a/A
12.881(2)
9.6559(16)
13.088(2)
110.357(3)
1526.2(4)
4
12.7580(10)
9.5881(8)
12.9304(11)
110.2330(10)
1484.1(2)
4
12.7225(19)
9.5748(14)
12.897(2)
110.205(2)
1474.4(4)
4
12.6365(6)
9.5308(4)
12.8091(6)
110.1280(10)
1448.46(11)
4
ꢁ
b/A
ꢁ
c/A
b/
Volume/A
ꢁ
ꢁ
3
Z
r calc/Mg m
ꢀ
3
2.794
1
Absorption coefficient/mm 5.736
2.891
6.707
2.941
7.186
3.049
8.253
ꢀ
F(000)
Crystal size
q range for data collection/ 1.91 to 26.39
1208
0.11 ꢂ 0.07 ꢂ 0.06
1224
0.60 ꢂ 0.40 ꢂ 0.10
1232
0.12 ꢂ 0.07 ꢂ 0.06
1248
0.11 ꢂ 0.07 ꢂ 0.04
ꢁ
1.93 to 26.37
1.94 to 26.37
1.95 to 25.03
Reflections collected/unique 120 24/3091 [R(int) ¼ 0.0435] 11 810/3012 [R(int) ¼ 0.0351] 11 438/2977 [R(int) ¼ 0.0491] 10 285/2549 [R(int) ¼ 0.0436]
R(int)]
Completeness (%)
Data/restraints/parameters 3091/0/226
[
99.00%
99.50%
99.20%
99.40%
3012/0/226
1.376
2977/0/208
1.314
2549/0/236
1.272
2
Goodness-of-fit on F
R1 [I > 2s(I)]
wR2[I >2s(I)]
R1 (all data)
wR2 (all data)
1.178
R1 ¼ 0.0343
wR2 ¼ 0.0731
R1 ¼ 0.0471
wR2 ¼ 0.0758
R1 ¼ 0.0356
wR2 ¼ 0.0907
R1 ¼ 0.0429
wR2 ¼ 0.0925
1.615 and ꢀ1.251
R1 ¼ 0.0696
wR2 ¼ 0.1961
R1 ¼ 0.0836
wR2 ¼ 0.2002
5.802 and ꢀ2.565
R1 ¼ 0.0365
wR2 ¼ 0.0821
R1 ¼ 0.0489
wR2 ¼ 0.0850
1.648 and ꢀ1.319
Largest diff. peak and hole 1.457 and ꢀ1.131
which run along the b and c directions in a crossing way, are
joined by the sulfate group, which is coordinated to four different
metallic centres at distances and angles corresponding to its
tetrahedral conformation (ESI†, Fig. S2). In this way, a totally
inorganic 3D net would be formed. Besides, there are other
connections along the [100] and [101] directions, through the (B)
and (A) succinate linkers, respectively. Additionally, two intra-
molecular hydrogen bonds between the coordination water and
the oxygen atoms of the sulfate and carboxylate groups were
found (ESI†, Fig. S3).
For topological study of the 3D net, after trying the rod-shape
24
packing method and in order to obtain the simplest net the
model has been simplified as follows: the chain nodes were placed
Fig. 1 ORTEP drawing of the asymmetric unit for the [La
2 2 4-
(C H
C
2
O
4
)
2
(SO )(H O) ] compound; ellipsoids are displayed at the 50%
2
4
2
4
in the Ln/Ln distances middle points, so that SO anions
probability level and some hydrogen atoms are omitted for clarity.
become bi-connected, and thus, they do not represent nodes, but
linkers. Sulfate linkers play an essential role in the structure,
since they join the crossing chains, forming the main scaffold.
The succinate ligands are the other linkers that join the parallel
chain centroids. As a result of this simplification, a 3D uninodal
six connected net of type pcu alpha-Po primitive cubic is found
Symmetry transformations used to generate equivalent atoms: (a) ꢀx + 1,
ꢀ
y + 1, ꢀz + 1; (b) ꢀx + 1, ꢀy + 2, ꢀz; (c) ꢀx + 3/2, y ꢀ 1/2, ꢀz + 1/2; (d)
x + 1/2, ꢀy + 3/2, z + 1/2; (e) x ꢀ 1/2, ꢀy + 3/2, z ꢀ 1/2; (f) ꢀx + 3/2,
y + 1/2, ꢀz + 1/2; (g) x, y, z + 1; (h) x, y, z ꢀ 1.
12
3
(
Fig. 2), with a point (Schlafli) symbol (4 ꢂ 6 ), which agrees
ꢁ
ꢁ
(
O3C8C3C4) ¼ 61.4 for (A) succinate, for (B) one is smaller,
25
with the program TOPOS analysis.
(
O1C1C2C6) ¼ 3.27 while the other is in the same order of those
ꢁ
of (A): (O1C7C6C2) ¼ 73.8 . These differences are due to the
type of coordination and to the succinate flexibility, which allow
the formation of the three-dimensional network. In both cases,
the carboxylate anions are bound in the uncommon chelating
Thermal stability study
All the compounds present similar decomposition curves (Fig. 3);
ꢁ
2
1
2
1
bridge mode h m –h m , simplified as m :m :h (Fig. 2).
2
the compounds are stable up to ꢃ260 C. Decomposition begins
2
2
This arrangement gives rise to infinite chains of LnO sharing
9
with the loss of coordinated water and follows with the total
ꢁ
edges polyhedra, with Ln1/Ln2 distances in the chain of (La1/
decomposition, losing the ligand molecule around ꢃ500
C
La2) ¼ 4.3817(8), (Pr1/Pr2) ¼ 4.3327(8), (Nd1/Nd2) ¼4.324
(% water mass calculated: compound 1: 5.8; 2: 5.57; 3: 5.52; and
4: 5.41). Decomposition products for the four compounds were
ꢁ
(
1) and (Sm1/Sm2) ¼ 4.2972(8) A (ESI†, Fig. S2). These chains,
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Fig. 2 Representation of: (up) primary building unit (PBU), secondary building unit, and linkers; (middle) structure formation; (down) schematic
3
12
description of the 4-connected net (4 ꢂ 6 ) topology in RPF-16.
2
6
[
(LnO)
2
SO
4
] (ICSD 66823), confirmed in all the cases by X-ray
compounds are good catalysts if the Ln coordination number is
eight or lower, no matter either the kind of lanthanide present or
the existence of potentially displaceable coordinated water
powder diffraction of the TG residue.
IR spectra
The FT-IR spectra of all RPF-16 show the same profile (Fig. 4a).
The O–H vibration of the coordination water can be identified as
one broad and asymmetrical band that involves two components
ꢀ1
centred at ꢃ3400 and ꢃ3255 cm , belonging to the two coor-
dination water molecules. Two low intensity bands located
at ꢃ1662 and ꢃ1675 are associated with the water bending
ꢀ1
vibration mode. The band sited at 1530–1565 cm is assigned
ꢀ1
to n (OCO) and the band at 1328 cm is associated with
as
n
s
(OCO) mode.
The absence of COOH bands indicates that the initial succinic
acid is not present in the final product. Low intensity bands
corresponding to S–O vibration of the sulfate anion appear in the
ꢀ1
ꢀ1
region of 1000–942 cm (942, 957, 985 and 1000 cm ).
Catalytic properties
From previous catalytic studies of Ln sulfonate MOFs, which
presented both redox and acid sites, we learnt that in those
reactions, whose mechanism implies the approximation of the
intermediate species to the lanthanide coordination sphere, the
Fig. 3 TGA of compounds 1–4, where the decomposition products are
[(LnO)
2
SO
4
] (Ln ¼ La, Pr, Nd, and Sm).
1
194 | J. Mater. Chem., 2012, 22, 1191–1198
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Table 2 Hydrogenation of nitrobenzene for compounds 1–4
a
Yield (%) (h)
b
TOF /h
ꢀ1
Compound
c
1
100 (6)
100 (1.5)
100 (2)
100 (1.5)
100 (2)
29.7
175.2
161.2
146.8
126.1
134
d
2
d
3
d
4
nd
rd
th
de
2
3
4
cycle
de
cycle
cycle
100 (2.5)
100 (2.5)
de
94
a
c
b
Yield determined by GC-MS. TOF: mmol of substrate/mmol cat. H.
d
Reaction performed with 1 mol% cat. Reaction performed at 0.5 mol
e
cat. Compound 2.
%
Fig. 4 FT-IR for compounds RPF-16.
a catalyst/substrate ratio of 1/200 (0.5 mol% catalyst), and
compounds 2–4 showed a higher activity than 1, with TOF values
2
molecules. Examples of such reactions, in which the bifunctional
character of the materials as catalyst has been tested, are the
of ꢃ10 (Table 2). This behaviour is in accordance with the
significant difference in the Lewis acidity of the corresponding
35
Ln–metal [La(III) vs. Pr(III), Nd(III) and Sm(III)]. This hydro-
27,28
7,8,29
transformation of linalool,
or the sulfide oxidation.
In
order to confirm this fact, the RPF-16 materials were used as
catalysts in these reactions. As was expected, since the Ln
coordination number in these materials is nine, none of the
compounds were active as catalysts in these reactions.
genation reaction is favoured by the presence of acid centres,
because they promote the heterolytic rupture of the hydrogen
molecule to form an Ln–hydride intermediate and then the
36–39
interaction with the substrate (nitro group).
Fig. 5 shows the
A mechanism for the lanthanide MOF-mediated hydrogena-
kinetic profile for the Ln-catalyzed hydrogenation of nitroben-
zene; as can be seen the activity increases in the order La ꢄ Sm <
Nd < Pr, which is in agreement with the increasing of their Lewis
2
tion, based on the H heterolytic cleavage, could be analogous to
that proposed in which the catalytic activity is explained by the
formation of metal–hydride intermediate species. Thinking of
a better approximation of the small H2 molecule to a nine-
coordinated lanthanide ion followed by the water molecule
displacement, the new series of catalysts was tested for hydro-
genation of nitroaromatics.
35,36
acidity.
Under the optimized reaction conditions, the scope of the
reaction was explored with structurally and electronically
different nitroarenes (Table 3). Substrates with electron-donor
(–NH and –CH CN) and electron-acceptor groups (–CHO, Br
2
2
Nitroaromatics are very important in industry, since these
compounds are good starting products due to easy introduction
of the nitro group in the aromatic ring. The reduction of simple
nitro compounds is readily carried out with various commer-
and I) were tested. In fact, the selective reduction of the nitro
groups is practically independent of the substitute. The amines
are obtained with high selectivity and at conversion levels about
30
cially available products (supported copper, cobalt, palladium
and nickel). In MOF materials, to our knowledge, only one based
31
on In has been tested in this reaction. With the new RPF-16 we
want to explore, the selective reduction of a nitro group with H2,
when other reducible groups are present in the same molecule.
Functionalized anilines are important intermediates for agro-
chemicals, pharmaceuticals, dyestuffs, urethanes and other
32–34
industrially important products and fine chemicals,
and there
is a strong incentive to develop chemoselective catalysts for the
reduction of nitro groups.
In this sense, our aim was to test the capacity of the rare-earth
MOFs to promote the hydrogenation of nitroaromatics as
a heterogeneous catalyst. The first step was to find the conditions
under which the compounds were active; different conditions
were tested (temperature, hydrogen pressure and catalyst
amount). It was observed that the compounds were only active at
5
3
63 K at 5 ꢂ 10 Pa of H pressure in toluene as solvent. The
2
results are presented in Table 2. Compound 1 showed the lowest
ꢀ1
activity with a TOF value of 29.7 h . Compounds 2, 3 and 4
Fig. 5 Kinetic profile for compound (1–4)-catalyzed hydrogenation of
were more active as catalysts. Reactions were carried out with
nitrobenzene.
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Table 3 Hydrogenation of para-nitroarenes for compound 2
a
Yield (%) (h)
b
TOF /h
ꢀ1
Substrate
1
1
00 (1.5)
00 (1.5)
175.2
147.3
1
1
00 (5)
232.0
200.7
29.4
00 (1.5)
Fig. 6 Kinetic profile in four consecutive reaction cycles employing
compound 2 as a catalyst, the test of solubility of the catalyst and the
X-ray powder pattern before and after catalysis.
6
0.5 (5)
c
00 (22)
1
10.8
the corresponding secondary amine (N-heptylaniline). Fig. 7
shows the kinetic profile for this reaction.
a
b
Yield determined by GC-MS. TOF: mmol of substrate/mmol cat. H.
The reaction rate with the current catalyst is lower than with
c
31
ꢁ
30% of nitrobenzene and 70% of aniline were formed.
In-MOF (yield 100%, 0.1% catalyst, 4 hours, 40 C, ethanol).
These results can be explained on the basis of the relative acidity of
the metallic ions. In being the most acidic, the relative order with
35
the lanthanide in RPF-16 is La 0 Pr < Nd < Sm 00 In.
All in all, RPF-16 hydrogenates selectively the NO group in
1
00%. Dehalogenation was only observed for 4-iodo-
2
40
nitrobenzene.
the presence of other reducible groups in the same molecule.
Luminescence measurements
Recycling of catalysts
We give now a general overview of the room-temperature lumi-
nescence behaviour of the new RPF-16 Ln materials (Fig. 8). An
intense Raman band associated with C–H vibrations (Raman
The heterogeneity of the catalyst is also evaluated to study
whether, using compound 2 as catalyst, the reaction occurred on
the solid or was catalyzed by species in the liquid phase. To
address this issue, we conducted two separate experiments. In the
first experiment, the reaction was concluded after 1 h, and the
conversion of nitrobenzene was found to be ꢃ60%. At this stage,
the catalyst was separated from the reaction mixture and the
reaction was continued with the filtrate for an additional 3 h and
the conversion remained almost unchanged. In a second exper-
iment, the solid recovered was used again in another reaction by
adding new reagents and as shown in Fig. 6 catalytic activity
remains unaltered.
ꢀ1
shift around 3000 cm ) overlaps with the emission spectra.
There is no emission band that could be associated with the
ligand. The Pr-compound shows a narrow band at 608 nm cor-
1
3
responding to the D / H , and a well-split band around
2
4
3
3
6
40 nm given by the P / H transition. The bands observed in
0 6
the high-energy side of the 608 nm band are Raman vibrations
also observed in the other compounds. The spectrum of the
Nd-compound shows several bands in the range 850–920 nm
4
4
associated with the F3/2 / I9/2 transition. The Sm-compound
presents the strongest emission (an order of magnitude higher
Ln catalysts could be recovered for recycling, and reused
retaining most of their catalytic activity, and the activity of the
recovered catalyst does not decrease after four cycles at least.
Analyses of the reused catalysts, separated from the reaction
media, showed that the overall catalyst structure was largely
maintained (Fig. 6 and Table 3).
We evaluate also the possible bifunctional character of RPF-
1
6 for the one-pot reductive amination of carbonyl compounds
(
Scheme 1) because we expected that the Lewis acid character of
these compounds may favour the acid-catalyzed step of the
reaction. To check this hypothesis, compound 2 was tested as
a catalyst for the reaction between nitrobenzene and heptanal.
Nitrobenzene is first hydrogenated into the corresponding
amines, which can be condensed with heptanal in a second step to
yield N-heptylideneaniline; this imine is subsequently reduced to
Scheme 1 One-pot synthesis of substituted aromatic amines.
This journal is ª The Royal Society of Chemistry 2012
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196 | J. Mater. Chem., 2012, 22, 1191–1198

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simplification of RPF-16 isostructural frameworks gives rise to
a 3D uninodal six connected net of type pcu alpha-Po primitive
cubic. Nitro groups of different compounds with carbonyl, nitrile
or halide groups have been successfully hydrogenated with
RPF-16 as catalysts. The used Ln-MOFs RPF-16 show high
chemoselectivity towards reduction of the nitro group at near-
complete conversion of the substrate; these catalysts can be used
in successive cycles. The bifunctional character for one-pot
reductive amination of carbonyl compounds was tested and
confirmed. Room-temperature luminescence measurements
show that the Sm-compound presents the strongest emission
(
an order of magnitude higher than that of Pr) of the series.
Abbreviations
MOF
RPF-16
Succ
Metal Organic Framework
Rare-earth Polymer Framework
Succinate
Fig. 7 Kinetic profile for the one-pot reductive amination of heptanal.
Acknowledgements
The authors thank A. E. Platero-Prats for her comments on the
topological studies. R.D. acknowledges a FPI scholarship from
Spanish Ministry of Science and Innovation (MICIN), and
Fondo Social Europeo from the EU. This work has been sup-
ported by the Spanish MCYT Project Mat 2007-60822 FAMA
and Consolider-Ingenio CSD2006-2001. S.A.G. thanks the CSIC
for a postdoctoral contract (Program JAE-DOC).
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