Bifunctional iridium-(2-aminoterephthalate)-Zr-MOF chemoselective catalyst for the synthesis of secondary amines by one-pot three-step cascade reaction

Article abstract of DOI:10.1016/j.jcat.2012.12.004

A simple and efficient method for the synthesis of N-alkyl amines via a cascade reaction (hydrogenation + reductive amination) using a new recyclable hybrid catalyst that combines the catalytic power of transition metal complexes with the architecture of metal organic frameworks (MOFs).

Full text of DOI:10.1016/j.jcat.2012.12.004

Journal of Catalysis 299 (2013) 137–145
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Bifunctional iridium-(2-aminoterephthalate)–Zr-MOF chemoselective catalyst
for the synthesis of secondary amines by one-pot three-step cascade reaction q
a,
Mercedes Pintado-Sierra a,c, Antonia M. Rasero-Almansa a, Avelino Corma b, , Marta Iglesias
,
⇑
⇑
Félix Sánchez c
a Instituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain
b Instituto de Tecnología Química, CSIC–UPV, Avenida de los Naranjos s/n, 46022 Valencia, Spain
c Instituto de Química Orgánica General, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and efficient method for the synthesis of N-alkyl amines via a cascade reaction (hydrogenation +
reductive amination) using a new recyclable hybrid catalyst that combines the catalytic power of transi-
tion metal complexes with the architecture of metal organic frameworks (MOFs).
Ó 2012 Elsevier Inc. All rights reserved.
Received 9 August 2012
Revised 5 December 2012
Accepted 6 December 2012
Keywords:
MOF
Post-functionalization
One-pot
Domino
Reductive amination
Heterogenized catalyst
1. Introduction
and storage [5,6], sensors, and catalysis [7]. The most remarkable
features of these networks as catalysts are the easy recovery and
Iridium-transition metal complexes are successful homoge-
neous catalysts, and their heterogenization over a solid matrix
leads to the formation of solid recyclable molecular catalysts with
well-defined active centers. In this sense, much effort has been
done to design and develop adequate supports and heterogeniza-
tion approaches that preserve the properties of the homogeneous
catalyst and also improve their behavior by a cooperative effect be-
tween the metal complex and the support [1]. Another approach to
transform a soluble catalyst (transition metal complex) into solid
heterogeneous catalyst involves the synthesis of hybrid organic–
inorganic structured porous materials [2] and coordination
polymers [3]. Metal organic frameworks (MOFs) are one type of
coordination polymers in which metal nodes are connected by or-
ganic linkers through strong bonds, forming crystalline hybrid
microporous materials with a tridimensional network. In the last
years, porous metal organic frameworks (MOFs) have aroused
great interest for their potential application in gas separation [4]
reusability and structure-dependent catalysis [8]. However,
recently networks have been shown present unique active site
structures and adequate porosity for shape and size selectivity
and for regio- and enantioselective reactions. Although several ser-
ies of MOFs have shown unusual thermal and chemical stability,
their use in catalysis and gas separations is still limited by the
lack of functional and selective sites in the most stable MOF
frameworks [9]. Among the possible routes to create functional
networks, we can highlight the direct synthesis and post-synthetic
functionalization (PSM) [10], grafting of active groups on the open
metal sites [11], and encapsulation of active species [12].
In the present work, we have prepared by PSM, multisite MOF
catalysts starting from amine-functionalized Zn-(IRMOF-3) [13]
and stable Zr-based MOFs (UiO66-NH2) [14]. These new materials
can act as heterogeneous catalysts that combine the properties of
soluble organometallic complexes with those of the MOF as support.
The high stability of the Zr-MOF (UiO66-NH2) compared with the IR-
MOF-3 is an incentive to use the former for heterogeneous catalysis.
One-pot type reactions, in which several steps are performed in
the same reaction vessel, are of much interest to achieve more sus-
tainable process [15] by avoiding intermediate separation and
purification steps and minimizing energy consumption [16]. These
transformations known as tandem, domino, or cascade reactions
[17] have become an important area of research in organic
q
This work describes the one-pot direct reductive amination of carbonyl
compounds with nitroarenes using new stable iridium-(2-aminoterephthalate)–
Zr-MOF catalyst prepared by post-functionalization, employing molecular hydro-
gen as the reductant.
⇑
Corresponding authors. Fax: +34 96 38794 44 (A. Corma).
E-mail address: acorma@itq.upv.es (A. Corma).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.12.004

138
M. Pintado-Sierra et al. / Journal of Catalysis 299 (2013) 137–145
chemistry [18,19]. Compounds containing a nitro group are of
interest as building blocks for agrochemicals, dyes, pharmaceuti-
cals, and ligands [20]. In particular, nitroarenes are one of the most
readily available starting materials in organic synthesis since they
can be produced from a range of aromatic starting materials [21].
One example is the hydrogenation of nitroarenes into anilines,
and further transformation by alkylation, reductive alkylation,
hydroamination, etc., to obtain their derivatives including hetero-
cyclic compounds [22]. In the present work, we will show the prep-
aration of stable Zr-MOF-Ir for the synthesis of mono N-alkyl
amines through reductive amination of aldehydes with nitroa-
renes, in a three-step cascade reaction. The recycling of the
catalytic materials has been studied and shows that Zr-MOF-Ir-
catalysts are stable after multiple reuses.
[31], acting as an acid–base type catalyst for cross-aldol condensa-
tion reactions [14c]. Here, we present the importance of selecting
properly the starting MOF material to be used as support for the
preparation of heterogenized catalysts.
IRMOF-3 and UiO-66-NH2 were reacted with 6-((diisopropyl-
amino)methyl)picolinaldehyde as has been previously reported
[13b]. The starting MOF was suspended in CH2Cl2 and treated with
the aldehyde (Scheme 1) at room temperature for 30 min for IR-
MOF-3 (to prevent the loss of crystallinity) and for 24–48 h for
the highly stable UiO66-NH2. The reaction was then stopped by re-
peated washings of the solid material with CH2Cl2 and drying un-
der vacuum. The contact time of this reaction is important for
IRMOF-3, since it was observed that for contact times longer than
30 min the material obtained shows low crystallinity. According to
the elemental analysis of the products obtained, a level of function-
alization of 2% and 5% of the total ANH2 groups for IRMOF-3 and
UiO-66-NH2, respectively, was achieved without losing the frame-
work integrity. Meanwhile, iridium complexation by the imino-
pincer complex was almost quantitative. Increasing the degree of
functionalization with imine for IRMOF-3 required longer contact
times. Unfortunately, this caused a severe decrease in the crystal-
linity of the final material, as commented above. Since the consec-
utive iridium complexation is quantitative, a 3% functionalization
of the imine allows introducing up to 1.4 wt% and 3 wt% of Ir in IR-
MOF-3 and UiO-66-NH2, respectively. The values obtained here are
typical values for metal loading in useful heterogenized catalysts. It
appears then that the sterically demanding 6-((diisopropyl-
amino)methyl)picolinaldehyde reacts preferentially at the surface
and at the channel entrance, showing the characteristics of molec-
ular sieves for MOFs.
Since the reaction is not complete, the FTIR spectra of the result-
ing samples (see Figs. S4 and S9 in Supplementary) are still domi-
nated by the bands assigned to the parent groups and by the
skeletal modes of the MOFs, mainly due to the presence of the organ-
icaromaticligands(i.e., bandsat 1600–1585 cmꢁ1, 1500–1430 cmꢁ1
and 700 cmꢁ1). The carboxylate anion has two strongly coupledCAO
bonds with bond strengths between C@O and CAO. Bands between
1650 and 1550 cmꢁ1 (s) correspond to asymmetrical stretching for
the carboxylates, and the band near 1400 cmꢁ1 (w) corresponds to
symmetrical stretching. The presence of functional groups from
post-synthetic modification of the starting IRMOF-3 or UiO-66-
NH2 can be observed in the full spectral range, being the intensity
proportional to the level of conversion. The fraction of PSM was
determined by normalizing FTIR spectra to the band at 760 cmꢁ1, as-
signed to the CAC vibrational mode in aromatic compounds (this
band is relatively little affected by ring substituents) [14b], and
whichdoes not overlap with other vibrational modes. These samples
will be hereafter referred to as IRMOF-3-L and UiO66-NH2-L. The fi-
nal step to prepare the Ir(I)-containing materials, IRMOF-3-LIr,
UiO66-NH2-LIr, consisted in reacting a suitable iridium precursor,
[IrCl(cod)]2, or [Ir(cod)(THF)2]BF4 with the imine-modified material.
The preparative procedure is summarized in Scheme 1, while a de-
tailed description of each step is given in the experimental section.
The reaction with the aldehyde modifies UiO-66-NH2 without deg-
radation of the framework, even after extended reaction times and
conversions, as indicated by the XRD results (see Fig. 1).
2. Results and discussion
The starting materials for this study were IRMOF-3 [13] (used as
reference catalyst) and UiO66-NH2 [14], since the presence of NH2
groups in the linker (2-aminoterephthalic acid) of the MOFs allows
to functionalize the material by conventional reactions, and PSM-
MOFs can be prepared according to the well-established proce-
dures. IRMOF-3 consists of a Zn4O tetrahedron connected with
six carboxylates to give a three-dimensional cubic porous material
(Zn4O(BDC-NH2)3); it has been prepared, in the last years, by differ-
ent groups [23,13a] with different textural properties. Rowsell
et al., have synthesized IRMOF-3 solvothermally with BDC-NH2
(BDC-NH2 = 2-aminoterephthalic acid) and Zn(NO3)2ꢀ4H2O in
N,N-diethylformamide with a BET surface area of 2446 m2 gꢁ1
.
Gascon et al. obtain a high BET surface area of 3130 m2 gꢁ1 in the
N2 protection [23b]. When IRMOF-3 was synthesized by a ‘‘direct
mixing’’ synthesis strategy at room temperature, samples with a
BET surface area of 750 m2 gꢁ1 were obtained [13b,24]. This last
strategy, using dimethylformamide (DMF) as solvent, was em-
ployed in this work, also obtaining IRMOF-3 with 750 m2 gꢁ1 BET
surface area and pores of ꢂ10 Å [13b]. Although the Zn(II)-based
MOFs have many favorable aspects, their poor stability to moisture
and protic solvents is likely to limit their applications. This
prompted us to use here more stable, chemically resistant frame-
works such as Zr(IV)-based MOFs [25]. UiO-66 (UiO for University
of Oslo) consists of
a Zr(IV)-carboxylate cluster Zr6(l3-O)4
(l3-OH)4(CO2)12, which works as the secondary building unit
(SBU) for the framework. The UiO-66 [Zr6O4(OH)4(BDC)6]
(BDC = 1,4-benzenedicarboxylate) shows high thermal stability
due to the presence of an inner Zr6O4(OH)4 core in which the
triangular faces of the Zr6-octahedron are alternatively capped
with
l3-O and l3-OH groups. Cohen and Lillerud independently
discovered that the UiO-66 motif was very tolerant to functional-
ized ligands, allowing the synthesis of UiO-66 MOFs and the
amino-functionalized UiO-66-NH2 derivatives with the same
topology [26]. IRMOF-3 materials were obtained starting from
Zn(NO3)2ꢀ6H2O and BDC-NH2 in DMF at 105 °C (BET surface
area = 750 m2 gꢁ1) [13b]. UiO-66-NH2 was prepared by mixing
ZrCl4 and BDC-NH2 in DMF, crystallization at 120 °C for 12–24 h,
and activation by DMF exchange with ethanol followed by drying
under vacuum (BET surface area = surface area of 800 m2 gꢁ1).
MOFs can easily be modified by post-synthesis treatments with
the aim of introducing functional groups that can act or be trans-
formed into catalytic active sites [27]. Chelating groups, which
can coordinate with a transition metal, can easily be introduced
in MOFs [28]. For instance, IRMOF-3 reacts with salicylaldehyde
to form the iminophenol ligand which reacts with Au(III) and
V(V) complexes to give active catalysts in the hydrogenation of
butadiene and in the oxidation of cyclohexene [13b,29]. UiO66-
NH2 has also been modified by reaction with salicylaldehyde (va-
por-phase post-synthetic modification) [30] and with anhydrides
The functionalization process was also studied, by ESI-MS anal-
ysis (see Supporting material) on digested samples of UiO-66-NH2
and PSM-derivatives. Due to the stability of UiO-66 MOFs in acidic
media, hydrofluoric acid (HF) was chosen to digest the materials,
taking into account the high affinity of Zr for fluoride. The starting
UiO-66-NH2 was also digested and used as reference. The negative
mode mass spectra obtained showed a base peak at m/z 180 that
corresponds to H2BDC-NH2 (Fig. S15a, [H2BDC-NH2 + H]ꢁ). The
negative mode mass spectra obtained for H2BDC-NH2L clearly
show a base peak at m/z 383 (Fig. S15b, [H2BDC-L + H]ꢁ), which

M. Pintado-Sierra et al. / Journal of Catalysis 299 (2013) 137–145
139
M
O
M
O
M
O
M
O
M
O
M
O
N
H
N
N
N
O
N
NH2
N
[IrCl(cod)]2
N
Ir
N
CH2Cl2
Cl
O
M
O
M
O
M
O
M
O
M
O
M
IRMOF-3L
UiO-66-NH2L
IRMOF-3LIr
UiO-66-NH2LIr
O
O
= [Zn4O]6+
NH2
IRMOF-3
=
= [Zr6O4(OH)4]12+
UiO-66-NH2
O
O
Scheme 1. Post-modification of IRMOF-3 and UiO-66-NH2.
comparable to that of UiO66-NH2 with decomposition tempera-
tures near 350 °C in air (Fig. S16).
The N2 adsorption–desorption isotherms obtained at 77 K on
the functionalized UiO66-NH2-MOF are type-I isotherms (Fig. S17).
And it indicates that most of the porosity of the original material is
retained after functionalization of the linker. Indeed, BET surface
area of 637 m2/g, total pore volume of 0.28 cm3 gꢁ1, and pore
diameter of 2.7 nm are obtained for UiO-66-L, while for UiO-66-
[LIr]BF4 they are 568 m2/g, 0.25 cm3 gꢁ1, and 2.71 nm, respec-
tively; decrease in surface area can be related to the presence of
a large ligand and the corresponding iridium complex.
UiO66-fresh
UiO66-L
UiO66-LIr
All UiO-66-NH2-modified samples remain crystalline after func-
tionalization (Fig. 1). All XRD peaks observed for the as-synthe-
sized UiO-66-NH2 are also observed with the post-synthetically
modified materials. XRPD pattern of UiO-66-NH2-modified materi-
als remains unaltered when heated up to 300 °C in air, demonstrat-
ing that the materials are thermally stable.
0
5
10
15
20
25
30
2 Theta (°)
Finally, scanning electron microscopy (SEM) experiments con-
firmed that the particle size remains unchanged during ligand for-
mation, as no significant particle size increase or decrease was
observed via this technique (Fig. 2)
Fig. 1. X-ray diffraction patterns of UiO-66-NH2, UiO-66-L, UiO-66-[LIr]BF4.
corresponds to the modified ligand. The spectra of H2BDC-[LIr]
show a peak at m/z 677 (Fig. S15c, {[H2BDC-[LIr(MeOH)]BF3 + H}ꢁ).
Last, HF cleaves also the Zr cluster itself, and the fluoro complex
[ZrF5]ꢁ is readily discerned (Fig. S15). 1H NMR spectra after diges-
tion of the MOFs under acidic conditions indicate that the ligands
remained in UiO-66-L and UiO-66-[LIr]BF4 materials (Fig. S14).
The stability of the modified UiO66-NH2-MOFs was examined
by means of TGA. The modified samples show thermal stabilities
2.1. Iridium MOF catalyst for reductive amination of aldehydes:
comparison with homogeneous catalysts
Development of catalysts that allow performing one-pot multi-
step reactions is of interest since it avoids intermediate separation
and purification steps with the corresponding benefits in energy
(a)
(b)
Fig. 2. SEM images for UiO-66-L (a) and UiO66-NH2-[LIr]BF4 (b).

140
M. Pintado-Sierra et al. / Journal of Catalysis 299 (2013) 137–145
saving and minimization of waste. In this sense, the reductive ami-
nation reaction is one important transformation that allows the di-
rect conversion of carbonyl compounds into amines using simple
operations [32]. The reaction offers compelling advantages over
other amine syntheses, including shortness, wide commercial
availability of substrates, mild reaction conditions, and no need
to isolate the imine intermediate. The resulting secondary amines
and their derivatives are highly versatile building blocks for vari-
ous organic substrates and are extremely important pharmaco-
phores in numerous biologically active compounds owing to
their interesting physiological activities [33]. Several reagents for
reductive amination have recently been reported [34]. Although
the reductive alkylation of amines with aldehydes is a common
process, only few papers describe the reaction under hydrogena-
tion conditions, and for example, a Pd/C [35,36] catalyst has been
reported to be active though the reported substrate scope is lim-
ited to aliphatic aldehydes. Imine formation from aldehydes and
nitroarenes has been reported with Au/TiO2 catalysts [37] or Au/
Fe2O3 [38]. More recently, it has been reported that gum acacia-
supported palladium nanoparticles [39] and a polymer-supported
palladium catalyst [40] promote reductive N-alkylation of nitroa-
renes using molecular hydrogen as reductant and methanol as sol-
vent. In the present work, we report on the application of Ir-MOF
catalyst for the atom economic synthesis of mono N-alkyl amines
through reductive amination of aldehydes with nitroarenes in the
presence of hydrogen. The process involves firstly a chemoselective
reduction of the nitro compound to the corresponding amine with
hydrogen in the presence of the carbonyl substrate on the iridium
site. Then, while in a consecutive step, the condensation between
the aromatic amine and the carbonyl group would occur on the
acid sites of the catalyst (MOF) (Scheme 2). Finally, the hydrogena-
tion of the resulting imine with the Ir-complexes will give the sec-
ondary amine.
Catalytic evaluation of both Ir-MOF-based catalysts was per-
formed by mixing them with the nitroarenes and carbonyl com-
pounds in isopropanol in a batch reactor. A series of initial
experiments using benzaldehyde and nitrobenzene as model reac-
tants has allowed us to establish the adequate reaction conditions
for obtaining secondary amines in high yields within a reasonable
reaction time. After that, the reaction has been extended to differ-
ent aldehydes and nitroarenes. Under optimized reaction condi-
tions, the scope of the reaction was explored with structurally
and electronically diverse aldehydes and different nitroarenes. As
shown in Table 1, various aldehydes were reductively aminated
with nitrobenzene under 6 bar pressure of molecular hydrogen.
The aldehydes used for this study included aromatic and aliphatic
aldehydes. Irrespective of the electronic nature of the substituent,
aromatic aldehydes react well to give the corresponding products
in good yields (Table 1, entries 1–6). On the other hand, aliphatic
aldehydes underwent the reductive amination rapidly and gave
NH2
O
NO2
O
Metal
R2
H
R2
H
R1
H2
R1
acid
R1
Metal
H2O
R1
R2
N
R2
N
H
H2
Scheme 2. One-pot synthesis of secondary amines by using [Ir]-catalysts.
Table 1
MOF-[LIr]-catalyzed one-pot reductive amination of aldehydes with nitrobenzene under H2 atmosphere in isopropanol.a
Entry
Catalyst
R
R0
Conv. (%)b
Sel. (%) amine (B)
1
2
3
4
5
6
7
8
9
LIr
Ph
Ph
Ph
Ph
H
H
H
4-I
4-Br
H
100
98
99
90
80 (95, 48 h)
99
99
98
66
99
99
IRMOF-3-LIr
UiO-66-LIr
UiO-66-LIr
UiO-66-LIr
UiO-66-LIr
UiO-66-LIr
UiO-66-LIr
UiO-66-LIr
70 (98, 48 h)
65 (97, 48 h)
99
99
100
99
Ph
OMePh
C6H13
C6H13
C6H13
H
4-I
o-OCH3
100
a
Reaction conditions: nitrobenzene (1.0 mmol), aldehyde (1.5 mmol), [Ir] (1 mol%), isopropanol (1 ml), PH2: 6 bar, T: 100 °C, time: 24 h.
Referred to nitrobenzene.
b

M. Pintado-Sierra et al. / Journal of Catalysis 299 (2013) 137–145
141
Table 2
In both cases, the conversion within the filtrates remained almost
unchanged. Ir was not detected in the filtrate in either experiment
by ICP-AES. These studies demonstrate that the Ir bound to the
support during the reaction is active and the reaction proceeds
on the solid.
As can be seen from the results in Table 2, UiO-66-LIr can be
recovered by simple filtration in air and reused without significant
loss of catalytic activity, at least after four cycles, whereas IRMOF-
3-LIr and soluble complexes could be used only once, since they
were completely deteriorated at the end of the first catalytic run
(Fig. S14).
Recycling experiments with MOF-based-[LIr]+ catalysts.
Cycle
Catalyst
t (h)
Conv. (%)
Sel. (%) amine (B)
1
2
3
4
1
2
UiO-66-LIr
24
24
24
24
24
24
99
99
99
99
98
–
99
99
99
99
99
–
IRMOF-3-LIr
Cat. 1 mol% based on Ir, T: 100 °C, PH2: 6 bar.
Hot filtration tests for UiO-66-LIr proved that the catalytic
activity was associated with the solid exclusively, and no reaction
occurred when the catalyst was removed from the reaction media.
The overall structure of the material remained intact after the cat-
alytic experiments (Fig. 3), and we can consider that the catalyst is
stable under reaction conditions. SEM images for fresh and recov-
ered materials show, also, that structure was maintained after the
reaction (Fig. 4). Fig. 5 shows the representative TEM images con-
ducted to analyze whether particles are formed during catalysis,
and we see no evidence for the formation of nanoparticles. Never-
theless, it should be considered that the content of Ir in the sample
is very low.
the products in excellent yield without the formation of byprod-
ucts (Table 1, entries 7–9).
2.2. Recycling experiments and heterogeneity
To study whether the reaction using solid [Ir]-MOF catalysts oc-
curred on the solid or was catalyzed by Ir species potentially lea-
ched in the liquid phase, two separate experiments were
conducted with benzaldehyde and nitrobenzene. In the first exper-
iment, the reaction catalyzed by UiO-66-LIr was terminated after
7 h, and the conversion of nitrobenzene was found to be 20%. At
this point, the catalyst was separated from the reaction mixture
and the reaction was continued with the filtrate for an additional
24-h. period. In the second experiment, the reaction was termi-
nated after 7 h at 20% conversion and the catalyst removed. The
reaction was then continued with the filtrate for additional 17 h.
3. Conclusions
We have developed a simple and efficient method for the syn-
thesis of N-alkyl amines via reductive amination in the presence
of hydrogen using a new readily recoverable hybrid catalyst that
combines the catalytic activity of transition metal complexes with
the architecture of metal organic frameworks (MOFs). This proto-
col can be used to generate a diverse range of N-alkyl amines in
good to excellent yields. The simple procedure for catalyst prepara-
tion, easy recovery, and reusability of the catalyst is expected to
contribute to its utilization for the development of benign chemical
processes and products.
UiO66-LIr fresh
UiO66-LIr 1 run
UiO66-LIr 4run
4. Experimental section
Starting materials were purchased and used without further
purification from commercial suppliers (Sigma–Aldrich and Alfa
Aesar). Dried, distilled, and deoxygenated solvents were used.
4.1. Materials preparation
5
10
15
20
25
30
The Zn and Zr-MOFs were prepared according to the corre-
sponding procedures reported in the original references: IRMOF-
2 Theta (°)
3 [12], UiO-66-NH2 [13a] X-ray diffraction (Phillips X’Pert, Cu K
radiation) was used to confirm the expected crystalline structure
a
Fig. 3. XRPD of UiO66-[LIr]BF4 after four runs.
(a)
(b)
Fig. 4. SEM images for UiO66-NH2-[LIr]BF4 fresh (a) and UiO66-NH2-[LIr]BF4 recycled (b).

142
M. Pintado-Sierra et al. / Journal of Catalysis 299 (2013) 137–145
(a)
(b)
Fig. 5. TEM images of UiO66-NH2-[LIr]BF4: (a) fresh and (b) recovered after reaction.
of the materials. All substances and reagents used were commer-
cially available and used as received. The reaction and analytic
methodology are given in Supplementary material.
stirred for an additional hour. The sample was collected by centri-
fugation, washed twice with CH2Cl2, and dried in air at 60 °C.
Elemental analysis was performed on samples outgassed under
vacuum (100 °C, 12 h). Zn4O(BDC-NH2)2.94(BDC-LIrCl)0.06: Calcu-
lated: C, 35.25; H, 1.94; N, 5.06%. Found: C, 33.62; H, 2.49; N,
4.99%. The amount of iridium in the final solid was determined
by ICP-AES. Calculated: Ir, 1.39%. Found: Ir, 1.38%. 13C NMR (CP
MAS): 19.0 (CH3iPr), 30.8, 51.4 (CHiPr), 118.2, 125.8, 132.8, 138.4,
150.3, 162.4 (imine C@N), 175.6 (COO) (see Figs. S3 and S5).
4.2. Synthesis of IRMOF-3, [Zn4O(BDC-NH2)3]. (BDC-NH2 = 2-aminote-
rephthalate)
Triethylamine (1.6 g, 15.81 mmol) was dropwise added to a
mixture of Zn(NO3)2ꢀ6H2O (1.21 g, 4.07 mmol) and BDC-NH2
(0.371 g, 2.05 mmol) in DMF (40 ml) and stirred for 2 h at room
temperature. The resulting pale yellow solid was collected by cen-
trifugation and washed twice with DMF. The solid was thrice im-
mersed in CHCl3 during 3 days, and the solid was finally dried in
air at 60 °C. This washing procedure removes most of the DMF sol-
vent molecules, although the presence of an IR absorption band at
1660 cmꢁ1 in the spectrum of the washed sample indicates that
some adsorbed DMF still remains inside the pores. The structure
of the material was confirmed by X-ray diffraction, prior and after
washing with CHCl3.
4.5. Synthesis of Zr(BDC-NH2) (UiO-66-NH2)
Synthesis of Zr-based metal organic framework was performed
in a 250-ml round-bottomed flask using a procedure similar to that
previously described [13a]. ZrCl4 (0.4 g, 1.7 mmol) in DMF (75 ml)
was dispersed by ultrasound at 50–60 °C, and acetic acid (2.85 ml,
850 mmol) was added. A DMF solution (25 ml) of the linker, 2-
aminoterephthalic acid (0.311 g, 1.7 mmol), was added to the clear
solution; finally, water (0.125 ml, 0.007 mmol) was added to the
solution. The tightly capped flask is sonicated at 60 °C and kept
in a bath at 120 °C under static conditions for 24 h. After 24 h,
the solutions were cooled to room temperature and the precipitate
was isolated by centrifugation and washed with DMF (10 ml).
The suspension was centrifuged and the solvent was decanted
off. The obtained particles were washed with ethanol several times
in the same way as described for washing with DMF. Finally, the
solid was dried under reduced pressure (80 °C, 3 h). This standard
washing procedure yielded the materials denoted with ‘‘as-synthe-
sized’’. (Yield: 577 mg). Zr6O4(OH)4(BDC-NH2)6: Calculated: C,
32.87; H, 1.95; N, 4.79. Found: C, 32.09; H, 2.34; N, 4.83%.
Elemental analysis was performed on samples outgassed under
vacuum (100 °C, 12 h). Zn4O(BDC-NH2)3: Calculated: C, 35.35; H,
1.84; N, 5.16%. Found: C, 34.61; H, 1.87; N, 5.14%. 13C NMR (CP
MAS): 116, 126, 132, 137, 149, 175 (COO) ppm (see Fig. S1).
4.3. Post-synthetic modification of IRMOF-3 IRMOF-3-L, [Zn4O(BDC-
NH2)3ꢁx(BDC-L)x] (L = 6-((diisopropylamino)methyl)picolinaldehyde)
In
a typical procedure, freshly prepared IRMOF-3 (1 g,
1.2 mmol) was dispersed in 15 ml CH2Cl2. To this slurry, a solution
of aldehyde (500 mg, 4.1 mmol) in CH2Cl2 (15 ml) was dropwise
added at room temperature, and the mixture was stirred for addi-
tional 30 min. The sample was collected by centrifugation, washed
twice with CH2Cl2, and dried in air at 60 °C. Elemental analysis was
performed on samples outgassed under vacuum (100 °C, 12 h).
Zn4O (BDC-NH2)2.94(L)0.06 (corresponding to 2% amine functionali-
zation): Calculated: C, 35.84; H, 1.97; N, 5.15%. Found: C, 32.40; H,
2.27; N, 5.04%. 13C NMR (CP MAS): 18.2 (CH3iPr), 30.9, 51.6 (CHiPr),
117.3, 123.8, 132.9, 137.7, 150.0, 164.0 (imine C@N), 174.6 (COO)
ppm (see Figs. S2 and S4).
4.6. Synthesis of UiO-66-NH2-L, [Zr6O4(BDC-NH2)6ꢁxLx] (L = 6-((diiso-
propylamino)methyl) picolinaldehyde)
UiO-66-NH2 (1.5 g, 1.6 mmol) was dispersed in 15 ml CH2Cl2. To
this slurry, a solution of aldehyde (375.0 mg, 1.7 mmol) in CH2Cl2
(5 ml) was dropwise added at room temperature, and the mixture
was stirred for additional 6 h. The sample was collected by centri-
fugation, washed twice with CH2Cl2, and dried in air at 70 °C. Ele-
mental analysis was performed on samples outgassed under
vacuum (100 °C, 12 h). Zr6O4(OH)4(BDC-NH2)5.7(BDC-L)0.3: (corre-
sponding to ꢂ5% amine functionalization): C, 34.35; H, 2.19; N,
5.09; Calculated: C, 34.35; H, 2.19; N, 5.09%. Found: C, 34.45; H,
2.64; N, 5.07%. 13C NMR (CP MAS): 17.8 (CH3iPr), 45.8 (CHiPr),
4.4. Synthesis of IRMOF-3-LIr, [Zn4O(BDC-NH2)3ꢁx([LIrCl]x)
Sample IRMOF-3-L (1 g) was dispersed in 15 ml CH2Cl2. To this
slurry, a solution of [IrCl(cod)]2ꢀ(200 mg, 0.3 mmol) in 15 ml CH2Cl2
was dropwise added at room temperature, and the mixture was

M. Pintado-Sierra et al. / Journal of Catalysis 299 (2013) 137–145
143
50.6 (CH2), 116.1, 123.0, 132.1, 138.5, 151.4, 162.8 (imine C@N),
mC@N; 1454 (m), 1382 (m), 1363 (s) dCH, 1205 (m)
m
CAC. 1H NMR
171.2 (COO) (see Fig. S7).
(300 MHz, CDCl3, 25 °C, ppm): d = 8.32 (s, 1H, N@CH); 7.84 (t,
1HPy, JHH = 4.4 Hz); 7.64 (d, 2HPy, JHH = 4.7 Hz); 3.80 (m, 2H, ACH2A);
3.03 (sept, 2H, 2 ACHiPr, JHH = 6.6 Hz); 1.28 (s, 9H, 3 ACH3tBu); 1.00
(d, 12H, 4 ACH3iPr, JHH = 6.6 Hz). 13C NMR (125 MHz, CDCl3, 25 °C,
ppm): d = 163.7 (CPy); 156.8 (N@CH); 154.3 (CPy); 136.7 (CHPy);
122.6 (CHPy); 118.2 (CHPy); 57.7 (CtBu); 51.4 (ACH2A); 48.9 (2C, 2
ACHiPr); 29.6 (3 ACH3tBu); 20.7 (4 ACH3iPr).
4.7. Synthesis of UiO-66-NH2-LIr, [Zr6O4(BDC-NH2)6ꢁx(BDC-LIrCl)x]
UiO-66-NH2-L (1 g) was dispersed in 15 ml CH2Cl2. To this slur-
ry, a solution of [IrCl(cod)]2ꢀ(200 mg, 0.3 mmol) in 15 ml CH2Cl2
was added at room temperature, and the mixture was stirred at re-
flux for 12 h. The sample was collected by centrifugation, washed
twice with CH2Cl2, and dried in air at 70–80 °C. Elemental analysis
was performed on samples outgassed under vacuum (100 °C, 4 h).
Zr6O4(OH)4(BDC-NH2)5.7([BDC-LIrCl])0.3: Calculated: C, 32.48; H,
2.15; N, 4.82%. Found: C, 34.12; H, 2.39; N, 4.82%. The amount of
iridium in the final solid was determined by ICP-AES. Calculated:
Ir, 3.15%; Found: Ir, 3.11%. 13C NMR (CP MAS): 17.9, 21.2 (CH3iPr),
45.7 (CHiPr), 116.9, 123.2, 131.7, 138.5, 151.4, 165.6 (imine C@N),
171.4 (COO) (see Figs. S8 and S9).
4.10.3. Synthesis of [IrLCl] (LIr)
In a 100-ml round-bottomed Schlenk flask, 50 mg (0.18 mmol)
of the ligand (L) was dissolved in 5 ml of CH2Cl2 under N2. This
solution was then treated with
a red solution of 60.6 mg
(0.091 mmol) [Ir(C8H12)Cl]2 in 20 ml CH2Cl2. The solution was stir-
red for an additional hour. The solvent was removed in vacuum un-
til 5 ml, pentane was added to the mixture and an orange solid was
obtained. After drying the residue for several hours in vacuum, the
analytically pure product was obtained in 70% yield (63.9 mg;
0.13 mmol). Anal. Calc. for C25H41ClIrN3 (611.3): C, 49.1; H, 6.8;
N, 6.9; Ir, 31.4; Found: C, 48.5; H, 6.4; N, 6.4 Ir, 30.5%. MS (ES+,
m/z): 612.3 (M+, 10), 576.3 ([Ir(cod)(L)]+, 40), 276 (L+, 100). IR
(KBr, cmꢁ1): mCHarom 3092 (w); mCHalip 2974 (s), 2937 (m) 2882
4.8. Synthesis of UiO-66-NH2-[LIr]+, [Zr6O4(OH)4(BDC-NH2)6ꢁx([LIr]BF4)x]
AgBF4 (8.7 mg, 0.044 mmol) was added to
a solution of
[IrCl(cod)]2ꢀ(15 mg, 0.022 mmol) in 10 ml THF at room tempera-
ture, and the mixture was stirred for 1 h; then, AgCl was filtered
and the solution containing [Ir(COD)]BF4 added to a suspension
of UiO-66-NH2-L (100 mg) in 10 ml THF. The mixture was stirred
for 10 h; the solid was collected by centrifugation, washed twice
with THF, and dried in air at 70 °C. Elemental analysis was
performed on samples outgassed under vacuum (100 °C, 4 h).
Zr6O4(OH)4(BDC-NH2)5.7L0.1([LIr]BF4)0.2. Calculated: C, 32.70;
H, 2.17; N, 4.85%. Found: C, 32.92; H, 2.56; N, 4.54%. The amount of
iridium in the final solid was determined by ICP-AES. Calculated: Ir,
2.11%; Found: Ir, 1.85%.
(m);
(s), 1379 (s), 1367 (s);
806 (m), 776 (m);
d = 8.89 (s, 1H, N@CH); 8.13 (d, 1HPy, J = 7.9 Hz,); 7.79 (t, 1HPy
J = 7.7 Hz); 7.48 (d, 1HPy J = 7.5 Hz,); 5.28 (d, 1H, ACH2A,
m
CAC,
m
CAN 1700 (m), 1635 (s) 1594 (s), 1465 (s); dCH 1397
CAC, 1209 (s), 1153 (s), 1095 (m); dCHout plane
556 (m), 503 (m). 1H NMR (300 MHz, CDCl3):
m
q
CH
,
,
JHH = 18.4 Hz); 4.41 (d, 1H, ACH2A, JHH = 18.4); 3.40 (br); 3.15 (q,
2H, ACHiPr, JHH = 6.6 Hz); 2.32 (br); 1.75 (s, 9H, 3 ACH3tBu); 1.12
(d, 6H, ACH3iPr, JHH = 2.2 Hz); 1.09 (d, 6H, ACH3iPr, JHH = 2.1 Hz);
13C NMR (125 MHz, CDCl3, 25 °C, ppm): d = 166.7 (N@CH); 160.2
(CPy); 154.4 (CPy); 136.3 (CHPy); 125.3 (CHPy); 123.8 (CHPy); 61.7
(CtBu); 53.6 (ACH2A); 48.9 (2C, 2 ACHiPr); 31.4 (3 ACH3tBu);
21.05, 20.08 (4 ACH3iPr).
4.9. Catalyst characterization
4.10.4. Synthesis of [IrL]X (X = BF4)
General considerations and characterization methods are pre-
sented in supporting information.
A solution of 70 mg (0.11 mmol) of the complex [Ir(C8H12)Cl]2 in
15 ml THF was placed in Schlenk tube, AgBF4 (22.3 mg, 0.11 mmol)
was added and immediately AgCl appears in the reaction media.
The mixture was stirred for 1 h, AgCl was removed, and ligand L
(30.3 mg, 0.11 mmol) was added to the filtrate upon which the col-
or of the solution changed to orange. After 3 h at 40 °C, the solvent
was removed in vacuum and the residue was washed in portions
with 15 ml of pentane. After 5-h drying in vacuum, the orange
product was obtained in 90% yield (68 mg, 0.10 mmol). MS (ES+,
m/z): 659.3 [M+, [Ir(cod)(L)]BF4, 20), 592.3 ([Ir(cod)(L)(H2O)], 10),
276 (L+, 80).
4.10. Preparation of homogeneous [IrL] complex
4.10.1. Synthesis of the ligand
4.11. Catalytic measurements
Selective domino hydrogenation of nitroaromatics in the pres-
ence of aldehydes was performed in a closed glass microreactor
(2.0 ml, SUPELCO) equipped with a magnetic bar, and sensors for
both temperature and pressure control. The reactor had also con-
nections to allow gas supply, and also an outlet for samples to be
a: K2CO3/NR1R2; b: (COCl)2, DMSO, ꢁ50 °C; c: t-BuNH2/AcN.
4.10.2. (E)-2-(tert-Butyliminomethyl)-6-diisopropylaminomethylpyri-
dine, (L)
taken at different time intervals. The reactants nitroarene (2.2
ll
To a stirred solution of 2-formyl-6-diisopropylaminomethyl-
pyridine (440 mg, 2 mmol) in ethanol (25 ml) was added tert-
butylamine (230 ll, 2.2 mmol) and freshly activated molecular
sieves 4 Å (2 g), after stirring for 14–16 h at room temperature,
the reaction mixture was filtered through celite, concentrated un-
der reduced pressure to yield pure (E)-2-(tert-butyliminomethyl)-
6-diisopropylaminomethylpyridine (545 mg, 99%). Anal. Calc. for
for nitrobenzene, 0.02 mmol)) and aldehyde (3.5 l for benzalde-
l
hyde, 0.03 mmol) were added to the suspension of catalyst
(2.4 mg, 2 ꢃ 10ꢁ4 mmol iridium) in isopropanol (1 ml). The reactor
was hermetically sealed, pressurized with hydrogen at six bars,
and heated at 100 °C under continuous stirring. Small liquid ali-
quots (ꢄ100
ll) were taken. The progress of the reaction was mon-
itored by GC–MS. The reaction mixture was filtered, and the
solvent was removed under reduced pressure to give the crude
product. It was purified by column chromatography on silica gel
using hexane-ethyl acetate mixture as eluent.
C
17H29N3 (275.4): C, 74.1; H, 10.6; N, 15.3. Found: C, 74.3; H,
10.8; N, 15.6%. MS (m/z): 275 [M+], 218 [M+-t-Bu], 176 [M+-Et2N],
162, 120. IR (KBr, cmꢁ1):
CHalip 2967 (vs), 1645 (m), 1589 (m) C@C,
m
m

144
M. Pintado-Sierra et al. / Journal of Catalysis 299 (2013) 137–145
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(see Fig. S14), whereas UiO66-LIr was recovered unaltered after
at least four run (Fig. S15).
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We thank CONSOLIDER-INGENIO 2010-(CSD-0050-MULTICAT)
and the MINECO of Spain (Project MAT2011-29020-C02-02) for
financial support.
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Supplementary data associated with this article can be found, in
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