A Mesoporous Indium Metal-Organic Framework: Remarkable Advances in Catalytic Activity for Strecker Reaction of Ketones

Article abstract of DOI:10.1021/jacs.6b05706

With the aim of developing new highly porous, heterogeneous Lewis acid catalysts for multicomponent reactions, a new mesoporous metal-organic framework, InPF-110 ([In₃O(btb)₂(HCOO)(L)], (H₃btb = 1,3,5-tris(4-carboxyphenyl)

Full text of DOI:10.1021/jacs.6b05706

Communication
pubs.acs.org/JACS
A Mesoporous Indium Metal−Organic Framework: Remarkable
Advances in Catalytic Activity for Strecker Reaction of Ketones
Daniel Reinares-Fisac, Lina María Aguirre-Díaz, Marta Iglesias, Natalia Snejko, Enrique Gutierrez-Puebla,
́
́
M. Angeles Monge,* and Felipe Gandara*
́
Departamento de Nuevas Arquitecturas en Química de Materiales − Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC),
Sor Juana Ines de la Cruz 3, Cantoblanco 28049, Madrid, Spain
́
S
* Supporting Information
highly active and selective catalysts to avoid the formation of
byproducts as a result of side reactions. Furthermore, the
presence of sufficiently large pore and aperture dimensions to
allow the diffusion of the involved molecules is a main goal in the
design of a new catalyst.
ABSTRACT: With the aim of developing new highly
porous, heterogeneous Lewis acid catalysts for multi-
component reactions, a new mesoporous metal−organic
framework, InPF-110 ([In3O(btb)2(HCOO)(L)], (H3btb
= 1,3,5-tris(4-carboxyphenyl)benzene acid, L = methanol,
water, or ethanol), has been prepared with indium as the
metal center. It exhibits a Langmuir surface area of 1470
m2 g−1, and its structure consists of hexagonal pores with a
2.8 nm aperture, which allows the diffusion of multiple
substrates. This material presents a large density of active
metal sites resulting in outstanding catalytic activity in the
formation of substituted α-aminonitriles through the one-
pot Strecker reaction of ketones. In this respect, InPF-110
stands out compared to other catalysts for this reaction
due to the small catalyst loadings required, and without the
need for heat or solvents. Furthermore, X-ray single crystal
diffraction studies clearly show the framework−substrate
interaction through coordination to the accessible indium
sites.
The Strecker6 reaction is used to prepare α-aminonitriles with
the combination of a carbonyl, an amine, and a cyano derivative
(Scheme 1).
Scheme 1. 3C Strecker Type Reaction
This approach offers high atom economy, and the possibility of
incorporating additional functional groups through the choice of
the corresponding carbonyl precursor.7 However, poor con-
versions are typically obtained when ketones are employed,
limiting their use in this reaction. Thus, new active catalysts are
required to allow the use of substituted ketones.
Herein we report the synthesis, crystal structure, gas sorption
characterization, and catalytic activity in the one-pot three-
component Strecker reaction of a new mesoporous MOF,
[In3O(btb)2(HCOO)(L)], (H3btb = 1,3,5-tris(4-carboxy-
phenyl)benzene acid, L = methanol, water, or ethanol),
constructed with indium as the metal center (Figure 1). This
new material, denoted InPF-110 (InPF = indium polymeric
framework), has a structure with hexagonal channels of ca. 3 nm
aperture, and it shows remarkable catalytic activity in the Strecker
reaction with the use of ketones. The activity of InPF-110 is wide
in the scope of this reaction, showing good to excellent activity
when used with up to eight different ketones and six different
amines.
he development of new heterogeneous Lewis acid catalysts
Tis important because a large number of organic trans-
formations of interest require the use of such species.1
Heterogeneous catalysts offer clear advantages regarding their
reuse and recyclability. However, they typically display lower
activity than their homogeneous counterparts.2 Porous solids are
used to enhance the number of active sites accessible to the
substrates. This is the case of metal−organic frameworks, MOFs,
a class of materials constructed by the joining of metal clusters,
denoted secondary building units (SBUs), through organic
linkers, to produce crystalline solids with potential porosity.3
MOFs have been synthesized with a plethora of metal elements.
This fact, along with the high surface area values that they might
exhibit, makes MOFs very promising heterogeneous Lewis acid
catalysts. In particular, MOFs constructed with group 13
elements have been proven to be very efficient Lewis acid
catalysts in various organic transformations.4 Recently, we
reported the catalytic activity of a series of isostructural MOFs
in the Strecker multicomponent reaction (MCR), which was
carried out in one pot, finding that the solids’ activity can be tuned
to address the different steps involved in the reaction and,
therefore, to obtain the desired product.5 Thus, although one-pot
MCRs offer advantages in atom economy, process simplification,
purification steps, and waste reduction, they require the use of
Reaction of In(NO3)3 with H3btb in DMF in the presence of
nitric acid at 150 °C affords crystals of InPF-110 (see section 1 of
Supporting Information (SI) for detailed conditions; CCDC
number 1483220). Single crystal X-ray diffraction analysis
indicates that the compound crystallizes in the hexagonal system,
space group P62c, with lattice parameters a = 31.8903(7) Å and c
̅
= 17.1915(5) Å. The inorganic SBUs are formed by three indium
atoms in an octahedral coordination environment, sharing a
central oxygen atom, and coordinated to six carboxylate groups
from the linkers (Figure 1). This type of SBU is commonly found
Received: June 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b05706
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Journal of the American Chemical Society
Communication
possesses two types of interconnected pores, with 55.8% of
accessible void space, according to PLATON.12 Hexagonal
channels run along the c axis, with a maximum dimension of 2.8
nm. Perpendicularly are located microporous cavities with a
diameter of 3.6 Å that can be reached from the main pore and
offer access to the active indium centers. The permanent porosity
of InPF-110 was proven by N2 adsorption isotherms performed at
77 K. Prior to sorption analysis, the material was activated by
solvent exchange with methanol (3 times) followed by heating at
200 °C under dynamic vacuum. The isotherm profile (Figure 3a)
shows a clear step at P/P0 = 0.11, indicative of the presence of
mesopores. The calculated Brunauer−Emmett−Teller surface
area is 1125 m2 g−1 (1470 m2 g−1 Langmuir).
Figure 1. H3btb linker is combined with a trimeric In-SBU to produce
InPF-110. C is black, O is red, and In atoms are blue polyhedra. L =
solvent ligand (water, methanol, ethanol).
in other trivalent-metal based MOFs.8 The coordination of the
SBU is completed by a solvent ligand, and by two formate anions,
which connect to two adjacent SBUs (Figure 2a). Since no formic
Figure 3. (a) N2 adsorption isotherm of InPF-110. Empty and filled
symbols represent adsorption and desorption points, respectively. (b)
Calculated and experimental PXRD patterns for InPF-110 calcd (gray),
InPF-110 expt (blue).
During the optimization of the reaction conditions we
frequently noticed the presence of crystals belonging to a
different phase, which we denote InPF-50 (section 4 of SI;
CCDC number 1483221). Pure InPF-110 was obtained from a
mixture of 0.33 mmol of In(NO3)3, 0.15 mmol of H3btb, 2.5 mL
of DMF, and 0.8 mL of nitric acid, heated at 150 °C for 4 h. Use of
a higher temperature results in the formation of In(OH)3, while
reducing the temperature (130 °C) promotes the appearance of
InPF-50. Furthermore, to ensure reproducibility it was necessary
to dry the commercial indium nitrate by heating it at 105 °C for an
overnight period, after observing that the amount of water
influences the appearance of indium hydroxide. Phase purity was
determined by PXRD comparison with the calculated pattern
(Figure 3b).13
The catalytic activity of InPF-110 was studied with a three-
component Strecker reaction among acetophenone, aniline, and
trimethylsilyl cyanide (TMSCN) (see section 6 of SI for details).
The selected standard conditions allow comparison with
previously reported data, regarding the catalytic activity of
indium MOFs synthesized by our group.5 Other catalysts
previously used in this same reaction include In-MOF 1 and
In-MOF 2,14 which were used with a 10 times higher catalyst
loading (see Tables S5.1 and S5.2). In other cases, the use of
solvents as dichloromethane, toluene or chloroform, and
temperatures higher than rt was required. Similarly, soluble
catalysts,15 such as o-benzenedisulfonimide, and sulfonic acid
based nanoreactors16 have also been employed in larger catalyst
loadings. InPF-110, even without any solvent, also outperforms
Figure 2. (a) The trimeric inorganic SBUs in InPF-110 are disposed
forming chains by the direct linkage through formate anions. (b)
Consideration of the chain as a rod-shaped SBU results in a topologically
simplified npo-e network.
acid was added in the synthesis, these formate anions are likely
produced by decomposition of DMF molecules during the MOF
formation reaction. Each SBU is connected to six btb3− linkers. If
we do not consider the direct connection of the SBUs through the
formate linkers, the structure of InPF-110 presents a binodal
topology denoted 3,6T60,9 with the presence of 3 and 6
coordinated nodes. This structure type has been previously
described for a related scandium MOF, with a cationic structure
wherein the trimers are not directly connected.10 In the case of
InPF-110, if we consider the direct linkage among inorganic
SBUs through the formate anions, a different topological
simplification can be made consisting of a rod-shaped SBU
with a node at the position of the formates, and another node at
the middle point between two btb3− linkers that are crystallo-
graphically independent. This simplification results in a binodal
4- and 6-connected network with an npo-e topology, which
resembles a kagome
́
net (Figure 2b).11 The InPF-110 structure
B
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Journal of the American Chemical Society
Communication
previously reported MOFs tested in the same reaction, as shown
in SI Table 5.2.
Table 1 (entry 1) shows that the product is obtained with a
99% yield in 4 h, using a catalyst loading of 0.5 mol % (other
Table 2. InPF-110-Catalyzed Strecker One-Pot Three
Component Reaction among Acetophenone, Aromatic
a
Anilines, and TMSCN
Table 1. InPF-110-Catalyzed Strecker One-Pot Three
a
Component Reaction
a
Reaction conditions: acetophenone, amine, TMSCN (1.1:1:1.1), 0.5
mol % catalyst (from [In3O(btb)2(HCO2)(L)]), N2 atmosphere, 25
b
1
°C; catalyst washed with methanol or ethanol. Yield by H NMR or
c
GC-MS, from reaction crude. TON = (mmol substrate/mmol
d
catalyst). Double amount of acetophenone and TMSCN was added.
A scale-up experiment with a 5-fold larger amount was also
performed, affording a 98% yield after 5 h. Also, the catalyst was
reused in up to 10 cycles without significant loss of activity (see
section 7 of SI). The heterogeneous nature of the catalyst was
assessed by performing a leaching test, where the solid was
separated from the reaction mixture after 5 min, and then the
mother liquid was transferred to an empty Schlenk flask. After 24
h, reaction progress was not observed, which proves that the
catalyst is necessary to quantitatively yield the Strecker product.
As we previously described, we noticed that the selection of the
solvent employed for the MOF activation influences the material
catalytic activity. It should be noted that there is a potential open
metal site in the inorganic SBU of InPF-110, which is occupied by
this solvent molecule. Indeed, single-crystal X-ray diffraction
studies showed the presence of ethanol at such positions, which in
addition are forming several hydrogen bonds with other solvent
ligand molecules of adjacent SBUs. The differences in the
catalytic activity indicate that this metal site is less accessible when
ethanol is employed, probably due to its larger bulkiness
compared to water or methanol, and/or to a more efficient
packing of ethanol ligands in the micropore that prevents its
displacement by the substrate molecules. This observation also
indicates that, as expected, the MOF catalytic activity is strongly
dependent on the metal coordination environment. Thus, a
pentacoordinated indium cation, as the one generated after
elimination of the adsorbed ligand, should be highly active,
although this position is likely occupied by either the remaining
solvent ligand or water molecules from moisture. To gain more
insight into the interaction between the substrate and catalyst, we
performed a single crystal X-ray diffraction study with an MOF
crystal that was soaked in a mixture of aniline and acetophenone.
The analysis of the diffraction data clearly shows the presence of
aniline coordinated to the indium atom with the accessible metal
site, partly replacing the solvent ligand (Figure 4). Other areas
with high electron density were located inside the channels of the
MOF in the vicinity of the SBUs, resembling a phenyl ring.
a
Reaction conditions: ketone, aniline, TMSCN (1.1:1:1.1), 0.5 mol %
catalyst (from [In3O(btb)2(HCO2)(L)]), N2 atmosphere, 25 °C;
catalyst washed with methanol, ethanol, or water. Yield by H NMR
or GC-MS, from reaction crude. TON = (mmol substrate/mmol
catalyst).
b
1
c
catalyst loadings were also tested; see Table S6 of the SI). When
the InPF-110 catalyst was washed with ethanol instead of
methanol, longer reaction times (10 h) were required to reach the
same yield, while the reaction time was similar in the case of
washing with water compared to methanol (Table 1, entries 2 and
3), indicating that the accessibility to the active sites is more
hindered when larger solvent ligands such as ethanol are present.
The scope of the reaction was evaluated with substituted ketones
(Table 1) showing that InPF-110 displays very good activity;
even linear ones yield the corresponding Strecker product at very
short reaction times (0.85 h for 2-hexanone, entry 9). Cyclic
ketones need longer reaction times (Table 1, entries 10, 13, and
14), and differently substituted acetophenones present only some
slight modifications compared to the standard reaction, requiring
a longer time than the case with a methyl group in the p-position
(Table 1, entry 5).
The influence of the substituent in aromatic amines has been
also studied (Table 2), with excellent yields (≥90%) found for all
the explored substrates, including p-phenyldiamine (entry 21).
When nonquantitative yields were obtained, only unreacted
substrates (entries 16) and/or imine were detected (entries 18,
20, and 22) (see SI Scheme S1). These results demonstrate the
large scope of use of InPF-110 in the formation of substituted α-
aminonitriles.
C
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Journal of the American Chemical Society
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D.R.-F. acknowledges an FPU scholarship from the Spanish
Ministry of Education, Culture and Sport. Financial support by
Fundacion
edged (F.G.).
́
General CSIC (Programa ComFuturo) is acknowl-
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: amonge@icmm.csic.es.
*E-mail: gandara@icmm.csic.es.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been supported by the Spanish Ministry of
Economy and Competitiveness (MINECO) Projects MAT2013-
45460-R, MAT2014-52085-C2-2, and CTQ2014-61748-EXP.
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DOI: 10.1021/jacs.6b05706
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