Polyoxopalladate-Loaded Metal-Organic Framework (POP?MOF): Synthesis and Heterogeneous Catalysis

Article abstract of DOI:10.1021/acs.inorgchem.0c00875

We report on the synthesis and characterization of the first polyoxo-noble-metalate-containing metal-organic framework (MOF) material, wherein the preformed MIL-101 has been impregnated with the discrete, cuboid-shaped polyoxopalladate [Pd13Se8O32]6- (Pd13Se8), leading to the composite Pd13Se8?MIL-101. This material was characterized by FTIR, TGA, elemental analysis, powder-XRD, N2 sorption (BET), SEM-EDX, and XPS. Furthermore, the Pd13Se8?MIL-101 composite was shown to be an effective, stable, and recyclable heterogeneous precatalyst for the Suzuki-Miyaura cross-coupling reaction at room temperature utilizing environmentally benign solvents, such as water and methanol.

Full text of DOI:10.1021/acs.inorgchem.0c00875

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Polyoxopalladate-Loaded Metal−Organic Framework (POP@MOF):
Synthesis and Heterogeneous Catalysis
Saurav Bhattacharya, Wassim W. Ayass, Dereje H. Taffa, Talha Nisar, Torsten Balster, Andreas Hartwig,
Veit Wagner, Michael Wark, and Ulrich Kortz*
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ABSTRACT: We report on the synthesis and characterization of the first
polyoxo-noble-metalate-containing metal−organic framework (MOF)
material, wherein the preformed MIL-101 has been impregnated with
the discrete, cuboid-shaped polyoxopalladate [Pd13Se8O32]6− (Pd13Se8),
leading to the composite Pd13Se8@MIL-101. This material was
characterized by FTIR, TGA, elemental analysis, powder-XRD, (N2
sorption) BET, SEM-EDX, and XPS. Furthermore, the Pd13Se8@MIL-
101 composite was shown to be an effective, stable, and recyclable
heterogeneous precatalyst for the Suzuki−Miyaura cross-coupling
reaction at room temperature utilizing environmentally benign solvents,
such as water and methanol.
cally active POMs on suitable support materials, such as
mesoporous transition-metal oxides, mesoporous silica-based
materials, polymers, magnetic nanoparticles, and carbon-based
materials.8b,10 However, some of these composites also suffer
from disadvantages, such as low POM loading, POM leaching,
and nonuniform distribution of active sites.
INTRODUCTION
■
Polyoxometalates (POMs) are a large group of discrete,
anionic, and soluble metal−oxo cages, which are generally
formed by oxo-bridged d-block metal ions in high oxidation
states (e.g., VV, MoVI, WVI), and they exhibit exceptional
diversity in the shape, size (micro- to nanometer scale), and
composition.1 The diversity in the physical and chemical
properties of POMs coupled with their structural and
compositional tunability and high solubility in various solvents
have rendered them promising candidates for antibacterial,
antifungal, and anticancer applications,2 magnetic and high
density data storage devices,3 dye-sensitized solar cells,4
photoelectrochemical applications,5 and macromolecular crys-
tallography.6 Apart from these, the catalytic activity of POMs
has also been thoroughly investigated over the years owing to
their relatively high thermal stability, ability to undergo
multielectron redox transformations without any structural
changes, and sensitivity to light and electricity.7 POMs,
however, usually function as homogeneous catalysts and
therefore suffer from certain disadvantages, such as low
recoverability and recyclability as well as self-aggregation,
leading to shielding of the catalytically active sites,
subsequently lowering the catalytic efficiency.8 Thus, the
search for stable POM-based heterogeneous catalysts with high
surface areas and better recyclability has attracted a lot of
interest in recent years. Several strategies have been employed
by researchers in order to introduce heterogeneity in POMs,
such as (a) precipitation using alkali metal ions,9,10b (b)
encapsulation of POMs by organic surfactants or ionic
liquids,10 and (c) encapsulation or immobilization of catalyti-
In this context, the encapsulation or immobilization of
POMs within metal−organic framework materials (MOFs)
(POM@MOFs) or the synthesis of MOFs with POMs as
linkers (POM-MOFs), have gained ground as effective
strategies to construct POM-based heterogeneous catalysts.11
MOFs are a class of crystalline porous materials comprising of
a connectivity between inorganic secondary building units
(SBUs) (metal ions or their clusters) and organic linkers
(carboxylates, sulfonates, phosphonates, heterocyclic organic
compounds, etc.) that lead to three-dimensional open
framework structures with well-defined pores, large surface
areas, as well as structural tunability and flexibility.12 MOFs, as
encapsulating/immobilizing hosts, have several distinct advan-
tages over traditional host matrices, such as zeolites and
carbon-based materials. (a) The inner pore surfaces of MOFs
can be suitably fine-tuned either by replacing the metal ions in
the inorganic SBUs or by modifying the organic linkers; (b)
Received: March 24, 2020
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depending on the size of the guest molecules, the pore sizes
can be suitably modified by changing the length and rigidity of
the organic linkers. (c) And, certain mesoporous MOFs, such
a series of derivatives of the type [MPd12O8(LXO3)8]n−
(MPd12).16 In addition, other POPs with different structures
are also known, such as the star-shaped assembly
[PdII15O10(LXO3)8]n− (Pd15) (X = AsV, L = Ph; X = PV, L =
O; X = SeIV, L = lone pair),17 as well as the double-cuboid
[Cu2PdII22P12O60(OH)8]20− (Cu2Pd22P12)18 and the bowl-
shaped [PdII VV O24(OH)2]6− (Pd7V6).19 POPs have exhibited
as the chromium terephthalate-based MIL-101 (MIL =
13
́
Material Institut Lavoisier), have large pore sizes albeit
with smaller pore windows (pore diameters ca. 2.9 and 3.4 nm
and pore window diameters ca. 1.2 and 1.6 nm, respectively).
The large cavities render them ideal candidates as host
materials for nanosized guests, such as POMs, whereas the
small pore windows prevent the leaching and aggregation of
the guest molecules.11h This allows for suitable isolation of the
catalytically active sites, and the presence of connecting pore
windows allows for proper diffusion of substrates into the
pores to reach the active sites, thereby increasing catalytic
efficiency. For example, Kholdeeva’s group made a detailed
comparison of the catalytic activity of POMs supported on
MIL-101 (POMs@MIL-101) versus POMs supported on
mesoporous silica and carbon materials for liquid phase
oxidation reactions.11a It was observed that POMs@MIL-101
exhibit better activity and selectivity as compared to the other
composites.
7
6
immense potential as catalysts in the homogeneous oxidation
of alcohols and hydrogenation reactions.14b,16c,17a Palladium-
based materials, especially PdII/Pd0 incorporated in MOFs,
have been utilized extensively for catalyzing several important
reactions, such as C−C cross-coupling reactions,20a CO
oxidation,20b as well as for H2 storage.21a,b However, the
precise structural determination of Pd nanoparticle clusters
inside the pores of MOFs using conventional characterization
techniques is difficult and involves extensive theoretical
modeling.21c−f In this regard, POPs with electronic structures
established using single-crystal X-ray diffraction have been
rated highly as important structural models to decipher the
molecular mechanism of noble-metal-based catalysis.14c−f
However, as with conventional POMs, POPs also suffer from
low surface areas, catalytic leaching, and inadequate recycla-
bility due to high solubility. Thus, a logical approach to realize
stable, effective, and recyclable heterogeneous catalysts based
on POPs would be to synthesize POP-based MOF composites.
Recently, we have isolated for the first time a POP-MOF (JUB-
1), wherein a discrete cuboid POP acts as a SBU and is linked
by organic linkers to form a three-dimensional open framework
structure that shows promise as a heterogeneous catalyst for
the Suzuki−Miyaura cross-coupling reaction.22
Primarily, two strategies have been employed to realize
POM-containing MOF composite materials, namely, (a) the
self-immobilization process wherein the POMs are organically
functionalized and utilized as building blocks in the
construction of porous crystalline MOFs (POM-MOF) and
(b) encapsulation of POMs inside the cavities of MOFs
(POM@MOFs) either by in situ assembly of MOFs around
the POMs, in situ synthesis of POMs in the cavities of
preformed MOFs, or the insertion of POM clusters in the
cavities of preformed MOFs by the solution impregnation
method.11 These POM-containing MOF composites have
been extensively utilized in recent years as heterogeneous
catalysts for organic oxidation reactions, hydrolysis reactions,
dye degradation, photocatalysis, and electrocatalysis.11 How-
ever, from a survey of the literature, we find that almost all
such POM-containing MOF materials involve the use of
conventional POMs such as heteropoly- and isopolytungstates,
-molybdates, and -niobates of the Keggin and the Wells−
Dawson types.11b−e
Here, we report on the synthesis of the first POP-containing
MOF material (POP@MOF) and its catalytic activity.
EXPERIMENTAL SECTION
Synthesis. All chemicals were purchased commercially and
utilized without further purification.
■
[Cr3(H2O)3(OH)(C8H4O4)3](NO3)2·17H2O (MIL-101). The syn-
thesis of MIL-101 was done using a modified, fluoride-free synthetic
procedure.23 A solid mixture of Cr(NO3)3·9H2O (2.00 mmol, 0.80 g),
1,4-benzenedicarboxylic acid (1,4-bdc, 2.00 mmol, 0.332 g), and
HNO3 (2.00 mmol, 140 μL of 65% technical grade) was dispersed in
deionized water (10 mL) and stirred for 30 min. The dark blue−green
suspension was then transferred to a Teflon-lined stainless-steel
autoclave and heated at 220 °C for 8 h. After slow cooling of the
autoclave to room temperature, the green powder was filtered off from
the solution and washed with deionized water and ethanol.
Subsequently, the powder was immersed in a solvent mixture of
ethanol and water (10:1), and the suspension was heated at 80 °C for
1 day. Finally, the green powder was filtered and dried at 150 °C
under vacuum overnight. FT-IR (KBr pellet method, cm−1): 3600−
3300 (s), 3053 (m), 2932 (m), 1668 (s), 1632 (s), 1549 (w), 1507
(w), 1502 (w), 1438 (s), 1400 (s), 1309 (w), 1254 (w), 1163 (w),
1108 (w), 1016 (w), 888 (w), 833 (w), 751 (m), 659 (m), 586 (m).
Elemental analysis (%) calcd for [Cr3(H2O)3(OH)(C8H4O4)3]-
(NO3)2·17H2O (MIL-101): C 25.1, H 4.7, N 2.43, Cr 13.56;
found: C 25.9, H 5.9, N 2.39, Cr 13.61.
Na6[Pd13Se8O32]·10H2O (Pd13Se8). The synthesis of the Pd13Se8
was carried out following a reported procedure.15 Pd(CH3COO)2
(0.62 mmol, 0.14 g) and SeO2 (0.63 mmol, 0.07 g) were dissolved in
5 mL of a 0.5 M NaCH3COO solution at pH 7.0, and the resulting
solution was stirred and heated at 80 °C for 30 min. The solution was
then cooled to room temperature, and the pH was adjusted to 6.4 by
the addition of dilute aqueous NaOH solution. The solution was then
stirred and heated again at 80 °C for 1 h. The light-brown precipitate
that formed was filtered off after the solution cooled, and the filtrate
was kept at room temperature for several days, leading to the
Noble-metal-based POMs, a relatively newly discovered
subarea of POMs, that self-assemble in aqueous medium
exclusively through PdII or AuIII-oxo units in the presence of
tetrahedral oxyacid heterogroups (e.g., PO43−, AsO43−), are
currently progressing rapidly and have garnered the interest of
researchers owing to their structural and compositional
novelty, stability in solution, and promising applications as
noble-metal-based catalysts.14 Our group pioneered the area of
polyoxopalladates (POPs) when they isolated the first discrete
PdII-oxo cluster, [PdII13AsV O34(OH)6]8− (Pd13As8),14b as well
8
as the first AuIII-based oxo-cluster, [AuIII4AsV4O20]8−
(Au4As4).14d In particular, POPs are the most studied among
the known compounds of this family, and there has been
continuous development in this area over the years.14e
Utilizing a similar self-condensation process as used for the
Pd13As8, an entire family of cuboid-shaped polyoxo-13-
palladates (edge length of about 1 nm) with the general
formula [PdII13O8(LXO3)8]n− (Pd13) was isolated, involving
different capping groups (X = AsV, L = O, Ph; X = SeIV, L =
lone pair), by a facile one-pot reaction of PdII salts with As2O5,
PhAsO3H2, and SeO2, respectively, in aqueous solution.15 Our
group was also successful in replacing the central PdII ion in
the 13-palladate cube with various guest metal ions, resulting in
B
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formation of dark-red hexagonal crystals, which were filtered, washed
with a mixture of acetonitrile and diethyl ether (1:1), and dried in air.
FT-IR (KBr pellet method, cm−1): 3550−3340 (s), 1622 (m), 796
(m), 721 (s), 620 (w), 550 (s).
integration. The catalyst was washed with water and acetonitrile
and dried under compressed air. A Shimadzu GC-2010 equipped with
a flame ionization detector (FID) was used to measure the substrate
conversion and selectivity of the obtained products via a HP-5 column
(15 m × 0.25 mm, i.d. 0.25 μm), providing a good separation of the
reaction products using the following temperature program: (a) 40 °C
for 2.5 min, (b) 25 °C/min ramp to 60 °C, (c) isothermal at 60 °C
for 7 min. The carrier gas was He. A Varian GC-MS was used to
confirm the GC results. ICP-MS analysis on the filtrate after catalysis
was conducted on a PerkinElmer NexION 350X instrument in the
KED mode, using Rh as a standard. The calibration was done with a
linear correlation between the three standards (0.5, 2.0, and 4.0 ppb).
[Cr3O(H2O)3(C8H4O4)3]·0.083[Pd13Se8O32]·0.502NO3·12H2O
(Pd13Se8@MIL-101). MIL-101 (0.15 mmol, 172 mg) was taken in a
glass vial and dehydrated at 70 °C under vacuum for 1 day. A solution
of Pd13Se8 (0.075 mmol, 214 mg) in 8 mL of water was added to the
vial, and the mixture was stirred at room temperature for 1 day, with
the vial capped. A reddish-green precipitate resulted, which was
filtered, sonicated thoroughly with water to remove any excess
Pd13Se8 that might be adhered to the surface, and dried under vacuum
at 70 °C. FT-IR (KBr pellet method, cm−1): 3600−3300 (s), 3054
(m), 2930 (m), 1672 (s), 1626 (s), 1553 (w), 1507 (w), 1502 (w),
1434 (s), 1397 (s), 1306 (w), 1260 (w), 1165 (w), 1104 (w), 1013
(w), 885 (w), 830 (w), 750 (m), 722 (w), 658 (m), 585 (m), 548
(w). Elemental analysis (%) calcd for [Cr3O(H2O)3(C8H4O4)3]·
0.083{Pd13Se8O32}·0.502(NO3)·12H2O (Pd13Se8@MIL-101, ∼10 wt
% Pd loading): C 24.52, H 3.6, N 0.59, Pd 9.76, Se 4.5, Cr 13.2;
found: C 24.88, H 3.52, N 0.41, Pd 9.32, Se 5.1, Cr 12.5.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Pd13Se8@MIL-101
Catalyst. The precursor materials [Pd13Se8O32]6− (Pd13Se8)
guest and MIL-101 host, were synthesized utilizing literature
procedures.15,23 We have successfully immobilized the discrete
polyoxopalladate (POP) Pd13Se8 onto the host matrix MIL-
101 (Figure 1) utilizing the solution impregnation method,
Characterization. Fourier-transform infrared spectra (FTIR, KBr
pellets) were recorded with a Nicolet-Avatar 370 spectrometer
(4000−400 cm−1). Thermogravimetric analyses (TGA) were
performed utilizing a TA Instruments Q600 device at a heating rate
of 5 °C min−1 under nitrogen (from room temperature to 850 °C).
Elemental analyses were performed at Analytische Laboratorien,
Industriepark Kaiserau, Lindlar, Germany. Powder X-ray diffraction
patterns were recorded with an Empyrean diffractometer (PAN-
alytical, The Netherlands) equipped with a 1D pixel detector in the
θ−2θ configuration. A Cu anode with Kα radiation (λ = 1.540598
nm) was utilized as the X-ray source, operated at 40 kV and 40 mA (θ
= 5−30°). The Brunauer−Emmett−Teller (BET) surface areas of the
materials were calculated from nitrogen physisorption isotherms,
which were measured at 77 K utilizing a Quantachrome AUTOSORB
1 apparatus. The samples were degassed at 70 °C for 1 day under
vacuum. The specific surface areas were evaluated with the BET
method in the P/P0 range of 0.05−0.3. Scanning electron microscopy
(SEM) was performed with a LEO1530 Gemini microscope from
Zeiss by placing the samples on electrically conducting polymer films.
Energy dispersive X-ray analysis (EDX) mapping was performed with
a Helios Nanolab 600i (FEI) scanning electron microscope utilizing
the Octane Super SDD energy dispersive spectroscopy with a Hikari
Super 1400pps electron backscatter diffraction (EBSD) detector. The
oxidation states of the elements in the catalyst before and after
catalysis was ascertained using X-ray photoelectron spectroscopy
(XPS) measurements. The samples were dispersed in acetone and
spin-coated onto the as prepared substrates at a rotation speed of
1000 rpm. The ultrahigh vacuum vessel, which had a vacuum pressure
of ∼1 × 10−8 mbar, was equipped with a water-cooled X-ray gun with
a double Mg/Al anode (Specs XR 50) and a hemispheric electron
analyzer (Specs Phoebos 100) with an angle of 45° between the X-ray
gun and the direction of the analyzer. Mg Kα1,2 radiation (E = 1253.6
eV) was used as a source of excitation. The photoelectrons were
detected in the large area lens mode and fixed analyzer transmission at
a pass energy of 50 eV. The measured data were analyzed using the
CASA-XPS software. The positions of the peaks were in the expected
regions.24a,b
Figure 1. (a) Ball-and-stick representation of the polyoxopalladate(II)
[Pd13Se8O32]6− (Pd13Se8) guest. Color code: PdII purple, SeIV blue, O
red. (b) The porous MIL-101 host MOF. Color code: CrIIIO6 green
octahedra, C gray, O red, and the central pore with a hexagonal
window is highlighted by a transparent light-yellow sphere.
wherein the Pd13Se8@MIL-101 composite material was
obtained by adding dehydrated MIL-101 to an aqueous
solution of Pd13Se8 and stirring the mixture for 1 day at room
temperature. The solution impregnation strategy for the
synthesis of conventional heteropolyanion-loaded MIL-101
́
materials (POM@MIL-101) was first reported by Ferey’s
group in 200513 and has been utilized subsequently by other
groups for different POMs.25 This strategy involves the
exchange of the counteranions of the MOF by POMs. In
our case, elemental analysis for the as-prepared MIL-101
indicated the presence of nitrate as counteranions. The
structure of MIL-101 in the Pd13Se8@MIL-101 composite
material comprises Cr3 trinuclear clusters that consist of three
Cr3+ ions, each in an octahedral environment and coordinated
by four terephthalate carboxylate oxygens, one μ3-oxo, and one
Suzuki−Miyaura Cross-Coupling Catalysis. In a typical
reaction, a vacuum oven-dried round-bottom flask was charged with
the aryl halide (0.50 mmol), phenylboronic acid (0.75 mmol),
anhydrous K2CO3 (1 mmol), dehydrated Pd13Se8@MIL-101 (11 mg,
2 mol % Pd basis w.r.t the limiting reagent bromobenzene), dry
MeOH (2 mL), and deionized H2O (2 mL). The vessel was purged
with N2 and sealed, and then, the reaction mixture was stirred
vigorously at room temperature for 3 h. The solution was then
centrifuged and filtered to remove the catalyst (to be utilized for
subsequent cycles). The filtrate was separated utilizing a separatory
funnel employing water and hexane as solvents. The organic phase
was then collected and dried over anhydrous Na2SO4. The crude
product was quantified by GC/GC-MS analysis via peak-area
oxygen from the terminal water ligand. In their original paper,
13
Ferey et al. reported that one of the Cr3+ ions has a terminal
́
F− while the other two have a terminal water molecule each.
Since we adopted a fluoride-free synthesis,23 the terminal F− is
replaced by a water molecule. This also explains the presence
of NO3− as counteranion, which could be partially replaced by
the Pd13Se8 anions, leading to the isolation of the composite
impregnated material Pd13Se8@MIL-101. Similar observations
have been reported earlier for Co-containing heteropolyanions
immobilized in the MIL-101.25d Elemental analysis of the
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Figure 2. IR spectra of the discrete polyanion Pd13Se8 (pink curve), the host MIL-101 (black curve), and the Pd13Se8@MIL-101 composite
material before and after catalysis (red and blue curves, respectively). The left figure shows the IR spectra for the entire range (4000−400 cm−1),
whereas the right figure shows the magnified representation of the fingerprint region from 775−425 cm−1 (highlighted in green in the left figure).
The labels * and # indicate the most intense IR peaks corresponding to the Pd13Se8 POP.
Pd13Se8@MIL-101 material indicated the absence of Na+ ions
and the presence of N, which further reaffirms that the POP
has partially replaced the nitrates. Elemental analysis also
indicated a ∼10 wt % loading based on palladium, and this
product was utilized further for complete characterization and
catalytic studies.
MIL-101, indicating that the framework structure of MIL-101
was maintained upon loading of the Pd13Se8 guest.
An examination of the thermogravimetric analysis (TGA)
curves of fresh MIL-101 and Pd13Se8@MIL-101 (Figure S1a)
revealed the following: (a) Firstly, both exhibit similar thermal
stability characteristics (from RT up to 280 °C), indicating
that the MIL-101 framework structure is maintained after
loading with Pd13Se8; (b) secondly, the TGA curve of fresh
MIL-101 showed ca. 11% excess weight loss in the temperature
range of 280−600 °C as compared to Pd13Se8@MIL-101,
supportive of POP loading in the MOF. Dinitrogen adsorption
measurements were performed on both fresh MIL-101 and the
Pd13Se8@MIL-101 composite to gauge the difference in BET
surface areas before and after POP loading (Figure 4). The
The immobilization of Pd13Se8 on MIL-101 was first
followed by FTIR spectroscopy (Figure 2). Strong bands
were observed at ∼721 cm−1 and at ∼550 cm−1 for the as-
prepared Pd13Se8 POP precursor, which corresponds to the
Pd−O bond stretching vibrations. These bands were also
observed, albeit with weaker intensities, in the FTIR spectrum
of the Pd13Se8@MIL-101 composite material, which indicates
that Pd13Se8 has been loaded successfully in MIL-101. All
vibrational bands corresponding to pure MIL-101 remained
unchanged in the composite Pd13Se8@MIL-101, indicating the
structural integrity of MIL-101. The stability of MIL-101 after
the Pd13Se8 loading was further monitored by powder X-ray
diffraction (PXRD) studies. The PXRD pattern of the as-
prepared fresh MIL-101 sample matched with the PXRD
pattern reported in the literature,13 indicating bulk purity of
the MIL-101 host material (Figure 3). In addition, no obvious
changes were detected for the Bragg peaks in the PXRD
spectrum of Pd13Se8@MIL-101 with respect to that of pure
Figure 4. Comparison of the N2-adsorption isotherms of fresh MIL-
101 (black curve) and Pd13Se8@MIL-101 before (red curve) and after
(blue curve) catalysis.
adsorption isotherms of both samples show stepwise N2-uptake
up to p/p0 values of 0.3, which can be attributed to the
existence of different types of cages in the MIL-101. However,
the position of the N2-uptake steps for the Pd13Se8@MIL-101
composite does not show a shift toward left that would have
been indicative of the insertion of the POP inside the pores,
which means that the POPs are immobilized on the surface of
the MIL-101 support matrix. This fact is supported by the
pore-size distribution curve, which indicates that the
impregnation of the MIL-101 with the POP does not affect
the signal corresponding to the large cavities but only restricts
Figure 3. PXRD patterns of the fresh MIL-101 (red curve) and of the
Pd13Se8@MIL-101 composite material before and after catalysis (blue
and pink curves, respectively) compared with the PXRD patterns of
MIL-101 obtained from literature (black curve).
D
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Figure 5. SEM images of (a) fresh MIL-101, (b) Pd13Se8@MIL-101 before catalysis, and (c) Pd13Se8@MIL-101 after catalysis. EDX mapping
images of Pd13Se8@MIL-101 (d) before and (e) after catalysis, indicating a uniform distribution of the composing elements (Cr, Pd, and Se).
the availability of the smaller cavities (Figure S1b). The BET
surface area and total pore volume for fresh MIL-101 were
found to be ca. 2427 m2/g and ca. 1.489 cc/g, respectively. On
the other hand, the N2-adsorption isotherm of the Pd13Se8@
MIL-101 composite showed a much lower BET surface area of
ca. 1596 m2/g with a total pore volume of ca. 0.96 cc/g,
indicating that the pore windows are partially blocked by the
Pd13Se8 moieties.
organic synthesis. In recent years, extensive research has been
undertaken by researchers to study the use of PdII-based
complexes or PdII/Pd0@porous matrices as homogeneous and
heterogeneous catalysts, respectively, for the Suzuki−Miyaura
C−C coupling reactions.20a,26 Therefore, after having immo-
bilized Pd13Se8 onto the MIL-101 matrix, we attempted to
utilize the resulting Pd13Se8@MIL-101 composite material as a
heterogeneous catalyst for the Suzuki−Miyaura C−C coupling
reaction. A general procedure for the C−C coupling reaction is
shown in the Experimental Section. In order to optimize the
reaction conditions, the reactivity of bromobenzene and
phenylboronic acid was investigated in the presence of
different bases, solvents, and catalysts and by employing
different reaction times (Tables S1−S4). All reactions were
performed by utilizing a 2 mol % Pd basis of the catalyst and
stirring the reaction mixtures vigorously at room temperature.
The efficiency of Suzuki−Miyaura reactions is greatly affected
by the nature of the base employed.27 In general, the reactivity
trend of different bases in coupling reactions follows the order
In order to monitor possible changes in the morphologies of
the microcrystallites accompanying the POP@MOF immobi-
lization process, SEM experiments were carried out on fresh
MIL-101 and the Pd13Se8@MIL-101 composite (Figure 5).
The SEM image of fresh MIL-101 indicates a distinct
octahedral morphology of the microcrystallites with sizes
ranging from 0.2−1 μm. An examination of the SEM image of
the Pd13Se8@MIL-101 material reveals that POP impregnation
neither changes the octahedral morphology nor the size of the
microcrystallites. An SEM-EDX mapping of the Pd13Se8@MIL-
101 composite material indicated a uniform distribution of the
elements Pd and Se throughout the sample with a Pd/Se
elemental ratio of ca. 1.8:1, which is close to the ratio (1.63:1)
expected for a uniform loading of Pd13Se8 onto MIL-101
(Figure 5, Figure S2). This means that ionic exchange of the
POPs with the nitrate anions leads to a good dispersion of the
catalytic centers on the surface of the MIL-101.11l An XPS
analysis on the Pd13Se8@MIL-101 composite sample exhibited
a Pd 3d5/2 peak at ∼337.9 eV, which is typical of Pd in the +2
oxidation state (Figure S3a,).24a Also observed are a Se 3d5/2
peak and a Cr 2p3/2 peak at ∼58.3 eV and ∼577.5 eV,
respectively, which are typical for Se in the +4 oxidation state
and Cr in the +3 oxidation state (Figures S3b and S3c).24a
Similar peaks were observed in the XPS spectra of the freshly
prepared Pd13Se8 POP (Figures S3d and S3e, Table S6). These
results indicate that the impregnation of the POP in the MOF
does not involve any redox transformations.
OH− > CO3 > PO4 > AcO−. We also used such bases in
our work and indeed observed a similar trend. The use of
NaOH led to quantitative conversion, whereas K2CO3, K3PO4,
CsF, and NaOAc gave progressively lower conversions (∼94%,
∼75%, ∼52%, and 23%, respectively). However, it was found
that the MIL-101 support is unstable under strongly alkaline
conditions obtained when using aq. NaOH.28 This prompted
us to use K2CO3 as base for the coupling reactions, because of
(i) the high conversion of ∼94% (from GC analysis) and (ii)
the MIL-101 support was found to be stable under such weakly
alkaline conditions. This was corroborated by FTIR, PXRD,
and sorption studies on the catalyst after catalysis (Figures
2−4). The FTIR study confirmed the presence of the
stretching frequencies corresponding to both the Pd13Se8
guest and the MIL-101 host after catalysis (Figure 2). The
PXRD studies indicated that the Bragg peaks corresponding to
the MIL-101 remain largely intact after catalysis (Figure 3),
and the N2-adsorption studies indicated only a marginal
decrease in the surface area of Pd13Se8@MIL-101 after catalysis
(Figure 4). The pore-size distribution curve indicates that the
2−
3−
Suzuki−Miyaura Cross-Coupling Reaction with
Pd13Se8@MIL-101. The Suzuki−Miyaura cross-coupling
reaction is one of the most versatile and robust methods for
C−C bond formation and is, therefore, particularly useful for
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access to the smaller cavities is still hindered as was observed
for the as-prepared Pd13Se8@MIL-101, which may explain that
the POP moieties do not leach out during catalysis (Figure
S1b). An examination of the SEM image of Pd13Se8@MIL-101
after catalysis reveals that the catalytic reaction does not
change the octahedral morphology or the sizes of the
microcrystallites, further indicating the stability of the support
MIL-101 (Figure 5).
Our aim was to utilize environmentally benign solvents for
the catalytic reactions, and therefore, we selected H2O, MeOH,
and EtOH as test solvents. Our studies indicated that
Pd13Se8@MIL-101 performed best as the catalyst for the
Suzuki−Miyaura cross-coupling reactions in a 1:1 H2O/
MeOH solvent mixture (Table S2). The effectiveness of the
Pd13Se8@MIL-101 catalyst was reiterated by the fact that in
the absence of any catalyst or in the presence of MIL-101
alone, no conversions were observed at similar catalytic
conditions, whereas in the presence of Pd13Se8 alone (2 mol
% Pd basis), only ∼7% conversion was observed (Table S3,
Figure S4c). It should be noted, however, that a ∼100%
conversion was observed when Pd13Se8 was utilized as the
catalyst and that the catalysis was done at 90 °C for 2 h (Figure
S4d). In pure water, the salt of Pd13Se8 is highly soluble,
whereas in a mixture of water and methanol that we used for
the catalysis, Pd13Se8 is only sparingly soluble at room
temperature. However, upon heating the same to 90 °C, it
becomes soluble. XPS analysis of Pd13Se8 after catalysis at
room temperature and at 90 °C revealed that the oxidation
state of Pd remained +2 after catalysis at room temperature,
whereas a partial reduction of Pd2+ to Pd0 was seen after
catalysis at 90 °C (Figure S3f). Thus, it can be concluded that
for the Pd13Se8 POP, a higher temperature is needed to
activate the catalyst under these catalytic conditions. However,
for the Pd13Se8@MIL-101 composite material, the catalytic
activation occurs at room temperature. We believe that MIL-
101 effectively separates Pd13Se8 from its counter-cations Na+
(no Na+ was found by elemental analysis and EDX of
Pd13Se8@MIL-101), thereby enabling the activation of the
catalyst at room temperature in the presence of methanol. The
impregnation of the MIL-101 with the Pd13Se8 may also create
local distortions in the structure of the POP, which may lead to
an increased catalytic activity at lower reaction temperatures.
Moreover, utilizing bromobenzene and phenylboronic acid as
model substrates, K2CO3 as the base, and H2O/MeOH (1:1)
as the solvent, a reaction time of 3 h was found to be optimal
when operating at room temperature (Figure 6, Table S4). In
order to demonstrate further the stability of the Pd13Se8@MIL-
101 catalyst, a leaching test was performed using the hot-
filtration method, wherein the catalytic reaction was performed
in the presence of the Pd13Se8@MIL-101 catalyst for 15 min,
after which the mixture was centrifuged and filtered and the
filtrate was further stirred for 165 min, with aliquots taken at
periodic intervals and GC spectra measured of the same
samples. No further conversion was observed, indicating a true
heterogeneous nature of the catalyst (Figure 6). Moreover, an
ICP-MS analysis of the filtrate after catalysis indicated a Pd-
content of only 0.2 ppm, suggesting negligible POP leaching.
An SEM-EDX mapping of Pd13Se8@MIL-101 after catalysis
indicated a uniform distribution of the elements Pd and Se
throughout the sample with a Pd/Se elemental ratio of 1.95:1,
which is close to the ratio (1.63:1) expected for a uniform
loading of Pd13Se8 onto MIL-101 (Figure 5, Figure S2).
Recyclability experiments were performed by isolating the solid
Figure 6. Substrate conversion plotted against reaction time in the
reaction of bromobenzene with phenylboronic acid (black) and the
leaching test utilizing the hot filtration method (red).
catalyst after each run by centrifugation, subsequently washing
it with water and acetonitrile, and drying it under vacuum at
room temperature, only for it to be reused in subsequent runs.
No significant loss in activity was observed for four reaction
cycles (Figure 7). Following the optimization studies, various
Figure 7. Recyclability of the Pd13Se8@MIL-101 as a catalyst for the
Suzuki−Miyaura coupling reaction.
substituted bromobenzenes along with phenylboronic acid
were utilized as substrates to study the efficacy of Pd13Se8@
MIL-101 as a catalyst for the Suzuki−Miyaura cross-coupling
reaction. High conversions (86−100%) were observed (Table
1, Figure S4), especially for bromobenzenes substituted with
electron-withdrawing groups (highest yields were observed for
cyano- and carbonyl-functionalized bromobenzenes).20a How-
ever, for the bulky substrate 5-bromoacenaphthene, a low
catalytic conversion of ∼29% was observed, indicating a
substrate-size dependence of the catalysis. The catalytic activity
of Pd13Se8@MIL-101 was also found to be comparable to that
of other known Pd-based catalysts under aqueous conditions
with our catalyst having the advantage of being operable at
room temperature (Table S5). It is known that Pd(0) is the
catalytically active site for Suzuki−Miyaura cross-coupling
reactions.29a Therefore, we decided to investigate if Pd13Se8@
MIL-101 undergoes in situ partial reduction of the POP moiety
(comprising PdII ions). It was shown before that methanol can
reduce Pd2+ to Pd(0).29b An XPS study on the Pd13Se8@MIL-
101 catalyst, upon immediate isolation after catalysis and
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a
Table 1. Suzuki−Miyaura Coupling Reactions Utilizing Pd13Se8@MIL-101 as a Catalyst
a
The general procedure of the catalytic reactions is given in detail in the Experimental Section.
drying under vacuum, indeed indicated partial reduction of the
Pd2+ ions of the POP moieties (ca. ∼50%, Figures S3a and S5,
Table S6), keeping the oxidation states of the Se and Cr intact
(+4 and +3, respectively). Interestingly, reoxidation of the
Pd(0)-centers is observed upon exposure to compressed air,
which leads us to believe that the POP moiety is stable enough
to structurally sustain partial reduction−reoxidation steps.
Some earlier work is supportive of our mechanistic
hypothesis.29c−e Thus, we conclude that Pd13Se8@MIL-101
can be considered as a competent precatalyst for the Suzuki−
Miyaura cross-coupling reaction.29c,f,g
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00875.
■
sı
Optimization of the reaction parameters of the catalytic
reaction, tabulated comparison of the heterogeneous
catalytic activity of Pd13Se8@MIL-101 with other Pd-
based catalysts, catalytic activity of the Pd13Se8 POP
alone, TGA curves, EDX spectra, XPS spectra, GC
chromatograms, and proposed catalytic reaction mech-
anism (PDF)
CONCLUSIONS
AUTHOR INFORMATION
Corresponding Author
■
■
In conclusion, we have synthesized the first polyoxo-noble-
metalate-containing MIL-101 material, wherein the discrete,
cuboid-shaped 13-palladium(II)-8-selenite(IV) [Pd13Se8O32]6−
(Pd13Se8) is immobilized onto the preformed MOF, MIL-101,
leading to the composite Pd13Se8@MIL-101. This material was
characterized by elemental analysis, FTIR spectra, TGA
analysis, PXRD studies, N2 sorption (BET analysis), SEM
and EDX mapping, and XPS spectra analysis and has been
shown to be an effective precatalyst for the Suzuki−Miyaura
cross-coupling reaction under ambient conditions. The
importance of this work lies in the fact that we can now
potentially utilize the plethora of known discrete POPs of
various sizes, shapes, and compositions,14f as well as
polyoxoaurates(III),14d and utilize them as guests in porous
host MOFs to realize a series of functionally interesting
polyoxo-noble-metalate@MOF materials. Such research works
are ongoing in our laboratory.
Ulrich Kortz − Department of Life Sciences and Chemistry,
Jacobs University, 28759 Bremen, Germany; orcid.org/
0000-0002-5472-3058; Email: u.kortz@jacobs-university.de
Authors
Saurav Bhattacharya − Department of Life Sciences and
Chemistry, Jacobs University, 28759 Bremen, Germany;
orcid.org/0000-0002-3396-8312
Wassim W. Ayass − Department of Life Sciences and Chemistry,
Jacobs University, 28759 Bremen, Germany; orcid.org/
0000-0003-2902-2142
Dereje H. Taffa − Institute of Chemistry, Carl von Ossietzky
University Oldenburg, 26129 Oldenburg, Germany;
orcid.org/0000-0002-1778-8223
Talha Nisar − Department of Physics and Earth Sciences, Jacobs
University, 28759 Bremen, Germany; orcid.org/0000-0003-
0091-362X
G
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Article
reactor design. Dalton Trans. 2016, 45, 16716−16726. (c) Weinstock,
I. A.; Schreiber, R. E.; Neumann, R. Dioxygen in Polyoxometalate
Mediated Reactions. Chem. Rev. 2018, 118, 2680−2717. (d) Suzuki,
K.; Mizuno, N.; Yamaguchi, K. Polyoxometalate Photocatalysis for
Liquid-Phase Selective Organic Functional Group Transformations.
ACS Catal. 2018, 8, 10809−10825. (e) Han, Q.; Ding, Y. Recent
advances in the field of light-driven water oxidation catalyzed by
transition-metal substituted polyoxometalates. Dalton Trans. 2018, 47,
8180−8188.
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Multicomponent Polyoxometalates in Liquid-Phase Reactions. Chem.
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progress on supported polyoxometalates for biodiesel synthesis via
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Torsten Balster − Department of Physics and Earth Sciences,
Jacobs University, 28759 Bremen, Germany
Andreas Hartwig − Fraunhofer Institute for Manufacturing
Technology and Advanced Materials IFAM, 28359 Bremen,
Germany; University of Bremen, Department 2 Biology/
Chemistry, 28359 Bremen, Germany; orcid.org/0000-
0002-3320-6414
Veit Wagner − Department of Physics and Earth Sciences, Jacobs
University, 28759 Bremen, Germany; orcid.org/0000-0002-
6831-8600
Michael Wark − Institute of Chemistry, Carl von Ossietzky
University Oldenburg, 26129 Oldenburg, Germany;
orcid.org/0000-0002-8725-0103
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00875
Notes
Note: Figure 1 was generated with Diamond, version 3.2
(Crystal Impact GbR).
(10) (a) Qi, W.; Wang, Y.; Li, W.; Wu, L. Surfactant-Encapsulated
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
U.K. thanks the German Research Council (DFG, KO-2288/
26-1), Jacobs University, and CMST COST Action CM1203
(PoCheMoN) for support. D.T. and M.W. thank the German
Research Council (DFG) for financial support for the X-ray
diffraction setup (INST 1841154-1FUGG).
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