The MOF-driven synthesis of supported palladium clusters with catalytic activity for carbene-mediated chemistry

Article abstract of DOI:10.1038/nmat4910

The development of catalysts able to assist industrially important chemical processes is a topic of high importance. In view of the catalytic capabilities of small metal clusters, research efforts are being focused on the synthesis of novel catalysts bearing such active sites. Here we report a heterogeneous catalyst consisting of Pd₄ clusters with mixed-valence 0/+1 oxidation states, stabilized and homogeneously organized within the walls of a metal-organic framework (MOF). The resulting solid catalyst outperforms state-of-the-art metal catalysts in carbene-mediated reactions of diazoacetates, with high yields (>90%) and turnover numbers (up to 100,000). In addition, the MOF-supported Pd₄ clusters retain their catalytic activity in repeated batch and flow reactions (>20 cycles). Our findings demonstrate how this synthetic approach may now instruct the future design of heterogeneous catalysts with advantageous reaction capabilities for other important processes.

Full text of DOI:10.1038/nmat4910

ARTICLES
PUBLISHED ONLINE: 12 JUNE 2017 | DOI: 10.1038/NMAT4910
The MOF-driven synthesis of supported palladium
clusters with catalytic activity for
carbene-mediated chemistry
Francisco R. Fortea-Pérez1†, Marta Mon1†, Jesús Ferrando-Soria1, Mercedes Boronat2,
Antonio Leyva-Pérez2 , Avelino Corma2 , Juan Manuel Herrera3, Dmitrii Osadchii4, Jorge Gascon4,
*
*
Donatella Armentano5 and Emilio Pardo1
*
*
The development of catalysts able to assist industrially important chemical processes is a topic of high importance. In view of
the catalytic capabilities of small metal clusters, research eꢀorts are being focused on the synthesis of novel catalysts bearing
such active sites. Here we report a heterogeneous catalyst consisting of Pd4 clusters with mixed-valence 0/+1 oxidation states,
stabilized and homogeneously organized within the walls of a metal–organic framework (MOF). The resulting solid catalyst
outperforms state-of-the-art metal catalysts in carbene-mediated reactions of diazoacetates, with high yields (>90%) and
turnover numbers (up to 100,000). In addition, the MOF-supported Pd4 clusters retain their catalytic activity in repeated
batch and flow reactions (>20 cycles). Our findings demonstrate how this synthetic approach may now instruct the future
design of heterogeneous catalysts with advantageous reaction capabilities for other important processes.
ltrasmall metal clusters1–5 have attracted a great deal of form and transfer carbenes21. Despite Pd compounds being found as
interest as extremely active and selective catalysts for organic catalysts for a plethora of fundamental transformations in organic
U
reactions6,7. Despite the efforts made, there is yet more work synthesis, such as hydrogenations, carbon–carbon couplings, and
to be done to fully understand the exact nature of the catalytic many others22, they have been only occasionally employed as cat-
species, their reactivity, structure, shape and nuclearity8,9. Ligand alysts in carbene-transfer processes23–27
stabilization10 and matrix-supported11,12 are the most efficient Here we report the preparation of Pd04/1+ clusters stabilized by
.
strategies to synthesize and characterize metal clusters. While the the anionic framework of a MOF. The material is able to catalyse
former allows structure determination of the cluster metal complex representative carbene-mediated reactions such as the inter- and
by single-crystal X-ray crystallography, these compounds rarely find intramolecular Buchner ring expansion reaction, the alcohol inser-
applications in catalysis13 since ligand exchange with the reactants tion and the dimerization of diazocompounds, to give some unique
triggers decomposition of the cluster. In clear contrast, clusters naturally occurring and synthetically useful products28–30. This new
supported on solids are stable catalytic species with highly reactive catalytic behaviour of Pd emerges due to the stabilization of interme-
uncoordinated atoms1, but only nuclearity and oxidation state of diate Pd+ valence within the supported-MOF cluster, which enables
the metal by X-ray absorption spectroscopies combined with high- carbene bonding to give a catalytic activity that exceeds by one order
resolution microscopies can be addressed. Thus, the synthesis of of magnitude that of the state-of-the-art catalysts for these reactions.
supported metal clusters with precise topological and electronic In addition, the solid catalyst can be recovered and reused up to
properties, as occurs in metal complexes, would be of much interest. 20 times in batch without any reactivation treatment or depletion
In this context, metal–organic frameworks (MOFs)14 have of the catalytic activity, or operated in flow with solvent recycling
emerged15 as very versatile materials for encapsulating catalytic during hours.
nanosized species within their channels16. In addition, the regular
and well-defined channels in MOFs appear, theoretically, as the Synthesis and characterization of Pd4-MOF
perfect hosts to obtain information about the nature of the metallic Herein, we take advantage of the emerging post-synthetic
catalyst species by means of single-crystal X-ray crystallography. methods31,32 to synthesize (see Methods), in three steps, [Pd4]2+
However, after a number of studies on this topic showing small clusters from a preformed highly porous anionic MOF of
1–2 nm metal nanoparticles17–20, it is difficult to find reports of metal formula MgII{MgII[CuII(Me3mpba)2]3}·45H2O (1) [Me3mpba4−
=
2
4
2
clusters below the nanometre size within MOF matrices.
N,N0-2,4,6-trimethyl-1,3-phenylenebis(oxamate)] (Fig. 1a) by: the
Metal carbenoids are powerful catalytic and synthetic intermedi- already reported transmetallation32 of 1 to yield the more robust
ates in organic synthesis, and different metals have been reported to MOF NiII{NiII[CuII(Me3mpba)2]3}·54H2O (2) (Fig. 1b); the
2
4
2
1Departamento de Química Inorgánica, Instituto de Ciencia Molecular (ICMOL), Universidad de Valencia, 46980 Paterna, Valencia, Spain. 2Instituto de
Tecnología Química (UPV-CSIC), Universidad Politècnica de València-Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos s/n, 46022
Valencia, Spain. 3Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18071 Granada, Spain.
4Catalysis Engineering-Chemical Engineering Dept, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands. 5Dipartimento di
Chimica e Tecnologie Chimiche, Università della Calabria, 87030 Rende, Cosenza, Italy. †These authors contributed equally to this work.
*e-mail: anleyva@itq.upv.es; acorma@itq.upv.es; donatella.armentano@unical.it; emilio.pardo@uv.es
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NATURE MATERIALS DOI: 10.1038/NMAT4910
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a
b
c
d
b
a
b
b
b
a
a
a
e
f
b
a
b
c
Figure 1 | X-ray crystal structure. a–d, Design approach showing the structures of 1–4 determined by single-crystal X-ray diꢀraction from the three-step
post-synthetic process consisting of a transmetallation 1 (a) to give 2 (b), an exchange of the NiII cations of the pores by [PdII(NH3)4]2+ ones yielding 3 (c)
and the final reduction process aꢀording 4 (d). e, Perspective views of a channel of 4 along the c (left) and a (right) axis. f, Detail of the proposed
self-assembly after reduction process of the PdII units in 3 (left) to give the Pd4 cluster in 4 (right). Copper, magnesium and nickel atoms from the network
are represented by cyan, orange and yellow polyhedral, respectively, whereas organic ligands are depicted as sticks. Orange, yellow, pink, dark blue, pale
blue and red spheres represent Mg, Ni, Na and Pd atoms and NH3 and O2− molecules/ions, respectively. Dashed and dotted lines represent the Pd···O
and Na···O weak supramolecular interactions, respectively.
exchange of the nickel(II) cations hosted in the pores of 1 by
[PdII(NH3)4]2+ ones yielding the novel compound [PdII(NH3)4]
[PdII(µ-O)(NH3)6(NH4)2]0.5{NiII[CuII(Me3mpba)2]3}·52H2O (3)
2
4
(Fig. 1c); and the reduction proces2s to give the final compound
[Pd4]0.5@Na3{NiII[CuII(Me3mpba)2]3}· 56H2O (4) (Fig. 1d).
Overall, the4 presence of PdII cations in
a controlled
2
stoichiometry33,34 together with the confined state provided
by the anionic porous network seems to play a key role in gaining
control of the size and mixed-valence nature of metal particles
and thus, such small clusters as [Pd4]2+ units were formed. The
robustness of the three-dimensional network allowed the resolution
of the crystal structure of every single material to unambiguously
visualize the single-crystal to single-crystal guest inclusion and
the final observation of quasi-linear [Pd4]2+ clusters (Figs 1 and 2
and Supplementary Figs 1–6), reminiscent of ligand-stabilized
palladium sandwich complexes reported before10,35,36, which are
undoubtedly stabilized by the network (Figs 1e and 2 and structural
details section in the Supplementary Information).
c
a
c
a
The nature of the Pd4–MOF compound 4 was established by
the combination of a variety of techniques including: density
functional theory calculations, inductively coupled plasma–mass
spectrometry, elemental, powder X-ray diffraction (PXRD)
and thermo-gravimetric analyses, Fourier transform infrared
(FTIR) under CO and X-ray photoelectron (XPS) spectroscopies
and scanning (SEM) and high-resolution transmission electron
(HR-TEM) microscopies, including high-angle annular dark-
field scanning transmission electron microscopy (HAADF-STEM)
Figure 2 | Cluster structure. Perspective view along the b crystallographic
axis of two portions (top and bottom) of the crystal structure of 4 enclosing
the two crystallographic not equivalent ligand-stabilized quasi-linear
[Pd4]2+ NCs. MOF structure is depicted as gold sticks and Pd atoms are
represented as purple spheres. Purple dotted lines show some of the
cluster–network interactions.
measurements (Fig. 3, Supplementary Figs 7–17 and Supplementary Figs 1, 2 and 4a) with no evidence of remarkable changes in
Table 1). The N2 adsorption isotherm at 77 K confirmed the structural parameters (Supplementary Table 2)32. The arrangement
permanent porosity of 4 (Supplementary Fig. 18). Finally, the of Pd2+ moieties in 3 and that of Pd nanoclusters (NCs) in 4,
resolution of the crystal structure of 4 allowed an unambiguous confined into the channels, are strictly related. In 3 they exhibit
observation of such small Pd entities in 4.
either mononuclear, [PdII(NH3)4]2+, or dinuclear complexes of
The anionic NiI4ICu6II open-framework structure displays the type [PdII(µ-O)(NH3)6]2+ (Supplementary Fig. 3), whereas
2
hydrophilic octagonal pores, exhibiting a virtual diameter of 2.2 nm, in 4, after reduction, they finally assemble in centrosymmetric
that accommodate free nickel(II) cations that can be exchanged tetranuclear Pd4 NCs (Fig. 1 and Supplementary Figs 1b, 2 and 4).
afterwards to give 3 and 4 (Figs 1b–d and 2, and Supplementary The Pd2+ ions from the oxo-bridged dinuclear entities in 3
2
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a
b
2.30
2.37
2.30
2.69
2.65
2.69
2.37
c
d
2.44
2.73
2.44
3.04
3.04
2.82
3.04
2.92
2.51
2.16
2−
Figure 3 | Density functional theory calculations. Optimized geometry of Pd4 in a Pd4{Ni4[Cu2(Me3mpba)2]3} model. a–d, The two equivalent
structures separated (a), overlapped (b) and averaged (c) are shown, together with the XRD structure (d). Pd–Pd and Pd–O (italics) distances are given in
Å. Pd, Cu, Ni, N, O, C and H atoms are depicted in blue, green, pink, purple, red, orange and white, respectively.
(Supplementary Fig. 3) are tetracoordinated, in a distorted square precursors of the central NCs core, whereas the mononuclear Pd2+
planar geometry, with Pd–NH3 and Pd–O bond distances similar cations become the terminal atoms in the Pd4 NCs (Supplemen-
to those found in the literature37. The Pd atoms in 4 are two- tary Fig. 3e). Finally, hydrated charge-counterbalancing alkali Na+
or one-coordinated, respectively in a slightly bent geometry cations are present in 4 as a result of the reduction with NaBH4. They
[Pd–Pd–Pd angles of 155(1)◦ and 169(1)◦] (Supplementary Fig. 4) are distributed within the channels, further stabilizing the whole net
growing tetranuclear NCs. The inner Pd–Pd bond length values (Supplementary Fig. 5).
[2.57(3)/2.44(3) Å] are significantly shorter than the relatively
The Pd2+-based guest molecules in 3 are efficiently organized
long outer ones [3.16(2)/2.91(3) Å] although both falling in the within the MOF pore, where they can react, aggregating in ordered
range of Pd–Pd bonding interactions10,35,36,38. This asymmetry dimers. This suggests that the crystalline porous host aligns the
of bond lengths within the [Pd4]2+ moiety is a striking feature, molecules within its cavities through a high degree of molecular
because despite the similarity in the shape of the Pd4 cluster, such recognition. While in 3, [PdII(µ-O)(NH3)6]2+ dimers interact with
2
a difference was not observed in the two previously reported the net through –NH3 ··· O hydrogen bonds (Supplementary
ligand-stabilized tetrapalladium compounds, where Pd–Pd Fig. 6), in 4, tetranuclear [Pd4]2+ clusters are most likely stabilized
separation varies in the range 2.538(1)–2.563(1) Å (ref. 35) and either by the network, through weak supramolecular interactions
2.571(1)–2.622(1) Å (ref. 36).
between Pd and oxamate moieties, or through a probable hydrated
The possibility to have disordered [Pd2] instead of [Pd4] clus- primary coordination sphere around Pd atoms, hydrogen-bonded to
ters might be consistent and it has been considered. However, the net (Figs 1f and 2 and Supplementary Fig. 4). Despite the typical
diverse factors determine that Pd4 clusters are, by far, the most structural uncertainty related to the difficulty to determine the po-
probable resolution (see structural details section in the Supple- sitions of solvent molecules in porous materials, the most plausible
mentary Information). To support the nature of these clusters, [Pd4]2+ cluster structure is solvated and stabilized by a synergic
theoretical calculations were carried out using a simplified model effect between the network and the solvent molecules surrounding
2−
of a Pd4{Ni4[Cu2(Me3mpba)2]3} unit (Fig. 3 and Supplementary clusters. The large accessible free voids (56% of the total cell volume)
Fig. 7). When Pd atoms are allowed to move without restrictions allow the guest to become more than an innocent bystander. Such a
within the octagonal ring model, they approach the donor oxygen vastly solvated nano-confined space stabilizes catalytic NCs without
atoms of the wall forming bonds with typical Pd–O distances of protective groups while retaining access to reactant species.
2.30/2.37 Å, and optimized Pd–Pd distances of 2.69 and 2.65 Å
PXRD patterns of 3 and 4 (Supplementary Fig. 8) confirm the
for the outer and inner bonds, respectively (Fig. 3a). Despite this crystallinity and pureness of the bulk samples with no trace of the
result apparently contradicting the quasi-linear arrangement of the typical peaks of Pd NPs, not even after catalysis.
[Pd4]2+ moiety observed by XRD, averaging of the two equivalent
FTIR spectroscopy of adsorbed CO (ref. 39) is able to
structures obtained from optimization yields a slightly bent, Pd4 unit differentiate between supported Pd0, Pd+ and Pd2+. The spectrum
with longer outer Pd–Pd distances (Fig. 3b,c), in agreement with of 4 (Supplementary Fig. 10a) shows absorption bands at 2,140 cm−1
experiment (Fig. 3d) where the positions of Pd atoms were refined and 1,882 cm−1, which correspond to linearly adsorbed CO to Pd+
with considerably large thermal motion. Analysis of charge distribu- and Pd0, respectively, together with some weak bands assignable
tion reveals a mixed-valence state of the type Pd+Pd0Pd0Pd+, with to Pd2+ metal cations39–41. For the sake of comparison, the FTIR
a calculated net atomic charge of 0.34 and 0.18 e on the outer and spectrum of adsorbed CO on the mono-directional zeolite LH after
inner Pd atoms, respectively.
impregnation and reduction of Pd2+ was recorded (Supplementary
Figure 1f suggests a tentative formation mechanism of the Pd4 Fig. 10b), showing that the mono-directional zeolite is able to
NCs by proposing that the [PdII(µ-O)(NH3)6]2+ complexes act as stabilize Pd+ to a much lesser extent.
2
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deconvolution of the Pd3d line, shows the typical peak for Pd0 at
a binding energy of BE(Pd3d5/2) = 335.7 eV together with a non-
ordinary signal at BE(Pd3d5/2) =337.0 eV, an energy between that of
Pd0 and the residual signals of Pd2+ at BE(Pd3d5/2) = 338.7 eV, and
with a 1:1 ratio with respect to Pd0, which, even if distinguishing
between Pd0 and Pd+ is challenging, can be attributed to Pd+, as
previously reported42.
Table 1 | Catalytic activity and structures of Pd complexes.
Catalyst
Catalyst
(1 mol%)
(5 mol%)
+ N2
COOEt
COOEt
O
CHCl3
Reflux, 2 h
O
6a (0.02 M) Reflux, 2 h
7a
N2
5a
8 (0.1 M)
9
All these results, together, further support that both Pd+ and
Pd0 can be found in 4 and that the Pd+Pd0Pd0Pd+ arrangement
predicted by theoretical calculations is the most probable electronic
state of the cluster.
2+
OH
Ph
P
Ph
Pd
N
L
N
L
Cl
Cl
Cl
N
Pd
Pd N
Pd·Pd
Fe
Pd
Pd
2
P
Ph
Cl
Ph
N
Aberration-corrected HR-TEM images of 4 (Supplementary
Figs 12a, 13 and 14) reveal no aggregation of the Pd4 NCs and their
uniform distribution throughout the highly crystalline framework.
Analysis of these images also directly visualizes the lattice fringes of
the net, where the interplanar spacing is 0.248 nm (Supplementary
Fig. 12a), corresponding to (110) planes of the MOF, and confirms
its retained crystallinity. Both SEM and HAADF-STEM imaging
and their corresponding EDX elemental mappings for Cu, Ni, Pd
and Na elements (Supplementary Figs 12b,d and 15–17) confirm
a homogeneous spatial distribution of the Pd4 NCs through the
MOF crystal.
OH
2BArf2−
10a
10b
10c
10d
OMe
OMe
PhosS Pd1 Pd1 SPhos 10e
tBuP Pd1 Pd1 PtBu 10f
iPr
SPhos =
P
L =
iPr
Entry
Catalyst
Yield 7a (%)
Yield 9 (%)
1
2
3
4
5
6
7
Pd(OAc)2
PdCl2
9
15
<5
7
<5
11
<1
<1
<1
<1
<1
<1
4
PdCl2(NH3)4 ·H2O
Pd(OAc)2phen
PdCl2(PhCN)2
PdCl2(PPh3)2
Pd2dba3
Pd4-MOF catalyses carbene-transfer reactions
Table 1 shows the catalytic results of different Pd compounds
for the Buchner reaction of benzene derivatives to give naturally
occurring and synthetically useful cycloheptatrienes28. This unique
transformation is catalysed only by dirhodium salts29 or particular
copper complexes30. Here, from the different Pd compounds tested
that include salts, complexes, pure metallic forms, and different
Pd supported species (entries 1–26), only 3 and 4 (entries 27–28)
catalyse efficiently the intermolecular Buchner reaction of benzene
5a and ethyl diazoacetate 6a (Table 1 left) and the intramolecular
reaction of 8 (Table 1 right).
17
8
9
10
11
Nájera’s complex 10a
Ferrocene–Pd2+10b
Pd–NHC complex 10c
Pd0 black
<5
<5
<5
9
<1
<1
<1
<1
<1
<1
<1
<1
<1
1
<1
<1
<1
<1
<1
3
4
1
2
17
57
<1
<1
<1
39 (71)
83 (99)
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Pd2+–C (5 wt%)
Pd0–C (5 wt%)
Pd0–Al2O3
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
14
34
18
26
68
75
Pd2+–MgO
Kinetic analysis showed that the green solid 3 gives an induction
time of ∼30 min after which the intermolecular reaction starts
and proceeds (Supplementary Fig. 19), and in situ magic angle
spinning–solid nuclear magnetic resonance (MAS–NMR) of a
sample of 3 treated with an equimolecular amount respect to Pd of
isotopically labelled 6a N123CH–COOEt (refs 43,44), in gas phase,
shows the immediate transformation of 6a to the corresponding
dimer (Supplementary Fig. 20) with the recovered solid active for
the Buchner reaction without induction period. These results point
to diazoacetate 6a as a reducing agent of Pd2+ in 3, to give 4 under
reaction conditions, as NaBH4 does externally. The solid precursors
1 and 2 do not show any catalytic activity, which confirms that the
catalytic activity comes from [Pd4]2+ and not from any defects on
the MOF structure.
Pd0–MgO
Pd2+–HY
Pd2+–NaY
[Pd(NH3)4]2+–NaY
Pd0–NaY
[Pd(NH3)4]2+–ZSM5
Pd0–HZSM5
Pd2+–KL
Pd0–KL
Pd2+–Mordenite
Pd0–Mordenite
3
4
Zeolites able to stabilize Pd+ cluster species within their anionic
zeolite framework, such as KL and mordenite, give significant
yields of 7a (entries 23–26), which supports the active role of
Pd+ in the reaction. In contrast, ligand-stabilized complexes such
as a perylene-stabilized linear [Pd4]2+ complex (10d; refs 35,45)
and two commercially available phosphine-stabilized Pd+ dimer
complexes, 10e (ref. 46) and 10f (ref. 47), were prepared and
evaluated catalytically (entries 29–31). In all cases, product 7a
was not observed, which can be easily understood taking into
account that the labile perylene ligand (10d) is also a potential
reactant in the Buchner reaction and that 10d–f are temperature-
sensitive compounds.
Perylene–Pd24+10d
Barder’s complex 10e
(tBu3PPdI)210f
4 (0.5 mol%)
Rh2(OAc)4 (0.5 mol%)
<5
<5
<5
53 (91)
52 (71)
∗
32
33
Reaction conditions: 6 (15 µl, 0.14 mmol, inter-) or 8 (100 mg, 0.60 mmol, intra-) in the
corresponding solvent (6 ml) at reflux temperature with the Pd catalyst (5 and 1 mol%,
respectively) for 2 h. Under the indicated reaction conditions, conversion of the diazoacetate is
always 100%, either to the desired products, alkene dimers, protonation to the acetate, and/or
unidentified decomposition products. Reaction yields were triple-checked by in situ 1H–NMR
analysis in deuterated solvents after calibration with the pure products, GC–MS analysis after
derivatization to the more stable conjugated isomers with CF3COOH or Et3N and calibration
with the pure products49, and isolated yields. The given yields refer to the average with a ±5%
error. ∗6 h reaction time; pump addition of the diazocompound in brackets.
The catalytic amount of 4 could be lowered to 0.5 mol% and
reused up to 20 times without a significant depletion of its
The XPS spectrum of compound 3 is shown in Supplementary inherent catalytic activity for the intermolecular reaction (entry 32,
Fig. 11a. The Pd3d line consists of only one doublet with binding Supplementary Fig. 21). The process could also be run in flow
energy (BE) of Pd3d5/2 peak equal to 338.6 eV, corresponding to (Supplementary Fig. 22) by adding 6a with a syringe pump to a fixed
Pd(II) (ref. 42). In turn, the XPS of 4 (Supplementary Fig. 11b), after amount of catalyst 4 under recirculating benzene 5a. In this way,
4
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Table 2 | Catalytic activity and substrate scope.
R1
R1
O
4 (0.5 mol%)
80 °C, 8 h
+
N2
COOR3
OR3
R2
R2
5a−g
6a−c
7a−l
(syringe pump)
R1
R2
R3
Product
Yield (%)
R1
R2
R3
Product
Yield (%)
H
H
H
Me
H
H
Et
Et
Et
Et
7a
7b
7c
7d
7e
7f
91
OMe
OMe
H
Me
OMe
F
H
H
H
H
H
H
Et
7g
7h
7i
7j
7k
7l
92
46
73
70
95
65
Me
Me
F
CF3
H
57
64
78
48
32
tBu
Bn
Bn
Bn
Bn
Et
H
tBu
In R4OH
11a−b
R4
O
COOR3
12a−f
Catalyst
N2
COOR3
80 °C
R3OOC
COOR3
13a−d
6a−c (0.5 M)
In toluene 5b
Time (min)
R3
R4
Catalyst (mol%)
Product
Yield (%)
E/Z
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
-
Me
-
Me
-
Me
-
Pd2dba3 (0.5)
10
12a
13a
12a
13a
12a
13a
12a
13a
12a
72
10
59
15
53
21
-
40/60
-
47/53
-
52/48
-
50/50
-
- (TON = 49,500)
- (TON = 65,000)
50/50
50/50
50/50 (TON = 93,000)
10f (0.5)
Pd0-KL (0.5)
Pd0-Mor (0.5)
44
6
Me
4(0.5)
98
99
65
95
96
93
74
88
96
85
99
78
97
98
4(0.005)
4(0.001)
4(0.5)
4(0.005)
4(0.001)
4(0.005)
120
Me
-
10
120
13a
Me
tBu
Bn
Bn
8
tAmyl
Me
12b
12c
12d
12e
12f
13b
13c
13d
Me
tAmyl
CD3OD
4(0.5)
4(0.005)
-
tBu
Bn
8
-
-
-
50/50
65/35
50/50
4(0.05)
Top, scope for the Pd4-catalysed intermolecular Buchner reaction. Bottom, catalyst comparison and substrate scope for other Pd4-catalysed carbene-mediated reactions of diazoacetates. Under the
indicated reaction conditions, conversion of the diazoacetate is always 100%, either to the desired products, protonation to the acetate, and/or unidentified decomposition products.
a solution of ∼1 g of 7a in 100 ml of benzene (∼0.1 M) could be 4-methyl substituted products in 7b changes from ∼1:1:2.5 with
obtained, which not only saves fivefold amounts of benzene but also Rh2(OAc)4 or ∼1:1:6 with Rh2(OAc3-F)4 (ref. 29) to 0:1:1 with 4,
enables a safer handling of 6a for industrial applications30,48. Besides, indicating that the MOF structure imposes shape selectivity effects
the recovered catalyst 4 completely retained its intrinsic catalytic during the catalytic process. Indeed, the intramolecular Buchner
activity after working in flow mode for 24 h (Supplementary Fig. 23). reaction of the bulkier diazoacetate 8 (ref. 49) proceeds in 57% yield,
A turnover number (TON) > 2,000 was obtained for 4 either in a significantly lower value than the intermolecular reaction, since
batch or in flow, a value one order of magnitude higher than the the channel of 4 probably restricts sterically the transition state. Note
state-of-the-art catalyst Rh2(OAc)4 (TON = 104, entry 33), which that no conversion of 8 was found with KL and mordenite with
rapidly decomposes to inactive species after the first use29.
smaller ∼7 Å2 channels.
Table 2 shows that 4 catalyses the intermolecular Buchner
The reaction rate for the intermolecular Buchner reaction of
reaction of different arenes and diazoacetates in yields compara- 5a (Supplementary Fig. 24) follows a first-order kinetics with
ble to Rh2(OAc)4 (ref. 29). In contrast to the soluble catalyst, the respect to both reactants and catalyst to give the equation
isomeric distribution obtained with 4 for products 7a–l shifts sig- rate v0 = k[cat][5a][6a] (k = observable kinetic constant) at
nificantly towards less hindered products (Supplementary Table 3). low concentrations of 6a. The participation of a Pd carbenoid
For instance, the isomeric ratio at full conversion for the 2-, 3- and in the rate-determining step of the reaction is supported by:
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NATURE MATERIALS DOI: 10.1038/NMAT4910
ARTICLES
ROOC
ROOC
ROOC
COOR
N
N
N
N
COOR
2+
+
2+
Pd4
Pd4
−N2
Pd4
6π disrotatory
MOF
MOF
MOF
Cyclopropanation
Electrocyclic
ring opening
Pd carbene formation
Figure 4 | Proposed mechanism for Pd4-catalysed intermolecular Buchner reaction.
the similar rates found for N2 liberation and formation of 4. Wang, N. et al. In situ confinement of ultrasmall Pd clusters within nanosized
cycloheptatriene 7a; values for 1H= and 1S= of 19.0(8) kcal mol−1
silicalite-1 zeolite for highly efficient catalysis of hydrogen generation. J. Am.
and −9.9(5) kcal mol−1 K−1, respectively, obtained by an Eyring
Chem. Soc. 138, 7484–7487 (2016).
5. Yamamoto, K. et al. Size-specific catalytic activity of platinum clusters
plot of the intermolecular reaction and which agree with previous
enhances oxygen reduction reactions. Nat. Chem. 1, 397–402 (2009).
reports on associative transition states during this reaction50;
6. Boronat, M., Leyva-Pérez, A. & Corma, A. Theoretical and experimental
and the structure of the conjugated cycloheptatrienes obtained
here can only come, in principle, from the classically accepted
cyclopropanation/electrocyclic ring opening route, since other
potential ways of coupling/opening should lead to different
products. Thus, a plausible mechanism for the Pd4-catalysed
Buchner reaction can be proposed (Fig. 4). The reaction would
start with the formation of a Pd carbenoid, which enables the
cyclopropanation of the arene and the 6π disrotatory electrocyclic
ring opening of the so-formed norcaradiene, to give the final
cycloheptatriene product.
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attempts to predict reactivity with clusters and nanoparticles of gold.
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Finally, Table 2 also compares the catalytic activity of different
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yield of products 12a–f and 13a–d with TONs approaching 100,000
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Pd catalysts but also the state-of-the-art metal catalysts for these
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Conclusions
The confinement and protection provided by the MOF together
with an exquisite control of the stoichiometry of the Pd2+ cations
has allowed the multigram-scale synthesis of linear Pd04/1+ clusters,
homogeneously distributed and stabilized along the MOF channels
and fully characterized with X-ray crystallography. These ultrasmall
clusters catalyse very efficiently representative carbene-mediated
reactions such as the inter- and intramolecular Buchner reaction,
the O–H insertion and the dimerization of diazocompounds, with
TONs at least one order of magnitude higher than the state-of-
the-art most active catalysts. The solid Pd04/1+–MOF catalyst is very
stable under reaction conditions, recoverable and reusable, and can
work in a batch or in flow with no observable loss of activity with
time. These results bring Pd to the selected group of metals that
catalyse carbene-mediated reactions.
18. Liu, H. et al. Controllable encapsulation of ‘clean’ metal clusters within MOFs
through kinetic modulation: towards advanced heterogeneous nanocatalysts.
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19. Liu, L. et al. Size-confined growth of atom-precise nanoclusters in
metal–organic frameworks and their catalytic applications. Nanoscale 8,
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20. Yang, Q., Xu, Q., Yu, S.-H. & Jiang, H.-L. Pd Nanocubes@ZIF-8: integration of
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22. Wu, X.-F., Anbarasan, P., Neumann, H. & Beller, M. From noble metal to nobel
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and their subsequent hydrolysis. Angew. Chem. Int. Ed. 41, 2363–2366 (2002).
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the carbenoid reactivity of sp 3 - g em -Organodimetallic
Methods
Methods, including statements of data availability and any
associated accession codes and references, are available in the
online version of this paper.
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Received 30 November 2016; accepted 20 April 2017;
published online 12 June 2017
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Acknowledgements
This work was supported by the MINECO (Spain) (Projects CTQ2013-46362-P,
CTQ2014-56312-P and Excellence Units ‘Severo Ochoa’ and ‘Maria de Maeztu’
SEV-2012-0267 and MDM-2015-0538), the Generalitat Valenciana (Spain) (Project
PROMETEOII/2014/070), the Ministero dell’Istruzione, dell’Università e della Ricerca
(Italy) and the Junta de Andalucía (FQM-195 and P11-FQM-7756). M.M. thanks the
MINECO for a predoctoral contract. Thanks are also extended to the Ramón y Cajal
Program and the ‘Convocatoria 2015 de Ayudas Fundación BBVA a Investigadores y
Creadores Culturales’ (E.P., A.L.-P. and J.F.-S.). J.G. acknowledge the financial support of
the European Research Council under the European Union’s Seventh Framework
Programme (FP/2007-2013)/ERC Grant Agreement no. 335746, CrystEng-MOF-MMM.
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palladium-palladium bonds. Coord. Chem. Rev. 231, 207–228 (2002).
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Author contributions
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work; F.R.F.-P. and M.M. performed synthetic work; D.A. performed powder and
single-crystal XRD characterization and analysed data; A.L.-P. carried out the catalytic
experiments; J.G. and D.O. performed spectroscopic characterization and analysed data;
J.M.H. performed microscopy measurements; M.B. carried out the theoretical
calculations; E.P., A.L.-P., A.C., D.A., J.F.-S., M.B. and J.G. wrote and revised the paper.
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Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints. Publisher’s note:
Springer Nature remains neutral with regard to jurisdictional claims in published maps
and institutional affiliations. Correspondence and requests for materials should be
addressed to A.L.-P., A.C., D.A. or E.P.
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Competing financial interests
The authors declare no competing financial interests.
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NATURE MATERIALS DOI: 10.1038/NMAT4910
ARTICLES
Methods
been carried out in a home-made IR cell able to work in the high- and low- (77 K)
temperature range. Further experimental details can be found in the
Supplementary Methods.
Materials. All chemicals were of reagent-grade quality. They were purchased from
commercial sources and used as received. Crystals of
MgII{MgII[CuII(Me3mpba)2]3}· 45H2O (1), NiII{NiII[CuII(Me3mpba)2]3}· 54H2O
4
2
2
(2) 2and [Pd(N2H3)4]Cl2 were prepared as previously 4reported (see Supplementary
Microscopy. High-resolution transmission electron microscopy (HR-TEM),
high-angle annular dark-field scanning transmission electron microscopy
(HAADF-STEM) and energy-dispersive X-ray analysis (EDX) characterizations
were done using a HAADF-FEI-TITAN G2 electron microscope. Five micrograms
of the material was re-dispersed in 1 ml of absolute ethanol. Carbon-reinforced
copper grids (200 mesh) were submerged into the suspension 30 times and then
allowed to dry in air for 24 h.
Methods). Alternatively, a large-scale synthesis of NiII{NiII[CuII(Me3mpba)2]3}·
2
4
2
54H2O (2) was carried out by direct reaction of two aqueous solutions (250 ml
each) of Na4[Cu2(Me3mpba)2] · 4H2O (8.71 g, 0.010 mol) and Ni(NO3)2 ·6H2O
(3.88 g, 0.013 mol) and subsequent addition, after filtration and re-suspension in
150 ml water of the resulting compound, of 1.94 g (0.0067 mol) of Ni(NO3)2 ·6H2O
(yield 99%).
Scanning electron microscopy coupled with energy-dispersive X-ray
(SEM/EDX) was carried out with an XL 30 ESEM (PHILIPS) microscope equipped
with a home-made EDX detector.
Syntheses. For [PdII(NH3)4][PdII(µ-O)(NH3)6(NH4)2]0.5{NiII[CuII(Me3mpba)2]3}·
2
2
52H2O (3), well-formed deep green prisms of 3, which were4suitable for XRD, were
obtained by immersing crystals of 2 (about 5 mg, 0.0015 mmol) for 48 h in 5 ml of a
[Pd(NH3)4]Cl2 aqueous solution (0.004 mmol). Aiming at industrial applications, a
multigram-scale procedure was also carried out by using the same synthetic
procedure but with greater amounts of both, a powder sample of compound 2 (20 g,
5.8 mmol) and [Pd(NH3)4]Cl2 (3.43 g, 14.0 mmol), with the same successful results
and a very high yield (20.33 g, 96%). Anal.: calcd (%) for Cu6Ni4Pd2C78H189N20O88.5
(3,652.3): C, 25.65; H, 5.22; N, 7.67. Found: C, 25.68; H, 5.13; N, 7.33. IR (KBr):
ν =3,014, 2,951 and 2,913 cm−1 (C–H), 1,607 cm−1 (C=O).
X-ray photoelectron spectroscopy. Samples were prepared by dropping a solid
water suspension onto a molybdenum plate followed by air drying, and then
measurements were performed on a SPECS spectrometer equipped with a Phoibos
150 MCD-9 analyser using a non-monochromatic Mg KR (1,253.6 eV) X-ray
source working at 50 W. As an internal reference for the peak positions in the XPS
spectra, the C1s peak has been set at 284.5 eV.
For [Pd4]0.5@Na3{NiII[CuII(Me3mpba)2]3}· 56H2O (4), both, crystals (about
Magic angle spinning–solid-state NMR. Fifty micrograms of catalyst 3
was introduced into glass inserts and dehydrated at 80◦C for 18 h. Then,
4.7 µmol of isotopically labelled 6a N213CH–COOEt, corresponding to
Pd/6a = 1, was introduced onto the dehydrated material. The glass inserts
were sealed while they were immersed in liquid nitrogen. In situ 13C solid-state
NMR spectra were recorded at room temperature with a Bruker AVIII HD
400 WB spectrometer. The glass inserts were fitted into 7 mm rotors and
were spun at 5 kHz in a Bruker BL7 probe. 13C CP/MAS NMR spectra were
recorded with proton decoupling, with a 1H 90◦ pulse length of 5 µs, and a
recycle delay of 3 s.
4
2
5 mg) and a powder polycrystalline sample of 3 (about 10 g), were suspended in
50 ml of a H2O/CH3OH (1:2) solution to which an excess of NaBH4, divided into
26 fractions (each fraction consisting of 1 mole of NaBH4 per mole of 3 to give a
final NaBH4/MOF molar ratio of 26 or, which is the same, NaBH4/Pd atom
molar ratio of 13), was added progressively in the space of 72 h. After each
addition, the mixture was allowed to react for 1.5 h. After this period, samples were
gently washed with a H2O/CH3OH solution and filtered on paper giving excellent
yields (98%). Anal.: calcd (%) for Cu6Ni4Pd2Na3C78H172N12O92 (3,648.1): C, 25.68;
H, 4.75; N, 4.61. Found: C, 25.68; H, 4.63; N, 4.65. IR (KBr): ν =3,013, 2,964 and
2,914 cm−1 (C–H), 1,603 cm−1 (C=O).
Computational details. All calculations are based on density functional theory
and were carried out with the Gaussian 09 program package, using the hybrid
B3PW91 functional, the LANL2DZ basis set for Pd, Cu and Ni atoms, and the
standard 6-311G(d,p) basis set by Pople for O, N, C and H atoms. Atomic
charges were calculated using the natural bond order (NBO) approach.
A detailed description of models and methods is given in the Supplementary
Information.
Single-crystal X-ray diffraction. Crystal data for 3 and 4: tetragonal, space group
P4/mmm, T =90(2) K, Z =4; 3: C78H189Cu6N20Ni4O88.5Pd2, a=35.920(2) Å,
c =15.3561(9) Å, V =19,813(2) Å3, ρcalc =1.224 g cm−3, µ=1.259 mm−1
4: C78H172Cu6N12Na3Ni4O92Pd2, a=35.794(13) Å, c =15.063(5) Å,
;
V =19,298(16) Å3, ρcalc =1.256 g cm−3, µ=1.299 mm−1. Further details can be
found in the Supplementary Information.
X-ray powder diffraction. PXRD patterns of solid polycrystalline samples 3 and 4,
before and after catalysis, were obtained with a Empyrean PANalytical powder
diffractometer, using Cu Kα radiation (λ=1.54056 Å) at room temperature
(2θ =2◦–60◦).
Catalytic experiments. A detailed description of all the catalytic experiments is
given in the Supplementary Information.
Data availability. All data supporting the findings of this work are available from
the corresponding authors on request. Crystallographic data for the structure
reported in this paper have been deposited in the Cambridge Structural Database
with CCDC numbers 1517224-1517225.
FTIR spectroscopy of adsorbed CO. The spectra were recorded on a Biorad
FTS-40A spectrometer equipped with a DTGS detector. The experiments have
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