Silylene-Bridged Tetranuclear Palladium Cluster as a Catalyst for Hydrogenation of Alkenes and Alkynes

Article abstract of DOI:10.1002/cctc.202001387

A planar tetranuclear palladium cluster was obtained from the reaction of a cyclotetrasilane with [Pd(CN^tBu)₂]₃. Single-crystal X-ray diffraction analysis and DFT calculations revealed that the tetranuclear framework of the cluster was supported effectively by the bridging organosilylene ligand. Although [Pd(CN^tBu)₂]₃ as well as mononuclear palladium bis(silyl) complex, cis-Pd(SiMePh₂)₂(CN^tBu)₂, do not act as the effective catalyst, the planar tetranuclear palladium cluster acts as an efficient catalyst for the hydrogenation of alkenes and alkynes including sterically hindered tri- and tetra-substituted alkenes.

Full text of DOI:10.1002/cctc.202001387

The European Society Journal for Catalysis
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Accepted Article
Title: Silylene-Bridged Tetranuclear Palladium Cluster as a Catalyst for
Hydrogenation of Alkenes and Alkynes
Authors: Chikako Yanagisawa, Seiji Yamazoe, and Yusuke Sunada
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01/2020

10.1002/cctc.202001387
ChemCatChem
COMMUNICATION
Silylene-Bridged Tetranuclear Palladium Cluster as a Catalyst for
Hydrogenation of Alkenes and Alkynes
Chikako Yanagisawa,[b] Seiji Yamazoe,[c] and Yusuke Sunada*[a,b]
[a]
Prof. Dr. Y. Sunada
Institute of Industrial Science
The University of Tokyo
4-6-1 Meguro-ku, Komaba, Tokyo (Japan)
E-mail: sunada@iis.u-tokyo.ac.jp
C. Yanagisawa, Prof. Dr. Y. Sunada
Department of Applied Chemistry, School of Engineering
The University of Tokyo
[b]
[c]
4-6-1 Meguro-ku, Komaba, Tokyo (Japan)
Prof. Dr. S. Yamazoe
Department of Chemistry, Graduate School of Science
Tokyo Metropolitan University
1-1 Minami-Osawa, Hachioji-shi, Tokyo, 192-0397 (Japan)
Supporting information for this article is given via a link at the end of the document.
Abstract: A planar tetranuclear palladium cluster was obtained from
the reaction of a cyclotetrasilane with [Pd(CNtBu)2]3. Single-crystal X-
ray diffraction analysis and DFT calculation revealed that the
tetranuclear framework of the cluster was supported effectively by the
bridging organosilylene ligand. Although [Pd(CNtBu)2]3 as well as
mononuclear palladium bis(silyl) complex, cis-Pd(SiMePh2)2(CNtBu)2,
do not act as the effective catalyst, the planar tetranuclear palladium
cluster acts as an efficient catalyst for the hydrogenation of alkenes
and alkynes including sterically hindered tri- and tetra-substituted
alkenes.
Pd(IPr)[P(cyclohexyl)3](H)2, was successfully isolated from the
reaction of Pd(IPr)[P(cyclohexyl)3] with molecular dihydrogen.[7]
In light of the high catalytic activity of palladium nanoparticles,
the construction of molecular palladium clusters would seem to
represent an effective way to synthesize active catalysts. Indeed,
palladium clusters of the type [Pd5(PPh)2]n have shown catalytic
performance in the hydrogenation of various alkenes and alkynes,
although experimental data regarding their structure has not yet
been reported.[8a-c] Harvey et al. reported that the tetranuclear
palladium
cluster
[Pd4(dppm)4(H)2]2+
(dppm
=
bis(diphenylphosphino)methane) acts as a precatalyst in the
hydrogenation of alkynes. However, the true catalytically active
species in this reaction was reported to be dinuclear.[8a,b,d] Thus,
the development of well-defined molecular palladium clusters with
high catalytic activity in the hydrogenation of alkenes remains to
be explored.
We hypothesized that the synthesis of planar molecular
palladium clusters with a rigid cluster framework could be a
straightforward strategy to construct highly active catalysts. We
have recently focused on the use of organosilicon compounds
with multiple Si–Si bonds as templates to construct transition-
metal clusters with strongly bound organosilicon ligands.[9] For
instance, we found that the planar tetranuclear palladium cluster
3a can be synthesized in one step via the reaction of isopropyl-
substituted cyclotetrasilane (iPr2Si)4 (1a) with a palladium(0)
isocyanide precursor.[9c] In the present paper, we report the facile
synthesis of the analogous cluster 3b by judiciously changing the
substituents on the silicon atoms. The obtained planar cluster 3b,
as well as 3a, showed good catalytic performance in the
hydrogenation of alkenes and alkynes.
Molecular transition-metal clusters have recently attracted great
attention in organometallic and inorganic chemistry,[1] because
they can serve as structural and functional models of catalytically
active nanosized compounds such as metal nanoparticles.[2]
Molecular metal clusters that consist of multiple metal atoms in a
planar arrangement are expected to most closely mimic the
structure of the active sites of heterogeneous catalysts. In fact,
recent studies have revealed that some planar molecular metal
clusters can effectively catalyze cross-coupling,[3a-d] C–C bond
formation,[3e,f] and reduction reactions[3g-j]
.
The catalytic hydrogenation of alkenes is one of the most
fundamental and useful organic transformations that can be
mediated by transition-metal catalysts.[4] The catalytic
hydrogenation of alkenes mediated by molecular-based catalysts
is mainly achieved using rhodium and iridium complexes as
catalysts, while in heterogeneous catalysis, palladium catalysts,
such as palladium nanoparticles embedded on carbon (Pd/C),
have been among the most widely used catalysts. However,
typical palladium complexes have been reported to be less
reactive, and examples of highly active palladium complex
catalysts are quite limited.[5] One exception has been reported by
Cazin et al.,[6] who discovered that the mononuclear palladium(0)
As described in our previous paper, the tetranuclear cluster
3a can be obtained from the reaction of 1a with palladium(0)
complex 2 in toluene at 65 °C. Because the Si–Si bonds in 1a are
i
sterically protected by the bulky Pr substituents, heating was
required in this reaction. We hypothesized that a cyclotetrasilane
with sterically less hindered substituents would allow the reaction
to proceed under milder conditions. Based on this hypothesis, the
cyclopentyl group was chosen as the substituent on the silicon
atom to provide a sterically less encumbered environment.[10]
complex Pd(SIPr)[P(cyclohexyl)3] (SIPr
diisopropylphenyl)imidazolidine) efficiently
=
N,Nʹ-(bis(2,6-
promotes the
hydrogenation of various alkenes. In addition, they have reported
that the mononuclear dihydride complex having IPr (IPr = N,N’-
bis-(2,6-(diisopropyl)phenyl)imidazol-2-ylidene) ligand, trans-
1
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10.1002/cctc.202001387
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COMMUNICATION
Thus, 1b was synthesized as shown in Scheme 1 and isolated in
78 % yield. The molecular structure of 1b, as well as that of 1a,
shows that the steric environment around the Si–Si bonds is
slightly different, as can be seen from the space-filling models of
1a and 1b (for details, see Figure S7 in the Supporting
Information).
this reaction proceeded effectively at room temperature to afford
the corresponding tetranuclear Pd cluster (3b) in 77% yield. The
molecular structure of 3b (for details, Supporting Information)
revealed that four Pd atoms are located on a plane. Similar
features were observed for the solid-state structure of 3a.[9c] In the
1H and 13C NMR spectra of 3b, a set of signals originating from
the cyclopentyl groups on the silicon atoms and a single signal
originating from the tBu group of the CNtBu ligand were observed
(for details, Supporting Information).[12]
To gain more insight into the bonding interactions in 3b, DFT
calculations were carried out using the PBE0, B3PW91, M06, and
B3LYP functionals. The optimized structural parameters obtained
using the PBE0 functional were in good agreement with the data
obtained from the XRD analysis (for details, see the Supporting
Information). The calculated Wiberg bond index (WBI) for the Pd–
Pd bonds ranged from 0.178 to 0.191, which supports the
presence of bonding interactions. The WBI of the Pd(4)–Si bonds
was ~0.6 (Pd(4)–Si(1) = 0.6312; Pd(4)–Si(2) = 0.6275; Pd(4)–
Si(3) = 0.6434), while the WBIs of the other Pd–Si bonds fell in
the range of 0.25–0.47. These parameters suggest that all four
palladium atoms are effectively supported by the three silylene
moieties. The LUMO of 3b contains an empty d-orbital component
of Pd(1), whereas the HOMO-4 consists of the d-orbital of the
central palladium atom (Pd(4)). The HOMO-14, HOMO-16,
HOMO-17, and HOMO-18 are located on the Pd–Si bonds,
suggesting that these bonds contribute to the stabilization of the
Pd4Si3 core (for selected molecular orbitals, see the Supporting
Information).
Then, we examined the catalytic performance of 2, 3a, and 3b
in the hydrogenation of alkenes and alkynes (Table 1). 3a and 3b
showed good catalytic activity in the hydrogenation of styrene in
toluene at room temperature under 1 atm of H2 with a catalyst
loading of 2 mol% (relative to Pd) (entries 1 and 2), while the
styrene conversion reached only 23 % when 2 was used as the
catalyst under otherwise identical conditions (entry 3). The
hydrogenation of styrene was also carried out in the presence of
Scheme 1. Synthesis of 1b.
Palladium(0) isocyanide complexes of the type Pd(CNR)2
reportedly to have planar trinuclear structures with three bridging
and three terminal isocyanide ligands in the solid state. However,
the details about the molecular structure of Pd(CNR)2 in the
solution state is unexplored yet. For instance, the molecular
structure of the planar triangular structure of [Pd(CN-
cyclohexyl)2]3 was unequivocally determined using single-crystal
XRD analysis.[11] The Pd–Pd, Pd–C (terminal isocyanide), and
Pd–C (bridging isocyanide) bond distances in this complex are
2.651(2), 2.004(14), and 2.072(13) Å, respectively. Although the
single-crystal XRD analysis of 2 was hampered by disorder in the
refinements, these previous results suggest that the solid-state
structure of 2 should be triangular. Subsequently, to obtain some
insight into the molecular structure of 2 in the solution state, 2 was
subjected to a Pd K-edge X-ray absorption spectroscopy (XAS)
analysis. The X-ray absorption near edge structure (XANES)
spectrum of a powder sample of 2 was essentially identical to that
of a toluene solution of 2, indicating that the solid-state structure
and electronic state were maintained in solution. A curve-fitting
analysis of the Pd K-edge Fourier transform extended X-ray
absorption fine structure (FT-EXAFS) spectrum revealed that
each of the palladium atoms is bound to adjacent palladium
atoms; the Pd–Pd and Pd–C (isocyanide) bond distances were
estimated to be ca. 2.66 Å and 2.04 Å, respectively (for details,
see the Supporting Information). These bond distances are in
good accordance with those of [Pd(CN-cyclohexyl)2]3,[11]
indicating that the planar triangular framework of 2 is maintained
in solution.
the
mononuclear
bis(silyl)
palladium
complex
cis-
Pd(SiMePh2)2(CNtBu)2 (4),[9d] albeit that styrene conversion was
not observed (entry 4). These results suggest that palladium
cluster 3 in which four palladium atoms were supported by
bridging silylene ligands showed good catalytic performance
toward hydrogenation of styrene. It should be noted that complex
3b showed good catalytic performance even at low catalyst
loadings. In a typical example, the hydrogenation of styrene
catalyzed by 0.02 mol% 3b proceeded smoothly at room
temperature under 1 atm of H2 to provide ethylbenzene in
quantitative yield with a TON of 5000 (entry 7).
Then the scope of the substrate with respect to different
alkenes was examined. The hydrogenation of alkenes with ester
groups proceeded effectively, whereby the ester group remained
intact (entry 9). 1,2-Disubstituted alkenes also undergo
hydrogenation under the same reaction conditions as terminal
alkenes (entries 11 and 12). It should be noted that the
hydrogenation of trans-stilbene catalyzed by 3b completed within
2h, in contrast, hydrogenation of cis-stilbene required prolonged
reaction time (18h, entry 13), and no conversion of cis-stilbene
was confirmed after 2 h (see the Supporting Information). Notably,
the hydrogenation of trans-stilbene mediated by 3b furnishes the
corresponding alkane quantitatively, while the reaction catalyzed
by 2 does not afford the product (entry 10).
Scheme 2. Synthesis of tetranuclear palladium clusters 3a and 3b. The
numbering of the palladium atoms is shown in the figure.
Subsequently, cyclotetrasilane 1b was treated with
i
[Pd(CNtBu)2]3 (2) in toluene. While the synthesis of the Pr-
analogue 3a required a higher reaction temperature (65 °C),[9c]
2
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In contrast, heterogeneous palladium catalysts such as Pd/C can
realize the hydrogenation of such substrates. In addition, Cazin’s
mononuclear palladium catalyst, (SIPr)PdPCy3 (SIPr = N,Nʹ-
(bis(2,6-diisopropylphenyl)imidazolidine), shows good catalytic
performance toward unfunctionalized tri- and tetra-substituted
alkenes. These results prompted us to test the performance of
cluster 3b in the hydrogenation of sterically hindered, largely
unfunctionalized alkenes. Although the conversion of 1-
methylcyclohexene was relatively low (entry 14), the
hydrogenation of tri-substituted alkenes such as trans--
methylstilbene (entry 15) and ethyl-3,3-dimethylacrylate (entry
16) proceeded effectively under mild reaction conditions to give
the corresponding alkanes quantitatively within 18 h using 2 mol%
of 3b. However, the catalytic performance of 3b in the
hydrogenation of tetra-substituted alkenes was insufficient
(entries 17 and 18). For instance, the hydrogenation of diethyl
isopropylidenemalonate proceeded only using a higher catalyst
loading (6 mol% of 3b) at room temperature under 1 atm of H2 to
give the corresponding alkane in 72% yield (entry 17). Notably,
the catalytic activity of 3b was still higher than that of 2 and
conventional Pd/C (for details, see the Supporting Information).
The hydrogenation of alkynes can also be achieved using 3b;
for example, 1-phenyl-1-propyne underwent hydrogenation to
quantitatively afford the corresponding alkane. Interestingly,
diphenylacetylene was semi-hydrogenated selectively to cis-
stilbene when the reaction was performed under 1 atm of H2 for 2
h. This is consistent with the result shown in entry 13 in Table 1
that the hydrogenation of cis-stilbene required 18 h to complete
the reaction. Complete hydrogenation occurred when the reaction
time was prolonged to 18 h, which furnished dibenzyl as the sole
product (Scheme 3).
Table 1. Catalytic hydrogenation of alkenes catalyzed by 2, 3a and 3b.[a]
cat. loading
(mol% for Pd)
time
(h)
Yield
(%)[b]
entry
cat.
alkene
1
2
3a
3b
2
2
2
2
2
styrene
styrene
styrene
styrene
styrene
styrene
styrene
1-octene
>99
>99
23
3
2
2
4
4
2
2
<1
5[c]
6[c]
7[d]
8
2
2
18
18
18
18
<1
3b
3b
3b
2
>99
>99
>99
0.02
2
9
3b
2
2
>99
10
11
2
2
2
2
2
<1
3b
>99
12
13
3b
3b
2
2
18
18
>99
>99
14
15
16
3b
3b
3b
2
2
2
18
18
18
62
>99
>99
17
18
3b
3b
6
2
18
18
72[e]
17[e]
Scheme 3. Hydrogenation of alkyens catalyzed by 3b.
[a] All reactions were carried out using 1 mmol of the alkene in the presence of
a catalytic amount of the Pd catalyst in toluene (2 mL). bThe product yield was
determined by GC in the presence of an internal standard (anisole), which was
used to determine the conversion of the starting material. cThe reaction was
carried out in the presence of thiophene (2 equiv. relative to Pd). d3 mmol of
alkene was used. eThe product yield was determined by 1H NMR spectroscopy
in the presence of an internal standard (anisole).
It is known that some palladium-catalyzed reactions are
mediated by in situ generated palladium nanoparticles or
colloids.[15] The addition of strongly donating ligands such as
thiophene is an effective method to distinguish whether a catalytic
reaction involves a molecular-based catalysts or heterogeneous
catalyst such as metal nanoparticles and colloids. If the catalyst
can be poisoned completely with < 1.0 eq. of ligand per metal
atom, it is most likely a heterogeneous catalyst.[16] For instance,
we confirmed that deactivation was observed in the reactions
mediated by the conventional heterogeneous catalyst Pd/C (see
entry 11 in Table S1-1 in the Supporting Information). In contrast,
the hydrogenation of styrene proceeded smoothly in the presence
of 2 mol% of 3b and thiophene (2 equiv. relative to Pd) (Table 1
The hydrogenation of sterically hindered, largely
unfunctionalized alkenes under mild reaction conditions remains
a challenge in modern organometallic chemistry.[13] Several
iridium-based complexes, such as Crabtree’s catalyst,
[(cod)IrPCy3(pyridine)]+ (cod
=
1,5-cyclooctadiene; Cy
=
cyclohexyl), are effective for this transformation.[14] However,
molecular-based complex catalysts based on elements other than
iridium exhibit relatively low catalytic performance in this reaction.
3
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COMMUNICATION
1915-1925; g) Y. Du, H. Sheng, A. Astruc, M. Zhu, Chem. Rev. 2020,
120, 526-622.
entry 6). These results suggest that the active catalytic species in
the 3b-mediated reaction seems to be homogeneous. Meanwhile,
the addition of thiophene (2 equiv. relative to Pd) also completely
deactivated 2 (entry 5). This result may indicate that the trinuclear
framework of 2 is not sufficiently supported under the applied
hydrogenation conditions.
In this study, we found that cluster 3b acts as an effective
catalyst for hydrogenation of various alkenes. In contrast, the
mononuclear palladium bis(silyl) complex 4 did not exhibit any
catalytic activity under the reaction condition shown in entry 4 in
Table 1. This result suggests that the tetranuclear cluster
framework is crucial to the catalytic activity. The tetranuclear
[2]
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framework in
3 is effectively supported by the bridging
organosilylene ligands, which leads to high catalytic performance.
The bridging organosilicon ligands may also play a key role in
activating the molecular hydrogen via -bond metathesis assisted
by the Pd–Si -bonds. The metal–silicon bond has previously
been reported to play an important role in the activation of
molecular hydrogen as well as the transfer of the activated
hydrogen atoms to the C=C bond.[17] One possible reaction
mechanism for the 3-catalyzed hydrogenation would involve the
coordination of molecular hydrogen to the palladium atom (Pd(1)
in Figure S6 and S10 in the Supporting Information) through the
LUMO orbital localized on the palladium atom. Activation of the
H–H bond through the adjacent Pd–Si bond could then occur,
followed by the hydrogenation of the alkene or alkyne. Efforts to
elucidate the details of the underlying reaction mechanism are
currently in progress in our laboratories.
In summary, we have synthesized the planar tetranuclear
palladium cluster 3, wherein the cluster skeleton is effectively
supported by the bridging organosilylene ligands, which enables
3 to act as the active catalyst for the hydrogenation of alkenes.
Compound 3 is an addition to the family of catalytically active
transition-metal clusters, which may bridge molecular-based
complex catalysts and heterogeneous catalysis such as
palladium nanoparticles. Efforts to expand the scope of
applications of such planar palladium clusters are currently in
progress.
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Acknowledgements
This work was supported by Grant in Aid for Scientific Research
(B) (No. 16H04120 and 20H02751) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and Yazaki
Memorial Foundation for Science and Technology. The
synchrotron radiation experiments were performed at the BL01B1
of SPring-8 with the approval of the Japan Synchrotron Radiation
Research Institute (JASRI) (Proposal No. 2018B1422).
[10] S. Nakanishi, M. Kawamura, H. Kai, R. -H. Jin, Y. Sunada, H. Nagashima,
Chem. Eur. J. 2014, 20, 5802-5814.
[11] C. G. Francis, S. I. Khan, P. R. Morton, Inorg. Chem. 1984, 23, 3680-
3681.
[12] As described in our previous paper, variable-temperature (VT) 1H NMR
spectra of 3a revealed that its tetranuclear framework is maintained in
solution.9c Unfortunately, the presence of complex multiplet signals for
the cyclopentyl groups prevented us from examining the dynamic
behavior of 3b using VT 1H NMR. Considering the structural similarity
between 3a and 3b, we thus assumed that the tetranuclear framework in
3b is also maintained in solution.
Keywords: palladium • cluster • catalyst • hydrogeneation •
silylene ligand
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10.1002/cctc.202001387
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[15] The addition of mercury, the so-called mercury test, is a well-established
test to identify heterogeneous catalysts such as metal nanoparticles and
colloids, which are poisoned via the formation of an amalgam or
adsorption on the metal surface. Therefore, we examined the 3b-
catalyzed reaction in the presence of an excess of mercury. We found
that the reaction occurred, albeit that the conversion reached only 21%
(entry 9 in Table S1-1 in the Supporting Information). At the first glance,
this result seems to suggest the involvement of palladium nanoparticles
in the 3b-catalyzed reaction. However, we discovered that this low
conversion can be rationalized by the decomposition of 3b in the
presence of metallic mercury,[18] which was confirmed by the crude 1H
NMR spectrum of the reaction of 3b with mercury (for details, see the
Supporting Information).
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Eremin, G. S. Ranny, V. N. Khrustalev, V. P. Ananikov. ACS Catal. 2019,
9, 2984-2995; b) O. N. Gorunova, M. V. Livantsov, Y. K. Grishin, M. M.
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5
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COMMUNICATION
Entry for the Table of Contents
A planar tetranuclear palladium cluster synthesized by the reaction of a cyclotetrasilane with [Pd(CNtBu)2]3 showed good catalytic
performance in the hydrogenation of alkenes and alkynes.
6
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