Transfer hydrogenation of alkenes using Ni/Ru/Pt/Au heteroquatermetallic nanoparticle catalysts: Sequential cooperation of multiple nano-metal species

Article abstract of DOI:10.1039/c4cc04559a

Quatermetallic alloy nanoparticles of Ni/Ru/Pt/Au were prepared and found to promote the catalytic transfer hydrogenation of non-activated alkenes bearing conjugating units (e.g., 4-phenyl-1-butene) with 2-propanol, where the composition metals, Ni, Ru, Pt, and Au, act cooperatively to provide significant catalytic ability. This journal is

Full text of DOI:10.1039/c4cc04559a

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Transfer hydrogenation of alkenes using
Ni/Ru/Pt/Au heteroquatermetallic nanoparticle
catalysts: sequential cooperation of multiple
nano-metal species†
Cite this: Chem. Commun., 2014,
50, 12123
Received 16th June 2014,
Accepted 15th August 2014
Yoshikazu Ito,a Hidetoshi Ohta,b Yoichi M. A. Yamada,b Toshiaki Enokia and
DOI: 10.1039/c4cc04559a
Yasuhiro Uozumi*abc
www.rsc.org/chemcomm
Quatermetallic alloy nanoparticles of Ni/Ru/Pt/Au were prepared
and found to promote the catalytic transfer hydrogenation of non-
activated alkenes bearing conjugating units (e.g., 4-phenyl-1-
butene) with 2-propanol, where the composition metals, Ni, Ru,
Pt, and Au, act cooperatively to provide significant catalytic ability.
Nanoparticles of transition metals have attracted increasing interest
because of their physical and chemical properties. In particular,
various transition metal nanoparticles have been found to exhibit
unique catalytic activity in organic molecular transformations.
Whereas the Enoki group has studied the physical properties of
multiple-metallic nanoparticles,1 the Uozumi group has concen-
trated on the catalytic utility of single-metallic nanoparticles.2
However, during our investigations, we became intrigued by the
catalytic utility of multiple-metallic nanoparticles. We thought that if
the nanoparticles of a multiple-metallic alloy showed catalytic
performance whereby the unique catalytic properties of the indivi-
dual composition metal species were sequentially combined, then
multiple-metallic nanoparticles, due to their limitless possible
combinations, should find a wide range of catalytic utilities. Given
the growing emphasis on green and safe chemical processes,
catalytic transfer hydrogenation is emerging as a viable alternative
to the conventional reduction processes that rely primarily on more
hazardous molecular hydrogen or metal hydride reagents.3 Herein
we report that the transfer hydrogenation of non-activated alkenes is
efficiently catalyzed by Ni/Ru/Pt/Au heteroquatermetallic alloy nano-
particles, which was not promoted by uni-, bi-, or trimetallic
nanoparticles of Ni, Ru, Pt, and/or Au.
Scheme 1 Preparation of quatermetallic alloy nanoparticles.
were prepared in a parallel combinatorial manner. Thus, a mixture
of NiCl2, RuCl3, H2PtCl6, and KAuCl4 was treated with an excess
amount of LiBEt3H in THF at 24 1C in the presence of TOPO for 2 h.
The reaction mixture was quenched with ethanol to give agglo-
merates of TOPO-capped quatermetallic nanoparticles [Ni/Ru/Pt/Au]nano
(Scheme 1).4 The agglomerates were collected by filtration,
rinsed with ethanol, and stored as an ethanolic suspension. A trans-
mission electron microscopic study confirmed their nanostructures
with an average diameter of 3.0 Æ 0.25 nm (Fig. 1). The ratio of Ni/Ru/
Pt/Au was determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) analysis to be 1.1/1.0/1.1/1.0. All
combinations of uni-, bi-, and trimetallic nanoparticles were pre-
pared under similar conditions with average diameters of ca. 3 nm.
Various uni-, bi-, tri-, and quatermetallic alloy nanoparticles
modified with a capping agent of trioctylphosphine oxide (TOPO)
a Department of Chemistry, The Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1 Ookayama, Tokyo 152-8551, Japan
b RIKEN Center for Sustainable Resource Science, Hirosawa, Wako 351-0198, Japan
c Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan.
E-mail: uo@ims.ac.jp; Fax: +81-564-59-5574
† Electronic supplementary information (ESI) available: Experimental details,
TEM images of nanoparticle catalysts, GC & GC-MS charts and 1H & 13C NMR
spectra of the products. See DOI: 10.1039/c4cc04559a
Fig. 1 A transmission electron microscopic (TEM) image and a histogram
of the size distribution of [Ni/Ru/Pt/Au]nano
.
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Table 1 Transfer hydrogenation of 4-phenyl-1-butene (1) with various
uni-, bi-, tri-, quatermetallic nanoparticlesa
Yield of
Yield of
Entry
Catalystb
mol%
2c [%]
3c [%]
1
2
3
4
5
6
7
8
[Ni0.24/Ru0.26/Pt0.26/Au0.24
[Ni]
[Ru]
[Pt]
]
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.4
0.5
0.3
0.4
0.7
0.7
2.0
0.5
0.3
0.5
98
2
o1
o1
o1
o1
o1
o1
o1
o1
14
o1
o1
o1
o1
o1
5
2
1
1
[Au]
[Ni] + [Ru] + [Pt] + [Au]
[Ni0.54/Ru0.46]
[Ni0.61/Pt0.39
[Ni0.51/Au0.49
]
]
9
10
11
12
13
14
15
16
17
[Ru0.43/Pt0.57
[Ru0.43/Au0.57
[Pt0.49/Au0.51
[Ni0.36/Ru0.35/Pt0.25
[Ni0.36/Ru0.35/Pt0.25
]
5
7
]
]
o1
o1
]
]
4
16
Scheme 2 Reaction pathway of the transfer hydrogenation of alkenes.
16
o1
2
o1
33
22
20
3
[Ni0.38/Ru0.27/Au0.35
[Ni0.34/Pt0.25/Au0.41
[Ru0.35/Pt0.43/Au0.22
]
]
amount of the olefin-migrated products 3 was obtained with
bi- and trimetallic nanoparticle catalysts containing nickel as a
composition element (entries 9, 13–16).
]
a
All reactions were carried out with 4-phenyl-1-butene (1, 1.0 mmol) in
aqueous 2-propanol (3.3 mL, H2O/2-propanol = 3/10) in the presence of
a nanometallic catalyst at 100 1C for 24 h. A ratio of composition
metals is described in subscript letters. GC yield.
b
A generally accepted reaction pathway of the heterogeneous
transition metal-catalyzed transfer hydrogenation of alkenes is out-
lined in Scheme 2.6 The reaction proceeds via (i) abstraction of
hydrogen from an alcohol (alcohol oxidation) to generate the metal–
hydrogen species (step I), (ii) insertion of an alkene between a
metal–hydrogen bond to form a metal(hydrogen)alkyl intermediate
(step II), and (iii) reductive elimination of the intermediate to release
the product alkane (step III). We hypothesized that the composition
elements (Ni, Ru, Pt, and Au) of the quatermetallic nanocatalyst
[Ni/Ru/Pt/Au]nano should promote the steps of the above mentioned
reaction pathway sequentially, and complete the catalytic cycle
cooperatively.
First, oxidation of 2-octanol was examined using the unim-
etallic nanocatalysts to evaluate their catalytic ability for step I
(Scheme 3). The reaction was carried out in refluxing water
under molecular oxygen for 5 h in the presence of 1 mol% of
the unimetallic nanocatalysts. It was found that [Pt]nano catalyzed
the oxidation to give 2-octanone in 31% yield with high reaction
selectivity,2a,e whereas [Ni]nano, [Ru]nano, and [Au]nano were not
effective for the oxidation and only afforded 2-octanone in 0%,
c
To explore the catalytic potential of the nanoparticles, we
elected to study the transfer hydrogenation of alkenes, which
has not been studied as extensively as that of carbonyl com-
pounds.3 After thorough reaction screening, we were pleased to
find that the quatermetallic nanoparticles [Ni/Ru/Pt/Au]nano
exhibited significantly higher catalytic performance for the transfer
hydrogenation of 4-phenyl-1-butene (1)5 with 2-propanol than the
other nanoparticles tested, and afforded a quantitative yield of
butylbenzene (2) (Table 1). Thus, a transfer hydrogenation reaction
of 4-phenyl-1-butene (1) was carried out in the presence of
0.5 mol% (total metal residue) of [Ni/Ru/Pt/Au]nano in aqueous
2-propanol (iPrOH/H2O = 10/3) at 100 1C for 24 h under atmo-
spheric conditions to give butylbenzene (2) in 98% yield (Table 1,
entry 1). Unimetallic [Ni]nano, [Ru]nano, [Pt]nano, and [Au]nano did
not catalyze the transfer hydrogenation at all (entries 2–5).
Furthermore, the reaction hardly proceeded even with 1 mol%
each of unimetallic nanoparticles [Ni]nano, [Ru]nano, [Pt]nano, and
[Au]nano (total metal residue = 4 mol%) under similar conditions
(entry 6). These results indicate that the high catalytic ability of
the quatermetallic [Ni/Ru/Pt/Au]nano catalyst is induced in a
cooperative manner by the combination of multiple metal
species. In order to determine the essential metal elements for
this catalysis, bi- and trimetallic nanoparticles were tested in the
transfer hydrogenation under similar conditions (entries 7–17).
However, none of the bi- or trimetallic nanoparticles efficiently
promoted the hydrogenation. It is noteworthy that a considerable
Scheme 3 Oxidation of 2-octanol with various nanocatalysts.
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Scheme 4 Olefin migration via insertion–elimination steps with various
nanocatalysts.
6%, and 1% yield, respectively. It should be noted that the
bimetallic [Pt/Au]nano promoted the oxidation more effectively
under identical conditions to give 46% yield of 2-octanone.7
Thus, both [Pt]nano and [Pt/Au]nano were identified as effective
catalysts for the hydrogen abstraction from alcohols (step I).9
As can be seen in Table 1, particularly in entries 9 and 13–16,
the olefin migration products 3 were observed when bi- and
trimetallic nanoparticles containing nickel were used as catalysts.
Olefin migration occurs via an olefin insertion/b-hydride elimina-
tion pathway (Scheme 2, step IV). Indeed, among the unimetallic
catalysts [Ni]nano, [Ru]nano, [Pt]nano, and [Au]nano, only [Ni]nano
promoted olefin migration of 4-phenyl-1-butene (1) with acetic
acid, where a protonated metal species, M(OAc)H, was generated
in situ (Scheme 4).8 Thus, the Ni–H species is somewhat more
active than the other metals for olefin insertion to form the
Ni-alkyl intermediate, thereby facilitating step IV and step II, as
shown in Scheme 2. [Ni/Au]nano also showed moderate catalytic
activity for olefin migration under acidic conditions.9
Hydrogenation of the olefin must include the reductive
elimination step (step III) from a M(H)alkyl intermediate. When
b-methylstyrene (4) was subjected to hydrogenation under molec-
ular hydrogen (1 atm) in refluxing water with 1 mol% metal of
unimetallic ([Ni]nano, [Ru]nano, [Pt]nano, and [Au]nano) as well as
bimetallic [Ni/Ru]nano catalysts, 20% and 25% yields of propyl-
benzene (5) were obtained using the [Ru]nano and [Ni/Ru]nano
catalysts, respectively. The [Ni]nano, [Pt]nano, and [Au]nano catalysts
were much less active for the hydrogenation under these
conditions.9
Scheme 6 Sequential cooperation of multiple metals to promote alkene
transfer hydrogenation.
Considering the individual reactivity of [Ni]nano, [Ru]nano
,
[Pt]nano, and [Au]nano, shown in Schemes 3–5, the significant
catalytic ability of the quatermetallic nanoparticles [Ni/Ru/Pt/Au]nano
in the transfer hydrogenation of alkenes was possibly induced by
the sequential cooperation of the composition metals. Although the
reaction mechanism is still unclear, we propose the inter-metal-
ligand relay sequence as a plausible reaction pathway, namely (i)
[Pt/Au]-promoted alcohol oxidation to generate a metal-hydride
species and relay of the hydrogen ligand onto Ni and Ru, (ii)
[Ni]-catalyzed olefin migration via an insertion–elimination
pathway to afford a reactive benzyl-metal intermediate and
relay of the benzylic ligand onto Ru, and (iii) the hydrogenation
(reductive elimination) from a Ru-benzyl intermediate (Scheme 6).
The substrate scope of the transfer hydrogenation protocol
using the quatermetallic nanoparticles [Ni/Ru/Pt/Au]nano was
examined for the reduction of various alkenes, representative
results of which are shown in Scheme 7. Reduction of b-methyl-
styrene (4) proceeded smoothly with [Ni/Ru/Pt/Au]nano under
similar transfer hydrogenation conditions to give propylbenzene
(5) in 98% yield. 4-(2-Methylphenyl)-2-pentene (6) and ethyl
1-pentenoate (8) also underwent catalytic reduction to afford
2-(n-pentyl)toluene (7) and ethyl pentanoate (9) in 93% and 98%
isolated yields, respectively. It is interesting that the reduction
of 1-octene (10), which lacks a conjugating group (e.g., aryl and
carbonyl), did not proceed under similar conditions and gave
only olefin-isomerized products 11 (97%); 6 isomers were
observed in the GC-MS analysis. Thus, the reductive elimina-
tion step forming the C–H bond must occur from the corre-
sponding benzylic or p-carbonyl alkyl metal intermediates
generated in situ via nickel-promoted olefin migration (insertion–
elimination process).
Scheme 5 Hydrogenation of b-methylstyrene via the reductive elimina-
tion step with various nanocatalysts.
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2 For studies on the catalytic properties of unimetallic nanoparticles
from the author’s group, see: (a) Y. M. A. Yamada, T. Arakawa,
H. Hocke and Y. Uozumi, Chem. – Asian J., 2009, 4, 1092; (b) Y. Uozumi
and Y. M. A. Yamada, Chem. Rec., 2009, 9, 51; (c) Y. M. A. Yamada and
Y. Uozumi, Tetrahedron, 2007, 63, 8492; (d) Y. Uozumi, R. Nakao and
H. Rhee, J. Organomet. Chem., 2007, 692, 420; (e) Y. M. A. Yamada,
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46, 704; ( f ) R. Nakao, H. Rhee and Y. Uozumi, Org. Lett., 2005, 7, 163;
(g) Y. Uozumi and R. Nakao, Angew. Chem., Int. Ed., 2003, 42, 194.
3 For reviews, see: (a) M. Kitamura and R. Noyori, ‘‘Hydrogenation and
Transfer Hydrogenation’’, in Ruthenium in Organic Synthesis, ed. S.-I.
Murahashi, Weinheim, Wiley-VCH, 2004, pp. 3–52; (b) F. Alonso,
P. Riente and M. Yus, Acc. Chem. Res., 2011, 44, 379; (c) J. M. Brunel,
Tetrahedron, 2007, 63, 3899.
4 The method for the preparation of the quatermetallic nanoparticles
[Ni/Ru/Pt/Au]nano is given in the ESI†.
5 It has been reported that the transfer hydrogenation of non-activated
alkenes often required 10–30 mol% of RANEYs Ni or homogeneous
phosphine–Ru complexes under thermal conditions. See ref. 3 and 6,
and references cited therein.
6 (a) G. P. Boldrini, D. Savoia, E. Tagliavini, C. Trombini and A. Umani-
Ronchi, J. Org. Chem., 1985, 50, 3082; (b) J. Yu and J. B. Spencer,
Chem. – Eur. J., 1999, 5, 2237.
Scheme 7 Transfer hydrogenation of olefins with [Ni/Ru/Pt/Au]nano
.
7 For bimetallic nanocatalysts whose catalytic performance was
improved by the bimetallic combination, see: (a) D. I. Enache, J. K.
Edwards, P. Landon, B. Solsona-Espriu, A. F. Carley, A. A. Herzing,
M. Watanabe, C. J. Kiely, D. W. Knight and G. J. Hutchings, Science,
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( f ) W.-J. Yoo, H. Miyamura and S. Kobayashi, J. Am. Chem. Soc., 2011,
133, 3095.
In conclusion, we have developed a novel protocol for the hetero-
geneous catalytic transfer hydrogenation of alkenes which was
promoted by the quatermetallic alloy nanoparticles [Ni/Ru/Pt/Au]nano
via sequential cooperation of the component metal elements. Along
this line, various types of catalysts are being developed by combining
the unique chemical properties of transition metals in our laboratory
and will be reported in due course.
We acknowledge the JSPS (Grant-in-Aid for Scientific Research
on Innovative Area #2105) for partial financial support of this
work. Y. I. was supported by the JSPS Fellowship program.
8 For an example, see: H. Bricout, A. Mortreux and E. Monflier,
J. Organomet. Chem., 1998, 553, 469.
9 Catalytic ability of the heteroquatermetallic [Ni/Ru/Pt/Au]nano was
also examined for the aerobic oxidation of 2-octanol (cf. Scheme 3),
olefin migration of 4-phenylbut-1-ene (cf. Scheme 4) and hydrogenation
of b-methylstyrene (cf. Scheme 5), where 2-octanol, olefin-isomerized
products 3 (4-phenylbut-2-ene and 1-phenylbut-1-ene (b-ethylstyrene)),
and phenylpropane (5) were obtained in 63%, 13%, and 99% yields,
respectively.
Notes and references
1 For studies on the physical properties of multiple-metallic nano-
particles from the author’s group, see: (a) Y. Ito, K. Takai and
T. Enoki, J. Phys. Chem. C, 2011, 115, 8971; (b) A. Miyazaki, Y. Ito
and T. Enoki, Eur. J. Inorg. Chem., 2010, 4279; (c) Y. Ito, A. Miyazaki,
12126 | Chem. Commun., 2014, 50, 12123--12126
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