Synthesis of small palladium nanoparticles stabilized by bisphosphine BINAP bearing an alkyl chain and their palladium nanoparticle-catalyzed carbon-carbon coupling reactions under room-temperature

Article abstract of DOI:10.1039/b605390b

A new bisphosphine ligand, C₈-BINAP, and C₈-BINAP- stabilized palladium nanoparticles have been prepared; C₈-BINAP was found to be an effective protecting ligand for preparing and stabilizing palladium nanoparticles with v

Full text of DOI:10.1039/b605390b

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COMMUNICATION
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Synthesis of small palladium nanoparticles stabilized by bisphosphine
BINAP bearing an alkyl chain and their palladium nanoparticle-
catalyzed carbon–carbon coupling reactions under room-temperature
Ryouta Tatumi,a Tomoki Akitab and Hisashi Fujihara*a
Received (in Cambridge, UK) 13th April 2006, Accepted 5th June 2006
First published as an Advance Article on the web 23rd June 2006
DOI: 10.1039/b605390b
(C8-BINAP) which is a potential candidate as a protective agent
to control, not only the particle size, but also the interfacial
reactivity of metal nanoparticles. We have found that the
C8-BINAP-stabilized palladium nanoparticles (C8-BINAP–Pd)
have a small core size and narrow size distribution (1.2 ¡
0.2 nm) and are remarkably stable. Moreover, we have found that
C8-BINAP–Pd nanoparticles can efficiently catalyze Stille
and Suzuki coupling reactions at room temperature, in which
the C8-BINAP–Pd nanoparticles can be recovered after the
reaction and reused. Palladium nanoparticles have been of
increasing scientific interest as a catalyst for organic carbon–
carbon bond formation reactions such as Stille and Suzuki cross-
coupling reactions.2,6–9 In general, palladium complexes and
palladium nanoparticles have been used as catalysts for the
carbon–carbon couplings which are among the most powerful
methods in organic synthesis; the reactions have been performed
by heating or refluxing.2,6–9 Much less is known about the room-
temperature carbon–carbon coupling reactions catalyzed by
phosphine-stabilized palladium nanoparticles. This communica-
tion reports the synthesis of a new type of bisphosphine protective
ligand, C8-BINAP, and its ligand-stabilized palladium nanoparti-
cles (C8-BINAP–Pd), and room-temperature Stille and Suzuki
coupling reactions catalyzed by the C8-BINAP–Pd nanoparticles.
A new bisphosphine (C8-BINAP) was synthesized according to
Scheme 1 using 6-hydroxymethyl-2,29-bis(methoxymethyl)-1,19-
binaphthol (7)10 obtained by the reported method.11 A typical
procedure for the preparation of C8-BINAP–Pd nanoparticles is as
follows. To a vigorously stirred solution of K2PdCl4 (65 mg,
0.2 mmol) in 5 mL of MeOH, 10 mL of deionized water, and
30 mL of THF was added bisphosphine C8-BINAP (156 mg,
0.2 mmol) in 20 mL of THF. NaBH4 (76 mg, 2 mmol) in 5 mL of
deionized water was then added. The mixture was stirred for 1 h at
room temperature. After the reaction, the filtrate was evaporated
in vacuo to yield C8-BINAP–Pd nanoparticles. Purification
(dichloromethane–n-hexane) of C8-BINAP–Pd was repeated until
A new bisphosphine ligand, C8-BINAP, and C8-BINAP-
stabilized palladium nanoparticles have been prepared; C8-
BINAP was found to be an effective protecting ligand for
preparing and stabilizing palladium nanoparticles with very
small core size and narrow size distribution and the C8-
BINAP–Pd nanoparticles behave as an efficient catalyst for
carbon–carbon coupling reactions at room temperature.
Metal nanoparticles have attracted a remarkable academic and
industrial research effort due to their potential applications,
ranging from fundamental studies in quantum physics to
catalysts.1 In recent years, great interest has been placed on the
use of metal nanoparticles for nanocatalysis.1,2 Control of the
surface properties and reactivities of metal nanoparticles is an
important aspect of developing nanomaterial applications. The
size, shape, and surface properties of metal nanoparticles are
crucially controlled by the nature of protective ligands. The
physical and chemical properties of metal nanoparticles can be
tuned by variations in both the nature of the ligand shell, and the
size of the metal core. Generally, transition metal nanoparticles
with small particle size (1–3 nm) have high catalytic activity and
obvious size-dependent properties. For the preparation of metal
nanoparticles with small sizes, narrow size distributions, and
stability, the use of various stabilizers such as amines, phosphines,
and thiols, has been extensively studied.1,3 Although a number of
thiol-stabilized metal nanoparticles have been reported,1,3d,e metal
nanoparticles stabilized by bisphosphines have received much
less attention.4 Previously we reported that optically active
bisphosphine,
2,29-bis(diphenylphosphino)-1,19-binaphthyl
palladium nanoparticles catalyzed
[BINAP]-stabilized
asymmetric hydrosilylation of styrene with trichlorosilane under
mild conditions.5
In our recent research efforts, special emphasis has been placed
on the elucidation of structural driving forces in the control of
particle size and shape, and the reactivity of metal nanoparticles
stabilized by bisphosphines such as BINAP-derivatives. The key
structural elements of a new bisphosphine, 6-octyl-substituted
BINAP (C8-BINAP), as a protective ligand in this study consist of
the BINAP phosphine for the coordination site and an alkyl chain
for the stabilization of nanoparticles via van der Waals interactions
1
no free phosphine remained, as detected by TLC, H and 13C
(Fig. 1). We have synthesized
a new BINAP derivative
aDepartment of Applied Chemistry, Kinki University, Kowakae,
Higashi-Osaka 577-8502, Japan. E-mail: h-fuji@apch.kindai.ac.jp
bNational Institute of Advanced Industrial Science and Technology
(AIST), Ikeda, Osaka 563-8577, Japan
Fig. 1 Structure of C8-BINAP. Bisphosphine (C8-BINAP) used to derive
the bisphosphine-stabilized palladium nanoparticles (C8-BINAP–Pd).
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Scheme 2
plasmon band in the UV-vis spectrum of C8-BINAP–Pd in
CH2Cl2 [Fig. 2(d)]. The absence of a plasmon band for
C8-BINAP–Pd is in accord with theoretical predictions13 and
experimental observations for 2.2 nm alkanethiolate-stabilized Pd
nanoparticles,14 but not with the report of a 302 nm surface
plasmon band for 2.2 nm octadecanethiolate-protected Pd
nanoparticles.15
Scheme 1 Reagents and conditions. X = –CH2OMe, R = –(CH2)7Me, Tf
= CF3SO2; (i) CBr4, PPh3, THF, rt, 30 min; (ii) Me(CH2)7SH, NaH, THF,
rt, 3 h; (iii) i-PrOH, HCl, 40 uC, 6 h; (iv) (CF3SO2)2O, pyridine, CH2Cl2,
0 uC, 3 h; (v) [1,2-bis(diphenylphosphino)ethane]dichloronickel(II), 1,4-
diazabicyclo[2.2.2]octane, Ph2PH, anhydrous DMF, 100 uC, 3 days.
Thermal gravimetric analysis was carried out to estimate the
total organic content of C8-BINAP–Pd. On heating to 720 uC,
C8-BINAP–Pd displays a 77% mass loss, corresponding to 25
C8-BINAP ligands on an assumed 55-atom palladium cluster.
The catalytic properties of C8-BINAP–Pd nanoparticles dis-
persed in organic solvent were tested in two very important
carbon–carbon bond-forming reactions, the Stille and Suzuki
coupling reactions. It has been known that Stille and Suzuki
coupling reactions were carried out under heating.8,9 Significantly,
however, C8-BINAP–Pd nanoparticle-catalyzed cross-coupling
reactions, Stille reaction of methyl 2-iodobenzoate (1) with 2-
(tributylstannyl)thiophene (2) and Suzuki reaction of 4-bromoto-
luene (4) with phenylboronic acid (5), were found to proceed
smoothly at room temperature to give the corresponding coupling
products in good yields (Scheme 2 for Stille reaction and Scheme 3
for Suzuki reaction). Typically, a mixture of 1 (0.15 mL, 1 mmol)
and 2 (0.32 mL, 1 mmol) in the presence of C8-BINAP–Pd (20 mg,
0.06 mol%) in anhydrous THF was stirred at room temperature
under argon for 4 h. After work-up, the crude products were
purified by silica-gel column chromatography (n-hexane:benzene =
1:1) to give the coupling product (3) in 91% yield.16 The catalytic
activity of C8-BINAP–Pd was preserved in the presence of a thio-
containing substrate which is known to poison colloidal palladium
nanoparticle catalysts.17 Separation of the products from the
catalyst is easy, and the C8-BINAP–Pd recovered from the
reaction was freely soluble in organic solvents, indicating that
the palladium core had not agglomerated under the reaction
conditions. More interestingly, in the second recycle reaction using
the recovered C8-BINAP–Pd, the coupling product 3 was obtained
in 90% yield under the same conditions. This finding indicates that
the nanocatalyst, C8-BINAP–Pd, can be reused under the same
conditions with essentially no loss in activity. Thus, the bispho-
sphine (C8-BINAP), unlike polymer, serves as the supporting
material for keeping the nanoparticles from aggregation and
facilitates the synthesis and reuse of the catalysis. Generally,
polymeric materials and dendrimers have been used as stabilizers
(protective ligands) of recyclable metal nanoparticles in catalysis.7
A small amount of C8-BINAP–Pd (0.02 mol%) was also
effective for a Suzuki coupling reaction under room temperature
(Scheme 3), i.e., the reaction of 4 (171 mg, 1 mmol) with 5 (186 mg,
NMR spectroscopy. C8-BINAP–Pd nanoparticles are remarkably
stable both in solution, as well as in the solid state.
The particle size and size distribution of C8-BINAP–Pd were
analyzed by transmission electron microscopy (TEM) and annular
dark-field scanning transmission electron microscopy (ADF-
STEM). As shown in Fig. 2(a–c), narrowly dispersed palladium
nanoparticles were produced in the one-pot synthesis. The core size
of C8-BINAP–Pd is very small 1.2 ¡ 0.2 nm. The particle size of
C8-BINAP–Pd was also obtained using small angle X-ray
scattering (SAXS). From SAXS analysis of C8-BINAP–Pd an
average particle size of 1.2 nm was determined.12 The particle size
of C8-BINAP–Pd is much smaller than that of BINAP-stabilized
palladium nanoparticles (2.0 ¡ 0.5 nm).5
The X-ray photoelectron spectroscopy (XPS) spectrum of
C8-BINAP–Pd displayed that the binding energies for the Pd 3d
doublet are 336.0 and 341.3 eV. There was a feature at 285 eV due
to C 1s and 132 eV due to P 2p. There is no obvious surface
Fig. 2 (a) TEM and (b) ADF-STEM micrographs, (c) histogram of size
distribution for C8-BINAP–Pd. (d) UV-vis spectrum of C8-BINAP–Pd in
CH2Cl2.
Scheme 3
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638; (d) D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed.,
2005, 44, 7852 and references therein.
1.5 mmol) in the presence of C8-BINAP–Pd (5 mg, 0.02 mol%)
and KF (174 mg, 3 mmol) in THF (1.5 mL) at room temperature
for 24 h afforded the coupling product (6) in 83% isolated yield.
These results demonstrate that C8-BINAP–Pd nanoparticles were
shown to be an effective nanocatalyst for the Stille and Suzuki
coupling reactions. The nanocatalyst was found to be very stable,
as evidenced by TEM and XPS analysis, and did not show any
decease in activity with time.
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In summary, a new protective ligand, C8-BINAP, bearing an
octyl group as a tuning site, induced the formation of remarkably
stable palladium nanoparticles (C8-BINAP–Pd) with very small
core size and narrow size distribution. The C8-BINAP–Pd
nanoparticles were found to be a versatile catalyst for carbon–
carbon coupling reactions of haloarenes at room temperature. The
recycling of C8-BINAP–Pd is possible without loss of activity. The
work reported here points to a new direction to design protective
ligands and nanocatalysts for carbon–carbon coupling reactions at
room-temperature. Further work is currently in progress in this
and related areas.
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11 C8-BINAP: 1H NMR (CDCl3) d 0.78 (t, 3H, CH3), 1.13–1.32 (m, 10H,
CH2), 1.47 (m, 2H, –SCH2CH2), 2.29 (t, 2H, –SCH2), 3.65 (s, 2H,
ArCH2S–), 6.65–7.14 (m, 24H, Ar–H), 7.22–7.60 (m, 4H, Ar–H), 7.72–
7.81 (m, 3H, Ar–H); 13C NMR (CDCl3) d 14.1, 22.6, 28.9, 29.1, 29.2,
31.3, 31.8, 36.3, 125.8–145.4 (44C, Ar); MS (FAB) m/z 781 (MH+).
12 Measurements were performed on a Rigaku RINT-2500.
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We thank Dr T. Kubo and Mr Y. Takatsuru for analysis of the
SAXS data. This work was supported in part by the Grant-in-Aid
for Scientific Research on Priority Areas (Nos. 17036067 and
18033055 ‘‘Chemistry of Coordination Space’’) from the Ministry
of Education, Science, Sports and Culture, Japan.
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16 In contrast to C8-BINAP–Pd, when a Pd complex, (Ph3P)2PdCl2
(5 mol%), was used for the Stille reaction of 1 with 2 in THF, the
reaction required refluxing for 20 h: T. R. Bailey, Tetrahedron Lett.,
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