1838
Y. Li et al.
palladium (Pd) complex of phosphines, amines or hetero-
cyclic carbenes in homogeneous solutions under inert
atmosphere with good synthetic yields [2]. However, these
Pd-complex catalysts do have their limitations. They are
only active when the Pd(II) is reduced to Pd(0), which
requires the reaction to proceed under a non-oxidizing
environment; the active Pd(0) intermediate species are
unstable in the reaction conditions and tend to aggregate
into inactive components, making it impossible to recycle
them for further use; the Pd(II) and Pd(0) as well as the
specially chosen ligands contaminate the reaction product
and must be removed from the product via tedious sepa-
ration process before the newly prepared organic com-
pounds can be used for further applications [3]. To
overcome these problems associated with the previous
catalysts, novel catalyst systems are also developed and
tested.
ability of PdSn NPs is mainly due to the unique charac-
teristics of tin. And the unique characteristics of tin are
probably attributed to the strong metal-support interaction
and/or the alloy formation between tin and precious metal,
such as Pd and Pt [20]. In other words, some electronic
effects are of interesting importance for the enhanced
performance of tin. For instance, in electronic effects, tin
could modify the electronic density of Pt due to positive
charge transfer from Snn ? species to noble metals, such
as Pd and Pt, and the formation of different alloys of Pd-Sn
and Pt–Sn. These modifications may be responsible for
some changes in the heat of adsorption of different
adsorbates, facilitating the reaction [21, 22]. Previous
studies also indicated that the compositions or morpholo-
gies of PdSn alloy NPs can be controlled by tuning the
amount of the precursors. In the literature, PdSn NPs were
synthesized by traditional co-reduction of Pd and Sn salts
directly on various supporting materials such as amorphous
carbon [23]. However, all the reported syntheses did not
provide sufficient control over the morphology and com-
position of the NPs simultaneously to meet the high stan-
dard of a catalyst [24].
These novel catalysts are either Pd complexes or Pd-
based elemental, alloy and core/shell nanoparticles (NPs)
supported on various forms of carbons, oxides, polymer
resins, or even magnetic NPs [4–9]. Mahmoud Nasrol-
lahzadeh has prepared Pd/CuO NPs for Heck coupling
reactions with the high yields, simple methodology, easy
preparation and handling of the catalyst and also easy work
up [10]. Pd NPs also can be supported on pectin and
gelatin, respectively to exhibit a high activity toward Heck
reaction, and the catalysts can be recycled for several runs
without any losses of catalytic ability [11, 12]. Pd NPs on
amino-vinyl silica functionalized Fe3O4 magnetic NPs
were also applied as nanocatalysts for Heck reaction and
the catalysts were recoverable magnetically and could be
reused for five runs without significant loss of catalytic
activity [13]. Among a great number of Pd based NPs for
Heck reaction, bimetallic Pd alloy NPs are particularly
important in the area of catalysis because they often
exhibited high catalytic ability than their monometallic
counterparts [14]. For Pd alloy NPs, mixing a second non-
precious element with Pd metal is an effective way to
reduce the precious metal usage and may enhance its
internal properties due to synergistic effects and the rich
diversity of the compositions [15, 16]. Seong-Ho Choi
synthesized a series of Pd alloy NPs, such as PdAg, PdNi,
and PdCu NPs for Heck reactions, and found the NPs
showed excellent capabilities as catalysts for Heck reac-
tions with the comparable catalytic ability of Pd NPs [17].
Palladium tin (PdSn) alloy nanostructures have attracted
intense attention in both synthesis and catalysis fields.
Earlier studies have shown that PdSn is an active catalyst
for hydrogenation of chloro(Cl)-benzene as Sn on PdSn has
higher affinity for the chloride, stabilizing Pd against oxi-
dation by Cl [18]. PdSn alloys are also active for
hydrogenolysis reactions and their activity are Pd/Sn
composition dependent [19]. The reason for high catalytic
Recently a facile synthetic approach of monodisperse Pd
NPs, Pd alloy and PtSn NPs was developed systematically
[25, 26]. We found that the synthesis could be extended to
PdSn as well and monodisperse PdSn alloy NPs with
compositions and sizes controlled. Herein, we report the
one-pot preparation of monodisperse PdSn alloy NPs and
study their catalytic abilities for Heck reaction between
aryl halides and styrene. We studied the catalytic properties
of Pd77Sn23, Pd67Sn33, Pd63Sn37, Pd52Sn48 and Pd49Sn51 for
model Heck reaction and found their catalytic abilities are
highly composition dependent. They were indeed more
active and stable catalyst than pure Pd NPs with Pd63Sn37
NPs being the most active one due to its high catalytic
ability, low active species leaching and high stability dur-
ing the recycle Heck reactions. In addition, Pd63Sn37 NPs
can be extended to be applied in a number of similar Heck
reactions, indicating that this kind of nanocatalysts provide
a new synthetic approach for carbon coupling reactions.
2 Experimental Section
2.1 Materials and Characterization Techniques
Palladium(II) bromide (PdBr2, 99 %), Tin(II) acetate
(Sn(ac)2, 99 %), oleylamine (OAm, technical grade, 70 %),
trioctylphosphine (TOP, technical grade, 90 %), acetic
acid, tributylamine borane complex (TBAB), Et3N, Na2-
CO3, NaHCO3, Na3PO4, chlorobenzene, bromobenzene,
iodobenzene, N-methylpyrrolidone (NMP), N,N-dimethy-
lacetamide (DMAc), N-dimethylformide (DMF), and
123