342
A. Gniewek et al. / Journal of Catalysis 229 (2005) 332–343
Scheme 3.
4. Summary
pounds was proposed or experimentally proved. On the
other hand, however, palladium leaching from the Pd/C cat-
alyst used for the Heck reaction in ionic liquids was not
found [42].
– Pd-PVP colloid (19.8-nm diameter) in [Bu4N]Br medi-
um is catalytically active in Heck coupling of bro-
mobenzene with butyl acrylate and methoxycarbonyla-
tion of iodobenzene.
– By two independent methods, TEM and XPS, it was
found that Pd-PVP colloid reacted with aryl halides
(PhX, X = Br, I) to form Pd(II) complexes containing
a X-Pd(II)-Ph fragment as a result of oxidative addition.
This reaction caused a small decrease in Pd nanoparticle
size, from 19.8 to 17.5 nm.
– The same reaction of Pd-PVP colloid carried out in
the presence of [Bu4N]Br led to the formation of
soluble [Bu4N]2[Pd(Ph)Br3]- or [Bu4N]2[PdBr4]-type
complexes, which then most probably participated in
a catalytic reaction in the solution. The formation of
these products was additionally proved by UV–vis spec-
troscopy.
– The appearance of new, very small crystallites in Pd-
PVP colloid samples after reaction with PhX + [Bu4N]X
(Fig. 3d) may be explained by the reduction of Pd(II)
complexes of the [Pd(Ph)X3]2− or [PdX4]2− type.
Methanol, always present as a solvent, could serve as
a reducing agent for Pd(II).
– Our studies clearly show that Pd-PVP colloid is a pre-
cursor of soluble forms of the catalyst, which subse-
quently are again reduced to colloidal forms of lower
size.
Acknowledgments
Financial support by the State Committee for Scientific
Research KBN (Poland) (Grant 3T09A 115 26) is gratefully
acknowledged. The authors are indebted to Jean-Charles
Wypych at Clermont-Ferrand and Dr. Ewa Mieczyn´ska for
technical assistance.
– TEM measurements of the Pd-PVP colloid after the
reaction with PhX and [Bu4N]X showed a significant
reduction in the Pd nanoparticle size as a result of pal-
ladium oxidation and solubilization. To our knowledge
this is the first reported evidence of Pd nanoparticle re-
dispersion during a reaction.
References
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was observed when Pd-PVP colloid was heated with PhX +
[Bu4N]X (X = Br, I), and most probably the same phenom-
enon will happen with any other aryl halide and ammonium
salt.
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