R. Cao et al.
responding to ꢀ35.5 ppm).[21] The inductively coupled
plasma emission spectrometry (ICP) results of the filtrates
with cat.-1 and cat.-2 showed Pd concentrations higher than
200 ppm after the first run, which were greatly higher than
the threshold value (35.5 ppm), and thus the formation of
palladium black was visible and the Heck reaction ceased in
these two systems. However, the Pd concentrations slowly
released from cat.-3–5 were generally comparable or lower
than 35.5 ppm (Table 3), and thus the formation of Pd-black
could be dramatically suppressed. Moreover, the coordina-
tion effect of Me10CB[5] to Pd species prevented the ag-
glomeration for Pd0 species. Such synergistic effects actually
lead to the formation of catalytically activated Pd NPs.
Although Pd NPs have been previously observed in Heck
reactions catalyzed by PdII sources,[22] it is not entirely clear
as to whether the Pd NPs contribute in the catalytic activity.
The concentrations of Pd species in filtrates were dramati-
cally decreased following the third cycle (Table 3) and thus,
if solely dependent on Pd species in reaction solution, one
would expect the Heck reaction to proceed in very low
yields. No dramatic decrease in yield is however observed at
this point and it is thus proposed that the Pd NPs prolong
the catalytic activity. The slight decrease in yield was ob-
served following several successive cycles (Table 3) and it is
postulated that this is due to an increase in Pd NP size (Fig-
ure 8c) and thus a decrease in surface area available for cat-
alysis. Mechanistically, the hybrid catalysts cat.-3–5 slowly
release the PdII species (with detectable concentrations
below 40 ppm) at the very initial period and catalyze the
Heck reaction in a homogeneous fashion; the PdII species
being released transform into Pd NPs and grow on the sur-
face of solid catalysts cat.-3–5, which can be considered as
a reservoir for the actual catalytically active Pd0 species at
later stages in a heterogeneous fashion;[23] the balancing be-
tween PdII being released and Pd0 attached to the surface of
the solid catalysts plays a subtle role in the transfer from ho-
mogeneous to heterogeneous catalytic reactions. Thus the
crystalline hybrid catalysts (cat.-3–5) function as precatalysts
to launch the Heck cross-coupling reactions, at the same
time, as supports for the actual catalytically active Pd NPs
and carry on the reactions in a recyclable way. Different
from the heterogeneous Pd nanoparticles immobilization on
different supports,[24] such Pd crystalline solids precatalysts
possess high catalytic activity of homogeneous catalysts as
well as good reusability of heterogeneous catalysts. More-
over, the in situ-generated Pd NPs can be stabilized by
Me10CB[5] ligand without addition of any surfactants or
polymer to protect the Pd NPs from aggregations.
Therefore, the release of the PdII species from cat.-1 and
cat.-2 hybrid catalysts at the initial stage of the catalytic re-
actions was more easy than that from cat.-3–5 and generated
palladium black. Alternatively, the Pd releasing rates may
also be responsible for the stability of the hybrid catalysts,
that is, the slower the Pd releasing rate, the more stable the
catalyst. Of note is that cucurbit[n]urils (CB[n], n=5–8)
have been widely used in the fabrication of noble metal NPs
in which the CB[n]s function as protecting agents.[25] In this
current catalytic systems, the Me10CB[5] may play triple
roles: 1) as a delivery vehicle to carry and release catalyti-
cally active Pd species to the Heck reaction systems; 2) as
a regulator to control the releasing rate and concentration
of Pd species from the crystalline hybrid precatalysts; 3) as
a stabilizer to restrict the over-growth of the catalytically
active Pd NPs.
Conclusion
A series of crystalline hybrid solid materials assembled from
Me10CB[5] and [PdCl4]2À anions in the presence of different
alkali metal cations have been successfully synthesized for
À
the first time by means of a simple diffusion method (M
À
Pd Me10CB[5]; M=Li, cat.-1; Na, cat.-2; K, cat.-3; Rb, cat.-
4; and Cs, cat.-5). The as-synthesized hybrid solids have
been examined as precatalysts for Heck cross-coupling reac-
tions and indeed exhibited good catalytic activities of a wide
range of reaction substrates. It has also been rationalized
that the introduction of various alkali metals afforded diver-
sified crystal structures of the hybrid precatalysts, which
could be responsible for the different stability and reusabili-
ty of the hybrid precatalysts in Heck reactions. In the pro-
cess of the catalytic reactions, the catalytically active Pd spe-
cies were released from the crystalline hybrid catalysts, de-
livered to the reaction systems, and then transformed into
catalytic active Pd NPs. The hybrid precatalysts cat.-3–5
could be easily recycled for several times through simple
centrifugation, without dramatically losing their catalytic ac-
tivities. In conclusion, the crystalline hybrid precatalysts
À
cat.-3–5 developed a new and unusual system for the C C
coupling reactions. Moreover, the synthesis of noble metal/
Me10CB[5] crystalline hybrid materials opened up a new di-
rection for design of CB[n] catalysts in the supramolecular
chemistry field.
Experimental Section
À
À
It is also reasonable that the M Pd Me10CB[5] catalysts
behave differently in the catalytic reactions from a structural
Materials and instrumentation: All chemicals were commercially pur-
chased and used without purification. Me10CB[5] was synthesized accord-
ing to the process reported in the literature.[26] Elemental analyses (C, H,
and N) were carried on an Elementar Vario EL III analyzer. Metal anal-
ysis was determined by using an inductively coupled plasma emission
spectrometer (ICP) Jobin Yvon Ultima 2. X-ray powder diffractions
(XRPD) were performed with a Rigaku DMAX 2500 diffractometer.
TGA was performed under a flow of nitrogen at a rate of 108C minÀ1, by
using a TA SDT-Q600 instrument. The TEM images were taken on
À
À
point of view. In the structures of M Pd Me10CB[5] (cat.-
1 and cat.-2) hybrid catalysts, the [PdCl4]2À anions interact
with the (M2Me10CB[5]) molecular capsules through weak
supramolecular H-bond interactions; whereas, in the struc-
À
À
tures of M Pd Me10CB[5] (cat.-3–5) hybrid catalysts, the
[PdCl4]2À anions covalently bound to the (M2Me10CB[5])
molecular capsules through stronger coordinative bonds.
15666
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15661 – 15668