Pd Loading on MIL-101Cr-NH for Suzuki–Miyaura Reactions
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FULL PAPER
[
33]
these mild reaction conditions. Thus, we decided to con-
tinue our investigation by using pinacolate ester 2b. Further-
more, the use of compound 2b compensated for the loss of
reactivity that was observed when Cs CO was replaced by
only 2 h in water/EtOH, in which the solubility of the sub-
strate was increased (Table 3, entry 4). Interestingly, the re-
action of 9-bromoanthracene (1e) proceeded very slowly,
even in water/EtOH after a prolonged reaction time
(Table 3, entry 5). This result may be due to the large size of
this substrate, which results in limited access to 8wt%Pd@
2
3
K CO and quantitative yields could be obtained with the
latter base by using only 1.2 equivalents of PhBpin (Table 2,
entry 19).
2
3
MIL-101Cr-NH , and the catalysis mainly occurred at the Pd
2
nanoparticles on the external surfaces of the MOF catalyst
particles. Importantly, heteroaromatic substrates 1 f and 1g
could also be coupled in high yields (Table 3, entries 6–7).
However, a lower yield was obtained for 2-bromopyridine
(1h; Table 3, entry 8). Slightly higher leaching of palladium
was observed for substrates that contained heteroatoms:
3.10 ppm Pd was found in the filtrate after the reaction of
bromopyridine (1g), whereas only 0.17 ppm was found after
the coupling of compound 1a (see above). The coupling re-
actions of aryl chlorides required more elevated tempera-
tures, which, disappointingly, were more favorable for the
formation of byproducts (that is, dehalogenation (ArH) or
homocoupling (ArÀAr) products). Activated aryl chlorides
Comparison of Pd@MIL-101Cr-NH catalysts with different
Pd loadings: With the optimized conditions in hand, we
compared the activities of the 4-, 8-, 12-, and
2
1
2
6wt%Pd@MIL-101Cr-NH2 catalysts (Table 2, entries 20–
3). The reactions were analyzed before full conversion had
occurred (3 h) by using a Pd loading of 3 mol%. Whereas
the reactions with 4- and 8wt%Pd@MIL-101Cr-NH afford-
2
ed very similar and satisfactory yields (Table 2, entries 20
and 21), the reactions with 12- and 16wt%Pd@MIL-101Cr-
NH afforded significantly lower yields (Table 2, entries 22
2
and 23). This behavior was in agreement with the TEM and
PDF analyses, which showed that the nanoparticles in the
12- and 16wt%Pd@MIL-101Cr-NH2 catalysts were larger
1i and 1j successfully reacted in water and they returned
good-to-excellent yields with little or no formation of unde-
sired byproducts (Table 3, entries 9 and 10). However, if
EtOH was used as a co-solvent, both activated and inacti-
vated substrates 1j–1m returned very good conversions but
only fair yields, with the concomitant formation of signifi-
cant amounts of homocoupling and dehalogenation products
(Table 3, entries 10–13).
with broader size distributions. The poorer performance of
the MOFs with higher Pd loadings may be explained by an
increased difficulty in the diffusion of the substrates and the
products through these materials, owing to pore blocking.
Leaching tests: The stability and performance of the immo-
bilized nanoparticles were evaluated according to the ap-
[34]
proach reported by Sheldon et al. A hot-filtration test was
performed by stirring the 8wt%Pd@MIL-101Cr-NH cata-
Recycling tests: The recyclability of 8wt%Pd@MIL-101Cr-
NH2 was investigated in the reaction of p-bromotoluene.
The experiments were performed on a larger scale (1 mmol)
than in previous investigations to facilitate separation and
minimize handling errors. After each run, the solid catalyst
was separated by centrifugation, washed with water and
EtOH (twice each), dried under vacuum, and reused. As
shown in Figure 6, the catalyst remained highly active, even
2
lyst in boiling water for 15 min, followed by filtration
through a pad of Celite. The filtrate was subsequently used
as the solvent for the reaction of compound 1a with com-
pound 2a under similar reaction conditions to those in
Table 2, entry 1, but in the absence of the palladium catalyst.
1
After 6 h, only 3% conversion was detected by H NMR
spectroscopy. The Pd leached into solution after catalysis
when carbonates were used as the base was only 0.17 ppm
[35]
after 10 cycles. Interestingly, the PXRD pattern after the
(
see above). Together with the high recyclability of the cata-
tenth cycle was different from that of MIL-101Cr-NH , but
2
lyst (see below), these results clearly demonstrate the stabil-
ity of the Pd nanoparticles in MIL-101Cr-NH2.
similar to that of another phase, presumably MIL-88B-NH
2
[36]
(Supporting Information, Figure S9). This phase-transfor-
mation phenomenon is currently under investigation in our
laboratories. TEM analysis showed the presence of some
limited aggregation of Pd nanoparticles on the surfaces of
the MOF catalysts after the 10th cycle (Supporting Informa-
tion, Figure S10). Because the size of the nanoparticles re-
mained unchanged, the aggregation did not seem to signifi-
cantly affect the efficiency of the catalyst. Elemental analy-
sis of the recycled 8wt%Pd@MIL-101Cr-NH2 material
showed an overall loss of 0.96 wt% Pd after 10 runs. Yields
of between 97% and >99% were obtained in the recycling
runs. Based on these excellent results, we decreased the Pd
loading by 20 times to 0.15 mol% and performed the reac-
tions on the same scale and under similar conditions to
those in Table 2, entry 19, and the reaction proceeded to
completion within 4 h. This result indicates that Pd@MIL-
Substrate scope: Based on this series of observations
(
Table 2), we screened
a variety of substrates with
8
wt%Pd@MIL-101Cr-NH as the catalyst (3 mol% Pd load-
2
ing) and K CO as the base, because K CO is environmen-
2
3
2
3
tally benign and about 20-times cheaper than Cs CO . More-
2
3
over, potassium has been shown to be the countercation
that infers the least-intense decelerating effect in the trans-
[31]
metalation step. These results are shown in Table 3. The
reactions were either performed in water or in water/EtOH
mixtures (1:1 v/v). Aryl bromides 1a–1c were easily coupled
within short reaction times (6 h in water, 30 min in water/
EtOH) at room temperature (Table 3, entries 1–3). 2-
Bromo ACHTUNGTRENNUNGn aphthalene (1d), which showed very low solubility
in water at ambient temperatures, afforded compound 3d in
only 22% yield. However, 97% yield was obtained after
101Cr-NH is a highly efficient catalyst.
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Chem. Eur. J. 2013, 19, 17483 – 17493
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
17489