G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
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Table 2
Optimization of the catalyst amount in Suzuki-Miyaura coupling reaction promoted
by Fe3O4@La-MOF-Schiff base-Pd.
Entry
Catalyst (mg)
Yield (%)
1
2
3
4
5
6
3
5
8
10
15
20
95
98
>99
>99
>99
>99
Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), ethanol (6 mL), catalyst Fe3O4@La-MOF-Schiff-Pd, temperature
80 °C, time 0.5 h.
Table 3
Evaluation of main reaction parameters (temperature and time) for the Suzuki-
Miyaura coupling reaction with the Fe3O4@La-MOF-Schiff base-Pd catalyst.
Fig. 8. Recycling of Fe3O4@La-MOF-Schiff base-Pd in Suzuki-Miyaura cross-cou-
pling (under optimal reaction conditions: Table 1, entry 1).
Entry
T (°C)
Time (h)
Yield (%)
1
2
3
4
5
40
60
80
80
80
1/2
1/2
1/6
1/3
1/2
52
89
93
95
>99
Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), ethanol (6 mL), catalyst (8 mg).
the 88% yield was obtained, corresponding to a high turnover num-
ber (TON) of 21,443 and turnover frequency (TOF) of 42,886 hꢁ1
.
Both values are by far superior to other heterogeneous based-
MOFs Pd catalysts (e.g. TON of 15,800 and TOF of 1317 for the MoII
cluster coordination polymer/PdIIPd00.6; TOFs of: 396 for Pd/MIL-53
(Al)-NH2, 2190.5 for Pd/UiO-66-NH2, 2037 for IRMOF-3-PI-Pd, 1235
for UiO-67-3-PI-Pd) [29]. More importantly, our TOF ranks second
among highest values communicated for crystalline MOF-Pd cata-
lysts in the Suzuki-Miyaura cross-coupling of bromobenzene and is
very close to the best so far reported (43,890, for Pd/AZC) [30].
A heterogeneity test was carried out as well. First, the reaction
between bromobenzene and phenylboronic acid was run under
above optimal conditions giving >99% yield. The supernatant was
then added to the regular mixture of p-bromotoluene and phenyl-
boronic and let to react without further addition of catalyst. As
expected, no product was detected in this reaction. Consequently,
the reaction proceeds solely in heterogeneous mode, with no
leaching of Pd ions in the supernatant.
Recyclability of the catalyst was tested in the reaction of
iodobenzene and phenylboronic acid, in consecutive runs, separat-
ing the catalyst with an external magnet after each run (Fig. 8). It
was found that the Fe3O4@La-MOF-Schiff base-Pd could be reused
twelve times without a significant decrease in the yield. The ele-
mental mappings and TEM images of the recovered catalyst show
that palladium ions were still evenly distributed in the composite
(Figs. 6 and 7). The valence +2 of Pd was maintained (from XPS),
indicating that no Pd(0) nanoparticles have been generated during
the catalytic process (Fig. S10). According to PXRD tests (Fig. 2) the
crystalline phase and catalyst structure were fully preserved in the
used catalyst. Therefore, Fe3O4@La-MOF-Schiff base-Pd proved to
be a quite robust, high-performance heterogeneous catalyst for
activating aromatic CAX (X = Br, I) bonds in Suzuki-Miyaura
cross-coupling reactions.
Scheme 2. Proposed interaction of La, organic linker, solvent and base within the
Pd-catalytic cycle.
effect, La nodes could as well accelerate the reductive elimination
step, favouring generation of Pd(0) species able to resume a new
catalytic cycle. This kinetic behaviour has been also evidenced in
previous reports on La-MOF-Pd catalyzed cross-couplings.
[18b,19b] Scheme 2 gives insight into the role of La and organic
linker within the Pd catalytic cycle, essentially distinct from that
of the substrate, base and solvent [21b–21d].
Along with the kinetic effect, a stronger N-Pd bond that
increases the stability of the Pd complex, may substantially dimin-
ish Pd leaching in the overall catalytic process. Furthermore, La
ions foster a tridimensional MOF architecture with channels and
pores favouring decoration with Fe3O4 NP. Post-synthetic modifi-
cation via a Schiff-base to coordinatively bind Pd ions in a uniform
and stable arrangement results in an innovative, highly active
catalyst.
4. Conclusions
To account for the superior behaviour of our catalyst we assume
that La ions, due to their electropositive propensity, hard Lewis
acid character and their key role in building a rigid 3D configura-
tion by multiple coordination modes, enhance the stability of the
Pd complex. The latter process occurs via a synergistic cooperation
of the oxophilic La ions and the aromatic organic linker in transfer-
ring charge density to the N-Pd bond (Fig. S7). By this electronic
This catalyst demonstrated important beneficial attributes: (a)
accessibility from largely available reagents, by a straightforward
synthetic protocol which leaves only innocuous residues; (b)
robustness of the La-MOF template due to a rigid configuration
ensured by the oxophilic lanthanide nodes and the nucleophilic
organic linker; (c) superior catalytic activity as evidenced by the
quantitative yields and high TONs and TOFs attained at low