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catalysts is mainly determined during the MW synthesis of the
catalysts which involves the simultaneous nucleation of the Pd
and Fe3O4 nanoparticles and reduction of GO. The non-equilibrium
dielectric heating during the MW-assisted chemical reduction of
GO leads to structural defects in the reduced GO sheets. These defect
sites, consisting of large vacancies and missing atoms, act as nucle-
ation centers for the Pd-Fe3O4 composite nanoparticles which can
be anchored to the graphene sheets thus decreasing their mobility
under typical reaction conditions and minimizing the potential of
their agglomeration and the subsequent decrease in the catalytic
activity. It appears that catalyst 3 with the most reduced GO, as
evident by the largest C/O ratio, may include more defect sites
than catalyst 4 and this may explain the better anchoring of the Pd
and Fe3O4 nanoparticles to the graphene surface in catalyst 3.
typically performed in the presence of homogeneous palladium
catalyst using appropriate ligands, the use of the Pd/Fe3O4/G cat-
alyst under ligand-free microwave irradiation conditions could
provide an efficient and facile approach for the synthesis of key
pharmaceutical compounds. As illustrated in Table 5, the Heck
reactions of aryl bromides with diverse range of substituted olefin
were studied using 0.3 mol% of catalyst 3, and potassium carbonate
(3 eq.) in a mixture of H2O:EtOH (1:1) at 150 ◦C for 10 min under
microwave heating. Both electron-rich (2b) and electron-poor (2a,
2d, 2f) aryl bromide substrates can easily undergo the Heck reac-
tions with substituted alkenes affording the coupling products in
excellent yields.
The above results clearly demonstrate the high activity and
excellent recyclability of the Pd/Fe3O4/graphene catalysts as truly
heterogeneous catalysts for the Suzuki and Heck coupling reac-
tions. The origin of the enhancement in activity and stability of
these graphene-supported catalysts is not fully understood. Tradi-
tional surface area and dispersion factors may not account for this
activity, and the emergence of electronic factors that fundamen-
tally alter interactions with the support need to be investigated. We
postulate that the defect sites on the surface of graphene provide
an excellent environment for the nucleation of surface active metal
nanoparticles and as a result, play a major role in imparting excep-
catalysts. It is now well established that the chemical reduction of
GO results in the formation of graphene nanosheets with a signifi-
edges, and others [27–29]. The use of MWI in our synthesis of the
graphene-supported catalysts is expected to enhance the formation
of graphene defects especially in the presence of metal nanopar-
ticles due to the non-equilibrium heating and the formation of
energetic hot spots [35–38].
4.3. Catalytic activity of the Pd/Fe3O4/graphene catalyst for
The catalytic activity of the 7.6 wt% Pd/Fe3O4/G catalyst was
further examined for other Suzuki cross coupling reactions employ-
ing a diverse range of functionalized aryl halide and aryl boronic
acid substrates. As shown in Table 4, the reactions of aryl bro-
mide with aryl boronic acid were carried out in the presence of
0.3 mol% of catalyst 3 using potassium carbonate (3 eq.) as the
base in H2O:EtOH (1:1), an environmentally benign solvent system.
All reactions were resulted in the formation of the correspond-
ing biphenyl products in a high yield. Diversity of the functional
groups on the phenyl ring can be modified from both aryl bro-
mide and phenyl boronic acid substrates. For instance, using aryl
halides containing aldehyde (1a), ketone (1e), nitro (1b), and nitrile
(1c) substituents afforded the corresponding Suzuki products in
high yields. Notably the reactions with electron withdrawing sub-
stituents such as aldehyde (1a) and nitro (1b) groups can be
performed at room temperature within 30 min. It should be noted
that the chloro- substituted aryl halide can also undergo Suzuki
reaction in a relatively good yield (1d). In addition, the aryl boronic
acid bearing different functionality such as dimethyl amino (1b),
thiomethyl (1a), 4-amino carbonyl (1d), and p-methoxy (1e) can
be effectively applied in these coupling reactions. The remarkable
reactivity of 7.6 wt% Pd/Fe3O4/G nanoparticles in Suzuki cross cou-
pling reactions with various functionalized substrates also proved
to exhibit a significant impact on the turn over number (TON) and
turn over frequency (TOF) for these reactions. As such, perform-
ing the reaction of (1a) at lowest catalyst loading of 0.008 mol% at
80 ◦C for 5 min under microwave irradiation afforded 65% conver-
sion to the product using H2O:EtOH as the solvent system. These
results led to a turn over number (TON) of 8100 and a turn over
frequency (TOF) of 97,500 h−1 for the formation of (1a). Likewise,
the TON and TOF numbers were also examined for other elec-
tron withdrawing (1b) and electron rich (1f) aryl halides under
the similar reaction conditions using 0.008 mol% of the catalyst.
These reactions also afforded conversions of 70% and 78% at 80 ◦C
microwave irradiation for 5 min for 1b and 1f, respectively, which
resulted in high TON/TOF numbers of 8750/105,000 h−1 for 1b and
9750/117,000 h−1 for 1f. The high TON/TOF numbers obtained in
these reactions provide strong evidence for the superior catalytic
activity of the 7.6 wt% Pd/Fe3O4/G catalyst in Suzuki cross cou-
pling reactions involving different functionalized substrates under
microwave heating at low catalyst concentration and in a short
reaction time.
5. Conclusions
In summary, we have developed an efficient method to gener-
ate highly active Pd/Fe3O4 composite nanoparticles supported on
graphene (Pd/Fe3O4/G) by microwave assisted chemical reduction
of the corresponding aqueous mixture of palladium nitrate, ferric
nitrate and dispersed graphene oxide nanosheets. We have shown
that the palladium/ferric nitrate concentration ratio used in the
graphene oxide solution has intrinsic effects on the particle size,
distribution, the degree of reduction of both the Pd ions and GO,
and subsequently on the catalytic activity of supported catalysts.
The Pd/Fe3O4/G catalyst containing 7.6 wt% Pd exhibits excellent
catalytic activity toward the Suzuki cross coupling reaction with a
high turnover number (TON) of 9250 and turnover frequency (TOF)
of 111,000 h−1. This catalyst offers a number of advantages includ-
ing high reactivity, recyclability of up to ten times, mild reaction
conditions, and short reaction times in an environmentally benign
solvent system. Furthermore, the magnetic properties imparted by
the Fe3O4 component of the catalyst enables the catalyst to be easily
isolated and recycled, thus greatly simplifying the ability to purify
the reaction products and increasing the economic value of the
catalyst. This catalyst also provided excellent yields over a broad
range of highly functionalized substrates in both Suzuki and Heck
coupling reactions.
Supplementary material
4.4. Catalytic activity of the Pd/Fe3O4/graphene catalyst for Heck
coupling reactions
Magnetic properties of the 7.6 wt% Pd/Fe3O4/G catalyst (Fig. S1),
photographs showing the sequence of magnetic separation of the
catalyst from the reaction mixture (Fig. S2), and NMR data for iden-
tification of the reaction products.
The utility of the 7.6 wt% Pd/Fe3O4/G catalyst was further eval-
uated for the Heck coupling reactions. While Heck reactions are