E. Rafiee, M. Kahrizi / Journal of Molecular Liquids 218 (2016) 625–631
629
Table 2
Electrochemical data.
The CV responses of Mo10V2-PyPS, Fe3O4@OA–Pd and mixture of both
component modified glassy carbon electrode in a phosphate buffer so-
lution of pH = 7 are shown in Fig. 3. In the potential range between
E = −0.34 and E = +1.58 V, three pairs of redox peaks were observed
for Mo10V2-PyPS modified electrode (Fig. 3a). It can be attributed to the
electron-transfer reaction of Mo(V)/Mo(IV), Mo(IV)/Mo(III) and V(IV)/
V(V) couples. Also, the oxidation behavior of Fe3O4@OA–Pd was inves-
tigated by means of CV and the oxidation of Pd(0)/Pd(II) was identified
with its characteristic peak located at E = +0.03 V (Fig. 3b) [39, 40]. The
peaks at potential 0.16–0.42 are attributed to Fe(II)/Fe(III) nanocrystals
[41]. In addition, to explore the interaction between Fe3O4@OA–Pd and
Mo10V2-IL, we investigate the CV of mixed Fe3O4@OA–Pd and Mo10V2-IL
glassy carbon electrode. As shown in Fig. 3c, in the potential range
between E = −0.33 and E = +1.42 V, all the redox peaks of
Mo10V2-PyPS and Fe3O4@OA–Pd were observed. Also the shift in
the potential of characteristic peak of Fe3O4@OA–Pd and Mo10V2-PyPS
in glassy carbon electrode, clearly indicates the interaction between
Fe3O4@OA–Pd and Mo10V2-PyPS. The amount of the shifts in different
peaks is reported in Table 2.
To establishing the best reaction conditions, firstly the model reac-
tion was carried out in the absence of the Mo10V2-PyPS, the reaction
did not proceed at all and the starting materials were remained intact
after 24 h (Table 3, entry 1). Increasing the quantity of Mo10V2-PyPS
from 0.13 mol% to 0.26 mol% decreases the reaction time but more
than this amount had no significant effect on the reaction time and
yield (entries 2–4). Improvement in time of the reaction was observed
as the quantity of Fe3O4@OA–Pd as catalyst was increased from
0.188 mol% to 0.376 mol%. Further increasing in the Fe3O4@OA–Pd
quantity showed no improvement in the yield or reaction time (en-
tries 4–6). Therefore, 0.26 mol% of the Mo10V2-PyPS and 0.38 mol%
of Fe3O4@OA–Pd were chosen as the optimum amounts of the cata-
lytic system for further reactions. Moreover the effects of various sol-
vents, such as H2O, dimethylacetamide (DMAc), toluene and DMF
have been studied (Table 3, entries 5, 7–9). DMF at 120 °C is found
to be the choice in terms of yield and reaction time (entry 5).
This catalytic system is efficient for a variety of aromatic bromides
and chlorides (Table 4). The results showed acceptable yields of the
corresponding products using aryl bromides with electron-rich or
electron-poor substituents, except in the case of 4-bromoaniline and
4-chloroaniline, where 43 and 56% yields were obtained (entries 3, 7).
Most reactions using styrene and aryl bromides gave well to excellent
yields regardless of steric effect of the substrates. This catalytic system
showed high selectivity for the trans-configured products in all cases,
and no cis/gem olefin products were observed.
Catalyst
EPc (V)
EPa (V)
Potential shift
Assignment peak
ΔEPc (V)
ΔEPa (V)
Fe3O4@OA–Pd
+0.30
N.A.
+0.21
+0.03
+1.42
+0.00
−0.14
+1.23
+0.28
+0.1
−
−
−
−
Fe(II)/Fe(III)
Pd(0)/Pd(II)
V(IV)/V(V)
Mo(IV)/Mo(III)
Mo(V)/Mo(IV)
V(IV)/V(V)
Fe(II)/Fe(III)
Pd(0)/Pd(II)
Mo(IV)/Mo(III)
Mo(V)/Mo(IV)
Mo10V2-PyPS
+1.46
+0.07
−0.07
+1.30
+0.40
N.A.
−
Mixture of both
components
−0.16
+0.10
N.A.
+0.03
+0.04
−0.19
+0.07
+0.07
+0.02
+0.02
+0.10
−0.03
+0.02
−0.12
oxidative addition and reductive elimination steps in the Heck reaction.
A possible mechanism for the Heck reaction in the presence of Fe3O4@
OA–Pd and Mo10V2-ILs is proposed in Scheme 2. In the first step the
C–X bond of an aryl halide (X = Br, Cl) is oxidatively added to the pal-
ladium atom. At this point, the oxidation of palladium (0) is facilitated
by Mo10V2-ILs. Palladium then forms a π complex with the alkene and
next, the alkene inserts itself in the palladium–carbon bond (Pd\\Ar).
In the next step, β-hydride elimination causes the formation of a new
palladium–alkene π complex. Finally this complex is destroyed and
the desired product is released. As shown in the highlighted part of
Scheme 2, in the reductive elimination step, the Pd–X bond is activated
by Mo10V2-IL acidic hydrogens. H–X is a very good leaving group and
this intermediate is very susceptible to loss of H–X group. Finally, in
the reductive elimination step a formed polyoxoanion with taking
hydrogen in Pd\\H group returns to acidic form and palladium reduces
to palladium (0). Further research is under investigation in our
laboratory.
The difference of IL parts in Mo10V2-IL series, apparently influenced
the acidity of the Mo10V2-ILs, which was probed by a potentiometric
titration with an organic base [38].
As shown in Fig. 2, all kinds of Mo10V2-ILs presented very strong
acid sites, Ei N 100 mV. The presence of pyridine as an electron
withdrawing group in Mo10V2-IL structures was accompanied by a
gradual increase in acidic strength active sites. Strength of acidic
sites of different hybrid materials is as the following trends: PyPS-
Mo10V2 N PyBS-Mo10V2 N TMABS-Mo10V2 N TEABS-Mo10V2 N TBABS-
Mo10V2 (Ei values for these compounds are 345, 303, 253, 245, and
238 mV respectively). As a result, it seems that higher activity of
Mo10V2-PyPS is related to higher acidic strength of this catalyst because
the number of acidic sites in all kinds of co-catalyst is approximately
similar.
In order to probe the electron transfer behavior of Mo10V2-ILs and
Fe3O4@OA–Pd, we decided to investigate this property of Mo10V2-ILs
and Fe3O4@OA–Pd by means of cyclic voltammetry (CV) measurements.
As expected, reactions with aryl chlorides gave slightly lower
yields and higher reaction time leaving some unreacted starting
materials. It should be noted that, aryl halides containing electron-
withdrawing functional groups give higher yields with lower reac-
tion times compared to those containing electron-donating func-
tional groups.
Table 3
Effect of the POM on the Heck coupling reaction.a
Entry
Catalyst
Solvent
Time (h)
Yield (%)b
TON
Fe3O4@OA–Pd (mol%)
Mo10V2-IL
1
2
3
4
5
6
7
8
9
0.19
0.19
0.19
0.19
0.38
0.56
0.38
0.38
0.38
–
DMF
DMF
DMF
DMF
DMF
DMF
H2O
24
12
9
9
7
0
94
89
93
96
90
32
8
0
500
473
494
251
159
85
Mo10V2-PyPS (0.13 mol%)
Mo10V2-PyPS (0.26 mol%)
Mo10V2-PyPS (0.39 mol%)
Mo10V2-PyPS (0.26 mol%)
Mo10V2-PyPS (0.26 mol%)
Mo10V2-PyPS (0.26 mol%)
Mo10V2-PyPS (0.26 mol%)
Mo10V2-PyPS (0.26 mol%)
7
24
24
24
Toluene
DMAc
21
207
78
a
Reaction conditions: styrene (4 mmol), bromobenzene (4 mmol), TBAB (4 mmol) at 120 °C.
Isolated yield.
b