4
G.K. Rao et al. / Journal of Organometallic Chemistry 749 (2014) 1e6
Table 2
both are contributing to CeC coupling [53]. Thus, it appears that in
a
Results of catalysis of Suzuki Miyaura coupling by nanoparticles.
the case of 1 coupling is catalyzed with a contribution of nano-sized
Pd species homogeneously as well as heterogeneously, and such a
possibility has been recently described as a “cocktail” like mixture
of the catalysts [53].
Entry no.
Aryl halide
Pd
1
3
Te
2
obtained from
Yieldb
Mol%
The activity of Pd
3 2
Te nanoparticles (isolated) is less than those
1
2
3
4
5
4-Bromobenzaldehyde
Bromobenzene
4-Bromoanisole
1-Bromo-4-nitrobenzene
4-Bromobenzaldehyde
2.0
2.0
2.0
3.0
3.0
86
69
e
of Pd16 [43] and Pd17Se15 [44] which give comparable conversions
S
7
at 0.1 and 0.5 mol% loading and also convert electronically
deactivated aryl bromide such as 4-bromoanisole. However, 4-
bromoanisole does not give cross-coupled product even in the
c
72
75
c
a
Reaction conditions: 1.0 equiv of aryl or heteroaryl bromide, 1.2 equiv of phe-
presence of 2 mol% loading of Pd
palladium chalcogenides are different in size (2 nm: Pd16
8 nm: Pd17Se15 [44] and w1ꢀ2 nm: Pd Te ). Though the NP’s of
Pd Te are smallest in size, they show the least activity. Hence it is
Te
2
. The nanoparticles of these
3
nylboronic acid and 2.0 equiv of base (K
2
CO
3
), 4 mL of aqueous DMF as solvent,
S
7
[43],
ꢁ
temperature of bath 100 C and reaction time 15 h.
b
3
2
Isolated yield.
4
c
-Formylphenylphenylboronic acid was used.
3
2
unfair to attribute the difference in catalytic activity only to size of
NPs alone.
purely homogeneous or purely heterogeneous systems as well as
“cocktail” like mixtures of the catalysts, are possible [53,54].
3. Conclusion
These reports restricted us to rule out the possibility of molec-
ular catalysis and hence mercury or PPh
3
poisoning tests [55]
In summary Te ligated palladacycle 1 acts as a pre-catalyst for
(
Supplementary information: I and II) were performed to ascer-
CeC coupling of aryl bromides (having substituents: eNO
) with phenylboronic acid. It dispenses Pd
and the activity of 1 is through them. The thermal stability, aerobic
and moisture insensitivity are additional advantages of pre-
catalyst 1.
2
, eCN, e
Te NPs
tain the nature of catalytic activity i.e. homogeneous vs heteroge-
neous. The suppression of catalysis by Hg (which is usually
considered as evidence for heterogeneous catalysis [55] or catalysis
via a Pd(0) intermediates [40,56]) was quite significant and <10%
conversion was observed in the reaction. Recently it has been re-
ported that the efficient inhibition of catalysis by Hg may be caused
not only by the amalgamation of heterogeneous or soluble Pd(0)
species [40,56], but also by the decomposition of the homogeneous
Pd(II) containing catalyst, due to its interaction with Hg(0). A pal-
CHO, ꢀCOCH
3
3
2
4. Experimental
The solvent, metal/ligand precursors other reagents and chem-
ical are described in Supplementary information. The various
techniques used to characterize newly formed compound are also
ladacycle has been found [57] to lead the formation (due to this
interaction with Hg) of a redox-transmetallation product [{
2
k
(N,C)-
3
described in Supplementary information. Poisoning (Hg/PPh ) and
L}HgCl] which has been characterized by spectral and X-ray
diffraction studies. Thus no conclusive comment about the nature
of catalytic activity is possible with the help of Hg poisoning test.
NPs may contribute in a homogeneous as well as heterogeneous
fashion. The possibility of homogeneous catalysis may arise due to
leaching of surface Pd atoms of heterogeneous nanoparticles of
two-phase tests are also detailed in Supplementary information.
4.1. Synthesis of L
The MeO-4-C
6
H
4
Teꢀ(CH
2
)
3
eN]C(Ph)C
6
H
4
ꢀ2ꢀOH (0.473 g,
1.0 mmol), prepared by reported methods [39] and NaBH
4
Pd
3
Te
2
in solution (to form a soluble Pd(II) intermediate such as
(0.0416 g, 1.1 mmol) were stirred for 8 h in 100 mL of dry ethanol.
The solvent was removed on a rotary evaporator. The ligand L was
dissolved into 20 mL of dry chloroform and filtered through Celite.
The solvent was removed with a rotary evaporator, and ligand L
ArꢀPdeBr by oxidative addition). A two-phase test (Scheme 3) was
performed to understand further catalytic processes [58e60].
However this test cannot conclusively indicate the colloidal or
molecular nature of the catalyst. This test (called a three-phase test
when the catalyst is a solid phase) was developed by Rebek and co-
workers [58,59]. If the activity of the catalyst is fully heterogeneous
in nature, the supported aryl halide is not expected to be converted
to a coupled product. However, the homogeneous activity of the
catalyst due to leaching of Pd in solution will lead to the conversion
of the supported substrate to product. Further, the colloidal catalyst
being in solution can always cause some conversion. A two-phase
test (Scheme 3) was conducted utilizing the reaction of a mixture
of 4-bromoacetophenone and immobilized 4-bromobenzoic acid
1
was obtained as a dark red liquid. Yield: 0.351 g (74%). H NMR
ꢁ
(300 MHz, CDCl
1.99 (m, 2H, H
(s, 1H, H ), 6.71e6.92 (m, 5H, H
J ¼ 7.2 Hz,1 H, H12), 7.26e7.39 (m, 5H, H16 þ H17 þ H18), 7.63 (d,
3
, 25 C, TMS):
d
(ppm): 1.22 (s, 2H, NH þ OH),1.94e
þ H ), 3.79 (s, 3H, OMe), 4.87
þ H11 þ H13 þ H14), 7.14 (t,
6
), 2.90e3.04 (m, 4H, H
5
7
8
2
1
3
1
ꢁ
J ¼ 7.5 Hz, 2H, H
(ppm): 6.1 (C ), 30.9 (C
(C ), 115.4 (C ), 115.8 (C11), 118.9 (C13), 127.0 (C14), 127.4 (C17), 127.9
(C18), 128.1 (C
(C10), 159.2 (C
(ppm) 456.2.
3
). C{ H} NMR (75 MHz, CDCl
3
, 25 C, TMS):
d
6
5
), 48.8 (C ), 55.0 (OMe), 63.6 (C
7
8
), 100.8
4
2
9
), 128.2 (C12), 128.3 (C16), 139.6 (C
3
), 143.1 (C15), 156.1
125
1
ꢁ
1
). Te{ H} NMR (94.69 MHz, CDCl
3
, 25 C, Me
2
Te):
(
as amide) with phenylboronic acid under optimum reaction con-
d
ditions. The soluble part was separated by filtration and analyzed
after workup with 1H NMR to determine the yield of the cross-
coupled product (4-acetylbiphenyl), which was found to be
w91%. The solid phase was hydrolyzed, and analysis of the resulting
4.2. Synthesis of complex 1
The Na PdCl (0.205 g, 0.7 mmol) was dissolved in 5.0 mL of
2
4
1
products (after workup) with H NMR revealed w21% conversion
water. The solution of ligand L (0.237 g, 0.5 mmol) made in 10 mL
of acetone was added to it with vigorous stirring. The mixture
was further stirred for 2 h. The orange red solution was extracted
with chloroform (2 ꢂ 100 mL). The chloroform layer was washed
with water (2 ꢂ 100 mL). It was separated, dried with anhydrous
of the immobilized 4-bromobenzoic acid (as amide) to the
cross-coupled product (biphenyl-4-carboxylic acid), and w79%
remained unreacted. These results indicate that there is a signifi-
cant contribution of homogeneous Pd species (molecular or
colloidal) to catalysis. Either the palladium species (responsible for
homogeneous catalysis) is the palladium atoms leached from the in
2 4
Na SO and filtered. The solvent was reduced up to 5 mL on a
rotary evaporator and 100 mL of hexane was added to this
situ generated nanoparticles of Pd
Te
3 2
, or the molecular complex or
concentrate. The precipitate thus obtained was filtered under
Rao, Gyandshwar Kumar
