N.Y. Baran et al. / Journal of Organometallic Chemistry 900 (2019) 120916
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3293 and 3080 cmꢂ1 representing stretching of OH bonded to Al
[43]. The peak at 1342 cmꢂ1 is associated to nitrate stretch vibra-
tion. The bands at 1160 and 1065 cmꢂ1 correspond to symmetrical
bending vibrations of OHeOH hydrogen binding [44]. The peak at
725 cmꢂ1 is linked to vibration of AleO [22]. The small peaks at
1788 and 834 cmꢂ1 can be explained by nitrate impurities [45]. FT-
IR results of boehmite particles is in accord with previous studies,
confirming the successful synthesis of boehmite. When NH2-
boehmite FT-IR spectrum was examined, beside characteristic
peaks of boehmite nanoparticles, newer peaks appeared at 3353
and 3292 cmꢂ1 (NeH asymmetrical and symmetrical stretching),
2929 and 2864 cmꢂ1 (aliphatic CeH stretching), 1592 cmꢂ1 (NH2
deformation vibration), and 1006 cmꢂ1 (SieO stretching) [46,47].
These important peaks affirmed that the amino groups were suc-
cessfully grafted on to boehmite. When compared to FT-IR spectra
of Sch-boehmite with NH2-boehmite, a new peak appeared at
1635 cmꢂ1 which is confirming formation of Schiff base. Addi-
tionally, we observed two peaks at 1605 and 1539 cmꢂ1 which were
attributed to stretching of C]C aromatic ring. These bands showed
that Schiff base formation was successfully performed. There is a
significant change in the Pd NPs@Sch-boehmite catalyst FT-IR
spectrum when compared to Sch-boehmite. However, it was
observed that the functional bands shifted to a lower wavelength
which can be explained by interactions of Sch-boehmite and
palladium ions.
Thermal durability of boehmite particles, NH2-boehmite, Sch-
boehmite and Pd NPs@Sch-boehmite catalyst were examined and
their TGA diagrams are provided in Fig. 2. While degradation
temperature maximum (Tmax) of boehmite particles was recorded
as 407.5 ꢀC, Tmax of NH2-boehmite, and Pd NPs@Sch-boehmite were
recorded as 327 ꢀC and 295 ꢀC, respectively. TGA diagram of Pd
NPs@Sch-boehmite catalyst indicated that it was stable up to
404.4 ꢀC. This result revealed that Pd NPs@Sch-boehmite catalyst
are more durable than its support.
Fig. 3 displays FE-SEM images of boehmite particles and Pd
NPs@Sch-boehmite catalyst which revealed their cubic ortho-
rhombic structures; surface of boehmite particles were covered
with Pd NPs. As seen in Fig. 3c, Pd NPs were quite homogeneously
dispersed and their average particle size was determined to be
30 nm. Additionally, EDS analyses were carried out for every
chemical modifications and it was found that chemical alterations
were successfully accomplished (Fig. 4).
XRD analyses of boehmite particles and Pd NPs@Sch-boehmite
catalyst (Fig. 5) were performed and the indicated peaks corre-
sponding to the characteristic peaks of boehmite particles were
confirmed (Fig. 5a). In case of Pd NPs@Sch-boehmite catalyst, it
contains a series of peaks at 39.1ꢀ, 45.4ꢀ and 66.26ꢀ which can be
ascribed to diffraction from the (1 1 1), (2 0 0) and (2 2 0) planes of
face centered cubic of Pd NPs on the boehmite surface [11,48,49].
Fig. 3. FE-SEM images of boehmite particles (a, b) and Pd NPs@Sch-boehmite catalyst
(c).
3.2. Catalytic studies of Pd NPs@Sch-boehmite
3.2.1. Fabrication of biaryls via suzuki cross-coupling reaction
An efficient method is presented for the formation of biaryls via
microwave (MW)-assisted Suzuki coupling reaction of phenyl-
boronic acid with aryl halides. Initially, Pd NPs@-Sch-boehmite was
evaluated as heteregeonus catalyst for the biaryls preparation. The
catalytic tests were performed under greener solvent-free condi-
tions using MW irradiation. To find the ideal reaction parameters,
different parameters such as catalyst loading and base-type were
studied on p-iodide anisole and phenylboronic acid as the model
reactants; highest reaction yield was attained with 0.005 mol% Pd
NPs@-Sch-boehmite catalyst, 6 min reaction time and K2CO3 as
base. To assess the functional group tolerance, a range of aryl ha-
lides were used (Table 1) and the Pd NPs@-Sch-boehmite catalyst
converted aryl iodides to desired biaryls with excellent reaction
yields (entries 1e6); 1-iodo-3-nitrobenzene was converted to cor-
responding product with reaction yield of 94%. On the other hand,
excellent yields were achieved when aryl bromides were applied as
the substrate (entries 7e15); 4-bromobenzonitrile afforded
outstanding reaction yield of 98%. Based on the encouraging find-
ings, the scope of Pd NPs@-Sch-boehmite was expanded for aryl
chlorides which have strong CeCl bond and good yields were ob-
tained (Table 1, entries 16e21). The reaction was tolerated by
diverse functional groups such as nitro, methyl, methoxy, amino,
and nitrile functionalities; the nature of the substituent on the ring
of aryl halides did not affect the reaction time, even for compounds