The synthesis of biphenyl through C-H bond activation in benzene over a Pd catalyst supported on graphene oxide

Article abstract of DOI:10.1039/d0nj02230f

This is the first report on carbon-hydrogen (C-H) bond activation in benzene over a palladium catalyst supported on graphene oxide (GO) leading to the sole formation of biphenyl with a yield of 78%. The reaction was performed for 12 h in the presence of acetic acid and oxygen at 80 °C. XPS studies indicated that in the initial catalyst, Pd is mainly present as Pd(ii) species on the GO surface. The interaction of these species with acetic acid during the reaction generates Pd acetate species. Density functional theory (DFT) studies revealed that the adsorption of the first benzene molecule on the Pd acetate is weak (0.15 eV) and the energy barrier of the following C-H bond scission is high, equal to 1.67 eV. The adsorption of the second benzene molecule is relatively strong (0.40 eV); acetic acid molecules are then released, leaving the biphenyl Pd intermediate, which enables biphenyl molecule formation. The presence of oxygen and acetic acid is needed for closing the catalytic cycle via the regeneration of the reactive Pd acetate.

Full text of DOI:10.1039/d0nj02230f

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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Synthesis of biphenyl through the C-H bond activation in benzene
over a Pd catalyst supported on graphene oxide
Received 00th January 20xx,
Accepted 00th January 20xx
Deepika Sharmaa, Lyubov G. Bulushevab,c, Dmitri A. Bulushevd and Neeraj Guptaa,e*
DOI: 10.1039/x0xx00000x
This seems a first report on the Carbon-Hydrogen (C-H) bond activation in benzene over a Palladium catalyst supported on
graphene oxide (GO) leading to the formation of solely biphenyl with the yield of 78%. The reaction was performed for 12 h
in the presence of acetic acid and oxygen at 80 °C. An XPS study indicated that in the initial catalyst Pd is mainly present as
Pd(II) species on the GO surface. The interaction of these species with acetic acid during the reaction generates Pd acetate
species. A density functional energy (DFT) study revealed that the adsorption of the first benzene molecule on the Pd acetate
is weak (0.15 eV) and the energy barrier of the following C-H bond scission is high and equal to 1.67 eV. The adsorption of
the second benzene molecule is relatively strong (0.40 eV), the acetic acid molecules release leaving the biphenyl Pd
intermediate which gives further the biphenyl molecule formation. The presence of oxygen and acetic acid is needed for
closing the catalytic cycle by regeneration of the reactive Pd acetate.
The C-H bond activation is one of the most popular strategies for
the synthesis of organic molecules.15, 16 It refers to a chemical process,
where the C-H bond is activated, cleaved and then replaced with
Introduction
Synthesis of biphenyl compounds has attracted enormous attention
in the last ten years. Biphenyl is an important intermediate1 or
constituent in wide range of agrochemicals, natural products,
pharmaceuticals and many other fine chemicals. Naturally it occurs
in crude oil and coal tar.2 Activated halogenated substrates such as
iodobenzene, chlorobenzene, bromobenzene, 4-iodotoluene, 4-
chloronitrobenzene, 4-bromonitrobenzene and 4-bromotoluene are
the first choice of synthetic organic chemists and other fine
chemicals professionals for the synthesis of biphenyl.3-7 It is due to
the fact that the C-H bond cleavage in aromatic hydrocarbons such
as benzene is very hard due to its high bond energy. In general, the
decreasing order of bond energy8 (C-H > C-F > C-Cl > C-Br > C-I) clearly
shows that C-H bond is very hard to break9 in comparison to carbon
halogen bond. The synthesis of biphenyl has been carried out10 by
Fujiwara et al. in 1970, where they have synthesized this compound
from benzene by using olefin-palladium chloride complex in the
presence of silver nitrate. The reaction was proposed to proceed via
a sigma bonded benzene palladium complex. Typically, biaryl
compounds are synthesized by conventional Suzuki and Heck
reactions,11, 12 which involve the use of aryl halides as substrates.
Direct use of benzene for C-C coupling reaction can minimize the use
of halogenated substrates for this particular synthesis as they are not
environmentally friendly. Therefore, synthesis of biphenyls from
benzene presents a great opportunity for their production from non-
halogenated substrates and also opens up a gateway to probe the
mechanism of the C-H bond activation.13, 14
carbon-X bond (X is carbon, oxygen or nitrogen).17 Mostly, transition
metals are involved in the C-H bond cleavage processes.18 These
metals enable the use of easily accessible feedstock for molecular
synthesis that includes most of the organic substrates including
aliphatic and aromatic hydrocarbons.19 Recently, metals such as
palladium (Pd)20, 21, rhodium (Rh)22, ruthenium (Ru)23, copper (Cu)24
and gold (Au)25 have been used for the C-H bond activation. The C-H
bond activation in hydrocarbon involves metal insertion in the
activated [C-H] bond leading to an organometallic complex, where
the hydrocarbon is coordinated to metal, either via an intermediate
or a transition state, which finally undergo subsequent reaction with
another substrate to produce the end product.26 Yokota et al. have
performed synthesis of biphenyl with the help of homogeneous Pd
acetate catalyst and molybdovanadophosphoric acid as a co-catalyst
in the presence of acetic acid and molecular oxygen.21 But the
homogeneous catalytic pathways are not much preferred due to
difficulties in recovery of expensive metal ions used in the catalytic
process, therefore heterogeneous catalysts offer easy recovery and
recycling of these metals used during the reaction.
Nano-carbon materials27 such as GO are chemically inert and
provide an opportunity to design metal based heterogeneous
catalysts due to their good electrical conductivity, tuneable surface
functional groups and strong interaction with metal clusters.
Palladium supported on GO28 was used as an efficient catalyst for the
acetoxylation of 2-(o-tolyl)pyridine through the activation of C-H
bond. The turnover frequency of the catalyst was higher (24.8 h−1)
than that of Pd(OAc)2 (19.0 h−1) indicating the high efficiency for the
C-H bond activation. In fact, biphenyls have been synthesized by GO
based metal catalysts. PdNPs on GO and functionalized graphene
derivatives were used for the Suzuki-Miyaura coupling of 4-bromo-
1,2-difluorobenzene with 4-methoxyphenylboronic acid leading to
the formation of biphenyl.29 The problem associated with the
method is the use of activated halogenated substrates. The highest
activity during the reaction was observed for Pd(II)-GO with TOF of
39000 h-1. Also, Pd-nanoparticles supported on graphene were
a. School of Chemistry, Faculty of Sciences, Shoolini University, Solan (H.P.) India -
173229, E-Mail: neeraj.gupta@shooliniuniversity.com
b. Nikolaev Institute of Inorganic Chemistry, Novosibirsk 630090, Russia
c. Novosibirsk State University, Novosibirsk 630090, Russia
d. Boreskov Institute of Catalysis, Novosibirsk 630090, Russia
e. Department of Chemistry and Chemical Sciences, Central University of Himachal
Pradesh, Dharamshala (H.P.) India – 176206 E-mail: gupta_nrj@yahoo.co.in
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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developed for the synthesis of biphenyl through C-C cross coupling
of bromobenzene and phenylboronic acid with a turnover frequency
of 108000 h-1.30 Pd nanoparticles supported on β-cyclodextrin@GO
were used for the Suzuki-Miyaura cross coupling reaction of aryl
halides and boronic acid.31 This nanocatalyst was efficient in coupling
reaction for the direct formation of the biphenyl as well as
benzophenone derivatives. These examples clearly show that
halogenated substrates were used for synthesising biphenyl with GO
supported catalysts. Therefore, we envisioned the synthesis of
biphenyl through the non-halogenated substrate – benzene with the
help of metal supported GO catalyst. The catalyst Pd(II)@GO had
displayed good catalytic activity for the C-H bond activation of
aromatic substrates particularly benzene to produce biphenyls under
mild reaction conditions. It also simplifies the recovery of the metal
without any further treatment and represents the first report for the
use of Pd supported on GO catalyst for the C-H bond activation of
aromatics for synthesis of biphenyls.
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DOI: 10.1039/D0NJ02230F
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Figure 1. (a) C 1s and (b) Pd 3d XPS spectra of the Pd(II)@GO catalyst.
The morphology of the catalyst was determined by
transmission electron microscopy (TEM). The TEM images at
different resolutions are shown in Figure 2. TEM images displayed
flat sheets of GO with few dark spots on it (shown under white
circles). Clear-cut boundaries and nature of these particles were hard
to determine, and they may probably be due to presence of small GO
flakes or formed due to accumulation of palladium oxide/hydroxide
particles.
Results and discussion
Catalyst Characterization
The Pd(II)@GO catalyst was prepared by a procedure where a
definite volume of a 0.069 M Pd(NO3)2 solution was adsorbed on the
GO surface by wet impregnation method and then the material was
calcined in air at 150 °C (see experimental). According to thermo-
gravimetric analysis data32 the decomposition of Pd nitrate gives Pd
oxide in the same temperature range 100-160 °C. This Pd oxide is
most probably hydrated due to low temperature of calcination and
contact of the sample with wet atmosphere. Partial hydrolysis of the
initial Pd nitrate may take place also in the solution.32, 33 Inductively
Coupled Plasma Mass-Spectrometry analysis (ICP-MS) was
performed to quantify the amount of Pd metal present on the
catalyst surface and it confirmed that 3.8 wt% of palladium was
present on its surface.
The nature of Pd species and surface functional groups34, 35 was
determined by X-ray photoelectron spectroscopy (XPS) (Figure 1 and
Figure S1). The deconvoluted C 1s spectrum exhibited the peaks at
285.1, 287.2 and 289.0 eV that were assigned to C-C species (47.5%),
C-O (epoxy and hydroxyl groups) (48.3%) and COOH (carboxyl groups
(4.1%)) species, respectively.36, 37 To check the presence of nitrate
species from Pd nitrate the N 1s spectrum was plotted (Figure S1 and
S2). It showed that the sample contains nitrogen species giving the
line at 402.1 eV. The peak intensity of nitrogen was significantly more
intensive in comparison to pristine GO, indicating that additional
nitrogen atoms have been deposited on its surface during the
catalyst preparation. The peak at 402.1 eV may not be assigned to
nitrate species as those should be responsible for the lines in a much
higher binding energy region (>406.0 eV). Hence, the nitrogen
present in the catalyst could be attached to the GO support.
Figure 2. (a), (b) and (c) TEM images of Pd(II)@GO at different
resolutions (d) Small spots visible on catalyst surface are highlighted
in circle.
X-ray diffraction powder analysis (XRD) was further performed
in attempt to confirm the presence of Pd oxide. A comparison of XRD
patterns of Pd(II)@GO and GO (Figure 3) exhibited only a small and
broad peak at about 34° which could be related to (101) plane of
hydrated Pd oxide.38,39 This line is the most intensive among the
other lines of these species which are not clearly observed, but their
possible positions are indicated with ticks. High dispersion of the Pd
oxide could be a reason that the only one line was observed. Metallic
Pd lines are not seen in accordance with the XPS data. GO is
presented by two typical sharp lines at 12.7° (001, not shown) and
42.4° (100).40
The nature of Pd was probed by the Pd 3d spectrum that
exhibited two pronounced peaks at 338.1 and 343.5 eV
corresponding to the 3d5/2 and 3d3/2 lines assigned to Pd2+ ions
adsorbed on the GO surface. These ions could correspond to
expected hydrated Pd oxide. The atomic concentrations for C, O, N
and Pd in the Pd(II)@GO sample are equal to 73.2, 25.4, 1.0 and 0.4
at%, respectively.
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DOI: 10.1039/D0NJ02230F
Catalytic Efficiency
In a typical experiment, 200 mg benzene was dissolved in acetic acid
followed by the addition of Pd(II)@GO (20 mg) catalyst. The contents
were stirred at 80 C for 12 h in presence of 1 bar of oxygen gas and
the solid product was isolated in 78 % yield after the separation of
catalyst (see Experimental Section for details).
(100) GO
The formation of biphenyl was confirmed by Nuclear Magnetic
Resonance (NMR) spectroscopy. A H-NMR spectrum of benzene
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shows a singlet44 due to equivalent protons at δ 7.33 and if it is
converted to biphenyl the singlet should convert into a multiplet
showing three sets of equivalent protons in the range of δ 7-8. In fact,
three peaks between δ 7.05 - 7.46 were observed (Figure S3) which
indicates that the product formed is different from benzene. The
physical state of the product is solid and different from benzene,
which is liquid. A final confirmation was also obtained using
differential scanning calorimetry (DSC), which exhibited a peak, at
66.8 ℃ (Figure 5b). The melting point observed for the isolated solid
was 69.1 ℃, which was found to be consistent with that reported in
the literature.45 All these analyses confirmed the synthesis of
biphenyl as the end product of this reaction (Figure 5a). The DSC
analysis of crude solid clearly indicates that biphenyl is the only
product formed in the reaction. Has there been any other product
formed that should have been indicated by a separate peak in the
DSC curve. The 1H NMR spectrum also confirmed that the selectivity
of the biphenyl product was 99%.
After the isolation and characterisation of biphenyl, we studied
the effects of the reaction time, temperature, catalyst amount, acetic
acid and oxygen contents on the yield of the reaction product. Thus,
an increase in the yield of the product with increasing the reaction
time was observed (Figure 5c). The presence of induction period may
indicate the initial slow formation and accumulation of some
important intermediates of the reaction. For example, unreactive
hydrated Pd oxide could transform to Pd acetate due to interaction
with acetic acid.
The reaction was performed at different temperatures (Table
1) and there was a decrease in the yield of biphenyl at lower
temperature (70 °C) and no product was formed at 40 °C. These
changes may indicate to the high apparent activation energy of the
rate-determining step of the reaction. Therefore, an elevated
temperature is required for benzene to show the intermolecular
condensation to form biphenyl.
The catalyst loading effect was studied at 80 °C for 12 h in the
presence of acetic acid and O2 (Figure 5d). We have used 1, 3, 5 and
10 % contents of the catalyst in the reaction mixture. It was found
that the yield of the product increased with the increase in the
amount of the catalyst as was expected.
GO support
(101)
PdOxH2O
Pd(II)@GO
30
40
50
60
70
2 (degree)
Figure 3. XRD patterns of the Pd(II)@GO catalyst and pristine GO.
Figure 4: Comparison of FTIR spectra of Pd(II)@GO and GO
The IR spectrum of Pd(II)@GO was compared with that of pristine
GO to observe the changes on the surface (Figure 4). The IR spectrum
of Pd(II)@GO demonstrates the main peaks41, 42 at 3434 (not shown),
1782, 1720, 1570, 1386, 1232 and 1043 cm-1. The peaks were
assigned to –OH, -C=O, -C-OH and C-O-C linkages. The results are in
agreement to the XPS results that show the presence of carboxylic,
hydroxyl, aldehyde/ketone, and ether linkage on the surface of the
catalyst. The peaks for the same functional groups at 3352 (not
shown), 1718, 1346, 1215 and 1030 cm-1 were present in the
spectrum of the GO sample. The peaks at 1782 and 1386 cm-1 were
found to be missing. The appearance of the new peak at 1782 and
1386 cm-1 in Pd(II)@GO indicated that there is formation of a new
type of species with C=O/-COOH groups on its surface. These may be
formed due to the interaction of Pd(II) ions present on its surface.
Thus the structural features of our sample are very much close
to those of the Pearlman’s catalyst (Pd(OH)2/C). Literature reports 43
demonstrate that the Pearlman’s catalyst as prepared and after
drying consists of carbon supported (mostly) nano-particulate
hydrous palladium oxide capped with a monolayer of hydroxyls
hydrogen-bonded to a few layers of water.
To understand the role of oxygen, the reaction was performed
in atmospheric air instead of pure oxygen at the same other
conditions. There was a significant decrease in biphenyl yield to 26%
in this case (Table 1) which was provided by the decreased partial
pressure of oxygen by a factor of 5. This proves that the presence of
oxygen is very important for the synthesis of biphenyl. Oxygen is
probably needed for oxidation of Pd atoms in the course of the
reaction as was proposed earlier for aerobic homogeneous catalytic
coupling reactions.11, 12, 21
Once the nature of the catalyst was probed, it was further
applied for the C-H activation reaction of benzene.
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evaporated the filtrate and then added all the reactants (beside the
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catalyst) to the same flask. The contents were heated at optimized
DOI: 10.1039/D0NJ02230F
condition for 12 h, but in this case, no product was isolated. This
clearly shows that filtrate does not have any significant amount of Pd
species necessary for this catalytic reaction.
The efficiency of the catalyst for the formation of other biphenyl
products was checked by formation of three more derivatives by
using anisole, toluene and nitrobenzene as reactants. The reactions
were performed under optimised condition for 12 h and the product
formation was ascertained by comparing the melting point of the
product with those reported commercially (see supporting
information). It is seen in Table 2 that the approach worked well for
all reactants giving the highest yield with nitrobenzene (88%). This
yield can be well explained due to the effect of nitro groups, which
probably help the metal in the CH scission step.
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Table 2. Derivatives study with different aromatic
compounds
Reactants
Product (Melting
point, °C)
Yield (%)
Anisole
Toluene
4,4’-dimethoxy-1,1’-
biphenyl (172)
4,4’-dimethyl-1,1’-biphenyl
(116)
4,4’-dinitro-1,1’-biphenyl
(238)
78
82
88
Figure 5. (a) C-H bond activation reaction of benzene to biphenyl, (b)
Thermal gravimetric analysis (TGA)-DSC analysis of biphenyl, (c) Yield
(%) vs reaction time (h) for C-H bond activation reaction of benzene
to biphenyl by using the Pd(II)@GO catalyst, (d) Plot of the catalyst
performance against the loading of catalyst (%) at the same reaction
conditions and (e) Stability tests using the Pd(II)@GO catalyst.
Reaction condition: benzene(200 mg), 20 mg catalyst, acetic acid (1
mL), O2 atmosphere at 80°C temperature.
Nitrobenzene
Reaction conditions: Reactants (200 mg), 20 mg catalyst,
acetic acid (1 mL), O2 atmosphere and 80°C for 12 h.
The performance of the catalyst was compared with some
literature data (Table S1). It is found to be at par or better than those
discussed in the literature. Previously, the carbon supports such as
activated carbon and even GO had been used as supports for metal
catalysts for the synthesis of biphenyls.10,46-50 The catalyst described
in this work provides a better yield in comparison to earlier studied
activated carbon and GO based catalysts. Moreover, the proposed
system works effectively without the use of any other co-catalyst and
co-solvent, and provides easy separation of the product from the
reaction mixture simply by the addition of water.
To check the necessity of acetic acid, it was substituted by
chloroform. It is important to note that no biphenyl formation was
observed in this case (Table 1). This result indicates that acetic acid
is vital for the reaction. This acid should provide the formation of
some related intermediate species such as Pd acetate during the
reaction from the oxidized Pd, which, as will be shown below, is the
important active species for the C-H bond scission.
Table 1. Effect of reaction conditions on the yield of
biphenyl
DFT Calculations
DFT calculations were performed to probe the mechanism of the
reaction. We have observed that the reaction proceeds well in acetic
acid, but not in chloroform. The first step should involve the
interaction of acetic acid with the surface Pd(II) ions (Figure 6a).
Moreover, palladium acetate that preferably exists in a state51 shown
in Figure 6b (1) did not show any tendency to react with benzene.
Owing to the interaction of Pd(II) with the GO support the acetic acid
may interact transforming it into another state where the Pd atom is
attached to each acetate via a single oxygen atom [Figure 6b(2)].
Benzene molecule adsorbs on this state (2) with Ebind=0.154 eV. Then,
the new state moves through the transition state (3), where a carbon
atom of benzene is coordinated by Pd, further to the state (4). The
energy barrier (Ea) for this step is high (1.666 eV). The C-H bond
scission occurs with participation of the acetate oxygen atom. As a
result, the C-H bond in benzene is broken, phenyl is stabilized by the
Pd atom and the O-H bond in the adsorbed acetic acid molecule is
formed. The state 4 has a higher energy than the state (2) by 0.904
eV. Then, the adsorption of another benzene molecule takes place.
It is stronger than that of the first molecule with Ebind= 0.403 eV
forming the state (5). When the second phenyl interacts with the Pd
Conditions
Temperature
(°C)
Yield
(%)
TON
TOF
(h-1)
O2+ CH3COOH
O2+ CH3COOH
O2+ CH3COOH
Air+CH3COOH
O2+CHCl3
40
70
80
80
80
0
-
-
9.7
11.0
3.6
-
69
78
26
0
58.6
66.1
22.0
-
Reaction conditions: benzene (200 mg), 20 mg catalyst
(3.8 wt% of Pd), acetic acid or CHCl3 (1 mL), 12 h, The TON
is defined as TON= 2 × mole (biphenyl)/mole (Pd) and TOF
= TON/Hour.
Finally, the stability/recyclability of Pd(II)@GO catalyst was
tested through a recycling study (Figure 5e). The results have shown
that there is some decrease in the product yield under identical
reaction conditions observed during the first runs. Then, the activity
stabilizes. The initial loss may occur due to leaching of some active
metal ions into the solution or sintering of Pd species. In order to
ascertain if the metal is leaching into the solution or not, we have
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atom, two acetic acid molecules release from the complex in the Green, red, grey, and yellow balls correspond to Pd, O,VCieawnAdrtHicleaOtonmlines,
form of dimer and the biphenyl Pd molecule is formed (state (6)). The respectively.
DOI: 10.1039/D0NJ02230F
energy of this step is lower by 0.113 eV. E.A. Hausman and co-
workers have also reported52 the formation of palladium diacetate
(without the formation of palladium hydroxyacetate) by hydrous
palladium oxide with acetic acid. Moreover, this complex was
reported to be soluble in benzene, which provides a strong evidence
3. Conclusion
for existence of analogous species as reported in this work.
Hence, the DFT calculation has helped us to explain the
adsorption of benzene molecules and the C-H bond scission on the
Pd acetate. The obtained biphenyl Pd molecule can be an
intermediate in the biphenyl molecule formation. Molecular oxygen
oxidizes the Pd atom. The obtained intermediate further interacts
with acetic acid present in the solution in accordance with
experimental data (Table 1) providing the formation of the Pd
acetate able to interact with benzene again (state (2)). This closes the
catalytic cycle.
Biphenyl has been successfully synthesized from benzene with a high
yield (78%) during 12 h over a Pd catalyst supported on graphene
oxide in the presence of acetic acid and molecular oxygen at 80 C.
The reaction did not involve the use of any halogenated precursor as
required by Heck or Suzuki coupling. Supported Pd acetate is an
important intermediate in this reaction able to break the C-H bond
in benzene. It transforms to Pd biphenyl after interaction with
benzene molecules. Then, releasing of biphenyl takes place, followed
by oxidation of Pd by oxygen and formation of Pd acetate by
interaction with acetic acid, thus, closing the catalytic cycle.
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Experimental Section
Synthesis of Pd@GO catalyst
A stock solution of 0.069 M palladium nitrate in water was prepared
and 628 µL of this solution was added to acetone (20 mL) to make a
homogeneous mixture. Commercial GO (100 mg) with a surface area
of 210 m2/g was added to it. The content was stirred for 3 h at room
temperature. Afterwards, the solvent was evaporated at 55 C under
vacuum and the wet solid material obtained was heated in an oven
at 150 C in air for 10 min. The oven was allowed to cool to 40-45 oC
and material was kept at this temperature for 12 h. It was unloaded
and stored in a desiccator for further use.
Catalytic Test
Benzene (200 mg), acetic acid (1 mL) and Pd(II)@GO catalyst (20 mg)
were placed in a round bottom flask equipped with a bottle filled
with oxygen gas at 1 bar pressure. The mixture was allowed to react
under stirring at 80 C for up to 12 h. After that the reaction mixture
was cooled and filtered. The filtrate was neutralized by NaOH
solution (0.1 N) and extracted with ethyl acetate (10 ×3 mL). The
combined organic layer was evaporated under vacuum and the
residue was finally dried in hot air oven at 45C to get the white
crystalline solid. The yield was calculated by a formula (Actual yield /
1
Theoretical yield) × 100 %]. H-NMR spectroscopy confirmed the
1
structure of synthesized biphenyl (Figure S2). H-NMR spectrum of
biphenyl exhibits the peaks at δ 7.05 ppm (m, 2H), δ 7.33 ppm (m,
4H), δ 7.46 ppm (m, 4H).
Recycling Test
Each catalytic run was carried out under the following reaction
conditions: benzene (200 mg), catalyst loading 20 mg, acetic acid (1
mL), O2 atmosphere and 80°C temperature for maximum of 12 h.
After the completion of the reaction the catalyst was filtered and
used as such without any purification.
Figure 6. (a) Plausible mechanism for the C-H activation of benzene
towards biphenyl formation. (b) Structures of different
intermediates used during DFT calculation. The number in the
parenthesis is used to designate the intermediate or transition state.
The original state of palladium acetate is shown separately in a box.
DFT calculations
Calculations were carried out using a gradient-corrected functional
of Perdew-Burke-Ernzerhof (PBE) with local and nonlocal exchange
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank Indian Institute of Technology, Mandi (H. P.) India
for extending the analysis facility. D.A.B. and L.G.B. thank the support
of the budget projects for Boreskov Institute of Catalysis and Nikolaev
Institute of Inorganic Chemistry SB RAS (Russia).
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245x142mm (150 x 150 DPI)