Dehydrogenative homocoupling of tetrafluorobenzene on Pd(111): Via para -selective C-H activation

Article abstract of DOI:10.1039/c7cc01476g

Aryl homocoupling reactions via meta- and ortho-selective C-H activation have been achieved on surfaces, but the highly important para-selective C-H activation has not been reported yet. Combined with scanning tunneling microscopy, time-of-flight secondar

Full text of DOI:10.1039/c7cc01476g

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DOI: 10.1039/C7CC01476G
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Dehydrogenative homocoupling of tetrafluorobenzene on Pd(111)
via para‐selective C‐H activation
Cheng‐Xin Wang,a Qiao Jin,a Chen‐Hui Shu,a Xin Hua,a Yi‐Tao Longa and Pei‐Nian Liu*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Aryl homocoupling reactions via meta‐ and ortho‐selective C‐H intermediates, and products of the organic reactions are all
activation have been achieved on surfaces, but the highly adsorbed on surfaces and the reactions proceed within a two‐
important para‐selective C‐H activation has not been reported yet. dimensional space. The synergy of surface morphology and
Combined with the scanning tunneling microscopy, time‐of‐flight catalytic reactivity provide a unique approach for some
secondary ion mass spectrometry and density functional theory, inefficient reactions in solution to proceed efficiently on a
here we describe dehydrogenative homocoupling of surface.6,7
tetrafluorobenzene on Pd(111) via para‐selective C‐H activation to
form perfluorinated oligo(p‐phenylene) oligomers.
deposited at 160 oC for 15 min,
annealed at 200 oC for 15 min
F
F
F
F
F
F
F
F
F
F
H
H
H
H
Pd(111)
F
F
F
F
F
F
F
F
F
F
C–H bond activation and functionalization is an extremely
attractive synthetic method because the atom‐economical
pathway produces H2 as the only by‐product. Functionalization
of the C–H bond instead of a prefunctionalization approach
reduces the number of synthetic steps, providing an attractive
alternative to classical coupling reactions.1 Over the past
decades, the development of C–H bond functionalization
reactions in organic solution has experienced an explosion of
creative advances in substrate and catalyst design to meet the
challenges of selectivity and reactivity.2 Despite these
advances, the selectivity for the aromatic C‐H activation is still
one of the most challenging topics in organic synthesis, due to
the high bond dissociation energy of C‐H bond and the
existence of multiple reaction sites. Nevertheless, direct C−H
functionalization of polyfluoroarenes has been achieved in
solution in various cross‐coupling reactions catalyzed by
transition metals, affording versatile fluorine‐containing
compounds useful as pharmaceuticals, agrochemicals and
organic functional materials.3
2
1
Figure
1.
Para‐selective homocoupling of 1,2,4,5‐
tetrafluorobenzene on Pd(111) via C‐H activation.
Some coupling reactions have been investigated at atomic
resolution on surfaces using scanning tunneling microscopy
(STM), including Ullmann coupling of aryl halides,4,6,8 Glaser
couplig of alkynes,9‐12 homocoupling of terminal alkynyl
bromides13, dimerization of alkenes,14,15 decarboxylative
polymerization,16,17 and cross coupling of porphyrin bromide
with aryl bromide.18 Ullmann reactions of aryl halides and
other aryl couplings have been used to generate diverse
macromolecular systems, including polymeric chains,19
hyperbranched oligomers,20 graphene ribbons,21,22,23 porous
molecular networks,24 super honeycomb frameworks,25 and
other stuctures.26
Coupling of aryl compounds via C‐H activation is preferable
to Ullmann coupling of aryl halides because it produces H2 as
the only by‐product, thereby avoiding halide contamination on
the surface. Aryl coupling via C‐H activation of benzene has
been achieved on various surfaces with meta‐ and ortho‐
selectivity.27‐32 However, the highly important para‐selective C‐
H activation of benzene which can afford poly(para‐phenylene)
(PPP) has not been reported.
After the pioneering work by Grill et al. in 2007,4 on‐surface
reactions, which take place on solid surfaces under ultra‐high
vacuum (UHV), have attracted tremendous attention owing to
their significant potentials for fabrication of nanostructures
and nanomaterials.5 In such processes, reactants,
Conjugated polymer PPPs, consisting of phenylene rings
sequentially connected at the 1‐ and 4‐positions, have
received considerable attention from both the academic and
industrial communities due to their numerous applications in
low‐cost organic electronic devices such as light‐emitting
diodes, field‐effect transistors, photodetectors, nonvolatile
memory, batteries, supercapacitors, solar cells and
a. Shanghai Key Laboratory of Functional Materials Chemistry, State Key Laboratory
of Chemical Engineering and School of Chemistry & Molecular Engineering, East
China University of Science and Technology, 130 Meilong Road, Shanghai,
200237, China
Electronic Supplementary Information (ESI) available: [General procedure for the
calculation, reagent source, more calculation results and the general procedure for
the ToF‐SIMS measurement]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 1
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thermoelectric generators.33‐37 Perfluorinated PPPs have
excellent electron‐transport abilities and therefore work as
efficient n‐type semiconductors, which have been applied
extensively in organic light‐emitting diodes and field‐effect
transistors.37‐39 Here we report the first on‐surface aryl
homocoupling via para‐selective C‐H activation, using 1,2,4,5‐
tetrafluorobenzene monomers and a crystalline surface of
Pd(111) to generate perfluorinated oligo(p‐phenylene)
oligomers with good control of product length. The para‐
selectivity of the C‐H activation originates from the different
chemical properties of the C‐H bond and C‐F bond. This is a rare
example of using the surface of Pd for on‐surface synthesis,40
although Pd is the most frequently used coupling catalyst in
solution.
1,2,4,5‐tetrafluorobenzene molecules were coupled via C‐H
activation with para‐selectivity to affo
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amount was over 90% on the surface. It is noteworthy that the
sticks adsorbed along the [ ] or equivalent directions of the
substrate were brighter than those adsorbed along the [ 01]
121
1
or equivalent directions, and the difference of the height
between two kinds of sticks is about 0.5 Å (Fig. 2b). The reason
for the impact of the adsorption direction to the STM
appearances of the molecule is not clear at this stage. Some
bent‐chain by‐products were observed, likely caused by radical
migration. No organometallic intermediate containing a C‐Pd‐C
moiety was observed.
Figure 3. ToF‐SIMS analysis showing the negative ions formed from
species on Pd(111). Peaks corresponding to coupling products are
labeled.
To confirm the identity of product 2, ex situ time‐of‐flight
secondary ion mass spectrometry (ToF‐SIMS) was performed.
The mass spectrum of negative ions with an m/z range from
240 to 660 amu is shown in Figure 3. The peak at m/z = 592.95
amu corresponds to the quasi‐molecular ion peak of
2
‐
[(PhF4)4H2] (C24HF16 , [M‐H]‐, calculated value is 592.98). It is
noteworthy that the peaks at m/z = 574.96 and 554.95 amu
correspond to [M‐F]‐ of
2
(C24H22F15‐, [M‐F]‐, calculated value is
574.99) and [M‐HF‐F]‐ of
2
(C24HF14‐, [M‐HF‐F]‐, calculated
value is 554.99). The peaks at m/z = 444.96 and 296.98 amu
correspond to the [M‐H]‐ of terphenyl product [(PhF4)3H2]
(calculated m/z for C18HF12‐: 444.99) and biphenyl product
‐
[(PhF4)2H2] (calculated m/z for C12HF8 : 296.99), respectively.
Figure 2a. Homocoupling via para‐selective C‐H activation on a
Pd(111) surface after depositing 1,2,4,5‐tetrafluorobenzene at 160
oC and annealing at 200 oC (‐1.5 V, ‐0.06 nA). 2b. Height
measurement of two kinds of sticks marked in a.
These perfluorinated terphenyl and biphenyl products may
have been generated as fragments via cleavage of
phase during mass spectrometry. Alternatively, they may have
been produced from the coupling of 1,2,4,5‐
tetrafluorobenzene via C‐H activation, but their high mobility
made them invisible in STM. Due to the results, the C‐F
cleavage can be excluded because the quasi‐molecular ion
peaks of such products do not exist.
2 in the gas
In a commercial UHV system (base pressure, ~3 × 10‐10
mbar) equipped with a variable‐temperature STM (SPECS,
Aarhus 150), 1,2,4,5‐tetrafluorobenzene
1 was deposited
(deposition condition: 1 x10‐9 mbar) onto clean Pd(111) held at
room temperature, then annealed to 200 oC. The STM analysis
of the sample only showed the surface of Pd(111) and no
coupling products were detected (scanning temperature: 110
K), indicating that the monomer desorbed from the surface.
This result is coincident with the DFT calculation of the low
In order to obtain longer chains of perfluorinated poly(p‐
phenylene), 1,2,4,5‐tetrafluorobenzene
1 was deposited
(deposition condition: 5 x10‐9 mbar) onto Pd(111), and the
sample was annealed to 500 ooC in 20 min and then cooled
down. Note that chamber pressure was kept at 5 x10‐9 mbar
by dosing molecule
process and
after the annealing. This ensured sufficient molecules of
1
continuously throughout the annealing
then shut off dosing
on
adsorption energy of 0.96 eV for 1. Then we tried to deposit
1,2,4,5‐tetrafluorobenzene
1
onto clean Pd(111) held at 160 oC
o
1
for 15 min, followed by annealing to 200 C for 15 min. STM
images showed that short sticks formed on the Pd(111) surface
(Fig. 2a), although the coverage of the product was rather low.
Most sticks were 18.0±0.2 Å long, consistent with the 18.0 Å
predicted by density functional theory (DFT) calculation for
the surface despite the remarkable desorption from the hot
surface.
STM showed that this procedure generated longer chains of
with 5‐9 units through coupling via similar C‐H activation
with para‐selectivity (Fig. 5). Products with 7‐8 units are
1
perfluorinated quaterphenyl 2. These results suggest that four
2 | J. Name., 2012, 00, 1‐3
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predominated (60‐70%) in all products. Chains grew primarily
along the ꢀ011ꢁ ꢀ112ꢁ ꢀ101ꢁ and equivalent directions of the
The adsorption energy of the dimer of
1.03 eV and that for monomer
1
was calculated to be
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is D0O.I9:610.e10V3,9/tCh7eCCs0i1m47il6aGr
,
,
1
surface. The dim, stripe‐like features in the centre of the image
rather seem related to contamination due to the strong
adsorption property of the surface of Pd(111). Once again, no
organometallic intermediate containing C‐Pd‐C was observed.
calculated adsorption energies may indicate some reduced
interaction of the dimer to the surface, presumably due to an
interphenylene rotation (34o). The calculated adsorption
energy of the tetramer
adsorption energy of longer chains is much higher. So, the
tetramer can be detected in the experiment and the shorter
2 was 2.12 eV, which indicate the
F
F
F
F
F
F
F
F
annealed to 500 oC in 20 min
Pd(111)
2
H
H
H
H
oligomers may desorb from the surface during the reaction
under high temperature. On the other hand, due to the low
concentration and low mobility of tetramer, the further
growth may be more difficult.
n
F
F
F
F
F
F
F
F
1
3
(n = 3-6)
Figure 4. Para‐selective coupling of 1,2,4,5‐tetrafluorobenzene via
C‐H activation on Pd(111).
Figure 5. 1,2,4,5‐tetrafluorobenzene on a Pd(111) surface
annealed to 500 oC in 20 min (26 nm × 26 nm, 2.0 V, 0.07 nA).
To elucidate the mechanism of the homocoupling of 1,2,4,5‐
tetrafluorobenzene via para‐selective C‐H activation, we
performed DFT calculations. The dehydrogenation reaction
and the coupling of the resulting intermediate were analyzed
using the climbing image nudged elastic band (CI‐NEB)
combined with the dimer method, as implemented in the
plane wave‐based Vienna ab initio simulation package (VASP)41.
The calculated reaction barrier (Ebarrier) and reaction energy
(Ereact) for the dehydrogenation of 1,2,4,5‐tetrafluorobenzene
on Pd(111) were 1.51 eV and 1.06 eV (Fig. 6), which were
comparable to the dehydrogenation of benzene on Cu(110).27
Figure 6. DFT‐calculated energy diagrams for (a)
The results indicate that dehydrogenation is highly dehydrogenation of 1,2,4,5‐tetrafluorobenzene on Pd(111)
and (b) coupling of two dehydrogenated 1,2,4,5‐
tetrafluorobenzene molecules on Pd(111). Ebarrier and Ereact
were defined, respectively, as (ETS ‐ EIS) and (EFS ‐ EIS). Top and
side views of the initial state (IS), transition state (TS) and final
state (FS) of the reactions are shown below the energy
diagrams.
endothermic, and that the resulting dehydrogenated
intermediate is very unstable. In the second step of the
reaction, two dehydrogenated 1,2,4,5‐tetrafluorobenzene
intermediates coupled para‐selectively to give the dimer with
Ebarrier = 0.46 eV and Ereact = ‐1.73 eV, indicating a highly
exothermic process. Note that the dihedral angle of the two
benzenes is 34o in FS (see supporting information, Fig. S1).
The DFT‐calculation of dehydrogenation of the dimer of
1,2,4,5‐tetrafluorobenzene on Pd(111) gave out Ebarrier of 1.41
eV and Ereact of 0.88 eV (see supporting information, Fig. S2),
although the calculation of dehydrogenation of longer chains
was not carried out due to the resource limitation. The results
demonstrated that the growth of dimer would also be smooth.
In summary, we report a dehydrogenative coupling of 1,2,4,5‐
tetrafluorobenzene on Pd(111) to form perfluorinated oligo(p‐
phenylene) chains as the first example of aryl homocoupling via
para‐selective C‐H activation. Chain length can be varied by
modifying reaction conditions. The reaction was studied at
single‐molecule resolution using UHV‐STM as well as ToF‐SIMS
analysis. DFT calculations suggest that the reaction proceeds via
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18 J. Adisoejoso, T. Lin, X. S. Shang, K. J. Shi, A. Gupta, P. N. Liu
dehydrogenation of 1,2,4,5‐tetrafluorobenzene followed by
coupling of the resulting intermediates. This reaction generates
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and N. Lin, Chem. Eur. J., 2014, 20, 4111.
DOI: 10.1039/C7CC01476G
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We greatly appreciate the insightful comments of the
reviewers in helping us to improve this paper. This work was
supported by the National Natural Science Foundation of
China (Project Nos. 21672059, 21421004, 21561162003 and
21372072), the Program for Eastern Scholar Distinguished
Professor, the Fundamental Research Funds for the Central
Universities and the Programme of Introducing Talents of
Discipline to Universities (B16017).
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Graphic abstract
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
annealing on Pd(111)
H
H
H
H
n
F
(n = 2-6)