Cross-coupling of aryl-bromide and porphyrin-bromide on an Au(111) surface

Article abstract of DOI:10.1002/chem.201501095

Cross-coupling is of great importance in organic synthesis. Here it is demonstrated that cross-coupling of aryl-bromide and porphyrin-bromide takes place on a Au(111) surface in vacuo. The products are oligomers consisting of porphyrin moieties linked by

Full text of DOI:10.1002/chem.201501095

DOI: 10.1002/chem.201501095
Communication
&
Surface Chemistry
Cross-Coupling of Aryl-Bromide and Porphyrin-Bromide on an
Au(111) Surface
Guowen Kuang,[a] Qiushi Zhang,[a] Deng Yuan Li,[b] Xue Song Shang,[b] Tao Lin,[a]
Pei Nian Liu,*[b] and Nian Lin*[a]
share fundamental similarities with their counterpart reactions
Abstract: Cross-coupling is of great importance in organic
synthesis. Here it is demonstrated that cross-coupling of
in solution. Nevertheless, on-surface reactions can also follow
a significantly different path or mechanism. The special condi-
aryl-bromide and porphyrin-bromide takes place on
tions of surface chemistry can be used to synthetic advantage,
a Au(111) surface in vacuo. The products are oligomers
as on-surface reactions under ultrahigh vacuum (UHV) can be
consisting of porphyrin moieties linked by p-phenylene at
conducted under a much wider range of temperatures than re-
porphyrin’s meso-positions. The ratio of the cross-coupled
actions in solution, and two-dimensional confined geometry
versus homocoupled bonds can be regulated by the reac-
can invoke reactions not accessible in three dimensions.
tant concentrations. Kinetic Monte Carlo simulations were
On-surface synthesis has been used to generate diverse
applied to determine the activation barrier. It is expected
organic systems, including macromolecules,[18] polymeric
that this reaction can be employed in other aryl-bromide
chains,[17,19,26,38] two-dimensional organic networks,[20,22,24,27,28]
precursors for designing alternating co-polymers incorpo-
and graphene ribbons.[21]
rating porphyrin and other functional moieties.
To date, carbonÀcarbon bond formation by homocoupling
reactions has been demonstrated in on-surface synthesis.
Cross-coupling reactions, however, have rarely been demon-
strated on-surface,[39] despite their significance in organic syn-
Cross-coupling is a powerful tool available to synthetic chem-
ists in their quest to create new carbonÀcarbon bonds, while
thesis. Here we report on cross-coupling of aryl-bromide (1)
at the same time introducing new carbon- and heteroatom-
based functional groups.[1,2] Cross-coupling reactions allow
chemists to design and manipulate delicate and complex mol-
ecules.[3–8] These reactions have been proved useful for the
synthesis of many important products, such as drugs, materi-
als, and optical devices.[9–11] Recently, it has been demonstrated
that coupling reactions can take place on solid surfaces,
known as on-surface synthesis.[12–16] In an on-surface reaction,
covalent bonds form between molecular precursors that
adsorb on a surface; during this process, reactants, intermedi-
ates, catalysts and products are confined to a two-dimensional
space defined by the surface. Various on-surface reactions, in-
cluding Ullmann coupling,[17–22] Glaser coupling,[23–25] alkane
polymerization,[26] boronic acid condensation,[27,28] decarboxyla-
tive polymerization,[29] imine coupling,[30,31] acylation reac-
tion,[32,33] dimerization of N-heterocyclic carbenes,[34] azide–
alkyne cycloadditions,[35,36] and Bergman cyclizations[37] have
been demonstrated. To a certain extent, on-surface reactions
and porphyrin-bromide (2; Scheme 1) on Au(111) surface. This
reaction, to our knowledge, has not been reported in organic
synthesis. As illustrated in Scheme 1, 1 undergoes homocou-
pling by Ullmann reaction, but steric hindrance inhibits homo-
coupling of 2. Mixing of 1 and 2 generates cross-coupled prod-
ucts of p-phenylene-linked porphyrin oligomers. Through ana-
lysing the yields of homocoupled and cross-coupled bonds,
we quantitatively evaluated the activation barriers of the two
coupling reactions, aided by kinetic Monte Carlo simulations.
The yield of the cross-coupled bonds could be boosted with
excess amount of 2 in the reactant mixture. We found that at
a ratio of [2]/[1]=13 ([2] and [1] stand for molecular dosage of
2 and 1, respectively, in a unit surface area), 95% of all formed
bonds are cross-coupled ones and the products are oligomers
with alternating (12)n morphology.
Homocoupling of molecules of 1 on an Au(111) surface by
Ullmann reaction occurs at 1808C, forming polymeric chains
consisting of alternating porphyrin and biphenyl moiet-
ies.[17,40,41] Figure 1a shows a 9-member chain and the inset
shows an unreacted monomer. The scanning tunneling micros-
copy (STM) topograph of the molecules in the chain retains
the features of the unreacted monomer, displaying a square-
shape morphology with a depression trough in the middle, as
marked by the dashed lines in Figure 1a. The molecules thus
exhibit twofold symmetry which reflects a nonplanar porphyrin
core. This feature corroborates the previous studies reporting
the porphyrin core is distorted to a saddle-shape conformation
when adsorbed on a metal surface.[42] Adjacent molecules in
the chains are 1.73 nm apart; this is in good agreement with
[a] G. Kuang, Q. Zhang, Dr. T. Lin, Prof. Dr. N. Lin
Department of Physics
The Hong Kong University of Science and Technology
Clear Water Bay (Hong Kong)
E-mail: phnlin@ust.hk
[b] D. Y. Li, X. S. Shang, Prof. Dr. D. N. Liu
Shanghai Key Laboratory of Functional Materials Chemistry and
Institute of Fine Chemicals, East China University of Science and
Technology, Meilong Road 130, Shanghai (P. R. China)
E-mail: liupn@ecust.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501095.
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Figure 1. a) STM topograph of a polymeric chain formed by Ullmann cou-
pling of 1. The scale bar is 1 nm. Inset (2.24”2.58 nm2): an unreacted mono-
mer of 1 with the structural model overlaid (the bromine ends are highlight-
ed by dots). b) Structural model of the polymeric chain illustrating the
saddle-shaped porphyrin core and the bending morphology.
Scheme 1. Cross-coupling of aryl-bromide (1) and porphyrin-bromide (2).
Homocoupling of porphyrin-bromide is inhibited by steric hindrance.
the structural model shown in Figure 1b. Note that the 9-
member chain is not perfectly straight: the chain bends 268
wherever the troughs of two neighbouring molecules are not
parallel. We propose that this non-straight morphology is asso-
ciated with the distorted porphyrin core. The model in Fig-
ure 1b highlights the saddle-shaped conformation: two up-
titled pyrrole moieties are shaded and the troughs are repre-
sented by the dashed lines. It has been reported that four
phenyl moieties at the meso-positions of a saddle-shaped por-
phyrin core point to different orientations.[42] Accordingly, the
two opposite phenyl moieties in a molecule are not co-linear,
as drawn schematically in the model. With such a molecular
conformation, the chain is bent where two adjacent molecules
have differently orientated troughs, as illustrated in Figure 1b.
Molecules of 2 self-assemble as closely packed molecular is-
lands on Au(111) as prepared at room temperature and after
1808C annealing, as shown in Figure 2a and b, respectively.
The inset in Figure 2b features a single molecule, displaying
a depressed central region and two protruding ends that can
be assigned to two phenyl moieties. A structural model is over-
laid on the STM topograph. Presumably, the porphyrin core is
closer to the substrate, whereas the phenyl moieties rotate out
of the molecular plane with a large dihedral angle, giving rise
to the bright ends. We use dotted ellipses to mark out single
molecules in the closely packed molecular islands in Figure 2a
and b. Close inspection of Figure 2a and b reveals that the
molecules arrange differently before and after 1808C anneal-
ing. Each molecule occupies 1.48 nm2 (1.35 nm2) before (after)
the annealing. So the molecules are packed slightly denser
after the annealing. The structural change hints that the an-
nealing has altered intermolecular interactions. We speculate
this change is associated with debromination of 2, which has
taken place at 1808C annealing. The molecular models of the
two closely packed islands are drawn in Figure 2a and b, re-
spectively. In an attempt to use catalysts to promote homo-
coupling of 2, we deposited Cu and 2 on Au(111). After anneal-
ing at 2008C, chain-like structures formed. The chains comprise
an organometallic compound in which neighbouring porphy-
rin molecules are linked by CÀCuÀC bonds (see the detailed
description in the Supporting Information). Apparently, unlike
1, 2 does not undergo homocoupling even when the bromine
atoms cleaved off. We further annealed this sample at up to
2508C, and observed that the molecules still formed islands of
the same structure shown in Figure 2b, but not any covalently
linked chain-like structures. So we conclude that homocou-
pling of 2 does not occur on Au(111) surface up to 2508C with
or without Cu deposition.
Considering that Cu surface is more reactive than Au in acti-
vating on-surface Ullmann coupling,[43–45] we deposited 2 on
a Cu(111) surface held at room temperature. Figure 2c shows
that the molecules organize as linear chain-like structures. The
adjacent molecules in a chain are spaced at 1.20 nm. This dis-
tance agrees well with an organometallic compound in which
neighbouring porphyrin moieties are linked by CÀCuÀC
bonds,[38] as illustrated by the model in Figure 2d, with a CÀCu
distance of 2.5 Š. Because formation of the organometallic
compounds associates with debromination, debromination of
2 must take place at room temperature on Cu(111). As dis-
cussed above, the same reaction is activated by 1808C anneal-
ing on Au(111). This contrast indicates that the Cu surface cat-
alyses debromination of 2. Annealing this sample at 2008C dis-
solved the organometallic structures, however, did not result
in covalently linked structures. Instead, individual molecules
are scattered on the surface (Figure 2e). We speculate that the
molecules of 2 undergo conformational distortion at high tem-
perature and consequently bind the substrate Cu atoms
strongly. This molecule–substrate bonding prevents the mole-
cules from forming the organometallic chains or the ordered
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always connected with those of motif-1, whereas the motif-
1 species can be connected with both types. The distance be-
tween two adjacent motif-1 species is 1.73 nm, while the dis-
tance between a motif-1 and a motif-2 species is 1.30 nm.
Based on the spacing and their characteristic shapes, we
assign the motif-1 species to 1, and the motif-2 species to 2.
The oligomer shown in Figure 3b is thus a chain of 1112 se-
quence. In the same way, we are able to unambiguously deter-
mine the sequence of any oligomers. Figure 3c shows some
examples of the oligomers featuring 2112, 12, 112, and
211112 sequences. Apparently, both homocoupling of 1 and
cross-coupling between 1 and 2 happened in the on-surface
reactions.
Figure 2. STM topographs (8”8 nm2) showing closely packed molecular is-
lands assembled by 2 on Au(111): a) at room temperature, and b) after an-
nealing at 1808C. Inset (b; 2.5”2.5 nm2): a single molecule with a structural
model overlaid. Dotted ellipses mark out single molecules. c) Organometallic
chains formed by 2 at room temperature on Cu(111) (10”6.5 nm2). d) Struc-
tural model of the organometallic chains (dots represent Cu atoms). e) Mole-
cules of 2 scattered on Cu(111) after annealing at 2008C (30”30 nm2). Inset
(2.5”2.5 nm2): a single molecule with a model overlaid showing metalation
of the porphyrin core.
molecular islands. Note that the molecules now appear bright-
er at the centre (Figure 2e, inset), presumably due to metala-
tion of the porphyrin core by Cu atoms,[46–48] as illustrated by
the overlaid molecular model in Figure 2e (inset). In conclu-
sion, homocoupling of 2 does not take place on Cu(111) sur-
face either. We attribute this behaviour to steric hindrance. It
has been reported that derivatives of 2 can be covalently
linked as oligomers in solution phase.[49–51] In these oligomers,
the neighbouring porphyrin moieties do not align in co-planar
conformation, but have a nearly 908 dihedral angle. As con-
fined on a surface, however, the porphyrin cores are parallel to
the surface. Supposing that two co-planar porphyrin cores are
linked by a CÀC bond at their meso-position, the hydrogen
atoms at the beta-positions will overlap in space. We propose
that steric repulsion inherently hinders on-surface homocou-
pling of 2.
To achieve cross-coupling of 1 and 2, both species were de-
posited on the Au(111) surface at room temperature, after
which the sample was annealed at 1808C. STM (Figure 3a) re-
veals closely packed ribbon-shaped islands composed of mole-
cules of 2 (see detailed description in the Supporting Informa-
tion) as pointed by the arrow, and many oligomer structures.
An example of these oligomers is shown in Figure 3b. One can
see that the oligomer consists of wide and narrow motifs, de-
noted as motif-1 and motif-2, respectively, hereafter. Motif-
1 has a shape resembling 1 (Figure 1a) whereas motif-2 resem-
bles 2 (Figure 2b). We found that the motif-2 species are
Figure 3. a) STM topograph (50”50 nm2) showing the oligomers formed out
of a mixture of 1 and 2. b) A magnified STM topograph of a 1112 oligomer
with a structural model. c) Selected oligomers formed in sample 1 with their
sequences marked. d) Selected oligomers formed in sample 3 exhibiting al-
ternating sequences. The images in c) and d) are in the same scale as b).
We conducted statistical analysis to quantify the yields of
the homocoupling and cross-coupling reactions (Table 1). In
sample 1, at the concentration ratio of the two molecules [2]/
[1]=1.6, 85% of molecules 1 and 29% of molecules 2 are cou-
pled. Among the formed bonds, 40% are homocoupled and
60% are cross-coupled. We tested how reactant concentration
may affect the yields of homo- and cross-coupling. In sample
2, the molecule dosage was increased 2.5-fold while the ratio
of [2]/[1] remained at 1.6. After the same annealing process as
conducted on sample 1, 75% of molecules 1 and 24% of mol-
ecules 2 are coupled; 36% of the bonds formed are the result
of homocoupling and 64% the result of cross-coupling
(Table 1). To conclude, varying the concentration but not the
ratio of the two reactants does not alter the yield of the cross-
coupling significantly. To boost the cross-coupling yield, we
raised the ratio [2]/[1] to 13 in sample 3, while maintaining the
total molecule concentration as in sample 1. We found that
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95% of the coupled bonds are the result of cross-coupling
(Table 1), indicating that cross-coupling predominates over ho-
mocoupling. Some examples of the oligomers formed in
sample 3 are shown in Figure 3d, exhibiting 212, 21212,
212112 and 121212 sequences. These examples demonstrate
that the cross-coupling may generate oligomers with alternat-
ing morphology at this unbalanced reactant concentration.
The drawback of this protocol is that a large amount (~90%)
of molecules of 2 are not used.
Table 1. Statistical analysis of homocoupled and cross-coupled bonds
that formed in three samples with different reactant concentrations.[a]
Sample 1
Sample 2
Sample 3
[1] per 100 nm2
[2] per 100 nm2
[2]/[1]
[coupled 1]/[1]
[coupled 2]/[2]
[homocoupled]
[cross-coupled]
4.6
7.5
1.6
85%
29%
40%
60%
11
18
1.6
75%
24%
36%
64%
0.8
10
13
95%
11%
5%
95%
Figure 4. a) Chain-length distribution histogram of sample 1 and 3. b) kMC
simulated chain-length distribution. c) kMC simulated structures with [2]/
[1]=1.6. The crosses containing dark and grey bars represent 1 and 2, re-
spectively, the white tips stand for the bromine ends. The black bars indicate
the CÀC bonds formed in the coupling reaction. d) Yield of cross-coupled
bonds as a function of [1]/([1]+[2]).
[a] Molecules counted: 2656 (1) and 4320 (2) in sample 1, 1357 (1) and
2209 (2) in sample 2, and 1612 (1) and 21278 (2) in sample 3.
simulated yields of the cross-coupled bond as a function of
[1]/([1]+[2]) are plotted in Figure 4d (the crosses mark the ex-
perimental values in samples 1 and 3, respectively). The linear
decay trend shows that the concentration of 1 in the reactant
mixture regulates the yield of the cross-coupled bonds. More-
over, the chain-length distributions of the simulated chains at
the two experimental conditions (Figure 4b) reproduce the ex-
perimental data.
Furthermore, we analysed the length of the alternating
chains formed in the three samples and obtained chain-length
distributions. Polymerization is generally categorized as step-
growth, which results in a monotonous decay length distribu-
tion, or chain-growth, which results in a Poisson-type length
distribution.[52] As shown in Figure 4a, the chains formed in
sample 1 and 2 ([2]/[1]=1.6) exhibit a monotonous decay
chain-length distribution, suggesting that the cross-coupling
reaction leads to a typical step-growth polymerization. The
average chain length is 2.75 monomers. Interestingly, the
chain-length distribution of sample 3 ([2]/[1]=13) deviates
from a monotonous decay, showing that the odd-numbered
chains become more abundant as compared with the shorter
even-numbered ones. Presumably, this is due to the effect that
excessive amount of molecules 2 terminate both ends of the
chains, thus resulting in odd-numbered chains. The average
chain length in this sample is slightly longer, 2.94 monomers.
We performed kinetic Monte Carlo (kMC) simulations to eval-
uate the activation barriers, EH and EC, of the homocoupling
and the cross-coupling reactions, respectively. The details of
the kMC simulation algorithm can be found in the Supporting
Information. In the simulations, the total concentration of the
two species, [1]+[2], was set in accordance with the experi-
mental samples 1 and 3. Figure 4c shows the simulated struc-
tures formed with [2]/[1]=1.6. The crosses with dark and grey
bars represent molecules of 1 and 2, respectively. The white
tips represent bromine ends, and the black bars the CÀC
bonds formed in the coupling reaction. We found that by set-
ting EH =1.20 eV[41] and EC =1.24 eV, the outcome of the simu-
lations is in good agreement with the experimental results, in-
cluding the percentages of 1 and 2 molecules that are cou-
pled, the yields of homocoupled and cross-coupled bonds. The
In summary, we have demonstrated that cross-coupling of
aryl-bromide and porphyrin-bromide can be achieved on
a Au(111) surface. The products are oligomers of porphyrin
linked by p-phenylene moiety at meso-positions. The detailed
mechanism of this reaction is subject for further study. We
expect that this reaction can be employed in other aryl-bro-
mide precursor molecules for designing alternating co-poly-
mers incorporating porphyrin and other functional moieties.
Experimental Section
Experiments were performed in an ultrahigh-vacuum system (Omi-
cron Nanotechnology) with base pressure below 5”10À10 mbar. A
single-crystalline Au(111) substrate was cleaned by Argon-ion sput-
tering and annealing to approximately 6308C. Molecules 1 and 2
were thermally evaporated by a molecular beam evaporator and
deposited onto the Au(111) substrate which was held at room tem-
perature. The evaporation temperatures for molecules 1 and 2
were 325 and 2858C, respectively. The STM images were acquired
at 77 K and the STM imaging parameters were U=À1.00 V, I =
0.30 nA.
The kinetic Monte Carlo (kMC) simulations were performed on
a 100”100 square lattice. Periodic boundary condition was ap-
plied. Molecule 1 and 2 take a cross shape with two opposite tips
as Br atoms. Each molecule occupies 3”3 sites, so, 33”33 mole-
cules fully cover the substrate lattice sites. Molecule 1 and 2 with
defined ratio are deposited randomly onto the substrate lattice.
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ˇ
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This work was supported financially by the Hong Kong Re-
search Grants Council (project no. 603213), the National Natu-
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21190033 and 21372072) and the Science Fund for Creative Re-
search Groups (21421004).
Keywords: cross-coupling
· gold · kinetic Monte Carlo
simulation · scanning tunneling microscopy · surface chemistry
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Received: March 20, 2015
Published online on April 16, 2015
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8032
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