Glaser coupling at metal surfaces

Article abstract of DOI:10.1002/anie.201208597

On-surface synthesis is a promising approach for constructing covalently bound nanostructures. However, the number of reliable chemical reactions suitable for on-surface chemistry is very limited. Arylalkynes can be coupled at various surfaces in a novel

Full text of DOI:10.1002/anie.201208597

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DOI: 10.1002/anie.201208597
Surface Chemistry
Glaser Coupling at Metal Surfaces**
Hong-Ying Gao, Hendrik Wagner, Dingyong Zhong, Jçrn-Holger Franke, Armido Studer,* and
Harald Fuchs*
The direct “bottom-up” preparation of innovative nano-
scaled systems at surfaces is of great interest because of the
potential applications in materials science and molecular
electronics.[1] In this regard, on-surface synthesis, by which
defined nanostructures are constructed at surfaces through
the covalent coupling of organic precursor molecules, is a key
technology that constitutes a young research field. Such
processes (two-dimensional or 2D reactions) are generally
carried out under ultra-high-vacuum (UHV) conditions and
the success of the reaction is conveniently monitored by
scanning tunneling microscopy (STM). Using this approach
the synthesis of robust functional materials that cannot be
prepared by “classical” solution-phase synthesis is possible.[1a]
To date, the most frequently applied 2D reaction is the
Ullman coupling of aryl halides at various metal surfaces.
Based on early contributions of Hla et al.,[2] Hecht and Grill
developed a versatile process for the on-surface homocou-
pling of aryl bromides for the preparation of linear and 2D
oligomers.[3] Mꢀllen and Fasel further developed that surface
reaction and successfully constructed conjugated polymeric
materials and graphene nanoribbons.[4] A “state-of-the-art”
application of the 2D Ullman coupling was reported by Hecht
and Grill recently.[5] It was shown that orthogonal coupling of
aryl bromides and iodides at the surface resulting in
conjugated polymeric networks is possible. Besides Ullman
coupling, other chemical reactions such as imine bond
formation,[6] dehydration and esterification of boronic
acids,[7] thermal dimerization of N-heterocyclic carbenes,[8]
and acylation reactions[9] have been applied at surfaces. By
using a geometric constraint, the dehydrogenative coupling of
inert alkanes at anisotropic Au(110) surfaces was recently
achieved, which also convincingly demonstrates the potential
of on-surface synthesis.[10]
Herein, we present the on-surface coupling of various
arylalkynes (Glaser coupling),[11] which is halogen free and
without “capping” ions, as an efficient 2D reaction at various
metal surfaces (Scheme 1a). Arylalkynes are readily acces-
sible starting materials and if the 2D Glaser coupling is
conducted with bisethynylarenes (Scheme 1b), the resulting
conjugated p-systems should show interesting physical prop-
erties.[12]
As monomers for initial investigations we chose the
diethynyl-substituted p-system 1 (Figure 1a; for the prepara-
tion of 1 see the Supporting Information). We first studied the
on-surface Glaser coupling of 1 on an Au(111) surface.
Experiments were performed with an integrated UHV sur-
face-analysis system equipped with an STM (Omicron).
Compound 1 was deposited on an Au(111) surface at room
temperature to form a clean monomolecular monolayer on
[*] Dr. H.-Y. Gao,[+] Dr. D. Zhong, Dr. J.-H. Franke, Prof. Dr. H. Fuchs
Physikalisches Institut
Westfꢁlische Wilhelms-Universitꢁt Mꢂnster
Wilhelm-Klemm-Strasse 10, 48149 Mꢂnster (Germany)
and
Center for Nanotechnology
Heisenbergstrasse 11, 48149 Mꢂnster (Germany)
E-mail: fuchsh@uni-muenster.de
Prof. Dr. H. Fuchs
Institute for Nanotechnology
Karlsruhe Institute of Technology
76344 Karlsruhe (Germany)
Dr. H. Wagner,[+] Prof. Dr. A. Studer
Organisch-Chemisches Institut
Westfꢁlische Wilhelms-Universitꢁt Mꢂnster
Corrensstrasse 40, 48149 Mꢂnster (Germany)
E-mail: studer@uni-muenster.de
[+] These authors contributed equally to this work.
Figure 1. a) Molecular structure of alkyne 1. b) High-resolution STM
image of the adsorbed alkyne 1 on Au(111) (sample voltage À0.5 V,
tunnel current 100 pA, 6 nmꢀ6 nm). c) Oligomers formed from alkyne
1 on Au(111) after annealing at 123–1408C (À2 V, 20 pA,
[**] We thank the Deutsche Forschungsgemeinschaft (SFB 858, TRR 61)
for financial support and Prof. Lifeng Chi for helpful discussions. H.-
Y.G. was supported by the Alexander von Humboldt-Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208597.
40 nmꢀ40 nm). d) High-resolution STM image of oligomers formed
from alkyne 1 on Au(111) (À0.2 V, 10 pA, 15 nmꢀ15 nm).
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the successful bond-forming events to the
different types of reactions (Figure 2).
Since it was not possible to unambigu-
ously distinguish between reactions I and II,
as well between IV and V in some cases, we
combined the different reaction pathways to
the pairs I/II and IV/V for the statistical
analysis. However, it is important to note
that within the I/II pair the main reaction
type is the targeted linear homocoupling (I,
Glaser reaction). The I/II pathways occurred
with a proportion of (26 Æ 3)% on average
over four STM images analyzed (such as
Figure 1c) on the Au(111) surface. Reaction
pathway III could be identified with a fre-
quency of (10.1 Æ 1.7)% and pathways IV/V
Scheme 1. a) General outline of the 2D Glaser coupling at surfaces. b) On-surface Glaser
coupling for the construction of conjugated chains.
the surface, as analyzed by UHV-STM (Figure 1b). The
length of the molecule along the alkyne axis accounted for
2.43 nm and the length along the alkyl chains was 2.01 nm
(periodic cell: a 1.99 nm, b 1.36 nm, angle 1168, as shown in
Figure 1b). Subsequent annealing of the 1-covered Au(111)
surface (ca. 1258C for 30 min) led to oligomerization of
1 (formation of short chains) through the covalent coupling of
the alkyne functionalities (Figure 1c; temperatures between
1238C and 1408C led to the same reaction outcome).
were most abundant, occurring with a frequency of (50 Æ
3.6)%. The trimerization process (VI) occurred with a pro-
À
The success of the C C bond formation was unambigu-
ously proved by moving oligomers with the STM tip without
destroying the connections between the individual monomer
units (see the Supporting Information). Moreover, we com-
pared the experimental center-to-center distance between
two linked linear alkyne moieties with the theoretical bond
length obtained by DFT calculations in the gas phase. The
data are in good agreement (experimental value: (2.36 Æ
0.02) nm; theoretical value: 2.32 nm; see the Supporting
Information). Therefore, an alternative conjugation mode
where an Au atom links two alkyne moieties for which we
calculated a center-to-center distance of 2.58 nm can be
unambiguously excluded. Along with the targeted 2D Glaser
coupling at surfaces, STM images also clearly indicated the
occurrence of side reactions which led to chain branching (for
further comparison of experimental and theoretical center-to-
center distances see the Supporting Information). We iden-
tified several types of side reactions: apart from the desired
Glaser coupling (Figure 2aI) we observed formal hydroalky-
nylation of the terminal alkyne functionality either at the a-
or b-position (Figure 2aII,aIII). We additionally observed the
formation of dienyne products (Figure 2aIV) and hydro-
alkynylation also occurred at the Glaser product leading to an
enediyne moiety (Figure 2aV). Interestingly, we also
observed alkyne trimerization providing a branching point
with an aromatic core structure (Figure 2aVI). Furthermore,
two rare types of branching points with diene–diyne moieties
could also be identified as others (see the Supporting
Information).
For a more detailed study of the selectivity of the on-
surface Glaser coupling we performed a statistical analysis
with respect to the relative frequency of occurrence of the
reaction pathways I–VI. To this end, we analyzed the
connections between independent molecules and assigned
Figure 2. a) Overview of the STM images (À1 V, 100 pA, 6 nmꢀ6 nm)
after on-surface oligomerization. Rationalization of the different
observed reaction pathways (annealing temperature 123–1408C) and
the corresponding molecular structures. b) Statistical analysis for the
distribution of the observed products on Au(111).
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portion of (10.7 Æ 3.1)% and bond formation according to
“other” pathways occurred to a small extent ((3.6 Æ 2.4)%).
All identified reaction pathways may lead to the con-
struction of novel interfacial networks. However, here we
focus on the optimization of the process towards highly
selective Glaser coupling (pathway I) for the controlled on-
surface synthesis of linear conjugated polymer chains.
We first investigated the influence of the structure of the
metal surface on the oligomerization of 1 by switching to the
Au(100) surface to achieve enhanced selectivity for formation
of long polymer chains (suppression of chain branching).
However, after the substrate had been covered with alkyne
1 and annealed, the surface reaction did not show a better
selectivity and shorter chains were formed (for STM images
see the Supporting Information).
Other substrates such as Cu(111) and Ag(111) were also
investigated. As the Glaser coupling in solution is mediated
by copper complexes, it is worth considering Cu(111) as
a solid substrate for the 2D Glaser coupling. We observed the
desired homocoupling reaction for alkyne 1 and obtained
oligomeric chains (see the Supporting Information). How-
ever, selectivity towards the linear chains was low and many
branching points were identified. In contrast, we found that
the Ag(111) surface exhibits high selectivity towards Glaser
coupling. Alkyne 1 deposited on the Ag(111) surface formed
highly ordered monolayers. Subsequent annealing of the 1-
covered Ag(111) surface provided oligomeric structures with
up to 15 monomer units in a row (Figure 3a). For a more
quantitative analysis of the reaction outcome we performed
a statistical analysis of the different reaction modes as
described above (Figure 3b).
We found that the two-unit linear coupling I/II at Ag(111)
showed a relative abundance of up to (64.3 Æ 8.1)%. This was
far higher than the proportion obtained on the Au(111)
surface. Consequently, the occurrence of reaction pathways
III, IV/V, VI, and “other” was significantly reduced to (4.6 Æ
0.6)%, (19.7 Æ 5.7)%, (6.8 Æ 1.9)%, and (4.6 Æ 0.9)%, respec-
tively. Hence, oligomerization of 1 for the generation of p-
conjugated linear chains by Glaser coupling is much more
efficient on Ag(111) than on Au(111) surfaces.
To further improve the selectivity towards reaction
pathway I, we next varied the substitution pattern of the
bisalkynylarene monomer. An ortho substituent next to the
alkyne functionality should slow down the side reactions for
steric reasons and we therefore prepared alkyne 2 (Figure 4a;
for the preparation see the Supporting Information). Depo-
sition of alkyne 2 on Au(111) gave a highly ordered
monolayer on the metal interface over a large area (Fig-
ure 4b) with the alkyne groups of isolated molecules in close
proximity.
The measured length along the alkyne axis was 1.00 nm
and length along the alkyl side chains was 2.01 nm. The
parameters for the periodic cell were: a 1.37 nm, b 1.03 nm
with an angle of 1018. Annealing of the molecular layer of 2
on Au(111) led to 2D homocoupling yielding short linear
polymer chains as shown in Figure 4c. Moreover, we com-
pared the experimental center-to-center distance of linear
linked molecules of type 2 with the theoretical bond length
obtained by DFT calculations. The resulting distances are in
very good agreement (experimental value: (0.93 Æ 0.01) nm;
theoretical value: 0.95 nm). Again, the possible C-Au-C
linkage resulting in a calculated center-to-center distance of
1.20 nm could be excluded.
In analogy to the study of the reaction outcome with
alkyne 1, we compiled a statistical distribution for all
reactions occurring on the Au(111) surface by analyzing
intermolecular connections (Figure 4c,d). The coupling of 2
on Au(111) showed a strongly enhanced selectivity towards
the formation of the targeted linear chains with a frequency of
(92.1 Æ 5.2)%, while reactions following pathways IV/V were
completely suppressed (Figure 5a). Reactions III ((2.6 Æ
0.9)%) and VI ((2.2 Æ 2.7)%) were also largely reduced.
This statistical analysis indicates that when one ortho position
next to the reacting alkyne moiety is blocked with an alkyl
substituent, the Glaser coupling becomes the major reaction
pathway and good selectivity results.
A further improvement of the on-surface oligomerization
was achieved by performing the alkyne coupling reaction with
2 on an Ag(111) surface (Figure 4e,f). In this case, both the
structure of the molecules and the effect of the surface made
the Glaser coupling the preferred reaction among the various
processes shown in Figure 2a. Annealing of the Ag(111)
surface covered with 2 at 1258C led to gradual chain growth
and within 30 min we observed polymeric chains with high
selectivity; the longest chains contain 59 precursor units (little
branching, Figure 4e). Growth of monomer 2 at the hot clean
surface could further lead to larger conjugated polyalkyne
islands. The main reaction pathway was the Glaser homo-
Figure 3. a) STM images of alkyne 1 on Ag(111) after on-surface
oligomerization (À2 V, 10 pA, 40 nmꢀ40 nm, area of inset
5 nmꢀ5 nm). b) Statistical analysis for the distribution of the observed
products on Ag(111).
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Figure 5. Statistical analysis for the distribution of observed products
from reactions of 2 on a) Au(111) and b) Ag(111).
chains in spatial confinement at surfaces, which is impossible
in conventional solution schemes. The generation of p-
conjugated linear polymers directly at the interface is possible
by using appropriate precursors and subsequent on-surface
coupling. It turned out that the Glaser coupling is more
efficient on Ag(111) surfaces than on Au(111) and Cu(111).
The surface acts as a 2D supporting system allowing the
molecules to adsorb and orient in two-dimensional space, and
it also likely mediates the reactions. The substitution pattern
of the organic precursor molecules is also of great importance
and simple steric shielding of the alkyne led to the suppres-
sion of side reactions. We believe that the approach presented
herein is not restricted to the formation of p-conjugated linear
chains but is also appropriate for the formation of defined p-
conjugated two-dimensional networks.
Figure 4. a) Molecular structure of alkyne 2. b) High-resolution STM
image of alkyne 2 on Au(111) (0.5 V, 100 pA, 6 nmꢀ6 nm). c) and
d) STM image and high-resolution image of 2 on Au(111) after
annealing (0.5 V, 100 pA, 25 nmꢀ25 nm and 5 nmꢀ5 nm). e) and
f) STM image and high-resolution image of 2 on Ag(111) after
annealing (0.5 V, 50 pA, 40 nmꢀ40 nm and 5 nmꢀ5 nm).
coupling occurring with a frequency of (96.4 Æ 1.7)%. The
abundance of reaction pathways III and VI was further
reduced to (0.8 Æ 0.4)% and (1.7 Æ 0.8)%, respectively (Fig-
ure 5b).
The optimization of the 2D Glaser coupling indicated that
both the ortho substituents at the arene next to the alkyne
moiety in the organic template and also Ag(111) as a solid
substrate are important for achieving the selective and
efficient coupling of alkynes at the interface. Based on
generally accepted mechanisms of Glaser-type couplings,[12a]
we assume that annealing of the surface-assembled mono-
mers leads to alkynyl–metal intermediates at the interface,
which undergo reductive coupling to give the Glaser product.
We believe that the different binding energies of the
precursor molecules to the solid substrate rationalize the
surface-dependent selectivity towards the Glaser coupling
reaction of the alkynes.
Experimental Section
Experiments were performed with a UHV low-temperature STM
(Omicron) at a base pressure of 1 ꢁ 10À10 mbar, which operates at
78 K in the constant-current topographic mode. The bias is the sample
voltage with respect to the STM tip. Atomic flat single-crystalline
metal surfaces were cleaned by cycles of ion sputtering and annealing.
Alkyne 1 was evaporated in UHV by sublimation from a Knudsen cell
at roughly 1508C onto the metal surface by using organic molecular
beam deposition (OMBD). The deposition rate, approximately
0.04 MLminÀ1 (monolayers per minute), was calibrated from the
STM images. Alkyne 2 with a relatively high vapor pressure at room
temperature was deposited onto the metal surface by free diffusion
using a custom-designed funnel in an isolated chamber (the funnel
was made of aluminum foil to hold a small quartz tube containing 2).
In summary, we have showed that the Glaser coupling can
be used highly efficiently to generate linear oligomer/polymer
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The deposition rate, 1.2 MLminÀ1, was calibrated from the STM
images. The annealing process for the coupling reaction was
monitored by an IR thermometer.
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Received: October 25, 2012
Published online: February 19, 2013
À
Keywords: C C coupling · conjugation · polymerization ·
scanning tunneling microscopy · surface chemistry
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