Controlling selectivity in the Ullmann reaction on Cu(111)

Article abstract of DOI:10.1039/c7cc02901b

Using a surface science approach, the selectivity in the Ullmann cross-coupling of aryl halides on Cu(111) has been understood and controlled. The binding strength of the reactants and repulsion between them dictates which organometallic intermediates form, and hence the product distribution. Cross coupling can be maximized at low reactant concentrations.

Full text of DOI:10.1039/c7cc02901b

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Controlling Selectivity in the Ullmann Reaction on Cu(111)
†
†
E. A. Lewis , M. D. Marcinkowski , C. J. Murphy, M. L Liriano, A. J. Therrien, A. Pronschinske and
E.C.H. Sykes
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Using a surface science approach, the selectivity in the Herein we report a combined scanning tunneling microscopy (STM)
Ullmann cross-coupling of aryl halides on Cu(111) has been and temperature programmed desorption (TPD) study of the
understood and controlled. The binding strength of the reactants coupling reaction of bromobenzene (BB) and 4-ethylbromobenzene
and repulsion between them dictates which organometallic (EBB) that leads to three products: biphenyl (B-B), 4-ethylbiphenyl
intermediates form, and hence the product distribution. Cross (B-EB), and 4,4-diethylbiphenyl (EB-EB) (Scheme 1). By working with
coupling can be maximized at low reactant concentrations.
a model system in ultra-high vacuum, we are able to quantify
reactants, intermediates, and products and correlate reactivity and
The Ullmann reaction, in which two aryl halides are selectivity as a function of surface composition visualized at the
coupled over a Cu catalyst to form a biaryl molecule, is one of the atomic scale. Our initial STM and TPD results of BB and EBB co-
1
,2
oldest metal-catalyzed synthetic methods for forming C-C bonds.
deposition showed an unprecedented selectivity toward the
Although the versatility of Pd- and Ni-catalyzed reactions in recent homocoupled EB-EB product, which was due to stronger binding of
3
years has overshadowed the Ullmann reaction , it has gained EBB to the Cu than BB. However, with this knowledge, we were able
renewed interest due to the low cost and toxicity of Cu-based to control the selectivity to favor the cross-coupled product by
2
catalysts. One of the major drawbacks to-date in developing Cu- altering the reactant ratios and their absolute amount with respect
catalyzed Ullmann synthetic methods has been
a lack of to the number of surface active sites.
understanding of the reaction mechanism, and despite the broad
literature on the topic, the method is still known for unpredictable
selectivity, sensitivity to functional groups, and the need for
1
,2
extreme reaction conditions. A better understanding of the
underlying mechanism would help to mitigate these challenges and
potentially make Cu-catalyzed Ullmann synthesis more widely
applicable.
Scheme 1. Schematic showing the Ullmann coupling reaction pathway
between bromobenzene (BB) and 4-ethylbromobenzne (EBB) on a Cu(111)
surface. Heating the aryl halide reactants on Cu(111) yields organometallic
Surface scientists have extensively examined the Ullmann
coupling reaction over the past few decades. Bent and co-workers
laid the ground work for this field of study, examining the chemistry
intermediates (B-Cu-B, B-Cu-EB, and EB-Cu-EB) in which a single Cu atom
4
,5
of iodobenzene on Cu(111) . Recently, there has been a number of
studies that use the Ullmann reaction as a means to fabricate
25–28
extracted from the surface is complexed by two phenyl groups.
Further
heating induces C-C bond formation that yields the coupled products
biphenyl (B-B), 4-ethylbiphenyl (B-EB), and 4,4-diethylbiphenyl (EB-EB).
6
–22
surface bound nanostructures such as polymers and graphene.
This body of work has revealed a plethora of information on the
6
,23–25
Ullmann reaction including the importance of defect sites
, the
We first examined the behavior of the two aryl halides
formation of organometallic intermediates involving abstracted deposited individually using TPD. The TPD traces for the two species
1
2,15,17,20,22,25–28
metal adatoms
adsorbed halogens
, the stabilizing effect of the co- are very similar, although overall EBB binds more strongly to the Cu
and the dependence of the formation of surface compared to BB (Figure 1A and B). At low exposures, no
1
0,18,21,23
8
–14,16,18–21
the coupling products on the metal, facet, and reactants.
intact desorption is observed for either molecule, and only coupling
Especially relevant to this study is work by Lafferentz et al. that products are seen. As the exposure increases, we observe three
1
6
examines the effect of cross coupling in polymer synthesis.
distinct desorption peaks for the reactants that appear at 165 K,
Motivated by this work, we sought to understand and 175 K, and 240 K for BB (Figure 1A) and at 185 K, 195 K, and 270 K
control the selectivity of the Ullmann reaction for cross-coupling. for EBB (Figure 1B). We assign the lowest temperature peaks for
each molecule to desorption from multilayers as they exhibit zero
order kinetics and do not saturate. The intermediate temperature
peaks are consistent with desorption from vertically standing
Department of Chemistry, Tufts University, 62 Talbot Ave., Medford, MA 02155
USA. E-mail: Charles.sykes@tufts.edu
† Authors contributed equally to the work.
Electronic Supplementary Information (ESI) available: Experimental methods
benzene, similar to results observed for iodobenzene, benzene, and
4
,5,29–31
toluene at high coverage.
For the purposes of calculating
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relative coverages for the TPD experiments, we assign the Cu surfaces the production of both B-B from BB and EB-EB from EBB
saturation of this peak to 1 monolayer (ML). We assign the high increases significantly. In addition, we no longer detect the
temperature peaks at
desorption of intact BB or EBB from the flat-lying species, indicating
that nearly all molecules that participate in the reaction do so on
these roughened, highly stepped surfaces created by sputtering.
While the steps are known to facilitate the formation of the
organometallic (R-Cu-R) intermediates and products (R-R) in
Ullmann coupling, it is also possible they facilitate the cleavage of
the C-Br bond, which leads to the formation of intermediates.
Consistent with this, it should also be noted that unlike the case for
2
9–31
benzene and toluene
, we see no evidence of intact BB or EBB
desorption from step sites even on the pristine Cu(111) surface,
which indicates that any molecules that bind to these step sites are
completely converted to products.
Having characterized the behavior of the two molecules
individually with TPD in this current study and with STM in previous
2
5,28
studies
, we then examined their behavior when co-deposited on
the surface. Using STM, we followed the reaction of BB and EBB
deposited in equal amounts on the Cu(111) surface, which in
contrast to what we expected, did not provide the greatest
selectivity toward cross-coupled products, but formed mostly the
Figure 1. TPD traces obtained after depositing BB or EBB on Cu(111) and a EB-EB product (Figure 2). In these experiments, BB was deposited
roughened (sputtered) Cu surface. (A) Uptake of BB and (B) EBB on Cu(111) first and annealed to 80 K for ease of imaging and quantification of
showing traces for the desorption of intact molecules and coupling
products. (C) TPD of BB and (D) EBB comparing the adsorption of each
molecules formed small clusters, and the surface coverage was
molecule on Cu(111) (black) and sputtered Cu (red).
its concentration on the surface (Figure 2A). The intact BB
about 0.2 ML, which was calculated from comparison to images of
full monolayer coverage of flat-lying molecules. EBB was then
2
40 K for BB and 270 K for EBB to desorption from a low coverage
deposited onto the surface at an equal surface coverage, 0.2 ML,
which was similarly determined from STM images (Figure 2B). With
high-resolution STM tip states, the ethyl tail on some of the EBB
molecules could be distinguished. After annealing this surface to
flat lying configuration of the molecule. Both peaks broaden and
shift toward lower temperature with increasing coverage, similar to
TPD traces for benzene and toluene uptakes on Cu(111).
on this result we predict that the intact molecules in the monolayer
rest with their phenyl ring flat on the surface and begin to tilt and
stand up near monolayer saturation, similar to the studies of other
29–31
Based
2
20 K, the BB and EBB reactants either desorbed or converted into
the organometallic reaction
2
9–31
aromatics on Cu(111)
of BB and EBB.
and in line with our previous STM studies
2
5,28
It is well known that Cu(111) is capable of breaking C-X
bonds in aryl halides to form phenyl species that undergo Ullmann
4
,5,25,28
coupling.
As previously mentioned, in addition to desorption
of intact BB and EBB, we also observe the coupling products for
each species desorbing at high temperature. Exposing the surface
to BB results in the production of B-B from the Ullmann coupling of
phenyl groups (Figure 1A), which desorbs in a broad peak between
4
00-450 K, similar to B-B production when iodobenzene is
4
,5,25
deposited on Cu(111).
The production of B-B saturates
coincidentally with the saturation of the BB peak corresponding to
flat lying phenyl groups at 240 K, indicating the standing BB
molecules and multilayers do not participate in the coupling
reaction. Exposing the surface to EBB yields EB-EB, produced
through the coupling of 4-ethylphenyl groups, which desorbs at 430
2
8
K (Figure 1B). Similar to the BB system, production of EB-EB
saturates at the same coverage as the EBB peak at 270 K. The
desorption of the Br atoms left behind by the aryl halides
3
2
dissociation is not observed until 990 K.
It was previously demonstrated that aryl halides extract
Cu atoms from step edges forming a surface-bound organometallic
1
2,15,17,20,22,25–28
intermediate of two phenyl groups and one Cu atom.
Figure 2. STM images demonstrating the preferential formation of EB-EB in
It is also known that step edges play an important role in promoting cross coupling experiments. STM scale bars = 3 nm. A.) Intact BB on Cu(111)
6
,23–25
Ullmann coupling.
The intermediate facilitates the coupling of following an 80 K anneal to order the species. B.) STM image following
the two phenyl groups at the step edges, where the Cu atom is deposition of EBB on the surface shown in A, giving a 1:1 BB:EBB ratio. C.)
2
5
After annealing the sample to 220 K, reaction intermediates form and intact
BB desorbs. The dominant species is EB-Cu-EB. D.) After annealing the
sample to 500 K, mostly EB-EB products are observed on the surface.
redeposited once coupling occurs. Therefore, we performed
experiments on sputtered Cu surfaces, which have a much higher
3
3
step density than Cu(111) (Figure 1C and D). On these sputtered
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Figure 3. (Top) TPD traces of different ratios of BB and EBB co-deposited on the Cu(111) surface. (Bottom) Bar graph showing the
amount of each coupling product produced at the given reactant ratio. Values reported in the bar graph are the average of the
integrated areas of five different TPD traces and the error bars are the standard deviation of the average. The values are also
corrected for the fragmentation patterns of the coupling products, their ionization cross sections, which were calculated assuming
3
6
additivity of cross sections , and the decrease of quadrupole mass spectrometer sensitivity with increasing mass.
intermediates. Figure 2C shows that EB-Cu-EB intermediates are unreactive BB orientated vertical to the surface at 175 K; the peak
preferentially formed, representing 72% of the surface species at 240 K for flat lying BB is absent. On the other hand, the trace for
(excluding the Br atoms). As the ethyl tail in these structures images EBB is unaffected and both peaks at 195 K and 270 K are present.
as a bright protrusion, the EB intermediate species were easily Similar results are observed when benzene and toluene are co-
distinguished from the phenyl groups formed from BB in the STM deposited on Cu(111), where toluene forces benzene into its
3
1
images. We previously demonstrated that the ethyl tail in the B-Cu- vertical, more weakly adsorbed configuration. It seems likely that
EB organometallic intermediates is able to rotate, as the phenyl due to its stronger binding to the surface, EBB prevents BB from
groups in the organometallic intermediates are angled away from accessing the step edges, where we have hypothesized the key
28
the surface. This means that the phenyl group region of the reaction steps occur, including dissociation of the C-Br bond,
intermediates formed from EBB are brighter than those formed production of the organometallic intermediates, and the coupling
from BB. The STM images in Figure 2C also show that the ratio of reaction. Studies examining the coupling of alcohols on oxygen
B:EB has dropped significantly at this stage to 1:5. Our TPD results covered Au showed a similar result. Larger alcohols react and
demonstrate that BB starts to desorb as low as 200 K, so this drop displace methoxy species from the surface and outcompete it for
in coverage is likely due to desorption of un-activated, intact BB. By reaction sites on the surface, making it necessary to use ratios of
annealing this sample to 500 K, all of the surface species have ~9:1 methanol:ethanol in order to achieve optimal cross
3
4,35
reacted leaving EB-EB as the dominant species on the surface coupling.
However, even when a 13:1 ratio of BB:EBB is used,
(Figure 2D). Statistical counting of the products using STM could not selectivity to the cross coupling product remains low at 20%. We
be carried out as our TPD results show that B-B and B-EB begin to still do not observe any intact BB desorbing at 240 K. At this ratio
desorb at a lower temperature than EB-EB (Figure 3); however, the production of B-B has increased significantly, and it is now the
relative abundances of the products could be quantified with TPD dominant product, indicating BB has access to steps and B-Cu-B
experiments as discussed next.
intermediates form. EB-EB production is also heavily favored at this
TPDs of the cross coupling reaction at different ratios of ratio. This result indicates there is a strong preference to form
BB to EBB on Cu(111) are displayed in Figure 3. In the cases homocoupling products, possibly due to repulsion and segregation
presented here, BB was always deposited first followed by EBB, between EBB and BB. With this in mind we performed an
although the order of deposition did not make a difference as experiment where we exposed the Cu(111) to very small amounts
confirmed by our control experiments. As expected from our STM of BB and EBB in equal portions. The total coverage on the surface
results, when the ratio of BB:EBB is 1:1 at a total combined surface in this experiment was <0.01 ML. Under these conditions selectivity
coverage greater than the saturation of the peak corresponding to to the cross-coupling product B-EB was 51%, and the coupling
the flat lying alkyl halides, the homocoupling product EB-EB is products appear in roughly the expected 1:2:1 ratio for random
heavily favored and the selectivity to the cross coupling product is statistical scrambling. The cross coupling product B-EB desorbs in a
only 27%. Production of B-B is also unfavorable. In the TPD trace for broad peak centered at 430 K. Based on our results, we propose
this experiment, all the intact BB desorbs from the peak assigned to that at this very low coverage both BB and EBB have equal access to
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active sites at the step edges, and the presence of repulsive forces
between the molecules is low, leading to increased selectivity
toward cross coupling.
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We have discovered a way to control the selectivity of
Ullmann cross coupling of alkyl halides on a Cu surface. BB and EBB
both desorb from Cu(111) in three phases: multilayers, a denser
phase with tilted phenyl groups, and a phase with flat lying phenyl
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