Nanoscale insight into C-C coupling on cobalt nanoparticles

Article abstract of DOI:10.1039/c4cc03678f

The Ullmann coupling of bromobenzene to biphenyl on Co nanoparticles proceeds below room temperature via an intermediate in which phenyl groups are bound directly to metallic Co. A similar surface-bound benzyl intermediate is observed for coupling of benzylbromide to bibenzyl on Co. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03678f

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Nanoscale insight into C-C coupling on cobalt
nanoparticles
Cite this: DOI: 10.1039/x0xx00000x
E. A. Lewis, C. J. Murphy, A. M. Pronschinske, M. L. Liriano and E. C. H. Sykes*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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The Ullmann coupling of bromobenzene to biphenyl on Co aryl and vinyl Grignard reagents to 1-bromo glycosides, forming and
7
nanoparticles proceeds below room temperature via an array of C-aryl or C-vinyl glycosides with high selectivity. Finally CoBr
2
intermediate in which phenyl groups are bound directly to catalysts used with allylchloride and reduced with Zn powder were
metallic Co. A similar surface-bound benzyl intermediate is also shown to effectively couple aryl halides and benzyl halides to
observed for coupling of benzylbromide to bibenzyl on Co.
diarylmethanes with high yield and excellent functional group
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tolerance.
The formation of carbon-carbon bonds is one of the most important
synthetic processes, as it has a wide range of pathways and
applications. The Ullmann reaction, the coupling of two aryl halides
over a Cu catalyst, is one of the oldest transition metal-catalysed
reactions for forming C-C bonds between aryl groups, but it is
notorious for its harsh conditions and intolerance to certain
We and others have examined the surface chemistry of Cu-
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catalysed coupling reactions.
Specifically, we elucidated the
reaction intermediate in the Ullmann coupling of bromobenzene to
biphenyl in a Cu(111) surface. By depositing the bromobenzene on a
Cu(111) surface, annealing the sample to progressively higher
temperatures and cooling to 5 K for high resolution imaging, we
showed that the reaction proceeds via an organometallic
intermediate in which a Cu atom is extracted from the surface and
bound between two phenyl groups. These intermediates are highly
mobile on the Cu surface at 80 K and prefer to associate with surface-
bound Br atoms that result from the dissociation of the aryl halide.
Following the recent interest in Co catalysts for solution-phase
coupling reactions, we have used low temperature scanning
tunnelling microscopy (STM), to track the reaction of both
bromobenzene and benzylbromide on Co surfaces. Using Co
1
,2
functionalities. As such, gentler more reliable pathways have been
developed for forming C-C bonds that use palladium or nickel as the
catalyst. These metals have found much popularity in the synthetic
community and are widely used as the catalyst of choice in reactions
3
such as the Suzuki, Stille, Heck, and Negishi cross couplings.
However, while these metals can be used at the commercial scale,
their cost (Pd) or toxicity (Ni) impose limitations on their use.
Recently, a number of research groups have explored Co as a
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catalyst for cross coupling reactions, as Co is a fraction of the cost of
Pd and is thought to be less carcinogenic than Ni-containing
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4,15
nanoparticles grown on Cu(111) (Fig. 1G),
we are able to visualize
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compounds. These studies have typically employed Co-halides that
key steps in the coupling of these molecules from intact surface-
bound starting materials, through dissociated intermediates, to
products on Co surfaces. Our STM images provide single molecule
resolution of the C-C bond forming process, which would not be
possible under dynamic catalytic conditions. We compare our results
to what is known for Cu and demonstrate that the intermediates have
a different structure to those on Cu, providing insight into the
favourable reactivity of Co-catalysed C-C coupling.
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are either reduced in situ with a reducing agent (Zn or Mn) or used in
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conjunction with a Grignard reagent. It was found that, in addition
to their attractive cost and non-toxic nature, these Co catalysts are
interesting for a number of synthetic reasons. For example CoBr
2
,
used in combination with PPh and reduced with Mn powder, showed
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high selectivity towards cross-coupled (vs. homocoupled) products
and high tolerance toward a variety of functional groups, including
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esters, nitriles and methoxy groups. The same catalyst was also able
Upon deposition at 5 K, bromobenzene is equally distributed
across the Co/Cu(111) surface (Fig. 1A). The intact molecules appear in
STM images as mushroom-shaped features (Fig. 1D), and our previous
work, as well as others’, shows that the smaller tails are the Br atoms
to activate the previously unreported cross-coupling of aryl chlorides,
as well as the cross coupling of aryl bromides with heteroaryl chlorides
3
and bromides. In another study a Co(acac) catalyst, in combination
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0,16,17
with TMEDA (N,N’-tetramethylethylenediamine), was used to couple
based on their preference to associate.
After annealing the
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After annealing the sample to 160 K, the reaction to form C-C
bonds is complete, and all of the single phenyl species on the Co
surface have coupled to biphenyl (Fig. 1C and F). The biphenyl species
is 0.69 ± 0.04 nm in length, which agrees well with the expected value
of 0.71 nm. Using dI/dV spectroscopy, we were able to further
distinguish the identity of biphenyl molecules from single phenyl
groups, as the unoccupied states of the two species are distinctly
different (spectra in ESI†). This low temperature conversion to
biphenyl indicates that that barrier to C-C bond formation is very low
on Co, in line with the relatively low temperature solution-phase
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synthetic procedures (<50°C). The same conversion to products was
not observed on the Cu(111) surface until the sample was heated to
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50 K. Full desorption of biphenyl from the Co/Cu(111) surface is
observed after a 360 K anneal. Only Br atoms remain on the Co
surfaces, indicating that Co does not decompose the biphenyl
products and that the reaction is selective to the coupled product.
To further examine the activity of Co as a catalyst for commonly
used C-C couplings, we explored the formation of benzylic C-C bonds
from benzylbromide. Following the same procedure as the
bromobenzene experiments, we deposited benzylbromide onto the
Co/Cu(111) surface at 5 K (Fig. 2A and D). After annealing the sample
to 80 K, the benzylic C-Br bond breaks, similar to the phenylic C-Br
bond (Fig. 2B and E); however, while most of the benzyl groups exist
on Co, the Cu terraces are more highly populated compared to the
bromobenzene system. This difference occurs because the benzylic C-
Br bond also breaks at ~80 K on Cu, which suggests that some of the
benzylbromide dissociates on Cu and the resulting intermediates
diffuse to the Co. In our high resolution STM images, the benzyl
intermediates appear as dimmer mushroom-shaped features, with
the CH
2
group visible as a more pronounced tail (Fig. 2E) in
sample to 80 K, all of the bromobenzene molecules diffuse to the Co comparison to the surface-bound phenyl groups (Fig. 1E). The Br
nanoparticles where they dissociate (Fig. 1B and E). The resulting atoms again appear as bright, circular protrusions. An attractive
phenyl groups and Br atoms appear as tear-drop shaped features and interaction between the Br atoms and the aromatic rings leads to the
bright circular protrusions, respectively, and are also distinguished by formation of ordered structures, in which the Br atoms surround the
different differential conductance (dI/dV) spectra (spectra in ESI†). aryl groups. These intermediates are stable on the Co surface in these
There appears to be an attractive interaction between the phenyl structures up to ≥325 K, indicating there is a higher barrier to forming
species and the Br atoms, as the two intermediates are closely the benzylic C-C bond on Co.
associated on the surface (Fig. 1E). At higher coverage, the two
species assemble into complex networks (high coverage image in
ESI†). Importantly, this result indicates that bromobenzene
dissociates on metallic Co sites at temperatures ≤80 K, as compared
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0
to Cu sites on which the C-Br bond remains intact up to 160 K.
In comparison to Cu, there are two additional differences to note
regarding the activity of Co. First, not only does the C-Br bond
dissociate at 80 K, but at this low temperature, C-C bonds begin to
form on the Co surface (Fig. 1E). While biphenyl is not the majority
species on the Co nanoparticles after annealing to this temperature, a
small percentage of the phenyl groups have coupled on the surface.
The second comparison of interest is that on Co, the phenyl
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intermediates are bound directly to the Co surface; whereas on Cu,
two phenyl groups extract a Cu atom from the surface and form a
mobile organometallic intermediate. This difference can be
understood in terms of Co being a more catalytically active metal,
binding reactants more strongly and having lower activation barriers
than Cu, on which the C-C coupling proceeds via a different Ph-Cu-Ph
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0
intermediate.
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After annealing to 450 K the benzyl groups couple to form do not require the same high temperatures as traditional Cu catalysts.
bibenzyl (1,2-diphenylethane; Fig. 2C and F). Length measurements We also report the selective low temperature formation of biphenyl
across the bibenzyl species yield an average of 0.8 ± 0.2 nm, from bromobenzene. Our results on well-defined metallic Co surfaces
consistent with the anti or eclipsed forms of surface-bound bibenzyl, indicate that heterogeneous Co nanoparticle catalysts are interesting
which have an expected length of 0.94 nm and 0.86 nm, respectively.
candidates to explore for the development of new efficient and
To further interpret our experimental results, we evaluated the selective C-C coupling procedures.
kinetics and thermodynamics of the two coupling reactions by
The authors would like to thank Prof. Clay Bennett of Tufts
examining the relevant bond dissociation energies (BDEs) of each step University for fruitful discussions. The U.S. Department of Energy
19,20
(
Fig. 3).
In the first step of both the bromobenzene and supported this work (Grant No. FG02-10ER16170). M.L.L. thanks the
benzylbromide coupling reactions, C-Br bonds are broken while C-Co NSF for a Graduate Research Fellowship.
and Br-Co bonds are formed. The strong Br-Co bond drives this first Notes and references
2
0
Department of Chemistry, Tufts University, 62 Talbot Ave., Medford,
MA 02155, USA. E-mail: charles.sykes@tufts.edu
reaction step. The difference between the benzylic and phenylic C-
1
9
Br bond strengths is ~100 kJ/mol; however, based on studies of alkyl
†
Electronic Supplementary Information (ESI) available: [experimental
2
1
and vinyl C-Co bonds in methyl- and vinyl(pyridine)cobaloxime, this
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final product in the second reaction step: since the BDE is ~220 kJ/mol
greater for the phenylic C-C bond vs. the benzylic C-C bond in the
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