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Atomic-scale insight into the formation, mobility and reaction of Ullmann coupling intermediates

Authors:
Emily A Lewis
Emily A Lewis
Emily A Lewis
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Colin James Murphy at Chalmers University of Technology
Colin James Murphy
Melissa Lizette Liriano at Tufts University
Melissa Lizette Liriano
Charles Sykes at Tufts University
Charles Sykes
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Abstract and Figures

The Ullmann reaction of bromobenzene, the simplest coupling reagent, to form biphenyl on a Cu surface proceeds via a highly mobile organometallic intermediate in which two phenyl groups extract and bind a single surface Cu atom.
STM images displaying the progression of the Ullmann reaction on Cu(111). All imaging at 5 K; all scale bars = 3 nm. (a) Intact bromobenzene on Cu(111) that has been annealed to 80 K. Clusters of 3, 4, and 5 molecules dominate. Insets: high resolution images of the clusters taken after deposition at 5 K (before the 80 K anneal). Top = 1.45 Â 1.47 nm 2 ; bottom = 1.79 Â 1.95 nm 2 . (b) After annealing to 160 K, clusters of the organometallic Ullmann coupling intermediates and Br atoms are present on the surface. (c) Annealing the sample to 350 K results in the formation of biphenyl. Insets: high resolution images illustrating the different appearance of the three-lobed phenyl–Cu–phenyl intermediate (top = 1.52 Â 1.47 nm 2 ) and the two-lobed biphenyl product (bottom = 1.19 Â 1.09 nm 2 ).
STM images displaying the progression of the Ullmann reaction on Cu(111). All imaging at 5 K; all scale bars = 3 nm. (a) Intact bromobenzene on Cu(111) that has been annealed to 80 K. Clusters of 3, 4, and 5 molecules dominate. Insets: high resolution images of the clusters taken after deposition at 5 K (before the 80 K anneal). Top = 1.45 Â 1.47 nm 2 ; bottom = 1.79 Â 1.95 nm 2 . (b) After annealing to 160 K, clusters of the organometallic Ullmann coupling intermediates and Br atoms are present on the surface. (c) Annealing the sample to 350 K results in the formation of biphenyl. Insets: high resolution images illustrating the different appearance of the three-lobed phenyl–Cu–phenyl intermediate (top = 1.52 Â 1.47 nm 2 ) and the two-lobed biphenyl product (bottom = 1.19 Â 1.09 nm 2 ).
… 
STM images and model showing the assembly and high-resolution details of the organometallic intermediate. All imaging at 5 K. (a) Clusters of the organometallic intermediates and Br atoms. Scale bar = 2 nm. (b) 3D rendering of a single phenyl–Cu–phenyl intermediate, illustrating the shape of the species. (c) Model showing the proposed bonding configu- ration of the phenyl species to the Cu atom.
STM images and model showing the assembly and high-resolution details of the organometallic intermediate. All imaging at 5 K. (a) Clusters of the organometallic intermediates and Br atoms. Scale bar = 2 nm. (b) 3D rendering of a single phenyl–Cu–phenyl intermediate, illustrating the shape of the species. (c) Model showing the proposed bonding configu- ration of the phenyl species to the Cu atom.
… 
STM movie clips demonstrating the mobility of the organometallic intermediate, phenyl–Cu–phenyl, on the Cu surface at 80 K. The full movie is provided in the ESI. † Inset: high resolution 5 K image of a cluster similar to the immobile structure in the lower right corner of the images. Scale bar = 3 nm; inset = 4.79 Â 4.45 nm 2 .
STM movie clips demonstrating the mobility of the organometallic intermediate, phenyl–Cu–phenyl, on the Cu surface at 80 K. The full movie is provided in the ESI. † Inset: high resolution 5 K image of a cluster similar to the immobile structure in the lower right corner of the images. Scale bar = 3 nm; inset = 4.79 Â 4.45 nm 2 .
… 
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This journal is ©The Royal Society of Chemistry 201 4 Chem. Commun.
Cite this: DOI: 10.1039/c3cc47002d
Atomic-scale insight into the formation, mobility
and reaction of Ullmann coupling intermediates†
Emily A. Lewis, Colin J. Murphy, Melissa L. Liriano and E. Charles H. Sykes*
The Ullmann reaction of bromobenzene, the simplest coupling
reagent, to form biphenyl on a Cu surface proceeds via a highly
mobile organometallic intermediate in which two phenyl groups
extract and bind a single surface Cu atom.
The Cu-catalysed coupling of two aryl halides to form a biaryl
molecule, commonly known as the Ullmann reaction, is one of
the oldest organic reactions promoted by a transition metal, and it is
still used today for a number of synthetic procedures.
1,2
However,
unlike other metal-mediated reactions, the Ullmann mechanism is
still relatively unclear. The original Ullmann reaction required
stoichiometric amounts of Cu to proceed, but modern iterations
of the reaction utilize catalytic amounts of Cu bound to ligands. The
variety of Cu species that are known to promote the reaction, in
addition to the multiple oxidation states in which Cu can exist, has
prevented the absolute determination of the catalytic species.
Early surface science studies attempted to approach this issue by
using temperature programmed desorption and high-resolution
electron energy loss spectroscopy to study iodobenzene on Cu(111)
surfaces.
3,4
This work suggested that coupling of the two phenyl
groups is the rate limiting step (RLS) and indicated that the
intermediate phenyl species are aligned roughly parallel to the
surface. Scanning tunnelling microscope (STM) studies built upon
this work, demonstrating that phenyl intermediates are stabilized at
Cu(111) steps,
5–7
and using p-diiodobenzene, McCarty et al. showed
that the intermediate phenyl species align end to end with their
rings parallel to the surface forming a chain.
8
More recent STM studies have taken advantage of this chain
formation, using complex halo-aromatic compounds and thermal
anneals to create polymer networks on a variety of surfaces.
9–18
These studies have noted a number of intermediate structures for
the surface polymerizations, and it is debated whether a metal atom
is incorporated into the complexes, resulting in an organometallic
structure. While it is well known that metal surfaces are not static
and that surface atoms can be extracted by adsorbates,
19–26
confirmation of a Cu organometallic intermediate in the
Ullmann coupling would provide valuable insight into the reaction
mechanism. Here, using the simplest Ullmann reagent, bromoben-
zene, in a model system amenable to study by high-resolution STM,
we demonstrate the formation, assembly and reaction of a
highly mobile organometallic intermediate comprised of two
phenyl groups and a surface Cu atom.
We studied the progression of the Ullmann reaction from weakly
adsorbed reactants, through intermediates to the biphenyl product
by depositing bromobenzene onto a Cu(111) surface held at 5 K.
Thermally annealing the sample to a wide range of tempera-
tures (5–350 K) and cooling back to 5 K for high-resolution
imaging enabled us to characterize the relevant steps of the
reaction with atomic-scale resolution as shown in Fig. 1.
Upon deposition at 5 K, the bromobenzene weakly adsorbs on the
Cu surface and remains intact up to 80 K (Fig. 1a). The adsorbates
assemble into clusters of 3, 4, or 5 molecules with ‘like’ ends pointing
toward the centre of the structures, as shown in the inset. Although
we cannot determine from our images which ends of the molecules
assemble in the cluster centres, previous studies indicate that the Br
ends of the molecules should attractively interact.
13,16,27
Annealing the sample to 160 K induces C–Br bond dissociation,
and an intermediate structure is formed (Fig. 1b). We propose
that these structures are the organometallic intermediates of
the Ullmann reaction and consist of two phenyl groups bound
to a central Cu atom that has been extracted from the surface
(Fig. 2c). These intermediates image as tri-lobed structures with
the central Cu atom appearing rounder and slightly more
pronounced than the two phenyl species (Fig. 2a and b). By
taking line scans along the length of these features and
measuring the distance between the first and third peak, we
determined that the length of the structure is 1.01 0.06 nm.
This value agrees well with the theoretical length of the inter-
mediate, 0.98 nm, corresponding to two Cu–C bonds
16
and two
phenyl diameters. Small round protrusions are located beside
the phenyl–Cu–phenyl intermediate structures, and consistent
Department of Chemistry, Tufts University, 62 Talbot Ave., Medford, MA 02155,
USA. E-mail: charles.sykes@tufts.edu; Fax: +1 617-373-3443; Tel: +1 617-373-3773
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc47002d
Received 12th September 2013,
Accepted 28th November 2013
DOI: 10.1039/c3cc47002d
www.rsc.org/chemcomm
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with other work,
9,16,17
we assign them as Br atoms. The ratio of
the Br atoms to phenyl species is 1 : 1, further supporting that
bromobenzene has dissociated.
To confirm that the central Cu atom in the organometallic
species is fully extracted rather than just raised
28
from the surface,
we examined the mobility of the intermediate species at 80 K. Fig. 3
shows a series of STM images taken over 100 minutes, forming a
time-lapse movie (full movie in ESI†). It is apparent that the majority
of species are mobile on the surface except for a small cluster that is
stabilized by a Cu surface defect in the lower right corner for
reference. The diffusion of the intermediate indicates that the Cu
atom must be fully extracted from the surface. Additionally,
although we cannot image the Br atoms at 80 K, we expect that
they play a role in stabilizing the transient clusters;
29
the assemblies
of intermediates that dynamically form and disperse (Fig. 3)
are similar to those observed at low temperature containing Br
atoms(Fig.2).Furthermore,theabsence of pure Br islands supports
that the Br atoms are driven energetically to mix with the phenyl–
Cu–phenyl intermediate structures.
The mobility of the organometallic intermediate also gives
insight into the mechanism of the Ullmann reaction on Cu(111).
Since the barrier is 50% greater to remove a Cu atom from the (111)
terrace vs. astep,
30
and since there is not an appreciable 2D gas of
Cu adatoms present at 160 K,
31
we postulate that the Cu atom in the
intermediate must be extracted from the step. The fact that we do
not observe an increase in Cu surface defects compared to the clean
Cu crystal, and that the number of phenyl and Br species doubles in
the clusters of intermediates relative to the intact bromobenzene
clusters, further supports that Cu atoms are removed from the step
rather than the initial adsorption site on the terrace. We also
demonstrate that intact bromobenzene diffuses on the surface at
80 K, and since the diffusion barriers of Br atoms and phenyl groups
on the surface were calculated to be 60 meV
32
and 90 meV,
28
respectively,weconcludethatphenyl groups are able to diffuse to
the most easily extracted Cu atoms at the steps. Due to the mobility
of both the intact bromobenzene and the dissociation products, we
cannot determine upon annealing whether two bromobenzene
molecules dissociate at a step andextractaCuatomorwhether
the C–Br bond dissociation occurs on the terrace leaving phenyl
groups to diffuse to the steps and form the intermediate. We can,
however, rule out that the STM tip is participating in the Cu atom
extraction, as phenyl–Cu–phenyl intermediates are observed on every
new area of the surface scanned and their structure does not change
with repeated imaging.
After a 350 K anneal, we find that the reaction progresses to
completion and biphenyl begins to form (Fig. 1c), in agreement
with previous reports.
33,34
By taking line scan measurements
across the length of the molecules, we find that they are 0.63
0.03 nm, which agrees well with the theoretical value of 0.71 nm
for biphenyl. The exclusive presence of biphenyl on the Cu step
edges suggests that it is formed at these sites, indicating that
Fig. 1 STM images displaying the progression of the Ullmann reaction on Cu(111). All imaging at 5 K; all scale bars = 3 nm. (a) Intact bromobenzene on
Cu(111) that has been annealed to 80 K. Clusters of 3, 4, and 5 molecules dominate. Insets: high resolution images of the clusters taken after depositionat
5 K (before the 80 K anneal). Top = 1.45 1.47 nm
2
; bottom = 1.79 1.95 nm
2
. (b) After annealing to 160 K, clusters of the organometallic Ullmann
coupling intermediates and Br atoms are present on the surface. (c) Annealing the sample to 350 K results in the formation of biphenyl. Insets: high
resolution images illustrating the different appearance of the three-lobed phenyl–Cu–phenyl intermediate (top = 1.52 1.47 nm
2
) and the two-lobed
biphenyl product (bottom = 1.19 1.09 nm
2
).
Fig. 2 STM images and model showing the assembly and high-resolution
details of the organometallic intermediate. All imaging at 5 K. (a) Clusters of
the organometallic intermediates and Br atoms. Scale bar = 2 nm. (b) 3D
rendering of a single phenyl–Cu–phenyl intermediate, illustrating the
shape of the species. (c) Model showing the proposed bonding configu-
ration of the phenyl species to the Cu atom.
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the organometallic intermediates release the Cu atoms at the
step edge/kink sites during the formation of biphenyl. Although
we cannot exclude the possibility that the biphenyl product has
simply diffused to these sites following its formation, this
proposed mechanism is consistent with the Evans–Polanyi
principle.
35
Returning the Cu atom to a step site yields a more
exothermic reaction as compared to leaving the Cu atom in a
more under-coordinated state on a terrace; therefore the activa-
tion barrier to form biphenyl (which our experiments suggest is
the RLS of the surface Ullmann reaction) at the step edge/kink
site will be lower and its formation at this site will be favoured.
In contrast to the reaction-rate-limited description of the surface
Ullmann reaction put forth by Bent and coworkers,
3,4
our direct
observation of biphenyl on the surface
33
indicates that its
formation is, in fact, desorption rate limited.
Using a well-characterized model system, we have provided
insight into the Ullmann reaction mechanism. First, we find
that the intermediate of the reaction is organometallic in
nature with a removed Cu atom being fully incorporated into
the structure. The low temperature at which the phenyl–
Cu–phenyl complex forms is consistent with the RLS for the
reaction being the formation of biphenyl, not the extraction of a
surface Cu atom or formation of the intermediate. Additionally,
the incorporation of single Cu atoms into the intermediate
structure may lower the barrier of biphenyl formation. These
results provide new insight into the reaction pathway of the
heterogeneously catalysed Ullmann reaction. This information
about the formation and mobility of the organometallic inter-
mediate will also enable us to explore and possibly control
selectivity in cross-coupling reactions and adds to the current
knowledge of using Ullmann-type reactions for the formation of
2D surface networks.
E.A.L. and E.C.H.S. thank the U.S. Department of Energy
(Grant No. FG02-10ER16170) and C.J.M. and M.L.L. thank the
U.S. National Science Foundation (Grant No. CBET-1159882)
for their support.
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Fig. 3 STM movie clips demonstrating the mobility of the organometallic
intermediate, phenyl–Cu–phenyl, on the Cu surface at 80 K. The full
movie is provided in the ESI.†Inset: high resolution 5 K image of a cluster
similar to the immobile structure in the lower right corner of the images.
Scale bar = 3 nm; inset = 4.79 4.45 nm
2
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Article
  • Mar 2022
Various methodologies have been well established to construct self-assembled structures on surfaces that are aesthetic and eye-catching. How to practically apply these self-assemblies still remains a great challenge. In particular, how to develop the surface molecular self-assembly into a new and applicable strategy to control on-surface reactions plays a crucial role in the construction of covalently bonded structures on surfaces. In this chapter, recent progress in the development of such a self-assembly strategy is overviewed. After a brief description of the fundamentals in surface reaction kinetics, a series of typical proof-of-principle case studies are summarised, focusing on the application of the self-assembly strategy to control on-surface reactions. Such a strategy may be exploited as an efficient bottom-up approach to constructing covalently bonded and thermally stable structures that are urgently needed in nanoscience and nanotechnology. In specific, the emphasis is to utilise the molecular self-assembly strategy to steer on-surface reactions via mediations of the reaction pathway, selectivity, and site. Some remaining challenges and perspectives relating to the self-assembly strategy are also provided for future explorations.
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A Fundamental Role of the Molecular Length in Forming Metal–Organic Hybrids of Phenol Derivatives on Silver Surfaces
Article
  • Feb 2021
In on-surface chemistry, the efficient preparation of metal-organic hybrids is regarded as a primary path to mediate controlled synthesis of well-ordered low-dimensional organic nanostructures. The fundamental mechanisms in forming these hybrid structures, however, are so far insufficiently explored. Here, with scanning tunneling microscopy, we studied the bonding behavior of the adsorbed phenol derivatives with different molecular lengths. We reveal that shorter molecules favor bonding with extracted metal adatoms and result in metal-organic hybrids, whereas longer molecules prefer to bond with lattice metal atoms. The conclusions are further confirmed by density functional theory calculations.
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Reply to “Comment on ‘Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization’”
Article
  • Aug 2013
  • ACS NANO
We provide insight into surface-catalyzed dehalogenative polymerization, analyzing the organometallic intermediate and its evolution into planar polymeric structures. A combined study using scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and first-principles calculations unveils the structural conformation of substrate-bound phenylene intermediates generated from 1,4-dibromobenzene precursors on Cu(110), showing the stabilizing role of the halogen. The appearance of covalently bonded conjugated structures is followed in real time by fast-XPS measurements (with an acquisition time of 2 s per spectrum and heating rate of 2 K/s), showing that the detaching of phenylene units from the copper substrate and subsequent polymerization occur upon annealing above 460 ± 10 K.
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Gold adatom as a key structural component in self-assembled monolayers of organosulfur molecules on Au(1 1 1)
Article
  • May 2010
  • PROG SURF SCI
Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent formation of the self-assembled structure. Gold adatom chemistry is proposed here to be a unifying theme that explains various aspects of the alkanethiol self-assembly and reconciles experimental evidence provided by scanning probe microscopy and spectroscopic methods of surface science. While several features of alkanethiol self-assembly have yet to be revisited in light of the new adatom-based models, the successes of alkanethiol SAMs suggest that adatom-mediated surface chemistry may be a viable future approach for the construction of self-assembled monolayers involving molecules which do not contain sulfur.
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Direct STM evidence for Cu-benzoate surface complexes on Cu(1 1 0)
Article
  • Oct 2006
  • SURF SCI
The chemisorption of benzoate on a Cu(110) crystal at room temperature was studied using low temperature scanning tunneling microscopy. STM images, obtained at 5K for low benzoate coverage, show isolated surface species that consist of a single Cu adatom stabilizing two benzoate molecules in a flat orientation. These species are discussed in relation to other known metal-organic surface compounds. At higher coverage the 4-315overlayer, called the α-phase, was also observed at 5K and found to contain features attributable to two Cu adatoms associated with two pairs of non-equivalent benzoate species. The observed topographic features are used to suggest refinements of the structural model of the ordered α-phase overlayer.
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Face specificity and the role of metal adatoms in molecular reorientation at surfaces
Article
  • Jul 1998
  • SURF SCI
Using reflection absorption infrared spectroscopy (RAIRS), the coverage-dependent reorientation of the benzoate species on the (110) and (111) faces of copper is compared and contrasted. Whereas on Cu(110) benzoate reorients from a flat-lying to an upright orientation with increasing coverage, on Cu(111), at all coverages, benzoate is aligned normal to the surface. The formation of periodic, flat-lying copper–benzoate structures has been attributed to the availability of metal adatoms, which differs dramatically between the (111) and (110) faces. We discuss the face specificity of molecular orientation by comparing calculated formation energies of adatom vacancies from ledges and kink sites on (100), (110) and (111) faces. Further support for this model is given by the evaporation of sodium, either by pre- or post-dosing, onto low-coverage benzoate/Cu(111), which induces benzoate to convert from a perpendicular to a parallel orientation. Likewise, coevaporation of Cu while dosing benzoic acid onto the Cu(111) surface also results in a majority of flat-lying benzoate species. Finally, for adsorption on the p(2×1)O/Cu(110) reconstruction, benzoate occurs only as the upright species, which is consistent with reducing the copper mobility and availability on the (110) face. We therefore suggest the possible role of metal adatoms as a new mechanism in controlling adsorbate orientation and therefore face specificity in surface reactions.
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Bromine atom diffusion on stepped and kinked copper surfaces
Article
  • May 2006
  • SURF SCI
The rates of Br atom diffusion on several single crystalline Cu surfaces have been studied because of the potential impact of Br diffusion on the selectivity of alkyl bromide surface chemistry on Cu. Density functional theory (DFT) has been used to study the diffusion of isolated bromine atoms on a flat Cu surface, Cu(1 1 1), two Cu surfaces with straight steps, Cu(2 2 1) and Cu(5 3 3), and two kinked Cu surfaces, Cu(6 4 3) and Cu(5 3 1). Bromine diffusion is rapid on the flat Cu(1 1 1) surface with a barrier of DeltaEdiff = 0.06 eV and a hopping frequency of nu = 4.8 × 1010 s-1 at 150 K. On the stepped and kinked surfaces the effective diffusion barriers lie in the range DeltaEdiff = 0.18 0.31 eV. Thus the rates of diffusion are many orders of magnitude slower on stepped and kinked Cu surfaces than on the Cu(1 1 1) surface. Nonetheless, at temperatures relevant for alkyl bromide debromination on Cu surfaces, bromine atoms remain sufficiently mobile that they can explore all available binding sites on the timescale of the debromination reaction.
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Coexisting superstructures of iodobenzene on Cu( 1 1 1 ) near saturation coverage
Article
  • Jan 2003
  • SURF SCI
Low-temperature scanning tunnelling microscopy reveals seven coexisting structures of iodobenzene (IC6H5) adsorbed on Cu(111) at ≈55 K. Only the least dense structure reflects the threefold symmetry of the surface. The other structures show p2-symmetry. In the densest structure molecules adsorb with their π system perpendicular to the surface. We identify as a common driving force for ordering the tendency of the phenyl rings (C6H5) to encapsulate the iodine.
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Surface-Assisted Organic Synthesis of Hyperbenzene Nanotroughs
Article
A hexagonal macrocycle consisting of 18 phenylene units (hyperbenzene) was synthesized on a Cu(111) surface in ultrahigh vacuum by Ullmann coupling of six 4,4''-dibromo-m-terphenyl molecules. The large diameter of 21.3 Å and the ability to assemble in arrays makes hyperbenzene an interesting candidate for a nanotrough that could enclose metallic, semiconducting, or molecular quantum dots.
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Imaging Benzene Molecules and Phenyl Radicals on Cu{111}
Article
  • Feb 1998
In two sets of experiments built on the work of Brian Bent, we imaged benzene molecules and phenyl radicals on the Cu{111} surface at low coverage and low temperature.1 The experiments allowed us to see how mobile molecules on surfaces probe the electronic structure of the surface. Bare terraces of the Cu{111} surface appear extremely flat in scanning tunneling microscope images. We are thus able to image the perturbations to the electronic structure caused by steps, defects, and adsorbates. These perturbations determine the structure and dynamics of the adsorbates. Benzene forms ordered structures along step edges at even very low coverages. Adsorbed phenyl radicals form complex pairs, aligned so as to be able to couple to form biphenyl at higher temperature. We discuss the chemical consequences of such substrate-mediated interactions.
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Material- and Orientation-Dependent Reactivity for Heterogeneously Catalyzed Carbon−Bromine Bond Homolysis
Article
  • Jul 2010
Adsorption of the brominated aromatic molecule 1,3,5-tris(4-bromophenyl)benzene on different metallic substrates, namely Cu(111), Ag(111), and Ag(110), has been studied by variable-temperature scanning tunneling microscopy (STM). Depending on substrate temperature, material, and crystallographic orientation, a surface-catalyzed dehalogenation reaction is observed. Deposition onto the catalytically more active substrates Cu(111) and Ag(110) held at room temperature leads to cleavage of carbon−bromine bonds and subsequent formation of protopolymers, i.e., radical metal coordination complexes and networks. However, upon deposition on Ag(111) no such reaction has been observed. Instead, various self-assembled ordered structures emerged, all based on intact molecules. Also sublimation onto either substrate held at 80 K did not result in any dehalogenation, thereby exemplifying the necessity of thermal activation. The observed differences in catalytic activity are explained by a combination of electronic and geometric effects. A mechanism is proposed, where initial charge transfer from substrate to adsorbate, followed by subsequent intramolecular charge transfer, facilitates C−Br bond homolysis.
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