An Electron-Rich Cyclic (Alkyl)(Amino)Carbene on Au(111), Ag(111), and Cu(111) Surfaces

Article abstract of DOI:10.1002/anie.201915618

The structural properties and binding motif of a strongly σ-electron-donating N-heterocyclic carbene have been investigated on different transition-metal surfaces. The examined cyclic (alkyl)(amino)carbene (CAAC) was found to be mobile on surfaces, and mo

Full text of DOI:10.1002/anie.201915618

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Accepted Article
Title: An Electron-rich Cyclic (Alkyl)(Amino) Carbene on Au(111),
Ag(111) and Cu(111) Surfaces
Authors: Anne Bakker, Matthias Freitag, Bertram Schulze Lammers,
Elena Kolodzeiskia, Alexander Timmer, Jindong Ren, Daniel
Moock, Herbert Roesky, Harry Mönig, Saeed Amirjalayer,
Harald Fuchs, and Frank Glorius
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915618
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10.1002/anie.201915618
Angewandte Chemie International Edition
COMMUNICATION
An Electron-rich Cyclic (Alkyl)(Amino) Carbene on Au(111),
Ag(111) and Cu(111) Surfaces
Anne Bakker‡[a,b], Matthias Freitag‡[c], Elena Kolodzeiski[a,b,d], Peter Bellotti[c], Alexander Timmer[a,b]
,
Jindong Ren[a,b], Bertram Schulze Lammers[a,b], Daniel Moock[c], Herbert W. Roesky[f], Harry Mönig[a,b]
Saeed Amirjalayer[a,b,d], Harald Fuchs*[a,b], Frank Glorius*[c]
,
Dedicated to Prof. Albert Eschenmoser on the occasion of his 95th birthday
[a]
[b]
[c]
[d]
[e]
[f]
Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany.
Center for Nanotechnology, Heisenbergstraße 11, 48149 Münster, Germany.
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität, Corrensstraße 40, 48149 Münster, Germany.
Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität, Corrensstraße 40, 48149 Münster, Germany.
nanoAnalytics GmbH, Heisenbergstraße 11, 48149 Münster, Germany.
Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammannstraße 4, 37077, Göttingen, Germany.
Email: fuchsh@uni-muenster.de, glorius@uni-muenster.de
Supporting information for this article is given via a link at the end of the document
DiMeCAAC[10] (Fig.1.A) which is a rather small and structurally
simple CAAC. To avoid contamination of the surface by salts and
solvent the CO2-adduct was used.[[9a,9d] DiMeCAAC • CO2 was
synthesized via the formation of the free carbene and subsequent
treatment with dry CO2, which led in precipitation of the pure
material. GC-MS measurements revealed that the free CAAC can
be generated without destruction.
Abstract: The structural properties and binding motif of a strongly
sigma-electron donating N-heterocyclic carbene are investigated on
different transition metal surfaces. The examined cyclic (alkyl)(amino)
carbene (CAAC) is found to be mobile on surfaces and molecular
islands with short-range order could be found at high coverage. By
combining scanning tunneling microscopy (STM), X-ray photoelectron
spectroscopy (XPS) and density functional theory (DFT) calculations
our study highlights how CAACs bind to the surface, which is of
tremendous importance to gain understanding for heterogeneous
catalysts bearing CAACs as ligands.
After vacuum depositing DiMeCAAC for 10 minutes at 60 °C on a
Au(111) crystal which was kept at room temperature, an almost
In recent years, N-heterocyclic carbenes (NHCs) have gained
great attention in a broad range of applications as catalysts and
as ligands for p-block elements and transition metals.[1] Due to
their strong σ-donating properties and their tunable side-groups,
NHCs are privileged ligands for NHC-metal complexes and the
stabilization and functionalization of nanoparticles[2] and
surfaces[3]. Cyclic (alkyl)(amino) carbenes (CAAC) are
a
prominent class of NHCs which exhibit even stronger σ-donating
properties than imidazole-based NHCs.[4] Therefore, CAACs are
especially valuable ligands for Rh-based hydrogenation catalysts,
which can be used to hydrogenate highly functionalized
(hetero)aromatic rings with excellent functional group tolerance
and conformational control.[4c,5] Moreover, they are also used as
ligands for metathesis catalysts and are even able to activate
small molecules like H2, CO, NH3, silanes, phosphenes and
organoboron species due to their high nucleophilicity.[6] On
heterogeneous surfaces, Johnson and coworkers showed that
CAACs can insert into Si-H bonds, which does not result in a
binding mode comparable to a single carbene bond in CAAC-
metal complexes.[7] Bullock and coworkers further studied their
behavior in Rh-catalyzed hydrogenation reactions and exclusively
found the protonated CAAC coordinating to the surface.[7,8] Over
the last years, several studies of NHCs on surfaces have been
reported including STM and XPS investigations.[3b,9] The
formation of ultra-stable self-assembled monolayers was shown,
as well as the mobility of some NHCs due to the formation of
ballbot-like species and the influence of the N-substituents on the
binding mode to the surface.[3b,9a,9b,9d] However, the fundamental
behavior of CAACs is only rarely explored and there is no report
of CAACs on transition metal surfaces. Furthermore,
investigations of CAACs with molecular resolution, which would
be necessary to design more efficient and selective catalysts, are
not reported. Thus, we investigated the on-surface behavior of
Figure 1. A) The deposition procedure for DiMeCAAC. B)
Overview STM image of a full layer of DIMeCAAC on Au(111) (-
2.0V; 25pA). C) Small scale STM image where the individual
molecular features appear as a bright ellips (methyl groups
pointing upwards) and a dimmer shadow (Dipp group) (-2.0V;
25pA). D, E) Side- and top-view of the DFT-optimized geometry
of the upstanding binding mode of DiMeCAAC on Au(111).
1
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completely covered sample was obtained (Fig.1.B). In the
molecular monolayer three preferential directions of short-range
order can be recognized as is indicated in Figure 1.B. However,
the long-range packing is not well-ordered, which is caused by the
multiple different orientations the molecular features adopt on the
surface.
clearly indicate (Fig.1.B). In principle, the atomic lattice of the
surface is driving the organization of DiMeCAAC-Au species on
the surface due to the prefered adsorption at the hollow sites.
Since the main interaction of these species with the surface is via
the gold atom, molecules can arrive in any orientation at an island
of molecules. However, due to the bulky Dipp group and the
asymmetric structure of the molecule, intermolecular interactions
are possible and rotation of individual ballbots in a molecular
island is limited. Combined with the mobility of the molecules and
the required high coverage for STM imaging this makes it more
difficult to reach the optimized self-assembled structure. Still, by
positioning the atomic lattice over the STM images, preferred
adsorption at or very close to the hollow site can be observed for
the majority of the molecules (See SI). Attempts to deposit slower
(70 min at 45 °C) did not result in an improved order in the
molecular layer. Also annealing of the sample after deposition
could not induce a better odered structure, but rather leads to
desorption of the molecules. Additionally the calculated
DiMeCAAC-Au configuration (Fig.2.E) allows for slight tilting to
the sides when the distance to neighbouring molecules is larger.
This explains the differences in brightness observed in Fig.1.C.
DiMeCAAC was also investigated on Ag(111) and Cu(111), which
gave similar results as on Au(111), as can be seen from the STM
images in the SI.
The same repeating shape can be recognized in the STM
contrast, represented by a bright ellipse with a dimmer shadow
(solid circle in Fig.1.C). The distance between the different bright
spots is measured to be 1.02±0.09 nm. This value is the average
of distances measured along all three directions as indicated in
Fig.1.B where obvious defects are ignored. Since DiMeCAAC
forms no well-ordered self-assembly, a unit cell cannot be
defined. However, this consistent value for the intermolecular
distance indicates that the packing is not completely disordered.
DFT simulations predicted an upright binding mode for
DiMeCAAC on Au(111) (Figure 1.D) with the di-isopropylphenyl
(Dipp) group perpendicular to the CAAC ring and parallel to the
surface, which minimizes steric hindrance. Considering this
optimized structure, the two methyl groups in the backbone point
upwards and should thus give the brightest STM contrast. The
bright dots in Fig.1.C are therefore assigned to these groups.
Based on these considerations, the Dipp group is attributed to the
dimmer shadow. The distance between these two features (arrow
in Fig.1.C) is measured to be 0.44±0.02 nm, which is in good
agreement with the DFT-calculated distance of 0.46 nm between
the upstanding methyl groups and the edge of the Dipp group. Not
all molecular features appear equally bright (dotted circle in
Fig.1.C), which is attributed to a possible tilting in the adopted
binding configuration (see below).
When a lower coverage (~80%) of DiMeCAAC is deposited on
Au(111), two observations can be made. Firstly, an adsorption
preference at the elbow positions and the fcc phases of the
herringbone reconstruction is observed (Fig.2.A and S.1), which
is in agreement with the behavior of other NHCs.[9a] Secondly, in
between the islands the STM contrast is blurred and the visibility
of the Au(111) herringbone reconstruction reduced. This is an
indication of mobility of the molecules on the surface. To illustrate
this, series of STM images are recorded on a low coverage
(~20%) sample (Fig.2.B-D). Small molecular islands have
nucleated from the elbow positions of the herringbone
reconstruction (Fig.2.B). The shape of the islands in this image is
transferred to the subsequent STM images in Fig.2.C and D. Here
it can be observed clearly how molecules appear and disappear,
thereby changing the shape of the island in between subsequent
images. The adsorption energy of DiMeCAAC on Au(111) is
calculated to be Eads=2.49 eV, which indicates a strong binding.
This energy is comparable to the adsorption energy calculated for
a mobile NHC (IPr, 2.78 eV) observed earlier.[9a] Here it was
shown that NHCs, after binding to a gold atom, can extract this
atom from the surface and thereby forming a so-called ballbot
species. The observed mobility of DiMeCAAC at low coverage
indicates that CAACs are also capable of forming such species
(Fig.2.E). The adsorption energy of such a DiMeCAAC-Au
ballbot-like species was calculated to be 4.11 eV, which is
significantly higher than the adsorption energy of the surface-
bound species. Furthermore, the mobility of such a DiMeCAAC-
Au species was investigated by DFT calculations. The differences
in binding energy along the lateral diffusion pathway were found
to be smaller than 0.04 eV (See SI) which indicates low diffusion
barriers.
Figure 2. A) Overview STM image of a ~80% coverage of
DiMeCAAC on Au(111) showing a preferred adsorption at the fcc
phase of the herringbone reconstruction (-2.0V; 16pA). B-D)
Subsequent STM images of the same area of a low coverage
(~20%) sample showing a changing size of the molecular islands
due to appearing and disappearing molecules (-2.0V; 17pA). E)
DFT-optimized geometry of a DiMeCAAC-Au ballbot species on
the surface. F) A theoretical close-packed self-assembly of
DiMeCAAC-Au ballbot-like species on Au(111).
The preferred adsorption position of a DiMeCAAC-Au species is
calculated to be at the hollow site of the Au(111) surface (See SI).
Based on this,
a possible, theoretical close-packed self-
assembled structure was calculated (Fig.2.F). The distance
between the adatoms in this image is 0.89 nm, which is smaller
than the experimentally determined average value of 1.02±0.09
nm. This shows that DiMeCAAC can not easily assemble in a
well-ordered, close-packed self-assembly as the STM images
2
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To confirm the intact deposition of DiMeCAAC and to obtain more
chemical information about the species on the surface, GC-MS
measurements of DiMeCAAC • CO2 and XPS measurements of
DiMeCAAC on Au(111) were performed. A requirement for GC-
MS analysis is the intact evaporation of the molecule. At inlet
temperatures of more then 200 °C the molecule will
decarboxylate as expected and releases the free CAAC, which
resulted in a single peak with the mother ion peak at m/z = 285.30
(See SI). This is in agreement with the mass of the free CAAC
within the error of the quadrupole mass analyzer used. Since
DiMeCAAC only has one nitrogen atom we used N 1s XPS
analysis as a marker to investigate the intact nature of DiMeCAAC
on the surface. The resulting XPS spectrum is plotted in Figure
3.A. At 400.7 eV a clear single peak is observed as is expected
for the deposition of intact molecules. XPS literature for CAACs
bound to transition metals is not available. However, for NHCs
bound to Au(111) surfaces or nanoparticles typically N 1s binding
energies around 400 to 401 eV are reported.[3b,9d,9e,11] We
therefore conclude that the peak at 400.7 eV corresponds to
surface-bound CAAC molecules. Additionally the O 1s XPS
spectrum shows negligible traces of oxygen on the surface,
confirming the removal of CO2 during sample preparation.
Keywords: Au(111) surface • CAAC • Scanning Probe
Microscopy • Ligand • X-ray photoelectron spectroscopy
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In summary we showed the successful deposition of DiMeCAAC
on different transition metal surfaces via evaporation of the CO2
adduct in UHV. Molecular islands with short-range order were
found, however due to the unsymmetric nature and bulky side
group of the molecule a well-ordered, close-packed self-
assembled structure cannot be obtained. At lower coverage
prefered adsorption at the fcc phase of the herringbone
reconstruction was observed. Furthermore series of STM images
showed that DiMeCAAC is mobile on the surface, which can be
explained by the formation of ballbot-like species as observed
previously for other NHCs. Furthermore, the intact deposition of
DiMeCAAC was confirmed by a single peak in the N 1s XPS
spectrum and the removal of the CO2 groups has been confirmed
by the O 1s XPS spectrum. This investigation of a CAAC on
different surfaces opens the way for further studies on more
complex (chiral, functional) CAAC ligands.
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Acknowledgements
This work was generously supported by the Deutsche
Forschungsgemeinschaft through the collaborative research
center SFB 858 (project B03), and through projects AM 460/2-1,
MO 2345/4-1 and FU 299/19. Furthermore, we thank
nanoAnalytics GmbH for technical support.
3
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Entry for the Table of Contents
Served on a (golden) tray! A highly reactive and electron-rich CAAC derivative is deposited on a surface after which its binding
mode and ordering are investigated for the first time. Mobile CAAC-Au species are found which form islands on the surface. These
results open the door for more on-surface studies of the highly relevant class of CAACs.
4
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