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RESEARCH ARTICLE
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N-Heterocyclic Olefins on a Metallic Surface – Adsorption,
Orientation, and Electronic Influence
Felix Landwehr, Mowpriya Das, Sergio Tosoni,* Juan J. Navarro, Ankita Das,
Maximilian Koy, Markus Heyde,* Gianfranco Pacchioni, Frank Glorius,*
and Beatriz Roldan Cuenya
N-Heterocyclic olefins (NHOs), possessing highly polarizable and remarkably
electron-rich double bonds, have been effectively utilized as exceptional
anchors for surface modifications. Herein, the adsorption, orientation, and
electronic properties of NHOs on a metal surface are investigated. On
Cu(111), the sterically low-demanding IMe-NHO is compared to its analogous
IMe-NHC counterpart. High-resolution electron energy-loss spectroscopy
(HREELS) measurements show for both molecules a flat-lying ring adsorption
configuration. While the NHC adopts a dimer configuration including a Cu
adatom, the NHO chemisorbs over a C–Cu bond perpendicular to the surface.
This distinct difference leads for the IMe-NHOs to have a higher thermal
stability on the surface. Moreover, IMe-NHOs introduce a higher net electron
transfer to the surface compared to the IMe-NHCs, which results in a
stronger effect on the work function. These results highlight the role of NHOs
in surface science as they extend the functionalization capabilities of NHCs
into stronger electronic modification.
1. Introduction
Although the inception of N-heterocyclic olefins (NHOs) (ene-
1,1-diamines) in literature dates back several decades,[1]those
F. Landwehr, J. J. Navarro, M. Heyde, B. R. Cuenya
Department of Interface Science
Fritz-Haber Institute of the Max Planck Society
Faradayweg 4–6, 14195 Berlin, Germany
E-mail: heyde@fhi.mpg.de
M. Das, A. Das, M. Koy,F. Glorius
Organisch-Chemisches Institut
WestfälischeWilhelms-Universität
Corrensstraße 40, 48149 Münster, Germany
E-mail: glorius@uni-muenster.de
S. Tosoni, G. Pacchioni
Dipartimento di Scienza dei Materiali
Università di Milano-Bicocca
Via Cozzi 55, Milano 20125, Italy
E-mail: sergio.tosoni@unimib.it
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/admi.202400378
© 2024 The Author(s). Advanced Materials Interfaces published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution
and reproduction in any medium, provided the original work is properly
cited.
DOI: 10.1002/admi.202400378
compounds have remained relatively un-
explored despite their intriguing nature.
However, in recent years, they have been
emerging as a captivating class of lig-
ands in organo-catalysis and polymeriza-
tion reaction.[2]This development paral-
lels the emergence of N-heterocyclic car-
benes (NHCs), from which NHOs can
formally be derived by addition of a termi-
nal alkylidene moiety.[3]The electron-rich
and highly polarizable double bond of
NHOs leads to elevated nucleophilicity at
the exocyclic carbon center (Cexo), which
can be rationalized by the ylidic character
of NHOs (Scheme 1a).[4]This ylidic char-
acter leads to a change in the bonding
properties in metal complexes making
NHOs excellent at binding to electron-
rich, low-oxidation state metal species.[5]
After their successful isolation in
1993, NHOs found utility in stabilizing a wide range of transition
metals and main group species due to their strong Lewis basic
site, thereby pioneering their application in catalysis.[2,3,6–10]Fur-
thermore, NHOs exhibit a structural resemblance to the “deoxy-
Breslow” intermediates that arise in specific NHC-organo-
catalyzed reactions.[11–15]Initial investigations demonstrate the
robust 𝜎-donor capacity of NHOs, accompanied by the absence
of 𝜋-backbonding characteristics.[5,16,17]
Conversely, in recent years NHCs (Scheme 1b) have garnered
significant scientific interest as surface modifications in the fields
of material science and catalysis,[18–35]with many studies focus-
ing on the binding and adsorption geometry of NHCs on well-
prepared surfaces, such as different crystal facets of Au,[36–45]
Si,[46 ]or Ag.[37]Our groups contributed to this field by elucidat-
ing the adsorption geometry of a di-isopropylphenyl-substituted
NHC (IPr-NHC) on Cu and oxidized Cu surfaces.[37,47–50]How-
ever, surface modification using NHO ligands has remained un-
explored on Cu surfaces, presenting an intriguing avenue for in-
vestigation.
Given the distinct geometric and electronic properties of
NHOs in comparison to NHCs, it is anticipated that surface mod-
ifications of NHOs could yield novel properties and assembly be-
havior which is essential for the potential applications of these
promising ligands in material science and catalysis.
Recently, the deposition of NHO species on a Au(111) and
a Si(111) surface have been reported.[51,52]These studies have
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Scheme 1. NHCs and NHOs on metal surfaces a) Mesomeric representation of the NHOs. b) Schematic of a free NHC. These molecules have been
discussed with different N-substituents on various surfaces in the literature (see main text). c) The process of the UHV introduction and subsequent
deposition of 1,3-dimethyl-2,3-dihydro-1H-imidazole (IMe-NHC) and 1,3-dimethyl-2-methylidene-2,3-dihydro-1H-imidazole (IMe-NHO) on Cu(111) is
illustrated.
shown that NHOs bind to the respective surfaces in different
ways: while NHOs bind directly to the Si(111) surface, on Au(111)
they are believed to adopt an adatom configuration similar to pre-
viously reported NHCs.[36,37,47]
In this study, we focused on the deposition of NHC and NHO
molecules with small methyl substituents (IMe-NHC and IMe-
NHO, respectively) on an ultrahigh vacuum (UHV)-prepared
Cu(111) single crystal (Scheme 1c). Cu has been chosen as a sub-
strate because copper has seen an increasing interest in recent
years in fields such as microelectronics,[53]photocatalysis[54 ]and
electrocatalysis[55]for its relative abundance and low cost com-
pared to more established materials.[56]Thus, the investigation
of the molecule–surface interaction on copper is crucial to de-
velop surface modifiers for practical applications in the men-
tioned fields.
To ensure stability and ease of handling, we utilized the CO2
adducts of the molecules as precursors, a common approach in
surface science studies involving NHCs.[36,38,46–48]The applica-
tion of thermal energy would trigger decarboxylation, resulting
in the release of the free molecules for vapor deposition onto the
intended surface yielding clean, salt-free NHC and NHO under
UHV conditions.
We employed low-temperature scanning tunneling mi-
croscopy (LT-STM) to investigate the molecular arrangement,
high-resolution electron energy-loss spectroscopy (HREELS)
to investigate the adsorption geometry and orientation, and
X-ray photoemission spectroscopy (XPS) to gain insight into
the electronic properties, complemented by density-functional
theory (DFT) calculations.
2. Results and Discussion
As a starting point, IMe-NHC and IMe-NHO were deposited on
Cu(111) following the procedure described above to obtain low-
coverage films (for details, see Section 1, Supporting Informa-
tion). O1s XPS data as well as push-rod mass spectrometry data
confirm that the CO2-adduct can be used for both IMe-NHC and
IMe-NHO (Figures S3 and S7, Supporting Information), as evi-
denced by the absence of any oxygen signal in the XPS spectra
and the absence of the [NHO+CO2]+mass fragment.
In the case of IMe-NHC, LT-STM images reveal a nonordered
arrangement of the molecules (Figure 1a). At full monolayer
and multilayer coverage IMe-NHC adsorbs in ordered domains
on Cu(111).[37]This is not the case at lower coverages as
shown in Figure 1a. This leads us to believe that the reported
ordering observed at higher monolayer and multilayer cover-
ages is likely due to constraint effects. However, we are able
to observe elongated and triangular structures (1b). This in-
dicates the formation of dimeric and trimeric adatom com-
plexes, as reported in the literature,[36,37]even at low molec-
ular coverages. DFT structural optimization of IMe-NHC on
Cu(111) also show that the (NHC)2-Cuad/Cu(111) is energetically
favored.
In the case of IMe-NHO, a similar arrangement with almost no
long-range or short-range ordering was observed (Figure 1d,e).
DFT calculations reveal an elongation of the C–C bond by 0.06 Å
upon binding and a C–Cu bond length of 2.11 Å. The Cexo atom
adopts the geometry of a distorted tetrahedron with the H–C–H
angle at 113.1°and the H–C–Cu angle at 105.4°. This shows a sp2
to sp3transition of the Cexo, indicating IMe-NHOs chemisorbs to
the surface. It is important to note that IMe-NHO at this point
does not seem to adopt the adatom configuration that has been
observed for IMe-NHC as confirmed by DFT structural optimiza-
tion of IMe-NHO on Cu(111). Here, the formation of a (NHO)-
Cuad/Cu(111) adatom complex was found to be thermodynam-
ically unfavorable and kinetically hindered by 1.10 and 1.29 eV,
respectively. Moreover, the formation of (NHO)2-Cuad/Cu(111)
dimer complexes could not be observed at all due to sterical hin-
derance. More details can be found in Figure S9 and Tables S3
and S4, Supporting Information.
These results indicate that there is no intrinsic driving force
for self-assembly at low coverage for both molecules, which dif-
fers from other NHCs that have been reported[47](for further de-
tails, see Sections 3and 4, Supporting Information). The lack of
self-assembly was also true at elevated temperatures (Figure S6,
Supporting Information).
Further analysis based solely on LT-STM data proved to be chal-
lenging as commonly employed strategies, such as comparing
apparent height of the observed molecules did not lead to any
conclusive results.
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Figure 1. LT-STM (T =5K)50nm×50 nm images on Cu(111) of high-sub-monolayer coverages for a) IMe-NHC (Vs=−1.7 V and It=50 pA) and
d) IMe-NHO (Vs=−1.2 V and It=35 pA). The white lines represent the orientation of the Cu(111) crystal. b, e) Zoomed in images of IMe-NHC and
IMe-NHO, respectively. c, f) A schematic representation of the binding configurations on the Cu(111) surface.
To further deepen our understanding of the binding con-
figurations of IMe-NHO in comparison to IMe-NHC we em-
ployed HREELS vibrational measurements. Among other UHV
compatible vibrational spectroscopy methods, such as infrared
reflection–absorption spectroscopy (IRAS) or helium atom scat-
tering (HAS), HREELS stands out by being able to provide good
energy resolution over a wide spectral range with an intrinsic sen-
sitivity to the orientation of vibrational modes relative to the sur-
face via different scattering mechanisms. By comparing specular
and off-specular measurements, dipole-active vibrations that pos-
sess a dynamic dipole moment perpendicular to the surface can
be identified, providing experimental information on the adsor-
bate orientation.
The obtained spectra are shown in Figure 2a,b together
with DFT-simulated spectra based on the adsorption geometries
which are shown in Figure 2c,d.Table 1also lists the main con-
tributing dipole active modes as well as their assignment based
on DFT calculations. For the assignment of vibrational modes,
multiple adsorption geometries were considered. However, there
were significant discrepancies between the simulated spectra of
other adsorption geometries and our experimental results. A
more detailed assignment can be found Section 7 in the Support-
ing Information.
The spectra obtained from IMe-NHC reveal only a single
clearly dipole-active signal at 716 cm−1, which is assigned to the
C–H wagging of the backbone, consistent with reports on vibra-
tional modes of other NHCs.[57]
The signals at 2972 and 3145 cm−1can be attributed
to symmetric and asymmetric C–H stretching vibrations of
the methyl groups and the molecular backbone. The fea-
tures between 1000 and 1500 cm−1correspond to various C–
N, C–C, and C=C stretching and C–H bending vibrations
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Figure 2. Binding geometry of IMe-NHC and IMe-NHO. Vibrational HREEL spectra in specular (black) and off-specular (red) scattering geometry of
sub-monolayer coverages of a) IMe-NHC and b) IMe-NHC together with DFT-calculated intensities and frequencies of the optimized binding geometry
produced by impact scattering (blue) and dipole scattering (grey) mechanism. The impact scattered signals were arbitrarily set at 50% of the dipole
scattered signal for the highest peak. The off-specular spectra were measured at analyzer angles of either 4.4°(IMe-NHC) or 4.1°(IMe-NHO). The
FWHM (full-width at half maximum) of the elastic scattering peak is given as a measurement of resolution. The DFT-optimized binding geometries of
c) IMe-NHC ((IMe-NHC)2-Cuad/Cu(111) and d) IMe-NHO ((IMe-NHO)-Cu(111)) each in top-view (top) and side-view (bottom) are also displayed.
of the molecular backbone. The experimental spectrum is
in good agreement with the simulated spectra of the (IMe-
NHC)2-Cuad/Cu(111) dimer configuration, an adsorption con-
figuration previously reported for this molecule (Figure 2a).
In this configuration, the molecular backbone is parallel to
the surface and the molecules are bound to a Cu adatom
(Figure 2c).
The high relative intensity of the spectrum compared to the
dipole-active 𝛿ring vibrations can be explained by the molecule
adopting a nonperfectly flat adsorption geometry, binding at an
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Tabl e 1 . Main dipole active vibrational modes (cm−1) contributing to the
HREELS spectra of IMe-NHC and IMe-NHO together with DFT calculated
values and the assigned vibrational mode. The abbreviation oop is used
for out-of-plane modes.
# IMe-NHC IMe-NHO Mode
exp.theo.exp.theo.
I 229 220 buckling
II 425a) 381 C–Cu stretching
III 630 625, 630 ring oop-deformation
IV 716a) 701-703 677a) 650-676 C–H wagging, oop-deformation
V 836a) 875 CH2wagging
VI 1024 1098 C–H bending
VII 1445 1418 1434 1404 C–H scissoring
VIII 1542 1505 C–C stretch
IX 2972 2922 2941 3042 sym. C–H stretch
X 3145 3027-3035 asym. C–H stretch
a)Main peaks visible in the spectra.
angle to the surface plane. This results in vibrations with a dy-
namic dipole moment in the molecular plane, exhibiting a partial
dipole-active character.
Another interpretation is the co-existence of upright and flat-
lying species at these low coverages, as reported for similar NHCs
with methyl side-chains.[57]
Significant differences are observed in the spectra recorded
for IMe-NHO. Three clearly dipole-active vibrations are observed
at 425, 677, and 836 cm−1. The first one, marked in blue, is
in good agreement with reported 𝜈(NHC-M) metal stretching
vibrations.[57–59]Themodeis,therefore,assignedtoaC–Cu
stretching vibration, suggesting a Cu–C bond perpendicular to
the surface, indicating a binding geometry with the molecule co-
valently bound to the surface and the molecular plane parallel to
the surface ((IMe-NHO)-Cu(111), Figure 2d).
The other vibrations can be assigned to the ring deformation
and C–H wagging mode at 677 cm−1similar to IMe-NHC and
a C–H wagging mode at 836 cm−1marked in orange. The lat-
ter originates from the terminal CH2group which is unique to
the IMe-NHO molecule compared to IMe-NHC. These differ-
ences in the HREELS spectra support the DFT findings that the
NHO molecule chemisorbs on the surface via the ylidic form
through a C–Cu bond perpendicular to the surface and with the
N-heterocylic backbone parallel to the surface. It further indicates
that the NHO remains intact during the evaporation and adsorp-
tion process.
The different binding configurations of IMe-NHC and IMe-
NHO on Cu(111) is also manifested in their thermal stability.
NHCs have been known for their high thermal stability on metal
surfaces, a key factor in establishing them as alternatives to thiol-
based self-assembled monolayers.[39,58]To establish the thermal
stability of the NHO binding configuration in comparison to
NHCs, we deposited both molecules on Cu(111) and monitored
the molecule density on flat terraces through LT-STM measure-
ments for several annealing steps. The associated images are
found in Figure 3. IMe-NHC only experiences a small decrease in
molecular density until 150 °C. Further annealing leads to a rapid
decline in molecule density. At 300 °C, no molecules can be found
on the surface and only the step edges of the Cu terraces show
minimal organic residue. On the other hand, IMe-NHO shows
only minor loss in density all the way up to 200–250 °Cbefore
molecules’ density sharply drops with increasing temperature.
At 350 °C, no molecules can be observed on the Cu terraces. Un-
like the IMe-NHC case, more residues can be seen not only at
the step edges but also extending onto the terraces. We attribute
this behavior to IMe-NHO directly binding to the surface, while
IMe-NHC forms the (IMe-NHC)2-Cuad dimer complexes that are
much weaker bound to the surface.
The different binding mode of the NHO molecule also raises
the question of how it influences the charge transfer during the
binding process. NHO ligands have been found to be stronger
electron donors as compared to NHCs in transition metal com-
plexes, which is remarkable considering that NHCs themselves
are known for their strong electron-donating capabilities.[16,17]
To investigate whether this holds true on a Cu(111) surface, we
have employed C1s and N1s XPS measurements, as the binding
energy is directly related to the electron density.
For IMe-NHC, XPS data exist for multiple surfaces includ-
ing Si(111),[46]Au(111),[37,38]and Cu(111).[37 ]Studies by Franz
et al. demonstrated that the electron transfer from IMe-NHC
to the substrate is very similar on Au(111) (0.29 e) and Si(111)
(0.26 e).[46]
Our calculated Bader charges evidence a very small charge
transfer from IMe-NHC to Cu(111) (0.13 e). Additionally, Jiang
et al.[37]have shown that the C1s XPS spectra for IMe-NHC
are virtually identical when comparing Au(111) and Cu(111).
These findings suggest a similar amount of electron transfer on
Cu(111).
The C1s XPS spectra of IMe-NHC and IMe-NHO on Cu(111)
are shown in Figure 4aand b, respectively. Annealing to
100 °C prior to XPS measurements to remove any physisorbed
species resulted in a slight sharpening and minor shift of the N1s
peak but significant change in the C1s peak. The spectrum of
IMe-NHC shows a single peak at 286.4 eV with a slight asymme-
try on the lower binding energy side. In contrast, the spectrum of
IMe-NHO shows a peak with a significant shoulder on the lower
binding energy side and the main peak is shifted significantly to
287.2 eV compared to IMe-NHC. This already indicates a signif-
icant difference in the electronic structure upon binding of IMe-
NHO compared to IMe-NHC. The appearance of the C1s peak is
in agreement with C1s data published for IMe-NHO adsorbed on
Au(111)[52]and Si(111).[51]
Further analysis involves deconvoluting the C1s XPS spec-
tra. Because resolution of the C1s spectra is limited due to the
nonmonochromated X-ray source, DFT calculated peak positions
have been used to deconvolute the spectra. The spectrum of IMe-
NHC (Figure 4a) can be separated into the expected three signals
for the molecule. The C1and C2signals are found at 286.5 and
285.7 eV, respectively, in good agreement with literature values
of carbon atoms bonded to N atoms.
The surface-bound carbene C3atom is observed at 287.2 eV,
which corresponds to a C atom bound to two N atoms. In this
literature, this is attributed to a shift of +1 to 1.5 eV compared to
C atoms bound to one N atom.[37]
The spectra of IMe-NHO (Figure 4b) exhibit noticeable differ-
ences compared to IMe-NHC.
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Figure 3. LT-STM images of a) IMe-NHO and b) IMe-NHC on Cu(111) after successive annealing steps at different annealing temperatures. Below a
quantitative analysis of the coverage behavior of IMe-NHC and IMe-NHO on Cu(111) depending on annealing temperature is plotted. The images and
corresponding coverages were taken by consecutively annealing the initially prepared surface for 60 s at each temperature step. Intermediate steps can
be found in Figure S6, Supporting Information.
The C3peak is shifted to 288.5 eV due to the absence of a metal
center, while the new metal-bound C4appears at 284.7 eV. The
C1and C2signals are found at 287.1 and 285.9 eV, respectively.
The observed shifts are in qualitative agreement with initial state
core level shift calculations, and the shifts with respect to the
C1of IMe-NHC are summarized in Table 2. It should be noted
that while the deconvolution into the individual components are
prone to a fitting error, the main peak positions show the same
trend with a significant difference of 0.8 eV between IMe-NHC
and IMe-NHO that is not affected by the fitting error.
Interestingly, a shift to higher binding energy can be observed
for C1when going from IMe-NHC to IMe-NHO. This shift indi-
cates a lower electron density in the imidazole ring for IMe-NHO
compared to IMe-NHC. This conclusion is further supported by
the N1s XPS spectra (Figure 4c), where an increase from 401.1 eV
for IMe-NHC to 401.2 eV for IMe-NHO can be observed. This
observation is in accordance with previous studies of IMe-NHC
and IMe-NHO on Au(111) and a modified Si(111) surface where
a N1s shift from 401.2 to 402.2 eV and 401.0 to 402.1 eV has been
observed.[51,52]
A DFT analysis of the Bader charges supports the shift in elec-
tron density, demonstrating that the net electron transfer from
IMe-NHO to the surface is more than that of IMe-NHC (0.33 e
compared to 0.13 e), which has already been shown to trans-
fer electron density upon binding to well-prepared Si and Au
surfaces.[46]This not only provides additional evidence for the
bond of the ylidic form of IMe-NHO to the Cu(111) surface but
also highlights the potential of the NHO compound class as a
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Figure 4. C1s and N1s XPS spectra of IMe-NHC and IMe-NHO. Fitted C1s XPS peaks of a) IMe-NHC and b) IMe-NHO. The underlying subspectral peaks
are area-constrained to reflect stoichiometric ratios 2:2:1 and 2:2:1:1, respectively. The main peaks are located at 286.4 and 287.2 eV, respectively. Their
positions have been marked in both graphs. c) Background-subtracted and intensity normalized N1s XPS spectra of IMe-NHC (black) and IMe-NHO
(blue) on Cu(111). The respective peak positions have been marked in the graphs.
whole for surface modifications. The almost cationic character
observed is an inherent property confined to the surface, aris-
ing from charge separation upon binding and the subsequent
“lock-in” of the ylidic form due to the separation of the 𝜋-system
of the molecular backbone from the surface. This property dis-
tinguishes NHOs from well-established NHCs and could pro-
vide a complementary method for modifying surfaces using or-
ganic molecules.
An increased charge donation from the adsorbed molecules
to the Cu surface could affect a change of the work function. To
investigate this aspect, we measured the secondary electron on-
sets for Cu(111) surfaces with different coverages of IMe-NHO
and IMe-NHC (Figure S7h, Supporting Information). The result-
ing reduction in the work function can be seen in Figure 5a.In
Figure 5b, the DFT calculated work functions for the (IMe-NHO)-
Cu(111), (IMe-NHC)-Cu(111) and the (IMe-NHC)2-Cuad/Cu(111)
configuration are displayed.
For a given coverage, the shift in the wok function is higher
for IMe-NHO than for IMe-NHC. This observation is in agree-
Tabl e 2 . Experimentally and theoretically determined B.E. positions of the
C1s and N1s peaks for IMe-NHC and IMe-NHO on Cu(111). For easy com-
parison, the values for the C1of IMe-NHC have been aligned. All other
values are given in relation to the C1values and N1, respectively. All shifts
are given in units of eV. The absolute values can be found in Table S1,
Supporting Information.
IMe-NHC IMe-NHO IMe-NHC IMe-NHO
(exp.)a) (theo.)b)
C10+0.6 0 +0.1
C2−0.8 −0.6 −0.1 0
C3+0.7 +2.0 +0.5 +1.5
C4–−1.8 – −1.1
N0 +0.1 0 +0.2
a)Values referenced to IMe-NHC (exp.) C1and N1;b)Values referenced to IMe-NHC
(theo.) C1and N1.
ment with a higher net electron transfer to the surface for
IMe-NHO, leading to an increased dipole moment perpendic-
ular to the surface compared to IMe-NHC (Table S2, Support-
ing Information) and subsequently a higher shift in work func-
tion. Note that for the upright binding configuration (IMe-
NHC)-Cu(111), the change in dipole moment perpendicular
to the surface and the subsequent work function shift would
be expected to be very similar to the (IMe-NHO)-Cu(111)
configuration.
3. Conclusion
In this study, we investigated the adsorption properties of
submonolayer coverages of the sterically low-demanding
IMe-NHO on a well-defined Cu(111) surface under UHV
conditions and compared it to the respective IMe-NHC
derivative.
Our HREELS data clearly demonstrated that IMe-NHO
chemisorbs to the Cu(111) surface “via” a C–Cu bond per-
pendicular to the surface. Both IMe-NHO and IMe-NHC ad-
sorb with their N-heterocyclic backbones parallel to the sur-
face. Moreover, both IMe-NHO and IMe-NHC did not show
any short-range or long-range order on the surface at the in-
vestigated submonolayer coverage, as revealed by LT-STM
measurements.
One of the significant findings of this study is that the net
electron transfer from the molecule to the surface is signif-
icantly higher for IMe-NHO compared to IMe-NHC, as evi-
denced by the higher binding energies observed in the C1s
and N1s XPS data. This is noteworthy considering the well-
established status of NHCs as strong electron donors in surface
science.
These differences in adsorption between IMe-NHC and IMe-
NHO lead to a stronger adsorption to the surface of IMe-NHO
and a resulting increased thermal stability. Moreover, IMe-NHO
induces a stronger reduction of the surfaces’ work function.
These findings suggest that NHOs could be viable alternatives
to NHCs in surface modification research.
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Figure 5. a) Experimentally determined work function reduction for lay-
ers of IMe-NHC and IMe-NHO with varying coverage. b) Calculated val-
ues of work function reduction for layers of IMe-NHC and IMe-NHO with
varying coverage. For IMe-NHC, two different adsorption geometries were
considered.
4. Experimental Section
Molecule Synthesis:Detailed synthetic procedure and characterization
for the CO2-adduct of IMe-NHO is included in the Supporting Informa-
tion section.
Sample Preparation:As precursor for IMe-NHC, bench-stable 1,3-
dimethyl-1H-imidazol-3-ium-2-carboxylate (IMe-NHC-CO2) was used that
is known to generate the free IMe-NHC under heating in ultrahigh vacuum
with only gaseous CO2as a by-product, enabling a clean NHC deposition
on the surface. For IMe-NHO, a similar route was employed, using 1,3-
dimethyl-1H-imidazol-3-ium-2-yl-acetate (IMe-NHO-CO2) as a precursor.
For details on the synthesis and characterization by nuclear magnetic res-
onance spectroscopy, see Section 1in the Supporting Information. The Cu
surfaces (99.99 %, MaTeck) were prepared in UHV chambers with a back-
ground pressure <5×10−10 mbar combining Ar+bombardment cycles at
1 kV and annealing at 900–950 K. The molecules were evaporated from a
heated quartz crucible in a “Kentax” evaporator at 318–328 K.
Computational Methods:All calculations were done with the code
VASP 6.[60,61]The core electrons were modeled with the Projector Aug-
mented Wave (PAW) method,[62,63]while H(1s), C(2s,2p), N(2s,2p),
O(2s,2p) and Cu(3d,4s) electrons were treated explicitly with a set of
plane waves expanded up to a kinetic energy cutoff of 400 eV. The PBE
exchange-correlation functional[64]was adopted, including the long-range
dispersion according to the DFT+D2′scheme.[65,66]The IMe-NHC-Cu-
IMe-NHC dimer structure (Figure S4, Supporting Information) was gen-
erated by adding an extra-lattice Cu atom on a three-folded hollow site
on Cu(111). The adsorption energy, De, was defined as the energy of the
molecule/substrate adduct with respect to the energy of its separated com-
ponents:
De=E(IMe-NHX∕Cu) −[E(IMe-NHX) +E(Cu)] (1)
where IMe-NHX stands for either IMe-NHC or IMe-NHO. Negative val-
ues of Deimply stable bonding. In the case of dimers, Deis reported per
molecule. Further details on the numerical tolerances, the construction of
the supercells, and the simulation of the vibrational spectra are reported
in the Supporting Information.
UHV Techniques:LT-STM measurements were conducted in a self-
designed low-temperature scanning tunneling microscope operating at
5 K using a PtIr tip. HREELS measurements were conducted at T <80 K
in a Delta 0.5 (VSI) spectrometer. The recorded spectra exhibited a full
width at half-maximum resolution of <36 cm−1(4.5 meV) for the elas-
tic peak in specular geometry. XPS measurements were performed using
a SPECS X-ray source with an Al anode operating at 300 W and a Phoi-
bos100 analyzer. The background of the spectra were subtracted through
the Shirley method. For the fittings, Gaussian–Lorentzian functions were
used. Further experimental details can be found in Section 1in the Sup-
porting Information.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
F.L., M.D., and S.T. contributed equally to this work. F.L., J.J.N., and M.H.
thank Helmut Kuhlenbeck for advice on experimental HREELS setup and
scientific discussion. J.J.N. thanks the Alexander von Humboldt Founda-
tion for the generous financial support. S.T. and G.P. acknowledge the
financial support from the Italian Ministry of University and Research
(MIUR) through the PRIN Project 20179337R7. S.T. and G. P. acknowl-
edge the access to the CINECA supercomputing resources was granted
via ISCRAB program. F.G., M.D., A.D., and M.K. gratefully acknowledge
generous financial support of the Deutsche Forschungsgemeinschaft (SFB
858 and SFB 1459) and through the International Graduate School for
Battery Chemistry, Characterization, Analysis, Recycling and Application
(BACCARA), funded by the Ministry for Culture and Science of North Rhine
Westphalia, Germany.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
Keywords
high-resolution electron energy-loss spectroscopy, N-heterocyclic car-
benes, N-heterocyclic olefins, scanning tunneling microscopy, X-ray pho-
toemission spectroscopy
Received: May 2, 2024
Published online: June 10, 2024
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