N -Heterocyclic carbenes on close-packed coinage metal surfaces: Bis-carbene metal adatom bonding scheme of monolayer films on Au, Ag and Cu

Article abstract of DOI:10.1039/c7sc03777e

By means of scanning tunnelling microscopy (STM), complementary density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) we investigate the binding and self-assembly of a saturated molecular layer of model N-heterocyclic carbene (NHC) on Cu(111), Ag(111) and Au(111) surfaces under ultra-high vacuum (UHV) conditions. XPS reveals that at room temperature, coverages up to a monolayer exist, with the molecules engaged in metal carbene bonds. On all three surfaces, we resolve similar arrangements, which can be interpreted only in terms of mononuclear M(NHC)₂ (M = Cu, Ag, Au) complexes, reminiscent of the paired bonding of thiols to surface gold adatoms. Theoretical investigations for the case of Au unravel the charge distribution of a Au(111) surface covered by Au(NHC)₂ and reveal that this is the energetically preferential adsorption configuration.

Full text of DOI:10.1039/c7sc03777e

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N-Heterocyclic carbenes on the close packed coinage metal
surfaces: Bis-carbene metal adatom bonding scheme of
monolayer films on Au, Ag and Cu
Received 00th January 20xx,
Accepted 00th January 20xx
Li Jiang,a Bodong Zhang, a Guillaume Médard,b Ari Paavo Seitsonen,c Felix Haag,a Francesco
Allegretti,a Joachim Reichert,a Bernhard Kuster,b Johannes V. Barth,a and Anthoula C.
Papageorgioua*
DOI: 10.1039/x0xx00000x
www.rsc.org/
By means of scanning tunnelling microscopy (STM), complementary density functional theory (DFT) and X-ray
photoelectron spectroscopy (XPS) we investigate the binding and self-assembly of a saturated molecular layer of model N-
heterocyclic carbene (NHC) on the Cu(111), Ag(111), and Au(111) surfaces under ultra-high vacuum (UHV) conditions. XPS
reveals that at room temperature coverages up to a monolayer exist, with the molecules engaged in metal carbene bonds.
On all three surfaces, we resolve similar arrangements, which can be interpreted only in terms of mononuclear M(NHC)2
(M=Cu, Ag, Au) complexes, reminiscent of the paired bonding of thiols to surface gold adatoms. Theoretical investigations
for the case of Au, unravel the charge distribution of a Au(111) surface covered by Au(NHC)2 and reveal that this is the
energetically preferential adsorption configuration.
at low coverages12 and such complexes have been isolated
after reacting NHCs with Au nanoparticles.13-15 These
microscopy studies were carried out on the technologically
Introduction
Carbenes are currently considered a promising alternative to
2
relevant Au(111) surface, but although they assign protrusions
of molecular dimensions in the STM images to upstanding NHC
ligands, the overall surface morphology and self-assembly is
markedly different, bedevilling the drawing of generalised
conclusions. The samples prepared from solution showed
significant surface pitting under the molecular layer, similar
with thiol SAMs on Au(111), and in one case, short-range
ordering of protrusions as close as 0.6 nm apart to each other
appeared.13 The samples prepared in ultra-high vacuum (UHV)
revealed high surface mobility at room temperature even for
coverages close to monolayers11 and regular long-range
thiols for self-assembled monolayers (SAMs)1, as well as
metal-molecule junctions.3 They are thermally robust entities4
with versatile chemical properties5, and predicted to have
6
superior electric conductivity over thiols.3 Accordingly, an
atomistic scale understanding of the adsorption geometry on
solid surfaces and their mobility is essential for the
development of nanotechnology or molecular electronics
applications. In particular, N-heterocyclic carbenes (NHCs)7-9
are persistent carbenes which have enabled an acceleration in
the aforementioned fields10 and are developing in a very active
research domain.
ordering at 77 K.12
Scheme 1. Decomposition of 1,3-dimethyl-1H-imidazol-3-ium-2-carboxylate to 1,3-
dimethyl-1H-imidazol-3-ium-2-ide (IMe) and carbon dioxide was performed under UHV
by heating to 393 K.
Hitherto there is merely a handful of microscopy studies of
NHC ligands on well-defined metal surfaces, and only two of
these were able to address the assembly at the single-
molecule level.1,
11, 12
The current understanding is that a
surface covered by NHCs will have the carbene-metal bond
perpendicular to the surface plane.2, 10 At the same time there
are reports that on Au surfaces bis-NHC-Au complexes formed
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Fig. 1
STM observations of IMe on Cu(111). (a) STM image (TSTM ~ 100 K, Us = 1.58 V, It = 0.11 nA) of a densely packed layer of dimeric NHC-Cu-NHC complexes
ꢂ
ꢀ
ꢄ
(examples outlined in yellow) on Cu(111). The unit cell of the overlayer structure is shown in green. The Cu ꢁꢁꢃ direction is indicated. (b) Line profile across a dimer
– see blue line in (a) – compared with the ball-and-stick models of a tetraazafulvalene (top) and a Cu(IMe)2 (bottom) unit. (c) Zoom in STM image (4.5×4.5 nm2) of the
left domain in (a) superposed with molecular models, the underlying Cu(111) lattice (thin lines), the overlayer lattice (thick rectangle) and the Cu adatom lattice
(dotted line). C, N, H, and Cu atoms are represented by black, blue, white, and orange spheres, respectively.
Here we aim to systematically elucidate the chemical Cu(NHC)2 compound. The former is a rather elusive, non-
nature of these surface ligands by studying a prototypical NHC planar species,19 whereas complexes of Cu and NHC have been
on the close-packed surfaces of the coinage metals (Cu, Ag and obtained by solution chemistry and found to have the
Au), under UHV conditions with a combination of scanning imidazole rings nearly coplanar, arranged opposite each
probe microscopy, X-ray spectroscopy and computational other.21 The room temperature incorporation of copper
modelling. We employed 1,3-dimethyl-1H-imidazol-3-ium-2- substrate atoms in the formation of metal-organic complexes
carboxylate as the precursor of the NHC 1,3-dimethyl-1H- and/or networks originating from carboxylates,22 halogenated
28
imidazol-3-ium-2-ide (IMe, Scheme 1), which (as recently precursors,23-25 thiolates,26 and pyridyl moieties,27,
is well
demonstrated)12 in UHV produces free IMe after heating of the documented on the Cu(111) surface. The difference in the
solid powder to 393 K. The carbene molecule was chosen so as length of the two linked IMe moieties in the case of formation
to minimize the impact of possible steric hindrance from the of a direct carbene-carbene bond, or a carbene-Cu-carbene
side groups (here methyl, instead of phenyl, mesityl or bond is approximately 0.25 nm. The resulting length of the
isopropylphenyl which were also explored in the afore- dimer would be ~ 0.7 nm in case of tetraazafulvalene
mentioned studies)16 as well as being able to discriminate formation vs. ~ 1.0 nm in the case of Cu(IMe)2. Fig. 1b
better the C 1s core level binding energy of the reactive carbon compares a line profile across a dimer which measures 1.0 nm,
atom of the carbene (corresponding to 20% of the total C 1s with the structural models of a tetraazafulvalene29 and a
signal against 4.7-6.6% for the other NHCs investigated by Cu(IMe)2,30 showing that the latter provides a much better fit.
STM).
The observed STM contrast, whereby the inserted Cu atom is
not particularly bright in comparison to the NHC, is consistent
with observations of Cu coordination with carbonitrile
32
34
groups31,
deprotonated amine groups,33,
and within
Results and Discussion
36
tetrapyrrole compounds.35,
We further note that a
Self-assembly of IMe on Cu(111)
spontaneous imaging change was occasionally observed,
whereby the described dimer appeared as single protrusions,
i.e. imaging of the central Cu atoms instead (see STM data of
the same area in ESI Fig. S1), which effect is associated with a
tip change.
After deposition of IMe on Cu(111) at room temperature, a
densely packed layer appeared on the surface as shown in Fig.
1a. One can identify bright round protrusions arranged in
dimers connected with a thin, dimmer waist (examples
outlined in yellow). Each protrusion is consistent with the
expected dimensions of a single planar IMe. On metallic
surfaces, NHCs are generally reported to form metal carbenes
directed perpendicular to the surface plane.1, 10-12, 17, 18 Beside
the two unpaired electrons, the free NHC employed contains
no other moieties which could be anticipated to give rise to
directional attractive interactions, such as hydrogen bonding.
Two different possibilities may account for the observed
The unambiguous confirmation of the Cu-carbene bond
can be found in the respective C 1s core level. The C 1s
signature appears as single peak with full width at half
maximum of 1.4 eV (green line in Fig. 2a). No clear
discrimination can be made between the different C atoms
within this peak. The observed binding energy of ~ 286.3 eV is
in good agreement with literature values of C atoms bonded to
N atoms.37 However, the reactive C atom of the carbene, being
connected to two N atoms, should have a higher binding
energy by ~1-1.5 eV with respect to the C atoms connected to
a single N atom.38, 39 On the other hand, the formation of a C-
dimerization. Either the carbene moieties react to yield a
tetraazafulvalene19,
20
(Fig. 1b top) or a surface Cu adatom
reacts with two carbene molecules to form an organometallic
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metal adatom bond is expected to result in a downshift of ~1 imaged in Fig. 1a (indicated in green). Note that we also
eV.40-42 Thus a metal coordinated C atom in IMe could be observed single domains extending over the complete
reasonably expected to have a shift of ~0.5 eV or less towards atomically flat areas (as shown in ESI Fig. S1).
higher binding energy. Such a small chemical shift of 20% of
Self-assembly of IMe on Ag(111)
the signal intensity would not be clearly discernible from the
other C 1s contributions at the energy resolution of our XPS A similar zig-zag arrangement of molecular features wets the
apparatus, whereas in the absence of the C-metal bond, the surface after room temperature deposition of IMe on Ag(111)
contribution of the reactive
C atom would be clearly (Fig. 3a). The molecular features in this case appear as
separated. The appearance of a single peak is therefore elongated protrusions with a brighter centre. The dimensions
indicative of the formation of an organometallic bond with the of these elongated protrusions are consistent with the
reactive C atom of the carbene, corroborating our assignment dimensions of Ag(IMe)2,46 as can be seen by overlying the
of the dimers on Cu(111) to Cu(NHC)2 rather than molecular structure to the STM zoom in image (Fig. 3b). The
tetraazafulvalene. bis-carbene complex appears brighter at its centre, where the
Ag adatom is located. The relative apparent height of the Ag
adatom and the ligand is similar to earlier observations of Ag
organometallic chains47 and complexes48 forming on Ag(111),
whereupon the Ag atoms appear significantly brighter.
Here the dense packed Ag(IMe)2 units run along the Ag
〉
ꢂ
〈
110 directions and their average separation is 1.0-1.1 nm. As
the Ag(IMe)2 units appear with very similar apparent heights,
we expected to be able to fit this average separation with an
integer number of Ag atoms in the underlying lattice.
However, this comes to be between three (0.87 nm) and four
(1.16 nm) substrate atomic lattice distances, it is thus an
incommensurate lattice to the Ag substrate. Only three
different orientations of domains were distinguished. At ~ 90°,
Fig. 2
XP spectra of IMe on Cu(111) (green lines) and on Au(111) (dark purple lines).
(a) C1s and
comparison.
N 1s core levels. The spectra are normalized in intensity for easier
ꢈꢈꢈꢈ
〈
〉
along the Ag 112 , the average separation of Ag(IMe)2 units is
The N 1s spectrum (green in Fig. 2b) shows a peak with
ꢂ
ꢂ
ꢄ
)
ꢀ
ꢄ
ꢀ
1.9-2.0 nm which fits well with the registry of (4 110 +8 011
binding energy of 401.1 eV. This is intermediate between the
43, 44
over the Ag substrate. Moreover, the surface density of the
M(IMe)2 is essentially the same as found on the Cu(111)
surface within the accuracy of our measurements.
binding energies of a neutral (399.2-400.5 eV)37,
and a
protonated (402.0 eV)37 amino group on a metal surface. It can
therefore be deduced that some positive charge is present in
the N atoms in this configuration.
The Cu(IMe)2 units arrange in a zig-zag fashion to form a
two-dimensional crystalline layer (Fig. 1a). This is characterized
by a rectangular unit cell which contains two dimers (indicated
in green). The dimensions of the unit cell are measured to be
approximately 0.9-1.1 nm and 1.7-1.9 nm, whereas the angle
between the unit cell vectors is 85°-90°. The long axis of the
overlayer unit cell was estimated to be rotated by ~ 8° with
respect to the underlying Cu(111) high symmetry axis. The
dimers appear with equal apparent height, hinting at
equivalent adsorption sites.45 Considering that all the centres
of the dimers (i.e. the Cu adatoms) are located in equivalent
adsorption sites, within the range and accuracy of our
measurements the monolayer structure of Cu(IMe)2 can be
ꢈ
ꢀ ꢄ
Typical STM observations of IMe on Ag(111) with the Ag 110 symmetry
Fig. 3
direction indicated. (a) Overview image of a saturation coverage of IMe following
deposition at room temperature (TSTM ~ 125 K, Us -1.17 V, It = 0.09 nA). (b) Zoom in
=
(TSTM ~ 125 K, Us = -1.58 V, It = 0.08 nA) overlaid with the unit cell of the overlayer
structure (blue solid lines) and the proposed model (C, N, H, and Ag atoms in bis-
carbene complexes are represented by black, blue, white, and grey spheres,
respectively).
7
ꢆ1
described by the epitaxy matrix
ꢅ
ꢇ. Fig. 1c shows the
3
5
proposed epitaxial model of Cu(IMe)2 superposed on the STM
image. The Cu(IMe)2 complexes are placed at equivalent
adsorption sites, and the angle between the long axes of the
dimers was set to ~ 80°; though reasonably good matching can
be obtained for angles in the range of 60° to 90°. At the
borders of the domains, dimers of fainter contrast are
observed, associated with mobility under measurement
conditions (with a substrate temperature of 100 K). Three
symmetrically equivalent unit cells with respect to the
underlying Cu lattice exist on the same atomically flat terrace
Self-assembly of IMe on Au(111)
In light of the preferred formation of organometallic NHCs
dimers incorporating Cu or Ag adatoms, we revisit the
interpretation of the experiment featuring the same NHC
molecule on Au(111). Fig. 4a is an overview of a surface
saturated with IMe at room temperature, the imaging being
consistent with previous work.12 The chevron structure of the
Au(111) substrate underneath is clearly discernible. The
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periodicity of the fcc-hcp domain walls along the 110 Au
Journal Name
ꢂ
ꢀ
ꢄ
After mild annealing to ~ 330 K, we notice an improved
direction is noticeably affected. In our data it was measured to ordering of both the Au surface and the surface overlayer (Fig.
vary between 6.3 and 7.9 nm, instead of 6.3 nm for the 4b). The chevron reconstruction is more regular, with a
pristine Au(111) surface in vacuum.49 Such gradual lifting of periodicity of ~ 6.3 nm, and no pitting appears at the elbow
the Au(111) chevron reconstruction concomitant with the sites, whereas the molecular layer shows single domains
ejection of the extra Au surface atoms has been observed homogeneously covering different orientational domains of
50
52
upon adsorption of NO2 and trimethylphosphine,51,
the substrate reconstruction. This indicates that the unit cell
respectively. However, in contrast to earlier works, as well as has a certain degree of flexibility, so as to maintain its
for another NHC on Au(111),11 no islands related to these extra
Au adatoms were found. For example, the periodicity of the
fcc-hcp domain walls in Fig. 4a is ~7.5 nm. If in Fig. 4a a dense
packed island was resulting from the extra adatoms of the
reduced reconstruction, this would be expected to have a
diameter of ~2.4 nm (see details of this calculation in the ESI).
Depressions also appear commonly on the elbow sites (see Fig.
4a). We note that the Au adatoms originating from the ejected
atoms of the relaxed periodicity of the domain walls and the
elbow sites are reasonably expected to be incorporated in
direct bonding with NHC units; another possible source of Au
adatoms is the intrinsic adatoms, e.g., released from atomic
steps due to thermal energy. The unit cell of the IMe assembly
was measured to be 1.1-1.2 nm by 1.6-1.8 nm and an angle of
87° to 90°.
1
5
ꢆ5
3
direction. The epitaxy matrix
ꢅ
ꢇ on reconstructed
Au(111) results in inequivalent unit cells, depending on the
substrate arrangement (cf. quasihexagonal patterns formed by
alkali metal adsorption).53 The pertaining unit cells have sides
within the range of 1.2-1.3 nm by 1.5-1.6 nm, enclosing angles
of 85°-90°. This epitaxy matrix is consistent with the observed
orientations of the domains and accounts for each bright
protrusion appearing at an equivalent binding site.
Fig. 5
(a) Zoom in STM image of a saturation coverage of IMe on Au(111) (TSTM ~
155 K, Us = -1.79 V, It = 0.03 nA) overlaid with proposed molecular model of Au(IMe)2
analogous to the observations of Cu(IMe)2 on Cu(111) and Ag(IMe)2 on Ag(111). The
solid lines indicate the overlayer unit cell and the dotted lines the Au adatom lattice. (b)
Unit cell and top view of the previously proposed molecular model of the same
assembly,12 drawn in the same scale as for our work. (c) Top view of DFT optimised
geometry of Au(IMe)2 in the unit cell given by the epitaxy matrix deduced here
(periodic unit cell of the calculation indicated in purple). C, N, H, and Au atoms are
represented by black, blue, white, and yellow spheres, respectively. In the DFT model
the Au(IMe)2 molecules are outlined for clarity.
By contrast, in the previous work, the individual
protrusions were attributed to an upstanding mono-carbene
metal complex, Au(IMe), which would diffuse freely onto the
surface (Fig. 5b).12 On all three surfaces investigated in the
present work, the unit cells have two carbene metal
complexes. At this point it is important to compare the
average surface footprints of the carbene metal complexes on
Fig. 4
Overview STM images of a saturation coverage of IMe on the Au(111) surface
(a) following deposition at room temperature (TSTM = 100 K, Us = 1.63 V, It = 0.08 nA) and Cu, Ag and Au: 1.08 nm2, 1.02 nm2, and 0.96 nm2, respectively.
(b) annealing to ~ 330 K (TSTM = 150 K, Us = 1.44 V, It = 0.09 nA). The unit cell of the
These differ by less than 15%. So it is clear, that if Au(IMe)
ꢈ
ꢀ
ꢄ
overlayer structure is shown in blue and the Au 110 symmetry direction is indicated.
complexes formed, the monolayer on Au would contain
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approximately half the surface density of NHCs than the indicates an electron donation of 0.38 e- from each NHC ligand
monolayers on Cu and Ag, which are decorated by M(IMe)2. mainly towards the Shockley-like surface state (Au pz orbitals
Hence the upstanding configuration does not seem to enhance close to the Fermi level),61 while other contributions can be
the packing density,
a driving force for upstanding found at lower energies and other p-type orbitals. Moreover,
configuration in self-assembly,54 weakening the plausibility of the Au atom in the complex is slightly positive by 0.23 e-, in
the previously proposed model.12 The results point to Au(IMe)2 agreement with the calculated core level shift (see ESI).
formation, like the corresponding Cu(IMe)2 on Cu(111) and
We further went to consider a Bader charge transfer
Ag(IMe)2 on Ag(111). This interpretation fits better the high analysis before and after the surface anchoring. We
resolution STM images shown in Fig. 5a. Moreover, it is in considered two separate cases of charge transfer:
good agreement with the reported reaction of an NHC with (a) n(Au(IMe)2 on Au(111)) – n(IMe) – n(Au(111) + adatoms)
gold nanoparticles which results in bis-carbene gold(I) (shown in Fig. 6a) and
complexes.13 Parallels can be drawn with the formation of (b) n(Au(IMe)2 on Au(111)) – n(Au(IMe)2) – n(Au(111)) (shown
“staple” motifs (i.e. the stapling of a Au atom by two opposing in Fig. 6b)
ligands) with thiolates. For the least sterically imposing where n is the electron density.
thiolate, methyl thiolate, the staple motif is consistently found We notice that irrespectively of whether we consider the Au
to be energetically favoured on the Au(111) facets.55-57 The atoms bonding directly to the NHC ligands as part of the
same bonding scheme was also confirmed experimentally for surface or the molecule, the ligands or the complexes donate
longer alkene thiols (e.g. propane-1-thiol55 and hexanethiol)58 electrons to the surface and the density differences are
as well as for benzenethiol.59
negative close to the molecule, consistent with the overall
Finally, our interpretation is fully supported by DFT electron densities above. Moreover, in both cases an electron
modelling. Fig. 5c shows the stable structure resulting by an depletion zone exists close to the N atoms (Fig. 6), which is in
energy minimisation of two Au(IMe)2 complexes in a unit cell agreement with the N 1s binding energy. This feature was not
described with the aforementioned epitaxy on the idealised evident in the earlier DFT study which considered an
non-reconstructed Au(111) substrate. The Au(IMe)2 complexes upstanding IMe directly bonded to a surface atom.60
preferentially bind near the bridge positions. To compare it
with the previously proposed model of an upstanding mono-
carbene complex in the same unit cell,9 we considered the
relative energies of one unit cell with two Au(IMe)2 plus one
unit cell with adatoms only compared with two unit cells
containing two Au(IMe) each. The former is energetically
favourable by 0.10 eV. Interestingly, we also found that a
single Au(IMe)2 in the unit cell is even more stable as a
structure than the average of two Au(IMe)2 plus a cell without
the adsorbates by 0.30 eV, explaining the observed high
mobility even at coverages close to monolayer, whereas
isolated Au(IMe)2 complexes were observed by STM at low
coverages.12
The interaction of the IMe with the Cu and Au surfaces is
further compared with the spectroscopic signatures of IMe on
the two surfaces (Fig. 2). The purple lines in Fig. 2 correspond
to the C 1s and N 1s core levels of a saturation coverage of IMe
on Au(111) and are plotted on the same graph as the
respective ones on Cu(111) (green lines) to facilitate easier
comparison. We note that recent DFT calculations show that
NHCs act as electron donors to the Au(111) surface,60 which is
consistent with the binding energies observed for the N 1s, as
described earlier. The direction of the charge transfer
predicted by DFT is independent of whether Au(IMe) or
Au(IMe)2 forms on Au(111) (see calculated work function
values in ESI). Note that the core levels have the same binding
energy (within 0.1-0.2 eV) and shape on both Au(111) and
Cu(111), as expected for molecules showing the same
interaction pattern with the supporting surfaces, further
supporting our proposed model.
Fig. 6 Electron density differences (Δn) of the Au(IMe)2 adsorbed on Au(111) from (a)
the free biradical IMe and the Au(111) with the respective adatoms or (b) the isolated
Au(IMe)2 complex and the pristine Au(111) surface. Left: One-dimensional
representation averaged along the surface plane. Note that
H and N atoms are
vertically displaced for clarity. Here z is the distance from the Au(111) surface. Right:
Three dimensional representations with red and blue representing regions of
enhancement and depletion of electron density, respectively.
Conclusions
In order to gain further insight into the electronic structure
and the bonding to the surface of this energetically favoured
bonding scheme, we performed a Bader analysis, which
In summary, we investigated saturated monolayers of a steri-
cally not imposing NHC on atomistically well-defined Cu(111),
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Ag(111), and Au(111) single crystal surfaces under UHV Sample preparation. The atomically flat and clean Cu(111),
conditions. Upon room temperature deposition the free Ag(111), and Au(111) single crystal surfaces were prepared by
carbene forms clearly identifiable dimers on Cu(111), which cycles of Ar+ sputtering and annealing to ~ 725 K, 700 K, and
are assigned to planar NHC fused by a Cu adatom as indicated 600 K, respectively. Excess of IMe was dosed on the surfaces
by the C 1s signal. On Ag(111) connatural dimers featuring a held at room temperature by sublimation of the solid powder
bright central protrusion (at the Ag adatom position) are in preparation chambers (base pressure ~ 4×10-10 mbar)
clearly identified in the STM images and exhibit a closely attached to the analysis chambers at with organic molecular
related surface arrangement in the monolayer. A remarkably beam epitaxy (OMBE). Pressure during the OMBE of the NHCs
similar complex formation and packing of IMe is demonstrated was ~ 10-7 mbar due to a partial pressure of CO2. Carbon diox-
on the technologically important Au(111) surface, with imaging ide does not adsorb on Cu(111)64 or on Ag(111)65 at room
characteristics akin to those on Ag(111), whereby virtually temperature, whereas reactively formed CO2 desorbs from
identical spectroscopic signatures of the C 1s and N 1s core Au(111) already at 77 K.66 No oxygen 1s signal was detected in
levels of the IMe as on Cu(111) are recorded. We therefore the XPS studies presented above, consistent with the CO2
deduce that the planar bis-carbene metal formation is group being abstracted from the IMe precursor and not
preferential even at high coverages and thus represents a interacting with the surfaces. At room temperature only
universal bonding scheme of the prototypical IMe to coinage molecules in direct contact with the metal surface condensed
metal surfaces. Model calculations demonstrate that the based on the observed saturated C 1s signal recorded after
organometallic dimer is energetically preferred to a previously varying the dosing time. In all cases, we investigated the dense
invoked mono-carbene Au complex. We further show that packed interfacial structures resulting from
there is no attractive interaction between the bis-carbene Au coverage of IMe.
a saturation
complexes formed, thus explaining the observed high mobility STM. STM was performed with an Aarhus-type variable-
and the ease of leaching of such complexes in solution. temperature STM housed in the analysis chamber of a home-
It is thus appropriate to draw attention to the formation of made UHV system at
2×10-10 mbar base pressure. The tip
≤
such bis-carbene motifs at high coverages and expect a consisted of a chemically etched tungsten wire which was held
complex landscape for the exact control of the adsorption at 273 K. The tunnelling bias, Us, was applied to the sample.
geometry of carbenes on solid surfaces, kindred to the The topography images have been processed by WSxM.67
adsorption of thiolates on Au(111). We further provide a direct XPS. XPS measurements were carried out in a SPECS GmbH
rationalisation of why this type of NHCs (without side groups UHV system (base pressure of 2×10-10 mbar) at the Technical
to enable attractive lateral interactions) might not stabilise University of Munich – Walter Schottky Institut laboratory. A
metal nanoparticles and exhibit high surface mobility in the XR50 X-ray source with ellipsoidal crystal FOCUS 500
respective SAMs. We envisage that instead, such NHCs which monochromator provided monochromatized Al Kα radiation
can be reacted with solid surfaces to form selective, well- (h = 1486.74 eV). Spectra were recorded with a PHOIBOS 150
ν
defined and robust organometallic junctions on the surface hemispherical analyser in a normal emission geometry with
plane are suitable for engineering isolated compounds and the samples held at room temperature. The binding energy
low-dimensional architectures. As carbene ligands can bind scale of the spectra was calibrated by setting the Cu 3p3/2 or
and stabilize a plethora of metals and p-block elements, they Au 7f7/2 core level at 75.1 and 84.0 eV, respectively. Any
are currently recognised as promising protean surface anchors possible change to these core levels due to the contribution of
with widespread relevance for interfacial functionalization.
the metal atoms in the bis-carbene complexes was
overshadowed by the bulk and surface atoms. A Shirley
background was usually subtracted from the raw data, with
the exception of the N 1s signal from IMe on Au(111). In this
case a linear background was subtracted, as the peak overlaps
with the tail of the Au 4d core level.
Methods
Synthesis. 1,3-dimethyl-1H-imidazol-3-ium-2-carboxylate was
synthesized by reacting 1-methylimidazole (2 mL) with neat
dimethyl carbonate (3 mL) over a week-end at 95 °C. The
precipitate was filtered out from the yellow supernatant and
washed with dichloromethane (thrice 10 mL), acetone (thrice
10 mL) and diethyl ether (thrice 10 mL) to yield a very pale
pink solid. When carried under microwave irradiation at 120
°C (temperature suggested by Holbrey et al.)62 for 15 hours,
the reaction yielded a precipitate which was identified as the
4-carboxylate isomer (1,3-dimethyl-1H-imidazol-3-ium-4-
carbo-xylate). This temperature-dependent stereoselectivity
was also observed by Voutchkova et al.,63 who preferred a
reaction temperature of 90 °C. In our hands the reaction
proceeded very sluggishly at 90 °C and we needed to increase
the temperature to 95 °C.
Structural modelling of molecular compounds. The molecular
models proposed here are substructures of compounds with
reported single crystal data. The atomic coordinates of the C, N
and metal atoms in the tentative molecular models presented
are based on the respective reported single crystal data, while
the positions of the hydrogen atoms are estimated with a C-H
bond distance of 0.1 nm. The single crystal data used were
retrieved
from
http://www.crystallography.net
and
correspond
to 5,5'-6,6'-tetramethoxy-1,1'-3,3'-
tetramethylbibenzimidazoli-nylidene29 for the tetraazafulvane,
N,N′-bis[2,6-(di-isopropyl) phenyl] imidazol-2-ylidene-N,N′-
(dicyclohexyl)imidazol-2-ylide-ne Copper(I)30 for the Cu(IMe)2,
[Bis(1-ethyl-3-methylimidazol-2-ylidene)silver(I)]
6 | J. Name., 2012, 00, 1-3
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[bis(trifluoromethylsulfonyl)amide]46 for Ag(IMe)2 and
[Au(IMe)2]PF668 for Au(IMe)2 and Au(IMe).
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Chem., 2009, 33, 1837-1840.
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17 P. Lara, O. Rivada-Wheelaghan, S. Conejero, R. Poteau, K.
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18 A. V. Zhukhovitskiy, M. G. Mavros, T. Van Voorhis and J. A.
Johnson, J. Am. Chem. Soc., 2013, 135, 7418-7421.
19 P. I. Jolly, S. Zhou, D. W. Thomson, J. Garnier, J. A. Parkinson, T.
Tuttle and J. A. Murphy, Chem. Sci., 2012, 3, 1675-1679.
20 M. Matena, T. Riehm, M. Stöhr, T. A. Jung and L. H. Gade,
Angew. Chem., Int. Ed., 2008, 47, 2414-2417.
Density functional theory simulations. The calculations were
performed on Au(111) using the Quantum ESPRESSO
71
approximation to the
package.69 The vdW-DF2-B86r70,
exchange-correlation term was applied. Five layers of
substrate were considered, with two bottom-most layers fixed
at the bulk-terminated positions. Optimised lattice constant of
4.1325 Å, 4×4 k points, Fermi-Dirac smearing of occupation
numbers with a width of 50 meV, projector augmented wave
(PAW) data sets for the pseudisation of the core electrons,
surface-dipole correction, and cut-off energy in plane wave
expansion of wave functions of 40 Ry and of density of 300 Ry
were used
.
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Organometallics, 2004, 23, 755-764.
Acknowledgements
This work was supported by the ERC Advanced Grant MolArt
(No. 247299) and the Munich-Centre for Advanced Photonics
(MAP). L.J. and B.Z. acknowledge funding from the China
Scholarship Council (CSC). A.P.S. acknowledges Centro Svizzero
di Calcolo Scientifico (CSCS), Lugano, Switzerland, for super-
computing resources.
22 N. Lin, D. Payer, A. Dmitriev, T. Strunskus, C. Wöll, J. V. Barth
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8 | J. Name., 2012, 00, 1-3
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N-heterocyclic carbenes wet the solid surfaces of group 11 metals with the energetically favourable
electron donating biscarbene metal complexes.