BN-Patterning of Metallic Substrates through Metal Coordination of Decoupled Borazines

Article abstract of DOI:10.1002/chem.201800849

We report on the synthesis of pyridine-terminated borazine derivatives, their molecular self-assembly as well as the electronic properties investigated on silver and copper surfaces by means of scanning tunneling microscopy and X-ray photoelectron spectro

Full text of DOI:10.1002/chem.201800849

DOI: 10.1002/chem.201800849
Full Paper
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Surface Chemistry |Hot Paper|
BN-Patterning of Metallic Substrates through Metal Coordination
of Decoupled Borazines
Martin Schwarz+,[a] Manuela Garnica+,[a] Francesco Fasano+,[b] Nicola Demitri,[c]
Davide Bonifazi,*[b] and Willi Auwꢀrter*[a]
Abstract: We report on the synthesis of pyridine-terminated
borazine derivatives, their molecular self-assembly as well as
the electronic properties investigated on silver and copper
surfaces by means of scanning tunneling microscopy and X-
ray photoelectron spectroscopy. The introduction of pyridine
functionalities allows us to achieve distinct supramolecular
architectures with control of the interdigitation of the mole-
cules by surface templating. On silver surfaces, the borazine
derivatives arrange in a dense-packed hexagonal structure
through van der Waals and H-bonding interactions, whereas
on Cu(111), the molecules undergo metal coordination. The
porosity and coordination symmetry of the reticulated struc-
ture depends on the stoichiometric ratio between copper
adatoms and the borazine ligands, permitting an unusual
three-fold coordinated Cu–pyridyl network. Finally, spectros-
copy measurements indicate that the borazine core is elec-
tronically decoupled from the metallic substrate. We thus
demonstrate that BNC-containing molecular units can be in-
tegrated into stable metal-coordination architectures on sur-
faces, opening pathways to patterned, BN-doped sheets
with specific functionalities, for example, regarding the ad-
sorption of polar guest gases.
Introduction
gases like CO2 and CO, which can in principle interact with BN
bonds through dipolar interactions, thus making BN-doped
materials very good candidates for gas adsorption.[10] Together
with the possibility of tuning the molecular bandgap into the
visible range of the solar spectrum, these structures could
emerge as unique photoactive materials triggering photo-
chemical transformations. Given these premises, we conjec-
tured that two-dimensional structures formed through non-co-
valent interaction on a surface could act as unique model ar-
chitectures to study the self-assembly and recognition proper-
ties of BN-materials at the molecular level through scanning
probe microscopy (STM).
Borazine derivatives have recently attracted increasing interest
as molecular building blocks,[1] due to their high potential for
applications in the fields of electronics,[2,3–5] and non-linear
optics.[6] In particular hybrid h-BNC nanostructures, where
carbon-carbon bonds are replaced by isoelectronic and iso-
structural BN couples are emerging as a new route to function-
alize polycyclic aromatic hydrocarbons without a significant
structural perturbation of the molecular periphery and of its
skeleton.[5,7] The presence of BN bonds imparts strong local
dipole moments that can tailor both, the optoelectronic prop-
erties and the self-assembly behavior of the molecule.[8,9] For
instance, one can conjecture that the polar BN bonds could
serve as anchoring point for non-covalent adsorption of polar
In particular, precisely tailored substituents of molecules can
be utilized to realize targeted adsorption geometries, as well
as to preserve the intrinsic properties of molecules upon ad-
sorption on a metal support.[11] Recently, it has been proposed
that the borazine scaffold of a molecule can be protected and
electronically decoupled from the conductive substrate by di-
methylphenyl terminal groups through steric hindrance with-
out affecting the possibility of the molecule itself to adsorb
and self-assemble.[12] Additionally, the selection of the substitu-
ent’s termination (e.g. -carbonitrile (CN), -carboxylate, -pyridyl)
steers the supramolecular interactions, driving for example the
formation of two dimensional metal-organic coordination net-
works (2D-MOCNs) in the presence of metal adatoms. The for-
mation of such a 2D-MOCN combining borazine and metal
adatoms has not been reported to date, although those
porous architectures are highly appealing as they can act as
templates featuring cavities and molecular units on regular
and well-defined sites linked by metallic nodes.[13] Indulging
this line of thought, here we describe the synthesis and the
[a] M. Schwarz,+ Dr. M. Garnica,+ Prof. Dr. W. Auwꢀrter
Physics Department
Technical University of Munich
85748 Garching (Germany)
E-mail: wau@tum.de
[b] F. Fasano,+ Prof. Dr. D. Bonifazi
School of Chemistry
Cardiff University
Park Place Main Building, Cardiff CF10 3AT (United Kingdom)
E-mail: bonifazid@cardiff.ac.uk
[c] Dr. N. Demitri
Elettra–Sincrotrone Trieste
S.S. 14 Km 163.5 in Area Science Park, 34149 Basovizza, Trieste (Italy)
[+] These authors contributed equally.
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201800849.
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self-assembly properties of borazine derivatives on Cu(111),
Ag(100) and Ag(111) surfaces that, exposing peripheral pyridyl
groups, can undergo metal-coordination in the presence of
copper adatoms to yield porous patterns. Specifically, STM
studies show that the molecules behave differently on the two
metals, with those deposited on Ag undergoing self-assembly
through weak interactions (van der Waals (vdW) and H-bond-
ing), whereas on Cu the molecules engage in metal-coordina-
tion. Using scanning tunneling spectroscopy (STS), we also
provide evidence that the borazine core is electronically de-
coupled from the conductive metal substrate. The architec-
tures on Cu(111) substrates were exposed to CO gas and their
chemical properties investigated by XPS. Additionally, we ex-
plored the thermal stability of the network on Cu(111) at room
temperature, and the effect of post-annealing on both metal
substrates by STM.
Figure 1. a) Structure of BNPPy determined by X-ray diffraction. STM images
of an individual BNPPy molecule on b) Cu(111) (Ub =1.9 V, It =170 pA) and
c) Ag(100) (Ub =0.5 V, It =81 pA). The superimposed dashed triangle in b) in-
dicates the orientation of the molecule.
mediate was reacted with trimethylsilyl (TMS)-protected ArLi
derivative 5, TMS-protected borazine derivative 6 could be pre-
pared in 71% yield (Scheme 2). Removal of the silyl protecting
group with TBAF yielded phenylacetylenborazine derivative 7,
that could be transformed into borazine 8 (BNAPy) by a Sono-
gashira cross-coupling reaction with 4-iodo-aniline (see Sup-
porting Information for the characterization).
Synthesis of BNPPy and BNAPy
The pyridine-bearing borazines have been prepared following
the BCl3 condensation protocol (see Supporting Information
for synthesis procedure).[3,4] Starting from aniline, which was re-
acted with BCl3 under refluxing conditions, the B,B’,B“-trichloro-
N,N’,N”-triphenyl borazine intermediate was obtained. Upon
subsequent treatment with tert-butyldimethylsilyl (TBDMS)-ar-
yllithium (ArLi) 1, TBDMS protected borazine 2 could be pre-
pared in 78% yield (Scheme 1). Removal of the TBDMS protect-
Scheme 2. Synthesis of BNAPy 8: a) BCl3, toluene, reflux, 18 h; b) 5, tBuLi,
THF, À848C, 16 h, 71%; c) TBAF, THF, 08C, 2 h, 94%; d) 4-iodo-pyridine,
[PdCl2(PPh3)2], PPh3, CuI, NEt3, 758C, 17 h, 70%.
Results and Discussion
High-resolution STM images of individual BNPPy molecules on
Cu(111) and Ag(100) show sub-molecular features with a very
similar appearance (Figure 1b,c). The central part of the mole-
cule appears as three bright lobes pointing along the axes de-
fined by the pyridyl-terminated substituents, which are imaged
dimmer. The molecular contrast does not change for moderate
bias voltages of both polarities, suggesting that it reflects the
molecular conformation. We simulated STM images for differ-
ent adsorption geometries using the extended Hꢂckel method
(see Supporting Information, Figure S25). The best agreement
between experiment and simulation is obtained for a BNPPy
geometry where the borazine core and the peripheral pyridyl-
groups are aligned (nearly) parallel to the surface plane while
all phenyl rings are rotated out of this plane. Specifically, the
three prominent lobes observed in STM can be reasonably as-
sociated with the bulky di-methyl-bearing aryl rings. This as-
signment is in line with STM observations and complementary
molecular dynamics (MD) modeling reported for dimethyl-
bearing aryl borazine derivatives on Au(111) and Cu(111).[12]
Consistently, we observe a very similar intramolecular contrast
featuring three bright protrusions for BNAPy on Cu(111) (see
Figure 4).
Scheme 1. Synthesis of BNPPy 4: a) BCl3, toluene, reflux, 18 h; b) 1, tBuLi,
THF, À848C, 2 h, 78%; c) TBAF, THF, 08C, 2 h; d) Tf2O, pyridine, r.t., 16 h,
78%; e) 4-pyridinphenylboronic acid, [Pd(PPh3)4], K2CO3, dioxane/H2O (3:1),
1058C, 18 h, 87%.
ing groups with TBAF and successive esterification with Tf2O in
pyridine gave tri-triflate borazine 3 in 78% yield. Final Suzuki
cross-coupling reaction between molecule 3 and pyridylphe-
nylboronic acid led to borazine 4 (BNPPy) in 87% yield. Crys-
tals suitable for X-ray diffraction analysis were obtained by
slow diffusion of pentane in a CHCl3 solution of 4 (Figure 1a).
The crystal packing shows hydrophobic contacts between
neighboring molecules, with partial p–p stacking involving the
aryl arms. The effective packing of BNPPy in the solid phase
gives rise to a distorted planar arrangement of the molecules,
piled up along the crystallographic c axis and with partial over-
laps of outer pyridine moieties of neighboring molecules (the
angle between neighboring borazine ring planes is 538). Simi-
larly, when the B,B’,B“-trichloro-N,N’,N”-triphenyl borazine inter-
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The molecular structure determined by X-ray diffraction on
BNPPy crystals shows that the expected three-fold symmetry
of the molecule can be disturbed due to the flexibility of the
substituents (Figure 1a). Upon adsorption, a similar behavior is
observed, as can be seen in Figures 1b,c. The pyridyl-terminat-
ed substituents appear bent in the surface plane, resulting in
angles enclosed by them deviating from 1208.
Borazine derivative BNPPy on Ag surfaces
Extended, highly ordered molecular islands of BNPPy were ob-
served on silver samples with different surface terminations,
namely a Ag(100) single crystal and a Ag(111) film (see Fig-
ures 2 and 5c, respectively). The molecules, deposited onto
the sample held at room temperature, arrange in six-mem-
bered rings and form porous honeycomb networks. A similar
self-assembly was observed for related molecules on the
Au(111) surface.[12] Molecular domains with sizes of several hun-
dreds of nm and arbitrary orientation with respect to the crys-
tal high-symmetry directions were present on the samples.
A high-resolution close-up view (Figure 2b) reveals that
each molecule in the honeycomb structure is surrounded by
three neighbors and three pores. The rhombic unit cell (white
lines in Figure 2b) has the parameters a=b=(26.0Æ0.5) ꢃ and
q=(60Æ3)8, which results in a packing density of 0.34 mole-
cules/nm2. The network is stabilized by intermolecular vdW
and H-bonding interactions between the aryl moieties decorat-
ing the BN core. The small molecule-molecule distance sug-
gests that the flexible substituents are bent to reduce steric
hindrance (see Figure 2c).
Nevertheless, STM-based manipulation experiments indicate
weak intermolecular attraction as individual molecules can be
removed from the edges of the islands in a controlled fashion
without significantly perturbing the self-assembled network
(Figure 2d,e). For this purpose, the STM tip is positioned above
a rim molecule, which is dragged out to the bare Ag terrace
(indicated by the white dashed lines) applying a bias voltage
Ub =50 mV and a tunneling current It =20 nA. Furthermore, the
molecules can be rotated by STM manipulation, changing the
orientation of the pyridyl-terminated substituents with respect
to the molecular axes (Figure 2 f,g). The flexibility of the sub-
stituents is clearly revealed in Figure 2 f where apparent angles
between 908 and almost 1808 are observed between adjacent
aryl substituents.
Figure 2. STM images of BNPPy on Ag(100). a) Large-scale image of the
dense-packed hexagonal network of BNPPy molecules. Scan parameter-
s: Ub =1.0 V, It =64 pA. b) High-resolution STM image (unit cell highlighted
by white rhombus). Scan parameters: Ub =0.2 V, It =40 pA. c) STM image
overlaid with a tentative structural model of the molecular assembly. d) Indi-
vidual molecules can be manipulated by STM in a controlled way (dotted
white lines represent the tip path). e) Three BNPPy molecules were detach-
ed from the dense-packed island. (Scan parameters, d,e): Ub =0.5 V,
It =81 pA). f,g) Additionally, the orientation of the protruding substituents
can be changed as indicated by the white arrows in (f). (Scan parameters,
f,g): Ub =1.7 V, It =81 pA).
In addition to the most abundant arrangement in six-mem-
bered rings, a second dense-packed phase (packing density
0.41 molecules/nm2) embedded in the honeycomb network is
occasionally observed (Figure S26). In this phase, BNPPy are as-
sembled in rows consisting of pairs of molecules oriented in
opposite directions. This pair of molecules constitutes the base
unit observed in both structures on the Ag substrates, sug-
gesting that the molecular packing is governed by the same
intramolecular interactions and the concentration of molecules
on the surface (see Figures 2 and S27). Large coverage deposi-
tion promotes the formation of the second phase, which pres-
ents a more compact network.
After annealing the sample to 570 K, the hexagonal arrays
still occur, including some dislocation lines. Additionally, disor-
dered regions are observed (Figure S27b), where covalent
structures, formed through thermal activation of CÀH bonds,
are presumably present.
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Figure 4. STM image of BNAPy on Cu(111) showing the self-assembly in mo-
lecular chains, connected through Cu-coordinated substituents. The trape-
zoidal unit cell has the parameters a=(32.6Æ2.6) ꢃ, b=(19.2Æ2.4) ꢃ, and
q=(75Æ4)8. Scan parameters: Ub =1.0 V, It =100 pA.
Figure 3. STM images of BNPPy on Cu(111). a) Upon room temperature dep-
osition, the BNPPy molecules form chains that are interconnected by the
protruding substituents coordinated to a Cu adatom. Scan parameter-
s: Ub =1.2 V, It =100 pA. b) Deposition of additional Cu atoms with the
sample held at 420 K leads to a structural transformation and patches of
molecules with all three substituents coordinated to Cu atoms are formed.
Scan parameters: Ub =1.0 V, It =64 pA. c) With sufficient Cu adatoms avail-
able, extended islands with fully three-fold coordinated BNPPy molecules
are observed. Scan parameters: Ub =0.96 V, It =40 pA. d,e) Tentative structur-
al models of the chain-like network and the fully reticulated network super-
imposed on STM images. f–j) A series of STM images acquired at 300 K
shows that the chain-like network is observable at room temperature. Con-
secutive scans of the same area reveal that individual molecules at the edge
of the molecular islands are mobile (highlighted by black circles). Scan pa-
rameters: Ub =1.0 V, It =110 pA.
coordinated to a copper atom. Notably, due to the flexibility of
the lateral aryl substituents, the molecules are not necessarily
connected in a straight line. This results in the formation of a
porous network featuring voids of unequal size (Figure S29).[14]
Deposition of additional copper atoms with the sample kept
at 420 K gives rise to a structural transformation of the chain-
like network: patches of fully Cu-coordinated hexagonal arrays
embedded in the former assembly of BNPPy on Cu(111)
emerge (Figure 3b). Annealing to this temperature in the ab-
sence of extra copper atoms or deposition of the molecules at
420 K does not trigger the formation of this phase. In this new
network structure, three BNPPy molecules are coordinated
with the N atoms of their terminal pyridyl groups to a node
that presumably consists of a single Cu atom (see discussion
below). As in the previous chain-like assembly, the individual
Cu atoms are not resolved in the STM images. Increasing the
dose of additional Cu atoms completely transforms the chain-
like architecture into a three-fold coordinated, fully reticulated
metal-organic network (Figure 3c,e and Figure S28b). The tra-
pezoidal unit cell has the parameters a=(25.5Æ1.7) ꢃ, b=
(24.2Æ1.6) ꢃ with an internal angle q=(60Æ2)8, resulting in a
packing density of 0.18 molecules/nm2. Notably, the nanostruc-
ture does not align with the high symmetry directions of the
crystal surface.
Borazine derivative BNPPy on Cu(111)
The self-assembly behavior of BNPPy molecules on Cu(111) is
summarized in Figure 3a. A highly ordered porous 2D network
with molecules arranged in interconnected chains (Figure 3d)
is formed after deposition of the molecules onto the sample at
room temperature. The trapezoidal unit cell of the network has
the parameters a=(38.4Æ2.9) ꢃ, b=(26.5Æ2.3) ꢃ, and q=
(75Æ5)8. The packing density corresponds to 0.20 molecules/
nm2 and the short axis of the unit cell is aligned parallel to
¯
one of the <110> directions of the Cu(111) surface. The
growth of the network initiates at step edges. Subsequently,
the anisotropic islands expand onto the terraces (Figure S28a).
A similar self-assembled chain-like structure was observed for
the borazine derivative BNAPy on the same support (see
Figure 4). Due to the reduced length of the aryl substituents,
the BNAPy network presents a smaller trapezoidal unit cell
with the parameters a=(32.6Æ2.6) ꢃ, b=(19.2Æ2.4) ꢃ, and
q=(75Æ4)8.
Further annealing of either network to temperatures higher
than 470 K results in a polymeric phase. Contrary to a recent
study employing bromine-substituted borazine derivatives
aiming for highly regular polymeric nanostructures,[15] only dis-
ordered structures were observed. Molecules have presumably
lost part of their peripheral substituents and are bond cova-
lently (Figure S27a). A similar observation was made for the
dense-packed structure on Ag, when it was annealed to 570 K
(Figure S27b).
Along the rows, the molecules are alternately rotated by
(180Æ6)8 and remarkably close-packed. The center-to-center
distance ((20.5Æ0.5) ꢃ for BNPPy and (17.9Æ0.4) ꢃ for BNAPy,
respectively) suggests that the substituents are slightly bent
(Figure 3d). Adjacent rows are connected via opposing sub-
stituents in BNPPy and BNAPy networks with a center-to-
center distance of (32.2Æ0.6) ꢃ and (27.2Æ0.5) ꢃ, respectively.
This long distance suggests a pyridyl-Cu-pyridyl coordination
motif, where the N atoms of the peripheral pyridyl groups are
STM measurements performed at 300 K (Figure 3 f–j) reveal
that the chain-like BNPPy network, comprehensively character-
ized at 5 K (Figure 3a), prevails at room temperature. However,
consecutive scans indicate that borazine modules at the edge
of molecular islands are mobile, while the bulk of the self-as-
sembled architecture remains mostly unperturbed. Likely, this
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is a consequence of the weak molecule-substrate interaction
combined with the high mobility of Cu adatoms at room tem-
perature that, enabling diffusion of the molecules, allow the
dynamic formation and rupture of coordination bonds at the
periphery of the self-assembled islands.
Finally, we have carried out XPS measurements on the
chain-like network on Cu(111) in order to investigate the ability
of the BNPPy molecule to adsorb CO through its central bora-
zine core, thus exploring the potential binding of polar guest
gases alluded to in the introduction. XP core-level spectra of a
sub-monolayer of BNPPy on Cu(111) before dosing CO are
shown in Figure S30. The main component of the C 1s core
level is observed at a binding energy Eb =284.8 eV. The N 1s
core level is found at a binding energy Eb =399.2 eV. Due to its
low cross-section, the B 1s core-level could not be resolved for
sub-monolayer coverage. In BNPPy multilayers on Cu(111), the
C 1s and N 1s core levels (Figure S31) reveal a slight upshift by
0.4 and 0.2 eV, respectively. This is attributed to a suppressed
polarization screening by the metallic substrate in the case of
the multilayer.
Figure 5. STM images of BNPPy on a) Cu(111) and c) a Ag(111) film (Scan pa-
rameters in both images: Ub =1.0 V, It =100 pA). The corresponding STS
spectra recorded on characteristic positions of the respective assembly are
shown in (b) and (d). e–g) Voltage series of BNPPy on Ag(111). (Scale
bar: 1 nm. Set point: It =200 pA).
After a CO dosage of 60 L at room temperature and a dose
of 6 L at ꢀ100 K, no changes in either core level have been de-
tected in XPS for sub-monolayer coverage. Also in STM meas-
urements no structural changes were detected after an in situ
dose of 10 L of CO at temperatures below 15 K. Therefore, we
infer that the CO is not adsorbed on the self-assembled net-
work. Presumably, this is caused by the presence of the steri-
cally-hindering methyl substituents that, shielding the borazine
core, impede the formation of favorable dipolar interactions
between the BN bonds and CO gas molecules.
is also detected on the core of the molecule (red and yellow
spectrum). However, for spectra taken at the center of the
pore (Figure 5c, blue dot), the surface state feature is not ob-
served. This can be tentatively explained by the presence of
the terminal pyridyl rings (see Figure 2c), which are clearly vi-
sualized in STM images only at certain bias voltage. The real
pore size is drastically reduced compared to that appearing at
low bias voltages. Indeed, this bias dependent contrast is re-
vealed in an STM voltage series of BNPPy on Ag(111) displayed
in Figures 5e–g. At low bias voltages, the contrast of the
BNPPy molecule is dominated by the three bright protrusions
in the center, while the peripheral substituents are only faintly
visible (Figure 5e, Ub =0.6 V). At higher bias voltages, the ter-
minal pyridyl groups of the substituents are imaged brighter
than the molecular core (Figure 5g, Ub =2.4 V), indicative of
unoccupied molecular resonances located on the terminal moi-
eties.
Spectroscopic characterization of BNPPy
Scanning tunneling spectroscopy (STS) data of BNPPy on a
Cu(111) single crystal and a Ag(111) film are summarized in
Figure 5. In the case of Cu(111), the spectra taken in the pore
of the chain-like assembly (Figure 5b, blue spectrum) show a
pronounced feature at ꢀÀ400 mV, which is attributed to the
increase of the local density of states due to the surface state
of Cu(111). This feature is still detected in the spectra taken at
the center (red spectrum) and at the bright lobe (yellow spec-
trum) of the molecule as indicated in Figure 5a. In analogy to
reports on other physisorbed adsorbates,[16] the persistence of
the surface state feature signals that the borazine core is only
weakly interacting with the substrate. Moreover, an additional
broad resonance is observed at 2.3 V, reflecting an electronic
feature of the BNPPy molecule, as no such signature is ob-
served on the bare metal. Spectra taken at the coordinated
pyridyl substituent (Figure 5b, green spectrum) show a pro-
nounced resonance shifted by ꢀ350 mV toward lower energy
compared to the spectral feature observed on the molecular
core. Indeed, the alignment and spatial distribution of molecu-
lar orbitals can be affected by the site-specific bonding with
metal adatoms as previously reported.[17,18]
Discussion
Using high-resolution STM imaging and X-ray diffraction, we
show that individual BNPPy molecules have a remarkable flexi-
bility, which is expressed by the possibility of their pyridyl-ter-
minated substituents to bend in-plane. The terminal pyridyl
groups drive the self-assembly, which can be controlled by the
choice of the metal substrate. On Ag(111) and Ag(100) surfaces,
the molecule-substrate interaction is weak and no influence of
the surface termination on the network architecture was ob-
served. The self-assembly is driven by intermolecular vdW
forces and H-bonding leading to a dense-packed network
(0.34 molecules/nm2) and no indication for metal coordination
has been observed. Previous studies have been devoted to the
coordination chemistry of pyridyl-functionalized molecules in
In the case of BNPPy on the Ag(111) film, the lowest unoc-
cupied molecular resonance is found at ꢀ2 eV and the related
surface state feature at ꢀÀ50 meV (Figure 5d, black spectrum)
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the quest to achieve supramolecular networks.[19–26] In particu-
lar, utilizing Cu atoms as metallic coordination nodes, two-fold
pyridyl-Cu-pyridyl coordination has been reported on Ag(111)
and Cu(111).[18,19,21] However, a coordinated BNPPy network
could not be achieved on silver surfaces, neither by annealing
nor by providing additional Cu atoms within the experimental
parameter space that was explored.
substituents, a fact which has been anticipated for similar bor-
azine derivatives in a previous report.[12] Additionally, the XPS
binding energy of the N 1s line is at 399.2 eV, comparable to
the value reported for other borazine derivatives on noble
metal supports,[9] and exceeding the value for h-BN/Cu(111) by
about 1 eV.[35] The different contributions of the N atoms locat-
ed in the borazine core and in the pyridyl moieties could not
be resolved with our lab-based XPS setup.
Contrary to borazine derivates terminating with phenyl moi-
eties,[12] BNPPy forms a highly ordered assembly on Cu(111), in
which chain-like structures are formed involving Cu coordina-
tion. The resulting porous network features a packing density
(0.20 molecules/nm2) clearly reduced compared to that formed
by BNPPy on Ag(111) and Ag(100), as well as compared to sim-
ilar phenyl-terminated borazine derivatives on Cu(111)
(0.33 molecules/nm2).[12] Thus, pyridyl-mediated metal-coordi-
nation allows for the fabrication of stable borazine arrays with
unprecedented pore sizes. Interestingly, this network does not
reflect the three-fold symmetry of BNPPy, but bases on inter-
connected chains where only one of the three pyridyl-substitu-
ents seems to engage in Cu coordination. Even if we cannot
exclude the presence of some coordinative interactions along
the chains, our data suggest the simultaneous expression of
metal-organic and organic bonding motifs.[27,28] The measured
center-to-center distance between BNPPy units along the co-
ordinated substituents, that is, perpendicular to the chain di-
rection, is (32.2Æ0.6) ꢃ. With a pyridyl-substituent length of
14.4 ꢃ, extracted from the structural model (after geometry op-
timization with the semi-empirical AM1 method in Hyper-
Chem)[29], this results in a projected pyridyl-Cu-pyridyl bond
length of 3.4 ꢃ, slightly reduced compared to the 3.6 ꢃ report-
ed in the literature.[20,23,24,30] This can be rationalized by the in-
plane bending of the substituents. Accordingly, the projected
NÀCu bond length corresponds to 1.7 ꢃ.
Conclusion
In summary, we combined pyridyl-functionalized borazine de-
rivatives with selected substrates to achieve distinct network
architectures, exploiting the remarkable flexibility of the sub-
stituents. While the BNPPy molecules form a dense-packed
hexagonal network on Ag substrates, a porous network
evolves for BNPPy and BNAPy on Cu(111) with stability up to
room temperature. The deposition of additional Cu atoms
yields a structural transformation of the metal-organic architec-
ture on Cu(111), which leads to a fully reticulated network with
a three-fold pyridyl–Cu coordination motif. Following this ap-
proach, the molecular density could be varied from 0.20 to
0.18 molecules/nm2, expanding the corresponding pore size
from 0.7 to 6.0 nm2. These findings thus provide unprecedent-
ed metal-organic coordination architectures on surfaces based
on BNC-containing molecules. Our experiments likely suggest
that the presence of the sterically shielding methyl groups on
the aryl B-bearing substituents prevents the adsorption of CO
on the BN core, notwithstanding, they electronically decouple
the BN core from the conducting substrate. This findings pro-
vide valuable insight for the design of borazine derivatives tar-
geting the anchoring of CO in functional nanostructures com-
prising for example functionalized hybrid BNC polyphenylenes
and graphene-like structures.[1]
A fully reticulated coordination network with a three-fold
symmetry was achieved by the deposition of additional Cu
atoms. Three-fold Cu coordination has previously been report-
ed for CN end groups,[30] bipyridyl molecules,[31] and pyridyl-ter-
minated tectons.[20,27,32] However, for the latter case, reports on
two-fold pyridyl-Cu-pyridyl coordination clearly prevail.[19–24,26,33]
In the present system, the NÀCu bond distance in the three-
fold node ((3.0Æ0.5) ꢃ) significantly exceeds the theoretically
predicted value of 1.6 ꢃ[23] and the 1.7 ꢃ characteristic for the
two-fold motif, which might reflect steric hindrance between
the (nearly) co-planar pyridyl rings. Nevertheless, we cannot
exclude a minor rotation of the terminal pyridyl moieties out
of the surface plane. For instance, such rotations can enable a
four-fold coordination of tetra-pyridyl-porphyrins to mononu-
clear centers.[34] We assign the coordination center to a single
Cu atom (see Figure 3e), as no protrusion is apparent at the
three-fold node. Protrusions were previously observed in
three-fold motifs featuring Cu dimers coordinated to pyridyl
termini.[20]
Experimental Section
Most STM experiments were carried out in a custom-designed
ultra-high vacuum (UHV) chamber housing a CreaTec STM operat-
ed at 5 K. Additional STM and XPS experiments were conducted at
room temperature in a second UHV chamber equipped with a
SPECS X-ray source with an Al anode, a SPECS PHOIBOS 100 elec-
tron analyzer and a CreaTec room temperature STM. The base pres-
sure during all experiments was <5ꢄ10À10 mbar. The Cu(111) and
the Ag(100) single crystals were prepared by repeated cycles of
Ar+ ion sputtering and annealing to 775 and 725 K, respectively.
Ag(111) films were prepared by e-beam evaporation of several
layers of Ag on a Cu(111) crystal, which was held at 575 K as de-
tailed in ref. [36]. The borazine derivatives BNPPy and BNAPy were
dosed from a thoroughly degassed quartz crucible held at 600 K
onto the sample held at room temperature. All STM images were
recorded in constant-current mode using an electrochemically
etched tungsten tip. The WSxM software was used to process the
STM raw data.[37] All XP core-level spectra were excited with the
AlKa photon energy of 1486 eV. A Shirley-type background was
subtracted and Voigt profiles were used to model the data.
Our STS data on BNPPy and BNAPy molecules (Figures 5
and S32) reveal that the surface state feature is still detected
on the molecular core on both the Cu(111) and the Ag sub-
strates. This observation shows that the borazine core is de-
coupled from the metallic substrates by means of the dimethyl
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Full Paper
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Acknowledgements
This work is supported by the European Research Council Con-
solidator Grant NanoSurfs (No. 615233) and the Munich-Center
for Advanced Photonics (MAP). M.G. would like to acknowl-
edge the H2020-MSCA-IF-2014 programme. W.A. acknowledges
funding by the Deutsche Forschungsgemeinschaft via a Hei-
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Conflict of interest
The authors declare no conflict of interest.
Keywords: borazines · coinage metals · scanning tunneling
microscopy · self-assembly · surface chemistry
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Revised manuscript received: April 13, 2018
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Accepted manuscript online: April 19, 2018
Version of record online: && &&, 0000
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FULL PAPER
Borazine derivatives on coinage
metals: Distinct network architectures
are achieved by self-assembly of func-
tionalized BNC-based tectons on Ag and
Cu surfaces, featuring borazine moieties
electronically decoupled from the sup-
port. On Ag, dense-packed arrays
evolve. On Cu, metal-coordination
steers the assembly of stable chain-like
structures that transform to fully coordi-
nated networks upon deposition of Cu
atoms.
&
Surface Chemistry
M. Schwarz, M. Garnica, F. Fasano,
N. Demitri, D. Bonifazi,* W. Auwꢀrter*
&& – &&
BN-Patterning of Metallic Substrates
through Metal Coordination of
Decoupled Borazines
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Chem. Eur. J. 2018, 24, 1 – 8
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ꢁ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!