The self-assembly and metal adatom coordination of a linear bis-tetrazole ligand on Ag(111)

Article abstract of DOI:10.1039/C8CC04323J

We employ a linear linker molecule consisting of a benzene functionalised with two tetrazole moieties at para positions. Its self-assembly and coordination with the native silver adatoms and codeposited Fe adatoms on a Ag(111) surface under ultra high vacuum conditions are investigated by means of scanning tunnelling microscopy and X-ray photoelectron spectroscopy. We discover a rich spectrum of room-temperature stable Ag and Fe²⁺ coordination nodes depending on the formation temperature.

Full text of DOI:10.1039/C8CC04323J

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Commun., 2018, DOI: 10.1039/C8CC04323J.
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The self-assembly and metal adatom coordination of a
linear bis-tetrazole ligand on Ag(111)†
Peter Knecht, Nithin Suryadevara, Bodong Zhang, Joachim Reichert,a Mario
a
b
a
b,c
a
b
a
Ruben, Johannes V. Barth, Svetlana Klyatskaya* and Anthoula C. Papageorgiou*
We employ a linear linker molecule consisting of a ben-
zene functionalised with two tetrazole moieties at para po-
sitions. Its self-assembly and coordination with the native
silver adatoms and codeposited Fe adatoms on a Ag(111)
surface under ultra high vacuum conditions are investigated
by means of scanning tunnelling microscopy and X-ray pho-
toelectron spectroscopy. We discover a rich spectrum of
room-temperature stable Ag and Fe2+ coordination nodes
depending on the formation temperature.
Fig. 1 Ball-and-stick model of 1,4-bis(1H-tetrazole-5-yl)benzene (BTB)
and possible coordination modes of the tetrazolate anion. Carbon, ni-
trogen and hydrogen atoms are black, blue and white, respectively. The
atom numbering is indicated.
The functionalisation of metal surfaces with azole compounds
and in particular triazole and tetrazole compounds has attracted
1
interest due to their corrosion inhibiting properties. More re-
nodes are reported to form under hydrothermal conditions and
have demonstrated SCO phenomena.3
cently, triazole and tetrazole ligands have been employed in the
formation of coordination polymers with functional properties
ranging from fluorescence and second harmonic generation to
To the best of our knowledge the surface coordination chem-
istry of tetrazoles is hitherto unexplored, although compounds
with the related triazole group have been studied. On the more
inert Au(111) hydrogen bonding drove the self-assembly of ben-
zotriazole.6 On Cu(111) triazoles are found to bind with a tri-
azole group either oriented towards the substrate or creating a
2
2
3
spin crossover (SCO) phenomena. The tetrazole group with its
four nitrogen atoms is known to show distinct types of coordina-
tion nodes for the formation of metal organic coordination net-
works. While the tetrazole can act as both a dinucleating ligand
and a trinucleating ligand, the deprotonated tetrazolate can en-
gage in coordination bonds with up to four metal atoms.4 Here
we investigate 1,4-di(1H-tetrazole-5-yl)benzene (BTB), consist-
ing of a benzene with tetrazole groups at the 1 and 4 positions
planar node with a native Cu adatom.7 On HOPG the addition of
2+ 8
Cu adatoms results in the formation of Cu
.
Here we investigate the afore described tetrazole linear linker
(
Fig. 1) under ultra-high vacuum conditions via scanning tun-
(
Fig. 1), as a promising ligand for metal-organic frameworks: it
nelling microscopy (STM) to obtain real space images of the
self-assembly with molecular resolution, and X-ray photoelectron
spectroscopy (XPS) to get information about the chemical state
of the elements on the surface. We thus gain the first experi-
mental atomistic scale insight of the binding of tetrazole moieties
on the Ag surface. Furthermore, we exploit our in-situ sample
preparation methodology for the formation of such metal-organic
networks at the vacuum-solid interface, which combines a clean
synthesis without any solvents with a great control of the thick-
couples the coordination properties of two tetrazole ligands and
offers tunability of size via implementation of additional back-
bone units. Metal-organic frameworks with Ag, Cu, and Fe3
5
5
a
Physics Department E20, Technical University of Munich, D-85748 Garching, Ger-
many. E-mail: a.c.papageorgiou@tum.de
b
Institute of Nanotechnology, Karlsruhe Institute of Technology, D-76344 Eggenstein-
Leopoldshafen, Germany. E-mail: svetlana.klyatskaya@kit.edu
c
9
ness by adjusting the dose of sublimed molecules. We note that
Institute de Physique et Chimie de Matériaux (IPCMS), CNRS-Université Strasbourg,
3 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France.
Electronic Supplementary Information (ESI) available: experimental procedures;
2
two dimensional (2D) metal organic coordination networks are
†
10
a current topic of intense investigation, and recently enhanced
BTB crystal structure; additional STM images, models and XPS data; quantitative
XPS analysis, illustrations of chiral recognition and switches; overview of polymor-
phism. See DOI: 10.1039/b000000x/
11
cooperativity for supported SCO networks was predicted.
Initially, we investigate the room temperature (RT, ∼ 300K)
1
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self-assembly of BTB on the Ag(111) surface, where mobile Ag
1
2
adatoms are known to be present. Shortly after the molecu-
lar deposition, elongated protrusions appear on the STM images
¯
along the [110] direction (Fig. 2), coexisting with a ’sea’ of diffus-
ing molecules. These protrusions are consistent with the molecu-
lar dimensions of BTB adsorbing planarly on the metallic surface.
The overview image in Fig. 2A evidences the formation of a self-
assembled close-packed two-dimensional island (phase α), which
is characterized by a commensurate unit cell (5, 0 | 2, 4), consist-
ing of two molecules (see Fig. 2B) and has been the sole structure
observed after annealing to temperatures of up to 390 K.
We propose that these islands consist of BTB in the trans con-
figuration (Fig. 2B). Here each unit cell contains two BTB trans
molecules, one in each surface enantiomeric form; that is the two
molecular trans configurations in this model result by a mirror
and translation operation and cannot arise from any combination
of rotation plus translation operations. The structure is stabilised
by the NH···N hydrogen bondings indicated by purple dotted
lines in Fig. 2B. The proposed model is in analogy to the hy-
drogen bonded structures found in single crystals of BTB (Fig.
Fig. 2 RT self-assembly of BTB on Ag(111) (phase α, observed after
annealing in the indicated temperature range). (A) Large scale STM im-
¯
age (−1.25 V, 90 pA), the Ag [110] direction is indicated. Phase α was
observed after annealing in the indicated temperature range. (B) De-
tailed STM image (1.25 V, 110 pA) overlayed with a model. The unit cell is
marked in yellow. The ball-and-stick model is based on the crystal struc-
ture.13 Black, blue, and white spheres represent carbon, nitrogen and
hydrogen, respectively. Grey spheres mark possible position of metal
adatoms (Ag and/or Fe). Empty grey circles represent substrate atoms.
Purple dotted lines indicate hydrogen bonding between nitrogen atoms.
1
3
S1), with the projected N···N separation measured as 2.9 Å vs.
2
.8 Å.13 We note that in the STM the two isomeric forms cannot
shows a splitting of the peak related to the tetrazole carbon.21
However, one can notice that the C 1s signal reveals only ∼50 %
of the tetrazole units to be deprotonated as the intensities of the
components at 287.0 eV (yellow in Fig. 3B) and 286.0 eV (green
in Fig. 3B corresponding to deprotonated nearest N neighbour)
are approximately equal. Therefore the total intensity of the N 1s
component at 399.6 eV has further contribution from a shift of the
iminic N components with two N nearest neighbours (for relative
intensities of different components see Table S1). Such a shift
would be consistent with a scenario of metal coordination with
Ag adatoms.21 These observations substantiate the model with
Ag adatoms in the densely packed phase α (Fig. 2B).
Upon annealing the sample at 430 K two additional phases, β
and γ, can be observed by STM measurements (see Fig. 4). The
amount of deprotonated tetrazoles over the surface is ∼60 % af-
ter the annealing (ratio of green component to total contributions
from tetrazole carbon atoms in Fig. 3C, left). We therefore infer
that the N1 of both tetrazole moieties of each molecule are de-
protonated in phases β and γ.
In phase β (green unit cell in Fig. 4A) the presence of Ag
adatoms is inferred based on the XPS evidence of Fig. 3C, which
shows the majority of N signal shifted to lower binding energy
as a combined effect of N deprotonation and N-Ag coordination.
STM images seldom evidenced the presence of single Ag adatoms
(Fig. S4). We propose two Ag adatoms between the tetrazole
groups of two molecules with a projected Ag-N distance of ∼
2.2 Å (indicated in Fig. 4B), which is in good accordance with re-
ported Ag-N bond lengths of tetrazole moieties.22 The molecules
assemble in a higher order coincidence superlattice (three differ-
ent adsorption sites on the Ag(111) substrate) with the epitaxy
matrix (8/3, 5/3 | 3, 6). In phase γ (yellow unit cell in Fig. 4A),
the presence of Ag adatoms can be identified in the STM image
as broad round protrusions. Close inspection of the unit cell di-
mensions reveals that each of these protrusions is best fitted with
be distinguished and it is also possible to model phase α solely
with cis isomers (Fig. S2). The spacing of the tetrazole units
along the Ag[110] and the epitaxial unit cell is consistent with a
¯
scenario whereupon Ag adatoms are located at bridge sites and
further stabilise the network by metal coordination (Fig. 2B). The
Ag adatom was not directly evidenced in our STM data, similarly
to Ag adatoms in coordination nodes of two ortho-benzoquinone
moieties.14 This type of on-surface coordination is not unprece-
dented on Ag(111) (coordination of four iminic N atoms with a
Ag adatom after annealing at 383-443 K was proposed recently,15
whereas free base porphyrins are complexing Ag after anneal-
ing above 530 K).16 However, this would be the first report of
such coordination on surfaces already at room temperature and
its plausibility will be discussed along with the XPS data.
Complementary XPS measurements confirmed that the intact
molecule was present on the Ag(111) following RT deposition.
The relevant C 1s spectrum (Fig. 3A, left) shows two main com-
ponents, which can be related to the carbon atoms of the ben-
zene (285.2 eV, blue) and the tetrazoles (287.3 eV, yellow), re-
spectively, with the expected ratio of ∼3:1. In the corresponding
N 1s spectrum, we also identify two peaks at 401.3 eV (blue) and
4
00.0 eV (violet) in a ratio of ∼3:1. The former corresponds to the
17,18
combined contribution of the aminic N
bound to two nitrogen atoms19, the latter to the iminic N atoms
bonded to a single N atom.
and iminic N atoms
1
9,20
XPS reveals that at RT the deprotonation of the N1 atoms
occurs slowly and concomitantly with N-Ag coordination at the
N2/N3 atoms. Fig. 3B shows measured spectra on the same sam-
ples as the one shown in Fig. 3A, albeit recorded ∼18 h later (see
Fig. S3 for spectra at intermediate time intervals). The lower
energy component of the N 1s region (Fig. 3B right) increases
in relative intensity, as a result of a shift of the N1 contribution
by ∼1.3 eV towards lower binding energy, commonly associated
1
7
with aminic N deprotonation. The C 1s spectrum (Fig. 3B, left)
2
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Fig. 4 Ag adatom coordination of BTB. (A) STM image (1.75 V, 110 pA)
of the structures formed after annealing, unit cells of phases β and γ in
green and yellow, respectively. Phases β and γ were observed after an-
nealing in the indicated temperature range. (B) Molecular model includ-
ing Ag adatoms (grey spheres). The shortest Ag-N bonds are depicted
in green and yellow for phases β and γ, respectively.
three Ag adatoms. Similar nanodots have been observed with Cu
2
3
adatoms in two dimensional coordination networks. Two ad-
ditional Ag adatoms per unit cell are evidenced under the same
imaging conditions as the adatoms in phase β (Fig. S4). We
therefore infer that single Ag adatoms are not necessarily visible
in STM images, whereas a trimer of Ag adatoms would be rou-
tinely discernible. The projected Ag-N distances between nitro-
gen atoms of the tetrazoles and silver adatoms are 2.3 Å. The unit
cell comprises two molecules and five Ag adatoms and is com-
mensurate: (5, 2 | 3, 6). Transitions between both structures, β
¯
and γ, can be observed along the Ag[121] directions and exhibit
organisational chiral recognition (Fig. S5).
Targeting metal-organic networks with magnetic properties,
such as SCO phenomena, we explored the coordination with Fe.
This occurs readily at RT after sublimation of BTB and Fe adatoms
on the clean Ag(111) surface, identified by an Fe oxidation state,
distinctively different than metallic Fe on Ag(111) (Fig. 3F) and
consistent with literature values of Fe2 /Fe
+
3+ 24
.
As under these
conditions, the STM images do not identify any other structure
formation than the densely packed phase α, the Fe is assumed to
be incorporated in that phase similarly as proposed in the model
in Fig. 2B (grey spheres). Since the molecular XPS signatures do
not change significantly in the presence of Fe adatoms, we pro-
pose that Fe displaces Ag in the coordination nodes. Based on
the number of tetrazole ligands and the Fe 2p binding energy we
ascribe the Fe oxidation state in these nodes to 2+. The projected
Fig. 3 XPS measurements. (A-E) C 1s and N 1s regions corresponding
to: (A) ∼1 monolayer of BTB on Ag(111) at RT showing intact molecules
after sublimation; (B) deprotonation of aminic N atoms and Ag coordina-
tion occuring slowly at RT (recorded 18 h after sublimation); (C) higher
degree of N deprotonation and Ag adatom coordination after annealing
at 450 K (∼ 4h after sublimation). (D) monolayer BTB and Fe atoms in a
ratio of ∼6:1 (Tables S1, S2) dosed sequentially on Ag(111) at RT evi-
dencing both deprotonation of aminic N and coordination of iminic N to
Fe (∼2 h after sublimation); (E) both of these processes are promoted
Fe-N distances in these structures (2.3 Å) are close to Fe-N bond
lengths (2.2 Å) in high spin Fe2
.
+ 3
Two distinct phases (δ and ε, Fig. 5) associated with the pres-
ence of Fe on the surface can be found in STM investigations of
Ag(111) surfaces prepared by sequential dosing of BTB and Fe
and subsequent annealing to 390 K and 430 K, respectively. In
phase δ (Fig. 5A,B) one can identify pairs of small round pro-
trusions in between molecular ligands, which can be tentatively
assigned to dimers of Fe atoms (separated by 3.3 Å, see also Fig.
S7A). The head-to-head Fe coordination of the tetrazole moieties
is reminiscent of the exo ligation reported for surface stabilized
by annealing to 400 K (∼4 h after sublimation). F: Fe 2p
spectra for
/2
3
Fe on the bare Ag(111) surface (blue, 707.2 eV) and Fe with excess BTB
on Ag(111) (red, 711.0 eV) show that all the Fe is coordinated with the
tetrazole moieties and shifts to a higher oxidation state.
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(
densely packed, phase α) of BTB on Ag(111) which can ac-
commodate metal adatoms coordinating the tetrazoles without
changing noticeably the positions of the molecular ligands, pre-
sumably mediated by the tetrazole deprotonation.
Financial support was provided by of German Research Foun-
dation (DFG) through the priority programme 1928 COORNETs.
B. Z. was funded by the China Scholarship Council (CSC). The
authors acknowledge the KNMF facility (KIT, Germany).
Conflict of interest
There are no conflicts to declare.
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magnetic properties of metal-organic complexes. Further ex-
periments are necessary to determine its spin state and whether
it exhibits SCO phenomena. At last, we identified a structure
4
|
1–5