Synthesis, Characterization, and Evaluation of Antibacterial Activity of Ferrocenyl-1,2,3-Triazoles, Triazolium Salts, and Triazolylidene Complexes of Gold(i) and Silver(i)

Article abstract of DOI:10.1002/ejic.202100024

1,2,3-triazoles, the corresponding triazolium salts, and the resulting mesoionic carbenes (MICs) of the 1,2,3-triazol-5-ylidene type are at the forefront of contemporary research. In this contribution, we present a comprehensive study involving eight tria

Full text of DOI:10.1002/ejic.202100024

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Very Important Paper
Synthesis, Characterization, and Evaluation of Antibacterial
Activity of Ferrocenyl-1,2,3-Triazoles, Triazolium Salts, and
Triazolylidene Complexes of Gold(i) and Silver(i)
[
a]
[b]
[a]
[c]
[a]
Carolin Hoyer, Peter Schwerk, Lisa Suntrup, Julia Beerhues, Maite Nössler,
[a]
[d]
[b]
[a, c]
Uta Albold, Jens Dernedde, Karsten Tedin, and Biprajit Sarkar*
1
,2,3-triazoles, the corresponding triazolium salts, and the
all ferrocenyl-containing compounds display a ferrocenyl-based
oxidation step. Additionally, in both the triazolium salts, and
the respective metal complexes, a ligand centered reduction
step is observed. All the complexes were tested for their
antibacterial properties against the Gram-negative bacterial
strains Salmonella typhimurium (S. typhimurium) and Escherichia
coli (E. coli). Many of the tested compounds display good
antibacterial activity against the rather resistant Salmonella
strain. To inhibit the growth of E. coli, higher concentrations of
the compounds tested were required. A preliminary structure-
activity relationship of this set of compounds is also reported.
resulting mesoionic carbenes (MICs) of the 1,2,3-triazol-5-
ylidene type are at the forefront of contemporary research. In
this contribution, we present a comprehensive study involving
eight triazoles, seventeen triazolium salts, two silver(i)- and four
gold(i)-MIC complexes. The substituents on the N1-atom of the
heterocycles are either ferrocenyl or phenyl and those on the
C4-atom bear various alkyl groups. All the compounds were
thoroughly characterized by a combination of multinuclear
NMR spectroscopy, ESI-MS and in many cases through single
crystal X-ray diffraction studies. In the electrochemical analysis,
[
4e,i,s,7]
Introduction
induced or redox-switchable catalysis.
Gold(i)-MIC com-
plexes display good to excellent catalytic activity in several
reactions containing alkynes as substrates.
plexes of 1,2,3-triazol-5-ylidenes have been popular because of
their relevance in the transmetallation route for synthesizing
metal complexes with MICs. Additionally, silver(i)-MIC com-
plexes have also found use in catalysis in their own right.
1,2,3-triazoles have relevance in medicinal chemistry.
Furthermore, metal complexes of N-heterocyclic carbenes
(NHCs) have recently been extensively tested for their biological
activity. In view of the above facts, we were interested in
probing the antibacterial effect of 1,2,3-triazoles, the 1,2,3-
[4s,7b,8]
The copper-catalyzed azide alkyne cycloaddition (CuAAC)
Silver(i) com-
[1]
reaction, one of the most popular examples of the so-called
[2]
click reaction, has impacted diverse branches of chemistry in
the recent past. In organometallic chemistry, the corresponding
[9]
[
10]
1
,2,3-triazol-5-ylidenes, which are derived from the 1,2,3-
[
11]
triazoles synthesized by this click reaction, have established
themselves as a popular class of mesoionic carbenes (MICs).
Metal complexes of such MICs have not only found use in
homogeneous catalysis,
as electro-active compounds, photochemistry and redox-
[
3]
[3d,4]
[12]
but also in various other fields such
[5]
[6]
triazolium salts, as well as the corresponding silver(i)- and gold
(
i)-MIC complexes. It should be mentioned here that a few
[
[
[
a] C. Hoyer, Dr. L. Suntrup, M. Nössler, Dr. U. Albold, Prof. Dr. B. Sarkar
Institut für Chemie und Biochemie, Freie Universität Berlin,
Fabeckstrasse 34–36, 14195 Berlin, Germany
[13]
1,2,3-triazolium salts and metal complexes with MIC ligands
of the 1,2,3-triazol-5-ylidene type
[7b,14]
have been tested in the
b] P. Schwerk, Dr. K. Tedin
recent past for their biological activity. In the following, we
present a comprehensive study that involves eight 1,2,3-
triazoles, seventeen 1,2,3-triazolium salts, two silver(i)- and four
gold(i) MIC complexes. All the compounds were thoroughly
characterized by a combination of multinuclear NMR spectro-
scopy, ESI-MS, and in several cases through single crystal X-ray
diffraction studies. The electrochemical properties of the
ferrocenyl derivatives are reported. The antibacterial activity of
the compounds against the Gram-negative bacterial strains S.
typhimurium and E. coli are presented. A first and preliminary
attempt is made at drawing a structure-activity correlation in
terms of antibacterial properties of these compounds.
Institut für Mikrobiologie und Tierseuchen, Freie Universität Berlin,
Robert-von-Ostertag-Str. 7–13, 14163 Berlin, Germany
c] J. Beerhues, Prof. Dr. B. Sarkar
Lehrstuhl für Anorganische Koordinationschemie, Institut für Anorganische
Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
E-mail: biprajit.sarkar@iac.uni-stuttgart.de
https://www.iac.uni-stuttgart.de/en/research/aksarkar/
d] Dr. J. Dernedde
[
Institut für Laboratoriumsmedizin, Klinische Chemie und Pathobiochemie,
Charité-Universitätsmedizin Berlin,
Corporate member of Freie Universität Berlin, Humboldt-Universität zu
Berlin, and Berlin Institute of Health,
Augustenburger Platz 1, 13353 Berlin, Germany
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202100024
©
2021 The Authors. European Journal of Inorganic Chemistry 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.
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Synthesis and Characterization
(1c–1e) in order to increase the water solubility of the
respective compounds, and to investigate the influence of a
proton source on their bioactivity. As the complex syntheses
with these ligand precursors failed and since the ferrocenyl
triazoles 1d and 1e themselves did not show significant
antibacterial activity against the tested bacterial strains (vide
The triazoles were synthesized via the CuAAC reaction from the
corresponding azides and alkynes in good yield (67–89%). The
[15]
[16]
compounds 1a and 5a–5c were previously reported and
were re-synthesized for the current work (Scheme 1). The
ferrocenyl group was chosen as a substituent on the N1-atom
of some of the triazoles (1a–1e). This was done as the
ferrocenyl group is often considered a privileged motif for
[16]
infra), only phenyl analogues 5a–5c were synthesized.
All the triazoles were methylated to the triazolium salts in
good yield (58–99%). For the methylation reactions, different
methylating reagents such as MeI, MeOTf and Meerwein salt
(Me OBF ) were used (Scheme 1). Such an approach allowed us
[17]
creating compounds with biological activity, and also for its
additional redox-activity.
3
4
[16]
For the triazoles 5a–5c the substituent at the N1-atom is
a phenyl group. This was done to have an aromatic, non-
ferrocenyl substituent in order to get a sound comparison with
the ferrocenyl substituted triazoles. Alkyl substituents were
chosen on the C4-atom of all the triazoles, as such substituents
to generate triazolium salts with different counter anions in
order to be able to test the effect of these on the biological
activity of the triazolium salts. The triazoles and the triazolium
1
13
1
salts were characterized by a combination of H and C{ H}
1
NMR spectroscopy and mass spectrometry. In the H NMR
[12c,13a,f]
showed positive effects
on the biological activity of
spectra of the triazoles, the resonance of the CÀ H triazole ring
related heterocycles. Additionally, it was planned to introduce
an alcohol or a carboxylic acid group on the alkyl substituents
proton appears downfield shifted between 7.50 and 8.22 ppm,
[
18]
and is very characteristic for this class of compounds. On
[
15]
[16]
Scheme 1. Synthetic route starting from the respective azides and alkynes. Compounds 1a and 5a–c are literature known.
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Figure 1. Molecular structures in the crystal of triazoles 1c and 1e and triazolium salts [2b](OTf), [2c](OTf) and [2d](OTf). Ellipsoids are rendered at 50%
probability level. Hydrogen atoms (except for the OH groups) and counter ions have been omitted for clarity.
Table 1. Selected bond lengths (Å) of 1c, 1e, [2b](OTf), [2c](OTf) and
methylation, the signal for the triazolium ring CÀ H proton is
[
2d](OTf).
further downfield shifted and appears between 9.00 and
Bond length [Å]
9
.60 ppm. The shift of this aforementioned signal is usually a
Bond
1c
1e
[2b](OTf)
[2c](OTf)
[2d](OTf)
[19]
good indication for the formation of the triazolium salts. In
the ESI-MS the triazoles usually delivered a peak in the
protonated form or with Na . For the triazolium salts the
N1À N2
N2À N3
N1À C1
1.351(2)
1.316(2)
1.354(2)
1.337(6)
1.328(6)
1.355(6)
1.325(3)
1.326(3)
1.355(3)
1.329(2)
1.329(2)
1.354(3)
1.321(3)
1.323(3)
1.364(3)
+
molecular peak corresponding to the triazolium cation was
clearly observed in the ESI-MS.
Single crystals suitable for X-ray diffraction studies could be
obtained for the triazoles 1c and 1e, and for the triazolium salts
The triazole and triazolium rings are twisted with respect to
the connected Cp-ring with a dihedral angle ranging from 3 to
46°. The two Cp rings in the compounds take up conformations
from almost perfectly eclipsed (1e and [2c](OTf)) to a more
staggered situation (1c, [2b](OTf) and [2c](OTf)).
[2b](OTf), [2c](OTf) and [2d](OTf) (Figure 1). As was reported
previously, in the molecular structures of the triazoles in the
crystal, the central N2À N3 bond is shorter compared to the two
[7a,20]
bonds that flank it (Table 1),
pointing to a localized bond
situation inside the triazole ring. On methylation to the
triazolium salts, the bond lengths within the triazolium ring
become more equal, indicating a more delocalized situation
The gold(i) complexes 3a, b and 7a, b were synthesized via
the Ag(i)-mediated transmetallation route in reasonable yield
(Scheme 1). The disappearance of the signal corresponding to
the ring CÀ H proton of the triazolium ring is a good indication
for the formation of such MIC complexes. The carbene-C signals
(Table 1).
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Figure 2. Molecular structures in the crystal of gold(i)-MIC complexes 3a, 7a and 7b. Ellipsoids are rendered at 50% probability level. Hydrogen atoms have
been omitted for clarity.
Table 2. Selected bond lengths (Å) of 3a, 7a and 7b.
1
3
1
in the C{ H} NMR spectra of the compounds were observed
Bond length [Å]
between 156.0 and 157.6 ppm. In the ESI-MS of 3a and 3b a
peak corresponding to the cation after loss of the Cl was
Bond
3a
7a
7b
À
N1À N2
N2À N3
N1À C1
AuÀ C1
AuÀ Cl
1.336(4)
1.323(4)
1.368(4)
1.986(3)
2.291(1)
1.330(4)
1.311(4)
1.373(4)
1.986(4)
2.297(9)
1.331(5)
1.328(5)
1.373(6)
1.971(5)
2.291(1)
observed. For the complexes 7a and 7b the peak for the
respective cationic complex with two triazolylidene ligands was
detected. Unfortunately, the isolation of the gold complexes via
the transmetallation route with ligands that contain additional
À OH or À COOH functionalities was not successful. Single
crystals suitable for X-ray diffraction studies were grown for 3a,
7
a and 7b (Figure 2). In the molecular structure in the crystal,
complexes were established in a similar way as reported for the
gold complexes above. Interestingly, for both complexes
coupling of the carbene-C to the two naturally occurring silver
the Au(i) center displays a near linear coordination, being
coordinated by the carbene-C of the MIC, and by an additional
chlorido ligand. The deviation from linearity in the three
compounds is between 2–3°. The bond lengths within the
1
07
109
isotopes,
Ag and
coupling at 165 ppm in the respective C{ H} NMR
07/109 Ag)
Ag, is observed as a doublet due to J
(C-
1
3
1
1
[12b,22]
triazolylidene ring, as well as the AuÀ C
and AuÀ Cl bond
spectrum.
Just like for the gold complexes, it was not
Carbene
[
7a,8b,e,14a,21]
lengths are all in the expected range (Table 2).
The
possible to isolate the silver complexes with ligands that
contain additional functionalities like À OH and À COOH. For
reasons that are not very clear to us, it was also not possible to
isolate the silver complexes with triazolylidene ligands that
contain a phenyl substituent at the N1-atom (Scheme 1). We
were able to obtain single crystals of [4a](OTf) and [4b](OTf)
that were suitable for single crystal X-ray diffraction analysis
(Figure 3). In both the compounds, the Ag(i)-center displays a
near linear coordination, being coordinated by a carbene-C
from each of the MIC ligands. The AgÀ Ccarbene bond lengths in
triazolylidene ring of 3a is twisted with respect to the
connected Cp-ring with a dihedral angle of 38°. The angle
between the phenyl substituent and the triazolylidene ring in
7
a and 7b varies from 46 to 56°.
The silver complexes [4a, b](OTf) were synthesized by the
reaction of Ag O and the respective triazolium salt [2a, b](OTf)
2
in the presence of KCl and a base (Scheme 1). Despite the
presence of excess KCl, we were only able to isolate the
homoleptic complexes. The purity and the identity of the silver
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Figure 3. Molecular structures in the crystal of silver(i)-MIC complexes [4a](OTf) and [4b](OTf). Ellipsoids are rendered at 50% probability level. Hydrogen
atoms and counter ions have been omitted for clarity.
[4h,23]
both compounds are in the expected range (Table 3).
The Electrochemistry and Antibacterial Studies
conformation of the Cp-rings in the ferrocenyl groups is nearly
eclipsed. In both the compounds, the ferrocenyl groups from
the two ligands are oriented anti to each other, likely because
of steric reasons (Figure 3).
All the ferrocenyl-containing compounds reported here were
subjected to cyclic voltammetry measurements in THF/0.1 m
Bu NPF . The compounds all display an oxidation step which is
4
6
ferrocenyl centered (Figure 4 and Figure S64–66). The substitu-
ent on the heterocyclic ring has only a very marginal influence
on the oxidation potentials (Table 4). For all the investigated
compounds the current heights for the forward and reverse
waves of the oxidation step are close to unity, indicating a high
degree of reversibility (Table 4). Interestingly, even though the
ratio of the current heights is close to unity for the oxidation
step, the difference in the potentials of the forward and the
Table 3. Selected bond lengths (Å) of [4a](OTf) and [4b](OTf).
Bond length [Å]
[4a](OTf)
Bond
[4b](OTf)
N1À N2
N2À N3
N1À C1
AgÀ C1
1.340(3)
1.323(3)
1.372(3)
2.076(2)
1.338(3)
1.320(3)
1.369(4)
2.094(3)
Figure 4. Comparison of the normalized cyclic voltammograms of triazole 1b, triazolium salt [2b](BF
4
), Au(i)-MIC complex 3b and Ag(i)-MIC complex
[
4b](OTf). All CVs were measured in abs. THF/0.1 m Bu NPF with a glassy carbon electrode at a scan rate of 100 mV/s and 295 K.
4
6
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[4e,i,7a]
ligands.
As expected, the reduction of the triazolium salts
Table 4. Oxidation potential (E1/2), peak-to-peak separation (DE) and peak
Red
Ox
current ratio I
p
/I
p
of the oxidation of all compound classes.
that contain a direct positive charge on the ring, is shifted
towards the positive direction in comparison to the metal
complexes in which the triazolylidene rings are formally neutral
Oxidation
Red Ox
Compound
E
1/2
DE
I
p
/I
p
(
Table S10).
1
1
1
1
1
a
b
c
d
e
0.17
0.16
0.18
0.18
0.17
0.29
0.30
0.32
0.32
0.30
0.21
0.23
0.21
0.21
0.31
0.17
0.10
0.15
0.32
0.14
0.09
0.15
0.10
0.09
0.13
0.07
0.08
0.09
1.06
1.06
1.01
0.97
0.99
1.07
0.94
1.03
0.97
1.01
0.93
0.94
0.94
0.91
All the compounds were tested in an initial screening for
their antibacterial activity against the Gram-negative bacterial
strain S. typhimurium by using a modified liquid microdilution
[24]
[
[
[
[
[
2a](BF
2b](BF
2c](OTf)
2d](OTf)
2e](BF
4
)
assay (CLSI, 2017; Supporting Information 6.1.1). After this
initial screening, all non-effective compounds were excluded
from further testing (Figure 5).
As the compounds are only poorly soluble in water and
partly unstable in DMSO, the experiments were performed
using ethanol as an additional solvent. A two-fold dilution
4
)
4
)
3
3
a
b
[
[
4a](OTf)
4b](OTf)
[24]
method (CLSI, 2017 ) was used resulting in an ethanol
concentration ranging from 10% v/v to 0.08% v/v and
À 1
À 1
compound concentrations between 0.5 mgmL to 3.9 μgmL .
In order to test the influence of EtOH on the inhibition of
bacterial growth, control experiments without the compound
but with the respective EtOH concentrations were performed.
Even though EtOH did display some inhibitory activity at the
applied concentrations, the growth inhibition in the presence
of some of the compounds was much higher compared to pure
EtOH. Thus, these results can be seen as a first qualitative trend.
For comparison, the graphics (Figures 6–8) show the untreated
growth (in 8 different wells, black dashed line, squares) and the
growth under the influence of the respective EtOH concen-
tration (grey dashed line, circles). Strictly speaking, the x-axis
scaling and labelling are not valid for these two graphs, as for
the untreated growth the compound- and EtOH-concentration
is 0 for all data points (black squares) and in the control
experiment the EtOH concentration varies from 10% v/v to
0.08% v/v (grey circles). The triazoles tested in this work did not
display any appreciable antibacterial activity against the Gram-
negative bacterial strain S. typhimurium. Therefore, the results
on the triazoles will not be discussed here any further. Under
the condition of our testing, a similar behaviour was observed
reverse peak vary quite a bit (Table 4). Ferrocenyl compounds
with large substituents often display a large reorganization
energy owing to different conformations in the non-oxidized
and oxidized forms. Such a conformational change is likely the
reason for a somewhat large difference in the forward and the
reverse potentials observed for many of the compounds here.
The oxidation potentials of the ferrocenyl units shift to the
positive direction on moving from the triazoles to the triazolium
salts. Such a shift can be attributed to the additional positive
charge on the triazolium rings (Table 4). Deprotonation of the
triazolium rings and complexation leads to a slight negative
shift of the oxidation potentials in comparison to the triazolium
salts.
The triazoles do not display any reduction steps within the
solvent window of THF. The triazolium salts as well as the metal
complexes display one or more irreversible reduction steps
(Figure 4). In comparison to related compounds reported earlier,
one irreversible reduction can be attributed to the reduction of
the triazolium unit, or to the reduction of the triazolylidene
Figure 5. Selected compound library.
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Figure 6. Effect of different compound concentrations of [2a](BF
4
),
4
Figure 8. Effect of different compound concentrations of [2b](BF ), 3b and
[4b](OTf) on relative growth of Salmonella typhimurium after 20 h at 37°C.
[
2b](BF ), 3a and 3b on relative growth of Salmonella typhimurium after
0 h at 37°C.
4
2
When comparing the compounds 3a, [4a](OTf) and 7a it is
seen that the compounds containing a phenyl substituent at
the C4-position of the triazolylidene ring display better
antibacterial activity compared to the compounds containing a
ferrocenyl substituent (Figure 7). Contrary to the actual assump-
tion, this indicates that the incorporation of the ferrocenyl
group in this case negatively affects the antibacterial activity.
Finally, a comparison among the triazolium salt [2b](BF4),
the gold(i) complex 3b and the silver(i) complex [4b](OTf)
demonstrates that the order of antibacterial activity for these
compound classes is triazolium<Au(i)<Ag(i) (Figure 8). Even
though for the compounds investigated here, the Ag(I) complex
outperforms the Au(I) complex in terms of antibacterial activity,
this does not seem to be a general trend, as an opposite trend
has also been observed previously. These results thus clearly
show the antibacterial activity of these compounds against the
relatively resistant Gram-negative bacterial strain S. typhimu-
rium, and also display the benefit of incorporating metals such
as Ag(i) and Au(i) for improving antibacterial activity.
Finally, we were also interested in testing the antibacterial
effect against the Gram-negative bacterial strain E. coli. In a
preliminary study, qualitative data on bacterial growth under
the influence of the compounds shown in Figure 5 were
Figure 7. Effect of different compound concentrations of 3a, [4a](OTf) and
7
a on relative growth of Salmonella typhimurium after 20 h at 37°C.
À 1
obtained by disc diffusion tests (c=1 to 0.25 mgmL ). For
for the triazolium salts affecting the bacterial growth only at
high compound concentrations. No obvious trend concerning
the counter ion or the aromatic substituent at N1 was
observable (Table S11). However, a comparison of [2a](BF4),
comparison purposes, we also tested the effect of the
established antibiotic kanamycin as well as of AgNO under the
3
same conditions. The EtOH concentration used did not
contribute to the inhibition of growth of the E. coli strain.
Similar to what has been observed for S. typhimurium, the
triazolium salts did no inhibit the bacterial growth at the tested
compound concentrations (Table S12). Only the silver(i) com-
plexes [4a, b](OTf) and the gold(i) complexes 7a, b showed a
clear zone of inhibition. As for the growth of S. typhimurium, the
[
2b](BF ), 3a and 3b (Figure 6) showed that the compounds
4
containing the longer pentyl chain display (within their
compound class) a slightly better activity compared to those
containing the propyl chain.
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silver(i) complexes have the most potent effect on E. coli
Table S12). For the most promising candidates, namely [4a,
effect of the tested compounds was found: 1,2,3-triazole!
(
1,2,3-triazolium salt<Au(i)-triazolylidene complexes<Ag(i)-tria-
zolylidene complexes. To inhibit the growth of Escherichia coli,
comparatively higher concentrations of the compounds studied
here were required. Future work could involve the development
of the triazolylidenes with functionalities such as À OH and
À COOH as viable ligands for transition metals. Finding new
synthetic strategies will be very important here, as these ligands
will potentially increase the water solubility of the resulting
metal complexes. Additionally, the resulting complexes can be
important compounds for the studies of proton-coupled
electron transfer (À OH/À COOH and ferrocenyl), with consequen-
ces for homogeneous catalysis. Further efforts will be necessary
to improve the stability of such metal complexes, and to
decipher the mode of action of their antibacterial activity. We
have presented here a first qualitative study on the antibacterial
activity of a set of 1,2,3-triazolium salts, and more important of
the gold(i)- and silver(i)-MIC complexes.
b](OTf) and 7a, antibacterial activity was further investigated
by liquid microdilution testing. In this case, lower compound
concentrations were chosen in order to avoid the impact of
colour change on optical density measurements after incuba-
tion (please compare SI 6.1.1). As can be seen from Figure 9, the
compounds inhibit bacterial growth only at a much higher
concentration compared to both kanamycin and AgNO3.
Interestingly, the concentrations at which the tested com-
pounds (particularly the metal complexes) display antibacterial
activity against S. typhimurium is lower compared to the
concentration at which the compounds affect the growth of E.
coli. This result should also be valid despite the small growth-
inhibiting effect of EtOH on the Salmonella strain.
Conclusion
In summary, we have presented here an investigation involving
eight triazoles, seventeen triazolium salts, four gold(i)- and two Experimental Section
silver(i) complexes. The synthesis and the thorough character-
ization of these compounds, including through single crystal X-
ray diffraction studies have been presented. The ferrocenyl-
containing compounds display a ferrocenyl-based oxidation
step and an irreversible triazolium or triazolylidene based
reduction step. The compounds were tested for their antibacte-
rial activity against the Gram-negative bacterial strains Salmo-
nella typhimurium and Escherichia coli. It was shown that
especially the triazolylidene complexes display good antibacte-
rial activity against the otherwise quite resistant Salmonella
strain. The following trend with regard to the growth-inhibiting
General Remarks and Instrumentation
All reactions, except CuAACs, were carried out using common
Schlenk-line techniques under an inert atmosphere of nitrogen
(Linde, HiQ Nitrogen 5.0, purity 99.999%). The solvents used for
methylations and metal complex syntheses were available from a
MBRAUN MB-SPS-800 solvent system. THF for cyclic voltammetry
was dried and distilled from sodium/benzophenone under nitro-
gen. All absolute solvents were degassed by common techniques
prior to use.
Commercially available chemicals were used as purchased.
[
25]
[26]
[27]
Bromoferrocene,
(
ferrocenyl azide,
phenyl azide,
AuCl
[28]
[29]
SMe2) and Ag O were prepared as described previously in the
2
literature.
Column chromatography was performed on Silica 60 M (0.04–
0.063 mm).
Solvents were removed by rotary evaporation at 40°C and
appropriate pressure or under reduced pressure by exposing to
À 3
vacuum (1×10 mbar).
1
13
1
H and C{ H} NMR spectra were recorded on Jeol ECS 400, Jeol
ECZ 400, Jeol ECP 500, Bruker Avance 500, Jeol ECZ 600 or Bruker
Avance 700 spectrometers at 20°C. Chemical shifts are reported in
parts per million (relative to the tetramethylsilane signal) with
reference to the residual solvent peaks. The multiplicities and
coupling constants are specified phenomenologically, i.e. according
to the actual appearance of the signal and not according to the
theoretically expected one. Multiplicities are reported as follows:
singlet (s), doublet (d), triplet (t), quartet (q), quintet (p), sextet
1
1
(
sext) and multiplet (m). Connectivity was determined by H,
H
1
13
1
13
COSY, H, C HMBC and H, C HMQC experiments and in analogy
to the literature.
Mass spectrometry was performed on an Agilent 6210 ESI-TOF.
X-ray data were collected on a Bruker D8 Venture diffractometer at
1
00(2) K using graphite monochromated Mo-Κα radiation (λ =
α
Figure 9. Optical Density (OD600) as measure for bacterial growth of E. coli
0.71073 Å). The strategy for the data collection was evaluated by
using the APEX2 (1c–d, [2b–d](OTf), 3a, [4a](OTf)) or APEX3
[
3
0
]
[
3
1
]
after 16 h at 37
[
treated).
°
C and treatment with different compound concentrations of
4a](OTf), [4b](OTf), 7a, AgNO
3
, Kanamycin and without antibiotic (un-
([4b](OTf), 7a, b) software. The data were collected by ω- and φ-
Eur. J. Inorg. Chem. 2021, 1373–1382
www.eurjic.org
1380
© 2021 The Authors. European Journal of Inorganic Chemistry published
by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/ejic.202100024
[
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scan techniques and were scaled and reduced using Saint+ and
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014/5 ([4a](OTf)), SHELXT-2017/1 ([4b](OTf), 7a) or Olex2[
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1c–d, [2b–d](OTf), 3a, [4a](OTf)), SHELXL-2017/1 ([4b](OTf), 7a)
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platinum wire as counter electrode and a coiled silver wire as a
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Decamethylferrocene was added as internal reference. Potentials
3
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4
A detailed description of the synthesis and characterization of all
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information.
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552919 (for [4a](OTf)), 2060754 (for [4b](OTf)), 1552963 (for 3a),
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Acknowledgements
We would like to thank the Core Facility BioSupraMol, supported
by the DFG, for assistance with NMR und ESI-MS measurements.
Open access funding enabled and organized by Projekt DEAL.
Conflict of Interest
2
017, 129, 9064–9066; j) E. Matteucci, F. Monti, R. Mazzoni, A. Baschieri,
The authors declare no conflict of interest.
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Keywords: Antibacterial agents · Carbene ligands · Ferrocenes ·
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© 2021 The Authors. European Journal of Inorganic Chemistry published
by Wiley-VCH GmbH