Iron in a Cage: Fixation of a Fe(II)tpy2 Complex by Fourfold Interlinking

Article abstract of DOI:10.1002/anie.202006340

The coordination sphere of the Fe(II) terpyridine complex 1 is rigidified by fourfold interlinking of both terpyridine ligands. Profiting from an octa-aldehyde precursor complex, the ideal dimensions of the interlinking structures are determined by reversible Schiff-base formation, before irreversible Wittig olefination provided the rigidified complex. Reversed-phase HPLC enables the isolation of the all-trans isomer of the Fe(II) terpyridine complex 1, which is fully characterized. While temperature independent low-spin states were recorded with superconducting quantum interference device (SQUID) measurements for both, the open precursor 8 and the interlinked complex 1, evidence of the increased rigidity of the ligand sphere in 1 was provided by proton T₂ relaxation NMR experiments. The ligand sphere fixation in the macrocyclized complex 1 even reaches a level resisting substantial deformation upon deposition on an Au(111) surface, as demonstrated by its pristine form in a low temperature ultra-high vacuum scanning tunneling microscope experiment.

Full text of DOI:10.1002/anie.202006340

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Accepted Article
Title: Iron in a cage: fixation of a Fe(II)tpy2 complex by fourfold
interlinking
Authors: Thomas Brandl, Sven Johannsen, Daniel Häussinger, Nithin
Suryadevara, Alessandro Prescimone, Stefan Bernhard,
Manuel Gruber, Mario Ruben, Richard Berndt, and Marcel
Mayor
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006340
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Iron in a cage: fixation of a Fe(II)tpy2 complex by fourfold
interlinking
Thomas Brandl,[a] Sven Johannsen,[b] Daniel Häussinger,[a] Nithin Suryadevara,[c] Alessandro
Prescimone,[a] Stefan Bernhard,[c] Manuel Gruber,[b] Mario Ruben,[d,e] Richard Berndt[b] and Marcel
Mayor*[a,d,f]
Dedicated to our joint friend and tutor Jean-Marie Lehn
Abstract: The coordination sphere of the Fe(II) terpyridine complex 1
is rigidified by fourfold interlinking of both terpyridine ligands. Profiting
from an octa-aldehyde precursor complex, the ideal dimensions of the
interlinking structures are determined by reversible Schiff-base
formation, before irreversible Wittig olefination provided the rigidified
complex. Reversed-phase HPLC enables the isolation of the all-trans
isomer of the Fe(II) terpyridine complex 1, which is fully characterized.
While temperature independent low-spin states were recorded with
superconducting quantum interference device (SQUID) measure-
ments for both, the open precursor 8 and the interlinked complex 1,
evidence of the increased rigidity of the ligand sphere in 1 was
provided by proton T2 relaxations in NMR experiments. The ligand
sphere fixation in the macrocyclized complex 1 even reaches a level
resisting substantial deformation upon deposition on a Au(111)
surface, as demonstrated by its pristine form in a low temperature
ultra-high vacuum scanning tunneling microscope experiment. The
observed two level fluctuation of the tunneling current at elevated bias
might even point at spin-switching in the deposited complex.
investigated at low temperature displayed bistability features
pointing at SCO events triggered by the distortion of the ligand
sphere. Examples are the mechanical spin-state manipulation of
a homoleptic Fe(II)tpy2 complex in a mechanically controlled
break junction (MCBJ),[7] or electric field (E-field) sensing
junctions with heteroleptic Fe(II)tpy2 complexes with push-pull
systems integrated in one tpy-ligand.[8] In the latter case, the
reliability of the E-field triggered switching was recently improved
by separating the active Fe(II) complex from the electrodes.[9]
While these MCBJ based single molecule experiments
showing the potential of Fe(II)tpy2 complexes as switchable
subunits were encouraging, scanning tunneling microscopy
(STM) may provide insights into the arrangement of the molecule
at the electrode surface and its electron transport. In a first
attempt, the arrangement of both tpy-ligands around the central
Fe2+ ion was stabilized by macrocyclization, providing chiral
Fe(II)tpy2 complexes.[10] Upon deposition on Au(111) in an ultra-
high vacuum (UHV) STM set-up however, the complex
disintegrated as clearly documented by the macrocycle spread on
the metal surface (Figure SI22).[11]
Switches are essential components in electronic devices and
driven by the ongoing miniaturization, molecular switches move
into the focus of interest.[1–3] While various structures triggered by
light, heat, or (electro) chemical potential have been reported,[4–6]
molecular switches responding on the applied potential and/or
mechanical pressure are less frequent, in spite of their better
miniaturization potential.
Here we report the next generation of Fe(II)tpy2 complexes
with rigidified coordination sphere. Fourfold interlinking of both
tpy-ligands freezes their spatial arrangement further improving
the stability of the central Fe(II)tpy2 complex in 1 (Figure 1).
Iron bis-terpyridine (Fe(II)tpy2) complexes are particularly
appealing as potential switching motives due to their coordination
geometry sensitive spin crossover (SCO) properties. And indeed,
single molecule junctions with tailor-made Fe(II)tpy2 complexes
[a]
Department of Chemistry, University of Basel, St. Johanns-Ring 19,
4056 Basel, Switzerland, E-mail: marcel.mayor@unibas.ch
Homepage: https://www.chemie.unibas.ch/~mayor/
[b]
[c]
[d]
Institut fꢀr Experimentelle und Angewandte Physik, Christian-
Albrechts-Universität zu Kiel, Leibnizstr. 19, 24098 Kiel, Germany
Department of Chemistry, Carnegie Mellon University, Pittsburgh,
Pennsylvania 15213, USA
Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT),
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen,
Germany, E-mail: marcel.mayor@kit.edu
[e]
[f]
Institut de Physique et Chimie des Matériaux de Strasbourg
(IPCMS), CNRS-Université de Strasbourg,
23, rue de Loess, BP 43, 67034 Strasbourg cedex 2, France
Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-
Sen University, Guangzhou 510275, China
Figure 1. Fourfold interlinked Fe(II) terpyridine complexes 1 and 2. While 1 was
isolated and characterized, 2 was observed by mass spectrometry and served
to identify linkers with dimensions suitable to bridge the two aldehydes.
Supporting information for this article is given via a link at the end of
the document.
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And indeed, the coordination sphere of the isolated target
complex 1 was considerably reinforced as displayed by Carr-
Purcell-Meiboom-Gill-NMR experiments. The improved structural
integrity of 1 even enabled its investigation by UHV STM
experiments.
All new compounds were fully characterized by 1H- and 13C-NMR
spectroscopy, and high resolution mass spectrometry. Single
crystals suitable for x-ray diffraction of the complex 8 were
obtained by diffusing diethyl ether vapor into an acetonitrile
solution of 8. The solid state structure (Figure 2) not only corro-
borated the identity of the complex, but also gave insight into the
intramolecular distances between the aldehyde groups. The inter-
aldehyde O-O spacing varied between 3.60 and 7.15 Å.
To explore a fourfold interlinking strategy, the octa-aldehyde
complex 8 (Scheme 1) moved into the focus of interest. The
symmetry of the compound facilitates its synthesis and the
exposed aldehyde functions enables a variety of potential inter-
linking reactions, like e.g. the McMurry reaction,[12] carbonyl-olefin
metathesis,[13] Schiff base condensation[14] or Wittig olefination.[15]
And indeed, as displayed in Scheme 1, the octa-aldehyde
Fe(II)tpy2 complex 8 was obtained in five steps. First the aldehyde
groups of commercially available 2-bromoisophthalaldehyde 3
were protected by refluxing 3 in toluene in the presence of
ethylene glycol and catalytic amounts of p-toluenesulfonic acid.
By removing the water with a Dean-Stark apparatus the reaction
was driven to completion and the bis-dioxolan species 4 was
isolated in quantitative yield. The boronic acid pinacol ester 5 was
obtained in 87% yield by exposing 4 to standard Miyaura boryl-
ation conditions. Thus 4 was kept for 18 h at 100 °C in 1,4-dioxane
with bis(pinacolato)diboron (Bpin)2 as borane source, bis(tri-
phenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2) as
catalyst and potassium acetate (KOAc) as base. The required
4,4''-dichloro-2,2':6',2''-terpyridine building block was assembled
following our already published protocol.[16] The catalytic system
reported by Guram and coworkers[17] enabled the twofold Suzuki-
Miyaura cross-coupling reaction with 5. Using potassium
carbonate (K2CO3) as base and bis(di-tert-butyl(4-dimethylamino-
pehnyl)phosphine) dichloropalladium (II) (Pd(amphos)Cl2) as
catalyst with a toluene/water solvent mixture in a sealed tube at
116 °C, provided the desired tpy-ligand 6 in 75% isolated yield
after column chromatography (CC). To liberate the aldehydes
prior to Fe2+ coordination, two reported protocols were com-
bined.[18,19] The tetra-aldehyde tpy ligand 7 was obtained in 85%
yield by treating 6 with pyridinium p-toluenesulfonate as proton
source in a mixture of acetone and water at 100 °C in a microwave
reactor. The homoleptic Fe(II) complex 8 was assembled by the
addition of iron (II) chloride (FeCl2) to a solution of the ligand 7 in
a dichloromethane (DCM) and methanol (MeOH) solvent mixture.
An immediate color change to deep purple indicated the complex
formation. After solvent removal and precipitation from a NH4PF6
water solution, the octa-aldehyde complex 8 was isolated in 94%
yield.
a)
b)
Figure 2. Solid state structure of complex 8 with rotation ellipsoids at 50%
probability. a) top view along a N-Fe-N axis and b) side view. Hydrogen atoms
-
and the PF6 counter ions were omitted for clarity. Color code: N: blue, Fe:
orange, C: gray, O: red.
To explore both, the feasibility of the fourfold interlinking strategy
and the ideal dimension of the interlinking units, the Schiff base
condensation was selected as test reaction. The reversibility of
the imine bond should enable to investigate the thermodynamic
stability and accessibility of the rigidified complex.
Initial attempts using ethylene diamine as linker (MM2 cal-
culated N-N distance: 3.80 Å) failed, the focus moved towards
aromatic diamines, hoping that the increased delocalization of the
imine π-systems additionally favors the interlinking. And indeed,
Scheme 1. Synthesis of the Fe(II)tpy cage complexes 2 and 1. Reagents and conditions: a) ethylene glycol, pTsOH.H2O, toluene, reflux, 12 h, 100%; (Bpin)2,
KOAc, Pd(PPh3)2Cl2, 1,4-dioxane, 100 °C, 18 h, 87%; c) K2CO3, Pd(amphos)Cl2, toluene/water (5:1), 116 °C, 18 h, 75%; d) PPTS, acetone/water (5:1), microwave
(MW), 80 °C, 3 h, 85%; e) 1.) FeCl2, DCM/MeOH (1:1), RT, 1 h, 2.) NH4PF6, water, 12 h, 94%; f) m-phenylenediamine, pTsOH.H2O, DCM to DCE, reflux, 10 d, n.d.;
g) (1,3-phenylenebis(methylene))bis(triphenylphosphonium) dibromide, DBU, DMF, -20 °C, 12 h, 4%.
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with meta-phenylenediamine (MM2 calculated N-N distance: 4.5
Å) the formation of an eightfold condensation product with 8 was
observed among a variety of possible condensation products by
direct injection electrospray mass spectrometry (DI-ESI-MS,
Figure SI18). While reaction conditions favoring the formation of
the fourfold interlinked complex 2 could be found (8 equivalents
of the diamine in refluxing 1,2-dichloroethane (83.5 °C) for 10
days, Figure SI19), all attempts to isolate the fourfold interlinked
structure 2 failed, pointing at the lability of the imine bonds. Also
numerous efforts to reduce the imine linkers to more robust amine
bonds employing a variety of reducing agents were not successful.
Thus, in spite the fact that the mass signal in the DI-ESI-MS
remains the only indication of the existence of the fourfold inter-
linked complex 2, the experiments were indicative to identify the
dimension of the ideal interlinking structure.
Figure 3. 1H-NMR of the all-trans isomer of 1 with assignment of the signals.
In order to improve the stability of the fourfold interlinked
complex the strategy was to replace the reversible and hydrolysis
sensitive imine-bonds of 2 by the olefins in 1. Like that the inter-
linking structure would be of comparable dimension and by using
Wittig chemistry, 1 should be accessible from the same octa-
aldehyde precursor Fe(II)tpy2 complex 8. And indeed, treating 8
with 10 equivalents of (1,3-phenylenebis(methylene))bis(triphen-
ylphosphonium) dibromide in -20 °C cold dimethylformamide
using 1,8-diazabicyclo[5.4.0]undec-7-ene as a base resulted in a
crude reaction mixture with the mass of the expected fourfold
interlinked complex 1 (1330 m/z) as main signal recorded in a
matrix-assisted laser desorption ionization mass spectrometer
(MALDI-MS). However, even though the interlinking reaction
proceeded smoothly, the purification remained challenging due to
the lack of conformational selectivity in the double bonds formed
by the Wittig reaction. With the D2h symmetry of 1, 64 different
isomers due to cis/trans configurations of the eight double bonds
are possible in theory. Even though not all of them are likely to be
formed, the uninterpretable NMR spectrum of the crude reaction
mixture points at several isomers being present. Thus the crude
material was subjected to reversed-phase HPLC and after two
runs, the all-trans isomer could be separated in 4% yield (Figure
SI23). The similar MALDI-MS signals of all isolated peaks
confirmed the hypothesis of various olefin isomers being formed
of the ligand sphere upon fourfold interlinking. The experiment
even allows to localize the mobility of individual subunits in both
complexes. While the protons of the central pyridine ring A are
the most static part, the ortho-positioned B rings are more mobile
in both compounds. Interestingly, the protons of the C and D rings
as subunits of the linker structure of 1 (all-trans isomer) displayed
flexibilities in between the values recorded for A and B, suggest-
ing a tight fixation of the interlinking bridges. In contrast, the C ring
of 8 displayed an up to three-fold T2 increase, as expected for the
higher degree of freedom of a rotating subunit. Another obser-
vation pointing at the rigidity of 1 is the increased uniformity of the
values recorded for T2 (max/min: 1.52) compared with the ones
observed for 8 (max/min: 3.04). While the molecular weight
increase from 8 to 1 is with 20.9 % too small to rationalize the
recorded differences in T2, an aggregation of 1 to oligomers might
be an alternative explanation for the observation. To disqualify
this hypothesis, the hydrodynamic radii of both complexes were
determined by diffusion ordered NMR spectroscopy (Fig. SI16
and SI17). The observed difference of 5.2 % in diffusion
coefficients corresponds to a mass increase of 16.4 % of 1
compared to 8, corroborating that both complexes were analyzed
as dissolved monomeric complexes.
Density functional theory (DFT) calculations were per-
formed to obtain a better understanding of the electronic structure
of 1 and to gauge the ring strain energy of the large ring system
surrounding the [Fe(tpy)2]2+ core of the complex. DFT calculations
were completed for compound 1 using a B3LYP functional and a
6-31G (d,p) basis set using the Gaussian 09 suite (Figure 4). An
unrestricted geometry optimization revealed that 1 optimized to
retain the D2d symmetry commonly observed for octahedral bis-
2,2’:6’,2”-terpyridine complexes. The flexibility of the surrounding
ring structure allowed for a slight bending; the direction of this
deformation was identical for the ring’s quarter-segments
diagonally opposed to each other. The electronic structure
retained some of the parent complex’ features: The low energy
pyridinic * orbitals were also observed for the terpyridine moiety
of 1 and covered the orbitals from LUMO to LUMO+7; at higher
energies the * orbitals of the ring structure started mixing in. The
HOMO of the parent [Fe(tpy)2]2+ is located on the Fe d orbitals
and is the origin of the complex’ well-studied MLCT transition. In
contrast, compound 1’s calculated HOMO is exclusively located
on the ring’s large  system and the first metal contributions do
not reveal itself until reaching the 1.5 eV more stable HOMO-8
and the degenerate pair HOMO-9 and HOMO-10. The reaction
1
during the interlinking Wittig reaction. The H-NMR spectrum of
the all-trans isomer of 1 is displayed in Figure 3. The rather simple
spectrum corroborates the identity of the highly symmetric
complex, with the 16.4 Hz coupling constants of both alkene
protons (magenta in Fig. 3) documenting the trans-connection of
the double bonds. The all-trans isomer of 1 was further
characterized by 1H-, 13C-, COSY-, ROESY-, HSQC- and HMBC-
NMR spectroscopy, UV-Vis spectroscopy, and high-resolution
electrospray ionization mass spectrometry (HR-ESI-MS, see
supporting information).
Of particular interest was the rigidity gained by the fourfold
interlinking in the all-trans isomer of 1, which was analyzed by the
comparison of the spin-spin relaxation times T2 for the protons of
1 (all-trans isomer) and the octa-aldehyde precursor complex 8.
In these complexes of comparable molecular weight, the spin-
spin relaxation correlates with the intramolecular mobility of the
spin and is thus a sensitive probe for the flexibility of the spin
bearing protons. Using a Carr-Purcell-Meiboom-Gill (CPMG) spin
echo pulse sequence, T2 was measured for all aryl protons (Fig.
SI15). And indeed, the recorded values display for every proton a
decreased T2 by a factor of 1.20 to 1.75 for the all-trans isomer of
1 compared to 8 (table SI3), pointing at a substantial rigidification
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enthalpy of an isodesmic reaction separating the peripheral ring
and the metal complex moiety was estimated by DFT calculations
and revealed that the overall ring strain energy for 1 is around 16
kcal/mol. The strain is distributed over 4 ring systems formed by
more than 70 atoms making it negligible. Details of these
calculations are documented in the Supporting Information (Fig.
SI24).
a ligand). In agreement with this simple model, the topograph
exhibits three protrusions with submolecular dimensions. The
distance between the outermost protrusions (1.6 nm) approxi-
mately matches the phenyl to phenyl distance of 1 (see scale bars
in Fig. 5 a) and b). The apparent height of the molecular adsorb-
ates (Fig. 5a) is with about 550 pm considerably higher than the
180 pm observed for the flattened terpyridine ligands of the first
generation of Fe(II)tpy2 complexes disintegrating upon deposition
on Au(111).[11] This observation is in agreement with a limited
adsorption-induced distortion of 1, further documenting the
structural integrity gained by fourfold interlinking of the tpy ligands.
The charge of 1 cannot be unambiguously clarified from the
present data.
Figure 4. Frontier molecular orbital diagram of
1 generated from DFT
calculations (B3LYP/6-31G (d,p)). The rendered 98% surfaces of selected
orbitals are also depicted. The first filled orbital with a significant metal
contribution is HOMO-8, which is in stark contrast to the metal-centred HOMO
observed for the parent [Fe(tpy)2]2+ complex.
Figure 5. a) Constant-current STM topograph of 1 on Au(111). Current I = 5 pA
and sample voltage V = -0.5 V. (b) Side-view model of 1. Orange, blue, dark
gray and light gray spheres represent iron, nitrogen, carbon and hydrogen
atoms, respectively. c) Time series of the tunneling current measured at V =
0.9 V with the tip position frozen over the center of 1.
The rigidity of 1 (all-trans isomer) was also observed in its
magnetic behavior recorded in a superconducting quantum inter-
ference device (SQUID) magnetometer. In a temperature range
between 5 K and 365 K, complex 1 remained in its low spin state.
While the observation supports the claim of a fixed coordination
sphere in the all-trans isomer of 1, it does not qualify to account
for an increased rigidity by fourfold interlinking, as also the open
octa-aldehyde 8 remains in its low-spin state in this temperature
range.
To our delight, the structural integrity of 1 was even pre-
served upon deposition on a gold surface in a STM experiment.
Employing an electrospray setup with in line mass selection[20] to
deposit 1 on an Au(111) surface (Fig. SI21) enabled its deposition
and subsequent investigation of the sample with a STM at 4.4 K
under UHV conditions. The electrospray deposition ensured the
absence of counter ions on the surface. A STM constant-current
topograph of a typical molecule is displayed in Figure 5a. The
adsorbate most probably correspond to intact 1, although a
degree of distortion may be present. The geometry of 1 on
Au(111) is most likely the one represented in the side view (Fig.
5b), exposing three dominant extrusions into vacuum (two phenyl
groups of interlinking subunits and the central pyridine moiety of
While spin switching of individual complexes has been evidenced
on partially insulating layers,[21–26] the switching on metal surfaces
remains a challenge with a few exceptions.[27,28] The strong
molecule-substrate interaction on metal surfaces often affects the
integrity and functionality of adsorbed complexes.[11,26] The
fixation of the ligands by fourfold interlinking not only increases
the structural integrity of 1, but also is expected to preserve the
functionality of the Fe(II)tpy2 complex caged in the center of the
macrocycle upon adsorption, which was not the case for less
tightly fixed Fe(II)tpy2 model complexes (Figure SI22). And indeed,
a two-level fluctuation of the tunneling current was recorded at
slightly elevated sample voltages (e.g., V = 0.9 V in Fig. 5c). Even
though the finding is consistent with structural and electronic
changes expected upon spin transitions in spin crossover
complexes, an unambiguous identification of the underlying
switching mechanism will require further experiments, most likely
in higher magnetic fields.
In summary, the tight fixation of the coordination sphere of
a Fe(II)tpy2 complex by fourfold interlinking of the suitably
decorated tpy ligands is reported. Profiting from the octa-
aldehyde precursor 8, the ideal dimensions of the interlinking
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[21]
[22]
T. G. Gopakumar, F. Matino, H. Naggert, A. Bannwarth, F. Tuczek, R.
Berndt, Angew. Chem. Int. Ed. 2012, 51, 6262–6266.
T. Miyamachi, M. Gruber, V. Davesne, M. Bowen, S. Boukari, L. Joly,
F. Scheurer, G. Rogez, T. K. Yamada, P. Ohresser, et al., Nat.
Commun. 2012, 3, 938.
M. Bernien, H. Naggert, L. M. Arruda, L. Kipgen, F. Nickel, J. Miguel,
C. F. Hermanns, A. Krüger, D. Krüger, E. Schierle, et al., ACS Nano
2015, 9, 8960–8966.
T. Jasper-Toennies, M. Gruber, S. Karan, H. Jacob, F. Tuczek, R.
Berndt, Nano Lett. 2017, 17, 6613–6619.
L. Kipgen, M. Bernien, S. Ossinger, F. Nickel, A. J. Britton, L. M.
Arruda, H. Naggert, C. Luo, C. Lotze, H. Ryll, et al., Nat. Commun.
2018, 9, 2984.
S. Rohlf, J. Grunwald, T. Jasper-Toennies, S. Johannsen, F.
Diekmann, M. Studniarek, R. Berndt, F. Tuczek, K. Rossnagel, M.
Gruber, J. Phys. Chem. C 2019, 123, 17774–17780.
K. Bairagi, O. Iasco, A. Bellec, A. Kartsev, D. Li, J. Lagoute, C.
Chacon, Y. Girard, S. Rousset, F. Miserque, et al., Nat. Commun.
2016, 7, 12212.
A. Köbke, F. Gutzeit, F. Röhricht, A. Schlimm, J. Grunwald, F. Tuczek,
M. Studniarek, D. Longo, F. Choueikani, E. Otero, et al.,Nat.
Nanotechnol. 2020, 15, 18–21.
subunits was determined by Schiff base condensation, while the
caged iron complex 1 was obtained by Wittig olefination. The all-
trans isomer of 1 was isolated by HPLC and fully characterized by
NMR spectroscopy, UV-Vis spectroscopy and mass spectrometry.
The rigidity of the ligand sphere was analyzed by CPMG-NMR
experiments and increased the integrity of the caged complex to
a level enabling to preserve its structure and functionality upon
adsorption on metal surfaces. Low-temperature STM experiments
evidenced preserved switching functionality of the complex.
The approach holds perspectives not only for spin-switch
functionality, but also to couple the spin of the complexes to more
reactive ferromagnetic substrates,[29] of relevance for molecular
spintronics. Currently, we are exploring both, the structural
diversity of caged complexes as well as their peripheral
functionalization.
[23]
[24]
[25]
[26]
[27]
[28]
[29]
S. Gueddida, M. Gruber, T. Miyamachi, E. Beaurepaire, W. Wulfhekel,
M. Alouani, J. Phys. Chem. Lett. 2016, 7, 900–904.
Acknowledgements
The authors gratefully acknowledge financial support by the
Swiss National Science Foundation (SNF, grant numbers:
200020‐178808). This project has received funding from the
European Union’s Horizon 2020 research and innovation
programme under grant no. 766726. M. M. acknowledges support
by the 111 project (90002-18011002). S.B. acknowledges support
by the US NSF (CHE-1764353). We thank Thomas Knaak for
providing some of the data shown in Fig. SI22.
Keywords: Fe terpyridine • Macrocyclic ligands • Wittig reactions
• Cage compounds • Hexadentate ligand
[1]
[2]
A. R. Pease, J. O. Jeppesen, J. F. Stoddart, Y. Luo, C. P. Collier, J. R.
Heath, Acc. Chem. Res. 2001, 34, 433–444.
A. S. Blum, J. G. Kushmerick, D. P. Long, C. H. Patterson, J. C. Yang,
J. C. Henderson, Y. Yao, J. M. Tour, R. Shashidhar, B. R. Ratna, Nat.
Mater. 2005, 4, 167–172.
[3]
S. Y. Quek, M. Kamenetska, M. L. Steigerwald, H. J. Choi, S. G.
Louie, M. S. Hybertsen, J. B. Neaton, L. Venkataraman, Nat.
Nanotechnol. 2009, 4, 230–234.
[4]
[5]
[6]
D. Čavlović, M. Juríček, Chim. Int. J. Chem. 2019, 73, 313–316.
G. Ke, C. Duan, F. Huang, X. Guo, InfoMat 2019, inf2.12068.
J. L. Zhang, J. Q. Zhong, J. D. Lin, W. P. Hu, K. Wu, G. Q. Xu, A. T. S.
Wee, W. Chen, Chem. Soc. Rev. 2015, 44, 2998–3022.
R. Frisenda, G. D. Harzmann, J. A. Celis Gil, J. M. Thijssen, M.
Mayor, H. S. J. van der Zant, Nano Lett. 2016, 16, 4733–4737.
G. D. Harzmann, R. Frisenda, H. S. J. van der Zant, M. Mayor,
Angew. Chem. Int. Ed. 2015, 54, 13425–13430.
T. Brandl, M. El Abbassi, D. Stefani, R. Frisenda, G. D. Harzmann, H.
S. J. van der Zant, M. Mayor, Eur. J. Org. Chem. 2019, 2019, 5334–
5343.
[7]
[8]
[9]
[10]
[11]
T. Brandl, V. Hoffmann, A. Pannwitz, D. Häussinger, M. Neuburger, O.
Fuhr, S. Bernhard, O. S. Wenger, M. Mayor, Chem. Sci. 2018, 9,
3837–3843.
T. Knaak, C. González, Y. J. Dappe, G. D. Harzmann, T. Brandl, M.
Mayor, R. Berndt, M. Gruber, J. Phys. Chem. C 2019, 123, 4178–
4185.
[12]
[13]
J. E. McMurry, M. P. Fleming, J. Am. Chem. Soc. 1974, 96, 4708–
4709.
M. R. Becker, R. B. Watson, C. S. Schindler, Chem. Soc. Rev. 2018,
47, 7867–7881.
[14]
[15]
[16]
H. Schiff, Ann. Chem. Pharm. 1864, 131, 118–119.
B. E. Maryanoff, A. B. Reitz, Chem. Rev. 1989, 89, 863–927.
G. D. Harzmann, M. Neuburger, M. Mayor, Eur. J. Inorg. Chem. 2013,
2013, 3334–3347.
[17]
[18]
A. S. Guram, A. O. King, J. G. Allen, X. Wang, L. B. Schenkel, J.
Chan, E. E. Bunel, M. M. Faul, R. D. Larsen, M. J. Martinelli, et al.,
Org. Lett. 2006, 8, 1787–1789.
Y. He, M. Johansson, O. Sterner, Synth. Commun. 2004, 34, 4153–
4158.
[19]
[20]
R. Sterzycki, Synthesis 2002, 1979, 724–725.
Chr. Hamann, R. Woltmann, I.-P. Hong, N. Hauptmann, S. Karan, R.
Berndt, Rev. Sci. Instrum. 2011, 82, 033903.
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10.1002/anie.202006340
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Caging an Fe(II) ion: Like a cube the
fourfold interlinked bisterpyridine
ligand surrounds the Fe(II) ion. The
post-coordinative macrocyclization
rigidifies the ligand sphere and
guaranties the preservation of the
complex’s properties.
Thomas Brandl,Sven Johannsen, Daniel
Häussinger, Nithin Suryadevara,
Alessandro Prescimone, Stefan
Bernhard, Manuel Gruber, Mario Ruben,
Richard Berndt and Marcel Mayor
Page No. – Page No.
Iron in a cage: fixation of a Fe(II)tpy2
complex by fourfold interlinking
This article is protected by copyright. All rights reserved.