Dynamic Complex-to-Complex Transformations of Heterobimetallic Systems Influence the Cage Structure or Spin State of Iron(II) Ions

Article abstract of DOI:10.1002/anie.201914629

Two new heterobimetallic cages, a trigonal-bipyramidal and a cubic one, were assembled from the same mononuclear metalloligand by adopting the molecular library approach, using iron(II) and palladium(II) building blocks. The ligand system was designed to readily assemble through subcomponent self-assembly. It allowed the introduction of steric strain at the iron(II) centres, which stabilizes its paramagnetic high-spin state. This steric strain was utilized to drive dynamic complex-to-complex transformations with both the metalloligand and heterobimetallic cages. Addition of sterically less crowded subcomponents as a chemical stimulus transformed all complexes to their previously reported low-spin analogues. The metalloligand and bipyramid incorporated the new building block more readily than the cubic cage, probably because the geometric structure of the sterically crowded metalloligand favours the cube formation. Furthermore it was possible to provoke structural transformations upon addition of more favourable chelating ligands, converting the cubic structures into bipyramidal ones.

Full text of DOI:10.1002/anie.201914629

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Accepted Article
Title: Dynamic Complex-to-Complex Transformations of
Heterobimetallic Systems Influence the Cage Structure or Spin
State of Iron(II) Ions
Authors: Matthias Hardy, Niklas Struch, Julian J. Holstein, Gregor
Schnakenburg, Norbert Wagner, Marianne Engeser,
Johannes Beck, Guido H. Clever, and Arne Lützen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914629
Angew. Chem. 10.1002/ange.201914629
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10.1002/anie.201914629
Angewandte Chemie International Edition
COMMUNICATION
Dynamic Complex-to-Complex Transformations of
Heterobimetallic Systems Influence the Cage Structure or Spin
State of Iron(II) Ions
Matthias Hardy,[a] Niklas Struch,[a]# Julian J. Holstein,[b] Gregor Schnakenburg,[c] Norbert Wagner,[c]
Marianne Engeser,[a] Johannes Beck,[c] Guido H. Clever,[b] Arne Lützen[a]*
Abstract: Two new heterobimetallic cages, a trigonal bipyramidal
and a cubic one, were assembled from the same mononuclear
metalloligand adopting the molecular library approach, using iron(II)
and palladium(II) building blocks. The ligand system was designed to
readily assemble through subcomponent self-assembly. It allowed
the introduction of steric strain at the iron(II) centres, which stabilizes
its paramagnetic high-spin state. This steric strain was utilized to
perform dynamic complex-to-complex transformations with both, the
metalloligand and heterobimetallic cages. Addition of sterically less
crowded subcomponents as a chemical stimulus transformed all
complexes to their previously reported low-spin analogues. The
metalloligand and bipyramid incorporated the new building block
more readily than the cubic cage, probably because the geometric
structure of the sterically crowded metalloligand favours the cube
formation. Furthermore it was possible to provoke structural
transformations upon addition of more favourable chelating ligands,
converting the cubic structures into bipyramidal ones.
irradiation12 were successfully used to perform complex-to-
complex transformations.
Aside from indisputably beautiful homometallic structures,
the subcomponent self-assembly approach also proved useful
for the formation of heterobimetallic cages within the meaning of
the complex-as-a-ligand strategy.13 Thus, it is possible to
transfer all advantages of the subcomponent self-assembly
approach to systems consisting of more than one type of metal
cation. However, to date only
a
few examples for
heterobimetallic complexes were reported that built up from
subcomponents.5c,13e,14
Previously, Drago et al. introduced a ligand system that can
be prepared from commercially available building blocks tris(2-
aminoethyl)amine
(TREN),
2-formylpyridine,
2-formyl-6-
methylpyridine and iron(II) salts.15 2-Formyl-6-methylpyridine
containing complexes show a stabilization of the high-spin state
of iron(II) cations at room temperature, due to steric strain
between the methyl groups and nearby pyridine rings, as well as
spin-crossover behaviour in solid state.15,16 These aldehydes
were also used to develop 3D architectures with different shapes
and sizes that show complex-to-complex transformations by
replacing 2-formyl-6-methylpyridine with less bulky 2-
formylpyridine. These transformations resulted in a change of
the spin state of iron(II) cations from high-spin to low-spin, due
to shortening of the Fe-N bond lengths.9b,17 The decrease of
steric strain might be responsible for the preferential
Metallosupramolecular chemistry1 has produced many
beautiful structures with defined shapes2 and fascinating
functionality.3 Within metallosupramolecular chemistry,
a
powerful tool is the subcomponent self-assembly approach, in
which aggregates emerge from assembling two ligand subunits
that form feasible ligands and their complexes in situ. These are
formed via the reversible formation of covalent bonds.4 This
approach was used to prepare numerous structures adopting
various properties and features,5 e.g. host-guest chemistry6 and
switchable systems.6c,7 The tuning of complex properties is often
facilitated by varying one distinct ligand subunit and exchanging
one building block.6b,8 In combination with the reversible
character of the covalent bonds, these properties can also allow
for building block exchanges in situ, resulting in complex-to-
complex transformations.7a,9 Besides subcomponent exchanges,
also ligand exchanges10, solvent dependencies11, or light
incorporation
methylpyridine.9b,17
In this work we investigated the dynamic behaviour of
heterobimetallic structures that assemble through
of
2-formylpyridine
over
2-formyl-6-
subcomponent self-assembly. These complexes contain
sterically strained iron(II) centres in the paramagnetic high-spin
state. We adopted previously reported subcomponent
exchanges and expanded these principles to heterometallic
systems. Chemical stimuli transferred the heterobimetallic
paramagnetic complexes to their diamagnetic analogues upon
reduction of steric strain. In addition, we could perform ligand
exchange reactions, resulting in structural transformations from
cubic to bipyramidal cages.
[a]
M.Sc. M. Hardy, Dr. N. Struch, PD. Dr. M. Engeser, Prof. Dr. A.
Lützen
Rheinische Friedrich Wilhelms-Universität Bonn
Kekulé-Institut für Organische Chemie und Biochemie
Gerhard-Domagk-Straße 1, 53121 Bonn (Germany)
E-mail: arne.luetzen@uni-bonn.de
Ditopic building block
1 was selected to achieve the
formation of C3 symmetric iron(II)-tris(pyridylimine) moieties18 as
well as C2 and C4 symmetric palladium(II)-pyridine moieties.
Reaction of 3 equivalents of aldehyde 1 with 1 equivalent of
[b]
[c]
Dr. J. Holstein, Prof. Dr. G. Clever
Technische Universität Dortmund
Fakultät für Chemie und Chemische Biologie
Otto-Hahn-Straße 6, 44227 Dortmund (Germany)
Dr. G. Schnakenburg, N. Wagner, Prof. Dr. J. Beck
Institut für Anorganische Chemie
TREN
2
and 1 equivalent of iron(II) tetrafluoroborate
hexahydrate in acetonitrile yielded the mononuclear complex
ML-1(BF4)2 as a bright red solid (Scheme 1). This mononuclear
complex was characterised by 1H-NMR- and UV-Vis
Rheinische Friedrich-Wilhelms-Universität Bonn
Gerhard-Domagk-Str. 1, 53121 Bonn (Germany)
spectroscopy,
ESI-MS spectrometry, vibrating sample
magnetometry and single crystal X-ray diffraction (see SI).
Mixing 1 equivalent of ML-1(BF4)2 with 1.5 equivalents of
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201914629
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Scheme 1. Stepwise self-assembly of BP-1(OTf)6(BF4)4 and CU-1(BF4)28 via formation of ML-1(BF4)2 from 1 and 2.
present in the unit cell as racemic mixture and is best described
as a trigonal bipyramid or helicate21 (for more details, see SI).
The iron cations are coordinated in an octahedral ligand
sphere with an average Fe-N bond length of 2.179 Å, consistent
with a paramagnetic high-spin iron(II) complex.22 The palladium
centres form the equatorial plane of the complex and are settled
in a square-planar ligand sphere, each coordinated by a
bidentate dppp ligand and two 4-pyridyl moieties.
The identity of the heterobimetallic cube CU-1(BF4)28 could
1
also be proven by H-NMR and UV-Vis spectroscopy and ESI-
MS (see SI). An Evans’ experiment revealed a magnetic
susceptibility of XmT = 24.1 cm3 K mol-1, proving its fully
paramagnetic character.20 Interestingly, the self-assembly of CU-
1(BF4)28 was completed after only 16 hours at 50 °C whereas
Figure 1. Structure of bipyramidal complex BP-1(OTf)6(BF4)4 as determined
by single crystal X-ray diffraction (hydrogen atoms, counter ions and solvent
molecules are omitted for clarity; colour code: grey – carbon, blue – nitrogen,
yellow-orange – phosphorus, dark orange – iron, petrol – palladium).
1,3-bis(diphenylphosphino)propane
([(dppp)Pd(OTf)2]) or 0.75 equivalents of tetrakis(acetonitrile)
palladium(II) tetrafluoroborate ([Pd(CH3CN)4](BF4)2) in
palladium(II)
triflate
acetonitrile resulted in the formation of heterometallic
pentanuclear bipyramidal assembly BP-1(OTf)6(BF4)4 or the
tetradecanuclear cubic assembly CU-1(BF4)28, respectively,
each as a bright red microcrystalline solid in high yields
(Scheme 1).
The identity and purity of bipyramid BP-1(OTf)6(BF4)4 were
1
proven by H-NMR and UV-Vis spectroscopy and ESI-MS (see
SI). Vibrating sample magnetometry and employing the Evans’
method19
revealed
a
magnetic
susceptibility
of
ΧmT = 6.0 cm3 K mol-1, which is in very good agreement with the
expected value for two uncoupled iron(II) cations in the high-spin
state (6.001 cm3 K mol-1).20 Slow diffusion of diethyl ether into a
concentrated solution of BP-1(OTf)6(BF4)4 in acetonitrile gave X-
ray diffraction quality single crystals that could be used to
determine its crystal structure (Figure 1). The mixed anionic
cage BP-1(OTf)6(BF4)4 crystallises in the monoclinic space
group C2/c, with both homochiral enantiomers [(Δ,Δ) and (Λ,Λ)]
Figure 2. Structure of cube CU-1 as determined by single crystal X-ray
diffraction using synchrotron radiation. Eight tetrafluoroborate anions occupy
the inner void. (hydrogen atoms, solvent molecules and counter ions outside
the cavity are omitted for clarity; colour code: grey – carbon, blue – nitrogen,
light green – fluorine, brown-beige – boron, dark orange – iron, petrol –
palladium).
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10.1002/anie.201914629
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COMMUNICATION
the self-assembly process of a previously reported diamagnetic
analogue13e of this heterobimetallic cube took 120 hours under
the same conditions. We attribute this tremendously shorter
reaction time of CU-1(BF4)28 to the larger opening angle of ML-
1(BF4)2, compared to the metalloligand without the additional
methyl groups on the pyridine binding to the iron(II) ion. Due to
the steric strain around the iron centre the opening angle
becomes larger, and therefore, should be better preorganized to
form the heterobimetallic cube.
Very slow evaporation of solvent from an acetonitrile solution
of CU-1(BF4)28 over 3 months yielded single crystals, which
diffracted up to 1.2 Å using synchrotron radiation23 and allowed
for unambiguous structure determination (Figure 2).
sterically less crowded 2-formyl-5-(4’-pyridyl)pyridine 3 (per
exchangeable methyl substituted aldehyde component) to the
complex solutions and were tracked by 1H-NMR spectroscopy.
In such a scenario a quantitative incorporation of 3 would lead to
a 50:50 or 1:1 ratio of both free aldehydes 1 and 3. Please note
that we had to analyse the signals of the individual aldehyde
subcomponents as probes here as integration of 1H-NMR
signals of paramagnetic species is generally not possible, if
mixed species with different magnetic properties are present.
Following this approach, addition of 6 equivalents of 3 to a
solution of ML-1(BF4)2 released free
3
equivalents of
subcomponent 1 when 3 was incorporated almost quantitatively
into the complex, resulting in the formation of previously
reported diamagnetic metalloligand ML-2(BF4)2 in a high yield
(Scheme 2).
̅
CU-1(BF4)28 crystallises in the cubic space group Fm-3c with
both homochiral enantiomers [all (Δ) and all (Λ)] present in the
unit cell as racemic mixture. Only one third of one ligand and
one iron ion is found in the asymmetric unit due to the 3-fold
symmetry axis. All three Fe-N bonds are crystallographically
equivalent and show a bond length of 2.105(11) Å, which is
consistent with a high-spin iron(II) complex. The tetravalent
palladium ions with their four coordinated pyridine moieties are
located on the 4-fold symmetry axes of the cubic cage. Eight
tetrafluoroborate anions occupy the void of the cage, adapting
the cubic arrangement themselves.
The steric strain in ML-1, BP-1 and CU-1 results in an
elongation of the Fe-N bond lengths and the stabilization of the
paramagnetic high-spin state.9b,15,17 Still the situation is
energetically unfavourable, and hence, can be used to perform
thermodynamically-driven complex-to-complex transformations
during which the steric strain is reduced. The transformation
reactions were performed by the addition of a twofold access of
Progress of this reaction was indicated by a colour change of
the solution from bright red to dark purple. After 16 hours at
40 °C the mixture reached equilibrium with free subcomponents
1
3 and 1 in a 56:44 ratio and a new set of signals in the H-NMR
spectrum referring to ML-2(BF4)2 (see SI).
The same chemical stimulus can be used to transfer
paramagnetic
BP-1(OTf)6(BF4)4
quantitatively
into
the
corresponding diamagnetic cage BP-2(OTf)6(BF4)4. After 16
hours at 40 °C the equilibrium ratio of 3 and 1 was determined to
be 1:1 and 1H-NMR signals arised that can be assigned to
BP-2(OTf)6(BF4)4 (see SI). Also the formation of BP-
2(OTf)6(BF4)4 could be shown by UV-Vis spectroscopy (see SI).
This transformation impressively demonstrates the strong driving
force of this subcomponent exchange and the highly dynamic
behaviour of this heterobimetallic system. To conduct this
exchange reaction one covalent imine bond, two coordinative
Scheme 2. Sterically driven complex-to-complex transformations. Red atoms mark iron(II) in the high-spin state, purple atoms mark iron(II) in the low-spin state.
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10.1002/anie.201914629
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Scheme 3. Structural transformations of cubic cages into bipyramidal ones, producing the metalloligands as side products.
Fe-N bonds and one coordinative Pd-N bond have to be broken,
the new building block has to align and to reform all formerly
cleaved bonds again for each exchanged subcomponent.
Similar treatment of cubic cage CU-1(BF4)28 with a higher
excess of 100 equivalents of 3 resulted in the transformation to
the analogues diamagnetic cube CU-2(BF4)28. Unlike with ML-
1(BF4)2 and BP-1(OTf)6(BF4)4, however, the subcomponent
exchange took noticeable more time. After three days at 65 °C
an equilibrium ratio between 1 and 3 of 30:70 was determined,
which is close to the expected 24:76 ratio. However, using only
48 equivalents of 3 did not lead to a complete transformation
(see SI). Therefore, these experiments show that incorporation
of 3 into the cubic assembly is slightly more favoured than
incorporation of 1, but less pronounced than in the case of the
bipyramidal assemblies.
Hence, in case of the cubic cage the sterically stressed
complex probably is not as disfavoured as for ML-1(BF4)2 and
BP-1(OTf)6(BF4)4. The larger opening angle of ML-1(BF4)2
caused by the additional methyl groups seems to be a better
preorganization to assemble cubic structures than the smaller
opening angle in ML-2(BF4)2. This assumption is corroborated
by the much faster formation of CU-1(BF4)28 compared to that of
CU-2(BF4)2 although it is difficult to dissect whether this is due to
the kinetics or the thermodynamics of the assembly process.
Besides sterically driven subcomponent exchange reactions
that influence the spin state of iron(II) cations, the system also
allows for structural conversions upon a chemical stimulus,
maintaining the initial spin state of the complexes (see SI). The
addition of dppp as a chelating ligand to cubic cages CU-1 and
CU-2 leads to complex-to-complex transformations, yielding the
bipyramidal complexes BP-1 and BP-2 and excess
metalloligands ML-1 and ML-2, respectively (Scheme 3, SI). In
these reactions two 4-pyridyl donors at palladium(II) cations are
replaced by one bidentate dppp ligand, leading to major
structural changes. Since the contingent of iron(II) cations in the
cubic assemblies is higher than in the bipyramidal cages, excess
metalloligand is produced as a side product in these
transformations. Addition of 6 equivalents of dppp transforms
one cubic cage into two bipyramidal assemblies and four free
metalloligands, increasing the overall entropy of the system,
which might be an additional driving force for this reaction
besides the enthalpically strongly preferred coordination of the
dppp ligand to palladium(II) cations to facilitate these
conversions.24
In conclusion, we presented the stepwise assembly of two
heterobimetallic cages BP-1 and CU-1 from the same C3-
symmetric metalloligand ML-1 adopting the well implemented
molecular library approach. Compared to previously reported
diamagnetic complexes ML-2, BP-2 and CU-213e the simple
addition of a methyl group in 6-position of the pyridyl ring leads
to a substantial amount of steric strain and changed the spin
state of the iron(II) cations from diamagnetic low to paramagnetic
high-spin. This impressively shows how rather small changes of
the ligand system can cause tremendous changes of the
complex properties.
Paramagnetic complexes ML-1 and BP-1 showed a very
high tendency to undergo subcomponent exchange reactions,
resulting
transformations
in
the
of
first
reported
complex-to-complex
complexes to
heterobimetallic
heterobimetallic complexes in which metal-bridging components
are exchanged. In this case, 2-formyl-6-methyl-5-(4’-pyridyl)-
pyridine 1 could be replaced by sterically less crowded 2-formyl-
5-(4’-pyrdiyl)pyridine 3, transforming the high-spin complexes
into their low-spin analogues.13e The cubic cage CU-1, however,
was found to exchange 1 much less readily, probably because
the opening angle of the sterically crowded metalloligand is
kinetically and thermodynamically favourable to assemble the
cubic structure.
Addition of bidentate dppp to cubic assemblies CU-1 and
CU-2 led to a ligand exchange reaction, in which two 4-pyridyl
donors around palladium(II) cations were replaced by the
chelating ligand. These exchanges resulted in complex-to-
complex transformations to the bipyramidal cages BP-1 and BP-
2, respectively, maintaining the spin state of iron(II). Since the
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10.1002/anie.201914629
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COMMUNICATION
iron(II) to palladium(II) ratio decreases from the cubic to the
bipyramidal cages, the corresponding free metalloligands ML-1
and ML-2 are formed as side products in these transformations.
The presented complex-to-complex transformations beautifully
show the highly dynamic behaviour of the shown
heterobimetallic systems, allowing the change of magnetic or
structural
properties
upon
certain
chemical
stimuli.
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M.H. thanks the Manchot Foundation for a doctoral scholarship.
N.S. is thankful to the Evonic Foundation for a doctoral grant.
Financial support from the DFG, SFB 813 “Chemistry at Spin
Centers” is gratefully acknowledged. G.H.C. and J.J.H. thank the
European Research Council (ERC Consolidator grant 683083,
RAMSES) for supporting this study. Diffraction data of
CU-1(BF4)28 was collected at PETRA III at DESY, a member of
the Helmholtz Association (HGF). The authors thank Sebastian
Günther for assistance in using synchrotron beam line P11 (I-
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[6]
[7]
Conflict of Interest
The authors declare no conflict of interest.
Keywords: supramolecular Chemistry • self-assembly •
subcomponent self-assembly • iron complexes • palladium
complexes
#
current address: Arlanxeo Netherlands B.V., Urmonderbaan 24, 6167
RD Geleen, The Netherlands
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COMMUNICATION
Lively Exchanges: Two new
heterobimetallic cages with
M. Hardy, N. Struch, J. J. Holstein, G.
Schnakenburg, N. Wagner, M. Engeser,
J. Beck, G. H. Clever, A. Lützen*
incorporated iron(II) cations in a
sterically strained coordination sphere
undergo complex-to-complex
transformation upon addition of
chemical stimuli. Subcomponent
exchanges by sterically less crowded
building blocks changed the spin state
of iron(II) from high to low-spin,
whereas ligand exchanges with
chelating ligands provoked major
structural transformations,
Page No. 1 – Page No. 5
Dynamic Complex-to-Complex
Transformations of Heterobimetallic
Systems Influence the Cage Structure
or Spin State of Iron(II) Ions
transforming cubic to bipyramidal
cages.
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