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10986 Chem. Commun., 2013, 49, 10986--10988 This journal is cThe Royal Society of Chemistry 2013
Cite this: Chem. Commun., 2013,
49, 10986
Room temperature switching of a neutral molecular
iron(II) complex†
Bernhard Scha
¨fer,
a
Cyril Rajna
´k,
a
Ivan S
ˇalitros
ˇ,
ab
Olaf Fuhr,
a
David Klar,
c
Carolin Schmitz-Antoniak,
c
Eugen Weschke,
d
Heiko Wende
c
and Mario Ruben*
ae
Abrupt room temperature switching (T
c
= 295 K with a 5 K hysteresis)
was achieved in a neutral Fe
II
complex based on a 2-(1H-pyrazol-1-yl)-
6-(1H-tetrazol-5-yl)pyridine ligand. Structural characterization and spin
crossover study (via SQUID magnetometry, photoexcitation and X-ray
absorption spectroscopy) in the solid state are described.
Spin-crossover (SCO) complexes can switch their intrinsic spin state
between the high spin (HS) state and the low spin (LS) state. This is
caused by external stimuli such as temperature, pressure, light,
electric or magnetic fields or charge flow.
1
Most of the known SCO
compounds are transition-metal complexes, especially with 3d
4
–3d
7
metal ions in (distorted) octahedral coordination geometry.
AmajorityisbasedonFe
II
stabilized in a coordination environment
of six nitrogen atoms of the ligand backbone. Molecular magnetic
switches have been intensely investigated and discussed, especially
in view of potential applications in devices for information storage
and processing.
2
Assemblies of such SCO complexes have been
synthesized and characterized in bulk, nanoscale, and surface
phases. Typical methods for preparing the latter include e.g.
Langmuir–Blodgett techniques, spin coating, and vacuum
deposition, leading to multi-, mono- or sub-monolayer coverage
of the substrate.
3
The change in the spin state can steer the
physical and chemical properties of the surface phase and can
be monitored by a wide variety of methods, for example, IR,
Mo
¨ssbauer, NMR, Raman, UV/vis, and X-ray absorption
spectroscopies (XAS), conductometry, dielectrometry, diffractometry,
refractometry, and magnetometry.
4–10
For the preparation of func-
tional devices it has to be considered that SCO complexes have to be
brought into contact with metal surfaces directly leading to a partial
loss of the switching properties.
10,11
That is why the SCO molecules
have to be decoupled from the underlying metallic surface, either by
athininsulator
10
such as Cu
2
N,
12
or by at least one sacrificial layer of
molecules.
10,11
However, in order to process the SCO compounds into functional
devicesitisrequiredthatthematerialshowsanabruptandcomplete
spin transition together with a wide thermal hysteresis behaviour at
around room temperature.
13,14
In recent years considerable effort
has been undertaken to develop SCO complexes matching these
requirements, but compounds with T
1/2
around room temperature
are still scarce.
15
Up to now it is still a great challenge to design and
realize systems that can be switched at room temperature.
In this communication, we report for the first time a newly
designed ligand LH (2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine)
and the unprecedented neutral Fe
II
complex 1,[Fe(L)
2
], Scheme 1.
We report the synthesis, the structural determination, and
the characterisation of the spectral and magnetic properties of
complex 1.Magneticstudiesof1revealed abrupt spin transition at
room temperature (T
1/2
= 295 K) with a hysteresis of 5 K which was
characterised by susceptibility measurements. The room tempera-
ture character of the spin crossover was further studied by XAS.
The synthesis of LH was accomplished within four steps
from commercially available starting materials in a yield of
Scheme 1 (a) Structural formula of 1, (b) illustration of a layer of 1investigated
by SQUID magnetometry, photoexcitation, and XAS, (c) wTproduct as a function
of temperature measured by SQUID magnetometry.
a
Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT),
Postfach 3640, 76021 Karlsruhe, Germany. E-mail: mario.ruben@kit.edu
b
Institute of Inorganic Chemistry, Technology and Materials,
Faculty of Chemical and Food Technology, Slovak University of Technology,
Bratislava, 81237, Slovak Republic
c
Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE),
University of Duisburg-Essen, Lotharstraße 1, 47048 Duisburg, Germany
d
Helmholtz-Zentrum Berlin fu
¨r Materialien und Energie (HZB),
Albert-Einstein-Str. 15, 12489, Berlin, Germany
e
Institut de Physique et Chimie des Mate
´riaux de Strasbourg (IPCMS),
CNRS-Universite
´de Strasbourg, 23, rue du Loess, BP 43, 67034,
Strasbourg cedex 2, France
† Electronic supplementary information (ESI) available. CCDC 956141. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc46624h
Received 29th August 2013,
Accepted 2nd October 2013
DOI: 10.1039/c3cc46624h
www.rsc.org/chemcomm
ChemComm
COMMUNICATION
This journal is cThe Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 10986--10988 10987
about 37%. The substitution of the central pyridine moiety in
2 and 6 positions with a pyrazolyl and a tetrazolyl ring yields a
ligand of asymmetric nature. The potassium salt of Lwas
formed in situ by reacting the tetrazolyl NH function of LH
with KOtBu. This salt chelated the Fe
II
ions to form a neutral
uncharged complex without any counter ions, Scheme 1.
The reaction solution was allowed to evaporate slowly and single
crystals suitable for X-ray diffraction were isolated after a few days.
The yellow block-shaped crystals changed their colour to red after
being removed from the mother solution. The molecular structure
of 1is shown in Fig. 1 (Fig. S1 and Tables S1 and S2, ESI†).
The unit cell of 1contains eight complex units [Fe(L)
2
] and
ten molecules of methanol. Six nitrogen atoms of two ligands
coordinate one Fe atom in a distorted octahedral way. At 180 K
the Fe–N bond lengths have typical values for an LS Fe(II) ion of
1.916(3)–1.971(4) Å. Other indicators of the spin state in related
Fe
II
-bis(pyrazol-1-yl)pyridine complexes are the N
pyrazole
–Fe–
N
pyrazole
angles (ca. 1601for LS and 1451for HS) and the
Sparameter (ca. 901for LS and 1601for HS).
16,17
According to
the determined N–Fe–N angles of N(7)–Fe(1)–N(1) 160.04(14)1,
N(14)–Fe(1)–N(8) 159.83(14)1and the calculated S= 87.11one
may conclude that compound 1is in the LS state at 180 K.
A detailed inspection of the crystal lattice of 1shows two types
of intermolecular interactions of neighbouring complexes: (A) a
p-stacking between two pyrazolyl units, and (B) an edge-to-face
p-stacking between a pyrazolyl ring and a tetrazolyl ring (Fig. 1
and Fig. S2–S4, ESI†). A more detailed description of both types
of intermolecular interactions is given under point 4 of ESI.†
The conformational change of 1due to the transition of the LS
state to the HS state and vice versa induces a change of these
intermolecular p–pinteractions which propagate over the com-
plete crystal lattice. Such weak intermolecular p–pinteractions
between the complexes in the crystal lattice are the reason for a
cooperativity effect which is parental for the observed hysteresis
behaviour (vide infra). The mono-anionic nature of Lgives rise
to the uncharged character of 1.
The temperature-dependent investigation of the magnetic
properties in the thermal range of 4–371 K revealed a room
temperature SCO of complex 1(Fig. 2a). The measurement
shows abrupt and complete thermal SCO centred at 294.5 K
from the HS state (wT= 3.15 cm
3
K mol
1
) to the LS state (wTo
0.2 cm
3
K mol
1
). Three subsequent cooling–heating cycles
revealed the presence of a stable thermal hysteresis loop with
5 K width (T
1/2k
= 292 K, T
1/2m
= 297 K) whereby 80% of iron(II)
atoms convert within 24 K upon cooling and within 21 K upon
heating respectively.
In the case of the photomagnetic experiments, the sample
was slowly cooled down to 10 K which results in the LS state of 1
and an external magnetic field of 0.1 T was applied. The subsequent
irradiation of 1using green laser light (l= 532 nm, laser intensity
was adjusted to 10 mW cm
2
) caused a significant increase of the
magnetic moment (Fig. 2a, blue triangles). After about 40 minutes
of irradiation, the wTproduct function reached saturation at
1.6 cm
3
K mol
1
and the irradiation was turned off. The heating
of the photoexcited sample caused an increase of wTup to
1.76 cm
3
K mol
1
(at 25 K; Fig. 2a, pink squares), which
Fig. 1 (a) Molecular structure of 1; crystal packing in the unit cell of 1, (b) H atoms and solvent molecules omitted, (c) with lattice-CH
3
OH in red.
Fig. 2 (a) Magnetic properties of 1(B= 0.1 T). Temperature dependence of the wTproduct (red circles) in darkness; laser excitation (l= 532 nm) at 10 K (blue
triangles); data recorded in the warming mode after irradiation (pink squares). Inset: temperature dependence of the first derivative of the wTproduct vs. temperature;
(b) X-ray absorption spectra at the Fe L
2,3
edge of 1(red: LS state at T= 5 K, black: HS state at 300 K).
Communication ChemComm
10988 Chem. Commun., 2013, 49, 10986--10988 This journal is cThe Royal Society of Chemistry 2013
corresponds to 56% of HS iron(II) ions. An increase of the
product function between 10 and 25 K is attributed to the zero-
field splitting of the metastable S= 2 state. The further increase
of the temperature above 25 K results in a decreased wTproduct
which finally undergoes a complete thermal relaxation in the
LS ground spin state. The T(LIESST) value, calculated from the
minimum in the @(wT)/@T vs. T curve, is 72 K (inset of Fig. 2a).
The two spin states of 1were also investigated by X-ray
absorption spectroscopy (XAS) of a 150 mm layer deposited on
the UHV tape. Fig. 2b shows the absorption spectra at the Fe
L
2,3
edges of 1at T= 300 K (black line) and T= 5 K (red line).
The spectrum at 300 K belongs to the first measurement after
transferring the sample to the UHV chamber, so the molecules
had no history concerning higher or lower temperatures. After-
wards the sample was cooled down to 5 K and the red spectrum
was recorded. In steps of 50 K we performed the X-ray absorp-
tion measurements until we reached 300 K again. Up to 300 K
the spectra were identical to the one recorded at 5 K. After
heating the sample to 340 K the spectrum revealed the same
structures as that of the initial one. With this temperature
dependent XAS measurement we confirm the high spin–low
spin transition with a sharp transition temperature in the
region of 300 K and a small thermal hysteresis. This results
in different spin states at 300 K depending on the history of the
sample, whether the molecules were cooled before or not. The
Fe L
3
edge of the molecules shows several features, consecu-
tively numbered from lower to higher energies (Fig. 2b). These
spectral differences appear clearly and permit distinguishing
between the LS state and the HS state of 1at the respective
temperatures. The shoulders 1 and 5 change their intensities,
absolutely as well as relatively to each other. Even more obvious
are the differences of the features 2, 3 and 4 of both spin states.
While 2 is clearly the highest peak in the HS state, it is lower in
intensity than 3 and 4 which are of similar intensity in the LS
state. These spectral changes originate from the rearrangement
and reoccupation of the Fe 3d-states along the HS–LS transition.
In the HS state peak 3 is not only clearly more intense than 4,
compared to the low spin state it is also shifted in energy.
Typically, the low energy peak (2) is the highest in intensity in
a HS state and decreases strongly in the LS state.
9,10,18
In conclusion, we have synthesized a neutral uncharged
mononuclear SCO complex which completely and abruptly
switches its spin state exactly at room temperature. To the best
of our knowledge this is the first example of an uncharged
tridentate pyridine based complex abstaining from charge-
balancing counter ions. The weak intermolecular p–pinter-
actions determined in the crystal lattice give rise to cooperativity
of the investigated crystalline material culminating in a small
hysteresis of 5 K. The uncharged molecular character of 1makes
it especially interesting for the sublimation process and the
future integration of SCO compounds into functional devices
for information storage, sensors and switchers which is currently
in progress.
19
We thank the HZB-BESSY II staff for their support during
synchrotron beamtime, the BMBF (05ES3XBA/5), the Marie
Curie RTN project No. PITN-GA-2009-238804, and the grant
agencies VEGA 1/0052/11, APVV-0014-11, APVV-0132-11 and
SK-GR-0022-11 for their financial support.
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