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Room temperature switching of a neutral molecular iron(II) complex


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Abrupt room temperature switching (Tc = 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.
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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
Cyril Rajna
Ivan S
Olaf Fuhr,
David Klar,
Carolin Schmitz-Antoniak,
Eugen Weschke,
Heiko Wende
and Mario Ruben*
Abrupt room temperature switching (T
= 295 K with a 5 K hysteresis)
was achieved in a neutral Fe
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.
Most of the known SCO
compounds are transition-metal complexes, especially with 3d
metal ions in (distorted) octahedral coordination geometry.
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.
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.
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,
¨ssbauer, NMR, Raman, UV/vis, and X-ray absorption
spectroscopies (XAS), conductometry, dielectrometry, diffractometry,
refractometry, and magnetometry.
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.
That is why the SCO molecules
have to be decoupled from the underlying metallic surface, either by
such as Cu
or by at least one sacrificial layer of
However, in order to process the SCO compounds into functional
spin transition together with a wide thermal hysteresis behaviour at
around room temperature.
In recent years considerable effort
has been undertaken to develop SCO complexes matching these
requirements, but compounds with T
around room temperature
are still scarce.
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
complex 1,[Fe(L)
], 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
= 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.
Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT),
Postfach 3640, 76021 Karlsruhe, Germany. E-mail:
Institute of Inorganic Chemistry, Technology and Materials,
Faculty of Chemical and Food Technology, Slovak University of Technology,
Bratislava, 81237, Slovak Republic
Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE),
University of Duisburg-Essen, Lotharstraße 1, 47048 Duisburg, Germany
Helmholtz-Zentrum Berlin fu
¨r Materialien und Energie (HZB),
Albert-Einstein-Str. 15, 12489, Berlin, Germany
Institut de Physique et Chimie des Mate
´riaux de Strasbourg (IPCMS),
´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/
Received 29th August 2013,
Accepted 2nd October 2013
DOI: 10.1039/c3cc46624h
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
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)
] 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
-bis(pyrazol-1-yl)pyridine complexes are the N
angles (ca. 1601for LS and 1451for HS) and the
Sparameter (ca. 901for LS and 1601for HS).
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 ppinteractions which propagate over the com-
plete crystal lattice. Such weak intermolecular ppinteractions
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
K mol
) to the LS state (wTo
0.2 cm
K mol
). Three subsequent cooling–heating cycles
revealed the presence of a stable thermal hysteresis loop with
5 K width (T
= 292 K, T
= 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
) 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
K mol
and the irradiation was turned off. The heating
of the photoexcited sample caused an increase of wTup to
1.76 cm
K mol
(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
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
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
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
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.
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 ppinter-
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.
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.
Notes and references
¨tlich, Y. Garcia and H. A. Goodwin, Chem. Soc. Rev., 2000, 29,
419–427; A. Hauser, Chem. Phys. Lett., 1986, 124, 543; (b) M. Marchivie,
D. Chasseau, J. Am. Chem. Soc., 2001, 124, 194; (c) K. Kato, M. Takata,
Y. Moritomo, A. Nakamoto and N. Kojima, Appl. Phys. Lett., 2007,
90, 201902; (d)A.Bousseksou,G.Molna
´.CodjoviandF.Varret,C. R. Chim., 2003, 6,329;(e) N. Baadji,
M.Piacenza,T.Tugsuz,F.D.Sala,G.MaruccioandS.Sanvito,Nat. Mater.,
2009, 8,813;(f)V.Meded,A.Bagrets,K.Fink,R.Chandrasekar,M.Ruben,
van der Zant, Phys. Rev. B, 2011, 83, 245415.
2(a) L. Cambi and L. Szego
¨,Ber. Dtsch. Chem. Ges. B, 1931, 64, 2591;
(b) J. S. Griffith and L. E. Orgel, Q. Rev., Chem. Soc., 1957, 11, 381;
(c) J.-F. Le
´tard, P. Guionneau and L. Goux-Capes, Spin Crossover in
Transition Metal Compounds III, Springer, Berlin, Heidelberg, 2004,
vol. 235, p. 221.
a, C. Mingotaud and P. Delhae
Adv. Mater., 1999, 11,382;(b)M.MatsudaandH.Tajima,Chem. Lett.,
2007, 700; (c)H.Naggert,A.Bannwarth,S.Chemnitz,T.vonHofe,
E. Quandt and F. Tuczek, Dalton Trans., 2011, 40, 6364; (d)T.
Palamarciuc, J. C. Oberg, F. El Hallak, C. F. Hirjibehedin, M. Serri,
S. Heutz, J.-F. Le
´tard and P. Rosa, J. Mater. Chem., 2012, 22, 9690.
¨tlich, A. Hauser and H. Spiering, Angew. Chem., Int. Ed. Engl.,
1994, 33, 2024.
´lix, K. Abdul-Kader, T. Mahfoud, I. A. Gural’skiy, W. Nicolazzi,
L. Salmon, G. Molna
´r and A. Bousseksou, J. Am. Chem. Soc., 2011,
133, 15342.
6 A. Rotaru, I. A. Gural’skiy, G. Molna
´r, L. Salmon, P. Demont and
A. Bousseksou, Chem. Commun., 2012, 48, 4163.
7 S. Bonhommeau, T. Guillon, L. M. Lawson Daku, P. Demont,
J. Sanchez Costa, J.-F. Le
´tard, G. Molna
´r and A. Bousseksou, Angew.
Chem., Int. Ed., 2006, 45, 1625–1629.
8 J. A. Real, I. Castro, A. Bousseksou, M. Verdaguer, R. Burriel,
M. Castro, J. Linares and F. Varret, Inorg. Chem., 1997, 36, 455.
9 M. Bernien, D. Wiedemann, C. F. Hermanns, A. Kru
¨ger, D. Rolf,
W. Kroener, P. Mu
¨ller, A. Grohmann and W. Kuch, J. Phys. Chem.
Lett., 2012, 3, 3431.
10 T. Miyamachi, M. Gruber, V. Davesne, M. Bowen, S. Boukari, L. Joly,
F. Scheurer, G. Rogez, T. K. Yamada, P. Ohresser, E. Beaurepaire and
W. Wulfhekel, Nat. Commun., 2012, 3, 938.
11 T. G. Gopakumar, F. Matino, H. Naggert, A. Bannwarth, F. Tuczek
and R. Berndt, Angew. Chem., Int. Ed., 2012, 51, 6262.
12 C. F. Hirjibehedin, Science, 2006, 312, 1021.
13 O. Kahn, Science, 1998, 279, 44.
14 M. Cavallini, I. Bergenti, S. Milita, G. Ruani, I. S
R. Chandrasekar and M. Ruben, Angew. Chem., Int. Ed., 2008, 47, 8596.
15 (a) A. Bousseksou, G. Molna
´r and G. Matouzenko, Eur. J. Inorg.
Chem., 2004, 4353; (b)I.S
ˇ, N. T. Madhu, R. Boc
ˇa, J. Pavlik and
M. Ruben, Monatsh. Chem., 2009, 140, 695; (c) J. Kro
¨ber, E. Codjovi,
O. Kahn, F. Grolie
`re and C. Jay, J. Am. Chem. Soc., 1993, 115, 9810;
(d) M. Ohba, K. Yoneda, G. Agustı
´, M. C. Mun
˜oz, A. B. Gaspar,
J. A. Real, M. Yamasaki, H. Ando, Y. Nakao, S. Sakaki and
S. Kitagawa, Angew. Chem., Int. Ed., 2009, 48, 4767; (e) L. Zhang,
G.-C. Xu, H.-B. Xu, T. Zhang, Z.-M. Wang, M. Yuan and S. Gao, Chem.
Commun., 2010, 46, 2554; ( f) B. Weber, W. Bauer and J. Obel, Angew.
Chem., Int. Ed., 2008, 44, 10098.
16 (a)I.S
ˇ, J. Pavlik, R. Boc
ˇa, O. Fuhr, C. Rajadurai and M. Ruben,
CrystEngComm, 2010, 12, 2361; (b)I.S
ˇ, O. Fuhr, R. Kruk,
J. Pavlik, L. Poga
´ny, B. Scha
¨fer, M. Tatarko, R. Boc
ˇa, W. Linert and
M. Ruben, Eur. J. Inorg. Chem., 2013, 1049.
17 P¼P
is the value of the N–Fe–N octahedron angle.
18 B. Warner, J. C. Oberg, T. G. Gill, F. El Hallak, C. F. Hirjibehedin,
M. Serri, S. Heutz, M.-A. Arrio, P. Sainctavit, M. Mannini, G. Poneti,
R. Sessoli and P. Rosa, J. Phys. Chem. Lett., 2013, 4, 1546.
19 (a)M.Ruben,Angew. Chem., Int. Ed., 2005, 44, 1594; (b)N.Lin,
P. Mu
¨ller,A.LandaandM.Ruben,Dalton Trans., 2006, 2794–2800.
ChemComm Communication
... Over the past decades, great efforts dedicated to achieving SCO materials with an abrupt and hysteretic thermal loop centered near room temperature have been made, [15][16][17][18][19][20][21][22][23] as the near room temperature transition is a prerequisite for a realistic application. Interestingly, it has been demonstrated that the profiles of a spin transition curve and critical temperature are associated with the cooperative effect of individual SCO centers. ...
... This suggests that a complete and abrupt spin conversion from the HS to LS state is achieved at a T 1/2 cooling of 278 K. Heating the sample leads to another abrupt spin conversion at 286 K (T 1/2 warming ), giving a narrow thermal hysteresis loop of 8 K, centered (T c ) at 282 K (9°C). It should be pointed out that 3 belongs to one of the very particular SCO complexes that show a very abrupt spin transition with the hysteresis loop near the RT region, 15,16,[18][19][20][21][22][64][65][66] which makes it an excellent candidate for molecular switches and memory devices in practical applications. A unique feature of compounds 1-4 lies in the absence of solvents in the lattice, which can be demonstrated by the single crystal structures, TGA and elemental analyses. ...
Full-text available
Spin-crossover (SCO) active compounds have received much attention due to their potential application in molecular devices. Herein, a family of solvent-free FeII compounds, formulated as (A)2[FeL2], (H2L = pyridine-2,6-bi-tetrazolate, A = (Me)4N+1, Et2NH2+2, iPr2NH2+3 and iPrNH3+4), were synthesized and characterized. Single-crystal X-ray diffraction studies reveal that 1-4 are all supramolecular frameworks containing the same [FeL2]2- center, which is arranged into two packing modes via inter-molecular interactions, that is, a 3D architecture in 1 and 1D chain in 2-4. The spin states of 1-4 at different temperatures are assigned on the basis of the single-crystal X-ray diffraction data. Solid state magnetic investigations indicate that 1 and 4 exhibit a low spin state (below 350 K) and high spin state (2-400 K), respectively. 2 and 3 display clear SCO behavior in the measured temperature, but with different profiles and critical temperatures. 2 undergoes a complete gradual SCO with a critical temperature of T1/2 = 260 K. 3 has an abrupt near room temperature transition between T1/2 cooling = 278 K and T1/2 warming = 286, centered at 282 K (9 °C). This study reveals the importance of organic cations in the modulation of SCO behavior and offers a new insight for the design of SCO compounds with near room temperature spin transitions.
... 24,29 Molecular complexes featuring abrupt and hysteretic spin-state switching, that is, bistable SCO, are especially suited for switching and memory applications. [30][31][32][33] Iron(II) complexes composed of all nitrogen-based BPP (BPP = 2,6bis(pyrazol-1-yl)pyridine) ligand systems are a prominent example of SCO complexes exhibiting bistable SCO. [34][35][36][37][38][39] Various parameters, for example, intermolecular interactions, molecular packing, electronic substituent effects, governing the occurrence and nature of SCO-gradual, abrupt, and bistable-in [Fe(BPP-R)] 2+ complexes have been elucidated. ...
Full-text available
Spin-crossover (SCO) active transition metal complexes are a class of switchable molecular materials. Such complexes undergo hysteretic high-spin (HS) to low-spin (LS) transition, and vice versa, rendering them suitable for the development of molecule-based switching and memory elements. Therefore, the search for SCO complexes undergoing abrupt and hysteretic SCO, that is, bistable SCO, is actively carried out by the molecular magnetism community. In this study, we report the bistable SCO characteristics associated with a new series of iron(ii) complexes-[Fe(BPP-CN)2](X)2, X = BF4 (1a-d) or ClO4 (2)-belonging to the [Fe(BPP-R)2]2+ (BPP = 2,6-bis(pyrazol-1-yl)pyridine) family of complexes. Among the complexes, the lattice solvent-free complex 2 showed a stable and complete SCO (T1/2 = 241 K) with a thermal hysteresis width (ΔT) of 28 K-the widest ΔT reported so far for a [Fe(BPP-R)2](X)2 family of complexes, showing abrupt SCO. The reproducible and bistable SCO shown by the relatively simple [Fe(BPP-CN)2](X)2 series of molecular complexes is encouraging to pursue [Fe(BPP-R)2]2+ systems for the realization of technologically relevant SCO complexes.
Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (Δ T 1/2 ) spin-state switching are desirable for molecule-based switching and memory applications. In this study, we report on the structural facets governing hysteretic SCO in a set of iron(II)-2,6-bis(1H-pyrazol-1-yl)pyridine) (bpp) complexes-[Fe(bpp-COOEt) 2 ](X) 2 ·CH 3 NO 2 (X = ClO 4 , 1 ; X = BF 4 , 2 ). Stable spin-state switching- T 1/2 = 288 K; Δ T 1/2 = 62 K-is observed for 1 , whereas 2 underwent above room-temperature lattice-solvent content-dependent SCO- T 1/2 = 331 K; Δ T 1/2 = 43 K. Variable temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations-from the Jahn-Teller distorted HS-state to less distorted LS-state-and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2 . Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements.
A family of ditopic hexadentate ligands based on the parent compound 2,6‐bis(6‐(pyrazol‐1‐yl)pyridin‐2‐yl)‐1,5‐dihydrobenzo[1,2‐d:4,5‐d']diimidazole ( L ) was developed and synthesized by using a straightforward condensation reaction, which forms the interlinking central benzo[1,2‐d:4,5‐d']diimidazole bridge in the ligand backbone. The two secondary amine groups of the benzodiimidazole unit tautomerize and allow the formation of two tauto‐conformers, which upon treatment with metal salts forms different isomeric coordination complexes. Here we report six new derivatives ( 1 ‐ 6 ) that can tautomerize (varying the pyrazolylpyridine part) and 14 derivatives ( 7 ‐ 13 ) with different alkyl and benzyl substitution on secondary amino groups (of L ) that prevent the tautomerization. In this way, it is possible to study the properties of isomeric coordination complexes and their intrinsic cooperativity by the example of [2x2] grid complexes in the future. A [2x2] Zn 4 complex of the ligand L was synthesized and structurally characterized.
Full-text available
In this work, a three-stage and easily scalable synthesis of 2,6-dicyano-4-pyrone (overall yield of 45%) as a new convenient building block has been developed from diethyl acetonedioxalate. It was shown that the transformation with hydroxylamine and [3 + 2]-cycloaddition, in contrast to the reactions with hydrazines, selectively proceed through the attack at the cyano groups without the pyrone ring-opening to give symmetrical and unsymmetrical pyrone-bearing heterocyclic triads containing 1,2,4- and 1,3,4-oxadiazoles as well as tetrazole moieties. The reaction of 2,6-bis(hetaryl)-4-pyrones with ammonia afforded 2,6-bis(hetaryl)pyridines in 63–87% yields. The 4-pyridone/4-pyridinol tautomerism of 2,6-bis(hetaryl)pyridinols and the influence of the nature of adjacent azolyl moieties on this equilibrium have been discussed.
Recently, ligand systems with straightforward synthesis, easily tunable and better stability are becoming an attractive research area in coordination chemistry where hydrazone based ligand systems are one of them. In spin crossover (SCO) chemistry, hydrazone based ligand systems has been grouped under the banner of imine based ligands in spite of their distinct advantages accomplished by the hydrazone moiety leading to compatibility towards multi-functional spin state switching modulated by different physico-chemical intermolecular interactions. The present focus review presents the most notable hydrazone based ligand systems having N6, N4O2, N4S2, N2O2S2 and N3O2S coordination sphere employed for constructing mono and multi nuclear Fe(II/III), Co(II) and Mn(III) complexes exhibiting an SCO event reported so far with their structural and magnetic explanations in detail. Moreover, the present review is systematically structured to provide not only to showcase the extra flexibility of hydrazone based ligand systems but also to provide a separate condensed library of these ligand systems demonstrated in the field of SCO for the first time. We hope that this focus review will lead to a divergence of these ligand systems from the general imine based ligand system counterparts which could open a new research pathway in SCO chemistry.
Bi-stable charge-neutral iron(ii) spin-crossover (SCO) complexes are a class of switchable molecular materials proposed for molecule-based switching and memory applications. In this study, we report on the SCO behavior of a series of iron(ii) complexes composed of rationally designed 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine (ptp) ligands. The powder forms of [Fe2+(R-ptp-)2]0 complexes tethered with less-bulky substituents-R = H (1), R = CH2OH (2), and R = COOCH3 (3; previously reported)-at the 4-position of the pyridine ring of the ptp skeleton showed abrupt and hysteretic SCO at or above room temperature (RT), whereas complex 5 featuring a bulky pyrene substituent showed incomplete and gradual SCO behavior. The role of intermolecular interactions, lattice solvent, and electronic nature of the chemical substituents (R) in tuning the SCO of the complexes is elucidated.
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The Spin-crossover (SCO) phenomenon is one of the most prominent example of bi-stability in molecular chemistry, and the SCO complexes are proposed for nanotechnological applications such as memory units, sensors, and displays. Since the discovery of the SCO phenomenon in tris(N,N-dialkyldithiocarbamato)iron(III) complexes, numerous investigations are made to obtain bi-stable SCO complexes undergoing spin-state switching at or around room temperature (RT). Valiant efforts are also made to elucidate the structure-property relationship in SCO complexes to understand the factors—such as ligand-field strength, molecular geometry, and intermolecular interactions—governing the SCO. Schiff base ligands are an important class of nitrogen-rich chelating ligands used to prepare SCO complexes, because the Schiff base ligands are easy to synthesize and tailor with additional functionalities. Iron(II)-Schiff base SCO complexes are a well-studied class of SCO active complexes due to the propensity of the complexes to undergo bi-stabe SCO. In this context, this perspective attempts elucidate structure-SCO property relationships governing SCO in selected mono-, bi-, and multi-nuclear iron(II)-Schiff base complexes.
A mononuclear Fe(II) complex, prepared with a Brønsted diacid ligand, H2L (H2L = 2‐[5‐phenyl‐1H‐pyrazole‐3‐yl] 6‐ benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [FeII(H2L)2](BF4)2 (1A), exhibits abrupt spin transition at T1/2 = 258 K, and treatment with base yields a deprotonated analogue [FeII(HL)2] (1B), which shows gradual SCO above 350 K. A range of ferric analogues were also characterized. [FeIII(HL)(H2L)](BF4)Cl (1C) has an S = 5/2 spin state, while the deprotonated complexes [FeIII(L)(HL)], (1D), and (TEA)[FeIII(L)2], (1E) exist in the low‐spin S = 1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid‐state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.
A mononuclear Fe(II) complex, prepared with a Brønsted diacid ligand, H2L (H2L = 2‐[5‐phenyl‐1H‐pyrazole‐3‐yl] 6‐ benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [FeII(H2L)2](BF4)2 (1A), exhibits abrupt spin transition at T1/2 = 258 K, and treatment with base yields a deprotonated analogue [FeII(HL)2] (1B), which shows gradual SCO above 350 K. A range of ferric analogues were also characterized. [FeIII(HL)(H2L)](BF4)Cl (1C) has an S = 5/2 spin state, while the deprotonated complexes [FeIII(L)(HL)], (1D), and (TEA)[FeIII(L)2], (1E) exist in the low‐spin S = 1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid‐state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.
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We report on the synthesis of spin transition compounds 1, 2 of formula [Fe(L)2](A)2 (where L ¼ 20,60- bis(pyrazol-1-yl)-3,40-bipyridine, A ¼ ClO4 �—compound 1; A ¼ BF4 �—compound 2) and compound 3 of formula [Fe(L)(LH)](BF4)3$H2O$CH3CN (where LH ¼ 3-(2,6-bis(pyrazol-1-yl) pyridine-4-yl)- pyridinium(+)). Compounds 1, 2 and 3 were characterized by single-crystal X-ray diffraction, ESI-ToF mass spectrometry, 1H NMR and elemental analysis. The single-crystal X-ray diffraction study of the counter anion analogues 1 and 2 reveals almost identical molecular structures without any significant presence of intermolecular interactions. However, in the case of compound 3, the crystal structure reveals supramolecular interactions involving molecular cations, BF4 � anions and, most importantly, lattice solvent molecules. The presence of solvent water molecules induces the presence of two different types of hydrogen bonding: (i) water molecules interacting with the fluorine atoms of BF4 � anions and (ii) water molecules interconnecting protonated and nonprotonated nitrogens of pyridine-3-yl substituents of neighboring complex cations. These overall hydrogen bonding pattern between the neighboring iron(II) complex cation moieties is responsible for the formation of a one dimensional (1D) hydrogen bonded zig-zag chain. The magnetic investigations elucidate high temperature spin transition behavior for both anion analogues 1 and 2, while compound 3 exhibits a lattice-solvent dependency of the temperature-driven spin transition accompanied with stepwise solvent liberation above room temperature. After complete solvent removal the solvent-free compound 3d, [Fe(L)(LH)](BF4)3, shows an abrupt spin transition accompanied with thermal hysteresis loop; T1/2([) ¼ 240 K and T1/2(Y) ¼ 231 K, DT1/2 ¼ 9 K. The Ising-like model that includes two vibrational modes has been applied in a direct fitting of magnetic data. The model recovers the temperature evolution of the cT product functions for all compounds under study, involving also compound 3d with the thermal hysteresis.
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We report clean evaporation under ultra-high vacuum conditions of two spin crossover materials, yielding either microcrystallites or homogeneous thin films. Magnetic and photomagnetic studies show that thermal and light-induced spin crossover properties are preserved. Preliminary STM imaging of sub-monolayers indicates that the deposited molecules remain intact on the surface.
Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by Mössbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping—acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged.
Using X-ray absorption techniques, we show that temperature- and light-induced spin crossover properties are conserved for a submonolayer of the [Fe(H2B(pz)2)2(2,2′-bipy)] complex evaporated onto a Au(111) surface. For a significant fraction of the molecules, we see changes in the absorption at the L2,3 edges that are consistent with those observed in bulk and thick film references. Assignment of these changes to spin crossover is further supported by multiplet calculations to simulate the X-ray absorption spectra. As others have observed in experiments on monolayer coverages, we find that many molecules in our submonolayer system remain pinned in one of the two spin states. Our results clearly demonstrate that temperature- and light-induced spin crossover is possible for isolated molecules on surfaces but that interactions with the surface may play a key role in determining when this can occur.
Spin-state switching of transition-metal complexes (spin crossover) is sensitive to a variety of tiny perturbations. It is often found to be suppressed for molecules directly adsorbed on solid surfaces. We present X-ray absorption spectroscopy measurements of a submonolayer of [FeII(NCS)2L] (L: 1-{6-[1,1-di(pyridin-2-yl)ethyl]-pyridin-2-yl}-N,N-dimethylmethanamine) deposited on a highly oriented pyrolytic graphite substrate in ultrahigh vacuum. These molecules undergo a thermally induced, fully reversible, gradual spin crossover with a transition temperature of T1/2 = 235(6) K and a transition width of ΔT80 = 115(8) K. Our results show that by using a carbon-based substrate the spin-crossover behavior can be preserved even for molecules that are in direct contact with a solid surface.
Thin film of a spin crossover complex [Fe(dpp)(2)](BF4)(2) (dpp = 2,6-di(pyrazollyl)pyridine) has been fabricated by a spin-coating method. The obtained film exhibits spin transition at around 260 K. The absorption spectra and the electrical resistance change with the spin transition. These phenomena are reproducible through both cooling and heating process.
We have studied the static and dynamic effects of pressure on the spin-transition in the temperature range of the thermal hysteresis loop for the compound Fe(phen)2(NCS)2. The high-spin fraction (nHS) as a function of pressure and temperature has been determined by optical reflectivity. In this compound, the pressure was found to upward shift the spin-transition temperature by 23 K per kbar. During the dynamic pressure pulse, a decrease in nHS is observed, with an irreversible (reversible) character in the descending (ascending) branch of the hysteresis loop. In this respect, pressure has a ‘mirror effect’ compared to the application of an intense and pulsed magnetic field, for which – as reported previously – an increase in nHS is observed, with an irreversible (reversible) character in the ascending (descending) branch of the hysteresis loop. To cite this article: A. Bousseksou et al., C. R. Chimie 6 (2003).
Fe(ptz) 6(BF 4) 2 (ptz = 1-propyltetrazole) is an iron(II) spin-crossover system which shows light-induced excited spin state trapping. In this paper we show that (a) the same phenomenon can also be observed in Zn 1- xFe x(ptz) 6(BF 4) 2 ( x ≈ 0.1) and is therefore basically a single-ion property, and (b) that the phenomenon is reversible. The efficiency of the light-induced spin crossover is of the order of 0.5% in the forward direction and 0.1% in the reverse direction.
A mononuclear iron(II) compound 1 of the general formula [Fe(L)(2)] (ClO4)(2) {L = 4-[2,6-bis(pyrazol-1-yl)pyridin-4-yl]-benzaldehyde} was prepared and structurally characterised. Single-crystal X-ray structure analysis revealed the presence of a complex dication [Fe(L)(2)](2+) and two ClO4- counteranions within the unit cell. The bond lengths and angles within the coordination polyhedron FeN6 indicate the low-spin state of the central iron(II) metal ion at T = 180 K. Magnetic investigations elucidate spin crossover with T-1/2 = 285 K. The experimental magnetic susceptibility data could be satisfactorily fitted with the Curie law in combination with the Ising-like model. The room-temperature character of the spin crossover was further studied by variable-temperature far-IR, Vis and Mossbauer spectroscopy. Laser irradiation of 1 carried out at 10 K gives rise to a complete low-spin to high-spin photoconversion. A subsequent temperature-dependent investigation revealed the existence of a photoexcited metastable HS state up to T-LIESST = 70K (LIESST = light-induced excited-spin-state trapping) as well as the presence of a light-induced thermal hysteresis loop with a width of 10 K.