Photomagnetic effect in a cyanide-bridged mixed-valence {FeII2FeIII2} molecular square

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ChemComm
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Chemical Communications
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Volume 46 | Number 32 | 28 August 2010 | Pages 5813–5976
1359-7345(2010)46:32;1-H
Volume 46 | Number 32 | 2010
ChemComm

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COMMUNICATION
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Chemical Communication
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Photomagnetic effect in a cyanide-bridged mixed-valence {FeII
molecular square†
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COMMUNICATION
This journal is © The Royal Society of Chemistry [year]
[Chem. Commun.], [year], [vol], 00–00 | 1
2FeIII
2}
Abhishake Mondal, a Yanling Li,a Patrick Herson,a Mannan Seuleiman,a Marie-Laure Boillot,b Eric
Rivière,b Miguel Julve,c Lionel Rechignat,d Azzedine Bousseksou d and Rodrigue Lescouëzec*,a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
The self-assembly of [FeIII(Tp)(CN)3]- and [FeII(bik)2(S)2]2+
affords the cyanide-bridged mixed valence {FeIII
molecular square, which exhibits photomagnetic effect under
laser light irradiation at low temperature and also shows
thermal spin-state conversion near ambient temperature.
2FeII
2}2+
10
Switchable molecular magnetic materials (SMMMs) have
become an outstanding research topic because of their potential
applications as molecular memories, switches or sensors.1 The
spin-transition systems, which can provide access to room
temperature bistability and photomagnetic effects are among the
most promising SMMMs.2 A great number of polynuclear
cyanide-based spin-transition systems have been published in the
last decade, most of them being coordination polymers, where
diamagnetic cyanide complexes are used as bridging ligands to
connect iron(II) spin-crossover (SCO) ions.3 Until now, less
interest has been devoted to cyanide-based discrete compounds.
To the best of our knowledge, only three square complexes
having the same {FeII
systems, diamagnetic low-spin (LS) cyanide-iron(II) units
connect the Fe(II) spin-crossover centres.
Following the pioneering
photomagnetic Prussian blue analogues (PBAs) by Hashimoto et
al,1a some of us reported molecular derivatives of these high
dimensional systems which
{FeII
systems, the tuning of the ligand field on the metal centres allows
to adjust the electronic properties of the Fe-Co pair.6 This
strategy led to {Fe2Co2} squares whose magnetic properties can
be switched by either a thermo- or a photo-induced electron
transfer: a diamagnetic FeII
converted into a paramagnetic FeIII
generally, the cyanide-based square complexes {M2M’2} are
currently attracting a strong interest as discrete molecular models
of the magnetic PBAs.7 The versatility of these square
compounds can also be very useful in designing spin-transition
complexes, as the surrounding of the cobalt ion is very close to
that observed in the above mentioned SCO squares.4 We have
therefore decided to extend our investigation toward the study of
analogous {FeIII
It is worth noting that other SCO iron square complexes have
been recently published.8 However, these systems are usually
obtained through the self-assembly of compartmental polypyridyl
15
20
2FeII
2} motif have been reported.4 In these
25
work on the iron-cobalt
consist of cyanide-bridged
30
2CoIII
2} and {FeIII
2CoII
2} squares complexes.5 In these later
35
LS-CN-CoIII
LS pair being reversibly
LS-CN-CoII
HS pair.5a More
40
2FeII
2} molecular squares.
45
ligands and FeII salts. Alternatively, Zheng et al have also
obtained such a square motif by self-assembling partially blocked
FeII complexes with the dicyanamide bridging ligand.8d In
contrast, we explore here the “complex as ligand” strategy by
using the stable paramagnetic [FeIII(Tp)CN)3]- unit to connect
partially blocked iron(II) complexes. Herein, we report the
synthesis, crystal structure, spectroscopic characterization and
magneto-structural study of the first example of a cyanide-based
mixed valence {FeIII
effect.
50
55
2FeII
2} square, exhibiting photomagnetic
Fig. 1 View of the cyanide-bridged {FeIII2FeII2} square unit with the atom
labelling for the metal environments. The hydrogen atoms are omitted for
clarity. C-gray, N-blue, O-red, B-pale blue, FeIII-yellow, FeII-orange.
[symmetry transformation (a): -x, -y, -z]
60
The complex of formula {[FeIII(Tp)(CN)3]2[FeII(bik)2]2}
[FeIII(Tp)(CN)3]2 . 18H2O . 4CH3OH (1.18H2O.4CH3OH) [Tp =
trispyrazolylborate, and bik = bis(1-methylimidazol-2-yl)ketone]
crystallizes as orange-red plate-like crystals by the reaction of
PPh4[FeIII(Tp)(CN)3].H2O with FeCl2
methanol. Its crystal structure has been determined at 200 K and
it consists of centrosymmetric cyanide-bridged {FeIII
cationic squares, free [FeIII(Tp)(CN)3]- anions and crystallization
solvent molecules, which are interlinked by hydrogen bonds and
Van der Waals interactions.
.
65
.4H2O and the bik ligand in
2FeII
2}
70
N2
Fe1
Fe2
N1
N11
N15
N13
N19
N17
N9
N7
C3
C1
C2
N3
Fe1a
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The two [FeIII(Tp)(CN)3]- building-blocks in the tetranuclear
complex act as bis-monodentate ligands toward two {FeII(bik)2}2+
units through two cyanide groups. The remaining terminal
cyanide ligand adopts an anti orientation relative to the {Fe4(µ-
CN)4} skeleton (Figure 1). The FeIII-CN-FeII edges are quasi-
identical [ca. 4.96(1) Å] and the angles at the corners are close to
orthogonality [88.7(1) and 91.2(1)° at Fe1 and Fe2, respectively].
Three nitrogen atoms of the tridentate Tp- ligand and three
cyanide-carbon atoms [at Fe1] and four nitrogen atoms from two
bidentate bik molecules plus two cyanide-nitrogen atoms [at Fe2]
build somewhat distorted six-coordinated Fe1N3C3 and Fe2N6
surroundings. The cyanide bridges are slightly bent on the FeII
side [Fe2–N1–C1 = 171.8(2)° and Fe2–N2–C2 = 176.3(2)°], but
they remain close to linearity on the FeIII side [177.9(2)-
178.4(2)°]. The Fe-Ccyano bond lengths in the [Fe(Tp)(CN)3]- unit
[values in the range 1.906(2)-1.909(3) Å] and those of the Fe-Nbik
[1.964(2)-1.971(3) Å] and Fe-Ncyanide distances [1.919(2) and
1.924(2) Å] are in agreement with a low-spin iron(III) [at Fe1]
and low-spin iron(II) in a N6 surrounding [at Fe2] linked by a
single cyanide bridge.4,9 The valence bond calculations and the
spectroscopic and magnetic data also support the occurrence of
low-spin {FeIII
molecular squares are well isolated from each other, the shorter
intersquare metal-metal bond distances being 8.325(1) Å. The
terminal cyanide groups are connected to the [FeIII(Tp)(CN)3]-
counterions through two hydrogen-bonded water molecules along
a CN…Ow100…Ow200…NC path (Fig. S1, ESI†).
FT-IR spectra of 1 have been recorded on both fresh and
dehydrated samples.10 Three cyanide stretching vibrations are
observed at 2123, 2147, 2160 (fresh) and 2120, 2137, 2150 cm-1
(dehydrated). These vibrations can be used for probing the
oxidation state of the iron ion linked to the carbon atom and also
the coordination mode of the cyanide.11 In particular, the lack of
vibration below 2100 cm-1 allows to discard the presence of the
FeII
stretching vibration can be ascribed to terminal FeIII-CN groups
while those at higher wavenumbers are typical of the FeIII-CN-M
cyanide bridges.5,11,12 Similarly, only small shifts have been
observed on the FT-IR spectra recorded between 315 and 180 K,
confirming thus the occurrence of the FeIII-CN oxidation state in
this temperature range (Fig. S3 ESI†).
The magnetic properties of 1 have been investigated in the 2.0-
400 K range by measuring the thermal dependence of the χMT
product for a fresh sample before and after dehydration in the
magnetometer and for a previously dehydrated sample, χM is the
molar magnetic susceptibility per six iron atoms (Fig. 2). The
observed magnetic behaviour significantly depends on the
hydration of the sample and on the measurement procedure. The
χMT product measured at 200 K for a freshly prepared hydrated
sample, wrapped in a polyethylene film that was introduced in the
magnetometer at low temperature, is ca. 2.5 cm3 mol-1 K. This
value is close to that expected for four low-spin FeIII units (SFe =
½) with significant orbital contributions (χMT = 4 x (Nβ2g2/3k)S(S
+ 1) = 2.52 cm3 mol–1 K with g = 2.6)6a,13 and two low-spin FeII
(SFe = 0) centres. Upon warming from low temperatures, the χMT
value first increases smoothly up to 280 K and then more
abruptly to reach almost a plateau at 370 K, with χMT = 9.93 cm3
mol-1 K. This value agrees well with that expected for a set of
5
10
15
20
LS-CN-FeII
LS} pair below 300 K (see below). The
25
30
LS-CN unit. Indeed, for both samples, the lowest cyanide
35
40
45
50
55
non-interacting four low-spin iron(III) ions with significant
orbital contribution and two high-spin iron(II) ions (χMT = 2 x
3.63 = 7.26 cm3 mol–1 K with g = 2.2). This increase is
irreversible and it has not been observed for the dehydrated
phase. So, it can be ascribed to a spin change of the two Fe(II)
centres (centred at T1/2↑ ≈ 330 K) related to the dehydration
process occurring in the magnetometer under helium atmosphere.
After dehydration in the magnetometer, 1 exhibits a smoother and
reversible spin crossover shifted toward lower temperatures, with
T1/2↓↑ ≈ 240 K. This transition matches well with that observed
for the previously dehydrated sample (Fig. 2). Finally, the χMT
value of the dehydrated phase at 200 K (ca. 3.79 cm3 mol–1 K)
suggests the presence of residual high-spin Fe(II) ions, which
may also be responsible for the small but significant decrease of
χMT occurring upon cooling as an antiferromagnetic interaction is
expected between the paramagnetic ions in the {FeII
square unit.14
60
65
70
2FeIII
2}
75
Fig. 2 Graph of χMT vs. T for 1 under an applied magnetic field of 1 T: (i)
hydrated sample introduced at 200 K (circles); (ii) dehydrated sample
introduced at 300 K (crosses); (iii) dehydrated sample irradiated at 750
nm (7 mW/cm2) during one hour (triangles).
The photo-sensitivity of 1 has been checked at 10 K by using
tuneable laser light (7 mW/cm2) in the range 400-1310 nm (Fig.
S4 ESI†). Whereas no effect has been detected for the hydrated
sample, the dehydrated sample undergoes a significant increase
of the magnetization whose maximum effect occurs in the 700-
900 nm range, the value of χMT reaching 7.55 cm3 mol-1 K after
20 min when using a 750 nm laser light (Fig. 2). This value is not
far from the expected for an antiferromagnetically coupled
{FeIII
cm3 mol–1 K with S = 3 and g = 2.2) with an additional
contribution of the two paramagnetic FeIII
orbital contribution is quenched at low temperature (χMT ≈ 2 x
0.4 = 0.8 cm3 mol–1 K). Upon heating the sample at 0.3 K/min,
the light-induced excited spin state persists up to TLIESST = 45 K
(Fig. 2).15
Preliminary solid-state UV-Vis measurements on 1 in the 100-
300 K range exhibit a temperature-dependent metal-to-ligand
charge transfer (MLCT) band of the FeII ion, which is consistent
with the electronic rearrangement occurring in this chromophore
(Fig. S5 ESI†). Moreover, an intense band located at 1300 nm,
80
85
LSFeII
HS}2 square complex (χMT = (Nβ2g2/3k)S(S + 1) = 7.26
LS counterions whose
90
95
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Journal Name, [year], [vol], 00–00 | 3
which can be ascribed to an intervalence charge transfer (CT)
band, shows a significant increase upon cooling.

electronic states of the iron ions. The spectra of dehydrated 1
were recorder at 80 and 300 K (Fig. 3). Two quadrupole doublets
with parameters typical for LS-FeIII (δ = 0.00 mm s-1, ∆EQ = 0.88
mm s-1)6b and HS-FeII (δ = 0.93 mm s-1, ∆EQ = 1.0 mm s-1)16 are
observed at room temperature. Upon cooling, the doublet
corresponding to the LS-FeIII does not significantly evolve (δ =
0.06 mm s-1, ∆EQ = 0.82 mm s-1) whereas that of the HS-FeII
disappears leaving place to new doublet which at 80 K is
assigned to a LS- FeII (δ = 0.52 mm s-1, ∆EQ = 0.19 mm s-1).16 No
residual HS-FeII could be detected under these experimental
conditions. The relative amount of FeII/FeIII ions derived from the
Mössbauer studies at 80 (38%/62%) and 300 K (29%/71%) are in
agreemment with the unit formula: {FeIII
the cyanide [FeIIITp(CN)3]- unit remaining low-spin in the whole
temperature range.

57Fe Mössbauer spectroscopy has been used to check the
5
10
15
2(LS)FeII
2(LS/HS)}FeIII
2(LS),
Fig. 3 Mössbauer spectra of desolvated 1 at 80 (a) and 300 K (b). The red
and green lines represent the quadrupole doublets of the FeIII and FeII ions,
respectively (the speed is relative to iron metal).
20
In conclusion, a new cyanide-based {FeIII
square complex has been obtained by a programmed self-
assembly of preformed building blocks. This complex exhibits a
thermally induced spin transition near room temperature and
photomagnetism at low temperatures. The photo-induced
metastable state keeps up to 45 K. This mixed valence complex
also exhibits metal-metal charge transfer in the near infrared
region. The present {FeIII
recently published {FeIII
exhibit charge transfer-induced spin transition (CTIST). These
versatile square systems represent interesting molecular model
compounds to investigate the role of both the electronic and
structural parameters on CT and ST and their possible synergy.
This work was supported by the Ministère de l’Enseignement
Supérieur et de la Recherche (France), and the Agence Nationale
de la Recherche (Project : ANR-08-BLAN-0186-01), Erasmus
mundus (lot 13) and the Ministerio Español de Ciencia e
Innovación (Project CTQ 2010-15364).
2FeII
2} molecular
25
2FeII
2CoII
2} square motif is reminiscent of the
2} photomagnetic analogues, which
30
35
40
Notes and references
a Institut Parisien de Chimie Moléculaire, Université Pierre et Marie
Curie- Paris 6, UMR 7201, F-75252 Paris cedex 05 France. Fax: 0033
(0)1 4427 3841; Tel: 0033 (1) 4427 3075; E-mail:
rodrigue.lescouezec@upmc.fr
45
b Institut de Chimie Moléculaire et des Matériaux d’Orsay, UMR 8182,
bât. 420, Université Paris-Sud 11, 11 rue George Clémenceau, 91405
Orsay cedex, France.
c Departament de Química Inorgànica, Instituto de Ciencia Molecular
(ICMOL). Universitat de València. 46980 Paterna, València, Spain.
d Laboratoire de Chimie de Coordination, CNRS UPR-8241, Route de
Narbonne, 31077 Toulouse cedex 04, France
† Electronic Supplementary Information (ESI) available: Experimental
section, Physical characterization data, additional Fig. S1-S5 and
crystallographic refinement details for 1 CCDC: 856634. For ESI and
crystallographic data in CIF
DOI: 10.1039/b000000x/
50
55
or other electronic format
Notes and References
1
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M.C. Muñoz, J.A. Réal, Coord. Chem. Rev., 2011, 255, 2068, and
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10 The FT-IR spectrum of the dehydrated sample has been recorded
after a thermogravimetric analysis (Fig. S2 ESI†). This spectrum
does not evolve when leaving the sample in air.
11 K. Nakamoto, Infrared and Raman spectra of Inorganic and
Coordination Compounds, 5th edition, J. Wiley and sons, Inc., New-
York, 1997, pp 105-113.
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