Heat- and Light-Induced Spin Transition of an Iron(II) Polymer Containing the 1,2,4,5-Tetrakis(diphenylphosphanyl)benzene Ligand

Article abstract of DOI:10.1002/ejic.200400284

A new iron(II) spin-crossover compound containing a phosphane ligand has been synthesized and characterized by magnetic susceptibility, Moessbauer spectroscopy, and heat capacity and photomagnetic experiments. The [Fe(tppb)Br2] complex [tppb = 1,2,4,5-tetrakis(diphenylphosphanyl)benzene] exhibits a gradual spin conversion at T_1/2 = 184 K, and displays at low temperature a partial photoconversion of the low-spin (LS) state into the metastable high-spin (HS) state by irradiating the sample at 530 nm.

Full text of DOI:10.1002/ejic.200400284

SHORT COMMUNICATION
Heat- and Light-Induced Spin Transition of an Iron(II) Polymer Containing the
1,2,4,5-Tetrakis(diphenylphosphanyl)benzene Ligand
Patrick Rosa,[a] Amy Debay,[a] Laurence Capes,[b] Guillaume Chastanet,[a]
[c]
[d]
[a]
´
Azzedine Bousseksou, Pascal Le Floch, and Jean-Francois Letard*
¸
Dedicated to Philipp Gütlich in the occasion of his 70th birthday
Keywords: Spin crossover / Phosphane ligands / Iron / Photomagnetism / Coordination polymers
A new iron(II) spin-crossover compound containing a phos-
phane ligand has been synthesized and characterized by
magnetic susceptibility, Mössbauer spectroscopy, and heat
capacity and photomagnetic experiments. The [Fe(tppb)Br2]
complex [tppb = 1,2,4,5-tetrakis(diphenylphosphanyl)ben-
displays at low temperature a partial photoconversion of the
low-spin (LS) state into the metastable high-spin (HS) state
by irradiating the sample at 530 nm.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
zene] exhibits a gradual spin conversion at T1/2 = 184 K, and Germany, 2004)
Introduction
dppen ϭ cis-1,2-bis(diphenylphosphanyl)ethylene, X ϭ Cl
or Br, and solvent molecules (S) ϭ CHCl3, CH2Cl2, or
(CH3)2CO.[3Ϫ7]
In the particular case of
[Fe(dppen)2Cl2]·2(CH3)2CO, single-crystal structures deter-
Iron() spin-crossover (SC) compounds have numerous
potential applications as display devices, optical switches,
and magneto-optical storage systems, since heat- and light-
induced spin transitions occur in the solid state without any
fatigue.[1] The possibility to convert the spin state by irradi-
ating with light is now reported in a large number of spin-
crossover complexes. The major drawback of this process
up to now is that optical switching requires that it be car-
ried out at very low temperatures. Nevertheless, it has been
theoretically predicted and experimentally confirmed that
HS Ǟ LS relaxation rates kHL can at least be controlled by
the energy difference ∆EHL° between the lowest vibrational
mined both in HS and LS states showed
a large
[4]
˚
∆rHL(FeϪP) variation (0.3 A), and relaxation kinetics of
the HS Ǟ LS conversion that occurs after the LIESST ef-
fect were slower than for classical FeN6 complexes.[7]
In this work, we report the preparation of a new iron()
spin-crossover complex containing phosphorus atoms with
the 1,2,4,5-tetrakis(diphenylphosphanyl)benzene (tppb) li-
gand (Figure 1). The thermal spin-crossover properties were
studied by magnetic susceptibility measurements,
levels of HS and of LS states, and by the change of the
metalϪligand bond length ∆rHL
[2]
.
In this particular con-
text, iron() spin-crossover compounds containing phos-
phorus atoms are very attractive since a large ∆rHL is ex-
pected. Unfortunately, to date, spin transition has only been
reported for solvated [FeX2(dppen)2]·nS compounds with
[a]
´
Groupe des Sciences Moleculaires, Institut de Chimie de la
`
´
Matiere Condensee de Bordeaux, UPR 9048 CNRS
Ϫ
´
Universite de Bordeaux 1,
87 Av. du Doc. A. Schweitzer, 33608 Pessac, France
Fax: (internat.) ϩ 33-1-40002649
E-mail: letard@icmcb-bordeaux.cnrs.fr
DRECAM/SCM, CEA Saclay,
[b]
[c]
[d]
91191 Gif-sur-Yvette Cedex, France
Laboratoire de Chimie de Coordination, CNRS UPR 8241,
31077 Toulouse Cedex, France
Figure 1. Top left: schematic view of ligand tppb; top right: optim-
ized structure[8] of complex [Fe(tppb)Br2]n; bottow: molar magnetic
susceptibility χM vs. temperature T before (filled squares), during
(unfilled circles) and after (unfilled squares) light irradiation
´ ´ ´ ´
Laboratoire Heteroelements et Coordination, UMR 7653
CNRS Ϫ Ecole Polytechnique,
91128 Palaiseau Cedex, France
Eur. J. Inorg. Chem. 2004, 3017Ϫ3019
DOI: 10.1002/ejic.200400284
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3017

´
J.-F. Letard et al.
SHORT COMMUNICATION
Mössbauer spectroscopy, and heat capacity experiments. confirms the occurrence of a temperature-controlled singlet/
Photomagnetic investigations are also reported.
quintet equilibrium with complete conversion. The single
doublet recorded at room temperature is typical of iron()
in the HS (S ϭ 2) ground state with a quadrupole splitting
of ∆EQ ϭ 2.773 Ϯ 0.007 mm·sϪ1 and an isomer shift of δ ϭ
Results and Discussion
0.663
Ϯ
0.004 mm·sϪ1 relative to natural iron
(Ϫ0.13 mm·sϪ1). In contrast, at 80 K, the unique doublet
found with ∆EQ ϭ 0.740 Ϯ 0.002 mm·sϪ1 and δ ϭ 0.396 Ϯ
0.001 mm·sϪ1 is typical of iron() in the LS (S ϭ 0) ground
state. At an intermediate temperature (180 K), the inner
and outer doublets are observed, as expected. This thermal
spin transition was also investigated by DSC techniques.
The temperature dependence of the molar heat capacity Cp
in the cooling and warming modes was examined in the
120Ϫ290 K temperature range. The enthalpy variation as-
sociated with the spin transition is ∆H ϭ 5.6 kJ·molϪ1, and
the entropy change is ∆S ϭ 31 J·molϪ1.KϪ1. The value for
∆S is smaller than those obtained in strongly cooperative
Complex [Fe(tppb)Br2]n was obtained by mixing equimo-
lar amounts of FeBr2 and tppb in a mixture of chloroform
and absolute ethanol (see Exp. Sect.). The elemental analy-
sis (C, H, Cl and Fe) was consistent with the expected com-
position of a polymeric structure. Crystallization was at-
tempted several times by slow diffusion of solutions of
FeBr2 and tppb in ethanol into an H-shaped double-tube
glass. An amorphous powder was always obtained, cer-
tainly due to the expected low solubility of the polymeric
structure. Figure 1 shows a polymeric model built up with
the CERIUS 4.0 program.[8] Energy minimization sug-
gests at first a linear organization of the iron() atoms along
˚
compounds (50Ϫ80 J·molϪ1·K
)
Ϫ1 [11]. Since it is very close
the polymeric chain, with FeϪP bond lengths of 2.5 A,
˚
to the electronic-only value (∆Sel) of 13.4 J·molϪ1 KϪ1 [1]
,
which are within the 2.3Ϫ2.6 A range reported for
[Fe(dppen)2Br2]·2CHCl3 by X-ray diffraction.[4] It also
the weakness of the vibrational component ∆Svib supports
the poorly cooperative behavior of [Fe(tppb)Br2]n.
shows that the FeϪFe distance along the polymer chain is
˚
9 A, which is longer than that in iron()Ϫtriazole polymers
Photomagnetic properties were investigated by irradiat-
ing the sample in a SQUID cavity coupled to a Krypton
Laser through an optical fiber. The compound was first
slowly cooled in the dark from room temperature to 10 K,
then irradiated for 1 h. The magnetic response was found
to increase rapidly until a value of about 1.8 cm3·K·molϪ1
was reached (Figure 1). This indicates a partial conversion
(50%) of the LS state into the metastable HS state accord-
ing to the Light-Induced Excited-Spin-State Trapping
(LIESST) process.[12] To increase the level of photoexcit-
ation different experiments were performed: an increase in
the irradiation time, an increase in the power (up to
30 mW·cmϪ2), and/or a change in the excitation wavelength
(488.0Ϫ514.5 nm, 647.1Ϫ676.4 nm, 752.5Ϫ799.3 nm). Un-
fortunately, the highest photoinduced LS Ǟ HS conversion
rate remained at around 50%. The light irradiation was then
switched off and the critical LIESST temperature value
T(LIESST) was recorded.[13] In fact, we recently proposed
that the capacity of a compound to retain the light-induced
HS information may be estimated through the determi-
nation of the T(LIESST) value.[14] With this procedure,
χMT was found to remain constant at low temperature (tun-
neling domain) and to drop within a few Kelvin when the
thermally activated region was reached. The value of
T(LIESST), determined as the minimum of the derivative
ѨχMT/ѨT, was estimated as 46 K (Figure 1). Such a value,
in comparison to the other iron() spin-crossover com-
plexes from our database,[14] is unfortunately not excep-
tional. A study of the kinetics in the vicinity of T(LIESST)
showed that the relaxation curves strongly deviate from sin-
gle exponentials, with a fast initial decay and a long tail
with a much slower decay, typical of a relaxation following
a stretched exponential law. This behavior has, in fact, been
reported in many other noncooperative spin-crossover com-
plexes and has been attributed to a distribution of acti-
vation energies due to some local inhomogeneity of the
(3.6 A[9,10]) since the bridging phosphorus atoms are kept
˚
apart by the rigid phenylene structure.
The magnetic properties of [Fe(tppb)Br2]n are presented
in Figure 1. The value of the molar magnetic susceptibility
product with temperature (χMT) varies from 3.8 cm3·
K·molϪ1 at room temperature to nearly 0 cm3·K·molϪ1 at
80 K. The room-temperature value is consistent with a
5
high-spin FeII complex with a T2 quintet (assuming octa-
hedral symmetry) ground state. At low temperature, the
χMT value corresponds to a low-spin FeII complex with a
1A1 singlet ground state. The χMT curves recorded in the
cooling and warming modes are identical, and no hysteresis
is detected. The temperature at which there are equal
amounts of HS and LS complexes T1/2 is 184 K.
Figure 2 displays the Mössbauer spectra recorded at tem-
peratures of 293 K, 180 K and 80 K. This series of data
Figure 2. 57Fe Mössbauer spectra for a powder sample of complex
2; an HS doublet is observed at 293 K, whereas cooling the sample
to 80 K gives an LS doublet; at 180 K, both doublets are present
with 37% of the HS state
3018
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 3017Ϫ3019

Heat- and Light-Induced Spin Transition of an Iron(II) Polymer
SHORT COMMUNICATION
iron() surroundings.[15] In our case, this finding is in good
agreement with the gradual character of the thermal spin
transition and proves once more the absence of significant
cooperativity in the [Fe(tppb)Br2]n polymer, certainly be-
cause of the long FeϪFe distances along the chain.
{[Fe(tppb)Br2]·0.16CHCl3}n: The complex was prepared from a
solution of tppb (0.1 g, 0.12 mmol) in CHCl3 (20 mL), which was
slowly added to a solution of FeBr2 (0.026 g, 0.12 mmol) in warm
(45 °C) absolute ethanol (10 mL) in the presence of a few crystals
of ascorbic acid (to prevent oxidation of FeII to FeIII). Upon ad-
dition of the ligand, the reaction mixture changed immediately
from an orange color to a cloudy mixture, which yielded a yellow
precipitate. The powder was then filtered off, washed with a few
milliliters of absolute ethanol, and dried under a stream of nitrogen
(yield 106 mg, 84%). C54.16H42.16Br2Cl0.48FeP4 (1049.56): calcd. C
61.98, H 4.05, Cl 1.62, Fe 5.32; found C 61.86, H 4.11, Cl 1.55,
Fe 4.93.
Conclusion
In this work, we have reported the heat- and light-in-
duced spin-crossover properties of a new iron() polymeric
complex based on a tetraphosphane. This polymer rep-
resents an important step towards the design of polynuclear
systems combining magnetic interaction and spin conver-
sion under light irradiation. Moreover, it is also a new ex-
ample of a spin-crossover material involving phosphorus
atoms. Synthetic efforts are currently under way to investi-
gate the properties of other complexes based on di- and
tetraphosphane ligands.
Acknowledgments
The authors would like to acknowledge the Aquitain region for
funding.
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1,2,3,4-Tetrakis(diphenylphosphanyl)benzene: The synthesis of
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from Ph2PCl (6 mL, 32 mmol) in THF (50 mL) and Na (1.5 g,
65.2 mmol) while monitoring by 31P NMR spectroscopy. After 10 h
at room temperature, the excess sodium was removed and 1,2,4,5-
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°C and one night at room temperature, 31P NMR spectroscopic
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with dichloromethane and water. The organic layer was dried with
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Received April 6, 2004
Eur. J. Inorg. Chem. 2004, 3017Ϫ3019
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3019