Fe(II) High Spin ↔ Low Spin Transition
in [FeII(isoxazole)6](X)2, X = BF4, ClO4
A. Bhattacharjee, P. Gütlich et al.
Department of Physics, Visva-Bharati University, Santiniketan
731235, India, E-mail: ashis@vbphysics.net.in
Department of Chemistry, University of Mainz,
55099, Mainz, Germany, E-mail: guetlich@uni-mainz.de
Spin Crossover
E
Research activities in the area of Fe(II) high spin
low spin transition
materials have been stimulated in recent years by the objective of developing
molecular switches or sensors for molecular electronics. In this context hexacoordinated Fe(II) spin crossover materials have been widely studied, which
has been motivated by their favourable response towards a change in
temperature or pressure, as well as upon light irradiation as they undergo
interconversion from a low spin (S = 0) diamagnetic state to a high spin (S = 2)
paramagnetic state under the influence of these external perturbations.
e*g e
....
g
t
2g t
....
2g
LS))
LowLow-Spin
Spin (1(A
1
1
A1
S = 0, Diamagnetic
(HS() 5T
High-Spin
High Spin
5T
HS
LS
2)
EHL  kBT
S = 2,2 Paramagnetic
rHL
rFe-N
For details see:
Gütlich, P. and Goodwin, H. A. (Editors) Topics in Current Chemistry (Berlin: Springer-Verlag 2004) Vol. 233-235.
[Fe(II)(isoxazole)6](X)2, X = BF4, ClO4
[Fe(II)(isoxazole)6](BF4)2
These mononuclear compounds have been
studied by 57Fe Mössbauer spectroscopy to
reveal the thermal spin crossover of Fe(II)
between low spin (S = 0) and high spin (S = 2)
states upon changing temperature as well as the
effect of different solvent molecules on the
nature of the spin transition.
Hibbs, W., et al., Inorg. Chem. 42 (2003) 5645.
[Fe(II)(isoxazole)6](ClO4)2
Mössbauer spectra of [Fe(II)(isoxazole)6](BF4)2
100
100
220 K
200 K
95
95
Rel. Transmission (%)
95
HS-B
HS-A
95
LS
90
160 K
170 K
90
85
100
100
95
95
100 K
110 K
90
Rel. Transmission (%)
100
100
90
85
100
100
95
95
90 60 K
50
100
40
-4
-2
0
2
v (mm/s)
4
-4
-2
0
2
4
v (mm/s)
HS-A
LS
HS-B
(a)
80
HS-A
60
40
Heating
LS
20
0
HS-B
1.0
Cooling
Heating
0.6
(b)
0.4
0.2
0.0
50
Bhattacharjee, A., et al., J. Phys.: Condens. Matter. 19 (2007) 406202.
250
Cooling
0
100
HS
85
200
20
0.8
85
150
60
90
70 K
100
80
Area Fraction (%)
Heating
Cooling
A temperature-dependent spin transition curve has
been constructed with the least-squares fitted data
obtained from the Mössbauer spectra measured at
various temperatures in the 240 – 60 K range during
cooling and heating cycle. The compound exhibits a
temperature-dependent two-step spin transition
phenomenon with TSCO(step 1) = 92 and TSCO(step 2) =
191 K.
T (K)
TSCO(2)
TSCO(1)
100
150
T (K)
200
250
Nature of Spin Transition in [Fe(II)(isoxazole)6](BF4)2
Area Fraction (%)
80
LS
60
40
HS-A'
20
HS-B
HS-A
0
50
100
150
T (K)
200
Bhattacharjee, A., et al.,
J. Phys.: Condens. Matter. 19 (2007) 406202
250
The compound has three high spin Fe(II)
sites at the highest temperature of study,
two of them have slightly different
coordination environment. These two Fe(II)
sites are found to undergo spin transition,
while the third Fe(II) site retains the high
spin state over the whole temperature range.
The possibility of existence of similar
structures in the high and low temperature
phases along with an intermediate structure
in the intermediate temperature range may
be another viable origin of the two-step spin
transition. The Mössbauer spectroscopic
study sheds new light on the mechanism of
thermal spin crossover in a system with
complex interplay of temperature dependent
spin
transition
and
structural
rearrangements.
Mössbauer spectra of [Fe(II)(isoxazole)6](ClO4)2
-1
-1
99
-2
0
v / mm.s
2
4
-4
-2
0
2
4
75
100
HS-A
otal
-4
100
100
Heating
Cooling
v / mm.s
LS
98
HS-C
230 K
230 K
98
50
100
96
100
25
98
98
97
96
213 K
223 K
HS-B
96
213 K
220 K
94
94
92
100
92
100
98
98
96
94
96
200 K
213 K
94
92
92
90
100
90
100
HS-B'
96
92
96
100 K
100 K
92
88
-4
88
-2
0
-1
v / mm.s
2
4
-4
-2
0
50
96
0
-1
v / mm.s
2
4
Rel. Transmission (%)
Rel. Transmission (%)
95
LS-cool
HS-cool
LS-heat
HS-heat
100
150
200
250
300
T/K
57Fe
Mössbauer spectroscopy has been performed in
the 270 – 84 K range during cooling and heating
cycle to reveal the thermal spin crossover of Fe(II)
between low spin (S = 0) and high spin (S = 2)
states. A temperature-dependent spin transition
curve constructed with the least-squares fitted data
exhibits an unusual temperature-dependent spin
transition behaviour with TSCO(↓) = 223 and TSCO(↑)
= 213 K.
Nature of Spin Transition in [Fe(II)(isoxazole)6](ClO4)2
The [Fe(II)(isoxazole)6](ClO4)2 compound
has three high spin Fe(II) sites at the highest
temperature of study. Two of these sites are
found to undergo spin transition. The
compound seems to undergo structural phase
transition around the spin transition
temperature which plays a significant role in
the magnetic behaviour of the compound at
temperatures below TSCO.
3.75
EQ / mm.s-1
3.00
HS-A (cool)
HS-B (cool)
HS-C (cool)
LS (cool)
HS-A (heat)
HS-B (heat)
HS-C (heat)
LS (heat)
2.25
1.50
0.75
0.00
80
120
160
200
240
T/K
Bhattacharjee, A., et al., unpublished data (2007)
280
Comparing the spin transition curves
obtained through the Mössbauer
spectroscopy with that obtained from the
magnetic measurements, it is found that
Mössbauer spectroscopy observes the spin
transition “earlier” than the magnetic
measurement. This is attributable to the
existence of a definite time window for the
spectroscopic method while bulk magnetic
measurements at thermodynamic
equilibrium correspond to no time window.