Chemistry Reference
In-Depth Information
dinuclear units becomes possible and the nature of the plateau in Fig.
2.22
b has
been clarified: it is predominantly due to the presence of [HS-LS] pairs [
34
,
35
].
The reason for the sharp decrease of v
M
T vs. T below ca. 30 K (Fig.
2.21
b) has
also become clear with the help of Mössbauer spectroscopy. It is due to zero-field
splitting (ZFS) of the remaining HS-Fe
II
ions and definitely not a continuation of
ST upon further cooling; the Mössbauer spectrum recorded at temperatures below
the plateau is nearly the same as that recorded in the region of the plateau
(Fig.
2.22
a).
Very
Fe
II
similar
studies
were
carried
out
with
the
dinuclear
complex
2
phdia (phdia = 4,7-phenanthroline-5,6-diamine), which
exhibits an almost complete two-step thermally induced ST with a plateau around
100 K in the v
M
T vs. T magnetization curve [
38
]. After rapidly cooling the sample
from the plateau down to 4.2 K the Mössbauer spectrum was recorded at that
temperature in applied magnetic field of 5 T. The analysis of the magnetically
perturbed spectrum indicated that the plateau consisted mainly of [HS-LS] pairs.
In another Mössbauer effect study the mechanism of the half-way ST in the
dinuclear
Fe phdia
ð
Þ
NCS
ð Þ
2
BF
ð Þ
4
DMF with pmatrz = 4-
amino-3,5-bis{[(2-pyridyl-methyl)amino]methyl}-4H-1,2,4-triazole could be elu-
cidated by recording the spectra solely as a function of temperature in zero field.
The results are in full agreement with the analysis of the structure determination
[
39
]. The structure of this dinuclear compound is shown in Fig.
2.23
[
39
]. The Fe
II
ions are in the HS state (shown in black) at room temperature as expected for [HS-
HS] pairs. On cooling, only half of the Fe
II
ions switch to the LS state as confirmed
by the magnetic susceptibility measurements (Fig.
2.23
). This is possible in two
ways: (1) Both Fe
II
complex ions have undergone ST but in only 50 % of the
original [HS-HS] pairs; (2) Only one Fe
II
center in all [HS-HS] pairs has swit-
ched. Mössbauer spectroscopy has confirmed that the second switching manner
holds, viz. a ST between [HS-HS] to [LS-HS] pairs. Such a ST from [HS-HS] to
[LS-HS] pairs on cooling is in agreement with the temperature dependence of the
magnetic properties (Fig.
2.23
). The HS to LS transition of half of the Fe
II
ions
occurs around 225 K to reach a plateau of very large length. The drop in v
M
T at
very low temperature is definitely not due to a further HS to LS transition, because
the Mössbauer spectrum recorded at 4.2 K shows a 50 % HS population. The
sharp decrease of v
M
T at low temperatures is due to ZFS of HS Fe
II
ions [
40
]. The
Mössbauer spectra recorded as a function of temperature and shown in Fig.
2.24
not only confirm a 50 % spin state conversion, but also give insight into structural
distortion that accompanies the spin transition (Fig.
2.23
). The quadrupole doublet
measured at 298 K refers to the original [HS-HS] pairs. At 240 K new signals
appear corresponding to LS Fe
II
ions (blue) in [HS-LS] pairs and HS Fe
II
ions
(dark red) in [HS-LS] pairs, both with the same area fraction. Upon further cooling
to 180 K, a complete transition of HS ions in [HS-HS] pairs to a 50 % mixture of
LS and HS ions in [HS-LS] pairs is identified, for the first time in zero-field
Mössbauer spectroscopy [
41
].
Fe
I
2
pmatrz
complex
of formula
ð
Þ
2
