Chemistry Reference
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CDW state at room temperature [
34
]. Adjacent Pt
2
(CS
2
)
4
units of Pt1-Pt2 and
Pt3-Pt3
0
in 4 are twisted by ca. 14
from the eclipsed arrangement. In addition,
the
n
-butyl groups of the dithiopentanato ligands bonded to the Pt3-Pt3
0
unit are
disordered over two sites. Therefore, the origin of threefold periodic structure
cannot be attributed to the valence ordering of the platinum atoms but to both the
twist of the adjacent diplatinum units and the difference in the conformation of
the dithiopentanato ligands. The shortest interchain S
S distance is S(2)
S(2)
˚
, indicating the absence of interchain
S
S contact. With the elongation of the alkyl chains in dithiocarboxylato ligands,
the interchain distances of 3-5 increase to 8.933, 9.563, and 10.082
˚
, respectively.
ð
1
=
2
x;
1
=
2
y;
1
=
2
zÞ¼
5
:
121
ð
4
Þ
HT Phases of [Pt
2
(RCS
2
)
4
I]
1
(R
¼ n
-Pr (3),
n
-Bu (4), and
n
-Pen (5))
With the RT-HT phase transition, the periodicities of the crystal lattices in the 1D
chain direction in the HT phases of 3 and 4 change to onefold of a -Pt-Pt-I- period
while keeping the same tetragonal space group
I
4/
m
[
34
,
35
]. The 1D chain
structures of 3 and 4 in the HT phases are shown in Fig.
9.6
.
The structural differences of the dithiocarboxylato ligands between adjacent
diplatinum units observed in the RT phases of 3 and 4 are removed in the HT
phases. The Pt1-Pt1
0
distance of 4 is 2.6930 (5)
˚
, which is 0.22
˚
shorter than the
distance between the mean planes defined by the four sulfur atoms (2.916 (3)
˚
).
Crystallographic mirror planes perpendicular to the 1D chain exist on the I1 atoms
and the midpoint of Pt1 and Pt1
0
atoms (i.e.,
z ¼
0, 0.5, 1). Therefore, the bridging
iodine atom exists at the midpoint of the diplatinum units (Pt1-I1
2.9557 (5)
˚
),
and two PtS
4
planes are disordered on two positions with the twist angle of
¼
18.41 (9)
.
On the other hand, the structure of 5 in the HT phase adopts the same space
group
I
4/
m
and keeps a similar threefold periodic structure as its RT phase [
35
].
Similar to the RT phase, the ligand moieties including sulfur atoms of Pt3-Pt3
0
units in 5 are disordered on two positions with the twist angle of
21.5(4)
due to
the crystallographic mirror planes perpendicular to the 1D chain existing on the
midpoint of Pt3 and Pt3
0
atoms. Furthermore, there is a distinct structural difference
between the RT and HT phases. It is found that two PtS
4
planes of the Pt1-Pt2
diplatinum unit are disordered over two positions with the ratio of 1:1 in the HT phase.
These twist angles between two PtS
4
planes are 31.7 (4)
and 11.6 (5)
, respec-
tively. In the case of 3 and 4, the unit cell dimension
c
along the 1D chain direction
is onefold of a -Pt-Pt-I- period in the HT phase, and the two PtS
4
planes of all the
diplatinum units are disordered in two positions. Therefore, the RT-HT phase
transition found in 5 is believed to originate from the fact that the two PtS
4
planes
in all the diplatinum units are disordered in two positions in the HT phase. Ikeuchi
and Saito have revealed from the detailed analysis of the heat capacity of 3, 4, and 5
that the entropy (disorder) once gained in alkyl chains in the RT phase is transferred
to the dithiocarboxylato group upon the RT-HT phase transition [
50
-
52
]. To clarify
the origin of the difference in the lattice periodicities, the
c
axis projection of 5 in
the RT phase is shown in Fig.
9.7
, together with those of 3 and 4.
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