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4H 2 O and (NC 4 N) 2 [Pt 2 (pop) 4 I] is only approximately 150 ˚ 3 .
Therefore, the vacancy of the water molecules is not filled perfectly in
(NC 4 N) 2 [Pt 2 (pop) 4 I]. This is probably the reason why the compound
(NC 4 N) 2 [Pt 2 (pop) 4 I] is able to absorb the water molecules in humid atmosphere.
(NC 4 N) 2 [Pt 2 (pop) 4 I]
10.3.2 Single-Crystal-to-Single-Crystal Transformation
of K 2 (NC 3 N)[Pt 2 (pop) 4 I] 4H 2 OtoK 2 (NC 3 N)[Pt 2 (pop) 4 I]
While the single crystal of dehydrated complex, (NC 4 N) 2 [Pt 2 (pop) 4 I] was
synthesized directly, more robust crystal which retains single crystallinity under
dehydration condition has been required. MMX chains with binary countercations
mentioned in Sect. 10.2 are promising candidates, because the multiple coordina-
tion bond and hydrogen bond networks among 1D chains can support
the
frameworks.
Recently, desired single-crystal-to-single-crystal
transformation of hydrated
complex, K 2 (NC 3 N)[Pt 2 (pop) 4 I]
4H 2 O, to dehydrated complex, K 2 (NC 3 N)
[Pt 2 (pop) 4 I], was achieved [ 46 ]. The crystal structure and chain structure are
shown in Fig. 10.12 . ACP-like distortion disappears, and there is only one
d (Pt-I-Pt) and d (Pt-I) in dehydrated state. In other words, K 2 (NC 3 N)[Pt 2 (pop) 4 I]
is no longer in an ACP + CDW state. The d (Pt-I-Pt) of K 2 (NC 3 N)[Pt 2 (pop) 4 I]
(5.689(2) ˚ ) is the shortest of all MMX chains thus far reported, and iodide ion is at
the midpoint between neighboring [Pt 2 (pop) 4 ] units without disorder. These struc-
tural features of K 2 (NC 3 N)[Pt 2 (pop) 4 I] are characteristic of the AV state. However,
there is still a possibility that the disorder of the bridging iodide ion is beyond the
resolution of the X-ray single-crystal analysis. The E CT determined from the peak in
the optical conductivity spectrum decreases from 0.85 to 0.45 eV by the dehydra-
tion, indicating a decrease in band gap. Although the peaks are in infrared region
not in visible region, this spectral change is also a kind of vapochromism.
Figure 10.13 shows 31 P MAS NMR spectra of K 2 (NC 3 N)[Pt 2 (pop) 4 I]·4H 2 O and
K 2 (NC 3 N)[Pt 2 (pop) 4 I]. As reported by Kimura et al [ 67 ], the 31 P chemical shift ( d )
is larger for P atoms coordinated to Pt ions in a lower oxidation state. Four intense
peaks with the satellite peaks due to the coupling between
31 P and
195 Pt are
31 P MAS NMR spectrum of K 2 (NC 3 N)[Pt 2 (pop) 4 I]
observed in the
4H 2 O,
attributed to the four inequivalent P atoms in the crystal. On the other hand, only
two intense peaks were observed in the spectrum of K 2 (NC 3 N)[Pt 2 (pop) 4 I],
indicating the existence of two sets of inequivalent P atoms. This large difference
of
between the peaks (26 ppm) derives from Pt ions in different oxidation states.
In addition, two peaks in the polarized Raman spectrum of K 2 (NC 3 N)[Pt 2 (pop) 4 I]
indicates the existence of two different [Pt 2 (pop) 4 ] units [ 46 ]. Therefore, the
electronic state of K 2 (NC 3 N)[Pt 2 (pop) 4 I] can be concluded as a CDW state. Curi-
ously, the broadened peaks in the 31 P MAS NMR spectrum suggest the generation
of paramagnetic spins, which should not exist in nonmagnetic CDW state.
d
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