Predominance of covalency in water-vapor-responsive MMX-type chain complexes revealed by 129I Moessbauer spectroscopy

Article abstract of DOI:10.1039/c4dt00627e

¹²⁹I Moessbauer spectroscopy was applied to water-vapor-responsive MMX-type quasi-one-dimensional iodide-bridged Pt complexes (MMX chains) in order to investigate their electronic state quantitatively. Two sets of octuplets observed in K₂(H ₃NC₃H₆NH₃)[Pt₂(pop) ₄I]·4H₂O (2·4H₂O) and one octuplet observed in K₂(cis-H₃NCH₂CHCHCH ₂NH₃)[Pt₂(pop)₄I]·4H ₂O (1·4H₂O) and dehydrated complexes (1 and 2) indicate a unique alternating charge-polarization + charge-density-wave (ACP + CDW) electronic state and a charge-density-wave (CDW) electronic state, respectively. These spectra correspond to their crystal structure and the change of electronic states upon dehydration. Since these complexes consist of an alternating array of positively charged and negatively charged layers, the charge on the iodide ion (ρ_IS) was discussed on the basis of the isomer shift (IS). The ρ_IS of the water-vapor-responsive MMX chains was mainly -0.13 to -0.21, which are the smallest of all MMX chains reported so far. Hence, it indicates that the negative charge on the iodide ion is strongly donated to the Pt ion in these complexes. This covalent interaction predominates in the ACP + CDW state as well as in the CDW state. Therefore, the ACP + CDW state is in fact the CDW state with the ACP-type lattice distortion. Because the ρ_IS became smaller with the decreasing Pt-I-Pt distance, it can be concluded that the covalent interaction plays an important role in determining the electronic states of the MMX chains with pop (= P ₂H₂O₅^2-) ligands.

Full text of DOI:10.1039/c4dt00627e

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Predominance of covalency in water-vapor-
responsive MMX-type chain complexes revealed
Cite this: Dalton Trans., 2014, 43,
8767
by 129I Mössbauer spectroscopy†
Hiroaki Iguchi,*a,b Shinji Kitao,c Makoto Seto,c Shinya Takaishia,b and
Masahiro Yamashita*a,b
129I Mössbauer spectroscopy was applied to water-vapor-responsive MMX-type quasi-one-dimensional
iodide-bridged Pt complexes (MMX chains) in order to investigate their electronic state quantitatively. Two
sets of octuplets observed in K2(H3NC3H6NH3)[Pt2(pop)4I]·4H2O (2·4H2O) and one octuplet observed in
K2(cis-H3NCH2CHvCHCH2NH3)[Pt2(pop)4I]·4H2O (1·4H2O) and dehydrated complexes (1 and 2) indicate
a unique alternating charge-polarization + charge-density-wave (ACP + CDW) electronic state and a
charge-density-wave (CDW) electronic state, respectively. These spectra correspond to their crystal struc-
ture and the change of electronic states upon dehydration. Since these complexes consist of an alternat-
ing array of positively charged and negatively charged layers, the charge on the iodide ion (ρIS) was
discussed on the basis of the isomer shift (IS). The ρIS of the water-vapor-responsive MMX chains was
mainly −0.13 to −0.21, which are the smallest of all MMX chains reported so far. Hence, it indicates that
the negative charge on the iodide ion is strongly donated to the Pt ion in these complexes. This covalent
interaction predominates in the ACP + CDW state as well as in the CDW state. Therefore, the ACP + CDW
state is in fact the CDW state with the ACP-type lattice distortion. Because the ρIS became smaller with
the decreasing Pt–I–Pt distance, it can be concluded that the covalent interaction plays an important role
in determining the electronic states of the MMX chains with pop (= P2H2O52–) ligands.
Received 28th February 2014,
Accepted 8th April 2014
DOI: 10.1039/c4dt00627e
www.rsc.org/dalton
tion of crystallinity9–11 and with the change of the electronic
states,11 indicating that they are promising multifunctional
Introduction
The electronic states in the one-dimensional (1D) electron vapor-induced switching materials.
system play a key role in bringing about a variety of attractive
Quasi-1D halogen-bridged metal complexes consisting of
physical properties such as high conductivity,1,2 large optical paddle-wheel-type dinuclear complexes, the so-called MMX
nonlinearity,3,4 Luttinger liquid behavior,5 ultra-fast optical chains, have higher degrees of freedom of the electrons than
responses,6 etc. Quasi-1D halogen-bridged metal complexes, those consisting of mononuclear complexes (MX chains), indu-
which also show attractive physical properties originating from cing a larger variety of electronic states in MMX chains. On the
the 1D electron system,4,6–8 have a robust chain structure basis of theoretical calculations12 and experimental data, the
derived from the coordination bonds between the metal ions electronic states of the MMX chains have been classified into
(M) and the bridging halide ions (X). Some of them show the the following four states:
reversible adsorption and desorption of molecules with reten-
(a) Average-valence (AV) state
–M2:5þ–M2:5þ–X–M2:5þ–M2:5þ–X–
aDepartment of Chemistry, Graduate School of Science, Tohoku University,
6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan.
(b) Charge-density-wave (CDW) state
ꢀ ꢀ ꢀ M2þ–M2þ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ X–M3þ–M3þ–X ꢀ ꢀ ꢀ
E-mail: h-iguchi@m.tohoku.ac.jp, yamasita@agnus.chem.tohoku.ac.jp;
Fax: +81-22-795-6548; Tel: +81-22-795-7643
bCore Research for Evolutional Science and Technology (CREST), Japan Science and
Technology Agency (JST), 4-1-8 Kawaguchi, Saitama, 332-0012, Japan
cResearch Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka, 590-0494,
(c) Charge-polarization (CP) state
ꢀ ꢀ ꢀ M2þ–M3þ–X ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ M2þ–M3þ–X ꢀ ꢀ ꢀ
Japan
†Electronic supplementary information (ESI) available: Analysis of 129I Möss-
bauer spectra and other detailed discussions including Fig. S1. See DOI:
10.1039/c4dt00627e
(d) Alternating charge-polarization (ACP) state
ꢀ ꢀ ꢀ M2þ–M3þ–X–M3þ–M2þ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ X ꢀ ꢀ ꢀ
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These electronic states are strongly correlated with the posi-
Estimating the charge on the metal or halide ion in MMX
tion of the bridging halide ion, which is at the midpoint chains is quite important to understand the electronic states.
between two metal ions in the AV state and is close to the 129I Mössbauer spectroscopy is a powerful tool for determining
higher-oxidized metal ion in other states. Strictly, formal oxi- the charge on the iodide ion quantitatively. So far,
dation numbers, 3+ and 2+, should be represented as (3 − δ)+ [Pt2(MeCS2)4I] and [Pt2(EtCS2)4I] forming the ACP state,
and (2 + δ)+, respectively. The AV state is a Robin–Day class III (H3NC6H12NH3)2[Pt2(pop)4I] forming the CDW state,
system, whereas the other three are class II systems.13
(Et2NH2)4[Pt2(pop)4I] forming the CP state, and their precursor
MMX chains synthesized to date are classified into two cat- Pt(III)2 complexes have been studied by 129I Mössbauer spectro-
egories on the basis of their ligands. A high conductivity,14 scopy.14d,19 Herein, we study the characteristics of the new
metal–insulator transition,14 and the formation of nanowires ACP + CDW state in 2·4H2O and its change upon dehydration
on a substrate15 have been reported in a dithioacetate (dta) by comparing the similar complex, 1·4H2O, and other MMX
system, [M2(RCS2)4I] (M = Ni, and Pt; R = alkyl chain group). chains in the pop system. In addition, we discuss the origin of
However, the adjustable components are limited to the metal the charge variation in the pop system.
ions (Ni and Pt) and the alkyl chain groups in the dta system.
On the other hand, in
a
diphosphite (pop) system,
alkali
Y4[Pt2(pop)4X]·nH2O and Y′ [Pt2(pop)4X]·nH2O (Y
=
2
Experimental section
Materials
metal, alkyl ammonium, etc.; Y′ = alkyldiammonium; X = Cl−,
Br−, and I−; pop = P2H2O52−),11,16 the negatively charged
chains require countercations, of which there are a huge
All of the commercially available chemicals were of reagent
number of possible candidates. Furthermore, removing lattice grade and used as received. The starting Pt(II)2 complex,
water molecules can change the electronic states of the MMX K4[Pt2(pop)4]·2H2O, was synthesized from K2PtCl4 and H3PO3
following the reported procedures.20
chains.11 Therefore, the pop system is a more promising target
to realize the new electronic states and to study the change of
electronic states upon chemical external stimuli such as
adsorption–desorption of molecules.
Synthesis of 129I-enriched MMX chains
The starting Pt(III)2 complex, 129I-enriched K4[Pt2(pop)4I2], was
synthesized at Kyoto University Reactor (KUR) using 5 mL of
Na129I aqueous solution containing 36 mg (0.28 mmol) of 129I
with 105 mg (0.74 mmol) of Na2SO3 and 5 mg (0.17 mmol) of
formaldehyde. The impurities were removed and 129I-enriched
K4[Pt2(pop)4I2] was synthesized by the following chemical reac-
tions. The reaction temperature was roughly controlled using a
hotplate.
Recently, we rationally designed more robust MMX chains
with binary countercations, A2B[Pt2(pop)4I]·nH2O (A+ = K+ and
Rb+; B2+ = aliphatic diammonium ion; n = 2 and 4),17 which
show reversible dehydration/rehydration when B2+ is
2+
H3NCH2CHXCH2NH3
(X
.
=
H, Me, Cl) or cis/trans-
Although the electronic state
2+ 17b,c
H3NCH2CHvCHCH2NH3
of K2(cis-H3NCH2CHvCHCH2NH3)[Pt2(pop)4I]·4H2O (1·4H2O)
is the typical CDW state,17c K2(H3NC3H6NH3)[Pt2(pop)4I]·4H2O
Reaction I
(2·4H2O) forms a new ACP + CDW electronic state (⋯Pt(2+a)+
–
2Na129I þ xNa2SO3 þ yHCHO þ xCaðNO3Þ2
Pt(2+b)+⋯I–Pt(3−c)+–Pt(3−d)+–I⋯; 0 < a, b, c, d < 0.5; a, c < b,
d).17b Dehydration of 2·4H2O changes the structure as shown
in Fig. 1 and also changes the electronic state from the ACP +
CDW state to the narrow-gapped CDW state,17b which contains
thermally activated Pt3+ spins as mobile solitons.18
! 2Na129I þ xCaSO3 # þ2xNaNO3 þ yHCHO
Reaction II
2Na129I þ 2xNaNO3 þ yHCHO þ PbðNO3Þ2
! Pb129I2 # þð2x þ 2ÞNaNO3 þ yHCHO
Reaction III
Pb129I2 þ K2CO3 ! 2K129I þ PbCO3 #
Reaction IV
K
129I þ 127I2 ! K129I127I2
Reaction V
K
129I127I2 þ K4½Pt2ðpopÞ4ꢁꢀ2H2O ! K4½Pt2ðpopÞ4I2ꢁ #
þ KI þ 2H2O
Reactions I–V. A Heated aqueous solution (5 mL) containing
Na129I (0.28 mmol) was added to 0.17 mL of 2.53 M Ca(NO3)2
aqueous solution (102 mg, 0.43 mmol), followed by filtration
of the resulting white powder. The filtrate was heated at
30–40 °C overnight to slowly concentrate the solution by half.
Fig. 1 Crystal structure, Pt–I and Pt–I–Pt distance of (a) 2·4H2O and (b)
2·H2O molecules and H3NC3H6NH3 ions are omitted for clarity. Iodide
ions of 2·4H2O and K+ ions of 2 are disordered. Pt yellow, I purple,
2+
K brown, P orange, O red.
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After being cooled to room temperature, the colorless crystals spectrometer with a NaI(Tl) scintillation counter. The source
(CaSO3) were separated by decantation, and then the solution and the absorber were cooled with a home-made closed-cycle
was added to 0.11 mL of 1.25 M Pb(NO3)2 aqueous solution refrigerator system using helium gas as the working medium.
(46 mg, 0.14 mmol). The resulting yellow powder (Pb129I2) was
Analysis of 129I Mössbauer spectra
filtered and washed three times with methanol. Pb129I2 was
again dissolved in hot water (20 mL) and was added to
The spectra were analyzed by the least-squares fitting with Lor-
entzian lines using the MossWinn software.21 The detailed
explanation of 129I Mössbauer spectra is given in some refer-
ences.19,22 In Mössbauer spectroscopy, the energy of the γ-ray
is controlled by changing the relative velocity of the source.
The difference between the nuclear transition energy of the
source and that of the absorber is called the isomer shift (IS)
and is customarily denoted by the relative velocity to the ZnTe
source. The quadrupole coupling constant (QCC), which is a
direct measure of the electric field gradient (EFG) at the iodine
nucleus, is the other important parameter available from the
Mössbauer spectra. The charge on the 129I can be calculated
from IS (ρIS) or QCC (ρQCC) as described in the ESI.†
0.145 mL of 1.25
M K2CO3 aqueous solution (25 mg,
0.18 mmol) to form a white suspension. After the centrifu-
gation and the filtration, the clear solution was heated at
60–70 °C to reduce the volume to about 5 mL, cooled to room
temperature, and 127I2 (85.3 mg, 0.336 mmol) was added. To
the resulting homogeneous solution, K4[Pt2(pop)4]·2H2O
(391 mg, 0.338 mmol) was added and heated at 30–40 °C for
2 h, followed by addition of methanol (approx. 50 mL) to pre-
cipitate 129I-enriched K4[Pt2(pop)4I2]. The brown solid was
washed with methanol and dried by heating (331 mg,
0.24 mmol, 85%).
129I-enriched K2(cis-H3NCH2CHvCHCH2NH3)[Pt2(pop)4I]·
4H2O (1·4H2O). The synthesis of 1·4H2O was carried out by
following the reported procedure for the non-enriched com-
plex.17c 21.5 mL of the Pt2 complexes in water (containing
165 mg (0.14 mmol) of K4[Pt2(pop)4]·2H2O and 168 mg
(0.12 mmol) of K4[Pt2(pop)4I2]) and 21 mL of the aqueous salt
solution (containing 42 mg (0.20 mmol) of cis-
H3NCH2CHvCHCH2NH3(NO3)2 and 808 mg (8.00 mmol) of
KNO3) were mixed and allowed to stand for 2 days. 1·4H2O was
obtained as glossy brown cubic crystals (217 mg, 68%).
129I-enriched K2(H3NC3H6NH3)[Pt2(pop)4I]·4H2O (2·4H2O).
The synthesis of 2·4H2O was carried out by following the
reported procedure for the non-enriched complex.17b 10.8 mL
of the Pt2 complexes in water (containing 81 mg (0.070 mmol)
of K4[Pt2(pop)4]·2H2O and 80 mg (0.058 mmol) of
K4[Pt2(pop)4I2]), 0.72 mL of 250 mM H3NC3H6NH3SO4
aqueous solution (31 mg, 0.18 mmol), and 4.00 mL of 900 mM
KNO3 aqueous solution (364 mg, 3.6 mmol) were mixed and
allowed to stand for 2 days. 2·4H2O was obtained as glossy
brown cubic crystals (144 mg, 85%).
Results and discussion
129I Mössbauer spectra of 1·4H2O, 2·4H2O and their
dehydrated states
129I Mössbauer spectra of 1·4H2O and 1 at 14 K are shown in
Fig. 2. The best fit for each spectrum was obtained with one
octuplet. Therefore, there is only one chemically independent
iodide site in both 1·4H2O and 1, consistent with their CDW
electronic state. As shown in Table 1, the Mössbauer para-
meters of 1·4H2O and 1 were nearly identical, indicating that
the electronic state was not changed by the dehydration. The
129I Mössbauer spectra of 2·4H2O and 2 at 14 K are shown in
Fig. 3. Apparently, the spectrum of 2·4H2O consists of two sets
of octuplets while that of 2 consists of one octuplet. These
spectra correspond to the crystal structure.17b Since 2·4H2O is
in the ACP + CDW state, two chemically independent iodide
ions exist due to the ACP-like distortion. This distortion dis-
Dehydration method
For dehydration, 1·4H2O was heated at 75 °C under vacuum
(8 × 10−2 Pa) for 17 h, and 2·4H2O was heated at 70 °C under
vacuum (8 × 10−2 Pa) for 19 h. This condition is stronger than
the previous one, which can complete dehydration.17b,c Each de-
hydrated sample 1 and 2 was put into the plastic bag and sealed
with a sealer in order to avoid exposure to air. The sealing was
carried out in a glove bag filled with dry nitrogen gas.
129I Mössbauer spectroscopy
The 27.7 keV γ-ray generated from 129Te was used for obtaining
the 129I Mössbauer spectrum. In this study, a 129Te source was
obtained by neutron irradiation of enriched 66Zn128Te in the
nuclear reaction 128Te(n,γ)129Te at KUR. Since the half-life of
129Te is 69.6 minutes, the measurement of 129I Mössbauer
spectra (typically continued for four hours) was accumulated
four to six times with the repeatedly irradiated 129Te source in
Fig. 2 129I Mössbauer spectra of 1·4H2O and 1 measured at 14 K. The
order to obtain distinct spectra using a constant-acceleration solid lines were fitted with the data.
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Table 1 Mössbauer parameters and Pt–I–Pt distance (d(Pt–I–Pt)) for MMX chains and their precursors
ISa/mm s−1
QCCb/MHz
ρIS
ρQCC
Ref.
d(Pt–I–Pt)/ Å
c
d
Electronic state
T/K
pop system
1·4H2O
1
2·4H2O-Ia
2·4H2O-Ib
2
(H3NC6H12NH3)2[Pt2(pop)4I]
(Et2NH2)4[Pt2(pop)4I]
K4[Pt2(pop)4I2]
CDW
CDW
ACP + CDW
14
14
14
0.751(15)
0.759(15)
0.639(13)
0.359(7)
0.757(15)
0.47(8)
−1506(45)
−1512(45)
−1625(49)
−1327(40)
−1471(44)
−1680(50)
−1567(47)
−1530(46)
−0.14(2)
−0.13(2)
−0.21(2)
−0.40(2)
−0.14(2)
−0.33(5)
−0.43(4)
−0.54(4)
−0.34(2)
−0.34(2)
−0.29(2)
−0.42(2)
−0.36(2)
−0.27(2)
−0.32(2)
−0.33(2)
This work
This work
This work
5.7623(18)e
5.679f
5.6305(8)g
5.9786(9)g
5.689(2)g
5.916h
CDW
CDW
CP
14
15
15
16
This work
19b
19b
0.31(7)
0.15(7)
6.665i
Discrete
19b
dta system
[Pt2(MeCS2)4I]-Ia
[Pt2(MeCS2)4I]-Ib
[Pt2(EtCS2)4I]-Ia
[Pt2(EtCS2)4I]-Ib
[Pt2(MeCS2)4I2]
[Pt2(EtCS2)4I2]
ACP
ACP
16
11
0.28(8)
0.56(8)
0.35(8)
0.50(8)
0.17(7)
0.22(7)
−1180(36)
−1380(40)
−1213(36)
−1432(43)
−1172(35)
−1177(35)
−0.45
−0.49
14d, 19a
19b
5.914(2)j
5.914(2)j
5.860(2)k
5.917(2)k
−0.27
−0.40
−0.41(5)
−0.31(5)
−0.53(4)
−0.49(4)
−0.47(2)
−0.38(2)
−0.49(2)
−0.49(2)
Discrete
Discrete
16
16
19b
19b
a Referenced to ZnTe. b Converted to QCC value of 127I. c Calculated from eqn (2) in the ESI. d Calculated from eqn (4) in the ESI. e At 100 K, see
ref. 17c. f Estimated from the lattice constant obtained from the powder X-ray diffraction pattern (lattice constant along the chain axis of 1 is
0.01 Å shorter than that of 2). g At 100 K, see ref. 17b. h At room temperature, see ref. 23. i At 93 K, see ref. 23. j Calculated from the lattice
constant along the chain axis and Pt–Pt distance at 91 K, see ref. 24. k At 48 K, see ref. 14f.
same ab plane, named “synchronized” packing17c (e.g. the
structure of 1·4H2O is shown in Fig. 4). Therefore, an iodide
ion, two K+ ions and four H2O molecules (if hydrated) per
formula are located in the same layer, which has a positive
charge in average. On the other hand, the other layer consist-
ing of a [Pt2(pop)4]3− unit and an aliphatic diammonium ion
per formula has a negative charge in average. In other words,
the iodide ions and the Pt ions are in the positively charged
and negatively charged layer, respectively (Fig. 4). Although the
Pt ions along the c axis (chain axis) mainly contribute to the
EFG at the iodine nucleus, the arrangement of the charged
layers should decrease the EFG, resulting in a smaller QCC
Fig. 3 129I Mössbauer spectra of 2·4H2O and 2 measured at 14 K. The
red and blue solid lines indicate the Ia and Ib components, respectively.
The black solid lines were fitted with the data.
appears by the dehydration; therefore, the change of the 129I
Mössbauer spectra from 2·4H2O to 2 supports the change of
the electronic state from the ACP + CDW state to the CDW
state.
Reliability of ρQCC in 1·4H2O, 2·4H2O and their dehydrated
states
It has been reported that the arrangement of cations and
anions around the iodide ions occasionally affects the QCC
19b
value and induces inaccurate ρQCC
.
In 1·4H2O, 1, 2·4H2O
and 2, the ρQCC is significantly larger than the ρIS as shown in
Table 1. This difference is attributable to the characteristic
molecular packing in these complexes. The three-dimensional
hydrogen-bond and coordination-bond networks in present
Fig. 4 Schematic representation of the charged layer in 1·4H2O. H2O
molecules and hydrogen atoms are omitted for clarity. Iodide ions, K+
2+
ions and cis-H3NCH2CHvCHCH2NH3 ions are disordered. Pt yellow,
complexes constrain each [Pt2(pop)4] unit to arrange in the I purple, K brown, P orange, O red, N blue, C black.
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and a larger ρQCC observed in the present complexes. Conse- iodide ions to Pt ions. The short d(Pt–I–Pt) in MMX chains
quently, it can be concluded that the ρQCC is not appropriate with binary countercations originates from the coordination
to discuss the quantitative value of the iodine charge in the bonds between K+ and oxygen atoms of pop ligands.17c The
pop system. This high susceptibility of QCC to the surrounding calculated small ρIS values in the present complexes are valid
charges is also supported by the relationship between QCC even under the predominance of the covalent interaction as
and IS as discussed in the ESI† and Fig. S1 therein. Therefore described in the ESI.†
we focus on the ρIS from now on.
Dependence of the iodine charge on the Pt–I–Pt distance
Iodine charge of 1·4H2O, 2·4H2O and their dehydrated states
As discussed above, smaller ρIS values observed in 1·4H2O,
As shown in Table 1, the ρIS values of 1, 1·4H2O, and 2 are
2·4H2O and their dehydrated states indicate that the covalent
−0.13(2), −0.14(2), and −0.14(2), respectively, which are the
interaction is stronger in MMX chains with binary counter-
smallest of all MMX chains. So far, the smallest values of ρIS
cations. It is more clearly expressed by the relationship
are −0.27 for [Pt2(MeCS2)4I] in the dta system14d,19a and
between ρIS and d(Pt–I–Pt) as shown in Fig. 5. Kobayashi et al.
−0.33(5) for (H3NC6H12NH3)2[Pt2(pop)4I] in the pop system.19b
reported that the ρIS of the discrete Pt(III)2 complex in the pop
Accordingly, the negative charge on the iodide ion is strongly
donated to the Pt ion in the case of 1·4H2O, 1 and 2. This
system, K4[Pt2(pop)4I2], is −0.54(4).19b This negative charge is
donated to the other Pt ion at the opposite side when the
result corresponds to the previous report that the covalent
iodide ion bridges two [Pt2(pop)4] units in the 1D chain. There-
interaction is more dominant in the CDW state compared with
fore, the decrease of the d(Pt–I–Pt) increases the charge
the Coulomb interaction.19b
donation from iodide ions to Pt ions, resulting in smaller ρIS.
The two chemically independent iodide ions in 2·4H2O are
The strength of the covalent interaction determines the elec-
denoted by Ia and Ib. The Ia is located between higher oxidized
tronic states of the MMX chains in the pop system.
Pt ions with shorter Pt–I–Pt distance (d(Pt–I–Pt)), and the Ib is
located between lower oxidized Pt ions with longer d(Pt–I–Pt).
On the other hand, the difference in ρIS between Ia and Ib is
relatively large in the dta system, though the difference in
The smaller ρIS of −0.21(2) can be assigned as the value
d(Pt–I–Pt) is very small or smaller than the resolution limit of
derived from the Ia in 2·4H2O, because the environment
the single crystal X-ray structural analysis. Thus, there is no
around the iodide ion in 2 (e.g. ρIS = −0.14(2), d(Pt–I–Pt) =
5.689(2) Å) is close to that around Ia in 2·4H2O (e.g. d(Pt–Ia–Pt) =
5.6305(8) Å) rather than that around Ib in 2·4H2O (e.g. d(Pt–Ib–
Pt) = 5.9786(9) Å). Thus, the ρIS of Ib in 2·4H2O is −0.40(2).
relationship between ρIS and d(Pt–I–Pt) in the dta system,
which is consistent with the previous report that the Coulomb
interaction is more dominant in the ACP state in the dta
system.19b This difference between the pop and the dta system
This assignment is in contrast to the previous results that Ia is
is probably related to the charge of each ligand and the pres-
more negative than Ib, observed in [Pt2(MeCS2)4I]14d,19a and
ence or absence of countercations. Since the monovalent
[Pt2(EtCS2)4I],19b which forms the ACP state. In the point-
anionic ligand (RCS2−) and the neutral 1D chain in the dta
charge model, the short d(Pt–I–Pt) destabilizes the 5dz2 orbi-
tals of Pt ions and stabilizes the 5pz orbital of iodide ions.
system produce a 1D electron system without perturbation,
various electronic states are observed in the same compound
Therefore, the more negative iodide ions and more positive Pt
in the dta system (e.g. the AV state above 300 K, the CP
ions are preferred in shorter d(Pt–I–Pt) if the Coulomb inter-
state from 300 K to 80 K, and the ACP state below 80 K for
action is dominant. On the other hand, the overlap integral
between these orbitals should be larger and the charge
donation from the iodide ion to Pt ions is enhanced in shorter
d(Pt–I–Pt) if the covalent interaction is dominant. Since Ia is
more positive than Ib in 2·4H2O, it can be concluded that the
covalent interaction predominates in the ACP + CDW state as
well as the CDW state,19b whereas the Coulomb interaction pre-
dominates in the ACP state. This is also supported by the
reported optical conductivity spectra, which reveal that the
electronic structure in the ACP + CDW state is similar to that
in the CDW state.17c Consequently, the ACP + CDW state can
be regarded as the CDW state with the ACP-type lattice
distortion.
The origin of the opposite result is probably the quite short
d(Pt–I–Pt) in 2·4H2O. The d(Pt–Ia–Pt) in 2·4H2O (5.6305(8) Å) is
0.23 Å shorter than that in [Pt2(EtCS2)4I] (5.860(2) Å).19b In
addition, the shortest Pt–I distance (d(Pt–I)) in 2·4H2O
(2.641(3) Å; Fig. 1) is 0.29 Å shorter than that in [Pt2(EtCS2)4I]
(2.93 Å). Consequently, short d(Pt–I–Pt) and d(Pt–I) enhance
the overlap between the orbitals and charge donation from extreme value of ρIS in MMX chains (d(Pt–I–Pt) = ∞).
Fig. 5 Relationship between d(Pt–I–Pt) and ρIS of MMX chains in the
pop system shown in Table 1. Dashed line indicates the ρIS of Pt(III)2
complex, K4[Pt2(pop)4I2], (ρIS = −0.54(4)).19b It can be regarded as the
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[Pt2(MeCS2)4I]14d). This multistability is intrinsic to the 1D
electron system. In contrast, the anionic 1D chain in the pop
Acknowledgements
system requires countercations, which induce the short d(Pt–I– This work was partly supported by a Grant-in-Aid for Scientific
Pt) and strong covalent interaction and thus suppress the Research (S) (grant no. 20225003) from the Ministry of Edu-
appearance of the ACP state in 1·4H2O, 2·4H2O and dehydrated cation, Culture, Sports, Science, and Technology, Japan (M. Y.),
complexes. Furthermore, divalent anionic ligands (pop2−) and by JSPS Research Fellowships for Young Scientists (H. I.), and
countercations can be regarded as the periodic potentials, by the Tohoku University Institute for International Advanced
which interfere with the Coulomb interaction between Pt ions Research and Education (H. I.). This work has been carried out
and iodide ions and thus bring about the dominance of the in part under the Visiting Researchers Program of Kyoto Uni-
covalent interaction. Therefore, it can be concluded that the versity Research Reactor Institute.
covalent interaction plays a crucial role in the pop system and
the Coulomb interaction plays a crucial role in the dta system.
Notes and references
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The electronic states of water-vapor-responsive MMX chains in
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formal charge of Pt ions can be represented as follows:
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⋯[Pt2+–Pt2+]⋯⋯I0.14−–[Pt3+–Pt3+]–I0.14−⋯ for the CDW state in
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