Bromide-bridged palladium(iii) chain complexes showing charge bistability near room temperature

Article abstract of DOI:10.1039/c4cc02547d

We synthesized and characterized bromide-bridged Pd(iii) chain complexes, [Pd(en)₂Br](MalC_n-Y)₂·H₂O (en = ethylenediamine; MalC_n-Y = dialkyl sulfomalonate; n: the number of carbon atoms) (n = 7 and 12

Full text of DOI:10.1039/c4cc02547d

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Bromide-bridged palladium(III) chain complexes
showing charge bistability near room temperature†
Cite this: Chem. Commun., 2014,
0, 8382
5
ab
a
a
c
Shohei Kumagai, Shinya Takaishi,* Brian K. Breedlove, Hiroshi Okamoto,
d
d
abe
Received 7th April 2014,
Accepted 5th June 2014
Hisaaki Tanaka, Shin-ichi Kuroda and Masahiro Yamashita*
DOI: 10.1039/c4cc02547d
www.rsc.org/chemcomm
We synthesized and characterized bromide-bridged Pd(III) chain
complexes, [Pd(en) Br](MalC –Y) (en ethylenediamine;
ÁH
MalC –Y = dialkyl sulfomalonate; n: the number of carbon atoms)
n = 7 and 12). The compound with n = 7 showed charge-bistability
2
n
2
2
O
=
n
(
near room temperature. In addition, it is shown that the Pd(III) state
is maintained in the thin film state.
À À
n n
Scheme 1 Chemical structure of (a) SucC –Y and (b) MalC –Y .
Bistable molecules, in which two stable states can be switched
by external stimuli, such as temperature, light, electric and
magnetic fields, and pressure, are promising for a variety of integrals (T) compete with each other. In particular, the com-
1a–d
1e–f
1g
applications, such as memory devices,
sensors,
actuators,
petition between S and U is the main factor that determines the
etc. Furthermore, condensed molecular solids showing cooperative electronic state. Thus, all Ni complexes should be in AV states,
bistability can exhibit gigantic stimulus response, although such whereas Pd and Pt complexes should be in MV states due to the U
3
4
materials are rare. Quasi-one-dimensional (Quasi-1D) halogen- values of each M ion (UNi E 6.0 eV; UPd E 1.5 eV; UPt E 1.0 eV,
bridged metal complexes (MX complexes) are aggregated bistable ref. 2). Although MX complexes have the potential for charge
materials, which can show charge bistability, such as the averaged bistability, until recently, there were no reports of a material
valence (AV) or the Mott-insulating state and the mixed valence showing both states. However, our research group have recently
(
MV) or the charge-density-wave state. MX complexes have 1D reported a series of Pd–Br complexes, [Pd(en) Br](SucC –Y) ÁH O
2
n
2
2
2
electron systems composed of dz orbitals of the metal ions (M = (Suc-Cn; en = ethylenediamine; SucC
Ni, Pd and Pt) and the p orbitals of bridging halides (X = Cl, Br (Scheme 1a); n: the number of carbon atoms), that show charge
and I), forming an MV state, represented as –M –XÁÁÁM ÁÁÁX–M –, bistability due to the introduction of alkyl chains as their counter-
n
–Y = dialkyl sulfosuccinate
z
IV
III
II
III
IV
III
5
or an AV state, represented as –M –X–M –X–M –. These ions. For example, Suc-C5 undergoes an MV-to-AV phase transi-
2
electronic states are thought to be Peierls–Hubbard systems,
C
tion at a critical temperature (T ) of 206 K.
in which electron–lattice interactions (S), on- and inter-site
With MX complexes, it should be possible to optically switch
Coulomb repulsions (U and V, respectively), and transfer the electronic states via charge-transfer (CT) transitions from X to M
or from M to a neighboring M. In fact, optical switching, such as
6
7
a
MV-to-AV conversion, and AV-to-metal one and gigantic third-order
Department of Chemistry, Graduate School of Science, Tohoku University,
-3 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan.
8
6
optical susceptibility have been reported for Pd–Br and Ni–Br
E-mail: yamasita@agnus.chem.tohoku.ac.jp; Tel: +81-22-795-6544
WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira,
Aoba-ku, Sendai 980-8577, Japan
complexes, respectively. Most recently, we have reported that
b
c
Suc-C5 undergoes an excitation-photon-energy-selective conver-
9
sion from an AV to a metal or an MV state at low temperature.
Department of Advanced Material Science, Graduate School of Frontier Sciences,
The University of Tokyo, Kashiwa 277-8561, Japan
However, for practical applications, the material has to be in an
AV state at room temperature (RT). In other words, TC should
be near or greater than RT. In the present study, we designed a
d
Department of Applied Physics, Graduate School of Engineering,
Nagoya University, Furocho, Chikusa-ku, Nagoya 464-8603, Japan
CREST, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Electronic supplementary information (ESI) available: VT ESR, Raman, reflec-
e
new counterion, MalC
to control T and prepared a new series of Pd–Br complexes
Pd(en) Br](MalC –Y) O (Mal-Cn). Mal-C7 was shown to be
n
–Y (dialkyl sulfomalonate) (Scheme 1b),
†
C
tivity and optical conductivity spectra, electrical resistivity and photographs of
thin films. CCDC 989698. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc02547d
[
2
n
2
ÁH
2
in an AV state at RT by using X-ray, magnetic, electric and
8
382 | Chem. Commun., 2014, 50, 8382--8384
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m
Fig. 2 Temperature dependence of (a) w and (b) r for Mal-C7.
À6
À6
À1
wm drastically changed between 5 Â 10 and 1 Â 10 esu mol
at 295 and 330 K, respectively. This result indicated that the
Fig. 1 (a) Crystal structure of Mal-C7 (293 K). Hydrogen atoms and water electronic state changed from AV to MV states with an increase in
molecules are omitted for clarity. Green, Pd; brown, Br; blue, N; yellow, S; temperature with T
E 310 K. Furthermore, w gradually decreased
C
m
red, O; gray, C. Blue dotted lines indicate the unit cell. (b) Crystal packing.
c) Schematic illustration of a 1D chain motif.
with a decrease in temperature below 200 K, which was probably
due to a spin-Peierls transition, which has been observed
(
1
0
2 2
for [Ni(chxn) Br]Br (chxn = 1R,2R-diaminocyclohexane) and
1
1
12
3
spectroscopic analyses. In addition, we were able to perform other materials, such as organic conductors and CuGeO .
single-crystal X-ray diffraction analysis on Mal-C7, resulting in This phenomenon is currently being study.
the first crystal structure of a Pd(III) complex in an AV state at
Next, we conducted electrical conductivity measurements. In
RT. Finally, we prepared a thin film for use in an optical device. Fig. 2b, the temperature dependence of the electrical resistivity
A crystal structure of Mal-C7 at RT is shown in Fig. 1a. Mal-C7 (r) along the a axis (1D chain direction) is shown. The r value
has a 1D chain structure of infinitely alternating Pd and Br ions. was ca. 10 MO cm at 100 K, and semiconducting behaviour was
The 1D chain structure is cooperatively maintained by N–HÁÁÁO observed when temperature was increased. Then r started to
hydrogen bonds among the en ligands and oxygen atoms of the increase at 310 K, indicating a conversion from AV to MV states.
sulfonate groups of counterions. In addition, water molecules The value of r continued to increase steeply until the temperature
contribute to hydrogen bonding and support of the 1D chain reached 350 K. Such behavior has also been observed for Suc-C5,
structure (water molecules are omitted for clarity in Fig. 1). As suggesting a charge fluctuation due to a AV–MV conversion
6
shown in Fig. 1b, Mal-C7 has a lamellar structure, in which the accompanied by CT. Therefore, at RT, Mal-C7 is in an AV state.
hydrophilic 1D space is separated from the hydrophobic space Differed from the ESR study, the electrical properties during
by counterions. These structural features closely resemble the heating and cooling processes did not correlate with each
5
those of Suc-C5.
other (see ESI†), most likely due to deterioration of the crystal
In 1D MX chains, the position of bridging halides is strongly surface at high temperature. Thus, we cannot discuss the
related to the electronic state. In other words, if each bridging differences in r during heating and cooling.
halide is located at the midpoint between neighboring M ions,
Polarized Raman and IR spectra were acquired to obtain further
the MX complexes are in AV states. In Mal-C7, the Pd–Br bond information about the electronic state. In Raman spectroscopy, the
IV
IV
lengths were determined to be 2.6001(18) and 2.6065(18) Å Br–Pd –Br symmetrical stretching mode (n(Br–Pd –Br)) is allowed
Fig. 1c), indicating that all bridging Br ions were located at the in an MV state, whereas it is forbidden in an AV state. A Raman peak
(
À1
IV
midpoints of neighboring Pd ions. Furthermore, the neighbor- at ca. 125 cm assigned to n(Br–Pd –Br) gradually appeared above
ing PdÁ Á ÁPd distance was 5.207(3) Å, which was shorter than 310 K (see ESI†). In addition, in IR spectroscopy, the vibrational
5
.26 Å which was the boundary between the MV and AV states energy of the N–H symmetrical stretching mode (n(N–H)) of the
5
reported previously. Thus, Mal-C7 was structurally expected to be in in-plane ligands depends on the valence of the metal ion to which it
an AV state at RT. Furthermore, while we could not succeed in is coordinated. In other words, two kinds of n(N–H) corresponding
determination of the crystal structure at high temperature, the diffuse to Pd and Pd are observed in an MV state, whereas only one
scattering arising from a disorder or a distortion of bridging X was n(N–H) assigned to Pd is observed in an AV state. For Mal-C7,
observed in the X-ray oscillation patterns above 308 K, implying that n(N–H) were observed in the range of 3080–3150 cm (see ESI†).
Mal-C7 converted from an AV state to an MV state (see ESI†).
13
II
IV
III
À1
À1
Below 310 K, only one peak at 3115 cm was observed. And this peak
In order to confirm that Mal-C7 was in an AV state at RT, and split into two peaks at 3100 and 3130 cm above 310 K. In other
À1
that it was in an MV state at high temperature, variable-temperature words, the electronic state changed from an AV to an MV state.
(VT) electron spin resonance (ESR) spectroscopy was performed.
Finally, the temperature dependence of the CT energy (ECT),
Because AV and MV states should display paramagnetic (antiferro- which was estimated from polarized reflectivity spectra and
magnetic) and diamagnetic behaviors, respectively, the magnetic optical conductivity spectra (see ESI†), is shown in Fig. 3a. ECT
susceptibilities should be different for both states. VT ESR spectra for Mal-C7 was ca. 0.6 eV below RT. This value and the
are shown in ESI.† The temperature dependence of the spin temperature dependence are consistent with those for Suc-C5
5
susceptibility (w
m
) obtained by integrating the first derivative in an AV state. In the AV state, the lowest electronic transition
of the ESR signal twice is shown in Fig. 2a. Near 310 K, was assigned to a CT transition from the lower Hubbard band
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which was in an AV state at RT. The electronic ground state
was characterized by using both structural and spectroscopic
studies. To the best of our knowledge, for the first time, we
determined the crystal structure of a Pd(III) chain complex in an
AV state at RT. In addition, we prepared a thin film of Mal-C12
for use in practical applications and are currently studying its
optical switching abilities.
This work was partly supported by a Grant-in-Aid for Creative
Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology.
Fig. 3 (a) Temperature dependence of ECT for Mal-C7. (b) Schematic
illustration of the band structures of Pd–Br AV (left) and MV (right) states.
Notes and references
(
c) Optical conductivity spectrum of Mal-C7 (black line) and UV-vis-NIR
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2
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4
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CT
dent in an MV state. Thus, on the bases of these results, we
concluded that Mal-C7 was in an AV state at RT.
To test the possibility for practical applications, we fabricated a
thin film of Mal-Cn and studied its optical absorption properties.
Herein, for simplification, we call an aggregate of nanocrystals
deposited on a transparent substrate a ‘‘thin film’’. In the case of
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thin films: (1) vacuum deposition methods are unavailable, and
6
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(
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8
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14
in-plane ligands or counterions due to the hydrophobic nature of
the alkyl chains. We believed that Mal-Cn was suitable for fabricat-
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hydrophobic counterions. By elongation of alkyl chains, a thin film
of Mal-C12 (TF-C12) was actually prepared using poly(methyl
methacrylate) (PMMA) as a matrix polymer (see ESI†). In Fig. 3c,
photon-energy dependence of absorbance for several Pd com-
pounds is shown. Single crystals of Mal-C7 (black line), TF-C12
9
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(red line) and KBr pellets of Mal-C7 and Mal-C12 (green and blue
lines, respectively) displayed a peak at 0.6 eV, indicating their
electronic states of AV, whereas the KBr pellet of Suc-C5 (purple
line) which was in an MV state displayed a peak at 1.0 eV. Hence,
Mal-C12 is in an AV state even in the thin film state, and thus
TF-C12 is promising for optical switching.
1
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In summary, we designed a new counterion and used it to
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384 | Chem. Commun., 2014, 50, 8382--8384
This journal is ©The Royal Society of Chemistry 2014