DOI: 10.1002/slct.201600065

Communicationsz Inorganic Chemistry

Optically Visible Phase Separation between Mott-Hubbard
and Charge-Density-Wave Domains in a Pd-Br Chain
Complex
Takefumi Yoshida,[a] Shinya Takaishi,*[a] Hiroaki Iguchi,[a] Hiroshi Okamoto,[b] Hisaaki Tanaka,[c]Shin-ichi Kuroda,[c] Yuka Hosomi,[d] Shoji Yoshida,[d] Hidemi Shigekawa,[d] Tatsuhiro Kojima,[e]Hiroyoshi Ohtsu,[e] Masaki Kawano,[e] Brian K. Breedlove,[a] Laurent Guérin,[f] andMasahiro Yamashita*[a]We synthesized a bromo-bridged Pd complex, which underwent as lowering the temperature a charge-density-wave(CDW) to Mott-Hubbard (MH) phase transition, as confirmed byusing X-ray diffraction analysis, ESR, FT-IR and Raman spectroscopies. The complex showed a phase-separation phenomenon
with a striped pattern over a wide temperature range, and domain propagation was observed using an optical microscope.
Moreover, we observed changes in the local electronic state accompanying the phase transition by using scanning tunnelingmicroscopy (STM)

Charge bistability in solid-state complexes is a key property for
functional materials. There has been great effort put into
switching the properties, such as charge-transfer phase transitions (CTPT), by using optical,[1] thermal,[2] and electrical[3]stimuli. However, there are still many challenges in designing
materials which undergo CTPT by controlling molecular charge[a] T. Yoshida, Dr. S. Takaishi, Dr. H. Iguchi, Dr. B. K. Breedlove,
Prof. M. Yamashita
Department of Chemistry, Graduate School of Science, Tohoku University,
6-3 Aza-aoba, Aramaki, Sendai 980-8578, Japanand
WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1
Katahira, Sendai 980-8577, Japan
E-mail: takaishi@mail.tains.tohoku.ac.jp
yamasita@agnus.chem.tohoku.ac.jp[b] Prof. H. Okamoto
Department of Advanced Materials Science, Graduate School of Frontier
Sciences, The University of Tokyo, Kashiwa 277-8561, Japan[c] Dr. H. Tanaka, Prof. S.-i. Kuroda
Department of Applied Physics, Graduate School of Engineering, Nagoya
University, Furocho, Chikusa-ku, Nagoya 464-8603, Japan[d] Y. Hosomi, Dr. S. Yoshida, Prof. H. Shigekawa
Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 3058573, Japan[e] Dr. T. Kojima, Dr. H. Ohtsu, Prof. M. Kawano
Division of Advanced Materials Science, Pohang University of Science andTechnology (POSTECH), RIST Building 3-3390, 77 Cheongam-Ro, Nam-Gu,
Pohang, Gyeongbuk 790-784, South Korea[f] Dr. L. Gurin
Universit de Rennes 1, Institut de Physique de Rennes, 263 Av. du Gnral Leclerc, 35042 Rennes cedex, France
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/slct.201600065

ChemistrySelect 2016, 2, 259 – 263

bistability and its cooperativity. Quasi-one-dimensional halogen-bridged metal complexes are good candidates for thesetypes of materials because they possess charge bistability
based on the oxidation states of the metal ions as well as highly isolated one-dimensional (1D) electronic system composedof dz2 orbital of metal ions (M) and pz orbital of bridging halideions (X). In other words, they can exhibit averaged valenceMott-Hubbard (MH) and mixed valence charge-density-wave(CDW) states. Ni complexes are primarily in an MH state with a–X–MIII–X–MIII–X– linear chain structure, in which bridging halides locate at the midpoint between the neighboring metal
ions,[4] thus they are classified as Robin-Day class III complexes.[5] They show marked physical properties due to strong
electron correlation of the NiIII states.[6] On the other hand, Pd
and Pt complexes are known to be in a CDW state(–X···MII···X–MIV–X···), in which bridging halides displaced fromthe midpoint, and are classified Robin-Day class II type complexes.[5] They show characteristic mixed valence state properties, such as luminescence with large Stokes shifts,[7] overtone
progressions in the resonant Raman spectra,[8] and intervalencecharge transfer transitions.[9] Although over 300 complexes
have so far been reported since Wolfram reported the firstchloro-bridged Pt complex in 1990,[10] no exceptions were reported before our recent research.
Recently, we have reported the first example of a Pd complex, [Pd(en)2Br](C5-Y)2·H2O (en = ethylenediamine; C5-Y = dipentylsulfocuccinate), exhibiting CTPT between CDW and MH
states at 205 K.[11] This CTPT was related to large temperaturedependence in Pd···Pd distance (from 5.31 (300 K) to 5.21 (100 K)). The PdIII MH state was shown to be stabilized by short
Pd···Pd distance and resultant single well potential of the
brdging Br ion (Figure 1(b)).[11c] The CTPT was classified into
the first-order transition with a hysteresis, because this phasetransition is accompanied by large structural change (e. g.
Pd···Pd distance jumps by ca. 0.03 ). This is probably because
the phase transition is coupled with the alkyl chain dynamics.
One of our goals is to explore the boundary region between
CDW state with double well potential (Figure 1a) and MH statewith single well potential (Figure 1b). From this point of view,[Pd(en)2Br](C5-Y)2·H2O is not suitable because the first orderphase transition is accompanied by large structural changewith the formation of a 3D ordering of the CDW state, there259

 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications

Figure 1. Schematic illustration of potential curve of brdging Br ions in (a) a
CDW state and (b) an MH state. Blue and red dotted lines represent the potential curves attributable to PdIV-Br or PdIII-Br sub-structures. Black solid lines
express the potential curves of Pd-Br-Pd structure (sum of the red and blue
dotted lines).

fore limiting the domain of coexistence of the two phases. In
order to explore the boundary region, in this work, we developed a new bromo-bridged Pd complex, [Pd(cptn)2Br]Br2 (1),(cptn = 1R,2R-diaminocyclopentane) without alkyl chains tomake crystalline lattice rigid, which could afford less structuraldifference between two phases. We found that 1 showed avery peculiar CDW-MH CTPT as well as coexistence of bothphases in a single crystal with a stripe patterned phase separation over a wide temperature range, which could be observedwith an optical microscope. Here we report its crystal structure,
its physical properties including the phase separation phenomenon and the local electronic state visualized by STM.
The crystal structure and crystallographic data of 1 at 93 K
are shown in Figure 2 and Table S1, respectively. CCDC 1449578
contains the supplementary crystallographic data of 1. For
comparison, we re-analyzed the crystal structure of [Pd(chxn)2Br]Br2 (chxn = 1R,2R-diaminocyclohexane), which has reported
to be in a CDW state,[12] at 93 K. The Pd ion was found to havean elongated octahedral geometry. Square planar Pd(cptn)2
moieties are bridged by Br– anions, forming a linear chain along
a-axis. These linear chains and free Br– counteranions are connected by N–H···Br···H–N hydrogen bonds along b-axis to forma two-dimensional sheet (Figure 2a). On the other hand, there
are no chemical bonds other than van der Waals interaction between the sheets (Figure 2c). The structural features of 1, including space group (I222), are similar to those of [Pd(chxn)2Br]
Br2. However, there are several subtle differences as described
below. First, the Pd···Pd distances along the 1D chain (5.2223(4) at 93 K) are slightly shorter than those of [Pd(chxn)2Br]Br2(5.2784(4)  at 93 K). Temperature dependency of Pd···Pd distances in both compounds are plotted in Figure S1. In a previous report, we have proposed that the Pd···Pd distances areimportant for determining the electronic state of the Pd complexes. In the case of bromo-bridged Pd complexes, we haveproposed that MH and CDW states occur when the Pd···Pd distances are shorter and longer than ca. 5.26 , respectively. In
fact, [Pd(chxn)2Br]Br2 with longer Pd···Pd distances than 5.26 
is known to have CDW state at any temperature. On the other
hand, Pd···Pd distances of 1 at 93 K is shorter than the boundary distance. Thus, we expected that 1 is in an MH state at
ChemistrySelect 2016, 2, 259 – 263

Figure 2. Perspective views of the crystal packings along c-axis (a and b) anda-axis (c and d), and ORTEP drawings with thermal ellipsoids at 50 % probability in 1 and [Pd(chxn)2Br]Br2, respectively (T = 93 K). Gray; Pd, brown; Br,blue; N, black; C, pink; H. Hydrogen atoms are partly omitted for clarity.

this temperature. The subtle difference (ca. 0.05 ) in Pd···Pddistances between the two compounds could be because cycloalkane rings existing between the 2D sheets in [Pd(chxn)2Br]Br2 sterically more crowded compared to 1 (Figure 2c and d),that makes Pd···Pd distances longer. Hirshfeld surface and fingerprint plots[13] for H···H contact of 1 and [Pd(chxn)2Br]Br2 (Figure S2) suggested that 1 has weaker intermolecular H···H contacts than [Pd(chxn)2Br]Br2 (less red area). This finding supports
the aforementioned hypothesis. Second, the thermal ellipsoidsof the bridging halides of [Pd(chxn)2Br]Br2 were elongatedalong the a-axis, whereas those of 1 were mostly spherical. The
elongated thermal ellipsoids correspond to a disordered atom
at the displaced position from the midpoint due to the double
well potential. Thus, the spherical ones suggest that 1 is in an
MH state at this T. The actual electronic state cannot be determined by conventional X-ray crystal structure analysis. Third,
the thermal ellipsoid of C3 in the cptn ligand is exceptionallylarge due to a flipping motion of the C3 atom. However, therelationship between this flipping motion and electronic stateof 1 has not yet been determined.
In order to determine the electronic states of 1 at various T,
we acquired X-ray diffraction (XRD) photographs (Figure S3). At
220 K, we observed diffuse peaks at h = m + 1/2 (m is an integer), which correspond to a cell doubling along the chain associated to the CDW state. These superlattice peaks were
found to be rod-shaped diffuse scatterings from the reconstructed diffraction image of the reciprocal lattice (Figure S3). These diffuse scatterings are due to the long range cor260

 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications
relation of CDW phase along a and b-axis and very small correlation along c-axis. Therefore, the structural state of 1 at 220 K
consists of a stacking of almost uncorrelated 2D CDW longrange order sheets. The intensity of the diffuse scatterings
gradually weakened upon cooling and mostly disappeared at
100 K, indicating that the electronic state of 1 is changing from
CDW to MH state gradually. This is in contrast to the case of [Pd(en)2Br](C5-Y)·H2O, which showed abrupt transition from the 3D
CDW to the MH state signed by the disappearance of the superlattice Bragg peaks at Tc = 205 K.[11a] In the spin susceptibility
measurements, phase transition phenomena between diamagnetic CDW state and paramagnetic MH state was observed
but its change is very gradual compared to [Pd(en)2Br](C5-Y)·H2
O (see Figure S4). Once again, this gradual change of the spin
susceptibility in 1 can be explained by the 2D character of thetransition as opposed to the 3D ordering of the CDW state in[Pd(en)2Br](C5-Y)·H2O.
Optical images of a single crystal of 1 at various T are
shown in Figure 3 and S5. This sample had a reddish luster atsult clearly indicates that some phase separation is occurring in
1 in a wide T range.
In order to determine the electronic state of each domain,
we acquired Raman spectra of 1 at various T using Raman microscope (beam size  1 mm). It has been established that Br
PdBr symmetrical stretching mode is Raman active and observed at around 130 cm–1 in CDW states,[11a] whereas it is inactive in MH states. As shown in Figure 4, clear Raman peaks

Figure 4. Raman spectra of 1 at various T.

Figure 3. Optical images of the single crystal of 1 at (a) 130 K, (b) 120 K, (c)100 K and (d) 50 K, and (e) optical reflectivity spectra (E // chain). Inset shows
optical conductivity spectra of 1 obtained by Kramers-Kronig transformationof the optical reflectivity spectra.

room T. The color did not change even after cooling to 130 K(Figure 3(a)). Below 130 K, on the other hand, a yellowish domain appeared and showed a striped pattern parallel to the (11 0) or (–1 1 0) directions. At 50 K, most of the crystal had ayellowish luster. Although texture of domain propagation depended on the sample batches, the change in color could be
repeated with the same crystal by cooling and heating. This reChemistrySelect 2016, 2, 259 – 263

were observed at ~ 90 and 130 cm–1 above 150 K. On the basisof a previous study, we assigned the latter peak to the BrPd
Br symmetrical stretching mode, although the former could not
be assigned at present. Below 100 K, both peaks disappeared.
In other words, 1 underwent a phase transition to an MH state
below 100 K. At 125 K, Raman spectra for the reddish and yellowish domains were acquired. For the reddish domain, a spectrum was characteristic of high temperature (CDW) phase,whereas the spectrum for the yellowish domain was corresponding to an MH state. Thus, we concluded 1 showed phaseseparation phenomenon between CDW and MH phases at wide
T range.
Besides XRD and Raman spectroscopy, FT-IR spectroscopy
can be used to determine the electronic state of MX complexes
because the NH symmetrical stretching mode (n(NH)) shows
single peak in an MH state and double peaks in a CDW state.[14]We measured the FT-IR spectra of a single crystal of 1 using an
FT-IR microscope (aperture size = 130 3 130 mm) at 100 K
where the phase separation occurs. FT-IR spectra of single crystal at different position (area 1 and area 2 in inset of Figure 5)of 1 at 100 K are shown in Figure 5. As can be seen, area 1 ismostly composed of yellowish domain whereas area 2 is mostly
composed of reddish domain. The FT-IR spectra showed single
and double peak at n(NH) region at area 1 and area 2, respectively. This result is another evidence of the charge separation phenomenon between CDW and MH domains.
STM is a powerful technique for visualizing the local electronic structure. In the previous paper, we have reported the
STM image for isostructural [M(chxn)2Br]Br2 (M = Ni and Pd).[15]
Ni and Pd compounds afforded bright spots approximately every 5  and 10  along the chain, respectively, which reflects
the periodicity of the charge arrangements in MH and CDW
261

 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications

Figure 5. FT-IR spectra (E ? chain) on the different sample position at 100 K.

states. In this study, we cleaved the single crystal of 1 using adhesive tape so as to expose flat surface of ab plane after
mounted to the sample stage with carbon paste. STM imagesof 1 in the ab plane are shown in Figure 6. At 114 K, brightspots were observed approximately every 10  along the chain,indicating that 1 was in a CDW state at this T. At 106 K, on theother hand, the bright spots were observed approximately every ~ 5  along the chain, confirming that 1 was in an MH stateat this T. Because these images were acquired with positive
sample bias, bright spots represents tunnel current from theFermi energy of the tip to the conduction band of the sample,
which are the upper Hubbard dz2 band of Pd3 + for MH stateand dz2 band of Pd4 + for CDW state. To the best our knowledge, this is the first observation of a change in the local electronic state with molecular spatial resolution, accompanied bya CTPT, using STM.
In conclusion, we synthesized the bromo-bridged Pd complex [Pd(cptn)2Br]Br2. This complex showed CDW-to-MH CTPT
accompanied by phase separation phenomena over a wide
temperature range, which was confirmed by using X-ray diffraction, ESR, Raman, optical reflectivity, and FT-IR spectroscopies. In addition, we determined the local electronic state byusing temperature-dependent STM. To the best of our knowledge, this is the first observation to detect the change in local
electronic state accompanied by a CTPT with molecular spatial
resolution. The mechanism for the phase separation phenomenon is currently under investigation.

Figure 6. STM topographic images of 1 at (a) 114 K and (c) 106 K. (b) and (d)shows height profiles on the black lines drawn on the images (a) and (c),respectively.

Supporting Information

Acknowledgements

Experimental Section, Crystallographic parameters(Table S1),
Temperature dependence of PdPd distance (Figure S1), Hirshfeld surface plots (Figure S2), X-ray reconstructed photographs(Figure S3), ESR spectra (Figure S4), and optical images (Figure S5) of 1.

This work was partly supported by a JSPS KAKENHI grant (A)26248015, a Grant-in-Aid for Young Scientist (B) 25810032, andCREST, JST.

ChemistrySelect 2016, 2, 259 – 263

Keywords: charge-transfer
phasetransition·
phase
separation · local electronic states · charge bistability · quasione-dimensional halogen-bridged metal complex

262

 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications[1] a) S. Koshihara, Y. Tokura, T. Mitani, G. Saito, T. Koda, Phys. Rev. B 1990,
42, 6853–6856; b) S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida,
H. Okamoto, Y. Tokura, Phys. Rev. Lett. 2003, 91, 057401; c) H. Uemura,
H. Okamoto, Phys. Rev. Lett. 2010, 105, 258302; d) Y. Tokura, J. Phys. Soc.
Jpn. 2006, 75, 011001; e) H. Ichikawa, S. Nozawa, T. Sato, A. Tomita, K.
Ichiyanagi, M. Chollet, L. Guerin, N. Dean, A. Cavalleri, S. Adachi, T. H.
Arima, H. Sawa, Y. Ogimoto, M. Nakamura, R. Tamaki, K. Miyano, S. Y.
Koshihara, Nature Mater. 2011, 10, 101–105. f) M. Dressel, Naturwissenschaften 2007, 94, 527–541.
[2] a) J. B. Torrance, A. Girlando, J. J. Mayerle, J. I. Crowley, V. Y. Lee, P.
Batail, Phys. Rev. Lett. 1981, 47, 1747–1750. b) J. B. Torrance, A. Girlando,
J. J. Mayerle, J. I. Crowley, V. Y. Lee, P. Batail, Phys. Rev. Lett. 1981, 47,
1747–1750. c) J. B. Torrance, J. E. Vazquez, J. J. Mayerle, V. Y. Lee, Phys.
Rev. Lett. 1981, 46, 253–257; d) Y. Iwasa, T. Koda, Y. Tokura, A. Kobayashi, N. Iwasawa, G. Saito, Phys. Rev. B 1990, 42, 2374–2377; e) H. Tokoro, S.-i. Ohkoshi, T. Matsuda, K. Hashimoto, Inorg. Chem. 2004, 43,
5231–5236; f) H. Kitagawa, N. Onodera, T. Sonoyama, M. Yamamoto, T.
Fukawa, T. Mitani, M. Seto, Y. Maeda, J. Am. Chem. Soc. 1999, 121,
10068–10080; g) N. Kojima, W. Aoki, M. Itoi, M. Seto, Y. Kobayashi, Y.
Maeda, Solid State Commun. 2001, 120, 165–170.
[3] a) Y. Iwasa, T. Koda, S. Koshihara, Y. Tokura, N. Iwasawa, G. Saito, Phys.
Rev. B 1989, 39, 10441–10444; b) R. Kumai, Y. Okimoto, Y. Tokura, Science, 1999, 284, 1645–1647; c) F. Sawano, I. Terasaki, H. Mori, T. Mori,
M. Watanabe, N. Ikeda, Y. Nogami, Y. Noda, Nature 2005, 437,
522–524.
[4] K. Toriumi, Y. Wada, T. Mitani, S. Bandow, M. Yamashita, Y. Fujii, J. Am.
Chem. Soc. 1989, 111, 2341–2342.
[5] M. B. Robin, P. Day, Adv. Inorg. Radiochem. 1967, 10, 247–422.
[6] a) H. Kishida, H. Matsuzaki, H. Okamoro, T. Manabe, M. Yamashita, Y.
Taguchi, Y. Tokura, Nature 2000, 405, 929–932; b) S. Takaishi, Y. Tobu, H.
Kitagawa, A. Goto, T. Shimizu, T. Okubo, T. Mitani, R. Ikeda, J. Am.
Chem. Soc. 2004, 126, 1614–1615. c) S. Takaishi, M. Yamashita, H. Matsuzaki, H. Okamoto, H. Tanaka, S. Kuroda, A. Goto, T. Shimizu, T. Takenobu, Y. Iwasa, Chem. Euro. J. 2008, 14, 472–477.

ChemistrySelect 2016, 2, 259 – 263[7] H. Tanino, K. Kobayashi, J. Phys. Soc. Jpn. 1983, 52, 1446–1456.
[8] a) R. J. H. Clark, M. L. Franks, W. R. Trumble, Chem. Phys. Lett. 1976, 41,
287–292. b) R. J. H. Clark, M. Kurmoo, D. N. Mountney, H. Toftlund, J.
Chem. Soc., Dalton Trans. 1982, 1982, 1851–1860; c) R. J. H. Clark, Chem.
Soc. Rev. 1990, 19, 107–131.
[9] a) M. Tanaka, S. Kurita, T. Kojima, Y. Yamada, Chem. Phys. 1984, 91,
257–65; b) Y. Wada, T. Mitani, M. Yamashita, T. Koda, J. Phys. Soc. Jpn.
1985, 54, 3143–3153.
[10] H. Wolfram, Dissertation 1900, Kçnigberg.
[11] a) S. Takaishi, M. Takamura, T. Kajiwara, H. Miyasaka, M. Yamashita, M.
Iwata, H. Matsuzaki, H. Okamoto, H. Tanaka, S.-i. Kuroda, H. Nishikawa, H. Oshio, K. Kato, M. Takata, J. Am. Chem. Soc. 2008, 130,12080–12084; b) S. Kumagai, S. Takaishi, B. K. Breedlove, H. Okamoto,H. Tanaka, S. Kuroda, M. Yamashita, Chem. Commun. 2014, 50,
8382–8384; c) M. Yamashita, S. Takaishi, Chem. Commun. 2010, 46,
4438–4448.
[12] A. Hazell, Acta Cryst. C, 1991, 47, 962–966.
[13] a) M. A. Spackman and D. Jayatilaka, CrystEngComm, 2009, 11, 19–32;b) M.A. Spackman, J. J. McKinnon, CrystEngComm, 2002, 4 ,378-392.
[14] a) K. Okaniwa, H. Okamoto, T. Mitani, K. Toriumi, M. Yamashita, J. Phys.
Soc. Jpn. 1991, 60, 997–1004.
[15] a) S. Takaishi, D. Kawakami, M. Yamashita, M. Sasaki, T. Kajiwara, H.
Miyasaka, K.-i. Sugiura, Y. Wakabayashi, H. Sawa, H. Matsuzaki, H.
Kishida, H. Okamoto, H. Watanabe, H. Tanaka, K. Marumoto, H. Ito, S.i. Kuroda, J. Am. Chem. Soc. 2006, 128, 6420–6425. b) S. Takaishi, H.
Miyasaka, K. Sugiura, M. Yamashita, H. Matsuzaki, H. Kishida, H. Okamoto, H. Tanaka, K. Marumoto, H. Ito, S.-i. Kuroda, T. Takami, Angew.
Chem. Int. Ed. Engl. 2004, 24, 3171–3175.

Submitted: January 27, 2016
Accepted: January 29, 2016263

 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim