One-Dimensional Bromo-Bridged NiIII Complexes [Ni(S,S-bn)2Br]Br2 (S,S-bn=2S,3S-diaminobutane): Synthesis, Physical Properties, and Electrostatic Carrier Doping

Dimensionally Extending from 1D MX-Chain to Ladder and Nanotube Systems: Structural and Electronic Properties

Molecular one-dimensional (1D) electron systems have attracted much attention due to their unique electronic state, physical and chemical properties derived from high-aspect-ratio structures. Among 1D materials, mixed-valence halogen-bridged transition-metal chain complexes (MX-chains) based on coordination assemblies are currently of particular interest because their electronic properties, such as mixed-valence state and band gap, can be controlled by substituting components and varying configurations. In particular, chemistry has recently noted that dimensionally extending MX-chains through organic rung ligands can introduce and modulate electronic coupling of metal atoms between chains, i. e., interchain interactions. In this review, for the first time, we highlight the recent progress on MX systems from the viewpoint of dimensionally extending from 1D chain to ladder and nanotube, mainly involving structural design and electronic properties. Overall, dimensional extension can not only tune the electronic properties of MX-chain, but also build the unique platform for studying transport dynamics in confined space, such as proton conduction. Based on these features, we envision that the MX-chain systems provide valuable insights into deep understanding of 1D electron systems, as well as the potential applications such as nanoelectronics.

Chemical pressure-induced PtIII-I Mott-Hubbard nanowire, [Pt(en)2I](Asp-Cn)2·H2O (13 ≤ n), detected via polarized infrared spectroscopy

The electronic states of iodo-bridged platinum nanowire complexes have been studied using polarized FT-IR spectroscopy. The N-H symmetrical stretching mode was found to be highly sensitive to the electronic states, distinguishing mixed-valence (MV) and averaged-valence (AV) states. The first Pt(III) nanowire complexes have been realized by the chemical pressures of the counter-anions.

Porous Mn2+ Magnet with a Pt-Cl Framework: Correlation between Water Vapor Adsorption/Desorption and Slow Magnetic Relaxation

We report the water adsorption/desorption behavior and dynamic magnetic properties of the Pt-Cl chain complex [{[Pt(en)₂ ][PtCl₂ (en)₂ ]}₃ ][{(MnCl₅ )Cl₃ }₂ ] ⋅ 12H₂ O (1). Upon heating 1 in a vacuum, we obtained the dehydrated form [{[Pt(en)₂ ][PtCl₂ (en)₂ ]}₃ ][{(MnCl₅ )Cl₃ }₂ ] (1DH). The framework structures of 1 and 1DH are identical, and both complexes underwent slow magnetic relaxation. However, the magnetic relaxation times for 1DH were shorter than those for 1, meaning that the dynamic magnetic properties were controlled upon water vapor adsorption/desorption. From detailed analyses of the dynamic magnetic behavior, a phonon-bottleneck effect contributes to the magnetic relaxation processes. We discuss the mechanism for the changes in the magnetic relaxation processes upon dehydration in terms of the heat capacity and thermal conductivity.

Hydrogen Bonding Propagated Phase Separation in Quasi-Epitaxial Single Crystals: A Pd-Br Molecular Insulator

In condensed matter, phase separation is strongly related to ferroelasticity, ferroelectricity, ferromagnetism, electron correlation, and crystallography. These ferroics are important for nano-electronic devices such as non-volatile memory. However, the quantitative information regarding the lattice (atomic) structure at the border of phase separation is unclear in many cases. Thus, to design electronic devices at the molecular level, a quantitative electron-lattice relationship must be established. Herein, we elucidated a Pd^II-Pd^IV/Pd^III-Pd^III phase transition and phase separation mechanism for [Pd(cptn)₂Br]Br₂ (cptn = 1R,2R-diaminocyclopentane), propagated through a hydrogen-bonding network. Although the Pd···Pd distance was used to determine the electronic state, the differences in the Pd···Pd distance and the optical gap between Mott-Hubbard (MH) and charge-density-wave (CDW) states were only 0.012 Å and 0.17 eV, respectively. The N-H···Br···H-N hydrogen-bonding network functioned as a jack, adjusting the structural difference dynamically, and allowing visible ferroelastic phase transition/separation in a fluctuating N₂ gas flow. Additionally, the effect of the phase separation on the spin susceptibility and electrical conductivity were clarified to represent the quasi-epitaxial crystals among CDW-MH states. These results indicate that the phase transitions and separations could be controlled via atomic and molecular level modifications, such as the addition of hydrogen bonding.

Ni(III) Mott-Hubbard-like State Containing High-Spin Ni(II) in a Semiconductive Bromide-Bridged Ni-Chain Compound

Halogen-bridged linear chain metal complexes (MX-Chains) are fascinating compounds that have a quasi-one-dimensional (1D) electronic system. In this study, we synthesized the first Ni-based MX-Chain compound having hydroxy groups, i.e., [Ni(dabdOH)₂Br]Br₂·[Ni(dabdOH_x)₂Br]_0.5·(2-PrOH)_0.25·(MeOH)_0.25 (1·solvent, x = ∼0.6, dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol). Single-crystal X-ray diffraction revealed that the MX-Chains in 1·solvent formed sheets and single-chain structures in the superlattice. It suggested an MH-like state, whereas the polarized reflection and Raman spectra suggested a CDW-like state. Magnetic and electron spin resonance measurements revealed that both high-spin Ni(II) (∼15%) and low-spin Ni(III) (∼85%) sites are present in the chain structures, i.e., the metal sites show mixed valency. Therefore, we concluded that 1·solvent adopts an intermediate state between the MH and CDW states. Moreover, a single crystal of 1·solvent exhibited semiconductive characteristics along the chain direction. This finding represents a new structural and electronic state of 1D electronic systems as well as MX-Chains.

Macro- and atomic-scale observations of a one-dimensional heterojunction in a nickel and palladium nanowire complex

The creation of low-dimensional heterostructures for intelligent devices is a challenging research topic; however, macro- and atomic-scale connections in one-dimensional (1D) electronic systems have not been achieved yet. Herein, we synthesize a heterostructure comprising a 1D Mott insulator [Ni(chxn)₂Br]Br₂ (1; chxn = 1R-2R-diaminocyclohexane) and a 1D Peierls or charge-density-wave insulator [Pd(chxn)₂Br]Br₂ (2) using stepwise electrochemical growth. It can be considered as the first example of electrochemical liquid-phase epitaxy applied to molecular-based heterostructures with a macroscopic scale. Moreover, atomic-resolution scanning tunneling microscopy images reveal a modulation of the electronic state in the heterojunction region with a length of five metal atoms (~ 2.5 nm), that is a direct evidence for the atomic-scale connection of 1 and 2. This is the first time that the heterojunction in the 1D chains has been shown and examined experimentally at macro- and atomic-scale. This study thus serves as proof of concept for heterojunctions in 1D electronic systems.

Low-dimensional heterojunctions are interesting for fundamental studies and applications, but their demonstrations have been limited to planar structures. Here the authors report synthesis and macro- and atomic scale characterization of a one-dimensional heterojunction formed by halogen-bridged metal-organic nanowires.

Thermally Induced Electron-Hole Dissociation Dynamics in Quasi-One-Dimensional Bromo-Bridged Palladium(III) Mott-Insulator [Pd(en)2Br](Suc-Cn)2·H2O (Cn-Y = dialkylsulfosuccinate; n = 5 and 6)

Current-voltage characteristics and dielectric properties were studied on bromo-bridged one-dimensional compounds, [Pd(en)2Br](Suc-C5)2·H2O, exhibiting mixed-valence and averaged valence (MV-AV) phase transition. In the AV phase, a clear nonlinear current-voltage characteristics was...

Surface Ohmic Conductivity on a Mott Insulator Based on a One-dimensional Bromide-bridged Nickel(III) Complex

Ohmic contacts are of critical importance to improve the performance of semiconductor devices as well as those of low-dimensional materials. Halogen-bridged metal complexes (MX-Chains) are fascinating quasi-one-dimensional (1D) electronic materials. However, reports on the electrical characteristics of MX-Chains remain elusive. Herein, we report the electrical characteristics of single crystals of [Ni(chxn)₂ Br]Br₂ (chxn: (1R,2R)-(-)-1,2-cyclohexanediamine), which is a 1D Mott insulator, using different probe arrangements. [Ni(chxn)₂ Br]Br₂ was shown for the first time to exhibit Ohmic characteristics on the crystal surface, albeit that the conductivity behavior of the interior of the crystal is nonlinear due to the Joule heating effect. The current flow on the surface is shallow (a few micrometers in depth) despite the millimeter-scale crystal size; this phenomenon is strongly connected to its structural anisotropy. The Ohmic contact revealed in this work should be valuable for the application of MX-Chains as electronic devices in the near future.

An unusual Pd(III) oxidation state in the Pd-Cl chain complex with high thermal stability and electrical conductivity

The Pd(iii) oxidation state is unusual and unstable since it strongly tends to disproportionate. We synthesized the quasi-one-dimensional (1D) halogen-bridged Pd(iii)-Cl complex [Pd(dabdOH)2Cl]Cl2 (1-Cl; dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol) with multiple hydrogen bonds. From single-crystal X-ray diffraction, the bridging Cl- ions were located at the midpoint of the Pd-Cl-Pd moieties in the 1D chains, indicating that the Pd ions are in a Pd(iii) average valence (AV) state. Moreover, bright spots for the Pd(iii) dz2 orbitals in the upper Hubbard band above the Fermi level were observed every ∼5 Å using scanning tunnelling microscopy. These results clearly indicate that the Pd ions are in a Pd(iii) AV state in 1-Cl. In addition, 1-Cl has the highest thermal stability (470 K) among the Pd(iii) complexes reported and the highest electrical conductivity (0.6 S cm-1 at 300 K) among the 1D Pd-Cl chains reported so far.

Correlation between Chemical and Physical Pressures on Charge Bistability in [Pd(en)2Br](Suc-Cn)2·H2O

Hydrostatic (physical) pressure effects on the electrical resistivity of a bromido-bridged palladium compound, [Pd(en)₂Br](Suc-C₅)₂·H₂O, were studied. The charge-density-wave to Mott-Hubbard phase transition temperature (T_PT) steadily increased with pressure. By a comparison of the effects of the chemical and physical pressures on T_PT, it was estimated that the chemical pressure by unit alkyl chain length, i.e., the number of carbon atoms in the alkyl chains within the counterion, corresponded to ca. 1.3 kbar of the physical pressure.

Charge-bistable Pd(iii)/Pd(ii,iv) coordination polymers: phase transitions and their applications to optical properties

This paper reviews the recent progress in charge-bistable Pd(iii)/Pd(ii,iv) coordination polymers (MX chains). The temperature-, pressure- and photo-induced phase transitions have been demonstrated, proposing MX chains as a material for optical switching devices.

Stabilization of Pd(III) states in nano-wire coordination complexes

In spite of a long history of quasi-one-dimensional halogen-bridged complexes (MX-chains), all Pt and Pd complexes form the charge-density wave (CDW) state (-X...M(II)...X-M(IV)-X...), while all Ni complexes form Mott-Hubbard (MH) states (-X-Ni(III)-X-Ni(III)-X-), without exception. We have succeeded in stabilizing the Pd(iii) MH state for the first time by utilizing the following two methods: partial substitution with Ni(iii) ions, [Ni(1-x)Pd(x)(chxn)(2)Br]Br(2), and chemical pressure via long alkyl chains introduced as counter-anions, [Pd(en)(2)Br](C(n)-Y)(2).H(2)O. In both systems, it has been revealed that the electronic state of bromo-bridged Pd compounds are determined by the PdPd distances, in other words, CDW and MH states are stabilized when PdPd distances are longer and shorter than 5.26 A, respectively.

Effect of an in-plane ligand on the electronic structures of bromo-bridged nano-wire Ni-Pd mixed-metal complexes, [Ni(1-x)Pd(x)(bn)2Br]Br2 (bn = 2S,3S-diaminobutane)

Single crystals of quasi-one-dimensional bromo-bridged Ni-Pd mixed-metal complexes with 2S,3S-diaminobutane (bn) as an in-plane ligand, [Ni(1-x)Pd(x)(bn)(2)Br]Br(2), were obtained by using an electrochemical oxidation method involving mixed methanol/2-propanol (1:1) solutions containing different ratios of [Ni(II)(bn)(2)]Br(2) and [Pd(II)(bn)(2)]Br(2). To investigate the competition between the electron-correlation of the Ni(III) states, or Mott-Hubbard states (MH), and the electron-phonon interaction of the Pd(II)-Pd(IV) mixed valence states, or charge-density-wave states (CDW), in the Ni-Pd mixed-metal compounds, X-ray structure analyses, X-ray oscillation photograph, and Raman, IR, ESR, and single-crystal reflectance spectra were analyzed. In addition, the local electronic structures of Ni-Pd mixed-metal single crystals were directly investigated by using scanning tunneling microscopy (STM) at room temperature and ambient pressure. The oxidation states of [Ni(1-x)Pd(x)(bn)(2)Br]Br(2) changed from a M(II)-M(IV) mixed valence state to a M(III) MH state at a critical mixing ratio (x(c)) of approximately 0.8, which is lower than that of [Ni(1-x)Pd(x)(chxn)(2)Br]Br(2) (chxn = 1R,2R-diaminocyclohexane) (x(c) approximately 0.9) reported previously. The lower value of x(c) for [Ni(1-x)Pd(x)(bn)(2)Br]Br(2) can be explained by the difference in their CDW dimensionalities because the three-dimensional CDW ordering in [Pd(bn)(2)Br]Br(2) observed by using X-ray diffuse scattering stabilizes the Pd(II)-Pd(IV) mixed valence state more than two-dimensional CDW ordering in [Pd(chxn)(2)Br]Br(2) does, which has been reported previously.