Optically Visible Phase Separation between Mott-Hubbard and Charge-Density-Wave Domains in a Pd-Br Chain Complex

Superatomic Stabilization of Dinuclear Platinum(III) through Iodide-Bridged Five-Center Ten-Electron Bonding

One of the goals in synthetic chemistry is to obtain compounds featuring unusual valence states that are stable under ambient conditions. At present, stabilizing unusual Pt(III) states is considered difficult, except through direct Pt-Pt bonding such as that in platinum-blues or organometallization using bulky ligands. Pt(III) stabilization is also very difficult in halogen-bridged metal complex chains (MX-Chains). Herein, the iodide-bridged Pt(III) dimer compound [Pt2(en)4I3]I3 (en = ethylenediamine), which is prepared by the iodine oxidation of [PtII(en)2]I2, has been successfully synthesized and characterized. This compound is stable and is obtained as diamond-shaped single crystals with a lustrous emerald-green color under reflected light and a red color under transmitted light. The Pt(III) state is stabilized by the five-center ten-electron (5c-10e) bonding in the I-Pt-I-Pt-I core, in addition to the very strong antiferromagnetic state. The stabilization mechanism of Pt(III) through a 5c-10e bonding is considered a superatom complex; thus, this work provides new insight for stabilizing the unusual Pt(III) state.

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.

Elucidating 2D Charge-Density-Wave atomic structure in an MX-chain by the 3D-ΔPair Distribution Function method

Many solids, particularly low-dimensional systems, exhibit charge density waves (CDWs). In one dimension, charge density waves are well understood, but in two dimensions, their structure and their origin are difficult to reveal. Here, the 2D Charge-Density-Wave atomic structure and stabilization mechanism in the bromide-bridged Pd compound [Pd(cptn) 2 Br]Br 2 (cptn = 1 R ,2 R -diaminocyclopentane) is investigated by means of single-crystal X-ray diffraction employing the 3D-ΔPair Distribution Function (3D-ΔPDF) method. Analysis of the diffuse scattering using 3D-ΔPDF shows that a 2D-CDW is stabilized by a hydrogen-bonding network between Br - counteranion and the amine (NH 2 ) group of the cptn in-plane ligand, and that 3D ordering is prevented due to a weak plane to plane correlation. We extract the effective displacements of the atoms describing the atomic structure quantitatively and discuss the stabilization mechanism of the 2D-CDW. Our study provides a method to identify and measure the key interaction responsible for the dimensionality and stability of the CDW that can help further progress of rational design.

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...

Interdigitated Pt-Br chains with π-stacking: an approach toward Robin-Day class I mixed valency in MX-chain complexes

The first interdigitated MX-type chain complex with infinite π-stacked arrays was synthesized. The synchronization between a Pt-Br⋯ chain and π-stacking periodicities led to the longest M-X-M distance (6.6978(15) Å) and nil or negligible intervalence charge transfer, which is essential to realize the Robin-Day class I mixed valence state in MX chains.

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.

Conductive zigzag Pd(iii)–Br chain complex realized by a multiple-hydrogen-bond approach

Coexistence of zigzag structure and the uncommon Pd(iii) oxidation state in quasi-1D halogen-bridged metal complexes was realized in a conductive Br-bridged Pd chain complex, [Pd(dabdOH)₂Br]SO₄·3H₂O (2), for the first time.

Observation of charge bistability in quasi-one-dimensional halogen-bridged palladium complexes by X-ray absorption spectroscopy

We performed X-ray absorption fine structure (XAFS) measurements on three representative bromo-bridged palladium compounds. In the X-ray absorption near-edge structure (XANES) spectra, the averaged-valence (AV) compound, [Pd3+(dabdOH)2Br]Br2 (1: dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol), and the mixed-valence (MV) compound, [Pd2+(en)2][Pd4+(en)2Br2](ReO4)4 (2), showed significant differences in their spectra. In [Pd(en)2Br](Suc-C5)2·H2O (3: en = ethylenediamine, Suc-C5 = dipentylsulfosuccinate), which exhibits an MV-AV phase transition, on the other hand, the spectroscopic difference between below and above the phase transition temperature was hardly observed due to the subtle difference in the oxidation states. In the extended X-ray absorption fine structure (EXAFS) spectra, a clear difference in the Pd-Br correlation region was observed upon the phase transition.

MX-type single chain complexes with an aromatic in-plane ligand: incorporation of aromatic interactions for stabilizing the chain structure

MX-type one-dimensional complexes [Pt^IV(amp)₂Br₂][Pt^II/IV(amp)₂Br]₂(HSO₄)₂(SO₄)₂·13H₂O (3) and [Pt^IV(amp)₂Br₂][Pt^II/IV(amp)₂Br]₂(H₂PO₄)₆·8H₂O (4) were synthesized as the first analogue containing only an aromatic in-plane ligand. The Pt-Br chain structures of 3 and 4 are stabilized by both the hydrogen-bond network along the chain and the π-stacking via intercalated Pt(iv) complexes. Structural and spectroscopic studies indicated that both 3 and 4 form the Pt(ii/iv) mixed valence state.