Solid-State Electrochemistry of a Semiconducting MMX-Type Diplatinum Iodide Chain Complex

Electron-transfer-facilitated dissolution, ion insertion, and desorption associated with an MMX-type quasi-one-dimensional iodide-bridged dinuclear Pt complex (MMX chain) have been investigated for the first time. K₂(NC₃N)­[Pt₂(pop)₄I]·4H₂O (<b>1</b>) (NC₃N²⁺ = (H₃NC₃H₆NH₃)²⁺; pop = P₂H₂O₅^2–) is a semiconductor with a three-dimensional coordination-bond and hydrogen-bond network included in the chain. The cyclic voltammetry of <b>1</b> was studied by using <b>1</b>-modified electrodes in contact with acetonitrile solutions containing electrolyte. The chemical reversibility for oxidation of <b>1</b> depended on the electrolyte cation size, with large cations such as tetrabutylammonium (Bu₄N⁺) being too large to penetrate the pores formed by the loss of K⁺ and NC₃N²⁺ upon oxidation. The potential for reduction of <b>1</b> decreased as the cation size increased. The presence of the acid induced additional well-defined processes but with gradual solid dissolution, attributed to the breaking of the coordination-bond networks

MX-Chain Compounds with ReO₄ Counterions: Exploration of the Robin–Day Class I–II Boundary

MX chains have been widely studied as a 1D mixed-valence system. Although there have been a large number of studies on the boundary between class II and III materials of the Robin–Day classification, there are few studies of compounds at the boundary between classes I and II. In this study, we synthesized a series of Pt- and Pd- MX-chain compounds with perrhenate counterions, [M­(en)₂]­[M­(en)₂X₂]­(ReO₄)₄ (X = Br for M = Pd and X = Cl, Br, and I for M = Pt). All compounds were isostructural, and the metal–metal distances within the chain exceed 6 Å, which is the longest among MX-chain compounds thus far reported. For [Pt­(en)₂]­[Pt­(en)₂Cl₂]­(ReO₄)₄ (<b>PtCl</b>), an intervalence charge transfer (IVCT) transition was observed in the UV region at 335 nm (3.7 eV), which is the shortest wavelength for the MX-chain compounds thus far reported, indicating that <b>PtCl</b> is the closest to the Robin–Day class I limit

Controlling the Electronic States and Physical Properties of MMX-Type Diplatinum-Iodide Chain Complexes via Binary Countercations

MMX-type quasi-one-dimensional iodide-bridged dinuclear Pt complexes (MMX chains) with binary countercations show a new alternating charge-polarization + charge-density-wave (ACP+CDW) electronic state and reversible switching of the electronic states and physical properties upon dehydration and rehydration process. By comparing several MMX chains with various binary countercations with previous chains, we found that the short backbone of the aliphatic diammonium ion was indispensable for realizing the ACP+CDW state because it induces a 2-fold periodicity along the chain axis via twisting of the ligands. Moreover, the reversibility of the changes in the structure and electrical conductivity upon dehydration and rehydration depend on the length of aliphatic diammonium ion. Short diammonium ions support a robust framework, which undergoes reversible structural changes. On the other hand, long and bent aliphatic diammonium ions weaken the framework, which causes partial degradation of the crystal and a decrease in the electrical conductivity when the structure changes. However, the decrease in the activation energy of the electrical conductivity after the dehydration process is independent of the robustness of the complex, indicating that the orbital overlap in MMX chains with binary countercations increases upon dehydration. Controllable electronic states and physical properties provide a platform for designing the multifunctional materials based on MMX chains

Controlling the Electronic States and Physical Properties of MMX-Type Diplatinum-Iodide Chain Complexes via Binary Countercations

MMX-type quasi-one-dimensional iodide-bridged dinuclear Pt complexes (MMX chains) with binary countercations show a new alternating charge-polarization + charge-density-wave (ACP+CDW) electronic state and reversible switching of the electronic states and physical properties upon dehydration and rehydration process. By comparing several MMX chains with various binary countercations with previous chains, we found that the short backbone of the aliphatic diammonium ion was indispensable for realizing the ACP+CDW state because it induces a 2-fold periodicity along the chain axis via twisting of the ligands. Moreover, the reversibility of the changes in the structure and electrical conductivity upon dehydration and rehydration depend on the length of aliphatic diammonium ion. Short diammonium ions support a robust framework, which undergoes reversible structural changes. On the other hand, long and bent aliphatic diammonium ions weaken the framework, which causes partial degradation of the crystal and a decrease in the electrical conductivity when the structure changes. However, the decrease in the activation energy of the electrical conductivity after the dehydration process is independent of the robustness of the complex, indicating that the orbital overlap in MMX chains with binary countercations increases upon dehydration. Controllable electronic states and physical properties provide a platform for designing the multifunctional materials based on MMX chains

Negative Differential Resistance in MX- and MMX-Type Iodide-Bridged Platinum Complexes

Negative differential resistance (NDR) was discovered in MX- and MMX-type iodide-bridged platinum complexes for the first time. The low resistance of the complex observed under the large current cannot be explained only by the Joule heat. The intrinsic charge-ordering states are considered to play an important role in the NDR of these compounds

Correlation between Chemical and Physical Pressures on Charge Bistability in [Pd(en)₂Br](Suc‑C_<i>n</i>)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 (<i>T</i>_PT) steadily increased with pressure. By a comparison of the effects of the chemical and physical pressures on <i>T</i>_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

Continuous Control of Optical Gaps in Quasi-One-Dimensional Bromide-Bridged Platinum Complexes by Utilizing Chemical Pressure

The optical gap in a series of bromo-bridged platinum chain complexes, [Pt­(en)₂Br]­(C_<i>n</i>–Y)₂·H₂O (en = ethylenediamine; C_<i>n</i>–Y = dialkyl sulfosuccinate; <i>n</i> = the number of carbon atoms), was controlled by using chemical pressure. From the single-crystal structure, [Pt­(en)₂Br]­(C₆–Y)₂·H₂O is in a mixed-valence state at 200 K. In addition, Pt–Pt distances decreased with an increase in <i>n</i> or with a decrease in the temperature. Continuous decreases in the optical gaps upon cooling were observed for <i>n</i> = 5, 7. The smallest gap of 1.20 eV was observed for <i>n</i> = 7 at 50 K. For <i>n</i> = 12, the complex was still in a mixed-valence state at 5 K, although the Pt–Pt distance was quite short. This is probably because of the energetic mismatch between 5d_<i>z</i>² orbitals of the Pt ions and 4p_<i>z</i> orbitals of the Br ions

Multiple-Hydrogen-Bond Approach to Uncommon Pd(III) Oxidation State: A Pd–Br Chain with High Conductivity and Thermal Stability

A Br-bridged Pd chain complex with the Pd ion in an uncommon +3 oxidation state, [Pd­(dabdOH)₂Br]­Br₂ (<b>3</b>), was prepared using a new method involving multiple hydrogen bonds. The PdBr chain complex exhibited superior electrical conductivity and thermal stability. An in-plane ligand with an additional hydrogen donor group (hydroxy group), (2<i>S</i>,3<i>S</i>)-2,3-diaminobutane-1,4-diol (dabdOH), was used to create a multiple-hydrogen-bond network, which effectively shrinks the Pd–Br–Pd distance, stabilizing the Pd­(III) state up to its decomposition temperature (443 K). <b>3</b> shows semiconducting behavior with quite high electrical conductivity (3–38 S cm^–1 at room temperature), which is 10⁶ times larger than the previous record for analogous PdBr chains. Indeed, <b>3</b> is the most conductive MX-type chain complex reported so far. The precise positional control of ions via a multiple-hydrogen-bond network is a useful method for controlling the electronic states, thermal stability and conductivity of linear coordination polymers