Crystal Structures and Properties of MMX-Chain Compounds Based on Dithiocarboxylato-Bridged Dinuclear Complexes

Abstract

In this chapter, a comprehensive study of the syntheses, crystal structures, and properties of the series of one-dimensional (1D) halogen-bridged mixed-valence dimetal complexes, MMX-chain compounds, based on the dithiocarboxylato-bridged dinuclear complexes, [Pt₂(RCS₂)₄I]_∞ (R = Me (1), Et (2), n-Pr (3), n-Bu (4), n-Pen (5), and n-Hex(6)) and [Ni₂(RCS₂)₄I]_∞ (R = Me (7), Et (8), n-Pr (9), and n-Bu (10)) are described. The evolution from 1D halogen-bridged metal complex, MX-chain compounds, to MMX-chain compounds has produced a variety of electronic states and subtle balance of solid-state properties originating from the charge–spin–lattice coupling and the fluctuation of these degrees of freedom. With increasing the internal degrees of freedom originating from the mixed-valence diplatinum unit, the Pt MMX-chain compounds except for 3 show relatively high electrical conductivity of 0.84–43 S cm^–1 at room temperature and exhibit metallic conducting behavior with T _M–S = 205–324 K. These compounds at room temperature are considered to take the valence-ordered state close to an averaged-valence (AV) state of –Pt^2.5+–Pt^2.5+–I^––. The analyses of the diffuse scattering observed in the metallic state of 2 revealed that the metallic state has appeared by the valence fluctuation accompanying the dynamic valence-ordering state of the charge-density-wave (CDW) type of –Pt²⁺–Pt²⁺–I^––Pt³⁺–Pt³⁺–I^––. On the other hand, the metallic Pt MMX-chain compounds become insulators with lowering temperature due to the lattice dimerization originating from an effective half-filled metallic band. The synchrotron radiation crystal structure analysis of 2 at 48 K revealed that the valence-ordered state in the LT phase is the alternate charge-polarization (ACP) state of –Pt²⁺–Pt³⁺–I^––Pt³⁺–Pt²⁺–I^––. Furthermore, the elongation of the alkyl chains introduces increasing motional degrees of freedom in the system. Interplay between electronic degrees of freedom and molecular dynamics is also expected to cause an intriguing structural phase transition accompanying an electronic and/or magnetic transition never observed for [M₂(MeCS₂)₄I]_∞ (M = Pt (1), Ni (7)). With the elongation of alkyl chains in dithiocarboxylato ligands, the compounds 3–5 undergo two phase transitions at near 210 K and above room temperature, indicating the existence of the LT, RT, and HT phases. The periodicity of crystal lattice in the RT phase of 3–5 along 1D chain is threefold of a –Pt–Pt–I– unit, and the structural disorders have occurred for the dithiocarboxylato group and the alkyl chain belonging to only the central dinuclear units in the threefold periodicity. In the HT phase, the dithiocarboxylato groups of all the dinuclear units in 3–5 are disordered and the lattice periodicities in 3 and 4 change to onefold of a –Pt–Pt–I– period. Ikeuchi and Saito have revealed from the heat capacity measurements that the entropy (disorder) reserved in alkyl groups in the RT phase is transferred to the dithiocarboxylate groups with the RT–HT phase transition [50–52]. Whereas, the lattice periodicity of 4 in the LT phase changes to twofold periodicity being assigned to the ACP state similar to the LT phase of the compound 2 and the dithiocarboxylate groups of all the diplatinum units are ordered. Furthermore, accompanying to the RT–LT phase transition, the compound 4 exhibits the paramagnetic–nonmagnetic transition originating from the regular electronic Peierls transition. These facts suggest that the dynamics (motional degrees of freedom) of the dithiocarboxylato ligands and bridging iodine atoms affects the electronic and magnetic systems through the electron–lattice interaction.

On the other hand, unlike the metallic Pt MMX-chain compounds, all the Ni MMX-chain compounds are Mott–Hubbard semiconductor due to the strong on-site Columbic repulsion on the nickel atom. The room-temperature crystal structures of the compounds 7–10 indicate their valence states to be an averaged-valence (AV) state or a charge-polarization (CP) state of –Ni^(2.5−δ)+–Ni^(2.5+δ)+–I⁻–Ni^(2.5−δ)+–Ni^(2.5+δ)+–Ni^(2.5−δ)+–I⁻– ($\delta \ll 0.5$) close to an averaged-valence state. With the elongation of the alkyl chains in dithiocarboxylato ligands, the periodicity of crystal lattice in 9 and 10 along 1D chain in the RT phase is threefold of a –Ni–Ni–I– unit by the same origin as the diplatinum compounds 3–5, and furthermore, the lattice periodicity of 9 changes to onefold in the LT phase with a first-order phase transition at 205.6 K. The high temperature magnetic susceptibilities of 8–10 can be described by an S = 1/2 1D Heisenberg antiferromagnetic chain model with the very large exchange coupling constant |J|/k _B ranging from 898(2) to 939(3) K. Furthermore, the compounds 8 and 9 undergo a spin-Peierls (SP) transition at relatively high T _sp = 47 and 36 K, respectively, which are accompanied by superlattice reflections corresponding to twofold of a –Ni–Ni–I– period below T _sp. The synchrotron radiation crystal structure analysis of 8 at 26 K revealed that the valence-ordered state changes from the CP state in the RT phase to the ACP state in the SP phase. These facts demonstrate that the electronic system of the Ni MMX-chain compounds in which the on-site Columbic repulsion U plays a dominant role in determining the electronic system is hardly affected by the molecular dynamics.