Recent progress on unusual halogen-bridged Pd(III) chain compounds realized by weak ligand field and triple hydrogen bond approaches

The development of new halogen-bridged Pd chain complexes with molecular precision offers a promising strategy to stabilize Pd(III) ions, which are being considered as future optical, magnetic, dielectric, and conducting materials. More recently, uncommon and rare electronic states of Pd(III) ions in halogenbridged chain complexes have been reported. In these systems, four different designed strategies have been adopted to stabilize Pd(III) oxidation states to date. This review focuses on the new strategies for the realization of Pd(III) halogen-bridged metal complexes (MX-Chains) using (1) weak ligand field and (2) triple hydrogen bond approaches. The resulting MX-Chains show clear evidence of the Pd(III) averaged-valence (AV) state rather than the conventional Pd(II)/Pd(IV) mixed-valence (MV) state. The relationships between the molecular structures, electronic states, and properties of these MX-Chains strongly depend on the in-plane ligands, counteranions and their surrounding environments. Furthermore, Pd(III) states have been found in the MX-Chains even above room temperature. Understanding and controlling the Pd(III) states in halogen-bridged chain complexes is a critical issue for development of next-generation devices, such as molecular electrical nano-wires. Overall, this review establishes halogen-bridged Pd(III) complexes as highly effective solid-state materials and provides detailed insight into their structure-property relationships, laying the groundwork for future developments in this new class of advanced materials.

Automated summary

The development of new halogen-bridged Pd chain complexes with molecular precision is a promising strategy to stabilize Pd(III) ions for future advanced materials. This review focuses on the new strategies using weak ligand field and triple hydrogen bond approaches to realize Pd(III) halogen-bridged metal complexes (MX-Chains). The resulting MX-Chains exhibit the Pd(III) averaged-valence state and have potential applications in next-generation devices.