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

Abstract

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.