An unusual Pd(iii) oxidation state in the Pd–Cl chain complex with high thermal stability and electrical conductivity

Abstract

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) d_z² 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.