Cs₂PdSe₈: A Unique Open Framework Structure with Double Helical Assemblies of [Pd(Se₄)₂]^2-**

Cs₂PdSe₈(1), a novel palladium polyselenide with a three-dimensional open framework structure, has been synthesized via a solvothermal reaction using ethylenediamine as solvent. Its crystal structure has been determined by single crystal X-ray diffraction. The structure belongs to the tetragonal space groupI4₁/acd(No. 142),a=14.935(2),c=12.907(3) Å,V=2897.0(8) ų,Z=8,R₁=0.044, andwR₂=0.063, for 640 unique reflections. The unique three-dimensional network is composed of two³ _∞[Pd(Se₄)₂] ^2-frameworks that are topologically identical and mutually insertional. Each framework is built upon by connecting the neighboring square-planar [Pd(Se₄)₂]^2-units to generate an infinite assembly resembling alternating right- and left-handed helices running along thec-axis. Thermal analysis (TGA) showed that compound 1 is stable up to 320 °C. Optical studies performed on the powder sample of 1 suggested that the compound is a semiconductor and a band gap of 1.5 eV was estimated from the diffuse reflectance data.