A novel layered Rb-Pd polyselenide has been synthesized by methanothermal reaction of Pd2+ with Se(x)2- (from Se2- + Se) in the presence of Rb2CO3. Rb2[Pd(Se4)2] · Se8 (I) reveals a unique structure with infinite [Pd(Se4)2]2- sheet polyanions acting as a host for neutral Se8 rings. The black shiny rectangular crystals of Rb2[Pd(Se4)2] · Se8 are insoluble in H2O and common organic solvents. The compound crystallizes in the noncentrosymmetric space group P4b2 (No. 117) with a = 12.6748(8) Å, c = 6.9273(7) Å, Z = 2, and V = 1112.9(2) Å3. The [Pd(Se4)2]2- sheet anion contains Pd2+ ions with a distorted square planar coordination geometry to four terminal Se atoms of Se4 2- chains. The cocrystallized neutral Se8 rings have a crown-like conformation as known from the elemental Se8 forms. Through coordination to Rb+ ions they build [Rb(Se8)+](n) 'cationic chains' running in the c direction. Rb2[Pd(Se4)2] · Se8 therefore differs fundamentally from its 'neighboring' alkali metal Pd polyselenides, K2PdSe10 and Cs2PdSe8, which each possess a three- dimensional structure with two interpenetrating [Pd(Se(x))2]2- frameworks. Optical spectroscopic data and the thermal stability of the compound are discussed.
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