Condensation of pyramidal [AsSe₃]^3- anions for the construction of polymeric networks: Solventothermal synthesis of K₃AgAs₂Se₅O25SMeOH, K₂AgAs₃Se₆, and Rb₂AgAs₃Se₆

Three novel quaternary selenoarsenates could be synthesized by the solventothermal reaction of [AsSe₃]^3- anions with AgBF₄ in the presence of the respective alkali metal cation. Orange crystals of K₃AgAs₂Se₅0.25MeOH (I) are formed after reaction in MeOH at 130 °C for ∼2 h. Its structure is built up of ψ-bitetrahedral [As₂Se₅]^4- units, which are connected by Ag⁺ ions to form anionic one-dimensional chains. These chains are separated from each other by the K⁺ cations. Longer reaction time, i.e., 3 days, at the same temperature provides red crystals of K₂AgAs₃Se₆ (II) and, correspondingly, the isostructural Rb₂AgAs₃Se₆ (III), which possess two-dimensional (2D) polyanions [AgAs₃Se₆]^2-. They consist of double layers of strongly waved, infinite [AsSe₂]^- chains, which are interconnected by tetrahedral Ag⁺ ions. The structure can also be described as [Ag₂As₆Se₁₄]^8- cores, which are connected by bridging Se atoms to build the sheet anion. Both II and III possess Ag₂Se₂ four-membered rings, which lead to AgAg distances between 3 and 3.3 Å. The thermal, optical, and spectroscopic properties of the compounds are reported. They melt incongruently between 259 and 284 ̊C and are wide band gap semiconductors with values between 1.9 and 2.1 eV. Raman spectroscopic data show typical patterns for the respective selenoarsenate anionic building units in the range 100-300 cm^-1. Crystal data: K₃AgAs₂Se₅0.25MeOH (I), monoclinic, P2₁/c, a = 9.229(1) Å, b = 11.508(2) Å, c = 12.905(2) Å, β= 104.272(2)°, Z = 4; K₂AgAs3Se₆ (II), monoclinic, P2₁/n, a = 12.5994(6) Å, b = 7.4980(4) Å, c = 14.2648(7) Å, β= 96.625(2)°, Z = 4; Rb₂-AgAs₃Se₆ (III), monoclinic, P2₁/n, a = 12.717(2) Å, b = 7.7329(8) Å, c = 14.368(2) Å, β= 95.745(2)°, Z = 4.