Encapsulation of Cyclooctasulfur Molecules in an Open Metal-Sulfide Framework. Isolation of the Host-Guest Complex Cs₂Sn₃S₇•1/2S₈ from Molten Cesium Polysulfide Fluxes

Cs₂Sn₃S₇•1/2S₈ was synthesized in 50-60% yield by reacting Sn with a Cs₂S_x flux at temperatures from 400 to 500 °C. The new greenish yellow ternary sulfide Cs₂Sn₃S₇•1/2S₈ crystallizes in the monoclinic C2/c space group (No. 15) with a = 22.350(6) Å, b = 13.238(2) Å, c = 16.301(4), β = 124.424(2)°, V = 3978.2(3.5) ų, Z = 8. D_calc = 3.25g/cm³, number of data with F₀² > 3σ(F₀²) 3320, number of variables 182. The final R/R_w = 3.8/4.9. The structure of Cs₂Sn₃S₇•1/2S₈ consists of Sn₃S₄ defect cubane units connected by (µ-S)₂ bridges forming large open rings in Sn₃S₇^2- anionic layers. These layers are stacked along the c direction so that the open rings are aligned forming channels. Cs⁺ cations are located between the layers and cyclooctasulfur molecules are found to statistically occupy overlapping conformations within the channels. Raman and vibrational spectroscopic data confirm the presence of the S₈ molecules. Single-crystal optical spectra indicate that this semiconductor has an indirect bandgap of 2.64 eV.