Arsenic-Containing Chalcophosphate Molecular Anions

We report five new discrete molecular arsenic-based chalcophosphates, K₇As₃(P₂Se₆)₄ (1), K₆As₂(P₂Se₆)₃ (2), Cs₆As₂(P₂Se₆)₃ (3), and Cs₅As(P₂Q₆)₂ [Q = Se (4a) and S (4b)]. Each of the compounds contains unique complex anions comprised of common building blocks that have condensed to produce these anions. Phosphorus forms well-known [P₂Q₆]⁴⁻ moieties in all of the compounds that are bridged by arsenic trigonal pyramids in 1 and 2 and distorted octahedra in 3, 4a, and 4b. Although 2 and 3 have the same molecular formula, the structural difference between the two salts is attributed to the size of the alkali metal. The influence of flux basicity also seems to play a role in the formation of the molecular anion in 4a and 4b, which has been observed with other trivalent main-group elements at the octahedral position but only with the highly basic cesium alkali metal as the counterion. All structures were determined by single-crystal X-ray diffraction and are discussed along with phase-purity powder X-ray diffraction, thermal analyses, electronic absorption, and Raman spectroscopy.