Benzothiarsolium cations: evidence for pπ-bonding between arsenic and sulfur in the first structurally characterized arsenium system

Tetrachloroaluminate salts of the benzothiazarsolium and toluodithiarsolium heteronaphthalenic cations have been prepared from the corresponding chloroarsoles by a routine halide ion abstraction procedure. Characterization data are presented, including the X-ray crystal structure of the thiaza derivative. (Crystal data for C₆H₅AlAsCl₄NS: monoclinic, space group P2₁/c, a = 6.4259(5) Å, b = 23.654(3) Å, c = 8.5859(7) Å, β = 98.203(6)°, V = 1291.7(2) ų, Z = 4, R = 0.054.) A planar bicyclic cationic unit is observed, typical of other heteronaphthalenic systems. The crystal structure of 2-chlorobenzo-1,3,2-thiazarsole has been obtained for direct comparison, and also has a close to planar heterobicyclic moiety. (Crystal data for C₆H₅AsClNS: monoclinic, space group P2₁/c, a = 8.370(3) Å, b = 10.829(3) Å, c = 8.756(2) Å, β = 95.99(2)°, V = 789.2(3) ų, Z = 4, R = 0.025.) The planarity and short heterocyclic bond lengths observed in the two compounds imply a significant degree of pπ bonding, which is unique between sulfur and arsenic. In addition, the cation is a rare example of a structurally characterized cationic dicoordinate arsenic, or arsenium, system. Keywords: arsenic, arsolium, arsenium, pπ-bonding, sulfur, heteronaphthalenic.