Aza- and thiaarsolidinium cations: Novel structural features for carbene analogues

Article abstract of DOI:10.1021/ja00047a024

The preparation, isolation, and structural characterization of salts containing the first arsolidinium cations are reported. Reactions between equimolar amounts of 2-chloro-1,3-dimethyl-1,3-diaza-2-arsolidine or 2-chloro-1,3-dithia-2-arsolidine with AlCl₃ or GaCl₃ result in quantitative chloride ion abstraction to give the corresponding cations [N(Me)CH₂CH₂N(Me)As]⁺ and [SCH₂CH₂SAs]⁺ as tetrachloroaluminate and gallate salts. X-ray crystallography reveals a centrosymmetric dimeric arrangement for three derivatives in which the cations are bound together by relatively weak As-N or As-S interactions. A complex between the respective chloroarsolidine and arsolidinium cation is formed in reaction mixtures which provide appropriate stoichiometry. The X-ray structure of the thia derivative, [SCH₂CH₂SAsSAs(Cl)SCH₂CH₂] [GaCl₄], shows a single, noticeably weaker interring connection, but otherwise the compound exhibits similar structural features. The aza derivative of the complex can be readily interconverted with the dicationic diazarsolidinium upon addition of a Lewis acid. Variable temperature ¹H NMR spectroscopy shows both the dication dimers and the complexes to be fluxional in solution, and a dissociative process is envisaged. Consequently, the cations are viewed as examples of stable arsenium (carbene analogue) units, unique in the absence of a steric shield or Hückel T-delocalization. The weakly associated structures represent alternatives to alkenic dimerization, typical of carbenes.