Tertiary arsine-stabilised arsenium salts: Syntheses and comparisons with phosphine analogues

The first tertiary arsine-stabilised arsenium salts, [(L)AsMePh]OTf (L = Ph₃As, Me₂PhAs, [2-(MeOCH₂)C₆H₄]Ph₂As, [2-(MeOCH₂)C₆H₄]Me₂As), have been prepared by chloride abstraction from chloromethylphenylarsine with trimethylsilyl triflate in the presence of the arsine. The complexes have been characterised by crystallography and ¹H NMR spectroscopy. The chiral cations in the complexes have structures based on the trigonal pyramid in which the arsine is coordinated orthogonally to the prochiral, six-electron MePhAs⁺ ion that forms the base of the pyramid. The NMR data for the complexes in dichloromethane-d₂ are consistent with rapid exchange of the arsine on the arsenium ion, even at 183 K. The corresponding phosphine-stabilised complexes are considerably more stable than their arsine counterparts in dichloromethane-d₂ with the free energy of activation ΔG^‡_c = ca. 60 kJ mol^-1 being calculated for phosphine exchange in [(Me₂PhP)AsMePh]OTf at 281 K; for [(Me₂{2-(MeOCH₂)C₆H₄}P)AsMePh]OTf in the same solvent, ΔG^‡_c = ca. 70 kJ mol^-1 at 323 K.