Phosphine-stabilized arsenium salts: Water-stable, labile, coordination complexes

A series of air- and water-stable tertiary phosphine-stabilized arsenium salts of the type R₃P→AsR₂⁺PF₆^- has been isolated. In the crystal structures of two chiral triarylphosphine complexes of prochiral methylphenylarsenium hexafluorophosphate, the stereochemistry around arsenic is trigonal pyramidal with the phosphorus atom occupying the apical position, the As-P bond being orthogonal to the plane of the trigonal (lone-pair included) arsenium ion: Ph₃P→AsMePh⁺PF₆^-, P2₁/c, a = 10.7775(2) Å, b = 17.7987(3) Å, c = 13.3797(2) Å, β = 109.066(1)°, V = 2425.78(7) ų, T = 200 K, Z = 4; Ph₂(2-MeOC₆H₄)P→AsMePh⁺ PF₆^-, P1̄, a = 10.8077(2) Å, b = 10.9741(2) Å, c = 13.5648(2) Å, α = 99.0162(9)°, β = 105.2121(9)°, γ = 116.4717(9)°, V = 1318.11(5) ų, T = 200 K, Z = 2. The arsenium ion in each case appears to be further stabilized by conjugation of the lone pair with the phenyl group, with which the arsenic and methyl-carbon atoms are almost coplanar. In the crystal structure of the 2-(methoxymethylphenyl)diphenylphosphine adduct of methylphenylarsenium hexafluorophosphate, there operates a counteractive chelate effect in which anchimeric oxygen coordination to arsenic destabilizes the arsenic-phosphorus bond in the six-membered chelate ring. Although they are stable, phosphine-stabilized arsenium salts undergo rapid phosphine exchange and attack at arsenic by anionic carbon and oxygen nucleophiles to give tertiary arsines and arsinous acid esters, respectively, with liberation of the phosphine.