Synthesis,Structure, Reactivity and Catalytic Implications of a Cationic,Acetylide-Bridged Trigold–JohnPhos Species

The cationic complex (JohnPhos–Au)₃(acetylide)]SbF₆] (JohnPhos=(2-biphenyl)di-tert-butylphosphine, L1) has been characterised structurally and features an acetylide–trigold(I)–JohnPhos system; the trinuclear–acetylide unit, coordinated to the monodentate bulk phosphines, adopts an unprecedented μ,η¹,η²,η¹ coordination mode with an additional interaction between distal phenyl rings and gold centres. Other cationic σ,π-(gold(I)L1)₂] complexes have also been isolated. The reaction of trimethylsilylacetylene with various alcohols (iPrOH, nBuOH, n-HexOH) catalysed by cationic Au^IL1]SbF₆] complexes in CH₂Cl₂ at 50 °C led to the formation of acetaldehyde acetals with a high degree of chemo- and regioselectivity. The reaction mechanism was studied, and several organic and inorganic intermediates have been characterised. A comparative study with the analogous cationic Cu^IL1]PF₆] complex revealed different behaviour; the copper metal is lost from the coordination sphere leading to the formation of cationic vinylphosphonium and copper nanoparticles. Additionally, a new catalytic approach for the formation of this high-value cationic vinylphosphonium has been established.