Metal-free pincer ligand chemistry polycationic phosphonium Lewis acids

Oxidation of 2,6-bis(diphenylphosphine)methyl pyridine with MeO₃SCF₃, yields the dication [2,6-(CH₂PMePh₂)₂C₅H₃N][CF₃SO₃]₂2, while subsequent treatment with two equivalents of [K(THF)_n][B(C₆F₅)₄] (n = 1.3-1.5) afforded [2,6-(CH₂PMePh₂)₂C₅H₃N][B(C₆F₅)₄]₂3. Analogous treatment using XeF₂ afforded 2,6-(CH₂PF₂Ph₂)₂C₅H₃N 4, while subsequent fluoride abstractions afford the corresponding [BF₄], [CF₃SO₃] and [B(C₆F₅)₄] salts, 5-7, respectively. Compound 4 reacts with an excess of MeO₃SCF₃ to give [2,6-(CH₂PF₂Ph₂)₂C₅H₃N(CH₃)][CF₃SO₃] 8 while subsequent reaction with [Et₃Si·2C₇H₈][B(C₆F₅)₄] gave the related tricationic species, 9. The Lewis acidity of these di- and triphosphonium cations 3, 7 and 9 was examined and their utility as Lewis acid catalysts was assessed in the dimerization of 1,1-diphenylethylene, the hydrodefluorination of 1-fluoroadamantane, and the dehydrocoupling of phenol and silane.