Cyclic and bicyclic poly(phosphane)borane cations

Article abstract of DOI:10.1002/cber.19971301009

The reactions of (Me₂S)BH₂Br and (Me₂S)BHBr₂ with equimolar quantities of l,2-bis(diphenylphosphanyl)ethene (1) or -benzene (2) lead to cyclic, cationic bis(phosphane)boranes {4: [(1)BH₂]⁺ [(1)BH₂]; 5: [(l)BHBr]⁺ [(1)BH₂]; 6: [(2)BH₂]⁺ Br^-; 7: [(2)BHBr]⁺ The (1)BH₂]⁺ counterions can be exchanged in metathesis reactions (e.g. with AgBF₄ to afford 4a). The tritertiary phosphane bis[2-(diphenylphosphanyl)phenyl]phenylphosphane (3), reacts with (Me₂S)BHBr₂ to give bicyclic dicationic tris(phosphane)borane 8, [(3)BH]²⁺-2 Br^-, which can be converted into the bis(hexafluorophosphate) 8a using NH₄PF₆. All compounds have been characterized by analytical and NMR-spectroscopic data. The crystal and molecular structures of 5-7 have been determined by single-crystal Xray diffraction.The five-membered rings C₂P₂B have an envelope conformation and show no sign of electron delocalization. Attempts to deprotonate the B-H functions in 5-8 have not been successful. Clearly, the two (5-7) or three phosphonium centers (8) adjacent to the boron atom are not sufficient to induce an "umpolung" of the B-H group. WILEY-VCH Verlag GmbH,.