CYCLIC AND BICYCLIC POLY(PHOSPHANE)BORANE CATIONS

The reactions of (Me2S)BH2Br and (Me2S)BHBr2 with equimolar quantities of 1,2-bis(diphenylphosphanyl)ethene (1) or -benzene (2) lead to cycl ic, cationic bis(phosphane)boranes {4: [(1)BH2](+) Br-; 5: [(1)BHBr]() Br-; 6: [(2)BH2](+) Br-; 7: [(2)BHBr](+) Br-}. The Br- counterions c an be exchanged in metathesis reactions (e.g. with AgBF4 to afford 4a) . The triter tiary phosphane bis[2-(diphenylphosphanyl)phenyl]phenylph osphane (3), reacts with (Me2S)BHBr2 to give bicyclic dicationic tris( phosphane)borane 8, [(3)BH](2+) . 2 Br-, which can be converted into t he bis(hexafluorophosphate) 8a using NH4PF6. All compounds have been c haracterized by analytical and NMR-spectroscopic data. The crystal and molecular structures of 5-7 have been determined by single-crystal Xr ay diffraction. The five-membered rings C2P2B have an envelope conform ation and show no sign of electron delocalization. Attempts to deproto nate the B-H functions in 5-8 have not been successful. Clearly, the t wo (5-7) or three phosphonium centers (8) adjacent to the boron atom a re not sufficient to induce an ''umpolung'' of the B-H group.