Synthetic and structural studies of phosphine coordinated boronium salts

The reaction of BrH₂B·SMe₂ with primary and secondary phosphines affords a range of boronium salts of the form [H₂B(PR₃)₂]Br (PR₃ = PHCy₂1; PHPh₂, 2; PH₂Cy, 3), which have been fully characterised including solid-state determinations. Reactions of bulky tertiary phosphines, e.g., PCy₃ and PPh₃, with BrH₂B·SMe₂ do not proceed beyond the phosphine-stabilised bromoborane adducts, however, the smaller tertiary phosphine PMe₂Ph readily proceeds to form [H₂B(PMe₂Ph)₂]Br (4). The formation of the unsymmetrical boronium salts [H₂B(PH₂Cy)(PHCy₂)]Br (5) and [H₂B(PHCy₂)(PHPh₂)]Br (6) was observed by in situ NMR spectroscopy, however, the compounds were found to spontaneously disproportionate to their respective homophosphine boronium cations, even on prolonged storage in solution at -78 °C. Di- and triphosphines were found to form ring-closed boronium salts to afford [H₂B(κ²-P,P'-diphosphine)]Br (diphosphine = dppe, 7; dcpe, 8; dmpe, 9; dppf, 10; dppf with [AsF₆]^- counterion, 11; amphos, 12). The analogous methodology with Br₂HB·SMe₂ proved less generally applicable due to an accessible decomposition pathway being available to boronium salts bearing primary and secondary phosphines, leading to the formation of phosphonium salts, although [BrHB(dcpe)]Br (13), [BrHB(PMe₂Ph)₂]Br (14) and [BrHB(amphos)]Br (15) were synthesised with varying degrees of success. Reaction of BrH₂B·SMe₂ with diphars afforded the boronium salt [H₂B(κ²-P,P'-diphars)]Br (16), which featured two pendant arsine arms. Similarly, triphos was found to react with BrH₂B·SMe₂ to give [H₂B(κ²-P,P'-triphos)]Br (17), which featured a pendant phosphine arm. Substitution of the bromide counter anion with either hexafluoroarsenate or hexfluoroantimonate anions revealed weak hydrogen bonds between the P-H bonds of the boronium cations and the anions, that appeared through NMR studies to be retained in solution (where hydrogen bonding order was determined to be Br^- > [SbF₆]^-/[AsF₆]^-). This was further demonstrated by comparison of solid-state structures and solution NMR data of 1 with [H₂B(PHCy₂)₂][SbF₆] (18), 4 with [H₂B(PMe₂Ph)₂][AsF₆] (19) and 17 with [H₂B(triphos)][AsF₆] (20).