Synthetic and structural studies of phosphine coordinated boronium salts.

Abstract

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).