Synthetic and structural studies of phosphine coordinated boronium salts

The reaction of BrH 2 B·SMe 2 with primary and secondary phosphines affords a range of boronium salts of the form [H 2 B(PR 3 ) 2 ]Br (PR 3 = PHCy 2 1; PHPh 2 , 2; PH 2 Cy, 3), which have been fully characterised including solid-state determinations. Reactions of bulky tertiary phosphines, e.g., PCy 3 and PPh 3 , with BrH 2 B·SMe 2 do not proceed beyond the phosphine-stabilised bromoborane adducts, however, the smaller tertiary phosphine PMe 2 Ph readily proceeds to form [H 2 B(PMe 2 Ph) 2 ]Br (4). The formation of the unsymmetrical boronium salts [H 2 B(PH 2 Cy)(PHCy 2 )]Br (5) and [H 2 B(PHCy 2 )(PHPh 2 )]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 [H 2 B(κ 2 -P,P'-diphosphine)]Br (diphosphine = dppe, 7; dcpe, 8; dmpe, 9; dppf, 10; dppf with [AsF 6 ] - counterion, 11; amphos, 12). The analogous methodology with Br 2 HB·SMe 2 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(PMe 2 Ph) 2 ]Br (14) and [BrHB(amphos)]Br (15) were synthesised with varying degrees of success. Reaction of BrH 2 B·SMe 2 with diphars afforded the boronium salt [H 2 B(κ 2 -P,P'-diphars)]Br (16), which featured two pendant arsine arms. Similarly, triphos was found to react with BrH 2 B·SMe 2 o give [H 2 B(κ 2 -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 - > [SbF 6 ] - /[AsF 6 ] - ). This was further demonstrated by comparison of solid-state structures and solution NMR data of 1 with [H 2 B(PHCy 2 ) 2 ][SbF 6 ] (18), 4 with [H 2 B(PMe 2 Ph) 2 ][AsF 6 ] (19) and 17 with [H 2 B(triphos)][AsF 6 ] (20).