Bis(phosphine)boronium salts. Synthesis, structures and coordination chemistry

The synthesis of a range of bis(phosphine)boronium salts is reported [(R2HP)(2)BH2][X] (R = Ph, Bu-t, Cy) in which the counter anion is also varied (X- = Br-, [OTf](-), [BAr4F](-), Ar-F = 3,5-(CF3)(2)C6H3). Characterization in the solid-state by X-ray diffraction suggests there are weak hydrogen bonds between the PH units of the boronium cation and the anion (X- = Br-, [OTf](-)), while solution NMR spectroscopy also reveals hydrogen bonding occurs in the order [BAr4F](-) < [OTf](-) < Br-. [(Ph2HP)(2)BH2][BAr4F] reacts with RhH-(PPh3)(3), by elimination of H-2, forming [Rh(kappa(1),eta-PPh2BH2 center dot PPh2H)(PPh3)(2)][BAr4F] which shows a beta-B-agostic interaction from the resulting base stabilised phosphino-borane ligand. Alternatively such ligands can be assembled directly on the metal centre by reaction of in situ generated {Rh(PPh3)(3)}(+) and Ph2HP center dot BH3 to afford [Rh(kappa(1),eta-PPh2BH2 center dot PPh3)(PPh3)(2)][BAr4F], which was characterised by X-ray crystallography. Addition of H3B center dot PPh2H to the well-defined 16-electron T-shaped complex [Rh(PiBu(3))(2)(PPh3)][BAr4F] (characterised by X-ray crystallography) formed of a mixture of base-stabilised phosphino borane ligated complexes [Rh(kappa(1),eta-PR2BH2 center dot PR3)(PR3)(2)][BAr4F] (R = Bu-i or Ph). These last observations may lend clues to the formation of bis(phosphine) boronium salts in the catalytic dehydrocoupling reaction of phosphine boranes as mediated by Rh(I) compounds.