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

The synthesis of a range of bis(phosphine)boronium salts is reported [(R₂HP)₂BH₂][X] (R = Ph, ^tBu, Cy) in which the counter anion is also varied (X⁻ = Br⁻, [OTf]⁻, [BAr^F₄]⁻, Ar^F = 3,5-(CF₃)₂C₆H₃). 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 [BAr^F₄]⁻ < [OTf]⁻ < Br⁻. [(Ph₂HP)₂BH₂][BAr^F₄] reacts with RhH(PPh₃)₃, by elimination of H₂, forming [Rh(κ¹,η-PPh₂BH₂·PPh₂H)(PPh₃)₂][BAr^F₄] which shows a β-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·BH₃ to afford [Rh(κ¹,η-PPh₂BH₂·PPh₃)(PPh₃)₂][BAr^F₄], which was characterised by X-ray crystallography. Addition of H₃B·PPh₂H to the well-defined 16-electron "T-shaped" complex [Rh(PⁱBu₃)₂(PPh₃)][BAr^F₄] (characterised by X-ray crystallography) formed of a mixture of base-stabilised phosphino borane ligated complexes [Rh(κ¹,η-PR₂BH₂·PR₃)(PR₃)₂][BAr^F₄] (R = ⁱBu 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.