Boron–boron bond oxidative addition to rhodium(I) and iridium(I) centres

Abstract

The reaction between the diborane(4) compound B₂(1,2-O₂C₆H₄)₂ and either of the rhodium(I) complexes [RhCl(PPh₃)₃] or [{Rh(µ-Cl)(PPh₃)₂}₂] afforded the colourless rhodium(III) bis(boryl) species [RhCl(PPh₃)₂{B(1,2-O₂C₆H₄)}₂]. Similar reactions have been carried out with the diborane(4) compounds B₂(1,2-O₂-4-Bu^tC₆H₃)₂, B₂(1,2-O₂-3,5-Bu^t₂C₆H₂)₂, B₂(1,2-O₂-3-MeC₆H₃)₂, B₂(1,2-O₂-4-MeC₆H₃)₂, B₂(1,2-O₂-3-MeOC₆H₃)₂, B₂(1,2-S₂C₆H₄)₂, B₂(1,2-S₂-4-MeC₆H₃)₂ and B₂[R,R-1,2-O₂CH(CO₂Me)CH(CO₂Me)]₂ affording analogous rhodium complexes all of which have been characterised spectroscopically. The complexes derived from the reactions with B₂(1,2-O₂C₆H₄)₂ and B₂(1,2-O₂-3-MeC₆H₃)₂ have also been characterised by X-ray crystallography, the structures comprising a five-co-ordinate rhodium centre with a square-based-pyramidal geometry in which the apical site is occupied by a boryl group and the phosphines are mutually trans in basal positions. Reactivity studies have also been carried out for [RhCl(PPh₃)₂{B(1,2-O₂C₆H₄)}₂]. Hydrolysis or alcoholysis with catechol afforded [RhH₂Cl(PPh₃)₃] and either B₂(1,2-O₂C₆H₄)₂(µ-O) or B₂(1,2-O₂C₆H₄)₃ and addition of the phosphines PMe₃, PEt₃ and PMe₂Ph afforded the new bis(boryl) compounds cis,mer-[RhCl(PMe₃)₃{B(1,2-O₂C₆H₄)}₂], [RhCl(PEt₃)₂{B(1,2-O₂C₆H₄)}₂] and cis,mer-[RhCl(PMe₂Ph)₃{B(1,2-O₂C₆H₄)}₂], the PEt₃ complex having been characterised by X-ray crystallography and shown to be similar to the PPh₃ complex. The iridium analogue [IrCl(PEt₃)₂{B(1,2-O₂C₆H₄)}₂] was also prepared from the reaction between [IrCl(PEt₃)₃] and B₂(1,2-O₂C₆H₄)₂ and shown by X-ray crystallography to be isomorphous with the rhodium complex. Reactions between [RhCl(PPh₃)₂{B(1,2-O₂C₆H₄)}₂] and the phosphines PPrⁱ₃, P(C₆H₁₁)₃, 1,2-bis(diphenylphosphino)ethane (dppe) and 1,2-bis(dicyclohexylphosphino)ethane (dcpe) are also described although these do not result in new rhodium boryl complexes. The reaction between [{RhCl(dppe)}₂] and B₂(1,2-O₂C₆H₄)₂ afforded a compound tentatively assigned as [Rh(dppe)₂{B(1,2-O₂C₆H₄)}] with analogous compounds being formed with the diborane(4) compounds B₂(1,2-O₂-3-MeC₆H₃)₂ and B₂(1,2-O₂-4-MeC₆H₃)₂. Finally, the reaction between [Rh(PMe₃)₄]Cl and the diborane(4) compound B₂(1,2-O₂C₆H₄)₂ is described which affords cis,mer-[RhCl(PMe₃)₃{B(1,2-O₂C₆H₄)}₂]. Analogous reactions with B₂(1,2-O₂-3,5-Bu^t₂C₆H₂)₂, B₂(1,2-O₂-3-MeC₆H₃)₂ and B₂[R,R-1,2-O₂CH(CO₂Me)CH(CO₂Me)]₂ afforded similar products.