The electron-poor phosphines P{C6H3(CF3)2-3,5}3 and P(C6F5)3 do not mimic phosphites as ligands for hydroformylation. A comparison of the coordination chemistry of P{C6H3(CF3)2-3,5}3 and P(C6F5)3 and the unexpectedly low hydroformylation activity of their rhodium complexes

Abstract

The fluoroaryl phosphines P{C₆H₃(CF₃)₂-3,5}₃ (L_aa) and P(C₆F₅)₃ (L_bb) form the complexes trans-[MCl₂(L_aa)₂] and trans-[MCl₂(L_bb)₂] (M = Pd or Pt) which have been isolated and fully characterised. ³¹P NMR studies of competition experiments show that the stability of trans-[PdCl₂L₂] is in the order L = L_bb < L_aa < PPh₃. The crystal structure of trans-[PtCl₂(L_aa)₂] is reported and reveals that the Pt–P bond lengths in trans-[PtCl₂L₂] are in the order L = L_bb < L_aa < PPh₃. The equilibria established when [Pt(norbornene)₃] is treated with L_aa or L_bb are investigated by ³¹P and ¹⁹⁵Pt NMR spectroscopy and the species [PtL_n(norbornene)_3−n] (n = 1–3) identified. Ligands L_aa and L_bb appear to have similar affinities for platinum(0). The complexes trans-[MCl(CO)(L_aa)₂] and trans-[MCl(CO)(L_bb)₂] (M = Rh or Ir) have been synthesised and fully characterised; the values of ν_CO are comparable with those for analogous phosphite complexes. The ligands L_aa, L_bb, P(C₆H₂F₃-3,4,5)₃ (L_ccc), P{C₆H₄(CF₃)-2}₃ (L_dd), PPh₃ and P(OPh)₃ have been tested in rhodium-catalysed hydroformylation of 1-hexene and L_aa, L_bb, and PPh₃ have been tested in rhodium-catalysed hydroformylation of 4-methoxystyrene. Ligands L_aa, and L_bb, have been shown to be stable under the hydroformylation catalysis conditions. For the 1-hexene reaction, the activity and selectivity for L_aa and L_ccc are very similar to the PPh₃ catalyst (TOF ca. 400 h⁻¹; n : iso 2.5–3.0) but for the sterically demanding L_bb and L_dd the activity and selectivity was much lower than with PPh₃ (TOF ca. 15, n : iso ratio 0.6). Thus, the yield of heptanals obtained with the catalyst derived from L_aa is 94% while under the same conditions with L_bb only 6%. The TOF for the L_aa/Rh catalyst was 5 times lower than for the P(OPh)₃/Rh catalyst despite the superficially similar ligand electronic characteristics for L_aa and P(OPh)₃.