One electron oxidation of chromium N,N-bis(diarylphosphino)amine and bis(diarylphosphino)methane complexes relevant to ethene trimerisation and tetramerisation

Abstract

Complexes of the type [(diphosphine)Cr(CO)₄] (diphosphine = Ph₂PN(iPr)PPh₂, Ar₂PN(Me)PAr₂ or Ar₂PCH₂PAr₂ (Ar = 2-C₆H₄(MeO)) have been synthesised. In the solid state, these complexes show tight phosphine bite angles in the range 67.82(4)° to 71.52(5)° and the nitrogen atom in N,N-bis(diarylphophino)amine ligands adopts an almost planar (sp²) geometry. All of the complexes are readily oxidised electrochemically or chemically to corresponding Cr(I) species. There is no evidence for coordination of the pendant ether group in derivatives with Ar = 2-MeO-C₆H₄ in either Cr(0) or Cr(I) species. Treatment of the [(diphosphine)Cr(CO)₄] complexes with [NO]BF₄ yields [(diphosphine)Cr(NO)(CO)₃]BF₄. Removal of CO ligands to generate an oligomerisation-active species is not observed with amine oxides but triethyl aluminium is effective in this role, and active catalysts can be produced. The use of weakly coordinating anions seems crucial in achieving oligomerisation catalysis.