Comparison of the Thermal and Reductive Decarbonylation of a Rhodium Trifluoroacetyl Diphosphine Complex

Acyl complexes of rhodium(III) with chelating diphosphine ligands are well known for their stability toward decarbonylation. However, we have found that the corresponding perfluoro complexes do decarbonylate under mild conditions. In this report we address specifically Rh(dppp)-(COCF3)I-2 (dppp = 1,3-bis(diphenylphosphino)propane). Another difference between this compound and rhodium acyl complexes containing monodentate phosphines is that it does not undergo reductive elimination of alkyl halide after decarbonylation. Instead, slow CO dissociation from Rh(dppp)(CO)(CF3)I-2 occurs to yield Rh(dppp)(CF3)I-2 as the final product. Reduction of the trifluoroacetyl complex is a net two-electron process that also involves decarbonylation, but through a quite different mechanism. The products consist of Rh(dppp)(CO)I, CF3- anion, and iodide. X-ray crystal structures of both Rh(dppp)(COCF3)I-2 and Rh(dppp)(CF3)I-2 have been obtained and show the expected square-pyramidal geometry with the acyl/alkyl group in the apical position.