Decarbonylation, reductive electrochemistry and x-ray crystal structures of some rhodium diphosphine acyl complexes.
Acyl complexes of rhodium(III) with chelating diphosphine ligands (P-P = 1,3- bis(diphenylphosphino)propane and others) are well known for their stability toward decarbonylation. Various rhodium diphosphine acyl complexes were synthesized and characterized by IR, NMR (¹H, ¹⁹F, and ³¹P), cyclic voltammetry, elemental analysis and X-ray crystallography. The chemical and electrochemical reduction of the rhodium diphosphine acyl complexes Rh(P-P)(COR)I₂ involves a net two-electron transfer yielding Rh(P-P)(CO)I, alkyl anion and iodide. The mechanism involves an initial one-electron transfer followed by the liberation of one of the iodides. Then a second electron transfer with the migration of alkyl group takes place yielding the 18-electron complex [Rh(P-P)(CO)(R)I]⁻. This 18-electron complex loses the alkyl group as the anion, producing Rh(P-P)(CO)I as the final product. We observed a difference in thermal stability between the acetyl and trifluoroacetyl complexes. Others have found that the acetyl complex is very stable in terms of alkyl migration while the monodentate phosphine analog of this complex undergo alkyl migration followed by the loss of alkyl halide. We discovered that when the acetyl group is replaced by a trifluoroacetyl group the resulting complex is unstable in terms of alkyl migration. It slowly changes from the acyl complex to the alkyl complex in solution at room temperature. If the resulting solution is allowed to stand for a long period of time, ca. 20 days or more, it gives the decarbonylated product Rh(P-P)(CF₃)I₂. When the trifluoroacetyl group is replaced by a difluoroacetyl group the complex does not undergo alkyl migration while replacing it with a chlorodifluoroacetyl group increases the rate of alkyl migration. The pentafluoropropionyl complex also undergoes alkyl migration. X-ray crystal structures of 19 rhodium diphosphine complexes were measured and their geometric parameters are compared with related structures. All five-coordinate complexes have square pyramidal geometries with the acyl group occupying the apical position.