Copper(I) diphosphine catalysts for C-N bond formation: Synthesis, structure, and ligand effects

A series of copper(I) complexes containing bis(diarylphosphino)propane, bis(diarylphosphino)ethane, bis(diarylphosphino)methane, and N,N-bis(diarylphosphino)amine ligands (Aryl = Ph, 2-C6H4(Me) or 2-C6H4(i-Pr)) has been synthesized. Crystal structures of selected chloride derivatives are reported. The complex structures proved to be very sensitive to both the backbone and P-substituents of the chelating ligand. The complexes have been screened for catalytic amidation reactions. Although in most cases only very low activity is observed, comparable with simple copper halide salts, notable exceptions are catalysts based on N,N-bis(diphenylphosphino)amine ligands, where a significant improvement in catalyst efficiency is observed. We propose the unusual electronic properties of these ligands may be the cause of their distinctive performance in these and other catalytic reactions where hard donor ligands have previously been employed.