Tris(pyrazolyl)phosphine Oxides. Synthesis and Coordination Chemistry with Copper(I)

A set of substituted tris(pyrazolyl)phosphine oxides (OP(pz(x))(3)) has been prepared in high yield and applied as neutral scorpion-type ligands. The P apex provides a convenient spectroscopic handle. Substitution at the 3-position of the pyrazolyl ring influences the steric demands of the ligand, while substitution at the 5-position enhances the stability. Copper(I) acetonitrile complexes of the OP(pz(x))(3) ligands were prepared and tested in ligand exchange reactions with PPh3 and CO. The v(CO) values of the carbonyl complexes demonstrate the electron-withdrawing properties of the ligands. These observations show that OP(pz(x))(3) ligands are an interesting extension of the widely used scorpion-type ligands.