Thiolato-Ligand Substitution Reaction with Halide Ions of Five-Coordinate Trigonal-Bipyramidal Palladium(II) Complexes with Tris(2-(diphenylphosphino)ethyl)phosphine. Electronic and Steric Effects on the Reaction Mechanism

Abstract

Abstract Mononuclear and dinuclear thiolato complexes with five-coordinate trigonal-bipyramidal palladium(II), [Pd(pt or tt)(pp3)](BF4) and [Pd2(pdt)(pp3)2](BF4)2 (pp3 = tris(2-(diphenylphosphino)ethyl)phosphine, pt = 1-propanethiolate, tt = α-toluenethiolate, pdt = 1,3-propanedithiolate) have been synthesized. The solid-state structure of the dinuclear pdt complex was confirmed by an X-ray crystal structure analysis and the structures of the pt, tt, and pdt complexes in solution were characterized by 31P NMR spectroscopy. It is indicated from the 31P NMR chemical shifts of the bound pp3 ligand that the axially coordinated thiolato ligands are strong σ and π donors compared with the halo ligands in the axial position of the corresponding trigonal-bipyramidal palladium(II) complexes, [PdX(pp3)]+ (X- = Cl-, Br-, I-). The kinetic parameters for the one-step and successive two-step thiolato-ligand substitution reactions with halide ions in chloroform were obtained for the mononuclear and dinuclear complexes, respectively. These results have revealed that the reaction mechanism of the present thiolato-ligand substitutions is much more dissociative than that of the corresponding halo-ligand substitutions using trimethyl phosphite as the entering ligand, and that the reaction mechanism of the two-step substitution of the dinuclear thiolato complex is more dissociative than that of the mononuclear complexes. It has been concluded that the reaction mechanism of the five-coordinate trigonal-bipyramidal palladium(II) complexes with the 18-electron ground state is quite sensitive to the electronic properties of the entering and leaving ligands and the steric environment of the reaction site compared with that of four-coordinate square-planar palladium(II) complexes with the 16-electron ground state, which is generally associatively activated.