Stereoselective Preparation, Structures, and Reactivities of Phosphine-Bridged Mixed-Metal Trinuclear and Pentanuclear Complexes with Tris[2-(diphenylphosphino)ethyl]phosphine

The phosphine-bridged linear trinuclear and pentanuclear complexes with Pd(II)−Pt(II)−Pd(II), Ni(II)−Pt(II)−Ni(II),
and Rh(III)−Pd(II)−Pt(II)−Pd(II)−Rh(III) metal-ion sequences were almost quantitatively formed by the stepwise
phosphine-bridging reaction of the terminal phosphino groups of tris[2-(diphenylphosphino)ethyl]phosphine (pp3),
which is the tetradentate bound ligand of the starting Pd(II) and Ni(II) complexes. The solid-state structures of the
trinuclear complexes were determined by X-ray structural analyses, and the structures of the polynuclear complexes
in solution were characterized by NMR spectroscopy. The trans and cis isomers of the trinuclear and pentanuclear
complexes, which arise from the geometry around the Pt(II) center, were selectively obtained simply by changing
the counteranion of the starting complexes:  the tetrafluoroborate salts, [MX(pp3)](BF4) [M = Pd(II) or Ni(II), X =
Cl- or 4-chlorothiophenolate (4-Cltp-)], gave only the trans isomers, and the chloride salt, [PdCl(pp3)]Cl, gave only
the cis isomers. The formation of the trinuclear complex with the 4-Cltp- and chloro ligands, trans-[Pt(4-Cltp)2{PdCl(pp3)}2](BF4)2, proceeded with exchange between the thiolato ligand in the starting Pd(II) complex, [Pd(4-Cltp)(pp3)](BF4), and the chloro ligands in the starting Pt(II) complex, trans-[PtCl2(NCC6H5)2], retaining the trans
geometry around the Pt(II) center. In contrast, the formation reaction between [PdCl(pp3)]Cl and trans-[PtCl2(NCC6H5)2]
was accompanied by the trans-to-cis geometrical change on the Pt(II) center to give the trinuclear complex, cis-[PtCl2{PdCl(pp3)}2]Cl2. The mechanisms of these structural conversions during the formation reactions were elucidated
by the 31P NMR and absorption spectral changes. The differences in the catalytic activity for the Heck reaction
were discussed in connection with the bridging structures of the polynuclear complexes in the catalytic cycle.