Synthesis and characterization of homonuclear and heteronuclear complexes of platinum and palladium containing the linear tetraphosphine bis[(diphenylphosphinoethyl)phenylphosphino]methane

Transition metal complexes are used as catalysts in various reactions of industrial importance. A number of bimetallic complexes have been shown to exhibit higher catalytic activity than analogous species containing a single metal center. The reason for this improvement has been attributed to the cooperative effect both metals can have on each other during a catalytic cycle.; The bis[(diphenylphosphinoethyl)phenylphosphino]methane (DPPEPM) ligand has both bridging and chelating characteristics and can bind strongly to two metal centers. Due to presence of two chiral centers in the molecule, it exists as meso and racemic diastereomers. The ligand has been synthesized and separated into its two diastereomers. Symmetrical diplatinum and dipalladium complexes of the types [M₂Cl₄(μ-DPPEPM)] and [Pt₂R ₄(μ-DPPEPM)] (R = alkyl, aryl) have been synthesized, and characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The solid state structures reveal metal-metal distances of ca 6 Å. Using a similar approach, a series of chloride-bridged complexes, [M₂R₂(μ-Cl)(μ-DPPEPM)]PF ₆, in which the metal centers should be held closer to each other, were also synthesized. Studies involving substitution reactions with the latter complexes showed a facile opening and closing of the bridge, and this observation could have implications in catalytic processes. We have also used the DPPEPM ligand to prepare unsymmetrical diplatinum and heterobimetallic (Pd-Pt) complexes as well as some unexpected trimetallic derivatives.; DPPEPM-bridged diplatinum complexes, with tin(II) chloride, were employed as catalytic precursors for alkene hydroformylation, and certain dipalladium complexes were studied as catalysts for the Heck reaction. In both cases, the complexes exhibited moderate catalytic activity. We have attempted to separate the enantiomers of the racemic form of DPPEPM by complexation to a chiral palladium species. A partial separation (ca 65%) of one of the enantiomers from the 1:1 racemic mixture has been achieved.