Octaethyldiphosphaferrocene:  An Efficient Ligand in the Palladium-Catalyzed Suzuki Cross-Coupling Reaction

Syntheses of octaethyldiphosphaferrocene 4 and tetraethylphosphaferrocene 5 are presented. Tetraethylzirconacyclopentadiene 1 reacts with PCl3 in dichloromethane to yield the 1-chlorotetraethylphosphole 2, which upon reduction by lithium in excess, yields the teraethylphospholide anion 3. Anion 3 was subsequently converted into 4 or 5 by treatment with FeCl2 or with [Fe(η6-C9H12)(η5-C5H5)][PF6] respectively. The structure of diphosphaferrocene 4 was determined. Two conformations are present in the cell:  a C2 conformation (α = 51.1°) in which the P atom of each ring is located above the α-carbon atom of the other ring, and a C2h conformation (α = 180°) in which P atoms point in opposite directions. Reaction of ligand 4 with [Pd(dba)2] yields the bis(octaethyldiphosphaferrocene)palladium(0) complex 6. An X-ray crystallographic study reveals that the overall geometry around palladium is nearly tetrahedral and that both ligands, whose geometry is not significantly perturbed, adopt a bridging mode involving a...