Chemistry of bridging phosphanes:: PdI dimers bearing 2,5-dipyridylphosphole ligands

Two synthetic routes to Pd-I dimers that feature a bridging 1-phenyl- and 1-cyclohexyl-2,5-di(2-pyridyl)-phosphole ligand, 3a and 3b, respectively, are described. The first involves a conproportionation process between Pd-II and Pd-0 complexes, while the second involves ligand displacement from a preformed Pd-I dimer. Both routes are operable for 1-phenylphosphole 1a, whereas the former failed with 1-cyclohexylphosphole 1b. A mechanistic study revealed that the conproportionation pathway implies a reversible oxidative addition of the P-C(phenyl) bond of Pd-II-coordinated 1a to Pd-II leading to a bimetallic Pd-II complex 5. The structures of complexes 3a and 3b were studied by means of X-ray diffraction. The similarity of these solid-state structures suggests that the bridging mode of the P atom is due to mu-1kappaN:1,2kappaP:2kappaN coordination of ligands 1a, b. The electrochemical behaviour and UV/Vis absorption properties of complexes 3a, b are reported. Complex 3a is inert towards CO, PPh3 and 1,3-dipoles. It reacted with dimethylacetylene dicarboxylate to give complex 6 as a result of insertion of the alkyne into the Pd-Pd bond. X-ray diffraction studies of complexes 5 and 6 are also presented.