Chemistry of Bridging Phosphanes: Pd I 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, 3 a and 3 b , 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 1 a , whereas the former failed with 1‐cyclohexylphosphole 1 b . A mechanistic study revealed that the conproportionation pathway implies a reversible oxidative addition of the PC(phenyl) bond of Pd II ‐coordinated 1 a to Pd 0 leading to a bimetallic Pd II complex 5 . The structures of complexes 3 a and 3 b 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 μ ‐1 κN :1,2 κP :2 κN coordination of ligands 1 a , b . The electrochemical behaviour and UV/Vis absorption properties of complexes 3 a , b are reported. Complex 3 a is inert towards CO, PPh 3 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.