Palladium(II) Complexes with the 4,5‐Bis(diphenylphosphino)acenaphthene Ligand and Their Reactivity with Ethylene

The last two decades have witnessed the development of homogeneous catalysts for ethylene homo‐ and co‐polymerization reactions based on late transition metals. When Pd(II) is the metal of choice, the best ligand‐metal combination deals with either bidentate nitrogen‐donor molecules or phosphinobenzene sulfonate derivatives. In this contribution we have investigated the coordination chemistry to Pd(II) of a bidentate phosphorus ligand, namely 4,5‐bis(diphenylphosphino)acenaphthene (1). Starting from the neutral complex, [Pd(1)(CH3)Cl], we obtained the cationic derivatives [Pd(1)(CH3)(L)][SbF6], with L being either CH3CN or 3,5‐lutidine. Using in situ NMR spectroscopy we investigated the reaction of [Pd(1)(CH3)(NCCH3)][SbF6] with ethylene, at room temperature, and ambient ethylene pressure. We discovered that [Pd(1)(CH3)(NCCH3)][SbF6] acts as a catalyst for butenes and hexenes synthesis with the relevant Pd‐ethyl intermediate as the catalyst resting state. At the same time the color of the solution turned from pale yellow to light red due to the formation of the dinuclear species [Pd(μ‐η2−C6H5)PPh)−PPh2]2[SbF6]2. Both the neutral Pd(II) complex, activated in situ by NaSbF6, and the monocationic acetonitrile derivative were tested in the ethylene homopolymerization reaction at high pressure, leading to low molecular weight, branched, polyethylene. A Pd(II) complex with a diphosphine ligand (P−P) having an acenaphthene skeleton has been studied as catalyst for ethylene oligomerization. Mechanistic investigations on the reaction of the Pd−(P−P) complex with ethylene point out the formation of butenes, internal hexenes, a dimeric Pd(I) species and a Pd‐ethyl intermediate, the latter being recognized as the catalyst resting state.