A mixed phosphine-iminophosphorane tetradentate ligand: Synthesis, coordination to group 10 metal centers, and use as catalyst in Suzuki-Miyaura coupling

The novel tetradentate ligand 1, associating two phosphines and two iminophosphorane moieties, was easily synthesized from commercially available bis(diphenylphosphino)methane and ethylenediamine via the preparation of the bis(phosphine-aminophosphonium) adduct 1-(HBr)₂. Deprotonation of this salt afforded the corresponding tetradentate P₂N₂ phosphine-iminophosphorane ligand 1. Coordination to Ni(II) and Pd(II) metal centers afforded complexes 2 and 3, respectively, characterized by multinuclear NMR and X-ray analysis. Both cationic complexes adopt a square-planar geometry around the metal, all phosphorus and nitrogen atoms being coordinated. In contrast to most iminophosphorane complexes, 2 and 3 could be handled in water without any apparent decomposition. Their catalytic activity was evaluated in Suzuki-Miyaura coupling. These reactions were conducted in toluene, water, or a biphasic toluene/water medium in the presence of a phase-transfer agent. Under these last conditions, the Pd catalyst 3 (0.1 mol % catalyst loading) could be reused for five cycles without noticeable loss of activity.