The synthesis of an anionic, tetraphenylborate-functionalized, [P,N]-hybrid phosphinobenzimidazole ligand and its hemilabile behaviour in ruthenium zwitterion chemistry

A new anionic [P,N]-hybrid ligand based on a phosphinobenzimidazole scaffold and functionalized with a tetraphenylborate substituent is reported. This new anionic ligand readily chelates to a variety of ruthenium-cyclopentadienyl and -pentamethylcyclopentadienyl precursors to form a series of zwitterionic ruthenium piano-stool complexes (η5-C5R5)Ru(L)(κ2-P,N) (R = H or Me; L = CO or PPh3). In the presence of excess CO or 1-alkynes, the chelate complexes undergo ring-opening of the κ2-P,N ligand at the ruthenium-nitrogen bond (in some cases reversibly) under relatively mild conditions. In particular, the reactions with 1-alkynes proceed via vinylidene intermediates which subsequently insert into the ruthenium-nitrogen bond of the κ2-P,N ligand.