A new class of anionic phosphinooxazoline ligands in palladium and ruthenium complexes: catalytic properties for the transfer hydrogenation of acetophenone

The synthesis of the cationic Pd complex [Pd(dmba)(PCH2-oxazoline)]Cl [PCH2-oxazoline = 2-oxazoline-2-ylmethyl)diphenylphosphine] (2) has allowed for the first time the observation of hemilabile behaviour for a phosphinooxazoline ligand. This molecular dynamics is stopped upon removal of the chloride anion, by reaction with either NH4PF6, which retains the cationic nature of the complex, or (BuOK)-O-t. The latter reaction leads to the formation of the first example of a Pd complex bearing an anionic phosphinooxazoline ligand, [Pd(dmba){Ph2PCH -C(-N)CH2CH2O}], abreviated [Pd(dmba)(PCH-oxazoline)] (7), the anionic charge resulting from the monodeprotonation of the PCH2 group. Following this methodology, the Ru complex [RuCl(p-cymene){Ph2PCH -C(-N)CH2CH2O}], abreviated [RuCl(eta (6)-p-cymene)(PCH-oxazoline)] (8) and containing this four-electron chelating anionic ligand, was prepared and shown to be more reactive for the catalytic transfer hydrogenation of acetophenone in propan-2-ol than the analogous complex bearing the neutral phosphinooxazoline ligand PCH2-oxazoline. The crystal structure of cis-[Pd{Ph2PCH -C(-N)CH2CH2O}(2)], abbreviated cis-[Pd(PCH-oxazoline)(2)] (6), was determined by X-ray diffraction.