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

Abstract

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 Bu^tOK. The latter reaction leads to the formation of the first example of a Pd complex bearing an anionic phosphinooxazoline ligand, [Pd(dmba){Ph₂PCHC(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){Ph₂PCHC(N)CH₂CH₂O}], abreviated [RuCl(η⁶-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{Ph₂PCHC(N)CH₂CH₂O}₂], abbreviated cis-[Pd(PCH-oxazoline)₂] (6), was determined by X-ray diffraction.