Electron-Rich, Bulky Ruthenium PNP-Type Complexes. Acceptorless Catalytic Alcohol Dehydrogenation

Reaction of the electron-rich, bulky tridentate PNP ligand (2,6-bis-(di-tert-butylphosphinomethyl)pyridine) with Ru(PPh3)3Cl2 at 65 °C resulted in formation of a solution containing the dinitrogen monomeric Ru(II) complex 1a and the N2-bridged dinuclear Ru(II) complex 1b, which can be interconverted. Passing argon through the solution results in formation of pure 1b. The Ru(II) hydride dinitrogen complex 2 was obtained by the reaction of complex 1b with 2 equiv of NaBEt3H. Complex 1b reacted with 4 equiv of AgOCOCF3 to yield [Ru(PNP)(CF3COO)2], 3. The Ru(II) carbonyl hydride complex 4 was obtained by the reaction of PNP and Ru2(OAc)4 in methanol as a result of O−H activation and decarbonylation of methanol. Complexes 1b, 2, and 4 were structurally characterized by X-ray crystallography. Complexes 1b and 2 catalyze the dehydrogenation of secondary alcohols to the corresponding ketones with good yields and high selectivity accompanied with the evolution of dihydrogen in a homogeneous system without a need for a hydrogen acceptor. The presumed intermediate Ru dihydride complex is generated in situ by reaction 1b or 2 with a base (1 equiv for each Ru−Cl bond), and the reaction can proceed in the absence of excess base or acid.