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 la and the N ₂-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 AgOCOCF ₃ to yield [Ru(PNP)(CF₃COO)₂], 3. The Ru(II) carbonyl hydride complex 4 was obtained by the reaction of PNP and Ru ₂(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.