Electron-rich, bulky PNN-type ruthenium complexes: synthesis, characterization and catalysis of alcohol dehydrogenation

Reaction of the electron-rich, bulky tridentate PNN ligand (PNN = 2-di-tert-butylphosphinomethyl-6-diethylaminomethylpyridine) with Ru(PPh3)(3)Cl-2 at 65 degrees C resulted in formation of the mononuclear dinitrogen complex (PNN)Ru(Cl)(2)N-2 1a (minor) and the N-2 bridged Ru(II) dinuclear complex [(PNN)Ru(Cl)(2)](2)(mu-N-2) 1b (major). These complexes can be interconverted; passing argon through a solution of the mixture resulted in formation of pure 1b. The cationic square-pyramidal [(PNN)Ru(PPh3)Cl]OTf 2 was obtained by the reaction of complex 1b with silver triflate followed by PPh3. Reaction of complex 1b with CO yielded (PNN)Ru(CO)Cl-2 3, which upon reaction with one equiv. of AgBF4 gave the cationic [(PNN)Ru(CO)Cl]BF4 4. The dicationic [(PNN)Ru(CO)(H2O)(acetone)](BF4)(2) 5 was obtained from 3 with 2 equiv. of AgBF4 in acetone solution. Complexes 1b, 2 and 5 were structurally characterized by X-ray crystallography. Complexes 1b and 3, upon addition of an equivalent of base, catalyzed the dehydrogenation of secondary alcohols to the corresponding ketones and primary alcohols to esters in good yields and high selectivity accompanied with the evolution of hydrogen gas.