Pincer CNN Ruthenium(II) Complexes with Oxygen-Containing Ligands (O2CR, OAr, OR, OSiR3, O3SCF3): Synthesis, Structure, and Catalytic Activity in Fast Transfer Hydrogenation

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The pincer complexes [RuX(CNN)(dppb)] ( 5 − 11: X = carboxylate, phenoxide, alkoxide, silanolate, triflate; HCNN = 1-[6-(4′-methylphenyl)pyridin-2-yl]methanamine; dppb = Ph 2 P(CH 2 ) 4 PPh 2 ), containing the Ru−NH 2 functionality and a monodentate oxygen donor ligand, have been prepared in high yield starting from [RuCl(CNN)(dppb)] ( 1 ) via substitution of the chloride with sodium or thallium compounds or protonation of the alkoxide [Ru(O i Pr)(CNN)(dppb)]· n ( i PrOH) ( 3 ). In the solid state the formate 5 shows intermolecular NH···O hydrogen bonds, whereas the acetate 6 displays an intramolecular interaction, as inferred from X-ray studies. Addition of phenol and alcohol compounds to the phenoxide and alkoxide complexes leads to fast ligand exchange through hydrogen bond interactions. The corresponding pincer complexes [RuX(CNN′)(dppb)] ( 12, 13b: X = phenoxide, alkoxide; HCNN′ = N, N -dimethyl-1-[6-(4′-methylphenyl)pyridin-2-yl]methanamine), containing the Ru−NMe 2 moiety, have been obtained from [RuCl(CNN′)(dppb)] ( 4 ) and sodium phenoxide or alkoxide. All the NH 2 complexes 5 − 11 are highly active catalysts in the transfer hydrogenation of ketones in 2-propanol with NaO i Pr, affording complete conversion in a few minutes and achieving TOF values of up to 3.8 × 10 6 h −1 with 0.005 mol % of catalyst.