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

The pincer complexes [RuX(CNN)(dppb)] (5−11: X = carboxylate, phenoxide, alkoxide, silanolate, triflate; HCNN = 1-[6-(4′-methylphenyl)pyridin-2-yl]methanamine; dppb = Ph2P(CH2)4PPh2), containing the Ru−NH2 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(OiPr)(CNN)(dppb)]·n(iPrOH) (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−NMe2 moiety, have been obtained from [RuCl(CNN′)(dppb)] (4) and sodium phenoxide or alkoxide. All the NH2 complexes 5−11 are highly active catalysts in the transfer hydrogenation of ketones in 2-propanol with NaOiPr, affording complete conversion in a few minutes and achieving TOF values of up to 3.8 × 106 h−1 with 0.005 mol % of catalyst.