Novel neutral phosphinite bridged dinuclear ruthenium(II) arene complexes and their catalytic use in transfer hydrogenation of aromatic ketones: X-ray structure of a new Schiff base, N3,N3′-di-2-hydroxybenzylidene-[2,2′]bipyridinyl-3,3′-diamine

Abstract

A novel Schiff base N3,N3′-di-2-hydroxybenzylidene-[2,2′]bipyridinyl-3,3′-diamine, 1 was synthesized from condensation of salicylaldehyde with 3,3′-diamino-2,2′-bipyridine. Reaction of 1 with two equivalents of PPh 2Cl in the presence of Et 3N proceeds in toluene to give N3,N3′-di-2-(diphenylphosphino)benzylidene-[2,2′]bipyridinyl-3,3′-diamine, 2 in quantitative yield. Ruthenium(II) dimers [Ru(η 6-arene)(μ-Cl)Cl] 2 readily react with phosphinite ligand [(Ph 2PO) 2-C 24H 16N 4], 2 in toluene at room temperature, to afford the neutral derivatives [C 24H 16N 4{OPPh 2–Ru(η 6-arene)Cl 2} 2] {arene: benzene 3, p-cymene, 4}. All the complexes were fully characterized by analytical and spectroscopic methods. 31P–{ 1H} NMR, 1H– 13C HETCOR or 1H– 1H COSY correlation experiments were used to confirm the spectral assignments. Molecular structure of the Schiff base, 1 was also determined by X-ray single crystal diffraction study. The catalytic activity of complexes 3 and 4 in the transfer hydrogenation of acetophenone derivatives was tested. Stable ruthenium(II)–phosphinite complexes were found to be efficient catalysts in the transfer hydrogenation of aromatic ketones in excellent conversions up to 99% (up to 530 per hour) in the presence of iso-PrOH/KOH.