Utilization of In Situ Generated Ruthenium(0) for O−H and C−H Bond Activation of N-(Aryl)salicylaldimines: Formation of New Complexes and Their Application in Catalytic Transfer Hydrogenation

A group of four N-(4-R-phenyl)salicylaldimines (abbreviated in general as L¹-R; where R=OCH₃, CH₃, H and Cl) were found to undergo activation of the phenolic O−H bond upon their reaction with [Ru(PPh₃)₂(CO)₂Cl₂] in refluxing toluene in the presence of triethylamine to afford hydrido complexes of type [Ru(PPh₃)₂(CO)(NO−R)(H)], 1-R, where NO−R depicts the anionic N,O-donor ligand derived from L¹-R. This reaction was realized to proceed via the intermediacy of an in situ generated ruthenium(0) species of type [Ru(PPh₃)₂(CO)₂]. Formation of 1-R goes via isomerization of an initially formed species 1-R′ with mutual change of disposition of CO and hydride. Utilizing the appropriate orientation of the Ru-bound hydride in the initial product (1-R′), cycloruthenated complexes [Ru(PPh₃)₂(CO)(CNO²)], 2, and [Ru(PPh₃)₂(CO)(CNO³)], 3, were synthesized by using N-(naphthyl)salicylaldimine (L²) and N-(pyrenyl)salicylaldimine (L³), where CNO² and CNO³ depict the dianionic CN,O-donor ligands derived from L² and L³ respectively. Crystal structures of the 1-OCH₃, 1-Cl, 2 and 3 complexes have been determined. Formation mechanism of the complexes has also been proposed. All the complexes show intense absorptions in the visible and ultraviolet regions, which have been analyzed by TDDFT calculations. The 1-R complexes are found to efficiently catalyze transfer hydrogenation of aldehydes, ketones to the corresponding alcohols, and alkynes to the corresponding alkenes.