Utilization of ruthenium(0) for σ-bond activation: Formation of organoruthenium complexes and emergence of their catalytic activity via in situ Ru-C bond cleavage

In this paper we report the successful utilization of ruthenium(0) for phenolic OH followed by aryl CH bond activations leading to the formation of ruthenium-hydrido and cycloruthenated complexes; and catalytic efficiency of these complexes towards transfer-hydrogenation (TH) of selected substrates. Reaction of [Ru⁰(CO)₃(PPh₃)₂] with 2-(arylazo)phenol (L-R; R = OCH₃, CH₃, H, Cl and NO₂) in refluxing toluene affords two products: a Ru-hydrido complex (depicted as 1-R) formed via activation of the phenolic OH bond; and a cycloruthenated complex (depicted as 2-R) via successive phenolic OH and aryl CH bond activations. The formation mechanism of the two types of complexes was probed with DFT calculations, which revealed that initial phenolic OH bond activation by Ru(0) center produces 1-R. Then 1-R converts, via absorption of thermal energy provided by the refluxing solvent, into its geometrical isomer with change in mutual disposition of the coordinated carbonyl and hydride. Finally, aryl CH bond activation takes place, assisted by the coordinated hydride, to form the cycloruthenated complex 2-R via elimination of molecular hydrogen. Exclusive formation of 1-R or 2-R complexes could also be achieved by carrying out the synthetic reaction respectively in refluxing acetone and ortho-xylene. Crystal structures of selected members from both the 1-R and 2-R families have been determined by X-ray diffraction method. The hydrido (1-R) complexes were found to efficiently catalyze TH of aryl/alkyl aldehydes and ketones to the corresponding alcohols, using 2-propanol as the provider of hydrogen. A solvent coordinated hydrido species (A), generated in situ via displacement of a PPh₃ from 1-R by 2-propanol, is believed to function as the catalytically active species. The cycloruthenated (2-R) complexes, which do not contain any Ru-H moiety in them, are also found to catalyze similar TH with equal efficiency. Through electronic spectral studies it has been realized that the same catalytically active Ru-hydrido species (A) is generated in situ from the 2-R complexes via 2-propanol assisted cleavage of the Ru-C bond.