ИСТИНА

We have recently reported the synthesis and chemical properties of a series of half-sandwich arene ruthenium iminophosphonamide (NPN) complexes [(arene)Ru{(R2P(NR’)2}(X)] (X = Cl-, PF6-), in which a strong σ,-donating zwitterionic NPN ligand can readily stabilize the coordinatively unsaturated 16ē complexes [1]. We showed that [(p-cymene)Ru{(Ph2P(NTol)2}(X)] (1, X = Cl) can efficiently catalyze transfer hydrogenation of acetophenone in isopropanol when activated with base by generating the active hydride complex (X = H). The catalytic mechanism was thoroughly scrutinized by experimental kinetics investigations as well as DFT calculations to propose a catalytic cycle involving an outer-sphere hydride transfer to ketone via a zwitterionic intermediate [2]. Here we present the arene and N-, P- substituent effect on activity and stability in the transfer hydrogenation of acetophenone catalyzed by 2-6. Hexamethylbenzene significantly improves the catalyst long-term stability. Electron-releasing groups at both N- and P-atoms enhance the catalyst activity (4,5) at the expense of its stability in isopropanol. Surprisingly, complex 6 having different N- alkyl/aryl substituents is both highly active, even in the absence of strong base, and has good long-term stability. The reasons for these trends are discussed.