Tridentate and Tetradentate Iminophosphorane-Based Ruthenium Complexes in Catalytic Transfer Hydrogenation of Ketones

Tetradentate iminophosphorane complexes [RuH(PNNP)][BArF4] (5) and [RuHCl(PNNP)] (6) (PNNP = PPh2CH2PPh2NCH2CH2NPPh2CH2PPh2), prepared by in situ deprotonation of the aminophosphonium ligands in the presence of the dihydrogen complex RuH(H2)(Cl)(PCy3)2, are active precatalysts for the catalytic transfer hydrogenation of ketones at 80 °C under basic conditions in HOiPr. Complex 6 was characterized by X-ray diffraction. With a predominant focus on acetophenone, a comparative study with [RuH(H2NNPP)(PCy3)][BArF4] (3) incorporating a tridentate ligand (H2NNPP = H2N(C6H4)NPPh2CH2PPh2) does not indicate an NH effect. It is postulated that the pentacoordinated configuration around Ru with a hydride trans to a vacant site is essential in the catalyst resting state; however, the stability of this structural arrangement is disadvantageous for high activities. Further mechanistic studies, including labeling experiments, indicate facile protonation of these systems and, in addition, structural variations as a function of protonating agent (HOiPr, HX, H2O). The hydrido(chloro) complex RuH(Cl)(HNP)(NP) (7) (NP = Ph2PCH2PPh2NCH2tBu) was also isolated and characterized by X-ray diffraction, and a hydrogen bond between the chlorine and the NH group of the pendant ligand was ascertained by DFT calculations.