Tridentate and tetradentate iminophosphorane-based ruthenium complexes in catalytic transfer hydrogenation of ketones

Tetradentate iminophosphorane complexes [RuH(PNNP)][BAr^F ₄] (5) and [RuHCl(PNNP)] (6) (PNNP = PPh₂CH ₂PPh₂NCH₂CH₂NPPh₂CH ₂PPh₂), prepared by in situ deprotonation of the aminophosphonium ligands in the presence of the dihydrogen complex RuH(H ₂)(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(H ₂NNPP)(PCy₃)][BAr^F₄] (3) incorporating a tridentate ligand (H₂NNPP = H₂N(C ₆H₄)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 = Ph₂PCH₂PPh₂NCH ₂^tBu) 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.