Coordination of NNN iminophosphorane ligands to Mn^II, Co^II, and Fe^II and application in the transfer hydrogenation of ketones

Tridentate NNN ligands combining three different N-coordinating groups; a pyridine, a secondary amine, and an iminophosphorane were synthesized (L^R, R = Et, Cy, Ph). They were coordinated to Fe^II, Co^II, and Mn^II to yield high spin complexes as indicated by the magnetic moment measured in solution. They were also characterized by ¹H NMR spectroscopy, HR-MS, elemental analysis, as well as X-ray diffraction for some of them. Depending on the nature of the P-substituent, ligand exchange occurs in solution leading to octahedral complexes featuring two L^Cy ligand coordinated in a κ² mode. The latter were also independently prepared. All the synthesized complexes were compared in the catalytic transfer hydrogenation of ketones. L^Et supported complexes performed better and the reaction was optimized for the Co^II and Mn^II complexes because these metals are more rarely employed for this reaction than Fe^II. Both complexes proved able to efficiently reduce a range of aromatic and aliphatic ketones within 24 h at 70 °C using 1 mol% catalyst in presence of 20 mol% of ^tBuOK. The reaction is facilitated by the presence of electron-withdrawing and the absence of coordinating atom in the substrate. Moreover, for aliphatic ketones, while acyclic derivatives and linear methylketones convert very efficiently, substrates with long linear alkyl chain did not react. Regarding the mechanism, some additional experiments allow to suggest, despite the paramagnetic nature of the precatalysts, the involvement of M-H species.