The ER has started his project by synthesizing rhodium and iridium complexes previously used in the group as pre-catalysts in ketone hydrogenation. These complexes contain a bidentate phosphine-thioether ligand (PS ligand) based on the ferrocene scaffold with planar chirality. He has tested these complexes as catalysts in transfer hydrogenation of the same substrates where the source of hydrogen is now isopropanol rather than gaseous H2. He has also used the same compounds in the catalytic redox isomerization of allylic alcohols. A striking results of this work is that, although the precatalysts need hydrogen from H2 or from isopropanol for activation and these conditions ultimately lead to hydrogenation of the isomerized ketone product, the allylic alcohol isomerization occurs much more rapidly than the subsequent reduction, allowing the ketone intermediates to be obtained in good selectivities, especially under transfer hydrogenation conditions (absence of gaseous H2). The Rh precatalysts were more active than the Ir analogues. All these processes need a strong base for activity.
According to previous work in the group, the active form of the catalyst for the iridium system in ketone hydrogenation seems to be an anionic hydride, [IrH4(L)2]- (L = ligand) involving a cooperative effect of the counter-cation for the ketone activation. The role of the strong base seems that of allowing deprotonation of intermediate alcohol complexes, followed by beta-H elimination and stabilization of the anionic hydride systems. Therefore, a considerable effort has been made in synthesizing, isolating and characterizing models of this anionic complex and in studying the stoichiometric steps leading to these species from the pre-catalyst. Using PPh3 as ligand L, only neutral hydride systems could be obtained starting from the common precatalyst [Ir(COD)Cl]2 (COD = 1,5-cyclooctadiene). This is perhaps related to the monodentate nature of the ligand, allowing stabilization as neuitral [IrH3(L)3]. This work has nevertheless allowed to confirm that the pre-catalyst can be activated under transfer hydrogenation conditions by the strong base, i.d. H2 is not necessary, and that the cyclooctadiene ligand is lost during the process. This point was previously controversial. This work is now published (J. Organometal. Chem. 2017, 829, 14-21).
Using diphenylphosphinoethane (dppe) as a bidentate ligand (L2), the ER has indeed been able to obtain evidence that an anionic hydride is formed. However, this compound is too insoluble and air sensitive for a spectroscopic characterization and decomposes in strongly coordinating solvents (a crystal of [IrH4(dppe)(DMSO)] has been obtained from a crystallization from dimethylsulfoxide). Its catalytic activity in ketone transfer hydrogenation has been demonstrated. It has also been shown that the nature of the cation (Na vs. K) and the presence of chelators for the cation (e.g. crown ethers) affect the catalytic performance, demonstrating the importance of the Ir-cation cooperativity.
During the secondment period at the University of York, the ER has investigated the mechanism of the H2 addition to the Ir complex [Ir(COD)(L2)]+, with L2 = ferrocene PS ligand, by using the NME technique with para-hydrogen at various temperatures. The study has revealed an extremely complex mechanism, with many intermediates. The study is now being continued on a collaborative basis by the York group, after the return of the ER to the main host institution in Toulouse, using simpler ligands (e.g. dppe).
