Experimental and theoretical studies on the mechanism of hydrogenation by η6-arene β-diketiminato -ruthenium and -osmium complexes

The β-diketiminate class of ligands is renowned for the stabilization of coordinatively unsatd. main-group and transition metal complexes. Moreover, the application and scope of transition metal β-diketiminate complexes in the catalytic transformation of org. substrates is rapidly increasing. We recently reported on the synthesis, characterization and hydrogenation capabilities of a series of cationic η6-arene Ru- and Os-β-diketiminate complexes, 1. This family of complexes readily undergoes thermo-reversible [4+2] cycloaddn. with alkenes, 2 alkynes and H2 3. DFT calcns., in adjunction with high pressure NMR-deuterium labeling expts., reveal a concerted heterolytic cleavage mechanism with a low energy barrier. Now, a complete mechanistic pathway for alkene hydrogenation for complexes such as 1 has been modeled and supported by exptl. results. Furthermore, in situ NMR studies show that η6-arene hydrogenation is essential for the formation of the active catalytic species. Through steric and electronic variation of both the arene and β-diketiminate ligand, the efficiency of styrene, cyclohexene and 1-methylcyclohexene hydrogenation has been shown to be tunable, with a high turnover rate obsd. Lastly the dehydrogenation application of this family of complexes towards other types of catalysis and substrates will be discussed.