Counterion Effect in Cobaltate-Catalyzed Alkene Hydrogenation

We show that countercations exert a remarkable influence on the ability of anionic cobaltate salts to catalyze challenging alkene hydrogenations. An evaluation of the catalytic properties of [Cat][Co(η⁴-cod)₂] (Cat=K (1), Na (2), Li (3), (^Depnacnac)Mg (4), and N(ⁿBu)₄ (5); cod=1,5-cyclooctadiene, ^Depnacnac={2,6-Et₂C₆H₃NC(CH₃)}₂CH)]) demonstrated that the lithium salt 3 and magnesium salt 4 drastically outperform the other catalysts. Complex 4 was the most active catalyst, which readily promotes the hydrogenation of highly congested alkenes under mild conditions. A plausible catalytic mechanism is proposed based on density functional theory (DFT) investigations. Furthermore, combined molecular dynamics (MD) simulation and DFT studies were used to examine the turnover-limiting migratory insertion step. The results of these studies suggest an active co-catalytic role of the counterion in the hydrogenation reaction through the coordination to cobalt hydride intermediates.