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(η ⁴ -cod) ₂ ] (Cat=K (1), Na (2), Li (3), ( ^Dep nacnac)Mg (4), and N( ⁿ Bu) ₄ (5); cod=1,5-cyclooctadiene, ^Dep nacnac={2,6-Et ₂ C ₆ H ₃ NC(CH ₃ )} ₂ CH)]) 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.
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