Disilaruthena- and Ferracyclic Complexes Containing Isocyanide Ligands as Effective Catalysts for Hydrogenation of Unfunctionalized Sterically Hindered Alkenes

Disilaferra- and disilaruthenacyclic complexes containing mesityl isocyanide as a ligand, 3′ and 4′, were synthesized and characterized by spectroscopy and crystallography. Both 3′ and 4′ showed excellent catalytic activity for the hydrogenation of alkenes. Compared with iron and ruthenium carbonyl analogues, 1′ and 2′, the isocyanide complexes 3′ and 4′ were more robust under the hydrogenation conditions, and were still active even at higher temperatures (∼80 °C) under high hydrogen pressure (∼20 atm). The iron complex 3′ exhibited the highest catalytic activity toward hydrogenation of mono-, di-, tri-, and tetrasubstituted alkenes among currently reported iron catalysts. Ruthenium complex 4′ catalyzed hydrogenation under very mild conditions, such as room temperature and 1 atm of H₂. The remarkably high catalytic activity of 4′ for hydrogenation of unfunctionalized tetrasubstituted alkenes was especially notable, because it was comparable to the activity of iridium complexes reported by Crabtree and Pfaltz, which are catalysts with the highest activity in the literature. DFT calculations suggested two plausible catalytic cycles, both of which involved activation of H₂ assisted by the metal-silicon bond through σ-bond metathesis of late transition metals (oxidative hydrogen migration). The linear structure of M C≡N - C (ipso carbon of the mesityl group) played an essential role in the efficient hydrogenation of sterically hindered tetrasubstituted alkenes.