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.