Catalytic Hydrogenation of Alkenes and Alkynes by a Cobalt Pincer Complex: Evidence of Roles for Both Co(I) and Co(II)

The Co(I) complex, [Co(N-2)((PNP)-P-Cy)] ((PNP)-P-Cy = anion of 2,5-bis-(dicyclohexylphosphinomethyl)pyrrole), is active toward the catalytic hydrogenation of terminal alkenes and the semi-hydrogenation of internal alkynes under 2 bar of H-2 (g) at room temperature. The products of alkyne semi-hydrogenation are a mixture of E : and Z-alkenes. By contrast, use of the related cobalt(I) precatalyst, [Co(PMe3)-((TNP)-T-Cy)], results in formation of exclusively Z-alkenes. A semi-stable Co(II) species, [CoH((PNP)-P-Cy)], can also be generated by treatment of degassed solutions of [Co(N-2)((TNP)-T-Cy)] with H-2. The Co-II-hydride displays activity toward both alkene hydrogenation and isomerization, but its instability hampers implementation as a catalyst. Several species relevant to potential catalytic intermediates have been isolated and detected in solution. These compounds include alkene and alkyne adducts of Co(I) as well as a Co(III) dihydride species. Catalytic results with the compounds examined are most consistent with a process involving shuttling between Co(I) and Co(III) states. However, generation of small quantities of Co(II) during catalytic turnover appears to be responsible for the isomerization observed for alkyne semi-hydrogenation. The interplay of cobalt oxidation states within the same catalyst system is discussed in the context of mechanistic scenarios for catalytic hydrogenation.

Automated summary

This study investigates the activity of cobalt complexes in catalyzing the hydrogenation of alkenes and semi-hydrogenation of alkynes, highlighting the importance of the shuttling between Co(I) and Co(III) states in the reaction mechanism. The generation of small quantities of Co(II) during catalytic turnover is found to be responsible for the observed isomerization in alkyne semi-hydrogenation.