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.