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

Abstract

The Co(I) complex, [Co(N 2 )( Cy PNP)] ( Cy PNP = 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(PMe 3 )-( Cy PNP)], results in formation of exclusively Z-alkenes. A semi-stable Co(II) species, [CoH( Cy PNP)], can also be generated by treatment of degassed solutions of [Co(N 2 )( Cy PNP)] 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.

Cobalt Complexes of Bulky PNP Ligand: H2 Activation and Catalytic Two-Electron Reactivity in Hydrogenation of Alkenes and Alkynes

“…For example, Kirchner and co-workers recently reported Mn and Fe pincer catalysts for hydrogenation of alkenes and semihydrogenation of alkynes, respectively. Cobalt pincer complexes were reported to be active in hydrogenation of alkenes and semihydrogenation of alkynes . A recent extensive review by Beller and coauthors describes the importance and activity of cobalt pincer complexes in catalysis …”

“…Cobalt pincer complexes were reported to be active in hydrogenation of alkenes and semihydrogenation of alkynes. 28 A recent extensive review by Beller and coauthors describes the importance and activity of cobalt pincer complexes in catalysis. 29 The basic organometallic reactivity of classical pyridinebased PNP Co complexes has also been studied by the Milstein and Chirik groups.…”

E-Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H2 Directly Employed or In Situ-Generated

“… 12 During the last decade, the concerns with regard to green chemistry principles have been grown rapidly, 13 and the development of catalysts based on earth-abundant metals for organic transformations has become very important. 14 In recent years, several reports on selective alkyne semihydrogenations using catalysts based on Cr, 15 Mn, 16 Fe, 17 Co, 18 Ni, 19 and Cu 20 are described.…”

Bench‐Stable Dinuclear Mn(I) Catalysts in E‐Selective Alkyne Semihydrogenation: A Mechanistic Investigation**

“…Significant work remains to uncover the nature of Mncatalyzed hydrogenation reaction mechanisms, and dinuclear catalysts have not been explored in detail. Furthermore, the importance of E-SASH to prepare biologically active compounds [16][17][18][19] using first row metals [20][21][22][23][24][25][26] holds promise for an applications-based outcome as a longer-term goal in sustainable metal catalysis. [27] As shown by us previously, bench-stable complexes such as 1 can be prepared in a single step from commercially available starting materials, namely HPR 2 and Mn 2 (CO) 10 (Scheme 1 and TOC graphic).…”