CpCo Complexes Research Articles

Silicon−Hydrogen Bond Activation and Hydrosilylation of Alkenes Mediated by CpCo Complexes: A Theoretical Study

Using DFT techniques, we show that triplet cyclopentadienylcobalt activates Si-H bonds to generate singlet silylcobalt hydrides without the intervention of sigma-silanes. The cobalt is configurationally unstable, as evidenced by the diastereoisomerization of derivatives bearing chiral silyl ligands. Inversion at the metal proceeds in the singlet state via a bridging hydride. We demonstrate that a two-state mechanism for the transformation of silyl hydride cobalt complexes into disilyl dihydride cobalt species is feasible. Our calculations predict that catalytic hydrosilylation of alkenes should be achievable in the coordination sphere of cyclopentadienylcobalt.

Cobalt-Catalyzed Cyclotrimerization of Alkynes: The Answer to the Puzzle of Parallel Reaction Pathways

To understand some experimental data at odds with the computed mechanism of the CpCo(L2)-catalyzed [2 + 2 + 2] cyclotrimerization of ethyne, DFT computations were carried out following the fate of methyl- and hydroxycarbonyl-substituted alkynes to give the corresponding arenes. The key intermediate in all cases is a triplet cobaltacyclopentadiene obtained by oxidative coupling of the corresponding CpCo(bisalkyne) complex and subsequent spin change via a minimum energy crossing point (MECP). From that species, two different catalytic cycles lead to an arene product, depending on the nature of the alkyne and other ligands present: either alkyne ligation to furnish a cobaltacyclopentadiene(alkyne) intermediate or trapping by a sigma-donor ligand to generate a coordinatively saturated cobaltacyclopentadiene(PR3) complex. The former leads to the CpCo-complexed arene product via intramolecular cobalt-assisted [4 + 2] cycloaddition, whereas the latter may, in the case of a reactive dienophile (butynedioic acid), undergo direct intermolecular [4 + 2] cycloaddition to generate a cobaltanorbornene. The bridgehead cobalt atom is then reductively eliminated after another change in spin state from singlet to triplet. The necessary conditions for one or the other mechanistic pathway are elaborated.