Bimetallic, Silylene-Mediated Multielectron Reductions of Carbon Dioxide and Ethylene

A metal/ligand cooperative approach to the reduction of small molecules by metal silylene complexes (R₂ Si=M) is demonstrated, whereby silicon activates the incoming substrate and mediates net two-electron transformations by one-electron redox processes at two metal centers. An appropriately tuned cationic pincer cobalt(I) complex, featuring a central silylene donor, reacts with CO₂ to afford a bimetallic siloxane, featuring two Co^II centers, with liberation of CO; reaction of the silylene complex with ethylene yields a similar bimetallic product with an ethylene bridge. Experimental and computational studies suggest a plausible mechanism proceeding by [2+2] cycloaddition to the silylene complex, which is quite sensitive to the steric environment. The Co^II /Co^II products are reactive to oxidation and reduction. Taken together, these findings demonstrate a strategy for metal/ligand cooperative small-molecule activation that is well-suited to 3d metals.