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

Abstract

A metal/ligand cooperative approach to the reduction of small molecules by metal silylene complexes (R 2 Si=M) is demonstrated, whereby silicon activates the incoming substrate and mediates net two-electron transformations by oneelectron redox processes at two metal centers. An appropriately tuned cationic pincer cobalt(I) complex, featuring a central silylene donor, reacts with CO 2 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.

Strongly Polarized Iridiumδ−–Aluminumδ+ Pairs: Unconventional Reactivity Patterns Including CO2 Cooperative Reductive Cleavage

“…On the other hand, the reactivity is slightly reminiscent of what is observed in f-element chemistry, ,− in particular with the formation of an intermediate where CO 2 is inserted between Ir and Al, but the protonation of CO 2 and the formation of a terminal hydroxyl unit are unique. Conceptually, the chemistry observed here is related to the heteroallene dipolar addition chemistry in Fisher carbene, − silylene, − or boryl complexes, for instance in the case where CO 2 is added across a IrC­(H)­(OR) fragment, which is nucleophilic at Ir and electrophilic at C, leading to an iridium carbonyl adduct and HC­(O)­OR …”

Carbon Dioxide Reduction with Dihydrogen and Silanes at Low-Valent Molybdenum Terphenyl Diphosphine Complexes: Reductant Identity Dictates Mechanism

“…However, higher silane concentration impeded the reaction, suggesting an abnormal CO 2 reduction mechanism ( vide infra ). While cooperative M/Si cleavage of CO 2 is known, − this reactivity would proceed from a Mo silyl complex and is therefore inconsistent with the observed inhibition (Scheme B, left). Additionally, increased steric bulk at silicon resulted in faster reactivity, with 6 forming 8 more slowly than 6′ , further supporting the premise that CO 2 reactivity requires silane dissociation.…”

Metal complexes containing silicon‐based pincer ligands: Reactivity and application in small molecule activation

“… 26 Although the ligand contained an anionic silyl donor, an alternative reaction pathway involving silylene was also suggested. A more recently reported silylene‐based cobalt(I) complex [( i Pr P 2 Si═)Co(CO) 2 ] could also activate CO 2 and liberate CO 147 …”