Silicon chemistry has become amongst the most practical in the 21st century, with uses spanning from semiconductors to composite ceramics and polymers. Most of these applications are based on materials containing heteronuclear Si–E (E = C, N, O) bonds with suitable physical and chemical properties (e.g. chemical inertness, thermal stability and good conductive properties) to allow for application in industry. However, the strength of the Si–E bond results in persistent and difficult to recycle materials. Hence, the synthesis and activation of monomeric precursors with Si–E bonds and their polymer counterparts has gained huge interest within the last decade. We have recently reported the straightforward dehydrocoupling reaction of silanes with amines, phosphines and alcohols using three-coordinate iron(II) β-diketiminate pre-catalysts, 1 (Scheme 1, right) to produce a variety of Si–E-type monomers and some Si–N and Si–O backbone polymers. Further investigations into silicon transformations found that 1 (in the presence of a mild reductant, pinacol borane) could also activate Si–O and Si–N bonds in siloxane and silazane monomers and polymers to perform Si–E bond-breaking desilylations. Here we discuss the expansion of the Si–E polymer scope through dehydrocoupling and the chemical recycling of these products by desilylation using 1. Our study represents one of the few examples of catalytic Si–E bond activation, complete with mechanistic and kinetic data. We have adapted the iron(II) mediated dehydrocoupling and desilylation processes to a circular approach for Si-based polymers, where successful polymerization and depolymerization of Si–E polymers was achieved. Useful borate esters or amine-boranes were obtained, and the system demonstrated accessible chemical recycling of heavily persistent silicon materials.

Polymerizing, depolymerizing and repurposing using iron catalysis

ACS Spring 2022

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