CO2 Reduction at a Borane-Modified Iron Complex: A Secondary Coordination Sphere Strategy

Abstract

This work addresses fundamental questions that deepen our understanding of secondary coordination sphere effects on carbon dioxide (CO₂) reduction using derivatized hydride analogues of the type, [Cp*Fe(diphosphine)H] (Cp* = C₅Me₅⁻) – a well-studied family of organometallic complex – as models. More precisely, we describe the general reactivity of [(Cp*-BR₂)Fe(diphosphine)H], which contains an intramolecularly positioned Lewis acid, and its cooperative reactivity with CO₂. Control experiments underscore the critical nature of borane incorporation for transforming CO₂ to reduced products, a reaction that does not occur for unfunctionalized [Cp*Fe(diphosphine)H]. Additional experiments highlight relevance of borane hybridization and substituent effects. Mechanistic studies performed in the presence and absence of CO₂ emphasize the significance of carbonyl substrate to catalyst longevity. Lessons from these reactions were also transferable – with such borane-containing complexes enabling the chemoselective reduction of aldehydes in the presence of alkenes. These findings provide valuable insights into metal-ligand cooperative design strategies for carbonyl reduction and illustrate the versatility of intramolecularly positioned Lewis acids for otherwise challenging chemical transformations.