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 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.