CO2reduction by Fe(i): solvent control of C–O cleavage versus C–C coupling

Abstract

This manuscript explores the product distribution of the reaction of carbon dioxide with reactive iron(I) complexes supported by tris(phosphino)borate ligands, [PhBP^R₃]⁻ ([PhBP^R₃]⁻ = [PhB(CH₂PR₂)₃]⁻; R = CH₂Cy, Ph, ⁱPr, mter; mter = 3,5-meta-terphenyl). Our studies reveal an interesting and unexpected role for the solvent medium with respect to the course of the CO₂ activation reaction. For instance, exposure of methylcyclohexane (MeCy) solutions of to CO₂ yields the partial decarbonylation product . When the reaction is instead carried out in benzene or THF, reductive coupling of CO₂ occurs to give the bridging oxalate species . Reaction studies aimed at understanding this solvent effect are presented, and suggest that the product profile is ultimately determined by the ability of the solvent to coordinate the iron center. When more sterically encumbering auxiliary ligands are employed to support the iron(I) center (i.e., [PhBP^Ph₃]⁻ and [PhBP^iPr₃]⁻), complete decarbonylation is observed to afford structurally unusual diiron(II) products of the type {[PhBP^R₃]Fe}₂(μ-O). A mechanistic hypothesis that is consistent with the collection of results described is offered, and suggests that reductive coupling of CO₂ likely occurs from an electronically saturated “Fe^II–CO₂˙⁻” species.