CO_2 reduction by Fe(I): solvent control of C-O cleavage versus C-C coupling

This manuscript explores the product distribution of the reaction of carbon dioxide with reactive iron(I) complexes supported by tris(phosphino)borate ligands, [PhBP^R_3]- ([PhBP^R_3]- =[PhB(CH_2PR_2)_3]-; R = CH_2Cy,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 CO_2 activation reaction. For instance, exposure of methylcyclohexane (MeCy) solutions of [PhBP^(CH_2Cy)_3 ]Fe(PR’_3) to CO_2 yields the partial decarbonylation product {[PhBP^(CH_2Cy)_3 ]Fe}_2(µ-O)(µ-CO). When the reaction is instead carried out in benzene or THF, reductive coupling of CO_2 occurs to give the bridging oxalate species {[PhBP^(CH_2Cy_3 ]Fe}_2(µ- κOO’: κOO’-oxalato). 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^(Ph)_3]- and [PhBP^(iPr)_3 ]-), complete decarbonylation is observed to afford structurally unusual diiron(II) products of the type {[PhBP^R_3]Fe}_2(µ-O). A mechanistic hypothesis that is consistent with the collection of results described is offered, and suggests that reductive coupling of CO_2 likely occurs from an electronically saturated “Fe^(II)–CO_2-” species