Carbon dioxide reduction by lanthanide(iii) complexes supported by redox-active Schiff base ligands

Abstract

Here we have explored the ability of Schiff bases to act as electron reservoirs and to enable the multi-electron reduction of small molecules by lanthanide complexes. We report the reductive chemistry of the Ln(III) complexes of the tripodal heptadentate Schiff base H₃trensal (2,2′,2′′-tris(salicylideneimino)triethylamine), [Ln^III(trensal)], 1-Ln (Ln = Sm, Nd, Eu). We show that the reduction of the [Eu^III(trensal)] complex leads to the first example of a Eu(II) Schiff base complex [{K(μ-THF)(THF)₂}₂{Eu^II(trensal)}₂], 3-Eu. In contrast the one- and two-electron reduction of the [Nd^III(trensal)] and [Sm^III(trensal)] leads to the intermolecular reductive coupling of the imino groups of the trensal ligand and to the formation of one and two C–C bonds leaving the metal center in the +3 oxidation state. The resulting one- and two electron reduced complexes [{K(THF)₃}₂Ln₂(bis-trensal)], 2-Ln, and [{K(THF)₃}₂{K(THF)}₂Ln₂(cyclo-trensal)], 4-Ln (Ln = Sm, Nd) are able to effect the reductive disproportionation of carbon dioxide by transferring the electrons stored in the C–C bonds to CO₂ to selectively afford carbonate and CO. The selectivity of the reaction contrasts with the formation of multiple CO₂ reduction products previously reported for a U(IV)-trensal system.