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Carbon Dioxide Reduction by Multimetallic Uranium(IV) Complexes Supported by Redox-Active Schiff Base Ligands

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journal contribution
posted on 26.02.2020, 22:47 by Nadir Jori, Marta Falcone, Rosario Scopelliti, Marinella Mazzanti
The synthesis, structure, and reactivity with CO2 and CS2 of new U­(IV) complexes with a redox-active Schiff base are reported. The reaction of UI3 with the heptadentate Schiff base ligand 2,2′,2″-tris­(salicylideneimino)­triethylamine (trensal) did not lead to the formation of a U­(III) complex but to the reductive coupling and C–C bond formation between two imino groups of the Schiff base, yielding the U­(IV) complex [U2(bis-trensal)], 1. Further reduction of 1 led to the dinuclear macrocyclic complex [{K­(THF)3}2U2(cyclo-trensal)], 3-THF, through a second C–C bond formation reaction between two additional imino groups. Complexes 1 and 3 are oxidized by AgOTf resulting in the cleavage of the C–C bonds and leading to the formation of the U­(IV) complex [U­(trensal)]­OTf, 2. Complex 1 does not reduce CO2 or CS2 but undergoes insertion of CO2 into one of the U–N bonds. In contrast, the reaction of 3 with 2 equiv of CO2 leads to the reductive disproportionation of CO2 to afford carbonate in 80% yield. In the presence of a large excess of CO2 multiple reactions take place, as supported by the isolation of the crystals of [{K­(THF)3}­U2(μ-O)­(CO2–CO-cyclo-trensal)­(U­(trensal))], 4. The higher reductive activity toward CO2 of complex 3 compared to previously reported U­(IV) complexes of reduced Schiff bases is interpreted in terms of its redox properties.