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

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 [U-2(bis-trensal)], 1. Further reduction of 1 led to the dinuclear macrocyclic complex [{K(THF)(3)}(2)U-2(cyclotrensal)], 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)}U-2(mu-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.