Controlled and sequential single-electron reduction of the uranyl dication

A flexible tripodal pyrrole-imine ligand (H₃L) has been used to facilitate the controlled and sequential single-electron reductions of the uranyl dication from the U(VI) oxidation state to U(V) and further to U(IV), processes that are important to understanding the reduction of uranyl and its environmental remediation. The uranyl(VI) complexes UO₂(HL)(sol) (sol = THF, py) were straightforwardly accessed by the transamination reaction of H₃L with UO₂{N(SiMe₃)₂}₂(THF)₂ and adopt 'hangman' structures in which one of the pyrrole-imine arms is pendant. While deprotonation of this arm by LiN(SiMe₃)₂ causes no change in uranyl oxidation state, single-electron reduction of uranyl(VI) to uranyl(V) occurred on addition of two equivalents of KN(SiMe₃)₂ to UO₂(HL)(sol). The potassium cations of this new [U^VO₂(K₂L)]₂ dimer were substituted by transmetalation with the appropriate metal chloride salt, forming the new uranyl(V) tetra-heterometallic complexes, [U^VO₂Zn(L)(py)₂]₂ and [U^VO₂Ln(Cl)(L)(py)₂]₂ (Ln = Y, Sm, Dy). The dimeric uranyl(V)-yttrium complex underwent further reduction and chloride abstraction to form the tetrametallic U(IV) complex [U^IVO₂Y^III(py)]₂, so highlighting the adaptability of this ligand to stabilise a variety of different uranium oxidation states.