Oxo-Functionalization and Reduction of the Uranyl Ion through Lanthanide-Element Bond Homolysis: Synthetic, Structural, and Bonding Analysis of a Series of Singly Reduced Uranyl–Rare Earth 5f¹‑4f^<i>n</i> Complexes

The heterobimetallic complexes [{UO₂Ln­(py)₂(L)}₂], combining a singly reduced uranyl cation and a rare-earth trication in a binucleating polypyrrole Schiff-base macrocycle (Pacman) and bridged through a uranyl oxo-group, have been prepared for Ln = Sc, Y, Ce, Sm, Eu, Gd, Dy, Er, Yb, and Lu. These compounds are formed by the single-electron reduction of the Pacman uranyl complex [UO₂(py)­(H₂L)] by the rare-earth complexes Ln^III(A)₃ (A = N­(SiMe₃)₂, OC₆H₃Bu^t₂-2,6) via homolysis of a Ln–A bond. The complexes are dimeric through mutual uranyl <i>exo</i>-oxo coordination but can be cleaved to form the trimetallic, monouranyl “ate” complexes [(py)₃LiOUO­(μ-X)­Ln­(py)­(L)] by the addition of lithium halides. X-ray crystallographic structural characterization of many examples reveals very similar features for monomeric and dimeric series, the dimers containing an asymmetric U₂O₂ diamond core with shorter uranyl UO distances than in the monomeric complexes. The synthesis by Ln^III–A homolysis allows [5f¹-4f^<i>n</i>]₂ and Li­[5f¹-4f^<i>n</i>] complexes with oxo-bridged metal cations to be made for all possible 4f^<i>n</i> configurations. Variable-temperature SQUID magnetometry and IR, NIR, and EPR spectroscopies on the complexes are utilized to provide a basis for the better understanding of the electronic structure of f-block complexes and their f-electron exchange interactions. Furthermore, the structures, calculated by restricted-core or all-electron methods, are compared along with the proposed mechanism of formation of the complexes. A strong antiferromagnetic coupling between the metal centers, mediated by the oxo groups, exists in the U^VSm^III monomer, whereas the dimeric U^VDy^III complex was found to show magnetic bistability at 3 K, a property required for the development of single-molecule magnets