Chelate-Free Synthesis of the U(II) Complex, [(C5H3(SiMe3)2)3U]1-, Using Li and Cs Reductants and Comparative Studies of La(II) and Ce(II) Analogs

To expand the range of synthetic options for generating complexes of the actinide metals in the +2 oxidation state, reduction of Cp '' U-3 (Cp '' = C5H3(SiMe3)(2)) and the lanthanide analogs, Cp '' La-3 and Cp '' Ce-3 with lithium in the absence of crown ether and cryptand chelates was explored. In each case, crystallographically characterizable [Li(THF)4][Cp '' M-3] complexes were obtainable in yields of 70-75% for M = La and Ce and 45-50% for M = U, that is, chelating agents are not necessary to sequester the lithium countercation to form isolable crystalline M(II) products. Reductions using Cs were also explored and X-ray crystallography revealed the formation of an oligomeric structure, [Cp '' U(mu-Cp '')(2)Cs(THF)(2)](n), involving Cp '' ligands that bridge (Cp '' U-II)(1+) moieties to [Cp '' Cs-2(THF)(2)](1-) units.