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″3U (Cp″ = C5H3(SiMe3)2) and the lanthanide analogs, Cp″3La and Cp″3Ce with lithium in the absence of crown ether and cryptand chelates was explored. In each case, crystallographically characterizable [Li(THF)4][Cp″3M] 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(μ-Cp")2Cs(THF)2] n, involving Cp″ ligands that bridge "(Cp″UII)1+" moieties to "[Cp″2Cs(THF)2]1-" units.