Chelate-Free Synthesis of the U(II) Complex, [(C 5 H 3 (SiMe 3 ) 2 ) 3 U] 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″ 3 U (Cp″ = C 5 H 3 (SiMe 3 ) 2 ) and the lanthanide analogs, Cp″ 3 La and Cp″ 3 Ce with lithium in the absence of crown ether and cryptand chelates was explored. In each case, crystallographically characterizable [Li(THF) 4 ][Cp″ 3 M] 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”) 2 Cs(THF) 2 ] n, involving Cp″ ligands that bridge “(Cp″U II ) 1+ ” moieties to “[Cp″ 2 Cs(THF) 2 ] 1– ” units.