Electrochemical Studies of Selected Lanthanide and Californium Cryptates

Efforts to quantitatively reduce Cf^III → Cf^II in solution as well as studies of its cyclic voltammetry have been hindered by its scarcity, significant challenges associated with manipulating an unusually intense γ emitter, small reaction scales, the need for nonaqueous solvents, and its radiolytic effects on ligands and solvents. In an effort to overcome these impediments, we report on the stabilization of Cf^II by encapsulation in 2.2.2-cryptand and comparisons with the readily reducible lanthanides, Sm³⁺, Eu³⁺, and Yb³⁺. Cyclic voltammetry measurements suggest that Cf^III/II displays electrochemical behavior with characteristics of both Sm^III/II and Yb^III/II. The °E_1/2 values of -1.525 and -1.660 V (vs Fc/Fc⁺ in tetrahydrofuran (THF)) for [Cf(2.2.2-crypt)]^3+/2+ and [Sm(2.2.2-crypt)]^3+/2+, respectively, are similar. However, the ΔE values upon complexation by 2.2.2-cryptand for Cf^III/II more closely parallels Yb^III/II with postencapsulation shifts of 705 and 715 mV, respectively, whereas the shift of Sm^III/II (520 mV) mirrors that of Eu^III/II (524 mV). This suggests more structural similarities between Cf^II and Yb^II in solution than with Sm^II that likely originates from more similar ionic radii and local coordination environments, a supposition that is corroborated by crystallographic and extended X-ray absorption fine structure measurements from other systems. Competitive-ion binding experiments between Eu^III/II, Sm^III/II, and Yb^III/II were also performed and show less favorable binding by Yb^III/II. Connectivity structures of [Ln(2.2.2-cryptand)(THF)][BPh₄]₂ (Ln = Eu^II, Sm^II) are reported to show the important role that THF plays in these redox reactions.