Electrochemical Studies of Selected Lanthanide and Californium Cryptates.

Efforts to quantitatively reduce Cf → Cf 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 by encapsulation in 2.2.2-cryptand and comparisons with the readily reducible lanthanides, Sm, Eu, and Yb. Cyclic voltammetry measurements suggest that Cf displays electrochemical behavior with characteristics of both Sm and Yb. The ° values of -1.525 and -1.660 V (vs Fc/Fc in tetrahydrofuran (THF)) for [Cf(2.2.2-crypt)] and [Sm(2.2.2-crypt)], respectively, are similar. However, the Δ values upon complexation by 2.2.2-cryptand for Cf more closely parallels Yb with postencapsulation shifts of 705 and 715 mV, respectively, whereas the shift of Sm (520 mV) mirrors that of Eu (524 mV). This suggests more structural similarities between Cf and Yb in solution than with Sm 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, Sm, and Yb were also performed and show less favorable binding by Yb. Connectivity structures of [Ln(2.2.2-cryptand)(THF)][BPh] (Ln = Eu, Sm) are reported to show the important role that THF plays in these redox reactions.