Electrochemical Studies of Selected Lanthanide and Californium Cryptates.

Efforts to quantitatively reduce CfIII → CfII 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 CfII by encapsulation in 2.2.2-cryptand and comparisons with the readily reducible lanthanides, Sm3+, Eu3+, and Yb3+. Cyclic voltammetry measurements suggest that CfIII/II displays electrochemical behavior with characteristics of both SmIII/II and YbIII/II. The °E1/2 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 CfIII/II more closely parallels YbIII/II with postencapsulation shifts of 705 and 715 mV, respectively, whereas the shift of SmIII/II (520 mV) mirrors that of EuIII/II (524 mV). This suggests more structural similarities between CfII and YbII in solution than with SmII 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 EuIII/II, SmIII/II, and YbIII/II were also performed and show less favorable binding by YbIII/II. Connectivity structures of [Ln(2.2.2-cryptand)(THF)][BPh4]2 (Ln = EuII, SmII) are reported to show the important role that THF plays in these redox reactions.

Understanding the Scarcity of Thorium Peroxide Clusters.

The reaction of Th(NO3)4·5H2O with 3 equiv of 2,2',6',2″-terpyridine (terpy) in a mixture of acetonitrile and methanol results in formation of the trinuclear thorium peroxide cluster [Th(O2)(terpy)(NO3)2]3. This cluster is assembled via bridging by µ-η2:η2 peroxide anions between thorium centers. It decomposes upon removal from the mother liquor to yield Th(terpy)(NO3)4 and Th(terpy)(NO3)4(EtOH). The peroxide formation appears to be radiolytic in origin and is, most likely, generated from radiolysis of water by short-lived daughters generated from 232Th decay. This cluster does not form when freshly recrystallized Th(NO3)4·5H2O is used as the starting material and requires an aged source of thorium. Analysis of the bonding in these clusters shows that, unlike uranium(VI) peroxide interactions, thorium(IV) complexation by peroxide is quite weak and largely ionic. This explains its much lower stability, which is more comparable to that observed in similar zirconium(IV) peroxide clusters.

A series of dithiocarbamates for americium, curium, and californium.

Characterizing how actinide properties change across the f-element series is critical for improving predictive capabilities and solving many nuclear problems facing our society. Unfortunately, it is difficult to make direct comparisons across the 5f-element series because so little is known about trans-plutonium elements. Results described herein help to address this issue through isolation of An(S2CNEt2)3(N2C12H8) (Am, Cm, and Cf). These findings included the first single crystal X-ray diffraction measurements of Cm-S (mean of 2.86 ± 0.04 Å) and Cf-S (mean of 2.84 ± 0.04 Å) bond distances. Furthermore, they highlight the potential of An(S2CNEt2)3(N2C12H8) for providing a test bed for comparative analyses of actinide versus lanthanide bonding interactions.