In this week’s issue of Nature magazine Arnold et al. have published an intriguing reactive complex of the notoriously stable uranyl dication [UO₂]⁺ that cleaves C-Si bonds and yields a functionalized U=O bond. This was accomplished by first encapsulating the UO₂ into a cyclic macrocycle, followed by the addition of the metal (either iron or zinc bound to the inner U=O bond) with a silylamide base. The major product is shown in the figure below.

Reprinted by permission from Macmillan Publishers Ltd: Nature, advance online publication, 17 January 2008 (doi:10.1038/nature06467) The crystal structure was also taken and is shown above. This reactivity was explained by postulating an U(VI) K₂ intermediate not observed. Postulated mechanism below.

Reprinted by permission from Macmillan Publishers Ltd: Nature, advance online publication, 17 January 2008 (doi:10.1038/nature06467)

A solution to the problem of determining the presence of the postulated intermediate might simply be to take mass specs along the course of the reaction; as one described prep takes 42 hours. In the end, this work literally builds the scaffold for future chemists to begin functionalizing uranyl. Although, no mention is given how to un-encapsulate the newly derivatized uranium.

Note 1: Link to article — Reduction and selective oxo group silylation of the uranyl dication

Mitch

Earlier this week a paper by L. Soderholm et al. in Angewandte Chemie may have solved the great plutonium polymer mystery. Plutonium polymer is the ubiquitous noun often spoken by plutonium chemists in regards to the un-extractable ill-defined hydrous oxides of plutonium that will form in any solution of aqueous plutonium lying about the bench top. Often plutonium polymerization can be inhibited by storing aqueous plutonium solutions at high acid concentrations. It was thought to form from a series of olation reactions:

Plutonium Olation Reaction: Pu—OH + Pu—OH₂ —> Pu—OH—Pu + H₂O

This old hypothesis is put to rest with the isolation of Li₁₄(H₂O)_n[Pu₃₈O₅₆Cl₅₄(H₂O)₈]. This occurred after repeated anion-exchange with an acidified alkaline peroxide solution of plutonium, that then crystallized in the presence of aqueous LiCl. This type of workup is common for samples containing plutonium polymer. The crystal is reported to have the same intracluster packing and structural topology as bulk PuO₂. The crystal structure of the [Pu₃₈O₅₄(H₂O)₈]⁴⁰⁺ is shown below.

Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.: Angewandte Chemie International Edition (Dec 2007).

The Plutonium is in green, oxygen from the oxide in red, and oxygen from water in blue. With this and other evidence of well defined Pu—O clusters and the lack of hard evidence for oxyhydroxides they expect plutonium to condensate through an oxolation reaction.

Plutonium Oxolation Reaction: 2Pu—OH —> Pu—O—Pu + H₂O

The one caveat with this work is that it was performed with the more stable plutonuium-242 (t_1/2=3.7 x 10⁵ y) and not the typical reactor plutonium-239 (t_1/2=2.4 x 10⁴ y). Perhaps in the presence of the >10x more radioactive Pu-239 the nanoclusters would become either too structurally damaged to resolve nice crystalline structures, or more chemically reactive towards hydrous oxide formation or oxyhydroxide formation. Regardless, this work may still lead to better methods of extracting plutonium out of the nuclear fuel cycle and represents a nice resolution to the nebulous plutonium polymer conundrum.

Link to Paper: http://dx.doi.org/10.1002/anie.200704420

Mitch