The effect of ancillary ligands on hydrocarbon C-H bond functionalization by uranyl photocatalysts.

The aqueous uranyl dication has long been known to facilitate the UV light-induced decomposition of aqueous VOCs (volatile organic compounds), via the long-lived highly efficient, uranyl excited state. The lower-energy visible light excited uranyl ion is also able to cleave unactivated hydrocarbon C-H bonds, yet the development of this reactivity into controlled and catalytic C-H bond functionalization is still in its infancy, with almost all studies still focused on uranyl nitrate as the precatalyst. Here, hydrocarbon-soluble uranyl nitrate and chloride complexes supported by substituted phenanthroline (Ph2phen) ligands are compared to each other, and to the parent salts, as photocatalysts for the functionalization of cyclooctane by H atom abstraction. Analysis of the absorption and emission spectra, and emission lifetimes of Ph2phen-coordinated uranyl complexes demonstrate the utility of the ligand in light absorption in the photocatalysis, which is related to the energy and kinetic decay profile of the uranyl photoexcited state. Density functional theory computational analysis of the C-H activation steps in the reaction show how a set of dispersion forces between the hydrocarbon substrate and the Ph2phen ligand provide control over the H atom abstraction, and provide predictions of selectivity of H atom abstraction by the uranyl oxo of the ring C-H over the ethyl C-H in an ethylcyclohexane substrate.

Current and emerging technologies for the remediation of difficult-to-measure radionuclides at nuclear sites.

Difficult-to-measure radionuclides (DTMRs), defined by an absence of high energy gamma emissions during decay, are problematic in groundwaters at nuclear sites. DTMRs are common contaminants at many nuclear facilities, with (often) long half-lives and high radiotoxicities within the human body. Effective remediation is, therefore, essential if nuclear site end-state targets are to be met. However, due to a lack of techniques for in situ DTMR detection, technologies designed to remediate these nuclides are underdeveloped and tend to be environmentally invasive. With a growing agenda for sustainable remediation and reduction in nuclear decommissioning costs, there is renewed international focus on the development of less invasive technologies for DTMR clean-up. Here, we review recent developments for remediation of selected problem DTMRs (129I, 99Tc, 90Sr and 3H), with a focus on industrial and site-scale applications. We find that pump and treat (P&T) is the most used technique despite efficacy issues for 129I and 3H. Permeable reactive barriers (PRBs) are a less invasive alternative but have only been demonstrated for removal of 99Tc and 90Sr at scale. Phytoremediation shows promise for site-scale removal of 3H but is unsuitable for 129I and 99Tc due to biotoxicity and bioavailability hazards, respectively. No single technique can remediate all DTMRs of focus. Likewise, there has been no successful site-applied technology with high removal efficiencies for iodine species typically present in groundwaters (iodide/I-, iodate/IO3- and organoiodine). Further work is needed to adapt and improve current techniques to field scales, as well as further research into targeted application of emerging technologies.

Developing field-scale, gentle remediation options for nuclear sites contaminated with 137Cs and 90Sr: The role of Nature-Based Solutions.

The remediation of contaminated land using plants, bacteria and fungi has been widely examined, especially in laboratory or greenhouse systems where conditions are precisely controlled. However, in real systems at the field scale conditions are much more variable and often produce different outcomes, which must be fully examined if 'gentle remediation options', or GROs, are to be more widely implemented, and their associated benefits (beyond risk-management) realized. These secondary benefits can be significant if GROs are applied correctly, and can include significant biodiversity enhancements. Here, we assess recent developments in the field-scale application of GROs for the remediation of two model contaminants for nuclear site remediation (90Sr and 137Cs), their risk management efficiency, directions for future application and research, and barriers to their further implementation at scale. We also discuss how wider benefits, such as biodiversity enhancements, water filtration etc. can be maximized at the field-scale by intelligent application of these approaches.

Towards the application of electrokinetic remediation for nuclear site decommissioning.

Contamination encountered on nuclear sites includes radionuclides as well as a range of non-radioactive co-contaminants, often in low-permeability substrates such as concretes or clays. However, many commercial remediation techniques are ineffective in these substrates. By contrast, electrokinetic remediation (EKR), where an electric current is applied to remove contaminants from the treated media, retains high removal efficiencies in low permeability substrates. Here, we evaluate recent developments in EKR for the removal of radionuclides in contaminated substrates, including caesium, uranium and others, and the current benefits and limitations of this technology. Further, we assess the present state of EKR for nuclear site applications using real-world examples, and outline key areas for future application.

Thermal and Photochemical Reduction and Functionalization Chemistry of the Uranyl Dication, [UVIO2]2.

The uranyl ion, [UVIO2]2+, possesses rigorously trans, strongly covalent, and chemically robust U-oxo groups. However, through the use of anaerobic reaction techniques, both one- and two-electron reductive functionalization of the uranyl oxo groups has been discovered and developed. Prior to 2010, this unusual reactivity centered around the reductive silylation of the uranyl ion which entailed conversion of the oxo ligands into siloxy ligands and reductive metalation of the uranyl oxo with Group 1 and f-block metals. This review surveys the large number of new examples of reductive functionalization of the uranyl ion that have been reported since 2010, including reductive borylation and alumination, metalation with d- or f-block metals, and new examples of reductive silylation. Other examples of oxo-group functionalization of [UVIO2]2+ that do not involve reduction, mainly with Group 1 cations, are also covered, along with new advances in the photochemistry of the uranyl(VI) ion that involve the transient formation of formally uranyl(V) [UVO2]+ ion.

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study.

The uranyl(vi) 'Pacman' complex [(UO2)(py)(H2L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp2Ti(η2-Me3SiC[triple bond, length as m-dash]CSiMe3) and [Cp2TiCl]2 to oxo-titanated uranyl(v) complexes [(py)(Cp2TiIIIOUO)(py)(H2L)] 1 and [(ClCp2TiIVOUO)(py)(H2L)] 2. Combination of ZrII and ZrIV synthons with A yields the first ZrIV-uranyl(v) complex, [(ClCp2ZrOUO)(py)(H2L)] 3. Similarly, combinations of Ae0 and AeII synthons (Ae = alkaline earth) afford the mono-oxo metalated uranyl(v) complexes [(py)2(ClMgOUO)(py)(H2L)] 4, [(py)2(thf)2(ICaOUO)(py) (H2L)] 5; the zinc complexes [(py)2(XZnOUO)(py)(H2L)] (X = Cl 6, I 7) are formed in a similar manner. In contrast, the direct reactions of Rb or Cs metal with A generate the first mono-rubidiated and mono-caesiated uranyl(v) complexes; monomeric [(py)3(RbOUO)(py)(H2L)] 8 and hexameric [(MOUO)(py)(H2L)]6 (M = Rb 8b or Cs 9). In these uranyl(v) complexes, the pyrrole N-H atoms show strengthened hydrogen-bonding interactions with the endo-oxos, classified computationally as moderate-strength hydrogen bonds. Computational DFT MO (density functional theory molecular orbital) and EDA (energy decomposition analysis), uranium M4 edge HR-XANES (High Energy Resolution X-ray Absorption Near Edge Structure) and 3d4f RIXS (Resonant Inelastic X-ray Scattering) have been used (the latter two for the first time for uranyl(v) in 7 (ZnI)) to compare the covalent character in the UV-O and O-M bonds and show the 5f orbitals in uranyl(vi) complex A are unexpectedly more delocalised than in the uranyl(v) 7 (ZnI) complex. The Oexo-Zn bonds have a larger covalent contribution compared to the Mg-Oexo/Ca-Oexo bonds, and more covalency is found in the U-Oexo bond in 7 (ZnI), in agreement with the calculations.

Complexes of Group 2 dications with soft thioether- and selenoether-containing macrocycles.

A new route to cationic complexes of Mg, Ca, Sr and Ba with 18-membered ring O4S2, O4Se2 and O2S4 donor macrocycles from metal acetonitrile complexes with weakly coordinating [BAr(F)](-) anions is described. The precursors used were [M(MeCN)x][BAr(F)]2 (M = Mg, x = 6; M = Ca, x = 8) and [M'(acacH)(MeCN)5][BAr(F)]2 (M' = Sr or Ba). Reaction of these with the heterocrowns, [18]aneO4S2 (1,4,10,13-tetraoxa-7,16-dithiacyclooctadecane), [18]aneO4Se2 (1,4,10,13-tetraoxa-7,16-diselenacyclooctadecane) or [18]aneO2S4 (1,10-dioxa-4,7,13,16-tetrathiacyclooctadecane) in anhydrous CH2Cl2 solution gave [M(heterocrown)(MeCN)2][BAr(F)]2 for M = Mg, Ca or Sr, whilst the larger Ba forms [Ba(heterocrown)(acacH)(MeCN)][BAr(F)]2. The complexes have been characterised by microanalysis, IR, (1)H and (13)C{(1)H} NMR spectroscopy. X-ray crystal structures are reported for [Ca([18]aneO2S4)(MeCN)2][BAr(F)]2, [Ca([18]aneO4Se2)(MeCN)2][BAr(F)]2, [Sr([18]aneO4S2)(MeCN)2][BAr(F)]2, and [Sr([18]aneO4Se2)(MeCN)2][BAr(F)]2 which contain 8-coordinate metal centres with trans-nitrile ligands and κ(6)-heterocrowns, and for the 9-coordinate [Ba([18]aneO4Se2)(acacH)(MeCN)][BAr(F)]2. Adventitious hydrolysis of the magnesium complexes in solution results in six-coordinate complexes, [Mg(κ(3)-[18]aneO4Se2)(OH2)2(MeCN)][BAr(F)]2 and [Mg(κ(3)-[18]aneO4S2)(OH2)2(MeCN)][BAr(F)]2, whose structures were determined. X-ray crystal structures are also reported for [Mg(MeCN)6][BAr(F)]2, [M(MeCN)8][BAr(F)]2 (M = Ca, Sr) and [Ca(18-crown-6)(MeCN)2][BAr(F)]2.