A series of R2Sn (R = Me, Ph) complexes of the Schiff’s base salicylaldehyde acyldihydrazone with a methylene spacer of variable length have been structurally characterized in order to explore the prevalence of Sn···O noncovalent interactions. Structural studies show that these can exist, with the shortest Sn···O distance of 3.480(2) Å in this library of compounds, significantly shorter than the sum of the van der Waals radii (3.92 Å). Crystallographic studies also show that steric effects are important, and these interactions are seen only for compounds with Me2Sn while they are not observed for Ph2Sn. However, this is not simply a steric effect, and C–H···O interactions can compete with these Sn···O interactions. A computational study, in combination with the quantum theory of atoms in molecules, shows that these interactions are mostly electrostatic in origin with little evidence of covalency.

A series of complexes [Et4N][Ln(NCS)4(H2O)4] (Ln = Pr, Tb, Dy, Ho, Yb) have been structurally characterized, all showing the same structure, namely a distorted square antiprismatic coordination geometry, and the Ln–O and Ln–N bond lengths following the expected lanthanide contraction. When the counterion is Cs+, a different structural motif is observed and the eight-coordinate complex Cs5[Nd(NCS)8] isolated. The thorium compounds [Me4N]4[Th(NCS)7(NO3)] and [Me4N]4[Th(NCS)6(NO3)2] have been characterized, and high coordination numbers are also observed. Finally, attempts to synthesize a U(III) thiocyanate compound has been unsuccessful; from the reaction mixture, a heterocycle formed by condensation of five MeCN solvent molecules, possibly promoted by U(III), was isolated and structurally characterized. To rationalize the inability to isolate U(III) thiocyanate compounds, thin-layer cyclic voltammetry and IR spectroelectrochemistry have been utilized to explore the cathodic behavior of [Et4N]4[U(NCS)8] and [Et4N][U(NCS)5(bipy)2] along with a related uranyl compound [Et4N]3[UO2(NCS)5]. In all examples, the reduction triggers a rapid dissociation of [NCS]− ions and decomposition. Interestingly, the oxidation chemistry of [Et4N]3[UO2(NCS)5] in the presence of bipy gives the U(IV) compound [Et4N]4[U(NCS)8], an unusual example of a ligand-based oxidation triggering a metal-based reduction. The experimental results have been augmented by a computational investigation, concluding that the U(III)–NCS bond is more ionic than the U(IV)–NCS bond.

The solid state structure of 1,1,1,2,2,3,3-heptachloropropane is reported. This molecule features numerous type I and type II Cl⋯Cl interactions in the solid state as well as Cl⋯H hydrogen bonds. A Hirshfeld surface analysis is also presented.The crystal structure of 1,1,1,2,2,3,3-heptachloropropane shows type I and type II Cl⋯Cl interactions in the solid state.

A series of 10-[(4-halo-2,6-diisopropylphenyl)imino]phenanthren-9-ones and derivatives of the phenanthrene-9,10-dione ligand have been synthesised and structurally characterised to explore two types of noncovalent interactions, namely the influence of the steric bulk upon the resulting C–H···π and π-stacking interactions and halogen bonding. Selected noncovalent interactions have additionally been analysed by DFT and AIM techniques. No halogen bonding has been observed in these systems, but X lone pair···π, C–H···O=C and C–H···π interactions are the prevalent ones in the halogenated systems. Removal of the steric bulk in N-(2,4,6-trimethylphenyl)-9,10-iminophenanthrenequinone affords different noncovalent interactions, but the C–H···O=C hydrogen bonds are observed. Surprisingly, in N-(2,6-dimethylphenyl)-9,10-iminophenanthrenequinone and N-(phenyl)-9,10-iminophenanthrenequinone these C–H···O=C hydrogen bonds are not observed. However, they are observed in the related 2,6-di-tert-butylphenanthrene-9,10-dione. The π-interactions in dimers extracted from the crystal structures have been analysed by DFT and AIM. Spectroscopic investigations are also presented and these show only small perturbations to the O=C–C=N fragment.

The reaction of a number of uranyl minerals of the (oxy)hydroxide, phosphate and carbonate types with Eu(III), as a surrogate for Am(III), have been investigated. A photoluminescence study shows that Eu(III) can interact with the uranyl minerals Ca[(UO2)6(O)4(OH)6]·8H2O (becquerelite) and A[UO2(CO3)3]·xH2O (A/x = K3Na/1, grimselite; CaNa2/6, andersonite; and Ca2/11, liebigite). For the minerals [(UO2)8(O)2(OH)12]·12H2O (schoepite), K2[(UO2)6(O)4(OH)6]·7H2O (compreignacite), A[(UO2)2(PO4)2]·8H2O (A = Ca, meta-autunite; Cu, meta-torbernite) and Cu[(UO2)2(SiO3OH)2]·6H2O (cuprosklodowskite) no Eu(III) emission was observed, indicating no incorporation into, or sorption onto the structure. In the examples with Eu3+ incorporation, sensitized emission is seen and the lifetimes, hydration numbers and quantum yields have been determined. Time Resolved Laser Induced Fluroescence Spectroscpoy (TRLFS) at 10 K have also been measured and the resolution enhancements at these temperatures allow further information to be derived on the sites of Eu(III) incorporation. Infrared and Raman spectra are recorded, and SEM analysis show significant morphology changes and the substitution of particularly Ca2+ by Eu3+ ions. Therefore, Eu3+ can substitute Ca2+ in the interlayers of becquerelite and liebigite and in the structure of andersonite, whilst in grimselite only sodium is exchanged. These results have guided an investigation into the reactions with 241Am on a tracer scale and results from gamma-spectrometry show that becquerelite, andersonite, grimselite, liebigite and compreignacite can include americium in the structure. Shifts in the UO and C–O Raman active bands are similar to that observed in the Eu(III) analogues and Am(III) photoluminescence measurements are also reported on these phases; the Am3+ ion quenches the emission from the uranyl ion.

The solid-state structures of CF3(CF2)5CH2CO2H and a fluorous triazole are reported, both of which display a wide variety and large number of noncovalent interactions in their packing. The solid-state structure of CF3(CF2)5CH2CO2H is stabilized by multiple F···F contacts but only one C–H···F–C interaction, as well as O–H···O and C–H···O hydrogen bonds. In contrast to other reported structures, the torsion angles in the fluorous chain are close to 180°, which means that the fluorine atoms are eclipsed. A DFT study of the interactions in both compounds show that F···F interactions, along with stacking and C–H···F and C–H···O contacts, are individually weakly energetically stabilizing, but collectively, they can give rise to interaction energies of up to 13 kcal mol–1. A topological approach to the interactions using atoms-in-molecules (AIM) theory reveals that there are bond critical points between the C–F···F–C interactions as well as C–F···H–C interactions that are not recognized when using only the van der Waals distances.

•Spectroscopic, structural and theoretical methods for characterisation uranyl minerals reviewed.•Reactivity of uranyl minerals towards common fission products reviewed.•Reactivity of uranyl minerals towards transuranics, especially Np, reviewed.The storage of spent nuclear fuels for long periods of time in geological repositories is one proposed solution to the stewardship of legacy, current and future nuclear waste. Recent studies have shown that UO2, the major component of spent nuclear fuel, can oxidise under repository conditions to a number of secondary phases. The weathering of naturally occurring uranium minerals can give an insight into the behaviour of SNF and this review highlights the structural and spectroscopic characterisation of a number of relevant minerals. Conversely, reducing conditions could also be possible and these phase alterations are also discussed. Furthermore, the interaction of these minerals with the common fission products caesium, strontium, technetium, iodine, selenium and the transuranic elements (Np, Pu, Am, Cm) is reviewed, as these minerals may provide a mechanism for the retardation of the mobility of these radioisotopes.

A comprehensive study of the complexes A4[U(NCS)8] (A = Cs, Et4N, nBu4N) and A3[UO2(NCS)5] (A = Cs, Et4N) is described, with the crystal structures of [nBu4N]4[U(NCS)8]·2MeCN and Cs3[UO2(NCS)5]·O0.5 reported. The magnetic properties of square antiprismatic Cs4[U(NCS)8] and cubic [Et4N]4[U(NCS)8] have been probed by SQUID magnetometry. The geometry has an important impact on the low-temperature magnetic moments: at 2 K, μeff = 1.21 μB and 0.53 μB, respectively. Electronic absorption and photoluminescence spectra of the uranium(IV) compounds have been measured. The redox chemistry of [Et4N]4[U(NCS)8] has been explored using IR and UV–vis spectroelectrochemical methods. Reversible 1-electron oxidation of one of the coordinated thiocyanate ligands occurs at +0.22 V vs Fc/Fc+, followed by an irreversible oxidation to form dithiocyanogen (NCS)2 which upon back reduction regenerates thiocyanate anions coordinating to UO22+. NBO calculations agree with the experimental spectra, suggesting that the initial electron loss of [U(NCS)8]4– is delocalized over all NCS– ligands. Reduction of the uranyl(VI) complex [Et4N]3[UO2(NCS)5] to uranyl(V) is accompanied by immediate disproportionation and has only been studied by DFT methods. The bonding in [An(NCS)8]4– (An = Th, U) and [UO2(NCS)5]3– has been explored by a combination of DFT and QTAIM analysis, and the U–N bonds are predominantly ionic, with the uranyl(V) species more ionic that the uranyl(VI) ion. Additionally, the U(IV)–NCS ion is more ionic than what was found for U(IV)–Cl complexes.

The solid-sate structures of the two uranyl peroxides studtite, [UO2(η2-O2)(H2O)2]·2H2O, and metastudtite [UO2(η2-O2)(H2O)2] have been determined by U–L3 edge extended X-ray absorption fine structure (EXAFS) spectroscopy and show that upon removal of the interstitial water in studtite there are structural changes with a small shortening of the U–Operoxo and small lengthening of the U–Oyl bonds. High-energy resolution X-Ray absorption near edge structure (HR-XANES) spectroscopy has been used to probe the differences in the local electronic structure and, supported by ab initio FEFF9.5.1 calculations, dehydration causes a shift to higher energies of the occupied O p-DOS and U d- and f-DOS of metastudtite. The HR-XANES spectrum of schoepite, [(UO2)4O(OH)6]·6H2O, has been measured as the White Line intensity can give information on the mixing of metal and ligand atomic orbitals. There is an indication for higher degree of ionicity for the U–OH bond in schoepite compared to the U–O2 bond in studtite.

The uranyl aryloxide [UO2(OAr)2(THF)2] (Ar = 2,6-tBu2-C6H2) is an active catalyst for the ring-opening cyclo-oligomerization of ε-caprolactone and δ-valerolactone but not for β-butyrolactone, γ-butyrolactone, and rac-lactide. 1H EXSY measurements give the thermodynamic parameters for exchange of monomer and coordinated THF, and rates of polymerization have been determined. A comprehensive theoretical examination of the mechanism is discussed. From both experiment and theory, the initiation step is intramolecular and in keeping with the accepted mechanism, while computational studies indicate that propagation can go via an intermolecular pathway, which is the first time this has been observed. The lack of polymerization for the inactive monomers has been investigated theoretically and C–H···π interactions stabilize the coordination of the less rigid monomers.

The solid-state structure of the known complex [Et4N][U(NCS)5(bipy)2] has been re-determined and a detailed spectroscopic and magnetic study has been performed in order to confirm the oxidation states of both metal and bipy ligand. Electronic absorption and infrared spectroscopy suggest that the uranium is in its +4 oxidation state and this has been corroborated by emission spectroscopy and variable temperature magnetic measurements, as well as theoretical calculations. Therefore the bipy ligands are neutral, innocent ligands and not, as would be inferred from just a solid state structure, radical anions.

The coordination and organometallic chemistry of U(III) has been revolutionised by the preparation of the key starting materials [UI3(solv)4] (solv = THF, py, 1/2DME), [U{N(SiMe3)2}3] and [U(OTf)3] and fascinating transformations have resulted. Herein the chemistry of these compounds is reviewed focussing on the initial reactivity of these species; further conversions are not discussed. Salt elimination and 1, 2 or 3 electron oxidation of the uranium centre based upon the ligands is described.Graphical abstractHighlights► Metathesis and 1, 2 or 3 electron oxidation chemistry of [UI3(THF)4] reviewed. ► Protonolysis and 1, 2 or 3 electron oxidation chemistry of U{N(SiMe3)2}3 reviewed. ► Use of [U(OTf)3] as a starting material reviewed.

The uranyl aryloxide, [UO2(OAr)2(THF)2], and uranyl chloride, [UO2Cl2(THF)3] or [UO2Cl2(THF)2]2 act as pre-catalysts for the ring opening polymerization of propylene oxide and cyclohexene oxide. Coordination of the monomers has been investigated using 1H EXSY spectroscopy and kinetic and thermodynamic parameters reported. NMR analyses of the polymers suggest a bimetallic mechanism for the polymerization.

The unusual uranyl peroxide studtite, [UO2(η2-O2)(H2O)2]·2H2O, is a phase alteration product of spent nuclear fuel and has been characterized by solid-state cyclic voltammetry. The voltammogram exhibits two reduction waves that have been assigned to the UVI/V redox couple at −0.74 V and to the UV/IV redox couple at −1.10 V. This potential shows some dependence upon the identity of the cation of the supporting electrolyte, where cations with larger ionic radii exhibit more cathodic reduction potentials. Raman spectroelectrochemistry indicated that exhaustive reduction at either potential result in a product that does not contain peroxide linkers and is likely to be UO2. On the basis of the reduction potentials, the unusual behavior of neptunium in the presence of studtite can be rationalized. Furthermore, the oxidation of other species relevant to the long-term storage of nuclear fuel, namely, iodine and iodide, has been explored. The phase altered product should therefore be considered as electrochemically noninnocent. Radiotracer studies with 241Am show that it does not interact with studtite so mobility will not be retarded in repositories. Finally, a large difference in band gap energies between studtite and its dehydrated congener metastudtite has been determined from the electronic absorption spectra.

A comprehensive computational study on the ring-opening polymerization of propylene oxide catalyzed by uranyl chloride [UO2Cl2(THF)3] and the uranyl aryloxide [UO2(OAr)2(THF)2] (Ar = 2,6-tBu2C6H3) is reported. The initiation and propagation steps have been probed and significant differences between the two catalysts discovered. The initiation step involving uranyl chloride is an intermolecular process because the orientation of the lone pair on the initiating chloride nucleophile is optimally oriented toward the empty σ*-antibonding orbital of the epoxide, which lowers the activation barrier by 22 kcal mol–1. Thus, initiation is orbitally controlled. Propagation occurs through a dimeric species, and low-temperature fluorescence spectroscopy has been used to probe this experimentally. In contrast the initiation step for the uranyl aryloxide catalyzed mechanism is intramolecular because of the steric constraints imposed by the bulky substituents on the aryl ring and the fact that the lone pair on the nucleophile is able to approach the propylene oxide coordinated to the same uranium center. Thus, initiation is principally sterically controlled. Propagation is, however, intermolecular, and this can be traced to steric effects. Experimental evidence in the form of fluorescence spectroscopy and diffusion NMR has been used to explore the propagation process in solution.

The synthesis of highly fluorinated zinc carboxylates [{CF3(CF2)5CH2CH2CO2}2Zn], and alkoxides [{CF3(CF2)5CH2CH2O}2Zn(OEt2)2] and their use as catalysts for the ring opening polymerisation of ɛ-caprolactone are described. Quenching the polymerisation reaction with fluorous acids or alcohols regenerates the catalyst, which can be recovered by fluorous solvent extractions, and the catalytic activity is retained for three cycles. The superior recyclability of the alkoxide to the carboxylate zinc compound is due to the greater partition coefficient between fluorous and organic solvents. Also investigated is the well defined aryloxide compound [(ArO)2Zn(THF)2] which yields very well controlled polymerisation, but cannot be recycled by quenching with a fluorous alcohol.Graphical abstractRecycling strategies for a zinc catalysed ring opening polymerisation of caprolactone have been explored. The use of fluorous alkoxide compounds of zinc and quenching with a fluorous alcohol allows the catalyst to be recycled three times before loss of activity.Highlights► Fluorinated zinc alkoxides can be readily prepared. ► Ring opening polymerisation of caprolactone catalysed by zinc alkoxides. ► Catalyst recovery using a fluorous quench technique.

The solid state structures of three compounds that contain a perfluorinated chain, CF3(CF2)5CH2CH(CH3)CO2H, CF3(CF2)5(CH2)4(CF2)5CF3 and {CF3(CF2)5CH2CH2}3P═O have been compared and a number of C–F···F–C and C–F···H–C interactions that are closer than the sum of the van der Waals radii have been identified. These interactions have been probed by a comprehensive computational chemistry investigation and the stabilizing energy between dimeric fragments was found to be 0.26–29.64 kcal/mol, depending on the type of interaction. An Atoms-in-Molecules (AIM) study has confirmed that specific C–F···F–C interactions are indeed present, and are not due simply to crystal packing. The weakly stabilizing nature of these interactions has been utilized in the physisorption of a selected number of compounds containing long chain perfluorinated ponytails onto a perfluorinated self-assembled monolayer, which has been characterized by IRRAS (Infrared Reflection Absorption Spectroscopy).

A series of highly fluorinated compounds of the type {CF3(CF2)5CH2CH2}3P=O, [{CF3(CF2)5CH2CH2}2P(E)CH2CH2P(E){CH2CH2(CF2)5CF3}] and {CF3(CF2)5CH2CH2}2C=E (E = O or S) have been examined for their ability to extract gold(III) from aqueous solutions. The phosphine oxides have been studied under liquid–liquid extraction conditions from water into perfluorohexane and found to give poor distribution ratios. The bidentate phosphine oxide, ketone and thioketone were studied under solid–liquid extraction conditions and were substantially better with extraction of up to 80% of Au(III). In addition, the crystal structure of {CF3(CF2)5CH2CH2}3P=O has been determined.

The preparation of a ketone with two long chain perfluoroalkyl groups is reported via the coupling reaction of a perfluorinated alkylzinc reagent and a perfluoro-acid chloride. This ketone has been investigated in the heterogeneous removal of heavy metals M2+ (M = Sn, Cd, Pb, Hg) and As5+ from aqueous solutions and removal of these metals from organic solvents using the unique thermomorphic properties of the fluorous ketone. In addition, a comprehensive 13C NMR study of one of the intermediates in the synthesis, 2H,2H,3H,3H-perfluoronanoic acid, has allowed the determination of all 1JC–F and 2JC–F coupling constants. Also reported is the crystal structure of the acid CF3(CF2)5CH2CH2CO2H.A ketone with perfluoroalkyl chains has been developed for the extraction of M(II) (M = Sn, Cd, Hg, Pb) and As(V) from aqueous and organic solvents under heterogeneous and homogeneous conditions using the unique thermomorphic properties of these ligands.

New highly fluorinated monodentate and bidentate phosphine oxide compounds of the type {CF3(CF2)nCH2CH2}3PO (n = 5, 9) and [{CF3(CF2)5CH2CH2}2P(O)CH2CH2P(O){CH2CH2(CF2)5CF3}] have been prepared. Their ability to extract a number of metals and radionuclides from aqueous solutions into perfluorinated solvents has been established and the extractable species investigated. All extractants extract the metals As(V), Cd(II), Co(II), Cr(VI), Hg(II), Pb(II), and Sn(II) with >75% removal. In addition, the radioisotopes 90Sr(II), 133Ba(II), and U(VI) have been investigated, whilst 59Fe(III) has been used to model the extraction of plutonium. 133Ba(II) shows a high distribution ratio for monodentate phosphine oxides, whilst for UO22+ and 59Fe(III) bidentate phosphine oxides are superior.Download full-size imageHighlights► Highly fluorinated phosphine oxides are studied as extractants for toxic metals and radioisotopes. ► Excellent extraction is seen for a number of toxic metals. ► Bidentate phosphine oxides are good for extracting uranium and Fe(III) {as a model for Pu(IV)}.

The solid-sate structures of the two uranyl peroxides studtite, [UO2(η2-O2)(H2O)2]·2H2O, and metastudtite [UO2(η2-O2)(H2O)2] have been determined by U–L3 edge extended X-ray absorption fine structure (EXAFS) spectroscopy and show that upon removal of the interstitial water in studtite there are structural changes with a small shortening of the U–Operoxo and small lengthening of the U–Oyl bonds. High-energy resolution X-Ray absorption near edge structure (HR-XANES) spectroscopy has been used to probe the differences in the local electronic structure and, supported by ab initio FEFF9.5.1 calculations, dehydration causes a shift to higher energies of the occupied O p-DOS and U d- and f-DOS of metastudtite. The HR-XANES spectrum of schoepite, [(UO2)4O(OH)6]·6H2O, has been measured as the White Line intensity can give information on the mixing of metal and ligand atomic orbitals. There is an indication for higher degree of ionicity for the U–OH bond in schoepite compared to the U–O2 bond in studtite.

The reaction of a number of uranyl minerals of the (oxy)hydroxide, phosphate and carbonate types with Eu(III), as a surrogate for Am(III), have been investigated. A photoluminescence study shows that Eu(III) can interact with the uranyl minerals Ca[(UO2)6(O)4(OH)6]·8H2O (becquerelite) and A[UO2(CO3)3]·xH2O (A/x = K3Na/1, grimselite; CaNa2/6, andersonite; and Ca2/11, liebigite). For the minerals [(UO2)8(O)2(OH)12]·12H2O (schoepite), K2[(UO2)6(O)4(OH)6]·7H2O (compreignacite), A[(UO2)2(PO4)2]·8H2O (A = Ca, meta-autunite; Cu, meta-torbernite) and Cu[(UO2)2(SiO3OH)2]·6H2O (cuprosklodowskite) no Eu(III) emission was observed, indicating no incorporation into, or sorption onto the structure. In the examples with Eu3+ incorporation, sensitized emission is seen and the lifetimes, hydration numbers and quantum yields have been determined. Time Resolved Laser Induced Fluroescence Spectroscpoy (TRLFS) at 10 K have also been measured and the resolution enhancements at these temperatures allow further information to be derived on the sites of Eu(III) incorporation. Infrared and Raman spectra are recorded, and SEM analysis show significant morphology changes and the substitution of particularly Ca2+ by Eu3+ ions. Therefore, Eu3+ can substitute Ca2+ in the interlayers of becquerelite and liebigite and in the structure of andersonite, whilst in grimselite only sodium is exchanged. These results have guided an investigation into the reactions with 241Am on a tracer scale and results from gamma-spectrometry show that becquerelite, andersonite, grimselite, liebigite and compreignacite can include americium in the structure. Shifts in the UO and C–O Raman active bands are similar to that observed in the Eu(III) analogues and Am(III) photoluminescence measurements are also reported on these phases; the Am3+ ion quenches the emission from the uranyl ion.

The synthesis of a series of uranyl compounds via oxidation of [Li(THF)4][UX5(THF)] (X = Cl, Br, I) in the presence of Ph3PO is described. The solid state structures of [Li(OPPh3)(MeCN)2]2[UO2Cl4], [UO2Br2(OPPh3)2] and [Li(OPPh3)4][I3], formed as a by-product from the oxidation of [Li(THF)4][UI5(THF)], is reported. The electronic absorption spectra and photoluminescence spectra of [UO2X2(OPPh3)2] (X = Cl, Br, I) have been measured and no significant changes in the position of the emission (515–530 nm) or the lifetimes (ca. 1 μs) are observed in this series. However a bathochromic shift is observed in the U–X LMCT band in the electronic absorption spectrum.

The crystal structure of [Ph4P][NCS]·HNCS is reported. This is the first structural determination of isothiocyanic acid and hydrogen bonding between the NCS anion and HNCS fragment explored using computational chemistry.

The uranyl aryloxide, [UO2(OAr)2(THF)2], and uranyl chloride, [UO2Cl2(THF)3] or [UO2Cl2(THF)2]2 act as pre-catalysts for the ring opening polymerization of propylene oxide and cyclohexene oxide. Coordination of the monomers has been investigated using 1H EXSY spectroscopy and kinetic and thermodynamic parameters reported. NMR analyses of the polymers suggest a bimetallic mechanism for the polymerization.

The solid-state structure of the known complex [Et4N][U(NCS)5(bipy)2] has been re-determined and a detailed spectroscopic and magnetic study has been performed in order to confirm the oxidation states of both metal and bipy ligand. Electronic absorption and infrared spectroscopy suggest that the uranium is in its +4 oxidation state and this has been corroborated by emission spectroscopy and variable temperature magnetic measurements, as well as theoretical calculations. Therefore the bipy ligands are neutral, innocent ligands and not, as would be inferred from just a solid state structure, radical anions.