Tailoring the pore size of expanded porphyrinoids for lanthanide selectivity.

Despite increase in demand, capacity for the recycling of rare earth elements remains limited, partly due to the inefficiencies with processes currently utilised in the separation of lanthanides. This study highlights the potential use of expanded porphyrinoids in lanthanide separation through selective binding, dependent on the tailored pore size of the macrocycle. Each emerging trend is subjected to multi-factored analysis to decompose the underlying source. Results promote the viability of size-based separation with preferential binding of larger lanthanum(iii) ions to amethyrin and isoamethyrin macrocycles, while smaller macrocycles such as pentaphyrin(0.0.0.0.0) present a preferential binding of lutetium(iii) ions. Additionally, the porphyrin(2.2.2.2) macrocycle shows a selectivity for gadolinium(iii) ions over both larger and smaller ions. An upper limit of applicable pore size is shown to be ≈2.8 Å, beyond which the formed complexes are predicted to be less stable than the corresponding nitrate complexes.

Bounding [AnO2]2+ (An = U, Np) covalency by simulated O K-edge and An M-edge X-ray absorption near-edge spectroscopy.

Restricted active space simulations are shown to accurately reproduce and characterise both O K-edge and U M4,5-edge spectra of uranyl in excellent agreement with experimental peak positions and are extended to the Np analogue. Analysis of bonding orbital composition in the ground and O K-edge core-excited states demonstrates that metal contribution is underestimated in the latter. In contrast, An M4/5-edge core-excited states produce bonding orbital compositions significantly more representative of those in the ground state. Quantum Theory of Atoms in Molecules analysis is employed to explain the discrepancy between K- and M-edge data and demonstrates that the location of the core-hole impacts the pattern of electron localisation in core-excited states. An apparent contradiction to this behaviour in neptunyl is rationalised in terms interelectronic repulsion between the unpaired 5f electron and the excited core-electron.

The role of covalency in enhancing stability of Eu and Am complexes: a DFT comparison of BTP and BTPhen.

We compare the stabilities and bonding nature of [Eu/Am(BTPhen)2(NO3)]2+ complexes to those previously reported for [Eu/Am(BTP)3]3+, and investigate whether more accurately reflecting the reaction conditions of the separation process by considering [Eu/Am(NO3)3(H2O)x] (x = 3, 4) complexes instead of aquo complexes increases the selectivity of the separation ligands BTP and BTPhen for Am over Eu. The geometric and electronic structures of [Eu/Am(BTPhen)2(NO3)]2+ and [Eu/Am(NO3)3(H2O)x] (x = 3, 4) have been evaluated using density functional theory (DFT) and used as the basis for analysis of the electron density through the application of the quantum theory of atoms in molecules (QTAIM). Increased covalent bond character for the Am complexes of BTPhen over Eu analogues was found, with this increase more pronounced than that found in BTP complexes. BHLYP-derived exchange reaction energies were evaluated using the hydrated nitrates as a reference and a favourability for actinide complexation by both BTP and BTPhen was found, with the BTPhen ligand found to be more selective, with relative stability ≈0.17 eV greater than BTP.

Exploring the Electrochemistry of Iron Dithiolene and Its Potential for Electrochemical Homogeneous Carbon Dioxide Reduction.

In this work, the dithiolene complex iron(III) bis-maleonitriledithiolene [Fe(mnt)2] is characterised and evaluated as a homogeneous CO2 reduction catalyst. Electrochemically the Fe(mnt)2 is reduced twice to the trianionic Fe(mnt)2 3- state, which is correspondingly found to be active towards CO2. Interestingly, the first reduction event appears to comprise overlapping reversible couples, attributed to the presence of both a dimeric and monomeric form of the dithiolene complex. In acetonitrile Fe(mnt)2 demonstrates a catalytic response to CO2 yielding typical two-electron reduction products: H2, CO and CHOOH. The product distribution and yield were governed by the proton source. Operating with H2O as the proton source gave only H2 and CO as products, whereas using 2,2,2-trifluoroethanol gave 38 % CHOOH faradaic efficiency with H2 and CO as minor products.

Does Reduction-Induced Isomerization of a Uranium(III) Aryl Complex Proceed via C-H Oxidative Addition and Reductive Elimination across the Uranium(II/IV) Redox Couple?

Reaction of the uranium(III) bis(amidinate) aryl complex {TerphC(NiPr)2}2U(Terph) (2, where Terph = 4,4″-di-tert-butyl-m-terphenyl-2'-yl) with a strong reductant enabled isolation of isomeric uranium(III) bis(amidinate) aryl product {TerphC(NiPr)2}2U(Terph*) (3, where Terph* = 4,4″-di-tert-butyl-m-terphenyl-4'-yl). In terms of connectivity, 3 differs from 2 only in the positions of the U-C and C-H bonds on the central aryl ring of the m-terphenyl-based ligand. A deuterium labeling study ruled out mechanisms for this isomerization involving intermolecular abstraction or deprotonation of the ligand C-H bonds activated during the reaction. Due to the complexity of this rapid, heterogeneous reaction, experimental studies could not further distinguish between two different intramolecular C-H activation mechanisms. However, high-level computational studies were consistent with a mechanism that included two sets of unimolecular, mononuclear C-H oxidative addition and reductive elimination steps involving uranium(II/IV).

Photoluminescence of Pentavalent Uranyl Amide Complexes.

Pentavalent uranyl species are crucial intermediates in transformations that play a key role for the nuclear industry and have recently been demonstrated to persist in reducing biotic and abiotic aqueous environments. However, due to the inherent instability of pentavalent uranyl, little is known about its electronic structure. Herein, we report the synthesis and characterization of a series of monomeric and dimeric, pentavalent uranyl amide complexes. These synthetic efforts enable the acquisition of emission spectra of well-defined pentavalent uranyl complexes using photoluminescence techniques, which establish a unique signature to characterize its electronic structure and, potentially, its role in biological and engineered environments via emission spectroscopy.

Systematic Investigation of the Molecular and Electronic Structure of Thorium and Uranium Phosphorus and Arsenic Complexes.

In continuing to examine the interaction of actinide-ligand bonds with soft donor ligands, a comparative investigation with phosphorus and arsenic was conducted. A reaction of (C5Me5)2AnMe2, An = Th, U, with 2 equiv of H2AsMes, Mes = 2,4,6-Me3C6H2, forms the primary bis(arsenido) complexes, (C5Me5)2An[As(H)Mes]2. Both exhibit thermal instability at room temperature, leading to the elimination of H2, and the formation of the diarsenido species, (C5Me5)2An(η2-As2Mes2). The analogous diphosphido complexes, (C5Me5)2An(η2-P2Mes2), could not be synthesized via the same route, even upon heating the bis(phosphido) species to 100 °C in toluene. However, they were accessible via the reaction of dimesityldiphosphane, MesP(H)P(H)Mes, with (C5Me5)2AnMe2 at 70 °C in toluene. When (C5Me5)2AnMe2 is reacted with 1 equiv of H2AsMes, the bridging µ2-arsinidiide complexes [(C5Me5)2An]2(µ2-AsMes)2 are formed. Upon reaction of (C5Me5)2UMe2 with 1 equiv of H2PMes, the phosphinidiide [(C5Me5)2U(µ2-PMes)]2 is isolated. However, the analogous thorium reaction leads to a phosphido and C-H bond activation of the methyl on the mesityl group, forming {(C5Me5)2Th[P(H)(2,4-Me2C6H2-6-CH2)]}2. The reactivity of [(C5Me5)2An(µ2-EMes)]2 was investigated with OPPh3 in an effort to produce terminal phosphinidene or arsinidene complexes. For E = As, An = U, a U(III) cation-anion pair [(C5Me5)2U(η2-As2Mes2)][(C5Me5)2U(OPPh3)2] is isolated. The reaction of [(C5Me5)2Th(µ2-AsMes)]2 with OPPh3 does not result in a terminal arsinidene but, instead, eliminates PPh3 to yield a bridging arsinidiide/oxo complex, [(C5Me5)2Th]2(µ2-AsMes)(µ2-O). Finally, the combination of [(C5Me5)2U(µ2-PMes)]2 and OPPh3 yields a terminal phosphinidene, (C5Me5)2U(═PMes)(OPPh3), featuring a short U-P bond distance of 2.502(2) Å. Electrochemical measurements on the uranium pnictinidiide complexes demonstrate only a 0.04 V difference with phosphorus as a slightly better donor. Magnetic measurements on the uranium complexes show more excited-state mixing and therefore higher magnetic moments with the arsenic-containing compounds but no deviation from uncoupled U(IV) behavior. Finally, a quantum theory of atoms in molecules analysis shows highly polarized actinide-pnictogen bonds with similar bonding characteristics, supporting the electrochemical and magnetic measurements of similar bonding between actinide-phosphorus and actinide-arsenic bonds.

Energy-Degeneracy-Driven Covalency in Actinide Bonding.

Evaluating the nature of chemical bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic density functional theory study that quantitatively evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2 g*/eg *), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amount of M-Cl orbital mixing, such that AnIV 6d - and Cl 3p-mixing decreased and metal 5f - and Cl 3p-orbital mixing increased across this series.

Coordination Chemistry and QTAIM Analysis of Homoleptic Dithiocarbamate Complexes, M(S2CNiPr2)4 (M = Ti, Zr, Hf, Th, U, Np).

In a systematic approach to comparing the molecular structure and bonding in homoleptic transition-metal and actinide complexes, a series of dithiocarbamates, M(S2CNiPr2)4 (M = Ti, Zr, Hf, Th, U, Np), have been synthesized. These complexes have been characterized through spectroscopic and X-ray crystallographic analysis, and their bonding has been examined using density functional theory calculations. Computational results indicate that the covalent character associated with the M-S bonds shows the trend of Hf < Zr < Th < Ti < U ≈ Np.

How to Bend the Uranyl Cation via Crystal Engineering.

Bending the linear uranyl (UO22+) cation represents both a significant challenge and opportunity within the field of actinide hybrid materials. As part of related efforts to engage the nominally terminal oxo atoms of uranyl cation in noncovalent interactions, we synthesized a new uranyl complex, [UO2(C12H8N2)2(C7H2Cl3O2)2]·2H2O (complex 2), that featured both deviations from equatorial planarity and uranyl linearity from simple hydrothermal conditions. Based on this complex, we developed an approach to probe the nature and origin of uranyl bending within a family of hybrid materials, which was done via the synthesis of complexes 1-3 that display significant deviations from equatorial planarity and uranyl linearity (O-U-O bond angles between 162° and 164°) featuring 2,4,6-trihalobenzoic acid ligands (where Hal = F, Cl, and Br) and 1,10-phenanthroline, along with nine additional "nonbent" hybrid materials that either coformed with the "bent" complexes (4-6) or were prepared as part of complementary efforts to understand the mechanism(s) of uranyl bending (7-12). Complexes were characterized via single crystal X-ray diffraction and Raman, infrared (IR), and luminescence spectroscopy, as well as via quantum chemical calculations and density-based quantum theory of atoms in molecules (QTAIM) analysis. Looking comprehensively, these results are compared with the small library of bent uranyl complexes in the literature, and herein we computationally demonstrate the origin of uranyl bending and delineate the energetics behind this process.

Author Correction: The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes.

This corrects the article DOI: 10.1038/ncomms14137.

Unravelling the electronic structure and dynamics of an isolated molecular rotary motor in the gas-phase.

Light-driven molecular motors derived from chiral overcrowded alkenes are an important class of compounds in which sequential photochemical and thermal rearrangements result in unidirectional rotation of one part of the molecule with respect to another. Here, we employ anion photoelectron spectroscopy to probe the electronic structure and dynamics of a unidirectional molecular rotary motor anion in the gas-phase and quantum chemistry calculations to guide the interpretation of our results. We find that following photoexcitation of the first electronically excited state, the molecule rotates around its axle and some population remains on the excited potential energy surface and some population undergoes internal conversion back to the electronic ground state. These observations are similar to those observed in time-resolved measurements of rotary molecular motors in solution. This work demonstrates the potential of anion photoelectron spectroscopy for studying the electronic structure and dynamics of molecular motors in the gas-phase, provides important benchmarks for theory and improves our fundamental understanding of light-activated molecular rotary motors, which can be used to inform the design of new photoactivated nanoscale devices.

Actinide covalency measured by pulsed electron paramagnetic resonance spectroscopy.

Our knowledge of actinide chemical bonds lags far behind our understanding of the bonding regimes of any other series of elements. This is a major issue given the technological as well as fundamental importance of f-block elements. Some key chemical differences between actinides and lanthanides-and between different actinides-can be ascribed to minor differences in covalency, that is, the degree to which electrons are shared between the f-block element and coordinated ligands. Yet there are almost no direct measures of such covalency for actinides. Here we report the first pulsed electron paramagnetic resonance spectra of actinide compounds. We apply the hyperfine sublevel correlation technique to quantify the electron-spin density at ligand nuclei (via the weak hyperfine interactions) in molecular thorium(III) and uranium(III) species and therefore the extent of covalency. Such information will be important in developing our understanding of the chemical bonding, and therefore the reactivity, of actinides.

Ligand size dependence of U-N and U-O bond character in a series of uranyl hexaphyrin complexes: quantum chemical simulation and density based analysis.

A series of uranyl complexes with hexaphyrin ligands are investigated at the density functional level of theory and analysed using a variety of density-based techniques. A relationship is identified between the size of the ligand and the stability of the complex, controlled by the presence of meso-carbon centres in the porphyrin ring. The complex with the smallest ligand, cyclo[6]pyrrole, is found to have enhanced covalent character in equatorial U-N bonds as defined by the quantum theory of atoms in molecules (QTAIM), as well as enhanced stability, compared to the larger complexes. QTAIM data are supported by electron density difference distributions, integrated electronic properties and analysis of the reduced density gradient (RDG), which all show unambiguous evidence of electron sharing in all U-N bonds. In all complexes, a weakening of the covalent axial U-Oyl interaction in comparison to free uranyl is found, with evidence for a separation of electronic charge resulting in a more ionic interaction. A relationship between covalent character in the U-N bonds and the magnitude of uranyl charge redistribution is identified, where the greater the covalent character of the U-N interaction, the more ionic the U-Oyl interaction appears. The complex with the largest ligand, hexaphyrin(1.1.1.1.1.1), is found to have additional interactions with the uranyl oxygen centres, perturbing the U-Oyl interaction.

The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes.

Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal-ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle.

Double Reduction of 4,4'-Bipyridine and Reductive Coupling of Pyridine by Two Thorium(III) Single-Electron Transfers.

The redox chemistry of uranium is burgeoning and uranium(III) complexes have been shown to promote many interesting synthetic transformations. However, their utility is limited by their reduction potentials, which are smaller than many non-traditional lanthanide(II) complexes. Thorium(III) has a greater redox potential so it should present unprecedented opportunities for actinide reactivity but as with uranium(II) and thorium(II) chemistry, these have not yet been fully realized. Herein we present reactivity studies of two equivalents of [Th(Cp'')3 ] (1, Cp''={C5 H3 (SiMe3 )2 -1,3}) with 4,4'-bipyridine or two equivalents of pyridine to give [{Th(Cp'')3 }2 {µ-(NC5 H4 )2 }] (2) and [{Th(Cp'')3 }2 {µ-(NC5 H5 )2 }] (3), respectively. As relatively large reduction potentials are required to effect these transformations we have shown that thorium(III) can promote reactions that uranium(III) cannot, opening up promising new reductive chemistry for the actinides.

Ionic adsorption on the brucite (0001) surface: A periodic electrostatic embedded cluster method study.

Density functional theory (DFT) at the generalised gradient approximation level is employed within the periodic electrostatic embedded cluster method (PEECM) to model the brucite (0001) surface. Three representative studies are then used to demonstrate the reliability of the PEECM for the description of the interactions of various ionic species with the layered Mg(OH)2 structure, and its performance is compared with periodic DFT, an approach known to be challenging for the adsorption of charged species. The adsorption energies of a series of s block cations, including Sr2+ and Cs+ which are known to coexist with brucite in nuclear waste storage ponds, are well described by the embedded cluster model, provided that basis sets of triple-zeta quality are employed for the adsorbates. The substitution energies of Ca2+ and Sr2+ into brucite obtained with the PEECM are very similar to periodic DFT results, and comparison of the approaches indicates that two brucite layers in the quantum mechanical part of the PEECM are sufficient to describe the substitution. Finally, a detailed comparison of the periodic and PEECM DFT approaches to the energetic and geometric properties of differently coordinated Sr[(OH)2(H2O)4] complexes on brucite shows an excellent agreement in adsorption energies, Sr-O distances, and bond critical point electron densities (obtained via the quantum theory of atoms-in-molecules), demonstrating that the PEECM can be a useful alternative to periodic DFT in these situations.

Concomitant Carboxylate and Oxalate Formation From the Activation of CO2 by a Thorium(III) Complex.

Improving our comprehension of diverse CO2 activation pathways is of vital importance for the widespread future utilization of this abundant greenhouse gas. CO2 activation by uranium(III) complexes is now relatively well understood, with oxo/carbonate formation predominating as CO2 is readily reduced to CO, but isolated thorium(III) CO2 activation is unprecedented. We show that the thorium(III) complex, [Th(Cp'')3 ] (1, Cp''={C5 H3 (SiMe3 )2 -1,3}), reacts with CO2 to give the mixed oxalate-carboxylate thorium(IV) complex [{Th(Cp'')2 [κ2 -O2 C{C5 H3 -3,3'-(SiMe3 )2 }]}2 (µ-κ2 :κ2 -C2 O4 )] (3). The concomitant formation of oxalate and carboxylate is unique for CO2 activation, as in previous examples either reduction or insertion is favored to yield a single product. Therefore, thorium(III) CO2 activation can differ from better understood uranium(III) chemistry.

Topological Study of Bonding in Aquo and Bis(triazinyl)pyridine Complexes of Trivalent Lanthanides and Actinides: Does Covalency Imply Stability?

The geometrical and electronic structures of Ln[(H2O)9]3+ and [Ln(BTP)3]3+, where Ln = Ce-Lu, have been evaluated at the density functional level of theory using three related exchange-correlation (xc-)functionals. The BHLYP xc-functional was found to be most accurate, and this, along with the B3LYP functional, was used as the basis for topological studies of the electron density via the quantum theory of atoms in molecules (QTAIM). This analysis revealed that, for both sets of complexes, bonding was almost identical across the Ln series and was dominated by ionic interactions. Geometrical and electronic structures of actinide (An = Am, Cm) analogues were evaluated, and [An(H2O)9]3+ + [Ln(BTP)3]3+ → [Ln(H2O)9]3+ + [An(BTP)3]3+ exchange reaction energies were evaluated, revealing Eu ↔ Am and Gd ↔ Cm reactions to favor the An species. Detailed QTAIM analysis of Eu, Gd, Am, and Cm complexes revealed increased covalent character in M-O and M-N bonds when M = An, with this increase being more pronounced in the BTP complexes. This therefore implies a small electronic contribution to An-N bond stability and the experimentally observed selectivity of the BTP ligand for Am and Cm over lanthanides.

Assessing covalency in equatorial U-N bonds: density based measures of bonding in BTP and isoamethyrin complexes of uranyl.

Calculations performed at the density functional level of theory have been used to investigate complexes of uranyl with the expanded porphyrin isoamethyrin and the bis-triazinyl-pyridine (BTP) ligands, the latter of which is well-known to be effective in the separation of trivalent lanthanides and actinides. Analysis has been performed using a range of density-based techniques, including the Quantum Theory of Atoms in Molecules (QTAIM), the Electron Localisation Function (ELF) and the reduced density gradient (RDG). The effects of peripheral alkyl substituents on UO2-isoamethyrin, known to be vital for proper replication of the experimental geometry, are considered. Evidence for comparable amounts of covalent character has been found in the largely ionic U-N bonds of UO2-isoamethyrin and [UO2(BTP)2](2+) and examination of the variation in the electronic characteristics of the uranyl unit upon complexation in both of these cases reveal striking similarities in the nature of the U-N bonding and the effect of this bonding on the U-Oyl interaction, as well as evidence of donation into the U-N bonding region from the uranyl unit itself.