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Structural Variability of 4f and 5f Thiocyanate Complexes and Dissociation of Uranium(III)–Thiocyanate Bonds with Increased Ionicity
Abstract
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.
... We have long been interested in the solid-state assembly of monomeric [UO 2 X 4 ] 2À units in the presence of organic cations capable of participating in non-covalent interactions (NCIs) such as hydrogen-bonding, halogen-bonding, π-π stacking, O yl ···π, X UO 2 X 4 ½ � 2À ···π and X cation ···O yl interactions. [21,32,33] As our group [21] and others [34][35][36][37][38][39][40][41][42] have shown, the nature and strength of NCIs between [UO 2 X 4 ] 2À and the counter-cation are indeed reflected in the photophysical properties, and by extension, electronic structure of the materials. In other words, the secondary coordination sphere influences the deactivation pathways of the excited ([UO 2 X 4 ] 2À )* ion. ...
A series of compounds of the form [HAr]2[UO2X4] is reported here, wherein Ar is systematically varied between pyridine (1‐X), quinoline (2‐X), acridine (3‐X), 2,5‐dimethylpyrazine (4‐X), quinoxaline (5‐X), and phenazine (6‐X), and X=Cl or Br. With greater conjugation in the organic cation, a larger quenching in uranyl luminescence is observed in the solid state. Supporting our luminescence experiments with computation, we map out the potential energy diagrams for the singlet and triplet states of both the [HAr]⁺ cations and [UO2Cl4]²⁻ anion in the crystalline state, and of the assembly. The distinct energy transfer pathways in each compound are discussed.
... When reducing an actinide atom to the + 3o xidation state, the degree of covalency is decreased even further,a nd assessing bond covalency becomes even more challenging. [13] Furthermore, characterization of heaviera ctinides is intrinsically complicated; firstly,o wing to the actinidec ontraction (i.e.,d ecreasing 5f orbital extension when traversing the actinides eries) and therefore the unresolved role of 5f orbitals in bonding and, secondly,asaresult of their radiotoxicity, which makes experimental manipulation of them very demanding owing to their short half-lives along with the great difficulties for many researchers to obtain such elements for their research pro-gram.T his makes validation of the results more challenging as only al imited number of laboratories have access to heavy actinidess ources. Under these circumstances, computational methodsa ssume significant importance in studying actinide complexes and overcoming experimental difficulties in working with them. ...
Covalency in actinides has emerged as a resounding research topic on account of the technological importance in separating minor actinides from lanthanides for spent nuclear fuel processing, and utilization of their distinct bonding properties has been realized as a route towards overcoming this challenge. Because of the limited radial extent of the 4f orbitals, there is almost no 4f electron participation in bonding in lanthanides; this is not the case for the actinides, which have extended 5f orbitals that are capable of overlapping with ligand orbitals, although not to the degree of overlap as in the d orbitals of transition metals. In this concept paper, a general description of covalency in actinide compounds is provided. After introducing two main approaches to enhance covalency, either by exploiting increased orbital overlap or decreasing energy differences between the orbitals causing orbital energy degeneracy, the current state of the field is illustrated by using several examples from the recent literature. This paper is concluded by proposing the use of actinide chalcogenides as a convenient auxiliary tool to study covalency in actinide compounds.
... However, electrochemical oxidation of the uranyl penta-thiocyanate ([UO 2 (NCS) 5 ] 3À ) complex results in oxidation of thiocyanate ligands and triggers a subsequently unprecedented uranyl(VI) reduction. [29] Oxidation of uranyl(VI) compounds containing multi-dentate highly conjugated organic ligands is primarily expected to produce ligand radicals. Indeed, this is quite obvious because there are no f-electrons present on U in its highest oxidation state. ...
Computational methods have been applied to understand the reduction potentials of [UO2‐salmnt‐L] complexes (L=pyridine, DMSO, DMF and TPPO), and their redox behavior is compared with previous experiments in dichloromethane solution. Since the experimental results were inconclusive regarding the influence of the uranyl‐bound tetra‐dentate ‘salmnt’ ligand, here we will show that salmnt acts as a redox‐active ligand and exhibits non‐innocent behavior to interfere with the otherwise expected one‐electron metal (U) reduction. We have employed two approaches to determine the uranyl (VI/V) reduction potentials, using a direct study of one‐electron reduction processes and an estimation of the overall reduction using isodesmic reactions. Hybrid density functional theory (DFT) methods were combined with the Conductor‐like Polarizable Continuum Model (CPCM) to account for solvation effects. The computationally predicted one‐electron reduction potentials for the range of [UO2‐salmnt‐L] complexes are in excellent agreement with shoulder peaks (∼1.4 eV) observed in the cyclic voltammetry experiments and clearly correlate with ligand reduction. Highly conjugated pi‐bonds stabilize the ligand based delocalized orbital relative to the localized U f‐orbitals, and as a consequence, the ligand traps the incoming electron. A second reduction step results in metal U(VI) to U(V) reduction, in good agreement with the experimentally assigned uranyl (VI/V) reduction potentials.
... We have an interest in thorium(IV) and uranium(IV) thiocyanate complexes as a platform to study how photoluminescence spectroscopy can delineate oxidation states. 9 We have recently shown that the -donor ability of the [NCS]ion does not stabilise the U III oxidation state; 10 (Fig. 2). In 1 and 2, the N=C and C=S bonds are invariant irrespective of oxidation state, consistent with our previous studies on U IV and U VI thiocyanate complexes. ...
Oxidation of Cs 4 [U(NCS) 8 ] in MeCN or DMF affords structurally characterised examples of the mixed-valent UIV/VI compound Cs 14 [{U(NCS) 8 } 3 {UO 2 (NCS) 4 (H 2 O)]·4.5H 2 O, or the [UIV-UV-UIV][UVI] species [U(DMF) 8 (μ-O)U(NCS) 5 (μ-O)U(DMF) 7 (NCS)][UO 2 (NCS) 5 ]. Vibrational and magnetism data support their oxidation state...
Understanding the nature of metal–ligand bonding is a major challenge in actinide chemistry. We present a new experimental strategy for addressing this challenge using actinide 3d4f resonant inelastic X-ray scattering (RIXS). Through a systematic study of uranium(IV) halide complexes, [UX6]2–, where X = F, Cl, or Br, we identify RIXS spectral satellites with relative energies and intensities that relate to the extent of uranium-ligand bond covalency. By analyzing the spectra in combination with ligand field density functional theory we find that the sensitivity of the satellites to the nature of metal–ligand bonding is due to the reduction of 5f interelectron repulsion and 4f-5f spin-exchange, caused by metal–ligand orbital mixing and the degree of 5f radial expansion, known as central-field covalency. Thus, this study furthers electronic structure quantification that can be obtained from 3d4f RIXS, demonstrating it as a technique for estimating actinide-ligand covalency.
Metal thiocyanates were some of the first pseudohalide compounds to be discovered and adopt a diverse range of structures. This review describes the structures, properties, and syntheses of the known binary and ternary metal thiocyanates. It provides a categorization of their diverse structures and connects them to the structures of atomic inorganic materials. In addition to this description of characterized binary and ternary thiocyanates, this review summarizes the state of knowledge for all other binary metal thiocyanates. It concludes by highlighting opportunities for future materials development.
A series of compounds of the form [HAr]2[UO2X4] is reported here, wherein Ar is systematically varied between pyridine (1‐X), quinoline (2‐X), acridine (3‐X), 2,5‐dimethylpyrazine (4‐X), quinoxaline (5‐X), and phenazine (6‐X), and X = Cl or Br. With greater conjugation in the organic cation, a larger quenching in uranyl luminescence is observed in the solid state. Supporting our luminescence experiments with computation, we map out the potential energy diagrams for the singlet and triplet states of both the [HAr]+ cations and [UO2Cl4]2− anion in the crystalline state, and of the assembly. The distinct energy transfer pathways in each compound are discussed.
A new uranyl tetrachloride salt with chemical formula, (NH4)2[UO2Cl4]·2H2O, namely, diammonium uranyl tetrachloride dihydrate, 1, was prepared and crystallized via slow evaporation from a solution of 2 M hydrochloric acid. As confirmed by powder X-ray diffraction, the title compound crystallizes with an ammonium chloride impurity that formed as a result of the breakdown of a triazine precursor. The (UO2Cl4)²⁻ dianion is charge balanced by ammonium cations, while an extensive hydrogen-bond network donated from structural water molecules stabilize the overall assembly. Compound 1 adds to the extensive collection of actinyl tetrachloride salts, but it represents the first without an alkali cation for purely inorganic compounds. Diffuse reflectance and luminescence spectra show typical absorption and emission behavior, respectively, of uranyl materials.
Separation and purification of rare earth elements (REEs) from each other has been a challenging task and a topic of immense interest across the world. The separation of high purity yttrium from chemically similar heavy rare earths (HREs) could be possible due to its differential behavior in thiocyanate & nitrate media with quaternary amine type of extractant Aliquat336. In case of a hard donor like nitrate ion, Y behaves like heavy rare earths (HERs), more or less in accordance with its ionic radii. On the contrary, in thiocyanate medium, the extraction properties of Y follows the light rare earths (LREs) which makes its separation from Er and other HREs effective. In the present investigation, a microscopic analysis employing computational tools has been utilized to offer an explanation for this curious behaviour of yttrium. The computational results bring out the striking fact that while the behavior of Y remains unchanged in two mediums, it is the trend within the lanthanide series that changes between the two mediums. It is observed that the size of the REE nitrates follow a decreasing trend along the lanthanide series, where Y nitrate shows comparable bond lengths with Er nitrate. On the other hand REE thiocyanates follow an increasing trend in size, where Y thiocyanate bond lengths are closer to the La thiocyanate. Similar trend is observed for the partial charges on the outer atoms of REE nitrates and thiocyanates, which affects the electrostatic binding interaction with the extractant Aliquat336. The contrasting trend among the lanthanide series is further explained in terms of spin density analysis shows that, in case of a hard donor like nitrate, f-electron delocalization takes place into the ligand and the size of the REE nitrates follow the lanthanide contraction. Y in this case behaves like HREs. On the contrary, in thiocyanate medium there is no delocalization of f-orbital in inner-sphere thiocyanate complex and due to absence of f orbital in Y, it occupies position with light rare earths as extraction follows the trend of electrostatic attraction between cation and ligand. Understanding of this extractant interaction is expected to provide a knowledge based approach for the development of new extractant molecules with better binding capacity for specific REE of interest.
We report the synthesis and characterization of a family of UO22+/Co2+ isothiocyanate materials containing [UO2(NCS)5]3- and/or [Co(NCS)4]2- building units charged balanced by tetramethylammonium cations and assembled via SS or SOyl non-covalent interactions (NCIs), namely (C4H12N)3[UO2(NCS)5], (C4H12N)2[Co(NCS)4], and (C4H12N)5[Co(NCS)4][UO2(NCS)5]. The homometallic uranyl phase preferentially assembles via SS interactions, whereas in the heterometallic phase SOyl interactions are predominant. The variation in assembly mode is explored using electrostatic surfaces potentials, revealing that the pendant -NCS ligands of the [Co(NCS)4]2- anion is capable of outcompeting those of the [UO2(NCS)5]3- anion. Notably, the heterometallic phase displays atypical blue shifting of the uranyl symmetric stretch in the Raman spectra, which is in contrast to many other compounds featuring non-covalent interactions at uranyl oxygen atoms. A combined experimental and computational (density functional theory and natural bond orbital analyses) approach revealed that coupling of the uranyl symmetric stretch with isothiocyanate modes of equatorial -NCS ligands was responsible for the atypical blue shift in the heterometallic phase.
This article provides broad coverage of the key developments in lanthanide coordination chemistry during the period 2003–19. The term lanthanide covers the elements in the series from cerium to lutetium, but does not include very detailed discussion of coordination compounds of the other three rare earth elements, namely scandium, yttrium and lanthanum. The article places some emphasis on unconventional ligand types that have not previously been included in the articles on the lanthanides earlier editions of Comprehensive Coordination Chemistry, notably hydride and ligands in which the donor atoms include p-block metalloid and metallic elements, such as the heavier Group 13 elements and the heavier Group 14 elements. Bonds between lanthanides and transition metals are also reviewed. Complexes that can readily be identified as belonging to the ‘organometallic’ family are mentioned in cases where the chemistry represents an important development for the lanthanide elements in general, notably the discovery of new oxidation states, single-molecule magnets and small-molecule activation. More traditional Werner-type coordination compounds form the bulk of the article, including vast numbers of N- and O-donor ligands, including many beautiful lanthanide complexes of multidentate and macrocyclic ligands. The applications of such complexes in luminescence and as materials for imaging are summarized, as are their molecular magnetic properties. Whilst every effort has been made to provide comprehensive coverage of topics in lanthanide chemistry, the sheer volume of research activity in this vibrant field makes citation of every study practically impossible. Many more fascinating studies in lanthanide chemistry can be accessed via the extensive list of review articles cited in the following pages.
The electrochemical properties of U(III)-in-crypt (crypt = 2.2.2-cryptand) were examined in dimethylformamide (DMF) and acetonitrile (MeCN) to determine the oxidative stability offered by crypt as a ligand. Cyclic voltammetry revealed a U(III)/U(IV) irreversible oxidation at EPA= -0.49 V (vs Fe(C5H5)2+/0) in DMF and at EPA= -0.31 V (vs Fe(C5H5)2+/0) in MeCN. The electrochemistry of U(III)-in-crypt complexes in the presence of water was also examined. These studies are supported by crystallographically characterized examples of U(III)-in-crypt complexes as DMF, MeCN, and water adducts.
The trivalent actinide thiocyanate complexes of Am and Cm are synthesized for the first time along with their lanthanide homologues allowing comparison of the lanthanide and actinide bonding.
Behavior of UVI, NpVI and PuVI in water‐acetonitrile solutions was studied spectrophotometrically with the successive addition of the polar organic ligands (dimethyl sulfoxide or hexamethylphosphoric triamide) and the NCS– ion. The detected spectral effects – changes in the absorption intensity, bathochromic shifts in the absorption bands, the absence of isosbestic points, a change in the color of the solution – indicate complex competitive processes occurring in the studied solutions. In the case of NpVI, its partial reduction to NpIV by NCS– ion is observed. Solid UVI complex, [UO2(HMPA)2(NCS)2], was isolated, its crystal structure was determined using X‐ray diffraction. In contrast to known AnO22+ compounds with the NCS– ion, this complex exhibits tetragonal bipyramidal environment of the U atom. [UO2(HMPA)2(NCS)2] is also characterized by UV/Vis, IR and luminescence spectroscopy.
The hydrothermal crystal growth of a new uranium phosphite, U(HPO3)2, is reported. This material was found to exhibit optical and magnetic properties not commonly observed in U(IV) containing materials, specifically purple coloration and low temperature magnetism. We discuss the synthesis, structure determination, and characterization of the title compound and comment on its optical, thermal, and magnetic properties. The magnetic behavior is consistent with frustrated antiferromagnetism that arises from the hexagonal honeycomb lattice of U(IV) ions. This material gives rare evidence for U(IV) ions participating in magnetic order.
A series of uranyl thiocyanate and selenocyanate of the type [R4N]3[UO2(NCS)5] (R4=nBu4, Me3Bz, Et3Bz), [Ph4P][UO2(NCS)3(NO3)] and [R4N]3[UO2(NCSe)5] (R4= Me4,nPr4, Et3Bz) have been prepared and structurally characterized. The resulting noncovalent interactions have been examined and compared to other examples in the literature. The nature of these interactions is determined by the cation so that when the alkyl groups are small, chalcogenide···chalcogenide interactions are present, but this "switches off" when R =nPr and charge assisted U═O···H-C and S(e)···H-C hydrogen bonding remain the dominant interaction. Increasing the size of the chain tonBu results in only S···H-C interactions. The spectroscopic implications of these chalcogenide interactions have been explored in the vibrational and photophysical properties of the series [R4N]3[UO2(NCS)5] (R4= Me4, Et4,nPr4,nBu4, Me3Bz, Et3Bz), [R4N]3[UO2(NCSe)5] (R4= Me4,nPr4, Et3Bz) and [Et4N]4[UO2(NCSe)5][NCSe]. The data suggest that U═O···H-C interactions are weak and do not perturb the uranyl moiety. While the chalcogenide interactions do not influence the photophysical properties, a coupling of the U═O and δ(NCS) or δ(NCSe) vibrational modes is observed in the 77 K solid state emission spectra. A theoretical examination of representative examples of Se···Se, C-H···Se, and C-H···O═U by molecular electrostatic potentials and NBO and AIM methodologies gives a deeper understanding of these weak interactions. C-H···Se are individually weak but C-H···O═U interactions are even weaker, supporting the idea that the -yl oxo's are weak Lewis bases. An Atoms in Molecules study suggests that the chalcogenide interaction is similar to lone pair···π or fluorine···fluorine interactions. An oxidation of the NCS ligands to form [(UO2)(SO4)2(H2O)4]·3H2O was also noted.
The electronic structure of f-element compounds is complex due to a combination of relativistic effects, strong electron correlation and weak crystal field environments. However, a quantitative understanding of bonding in these compounds is becoming increasingly technologically relevant. Recently, bonding interpretations based on analyses of the physically observable electronic density have gained popularity and, in this Feature Article, the utility of such density-based approaches is demonstrated. Application of Bader’s Quantum Theory of Atoms in Molecules (QTAIM) is shown to elucidate many properties including bonding trends, overlap and degeneracy-driven covalency, oxidation state identification and bond stability, demonstrating the increasingly important role that simulation and analysis play in the area of f-element bond characterisation.
The first hydrophilic, 1,10-phenanthroline derived ligands consisting of only C,H,O and N atoms for a selective Am(III) extraction process are reported herein. One of these 2,9-bis-triazolyl-1,10-phenanthroline (BTrzPhen) ligands was found to exhibit process-suitable selectivity for Am(III) over Eu(III) and Cm(III), providing a clear step forward.
The electronic structure of a series of uranium and cerium hexachlorides in a variety of oxidation states was evaluated at both the correlated wavefunction and density functional (DFT) levels of theory. Following recent experimental observations of covalency in tetravalent cerium hexachlorides, bonding character was studied using topological and integrated analysis based on the quantum theory of atoms in molecules (QTAIM). This analysis revealed that M–Cl covalency was strongly dependent on oxidation state, with greater covalency found in higher oxidation state complexes. Comparison of M–Cl delocalisation indices revealed a discrepancy between correlated wavefunction and DFT-derived values. Decomposition of these delocalisation indices demonstrated that the origin of this discrepancy lay in ungerade contributions associated with the f-manifold which we suggest is due to self-interaction error inherent to DFT-based methods. By all measures used in this study, extremely similar levels of covalency between complexes of U and Ce in the same oxidation state was found.
We report the first examples of hydrophilic 6,6’-bis(1,2,4-triazin-3-yl)-2,2’-bipyridine (BTBP) and 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen) ligands, and their applications as actinide(III) selective aqueous complexing agents. The combination of a hydrophobic diamide ligand in the organic phase and a hydrophilic tetrasulfonated bis-triazine ligand in the aqueous phase is able to separate Am(III) from Eu(III) by selective Am(III) complex formation across a range of nitric acid concentrations with very high selectivities, and without the use of buffers. In contrast, disulfonated bis-triazine ligands are unable to separate Am(III) from Eu(III) in this system. The greater ability of the tetrasulfonated ligands to retain Am(III) selectively in the aqueous phase than the corresponding disulfonated ligands appears to be due to the higher aqueous solubilities of the complexes of the tetrasulfonated ligands with Am(III). The selectivities for Am(III) complexation observed with hydrophilic tetrasulfonated bis-triazine ligands are in many cases far higher than those found with the polyaminocarboxylate ligands previously used as actinide-selective complexing agents, and are comparable to those found with the parent hydrophobic bis-triazine ligands. Thus we demonstrate a feasible alternative method to separate actinides from lanthanides than the widely studied approach of selective actinide extraction with hydrophobic bis-1,2,4-triazine ligands such as CyMe4-BTBP and CyMe4-BTPhen.
It has been shown that modification of the phenanthroline backbone of CyMe4-BTPhen leads to subtle electronic modulation, permitting differential ligation of Am(III) and Cm(III) resulting in separation factors up to 7
The new computer program SHELXT employs a novel dual-space algorithm to solve the phase problem for single-crystal reflection data expanded to the space group P1. Missing data are taken into account and the resolution extended if necessary. All space groups in the specified Laue group are tested to find which are consistent with the P1 phases. After applying the resulting origin shifts and space-group symmetry, the solutions are subject to further dual-space recycling followed by a peak search and summation of the electron density around each peak. Elements are assigned to give the best fit to the integrated peak densities and if necessary additional elements are considered. An isotropic refinement is followed for non-centrosymmetric space groups by the calculation of a Flack parameter and, if appropriate, inversion of the structure. The structure is assembled to maximize its connectivity and centred optimally in the unit cell. SHELXT has already solved many thousand structures with a high success rate, and is optimized for multiprocessor computers. It is, however, unsuitable for severely disordered and twinned structures because it is based on the assumption that the structure consists of atoms.
[U(Tp(Me2))2(bipy˙)], a uranium(iii) complex with a radical bipyridine ligand which has magnetic properties with contributions from both the ligand and the metal, presents slow relaxation of the magnetisation at low temperatures, already under zero static magnetic field, and energy barriers slightly above the non-radical analogues.
The first anhydrous molecular complexes of uranium(iii) chloride, soluble in polar aprotic solvents, are reported, including the structures of the dimeric [UCl3(py)4]2 and the trimetallic [UCl(py)4(μ-Cl)3U(py)2(μ-Cl)3UCl2(py)3].
Bipyridyl thorium metallocenes [η5-1,2,4-(Me3C)3C5H2]2Th(bipy) (1) and [η5-1,3-(Me3C)2C5H3]2Th(bipy) (2) have been investigated by magnetic susceptibility and computational studies. The magnetic susceptibility data reveal that 1 and 2 are not diamagnetic, but they behave as temperature independent paramagnets (TIPs). To rationalize this observation, density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations have been undertaken, which indicated that Cp′2Th(bipy) has indeed a Th(IV)(bipy2−) ground state (f0d0π*2, S = 0), but the open-shell singlet (f0d1π*1, S = 0) (almost degenerate with its triplet congener) is only 9.2 kcal mol−1 higher in energy. Complexes 1 and 2 react cleanly with Ph2CS to give [η5-1,2,4-(Me3C)3C5H2]2Th[(bipy)(SCPh2)] (3) and [η5-1,3-(Me3C)2C5H3]2Th[(bipy)(SCPh2)] (4), respectively, in quantitative conversions. Since no intermediates were observed experimentally, this reaction was also studied computationally. Whereas coordination of Ph2CS to 2 in its S = 0 ground state is not possible, Ph2CS can coordinate to 2 in its triplet state (S = 1) upon which a single electron transfer (SET) from the (bipy2−) fragment to Ph2CS followed by C–C coupling takes place.
The comparative study of chemical reactivity between trivalent late actinides (An) and the lanthanides (Ln) has always been a challenging issue. For that purpose, soft donor ligands (containing for example sulfur atoms or more borderline N atoms) are known to have a relative tendency for higher interaction with the An(III) than with the Ln(III) metal cations. Consequently, thiocyanates have been envisioned as possible ligands for the selective complexation of heavy actinides (namely, Am and Cm) over lanthanides. This paper is an illustration of this approach and reports on the thiocyanate chemistry of lutetium. Three new complexes, 1, [n-(C4H9)4N]3[Lu(NCS)4(NO3)2], 2, K4[Lu(NCS)4(H2O)4](NCS)3(H2O)2 and for comparison 3, K4[Nd(NCS)4(H2O)4](NCS)3(H2O)2 have been characterized by X-ray single crystal diffraction. In addition, dissolution of the nitrato lutetium adduct (1) in wet ethanolsolvent brings some valuable information about the structure of the cation coordination sphere in solution compared to the solid crystalline state. Another point of comparison comes from the dissolution of lutetium nitrate also in wet ethanol. In both cases, the cation's coordination sphere has been probed by IR and EXAFS at the Lu L3 edge. Additional comparison with molecular dynamic simulations of the lutetium-nitrate–ethanol (wet) system has been performed and coupled to the EXAFS data fitting. Upon dissolution of 1 as well as of lutetium nitrate, a decrease of the number of nitrate ligands has been observed. In the case of 1, a clear decrease of the number of thiocyanate ligand coordination has also been observed, leading to a strong rearrangement of the cation polyhedron from solid state to solution.
The used fuel discharged from nuclear power plants constitutes the main contribution to nuclear waste in countries which do not undertake reprocessing. As such, its disposal requires isolation from the biosphere in stable deep geological formations for long periods of time (some hundred thousand years) until its radioactivity decreases through the process of radioactive decay. Ways for significantly reducing the volumes and radiotoxicities of the waste and to shorten the very long times for which the waste must be stored safely are being investigated. This is the motivation behind the partitioning and transmutation (P&T) activities worldwide. This paper addresses the potential impact of P&T on the long-term disposal of nuclear waste. In particular, it evaluates how realistic P&T scenarios can lead to a reduction in the time required for the waste to be stored safely. The calculations have been done independently by three research groups: ITU and FZK in Germany, and by the CEA in France.
The reaction of UI3 and KCpRR′ (CpRR′ = pentamethylcyclopentadienyl, trimethylsilylcyclopentadienyl or tetramethylcyclopentadienyl) in diethyl ether results in the two-electron reduction of the solvent to form trimetallic, mixed valence uranium oxo species.
The spectroscopic, magnetic, and bonding properties of a series of tri- and tetravalent hexachloride actinide complexes are investigated using correlated electronic structure calculations together with ab initio ligand-field theory. Despite the predominately ionic character of An−Cl bonds, they have been found to exhibit a nonnegligible level of f covalency compared to their lanthanide analogues.
Reaction of trivalent [(((Ad,tBu)ArO)3tacn)U] (1) with 2,2'-bipyridine (bipy) yields [(((Ad,tBu)ArO)3tacn)U(bipy)] (2) and subsequent reduction of 2 with KC8 in the presence of Kryptofix222 furnishes [K(2.2.2-crypt)][(((Ad,tBu)ArO)3tacn)U(bipy)] (3). Alternatively, complex 3 can be synthesized from 1 by addition of [K(bipy)] in the presence of the cryptand. New complexes 2 and 3 are characterized by a variety of spectroscopic, electrochemical, and magnetochemical methods, single-crystal X-ray diffraction, computational methods, and CHN elemental analysis. Structural analyses reveal a bipyridine radical (bipy(•-)) ligand in 2 and a dianionic (bipy(2-)) species in 3. Complex 3 represents a rare example of an isolated and unambiguously characterized bipy(2-) ligand coordinated to a uranium ion. The electronic structure assignments are supported by UV/vis/NIR and EPR spectroscopy, as well as SQUID magnetometry. The results of CASSCF calculations indicate multiconfigurational ground states for complexes 2 and 3. The electronic ground state for 2 consists of an open-shell doublet U(4+)(bipy(•-)) state (91%) and a closed-shell doublet U(5+)(bipy(2-)) state (9%). The almost degenerate multiconfigurational ground state for 3 was found to be composed of an open-shell singlet and pure triplet state 0.06 eV higher in energy, both resulting from the U(4+)(5f(2)) (bipy(2-)) configuration.
The chemical bonding in actinide compounds is usually analysed by inspecting the shape and the occupation of the orbitals or by calculating bond orders which are based on orbital overlap and occupation numbers. However, this may not give a definite answer because the choice of the partitioning method may strongly influence the result possibly leading to qualitatively different answers. In this review, we summarized the state-of-the-art of methods dedicated to the theoretical characterisation of bonding including charge, orbital, quantum chemical topology and energy decomposition analyses. This review is not exhaustive but aims to highlight some of the ways opened up by recent methodological developments. Various examples have been chosen to illustrate this progress.
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.
The first examples of 4,7-disubstituted 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzo-triazin-3-yl)-1,10-phenanthroline (CyMe4-BTPhen) ligands are reported herein. Evaluating the kinetics, selectivity and stoichiometry of actinide(iii) and lanthanide(iii) radiotracer extractions has provided a mechanistic insight into the extraction process. For the first time, it has been demonstrated that metal ion extraction kinetics can be modulated by backbone functionalisation and a promising new CHON compliant candidate ligand with enhanced metal ion extraction kinetics has been identified. The effects of 4,7-functionalisation on the equilibrium metal ion distribution ratios are far more pronounced than those of 5,6-functionalisation. The complexation of Cm(iii) with two of the functionalised ligands was investigated by TRLFS and, at equilibrium, species of 1 : 2 [M : L] stoichiometry were observed exclusively. A direct correlation between the ELUMO-EHOMO energy gap and metal ion extraction potential is reported, with DFT studies reaffirming experimental findings.
It is intriguing how nature attains recognition specificity between molecular interfaces where there is no apparent scope for classical hydrogen bonding or polar interactions. Methionine aminopeptidase (MetAP) is one such enzyme where this fascinating conundrum is at play. In this study, we demonstrate that a unique C-HS hydrogen bond exists between the enzyme methionine aminopeptidase (MetAP) and its N-terminal-methionine polypeptide substrate, which allows specific interaction between apparent apolar interfaces, imposing a strict substrate recognition specificity and efficient catalysis, a feature replicated in Type I MetAPs across all kingdoms of life. We evidence this evolutionarily conserved C-HS hydrogen bond through enzyme assays on wild-type and mutant MetAP proteins from Mycobacterium tuberculosis that show a drastic difference in catalytic efficiency. The X-ray crystallographic structure of the methionine bound protein revealed a conserved water bridge and short contacts involving the Met side-chain, a feature also observed in MetAPs from other organisms. Thermal shift assays showed a remarkable 3.3 °C increase in melting temperature for methionine bound protein compared to its norleucine homolog, where C-HS interaction is absent. The presence of C-HS hydrogen bonding was also corroborated by nuclear magnetic resonance spectroscopy through a change in chemical shift. Computational chemistry studies revealed the unique role of the electrostatic environment in facilitating the C-HS interaction. The significance of this atypical hydrogen bond is underscored by the fact that the function of MetAP is essential for any living cell.
After more than 50 years, the synthesis and electronic structure of the first and only reported “U ⁰ complex” [U(bipy) 4 ] ( 1 ) has been reinvestigated. Additionally, its one‐electron reduced product [Na(THF) 6 ][U(bipy) 4 ] ( 2 ) has been newly discovered. High resolution crystallographic analyses combined with magnetic and computational data show that 1 and its derivative 2 are best described as highly reduced species containing mid‐to‐high‐valent uranium ligated by redox non‐innocent ligands.
The data on homoand heteroleptic molecular and ionic Sc, Y, and lanthanide thiocyanates are systematized and generalized. The influence of the metal cation, its coordination number, and the ligand environment on the formation of the coordination sphere and stereochemistry of rare-earth metal thiocyanates is studied. The bibliography consists of 62 references.
The synthesis, X-ray crystal structure, vibrational and optical spectroscopy for the eight-coordinate thiocyanate compounds, [Et4N]4[PuIV(NCS)8], [Et4N]4[ThIV(NCS)8], and [Et4N]4[CeIII(NCS)7(H2O)] are reported. Thiocyanate was found to rapidly reduce plutonium to PuIII in acidic solutions (pH<1) in the presence of NCS−. The optical spectrum of [Et4N][SCN] containing PuIII solution was indistinguishable from that of aquated PuIII suggesting that inner-sphere complexation with [Et4N][SCN] does not occur in water. However, upon concentration, the homoleptic thiocyanate complex [Et4N]4[PuIV(NCS)8] was crystallized when a large excess of [Et4N][NCS] was present. This compound, along with its UIV analogue, maintains inner-sphere thiocyanate coordination in acetonitrile based on the observation of intense ligand-to-metal charge-transfer bands. Spectroscopic and crystallographic data do not support the interaction of the metal orbitals with the ligand π system, but support an enhanced AnIV–NCS interaction, as the Lewis acidity of the metal ion increases from Th to Pu.
Solvent extraction of Y, Am(III) and some lanthanides from thiocyanate, selenocyanate, cyanate and azide solutions by tri-n-butyl phosphate was studied. It has been found that the distribution ratio, D, is high for thiocyanates and selenocyanates and very low for azides and cyanates. At the same time the separation factor DAm/DY is about 12 for the NCS- and NCSe-ligands, while for the NCO- and N3- ligands the separation factors between Am and Y, and between all the elements studied are very close to one. The division of the isoelectronic pseudohalide ligands into two classes with respect to D and to separation factors corresponds with the division of the respective hydracids into moderately strong (HNCS and HNCSe) and weak (HNCO and HN3). The results are discussed in terms of the effect of the pseudohalide anion, X-, on the stability and transfer of the innersphere MX3 complexes, and on the equilibrium between the inner-and outer-sphere complexes in the aqueous phase.
The title new complexes were synthesized and their crystal and molecular structures were determined by the single-crystal X-ray diffraction method. The crystal form of [(C2H5)4N]3[Yb(NCS)6]·(C6H6), for example, is monoclinic, space group P21⁄c, a=19.458(12), b=14.046(6), c=18.288(7) A, β=91.28(4)°, Z=4, and the final R value obtained was 0.064. The other seven crystals are isomorphous with each other. The central metal atom is in an octahedral hexa-coordination geometry: six nitrogen atoms of isothiocyanate ions are ligated to each metal atom. Each solvent molecule is arranged betweeen each pair of neighboring metal atoms along the [1 \bar1 0] axis in each crystal. The cations are arranged approximately between each pair of metal atoms along the [0 1 1], [0 \bar1 1], and [1 1 0] axes, respectively. They were stable for more than a few weeks at room temperature; however, when heated at about 80–90 °C, their guest molecules were lost.
The crystals of the title new lanthanum(III) complex, LaC45H86N11S7, F.W. 1144.59 are orthorhombic space group Bm21b, a=17.438(7), b=21.507(9), c=16.616(6) A, U=6232(4) A3, Dm=1.21(3), Dx=1.22 g cm−3, and μ(Mo Kα)=9.63 cm−1. The central lanthanum(III) atom has a unicapped trigonal-prism 7N-hepta-coordination geometry: seven thiocyanato nitrogen atom are ligated. There are no bridgings between the complexes and/or counter cations. Each benzene molecule is surrounded by the thiocynate ions and ethyl groups of the cations. Although these crystals lose benzene at about 60 °C, they are stable for more than several weeks ambient temperature, and can be regarded as a kind of inclusion compound. The isomorphous praseodymium(III) complex was also obtained.
The crystal and molecular structures of the title new complexes were determined by means of a single-crystal X-ray diffraction method. The crystals of the neodymium(III) complex, NdC12H28N5O4S4, F. W. 578.87, are triclinic, space group P\bar1, a=14.13(2), b=15.39(1), c=12.92(1) Å, α=106.81(8), β=114.70(9), γ=90.09(10)°, U=2418(5) Å3, Z=4, Dm=1.57(3), Dx=1.59 g cm−3, μ(MoKα)=25.22 cm−1. The isomorphous europium(III) complex was also obtained: a=14.069(9), b=15.328(9), c=12.865(9) Å, α=106.98(5), β=114.57(5), γ=90.07(6)°, U=2390(3) Å3, Dm=1.60(3), Dx=1.63 g cm−3, μ(Mo Kα)=30.04 cm−1. Two crystallographically independent neodymium(III) atoms in a unit cell have approximately the same coordination geometry: square antiprism 4O,4N-octa-coodination. A pair of the O atoms as well as N atoms are in cis positions on the top square and in trans positions on the bottom square of each polyhedron, where the lines connecting both pairs of the N atoms on both squares are approximately parallel.
The synthesis and characterization of (bipy)2U(N[t-Bu]Ar)2 (1-(bipy)2, bipy = 2,2’-bipyridyl, Ar = 3,5-C6H3Me2), (bipy)U(N[1Ad]Ar)3 (2-bipy), (bipy)2U(NC[t-Bu]Mes)3 (3-(bipy)2, Mes = 2,4,6-C6H2Me3), and IU(bipy)(NC[t-Bu]Mes)3 (3-I-bipy) are reported. X-ray crystallography studies indicate that bipy coordinates as a radical anion in 1-(bipy)2 and 2-bipy, and as a neutral ligand in 3-I-bipy. In 3-(bipy)2, one of the bipy ligands is best viewed as a radical anion, the other as a neutral ligand. The electronic structure assignments are supported by NMR spectroscopy studies of exchange experiments with 4,4’-dimethyl-2,2’-bipyridyl and also by optical spectroscopy. In all complexes, uranium was assigned a +4 formal oxidation state.
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 UO2
2+. 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.
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A new Np(IV) complex, [N(CH3)4]4[Np(NCS)8], was prepared. Its crystal structure was determined, and the absorption spectra in the IR and near IR ranges were measured. Crystal data: a = 27.280(6), b = 12.288(3), c = 13.493(3) Å, space group Pna21, Z = 4, V = 4523(2) A3, R = 0.044, wR(F
2) = 0.091. The crystal structure of the compound consists of [Np(NCS)8]4- anions and N(CH3)4+ cations. The coordination polyhedron of the Np atom is a distorted tetragonal antiprism formed by the nitrogen atoms of eight NCS- ions.
The X-ray crystal structures of [Th(NCS)4(DIPIBA)3] (1) [DIPIBA Me2CH CON(CHMe2)2] and [Th(NCS)2Cl2(DCA)3](2) [DCA MeCON(cycloC6H11)2] have been determined from three-dimensional X-ray diffraction data. The different steric constraints of the ligand are not sufficient for different coordination geometries. In 1 and 2 the coordination polyhedron about the seven-coordinate thorium atom is a pentagonal bipyramid geometry which is becoming rather common for thorium(IV) compounds. In these compounds two chlorine (2) and two thiocyanate (1) anions are in axial positions with a significant shortening of the axial Th—NCS bonds with respect to the equatorial ones, while in the equatorial plane a neutral ligand lies between two anionic ligands, the remaining coordination positions being occupied by two other neutral ligands. The average bonding distances are ThO = 2.34(1)Å, ThNax = 2.42(1) and ThNeq = 2.46(1)Åin 1, and ThO = 2.357(5), ThNeq = 2.479(6) and ThClax = 2.695(3)Åin 2.
The presence of covalency in complexes of the 4f and 5f elements has been a source of intense research and controversy. In addition to academic interest in this debate, there is an industrial motivation for better understanding of bonding in f-element complexes due to the need to separate trivalent trans-plutonium elements from trivalent lanthanide fission products in advanced nuclear fuel cycles. This review discusses the key evidence for covalency in f-element bonds derived from structural, spectroscopic and theoretical studies of some selected classes of molecules, including octahedral hexahalides, linear actinyl and organometallic sandwich complexes. This evidence is supplemented by a discussion of covalency, including the possibility of both overlap and near-degeneracy driven covalency and the need to quantify their relative contributions in actinide metal–ligand bonds.
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.
A series of ammonium and tetraalkylammonium metal isothiocyanate salts of the type Qy[M(NCS)x] (M = Cr(iii), x = 6, y = 3, Q = NH4(+), Me4N(+), Et4N(+), n-Bu4N(+); M = Mn(ii), x = 6, y = 4, Q = Me4N(+); M = Mn(ii), x = 5, y = 3, Q = Et4N(+); M = Fe(iii), x = 6, y = 3, Q = Me4N(+), Et4N(+), n-Bu4N(+); M = Co(ii), x = 4, y = 2, Q = n-Bu4N(+); M = Eu(iii), Gd(iii), Dy(iii), x = 6, y = 3, Q = Bu4N(+)) has been synthesized and structurally characterized. The octahedral Cr(iii) salts are isostructural to previously and newly reported Fe(iii) salts. The Ln(iii)-containing anions are also octahedral. For Mn(ii), although the Me4N(+) salt is octahedral, the Et4N(+) salt [Mn(NCS)5](3-) possesses a distorted trigonal bipyramidal geometry. For (Et4N)3[Fe(NCS)6] and (n-Bu4N)3[Fe(NCS)6], a solid-state size-dependent change in colour from red to green was observed and was attributed to a light scattering phenomenon in the crystalline samples.
If nuclear power becomes a sustainable source of energy, a safe, robust, and acceptable solution must be pursued for existing and projected inventories of high-activity, long-lived radioactive waste. Remarkable progress in the field of geological disposal has been made in the last two decades. Some countries have reached important milestones, and geological disposal (of spent fuel) is expected to start in 2020 in Finland and in 2022 in Sweden. In fact, the licensing of the geological repositories in both countries is now entering into its final phase. In France, disposal of intermediate-level waste (ILW) and vitrified high-level waste (HLW) is expected to start around 2025, according to the roadmap defined by an Act of Parliament in 2006. In this context, transmutation of part of the waste through use of advanced fuel cycles, probably feasible in the coming decades, can reduce the burden on the geological repository. This article presents the physical principle of transmutation and reviews several strategies of partitioning and transmutation (P&T). Many recent studies have demonstrated that the impact of P&T on geological disposal concepts is not overwhelmingly high. However, by reducing waste heat production, a more efficient utilization of repository space is likely. Moreover, even if radionuclide release from the waste to the environment and related calculated doses to the population are only partially reduced by P&T, it is important to point out that a clear reduction of the actinide inventory in the HLW definitely reduces risks arising from less probable evolutions of a repository (i.e., an increase of actinide mobility in certain geochemical situations and radiological impact by human intrusion).
The title complexes were synthesized and their crystal and molecular structures were determined by the X-ray diffraction method using their single crystals. The crystals of the former ten hexakis(isothiocyanato) complexes of lanthanum through dysprosium(III) and erbiun(III) are isomorphous with each other: monoclinic, space group P21, Z=2. The central metal atom is octa-coordinated, in a square-antiprism geometry, to six nitrogen atoms of thiocyanato (SCN) ions and two oxygen atoms of methanol and water molecules. The two oxygen atoms take the positions of the most approaching apexes of two squares. The latter three heptakis(isothiocyanato) complexes of dysprosium, erbium, and ytterbium are in another isomorphous form with each other: orthorhombic, space group P21nb, Z=4. The central metal atom is hepta-coordinated, in a deformed pentagonal-bipyramidal geometry, to seven nitrogen atoms of SCN ions.
[C 16 H 36 N] 3 [Er(NCS) 6 ] cristallise dans A1 - avec a=22,72, b=16,80, c=18,87 A, α=88,00, β=89,15, γ=92,47°, Z=4; affinement jusqu'a R=0,076. La maille asymetrique comprend un ion complexe, contenant un atome Er qui est coordine octaedriquement a six coordinats thiocyanate par les atomes N, et trois cations tetra-n-butylammonium
The hydrogen-bond acceptor ability of divalent sulfur in Y-S-Z systems, Y, Z = C, N, O or S, and the donor ability of thiol S-H have been studied using crystallographic data retrieved from the Cambridge Structural Database. Of 1811 Y-S-Z substructures that co-occur with N-H or O-H donors, only 86 (4.75%) form S⋯H-N,O bonds within S⋯H < 2.9 Å. In dialkylthioethers, the frequency of S⋯H bond formation is 6.24%, but drops below 3% when the alkyl groups are successively replaced by Csp2 centres. This parallels an increasing δ-positivity of S as calculated using ab initio methods. A similar frequency trend is observed for O⋯H-N,O bond formation by analogous oxyethers. Mean intermolecular >S⋯H distances for O-H [2.67 (3) Å] and N-H [2.75 (2) A] donors (with H positions normalized to neutron values) are ca 0.25 Å longer than in C=S⋯H-N,O systems, indicative of very weak hydrogen bonding to >S. Intramolecular >S⋯H are slightly more frequent (8.56%), with S⋯H slightly shorter than for the intermolecular case. In contrast, 26 (70.3%) out of 37 S-H donors that co-occur with suitable acceptors form X⋯H-S bonds. The C=O⋯H-S system is predominant with a mean O⋯H distance of 2.34 (4) Å, considerably longer (weaker) than in C=O⋯H-O systems.
Treatment of LnI3 (Ln = La, Ce) or [UI3(py)4] with 3 equivalents of terpy in acetonitrile gave the tris(terpy) complexes [M(terpy)3]I3. Addition of 3 equivalents of Rbtp (2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine) to MX3 (X = I or OSO2CF3) in pyridine or acetonitrile afforded the tris(Rbtp) compounds [M(Rbtp)3]X3. By comparison with terpy, the Rbtp ligand has a better affinity for the 4f and 5f ions and is more selective for U(III) than for Ce(III) or La(III). This trend has been revealed by 1H NMR competition experiments and X-ray crystallographic studies which show that in the [M(terpy)3]3+ and [M(Rbtp)3]3+ cations, the M–N(Rbtp) bond lengths are shorter than the M–N(terpy) bond lengths, and the U–N bond lengths are shorter than the
corresponding Ce–N or La–N bond distances.
The U4+ cyclooctatetraenyl complex, [(C5Me5)(C8H8)U]2(μ-C8H8), 1, reacts with two equiv of 4,4′-dimethyl-2,2′-bipyridine (Me2bipy) and 2 equiv of 2,2′-bipyridine (bipy) to form 2 equiv of (η5-C5Me5)(η8-C8H8)U(Me2bipy-κ2N,N′) and (η5-C5Me5)(η8-C8H8)U(bipy-κ2N,N′), respectively. X-ray crystallography, infrared spectroscopy, and density functional theory calculations indicate that the products are best described as U4+ complexes of bipyridyl radical anions. Hence, only one of the (C8H8)2− ligands in 1 acts as a reductant and delivers 2 electrons per equiv of 1. Since the reduction potentials of uncomplexed (C8H8)2−, Me2bipy, and bipy are −1.86, −2.15, and −2.10V vs SCE, respectively, it is likely that prior coordination of the bipyridine reagents enhances the electron transfer.
Reactions of tetra-n-butylammonium thiocyanate with the appropriate lanthanide nitrate polyhydrate yield complexes of the general type [(n-C4H9)4N]3[Ln(NCS)x(NO3)y]. [(n-C4H9)4N]3[Pr(NCS)2(NO3)4] (I), [(n-C4H9)4N]3[Nd(NCS)4(NO3)2] (II), and [(n-C4H9)4N]3[Dy(NCS)4(NO3)2] (III) were crystallized from ethanol and characterized by single-crystal X-ray crystallography, IR spectroscopy, and other physical determinations. The coordination about the central lanthanide ion varied. For complex I, the geometric polyhedron is a bicapped square antiprism (D4d), while those for complexes II and III are best described as a dodecahedral prism (D2d). Details of the synthesis and selected bond distances and angles are presented and discussed.
The structural analysis of [(n-C4H9)4N]3[Lu(NCS)6] (I) using single-crystal diffraction data and full-matrix least-squares refinement has yielded reliability factors of R=0.060 and Rw=0.071 based on 5578 observed reflections. The hexaisothiocyanate complex crystallizes in the centrosymmetric triclinic space group P1̄ with unit cell dimensions a=12.405(2), b=12.830 (2), c=22.699(2) Å, α=90.87(1), β=92.10(1), γ=96.67(1)°, V=3585.1(8) Å3 and Z=2. The observed density is 1.16(1) g cm−3 (Dcalc=1.159 g cm−3). The molecular unit consists of three separate cationic tetra-n-butylammonium groups and an independent hexakisisothiocyanatolutetium anionic group in which the six thiocyanate ligands octahedrally coordinate through the N atom to the Lu central atom. Selected bond distances and angles are presented as well as references to the synthesis of I and peripheral studies.
Two Cd(II)-tppda (tppda = N,N,N',N'-tetrakis(2-pyridyl)-2,6-pyridinediamine) coordination polymers have been prepared via variations of the templating solvent. The solvated complex [Cd(2)(tppda)(DMF)(2)(mu-SCN)(2)(SCN)(2)](n) (1) exhibits a 1-D pseudo-helical structure, whereas the unsolvated complex [Cd(2)(tppda)(mu-SCN)(4)](n) (2) features a 2-D metal-organic framework (MOF). Both are luminescent in the solid state, with emission maxima 444 and 435 nm for 1 and 2, respectively. (c) 2006 Elsevier B.V. All rights reserved.
A novel 3D coordination polymer, [Cd(diec)2(NCS)2]n (1) (diec=3,6-diimidazolyl-9-ethylcarbazole) has been prepared by reacting Cd(SCN)2 with the diec ligand. Single crystal X-ray analysis has revealed that the 3D framework of the title compound is formed by significant C–H⋯S hydrogen bonds and π–π interactions. The solid-state fluorescent determinations show that the complex exhibits a strong emission band at 403nm.
Crystal Structure of the Isothiocyanato Complex [Ph3PNH2(OEt2)][Sm(NCS)4(DME)2]Colourless single crystals of [Ph3PNH2(OEt2)][Sm(NCS)4(DME)2] (1) have been obtained besides of Ph3PS from the reaction of the homoleptic phosphorane iminato complex [Sm(NPPh3)3]2 with carbon disulfide in THF solution, followed by recrystallisation from DME/Et2O. According to the crystal structure analysis 1 consists of [Ph3PNH2]+ cations with the diethylether molecule forming a N–H…O hydrogen bridge, and anions [Sm(NCS)4(DME)2]–. Sm3+ realizes coordination number eight by four nitrogen atoms of the isothiocyanato ions and by four oxygen atoms of the DME chelates.1: Space group P 1, Z = 4, lattice dimensions at 193 K: a = 919.0(1), b = 1965.2(2), c = 2401.3(2) pm, α = 96.748(11)°, β = 94.827(10)°, γ = 91.720(11)°, R = 0.029.
The liquid-liquid biphasic chemistry of the system quaternary alkylammonium salt-o-xylene-water-trivalent metal cation (actinide or lanthanide)-thiocyanate-nitrate anions has been studied with the purpose to obtain both information on the aqueous complex formation constants of these elements and parametric equations useful to process chemistry design.The aqueous equilibrium constants between Eu+3, Ce+3, Am+3, Cm+3, Cf+3 and SCN− and NO3− have been obtained at the constant ionic strength of 2·01 M and 25°C, by using a liquid anion exchanger (tricaprylmethylammonium thiocyanate in o-xylene). In the case of the SCN− anion the actinide series shows a tendency to form complexes greater than the lanthanides, while this is approximately the same with the NO3− anion. For the actinide elements the tendency to form complexes is greater for SCN− than for NO3−.As far as the technological applications are concerned the possibility of performing actinides-lanthanides separation appears promising.
A single crystal X-ray structure determination shows that in [(C2H5)4N]4[U(NCS)8] the eight N-bonded ligands are disposed at the vertices of a cube. Within experimental accuracy no deviations from regular cubical 8-coordination are found.
Stability constants of Am(II) and Eu(III) complexes with chloride, nitrate and thiocyanate ions have been determined in lithium, hydrogen, sodium and ammonium ion media at 30±0·1°C and unit ionic strength, by a solvent extraction method. For chloride complexes, the first stability constant K1 has been found to increase regularly with decreasing hydration of the medium cations. The effect of medium cations on the stabilities of nitrate and thiocyanate complexes has been found to be much smaller.