Abstract Luminescence spectroscopy is a powerful method for probing the chemical and physical properties of metal ions, as perturbation of electronic states due to ligand–field effects yields changes in emissive behavior. This sensitivity is particularly valuable for the trivalent actinides, whose chemical bonding is still not well understood. Here, we report the synthesis and characterization of an americium β-diketonate, [Am(acac)2(μ-acac)(H2O)]2 (Am2); acac = acetylacetonate. Am2 was prepared via slow evaporation of a methanolic americium–acac solution and characterized via single-crystal X-ray diffraction, Raman, UV–vis–NIR absorbance, and luminescence spectroscopies. The structure consists of acac-bridged Am dimers with both Am(III) metal centers adopting a distorted square antiprismatic coordination geometry. Am2 represents a rare example of ligand-sensitized Am(III) emission in the solid state, with ligand excitation at 320 nm resulting in characteristic 5f–5f emission bands at ca. 699, 850, and 1060 nm that correspond to the 5D1′ → 7F1–3′ transitions. The absence of the 5D1′ → 7F0′ transition is consistent with the C1 symmetry about the metal center established from structural studies. The observed peak splitting and bathochromic shifting can be attributed to crystal-field effects and a limited degree of covalency in Am–acac bonding.

Metal ion hydrolysis and condensation reactions are critical to describing the chemical behavior of the tetravalent actinides (An) due to their high charge density. This recognition has fueled synthetic efforts targeting polynuclear actinide-oxo clusters. Oligomers ranging from trimers to octatriacontamers have been reported, with the hexameric unit, which typically exhibits a [An6(OH)4O4]12+ core, representing the most pervasive cluster. Hexamers decorated by a range of ligands, including carboxylates, sulfates, and chlorides, have been described. Previous reports have demonstrated the formation of hexamers for Th, U, Np, and Pu both in solution and the solid state, yet little work has focused on the synthesis and properties of structurally analogous clusters across the early An series using the same complexing ligand. Here, a series of benzoate (Bz) decorated actinide-oxo/hydroxo hexamers of the same general formula [An6O4(OH)4(Bz)12(H2O)n], where An = Th, U, Np, Pu and n = 6 for Th and 4 for U-Pu is reported. The title compounds were characterized by X-ray diffraction, UV-vis-NIR absorbance, Raman, and infrared spectroscopy. Notably isolation of these phases and elucidation of the parameters that underpin their formation provides insight into the ways differences in metal ion charge density manifest across the early tetravalent actinides, both in their synthetic and structural chemistry.

The synthesis, structural characterization, and spectroscopic properties of five tetravalent uranium (U) phases including Li6[U4(μ3-O)2Cl18(H2O)2]·10H2O (1), [U(H2O)4Cl4] (2), [U(H2O)4Cl4]·KCl (3), Rb2UCl6 (4), and Cs2UCl6 (5) are reported. Notably, a change in the U4+ solid-state structural unit was observed based on the identity of the alkali counterion used in the synthesis. Li1+ yielded a tetranuclear oxo-bridged cluster, [U4(μ3-O)2Cl18(H2O)2]6-, Na1+ and K1+ yielded two structurally distinct [U(H2O)4Cl4] complexes, and Rb1+ and Cs1+ resulted in [UCl6]2- as the dominant phases. The spectroscopic properties of the compounds were analyzed using Raman and UV-vis-NIR absorption spectroscopy. The UV-vis-NIR spectra of compounds 1-5 exhibited transitions consistent with uranium in the +4 oxidation state. Clear differences in the absorption band splitting were observed and are likely attributed to differences in metal ion coordination, crystal field effects, and outer sphere interactions Overall, this work demonstrates the utility of noncovalent interactions in tuning the crystallization of various metal complexes from otherwise identical reaction solutions and provides further evidence that counterions impact the composition and structure of actinide complexes isolated in the solid state. In this way, this work affords important insight into directing and controlling the structure of actinide complexes and clusters.

The lanthanides form a chemically cohesive series, and identifying differences that can be leveraged to separate near neighbors remains an ongoing need. Here, the synthesis, structural chemistry, and spectroscopic properties of a neodymium-acetylacetonate monomer, [Nd(acac)3(H2O)2] (Nd-1), and an acac-bridged Nd-dimer, [Nd2(acac)6(H2O)2] (Nd-2), are described. Interestingly, the Nd-dimer readily precipitates from solution; however, efforts to crystallize the Dy analog using the same synthetic approach are unsuccessful. Yet heterometal dimers, [Nd2-xDyx(acac)6(H2O)2] (NdDy-2), can be prepared from mixed-metal solutions. To understand these differences in crystallization behavior of Nd- and Dy-acac complexes, solution speciation of Ln-acac systems is examined using nano-electrospray ionization-mass spectrometry (nESI-MS). Both homometal and heterometal solutions are prepared (Ln = Nd, Dy, Nd/Dy). nESI-MS of the solutions showed presence of homometallic dimeric species for Nd and Dy and peaks consistent with both homometallic and heterometallic complexes in mixed-metal solutions. Density functional theory is used to further understand the intrinsic differences in nucleation energetics in dimeric homo- and heterometallic species. Overall, this work provides fundamental insight into the correlation between solution- and solid-state structural units and differences in the crystallization behavior of Ln complexes; the latter is particularly relevant to separating metal ions with otherwise very similar chemical behavior.

Nitrate-decorated hexamers with a [Ln6(μ6-O)(μ3-OH)8]8+ core have been reported for nearly every lanthanide ion and are used as precursors for the assembly of functional metal-organic frameworks. Yet, few studies have examined the correlation between the solution and solid-state species, and the formation of mixed-metal clusters. Toward this end, a series of homo- and heterometal lanthanide nitrate hexamers was prepared via pH adjustment of aqueous lanthanide nitrate solutions. Examination of the homometallic europium solutions using Small Angle X-ray Scattering and nESI-MS showed that lower order complexes dominate lanthanide speciation in nitrate media. Yet, powder X-ray diffraction data of the precipitated phase confirmed the formation of [Ln6(μ6-O)(μ3-OH)8(NO3)6(H2O)12]·2(NO3)·n(H2O), Ln6, for Ln = Eu and Tb. For heterometal systems, analysis of the solid-state product by ICP-MS showed the selective incorporation of the heavier rare earths into Ln6. Selectivity was quantified by calculating an average separation factor, which is defined as the ratio of recovery factors of both metals. Further examination of the luminescence behavior of mixed metal [Tb6-xEux(μ6-O)(μ3-OH)8(NO3)6(H2O)12]·2(NO3)·n(H2O), with x = 1.1-3.6, showed that the relative intensities of the peaks at 489 nm (terbium, 5D4 → 7F6) and 690 nm (europium, 5D0 → 7F4) trend with the percent incorporation of europium and terbium into the cluster.

The solvothermal synthesis of a series of Nd dimers decorated with various chalcogenophene carboxylates and 2,2 ':6 ',2 ''-terpyridine of the general formula, [Nd2(mu-XC5H3O2)2(XC5H3O2)4(N3C15H11)2(H2O)2] where X = O, S, Se, and Te, is reported. The solid-state structures were characterized using single-crystal X-ray diffraction (scXRD) and all the complexes are isomorphous, despite substitution of the heterocyclic chalcogen; phase purity was confirmed via powder X-ray diffraction (pXRD). Vibrational spectroscopy was collected and correlations between chalcogen identity and the binding strength of the carboxylate groups of the chalcogenophene ligands with each metal center were shown to be independent of chalcogen identity. All four complexes displayed Nd(III)-based near-IR luminescence and exhibited ligand-sensitized emission. Varying the chalcogenophene chromophore enabled tuning of the sensitizing triplet state energy level, as evidenced by an 8-fold increase in the sensitization of the TeCA-decorated dimer relative to the other chalcogenophene congeners. This behavior was rationalized by comparing the Nd(III) acceptor and ligand donor states across the series. The donor triplet state of each ligand was estimated via low-temperature (77 K) phosphorescence measurements from 1:1 mixtures with Gd(III); these were found to be 24,631 cm-1 for furan-2-carboxylic acid (FCA), 23,764 cm-1 for thiophene-2-carboxylic acid (TCA), 22,548 cm-1 for selenophene-2-carboxylic acid (SeCA), and 21,186 cm-1 for tellurophene-2-carboxylic acid (TeCA). The greater sensitization efficiency of TeCA is the result of well-matched ligand donor and metal acceptor levels and thus suppression of nonradiative back-energy transfer. More broadly, triplet energy level information for these ligands serves as a guide for future application to other target metals based on the electronic properties necessary to effect efficient sensitization.

Three novel bismuth-organic compounds, with the general formula [Bi-2(HPDC)(2)(PDC)(2)](arene)2H(2)O (H2PDC = 2,6-pyridinedicarboxylic acid; arene = pyrene, naphthalene, and azulene), that consist of neutral dinuclear Bi-pyridinedicarboxylate complexes and outer coordination sphere arene molecules were synthesized and structurally characterized. The structures of all three phases exhibit strong pi-pi stacking interactions between the Bi-bound PDC/HPDC and outer sphere organic molecules; these interactions effectively sandwich the arene molecules between bismuth complexes and thereby prevent molecular vibrations. Upon UV irradiation, the compounds containing pyrene and naphthalene displayed red and green emission, respectively, with quantum yields of 1.3(2) and 30.8(4)%. The emission was found to originate from the T-1 -> S-0 transition of the corresponding arene and result in phosphorescence characteristic of the arene employed. By comparison, the azulene-containing compound displayed very weak blue-purple phosphorescence of unknown origin and is a rare example of T-2 -> S-0 emission from azulene. The pyrene- and naphthalene-containing compounds both display radioluminescence, with intensities of 11 and 38% relative to bismuth germanate, respectively. Collectively, these results provide further insights into the structure-property relationships that underpin luminescence from Bi-based materials and highlight the utility of Bi-organic molecules in the realization of organic emission.

Recent advances enabled the discovery of heterometallic molecules for many metals: main group, d-block, lanthanides, and some actinides (U, Th). These complexes have at least two different metals joined by bridging ligands or by direct metal-metal bonding interactions. They are attractive because they can enable chemical cooperativity between metals from different parts of the periodic table. Some heterometallics provide access to unique reactivity and others exhibit physical properties that cannot be accessed by homometallic species. We envisioned that transuranic heterometallics might similarly enable new transuranic chemistry, though synthetic routes to such compounds have yet to be developed. Reported here is the first synthesis of a molecular transuranic complex that contains plutonium (Pu) and cobalt (Co). Our analyses of PuCl3{CoCp[OP(OEt)(2)](3)} showed Pu(IV) and Co(III) were present and suggested that the Pu(iv) oxidation state was stabilized by the electron donating phosphite ligands. This synthetic method - and the demonstration that Pu(iv) can be stabilized in a heterobimetallic molecular setting - provides a foundation for further exploration of transuranic multimetallic chemistry.

A series of fifteen tetravalent thorium phases were prepared. The compounds were isolated from acidic aqueous nitrate solutions using protonated nitrogen heterocycles of varying hydrogen-bond donation strength. Structural analysis via single crystal X-ray diffraction showed that the structures are built from pentanitrato, [Th(NO3)(5)(H2O)(2)](1-), and hexanitrato, [Th(NO3)(6)](2-), molecular units, with the latter being far more prevalent in the solid state. The vibrational properties of the compounds were examined using Raman and IR spectroscopy; the spectra are dominated by stretches characteristic of nitrate and the organic ions. The relative energetics of nitrate complexation was examined using electronic structure theory. These results confirmed that there are clear thermodynamic sinks for the penta- and hexanitrato structural units that were observed experimentally. Additionally, electrostatic surface potentials (ESPs) were calculated in an effort to better understand the counterion stabilization of the complexes. The ESP surfaces showed that the position of the water and nitrate molecules and the coordination geometry of the metal complex had a clear effect on the polarizability of the two structural motifs. Despite limited speciation of the Th-nitrate structural units, the compounds exhibit rich supramolecular chemistry resulting from hydrogen bonding of the Th complexes with the organic N-H donors and pi-pi stacking interactions from the protonated N-heterocycles.

Four cerium compounds - (HPy)2[CeCl6]2(HPyCl) (Ce1-1), (HPy)2[CeCl6] (Ce1-2), (HPy)m[Ce38O56-x(OH)xCl50(H2O)12]nH2O (Ce38), and (HPy)m[Ce52O80-x(OH)xCl59(H2O)17]nH2O (Ce52) - were crystallized from acidic aqueous solutions using pyridinium (HPy) counterions. The latter consists of two unique cerium oxide nanoclusters that are built from 52 metal ions and represents the largest chloride capped {CeIII/IVO} and/or {MIVO} (M = Ce, Th, U, Np, Pu) nanocluster that adopts the fluorite-type structure of MO2 that has been reported. A chloride/water terminated Ce-oxo cluster built from 52 metal ions, with the longest Ce-Ce distance measuring roughly 1.6 nm, was isolated from acidic aqueous solution using pyridinium counterions.

Five bismuth-organic materials that exhibit fluorescence or phosphorescence and radioluminescence are reported, with the photophysical behavior dependent on both the identity of the outer sphere fluorophore and noncovalent interactions.

A new bismuth-organic compound containing 1,10phenanthroline (phen) and 2,5-pyridinedicarboxylic acid (PDC) was synthesized and structurally characterized by single-crystal Xray diffraction. The structure consists of 2-D {Bi(phen)(HPDC)(PDC)}n sheets wherein the PDC ligands bridge metal centers via three unique bonding modes. The 2-D sheets are further connected through strong hydrogen-bonding interactions to form a 3-D supramolecular network. The parent compound displayed yellow photoluminescence in the solid state at room temperature. Doping studies were undertaken to incorporate Eu3+ into the structure, statistically replacing Bi3+ in small quantities (1, 5, and 10 mol % Eu3+ relative to Bi3+). All three compounds displayed characteristic Eu3+ emission, with total quantum yields as high as 16.0% and sensitization efficiencies between 0.21 and 0.37 depending on the Eu3+ doping percentage.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1–) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1–-decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1–, OH1–, MeO1–, and O2– ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.

Modern molten salt reactor design and the techniquesof electrorefiningspent nuclear fuels require a better understanding of the chemicaland physical behavior of lanthanide/actinide ions with different oxidationstates dissolved in various solvent salts. The molecular structuresand dynamics that are driven by the short-range interactions betweensolute cations and anions and long-range solute and solvent cationsare still unclear. In order to study the structural change of solutecations caused by different solvent salts, we performed first-principlesmolecular dynamics simulations in molten salts and extended X-rayabsorption fine structure (EXAFS) measurements for the cooled moltensalt samples to identify the local coordination environment of Eu2+ and Eu3+ ions in CaCl2, NaCl, andKCl. The simulations reveal that with the increasing polarizing theouter sphere cations from K+ to Na+ to Ca2+, the coordination number (CN) of Cl- inthe first solvation shell increases from 5.6 (Eu2+) and5.9 (Eu3+) in KCl to 6.9 (Eu2+) and 7.0 (Eu3+) in CaCl2. This coordination change is validatedby the EXAFS measurements, in which the CN of Cl- around Eu increases from 5 in KCl to 7 in CaCl2. Oursimulation shows that the fewer Cl- ions coordinatedto Eu leads to a more rigid first coordination shell with longer lifetime.Furthermore, the diffusivities of Eu2+/Eu3+ arerelated to the rigidity of their first coordination shell of Cl-: the more rigid the first coordination shell is, theslower the solute cations diffuse. First-principlemolecular dynamics simulations for moltensalts and EXAFS measurements for the cooled molten salt samples toidentify the local coordination environment of Eu2+ andEu(3+) ions in molten salts of CaCl2, NaCl, andKCl, which help us to understand the changes of coordination environmentof Eu, the rigidity of the first coordination shell, and their impacton the diffusivity in the melts.

Molten salts have found use as solvents in numerous applications including nuclear reactors, batteries, and the extraction and purification of various metals. Unfortunately, understanding of the chemistry of molten salt solutions is limited. In this presentation we explore the use of molten salts as a testbed for understanding both outer and inner coordination sphere effects on dissolved metal ions. The electron transfer reactions available to lanthanides (Eu3+, Sm3+, and Yb3+) and actinides (U3+, U4+, and Th4+) were explored in a series of alkali and alkaline earth halide salts. We present electrochemical data that demonstrate significant shifts in the reduction potentials of these metal ions as a function of the anion and cation identities of the molten salt solvent. We hypothesize that effects on the reduction potential of these metals come from two sources: (1) the primary coordination sphere and (2) the secondary coordination sphere. The influence from the primary coordination sphere is dominated by the degree of covalency in the coordination bonds between the Lnn+ and Ann+ cations and the molten salt anions. The influence of the secondary coordination sphere is dominated by the electron-withdrawing character of the salt cations. EXAFS data and computational results that support these hypotheses are presented. Further, we provide insight into electrodeposition of the An0 metals under these conditions and highlight temperature and molten salt effects that influence these electrodepositions. Specifically, we propose that increased mobility of solid-state atoms at high temperature (> 800°C) influence the properties of electrodeposited metals.

A cerium-oxo nanocluster capped by chloride ligands, [CeIV38-nCeIIInO56-(n+1)(OH)n+1Cl51(H2O)11]10- (n = 1-24), has been isolated from acidic chloride solutions by using potassium counterions. The crystal structure was elucidated using single crystal X-ray diffraction. At the center of the cluster is a {Ce14} core that exhibits the same fluorite-type structure as bulk CeO2, with eight-coordinate Ce sites bridged by tetrahedral oxo anions. The {Ce14} is further surrounded by a peripheral shell of six tetranuclear {Ce4} subunits that are located on each of the faces of the core to yield the {Ce38} cluster. The surface of the cluster is capped by 51 bridging/terminal chloride ligands and 11 water molecules; the anionic cluster is charge balanced by potassium counterions that exist in the outer coordination sphere. While assignment of the Ce oxidation state by bond valence summation was ambiguous, Ce L3-edge X-ray absorption, X-ray photoelectron, and UV-vis-NIR absorption results were consistent with a CeIII/CeIV cluster. Systematic changes in the XANES and UV-vis-NIR absorption spectra over time pointed to reactivity of the cluster upon exposure to air. These changes were examined using single crystal X-ray diffraction, and a clear single-crystal-to-single-crystal transformation was captured; an overall loss of surface-bound chlorides and water molecules as well as new μ2-OH sites was observed on the cluster surface. This work provides a rare snapshot of metal oxide cluster reactivity. The results may hold implications for understanding the physical and chemical properties of ceria nanoparticles and provide insight into the behavior of other metal-oxo clusters of significant technological and environmental interest.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The synthesis and structural chemistry of two tetravalent thorium compounds precipitated from acidic bromide solutions are described. One of the phases, [Th(H2O)(10])Br-4 (1), has been previously reported. The other phase, [Th(H2O)(4)Br-4](HPy.Br)(2) (2), is a novel compound and exhibits Th-Br coordination. While complexes with such Th-Br bonding have been observed in the solid state, most have been isolated from nonaqueous solutions. Notably, the two compounds crystallize from HBr(aq) solutions containing pyridinium (HPy+) counterions; in the absence of HPy+, only 1 is observed. The reaction energies of stepwise bromide addition were predicted using electronic structure theory. These calculations highlighted an overall thermodynamic driving force for Th-Br complexation up to the addition of four bromide ions. Electrostatic potential (ESP) surfaces were calculated to better understand the role of HPy+ in the isolation of 2. Consistent with our previous work, the ESP surfaces point to the importance of the pK(a) of the organic cation in determining the distribution of the Br- ligands about the Th metal center.

The luminescence properties of two divalent europium complexes of the type Eu[N(SPPh2)2]2(THF)2 (1) and Eu[N(SePPh2)2]2(THF)2 (2) were investigated. The first complex, Eu[N(SPPh2)2]2(THF)2 (1), was found to be isomorphous with the reported structure of complex 2 and exhibited room temperature luminescence with thermochromic emission upon cooling. We found the complex Eu[N(SePPh2)2]2(THF)2 (2) was also thermochromic but the emission intensity was sensitive to temperature. Both room temperature and low temperature (100 K) single crystal X-ray structural investigation of 1 and 2 indicate geometric distortions of the metal coordination, which may be important for understanding the thermochromic behavior of these complexes. The trivalent europium complex Eu[N(SPPh2)2]3 (3) with the same ligand as 1 was also structurally characterized as a function of temperature and exhibited temperature-dependent luminescence intensity, with no observable emission at room temperature but intense luminescence at 77 K. Variable temperature Raman spectroscopy was used to determine the onset temperature of luminescence of Eu[N(SPPh2)2]3 (3), where the 615 nm (5D0 → 7F2 transition) peak was quenched above 130 K. The UV-visible diffuse reflectance of 3 provides evidence of an LMCT band, supporting a mechanism of thermally activated LMCT quenching of Eu(III) emitting states. A series of ten isomorphous, trivalent lanthanide complexes of type Ln[N(SPPh2)2]3 (Ln = Eu (3) Pr (4), Nd (5), Sm (6), Gd (7), Tb (8)) and Ln[N(SePPh2)2]3 (Ln = Pr (9), Nd (10, structure was previously reported), Sm (11), and Gd (12) for Q = Se) were also synthesized and structurally characterized. These complexes for Ln = Pr, Nd, Sm, and Tb exhibited room temperature luminescence. This study provides examples of temperature-dependent luminescence of both Eu2+ and Eu3+, and the use of soft-atom donor ligands to sensitize lanthanide luminescence in a range of trivalent lanthanides, spanning near IR and visible emitters.