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Coordination Chemistry in the Structural and Functional Exploration of Actinide-Based Metal-Organic Frameworks
Abstract
The coordination chemistry between inorganic and organic species can be optimally exemplified by metal–organic frameworks (MOFs), whose structures and functionalities can be rationally designed from these highly tunable building blocks. The high porosity, stability, and versatile functionalities of MOFs have attracted wide-spread attention from energy-related research and pollution remediation to biomedical applications. A unique and underexplored subset of these materials are MOFs based on actinide nodes; these MOFs have distinguished themselves as a unique platform for investigating the versatile oxidation states, reactivity, and coordination chemistry of actinides. Herein, we will focus on the rational design and synthesis of actinide-based MOFs under the general guidelines of coordination chemistry for their structural and functional explorations. The dimensionality, topology, and structures of actinide-based MOFs can be controlled by selecting pre-designed building blocks of actinide-based nodes and organic linkers with certain desired coordination geometries and functionalities. These unique actinide-based MOFs have shown promise for applications in nuclear waste mitigation, pollution control, and catalysis.
... Meanwhile, Shustova presented a seminal overview of the structural motifs of Th-, U-MOFs, and MOFs for radionuclide immobilization [26]. Recently, Zhang recorded the structure and functionality of An-MOFs prepared by the Farha group [27] meanwhile the Zhou group summarized MOFs based on group 3 and 4 metals, including several Th-, Np-, and Pu-MOFs [28]. The Park group discussed the coordination environments and chemical behaviors of a portion of An-MOFs, focusing their implications for nuclear industry [29]. ...
Actinide metal-organic frameworks (An-MOFs) consisting of actinide nodes and organic linkers represent an underexplored category of coordination polymers due to challenges in their synthesis and characterization. The unparalleled coordination chemistry of actinide elements confers a huge opportunity to explore the rational design, chemical reactivity, and versatile properties of An-MOFs as one of the most intriguing class of metal-organic frameworks (MOFs). Significant advances in this “juvenile” MOF research field have been witnessed in recent years and progress in the An-MOFs area since 2003 has been reviewed from the aspects of the synthesis, structure, and applications. The preparative handling and synthetic strategies implemented in constructing An-MOFs are illustrated. Their structure motifs are then classified and expounded by actinide building blocks and organic linkers. The modularity, topology, and porosity of An-MOFs are specified to highlight a great potential to tune their electronic structures and ensuing properties. Ultimately, applications of An-MOFs as selective adsorbents, heterogenous catalysts, luminescent sensors, conducting, and semiconducting materials, and nuclear targets are underlined. This updated review is envisaged to guide in-depth investigation of largely elusive transuranium MOFs and the development of thorium or uranium-based MOFs towards practical applications.
Protection of enzymes with synthetic materials is a viable strategy to stabilize, and hence to retain, the reactivity of these highly active biomolecules in non-native environments. Active synthetic supports, coupled to encapsulated enzymes, can enable efficient cascade reactions which are necessary for processes like light-driven CO2 reduction, providing a promising pathway for alternative energy generation. Herein, a semi- artificial system - containing an immobilized enzyme, formate dehydrogenase, in a light harvesting scaffold - is reported for the conversion of CO2 to formic acid using white light. The electron-mediator Cp*Rh(2,2’-bipyridyl-5,5’-dicarboxylic acid)Cl2 was anchored to the nodes of the metal-organic framework NU- 1006 to facilitate ultrafast photo-induced electron transfer when irradiated, leading to the reduction of the coenzyme nicotinamide adenine dinucleotide at a rate of about 28 mM h-1. Most importantly of all, the immobilized enzyme utilizes the reduced coenzyme to generate formic acid selectively from CO2 at a high turnover frequency of about 865 h-1 in 24 hours. The outcome of this research is the demonstration of a feasible pathway for solar-driven carbon fixation.
Previous work has shown that introduction of hexafluoroacetylacetonate (Facac) units as nonstructural ligands for the zirconia-like nodes of the eight-connected metal−organic framework (MOF), NU-1000, greatly alters the selectivity of node-supported oxy-nickel clusters for ethylene dimerization vs. oligomerization. Here we explore a related concept: tuning of support/catalyst interactions, and therefore, catalyst activity, via parallel installation of organic modifiers on the support itself. As modifiers we focused on para-substituted benzoates (R-BA⁻; R = –NH2, –OCH3, –CH3, –H, –F and –NO2) where the substituents were chosen to present similar steric demand, but varying electron-donating or electron-withdrawing properties. R-Benzoate-engendered shifts in the node-based aqua O-H stretching frequency for NU-1000, as measured by DRIFTS (diffuse-reflectance infrared Fourier-transform spectroscopy), together with systematic shifts in Ni2p peak energies, as measured by X-ray photo-electron spectroscopy, show that the electronic properties of the support can be modulated. The vibrational and electronic peak shifts correlate with the putative electron-withdrawing vs. electron-donating strength of the para-substituted benzoate modifiers. Subsequent installation of node-supported, oxy-Ni(II) clusters for ethylene hydrogenation yield a compelling correlation between log (catalyst turnover frequency) and the electron donating or withdrawing character of the substituent of the benzoate units. Single crystal X-ray diffraction measurements reveal that each organic modifier makes use of only one of two available carboxylate oxygens to accomplish grafting. The remaining oxygen atom is, in principle, well positioned to coordinate directly to an installed Ni(II) ion. We believe that the unanticipated direct coordination of the catalyst by the node-modifier (rather than indirect modifier-based tuning of support(node)/catalyst electronic interactions) is the primary source of the observed systematic tuning of hydrogenation activity. We suggest, however, that regardless of mechanism for communication with active-sites of MOF-supported catalysts, intentional elaboration of nodes via grafted, nonstructural organic species could prove to be a valuable general strategy for fine-tuning supported-catalyst activity and/or selectivity.
Reticular chemistry, the linking of molecular building units by strong bonds to make crystalline, extended structures such as metal-organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), and covalent organic frameworks (COFs), is currently one of the most rapidly expanding fields of science. In this contribution, we outline the origins of the field; the key intellectual and practical contributions, which have led to this expansion; and the new directions reticular chemistry is taking that are changing the way we think about making new materials and the manner with which we incorporate chemical information within structures to reach additional levels of functionality. This progress is described in the larger context of chemistry and unexplored, yet important, aspects of this field are presented.
Metal-organic frameworks--a class of porous hybrid materials built from metal ions and organic bridges--have recently shown great promise for a wide variety of applications. The large choice of building blocks means that the structures and pore characteristics of the metal-organic frameworks can be tuned relatively easily. However, despite much research, it remains challenging to prepare frameworks specifically tailored for particular applications. Here, we have used computational modelling to design and predictively characterize a metal-organic framework (NU-100) with a particularly high surface area. Subsequent experimental synthesis yielded a material, matching the calculated structure, with a high BET surface area (6,143 m(2) g(-1)). Furthermore, sorption measurements revealed that the material had high storage capacities for hydrogen (164 mg g(-1)) and carbon dioxide (2,315 mg g(-1))--gases of high importance in the contexts of clean energy and climate alteration, respectively--in excellent agreement with predictions from modelling.
Accurately characterizing actinide oxides bound to metal–organic frameworks (MOFs) is important for designing MOFs as radioactive waste sorbents and catalytic supports. In this work, the zirconium MOF NU-1000 was post-synthetically modified through solvothermal deposition to include the uranyl (UO2²⁺) ion and characterized via single-crystal X-ray diffraction. Bond lengths derived from the diffraction pattern and Raman spectroscopy indicate that UO2²⁺ maintains its chemical properties upon deposition, while alcohol oxidation photocatalysis reveals photo-interactions between the pyrene linkers and the UO2²⁺ ion.
An isostructural series of Np(V) MOFs with shp-topology were synthesized and characterized. X-ray diffraction data revealed an unusual wheel-shaped node of eighteen neptunyl polyhedra stabilized in the framework. Strong distortion in local coordination of the neptunium atoms is evidenced by Np-Oyl bond lengths that lie outside the typical range for Np(V). The structure was further interrogated by Raman spec-troscopy and density functional theory calculations to assign the vibrational modes.
The understanding of the catalyst-support interactions has been an important challenge in heterogeneous catalysis since the supports can play a vital role in controlling the properties of the active species and hence their catalytic performance. Herein, a series of isostructural mesoporous metal-organic frameworks (MOFs) based on transition metals, lanthanides, and actinides (Zr, Hf, Ce, Th) were investigated as supports for a vanadium catalyst. The vanadium species was coordinated to the oxo groups of the MOF node in a single-ion fashion, as determined by single-crystal X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, and diffuse reflectance UV-vis spectroscopy. The support effects of these isostructural MOFs were then probed using the aerobic oxidation of 4-methoxybenzyl alcohol as a model reaction. The turnover frequency was found to be correlated with the electronegativity and oxidation state of the metal cations on the supporting MOF nodes, highlighting an important consideration when designing catalyst supports.
Control over the assembly of thorium-based polyoxo-clusters remains a great challenge because of their quick olation and oxolation reactions. Here, we report the synthesis and isolation of two pure phase thorium polyoxo-cluster-based metal–organic frameworks (Th-MOFs), Th-NU-1008 and Th-NU-1011. Crystal structures of these Th-MOFs reveal two distinct 8-connected polyoxo-clusters: a hexanuclear cluster [Th6(μ3-O)4(OH)4(H2O)6(HCOO)4] was observed in Th-NU-1008 and a tetranuclear cluster [Th4(μ3-O)2(OH)4(DMF)2(H2O)4] in Th-NU-1011 which has never been reported before among thorium species. In both Th-NU-1008 and Th-NU-1011, the tetratopic linker, 1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl) benzene, joins the clusters together, forming two three-periodic networks, each with csq topology. The Brunauer–Emmett–Teller (BET) surface areas calculated from the Ar isotherms are approximately 800 and 700 m2 g–1 for Th-NU-1008 and Th-NU-1011, respectively. Because the linkers are oriented orthogonal to each other in the two respective MOFs, a 32 Å hexagonal channel is observed in Th-NU-1008 while a 15 Å channel is observed in Th-NU-1011. This work opens a new avenue to construct unexplored Th-MOF materials with targeted polyoxo-cluster structures
The crystal structures of thorium clusters are important for understanding the formation and transformation mechanisms of actinide species in solution, which can in turns benefit nuclear waste processing and management. However, stabilizing thorium clusters in aqueous solution is quite challenging because of their fast olation and oxolation reactions. Here, we report a thorium-based metal–organic framework, NU-905, with the formula [Th6(μ3-O)2(HCOO)4(H2O)6(TCPP)4] [TCPP = tetrakis(4-carboxyphenyl)porphyrin], synthesized by a solvothermal reaction in N,N-dimethylformamide and water at 120 °C. NU-905 contains a hexanuclear secondary building unit (SBU), [Th6(μ3-O)2(HCOO)4(H2O)6], which has never been reported previously. The SBUs are capped and bridged by the tetratopic linker TCPP to form a three-dimensional network with scu topology. The activated NU-905 exhibits permanent porosity and shows high catalytic activity for the selective photooxidation of a mustard gas simulant.
Perovskite ceramics have been extensively studied as host matrixes for radionuclide entrapment for nuclear waste disposal. As an expansion of these investigations, cerium, neodymium, and plutonium were incorporated into a perovskite phase, ACu3FeTi3O12 (A = Nd, Ce, Pu), using sol-gel methods under oxidizing and reducing atmospheres. The targeted materials contained varying levels of Ce3+ and Nd3+ on the A site, yielding potential compositions of Nd1- xCe xCu3FeTi3O12 ( x = 0, 0.1, 0.2, 0.3, 0.4, 0.8). However, interrogation of these materials shows that the maximum Ce3+ loading is achieved near x ≈ 0.2. A single composition with plutonium was targeted, Nd0.9Pu0.1Cu3FeTi3O12, in order to properly model more realistic loading levels for a repository-destined material. These compounds were characterized using powder X-ray diffraction with Rietveld refinements of the structures and by a variety of spectroscopic techniques. The data suggest that, in order to achieve Pu3+ substitution onto the A sites in the Nd0.9Pu0.1Cu3FeTi3O12, a reducing atmosphere must be employed. Otherwise, the redox activity of plutonium results in substitution onto multiple sites in the material as well as the formation of secondary phases such as TiO2.
Two-dimensional metal-organic frameworks (2D-MOFs) have shown promise in gas storage and separation applications due to their structural isomerism in response to external stimuli such as temperature, mechanical pressure, and/or guest molecules. Here, we report the guest-dependent phase transitions of a uranyl-based 2D-MOF, NU-1302, observed as single-crystal-to-single-crystal transformations. Different stacking configurations of the same structure were observed in DMF and ethanol, and after supercritical CO2 activation. The structural isomerism upon exposure to different solvents and when solvent-free demonstrated the ability of this system to respond to guests by shifting neighboring 2D sheets, resulting in the expansion or contraction of one-dimensional channels.
Isostructural metal-organic frameworks (MOFs) have been prepared from a variety of metal-oxide clusters, including transition metals, lanthanides, and actinides. Experimental and calculated shifts in O-H stretching frequencies for hydroxyl groups associated with the metal-oxide nodes reveal varying electronic properties for these units, thereby offering opportunities to tune support effects for other materials deposited onto these nodes.
Transition metal ions as a template method has been widely used in the field of supramolecular chemistry. The metal complexation is advantageous in making complex supramocluar architectures because it pre-organizes the ligands into a desirable orientation which faciliatates the following ring-closing reaction, with shorter synthetic steps and generally higher yield. In a similar fashion, this synthetic strategy has recently been adopted to make extended materials by linking the metal-coordinated building blocks with design principles of reticular synthesis. Individual building units are stitched together through strong covalent bond formation to yield long covalent molecular threads that are woven two- or three-dimensionally (2D or 3D), at regular intervals templated by the metal ions. For example, by linking functionalized tetrahedrally-shaped metal complexes with linear links through reversible imine bond formation, crystalline 3D covalent organic frameworks with diamond topology, COF-505 and COF-112, have been constructed by design. In particular, the metal templates can be post-synthetically removed so that the threads have high degrees of freedom to move in respect to each other, which leads to unusal mechanical properties of the woven materials.
Metal-organic frameworks (MOFs) are increasingly being used for the sorption of dye molecules. Researchers have demonstrated the high sorption performance of several MOFs; however, the structure-property relationships have yet to be fully elucidated. Furthermore, the lack of a generalized deposition method for the fabrication of MOF membranes limits the industrial-scale application of MOF-based sorbents. In this work, we used a polymorphic system with three MOFs, ZIF-8, ZIF-L, and dia(Zn), to identify the key factor affecting dye sorption performance. We found that ZIF-L, which possesses free imidazole molecules that leach into water during sorption, had the highest sorption capacity for the dye molecule Rose Bengal. We developed a wet deposition method for the creation of ZIF-L membranes on a porous alumina substrate using stabilized suspensions of ZIF-L. Tubular alumina supports cast with ZIF-L were used as a membrane adsorber for the removal of Rose Bengal under continuous-flow conditions. Finally, we investigated the influence of membrane adsorber synthesis conditions on the sorption characteristics.
Abundant bimetallic electrocatalysts for oxygen evolution reaction (OER) have been developed recently due to the superior performance. However, the in‐depth understanding of the performance improvement in bimetallic electrocatalysts remains a huge challenge. Here, we designed and synthesized a series of stable metal‐organic frameworks (MOFs: NNU‐21~24) based on trinuclear metal carboxylate clusters and tridentate carboxylate ligands. The precise structures and excellent stability of MOFs contribute to the investigation of their OER performance. Among these stable MOFs, NNU‐23 exhibits the best OER performance; specially, compared with monometallic MOFs, all the bimetallic MOFs display the improved OER activity. The experimental results are consistent with DFT theoretical computation, which demonstrates the introduction of the second metal atom can improve the activity of the original atom. This work is meaningful for energy storage and conversion to design more bimetallic catalysts and explore the catalytic mechanism.
In this review, we highlight how recent advances in the field of actinide structural chemistry of metal-organic frameworks (MOFs) could be utilized towards investigations relative to efficient nuclear waste administration, driven by the interest towards development of novel actinide-containing architectures as well as concerns regarding environmental pollution and nuclear waste storage. We attempt to perform a comprehensive analysis of more than 100 crystal structures of the existing An (U,Th)-based MOFs to establish a correlation between structural density and wt% of actinide and bridge structural motifs common for natural minerals with ones typically observed in the solution chemistry of actinides. In addition to structural considerations, we showcase the benefits of MOF modularity and porosity towards the stepwise building of hierarchical material complexity and the capture of nuclear fission products, such as technetium and iodine. We expect that these facets not only contribute to the fundamental science of actinide chemistry, but also could foreshadow pathways for more efficient nuclear waste management.
Intricacy anchored by uranium
Metal-organic frameworks generally have one level of assembly complexity: Organic linkers join inorganic nodes in a repeating lattice. Li et al. created a structure composed of cuboctahedra, assembled from uranium cations and organic linkers, that shared triangular faces to form prisms. These structures formed cages, which in turn joined to make tetrahedra that assembled with a diamond-lattice topology. This hierarchical open structure generated a huge unit cell with more than 800 nodes and linkers, containing internal cavities with diameters of 5 and 6 nm.
Science , this issue p. 624
Highly-connected and edge-transitive nets are of prime importance in crystal chemistry, and regarded as ideal blueprints for the rational design and construction of metal-organic frameworks (MOFs). We report the design and synthesis of highly-connected MOFs based on reticulation of the sole two edge-transitive nets with a vertex-figure as double six-membered ring (d6R) building-unit, namely (4,12)-coordinated shp net (square and hexagonal-prism) and (6,12)-coordinated alb net (aluminium diboride, hexagonal-prism and trigonal-prism). Decidedly, the combination of our recently isolated 12-connected rare-earth (RE) nonanuclear [RE9(μ3-OH)12(μ3-O)2(O2C−)12] carboxylate-based cluster, points of extension matching the 12 vertices of hexagonal-prism (d6R), with 4-connected square porphyrinic tetra-carboxylate ligand led to the formation of the targeted RE-shp-MOF. This is the first time that RE-MOFs based on 12-connected molecular building blocks (MBBs), d6R building-units, are deliberately targeted and successfully isolated, paving the way for the long-awaited (6,12)-c MOF with alb topology. Certainly, a custom-designed hexa-carboxylate ligand and its combination with RE salts led to the formation of the first related alb-MOF, RE-alb-MOF. Intuitively, we successfully transplanted the alb topology to another chemical system and constructed the first indium-based alb-MOF, In-alb-MOF, by employing trinuclear [In3(μ3-O)(O2C−)6] as the requisite 6-connected trigonal-prism and purposely-made dodecacarboxylate ligand as a compatible 12-c MBB. Prominently, the dodecacarboxylate ligand was employed to transplant shp topology into copper-based MOFs by employing copper-paddlewheel [Cu2(O2C−)4] as the complementary square building-unit and affording the first Cu-shp-MOF. We revealed that highly-connected edge-transitive nets such shp and alb are ideal for topological transplantation and deliberate construction of related MOFs based on minimal edge-transitive nets.
Ionic metal-organic frameworks (MOFs) are a subclass of porous materials that have the ability to incorporate different charged species in confined nanospace by ion-exchange. To date, however, very few examples combining mesoporosity and water stability have been realized in ionic MOF chemistry. Herein, we report the rational design and synthesis of a water-stable anionic mesoporous MOF based on uranium and featuring tbo-type topology. The resulting tbo MOF exhibits exceptionally large open cavities (3.9 nm) exceeding those of all known anionic MOFs. By supercritical CO2 activation, a record-high Brunauer-Emmett-Teller (BET) surface area (2100 m(2) g(-1) ) for actinide-based MOFs has been obtained. Most importantly, however, this new uranium-based MOF is water-stable and able to absorb positively charged ions selectively over negatively charged ones, enabling the efficient separation of organic dyes and biomolecules.
Metal-organic frameworks (MOFs) are being increasingly studied as scaffolds and supports for catalysis. The solid-state structures of MOFs, combined with their high porosity, suggest that MOFs may possess advantages shared by both heterogeneous and homogeneous catalysts, with few of the shortcomings of either. Herein, efforts to create single-site catalytic metal centers appended to the organic ligand struts of MOFs will be discussed. Reactions important for advanced energy applications, such as H2 production and CO2 reduction, will be highlighted. Examining how these active sites can be introduced, their performance, and their existing limitations should provide direction for design of the next generation of MOF-based catalysts for energy-relevant, small-molecule transformations. Finally, the introduction of second-sphere interactions (e.g., hydrogen bonding via squaramide groups) as a possible route to enhancing the activity of these metal centers is reported.
Covalent organic frameworks (COFs) formed by connecting multidentate organic building blocks through covalent bonds provide a platform for designing multifunctional porous materials with atomic precision. As they are promising materials for applications in optoelectronics, they would benefit from a maximum degree of long-range order within the framework, which has remained a major challenge. We have developed a synthetic concept to allow consecutive COF sheets to lock in position during crystal growth, and thus minimize the occurrence of stacking faults and dislocations. Hereby, the three-dimensional conformation of propeller-shaped molecular building units was used to generate well-defined periodic docking sites, which guided the attachment of successive building blocks that, in turn, promoted long-range order during COF formation. This approach enables us to achieve a very high crystallinity for a series of COFs that comprise tri-and tetradentate central building blocks. We expect this strategy to be transferable to a broad range of customized COFs.
Successful implementation of reticular chemistry using a judiciously designed rigid octatopic carboxylate organic linker allowed the construction of expanded HKUST-1-like tbo-MOF series with intrinsic strong CH4 adsorption sites. The Cu-analogue displayed a concomitant enhancement of the gravimetric and volumetric surface area with the highest reported CH4 uptake among the tbo family, comparable to the best performing MOFs for CH4 storage. The corresponding gravimetric (BET) and volumetric surface area of 3971 m2 g-1 and 2363 m2 cm-3 represent an increase of respectively 115 % and 47 % in comparison to the corresponding values for the prototypical HKUST-1 (tbo-MOF-1), and 42 % and 20 % higher than tbo-MOF-2. High pressure methane adsorption isotherms revealed a high total gravimetric and volumetric CH4 uptakes, reaching 372 cm3 (STP) g-1 and 221 cm3 (STP) cm-3 respectively at 85 bar and 298 K. The corresponding working capacities between 5-80 bar were found to be 294 cm3 (STP) g-1 and 175 cm3 (STP) cm-3 and are placed among the best performing MOFs for CH4 storage particularly at relatively low temperature (e.g. 326 cm3 (STP) g-1 and 194 cm3 (STP) cm-3 at 258 K). To better understand the structure-property relationship and gain insight on the mechanism accounting for the resultant enhanced CH4 storage capacity, molecular simulation study was performed and revealed the presence of very strong CH4 adsorption sites at the vicinity of the organic linker with similar adsorption energetics as the open metal sites. The present findings supports the potential of tbo-MOFs based on the supermolecular building layer (SBL) approach as an ideal platform to further enhance the CH4 storage capacity via expansion and functionalization of the quadrangular pillars.
A series of thorium-based terephthalates have been solvothermaly synthesized in N,N-dimethylformamide (DMF) with different amounts of water and various temperatures (100-150 °C). Without the addition of water, the Th-H2bdc-DMF system gives rise to the formation of two phases, Th(bdc)2(DMF)2 (1) and Th6O4(OH)4(H2O)6(bdc)6·6DMF·12H2O (3) (bdc = 1,4-benzenedicarboxylate or terephthalate). Their structures are built up of isolated thorium centers ThO8(DMF)2 for (1) and the hexanuclear core Th6O4(OH)4(H2O)6 for (3). The latter adopts the UiO-66 metal-organic framework topology and exhibits a very high porosity for an actinides-based porous material (BET surface up to 730(6) m(2)·g(-1)). The synthesis of (3) is also favored upon adding water. However, for pure aqueous solutions or for a very low amount of water, a third solid Th(bdc)2 (2) crystallizes and contains thorium monomers ThO8. The main similitude with the parent system dedicated to tetravalent uranium concerns the possibility to stabilize the An6O8(H2O)6 core by terephthalate linkers and to reproduce An(bdc)2(DMF)2 for both actinides U(4+) and Th(4+). The thermal treatment of the latter shows a structural transition into the crystalline Th(bdc)2 (2) solid.
A mixed hydroxo/oxo plutonium(IV) carboxylate resulting from the hydrolysis and condensation of Pu(IV) in an acidic aqueous solution has been isolated. The structure of Li6[Pu6(OH)4O4(H2O)6(HGly)12]Cl18·10.5H2O (1) consists of a cationic [Pu6(OH)4O4](12+) core that is decorated by glycine ligands. The synthesis, structure, and characterization of the hexanuclear unit, which represents the first example of a Pu(IV) polynuclear complex containing both hydroxo- and oxo-bridging ligands, are described herein.
The cryst. oxozirconium methacrylate clusters Zr6(OH)4O4(OMc)12 and Zr4O2(OMc)12 were obtained by reaction of Zr(OPr)4 with an excess of methacrylate and were analyzed by x-ray diffraction. The oxide and hydroxide groups are in a μ3-bridging mode in both structures. The methacrylate ligands are chelating or bridging. The Zr atoms in Zr6(OH)4O4(OMc)12 form an octahedron, the cluster having C3v symmetry [hexagonal, space group P63mc; Z = 6; a 1740.0(3), c 1820.0(4) pm, V = 4772(1)·106 pm3]. Each metal atom is square-antiprismatically coordinated by 8 O atoms. In Zr4O2(OMc)12, the Zr atoms have a distorted butterfly arrangement [monoclinic, space group P21/c; Z = 4; a 1710.11(2), b 1736.72(1), c 1977.88(1) pm, β 90.086(1)°, V = 5874.3(1)·106 pm3]. Their coordination geometry is square-antiprismatic or capped octahedral. [on SciFinder(R)]
The structures of 59 inorganic Np(5+) and Np(6+) neptunyl compounds are examined and placed within a structural hierarchy, with special attention to the relationships of these structures with U(6+) uranyl compounds. Forty-three Np(5+) neptunyl compounds containing square, pentagonal and hexagonal bipyramids have structural units consisting of isolated polyhedra (2), finite clusters of polyhedra (1), chains of polyhedra (12), sheets of polyhedra (16), and frameworks of polyhedra (12). Cation-cation interactions occur in 18 of these, resulting in a dramatic departure of their topologies from those of U(6+) uranyl compounds. Compounds with cation-cation interactions have structural units consisting of chains (4), sheets (4) and frameworks (10) of polyhedra. Those Np(5+) compounds that lack cation-cation interactions exhibit topologies that are similar or identical to those of U(6+) compounds. Sixteen Np(6+) neptunyl compounds have structural units consisting of isolated polyhedra (1), finite clusters of polyhedra (3), chains of polyhedra (4), and sheets (8) of polyhedra. None of these structures exhibit cation-cation interactions, and their topologies are similar to U(6+) uranyl compounds. Geometries of Np(5+) and Np(6+) polyhedra are examined, and updated bond-valence parameters, R(o) = 2.035 and b = 0.422, are provided for Np(5+).
A study was conducted to demonstrate solution and solid-state structural chemistry of actinide hydrates and their hydrolysis and condensation products. The investigations focused on providing a detailed metrical description of a hydrated ion's coordination environment and how it changed as it further reacted with water through hydrolysis. The study emphasized on facilitating information transfer and comparisons between these two approaches to the same problem. It also focused on relating the metal-ligand correlated moieties and aggregates identified from thermodynamics with molecular level structures for which the theorist assessed the results. A number of An hydrates, mononuclear hydrolysis products, and polynuclear complexes were highlighted where polynuclear complexes resulted from metal-ion hydrolysis and condensation in aqueous and nonaqueous solution.
We discuss a recently developed approach to formalize the analysis of extended architectures by successive simplifications of a crystal structure perceived as a periodic net. The approach has been implemented into the program package TOPOS that allows one to simplify and classify coordination polymers of any complexity in an automated mode. Using TOPOS, we retrieved 6620 3-periodic coordination polymers from the Cambridge Structural Database and represented them in a standard way as underlying nets. The topological classification of both 975 interpenetrating and 5645 single 3-periodic underlying nets has been performed and compared. The up-to-date methods for prediction of the topology of underlying nets are discussed and the ways to develop reticular chemistry are outlined.
Both the science and technology of the actinides as we know them today owe much to separation science. Conversely, the field
of metal ion separations, solvent extraction, and ion exchange in particular, would not be as important as it is today were
it not for the discovery and exploitation of the actinides. Indeed, the synthesis of the actinides and the elucidation of
their chemical and physical features required continuous development and improvement of chemical separation techniques. Furthermore,
the diverse applications of solvent extraction and ion exchange for metal ion separations as we know them today received significant
impetus from Cold War tensions (and the production of metric tons of plutonium) and the development of nuclear power for peaceful
uses.
Solvent extraction, precipitation/coprecipitation, and ion exchange procedures have played a central role in the discovery
and characterization of the 5f transition elements. Each of these separations techniques likewise has shaped progress in technological
applications of actinides for electricity production and for nuclear weapons. Recent decades have seen the rise of pyroelectrometallurgical
separations, wherein the long-term future of actinide separations may lie.
Porous crystals are strategic materials with industrial applications within petrochemistry, catalysis, gas storage, and selective separation. Their unique properties are based on the molecular-scale porous character. However, a principal limitation of zeolites and similar oxide-based materials is the relatively small size of the pores, typically in the range of medium-sized molecules, limiting their use in pharmaceutical and fine chemical applications. Metal organic frameworks (MOFs) provided a breakthrough in this respect. New MOFs appear at a high and an increasing pace, but the appearances of new, stable inorganic building bricks are rare. Here we present a new zirconium-based inorganic building brick that allows the synthesis of very high surface area MOFs with unprecedented stability. The high stability is based on the combination of strong Zr-O bonds and the ability of the inner Zr6-cluster to rearrange reversibly upon removal or addition of mu3-OH groups, without any changes in the connecting carboxylates. The weak thermal, chemical, and mechanical stability of most MOFs is probably the most important property that limits their use in large scale industrial applications. The Zr-MOFs presented in this work have the toughness needed for industrial applications; decomposition temperature above 500 degrees C and resistance to most chemicals, and they remain crystalline even after exposure to 10 tons/cm2 of external pressure.
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