Molecular and polymeric uranyl and thorium hybrid materials featuring methyl substituted pyrazole dicarboxylates and heterocyclic 1,3-diketones
Graphical abstract
A series of seven novel f-element bearing hybrid materials have been prepared from either methyl substituted 3,4 and 4,5-pyrazoledicarboxylic acids, or heterocyclic 1,3- diketonate ligands using hydrothermal conditions.
Introduction
Actinide hybrid materials are an area of continued interest and development due to their tendency for forming structurally diverse coordination polymers and molecular complexes, as well as their continued relevance to the nuclear fuel cycle [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. The majority of these crystalline hybrid materials have featured the divalent uranyl cation ([UO2]2+), which is known for its relative stability under ambient conditions and its unique chemistry that is a result of the largely terminal bonding arrangements of the trans [UO2]2+ bonds. Tetravalent actinides, including thorium(IV), have not been as widely utilized for the formation of actinide hybrid materials, despite their environmental relevance. This may be a result of their strong Lewis acidity, which often facilitates the formation of cluster species as a result of hydrolysis and condensation processes [11], [12], [13], [14], [15].
Some of our recent contributions concerning the synthesis and characterization of actinide hybrid materials have focused on supramolecular chemistry [16], [17], [18], [19], [20], [21], [22], [23] and bimetallic synthesis, via the use of metalloligands or post-synthetic metalation [24], [25], [26], as means to overcome hydrolysis-related synthetic challenges as both methods offer greater control over the metal cation first coordination sphere. Herein we utilized both ideas with two classes of underexplored ligands in the actinide hybrid material literature. Inclusion of the heterofunctional pyrazoledicarboxylic acid and β-diketonate ligands is a topic of great interest in lanthanide hybrid materials due to the wide array of potential applications for these materials, particularly luminescence [27], [28], [29], [30], [31], yet actinide hybrids featuring these same ligands have not garnered nearly as much attention [32], [33], [34]. As pyrazoledicarboxylic acid linkers feature both hard carboxylate moieties and softer pyrazole nitrogen atoms they have the potential to coordinate both the uranyl and thorium cation via an array of binding modes. β-diketonate ligands have been shown to ‘cap’ the uranyl cation [34], [35], similar to how we have previously used 1,10-phenanthroline and 2,2’:6′,2″-terpyridine in the same role [17], [18], [19], and herein we endeavored to continue exploration of this process with heterofunctional pyrazole 1,3-β-diketonate ligands and methyl-substituted pyrazoledicarboxylic acid ligands. Efforts to extend study of β-diketonate capping ligands to the Th(IV) cation where little is known, as evidenced by only nineteen results in the Cambridge Structural Database (CSD, V. 5.38, Nov. 2016) [36] featuring both Th4+ and 1,3-diketone moiety, did not yield single crystals which is likely a result of the two groups representing a poor hard-soft acid-base (HSAB) pairing.
Drawing inspiration from our previous work on uranyl bimetallics and f-element supramolecular assembly, we set out to explore the construction and assembly of actinide hybrid materials incorporating either 1-Methyl-1H-pyrazole-3,4- or 1-Methyl-1H-pyrazole-4,5-dicarboxylic acids (SVI-COOH 3,4; SVI-COOH 4,5) or heterocyclic 1,3-bis(4-N1-methyl-pyrazolyl)- or 1,3-bis(4-N1,3-dimethyl-pyrazolyl)propane-1,3-dione ligands (SVI-6; SVI-7) with [UO2]2+ and Th4+ cations. Herein we report the syntheses, crystal structures, modes of supramolecular assembly, and uranyl photophysical properties for a family of seven new actinide hybrid materials, five of which include the uranyl cation and two of which feature Th(IV) metal centers. Moreover, the materials described herein provide a platform for the comparison of the coordination environments of the UO22+ and Th4+ cations with similar ligands, thereby enhancing our understanding of effects of cation Lewis acidity on both hydrolysis and reactivity.
Section snippets
Materials and methods
Caution: Whereas the uranium nitrate hexahydrate [UO2(NO3)2]·6H2O used in this study consists of depleted U, standard precautions for handling radioactive and toxic substances should be followed. [UO2(NO3)2]·6H2O was crystallized from a mixture of uranyl nitrates and oxides dissolved in concentrated nitric acid. Additionally, standard precautions for handling radioactive substances should be followed when working with thorium(IV) nitrate tetrahydrate [Th(NO3)4·4H2O] which was purchased from
X-ray structure determination
Single crystals from each bulk sample were isolated and mounted on MiTeGen micromounts. Structure determination for each of the single crystals was achieved by collecting reflections using 0.5° ω scans on a Bruker SMART diffractometer furnished with an APEX II CCD detector using MoKα (λ = 0.71073 Å) radiation at 100(2) K. The data were integrated using the SAINT software package [40] contained within the APEX II software suite [41] and an absorption correction was performed using SADABS [42].
Description of structures
Single crystal X-ray crystallography analyses revealed five unique coordination environments in this family of actinide materials. Whereas compounds 1–4 feature the 1-Methyl-1H-pyrazole-3,4- and 1-Methyl-1H-pyrazole-4,5-dicarboxylic acid ligands (SVI-COOH 3,4 and SVI-COOH 4,5) and adopt polymeric and molecular structures with the uranyl and thorium(IV) cations, respectively, compounds 5–7 are molecular materials that feature the 1,3-bis(4-N1-methyl-pyrazolyl)- and 1,3-bis(4-N1
Conclusions
The syntheses and crystal structures of seven f-element bearing hybrid materials featuring either methyl substituted 3,4 and 4,5-pyrazoledicarboxylic acids, or heterocyclic 1,3- diketonate ligands are reported, and their means of assembly via hydrogen bonding have been detailed. These structural results indicate that methyl functionalized pyrazole dicarboxylic acids and diketonates feature a rich array of hydrogen bonding synthon sites, and a diverse range of interactions are observed in
Notes
The authors declare no competing financial interest.
Acknowledgement
This material was supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001089. K. P. C. would also like to acknowledge The George Washington University for a Presidential Merit Fellowship award.
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