Leveraging Nitrogen Linkages in the Formation of a Porous Thorium–Organic Nanotube Suitable for Iodine Capture

Abstract

We report the synthesis, characterization, and iodine capture application of a novel thorium−organic nanotube, TSN-626, [Th 6 O 4 (OH) 4 (C 6 H 4 NO 2 ) 7 (CHO 2 ) 5 (H 2 O) 3 ]•3H 2 O. The classification as a metal−organic nanotube (MONT) distinguishes it as a rare and reduced dimensionality subset of metal−organic frameworks (MOFs); the structure is additionally hallmarked by low node connectivity. TSN-626 is composed of hexameric thorium secondary building units and mixed O/N-donor isonicotinate ligands that demonstrate selective ditopicity, yielding both terminating and bridging moieties. Because hard Lewis acid tetravalent metals have a propensity to bind with electron donors of rival hardness (e.g., carboxylate groups), such Th−N coordination in a MOF is uncommon. However, the formation of key structural Th−N bonds in TSN-626 cap some of the square antiprismatic metal centers, a position usually occupied by terminal water ligands. TSN-626 was characterized by using complementary analytical and computational techniques: X-ray diffraction, vibrational spectroscopy, N 2 physisorption isotherms, and density functional theory. TSN-626 satisfies design aspects for the chemisorption of iodine. The synergy between accessibility through pores, vacancies at the metal−oxo nodes, and pendent N-donor sites allowed a saturated iodine loading of 955 mg g −1 by vapor methods. The crystallization of TSN-626 diversifies actinide−MOF linker selection to include soft electron donors, and these Th−N linkages can be leveraged for the investigation of metal-to-ligand bonding and unconventional topological expressions.

Selective Crystallization of a Highly Radiation-Resistant Isonicotinic Acid-Based Metal–Organic Framework as a Primary Actinide Waste Form

“…The presence of oxygen vacancies is indicated by the fitting of the O 1s peak (Figure S2b) and characteristic electron paramagnetic resonance (EPR) g -value (Figure S3), which presumably result from the dissociation of weakly coordinating DMF or H 2 O at the cap of a Th 6 hexamer. Hence, we attribute this unique PBU coordination environment to the terminating and bridging functions of INA ligands as well as the Th–N linkages . Besides the Th 6 cluster, multiple covalent bonding within the organic component can be verified by the Raman spectra (Figure S5, Table S5), while an intense ligand-to-metal charge transfer is implied by the UV–vis–NIR spectra (Figure S4).…”

“…Isonicotinic acid (INA) is a simple bifunctional N, O-donor ligand featuring hard (carboxylate group) and intermediate (nitrogen of the pyridine) bases that readily coordinate with hard acids (e.g., Th 4+ , UO 2 2+ ) and mild acids (e.g., Co 2+ , Ni 2+ ), respectively. Since it has a strong affinity toward actinide ions, it has been utilized to construct actinide-containing molecular complexes , or An-MOFs. − It has been widely adopted in the rational constructions of novel heterometallic MOFs via the one-pot − or postsynthetic route. , Starting from this prototype ligand, a variety of bifunctional N- and O-donor ligands can be taken into consideration for novel An-MOFs by altering the relative location of bifunctional sites, inserting substitution groups, or extending or modifying the pyridine skeletons. We investigate INA as the target ligand.…”

“…It is notable that Th 6 a is able to incorporate up to 16 INA ligands, which contributes to a tightly packed three-dimensional (3D) framework with a solvent-accessible void of 6.9% and accords with the Brunauer-Emmett-Teller (BET) surface area of 1.77 m 2 /g for the nonactivated powder sample (Figure S1). As opposed to the bidentate brigading azole in Azole-Th-1, Th-MOF-67(-68), and ECUT-68 with UiO-66-like structures, the monodentate N-donor of the pyridine, as well as uncoordinated pyridine, contribute to the formation of distinct thorium hexamers with low node connectivity observed in both Th-INA-1 and TSN-626 . The hexamers in Th-INA-1 bind more INA ligands than those in TSN-626, leading to the relatively small accessible micropore regime that needs to be explored in more detail.…”

“…The bond lengths of the [ThO 8 N] and [ThO 9 ] primary building units (PBUs) in Th-INA-1 are in accordance with those in these N, O-donor ligands-based thorium MOFs (Tables S2–S4). ,,, …”

A robust thorium–organic framework as a bifunctional platform for iodine adsorption and Cr(vi) sensitization

“…5 Moreover, Th 4+ is inclined to adopt a 9-coordinate capped square antiprism geometry, whose capping addenda can function as an extra bridging site for creating framework isomerism. [25][26][27] Herein, by simply modifying the reaction modulator, we obtained a new Th-MOF, [Th 6 (μ 3 -OH) 8 (H 2 O) 2 (TCBPA) 4 (CH 3 COO) 2 ]•(NO 3 ) 2 •(DMF) 12 (H 2 O) 25 (Th-TCBPA), which can function as a bifunctional platform for radioiodine adsorption and Cr(VI) sensitization. The porous nature of Th-TCBPA, in combination with its π-electron-rich organic linkers, gives rise to an adsorption capacity of 1.01 g g −1 for iodine vapor.…”