An fcu Th-MOF Constructed from In Situ Coupling of Monovalent Ligands

: Synthetic efforts targeting highly symmetrical metal–organic frameworks (MOFs) have always been relentless, for the symmetry of a MOF’s pore environment and overall crystal structure are relevant to the MOF’s properties and behavior. Herein, we report a novel Th-based MOF constructed from assembling highly symmetrical Th-oxo clusters via in situ reductive coupling of nitroso groups on the cluster surface. Nitroso groups have long been known to dimerize in a reversible fashion. Putting them on the monovalent ligands that decorate the Th-oxo clusters can facilitate a downstream assembly process that link the said clusters in a controllable and predictable manner, preserving the overall symmetry in the MOF product. Moreover, the assembly can be made permanent by reducing the azodioxy moiety to azo, effectively locking the symmetrical MOF form. We believe this process of assembling pre-formed Th-oxo clusters helps the overall MOF adopt a highly symmetrical topology (face-centered cubic, fcu ) resembling the well-known UiO series MOFs based on tetravalent Zr/Hf.


Introduction
For more than two decades, metal-organic frameworks (MOFs), as a burgeoning class of versatile materials, have profoundly enriched the knowledge of coordination chemistry and created opportunities in fields like gas storage and separation, catalysis, biomedicine, chemical sensing, and so on [1,2]. Generally, MOFs are synthesized using metal sources (salts etc.) and bridging organic ligands of various geometries that are often relevant to the topology of the final products. One can refer to resources such as the Reticular Chemistry Structure Resource (RCSR) for rational design and outcome predictions for MOFs [3].
Among the 100,000+ MOFs reported in the literature [4], structures with a higher degree of symmetry are particularly appealing from a fundamental viewpoint. Yet it remains challenging to develop straightforward synthetic routes for targeted highly symmetrical MOFs, especially for metals with a broad range of coordination numbers and diverse coordinative reactivities. For example, selective synthesis of MOFs with tetravalent and trivalent metals that are prone to premature hydrolysis and ill-controlled coordination environments are particularly difficult when using conventional synthetic protocols, hence the modulated synthesis (e.g., Zr/Hf-and rare earth-based MOFs) in which the addition of monovalent ligands regulates the specific coordination processes between metal cations and corresponding ligands, slowing down the formation of extended networks [5][6][7][8][9]. Besides coordinative modulation, another potential route to gain better control of the MOF crystallization process is through the introduction of reversible covalent bonding [10][11][12][13], as it also slows down the overall formation of extended framework structures. In fact, as the basis of covalent organic frameworks, reversible bonding is required to make the structure periodic and even highly crystalline in some cases [14,15].
Thorium is the most abundant radioactive element found in nature with an extremely long half-life (t 1/2 = 1.405 × 10 10 years) [16]. Th 4+ is a stable tetravalent cation with one of the richest coordination chemistries (coordination number ranging from 4 to 15) among all metals [17]. Thorium-based MOFs that have been reported so far encompass quite diverse coordinative environments for Th 4+ [18][19][20]. Thus, we propose to use a covalent bond-based synthetic protocol to control the formation of MOFs with targeted coordination chemistry for Th 4+ ; if proven helpful, the method might be seen as versatile and applicable to other metals.
Here, we report a covalently modulated synthesis of a highly symmetrical Th-MOF with fcu (face-centered cubic) topology enabled by homologous coupling of nitroso groups that undergo a reductive dimerization process, simultaneously regulating the coordination process and locking the targeted MOF structure. We hypothesize the process takes place in a stepwise manner: (1) formation of highly symmetrical Th-oxo clusters covered with monovalent ligands (i.e., 4-nitrosobenzoate, L1); (2) due to the symmetry and stability of said Th 6 cluster, the assembly must also be a selective process that leads to formation of a MOF with high symmetry; (3) nitroso dimers, which are the basis of the assembly, are reduced in situ, effectively locking the MOF product.

Results and Discussion
Several framework materials connected by covalent imine bonds formed between reactive groups (i.e., amine and aldehyde) that decorate metal clusters have been reported [10][11][12][13]22] in which the imine bonds are reversible to some extent. Here, we hypothesize that using ligands with functional groups that undergo homologous reversible coupling could enable and regulate the growth of MOFs. Nitroso compounds were known to dimerize reversibly to form cisor transazodioxides and were successfully used to synthesize covalently linked organic framework materials in the form of large single crystals [14]. Hence, we selected a monovalent ligand functionalized with a nitroso group to prepare a Th-oxo complex that served as a structural node in the subsequent construction of a Th-MOF via nitroso dimerization, all in a one-pot approach. We successfully obtained large single crystals of SUM-5 (SUM = Sichuan University Materials) with high symmetry (face-centered cubic, space group Fm3m), free of coordinative modulators. We note that in a recently reported isoreticular series of Th-MOFs with UiO-type structures, Lin et. al. employed a typical acid-modulated approach and prepared the MOFs with divalent carboxy ligands with a variety of lengths [23].
To the best of our knowledge, coordinative modulators are essential to successfully synthesize tetravalent Zr/Hf-based MOFs (e.g., UiO-type MOFs). Owing to their strong Lewis acidity, tetravalent metal cations have a high tendency to coordinate with polycarboxylate ligands, often giving rise to bulk polycrystalline or amorphous powders if the coordination process is not properly controlled (modulated). However, in the case of L1, which is a monovalent ligand, fast reaction with Th 4+ only leads to discrete complexes (clusters), and an extended network could only form when the nitroso groups dimerize, bringing neighboring clusters together. Therefore, the reversible dimerization of nitroso groups acts as a self-modulating process, facilitating the formation of large single crystals of SUM-5.
Single-crystal X-ray diffraction of a suitable crystal revealed that SUM-5 crystallizes in the cubic space group Fm3m (CCDC deposition number 2085727, asymmetric unit shown in Figure S6, for crystallographic refinement details see Table S1). SUM-5 conforms to the fcu topology, connecting Th 6 (µ 3 -O) 4 (µ 3 -OH) 4 Figure S7a) and octahedral cavities (Figure 1b and Figure S7b) with internal pore diameters of ca. 12.2 Å and 15.8 Å, respectively, which are connected by ca. 10.9 Å triangular windows, with van der Waals radii taken into consideration. Phase purity of SUM-5 was confirmed by comparing the experimental and simulated PXRD patterns (Figure 1c). The crystal structure of SUM-5 showed that the organic ligand was an azo compound instead of an azodioxide, which is the direct product of nitroso dimerization. The identity of the ligand was also confirmed by the 1 H NMR spectrum of digested SUM-5 ( Figures S2 and S3). The formation of the azo ligand is a result of an in situ reduction of azodioxides, most likely by dimethylamine, decomposition product of DMF, the solvent used. After activation, SUM-5 exhibited a classic type I isotherm with a maximum N 2 uptake of 338.4 m 3 ·g −1 at 77 K (Figure 2a), corresponding to a Brunauer-Emmett-Teller (BET) surface area of 1410 m 2 ·g −1 and a pore volume of 0.5232 cm 3 ·g −1 . Moreover, pore size distribution showed that SUM-5 has micropores with diameters of 1.26 nm and 1.42 nm, respectively, which are consistent with the crystal structure determined by SC-XRD (Figure 2a inset). As shown in Figure S8, the FT-IR spectrum of SUM-5 shows peaks at 1590 and 1410 cm −1 , which correspond to the asymmetric ν asym (COO) and symmetric ν sym (COO) vibrations of carboxylates, respectively. The peak centered at 1498 cm −1 corresponds to the aromatic C=C vibrations [24]. The C-H out-of-plane vibration of the benzene ring was found at 791 cm −1 . In addition, SUM-5 was determined to be thermally stable up to 400 • C, as indicated in the TGA curve (Figure 2b). We note that the physical characteristics of SUM-5 (BET surface area, etc.) are consistent with the literature [23].

Conclusions
In conclusion, we have synthesized a Th-based MOF with highly symmetrical fcu topology in which the bridging ligands were formed in situ via homo-coupling and reductive dimerization of nitroso groups on discrete Th-oxo clusters. We argue that the use of monovalent ligands circumvents the fast and ill-controlled coordination of tetravalent cations with hard-base bridging polydentate ligands. Instead, the reversible covalent bonding becomes the regulating step, linking the Th-oxo clusters into an extended framework while preserving the symmetry, effectively constituting a self-modulating process that leads to the formation of large single crystals, free of extraneous modulators. Subsequently, the assembly was made permanent by reducing the azodioxy moiety to azo, locking the symmetrical MOF form. The nitroso dimerization-based strategy could be applicable in other systems, linking together various discrete metal clusters. Joining the other Th-MOFs, SUM-5 is potentially applicable as a candidate material for radiation detection, X-ray scintillation, radionuclide storage, etc.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.