Two-dimensional Zr/Hf-hydroxamate metal–organic frameworks†

Qiuxue Lai ^a, Zhao-Qin Chu ^b, Xinyi Xiao ^a, Dejun Dai ^c, Ting Song ^a, Tian-Yi Luo ^d, Wenlei Tang ^a, Xuan Feng ^a, Zhiyuan Zhang ^a, Tao Li ^c, Hai Xiao ^e, Jing Su *^b and Chong Liu *^a
aSchool of Chemical Engineering, Sichuan University, Chengdu, 610065, China. E-mail: liuchong@scu.edu.cn
^bCollege of Chemistry, Sichuan University, Chengdu, 610064, China. E-mail: jingsu@scu.edu.cn
^cSchool of Physical Science and Technology, ShanghaiTech University, Shanghai, 201210, China
^dInstitute for NanoBioTechnology, Johns Hopkins University, Baltimore, MD 21218, USA
^eDepartment of Chemistry, Tsinghua University, Beijing, 100084, China

First published on 26th January 2022

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Metal–organic frameworks (MOFs) are an ever-expanding class of highly diverse materials with impressive properties in multiple research and industrial scenarios,¹ yet their applications often face challenges due to their instability under harsh chemical and mechanical conditions.² Therefore, the stability issue of MOFs has been thoroughly reviewed and found to be multifaceted, with relevance to coordination, organic ligands, surface properties, etc.³ For example, as coordination is usually the basis for MOF construction, when coordinatively stable MOFs are desired, a rule of thumb is the Hard and Soft Acids and Bases (HSAB) theory. This is best represented by some of the well-known stable MOFs, including UiO-66,⁴ MIL-101,⁵ PCN-601,⁶ and the ZIF series.⁷

In addition, coordination itself is also a multifaceted issue. While approaches based on HSAB aim to boost individual coordination bond strengths, other strategies with even higher degrees of chemical tunability have also been developed to improve the overall stability of MOFs from the coordination perspective, including cantellation and postsynthetic ligand installation.⁸ In these reports, a bundle of ligands or a polyvalent ligand was used, in which a single coordination bond is associated with other bonds in said ligand(s), improving the overall structural stability. These examples represent a collective coordination strategy that is entropy-favored.

Chelation is another typical entropy-favored coordination process with polydentate ligands freeing previously coordinated (solvent) molecules. Stability enhancement of a complex as a result of chelation is quite common, demonstrated by artificially synthesized ethylenediaminetetraacetic acid⁹ (EDTA) as well as biologically produced siderophores¹⁰ that sequester soluble iron very effectively. Hydroxamate, as the main chelating group in many siderophores, has been used in MOF designs. Coupled with iron(III)¹¹ and titanium(IV),¹² crystalline products were synthesized and studied. Taking advantage of the stable metal-hydroxamate chelation, with fairly simple ligand designs and limited apparent coordination numbers, these MOFs exhibit very good chemical stability. Further, use of Fe³⁺ or Ti⁴⁺ also stabilizes the structures from an enthalpic perspective (i.e. HSAB theory). However, this MOF category is still underdeveloped, especially for other well-known hard metal cations (Zr⁴⁺/Hf⁴⁺), except for a postsynthetic attempt.¹³

Inspired by the development of ⁸⁹Zr complexes for positron emission tomography (PET) imaging using ligands with hydroxamate groups,¹⁴ which feature stable coordination and fast preparation (a prerequisite for ⁸⁹Zr with a t_1/2 = 78.4 hours), our synthesis toward Zr-hydroxamate MOFs was conventional to begin with. ZrCl₄ and benzene-1,4-dihydroxamic acid (H₂-BDHA, Fig. S1, ESI,† prepared according to literature¹⁵) were reacted solvothermally, crystallizing into ∼20 μm hexagonal prismatic blocks after a short reaction time of 10 min at 120 °C. Carefully prolonged reaction time of 40 min led to formation of ∼100 μm crystals (Fig. 1a and Fig. S2, ESI†). Determined with single crystal X-ray diffraction (SC-XRD), a layered two-dimensional (2D) MOF structure was resolved (for details, see ESI† Section 3.1), hereafter named SUM-1(Zr) (SUM = Sichuan University Materials). Like discrete complexes, Zr⁴⁺ is also chelated by four hydroxamates (Fig. 1a). In the structure of high symmetry (P6/mmm), asymmetric hydroxamates are perpendicular to mirror planes, leading to a two-site disorder of equal occupancies (Fig. S5, ESI†). Within the ab plane (Fig. 1b and Fig. S6, ESI†), there exists a hexagonal channel (d = circa 2.1 nm, with van der Waals radii taken into consideration) and a triangular channel (d = circa 0.99 nm). Rotating 90 degrees (Fig. 1c and Fig. S7, ESI†), the crystal structure comprises 2D layers stacking in an eclipsed AA mode with an interlayer distance of circa 0.64 nm, possibly accommodating solvent molecules. Overall, the MOF structure conforms to a kagome (kag) net (Fig. 1d), according to the Reticular Chemistry Structure Resource (RCSR).¹⁶ Similarly, starting with HfCl₄, an isostructural MOF SUM-1(Hf) was obtained (Fig. S3, ESI† Section 3.2). We note that in addition to Zr⁴⁺/Hf⁴⁺, synthetic attempts using Fe³⁺ and H₂-BDHA have been unsuccessful.

Now, we want to highlight that the 10-to-40 min reaction time that produces high quality, large-sized Zr-MOF crystals is impressive and uncommon,¹⁷ providing a convenient opportunity for mechanistic study of MOF crystallization. Empirically speaking, high valence metal cations hydrolyze quickly and the fast kinetics often leads to ill-controlled reaction routes, sometimes resulting in aperiodic structures instead of crystalline MOFs.¹⁸ Therefore, for certain cations, modulated synthetic procedures were a reasonable choice, using monovalent ligands (e.g. for Zr⁴⁺, benzoic acid, formic acid, etc.) in significant amounts in addition to bridging ligands to slow down the reaction, forcing the formation of targeted crystal structures.¹⁹ Here in the case of SUM-1(Zr), in the absence of modulators, we hypothesize that there exists an exclusive reaction route for Zr⁴⁺ with hydroxamates, and the sole product is an 8-coordinated Zr(hydroxamate)₄ complex, identical to previously reported discrete compounds.^14a,20 The hypothesis was proved using density functional theory (DFT) calculations (for details, see ESI† Section 4). In summary, unlike multi-nuclear Zr-MOF nodes such as UiO-66, in the Zr-hydroxamate system, the formation of a dinuclear species (en route to even higher nuclearity) from mononuclear species (as shown in Fig. 1a) is thermodynamically unfavored with a large positive ΔG of 18.9 kcal mol⁻¹ (ESI† Section 4.3), and also kinetically difficult with an activation energy barrier of 25.1 kcal mol⁻¹ (Fig. S14 and S15, ESI†). Also, an 8-O-coordinated Zr⁴⁺ is the only stable product, comparing to 7-coordinated and/or N-coordinated alternatives (Table S3 and Fig. S15, ESI†).

In addition, the kinetic dominance of the chelated mononuclear Zr(hydroxamate)₄ node in SUM-1(Zr) over the multi-nuclear node in the benchmark Zr-MOF UiO-66 was shown in a competing synthesis experiment, in which exclusive formation of SUM-1(Zr) was observed from a mixture of ZrCl₄ with both H₂-BDHA and terephthalic acid. Moreover, in the presence of HCl which can accelerate or catalyze the UiO-66 formation,²¹ still only SUM-1(Zr) crystallized from the mixture (for details, see ESI† Section 5). These results further demonstrated the fast kinetics of SUM-1(Zr) formation.

With a robust synthesis established, the prepared SUM-1 crystals were determined to be phase pure by comparing experimentally determined powder X-ray diffraction (PXRD) patterns with simulated patterns from SC-XRD data (Fig. 2a, Fig. S20 and S21, ESI†). The MOFs were further characterized with Fourier transform infrared (FTIR) spectroscopy (Fig. S22, ESI†) and thermogravimetric analysis (TGA) (Fig. S23 and S24, ESI†). Activated after acetone exchange, N₂ sorption measurements for SUM-1 were performed at 77 K (for details, see ESI† Section 6.4). Based on the isotherms (Fig. 2b, Fig. S25 and S26, ESI†), the Brunauer–Emmett–Teller (BET) surface area for SUM-1(Zr) and SUM-1(Hf) was calculated to be 1252 m² g⁻¹ and 1005 m² g⁻¹, respectively (Tables S7 and S8, ESI†). Pore size distribution of the two samples was estimated based on the non-local density functional theory (NLDFT) modeling, agreeing well with the crystal structures (Fig. 2b inset, Fig. S29 and S30, ESI†). We note that the N₂ sorption experiments on pristine crystals yielded isotherms with unexpected hysteresis loops. The hysteresis was likely a sign of desorption equilibrium not being reached because of sluggish diffusion kinetics. Therefore, it was proposed that upon removal of (most) interlayer (solvent) molecules, the non-covalent interactions which hold the layers in place could be significantly weakened, possibly leading to relative translation between layers and (partially) blocked channels²² (illustrated in Fig. S31, ESI†). Inspired by studies of other systems,²³ we propose that miniaturization of SUM-1(Zr), especially minimizing the number of layers in the axial direction should statistically decrease the occurrence of layer slippage as well as shorten the diffusion distances, which should improve the N₂ transport kinetics in return. Additionally, prior to miniaturization which may involve chemical treatments, stability of SUM-1 in various pH aqueous solutions needed to be assessed. PXRD data indicated that the MOF crystallinity maintained in a wide pH range of 2–11 (Fig. 2c, d and Fig. S32, S33, ESI†). However, activation of the treated samples for N₂ sorption measurements was unsuccessful. In addition, SUM-1 crystals could also resist decomposition by various coordinating organic solvents of different properties, as shown in Fig. S34 and S35 of ESI.†

For MOF miniaturization in general, common practices include induction of homogeneous nucleation and shortening of reaction time.²⁴ For the synthesis of SUM-1(Zr) that is already quite fast, we implemented a sacrificial modulation method. Prior to solvothermal reactions in an open vial, HCl/H₂O was added to help H₂-BDHA into full dissolution in N,N-dimethylformamide (DMF), promoting a more homogeneous nucleation (for details, see ESI† Section 7). Three different conditions with varied HCl/H₂O amounts led to formation of thinned SUM-1(Zr) with average thicknesses of 318 nm, 163 nm, and 51 nm, labeled as SUM-1(Zr)-a, -b, and -c, respectively, as shown in scanning electron microscopy (SEM) images and corresponding statistics (Fig. 3a–c, Fig. S36–S39, ESI†). For example, a thickness of 42 nm for SUM-1(Zr)-c corresponds to approximately 53 laminated layers (Fig. 3c). In contrast, for a crystal with a thickness of 100 μm (Fig. 1a), the number of layers is approximately 125000. Therefore, the difference between respective probabilities of interlayer translation and corresponding impacts on N₂ uptake behavior should be significant. In fact, when N₂ sorption experiment was performed on SUM-1(Zr)-c, an improved measurement time (∼50% faster) and disappearance of hysteresis loop (Fig. 3d and Fig. S43, ESI†) were observed, with a BET surface area of 1316 m² g⁻¹ (Table S9, ESI†). As shown in Fig. 3d inset, significant preferred orientation in the PXRD pattern of SUM-1(Zr)-c (for additional PXRD data, see Fig. S40–S42, ESI†) and the fact that SUM-1(Zr)-c exhibits the Tyndall effect when dispersed in methanol, indicate a predominant presence of nanoplates.

As miniaturization helped improve the interlayer slippage situation while maintaining the crystallinity, the chemical and structural stability of SUM-1(Zr) nanocrystals needed to be assessed. After being treated in aqueous solutions of various pH values, SUM-1(Zr)-c samples were activated for N₂ sorption measurements. The results clearly showed that both acid and base treatments led to loss of porosity (Fig. 3e, Fig. S45 and S46, ESI†), while only the H₂O-treated sample exhibited a N₂ uptake comparable to the pristine sample, with a BET surface area of 1328 m² g⁻¹ (Table S10, ESI†). We hypothesize that for the 2D MOF SUM-1(Zr), the chemical stability within layers mainly depends on coordinative factors such as chelation and HSAB. Between layers, however, the non-covalent interactions can be easily disrupted by imported species due to pH adjustments, hence the relative translation of layers and the lowered N₂ uptake due to impeded transport. The nano-sized crystallites would be even more susceptible to this effect. One extreme example is the isotherm of the sample treated in a pH = 1 solution (Fig. 3e and Fig. S45, ESI,† red curve), exhibiting a hysteresis loop typically corresponding to aggregated plate-like particles,^22a indicating delamination of SUM-1(Zr) in this case.

In conclusion, we have constructed 2D kagome MOFs with mononuclear Zr⁴⁺ or Hf⁴⁺ nodes chelated by hydroxamates, namely SUM-1. Theoretical studies confirm that the chelated Zr(hydroxamate)₄ unit, as the MOF's structural basis, is the sole thermodynamically favored product. Moreover, based on the facts that SUM-1 forms sizable crystals in a synthetic time scale of minutes and beats UiO-66 in a competing synthesis, the formation of SUM-1 is also kinetically favorable. Therefore, hydroxamate-chelated Zr⁴⁺ bestows the MOF with chemical stability from both the enthalpic and entropic perspectives. As a 2D MOF, pristine SUM-1(Zr) suffers from loss of permanent porosity due to interlayer translation. When miniaturized, SUM-1(Zr) nanoplates show improved N₂ uptake and transport while retaining the structural integrity and chemical stability. The nano-sized SUM-1(Zr) can be potentially developed for ⁸⁹Zr PET imaging.²⁵

This research was supported by the National Natural Science Foundation of China (22176135, C. L. and 22076130, J. S.). Additionally, this research was supported by the Fundamental Research Funds for the Central Universities in China (No. YJ201976, C. L. and 20826041D4117, J. S.) and start-up funds from the School of Chemical Engineering (C. L.) and College of Chemistry (J. S.), Sichuan University. We would like to thank the Center of Engineering Experimental Teaching, School of Chemical Engineering, Sichuan University for analytical instrumentation and Dr Meng Yang for the collection of SC-XRD data. The authors thank Ms Disi Wang for figure preparation.

Conflicts of interest

The authors declare no competing financial interest.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2121493 and 2121494. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d2cc00213b

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