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Cite this: Dalton Trans., 2024, 53, 6507 Received 19th January 2024, Accepted 11th March 2024 DOI: 10.1039/d4dt00169a

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Construction of zirconium/hafnium-oxo clusters based on a new protection-calix[8]arene†

Fei-Fei Wang *and Baoshan Hou*

Calix[8]arene has been used as a promising type of macrocyclic ligand for the construction of multinuclear metal-oxo clusters (MOCs), but not for zirconium/hafnium-oxo clusters (Zr/HfOCs). In this paper, we report the first series of ZrOCs (HfOCs) based on calix[8]arene: Zr, Zr, Hf, and Hf. Zr /Hf has a rhombohedral conformation and can be regarded as a derivative of the octahedral Zrcluster. Remarkably, I adsorption experiments indicate that Zr(Zr) adsorbs much faster than Hf(Hf). Density functional theory (DFT) calculations show that metallic Zr atoms interact more strongly with Ithan metallic Hf atoms. The successful application of calix[8]arene for the synthesis of well-defined ZrOCs (HfOCs) shows a bright future for MOCs protected by macrocyclic ligands.

Introduction

Metal-oxo clusters (MOCs) composed of metal ions belonging to group IV have attracted extensive research attention in science and industry due to their unique structural stability, reactivity and electronic properties. Therefore, they have a wide range of applications in different fields, such as adsorption, separation and catalysis. Zr and Hf are two common group IV metals, which are chemically similar with the same number of metal coordination sites and prefer 7–9 bonded coordination sites. This usually leads to the isomorphism of zirconium/hafnium-oxo clusters (Zr/HfOCs)

Their most common oxidation state is +4 making their cations strongly Lewis acidic, which facilitates their hydrolysis and formation of stable metal-oxo bonds. However, as ZrOCs and HfOCs prefer to retain their inorganic cores and have little ligand exchange, cluster collapse may often occur, and thus, ZrOCs (HfOCs) with good stability must urgently be developed.

To create stable ZrOCs (HfOCs), it is particularly important to consider protective ligands. Widely recognized candidates in this regard are carboxylic acids, carboxylates and inorganic anions.Compared to ligands with a single coordination site, macrocyclic ligands with multiple pre-organized coordination sites are more suitable for the construction of ZrOCs (HfOCs) due to enhanced coordination through the synergistic coordination of multiple binding groups present in them. As a macrocyclic octamer of phenol, p-tert-butylcalix[8]arene (TBC[8]) is considered important, and it is significantly more flexible than calix[4]arene due to ring inversion and can be an excellent ligand for multinuclear MOCs. Based on the multiple hydroxyl groups of TBC[8], Dalgarno, Liao and other groups have reported a series of MOCs, including Gd, Pr , LnCr, LnCo and Pb . In addition, bulky calixarenes provide a rigid shell to protect the metal core, and the continuous carbon backbone structure can increase the stability of the clusters.However, calixarene-protected ZrOCs (HfOCs) are rare, and the crystal structure of calix[8]arene-based ZrOCs (HfOCs) has not been reported. Although the hydroxyl coordination of TBC[8] facilitates the assembly of ZrOCs (HfOCs), the large backbone of TBC[8] also poses a great challenge for the growth of single crystals, which are extremely important for understanding structural details of the metal core and metal– ligand interface.

Several conformations of TBC[8] have been observed in the transition metal (TM) or lanthanide metal complexes based on TBC[8], including pleated-loop, double-cone, and inverted double-cone.In particular, TBC[8] that adopts the double-cone conformation can form the Ln-TBC[8] subunit upon reaction with Lnions. However, we do not know the conformation in which TBC[8] combines with Zr(Hf ) to

† Electronic supplementary information (ESI) available: Experimental details, structural figures, tables and additional characterization data, including PXRD patterns, IR spectra, TGA curves and UV-vis spectra. CCDC 2322411–2322414. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt00169a

aDepartment of Chemistry, Xinzhou Normal University, Xinzhou, 034000, China. E-mail: wangff718@nenu.edu.cn bKey Laboratory of Precise Synthesis of Functional Molecules of Zhejiang Province, Department of Chemistry, School of Science and Research Center for Industries of the Future, Westlake University, and Westlake Institute for Advanced Study, 600 Dunyu Road, Zhejiang, Hangzhou, 310030, China. E-mail: houbaoshan@westlake.edu.cn

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form the hierarchical structure. The formation of Zr (Hf) clusters in DMF-based solvent mixtures is highly predictable, and it is difficult to ascertain whether the binding of Zr (Hf ) to TBC[8] will be a derivative of Zr(Hf) or a new cluster arrangement will occur. Although the properties of the ligands and metal centers are well understood, the method of precisely determining the assembly of the final clusters formed remains unclear.

Here, the first series of TBC[8]-based ZrOCs (HfOCs) were synthesized, including Zr , Zr, Hf , and Hf (Scheme 1). We can learn from the crystal structure that the coordination mode of TBC[8] with Zr (Hf ) is different from that of TBC[8] with Ln or TM. By comparing the adsorption experiments of Hfand Zron I, we found that there is also a large difference in the adsorption rate of the two isomorphic MOCs on I. Furthermore, the order of binding abilities of metal atoms to I , i.e., Zr > Hf, was confirmed by density functional theory (DFT) calculations.

Results and discussion

Structural description

Single-crystal X-ray diffraction (SCXRD) analysis shows that Zr and Hf are isomorphic and characterized by two TBC[8]encapsulated octanuclear MOC cores. Hf differs from Zrin four-coordinated DMF molecules, which are replaced by N,Ndimethylcarbamate anions, and two CHOH molecules replace two H O molecules (Fig. S1†). Therefore, Zrwas chosen as an example to illustrate the structure. Zris monoclinic and crystallizes in the C2/m space group. As shown in Fig. S2a,† the asymmetric unit of Zrincludes three crystallographically independent Zr sites (Zr1–Zr3), half a TBC[8] ligand, a DMF, half a μ-O atom, 1.5 μ-O atoms, and half a μ-O atom

The bond valence sum (BVS) calculations indicate that all Zr atoms are tetravalent. The metal skeleton of Zr can be viewed as an octahedron with two caps. Two Zr2 occupy the top position of the octahedron, and four Zr1 occupy the square plane of the octahedron, while two Zr3 cover each of the two Zr1–Zr2–Zr1 triangular faces. μ-O connects Zr2 and Zr3, and the six exposed faces of the octahedron are covered by μ-O, while μ-O fills the central cavity of two hat-like tetrahedra, forming a centrosymmetric {ZrO } core (Fig. S2b†). The distance between Zr– Zr ranges from 3.436 to 3.654 Å, while the distance between Zr–O ranges from 2.028 to 2.365 Å. Zrcan also consist of two Zrunits, where Zris composed of two Zr1 and one each of Zr2 and Zr3. Although all three types of Zr are seven-coordinated, the coordination configurations are quite different. Zr1 and Zr2 are in a single-cap octahedral configuration, while Zr3 is in a pentagonal bipyramidal configuration. Two TBC[8] aromatic cavities capture two Zrunits in the same way, respectively (Fig. 1a). Describing one of them as an example, TBC[8] has eight phenolic hydroxyl groups, six of which are coordinated to Zr atoms in a monodentate mode and two of which bridge two Zr atoms each (two O4). The conformation adopted by the TBC[8] ligand can be described as a “halfway house” between a double cone and a pleated loop, where the four phenolic hydroxyl groups of the ligand are partially coneoriented (two O6 and two O9), while the other four are regular pleated loops (two O4 and two O10). The remaining metal coordination sites are in turn filled with DMF and water molecules, where four coordinated DMF molecules are inserted between two TBC[8] for coordination with Zr1, while oxygen atoms in HO are coordinated with two Zr3 (Fig. 1b and S3†).

SCXRD analysis shows that Zr and Hf are isomorphic and crystallize in the C2/m space group of the monoclinic crystal system. The following structural analysis takes Zras an example. Zris cationic, and the charge is balanced by Cl anions. In the asymmetric unit, there are two crystallographically independent metal centers comprising Zr (Zr1 and Zr2),

Scheme 1 Preparation process of Zr , Zr , Hf , and Hf .

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half of a TBC[8] ligand, one μ-OCH , 0.5 μ-O CH and 0.25 Clanion (Fig. S4a†). The bond valence sum (BVS) calculations indicate that all Zr atoms are tetravalent. Zr1 and Zr2 are sixcoordinated in an octahedral geometric configuration (Fig. S4b†). The coordination sphere of octahedral Zr1 is occupied by six oxygen atoms: three TBC[8] phenolic oxygen atoms (O3 and two O1), one O5 from the formate anion and two μmethoxy (O2). Similarly, Zr2 is coordinated by six oxygen atoms: three TBC [8] phenolic oxygen atoms (O6 and two O4), one O5 from the formate anion, and two μ-methoxy (O2). The distances of six Zr1–O and Zr2–O bonds are between 1.966 Å– 2.2 Å and 1.952 Å–2.236 Å, respectively.

We can describe this structure from two different perspectives

(i) Zr1 and Zr2 centers are covalently linked by two O2 from methoxy to form a central Zr(μ-O)four-membered ring (Fig. 2a). The formate anion (from the decarboxylation reaction of the DMF solvent under solvothermal reaction conditions) is then bridged between these two Zr atoms in the form μ–η:η. TBC[8] phenolic oxygen atoms coordinated to Zr1 and Zr2 from the central four-ring Zr (μ-O)come from two TBC[8], while the cavity of one TBC[8] also captures two Zr atoms from different Zr(μ-O)(also called Zr1 and Zr2). The structure consisting of two TBC[8] and two Zr(μ-O)fourmembered ring is Zr. (ii) In addition to two oxygen atoms (O8) in the middle of TBC[8], three phenols at their lower edges on each side adopt a 3/4-cone conformation binding a Zr atom, thus forming a Zr-TBC[8] subunit. Zr-TBC[8] SBU is copied and inverted to obtain Zr-TBC[8]. Regardless of the angle, Zr-TBC[8] is completely centrosymmetric (Fig. 2b and S5†).

Fig. 1 Molecular structure of Zr . (a) Top view of Zr building block and all Zr sites are in a seven-coordinated environment. (b) Ball-and-stick representation of the crystal structure of Zr . Color code: Zr, blue; O, red. C on two TBCs[8] are indicated by dark blue and purple, respectively. The H atom is omitted for clarity.

Fig. 2 Molecular structure of Zr . (a) A Zr sub-unit that forms the core of the Zr cluster. (b) Ball-and-stick representation of the crystal structure of Zr . Color code: Zr, blue; O, red. C on the TBCs[8] are indicated by dark blue and purple, respectively. The H atom is omitted for clarity.

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Crystallographic data tables and basic characterization of the four compounds are listed in Tables S1–S6 and Fig. S7–S18.† The stability of the clusters wrapped by TBC[8] was explored using Zrand Zras examples. Their crystallinity remained when they were placed in air for several months or in a solution with pH = 0.3/14 or different solvents for 24 h (Fig. S19–S22†).

Iodine capture

ZrOCs (HfOCs) have received very little attention in terms of potential applications for direct iodine (I ) capture. Researchers studying iodine capture by porous materials generally consider the effect of special functional groups, such as nitrogen-containing groups, benzene rings and triple bonds on iodine, but they rarely consider the effect of different metal sites on iodine interactions. Therefore, isomorphic Zr and Hfwere used as an example to study the effect of different metal centers on iodine adsorption. When the crystal sample (30 mg) was immersed in 5 mL of 2 mmol Liodine in cyclohexane solution, the crystals turned from white to tan, and the solution gradually changed from dark purple to light purple. The fading rate of the iodine solution with Zr was faster than that with Hf at the same time. To further investigate the kinetics of iodine absorption by these two crystals, the changes in the intensity of characteristic absorption peaks of iodine in solution were monitored by UV–Vis spectroscopy (Fig. 3a and S23†). As shown in Fig. 3b

the absorbance of I decreases with longer durations; the relative amount of I removed for Zrand Hf are 91.7% and 67.0% after 36 h, respectively. Further, the uptake rate of Ican be ranked as Zr > Hf , which is consistent with the trend of change in the solution color (Fig. 3c). The adsorption kinetics of Zrand Hf were investigated by the relationship between time and absorbance monitored at 523 nm. As shown in Fig. 3d, ln(A − A) was linear as a function of time at 523 nm, which can be attributed to a pseudo primary kinetic process. The reaction rate constant kvalues were 0.118 hand 0.083 h, respectively, which better illustrates the phenomenon that the adsorption rate of Zr is greater than that of Hf .

In addition, solid UV spectra illustrate the successful adsorption of iodine, and PXRD and IR spectra confirm the structural integrity of Zrand Hfbefore and after iodine adsorption (Fig. S24–29†). To determine the forms of adsorbed iodine species, the X-ray photoelectron spectroscopy (XPS) of Zrand Hf after Iadsorption was performed. As illustrated in Fig. 4, I 3d XPS of iodine-captured materials can be deconvoluted into two pairs of doublets: (1) centered at 618 eV for I 3dand 631 eV for I 3d, which were assigned to I; (2) centered at 620 eV for I 3d and 630 eV for I 3d, which were ascribed to I.The relative area of Ispecies indicated that Iwas dominant, and the adsorbed iodine species were mainly present in Iforms.

Iodine release

The iodine release process of I @Zrand I @Hfin acetonitrile was studied (Fig. 5a and Fig. S30†). When 5 mg of

Fig. 3 (a) Temporal evolution of UV–Vis absorption spectra of Zr captured by Iin an iodine/cyclohexane solution (2 × 10mol L). (b) Plots of time vs. absorbance for Zr and Hf were monitored at 523 nm of iodine at selected time intervals. (c) Photographs of the color change of cyclohexane when Zr and Hf crystals were immersed in cyclohexane solution of iodine, and the change of Zr crystals before and after iodine absorption. (d) Kinetic analysis of Zr and Hf monitored at 523 nm of iodine adsorption at different time intervals.

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Dalton Transactions

| Dalton Trans., 2024, 53, 6507–6514

This journal is © The Royal Society of Chemistry 2024

Published on 21 March 2024. Downloaded by Southwest Petroleum University on 5/23/2024 3:49:05 AM.

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