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Kazutaka Sonobe, [Satoshi Tominaka](https://orcid.org/0000-0001-6474-8665), Akihiko Machida

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[Experimental Elucidation of a Cubane Water Cluster in the Hydrophobic Cavity of UiO‐66](https://mdr.nims.go.jp/datasets/594a5646-b8f9-4447-a6eb-5f1f0b02b59b)

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Experimental Elucidation of a Cubane Water Cluster in the Hydrophobic Cavity of UiO‐66Experimental Elucidation of a Cubane Water Cluster in theHydrophobic Cavity of UiO-66Kazutaka Sonobe,[a] Satoshi Tominaka,*[a] and Akihiko Machida[b]Nanoscale water plays a pivotal role in determining the proper-ties and functionalities of materials, and the precise control ofits quantity and atomic-scale ordered structure is a focal pointin nanotechnology and chemistry. Several studies have theoret-ically discussed the nano-ordered ice within one- or two-dimensional space and without confinement through hydrogenbonds. In particular, the water cluster has been predicted toplay a significant role in biomolecules or functional nano-materials; however, there has been little experimental evidencefor their presence in hydrophobic cavities. In this study, thecubane water octamer – the most stable isomer among smallwater clusters – was detected within the hydrophobic cavitiesof UiO-66 metal–organic frameworks, revealing the presence ofthe smallest ice in their hydrophobic cavity, in the absence ofhydrogen bonding. This observation contrasts earlier examplesof water clusters confined within nanocavities through hydro-gen bonds and provides experimental evidence for water-cluster capturing within hydrophobic cavities. Consequently,our renewed understanding of hydrophilicity and hydrophobic-ity warrants a design re-evaluation of materials for chemicalapplications, including fuel cells, water harvesting, catalysts,and batteries.IntroductionNanotechnology has reached a pivotal stage where the precisecontrol over the presence and transport of water beyond theimplicit chemistry that governs processes such as proteinfolding[1–3] and low-dimensional fluid dynamics is essential.[4,5]For example, although the role of water in facilitating protontransport is critical for catalysis in fuel cells,[6,7] water can alsoobstruct the pathways necessary for oxygen and hydrogen toreach the catalysts. In addition, the presence of a few watermolecules in nanochannels can significantly accelerate thermalcatalytic reactions[8] and molecular transportation.[9,10] Todesign materials with reliable and reproducible properties, thepresence and behavior of water must be meticulouslycontrolled at the atomic level. Crucially, in material design,hydrophilicity and hydrophobicity are often considered as awater-attracting or -repelling behavior on a macroscopicinterface; however, this is not true at the nanoscale. Forinstance, one- or two-dimensional ice formations in hydro-phobic nanospaces, such as graphene[11,12] or carbon nanotubecavities,[10,13,14] are considered exceptional cases (Figure 1).However, the experimental evidence of water clusters inhydrophobic channels or more typically narrow cavities hasnot been revealed in atomic resolutions.From a classical thermodynamic perspective, water mole-cules in bulk liquid water benefit from the stabilizing effects ofstrong hydrogen bonding (around 0.46 eV per molecule)[15]and the high entropy of the liquid state. Therefore, to confinea water molecule within a cavity, a stronger attractive force isrequired to overcome these stabilizing factors. Traditionalviews suggest that van der Waals forces and other non-bonding interactions are simply too weak to achieve thisconfinement in nanoscale cavities.[10–14] In such cases, thepresence of water within hydrophobic cavities might beexplained by the formation of clusters that lack the extensivehydrogen bonding network observed in bulk liquid water.Unlike bulk water, where hydrogen bonds create extensivenetworks, water clusters are stabilized by closed-chain hydro-gen-bonding configurations,[16] enabling them to act inde-pendently of external materials and similarly to macromole-cules. Larger clusters, such as water octamers, have sufficientinternal hydrogen bonds that make them as stable as bulkwater. This is evidenced by the detection of water clusters[a] K. Sonobe, S. TominakaCenter for Basic Research on Materials, National Institute for MaterialsScience, Tsukuba, Ibaraki 305–0044, JapanE-mail: tominaka.satoshi@nims.go.jp[b] A. MachidaSynchrotron Radiation Research Center, National Institutes for QuantumScience and Technology (QST), SPring-8, Sayo, Hyogo 679-5148, JapanSupporting information for this article is available on the WWW underhttps://doi.org/10.1002/cphc.202400583© 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbH. This isan open access article under the terms of the Creative Commons AttributionLicense, which permits use, distribution and reproduction in any medium,provided the original work is properly cited.Figure 1. Structure of a low-dimensional ordered water isomer. (a) 0D water-cluster structures predicted by ab initio calculations. (b, c) Reported 1D and2D structures in the hydrophobic spaces of carbon nanotubes andgraphene.Wiley VCH Montag, 21.10.20242499 / 373627 [S. 1/7] 1ChemPhysChem 2024, e202400583 (1 of 6) © 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbHChemPhysChemwww.chemphyschem.orgResearch Articledoi.org/10.1002/cphc.202400583http://orcid.org/0000-0001-6474-8665https://doi.org/10.1002/cphc.202400583within cavities of organic molecules[17–21] and proteins;[22–24]these are often anchored to adjacent functional groups byhydrogen bonds containing heteroatoms such as oxygen andnitrogen, making the cavities partially hydrophilic. However,the observation of water clusters not stabilized by hydrogenbonding in hydrophobic environments is rare.In this study, we present the first experimental observationof an ordered water octamer within the hydrophobic nano-cavity of a UiO-66 metal-organic framework (MOF) at roomtemperature. Previous literature on confined water clusters inMOFs[18,25] and organic porous materials[19,20] has shown stronginteractions via hydrogen bonds with hydrophilic heteroa-toms. On the contrary, our findings suggest that water clusterscan even be stabilized by weak interactions, thus eliminatingthe need for strong hydrogen bonds with the framework. Thisdiscovery challenges macroscopic intuition for hydrophobicityand has significant implications for material design in variouschemical applications.To analyze water molecules within nanopores in detail, weemployed the host–guest interactions offered by MOFs. UiO-66 is a well-studied MOF system composed of zirconium oxideclusters (Zr6O4(OH)4) linked by terephthalate ligands.[26] Thisstructure creates an MOF that remains stable upon hydrationand boasts a high surface area with both hydrophobic andhydrophilic environments, making it ideal for investigatingwater behavior in nanoscale spaces. Unlike many other MOFsthat degrade upon hydration, UiO-66 is renowned for itsrobustness.[27–29] Although host–guest interactions – includingwater-framework interactions within UiO-66 – have beenpreviously reported[30] and are common with MOFs,[31,32] thereremains scope for further exploration of water moleculeswithin this framework. This is because the X-ray structuralanalyses of MOFs typically involve removing guest moleculesas much as possible to elucidate the structure of the hostframework. In contrast, our focus is on the structure and stateof the guest water molecules, similar to the crystalline spongemethod to focus guest material,[33,34] particularly those residingwithin the hydrophobic pores.Results and DiscussionUiO-66 samples were synthesized following established proce-dures described in the literature.[27,35] The quality of thesamples was confirmed using standard characterization tech-niques, such as powder diffraction. Full details on the synthesisand characterization can be found in the supporting informa-tion. We prepared UiO-66 powders with varying degrees ofhydration. Briefly, the synthesized crystalline powders wereactivated by vacuum treatment (Figure S1) to remove guestmolecules from the pores. Subsequently, the samples wereexposed to ambient atmosphere for water uptake (hydratedstate), partially dehydrated at 100 °C for 12 h, and fullydehydrated at 100 °C for 3 d. Thermogravimetric analysis (TGA)was employed to determine the framework composition,including linker molecule deficiencies, for the hydratedsample. The analysis revealed the formula[Zr6O4OH4(bdc)5.28(ac)1.44] (bdc=C6H4(COO)2, ac=CH3COO),which is consistent with the presence of residual acetateligands at linker-deficient sites based on the TGA results. TheTGA also identified two distinct types of water moleculeswithin the framework: weakly adsorbed water moleculesdesorbed at up to approximately 63 °C (8.77 molecules perframework unit) and strongly adsorbed water moleculesdesorbed at up to approximately 144 °C (4.0 molecules perframework unit).Experimentally observing water clusters within nanoporespresents a challenge – guest water molecules can eitheroccupy well-defined, crystallographically symmetric sites, iden-tifiable through X-ray crystallography, or adopt a moredisordered, amorphous arrangement. To account for bothscenarios in our investigation, we employed high-energysynchrotron X-ray scattering, which offers two key advantages– it provides reciprocal space data suitable for crystallographicanalysis (Figure 2a); and, real-space data through Fouriertransformation (Figure 2b) for a comprehensive understandingof the structure, including the distribution of amorphous watermolecules.[36] Furthermore, using the high-angle region of thereciprocal space data provides higher resolution in real-spaceanalysis. However, a trade-off exists – high-energy X-rays offerlower resolution in reciprocal space, hindering phase determi-nation. Therefore, we characterized the synthesized UiO-66using powder diffraction with Cu Kα radiation prior to thesynchrotron experiments (details in the supporting informa-tion). As shown in Figure S2, the hydration levels significantlyimpact the lattice vibrations or symmetry. These structuralchanges induced by hydration are discussed in the followingparagraphs.Rietveld analysis was used to characterize the fullydehydrated UiO-66 as a cubic phase (space group: F43 m, a=20.75497(14) Å, weighted R factor (Rwp)=2.33%) (Figure S4)exhibiting lower symmetry than typically reported (Fm3m) forpristine samples, which can be attributed to polyhedraldistortion and proton localization within the Zr6O8H4 clusters(Oh-to-Th point group changes).[37,38] The UiO-66 structurecomprises two distinct pore types, octahedral and tetrahedralnanocages, and we focused on the latter. These smallercavities, which host water molecules, were further categorizedFigure 2. Insertion of ordered water into hydrophobic spaces. (a) X-raydiffraction patterns of UiO-66 during dehydration. The differences in thediffraction intensity, which reveal the effect of the water content, areindicated with asterisks. (b) Pair distribution functions of the same data,revealing the ordered structure of water within the nanospaces of UiO-66.Wiley VCH Montag, 21.10.20242499 / 373627 [S. 2/7] 1ChemPhysChem 2024, e202400583 (2 of 6) © 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbHChemPhysChemResearch Articledoi.org/10.1002/cphc.202400583 14397641, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cphc.202400583 by National Institute For, Wiley Online Library on [28/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://chemistry-europe.onlinelibrary.wiley.com/action/rightsLink?doi=10.1002%2Fcphc.202400583&mode=based on the presence of OH groups within the Zr6O4(OH)4units.Interestingly, the hydration caused subtle changes in peakintensities, hinting at water molecules entering the dehy-drated crystal structure. The hydrated sample that wasprepared under 60% RH humidity level at 20 °C, where thewater adsorption saturates according to reported vapor waterisotherms,[30] was compared with a dried sample. The structuraldifference was reflected in the XRD patterns, particularly atlower angles (Figure 2a). Notably, these changes were not dueto framework decomposition, as confirmed by the PDFs(Figures 2b and S3). This suggests the formation of an orderedwater structure within the UiO-66 nanoscale pores.Subsequent water sorption led to noticeable changes inpeak intensities, and Rietveld refinement identified thehydrated UiO-66 as an orthorhombic structure (space group:Imm2, a=14.6729(2) Å, b=14.6729(2) Å, c=20.7506(3) Å,Rwp=3.82%) (Figures 3a and b). In addition to the four watermolecules in the OH-equipped tetrahedral pores, eight moremolecules were found within the remaining tetrahedral nano-cages, reducing the overall crystal symmetry. Similarly, theamount of water was the same as that of the weakly adsorbedwater measured by thermogravimetry (Figure S6). This distri-bution of water molecules is consistent with the reportedstepwise dehydration behavior of UiO-66.[39,40] The oxygen-to-oxygen distances, which were larger than 3.9 Å, eliminate thepossibility of hydrogen bonding with the zirconia cluster, thusindicating other stabilization mechanisms for these molecules.Compared with the known water clusters bound to the MOFnanocavities lining via hydrogen bonds,[20,25] this octamer lackssuch strong hydrogen bonds because of the absence ofnearby heteroatoms suitable for bonding, as observed in thenanospaces of nanocarbons (Figure S9).Within the tetrahedral cages, the eight water moleculesassemble into isolated octamers, forming a cubane structurestabilized by three hydrogen bonds (Figure 3c) for a watermolecule, which is comparable to the mean number ofhydrogen bonds of liquid water (2.2–3.4 bonds).[41] Vibrationrotation tunneling (VRT) spectroscopy study have reportedseveral isomers of this water octamer,[42] and the one with D2dsymmetry was the equilibrium configuration for the cubanecluster. Further, our findings revealed that the octamer withinUiO-66 undergoes a distortion (point group: Cs), breaking theimproper rotation symmetry (S4) in the D2d point group. Thisdistortion, coupled with the positioning of water oxygenatoms at symmetry locations within the framework, suggestsinteractions with the UiO-66 interior surface. The cubaneoctamer is recognized as the most stable water clustercomposed of fewer than ten water molecules, and itsstructural variations arising from the changes in protonconfigurations and hydrogen bonds, which result in multiplestable structures, have been well documented.[43–45]The adsorption energies of water on hydrophilic andhydrophobic cavities in several hydration states were com-pared (Figure 4). In addition to the hydrated model discoveredin this work, the partially hydrated model (Figure 4b), whichhas been revealed in a previous work, was observed. Thepartially hydrated sample maintained a similar cubic crystalstructure (space group: F43 m, a=20.75664(17) Å, Rwp=2.59%)(Figure S5) but contained four H2O molecules[30,36,46] in theFigure 3. Determination of the crystal structure of the water octamer withinUiO-66. (a) Rietveld fitting of the observed X-ray diffraction pattern forhydrated UiO-66. The inset shows fitting at higher angles. (b) Derived crystalstructural model, highlighting the hydrophobic cage in yellow. (c) Stablestructure of the water octamer within hydrophobic cages (red) and thatoutside cage (calculation in vacuum) (blue). Here, the hydrogen atoms wereadded based on the most stable positions of the cubane octamer. Notably,the S4 symmetry observed for the water octamer in a vacuum is disruptedwithin the hydrophobic cage.Figure 4. Crystal structure models (top: ab plane, bottom: bc plane)illustrating the stepwise desorption of water molecules from UiO-66. Thestructures represent the (a) hydrated, (b) partially hydrated, and (c)dehydrated states. Hydrophobic and hydrophilic cages are depicted in blueand pink, respectively, indicating the presence of water molecules. Emptycages are shown in gray. Quantum chemical simulations predicted thatweakly adsorbed water molecules, identified by X-ray analysis within thehydrophobic cages, possess a lower adsorption energy of approximately0.21 eV. In contrast, the stably adsorbed water molecules found in hydro-philic cages exhibit a higher adsorption energy of approximately 0.53 eV.Wiley VCH Montag, 21.10.20242499 / 373627 [S. 3/7] 1ChemPhysChem 2024, e202400583 (3 of 6) © 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbHChemPhysChemResearch Articledoi.org/10.1002/cphc.202400583 14397641, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cphc.202400583 by National Institute For, Wiley Online Library on [28/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://chemistry-europe.onlinelibrary.wiley.com/action/rightsLink?doi=10.1002%2Fcphc.202400583&mode=tetrahedral pore with OH groups. The amount of water wasthe same as that of the strongly adsorbed water measured bythermogravimetry (Figure S6). The distance between the oxy-gen atoms of water and the zirconia cluster was 2.8 Å, which istypical[47] for hydrogen-bonded oxygen-to-oxygen distances.Thus, the UiO-66 can take at least three different hydrationstates.The structural models for the three adsorption states werecorroborated by density functional theory (DFT) calculations(Figures 4 and S7), which confirmed the structures energeticfavorability, and were validated through linear-scaling DFTwith the VdW-DF-optB88 functional. This functional adeptlycapture the non-bonded dispersion interactions between thewater octamer and UiO-66. Before performing calculations onthe UiO-66 model, the cubane structure with a regularoctahedral shape was studied using various theoreticalmethods, including DFT (ωB97X-D3), the random phaseapproximation (RPA), and spin-component-scaled second-order Møller–Plesset perturbation theory (SCS-MP2). Theresults were consistent across these different theoreticalmethods, revealing that the observed distortions were intrinsicto the framework and not computational artifacts (Figure S8).The weak interactions between the water octamer andUiO-66 framework could be observed through reducedelectron density maps obtained from non-covalent interaction(NCI) analyses (Figure 5).[48] These analyses confirmed thehydrogen bonds between the four water molecules (asmentioned earlier) and the OH groups of the zirconia cluster(Figure 4a), contributing approximately 0.53 eV per moleculeto the desorption energy (Figure 4b). The presence of theseinteractions was further supported by peaks in the reduceddensity gradient plots (Figure 5e). Concerning the wateroctamer, the interaction zone extended across the entiretetrahedral cavity wall, indicating that van der Waals forces arethe primary driver of adsorption. Furthermore, desorptionenergy calculations estimated that desorbing the octamer as awhole requires approximately 0.21 eV per molecule, whereasdesorbing individual water molecules requires 0.63 eV permolecule (Figure 4a). Notably, considering the evaporationenergy of liquid water (0.46 eV), the formed octamer withinthe cavity might exhibit comparable stability (although theaccuracy of simulations may not perfectly match measuredheat values). Additionally, NCI analysis suggested a possiblesecondary role played by weak OH� π interactions with thephenylene rings of UiO-66, similar to the interactions observedin cubane-benzene clusters.[49]Moreover, these findings align with the thermogravimetricresults (Figure S6), which demonstrated that the octamerevaporates at a lower temperature than liquid water (<63 °C).[50] The TGA analysis reveals a distinct, sharp step for thedesorption of cubane water, preceding the gradual desorptionof hydrogen-bonded water. It’s worth noting that waterdesorption from the grain surface often appears as a gradualloss below 100 °C. Consequently, the calculated weakeradsorption energy of water octamers is consistent with theexperimental TGA data. Both theoretical calculations andexperimental results thus support the evidence for thepresence of water octamers within the cavities.ConclusionsThe crystallographic analysis in this study revealed the firststable water octamer within a hydrophobic nanocavity thatwas not anchored by strong hydrogen bonds. This discoveryhighlighted the water cluster behavior, different from themacroscopic view of hydrophobicity, as a repelling behavior;moreover, the typical synthetic view emphasized the necessityof a polar functional group for water to capture water. Further,we demonstrated that water molecules can infiltrate extremelynarrow hydrophobic nanospaces, even when larger poresremain unoccupied, without interacting with heteroatoms.These findings emphasize the significant role of clusterchemistry, indicating that the fundamental unit influencingthe chemical properties (even for well-known molecules suchas water) may extend beyond the individual molecule. Thisinsight paves the way for an advanced nanospace design,thereby enabling the manipulation of water molecules at thenano- and molecular scales and fostering innovation in fieldsFigure 5. NCI analysis of water adsorption on UiO-66. (a, b) Van der Waals(VdW) interaction surfaces between the water octamer and nanocage areshown in green. (c, d) Hydrogen bond interaction surfaces between waterand the OH groups of Zr6O8H4 are shown in blue and red. (e) Reduceddensity gradient NCI plot corresponding to the interactions in panels “a” to“d”, where the differences in interaction strength at each coordination incrystal is shown as the difference between value of sign(λ2) ρ.Wiley VCH Montag, 21.10.20242499 / 373627 [S. 4/7] 1ChemPhysChem 2024, e202400583 (4 of 6) © 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbHChemPhysChemResearch Articledoi.org/10.1002/cphc.202400583 14397641, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cphc.202400583 by National Institute For, Wiley Online Library on [28/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://chemistry-europe.onlinelibrary.wiley.com/action/rightsLink?doi=10.1002%2Fcphc.202400583&mode=such as fuel cells, water harvesting, catalysts, and batteries bydispelling misconceptions about hydrophobicity.Supporting Information SummaryThe authors have cited additional references within theSupporting Information (Ref. [30,31]).AcknowledgementsThis study was supported in part by JSPS KAKENHI (GrantNumber 22K14712) and by NEDO JPNP20003. We thank Y.Yoshida (NIMS) for the experimental support and Dr. K.Kawakami (NIMS) for the TGA experiments. Synchrotronradiation experiments at BL22XU of SPring-8 were performedwith approval from the Japan Synchrotron Radiation ResearchInstitute (JASRI) (Proposal No. 2022A3751).Conflict of InterestsThe authors declare no conflict of interest.Data Availability StatementWe will deposit the data in MRD (a public database in ourinstitute, NIMS) when this paper got accepted.Keywords: Cubane water cluster · Hydrogen bonds ·Hydrophobic nanocage · Metal-organic frameworks ·Nanotechnology[1] Y. Levy, J. N. Onuchic, Proc. Nat. Acad. Sci. 2004, 101, 3325–3326.[2] B. W. Matthews, L. Liu, Protein Sci. 2009, 18, 494–502.[3] S. Hong, D. Kim, Proteins Struct. Funct. Bioinf. 2016, 84, 43–51.[4] L. V. Mirantsev, Phys. Rev. E 2019, 100, 023106.[5] M. Trushin, A. Carvalho, A. H. Castro Neto, Commun. Phys. 2023, 6, 162.[6] A. Forner-Cuenca, J. Biesdorf, L. Gubler, P. M. Kristiansen, T. J. Schmidt,P. Boillat, Adv. Mater. 2015, 27, 6317–6322.[7] K. D. Kreuer, S. J. Paddison, E. Spohr, M. Schuster, Chem. 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ChemPhysChem published by Wiley-VCH GmbHChemPhysChemResearch Articledoi.org/10.1002/cphc.202400583 14397641, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cphc.202400583 by National Institute For, Wiley Online Library on [28/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://doi.org/10.1016/j.cpc.2013.10.026https://doi.org/10.1016/j.cpc.2013.10.026https://doi.org/10.1021/ja100936whttps://doi.org/10.1016/j.cpc.2013.10.026https://doi.org/10.1016/j.cpc.2013.10.026https://doi.org/10.1021/cm1022882https://chemistry-europe.onlinelibrary.wiley.com/action/rightsLink?doi=10.1002%2Fcphc.202400583&mode=RESEARCH ARTICLEThis work highlights two intuitivelydifferent hydrophobicity betweenmacroscopic and nanoscale. The ad-sorption of water cluster on hydro-phobic nanocage without linked tohost materials by hydrogen bondswas elucidated experimentally,although hydrophobicity is regardedas repellent behavior in macroscale.This nanoscale water behaviorproposes necessary of reconsiderationfor design of hydrophobicity and hy-drophilicity of functional materials.K. Sonobe, S. Tominaka*, A. Machida1 – 7Experimental Elucidation of aCubane Water Cluster in the Hydro-phobic Cavity of UiO-66Wiley VCH Montag, 21.10.20242499 / 373627 [S. 7/7] 1 14397641, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cphc.202400583 by National Institute For, Wiley Online Library on [28/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://chemistry-europe.onlinelibrary.wiley.com/action/rightsLink?doi=10.1002%2Fcphc.202400583&mode= Experimental Elucidation of a Cubane Water Cluster in the Hydrophobic Cavity of UiO-66 Introduction Results and Discussion Conclusions Supporting Information Summary Acknowledgements Conflict of Interests Data Availability Statement