Consecutive Single-Crystal-to-Single-Crystal Isomerization of Novel Octamolybdate Anions within a Microporous Hybrid Framework with Robust Water Sorption Properties

Water Sorption Hot Paper Consecutive Single-Crystal-to-Single-Crystal Isomerization of Novel Octamolybdate Anions within a Microporous Hybrid Framework with Robust Water Sorption Properties Estibaliz Ruiz-Bilbao, Amaia Iturrospe, Santiago Reinoso, Beñat Artetxe,* Garikoitz Beobide, Leire San Felices, Luis Lezama, Juan M. Gutiérrez-Zorrilla, Shaza Darwish, Debobroto Sensharma, and Michael J. Zaworotko Angewandte Chemie Research Articles www.angewandte.org How to cite: Angew. Chem. Int. Ed. 2023,62, e202307436 doi.org/10.1002/anie.202307436 Angew. Chem. Int. Ed. 2023,62, e202307436 (1 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Chemie Abstract: The 3D hybrid framework [{Cu(cyclam)}3(kMo8O27)]·14H2O (1) (cyclam=1,4,8,11-tetraazacyclotetradecane) undergoes sequential single-crystal-to-singlecrystal transformations upon heating to afford two different anhydrous phases (2a and 3a). These transitions modify the framework dimensionality and enable the isomerization of k-octamolybdate (k-Mo8) anions into λ(2a) and μ (3a) forms through metal migration. Hydration of 3a involves condensation of one water molecule to the cluster to afford the γ-Mo8isomer in 4, which dehydrates back into 3a through the 6a intermediate. In contrast, 2a reversibly hydrates to form 5, exhibiting the same Mo8cluster as that of 1. It is remarkable that three of the Mo8clusters (k,λand μ) are new and that up to three different microporous phases can be isolated from 1(2a,3a, and 6a). Water vapor sorption analyses show high recyclability and the highest uptake values for POM-based systems. The isotherms display an abrupt step at low humidity level desirable for humidity control devices or water harvesting in drylands. Introduction The development of porous materials with capacity to adsorb gases and vapors represents a potential solution to current global challenges such as atmospheric pollution[1] and scarcity of freshwater.[2] Indeed, atmospheric water harvesting not only has the potential to address the global water shortage crisis, but also provides opportunities to fabricate practical devices for water purification,[3] dehumidification,[4] or indoor humidity control.[5] In this sense, Metal-Organic Frameworks (MOFs) are highly promising materials for gas sorption and separation,[6–7] because their modular constitution allows the fine-tuning of pore size and chemistry. However, water sorption could be challenging due to the limited hydrolytic stability displayed by some MOF families.[8–9] Incorporation of rigid, inorganic building-blocks that expose electronegative atoms toward the framework pores might improve the stability and selectivity toward polar adsorbates like water.[10] In the past few years, polyoxometalate (POM)-based porous crystalline solids[11–12] have attracted great attention due to the possibility of combining the intrinsic multifunctionality of these clusters (e.g. catalysis, magnetism, or luminescence)[13] with the properties of extended open-framework architectures. Insoluble ionic crystals in which discrete metal-organic macrocations are combined with Keggin-type heteroPOMs [XW12O40]n(X=CoII, BIII, SiIV, PV)[14] have exhibited selective water sorption properties,[15–16] because the non-efficient packing of such macro-ionic entities renders meaningful porosity to the structure. Another strategy involves the assembly of POMs with transition metal complexes acting as linkers. Some of us recently studied the use of CuII complexes of macrocyclic polyamines in which equatorial positions on the metal center are blocked by the multidentate ligand, leaving axial positions available for linking contiguous POM units. This approach led to different multifunctional materials, including the robust supramolecular framework [Cu(cyclam)][{Cu(cyclam)}2- (V10O28)]·10H2O (cyclam=1,4,8,11-tetraazacyclotetradecane), which selectively adsorbs CO2over N2and catalyzes the heterogeneous oxidation of adamantane,[17] as well as the [{Cu(cyclam)}3(W7O24)]·15.5H2O covalent hybrid with gas sorption capacity and the ability to undergo thermallytriggered single-crystal-to-single-crystal (SCSC) transformations.[18] SCSC transformations allow for monitoring and gaining insight into how the location of atoms and molecules varies during a solid-state phase transition. The structural changes induced by external stimuli may modify key properties of the parent structure (e.g. magnetic, optical, or adsorptive) and lead to applications in sensing, selective storage, and molecular switches, among others. Furthermore, SCSC transformations can afford compounds showing peculiar architectures different from those obtained by conventional synthetic routes. Although this property is difficult to achieve without the material losing crystallinity, tens of examples of SCSC transformations have been reported to date for coordination networks.[19] In comparison, examples are scarce for POMbased systems,[20] despite the fact that the largest change in a cell volume (ca. 2170 Å3) observed for such kind of transformation corresponds to a solvent-mediated transition of the Li9K7WCo10[H2P8W48O186] salt.[21] The relative arrangement of POM units or counterions is usually modified during these [*] E. Ruiz-Bilbao, Dr. B. Artetxe, Dr. G. Beobide, Prof. L. Lezama, Prof. J. M. Gutiérrez-Zorrilla Departamento de Química Orgánica e Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU P.O. Box 644, 48080 Bilbao (Spain) E-mail: [email protected] Dr. A. Iturrospe Centro de Física de Materiales (CSIC, UPV/EHU) Paseo Manuel Lardizabal 5, 20018 Donostia-San Sebastián (Spain) Dr. S. Reinoso Departamento de Ciencias & Institute for Advanced Materials and Mathematics (InaMat2), Universidad Pública de Navarra (UPNA) Campus de Arrosadia, 31006 Pamplona (Spain) Dr. G. Beobide, Prof. J. M. Gutiérrez-Zorrilla BCMaterials Edificio Martina Casiano, 3rd Floor, UPV/EHU Science Park, Barrio Sarriena s/n, 48940 Leioa (Spain) Dr. L. San Felices Servicios Generales de Investigación SGIker, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU P.O. Box 644, 48080 Bilbao (Spain) Dr. S. Darwish, Dr. D. Sensharma, Prof. M. J. Zaworotko Bernal Institute, Department of Chemical Sciences, University of Limerick Limerick V94 T9PX (Ireland) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is noncommercial and no modifications or adaptations are made. Angewandte Chemie Research Articles Angew. Chem. Int. Ed. 2023,62, e202307436 (2 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 42, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202307436 by Universidad Del Pais Vasco, Wiley Online Library on [15/12/2023]. 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 License solid state reactions,[22] but no major skeletal modification of the metal-oxo cluster has been observed to date with the exception of i) an alkylammonium salt of [γ-SiV2W10O39]4, which transforms its linear (μ-oxo)-divanadium(V) core into a bis(μ-hydroxo)-divanadium moiety when exposed to moisture;[23] and ii) the isomerization of [B-H2As2Mo6O26- (H2O)]4to [A-H2As2Mo6O26]4within a POM/metal–organic framework.[24] The latter example belongs to our systematic studies on the {Cu(cyclam)}2+/POM family, which has proven suitable for preparing hybrid coordination networks able to undergo SCSC transformations.[25] As part of our continuing investigations on the interaction between {Cu(cyclam)}2+and isopolymolybdates,[26] here we report the hybrid coordination network [{Cu(cyclam)}3(k-Mo8O27)]·14H2O (1), which contains a novel octamolybdate (Mo8) cluster (labeled k) and undergoes sequential, thermally-induced SCSC transformations to afford two different anhydrous phases (2a and 3a). Beyond relevant rearrangement of the CuO bonding scheme, these transitions involve isomerization of k-Mo8into λ(2a) and μ (3a) forms through migration of Mo centers with variation of their coordination numbers. The three Mo8 clusters are novel, thereby enlarging the catalog of as many as 9 different isomers reported to date.[27–29] The reversibility of these phase transitions has been studied and water vapor sorption properties analyzed for the microporous phases 2a and 3a. Results and Discussion Compound 1was first obtained as a purple crystalline powder in ca. 15% yield from the reaction of commercial (NH4)6- [Mo7O24] and the in situ prepared {Cu(cyclam)}2+complex (1:3 stoichiometric ratio) in water at pH=6.5, followed by addition of 1,4-dioxane. Samples suitable for single-crystal Xray diffraction (SCXRD) were alternatively obtained by using hydrothermal methods. The infrared spectrum and powder X-ray diffraction (PXRD) pattern of the resulting prismatic crystals both fit well with those obtained for the polycrystalline samples (Figures S1–S3). For a more detailed Experimental Section, see the Supporting Information. Motivated by the wide thermal stability range (ca. 100– 230°C) found in the thermogravimetric (TGA) curve of 1 upon dehydration (Figures S4–S5), variable temperature (VT) PXRD patterns were recorded from room temperature (RT) to 600°C every 20°C (Figure S6) to explore its thermostructural behavior, i.e. whether 1shows crystallinity retention over amorphization with increasing temperature, and in the former case, whether it displays framework dynamism vs robustness. Results revealed that the hybrid coordination network retains crystallinity throughout the whole dehydration process and beyond up to 230°C, in line with the thermal stability range found in the TGA curve. Upon heating, 1 rapidly transforms into a new crystalline phase (2a) as indicated by major modifications in both positions and relative intensities of the main diffraction peaks. In particular, the two intense maxima at 2θ=7.1°and 7.4°positions in 1 split into three different signals located at 7.3, 7.5, and 7.7°in 2a (Figure S7). Moreover, the peak at 10.5°vanishes and new signals appear at 11.3, 22.5, and 23.2°. Considering that this transformation takes place at values close to RT, additional PXRD patterns were collected from 30 to 46°C every 2°C to explore this phase transition in depth (Figure S8). These experiments revealed that 2a is already fully formed at 36°C, and according to the TGA results, should correspond to a partially dehydrated phase. However, no mass loss associated with the release of H2O molecules was observed in the TGA analyses carried out for a sample of 1previously kept at 40°C for 1 h to ensure its full conversion into 2a. These observations suggest slow kinetics governing the removal of guest solvent molecules from 1, and confirm the anhydrous nature of 2a. Phase 2a remains stable until 110°C, but undergoes an additional transition above this temperature, transforming into a high-temperature anhydrous phase (3a), as indicated by the emergence of a new pair of most intense maxima at 7.4 and 7.8°with approx. 3:1 intensity ratio. SCXRD data (Table S1) show that 1crystallizes in the monoclinic space group P21/nwith an asymmetric unit (ASU) containing one [k-Mo8O27]6anion (k-Mo8), a total of three {Cu(cyclam)}2+complexes, of which two (Cu1A, Cu1B) are located in general positions and another two on centers of symmetry (Cu1C, Cu2C), and 14 H2O molecules disordered over 20 sites (Figure S9). The k-Mo8anion is novel, thereby enlarging the current catalog of Mo8clusters in the literature (Table S2). It can be best described as one {Mo4O16} tetramer and one {Mo3O13} trimer fused by sharing one face and one corner between MoO6octahedra, with the eighth Mo center exhibiting square pyramidal geometry. The latter is connected to the tetramer by edge-sharing and to the trimer by sharing two corners (Figure 1a). To our knowledge, the Mo8species reported to date comprise up to nine different structural isomers for the [Mo8O26]4anion, namely α,β,γ,δ,ɛ,ζ,η,θ and ι,[27–29] and a single isomeric form (γ) for the {Mo8O28}6 counterpart. The latter is a building block that can be found stabilized either by protonation in the [Mo8O26(OH)2]6 anion,[30] or by polymerization,[26] resulting in {Mo8O27}nand {Mo8O26}nchain-like assemblies depending on the number of corners shared to polymerize. Despite the variety of MoOx units (x=4–6) and polyhedral connectivity shown in Table S2 for Mo8isomers, the face-sharing feature of k-Mo8is structurally novel. All CuII centers display octahedral CuN4O2geometry with cyclam N atoms in equatorial positions and terminal OPOM atoms occupying both axial positions. Significant Jahn–Teller distortion is present in all complexes, with the longest Cu1AOPOM bond length of 2.876(5) Å being close to semicoordination (Table S3). The crystal packing (Figure S10) reveals a 3-dimensional porous coordination network, in which hybrid layers of k-Mo8clusters linked to four neighbors through four bridging complexes (Cu1A, Cu1B, Cu1C, Cu2C) can be identified in the (1� 10) plane (Figure 1b). This pattern results in rhombus-like grids with two types of voids arranged alternately along the crystallographic zaxis. These grids are further connected through coordination of two additional complexes (Cu1Ai, Cu1Bii; symmetry codes in Figure S10) to afford an open-framework architecture in which multiple CH···OPOM and NH···OPOM contacts estabAngewandte Chemie Research Articles Angew. Chem. Int. Ed. 2023,62, e202307436 (3 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 42, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202307436 by Universidad Del Pais Vasco, Wiley Online Library on [15/12/2023]. 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 License lished between cyclam ligands and POM surfaces play a relevant reinforcing role (Table S4). The structure exhibits a 3-dimensional system of intersected, water-filled channels with cavities located at the space between 8 neighboring clusters and connected through narrow necks defined by 4 neighboring complexes (Figure 1c). The channels run along the [001], [1� 10] and [110] directions, their necks show crosssections of ca. 9.7×9.1 and 9.6×9.2 Å (interatomic distances between opposite N atoms of the grid voids), and their total solvent accessible space accounts for 32% of the unit cell volume. To explore the structural changes that the thermallytriggered transitions above imply, SCXRD experiments were conducted on crystals of 1heated for 1 h at 50 and 120°C to ensure full transformation into the anhydrous 2 a and 3a phases, respectively. For 2a, dehydration results in a decrease of the unit cell volume to less than one-half of that of 1, which is associated with the reduction of the cell symmetry to the triclinic P� 1space group. The ASU (Figure S11) now contains one [λ-Mo8O27]6(λ-Mo8) cluster and a total of three {Cu- (cyclam)}2+moieties: one located in general positions (Cu1C) and four halves of centrosymmetric complexes (Cu1A, Cu2A, Cu1B, Cu2B). The Mo8cluster undergoes kto λisomerization during the phase transition. This isomerization implies rupture of the face-sharing connection between {Mo4O16} and {Mo3O13} fragments observed in the kform and formation of the edgesharing {Mo6O22} moiety exhibited by the new λ-type anion, which is similar to the cap displayed by the well-known [βMo8O26]8species (Figure 2a). This rearrangement proceeds via the rupture of two Mo-μ3O bonds and the following consequent modifications: i) the seventh {MoO6} octahedron goes from exclusively sharing edges in the kform to exclusively sharing corners in the λisomer, and ii) the eighth Mo center decreases its coordination number from five to four (tetrahedral geometry) and becomes linked to each of the fragments above by sharing one corner. Thus, solid-state migrations of some of the Mo centers take place within the cluster skeleton (Table S5 and Supporting Movies). Regarding the crystal packing, hybrid layers in the (01� 1) plane are analogous to those described for 1but now the rhombus-like grid shows two void types of distinct size: those delimited by Cu1B and Cu1C, which are comparable in size to the voids in 1(9.2×9.4 Å), and those defined by Cu2A and Cu1C with much smaller dimensions of 7.5×8.6 Å (Figure 2b). The connection between sheets is also different from that in 1 because only half of the {Cu(cyclam)}2+moieties located in the interlamellar space retain elongated octahedral geometry (Cu2B), whereas the other half becomes square planar (Figures S12–S13). The latter Cu1A complexes lose their character as covalent linkers, but still play a significant role in stabilizing the stacking of hybrid grids by establishing hydrogen bonding connectivity with tetrameric faces of λ-Mo8 clusters from adjacent sheets (Figure S12, Table S4). The connection between interlamellar cavities located at the space among clusters is disrupted through constrictions involving Cu1A and Cu2A fragments; hence, the 3-dimensional solvent-accessible network found in 1is dismantled in 2a. Figure 1. a) Polyhedral view of [k-Mo8O27]6(k-Mo8) ({Mo4O16} tetramer, pink; {Mo3O13}, light blue; {MoO5}, gray). b) Projection of a hybrid layer of 1on the (1� 10) plane ({MoO5} polyhedra of k-Mo8, dark gray; {CuN4O2} octahedra, blue; OWatoms, red spheres). c) Surface representation of interconnected water-filled channels in 1. Figure 2. a) Polyhedral view of the λ-Mo8isomer ({Mo6O22} hexamer, lilac; seventh {MoO6}, pink; tetrahedral {MoO4}, gray). b) Projection of a hybrid layer of 2a on the (01� 1) plane. c) Polyhedral view of the μ-Mo8 isomer ({Mo7O24} heptamer, light gray; tetrahedral {MoO4}, dark gray). d) Projection of a supramolecular layer of 3a on the (101) plane. Angewandte Chemie Research Articles Angew. Chem. Int. Ed. 2023,62, e202307436 (4 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 42, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202307436 by Universidad Del Pais Vasco, Wiley Online Library on [15/12/2023]. 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 License The resulting cavities nevertheless remain connected through strings of alternate wider and narrower windows arranged along the yaxis in a zig-zag mode, giving rise to a porous system of parallel channels that diagonally intersect the stacking of hybrid layers (Figure S14). These pores are empty in 2a and correspond to 24% of the unit cell volume. The second high-temperature anhydrous phase (3a) crystallizes in the same space group as 1with the ASU (Figure S15) formed by one [μ-Mo8O27]6(μ-Mo8) cluster, two {Cu(cyclam)}2+moieties in general positions (Cu1A, Cu1B), and another two halves of centrosymmetric complexes (Cu2A, Cu1C). The phase transition from 2a to 3a involves further isomerization of the Mo8cluster from the λ to the new μ form (Figure 2c). This process implies full edgesharing condensation of the seventh {MoO6} unit in the λ isomer to the hexameric cap, resulting in a {Mo7O24} fragment analogous to that displayed by [β-Mo8O26]8. As a result, the eighth Mo center becomes a tetrahedral {MoO4} antenna unit attached by a single corner-sharing linkage. It is noteworthy that the three Mo8isomers reported in this work are novel, evidencing that solid-state transformations can afford POMs that cannot be accessed by conventional solvent-mediated synthetic routes. Compared to 1and 2a, the covalent 3-dimensional network is broken in 3a, because the intralamellar moiety Cu1B loses its linking character and becomes a pentacoordinate appended unit with square-pyramidal geometry. Conversely, complexes Cu2A and Cu1C retain their bridging role in such a way that the μ-Mo8anions are covalently linked into zig-zagging ribbons along the yaxis (Figure 2d). The appended complexes nevertheless still play a key role in assembling such ribbons into supramolecular hybrid layers in the (101) plane through a massive hydrogen bonding network with the tetrameric faces of μ-Mo8clusters (Figure S16, Table S4). The stacking of these supramolecular assemblies is facilitated by interlamellar Cu1A complexes, which in contrast to what observed for 2a, are all octahedral covalent linkers (Figure S17). The porous network in 3a consists also of a system of parallel channels, but in this case, the interlamellar cavities appear connected along the stacking [101] direction via the wide necks with ca. 8.6×8.4 Å crosssection that the formation of the pentacoordinate appended units opens. These empty pores account for 23% of the unit cell volume. To study the reversibility of the SCSC transitions above, samples of 2a and 3a were exposed to the laboratory atmosphere (LA: 15–20°C; 40–50% relative humidity, RH) and monitored by a combination of TGA and PXRD. With respect to 3a, TGA analyses recorded after 1, 3, and 7 d of air exposure (Figures S18–S19) revealed that the anhydrous phase captures up to 12 H2O molecules per POM after one week (%m, calcd 9.8; found 9.6), of which as many as 10 are adsorbed during the first 24 h. No additional H2O molecules were adsorbed after 7 d of air exposure and the hydration/ dehydration cycles in the resulting samples proved to be fully reversible. These results are in line with those from PXRD studies, which indicate that a new crystalline phase (4) different from the parent 1starts forming when 3a hydrates, but it is not fully formed until the seventh day of air exposure (Figure S20). In an attempt to promote a faster transformation, crystals of 3a were placed in a more humid atmosphere and additional PXRD analyses demonstrated that formation of 4could reach completion in just 24 h (Figure S21). In contrast, soaking the crystals in water for 1 h appeared to imply slight degradation, as evidenced by some additional poorly resolved maxima found in the experimental diffraction patterns (Figure S22). The reversibility of such transition was corroborated through VT-PXRD on 4(Figure S23), which showed an intermediate phase (6a) in the transition from 4to 3a at 120°C upon heating. Considering the intricate crystal dynamics shown by this system, we tested whether the structure of 4could be elucidated by SCXRD. Fortunately, the data acquired on a crystal of 3a exposed to air for 7 days were suitable for a full structural resolution. Unluckily, we were not able to collect any data set of good quality for the intermediate 6a despite testing several different crystal batches. Compound [{Cu- (cyclam)}3{γ-Mo8O26(OH)2}]·11H2O (4) crystallizes in the triclinic P� 1space group and its ASU contains one-half of the well-known {γ-Mo8O26(OH)2}6species,[30] three halves of crystallographically independent {Cu(cyclam)}2+moieties and 5.5 H2O molecules over 9 sites (Figure S24). Formation of [γ-Mo8O26(OH)2]6in 4(Figure 3a) implies that one H2O molecule condenses to the POM skeleton and further dissociates along the hydration process. Protonation sites have been unequivocally assigned on the basis of Bond Valence Sum calculations (Table S6).[31] The isomerization of the Mo8anion to the more condensed, thermodynamically favored γform after a full dehydration-phase transitionFigure 3. a) Polyhedral view of [γ-Mo8O26(OH)2]6(protonated O atoms, maroon spheres). b) Projection of a hybrid layer of 4on the (001) plane. c) Surface representation of interconnected water-filled channels in 4. Angewandte Chemie Research Articles Angew. Chem. Int. Ed. 2023,62, e202307436 (5 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 42, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202307436 by Universidad Del Pais Vasco, Wiley Online Library on [15/12/2023]. 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 License hydration cycle suggests that all the three previous isomers in 1,2a and 3a are obtained only because of the stabilization provided by the hydrogen bonding network established between cyclam ligands and surface OPOM atoms. Indeed, these weak but cooperative forces, together with the plasticity of CuII centers, might well be the key factor in allowing such a string of consecutive SCSC transformations in this system.[22] The transformation of 3a into 4upon hydration implies that all {Cu(cyclam)}2+moieties regain their octahedral geometry and bridging role. Thus, each γ-Mo8unit is connected to six neighbors to give rise to a 3-dimensional covalent network reminiscent of that of 1in which hybrid grids with square-like voids of cross-section 9.1×9.0 Å stack along the [001] direction (Figures 3b, S25). As in 1, this architecture is pierced by a system of interconnected channels along the three crystallographic axes that host all the H2O molecules (Figure 3c). The necks that communicate the wide cavities at the space between 8 neighboring clusters are somewhat narrower than those in 1, hence the solventaccessible space is slightly reduced to roughly 30% of the unit cell volume. In contrast to 3a, hydration of 2a upon air exposure was fully achieved in just 24 h (Figures S19, S26). Neither the number of adsorbed H2O molecules calculated from TGA, nor the PXRD patterns of the resulting samples, matched any of the previous crystalline phases. This confirms the irreversibility of the transition from the parent hydrated phase 1to the low-temperature anhydrous derivative 2a. In fact, VTPXRD patterns recorded every 2°C for the cooling process of 2a from 46 to 30°C showed that a new phase (5) starts forming at RT within the experimental time-scale (Figure S27). The transition from 2a to 5is fully reversible as indicated by PXRD analyses carried out for 5when heating from 30 to 100°C every 5°C (Figure S28). The structure of the new hydrated phase 5was determined on crystals of 2a exposed to LA for 24 h to ensure full hydration. Compound [{Cu(cyclam)}3(k-Mo8O27)]·13H2O (5) crystallizes in the triclinic P� 1space group with one k-Mo8 anion, a total of three {Cu(cyclam)}2+complexes (Cu1B in general position; Cu1A, Cu2B, Cu1C, and Cu2C on centers of inversion), and 13 H2O molecules over 16 sites in the ASU (Figure S29). It is noteworthy that 5exhibits the same k-Mo8 isomer as 1, hence the kto λisomerization that takes place when going from 1to 2a is reversible despite such phase transition being irreversible. On the other hand, 5displays the same space group as 2 a with very similar unit cell parameters, and therefore, this hydrated phase might be viewed as a transient state between parent 1and anhydrous 2a. In fact, preliminary experiments on the crystal dynamics triggered by vacuum in this system show that 5generates upon dehydration of 1in these conditions and subsequent partial hydration. Further studies on this matter will be reported in due course. Regarding the crystal packing, the arrangement of POMs and bridging complexes within the hybrid layers in 5is virtually identical to that in 2a (Figure S30), but with the two distinct grid voids somewhat enlarged (9.1×8.7 and 9.6×9.4 Å) probably due to the breathing effect provided by the incorporation of H2O molecules. In contrast to 2a, no squareplanar complexes are found in the interlamellar space as the stacking of hybrid layers along the [01� 1] direction is exclusively due to octahedral complex linkers. The resulting open framework displays a 2-dimensional interlamellar system of intertwined channels running along the [011] and [100] directions that host all H2O molecules. These solventaccessible regions are connected along the stacking direction only through the wider grid voids defined by Cu1B and Cu2C (Figure S31), accounting for 29% of the unit cell volume. Figure 4 summarizes the intricate thermally-induced crystal dynamics shown by the title {Cu(cyclam)}2+/Mo8system and the collection of SCSC transitions that relate the five crystal phases described in this work.[32] Considering that 1retains microporosity upon thermal removal of guest H2O molecules, and that the pore windows are a priori wide enough to enable the diffusion of small molecule adsorbates, the gas sorption properties of the two different anhydrous phases in this work were evaluated. Figure 5 displays the results from N2(77 K) and CO2(195 K) sorption experiments carried out on crystalline samples of 1 activated at 50 and 120°C under vacuum to ensure full conversion into 2a and 3a, respectively. The N2adsorption curves correspond to type Ia isotherms typical of microporous materials with narrow pores (<1 nm). In both cases, micropores are filled rapidly at low relative pressures (p/p0<0.1) to achieve saturation that extends up to p/p0=1. The maximum gas uptake values for 2a and 3a (72 and 76 cm3(STP) g1, respectively) account for 6.4 and 6.8 adsorbed N2molecules per Mo8unit. The adsorption processes were observed to be completely reversible, and hence, all the gas molecules are desorbed at p/p0=0. Fitting of both adsorption branches to the Brunauer–Emmett–Teller (BET) model[33] affords similar values of BET surface areas for 2a and 3a (302 and 298 m2g1). Although these values are much lower than the highest areas reported for POM-loaded MOFs (>1000 m2g1),[34–35] our results are comparable to those displayed by some other POM-metalorganic frameworks, such as [Cu(cyclam)][{Cu(cyclam)}2(V10O28)] (205 m2g1),[16] or (TBA)2[Cu(BBTZ)2(α-Mo8O26)] Figure 4. Scheme of the SCSC transformations reported in this work. Angewandte Chemie Research Articles Angew. Chem. Int. Ed. 2023,62, e202307436 (6 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 42, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202307436 by Universidad Del Pais Vasco, Wiley Online Library on [15/12/2023]. 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 License (773 m2g1; BBTZ=1,4-bis(1,2,4-triazol-1-ylmethyl)- benzene).[36] In regard to the CO2uptake isotherms, saturation is roughly achieved at much higher relative pressures (p/p0= 1.0). The adsorption processes were also observed to be fully reversible and the maximum uptakes were also found to reach very similar values for 2a and 3a (66 and 64 cm3 (STP) g1), accounting for 5.9 and 5.7 adsorbed CO2molecules per Mo8unit. In contrast to what we observed in related POM/Cu(cyclam) porous frameworks, in which N2adsorption was negligible[16] or the maximum uptake accounted for only 30% of the simulated value,[17] both N2and CO2are readily adsorbed by 2a and 3a. This fact encouraged us to carry out additional experiments at different temperatures to gain a better understanding of the adsorptive properties of such anhydrous phases and to test their performance in purification or selective capture of small molecule gas mixtures. These results will be reported elsewhere in due course. The reversible nature of the transitions from 2a to 5and from 3a to 4motivated us to study the water-vapor sorption properties of our thermally-activated microporous materials. Dynamic vapor sorption (DVS) experiments were conducted at 298 K from vacuum to 90% RH for both anhydrous phases. For 2a, the adsorption isotherm indicates that pores remain virtually empty up to almost 20% of RH. After this point, a sudden increase of the water uptake occurs with a steep step that reaches ca. 70% of the total capacity at 35% RH. The maximum uptake at 90% RH represents an increase of 16.4% of the total mass with respect to that of 2a (Figure 6a). Small differences between the calculated mass for the 13 H2O molecules in the resulting hydrated phase 5 and the values found experimentally (Table S7) could arise from the surface condensation that can take place in the intergranular space.[2] These values result in a maximum working capacity of 222 cm3g1for 2a, which is one of the highest values observed to date for POM-based water sorbents (see Table S8 for literature uptake values of related compounds and associated discussion, including a brief comparison to zeolites and MOFs). The desorption process is almost superimposable to the adsorption, which suggests full reversibility. In order to confirm its cyclability, a second isotherm was measured under the same conditions and the working capacity remained virtually identical (218 cm3g1, 16.2% change in mass at 90% RH). PXRD analyses on 2 a verified retention of crystallinity after the adsorption/desorption processes (Figure S32). With respect to 3a, this phase does not adsorb any significant amount of water below 15% RH. From this point, a steep water uptake is observed up to 35% RH, after which the slope of the adsorption profile decreases. This slower step extends up to 50% RH and accounts for the mass of a single H2O molecule. This observation could result from the structural transition from 3a to 4associated with the incorporation of one H2O molecule to the POM skeleton and subsequent structural rearrangement from [μ-Mo8O27]6to [γMo8O26(OH)2]6(Figure 6b). After reaching a maximum working capacity of 235 cm3g1(17.4% change in mass) at 90% RH, the desorption process suggests that the [γ-Mo8O26- (OH)2]6anion remains stable upon dehydration because one H2O molecule is retained in the active material. PXRD analyses (Figure S33) confirmed that 4does not revert back to 3a under vacuum and indicate that the third anhydrous phase (6a) could be generated as a result. The PXRD pattern of this phase fits well with that observed for a sample of 4 treated under vacuum (Figure S34), as well as, to that of the intermediate phase found in the thermal transition from 4to 3a upon heating (Figure S23). FTIR spectra suggest the presence of the γ-Mo8isomer in 6a, albeit inconclusively (Figure S35). The distinct adsorption/desorption processes made us record additional cycles (Figure 6c). The reversibility of the water uptake between phases 4and 6a was confirmed because both curves were virtually identical and they showed a maximum working capacity of 223 cm3g1at 90% RH (16.5% change in mass). Besides the total capacity, the cyclability of the system and sorption kinetics are also important factors that should be taken into account for utility. Considering that i) the steep uptake takes place at 20–35% RH range in all cases, and ii) 80% of the total capacity is reached at 60% RH (standard working conditions for some common commercial Figure 5. N2and CO2isotherms for a) 2a and b) 3a. Filled and empty markers correspond to adsorption and desorption branches, respectively. Angewandte Chemie Research Articles Angew. Chem. Int. Ed. 2023,62, e202307436 (7 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 42, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202307436 by Universidad Del Pais Vasco, Wiley Online Library on [15/12/2023]. 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 License desiccants),[37] the kinetics of these two processes were studied at a constant vapor flow rate of 30 and 60% RH, respectively. Under low humidity vapor flow swing (0–30–0% RH), water uptake took as long as 100 min to be completed (Figures S36–S37), whereas both adsorption and desorption processes are achieved in less than 15 min at a RH swing of 0–60– 0% for both phases at RT. The durability and recyclability of the adsorbents were evaluated by 100 cycles of water adsorption-desorption experiments at 298 K and a constant vapor flow rate of 0–60– 0% RH. Plots in Figures S36 and S37 show an average water uptake of 14% and 13% for 2a and 6a, respectively, values that are very close to those observed in the initial isotherms. Thus, the regeneration of the materials occurs after 100 cycles under humidity swing conditions. Materials with this type of relatively fast, reversible behavior are suited for their use in heat pumps or air conditioners.[38] Moreover, the fast kinetics and reproducibility of the adsorption-desorption process make these materials potentially suitable for dehumidification in confined spaces, as well as, for water harvesting and purification in drylands.[39–41] Conclusion This work nicely exemplifies the potential of solid-state transitions to afford novel POM clusters that cannot be accessed by classical solvent-mediated synthesis. The extended framework [{Cu(cyclam)}3(k-Mo8O27)]·14H2O (1) represents one of the rare examples of POM-based systems that can undergo up to four SCSC transformations related to thermal dehydration-hydration processes. Removal of H2O molecules sequentially affords the anhydrous [Cu(cyclam)]- [{Cu(cyclam)}2(λ-Mo8O27)] (2a) and [{Cu(cyclam)}3(μMo8O27)] (3a) phases upon heating. These transitions imply two consecutive isomerizations of Mo8anions through solidstate metal migration from a kform with unusual face-sharing connectivity between {MoO6} octahedra to a μ form reminiscent of the most usual βisomer but with an appended {MoO4} tetrahedral unit. It is noteworthy that the three k-, λ-, and μMo8isomers in this work have never been reported before. Hydration of 3a upon air exposure involves condensationdissociation of one H2O molecule to the POM cluster to result in the known γisomer that is found in [{Cu(cyclam)}3{γMo8O26(OH)2}]·11H2O (4), which dehydrates into the third anhydrous phase 6a before reverting back to 3 a. In contrast, 2a hydrates to form [{Cu(cyclam)}3(k-Mo8O27)]·13H2O (5), which displays the same Mo8cluster as 1, proving the reversibility of the kto λisomerization. Water vapor sorption isotherms of the anhydrous phases display an abrupt step at low relative humidity level (20–30%) that, together with a high cyclability (>100 cycles), could be desirable for humidity control devices in confined spaces, or water harvesting and purification systems in drylands. Their associated uptake values are among the highest reported to date for POMbased systems. The fact that 1can afford three different microporous phases (2a,3a, and 6a) could endow this system with the capacity to selectively adsorb different molecules depending on its activation procedure. Studies on the ability of these phases to undergo selective adsorption from mixtures of gases will be carried out in the near future. Acknowledgements Funded by Eusko Jaurlaritza/Gobierno Vasco (EJ/GV, grants IT1722-22 and KK-2022/00045). E.R.B. thanks EJ/ GV for her doctoral fellowship (PRE_2018_1_0143). TechFigure 6. DVS isotherms from 0 to 90% relative humidity at 298 K for a) 2a (2 cycles), and 3a, b) first cycle leading to 6a, c) second and third cycles. Angewandte Chemie Research Articles Angew. Chem. Int. Ed. 2023,62, e202307436 (8 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 42, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202307436 by Universidad Del Pais Vasco, Wiley Online Library on [15/12/2023]. 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 License nical and human support from SGIker (UPV/EHU) is gratefully acknowledged. Conflict of Interest The authors declare no conflict of interest. Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. Keywords: Microporous Materials ·Polyoxometalates · Single-Crystal-to-Single-Crystal ·Solid-State Isomerization · Water Sorption [1] Ed. S. K. Ghosh, Metal-Organic Frameworks (MOFs) for Environmental Applications, Elsevier, Amsterdam 2019. [2] J. Canivet, A. Fateeva, Y. Guo, B. 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Manuscript received: May 26, 2023 Accepted manuscript online: June 15, 2023 Version of record online: June 30, 2023 Angewandte Chemie Research Articles Angew. Chem. Int. Ed. 2023,62, e202307436 (9 of 9) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 42, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202307436 by Universidad Del Pais Vasco, Wiley Online Library on [15/12/2023]. 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 License