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Yuudai Iwai, Saaya Kimura, [Manabu Nakaya](https://orcid.org/0000-0001-8483-8131), [Takanori Nakane](https://orcid.org/0000-0003-2697-2767), [Akihiro Kawamoto](https://orcid.org/0000-0002-7380-0127), [Genji Kurisu](https://orcid.org/0000-0002-5354-0807), [Yuta Tsuji](https://orcid.org/0000-0003-4224-4532), [Kenji Hirai](https://orcid.org/0000-0003-3307-3970), [Koji Kimoto](https://orcid.org/0000-0002-3927-0492), [Ovidiu Cretu](https://orcid.org/0000-0002-1822-8172), [Fumitaka Takeiri](https://orcid.org/0000-0001-9839-5669), [Kunihisa Sugimoto](https://orcid.org/0000-0002-0103-8153), [Benjamin Le Ouay](https://orcid.org/0000-0001-7053-4786), [Masaaki Ohba](https://orcid.org/0000-0001-9268-3512), [Ryo Ohtani](https://orcid.org/0000-0003-4840-3338)

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[Reversible and Massive Structural Transformation in Meltable Cyanido‐bridged Coordination Polymer Crystals](https://mdr.nims.go.jp/datasets/bedf2a73-b9ed-44f7-996f-a552a8cc6dea)

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Reversible and Massive Structural Transformation in Meltable Cyanido‐bridged Coordination Polymer CrystalsChemEurJResearch Articledoi.org/10.1002/chem.202502640www.chemeurj.orgReversible andMassiveStructural Transformation inMeltableCyanido-bridgedCoordinationPolymerCrystalsYuudai Iwai,[a] Saaya Kimura,[a] Manabu Nakaya,[b] Takanori Nakane,[c, d]Akihiro Kawamoto,[c, d] Genji Kurisu,[c, d] Yuta Tsuji,[e] Kenji Hirai,[f] Koji Kimoto,[g]Ovidiu Cretu,[g] Fumitaka Takeiri,[h] Kunihisa Sugimoto,[h] Benjamin Le Ouay,[a]Masaaki Ohba,*[a] and Ryo Ohtani*[a]Cyanido (CN−)-bridged coordination polymers (CP) have beenextensively studied as molecular-based functional materials.However, synthesizing 3D compounds composed only of metalions and CN−—without bulky organic groups—and that meltbefore decomposing remains a considerable challenge. This dif-ficulty arises because CN− strongly interconnect metal ions,forming rigid, dense frameworks with high melting points. Inthis study, we successfully synthesized a melting compositeconsisting of 3D KCd[Cu(CN)2]3 and 2D K2Cu3(CN)5 by dehy-drating K2Cd(H2O)Cu4(CN)8·1.5H2O. Remarkably, nanodomains ofthese two compounds coexisted within single particles, allow-ing their crystal structures to be independently determinedby 3D electron diffraction (MicroED) of the resulting pow-ders. Each compound melted at its respective melting point,around 559 K. Notably, the melting point of KCd[Cu(CN)2]3 isunusually low for a 3D dense coordination framework. Thisatypically low melting point results from a combination ofcrystalline surface effects, and the entropy contribution ofthe dynamic, labile two-coordinate Cu centers in the frame-work. Additionally, we demonstrated a reversible transforma-tion between the dehydrated mixture and the hydrated parentcompound through exposure to water vapor, highlighting thedynamic and responsive nature of these CN−-based solid-statematerials.1. IntroductionPhase transitions in inorganic solid-state materials present newopportunities for their development and applications. Over thepast decade, the melting behavior of coordination frameworks—such as coordination polymers (CPs) and metal–organic frame-works (MOFs)—has been extensively studied, focusing notonly on fundamental insights but also on practical materialapplications.[1–3] Among these, the most thoroughly investi-gated melting MOFs are the zeolitic imidazolate frameworkseries,[4–7] followed by phosphoric acid-based CPs[8–11] and car-boxylic acid-bridged MOFs.[12] These studies have indicatedthat using bulky ligands with delocalized charges and/or longaliphatic chains was effective for producing melting MOFsbecause this promotes weak coordination bonds between metalions and ligands and/or increases entropy through kinetic, con-formational, and configurational effects. In this context, smallligands such as cyanido (CN−), which possess a strong dipole,completely defy these design principles. Consequently, melt-ing 3D CN−-bridged CPs have not been observed, and theirsynthesis remains a considerable challenge because CN− tendto decompose at relatively low temperatures—that is, theirdecomposition temperature is generally lower than their meltingpoint.[a] Y. Iwai, S. Kimura, B. Le Ouay, M. Ohba, R. OhtaniDepartment of Chemistry, Faculty of Science, Kyushu University, 744Motooka, Nishi-ku, Fukuoka 819-0395, JapanE-mail: ohba@chem.kyushu-univ.jpohtani@chem.kyushu-univ.jp[b] M. NakayaDepartment of Chemistry and Biological Science, Faculty of Science, JosaiUniversity, 1-1 Keyakidai, Sakado, Saitama 350-0295, Japan[c] T. Nakane, A. Kawamoto, G. KurisuInstitute for Protein Research, The University of Osaka, 3-2 Yamadaoka, Suita,Osaka 565–0871, Japan[d] T. Nakane, A. Kawamoto, G. KurisuJEOL YOKOGUSHI Research Alliance Laboratories, Graduate School ofFrontier Biosciences, The University of Osaka, 1–3 Yamadaoka, Suita, Osaka565–0871, Japan[e] Y. TsujiFaculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka816–8580, Japan[f ] K. HiraiDivision of Photonics and Optical Science, Research Institute for ElectronicScience (RIES), Hokkaido University, North 20 West 10, Kita ward, Sapporo,Hokkaido 001–0020, Japan[g] K. Kimoto, O. CretuCenter for Basic Research on Materials, 1-1 Namiki, Tsukuba, Ibaraki305-0044, Japan[h] F. Takeiri, K. SugimotoDepartment of Chemistry, Kindai University, 3-4-1 Kowakae, Higashi-osaka,Osaka 577–8502, JapanSupporting information for this article is available on the WWW underhttps://doi.org/10.1002/chem.202502640© 2025 The Author(s). Chemistry – A European Journal published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properly cited.Chem. Eur. J. 2025, 31, e02640 (1 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbHwww.chemeurj.orghttps://orcid.org/0000-0001-8483-8131https://orcid.org/0000-0003-2697-2767https://orcid.org/0000-0002-7380-0127https://orcid.org/0000-0002-5354-0807https://orcid.org/0000-0003-4224-4532https://orcid.org/0000-0003-3307-3970https://orcid.org/0000-0002-3927-0492https://orcid.org/0000-0002-1822-8172https://orcid.org/0000-0001-9839-5669https://orcid.org/0000-0002-0103-8153https://orcid.org/0000-0001-7053-4786https://orcid.org/0000-0001-9268-3512https://orcid.org/0000-0003-4840-3338mailto:ohba@chem.kyushu-univ.jpmailto:ohtani@chem.kyushu-univ.jphttps://doi.org/10.1002/chem.202502640http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fchem.202502640&domain=pdf&date_stamp=2025-10-25ChemEurJResearch Articledoi.org/10.1002/chem.202502640CN−-bridged CPs exhibit a wide range of functionali-ties, including magnetic,[13–16] electronic,[17–20] and adsorptionproperties.[21–23] Prussian blue, first synthesized in 1704 and his-torically used as a blue pigment, is often considered the pro-totype of CPs and MOFs. The relationship between structureand properties—encompassing metal species as well as thedimensionality and topology of frameworks—has long been akey focus for advancing CN−-based solid-state materials. Morerecently, the structural complexity of seemingly simple CN−-bridged frameworks has been explored[24,25]; correlated disor-der in the Prussian blue analog (PBA) lattice and the glassyphase of PBA were identified, further improving our under-standing of defects for material applications. Since the 20thcentury, researchers have synthesized numerous related com-pounds by combining metal ions with cyanometallate units suchas [M(CN)6]n− (n = 3, 4), [M(CN)8]n− (n = 3, 4), [M(CN)4]2−,and [M(CN)2]− (Figure S1).[26–29] These efforts have revealedintriguing properties beyond melting, including negative ther-mal expansion[30–33] and negative linear compression.[32,34] Thishighlights the need for new synthetic strategies to exploreyet-undiscovered 3D-melting metal–CN− frameworks.In this study, we demonstrated the synthesis of the 3Dcompound KCd[Cu(CN)2]3 and its melting behavior, attributed todistinctive crystal-interface and structural effects. KCd[Cu(CN)2]3is an analog of the well-known wine-rack-type KCd[M(CN)2]3(M = Au and Ag)[31,32] and represents the first melting 3D metal–CN− framework reported. This compound was synthesizedthrough a substantial structural transformation and phase sep-aration of K2Cd(H2O)Cu4(CN)8·1.5H2O via dehydration. Uniquely,a single dehydrated particle contained not only KCd[Cu(CN)2]3but also minor amounts of a melting 2D K2Cu3(CN)5 phase.Detailed investigation of the phase transition showed thateach compound in the composite melted at its own meltingpoint, approximately 559 K. The relatively low melting points,compared to other 3D-melting MOFs, resulted from surfaceinteractions between the crystalline nanodomains ofKCd[Cu(CN)2]3 and K2Cu3(CN)5. Additionally, the mix-ture could be reversibly converted back to the parentK2Cd(H2O)Cu4(CN)8·1.5H2O through water adsorption. Theseunusual dynamic behaviors were further attributed to the flex-ibility of the low-coordinate copper nodes, as supported bymolecular dynamics simulations.2. Results and DiscussionColorless block single crystals of K2Cd(H2O)Cu4(CN)8·1.5H2O33 (1)were obtained using aqueous solutions of K2Cd(CN)4, K[Ntf2](Ntf2 = bis(trifluoromethanesulfonyl)imide), and CuCN (Figure 1a,1e and Table S1). This compound has a 3D CN−-bridged anionicframework. The Cu ions are planar and three-coordinated, with abond distance to C/N in the first coordination sphere of approx-imately 1.9 Å. The Cd ions form three-way bipyramidal nodescoordinated by four CN− groups and one water molecule. Thebond lengths were measured as 2.175 Å for Cd─N and 2.559 Åfor Cd─O(H2O). Infrared (IR) spectroscopy revealed a stretchingvibration peak of the CN group at 2112 cm−1 and characteris-tic peaks from water molecules around 3600 cm−1 (Figure S1).Thermogravimetric analysis (TGA) of 1 indicated the loss ofboth crystalline and coordinated water molecules up to 450 K(Figure S2). The dehydrated sample (CdCudehyd) remained stableup to approximately 600 K. The powder X-ray diffraction (PXRD)pattern of CdCudehyd—obtained by heating 1 at 450 K—differedfrom that of the original compound (Figure S3).We found that CdCudehyd was a mixture of two crystallineCN−-bridged CPs—KCd[Cu(CN)2]3 (2) and K2Cu3(CN)5 (3)[34]—based on 3D electron diffraction (MicroED) measurements ofthe dehydrated powder at 79 K (Figure 1b, 1c and S4, S5 andTables S2–S4). Compound 2 crystallizes in the P-31m spacegroup. The Cd ions adopt an octahedral geometry by bridgingwith slightly bent Cu(CN)2 units, forming a 3D wine-rack-typeframework with K+ ions located in the framework’s interstitialspaces. Notably, 2 is isostructural with its well-known analogsKCd[M(CN)2]3 (M = Ag, Au).[31,32] The Cd─N bond length of 2.34 Åis comparable to those of the Ag (2.332 Å) and Au (2.331 Å)analogs. The distance between the closest Cu atoms along thea-axis is 3.39 Å, slightly longer than 3.376 Å in KCd[Ag(CN)2]3 and3.318 Å in KCd[Au(CN)2]3 (Figure S6). In contrast, compound 3crystallizes in the C2/c space group, forming an anisotropic lay-ered structure of [Cu3(CN)5]2− with three-coordinate Cu nodes,while K+ ions occupy the interlayer spaces. Compound 3 has anincomplete pentagonal network owing to a single nonbridgingCN− ligand. The PXRD pattern of CdCudehyd can be interpretedas a combination of the simulated PXRD patterns of 2 and 3(Figure 1d). Based on the peak intensity ratio, 2 is the predom-inant phase in CdCudehyd. The IR spectra of CdCudehyd exhibitedCN stretching mode peaks at 2145 and 2103 cm−1, along with twoshoulders at 2087 and 2122 cm−1 (Figure S1). Comparing these tothe IR spectra of KCd[Ag(CN)2]3 and KCd[Au(CN)2]3, which show aCN stretching mode at 2156 cm−1, the peak at 2145 cm−1 can beattributed to 2. Therefore, the remaining three peaks correspondto 3, consistent with the presence of three crystallographicallyindependent CN groups.Interestingly, the formation of 2 and 3 through the dehydra-tion of 1 involved extensive structural changes while preservingthe overall crystal morphology, despite the appearance of smallholes on the crystal surface (Figures 1d, 1e, and S7). This find-ing confirms the coexistence of the two crystalline domains, 2and 3, in a single composite particle. However, no combina-tion of stoichiometric ratios of 2 and 3 can fully represent theformula of 1, indicating that CdCudehyd also contains an amor-phous component. This conclusion is supported by transmis-sion electron microscopy–energy-dispersive X-ray spectroscopy(TEM–EDX), which detected amorphous CuCN alongside thecrystalline nanodomains of 2 and 3 (Figure S8 and Table S5).However, the IR spectra showed no peaks corresponding toCuCN (2162 cm−1) (Figure S1), confirming that the amount ofCuCN residue in CdCudehyd was negligible. The intracrystallinephase separation into 2 and 3 from 1 indicates that a dehy-drated 1 is unstable, likely due to the pentacoordinate Cdcenters within the network, and it subsequently transformsinto the thermodynamically more stable hexacoordinate Cd-based compound of 2. Subsequently, 3 and an amorphousChem. Eur. J. 2025, 31, e02640 (2 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 66, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202502640 by National Institute For, Wiley Online Library on [28/11/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202502640Figure 1. Crystal structures of a) 1, b) 2, and c) 3. Color code: light yellow; Cd, orange; Cu, light blue; N, gray; C, red; O, purple; K. d) powder X-raydiffraction (PXRD) pattern of CdCudehyd and simulation patterns of 2 and 3. λ = 1.08 Å. e) Scanning electron microscopy images of 1 and CdCudehyd . Redallows indicate small holes on the surface.phase derived from the remaining components are formedsequentially.Differential scanning calorimetry (DSC) results revealed thatCdCudehyd melted (Figure 2a, 2b). The DSC curves of 1 exhibited asmall peak at 470.5 K followed by a large endothermic peak witha minor shoulder near 559 K during heating at 5 K/minute. In situimaging showed that the first peak corresponds to dehydrationforming CdCudehyd, while the second peak indicates melting ofthe composite. During the subsequent cooling, an exothermicsolidification peak was observed at 556.4 K.To elucidate the melting behavior of CdCudehyd, which con-tains 2 and 3, variable-temperature powder X-ray diffraction (VT–PXRD) measurements were performed under vacuum (Figures 2c,S9–S18). During heating, the diffraction pattern of 3 disappearedat 565 K, consistent with its melting at this temperature, whilethe pattern of 2 remained but with reduced intensity (Figures 2c,S9–S11, and S18a). At 570 K, the diffraction pattern of 2 alsodisappeared (Figures 2c, S12, and S13), indicating that the melt-ing temperature of 2 was higher than that of 3 in CdCudehyd.Notably, small new peaks emerged at 12.1° and 15.5° followingthe melting of 3 at 565 K (marked by green squares in Figure 2c).After 2 melted at 570 K, an additional diffraction peak appearedat 18.7° (orange squares in Figures 2c and S13). The peaks at12.1° and 15.5° and the peak at 18.7° are consistent with the for-mation of Cu3N (peaks at 12.1° and 15.5°)[35] and CdO (peak at18.7°)[36] from reactions of the molten 2 and 3 with trace O2Chem. Eur. J. 2025, 31, e02640 (3 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 66, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202502640 by National Institute For, Wiley Online Library on [28/11/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202502640Figure 2. a) DSC curves of 1, with enthalpy values calculated based on itsmolecular weight. b) Photographs of CdCudehyd at 561.08–562.26 K duringDSC measurements. c) VT–PXRD results of 1 during heating at 10 K/minute(λ = 0.799 585 Å). Red circles, blue triangles, green squares, and orangesquares mark the diffraction peaks of 2, 3, Cu3N, and CdO, respectively.and N2. The VT–PXRD cooling process confirmed the reversiblephase transition of 2 and 3, despite changes in peak intensityratios owing to recrystallization and minor sample loss from sidereactions. VT–IR measurements showed that the liquid phaseexhibited a single CN stretching mode at 2104 cm−1 (Figure S19),indicating the formation of a homogeneous ionic liquid statethrough the breaking of CN− bridges during melting. VT–PXRDmeasurements from 100 to 450 K additionally provided character-istic anisotropic thermal expansion behavior for 2, yielding linearcoefficients of αa , b = 41.8 (16) MK−1 (M = 106), αc = −47.1 (6)MK−1, and αV = 35.2 MK−1. A comprehensive discussion of thisbehavior is depicted in Figures S20, S21 and Tables S6, S7. Finally,TEM–EDX analysis also identified the presence of CdO in thesample after melting (Figure S22 and Table S8); it is noteworthythat Cu3N proved challenging to detect owing to interferencefrom the instrument’s copper components.It is important to note that 2 cannot be obtained using[Cu(CN)2]− in the synthetic protocol for KCd[M(CN)2]3 (M = Agand Au). In contrast, although 3 was a known entity, its ther-modynamic behavior had not been elucidated. Therefore, weinvestigated the phase transition behavior of 3 by synthesizingits powders via a reported method[34] (Figure S23) and perform-ing TG-DTA measurements (Figure S24a). Our findings indicatethat a single phase of 3 melts at 600 K, a temperature exceedingits melting point when incorporated in CdCudehyd. The decreasein the melting points of 2 and 3 in CdCudehyd would be partlyattributed to lattice defects, tiny crystal domains, and amor-phous residues which were generated by the large structuraltransformation via dehydration. On the other hand, this obser-vation also implies that the distinctive low melting point ofCdCudehyd arises from a nanoscale crystal-interface interactionbetween 2 and 3. Further evidence for this melting point depres-sion was provided by solid-state mixing experiments involvingCdCudehyd and 3 powders. The melting temperatures of theresulting mixtures changed as the mixing ratio of 3 increased(Figure S24b and Table S9). This observation suggests thatdespite 2 and 3 possessing completely distinct crystal phases,the CN− groups on their respective crystal surfaces interactwith the metal ions of neighboring crystals during solid mix-ing, thereby affecting the thermodynamic properties of thecomposite materials.Conventional strategies for synthesizing meltable MOFs andCPs have typically used bulky and electrodelocalized ligands,including imidazolate cations,[4–7,37–39] [Ntf2] anions,[40–42] phos-phoric acid,[8–11] and methanetricarbonitrile.[39,43,44] These largermolecular components reduce the number of coordinationbonds, thereby contributing enthalpically to lower melting tem-peratures, and increase conformational entropy, contributingentropically. In this context, it is notable that 2, a 3D CN−-bridged framework composed only of small, anisotropic CN−linkers, melts at approximately 559 K (Figures S25, S26, and TableS10). Notably, the analogous compounds, KCd[M(CN)2]3 (M = Agand Au), do not exhibit melting behavior (Figure S27). This obser-vation implied the role of Cu nodes in the melting of CdCudehyd,in addition to the previously discussed crystal-interface effects.Therefore, the unique low-temperature melting of 2 was theoret-ically explored via molecular dynamics simulations to elucidatethe impact of metal species at the two-coordinate site on thedynamic behavior of the overall KCd[M(CN)2]3 network (M = Cu,Ag, and Au). Notably, the simulation results for 2 revealed a sub-stantial fluctuation of Cu nodes, coupled with changes in theircoordination number, at high temperatures in 1500 fs (Figure S28and Supplementary movie 1). Additionally, the radial distribu-tion function (RDF) of 2 was examined across time frames suchas 0–500 fs and 500–1000 fs, and 1000–1500-fs segments, reveal-ing a broadening of the peak for the first coordination sphereof the Cu center with increasing time (Figure S29). These dras-tic changes in the coordination sphere and RDF peaks wereabsent in KCd[Ag(CN)2]3 and KCd[Au(CN)2]3 when subjected tothe same conditions. Furthermore, the enthalpies of formationfor these three compounds were confirmed to exhibit only minordifferences (Table S11). Therefore, we conclude that the flexibleand labile nature of the two-coordinate Cu nodes increases theentropy change upon melting, leading to a reduced meltingpoint (�H/�S).Remarkably, CdCudehyd, comprising 2 and 3, transformed backinto 1 (CdCurehyd) upon exposure to water vapor. Analytical datafrom PXRD, IR, TGA, and elemental analysis of CdCurehyd werefound to be consistent with those of precursor 1 (Figures 3a, S30,and Scheme 1), despite the minor presence of CdO detected viaChem. Eur. J. 2025, 31, e02640 (4 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 66, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202502640 by National Institute For, Wiley Online Library on [28/11/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202502640Figure 3. a) PXRD results showing the reversible structural transformationof CdCudehyd to CdCurehyd upon exposure to water vapor (λ = 1.54 Å). Thepowder pattern of CdCurehyd matches that of 1. b) Water adsorptionisotherm of CdCudehyd at 298 K.Scheme 1. Overview of the structural conversion and phase transition ofthe system.TEM–EDX (Figure S31 and Table S12). This distinctive rehydrationbehavior further substantiates that the separated nanodomainsof 2 and 3 possess robust interfacial connections in each par-ticle, allowing them to collectively react to water vapor andregenerate the parent 1 as if they constitute a single mate-rial. Importantly, the crystal morphology remained intact afterrehydration (Figures S7 and S32), indicating a solid–solid transfor-mation despite substantial structural changes. To further eluci-date this reversible interconversion between CdCudehyd and 1, thewater adsorption isotherm of CdCudehyd was measured at roomtemperature. Observations indicated gate-opening type adsorp-tion behavior starting at approximately 0.5 relative pressure, witha total uptake of approximately 2.8 water molecules (Figure 3b).The manifestation of this gate-opening behavior strongly sup-ports the occurrence of a substantial structural transformationduring the interconversion of CdCudehyd and 1.3. ConclusionSimple dehydration of K2Cd(H2O)Cu4(CN)8•1.5H2O resultedin unique melting crystals containing discrete domains ofKCd[Cu(CN)2]3 and K2Cu3(CN)5. In particular, KCd[Cu(CN)2]3 isa new melting analog in the well-established family of 3Dwine-rack-type CN−-bridged structures. We have demonstratedthat both crystal-interface interactions and the presence oflow-coordinate Cu nodes exert substantial effects on the phasetransition. Accordingly, this study introduces a new designparadigm for constructing meltable coordination frameworks,emphasizing the tuning of crystalline surfaces and the useof low-coordinate metal nodes. It is noteworthy that while[Cu(CN)2]− appears to be a straightforward building unit, its usein the synthesis of coordination frameworks has not been previ-ously reported. Therefore, this study shows that the synthesis ofnovel compounds through the conversion of hydrated parentsinto distinct crystalline phases offers a simple and promisingavenue for obtaining unexpected materials, often unattainablevia conventional synthetic routes. Although the dehydrationprocess can entail complex intracrystalline reactions, frequentlyyielding mixed-phase products, careful characterization of thesecompounds and comprehensive investigation of their physicalproperties through techniques such as electron microscopyand various spectroscopies can reveal new materials, therebyadvancing the field of materials chemistry.Supporting InformationThe Supporting Information is available free of charge at Exper-imental Section, crystal parameters, VT-PXRD patterns, VT-IRspectra, TG-DTA curves, TEM-EDX results, microscopic images,DSC curves, and MD simulation resultsAccession codeMicroED raw images have been deposited to XRDa (accessioncode XRD-283). Refined coordinates of compounds have beendeposited in CCDC 2414728 (1), 2414729 (2), and 2414730 (3) andCOD 5000584 (2) and 5000585 (3). Scripts for MicroED datacollection and processing are available at https://github.com/GKLabIPR/MicroED.AcknowledgmentsThis work was supported by the Grant-in-Aid for TransformativeResearch Areas (A) “Supra-ceramics” (JSPS KAKENHI grant num-ber JP22H05144, JP22H05145, and JP22H05146) and JST, PRESTOGrant Number JPMJPR24M2. This work was also supported by theJSPS KAKENHI grant number JP24K21784, JP24K01457, JP21K18936,JP24K01499, JP22K19052, and JP24K08447. This work was partiallysupported by the Cooperative Research Program of “NetworkJoint Research Center for Materials and Devices”. Kyushu Syn-chrotron Light Research Center (SAGA-LS) 2311074P. Synchrotronradiation experiments at SPring-8 were approved by the JapanSynchrotron Radiation Research Institute (2024B1687). The com-putations in this work were performed using the computerfacilities at the Research Institute for Information Technology,Kyushu University, at the Supercomputer Center, the Institute forSolid State Physics, the University of Tokyo, and at CyberscienceCenter, Tohoku University. MicroED was performed under theCollaborative Research Program of Institute for Protein Research,Osaka University (MEDCR-24–02). The electron microscope wasChem. Eur. J. 2025, 31, e02640 (5 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 66, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202502640 by National Institute For, Wiley Online Library on [28/11/2025]. 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://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/chem.202502640https://github.com/GKLabIPR/MicroEDhttps://github.com/GKLabIPR/MicroEDChemEurJResearch Articledoi.org/10.1002/chem.202502640partly supported by Research Support Project for Life Scienceand Drug Discovery (BINDS) from AMED under Grant NumberJP24ama121001.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are availablefrom the corresponding authors upon reasonable request.Keywords: coordination polymer • melting • metal cyanido[1] T. D. Bennet, S. Horike, Nat. Rev. 2018, 3, 431.[2] S. Horike, S. S. Nagarkar, T. Ogawa, S. Kitagawa, Angew. Chem. Int. Ed.2020, 59, 6652 .[3] N. Ma, S. Kosasang, E. K. Berdichevsky, T. Nishiguchi, S. Horike, Chem.Sci. 2024, 15, 7474.[4] Y. Gong, H. Zhang, P. Li, Y. Bai, B. Yin, M. Ouyang, N. Zheng, X. Liu, Z.Zhao, J. Qiu, Z. Yang, G. Dong, Adv. Funct. Mater. 2024, 34.[5] H. Tao, T. D. Bennett, Y. Yue, Adv. Mater. 2017, 29.[6] T. D. Bennett, Y. Yue, P. Li, A. Qiao, H. Tao, N. G. Greaves, T. Richards,G. I. Lampronti, S. A. Redfern, F. Blanc, O. K. Farha, J. T. Hupp, A. K.Cheetham, D. A. Keen, J. Am. Chem. Soc. 2016, 138, 3484.[7] A. M. Bumstead, M. L. Ríos Gómez, M. F. Thorne, A. F. Sapnik, L. Longley,J. M. Tuffnell, D. S. Keeble, D. A. Keen, T. D. Bennett, CrystEngComm2020, 22, 3627.[8] M. Inukai, Y. Nishiyama, K. Honjo, C. Das, S. Kitagawa, S. Horike, Chem.Commun. 2019, 55, 8528.[9] T. Tiyawarakul, T. Imyen, K. Kongpatpanich, T. Watcharatpong, S. Horike,APL Mater. 2023, 11.[10] D. Umeyama, S. Horike, M. Inukai, T. Itakura, S. Kitagawa, J. Am. Chem.Soc. 2012, 134, 12780.[11] D. Umeyama, S. Horike, M. Inukai, T. Itakura, S. Kitagawa, J. Am. Chem.Soc. 2015, 137, 864.[12] M. Kim, H. S. Lee, D. H. Seo, S. J. Cho, E. C. Jeon, H. R. Moon, Nat.Commun. 2024, 15, 1174.[13] P. Dechambenoit, J. R. Long, Chem. Soc. Rev. 2011, 40, 3249.[14] S. Ferlay, T. Mallah, R. Ouahes, P. Veillett, M. A. Verdaguer, Nature 1995,378, 701.[15] H. Tokoro, S. Ohkoshi, Dalton Trans. 2011, 40, 6825.[16] S. Ohkoshi, K. Arai, Y. Sato, K. Hashimoto, Nat. Mater. 2004, 3,857.[17] Z. Liang, F. Tian, G. Yang, C. Wang, Nat. Commun. 2023, 14, 3591.[18] Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Chem. Commun. 2012, 48,6544.[19] Y. Zeng, X. F. Lu, S. L. Zhang, D. Luan, S. Li, X. W. Lou, Angew. Chem. Int.Ed. 2021, 60, 22189.[20] S. Ohkoshi, K. Nakagawa, K. Tomono, K. Imoto, Y. Tsunobuchi, H.Tokoro, J. Am. Chem. Soc. 2010, 132, 6620.[21] M. Ishizaki, S. Akiba, A. Ohtani, Y. Hoshi, K. Ono, M. Matsuba, T. Togashi,K. Kananizuka, M. Sakamoto, A. Takahashi, T. Kawamoto, H. Tanaka, M.Watanabe, M. Arisaka, T. Nankawa, M. Kurihara, Dalton Trans. 2013, 42,16049.[22] S. S. Kaye, J. R. Long, J. Am. Chem. Soc. 2005, 127, 6506.[23] Y. Xie, R. B. Lin, B. Chen, Adv. Sci. 2022, 9, 2104234.[24] J. Cattermull, M. Pasta, A. L. Goodwin, Mater. Horiz. 2021, 8, 3178.[25] N. Ma, R. Ohtani, H. M. Le, S. S. Sørensen, R. Ishikawa, S. Kawata, S.Bureekaew, S. Kosasang, Y. Kawazoe, K. Ohara, M. M. Smedskjaer, S.Horike, Nat. Commun. 2022, 13, 4023.[26] R. Ohtani, H. Matsunari, T. Yamamoto, K. Kimoto, M. Isobe, K. Fujii,M. Yashima, S. Fujii, A. Kuwabara, Y. Hijikata, S. I. Noro, M. Ohba, H.Kageyama, S. Hayami, Angew. Chem. Int. Ed. 2020. 59, 19254.[27] J. Lefebvre, S. Trudel, R. H. Hill, D. B. Leznoff, Chem. Eur. J. 2008, 14, 7156.[28] B. F. Hoskins, R. Robson, N. V. Y. Scarlett, J. Chem. Soc. Chem. Commun.1994, 2025.[29] W. Dong, L. N. Zhu, Y. Q. Sun, M. Liang, Z. Q. Liu, D. Z. Liao, Z. H. Jiang,S. P. Yan, P. Cheng, Chem. Commun. 2003, 2544.[30] A. L. Goodwin, M. Calleja, M. J. Conterio, M. T. Dove, J. S. O. Evans, D.A. Keen, L. Peters, M. G. Tucker, Science 2008, 8, 794.[31] J. L. Korcok, M. J. Katz, D. B. Leznoff, J. Am. Chem. Soc. 2009, 131, 4866.[32] A. B. Cairns, J. Catafesta, P. Hermet, J. Rouquette, C. Levelut, D. Maurin,A. van der Lee, V. Dmitriev, J. L. Bantignies, A. L. Goodwin, J. Haines, J.Phys. Chem. C Nanomater Interfaces 2020, 124, 6896.[33] S.-i. Nishikiori, J. Coord. Chem. 2006, 37, 23.[34] A. H. Pohl, A. M. Chippindale, S. J. Hibble, Solid State Sci. 2006, 8, 379.[35] R. Juza, H. Hahn, Z. Anorg. Allg. Chem. 2004, 239, 282.[36] J. Zhang, Phys. Chem. Miner. 1999, 26, 644.[37] T. Ji, Y. Song, J. Yan, Y. Sun, C. Wang, S. Chen, Y. Liu, Chem. Mater. 2022,34, 7878.[38] T. Mochida, Y. Qiu, Y. Funasako, M. Inokuchi, M. Noguchi, H. Fujimori, Y.Furushima, Chem. Commun. 2022, 58, 6725.[39] T. Mochida, Y. Qiu, R. Sumitani, H. Kimata, Y. Furushima, Inorg. Chem.2022, 61, 14368.[40] M. Moriya, D. Kato, W. Sakamoto, T. Yogo, Chem. Eur. J. 2013, 19, 13554.[41] Y. Hirai, T. Nakanishi, Y. Kitagawa, K. Fushimi, T. Seki, H. Ito, H. Fueno,K. Tanaka, T. Satoh, Y. Hasegawa, Inorg. Chem. 2015, 54, 4364.[42] Y. Ohara, T. Nishiguchi, X. Zheng, S.-i. Noro, D. M. Packwood, S. Horike,Chem. Commun. 2024, 60, 9833.[43] C. Das, T. Ogawa, S. Horike, Chem. Commun. 2020, 56, 8980.[44] H. Kimata, T. Mochida, Chem. Eur. J. 2019, 25, 10111.Manuscript received: August 24, 2025Revised manuscript received: October 7, 2025Version of record online: October 25, 2025Chem. Eur. J. 2025, 31, e02640 (6 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 66, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202502640 by National Institute For, Wiley Online Library on [28/11/2025]. 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 t par 
elax Reversible and Massive Structural Transformation in Meltable Cyanido-bridged Coordination Polymer Crystals 1. Introduction 2. Results and Discussion 3. Conclusion Supporting Information Accession code Acknowledgments Conflict of Interest Data Availability Statement