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[Nattapol Ma](https://orcid.org/0000-0002-6162-1834), Soracha Kosasang, Ellan K. Berdichevsky, Taichi Nishiguchi, Satoshi Horike

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Functional metal–organic liquidsChemicalScienceREVIEWOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueFunctional metalNattapol MaNiJadiRg2I(rfcmetal–organic framework glasses.aInternational Center for Young ScientistsScience, 1-1 Namiki, Tsukuba, Ibaraki, 3nims.go.jpbCentre for Membrane Separations, AdsorptKU Leuven, Celestijnenlaan 200F, 3001 LeucDepartment of Chemistry, Graduate SKitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto3r@kyoto-u.ac.jpdDepartment of Synthetic Chemistry and BEngineering, Kyoto University, Katsura, NisheInstitute for Integrated Cell-Material ScienUniversity, Yoshida-Honmachi, Sakyo-ku, KyfDepartment of Materials Science and EnginEngineering, Vidyasirimedhi Institute of SThailandCite this: Chem. Sci., 2024, 15, 7474Received 17th March 2024Accepted 30th April 2024DOI: 10.1039/d4sc01793ersc.li/chemical-science7474 | Chem. Sci., 2024, 15, 7474–7–organic liquidsNattapol Ma, *ab Soracha Kosasang, c Ellan K. Berdichevsky,d Taichi Nishiguchidand Satoshi Horike *cefFor decades, the study of coordination polymers (CPs) and metal–organic frameworks (MOFs) has been limitedprimarily to their behavior as crystalline solids. In recent years, there has been increasing evidence that they canundergo reversible crystal-to-liquid transitions. However, their “liquid” states have primarily been consideredintermediate states, and their diverse properties and applications of the liquid itself have been overlooked. Aswe learn from organic polymers, ceramics, and metals, understanding the structures and properties of liquidstates is essential for exploring new properties and functions that are not achievable in their crystalline state.This review presents state-of-the-art research on the liquid states of CPs and MOFs while discussing thefundamental concepts involved in controlling them. We consider the different types of crystal-to-liquidtransitions found in CPs and MOFs while extending the interpretation toward other functional metal–organicliquids, such as metal-containing ionic liquids and porous liquids, and try to suggest the unique features ofCP/MOF liquids. We highlight their potential applications and present an outlook for future opportunities.attapol Ma obtained his PhDn 2023 from Kyoto University,apan, with Prof. Satoshi Horikend received an FWO post-octoral fellowship for conduct-ng research in the group of Prof.ob Ameloot at KU Leuven, Bel-ium (2023–2024). In April024, he joined the Nationalnstitute for Materials ScienceNIMS), Japan, as an ICYSesearch fellow. His researchocuses on charge transport inoordination polymer and(ICYS), National Institute for Materials05-0044, Japan. E-mail: ma.nattapol@ion, Catalysis and Spectroscopy (cMACS),ven, Belgiumchool of Science, Kyoto University,606-8502, Japan. E-mail: horike.satoshi.iological Chemistry, Graduate School ofikyo-ku, Kyoto 615-8510, Japances, Institute for Advanced Study, Kyotooto 606-8501, Japaneering, School of Molecular Science andcience and Technology, Rayong, 21210,5011. IntroductionSolid-to-liquid transition, a ubiquitous yet complex phenom-enon, is oen described as one of the most fundamental andpivotal processes in materials science.1 It offers opportunitiesfor material processing and the realization of exotic function-alities. For example, the melt growth process from their liquidstates allows the production of large single crystals of Si andLiNbO3, essential materials in semiconductor technology andferroelectric applications.2–4 The quenching of liquid states alsoyields glass, a transformation that is signicant in optical andmaterials engineering.5 Accessing melting involves the strategicloosening of cohesive forces at melting temperature (Tm, Fig. 1).For example, Ga melts at low Tm due to its large atomic sizeSoracha KosasangSoracha Kosasang obtained herPhD in 2022 from VISTEC,Thailand. She is currently a JSPSpostdoctoral fellow in the groupof Prof. Satoshi Horike at KyotoUniversity, Japan. She was alsoa visiting researcher in the samegroup (2019–2020) and in thegroup of Prof. Maria Lukatskayaat ETH Zurich (2020–2021). Herresearch focuses on multiphasephoto- and electro-catalysis.© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://crossmark.crossref.org/dialog/?doi=10.1039/d4sc01793e&domain=pdf&date_stamp=2024-05-18http://orcid.org/0000-0002-6162-1834http://orcid.org/0009-0006-5254-2890http://orcid.org/0000-0001-8530-6364http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793ehttps://pubs.rsc.org/en/journals/journal/SChttps://pubs.rsc.org/en/journals/journal/SC?issueid=SC015020Fig. 1 Approximate melting temperature range in organic ionic liquid,covalent polymer, CP/MOF, metal, and ceramic/oxide.Ellan K: BerdichevskyEllan K. Berdichevsky receivedhis MSc in Chemistry in 2022 atMemorial University ofNewfoundland, Canada, with DrMichael Katz. He is now a PhDstudent in the group of Prof.Satoshi Horike. His researchinterests include exploring theconductive and mechanicalproperties of CP/MOFs.Taichi NishiguchiTaichi Nishiguchi received hisbachelor's degree in 2023 at theFaculty of Engineering, KyotoUniversity. He is currentlya master's student working withProf. Satoshi Horike at KyotoUniversity. His research inter-ests focus on the properties ofliquid and glassy states of coor-dination polymers and MOFs.© 2024 The Author(s). Published by the Royal Society of ChemistryReview Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online(weakmetallic bond) and unusual crystal structure.6 For organicpolymers, factors like molecular weight, degree of cross-linking,and composition all play pivotal roles in controlling the Tm.7Reducing the network size in silica glass decreases the pro-cessing temperature (Fig. 6).8 The reduction of Tm in ionicliquids is predominantly driven by entropy changes.9 Theseexamples highlight diverse mechanisms involved in controllingmelting behavior across different material classes and oppor-tunities within this solid–liquid transition in materials science.Coordination polymers (CPs) and metal–organic frameworks(MOFs) are solid materials with repeating coordinationentities.10–14 They are assembled frommetal nodes, commonly d-or f-block metal cations or clusters (secondary building units,SBUs), and linkers capable of bridging metal nodes via coordi-nation bonds to generate polymeric arrays extended in one, two,or three dimensions.15 Robust and predictable coordinationbonds enable the precise positioning of atoms in three-dimensional (3D) space, while the presence of organic linkersallows chemists to pre-program functional side groups. Theseimportant features of CPs/MOFs allow the control of structuresand properties to be feasible even at the molecular level.12,13 Theirfunctions have covered a wide range of elds, such as gas sepa-ration and storage, catalysis, optics, charge and mass transport,magnetics, and more.16–27 However, the development progresshas predominantly revolved around the crystalline phase.In recent years, the concept of multistability within crystallineCPs/MOFs has expanded beyond the transformation between twostable crystalline states, observed in so porous crystals,55–66 tofurther include the complete structural transformations in stableliquid and glass states.31,54,67 The emergence of liquid and glassCPs/MOFs presents opportunities for material processing and theexploration of novel or improved features inherent to the originalcrystals. Although melting and vitrication are common in manymaterial families, they remain exotic within the context of CPs/MOFs since most tend to decompose upon heating due to theirreversible decomposition prior to reaching the Tm. This led to theexploration of alternative methods that circumvent the need forSatoshi HorikeSatoshi Horike received his PhDin chemistry from Kyoto Univer-sity in 2007, followed by a post-doctoral fellowship at theUniversity of California, Berke-ley, from 2007 to 2009. Hereturned to Kyoto in 2009 as anAssistant Professor and has beenan Associate Professor/PI atKyoto University since 2017. Heis currently a Professor at theGraduate School of Science,Kyoto University, and theVidyasirimedhi Institute ofScience and Technology, Thailand. His research interest is thesynthesis of glass and liquid states of molecular frameworkmaterials.Chem. Sci., 2024, 15, 7474–7501 | 7475http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eChemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinemelting, including mechanical vitrication,68–72 directsynthesis,73,74 and dehydration.75,76 Since the rst observation ofstable liquid states34 and vitrication,31,54,67 examples of CPs/MOFsexhibiting crystal melting have recently expanded beyond d10metal ions to include a broader range of transition metals andamore comprehensive selection of linkers (Fig. 2 and 3).44,45,48,53,77,78Before getting into the details, it is crucial to establish a clearunderstanding of the term “metal–organic liquid” and describethe scope of this review. Broadly, this term encompasses liquidmaterials comprising metal ions and organic components thatlink into repeating entities under certain states. The termincludes a diverse array of entities known to exhibit a liquidstate, ranging from metallo-supramolecular polymers,79–82metal–organic cages/polyhedra,83–86 metal complexes,87–89 andcertain metal-containing ionic liquids,88,90–96 among others.Many were known before the observation of melting behavior inCPs/MOFs. The distinctions between these systems becomeblurred upon the transition from solid to liquid. For instance,while CPs/MOFs exhibit extended coordination bondingnetworks in their crystalline state, such characteristics may notpersist upon melting. The behavior of CPs/MOFs upon meltingvaries across individual systems; some undergo completeFig. 2 Three main categories of melting behaviors found in CPs/MOFs:tures, represented by Zn(H2PO4)2(HTr)2 (HTr = 1,2,4-triazole).28 (B) Partiacleavage of coordination bonds as a polymer-forming liquid, representrepresented by dark blue, gray, light blue, red, orange, and brown, respe7476 | Chem. Sci., 2024, 15, 7474–7501dissociation of coordination bonds, resembling discrete metalcomplexes in their liquid state,28 while others exhibit visco-elastic proles similar to ionic liquids31 or retain polymericcharacteristics with coordination bonds intact across allstates.30 This review predominantly discusses the fundamentalconcepts and recent advancements in the design of stable liquidstates for CPs/MOFs, particularly highlighting their meltingbehavior, structures, and properties. Our focus is on thesystems that maintain well-dened, extending coordinationbonding networks in specic states. Additionally, strategies forachieving thermodynamically stable liquid states are discussed,including insights from neighboring systems such as metal-containing ionic liquids, which possess well-dened struc-tures but melt below room temperature, and porous liquids.2. Metal–organic liquids andsupercooled liquids2.1 Thermodynamics of phase change and meltingtemperatureStabilizing a liquid state involves balancing melting (Tm) anddecomposition (Td) temperatures, usually under atmospheric(A) non-preservation of coordination bonds or ionic liquid-like struc-l dissociation of coordination bonds, represented by ZIF-4.29 (C) Non-ed by Cu(2-isopropylimidazolate).30 Zn, C, N, O, P, and Cu atoms arectively. Isopropyl groups and H atoms are omitted for clarity.© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 3 Examples of CPs and MOFs known to melt and their melting temperature (Tm). Phosphate-azole from left to right: [Zn3(H2PO4)6(-H2O)3](azole) (azole = HmbIm = 2-methylbenzimidazole,31 BTA = benzotriazole,32 HbIm = benzimidazole).31[Zn2(HPO4)2(H2PO4)](ClbImH+)2(H2PO4−)(MeOH) (ClbIm = 5-chloro-1H-benzimidazole).33 [Zn(HPO4)(H2PO4)2](H2Im)2 (HIm = imidazole).34[Zn(H2PO4)2(HPO4)](H2dmbIm)2 (HdmbIm = 5,6-dimethylbenzimidazole).35,36 Zn(HTr)2(H2PO4)2 (HTr = 1,2,4-triazolate).31 Nitrile and thiocya-nate: Li[N(SO2F)](NCCH2CH2CN)2.37 [Emim][K(tricyanomethanide)2] (Emim= 1-ethyl-3-methylimidazolium).38,39 Cu8(SCN)12(Phbpy)4 (Phbpy= 1-phenyl-[4,40-bipyridin]-1-ium).40 [Ag(pL2)(CF3SO3)]$2C6H6 (pL2 = 1,3,5-tris(4-cyanophenylethynyl)benzene).41 Ag(L1)(CF3SO3)2 (L1 = 4,40-biphenyldicarbonitrile).42 Diamine, bis(acetamide), dicyanamide, and carboxylate: [Ag(ethylenediamine)][Tf2N].43 Fe(eba)3[ZnCl4] (eba = ethyl-enebis(acetamide)).44 Mg(bba)3Cl2 (bba = butylenebis(acetamide)).45 (TPrA)[M(dca)3] (TPrA = tetrapropylammonium, dca = dicyanamide).46,47Mg4(adipate)4(DMA)(H2O).48 Imidazolate: Cu(isopropylimidazolate).30,49 Linker exchanged ZIF-8 (Zn(mIm)2x(Im)2y(mbIm)2z, (x + y + z = 1), mIm =methylimidazolate, Im = imidazolate, bIm = benzimidazolate).50 ZIF-62 (M(Im)2−x(bIm)x, M = Zn2+ or Co2+).51,52 MUV-24 (Fe(Im)2).53 ZIF-4(Zn(Im)2).54 H are omitted for clarity.Review Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinepressure, which is achieved by either lowering Tm or raising Td.Melting is understood as the transition of particles between twoor more equilibrium phases at a well-dened temperature andpressure and is described in the context of chemical potential(m). m is dened as the partial deviation of G with respect to theamount of substance (n): m = (vG/vn)P,T. The differential of© 2024 The Author(s). Published by the Royal Society of ChemistryGibbs energy is written as dG = VdP − SdT + mdn. The differ-ential of Gibbs energy is also given by dG = ndm + mdn. Hence,ndm = VdP − SdT is derived. This equation is also expressedusing molar volume Vm and molar entropy Sm: dm = VmdP −SmdT and is known as the Gibbs–Duhem equation. Sincemelting involves the migration of particles between solid andChem. Sci., 2024, 15, 7474–7501 | 7477http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eChemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineliquid phases in equilibrium, the deviation of the chemicalpotential of each phase is balanced as dmsolid = dmliquid. Byemploying the Gibbs–Duhem equation, the equation Vm,soliddP− Sm,soliddT = Vm,liquiddP − Sm,liquiddT is obtained. Transitionentropy DtrSm and transition volume DtrVm are dened as thedifference of entropy and volume of each phase, and dP/dT =DtrSm/DtrVm is derived. The Tm is dened as the ratio of meltingenthalpy and melting entropy: Tm = DHfus/DSfus. Therefore, Tmis minimized by minimizing the DHfus while maximizing theDSfus (Fig. 4).Minimizing DHfus involves minimizing cohesive chemicalinteractions among the components, including coordinationbonds, hydrogen bonds, electrostatic interactions, and van derWaals interactions. Upon melting, the system should allowa certain degree of dissociation, which later translates intomicro uctuation and macroscopic uidity in the liquid state(Fig. 2). This involves total coordination bond dissociation,28,31partial coordination bond dissociation,29 or the dissociation ofvan der Waals interactions while maintaining the coordinationbond.30 For example, pairing a d10 transition metal ion withlower crystal eld stabilization energy with organic linkerforming of weak coordination bonds would indeed requirecomparatively lower energy to dissociate the coordination bond.On the other side, the entropy of extended systems is inter-preted as the summation of rotational, vibrational, andcongurational terms, ignoring electronic or spin states. DSfusreects the freedom of structure and mobility. Designing CPs/MOFs with low-symmetry, high-exibility linkers in whichmobility is restricted in the crystalline state should give rise tothe term DSfus since diverse conformations are only accessibleupon melting. Chemical component paring and resultingcrystal structures inuence these thermodynamic terms. WeakFig. 4 (A) Comparison of the difference in enthalpy of fusion (DHfus), theand the melting temperature (Tm) of selected CP/MOF glasses. Each syrepresents Tm. Data are taken from the following references: Zn–phosphtripodal nitriles,41,69 ZIF-62,52,97,98 ZIF-4,54 and HOIPs.47 Adapted with permCreative Commons license CC-BY-NC-ND 4.0. https://creativecommonliquid transition.7478 | Chem. Sci., 2024, 15, 7474–7501coordination bonds between cations and ligands, as well asweak electrostatic interactions between cations and anions,together minimize DHfus. The conformational exibility of thebridging ligands also helps maximize DSfus.The next question is: which of enthalpy or entropy contrib-utes more to lowering the Tm? To investigate this, we turn ourattention to a study that delves into the origins of Tm in ionicliquids (ILs).9 Comparing DHfus and DSfus of 20 alkali halidesand 257 ILs shows that DSfus plays a more critical role (2.67times larger in ILs) compared to DHfus (0.85 times larger) on Tmlowering. Even in the case where conformational entropy in thecation and the anion is absent in ILs, a larger DSfus stillcontributes more to reducing the Tm. To further quantify theorigin of the large entropic contribution, the DSfus ofimidazolium-based ionic liquids (imILs) was decomposed intokinetic (Skin) and structural (Sstr) entropies. The former includestranslational (Stra), rotational (Srot), and intramolecular vibra-tion (Svib). The latter comprises conformational or intra-molecular (Sconf) and congurational or intermolecular (Scong)entropies. The DSkin of imILs is smaller than NaCl, with DSvibremaining close to zero or even showing a negative value. Thisbehavior is attributed to the lack of signicant changes inintramolecular vibration during the melting process and thepresence of inactive diffusive motions resulting from the lowerTm of ILs. A large DSstr, especially driven by a considerableDScong, signicantly contributes to the reduction of Tm. Theorigin of DScong is associated with the existence of multiplecongurations, characterized by the presence of delocalizedcharges and an asymmetric ion structure (Fig. 4B).The elongation of alkyl chains on the cation of ILs is directlyassociated with an increase in DSconf and is concurrent with anelevation in DSfus, while having minimal impact on bothdifference in entropy of fusion (DSfus) between solid and liquid phases,mbol shape denotes a series of compounds, while the symbol colorate-azole,28,31,32,34 metal-bis(acetamides),45 copper thiocyanates,40 Ag-ission from ref. 78. Copyright 2022 American Chemical Society unders.org/licenses/by/4.0/. (B) Difference in entropy (DS) upon crystal-to-© 2024 The Author(s). Published by the Royal Society of Chemistryhttps://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 5 Degrees of thermal vibration of oxygen atoms around the Zn2+.O1 (blue circles), O3 (green triangles), O5 (purple diamonds), and O9(red squares) and the ORTEP model and the H-bond distances of[Zn(HPO4)(H2PO4)](H2Im)2 (ZnPIm) at −50 °C. The Zn, P, O, N, and Catoms are shown in purple, yellow, red, blue, and gray, respectively; Hatoms are omitted. Adapted with permission from ref. 31. Copyright2015 American Chemical Society.Review Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineDScong and DSkin.99 This behavior is illustrated in 1-alkyl-3-methylimidazolium bis(triuoromethylsulfonyl)amide([Cnmim][NTf2], where n = 2, 4, 6, 8, and 10). When comparingthe molecular dynamics (MD) simulations between liquid andgas states, it was also found that the population of transconformations for the cation is more preferred in liquid thanthat in gas. The higher stability of trans conformers is due to (1)van der Waals interactions between alkyl chains and (2)coulombic interactions between the cations and anions that arenot disturbed in these conformers. The populations of transconformers for anion are, on the other hand, almost identicalfor both states. Apart from introducing linear n-alkyl chains, theintroduction of branched alkyl substituents simulates a reduc-tion in the melting point (Tm) of C60 derivatives.100,101 This isattributable to the presence of additional bulky, exible groups,which contribute to an increase in DSfus. Simultaneously, theintroduction of these groups disturbs the p–p interactionsbetween the p-conjugated cores, leading to a decrease in DHfus.Lindemann's rule is another important parameterdescribing the origin of melting behavior in crystalline solids.102It predicts that melting occurs when the root mean squaredisplacement of particles due to thermal vibration exceedsa certain percentage of the interparticle spacing.103–105 Asa function of temperature, a measure of the stability of coor-dination bonds, or the Lindermann ratio, is quantied by thevariance in metal–ligand thermal vibration (u) compared to theinter-atomic distance d.31,102 u is the square root of the Debye–Waller factor and is obtained directly from a single crystal XRD.The coordination bond is considered unstable at a criticalLindemann ratio (f = u/d) of around 0.1 but is varied dependingon structure and bonding.The determination of the Lindemann ratio is benecial toclarifying the melting mechanism. An example illustrated inone-dimensional (1D) CP [Zn(HPO4)(H2PO4)2]$2H2Im (referredto as ZnPIm).31 The melting behavior is hypothesized to involvecoordination bond dissociation within the 1D zinc–phosphate,ion-pair formation, and stabilization by its high ionicity. Toclarify the melting event, the thermal vibration of oxygen atomsthat coordinate with Zn2+ is analyzed (Fig. 5). Measuring single-crystal XRD under variable temperatures shows that the f valueof all O atoms approaches 0.1 to 0.13 near Tm. The coordinatedoxygen atom of the H2PO4 ligand labeled as O9 exhibits a higherdegree of thermal vibration than the other coordinated oxygenatoms (O1, O3, and O5). The H2PO4-contained O9 interacts withthe imidazolium cation via hydrogen bonding, in which theimidazolium cation is rotatable at high temperatures, leading toits higher f value. The f value of O9 is 0.12, while O1, O3, and O5are less than 0.10 at 140 °C, just below Tm, suggesting that themelting event starts from the bond dissociation between Zn2+and O9. The other Zn–O bonds dissociate aer breaking thestable tetrahedral arrangement of the Zn2+ ion. The zinc–phosphate chains' loosening is induced by the mobility ofimidazolium cations.While most CPs/MOFs experience a complete transitionfrom a crystalline solid to a liquid at a single temperature,referred to as congruent melting, some exhibit multistep tran-sitions (incongruent melting), leading to the formation of© 2024 The Author(s). Published by the Royal Society of Chemistrya solid–liquid mixture (Table 1).38,39,95,107 In such cases, thetemperature at which the compound begins to melt is termedthe solidus temperature. The point at which complete lique-faction is achieved is referred to as the liquidus temperature. Acompound melts congruently when its composition in theliquid state matches its original solid state. Some compoundsbecome unstable during the transformation into a liquid,leading them to melt incongruently into their componentsinstead of retaining their original composition.CPs with a composition of [Ru(Cp)(C6H5R)][M{C(CN)3}2] (R=Me, Et; M = K, Rb; Cp = C5H5) melt incongruently, forminga mixture of solid M[C(CN)3] salts and ionic liquids[Ru(Cp)(C6H5R)][C(CN)3].95 This occurs due to the low solubilityof M[C(CN)3] in the ionic liquids. The melt states of thesecompounds are relatively unstable and show partial decompo-sition of the liquid phase. Replacing organometallic cationswith Emim+ (1-ethyl-3-methyl-imidazolium), a representativeionic liquid component, improves the thermal stability of liquidstates while maintaining incongruent melting behavior.38Melting of [Emim][K(TCM)2] (TCM = tricyanomethanide)begins at ca. 112 °C, accompanied by an immediate growth anddeposition of microcrystals. XRD and Raman conrm thecomposition of the solid–liquid mixture to be K[TCM] micro-crystals and [Emim][TCM] ionic liquid. A uniform liquid isobserved aer the temperature reaches 240 °C. Correlationsbetween CPs with incongruent melting behavior and theirconstituents are generalized in a series of CPs synthesized fromonium ionic liquids and K[TCM].39 The Tm of CPs was linearlycorrelated with the Tm of ILs. The cooling rate required forvitrication was correlated with the exibility of cations, withChem. Sci., 2024, 15, 7474–7501 | 7479http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eTable 1 Selected CPs/MOFs systems showing incongruent melting behavioraMaterials Note Tsolidus/°C Tliquidus/°C Ref.[Ru(Cp)(C6H5C2H5)][Rb{C(CN)3}2] 3D 102.9 220 95[Ru(Cp)(C6H5C2H5)][K{C(CN)3}2] 3D 132.9 — 95[Ru(Cp)(C6H5CH3)][K{C(CN)3}2] 3D 214.6 — 95[Emim][K(TCM)2] 2D 112.3 240 38 and 39[EtPy][K(TCM)2] 2D 107.7 — 39[PrPy][K(TCM)2] 2D 89.9 241 39[N(C4H8)2][K(TCM)2] 2D 191.6 233 39[SEt3][K(TCM)2] 2D 73.3 — 39[NEt4][K(TCM)2] 3D 130.6 — 39[CsHSO4]x[ZnPIm]1−x Binary system 71.3–85.8 82.5–155 107a Emim+, Etpy+, PrPy+, N(C4H8)2+, SEt3+, and NEt4+ are 1-ethyl-3-methyl-imidazolium, 1-ethylpyridinium, 1-propylpyridinium, 5-azaspiro[4.4]nonan-5-ium, triethylsulfo-nium, and tetraethylammonium, respectively. x represents the molar fraction of CsHSO4 in the binary system. Note that theTliquidus of some systems is not reported.Fig. 6 Viscosity versus temperature curve of various glasses andviscosity reference temperature. Data are taken from ref. 106.Chemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehigher exibility resulting in easier vitrication abilities. Inaddition to these pure compounds, a binary system composedof CP and CsHSO4 also shows incongruent melting and eutecticbehavior.107 Detailed discussions are provided in the nextsection.The transition from a glassy state to a liquid state is morecomplex. The glass transition temperature (Tg) marks the pointat which there is a noticeable change in the temperature-dependent thermodynamic properties, shiing from valuesresembling those of a crystal to those of a liquid.108 Below the Tg,materials exist in a glassy state. When heated above its Tg butstill below the liquidus point, these glasses undergo a trans-formation from a rigid, amorphous solid to a viscoelastic,supercooled liquid state. In this state, the supercooled liquiddisplays a viscous response, making it suitable for shaping.Once it cools down again to below Tg, the supercooled liquidstructure freezes to a solid glassy state, and the shape ismaintained. The viscosity at Tg is z 1012.5 Pa s (Fig. 6). Whenobserving the viscoelastic behavior using dynamic mechanicalanalysis, the point at which the loss modulus (G00) reaches itsmaximum (relaxation temperature) signies the onset ofa supercooled liquid state. Heating the supercooled liquidabove the liquidus point (oen equivalent to Tm) results in thetransition to the liquid phase (viscosity < 101 Pa s). This viscosityis suitable for the melt to provide good homogeneity within themelting container, and air pockets are removed by vacuum.Handling some liquids might require an inert atmosphere toavoid oxidation or degradation.For glass-forming liquids, the concept of liquid fragility hasbeen used to classify the viscosity/temperature relations, whichare represented by the fragility diagram (Fig. 7).5,109,110 A liquid isconsidered “strong” when it exhibits near-Arrhenian behaviorover the entire viscosity range. In contrast, a liquid that displaysa large degree of curvature is termed “fragile.” Generally, strongliquids maintain a high degree of short-range order and onlyallow minor dissociation of bonds upon increasing tempera-ture.111 As a result, only a small change in heat capacity isobserved when passing through the Tg. On the other hand,fragile liquids tend to have less well-dened short-range order,and their structures disintegrate rapidly with temperatures7480 | Chem. Sci., 2024, 15, 7474–7501increasing above Tg with a large change in heat capacity. Inaddition, the liquid fragility is usually quantied by the fragilityindex, m, where m = [d(log h)/d(Tg/T)]T=Tg, which describes theslope of viscosity (h) with temperature as it approaches Tg.112 Forexample, SiO2 has a m value of ca. 20 and is considered a strongliquid, while the m value of fragile liquids usually rangesbetween 40 and 50.2.2 Melting behaviors and structuresDetermining average structures in liquids and glasses isimportant for understanding their behavior and achievingfunctionality. It requires the integration of various techniques,such as the pair-distribution function (PDF), solid-state NMR,extended X-ray absorption ne structure (EXAFS), and atomisticmodeling.78 The building blocks of CPs/MOFs, consisting ofmetal nodes and bridging linkers, establish interconnectingnetworks through coordination bonds in their crystalline state.When it is subjected to melting, these connections display© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 7 Fragility plot (Angell's plot) of selected glass-forming systems.Data are taken from ref. 5, 54, 97 and 113–115. HDA and LDA are thehigh-density and low-density amorphous phases of ZIF-4, respec-tively. Red, blue, green, orange, and black lines and symbols presentinorganic, organic, metallic, water, and metal–organic systems,respectively.Review Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinediverse behaviors that fall into three main categories observedthus far (Fig. 2): (1) non-preservation of coordination bonds orionic liquid-like structures, (2) partial dissociation of coordi-nation bonds, and (3) non-cleavage of coordination bonds asa polymer-forming liquid.The foremost behavior is observed in two-dimensional (2D)Zn(H2PO4)2(HTr)2 (HTr = 1,2,4-triazole) (Fig. 2A).28 The CPconsists of octahedral (Oh) Zn2+ with monocoordinated ortho-phosphate (axial position) and bridging 1,2,4-triazole, whichform extended arrays of 2D sheets parallel to the ab plane. Itmelts at 184 °C. This initiates a local geometry transformationaround Zn2+ from an octahedral (Oh) in crystal to a tetrahedral(Td) arrangement in liquid. The change drives the structuraltransformation from 2D to 0D (discrete molecular fragment)and persists even aer cooling in the glassy state. The emer-gence of these discrete molecular fragments highlights thesimilarity between the melting of CPs/MOFs and the behavior ofionic liquids. Conventionally, the network preservation betweencrystal and glass states in CP/MOF is rationalized by consid-ering that the lattice enthalpies must be comparable in bothstates. This must be an exceptional case where the latticeenthalpies between the two states are comparable despitehaving very different structures.116Slight variations in behavior are also observed in the case of1D Zn–phosphate-azole: ZnPIm.31 A single sharp peak for theliquid state (160 °C) compared to the broad peak of glass andcrystal displayed in the 31P solid-state NMR spectra conrms thediscrete molecular fragments of Zn2+, phosphate, and imida-zolium ions without coordination bond preservation, referringto an ionic liquid-like structure. A dynamic mechanical analysis(DMA) displays an immediately higher value of the loss© 2024 The Author(s). Published by the Royal Society of Chemistrymodulus than the storage modulus (G00 > G0) above 30 °C (Tg),which is a typical prole of viscoelastic uids. Suppose thecoordination bonds are preserved upon melting, as in linearorganic polymers. In that case, the effect of entangled chainsshould be presented as a rubbery plateau regime (G0 > G00). Thissupports the cleaved-bond model without preserving coordi-nation bonds. The viscosity of the liquid state, derived from theshear modulus, also follows the typical prole for ionic liquids.Despite the coordination bonds being cleaved and behaving likeionic liquids upon melting, PDF conrms that its 1D polymericstructure re-establishes upon cooling below the Tg.The melting process of Zn(Imidazolate)2 (ZIF-4) leads to thepartial dissociation of coordination bonds (Fig. 2B).29 Thecompound consists of Zn2+ and imidazolate groups in a tetra-hedral coordination arrangement, forming a 3D-crystallinenetwork with a maximum cavity diameter of 4.9 Å. Note thatZIF-4 undergoes amorphization and recrystallization to becomenonporous ZIF-zni upon heating before the melting process.54Through rst-principle molecular dynamics (FPMD) simula-tions, over 94% of Zn2+ maintains the ideal coordinationnumber of 4, where Zn2+ coordinates with the N atom of fourimidazole linkers just below the Tm. The number of under-coordinations of Zn2+ in addition to 4-fold coordination abovethe Tm, such as tri- and bi-coordination, arises due to the partialdissociation of Zn2+–N bonds. The snapshots of microscopicevolution during melting (FPMD at 1227 °C) reveal the breakingof Zn2+–N bonds and the reorientation of imidazolate linkersbetween adjacent coordination sites within picoseconds. Notethat the higher temperature ranges in FPMD are not physicallyrelevant for the experimental system but are necessary, due tothe short time explored in FPMD, to gather statistics on rela-tively rare events and high thermodynamic barriers. A relativelysmall congurational difference between the crystalline stateand its corresponding liquid translates into a small entropicdifference when compared to other systems, together witha relatively low fragility index. The ZIF-4 melts also sharea common feature with ionic liquids, where their constituentsshow comparable translational diffusion between imidazolateand Zn2+ of 7.7 × 10−10 and 6.5 × 10−10 m2 s−1, respectively.An identical feature in melts has been observed in metal-bis(acetamide) frameworks, such as Co(N,N0-1,4-butylenebis(acetamide))3[CoCl4], in which the coordinationnumber around Co2+ centers decreases by ca. 20% uponmelting.45 In the molten state, the average coordination numberis approximately 4.8, which signicantly surpasses the bondpercolation threshold of 2.4 required for a 3D aperiodicnetwork. PDF analysis showcases the insignicant alteration ofpair distances up to ca. 4 Å in the melt compared to the crys-talline state and, more importantly, quasiperiodic oscillationsextending up to 80 Å. The latter points to the existence of bothtopological and chemical ordering, thus verifying the partiallyretained extended-range order connectivity within the liquid.The third type emerges when melting occurs without thedissociation of any bonds, resembling the melting characteris-tics of organic polymers (Fig. 2C). One example of a polymer-type forming liquid is found in 1D Cu(2-isopropylimidazolate)with a Tm of 143 °C.30,49 The initial crystal structure comprisesChem. Sci., 2024, 15, 7474–7501 | 7481http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eChemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online6.5 Cu2+-isopropylimidazolate units, each of which is crystallo-graphically distinct. These 1D chains are assembled throughvan der Waals interactions. Heating above its Tm, thecompound maintains its coordination environment and intra-molecular connectivity (N–Cu+–N). The main PDF peaks of thecrystalline state (30 °C) at 6.1, 11.6, and 17.1 Å correspond to thenearest, second, and third neighbor correlations of two intra-chain Cu+ ions. The peak at 6.1 Å remains mostly unchangedeven above the Tm, indicating the preservation of Cu+-iso-propylimidazolate-Cu+ bridging within its liquid state. Incontrast, the reduced intensity of peaks at 11.6 and 17.1 Åsuggests lower structural periodicity. An insignicant change incoordination number from 2 in the crystalline state to 1.97 inthe liquid state also supports the retention of the 1D chainstructure model without bond breaking. 1H MAS solid-stateNMR of the melt at 157 °C shows peaks narrowing due tohigher molecular dynamics compared to the parent crystal. Theretention of spinning side bands suggests the presence of weakanisotropic nuclear spin interactions between the ligands. Abroader distribution of intermolecular Cu+–Cu+ correlation andthe relatively high viscosity with storage modulus above the Tmsuggest that the melting is due to the dissociation of chainpacking and entangled networks of stiff chains rather than thedissociation of coordination bonds as observed in previousexamples.Viscosity modulation of liquid CPs/MOFs through theincorporation of network modiers has been a longstandingtechnique to regulate the processing temperature of glass (Fig. 6and 8A).111 Specically, pure silica glass (quartz glass) composedof SiO2 typically demands temperatures as high as 1800 °C toachieve a viscosity appropriate for processing, owing to thepresence of extended Si–O–Si connectivity. The viscosity of themelts is reduced by adding network modiers such as Na2O,CaO, or MgO. Introducing terminal oxygen to reduce networkconnectivity results in a lower viscosity (lower Tm and Tg) andthus lower processing costs. The concept is also known as chainscissions, but it is less known for meltable CPs/MOFs. The sameprinciple is also applicable for controlling the thermal behaviorof polymers since their properties are dictated by molecularweight (Fig. 8B).7A superionic solid acid, CsHSO4, functions as a networkmodier when introduced to ZnPIm.107 As previouslymentioned, ZnPIm consists of a 1D Zn–phosphate coordinationchain and non-coordinated H2Im molecules. The physicalmixing of ZnPIm (Tm= 154 °C) with CsHSO4 (Tm= 206 °C) leadsto changes in the mixture's Tm, revealing composition-dependent melting behavior (Fig. 8C). For example, the DSCsof the mixture [CsHSO4]x[ZnPIm]1−x with equivalent mol frac-tion (x = 0.5) show rst endothermic peaks with an onsettemperature of 85.8 °C of crystal melting. The value is muchlower for both constituents. The melting process remainsincomplete, and a portion of the crystalline domain persistsuntil it completely melts beyond 140 °C. This behavior isreferred to as incongruent melting. The solidus point marks thetemperature at which the mixture starts to melt (85.8 °C), whilethe liquidus point signies the temperature at which themixture achieves complete melting (140 °C). The behavior is7482 | Chem. Sci., 2024, 15, 7474–7501also observable through variable-temperature XRD and SEM. Ateutectic composition (xz 0.75), only a single endothermic peakis observed, showing the lowest Tm and dening the eutecticpoint. This is described as the rst example of eutectic meltingobserved in the system of CPs/MOFs. In addition to exhibitingeutectic behavior, cooling the melts back also results in a binaryglass formation that helps preserve the superprotonic conduc-tivity of CsHSO4 and will be later discussed in Section 4.1. Theenhanced conductivity within the binary system arises fromchanges in their structure and the role of CsHSO4 as a networkmodier for ZnPIm. 31P magic-angle spinning (MAS) solid-stateNMR reveals a partial reorientation of bridging phosphate toa monodentate type due to the oxyanion exchanged. The tetra-hedral coordination of Zn2+ is lled by HSO4− from CsHSO4,leading to chain scission of the primary 1D chain of ZnPIm.This phenomenon reduces the overall viscosity (h < 103 Pa s at65 °C) of the system while simultaneously enhancing themolecular mobility within the system, resembling silica glasswhen network modiers are introduced (Fig. 8D).Water also acts as a network modier to break the contin-uous network of a meltable Co(hmba)3[CoBr4], where hmbarepresents N,N0-1,6-hexamethylene-bis(acetamide).118 In thecrystalline state, Co2+ nodes are coordinated in an octahedralmanner to hmba, forming a 2D layered-like structure. Non-coordinate [CoBr4]2− units are situated between these layersto balance the overall charge. By carefully controlling theaddition of water, it's possible to induce alterations in both theTm and Tg of the glass. The Tm decreases from 110 °C to ∼70 °Cby adding ∼7 wt% water (∼4.6 water molecules per bridgingCo2+), whereas the Tg is also lowered to−20 °C compared to 20 °C of the pristine compound. Note that the amount of addedwater is within the stability limit, and excessive water additionleads to the complete dissociation of the coordination network.Similar effects of decreasing Tm are also observed whenextending toward 2D Mn(hmba)3[MnBr4] and 3D Co(hmba)3[-Co(SCN)4]. Adding water also promotes the glass-forming abilityof Mn(hmba)3[MnBr4] by suppressing the reformation of theinitial network-connecting species. X-ray analyses conducted onanhydrous and hydrated Co(hmba)3[CoBr4] also indicate thatthe presence of water triggers partial decoordination of linkersby forming coordination bonds with Co2+ nodes, even beforethe Tm is reached. To further understand the mechanism, themean square displacement (MSD) and coordination number(CN) of Co atoms were calculated via ab initio MD (Fig. 8E). At727 °C, a temperature corresponding to the melting event (asindicated by the Lindemann ratio between 1.0–1.5), the Co MSDincreases more rapidly with time in the presence of water. Thissuggests that water accelerates the melting process ofCo(hmba)3[CoBr4]. This effect becomes even clearer whenexamining the change in CN. The results show that the presenceof water induces a shi in the melting behavior by promotingthe decoordination of hmba ligands. This leads to a lower Tm.The impact of water is evident in the accelerated decrease of theCN only for the networking Co–O interactions, while the Co–Brinteractions of the anion remain mostly unaffected. Thesemechanisms observed in the two examples here resemble those© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 8 Modification of melting behavior and properties of melts through the addition of network modifiers. (A) Schematic diagram depicting theprocess of soda-lime silica glass formation by introducing network modifiers like Na2O and CaO, accompanied by their correspondingtemperature-dependent viscosity profiles. (B) Approximate relation between molecular weight, glass transition temperature (Tg, blue line),melting point (Tm, red line), and polymer properties.117 (C) A diagram demonstrating composition-dependent solidus and liquidus points of theCsHSO4–ZnPIm binary system at ambient pressure. Blue hexagons and red circles represent the invariant point and temperature of the liquidus,respectively. The diagram has three regions: mixed solids (blue), incongruent melting (purple), and liquid phase (red). (D) Temperature-dependent viscosity of CsHSO4, ZnPIm-g, and [CsHSO4]0.5[ZnPIm]0.5-g over heating scan. (E) Simulated average coordination number of theCo–O correlation in Co(hmba)3[CoBr4] structures containing 0, 1, 2, or 3 water molecules. (B and C) are adapted with permission from ref. 107.Copyright 2022 American Chemical Society. (D) is adapted with permission from ref. 118. Copyright 2023 John Wiley & Sons, Inc. under CreativeCommons license CC BY 4.0. https://creativecommons.org/licenses/by/4.0/.Review Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineof network modiers used in traditional oxide glass-formingsystems to design and tune the liquid and glass properties.Modication of structural composition leads to an accessibleTm in non-melt CP/MOFs. Unlike ZIF-4, the organic linker inZn(2-methylimidazole)2 (ZIF-8) faces instability when the Zn2+–N bonds partially dissociate.119 This instability hinders ZIF-8from undergoing a melting process and instead leads to itsdecomposition upon heating (Tm > Td = 550 °C). This is due toa large bond cleavage activation energy difference and becausethe nearby coordination sites cannot promptly accommodatethe departing linkers due to their low density. Stabilizingagents, such as ionic liquid, are an option to counteract thischallenge and help stabilize the rapidly dissociating linkers.120Incorporating 1-ethyl-3-methylimidazolium bis(tri-uoromethanesulfonyl)imide ionic liquid, [EMIM][TFSI] (IL),into the pores of ZIF-8 (IL@ZIF-8) stabilizes the frameworks,resulting in an accessible Tm (below Td). The IL@ZIF-8composite shows a small endothermic peak (Tm) at 381 °Cbefore decomposing at ca. 412 °C. The results suggest thatelectrostatic interaction between ZIF-8 and the partiallydecomposed IL fragments or IL itself helps stabilize the disso-ciating ZIF-8 components upon melting.Structural transformation to a denser phase upon heatingcan lead to a meltable phase. ZIF-4 undergoes a multiple-phasetransition, rst to amorphous phases, and then transformsagain to a denser zni topology beforemelting at 590 °C (Td= 600© 2024 The Author(s). Published by the Royal Society of Chemistry°C).54 A meltable Fe2+-based ZIF Fe(Im)2 is obtained fromthermal treatment of Fe3(im)6(Him)2 at 283 °C to removeneutral imidazole.53 Further heating to 417 °C induces anothertransformation to denser zni phases before melting at 482 °C.The liquid state is stable until 550 °C. Although ZIF-4 with Co2+shows identical transformation to the zni phase, further heatingresults in decomposition without melting.52Another approach to creating a meltable version of ZIF-8(Fig. 9A) involves a process called solvent-assisted linkerexchange (SALE) with the incorporation of two additionalorganic linkers.50 In this method, partial exchange of the 2-methylimidazole linker (mim−) with a smaller and weaklycoordinating imidazolate linker (im−) helps facilitate the bond-breaking process. Simultaneously, introducing a larger benzi-midazolate (bim−) linker stabilizes the resulting melts whilepreventing the dense ZIF from crystallizing. By varying themolar ratios of these three linkers, a range of ZIF-8-mimximy-bimz derivatives are produced. Some of these derivatives retainthe same sod topology as ZIF-8. The inclusion of im− aloneleads to the formation of a small amount of zni phase (relatingto ZIF-61).121 The introduction of bim− into the mix reduces theoccurrence of ZIF-61-like structures. As the bim− ratio increasesbeyond 0.65, it results in the formation of either a cubic sod ora heavily distorted rhombohedral or triclinic sod structure. Thiscomposition-dependent behavior leads to a range of thermalcharacteristics, and the melting behavior is summarized inChem. Sci., 2024, 15, 7474–7501 | 7483https://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 9 (A) Crystal structure of ZIF-8. Zn, C, and N atoms are presentedin purple, grey, and blue. H atoms are omitted for clarity. (B) Ternaryphase diagram based on thermal analysis and XRPD data of 50 deri-vates of ZIF-8-mimximybimz along with literature data of ZIF-4 andZIF-62-bimx (orange line), with the blue area being the non-meltingregion, the area between the two dashed lines being the incongruentmelting region, and the orange area being the melting region(excluding the blue line of ZIF-8-mimximy). Adapted from ref. 50 underCreative Commons license CC BY 4.0. https://creativecommons.org/licenses/by/4.0/.Chemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinea ternary phase diagram, including the regions of congruentmelting, incongruent melting, or no melting at all (Fig. 9B).Cooling of samples that melt congruently results in glass withTg between 334 and 361 °C. On the other hand, samplesshowing incongruent melting provide crystal-glass compositesaer cooling consisting of crystalline ZIF-61 or ZIF-8.2.3 Metal-containing ionic liquidsMetal-containing ionic liquids predate the discovery of meltingbehavior in CPs/MOFs, with some examples exhibitingextended coordination networks upon crystallization butmelting at much lower temperatures. Considering the strongconnection between CPs/MOFs in their molten state and metal-containing ionic liquids,91 delving into the design principles ofthe latter could aid us in better understanding the factors thatgovern the melting behavior of CPs/MOFs. One could alsoconsider these metal-containing ionic liquids with well-denedstructures as CPs with Tm below room temperature. In the rstexample presented here, the inuence of hydrogen bonding on7484 | Chem. Sci., 2024, 15, 7474–7501Tm has been examined in lanthanide-containing ionic liquidsystems, such as [BMIM]x−3[Ln(NCS)x(H2O)y] (x = 6–8, y = 0–2,Ln= Y2+, La3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Ho3+, Er3+, andYb3+, with BMIM representing the 1-butyl-3-methylimidazoliumcation).90 These systems predominantly exist as transparent orsupercooled liquids at room temperature and tend to formglasses rather than crystals upon cooling. Selected fewcompounds with x = 6–8, y = 1–2, and Ln = La3+, Y3+, and Nd3+were observed to crystallize at approximately 16 °C from themolten state, exhibiting Tm ranging from 28 °C (Nd3+) to 39 °C(Y3+). Among these compounds, the crystal structure of[BMIM]4[La(NCS)7(H2O)] was determined using single-crystal X-ray diffractometry. The coordination number of the Ln3+ ion iseight, including seven coordinated isothiocyanate anions andone coordinated water, where a slightly distorted square anti-prism of the coordination polyhedron is observed. Each coor-dinated water molecule in [La(NCS)7(H2O)]4− forms stronghydrogen bonds to the isothiocyanate anion of the neighboringunit with d(O–H/S) 2.48 and 2.58 Å, resulting in a columnarstacking of these units along the direction of the a axis. Acidichydrogen atoms from four imidazolium cations surroundingeach [La(NCS)7(H2O)]4− moiety also form weak hydrogen bondsto the sulfur of the thiocyanate anions (C–H/S from 2.73 up to2.84 Å). The crystallization ability of metal-containing ionicliquids is attributed to hydrogen bonding formed by coordi-nated water molecules that leads to the polymeric stacking ofanions. In comparison, [BMIM]5[Ln(NCS)8] without O–H/Shydrogen bonding capability does not exhibit a solid state evenat −20 °C, where it behaves as highly viscous liquids.The Tm of Co2+, Ni2+, and Cu2+ containing ionic liquids,[M(AlkIm)n][Tf2N]2 (AlkIm = N-alkylimidazole; Tf2N = bis(tri-uoromethylsulfonyl)imide; n = 4 or 6), are inuenced by boththe length of the alkyl chain and the choice of cation on the N-alkylimidazole ligands (Fig. 10A).88 By adjusting the alkyl chainlength on the N-alkylimidazole ligands in the [Cu(AlkIm)4][Tf2N]2 complexes, it becomes possible to modulate the Tm andalter their crystal structure due to the entropic contributions.This variability allows for the manipulation of the Tm, rangingfrom 89 °C down to a liquid state below room temperature. Forexample, in the [Cu(AlkIm)4][Tf2N]2 series, the meltingdecreases from 89 °C for N-methylimidazole to 74 °C and 46 °Cfor N-ethylimidazole and N-butylimidazole to below roomtemperature for N-hexylimidazole. The choice of metal ion inthe octahedral [M(MeIm)6][Tf2N]2 ionic liquids has a signicantimpact on their Tm, where a lower Tm is observed when Cu2+ isutilized instead of Co2+ or Ni2+ due to the Jahn–Teller effect.A series of alkyl-phosphonium ionic liquids with rare-earthelements, specically [PR4]3[RECl6] (R = alkyl), behave differ-ently at room temperature depending on the choice of the alkylgroup.92 The alkyl-phosphonium cations include [P666 14]+,[P4448]+, and [P4444]+, while the rare-earth (RE) elements includeLa3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tm3+, Tb3+, Dy3+, Ho3+,Er3+, Yb3+, Lu3+, Y3+, and Sc3+. The [P666 14]3[RECl6] compoundscontaining rare-earth elements remain in a liquid state at roomtemperature with Tm between −58 and −40 °C, except for theLa-containing compound, which solidies at −1.6 °C. On theother hand, the [P4444]3[RECl6] compounds are solid andmelt at© 2024 The Author(s). Published by the Royal Society of Chemistryhttps://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 10 (A) View of the packing in the crystal structure of [Cu(N-ethylimidazole)4][Tf2N]2. (B) Packing in the large unit cell of[P4444]3[EuCl6]. (C) Reactions of [Ru(C5H5)(MeCN)3]X (X = FSA−, PF6−)and L. Conversion between [1]X and [2]X occurs for X = FSA−. (A) isadapted with permission from ref. 88. Copyright 2016 The RoyalSociety of Chemistry. (B) is adapted with permission from ref. 92.Copyright 2016 American Chemical Society. (C) is adapted withpermission from ref. 93. Copyright 2016 The Royal Society ofChemistry.Review Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinetemperatures between 43 and 103 °C. For the [P4448]+ cation, theheavier lanthanides have lower Tm between −6 and −48 °C,while the lighter lanthanides behave as supercooled liquidswhen cooled between 12 and 18 °C. These liquids are thermallystable up to 340–380 °C. The crystal structure of [P4444]3[EuCl6]contains [EuCl6]3− anions surrounded by three crystallograph-ically independent [P4444]+ cations. The packing arrangement inthe large cell is shown in Fig. 10B, with space group P412121 anddimensions a = 16.5192(6) Å, b = 16.5192(6) Å, and c =46.471(3) Å. X-ray absorption spectroscopy (XAS) of liquidsamples of [P666 14]+ cations and [LnCl6]3− anions (Ln = Nd3+,Eu3+, and Dy3+) conrmed that the lanthanide ions are hexa-coordinated by six chloride ligands in the liquid state. TheEXAFS measurements revealed that the Ln/Cl distancedecreases as the lanthanide ionic radius decreases: 2.70 Å (270pm) for Nd3+ (ionic radius = 98.3 pm), 2.66 Å (266 pm) for Eu3+(ionic radius = 94.7 pm), and 2.65 Å (265 pm) for Dy3+ (ionicradius = 91.2 pm). This decrease in distance is attributed to thestronger attraction between the lanthanide ion and the chlorideligands due to the increasing charge density.A series of metal-containing ionic liquids, composed mainlyof cationic organometallic sandwich complexes with nitrile-based anions, transform into CPs when exposed to light andrevert to a liquid state through melting (Fig. 10C).93–96© 2024 The Author(s). Published by the Royal Society of ChemistryCombining [Ru(C5H5)(MeCN)3]X (X = FSA− or PF6−) with 1,3,5-C6H3(OC6H12CN)3 ligands provides colorless liquid or yellowamorphous CP, depending on the synthetic condition.93Exposing the ionic liquid to UV light induces polymerization,while heating the solid at 90 °C for 30 min or 130 °C for 1 minreverses this process. Substituting the cation and anion with[Ru(C5H5)[C6H3(OC6H12CN)3]]+ and anionic covalent chain[CH2–CH(SO2N–SO2CF3)]n results in a photoresponsive poly(-ionic liquid). In this material, the ionic conductivity is reversiblycontrolled based on the mobility of cations, with polymerizedstates having lower mobility.96 [Ru(C5H5)(C6H5R)][B(CN)4] (R =butyl, ethyl, octyl) ionic liquids undergo polymerization trig-gered by UV irradiation.94 The process requires the eliminationof the arene ligand, resulting in the formation of an amorphousCP Ru(C5H5)[B(CN)4] with microporosity.2.4 Porous liquidGenerally, gases are stored in conventional liquids by dissolvingand residing within intermolecular voids (Fig. 11A). The solu-bility of CO2 in water is only 0.04 M, while values of 0.27 and6.97 M are achievable in acetonitrile and propylene carbonate,depending on the physical nature of the liquid hosts.122,123Substituting saline water with uorocarbons also increases O2solubility by at least ten times and is promising for use asarticial blood.124 In addition to small transient cavities orextrinsic porosity, liquids with well-dened cavities will furtherimprove gas solubility and, at the same time, selectivity. Thisconcept of liquids with permanent porosity was proposed in2007.125 Later on, the example of porous liquid was demon-strated in 2015.126 These microporous liquids possess intrinsicporosity, characterized by permanent, empty, and well-denedcavities within the molecules of the liquid or particlesdispersed within it. These cavities provide potential guests withaccess to the interior of the liquid. Initially, porous liquids wereclassied into three distinct types (Fig. 11A).125 The discovery ofmeltable CPs/MOFs that maintain microporosity in their liquidphase introduced a new porous liquid category, Type IV.Developing Type I CP/MOF-based porous liquids presentstwo primary challenges: lowering their Tm and preventing poreblockage by functional groups or guest molecules.128 Surfacemodication using liquefying agents such as polyethyleneglycols and imidazoliums lowers the Tm of CPs/MOFs to belowroom temperature. Ion-exchanging of Cl− in imidazolium-functionalized Deim-UiO-66 with a negatively charged poly(-ethylene glycol)-tailed sulfonate (PEGS) canopy yields a stableIm-UiO-PL porous CP/MOF liquid. In contrast, the same processdoes not apply to a neutral UiO-66, highlighting the importanceof cationic nature. The elongated carbon chains linked to theimidazolium groups functioned as protective coronas, ensuringthe host cavities remained unblocked and accessible. The Im-UiO-PL exhibits slightly higher Tg, Tc, and Tm values of −51 °C, −6 °C, and 28 °C compared to PEGS, which has corre-sponding values of −53 °C, −14 °C, and 23 °C, respectively. MDsimulations also supported the presence of permanent poros-ities ranging from 4 Å to 6 Å, which readily accommodate CO2molecules. DFT analysis showed that PEGS has dimensions ofChem. Sci., 2024, 15, 7474–7501 | 7485http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 11 (A) Four types of porous liquids compared to conventionalnon-porous liquids. Type I: neat liquid hosts that cannot collapse orinterpenetrate. Type II: empty hosts dissolved in sterically hinderedsolvents. Type III: framework materials dispersed in hindered solvents.Type IV: porosity retention in melt state of CPs/MOFs. (B) Sorptionisotherms of non-responsive (blue) and responsive (red) porous solids.(C) Isothermal gas uptake of porous liquids with non-responsive CP/MOF particles (blue) and responsive CP/MOF particles (red). Maximumadsorption pressure (pmax) andminimumdesorption pressure (pmin) fora pressure swing are indicated by dashed vertical grey lines. Adaptedwith permission from ref. 127. Copyright 2023 Springer Nature Limitedunder Creative Commons license CC BY 4.0. https://creativecommons.org/licenses/by/4.0/.Chemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online23.0 Å in length, 14.7 Å in width, and 20.2 Å in height, making ittoo large to enter Deim-UiO-66 cavities. As a result, Im-UiO-PLhas 14 times greater CO2 adsorption capacity than pure PEGS.Outer surface functionalization of crystalline ZIF-67(Co(2-methylimidazolate)) with N-heterocyclic carbene ligands(NHCs) yields a Type III porous liquid.129 ZIF-67 particles with anaverage size of 264 ± 54 nm were synthesized and then surfacegraed to the open metal site with 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene (IMes) and 1,3-bis(2,4,6-diisopropylphenyl)imidazole-2-ylidene (IDip) NHCs. Dispersing IDip-modied ZIF-67 in cyclohexane, cyclooctane, and mesitylene yields a Type IIIporous liquid in which the overall uptake capability per gram ofZIF-67 is maintained. Stable dispersion also leads to the oppor-tunity to co-process the ZIF-67 with the 6FDA-DAM polymer tocreate a mixed matrix membrane. A maximum ller loading ofup to 47.5 wt% is achievable while preventing agglomeration.7486 | Chem. Sci., 2024, 15, 7474–7501Instead of using sterically bulky solvents, a stable dispersionof CPs/MOFs in water is achieved while maintaining a dryinternal micropore.130 Several nanocrystalline zeolite and CPs/MOFs synthesized with hydrophobic pore surfaces excludedliquid water from their micropores at ambient temperature andpressure (entropically disfavored). In the case of zeolite, namelysilicate-1, optimizing the synthetic condition of nanocrystallinealone yields a stable, translucent aqueous colloidal solution(38 wt%). This is explained by the nature of silicate-1, which hashydrophobic internal and hydrophilic external surfaces. Thepresence of dry micropores has led to an order of magnitudehigher gas uptake: 26 ± 1 mmol O2 l−1 at 0.84 bar and 284 ±2 mmol CO2 l−1 at 0.67 bar, as compared to 1.1 mmol O2 l−1 and23 mmol CO2 l−1 for water under the same condition. Surfacefunctionalization of Zn(mIm)2 (ZIF-8, mIm = 2-methyl-imidazolate) and Co(mIm)2 (ZIF-67), with surface ligandspromotes dispersibility without blocking access to the micro-pore, leading to microporous water. Attaching bovine serumalbumin (BSA) globular water-soluble proteins onto the ZIF-8and ZIF-67 hydrophobic external surfaces could providea complete dispersion with 80% ± 9% of the O2 theoreticalcapacity for ZIF-67. Aside from the non-covalent approach,reacting ZIF-8 with methoxypolyethylene epoxide (mPEG; Mn =750 g mol−1 for PEG) results in an epoxy ring opening andmPEG functionalization. The measured O2 capacity of func-tionalized ZIF-8 (7.0 wt%) here is 96% ± 7% of the theoreticalamount.A dispersion of exible CPs/MOFs, capable of structuralchanges under varying gas adsorption pressures, within a bulkysolvent generates breathable porous liquids (Type III).127 Whilemany CPs/MOFs exhibit a microporous structure withLangmuir-shaped gas sorption isotherms, the transition froma contracted, minimally porous phase to an expanded, highlyporous phase in response to gas pressure results in sigmoidaladsorption proles. These sigmoidal adsorption proles persistin the porous liquid when breathable CPs/MOFs are dispersedin the bulky solvent (Fig. 11B). Responsive CPs/MOFs displaysignicant gas uptake variations within a narrow pressurerange, thus enhancing their working capacity. For instance, ZIF-7 and ZIF-9, known for their breathing behavior and featuringsubmicro- or nanoparticle sizes, were uniformly dispersed insilicone oil (1,3,3,5-tetramethyl-1,1,5,5-tetraphenyl-trisiloxane,Silicone 704, viscosity of 42 mPa s at 25 °C) to create breath-able porous liquids upon 15 minutes of sonication. In partic-ular, 5 wt% and 10 wt% ZIF-7 in Silicone 704 are denoted asPL7_5 and PL7_10, with the latter displaying particle sizes of469 ± 74 nm as determined by DLS analysis and a viscosity of107 mPa s at a shear rate of 10 s−1. ZIF-7 nanocrystals exhibita CO2 adsorption capacity of approximately 43 cm3 (STP) g−1 at1224 mbar and 25 °C, characterized by a sigmoidal sorptionisotherm, while Silicone 704 displays a CO2 uptake of 1.2 cm3(STP) g−1 under the same conditions. The CO2 sorptionisotherms of PL7_10 and PL7_5 also display sigmoidal charac-teristics, with CO2 uptakes of 5.24 cm3 g−1 and 3.12 cm3 g−1,respectively.Without any bulky solvent, porosity preservation in network-forming ZIF-4 above the Tm leads to a new category of porous© 2024 The Author(s). Published by the Royal Society of Chemistryhttps://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eReview Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineliquid (Type IV).29 Upon melting, the translational diffusion ofZn2+ and imidazolate ions is calculated to be 7.7 × 10−10 m2 s−1and 6.5 × 10−10 m2 s−1, respectively, at temperature of 1227 °C(FPMD). Note that the higher temperature in FPMD is notphysically relevant for the experimental system, as mentionedin an earlier section. The viscosity of 19 mPa s at 1227 °C isestimated from these diffusion coefficients. Extrapolating bythe Arrhenius law, the viscosity at the experimental Tm of 567 °Cis equivalent to 8000 mPa s. A slight variation in porositydevelopment is observed in the solid phase at temperaturesbelow 927 °C. When the system transitions to the liquid state athigher temperatures, the overall porosity remains relativelyconsistent, with a slight shi towards lower average porevolumes. 95% of the void space in ZIF-4 liquid at 1227 °C isaccessible porosity, while crystalline ZIF-4 voids are 74%accessible at 27 °C, suggesting a hybrid porous liquid even athigh temperatures (Fig. 12). The porosity of the ZIF-4 porousliquid is also larger than that of similar imidazolium ionicliquids, where the void space size distribution is typicallynegligible above 1 Å in radius. RMC modeling on X-ray totalscattering data was performed, and the internal surface of theglass was calculated using a standard probe diameter of 2.4 Å.The results suggest that the internal surface was enhanced to16.2% in the liquid phase at 583 °C from 4.8% in glass atambient temperature, referring to the intrinsically porousliquid of melt ZIF-4.In addition to meltable CP/MOF systems, retention ofpermanent porosity is observed in the liquid state of discretecoordination cages and potentially in metal–organic polyhedraFig. 12 (A) Temperature evolution of the distribution of the total porevolume, determined for a standard probe of radius 1.2 Å. The averagepore volume takes the following values: 52 cm3 kg−1 at 27 °C, 49 cm3kg−1 at 1727 °C, and 41 cm3 kg−1 at 1977 °C. (B) Atomic configuration ofthe ZIF melt, gained from reverse Monte Carlo modeling of the totalscattering data collected at 583 °C. Free volume is represented inorange, Zn atoms in green, N in blue, and C in grey. Adapted withpermission from ref. 29. Copyright 2017 Springer Nature Limited.© 2024 The Author(s). Published by the Royal Society of Chemistry(MOPs).84–86 Attaching long poly(ethylene glycol)-imidazoliumchains into the periphery of the parent Zn4L4 tetrahedron(methylated ligand) results in a room-temperature ionic liquidwith permanent porosity.84 A stable liquid is maintainedbetween −44 and 300 °C and has an average void diameter of6.29± 0.08 Å at 298 K (ortho-positronium lifetime of 2.34± 0.05ns), which accommodates gaseous chlorouorocarbon andnon-gaseous alcohol molecules. Surface graing of amine-terminated poly(ethylene glycol) onto carboxylic acid-functionalized Rh(II)-based MOPs via covalent amide forma-tion enables melting behavior (Tm = 47 °C).85 Without theintrusion of the surface polymer chains, the porosity of theMOPs is maintained in amorphous form. This allows the use ofmeltable MOP as a matrix for creating mixed-matrix compositelms with porous MOFs. Another example of melting behavioris enabled by graing MOP with tethered polymers onto openmetal sites (axial positions) through a coordination bond (Tm =47 °C).86 The polymer design inhibits the interpenetration ofpolymers into the MOP's internal cavity. One of the terminalscontains a Lewis-basic coordination site to bind with the MOP.The other side has a bulky functional group that is larger thanthat of the pore opening. Cooling the liquid state faster than30 °C min−1 results in a glass transition at −56 °C. Subsequentheating induces crystallization (Tc = −29 °C). Moreover, thethermal behaviors are controllable by tuning the polymer lengthand the polarity on the MOP surface (functional groupsubstituent). With Tm between 17 and 25 °C, they behave asporous liquids at room temperature with maintained porosityand gas-separating capability even in liquid forms.3. Crystal-liquid transitionReversible phase change is perhaps a more ubiquitousphenomenon than the thermodynamic well, which is the glassystate. Many of us are familiar with the melting and recrystalli-zation of simple organic molecules. In the case of melting CPs/MOFs, if the appropriate conditions are met, cooling the meltwill result in recrystallization of the pristine crystal structurerather than the trapping of the glassy state. From an applicationpoint of view, this provides CPs/MOFs with the capability toprocess and shape them into complex structures in the liquidphase, and then functional crystalline monoliths are later ob-tained by cooling below the crystallization temperature.Therefore, it is crucial to understand what inuences thereversibility of the crystal-liquid phase change in CPs/MOFs.A series of metal-bis(acetamide) frameworks has beeninvestigated to gain insight into their phase transitions.45 Theseligands are advantageous for producing melting CPs/MOFs asthey feature weaker, more labile coordination bonds (i.e., the Oatom of the acetamide moieties) and longer aliphatic bridgingsections that offer greater ligand exibility. These propertieswork together to decrease DH and increase DS, respectively,resulting in lowered Tm. The kinetics of recrystallization areaffected most noticeably by the Tm and viscosities of thematerials. Mn(bba)3[MnCl4], Mn(bba)3[ZnCl4], Mg(bba)3[-CoCl4], and Mg(bba)3[ZnCl4] all exhibit fast recrystallizationupon cooling of the melt, while at the same time, they haveChem. Sci., 2024, 15, 7474–7501 | 7487http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eChemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinesome of the highest Tm of the compounds in the study. Thisgives the compounds ample energy to overcome DHfus. Thecompound in this series with lower Tm (and relatively higherentropies) tended to show slow recrystallization kinetics, wherefast cooling resulted in the glassy state and isothermal hold wasneeded to recrystallize the compound completely. Co(hmba)3[-CoBr4] has the lowest Tm and high viscosity in the melt (1313mPa s at 120 °C, Fig. 13), which, coupled with retained anionsand cationic networks in the melt, hinders its ability to recrys-tallize completely. Varying M0 gave a more marked difference inTm than varying the corresponding [M00X4]2−, due to M0 beingthe main framework metal ions. Changing the linker from theshorter bba to the longer hmba expectedly increased the DSfus,as the longer linker is more exible, while also decreasing theTm.Ag2(L1)(CF3SO3)2 (L1 = 4,40-biphenyldicarbonitrile), a Ag+-based CP, shows Tm of 282 °C and crystallizes rapidly uponcooling to below 242 °C.42 The DHfus and DSfus of Ag2(L1)(CF3-SO3)2 were 11.4 kJ mol−1 and 20.5 J mol−1 K−1, respectively.Rheological measurements revealed that Ag2(L1)(CF3SO3)2 hasa viscosity of 98 mPa s at 282 °C (Tm), which is remarkably lowcompared to some other melting CPs/MOFs such as ZIF-62(108.1 mPa s) or the aforementioned Co(hmba)3[CoBr4] (1313mPa s). The high Tm, giving sufficient energy for nucleation,coupled with the low melt viscosity, are the major contributorsto the rapid recrystallization of [Ag2(L1)(CF3SO3)2]. Cyclingthrough melting and crystallization cycles results in morpho-logical changes, resulting in the evolution of melting endo-therms with continuous Tm lowering. Analyzing crystallizationkinetics using the Avrami equation yields Avrami exponentsfalling within the range of 2.27 to 2.32. The value suggests a 1Drod-like crystal growth mechanism arising from the sporadicnuclei, aligning with the observations in DSC, VT-PXRD, RamanFig. 13 Comparison of the viscosity at Tm and behavior upon coolingof different meltable CPs/MOFs.7488 | Chem. Sci., 2024, 15, 7474–7501spectroscopy, real-time hot-stage microscopy, and polarizedoptical microscopy.The phase-transition behavior is studied in a series of hybridorganic–inorganic crystals of the type (Me3NR)4[Ni(NCS)6],where R = ethyl, propyl, or butyl.131 The three compounds showmultiple solid–solid phase transitions in a wide temperaturerange from −113 to 140 °C, while the propyl and butylsubstituted compounds also exhibit melting and recrystalliza-tion behavior. The Hirshfeld surfaces of the various compoundsand their respective crystalline phases were examined to gaina deeper understanding of the underlying intermolecularinteractions responsible for the phase changes (Fig. 14). In thecrystal containing ethyl-substituted amines, the [Ni(NCS)6]4−anions interact with adjacent anions through S/S shortcontacts, adding to the overall attractive forces holding thecrystal together. The small relative size of Me3NEt+ alsoimproves attractive Coulomb interactions between the cationsand anions. The two melting crystals containing propyl or butylgroups did not have the same S/S short contacts as in the ethyl-substituted compounds. As such, only Coulomb interactionsneeded to be overcome in these compounds, which resulted inmelting.Fig. 14 The Hirshfeld surfaces mapped with dnorm and fingerprintplots for crystallographically independent [Ni(NCS)6]4− anions in(Me3NR)4[Ni(NCS)6], R = (A) ethyl, (B) propyl, and (c) butyl. Adaptedwith permission from ref. 131. Copyright 2023 John Wiley & Sons, Inc.© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eReview Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineCu(2-isopropylimidazolate) forms a melt state above 143 °Cand recrystallizes below 112 °C, showing good consistency inthe reversibility of this transition.30 The CP does not becomeglass, even upon rapid cooling to−196 °C from the melt state. Amajor contributing factor to this may be the molecular structureof liquid Cu(2-isopropylimidazolate). PDF analysis above the Tmsuggests retention of Cu+-isopropylimidazolate-Cu+ bridgingmotifs in the liquid phase. This is also supported by rheologicalstudies that show a large G0 (ca. 106 Pa) above the Tm, indicatinga high viscosity and preservation of 1D structure in the liquidstate (Fig. 15). Although high viscosity has been shown to limitrecrystallization in some CPs, the high viscosity in the case ofCu(2-isopropylimidazolate) is due to its retention of molecularstructure, so cooling of the melt results in less thermal motionof the 1D chains that again return into a crystalline solid.4. Functions and applications4.1 Ionic conductivityTransitioning between phases in CP/MOF yields distinctadvantages in terms of ionic conductivity, encompassingenhancements in molecular dynamics at the microlevel andfacilitating processability at macroscales. An early example ofenhanced conductivity aer phase transition was demonstratedin a non-melt 2D layered Cd(H2PO4)2(1,2,4-triazole)2.68 Thecrystal-to-glass transition via mechanical vitrication leads toenhanced anhydrous proton conductivity over two orders ofmagnitude, reaching 1 × 10−4 S cm−1 at 125 °C. The behaviorFig. 15 (A) Frequency sweep and (B) strain sweep viscoelasticmeasurements of 1D Cu(2-isopropylimidazolate) from 235 to 115 °Cunder N2 flow. The filled and opened circles represent G0 and G00,respectively. (C) Fibers of Cu(2-isopropylimidazolate) prepared by hot-pressing and collected by spinning around the glass tube. Adaptedwith permission from ref. 30. Copyright 2021 The Royal Society ofChemistry. https://creativecommons.org/licenses/by/3.0/.© 2024 The Author(s). Published by the Royal Society of Chemistryarises as the acidity and isotropy of H2PO4− increase, leading tohigher overall proton mobility. The event is reversible. Asheating continues above the crystallization temperature (142 °C), the conductivity value reverts as recrystallization occurs.Another advantage of the reversible solid–liquid transition isits shaping capability, which allows for versatile and adaptableformability. The 1D meltable and proton-conductive [Zn3(H2-PO4)6(H2O)3](1,2,3-benzotriazole) is an example of the case.32The compound exhibits melting behavior with a Tm of 114 °Cand is quenched to form glass upon subsequent cooling. Theglass behaves like a rigid solid below 90 °C, where its viscositylies above the Littleton soening point (106.6 Pa s). Above 90 °C,the viscosity drops to 42.8 Pa s (120 °C), which is below theworking point of the glass. This allows the compound to beshaped above 90 °C and used as a solid below that temperature.Note that the working point for soda-lime-silica glass is above1100 °C and is suitable for industrial processing.111 Increases inoverall proton conductivity were observed throughout themeasurement range (Fig. 16A). An approximately six-fold higherconductivity is observed at 60 °C, while the highest value of 8 ×10−3 S cm−1 is achieved at 120 °C. A prototype solid-state protonbattery using glass as a solid electrolyte shows an indistin-guishable electrode–electrolyte interface and the absence ofgrain-boundaries, conrmed by cross-sectional SEM images(Fig. 16B). The solid-state proton battery shows a dischargecapacity of 55.4 mA h g−1 at 25 °C and works up to 110 °C. Inanother work, the absence of grain boundaries also contributedto the enhanced ion transport of Li-ion electrolyte guests whencomparing the glass with crystalline ZIF-4 hosts.132Reversible crystal-liquid transition without forming a glassalso aids device fabrication. 3D tetrahedrally coordinated Li[N(SO2F)2](NCCH2CH2CN)2 conduct Li-ion with conductivityvalues of 1 × 10−4 S cm−1 at 30 °C and 1 × 10−5 S cm−1 at −20 °C with a transport number of 0.95.37 All solid-state Li-ionbatteries were fabricated upon melting and used aer crystal-lization (Fig. 16C–E). The fractures of the solid-electrolyte layerformed during operation are repairable by repeating the melt-crystallize cycle.Proton conductivity and viscosity are modulated throughcoordination network connectivity, as demonstrated in a seriesof demax[Zny(HnPO4)3] (Fig. 17A and B).73,74 They were directlysynthesized from (dema)(H2PO4) protic ionic liquid (dema =diethylmethyl ammonium), where the Zn2+ contents control thesize of the coordination network. At the highest Zn2+ concen-tration, the conductivity of (dema)0.35[Zn(H2.22PO4)3] reaches1.3 × 10−2 S cm−1 at 120 °C under anhydrous conditions, witha proton transport number of 0.94. This is because the coordi-nation network helps restrict the movement of counter anions.As the connectivity decreases, the viscosity decreases from 106to 101 Pa s. While their proton conductivities slightly increase ina narrow range up to 2.1 × 10−2 S cm−1 at 120 °C in(dema)0.45[Zn0.75(H2.35PO4)3]. A higher proton conductivity doesnot always translate to higher power delivery in H2/O2 fuel cells,as the fuel crossover cannot be efficiently mitigated due toinsufficient viscosity.The wide composition range and reversible phase transitionof ZnPIm are used as a suitable host matrix to maintain the highChem. Sci., 2024, 15, 7474–7501 | 7489https://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 16 (A) Arrhenius plots of the anhydrous H+ conductivity of a [Zn3(H2PO4)6(H2O)3](BTA) degas crystal (C) and MQG (:) under an Aratmosphere. (B) Cross-sectional SEM images (150× magnification) of the electrode–solid-state electrolyte interface. Adapted with permissionfrom ref. 32. Copyright 2021 The Royal Society of Chemistry. https://creativecommons.org/licenses/by/3.0/. (C) Cell configuration and (D) top-view photograph of Li-battery (LijLi[N(SO2F)2](NCCH2CH2CN)2jLiCoO2jAu). (E) Charge–discharge profiles at a current density of 1 mA cm−2.Adapted with permission from ref. 37. Copyright 2020 American Chemical Society.Chemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineconductivity of the guest molecules.107 CsHSO4 is a super-protonic solid acid known for its fast proton conduction, butonly above the transition temperature (Tc) of 141 °C. By forminga binary CsHSO4–ZnPIm glass system (discussed in Section 2.2),their anhydrous conductivities below Tc are over three orders ofmagnitude higher than the CsHSO4 without compromising theconductivity above Tc. At 180 °C, the conductivity reaches 6.3 ×10−2 S cm−1(Fig. 17C). The preservation of conductivity isattributed to the oxyanion exchange between HSO4− andbridging phosphate. The event induces chain scission, thusincreasing the overall molecular dynamics. Forming a binaryglass system also introduces processing capability to theCsHSO4, as a micrometer-scale thin lm was prepared witha transmittance over 85% between 380 and 800 nm.The presence of a liquid phase plays a crucial role in facili-tating the homogeneous distribution of guest molecules withinthe CPs/MOFs matrix. By adding triuoromethanesulfonic acid(15 mol%) to ZnPIm upon melting, the proton conductivity ofthe crystalized sample increases to 2.0 × 10−7 and 2.7 ×10−4 S cm−1 from 3.2 × 10−9 and 2.1 × 10−5 S cm−1 for thepristine sample at 30 and 110 °C, respectively.40 Doping a pho-toresponsive molecule, such as trisodium 8-hydroxy-1,3,6-pyrenetrisulfonate (pyranine), enables overall conductivitycontrol upon 365 nm light exposure. The system reaches equi-librium within ca. 50 min, where the overall resistance7490 | Chem. Sci., 2024, 15, 7474–7501decreases by 1.7 times. Pyranine releases protons upon irradi-ation, increasing the number of charge carriers in the systemand thus the conductivity. A reverse event occurs when theirradiation ends and overall conductivity decreases.An alternative approach to achieving control over protonconductivity under anhydrous conditions is by generatingproton-decient sites within the host matrix (Fig. 17D). Photo-excitation of the tris(bipyrazine)-ruthenium complex (Rubpz,0.1 mol%) leads to improved responsiveness and intensies theconductivity change between on and off states.133–135 The systemreaches the highest on/off ratio of over 180 times within 5 min,and the conductivity was controlled explicitly by continuouslycontrolling the light intensity and ambient temperature. Themetal-to-ligand charge transfer (MLCT) excitation of Rubpzinitiates the pKa changes, where the proton is transferred fromthe ZnPIm glass domain to Rubpz, thus generating proton-decient sites. The behavior leads to a lower energy barrier(Ea), from 0.76 to 0.30 eV, required to initiate proton migrationwithout disturbing the local structure of the glass.4.2 Gas absorption and permeabilityPermanent porosity in some melt-quenched ZIF glasses hasbeen reported.136 ZIF-76 glass structures, [Zn(Im)1.62(5-ClbIm)0.38] with 5-ClbIm representing 5-chlorobenzimidazolate(C7H4N2Cl−), maintain the porosity revealed by the positron© 2024 The Author(s). Published by the Royal Society of Chemistryhttps://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 17 (A) Simulated coordination network structures (RMC) of demax[Zny(HnPO4)3]. The relative position and size of each structure arearranged for clarity. The Zn (blue), P (orange), and O (red) were shown as ball and stick models. H atoms were omitted for clarity. (B) Walden plotsand the comparison with H3PO4 (85%) and protic ionic liquids. Adaptedwith permission from ref. 74. Copyright 2022 American Chemical Society.(C) Variable temperature anhydrous H+ conductivity of [CsHSO4]0.75[ZnPIm]0.25-g (blue), [CsHSO4]0.5[ZnPIm]0.5-g (green), [CsHSO4]0.5[ZnPIm]0.5(physically blended, beige), ZnPIm-g (red), and CsHSO4 (gray). Reprinted with permission from ref. 107. Copyright 2023 American ChemicalSociety. (D) Schematic illustrating photoinduced protonation of monoprotonated Rubpz, generating H+-deficient sites in Rubpz-ZnPIm-g, andphotoexcited H+ conductivity of Rubpz–ZnPIm-g at 30 °C under anhydrous condition. Light irradiation is demonstrated by blue highlighting,where the percentage indicates the light intensity controlled by the equipped continuous variable neutral density (ND) filter. Adapted withpermission from ref. 133. Copyright 2023 American Chemical Society.Review Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineannihilation lifetime spectroscopy (PALS) technique. ZIF-76glass features a single 5 Å pore that adsorbs over 4 wt% ofCO2 at 0 °C, but desorption requires prolonged equilibration, asseen in the hysteresis isotherm. This suggests limited guestmolecule diffusion in the intricate pore network due to thepartial collapse of distinct 5.7 Å and 15.7 Å cavities post-quenching. This aligns with low N2 and H2 adsorption at−196 °C. By adjusting linkers, matching pores and moleculesizes for gas separation were achieved. A methyl group in ZIF-76-mbIm glass anchors its structure through non-covalentinteractions with nearby ligands, maintaining an inter-connected network of pores and channels within the glass,leading to permanent porosity. ZIF-76-mbIm glass with twopore diameters of 4.8 Å and 7.2 Å displays reversible CO2 (0.12cm3 g−1 at 0 °C, 1 bar) and CH4 adsorption at 20 °C withminimal hysteresis, indicating low diffusion limitations. Yet, N2diffusion is hindered, and H2 adsorption shows hysteresis.Isosteric heats of adsorption suggest that vitrication createssites in the pore network with high CO2 affinity. The network ofchannels in ZIF-76-mbIm glass has remained stable for threemonths. These characteristics are benecial for gas absorptionand permeability applications.Melt-quenching of the solvothermal synthesized ZIF-62membrane on alumina support results in a monolithic© 2024 The Author(s). Published by the Royal Society of Chemistrymembrane without a grain boundary that minimizes undesiredinterparticle diffusion, a feature particularly advantageous forgas separation.137 Post-quenching, PALS shows that the micro-porosity of ZIF-62 glass increases its radius to 3.16 Å comparedto 2.66 Å found in the crystalline counterpart (Fig. 18A). At 20 °Cand 1 bar, CO2, CH4, and N2 gas uptake decreases from 18.5, 10,and 2.4 cm3 g−1 in the crystalline state to 11, 2.6, and 0.7 cm3g−1 in the glass state. The interaction between ZIF-62 and CO2strengthens aer converting to the glassy state, with an isostericheat of adsorption (Q0st) of 29 kJ mol−1 compared to the originalstate of 26 kJ mol−1. A composite membrane of CP/MOF glasswas created by melting a polycrystalline CP/MOF membrane ona porous ceramic alumina support at 440 °C for 15 minutes,enabling the molten substance to inltrate the nanopores of thesupport through capillary extrusion. The glass membrane onthe substrate is achieved aer rapid cooling, where the gaps,pinholes, and grain boundaries are absent in the glassy ZIF-62membrane (Fig. 18B). The ideal selectivity, or the ratios of thepermeability of faster-moving gas to slower-moving gas, in theglass membrane for H2/N2, H2/CH4, CO2/N2, and CO2/CH4 pairsat 25 °C reaches values of 53, 59, 23, and 26, respectively(Fig. 18C). These values were far above the Knudsen selectivities(3.7, 2.8, 0.8, and 0.6), determined by the inverse square root ofthe molecular masses. In the case of single gas permeation, theChem. Sci., 2024, 15, 7474–7501 | 7491http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 18 (A) Pore size distribution derived from PALS of ZIF-62 crystal and glass. (B) Schematic and top-view SEM images of polycrystalline ZIF-62membrane and ZIF-62 glass membrane. (C) Single gas permeability as a function of the gas kinetic diameter of the ZIF-62 glass membranes at25 °C and separation performance comparison with reported membranes. (D) CO2/CH4 separation performance comparison with reportedmembranes. Adapted with permission from ref. 137. Copyright 2020 John Wiley & Sons, Inc. (E) Single gas permeability as a function of the gaskinetic diameter from the agM–P-dmbIm glass membranes at 20 °C and 1 bar. M represents Zn2+, Cd2+, Cu2+, and Mn2+; P is phosphate; anddmbIm is 5,6-dimethylbenzimidazole. (F) The H2/CO2 selectivity of agZn–P-dmbIm during the cyclic healing process. Adapted with permissionfrom ref. 139. Copyright 2021 John Wiley & Sons, Inc.Chemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineactivation energy of permeation increases with gas moleculesizes, which are 4.0, 6.1, and 6.8 kJ mol−1 for H2, CH4, and N2,respectively. In contrast, the activation energy of CO2 perme-ation is −2.6 kJ mol−1 as the CO2 permeance decreased withincreasing temperature due to the high heat of adsorption forCO2 in ZIF-62 glass. For the binary gas mixture, including H2/CH4, CO2/N2, and CO2/CH4, the ZIF-62 glass membrane showsa promising gas separation ability. The selectivity of 50.7 isachieved for an equimolar mixture of H2/CH4 at 25 °C and 1 bar.Although the exibility of CPs/MOFs compromises molecularsieving, the H2/CH4 separation efficiency of glass CP/MOFmembranes exceeds that of the majority of reported poly-crystalline CP/MOF membranes (Fig. 18D). The exceptionalperformance of the ZIF-62 glass membrane exceeded Robeson'supper boundary,138 which represents the tread-off relationbetween permeability and selectivity. While zeolite and carbonmembranes exhibit strong H2/CH4 separation, they encounterissues such as physical aging and low reproducibility with grainboundary defects. The glass membranes offer comparableperformance without these drawbacks. For CO2/N2 and CO2/CH4 equimolar gas mixtures at 25 °C and 1 bar, the glassmembrane demonstrates CO2 selectivities of 34.5 and 36.6,respectively, surpassing ideal selectivity due to its greater CO2adsorption than N2 and CH4. The CO2 permeabilities of themembranes for CO2/N2 and CO2/CH4 are 2602 and 2638 Barrer.These values also surpass Robeson's upper bound. The glassmembrane exhibits resilience to water, although the gas per-meance diminishes upon exposure to water vapor, leading tocomplete lling of the micropores within 24 hours. The7492 | Chem. Sci., 2024, 15, 7474–7501restoration of empty micropores was achieved by subjecting themembrane to 180 °C for 2 hours, utilizing dry feeding gas andHe sweep gas. Apart from CP/MOF glasses, the promisingfeasibility of upscaling the production of glass membranes alsorelies on well-suited supports.While high vitrication temperatures and viscosity couldlimit the scalability of ZIF glasses, a series of low-viscositymeltable CPs (M–P-dmbIm) with Tm around 162–176 °C havebeen developed as free-standing glass membranes witha thickness of ca. 95 mm via the hot-casting method.139 Notethat M represents Zn2+, Cd2+, Cu2+, and Mn2+; P is phosphate;and dmbIm is 5,6-dimethylbenzimidazole. The viscosity valuesof Zn–P-dmbIm are 45.0 and 61.2 Pa s at Tm (176 °C) and Tg (112°C), respectively. This is much lower than ZIF-62, where muchhigher viscosity values of 105 and 1012 Pa s are observed at Tmand Tg. The low Tm, Tg, and low viscosity are a result of theformation of the framework through non-covalent interactions.The steric hindrance caused by the 5,6-dmbIm ligand alsocontributes to an exceptionally high glass forming ability (GFA)ranging from 0.85 to 0.94 in the M–P-dmbIm. This hinders themovement of atoms or ions, thus inhibiting the creation oforderly crystal formations aer glass membrane fabrication.The PALS reveals the pore size diameter of 2.93 Å and 4.87 Å ofZn–P-dmbIm glass membrane decreased from 3.4 and 5.5 Å ofits crystalline state, which is slightly larger than the H2 kineticdiameter (2.9 Å), while inhibiting larger gas penetration. The H2permeabilities at 1 bar pressure of Zn-, Cd-, Cu-, and Mn–P-dmbIm are calculated to be 62 000, 75 000, 99 000, and 42 000barrer, which are higher among other pure gases, including© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eReview Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineCO2, N2, and CH4 (Fig. 18E). Similar to ZIF-62 glass, the M–P-dmbIm glass membranes also demonstrate ideal selectivityexceeding Knudsen selectivity within binary gas mixtures. Theselectivity of Zn–P-dmbIm glass membrane for H2/CO2, H2/N2,and H2/CH4 is 92.7, 49.6, and 75.0, respectively, with H2permeability of 6.47 × 104 barrer. Their separation ability alsosurpasses the Robeson upper bound, pointing to their potentialapplicability for the integrated gasication combined cycleprocedure and the separation of H2 during ammonia produc-tion and methane reformation. The apparent activation ener-gies of permeation for H2 and CO2 are 3.8 and 7.0 kJ mol−1,along with deduced diffusion activation energies of 8.8 and 38.0kJ−1, conrming a more active diffusion process for H2 thanthat for CO2. Under a humidity of 60% in the feed gas mixture ofH2/CO2, the permeability of H2 and CO2 and H2/CO2 selectivitydisplay insignicant uctuation due to a limited water moleculeadsorption ability at relatively low humidity. Beyond itshumidity stability, Zn–P-dmbIm liquid also has the capability torepair cracks when it is melted. This unique property allows themembrane to restore its structural integrity aer being sub-jected to bending stress (Fig. 18F), despite Young's modulus of13.1 GPa indicating susceptibility to fragility upon bending, asdetermined through nanoindentation.Differences in thermal stability are benecial for creatingextra pore networks within a self-supported glass monolith(glass foam, Fig. 19).140 A mixture of low-molecular-weightpolyethylene-imine (PEI; Mw 300) and microcrystalline ZIF-62was pressed and heated to 440 °C under an inert atmosphere.Thermal decomposition of PEI (160 to 370 °C) upon heatinggenerates gases, including CO2, NH3, and H2O. These gaseswere then released upon cooling the ZIF-62 melts, thus intro-ducing a more interconnected microporosity. This results ina membrane with higher porosity and a faster gas diffusionkinetic compared to the conventional ZIF-62 glass. The methodprovides circular membranes with a 3.3 cm diameter anda thickness between 200 and 330 mm that were tested for CH4/N2separation. A high CH4 permeance of 30 000–50 000 GPU andpermeability of ca. 107 barrer, together with a high CH4/N2selectivity of 4–6, were achieved. The values here contrast withthe conventional ZIF-62 glass, as the CH4 permeance and CH4/N2 selectivity are 36.3 GPU and 1.1 due to the lack of poreconnectivity.Fig. 19 Formation mechanism of ZIF-62 glass foam. The PEIdecomposition to CO2, NH3, and H2O increases pore connectivity inthe glass membrane. Adapted with permission from ref. 140. Copyright2023 Springer Nature Limited.© 2024 The Author(s). Published by the Royal Society of Chemistry4.3 Optical propertiesThe photophysical properties of CPs/MOFs vary between theircrystalline and amorphous phases, and control over phasetransition behavior is desirable for optics applications. This wasexemplied in a series of isomeric copper CPs [Cu4I4L(MeCN)2]sol (L = N,N0-bis[2-(cyclohexylthio)-ethyl]pyromellitic diimide;sol = CH2Cl2, CHCl3, 0.5p-xylene, or nothing).141 The as-synthesized, crystalline CPs exhibit photoluminescencearising from the interaction between the Cu4I4 cluster and theligand, with emission maxima in the 583–604 nm range. Whensubjected to heat-induced amorphization, the luminescence ofthe material is lost. When the amorphous CP is subsequentlyrecrystallized through exposure to acetonitrile vapor, the emis-sion is once again observed. This illustrates a promisingapplication in vapochromic sensing.Melt-state processing is effectively used in the design ofoptical devices. Co2+-doped ZIF-62 exhibits broad emission inthe mid-IR region (lem = 1.5–4.8 mm), and photoluminescenceenhancement occurs upon formation of the melt-quenchedglass (Fig. 20A).142 While the native Zn–ZIF-62 does not showany absorption or emission in the visible to mid-IR regions,doping with Co2+ (10% or 50%) gave rise to absorption bands at570 and 1100 nm, consistent with Co2+ in a CoN4 tetrahedralenvironment as it would be bound by imidazole and benz-imidazole. The subsequent photoluminescence (lex = 980 nm)was assigned to the 4T1(4F) / 4T2(4F) and 4T2(4F) / 4A2(4F)transitions of the Co2+ center, with increasing intensity corre-lating with increased Co2+ doping. While the photo-luminescence intensity of the glassy state was greater than thatof the crystalline state, this is mainly attributed to differences insample morphology and scattering effects. The short-rangestructural order of ZIF-62 glass has been shown to stay intactupon melt-quenching, so the environment around the emittingCo2+ center is expected to be similar in both crystal and glass.This highlights the versatility of CP/MOF glasses, wherein theenvironment around the metal center is more or less retained.Towards applications in information science, another seriesof metal-bis(acetamide) frameworks was synthesized, this timeopting for the shorter N,N0-ethylenebis(acetamide) (eba) inM0(eba)3[M00Cl4] compounds (M0 = M00 = Mn2+, Fe2+, and M0 =Mn2+, Fe2+, Co2+, M00 = Zn2+).44 They were expected to havea higher Tg than CPs constructed from the longer hmba or bbalinkers, increasing their workable temperature range. Changesin optical properties upon phase transition (i.e., crystal to melt-quenched glass) were studied in pure samples and binarymixtures formed by mixing parent compounds in the liquidstate, followed by quenching to glass and thermal annealing tocrystallize the mixture (Fig. 20B). The mixture of Mn(eba)3[-ZnCl4] and Co(eba)3[ZnCl4] (1-Mn/Co(Zn)) was found to have themost pronounced differences between crystalline and glassystates, with the CP glass exhibiting a 5.4 times greater absorp-tion coefficient (a) and the crystalline state having 4.8 timesgreater normalized reectivity, both measured at 650 nm. Thedistinct optical differences in 1-Mn/Co(Zn) glassy and crystal-line states were attributed to different Co2+ coordination envi-ronments. The faint blue absorption in crystalline 1-Mn/Co(Zn)Chem. Sci., 2024, 15, 7474–7501 | 7493http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 20 (A) PL spectra of the crystalline ZIF-62:0.5Co2+ and glass ZIF-62:0.5Co2+ under excitation by 980 nm laser diodes. (B) High reflec-tivity contrast ratio of 4.8 at 650 nm (red dotted line) across a meltquench-annealing cycle of a 1-Mn/Co(Zn) film. The % normalizedreflectivity of the glass and crystalline film is shown in dark and lightblue solid lines, respectively. Inset: optical images of the measuredcrystalline film, from which a single grain with a width of ∼15 mm wasselected for data analysis. The scale bar corresponds to 10 mm. (A)reprinted with permission from ref. 142. Copyright 2019 AmericanChemical Society under Creative Commons license. (B) reprinted withpermission from ref. 44. Copyright 2022 American Chemical Society.Chemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineis attributed to d–d transitions of [CoCl4]2− tetrahedra, whiletransitions of the [CoO6]2+ octahedra are forbidden bysymmetry. In situ formation of [CoCl4]2− was further conrmedby EXAFS, as it was not present in either of the starting CPs. Theformation of the glass lowers the average coordination numberof [CoO6]2+ centers, breaking local symmetry and subsequentlyallowing the previously forbidden d–d transitions, resulting ina deep blue glass.4.4 Thermal energy storageThermal energy is stored in materials in various forms,including thermochemical, sensible, and latent heat. Amongthese, latent heat storage stands out for its relatively high energydensity and the added advantage of its isothermal nature.143This concept aligns with the reversible phase transformationobserved in metal–organic hybrid materials, where the transi-tion between crystalline solid and disordered liquid states playsa signicant role. Furthermore, strategic ligand design andprecise coordination bonds enable control over factors likedimensionality, entropy, and the strength of intermolecular andintramolecular interactions. This control, in turn, leads to7494 | Chem. Sci., 2024, 15, 7474–7501predictable thermodynamic and kinetic properties of phasetransitions.144The potential for manipulating order-to-disorder phasetransitions in metal–organic compounds to achieve thermalenergy storage has been showcased in a series of isostructuralmetal–organic coordination complexes labeled as [M(L)6]X2 (M= Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+; L = N-methylurea(MeUr) or acetamide (AcNH2) or AcNH2 with MeOH; X = Cl− orNO3−).89 Pairing metal halides with nitrate salts forms a seriesof octahedral complexes with directional intramolecularhydrogen bonds. These compounds show melting at 76.2–190.9 °C, and the melting enthalpy (DHfus) ranges from 155 to357 kJ L−1 depending on the choices of metal ions and ligands.The overall enthalpic change during the transition is primarilyinuenced by the density and strength of both hydrogen andcoordination bonds, which are highly dependent on the identityof metal cations and the orientation of counter-anions. Notablealterations within the rst coordination sphere during meltingalso signify DSfus, and thus DHfus, upon transition. This isbecause of the increased rotational, vibrational, and trans-lational degrees of freedom that become accessible to thedissociated ligands. Apart from melting behaviors, other typesof phase transitions, such as conformation changes in organicconstituents145 or spin state transitions of metal ions,146 havethe potential to be useful for storing and releasing thermalenergy.5. PerspectiveSimilar to the properties of crystals, the properties of CP/MOFliquids (melting point, viscosity, structure, etc.) are correlatedwith the combination of metal ions and ligands and the struc-tural dimensionality. A comparison between the metal–ligandbond strengths of different materials to gain additionalunderstanding of these melting behaviors will provide animportant guideline for designing new meltable CPs/MOFs.One of the fundamental challenges is controlling thedynamic structure inherent in CP/MOF liquids (Fig. 21A). Thereare questions: does it take on a network nature connected bycoordination bonds—on what timescale do the cleavage,reformation, and exchange of coordination bonds take place—are there phenomena such as dynamic heterogeneity?A better understanding of these fundamentals will open uppossibilities for material applications (Fig. 21B). One is thetransport function using the liquid state. The targets are smallmolecules, including gases, ions, and even electrons. Separa-tion by selective gas transport would be possible.124 If they showfast and selective ion transport, they are used in a variety ofelectrochemical devices. If a gradient of liquid structures iscreated, it could potentially give rise to anisotropic properties.An example here is unidirectional ion transport.147 Designingelectrically conductive liquids is more difficult, but the localcoordination and assembled structures can be controlled ina similar way to electrically conductive glasses.148 They could beplatforms for sensors, dielectrics, energy storage, etc. Liquidsalso have the advantage of forming a mutual interface withdifferent substances. Ionic liquids play an important role as© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eFig. 21 Schematic illustrations of future ideas. (A) Control of structural dynamics in liquid states. (B) Potential applications of CPs/MOFs in theirliquid state based on their isotropic/anisotropic natures and kinetics.Review Chemical ScienceOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineelectrolytes in iontronics due to their high capacitance andcarrier surface density.149–151 Liquid metal catalysts are alsoattracting attention,152 and CP/MOF liquids may also play a rolein such applications. In addition to transport, dissolution anddecomposition in liquids are also important functions. Forexample, highly active, unstable species and substances, suchas radioactive and volatile materials, can be dissolved andstored. Conversely, the decomposition of stable substances inthe medium may also be possible.Another challenge is the control of phase transitions. Inliquid–solid (crystals and glasses) and even liquid–gas transi-tions, parameters such as operating temperature, heat balance,volume change, domain size, and response time can be studiedfor applications. Reversible and fast liquid–solid transitions areused to develop functions for heat and data storage. Unstablematerials are encapsulated in a liquid medium, and uponsolidication (vitrication or crystallization) upon cooling, bothprotective functions and transparency can be achieved. Controlof the energy landscape also allows the creation of semi-melted(semi-crystalline) phases oen found in conventional organicpolymers. In other words, by designing domain structures inwhich the supercooled liquid phase and the crystalline phasecoexist in the material, dual properties of tough mechanics andphysical properties are expected. In addition to the liquid andsolid phases, if the gas phase can be introduced in the future,large-area crystal lms and giant single crystals can be© 2024 The Author(s). Published by the Royal Society of Chemistryproduced by dry processes such as CVD and ALD. The liquidchemistry and phase transitions of CP/MOFs with differentstructural order, composition, and dynamics have great poten-tial in various aspects, including energy harvesting, resourcerecycling, and scalability, as well as the fundamental develop-ment of novel disordered systems.Author contributionsThe manuscript was written with the contributions of allauthors. All authors have approved the nal version of themanuscript.Conflicts of interestThere are no conicts to declare.AcknowledgementsThis work was supported by the Japan Society of the Promotionof Science (JSPS) for a Grant-in-Aid for Scientic Research (B)(JP21H01950), Challenging Research (Exploratory)(JP19K22200), Transformative Research Areas (A) “Supra-ceramics” (JP22H05147) from the Ministry of Education,Culture, Sports, Science and Technology, Japan. N. M. and E. B.acknowledge the Japanese Government (MEXT) scholarship. S.Chem. Sci., 2024, 15, 7474–7501 | 7495http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793eChemical Science ReviewOpen Access Article. Published on 08 May 2024. Downloaded on 10/8/2024 1:32:20 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineK. acknowledges the JSPS postdoctoral fellowship. N. M.acknowledges the support from FWO for the Junior Post-doctoral Fellowship (1280924N) and ICYS research fellowship.References1 K. Mochizuki, M. Matsumoto and I. Ohmine, Defect pairseparation as the controlling step in homogeneous icemelting, Nature, 2013, 498, 350–354.2 B. T. Matthias and J. P. Remeika, Ferroelectricity in theIlmenite Structure, Phys. Rev., 1949, 76, 1886–1887.3 P. H. Keck and M. J. E. Golay, Crystallization of Silicon froma Floating Liquid Zone, Phys. Rev., 1953, 89, 1297.4 J. R. 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Sci., 2024, 15, 7474–7501 | 7501http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sc01793e Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids Functional metaltnqh_x2013organic liquids