Tuning the Negative Thermal Expansion Behavior of the Metal–Organic Framework Cu3BTC2 by Retrofitting

Compositional dependence of uniaxial zero thermal expansion and zero linear compressibility in metal-organic framework MIL-122 (Al, Ga, In)

The responses of MIL-122(Al, Ga, In) to pressure or temperature have been investigated. The findings suggest that the a-axis in the lattice exhibits minimal compressibility, particularly zero linear compressibility (ZLC) behavior observed in MIL-122(In), consistent with experimental reports. Additionally, as the radius of metal atoms increases, the compressibility of the b-axis and c-axis gradually strengthens. There is a notable compositional dependence on volume thermal expansion in MIL-122(Al, Ga, In), where an increase in the metal atom radius leads to gradual weakening of volume thermal expansion. In particular, MIL-122(In) demonstrates pronounced volume negative thermal expansion (NTE) behavior, with the a-axis displaying zero thermal expansion (ZTE) behavior and both the b-axis and c-axis exhibiting NTE behavior. The temperature-dependent relative change in the bulk modulus of MIL-122(Al, Ga, In) has also been explored, revealing abnormal thermal hardening specifically within MIL-122(Ga, In). We attribute these unique uniaxial ZTE and ZLC behaviors in MIL-122(In) to its distinctive wine-rack topology, anomalous phonons (negative Grüneisen parameters), and internal structural flexibility.

Significant Enhancement of Negative Thermal Expansion Under Low Pressure in Cu2P2O7

Much effort is made to achieve the negative thermal expansion (NTE) control, but rare methods reached the improvement of intrinsic NTE. In the present work, a significantly enhanced NTE is realized in Cu2P2O7 by applying low pressure. Especially, the volumetric coefficient of thermal expansion (CTE) of Cu2P2O7 reached to -50.0 × 10-6 K-1 (150-325K) under 0.25 GPa, which is increased by 47.5% compared to its NTE in a similar temperature range under atmosphere pressure. This character enables a more effective manifestation of the thermal compensation role of Cu2P2O7 in composites. The enhanced NTE mechanisms are analyzed by high pressure synchrotron X-ray diffraction, neutron diffraction at variable temperature and pressure, as well as density functional theory (DFT) calculations. The results show that applied pressure accelerates the contraction of the distance between adjacent CuO layers and CuO columns. Meanwhile, the low-frequency phonon contribution to NTE in α-Cu2P2O7 is improved. This work is meaningful for the exploration of methods to enhance NTE and the practical application of NTE materials.

Critical Role of Nonrigid Unit and Spiral Acoustical Modes in Designing Colossal Negative Thermal Expansion

Exploring the relationship between thermal expansion and structural complexity is a challenging topic in the study of modern materials where volume stability is required. This work reports a new family of negative thermal expansion (NTE) materials, AM(CN)4 with A = Li and Na and M = B, Al, Ga, and In. Here, the compounds of LiB(CN)4 and NaB(CN)4 were only synthesized; others were purely computationally studied. A critical role of nonrigid vibrational modes and spiral acoustical modes has been identified in NaB(CN)4. This understanding has been exploited to design the colossal NTE materials of NaM(CN)4 (M = Al, Ga, In). A joint study involving synchrotron X-ray diffraction, Raman spectroscopy, and first-principles calculations has been conducted to investigate the thermal expansion mechanism. It has been found that the A atoms can either increase the symmetry of the crystal structure, inducing stronger NTE, or lower the crystal symmetry, thus resulting in positive thermal expansion. Conversely, the M-site atoms do not affect the crystal structure. However, as the radius of the M atoms increases, the ionic nature of the C-M bonds strengthens and the CN vibrations become more flexible, thereby enhancing the NTE behavior. This study provides new insights to aid in the discovery and design of novel NTE materials and the control of thermal expansion.

Regulating the thermal expansion of a [FePt(CN)4] layer by axial coordination and dimensional reduction

Thermal expansion regulation by chemical decoration at a molecular level is of great technological value for materials science. Herein, we show that the spin crossover active compound Fe(pyz)Pt(CN)4 (pyz = pyrazine) shows a rare 2D negative thermal expansion (NTE) in the ab-plane. By introducing axial coordination iodine ions or reducing the framework dimension from 3D to 2D, the NTE behavior can be effectively switched to positive thermal expansion (PTE) or even zero thermal expansion (ZTE). Moreover, it is found that different spin states of Fe2+ also influence the magnitude of NTE. Compared with the low-spin (LS) sate, the high-spin (HS) state tends to enhance the magnitude of NTE. Combined in situ structural and Raman spectral analyses revealed that the NTE mainly originates from the transverse vibration of a bridging cyano group and the tailorable thermal expansion is closely related to the state of the Fe-CN-Pt linkage. The present study shows how the rational regulation of the building unit and framework dimensions can effectively control thermal expansion behaviors. This insight can serve as guidance for designing and synthesizing novel NTE materials.

Node Distortions in UiO-66 Inform Negative Thermal Expansion Mechanisms: Kinetic Effects, Frustration, and Lattice Hysteresis

In metal-organic frameworks (MOFs) the interplay between the dynamics of individual components and how these are constrained by the extended lattice can yield unusual emergent phenomena. For the archetypal Zr-MOF, UiO-66, we explore the cooperative dynamics of a Zr-node transformation that gives rise to negative thermal expansion (NTE). Using in situ synchrotron X-ray scattering, with powder diffraction and pair distribution function (PDF) analyses, we identify lattice hysteresis and a thermal ramp-rate-dependence of the thermal expansion. Specifically, kinetic trapping of distorted node states formed at high temperature, leads to broad variability in the apparent thermal expansion which ranges from large positive to large negative thermal expansion with coefficients of thermal expansion (CTE) from +45 to -80 × 10-6K-1. Time-resolved relaxation studies at selected temperatures suggest that when equilibrated UiO-66 is intrinsically NTE, with a CTE of -35 × 10-6K-1. Kinetic trapping of the node-distorted state following high temperature activation has broad implications for characterization and applications of these Zr-MOFs; the nonequilibrium node state depends on the thermal history of the sample with quench vs slow cooling likely to impact gas binding, pore volume, and accessible catalytic sites.

Giant Negative Thermal Expansion in Ultralight NaB(CN)4

Negative thermal expansion (NTE) is crucial for controlling the thermomechanical properties of functional materials, albeit being relatively rare. This study reports a giant NTE (αV ∼-9.2 ⋅ 10-5  K-1 , 100-200 K; αV ∼-3.7 ⋅ 10-5  K-1 , 200-650 K) observed in NaB(CN)4 , showcasing interesting ultralight properties. A comprehensive investigation involving synchrotron X-ray diffraction, Raman spectroscopy, and first-principles calculations has been conducted to explore the thermal expansion mechanism. The findings indicate that the low-frequency phonon modes play a primary role in NTE, and non-rigid vibration modes with most negative Grüneisen parameters are the key contributing factor to the giant NTE observed in NaB(CN)4 . This work presents a new material with giant NTE and ultralight mass density, providing insights for the understanding and design of novel NTE materials.

Colossal negative thermal expansion in a cucurbit[8]uril-enabled uranyl-organic polythreading framework via thermally induced relaxation

It is an ongoing goal to achieve the effective regulation of the thermal expansion properties of materials. In this work, we propose a method for incorporating host-guest complexation into a framework structure and construct a flexible cucurbit[8]uril uranyl-organic polythreading framework, U₃(bcbpy)₃(CB8). U₃(bcbpy)₃(CB8) can undergo huge negative thermal expansion (NTE) and has a large volumetric coefficient of -962.9 × 10^-6 K^-1 within the temperature range of 260 K to 300 K. Crystallographic snapshots of the polythreading framework at various temperatures reveal that, different from the intrinsic transverse vibrations of the subunits of metal-organic frameworks (MOFs) that experience NTE via a well-known hinging model, the remarkable NTE effect observed here is the result of a newly-proposed thermally induced relaxation process. During this process, an extreme spring-like contraction of the flexible CB8-based pseudorotaxane units, with an onset temperature of ∼260 K, follows a period of cumulative expansion. More interestingly, compared with MOFs that commonly have relatively strong coordination bonds, due to the difference in the structural flexibility and adaptivity of the weakly bonded U₃(bcbpy)₃(CB8) polythreading framework, U₃(bcbpy)₃(CB8) shows unique time-dependent structural dynamics related to the relaxation process, the first time this has been reported in NTE materials. This work provides a feasible pathway for exploring new NTE mechanisms by using tailored supramolecular host-guest complexes with high structural flexibility and has promise for the design of new kinds of functional metal-organic materials with controllable thermal responsive behaviour.

Node Distortion as a Tunable Mechanism for Negative Thermal Expansion in Metal-Organic Frameworks

Chemically functionalized series of metal-organic frameworks (MOFs), with subtle differences in local structure but divergent properties, provide a valuable opportunity to explore how local chemistry can be coupled to long-range structure and functionality. Using in situ synchrotron X-ray total scattering, with powder diffraction and pair distribution function (PDF) analysis, we investigate the temperature dependence of the local- and long-range structure of MOFs based on NU-1000, in which Zr₆O₈ nodes are coordinated by different capping ligands (H₂O/OH, Cl^- ions, formate, acetylacetonate, and hexafluoroacetylacetonate). We show that the local distortion of the Zr₆ nodes depends on the lability of the ligand and contributes to a negative thermal expansion (NTE) of the extended framework. Using multivariate data analyses, involving non-negative matrix factorization (NMF), we demonstrate a new mechanism for NTE: progressive increase in the population of a smaller, distorted node state with increasing temperature leads to global contraction of the framework. The transformation between discrete node states is noncooperative and not ordered within the lattice, i.e., a solid solution of regular and distorted nodes. Density functional theory calculations show that removal of ligands from the node can lead to distortions consistent with the Zr···Zr distances observed in the experiment PDF data. Control of the node distortion imparted by the nonlinker ligand in turn controls the NTE behavior. These results reveal a mechanism to control the dynamic structure of MOFs based on local chemistry.

Switch of Thermal Expansions Triggered by Itinerant Electrons in Isostructural Metal Trifluorides

Manageable thermal expansion (MTE) of metal trifluorides can be achieved by introducing local structure distortion (LSD) in the negative thermal expansion ScF₃. However, an open issue is why isostructural TiF₃, free of LSD, exhibits positive thermal expansion. Herein, a combined analysis of synchrotron X-ray diffraction, X-ray pair distribution function, and rigorous first-principles calculations was performed to reveal the important role of itinerant electrons in mediating soft phonons and lattice dynamics. Metallic TiF₃ demonstrates itinerant electrons and a suppressed Grüneisen parameter γ ≈ -20, while insulating ScF₃ absence of itinerant electrons has a considerable γ ≈ -120. With increasing electron doping concentrations in ScF₃, soft phonons become hardened and the γ is repressed significantly, identical to TiF₃. The presented results update the thermal expansion transition mechanism in framework structure analogues and provide a practical approach to obtaining MTE without inducing sizable structure distortion.

Integration of negative, zero and positive linear thermal expansion makes borate optical crystals light transmission temperature-independent

Negative and zero thermal expansion (NTE and ZTE) materials are widely adopted to eliminate the harmful effect from the "heat expansion and cool contraction" effect and frequently embrace novel fundamental physicochemical mechanisms. To date, the manipulation of NTE and ZTE materials has mainly been realized by chemical component regulation. Here, we propose another method by making use of the anisotropy of thermal expansion in noncubic single crystals, with maximal tunability from the integration of linear NTE, ZTE and positive thermal expansion (PTE). We demonstrate this concept in borate optical crystals of AEB₂O₄ (AE = Ca or Sr) to make the light transmission temperature-independent by counterbalancing the thermal expansion and thermo-optics coefficient. We further reveal that such a unique thermal expansion behavior in AEB₂O₄ arises from the synergetic thermal excitation of bond stretching in ionic [AEO₈] and rotation between covalent [BO₃] groups. This work has significant implications for understanding the thermal excitation of lattice vibrations in crystals and promoting the functionalization of anomalous thermal expansion materials.

Two-Dimensional Negative Thermal Expansion in a Facile and Low-Cost Oxalate-Based Metal-Organic Framework

Two-dimensional negative thermal expansion (NTE) is achieved in a tetragonal oxalate-based metal-organic framework (MOF), CdZrSr(C₂O₄)₄, within a temperature range from 123 to 398 K [space group I4̅m2, α_a = -2.4(7) M K^-1, α_c = 11.3(3) M K^-1, and α_V = 6.4(1) M K^-1]. By combining variable-temperature X-ray diffraction, a high-resolution synchrotron X-ray pair distribution function, and thermogravimetry-differential scanning calorimetry, we shows that NTE within the ab plane derives from the oriented rotation of an oxalate ligand in zigzag chains (-CdO₈-ox-ZrO₈-ox-)_∞. That is simplified to the Zr atom rotating with an unchanged Zr···Cd distance as the radius, which also gives rise to the deformation of a hingelike connection along the c axis and results in its positive thermal expansion. By virtue of the facile and low-cost oxalate ligand, the present NTE MOF may show application prospects in the future.

A neural network potential for the IRMOF series and its application for thermal and mechanical behaviors

Metal-organic frameworks (MOFs) with their exceptional porous and organized structures have been the subject of numerous applications. Predicting the bulk properties from atomistic simulations requires the most accurate force fields, which is still a major problem due to MOFs' hybrid structures governed by covalent, ionic and dispersion forces. Application of ab initio molecular dynamics to such large periodic systems is thus beyond the current computational power. Therefore, alternative strategies must be developed to reduce computational cost without losing reliability. In this work, we construct a generic neural network potential (NNP) for the isoreticular metal-organic framework (IRMOF) series trained by PBE-D4/def2-TZVP reference data of MOF fragments. We confirmed the success of the resulting NNP on both fragments and bulk MOF structures by prediction of properties such as equilibrium lattice constants, phonon density of states and linker orientation. The RMSE values of energy and force for the fragments are only 0.0017 eV atom^-1 and 0.15 eV Å^-1, respectively. The NNP predicted equilibrium lattice constants of bulk structures, even though not included in training, are off by only 0.2-2.4% from experimental results. Moreover, our fragment based NNP successfully predicts the phenylene ring torsional energy barrier, equilibrium bond distances and vibrational density of states of bulk MOFs. Furthermore, the NNP enables revealing the odd behaviors of selected MOFs such as the dual thermal expansion properties and the effect of mechanical strain on the adsorption of hydrogen and methane molecules. The NNP based molecular dynamics (MD) simulations suggest IRMOF-4 and IRMOF-7 to have positive-to-negative thermal expansion coefficients while the rest to have only negative thermal expansion at the studied temperatures of 200 K to 400 K. The deformation of the bulk structure by reduction of the unit cell volume has been shown to increase the volumetric methane uptake in IRMOF-1 but decrease the volumetric methane uptake in IRMOF-7 due to the steric hindrance. To the best of our knowledge, this study presents the first pre-trained model publicly available giving the opportunity for the researchers in the field to investigate different aspects of IRMOFs by performing large-scale simulation at the first-principles level of accuracy.

Understanding Large Negative Thermal Expansion of NdFe(CN)6 through the Electronic Structure and Lattice Dynamics

A large negative thermal expansion (NTE) (α_v = -4.1 × 10^-5 K^-1, 100-525 K) has been discovered in NdFe(CN)₆. Here, the synchrotron X-ray diffraction and lattice dynamics calculations using the density functional theory were conducted to understand the NTE in NdFe(CN)₆. The information obtained on the bond nature of the Nd-N≡C-Fe linkage and on the atomic thermal vibrations suggests that the transverse vibrations of the -N≡C- group, in particular from N atoms, produced the NTE in NdFe(CN)₆. This is corroborated by the calculated Grüneisen parameters, which confirm the relationship between NTE and CN atomic vibrations. The results provide a helpful contribution toward the realization of new materials with negative or controllable thermal expansion.

Chemical Diversity for Tailoring Negative Thermal Expansion

Negative thermal expansion (NTE), referring to the lattice contraction upon heating, has been an attractive topic of solid-state chemistry and functional materials. The response of a lattice to the temperature field is deeply rooted in its structural features and is inseparable from the physical properties. For the past 30 years, great efforts have been made to search for NTE compounds and control NTE performance. The demands of different applications give rise to the prominent development of new NTE systems covering multifarious chemical substances and many preparation routes. Even so, the intelligent design of NTE structures and efficient tailoring for lattice thermal expansion are still challenging. However, the diverse chemical routes to synthesize target compounds with featured structures provide a large number of strategies to achieve the desirable NTE behaviors with related properties. The chemical diversity is reflected in the wide regulating scale, flexible ways of introduction, and abundant structure-function insights. It inspires the rapid growth of new functional NTE compounds and understanding of the physical origins. In this review, we provide a systematic overview of the recent progress of chemical diversity in the tailoring of NTE. The efficient control of lattice and deep structural deciphering are carefully discussed. This comprehensive summary and perspective for chemical diversity are helpful to promote the creation of functional zero-thermal-expansion (ZTE) compounds and the practical utilization of NTE.

Perspectives on the Influence of Crystal Size and Morphology on the Properties of Porous Framework Materials

Miniaturization is a key aspect of materials science. Owing to the increase in quality experimental and computational tools available to researchers, it has become clear that the crystal size and morphology of porous framework materials, including metal-organic frameworks and covalent organic frameworks, play a vital role in defining the physicochemical behaviour of these materials. However, given the multiscale and multidisciplinary challenges associated with establishing how crystal size and morphology affect the structure and behaviour of a material–from local to global structural modifications and from static to dynamic effects–a comprehensive mechanistic understanding of size and morphology effects is missing. Herein, we provide our perspective on the current state-of-the-art of this topic, drawn from various complementary disciplines. From a fundamental point of view, we discuss how controlling the crystal size and morphology can alter the mechanical and adsorption properties of porous framework materials and how this can impact phase stability. Special attention is also given to the quest to develop new computational tools capable of modelling these multiscale effects. From a more applied point of view, given the recent progress in this research field, we highlight the importance of crystal size and morphology control in drug delivery. Moreover, we provide an outlook on how to advance each discussed field by size and morphology control, which would open new design opportunities for functional porous framework materials.

Mechanistic Insights into Nanoparticle Formation from Bimetallic Metal-Organic Frameworks

Understanding and controlling nanomaterial structure, chemistry, and defects represents a synthetic and characterization challenge. Metal-organic frameworks (MOFs) have recently been explored as unconventional precursors from which to prepare nanomaterials. Here we use in situ X-ray pair distribution function analysis to probe the mechanism through which MOFs transform into nanomaterials during pyrolysis. By comparing a series of bimetallic MOFs with trimeric node different compositions (Fe₃, Fe₂Co, and Fe₂Ni) linked by carboxylate ligands in a PCN-250 lattice, we demonstrate that the resulting nanoparticle structure, chemistry, and defect concentration depend on the node chemistry of the original MOF. These results suggest that the preorganized structure and chemistry of the MOF offer new potential control over the nanomaterial synthesis under mild reaction conditions. In the case of Fe₂Ni-PCN-250, selective extraction of one Ni ion from each node without collapsing the framework (i.e., node-ligand connectivity) leaves a metal-deficient MOF state that may provide a new route to post-synthetically tune the chemistry the MOF and subsequent nanomaterials.

What Lies beneath a Metal-Organic Framework Crystal Structure? New Design Principles from Unexpected Behaviors

The rational design principles established for metal-organic frameworks (MOFs) allow clear structure-property relationships, fueling expansive growth for energy storage and conversion, catalysis, and beyond. However, these design principles are based on the assumption of compositional and structural rigidity, as measured crystallographically. Such idealization of MOF structures overlooks subtle chemical aspects that can lead to departures from structure-based chemical intuition. In this Perspective, we identify unexpected behavior of MOFs through literature examples. Based on this analysis, we conclude that departures from ideality are not uncommon. Whereas linker topology and metal coordination geometry are useful starting points for understanding MOF properties, we anticipate that deviations from the idealized crystal representation will be necessary to explain important and unexpected behaviors. Although this realization reinforces the notion that MOFs are highly complex materials, it should also stimulate a broader reexamination of the literature to identify corollaries to existing design rules and reveal new structure-property relationships.

Structural flexibility in crystalline coordination polymers: a journey along the underlying free energy landscape

In this perspective, we discuss structural flexibility in crystalline coordination polymers. We identify that the underlying free energy landscape unites scientific disciplines, and discuss key areas to advanced the field.

Host-Guest Interactions in Metal-Organic Frameworks Doped with Acceptor Molecules as Revealed by Resonance Raman Spectroscopy

Metal-organic frameworks (MOFs) represent a class of porous materials whose properties can be altered by doping with redox-active molecules. Despite advanced properties such as enhanced electrical conduction that doped MOFs exhibit, understanding physical mechanisms remains challenging because of their heterogeneous nature hindering experimental observations of host-guest interactions. Here, we show a study of charge transfer between Mn-MOF-74 and electron acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ) and XeF₂, employing selective enhancement of Raman scattering of different moieties under various optical-resonance conditions. We identify Raman modes of molecular components and elucidate that TCNQ gets oxidized into dicyano-p-toluoyl cyanide (DCTC^-) while XeF₂ fluorinates the MOF upon infiltration. The framework's linker in both cases acts as an electron donor as deduced from blue shifts of the C-O stretching mode accompanied by the emergence of a quinone-like mode. This work demonstrates a generally applicable methodology for investigating charge transfer in various donor-acceptor systems by means of resonance Raman spectroscopy.

Design of Metal-Organic Framework Templated Materials Using High-Throughput Computational Screening

The ability to crosslink Metal-Organic Frameworks (MOFs) has recently been discovered as a flexible approach towards synthesizing MOF-templated “ideal network polymers”. Crosslinking MOFs with rigid cross-linkers would allow the synthesis of crystalline Covalent-Organic Frameworks (COFs) of so far unprecedented flexibility in network topologies, far exceeding the conventional direct COF synthesis approach. However, to date only flexible cross-linkers were used in the MOF crosslinking approach, since a rigid cross-linker would require an ideal fit between the MOF structure and the cross-linker, which is experimentally extremely challenging, making in silico design mandatory. Here, we present an effective geometric method to find an ideal MOF cross-linker pair by employing a high-throughput screening approach. The algorithm considers distances, angles, and arbitrary rotations to optimally match the cross-linker inside the MOF structures. In a second, independent step, using Molecular Dynamics (MD) simulations we quantitatively confirmed all matches provided by the screening. Our approach thus provides a robust and powerful method to identify ideal MOF/Cross-linker combinations, which helped to identify several MOF-to-COF candidate structures by starting from suitable libraries. The algorithms presented here can be extended to other advanced network structures, such as mechanically interlocked materials or molecular weaving and knots.

Large and tunable negative thermal expansion induced by a synergistic effect in M2II[MIV(CN)8] Prussian blue analogues

Designing negative thermal expansion (NTE) materials with a larger NTE coefficient and a wider temperature window is a great challenge nowadays, leading to the limitation of existing NTE materials such that only about 150 kinds of NTE materials have been discovered since 1996. Here, using first-principles calculations combined with the quasi-harmonic approximation (QHA), we find that the synergistic effect of different vibrational modes can significantly enhance the NTE in open framework compounds. We systematically investigate the NTE properties of the M₂^IIM^IV(CN)₈ (M^II = Ni, Co, Fe, and Mn; M^IV = Mo and W) family, which is the first kind of Prussian blue analogues (PBAs) with a 2D and 3D topology structure, to explore the synergistic enhancement effect in NTE. We reveal that both the optical modes of the rotational motion of [W(CN)₈] and [Ni(NC)₄] rigid units and the low frequency acoustic modes of the transverse vibration contribute significantly to the NTE. Furthermore, the coefficients of NTE increase monotonously with increasing ionic radius upon substituting Ni in M₂^IIW(CN)₈ with Co, Fe, or Mn, respectively. Analyzing the vibrational modes of the substituted systems indicates that the dramatic changes in NTE originate from a highly synergistic effect, in which all the frequencies of these NTE modes have the same trend, i.e. the lower the frequencies, the larger the coefficient of NTE. This work clearly presents a synergistic mechanism of enhancing NTE in PBA materials, and sheds light on designing new materials with better properties.

Electronic Structure Modeling of Metal-Organic Frameworks

Owing to their molecular building blocks, yet highly crystalline nature, metal-organic frameworks (MOFs) sit at the interface between molecule and material. Their diverse structures and compositions enable them to be useful materials as catalysts in heterogeneous reactions, electrical conductors in energy storage and transfer applications, chromophores in photoenabled chemical transformations, and beyond. In all cases, density functional theory (DFT) and higher-level methods for electronic structure determination provide valuable quantitative information about the electronic properties that underpin the functions of these frameworks. However, there are only two general modeling approaches in conventional electronic structure software packages: those that treat materials as extended, periodic solids, and those that treat materials as discrete molecules. Each approach has features and benefits; both have been widely employed to understand the emergent chemistry that arises from the formation of the metal-organic interface. This Review canvases these approaches to date, with emphasis placed on the application of electronic structure theory to explore reactivity and electron transfer using periodic, molecular, and embedded models. This includes (i) computational chemistry considerations such as how functional, k-grid, and other model variables are selected to enable insights into MOF properties, (ii) extended solid models that treat MOFs as materials rather than molecules, (iii) the mechanics of cluster extraction and subsequent chemistry enabled by these molecular models, (iv) catalytic studies using both solids and clusters thereof, and (v) embedded, mixed-method approaches, which simulate a fraction of the material using one level of theory and the remainder of the material using another dissimilar theoretical implementation.

Retrofitting metal-organic frameworks

The post-synthetic installation of linker molecules between open-metal sites (OMSs) and undercoordinated metal-nodes in a metal-organic framework (MOF) — retrofitting — has recently been discovered as a powerful tool to manipulate macroscopic properties such as the mechanical robustness and the thermal expansion behavior. So far, the choice of cross linkers (CLs) that are used in retrofitting experiments is based on qualitative considerations. Here, we present a low-cost computational framework that provides experimentalists with a tool for evaluating various CLs for retrofitting a given MOF system with OMSs. After applying our approach to the prototypical system CL@Cu₃BTC₂ (BTC = 1,3,5-benzentricarboxylate) the methodology was expanded to NOTT-100 and NOTT-101 MOFs, identifying several promising CLs for future CL@NOTT-100 and CL@NOTT-101 retrofitting experiments. The developed model is easily adaptable to other MOFs with OMSs and is set-up to be used by experimentalists, providing a guideline for the synthesis of new retrofitted MOFs with modified physicochemical properties.

Retrofitting is recognized as a powerful tool to control the structure and the corresponding functionality in MOFs. Here, the authors develop a low-cost computational framework to guide experimentalists for retrofitting experiments in MOFs and test it on the prototypical Cu3BTC2 MOF.

Accordion and layer-sliding motion to produce anomalous thermal expansion behaviour in 2D-coordination polymers

Solvent-free (1) and solvated (2) 2D-coordination polymers have been synthesised by varying the amount of solvent during crystallisation. 1 undergoes a unique accordion motion of 2D zig-zag interwoven layers whereas 2 experiences layer-sliding within 2D layers to produce anomalous thermal expansion behaviour.

Micro-spectroscopy of HKUST-1 metal–organic framework crystals loaded with tetracyanoquinodimethane: effects of water on host–guest chemistry and electrical conductivity

Guest@MOF materials have potential in next-generation materials for electroconductive devices. Micro-spectroscopy studies of TCNQ@HKUST-1 electroconductive composites revealed the effects of spatial distribution and water vapor on this material.