The amalgamation of different disciplines is at the heart of reticular chemistry and has broadened the boundaries of chemistry by opening up an infinite space of chemical composition, structure, and material properties. Reticular design has enabled the precise prediction of crystalline framework structures, tunability of chemical composition, incorporation of various functionalities onto the framework backbone, and as a consequence, fine-tuning of metal-organic framework (MOF) and covalent organic framework (COF) properties beyond that of any other material class. Leveraging the unique properties of reticular materials has resulted in significant advances from both a fundamental and an applied perspective. Here, we wish to review the milestones in MOF and COF research and give a critical view on progress in their real-world applications. Finally, we briefly discuss the major challenges in the field that need to be addressed to pave the way for industrial applications.

The Current Status of MOF and COF Applications

The variety of functionalities and porous structures inherent to metal-organic frameworks (MOFs) together with the facile tunability of their properties makes these materials suitable for a wide range of existing and emerging applications. Many of these applications are based on processes involving interaction of MOFs with guest molecules. To optimize a certain process or successfully design a new one, a thorough knowledge is required about the physicochemical characteristics of materials and the mechanisms of their interaction with guest molecules. To obtain such important information, various complementary analytical techniques are applied, among which vibrational spectroscopy (IR and Raman) plays an important role and is indispensable in many cases. In this review, we critically examine the reported applications of IR and Raman spectroscopies as powerful tools for initial characterization of MOF materials and for studying processes of their interaction with various gases. Both the advantages and the limitations of the technique are considered, and the cases where IR or Raman spectroscopy is preferable are highlighted. Peculiarities of MOFs interaction with specific gases and some inconsistent band assignments are also emphasized. Summarizing the broad analytical possibilities of the IR and Raman spectroscopies, we conclude that it can be applied in combinations with other techniques to explicitly establish the structure, properties, and reactivity of MOFs.

Power of Infrared and Raman Spectroscopies to Characterize Metal-Organic Frameworks and Investigate Their Interaction with Guest Molecules

Three-dimensional porous coordination polymers, which are known as metal–organic frameworks (MOFs), have drawn considerable attention owing to their properties, such as highly crystalline and ordered structures, inorganic–organic-hybrid nature, tunability of chemical functionality, high porosity and surface area and moderate-to-high stability. Due to these properties, MOFs are applied extensively as probes for the detection of large varieties of organic molecules. In this line, different MOF-based instrumental and material-based methods along with different strategies have been developed to study and extend our information about the capabilities of MOFs as sensing probes for the detection of organic molecules as well as improving and developing their detection limits, selectivity and sensitivity toward specific analytes. Considering these points, we have presented and classified the content of this review based on the chemical structures of organic analytes in three categories, including (I) nitroaromatic explosives and energetic materials, (II) small organic molecules, such as solvents and volatile organic compounds, and (III) organic amines. For each group of these analytes, different material and instrumental methods using MOFs have been explained, with illustrations of remarkable examples. Finally, we compare the methods for the detection of each group of analytes and discuss which instrumentation is more effective for each group of analytes.

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Sensing organic analytes by metal–organic frameworks: a new way of considering the topic

Post-synthetic modification (PSM) of metal–organic framework (MOF) compounds is a useful technique for preparing new MOFs that can exhibit or enhance many of the properties of the parent MOFs. PSM can be carried out by a number of approaches such as modifying the linker (ligand) and/or metal node, and adsorption/exchange of guest species. The surface environment of the MOF can be modified to increase structural stability as well as introducing desired properties. There is considerable scope in widening the applications of the MOF with compatible metal or ligand employing the PSM. This review focuses on the recent developments of modified materials through PSM, which augers well for the chemical modification and functionalization of MOFs. In this review, different types of PSM methods are presented in an orderly manner, and the diverse applications of resultant frameworks are described and discussed.

Post-Synthetic Modification of Metal–Organic Frameworks Toward Applications

Metal Organic Frameworks (MOFs) and ultrasound: A review.

Metal-organic frameworks (MOFs) can contain open metal sites (OMS) or coordinatively unsaturated sites (CUS) or open coordination sites (OCS) when vacant Lewis acid sites on the metal ions or cluster nodes have been generated. This review combines for the first time all aspects of OMS in MOFs, starting from different preparation strategies over theoretical studies on the effects of OMS with host-guest interactions up to distinct OMS-MOF applications. In the experimental part the focus of this review is on MOFs with proven OMS formation which are not only invoked but are clearly verified by analytical methods.

Coordinatively unsaturated metal sites (open metal sites) in metal–organic frameworks: design and applications

In this study, through introducing a new functional group into the structure, the performance and efficiency of MOFs as a sensor for heavy metal cations have been improved. It was observed that the N1,N3-di(pyridine-4-yl) malonamide ligand (–NH–CO–CH2–CO–NH–)(S), one of the pillar linkers, has not directly entered into the structure of the synthesized MOFs. To solve this issue, three new structures based on copper metal–organic frameworks and amide-functionalized pillar ligands (–NH–CO–), TMU-46, 47 and 48 have been synthesized under hydrothermal conditions. An exciting aspect of the acylamide pillar ligands is their efficient detection ability of Hg2+ (mercury ion) in the presence of other heavy metal cations such as Cd2+ (cadmium), Cu2+ (copper), and Cr3+ (chromium). Due to their chelating effect on heavy metal cations, we hypothesized that decoration of the MOF wall with both malonamide and acylamide struts would promote their Lewis basic properties, and improve the removal capacity of heavy metal ions. A new linker, containing suitable functional group malonamide (S) to enhance Hg2+ cation sensing, was successfully exchanged and the produced material was labeled TMU-46S, TMU-47S and TMU-48S. Designing dual-functionalized MOFs is our “design-for-purpose” approach for the decoration of MOF walls by suitable functional groups resulting in high removal capacity and sensitivity of heavy metal ions. To the best of our knowledge, this is the first report of a mixed amide-malonamide based MOF which provides a proper coordination site to coordinate strongly to Hg2+ ions, along with 714 mg g−1 maximum adsorption capacity and 186 087 M−1Ksv. Generally, we attributed the impressive Hg2+ sensing of TMU-48S to synergistic effects of both hydrophilic chelating malonamide and acylamide functional groups. Hence, the results represent an effective strategy in designing and developing multi-functional MOF-based materials and their application in removal processes and environmental protection efforts.

Selective detection and removal of mercury ions by dual-functionalized metal–organic frameworks: design-for-purpose

Ultrasound assisted synthesis of amide functionalized metal-organic framework for nitroaromatic sensing.

Designing adsorbents with accessible chelating sites and achieving high contaminant purification efficiency are still important to overcome environmental remediation challenges. As one of the significant global concerns, the presence of heavy metal ions in the environment has attracted increasing attention due to their toxicity, carcinogenicity, and bioaccumulation in the food chain. Herein, we performed a targeted design of a new dual-functionalized metal-organic framework (DF-MOF) by incorporating different percentages of the N1,N3-di(pyridine-4-yl) malonamide ligand (S) into urea-containing MOF (TMU-32); the produced material was labeled as TMU-32S (with 33%, 65%, and 100% incorporation percentages). Designing DF-MOF is our "design-for-purpose" approach for the decoration of MOF walls by suitable functional groups, resulting in high removal capacity of heavy metal ions. Among the TMU-32S series having different concentrations of the S ligand, TMU-32S-65% demonstrated exceptional Hg2+ ion selectively. To the best of our knowledge, this is the first report of mixed urea-malonamide-based MOF, which provides a proper coordination site to strongly coordinate with Hg2+ ions, along with 1428 mg g-1 maximum adsorption capacity. Generally, we attributed the impressive implementation of TMU-32S-65% to the synergistic effects of both hydrophilic chelating urea and the malonamide functional group. Hence, the results reported in this work indicate the exceptional potential of DF-MOFs for the high accomplishment of environmental remediation.

The targeted design of dual-functional metal-organic frameworks (DF-MOFs) as highly efficient adsorbents for Hg2+ ions: synthesis for purpose.

In recent years, functionalized pillar ligands have gained significant interests due to their important role in MOF structure and performance. The synthesis of MOF compounds with a particular functionalized ligand is not always successful, and sometimes, synthesis cannot be achieved easily or directly, even by employing several methods. However, this limitation can be overcome by applying a post-synthesis step that swaps the functional groups without changing the backbone of the pillar ligand. Solvent-assisted ligand exchange (SALE) is a post-synthesis method that has been used for confronting this challenge by replacing a functional group with an alternative. Through this investigation, we tried to improve the properties of MOF compounds and increase their catalytic efficiency by importing new functional groups into their structures. The N1,N3-di (pyridine-4-yl) malonamide linker (S) is a pillar ligand, which does not easily enter into the structure during the synthesis of MOF compounds. Therefore, to solve this issue, amide-functionalized, benzene-core ligand derivatives were designed as linkers to manufacture the new 3D structures [Co(oba)(bpta)]·(DMF)2 TMU-50 and [Co2(oba)2(bpfn)]·(DMF)2.5 TMU-51 and the novel 2D structure [Co(oba)(bpfb)]·(DMF)2 TMU-49. These structures were achieved by layering the compounds via hydrothermal reaction. Moreover, the ability of these structures to act as catalysts was evaluated using the methanolysis reaction of epoxides. To increase the MOF catalytic efficiency, we designed the N1,N3-di (pyridine-4-yl) malonamide linker (S) as a malonamide pillar ligand, which contains an acidic hydrogen that is suitable for catalyzing an epoxide ring-opening reaction and therefore enhancing the catalytic activity. As the synthesis of the MOF structure with this linker was not successful, we designed three new structures by incorporating different percentages of S linkers by exchanging the acylamide functional group with malonamide via the SALE pathway. The acylamide functional group was successfully replaced and produced daughter MOFs TMU-49S, TMU-50S and TMU-51S. PXRD and NMR spectroscopy confirmed that the S linker was incorporated into the acylamide-MOF structure. The obtained materials TMU-49S, TMU-50S and TMU-51S are isostructural with their parent frameworks. The S spacer significantly improved the catalytic properties of the MOF compounds in the ring-opening reaction of epoxides, with TMU-50S showing a 98% catalytic efficiency after incorporating the S linker. The catalysts could be recycled without any significant loss in the catalytic efficiency.

Solvent-assisted ligand exchange (SALE) for the enhancement of epoxide ring-opening reaction catalysis based on three amide-functionalized metal-organic frameworks.

Maniya Gharib

Papers

Coordinatively unsaturated metal sites (open metal sites) in metal–organic frameworks: design and applications

Selective detection and removal of mercury ions by dual-functionalized metal–organic frameworks: design-for-purpose

Ultrasound assisted synthesis of amide functionalized metal-organic framework for nitroaromatic sensing.

The targeted design of dual-functional metal-organic frameworks (DF-MOFs) as highly efficient adsorbents for Hg2+ ions: synthesis for purpose.

Solvent-assisted ligand exchange (SALE) for the enhancement of epoxide ring-opening reaction catalysis based on three amide-functionalized metal-organic frameworks.