**Preface to "Functional Coordination Polymers and Metal–Organic Frameworks"**

When I was asked to prepare a Special Issue of *Inorganics* focused on Coordination Polymers and Metal-Organic Frameworks, I could not imagine that it would have such a great success and participation. I am really pleased to have gathered so many excellent contributions coming from three different continents all over the World: America, Europe, and Asia. The topics covered are diverse, including synthesis, advanced characterization, and applications in the fields of CO<sup>2</sup> storage, water adsorption, heterogeneous catalysis, and luminescence. The incredible flexibility in CPs and MOFs design stemming from a virtually infinite combination of organic linkers of assorted nature (azolates, carboxylates) and metallic nodes containing transition metals with specific chemical properties opens wide horizons in the future development of these exciting materials. I hope that the *Inorganics* readers will enjoy reading this Special Issue as much as I did when I assembled it in its final stage. I must acknowledge once more all the contributors (some of them are also friends!) and the *Inorganics* Editorial Office for its valuable support in this happy-ending adventure.

> **Andrea Rossin** *Editor*

### *Editorial* **Editorial for Special Issue "Functional Coordination Polymers and Metal–Organic Frameworks"**

**Andrea Rossin**

Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy; arossin@iccom.cnr.it

Metal–Organic Frameworks (MOFs) and Coordination Polymers (CPs) are at the forefront of contemporary coordination chemistry research, as witnessed by the impressive (and ever-growing) number of publications appearing in the literature on this topic in the last 20 years (Figure 1), reaching almost 4000 papers in 2020. Among them, several recent review articles and books clearly illustrate the huge potential of this class of compounds [1–6].

**Figure 1.** The number of publications in the 2002–2020 time period whose titles or keywords contain "MOF". Data from Web of Science.

The virtually infinite combination of tailored organic linkers and inorganic metal nodes (in the form of isolated ions or more complex cluster structures) generates a wide variety of functional materials for assorted applications. With a specific application in mind, chemists and materials scientists devoted to MOFs synthesis "play LEGO©" [7] every day in their laboratories to reach their targets, choosing the most suitable pairs to create valuable MOFs.

The present Special Issue collects a total number of 10 papers: 8 original articles and 2 review articles covering multiple aspects of MOFs and CPs chemistry.

′ Two fundamental synthetic studies are part of this collection. Armelao et al. (Padova, Italy) [8] examined the solvent effects in the structures of CuII-4,4′ -bipyridyl CPs coupled to different coordinating solvents, such as DMA, DMF and DMSO. The CP dimensionality (either 2D sheet or 1D chain) strongly depends on the solvent of choice. Thus, the coordinating solvent approach (CSA) can be used as an effective tool to modulate and control the dimensionality, composition and network of CPs. Galli and co-workers (Como,

**Citation:** Rossin, A. Editorial for Special Issue "Functional Coordination Polymers and Metal–Organic Frameworks". *Inorganics* **2021**, *9*, 33. https:// doi.org/10.3390/inorganics9050033

Received: 12 April 2021 Accepted: 28 April 2021 Published: 3 May 2021

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Italy) [9] proved the versatility of the 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr2ad) linker in the synthesis of new CdII, CuII and ZnII CPs of assorted dimensionality (ranging from 1D to 2D) and featured by excellent thermal stability. The high thermal robustness is quite common in MOFs and CPs built with N-donors like pyrazolates, triazolates and imidazolates, and it stems from the strength of the metal-nitrogen coordination bond.

This Special Issue though is focused on applications, collecting contributions covering many different applicative contexts. In the field of heterogeneous catalysis, an excellent and comprehensive review article by Zhou et al. (College Station, TX, USA) [10] summarizes the most recent advances in organometallic functionalization of MOFs from both linker-centric and metal-cluster-centric perspectives. MOFs can function as a tailorable platform for traditional organometallic transformations, including reaction of alkenes, cross-coupling reactions and C–H activations.

The timely CO<sup>2</sup> adsorption topic [at the core of contemporary Carbon Capture and Sequestration (CCS) technology to reduce carbon dioxide levels in the Earth's atmosphere and concomitant greenhouse effect] is developed by Bonino and co-workers (Torino, Italy) [11]. The research team in Torino reports on the synthesis, spectroscopic characterization and adsorption properties of the CeIII MOF Ce5(BDC)7.5(DMF)4, containing the simple terephtalic acid (H2BDC) as linker. The coordinated DMF solvent can be removed from the metal nodes generating open metal sites, as confirmed by FTIR spectroscopy. The interaction of the desolvated material with CO<sup>2</sup> was characterized by volumetric and calorimetric measurements, finding a relevant heat of adsorption (Qst) for the very first dose, typical of MOFs with open metal sites. Another contribution focused on the same topic comes from Taddei et al. (Swansea, Wales, UK) [12]. The team reported a systematic approach aimed at identifying the optimal conditions for the compaction (i.e., pellets preparation) of MOF-801, a small-pore zirconium-based MOF containing fumaric acid as linker. The MOF pellets (prepared under different experimental pressure conditions and in the presence of assorted binders) were tested in CO<sup>2</sup> adsorption, retaining as much as 90% of the CO<sup>2</sup> working capacity of the powder, while displaying unaffected sorption kinetics. This work provides a starting point for future exploration of shaping of MOF powders, which is becoming a progressively more important aspect as MOFs move towards commercialization and employment in industrial applications.

Another application of growing interest for MOFs and CPs is luminescence. As chemical sensors, luminescent MOFs possess a number of advantages over other luminescent materials. Analyte adsorption within MOF pores allows for its pre-concentration, increasing sensor sensitivity. Selectivity in MOFs can be achieved by tuning pore dimensionality and/or by proper functionalization of the linkers. Porosity allows for adsorption of chromophores which can be luminescent themselves or act as antennas, this enlarging the number of mechanisms beyond luminescence. Given my long-lasting interest in the design and synthesis of thiazole-containing linkers and related MOFs/CPs in Firenze (Italy) [13–20], I have personally contributed to this Special Issue with a mini-review article describing the luminescent features of thiazole- and thiadiazole-based MOFs and CPs [21]. Thiazoles are intrinsically fluorescent heterocycles, and their exploitation in the construction of MOFs is still at its infancy. Within this Special Issue, there are two additional contributions on the luminescence topic. Cepeda, Rodríguez-Diéguez et al. (País Vasco/Granada, Spain) published two new ZnII and CdII CPs based on 1H-indazole-6-carboxylic acid, along with their photoluminescence (PL) properties [22]. Both CPs present emission spectra similar to the free ligand, confirming that the electronic transitions of the polymers are ligand-centered (of π–π \* type). This kind of emission is typical of MOFs and CPs built with an electronically inert metal ion (like ZnII or CdII, with a d<sup>10</sup> closed-shell electronic configuration). The Russian team of Artem'ev and colleagues (Novosibirsk, Russian Federation) has synthesized a 1D Ag<sup>I</sup> -based CP with 4,6-bis (diphenylphosphino)pyrimidine [23]. The material exhibits pronounced thermochromic luminescence, expressed by reversible changing of the emission chromaticity from yellow (at ambient temperature) to orange (at the liquid nitrogen temperature). The detailed temperature-dependent photophysical study has shown

that the ambient temperature photoluminescence may be tentatively ascribed to thermally activated delayed fluorescence (TADF) caused by formation of (M + L')LCT excited states. This class of materials find promising application in luminescent thermometry.

The water tolerance of MOFs is an essential feature for many practical applications when operating under ordinary humid atmosphere. Resistance toward hydrolysis in such environment is not common, given the intrinsically reactive metal-ligand coordination bond. The most important MOF family that is resistant to hydrolysis is that of the zirconium based "UiOs". Water stability opens greener synthetic routes, and aqueous synthesis of MOFs at room temperature offers many advantages such as reduction in the generation of toxic byproducts and operation costs, as well as increased safety in the material's production. The American team of Islamoglu and Farha (Chicago, US) has reported an aqueous solution-based synthesis of the robust zirconium MOF UiO-66-NO<sup>2</sup> at room temperature [24]. Water vapor sorption isotherms at room temperature indicated high uptake, suggesting the potential of this MOF for adsorption-based cooling applications or water harvesting systems. Water can also be considered a substrate for heterogeneous catalysis. In this regard, the team of Macchioni and Costantino (Perugia, Italy) has functionalized UiO-66 post-synthetically with the organometallic IrIII complex [Ir(HEDTA)Cl]Na, placing it on the defective MOF sites occupied by formate linkers [25]. Anchoring of the complex occurs through an exchange of formate with the free HEDTA carboxylate group. The modified material was tested as a heterogeneous catalyst for the water chemical oxidation reaction (water splitting with concomitant O<sup>2</sup> production) by using cerium ammonium nitrate as sacrificial agent.

In conclusion, I hope that these open-access contributions will serve as guiding lights for future MOFs and CPs development in still undiscovered applications. I thank the authors for their original contributions for the Special Issue, and I thank the reviewers for their tireless enthusiasm, insightful comments and revisions. Last but not least, my warmest acknowledgments go to the *Inorganics* Editorial Staff for its constant dedication, support and patience in collecting so many excellent papers over a time period of two years.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**


*Review*
