Tetrazine-based linkers as intrinsically tagged alternatives for click functionalization of metal-organic frameworks

Reticular chemistry has in the post-synthetic modification (PSM) of frameworks one of the most versatile tools to adapt the systems' physicochemical properties to the specific requirements which are imposed by their application in different contexts. We can safely say that PSM methodologies in all their variants are currently one of the main resources that reticular chemists turn to when they need to diversify a framework compositionally. Practically all these modifications require the integration of functional groups appended to the organic linkers in the framework, either by direct synthesis or by post-synthetic exchange. The reactivity of these tags allows, at a subsequent stage, covalent modification of the framework under conditions that ideally respect its structural integrity. In this perspective article we introduce the use of tetrazine-based linkers as intrinsically tagged alternatives to integrate PSM with click chemistry reactivity. This strategy is ideally suited to molecular frameworks, as it combines very mild modification conditions with direct control over the organisation of built-in appendices and the acknowledged potential of click chemistry to build framework libraries.

The post-synthesis modification (PSM) of MOFs for catalysis

While there are myriad ways to construct metal-organic framework (MOF) based catalysts, the introduction of catalytic functionality via covalent post-synthesis functionalization (PSM) offers multiple advantages: (i) a wide range of different catalyst types are generated from a handful of well-known parent MOFs, (ii) MOF catalyst properties can be systematically tuned while changing few variables, and (iii) catalytically active functional groups that would otherwise interfere with MOF assembly can be introduced facilely. This last advantage is particularly crucial for our quest to generate MOF active sites that are decorated with multiple functional groups capable of cooperative activity, analogous to enzyme active sites.

Bioinspired Structure Regulation of Apyrase-Like Nanozyme with Intracellular-Generated H2O2 for Tumor Catalytic Therapy

Adenosine triphosphate (ATP) is the major resource of energy supply in tumor activity. Therefore, improving ATP consumption efficiencies is a promising approach for cancer therapy. Herein, inspired by the H₂O₂-involved structure regulation effect during the catalysis of natural protein enzymes, we developed an artificial H₂O₂-driven ATP catalysis-promoting system, the Ce-based metal-organic framework (Ce-MOF), for catalytic cancer therapy. In the presence of H₂O₂, the hydrolysis ATP activity of Ce-MOF(H₂O₂) was enhanced by around 1.6 times. Taking advantage of the endogenous H₂O₂ in cancerous cells, catalytic hydrolysis for intracellular ATP of the Ce-MOF achieves the inhibition of cancerous cell growth, which involves damaged mitochondrial function and autophagy-associated cell death. Furthermore, in vivo studies suggest that the Ce-MOF has a good tumor inhibition effect. The artificial H₂O₂-driven ATP catalysis-promoting system not only demonstrates high catalytic ATP consumption efficiencies for cancer therapy but also highlights a bioinspired strategy to expedite nanozyme research in both design and applied sciences.

Tetrazine Linkers as Plug-and-Play Tags for General Metal-Organic Framework Functionalization and C60 Conjugation

The value of covalent post-synthetic modification in expanding the chemistry and pore versatility of reticular solids is well documented. Here we use mesoporous crystals of the metal-organic framework (MOF) UiO-68-TZDC to demonstrate the value of tetrazine connectors for all-purpose inverse electron-demand Diels-Alder ligation chemistry. Our results suggest a positive effect of tetrazine reticulation over its reactivity for quantitative one-step functionalization with a broad scope of alkene or alkyne dienophiles into pyridazine and dihydropyridazine frameworks. This permits generating multiple pore environments with diverse chemical functionalities and the expected accessible porosities, that is also extended to the synthesis of crystalline fulleretic materials by covalent conjugation of fullerene molecules.

Effect of Linker Distribution in the Photocatalytic Activity of Multivariate Mesoporous Crystals

The use of Metal-Organic Frameworks as crystalline matrices for the synthesis of multiple component or multivariate solids by the combination of different linkers into a single material has emerged as a versatile route to tailor the properties of single-component phases or even access new functions. This approach is particularly relevant for Zr₆-MOFs due to the synthetic flexibility of this inorganic node. However, the majority of materials are isolated as polycrystalline solids, which are not ideal to decipher the spatial arrangement of parent and exchanged linkers for the formation of homogeneous structures or heterogeneous domains across the solid. Here we use high-throughput methodologies to optimize the synthesis of single crystals of UiO-68 and UiO-68-TZDC, a photoactive analogue based on a tetrazine dicarboxylic derivative. The analysis of the single linker phases reveals the necessity of combining both linkers to produce multivariate frameworks that combine efficient light sensitization, chemical stability, and porosity, all relevant to photocatalysis. We use solvent-assisted linker exchange reactions to produce a family of UiO-68-TZDC_% binary frameworks, which respect the integrity and morphology of the original crystals. Our results suggest that the concentration of TZDC in solution and the reaction time control the distribution of this linker in the sibling crystals for a uniform mixture or the formation of core-shell domains. We also demonstrate how the possibility of generating an asymmetric distribution of both linkers has a negligible effect on the electronic structure and optical band gap of the solids but controls their performance for drastic changes in the photocatalytic activity toward proton or methyl viologen reduction.

Enhancing Magnetic Communication between Metal Centres: The Role of s-Tetrazine Based Radicals as Ligands

Although 1,2,4,5-tetrazines or s-tetrazines have been known in the literature for more than a century, their coordination chemistry has become increasingly popular in recent years due to their unique redox activity, multiple binding sites and their various applications. The electron-poor character of the ring and stabilization of the radical anion through all four nitrogen atoms in their metal complexes provide new aspects in molecular magnetism towards the synthesis of new high performing Single Molecule Magnets (SMMs). The scope of this review is to examine the role of s-tetrazine radical ligands in transition metal and lanthanide based SMMs and provide a critical overview of the progress thus far in this field. As well, general synthetic routes and new insights for the preparation of s-tetrazines are discussed, along with their redox activity and applications in various fields. Concluding remarks along with the limitations and perspectives of these ligands are discussed.