Regioselecitive Huisgen Cycloaddition within Porous Coordination Networks

Catalysis in Single Crystalline Materials: From Discrete Molecules to Metal-Organic Frameworks

Catalysis is one of the key techniques for people's modern life. It has created numerous essential chemicals such as biomedicines, agricultural chemicals and unique materials. Heterogeneous catalysis is the new emerging method with reusable catalysts. Among heterogenous catalysis patterns developed so far, single crystalline catalysis has become the promising one owing to its high catalytic density and selectivity resulted by the inherent porosity, orderliness of the lattices and permeability. These crystalline catalysts could be used in various reactions such as photo-dimerization, Diels-Alder reaction, CO₂ transformation and so on. In this review, we highlighted the reported works about the single crystalline catalysts. Both discrete small molecules and metal-organic frameworks (MOFs) have been used to prepare single crystals for catalysis. For discrete molecules based crystalline catalysts, coordinated and covalent molecules have been used. There were more catalytic modes in crystalline MOF catalysts. Three patterns were identified in this review: single crystalline MOFs i) without catalytic sites, ii) with inherent catalytic features and iii) with introducing catalytic units by post synthetic modification. Based on these examples, this review committed to provide the inspirations for the further design and application of single crystalline materials.

Crystalline Sponge Method: X-ray Structure Analysis of Small Molecules by Post-Orientation within Porous Crystals-Principle and Proof-of-Concept Studies

This Review discusses, along with the historical background, the principles as well as proof-of-concept studies of the crystalline sponge (CS) method, a new single-crystal X-ray diffraction (SCXRD) method for the analysis of the structures of small molecules without sample crystallization. The method uses single-crystalline porous coordination networks (crystalline sponges) that can absorb small guest molecules within their pores. The absorbed guest molecules are ordered in the pores through molecular recognition and become observable by conventional SCXRD analysis. The complex {[(ZnI₂ )₃ (tpt)₂ ]⋅x(solvent)}_n (tpt=tris(4-pyridyl)-1,3,5-triazine) was first proposed as a crystalline sponge and has been most generally used. Crystalline sponges developed later are also discussed here. The principle of the CS method can be described as "post-crystallization" of the absorbed guest, whose ordering is templated by the pre-latticed cavities. The method has been widely applied to synthetic chemistry as well as natural product studies, for which proof-of-concept examples will be shown here.

Reversible Diels-Alder and Michael Addition Reactions Enable the Facile Postsynthetic Modification of Metal-Organic Frameworks

Functionalization of metal-organic frameworks (MOFs) is critical in exploring their structural and chemical diversity for numerous potential applications. Herein, we report multiple approaches for the tandem postsynthetic modification (PSM) of various MOFs derived from Zr(IV), Al(III), and Zn(II). Our current work is based on our efforts to develop a wide range of MOF platforms with a dynamic functional nature that can be chemically switched via thermally triggered reversible Diels-Alder (DA) and hetero-Diels-Alder (HDA) ligations. Furan-tagged MOFs (furan-UiO-66-Zr) were conjugated with maleimide groups bearing dienophiles to prepare MOFs with a chemically switchable nature. As HDA pairs, phosphoryl dithioester-based moieties and cyclopentadiene (Cp)-grafted MOF (Cp-MIL-53-Al) were utilized to demonstrate the cleavage and rebonding of the linkages as a function of temperature. In addition to these strategies, the Michael addition reaction was also applied for the tandem PSM of IRMOF-3-Zn. Maleimide groups were postsynthetically introduced in the MOF lattice, which were further ligated with cysteine-based biomolecules via the thiol-maleimide Michael addition reaction. On the basis of the versatility of the herein presented chemistry, we expect that these approaches will help in designing a variety of sophisticated functional MOF materials addressing diverse applications.

Hypervalent organoiodine(v) metal-organic frameworks: syntheses, thermal studies and stoichiometric oxidants

Iodinated analogues of the highly porous IRMOF-9 and UiO-67 frameworks were prepared and post-synthetically oxidised with dimethyldioxirane (DMDO). Analysis by X-ray photoelectron spectroscopy (XPS) confirmed promotion to the iodine(v) state and detailed differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA) showed the hypervalent metal-organic frameworks (MOFs) undergo exothermic elimination at ∼200 °C with XPS showing hypervalency is maintained. The hypervalent MOFs are active heterogeneous reagents in sulfoxidation and alcohol oxidation reactions. The crystallinity and porosity of the MOFs were maintained following post-synthetic oxidation, thermolysis and after the heterogeneous reactions, as shown by powder X-ray diffraction (PXRD) and gas adsorption analyses. This work showcases the unique ability MOFs hold for studying chemical reactions and the potential for hypervalent organoiodine MOFs as reuseable oxidants.

The growing importance of crystalline molecular flasks and the crystalline sponge method

This article showcases recent advancements made using crystalline molecular flasks and the widening list of prospective applications for the crystalline sponge method. This expansion has coincided with an increasing number of materials termed crystalline sponges, and a report of a predictive means of identifying candidates from crystallographic databases. The crystalline sponge method's primary application has been determination of absolute configuration, and this has evolved from the analysis of carefully chosen planar aromatic guests to more diverse identification of natural products, biological metabolites, and analysis of volatile chemical components. However with time-resolved X-ray crystallography providing arguably the most informative atomic scale insights of dynamic chemical processes, this application of the crystalline sponge method may soon eclipse structural determination in terms of importance.

Kinetic assembly of coordination networks

Kinetic assembly is an important method for obtaining desired materials in chemical synthesis and materials science. We highlight the kinetic assembly of porous coordination networks, which promote the production of interactive pore sites. The interactive pore sites can activate or stabilize guest molecules. The properties of interactive pores are typified by iodine chemisorption and small sulfur encapsulation. Using interactive pores, we trapped small sulfur allotropes, such as S₂, cyclo-S₃, bent-S₃, and S₆, demonstrating the importance of interactive pore sites. Here, we address the important aspects of interactive pore sites created by kinetic assembly of porous coordination networks.

Coordination frameworks containing compounds as catalysts

This article describes a collection of MOF containing compounds as catalysts in a diverse range of organic transformation reactions.

Chemically reprogrammable metal organic frameworks (MOFs) based on Diels–Alder chemistry

We pioneer a new class of reprogrammable MOFs able to switch their interlattice chemistry via a facile Diels–Alder based cycloreversion process.

An Iodine-Vapor-Induced Cyclization in a Crystalline Molecular Flask

A vapor-induced cyclization has been observed in the host environment of a crystalline molecular flask (CMF), within which 1,8-bis(2-phenylethynyl)naphthalene (bpen), a diarenynyl system primed for cyclization, was exposed to iodine vapor to yield the corresponding indeno[2,1-α]phenalene species. The cyclization process, unique in its vapor-induced, solvent-free nature, was followed spectroscopically, and found to occur concurrently with the displacement of lattice solvent for molecular iodine in CMF⋅0.75 bpen⋅2.25 CHCl3 ⋅H2 O. The cyclization occurred under mild conditions and without the need to suspend the crystals in solvent. The ability of CMFs to host purely gas-induced reactions is further highlighted by the subsequent sequential oxidation reaction of cyclized 7-iodo-12-phenylindeno[2,1-α]phenalene (ipp) with molecular oxygen derived from air, yielding 12-hydroxy-7-iodo-2-phenylindeno[2,1-α]phenalen-1(12H)-one (hipp).

Synthesis of triazolo-fused benzoxazepines and benzoxazepinones via Passerini reactions followed by 1,3-dipolar cycloadditions

Azidobenzaldehydes can be used in Passerini three-component condensations to synthesize small collections of triazolo-fused heterocycles in an efficient and combinatorial fashion upon post-condensation azide-alkyne cycloadditions. Triazolo-fused benzoxazepinones were obtained in moderate to good overall yields with a concise two-step protocol. Triazolo-fused benzoxazepines were instead prepared by means of a longer, yet straightforward route comprising a Passerini reaction, hydrolysis of the ester moiety, O-alkylation with propargylic bromides, and 1,3-dipolar cycloaddition.

Single-crystal X-ray diffraction studies on structural transformations of porous coordination polymers

This review gives a brief overview of single-crystal X-ray diffraction studies and single-crystal to single-crystal transformations of porous coordination polymers.

Post-assembly functionalization of organoplatinum(II) metallacycles via copper-free click chemistry

We describe the use of a strain-promoted copper-free click reaction in the post-self-assembly functionalization of organoplatinum(II) metallacycles. The coordination-driven self-assembly of a 120° cyclooctyne-tethered dipyridyl donor with 60° and 120° di-Pt(II) acceptors forms molecular rhomboids and hexagons bearing cyclooctynes. These species undergo post-self-assembly [3+2] Huisgen cycloaddition with a variety of azides to give functionalized ensembles under mild conditions.

Crystalline molecular flasks

A variety of host compounds have been used as molecular-scale reaction vessels, protecting guests from their environment or restricting the space available around them, thus favouring particular reactions. Such molecular 'flasks' can endow guest molecules with reactivities that differ from those in bulk solvents. Here, we extend this concept to crystalline molecular flasks, solid-state crystalline networks with pores within which pseudo-solution-state reactions can take place. As the guest molecules can spontaneously align along the walls and channels of the hosts, structural changes in the substrates can be directly observed by in situ X-ray crystallography during reaction. Recently, this has enabled observation of the molecular structures of transient intermediates and other labile species, in the form of sequential structural snapshots of the chemical transformation. Here, we describe the principles, development and applications of crystalline molecular flasks.