Host-Guest Interactions Induced Enhancement in Oxidase-Like Activity of a Benzothiadiazole Dye Inside an Aqueous Pd8L4 Barrel.

The effect of host-guest interactions on the chemistry of encapsulated molecules is a fascinating field of research that has gained momentum in recent years. Much of the work in this field has been focused on the effect of such interactions on catalysis and photoluminescence of encapsulated dyes. However, the effect of such interactions on related photoinduced processes, such as photoregulated oxidase-mimicking activity, has not been explored much. Herein, we report a unique example of enhancement of oxidase-like activity of a benzothiadiazole dye (G1) in water through encapsulation within a M8L4 molecular barrel (1). Favorable host-guest interactions helped the encapsulated guest G1 to have better photoinduced electron transfer to molecular oxygen leading to increased production of superoxide radical anions and oxidase-like activity. Furthermore, encapsulation inside 1 also caused a change in the redox potentials of the guest (G1) which after photoinduced electron transfer produced a better oxidizing agent than free G1. These phenomena combined to enhance the oxidase-like activity of dye G1 upon encapsulation inside cage 1. The present report demonstrates a unique effect of host-guest chemistry on photoregulated processes.

Mechanically Interlocked Water-Soluble Pd6 Host for the Selective Separation of Coal Tar-Based Planar Aromatic Molecules.

Research on the synthesis of catenated cages has been a growing field of interest in the past few years. While multiple types of catenated cages with different structures have been synthesized, the application of such systems has been much less explored. Specifically, the use of catenated cages in the separation of industrially relevant molecules that are present in coal tar has not been explored before. Herein, we demonstrate the use of a newly synthesized interlocked cage 1 [C184H240N76O48Pd6] (M6L4), formed through the self-assembly of ligand L.HNO3 (tris(4-(1H-imidazole-1-yl)benzylidene)hydrazine-1-carbohydrazonhydrazide) with acceptor cis-[(tmchda)Pd(NO3)2] [tmchda = ±N,N,N',N'-tetramethylcyclohexane-1,2-diamine] (M). The interlocked cage 1 was able to separate the isomers (anthracene and phenanthrene) using a simple solvent extraction technique. Using the same technique, the much more difficult separation of structurally and physiochemically similar compounds acenaphthene and acenaphthylene was performed for the first time with 1 as the host. Other noninterlocked hexanuclear Pd6 cages having a wider cavity proved inefficient for such separation, demonstrating the uniqueness of the interlocked cage 1 for such challenging separation.

Uncovering tetrazoles as building blocks for constructing discrete and polymeric assemblies.

Metal-organic self-assembly with flexible moieties is a budding field of research due to the possibility of the formation of unique architectures. Tetrazole, characterised by four nitrogen atoms in a five-member ring, exhibits immense potential as a component. Tetrazole offers four coordination sites for binding to the metal centre with nine distinct binding modes, leading to various assemblies. This review highlights different polymeric and discrete tetrazole-based assemblies and their functions. The meticulous manipulation of stoichiometry, ligands, and metal ions required for constructing discrete assemblies has also been discussed. The different applications of these architectures in separation, catalysis and detection have also been accentuated. The latter section of the review consolidates tetrazole-based cage composites, highlighting their applications in cell imaging and photocatalytic applications.

Cavity-Shape-Dependent Divergent Chemical Reaction inside Aqueous Pd6L4 Cages.

Chemical reactions inside the confined pockets of enzyme-mimicking hosts, such as cages and macrocycles, have been an emerging field of interest over the past decade. Although many such reactions are known, the use of such cages toward the divergent synthesis of nonisomeric products has not been well explored. Divergent synthesis is a technique of forming two or more distinct products from the same reagents by changing the catalyst or reaction conditions. Changing the shape of the cage can also change the nature and magnitude of the host-guest interactions. Thus, is it possible for such changes to cause differences in the reaction pathways leading to formation of nonisomeric products? Herein, we report a divergent chemical transformation of anthrone [anthracen-9(10H)-one] inside different water-soluble M6L4 cages. When anthrone was encapsulated inside a newly synthesized M6L4 octahedral cage 1, it dimerized to form dianthrone [9,9'-bianthracen-10,10'(9H,9'H)-dione]. In contrast, when the same chemical reaction was performed inside a M6L4 double-square shaped cage 2, it was oxidized to form anthraquinone [anthracene-9,10-dione]. Similar results were obtained with a different set of isomeric aqueous Pd6 cages 3a (octahedral cage) and 3b (double-square cage), indicating the dependence of the shape of cavity on the divergent synthesis. The present report demonstrates a unique example of different outcomes/results of a reaction depending on the shape of the molecular container, which was driven by the host-guest interactions and the preorganization of the substrates.

Structural Switching of a Distorted Trigonal Metal-Organic Cage to a Tetragonal Cage and Singlet Oxygen Mediated Oxidations.

Construction of metal-organic cages with unique architecture and guest binding abilities is highly desirable. Herein, we report the synthesis of a distorted trigonal cage (1) from a twisted tetratopic ligand (L) and a PdII acceptor. Surprisingly, 1 exhibited a complete structural reorganization of its building units in the presence of C70 and C60 to form guest-encapsulated tetragonal cages, (C70 )2 @2 and (C60 )2 @2, respectively. These guest-bound cages were found to be potential 1 O2 generators, with the former effectively catalyzing two different varieties of 1 O2 -mediated oxidation reactions.

Molecular Barrels as Potential Hosts: From Synthesis to Applications.

Self-assembled discrete molecular architectures that show selective molecular recognition within their internal cavities are highly desirable. Such hosts often show guest recognition through several noncovalent interactions. This emulates the activity of naturally occurring enzymes and proteins. Research in the formation of 3D cages of different shapes and sizes has progressed rapidly since the development of coordination-driven self-assembly and dynamic covalent chemistry. Such molecular cages find applications in catalysis, stabilization of metastable molecules, purification of isomeric mixtures via selective encapsulation, and even in biomedical applications. Most of these applications stem from the ability of the host cages to bind guests strongly in a selective fashion, providing a suitable environment for the guests to perform their functions. Molecular cages having closed architectures with small windows either show poor encapsulation or inhibit easy guest release while those with wide open structures fail to form stable host-guest complexes. In this context, molecular barrels obtained by dynamic metal-ligand/covalent bond formation techniques possess optimized architectures. With a hollow-walled cavity and two large openings, molecular barrels satisfy the structural requirements for many applications. In this perspective, we will discuss in detail the synthetic strategies for obtaining barrels or barrel-like architectures employing dynamic coordination and covalent interactions, their structure-based classification, and their applications in catalysis, storing transient molecules, separation of chemicals, and photoinduced antibacterial activity. We aim to highlight the structural advantages of molecular barrels over other architectures for efficiently carrying out several functions and for the development of new applications.

Selective separation of planar and non-planar hydrocarbons using an aqueous Pd6 interlocked cage.

Polycyclic aromatic hydrocarbons (PAHs) find multiple applications ranging from fabric dyes to optoelectronic materials. Hydrogenation of PAHs is often employed for their purification or derivatization. However, separation of PAHs from their hydrogenated analogues is challenging because of their similar physical properties. An example of such is the separation of 9,10-dihydroanthracene from phenanthrene/anthracene which requires fractional distillation at high temperature (∼340 °C) to obtain pure anthracene/phenanthrene in coal industry. Herein we demonstrate a new approach for this separation at room temperature using a water-soluble interlocked cage (1) as extracting agent by host-guest chemistry. The cage was obtained by self-assembly of a triimidazole donor L·HNO3 with cis-[(tmeda)Pd(NO3)2] (M) [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine]. 1 has a triply interlocked structure with an inner cavity capable of selectively binding planar aromatic guests.

Recent trends in organic cage synthesis: push towards water-soluble organic cages.

Research on organic cages has blossomed over the past few years into a mature field of study which can contribute to solving some of the challenging problems. In this review we aim to showcase the recent trends in synthesis of organic cages including a brief discussion on their use in catalysis, gas sorption, host-guest chemistry and energy transfer. Among the organic cages, water-soluble analogues are a special class of compounds which have gained renewed attention in recent times. Due to their advantage of being compatible with water, such cages have the potential of showing biomimetic activities and can find use in drug delivery and also as hosts for catalysis in aqueous medium. Hence, the synthetic strategies for the formation of water-soluble organic cages shall be discussed along with their potential applications.

De novo approach for the synthesis of water-soluble interlocked and non-interlocked organic cages.

Research on self-assembled metallosupramolecular architectures has bloomed in recent times. Analogous metal-free organic architectures with water solubility are highly challenging. We report here a unique class of triazine based immidazolium water-soluble metal-free interlocked organic cage (1), which was synthesized in a one-pot reaction without using dynamic covalent chemistry and without any chromatographic separation. An analogous non-interlocked cage (2) was also successfully achieved by steric control using different positional isomers of the building blocks.

Self-Assembly of a Pd8 Macrocycle and Pd12 Homochiral Tetrahedral Cages Using Poly(tetrazolate) Linkers.

A two-dimensional molecular square (MC) was obtained by the self-assembly of a bis(tetrazole) linker, 4,4'-bis(1H-tetrazol-5-yl)-1,1'-biphenyl (H2L1), with a square-planar metal acceptor M [M = (tmeda)Pd(NO3)2, where tmeda = N,N,N',N'-tetramethylethane-1,2-diamine] in dimethyl sulfoxide (DMSO) followed by crystallization. The uncommon 2,3-binding mode through N atoms of the tetrazole rings in this assembly leads to the formation of an octanuclear molecular square. The molecular square MC [Pd8(L1)4(NO3)8] is unstable in DMSO and slowly converts to a dynamic mixture of a 3D tetrahedral cage T1 [Pd12(L1)6(NO3)12] and the macrocycle MC. A tetrahedral cage (T1) is formed by the usual 1,3-binding mode of the tetrazole rings. However, self-assembly of the T1 [Pd12(L1)6(PF6)12] was possible to access in the pure form in a less polar solvent like acetonitrile. The pure T1 [Pd12(L1)6(PF6)12] also converts to a mixture of T1 and MC in DMSO. Interestingly, when a tris(tetrazole) linker, tris(4-(1H-tetrazol-5-yl)phenyl)amine (H3L2), was treated with the acceptor M, it produced a tetrahedral nanocage T2 [Pd12(L2)4(NO3)12] through 1,3-binding mode of the tetrazole rings without any trace of an octahedral cage through 2,3-binding mode of the tetrazole moieties.