Molecular Barrels as Potential Hosts: From Synthesis to Applications

Abstract

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.

Highly Robust Metal–Organic Framework for Efficiently Catalyzing Knoevenagel Condensation and the Strecker Reaction under Solvent-Free Conditions

“…Among the solid catalysts, the employment of heterogeneous catalysts is more advantageous than the homogeneous catalysts due to having several superiorities such as (i) easier catalyst recovery by filtration from reaction mixtures , (ii) reusable for consecutive runs , and (iii) their excellent adaptability to continuous flow processes . , However, to obtain the maximum efficiency of heterogeneous catalysts, a high surface area is required for more favorable interactions between the active sites of employed insoluble solids and the substrates and reagents. In this regard, metal–organic frameworks (MOFs) are the superior candidate over other solid catalysts, as they possess distinct superior qualities such as a high surface area with well-defined porosity − and innumerable structural diversity with suitable framework tunability. − MOFs exhibit superior catalytic activity due to the presence of several active sites such as (i) open metal sites (OMSs) , (ii) structural defect formations , (iii) bared functional groups’ presence of the employed organic linkers, and so on . Moreover, the synthesis flexibility and various binding modes of the metal centers (which often generate OMSs by mild activation) made MOFs promising candidates for heterogeneous catalysis.…”

Guest Encapsulation Alters the Thermodynamic Landscape of a Coordination Host

“…Coordination hosts with specific architectures can be rationally designed , by judiciously selecting metal nodes and organic panels with specific structural and binding features. For metal ions, these characteristics include valency, coordination geometry, and the nature of ancillary ligands, and for panelssize, shape, and the number and spatial orientation of the donor atoms. − The architectures of coordination hosts determine , their applicability in catalysis, − separations, − and site-selective derivatization of the encapsulated guests, among other functions. , An important characteristic is the size of the largest window, which determines how readily the guest molecules can enter and escape from the host’s cavity. For example, hosts whose windows are small (compared to the guest size) can efficiently stabilize − molecules that would otherwise undergo decomposition.…”

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

“…Metal–organic cages have also been used as hosts for carrying out different organic reactions. , The reactions investigated include the Diels–Alder reaction, the Knoevenagel condensation, the Nazarov cyclization, the aza-Darzens Reaction, oxidation reactions − and many more. − However, although quite a few reactions inside metal–organic hosts are known, many of them face multiple complications, like product inhibition and cage degradation by the products . Probably for these reasons, most reactions inside cages are single-step; and multiple-step, cascade, or domino reactions are rare. , Similarly, cage dependent divergent reactions, other than regioselective ones, , are also unexplored inside cages.…”