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.

Self-assembly of naked Pd II ions separately with newly designed bis(3-pyridyl)benzothiadiazole (L1) and bis(3pyridyl)thiazolo [5,4-d]thiazole (L2) donors separately, under varying experimental conditions, yielded Pd 4 L 8 (L= L1 or L2) tetrahedral cages and their homologous Pd 3 L 6 (L= L1 or L2) double-walled triangular macrocycles. The resulting assemblies exhibited solvent, temperature, and counteranion induced dynamic equilibrium. Treatment of L1 with Pd(BF 4 ) 2 in acetonitrile (ACN) resulted in selective formation of a tetrahedral cage [Pd 4 (L1) 8 ]-(BF 4 ) 8 (1a), which is in dynamic equilibrium with its homologue triangle [Pd 3 (L1) 6 ](BF 4 ) 6 (2a) in dimethyl sulfoxide (DMSO). On the other hand, similar self-assembly using L2 instead of L1 yielded an equilibrium mixture of tetrahedral cage [Pd 4 (L2) 8 ]-(BF 4 ) 8 (3a) and triangle [Pd 3 (L2) 6 ](BF 4 ) 6 (4a) forms in both ACN and DMSO. The assembles were characterized by multinuclear NMR and ESI-MS while the structure of the tetrahedral cage (1a) was determined by single crystal X-ray diffraction. Existence of a dynamic equilibrium between the assemblies in solution has been investigated via variable temperature 1 H NMR. The equilibrium constant K = ([Pd 4 L 8 ] 3 /[Pd 3 L 6 ] 4 ) was calculated at each experimental temperature and fitted with the Van't Hoff equation to determine the standard enthalpy (ΔH°) and entropy (ΔS°) associated with the interconversion of the double-walled triangle to tetrahedral cage. The thermodynamic feasibility of structural interconversion was analyzed from the change in ΔG°, which suggests favorable conversion of Pd 3 L 6 triangle to Pd 4 L 8 cage at elevated temperature for L1 in DMSO and L2 in ACN. Interestingly, similar self-assembly reactions of L1 and L2 with Pd(NO 3 ) 2 instead of Pd(BF 4 ) 2 resulted in selective formation of a tetrahedral cage [Pd 4 (L1) 8 ](NO 3 ) 8 (1b) and double-walled triangle [Pd 3 (L2) 6 ](NO 3 ) 6 (4b), respectively.

Structural and functional modulation of three-dimensional artificial macromolecular systems is of immense importance. Designing supramolecular cages that can show stimuli mediated reversible switching between higher-order structures is quite challenging. We report here construction of a Pd6 trifacial barrel (1) by coordination self-assembly. Surprisingly, barrel 1 was found to exhibit guest-responsive behavior. In presence of fullerenes C60 and C70, 1 unprecedentedly transformed to its metastable higher homologue Pd8 tetrafacial barrel (2), forming stable host–guest complexes (C60)3⊂2 and (C70)2⊂2, respectively. Again, encapsulated fullerenes could be extracted from the cavity of 2 using 1,2-dichlorobenzene, leading to its facile conversion to the parent trifacial barrel 1. Such reversible structural interconversion between an adaptable molecular barrel and its guest stabilized higher homologue is an uncommon observation.