Fascination with supramolecular chemistry over the last few decades has led to the synthesis of an ever-increasing number of elegant and intricate functional structures with sizes that approach nanoscopic dimensions Today, it has grown into a mature field of modern science whose interfaces with many disciplines have provided invaluable opportunities for crossing boundaries both inside and between the fields of chemistry, physics, and biology This chemistry is of continuing interest for synthetic chemists; partly because of the fascinating physical and chemical properties and the complex and varied aesthetically pleasing structures that supramolecules possess For scientists seeking to design novel molecular materials exhibiting unusual sensing, magnetic, optical, and catalytic properties, and for researchers investigating the structure and function of biomolecules, supramolecular chemistry provides limitless possibilities Thus, it transcends the traditional divisional boundaries of science and represents a highly interdisciplinary field

In the early 1960s, the discovery of ‘crown ethers’, ‘cryptands’ and ‘spherands’ by Pedersen,1 Lehn,2 and Cram3 respectively, led to the realization that small, complementary molecules can be made to recognize each other through non-covalent interactions such as hydrogen-bonding, charge-charge, donor-acceptor, π-π, van der Waals, etc Such ‘programmed’ molecules can thus be self-assembled by utilizing these interactions in a definite algorithm to form large supramolecules that have different physicochemical properties than those of the precursor building blocks Typical systems are designed such that the self-assembly process is kinetically reversible; the individual building blocks gradually funnel towards an ensemble that represents the thermodynamic minimum of the system via numerous association and dissociation steps By tuning various reaction parameters, the reaction equilibrium can be shifted towards the desired product As such, self-assembly has a distinct advantage over traditional, stepwise synthetic approaches when accessing large molecules

It is well known that nature has the ability to assemble relatively simple molecular precursors into extremely complex biomolecules, which are vital for life processes Nature’s building blocks possess specific functionalities in configurations that allow them to interact with one another in a deliberate manner Protein folding, nucleic acid assembly and tertiary structure, phospholipid membranes, ribosomes, microtubules, etc are but a selective, representative example of self-assembly in nature that is of critical importance for living organisms Nature makes use of a variety of weak, non-covalent interactions such as hydrogen–bonding, charge–charge, donor–acceptor, π-π, van der Waals, hydrophilic and hydrophobic, etc interactions to achieve these highly complex and often symmetrical architectures In fact, the existence of life is heavily dependent on these phenomena The aforementioned structures provide inspiration for chemists seeking to exploit the ‘weak interactions’ described above to make scaffolds rivaling the complexity of natural systems

The breadth of supramolecular chemistry has progressively increased with the synthesis of numerous unique supramolecules each year Based on the interactions used in the assembly process, supramolecular chemistry can be broadly classified in to three main branches: i) those that utilize H-bonding motifs in the supramolecular architectures, ii) processes that primarily use other non-covalent interactions such as ion-ion, ion-dipole, π–π stacking, cation-π, van der Waals and hydrophobic interactions, and iii) those that employ strong and directional metal-ligand bonds for the assembly process However, as the scale and degree of complexity of desired molecules increases, the assembly of small molecular units into large, discrete supramolecules becomes an increasingly daunting task This has been due in large part to the inability to completely control the directionality of the weak forces employed in the first two classifications above Coordination-driven self-assembly, which defines the third approach, affords a greater control over the rational design of 2D and 3D architectures by capitalizing on the predictable nature of the metal-ligand coordination sphere and ligand lability to encode directionality Thus, this third strategy represents an alternative route to better execute the “bottom-up” synthetic strategy for designing molecules of desired dimensions, ranging from a few cubic angstroms to over a cubic nanometer For instance, a wide array of 2D systems: rhomboids, squares, rectangles, triangles, etc, and 3D systems: trigonal pyramids, trigonal prisms, cubes, cuboctahedra, double squares, adamantanoids, dodecahedra and a variety of other cages have been reported As in nature, inherent preferences for particular geometries and binding motifs are ‘encoded’ in certain molecules depending on the metals and functional groups present; these moieties help to control the way in which the building blocks assemble into well-defined, discrete supramolecules4

Since the early pioneering work by Lehn5 and Sauvage6 on the feasibility and usefulness of coordination-driven self-assembly in the formation of infinite helicates, grids, ladders, racks, knots, rings, catenanes, rotaxanes and related species,7 several groups - Stang,8 Raymond,9 Fujita,10 Mirkin,11 Cotton12 and others13,14 have independently developed and exploited novel coordination-based paradigms for the self-assembly of discrete metallacycles and metallacages with well-defined shapes and sizes In the last decade, the concepts and perspectives of coordination-driven self-assembly have been delineated and summarized in several insightful reviews covering various aspects of coordinationdriven self-assembly15 In the last decade, the use of this synthetic strategy has led to metallacages dubbed as “molecular flasks” by Fujita,16 and Raymond and Bergman,17 which due to their ability to encapsulate guest molecules, allowed for the observation of unique chemical phenomena and unusual reactions which cannot be achieved in the conventional gas, liquid or solid phases Furthermore, these assemblies found applications in supramolecular catalysis18,19 and as nanomaterials as developed by Hupp20 and others21,22

This review focuses on the journey of early coordination-driven self-assembly paradigms to more complex and discrete 2D and 3D supramolecular ensembles over the last decade We begin with a discussion of various approaches that have been developed by different groups to assemble finite supramolecular architectures The subsequent sections contain detailed discussions on the synthesis of discrete 2D and 3D systems, their functionalizations and applications

Supramolecular coordination: self-assembly of finite two- and three-dimensional ensembles.

Synthetic organic chemists enjoy the luxury of having a large collection of reliable reactions at their disposal for preparing small molecules, mesoscopic structures, and polymers. Coordination chemists, on the other hand, are faced with the fact that transition metal chemistry, when normalized for the number of transition metals, has relatively few high-yielding reactions, when compared to the chemistry of carbon, for preparing even small molecule structures. This lack of control is manifested, in large part, in the weak metal-ligand interactions found in coordination complexes as compared with the strong covalent bonds in organic compounds. Weak bonding often translates into many reaction pathways that are not substantially different from an energetic point of view, and therefore, results in poor selectivity. As a result, many coordination chemists in recent years have come to the realization that it may be easier and more productive to develop straightforward and reliable routes to mesoscopic supramolecular structures by capitalizing on the modest collection of high-yielding reactions in coordination chemistry, the directional bonding afforded by metal centers, and strategies aimed at taking advantage of the weak metal bonds found in coordination complexes. Three emerging synthetic strategies, the symmetry-interaction, directional-bonding, and weak-link synthetic approaches, all use metal centers as structural building blocks to rationally assemble molecular components into supramolecular metallocyclophanes. These three approaches are discussed herein, and the fundamental principles underlying each as well as their capabilities are compared and contrasted.

Strategies for the Construction of Supramolecular Compounds through Coordination Chemistry

A metal organic framework (MOF) represents a new class of hybrid material built from metal ions with well-defined coordination geometry and organic bridging ligands Over the past few years, MOFs have attracted a great deal of attention due to their intriguing framework architecture, topology, and optical properties MOFs have provided promising perspectives in various research fields, such as catalysis, energy storage, drug-delivery systems, non-linear optics, and gas storage Recently, the application of MOFs was further extended to cover new, interesting fields of sensing toward various target components, including small molecules, solvents, pesticides, explosives, and biological markers In this review, we focus on recent progress achieved in MOF research with respect to general sensing principles and analytical performance This review also discusses future industrial applications in sensing that are of immense research interest

Metal organic frameworks for sensing applications

An effective solvothermal procedure has been developed to synthesize the new three-dimensional metal−organic framework, [ZnF(AmTAZ)]·solvents, using either 3-amino-1,2,4-triazole (AmTAZ) or 3-amino-1,2,4-triazole-5-carboxylic acid (AmTAZAc) and a choice of several Zn(II) salts as starting materials. The three-dimensional structure displays open-ended, hollow nanotubular channels that are formed by hexanuclear metallamacrocyclic Zn6F6(AmTAZ)6 rings. The framework integrity is maintained to 350 °C, at which point most of the guest solvent molecules have been removed, as evidenced by single-crystal X-ray analyses, 1H solid-state NMR, and TGA measurements. At higher temperatures, the framework is converted either to zinc oxide (ZnO) when heated in air or to zinc cyanamide (ZnCN2) when heated in an inert atmosphere. In both cases, the as-grown, rodlike crystal shape is maintained during the solid-state transformation, suggesting a possible route for preparing one-dimensional crystalline nanomaterials.

Exceptionally stable, hollow tubular metal-organic architectures: synthesis, characterization, and solid-state transformation study.

Among the recent developments in metal-organic frameworks (MOFs), porous layered coordination polymers (CPs) have garnered attention due to their modular nature and tunable structures. These factors enable a number of properties and applications, including gas and guest sorption, storage and separation of gases and small molecules, catalysis, luminescence, sensing, magnetism, and energy storage and conversion. Among MOFs, two-dimensional (2D) compounds are also known as 2D CPs or 2D MOFs. Since the discovery of graphene in 2004, 2D materials have also been widely studied. Several 2D MOFs are suitable for exfoliation as ultrathin nanosheets similar to graphene and other 2D materials, making these layered structures useful and unique for various technological applications. Furthermore, these layered structures have fascinating topological networks and entanglements. This review provides an overview of different aspects of 2D MOF layered architectures such as topology, interpenetration, structural transformations, properties, and applications.

Two-Dimensional Metal-Organic Framework Materials: Synthesis, Structures, Properties and Applications.

The synthesis and characterization of a cadmium(II) coordination polymer, [Cd(C12H6N2O4)·H2O]n (1), is reported. A single-crystal X-ray analysis shows that compound 1 presents a noninterpenetrating three-dimensional porous host containing one-dimensional hydrophilic channels, where guest water molecules reside. The strategy in designing the 3-D framework architecture is based on a combination of two building subunits:  a porous two-dimensional grid of (4,4) topology and a metal dicarboxylate cluster chain. Both subunits are assembled from the coordination of a cadmium ion with a three-connecting organic modular ligand, 2,2‘-bipyridyl-4,4‘-dicarboxylic acid (H2bpdc). The results of thermogravimetric analysis and powder X-ray diffraction study show that the framework rigidity of compound 1 remains intact upon the removal of guest molecules, and maintains the thermal stability up to 440 °C. The second-row transition-metal ions are capable of engaging higher coordination modes (e.g., hepta- and octacoordinati...

[CdII(bpdc).H2O]n: A robust, thermally stable porous framework through a combination of a 2-D grid and a cadmium dicarboxylate cluster chain (H2bpdc = 2,2'-bipyridyl-4,4'-dicarboxylic acid)

A 0D discrete molecule [Co(3,5-pdc)(H2O)5]·2H2O (1) was obtained in quantitative yield from the reaction of CoCl2·6H2O and pyridine-3,5-dicarboxylate (3,5-pdc) in pure water solvent at ambient temperature. While a 1D zigzag chain species, [{Co(3,5-pdc)(H2O)4}·H2O]n (2), was produced in a water-rich environment, a 2D layer compound, [Co(3,5-pdc)(H2O)2]n (3), with a 63 topology was generated under a water-reduced condition and a 2D sheet structure, [{Cu(3,5-pdc)(py)2}·H2O·EtOH]n (4), was formed under a water-poor condition. Compounds 1, 2, and 4 were characterized by single-crystal X-ray diffraction analysis. The 1D zigzag chain 2 shows a recoverable collapsing property. Compound 4 adopts a 2D sheet structure with a 4·82 topology, observed for the first time for the 3,5-pdc-related metal−organic frameworks. Water content was found to be an important factor in determining the topologies of the products in the self-assembly of divalent metal ions (Co2+, Cu2+) and pyridine-3,5-dicarboxylate under mild conditions.

Influence of water content on the self-assembly of metal-organic frameworks based on pyridine-3,5-dicarboxylate

The one-pot synthesis of soluble, neutral, luminescent rectangular supramolecules in high yields by solvothermal methods and fine tuning of the molecular architecture in one dimension from ∼7 A to a nanometer scale, ∼21 A, have been achieved.

Novel one-pot synthesis of luminescent neutral rhenium-based molecular rectangles

Novel One-Pot Synthesis of Luminescent Neutral Rhenium-Based Molecular Rectangles.

We synthesized a new tetraphenylethylene-modified chitosan bioconjugate, CS-TPE, that shows the aggregation-induced emission effect. It can self-assemble into fluorescent polymeric nanoparticles in an aqueous solution at pH 5.3 either alone or with the water-soluble bowl-shaped six-fold carboxylated tribenzotriquinacene derivative TBTQ-C6 via host–guest binding. The spherical nanoparticles formed by CS-TPE amphiphiles or TBTQ-C6/CS-TPE supra-amphiphiles disintegrated under alkaline stimulation at pH 10.4 and the dispersion of the aggregates after the collapse in the presence of TBTQ-C6 was greatly improved. In addition, the fluorescence of CS-TPE was significantly enhanced by introducing TBTQ-C6, and remained relatively stable with variations in pH for both CS-TPE and TBTQ-C6/CS-TPE. Such pH-responsive supramolecular spherical nanoparticles with stable fluorescence emission based on CS-TPE or TBTQ-C6/CS-TPE may find applications in various fields, including the development of visual oral drug delivery systems.

Yi-Long Lu

Papers

[CdII(bpdc).H2O]n: A robust, thermally stable porous framework through a combination of a 2-D grid and a cadmium dicarboxylate cluster chain (H2bpdc = 2,2'-bipyridyl-4,4'-dicarboxylic acid)

Influence of water content on the self-assembly of metal-organic frameworks based on pyridine-3,5-dicarboxylate

Novel one-pot synthesis of luminescent neutral rhenium-based molecular rectangles

Novel One-Pot Synthesis of Luminescent Neutral Rhenium-Based Molecular Rectangles.

pH-Responsive fluorescent supramolecular nanoparticles based on tetraphenylethylene-labelled chitosan and a six-fold carboxylated tribenzotriquinacene