Tuning the Size and Geometry of Heteroleptic Coordination Cages by Varying the Ligand Bent Angle.
TL;DR: In this article, a similar correlation between the ligand bent angle and the nuclearity is observed, with larger angles leading to complexes with a higher nuclearity, n. It is well established that the bent angle, α, of a ligand is a decisive factor in the self-assembly process.
Abstract: Spherical assemblies of the type [Pdn L2n ]2n+ can be obtained from PdII salts and curved N-donor ligands, L. It is well established that the bent angle, α, of the ligand is a decisive factor in the self-assembly process, with larger angles leading to complexes with a higher nuclearity, n. Herein, we report heteroleptic coordination cages of the type [Pdn Ln L'n ]2n+ , for which a similar correlation between the ligand bent angle and the nuclearity is observed. Tetranuclear cages were obtained by combining [Pd(CH3 CN)4 ](BF4 )2 with 1,3-di(pyridin-3-yl)benzene and ligands featuring a bent angle of α=120°. The use of a dipyridyl ligand with α=149° led to the formation of a hexanuclear complex with a trigonal prismatic geometry; for linear ligands, octanuclear assemblies of the type [Pd8 L8 L'8 ]16+ were obtained. The predictable formation of heteroleptic PdII cages from 1,3-di(pyridin-3-yl)benzene and different dipyridyl ligands is evidence that there are entire classes of heteroleptic cage structures that are privileged from a thermodynamic point of view.
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TL;DR: Tetra-and hexanuclear coordination cages were obtained in reactions of [Pd(CH3CN)4]-BF4)2 with low-symmetry dipyridyl ligands as discussed by the authors.
21 citations
TL;DR: In this paper , a photoswitchable ligand and palladium(II) ions form a dynamic mixture of self-assembled metallosupramolecular structures, which can be reversibly pumped to a kinetic trap by small perturbations of the isomer distribution using non-destructive visible light.
Abstract: A photoswitchable ligand and palladium(II) ions form a dynamic mixture of self-assembled metallosupramolecular structures. The photoswitching ligand is an ortho-fluoroazobenzene with appended pyridyl groups. Combining the E-isomer with palladium(II) salts affords a double-walled triangle with composition [Pd3L6]6+ and a distorted tetrahedron [Pd4L8]8+ (1 : 2 ratio at 298 K). Irradiation with 410 nm light generates a photostationary state with approximately 80 % of the E-isomer of the ligand and results in the selective disassembly of the tetrahedron, the more thermodynamically stable structure, and the formation of the triangle, the more kinetically inert product. The triangle is then slowly transformed back into the tetrahedron over 2 days at 333 K. The Z-isomer of the ligand does not form any well-defined structures and has a thermal half-life of 25 days at 298 K. This approach shows how a thermodynamically preferred self-assembled structure can be reversibly pumped to a kinetic trap by small perturbations of the isomer distribution using non-destructive visible light.
18 citations
TL;DR: In this paper , the authors show examples of structure prediction and host-guest/catalytic property evaluation of metal-organic cages and highlight the importance of considering realistic, flexible systems.
16 citations
TL;DR: In this paper , a photoswitchable ligand based on azobenzene is self-assembled with palladium(II) ions to form a [Pd2(E•L)4]4+ cage.
Abstract: Abstract A photoswitchable ligand based on azobenzene is self‐assembled with palladium(II) ions to form a [Pd2(E‐L)4]4+ cage. Irradiation with 470 nm light results in the near‐quantitative switching to a monomeric species [Pd(Z‐L)2]2+, which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self‐assembly, and the thermal half‐life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.
14 citations
TL;DR: In this paper , the ways in which the cavities of metal-organic cages can be engineered at the molecular level are discussed, with a focus on the Pd2L4 class of assemblies.
9 citations
References
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TL;DR: In the early 1960s, the discovery of crown ethers and spherands by Pedersen, Lehn, and Cram3 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, hydrophilic and hydrophobic interactions to achieve these highly complex and often symmetrical architectures as mentioned in this paper.
Abstract: 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
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1,366 citations
TL;DR: It is demonstrated that a mixture of palladium ions and V-shaped bridging ligands can self-assemble into a hollow, nearly spherical polyhedron with 24 vertices and a central diameter of 4 nanometers.
Abstract: Self-assembly is a powerful technique for the bottom-up construction of discrete, well-defined nanoscale structures. Large multicomponent systems (with more than 50 components) offer mechanistic insights into biological assembly but present daunting synthetic challenges. Here we report the self-assembly of giant M24L48 coordination spheres from 24 palladium ions (M) and 48 curved bridging ligands (L). The structure of this multicomponent system is highly sensitive to the geometry of the bent ligands. Even a slight change in the ligand bend angle critically switches the final structure observed across the entire ensemble of building blocks between M24L48 and M12L24 coordination spheres. The amplification of this small initial difference into an incommensurable difference in the resultant structures is a key mark of emergent behavior.
669 citations
TL;DR: System Chemistry has arisen in recent years as a new discipline that aims to investigate complex mixtures of interacting molecules and can give rise to outstanding emergent properties as a result of the interaction of the individual components and cannot be ascribed to any of their components acting in isolation.
Abstract: Nature successfully manages under extremely adverse conditions to accomplish intricate functions responsible for the regulation and control of the vast majority of biological processes that eventually sustain life on our planet. Biological molecules are required to carry out selective functions while often being hindered by surrounding agents which are simultaneously competing to bind the same targets. This high degree of selectivity in nature ultimately depends on the “molecular instructions” encoded in the chemical structure of the interacting species responsible for every single recognition or discrimination event. The formation of the DNA double helix, for instance, requires the base-pairing (sorting) of complementary nitrogenous bases (adenine thymine (A T) and cytosine guanine (C G)). These high-fidelity recognition processes are crucial in the storage of genetic information used in the development and functioning of all known living organisms and some viruses. Other sophisticated superstructures such as microtubules, are built upon polymerization of dimers of two different globular proteins (Rand β-globulin), giving rise to cylindrical micrometric arrangements. The formation of heterodimers composed of two different proteins requires the self-discrimination of equals, and the simultaneous recognition of complementary units. In the final instance, the small molecules of life (e.g., sugars, amino acids and fatty acids) are able to assemble not only to form such abovementioned macromolecules, but also to self-sort in one of the most efficient and complex processes known in nature to build the functional basic unit of life: a cell. In a cell, multiple levels of compartmentalization arising from the self-sorting of their molecular components allow the coexistence of different functional architectures acting independently. This exceptional selectivity in nature makes possible the existence of life on our planet. Unlike the high complexity of natural or biological architectures, the majority of artificial self-assembled systems reported so far have been investigated in isolation. This has been mainly due to the lack of suitable characterization methods and technical or economic constraints, which far exceed the resources of most research institutes. However, the remarkable development of analytical tools is increasingly enabling scientists to pinpoint intractable problems associated to multicomponent mixtures. In this context, Systems Chemistry has arisen in recent years as a new discipline that aims to investigate complex mixtures of interacting molecules. 17 These mixtures can give rise to outstanding emergent properties as a result of the interaction of the individual components and cannot be ascribed to any of their components acting in isolation. Although this emerging discipline is still in its infancy, ongoing research advances are enabling current (supramolecular) chemists to unravel the behavior of individual molecules in multicomponent mixtures and to anticipate the reasons that lead artificial molecules to bind or ignore a specific partner in a complex multicomponent environment. In this review, wewill discuss the external variables and intrinsic factors (molecular codes) that influence the recognition or discrimination of supramolecularly interacting chemical species in solution. The comprehension of this “molecular programming” in artificial systems will define the variables that control self-sorting processes, and may ultimately contribute to a better understanding of the self-assembly pathways in natural systems. By restricting ourselves to noncovalent bonds and self-sorting in solution we will not cover self-assembly processes on solid surfaces and self-sorting phenomena based on reversible covalent bonds.However, excellent reviews have recently become available by De Feyter and Otto, which cover these topics.
595 citations
TL;DR: This tutorial review deals with the design, synthesis and host-guest chemistry of discrete coordination cages built according to the combination of pyridyl ligands and square-planar Pd(ii) or Pt(II) cations for the realization of supramolecular self-assemblies.
Abstract: The combination of pyridyl ligands and square-planar Pd(II) or Pt(II) cations has proven to be a very reliable recipe for the realization of supramolecular self-assemblies. This tutorial review deals with the design, synthesis and host–guest chemistry of discrete coordination cages built according to this strategy. The focus is set on structures obeying the formula [PdnL2n] (n = 2–4). The most discussed ligands are bent, bis-monodentate bridges having their two donor sites pointing in the same direction. The structures of the resulting cages range from simple globules over intertwined knots to interpenetrated dimers featuring three small pockets instead of one large cavity. The cages have large openings that allow small guest molecules to enter and leave the cavities. Most structures are cationic and thus favour the uptake of anionic guests. Some examples of host–guest complexes are discussed with emphasis on coencapsulation and allosteric binding phenomena. Aside from cages in which the ligands have only a structural role, some examples of functional ligands based on photo- and redox-active backbones are presented.
560 citations