Inflating face-capped Pd6L8 coordination cages

Tritopic metalloligands were used to form two Pd6L8-type coordination cages. With molecular weights of more than 15 kDa and PdPd distances of up to 4.2 nm, these complexes are among the largest palladium cages described to date.

Multicomponent Self-Assembly of Metallo-Supramolecular Macrocycles and Cages through Dynamic Heteroleptic Terpyridine Complexation

Spontaneous formation of the heteroleptic cadmium(II) bis(terpyridine) complex under ambient conditions can be achieved by a combination of 6,6''-di(2,6-dimethoxylphenyl)-substituted and unsubstituted terpyridine-based ligands. Building on this dynamic heteroleptic complexation, diverse metallo-supramolecular macrocycles and cages were readily assembled in quantitative yields from the predesigned multicomponent systems. The complementary ligation reinforced self-recognition to facilitate the shape-dependent self-sorting of a four-component dynamic library into two well-defined parallelograms. In addition, the subtle lability difference between homoleptic and heteroleptic complexes led to the site-selective Cd^II -Zn^II transmetalation in the Sierpiński triangle. Facile construction of a dodecanuclear tetrahedral metallocage was also realized by using two self-recognizable tritopic building blocks. The photophysical study of the metallo-supramolecules assembled from the d¹⁰ metal ions revealed intense ligand-based photoluminescence in solution. The self-assembly strategy described here provides an efficient methodology for building pre-programmable, sophisticated supramolecular architectures furnished with photoactivity.

Self-Assembly of a [1 + 1] Ionic Hexagonal Macrocycle and Its Antiproliferative Activity

A unique irregular hexagon was self-assembled using an organic donor clip (bearing terminal pyridyl units) and a complementary organometallic acceptor clip. The resulting metallamacrocycle was characterized by multinuclear NMR, mass spectrometry, and elemental analyses. Molecular modeling confirmed hexagonal shaped cavity for this metallamacrocycle which is a unique example of a discrete hexagonal framework self-assembled from only two building blocks. Cytotoxicity of the Pt-based acceptor tecton and the self-assembled Pt^II-based macrocycle was evaluated using three cancer cell lines and results were compared with cisplatin. Results confirmed a positive effect of the metallamacrocycle formation on cell growth inhibition.

Concentration dependent supramolecular interconversions of triptycene-based cubic, prismatic, and tetrahedral structures

New insight into the molecular fission–fusion process is obtained with the characterization of a stable intermediate prismatic cage.

Terpyridine-based metallosupramolecular constructs: tailored monomers to precise 2D-motifs and 3D-metallocages

Comprehensive summary of the recent developments in the growing field of terpyridine-based, discrete metallosupramolecular architectures.

Integrative self-sorting of coordination cages based on 'naked' metal ions

Coordination-driven self-assembly of metal ions and organic ligands has been extensively utilised over the past four decades to access a variety of nano-sized cage assemblies, with functions ranging from sensing and catalysis to drug delivery. Many of the reported examples, however, are highly symmetric architectures that contain one type of organic ligand carrying not more than a single functionality. This contrasts significantly with the level of structural and functional complexity encountered in biological macromolecular hosts, which are able to bind and chemically convert smaller molecules in their highly-decorated internal cavities. To address this disparity, rational approaches that facilitate heteroleptic assembly by regulating integrative self-sorting of metal ions and multiple ligand components have emerged. Among these, routes to access coordination cages from 'naked' metal cations that offer more than two coordination sites are still in early development, as the complexity of the self-sorted products in terms of composition and stereochemistry presents an entropic challenge. This feature article highlights recent progress in controlling integrative self-sorting of multi-component cage systems with a focus on structures composed of 'naked' metal cations and two different ligands. Once heteroleptic self-assembly strategies find a wider implementation in supramolecular design, the resultant interplay between tailored combinations of precisely positioned substituents promises enhanced functionality in nanoscale structures.

A Truncated Molecular Star

A pentanuclear coordination complex assembled from any palladium(II) component and non-chelating ligands is hitherto unreported. The pentanuclear complex [Pd₅ (L1)₅ (L2)₅ ](BF₄ )₁₀ , 1 reported here was prepared by the spontaneous complexation of [Pd(DMSO)₄ ](BF₄ )₂ with the non-chelating bidentate ligands 1,4-phenylenebis(methylene) diisonicotinate, L1 and 4,4'-bipyridine, L2 in a one-pot method at room temperature. The planar polycyclic complex 1 with outer diameters of ≈3 nm is termed as a "molecular star" owing to its resemblance with a pentagram shape. Interim paths leading to the star were also probed to decipher related dynamics of the system.

PdII2L4-type coordination cages up to three nanometers in size

The utilization of large ligands in coordination-based self-assembly represents an attractive strategy for the construction of supramolecular assemblies more than two nanometers in size. However, the implementation of this strategy is hampered by the fact that the preparation of such ligands often requires substantial synthetic effort. Herein, we describe a simple one-step protocol, which allows large bipyridyl ligands with a bent shape to be synthesized from easily accessible and/or commercially available starting materials. The ligands were used to construct PdII2L4-type coordination cages of unprecedented size. Furthermore, we provide evidence that these cages may be stabilized by close intramolecular packing of lipophilic ligand side chains. Packing effects of this kind are frequently encountered in protein assemblies, but they are seldom used as a design element in metallasupramolecular chemistry.

Design of a flexible organometallic tecton: host–guest chemistry with picric acid and self-assembly of platinum macrocycles

A new “flexible” and ditopic Pt(ii) organometallic compound is a tecton for the self-assembly of neutral metallacycles. It also exhibits significant binding affinity for picric acid.

Heteroleptic complexesviasolubility control: examples of Cu(ii), Co(ii), Ni(ii) and Mn(ii) complexes based on the derivatives of terpyridine and hydroxyquinoline

We describe the preparation of synthetically challenging heteroleptic complexes by considering the solubility properties of their corresponding favored homoleptic complexes.

Supramolecular five-component nano-oscillator

A five-component self-sorted metallo-supramolecular nano-oscillator was designed based on the full orthogonality of three different dynamic complexation motifs.

Five-State Rotary Nanoswitch

In our quest to develop artificial multistate devices, we synthesized the nanomechanical switch 1 that is characterized by a tetrahedral core equipped with four pending arms. The rotary arm with its azaterpyridine terminal is intramolecularly coordinated to a zinc(II) porphyrin station that is the terminus of another arm in 1. The two other arms carry identical sterically shielded phenanthroline stations. The 2-fold alternate addition of a copper(I) ion and [1,10]-phenanthroline (1 equiv each) results in the formation of five different switching states (State I→ State II→ State III→ State IV→ State V → State I), which force the toggling arm to move back and forth between the zinc(II) porphyrin and phenanthroline stations separated by a distance of 25 Å. All switching states constitute clean single species, except for State III, and thus are fully characterized by spectroscopic methods and elemental analysis. Finally, the initial state of nanoswitch was reset by addition of cyclam for complete removal of the copper(I) ions.

Photophysical Properties of Organoplatinum(II) Compounds and Derived Self-Assembled Metallacycles and Metallacages: Fluorescence and its Applications

Over the past couple of decades, coordination-driven self-assembly has evolved as a broad multidisciplinary domain that not only covers the syntheses of aesthetically pleasing supramolecular architectures but also emerges as a method to form new optical materials, chemical sensors, theranostic agents, and compounds with light-harvesting and emissive properties. The majority of these applications depend upon investigations that reveal the photophysical nature and electronic structure of supramolecular coordination complexes (SCCs), including two-dimensional (2D) metallacycles and three-dimensional (3D) metallacages. As such, well-defined absorption and emission spectra are important for a given SCC to be used for sensing, bioimaging, and other applications with molecular fluorescence being an important component. In this Account, we summarize the photophysical properties of some bis(phosphine)organoplatinum(II) compounds and their discrete SCCs. The platinum(II) based organometallic precursors typically display spectral red-shifts and have low fluorescence quantum yields and short fluorescence lifetimes compared to their organic counterparts because the introduction of metal centers enhances both intersystem crossing (ISC) and intramolecular charge transfer (ICT) processes, which can compete with the fluorescence emissions. Likewise ligands with conjugation can also increase the ICT process; hence the corresponding organoplatinum(II) compounds undergo a further decrease in fluorescence lifetimes. The use of endohedral amine functionalized 120°-bispyridyl ligands can dramatically enhance the emission properties of the resultant organoplatinum(II) based SCCs. As such these SCCs display emissions in the visible region (ca. 400-500 nm) and are significantly red-shifted (ca. 80-100 nm) compared to the ligands. This key feature makes them suitable as supramolecular theranostic agents wherein these unique emission properties provide diagnostic spectroscopic handles and the organoplatinum(II) centers act as potential anticancer agents. Using steady state and time-resolved-spectroscopic techniques and quantum computations in concert, we have determined that the emissive properties stem from the ligand-centered transitions involving π-type molecular orbitals with modest contributions from the metal-based orbitals. The self-assembly and the photophysics of organoplatinum(II) ← 3-substituted pyridyl based SCCs are highly diverse. Subtle changes in the ligands' structures can form molecular congener systems with distinct conformational and photophysical properties. Furthermore, the heterometallic SCCs described herein possess rich photophysical properties and can be used for sensing based applications. Tetraphenylethylene (TPE) based SCCs display emissions in the aggregated state as well as in dilute solutions. This is a unique phenomenon that bridges the aggregation caused quenching (ACQ) and aggregation induced emission (AIE) effects. Moreover, a TPE based metallacage exhibits solvatoluminescence, including white light emission in THF solvent, and can act as a fluorescence-sensor for structurally similar ester compounds.

Immobilizing Tetraphenylethylene into Fused Metallacycles: Shape Effects on Fluorescence Emission

Herein, we describe the selective formation of a discrete fused metallarhomboid and a triangle by the careful control of the shape and stoichiometry of the building blocks. A tetraphenylethylene (TPE)-based tetrapyridyl donor is exploited as the bridging component, and coordination immobilization of the TPE unit within the rigid metallacyclic frameworks efficiently suppresses its intramolecular rotational motions. As a result, the fused polygons are innately emissive in dilute solution, representing an alternative to aggregation-induced emission. Upon further molecular aggregation, these metallacycles display aggregation-induced enhanced emissions. Interestingly, the fused rhomboid 7 shows a weaker fluorescence in dilute solutions relative to that of the fused triangle 8, while a reversal of emission intensities was observed in the aggregated state. These markedly different fluorescence efficiencies are likely due to the differences in the shapes of the fused polygons. Thus, this work shows that the properties of supramolecular coordination complexes can be affected by subtle structural factors, which can be controlled easily and precisely at the molecular level.

Synthesis and characterization of metallo-supramolecular polymers

The incorporation of metal centers into the backbone of polymers has led to the development of a broad range of organometallic and coordination compounds featuring properties that are relevant for potential applications in diverse areas of research, ranging from energy storage/conversion to bioactive or self-healing materials. In this review, the basic concepts and synthetic strategies leading to these types of materials as well as the scope of available characterization techniques will be summarized and discussed.

A Four-Component Heterometallic Cu-Pt Quadrilateral via Self-Sorting

Herein we combine the subcomponent self-assembly and integrative self-sorting techniques with the well-established platinum(II)-pyridine coordination-driven self-assembly to report the quantitative synthesis and spectroscopic characterization of a heterometallic Cu^I -Pt^II quadrilateral QL that is formed from a total of twelve molecular components from four unique species, including 5-(pyridin-4-ylethynyl)picolinaldehyde (1), p-toluidine (2), [Cu(CH₃ CN₄ ](PF₆ ) (3), and cis-Pt(PEt₃ )₂ (OTf)₂ (4), in a 4:4:2:2 ratio. Despite the many different entities potentially forming from these four precursor units, the clean formation of QL is mainly guided by the different coordination preferences of the metal ions 3 and 4, and the design criteria inherent in compounds 1 and 2.

Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages

It is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic Fe(II)8L12 cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral Fe(II)4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of Pd(II)-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral Pd(II)4L8 assembly, whereas an octahedral Pd(II)6L12 cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the composition of dynamic mixtures of Pd(II) cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of Pd(II)4L8 and Pd(II)6L12 cages, whereas the other did not.

Pyrazine-based donor tectons: synthesis, self-assembly and characterization

Two donor tectons, each bearing a central pyrazine moiety covalently linked to two pyridine units, have been self-assembled with PtII2 acceptor units to yield ionic nanoscalar metallamacrocycles, having a convex hexagonal cavity.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.

Fully reversible three-state interconversion of metallosupramolecular architectures

The reversible switching of a sterically encumbered phenanthroline–Cu⁺–picolinaldehyde trio back and forth between homoleptic and heteroleptic coordination using the relative metal-ion to ligand ratio is the basis for an unprecedented cyclic three-state interconversion of metallacycles.

A Suite of Tetraphenylethylene-Based Discrete Organoplatinum(II) Metallacycles: Controllable Structure and Stoichiometry, Aggregation-Induced Emission, and Nitroaromatics Sensing

Materials that organize multiple functionally active sites, especially those with aggregation-induced emission (AIE) properties, are of growing interest due to their widespread applications. Despite promising early architectures, the fabrication and preparation of multiple AIEgens, such as multiple tetraphenylethylene (multi-TPE) units, in a single entity remain a big challenge due to the tedious covalent synthetic procedures often accompanying such preparations. Coordination-driven self-assembly is an alternative synthetic methodology with the potential to deliver multi-TPE architectures with light-emitting characteristics. Herein, we report the preparation of a new family of discrete multi-TPE metallacycles in which two pendant phenyl rings of the TPE units remain unused as a structural element, representing novel AIE-active metal-organic materials based on supramolecular coordination complex platforms. These metallacycles possess relatively high molar absorption coefficients but weak fluorescent emission under dilute conditions because of the ability of the untethered phenyl rings to undergo torsional motion as a non-radiative decay pathway. Upon molecular aggregation, the multi-TPE metallacycles show AIE-activity with markedly enhanced quantum yields. Moreover, on account of their AIE characteristics in the condensed state and ability to interact with electron-deficient substrates, the photophysics of these metallacycles is sensitive to the presence of nitroaromatics, motivating their use as sensors. This work represents a unification of themes including molecular self-assembly, AIE, and fluorescence sensing and establishes structure-property-application relationships of multi-TPE scaffolds. The fundamental knowledge obtained from the current research facilitates progress in the field of metal-organic materials, metal-coordination-induced emission, and fluorescent sensing.

Self-assembly of heteroleptic dinuclear metallosupramolecular kites from multivalent ligands via social self-sorting

A Tröger's base-derived racemic bis(1,10-phenanthroline) ligand (rac)-1 and a bis(2,2'-bipyridine) ligand with a central 1,3-diethynylbenzene unit 2 were synthesized. Each of these ligands acts as a multivalent entity for the binding of two copper(I) ions. Upon coordination to the metal ions these two ligands undergo selective self-assembly into heteroleptic dinuclear metallosupramolecular kites in a high-fidelity social self-sorting manner as evidenced by NMR spectroscopy and mass spectrometry.

Amino-ether macrocycle that forms CuII templated threaded heteroleptic complexes: a detailed selectivity, structural and theoretical investigations

Self-sorting behavior of a newly synthesized macrocycle with divalent metal ions and aromatic ligands via pseudorotaxane formation has been described.

Multicomponent reassembly of terpyridine-based materials: quantitative metallomacrocyclic rearrangement

Two heterometallic triangles were constructed in quantitative yield by the reorganization of triangular and tetrameric metallomacrocyclic species.

A sequential logic gate-based “smart probe” for selective monitoring of Cu2+, Fe3+ and CN−/F−via differential analyses

A sequential logic gate-based probe for the detection of Cu²⁺, Fe³⁺, CN⁻ and F⁻ at ppm levels in water.

Metal-coordination-driven mixed ligand binding in supramolecular bisporphyrin tweezers

The mixed ligand polymers of Mg(ii)bisporphyrin with heterogeneous guest pairs have been synthesized just by mixing the host and guests in one pot and structurally characterized. The guest ligands are bound selectively between the inside and the outside of the bisporphyrin cavity.

Size dependent effect of new organometallic triptycene tectons on the dimensions of self-assembled macrocycles

The effect of changing the dimension of triptycene-based acceptor building blocks on the shape and size of the resulting supramolecular framework is reported.

From self-sorted coordination libraries to networking nanoswitches for catalysis

This feature article sketches our long way from the development of dynamic heteroleptic coordination motifs to the self-sorting of multi-component libraries and finally the design of a new family of triangular nanomechanical switches, which are useful for ON–OFF control of catalysis and in bidirectional communication.

A trio of nanoswitches in redox-potential controlled communication

A potential-controlled two-step bidirectional communication protocol between the nanoswitches [Cu(1)](+), 2 and 3 is set up, in which ligand followed by metal-ion oxidation drives two subsequent metal ion translocations (self-sorting) changing the switching state at each switch. The communication is reset to its starting point by a two-electron reduction.