Highly Stable Spherical Metallo-Capsule from a Branched Hexapodal Terpyridine and Its Self-Assembled Berry-type Nanostructure

Hydrogenation Versus Hydrosilylation: The Substantial Impact of a Palladium Capsule on the Catalytic Outcome

A palladium capsule, made of three cavitands, namely P,P-dichlorido{5,17-bis[5-(diphenylphosphanyl)-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]arenyl-17-oxymthyl]-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]arene}palladium(II) (1), was synthetized by coordination of the corresponding diphosphinated ligand and the palladium precursor [PdCl2(PhCN)2] in 27% yield. The obtained P,P-chelate complex was fully characterized by elemental analysis, NMR and mass spectrometry. Molecular dynamics simulations carried out on the metallo-capsule showed the structure made by the three cavitands was slightly distorted over the 1 μs of the simulation. The evaluation of the palladium capsule 1 in the reaction between arylacetylenes and Et3SiH in undried conditions unequivocally demonstrates a drastic change in chemoselectivity, with the formation of the partially hydrogenation product rather than the hydrosilylation products observed with complexes whose active center is more accessible, for instance [PdCl2(PPh3)2].

Constructing Ultrastable Metallo-Cages via In Situ Deprotonation/Oxidation of Dynamic Supramolecular Assemblies

Coordination-driven self-assembly enables the spontaneous construction of metallo-supramolecules with high precision, facilitated by dynamic and reversible metal-ligand interactions. The dynamic nature of coordination, however, results in structural lability in many metallo-supramolecular assembly systems. Consequently, it remains a formidable challenge to achieve self-assembly reversibility and structural stability simultaneously in metallo-supramolecular systems. To tackle this issue, herein, we incorporate an acid-/base-responsive tridentate ligand into multitopic building blocks to precisely construct a series of metallo-supramolecular cages through coordination-driven self-assembly. These dynamic cagelike assemblies can be transformed to their static states through mild in situ deprotonation/oxidation, leading to ultrastable skeletons that can withstand high temperatures, metal ion chelators, and strong acid/base conditions. This in situ transformation provides a reliable and powerful approach to manipulate the kinetic features and stability of metallo-supramolecules and allows for modulation of encapsulation and release behaviors of metallo-cages when utilizing nanoscale quantum dots (QDs) as guest molecules.

Hourglass-Shaped Nanocages with Concaved Structures Based on Selective Self-Complementary Coordination Ligands and Tunable Hierarchical Self-Assembly

Three-dimensional (3D) structures constructed via coordination-driven self-assemblies have recently garnered increasing attention due to the challenges in structural design and potential applications. In particular, developing new strategy for the convenient and precise self-assemblies of 3D supramolecular structures is of utmost interest. Introducing the concept of self-coordination ligands, herein the design and synthesis of two meta-modified terpyridyl ligands with selective self-complementary coordination moiety are reported and their capability to assemble into two hourglass-shaped nanocages SA and SB is demonstrated. Within these 3D structures, the meta-modified terpyridyl unit preferably coordinates with itself to serve as concave part. By changing the arm length of the ligands, hexamer (SA) and tetramer (SB) are obtained respectively. In-depth studies on the assembly mechanism of SA and SB indicate that the dimers could be formed first via self-complementary coordination and play crucial roles in controlling the final structures. Moreover, both SA and SB can go through hierarchical self-assemblies in solution as well as on solid-liquid interface, which are characterized by transmission electron microscope (TEM) and scanning tunneling microscopy (STM). It is further demonstrated that various higher-order assembly structures can be achieved by tuning the environmental conditions.

Shape-Dependent Complementary Ditopic Terpyridine Pair with Two Levels of Self-Recognition for Coordination-Driven Self-Assembly

Molecular recognition in biological systems plays a vital role in the precise construction of biomacromolecules and the corresponding biological activities. Such recognition mainly relies on the highly specific binding of complementary molecular pairs with complementary sizes, shapes, and intermolecular forces. It still remains challenging to develop artificial complementary motif pairs for coordination-driven self-assembly. Herein, a series of shape-dependent complementary motif pairs, based on ditopic 2,2':6',2″-terpyridine (TPY) backbone, are designed and synthesized. The fidelity degrees of self-assemblies from these motifs are carefully evaluated by multi-dimensional mass spectrometry, nuclear magnetic resonance spectroscopy, and molecular modeling. In addition, two levels of self-recognition in both homoleptic and heteroleptic assembly are discovered in the assembled system. Through finely tuning the shape and size of the ligands, a complementary pair is developed with error-free narcissistically self-sorting at two levels of self-recognition, and the intrinsic principle is carefully investigated.

Tetratopic Terpyridine Building Unit as a Precursor to Wheel-Like Metallo-Supramolecules

In an effort to construct molecules with distinct shapes and functions, the design and synthesis of multitopic ligands are often able to play an important role. Here, we report the synthesis of a novel tetratopic organic ligand LA, which can be viewed as a bis-tenon with successive angular orientations in space. The particular ligand has been treated with different tailored metal-organic ligands to afford new members of the molecular wheel family (multi-rhomboidal-shaped wheel and bis-trapezium-shaped wheel) that show enhanced stability. Two-dimensional (2D) diffusion nuclear magnetic resonance (NMR) spectroscopy (DOSY), electrospray ionization (ESI) mass spectrometry, traveling wave ion mobility (TWIM), and gradient tandem mass spectrometry (gMS₂) experiments, as well as molecular modeling, have been employed to provide structural information and differentiate the isomeric separation process. In addition, considering that LA has rotational properties, it is expected to open the door to functional supramolecules and stimuli-responsive materials.

Design and Self-Assembly of Macrocycles with Metals at the Corners Based on Dissymmetric Terpyridine Ligands

Terpyridine-based discrete supramolecular architectures with metal ions in the corners have rarely been reported. Herein, we report two dissymmetric terpyridyl ligands LA and LB decorated at the 5-position and 4-position of terpyridine respectively. The complexes constructed by the self-assembly of LA and LB with Zn(II) exhibit hand-circle-like structures. Moreover, all Zn(II) are successfully fixed in the corners. A series of dimeric to hexameric macrocycles is obtained by head-to-tail connections with changing concentration. This work will pave the way for preparation of more elaborate self-assembled structures based on dissymetric ligands.

Terpyridine-Based 3D Discrete Metallosupramolecular Architectures

Terpyridine (tpy)-based 3D discrete metallosupramolecular architectures, which are often inspired by polyhedral geometry and the biological structures found in nature, have drawn significant attention from the community of metallosupramolecular chemistry. Because of the linear tpy-M(II)-tpy connectivity, the creation of sophisticated 3D metallosupramolecules based on tpy remains a formidable synthetic challenge. Nevertheless, with recent advancement in ligand design and self-assembly, diverse 3D metallosupramolecular polyhedrons, such as Platonic solids, Archimedean solids, prims as well as Johnson solids, have been constructed and their potential applications have been explored. This review summarizes the progress on tpy-based discrete 3D metallosupramolecules, aiming to shed more light on the design and construction of novel discrete architectures with molecular-level precision through coordination-driven self-assembly.

Self-Assembly Methods for Recently Reported Discrete Supramolecular Structures Based on Terpyridine

In this Review, self-assembly methods of discrete metallo-supramolecules based on 2,2' : 6',2''-terpyridine (tpy) are comprehensively summarized. With the development of self-assembly, strategies for discrete 2D and 3D supramolecular architectures have boomed, including the geometry-directed method, template-driven method, and stepwise method. Ligand geometry-directed method mainly depends on the geometry of ligands (i. e., angle, geometric strain, and rigidity), and it is suitable for dual-component systems, while the template-driven method can guide the self-assembly of predesigned supramolecules by the introduction of specific templates. Meanwhile, stepwise method, breaking the inherent self-sorting of ligands and reducing the probability of mismatch, is suitable for multicomponent systems to yield predesigned supramolecules. This review focuses on self-assembly methods and aims to provide a guideline for constructing supramolecular architectures using a suitable approach.

Designing narcissistic self-sorting terpyridine moieties with high coordination selectivity for complex metallo-supramolecules

Coordination-driven self-assembly is a powerful approach for the construction of metallosupramolecules, but designing coordination moieties that can drive the self-assembly with high selectivity and specificity remains a challenge. Here we report two ortho-modified terpyridine ligands that form head-to-tail coordination complexes with Zn(II). Both complexes show narcissistic self-sorting behaviour. In addition, starting from these ligands, we obtain two sterically congested multitopic ligands and use them to construct more complex metallo-supramolecules hexagons. Because of the non-coaxial structural restrictions in the rotation of terpyridine moieties, these hexagonal macrocycles can hierarchically self-assemble into giant cyclic nanostructures via edge-to-edge stacking, rather than face-to-face stacking. Our design of dissymmetrical coordination moieties from congested coordination pairs show remarkable self-assembly selectivity and specificity.

Balancing Ligand Flexibility versus Rigidity for the Stepwise Self-Assembly of M12 L24 via M6 L12 Metal-Organic Cages

Non-covalent interactions are important for directing protein folding across multiple intermediates and can even provide access to multiple stable structures with different properties and functions. Herein, we describe an approach for mimicking this behavior in the self-assembly of metal-organic cages. Two ligands, the bend angles of which are controlled by non-covalent interactions and one ligand lacking the above-mentioned interactions, were synthesized and used for self-assembly with Pd²⁺ . As these weak interactions are easily broken, the bend angles have a controlled flexibility giving access to M₂ (L1)₄ , M₆ (L2)₁₂ , and M₁₂ (L2)₂₄ cages. By controlling the self-assembly conditions this process can be directed in a stepwise fashion. Additionally, the multiple endohedral hydrogen-bonding sites on the ligand were found to play a role in the binding and discrimination of neutral guests.

A parallelogram metallomacrocycle bearing self-catenation and its derivative supramolecular isomerism

Macrocycle-based architectures are of importance in synthetic chemistry. Here, a novel parallelogram metallomacrocycle Fe2(LA)2 with reversible structural transformation was designed and synthesized. The template-free metalla[2]catenane [Fe2(LA)2]2 could be obtained by changing the concentration and has been monitored on the basis of NMR analysis. By redesigning the metallo-ligand, a catenane-like intersected parallelogram assembly with two shape-persistent supramolecular isomers was achieved. This work develops the field of terpyridine-based macrocycle system research and is valuable for obtaining other types of supramolecular isomerism.

Reinforced Topological Nanoassemblies: 2D Hexagon-Fused Wheel to 3D Prismatic Metallo-Lamellar Structure with Molecular Weight of 119 K Daltons

By a precise metallo-ligand design, the advanced coordination-driven self-assembly could succeed in the preparation of giant molecular weight of the metallo-architectures. However, the synthesis of a single discrete high-molecular-weight (>100 K Da) structure has not been demonstrated due to the insurmountable synthetic challenge. Herein, we present a two-dimensional wheel structure (W1) and a gigantic three-dimensional dodecagonal prism-like architecture (P1), which were generated by multicomponent self-assembly of two similar metallo-organic ligands and a core ligand with metal ions, respectively. The giant 2D-suprastructure W1 with six hexagonal metallacycles that fused to the central spoke wheel was first achieved in nearly quantitative yield, and then, directed by introducing a meta-substituted coordination site into the key ligand, the supercharged (36 Ru²⁺ and 48 Cd²⁺ ions) double-decker prismatic structure P1 with two wheel structure W1s serve as the surfaces and 12 <Tpy-Cd²⁺-Tpy> connectivities serve as the edges, where a molecular weight up to 119 498.18 Da was accomplished. The expected molecular composition and size morphology was unequivocally characterized by nuclear magnetic resonance, mass spectrometry, and transmission electron microscopy investigations. The introduction of a wheel structure is able to add considerable stability and complexity to the final architecture. These well-defined scaffolds are expected to play an important role in the functional materials field, such as molecular encapsulation and medicine sustained release.

Introducing Seven Transition Metal Ions into Terpyridine-Based Supramolecules: Self-Assembly and Dynamic Ligand Exchange Study

In coordination-driven self-assembly, 2,2':6',2″-terpyridine (tpy) has gained extensive attention in constructing supramolecular architectures on the basis of ⟨tpy-M-tpy⟩ connectivity. In direct self-assembly of large discrete structures, however, the metal ions were mainly limited to Cd(II), Zn(II), and Fe(II) ions. Herein, we significantly broaden the spectrum of metal ions with seven divalent transition metal ions M(II) (M = Mn, Fe, Co, Ni, Cu, Zn, Cd) to assemble a series of supramolecular fractals. In particular, Mn(II), Co(II), Ni(II), and Cu(II) were reported for the first time to form such large and discrete structures with ⟨tpy-M-tpy⟩ connectivity. In addition, the structural stabilities of those supramolecules in the gas phase and the kinetics of the ligand exchange process in solution were investigated using mass spectrometry. Such a fundamental study gave the relative order of structural stability in the gas phase and revealed the inertness of coordination in solution depending on the metal ions. Those results would guide the future study in tpy-based supramolecular chemistry in terms of self-assembly, characterization, property, and application.

Ditopic Chiral Pineno-Fused 2,2':6',2″-Terpyridine: Synthesis, Self-Assembly, and Optical Properties

The syntheses of 4'-substituted chiral 2,2':6',2″-terpyridine (tpy) ligands with predetermined configurations and directionalities are rather limited in the supramolecular chemistry field. In this study, a carbazole-linked ditopic chiral ligand L was synthesized using 4'-bromo-substituted pineno-fused tpy 5 as the precursor. Upon complexation with Cd(NO₃)₂·4H₂O and Zn(NO₃)₂·6H₂O, two enantiomerically pure metallosupramolecules, [Cd₃L₃] and [Zn₄L₄], have been self-assembled and characterized by NMR, electrospray ionization-mass spectrometry, traveling wave ion mobility-mass spectrometry, and DOSY analysis. In addition, their optical properties are characterized by UV-vis, fluorescence, circular dichroism, and circularly polarized luminescence, suggesting an efficiency transmission and amplification of chirality from the ligand to metal center via self-assembly.

Trefoiled Propeller-Shaped Spiral Terpyridyl Metal-Organic Architectures

Constructing exquisite and intricate molecular architectures is always the pursuit of chemists. In this report, the propeller-shaped trefoil structures S1 and S2 were successfully prepared by the stepwise self-assembly of predesigned tripodal metal-organic ligands, which consist of bis(terpyridine)s-Ru²⁺-tris(terpyridine)s connectivities for the following complexation with Fe²⁺. The complexes can be described as racemic spiral assemblies with three-fold spiralism. These unique discrete metal-organic architectures were fully characterized by ¹H NMR, 2D NMR spectroscopy (COSY and NOESY), diffusion-ordered NMR spectroscopy (DOSY), ESI-MS, TWIM-MS, and TEM, and their photophysical and electrochemical properties were also investigated. Further, hybrid trefoiled structure [Fe₃L1L2] was detected by taking advantage of the flexibility of metal-organic ligands.

Supramolecular Arene-Ruthenium Metallacycle with Thermotropic Liquid-Crystalline Properties

Functionalization of 1,4-di(4-pyridinyl)benzene with poly(arylester) dendrimers bearing cyanobiphenyl end-groups gives a bidentate dendromesogenic ligand (L) that exhibits thermotropic liquid-crystalline properties. Combination of the diruthenium complex [Ru₂(p-cymene)₂(donq)][DDS]₂ (M) with L, by coordination-driven self-assembly, affords the discrete and well-defined metallacycle M₂L₂. Like L, this supramolecular dendritic system displays mesomorphic properties above 50 °C. Both compounds L and M₂L₂ show smectic phases, characterized by a multilayered organization of the multiple components.

Geometrically Complementary Self-Assembly of a Hexarhomboid Architecture from Two Ruthenium(II)-Organic Building Blocks

A shape-persistent metallosupramolecular multirhomboid that inlays a hexarhomboid polygon in a three-lobed flat structure was prepared by means of coordination-driven self-assembly. The key ligands were synthesized by a "reaction on complex" strategy that becomes accessible to troublesome metalloorganic ligand L3. The formation here consists of four different starting components and two metal ions. Complementarity of the shape and size drives molecular puzzling and results in the multicomponent, quantitative self-assembled construct.

Terpyridine-metal complexes: Applications in catalysis and supramolecular chemistry

As an NNN-tridentate ligand, the 2,2':6',2"-terpyridine plays an important role in coordination chemistry. With three coordination sites and low LUMO, terpyridine and its derivatives are one of the typical Pincer ligand and/or non-innocent ligands in transition metal catalysis. Interesting catalytic reactivities have been obtained with these tpy-metal complexes targeting some challenging transformations, such as C-C bond formation and hydrofunctionalization. On the other hand, terpyridine ligands can form "closed-shell" octahedral complexes, which provide a linear and stable linkage in supramolecular chemistry. Numerous supramolecular architectures have been achieved using modified terpyridine ligands including Sierpiński triangles, hexagonal gasket and supramolecular rosettes. This review presents a summary of recent progress regarding transition metal-terpyridine complexes with the focus on their applications in catalysis and supramolecular structure construction. Facile synthesis of terpyridine derivatives is also described. We hope this article can serve to provide some general perspectives of the terpyridine ligand and their applications in coordination chemistry.

Electronic Peculiarities of a Self-Assembled M12L24 Nanoball (M = Pd+2, Cr, or Mo)

We use molecular mechanics and DFT calculations to analyze the particular electronic behavior of a giant nanoball. This nanoball is a self-assembled M₁₂L₂₄ nanoball; with M equal to Pd⁺²; Cr; and Mo. These systems present an extraordinarily large cavity; similar to biological giant hollow structures. Consequently, it is possible to use these nanoballs to trap smaller species that may also become activated. Molecular orbitals, molecular hardness, and Molecular Electrostatic Potential enable us to define their potential chemical properties. Their hardness conveys that the Mo system is less reactive than the Cr system. Eigenvalues indicate that electron transfer from the system with Cr to other molecules is more favorable than from the system with Mo. Molecular Electrostatic Potential can be either positive or negative. This means that good electron donor molecules have a high possibility of reacting with positive regions of the nanoball. Each of these nanoballs can trap 12 molecules, such as CO. The nanoball that we are studying has large pores and presents electronic properties that make it an apposite target of study.

Coordination-Driven Self-Assembled Metallacycles Incorporating Pyrene: Fluorescence Mutability, Tunability, and Aromatic Amine Sensing

Constructing polycyclic aromatics-based, highly emissive fluorophores with good solubility and tunable aggregated structures and properties is of great importance for film fabrication, solution processing, and relevant functionality studies. Herein, we describe a general strategy to endow conventional organic fluorophores with enhanced solubility and modulated fluorescent properties via their incorporation into coordination-driven self-assembled metallacycles. A widely used fluorophore, pyrene, was decorated with two pyridyl groups to yield functionalized pyrene 4. Mixing 4 with three aromatic dicarboxylates with different lengths and a 90° Pt(II) metal acceptor in a 2:2:4 stoichiometric ratio resulted in the formation of three metallacycles, 1, 2, and 3. The metallacycles display good solubility in polar organic solvents, highly aggregation-dependent fluorescence, and size-dependent emissions at higher concentrations. Moreover, metallacycle 2-based, silica-gel-supported film as fabricated not only is more emissive than the ligand 4-based one but also displays much improved sensing properties for amines in the vapor state, as demonstrated by significantly increased response speed and decreased recovery time. The enhanced solubility, unique fluorescence behavior, and multi-factor modulation character show that coordination-driven self-assembly can be utilized for the development of new fluorophores through simple modification of conventional fluorophores. The fluorophores synthesized this way possess not only complex topological structures but also good modularity and tunability in fluorescence behavior, which are important for grafting multi-stage energy-transfer systems necessary for the development of high-performance sensing materials.

Stepwise Self-Assembly and Dynamic Exchange of Supramolecular Nanocages Based on Terpridine Building Blocks

Coordination-driven self-assembly as a powerful bottom-up approach has been extensively used to construct multifarious supramolecular architectures with increasing complexity and functionality. Due to the unique cavity structures and precisely controllable dimensions, 3D supramolecules display unprecedented properties and functions in catalysis, sensing, gas storage, and smart materials. Herein, we have built two 3D nanocages with different sizes by changing the length of the organic ligand arms. The structures were characterized by 1D and 2D NMR spectroscopy, electrospray ionization-mass spectrometry (ESI-MS), traveling wave ion mobility-mass spectrometry (TWIM-MS), gradient tandem-mass spectrometry (gMS2 ), and transmission electron microscopy (TEM). Furthermore, the intermolecular dynamic exchange of two 3D nanocages was conducted to construct a series of hybrid 3D structures as evidenced by mass spectrometry.

Self-Assembly of Supramolecular Fractals from Generation 1 to 5

In the seeking of molecular expression of fractal geometry, chemists have endeavored in the construction of molecules and supramolecules during the past few years, while only a few examples were reported, especially for the discrete architectures. We herein designed and constructed five generations of supramolecular fractals (G1-G5) based on the coordination-driven self-assembly of terpyridine ligands. All the ligands were synthesized from triphenylamine motif, which played a central role in geometry control. Different approaches based on direct Sonogashira coupling and/or ⟨tpy-Ru(II)-tpy⟩ connectivity were employed to prepare complex Ru(II)-organic building blocks. Fractals G1-G5 were obtained in high yields by precise coordination of organic or Ru(II)-organic building blocks with Zn(II) ions. Characterization of those architectures were accomplished by 1D and 2D NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), traveling-wave ion mobility mass spectrometry (TWIM-MS), and transmission electron microscopy (TEM). Furthermore, the two largest fractals also hierarchically self-assemble into ordered supramolecular nanostructures either at solid/liquid interface or in solution on the basis of their well-defined scaffolds.

Truncated Sierpiński Triangular Assembly from a Molecular Mortise-Tenon Joint

The amalgamation of different components into a giant and intricate structure that makes quantitative and spontaneous assembly through molecular design is indispensable but challenging. To construct novel metallo-supramolecular architectures, here we present an architectural design principle based on multicomponent self-assembly. Using a carefully designed hexatopic terpyridine-based metallo-organic ligand (MOL), [Ru₂T₂K], we report on the formation of supramolecular trapezoid Zn₅[Ru₂T₂K]V₂, hollow hexagon Zn₁₅[Ru₂T₂K]₃K₃, and giant star-shaped supramolecule Zn₁₈[Ru₂T₂K]₃[Ru₂X₂V]₃, all of which were assembled by one-pot, nearly quantitative assembly of [Ru₂T₂K] with the ditopic 60°-directed bisterpyridine V, tetrakisterpyridine K, and MOL [Ru₂X₂V], respectively. The complementary ligands were selected on the basis of the size- and shape-fit principles, actually similar to the mortise-tenon joint that aligns and locks the two complementary wood components. This strategy is expected to open the door to sophisticated designer supramolecules and nonbiological materials. The multivalent connections within the mutual ligands give rise to the formation of stable assemblies, which are unambiguously characterized by NMR, ESI-MS, TWIM-MS, and TEM analyses.

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.