Self-Assembly Processes of Octahedron-Shaped Pd6L4 Cages

Cage Match: Comparing the Anion Binding Ability of Isostructural Versus Isofunctional Pairs of Metal-Organic Nanocages

Affinities of six anions (mesylate, acetate, trifluoroacetate, p-toluenecarboxylate, p-toluenesulfonate, and perfluorooctanoate) for three related Pt2+ -linked porphyrin nanocages were measured to probe the influence of different noncovalent recognition motifs (e. g., hydrogen bonding, electrostatics, π bonding) on anion binding. Two new hosts of M6 L3 12+ (1b) and M4 L2 8+ (2) composition (M=(en)Pt2+ , L=(3-py)4 porphyrin) were prepared in a one-pot synthesis and allowed comparison of hosts that differ in structure while maintaining similar N-H hydrogen-bond donor ability. Comparisons of isostructural hosts that differ in hydrogen-bonding ability were made between 1b and a related M6 L3 12+ nanoprism (1a, M=(tmeda)Pt2+ ) that lacks N-H groups. Considerable variation in association constants (K1 =1.6×103  M-1 to 1.3×108  M-1 ) and binding mode (exo vs. endo) were found for different host-guest combinations. Strongest binding was seen between p-toluenecarboxylate and 1b, but surprisingly, association of this guest with 1a was only slightly weaker despite the absence of NH⋅⋅⋅O interactions. The high affinity between p-toluenecarboxylate and 1a could be turned off by protonation, and this behavior was used to toggle between the binding of this guest and the environmental pollutant perfluorooctanoate, which otherwise has a lower affinity for the host.

Biasing the Formation of Solution-Unstable Intermediates in Coordination Self-Assembly by Mechanochemistry

Due to the reversible nature of coordination bonds and solvation effect, coordination self-assembly pathways are often difficult to elucidate experimentally in solution, as intermediates and products are in constant equilibration. The present study shows that some of these transient and high-energy self-assembly intermediates can be accessed by means of ball-milling approaches. Among them, highly aqueous-unstable Pd3 L11 and Pd6 L14 open-cage intermediates of the framed Fujita Pd6 L14 cage and Pd2 L22 , Pd3 L21 and Pd4 L22 intermediates of Mukherjee Pd6 L24 capsule are successfully trapped in solid-state, where Pd=tmedaPd2+ , L1=2,4,6-tris(4-pyridyl)-1,3,5-triazine and L2=1,3,5-tris(1-imidazolyl)benzene). Their structures are assigned by a combination of solution-based characterization tools such as standard NMR spectroscopy, DOSY NMR, ESI-MS and X-ray diffraction. Collectively, these results highlight the opportunity of using mechanochemistry to access unique chemical space with vastly different reactivity compared to conventional solution-based supramolecular self-assembly reactions.

Guest Encapsulation Alters the Thermodynamic Landscape of a Coordination Host

The architecture of self-assembled host molecules can profoundly affect the properties of the encapsulated guests. For example, a rigid cage with small windows can efficiently protect its contents from the environment; in contrast, tube-shaped, flexible hosts with large openings and an easily accessible cavity are ideally suited for catalysis. Here, we report a "Janus" nature of a Pd6L4 coordination host previously reported to exist exclusively as a tube isomer (T). We show that upon encapsulating various tetrahedrally shaped guests, T can reconfigure into a cage-shaped host (C) in quantitative yield. Extracting the guest affords empty C, which is metastable and spontaneously relaxes to T, and the T⇄C interconversion can be repeated for multiple cycles. Reversible toggling between two vastly different isomers paves the way toward controlling functional properties of coordination hosts "on demand".

Theoretical and computational methodologies for understanding coordination self-assembly complexes

This perspective highlights three theoretical and computational methods to capture the coordination self-assembly processes at the molecular level: quantum chemical modeling, molecular dynamics, and reaction network analysis. These methods cover the different scales from the metal-ligand bond to a more global aspect, and approaches that are best suited to understand the coordination self-assembly from different perspectives are introduced. Theoretical and numerical researches based on these methods are not merely ways of interpreting the experimental studies but complementary to them.

Exploring the Emergent Redox Chemistry of Pd(II) Nodes with Pendant Ferrocenes: From Precursors, through Building Blocks, to Self-Assemblies

Energy-relevant small molecule activations and related processes are often multi-electron in nature. Ferrocene is iconic for its well-behaved one-electron chemistry, and it is often used to impart redox activity to self-assembled architectures. When multiple ferrocenes are present as pendant groups in a single structure, they often behave as isolated sites with no separation of their redox events. Herein, we study a suite of molecules culminating in a self-assembled palladium(II) truncated tetrahedron (TT) with six pendant ferrocene moieties using the iron(III/II) couple to inform about the electronic structure and, in some cases, subsequent reactivity. Notably, although known ferrocene-containing metallacycles and cages show simple reversible redox chemistry, this TT undergoes a complex multi-step electrochemical mechanism upon oxidation. The electrochemical behavior was observed by voltammetric and spectroelectrochemical techniques and suggests that the initial Fc-centered oxidation is coupled to a subsequent change in species solubility and deposition of a film onto the working electrode, which is followed by a second separable electrochemical oxidation event. The complicated electrochemical behavior of this self-assembly reveals emergent properties resulting from organizing multiple ferrocene subunits into a discrete structure. We anticipate that such structures may provide the basis for multiple charge separation events to drive important processes related to energy capture, storage, and use, especially as the electronic communication between sites is further tuned.

Stochastic paths controlling speed and dissipation

Natural processes occur in a finite amount of time and dissipate energy, entropy, and matter. Near equilibrium, thermodynamic intuition suggests that fast irreversible processes will dissipate more energy and entropy than slow quasistatic processes connecting the same initial and final states. For small systems, recently discovered thermodynamic speed limits suggest that faster processes will dissipate more than slower processes. Here, we test the hypothesis that this relationship between speed and dissipation holds for stochastic paths far from equilibrium. To analyze stochastic paths on finite timescales, we derive an exact expression for the path probabilities of continuous-time Markov chains from the path summation solution to the master equation. We present a minimal model for a driven system in which relative energies of the initial and target states control the speed, and the nonequilibrium currents of a cycle control the dissipation. Although the hypothesis holds near equilibrium, we find that faster processes can dissipate less under far-from-equilibrium conditions because of strong currents. This model serves as a minimal prototype for designing kinetics to sculpt the nonequilibrium path space so that faster paths produce less dissipation.

Phenylthiol-BODIPY-based supramolecular metallacycles for synergistic tumor chemo-photodynamic therapy

The development of more effective tumor therapy remains challenging and has received widespread attention. In the past decade, there has been growing interest in synergistic tumor therapy based on supramolecular coordination complexes. Herein, we describe two triangular metallacycles (1 and 2) constructed by the formation of pyridyl boron dipyrromethene (BODIPY)-platinum coordination. Metallacycle 2 had considerable tumor penetration, as evidenced by the phenylthiol-BODIPY ligand imparting red fluorescent emission at ∼660 nm, enabling bioimaging, and transport visualization within the tumor. Based on the therapeutic efficacy of the platinum(II) acceptor and high singlet oxygen (¹O₂) generation ability of BODIPY, 2 was successfully incorporated into nanoparticles and applied in chemo-photodynamic tumor therapy against malignant human glioma U87 cells, showing excellent synergistic therapeutic efficacy. A half-maximal inhibitory concentration of 0.35 μM was measured for 2 against U87 cancer cells in vitro. In vivo experiments indicated that 2 displayed precise tumor targeting ability and good biocompatibility, along with strong antitumor effects. This work provides a promising approach for treating solid tumors by synergistic chemo-photodynamic therapy of supramolecular coordination complexes.

Computational Modeling of Supramolecular Metallo-organic Cages-Challenges and Opportunities

Self-assembled metallo-organic cages have emerged as promising biomimetic platforms that can encapsulate whole substrates akin to an enzyme active site. Extensive experimental work has enabled access to a variety of structures, with a few notable examples showing catalytic behavior. However, computational investigations of metallo-organic cages are scarce, not least due to the challenges associated with their modeling and the lack of accurate and efficient protocols to evaluate these systems. In this review, we discuss key molecular principles governing the design of functional metallo-organic cages, from the assembly of building blocks through binding and catalysis. For each of these processes, computational protocols will be reviewed, considering their inherent strengths and weaknesses. We will demonstrate that while each approach may have its own specific pitfalls, they can be a powerful tool for rationalizing experimental observables and to guide synthetic efforts. To illustrate this point, we present several examples where modeling has helped to elucidate fundamental principles behind molecular recognition and reactivity. We highlight the importance of combining computational and experimental efforts to speed up supramolecular catalyst design while reducing time and resources.

Unexpected Self-Assembly Pathway to a Pd(II) Coordination Square-Based Pyramid and Its Preferential Formation beyond the Boltzmann Distribution

Experimental and theoretical investigations of the self-assembly process of a Pd(II) coordination M₆L₄ square-based pyramid (SP) were conducted. It was found that the probable self-assembly pathway, in which the dimerization of M₂L₂ with two M leads to SP, expected from the connectivity of the building blocks is not a major self-assembly pathway to the M₆L₄ SP. Whether the M₆L₄ SP is assembled or M₂L₂ is trapped is determined by an inter- or intramolecular reaction in a chain-like M₂L₂X, where X is a leaving ligand. The kinetically trapped state where the M₆L₄ SP is produced from M₂L₂ beyond the Boltzmann distribution was realized by a concentration-induced process and was kept for at least 2 months at 298 K.

Controlled Self-Assembly Mediated by the Complexation of Calixpyridinium: Diverse Assembled Morphology, Solid-State Fluorescence, and Iodine Capture Capacity

It is a great challenge to precisely control the molecules that self-assemble into diverse shapes with diverse properties. Herein, the self-assembled behaviors between calixpyridinium and two pyrenesulfonate guests, 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (PyTS) and sodium 1-pyrenesulfonate (PS), were studied. The morphology and property of the two assemblies were quite different. PS guests self-assembled into spherical aggregates upon complexation with calixpyridinium, while the self-assembled rodlike aggregates were formed via the binding between calixpyridinium and PyTS guests. The calixpyridinium-PS supramolecular aggregates could not emit fluorescence in the solid state, while a strong green fluorescence was emitted by the calixpyridinium-PyTS supramolecular aggregates in the solid state. More interestingly and importantly, the solid calixpyridinium-PyTS supramolecular aggregates exhibited an adsorbent ability to iodine in both the aqueous solution and the vapor phase, while the solid calixpyridinium-PS supramolecular aggregates could not capture iodine. The diverse iodine capture capability of the two supramolecular aggregates was determined by the self-assembled structure at the molecular level.

Coordination-Driven Self-Assembly of Cyclometalated Iridium Squares Using Linear Aromatic Diisocyanides

Here, we demonstrate facile [4 + 4] coordination-driven self-assembly of cyclometalated iridium(III) using linear aryldiisocyanide bridging ligands (BLs). A family of nine new [Ir(C^N)₂(μ-BL)]₄⁴⁺ coordination cages is described, where C^N is the cyclometalating ligand-2-phenylpyridine (ppy), 2-phenylbenzothiazole (bt), or 1-phenylisoquinoline (piq)-and BL is the diisocyanide BL, with varying spacer lengths between the isocyanide binding sites. These supramolecular coordination compounds are prepared via a one-pot synthesis, with isolated yields of 40-83%. ¹H NMR spectroscopy confirms the selective isolation of a single product, which is affirmed to be the M₄L₄ square by high-resolution mass spectrometry. Detailed photophysical studies were carried out to reveal the nature of the luminescent triplet states in these complexes. In most cases, phosphorescence arises from the [Ir(C^N)₂]⁺ nodes, with the emission color determined by the cyclometalating ligand. However, in two cases, the lowest-energy triplet state resides on the aromatic core of the BL, and weak phosphorescence from that state is observed. This work shows that aromatic diisocyanide ligands enable coordination-driven assembly of inert iridium(III) nodes under mild conditions, producing supramolecular coordination complexes with desirable photophysical properties.

Porphyrin-based metal coordination polymers with self-assembly pathway-dependent properties for photodynamic and photothermal therapy

During the self-assembly process of porphyrin-based metal coordination polymers, two different nanoaggregates were discovered. The nanoaggregates exhibited pathway-dependent properties and showed different applications in phototherapy.

Coordination Self-Assembly Processes Revealed by Collaboration of Experiment and Theory: Toward Kinetic Control of Molecular Self-Assembly

The importance of the collaboration of experiment and theory has been proven in many examples in science and technology. Here, such a new example is shown in the investigation of molecular self-assembly process, which is a complicated multi-step chemical reaction occurring in the reaction network composed of a huge number of intermediates. An experimental method, QASAP (quantitative analysis of self-assembly process), developed by us and a numerical approach, NASAP (numerical analysis of self-assembly process), that analyzes the experimental data obtained by QASAP to draw detail molecular self-assembly pathways, which was also developed by us, are introduced, and their application to the investigation of Pd(II)-mediated coordination assemblies are presented. Further, the possibility of the prediction of the outcomes of molecular self-assembly by varying the reaction conditions is also demonstrated. Finally, a future direction in the field of artificial molecular self-assembly based on pathway-dependent self-assembly, that is, kinetic control of molecular self-assembly is discussed.

Towards kinetic control of coordination self-assembly: a case study of a Pd3L6 double-walled triangle to predict the outcomes by a reaction network model

Numerical analysis of self-assembly process (NASAP) was performed for a [Pd3L6]6+ double-walled triangle (DWT) complex. With a chemical reaction network and a parameter set of the reaction rate constants obtained from a numerical search in an eighteen-dimensional parameter space to obtain a good fit to the data from the experimental counterpart (quantitative analysis of self-assembly process, QASAP), a refined calculation resulted in a detailed time evolution of each molecular species. Analysis based on those clues revealed dominant self-assembly pathways and a balance between inter- and intramolecular reactions, and enabled prediction of the reaction outcomes depending on the initial stoichiometric ratio under kinetic control.

Nucleation of Tiny Silver Nanoparticles by Using a Tetrafacial Organic Molecular Barrel: Potential Use in Visible-Light-Triggered Photocatalysis

Coordination-driven self-assembly of discrete molecular architectures of diverse shapes and sizes has been well studied in the last three decades. Use of dynamic imine bonds for designing analogous metal-free architectures has become a growing challenge recently. This article reports an organic molecular barrel (OB4^R ) as a potential template for nucleation and stabilization of very tiny (<1.5 nm) Ag nanoparticles (AgNPs). Imine bond condensation of a rigid tetra-aldehyde with a flexible diamine followed by imine-bond reduction yielded the discrete tetragonal organic barrel (OB4^R ). The presence of a molecular pocket ornamented with eight diamine moieties gives the potential for encapsulation of silver(I). The organic barrel was finally used as a molecular vessel for the controlled nucleation of silver nanoparticles (AgNPs) with fine size tuning through binding of Ag^I ions in the confined space of the barrel followed by reduction. Transmission electron microscopy (TEM) analysis of the Ag⁰ @OB4^R composite revealed that the mean particle size is 1.44±0.16 nm. The composite material has approximately 52 wt % silver loading. The barrel-supported ultrafine AgNPs [Ag⁰ @OB4^R ] are found to be an efficient photocatalyst for facile Ullmann-type aryl-amination coupling of haloarenes at ambient temperature without using any additives. The catalyst was stable for several cycles of reuse without any agglomeration. The new composite Ag⁰ @OB4^R represents the first example of discrete organic barrel-supported AgNPs employed as a photocatalyst in Ullmann-type coupling reactions at room temperature.

Coordination-Assembled Water-Soluble Anionic Lanthanide Organic Polyhedra for Luminescent Labeling and Magnetic Resonance Imaging

Lanthanide-containing functional complexes have found a variety of applications in materials science and biomedicine because of their unique electroptical and magnetic properties. However, the poor stability and solubility in water of multicomponent lanthanide organic assemblies significantly limit their practical applications. We report here a series of water-stable anionic Ln_2nL_3n-type (n = 2, 3, 4, and 5) lanthanide organic polyhedra (LOPs) constructed by deprotonation self-assembly of three fully conjugated ligands (H₄L¹ and H₄L^2a/b) featuring a 2,6-pyridine bitetrazolate chelating moiety. The outcomes of the LOPs formation reactions were found to be very sensitive toward the reaction conditions including base, metal source, solvents, and concentrations as characterized by a combination of NMR, high-resolution ESI-MS and X-ray crystallography. Ligands H₄L^2a/b manifested an excellent sensitization toward lanthanide ions (Ln = Eu^III and Tb^III), with high luminescent quantum yields for Tb₈L^2a₁₂ (Φ = 11.2% in water) and Eu₈L^2b₁₂ (Φ = 76.8% in DMSO) measured in polar solvents. Furthermore, due to the giant molecular weight and rigidity of the polyhedral skeleton, Gd₈L^2b₁₂ showed a very high longitudinal relaxivity (r₁) of 400.53 mM^-1S^-1. The performance of Gd₈L^2b₁₂ as potential magnetic resonance imaging contrast agents (CAs) in vivo was evaluated with much longer retention time in the tumor sites compared with the commercial Gd^III-based CAs. Dual-modal imaging potential has also been demonstrated with the mixed Eu/Gd LOPs. Our results not only provide a new design route toward water-stable multinuclear lanthanide organic assemblies but also offer potential candidates of supramolecular-edifices for bioimaging and drug delivery.

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.

Bifurcation of self-assembly pathways to sheet or cage controlled by kinetic template effect

The template effect is a key feature to control the arrangement of building blocks in assemblies, but its kinetic nature remains elusive compared to the thermodynamic aspects, with the exception of very simple reactions. Here we report a kinetic template effect in a self-assembled cage composed of flexible ditopic ligands and Pd(II) ions. Without template anion, a micrometer-sized sheet is kinetically trapped (off-pathway), which is converted into the thermodynamically most stable cage by the template anion. When the template anion is present from the start, the cage is selectively produced by the preferential cyclization of a dinuclear intermediate (on-pathway). Quantitative and numerical analyses of the self-assembly of the cage on the on-pathway revealed that the accelerating effect of the template is stronger for the early stage reactions of the self-assembly than for the final cage formation step itself, indicating the kinetic template effect.