Controlled Construction of Heteroleptic [Pd2 (LA )2 (LB )(LC )]4+ Cages: A Facile Approach for Site-Selective endo-Functionalization of Supramolecular Cavities

Multicomponent Metallacages via the Integrative Self-Assembly of Pt(II) Nodes with Multiple Pyridyl and Carboxylate Ligands

ConspectusIn recent years, multicomponent self-assembly has emerged as a pivotal strategy in supramolecular chemistry, enabling the construction of artificial systems with enhanced functionalities that surpass those of individual molecular components. These assemblies have garnered significant interest due to their potential applications in molecular recognition, catalysis, and biomedical engineering. However, achieving precise control over the assembly process remains a significant challenge, as increased structural complexity often results in thermodynamic mixtures, limiting their practical applications. In this context, metal-coordination-driven multicomponent self-assembly has emerged as a promising strategy, as the moderate strength and good directionality of metal-ligand bonds ensure the formation of discrete supramolecular architectures with well-defined structures and geometries. Notably, the integration of pyridyl and carboxylate donors with cis-Pt(II) nodes offers an effective approach for constructing multicomponent metallacages. This method is particularly attractive due to its ability to enable precise heteroleptic assembly, along with the accessibility and tunability of the ligands, which impart desirable photophysical properties and potential anticancer activities.This Account provides a comprehensive overview of our work on the design, preparation, and functionalization of multicomponent metallacages via the heteroleptic assembly of pyridyl and carboxylate ligands with cis-Pt(II) nodes. By strategically tailoring the ligand structures and adjusting the number of coordination sites, we have successfully constructed multicomponent metallacages with diverse geometries, including tetragonal and hexagonal prisms, triple-cavity ladders, truncated octahedra, and cyclic bis[2]catenanes, etc. In particular, we highlight the use of functional pyridyl ligands, such as tetraphenylethylene, hexaphenylbenzene, perylene diimide, and porphyrin derivatives, to explore the applications of these metallacages. Tetraphenylethylene and hexaphenylbenzene derivatives are propeller-shaped molecules with aggregation-induced emission properties, enhancing the emission of the metallacages both in solution and the solid state. Perylene diimide derivatives, with their electron-deficient, planar conjugated structures, contribute to strong emission in solution, thereby improving the host-guest chemistry and luminescent properties of the metallacages. Porphyrin derivatives, owing to their planar structures and excellent photosensitivity, endow the metallacages with significant photocatalytic capabilities. Additionally, other ligands, such as phenanthroline and triazine derivatives, have been employed to impart antibacterial and SO2 adsorption properties, respectively, to the metallacages. Furthermore, metallacage-cross-linked supramolecular networks exhibit enhanced mechanical properties and superior processability, making them promising candidates for functional materials. At the conclusion of this Account, we address the challenges and future perspectives in the development of multicomponent metallacages. We believe that ongoing research in this field will significantly advance the understanding of metal-coordination chemistry, supramolecular chemistry, and the development of functional supramolecular materials.

Pathway Engineering in Pd-Based Supramolecular Cage Synthesis via Inner-Outer Steric Synergy

For artificial supramolecular architectures designed to mimic biological systems, achieving different pathway synthesis is challenging due to the requirement of multiple stable and interconvertible intermediates. Here, we propose a novel "inner-outer steric synergy" strategy and investigate controllable pathway engineering for the synthesis of specific structures. Firstly, three structures (Ring-Pd2LA 2, Bowl-Pd2LA 3 and Cage-Pd2LA 4) with interconversion properties were selectively formed by assembling externally modified ligand LA with Pd(II). Furthermore, Ring-Pd2LA 2 can further assemble with the ligand LB with inner steric hindrance to generate heteroleptic trans-Pd2LA 2LB 2 cage, while Bowl-Pd2LA 3, as an intermediate, can assemble with LB to form Pd2LA 3LB. It is noteworthy that Ring-Pd2LA 2, Bowl-Pd2LA 3, and Cage-Pd2LA 4 can interconvert under specific conditions, enabling the synthesis of Pd2LA 3LB and trans-Pd2LA 2LB 2 through 10 and 16 pathways, respectively. This research not only introduces a novel strategy for constructing heteroleptic cages but also demonstrates the achievement of pathway engineering.

Configurational control of low-symmetry heteroleptic metal-organic cages with asymmetric ligands

Low-symmetry metal-organic cages (MOCs) can better mimic the structure of biological enzymes compared to high-symmetry MOCs, due to their unique internal cavities that resemble the specialized and irregular active sites of enzymes. In this study, two low-symmetry heteroleptic MOCs with six Pd(ii) centers, Pd6LA 6LB 6 and Pd6LB 6LC 6, were successfully constructed by combining two strategies: asymmetric ligand assembly and multi-ligand co-assembly. Crystallographic characterization and analysis revealed that Pd6LA 6LB 6 is a mixture of potentially 16 isomers. Introducing a methyl group at the ortho position of the coordination site of ligand LC induced steric hindrance, driving Pd6LB 6LC 6 to undergo a structural transformation and selectively assemble into a single dominant configuration from 13 potential isomers. This work not only demonstrates the immense potential of integrating asymmetric ligand assembly with multi-ligand co-assembly strategies but also highlights the critical role of steric effects in guiding assembly pathways and achieving precise configurational control in low-symmetry MOCs.

Single-Step Synthesis of a Heterometallic [Cu2PdL4]2+ Hybrid Metal-Organic Coordination Cage

Traditional methods of assembling low-symmetry heterometallic cage architectures are limited to stepwise construction and combinations of inert and labile metal ions, affording complex, anisotropic cage structures by sacrificing synthetic ease. Herein, a heterometallic [Cu2PdL4]2+ lantern-type cage has been assembled in a single self-assembly step through the use of a heteroditopic ligand with two different metal-binding groups. The resultant cage complex is a fusion of two common lantern-type cage motifs-carboxylate-based metal-organic Cu4L4 cages and pyridyl-based Pd2L4 coordination cages. Evidence for heterometallic cage formation in solution was provided by 1H and diffusion-ordered NMR spectroscopy and electrospray ionization mass spectrometry (ESIMS) data, whereas circular dichroism (CD) spectra confirmed the helical nature of the assembly. The heterometallic cage was then exploited in binding heterotopic guests. It is envisioned that the simple design strategy presented herein will ease the assembly of other structurally complex, low-symmetry cage architectures.

Precise functionalization in nano-confinement: a bottom-up approach to the evolution of selective molecular receptors

Precise molecular recognition depends on the delicate interplay between a guest molecule and a host possessing complementary functional groups. The de novo design of selective artificial receptors remains a formidable challenge, given the complexity of predicting these interactions. We present herein a bottom-up approach to the evolution of selective molecular receptors through precise endo-functionalization of a supramolecular cage. Internal functional groups were introduced within the heteroleptic palladium coordination cage in a site-precise fashion. With just five different functional groups, we successfully created a library of 32 isoreticular nano-cages, each featuring a unique micro-environment, by varying the nature, location and combination of endo-functional groups. The nano-cage exhibited adaptive recognition ability towards guest molecules of distinct geometries and hydrogen bonding capabilities. Titration experiments demonstrated that the binding affinity for a specific guest can be finely tuned and optimized by changing the endo-functional groups. As a proof of principle, by strategically screening our nano-cage library, we identified a receptor with high affinity and specificity for the dihydrogen phosphate guest. X-ray analysis and DFT calculation highlighted the pivotal role of the synergistic interactions among distinct endo-functional groups in achieving high-fidelity molecular recognition. This study is expected to provide a versatile solution for the bottom-up construction of tailor-made molecular receptors.

Solvent-Controlled Separation of Integratively Self-Sorted Pd2LA 2LB 2 Coordination Cages

The integrative implementation of multiple different components into metallosupramolecular self-assemblies requires sophisticated strategies to avoid the formation of statistical mixtures. Previously, the key focus was set on thermodynamically driven reactions of simple homoleptic into complex heteroleptic structures. Using Pd2LA 2LB 2-type coordination cages, we herein show that integrative self-sorting can be reversed by a change of solvent (from DMSO to MeCN) to favor narcissistic re-segregation into coexisting homoleptic species Pd2LA 4 and Pd3LB 6. Full separation ("unsorting") back to a mixture of the homoleptic precursors was finally achieved by selective precipitation of Pd3LB 6 with anionic guest G1 from MeCN, keeping pure Pd2LA 4 in solution. When a mixture of homoleptic Pd3LB 6 and heteroleptic Pd2LA 2LB 2 is exposed to a combination of two different di-anions (G1 and G2) in DMSO, selective guest uptake gives rise to two defined coexisting host-guest complexes. A joint experimental and deep theoretical investigation via liquid-state integral equation theory of the reaction thermodynamics on a molecular level accompanied by solvent distribution analysis hints at solvent expulsion from Pd2LA 4 to favor the formation of Pd2LA 2LB 2 in DMSO as the key entropic factor for determining the solvent-specific modulation of the cage conversion equilibrium.

A Combined Experimental and Computational Exploration of Heteroleptic cis-Pd2L2L'2 Coordination Cages through Geometric Complementarity

Heteroleptic (mixed-ligand) coordination cages are of interest as host systems with more structurally and functionally complex cavities than homoleptic architectures. The design of heteroleptic cages, however, is far from trivial. In this work, we experimentally probed the self-assembly of Pd(II) ions with binary ligand combinations in a combinatorial fashion to search for new cis-Pd2L2L'2 heteroleptic cages. A hierarchy of computational analyses was then applied to these systems with the aim of elucidating key factors for rationalising self-assembly outcomes. Simple and inexpensive geometric analyses were shown to be effective in identifying complementary ligand pairs. Preliminary results demonstrated the viability of relatively rapid semi-empirical calculations for predicting the topology of thermodynamically favoured assemblies with rigid ligands, whilst more flexible systems proved challenging. Stemming from this, key challenges were identified for future work developing effective computational forecasting tools for self-assembled metallo-supramolecular systems.

Orthogonal Phase Transfer of Oppositely Charged FeII 4L6 Cages

Coordination cages and their encapsulated cargo can be manoeuvred between immiscible liquid layers in a process referred to as phase transfer. Among the stimuli reported to drive phase transfer, counterion exchange is the most widespread. This method exploits the principle that counterions contribute strongly to the solubility preferences of coordination cages, and involves exchanging hydrophilic and hydrophobic counterions. Nevertheless, phase transfer of anionic cages remains relatively unexplored, as does selective phase transfer of individual cages from mixtures. Here we compare the phase transfer behaviour of two FeII 4L6 cages with the same size and geometry, but with opposite charges. As such, this study presents a rare example wherein an anionic cage undergoes phase transfer upon countercation exchange. We then combine these two cages, and demonstrate that their quantitative separation can be achieved by inducing selective phase transfer of either cage. These results represent unprecedented control over the movement of coordination cages between different physical compartments and are anticipated to inform the development of next-generation supramolecular systems.

Reactivity Inside Molecular Flasks: Acceleration Modes and Types of Selectivity Obtainable

There is increasing interest in the discovery and application of molecular flasks-supramolecular host structures capable of catalyzing organic reactions. Reminiscent of enzymes due to possessing a host cavity akin to an active site, molecular flasks can exhibit complex catalytic mechanisms and in many cases provide selectivity not achievable in bulk solvent. In this Review, we aim to organize the increasingly diverse examples through a two-part structure. In part one, we provide an overview of the different acceleration modes that operate within molecular flasks, while in part two we showcase, through selected examples, the different types of selectivity that are obtainable through the use of molecular flasks. Particular attention is given to examples that are relevant to current challenges in synthetic organic chemistry. We believe that this structure makes the field more approachable and thus will stimulate the development of novel applications of molecular flasks.

Ring-Size Control and Guest-Induced Circularly Polarized Luminescence in Heteroleptic Pd3A3B3 and Pd4A4B4 Assemblies

Two new structural motifs within the class of heteroleptic PdnAnBn assemblies, namely syn-cis-Pd3A3B3 bowls and bowl- (syn) or saddle- (anti) shaped cis-Pd4A4B4 rings are introduced. All of the ten examples share a common longer fluorenone-based bis-monodentate ligand, equipped with meta-pyridine donor groups. The ring size (3- vs. 4-membered) and conformational preference (bowl vs. saddle) are controlled by the choice of the shorter ligand. These carry para-pyridine donors, different aromatic backbones (benzene, thiophene or selenophene) and either no or small or bulky endohedral substituents, serving to control the nuclearity of the heteroleptic rings through different effects (ligand angle, charge distribution or backbone bulk). Moreover, the luminescence of the fluorenone ligand is conserved in the formed architectures. Emission intensity as well as host-guest properties vary depending on the inward-pointing functions. All Pd3A3B3 assemblies are shown to bind chiral guest BINOL bis-sulfonate which imparts its chirality to the entire host-guest complex. This results in a guest-induced circular dichroism (CD) and circularly polarized luminescence (CPL) with dissymmetry factor glum up to 10-3.

Highly Anisotropic Pd2Lab 2Lcc 2 and Pd2Lab 2Lcd 2 Type Cages by Heteromeric Completive Self-Sorting

Pd(II)-based low-symmetry coordination cages possessing anisotropic cavities are of great interest. The common strategies employed to achieve such cages utilize either more than one type of symmetrical ligands (e.g., Laa, Lbb etc.) or only one type of unsymmetrical ligand (e.g., Lab). To significantly enhance the anisotropy, we have designed two unsymmetrical bidentate ligands i.e., Lab and Lcd, aiming at a low-symmetry Pd2Lab 2Lcd 2-type cage. It was accomplished by high-fidelity integrative self-sorting of two different low-symmetry cages having Pd2Lab 4 and Pd4Lcd 8-type architectures (homoleptic complexes of the designed ligands). Structural constraints and geometry complementarity in the ligand design drive the non-statistical exclusive self-assembly of the Pd2Lab 2Lcd 2-type cage. By taking advantage of the complemental geometries between ligands, a low-symmetry Pd2Lab 2Lcc 2-type cage was also obtained. Heteromeric completive self-sorting of three homoleptic assemblies (Pd2Lab 4, Pd4Lcc 8 and Pd4Lcd 8-type cages) into an exclusive mixture of Pd2Lab 2Lcd 2 and Pd2Lab 2Lcc 2-type mixed ligated assemblies was demonstrated through cage-to-cage transformations.

Shape-Complementary Multicomponent Assembly of Low-Symmetry Co(III)Salphen-Based Coordination Cages

While metal-mediated self-assembly is a popular technique to construct discrete nanosized objects, highly symmetric structures, built from one type of ligand at a time, are dominating reported systems. The tailored integration of a set of different ligands requires sophisticated approaches to avoid narcissistic separation or formation of statistical mixtures. Here, we demonstrate how the combination of three structure-guiding effects (metal-templated macrocyclization, additional bridging ligands and shape-complementarity) based on Co(III)salphen metal nodes allows for a rational and high-yielding synthesis of structurally complex, lantern-shaped cages with up to four differentiable bridges. Three new heteroleptic coordination cages based on dinuclear Co(III)salphen macrocycles were synthesized in a one-pot reaction approach and fully characterized, including single crystal X-ray analyses. One cage groups two of the same ligands, another two different ligands around a symmetric Co2-bis-salphen ring. In the most complex structure, this ring is unsymmetric, rendering all four connections between the two metal centers distinguishable. While heteroleptic assembly around Pd(II) nodes has been shown to be dynamic, beneficial for cage-to-cage transformations, assembly cascades and adaptive systems, the herein introduced cages based on kinetically more inert Co(III)salphen will be advantageous for applications in enzyme-like catalysis and molecular machinery that require enhanced structural and chemical stability.

Non-statistical assembly of multicomponent [Pd2ABCD] cages

Self-assembled hosts, inspired by biological receptors and catalysts, show application potential in sustainable synthesis, energy conversion and medicine. Implementing multiple functionalities in the form of distinguishable building blocks, however, is difficult without risking narcissistic self-sorting or a statistical mess. Here we report a systematic series of integratively self-assembled heteroleptic cages in which two square-planar PdII cations are bridged by four different bis-pyridyl ligands, A, B, C and D, via synergistic effects to exclusively form a single isomer-the lantern-shaped cage [Pd2ABCD]. This self-sorting goal-forming just one out of 55 possible structures-is reached under full thermodynamic control and can be realized progressively (by combining progenitors, such as [Pd2A2C2] with [Pd2B2D2]), directly from ligands and PdII cations or by mixing all four corresponding homoleptic cages. The rational design of complex multicomponent assemblies that enables the modular incorporation of diverse chemical moieties will advance their applicability in functional nanosystems.

Heteromeric Completive Self-Sorting in Coordination Cage Systems

Heteroleptic coordination cages, nonstatistically assembled from a set of matching ligands, can be obtained by mixing individual components or via cage-to-cage transformations from homoleptic precursors. Based on the latter approach, we here describe a new level of self-sorting in coordination cage systems, namely, 'heteromeric completive self-sorting'. Here, two heteroleptic assemblies of type Pd2A2B2 and Pd2A2C2, sharing one common ligand component A but differing in the other, are shown to coexist in solution. This level of self-sorting can be reached either from a statistical mixture of assemblies based on some ligands B and C or, alternatively, following a first step of integrative self-sorting giving a distinct Pd2B2C2 intermediate. While subtle enthalpic factors dictate the outcome of the self-sorting, we found that it is controllable. From a unique set of three ligands, we demonstrate the transition from strict integrative self-sorting forming a Pd2AB2C cage to heteromeric completive self-sorting to give Pd2A2B2 and Pd2A2C2 by variation of the ligand ratio. Cage-to-cage transformations were followed by NMR and MS experiments. Single crystal X-ray structures for three new heteroleptic cages were obtained, impressively highlighting the versatility of ligand A to either form a π-stacked trans-figure-of-eight arrangement in Pd2A2B2 or occupy two cis-edges in Pd2A2C2 or only a single edge in Pd2AB2C. This study paves the way toward the control of heteroleptic cage populations in a systems chemistry context with emerging features such as chemical information processing, adaptive guest selectivity, or stimuli-responsive catalytic action.

Triamine and Tetramine Edge-Length Matching Drives Heteroleptic Triangular and Tetragonal Prism Assembly

Heteroleptic metal-organic capsules, which incorporate more than one type of ligand, can provide enclosed, anisotropic interior cavities for binding low-symmetry molecules of biological and industrial importance. However, the selective self-assembly of a single mixed-ligand architecture, as opposed to the numerous other possible self-assembly outcomes, remains a challenge. Here, we develop a design strategy for the subcomponent self-assembly of heteroleptic metal-organic architectures with anisotropic internal void spaces. Zn6Tet3Tri2 triangular prismatic and Zn8Tet2Tet'4 tetragonal prismatic architectures were prepared through careful matching of the side lengths of the tritopic (Tri) or tetratopic (Tet, Tet') and panels.

Assembly of Six Types of Heteroleptic Pd2L4 Cages under Kinetic Control

Heteroleptic assemblies composed of several kinds of building blocks have been seen in nature. It is still unclear how natural systems design and create such complicated assemblies selectively. Past efforts on multicomponent self-assembly of artificial metal-organic cages have mainly focused on finding a suitable combination of building blocks to lead to a single multicomponent self-assembly as the thermodynamically most stable product. Here, we present another approach to selectively produce multicomponent Pd(II)-based self-assemblies under kinetic control based on the selective ligand exchanges of weak Pd-L coordination bonds retaining the original orientation of the metal centers in a kinetically stabilized cyclic structure and on local reversibility given in certain areas of the energy landscape in the presence of the assist molecule that facilitates error correction of coordination bonds. The kinetic approach enabled us to build all six types of Pd2L4 cages and heteroleptic tetranuclear cages composed of three kinds of ditopic ligands. Although the cage complexes thus obtained are metastable, they are stable for 1 month or more at room temperature.

Diastereoselective Self-Assembly of Low-Symmetry Pdn L2n Nanocages through Coordination-Sphere Engineering

Metal-organic cages (MOCs) are popular host architectures assembled from ligands and metal ions/nodes. Assembling structurally complex, low-symmetry MOCs with anisotropic cavities can be limited by the formation of statistical isomer libraries. We set out to investigate the use of primary coordination-sphere engineering (CSE) to bias isomer selectivity within homo- and heteroleptic Pdn L2n cages. Unexpected differences in selectivities between alternative donor groups led us to recognise the significant impact of the second coordination sphere on isomer stabilities. From this, molecular-level insight into the origins of selectivity between cis and trans diastereoisomers was gained, highlighting the importance of both host-guest and host-solvent interactions, in addition to ligand design. This detailed understanding allows precision engineering of low-symmetry MOC assemblies without wholesale redesign of the ligand framework, and fundamentally provides a theoretical scaffold for the development of stimuli-responsive, shape-shifting MOCs.

A Lantern-Shaped Pd(II) Cage Constructed from Four Different Low-Symmetry Ligands with Positional and Orientational Control: An Ancillary Pairings Approach

One of the key challenges of metallo-supramolecular chemistry is to maintain the ease of self-assembly but, at the same time, create structures of increasingly high levels of complexity. In palladium(II) quadruply stranded lantern-shaped cages, this has been achieved through either 1) the formation of heteroleptic (multi-ligand) assemblies, or 2) homoleptic assemblies from low-symmetry ligands. Heteroleptic cages formed from low-symmetry ligands, a hybid of these two approaches, would add an additional rich level of complexity but no examples of these have been reported. Here we use a system of ancillary complementary ligand pairings at the termini of cage ligands to target heteroleptic assemblies: these complementary pairs can only interact (through coordination to a single Pd(II) metal ion) between ligands in a cis position on the cage. Complementarity between each pair (and orthogonality to other pairs) is controlled by denticity (tridentate to monodentate or bidentate to bidentate) and/or hydrogen-bonding capability (AA to DD or AD to DA). This allows positional and orientational control over ligands with different ancillary sites. By using this approach, we have successfully used low-symmetry ligands to synthesise complex heteroleptic cages, including an example with four different low-symmetry ligands.