Self-assembled conjoined-cages

High-Throughput Computational Evaluation of Low Symmetry Pd2 L4 Cages to Aid in System Design*

Unsymmetrical ditopic ligands can self-assemble into reduced-symmetry Pd2 L4 metallo-cages with anisotropic cavities, with implications for high specificity and affinity guest-binding. Mixtures of cage isomers can form, however, resulting in undesirable system heterogeneity. It is paramount to be able to design components that preferentially form a single isomer. Previous data suggested that computational methods could predict with reasonable accuracy whether unsymmetrical ligands would preferentially self-assemble into single cage isomers under constraints of geometrical mismatch. We successfully apply a collaborative computational and experimental workflow to mitigate costly trial-and-error synthetic approaches. Our rapid computational workflow constructs unsymmetrical ligands and their Pd2 L4 cage isomers, ranking the likelihood for exclusively forming cis-Pd2 L4 assemblies. From this narrowed search space, we successfully synthesised four new, low-symmetry, cis-Pd2 L4 cages.

Discrete Self-Assembled Metallo-Foldamers with Heteroleptic Sequence Specificity

Discrete and structurally diverse foldamer sequences are constructed in both natural and abiotic systems primarily using inert connectivity with irreversible organic covalent bonds, serving to preserve the identity of the sequence. The formation of sequences under thermodynamic control using labile coordination bonds would be attractive for synthetic ease and modular capability, but this presents issues regarding sequence preservation. Here is presented an approach integrating palladium(II) metal ions into the sequence itself, with fidelity maintained through use of complementary pairings of ligand arrangements at the metal centre. This is accomplished using sites of different denticity and/or hydrogen bonding capability. In this fashion, discrete and ordered metallo-sequences are formed as thermodynamic products in a single step, and these then fold into defined conformations due to π-π interactions between electron-rich and -poor aromatic regions of the combined componentry.

Comparison of [Pd2L4][BF4]4 cages for binding of n-octyl glycosides and nitrate (L = isophthalamide or dipicolinamide linked dipyridyl ligand)

Two dipyridyl ligands were synthesized, where the pyridyl donor fragments were separated by an isophthalamide (1) or a dipicolinamide moiety (2). Both ligands formed [Pd2(Ligand)4][BF4]4 complexes in CD2Cl2 containing 5% dmso-d6. It was found that while [Pd2(1)4][BF4]4 readily binds to n-octyl glycosides and to nitrate anions, [Pd2(2)4][BF4]4 did not. The difference in binding properties could be rationalized based on the reduced flexibility and size of the [Pd2(2)4]2+ cage and/or stronger interior binding of a BF4- counter anion.

Tuning the Size and Geometry of Heteroleptic Coordination Cages by Varying the Ligand Bent Angle

Spherical assemblies of the type [Pd_n L_2n ]²ⁿ⁺ can be obtained from Pd^II salts and curved N-donor ligands, L. It is well established that the bent angle, α, of the ligand is a decisive factor in the self-assembly process, with larger angles leading to complexes with a higher nuclearity, n. Herein, we report heteroleptic coordination cages of the type [Pd_n L_n L'_n ]²ⁿ⁺ , for which a similar correlation between the ligand bent angle and the nuclearity is observed. Tetranuclear cages were obtained by combining [Pd(CH₃ CN)₄ ](BF₄ )₂ with 1,3-di(pyridin-3-yl)benzene and ligands featuring a bent angle of α=120°. The use of a dipyridyl ligand with α=149° led to the formation of a hexanuclear complex with a trigonal prismatic geometry; for linear ligands, octanuclear assemblies of the type [Pd₈ L₈ L'₈ ]¹⁶⁺ were obtained. The predictable formation of heteroleptic Pd^II cages from 1,3-di(pyridin-3-yl)benzene and different dipyridyl ligands is evidence that there are entire classes of heteroleptic cage structures that are privileged from a thermodynamic point of view.

Unique Dynamics of Hierarchical Constrained Macromolecular Ligands on Coordination Nanocage Surface Promotes Facile and Precise Assembly of Polymers

With access to the solution structures of nanocomposites of coordination nanocages (CNCs) via scattering and chromatography techniques, their mysterious solution dynamics have been, for the first time, resolved, and interestingly, the surface macromolecules can be substituted by extra free macromolecules in solutions. Obvious exchange of macromolecules can be observed in the solution mixtures of CNC nanocomposites at high temperatures, revising the understanding of the dynamics of CNC nanocomposites. Being distinct from nanocomposites of a simple coordination complex, the quantified solution dynamics of CNC nanocomposites indicates a typical logarithmic time dependence with the dissociation of surface macromolecules as the thermodynamically limiting step, suggesting strongly coupled and hierarchically constrained dynamics among the surface macromolecules. Their dynamics can be activated only upon application of high temperature or selected solvents, and therefore, the rational design of polymer assemblies, for example, hybrid-arm star polymers with precisely controlled compositions and reprocessable, robust CNC-cross-linked supramolecular polymer networks, is facilitated.

Mechanochemical Release of Non-Covalently Bound Guests from a Polymer-Decorated Supramolecular Cage

Supramolecular coordination cages show a wide range of useful properties including, but not limited to, complex molecular machine-like operations, confined space catalysis, and rich host-guest chemistries. Here we report the uptake and release of non-covalently encapsulated, pharmaceutically-active cargo from an octahedral Pd cage bearing polymer chains on each vertex. Six poly(ethylene glycol)-decorated bipyridine ligands are used to assemble an octahedral PdII6 (TPT)4 cage. The supramolecular container encapsulates progesterone and ibuprofen within its hydrophobic nanocavity and is activated by shear force produced by ultrasonication in aqueous solution entailing complete cargo release upon rupture, as shown by NMR and GPC analyses.

Increasing structural and functional complexity in self-assembled coordination cages

Progress in metallo-supramolecular chemistry creates potential to synthesize functional nano systems and intelligent materials of increasing complexity. In the past four decades, metal-mediated self-assembly has produced a wide range of structural motifs such as helicates, grids, links, knots, spheres and cages, with particularly the latter ones catching growing attention, owing to their nano-scale cavities. Assemblies serving as hosts allow application as selective receptors, confined reaction environments and more. Recently, the field has made big steps forward by implementing dedicated functionality, e.g. catalytic centres or photoswitches to allow stimuli control. Besides incorporation in homoleptic systems, composed of one type of ligand, desire arose to include more than one function within the same assembly. Inspiration comes from natural enzymes that congregate, for example, a substrate recognition site, an allosteric regulator element and a reaction centre. Combining several functionalities without creating statistical mixtures, however, requires a toolbox of sophisticated assembly strategies. This review showcases the implementation of function into self-assembled cages and devises strategies to selectively form heteroleptic structures. We discuss first examples resulting from a combination of both principles, namely multicomponent multifunctional host-guest complexes, and their potential in application in areas such as sensing, catalysis, and photo-redox systems.

Integrative Assembly of Heteroleptic Tetrahedra Controlled by Backbone Steric Bulk

A bent fluorenone-based dipyridyl ligand LA reacts with PdII cations to a solvent-dependent dynamic library of [PdnL2n] assemblies, constituted by a [Pd3LA6] ring and a [Pd4LA8] tetrahedron as major components, and a [Pd6LA12] octahedron as minor component. Introduction of backbone steric hindrance in ligand LB allows exclusive formation of the [Pd6LB12] octahedron. Combining equimolar amounts of both ligands results in integrative self-sorting to give an unprecedented [Pd4LA4LB4] heteroleptic tetrahedron. Key to the non-statistical assembly outcome is exploiting the structural peculiarity of the [Pd4L8] tetrahedral topology, where the four lean ligands occupy two doubly bridged edges and the bulky ligands span the four remaining, singly bridged edges. Hence, the system finds a compromise between the entropic drive to form an assembly smaller than the octahedron and the enthalpic prohibition of pairing two bulky ligands on the same edge of the triangular ring. The emission of luminescent LA is maintained in both homoleptic [Pd3LA6] and heteroleptic [Pd4LA4LB4].

Multi-stimuli Control over Assembly and Guest Binding in Metallo-supramolecular Hosts Based on Dithienylethene Photoswitches

It is difficult to assemble multi-component metallo-supramolecular architectures in a non-statistical fashion, which limits their development toward functional materials. Herein, we report a system of interconverting bowls and cages that are able to respond to various selective stimuli (light, ligands, anions), based on the self-assembly of a photochromic dithienylethene (DTE) ligand, L^a, with Pd^II cations. By combining the concept of “coordination sphere engineering”, relying on bulky quinoline donors, with reversible photoswitching between the ligand’s open (o-L^a) and closed (c-L^a) forms, a [Pd₂(o-L^a)₄] cage (o-C) and a [Pd₂(c-L^a)₃] bowl (c-B) were obtained, respectively. This structural rearrangement modulates the system’s guest uptake capabilities. Among three bis-sulfonate guests (G1, G2, and G3), the cage can encapsulate only the smallest (G1), while the bowl binds all of them. Bowl c-B was further used to synthesize a series of heteroleptic cages, [Pd₂L^A₃L^B], representing a motif never reported before. Additional ligands (L^c-f), with short or long arms, tune the cavity size, thus enabling or preventing guest uptake. Addition of Br^–/Ag⁺ makes it possible to change the overall charge, again triggering guest uptake and release, as well as fourth ligand de-/recomplexation. In combination, site-selective introduction of functionality and application of external stimuli lead to an intricate system of hosts with different guest preferences. A high degree of complexity is achieved through cooperativity between only a few components.

Identification of a Heteroleptic Pd6L6L'6 Coordination Cage by Screening of a Virtual Combinatorial Library

The design of structurally defined heteroleptic coordination cages is a challenging task, and only few examples are known to date. Here we describe a selection approach that allowed the identification of a novel hexanuclear Pd cage containing two types of dipyridyl ligands. A virtual combinatorial library of [Pd_nL_2n](BF₄)_2n complexes was prepared by mixing six different dipyridyl ligands with substoichiometric amounts of [Pd(CH₃CN)₄](BF₄)₂. Analysis of the equilibrated reaction mixture revealed the preferential formation of a heteroleptic [Pd₆L₆L'₆](BF₄)₁₂ assembly. The complex was prepared on a preparative scale by a targeted synthesis, and its structure was elucidated by single-crystal X-ray diffraction. It features an unprecedented trigonal-antiprismatic cage structure with two triangular Pd₃L₃ macrocycles bridged by six L' ligands. A related but significantly larger [Pd₆L₆L'₆](BF₄)₁₂ cage was obtained by using metalloligands instead of organic dipyridyl ligands.

Guest-Reaction Driven Cage to Conjoined Twin-Cage Mitosis-Like Host Transformation

We report here a guest-reaction-induced mitosis-like host transformation from a known Pd₄ L₂ cage 1 to a conjoined Pd₆ L₃ twin-cage 2 featuring two separate cavities. The encapsulation of 1-hydroxymethyl-2-naphthol (G1), a known ortho-quinone methide (o-QMs) precursor, within the hydrophobic cavity of cage 1 is found crucial to realize the cage to twin-cage conversion. Confined G1 molecules within the nanocavity undergo self-coupling dimerization reaction to form 2,2'-dihydroxy-1,1'-dinaphthylmethane (G2) which then triggers the cage to twin-cage mitosis. The same conversion also proceeds, in a much faster rate, via the direct templation of G2, confirming the induced-fit transformation mechanism. The structure of the (G2)₂ ⊂2 host-guest complex has been established by X-ray crystallographic study, where cis- to trans- conformational switch on one bridging ligand is revealed.

Backbone-Bridging Promotes Diversity in Heteroleptic Cages

The combination of shape-complementary bis-monodentate ligands L^A and L^B with Pd^II cations yields heteroleptic cages cis-[Pd₂ L^A ₂ L^B ₂ ] by self-sorting. Herein, we report how such assemblies can be diversified by introduction of covalent backbone bridges between two L^A units. Together with solvent and guest effects, the flexibility of these linkers can modulate nuclearity, topology, and number of cavities in a family of four structurally diverse assemblies. Ligand L^A1 , with flexible linker, reacts in CH₃ CN with its L^B counterpart to a tetranuclear dimer D1. In DMSO, however, a trinuclear pseudo-tetrahedron T1 is formed. The product of L^A2 , with rigid linker, looks similar to D1, but with a rotated ligand arrangement. In presence of an anionic guest, this dimer D2 transforms and a hexanuclear prismatic barrel P2 crystallizes. We demonstrate how controlling a ligand's coordination mode can trigger structural differentiation and increase complexity in metallo-supramolecular assembly.

Self-Assembly of a Redox Active, Metallosupramolecular [Pd3 L6 ]6+ Complex Using a Rotationally Flexible Ferrocene Ligand

A new ferrocene-containing [Pd₃ (L_4EFc )₆ ]⁶⁺ (X^- )₆ (C ⋅ BF₄ and C ⋅ SbF₆ where X=BF₄ ^- or SbF₆ ^- ) self-assembled double-walled triangle has been synthesized from the known, rotationally flexible, 1,1'-bis(4-pyridylethynyl)ferrocene ligand (L_4EFc ), and characterized by ¹ H, ¹³ C and diffusion ordered (DOSY) NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), X-ray crystallography and cyclic voltammetry (CV). The molecular structures confirmed that double-walled triangle cage systems (C ⋅ BF₄ and C ⋅ SbF₆ ) were generated. C ⋅ BF₄ was shown to interact with the anionic guest, p-toluenesulfonate. CV experiments revealed that the triangles were redox active, however addition of the guest did not influence the redox potentials.

Flexibility and anion exchange of [(X)@Pd2L4] cages for recognition of size and charge of polyatomic anions

The self-assembly of Pd(NO3)2 with L (L = 1,2-bis(dimethyl(pyridin-3-yl)silyl)ethane) gives rise to [PdL2](NO3)2 in high yields. Anion exchange of [PdL2](NO3)2 with X- (X- = BF4-, ClO4-, and PF6-) changes the skeleton into a cage of [(X)@Pd2L4](X)3. Successive anion exchange of [(X)@Pd2L4](X)3 (X- = BF4-, ClO4-, and PF6-) with X- (X- = ReO4- and SiF62-) produces [(ReO4)@Pd2L4](ReO4)3 and [(SiF6)@Pd2L4](SiF6), respectively, irrespective of anion charge. The flexible nature and conformation of cages are significantly dependent on the nestled polyatomic anions. Thus, this system can be used as a molecular recognizer of the size and charge of ubiquitous polyatomic anions.

Directional asymmetry over multiple length scales in reticular porous materials

In nature and synthetic materials, asymmetry is a useful tool to create complex and functional systems constructed from a limited number of building blocks. Reticular chemistry has allowed the synthesis of a wide range of discrete and extended structures, from which modularity permits the controlled assembly of their constituents to generate asymmetric configurations of pores or architectures. In this perspective, we present the different strategies to impart directional asymmetry over nano/meso/macroscopic length scales in porous materials and the resulting novel properties and applications.

Design strategies for the controlled assembly of discrete and extended reticular materials with asymmetric configurations of pores or architectures.

Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118-119). However, there is still a vivid interest in coordination compounds based on imine ligands. The aim of this paper is to review the most recent concepts on construction of homo- and hetero-oligonuclear Schiff base coordination compounds narrowed down to the less frequently considered complexes of platinum group metals (PGM). The combination of SB and PGM in oligonuclear entities has several advantages over mononuclear or polynuclear species. Such complexes usually exhibit better electroluminescent, magnetic and/or catalytic properties than mononuclear ones due to intermetallic interactions and frequently have better solubility than polymers. Various construction strategies of oligodentate imine ligands for coordination of PGM are surveyed including simple imine ligands, non-innocent 1,2-diimines, chelating imine systems with additional N/O/S atoms, classic N2O2-compartmental Schiff bases and their modifications resulting in acyclic fused ligands, macrocycles such as calixsalens, metallohelical structures, nano-sized molecular wheels and hybrid materials incorporating mesoionic species. Co-crystallization and formation of metallophilic interactions to extend the mononuclear entities up to oligonuclear coordination species are also discussed.

Exploiting the labile site in dinuclear [Pd2L2]n+ metallo-cycles: multi-step control over binding affinity without alteration of core host structure

Synthetic metallosupramolecular systems have generally been binary (on/off) when they have control over molecular recognition. This report details a dipalladium(ii) system with four-step graduated control over recognition for a guest.

Diverse anion exchange of pliable [X2@Pd3L4]4+ double cages: a molecular ruler for recognition of polyatomic anions

Reaction of Pd(BF4)2 with L (L = bis(pyridin-3-yl-propyl)pyridine-3,5-dicarboxylate) in the 1 : 2 mole ratio gives rise to a spiro-type [PdL2]·(BF4)2·2C6H6·2CH3CN, and further self-assembly of [PdL2]·(BF4)2·2C6H6·2CH3CN with Pd(NO3)2 in the 2 : 1 mole ratio in Me2SO at 90 °C produces a uniquely pliable double cage of [(NO3)2(H2O)2@Pd3L4](BF4)4·6C3H7NO. Both the encapsulated NO3- and the outside BF4- anions are exchanged by X- to form [(X)2@Pd3L4](X')4 (X- = PF6-, ClO4-, and/or NO3-; X'- = BF4-, PF6-, ClO4-, and NO3-) with all-inclusive pure or mixed anions. The pliable and characteristic properties of the double cages were confirmed by anion exchange of the nestled or outside anions in the present study. This system can be used as a ruler for recognition of ubiquitous polyatomic anions.