Self-assembly of M4L4tetrahedral cages incorporating pendant PS and PSe functionalised ligands

The synthesis and study of metal–organic tetrahedral cages incorporating flexible thio- and selenophosphate-based ligands.

Structure-switching M3L2 Ir(iii) coordination cages with photo-isomerising azo-aromatic linkers

Cyclotriguaiacylene has been functionalised with 3- or 4-pyridyl-azo-phenyl groups to form a series of molecular hosts with three azobenzene-type groups that exhibit reversible photo-isomerisation. Reaction of the host molecules with [Ir(C^N)2(NCMe)2]+ where C^N is the cyclometallating 2-phenylpyridinato, 2-(4-methylphenyl)pyridinato or 2-(4,5,6-trifluorophenyl)pyridinato results in the self-assembly of a family of five different [{Ir(C^N)2}3(L)2]3+ coordination cages. Photo-irradiation of each of the cages with a high energy laser results in E → Z photo-isomerisation of the pyridyl-azo-phenyl groups with up to 40% of groups isomerising. Isomerisation can be reversed by exposure to blue light. Thus, the cages show reversible structure-switching while maintaining their compositional integrity. This represents the largest photo-induced structural change yet reported for a structurally-integral component of a coordination cage. Energy minimised molecular models indicate a switched cage has a smaller internal space than the initial all-E isomer. The [Ir(C^N)2(NCMe)2]+ cages are weakly emissive, each with a deep blue luminescence at ca. 450 nm.

Coordination Cages Based on Bis(pyrazolylpyridine) Ligands: Structures, Dynamic Behavior, Guest Binding, and Catalysis

We describe here a family of coordination cages with interesting structural, guest-binding, and catalytic properties. Flexible bridging ligands containing two bidentate pyrazolylpyridine termini assemble with transition-metal dications to afford coordination cages containing a metal ion at each vertex, a bridging ligand spanning each edge, and a 2:3 metal:ligand ratio. This stoichiometry is expressed in structures ranging from M₄L₆ tetrahedra to M₁₆L₂₄ tetracapped truncated tetrahedra, which are stabilized by the formation of π-stacked arrays between electron-rich and electron-poor ligand segments that form around the cage periphery. In some cases concentration- and/or temperature-dependent equilibria between multiple cage structures occur, arising from a balance between entropy, which favors the formation of a larger number of smaller assemblies, and enthalpy, which maximizes both interligand aromatic stacking and solvophobic effects in the larger assembles. The cages are hollow and can accommodate guests-often anions or solvent molecules-in the central cavity. For one cage family, M₈L₁₂ species with an approximately cubic structure and a ca. 400 ų cavity, the guest binding properties have been studied extensively. This cage can accommodate a wide range of neutral organic guests, with binding in water being driven principally by the hydrophobic effect, which leads to binding constants of up to 10⁸ M^-1. The accumulation of a large amount of empirical data on guest binding in the M₈L₁₂ cage in water provided the basis for a predictive tool for in silico screening of potential guests using the molecular docking program GOLD; this methodology has allowed the identification of numerous new guests with accurately predicted binding constants and provides a transformative new approach to exploring the host/guest chemistry of cages. Binding of benzisoxazole inside the M₈L₁₂ cage results in substantial rate enhancements-by a factor of up to 2 × 10⁵-of the Kemp elimination, in which benzisoxazole reacts to give 2-cyanophenolate. Catalysis arises because the 16+ cage cation accumulates anions around the surface by ion pairing, leading to a high effective concentration of hydroxide ions surrounding the guest even when the bulk pH is modest. Thus, the catalysis relies on the operation of two orthogonal interactions that bring the reaction partners together: hydrophobic guest binding in the cavity, which is lined with CH groups from the ligands, and ion pairing around the highly cationic cage surface. A consequence of this is that under some conditions the product of the cage-catalyzed Kemp elimination (the 2-cyanophenolate anion) itself accumulates around the cage surface and deprotonates another benzisoxazole guest, perpetuating the reaction in an autocatalytic manner. Thus, different anions accumulating around the cage can act as partners for reaction with a cavity-bound guest, opening up the possibility that the M₈L₁₂ cage can act as a general catalyst for reactions of electrophilic guests with surface-bound anions.

Sulfate Donor Based Dinuclear Heteroleptic Triple-Stranded Helicates from Sulfite and Ditopic Nitrogen Donor Ligands and Their Transformation to Dinuclear Homoleptic Double-Stranded Mesocates

Sulfate donor based supramolecular coordination complexes [{ fac-Re(CO)₃}(μ-SO₄)(L ⁿ)₂{ fac-Re(CO)₃}] (1-3) were obtained using ditopic N donors (L ⁿ; n = 1-3), NaHSO₃, and Re₂(CO)₁₀ in a one-pot, multicomponent, coordination-driven self-assembly approach, in which SO₃^2- becomes oxidized to SO₄^2- during the reaction and acts as a building framework. Complexes 1-3 were characterized using IR, ESI-TOF-MS, and ¹H NMR spectroscopy. The structures of complexes 1-3 were confirmed using single-crystal X-ray diffraction analysis. The transformation of the dinuclear heteroleptic triple-stranded helicate to the dinuclear homoleptic double-stranded mesocate [{Re(CO)₃Cl}₂(L ⁿ)₂] (L ⁿ = L¹, L², L³; 4a-6a) was achieved by the addition of BaCl₂. The direct treatment of Re(CO)₅X (X = Cl, Br) with L¹/L²/L³ yielded the dinuclear homoleptic double-stranded helicates [{Re(CO)₃X}₂ (L ⁿ)₂] (4b-6b and 7-9).

Pre-programmed self-assembly of polynuclear clusters

This perspective reviews our recent efforts towards the self-assembly of polynuclear clusters with ditopic and tritopic multidentate ligands HL1 (2-phenyl-4,5-bis{6-(3,5-dimethylpyrazol-1-yl)pyrid-2-yl}-1H-imidazole) and H2L2 (2,6-bis-[5-(2-pyridinyl)-1H-pyrazole-3-yl]pyridine), both of which are planar and rigid molecules. HL1 was found to be an excellent support for tetranuclear [Fe4] complexes, [FeII4(L1)4](BF4)4 ([FeII4]) and [FeIII2FeII2(L1)4](BF4)6 ([FeIII2FeII2]). The homovalent system was found to exhibit multistep spin crossover (SCO), while the mixed-valence [FeIII2FeII2] complex shows wavelength-dependent tuneable light-induced excited spin state trapping (LIESST). For H2L2, a variety of polynuclear complexes were obtained through complexation with different transition metal ions, allowing the isolation of rings, grids, and helix structures. The rigidity of the ligand, difference in its coordination sites, and affinity for different metal ions dictates its coordination behaviour. In this paper, we summarise these ligand pre-programmed self-assembled clusters and their diverse physical properties.

Synthesis and Characterization of Ferrocene Based Hemicages

We present a series of tripodal ligands L1-3, which fold into hemicages C1-3 by using coordination-driven dynamic combinational chemistry. The identities of these hemicages were characterized using ¹H NMR, ¹H-¹H COSY, DOSY, and ESI-TWIM-MS. Free rotation of the ferrocene structural units in the ligands affords an adaptable directionality, which is essential for the construction of these hemicages. Encapsulation of adamantane by C2 indicates the presence of a well-defined inner cavity as the binding pocket.

Chiral metallosupramolecular architectures

Over the past few decades, supramolecular chirality in discrete metallosupramolecular architectures has received considerable attention. In this review, a comprehensive summary of discrete, chiral coordination-driven structures, including helices, metallacycles, metallocages, etc., is presented. Although chirality can be introduced prior to, during or even after the coordination self-assembly process, this review puts major emphasis on the more recent development of metallosupramolecular architectures from chiral components, where chirality arises from the enantiopure or racemic scaffolds (bridging or auxiliary ligand). Special attention will be paid to homochiral metallo-assemblies using achiral components where chirality is obtained as a consequence of the twisting of the ligands. Additionally, the potential applications of homochiral metallosupramolecular architectures are also discussed. We hope that this review will be of interest to researchers attempting to design new elaborate homochiral metallosupramolecular architectures with even greater complexity and potential for functions such as chiral recognition, enantiomer separation, asymmetric catalysis, nonlinear sensors, and devices.

Imido-P(v) trianion supported enantiopure neutral tetrahedral Pd(ii) cages

Charge-neutral chiral hosts are attractive due to their ability to recognize a wide range of guest functionalities and support enantioselective processes. However, reports on such charge-neutral cages are very scarce in the literature. Here, we report an enantiomeric pair of tetrahedral Pd(ii) cages built from chiral tris(imido)phosphate trianions and oxalate linkers, which exhibit enantioselective separation capabilities for epichlorohydrin, β-butyrolactone, and 3-methyl- and 3-ethyl cyclopentanone.

Catalysis in a Cationic Coordination Cage Using a Cavity-Bound Guest and Surface-Bound Anions: Inhibition, Activation, and Autocatalysis

The Kemp elimination (reaction of benzisoxazole with base to give 2-cyanophenolate) is catalyzed in the cavity of a cubic M₈L₁₂ coordination cage because of a combination of (i) benzisoxazole binding in the cage cavity driven by the hydrophobic effect, and (ii) accumulation of hydroxide ions around the 16+ cage surface driven by ion-pairing. Here we show how reaction of the cavity-bound guest is modified by the presence of other anions which can also accumulate around the cage surface and displace hydroxide, inhibiting catalysis of the cage-based reaction. Addition of chloride or fluoride inhibits the reaction with hydroxide to the extent that a new autocatalytic pathway becomes apparent, resulting in a sigmoidal reaction profile. In this pathway the product 2-cyanophenolate itself accumulates around the cationic cage surface, acting as the base for the next reaction cycle. The affinity of different anions for the cage surface is therefore 2-cyanophenolate (generating autocatalysis) > chloride > fluoride (which both inhibit the reaction with hydroxide but cannot deprotonate the benzisoxazole guest) > hydroxide (default reaction pathway). The presence of this autocatalytic pathway demonstrates that a reaction of a cavity-bound guest can be induced with different anions around the cage surface in a controllable way; this was confirmed by adding different phenolates to the reaction, which accelerate the Kemp elimination to different extents depending on their basicity. This represents a significant step toward the goal of using the cage as a catalyst for bimolecular reactions between a cavity-bound guest and anions accumulated around the surface.

Anion-Directed Metallocages: A Study on the Tendency of Anion Templation

Self-assembly of Cu(NO₃ )₂ ⋅3 H₂ O and di(3-pyridylmethyl)amine (dpma) with addition of different acids (HNO₃ , HOAc, HCl, HClO₄ , HOTf, HPF₆ , HBF₄ , and H₂ SO₄ ) afforded a family of anion-templated tetragonal metallocages with a cationic prismatic structure of [(G^n- )⊂{Cu₂ (Hdpma)₄ }]^(8-n)+ (G^n- =NO₃^- , PF₆^- , SiF₆^2- ) with different ligating anions/solvents (NO₃^- , Cl^- , ClO₄^- , OTf^- , H₂ O) outside the cage. Systematic competitive experiments have rationalized the tendency of anion templation towards the formation of metallocages [(G^n- )⊂{Cu₂ (Hdpma)₄ }]^(8-n)+ as occurring in the order SiF₆^2- ≈PF₆^- >NO₃^- >SO₄^2- ≈ClO₄^- ≈BF₄^- . This sequence is mostly elucidated by shape control over size selectivity and electrostatic attraction between the cationic {Cu₂ (Hdpma)₄ }⁸⁺ host and the anionic guests. In addition, these results have also roughly ranked the anion coordination ability in the order Cl^- , ClO₄^- , OTf^- >NO₃^- >BF₄^- , CH₃ SO₄^- . Magnetic studies of metallocages 1 t and 2-4 suggest that the fitted magnetic interaction, being weakly magnetically coupled overall, is interpreted as a result of the combination of intracage ferromagnetic coupling integrals and intercage antiferromagnetic exchange; both contributions are very weak and comparable in strength.

Synthesis, structure, and excited state kinetics of heteroleptic Cu(i) complexes with a new sterically demanding phenanthroline ligand

In this report we describe the synthesis of a new phenanthroline ligand, 2,9-di(2,4,6-tri-isopropyl-phenyl)-1,10-phenanthroline (bL2) and its use as the blocking ligand in the preparation of two new heteroleptic Cu(i)diimine complexes. Analysis of the CuHETPHEN single crystal structures shows a distinct distortion from an ideal tetrahedral geometry around the Cu(i) center, forced by the secondary phenanthroline ligand rotating to accommodate the isopropyl groups of bL2. The increased steric bulk of bL2 as compared to the more commonly used 2,9-dimesityl-1,10-phenanthroline blocking ligand prohibits intramolecular ligand-ligand interaction, which is unique among CuHETPHEN complexes. The ground state optical and redox properties of CuHETPHEN complexes are responsive to the substitution on the blocking ligand even though the differences in structure are far removed from the Cu(i) center. Transient optical spectroscopy was used to understand the excited state kinetics in both coordinating and non-coordinating solvents following visible excitation. Substitution of the blocking phenanthroline ligand has a significant impact on the ³MLCT decay and can be used to increase the excited state lifetime by 50%. Electronic structure calculations established relationships between ground and excited state properties, and general entatic state concepts are discussed for copper photosensitizers. This work contributes to the growing library of CuHETPHEN complexes and broadens the fundamental understanding of their ground and excited state properties.

Oriented assembly of anisotropic nanoparticles into frame-like superstructures

It is fascinating but challenging for nanoscientists to organize nanoparticles (NPs) into ordered architectures just as it is for chemists to manipulate atoms and molecules to form functional molecules and supramolecules. We explore a strategy to assemble anisotropic NPs into open frame-like superstructures via oriented attachment (OA), experimentally realizing a nanoscale analog to the bonding behavior in M₈L₁₂-type supramolecular cubes. We highlight the role of NP shape in the OA-involved assembly for constructing predictable superstructures. In addition, the frame-like superstructures can retain their basic structure when undergoing postcrystallization of the building blocks as well as annealing for conversion toward functional electrocatalytic materials. Our work enables fundamental insights into directional "bonding" among NPs and adds to the growing body of knowledge for bottom-up assembly of anisotropic NPs into sophisticated functional materials.

Design Principles for the Optimization of Guest Binding in Aromatic-Paneled FeII4L6 Cages

A series of aromatic-paneled Fe^II₄L₆ cages was synthesized through iron(II)-templated subcomponent self-assembly of 2-formylpyridine and C₂-symmetric diamine building blocks having differing geometries, including many with a large degree of lateral offset between metal-binding sites. The new cages were characterized using X-ray crystallography, NMR spectroscopy, and mass spectrometry. Investigations of the guest binding properties of the cages provided insights into the structural factors important for the observation of guest binding. Both the size and arrangement of the aromatic panels were shown to be crucial for achieving effective encapsulation of large hydrophobic guests, including fullerenes, polycyclic aromatic hydrocarbons, and steroids, with subtle differences in the structure of subcomponents resulting in incommensurate effects on the binding abilities of the resulting hosts. Cages with large, offset aromatic panels were observed to be the most effective hosts as a result of a preference for a ligand conformation where the aromatic panels lie tangent to the edges of the tetrahedron, thus maximizing cavity enclosure.

Homochiral Self-Sorted and Emissive IrIII Metallo-Cryptophanes

The racemic ligands (±)-tris(isonicotinoyl)-cyclotriguaiacylene (L1), or (±)-tris(4-pyridyl-methyl)-cyclotriguaiacylene (L2) assemble with racemic (Λ,Δ)-[Ir(ppy)2 (MeCN)2 ]+ , in which ppy=2-phenylpyridinato, to form [{Ir(ppy)2 }3 (L)2 ]3+ metallo-cryptophane cages. The crystal structure of [{Ir(ppy)2 }3 (L1)2 ]⋅3BF4 has MM-ΛΛΛ and PP-ΔΔΔ isomers, and homochiral self-sorting occurs in solution, a process accelerated by a chiral guest. Self-recognition between L1 and L2 within cages does not occur, and cages show very slow ligand exchange. Both cages are phosphorescent, with [{Ir(ppy)2 }3 (L2)2 ]3+ having enhanced and blue-shifted emission when compared with [{Ir(ppy)2 }3 (L1)2 ]3+ .

Sequence-selective encapsulation and protection of long peptides by a self-assembled FeII8L6 cubic cage

Self-assembly offers a general strategy for the preparation of large, hollow high-symmetry structures. Although biological capsules, such as virus capsids, are capable of selectively recognizing complex cargoes, synthetic encapsulants have lacked the capability to specifically bind large and complex biomolecules. Here we describe a cubic host obtained from the self-assembly of FeII and a zinc-porphyrin-containing ligand. This cubic cage is flexible and compatible with aqueous media. Its selectivity of encapsulation is driven by the coordination of guest functional groups to the zinc porphyrins. This new host thus specifically encapsulates guests incorporating imidazole and thiazole moieties, including drugs and peptides. Once encapsulated, the reactivity of a peptide is dramatically altered: encapsulated peptides are protected from trypsin hydrolysis, whereas physicochemically similar peptides that do not bind are cleaved.

End-to-end azido-pinned interlocking lanthanide squares

A self-assembled end-to-end azido-pinned interlocking lanthanide square displays a record energy barrier of 152(4) K among lanthanide azido-bridged SMMs in a zero dc field.

Hydrophobic metallo-supramolecular Pd2L4 cages for zwitterionic guest encapsulation in organic solvents

Hydrophobic metallo-supramolecular cages are selectively encapsulating hydrophilic zwitterionic guests in organic solvents via synergetic multivalent recognition.

Functional metallosupramolecular architectures using 1,2,3-triazole ligands: it's as easy as 1,2,3 “click”

Self-assembled metallosupramolecular architectures generated using “click” ligands have become an increasingly popular area of inorganic chemistry.

Photoinduced energy- and electron-transfer from a photoactive coordination cage to bound guests

The array of naphthyl chromophores in a self-assembled cage can effect photoinduced energy- or electron-transfer to guests in the central cavity.

Metallosupramolecular 3D assembly of dimetallic Zn4[RuL2]2 and trimetallic Fe2Zn2[RuL2]2

A 3D trismetallo-macromolecule was assembled with a stepwise synthesized key metallo-organic ligand, which was created by a reaction on complex strategy.

Rhenium(I)-Based Monocyclic and Bicyclic Phosphine Oxide-Coordinated Supramolecular Complexes

Neutral, flexible ditopic phosphine (P-P) or phosphine oxide (O=P-P=O) donors, rigid anionic bis-chelating oxygen donors, and Re₂(CO)₁₀ were used to assemble ten phosphine oxide (P=O)-donor-based neutral monocyclic M₂LL'-, bicyclic M₄L₂L″-, and bicyclic M₄LL'₂-type supramolecular coordination complexes (SCCs). A soft ditopic phosphine donor was transformed into a hard ditopic phosphine oxide donor, during the formation of the cyclic complexes 1-3, 5-6, and 9-10. Complexes 4, 7, and 8 were obtained using a hard P=O donor ligand. These SCCs were characterized using elemental analysis, FTIR, NMR, and single-crystal X-ray diffraction analysis. The absorption properties of 1-8 were studied using absorption UV-vis spectroscopic methods, and the results were analyzed using theoretical calculations. The results revealed that the neutral P=O donor significantly influenced the photophysical properties by enhancing the absorption coefficient in the visible region.

Immobilizing Tetraphenylethylene into Fused Metallacycles: Shape Effects on Fluorescence Emission

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