Guest-induced transformation of a porphyrin-edged Fe(II)4L6 capsule into a Cu(I)Fe(II)2L4 fullerene receptor

Guest-Induced Reversible Transformation between an Azulene-Based Pd2L4 Lantern-Shaped Cage and a Pd4L8 Tetrahedron

Azulene, a blue structural isomer of naphthalene, is introduced as the backbone for a new family of Pd(II)-based self-assemblies. Three organic ligands, equipped with varying donor groups, produce three [Pd2L4] cages of different cavity dimensions. Unexpectedly, the addition of organic disulfonate guests to the smallest lantern-shaped cage (featuring pyridine donors) led to a rapid and quantitative transformation to a distorted-tetrahedral [Pd4L8] species. On the contrary, [Pd2L4] cages formed from ligands with isoquinoline donors either just encapsulated the guests or showed no interaction. The tetrahedral species could be fully reverted back to its original [Pd2L4] topology by capturing the guest by another, stronger binding [Pd2L'4] coordination cage, narcissistically self-sorting from the first cage. The azulenes, serving as colored hydrocarbon backbones of minimal atom count, allow one to follow cage assembly and guest-induced transformation by the naked eye. Furthermore, we propose that their peculiar electronic structure influences the system's assembly behavior.

Recent update on the electroactive oligopyrrolic macrocyclic hosts with a Bucky-ball heart

Supramolecular chemistry is a multidisciplinary research area mostly associated with the investigation of host-guest interactions within intricate three-dimensional (3D) molecular architectures held together reversibly by various non-covalent interactions. Continuous efforts to develop such kinds of complex host-guest systems with designer oligopyrrolic macrocyclic receptors are a rapidly growing research domain, which is deeply involved in applied supramolecular chemistry research. These host-guest supramolecular complexes can be constructed by combining suitable electron-rich oligopyrrolic donors (as a host) with complementary electron-poor guests (as acceptors), held together by the ionic force of attraction triggered by intermolecular charge/electron transfer (CT/ET) transitions. Some of these resulting CT/ET ensembles are potential candidates for the construction of efficient optoelectronic materials, optical sensors, molecular switches, etc. In this Feature Article we aim to focus on these supramolecular ensembles composed by size and shape complementary electroactive oligopyrrolic molecular containers, which are suitable for spherical guest (e.g., buckminsterfullerene) complexation. We also provide a "state-of-the-art" overview on plausible applications of these particular host-guest systems. Our aim is to cover only specific electron-rich tetrathiafulvalene (TTF)-based oligopyrrolic receptors, e.g., TTF-calix[4]pyrroles, TTF-cryptands, TTF-porphyrins and exTTF-porphyrin-based molecular motifs reported to date, along with a brief outlining of their "functional behaviour" in materials chemistry research.

Synthesis of an Adaptable Molecular Barrel and Guest Mediated Stabilization of Its Metastable Higher Homologue

Structural and functional modulation of three-dimensional artificial macromolecular systems is of immense importance. Designing supramolecular cages that can show stimuli mediated reversible switching between higher-order structures is quite challenging. We report here construction of a Pd₆ trifacial barrel (1) by coordination self-assembly. Surprisingly, barrel 1 was found to exhibit guest-responsive behavior. In presence of fullerenes C₆₀ and C₇₀, 1 unprecedentedly transformed to its metastable higher homologue Pd₈ tetrafacial barrel (2), forming stable host-guest complexes (C₆₀)₃⊂2 and (C₇₀)₂⊂2, respectively. Again, encapsulated fullerenes could be extracted from the cavity of 2 using 1,2-dichlorobenzene, leading to its facile conversion to the parent trifacial barrel 1. Such reversible structural interconversion between an adaptable molecular barrel and its guest stabilized higher homologue is an uncommon observation.

Photo- and Temperature-Induced Reversible Structural Transformation between Dodecanuclear and Pentadecanuclear Gold(I) Sulfido Complexes

Stimuli-responsive structural transformation has attracted much attention for its potential to mimic the behavior of biological transformations and functions. Here, two unprecedented dodecanuclear and pentadecanuclear gold(I) sulfido clusters (denoted trans-Au₁₂ and trans-Au₁₅, respectively) with impressive stimuli-responsive interconversion have been obtained by taking advantage of the judiciously designed tridentate phosphine ligand L_trans as the building block. Both UV light and temperature can be applied to trigger the structural conversions between trans-Au₁₂ and trans-Au₁₅. In addition, NMR, high-resolution electrospray ionization mass spectrometry, and UV-vis absorption spectroscopy have been employed to monitor the transformation process and decipher the mechanism of structural conversion. This work not only provides a paradigm to investigate photo-induced cluster-to-cluster transformation based on polydentate phosphine ligands but also offers a new direction for the construction of the stimuli-responsive materials.

An Organic Molecular Nanobarrel that Hosts and Solubilizes C60

Organic cages have gained increasing attention in recent years as molecular hosts and porous materials. Among these, barrel-shaped cages or molecular nanobarrels are promising systems to encapsulate large hosts as they possess windows of the same size as their internal cavity. However, these systems have received little attention and remain practically unexplored despite their potential. Herein, we report the design and synthesis of a new trigonal prismatic organic nanobarrel with two large triangular windows with a diameter of 12.7 Å optimal for the encapsulation of C₆₀ . Remarkably, this organic nanobarrel shows a high affinity for C₆₀ in solvents in which C₆₀ is virtually insoluble, providing stable solutions of C₆₀ .

Gram-scale synthesis of a covalent nanocage that preserves the redox properties of encapsulated fullerenes

Discrete nanocages provide a way to solubilize, separate, and tune the properties of fullerenes, but these 3D receptors cannot usually be synthesized easily from inexpensive starting materials, limiting their utility. Herein, we describe the first fullerene-binding nanocage (Cage⁴⁺) that can be made efficiently on a gram scale. Cage⁴⁺ was prepared in up to 57% yield by the formation of pyridinium linkages between complemantary porphyrin components that are themselves readily accessible. Cage⁴⁺ binds C₆₀ and C₇₀ with large association constants (>10⁸ M⁻¹), thereby solubilizing these fullerenes in polar solvents. Fullerene association and redox-properties were subsequently investigated across multiple charge states of the host-guest complexes. Remarkably, neutral and singly reduced fullerenes bind with similar strengths, leaving their 0/1⁻ redox couples minimally perturbed and fully reversible, whereas other hosts substantially alter the redox properties of fullerenes. Thus, C₆₀@Cage⁴⁺ and C₇₀@Cage⁴⁺ may be useful as solubilized fullerene derivatives that preserve the inherent electron-accepting and electron-transfer capabilities of the fullerenes. Fulleride dianions were also found to bind strongly in Cage⁴⁺, while further reduction is centered on the host, leading to lowered association of the fulleride guest in the case of C₆₀²⁻.

This report describes the first gram-scale synthesis of a nanocage that can host fullerenes (C₆₀ and C₇₀). The redox properties of the fullerenes are preserved in this host, enabling characterization of complexes with fulleride anions and dianions.

Beyond Platonic: How to Build Metal-Organic Polyhedra Capable of Binding Low-Symmetry, Information-Rich Molecular Cargoes

The field of metallosupramolecular chemistry has advanced rapidly in recent years. Much work in this area has focused on the formation of hollow self-assembled metal-organic architectures and exploration of the applications of their confined nanospaces. These discrete, soluble structures incorporate metal ions as ‘glue’ to link organic ligands together into polyhedra.Most of the architectures employed thus far have been highly symmetrical, as these have been the easiest to prepare. Such high-symmetry structures contain pseudospherical cavities, and so typically bind roughly spherical guests. Biomolecules and high-value synthetic compounds are rarely isotropic, highly-symmetrical species. To bind, sense, separate, and transform such substrates, new, lower-symmetry, metal-organic cages are needed. Herein we summarize recent approaches, which taken together form the first draft of a handbook for the design of higher-complexity, lower-symmetry, self-assembled metal-organic architectures.

Subphthalocyanine capsules: molecular reactors for photoredox transformations of fullerenes

The internal cavity formed by a dimeric subphthalocyanine (SubPc) capsule (SubPc₂Pd₃, 2), ensembled by coordination of pyridyl substituents in the monomeric SubPc 1 to Pd centers, has proved an optimal space for the complexation of C₆₀ fullerene. Taking advantage of the intense absorption of green light of the SubPc component at around 550 nm, we have tested different green-light induced photoredox addition reactions over the double bonds of guest C₆₀. Both addition of amine radicals, generated by reductive quenching of the excited state of 2 by aromatic trimethylsilylamines, and addition of trifluoroethyl radicals, obtained from oxidative quenching of the photosensitizer, have successfully taken place with good yields in the 2:C₆₀ host:guest complex. On the other hand, both the photoredox reactions result in much lower yields when the monomeric pyridyl-SubPc is used as a photocatalyst, demonstrating that encapsulation results in a strong acceleration of the reaction. Importantly, this is the first example of the use of a confined microenvironment to trigger photoredox chemical transformations of fullerenes.

A photoredox cage built by coordination of two pyridyl-subphthalocyanines to Pd centers has proved versatile and efficient to catalyze photoredox addition reactions over encapsulated C₆₀.

Silver(I)-Carbene Bond-Directed Rigidification-Induced Emissive Metallacage for Picric Acid Detection

A new triphenylamine-based tetraimidazolium salt L was developed for silver(I)-carbene bond-directed synthesis of tetranuclear silver(I) octacarbene ([Ag₄(L)₂](PF₆)₄) metallacage 1. Interestingly, after assembly formation, metallacage 1 showed a nine-fold emission enhancement in dilute solution while ligand L was weakly fluorescent. This is attributed to the rigidity induced to the system by metal-carbene bond formation where the metal center acts as a rigidification unit. The enhanced emission intensity in dilute solution and the presence of the triphenylamine core made 1 a potential candidate for recognition of picric acid (PA). This recognition can be ascribed to the dual effect of ground-state charge-transfer complex formation and resonance energy transfer between the picrate and metallacage 1. For metallacage 1, a considerable detection limit toward PA was observed. The use of such metal-carbene bond-directed rigidification-induced enhanced emission for PA sensing is noteworthy.

A self-assembled M2L2 truncated square and its application as a container for fullerenes

An unprecedented bisthianthrene dipyridyl ligand was designed and synthesized for coordination driven self-assembly. The combination of this conformationally dynamic linker with a 90° convergent metal corner exclusively afforded a novel M₂L₂ truncated square-like metallamacrocycle. The single crystal X-ray structure reveals a belt-shaped geometry with a cavity diameter of ca. 13.7 Å. The breathable cavity and electron-rich thianthreno panels enable the highly efficient binding of the fullerenes C₆₀ (K_a = 5.1 × 10⁶ M^-1) and C₇₀ (K_a = 3.7 × 10⁶ M^-1). The encapsulation of C₆₀ is fully confirmed by NMR, mass spectroscopy and single crystal X-ray diffraction analyses. The cyclic voltammograms further reveal that the truncated square is redox active. The strong binding affinity, adaptive complexation, and redox activity of the thianthrene-incorporated metallamacrocycle provide new insights into the design of supramolecular hosts.

A Flexible Synthetic Strategy for the Preparation of Heteroleptic Metallacycles of Porphyrins

We present a stepwise synthetic strategy for the preparation of the unprecedented heteroleptic 2+2 neutral metallacycle [{t,c,c-RuCl₂(CO)₂}₂(4'cisDPyP)(3'cisDPyP)] (5), in which two different 5,10-meso-dipyridylporphyrins, 4'cisDPyP [i.e., 5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin] and 3'cisDPyP [i.e., 5,10-bis(3'-pyridyl)-15,20-diphenylporphyrin], are joined through equal 90°-angular Ru(II) connectors. The synthesis of 5 was accomplished through the preparation of a reactive ditopic intermediate in which one of the two pyridylporphyrins is linked to two neutral ruthenium fragments, each having one residual readily available coordination site (a dmso-O). Thus, compound 5 was obtained under mild conditions through two complementary routes: either by treatment of [{t,c,c-RuCl₂(CO)₂(dmso-O)}₂(4'cisDPyP)] (3) with 1 equiv of 3'cisDPyP or, alternatively, by treatment of [{t,c,c-RuCl₂(CO)₂(dmso-O)}₂(3'cisDPyP)] (4) with 1 equiv of 4'cisDPyP. Heteroleptic metallacycle 5 was isolated in pure form in acceptable yield and fully characterized. Spectroscopic data and a molecular model show that 5 has an L-shaped geometry, with the two porphyrins almost orthogonal to one another. The modular approach that we established is highly flexible and opens the way to several possible exciting developments.

A Self-Assembled Palladium(II) Barrel for Binding of Fullerenes and Photosensitization Ability of the Fullerene-Encapsulated Barrel

Fullerene extracts obtained from fullerene soot lack their real application due to their poor solubility in common solvents and difficulty in purification. Encapsulation of these extracts in a suitable host is an important approach to address these issues. We present a new Pd₆ barrel (1), which is composed of three 1,4-dihydropyrrolo[3,2-b]pyrrole panels, clipped through six cis-Pd^II acceptors. Large open windows and cavity make it an efficient host for a large guest. Favorable interactions between the ligand and fullerene (C₆₀ and C₇₀ ) allows the barrel to encapsulate fullerene efficiently. Thorough investigation reveals that barrel 1 has a stronger binding affinity towards C₇₀ over C₆₀ , resulting in the predominant extraction of C₇₀ from a mixture of the two. Finally, the fullerene encapsulated barrels C₆₀ ⊂1 and C₇₀ ⊂1 were found to be efficient for visible-light-induced singlet oxygen generation. Such preferential binding of C₇₀ and photosensitizing ability of C₆₀ ⊂1 and C₇₀ ⊂1 are noteworthy.

Controlled Self-Assembly and Multistimuli-Responsive Interconversions of Three Conjoined Twin-Cages

Stimuli-responsive structural transformations between discrete coordination supramolecular architectures not only are essential to construct smart functional materials but also provide a versatile molecular-level platform to mimic the biological transformation process. We report here the controlled self-assembly of three topologically unprecedented conjoined twin-cages, i.e., one stapled interlocked Pd₁₂L₆ cage (2) and two helically isomeric Pd₆L₃ cages (3 and 4) made from the same cis-blocked palladium corners and a new bis-bidentate ligand (1). While cage 2 features three mechanically coupled cavities, cages 3 and 4 are topologically isomeric helicate-based twin-cages based on the same metal/ligand stoichiometry. Sole formation of cage 2 or a dynamic mixture of cages 3 and 4 can be controlled by changing the solvents employed during the self-assembly. Structural conversions between cages 3 and 4 can be triggered by changes in both temperature/solvent and induced-fit guest encapsulations. Well-controlled interconversion between such topologically complex superstructures may lay a solid foundation for achieving a variety of functions within a switchable system.

Self-Assembly of Metallo-Supramolecules with Dissymmetrical Ligands and Characterization by Scanning Tunneling Microscopy

Asymmetrical and dissymmetrical structures are widespread and play a critical role in nature and life systems. In the field of metallo-supramolecular assemblies, it is still in its infancy for constructing artificial architectures using dissymmetrical building blocks. Herein, we report the self-assembly of supramolecular systems based on two dissymmetrical double-layered ligands. With the aid of ultra-high-vacuum, low-temperature scanning tunneling microscopy (UHV-LT-STM), we were able to investigate four isomeric structures corresponding to four types of binding modes of ligand LA with two major conformations complexes A. The distribution of isomers measured by STM and total binding energy of each isomer obtained by density functional theory (DFT) calculations suggested that the most abundant isomer could be the most stable one with highest total binding energy. Finally, through shortening the linker between inner and outer layers and the length of arms, the arrangement of dissymmetrical ligand LB could be controlled within one binding mode corresponding to the single conformation for complexes B.

Porphyrin bearing phenothiazine pincers as hosts for fullerene binding via concave–convex complementarity: synthesis and complexation study

Free base porphyrin hosts, m-(PTZ)4-H2P and p-(PTZ)4-H2P, tethered with four phenothiazine moieties at the meso-position via a flexible ethoxy phenyl linker were synthesized and used for the selective complexation of fullerenes, C60 and C70.

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.

A Fullerene-Based Molecular Torsion Balance for Investigating Noncovalent Interactions at the C60 Surface

To investigate the nature and strength of noncovalent interactions at the fullerene surface, molecular torsion balances consisting of C₆₀ and organic moieties connected through a biphenyl linkage were synthesized. NMR and computational studies show that the unimolecular system remains in equilibrium between well-defined folded and unfolded conformers owing to restricted rotation around the biphenyl C-C bond. The energy differences between the two conformers depend on the substituents and is ascribed to differences in the intramolecular noncovalent interactions between the organic moieties and the fullerene surface. Fullerenes favor interacting with the π-faces of benzenes bearing electron-donating substituents. The correlation between the folding free energies and corresponding Hammett constants of the substituents in the arene-containing torsion balances reflects the contributions of the electrostatic interactions and dispersion force to face-to-face arene-fullerene interactions.

An atropisomeric M2L4 cage mixture displaying guest-induced convergence and strong guest emission in water

Introduction of atropisomeric axes into a bent bispyridine ligand leads to the quantitative formation of a complex mixture of atropisomeric M2L4 cages upon treatment with metal ions. Whereas the isomer ratio of the obtained cage mixture, consisting of up to 42 isomers, is insensitive to temperature and solvent, the quantitative convergence from the mixture to a single isomer is accomplished upon encapsulation of a large spherical guest, namely fullerene C60. The observed isomerization with other guests depends largely on their size and shape (e.g., <10 and 82% convergence with planar triphenylene and bowl-shaped corannulene guests, respectively). Besides the unusual guest-induced convergence, the present cage mixture displays the strongest guest emission (Φ F = 68%) among previously reported M n L m cages and capsules, upon encapsulation of a BODIPY dye in water.

Selective guest recognition by a metallo-organic phthalocyanine-based host

In this manuscript, we have studied the selectivity in the complexation of fullerene species by a Fe[Formula: see text]Pc[Formula: see text] metallo-organic helicate (1) assembled using a bidentate phthalocyanine (Pc) as ligand. The large aromatic internal surface of this helicate shows a strong selectivity towards the encapsulation of C[Formula: see text] from a mixture of C[Formula: see text] and C[Formula: see text]. On the other hand, a bisimidazole-containing naphthalenediimide was used to perform guest exchange experiments over [fullerene [Formula: see text] 1] complexes, taking advantage of the strong coordination bond of the imidazole ring to the Zn centers in the Pc cavity.

Heterocomponent ternary supramolecular complexes of porphyrins: A review

Porphyrins are prominent host molecules which are widely used due to their structural characteristics and directional interaction sites. This review summarizes non-covalently bound ternary complexes of porphyrins, constructed from at least three non-identical species. Progress in supramolecular chemistry allows the creation of complex molecular machinery tools, such as rotors, motors and switches from relatively simple structures in a single self-assembly step. In the current review, we highlight the collection of sophisticated molecular ensembles including sandwich-type complexes, cages, capsules, tweezers, rotaxanes, and supramolecular architectures mediating oxygen-binding and oxidation reactions. These diverse structures have high potential to be applied in sensing, production of new smart materials as well as in medical science.

A pH-Switchable Triple Hydrogen-Bonding Motif

A stimuli responsive linear hydrogen bonding motif, capable of in situ protonation and deprotonation, has been investigated. The interactions of the responsive hydrogen bonding motif with complementary partners were examined through a series of 1H NMR experiments, revealing that the recognition preference of the responsive hydrogen bonding motif in a mixture can be switched between two states.

Selective detection of DABCO using a supramolecular interconversion as fluorescence reporter

The quantitative double self-sorting between the three-component rectangle [Cu₄(1)₂(2)₂]⁴⁺ and the four-component sandwich complex [Cu₂(1)(2)(4)]²⁺ is triggered by inclusion and release of DABCO (4). The fully reversible and clean switching between two multicomponent supramolecular architectures can be monitored by fluorescence changes at the zinc porphyrin sites. The structural changes are accompanied by a huge spatial contraction/expansion of the zinc porphyrin–zinc porphyrin distances that change from 31.2/38.8 Å to 6.6 Å and back. The supramolecular interconversion was used for the highly selective detection of DABCO in a mixture of other similar compounds.

Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls, and Heteroleptic Pills Binding One or Two Fullerenes

Fullerenes and their derivatives are of tremendous technological relevance. Synthetic access and application are still hampered by tedious purification protocols, peculiar solubility, and limited control over regioselective derivatization. We present a modular self-assembly system based on a new low-molecular-weight binding motif, appended by two palladium(II)-coordinating units of different steric demands, to either form a [Pd₂L¹₄]⁴⁺ cage or an unprecedented [Pd₂L²₃(MeCN)₂]⁴⁺ bowl (with L¹ = pyridyl, L² = quinolinyl donors). The former was used as a selective induced-fit receptor for C₆₀. The latter, owing to its more open structure, also allows binding of C₇₀ and fullerene derivatives. By exposing only a fraction of the bound guests’ surface, the bowl acts as fullerene protecting group to control functionalization, as demonstrated by exclusive monoaddition of anthracene. In a hierarchical manner, sterically low-demanding dicarboxylates were found to bridge pairs of bowls into pill-shaped dimers, able to host two fullerenes. The hosts allow transferring bound fullerenes into a variety of organic solvents, extending the scope of possible derivatization and processing methodologies.

Rare Example of Stereoisomeric 2 + 2 Metallacycles of Porphyrins Featuring Chiral-at-Metal Octahedral Ruthenium Corners

In this paper, we describe three new stereoisomers of the already known 2 + 2 metallacycle of porphyrins [ trans, cis, cis-RuCl₂(CO)₂(4' cisDPyP)]₂ (2, 4' cisDPyP = 5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin), namely [{ trans,cis,cis-RuCl₂(CO)₂}(4' cisDPyP)₂{ cis,cis,cis-RuCl₂(CO)₂}] (14) and [ cis,cis,cis-RuCl₂(CO)₂(4' cisDPyP)]₂ (15), in which the chiral { cis,cis,cis-RuCl₂(CO)₂} fragment has either a C or A handedness. The least abundant 15 exists as a mixture of two stereoisomers defined as alternate (15_alt, both porphyrins are trans to a Cl and a CO) and pairwise (15_pw, one porphyrin is trans to two chlorides and the other to two carbonyls), each one as a statistical mixture of meso ( AC) and racemic ( AA and CC) diastereomers. Remarkably, both 14 and 15 are-to the best of our knowledge-unprecedented examples of 2D metallacycles with octahedral chiral-at-metal connectors, and 14 is the first example of a 2 + 2 molecular square with stereoisomeric Ru(II) corners. Whereas 2 is selectively obtained by treatment of trans,cis,cis-RuCl₂(CO)₂(dmso-O)₂ (1) with 4' cisDPyP, 14 and 15 were obtained, together with 2 (major product), using stereoisomers of 1, either cis,cis,trans-RuCl₂(CO)₂(dmso-S)₂ (5) or cis,cis,cis-RuCl₂(CO)₂(dmso)₂ (6), as precursors. From a general point of view, this work demonstrates that-even for the smallest 2 + 2 metallacycle and using a symmetric organic linker-several stereoisomers can be generated when using octahedral metal connectors of the type {MA₂B₂} that are not stereochemically rigid. As a proof-of-concept, it also opens the way to new-even though challenging-opportunities: unprecedented and yet unexplored chiral metallosupramolecular assemblies can be obtained and eventually exploited (e.g., for supramolecular catalysis) by using stereogenic octahedral metal connectors amenable to become chiral centers.

Supramolecular chirality transformation driven by monodentate ligand binding to a coordinatively unsaturated self-assembly based on C 3-symmetric ligands

Monodentate ligand binding is facilitated by supramolecular chirality transformations from propeller-shaped chirality into single-twist chirality by altering the self-assembly of C 3-symmetric chiral ligands. The C 3-symmetric chiral ligands (Im R 3Bz and Im S 3Bz) contain three chiral imidazole side arms (Im R  and Im S ) at the 1,3,5-positions of a central benzene ring. Upon coordination to zinc ions (Zn2+), which have a tetrahedral coordination preference, the C 3-symmetric chiral ligands assemble, in a stepwise manner, into a propeller-shaped assembly with a general formula (Im( R or S ) 3Bz)4(Zn2+)3. In this structure each Zn2+ ion coordinates to the three individual imidazole side arms. The resulting assembly is formally coordinatively unsaturated (coordination number, n = 3) and capable of accepting monodentate co-ligands (imidazole: ImH2) to afford a coordinatively saturated assembly [(ImH2)3(Im R 3Bz)4(Zn2+)3]. The preformed propeller-shaped chirality is preserved during this transformation. However, an excess of the monodentate co-ligand (ImH2/Zn2+ molar ratio of ∼1.7) alters the propeller-shaped assembly into a stacked dimer assembly [(ImH2) m (Im R 3Bz)2(Zn2+)3] (m = 4-6) with single-twist chirality. This switch alters the degree of enhancement and the circular dichroism (CD) pattern, suggesting a structural transition into a chiral object with a different shape. This architectural chirality transformation presents a new approach to forming dynamic coordination-assemblies, which have transformable geometric chiral structures.

Supramolecular Self-Sorting Networks using Hydrogen-Bonding Motifs

A current objective in supramolecular chemistry is to mimic the transitions between complex self-sorted systems that represent a hallmark of regulatory function in nature. In this work, a self-sorting network, comprising linear hydrogen motifs, was created. Selecting six hydrogen-bonding motifs capable of both high-fidelity and promiscuous molecular recognition gave rise to a complex self-sorting system, which included motifs capable of both narcissistic and social self-sorting. Examination of the interactions between individual components, experimentally and computationally, provided a rationale for the product distribution during each phase of a cascade. This reasoning holds through up to five sequential additions of six building blocks, resulting in the construction of a biomimetic network in which the presence or absence of different components provides multiple unique pathways to distinct self-sorted configurations.

Size-Selective Hydroformylation by a Rhodium Catalyst Confined in a Supramolecular Cage

Size-selective hydroformylation of terminal alkenes was attained upon embedding a rhodium bisphosphine complex in a supramolecular metal-organic cage that was formed by subcomponent self-assembly. The catalyst was bound in the cage by a ligand-template approach, in which pyridyl-zinc(II) porphyrin interactions led to high association constants (>105  m-1 ) for the binding of the ligands and the corresponding rhodium complex. DFT calculations confirm that the second coordination sphere forces the encapsulated active species to adopt the ee coordination geometry (i.e., both phosphine ligands in equatorial positions), in line with in situ high-pressure IR studies of the host-guest complex. The window aperture of the cage decreases slightly upon binding the catalyst. As a result, the diffusion of larger substrates into the cage is slower compared to that of smaller substrates. Consequently, the encapsulated rhodium catalyst displays substrate selectivity, converting smaller substrates faster to the corresponding aldehydes. This selectivity bears a resemblance to an effect observed in nature, where enzymes are able to discriminate between substrates based on shape and size by embedding the active site deep inside the hydrophobic pocket of a bulky protein structure.

Photocatalytic Hydrogen Evolution by a Synthetic [FeFe] Hydrogenase Mimic Encapsulated in a Porphyrin Cage

The design of a biomimetic and fully base metal photocatalytic system for photocatalytic proton reduction in a homogeneous medium is described. A synthetic pyridylphosphole-appended [FeFe] hydrogenase mimic was encapsulated inside a supramolecular zinc porphyrin-based metal-organic cage structure Fe4 (Zn-L)6 . The binding is driven by the selective pyridine-zinc porphyrin interaction and results in the catalyst being bound strongly inside the hydrophobic cavity of the cage. Excitation of the capsule-forming porphyrin ligands with visible light while probing the IR spectrum confirmed that electron transfer takes place from the excited porphyrin cage to the catalyst residing inside the capsule. Light-driven proton reduction was achieved by irradiation of an acidic solution of the caged catalyst with visible light.

Functional Capsules via Subcomponent Self-Assembly

Coordination-driven self-assembly can produce large, symmetrical, hollow cages that are synthetically easy to access. The functions provided by these aesthetically attractive structures provide a driving force for their development, enabling practical applications. For instance, cages have provided new methods of molecular recognition, chirality sensing, separations, stabilization of reactive species, and catalysis. We have fruitfully employed subcomponent self-assembly to prepare metal-organic capsules from simple building blocks via the simultaneous formation of dynamic coordinative (N→metal) and covalent (N═C) bonds. Design strategies employ multidentate pyridyl-imine ligands to define either the edges or the faces of polyhedral structures. Octahedral metal ions, such as Fe^II, Co^II, Ni^II, Zn^II, and Cd^II, constitute the vertices. The generality of this technique has enabled the preparation of capsules with diverse three-dimensional structures. This Account highlights how fundamental investigations into the host-guest chemistry of capsules prepared through subcomponent self-assembly have led to the design of useful functions and new applications. We start by discussing simple host-guest systems involving a single capsule and continue to systems that include multiple capsules and guests, whose interactions give rise to complex functional behavior. Many of the capsules presented herein bind varied neutral guests, including aromatic or aliphatic molecules, biomolecules, and fullerenes. Binding selectivity is influenced by solvent effects, weak non-covalent interactions between hosts and guests, and the size, shape, flexibility, and degree of surface enclosure of the inner spaces of the capsules. Some hosts are able to adaptively rearrange structurally or express a different ratio of cage diastereomers to optimize the guest binding ability of the system. In other cases the bound guest can be either protected from degradation or catalytically transformed through encapsulation. Other capsules bind anions, most often in organic solvents and occasionally in water. Complexation is usually driven by a combination of electrostatic interactions, hydrogen bonding, and coordination to additional metal centers. Anion binding can also induce cage diastereomeric reconfiguration in a similar manner to some neutral guests, illustrating the general ability of subcomponent self-assembled capsules to respond to stimuli due to their dynamic nature. Capsules have been developed as supramolecular extractants for the selective removal of anions from water and as channels for transporting anions through planar lipid bilayers and into vesicles. Different capsules may work together, allowing for functions more complex than those achievable within single host-guest systems. Incorporation of stimuli-responsive capsules into multicage systems allows individual capsules within the network to be addressed and may allow signals to be passed between network members. We first present strategies to achieve selective guest binding and controlled guest release using mixtures of capsules with varied affinities for guests and different stabilities toward external stimuli. We then discuss strategies to separate capsules with encapsulated cargos via selective phase transfer, where the solvent affinities of capsules change as a result of anion exchange or post-assembly modification. The knowledge gained from these multicage systems may lead to the design of synthetic systems that can perform complex tasks in biomimetic fashion, paving the way for new supramolecular technologies to address practical problems.