Subcomponent Self-Assembly and Guest-Binding Properties of Face-Capped Fe4L48+ Capsules

A new nitrogen rich open chain diazine ligand system: synthesis coordination chemistry and magneto-structural studies

The synthesis and coordination chemistry of a new series of open chain diazine based ligands (L3aH₂, L3bH₂, and L3cH₂) is reported. The ligands comprise a central disubstituted bipyridine moiety with two bridging alkoxide oxygen donors, together with diazine and pyridine terminal groups strategically located to coordinate three metal centres. Reactions of L3aH₂ and L3bH₂ with CuX₂ (X^- = ClO₄, Cl, NO₃) yield a trinuclear complex (1) and 1-D copper chains (2, 3). In these complexes the ligands bind copper ions via N_bipyridne, trans N_diazine, O_hydrazone, and N_pyridne donors while vacant sites are occupied by counter ions or solvent molecules (methanol, water, acetonitrile). Reaction of L3cH₂ with MnCl₂ affords a linear trinuclear Mn complex (4), where the Mn(ii) ions are connected via μ₂-O_hydrazone linkers with no N-N bridging. Reaction of L3aH₂ with Fe(SO₃CF₃)₂ yields a tetranuclear mixed valence Fe complex (5), in which both trans N-N and O_hydrazone bridging is observed. Magnetic studies reveal the presence of moderate to strong antiferromagnetic interactions in complexes 1-4 while a mix of ferromagnetic and antiferromagnetic interactions is observed in complex 5.

Hierarchical self-assembly of a fluorescence emission-enhanced organogelator and its multiple stimuli-responsive behaviors

A discrete hexagonal metallacycle 1 decorated with tetraphenylethylene, amide groups and long hydrophobic alkyl chains was constructed via [3 + 3] coordination-driven self-assembly, from which the fluorescence emission-enhanced organogelator with multiple stimuli-responsiveness was successfully prepared via hierarchical self-assembly.

Metallo-organic ensembles of tritopic subphthalocyanine ligands

Organic building blocks containing amines and aldehydes can be used for the preparation of complex metallo-organic structures, such as M[Formula: see text]L[Formula: see text] triple helicates or face-capped M[Formula: see text]L[Formula: see text] tetrahedral cages, through the formation of both dynamic covalent and coordinative linkages during the self-assembly process. Herein we describe how the subcomponent self-assembly method can be succesfully applied over a triamine-functionalized subphthalocyanine (SubPc) ligand to build metallo-supramolecular helicates. Two isomeric SubPcs (C[Formula: see text]-SubPc1 and C[Formula: see text]-SubPc1) have been prepared from the corresponding C[Formula: see text]-SubPcI[Formula: see text] and C[Formula: see text]-SubPcI[Formula: see text] precursors under optimized Suzuki conditions. We selected the tritopic C[Formula: see text]-SubPc1 derivative as ligand for the subcomponent self-assembly experiments, which involved the reaction with 2-formylpyridine and different Fe(II) salts. The self-assembly process was mainly studied by mass spectrometry (ESI direct injection techniques), and in all the conditions applied, we could observe the formation of helicate-type Fe[Formula: see text]SubPc[Formula: see text] structures and/or Fe[Formula: see text]SubPc[Formula: see text] species, which can be considered as open precursors of Fe[Formula: see text]SubPc[Formula: see text] tetrahedral cages.

Excitation Energy Delocalization and Transfer to Guests within MII4L6 Cage Frameworks

We have prepared a series of M^II₄L₆ tetrahedral cages containing one or the other of two distinct BODIPY moieties, as well as mixed cages that contain both BODIPY chromophores. The photophysical properties of these cages and their fullerene-encapsulated adducts were studied in depth. Upon cage formation, the charge-transfer character exhibited by the bis(aminophenyl)-BODIPY subcomponents disappeared. Strong excitonic interactions were instead observed between at least two BODIPY chromophores along the edges of the cages, arising from the electronic delocalization through the metal centers. This excited-state delocalization contrasts with previously reported cages. All cages exhibited the same progression from an initial bright singlet state (species I) to a delocalized dark state (species II), driven by interactions between the transition dipoles of the ligands, and subsequently into geometrically relaxed species III. In the case of cages loaded with C₆₀ or C₇₀ fullerenes, ultrafast host-to-guest electron transfer was observed to compete with the excitonic interactions, short-circuiting the I → II → III sequence.

Supersnowflakes: Stepwise Self-Assembly and Dynamic Exchange of Rhombus Star-Shaped Supramolecules

With the goal of increasing the complexity of metallo-supramolecules, two rhombus star-shaped supramolecular architectures, namely, supersnowflakes, were designed and assembled using multiple 2,2':6',2″-terpyridine (tpy) ligands in a stepwise manner. In the design of multicomponent self-assembly, ditopic and tritopic ligands were bridged through Ru(II) with strong coordination to form metal-organic ligands for the subsequent self-assembly with a hexatopic ligand and Zn(II). The combination of Ru(II)-organic ligands with high stability and Zn(II) ions with weak coordination played a key role in the self-assembly of giant heteroleptic supersnowflakes, which encompassed three types of tpy-based organic ligands and two metal ions. With such a stepwise strategy, the self-sorting of individual building blocks was prevented from forming the undesired assemblies, e.g., small macrocycles and coordination polymers. Furthermore, the intra- and intermolecular dynamic exchange study of two supersnowflakes by NMR and mass spectrometry revealed the remarkable stability of these giant supramolecular complexes.

Electron-deficient trifluoromethyl-substituted sub-components affect the properties of M4L4 tetrahedral cages

Two supramolecular cages based on a trifluoromethyl-substituted pyridylimine ligand have been synthesised where the iron(ii) complex shows host–guest chemistry and complex-to-complex transformations.

Peripheral Templation Generates an M(II) 6 L4 Guest-Binding Capsule

Pseudo‐octahedral M^II₆L₄ capsules result from the subcomponent self‐assembly of 2‐formylphenanthroline, threefold‐symmetric triamines, and octahedral metal ions. Whereas neutral tetrahedral guests and most of the anions investigated were observed to bind within the central cavity, tetraphenylborate anions bound on the outside, with one phenyl ring pointing into the cavity. This binding configuration is promoted by the complementary arrangement of the phenyl rings of the intercalated guest between the phenanthroline units of the host. The peripherally bound, rapidly exchanging tetraphenylborate anions were found to template an otherwise inaccessible capsular structure in a manner usually associated with slow‐exchanging, centrally bound agents. Once formed, this cage was able to bind guests in its central cavity.

Kinetically Trapped Tetrahedral Cages via Alkyne Metathesis

In dynamic covalent synthesis, kinetic traps are perceived as disadvantageous, hindering the system from reaching its thermodynamic equilibrium. Here we present the near-quantitative preparation of tetrahedral cages from simple tritopic precursors using alkyne metathesis. While the cages are the presumed thermodynamic sink, we experimentally demonstrate that the products no longer exchange their vertices once they have formed. The example reported here illustrates that kinetically trapped products may facilitate high yields of complex products from dynamic covalent synthesis.

Metal-center exchange of tetrahedral cages: single crystal to single crystal and spin-crossover properties

Effective SCSC metal-center exchange was observed in a tetrahedral metal–organic cage, in which the metal centers can be induced to display spin crossover behaviors.

Towards design strategies for anion–π interactions in crystal engineering

This highlight article summarizes some of the fundamental aspects of the anion–π interaction leading to several design strategies for generating it in solids. In the main body we highlight some relevant examples that illustrate the viability of these strategies and the importance of anion–π interactions in crystal engineering.

Stacking Interactions Drive Selective Self-Assembly and Self-Sorting of Pyrene-Based M(II)4L6 Architectures

Subcomponent self-assembly of two isomeric bis(3-aminophenyl)pyrenes, 2-formylpyridine and the metal ions Fe(II), Co(II), and Zn(II) led to the formation of two previously unidentified structure types: a C2-symmetric M(II)4L6 assembly with meridionally coordinated metal centers, and a C3-symmetric self-included M(II)4L6 assembly with facially coordinated metal centers. In both structures the meta linkages within the ligands facilitate π-stacking between the pyrene panels of the ligands. A C2h-symmetric M(II)2L2 box was also obtained, which was observed to selectively bind electron-deficient aromatic guests between two parallel pyrene subunits. Similar donor-acceptor interactions drove the selective self-assembly of a singular M(II)4L4L'2 architecture incorporating both a pyrene-containing diamine and an electron-deficient NDI-based diamine. This heteroleptic architecture was shown to be thermodynamically favored over the corresponding homoleptic M(II)4L6 and M(II)4L'6 complexes, which were nonetheless stable in each others' absence. By contrast, an isomeric pyrene-based diamine was observed to undergo narcissistic self-sorting in the presence of the NDI-based diamine.

Controllable Coordination Self-Assembly Based on Flexible Tripodal Ligands: From Finite Metallocages to Infinite Polycatenanes Step by Step

This article describes the developments in coordination self-assembly based on flexible tripodal ligands with different metal species. Various finite metallocages such as M3 L2 , M6 L8 , M6 L4 , M4 L4 and different catenanes based on discrete metallocages constructed from flexible tripodal ligands with suitable metal species are presented here. Many M3 L2 metallocages based on ligands L(1) -L(12) and different two-coordinated metal species have been prepared, in which various Ag(I) salts and other metal species that have been protected by suitable groups, such as Zn(OAc)2 , ZnBr2 , and PdBr2 , have been used as effective acceptors. All of the M6 L8 -type metallocages are constructed from ligands L(2) or L(12) -L(20) and different four-coordinated metal species, such as various palladium(II) salts or NiCl2 , and have similar topological structures. Only a few discrete M6 L4 -type metallocages, based on ligands L(21) -L(24) , have been reported, using different strategies such as protecting groups and steric hindrance. All of the M4 L4 -type cages have similar topological structures and are constructed from ligands L(25) -L(29) with multiple donor sites. More intriguing interlocking ensembles constructed from discrete metallocages are also described here in detail, namely, three [2]catenanes based on ligands L(30) -L(32) and four polycatenanes based on ligands L(33) -L(34) .

DOSY NMR, X-ray Structural and Ion-Mobility Mass Spectrometric Studies on Electron-Deficient and Electron-Rich M6L4 Coordination Cages

A novel modular approach to electron-deficient and electron-rich M6L4 cages is presented. From the same starting compound, via a minor modulation of the synthesis route, two C3-symmetric ligands L1 and L2 with different electronic properties are obtained in good yield. The trifluoro-triethynylbenzene-based ligand L1 is more electron-deficient than the well-known 2,4,6-tri(4-pyridyl)-1,3,5-triazine, while the trimethoxy-triethynylbenzene-based ligand L2 is more electron-rich than the corresponding benzene analogue. Complexation of the ligands with cis-protected square-planar [(dppp)Pt(OTf)2] or [(dppp)Pd(OTf)2] corner-complexes yields two electron-deficient (1a and 1b) and two electron-rich (2a and 2b) M6L4 cages. The single crystal X-ray diffraction study of 1a and 2a confirms the expected octahedral shape with a ca. 2000 Å(3) cavity and ca. 11 Å wide apertures. The crystallographically determined diameters of 1a and 2a are 3.7 and 3.6 nm, respectively. The hydrodynamic diameters obtained from the DOSY NMR in CDCl3:CD3OD (4:1), and diameters calculated from collision cross sections (CCS) acquired by ion-mobility mass spectrometry (IM-MS) were for all four cages similar. In solution, the cage structures have diameters between 3.3 to 3.6 nm, while in the gas phase the corresponding diameters varied between 3.4 to 3.6 nm. In addition to the structural information the relative stabilities of the Pt6L4 and Pd6L4 cages were studied in the gas phase by collision-induced dissociation (CID) experiments, and the photophysical properties of the ligands L1 and L2 and cages 1a, 1b, 2a, and 2b were studied by UV-vis and fluorescence spectroscopy.

High fidelity sorting of remarkably similar components via metal-mediated assembly

Subtle differences in coordination angle and rigidity lead to narcissistic self-sorting between highly similar individual components upon metal-mediated assembly.

Subtle differences in ligand coordination angle and rigidity lead to high fidelity sorting between individual components displaying identical coordination motifs upon metal-mediated self-assembly. Narcissistic self-sorting can be achieved between highly similar ligands that vary minimally in rigidity and internal coordination angle upon combination with Fe(ii) ions and 2-formylpyridine. Selective, sequential cage formation can be precisely controlled in a single flask from a mix of three different core ligands (and 33 total components) differing only in the hybridization of one group that is uninvolved in the metal coordination process.

Recent advances in the construction of fluorescent metallocycles and metallocages via coordination-driven self-assembly

During the last few years, the construction of fluorescent metallocycles and metallocages has attracted considerable attention because of their wide applications in fluorescence detection of metal ions, anions, or small molecules, mimicking complicated natural photo-processes, and preparing photoelectric devices, etc. This perspective focuses on the recent advances in the construction of a variety of fluorescent metallocycles and metallocages via coordination-driven self-assembly. In addition, the fluorescence properties and the applications of these organometallic architectures have also been discussed.

Narcissistic self-sorting in self-assembled cages of rare Earth metals and rigid ligands

Highly selective, narcissistic self-sorting can be achieved in the formation of self-assembled cages of rare earth metals with multianionic salicylhydrazone ligands. The assembly process is highly sensitive to the length of the ligand and the coordination geometry. Most surprisingly, high-fidelity sorting is possible between ligands of identical coordination angle and geometry, differing only in a single functional group on the ligand core, which is not involved in the coordination. Supramolecular effects allow discrimination between pendant functions as similar as carbonyl or methylene groups in a complex assembly process.

Anisotropic expansion of an M2L4 coordination capsule: host capability and frame rearrangement

Anisotropic expansion of a spherical M2L4 coordination capsule through the elongation of the ligand led to a new M2L'4 capsule. The expanded capsule provides an elliptical cavity encircled by polyaromatic frameworks with large openings and thereby can encapsulate elliptical fullerene C70 and monofunctionalized fullerene C60 in high yields. In addition, selective formation of a new M2L2L'2 capsule occurs by mixing the original M2L4 and expanded M2L'4 capsules in a 1:1 ratio upon addition of C60 or monofunctionalized C60 as a template molecule.

Dynamic imine chemistry in metal–organic polyhedra

This review highlights the intercession of Schiff base ligands in the preparation of self-assembled architectures mainly metal–organic polyhedra and describes their unprecedented role in various key applications.

Indistinguishability and distinguishability between amide and ester moieties in the construction and properties of M6L8 octahedral nanocages

A series of octahedral nanocages with ester, amide, and mixed ester/amide spacers in construction and valuable functions have been investigated.

Ligand Constraints and Synthesis of Metal–Organic Polyhedra

Metal–organic polyhedra are three dimensional discrete structures typically constructed by the self-assembly of metal ions and ligands. The synthesis and geometry of discrete structures entirely rely on the choice of metal ions, ligand constraints such as steric bulk, bend angle, and functionalities, and the nature of applied solvents. As a result, they provide tailorable internal volume and usually hydrophobic nature to the cavity that in turn makes them one of the prominent host molecules for a range of applications. This review highlights the intervention of ligand constraints, precisely bend angle (0°, 60°, 120°, and 180°), hydroxyl functionalities, and the role of concepts such as molecular panelling and subcomponent self-assembly in the synthesis of polyhedra.

Supramolecular metallacycles with a ‘pseudo double-paracyclophane’ structure based on flexible π-conjugated linkers

Coordination-driven supramolecular synthesis of new supramolecular π-stacked metallacycles based on flexible π-conjugated linkers and having a ‘pseudo double-paracyclophane’ structure is presented.

Probing a hidden world of molecular self-assembly: concentration-dependent, three-dimensional supramolecular interconversions

A terpyridine-based, concentration-dependent, facile self-assembly process is reported, resulting in two three-dimensional metallosupramolecular architectures, a bis-rhombus and a tetrahedron, which are formed using a two-dimensional, planar, tris-terpyridine ligand. The interconversion between these two structures is concentration-dependent: at a concentration higher than 12 mg mL(-1), only a bis-rhombus, composed of eight ligands and 12 Cd(2+) ions, is formed; whereas a self-assembled tetrahedron, composed of four ligands and six Cd(2+) ions, appears upon sufficient dilution of the tris-terpyridine-metal solution. At concentrations less than 0.5 mg mL(-1), only the tetrahedron possessing an S4 symmetry axis is detected; upon attempted isolation, it quantitatively reverts to the bis-rhombus. This observation opens an unexpected door to unusual chemical pathways under high dilution conditions.

Solvent effects upon guest binding and dynamics of a Fe(II)4L4 cage

Solvent-dependent host-guest chemistry and favoring of otherwise disfavored conformations of large guests has been achieved with an adaptive, self-assembled Fe(II)4L4 coordination cage. Depending on the counterion, this face-capped tetrahedral capsule is soluble either in water or in acetonitrile and shows a solvent-dependent preference for encapsulation of certain classes of guest molecules. Quantitative binding studies were undertaken, revealing that both aromatic and aliphatic guests bind in water, whereas only aliphatic guests bind in acetonitrile. The flexibility of its subcomponent building blocks allows this cage to expand or contract upon guest binding, as studied by VT-NMR, thereby ensuring strong binding of both small and large guests. Upon encapsulation, large guest molecules can adopt conformations which are not thermodynamically favored in the free state. In addition, the chirotopic inner phase of the cage renders enantiotopic guest proton signals diastereotopic in specific cases.

AuToGraFS: automatic topological generator for framework structures

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are recently notable examples of highly porous polymer frameworks with a raft of potential applications. Synthesis of these compounds is modular, with "connectors" and "linkers" able to be replaced almost at will in the fabrication of isoreticular frameworks (frameworks with the same underlying topology). The range of components available to form such framework structures is vast, leading to a "combinatorial explosion" problem in predicting which framework compounds might have a set of desired properties. Computational investigations can be used in both predictive and explanatory roles in this research but rely on accurate structural models. In this work, we present our software, AuToGraFS, Automated Topological Generator for Framework Structures, and show some of its advanced functionality in "computational reticular chemistry". AuToGraFS is linked to a fully featured force field to produce fully optimized structures of arbitrary frameworks. AuToGraFS, including a graphical user interface, is publicly available for download.

Self-Assembly of Conjugated Units Using Metal-Terpyridine Coordination

Due to their inherently dynamic natures and fascinating photoluminescent/photoelectronic properties, coordination compounds of metal ions and conjugated terpyridine ligands have attracted considerable attention as functional materials for a variety of potential applications. In this feature article, a summary of recent work toward the development of one- (1D), two- (2D), and three-dimensional (3D) supramolecular polymers, networks, and metallomacrocycles based on zinc metal ion coordination of conjugated units bearing terpyridine ligands is presented, and it is shown how it fits within the overall framework of work in this field. Here, a sequential study from terpyridines as basic building blocks to their zinc-coordinated supramolecular 1D polymers, 2D macrocycles, and 2D and 3D networks is developed. These networks are compared with respect to their thermal stabilities, molecular organization, and linear and nonlinear optical properties. This work opens new prospects for the development of supramolecular chemistry of terpyridines and other transition metal ions, and also their application in future optoelectronic devices.

Stereochemistry in subcomponent self-assembly

CONSPECTUS: As Pasteur noted more than 150 years ago, asymmetry exists in matter at all organization levels. Biopolymers such as proteins or DNA adopt one-handed conformations, as a result of the chirality of their constituent building blocks. Even at the level of elementary particles, asymmetry exists due to parity violation in the weak nuclear force. While the origin of homochirality in living systems remains obscure, as does the possibility of its connection with broken symmetries at larger or smaller length scales, its centrality to biomolecular structure is clear: the single-handed forms of bio(macro)molecules interlock in ways that depend upon their handednesses. Dynamic artificial systems, such as helical polymers and other supramolecular structures, have provided a means to study the mechanisms of transmission and amplification of stereochemical information, which are key processes to understand in the context of the origins and functions of biological homochirality. Control over stereochemical information transfer in self-assembled systems will also be crucial for the development of new applications in chiral recognition and separation, asymmetric catalysis, and molecular devices. In this Account, we explore different aspects of stereochemistry encountered during the use of subcomponent self-assembly, whereby complex structures are prepared through the simultaneous formation of dynamic coordinative (N → metal) and covalent (N═C) bonds. This technique provides a useful method to study stereochemical information transfer processes within metal-organic assemblies, which may contain different combinations of fixed (carbon) and labile (metal) stereocenters. We start by discussing how simple subcomponents with fixed stereogenic centers can be incorporated in the organic ligands of mononuclear coordination complexes and communicate stereochemical information to the metal center, resulting in diastereomeric enrichment. Enantiopure subcomponents were then incorporated in self-assembly reactions to control the stereochemistry of increasingly complex architectures. This strategy has also allowed exploration of the degree to which stereochemical information is propagated through tetrahedral frameworks cooperatively, leading to the observation of stereochemical coupling across more than 2 nm between metal stereocenters and the enantioselective synthesis of a face-capped tetrahedron containing no carbon stereocenters via a stereochemical memory effect. Several studies on the communication of stereochemistry between the configurationally flexible metal centers in tetrahedral metal-organic cages have shed light on the factors governing this process, allowing the synthesis of an asymmetric cage, obtained in racemic form, in which all symmetry elements have been broken. Finally, we discuss how stereochemical diversity leads to structural complexity in the structures prepared through subcomponent self-assembly. Initial use of octahedral metal templates with facial stereochemistry in subcomponent self-assembly, which predictably gave rise to structures of tetrahedral symmetry, was extended to meridional metal centers. These lower-symmetry linkages have allowed the assembly of a series of increasingly intricate 3D architectures of varying functionality. The knowledge gained from investigating different aspects of the stereochemistry of metal-templated assemblies thus not only leads to new means of structural control but also opens pathways toward functions such as stereoselective guest binding and transformation.