Jack K. Clegg

A Self‐Assembled M8L6 Cubic Cage that Selectively Encapsulates Large Aromatic Guests

Biological encapsulants such as ferritin, lumazine synthase, and viral capsids achieve their selective separation and sequestration of substrates by providing: 1) a guest microenvironment isolated from the surroundings, 2) favorable interactions complementing a size and shape match with the encapsulated guests, and 3) sufficient flexibility to allow guests to be incorporated and released. These biological hosts self-assemble from multiple copies of identical protein subunits, the symmetries and connection properties of which dictate the hollow polyhedral structures of the encapsulant. In order to create abiological molecular systems that are capable of expressing functions of similar complexity to biological systems and to explore new applications of synthetic hosts, there is a need to create synthetic capsules capable of tightly and selectively binding large substrates. Taking inspiration from natural systems and from other previously reported metal–organic capsules, we report the design and synthesis of a series of metallo-supramolecular cage molecules capable of selectively encapsulating large aromatic guests. The necessary features to achieve this function are: 1) small pore sizes to isolate guests from the environment, 2) large cavity sizes to ensure sufficient volume for the guests of interest, 3) enough flexibility and lability to allow guests to enter and exit the host, and 4) regions of the cage walls rich in p-electron density to provide favorable interactions with targeted guests. The selective encapsulation of large aromatic molecules is an attractive goal since their physicochemical properties are similar, which can render their separation difficult. The higher fullerenes represent particularly attractive targets because their potential applications remain difficult to explore because of the challenges associated with their separation, despite recent advances. Employing principles of geometric analysis, we determined that combination of the C4-symmetric tetrakis-bidentate ligand shown in Figure 1 with the C3-symmetric iron(II) tris(pyridylimine) center would result in the formation of an O-symmetric cubic structure of general formula M8L6, in which the corners of the cube are defined by the metal centers and the faces by the ligands (Figure 1). This cage represents the first example of a new class of closed-face metallosupramolecular cubic hosts to be synthesized. In order to provide favorable binding sites for our target guests we incorporated porphyrin moieties, which have previously been demonstrated to interact with large aromatic molecules, into our design. This design also provides for small pore sizes and the potential to create new chemical functionality through the introduction of different metal ions into the centers of the N4 macrocycle and by substituting these metals axial ligands. We chose to employ labile iron(II) centers with pyridylimine ligands as chelating agents to allow for the formation of the ligand in situ through the subcomponent self-assembly approach. The reaction between tetrakis(4-aminophenyl)porphyrin (H2-tapp), 2-formylpyridine, and iron(II) trifluoromethanesulfonate (triflate, OTf ) in DMF produced cage [H21]·16OTf (Figure 1) as the uniquely observed product, as verified by NMR spectroscopy (Figure 3b), electrospray mass spectrometry (ESI-MS), and elemental analysis. Substitution of nickel(II) tetrakis(4-aminophenyl)porphyrin (Ni-tapp) or zinc(II) tetrakis(4-aminophenyl)porphyrin (Zn-tapp) for H2tapp under identical conditions yielded the nickel-containing (Ni-1) and zinc-containing (Zn-1) congeners of H2-1 (Figures S2a and S3a in the Supporting Information), respectively, suggesting the formation of such capsules to be a general feature of tetrakis(4-aminophenyl) porphyrins (Figure 1). Vapor diffusion of diethyl ether into a DMF/acetonitrile solution of Ni-1 resulted in the isolation of block-shaped dark purple crystals. Single-crystal X-ray diffraction revealed a solid-state structure (Figure 2) consistent with the O-symmetric NMR spectra recorded in solution. Each face of Ni-1 is covered by one porphyrin ligand and each corner is defined by a six-coordinate low-spin Fe ion. All of the Fe centers within each cage adopt the sameL or D configuration; both enantiomers of Ni-1 are present in the crystal lattice. The Ni–Ni distance between opposite faces is 15 , and the internal cavity volume is 1340 3 (Figure S2e). [*] W. Meng, Dr. B. Breiner, Prof. J. D. Thoburn, Dr. J. K. Clegg, Dr. J. R. Nitschke University of Cambridge, Department of Chemistry Lensfield Road, Cambridge, CB2 1EW (UK) E-mail: jrn34@cam.ac.uk Homepage: http://www-jrn.ch.cam.ac.uk/

Anion-induced reconstitution of a self-assembling system to express a chloride-binding Co10L15 pentagonal prism

Biochemical systems are adaptable, capable of reconstitution at all levels to achieve the functions associated with life. Synthetic chemical systems are more limited in their ability to reorganize to achieve new functions; they can reconfigure to bind an added substrate (template effect) or one binding event may modulate a receptors affinity for a second substrate (allosteric effect). Here we describe a synthetic chemical system that is capable of structural reconstitution on receipt of one anionic signal (perchlorate) to create a tight binding pocket for another anion (chloride). The complex, barrel-like structure of the chloride receptor is templated by five perchlorate anions. This second-order templation phenomenon allows chemical networks to be envisaged that express more complex responses to chemical signals than is currently feasible.

Selective anion binding by a “Chameleon” capsule with a dynamically reconfigurable exterior

A new class of tetrahedral metal–organic capsules that can incorporate up to twelve different externally-directed amine residues is reported, allowing for very large dynamic libraries to be formed from mixtures of amines. Selectivity is observed both externally—more electron-rich amines are incorporated in favour of electron-poor amines—and internally—PF6− is bound in preference to CF3SO3− or BF4−.

Subcomponent self-assembly and guest-binding properties of face-capped Fe4L48+ capsules

A general method for preparing Fe(4)L(4) face-capped tetrahedral cages through subcomponent self-assembly was developed and has been demonstrated using four different C(3)-symmetric triamines, 2-formylpyridine, and iron(II). Three of the triamines were shown also to form Fe(2)L(3) helicates when the appropriate stoichiometry of subcomponents was used. Two of the cages were observed to have nearly identical Fe-Fe distances in the solid state, which enabled their ligands to be coincorporated into a collection of mixed cages. Only one of the cages combined a sufficiently large cavity with the sufficiently small pores required for guest binding, taking up a wide variety of guest species in size- and shape-selective fashion.

Triangles and tetrahedra: metal directed self-assembly of metallo-supramolecular structures incorporating bis-β-diketonato ligands

The interaction of six aryl-linked bis-beta-diketones, including a new naphthylene linked species, with copper(II), iron(III) and, in one instance gallium(III), has been investigated with the aim of obtaining metallo-supramolecular assemblies exhibiting different geometries. New examples of two assembly types incorporating the above bis-beta-diketones (L) were generated. The first type is represented by a range of molecular triangles of formula [Cu(3)(L-H(2))(3)](solvent)(n) while the second is given by a corresponding selection of less-common neutral molecular tetrahedra of formula [Fe(4)(L-H(2))(6)](solvent)(n) as well as [Ga(4)(L-H(2))(6)].8.5THF.0.5H(2)O; an example of each type has been characterised by X-ray crystallography. A magnetochemical investigation of [Fe(4)(-H(2))(6)].6THF is reported. The susceptibility is Curie like and consistent with very weak coupling occurring between the iron(III) d(5)(high spin) centres. The X-ray structures of two trinuclear copper(II) as well as a tetranuclear iron(III) and a tetranuclear gallium(III) assembly confirm their discrete triangular and tetrahedral geometries, respectively. The structure of the gallium(III) species is closely related to that of the corresponding iron(III) species. The tetrahedral structures provide rare examples of such assemblies encapsulating guest solvent molecules--in each case tetrahydrofuran is incorporated in the central cavity.

A new FeII quaterpyridyl M4L6 tetrahedron exhibiting selective anion binding

A rigid linear bis-bidentate quaterpyridine undergoes metal directed self-assembly with iron(ii) salts yielding M(4)L(6) host-guest complexes; selective anion binding for PF(6)(-) over BF(4)(-) is observed.

Hierarchical Self-Assembly of a Chiral Metal–Organic Framework Displaying Pronounced Porosity†

[Extract] Significant recent attention has been devoted to the development of useful self-assembled hybrid materials.[1] This is particularly the case for metal–organic frameworks (MOFs), which display properties such as regularity, porosity, robustness, and high surface area that lead to potential applications in areas such as catalysis, gas separation, and storage.[2, 3] Our research groups and others have been developing new methods for the synthesis of both discrete and extended metal–organic materials, with particular interest in the controlled generation of increased structural complexity.[4] Herein we report a hierarchical self-assembly strategy which has been used to synthesize a new metal–organic framework. This strategy differs from the commonly employed molecular building block (MBB) and secondary building unit (SBU) approaches, where single metal ions or small inorganic clusters (polyhedra) are linked by bridging (often carboxylate) ligands in a one-pot reaction.[5] In these approaches, substantial pore volume is achieved principally through the enthalpically favorable formation of an open framework overcoming the entropic penalties associated with the entrapment of solvent guest molecules. Kinetic control over the formation of the framework is achieved largely through the trial-and-error optimization of synthetic conditions to prevent formation of unwanted kinetic intermediates.[6]

Self-assembled Metallo-supramolecular Systems Incorporating β-Diketone Motifs as Structural Elements

Publisher Summary This chapter discusses the synthesis and properties of metal-containing molecular architectures incorporating β-diketonate motifs. The chapter focuses on the systems in which the use of designed metal-ion-directed assembly is employed for constructing new supramolecular entities. The simplest β-diketone ligand is 2,4-pentanedione (acetylacetone, acacH). β-diketones such as acacH show a tendency to form neutral complexes—with the geometry adopted normally reflecting the preferred geometry of the metal ion involved. In the case of cobalt(II), tetrahedral complexes can also result if a bulky β-diketonate ion is used. The ability to form higher coordinate species can be tuned through variation of the terminal R groups on the β-diketone. Reflecting the available relative orientations of the β-diketonate groups in each ligand type, a range of both dinuclear (rectangular) and trinuclear (triangular) “platform-like” products are isolated and characterized. In the lanthanum-containing complex, the lanthanum(III) ion is eleven coordinate being bound by the six “internal” oxygens from the β-diketonate groups, the three pyridyl nitrogens, a tetrahydrofuran group, and a water molecule.

A stimuli responsive system of self-assembled anion-binding Fe4L68+ cages

A new cationic Fe4L6 cage molecule was synthesised from 4,4′-diaminobiphenyl, 2-formylpyridine and iron(II). The cage exists as a system of interconverting diastereomers in solution. The system adapts to the addition of anionic guest molecules, expressing a new combination of diastereomers that synergistically bind the guest molecules. Not only do the cage diastereomers interconvert, the volume of the individual cages adapts physically through the rotation of bonds, providing a tailored binding pocket for the guest lined with hydrogen-bond donors. A model for the resulting complex network of species was developed that allowed the system to be fully described. The anion binding constants and the kinetics of both diastereomer interconversion and guest exchange were measured.

Unprecedented encapsulation of a [FeIIICl4]− anion in a cationic [FeII4L6]8+ tetrahedral cage derived from 5,5′′′-dimethyl-2,2′:5′,5′′:2′′,2′′′-quaterpyridine

A unique example of incorporation of a tetrahalometalate anion in a small supramolecular cage is described in which a tetrahedral cage of type [Fe4L6]8+ selectively encapsulates a [FeIIICl4]− anion over a [FeIICl4]2− anion in its central cavity to yield a discrete, mixed oxidation state, Fe(II)/Fe(III) supramolecular assembly. This unusual outcome has been achieved using two alternative synthetic strategies.