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.     

Guanidinium binding modulates guest exchange within an [M4L6] capsule

Take it slow! A metal-organic container molecule has been shown to bind guanidinium cations (blue) between the sulfonate groups on its periphery, as well as accommodating guests such as cyclopentane and cyclohexane in its internal cavity (red). Kinetic studies on the system demonstrated a linear relationship between the amount of bound guanidinium ions and the rate of guest exchange.     

Oxidation triggers guest dissociation during reorganization of an FeII4L6 twisted parallelogram

A three-dimensional Fe^II₄L₆ parallelogram was prepared from ferrocene-containing ditopic ligands. The steric preference of the bulky ferrocene cores towards meridional vertex coordination brought about this new structure type, in which the ferrocene units adopt three distinct conformations. The structure possesses two distinct, bowl-like cavities that host anionic guests. Oxidation of the ferrocene Fe^II to ferrocenium Fe^III causes rotation of the ferrocene hinges, converting the structure to an Fe^II₁L₁⁺ species with release of anionic guests, even though the average charge per iron increases in a way that would ordinarily increase guest binding strength. The degrees of freedom exhibited by these new structures – derived from the different configurations of the three ligands surrounding a meridional Fe^II center and the rotation of ferrocene cores – thus underpin their ability to reconfigure and eject guests upon oxidation.

An oxidation-triggered twist in its ferrocene ligands causes an Fe₄L₆ parallelogram to release its guests and collapse into a high spin Fe₁L₁ structure.     

Diastereoselective formation and optical activity of an M4L6 cage complex

A chiral bridging ligand affords a single diastereoisomer of tetrahedral M4L6 cage complex in which the optical rotation of each ligand is increased by a factor of 5 on coordination.     

FeII4L4 tetrahedron binds and aggregates DNA G-quadruplexes

Since the discovery of the G-quadruplex (G4) structure in telomeres in 1980s, studies have established the role it plays in various biological processes. Here we report binding between DNA G4 and a self-assembled tetrahedral metal-organic cage 1 and consequent formation of aggregates, whereby the cage protects the DNA G4 from cleavage by S1 nuclease. We monitor DNA–cage interaction using fluorescence spectroscopy, firstly by quenching of a fluorescent label appended to the 5′ end of G4. Secondly, we detect the decrease in fluorescence of the G4-selective dyes thioflavin-T and Zn-PPIX bound to various DNA G4 sequences following the addition of cage 1. Our results demonstrate that 1 interacts with a wide range of G4s. Moreover, gel electrophoresis, circular dichroism and dynamic light scattering measurements establish the binding of 1 to G4 and indicate the formation of aggregate structures. Finally, we find that DNA G4 contained in an aggregate of cage 1 is protected from cleavage by S1 nuclease.

We find Fe^II₄L₄ binds to G-quadruplex and forms aggregates. G-quadruplex in the aggregates is protected from digestion by S₁ nuclease.     

Allosterically Regulated Guest Binding Determines Framework Symmetry for an FeII4L4 Cage

Self-assembly of a flexible tritopic aniline and 3-substituted 2-formylpyridine subcomponents around iron(II) templates gave rise to a low-spin Fe^II₄L₄ capsule, whereas a high-spin Fe^II₃L₂ sandwich species formed when a sterically hindered 6-methyl-2-formylpyridine was used. The Fe^II₄L₄ cage adopted a new structure type with S₄ symmetry, having two mer-Δ and two mer-Ʌ metal vertices, as confirmed by NMR and X-ray crystallographic analysis. The flexibility of the face-capping ligand endows the resulting Fe^II₄L₄ framework with conformational plasticity, enabling it to adapt structurally from S₄ to T or C₃ symmetry upon guest binding. The cage also displayed negative allosteric cooperativity in simultaneously binding different guests within its cavity and at the apertures between its faces.     

Selective Anion Extraction and Recovery Using a FeII4L4 Cage

Selective anion extraction is useful for the recovery and purification of valuable chemicals, and in the removal of pollutants from the environment. Here we report that Fe^II₄L₄ cage 1 is able to extract an equimolar amount of ReO₄^?, a high‐value anion and a nonradioactive surrogate of TcO₄^?, from water into nitromethane. Importantly, the extraction was efficiently performed even in the presence of 10 other common anions in water, highlighting the high selectivity of 1 for ReO₄^?. The extracted guest could be released into water as the cage disassembled in ethyl acetate, and then 1 could be recycled by switching the solvent to acetonitrile. The versatile solubility of the cage also enabled complete extraction of ReO₄^? (as the tetrabutylammonium salt) from an organic phase into water by using the sulfate salt of 1 as the extractant.     

Crystal structure and magnetic properties of an ionic C60 complex with decamethylcobaltocene: (Cp*2Co)2C60(C6H4Cl2, C6H5CN)2. Singlet-triplet transitions in the C60(2-) anion

The C(60) complex with decamethylcobaltocene, (Cp(2)Co)(2)C(60)(C(6)H(4)Cl(2), C(6)H(5)CN)(2) (1) (C(6)H(4)Cl(2) = 1,2-dichlorobenzene; C(6)H(5)CN = benzonitrile), has been obtained as single crystals by the diffusion method. The IR and UV-vis-NIR spectra show the presence of the C(60)(2)(-) and the Cp(2)Co(+) ions, which form a three-dimensional framework with channels accommodating solvent molecules. EPR and SQUID measurements show that C(60)(2)(-) has a diamagnetic singlet (S = 0) state in the 2-140 K range. The appearance of a broad EPR signal and the increase in magnetic susceptibility of 1 above 140 K are assigned to a thermal population of a close lying triplet (S = 1) state. The singlet-triplet energy gap for C(60)(2)(-) in solid 1 is estimated to be 730+/-10 cm(-)(1).     

Positron annihilation in C60 and K6C60

The positron density distributions in C₆₀ and K₆C₆₀ have been evaluated using the positron lifetime and Doppler-broadening spectroscopy for the annihilation radiation.In C₆₀, positrons are distributed in the interstitial sites between the C₆₀ molecules,which has been demonstrated by measurements of the temperature dependence of the Doppler-broadening of the annihilation radiation. On the other hand, the positron density distribution must be greatly changed in K₆C₆₀, because positrons are repelled by Coulomb interactions by the positively charged K atoms. It has been observed that there is an extremely short lifetime and a small Doppler-broadening component for the positron annihilation in K₆C₆₀. This component is considered to reflect the positron annihilation inside a C₆₀ molecule.     

An expanded neutral M4L6 cage that encapsulates four tetrahydrofuran molecules

A large 4,4'-biphenylene-spaced bis-β-diketone ligand is demonstrated to form a neutral tetrahedral M(4)L(6) metal-organic cage that encloses a volume of 844 ?(3) and encapsulates four tetrahydrofuran guest molecules.     

Spontaneous Resolution of an Electron‐Deficient Tetrahedral Fe4L4 cage

A highly electron‐deficient C₃‐symmetric tris(bipyridyl) ligand was prepared in four steps and used for the coordination of Fe(OTf)₂, thereby resulting in the homochiral assembly of a new family of robust tetrahedral M₄L₄ cages. This homochiral T‐symmetric cage containing a relatively large cavity of 330 ų is capable of encapsulating an anionic guest, as was determined by mass spectrometry, ¹⁹F NMR spectroscopy, and finally shown from its crystal structure. Moreover, crystallization of the cage from CH₃CN led to crystals containing both (ΔΔΔΔ and ΛΛΛΛ) enantiomers, while crystallization from CH₃OH resulted in crystals containing only the right‐handed (ΔΔΔΔ) cage. The difference in the crystal packing of the two crystal structures is discussed and a feasible explanation for the unique phenomenon among supramolecular cages—spontaneous resolution—is given.     

Tube and cage C(60)H(60): a comparison with C(60)F(60)

Tube C(60)H(60) (5) with fused five-membered rings is more stable than the cage isomer (1) with isolated five-membered rings. Introduction of endo C-H bonds into structure 5 results in further stabilization, but the most stable tube structure with four endo C-H bonds (7) is higher in energy than the most stable cage structure with ten endo C-H bonds (3) by 74.2 kcal/mol. A comprehensive comparison of C(60)H(60) with C(60)F(60) has been made.     

A Zn4L6 Capsule with Enhanced Catalytic C−C Bond Formation Activity upon C60 Binding

A redox‐switchable self‐assembled Zn^II₄L₆ cage was synthesized that contains naphthalenediimide (NDI) motifs. Its reduction lent these NDI panels persistent radical anion character. The redox activity of this cage allows it to act as a catalyst for the oxidative coupling of different tetraaryl borates to give biaryls. The catalytic activity of the cage was enhanced following its binding of C₆₀, which implies a mechanism that does not involve encapsulation of the substrate.     

Guest‐Induced Transformation of a Porphyrin‐Edged FeII4L6 Capsule into a CuIFeII2L4 Fullerene Receptor

The combination of a bent diamino(nickel(II) porphyrin) with 2‐formylpyridine and Fe^II yielded an Fe^II₄L₆ cage. Upon treatment with the fullerenes C₆₀ or C₇₀, this cage was found to transform into a new host–guest complex incorporating three Fe^II centers and four porphyrin ligands, in an arrangement that is hypothesized to maximize π interactions between the porphyrin units of the host and the fullerene guest bound within its central cavity. The new complex shows coordinative unsaturation at one of the Fe^II centers as the result of the incommensurate metal‐to‐ligand ratio, which enabled the preparation of a heterometallic cone‐shaped Cu^IFe^II₂L₄ adduct of C₆₀ or C₇₀.