Self-assembled Coordination Cages Research Articles

Remote chiral transfer into [2+2] and [2+4] cycloadditions within self-assembled molecular flasks

A self-assembled chiral coordination cage was prepared from triangular triazine-panel ligands and Pd(II) complexes with chiral diamine auxiliaries. The chiral environment of the cage is induced by the structural deformation of the triazine panels ascribed to the steric bulk of the substituents on the chiral auxiliaries. The chiral cage can accommodate a pair of two hydrophobic molecules to form a specific diastereomeric ternary complex. We succeeded in conducting unusual [2+2] and [2+4] asymmetric cycloadditions from the identical ternary complex including a maleimide derivative and aromatic compounds with 6–50% enantiomeric excess (ee). It is remarkable that the remote chirality on the auxiliaries is efficiently transmitted to the chiral orientation of achiral ligands, which define a chiral cavity, to induce up to 50% ee. The present strategy is widely applicable to cavity-directed asymmetric reactions and maintaining inherent properties of the cage.

Diels–Alder Reactions of Inert Aromatic Compounds within a Self‐Assembled Coordination Cage

A self-assembled coordination cage serves as a nanometer-sized molecular flask to promote the Diels-Alder reactions of aromatic hydrocarbons with N-cyclohexylmaleimide. The coordination cage accelerated the Diels-Alder reaction of anthracene at the electronically unfavorable, terminal benzene ring to give a compact, cavity-restrained syn-adduct. Activation-parameter measurements for the reactions revealed considerable reduction in the entropy cost, and preorganization of the substrates is a dominant factor in the enhanced reactivity. Owing to this entropy-cost reduction, otherwise-unreactive aromatic compounds, such as naphthalenes or triphenylene, also underwent Diels-Alder reactions in a regio- and stereocontrolled fashion. In the naphthalene Diels-Alder reaction, X-ray crystallographic analysis of the guest-inclusion complex clarified the reinforced orientation and proximity of the substrate pairs before the reaction. A perylene Diels-Alder adduct was stabilized inside the cage and protected from aerial oxidation.

Bent into shape

The composition and structure of self-assembled coordination cages are very sensitive to subtle changes in the geometry of the bent organic ligands that are used to stitch together the metal ions.

Both [2+2] and [2+4] additions of inert aromatics via identical ternary host–guest complexes

A self-assembled coordination cage enables inert aromatics to undergo both [2+2] and [2+4] cycloadditions by encapsulating them within the cavity. Aceanthrylene reacts with a maleimide derivative under thermal and photo-irradiation conditions to give a product as a single regio- and stereoisomer.

Engineering Stacks of Aromatic Rings by the Interpenetration of Self-Assembled Coordination Cages

One-step quantitative self-assembly of aromatic towers consisting of seven to nine discretely stacked aromatic rings is achieved from 25-27 multicomponents via the interpenetration of two identical coordination cages.

Highly Selective Photomediated 1,4‐Radical Addition to o‐Quinones Controlled by a Self‐Assembled Cage

Cavity control: A highly selective 1,4-addition reaction of a toluene derivative to o-quinones proceeds efficiently within a self-assembled coordination cage (see scheme). The cage accelerates the formation of the 1,4-adduct but suppresses all other reaction pathways that are normally favorable in the absence of the cage.

Unusual [2+4] and [2+2] Cycloadditions of Arenes in the Confined Cavity of Self-Assembled Cages

Unusual [2+4] and [2+2] cross-cycloadditions of arenes were efficiently promoted within a self-assembled coordination cage. For example, triphenylene underwent the Diels−Alder reaction with N-cyclo...

Self-assembled hetero-bimetallic coordination cage and cation-clusters with µ2-Cl bridging using a flexible two-arm ferrocene amide linker

One flexible, discrete coordination cage [Cu(2)(3-BPFA)(4)(H(2)O)(2)](ClO(4))(4).4CH(3)OH (), and two cation-clusters with micro(2)-Cl bridging [Ni(2)(micro-Cl)(3-BPFA)(4)(H(2)O)(2)](ClO(4))(3) () and [Co(2)(micro-Cl)(3-BPFA)(4)(H(2)O)(2)](ClO(4))(4).4CH(3)OH (), containing the ferrocenyl functionality were prepared via coordination-driven self-assembly and Cl-anion template from Cu(II), Ni(II) and Co(II) salts and a flexible two-arm molecule 1,1-bis[(3-pyridylamino)carbonyl]ferrocene (3-BPFA).

Manipulating the Through‐Space Spin–Spin Interaction of Organic Radicals in the Confined Cavity of a Self‐Assembled Cage

We show a new approach to manipulating the through-space spin-spin interaction by utilizing the confined cavity of a self-assembled M6L4 coordination cage. The coordination cage readily encapsulates stable organic radicals in solution, which brings the spin centers of the radicals closer to each other. In sharp contrast to the fact that the radical in solution in the absence of the cage is in a doublet state, in the presence of the cage through-space spin-spin interaction is induced through cage-encapsulation effects in solution as well as in the solid state, resulting in the triplet state of the complex. These results were confirmed by ESR spectroscopy and X-ray crystallography. The quantity of triplet species generated by encapsulation in the cage increases with increasing affinity of the radicals to the cage. We estimated the affinity between several types of guests and the cage in solution by cyclic voltammetry. We also demonstrate that the through-space interaction of organic radicals within the self-assembled coordination cage can be controlled by external stimuli such as heat or pH.

Self-assembled coordination cage derived from small-sized pyridinophane

The dithia[3.3]pyridinophane consisting of two pyridine rings has been found out to assume the syn-structure by the X-ray crystallography, meaning the two nitrogen atoms point in the same direction. From this cyclophane and cis-protected palladium(II), the self-assembled coordination molecular cage has been constructed.

Red-shifted luminescence from naphthalene-containing ligands due to π-stacking in self-assembled coordination cages

Incorporation of ligands containing substituted naphthalene cores into coordination cages results in extensive aromatic pi-stacking between ligands; this results in a red-shifted 'excimer-like' luminescence component from the naphthyl groups compared to the free ligands, which is diagnostic of, and can be used to monitor, cage assembly.

Control of molecular interactions by the hollow of coordination cages

Coordination self-assembly entities have been demonstrated to be useful and powerful alternatives for the construction of predefined and well-organized architectures. Self-assembled coordination cages, which possess an inner hydrophobic cavity, can be utilized to move substrates closer by encapsulation so enabling their interaction with each other and exhibiting of non-classical physical properties.

Inclusion of methano[60]fullerene derivatives in cavitand-based coordination cages

The X-ray study of self-assembled coordination cage 1 , constituted of two tetrapyridyl-substituted resorcin[4]arene cavitands coupled through four square-planar palladium complexes is reported. The coordination cage, embracing an internal cavity of ca. 840 Å 3, reveals to have the right size for the inclusion of large molecules such as fullerenes. Cage 1 forms 1:1 complexes with methano[60]fullerene derivatives 3 and 4 bearing a dimethyl and a diethyl malonate addend, respectively. Evidence for inclusion complexation was provided by 1H NMR spectroscopic studies and ESI-MS investigations, which unambiguously showed the formation of 1:1 fullerene–cage complexes. The association constants ( K a) were experimentally determined to be ca. 150 M −1 at 298 K in CD 2Cl 2. In both complexes 1· 3 and 1· 4 , the malonate residue is threaded through one of the four lateral portals, as clearly shown by docking simulations.

Selective Enclathration of Linear Alkanols by a Self-assembled Coordination Cage. Application to the Catalytic Wacker Oxidation of ω-Alkenols

Abstract Linear alkanols were efficiently enclathrated by a self-assembled cage (1) to give a 1:2 host–guest complex as confirmed by NMR and crystallographic analyses. Cage 1 also enclathrated an ω-alkenol, 8-nonen-1-ol, and, upon heating, Wacker-type oxidation took place with a catalytic amount of 1 to give 9-hydroxynonan-2-one in a good yield.

Large M4L4 (M = Al(III), Ga(III), In(III), Ti(IV)) Tetrahedral Coordination Cages: an Extension of Symmetry-Based Design

As an extension to a rational design for the formation of self-assembled coordination cages, the syntheses for very large M4L4 tetrahedra based on a hexadentate 3-fold symmetric ligand (1,3,5-tris(4'-(2' ',3' '-dihydroxybenzamido)phenyl)benzene (H6L2)) are described. Four tetrahedral M4L2(4) assemblies (M = Al(III), Ga(III), In(III), Ti(IV)), with cavity sizes of around 450 A3, have been characterized by elemental analysis, NMR spectroscopy, and high-resolution electrospray mass spectrometry. Differences in chiral resolution and dynamic behavior of host-guest interactions with previously reported tetrahedral M4L(N)6 and M4L1(4) architectures are highlighted for the ligands 1,5-bis(2',3'-dihydroxybenzamido)naphthalene (H4L(N)) and 1,3,5-tris(2',3'-dihydroxybenzamido)benzene (H6L1). An even larger 3-fold symmetric ligand, 1,3,5-tris(4'-(2' '',3' ''-dihydroxybenzamido)-1',1' '-biphenyl)benzene (H6L3) has been prepared but, due to increased flexibility and deviation from the intended 3-fold symmetry, does not undergo self-assembly to form the M4L3(4) structure.

Self-assembled coordination cage as a reaction vessel: triplet sensitized [2+2] photodimerization of acenaphthylene, and [4+4] photodimerization of 9-anthraldehyde

Inner cavity of Pd-nanocage has been used as a reaction vessel for performing triplet sensitized [2+2] photodimerization of acenaphthylene using water soluble xanthene dyes (Eosin Y and Rose Bengal) as sensitizers, and [4+4] photodimerization of 9-anthraldehyde. Although the [4+4] photodimerization of 9-anthraldehyde gave similar results to solution reaction, the xanthene dye sensitized [2+2] triplet state photodimerization of acenaphthylene encapsulated within Pd-nanocage yielded the syn dimer in quantitative yield. The results obtained from the triplet state [2+2] photodimerization of acenaphthylene within Pd-nanocage is remarkable given the fact that the photodimerization reaction when performed in methanol in the presence of Eosin Y and Rose Bengal gave the syn and anti dimers in the ratio 0.5 and 0.6, respectively. Preaggregation of molecules encapsulated inside Pd-nanocage in a syn fashion seems to be the governing factor for such a behavior.

Sequence-Selective Recognition of Peptides within the Single Binding Pocket of a Self-Assembled Coordination Cage

The single binding pocket of a self-assembled Pd6L4 coordination cage recognizes oligopeptides in a highly sequence-selective fashion. In particular, the Trp-Trp-Ala sequence is strongly bound by the cavity (Ka >/=106 M-1). Tripeptides possessing the same residues but in different sequences (i.e., Trp-Ala-Trp and Ala-Trp-Trp) show much poorer affinity. Even singly mutated tripeptides with aromatic-aromatic-aliphatic sequences of the residues (e.g., Trp-Trp-Gly and Trp-Tyr-Ala) are not recognized efficiently. X-ray analysis and NMR reveal that all residues of the Trp-Trp-Ala sequence cooperatively interact with the cage via CH-pi and pi-pi interactions.

Endohedral Clusterization of Ten Water Molecules into a “Molecular Ice” within the Hydrophobic Pocket of a Self-Assembled Cage

An adamantanoid (H2O)10 cluster is formed within the hydrophobic cavity of a self-assembled coordination cage. This cluster is termed "molecular ice" because it is the smallest unit of naturally occurring Ic-type ice. X-ray structural analysis, coupled with neutron diffraction study, reveals that the molecular ice is formed not by a simple space-filling effect but by efficient molecular recognition within the cage via H2O:...pi interaction.

Self-assembled coordination cage as a molecular flask

Abstract Cavity-directed chemical transformations represent one of the most important features in 3-D host chemistry, yet they are unexplored as synthetic receptors. We are developing such functions with the large cavity of self-assembled cages, particularly an M6L4-type cage. The photodimerization of olefins within this cage shows remarkable rate enhancement, perfect regio- and stereoselection, and high pairwise selection (when two different olefins are used) giving only the cross [2+2] adduct. Another intriguing property of the cavity is the stabilization of labile molecules by encapsulation. We succeeded in trapping labile molecules by in situ preparation from small components coming through small portals of the cage. For example, hydrolysis and condensation of PhSi(OMe)3, which ordinarily provides a sophisticated Si-O 3-D network (so-called sol-gel condensation), selectively gave a cyclic trimer, [PhSi(OH)O]3, in a "ship-in-a-bottle" fashion. Cavity-sensitized photochemical oxidation of alkanes within the cage is also discussed.

Cavity-Induced Spin−Spin Interaction between Organic Radicals within a Self-Assembled Coordination Cage

We present a new approach for manipulating spin-spin interaction by the self-assembled M6L4-coordination cage. Stable radicals are encapsulated by the cage to become a radical pair in triplet state in both solid and solution states, although the radicals themselves show doublet character without particular intermolecular interaction in solution. These results were confirmed by ESR spectroscopy and X-ray crystallography.