Guest Encapsulation within Self-Assembled Molecular Containers

Using Slippage to Construct a Prototypical Molecular "Lock & Lock" Box

We report a new slippage system based on p-tert-butylbenzyl-terminated imidazolium ions and di(ethylene glycol)-containing macrocycles and their use as linking units for the construction of a prototypical molecular "Lock & Lock" box from a resorcinarene-based cavitand "bowl" and a porphyrin "cover". The multivalent structure with four slippage linkers provided the molecular box with high stability, yet the system dissociated into its two components upon application of suitable external stimuli.

Design and Applications of Water-Soluble Coordination Cages

Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

A pentazolate-based bowl-shaped molecular container

A three-dimensional bowl-shaped molecular container based on pentazole was first synthesized. These containers are sealed and linked by the assembled "molecular plane". Each container has an ovoid cavity occupied by one DMSO guest molecule. The self-assembly of this molecular container will provide opportunities for the use of pentazole in supramolecular chemistry.

Hexaphenolic Rigid Cages Prepared by Self-Organization of C3 v Tridentates

Coordination cages were composed by self-organization of rigid C_{3 v}-symmetric heptaarene tridentates and Pd(II) precursors. The heptaarene framework involves one mesitylene, three phenol, and three pyridine moieties, which were connected by Suzuki coupling reactions. The treatment of the tridentates with Pd(dppp)(OTf)₂ or Pd(en)(NO₃)₂ in a 2:3 molar ratio furnished coordination cages, which was ascertained by crystallography, ¹H NMR and DOSY measurements, and ESI-TOFMS and UV-vis spectra. The cages have six phenolic hydroxy groups inside and were expected to incorporate hydrogen-bonding guest molecules such as saccharides. CD and DOSY measurements showed that octyl hexoside guests could be incorporated into the cage.

Anionic Hosts for the Incorporation of Cationic Guests

Pentaphosphaferrocene [Cp*Fe(η5 -P5 )] (1 a) represents an excellent building block for the template-directed synthesis of spherical supramolecules. Here, the self-assembly of 1 a with CuI and CuII halides in the presence of the template complexes [FeCp2 ][PF6 ], [CoCp2 ][PF6 ] and [CoCp2 ] is reported, testifying to the redox behavior of the formed supramolecules. The oxidation or reduction capacity of these reactive complexes does not inhibit their template impact and, for the first time, the cationic metallocene [CoCp2 ]+ is enclosed in unprecedented anionic organometallic hosts. Furthermore, the large variety of structural motifs, as icosahedral, trigonal antiprismatic, cuboidal and tetragonal antiprismatic arrangements of 1 a units are realized in the supramolecules [FeCp2 ]@[{1 a}12 (CuBr)17.3 ] (3), [CoCp2 ]+3 {[CoCp2 ]+ @[{1 a}8 Cu24.25 Br28.25 (CH3 CN)6 ]4- } (4), {[Cp2 Co]+ @[{1 a}8 (CuI)28 (CH3 CN)9.8 ]}{[Cp2 Co]+ @[{1 a)}8 Cu24.4 I26.4 (CH3 CN)8 ]2- } (5), and [{1 a}3 {(1 a)2 NH}3 Cu16 I10 (CH3 CN)7 ] (6), respectively.

Metal–organic redox vehicles to encapsulate organic dyes for photocatalytic protons and carbon dioxide reduction

By encapsulation of an organic dye, a supramolecular nickel–organic macrocycle for the photocatalytic reduction of protons and CO₂ has been reported.

Self-assembled copper(II) metallacycles derived from asymmetric Schiff base ligands: efficient hosts for ADP/ATP in phosphate buffer

Novel asymmetric Schiff base ligands 2-{[3-(3-hydroxy-1-methyl-but-2-enylideneamino)-2,4,6-trimethylphenylimino]-methyl}-phenol (H2L(1)) and 1-{[3-(3-hydroxy-1-methyl-but-2-enylideneamino)-2,4,6-trimethylphenylimino]-methyl}-naphthalen-2-ol (H2L(2)) possessing dissimilar N,O-chelating sites and copper(ii) metallacycles (CuL(1))4 (1) and (CuL(2))4 (2) based on these ligands have been described. The ligands and complexes have been thoroughly characterized by satisfactory elemental analyses, and spectral (IR, (1)H, (13)C NMR, ESI-MS, UV/vis) and electrochemical studies. Structures of H2L(2) and 1 have been unambiguously determined by X-ray single crystal analyses. The crystal structure of H2L(2) revealed the presence of two distinct N,O-chelating sites on dissimilar cores (naphthalene and β-ketoaminato groups) offering a diverse coordination environment. Metallacycles 1 and 2 having a cavity created by four Cu(ii) centres coordinated in a homo- and heteroleptic fashion with respective ligands act as efficient hosts for adenosine-5'-diphosphate (ADP) and adenosine-5'-triphosphate (ATP) respectively, over other nucleoside polyphosphates (NPPs). The disparate sensitivity of these metallacycles toward ADP and ATP has been attributed to the size of the ligands assuming diverse dimensions and spatial orientations. These are attuned for π-π stacking and electrostatic interactions suitable for different guest molecules under analogous conditions, metallacycle 1 offers better orientation for ADP, while 2 for ATP. The mechanism of the host-guest interaction has been investigated by spectral and electrochemical studies and supported by molecular docking studies.

Tunable porosities and shapes of fullerene-like spheres

The formation of reversible switchable nanostructures monitored by solution and solid-state methods is still a challenge in supramolecular chemistry. By a comprehensive solid state and solution study we demonstrate the potential of the fivefold symmetrical building block of pentaphosphaferrocene in combination with Cu(I) halides to switch between spheres of different porosity and shape. With increasing amount of CuX, the structures of the formed supramolecules change from incomplete to complete spherically shaped fullerene-like assemblies possessing an Ih -C80 topology at one side and to a tetrahedral-structured aggregate at the other. In the solid state, the formed nano-sized aggregates reach an outer diameter of 3.14 and 3.56 nm, respectively. This feature is used to reversibly encapsulate and release guest molecules in solution.

Gas-phase chemistry of molecular containers

The use of mass spectrometry for the investigation of supramolecular capsules and containers in solution and the gas-phase is reviewed.

A nano-sized supramolecule beyond the fullerene topology

The reaction of [Cp(Bn) Fe(η(5) -P5 )] (1) (Cp(Bn) =η(5) -C5 (CH2 Ph)5 ) with CuI selectively yields a novel spherical supramolecule (CH2 Cl2 )3.4 @[(Cp(Bn) FeP5 )12 {CuI}54 (MeCN)1.46 ] (2) showing a linkage of the scaffold atoms which is beyond the Fullerene topology. Its extended CuI framework reveals an outer diameter of 3.7 nm-a size that has not been reached before using five-fold symmetric building blocks. Furthermore, 2 shows a remarkable solubility in CH2 Cl2 , and NMR spectroscopy reveals that the scaffold of the supramolecule remains intact in solution. In addition, a novel 2D polymer [{Cp(Bn) Fe(η(5) -P5 )}2 {Cu6 (μ-I)2 (μ3 -I)4 }]n (3) with an uncommon structural motif was isolated. Its formation can be avoided by using a large excess of CuI in the reaction with 1.

Cerium-based M4L4 tetrahedra as molecular flasks for selective reaction prompting and luminescent reaction tracing

The application of metal-organic polyhedra as "molecular flasks" has precipitated a surge of interest in the reactivity and property of molecules within well-defined spaces. Inspired by the structures of the natural enzymatic pockets, three metal-organic neutral molecular tetrahedral, Ce-TTS, Ce-TNS and Ce-TBS (H6TTS: N',N'',N'''-nitrilotris-4,4',4''-(2-hydroxybenzylidene)-benzohydrazide; H6TNS: N',N'',N'''-nitrilotris-6,6',6''-(2-hydroxybenzylidene)-2-naphthohydrazide; H6TBS: 1,3,5-phenyltris-4,4',4''-(2-hydroxybenzylidene)benzohydrazide), which exhibit different size of the edges and cavities, were achieved through self-assembly by incorporating robust amide-containing tridentate chelating sites into the fragments of the ligands. They acted as molecular flasks to prompt the cyanosilylation of aldehydes with excellent selectivity towards the substrates size. The amide groups worked as trigger sites and catalytic driven forces to achieve efficient guest interactions, enforcing the substrates proximity within the cavity. Experiments on catalysts with the different cavity radii and substrates with the different molecular size demonstrated that the catalytic performance exhibited enzymatical catalytic mechanism and occurred in the molecular flask. These amides were also able to amplify guest-bonding events into the measurable outputs for the detection of concentration variations of the substrates, providing the possibility for metal-organic hosts to work as smart molecular flasks for the luminescent tracing of catalytic reactions.

Cerium-based M4L4 tetrahedrons containing hydrogen bond groups as functional molecular flasks for selective reaction prompting

Metal–organic tetrahedrons with abundant hydrogen bond groups work as “molecular flasks” to prompt Knoevenagel condensation and cyanosilylation reactions.

Capsular complexes of nonpolar guests with octa amine host detected in the gas phase

Nanocapsules, made up of the deep cavitand octa amine and several guests, were prepared in aqueous acidic solution and were found to be stable in the gas phase as detected by electrospray ionization mass spectrometry (ESI-MS). The observed gas phase host-guest complexes contained five positive charges and were associated with several acid molecules (HCl or HBr).

Templated construction of a Zn-selective protein dimerization motif

Here, we report that the approach of metal-templated ligand synthesis can be applied to construct a dimeric protein assembly ((BMOE)RIDC1(2)), which is stabilized by noncovalent interactions and flexible covalent cross-linkers around the Zn templates. Despite its flexibility, (BMOE)RIDC1(2) selectively binds Zn(II) over other divalent metals and undergoes dimerization upon metal binding. Such simultaneous fulfillment of plasticity and selectivity is a hallmark of cellular signaling events that involve ligand/metal-induced protein dimerization.

Metal-organic cyclohelicates as optical receptors for glutathione: syntheses, structures, and host-guest behaviors

Two trinuclear zinc-based cyclohelicates, Zn-PDB (PDB = [5-(dibenzylamino)-N1',N3'-bis(pyridin-2-ylmethylene)isophthalohydrazide]) and Zn-PMB (PMB = [5-(bodipy-oxy)-N1',N3'-bis(pyridin-2-ylmethylene)isophthalohydrazide]) containing dibenzylamino and BODIPY groups, respectively, were generated by incorporating two amide-containing tridentate chelators into meta-positions of a substituted phenyl ring. Single-crystal structure analysis and related spectroscopic characterizations demonstrated the formation of macrocyclic helicals both in the solid state and in solution. The host-guest behavior of the cyclohelical hosts towards γ-glutamyl-cysteinyl-glycine (GSH) and its component amino acids was investigated by spectroscopic titrations. UV/Vis absorption titration and NMR titrations of Zn-PDB and Zn-PMB upon addition of the above-mentioned guests suggested that the Glu residue of GSH was positioned within the cavity. The COO(-) groups interacted with metal ions through static interactions. The Cys moiety of GSH interacted with the amide groups sited in host molecules through hydrogen-bonding interactions to produce measurable spectral changes. Fluorescent titrations of Zn-PMB upon the addition of GSH and ESI-MS investigations of the titration solutions confirmed the host-guest interaction modes and revealed the possible 1:1 complexation stoichiometry. These results showed that the recognition of a substrate within the cavity of functionalized metal-organic cage-like receptors could be a useful method to produce supramolecular sensors for biomolecules.

Encapsulation of small gas molecules by cryptophane-111 in organic solution. 1. Size- and shape-selective complexation of simple hydrocarbons

The reversible trapping of small hydrocarbons and other gases by cryptophane-111 (1) in organic solution was characterized with variable-temperature (1)H NMR spectroscopy. Characteristic spectral changes observed upon guest binding allowed kinetic and thermodynamic data to be readily extracted, permitting quantification and comparison of different host-guest interactions. Previous work (J. Am. Chem. Soc. 2007, 129, 10332) demonstrated that 1, the smallest cryptophane to date, forms a complex with xenon with remarkably high affinity. Presently, it is shown that 1 also exhibits slow exchange dynamics with methane at reduced temperatures (delta(bound) = -5.2 ppm) with an association constant K(a) = 148 M(-1) at 298 K. In contrast, ethane and ethylene are poorly recognized by 1 with K(a) values of only 2 M(-1) and 22 M(-1), respectively; moreover, chloromethane (whose molecular volume is similar to that of xenon, approximately 42 A(3)) is not observed to bind to 1. Separately, molecular hydrogen (H(2)) gas is observed to bind 1, but in contrast to other ligands presently studied, H(2) complexation is spectrally manifested by fast exchange throughout virtually the entire range of available conditions, as well as by a complex dependence of the guest (1)H resonance frequency upon temperature and host concentration. Taken together, these results establish 1 as a selective host for small gases, with implications for the design of size- and geometry-selective sensors targeted for various gas molecules.

Design and synthesis of a stable supramolecular trigonal prism formed by the self-assembly of a linear tetrakis(Zn2+-cyclen) complex and trianionic trithiocyanuric acid in aqueous solution and its complexation with DNA (cyclen = 1,4,7,10-tetraazacyclododecane)

A new supramolecular complex, {(Zn(4)L(4))(3)-(TCA(3-))(4)}(12+), was designed and synthesized by the 3:4 self-assembly of a linear tetrakis(Zn(2+)-cyclen) complex (Zn(4)L(4))(8+) and trianionic trithiocyanurate (TCA(3-)) in aqueous solution (cyclen = 1,4,7,10-tetraazacyclododecane). The {(Zn(4)L(4))(3)-(TCA(3-))(4)}(12+) complex, which should have a trigonal prism configuration, was found to be very stable in aqueous solution at neutral pH and 25 degrees C, as evidenced by (1)H NMR titration, potentiometric pH and UV titrations, and MS measurements. The complex does not dissociate into the starting building blocks in the presence of Zn(2+)-binding anions such as phosphates and double-stranded DNA. The results of the competitive binding assays with ethidium bromide and calf-thymus DNA, thermal melting experiments, gel mobility shift assays, and dynamic light-scattering data strongly indicated that the trigonal prism functions as a polycationic template to induce the aggregation of double-stranded DNA.