Synthetic Molecular Machine Based on Reversible End-to-Interior and End-to-End Loop Formation Triggered by Electrochemical Stimuli

An electrochemically controlled supramolecular zip tie based on host-guest chemistry of CB[8]

Three molecular threads incorporating two viologen units attached by an oligo-ethylene glycol chain of variable length and an electron rich naphthalene moiety were prepared. The internal viologen unit is connected to the naphthalene by a rigid linker that prevents the simultaneous complexation of these systems. The axle with the shortest chain (14+) in the presence of CB[8] placed the chain inside the macrocycle in aqueous media. In contrast, the longer chains in 24+ and 34+ allow locating the terminal viologen and the electron rich unit inside the cavity. Upon reduction of the bipyridinium salts, the system behaves like a zip tie relaxing the chain as a consequence of the insertion of both radical cation moieties within the CB[8] and making these switches as potential components for molecular machinery.

Molecular Snuggle and Stretch of a Tetraammonium Chain in the Construction of a Hetero-[4]pseudorotaxane with CyclopentanoQ[6] and Classical Q[7]

A hetero-[4]pseudorotaxane was designed to perform a molecular machine function of contraction and expansion utilizing the binding features of CyP₆Q[6] and classical Q[7]. First, the effect on guest binding of equatorial substitution on Q[6]'s was examined by comparing Me₄Q[6] and CyP₆Q[6] against classical Q[6] using eight guest molecules varying in shape, size, neutrality, or cations. Second, the binding data provided optimal structural features for the design of a tetraammonium ion chain to effect the synthesis of the hetero-[4]pseudorotaxane. Finally, the hetero-[4]pseudorotaxane was constructed, and the order of component placement was examined for function and thermodynamic stability in relation to component order on the molecular axle.

Flexible Viologen Cyclophanes: Odd/Even Effects on Intramolecular Interactions

The ability of three bis-viologen cyclophanes to act as redox-triggered contractile switches is investigated. Odd/even effects in the formation of cyclic bis-viologens are circumvented by the use of a Zincke salt intermediate and a tetrathiafulvalene template to prepare a flexible cyclophane with hexyl linkers. Comparative spectro-electrochemical studies of this macrocycle with two other pentyl- or heptyl-linked cyclic bis-viologens show that the development of intramolecular interactions in aqueous solution depends on the length of the bridges. This dependence is confirmed by EPR and DFT studies of the magnetic coupling in the diradical dication species. The anti-ferromagnetic or ferromagnetic nature of the coupling depend, respectively, on the odd or even number of methylene groups in the spacer.

Recognition of silver cations by a cucurbit[8]uril-induced supramolecular crown ether

A supramolecular crown formed with the cucurbit[8]uril-induced intramolecular charge-transfer interaction is able to recognize silver cations.

Amino-ether macrocycle that forms CuII templated threaded heteroleptic complexes: a detailed selectivity, structural and theoretical investigations

Self-sorting behavior of a newly synthesized macrocycle with divalent metal ions and aromatic ligands via pseudorotaxane formation has been described.

Viologen cyclophanes: redox controlled host–guest interactions

Bis(viologen) cyclophanes with alkyl linkers demonstrate redox-dependent host–guest properties, including the formation of a crystalline mixed valence species.

Redox-induced Ru(bpy)(3)(2+)-methylviologen radical formation and its dimerization in cucurbit[8]uril

A redox-induced radical Ru(bpy)(3)(2+)-(CH(2))(n)-MV(+)˙ (n = 4, 7) and its dimerization in cucurbit[8]uril (CB[8]) have been observed concomitantly in aqueous solution, which depends heavily on the length of the carbon chain linkage, in the case of n = 4, the characteristic pattern for the radical dimer is predominating, while that for the radical becomes dominated for n = 7.

Highly stable tetrathiafulvalene radical dimers in [3]catenanes

Two [3]catenane 'molecular flasks' have been designed to create stabilized, redox-controlled tetrathiafulvalene (TTF) dimers, enabling their spectrophotometric and structural properties to be probed in detail. The mechanically interlocked framework of the [3]catenanes creates the ideal arrangement and ultrahigh local concentration for the encircled TTF units to form stable dimers associated with their discrete oxidation states. These dimerization events represent an affinity umpolung, wherein the inversion in electronic affinity replaces the traditional TTF-bipyridinium interaction, which is over-ridden by stabilizing mixed-valence (TTF)2•+ and radical-cation (TTF•+)2 states inside the 'molecular flasks.' The experimental data, collected in the solid state as well as in solution under ambient conditions, together with supporting quantum mechanical calculations, are consistent with the formation of stabilized paramagnetic mixed-valence dimers, and then diamagnetic radical-cation dimers following subsequent one-electron oxidations of the [3]catenanes.

Acyclic cucurbit[n]uril congeners are high affinity hosts

We present the design, synthesis via methylene bridged glycoluril tetramer building blocks, and charaterization of acyclic cucurbit[n]uril congeners that function as hosts for a wide variety of ammonium ions in water. The X-ray crystallographic characterization of the free host and its complexes with p-xylylenediamine and spermine establish the flexibility of the methylene bridged backbone of the acyclic cucurbit[n]uril congeners that allow them to adapt to the structural features of the guest. We find that the acyclic cucurbit[n]uril congeners-with their four contiguous methylene bridged glycoluril units and two aromatic o-xylylene walls bearing CO(2)H substituents-bind to ammonium ions in buffered water with values of K(a) ranging from approximately 10(5) M(-1) to greater than 10(9) M(-1). Similar to the cucurbit[n]uril family of hosts, we find that increasing the concentration of metal cations in the buffer reduces the affinity of the acyclic cucurbit[n]uril congener toward guests by competitive binding at the ureidyl C horizontal lineO portals. Although the acyclic cucurbit[n]uril congeners retain the ability to bind to ammonium ions with high affinity, they do so with lower selectivity than cucurbit[n]urils presumably do to the structural flexibility of the hosts. A methylene bridged glycoluril tetramer model compound that lacks the substituted o-xylylene walls is a much lower affinity host, which establishes the importance of these rings on the overall recognition behavior of the acyclic cucurbit[n]uril congeners. Overall, the results in this paper establish that acyclic cucurbit[n]uril receptors that contain four or more contiguous methylene bridged glycoluril units retain many of the excellent recognition properties of the cucurbit[n]uril family.

Biological catalysis regulated by cucurbit[7]uril molecular containers

We report the synthesis of two-faced inhibitors 1-5 that contain both enzyme inhibitor and cucurbit[n]uril binding domains. The enzyme binding domains of 1-5 bind to the active sites of bovine carbonic anhydrase (BCA) or acetylcholinesterase (AChE) and inhibit their catalytic activities. Addition of CB[7] to BCA*1 and BCA*2 results in the transient formation of the BCA*1*CB[7] and BCA*2*CB[7] ternary complexes that undergo rapid dissociation to form free catalytically active BCA along with CB[7]*1 and CB[7]*2. The on-off cycle can be performed repetitively by the sequential addition of competitive guest 8 and CB[7]. The detailed origins of this on-off switching of the catalytic activity of BCA is delineated by the combined inference of UV/vis catalytic assays, fluorescence displacement assays, (1)H NMR, along with measurement of the fundamental values of K(a), k(on), and k(off) for the various complexes involved. In contrast, addition of CB[7] to AChE*4(4) and AChE*5(4) results in the formation of thermodynamically stable ternary complexes AChE*4(4)*CB[7](4) and AChE*5(4)*CB[7](4) that are catalytically inactive. We highlight some of the advantages and disadvantages of the strategy, based on the direct competition between two receptors (e.g., enzyme and CB[7]) for a common inhibitor, used in this paper to control enzyme catalytic activity compared to the strategy employed by Nature involving the binding of an allosteric small molecule remote from the enzyme active site.

A new three-way supramolecular switch based on redox-controlled interconversion of hetero- and homo-guest-pair inclusion inside a host molecule

A novel three-way supramolecular switch based on the interconversion of hetero-guest-pair (D-A) and homo-guest-pair (D(2) or A(2)) inclusion inside cucurbit[8]uril is reported, which can be selectively controlled by chemical or electrochemical stimuli.