Reaction of Amino-Terminated PAMAM Dendrimers with Carbon Dioxide in Aqueous and Methanol Solutions.

Polyamines have been used as active materials to capture carbon dioxide gas based on its well-known reaction with amines to form carbamates. This work investigates the reactions between three amino-terminated poly(amidoamine) (PAMAM) dendrimers (G1, G3 and G5) and CO2(g) in aqueous (D2O) and methanolic (CD3OD) solutions. The reactions were monitored using 1H NMR spectroscopy, and yielded dendrimers with a combination of terminal carbamate and terminal ammonium groups. In aqueous media the reaction was complicated by the generation of soluble carbonate and bicarbonate ions. The reaction was cleaner in CD3OD, where the larger G5 dendrimer solution formed a gel upon exposure to CO2(g). All reactions were reversible, and the trapped CO2 could be released by treatment with N2(g) and mild heating. These results highlight the importance of the polyamine dendrimer size in terms of driving changes to the solution's physical properties (viscosity, gel formation) generated by exposure to CO2(g).

Cucurbiturils as Effectors on the Self-Assembly of Pd(II) and Pt(II) Metallacycles.

Four bidentate, dicationic ligands (L12+-L42+) were prepared and investigated as guests for binding by the cucurbit[7]uril (CB[7]) host and structural components for metal (Pd and Pt)-coordinated self-assembly into metallacycles. In aqueous solutions, all the ligands were found to form stable complexes of variable stoichiometries with CB[7], and only one (L22+) failed to self-assemble, induced by the presence of suitable Pd or Pt complexes, into metallacycles. Exposure of the Pd-based metallacycles to CB[7] led to their disassembly at room temperature, while the Pt-based metallacycles remained stable under these conditions. However, heating of the Pt metallacycles in the presence of CB[7] also led to their disassembly. This interplay between the interactions in aqueous media of the L12+, L32+, and L42+ ligands with the CB[7] host and Pd (or Pt) complexes suggests the possibility of using these or related systems for controlled drug delivery applications.

Kinetics and Thermodynamics of Binding between Zwitterionic Viologen Guests and the Cucurbit[7]uril Host.

The host-guest binding interactions between a series of zwitterionic, symmetric viologens, and the cucurbit[7]uril host were investigated using NMR and UV-vis spectroscopic techniques. The nature of the viologen side arms had strong effects on the kinetics of association and dissociation and weaker, more uniform influence on the thermodynamic stability of the final 1:1 inclusion complexes, which can also be characterized as pseudorotaxanes.

Visualization and quantitation of electronic communication pathways in a series of redox-active pillar[6]arene-based macrocycles.

While oxidized pillar[5]arenes with 1-5 benzoquinone units are known, very few examples of oxidized pillar[6]arenes have been reported. We describe here the synthesis, characterization and electrochemical behavior of a series of macrocyclic hosts prepared by the stepwise oxidation of 1,4-diethoxypillar[6]arene, resulting in high-yield and high-purity isolation of two constitutional isomers for each macrocycle, in which two, three or four 1,4-diethoxybenzene units are replaced by benzoquinone residues. A careful structural comparison with their counterparts in the pillar[5]arene framework indicates that the geometries of the macrocycles are better described as non-Euclidean hyperbolic hexagons and elliptic pentagons, respectively. A comprehensive computational study to determine anisotropic induced current density (ACID) allows us to visualize and quantify through-space and through-bond communication pathways along the macrocyclic belt. Experimental and simulated voltammetric data, as well as UV-vis spectra, of the new macrocycles afford insights into the various electronic communication pathways in these compounds.

Self-Assembly of Partially Oxidized Pillar[5]arene into Fibrous Structures.

Partial oxidation of the pillar[5]arene macrocycle with Fe(III) leads to the isolation of a fibrous purple solid with high porosity. Experimental and computational data indicate that the solid material is primarily composed of macrocycles in which two of the hydroquinone units have been oxidized to benzoquinone units. Self-assembly takes place through the formation of multiple quinhydrone charge-transfer complexes between hydroquinone and benzoquinone units on adjacent macrocycles, providing a rare example of self-assembly based on a pentagonal molecular symmetry.

Terminal Carboxylate Effects on the Thermodynamics and Kinetics of Cucurbit[7]uril Binding to Guests Containing a Central Bis(Pyridinium)-Xylylene Site.

A series of bis(pyridinium)-xylylene derivatives bearing carboxylate terminal groups were investigated as guests for the cucurbit[7]uril host in aqueous solution. While the presence of the terminal carboxylates has a modest effect on the thermodynamic stability of the complexes, the kinetics of complex association/dissociation is strongly affected. The relative position ( meta, para) of the carboxylate group in relation to the pyridinium nitrogen also exerts a considerable effect on the binding kinetics.

Cucurbit[8]uril (CB[8])-Based Supramolecular Switches.

The use of cucurbit[8]uril as a molecular host has emerged in the chemical literature as a reliable strategy for the creation of dynamic chemical systems, owing to its ability to form homo- and heteroternary complexes in aqueous media with appropriate molecular switches as guests. In this manner, CB[8]-based supramolecular switches can be designed in a predictable and modular fashion, through the selection of appropriate guests able to condition the redox, photochemical, or pH-triggered behavior of tailored multicomponent systems. Furthermore, CB[8] allows the implementation of dual/triple and linear/orthogonal stimuli-dependent properties into these molecular devices by a careful selection of the guests. This versatility in their design gives these supramolecular switches great potential for the rational development of new materials, in which their function is not only determined by the custom-made stimuli-responsiveness, but also by the transient aggregation/disaggregation of homo- or heteromeric building blocks.

Synthesis and Binding Properties of Monohydroxycucurbit[7]uril: A Key Derivative for the Functionalization of Cucurbituril Hosts.

We present a simple, direct method to prepare monohydroxylated cucurbit[7]uril (CB7-OH) through the direct oxidation of its precursor host, cucurbit[7]uril (CB7). Although the conversion takes place in low yield (14%), the isolation of CB7-OH from the reaction mixture is straightforward, and the unreacted CB7 can be easily recovered. ITC measurements with several selected guests confirmed that CB7-OH binds all of them in aqueous solution with similar, albeit slightly lower, binding affinities than those observed with the unmodified CB7 host. ESI mass spectrometric competition experiments are consistent with the ITC measurements. A variety of spectroscopic and voltammetric measurements also verify that the CB7-OH complexes exhibit properties essentially identical to those of the CB7 complexes. DFT computational data also confirm the similar thermodynamic stabilities and structures of the CB7-OH and CB7 inclusion complexes. Finally, the high thermodynamic stability of the CB7-OH complexes was used to improve on the extraction efficiency of stir bar sorptive extraction methods after suitable modification of the active coating with CB7-OH.

Through-Space Communication Effects on the Electrochemical Reduction of Partially Oxidized Pillar[5]arenes Containing Variable Numbers of Quinone Units.

The electrochemical reduction of five partially oxidized pillar[5]arenes containing variable numbers (1-5) of quinone units was investigated in acetonitrile and dichloromethane solutions. The cathodic voltammetric behavior of all the macrocyclic compounds containing two or more quinone units indicates the existence of detectable levels of electronic communication between them. We have also investigated these compounds and their reduced forms using DFT computations. Our computational results indicate the presence of through-space communication between quinone units.

pH Control on the Sequential Uptake and Release of Organic Cations by Cucurbit[7]uril.

Cucurbit[7]uril (CB7) is a macrocycle with the ability to form the most stable supramolecular complexes in water ever reported for an artificial receptor. Its use for the design of advanced functional materials is, however, very limited because there is no example of a fully reversible CB7 based supramolecular complex enabling repetitious dissociation/association controlled by external stimuli. We report the synthesis of a new ferrocene amino acid that forms with CB7 a 1:1 inclusion complex that is stable in submicromolar concentration at low pH but dissociates at high pH. This reversible process was used for the sequential uptake and release of bispyridinium and antraquinone guests by CB7, which is controlled by adjusting the pH of the solution.

Binding Interactions between a Ferrocenylguanidinium Guest and Cucurbit[n]uril Hosts.

The binding interactions between a novel ferrocenylguanidinium derivative (FcG+) and the macrocyclic hosts cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) were investigated in aqueous solution. 1H NMR spectroscopic experiments indicated that both hosts form stable 1:1 inclusion complexes with FcG+, in which the ferrocenyl group is engulfed by the host cavity. The stoichiometry of the CB7·FcG+ complex was also confirmed by electrospray mass spectrometric (ESI MS) experiments. The association equilibrium constants (K) were determined from NMR competition experiments. The measured K values were 3.5 × 109 and 2.5 × 108 M-1 for CB7 and CB8, respectively, in 50 mM sodium acetate-d3 D2O solution (pD 4.7). DFT computational studies confirmed the 1:1 stoichiometry and the inclusion character of both complexes. Voltammetric experiments were carried out to measure the complexation-induced shifts on the half-wave potentials for the one-electron oxidation of the ferrocenyl moiety. Complexation by CB7 led to a 12 mV anodic shift, while CB8 caused a larger 32 mV shift also in the positive direction. These potential shifts suggest that the delocalization of the positive charge on the side arm over the three nitrogens in the guanidinium unit results in electrochemical behavior similar to that observed with neutral ferrocene derivatives.

Cooperative Self-Assembly of a Quaternary Complex Formed by Two Cucurbit[7]uril Hosts, Cyclobis(paraquat-p-phenylene), and a "Designer" Guest.

The self-assembly in aqueous solution of the well-known cyclophane, cyclobis(paraquat-p-phenylene) (BB(4+) ), and two cucurbit[7]uril (CB7) hosts around a simple hydroquinol-based, diamine guest (GH2 (2+) ) was investigated by (1) H NMR and electronic absorption spectroscopies, electrospray mass spectrometry and DFT computations. The formation of a quaternary supramolecular assembly [GH2 (2+) ⋅BB(4+) ⋅ (CB7)2 ] was shown to be a very efficient process, which takes place not only because of the attractive forces between each of the hosts and the guest, but also because of the lateral interactions between the hosts in the final assembly. This complementary set of attractive interactions results in clear cooperative binding effects that help overcome the entropic barriers for multiple component assembly.

The Binding Interactions between Cyclohexanocucurbit[6]uril and Alkyl Viologens Give Rise to a Range of Diverse Structures in the Solid and the Solution Phases.

The binding interactions between the cyclohexanocucurbit[6]uril (Cy6CB6) host and a series of dialkyl-4,4'-bipyridinium (viologen) dicationic guests were investigated in the solution phase, using (1)H NMR spectroscopy, and in the solid phase, using X-ray diffraction methods. In D2O solution, methyl viologen (MV(2+)) and ethyl viologen (EV(2+)) form 1:1 complexes in which the bipyridinium aromatic nucleus is partially included inside the Cy6CB6 cavity. Propyl viologen (PV(2+)), butyl viologen (BV(2+)), pentyl viologen (FV(2+)), and heptyl viologen (HV(2+)) form 2:1 complexes with Cy6CB6, in which each of the viologen aliphatic chains is included by a host molecule. In the solid state, EV(2+) forms a polypseudorotaxane via pseudorotaxane interdigitation of Cy6CB6 hosts. The PV(2+) guest forms a dumbbell-shaped structure with a Cy6CB6 host residing over each of the terminal propyl groups of the guest. In contrast to this, the BV(2+) guest and Cy6CB6 form a different polypseudorotaxane structure in which dumbbell-shaped structures, formed by two host molecules interacting with a single guest, are interconnected through metal-host coordination.

Cathodic Voltammetric Behavior of Pillar[5]quinone in Nonaqueous Media. Symmetry Effects on the Electron Uptake Sequence.

The cathodic voltammetric behavior of pillar[5]quinone was investigated in dichloromethane solution. Our data show that the symmetry of the macrocycle has a pronounced effect on the electron uptake sequence. The uptake of the first five electrons follows a 2-1-2 pattern, and only a total of eight electrons could be injected into the macrocycle under our experimental conditions.

Electrochemistry of viologen dications in cholate media and competition between the cholate assemblies and the cucurbit[7]uril host.

The cathodic voltammetric behavior of N,N'-dimethyl-4,4'-bipyridinium (methylviologen, MV(2+)), N,N'-dipropyl-4,4'-bipyridinium (propylviologen, PV(2+)), N,N'-dibutyl-4,4'-bipyridinium (butylviologen, BV(2+)), and N-heptyl-N'-ethyl-4,4'-bipyridinium (heptyl-ethylviologen, HEV(2+)) was investigated in aqueous solution containing variable concentrations of sodium cholate. In general, the presence of cholate was found to solubilize the more hydrophobic forms of the viologen probes. Among the three accessible viologen oxidation states (V(2+), V(+•), and V), the intermediate cation radical (V(+•)) was preferentially stabilized by the cholate aggregates regardless of the nature of the N-alkyl substituents. This stabilization leads to anodic shifts in the first half-wave potential (V(2+)/V(+•)) and cathodic shifts in the second half-wave potential (V(+•)/V) for viologen reduction. Both potential shifts were considerably more pronounced as the hydrophobic character of the viologen probe increased. The presence of the cucurbit[7]uril host in the solution leads to the formation of very stable inclusion complexes with the viologen probes, which tend to eliminate or substantially decrease the interactions between the viologens and the cholate micellar aggregates.

Rotaxane formation by cucurbit[7]uril in water and DMSO solutions.

The cucurbit[7]uril (CB7) host forms rotaxane-type complexes with dumbbell-shaped, cationic guests bis(3,5-dimethoxybenzyl)-4,4'-bipyridinium (1(2+)) and bis(3,5-diethoxybenzyl)-4,4'-bipyridinium (2(2+)). The kinetics of complex formation is slower with the latter guest because of its bulkier end groups. Rotaxane formation was found to be thermodynamically more favorable and kinetically faster in D2O than in DMSO-d6 solution, which highlights the importance of hydrophobic interactions in the assembly of cucurbituril complexes.

Toward reversible control of cucurbit[n]uril complexes.

CONSPECTUS: The cucurbit[n]uril (CBn) host family consists of a group of rigid macrocyclic hosts with barrel-like shapes and limited solubility in aqueous media. These hosts are capable of reaching high binding affinities with positively charged hydrophobic guests. In optimum cases, equilibrium association constant (K) values as high as 10(17) M(-1) have been reported, exceeding the binding affinity of the avidin-biotin host-guest pair. The synthetic CBn receptors have shattered the notion that highly stable noncovalent complexes can form only when one of the partners is a molecule of biological origin. The work described in this Account is concerned with the development of methods geared toward the reversible modulation of the binding affinity of CBn inclusion complexes under mild conditions. A good fraction of the research work has dealt with redox active guests, such as 4,4'-bipyridinium (viologen), ferrocene, and cobaltocenium derivatives. Our experimental results show that the thermodynamics and kinetics of the electron transfer reactions of these compounds can be substantially altered by complexation with CBn hosts, and therefore, electron transfer reactions can be used to exert a measure of control on the overall binding affinity of the CBn complexes. We have also developed systems in which proton transfer reactions have a strong effect on the binding affinity. With more structurally elaborate guests containing more than one adjacent binding sites, proton transfer reactions may affect the average location of the CBn host within the complexes. A series of guest compounds containing paramagnetic 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) residues also exhibit interesting binding properties with CB7 and CB8. The latter host forms a very stable host-guest pair with TEMPO subunits, in which the nitroxide group resides inside the host cavity. Finally, with suitable ditopic guests, we have detected distinct microscopic complexes using experimental techniques with relatively slow time scales, such as NMR spectroscopy. These unusual findings are the result of the considerable thermodynamic and kinetic stability of CBn inclusion complexes.

Water-mediated inclusion of benzoates and tosylates inside the bambusuril macrocycle.

A supramolecular complex between benzoates and a bambusuril crystallizes out immediately after mixing in chloroform but only in the presence of residual water molecules. In this complex each of the two portals of the macrocycle is occupied by one benzoate. Carboxylate groups are connected through hydrogen bonding interactions with one molecule of water positioned between them in the center of the bambusuril cavity. Similar water assisted host-guest behavior was also observed when tosylates instead of benzoates were used.

Detection of isomeric microscopic host-guest complexes. A time-evolving cucurbit[7]uril complex.

The formation of inclusion complexes between the cucurbit[7]uril host and a cationic guest containing ferrocenylmethyl and adamantyl residues connected to an ammonium nitrogen initially leads to an ~1:1 mixture of two isomeric microscopic complexes, which evolves as a function of time toward the thermodynamically stable mixture, dominated by the adamantyl-included complex.

Cucurbituril hosts in real-life action.

Stick it to them! Underwater adhesion becomes possible by taking advantage of highly stable host-guest complexes formed between cucurbit[7]uril hosts and ferrocenyl residues. The formation of multiple noncovalent complexes between the two complementary surfaces gives rise to strong adhesive forces resulting in a "supramolecular velcro" system that operates efficiently in the presence of water.