Cationic gemini guests promote the efficient self-assembly of capsular entities in water at neutral pH

Abstract

The binding features of a sulfonato-calix[4]arene with pyridinium-based gemini guests have been investigated in water at physiological condition by NMR spectroscopy. Results provided converging evidence showing that guests having different size, shape and flexibility formed water-soluble homodimeric supramolecular capsules. The remarkably large encapsulation efficiency of the anionic calixarene host towards these bis(pyridinium) gemini guests results from the favourable combination of electrostatic and hydrophobic interactions. The introduction of short and rigid spacers between the two positively charged heads of the guest improves the efficiency of the capsule formation. The insertion of a substituent into the pyridinium ring of the guest dramatically affects both the association constants and the binding modes.     

Host-[2]rotaxane: advantage of converging functional groups for guest recognition

A host-[2]rotaxane was constructed by converting a diaminophenylcalix[4]arene into a [2]rotaxane using the DCC-rotaxane method (Zehnder, D.; Smithrud, D. B. Org. Lett. 2001, 16, 2485-2486). N-Ac-Arg groups were attached to the dibenzo-24-crown-8 ring of the rotaxane to provide a convergent functional group. To demonstrate the advantage provided by the rotaxane architecture for recognition of guests that contain a variety of functional groups, association constants (K(A)) for N-Ac-Trp, indole, N-Ac-Gly, fluorescein, 1-(dimethylamino)-5-naphthalenesulfonate, and pyrene bound to the [2]rotaxane were determined by performing (1)H NMR and fluorescence spectroscopic experiments. The host-[2]rotaxane had the highest affinity for fluorescein with a K(A) = 4.6 x 10(6) M(-)(1) in a 98/2 buffer (1 mM phosphate, pH 7)/DMSO solution. A comparison of K(A) values demonstrates that both the aromatic pocket and ring of the host-[2]rotaxane contribute binding free energy for complexation. Association constants were also derived for the same guests bound to the diaminophenylcalix[4]arene and to a diphenylcalix[4]arene that contained arginine residues displayed in a nonconvergent fashion. The host-[2]rotaxane provides higher affinity and specificity for most guests than the host with divergent N-Ac-Arg groups of the one that only has an aromatic pocket. For example, the K(A) for the complex of the host-[2]rotaxane and fluorescein in the DMSO/water mixture is more than 2 orders of magnitude greater than association constants derived for the other hosts.     

Introducing negative charges into bis-p-phenylene crown ethers: a study of bipyridinium-based [2]pseudorotaxanes and [2]rotaxanes

This paper describes novel host-guest systems comprising viologen cations (guests) and the derivatives of bis-para-phenylene-34-crown-10 (hosts) with anionic groups COO(-) or SO(3)(-). The structure of the resulting charge-compensated host-guest complexes, their association constants and their electrochemical behaviour have been studied. In the solid state, the viologen cations thread the negatively charged crown ethers forming electroneutral zwitterion-like [2]pseudorotaxane salts; in solution this threaded geometry is preserved. The association constants of [2]pseudorotaxane salts incorporating the 1,1'-diethylviologen moiety in solution are significantly higher than those of previously reported analogues. The extrapolated association free energies in non-aqueous media exceed -40 kJ mol(-1) at 25 degrees C. This significant increase of the interaction free energy makes these compounds stable even in aqueous solutions. The association constants of [2]pseudorotaxane salts incorporating sterically more hindered 1,1'-diethyl-3,3'-dimethylviologen moieties are significantly lower. Structurally related [2]rotaxane salts, in which the oppositely charged ionic components are mechanically interlocked, have been prepared in good yields. It has been shown that [2]rotaxane salts incorporating anti-isomers of bisfunctionalised crown ethers are cycloenantiomeric. In both [2]pseudorotaxane and [2]rotaxane salts, the electrostatic interactions between the viologen moieties and the negatively charged crown ethers lead to very significant negative shifts of viologen reduction potentials up to 450 mV. The findings of the present study are valuable for the design of nanoscale molecular electronic devices.     

Self-assembled hemicapsules with inherent functionalities: modeling of a supramolecular electrostatic self-assembly

An approach to functional self-assembled hemicapsules is described consisting of the use of multivalent (valency > or = 4) and divalent components, the functional groups of which have a relatively weak binding affinity. Electrostatic self-assembly of tetrakis(pyridiniummethyl)cavitand hemispheres (H) and doubly charged anions (A) in polar media gives rise to an equilibrium mixture that consists, as detected with ESI-MS, of hemicapsule H2A3, capsule H2A4, and other ion-pair associates. Fitting 1H NMR data with a model that includes (hemi)capsules and ion-pair associates gave an effective molarity (EM) for the intramolecular assembly of the host (H) with sulfate (A) of 0.19 +/- 0.02 M and binding constants of the functional [2 + 3] hemicapsules H2A3 and the [2 + 4] capsules H(2)A(4) in methanol of 3.25 x 10(12) M(-4) and 3.45 x 10(15) M(-5), respectively. A substantial amount of the functional [2 + 3] hemicapsules H2A3 with respect to [2 + 4] capsules H2A4 is present in solution, with ratios of H2A3 to H2A4 of 5.67-0.43 in the studied concentration range (0.1-25 mM of [H]tot). The [2 + 3] hemicapsules H2A3 built with sulfate linkers incorporate guests between the closely positioned pyridinium planes.     

Kinetics of the electron self-exchange and electron-transfer reactions of the (trimethylammonio)methylferrocene host-guest complex with cucurbit[7]uril in aqueous solution

The electron self-exchange rate constants for the (trimethylammonio)methylferrocene(+/2+) couple (FcTMA+/2+) have been measured in the absence and presence of the cucurbit[7]uril (CB[7]) host molecule in aqueous solution, using 1H NMR line-broadening experiments. The very strong binding of the ferrocene to CB[7] results in slow exchange of the guest on the NMR time scale, such that resonances for both the free and bound forms of the reduced ferrocene can be observed. The extents of line broadening in the resonances of the two forms of the guest in the presence of the FcTMA2+ species can be monitored independently, allowing for the determination of the rate constants for the possible self-exchange pathways involving the bound and free forms of both the oxidized and reduced members of the redox couple. The encapsulation of both the reduced and oxidized forms of the ferrocene increases the rate constant (25 degrees C) from (2.1+/-0.1)x10(6) M-1 s-1 (for FcTMA+/2+) to (6.7+/-0.7)x10(6) M(-1) s(-1) (for {FcTMA.CB[7]}+/2+), whereas inclusion of the reduced form only decreases the rate constant to (6+/-1)x10(5) M(-1) s(-1). The changes in the exchange rate constants upon inclusion of the reactants are related to the effects of CB[7] acting as an outer, second-coordination sphere and are compared to those observed previously for the electron-exchange process in the presence of beta-cyclodextrin and p-sulfonated calix[6]arene hosts. The binding of FcTMA+ and hydroxymethylferrocene to CB[7] significantly reduces the rate constants for their oxidations by the bis(2,6-pyridinedicarboxylato)cobaltate(III) ion (which does not bind to CB[7]) as a result of reduced thermodynamic driving forces and steric hindrance to close approach of the oxidant to the encapsulated ferrocenes.     

Binding selectivity of cucurbit[7]uril: bis(pyridinium)-1,4-xylylene versus 4,4'-bipyridinium guest sites

The binding interactions between the host cucurbit[7]uril (CB7) and a series of linear guests containing bis(pyridinium)-1,4-xylylene and/or 4,4'-bipyridinium residues were investigated by (1)H NMR spectroscopy. CB7 was found to exhibit considerable binding selectivity for bis(pyridinium)-1,4-xylylene over 4,4'-bipyridinium sites. New pseudo-rotaxane and rotaxane compounds were synthesized utilizing the host-guest interactions between CB7 and the surveyed guests. [structure: see text]     

Designed self-assembly of molecular necklaces

This paper reports an efficient strategy to synthesize molecular necklaces, in which a number of small rings are threaded onto a large ring, utilizing the principles of self-assembly and coordination chemistry. Our strategy involves (1) threading a molecular "bead" with a short "string" to make a pseudorotaxane and then (2) linking the pseudorotaxanes with a metal complex with two cis labile ligands acting as an "angle connector" to form a cyclic product (molecular necklace). A 4- or 3-pyridylmethyl group is attached to each end of 1,4-diaminobutane or 1,5-diaminopentane to produce the short "strings" (C4N4(2+), C4N3(2+), C5N4(2+), and C5N3(2+)), which then react with a cucurbituril (CB) "bead" to form stable pseudorotaxanes (PR44(2+), PR43(2+), PR54(2+), and PR53(2+), respectively). The reaction of the pseudorotaxanes with Pt(en)(NO(3))(2) (en = ethylenediamine) produces a molecular necklace [4]MN, in which three molecular "beads" are threaded on a triangular framework, and/or a molecular necklace [5]MN, in which four molecular "beads" are threaded on a square framework. Under refluxing conditions, the reaction with PR44(2+) or PR54(2+) yields exclusively [4]MN (MN44T or MN54T, respectively), whereas that with PR43(2+) or PR53(2+) produces exclusively [5]MN (MN43S or MN53S, respectively). The products have been characterized by various methods including X-ray crystallography. At lower temperatures, on the other hand, the reaction with PR44(2+) or PR54(2+) affords both [4]MN and [5]MN. The supermolecules reported here are the first series of molecular necklaces obtained as thermodynamic products. The overall structures of the molecular necklaces are strongly influenced by the structures of pseudorotaxane building blocks, which is discussed in detail on the basis of the X-ray crystal structures. The temperature dependence of the product distribution observed in this self-assembly process is also discussed.     

Monofunctionalized pillar[5]arene as a host for alkanediamines

Alkanediamines serve as neutral guests for the recently discovered host pillar[5]arene. The proposed [2]pseudorotaxane nature of the superstructure of the 1:1 host-guest complexes is supported by the template-directed synthesis of a related [2]rotaxane. A synthetic route to monofunctional pillar[5]arenes has also been developed, allowing for the creation of a fluorescent sensor for alkylamine binding. The precursors to this host could act as starting points for a large library of monofunctional pillar[5]arene macrocycles.     

Upper rim allyl- and arylazo-coupled calix[4]arenes as highly sensitive chromogenic sensors for Hg2+ ion

The syntheses and chromogenic properties of calix[4]arenes, carrying 5,17-bisallyl-11,23-bis(p-X-phenyl)azo 3a-c, 5,11,17-triallyl-23-(p-X-phenyl)azo 4a-c, and 5,17-bis(hydroxypropyl)-11,23-bis(p-X-phenyl)azo groups on the upper rims 5a,b, are described. Unexpectedly, UV/vis spectra of the very popular 4-(4-nitrophenyl)azophenol-coupled calix[4]arenes 3c and 4c did not show any shift in lambda(max) when 10 different metal perchlorates were added separately to the host in a methanol-chloroform (v/v = 1/399) cosolvent. In contrast, the absorption spectra of calix[4]arenes with either 4-methoxyphenylazo (3b-5b) or 4-phenylazo (3a-5a) on the upper rim showed substantial bathochromic shifts (Deltalambda = 128-162 nm) upon the addition of soft metal ions (such as Hg(2+), Cr(3+), and Cu(2+)). The 4-(4-methoxyphenyl)azophenol-coupled calix[4]arenes (the 3b-5b series) are found to be highly sensitive for mercury ion (Hg(2+)) among the 10 different metal ions screened. Strong interactions between Hg(2+) ion and the 4-(4-methoxyphenyl)azophenol(s) as well as the p-allyl groups are corroborated by the (1)H NMR studies of 3a,b.Hg(2+) complexes. Furthermore, Job's plots revealed 1:1 binding stoichiometry for all these p-allyl- and arylazo-coupled calix[4]arenes with transition metal ions, and Benesi-Hilderbrand plots were used for the determination of their association constants.     

Characterization of self-assembled supramolecular [Ga4L6] host-guest complexes by electrospray ionization mass spectrometry

Self-assembled supramolecular host-guest complexes have been characterized by electrospray ionization mass spectrometry. The spectra obtained by use of a Q-TOF instrument equipped with a Z-spray ion source show primarily the 3- and 4- charge states of the assemblies. The assemblies have the general formula [guest subset Ga4L6]11- where L represents the chelating bidentate catechol ligand 1,5-bis(2',3'-dihydroxy-benzamido)naphthalene and guests are tetramethyl ammonium (Me4N+), tetraethyl ammonium (Et4N+), tetra-n-propyl ammonium (Pr4N+) and decamethylcobaltocenium (Cp*2Co+) cations. For the first time, the mass spectrum of the empty assembly [Ga4L6]12- is reported. This article also reports that provided the electrospray ion source is capable of preserving noncovalent interactions, it is possible to observe host-guest complexes containing both weak binding guests as well as sterically demanding guests in the mass spectra. The present data suggest that electrospray mass spectrometry is a powerful tool for characterization of supramolecular host-guest complexes.     

A fluorophoric-axle-based, nonfluororescent, metallo anti-[3]pseudorotaxane: recovery of fluorescence by means of an axle substitution reaction

A Cu(2+)-templated, multinuclear, nonfluorescent, anti-[3]pseudorotaxane was synthesized on a fluorophoric axle. The Cu(2+)-templated [3]pseudorotaxane was characterized by the electrospray ionization mass spectroscopy (ESI-MS), UV/Vis and EPR spectroscopy, and single-crystal X-ray data. The ESI-MS showed peaks that support the formation of [3]pseudorotaxane. The UV/Vis spectrum of [3]pseudorotaxane in CH(3)CN showed a characteristic d-d band of a Cu(2+) complex at 650 nm. Further, the X-band in the EPR spectrum of [3]pseudorotaxane suggested a distorted square-pyramidal geometry of Cu(2+). Importantly, formation of the [3]pseudorotaxane was confirmed by the single-crystal X-ray structural analysis, which showed that one fluorophoric axle was threaded into two Cu(2+) macrocyclic wheels (MC-Cu(2+)) with an anti conformation. The UV/Vis and fluorescence titration experiments were carried out to follow the solution-state formation of [3]pseudorotaxane by MC-Cu(2+) and fluorophoric axle in CH(3)CN. In both studies, the sigmoidal curve fit supported the formation of 1:2 complex of the fluorophoric axle and MC-Cu(2+) complex. Secondly, the release of the fluorophoric axle from the nonfluorescent [3]pseudorotaxane through the formation of a [2]pseudorotaxane was demonstrated by titrating a solution of the [3]pseudorotaxane with a stronger bidentate chelating ligand, such as 1,10-phenanthroline (Phen). Substitution of the fluorophoric axle from the [3]pseudorotaxane with about 100% efficiency was achieved by the addition of approximately two equivalents of Phen, and the formation of a Phen-threaded [2]pseudorotaxane was established by ESI-MS of the resulting solution and a single-crystal X-ray study. Axle substitution was also confirmed by a fluorescence titration experiment, which showed a step-wise recovery of the fluorescence intensity of the fluorophoric axle. The association constants for the formation of the [3]- and [2]pseudrotaxanes were calculated from the fluorescence and UV/Vis data. In addition, 2,2'-bipyridine (BPy), which is a relatively weaker bidendate chelating ligand compared to Phen, showed an inefficient and incomplete axle substitution of the [3]pseudorotaxane, although BPy previously showed the formation of [2]pseudrotaxane with the MC-Cu(2+) wheel in solution and ESI-MS studies. In this context, the formation of a BPy-threaded [2]pseudrotaxane was further established by single-crystal X-ray diffraction study.     

Orientation of pyrylium guests in cucurbituril hosts

According to recent reports, supramolecular complexes of the pyrylium cation with cucurbit[x]urils (CB[x], x = 7, 8) show promising photoluminescence suitable for electroluminescent devices. In turn, photoluminescence seems to be related to the stereochemistry of the complexes; however, that has been controversial. Here, we report that in H(2)O, 2,6-disubsituted-4-phenyl pyryliums (Pylm) form dimers quantitatively (equilibrium constants >10(4) M(-1)), but they enter as such only in the larger CB[8]. In terms of orientation, (1)H NMR shows that Me-Pylm, Ph-Pylm, and t-Bu-Pylm insert their 4-phenyl groups in either the CB[7] or CB[8] cavity. The orientation of iPr-Pylm in the iPr-Pylm@CB[7] complex is similar. Experimental conclusions are supported by DFT calculations using the M062X functional and the 6-31G(d) basis set. In the case of (iPr-Pylm)(2)@CB[8], (1)H NMR of both the guest and the host indicates that both guests might enter CB[8] from the same side with their iPr groups in the cavity, but DFT calculations leave room for ambiguity. In addition to the size and hydrophobicity of the 2,6-substituents of the guests, as well as the size and flexibility of the hosts, theory reveals the importance of explicit solvation (H(2)O) and finite temperature effects (particularly for (1)H NMR shielding calculations) in the determination of the stereochemistry of those complexes.     

Thermodynamic origins of selective binding affinity between p-sulfonatocalix[4,5]arenes with biguanidiniums

The binding geometries, abilities and thermodynamic parameters for the intermolecular complexation of two water-soluble calixarenes, p-sulfonatocalix[4]arene (SC4A) and p-sulfonatocalix[5]arene (SC5A), with biguanidinium guests, metformin (MFM) and phenformin (PFM), were investigated by (1)H and 2D NMR spectroscopy, X-ray crystallography, and isothermal titration calorimetry (ITC). The obtained results show that biguanidinium guests are captured by calixarenes with the alkyl or aromatic portion immersed into the cavities and the guanidinium portion fixed at the upper-rims. At both acidic and neutral conditions, SC4A always presents stronger binding affinities to biguanidinium guests than SC5A. Moreover, SC4A prefers to include MFM rather than PFM. As a result, the binding selectivity of MFM is up to 44.7 times for the SC4A/SC5A hosts. The intrinsic relationship between binding structures and selectivities were comprehensively analyzed and discussed from the viewpoint of thermodynamics. Finally, the ITC measurements were further performed in phosphate buffer instead of aqueous solution, to examine the buffer effects, counterion effect, and the differences between thermodynamic and apparent association constants.     

Switching of pseudorotaxanes and catenanes incorporating a tetrathiafulvalene unit by redox and chemical inputs

An acyclic polyether 1a, incorporating a central tetrathiafulvalene (TTF) electron donor unit and two 4-tert-butylphenoxy groups at its termini, has been synthesized. Two macrocyclic polyethers containing two different electron donors, namely a TTF unit with, in one case, a 1,4-dioxybenzene ring (2a), and, in the other case (2b), a 1,5-dioxynaphthalene ring system, have also been synthesized. These two macrocyclic polyethers have been mechanically interlocked in kinetically controlled template-directed syntheses with cyclobis(paraquat-p-phenylene) cyclophane (3(4+)) to afford the [2]catenanes 2a/3(4+) and 2b/3(4+), respectively. X-ray crystallography reveals that the [2]-catenane 2b/3(4+) has the TTF unit of 2b located inside the cavity of 3(4+). The spectroscopic (UV/vis and 1H NMR) and electrochemical properties of compounds 1a, 2a, 2b, 2a/3(4+), and 2b/3(4+) and of the [2]pseudorotaxane 1a.3(4+) were investigated. The absorption and emission properties of the mono- and dioxidized forms of the TTF unit in these various species have also been studied. The results obtained in acetonitrile solution can be summarized as follows. (a) While TTF2+ exhibits a strong fluorescence, no emission can be observed for the TTF2+ units contained in the polyethers and in their pseudorotaxanes and catenanes. (b) A donor-acceptor absorption band is observed upon two-electron oxidation of the TTF unit in the macrocyclic polyethers 2a and 2b. (c) The spontaneous self-assembly of 1a and 3(4+) to give the [2]pseudorotaxane 1a.3(4+) is strongly favored (Kass. = 5 x 10(5) L mol-1) but slow (at 296 K, k = 11.3 L mol-1 s-1 and delta G++ = 15.9 kcal mol-1) because of the steric hindrance associated with the bulky end groups of 1a. (d) In the pseudorotaxane 1a.3(4+), the reversible displacement of the cyclophane from the TTF unit in the threadlike substrate occurs on oxidation/reduction of its electroactive components. (e) Switching between the two translational isomers of the catenanes 2a/3(4+) and 2b/3(4+) occurs by cyclic oxidation and reduction of the TTF unit contained in 2a and in 2b, respectively. (f) Addition of o-chloroanil to the pseudorotaxane 1a.3(4+) and to the catenanes 2a/3(4+) and 2b/3(4+) causes the displacement of the TTF unit from the cavity of the cyclophane 3(4+) because of the formation of an adduct between the TTF unit and o-chloroanil.     

Synthesis of a large amino-phenolic cage. Synthesis, crystal structures, and acid-base and coordination behavior toward cations and anions

The synthesis and characterization of the new polyaza-phenolic-macrobicycle 32-hydroxy-1,4,7,10,13,16,19,22-octaazatricyclo-[11.11.7.1(26,30)]-diatriconta-26,28,Delta(30,32)-triene (L) are reported. L incorporates a 2,6-dimethyl-phenolic unit bridging two opposite amine functions of the [24]aneN(8) polyazamacrocyclic base to obtain a large cage. The basicity and binding properties of L toward Cu(II), Zn(II), and Cl(-) were determined by means of potentiometric measurements in aqueous solution (298.1 +/- 0.1 K, I = 0.15 mol dm(-3)). L can add up to six acidic protons, yielding the H(5)L(5+) species or the H(6)L(6+) species, depending on the ionic medium used. The molecular topology of L permits the formation of a highly positive three-dimensional cavity in the polyprotonated species that is able to host the chloride anion. This was detected both using potentiometric data, log K = 41.33 for the reaction L + 6H(+) + Cl(-) = H(6)LCl(5+), and in (35)Cl NMR experiments that showed interactions also with the H(5)L(5+) and H(4)L(4+) species. The anion is probably hosted inside the three-dimensional cavity of L, and stabilized by H-bonding interactions with the ammonium groups, as depicted in the crystal structure of the H(6)L(6+) cation reported. L forms mono- and dinuclear complexes with all the metal ions investigated; the dinuclear species are the only existing species with an L:M(II) molar ratio of 1:2 at pH higher than 6. The phenolate oxygen atom coordinates the two metal ions in a bridged disposition, drawing them inside the macrobicyclic cavity. The two metals were found to be quite isolated by the medium, and were coordinated by all the amine groups of L, as shown by the crystal structure of the dinuclear [Zn(2)H(-1)L](3+) species. This species can bind guests such as hydroxide and phosphate anions. Studies of anion binding in aqueous solution using pyrochatecol violet as the sensing guest revealed that the [Zn(2)H(-1)L](3+) species is able to bind one phosphate at physiological pH.     

Computational studies of inclusion phenomena and synthesis of a novel and selective molecular receptor for 1,4-disubstituted benzenes and 4,4'-disubstituted biphenyls

A new tetracationic molecular receptor has been synthesized and studied by semiempirical molecular orbital theory. This novel macrocycle, 1(4+), derived from pentacyclo[5.0.0.0.(2,6)0.(3,10)0(5,9)]undecane-8,11-dione (PCU-8,11-dione), structurally resembles cyclobis(paraquat-p-phenylene), 2(4+), in which a xylyl group has been replaced by a PCU unit. This derivatization effectively increases the size and flexibility of 1(4+) and changes its electronic, dynamical, and binding properties. A conformational search using Osawa's corner flapping technique and the PM3 semiempirical method identifies eight unique and low-energy 1(4+) conformers. The principal regions of structural variation occur in the bipyridinium torsion and in the ethylene bridges between PCU and the tetracationic unit. The inclusion complexes of 1(4+) with 1,4-disubstituted benzenes and 4,4'-disubstituted biphenyls have been studied by PM3. The first shell of solvation is approximated by the explicit inclusion of 12 acetonitriles in the computed supramolecular complexes. Binding of 1,4-disubstituted benzenes and 4,4'-biphenol is shifted from the 1(4+) geometric center. From the computations, host 1(4+) is predicted to have an enhanced binding preference for benzidine over 4,4'-biphenol, as compared to 2(4+). For all guests computed, 1(4+) binds more strongly than 2(4+). These properties can be exploited in the future design of supramolecular systems with potential applications as nanoscale devices.     

Bis[1,1'-N,N'-(2-picolyl)aminomethyl]ferrocene as a redox sensor for transition metal ions

The compound bis[1,1'-N,N'-(2-picolyl)aminomethyl]ferrocene, L(1), was synthesized. The protonation constants of this ligand and the stability constants of its complexes with Ni(2+), Cu(2+), Zn(2+), Cd(2+) and Pb(2+) were determined in aqueous solution by potentiometric methods at 25 degrees C and at ionic strength 0.10 mol dm(-3) in KNO(3). The compound L(1) forms only 1:1 (M:L) complexes with Pb(2+) and Cd(2+) while with Ni(2+) and Cu(2+) species of 2 [ratio] 1 ratio were also found. The complexing behaviour of L(1) is regulated by the constraint imposed by the ferrocene in its backbone, leading to lower values of stability constants for complexes of the divalent first row transition metals when compared with related ligands. However, the differences in stability are smaller for the larger metal ions. The structure of the copper complex with L(1) was determined by single-crystal X-ray diffraction and shows that a species of 2:2 ratio is formed. The two copper centres display distorted octahedral geometries and are linked through the two L(1) bridges at a long distance of 8.781(10) Angstrom. The electrochemical behaviour of L(1) was studied in the presence of Ni(2+), Cu(2+), Zn(2+), Cd(2+) and Pb(2+), showing that upon complexation the ferrocene-ferrocenium half-wave potential shifts anodically in relation to that of the free ligand. The maximum electrochemical shift ([capital Delta]E(1/2)) of 268 mV was found in the presence of Pb(2+), followed by Cu(2+)(218 mV), Ni(2+)(152 mV), Zn(2+)(111 mV) and Cd(2+)(110 mV). Moreover, L(1) is able to electrochemically and selectively sense Cu(2+) in the presence of a large excess of the other transition metal cations studied.     

Binding modes of cucurbit[6]uril and cucurbit[7]uril with a series of bis-pyridinium compounds

Binding behaviors of cucurbit[6]uril (CB[6]) and cucurbit[7]uril (CB[7]) with a series of bis-pyridinium compounds N, N’-hexamethylenebis(1-alkyl-4-carbamoyl pyridinium bromide) (HBPB-n) (alkyl chain length, n = 6, 8 and 10) guests were investigated using ¹H-NMR, ESI–MS and single crystal X-ray diffraction methods. The results show that CB[6] and CB[7] can form [2]pseudorotaxanes with HBPB-n easily. When increasing the length of tail alkyl chain, the binding site of CB[6] at guest molecules changed from the tail to the middle part, while CB[7] remained located over the tail chain. As CB[6] and CB[7] were added in HBPB-8 aqueous solution, a [3]pseudorotaxane was formed by the inclusion of the internal middle site in CB[6] and the tail chain in CB[7].     

Inclusion of Carboxyl Function Inside of Cucurbiturils and its Use in Molecular Switches

We have prepared organic guest molecules in which two pyridinium rings are connected through an aromatic/aliphatic bridge bearing a carboxyl group. The supramolecular interactions between these guests and macrocyclic hosts cucurbit[7]uril ( CB7 ) and cucurbit[8]uril ( CB8 ) has been studied. We have demonstrated that the binding modes of the complexes depend on the type of central bridge present in the guest molecules and the size of the macrocycle. We have also showed that the binding mode between cucurbiturils and guests with aromatic bridges is pH independent. On the other hand, a guest containing an aliphatic bridge and CB7 formed a pseudorotaxane, which behaved as a pH‐driven molecular switch.     

Triple molecular recognition as a directing element in the formation of host-guest complexes with p-sulfonatocalix[4]arene and beta-cyclodextrin

We have investigated a mixture consisting of p-sulfonatocalix[4]arene (CX4), beta-cyclodextrin (beta-CD), and 2,3-diazabicyclo[2.2.2]oct-2-ene (1) and its bridgehead-substituted derivative (2) in the absence and presence of Zn(2+). In the absence of Zn(2+), four equally populated host-guest complexes exist in solution, as projected from their comparable binding constants (ca. 1000 M(-1)). However, upon the addition of Zn(2+), the formation of a ternary complex, CX4 x 1 x Zn(2+), is induced by a synergy of three supramolecular interactions (Coulombic, hydrophobic, and weak metal-ligand bonding). Concomitantly, the CX4 x 2 complex is destabilized by competitive binding, which drives the system toward a state where only two complexes predominate: namely, CX4 x 1 x Zn(2+) and beta-CD x 2. Known binding constants for the multiple equilibria were used to model the complex system, and the results were consistent with experimental data obtained from 1D and 2D NMR as well as induced circular dichroism (ICD) spectroscopy. The combined results demonstrate how a subtle interplay between cooperative and competitive binding can be exploited to design a complex multicomponent sorting system.