Competitive interactions of two ion-paired salts with a neutral host to form two non-ion-paired complexes

It is demonstrated that our reported equilibrium treatments that take into account ion-paired guest and non-ion-paired complexes can be applied to competitive complexations. Satisfactory results were obtained for a system with two cationic guests [N,N'-dimethyl-4,4'-biyridinium bis(hexafluorophosphate) (1) and dibenzylammonium hexafluorophosphate (2)] having a common counterion and a single neutral host dibenzo-24-crown-8 (3), even though for this system one exchange process is slow and the other fast on the 1H NMR time scale. The competitive complexation protocol presented here provides a convenient method for the determination of KapKipd (the product of the ion-pair dissociation constant of the guest salt and the association constant for the host with the resultant free cation) for new systems from ion-paired guests that form complexes that are not ion paired.     

Bis(m-phenylene)-32-crown-10-based cryptands, powerful hosts for paraquat derivatives

Four new bis(m-phenylene)-32-crown-10-based cryptands with different third bridges were prepared. Their complexes with paraquat derivatives were studied by proton NMR spectroscopy, mass spectrometry, and X-ray analysis. It was found that these cryptands bind paraquat derivatives very strongly. Specifically, a diester cryptand with a pyridyl nitrogen atom located at a site occupied by either water or a PF(6) anion in analogous complexes exhibited the highest association constant K(a) = 5.0 x 10(6) M(-1) in acetone with paraquat, 9000 times greater than the crown ether system. X-ray structures of this and analogous complexes demonstrate that improved complexation with this host is a consequence of preorganization, adequate ring size for occupation by the guest, and the proper location of the pyridyl N-atom for binding to the beta-pyridinium hydrogens of the paraquat guests. This readily accessible cryptand is one of the most powerful hosts reported for paraquats.     

Complexation between Methyl Viologen (Paraquat) Bis(Hexafluorophosphate) and Dibenzo[24]Crown‐8 Revisited

Paraquat bis(hexafluorophosphate) undergoes stepwise dissociation in acetone. All three species—the neutral molecule, and the mono‐ and dications—are represented significantly under the experimental conditions typically used in host–guest binding studies. Paraquat forms at least four host–guest complexes with dibenzo[24]crown‐8. They are characterized by both 1:1 and 1:2 stoichiometries, and an overall charge of either zero (neutral molecule) or one (monocation). The monocationic 1:1 host–guest complex is the most abundant species under typical (0.5–20 mM ) experimental conditions. The presence of the dicationic 1:1 host–guest complex cannot be excluded on the basis of our experimental data, but neither is it unambiguously confirmed to be present. The two confirmed forms of paraquat that do undergo complexation—the neutral molecule and the monocation—exhibit approximately identical binding affinities toward dibenzo[24]crown‐8. Thus, the relative abundance of neutral, singly, and doubly charged pseudorotaxanes is identical to the relative abundance of neutral, singly, and doubly charged paraquat unbound with respect to the crown ether in acetone. In the specific case of paraquat/dibenzo[24]crown‐8, ion‐pairing does not contribute to host–guest complex formation, as has been suggested previously in the literature.     

Two bis(p-phenylene)-34-crown-10-based cryptand constitutional isomers: different binding abilities induced by structural alterations

Two novel bis(p-phenylene)-34-crown-10-based cryptand constitutional isomers were prepared and their host–guest complexations with paraquat were studied by ESI-MS, UV–vis spectroscopy, ¹H NMR spectra, and X-ray crystal structures. Notably, though the only difference between the two hosts is the location of the nitrogen atom on the third arms, they exhibited quite different binding abilities with paraquat. Competitive complexation was carried out and it may provide a simple way to construct sophisticated supramolecular materials with reversibility and adaptability.     

Isomeric 2,6-pyridino-cryptands based on dibenzo-24-crown-8

Cryptands 4 and 5 were synthesized from cis- and trans-bis(hydroxymethylbenzo)-24-crown-8 by reaction with pyridine-2,6-dicarboxylic acid chloride in 42 and 48% yields, respectively. The new cryptands form pseudorotaxanes with the paraquat derivative N,N'-bis(beta-hydroxyethyl)-4,4'-bipyridinium bis(hexafluorophosphate) ("paraquat diol", 6): Ka=1.0x10(4) and 1.4x10(4) M-1, respectively. The cryptands also form complexes with ammonium hexafluorophosphate. Formation of the paraquat/cryptand-based pseudorotaxanes can be switched off and on in a controllable manner on the basis of the cryptands' abilities to complex KPF6 strongly, providing a new mechanism for control of molecular shuttles. K+ displaces paraquat diol from the cryptands, converting yellow-orange solutions to colorless; however, addition of 18-crown-6 binds the KPF6 and allows the colored cryptand-paraquat complex to reform. Crystal structures are reported for both cryptands, both paraquat diol-based pseudorotaxanes, both NH4PF6 complexes, and both KPF6 complexes.     

Crown ether-based cryptand/tropylium cation inclusion complexes

Host–guest complexation between crown ether-based cryptand hosts and a carbonium ion, tropylium hexafluorophosphate was studied. ¹H NMR, NOESY NMR, and electrospray ionization mass spectrometry were employed to characterize these inclusion complexes. The contrast tests of ¹H NMR and association constants indicated that cryptands are much better hosts for tropylium hexafluorophosphate than the corresponding simple crown ethers. C–H?O hydrogen bonding, face-to-face π-stacking interactions, and charge-transfer interactions are thought to be the main driving forces for the formation of these host–guest complexes. These multiple non-covalent interactions may jointly contribute to the complex formation and considerably reinforce the complex stability. Moreover, the complexation between dibenzo-24-crown-8-based cryptand 4 and tropylium hexafluorophosphate 7 can be reversibly controlled by adding KPF₆ and then DB18C6 in 1:1 acetonitrile/chloroform, providing a new cation-responsive host–guest recognition motif for supramolecular chemistry.     

Cooperative molecular recognition of dyes by dyad and triad cyclodextrin-crown ether conjugates

Three beta-cyclodextrin (beta-CyD) derivatives with crown ether units, that is N-(4'-benzo-15-crown-5)-6-imino-6-deoxy-beta-CyD (2), 6,6'-[N-(4,4'-dibenzo-18-crown-6)-imino]-bridged bis(beta-CyD)(3), and 2,2'-[O-(4',5'-benzo-15-crown-5)-ethyl]-bridged bis (beta-CyD)(5), were synthesized as cooperative recognition receptor models. Their molecular binding behavior with four representative fluorescent dyes, i.e., ammonium 8-anilino-1-naphthalenesulfonate (ANS), sodium-6-toluidino-2-naphthalene-sulfonate (TNS), Acridine Red (AR) and Rhodamine B (RhB), was investigated in buffer solutions (pH = 7.20) at 25 degreesC by means of circular dichroism, NMR and fluorescence spectroscopy. 2D-ROESY experiments showed that dyad host 2 and triad host 3 adopted a CyD-guest-crown ether binding mode, while triad host 5 adopted a CyD-guest-CyD binding mode, upon inclusion complexation with guest molecules. Therefore, hosts 2 and 3 showed high molecular recognition ability towards charged guests, giving an enhanced binding ability up to 115 times for ANS by 3 and fairly high molecular selectivity up to 1450 times for the ANS/AR pair by 2 as compared with native beta-CyD in an aqueous phosphate buffer solution. On the other hand, host 5 was found to be able to effectively recognize the shape of a guest molecule, showing significantly higher binding ability towards linear guests. The binding affinities and molecular recognition abilities of these CyD-crown ether conjugates towards guest molecules are discussed from the viewpoint of electrostatic and/or hydrophobic interactions, size/shape-fit concept, and multiple recognition mechanism between host and guest.     

2]Pseudorotaxanes based on the recognition of cryptands to vinylogous viologens

Host-guest complexation between two crown ether-based cryptands and two vinylogous viologens has been studied. Formation of [2]pseudorotaxanes from a dibenzo-24-crown-8-based cryptand and these vinylogous viologens can be reversibly controlled by adding and removing potassium cation in acetone. Furthermore, the complexation between a bis(m-phenylene)-32-crown-10-based cryptand and a vinylogous viologen exhibits a high association constant, 1.18 × 10(6) M(-1) in acetone, and leads to the formation of a supramolecular poly[2]pseudorotaxane in the solid state.     

A triptycene-based bis(crown ether) host: complexation with both paraquat derivatives and dibenzylammonium salts

[reaction: see text] A novel triptycene-based bis(crown ether) host (1) incorporating two dibenzo-24-crown-8 ether moieties has been synthesized. It can form not only a new bis[2]pseudorotaxane with dibenzylammonium salts but also stable clip-shaped complexes with paraquat derivatives. Moreover, the complexation process between 1 and the two classes of guests can be chemically controlled.     

Ion pairing and host-guest complexation in low dielectric constant solvents

We report an equilibrium treatment for complexation of ionic species in low dielectric constant media that explicitly includes ion pairing of one of the components. Experimental validation was achieved through study of pseudorotaxane formation between dibenzylammonium salts and dibenzo-24-crown-8. In particular, we show that concentration-dependent fluctuations in the apparent K(a,exp) values as usually reported are attributable to ion pairing, with dissociation constant K(ipd), and that the constant K(ap) for complexation of the free cationic guest species, G(+), by the host crown ether is independent of counterion. More generally, using a simple extension of our model, we show the ability to diagnose the relative extent of ion pairing of the complex, which may be readily applied to other host-guest systems involving ionic species.     

Cooperative host/guest interactions via counterion assisted chelation: pseudorotaxanes from supramolecular cryptands

Addition of di- or tritopic hydrogen bond accepting anions to solutions of bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10 and paraquat di(hexafluorophosphate) serves to enhance host/guest interaction. In particular, addition of Et4N+CF3COO- effectively boosts Ka 14-fold, as estimated by 1H NMR studies. Similar increases in apparent Ka values are observed upon addition of n-Bu4N+OTs-. Evidenced by crystal structures, the increased association results from chelation of the OH moieties of the crown by the di- or tritopic anions, forming supramolecular bicyclic macrocycles and stabilizing the complex in a cooperative manner.     

Selective binding behaviors of p-sulfonatocalixarenes in aqueous solution

The complex structures, binding abilities, molecular selectivities, and thermodynamic origin of p-sulfonatocalixarenes upon complexation with kinds of guests are outlined in this review article, including inorganic cations, organic ammonium cations, pyridiniums and viologens, neutral organic molecules, dye molecules, and others. Calorimetric and spectroscopic investigations afford the complex stability constants, thermodynamic parameters and binding manners of the inclusion complexation of p-sulfonatocalixarenes with guest molecules. The π-stacking, hydrophobic and charge interactions are the main driving-forces during the course of the host–guest inclusion complexation. The molecular binding abilities and selectivities are influenced by not only the frameworks of calixarene cavities, structures of guest molecules, and their binding manners but also the conditions of solutions (mainly pH), which are discussed from the correlation between the structural features and molecular-recognition abilities. Moreover, the further applications and potentials of p-sulfonatocalixarenes are briefly described.     

Complexation of polyvalent cyclodextrin ions with oppositely charged guests: entropically favorable complexation due to dehydration

Thermodynamic parameters for complexation of polyvalent cyclodextrin (CD) cation and anion with oppositely charged guests have been determined in D2O containing 0.02 M NaCl by means of 1H-NMR spectroscopy. Protonated heptakis(6-amino-6-deoxy)-beta-CD (per-NH3+-beta-CD) forms stable inclusion complexes with monovalent guest anions. The enthalpy (deltaH) and entropy changes (deltaS) for complexation of per-NH3+-beta-CD with p-methylbenzoate anion (p-CH3-Ph-CO2-) are 3.8 +/- 0.7 kJ mol(-1) and 88.6 +/- 2.2 J mol(-1) K(-1), respectively. The deltaH and deltaS values for the native beta-CD-p-CH3-Ph-CO2- system are -8.6 +/- 0.1 kJ mol(-1) and 15.3 +/- 0.7 J mol(-1) K(-1), respectively. The thermodynamic parameters clearly indicate that dehydration from both the host and guest ions accounts for the entropic gain in inclusion process of p-CH3-Ph-CO2- into the per-NH3+-beta-CD cavity. The fact that the neutral guests such as 2,6-dihydroxynaphthalene and p-methylbenzyl alcohol hardly form the complexes with per-NH3+-beta-CD exhibits that van der Waals and/or hydrophobic interactions do not cause the complexation of the polyvalent CD cation with the monovalent anion. The acetate anion is not included into the per-NH3+-beta-CD cavity, while the butanoate and hexanoate anions form the inclusion complexes. The complexation of the alkanoate anions is entropically dominated. Judging from these results, it may be concluded that Coulomb interactions cooperated with inclusion are required for realizing the large entropic gain due to extended dehydration. Entropically favorable complexation was also observed for the anionic CD-cationic guest system. The present study might present a general mechanism for ion pairing in water.     

A new functional bis(m-phenylene)-32-crown-10-based cryptand host for paraquats

The pseudorotaxane complex of the new hydroxymethyl cryptand 3 with N,N'-dimethyl-4,4'-bipyridinium bis(hexafluorophosphate), PQ(PF6)2, has an association constant of 2.0(+/-0.3) x 10(4) M(-1). In the crystal structure of 3 x PQ(PF6)2 one of the bonding elements appears to be an aromatic edge-to-face interaction of a paraquat beta-proton with the hydroquinone moiety; this is the first time this interaction has been reported between a cryptand and paraquat.     

Guest-dependent complexation of triptycene-based macrotricyclic host with paraquat derivatives and secondary ammonium salts: a chemically controlled complexation process

The triptycene-based macrotricyclic host containing two dibenzo-[24]-crown-8 moieties has been found to form stable 1:1 or 1:2 complexes in different complexation modes with different functional paraquat derivatives and secondary ammonium salts in solution and in the solid state. Consequently, the alkyl-substituted paraquat derivatives thread the lateral crown cavities of the host to form 1:1 complexes. It was interestingly found that the paraquat derivatives containing two beta-hydroxyethyl or gamma-hydroxypropyl groups form 1:2 complexes, in which two guests thread the central cavity of the host. Other paraquat derivatives containing terminal hydroxy, methoxy, 9-anthracylmethyl, and amide groups were included in the cavity of the host to form 1:1 complexes. Moreover, the host also forms a 1:2 complex with two 9-anthracylmethylbenzylammonium salts, in which the 9-anthracyl groups were selectively positioned outside the lateral crown cavities. The competition complexation process between the host and two different guests (the propyl-substituted paraquat derivative and a dibenzylammonium salt) could be chemically controlled.     

Effect of Alkylpyrazinium Counterions on the Host-guest Recognition Based on Dimethoxypillar[5]arenes

In this work, we synthesized n-octylpyrazinium bromide(G-Br) and n-octylpyrazinium hexafluorophosphate(G-PF₆) as model guests and studied their host-guest complexation with 1,4-dimethoxypillar[5]arene(DMP5A). Effect of alkylpyrazinium counterions on the host-guest recognition was investigated. Based on the ¹H NOESY spectra, the binding site of DMP5A with G-PF₆ is the same as that of DMP5A with G-Br. However, G-PF₆ forms a stronger complex with DMP5A than G-Br, owing to that hexafluorophosphate forms weaker doubly inonic H-bonds with ammonium cation than bromide ion in chloroform, which leads to some aggregates that could be dissociated with the addition of DMP5A.     

High-yield preparation of [2]rotaxanes based on the bis(m-phenylene)-32-crown-10-based cryptand/paraquat derivative recognition motif

Based on the complexation between bis(m-phenylene)-32-crown-10-based cryptands and a paraquat derivative, two [2]rotaxanes were synthesized by using a threading-followed-by-stoppering method. Due to the strong associations between the cryptands and the paraquat derivative, high yields were achieved even in dilute solution.     

Microcalorimetric determination of thermodynamic parameters for ionophore-siderophore host-guest complex formation

Thermodynamic parameters (delta H, delta S, and delta G) were determined by microcalorimetry in wet chloroform for host-guest assembly formation involving second-sphere complexation of the siderophore ferrioxamine B by crown ether (18-crown-6, cis-dicyclohexano-18-crown-6, benzo-18-crown-6) and cryptand (2.2.2 cryptand) hosts. Similar data were also collected for the same hosts with the pentylammonium ion guest, which corresponds to the pendant pentylamine side chain of ferroxamine B. Host-guest assembly formation constants (Ka) obtained from microcalorimetry agree with values obtained indirectly from chloroform/water extraction studies in those cases where comparable data are available. On the basis of a trend established by the pentylammonium guest, an enhanced stability relative to the crown ethers is observed for the assembly composed of ferrioxamine B and 2.2.2 cryptand that is due to entropic effects. Trends in delta H and delta S with changes in host and guest structure are discussed and attributed directly to host-guest complex formation, as solvation effects were determined to be insignificant (delta Cp = 0).     

Complexation of Neutral Molecules by Cyclophane Hosts

Since the discovery of the crown ethers by Pedersen twenty years ago, the chemistry of synthetic hosts for the selective complexation of organic and inorganic guests has seen an extraordinarily rapid development. This article discusses in particular the contributions provided by synthetic cyclophanes as hosts to the understanding of molecular complexation of neutral organic guest molecules in aqueous and organic solvents. In aqueous solution, cyclophanes form stoichiometric complexes with neutral aromatic guests which can approach enzyme-substrate complexes in their stability. Efficient molecular complexation is also observed in organic environments. Here, as a result of large solvation effects, the strength of complexation is strongly dependent on the nature of the organic solvent. Electron donor-acceptor interactions can contribute significantly to the stability of complexes formed between cyclophane hosts and aromatic guests. Force-field calculations together with computer graphics are powerful tools in the design of water-soluble, optically active hosts for chiral recognition of complexed racemic guests. Simple and selective functionalization of the cyclophane framework leads to stable, bioorganic catalysts. Like enzymes, these catalysts bind their substrates in a rapid equilibrium prior to the reaction steps. As a perspective, some fascinating research objectives in the field of molecular recognition and catalysis which can be targeted with designed cyclophane hosts are shown.     

Synthesis of novel bis(beta-cyclodextrin)s and metallobridged bis(beta-cyclodextrin)s with 2,2'-diselenobis(benzoyl) tethers and their molecular multiple recognition with model substrates

To investigate quantitatively the cooperative binding ability of beta-cyclodextrin dimers, a series of bridged bis(beta-cyclodextrin)s with 2,2'-diselenobis(benzoyl) spacer connected by different lengths of oligo(ethylenediamine)s (2-5) and their platinum(IV) complexes (6-9) have been synthesized and their inclusion complexation behavior with selected substrates, such as Acridine Red, Neutral Red, Brilliant Green, Rhodamine B, ammonium 8-anilino-1-naphthalenesulfonate, and 6-p-toluidino-2-naphthalenesulfonic acid, were investigated by means of ultraviolet, fluorescence, fluorescence lifetime, circular dichroism, and 2D-NMR spectroscopy. The spectral titrations have been performed in aqueous phosphate buffer solution (pH 7.20) at 25 degrees C to give the complex stability constants (K(S)) and Gibbs free energy changes (-DeltaG degrees ) for the inclusion complexation of hosts 2-9 with organic dyes and other thermodynamic parameters (DeltaH degrees and TDeltaS degrees ) for the inclusion complexation of 2-5with fluorescent dyes ANS and TNS. The results obtained indicate that beta-cyclodextrin dimers 2-5 can coordinate with one or two platinum(IV) ions to form 1:1 or 1:2 stoichiometry metallobridged bis(beta-cyclodextrin)s. As compared with parent beta-cyclodextrin (1) and bis(beta-cyclodextrin)s 2-5, metallobridged bis(beta-cyclodextrin)s 6-9 can further switch the original molecular binding ability through the coordinating metal to orientate two beta-cyclodextrin cavities and an additional binding site upon the inclusion complexation with model substrates, giving the enhanced binding constants K(S) for both ANS and TNS. The tether length between two cyclodextrin units plays a crucial role in the molecular recognition with guest dyes. The binding constants for TNS decrease linearly with an increase in the tether length of dimeric beta-cyclodextrins. The Gibbs free energy change (-DeltaG degrees ) for the unit increment per ethylene is 0.32 kJ.mol(-)(1) for TNS. Thermodynamically, the higher complex stabilities of both ANS and TNS upon the inclusion complexation with 2-5 are mainly contributed to the favorable enthalpic gain (-DeltaH degrees ) by the cooperative binding of one guest molecule in the closely located two beta-cyclodextrin cavities as compared with parent beta-cyclodextrin. The molecular binding ability and selectivity of organic dyes by hosts 1-9 are discussed from the viewpoints of the multiple recognition mechanism and the size/shape-fitting relationship between host and guest.