Synthesis of triptycene-derived macrotricyclic host containing two dibenzo-[18]-crown-6 moieties and its complexation with paraquat derivatives: Li(+)-ion-controlled binding and release of the guests in the complexes

A new triptycene-derived macrotricyclic host containing two dibenzo-[18]-crown-6 moieties was synthesized and shown to form 1:1 complexes with paraquat derivatives in solution, in which the guests all thread the central cavity of the host. However, it was interestingly found that, depending on the paraquat derivatives with different functional groups, the host can form stable 1:1 or 1:2 complexes in different complexation modes in the solid state, which is significantly different from those of the macrotricyclic host containing two dibenzo-[24]-crown-8 moieties. The formation of the complexes was also proved by the ESI MS and electrochemical experiments. Moreover, it was found that the binding and release of the guests in the complexes could be easily controlled by the addition and removal of lithium ions.     

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 in fast-exchange host-guest systems: concentration dependence of apparent association constants for complexes of neutral hosts and divalent guest salts with monovalent counterions

An equilibrium treatment of complexation of neutral hosts with dicationic guests having univalent counterions includes two possible modes: (1) dissociation of the ion pair prior to interaction of the free dication with the host to produce a complex that is not ion paired and (2) direct complexation of the ion pair to produce an ion paired complex. This treatment is easily modified for complexation of neutral guests by dianionic hosts, or divalent hosts by neutral guests. The treatment was tested by a study of fast-exchange host-guest systems based on paraquats or viologens (G(2+)2X(-)) and crown ethers (H). The bis(hexafluorophosphate) salts of viologens are predominantly ion paired in acetone; the value of the dissociation constant of paraquat bis(hexafluorophosphate) was determined to be 4.64 (+/- 1.86) x 10(-4) M(2). The complex based on dibenzo-24-crown-8 and paraquat bis(hexafluorophosphate) is not ion paired in solution, resulting in concentration dependence of the apparent association constant K(a,exp), (= [complex]/[H][G(2+)2X(-)]) which is well fit by the treatment, according to mode (1), yielding K(ap) = 106 (+/-42) M(-1). However, the four complexes of two different bis(m-phenylene)-32-crown-10 derivatives and bis(p-phenylene)-34-crown-10 with paraquat derivatives are all ion paired in solution and therefore K(a,exp) is not concentration dependent for these systems, mode (2). X-ray crystal structures support these solution-based assessments in that there is clearly ion pairing of the cationic guest with its PF(6)(-) counterions in the solid states of the latter four examples in which access of the counterions to the guests is granted by the relatively large cavities of the hosts and dispositions of the guest species within them.     

Li+-templated complexation of cylindrical macrotricyclic host with naphthalene diimide: Cation-controlled switchable complexation processes

Anthracene-based cylindrical macrotricyclic polyether (1) containing two dibenzo-30-crown-10 cavities has been proved to be an efficient host for the templated complexation with N,N’-dipropyl-1,4,5,8-naphthalenetetracarboxylic diimide in the presence of lithium ions in both solution and solid state. Host 1 could also form 1:1 complex with the bispyridinium salt with two β-hydroxyethyl groups in solution and in the solid state. Moreover, it was also found that the switchable complexation processes between the macrotricyclic host and two different kinds of guests could be chemically controlled by the addition and removal of lithium ions.     

Synthesis of a novel triptycene-based molecular tweezer and its complexation with paraquat derivatives

[reaction: see text] A novel triptycene-based molecular tweezer has been synthesized, and its complexation with paraquat derivatives in solution and in the solid state has been studied. Due to its electron-rich cavity, the molecular tweezer could form stable complexes with paraquat derivatives with different functional groups. Moreover, it was also found that formation of the complexes was caused by a charge transfer interaction and the complexes dissociated upon two one-electron reduction of the bipyridinium ring.     

Novel triptycene-based cylindrical macrotricyclic host: synthesis and complexation with paraquat derivatives

[reaction: see text] A novel triptycene-based cylindrical macrotricyclic polyether containing two dibenzo[24]crown-8 cavities has been synthesized and proved to be a highly efficient host for the complexation with paraquat derivatives. Consequently, a new kind of very stable pseudorotaxane-type complex was formed in solution and in the solid state.     

Conformationally regulated fluorescent sensors. Study of the selectivity in Zn versus Cd sensing

The Zn²⁺ and Cd²⁺ complexing properties of four ligands containing a 4,4′-substituted biphenyl moiety are described. Ligands 1 and 3, containing only one 1-aza-18-crown-6 cavity, lead to selective complexation of Cd²⁺ versus Zn²⁺. Ligand 4, with two crown cavities linked to a tetramethylbenzidine unit, is able to form 1:1 complexes with Zn²⁺ and Cd²⁺, showing a higher complexing constant with Zn²⁺ than with Cd²⁺, probably due to enthalpic factors. Several complementary experiments suggest that the 1:1 complexes formed by ligand 4 involve both crown cavities acting together to give rise to clamp structures. The formation of this type of zinc complex gives rise to red shifted emission bands and distinct quenching of the fluorescence. A similar situation is observed with cadmium but the change is then less pronounced. When mixtures of both salts are used, ligand 4 selectively responds to zinc. Finally, ligand 2, which also has two crown cavities but contains nitro rather than amino groups in the biphenyl moiety, shows no propensity to form clamp complexes and, for this reason, it complexes cadmium much more strongly than zinc and binds the former selectively when mixtures of both salts are used in complexing experiments.     

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.     

Cycloalkane and alicyclic heterocycle complexation by new switchable resorcin[4]arene-based container molecules: NMR and ITC binding studies

The synthesis and structural characterization of novel, "molecular basket"-type bridged cavitands is reported. The resorcin[4]arene-based container molecules feature well-defined cavities that bind a wide variety of cycloalkanes and alicyclic heterocycles. Association constants (K(a)) of the 1:1 inclusion complexes were determined by both (1)H NMR and isothermal titration calorimetry (ITC). The obtained K(a) values in mesitylene ranged from 1.7×10(2) M(-1) for cycloheptane up to 1.7×10(7) M(-1) for morpholine. Host-guest complexation by the molecular baskets is generally driven by dispersion interactions, C-H···π interactions of the guests with the aromatic walls of the cavity, and optimal cavity filling. Correlations between NMR-based structural data and binding affinities support that the complexed heterocyclic guests undergo additional polar C-O···C=O, N-H···π, and S···π interactions. The first crystal structure of a cavitand-based molecular basket is reported, providing precise information on the geometry and volume of the inner cavity in the solid state. Molecular dynamic (MD) simulations provided information on the size and conformational preorganization of the cavity in the presence of encapsulated guests. The strongest binding of heterocyclic guests, engaging in polar interactions with the host, was observed at a cavity filling volume of 63 ± 9%.     

Complexation between pentiptycene derived bis(crown ether)s and CBPQT4+ salt: ion-controlled switchable processes and changeable role of the CBPQT4+ in host-guest systems

The pentiptycene derived bis(crown ether)s with two 24-crown-8 moieties in the cis position could include the CBPQT(4+) ring inside their cavities to form 1:1 complexes, and the naphthalene groups connected in the crown ether moieties showed less effective complexation ability toward the CBPQT(4+) ring than the host containing two terminal benzene rings. This result was probably due to the stereohindrance effect of the naphthalene groups, and it was obviously different from that of the pentiptycene derived mono(crown ether)s. For the pentiptycene derived bis(crown ether) with two 24-crown-8 moieties in the trans position, it formed a 1:2 stable complex with the CBPQT(4+) salt in solution and in the solid state, in which the pentiptycene moiety played an important role in stabilizing the complex. Moreover, binding and release of the CBPQT(4+) ring in the complexes based on the pentiptycene-derived crown ethers could be chemically controlled by adding and removing potassium ions, in which the complexation modes played the key role. Interestingly, it was further found that switching the role of the CBPQT(4+) ring in host and guest systems based on the pentiptycene derived bis(crown ether)s was easily achieved, which represents a new kind of supramolecular system.     

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.     

Structural study of the inclusion compounds of thymol, carvacrol and eugenol in β-cyclodextrin by X-ray crystallography

The crystal structures of the inclusion compounds of thymol, carvacrol and eugenol, (components of essential oils of vegetable origin) in β-cyclodextrin have been determined. Thymol/β-CD crystallizes in the space group P1 containing two host molecules in its asymmetric unit whereas both carvacrol/β-CD and eugenol/β-CD complexes crystallize in the space group C2. In all three complexes two host molecules form head-to-head dimers their guest/host stoichiometry being: 1/2 (carvacrol/β-CD), 2/2 (thymol/β-CD) and 3/2 (eugenol/β-CD). In the cases of the thymol/β-CD and the carvacrol/β-CD complexes the β-CD dimers are arranged according to the channel packing mode. The accommodation of the geometrical isomer guests is performed solely by their hydrophobic groups revealing the leading role of the hydrophobic driving forces in the complexation process whereas the position of their hydroxyl group affects the stoichiometry of the formed dimeric complexes. In the case of the eugenol/β-CD dimeric complex one guest molecule is found lying between the β-CD groups in a sandwich fashion whereas the other two symmetry related guests protrude outwards the narrower rim of the hosts with only their hydrophobic allyl-chain located inside the hosts’ cavities. This arrangement prohibits the formation of a channel and the observed crystal packing is that of a Tetrad mode.     

Self-assembly of dendritic tris(crown ether) hexagons and their complexation with dibenzylammonium cations

The construction of a new series of dendritic tris(crown ether) hexagons via coordination-driven self-assembly is described. Combining 120° crown ether-containing diplatinum(II) acceptors with 120° dendritic dipyridyl donors in a 1:1 ratio allows for the formation of a new family of dendritic triple crown ether derivatives with a hexagonal cavity in quantitative yields. The number and the position of these pendant groups can be precisely controlled on the hexagonal metallacycle. The structures of all dendritic multiple crown ether hexgaons are confirmed by multinuclear NMR ((1)H and (31)P), ESI-MS and ESI-TOF-MS, and elemental analysis. The complexation of these dendritic trivalent receptors with dibenzylammonium cations was investigated by (1)H NMR titration experiments. The thermodynamic binding constants between the receptors and guests were established by using the nonlinear least-squares fit method based on (1)H NMR titration experiments. It was found that the association constants of each assembly decrease correspondingly upon the increase of the generation of the dendrons from [G0] to [G3], which might be caused by the steric effect of the dendrons on host-guest complexation.     

Spectrophotometric study on the thermodynamics of binding of alpha- and beta-cyclodextrin towards some p-nitrobenzene derivatives

Binding properties of native alpha- and beta-cyclodextrin towards some nitrobenzene derivatives have been studied by means of UV-vis spectrophotometry. The former host is able to form complexes having 1:1 and 1:2 stoichiometric ratios with these guests, while only 1:1 complexes are detected with the latter host. A careful analysis of the thermodynamic parameters for complexation equilibria, under the perspective of the enthalpy-entropy compensation effect, reveals that binding abilities of the two different hosts are subject to different features.     

Formation of ternary complexes between a macrotricyclic host and hetero-guest pairs: an acid-base controlled selective complexation process

A triptycene-based cylindrical macrotricyclic host can include diquat and electron-rich aromatics simultaneously to form stable ternary complexes, which is stabilized not only by a charge-transfer (CT) interaction between electron-rich and electron-deficient guests but also by the face to face pi-stacking interactions between the host and the guests. Moreover, a selective complexation process between a ternary complex containing benzidine and a binary complex can be effectively controlled by the use of acid and base.     

Synthesis of a symmetric cylindrical bis(crown ether) host and its complexation with paraquat

The first cylindrical host for paraquat derivatives was prepared and characterized by X-ray analysis. Its complex with paraquat has 1:2 stoichiometry. The complexation is statistical and strong as shown by proton NMR characterization, electrospray mass spectrometry, and X-ray analysis.     

Selective templated complexation of a cylindrical macrotricyclic host with neutral guests: three cation-controlled switchable processes

A novel triptycene-based cylindrical macrotricyclic host 1 containing an anthracene unit and two dibenzo [24]crown-8 moieties was synthesized, and its cation binding properties were studied. It was found that the host could not only form complex with the paraquat derivative 4, but also show selective templated complexation with pyromellitic diimide 2 and anthraquinone 3 in the presence of lithium and potassium ions, respectively. Consequently, two novel cascade complexes with neutral molecules as bridging species were formed in solution and in the solid state, which were structurally studied by NMR, MS spectra, and X-ray methods. Moreover, we also found that the association and dissociation of the complexes could be easily achieved by the addition and removal of lithium or potassium ions, which resulted in three cation-controlled switchable processes.     

Structure selective complexation of cyclodextrins with five polyphenols investigated by capillary electrokinetic chromatography

The complexation of five polyphenols, namely trans‐resveratrol, astilbin, taxifolin, ferulic acid, and syringic acid (guest molecules) with α‐, β‐, and γ‐cyclodextrin (host molecules), was investigated by capillary electrokinetic chromatography. The binding constants were calculated based on the effective electrophoretic mobility change of guests with the addition of cyclodextrins into the background electrolyte. Because of cavity size, cyclodextrins showed structure‐selective complexation property to different guest. The stability of the trans‐resveratrol complexes was in the order of β‐ > α‐ > γ‐cyclodextrin. The cavity size of α‐cyclodextrin was too small for astilbin and taxifolin molecules, and thus they could not form complexes. The molecular size of syringic acid was too big for all cyclodextrins cavity, and no cyclodextrin could form complexes with it. Temperature studies showed that the binding constants decreased with the rise of temperature. Enthalpy and entropy values were calculated and the negative values of these parameters indicated that the complexation process was enthalpy‐controlled. Van der Waals force and release of high‐enthalpy water molecules from the cyclodextrins cavity played important roles in the process.     

Complex formation of cyclodextrins with poly(propylene glycol) derivatives

The complex formation between cyclodextrins (CDs) and poly(propylene glycol) (PPG) derivatives is described. β‐CD and γ‐CD formed complexes with PPG derivatives such as 1‐naphthyl (1NA), 2‐naphthyl (2NA), 3,5‐dinitrobenzoyl, and 2,4‐dinitrophenyl PPG. α‐CD did not form complexes with these PPG derivatives. Although γ‐CD gave complexes with 9‐anthryl PPG (PPG9An), β‐CD did not efficiently form complexes with PPG9An. β‐CD did not form complexes with trityl PPG, demonstrating that trityl groups were too bulky to thread a β‐CD cavity. The emission spectra of the complexes showed that β‐CD bound a single 2NA moiety in its cavity and that γ‐CD included two 2NA moieties. In contrast, γ‐CD bound a single 1NA moiety in the cavity. X‐ray diffraction studies and ¹H NMR analysis showed that the CD molecules were stacked along a PPG chain to form a channel structure. The inclusion modes are discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4839–4849, 2000     

Second-Sphere Coordination of Ferrioxamine B and Association of Deferriferrioxamine B, CH(3)(CH(2))(4)NH(3)(+), NH(4)(+), K(+), and Mg(2+) with Synthetic Crown Ethers and the Natural Ionophores Valinomycin and Nonactin in Chloroform

The interaction of ferrioxamine B, FeHDFB(+), through a protonated amine side chain, with various host ionophore structures to form a host-guest complex in the second coordination shell has been investigated. Host-guest association constants (K(a)) in water saturated chloroform are reported for synthetic crown ethers with different cavity size and substituents (18-crown-6 and its dicyclohexano, benzo, and dibenzo derivatives; dibenzo and dicyclohexano derivatives of 24-crown-8; and dibenzo-30-crown-10). The natural ionophores valinomycin and nonactin were also found to form stable second-sphere complexes with ferrioxamine B in wet chloroform. Results are reported for both picrate and perchlorate salts of FeHDFB(+). Since the protonated amine side chain of ferrioxamine B may be viewed as a substituted amine, the host-guest association constants for FeHDFB(+) are compared to the interaction of Mg(2+), K(+), NH(4)(+), CH(3)(CH(2))(4)NH(3)(+), and H(4)DFB(+) with the same ionophores. This is the first report of nonactin complexation of this series of cations in an organic medium of low polarity and one of the few reports of valinomycin complexation. To the best of our knowledge these are the first reported stability constants for the association of (Mg(2+),2pic(-)) with natural and synthetic ionophores in chloroform. K(a) values for ferrioxamine B complexation by the synthetic crown ethers are influenced by ring size and substituent. Despite significant preorganization capabilities, the large cavities of valinomycin, nonactin and benzo-30-crown-10 do not form as stable host-guest assemblies with bulky substituted amine cations such as ferrioxamine B as does cis-dicyclohexano-18-crown-6.