Viologen-calix[6]arene pseudorotaxanes. Ion-pair recognition and threading/dethreading molecular motions

A calix[6]arene wheel, whose cavity has been extended and rigidified by N-phenylureido groups on the upper rim, forms pseudorotaxane species with molecular axles containing the viologen (4,4'-bipyridinium) unit in CH(2)Cl(2) solution. In these conditions, the self-assembly process is very efficient, with associated DeltaG degrees values of around -8 kcal mol(-1). The counteranions of the bipyridinium-based threads play indeed an important role in the formation of the complex. The use of either tosylate or hexafluorophosphate salts of the guests affects both the stability of the complexes and the rate of the threading process. Such effects have been interpreted in terms of ion-pair recognition, suggesting that coordination of the counteranions of the viologen thread by the ureido groups of the calixarene wheel is crucial for the breaking of tight ion pairs prior to threading. The rate constants of the threading/dethreading reactions coupled with the redox processes of the viologen unit of the axle have been obtained by means of cyclic voltammetry. The pseudorotaxane species undergo fast dethreading (submicrosecond time scale) on electrochemical reduction of the guest. The heterogeneous electron-transfer kinetics for the reduction of the viologen unit is slowed upon encapsulation into the calixarene cavity.     

Electrostatic barriers in rotaxanes and pseudorotaxanes

The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY²⁺) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5‐dioxynaphthalene (DNP) units flanking a central BIPY²⁺ unit in the dumbbell component and encircled by the cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) tetracationic cyclophane, has been synthesized employing a threading‐followed‐by‐stoppering approach. Variable‐temperature ¹H NMR spectroscopy reveals that the barrier to shuttling of the CBPQT⁴⁺ ring over the central BIPY²⁺ unit is in excess of 17 kcal mol^?1 at 343 K. Further information about the nature of the BIPY²⁺ unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY²⁺ moiety or a central DNP unit flanked by a BIPY²⁺ moiety. The threading and dethreading processes of the CBPQT⁴⁺ ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY²⁺ unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self‐folding (through donor–acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (Δ${G{{{\ne}\hfill \atop {\rm f}\hfill}}}The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY(2+)) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5-dioxynaphthalene (DNP) units flanking a central BIPY(2+) unit in the dumbbell component and encircled by the cyclobis(paraquat-p-phenylene) (CBPQT(4+)) tetracationic cyclophane, has been synthesized employing a threading-followed-by-stoppering approach. Variable-temperature (1)H?NMR spectroscopy reveals that the barrier to shuttling of the CBPQT(4+) ring over the central BIPY(2+) unit is in excess of 17 kcal mol(-1) at 343 K. Further information about the nature of the BIPY(2+) unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY(2+) moiety or a central DNP unit flanked by a BIPY(2+) moiety. The threading and dethreading processes of the CBPQT(4+) ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY(2+) unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self-folding (through donor-acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (ΔG(f)(++)) of the CBPQT(4+) for the case of the thread composed of a DNP flanked by two BIPY(2+) units was found to be as high as 21.7 kcal mol(-1) at room temperature. These results demonstrate that we can effectively employ the BIPY(2+) unit to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT(4+) ring in both mechanically interlocked and supramolecular systems.     

Polyrotaxanes based on polyethers and β‐cyclodextrin

A polyrotaxane in which β‐cyclodextrins (β‐CDs) are threaded onto a polyether chain was prepared by polycondensation of a β‐CD/bisphenol A (BPA) inclusion complex with aromatic dihalides. Two dihalides, with and without a side chain, were used. This polycondensation results in a polyrotaxane (or pseudopolyrotaxane for polymers without stoppers) with a 1:1 threading ratio when the side chain is present and 2:3 when there is none. The long side chain prevents dethreading of the macrocycles. The best yield and a good threading ratio were obtained when the polycondensation was performed by liquid?solid phase transfer catalysis without solvent (L/S PTC) using 2,5‐bi(iodomethyl)‐4‐methoxy‐(1‐octyloxy)benzene as dihalide. The ¹H NMR and FTIR spectra show that the products consist of β‐CD and polyether. The 2D NOESY NMR spectrum shows that the polyether chains are included in the β‐CD cavity. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4391–4399, 2009     

Synthesis of a polypseudorotaxane, polyrotaxane, and polycatenane using ‘click’ chemistry

The synthesis of a polypseudorotaxane, polyrotaxane, and polycatenane containing the electron-deficient cyclophane cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) subunit in the side chain is described. These interlocked supramolecular polymers have been prepared from an azide-functionalized polystyrene derivative and an acetylene-functionalized [2]rotaxane, [2]catenane and their parent tetracationic cyclophane via Cu(I)-catalyzed 1,3 dipolar cycloadditions (‘click chemistry’). The synthesis and characterization of the polymers and intermediates has been described using IR, ¹H NMR, UV spectroscopies, and voltammetry. We have shown that the CBPQT⁴⁺ unit of the side chain polystyrene derivative has the ability to reversibly undergo complexation with a complementary dialkoxynaphthalene derivative.     

Solvent‐ and Light‐Controlled Unidirectional Transit of a Nonsymmetric Molecular Axle Through a Nonsymmetric Molecular Wheel

The development of a pseudorotaxane motif capable of performing unidirectional threading and dethreading processes under control of external stimuli is particularly important for the construction of processive linear motors based on rotaxanes and, at least in principle, it discloses the possibility to access to rotary motors based on catenanes. Here, we report a strategy to obtain the solvent‐controlled unidirectional transit of a molecular axle through a molecular wheel. It is based on the use of appropriately designed molecular components, the essential feature of which is their non‐symmetric structure. Specifically they are an axle containing a central electron‐acceptor 4,4′‐bipyridinium core functionalized with a hexanol chain at one side, and a stilbene unit connected through a C6 chain at the other side, and a heteroditopic tris(phenylureido)‐calix[6]arene wheel. In apolar solvents the axle threads into the wheel from its upper rim and with the end carrying the OH group, giving an oriented pseudorotaxane structure. After a stoppering reaction, which replaces the small hydroxy group with a bulky diphenylacetyl moiety, and replacement of the apolar solvent with a polar one, dethreading occurs through the slippage of the stilbene unit from the lower rim of the wheel, that is, in the same direction of the threading process. The essential role played by the stilbene unit to achieve the unidirectional transit of the axle through the wheel, and to tune the dethreading rate by light is also demonstrated.     

Precision Molecular Threading/Dethreading

The general principles guiding the design of molecular machines based on interlocked structures are well known. Nonetheless, the identification of suitable molecular components for a precise tuning of the energetic parameters that determine the mechanical link is still challenging. Indeed, what are the reasons of the “all‐or‐nothing” effect, which turns a molecular “speed‐bump” into a stopper in pseudorotaxane‐based architectures? Here we investigate the threading and dethreading processes for a representative class of molecular components, based on symmetric dibenzylammonium axles and dibenzo[24]crown‐8 ether, with a joint experimental–computational strategy. From the analysis of quantitative data and an atomistic insight, we derive simple rules correlating the kinetic behaviour with the substitution pattern, and provide rational guidelines for the design of modules to be integrated in molecular switches and motors with sophisticated dynamic features.     

Study on the Influence of Chirality in the Threading of Calix[6]arene Hosts with Dialkylammonium Axles

The influence of chirality in calixarene threading has been studied by exploiting the “superweak anion approach”. In particular, the formation of chiral pseudo[2]rotaxanes bearing a classical stereogenic center in their axle and/or wheel components has been considered. Two kind of pseudo[2]rotaxane stereoadducts, the “endo-chiral” and “exo-chiral” ones, having the stereogenic center of a cationic axle inside or outside, respectively, the calix-cavity of a chiral calixarene were preferentially formed with specifically designed chiral axles by a fine exploitation of the so-called “endo-alkyl rule” and a newly defined “endo-α-methyl-benzyl rule” (threading of a hexaalkoxycalix[6]arene with a directional (α-methyl-benzyl)benzylammonium axle occurs with an endo-α-methyl-benzyl preference). The obtained pseudorotaxanes were studied in solution by 1D and 2D NMR, and in the gas-phase by means of the enantiomer-labeled (EL) mass spectrometry method, by combining enantiopure hosts with pseudoracemates of one deuterated and one unlabeled chiral axle enantiomer. In both instances, there was not a clear enantiodiscrimination in the threading process with the studied host/guest systems. Possible rationales are given to explain the scarce reciprocal influence between the guest and host chiral centers.     

A new class of aromatic polybenzoxazoles containing ortho-phenylenedioxy groups

A series of poly(o-hydroxy amide)s having both ether and ortho-catenated phenylene unit in the main chain were synthesized via the low-temperature solution polycondensation of 4,4^′-(1,2-phenylenedioxy)dibenzoyl chloride and 4,4^′-(4-tert-butyl-1,2-phenylenedioxy)dibenzoyl chloride with three bis(o-aminophenol)s including 4,4^′-diamino-3,3^′-dihydroxybiphenyl, 3,3^′-diamino-4,4^′-dihydroxybiphenyl, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane. The poly(o-hydroxy amide)s exhibited inherent viscosities in the range of 0.23-0.96 dl/g. Most of the poly(o-hydroxy amide)s were soluble in polar organic solvents such as N,N-dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP) and could afford flexible and tough films by solution casting. Subsequent thermal cyclodehydration of the poly(o-hydroxy amide)s afforded polybenzoxazoles. However, the polybenzoxazoles were organic-insoluble except for those with the hexafluoroisopropylidene group. The polybenzoxazoles exhibited glass-transition temperatures (T_g) in the range of 200-232 °C by DSC and softening temperatures (T_s) of 250-256 °C by thermomechanical analysis. Thermogravimetric analyses indicated that most polybenzoxazoles were stable up to 500 °C in air or nitrogen. The 10% weight loss temperatures were recorded in the ranges of 546-606 °C in air and 574-631 °C in nitrogen.     

Characterization of a slippage stopper for the 1,2-bis(pyridinium)ethane-[24]crown-8 ether [2]pseudorotaxane motif

Comparison of 1,2-bis(pyridinium)ethane axles containing iso-nicotinate esters (R=ethyl, iso-propyl, cyclohexyl, cycloheptyl) as the pyridinium unit shows that the cyclohexyl group provides a stopper size that allows slippage to be used as a method for pseudorotaxane/rotaxane formation employing dibenzo-24-crown-8 ether (DB24C8) as the wheel. Rates of complexation and decomplexation were measured in a number of solvent systems and the kinetic and thermodynamic parameters associated with the process reported.     

Novel microporous zirconium silicate (K2ZrSi3O9·2H2O) from high temperature phase transformation

A new microporous zirconosilicate K₂ZrSi₃O₉·2H₂O (AV-15) has been prepared by high-temperature phase transformation at 910 °C. Its structure has been determined ab initio from powder X-ray diffraction data. The unit cell is orthorhombic, space group C222₁ (no. 20), Z=4 with cell dimensions: a=8.105(3), b=10.684(5), c=12.030(5) Å, V=1041.76(7) Å³. The framework connection of AV-15 is essentially the same as the previously reported sodium stannosilicate AV-10 while the locations of potassium and water molecules in the former are quite different from those of the sodium and water molecules in AV-10. In AV-10 sodium and water molecules form a sinucoidal chain, while potassium and water molecules build up a linear chain in AV-15. The water molecules in AV-15 are lost on heating with a typical zeolitic behaviour. SEM shows that the particle sizes and habits of AV-15 and parent umbite material are the same. The ²⁹Si MAS NMR spectrum of AV-15 displays two resonances at ca. −89.4 and −90.1 ppm in a 1:2 intensity ratio. Thermogravimetry analysis confirms the existence of water in this material.     

Synthesis and properties of poly(aryl ether benzimidazole) copolymers for high-temperature fuel cell membranes

A series of poly(aryl ether benzimidazole) copolymers bearing different aryl ether linkage contents were synthesized by condensation polymerization in polyphosphoric acid (PPA) by varying the feed ratio of 4,4′-dicarboxydiphenyl ether (DCPE) to terephthalic acid (TA). As the ether unit content in the copolymer increased, the solubility of the copolymer in PPA and N,N′-dimethylacetamide/LiCl improved. For example 3–7 wt.% DMAc solution containing 2 wt.% of LiCl could be prepared from the copolymers. XRD studies revealed that the incorporation of flexible aryl ether linkages increased the chain d-spacings of the polymer backbones and decreased the crystallinity of the copolymers. Still, these copolymers having ether linkages showed reasonably good thermal/mechanical stability and high proton conductivity. For example, the copolymer with 30 mol% ether linkage had a tensile strength of 43 MPa (at 26 °C and 40% relative humidity) at an acid doping level of 7.5 mol H₃PO₄ and a proton conductivity of 0.098 S cm⁻¹ (at 180 °C and 0% relative humidity) at an acid doping level of 6.6 mol H₃PO₄.     

Self-Assembly of Novel Polyrotaxanes: Main-Chain Pseudopolyrotaxanes with Poly(ester crown ether) Backbones

Not threading rings on chains, but chains through rings results in a new class of main-chain polyrotaxanes. The cations of bipyridinium salts thread through the crown ether units of a poly(ester crown ether) chain (shown below). The threading efficiency m/n of the polyrotaxane is controlled by stoichiometry and temperature.     

Pseudo-crown ether having AIE and PET effects from a TPE-CD conjugate for highly selective detection of mercury ions

A new tetraphenylethylene-cyclodextrin（TPE-CD） conjugate with a linkage composed of long triethylene glycol chain and triazole ring on the CD rim has been designed and synthesized. The TPE-CD conjugate exists in a stretched form in DMSO and enhances its fluorescence after addition of a small amount of water due to aggregation-induced emission（AIE） effect. However, in the presence of a large amount of water, the TPE unit will enter the cyclodextrin cavity to form a folded self-inclusion compound....     

Synthesis, characterization and properties of heat-resistant and soluble poly(aryl ether)s containing s-triazine units in the main chain

Soluble and heat-resistant s-triazine-containing poly(aryl ether)s have been prepared for their potent utilities as high-temperature membranes and composite matrix materials. They have been synthesized by the nucleophilic displacement polymerization of 2,4-bis(4-fluorophenyl)-6-phenyl-1,3,5-triazine (BFPT) with each of resorcinol (RS), 4,4′-dihydroxydiphenyl sulfone (DS), and bisphenol-like 4-(4-hydroxylphenyl)(2H)-phthalazin-1-one (HP). The presence of meta-ether linkages, sulfone groups or phthalazinone moieties in the polymer chain results in an improvement in the solubility of s-triazine-containing poly(aryl ether)s in common organic solvents (e.g., N,N′-dimethylacetamide, N-methyl-2-pyrrolidinone). The new polymers are amorphous and exhibit excellent thermal stability. The apparent activation energy values (E_a) as determined by using Kissinger method are respectively 268.1, 245.9 and 215.1 kJ/mol under N₂ flow in dynamic heating conditions, for the first degradation stage of RS-PE, DS-PE and HP-PE, which are in well agreement with those values (272.6, 249.9 and 239.1 kJ/mol) determined by using Flynn-Wall-Ozawa method. The thermal stability classification among the polymers is made on the basis of the E_a values, and it follows the decreasing order: RS-PE > DS-PE > HP-PE. The properties of these polymers have been also compared with those of corresponding poly(aryl ether)s.     

AT‐Specific DNA Binding of Binuclear Ruthenium Complexes at the Border of Threading Intercalation

The binuclear ruthenium complex [μ‐bidppz(phen)₄Ru₂]⁴⁺ has been extensively studied since the discovery of its unusual threading intercalation interaction with DNA, a binding mode with extremely slow binding and dissociation kinetics. The complex has been shown to be selective towards long stretches of alternating AT base pairs, which makes it interesting, for example, as a model compound for anti‐malaria drugs due to the high AT content of the genome of the malaria parasite P. falciparum. We have investigated the effect of bridging ligand structure on threading intercalation ability and found that length and rigidity as well as the size of the intercalated ring system are all factors that affect the rate and selectivity of the threading intercalation. In particular, we discovered a new DNA‐threading compound, [μ‐dppzip(phen)₄Ru₂]⁴⁺, which appears to be just at the border of being capable of threading intercalation and displays even greater selectivity for AT‐DNA than the parent compound, [μ‐bidppz(phen)₄Ru₂]⁴⁺.     

Rational Design for Rotaxane Synthesis through Intramolecular Slippage: Control of Activation Energy by Rigid Axle Length

We describe a new concept for rotaxane synthesis through intramolecular slippage using π‐conjugated molecules as rigid axles linked with organic soluble and flexible permethylated α‐cyclodextrins (PM α‐CDs) as macrocycles. Through hydrophilic–hydrophobic interactions and flipping of PM α‐CDs, successful quantitative conversion into rotaxanes was achieved without covalent bond formation. The rotaxanes had high activation barrier for their de‐threading, so that they were kinetically isolated and derivatized even under conditions unfavorable for maintaining the rotaxane structures. ¹H NMR spectroscopy experiments clearly revealed that the restricted motion of the linked macrocycle with the rigid axle made it possible to control the kinetic stability by adjusting the length of the rigid axle in the precursor structure rather than the steric bulkiness of the stopper unit.     

Enantioselective synthesis of chiral porphyrin macrocyclic hosts and kinetic enantiorecognition of viologen guests

The construction of macromolecular hosts that are able to thread chiral guests in a stereoselective fashion is a big challenge. We herein describe the asymmetric synthesis of two enantiomeric C₂-symmetric porphyrin macrocyclic hosts that thread and bind different viologen guests. Time-resolved fluorescence studies show that these hosts display a factor 3 kinetic preference (ΔΔG^‡_on = 3 kJ mol⁻¹) for threading onto the different enantiomers of a viologen guest appended with bulky chiral 1-phenylethoxy termini. A smaller kinetic selectivity (ΔΔG^‡_on = 1 kJ mol⁻¹) is observed for viologens equipped with small chiral sec-butoxy termini. Kinetic selectivity is absent when the C₂-symmetric hosts are threaded onto chiral viologens appended with chiral tails in which the chiral moieties are located in the centers of the chains, rather than at the chain termini. The reason is that the termini of the latter guests, which engage in the initial stages of the threading process (entron effect), cannot discriminate because they are achiral, in contrast to the chiral termini of the former guests. Finally, our experiments show that the threading and de-threading rates are balanced in such a way that the observed binding constants are highly similar for all the investigated host–guest complexes, i.e. there is no thermodynamic selectivity.

Chiral guests display kinetic stereoselective threading through chiral porphyrin cages if their chirality is located at the chain ends and not in the centers, supporting the previously reported entron effect of threading.     

Stereoselective total synthesis of 4-((3S,5R)-3,5-dihydroxynonadecyl)phenol

A stereoselective total synthesis of 4-((3S,5R)-3,5-dihydroxynonadecyl)phenol has been accomplished in two different synthetic approaches. In the first approach, Prins cyclization has been successfully utilized to produce the anti-1,3-diol unit, which was further converted into a required syn-1,3-diol through Mitsunobu reaction. The side chain was constructed through cross metathesis and hydrogenation sequence. In the second approach, the chiral syn-1,3-diol was prepared by a sequence of reactions such as alkylation of 1,3-dithane with (R)-epichlorohydrin, ring opening of the epoxide with vinylmagnesium bromide, and 1,3-syn-reduction of the β-hydroxyketone with NaBH₄ in the presence of diethylmethoxyborane.     

Hydrothermal synthesis and structure of a novel 3D framework based on ξ-octamolybdate chains: [Cu2(quinoxaline)2Mo4O13]n

An unprecedented three-dimensional (3D) polymer [Cu₂(quinoxaline)₂Mo₄O₁₃]_n, formed from ξ-octamolybdate chains as building units and pairs of 1D [Cu(quinoxaline)]_nⁿ⁺ polymeric chains as linkers, provides the first example of an extended higher dimensional structure based on octamolybdate chain. The basic building block of the octamolybdate chain included in the title compound is first reported to be constructed from ξ-isomer of octamolybdate unit.     

Vibrational circular dichroism,absolute configuration and predominant conformations of volatile anesthetics: 1,2,2,2-tetrafluoroethyl methyl ether

Vibrational absorption and circular dichroism spectra of (−)-1,2,2,2-tetrafluoroethyl methyl ether have been measured in CCl₄ solution in the 2000–900 cm⁻¹ region. These spectra are compared with the ab initio predictions of absorption and VCD spectra obtained with density functional theory using B3LYP/6-31G* and B3PW91/6-311G(2d) basis sets for different conformers of (R)-1,2,2,2-tetrafluoroethyl methyl ether. The results suggest that the trans-conformer of 1,2,2,2-tetrafluoroethyl methyl ether is predominant in the solution phase and that the (−)-enantiomer has the R-configuration.