Octahydroxypyridine[4]arene self-assembles spontaneously to form hexameric capsules and dimeric aggregates

In recent years it has been observed that resorcin[4]arenes and pyrogallol[4]arenes form hydrogen-bonded hexameric capsules in nonpolar solvents. In the present study we have used NMR spectroscopy, with an emphasis on diffusion NMR, to investigate the self-assembly and the aggregation mode of solutions of octahydroxypyridine[4]arene (1 b) in chloroform. In spectroscopic studies, the hexameric capsule of C-undecylresorcin[4]arene (2 b) was used as a reference compound and in some cases also as an internal reference. The current diffusion NMR spectroscopy study shows, in contrast to a previous report, that compound 1 b self-assembles spontaneously into hexameric and dimeric aggregates in solutions in chloroform. The (1)H NMR and diffusion NMR spectroscopic studies on a solution of 1 b in CHCl(3) show the presence of new upfield-shifted peaks, which diffuse with the same diffusion coefficient as the hexameric peaks in the spectrum. Therefore, these new upfield-shifted peaks were attributed to encapsulated CHCl(3) molecules. Interestingly, diffusion NMR measurements showed that the addition of trifluoroacetic acid (6.7 equiv), which had no effect on the hexameric capsules of 2 b, led to the disassembly of the hexamer and the dimer of 1 b into its monomers. Therefore, we conclude that compound 1 b self-assembles spontaneously into hexameric capsules in nonpolar organic solvents, as do resorcin[4]arenes 2 b and 2 c and pyrogallol[4]arenes 3 a and 3 b.     

Diastereoselective self-assembly of chiral diamine-chelated aryllithiums to dimeric aggregates

Aryllithium compounds [LiC6H4(CH2N(Et)CH2CH2NEt2)-2]2 (2b), [LiC6H4(CH(Me)N(Me)CH2CH2NMe2-(R))-2]2 ((R)-3b), and [LiC6H4(CH(Me)N(Me)CH2CH2NMe2-(rac))-2]2 ((rac)-3b) were synthesized and characterized in the solid state and in solution. X-ray crystallographic studies of 2b and (R)-3b and molecular weight determinations of 2b, (R)-3b, and (rac)-3b by cryoscopy in benzene showed that, both in the solid state and in apolar, noncoordinating solvents such as benzene, these compounds exist as discrete dimeric aggregates. For (R)-3b and (rac)-3b the aggregation process of two monomeric aryllithium units to one dimer is highly diastereoselective.     

Sonication-induced self-assembly of flexible tris(ureidobenzyl)amine: from dimeric aggregates to supramolecular gels

Tris(ureidobenzyl)amine derivatives 1a,b form dimeric aggregates in apolar solution and in the solid state. Specifically, the meta-substituted tris(urea) 1a is able to transform into supramolecular gels in certain solvents via sonication.     

Understanding organic reactions in water: from hydrophobic encounters to surfactant aggregates

A crucial factor in realising a green chemical process in solution involves the choice of a safe, non-toxic and cheap solvent. Water is the obvious choice. Despite solubility problems, considerable interest has developed recently in organic chemistry in water. This interest also results from the fact that association and chemical reactions often benefit noticeably from the special properties of water, resulting mainly from its small molecular size, its three-dimensional hydrogen-bond network and hydrophobic interactions which are so unique for liquid water. Here we discuss organic reactions and assembly processes in water, largely taken from experiments performed in the authors' laboratories. We show that non-covalent interactions in water can be utilised for fine tuning organic reactions in aqueous media.     

Molecular density functional theory of solvation: from polar solvents to water

A classical density functional theory approach to solvation in molecular solvent is presented. The solvation properties of an arbitrary solute in a given solvent, both described by a molecular force field, can be obtained by minimization of a position and orientation-dependent free-energy density functional. In the homogeneous reference fluid approximation, limited to two-body correlations, the unknown excess term of the functional approximated by the angular-dependent direct correlation function of the pure solvent. We show that this function can be extracted from a preliminary MD simulation of the pure solvent by computing the angular-dependent pair distribution function and solving subsequently the molecular Ornstein-Zernike equation using a discrete angular representation. The corresponding functional can then be minimized in the presence of an arbitrary solute on a three-dimensional cubic grid for positions and Gauss-Legendre angular grid for orientations to provide the solvation structure and free-energy. This two-step procedure is proved to be much more efficient than direct molecular dynamics simulations combined to thermodynamic integration schemes. The approach is shown to be relevant and accurate for prototype polar solvents such as the Stockmayer solvent or acetonitrile. For water, although correct for neutral or moderately charged solute, it tends to underestimate the tetrahedral solvation structure around H-bonded solutes, such as spherical ions. This can be corrected by introducing suitable three-body correlation terms that restore both an accurate hydration structure and a satisfactory energetics.     

Thermochemistry of the initial steps of methylaluminoxane formation. Aluminoxanes and cycloaluminoxanes by methane elimination from dimethylaluminum hydroxide and its dimeric aggregates

Results are presented of ab initio studies at levels MP2(full)/6-31G* and MP2(full)/6-311G** of the hydrolysis of trimethylaluminum (TMA, 1) to dimethylaluminumhydroxide (DMAH, 2) and of the intramolecular 1,2-elimination of CH(4) from 2 itself to form methylaluminumoxide 3, from its dimeric aggregate 4 to form hydroxytrimethyldialuminoxane 5 and dimethylcyclodialuminoxane 6, and from its TMA aggregate 7 to form 8 and/or 9, the cyclic and open isomers of tetramethyldialuminoxane, respectively. Each methane elimination creates one new Lewis acid site, and dimethylether is used as a model oxygen-donor molecule to assess the most important effects of product stabilization by Lewis donor coordination. It is found that the irreversible formation of aggregate 4 (ΔG(298) = -29.2 kcal/mol) is about three times more exergonic than the reversible formation of aggregate 7 (ΔG(298) = -9.9 kcal/mol), that the reaction free enthalpies for the formations of 5 (ΔG(298) = -9.0 kcal/mol) and 6 (ΔG(298) = -18.8 kcal/mol) both are predicted to be quite clearly exergonic, and that there is a significant thermodynamic preference (ΔG(298) = -7.2 kcal/mol) for the formation of 6 over ring-opening of 5 to hydroxytrimethyldialuminoxane 10. The mechanism for oligomerization is discussed based on the bonding properties of dimeric aggregates and involves the homologation of HO-free aluminoxane with DMAH (i.e., 9 to 13), and any initially formed hydroxydialuminoxane 10 is easily capped to trialuminoxane 13. Our studies are consistent with and provide support for Sinn's proposal for the formation of oligoaluminoxanes, and in addition, the results point to the crucial role played by the kinetic stability of 5 and the possibility to form cyclodialuminoxane 6. Dialuminoxanes 9 and 10 are reversed-polarity heterocumulenes, and intramolecular O→Al dative bonding competes successfully with Al complexation by Lewis donors. Intramolecular O→Al dative bonding is impeded in cyclodialuminoxane 6, and the dicoordinate oxygen in 6 is a strong Lewis donor. Ethylene polymerization catalysts contain highly oxophilic transition metals, and our studies suggest that these transition metal catalysts should discriminate strongly in favor of cycloaluminoxane-O donors even if these are present only in small concentrations in the methylaluminoxane (MAO) cocatalyst.     

Contribution of electron-deficient platinum to benzene hydrogenation and its activation by water

The activity of metallic (Pt^o) and electron-deficient (Pt) platinum in Pt/Al₂O₃ catalysts has been studied in the low temperature hydrogenation of benzene. Reaction kinetics has been clarified. It is shown that water preadsorption does not affect the character of kinetic curves, but enhances the reaction rate due to an increase in the specific activity of Pt.

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(Pt^o) (Pt) Pt/Al₂O₃- . . , , Pt.

     

Macromolecular assembly: from irregular aggregates to regular nanostructures

Based on our long-term research on interpolymer complexation due to hydrogen bonding, we proposed several novel self-assembly approaches to polymeric micelles with regular structures. Differing from micelles of block and graft copolymers, our micelles don't have any chemical bonds between the core and shell. In addition, some of these approaches have been proved to be effective to fabricate hollow aggregates.     

Molecular dynamics simulation on ion-pair association of NaCl from ambient to supercritical water

Molecular dynamics (MD) simulations for aqueous NaCl solution were performed from ambient to supercritical conditions (25 °C, 1.0 g cm⁻³; 250–350 °C, 0.67–0.8 g cm⁻³; 380 °C, 0.2–0.8 g cm⁻³; and 400–600 °C, 0.4 and 0.7 g cm⁻³) in the canonical ensemble to examine how the hydration structure relates with the thermodynamics of the ion-pair association. Hydration structure and the potential of mean force (PMF) of Na⁺Cl⁻ ion-pair were calculated. Ion-pair association constants were also calculated from the PMFs. Energies and entropies of the ion-pair at arbitrary inter-ionic distances from 2.0 to 8.0 Å were evaluated from the temperature derivative of the PMFs. From the calculation of energies and entropies, Na⁺–Cl⁻ pair association was found to be endothermic and promoted by the entropy gain. PMFs had minimums and a slight maximum corresponding to CIP, SShIP, and the transition state between CIP and SShIP, and similar minimums and maximum were only observed for the energy term and not clearly observed for the entropy term. This result indicates that the shape of the PMF and stability of SShIP are determined by the energy of the system. Relationship between the hydration structure and the energy of the system was examined and it was confirmed that the hydration structure in the first hydration shell of the ion-pair was one of the important factor, which made the minimums and maximum in the energy terms and PMFs, and stabilized the SShIP structure.     

An evolutionary algorithm for the global optimization of molecular clusters: application to water, benzene, and benzene cation

We have developed an evolutionary algorithm (EA) for the global minimum search of molecular clusters. The EA is able to discover all the putative global minima of water clusters up to (H(2)O)(20) and benzene clusters up to (C(6)H(6))(30). Then, the EA was applied to search for the global minima structures of (C(6)H(6))(n)(+) with n = 2-20, some of which were theoretically studied for the first time. Our results for n = 2-6 are consistent with previous theoretical work that uses a similar interaction potential. Excluding the very symmetric global minimum structure for n = 9, the growth pattern of (C(6)H(6))(n)(+) with n ≥ 7 involves the (C(6)H(6))(2)(+) dimer motif, which is placed off-center in the cluster. Such observation indicates that potentials commonly used in the literature for (C(6)H(6))(n)(+) cannot reproduce the icosahedral-type packing suggested by the available experimental data.     

Synthesis of Glux based polymers for removal of benzene derivatives and pesticides from water

In this study, biomass‐derived Glux was used as a monomer in the polymerization of a novel semi‐aromatic polyethersulfone and polyester. The characterization of the synthesized polymers was carried out by means of MALDI‐TOF and NMR spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. These two polymers were tested as sorbent phases of two families of pollutants (benzene derivatives and pesticides) in water. The analysis of the target hydrophilic pollutants was successfully achieved using high performance liquid chromatography meanwhile gas chromatography was applied for the detection of the hydrophobic pollutants. The adsorption data demonstrate that the novel polyethersulfone based on Glux can effectively remove the studied organic pollutants (polar and non polar, 90% to 100%) at environmental concentration (tested concentration: 1 mg L^?1) simultaneously when present in mixtures.     

Multiparticle sintering dynamics: from fractal-like aggregates to compact structures

Multiparticle sintering is encountered in almost all high temperature processes for material synthesis (titania, silica, and nickel) and energy generation (e.g., fly ash formation) resulting in aggregates of primary particles (hard- or sinter-bonded agglomerates). This mechanism of particle growth is investigated quantitatively by mass and energy balances during viscous sintering of amorphous aerosol materials (e.g., SiO(2) and polymers) that typically have a distribution of sizes and complex morphology. This model is validated at limited cases of sintering between two (equally or unequally sized) particles, and chains of particles. The evolution of morphology, surface area and radii of gyration of multiparticle aggregates are elucidated for various sizes and initial fractal dimension. For each of these structures that had been generated by diffusion limited (DLA), cluster-cluster (DLCA), and ballistic particle-cluster agglomeration (BPCA) the surface area evolution is monitored and found to scale differently than that of the radius of gyration (moment of inertia). Expressions are proposed for the evolution of fractal dimension and the surface area of aggregates undergoing viscous sintering. These expressions are important in design of aerosol processes with population balance equations (PBE) and/or fluid dynamic simulations for material synthesis or minimization and even suppression of particle formation.     

Molecular cobalt pentapyridine catalysts for generating hydrogen from water

A set of robust molecular cobalt catalysts for the generation of hydrogen from water is reported. The cobalt complex supported by the parent pentadentate polypyridyl ligand PY5Me(2) features high stability and activity and 100% Faradaic efficiency for the electrocatalytic production of hydrogen from neutral water, with a turnover number reaching 5.5 × 10(4) mol of H(2) per mole of catalyst with no loss in activity over 60 h. Control experiments establish that simple Co(II) salts, the free PY5Me(2) ligand, and an isostructural PY5Me(2) complex containing redox-inactive Zn(II) are all ineffective for this reaction. Further experiments demonstrate that the overpotential for H(2) evolution can be tuned by systematic substitutions on the ancillary PY5Me(2) scaffold, presaging opportunities to further optimize this first-generation platform by molecular design.     

pH/potential-responsive large aggregates from the spontaneous self-assembly of a triblock copolymer in water

A simple triblock copolymer, mPEG750-aniline pentamer-mPEG750, was prepared by condensation polymerization. The solubility of aniline pentamer in this kind of copolymer was improved in common solvents especially in aqueous solution, and the electroactivity of this copolymer was confirmed by UV-vis and CV in aqueous solution. When aniline pentamer was in its emeraldine state, the copolymer spontaneously self-assembled into large spheres (with diameters up to 1000 nm) in acidic aqueous solution (pH < 3), and into microspheres (with diameters of about 300 nm) in alkali aqueous solution, while the size of the aggregates decreased with the increase of pH. For reversible transition between the large spheres and microspheres under the change of the pH and potential, which changed the doping state and the oxidation state, respectively, the copolymer has potential applications in sensors, controlled drug release, and so forth.     

Hydrophobic Molecular Similarity from MST Fractional Contributions to the Octanol/water Partition Coefficient

Summary The use of a recently proposed hydrophobic similarity index for the alignment of molecules and the prediction of their differences in biological activity is described. The hydrophobic similarity index exploits atomic contributions to the octanol/water transfer free energy, which are evaluated by means of the fractional partitioning scheme developed within the framework of the Miertus-Scrocco-Tomasi continuum model. Those contributions are used to define global and local measures of hydrophobic similarity. The suitability of this computational strategy is examined for two series of compounds (ACAT inhibitors and 5-HT₃ receptor agonists), which are aligned to maximize the global hydrophobic similarity using a Monte Carlo-simulated protocol. Indeed, the concept of local hydrophobic similarity is used to explore structure–activity relationships in a series of COX-2 inhibitors. Inspection of the 3D distribution of hydrophobic/hydrophilic contributions in the aligned molecules is valuable to identify regions of very similar hydrophobicity, which can define pharmacophoric recognition patterns. Moreover, low similar regions permit to identify structural elements that modulate the differences in activity between molecules. Finally, the quantitative relationships found between the pharmacological activity and the hydrophobic similarity index points out that not only the global hydrophobicity, but its 3D distribution, is important to gain insight into the activity of molecules. J.M.M. and S.P. have contributed equally to this study.     

Self-assembly of tris(2-ureidobenzyl)amines: a new type of capped, capsule-like dimeric aggregates derived from a highly flexible skeleton

A set of tris(2-ureidobenzyl)amines 3 was prepared and their dimerization processes thoroughly investigated. In spite of their inherent flexibility, tris(ureas) 3 form dimeric aggregates both in the solid state and in solution. Evidence for the existence of these dimeric species was provided by a combination of techniques (X-ray analysis, NMR and IR spectroscopy, and ESI-MS). The association constants and thermodynamic parameters for the dimerization processes of selected tris(ureas) were determined and show that they are enthalpically driven. Heterodimerization experiments in solution reveal a high degree of self-recognition or narcissistic self-sorting. On the other hand, desymmetrized tris(ureas) derived from 3 self-assemble with modest regioselectivities depending on the terminal substituent of every urea functionality.     

Molecular fragmentations: Part VII. Structures and energies of mass spectral fragments derived from benzene

Molecular structures and energies have been calculated, using MINDO/3, of the mass spectral ions arising from benzene: (C₆H₆)⁺ (three non-valence isomers); (C₆H₅⁺); (C₅H₃⁺) (four isomers); (C₄H₄)⁺ (three isomers); (C₄H₃)⁺ (two isomers); (C₄H₂)⁺ (four isomers); (C₃H₃)⁺; and (C₂H₂)⁺. Calculations have been made for the conjugate neutral fragments, allowing calculation of appearance potentials, and also for the ion (C₆H₇)⁺.     

Adsorption properties of the MOF-5 metal-organic framework in relation to water and benzene

Adsorption properties of synthesized metal-organic frameworks based on 1,4-dicarboxylate ligands and zinc ions were studied. It was shown that the adsorption properties of these metal-organic frameworks in relation to water and benzene are much higher than those for the known adsorbents: KT-1 and KT-2 coals, USY and ZSM-5 zeolites, and pentasil.     

Adsorption of copolymers aggregates: from kinetics to adsorbed layer structure

We examined the adsorption, on hydrophobic and hydrophilic surfaces, of 4 rake-type poly(dimethyl siloxane) (PDMS) copolymers varying the amount of poly(ethylene glycol) (PEG) graft arms from 41 to 72%. The copolymers formed large aggregates in solution, complicating their adsorption kinetics and layer structures. We found the adsorption process always to be dominated by the adsorption of large aggregates, with strongly bound layers resistant to rinsing in adsorbing buffer. Adsorbed amounts were nearly independent of the substrate. However, subtleties in the adsorption kinetics suggested different layer structures for the different systems. On hydrophilic silica, aggregates adsorbed at the transport limited rate until surface saturation, and associated interfacial structures were likely retained. On the hydrophobic surface, a subset of the copolymers exhibited retarded late stage adsorption kinetics suggestive of brush formation. This work demonstrates how subtle differences in adsorption kinetics provide insight into potential interfacial layer structures.