Reversible sol-gel transition of a tris-urea gelator that responds to chemical stimuli

Tris-urea 1 functioned as a low-molecular-weight gelator for a variety of polar organic solvents. An acetone gel of 1 became a homogeneous solution in response to the addition of anions. The minimum amount of anion necessary for the gel-sol transition was specific to the anion species. A linear relationship was demonstrated between the amount of anion required and the total binding constant of 1 with the anion. Re-gelation occurred by addition of BF₃·OEt₂ and with ultrasound irradiation of the acetone solution of 1 and F⁻.     

Hydrogen-bonding A(LS)2-type low-molecular-mass gelator and its thermotropic mesomorphic behavior

A unique cholesterol-based A(LS)₂-type gelator, which is a hydrogen-bonding complex based on an ALS-type non-gelator molecule 3-cholesteryl 4-(trans-2-(4-pyridinyl)vinyl)phenyl succinate and a counterpart 3-cholesteryloxycarbonylpropanoic acid, shows strong gelation ability in alcohol and aromatic solvents. The formed gel has a high T_g at low gelation concentration, and its xerogel shows fibrillar microstructure revealed by scanning electron microscopy (SEM). FTIR confirms the existence of intermolecular hydrogen bond in the gelator, and X-ray diffraction (XRD) analysis reveals that the gelator possesses a folded conformation in gel and self-assembles into the fibrillar structure mainly by van der Waals interaction between cholesteryl moieties of the gelator. Further more, the thermotropic behavior of the xerogel is studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), which shows typical optical textures of liquid crystals.     

A chelating beta-diketonate/phenoxide ligand and its coordination behavior toward titanium and scandium

Dibenzoylmethane derivatives with one (L1H2) or both (L2H3, L3H3) benzenes linked at their ortho positions to 4,6-di-tert-butylphenol moieties by two-carbon linkers have been synthesized. The mono-beta-diketone-monophenol ligand L1H2 is metalated by titanium alkoxides to form the homoleptic complex (L1)2Ti and heteroleptic complexes (L1)Ti([OCH2CH2]2NR) (R = H, CH3), and reacts with Cp3Sc to form CpSc(L1). These are the first examples of complexes of a beta-diketonate ligand which is further chelating to a single metal center. Crystallographic analysis of (L1)2Ti indicates that the 10-membered ring allows chelation of the phenoxide with little strain, and both fac and mer geometries are accessible in solution. Protonolysis of the second cyclopentadienyl ring of Cp3Sc appears to take place by an indirect, Cp3Sc-catalyzed pathway.     

Regioselective cleavage of the bis-benzylidene acetal of d-mannitol under oxidative and reductive conditions: a new approach to C2-symmetric chiral ligands

A highly regioselective oxidative cleavage of 1,3:4,6-di-O-benzylidene-d-mannitol was carried out using NBS and the resultant product was readily converted to the C₂-symmetric chiral ligand, (R,R)-3,4-dihydroxy-1,5-hexadiene. On the other hand, reductive cleavage of 1,3:4,6-di-O-benzylidene-d-mannitol was achieved in a highly regioselective manner using BF₃·OEt₂ and Et₃SiH to give a highly functionalized benzyl ether, which was converted to a synthetically useful C₂-symmetric bis-amino alcohol derivative.     

Amido-bridged Cu2N2 diamond cores that minimize structural reorganization and facilitate reversible redox behavior between a Cu1Cu1 and a class III delocalized Cu1.5Cu1.5 species

A novel Cu(2)N(2) diamond core structure supported by an [SNS](-) ligand (1) ([SNS](-) = bis(2-tert-butylsulfanylphenyl)amido) has been prepared. This dicopper system exhibits a fully reversible one-electron redox process between a reduced Cu(1)Cu(1) complex, [[SNS][Cu]](2) (2), and a class III delocalized Cu(1.5)Cu(1.5) state, [[[SNS][Cu]](2)][B(3,5-(CF(3))(2)C(6)H(3))(4)] (3). Structural snapshots of both redox forms have been obtained to reveal remarkably little overall structural reorganization. The Cu...Cu bond distance nonetheless undergoes an appreciable compression (approximately 0.13 A) upon oxidation, providing a Cu...Cu distance of 2.4724(4) A in the mixed-valence state that is virtually identical to the Cu...Cu distance observed in the reduced form of the Cu(A) site of thiolate-bridged cytochrome c oxidase. Despite the low structural reorganization evident between 2 and 3, the [SNS](-) ligand is quite flexible. For example, square-planar geometries can prevail for divalent copper ions supported by [SNS](-) as evident from the crystal structure of [SNS]CuCl (4). Physical characterization for the mixed valence complex 3 includes electrochemical, magnetic (SQUID), EPR, and optical data. The complex has also been examined by density functional methods. An attempt was made to measure the rate of electron self-exchange k(s) between the Cu(1)Cu(1) and the Cu(1.5)Cu(1.5) complexes 2 and 3 by NMR line-broadening analysis in dichloromethane solution. While the system is certainly in the fast-exchange regime, the exchange process is too fast to be accurately measured by this technique. The value for k(s) can be bracketed with a conservative lower boundary of > or =10(7) M(-1) s(-1), a value that appears to be larger than other low molecular weight copper model complexes for which similar data is available. The unusually large magnitude of k(s) likely reflects the minimal structural reorganization that accompanies Cu(1)Cu(1) <--> Cu(1.5)Cu(1.5) interchange.     

Synthesis and Cu(II) coordination chemistry of a patellamide derivative: consequences of the change from the natural thiazole/oxazoline to the artificial imidazole heterocycles

The synthesis and Cu(II) coordination chemistry of the cyclic pseudo-octapeptide H(4)pat(1), a dimethyl-imidazole analogue of naturally occurring cyclic peptides (patellamide A-F, ascidiacyclamide) is reported. Substitution of the oxazoline and thiazole heterocycles by dimethyl-imidazoles leads to a slightly different structure of the macrocycle in the solid state. The Cu(II) coordination chemistry of H(4)pat(1), monitored with high-resolution electrospray mass spectrometry, spectrophotometric titrations, and EPR spectroscopy, revealed the presence of both mono- and dinuclear Cu(II) complexes. The dimethyl-imidazole analogue shows a high cooperativity in Cu(II) coordination, that is, the preferred formation of dinuclear complexes. The dinuclear unbridged Cu(II) complexes of H(4)pat(1) have unusual EPR features, reminiscent of those of patellamide D: the dipole-dipole interaction of the Cu(II) centers is negligible due to the "magic angle" orientation of the two Cu(II) ions. Density functional theory calculations (DFT) are used to model the structures of the Cu(II) complexes, and the structural assignments from the spectroscopic investigations are supported by the optimized and by X-ray structures of the metal-free macrocycle and dinuclear Cu(II) complexes of H(4)pat(1). The rigidity of the dimethyl-imidazole rings has a significant effect on the structures of the metal-free ligands and Cu(II) complexes and therefore changes the properties of these compounds. This may explain why Nature has chosen the thiazole-oxazoline combination for the patellamides and ascidiacyclamide.     

A tandem oxidative dearomatization/intramolecular Diels-Alder reaction: a short and efficient entry into tricyclic system of maoecrystal V

A short and efficient route to tricyclic ring system containing bridged bicyclo[2.2.2]octanone annulated with a lactone ring present in the maoecrystal is described. In-situ generation of spiroepoxycyclohexa-2,4-dienone and intramolecular cycloaddition are the key features of our approach.     

Thermosensitive and control release behavior of poly(N-isopropylacrylamide-co-acrylic acid)/nano-Fe3O4 magnetic composite latex particle that is synthesized by a novel method

In this study, a novel method was used to synthesize the poly(N-isopropylacrylamide-co-acrylic acid)/Fe₃O₄ (poly(NIPAAm-AA)/Fe₃O₄) magnetic composite latex. The crosslinked poly(NIPAAm-AA) polymer latex particles were first synthesized by the method of soapless emulsion polymerization, then Fe²⁺ and Fe³⁺ ions were introduced to bond with the -COOH groups of AA segments in poly(NIPAAm-AA) polymer latex particles. Further by a reaction with NH₄OH, Fe₃O₄ nanoparticles were generated in situ. The concentrations of acrylic acid (AA), crosslinking agent (N,N′-methylene bisacrylamide (MBA)), and Fe₃O₄ nanoparticles were important factors to influence the morphology and lower critical solution temperature (LCST) of poly(NIPAAm-AA)/Fe₃O₄ magnetic composite latex particles. The poly(NIPAAm-AA)/Fe₃O₄ latex particles were used as a thermosensitive drug carrier to load caffeine. The control release of caffeine was studies. Morphology-based schematic models were proposed to explain the control release behavior of the composite particles with different compositions. Moreover, the protein (albumin, acetylated from bovine serum (BSA)) was bound on the surface of poly(NIPAAm-AA)/Fe₃O₄ composite latex particles. The effects of AA, crosslinking agent and Fe₃O₄ contents on the amount of BSA binding were investigated at different temperatures and pH values. The composition-morphology-BSA conjugation relationship was established.     

Compared behavior of 5-deoxy-5-iodo-D-xylo- and L-arabinofuranosides in the reductive elimination reaction: a strong dependence on structural parameters and on the presence of Zn2+. A combined experimental and theoretical investigation

In the framework of a project devoted to the chemical transformation of monosaccharides from hemicelluloses into higher added value materials, the zinc-induced reductive elimination from 5-deoxy-5-iodo derivatives of D-xylose and L-arabinose was carried out. This study gave us the opportunity to observe surprising behaviors. In particular, the reaction strongly depends on structural parameters (protecting group pattern, configuration at C-4) and on the presence of Zn2+ ions. Collaterally with the experimental work, water solvent PCM HF-DFT (MPW1K/LANL2DZ) computations were performed to obtain insight into the mechanism for the reductive part of the reaction sequence. Without Zn2+, the zinc insertion reaction was found to proceed through a concerted but non-synchronous process involving a relatively large energy barrier (32 kcal mol-1) that directly leads to the presumed organozinc intermediate. In the presence of Zn2+, a three-step mechanism was identified in which the cation coordinates the anomeric and ring oxygen atoms and also the sugar iodine atom, causing an activating effect on the zinc insertion process by facilitating the homolytic rupture of the C-I bond. Complexes between zinc and Zn2+ bound carbohydrates were characterized with large stabilization energies, suggesting that Zn2+ might enhance the affinity of the organic compound with the zinc metal surface.     

Ln2BaZnO5 and Ln2BaZn1−xCuxO5: A series of zinc oxides with zinc in a pyramidal coordination

A series of zinc oxides Ln₂BaZnO₅ has been synthesized for Ln = Sm, Eu, Gd, Dy, Ho, and Y. Theses phases are orthorhombic and isostructural with the copper compounds Ln₂BaCuO₅ previously described, as shown from the structural study of one member Y₂BaZnO₅. In this structure, whose framework is built up from edge- and face-sharing LnO₇ polyhedra, the Zn²⁺ ions exhibit an unusual pyramidal coordination ZnO₅. The solid solution Y₂BaZn_1?xCu_xO₅ has been studied by infrared spectroscopy and electron spin resonance (ESR). The distorted square-based pyramidal configuration of Zn²⁺ and Cu²⁺ is confirmed. The ESR spectra of diluted samples exhibit a hyperfine structure and are typical of individual Cu(II) ions. For higher Cu(II) contents, they exhibit an anisotropic broad signal which is interpreted in terms of CuCu interactions.     

A new macrocyclic cryptand with squaramide moieties: an overstructured Cu(II) complex that selectively binds halides: synthesis, acid/base- and ligational behavior, and crystal structures

The synthesis and characterization of the novel 24,29-dimethyl-6,7,15,16-tetraoxotetracyclo-[19.5.5.0(5,8).0(14,17)]-1,4,9,13,18,21,24,29-octaazaenatriaconta-Delta(5,8),Delta(14,17)-diene (L) are reported. Molecule L incorporates two squaramide functions in a overstructured chain connecting two opposite nitrogen atoms of the Me(2)[12]aneN(4) polyaza macrocyclic base to obtain a cage topology. The basicity and binding properties of L towards Cu(II) were determined by means of potentiometric measurements in aqueous solution (298.1+/-0.1 K, I=0.15 mol dm(-3)). Molecule L behaves as a diprotic base under the experimental conditions employed and forms only mononuclear Cu(II) complexes in which the squaramide moieties are not involved in the stabilization of the metal ion that is stabilized by the amine functions of the polyaza base inside the three-dimensional cavity. The [CuL](2+) species was tested as a host for the series of halide anions. UV-visible spectrophotometric experiments permitted the determination of the addition constants of halides to the Cu(II)-complexed species. The [CuL](2+) species binds the anions F(-), Cl(-), and Br(-) by forming the [CuLX](+) species, but does not bind the biggest I(-) anion. A trend of selectivity as a function of the hydrogen-bonding capability as well as the dimensions of the anion were established; the maximum value of selectivity was for addition of the F(-) anion (log K=4.8). This selectivity is due to the presence of the overstructured chain containing the squaramide groups up to the Me(2)[12]aneN(4) macrocyclic base. The squaramide groups, by providing hydrogen-bond contacts, permit the [CuL](2+) species to selectively bind these anions through the formation of a hydrogen-bond network with F(-) and Cl(-). The crystal structures of the [CuLF](+) and [CuLCl](+) cations support the results obtained in aqueous solution.     

Stereochemistry of base-induced cleavage of methoxide ion on cis- and trans-1,4-diphenylphosphorinanium salts. A different behavior with a phenyl substituent

The synthesis and characterization of pure cis- and trans-1,4-diphenyl-1-methoxy-phosphorinanium tetrafluoroborate salts 11a and 11b, molecules designed to evaluate the effect of a phenyl substituent at C-4 on the stereochemical course of base-induced nucleophilic displacement of the methoxy group at phosphorus, was accomplished. The presence of a phenyl substituent at C-4 changes the stereochemical course of these reactions, from complete inversion with alkyl groups to products where retention of configuration at phosphorus predominates. We suggest a mechanism involving Berry pseudorotations and provide evidence that the hydroxide ion attacks the phosphorus atom through experiments with NaOH enriched with ¹⁷O.     

A robust porous metal-organic framework with a new topology that demonstrates pronounced porosity and high-efficiency sorption/selectivity properties of small molecules

A robust porous metal-organic framework (MOF), [Co(3)(ndc)(HCOO)(3)(μ(3)-OH)(H(2)O)](n) (1) (H(2)ndc=5-(4-pyridyl)-isophthalic acid), was synthesized with pronounced porosity. MOF 1 contained two different types of nanotubular channels, which exhibited a new topology with the Schlafli symbol of {4(2).6(5).8(3)}{4(2).6}. MOF 1 showed high-efficiency for the selective sorption of small molecules, including the energy-correlated gases of H(2), CH(4), and CO(2), and environment-correlated steams of alcohols, acetone, and pyridine. Gas-sorption experiments indicated that MOF 1 exhibited not only a high CO(2)-uptake (25.1 wt % at 273 K/1 bar) but also the impressive selective sorption of CO(2) over N(2) and CH(4). High H(2)-uptake (2.04 wt % at 77 K/1 bar) was also observed. Moreover, systematic studies on the sorption of steams of organic molecules displayed excellent capacity for the sorption of the homologous series of alcohols (C(1)-C(5)), acetone, pyridine, as well as water.