Molecular-scale design of catalytically active structures at oxide surfaces is highlighted, focusing on our recent challenges for selective heterogeneous catalysis. We found novel structural transformations of supported metal complexes on oxide surfaces and achieved unique selective catalysis for various chemical syntheses. In this account, the advanced design of supported metal-complex catalysts on oxide surfaces and the in situ characterization of their structural kinetics, which means kinetics of structural changes of catalysts themselves, under catalyst-working conditions are reviewed.
Our recent research on advanced design of supported metal-complex catalysts on oxide surfaces and in situ characterization for structural kinetics of catalytically active species under catalyst-working conditions are highlighted.
Two isomeric PdII complexes, 4 and 5, of a β-tetraphenyl meso-hexakis(pentafluorophenyl) substituted hexaphyrin were isolated, both of which exhibited distinct diatropic ring currents and electronic absorption spectra characteristic of aromatic porphyrinoids. These complexes have been designated Möbius aromatic species on the basis of their twisted conformations and 28π-electronic networks.
PdII metallation of a β-tetraphenyl meso-hexakis(pentafluorophenyl) substituted hexaphyrin afforded two twisted Möbius aromatic complexes, which were well separated by column chromatography owing to conformational rigidity conferred by PdII-complexation.
A novel trigonal conjugate of glutathiones with a 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene core was synthesized and its self-assembling behavior was investigated in water. Three glutathione units were regulated to orient on the same side of the benzene ring, through steric repulsions between ethyl groups attached on the benzene core. Concentration dependence of 1H NMR chemical shifts in D2O revealed formation of molecular assemblies with two affinity constants (Ka = 4.75 × 102 and 6.76 × 104 M−1), which reflect stepwise assembly directed by electrostatic interactions, hydrophobic interactions, and hydrogen bonding. In scanning electron microscopy, hard spherical assemblies with the size of 310 ± 50 nm were observed at high concentration (10 mM), whereas slightly disordered spherical assemblies were obtained at lower concentrations. The spherical assemblies self-assembled from the conformation-regulated trigonal glutathiones showed regular morphology and enhanced rigidity compared to those formed from conformationally non-regulated trigonal glutathiones.
A comparative quasiclassical trajectory study of the H + FCl (v = 0–3,j = 0–3) → HCl + F reaction and its isotope variant has been carried out to investigate the isotope effect from a chemical stereodynamics view point. Employing the recent DHTSN PES of the ground 12A′ electronic state [Deskevich et al., J. Chem. Phys.2006, 124, 224303], the present study calculated the angular distributions and the four polarization-dependent differential cross sections characterizing the vector correlations among the product angular momentum j′ and the reagent and the product initial relative velocities k and k′. The investigated collision energies are 20 and 40 kcal mol−1. The study revealed stronger product polarization behaviors in the heavier isotope variant which can be explained with the proposed impulse model for triatomic collision systems.
The present study theoretically investigated hypervalent bonding systems with the skeleton of pentalene. Geometries and energetics were examined by density functional theory calculations with triple-zeta class basis sets. The bond energies of the O–X and N–X hypervalent three-center four-electron bonds were estimated. Furthermore, the relationships between the bond-switching equilibration reactions and the stabilities of the hypervalent bonding intermediates were examined.
The detection sensitivities of proton-transfer reaction mass spectrometry for isobaric 2-propanol and acetic acid were examined systematically at five different kinetic energies. The detection sensitivity for acetic acid was in good agreement with the sensitivities calculated from the experimentally and theoretically obtained rate constants for the reaction of H3O+ with acetic acid. However, the detection sensitivity for 2-propanol was quite small compared with the calculated values. In deuterium-labeling studies, the formation of H2DO+ ions at m/z 20 in the reaction of H3O+ with (CD3)2CHOH was clearly observed, whereas the formation of H2DO+ was not observed in the reaction of H3O+ with CD3C(O)OH. We concluded that the difference between the experimental and calculated detection sensitivities for 2-propanol was attributed to a reaction channel that reproduced H3O+ ion during the reaction of H3O+ with 2-propanol.
A new series of substituted hydrazone complexes, [Cu(Hpbph)I] (1), [Cu(Hpbph)PPh3]PF6 (2-PF6), [NiCl(Hpbph)]Cl (3-Cl), [PtCl(Hpbph)]ClO4 (4-ClO4), and [PtCl(pbph)] (4b) (Hpbph = 2-(diphenylphosphino)benzaldehyde 2-pyridylhydrazone) have been synthesized and characterized. X-ray crystallography revealed that the copper(I) complexes adopt pseudo-tetrahedral geometry, while the nickel(II) and platinum(II) complexes provide square-planar forms. All the complexes exhibit distinct color changes on the basis of the deprotonation/protonation on the ligand although their acid/base behaviors are largely different. The acidity constants (pKa) in methanol were determined to be 11.4 (1), 12.5 (2), 7.7 (3), and 6.7 (4). The results indicate that the dissociation of the proton on the ligand strongly depends on the ligand deformation controlled by the coordination geometry of the complexes and ancillary ligands also somewhat affect the acidity.
The abnormal aggregation of tau protein into paired helical filaments (PHFs) is one of the hallmarks of Alzheimer’s disease. Although some small molecules capable of inhibiting the aggregation have been discovered, knowledge of relevant mechanisms is fragmentary. In this paper, Alzheimer’s tau fragment R3 corresponding to the third repeat of microtubule-binding domain was selected as the tau model. We, for the first time, reported flavonoids for their ability to inhibit heparin-induced assembly of R3 by thioflavin S (ThS) fluorescence assay. Moreover, we further proposed the possible inhibition mechanism of six flavonoid compounds, applying tyrosine fluorescence quenching and circular dichroism (CD) methods. It was shown that flavonoids could inhibit fibril formation of R3 by deformation of the flexible extended structure, consequently losing its aggregation ability. The inhibitory ability is closely related to their binding modes and binding degree to R3. Finally a specific flavonoid structure was found to play an important role in inhibition.
4,4′-Azopyridine (2c) is used in conjunction with triphenylphosphine for the efficient conversion of carboxylic acids into amides via Mitsunobu reaction with primary and secondary aliphatic and aromatic amines. The highly selective amidation of only primary aromatic amines with new heterogeneous azo compounds based on benzothiazole 2d and isoxazole 2e is also described. These azo compounds 2c–2e can also be applied for selective mono-N-benzylation of primary aromatic amines. The solid side product heteroaromatic hydrazines obtained under the developed Mitsunobu conditions are easily separated by simple filtration and can be reoxidized to azo compounds for further use.
Transparent porous silica gel encapsulating ribonuclease A (RNase A) in nanoscale pores was prepared on the wall of a quartz cell. The UV and CD spectral changes upon heating showed that the protein unfolds in the pores with apparent heat denaturation temperatures (Tm) at 60 to 64 °C, which are slightly lower than that in solution. The unfolding was irreversible due probably to strong interaction of the unfolded species with the surface of the pores upon prolonged treatment at high temperatures. However, when the gel was subjected to repeated temperature-jump experiments between 40 and 75 °C, RNase A reversibly unfolded and refolded in the pores. The unfolding was not apparently decelerated but the refolding was remarkably slowed. The protein refolded in the porous gel exhibited enzymatic activity toward cytidine 2′,3′-cyclic monophosphate (c-CMP), indicating that small molecules, such as c-CMP and its hydrolyzed species, can go through the holes of the silica gel. These results demonstrated not only that the wet porous gel is useful for the heat denaturation study of proteins but also that the gel encapsulating a protein can be applied as a soft material with enzymatic function.
Construction of the ABC ring system of taxanes via one-pot three-component coupling and intramolecular alkylation is accomplished. The 1,4-addition of a protected cyanohydrin ether to 2-methyl-2-cyclohexenone and subsequent addition of the resulting enolate to formaldehyde proceeded stereoselectively to provide the AC ring in 90% yield. The stereoselective reduction of the 2-keto group was achieved by using hydroxy-directed hydride reduction with LiAlH4. The intramolecular alkylation of the protected cyanohydrin ether furnished the ABC ring system of taxanes in 43% yield.
A novel zinc porphyrin receptor has been synthesized that has two identical binding pockets surrounded by six phenyl rings on both sides of the porphyrin plane. The binding of amine guests to the zinc porphyrin receptor was studied by UV–vis titration experiments. Among the amine guests, 1,4-diazabicyclo[2.2.2]octane (DABCO) showed the highest binding affinity (ΔG = −36.6 kJ mol−1 at 298 K in toluene) thanks to close contacts of DABCO with the aromatic walls of the binding pocket. The binding of DABCO was further investigated by dynamic NMR experiments. DABCO was tightly bound in one binding pocket when less than 1 equivalent of DABCO was added, but it started a rapid exchange between the two binding pockets when exceeding 1 equivalent.
Carbon nanotube-supported Ru (Ru/CNT) catalysts were covered with silica layers to inhibit the aggregation of Ru particles as well as the detachment of Ru particles from the CNTs during catalytic reactions. Coverage of the Ru/CNT with silica was performed by the successive hydrolysis of 3-aminopropyltriethoxysilane and tetraethoxysilane in the presence of CNT-supported Ru metal precursors, followed by reduction with hydrogen at 623 K. The diameter of the Ru particles in the silica-coated Ru/CNT ranged from 1 to 3 nm. The Ru particles and the CNTs in the silica-coated catalysts were uniformly covered with silica layers a few nanometers thick. The size of the Ru particles in the silica-coated catalysts did not change appreciably after treatment at 873 K, while the metal particles in the Ru/CNT were seriously aggregated after treatment at 873 K. The silica-coated Ru/CNT catalyzed the CO hydrogenation and the 1-hexene hydrogenation, despite the coverage of Ru particles with silica. In addition, the silica-coated Ru catalysts showed high durability toward Ru detachment from the CNTs during the repeated hydrogenation of 1-hexene, whereas the catalysts without silica layers were deactivated because of Ru detachment. Therefore, coverage of the Ru/CNT catalysts with silica layers prevented the deactivation of the catalysts.
Poly(arylene ether)s containing superacid groups (FSPEs) were synthesized as proton conducting membranes for fuel cell applications. To obtain the title ionomers, a series of brominated poly(arylene ether)s were synthesized and perfluorosulfonated via Ullmann coupling. The chemical structure and the ion exchange capacity (IEC) of the FSPEs were characterized by 1H and 19F NMR spectra. Tough, flexible, and transparent membranes with the IEC ranging from 0.34 to 1.29 mequiv g−1 were obtained by solution casting. The FSPE membranes did not show obvious glass transition behavior up to the decomposition temperature (180 °C). Microscopic analyses revealed homogeneous and well-connected ionic clusters for the high IEC membrane. Compared to conventional sulfonated poly(arylene ether) membranes, the FSPE membranes showed much higher proton conductivity. The highest proton conductivity of 0.07 S cm−1 was achieved at 80 °C and 86% relative humidity (RH) with the IEC = 1.29 mequiv g−1 membrane. A fuel cell using the FSPE membrane showed comparable performance to that of a Nafion cell at 78% RH and 80 °C.