Pyridine-Stabilized Cationic Silanethione Tungsten Complexes: Synthesis,Structures, and Reactivity

Silanethione compounds, R₂Si=S, have been recognized as highly reactive species. One reliable way to stabilize silanethione is its coordination to transition metal fragments to convert silanethione-coordinated transition metal complexes. Herein, we report the synthesis, structure, and reactivity of a second cationic silanethione tungsten complex [Cp*(OC)₃W{S=SiR₂(py)}]TFPB (R=Me ( 5 a ), Ph ( 5 b ), Cp*: η⁵-C₅Me₅, py: pyridine, and TFPB⁻: [B{3,5-(CF₃)₂C₆H₃}₄]⁻). Complex 5 was obtained by H⁻ abstraction from the Si atom in the corresponding silylsulfanyl complex Cp*(OC)₃W(SSiR₂H) ( 4 ) with Ph₃CTFPB, followed by the addition of pyridine. The reaction of 5 with PhNCS and PMe₃ produced [Cp*(OC)₃W{SSiR₂N(Ph)C(PMe₃)₂}]TFPB (R=Me ( 6 a ), Ph ( 6 b )) via the elimination of pyridine and the addition of the 1,3-dipolar species PhNC(PMe₃)₂ ( A ) to the Si atom.     

Photoinduced Charge‐Transfer State of 4‐Carbazolyl‐3‐(trifluoromethyl)benzoic Acid: Photophysical Property and Application to Reduction of Carbon−Halogen Bonds as a Sensitizer

The photoinduced persistent intramolecular charge‐transfer state of 4‐carbazolyl‐3‐(trifluoromethyl)benzoic acid was confirmed. It showed a higher catalytic activity in terms of yield and selectivity in the photochemical reduction of alkyl halides compared to the parent carbazole. Even unactivated primary alkyl bromides could be reduced by this photocatalyst. The high catalytic activity is rationalized by considering the slower backward single‐electron transfer owing to the spatial separation of the donor and acceptor subunits.     

Selective inclusion of electron-donating molecules into porphyrin nanochannels derived from the self-assembly of saddle-distorted, protonated porphyrins and photoinduced electron transfer from guest molecules to porphyrin dications

A doubly protonated hydrochloride salt of a saddle-distorted dodecaphenylporphyrin (H2DPP), [H4DPPP]Cl2, forms a porphyrin nanochannel (PNC). X-ray crystallography was used to determine the structure of the molecule, which revealed the inclusion of guest molecules within the PNC. Electron-donating molecules, such as p-hydroquinone and p-xylene, were selectively included within the PNC in sharp contrast to electron acceptors, such as the corresponding quinones, which were not encapsulated. This result indicates that the PNC can recognize the electronic character and steric hindrance of the guest molecules during the course of inclusion. ESR measurements (photoirradiation at lambda>340 nm at room temperature) of the PNC that contains p-hydroquinone, catechol, and tetrafluorohydroquinone guest molecules gave well-resolved signals, which were assigned to cation radicals formed without deprotonation based on results from computer simulations of the ESR spectra and density functional theory (DFT) calculations. The radicals are derived from photoinduced electron transfer from the guest molecules to the singlet state of H4DPP2+. Transient absorption spectroscopy by femtosecond laser flash photolysis allowed us to observe the formation of 1(H4DPP2+)*, which is converted to H4DPP+. by electron transfer from the guest molecules to 1(H4DPP2+)*, followed by fast disproportionation of H4DPP+., and charge recombination to give diamagnetic species and the triplet excited state 3(H4DPP2+)*, respectively.     

Sesquiterpenoids and flavonoids from the aerial parts of Tithonia diversifolia and their cytotoxic activity

Cytotoxicity-guided fractionation of the 80% EtOH extract of Tithonia diversifolia has resulted in the isolation of twelve sesquiterpenoids (1-12), including three new ones (4, 10, 12), and three known flavonoids (13-15). The structures of the new compounds were determined by analysis of their spectroscopic data. The isolated compounds showed cytotoxic activity against HL-60 leukemia cells with IC(50) values ranging from 0.13 to 13.0 microM, when etoposide used as a positive control gave an IC(50) value of 0.43 microM. The cancer growth inhibitory property of 9, the main cytotoxic compound in T. diversifolia, was examined using a disease-oriented panel composed of 39 human cancer cell lines in the Japanese Foundation for Cancer Research.     

Orientational isomerism controlled by the difference in electronic environments of a self-assembling heterodimeric capsule

Tetrakis(4-hydroxyphenyl)-cavitand 1 and tetra(4-pyridyl)-cavitand 2 self-assemble into a heterodimeric capsule 1.2 via four ArOH...pyridyl hydrogen bonds in CDCl3. The 1.2 expresses the orientational isomerism of an encapsulated unsymmetrical guest with high orientational selectivity because the electronic environment of the 1 unit is different from that of the 2 unit. For p-ethoxyiodobenzene and 2-iodo-6-methoxynaphthalene encapsulated in 1.2, the iodo group is specifically oriented to the cavity of the 2 unit. The orientational isomeric selectivity for methyl p-acetoxybenzoate and methyl p-ethoxybenzoate within 1.2 is 1:0.11 and 1:<0.05, respectively, wherein the methyl ester group is preferentially oriented to the cavity of the 2 unit. The delicate balance among electrostatic potential repulsion, CH-pi interaction, or CH-halogen (halogen-pi) interaction, in 1.2-guest assembly influences the orientational isomeric selectivity of unsymmetrical guests within 1.2.     

Heparin bioconjugate with a thermoresponsive cationic branched polymer: a novel aqueous antithrombogenic coating material

With a view to reducing the thrombogenic potential of artificial blood-contact devices and natural tissues, we developed a novel aqueous antithrombogenic coating material, comprising a heparin bioconjugate that incorporated a thermoresponsive cationic polymer as a surfactant. The polymer was prepared by the sequential steps of initiator-transfer agent-terminator (iniferter)-based living radical photopolymerization of N-[3-(dimethylamino)propyl]acrylamide, followed by the polymerization of N-isopropylacrylamide from tetra(N,N-diethyldithiocarbamylmethyl)benzene as a multifunctional iniferter. The polymer obtained possessed four branched chains, each consisting of a cationic PDMAPAAm block (Mn: ca. 3000 g.mol(-1)) forming an inner domain for heparin binding and a thermoresponsive PNIPAM block (Mn: ca. 6000 g.mol(-1)) forming an outer domain for surface fixation; bioconjugation of the polymer with heparin occurred immediately upon simple mixing in an aqueous medium. Because the lower critical solution temperature of the heparin bioconjugate was approximately 35 degrees C, it could be coated from an aqueous solution at room temperature. The excellent adsorptivity and high durability of the coating below 37 degrees C was demonstrated on several generally used polymers by wettability measurement and surface chemical compositional analysis, and on collagen sheets and rat skin tissue by heparin staining. Blood coagulation was significantly prevented on the heparin bioconjugate-coated surfaces. The thermoresponsive bioconjugate developed therefore appeared to satisfy the initial requirements for a biocompatible aqueous coating material.     

Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

Fluorophotometric Determination of Hydrogen Peroxide with Fluorescin in the Presence of Cobalt (II) and Reaction Against Other Reactive Oxygen Species

A fluorophotometric method for the determination of hydrogen peroxide (H₂O₂) using fluorescin was developed. This method was based on the oxidative reaction of fluorescin, a colorless, non-fluorescent lactoid fluorescein, by H₂O₂ to give highly fluorescein fluorescence emission. In the determination of H₂O₂, the calibration curve exhibited linearity over the H₂O₂ concentration range of 1.5–310 ng mL⁻¹ at an emission wavelength of 525 nm with an excitation of 500 nm and with relative standard deviations (n = 6) of 2.51%, 2.48%, and 1.31% for 3.1 ng mL⁻¹, 30.8 ng mL⁻¹, and for 308 ng mL⁻¹ of H₂O₂, respectively. The detection limit for H₂O₂ was 1.9 ng mL⁻¹ six blank determinations was performed (ρ = 6). This proposed method was applied to detection of other reactive oxygen species and nitrogen species (ROS/RNS) such as singlet oxygen (¹O₂), hydroxyl radical (^•OH), peroxynitrite (ONOO⁻) etc., and it was possible to detect them with a high sensitivity. In addition, this proposed method was applied to the recovery tests of H₂O₂ in calf serum, human saliva, rain water, and wheat noodles; the results were satisfactory.     

Guest-encapsulation behavior in a self-assembled heterodimeric capsule

Tetra(4-pyridyl)-cavitand 1 and tetrakis(4-hydroxyphenyl)-cavitand 2 self-assemble into a heterodimeric capsule 1·2 via four PhOH?pyridyl hydrogen bonds in CDCl₃, wherein one molecule of 1,4-disubstituted-benzene as a guest is encapsulated to form a ternary complex, guest@(1·2). The X-ray crystallographic analysis of (methyl p-ethoxybenzoate)@(1·2) confirmed that the methyl ester and ethoxy groups of the encapsulated guest are oriented to the cavity ends of the 1 and 2 units, respectively. The scope and limitation of guest encapsulation in 1·2, including guest-binding selectivity and orientational isomeric selectivity, are described from the viewpoint of size complementarity and CH-π, CH-halogen, and halogen-π interactions between guest and the cavity of 1·2.