Recent studies on the azo-group-combined photochromic metal complexes are reviewed. These studies show unique trans–cis isomerization behaviors that have not been observed in regular organic azobenzenes. In the case of azobenzene-bound bis(terpyridine) complexes of transition metals, photoisomerization behavior depends strongly on the central metals, counterions, and solvents. Rh and Co complexes undergo photoisomerization smoothly, but the isomerization of Ru and Fe complexes is substantially retarded due to the energy transfer from the π–π* excited state to the MLCT transition. The photoluminescence property of an azobenzene-attached Pt complex is switched by photoisomerization of the azo moiety. Tris(azobenzene-bound bipyridine)cobalt undergoes reversible trans–cis isomerization using a combination of the CoIII/CoII redox change and a single UV light source exciting the π–π* transition. The trans–cis conversion yield is higher for the meta isomer (with respect to the position of the azo group against the pyridyl group) than for the para isomer. The trans–cis photoisomerization behavior of a 6,6′-dimethyl-substituted azobenzene-bipyridine ligand is synchronized with coordination of the bipyridine moiety to copper, and the trans/cis isomerization can be controlled reversibly through CuII/CuI redox and a single UV light irradiation. Photoreaction of azoferrocene occurs not only by UV-light irradiation but also by green light irradiation that excites the MLCT transition. 3-Ferrocenylazobenzene undergoes reversible trans–cis isomerization using a single green light source and the FeIII/FeII redox change. Azo-conjugated metalladithiolenes of NiII, PdII, and PtII with diphenylphosphinoethane as a co-ligand show facile photoisomerization. The energy of the reversible trans-to-cis photoisomerization is considerably lower than that of azobenzene. The thermal stability of the cis form is, however, much higher than that of the organic azobenzene derivatives showing similar low-energy trans-to-cis photoisomerization. A novel proton response of the azo group occurs, and the combination of photoisomerization and protonation leads to a novel proton-catalyzed cis-to-trans isomerization. These results indicate that several kinds of multi-photo-functionalities can be realized for the azo-conjugated transition metal complexes by tuning the interaction between the azo moiety and the metal complex unit.
In this Account, the syntheses, structures, and properties of novel aromatic species containing a heavier group 14 element were delineated with systematic comparison, focusing on their aromaticity. Several kinds of silaaromatic compounds, such as silabenzene, 1- and 2-silanaphthalenes, and 9-silaanthracene, which are a very interesting class of novel π-bonding systems containing a silicon atom, were successfully synthesized and isolated as stable compounds by taking advantage of the kinetic stabilization afforded by introduction of an efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt). A similar synthetic approach can be applied to the cases of the first stable germaaromatics, such as germabenzene and 2-germanaphthalene. The molecular structures of the newly obtained metallaaromatic species were systematically investigated based on X-ray crystallographic analysis and spectroscopic studies together with theoretical calculations. During this study, a variety of new reactivities for these metallaaromatic species were revealed and some new insights and deeper knowledge were obtained about the aromaticity of metallaaromatic compounds of heavier group 14 elements.
This account describes bioorganic studies on marine natural products with bioactivity, such as antitumor activity and feeding attractance. Chemical studies on the following bioactive compounds of marine origin have been carried out: 1) feeding attractants for the starfish Acanthaster planci, 2) potent toxins from shellfish of the genus Pinna that cause food poisoning, and 3) a variety of cytotoxic and antitumor compounds from marine invertebrates including sea hares, Dolabella auricularia and Aplysia kurodai. Their isolation, structure elucidation, synthesis, and some aspects of their biological activities are summarized.
The concept and examples of armed cyclens are described and some of their recent applications in molecular recognition and supramolecular chemistry are explained. Armed cyclens are characterized by a parent tetraamine ring and functionalized sidearms. They can act as three-dimensional ligands for octadentate metal complexation. They exhibit comparable logK values for Na+ complexation to bicyclic cryptands, though they have flexible, open-chained structures. Since the resulting octacoordinate metal complexes have quadruple helicated structures, they can work as unique building blocks having C4 symmetry for chiral supramolecular architecture. Cholesterol-armed cyclen typically formed a self-aggregate with the integrated chirality in aqueous solution, in which asymmetrically helicated cyclen complexes were arrayed on a supramolecular scale. The self-aggregate had asymmetrically ordered domains, in which achiral organic anions and racemic metal complexes were nicely accommodated in stereo-controlled fashions. Since the armed cyclens have broad structural variations, they are applicable as specific receptors for cation recognition at the molecular level and also as building blocks for supramolecular architecture.
Novel heterophene analogues of tetracyanodiphenoquinodimethane (TCNDQ) having three electron-withdrawing groups in one molecule, 2,6-dicyanomethylene-4-oxo-2,6-dihydrocyclopentadithiophene (CPDT-TCNQ: 1) and 2,6-dicyanomethylene-4-oxo-2,6-dihydrocyclopentadiselenophene (CPDS-TCNQ: 2), have been synthesized. CPDT-TCNQ and CPDS-TCNQ have a good coplanar conformation and have a fairly high electron-accepting ability due to the existence of the central carbonyl group in addition to the two terminal dicyanomethylene groups. The anion radical salts, Me4X(CPDT-TCNQ)2 (X = N, P, and As), Et4N(CPDT-TCNQ)2, and Me4X(CPDS-TCNQ)2 (X = P and As), showed a metallic conducting behavior (σrt = 260 to 42 S cm−1) down to 130–255 K. In the crystal structures of Me4X(CPDT-TCNQ)2 (X = N, P, and As), which are isostructural with each other, the acceptor molecules form rigid and tight two-dimensional networks consisting of strong S···N and O···H inter-column contacts in the side-by-side direction. However, these salts have an extremely one-dimensional electronic structure along the stacking direction. This fact indicates that the chalcogen atoms in the acceptor molecules do not increase the dimensionality of the electronic structures of the anion radical salts; that property is significantly different from the role of chalcogen atoms of TTF or BEDT-TTF type donor molecules. The existence of the central carbonyl group in CPDT-TCNQ and in CPDS-TCNQ plays a very important role to give metallic anion radical salts due to the rigid conformation. The phase transition at 130 K on Me4N(CPDT-TCNQ)2 is regarded as 2kF CDW instability. The origins of the phase transitions at 165 K for Me4P(CPDT-TCNQ)2 and 185 K for Me4As(CPDT-TCNQ)2 are regarded as 4kF CDW. Thus the phase transition mechanisms of these salts are significantly different from each other, although the crystal structures are isostructural.
Pressure effects on the phase transitions in [1H12] and [2H12]trans-stilbene crystals were studied. Pressure–frequency plots and pressure–half band width plots for the Raman bands due to the inter- and intramolecular vibrations of [1H12] and [2H12]trans-stilbene crystals showed inflectional variations of the slopes of the plots at about 2 and 4 GPa. The fact that no clear discontinuous changes of the slopes of the plots were observed indicates that the second-order phase transitions take place at about 2 and 4 GPa. The low-frequency Raman bands of [1H12] and [2H12]trans-stilbene crystals were reinvestigated and assignments to the intermolecular rotational vibrations about the x, y, and z axes were made on the basis of the isotopic effect on the vibrational frequency.
The behavior of the inclusion complex consisting of tetradecyltrimethylammonium bromide (TTAB) and β-cyclodextrin (β-CD) was studied using ion selective electrodes sensitive to surfactant ions. Two different methods were used in the construction of the ion selective electrode: (i) Membrane ion selective electrode (MISE) and (ii) Coating wire ion selective electrode (CWISE). The data obtained from two methods showed a consistency between two methods under all conditions. The CWISE method showed a faster response relative to the MISE ones. The experiments were carried out at different temperatures and in different concentrations of cyclodextrin. The data obtained indicate that the inclusion complexes S(CD) and S(CD)2 had formed between TTAB and β-cyclodextrin in an aqueous environment. In addition to the 1:1 complex, TTAB formed 1:2 complexes with β-cyclodextrin. Further investigation showed that K1 for S(CD) was greater than K2 for S(CD)2. Finally, thermodynamic parameters of the complexation, i.e., ΔH°, ΔG°, and ΔS° were also calculated.
Examination of the structure-dependence of the total π-electron energy leads to the equation F(G, x) = ln Z(G), where F(G, x) is the Coulson function (of the molecular graph G) and Z(G) is the corresponding topological index Z. The (positive and unique) solution of this equation is called the Z-point of G and is denoted by xH. By the analysis of the Z-points of trees and chemical trees the following generally valid regularities were established: (a) Not all trees have a Z-point, but all chemical trees have a Z-point. (b) The Z-points of all chemical trees (irrespective of their size and other structural features) are nearly equal; for all chemical trees, xH ≈ 1.2.
Continuous measurements of air pollutants (NOx, SO2, O3) by passive samplers and chemical components of rainwater in Kyoto were made from 1996 to 2002. An eruption on Miyake Island and urban climate changes were found to greatly affect the seasonal changes of air pollutants and chemical components of rainwater in 2000 and 2001. Several peaks of NO2 concentrations at mountains around the Kyoto basin occurred in the spring and summer of 2000 and 2001. The higher NO2 concentration in the summer of 2000 and 2001 may be attributed to the formation of a temperature-inversion layer. Atmospheric SO2 concentrations became abruptly higher in September 2000, and the concentration range of atmospheric SO2 in mountains around the Kyoto basin was 1.0–7.9 ppb from September 2000 to July 2001, which was about two-times higher than that until 1999. These results suggest that a large quantity of SO2 discharge by the eruption on Miyake Island may have affected the increase of atmospheric SO2 concentrations in Kyoto. The O3 concentrations in the summer of 2000 and 2001 were higher than that until 1999. The cause may be O3 production by a photochemical reaction with an increase of anthropogenic NOx in the warm months after 2000. The concentration of gaseous H2O2 was very low when the SO2 content was high in September, 2000, and in June and July, 2001, which may have been due to a large quantity of SO2 discharge that occurred as a result of the eruption on Miyake Island. These results indicate that gaseous H2O2 may largely determine the formation of aerosol sulfate in the warm months. Thus, the increase of anthropogenic or natural SO2 emission may significantly affect the increase of SO42− concentration in rainwater.
Four new quaternary Ag-containing group 5 chalcogenides, KAg2NbSe4 (1), KAg2TaSe4 (2), K3Ag3Nb2S8 (3), and K3Ag3Ta2S8 (4), have been prepared through the use of molten alkali metal polychalcogenides as reactive fluxes and structurally characterized by single-crystal X-ray diffraction techniques. The layer-type structures of 1 and 2 can be regarded as constructed from the basic building block of the incomplete cubane [Ag3MVSe3], which are corner-shared to form an infinite chain along the a direction. These incomplete cubane chains are interconnected and further bridged by Se atoms along the c direction, leading to a two-dimensional structure. The crystal structure of 3 and 4 consists of one-dimensional triple-metal [Ag3MV2S8]3− anionic chains seperated by K+ cations. The alternate packing of MVS4 and AgS4 tetrahedra via edge-sharing along the b direction leads to mixed-metal sub-chains, every two of which are further linked by AgS4 tetrahedra along the a direction through edge-sharing to the MVS4 tetrahedra, thus yielding the so-called triple-metal chains.
The ion-pair extraction of 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrinatocopper(II) ([CuII(tmpyp)]4+, CuP4+) with perchlorate ion has been studied for a water–propiononitrile (PN) system in the presence of sodium chloride. The [CuII(tmpyp)]4+ was extracted as [CuP·4ClO4] and dissociated to [CuP·2ClO4]2+ and [CuP·ClO4]3+ in the PN phase. The high concentration of NaCl in the aqueous phase suppressed the extraction of [CuP·nClO4](4−n)+ (n = 1, 2) due to the increased partition of ClO4− into the PN phase. The extraction constants of [CuII(tmpyp)]4+ with ClO4− into the PN phase at different concentrations of NaCl were found to be KexKdis1Kdis2Kdis3 (mol−1 dm3) = [CuP·ClO43+]o[ClO4−]o3[CuP4+]a−1[ClO4−]a−4 = 9.1 ± 1.9, (3.0 ± 0.2) × 10−1, and (2.5 ± 2.2) × 10−1 for 0, 0.5, and 1.0 mol dm−3 NaCl, respectively, and KexKdis1Kdis2 (mol−2 dm6) = [CuP·2ClO42+]o[ClO4−]o2[CuP4+]a−1[ClO4−]a−4 = 31 ± 18 for 1.0 mol dm−3 NaCl. The extraction constants of NaClO4 were found to be Kex(NaClO4) (mol−1 dm3) = [NaClO4]o[Na+]a−1[ClO4−]a−1 = (8.8 ± 1.6) × 10−2 and (5.1 ± 0.7) × 10−2 for 0.5 and 1.0 mol dm−3 NaCl, respectively. The dissociation constants of NaClO4 in the PN phase were determined: Kdis(NaClO4) (mol dm−3) = [Na+]o[ClO4−]o[NaClO4]o−1 = (1.5 ± 0.3) × 10−2 and (2.8 ± 0.4) × 10−2 for 0.5 and 1.0 mol dm−3 NaCl, respectively, and Kex(NaClO4)Kdis(NaClO4) = [Na+]o[ClO4−]o[Na+]a−1[ClO4−]a−1 = (2.37 ± 0.05) × 10−3 for 0 mol dm−3 NaCl. The detailed extraction mechanism of [CuII(tmpyp)]4+ is discussed by taking into account the chemical species of [CuP·nClO4](4−n)+ in the PN phase and the extraction of NaClO4 and comparing these with the results obtained for acetonitrile.
A divanadium-substituted derivative of 12-tungstosulfate(VI), [S(V2W10)O40]4− was prepared as the tetrabutylammonium (n-Bu4N+) salt by heating a 50 mM (M = mol dm−3) W(VI)–20 mM V(V)–0.5 M H2SO4–50% (v/v) CH3CN system at 70 °C for 24 h. The [S(V2W10)O40]4− anion was transformed spontaneously into a monovanadium-substituted derivative, [S(VW11)O40]3− in a 70% (v/v) CH3CN–1.0 M HCl system at 70 °C. On the other hand, [SW12O40]2− was precipitated as the n-Bu4N+ salt upon heating a 95% (v/v) CH3CN–0.5 M HCl system containing [S(V2W10)O40]4−. The single-crystal X-ray structural analysis revealed that the structure consisted of an α-Keggin-type [SW12O40]2− anion. The [SW12O40]2− anion was characterized by IR and UV–vis spectroscopy, and voltammetry.
There is a great deal of concern that wildlife and human health have been adversely affected by anthropogenic chemicals that can disrupt normal endocrine homeostasis. In order to identify the binding ability of chemicals to human estrogen receptors (hER), we have constructed a biosensor which carries the hER ligand-binding domain on the surface. The receptor was expressed as an in-frame fusion with ten consecutive histidine residues using a bacterial system, and then the recombinant protein was immobilized on an Au-electrode via Ni(II)-mediated chemisorption using the histidine tag and thiol-modified nitrilotriacetic acid. This receptor-modified electrode was used to define the binding capacity of several xenoestrogens and anti-estrogen to the hER. The biosensor showed a linear response to the chemicals in a concentration-dependent manner. The order of the detection limits was: 17β-estradiol (10−10 M) > diethylstilbestrol (DES; 10−8 M), ICI 182780 (10−8 M), dibutyl phthalate (10−8 M), bisphenol A (10−8 M) > p-nonylphenol (10−6 M), testosterone (10−6 M). Compared to the traditional binding assay, our method has the advantage of being more feasible, due to the high-throughput screening assay for evaluating the binding ability of chemicals to the receptors.
A novel dinuclear benzenedithiolatogold(I) complex was synthesized and characterized spectroscopically. This complex showed intense blue emission [quantum yield at 1.0 × 10−4 mol dm−3: Φ = 0.026(2)] at room temperature in fluid solvents. This emission originates from a metal-perturbed ligand-centered transition. The relatively long life time of the excited state (τ50 = ca. 2 μsec) and the large stokes shift suggest that the emission is phosphorescence. The X-ray single crystal structural determination showed that the complex has a distorted 12-membered macrocyclic structure bridged by two benzenedithiolate ligands, with no gold–gold interactions.
We revealed that Dionaea muscipula Ellis accumulates a great amount of cytotoxic naphthoquinone derivatives, of which the main component is plumbagin (1). It was also revealed that 1 showed strong antifeedant activity against Spodoptera litura. These results strongly suggest that 1 in D. muscipula operates as an antifeedant which prevents them from being consumed by predators. The accumulation of a great amount of 1 would be an important strategy of carnivorous plants for their survival. Also, we discuss the structure–activity relationship of the cytotoxicity of naphthoquinones using isolated naphthoquinones and their derivatives.
For lipase-catalyzed reactions of 2-(4-substitued phenoxy)propionic acids with alcohols in organic solvents containing a small amount of water, the increase of the lipase flexibility brought about by addition of water is found to be favorable for the induced-fit motion of the lipase for the correctly binding enantiomer of the substrate used, thus resulting in the improvement of the lipase enantioselectivity. In particular, for the reaction of the substrate with rich π electron density on its aromatic ring, the enantioselectivity was much more sensitive to the change of the lipase flexibility. Thus, in the induced fit motion, the CH···π association between amino acids side chains around the lipase’s active site and the aromatic ring of the correctly binding enantiomer is assumed to accelerate the accommodation of the substrate into the lipase’s active site and the stabilization of the complex between the enzyme and the substrate. This assumption is also supported by a discussion based on the value of the Michaelis constant obtained. Furthermore, on the basis of a model concerning the acyl-enzyme structure for the incorrectly binding enantiomer, the long alkyl chain alcohols as a nucleophile are found to improve the lipase enantioselectivity markedly.
A facile procedure for the conjugate reduction of α,β-unsaturated aldehydes, ketones, carboxylic acids, and esters is reported with nickel boride generated in situ from NiCl2·6H2O/NaBH4 in methanol–water at ambient temperature.
A novel quinoline synthesis starting with 2-isocyanostyrene derivatives is described. The treatment of 2-isocyanostyrene derivatives with aldehydes (or acetone) in the presence of a catalytic amount of diethyl ether–boron trifluoride afforded quinoline derivatives carrying a 1-hydroxyalkyl substituent at the 2-position. The use of acetaldehyde diethyl acetal or phenyloxirane as an electrophile under the same conditions gave the corresponding quinoline derivatives, carrying the 1-ethoxyethyl or 2-hydroxy-2-phenylethyl substituent at the 2-position, respectively. 2-Isocyanostyrene derivatives reacted with N,N-dimethyliminium salts without any catalyst to give 2-(1-dimethylaminoalkyl)quinolines.
The preparation and application of a novel class of chiral Lewis acid catalysts based on chiral 2,2′-binaphthyldiimine ligands are described. Among the binaphthyldiimine–metal complexes tested, N,N′-bis(2-quinolylmethylene)-1,1′-binaphthyl-2,2′-diamine–Ni(II) complex was found to be an efficient chiral Lewis acid catalyst for asymmetric Diels–Alder reactions (endo: up to 96% ee) between cyclopentadiene and 3-alkenoyl-2-oxazolidinones. This catalyst is easy to prepare and is efficient; that is, 1 mol% of the Ni(II) catalyst promoted a Diels–Alder reaction between cyclopentadiene and 3-acryloyl-2-oxazolidinone smoothly with high enantioselectivity (endo: 90% ee).
Melamine derivatives bearing thiourea and thiouronium ion were prepared as flavin receptors. The binding modes of the receptors for flavin mimics were evaluated from the binding constants and 1H NMR data in chloroform. The effects of the receptors on the oxidation of N-benzyl-1,4-dihydronicotinamide (BNAH) and benzenethiol by flavin mimics were kinetically investigated in chloroform.
In the presence of atmospheric oxygen, O-pentafluorobenzoyloximes having a (2,4-cyclohexadienyl)Fe(CO)3 moiety at the α-position cyclize smoothly to give trans-fused [(4,5,6,7-η)-3a,7a-dihydro-3H-indole]Fe(CO)3 complexes. The cyclization proceeds via radical chain mechanism initiated by molecular oxygen, in which pentafluorobenzoyloxyl radical plays a role as a chain carrier. Thus obtained trans-fused dihydroindole iron complexes readily isomerize to the cis-fused isomers on acidic silica gel or under basic conditions. Such isomers are converted to 3a,7a-dihydroindole or indole derivatives by the oxidative removal of the Fe(CO)3 group.
Silica-based mesostructured materials derived from a layered polysilicate kanemite were prepared by reactions with gemini-type diammonium (C16–3–16, C16–5–16, C16–3–1) surfactants. The surfactants were assembled in the two-dimensionally limited space between the silicate layers of kanemite, leading to the formation of lamellar, 2-d hexagonal, and disordered mesophases. Lamellar phases were formed by using C16–3–16 and C16–5–16 surfactants. An acid treatment of the lamellar C16–5–16–silicate complex induced the transformation into a 2-d hexagonal phase, whereas the C16–3–16–silicate complex retained its lamellar structure after the treatment. A disordered phase was obtained through the reaction of kanemite with C16–3–1 surfactants whose assembly shows a larger surface curvature. The formation of layered surfactant–silicate intermediates under various conditions and the transformation to other mesophases lead to the conclusion that the formation of ordered and disordered mesostructured materials derived from kanemite simply depends on the surfactant assemblies in the two-dimensionally limited space.
The thermal dehydration of poly(vinyl alcohol) (PVA) films containing diphenyliodonium trifluoromethanesulfonate (Ph2I+Tf−) was investigated in the temperature range of 150 to 190 °C by absorption spectroscopy and elemental analysis. The dehydration was greatly promoted in the presence of Ph2I+Tf−, which is known to be a thermally stable photoacid generator. The oxidation of thermally generated PVA radicals, α-hydroxyalkyl type radicals, by Ph2I+Tf− is considered to lead to the dehydration by cationic reactions. The dehydration yield, calculated from the elemental analysis results, became highest at 170 °C. A lowering of the dehydration yields at the higher temperatures, 180 and 190 °C, has been attributed to an elimination of the volatile H+Tf− from the films, resulting in the disappearance of the cationic intermediates participating in the dehydration.
Shifts of the UV–vis peak caused by deprotonation of 2-heptylbenzimidazole and sterically strained 2-heptyl-4,7-diphenyl-benzimidazole and poly(2-heptylbenzimidazole-4,7-diyl) (P[4,7-Bim(Hep)]) are discussed. The π–π* absorption band of the strained molecules showed a large shift to a longer wavelength on deprotonation with NaOH. P[4,7-Bim(Hep)] showed a new absorption peak at about 400 nm.