Synthesis and Photophysical Properties of Silver(I) Coordination Polymers Bridged by Dimethylpyrazine: Comparison of Emissive Excited States between Silver(I) and Copper(I) Congeners.

Highly luminescent silver(I) coordination polymers [Ag2X2(PPh3)2(Me2pyz)]n (X = I, Br, Cl; Me2pyz: 2,5-dimethylpyrazine) were prepared together with copper congeners [Cu2X2(PPh3)2(Me2pyz)]n (X = I, Br). All the complexes showed thermally activated delayed fluorescence from the charge-transfer states in the visible region, from blue to red. The isomorphous relationship among the complexes allowed a detailed discussion of the effect of halogenido ligands and crystal packing on their luminescence energy. The relaxation in the emissive excited states (ESs) was determined to be more remarkable in silver complexes than in copper complexes despite their isomorphous structures, and the electronic effect of halogenido ligands was comparable to the effect of relaxation in emissive ESs.

One-Pot Synthesis of Pentasubstituted Pyridines following the Gold(I)-Catalyzed Aza-Enyne Metathesis/6π-Electrocyclization-Aromatization Sequence.

The one-pot de novo synthesis of pentasubstituted pyridines was realized following the process of Au(I)-autotandem catalysis and subsequent aromatization. The process involves aza-enyne metathesis with aryl propiolates to yield 1-azabutadienes and their addition/6π-electrocyclization sequence with the other propiolate units. The resultant 1,4-dihydropyridines were aromatized to furnish the pyridines in the presence of atmospheric oxygen. The aryl propiolates were regioselectively incorporated into the ring system to afford 2-arylpyridines as the sole product.

Structure-activity relationship and mechanistic study on guggulsterone derivatives; Discovery of new anti-pancreatic cancer candidate.

Pancreatic cancer is one of the deadliest types of malignancies. A new intervention aiming to combat pancreatic cancer is targeting its extra-ordinary ability to tolerate nutrition starvation, a phenomenon known as "Austerity". As a part of a research program aiming to develop a new-generation of anticancer agents, known as "anti-austerity agents", guggulsterone derivatives (GSDs) were identified as unique anti-austerity agents in terms of potency and selectivity. These agents are able to exert preferential cytotoxic activity only under nutrient-deprived conditions with little or no toxicity under normal conditions. In the present study, a library of 14 GSDs was synthesized and screened against PANC-1 human pancreatic cells. Among tested compounds, GSD-11 showed the most potent activity with PC50 a value of 0.72 µM. It also inhibited pancreatic cancer cell migration and colony formation in a concentration-dependent manner. A mechanistic study revealed that this compound can inhibit the activation of the Akt/mTOR signaling pathway. Therefore, GSD-11 could be a promising lead compound for the anticancer drug discovery against pancreatic cancer.

Synthesis and Photophysical Properties of Emissive Silver(I) Halogenido Coordination Polymers Composed of {Ag2X2} Units Bridged by Pyrazine, Methylpyrazine, and Aminopyrazine.

Luminescent silver(I) coordination polymers having a {Ag2(µ-X)2} rhombic core (X = I, Br) were prepared using pyrazine (pyz), methylpyrazine (Mepyz), and aminopyrazine (ampyz) as bridging ligands. Photophysical measurements show that the complexes were strongly luminescent in the solid state at room temperature; further, the emissive excited state of the pyz and Mepyz complexes was a triplet charge-transfer (3CT) excited state, similar to that of their copper(I) congeners, whereas that of the ampyz complex was a intraligand (3IL) excited state. The energy of the 3CT excited state of a silver halogenido complex was revealed to be ca. 5000 cm-1 higher than that of the corresponding copper complex.

Control of Emissive Excited States of Silver(I) Halogenido Coordination Polymers by a Solid Solution Approach.

Luminescent silver(I) halogenido coordination polymers [Ag2X2(PPh3)2(bpy)] n (X = I, Br, Cl) have been prepared. The iodido and bromido complexes exhibit strong blue phosphorescence assignable to the 3π-π*-excited-state of bpy, whereas the chlorido complex shows luminescence thermochromism due to the π-π*-state of bpy and charge transfer from the {Ag2Cl2} core to the bpy π*-orbital. Taking advantage of their structural similarities, we prepared a series of mixed-halogenido silver(I) complexes [Ag2(X xX'(1- x))2(PPh3)2(bpy)] n (X, X' = I, Br, Cl) at varying molar fractions as solid solutions. The mixed-halogenido complexes are as strongly luminescent as their parent complexes. The detailed study of their structure and emissive properties revealed smooth energy migration between the luminescent units and modification of the luminescence properties based on the planarity of bpy.

Directional Energy Transfer in Mixed-Metallic Copper(I)-Silver(I) Coordination Polymers with Strong Luminescence.

Strongly luminescent mixed-metallic copper(I)-silver(I) coordination polymers with various Cu/Ag ratio were prepared by utilizing the isomorphous relationship of the luminescent parent homometallic coordination polymers (Φ(em) = 0.65 and 0.72 for the solid Cu and Ag polymers, respectively, at room temperature). The mixed-metallic polymer with the mole fraction of copper even as low as 0.005 exhibits a strong emission (Φ(em) = 0.75) from only the copper sites as the result of the efficient energy migration from the silver to the copper sites. The migration rates between the two sites were evaluated from the dependence of emission decays upon the mole fraction of copper.

Water-molecule-driven vapochromic behavior of a mononuclear platinum(II) system with mixed bipyridine and thioglucose.

A mononuclear platinum(II) complex with mixed bpy and H4tg(-) ligands, [Pt(H4tg-S)2(bpy)]·2H2O (1a; bpy = 2,2'-bipyridine and H5tg = 1-thio-ß-D-glucose), is photoluminescent in the solid state and undergoes facile and reversible removal of solvated water molecules accompanied by changes in its absorption and emission wavelengths. The analogous dmbpy complex, [Pt(H4tg-S)2(dmbpy)]·H2O (2a; dmbpy = 4,4'-dimethyl-2,2'-bipyridine), is also emissive in the solid state, but its absorption and emission spectra remain unchanged after dehydration. X-ray crystallographic analyses revealed that the difference in the chromism of 1a and 2a is due to the difference in their hydrogen-bonding networks, which involve solvated water molecules.

Ring expansion of spirocyclopropanes with stabilized sulfonium ylides: highly diastereoselective synthesis of cyclobutanes.

A novel method was devised for regioselective ring expansion of Meldrum's acid-derived spirocyclopropanes to spirocyclobutanes with stabilized sulfonium ylides, affording 1,2-trans-disubstituted 6,8-dioxaspiro[3.5]nonane-5,9-diones in up to 87% yields without the formation of any isomers. The aforementioned reaction was also applied to the barbituric acid-derived spirocyclopropane, resulting in the formation of the corresponding cyclobutanes.

New 19(10 → 9)abeo-euphane-type triterpenoids from Trichilia dregeana leaves and NO production inhibitory activities.

Phytochemical investigation of the EtOAc soluble fraction from leaves of Trichilia dregeana Sond. (Meliaceae) afforded naturally rare four new pentacyclic triterpenoids (1-4), together with five known pentacyclic analogs (5-8, and 11) and two steroids (9 and 10). Their structures were elucidated by extensive spectroscopic techniques such as 1D and 2D NMR and HRESIMS data analyses. The absolute configuration of 1 was determined by using the single-crystal X-ray diffraction analysis. The nitric oxide (NO) production inhibitory assay indicated that the EtOAc fraction as well as 4 and 7 inhibited the NO production in lipopolysaccharide (LPS)-stimulated RAW264.7 macrophage cells with the IC50 values of 83.53 µg/mL and 81.31 and 85.71 µM, respectively. Compounds 1-4 are rare 19(10 â†’ 9)abeo-euphane-type triterpenoids bearing a 3,10-ether bridge. To the best of our knowledge, this study is the first isolation of triterpenoids with the 3,10-ether bridge in their skeleton from the genus Trichilia, providing new insights into the chemodiversity of the terpenoids in T. dregeana.

Photo- and vapor-controlled luminescence of rhombic dicopper(I) complexes containing dimethyl sulfoxide.

Halide-bridged rhombic dicopper(I) complexes, [Cu2(µ-X)2(DMSO)2(PPh3)2] (X = I(-), Br(-); DMSO = dimethyl sulfoxide; PPh3 = triphenylphosphine), were synthesized, the iodide complex of which exhibited interesting photochromic luminescence driven by photoirradiation and by exposure to DMSO vapor in the solid state. Single-crystal X-ray diffraction measurements revealed that the iodo and bromo complexes (abbreviated Cu2I2-[O,O] and Cu2Br2-[O,O]) were isomorphous, and that the two DMSO ligands were coordinated to the Cu(I) ion via the O atom in both complexes. Both complexes exhibited bright blue phosphorescence at room temperature (λ(em) = 435 nm, Φ(em) = 0.19 and 0.14 for Cu2I2-[O,O] and Cu2Br2-[O,O], respectively) with a relatively long emission lifetime (τ(em) ~ 200 µs at 77 K) derived from the mixed halide-to-ligand and metal-to-ligand charge transfer ((3)XLCT and (3)MLCT) excited state. Under UV irradiation, the blue phosphorescence of Cu2Br2-[O,O] disappeared uneventfully and no new emission band appeared, whereas the blue phosphorescence of Cu2I2-[O,O] rapidly disappeared with simultaneous appearance of a new green emission band (λ(em) = 500 nm). On further irradiation, the green emission of the iodide complex gradually changed to bright yellowish-green (λ(em) = 540 nm); however, this change could be completely suppressed by lowering the temperature to 263 K or in the presence of saturated DMSO vapor. The initial blue phosphorescence of Cu2I2-[O,O] was recovered by exposure to DMSO vapor at 90 °C for a few hours. IR spectroscopy and theoretical calculations suggest that the DMSO ligand underwent linkage isomerization from O-coordination to S-coordination, and both the occurrence of linkage isomerization and the removal of DMSO result in contraction of the rhombic Cu2(µ-I)2 core to make the Cu···Cu interaction more effective. In the contracted core, the triplet cluster-centered ((3)CC) emissive state is easily generated by thermal excitation of the (3)XLCT and (3)MLCT mixed transition state, resulting in the green to yellowish-green emission. In contrast, the Cu···Cu distance in Cu2Br2-[O,O] is considerably longer than that of Cu2I2-[O,O], which destabilizes the (3)CC emissive state, resulting in the nonemissive character.

Bis(bipyridine)ruthenium(II) complexes with an aliphatic sulfinato donor: synthesis, characterization, and properties.

Treatment of a thiolato-bridged Ru(II)Ag(I)Ru(II) trinuclear complex, [Ag{Ru(aet)(bpy)(2)}(2)](3+) (aet = 2-aminoethanthiolate; bpy = 2,2'-bipyridine), with NaI in aqueous ethanol under an aerobic condition afforded a mononuclear ruthenium(II) complex having an S-bonded sulfinato group, [1](+) ([Ru(aesi-N, S)(bpy)(2)](+) (aesi = 2-aminoethanesulfinate)). Similar treatment of optically active isomers of an analogous Ru(II)Ag(I)Ru(II) trinuclear complex, Δ(D)Δ(D)- and Λ(D)Λ(D)-[Ag{Ru(d-Hpen-O,S)(bpy)(2)}(2)](3+) (d-pen = d-penicillaminate), with NaI also produced mononuclear ruthenium(II) isomers with an S-bonded sulfinato group, Δ(D)- and Λ(D)-[2](+) ([Ru(d-Hpsi-O,S)(bpy)(2)](+) (d-psi = d-penicillaminesulfinate)), respectively, retaining the bidentate-O,S coordination mode of a d-Hpen ligand and the absolute configuration (Δ or Λ) about a Ru(II) center. On refluxing in water, the Δ(D) isomer of [2](+) underwent a linkage isomerization to form Δ(D)-[3] (+) ([Ru(d-Hpsi-N,S)(bpy)(2)](+)), in which a d-Hpsi ligand coordinates to a Ru(II) center in a bidentate-N,S mode. Complexes [1](+), Δ(D)- and Λ(D)-[2](+), and Δ(D)-[3](+) were fully characterized by electronic absorption, CD, NMR, and IR spectroscopies, together with single-crystal X-ray crystallography. The electrochemical properties of these complexes, which are highly dependent on the coordination mode of sulfinate ligands, are also described.

An NAD+-type earth-abundant metal complex enabling photo-driven alcohol oxidation.

For the first time, an NAD+-type earth-abundant metal complex [Zn(pbn)2(H2O)](ClO4)2 (1) was found to exhibit photo-induced oxidizing ability to convert various primary and secondary alcohols to the corresponding aldehyde and ketone compounds. In addition, a two-electron-reduced Zn(II) complex [Zn(pbnH-pbnH)(ClO4)2] (1red) comprising the novel C-C coupling ligand, obtained by the photo-induced oxidation of alcohols by 1, was successfully isolated and completely characterized. We clarified that the photochemical oxidation of alcohols by 1 to produce 1red proceeds via an electron transfer followed by proton transfer mechanism as elucidated by kinetic analysis on the basis of absorption spectroscopic measurements.

Au(I) Catalyzed Synthesis of Densely Substituted Pyrazolines and Dihydropyridines via Sequential Aza-Enyne Metathesis/6π-Electrocyclization.

A gold(I) autotandem catalysis protocol is reported for the de novo synthesis of densely substituted pyrazolines and dihydropyridines from the corresponding imine derivatives in a highly regioselective fashion via a one-pot aza-enyne metathesis/6π-electrocyclization sequence. The substituents on the nitrogen atom of the imine perfectly control the reaction pathways from the pivotal 1-azabutadiene intermediate; thus, carbazates were converted into pyrazolines via 6π-electrocyclization of α,ß-unsaturated hydrazones, while aryl imines provided dihydropyridines via 6π-electrocyclization of 3-azahexatrienes.

Anti-Austerity Activity of Thai Medicinal Plants: Chemical Constituents and Anti-Pancreatic Cancer Activities of Kaempferia parviflora.

Human pancreatic tumor cells have an intrinsic ability to tolerate nutrition starvation and survive in the hypovascular tumor microenvironment, the phenomenon termed as "austerity". Searching for an agent that inhibits such tolerance to nutrient starvation and kills the pancreatic cancer cells preferentially in nutrient-starvation is a unique anti-austerity strategy in anti-cancer drug discovery. In this strategy, plant extracts and compounds are tested against PANC-1 human pancreatic cancer cell line under the conditions of nutrient-deprived medium (NDM) and nutrient-rich medium (DMEM), to discover the compounds that show selective cytotoxicity in NDM. Screening of twenty-five Thai indigenous medicinal plant extracts for their anti-austerity activity against the PANC-1 human pancreatic cancer cell line in nutrient deprived medium (NDM) resulted in the identification of four active plants, Derris scandens, Boesenbergia pandurata, Citrus hystrix, and Kaempferia parviflora, with PC50 values 0.5-8.9 µg/mL. K. parviflora extract also inhibited PANC-1 cancer cell colony formation. Phytochemical investigation of K. parviflora extract led to the isolation of fourteen compounds, including two polyoxygenated cyclohexanes (1 and 2), eleven flavonoids (3-13), and ß-sitosterol (14). Stereochemical assignment of compound 1 was confirmed through X-ray analysis. All isolated compounds were tested for their preferential cytotoxicity against PANC-1 cells. Among them, 5-hydroxy-7-methoxyflavone (3) displayed the most potent activity with a PC50 value of 0.8 µM. Mechanistically, it was found to induce apoptosis in PANC-1 cell death in NDM as evident by caspase cleavage. It was also found to inhibit PANC-1 cancer cell colony formation in DMEM. Therefore, compound 3 can be considered as a potential lead compound for the anticancer drug development based on the anti-austerity strategy.

Excited-state properties of octahedral hexarhenium(III) complexes with redox-active N-heteroaromatic ligands.

New sulfide-capped octahedral hexarhenium(III) complexes containing 4-phenylpyridine (ppy) or 1,2-bis(4-pyridyl)ethane (bpe) ((n-C(4)H(9))(4)N)[mer-{Re(6)S(8)Cl(3)(ppy)(3)}] ((Bu(4)N)[1]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(ppy)(2)}] ((Bu(4)N)(2)[2a]), ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(ppy)(2)}] ((Bu(4)N)(2)[2b]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(bpe)(2)}] ((Bu(4)N)(2)[3a]), and ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(bpe)(2)}] ((Bu(4)N)(2)[3b]) were prepared, and X-ray single-crystal structure determination was carried out for (Bu(4)N)(2)[2a] and (Bu(4)N)(2)[3a]. The photophysical properties of these complexes were studied both in acetonitrile at 298 K and in the solid state at 298 and 80 K, along with those of the known 4,4'-bipyridine (bpy) analogues ((n-C(4)H(9))(4)N)[mer-{Re(6)S(8)Cl(3)(bpy)(3)}] ((Bu(4)N)[4]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(bpy)(2)}] ((Bu(4)N)(2)[5a]), and ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(bpy)(2)}] ((Bu(4)N)(2)[5b]). The photophysical data of [5a](2-) and [5b](2-) in solution and in the solid state were significantly different from those of other complexes. On the basis of experimental observations of [2a](2-) and [5a](2-) and density-functional theory (DFT) calculations, it was concluded that [5a](2-) and [5b](2-) exhibited metal (Re(6)S(8) core)-to-ligand (bpy) charge transfer (MLCT) type emission. This is the first unambiguous demonstration of MLCT type emissions for the hexarhenium complexes. The MLCT components, where present, are only minor in the case of the emissions of [1](-), [2a](2-), [2b](2-), and [4](-); these can be explained primarily as the contributions of the intracore electronic transitions. The emissions of [3a](2-) and [3b](2-) can be assigned almost completely to the electronic transitions within the Re(6)S(8) core. The different emission characteristics of the bis(bpy) complexes ([5a](2-) and [5b](2-)) from the tris(bpy) complex ([4](-)) are a result of the increase in the number of nitrogen donors on the Re(6)S(8) core, which stabilizes the Re(6)S(8) core energy to a lower level than the energy of the bpy ligand pi* orbital. On the other hand, it has been shown that the emissions of the bis(ppy) ([2a](2-) and [2b](2-)) and bis(bpe) complexes ([3a](2-) and [3b](2-)) are best characterized by the higher pi* energy level of each N-heteroaromatic ligand, which lead to a stronger metal character in the emissive excited state of the complex.

Synthesis and linkage isomerization of thiolato-bridged Ru(II)Ag(I)Ru(II) trinuclear complex with D-penicillaminate.

The reaction of [Ru(solvent)(2)(bpy)(2)](2+) (bpy = 2,2'-bipyridine) with D-H(2)pen (D-penicillamine) in ethanol/water in the presence of Ag(+) gave a thiolato-bridged Ru(II)Ag(I)Ru(II) trinuclear complex, [Ag{Ru(D-Hpen)(bpy)(2)}(2)](3+) ([1](3+)), in which two octahedral [Ru(II)(D-Hpen)(bpy)(2)](+) units are linked by a linear Ag(I) ion. Of three possible isomers (Delta(D)Delta(D), Delta(D)Lambda(D), and Lambda(D)Lambda(D)), [1](3+) formed the Delta(D)Delta(D) and Lambda(D)Lambda(D) isomers that were separately isolated by fractional crystallization with the use of [Sb(2)(R,R-tartrato)(2)](2-). In [1](3+), each D-Hpen ligand chelates to a Ru(II) center through thiolate and carboxylate groups, while an amine group of D-Hpen is protonated and does not participate in the coordination. On refluxing in ethanol/water, the Delta(D)Delta(D) isomer of [1](3+) was converted to Delta(D)Delta(D)-[2](3+), in which each D-Hpen ligand chelates to a Ru(II) center through thiolate and amine groups with a non-coordinating carboxyl group. On the other hand, a similar thermal linkage isomerization was not noticed for the Lambda(D)Lambda(D) isomer of [1](3+) under the same conditions. The isolated Delta(D)Delta(D)-[1](3+), Lambda(D)Lambda(D)-[1](3+), and Delta(D)Delta(D)-[2](3+) were fully characterized by electronic absorption, CD, and NMR spectroscopies, along with single-crystal X-ray crystallography for Lambda(D)Lambda(D)-[1](3+) and Delta(D)Delta(D)-[2](3+).

A Novel Photo-Driven Hydrogenation Reaction of an NAD+-Type Complex Toward Artificial Photosynthesis.

The photocatalytic reduction of carbon dioxide (CO2) to value-added chemicals is an attractive strategy to utilize CO2 as a feedstock for storing renewable energy, such as solar energy, in chemical bonds. Inspired by the biological function of the nicotinamide adenine dinucleotide redox couple (NAD+/NADH), we have been developing transition-metal complexes containing NAD+/NADH-functionalized ligands to create electro- and/or photochemically renewable hydride donors for the conversion of CO2 into value-added chemicals. Our previous findings have provided insights for the development of photocatalytic organic hydride reduction reactions for CO2, however, further examples, as well as investigation, of these photo-driven NAD+/NADH-type hydrogenation and organic hydride transfer reactions are required not only to explore the mechanism in detail but also to develop a highly efficient catalyst for artificial photosynthesis. In this paper, we report the synthesis, characterization, and photo-induced NAD+/NADH conversion properties of a new ruthenium(II) complex, [Ru(bpy)2(Me-pn)](PF6)2 (1), which contains a new NAD+-type ligand, Me-pn (2-methyl-6-(pyridin-2-yl)-1,5-naphthyridine). In addition, we have succeeded in the isolation of the corresponding two-electron reduced ruthenium(II) complex containing the NADH-type ligand Me-pnHH (2-methyl-6-(pyridin-2-yl)-1,4-dihydro-1,5-naphthyridine), i.e., [Ru(bpy)2(Me-pnHH)](PF6)2 (1HH), by the photo-induced hydrogenation reaction of 1. Thus, in this study, a new photo-driven NAD+/NADH-type hydrogenation reaction for possible CO2 reduction using the NAD+/NADH redox function has been constructed.

Facile o-quinodimethane formation from benzocyclobutenes triggered by the Staudinger reaction at ambient temperature.

Electron-donating iminophosphoranes were found to significantly enhance 4π-ring opening of benzocyclobutenes to generate o-quinodimethanes at 20-25 °C. These iminophosphorane benzocyclobutenes can be conveniently generated from azide benzocyclobutenes and phosphines via the Staudinger reaction. Thus, Staudinger reaction-triggered sequential molecular transformations of the azide benzocyclobutenes have been established via o-quinodimethanes at ambient temperature, which is expected to exhibit potential for a wide range of applications.

Ring-opening cyclization of spirocyclopropanes with stabilized sulfonium ylides for the construction of a chromane skeleton.

Regioselective ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes with stabilized sulfonium ylides provided 2,3-trans-disubstituted 2,3,4,6,7,8-hexahydro-5H-1-benzopyran-5-ones in high yields without the formation of any isomers. The obtained product was readily converted into highly substituted chromane.

Reversible mechanochromic luminescence of [(C6F5Au)2(mu-1,4-diisocyanobenzene)].

Reversible mechanochromic luminescence of [(C6F5Au)2(mu-1,4-diisocyanobenzene)] is reported. Grinding of the complex induced a photoluminescent color change, which was restored by exposure to a solvent. This cycle was repeated 20 times with no color degradation in the emissions. Their optical properties, X-ray crystallographic analysis, IR, and XRD measurements strongly suggested that the change in the molecular arrangement is responsible for this mechanochromic property. Intermolecular aurophilic bondings presumably play a key role in the altered emission.