Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer

The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy)32+ photosensitizers and two peripheral triarylamine (TAA) donors was investigated by transient IR and UV–vis spectroscopies in the presence of 0.2 M p-toluenesulfonic acid (TsOH) in deaerated acetonitrile. In ∼15% of all excited pentad molecules, AQ is converted to its hydroquinone form (AQH2) via reversible intramolecular electron transfer from the two TAA units (τ = 65 ps), followed by intermolecular proton transfer from TsOH (τ ≈ 3 ns for the first step). Although the light-driven accumulation of reduction equivalents occurs through a sequence of electron and proton transfer steps, the resulting photoproduct decays via concerted PCET (τ = 4.7 μs) with an H/D kinetic isotope effect of 1.4 ± 0.2. Moreover, the reoxidation of AQH2 seems to take place via a double electron transfer step involving both TAA+ units rather than sequential single electron transfer events. Thus, the overall charge-recombination reaction seems to involve a concerted proton-coupled two-electron oxidation of AQH2. The comparison of experimental data obtained in neat acetonitrile with data from acidic solutions suggests that the inverted driving-force effect can play a crucial role for obtaining long-lived photoproducts resulting from multiphoton, multielectron processes. Our pentad provides the first example of light-driven accumulation of reduction equivalents stabilized by PCET in artificial molecular systems without sacrificial reagents. Our study provides fundamental insight into how light-driven multielectron redox chemistry, for example the reduction of CO2 or the oxidation of H2O, can potentially be performed without sacrificial reagents

Characterization and Reactivity Studies of Dinuclear Iridium Hydride Complexes Prepared from Iridium Catalysts with N,P and C,N Ligands under Hydrogenation Conditions

The dinuclear iridium hydride complexes [IrH­(CH₃CN)­(L1)­(μ-H)]₂­(BAr_F)₂ (<b>7</b>; L1 = (<i>S</i>)-2-(2-((di­phenyl­phos­phanyl)­oxy)­propan-2-yl)-4-isopropyl-4,5-dihydrooxazole, BAr_F = tetrakis­[3,5-bis­(tri­fluoro­methyl)­phenyl]­borate), [IrH­(CH₂Cl₂)­(L1)­(μ-H)]₂­(BAr_F)₂ (<b>8</b>), [IrH­(L2)­(μ-H)]₂­(BAr_F)₂ (<b>9a</b>; L2 = (<i>S</i>)-1-[2-(2-adamantan-2-yl-4,5-dihydro­oxazol-4-yl)-ethyl]-3-(2,6-diiso­propyl­phenyl)-1,2-dihydroimidazol-2-ylidene), and [IrH­(L3)­(μ-H)]₂­(BAr_F)₂ (<b>9b</b>; L3 = (<i>S</i>)-1-[2-(2-<i>tert</i>-butyl-4,5-dihydrooxazol-4-yl)-ethyl]-3-(2,6-diiso­propyl­phenyl)-1,2-dihydro­imidazol-2-ylidene) were prepared from the corresponding mononuclear [Ir­(COD)­(L)]­BAr_F precursors by treatment with H₂ and characterized by 2D NMR spectroscopy and X-ray diffraction. Conversion to a trinuclear iridium hydride complex, which is usually observed for N,P iridium hydride complexes, is inhibited by addition of 0.5 equiv of [H­(OEt₂)₂]­BAr_F or acetonitrile. Reactions with acetonitrile or 6,6′-bi-2-picoline afforded the mononuclear iridium dihydride complexes [Ir­(H)₂­(CH₃­CN)₂­(L1)]­BAr_F (<b>5</b>), [Ir­(H)₂­(CH₃­CN)₂­(L3)]­BAr_F (<b>10</b>), or [Ir­(H)₂­(6,6′-bi-2-picoline)­(L3)]­BAr_F (<b>11</b>). The CH₃CN complexes <b>7</b> and <b>10</b> are inactive as hydrogenation catalysts. In contrast, the coordinatively unsaturated dinuclear complexes <b>9a</b> and <b>9b</b> are active catalysts for the hydrogenation of (<i>E</i>)-1,2-diphenyl-1-propene at 50 bar hydrogen pressure

Domino Inverse Electron-Demand Diels–Alder/Cyclopropanation Reaction of Diazines Catalyzed by a Bidentate Lewis Acid

A domino inverse electron-demand Diels–Alder (IEDDA)/cyclopropanation reaction of diazines was discovered by applying electron-rich furans in the bidentate Lewis acid catalyzed IEDDA reaction. This process produces benzonorcaradienes in excellent yields with a low loading of a bidentate Lewis acid catalyst of 2 to 5 mol %. We demonstrate the broad applicability by 20 examples with different dienophiles and a variety of dienes. A detailed mechanism is proposed supported by DFT calculations

Photoinduced Electron Transfer in Rhenium(I)–Oligotriarylamine Molecules

Two molecular triads with an oligotriarylamine multielectron donor were synthesized and investigated with a view to obtaining charge-separated states in which the oligotriarylamine is oxidized 2-fold. Such photoinduced accumulation of multiple redox equivalents is of interest for artificial photosynthesis. The first triad was comprised of the oligotriarylamine and two rhenium­(I) tricarbonyl diimine photosensitizers each of which can potentially accept one electron. In the second triad the oligotriarylamine was connected to anthraquinone, in principle an acceptor of two electrons, via a rhenium­(I) tricarbonyl diimine unit. With nanosecond transient absorption spectroscopy (using an ordinary pump–probe technique) no evidence for the generation of 2-fold oxidized oligotriarylamine or 2-fold reduced anthraquinone was found. The key factors limiting the photochemistry of the new triads to simple charge separation of one electron and one hole are discussed, and the insights gained from this study are useful for further research in the area of charge accumulation in purely molecular (nanoparticle-free) systems. An important problem of the rhenium-based systems considered here is the short wavelength required for photoexcitation. In the second triad, photogenerated anthraquinone monoanion is protonated by organic acids, and the resulting semiquinone species leads to an increase in lifetime of the charge-separated state by about an order of magnitude. This shows that the proton-coupled electron transfer chemistry of quinones could be beneficial for photoinduced charge accumulation

Sir Thomas Frankland, Thirkleby, near York, to James Edward Smith

'Ulva rubra' just figured in "English Botany" is definitely the plant [William] Hudson named to him. Anxious to hear whether rumours of [James] Brodie's death are true or not. Archdeacon Pierson's herbarium did not sell so is now to be raffled with sixty guinea tickets. Bad harvest weather

Catalyst-Controlled Stereodivergent Synthesis of Atropisomeric Multiaxis Systems

Molecular scaffolds with multiple rotationally restricted bonds allow a precise spatial positioning of functional groups. However, their synthesis requires methods addressing the configuration of each stereogenic axis. We report here a catalyst-stereocontrolled synthesis of atropisomeric multiaxis systems enabling divergence from the prevailing stereochemical reaction path. By using ion-pairing catalysts in arene-forming aldol condensations, a strong substrate-induced stereopreference can be overcome to provide structurally well-defined helical oligo-1,2-naphthylenes. The configuration of up to four stereogenic axes was individually catalyst-controlled, affording quinquenaphthalenes with a unique topology

New species, additions and a key to the Brazilian species of the Geminata clade of Solanum L. (Solanaceae) in Brazil

Two additions and four new species are described from Brazil for the large Geminata clade (Solanum: Solanaceae) bringing the total diversity in the group to 149 species, with 44 of these occurring in Brazil. New species are described from Brazil: S. amorimii S.Knapp &amp; Giacomin, sp. nov. from Bahia and adjacent Minas Gerais states, S. filirhachis Giacomin &amp; Stehmann, sp. nov. from Espirito Santo, S. psilophyllum Stehmann &amp; Giacomin, sp. nov. from Minas Gerais and S. verticillatum S.Knapp &amp; Stehmann, sp. nov. from São Paulo, Rio de Janeiro and Minas Gerais. Modern character-rich descriptions and lectotypifications are provided for S. apiahyense Witasek and Solanum lacteum Vell. All are illustrated, mapped and assessed for conservation status. We also provide a brief analysis of the diversity and endemism of the Geminata clade in Brazil and a key to all 44 Brazilian species

Syntheses of Taiwaniaquinone F and Taiwaniaquinol A via an Unusual Remote C–H Functionalization

A protecting-group-free route to (−)-taiwaniaquinone F based on a ring contraction and subsequent aromatic oxidation of a sugiol derivative is reported. In addition, the first synthesis of (+)-taiwaniaquinol A is reported via short time exposure of (−)-taiwaniaquinone F to sunlight triggering a remote C–H functionalization. The hypothesis that the biogenesis of some methylenedioxy bridged natural products could proceed via similar nonenzymatic mechanisms is presented

A Strain Induced Change of Mechanism from a [2 + 2 + 2] to a [2 + 1 + 2 + 1] Cycloaddition Reaction

While investigating the [2 + 2 + 2] cycloaddition as a tool to build up strained oligophenyl systems with a diyne-ethylene glycol macrocyle, a surprising change of mechanism was observed. Instead of the expected [2 + 2 + 2] <i>para</i>-terphenyl, the <i>ortho</i>-terphenyl product explained by a formal [2 + 1 + 2 + 1] cycloaddition was formed. An η⁴-coordinated metal-cyclobutadiene is proposed as the key structure in the catalytic cycle, which is formed to release the induced strain. The optical properties of the <i>ortho</i>-terphenyl products have been measured as well as the coordination ability of Na⁺ and K⁺

Water-Soluble Co(III) Complexes of Substituted Phenanthrolines with Cell Selective Anticancer Activity

Transition metal complexes with substituted phenanthrolines as ligands represent potential anticancer products without the drawbacks of platinum complexes that are currently marketed. Here, we report the synthesis and cell selective anticancer activity of five new water-soluble Co­(III) complexes with methyl substituted phenanthroline ligands. The complexes were characterized by elemental analysis, NMR, FAB-mass spectrometry, FTIR, electronic spectroscopy, and single crystal X-ray diffraction. Possible interaction of these complexes with DNA was assessed by a combination of circular dichroism, UV–vis spectroscopy titration, and ethidium bromide displacement assay, and the results indicated that DNA interaction is weak for these complexes. Cellular uptake and cytotoxicity of complexes at low concentrations were assessed by flow cytometry on PC-3 cells, while their effect on intracellular mitochondrial function was measured by MTS assay on HeLa and PC-3 cell lines. These complexes showed selective cytotoxicity with a significantly higher effect on intracellular mitochondrial function in PC-3 cells than in HeLa cells. At low concentrations, complex <b>2</b> had the highest cytotoxic effect on PC-3 cells, inducing around 38% cell death, and the correlation of cytotoxicity of these complexes to their hydrophobicity indicates that an appropriate value of the hydrophobicity is essential for high antitumor activity