“Organic Metals”—The Intermolecular Migration of Aromaticity

The electrical conductivity of a wide variety of organic materials is reviewed and a comparison made between those materials which transport charge efficiently and those that do not. This comparison reveals that the most efficient conductors contain molecules (a) whose radical ions form a new aromatic sextet upon one-electron oxidation or reduction and (b) whose aromaticity can migrate by mixed-valence interaction. The electrical data are presented in light of the chemistry of these materials and the importance of chemical bonding interactions in the electron transport process. Several new examples are discussed and directions for further research explored.     

Interfaces between Molecular and Polymeric “Metals”: Electrically Conductive,Structure-Enforced Assemblies of Metallomacrocycles

The design, synthesis, characterization, and understanding of new molecular and macro-molecular substances with “metal-like” electrical properties represents an active research area at the interface of chemistry, physics, and materials science. An important, long-range goal in this field of “materials by design” is to construct supermolecular assemblies which exhibit preordained collective phenomena by virtue of “engineered” interactions between molecular building blocks. In this review, such a class of designed materials is discussed which, in addition, bridges the gap between molecular and polymeric conductors: assemblies of electrically conductive metallomacrocycles. It is seen that efforts to rationally construct stacked metal-like molecular arrays lead logically to structure-enforced macromolecular assemblies of covalently linked molecular subunits. Typical building blocks are robust, chemically versatile metallophthalocyanines. The electrical optical, and magnetic properties of these metallomacrocyclic assemblies and the fragments thereof, provide fundamental information on the connections between local atomic-scale architecture, electronic structure, and the macroscopic collective properties of the bulk solid.     

Stereochemistry of Reaction Paths as Determined from Crystal Structure Data—A Relationship between Structure and Energy

The three-dimensional structure of frequently occurring molecular fragments has been studied systematically. Molecular subunits which were examined include hydrogen bridges (O—H?O), triiodide anions (I^?₃), other linear triatomic fragments (Cl—Sb?Cl, S—S?S, Mo—O?Mo), tetrahedral ions (SO^2?₄, PO^3?₄, AlCl^?₄) and related species (MSO₃, etc.), molecules containing both keto- and amino-groups (mostly alkaloids), substituted [10]annulenes and cycloheptatrienes, organic five-membered rings, and five-coordinated metal and nonmetal atoms. The bond distances and angles describing the structure of a giving fragment cover a range that is many times larger than the range of the experimental standard deviations. The changes of the various structural parameters of a fragment are correlated with each other. The observed mutual dependence (structural correlation) may be described by means of Pauling's equation relating bond length r to bond number n:r = r₀ - clogn. The bond numbers n are expressed in terms of bond angles. The sum of bond numbers at a given atom is roughly constant and does not depend on the environment. “Standard bond lengths” of a fragment are determined by a least-squares procedure based on all available data. They are supplemented by curves that describe the observed distortions. The shape of these correlation curves is reminiscent of the structural changes occurring along the pathways of chemical reactions, e.g. nucleophilic substitution at tetrahedrally coordinated atoms (S_N1 and S_N2), nucleophilic addition to carbonyl groups, electrocyclic ring closure of polyenes, pseudo-rotation of five-membered rings or Berry pseudo-rotation. For many of these reactions approximate energy hypersurfaces have been obtained from quantum mechanical calculations, model force fields and from spectroscopic information (IR, NMR). Comparison between reaction pathways determined from structural correlations with those obtained from models of the energy surfaces shows fair agreement.     

“Superpermeability” and “pumping” of atomic hydrogen through palladium membranes

Permeation of atomic as well as molecular hydrogen through palladium membranes has been investigated experimentally in the temperature range from room temperature to 200 °C and at a higher incident flux of hydrogen atoms on palladium surface than in previous studies. The results demonstrate that phenomena of ‘superpermeability’ and ‘pumping’ of atomic gases through metal membranes are of a common nature. A theoretical model based on chemical thermodynamics and diffusion theory adequately describes the quantitative relationships observed in experiments. It was found that permeability of atomic hydrogen depends strongly on the magnitude of surface incident flux and membrane temperature.     

Diels–Alder “Click” Chemistry in Designing Dendritic Macromolecules

Simple, versatile and green : Diels–Alder “click” chemistry is a simple, versatile and “greener” approach in the design of a diverse range of dendritic macromolecules (see scheme).

     

“Breathing” in Adsorbate‐Responsive Metal Tetraphosphonate Hybrid Materials

Breathe easy : Reversible H₂O and NH₃ gas uptake by 2D calcium tetraphosphonates (see figure) is accompanied by framework structural changes similar to those previously reported for some carboxylate‐based hybrids. This breathing mechanism is accompanied by a volume increase of 55 %, while maintaining the topology and crystallinity of the material.

     

Kinetics of the “a” and “b” band emissions of mercury excimers with added gases

The kinetics of the “a” and “b” band emissions arising from the ¹Σ ← ³O_u and ¹Σ ← ³l_u transitions of the diatomic mercury molecule at λ_max ～ 4850 Å and 3350 Å, respectively, have been studied at low concentrations of mercury in the presence of N₂, C₂H₆, C₃H₈, and N₂O. Rate constant values have been obtained for the following reactions of the excimer molecule: Hg₂(³l_u) + N₂ → Hg₂(³O_u) + N₂ and Hg₂(³O_u) + RH → Hg₂(¹Σ) + RH, where RH = C₂H₆ or C₃H₈. From a consideration of the detailed kinetics of band emissions, it was also possible to derive rate constants for the quenching reactions of Hg(³P₀) atoms. These values are in reasonable agreement with those obtained previously from monitoring atom concentrations directly by 4047 Å absorbiometry.     

The Role of Aqueous Aerosols in the “Glyoxylate Scenario”: An Experimental Approach

The origin of life is one of the fundamental questions in science. Eschenmoser proposed the “glyoxylate scenario”, in which plausible abiotic synthesis pathways were suggested to be compatible with the constraints of prebiotic chemistry. In this proposal, the stem compound is HCN. In this work, we explore the “glyoxylate scenario” through several syntheses of HCN polymers, paying particular attention to the role of the aqueous aerosols, together with statistical methods, as a step to elucidate the synthetic problem of the origin of life. The soluble and insoluble HCN polymers synthetized were analyzed by GC‐MS. We identified, for the first time, glyoxylic acid in these polymers, together with some constituents of the reductive tricarboxylic acid cycle, amino acids and several N‐heterocycles. The findings presented herein, as the first global approach to the “glyoxylate scenario”, give full effect to this hypothesis and prove that aqueous aerosols could play an important role in this plausible scene of the origin of life.     

Reversible Hydrogenation of Graphene on Ni(111)—Synthesis of “Graphone”

Understanding the adsorption and reaction between hydrogen and graphene is of fundamental importance for developing graphene‐based concepts for hydrogen storage and for the chemical functionalization of graphene by hydrogenation. Recently, theoretical studies of single‐sided hydrogenated graphene, so called graphone, predicted it to be a promising semiconductor for applications in graphene‐based electronics. Here, we report on the synthesis of graphone bound to a Ni(111) surface. We investigate the formation process by X‐ray photoelectron spectroscopy (XPS), temperature‐programmed desorption (TPD), and density‐functional theory calculations, showing that the hydrogenation of graphene with atomic hydrogen indeed leads to graphone, that is, a hydrogen coverage of 1 ML (4.2 wt %). The dehydrogenation of graphone reveals complex desorption processes that are attributed to coverage‐dependent changes in the activation energies for the associative desorption of hydrogen as molecular H₂.     

“Truncated” Pagodanes — Synthesis of Functionalized [1.1.1.1] and [1.1.1.0] Pagodanes

“Truncated” [1.1.1.1] pagodanes like the [1.1.0.0] and [0.0.0.0] homologues3 and 4 (E_Str = 146.1 – 171.5 kcal mol^?1, MM2) are potential precursors of unusual unsaturated homoconjugated radical cations and σ-bishomoaromatic dications. Attempts are presented toward the synthesis of 3 - starting out from [1.1.1.1] pagodane-4,9-dione 9 and diaza [2.2.1.1] pagodadiene 11 by cycloelimination (unsuccessful) and by Favorskii-type ring contraction methodologies. Control experiments with the model diketone 19 documented the limitations for α,α′-difunctionalization of 9 as the prerequisite for one-pot double Favorskii-type ring contraction. A de novo synthesis for such α,α′-disubstituted derivatives of 9(54) was not sufficiently expeditious to open a practical access to 3. Sequential double bridgehead hydroxylation of 9, successfully practized with model 19, similarly suffered from inherent cage effects and allowed only limited yields of [1.1.1.0] pagodanone 10, the intermediate on the way to 3.     

New Fluorescence Domain “Excited Multimer” Formed upon Photoexcitation of Continuously Stacked Diaroylmethanatoboron Difluoride Molecules with Fused π‐Orbitals in Crystals

The crystal‐packing structures of seven derivatives of diaroylmethanatoboron difluoride ( 1 a – gBF₂ ) are characterized by no overlap of the π‐conjugated main units of two adjacent molecules (type I), overlap of the benzene ring π‐orbitals of two adjacent molecules (type II), and overlap of the benzene and dihydrodioxaborinine rings π‐orbitals of adjacent molecules (type III). The crystal‐packing structures govern the fluorescence (FL) properties in the crystalline states. The FL domain that is present in type I crystals, in which intermolecular orbital interactions are absent, leads to excited monomer‐like FL properties. In the case of the type II crystals, the presence of intermolecular overlap of the benzene rings π‐orbitals generates new FL domains, referred to as “excited multimers”, which possess allowed S₀–S₁ electronic transitions and, as a result, similar FL lifetimes at longer wavelengths than the FL of the type I crystals. Finally, intermolecular overlap of the benzene and dihydrodioxaborinine ring π‐orbitals in the type III crystals leads to “excited multimer” domains with forbidden S₀–S₁ electronic transitions and longer FL lifetimes at similar wavelengths as that in type I crystals.     

Structure and Evolution of the “Atmungsferment” Cytochrome c Oxidase

Approximately 50 years ago, Otto Warburg received the Nobel Prize for his fundamental work on the “Atmungsferment”. But not until the end of the fifties was it possible to isolate this complicated membrane enzyme, which is necessary for respiration and energy production in most living organisms on earth. Since then, intensive research has been performed to elucidate the mechanism of reduction of oxygen to water and the coupled translocation of protons across the membrane which is involved in ATP synthesis. Until now the results have been unsatisfactory because the four catalytic heavy-metal redox centers are bound to proteins, the structures of which have begun to be studied only recently. In the course of this research, it was discovered that cytochrome c oxidase from bacteria contains only two or three, whereas the enzyme complex from animals contains thirteen different protein components. The present article analyzes the possible functions of the various protein subunits and, using the example of cytochrome c oxidase, shows that biochemical evolution proceeds in such a way as to increase regulatory capacity.