Shape-persistant macrocycles with terpyridine units: synthesis,characterization, and structure in the crystal

The synthesis of a variety of shape-persistent macrocycles with either one (1a-d, 2) or two (opposing) terpyridine units (3, 4, 5a-c) and inner diameters of up to 2 nm is described. The sequences are mainly based on transition metal cross-coupling reactions and, whenever appropriate, compared with one another regarding their respective efficiency. Typical overall yields and amounts prepared range from 8% (4) to 27% (3) and 25 mg (1a) to 290 mg (1b), respectively. For solubility and processing of the targeted cycles, all precursors have already been decorated with flexible side chains (hexyloxy or hexyloxymethyl). The cycles' characterization is based on MALDI-TOF mass spectrometry, 2D NMR spectroscopy, and/or low-temperature single-crystal X-ray diffraction. Their packing in the crystal is discussed in terms of both number and length of side chains. Cycle 1d was physisorbed into an ordered structure at the solution-HOPG interface and investigated by scanning tunneling microscopy (STM).     

Die Kristallstruktur von SCl3[Re2Cl9] und ihre Verwandtschaft zum RuBr3‐Typ

The Crystal Structure of SCl₃[Re₂Cl₉] and its Relation to the RuBr₃ Type SCl₃[Re₂Cl₉] was obtained from the reaction of rhenium and SCl₂ at 400 °C. The X‐ray crystal structure determination revealed a monoclinic structure, a = 834.1 pm, b = 1053.3 pm, c = 866.1 pm, β = 91.90°, space group P2₁/m, R₁ = 0.058. The SCl₃⁺ and Re₂Cl₉^– ions have the known structures; the ReRe bond length in the face‐sharing bioctahedron is 272.2 pm. The crystal packing can be derived from the RuBr₃ structure type, which has infinite columns of face‐sharing octahedra; one quarter of the metal atoms are removed and another quarter are replaced by sulfur atoms. The chlorine atoms form a slightly distorted hexagonal closest‐packing. The symmetry relationships are shown in a family tree of group–subgroup relations.     

Factors Governing the Activity of α-MnO2 Catalysts in the Oxygen Evolution Reaction: Conductivity versus Exposed Surface Area of Cryptomelane

Cryptomelane (α-(K)MnO₂) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce⁴⁺ were compared. The decisive factors that explain this difference were identified in the catalysts’ microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H₂O₂ decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm⁻² at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions.