Preparation of an Organometallic Molecular Square by Self‐Assembly of Phosphorus‐Containing Building Blocks

Molecular squares are among the most common supramolecular architectures, but phospha‐organometallic complexes have not been used as building blocks for this type of structure. Herein we describe the formation of the molecular square [Au{Co(P₂C₂tBu₂)₂}]₄ ( 1 ) by the self‐assembly of anionic 1,3‐diphosphacyclobutadiene cobalt complexes and gold(I) cations. The X‐ray crystallographic determination of the molecular structure of 1 is complemented by solid‐state ³¹P and ¹³C NMR investigations. High‐level DFT calculations confirm the assignment of the ³¹P and ¹³C NMR resonances.     

Measurement of preferences with self-explicated approaches: A classification and merge of trade-off- and non-trade-off-based evaluation types

Self-explicated approaches are popular preference measurement approaches for products with many attributes. This article classifies previous self-explicated approaches according to their evaluation types, i.e. trade-off- versus non-trade-off-based, and outlines their advantages and disadvantages. In addition, it proposes a new method, the presorted adaptive self-explicated approach that is based on Netzer and Srinivasan’s (2011) adaptive self-explicated approach and that combines trade-off- and non-trade-off-based evaluation types. Two empirical studies compare this new method with the most popular existing self-explicated approaches, including the adaptive self-explicated approach and paired comparison preference measurement. The new method overcomes the insufficient discrimination between importance weights, as usually found in non-trade-off-based evaluation types; discourages respondents’ simplification strategies, as are frequently encountered in trade-off evaluation types; is easy to implement; and yields high predictive validity compared with other popular self-explicated approaches.     

Structural characterization of bioactive glasses by solid state NMR

Nuclear magnetic resonance (NMR) spectroscopy offers an element-selective, inherently quantitative and experimentally very flexible approach for the structural elucidation of non-crystalline materials. The present review introduces the basic concepts of this technique, highlighting the use of advanced NMR methodology for characterizing short- and intermediate range order in bioactive glass systems. The current state of the literature in this field is summarized in a comprehensive manner. NMR can give clear-cut and quantitative answers about the extent of network polymerization, the spatial distribution of the network former and network modifier species, and the structural roles of Group III elements introduced into these networks. These results facilitate our understanding of the influence of bioglass compositions upon the dissolution kinetics and bioactivities of these glasses. A particular mission of this review is to highlight the utility of non-routine, more advanced experimentation, in the hope of their increased usage and circulation in future applications.

Open image in new window

The main six nuclear isotopes used in obtaining high-resolution magic-angle spinning NMR spectra for the structural characterization of bioactive glasses

     

Transition of the electronic response from molecular-like to jellium-like in cold,small sodium clusters

Absolute photoabsorption cross sections for Na _n ⁺ (2≤n≤21) were measured in the visible energy range. The cluster ions were produced in a gas aggregation source and thus have a canonical distribution of internal energy corresponding to a temperature of ～ 105 K. The spectra for n≤9 and 11 exhibit between two and six absorption lines, and are in qualitative agreement with ab inito quantum chemical calculations. For n=15 and 21, the position of the resonances can be explained as excitations of a nearly free electron gas in a spheroidal container. An evolution is thus observed from molecular-like transitions to a giant collective resonance of the electron cloud. The integrated oscillator strength is 0.95 per 3s-electron in the energy range covered for n≥4, showing that the main excitations of the valence electrons have been found.     

The preparation and characterisation of ruthenium cyanovinylidene complexes

Reactions of half-sandwich ruthenium metal acetylide complexes with 1-cyano-4-dimethylaminopyridinium salts afford complexes containing mono- or di-cyanovinylidene ligands; the procedure can be adapted to permit the simple synthesis of a cyanoacetylide complex, via the in situ deprotonation of a primary cyanovinylidene complex.     

Local polarity in CO2-expanded acetonitrile: a nucleophilic substitution reaction and solvatochromic probes

Many processes that use highly tunable gas-expanded liquids (GXLs) rely on the fact that CO2 addition can greatly affect the polarity of the solvent. We have examined several measures of bulk and local polarity in CO2-expanded acetonitrile to enable more effective exploitation of these polarity changes. The rate of the nucleophilic substitution reaction of tributylamine with methyl p-nitrobenzenesulfonate has been analyzed as a function of solvent composition by using in situ high-pressure UV/vis spectroscopy. We have also measured solvatochromic properties including the Kamlet-Taft pi* parameter and Kosower's Z-value. We correlate these local polarity-based kinetic and solvatochromic measures to develop a better understanding of these property changes as a function of bulk and local solvent composition. The data suggest that local composition enhancement in CO2-expanded acetonitrile has a significant impact on the reaction kinetics.     

Variable temperature neutron diffraction and X-ray charge density studies of tetraacetylethane

Single crystal neutron diffraction data have been collected on a sample of enolized 3,4-diacetyl-2,5-hexanedione (tetraacetylethane, TAE) at five temperatures between 20 and 298 K to characterize the temperature-dependent behavior of the short, strong, intramolecular hydrogen bond. Upon decreasing the temperature from 298 K to 20 K, the O2-H1 distance decreases from 1.171(11) to 1.081(2) A and the O1...H1 distance increases from 1.327(10) to 1.416(6) A. The convergence of the C-O bond lengths from inequivalent distances at low temperature to identical values (1.285(4) A) at 298 K is consistent with a resonance-assisted hydrogen bond. However, a rigid bond analysis indicates that the structure at 298 K is disordered. The disorder vanishes at lower temperatures. Short intermolecular C-H...O contacts may be responsible for the ordering at low temperature. The intramolecular O...O distance (2.432 +/- 0.006 A) does not change with temperature. X-ray data at 20 K were measured to analyze the charge density and to gain additional insight into the nature of the strong hydrogen bond. Quantum mechanical calculations demonstrate that periodic boundary conditions provide significant enhancement over gas phase models in that superior agreement with the experimental structure is achieved when applying periodicity. One-dimensional potential energy calculations followed by quantum treatment of the proton reproduce the location of the proton nearer to the O2 site reasonably well, although they overestimate the O-H distance at low temperatures. The choice of the single-point energy calculation strategy for the proton potential is justified by the fact that the proton is preferably located nearer to O2 rather than being equally distant to O1 and O2 or evenly distributed (disordered) between them.     

A bridging hexazene (RNNNNNNR) ligand from reductive coupling of azides

This communication reports the first examples of transition metal complexes containing an RNNNNNNR 2- ligand. Addition of 1-azidoadamantane to the diiron(I) synthon LRFeNNFeL R (L R = HC[C(R)N(2,6- iPr 2C 6H 3)] 2; R = methyl, tert-butyl) leads to the diiron complexes L RFe(mu-eta2:eta2-AdN6Ad)FeLR, which are surprisingly thermally stable. Magnetic, M?ssbauer, and crystallographic data are consistent with pairs of high-spin iron(II) ions antiferromagnetically coupled through a dianionic AdN6Ad 2- bridge.     

Materials Chemistry in Münster

Ende August haben Sigma‐Aldrich High‐Technology und die NRW Graduate School of Chemistry ihren zweiten gemeinsamen Doktoranden‐Workshop in Materials Chemistry veranstaltet. Mehr als 60 Doktoranden aus dem In‐ und Ausland haben drei Tage in den Münsteraner chemischen Instituten über Materialwissenschaften diskutiert.     

Mathematics, Experiments, and Theoretical Physics: The Early Days of the Sommerfeld School

Arnold Sommerfeld (1868-1951) founded one of the most influential schools of twentieth-century theoretical physics. His favored specialty was atomic theory, and a world-wide community of physicists was introduced to this field by his legendary textbook, Atomic Structure and Spectral Lines. The names of his students read like a Who's Who of the pioneers in modern physics Peter Debye, Peter Paul Ewald, Wolfgang Pauli, Werner Heisenberg, Hans A. Bethe - to name only the most prominent. In retrospect, the success of Sommerfeld's school of modern theoretical physics tends to overshadow its less glorious beginnings. A century ago, theoretical physics was not yet considered as a distinct discipline. In this article I emphasize more the haphazard beginnings than the later achievements of Sommerfeld's school, which mirrored the state of theoretical physics before it became an independent discipline.