Structurally Defined Potassium‐Mediated Zincation of Pyridine and 4‐R‐Substituted Pyridines (R=Et,iPr, tBu,Ph, and Me2N) by Using Dialkyl–TMP–Zincate Bases

Two potassium–dialkyl–TMP–zincate bases [(pmdeta)K(μ‐Et)(μ‐tmp)Zn(Et)] ( 1 ) (PMDETA=N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine, TMP=2,2,6,6‐tetramethylpiperidide), and [(pmdeta)K(μ‐nBu)(μ‐tmp)Zn(nBu)] ( 2 ), have been synthesized by a simple co‐complexation procedure. Treatment of 1 with a series of substituted 4‐R‐pyridines (R=Me₂N, H, Et, iPr, tBu, and Ph) gave 2‐zincated products of the general formula [{2‐Zn(Et)₂‐μ‐4‐R‐C₅H₃N}₂ ? 2{K(pmdeta)}] ( 3 – 8 , respectively) in isolated crystalline yields of 53, 16, 7, 23, 67, and 51 %, respectively; the treatment of 2 with 4‐tBu‐pyridine gave [{2‐Zn(nBu)₂‐μ‐4‐tBu‐C₅H₃N}₂ ? 2{K(pmdeta)}] ( 9 ) in an isolated crystalline yield of 58 %. Single‐crystal X‐ray crystallographic and NMR spectroscopic characterization of 3 – 9 revealed a novel structural motif consisting of a dianionic dihydroanthracene‐like tricyclic ring system with a central diazadicarbadizinca (ZnCN)₂ ring, face‐capped on either side by PMDETA‐wrapped K⁺ cations. All the new metalated pyridine complexes share this dimeric arrangement. As determined by NMR spectroscopic investigations of the reaction filtrates, those solutions producing 3 , 7 , 8 , and 9 appear to be essentially clean reactions, in contrast to those producing 4 , 5 , and 6 , which also contain laterally zincated coproducts. In all of these metalation reactions, the potassium–zincate base acts as an amido transfer agent with a subsequent ligand‐exchange mechanism (amido replacing alkyl) inhibited by the coordinative saturation, and thus, low Lewis acidity of the 4‐coordinate Zn centers in these dimeric molecules. Studies on analogous trialkyl–zincate reagents in the absence and presence of stoichiometric or substoichiometric amounts of TMP(H) established the importance of Zn? N bonds for efficient zincation.     

An exact,closed form,solution for the flexural vibration of a thin annular plate having a parabolic thickness variation

An exact, closed form, solution is obtained for the transverse vibrations, with nodal diameters and circles, of a thin annular plate having a parabolic thickness variation. Representative numerical values for the frequency parameter and typical mode shapes are presented for three different combinations of simple boundary conditions. The corresponding exact solution for an aeolotropic annular plate of the same geometry is also presented. Aside from possible design applications, these exact, closed form, data can be used as test cases for assessing the accuracy of various approximate methods of solution. The analysis involves only the powers of the radius and is simpler than that for the constant thickness solution which involves Bessel functions.     

Minimal-energy clusters of hard spheres

What is the tightest packing ofN equal nonoverlapping spheres, in the sense of having minimal energy, i.e., smallest second moment about the centroid? The putatively optimal arrangements are described forN≤32. A number of new and interesting polyhedra arise.     

Structure and dynamics of a dihydrogen/hydride ansa molybdenocene complex

In contrast to [Cp(2)MoH(3)](+), which is a thermally stable trihydride complex, the ansa-bridged analogue [(eta-C(5)H(4))(2)CMe(2)MoH(H(2))](+) (1) is a thermally labile dihydrogen/hydride complex. Partial deuteration of the hydride ligands allows observation of J(H)(-)(D) = 11.9 Hz in 1-d(1) and 9.9 Hz in 1-d(2) (245 K), indicative of a dihydrogen/hydride structure. There is a slight preference for deuterium to concentrate in the dihydrogen ligand. A rapid dynamic process interchanges the hydride and dihydrogen moieties in complex 1. Low temperature (1)H NMR spectra of 1 give a single hydride resonance, which broadens at very low temperature due to rapid dipole-dipole relaxation (T(1) = 23 ms (750 MHz, 175 K) for the hydride resonance in 1). Low temperature (1)H NMR spectra of 1-d(2) allow the observation of decoalescence at 180 K into two resonances. The bound dihydrogen ligand exhibits hindered rotation with DeltaG(150) = 7.4 kcal/mol, but H atom exchange is still rapid at all accessible temperatures (down to 130 K). Density functional calculations confirm the dihydrogen/hydride structure as the ground state for the molecule and give estimates for the energy of two hydrogen exchange processes in good agreement with experiment. The presence of the C ansa bridge is shown to decrease the ability of the metallocene fragment to donate to the hydrogens, thus stabilizing the (eta(2)-H(2)) unit and modulating the barrier to H(2) rotation.     

Electrocatalytic effect of the oxide film at Pt anodes on Cl• recombination kinetics in chlorine evolution

In aqueous medium, Cl₂ evolution on noble metal anodes occurs on a surface oxide film, the properties of which, rather than those of the metal, determine the kinetics of the reaction. The kinetics of Cl₂ evolution on an oxidized Pt electrode in aqueous KCl are compared with those for Cl₂ evolution on a demonstrably unoxidized Pt surface in anhydrous trifluoroacetic acid (TFA) as solvent. Hence the effects of development of an oxide film on Pt in aqueous medium can be determined in relation to the kinetic behaviour of Cl₂ evolution on the unoxidized Pt metal surface. In both TFA and water, and their mixtures, it is shown that Cl₂ evolution kinetics are recombination-controlled and non-diffusion-controlled limiting currents, i_lim, are observed. The i_lim values in aqueous medium, i.e. for the oxide-covered surface, are some 45 times larger than on the free Pt surface in TFA; since the mechanism is shown to be the same in the two solvents, and their mixtures, this observation indicates a substantial electrocatalytic enhancement of the recombination rate for Cl^? on the oxide in comparison with the Pt metal surface. This is presumably due to weaker binding of Cl^? on oxidized Pt than on the “bare” Pt surface. Effects due to specific adsorption of Cl^? ion are also important and give rise to dependence of the recombination catalysis on Cl^? ion concentration. A method is proposed for analyzing the recombination kinetics by means of linear plots which give both the recombination rate constant and the quasi-equilibrium constant for the prior Cl^? ion discharge step as a function of water content.     

Electrochemical oxide film formation at noble metals as a surface-chemical process

The mechanisms of electrochemical oxide film formation at noble metals are described and exemplified by the cases of Pt and Au, especially in the light of recent experimentation by means of cyclic voltammetry, ellipsometry and vacuum surface-science studies using LEED and AES.

Unlike the mechanisms of base-metal oxidation, e.g., in corrosion processes, anodic oxide film formation at noble metals proceeds by surface chemical processes involving, initially, sub-monolayer, through monolayer, formation of 2-dimensional OH/O arrays. During such 2-d processes, place-exchange between electrosorbed OH or O species on the surface, and Pt or Au atoms within the surface lattice, takes place leading to a quasi-2-d compact film which then grows ultimately to a multilayer hydrous oxide film, probably by continuing injection of ions of the substrate metal and their migration through the growing film under the influence of the field.

The initial, sub-monolayer stage of electrosorption of OH involves competitive chemisorption by anions, e.g. HSO₄⁻, ClO₄⁻, Cl⁻, which inhibits onset of the first stage of surface oxidation. These processes are demonstrable in experiments on single-crystal surfaces. The combination of such anion effects with place-exchange during the extension of the film, leads to a general mechanism of noble metal oxide film formation.

The formation of the oxide films can be examined in detail by recording the distinguishable stages in the film's electrochemical reduction in linear-sweep voltammetry which is sensitive down to OH/O fractional coverages as low as 0.5% and over time-scales down to 50μs in experiments on time-evolution and transformation of the states of the oxide films.

By means of LEED, AES and STM or AFM experiments, the reconstructions and perturbations (e.g. generation of stepped terraces) which oxide films cause on singlecrystal surfaces can be followed.     

A rapid method for the determination of lasalocid in animal tissues and eggs by high performance liquid chromatography with fluorescence detection and confirmation by LC-MS-MS

A simple and rapid method has been developed for the extraction of lasalocid from chicken muscle, eggs and liver and kidney from chicken, pig, sheep and calf. This method allows the screening of a large number of samples, i.e. 30-40 within a working day, and has an overall analysis time of 90 min. Lasalocid standard solution can be detected at 1 ng ml-1 by both HPLC-fluorescence (HPLC-F) and LC-MS-MS; the limit of quantification in fortified samples by the described method is 1 ng g-1. Results show good repeatability and mean 'spiked' recoveries by HPLC-F in the range of 10 to 200 ng g-1 (ppb) of 103, 87, 107, 97, 97, 103, 93, 109 and 100% in chicken muscle, chicken liver, egg, pig liver, pig kidney, sheep liver, sheep kidney, calf liver and calf kidney, respectively. For concentrations between 1 and 6 ng g-1 of spiked lasalocid in eggs and chicken liver by LC-MS-MS, the average recoveries were 76 and 59%, respectively.