Halogenation of Lewis acid/base stabilised phosphanylboranes

The reaction pattern of the Lewis-acid/base stabilised phosphanylborane [(CO)₅W(H₂PBH₂ · NMe₃)] (1) with elemental halogens is comprehensively studied. The reaction with iodine and bromine leads to a selective halogenation at the tungstencarbonyl moiety under formation of [WX₂(CO)₄(H₂PBH₂ · NMe₃)] (X = I (2), Br (3)). Whereas 2 is a stable product the brominated compound 3 dimerises easily to [WBr₂(CO)₃(H₂PBH₂ · NMe₃)]₂ (4) under lost of CO. The CO elimination reaction of 3 is extensively studied. If 3 is reacted with [Et₄N][Br] the ionic compound [Et₄N][WBr₃(CO)₃(H₂PBH₂ · NMe₃)] (5) is formed. Otherwise, if 3 is combined with the donor ligand [H₂PBH₂ · NMe₃], the complex [WBr₂(CO)₃(H₂PBH₂ · NMe₃)₂] (6) is obtained. Compounds 2–6 are comprehensively characterised by X-ray diffraction analysis, NMR, and IR spectroscopy.     

New method to measure packing densities of self-assembled thiolipid monolayers

For a monolayer of 2,3-di-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-a-lipoic acid ester lipid (DPTL) self-assembled (SAM) at a gold electrode surface we propose a new method to determine the charge number per adsorbed molecule and the packing density (area per molecule) in the monolayer. The method relies on chronocoulometry to measure the charge density at the SAM covered gold electrode surface. Two series of measurements have to be performed. In the first series, charge densities are measured for a monolayer transferred from the air-solution to the metal-solution interface using the Langmuir-Blodgett (LB) technique. This series of measurements allows one to determine charge numbers per adsorbed DPTL molecule. The second series is performed using a gold electrode covered with a self-assembled monolayer. The charge densities obtained in this series are then used to calculate the packing density with the help of charge numbers per adsorbed DPTL determined in the first series. The area per adsorbed molecule determined by the new method was compared to the area per molecule determined by the popular reductive desorption method. The molecular area determined with the new method is about 20% larger than the area calculated from the van der Waals model, which is a physically reasonable result. In contrast, the popular reductive desorption method gives an area per molecule 20% lower than the minimum estimated based on a van der Waals model. This is a physically unreasonable result. It is also shown that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electrons participating in the reductive desorption step. An explanation of the origin of the "partial charge numbers" is provided. We recommend the new method be used in future studies of thiol adsorption at metal surfaces.     

Identification of material parameters for inelastic constitutive models: Stochastic simulations for the analysis of deviations

For parameter identification a distance function between the measured and the simulated data has to be minimized. Therefore, the influence of three different norms used in the definition of such a distance function is investigated. The nonlinear optimization problem is solved using a modified random search algorithm originally proposed by Price (1978). Next a stochastic model for the generation of artificial test data is presented. This model is used for a stochastic simulation of test data (constant strain rate tension with relaxation and creep). From these artificial data the material parameters of the model of Chan, Bodner and Lindholm are identified. To measure the quality of the identified material parameters their mean values and empirical standard deviations are computed. Furthermore, the coefficients of the empirical correlation matrix for the material parameters are computed. The model responses for tensile tests with the parameter vector generated from all tests and with the estimated parameters (from stochastic simulations) differ not considerably. However, for the creep tests the different parameter estimations lead to quite different model responses. Received October 22, 1999     

Magnetism and candidate muon-probe sites in RBa2Cu3O y

Key results of zero-field (ZF) and transverse-field (TF) muon-spin-relaxation (μSR) experiments on superconducting and insulating RBa₂Cu₃O _y (R123 _y , with R=Eu, Gd, Pr and Pr/Y:y=6, 7) are examined. The chemical behavior of the positive muon probe is addressed, and muon-oxygen bonding is shown to occur in all these cuprates. To explain magnetic fields at muon-probe sites in Pr _x Y_1−x Ba₂Cu₃O _y (0<=x<0.5,y=7 andx=0,y=6) samples, improvements on the reported magnetic structures from neutron diffraction are necessary. Cu magnetism in Pr123y (y=6,7) is observed belowT _N1, which is near RT. The magnetism seen belowT _N2 can be interpreted assuming an additional ordering in the Cutt-O chain layers. Alternatively, Pr ordering is also considered as the cause of the second phase transition. Considering the specific muon-probe location, a more detailed interpretation can be provided for the μSR parameters, measured in the normal and mixed states of these unconventional superconductors.     

The first example ot the benzo[b]pyrimido[4,5-f]azocine king system

Base catalyzed cyclization of ethyl 4-[2-(2-amino-4,5-dimethoxyphenyl)ethyl]-2-phenylpyrimidine-5-carboxylate, derived from ethyl 4-methyl-2-phenyl-5-pyrimidinecarboxylate and 3,4-dimethoxy-6-nitrobenz-aldehyde, gave 5,6-dihydro-8,9-dimethoxy-3-phenylbenzo[b]pyrimido[4,5-f]azocine-12(11H)-one, the first reported example of this ring system.     

The stabilization of monomeric parent compounds of phosphanyl- and arsanylboranes

The structures of the parent compounds of phosphanyl- and arsanylboranes, H(2)BPH(2) and H(2)BAsH(2), were calculated by DFT-B3LYP methods. Such compounds have not previously been obtained preparatively. By applying the concept of Lewis acid/base stabilisation, [(CO)(5)W(H(2)EBH(2).NMe(3))] (E=P (3), As (4)) derivatives have been synthesised by the metathesis reactions between Li[(CO)(5)WEH(2)] and ClH(2)BNMe(3) (E=P, As). Comprehensive thermodynamic studies on these systems verify the high stability of the Lewis acid/base stabilised complexes. Unexpected based on the thermodynamic calculations, UV radiation of the phosphanylborane 3 leads to the dinuclear phosphanido-bridged complex [(CO)(8)W(2)(mu-PHBH(2).NMe(3))(2)] (5) by H(2) and CO elimination.     

Synthesis and Crystal Structures of Novel Copper(I) Complexes of a Phosphanylborane

The novel copper iodide clusters [Cu₃(μ‐I)(μ₃‐I)₂(PH₂BH₂·NMe₃)₃] ( 2 ) and [Cu₄(μ‐I)₂(μ₃‐I)₂(PH₂BH₂·NMe₃)₃] ( 3 ) were synthesized by treating CuI with the primary phosphine (H₂PBH₂·NMe₃). The novel features of both compounds, which have been characterized by X‐ray crystallography, are the unsymmetrical constitution of the copper iodide core due to the influence of the monodentate phosphorus ligand. This results in copper atoms with different coordination numbers within the compound. Complex 2 , the major product of the reaction, contains a distorted octahedral Cu₃I₃‐core, in which one vertex is missing. Complex 3 was isolated as a by‐product and is composed of a Cu₄I₄‐core in a distorted octahedral coordination.     

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates

The electrochemical reduction of benzyl thiocyanate and p-nitrobenzyl thiocyanate was investigated in acetonitrile at an inert electrode. These two compounds reveal a change in the reductive cleavage mechanism, and more interestingly, they show a clear-cut example of a regioselective bond dissociation. Both phenomena may be understood on the basis of the dissociative ET theory and its extension to the formation/dissociation reactions of radical ions. While the effect of the standard oxidation potential of the leaving group seems to be predominant in understanding the change in the ET mechanism by changing the driving force, the regioselective cleavage is dictated by changes in the intrinsic barrier related to the nature of the substituent on the aryl moiety.     

Ring closure,of 2-benzylamino-1-phenylethanols to 4-phenyltetrahydroisoquinolines

The, facile preparation of 4-pbenyltetrabydroisoquinolines by the acid-catalyzed cyclization of 2-benzylamino-1-pbenyletbanols and related compounds is described along with a discussion of the scope, limitations, and possible mechanism of the reaction.