To what extent can cyclometalation promote associative or dissociative ligand substitution at platinum(II) complexes? A combined kinetic and theoretical approach

The ligand exchange rate constants for the reactions [Pt(bph)(SR2)2] + 2*SR2 --> [Pt(bph)(*SR2)2] + 2SR2 (bph = 2,2'-biphenyl dianion; R = Me and Et) and cis-[PtPh2(SMe2)2] + 2*SMe2 --> cis-[PtPh2(*SMe2)2] + 2SMe2 have been determined in CDCl3 as a function of ligand concentration and temperature, by 1H NMR isotopic labeling and magnetization transfer experiments. The rates of exchange show no dependence on ligand concentration and the kinetics are characterized by largely positive entropies of activation. The kinetics of displacement of the thioethers from [Pt(bph)(SR2)2] with the dinitrogen ligands 2,2'-bipyridine and 1,10-phenanthroline (N-N) to yield [Pt(bph)(N-N)], carried out in the presence of sufficient excess of thioether and N-N to ensure pseudo-first-order conditions, follow a nonlinear rate law k(obsd) = a[N-N]/(b[SR2] + [N-N]). The general pattern of behavior indicates that the rate-determining step for substitution is the dissociation of a thioether ligand and the formation of a three-coordinated [Pt(bph)(SR2)] intermediate. The value of the parameter a, which measures the rate of ligand dissociation, is constant and independent of the nature of N-N, and it is in reasonable agreement with the value of the rate of ligand exchange at the same temperature. Theoretical ab initio calculations were performed for both [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], and for their three-coordinated derivatives upon the loss of one SMe2 ligand. The latter optimize in a T-shaped structure. Calculations were performed in the HF approximation (LANL2DZ basis set) and refined by introducing the correlation terms (Becke3LYP model). The activation enthalpies from the optimized vacuum-phase geometries are 52.3 and 72.2 kJ moll compared to the experimental values in CDCl3 solution, 80 +/- 1 and 93 +/- 1 kJ mol(-1) for [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], respectively. The electrostatic potential maps of both parent compounds show a remarkable concentration of negative charge over the platinum atom which exerts a repulsion force on an axially incoming nucleophile. On the other hand, the strength of the organic carbanions trans to the leaving group and the stabilization of the T-shaped intermediate in the singlet ground state may also rationalize the preference for the dissociative mechanism. All of the kinetic and theoretical data support the latter hypothesis and indicate, in particular, that dissociation from the complex containing the planar 2,2'-biphenyl dianion is easier than from its analogue with single aryl ligands. Electron back-donation from filled d orbitals of the metal to empty pi* of the in-plane cyclometalated rings is weak or absent and is not operative in promoting an associative mode of activation.     

Steric course of some cyclopropanation reactions of L-threo-hex-4-enopyranosides

Completely protected 4-deoxy-α-L-threo-hex-4-enopyranosides 1c,d undergo the dichlorocarbene addition affording exclusively diastereomeric adducts 5c,d with the cyclopropane ring anti to the C-3 alkyloxy substituent, while the reaction with 3-unprotected derivatives 1a,b affords a mixture of syn and anti derivatives. Under the Simmons-Smith cyclopropanation adducts 2a-d with a syn stereochemistry are obtained. Starting from 5b, the cyclopropanated sugar 3b is obtained by reduction with LiAlH₄, thus the two diastereomers 2b and 3b can be stereoselectively obtained through the two different pathways. For a useful comparison, 4-deoxy-β-L-threo-hex-4-enopyranoside 1e was also subjected to the above two cyclopropanation methods affording the expected cycloadduct 2e and a diastereomeric mixture of dichlorocycloadducts 4e and 5e (4e/5e=2.8:1).     

The prediction of isobaric phase equilibria for non-ideal multicomponent mixtures from heat of mixing data

A method for predicting isobaric binary and ternary vapor—liquid equilibrium data using only isothermal binary heat of mixing data and pure component vapor pressure data is presented. Three binary and two ternary hydrocarbon liquid mixtures were studied. The method consists of evaluating the parameters of the NRTL equation from isothermal heat of mixing data for the constituent binary pairs. These parameters are then used in the multicomponent NRTL equation to compute isobaric vapor—liquid equilibrium data for the ternary mixture. No ternary or higher order interaction terms are needed in the ternary calculations because of the nature of the NRTL equation. NRTL parameters derived from heat of mixing data at one temperature can be used to predict vapor—liquid equilibrium data at other temperatures up to the boiling temperature of the liquid mixture.For the systems studied this method predicted the composition of the vapor phase with a standard deviation ranging from 1–8% for the binary systems and from 4–12% for the ternary systems.     

Indecomposable r-graphs and some other counterexamples

An r-graph is any graph that can be obtained as a conic combination of its own 1-factors. An r-graph G(V, E) is said to be indecomposable when its edge set E cannot be partitioned as E = E₁ ∪ E₂ so that G_i(V, E_i) is an r_i-graph for i = 1, 2 and, for some r₁, r₂. We give an indecomposable r-graph for every integer r ≥ 4. This answers a question raised in [Seymour, Proc London Math Soc 38 (1979, 423–460], and has interesting consequences for the Schrijver System of the T-cut polyhedron to be given in [Rizzi, 1997, to appear]. A graph in which every two 1-factors intersect is said to be poorly matchable. Every poorly matchable r-graph is indecomposable. We show that for every r ≥ 4 that “being indecomposable” does not imply “being poorly matchable.” Next we give a poorly matchable r-graph for every r ≥ 4. The article provides counterexamples to some conjectures of Seymour. © 1999 John Wiley & Sons, Inc. J Graph Theory 32: 1–15, 1999     

A short proof of König's matching theorem

We give a short proof of the following basic fact in matching theory: in a bipartite graph the maximum size of a matching equals the minimum size of a node cover. © John Wiley & Sons, Inc. J Graph Theory 33: 138–139, 2000     

Analysis of Bulk and Interface Phenomena in CdTe/CdS Thin-Film Solar Cells

CdTe layers have been grown on CdS layers to produce thin-film photovoltaic devices. Because of the large lattice mismatch of roughly 10%, CdTe and CdS can only be joined at the expense of a high density of misfit dislocations. Additionally, after deposition the CdTe layer contains submicrometer sized, [111] oriented, columnar grains with a high density of stacking faults and microtwins resulting in a poor electrical performance of the p-n junction. The performance of these cells can be improved by depositing a CdCl₂ layer on the CdTe absorber layer and subsequent annealing of the stack in air. This treatment induces interdiffusion of S and Te across the interface, which results in a better lattice match. During this anneal, CdTe is subject to grain growth, recovery and recrystallization. In samples annealed for different durations after different amounts of CdCl₂ were applied, grain growth is completed during the first minutes of annealing. Subsequent diffusion of Cl is detected along the CdTe grain boundaries. The presence of Cl enhances the recrystallization of the CdTe layer, starting from the CdTe surface, while recovery of the CdTe layer, mostly by the reduction of microtwins, takes place at the interface. The simultaneous occurrence of recrystallization and recovery leads to a preferred alignment of grain boundaries in CdTe parallel to the interface. Electron beam induced current measurements show the detrimental effect of these grain boundaries on the charge carrier collection efficiency of the cell. Based on these results, a modified growth procedure is proposed.     

Electroluminescence in vapor grown CuGaS2

Electroluminescence emission has been obtained in the compound CuGaS₂ with In and Au contacts. Such emission is mainly ascribed to the injection of minority carriers from the In contact. The electroluminescent spectrum presents some peaks at energies lower than gap energy showing that electroluminescence is mainly due to a recombination into levels localized in the gap. A weak emission at an energy near the gap has also been observed.     

Nuclear magnetic resonance of 1,4-benzodiazepines. II Stereochemistry of 1,3-dihydro-2H-1,4-benzodiazepin-2-ones by lanthanide shift reagents

The sterochemistry of some 1,3-dihydro-2H-1,4-benxodiazepin-2-ones, employed as psychotherapeutic agents, is deduced by proton magnetic resonance using the paramagnetic shift reagent Eu(fod)₃. the lanthanide induced shifts are computer simulated on the basis of the geometric parameters of the protons in different model structures, having intermediate conformations between a cycloheptadiene- and a cyclohepatatriene-like system. N-Desmethyldiazepam shows a conformational equilibrium between two pseudoboat forms, while the 1-alkyl substituted derivatives exist, at room temperature, in olny one boat cycloheptatriene-like conformation.     

Photoelectrical memory effect in ZnIn2Se4

The compound ZnIn₂Se₄ with n-type conductivity is shown to exhibit electro-optical memory effect and negative resistance effect similar to those previously reported for ZnIn₂S₄. At low temperature ZnIn₂Se₄ material will present a high conductivity during and after illumination by light of energy greater than the band gap. This high conductivity state can be quenched in three different ways (i) by heating the sample until it reaches room temperature, (ii) by illumination with monochromatic light of appropriate energy, (iii) by an electric field. In the latter case the material exhibits a negative resistance effect in the transition between the low and high conductivity conditions. These charge storage phenomena have been explained by assuming the presence of a level, localized in the forbidden gap, which is twofold negative charged and presents a repulsive barrier to the recombination of electrons.     

Constituents of Satureia calamintha. Application of Eu(fod)3 to the assignments of the methyl resonances of triterpenes related to 12-ursene

The n.m.r. spectra of triterpenoids related to 12-ursene have been examined using the shift reagent Eu(fod)₃. The assignment of all the methyl resonances has been achieved for ursolic acid derivatives and the corresponding compounds of the 3-epi series, which have not been investigated previously. The additive shielding effects of various substituents on the methyl resonance frequencies have been deduced from the results.