Thermal behaviour of the homopolynuclear glyoxylate complex combinations with Cu(II) and Cr(III)

Homopolynuclear complexes of Cu(II) respectively Cr(III) with the glyoxylate dianion, C₂H₂O₄ ^2-, have been studied in non-isothermal regime in air and nitrogen. The results of the non-isothermal analysis performed for the synthesised complexes, Cu(C₂H₂O₄)·0.5H₂O, respectively [Cr₂(OH)₂(C₂H₂O₄)₂(OH₂)₄]·2H₂O, correlated with the results of the IR and TG analysis of the compounds obtained by thermal treatment from the initial complexes and the results of the GLC and XR analysis have led to the establishment of the thermal decomposition mechanisms for the two studied complexes. The decomposition mechanisms confirm the stoichiometric and structural formulae proposed for the two synthesised homopolynuclear complexes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal and electrochemical C-X activation (X = Cl,Br, I) by the strong Lewis acid Pd3(dppm)3(CO)2+ cluster and its catalytic applications

The stoichiometric and catalytic activations of alkyl halides and acid chlorides by the unsatured Pd(3)(dppm)(3)(CO)(2+) cluster (Pd(3)(2+)) are investigated in detail. A series of alkyl halides (R-X; R = t-Bu, Et, Pr, Bu, allyl; X = Cl, Br, I) react slowly with Pd(3)(2+) to form the corresponding Pd(3)(X)(+) adduct and "R(+)". This activation can proceed much faster if it is electrochemically induced via the formation of the paramagnetic species Pd(3)(+). The latter is the first confidently identified paramagnetic Pd cluster. The kinetic constants extracted from the evolution of the UV-vis spectra for the thermal activation, as well as the amount of electricity to bring the activation to completion for the electrochemically induced reactions, correlate the relative C-X bond strength and the steric factors. The highly reactive "R(+)" species has been trapped using phenol to afford the corresponding ether. On the other hand, the acid chlorides react rapidly with Pd(3)(2+) where no induction is necessary. The analysis of the cyclic voltammograms (CV) establishes that a dissociative mechanism operates (RCOCl --> RCO(+) + Cl(-); R = t-Bu, Ph) prior to Cl(-) scavenging by the Pd(3)(2+) species. For the other acid chlorides (R = n-C(6)H(13), Me(2)CH, Et, Me, Pr), a second associative process (Pd(3)(2+) + RCOCl --> Pd(3)(2+.....)Cl(CO)(R)) is seen. Addition of Cu(NCMe)(4)(+) or Ag(+) leads to the abstraction of Cl(-) from Pd(3)(Cl)(+) to form Pd(3)(2+) and the insoluble MCl materials (M = Cu, Ag) allowing to regenerate the starting unsaturated cluster, where the precipitation of MX drives the reaction. By using a copper anode, the quasi-quantitative catalytic generation of the acylium ion ("RCO(+)") operates cleanly and rapidly. The trapping of "RCO(+)" with PF(6)(-) or BF(4)(-) leads to the corresponding acid fluorides and, with an alcohol (R'OH), to the corresponding ester catalytically, under mild conditions. Attempts were made to trap the key intermediates "Pd(3)(Cl)(+)...M(+)" (M(+) = Cu(+), Ag(+)), which was successfully performed for Pd(3)(ClAg)(2+), as characterized by (31)P NMR, IR, and FAB mass spectrometry. During the course of this investigation, the rare case of PF(6)(-) hydrolysis has been observed, where the product PF(2)O(2)(-) anion is observed in the complex Pd(3)(PF(2)O(2))(+), where the substrate is well-located inside the cavity formed by the dppm-Ph groups above the unsatured face of the Pd(3)(2+) center. This work shows that Pd(3)(2+) is a stronger Lewis acid in CH(2)Cl(2) and THF than AlCl(3), Ag(+), Cu(+), and Tl(+).     

Remarkable stereocontrol in the synthesis of 1,2,3,5-tetrasubstituted tetrahydropyrans via an asymmetric heterocycloaddition of a ketone-derived enol ether

The synthesis of chiral 1,2,3,5-substituted tetrahydropyrans has been realized via an asymmetric hetero Diels-Alder (HDA) reaction. The key step that involved a trisubstituted chiral enol ether derived from (R)-mandelic acid as the dienophile promoted the creation of three stereogenic centers with a remarkable and unprecedented endo and facial stereocontrol. The hydrogenation of the heteroadduct 2 was optimized by using Pd on charcoal and diisopropylethylamine, leading to a unique isomer. The chiral inductor was cleanly and stereoselectively removed via an acetal reduction, which demonstrated the potential of this methodology for the efficient construction of key intermediate of biologically active molecules.     

Determination of carbon in solidified sodium coolant using new ICP-OES methods

New methods for the determination of carbon in sodium using laser ablation inductively coupled plasma optical emission spectrometry (LA-ICP-OES) and ICP-OES with pneumatic nebulisation (PN-ICP-OES) were developed. The determination was required for the study of the carbon dioxide reaction with molten sodium at high temperatures (300–600°C). After exposition to CO₂, the solidified sodium sample was subjected to direct solid analysis by LA-ICP-OES and to solution analysis. For the determination of carbon in the sodium sample surface layer by LA-ICP-OES, three different matrices containing sodium were tested (NaCl, NaF, and Na₂B₄O₇ · 10H₂O) as calibration pellets. The calibration dependences were improved using sodium as the internal standard. Average carbon content in the sodium bulk sample was determined by PN-ICP-OES after the sample dissolution by water vapour.     

DFT study of structure–properties correlations in [MnTPP][TCNE] quasi-one-dimensional molecular magnets

We report the first band structure calculations of the quasi-one-dimensional [MnTPP][TCNE] compounds (TPP = meso-tetraphenylporphyrinato, TCNE = tetracyanoethylene), based on Density Functional Theory (DFT) methods, in order to interpret the magnetic ordering in these prototypic systems. We compare and contrast the results of broken-symmetry DFT calculations for extended systems, with periodic boundary conditions, and for finite systems, magnetic dimers modeling the actual molecular magnets. By varying systematically the main angles, we are able to determine the geometry dependence of the exchange interaction. Structure–properties correlations in these charge-transfer salts reveal the determinant role of the Mn-(N≡C)_TCNE bond angle on the strength of the ferrimagnetic coupling between the S ₁ = 2 spin located on the Mn^III-porphyrin donor and the S ₂ = 1/2 spin positioned on the cyanocarbon acceptor. When the Mn-(N≡C)_TCNE angle is decreased, the intrachain magnetic coupling strengthens, correlated with the increase in the d_z² - p* d_{{z^{2} }} - \pi * orbital overlap. The exchange coupling constants resulting from DFT calculations of extended systems, with periodic boundary conditions, were found to be consistent with those obtained for the dimers, but systematically smaller. The exchange constants vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the experimental mean-field critical temperatures. The coupling constant varies significantly with the type of broken-symmetry approach, depending on the overlap between magnetic orbitals, but weakly on the basis set once polarization effects are included. The electronic structure calculations for the extended systems provide a density of states consistent with the energy spectrum of the corresponding dimer, allowing for an intuitive explanation of the intrachain ferrimagnetic ordering.     

H-quadrupolar coupling-based analysis of stereochemical and regiochemical memory in the Pd-catalysed allylic alkylation of iso-cinnamyl type substrates employing the chiral monophosphine ligands ‘MOP’ and ‘MAP’

The reaction of iso-cinnamyl acetate with NaC(Me)(CO₂Me)₂, catalysed by Pd-‘MOP’ (MOP=2-methoxy-2′-diphenylphosphino-1,1′-binaphthalene) is known to proceed with a regiochemical memory effect that results in the predominant generation of the branched alkylation product. The analogous reaction employing ‘MAP’ as ligand (MAP=2-N,N-dimethylamino-2′-diphenylphosphino-1,1′-binaphthalene) proceeds with ‘normal’ regioselectivity to generate predominantly the linear isomer of product. A ²H-NMR based analysis, employing quadrupolar coupling in a chiral liquid crystal matrix, has been developed to facilitate the simultaneous study of the regiochemical and stereochemical outcome of the reaction of both enantiomers of iso-cinnamyl ester substrates in ²H-labelled but racemic samples. The analysis allows the comparison of relative rates of two competing isomerisation processes occurring in the π-allyl intermediates in the Pd-catalysed reaction, one of which facilitates asymmetric induction, the other resulting in loss of regiochemical memory. It is demonstrated that the two processes are partially coupled and that this then limits the attainment of high global enantiomeric excess in the branched product to reactions that proceed with low regiochemical retention. A key factor for the observation of high regiochemical memory is found to be the nucleophilicity of the malonate anion and the electrophilicity of the Pd-π-allyl intermediate with reduction in the reactivity of either partner resulting in the onset of substantial loss of memory.     

A joint theoretical and experimental investigation on the 13C and 1H NMR chemical shifts of coumarin derivatives

¹H and ¹³C NMR chemical shifts of coumarin derivatives have been determined using first principles approaches with and without accounting for the effects of the solvent and compared to experiment in order to assess their reliability. Good linear relationships are obtained between theory and experiment, which allows correcting the calculated values for systematic errors. This is particularly the case when using the PCM scheme to model the solvent effects because the δ values larger than 150 ppm are more difficult to reproduce. The final accuracy of the method amounts to about 1 ppm for ¹³C and 0.05 ppm for ¹H.     

Microscale imaging of the preservation state of 5,000-year-old archaeological bones by synchrotron infrared microspectroscopy

Archaeological bone materials record characteristic markers of life in prehistoric times (dating, climate, environment, diet, human migration) in their isotopic and chemical composition in addition to palaeontological, archaeozoological, anthropological and palaeogenetic information. Thus, the discovery and conservation of archaeological bone materials is of great importance to get access to this information. However, archaeological materials are altered by different postmortem processes and it appears necessary to estimate if the archaeological information is still reliable or if it has been modified during burial. As archaeological bone materials present a high structural hierarchy at the micro- and nanoscale, changes induced by diagenetic phenomena have to be observed at these scales. One method for revealing post mortem changes of the bone structure and composition at the microscale is synchrotron radiation micro-FTIR imaging (SR micro-FTIR). Thus, thin sections of about 5,000-year-old archaeological bones have been analysed in transmission mode at the IRIS beamline (BESSY II, HZB Berlin) to determine markers of the state of bone preservation at the microscale. The archaeological bone material comes from station 19 of the Neolithic site of the Chalain Lake. By using SR micro-FTIR it was possible to image characteristic bone structures, e.g. osteons (the constitutive histological unit of cortical bone), using the absorption band ratios corresponding to different chemical bone constituents (collagen content and quality, phosphate crystallinity, carbonate content). These data allow us to precisely evaluate the state of preservation of a 5,000-year-old bone at the histological level.

An oestrogen-derivative labeled with 99mTc and its radiopharmaceutical potential

An oestrogen derivative 3,17-a-oestradiolyl propyl 1,4,8,11-tetraazacyclotetradecanyl-1-(4-methylbenzoic acid)ester (ESTCPTA) that is 3,17-a-oestradiolyl propinol coupled to 1-(4-methylbenzoic acid)1,4,8,11-tetraazacyclotetradecane (CPTA) was synthesized in five steps. The product was purified by recrystallization in ethyl alcohol, and analysed by NMR and IR spectroscopy. ESTCPTA was labeled with ^99mTc and radio thin layer chromatography (RTLC) and radio-paper electrophoresis were used to determine the radiochemical yields. Specific activity was approximately 23.7 GBq/mmol and the labeling yield was over 95%. The biodistribution studies were performed on female Albino Wistar rats. The rats were sacricified by ether narcotization at certain time intervals and the activity of the organs was counted by a gamma counter. The activity per gram tissue was calculated and time activity curves were generated.