Electronic structure study of seven-coordinate first-row transition metal complexes derived from 1,10-diaza-15-crown-5: a successful marriage of theory with experiment

A detailed study of the electronic structure of seven-coordinate Mn(II), Co(II), and Ni(II) complexes with the lariat ether N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L(1)) is presented. These complexes represent new examples of structurally characterized seven-coordinate (pentagonal bipyramidal) complexes for the Mn(II), Co(II), and Ni(II) ions. The X-ray crystal structures of the Mn(II) and Co(II) complexes show C(2) symmetries for the [M(L(1))](2+) cations, whereas the structures of the Ni(II) complexes show a more distorted coordination environment. The magnetic properties of the Mn(II) complex display a characteristic Curie law, whereas those of the Co(II) and Ni(II) ions show the occurrence of zero-field splitting of the S = 3/2 and 1 ground states, respectively. Geometry optimizations of the [M(L(1))](2+) systems (M = Mn, Co, or Ni) at the DFT (B3LYP) level of theory provide theoretical structures in good agreement with the experimental data. Electronic structure calculations predict a similar ordering of the metal-based beta spin frontier MO for the Mn(II) and Co(II) complexes. This particular ordering of the frontier MO leads to a pseudodegenerate ground state for the d(8) Ni(II) ion. The distortion of the C(2) symmetry in [Ni(L(1))](2+) is consistent with a Jahn-Teller effect that removes this pseudodegeneracy. Our electronic structure calculations predict that the binding strength of L(1) should follow the trend Co(II) approximately Mn(II) > Ni(II), in agreement with experimental data obtained from spectrophotometric titrations.     

Development of an extraction method for the determination of zearalenone in corn using less organic solvents

A method for the determination of zearalenone in corn has been developed applying pressurised liquid extraction (PLE) and using environmentally acceptable and less noxious organic solvents. The extracted samples were analysed with liquid chromatography coupled to mass spectrometry (LC-MS) equipped with an electrospray (ESI) ionisation interface. The optimised extraction mixture was isopropanol and an aqueous solution of triethylamine (1%) 50:50 (v/v), which allowed to halve the use of organic solvent compared to the method proposed by ISO. When applying the optimised method to five different naturally contaminated corn samples the obtained concentrations were slightly increased compared to the analysis using the previously used extraction solvent (acetonitrile-methanol). The relative standard deviation (RSD, n = 3) varied between 4 and 10% depending on the concentration level of the target analyte in the test material.     

Designing binuclear transition metal complexes: a new example of the versatility of N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6

N,N'-Bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L) is a versatile receptor able to adapt to the coordinative preferences of different metal cation guests. With first-row transition metal ions, L tends to form binuclear complexes but, depending on the nature of the particular metal ion, the structure of the binuclear complex may be very different. Herein we report a study of the structure and magnetic properties of the corresponding nickel(II) and cobalt(II) complexes. The X-ray crystal structure of the nickel complex (1), with formula [Ni2(L)(CH3CN)4](ClO4)4.CH3CN, shows that this compound presents a symmetric coordination environment with L adopting an anti arrangement. Each Ni(II) ion is six-coordinate in a distorted octahedral environment, and both metal ions are quite far from each other. On the other hand, the X-ray crystal structure of the cobalt complex (2), with formula [Co(L)(mu-OH)Co(CH3CN)](ClO4)3, reveals a rather different structure. Coordination number asymmetry is found: one of the Co(II) is five-coordinate in a distorted trigonal-bipyramidal coordination environment, while the second Co(II) ion is six-coordinate in a distorted octahedral arrangement. Now L adopts a syn arrangement and a hydroxide group acts as a bridge between both cobalt ions. This hydroxo-bridged Co(II) binuclear complex shows structural features that mimic the active site of methionine aminopeptidases. The magnetic properties of 1 and 2 have been investigated in the temperature range 2.0-300 K. Whereas 1 displays a Curie law except for temperatures below 50 K where zero-field splitting of the S= 1 ground state is observed, antiferromagnetic exchange in the singular asymmetric binuclear Co(II) complex 2 has been observed. This magnetic behaviour has been fitted considering first-order spin-orbit coupling in the assumed axially distorted octahedral site and totally quenched orbital contribution in the five-coordinate site in which zero-field splitting of the S= 3/2 ground state is operative.     

Direct determination of silver in geological reference samples after aqua regia leach by graphite furnace atomic absorption spectrometry with matrix modification

Summary Silver was determined in 20 different geological samples by electrothermal AAS with and without (NH₄)₂HPO₄ matrix modifier after aqua regia leach. The results were compared with those obtained after total dissolution of samples.The ashing temperature can be 300 °C higher and the atomising temperature 100°C lower when matrix modifier is used. The sensitivity of the method for Ag was 25% better with modifier. The detection limit, calculated in relation to the original solid sample, was 0.01 g/g with modifier.The results were systematically lower with matrix modifier, the mean value of recovery being 91.2% with modifier and 97.2% without. The ranges of the recovery values were noticeably different: from 84.5 to 99.5 with matrix modifier and from 63.4 to 110.2 without. The matrix modifier tended to even out the differences in the effects of geological matrices.

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Direkte Silberbestimmung in geologischem Referenzmaterial mit Hilfe der Graphitofen-AAS mit Matrixmodifikation (nach Auslaugen mit Königwasser)

     

Trennung des Mono- Di- und Triaethylamins

Ohne Zusammenfassung     

Limits to time variation of fundamental constants from comparisons of atomic frequency standards

Comparisons between frequency standards based on optical and microwave transitionsin atoms and ions currently provide the most stringent laboratory limitson present-day local variation of the fundamental constants.This paper discusses the sensitivity of these frequency standards to variation ofthe electromagnetic and strong interactions, summarises the constraints onvariation of these interactions which can be obtained from currently-available data, and discusses the magnitude of improvements in these limits which is anticipated fromexperiments currently underway.     

Reaction auf Blaus?ure

Ohne Zusammenfassung     

Ueber die Einwirkung des Lichtes auf Chlor- und Bromsilber

Ohne Zusammenfassung     

Ueber die L?slichkeit einiger Silbersalze in L?sungen von citronensaurem Natron

Ohne Zusammenfassung