Über die maßanalytische Bestimmung kleiner Eisenmengen. II

Microchimica Acta - Aus den durchgeführten Versuchen geht hervor, daß sich 0,5 bis 4 mg Eisen(II) in 0,25- bis 1-n Schwefelsäurelösung, die 1 mg Kupfer, Kobalt und Mangan sowie...     

On the role of PIII ligands in the conjugate addition of diorganozinc derivatives to enones

A mechanistic study on the conjugate addition of diethylzinc to cyclohexenone catalyzed by various chiral PIII ligands, provides new insights into its mechanism. Complete in situ conversion of the catalytic amount of Cu(OTf)2 into CuI species by excess ZnEt2 is demonstrated by EPR spectroscopy. Experimental evidence is presented in favor of a critical ternary 1:1:1 complex between enone, Et2Zn and catalyst, supporting a rate-limiting reductive elimination or carbocupration from a preformed mixed Cu/Zn cluster carrying one- or two-ligand molecules. A crystal structure has been obtained for a CuIL2 complex, which shows catalytic turnover on addition of reagent and substrate. NMR spectroscopic analyses and VT experiments reveal that steric hindrance may prevent complexation of a second ligand molecule on the CuI cation, leading to extremely fast precatalysts based on CuX.L species.     

New,functionalised ionic liquids from Michael-type reactions--a chance for combinatorial ionic liquid development

We describe for the first time an alternative and far more efficient method to synthesise functionalised ionic liquids in a simple, straightforward, two-step synthesis.     

Electron density distributions of redox active mixed valence carboxylate bridged trinuclear iron complexes

The electron density distributions (EDD) of the redox active mixed valence trinuclear oxo-centered iron carboxylate, [Fe(3)O(CH(2)ClCOO)(6)(H(2)O)(3)].3H(2)O, 1, and the oxidized form of 1, [Fe(3)O(CH(2)ClCOO)(6)(H(2)O)(2)(CH(2)ClCOO)].1H(2)O, 2, as well as of [Fe(3)O(C(CH(3))(3)COO)(6) (NC(5)H(5))(3)], 3, have been determined from accurate single-crystal X-ray diffraction data measured at 100 K (1, 2) and from extensive synchrotron radiation X-ray diffraction data measured at 28 K (3). Analysis of the EDDs shows that the central oxygen atom has a very different EDD in the mixed valence complexes (1 and 3) compared with the oxidized complex (2). Furthermore, in 1 and 3 the chemical bonds between formally identical trivalent Fe atoms and the central oxygen are fundamentally different. This is in direct contrast to the Fe(IIImicro(3)-O) bonds in the oxidized complex, which are practically identical. Analysis of the d-orbital populations on the metal sites in the three complexes shows that the extra electron density on the Fe(II) site primarily is distributed in a d(yz) orbital (z-axis toward the central oxygen, y-axis perpendicular to the Fe(3)O-plane). Presence of extra charge in the d(yz) orbital correlates with a decrease in the d(xy) population, i.e., with a depletion of charge in the equatorial region of coordination to carboxylate oxygen. The d(xy) charge depletion appears to be of importance for determining the active versus trapped Fe(III) site, and the equatorial ligands therefore have a considerable influence on the ET process. Bader topological analysis of the EDDs corroborates the conclusions drawn from the orbital population analysis, but it also provides additional knowledge about the chemical bonding in the structures. For comparison with the X-ray results, theoretical calculations were carried out for 3 in the experimental geometry. The present information about ET processes in trinuclear oxo-centered iron complexes cannot be deduced from analysis of the molecular structures (i.e., bond lengths and angles), and thus it is demonstrated that X-ray charge density analysis is able to reveal subtle new features of significant physical and chemical importance on complex molecular systems.     

Abrasive stripping voltammetry — an electrochemical solid state spectroscopy of wide applicability

Abrasive stripping voltammetry is a new electroanalytical technique designed for the qualitative and quantitative analysis of solid materials. It is based upon a preliminary mechanical transfer of trace amounts of a solid sample onto the surface of an electrode. All conventional electrochemical measuring methods can be used. The technique is applicable in many fields of solid state analysis and for fundamental studies of the electrochemistry of solid compounds.     

Photochemically induced [4+2]- versus [3+2]-cycloreversion reaction of hexasubstituted 2,3-diaza-bicyclo[3.1.0]hex-2-enes

Hexasubstituted 2,3-diaza-bicyclo[3.1.0]hex-2-enes containing large substituents at C-4 and C-6 upon irradiation undergo a novel [4+2]cycloreversion reaction leading to 2,3-diazahexatriens besides the normal [3+2]cycloreversion.     

Thiocoumarin Caged Nucleotides: Synthetic Access and Their Photophysical Properties

Photocages have been successfully applied in cellular signaling studies for the controlled release of metabolites with high spatio-temporal resolution. Commonly, coumarin photocages are activated by UV light and the quantum yields of uncaging are relatively low, which can limit their applications in vivo. Here, syntheses, the determination of the photophysical properties, and quantum chemical calculations of 7-diethylamino-4-hydroxymethyl-thiocoumarin (thio-DEACM) and caged adenine nucleotides are reported and compared to the widely used 7-diethylamino-4-hydroxymethyl-coumarin (DEACM) caging group. In this comparison, thio-DEACM stands out as a phosphate cage with improved photophysical properties, such as red-shifted absorption and significantly faster photolysis kinetics.