Reinhard Stößer

A Dinuclear Nickel(I) Dinitrogen Complex and its Reduction in Single‐Electron Steps

Electron by electron: Beta-diketiminato nickel(I) complex fragments are capable of activating N(2) through coordination. The resulting complex can be reduced in two single-electron steps, which further activates the N-N bond. The picture shows the structure of the singly reduced complex with mu-eta(1):eta(1)-bound N(2).

Photoluminescence of atomic gold and silver particles in soda-lime silicate glasses

We report the chemistry and photophysics of atomic gold and silver particles in inorganic glasses. By synchrotron irradiation of gold-doped soda-lime silicate glasses we could create and identify unambiguously the gold dimer as a stable and bright luminescing particle embedded in the glassy matrix. The gold dimer spectra coincide perfectly with rare gas matrix spectra of Au(2). The glass matrix is, however, stable for years, and is hence perfectly suited for various applications. If the irradiated gold-doped sample is annealed at 550 degrees C a bright green luminescence can be recognized. Intense 337 nm excitation induces a decrease of the green luminescence and the reappearance of the 753 nm Au(2) emission, indicating a strong interrelationship between both luminescence centers. Time-dependent density functional theory (TD-DFT) calculations indicate that the green luminescence can be assigned to noble metal dimers bound to silanolate centers. These complexes are recognized as the first stages in the further cluster growth process, which has been investigated with small-angle x-ray scattering (SAXS). In silver-doped glasses, Ag(0) atoms can be identified with electron paramagnetic resonance (EPR) spectroscopy after synchrotron activation. Annealing at 300 degrees C decreases the concentration of Ag(1), but induces an intense white light emission with 337 nm excitation. The white luminescence can be decomposed into bands that are attributed to small silver clusters such as Ag(2), Ag(3) and Ag(4), and an additional band matching the green emission of gold-doped glasses.

In-situ electron spin resonance study of vanadium phosphate catalysts during the selective oxidation of n-butane to maleic anhydride

Abstract Differently disordered (VO) 2 P 2 O 7 phases and an amorphous V 3+ PO catalyst were studied by electron spin resonance (ESR) spectroscopy at various temperatures and in various atmospheres, as well as under catalytic conditions, using an in-situ ESR flow reactor operating in the X band. Exchange integrals and the second and fourth moments of the ESR signals were used to characterize spin-spin exchange in (VO) 2 P 2 O 7 , which depends on the degree of structural disorder. The catalytic activity of (VO) 2 P 2 O 7 and the maleic anhydride selectivity are enhanced with rising disorder. During butane oxidation a significant reversible alteration of the ESR line shape became evident. This unique effect can only be observed under working conditions and is discussed in terms of a perturbation of exchange interactions between neighbouring vanadyl centres near the surface, the oxidation state of which is assumed to fluctuate between +4 and +5 during the catalytic reaction. In contrast, heating in pure air improves the spin-spin exchange and lowers the disorder by healing lattice defects. The activity of the V 3+ PO catalyst in the title reaction “awoke” after about 20 min time on stream, simultaneous with the formation of a (VO) 2 P 2 O 7 -like local arrangement of V 4+ centres on the catalyst surface.

Four‐Coordinate Trispyrazolylboratomanganese and ‐iron Complexes with a Pyrazolato Co‐ligand: Syntheses and Properties as Oxidation Catalysts

A series of complexes of the type [(Tp(R1,R2))M(X)] (Tp = trispyrazolylborato) with R(1)/R(2) combinations Me/tBu, Ph/Me, iPr/iPr, Me/Me and for M = Mn or Fe coordinating [Pz(Me,tBu)](-) (Pz = pyrazolato) or Cl(-) as co-ligand X has been synthesised. Although the chloride complexes were very unreactive and stable in air, the pyrazolato series was far more reactive in contact with oxidants like O(2) and tBuOOH. The [(Tp(R1,R2))M(Pz(Me,tBu))] complexes proved to be active pre-catalysts for the oxidation of cyclohexene with tBuOOH, reaching turnover frequencies (TOFs) ranging between moderate and good in comparison to other manganese catalysts. Cyclohexene-3-one and cyclohexene-3-ol were always found to represent the main products, with cyclohexene oxide occasionally formed as a side product. The ratios of the different oxidation products varied with the reaction conditions: in the case of a peroxide/alkene ratio of 4:1, considerably more ketone than alcohol was obtained and cyclohexene oxide formation was almost negligible, whereas a ratio of 1:10 led to a significant increase of the alcohol proportion and to the formation of at least small amounts of the epoxide. Pre-treatment of the dissolved [(Tp(R1,R2))M(Pz(Me,tBu))] pre-catalysts with O(2) led to product distributions and TOFs that were very similar to those found in the absence of O(2), so that it may be argued that tBuOOH and O(2) both lead to the same active species. The results of EPR spectroscopy and ESI-MS suggest that the initial product of the reaction of [(Tp(Me,Me))Mn(Pz(Me,tBu))] with O(2) contains a Mn(III)(O)(2)Mn(IV) core. Prolonged exposure to O(2) leads to a different dinuclear complex containing three O-bridges and resulting in different TOFs/product distributions. Analogous findings were made for other complexes and formation of these overoxidised products may explain the deviation of the catalytic performances if the reactions are carried out in an O(2) atmosphere.

Direct and continuous determination of pH values in nontransparent w/o systems by means of EPR spectroscopy

Abstract The change of the pH value induced by the decomposition of hydrolyzable drugs with ester structure was monitored directly and continuously in nontransparent w/o systems by means of EPR spectroscopy using the pH sensitivity of the spectral parameters of imidazoline-derived spin probes. This nondestructive method makes it possible to perform measurements in nontransparent w/o systems without the need of any preparatory steps. While the hydrolysis of acetylsalicylic acid caused a rapid decrease of the pH value during 5 h at room temperature (25°C) within the emulsion, benzocain exhibited a relatively higher stability.

In vivo ESR studies on subcutaneously injected multilamellar liposomes in living mice

AbstractPurpose. An innovative, noninvasive, low-frequency electron spin resonance (ESR) spectroscopy method was applied and adapted to investigate the integrity of multilamellar liposomes from hydrogenated phospholipids after subcutaneous injection in living mice. Moreover, the fate of the injected liposomal preparations was examined, as well as the possibility to achieve a depot effect. Methods. Highly concentrated solutions of the spin probe 2,2,6,6-tetramethyl-4-trimethylammoniumpiperidine-1-oxyl-iodide (CAT-1; 138 mM) were encapsulated in liposomes. They were characterized by laser diffraction, and the liberation of spin probe was investigated by ESR spectroscopy. Results. Line shape changes allowed the differentiation between encapsulated and released CAT-1 after subcutaneous injection of liposomes. Multilamellar liposomes form a local depot at the site of injection. A sustained release of the spin probe from the depot was monitored by means of ESR. Whereas 40% of the spin probe was released within the first 96 h after administration, 60% remained in intact liposomes under the skin. No depot formation could be observed after injection of CAT-1 solutions, but a fast signal decrease due to systemic distribution and bioreduction of the nitroxide spin probe. Conclusions. Noninvasive analysis of liposomal integrity in living animals was successfully accomplished using a new L-Band ESR spectroscopy method. The liberation of CAT-1 from liposomes in vitro and in vivo was monitored by changes in the lineshape of ESR spectra and Heisenberg spin exchange. The significance of liposomal integrity for the formation of a localized drug depot effect was proved.

Copper–oxygen coordination in CuII-heteropolyanion compounds: electron paramagnetic resonance studies and CNDO/2 calculations

The coordination of CuII ions in Anderson and Keggin type heteropolyanion compounds in their polycrystalline and corresponding magnetically diluted forms {(NH4)4[Zn(OH)6Mo6O18]·nH2O/Cu2+; ((CH3)4N)6[ZnO4W12O36]·nH2O/Cu2+} have been investigated by means of chemical, spectroscopic (EPR, IR), diffraction (X-ray) and quantum-mechanical methods. Simple model compounds with known Cu—O coordination have been used for comparison. In both the Anderson ion [Cu(OH)6Mo6O18]4– and the Keggin ion [SiO4W11O30CuO5(OH2)]6– the copper coordination polyhedra consist of six oxygen atoms, forming a distorted octahedron. As expected, the magnetically diluted Anderson compound exhibits a pronounced dynamic Jahn–Teller effect.No arguments for the existence of a Keggin species with CuII ions on the central tetrahedral site [CuO4W12O36]6– could be derived from the experimental and theoretical investigations. The Cu2+ ions incorporated into the products resulting from different attempts to synthesize the Keggin structure with a central copper heteroatom appear to be [Cu(OH2)6]2+ units. Therefore, the calculations of selected molecular subunits for the assumed Keggin compounds did not yield the experimental EPR parameters.The g tensor calculations in the CNDO/2 approximation are consistent with the EPR spectra of the Cu Anderson compound.

Electron paramagnetic resonance and mass spectrometry: Useful tools to detect ultraviolet light induced skin lesions on a molecular basis – A short review

Ultraviolet radiation is considered responsible for sunburning, premature skin aging, and cancerogenesis through the production of free radical species. Therefore, the favoured possibility for direct detection of unpaired electrons - electron para- magnetic resonance spectroscopy - is predestinated for detection and structural and dynamic analysis of this kind of molecules. However, many of UV induced radicals in skin have a short lifetime at ambient conditions and possibilities for stabilisation or transformation into definite para- or diamagnetic products have to be found. On the other hand, diamagnetic products, po- tentially also originated by reporter molecules, which are not detectable by EPR, are target molecules for mass spectrometric analysis. In this review, potentials and limitations of both spectroscopic methods are reviewed, and the effect of ultraviolet radiation on human skin is discussed in particular. Suitable combinations of both techniques result in detailed information about photoproducts and processes taking place within skin during and after irradiation. The literature is viewed from a recent perspective; historical aspects were not in the scope of this paper.

Low-temperature photochemistry of nitro-substituted aromatic azides; subsequent reactions of intermediates

Abstract Triplet 4-nitrophenylnitrene ( 1B ) and triplet 4-nitrene-4′-nitrostilbene ( 2B ) are characterized by their low-temperature UV-Vis and ESR spectra as well as by their consecutive thermal and photochemical reactions. The main thermal reaction of 1B and 2B between 77 and 90 K is a stepwise, twofold hydrogen abstraction from the methyltetrahydrofuran (2-MTHF) matrix solvent, leading to the corresponding primary amines. The activation energy of the first hydrogen abstraction of 1B and 2B is not influenced by their structure and is about 30 kJ mol −1 . A similar amount of energy is required for the second hydrogen abstraction. Since the activation energy required for diffusion in a solid 2-MTHF matrix is about 87 kJ mol −1 , bimolecular processes that require diffusion are less efficient between 77 and 87 K. While 2B is photochemically stable, 1B reacts after optical excitation under hydrogen abstraction from the solvent to form an imino radical 1C . The photochemically induced hydrogen abstraction after the excitation of 1B and the absence of the analogous reaction after the excitation of 2B can be explained by means of two models; either as a hot ground state reaction or as a reaction starting from an electronically excited state of 1B . In the latter model, it is assumed that the activation energy of the hydrogen abstraction is lower in the first excited state than in the electronic ground state of 1B . Both models predict that the hydrogen abstraction is more efficient if the difference in energy between the electronic ground and electronically excited states is high enough. This is the case in 1B but not in 2B . The experimental results are discussed in connection with theoretical statements arising from quantum chemical calculations.

Comparison of malondialdehyde and hydrogen peroxide modified CuZnSOD by EPR spectroscopy.

CuZn superoxide dismutase (CuZnSOD) contributes to the regulation of the steady-state concentration of reactive oxygen species in cells and minimizes pathological consequences induced by these reactive oxygen species. During the exaggerated formation of reactive oxygen species, often resulting from an activation of phagocytotic cells, CuZnSOD is administered with a therapeutic purpose. But inhibition of the endogeneous or administered CuZnSOD by products generated during the process of formation of reactive oxygen species (H2O2, HOCl, · OH, products of lipid peroxidation) might intensify cell damage. In this study, we compared the influence of malondiadehyde (MDA, high reactive molecule formed in lipid peroxidation) and H2O2 (known to inhibit the CuZnSOD) on bovine CuZnSOD. MDA reacted with CuZnSOD. The reaction was found to be both time- and concentration-dependent, which was demonstrated by the formation of fluorophors. EPR spectroscopy revealed that this reaction had no influence on the activity of CuZnSOD since the catalytic centre of the CuZnSOD was not effected by MDA. In contrast, H2O2 modified the catalytic centre which caused an activity decrease.