R. Stösser

A Reduced β-Diketiminato-Ligated Ni3H4 Unit Catalyzing H/D Exchange

An investigation concerning the stepwise reduction of the β-diketiminato nickel(II) hydride dimer [LNi(μ-H)(2)NiL], 1 (L = [HC(CMeNC(6)H(3)(iPr)(2))(2)](-)), has been carried out. While the reaction with one equivalent of potassium graphite, KC(8), led to the mixed valent Ni(I)/Ni(II) complex K[LNi(μ-H)(2)NiL], 3, treatment of 1 with two equivalents of KC(8) surprisingly yielded in the trinuclear complex K(2)[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 4, in good yields. The Ni(3)H(4) core contains one Ni(II) and two Ni(I) centers, which are antiferromagnetically coupled so that a singlet ground state results. 4 represents the first structurally characterized molecular compound with three nickel atoms bridged by hydride ligands, and it shows a very interesting chemical behavior: Single-electron oxidation yields in the Ni(II)(2)Ni(I) compound K[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 5, and treatment with CO leads to the elimination of H(2) with formation of the carbonyl complex K(2)[LNi(CO)](2), 6. Beyond that, it could be shown that 4 undergoes H/D exchange with deuterated solvents and the deuteride-compound 4-D(4) reacts with H(2) to give back 4. The crystal structures of the novel compounds 3-6 have been determined, and their electronic structures have been investigated by EPR and NMR spectroscopy, magnetic measurements, and DFT calculations.

Local structural orders in nanostructured Al2O3 prepared by high-energy ball milling

Nanostructured Al2O3 powders were prepared by high-energy ball milling of corundum. Both the solid state nuclear magnetic resonance spectra of the Al3+ ions and the solid state electron paramagnetic resonance spectra of incorporated Fe3+ ions are governed by noticeable spectral changes dependent on the duration of the mechanical treatment. The quadrupolar parameters of the 27Al nuclei and the zero-field splitting parameters of the Fe3+ ions as well as their statistical distributions were determined as functions of the milling time. Structural changes of the Al2O3 matrix were also followed by powder x-ray diffraction and transmission electron microscopy measurements. Direct relations between the structural disorder as obtained by x-ray data and the spin Hamiltonian parameters of both ions could be established. These results suggest that the milled powders consist of nanocrystalline grains embedded in amorphous grain boundaries even for the longest milling time. The grains can be described in terms of ordered AlO6-octahedra as in the starting crystalline material exhibiting a slight rhombic distortion. The grain boundaries look like random arrangements of these octahedral units. The specific behaviour of the environment of the Fe3+ paramagnetic probe points out that such a point defect acts as an activation centre of the amorphization process.

Influence of irradiation sterilization on a semi-solid poly(ortho ester)

Viscous poly(ortho ester) (POE), a promising polymer for controlled release is being investigated as an injectable drug delivery system for peptides, for antiproliferative agents after glaucoma filtering surgery and for antibiotics in the treatment of periodontitis. Due to the chemical lability of POE, the strategies for obtaining a sterile product are limited to aseptic processing and terminal sterilization using high energy radiation. In the first part of the present investigation, we used electron-paramagnetic-resonance (EPR) spectroscopy to evaluate radical formation and radical-induced polymer degradation after irradiation treatment. Due to the viscous nature of POE, radicals were only found at low temperatures or by using the method of ‘spin-trapping’. Several radical species could be distinguished by a variation of the microwave power and the differences of the thermal stability of the radicals. The incorporation of 5-fluorouracil accelerates the degradation of the polymer. In the second part, we have compared the effects of the two commonly applied methods for irradiation sterilization (i.e. gamma and beta rays) on POE and on POE with incorporated 5-fluorouracil and compared these methods to aseptically prepared devices. In addition, we have checked the possibility of preventing radical-induced degradation using two different protecting agents: α-tocopherol at a concentration of 0.1% (w/w) and sterilization under nitrogen monoxide. The weight and number average molecular weight of POE decreased drastically after irradiation treatment and subsequent to irradiation, an accelerated degradation was observed. Generally it was found that higher molecular weight polymers are more affected and that gamma irradiation leads to more degradation than beta treatment. Also, the addition of protecting agents did not significantly prevent polymer degradation. Therefore, we have concluded that irradiation sterilization of POE is not a viable process and aseptic preparation is preferred. Without sterilization POE is stable for about 1 year when kept as monodoses at low temperatures.

Electron paramagnetic resonance investigations of α-Al2O3 powders doped with Fe3+ ions: experiments and simulations

Electron paramagnetic resonance (EPR) of Fe3+ ions in Al2O3 is studied in powder samples prepared by different routes and/or modified by thermal or mechanical treatments, with different doping levels and grain sizes. The measurements are performed in various frequency bands (S, X, K, Q and W) and with bimodal detection in X-band. Simulations of the spectra are achieved with a code designed for computing EPR powder spectra described by any spin Hamiltonian including second-, fourth-and sixth-order ZFS terms (S ≤ 7/2). The linewidths, intensities and lineshapes are accounted for. The lineshape is Gaussian at low Fe3+ concentration whereas it is Lorentzian for higher concentration. The linewidths are interpreted as the superimposition of three main contributions: intrinsic linewidth, dipolar broadening and broadening due to lattice imperfections. The latter is tentatively interpreted in terms of quadrupolar spin Hamiltonian parameter distributions treated using first-order perturbation theory. Whatever the sample, only the b22 spin Hamiltonian parameter is found to be distributed around a mean zero value which corresponds to rhombic distortions. Angle and bond length distributions are tentatively extracted from the b22 distributions which gives some insight into the local order around the spin probe in relation to the preparation and treatment of the samples.

Modelling of multifrequency ESR spectra of Fe3+ ions in crystalline and amorphous materials: A Simplified approach to determine statistical distributions of spin-spin coupling parameters

Focussed on the understanding of the complex Fe3+ electron spin resonance (ESR) spectra in crystalline and amorphous systems, the simulation program ESR-MAKRO-1 was developed, which allows the calculation of powder spectra of paramagnetic centers with 1/2 ≤S ≤7/2. For the reproduction of the ESR spectra of amorphous systems Gaussian distributions of the zero-field-splitting parameters of second and fourth order (D, E, a, F) were implemented into the program. Applications of ESR-MAKRO-l are given both for crystalline powders and for glassy systems. A stepwise analysis of the ESR spectra, allowing an approach to Fe3+ ESR spectra of amorphous matrices and showing the influence ofD, E, a, andF as well as their distributions, is given.

ESR evidence of order/disorder transitions in solids caused by dehydration

ESR spectroscopy was used to study order/disorder phenomena in paramagnetically doped solids. The dehydration process of aluminium fluoride and oxide hydrates and aluminosilicates was examined by ESRin situ measurements as well as using thermoanalytically quenched samples. Two types of amorphous states of solids have been studied: inorganic glasses in the conventional sense and amorphous states of solids which have not passed the liquid state. The complete ESR fine structure pattern of Fe3+ ions is susceptible with respect to order/disorder processes of the actual host matrix. Dehydration processes bear some model character in this field. A quantitative description of the disorder effects on the Fe3+ ESR could be given on the basis of a complete spin Hamiltonian using distributions of the fourth order spin coupling parameters.

Desilylation by ether cleavage

Abstract The electron-transfer-sensitized photolysis of substituted phenyl-(trimethylsilyl)methyl ethers was studied. N-Methylquinolinium perchlorate, p-tolyltropylium perchlorate and 9,10-dicyanoanthracene were used as electron acceptors. The main reaction of the stabilization of the radical cation of the ether (the intermediate formed by single electron transfer to the singlet-excited acceptor) is CO bond cleavage. With 9,10-dicyano-anthracene as acceptor 100% phenol is formed. The competing CSi bond cleavage is a minor reaction in the case of the quinolinium salt. This leads to addition of the ether fragment and the acceptor. The photoexcited tropylium salt does not induce any reaction of the added ether. The quantum efficiencies of ether cleavage are relatively high and depend on the donor capacity of the substituent in the phenyl ring. This is explained by competition between back electron transfer and bond cleavage in the radical pair formed. Electron paramagnetic resonance (EPR) was used to detect the intermediate radicals and flash photolysis to detect the anion radicals.

In Situ EPR Study of Chemical Reactions in Q-Band at Higher Temperatures: A Challenge for Elucidating Structure-Reactivity Relationships in Catalysis

For the first time, heterogeneous catalytic reactions have been monitored by in situ EPR spectroscopy in the Q-band using a homemade heatable probe head equipped with a flow reactor. The reactions of Al(2)O(3)-supported TEMPO with NO and H(2) as well as of SiO(2)/Al(2)O(3)-supported H(4)PVMo(11)O(40) with methanol and formaldehyde were studied up to 400 degrees C. TEMPO radicals are immobilized on the support in positions which impose a different reactivity to NO and H(2). This may be due to different accessibility, which changes during thermal treatment. By combined evaluation of anisotropic X- and Q-band spectra with a complex hyperfine structure (e.g., from VO(2+)), spin Hamiltonian parameters can be derived with higher precision, since limits of the specific resolution in both frequency bands are compensated for. In addition to VO(2+), Mo(5+) is formed above 180 degrees C depending on the O(2) content of the feed, which is easily discriminated in the Q-band but not in the X-band.

Structural phase transition in AlF3: An EPR study of doped powder samples

Searching for a microscopic description of structural phase transitions in AlF3, these transitions were followed by electron paramagnetic resonance (EPR) in situ measurements in X-band in the temperature range 290 K ≤T ≤ 800 K with Cr3+ — and Fe3+-doped powder samples of α-AlF3. For each temperature the zero-field splitting (zfs) parameters of the paramagnetic transition metal ions were determined by simulation. Starting from the axial zfs parametersb20(T) and bearing in mind the trigonal symmetry, the application of the superposition model allowed the calculation of the angle θ(T) (θ: <(c-axis, Al-F)) withb20(T) = 3b2(R)(3cos2θ(T) − 1) (c-axis ∥ 3-fold axis). Direct relations between θ and the rotational angle ω of the matrix in dependence on temperature could be obtained by geometrical considerations. Local displacements of the anions were taken into account in terms of the parameterx characterizing their lattice position in the hexagonal cell of aluminium fluoride.

Atomic hydrogen as spin probe in thermally and mechanically activated materials

Atomic hydrogen H˙ could be stabilized at T  293 K in specially prepared fluoride and oxide compounds of aluminium: {AlF3:OH} and {AlOOHx}. The formation of atomic hydrogen was accomplished using irradiation including γ rays, X rays and UV radiation, and even sunlight, that is those wavelengths in the region of λ ⩽ 254 nm. The hyperfine coupling constant of H˙@AlF3 could be precisely determined by using H˙@MeT8 as a standard to calibrate the B0 field. In the samples mentioned above H˙ acts a spin probe in two ways: (i) that it can be trapped in cages and stabilized with Pauli repulsion indicates sensitively and unambiguously that there are local host nano-cages present in the matrices and, moreover, (ii) that it acts as a spin probe for determining the matrix state of order with its spin dynamics as well as with the kinetic parameters of detrapping.