Accessing N‐Stereogenicity through a Double Aza‐Michael Reaction: Mechanistic Insights

Further development of the chemistry and applications of chiral compounds that possess configurationally stable stereogenic nitrogen atoms is hampered by the lack of efficient strategies to access such compounds in an enantiomerically pure form. Esters of propiolic acid and chiral alcohols were evaluated as cheap and readily available Michael acceptors in a diastereoselective synthesis of N‐stereogenic compounds by means of a double aza‐Michael conjugate addition. Diastereomeric ratios of up to 74:26 and high yields were achieved with (?)‐menthyl propiolate as a substrate. Furthermore, a detailed mechanistic investigation was undertaken to shed some light on the course of this domino transformation. Kinetic studies revealed that the protic‐solvent additive acts as a Brønsted acid and activates the ester toward the initial attack of the tetrahydrodiazocine partner. Conversely, acidic conditions proved unfavorable during the final cyclization step that provides the product.     

Fifty Years of Mössbauer Spectroscopy in Solid State Research – Remarkable Achievements,Future Perspectives

Mössbauer spectroscopy was founded more than fifty years ago based on an outstanding discovery by the young German physicist Rudolf Ludwig Mössbauer while working on his Ph.D. thesis. He discovered the recoilless nuclear resonance fluorescence of gamma radiation and was awarded the Nobel Prize in Physics in 1961 as one of the youngest recipients of this most prestigious award. His discovery led to the development of a new technique for measurements of hyperfine interactions between nuclear moments and electromagnetic fields. This method, with highest sharpness of tuning of 10^–13, yields information on valence state, symmetry, magnetic behavior, phase transition, lattice dynamics and other solid state properties.     

Mossbauer Study of the Effect of Intraligand Substituents on the Spin Crossover in Solid (Dithiocyanato)BIS(N-Substituted-Phenyl-2-Pyridinaldimine) Iron (II)

The spin transition of high-spin (^sT₂?)low-spin (¹A₁) of (dithiocyanato) (N-phenyl-2-pyridinaldimine) iron (II) complexes can be altered by substituents on the phenyl ring. Mössbauer spectra at 78K for the 4-substituted derivatives (with the exception of the 4-OH-substituted derivative) indicate that the fraction of low-spin states increases with decreasing substituent electron-withdrawing ability, as measured by the Hammet σ constant (4-OCH₃<4-CH₃CONH 4-C₆H₅<4-CH₃<4-H<4-Cl<4-NO₂). In addition, the effect of methyl-substitution at the ortho-, meta- or paraposition of the phenyl ring on the spin transition was examined. Mössbauer spectra of these methyl-substituted complexes reveal quite different spin equilibria.     

Mössbauer Spectroscopic Studies of Supported α-Fe2O3 and Fe Catalysts I: Particle Size Determination and Support Effect

The chemical state of particles of 5wt% Fe in α-Fe₂O₃ and the subsequently reduced iron particles supported on different particle size (50–200 mesh) of silica (SiO₂), alumina (Al₂O₃), magnesium oxide (MgO) and carbon (C) was examined by Mössbauer spectroscopy at various stages of calcination and reduction. The particle size of the α-Fe₂O₃ supported on different mesh sizes (50, 100, 140, 200 mesh) of SiO₂ has been determined. The strength of metal-support interaction with respect to the kind of support was found to be MgO>SiO₂>Al₂O₃>C.     

Mössbauer Spectroscopic Studies of the Hydroxofluorostannate(IV) Complexes

Mössbauer isomer shifts of ¹¹⁹Sn in a series of complexes K₂Sn(OH)_6-mF_m observed at 78 K were ?0.05, ?0.05, ?0.24, ?0.27 and ?0.40 mm s^?1, respectively, for m=0, 2, 4, 5 and 6. These IS values were linearly related to both m and (the average Pauling electronegativity of the ligands): . The IS straight line was compatible within experimental error with the one reported by Parish and Row-botham for the hexahalogenostannate complexes SnX₄Y₂², namely, The revised IS straight line including both series of complexes could be expressed by the equation Quadrupole splittings observed in the complexes of m = 2,4 and 5 were 1.16, 0.80 and 0.73 mm s^?1 respectively. They were linearly related to both m and .     

The Cycloalkyl Ring Effect on the Spin State of Di(Thiocyanato)Bis(N-Cycloalkyl-Substituted-2-Pyridinaldimine) Iron(II)

A new series of octahedral iron(II) complexes with the composition Fe(II) (N-R-2-pyridinaldimine)₂(NCS)₂, where R=cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl, have been synthesized and the spin states of the iron atom have been studied by means of Mössbauer spectroscopy and magnetic measurement.     

Mössbauer Spectroscopic Studies of Supported α-Fe2O3 and Fe Catalysts II: The Effect of the Second Metal Addition

The effect of the addition of a second metals such as Zn and Ni on the calcinaton and reduction of alumina, magnesia and silicasupported iron catalysis with total iron loading of 5wt% is investigated by Mössbauer spectroscopy. It is shown that the reducibility of supported α-Fe₂O₃ is gradually increased by adding the second metal. The values of the magnetic hyperfine field obtained from Mössbauer spectra for the Zn or Ni-added α-Fe₂O₃ or Fe catalysts decreased with increasing second metal loading.     

Mössbauer Spectroscopic Studies of Ferrocenylaldimines and their TCNQ Complexes

A series of ferrocenylaldimines (Fc-CH=N-R) has been prepared by Schiff base condensation of formylferrocene with polyacenylamines RNH₂ where R = naphthyl, 5,6,7,8-tetrahydronaphthyl, anthracenyl, and pyrenyl groups. This step was followed by oxidation with TCNQ to give [Fc-CH = N-R][TCNQ]² salts. The electronic state of iron in these compounds was investigated by means of⁵⁷Fe Mössbauer spectroscopy.     

A Doubly Zwitterionic Antiaromatic [28]Hexaphyrin Formed upon Deprotonation of 5,20‐Di(N‐methyl‐4‐pyridinium)‐Substituted [28]Hexaphyrin

A rectangular 5,20‐di(4‐pyridyl) [26]hexaphyrin was reduced with NaBH₄ to give the corresponding twisted Möbius aromatic [28]hexaphyrin. Subsequent double N‐methylation gave a dicationic 5,20‐di(N‐methyl‐4‐pyridinium) [28]hexaphyrin, which was converted to a doubly zwitterionic and Hückel antiaromatic [28]hexaphyrin upon deprotonation with sodium methoxide.     

Electrodeposition and Mössbauer Spectra of Fe-Zn Alloys on Aluminum Foil

By controlling the current density, pH and temperature of the electrodepositing iron-zinc bath, various compositions of electrodeposited Fe_1-xZn_x (0.95 > x > 0.15) alloys on Al-foil have been prepared. The electronic state, intermetallic phases and magnetic hyperfine interactions have been studied by means of ⁵⁷Fe Mössbauer spectroscopy at 298K for these electrodeposited Fe-Zn alloys. The isomer shifts obtained from the Mössbauer spectral of the alloys increased with increasing Zn content. An alloy containing above 50% Fe, exhibits a linear dependence of the internal magnetic field, Hin, on the Zn content.     

161Dy Time‐Domain Synchrotron Mössbauer Spectroscopy for Investigating Single‐Molecule Magnets Incorporating Dy Ions

Time‐domain synchrotron Mössbauer spectroscopy (SMS) based on the Mössbauer effect of ¹⁶¹Dy has been used to investigate the magnetic properties of a Dy^III‐based single‐molecule magnet (SMM). The magnetic hyperfine field of [Dy(Cy₃PO)₂(H₂O)₅]Br₃?2 (Cy₃PO)?2 H₂O?2 EtOH is with B₀=582.3(5) T significantly larger than that of the free‐ion Dy^III with a ⁶H_15/2 ground state. This difference is attributed to the influence of the coordinating ligands on the Fermi contact interaction between the s and 4f electrons of the Dy^III ion. This study demonstrates that ¹⁶¹Dy SMS is an effective local probe of the influence of the coordinating ligands on the magnetic structure of Dy‐containing compounds.