Bismuth radical catalysis in the activation and coupling of redox-active electrophiles

Radical cross-coupling reactions represent a revolutionary tool to forge C(sp3)–C and C(sp3)–heteroatom bonds, by means of transition metals, photoredox or electrochemical approaches. This study demonstrates how a low-valent bismuth complex is able to undergo one-electron oxidative addition with redox-active alkyl radical precursors in an autonomous manner, mimicking the behavior of first-row transition metals. This reactivity paradigm for bismuth gives rise to unique radical-equilibrium complexes, which could be fully characterized in solution and solid state. The resulting Bi(III)–C(sp3) intermediates display divergent reactivity patterns depending on the α-substituents of the alkyl fragment. Mechanistic investigations on this reactivity led to the development of a bismuth-catalyzed C(sp3)–N cross-coupling reaction that operates under mild conditions and accommodates synthetically relevant N-heterocycles as coupling partners

Multipurpose High Frequency Electron Spin Resonance Spectrometer for Condensed Matter Research

We describe a quasi-optical multifrequency ESR spectrometer operating in the 75-225 GHz range and optimized at 210 GHz for general use in condensed matter physics, chemistry and biology. The quasi-optical bridge detects the change of mm wave polarization at the ESR. A controllable reference arm maintains a mm wave bias at the detector. The attained sensitivity of 2x10^10 spin/G/(Hz)1/2, measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4 GHz and 300 K, is comparable to commercial high sensitive X band spectrometers. The spectrometer has a Fabry-Perot resonator based probe head to measure aqueous solutions, and a probe head to measure magnetic field angular dependence of single crystals. The spectrometer is robust and easy to use and may be operated by undergraduate students. Its performance is demonstrated by examples from various fields of condensed matter physics.Comment: submitted to Journal of Magnetic Resonanc

Redox and Chemical Activities of the Hemes in the Sulfur Oxidation Pathway Enzyme SoxAX

BACKGROUND: SoxAX enzymes initiate microbial oxidation of reduced inorganic sulfur compounds. Their catalytic mechanism is unknown. RESULTS: Cyanide displaces the CysS(-) ligand to the active site heme following reduction by S(2)O(4)(2-) but not Eu(II). CONCLUSION: An active site heme ligand becomes labile on exposure to substrate analogs. SIGNIFICANCE: Elucidation of SoxAX mechanism is necessary to understand a widespread pathway for sulfur compound oxidation. SoxAX enzymes couple disulfide bond formation to the reduction of cytochrome c in the first step of the phylogenetically widespread Sox microbial sulfur oxidation pathway. Rhodovulum sulfidophilum SoxAX contains three hemes. An electrochemical cell compatible with magnetic circular dichroism at near infrared wavelengths has been developed to resolve redox and chemical properties of the SoxAX hemes. In combination with potentiometric titrations monitored by electronic absorbance and EPR, this method defines midpoint potentials (E(m)) at pH 7.0 of approximately +210, -340, and -400 mV for the His/Met, His/Cys(-), and active site His/CysS(-)-ligated heme, respectively. Exposing SoxAX to S(2)O(4)(2-), a substrate analog with E(m) ~-450 mV, but not Eu(II) complexed with diethylene triamine pentaacetic acid (E(m) ~-1140 mV), allows cyanide to displace the cysteine persulfide (CysS(-)) ligand to the active site heme. This provides the first evidence for the dissociation of CysS(-) that has been proposed as a key event in SoxAX catalysis

Radical Activation of N–H and O–H Bonds at Bismuth(II)

The development of unconventional strategies for the activation of ammonia (NH3) and water (H2O) is of capital importance for the advancement of sustainable chemical strategies. Herein we provide the synthesis and characterization of a radical equilibrium complex based on bismuth featuring an extremely weak Bi–O bond, which permits the in situ generation of reactive Bi(II) species. The ensuing organobismuth(II) engages with various amines and alcohols and exerts an unprecedented effect onto the X–H bond, leading to low BDFEX–H. As a result, radical activation of various N–H and O–H bonds─including ammonia and water─occurs in seconds at room temperature, delivering well-defined Bi(III)-amido and -alkoxy complexes. Moreover, we demonstrate that the resulting Bi(III)–N complexes engage in a unique reactivity pattern with the triad of H+, H–, and H• sources, thus providing alternative pathways for main group chemistry

An improved coupling design for high-frequency TE011 electron paramagnetic resonance cavities

In high-frequency electron paramagnetic resonance (EPR) spectroscopy the sample is usually accommodated in a single-mode cylindrical TE011 microwave cavity. This cavity stands out in terms of flexibility for various types of EPR experiments due to convenient control of its resonance frequency and easy waveguide-to-cavity microwave coupling. In continuous wave and in pulsed EPR it is, however, essential to be able to vary the coupling efficiency over a large range. We present a new mechanical design to vary the microwave coupling to the cavity using a movable metal sphere. This coupling sphere is shifted in the plane of the iris wall inside the coupling waveguide. The design allows for a compact and robust construction of the EPR probehead that can be easily accommodated inside a limited space of helium flow cryostat. The construction details and characterization of the coupling element for 95 GHz (W-band) EPR as well as for 34 GHz (Q-band) are presented. © 2013 American Institute of Physics

Spectroscopic investigations of a semi-synthetic [FeFe] hydrogenase with propane di-selenol as bridging ligand in the binuclear subsite: comparison to the wild type and propane di-thiol variants

[FeFe] Hydrogenases catalyze the reversible conversion of H2 into electrons and protons. Their catalytic site, the H-cluster, contains a generic [4Fe–4S]H cluster coupled to a [2Fe]H subsite [Fe2(ADT)(CO)3(CN)2]2−, ADT = µ(SCH2)2NH. Heterologously expressed [FeFe] hydrogenases (apo-hydrogenase) lack the [2Fe]H unit, but this can be incorporated through artificial maturation with a synthetic precursor [Fe2(ADT)(CO)4(CN)2]2−. Maturation with a [2Fe] complex in which the essential ADT amine moiety has been replaced by CH2 (PDT = propane-dithiolate) results in a low activity enzyme with structural and spectroscopic properties similar to those of the native enzyme, but with simplified redox behavior. Here, we study the effect of sulfur-to-selenium (S-to-Se) substitution in the bridging PDT ligand incorporated in the [FeFe] hydrogenase HydA1 from Chlamydomonas reinhardtii using magnetic resonance (EPR, NMR), FTIR and spectroelectrochemistry. The resulting HydA1-PDSe enzyme shows the same redox behavior as the parent HydA1-PDT. In addition, a state is observed in which extraneous CO is bound to the open coordination site of the [2Fe]H unit. This state was previously observed only in the native enzyme HydA1-ADT and not in HydA1-PDT. The spectroscopic features and redox behavior of HydA1-PDSe, resulting from maturation with [Fe2(PDSe)(CO)4(CN)2]2−, are discussed in terms of spin and charge density shifts and provide interesting insight into the electronic structure of the H-cluster. We also studied the effect of S-to-Se substitution in the [4Fe–4S] subcluster. The reduced form of HydA1 containing only the [4Fe–4Se]H cluster shows a characteristic S = 7/2 spin state which converts back into the S = 1/2 spin state upon maturation with a [2Fe]–PDT/ADT complex

Essentialism in social representations of citizenship: an analysis of Greeks’ and migrants’ discourse

Following a Social Representations approach, the article examines the representations of citizenship held by both migrants and Greek citizens in Greece after the announcement of a heavily debated citizenship legislation. Essentialism, a way of representing social categories as holding an underlying essence that determines their characteristics, was used as an analytical tool to understand the inclusive or exclusive function of representations of citizenship towards migrants. Findings showed that Greeks construct representations based on ethnic, civic, and cultural ideas, while migrants construct representation of citizenship based on civic and cultural ideas. Essentialism was a way of constructing ethnic and cultural representations of citizenship and functioned in both exclusive and inclusive ways, but assimilatory terms accordingly. Civic and cultural representations of citizenship were constructed in nonessentialist ways and functioned in inclusive ways. However, from Greeks' perspective, civic inclusion was conditioned upon an often-questioned legality of migrants and upon cultural assimilation terms. Studying both the content and the essentialist/nonessentialist formulation of representations of citizenship is an important tool in understanding the politics of inclusion and exclusion of citizens in the social arena

Impact of disorder on formation of free radicals by gamma-irradiation: Multi-frequency EPR studies of trehalose polymorphs

Electron paramagnetic resonance (EPR) studies of the radiation-induced radicals in two anhydrous trehalose polymorphs, beta- crystalline (TREc) and glassy (TREg), were conducted with the aim to resolve whether different types of free radicals are induced in a differently disordered environment. A multifrequency approach (9.5 GHz, 94 GHz, and 244 GHz) was applied to improve the resolution of the complex EPR spectra. In addition, the thermal stability of the EPR spectra and the respective decay kinetics were analyzed in a series of thermal annealing studies in the temperature interval from 333 K to 363 K. It was found that in the crystalline matrix the transformation process of the induced radicals is more complex than in the glassy host matrix. Qualitative decomposition of the experimental spectra, assuming four contributing species, reproduced characteristic EPR spectral features in both matrices. These were interpreted as carbon-centered radicals while the possibility of the formation of alkoxy radicals due to the abstraction of a hydrogen atom could be ruled out. Only in one case, the tentative assignment of the EPR spectral components revealed the formation of the same radical species in both TREc and TREg. Furthermore, by thermal annealing TREg lost one of the radical species, whereas in TREc all 4 radical species pertained irrespective of the treatment. The results presented here, therefore, provide experimental evidence that the extent of disorder present in the material strongly affects the type and stability of radicals induced by ionizing radiation