Not Guilty on Every Count: The “Non‐Innocent” Nitrosyl Ligand in the Framework of IUPAC′s Oxidation‐State Formalism

Nitrosyl–metal bonding relies on the two interactions between the pair of N–O‐π* and two of the metal's d orbitals. These (back)bonds are largely covalent, which makes their allocation in the course of an oxidation‐state determination ambiguous. However, apart from M‐N‐O‐angle or net‐charge considerations, IUPAC′s “ionic approximation” is a useful tool to reliably classify nitrosyl metal complexes in an orbital‐centered approach.     

d–d Dative Bonding Between Iron and the Alkaline‐Earth Metals Calcium,Strontium, and Barium

Double deprotonation of the diamine 1,1′‐(tBuCH₂NH)‐ferrocene ( 1 ‐H₂) by alkaline‐earth (Ae) or Eu^II metal reagents gave the complexes 1 ‐Ae (Ae=Mg, Ca, Sr, Ba) and 1 ‐Eu. 1 ‐Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe???Mg distance in 1 ‐Mg is too long for a bonding interaction, but short Fe???Ae distances in 1 ‐Ca, 1 ‐Sr, and 1 ‐Ba clearly support intramolecular Fe???Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related ¹H NMR chemical‐shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1 ‐Ae complexes shows that the heavier species 1 ‐Ca, 1 ‐Sr, and 1 ‐Ba possess genuine Fe→Ae bonds which involve vacant d‐orbitals of the alkaline‐earth atoms and partially filled d‐orbitals on Fe. In 1 ‐Mg, a weak Fe→Mg donation into vacant p‐orbitals of the Mg atom is observed.     

Metastable P-Tellurium-Substituted Phosphaalkenes: Formation, 125Te- and 31P-NMR Spectroscopic Characterization,and Decomposition

Abstract

The formation and decomposition of P-tellurium-substituted phosphaalkenes was followed by ³¹P- and ¹²⁵Te-NMR spectroscopy. Acyclic compounds with C?P-Te moieties are in general thermally labile, but bulky substituents enhance the lifetime of a number of species. The P-chlorophosphaalkene (Me₃Si)₂C?PCl (1a) reacts with the disilyltelluride (iPrMe₂Si)₂Te (2) leading to the mixed-substituted telluride (Me₃Si)₂C?PTeSiMe₂iPr 3a which reacts with another equivalent of 1a furnishing the tellurobis(phosphaalkene) [(Me₃Si)₂C?P]₂Te (4a). 4a is a shortlived compound decomposing thermally with precipitation of elemental tellurium, leading to a known diphosphabicyclobutane 5a. In a similar way, the bulkier P-chlorophosphaalkene (iPrMe₂Si)₂C?PCl (1b) reacts with (iPrMe₃Si)₂Te furnishing [(iPrMe₂Si)₂C?P]₂Te (4b), which loses tellurium much more slowly than 4a and can be kept in cold solutions for an extended time. Reactions of in situ-prepared lithium aryltellurolates LiTeAr 6 – 9 [Ar?Ph: 6, Ar?2,4,6-Me₃Ph (?Mes): 7, Ar?2,4,6-iPr₃Ph (?TIP): 8, Ar?2,4,6-tBu₃Ph (?Mes*): 9] with 1a provide P-aryltellurophosphaalkenes 10 – 13, which decompose with the loss of diarylditellurides leading to 5a. After a 2 + 4 cycloaddition trapping experiment of 12 with cyclopentadiene, a metastable P-aryltelluro phosphanorbornene 14 was detected by ³¹P-NMR. Reactions of elemental tellurium with P-phosphanylphosphaalkenes (Me₃Si)₂C?PPR′R′;′ 15 – 17 (R′, R′′?iPr: 15; R′?iPr, R′′?tBu: 16; R′, R′′?tBu: 17) lead to metastable insertion products (Me₃Si)₂C?PTePR′R′′ 18 – 20 that decompose with formation of the tellurobisphosphanes (R′R′′P)₂Te 21 – 23, and of the bicyclic diphosphane 5a, which isomerises thermally to the diphosphabicyclooctane 24. The P-di-i-propylphosphanyl-phosphanorbornene 25 dismutates under the action of tellurium into the symmetric diphosphanes iPr₄P₂ and bis-phosphanorbornene 26. The tellurium-free products 24 and 26 were characterized by X-ray crystallography.     

Ultraviolet laser desorption/ionization mass spectrometry of single‐core and multi‐core polyaromatic hydrocarbons under variable conditions of collisional cooling: insights into the generation of molecular ions,fragments and oligomers

The ultraviolet laser desorption/ionization of polyaromatic hydrocarbons (PAHs) has been investigated under different background pressures of an inert gas (up to 1.2 mbar of N₂) in the ion source of a hybrid, orthogonal‐extracting time‐of‐flight mass spectrometer (oTOF‐MS). The study includes an ensemble of six model PAHs with isolated single polyaromatic cores and four ones with multiple cross‐linked aromatic and polyaromatic cores. In combination with a weak ion extraction field, the variation of the buffer gas pressure allowed to control the degree of collisional cooling of the desorbed PAHs and, thus, to modulate their decomposition into fragments. The dominant fragmentation channels observed are related to dehydrogenation of the PAHs, in most cases through the cleavage of even numbers of C―H bonds. Breakage of C―C bonds leading to the fragmentation of rings, side chains and core linkages is also observed, in particular, at low buffer gas pressures. The precise patterns of the combined fragmentation processes vary significantly between the PAHs. The highest abundances of molecular PAH ions and cleanest mass spectra were consistently obtained at the highest buffer gas pressure of 1.2 mbar. The effective quenching of the fragmentation pathways at this elevated pressure improves the sensitivity and data interpretation for analytical applications, although the fragmentation of side chains and of bonds between (poly)aromatic cores is not completely suppressed in all cases. Moreover, these results suggest that the detected fragments are generated through thermal equilibrium processes rather than as a result of rapid photolysis. This assumption is further corroborated by a laser desorption/ionization post‐source decay analysis using an axial time‐of‐flight MS. In line with these findings, covalent oligomers of the PAHs, which are presumably formed by association of two or more dehydrogenated fragments, are detected with higher abundances at the lower buffer gas pressures. Copyright © 2014 John Wiley & Sons, Ltd.     

Catalysis‐Based Total Synthesis of Putative Mandelalide A

A concise synthesis of the putative structure assigned to the highly cytotoxic marine macrolide mandelalide A ( 1 ) is disclosed. Specifically, an iridium‐catalyzed two‐directional Krische allylation and a cobalt‐catalyzed carbonylative epoxide opening served as convenient entry points for the preparation of the major building blocks. The final stages feature the first implementation of terminal‐acetylene metathesis into natural product synthesis, which is remarkable as this class of substrates was beyond reach until very recently; key to success was the use of the highly selective molybdenum alkylidyne complex 42 as the catalyst. Although the constitution and stereochemistry of the synthetic samples are unambiguous, the spectra of 1 as well as of 11‐epi‐ 1 deviate from those of the natural product, which implies a subtle but deep‐seated error in the original structure assignment.     

Probing Transient Conformational States of Proteins by Solid‐State R1ρ Relaxation‐Dispersion NMR Spectroscopy

The function of proteins depends on their ability to sample a variety of states differing in structure and free energy. Deciphering how the various thermally accessible conformations are connected, and understanding their structures and relative energies is crucial in rationalizing protein function. Many biomolecular reactions take place within microseconds to milliseconds, and this timescale is therefore of central functional importance. Here we show that R_1ρ relaxation dispersion experiments in magic‐angle‐spinning solid‐state NMR spectroscopy make it possible to investigate the thermodynamics and kinetics of such exchange process, and gain insight into structural features of short‐lived states.     

Counterions Control Whether Self‐Assembly Leads to Formation of Stable and Well‐Defined Unilamellar Nanotubes or Nanoribbons and Nanorods

Self‐assembly of the amphiphilic π‐conjugated carbenium ion ATOTA‐1⁺ in aqueous solution selectively leads to discrete and highly stable nanotubes or nanoribbons and nanorods, depending on the nature of the counterion (Cl^? vs. PF₆^?, respectively). The nanotubes formed by the Cl^? salt illustrate an exceptional example of a structural well‐defined (29±2 nm in outer diameter) unilamellar tubular morphology featuring π‐conjugated functionality and high stability and flexibility, in aqueous solution.     

Quantitative monitoring of yeast fermentation using Raman spectroscopy

Compared to traditional IR methods, Raman spectroscopy has the advantage of only minimal interference from water when measuring aqueous samples, which makes this method potentially useful for in situ monitoring of important industrial bioprocesses. This study demonstrates real-time monitoring of a Saccharomyces cerevisiae fermentation process using a Raman spectroscopy instrument equipped with a robust sapphire ball probe. A method was developed to correct the Raman signal for the attenuation caused by light scattering cell particulate, hence enabling quantification of reaction components and possibly measurement of yeast cell concentrations. Extinction of Raman intensities to more than 50 % during fermentation was normalized with approximated extinction expressions using Raman signal of water around 1,627 cm^?1 as internal standard to correct for the effect of scattering. Complicated standard multi-variant chemometric techniques, such as PLS, were avoided in the quantification model, as an attempt to keep the monitoring method as simple as possible and still get satisfactory estimations. Instead, estimations were made with a two-step approach, where initial scattering correction of attenuated signals was followed by linear regression. In situ quantification measurements of the fermentation resulted in root mean square errors of prediction (RMSEP) of 2.357, 1.611, and 0.633 g/L for glucose, ethanol, and yeast concentrations, respectively.