Evolution of Oxygen–Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X‐Ray Spectroscopy

Manganese oxide (MnO_x) electrocatalysts are examined herein by in situ soft X‐ray absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS) during the oxidation of water buffered by borate (pH 9.2) at potentials from 0.75 to 2.25 V vs. the reversible hydrogen electrode. Correlation of L‐edge XAS data with previous mechanistic studies indicates Mn^IV is the highest oxidation state involved in the catalytic mechanism. MnO_x is transformed into birnessite at 1.45 V and does not undergo further structural phase changes. At potentials beyond this transformation, RIXS spectra show progressive enhancement of charge transfer transitions from oxygen to manganese. Theoretical analysis of these data indicates increased hybridization of the Mn?O orbitals and withdrawal of electron density from the O ligand shell. In situ XAS experiments at the O K‐edge provide complementary evidence for such a transition. This step is crucial for the formation of O₂ from water.     

Multivalent Chelators for In Vivo Protein Labeling

With the advent of single‐molecule methods, chemoselective and site‐specific labeling of proteins evolved to become a central aspect in chemical biology as well as cell biology. Protein labeling demands high specificity, rapid as well as efficient conjugation, while maintaining low concentration and biocompatibility under physiological conditions. Generic methods that do not interfere with the function, dynamics, subcellular localization of proteins, and crosstalk with other factors are crucial to probe and image proteins in vitro and in living cells. Alternatives to enzyme‐based tags or autofluorescent proteins are short peptide‐based recognition tags. These tags provide high specificity, enhanced binding rates, bioorthogonality, and versatility. Here, we report on recent applications of multivalent chelator heads, recognizing oligohistidine‐tagged proteins. The striking features of this system has facilitated the analysis of protein complexes by single‐molecule approaches.     

Ligands Based on Phosphine-Stabilized Aluminum(I), Boron(I), and Carbon(0)

A systematic quantum chemical study of the bonding in d⁶-transition-metal complexes, containing phosphine-stabilized, main-group-element fragments, (R₃P)₂E, as ligands (E=AlH, BH, CH⁺, C), is reported. By using energy decomposition analysis, it is demonstrated that a strong M−E bond is accompanied by weak P−E bonds, and vice versa. Although the Al−M bond is, for example, found to be very strong, the weak Al−P bond suggests that the corresponding metal complexes will not be stable towards phosphine dissociation. The interaction energies for the boron(I)-based ligand are lower, but still higher than those for two-carbon-based ligands. For neutral ligands, electrostatic interactions are the dominating contributions to metal–ligand bonding, whereas for the cationic ligand a significant destabilization, with weak orbital and even weaker electrostatic metal–ligand interactions, is observed. Finally, for iron(II) complexes, it is demonstrated that different reactivity patterns are expected for the four donor groups: the experimentally observed reversible E−H reductive elimination of the borylene-based ligand (E=BH) exhibits significantly higher barriers for the protonated carbodiphosphorane (CDP) ligand (E=CH) and would proceed through different intermediates and transition states. For aluminum, such reaction pathways are not feasible (E=AlH). Moreover, it is demonstrated that the metal hydrido complexes with CDP ligands might not be stable towards reduction and isomerization to a protonated CDP ligand and a reduced metal center.     

Contrasting Effect of Additives on Photoinduced Reactions of SmI2

The work described herein compares the effect of additives (HMPA, methanol, ethylene glycol, pinacol, N-methylethanolamine) on thermal and photochemical reactions of samarium diiodide (SmI₂). In thermal reactions, additives that coordinate to SmI₂ induce a significant increase in reaction rate. In photochemical reactions, the presence of an electronegative atom with a highly localized negative charge on the substrate leads to a rate deceleration. In order to benefit from the columbic interaction with the positively charged samarium cation, these substrates react preferentially by an inner sphere reduction mechanism. The addition of ligands prevents this close interaction causing rate retardation. Furthermore, studies demonstrate that excited state quenching of Sm^II by ethylene glycol and other additives indicate that it is unlikely to be the major cause for the observed rate retardation. This effect provides a simple diagnostic tool to distinguish between an inner and an outer sphere reduction mechanism.     

Rate and Computational Studies for Pd-NHC-Catalyzed Amination with Primary Alkylamines and Secondary Anilines: Rationalizing Selectivity for Monoarylation versus Diarylation with NHC Ligands

The relative rates of arylation of primary alkylamines with different Pd-NHC catalysts have been measured, as have the relative rates of arylation of the secondary aniline product in an attempt to understand the key ligand design features necessary to have high selectivity for the monoarylated amine product. As the substituents on the N-aryl ring of the NHC increase in size, selectivity for monoarylation increases and this is further enhanced by chlorinating the back of the NHC ring. Computations have been performed on the catalytic cycle of this transformation in order to understand the selectivity obtained with the different catalysts.     

Analysis of curing reaction of liquid-crystalline epoxy compositions by using temperature-modulated DSC TOPEM®

Journal of Thermal Analysis and Calorimetry - The curing behaviour of the composition of a liquid-crystalline diepoxy monomer (LCEM) with the central triaromatic mesogenic group was studied using...     

Ion mobility augments the utility of mass spectrometry in the identification of human hemoglobin variants

The global dispersion of hemoglobin variants through population migration has precipitated a need for their identification. A particularly effective mass spectrometry (MS)-based procedure involves analysis of the intact globin chains in diluted blood to detect the variant through mass anomalies, followed by location of the variant amino acid residue by direct analysis of the enzymatically digested globins. Here we demonstrate the use of ion mobility separation in combination with this MS procedure to reduce mass spectral complexity. In one example, the doubly charged tryptic peptide from a low abundance variant (4%) occurred at the same m/z value as a singly and a doubly charged interfering ion. In another example, the singly charged tryptic peptide from an alpha-chain variant (26%) occurred at the same m/z value as a doubly charged interfering ion. Ion mobility was used to separate the variant ions from the interfering ions, thus allowing the variant peptides to be observed and sequenced by tandem mass spectrometry.     

A rational basis for the axial ligand effect in C-H oxidation by [MnO(porphyrin)(X)]+ (X = H2O, OH-, O2-) from a DFT study

Oxyl radical character in the MnO group of the title system is shown from a density functional theory study to be essential for efficient C-H cleavage, which is a key step in C-H oxidation. Since oxyl species have elongated Mn-O bonds relative to the more usual oxo species of type MnO, the normal expectation would be that high trans-influence ligands X should facilitate oxyl character by elongating the Mn-O bond and thus enhance both oxyl character and reactivity. Contrary to this expectation, but in line with the experimental data (Jin, N.; Ibrahim, M.; Spiro, T. G.; Groves, J. T. J. Am. Chem. Soc. 2007, 129, 12416), we find that reactivity increases along the series X = O(2-) < OH(-) < H2O for the following reasons. The ground-state singlet (S) is unreactive for all X, and only the higher-energy triplet (T) and quintet (Q) states have the oxyl character needed for reactivity, but the higher trans-influence X ligands are also shown to increase the S/T and S/Q gaps, thus making attainment of the needed T and Q states harder. The latter effect is dominant, and high trans-influence X ligands thus disfavor reaction. The higher reactivity in the presence of acid noted by Groves and co-workers is thus rationalized by the preference for having X = H2O over OH(-) or O(2-).     

Synthetic studies on Et-743. Assembly of the pentacyclic core and a formal total synthesis

A formal total synthesis of the potent anticancer agent Et-743 is described. The tetrahydroisoquinoline core is stereoselectively constructed using a novel radical cyclization of a glyoxalimine. Further elaboration of this core rapidly accessed the pentacyclic core of Et-743, but a mixture of regiosisomers was obtained in the key Pictet-Spengler ring closure. A known advanced intermediate in the synthesis of Et-743 was intercepted, constituting a formal synthesis of the molecule.