Spin crossover in a four-coordinate iron(II) complex

The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)(3)Fe-N═PPh(3) undergoes an S = 0 to S = 2 spin transition with T(C) = 81 K, as determined by variable-temperature magnetic measurements and Mo?ssbauer spectroscopy. Variable-temperature single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is associated with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.     

Synthesis,characterisation and chromatographic evaluation of polyether dendrimer stationary phases

Supercritical carbon dioxide has attracted attention as a potential replacement for traditional organic solvents due to its simplified workup procedures and reduced environmental impact—providing a green chemistry approach for organic solvent-free functionalisation. In addition to the environmental benefits, the enhanced diffusivity observed in supercritical solvents can often enhance reaction rates. We have applied these valuable features to the preparation of silica-bonded stationary phases and examined their potential in liquid chromatography. We report the successful preparation and characterisation of polyether silica based on Frechet dendrimers—this significantly enhances the range of stationary-phase chemistries that can be prepared in supercritical fluids. First- and second-generation polyether silicas were prepared, characterised, end-capped and evaluated for use as stationary phases for liquid chromatography. Figure SRM1649 on 2nd generation polyether silica Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

A Novel Approach for Incorporation of Mono‐Functional Bioactive Phenols into Polyanhydrides

Antiseptics based on phenol and phenolic derivatives were chemically incorporated into polyanhydrides as pendant groups via ester linkages. Polyanhydrides with antiseptic loadings of 46–58 wt.‐% were obtained with molecular weights ranging from 9 400–23 000. In general, polymers with the bulkier antiseptics were more difficult to polymerize and yielded lower molecular weights. All polyanhydrides were amorphous with glass transition temperatures ranging from 27–58 °C. Polymers were deemed noncytotoxic after culturing L929 mouse fibroblast cells in media containing the polymers at two concentrations (0.10 and 0.01 mg · mL⁻¹) over three days. In summary, mono‐functional bioactives can be chemically incorporated into noncytotoxic polyanhydrides.

     

EPR studies of the dynamics and phase behaviour of dithiadiazolyl radicals derived from mesogenic precursors

A series of liquid-crystalline materials based on 4-substituted cyanobiphenyls, RC(6)H(4)C(6)H(4)CN (R = C(5)H(11), C(6)H(13), C(7)H(15), C(8)H(17) and C(12)H(25), commonly referred to as 5CB, 6CB, 7CB, 8CB and 12CB, respectively) were functionalised to give the corresponding dithiadiazolyl organic radicals RC(6)H(4)C(6)H(4)CNSSN(*) (compounds 1(*), 2(*), 3(*), 4(*) and 5(*), respectively). EPR spectra of n-C(12)H(25)C(6)H(4)C(6)H(4)CNSSN(*) (5(*)) reveal that it adopts a dimeric diamagnetic structure in the solid state with a small number of paramagnetic defect sites. Variable temperature electron paramagnetic resonance (EPR) studies reveal a thermally activated rotation about the molecular long axis at temperatures above 314 K. The energy barrier to rotation about the phenylene-dithiadiazolyl bond was estimated as 25 kJ mol(-1) using density functional theory (DFT). At elevated temperatures thermal annealing was also observed, quenching the sample's paramagnetism.     

Functional Molecular Flasks: New Properties and Reactions within Discrete,Self‐Assembled Hosts

Insider dealing : Self‐assembled hosts applied as “molecular flasks” can alter and control the reactivity and properties of molecules encapsulated within their well‐defined, confined spaces. A variety of functional hosts of differing sizes, shapes, and utility have been prepared by using the facile and modular concepts of self‐assembly.

     

Strain-release rearrangement of N-vinyl-2-arylaziridines. Total synthesis of the anti-leukemia alkaloid (-)-deoxyharringtonine

Deoxyharringtonine (1) is among the most potent of the anti-leukemia alkaloids isolated from the Cephalotaxus genus. A convergent total synthesis of (-)-1 is reported, involving novel synthetic methods and strategies that include (1) the strain-release rearrangement of N-aryl-2-vinylaziridines for [3]benzazepine synthesis, (2) a vinylogous amide acylation-cycloaddition cascade for spiro-pyrrolidine construction, and (3) efficient acylation of the cephalotaxine core by alpha-(beta-lactone)carboxylic acid derivatives to access the biologically active cephalotaxus esters. These innovations should allow rapid access not only to other Cephalotaxus alkaloids but also to non-natural analogues of potential therapeutic utility.     

Deviation from the mobile proton model in amino-modified peptides: implications for multiple reaction monitoring analysis of peptides

The study of peptide fragmentation is important to the understanding of chemical processes occurring in the gas phase and the more practical concern of peptide identification for proteomic analysis. Using the mobile proton model as a framework, we explore the effect of amino-group modifications on peptide fragmentation. Three aldehydes are used to transform the peptides' primary amino groups into either a dimethylamino or a heterocyclic structure (five- or six-membered). The observed fragmentation patterns deviate strongly from those observed for the analogous underivatised peptides. In particular, the a1 ion is the base peak in most tandem mass spectra of the derivatised peptides. The a1 ion intensity depends strongly on the N-terminal amino acid, with tyrosine and phenylalanine having the strongest enhancement. Despite the change in fragmentation patterns of the derivatised peptides, they still provide high-quality tandem mass spectra that, in many cases, are more amenable to database searching than the spectra of underivatised peptides. In addition, the reliable presence of the a1 ion facilitates rapid quantitative measurements using the multiple reaction monitoring approach.     

Rotational reorientation dynamics of ionic dye solutes in polar solvents with the application of a general model for the solvation shell

The rotational reorientation times of two cationic dyes, nile blue A and oxazine 720 have been determined in various protic and aprotic polar solvents from the picosecond decay of their transient absorption. The results have shown a good agreement with those obtained from theoretical calculations based on Stokes–Einstein–Debye theory, using a simple model for the rotating species. In this model these large closely planar solutes are treated as oblate symmetric tops, their solvent shells are represented by layers with uniform thickness of γ · 2r_S, where 2r_S is the diameter of the solvent molecule, and γ is a common fitting parameter for all types of solvent.     

Accessing the dynamics of end-grafted flexible polymer chains by atomic force-electrochemical microscopy. Theoretical modeling of the approach curves by the elastic bounded diffusion model and Monte Carlo simulations. Evidence for compression-induced lateral chain escape

The dynamics of a molecular layer of linear poly(ethylene glycol) (PEG) chains of molecular weight 3400, bearing at one end a ferrocene (Fc) label and thiol end-grafted at a low surface coverage onto a gold substrate, is probed using combined atomic force-electrochemical microscopy (AFM-SECM), at the scale of approximately 100 molecules. Force and current approach curves are simultaneously recorded as a force-sensing microelectrode (tip) is inserted within the approximately 10 nm thick, redox labeled, PEG chain layer. Whereas the force approach curve gives access to the structure of the compressed PEG layer, the tip-current, resulting from tip-to-substrate redox cycling of the Fc head of the chain, is controlled by chain dynamics. The elastic bounded diffusion model, which considers the motion of the Fc head as diffusion in a conformational field, complemented by Monte Carlo (MC) simulations, from which the chain conformation can be derived for any degree of confinement, allows the theoretical tip-current approach curve to be calculated. The experimental current approach curve can then be very satisfyingly reproduced by theory, down to a tip-substrate separation of approximately 2 nm, using only one adjustable parameter characterizing the chain dynamics: the effective diffusion coefficient of the chain head. At closer tip-substrate separations, an unpredicted peak is observed in the experimental current approach curve, which is shown to find its origin in a compression-induced escape of the chain from within the narrowing tip-substrate gap. MC simulations provide quantitative support for lateral chain elongation as the escape mechanism.