Time filter

Source Type

Carpentier J.-F.,CNRS Chemistry Institute of Rennes
Macromolecular Rapid Communications | Year: 2010

Poly(β-hydroxyalkanoate)s (PHAs) are a class of aliphatic polyesters that can be efficiently synthesized by ring-opening polymerization (ROP) of β-lactones. The case of chiral racemic β-substituted β-lactones is particularly appealing since these monomers open the way to original tacticities and materials different from those biotechnologically produced. In this overview, after briefly surveying general considerations associated to the ROP of β-lactones and metal-based catalysts used in stereoselective ROP of racemic b-butyrolactone, special emphasis is given to discrete rare earth catalysts that have allowed the preparation of highly syndiotactic poly(3-hydroxybutyrate)s. Recent developments - such as preparation of stereocontrolled PHAs with pendant structural groups via (co)polymerization of functional bsubstituted β-lactones, and highly alternating copolymers obtained by ROP of mixtures of enantiomerically pure but different monomers - are also discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGa. Source

Mongin F.,CNRS Chemistry Institute of Rennes | Harrison-Marchand A.,INSA Rouen
Chemical Reviews | Year: 2013

The article presents generalities concerning the formation and main features of the Mixed AggregAtes (MAA). Alternative ways to metal organo, amido, and/or alkoxo ate compounds, such as magnesates, calciates, manganates, ferrates, nickelates, and zincates, use the corresponding monometallic compounds (cocomplexation approach). The syntheses of mixed-ligand organo, amido, and/or alkoxo heteroMAAs are in general performed by mixing precursors possessing different ligands in appropriate ratios. The structural data of these compounds have been summarized. It is important to note that, in the presence of oxygen during the preparation of metal ate compounds such as magnesates, aluminates, manganates, and zincates, complex structures with oxo or peroxo cores can be formed, and some of them were evidenced by X-ray crystallography. Source

Harrison-Marchand A.,INSA Rouen | Mongin F.,CNRS Chemistry Institute of Rennes
Chemical Reviews | Year: 2013

Organometallic chemistry is one of the key assets to successfully achieve complex organic transformations. Numerous tools have been conceived and developed to address problems related to the chemo-, regio-, and stereoselectivities of general reactions such as the deprotonation of prochiral species or the creation of C-C bonds. By way of announcement of the concept, it was decided to dedicate this preliminary review (part 1) on the structural knowledge, in the solid state and/or in solution, of oligo-, homo-, and heteroMAAs that are made of lithium, sodium, and potassium species, as well as magnesium, aluminum, and zinc derivatives. With the latter selection covering still a large panel of organo(bi)metallic combinations depicted in the literature, only systems that are the most encountered and discussed by organic chemists and that are either obtained directly by mixing two commercial organometallics routinely used in organic chemistry or very frequently synthesized and easy to access will be depicted. Source

The combination of monocrystalline silicon's well-defined structure and the ability to prepare hydrogen-terminated surfaces (Si-H) easily and reproducibly has made this material a very attractive substrate for immobilizing functional molecules. The functionalization of Si-H using the covalent attachment of organic monolayers has received intense attention due to the numerous potential applications of controlled and robust organic/Si interfaces. Researchers have investigated these materials in diverse fields such as molecular electronics, chemistry, and bioanalytical chemistry. Applications include the preparation of surface insulators, the incorporation of chemical or biochemical functionality at interfaces for use in photovoltaic conversion, and the development of new chemical and biological sensing devices. Unlike those of gold, silicon's electronic properties are tunable, and researchers can directly integrate silicon-based devices within electronic circuitry. Moreover, the technological processes used for the micro- and nanopatterning of silicon are numerous and mature enough for producing highly miniaturized functional electronic components. In this Account, we describe a powerful approach that integrates redox-active molecules, such as ferrocene, onto silicon toward electrically addressable systems devoted to information storage or transfer. Ferrocene exhibits attractive electrochemical characteristics: fast electron-transfer rate, low oxidation potential, and two stable redox states (neutral ferrocene and oxidized ferrocenium). Accordingly, ferrocene-modified silicon surfaces could be used as charge storage components with the bound ferrocene center as the memory element. Upon application of a positive potential to silicon, ferrocene is oxidized to its corresponding ferrocenium form. This redox change is equivalent to the change of a bit of information from the "0" to "1" state. To erase the stored charge and return the device to its initial state, a low potential must be applied to reduce the whole generated ferrocenium. In this type of application, the electron is transferred from the ferrocene headgroups to the underlying conducting silicon surface by a tunneling process across the monolayer. To produce a stable and reproducible electrical response, this process must be efficient, fast, and reversible. The stability, charge density, and capacitance performances of high-quality ferrocene-terminated monolayers could compete with those of the existing semiconductor-based memory devices, such as dynamic random access memories, DRAMs. Moreover, we provide experimental evidence that a series of immobilized ferrocene centers can efficiently communicate via a lateral electron hopping process. Using these modified interfaces, we demonstrate that the thin redox-active monolayer can behave as a purely conducting material, highlighting an unprecedented very fast electron communication between immobilized redox groups. Perhaps more importantly, the surface coverage of ferrocene allows us to precisely control the rate of this process. Such characteristics are relevant not only for electrocatalytic reactions but also for widening the potential applications of these assemblies to novel molecular electronic devices (e.g. chemiresistors, chemically sensitive field-effect transistors (CHEMFETs)) and redox chemistry on insulating surfaces. © 2010 American Chemical Society. Source

Geiger W.E.,University of Vermont | Barriere F.,CNRS Chemistry Institute of Rennes
Accounts of Chemical Research | Year: 2010

Electrochemistry is a powerful tool for the study of oxidative electron-transfer reactions (anodic processes). Since the 1960s, the electrolytes of choice for nonaqueous electrochemistry were relatively small (heptaatomic or smaller) inorganic anions, such as perchlorate, tetrafluoroborate, or hexafluorophosphate. Owing to the similar size-to-charge ratios of these "traditional" anions, structural alterations of the electrolyte anion are not particularly valuable in effecting changes in the corresponding redox reactions. Systematic variations of supporting electrolytes were largely restricted to cathodic processes, in which interactions of anions produced in the reactions are altered by changes in electrolyte cations. A typical ladder involves going from a weakly ion-pairing tetraalkylammonium cation, [N(CnH2n+1)4]+, with n ? 4, to more strongly ion-pairing counterparts with n < 4, and culminating in very strongly ion-pairing alkali metal ions. A new generation of supporting electrolyte salts that incorporate a weakly coordinating anion (WCA) expands anodic applications by providing a dramatically different medium in which to generate positively charged electrolysis products. A chain of electrolyte anions is now available for the control of anodic reactions, beginning with weakly ion-pairing WCAs, progressing through the traditional anions, and culminating in halide ions. Although the electrochemical properties of a number of different WCAs have been reported, the most systematic work involves fluoro- or trifluoromethyl-substituted tetraphenylborate anions (fluoroarylborate anions). In this Account, we focus on tetrakis(perfluorophenyl)borate, [B(C 6F5)4]?, which has a significantly more positive anodic window than tetrakis[(3,5-bis(trifluoromethyl)phenyl)] borate, [BArF24]?, making it suitable in a larger range of anodic oxidations. These WCAs also have a characteristic of specific importance to organometallic redox processes. Many electron-deficient organometallic compounds are subject to nucleophilic attack by the traditional family of electrolyte anions. With a view to testing the scope of the much less nucleophililic WCAs in providing a benign electrolyte anion for the generation of organometallic cation radicals, we carried out a series of studies on transition metal sandwich and half-sandwich compounds. The model compounds were chosen both for their fundamental importance and because their radical cations had been neither isolated nor spectrally characterized, despite many previous electrochemical investigations with traditional anions. The oxidation of prototypical organometallic compounds, such as the sandwich-structured ruthenocene and the piano-stool structured Cr(ν6-C 6H6)(CO)3, Mn(ν5-C 5H5)(CO)3, Re(ν5-C 5H5)(CO)3, and Co(ν5-C 5H5)(CO)2, gave the first definitive in situ characterization of their radical cations. In several cases, the kinetic stabilization of the anodic products allowed the identification of dimers or unique dimer radicals having weak metal?metal bonds and provided new preparative options for organometallic systems. In terms of thermodynamic effects, the lower ion-pairing abilities of WCAs and their good solubility in a broad range of solvents, including those of lower polarity, permitted a systematic study that yielded an integrated model of how to use solvent?electrolyte combinations to manipulate the E1/2 differences of compounds undergoing multiple electron-transfer reactions. Although the efficacy of WCA-based electrolytes in organometallic anodic chemistry is now established, WCAs might further expand applications of organic redox chemistry. Other WCAs, including those derived from carboranes and fluorinated alkoxyaluminates, merit additional studies. © 2010 American Chemical Society. Source

Discover hidden collaborations