Synthesis and reactivity of molybdenum organometallic complexes supported by amide ligands

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.Vita.Includes bibliographical references.Chapter 1. Synthesis and Reactivity of Molybdenum Alkyl Complexes Supported by a Diamidoamine Ligand. The synthesis of a new diamidoamine ligand, CH3N[CH2CH2NH(3-(CF3)C6H4)]2 (H2L) is reported. Molybdenum complexes of the type X[LMoC13] (X = Et3NH, Bu4N) are readily synthesized as purple air and moisture sensitive crystalline solids. They may be alkylated by Grignard reagents to yield the monoalkyl complex LMo(Cl)(CH2CMe3), the alkylidyne complex LMo(CCMe3)(CH2CMe3) via a,a - elimination of dihydrogen, and the stable complex LMo(CH2TMS)2 (TMS = SiMe3). LMo(CH2TMS)2 is readily oxidized by cyclohexene sulfide and hexachloroethane to yield, initially, molybdenum alkylidenes that readily undergo decomposition reactions to the bridging sulfide complex {LMoS}2 and [LMo(CTMS)Cl]2, respectively. However, the alkylidene complex LMo(CHTMS)(q2 - MeCCMe) may be isolated if LMo(CH2TMS)2 is heated in the presence of five equivalents of 2-butyne. It is a rare example of an alkylidene and alkyne in the coordination sphere of a group six metal. The relationship between oxidation and alkyne binding is discussed. Chapter 2. Early Transition Metal Pyrrolyl Complexes: Structure, Spectroscopy, Hapticity, and Fluxional Processes.(cont.) A review of group 4, 5,and 6 transition metal pyrrolyl complexes is presented. The focus is on the unique bonding and dynamic processes that the pyrrolyl (and related ligands) can undergo. It is concluded that 1 bound pyrrolyl ligands are poor it donors. Chapter 3. Synthesis and Characterization of Molybdenum Imido Alkylidene Bis(amide) Complexes. The synthesis of bis(amide) complexes Mo(NR)(CHR')(X)2 (R = Ar, Ad, 2,6-Br2-4-MeC6H2 X = N(t-Bu)Ar', NC4H4) from the bis(triflate) complexes Mo(NR)(CHR')(OTf)2(DME) (OTf = trifluoromethanesulfonate, DME = 1,2- dimethoxyethane) is reported. These complexes are of interest as potential precursors for the in situ generation of active and enantioselective metathesis catalysts. The tert-butyl anilide complexes are found to be unreactive towards enantiopure diols. The bis(pyrrolyl) complexes exist in rapid equilibrium between a dimeric form [Mo(NR)(CHR')(NC4H4)2]2 and one in which all pyrrolyl resonances are equilibrating rapidly. The nature of the bis(pyrrolyl) complexes was interrogated by variable temperature 1H and 13C NMR for R = Ar and Ad. The dimeric form was confirmed through an x-ray structural study of {Mo(NAr)(syn-CHCMe2Ph)(S -NC4H4)(r 1-NC4H14)} { Mo(NAr)(syn-CHCMe2Ph)(rq -NC4H4)2 }.(cont.) Chapter 4. Reactivity of Molybdenum Imido Alkylidene Bis(pyrrolyl) Complexes. The Lewis amphoteric nature of the bis(pyrrolyl) complexes reported in chapter 3 is examined by demonstrating that these complexes react with both trimethylphosphine (at the molybdenum center) and B(C6Fs)3 (at a q5 pyrrolyl nitrogen). A structure of a trimethylphosphine adduct is reported. The bis(pyrrolyl) complexes are found to serve as excellent precursors for the in situ generation of olefin metathesis catalysts at room temperature and millimolar concentration. Furthermore, catalysts not accessible via traditional routes may now be accessed from bis(pyrrolyl) precursors. The bis(pyrrolyl) complexes also react with simple olefins such as ethylene and isobutylene to yield what are proposed to be a bimetallic dimer [Mo(NAr)(NC4H4)2]2 and a 2-propylidene complex via olefin metathesis. The impact of in situ synthesis on syn and anti isomer ratios is discussed as is reactivity with protic reagents other than alcohols.Adam S. Hock.Ph.D

Atomic Layer Deposition of Tin Monosulfide Thin Films

Thin film solar cells made from earth-abundant, non-toxic materials are needed to replace the current technology that uses $Cu(In,Ga)(S,Se)_2$ and $CdTe$, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide ($SnS$). In this report, pure, stoichiometric, single-phase SnS films were obtained by atomic layer deposition (ALD) using the reaction of $bis(N,N′-diisopropylacetamidinato)tin(II)$ $[Sn(MeC(N-^{i}Pr)_{2})_{2}]$ and hydrogen sulfide $(H_2 S)$ at low temperatures (100 to 200 $°C$). The direct optical band gap of SnS is around 1.3 eV and strong optical absorption $(\alpha > 10^4 cm^{−1})$ is observed throughout the visible and near-infrared spectral regions. The films are p-type semiconductors with carrier concentration on the order of $10^{16} cm^{−3}$ and hole mobility $0.82–15.3cm^{2}V^{−1}s^{−1}$ in the plane of the films. The electrical properties are anisotropic, with three times higher mobility in the direction through the film, compared to the in-plane direction.Chemistry and Chemical Biolog

ALD of Tin Monosulfide, SnS

Earth and Planetary Science

Low Temperature Atomic Layer Deposition of Tin Dioxide, SnO2_2

Chemistry and Chemical Biolog

Bioinformatics process management: information flow via a computational journal

This paper presents the Bioinformatics Computational Journal (BCJ), a framework for conducting and managing computational experiments in bioinformatics and computational biology. These experiments often involve series of computations, data searches, filters, and annotations which can benefit from a structured environment. Systems to manage computational experiments exist, ranging from libraries with standard data models to elaborate schemes to chain together input and output between applications. Yet, although such frameworks are available, their use is not widespread–ad hoc scripts are often required to bind applications together. The BCJ explores another solution to this problem through a computer based environment suitable for on-site use, which builds on the traditional laboratory notebook paradigm. It provides an intuitive, extensible paradigm designed for expressive composition of applications. Extensive features facilitate sharing data, computational methods, and entire experiments. By focusing on the bioinformatics and computational biology domain, the scope of the computational framework was narrowed, permitting us to implement a capable set of features for this domain. This report discusses the features determined critical by our system and other projects, along with design issues. We illustrate the use of our implementation of the BCJ on two domain-specific examples

The Leptonic Higgs as a Messenger of Dark Matter

We propose that the leptonic cosmic ray signals seen by PAMELA and ATIC result from the annihilation or decay of dark matter particles via states of a leptonic Higgs doublet to $\tau$ leptons, linking cosmic ray signals of dark matter to LHC signals of the Higgs sector. The states of the leptonic Higgs doublet are lighter than about 200 GeV, yielding large $\bar{\tau} \tau$ and $\bar{\tau} \tau \bar{\tau} \tau$ event rates at the LHC. Simple models are given for the dark matter particle and its interactions with the leptonic Higgs, for cosmic ray signals arising from both annihilations and decays in the galactic halo. For the case of annihilations, cosmic photon and neutrino signals are on the verge of discovery.Comment: 34 pages, 9 figures, minor typos corrected, references adde

Brain areas with normatively greater cerebral perfusion in early life may be more susceptible to beta amyloid deposition in late life

Background: The amyloid cascade hypothesis characterizes the stereotyped progression of pathological changes in Alzheimer’s disease (AD) beginning with beta amyloid deposition, but does not address the reasons for amyloid deposition. Brain areas with relatively higher neuronal activity, metabolic demand, and production of reactive oxygen species in earlier life may have higher beta amyloid deposition in later life. The aim of this study was to investigate early life patterns of perfusion and late life patterns of amyloid deposition to determine the extent to which normative cerebral perfusion predisposes specific regions to future beta amyloid deposition. Materials and Methods: One hundred twenty-eight healthy, older human subjects (age: 56–87 years old; 44% women) underwent positron emission tomography (PET) imaging with [ 11 C]PiB for measures of amyloid burden. Cerebral perfusion maps derived from 47 healthy younger adults (age: 22–49; 47%) who had undergone single photon emission computed tomography (SPECT) imaging, were averaged to create a normative template, repre- sentative of young, healthy adults. Perfusion and amyloid measures were investigated in 31 cortical regions from the Hammers atlas. We examined the spatial relationship between normative perfusion patterns and amyloid pathophysiology. Results: The pattern of increasing perfusion (temporal lobe < parietal lobe < frontal lobe < insula/cingulate gyrus < occipital lobe; F(4,26) = 7.8, p = 0.0003) in young, healthy adults was not exactly identical to but approximated the pattern of increasing amyloid burden (temporal lobe < occipital lobe < frontal lobe < parietal lobe < insula/cingulate gyrus; F(4,26) = 5.0, p = 0.004) in older adults. However, investigating subregions within cortical lobes provided consistent agreement between ranked normative perfusion patterns and expected Thal staging of amyloid progression in AD (Spearman r = 0.39, p = 0.03). Conclusion: Our findings suggest that brain areas with normatively greater perfusion may be more susceptible to amyloid deposition in later life, possibly due to higher metabolic demand, and associated levels of oxidative stress and inflammation

Robust changes and sources of uncertainty in the projected hydrological regimes of Swiss catchments

Projections of discharge are key for future water resources management. These projections are subject to uncertainties, which are difficult to handle in the decision process on adaptation strategies. Uncertainties arise from different sources such as the emission scenarios, the climate models and their post-processing, the hydrological models and natural variability. Here we present a detailed and quantitative uncertainty assessment, based on recent climate scenarios for Switzerland (CH2011 data set) and covering catchments representative for mid-latitude alpine areas. This study relies on a particularly wide range of discharge projections resulting from the factorial combination of 3 emission scenarios, 10 to 20 regional climate models, 2 post-processing methods and 3 hydrological models of different complexity. This enabled us to decompose the uncertainty in the ensemble of projections using analyses of variance (ANOVA). We applied the same modeling setup to 6 catchments to assess the influence of catchment characteristics on the projected streamflow and focused on changes in the annual discharge cycle. The uncertainties captured by our setup originate mainly from the climate models and natural climate variability, but the choice of emission scenario plays a large role by the end of the century. The respective contribution of the different sources of uncertainty varied strongly among the catchments. The discharge changes were compared to the estimated natural decadal variability, which revealed that a climate change signal emerges even under the lowest emission scenario (RCP2.6) by the end of the century. Limiting emissions to RCP2.6 levels would nevertheless reduce the largest regime changes at the end of the 21st century by approximately a factor of two, in comparison to impacts projected for the high emission scenario SRES A2. We finally show that robust regime changes emerge despite the projection uncertainty. These changes are significant and are consistent across a wide range of scenarios and catchments. We propose their identification as a way to aid decision-making under uncertainty