201 research outputs found
Complexes of Schiff-base macrocycles and donor-expanded dipyrrins for catalysis and uranyl reduction
The modern world faces a number of challenges related to energy and the environment.
Atmospheric levels of carbon dioxide have now surpassed the 400 ppm
mark due to the burning of fossil fuels, yet despite its abundance and potential
use as a C1 feedstock for value-added products, there are both thermodynamic
and kinetic barriers associated with the strong carbon-oxygen bonds that preclude
its widespread deployment in industry. Nuclear energy is an alternative power
source that reduces carbon emissions by billions of tonnes each year, but there
are widespread concerns regarding the treatment of the radioactive waste that
it accrues (of which the main component is uranyl, [UO2]2+). Most of the work
presented in this thesis concerns the synthesis of transition-metal complexes, with
the aim of directing catalytic reactivity to convert CO2 to useful products. Part of
this thesis also concerns the synthesis of uranyl complexes and the study of uranyl
reduction chemistry, which is relevant to uranyl remediation and nuclear waste
treatment at a fundamental level. Making use of Earth-abundant metals to carry
out hydrocarbon oxidation catalysis is a further focus of this work, as the efficient
production of oxygenated compounds under mild conditions is of importance to
the fine-chemical industry.
Chapter 1 reviews important complexes reported in the literature that successfully
convert CO2 to useful products through molecular, homogenous electro-catalysis
and ring-opening copolymerisation catalysis. Reactions that exemplify a
two-electron reduction of uranyl (i.e. uranium(VI) to uranium(IV)) are reviewed,
along with uranyl complexes that undergo ligand-centred redox to give ligand-based
radicals. The state of the literature on hydrocarbon oxidation catalysis
is reviewed in the introduction. The development of multinuclear, macrocyclic
complexes and the reactivity of dinuclear Pacman complexes are also presented.
Chapter 2 reports the synthesis and characterisation of a new set of Schiff-base
macrocycles and acyclic dipyrrin ligands. A number of attempted synthetic
routes towards incorporating a dipyrrin coordination compartment in a macro-cyclic
setting are discussed. Differences in electronic structures between dipyrromethanes
and dipyrromethenes are also examined by theoretical and experimental methods.
Chapter 3 introduces the coordination chemistry of these new macrocycles
with zinc(II), where the isolation of dinuclear and tetranuclear complexes is
demonstrated using different zinc(II) precursors. Tetranuclear zinc-alkyl complexes
presented here are shown to be resistant to insertion chemistry with small molecules,
but readily form zinc-oxo, -hydroxyl and -alkoxide clusters upon protonolysis with
water and alcohols. These molecular clusters display reactivity towards CO2: a
zinc-hydroxyl complex precipitates ZnCO3 at high temperature; and zinc-alkoxide
complexes have been used to catalyse the copolymerisation reaction between CO2
and cyclohexene oxide to form polycarbonates.
Chapter 4 describes the synthesis of late-transition-metal complexes of macrocyclic
ligands and dipyrrins, and explores the relationship between macrocycle
geometry and electronic structure. Their reactivities towards CO2 are assessed
here, using cyclic voltammetry to assess the electro-catalytic activity of a number
of the complexes.
Chapter 5 reports the oxidation chemistry of hydrocarbon substrates catalysed
by copper(II) complexes. High-temperature catalysis occurs with bimetallic
copper(II) complexes, and this chapter describes how added FeCl3 acts as a co-catalyst,
leading to greater catalyst stability and allowing the catalytic reaction
to occur at room temperature. A range of analytical methods have been used to
deduce the catalytically active species, and chemical kinetic measurements have
been used to deduce a possible reaction mechanism.
Chapter 6 reports the synthesis of a uranyl(VI) dipyrrin complex and details
characterisation of its electronic structure by theoretical and experimental methods.
Theoretical modelling has indicated that the observed two-electron reduction of
uranium(VI) to uranium(IV) is facilitated by the dipyrrin ligand, representing a
novel uranyl reduction mechanism
Metal-Metal Bonding in Uranium-Group 10 Complexes
Heterobimetallic
complexes containing short uraniumâgroup
10 metal bonds have been prepared from monometallic IU<sup>IV</sup>(OAr<sup>P</sup>-Îș<sup>2</sup><i>O</i>,<i>P</i>)<sub>3</sub> (<b>2</b>) {[Ar<sup>P</sup>O]<sup>â</sup> = 2-<i>tert</i>-butyl-4-methyl-6-(diphenylphosphino)Âphenolate}.
The UâM bond in IU<sup>IV</sup>(ÎŒ-OAr<sup>P</sup>-1Îș<sup>1</sup><i>O</i>,2Îș<sup>1</sup><i>P</i>)<sub>3</sub>M<sup>0</sup>, M = Ni (<b>3âNi</b>), Pd (<b>3âPd</b>), and Pt (<b>3âPt</b>), has been
investigated by experimental and DFT computational methods. Comparisons
of <b>3âNi</b> with two further UâNi complexes
XU<sup>IV</sup>(ÎŒ-OAr<sup>P</sup>-1Îș<sup>1</sup><i>O</i>,2Îș<sup>1</sup><i>P</i>)<sub>3</sub>Ni<sup>0</sup>, X = Me<sub>3</sub>SiO (<b>4</b>) and F (<b>5</b>), was also possible via iodide substitution. All complexes were
characterized by variable-temperature NMR spectroscopy, electrochemistry,
and single crystal X-ray diffraction. The UâM bonds are significantly
shorter than any other crystallographically characterized dâf-block
bimetallic, even though the ligand flexes to allow a variable UâM
separation. Excellent agreement is found between the experimental
and computed structures for <b>3âNi</b> and <b>3âPd</b>. Natural population analysis and natural localized molecular orbital
(NLMO) compositions indicate that U employs both 5f and 6d orbitals
in covalent bonding to a significant extent. Quantum theory of atoms-in-molecules
analysis reveals UâM bond critical point properties typical
of metallic bonding and a larger delocalization index (bond order)
for the less polar UâNi bond than UâPd. Electrochemical
studies agree with the computational analyses and the X-ray structural
data for the UâX adducts <b>3âNi</b>, <b>4</b>, and <b>5</b>. The data show a trend in uraniumâmetal
bond strength that decreases from <b>3âNi</b> down to <b>3âPt</b> and suggest that exchanging the iodide for a fluoride
strengthens the metalâmetal bond. Despite short UâTM
(transition metal) distances, four other computational approaches
also suggest low UâTM bond orders, reflecting highly transition
metal localized valence NLMOs. These are more so for <b>3âPd</b> than <b>3âNi</b>, consistent with slightly larger UâTM
bond orders in the latter. Computational studies of the model systems
(PH<sub>3</sub>)<sub>3</sub>MUÂ(OH)<sub>3</sub>I (M = Ni, Pd) reveal
longer and weaker unsupported UâTM bonds vs <b>3</b>
Theory-guided enhancement of CO2 reduction to ethanol on Ag-Cu tandem catalysts via particle-size effects
In the CO2 reduction reaction, the design of electrocatalysts that selectively promote alcohols over hydrocarbons (e.g., ethanol over ethylene) hinges on the understanding of the pathways and specific sites that forms alcohols. Herein, theoretical considerations guide state-of-the-art synthesis of well-defined catalysts to show that higher selectivity toward ethanol is achieved on Cu(110) edge sites compared to Cu(100) terraces. Specifically, we study the catalytic behavior of Cu nano-cubes (Cucub) of different sizes in the framework of tandem catalysis with CO-producing Ag nanospheres. We predict and experimentally find that the smaller Cucub possess higher selectivity for ethanol in view of their larger edge-to-faces ratio and of the fact that ethylene is produced at terraces while ethanol is selectively produced at step edges. These results call for synthetic developments toward Cu nanostructures exposing only edge sites, such as hollow cubic nanocages, to further increase ethanol selectivity. More generally, this study encourages the application of well-defined nano catalysts as a bridge between theory and experiments in electrocatalysis.This work was financially supported by Gaznat S.A. J.R.P. acknowledges the H2020 Marie Curie Individual Fellowship grant SURFCAT with Agreement No. 837378. This publication was created as part of NCCR Catalysis, a National Centre of Competence in Research funded by the Swiss National Science Foundation. The theoretical effort was supported by Spanish MICIUNâs RTI2018-095460âB-I00, RamĂłn y Cajal RYC-2015-18996, and MarĂa de Maeztu MDM-2017-0767 Grants, and partly by Generalitat de Catalunya via 2017SGR13. M.J.K. and F.C.V. are thankful to Red Española de SupercomputaciĂłn (RES) for supercomputing time at SCAYLE (Projects QS-2019-3-0018, QS-2019-2-0023, and QCM-2019-1-0034). The use of supercomputing facilities at SURFsara was sponsored by NWO Physical Sciences
Polynuclear alkoxyâzinc complexes of bowl-shaped macrocycles and their use in the copolymerisation of cyclohexene oxide and CO2
The reactions between alcohols and the tetranuclear ethyl-Zn complexes of an ortho-phenylene-bridged polypyrrole macrocycle, Zn4Et4(L1) 1 and the related anthracenyl-bridged macrocyclic complex, Zn4Et4(THF)4(L2) 2 have been studied. With long-chain alcohols such as n-hexanol, the clean formation of the tetranuclear hexoxide complex Zn4(OC6H13)4(L1) 3 occurs. In contrast, the use of shorter-chain alcohols such as i-propanol results in the trinuclear complex Zn3(Ό2-OiPr)2(Ό3-OiPr)(HL1) 4 that arises from demetalation; this complex was characterised by X-ray crystallography. The clean formation of these polynuclear zinc clusters allowed a study of their use as catalysts in the ring-opening copolymerisation (ROCOP) reaction between cyclohexene oxide and CO2. In situ reactions involving the pre-catalyst 1 and n-hexanol formed the desired polymer with the best selectivity for polycarbonate (90%) at 30 atm CO2, whilst the activity and performance of pre-catalyst 2 was poor in comparison
Precise U-Pb zircon ages and geochemistry of Jurassic granites, Ellsworth-Whitmore terrane, central Antarctica
The Ellsworth-Whitmore Mountain terrane of central Antarctica was part of the early Paleozoic amalgamation of Gondwana, including a 13,000 m section of CambrianâPermian sediments in the Ellsworth Mountains deposited on Grenville-age crust. The Jurassic breakup of Gondwana involved a regional, bimodal magmatic event during which the Ellsworth-Whitmore terrane was intruded by intraplate granites before translation of the terrane to its present location in central Antarctica. Five widely separated granitic plutons in the Ellsworth-Whitmore terrane were analyzed for their whole-rock geochemistry (X-ray fluorescence), Sr, Nd, and Pb isotopic compositions, and U-Pb zircon ages to investigate the origins of the terrane magmas and their relationships to mafic magmatism of the 183 Ma Karoo-Ferrar large igneous province (LIP). We report high-precision (±0.1 m.y.) isotope dilutionâthermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from granitic rocks from the Whitmore Mountains (208.0 Ma), Nash Hills (177.4â177.3 Ma), Linck Nunatak (175.3 Ma), Pagano Nunatak (174.8 Ma), and the Pirrit Hills (174.3â173.9 Ma), and U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from the Whitmore Mountains (200 ± 5 Ma), Linck Nunatak (180 ± 4 Ma), Pagano Nunatak (174 ± 4 Ma), and the Pirrit Hills (168 ± 4 Ma). We then compared these results with existing K-Ar ages and Nd model ages, and used initial Sr, Nd, and Pb isotope ratios, combined with xenocrystic zircon U-Pb inheritance, to infer characteristics of the source(s) of the parent magmas. We conclude that the Jurassic plutons were not derived exclusively from crustal melts, but rather they are hybridized magmas composed of convecting mantle, subcontinental lithospheric mantle, and lower continental crustal contributions. The mantle contributions to the granites share isotopic similarities to the sources of other Jurassic LIP mafic magmas, including radiogenic 87Sr/86Sr (0.706â0.708), unradiogenic 143Nd/144Nd (ΔNd < â5), and Pb isotopes consistent with a low-” source (where ÎŒ = 238U/204Pb). Isotopes and zircon xenocrysts point toward a crustal end member of predominantly Proterozoic provenance (0.5â1.0 Ga; Grenville crust), extending the trends illustrated by Ferrar mafic intrusive rocks, but contrasting with the inferred Archean crustal and/or lithospheric mantle contributions to some basalts of the Karoo sector of the LIP. The Ellsworth-Whitmore terrane granites are the result of mafic rocks underplating the hydrous crust, causing crustal melting, hybridization, and fractionation to produce granitic magmas that were eventually emplaced as post-Ferrar, within-plate melts at higher crustal levels as the Ellsworth-Whitmore terrane rifted off Gondwana (47°S) before migrating to its current position (82°S) in central Antarctica
Pressure-induced inclusion of neon in the crystal structure of a molecular Cu2(pacman) complex at 4.67 GPa
Crystals of a Cu complex of the macrocyclic Schiff-base calixpyrrole or 'Pacman' ligand, Cu2(L), do not contain any solvent-accessible void space at ambient pressure, but adsorb neon at 4.67 GPa, forming Cu2(L)·3.5Ne
Towards dipyrrins:oxidation and metalation of acyclic and macrocyclic Schiff-base dipyrromethanes
Oxidation of acyclic Schiff-base dipyrromethanes cleanly results in dipyrrins, whereas the macrocyclic âPacmanâ analogues either decompose or form new dinuclear copper(ii) complexes that are inert to ligand oxidation.</p
Earth-Abundant Mixed-Metal Catalysts for Hydrocarbon Oxygenation
The
oxygenation of aliphatic and aromatic hydrocarbons using earth-abundant
Fe and Cu catalysts and âgreenâ oxidants such as hydrogen
peroxide is becoming increasingly important to atom-economical chemical
processing. In light of this, we describe that dinuclear Cu<sup>II</sup> complexes of pyrrolic Schiff-base macrocycles, in combination with
ferric chloride (FeCl<sub>3</sub>), catalyze the oxygenation of Ï-activated
benzylic substrates with hydroperoxide oxidants at room temperature
and low loadings, representing a novel design in oxidation catalysis.
Mass spectrometry and extended X-ray absorption fine structure analysis
indicate that a cooperative action between Cu<sup>II</sup> and Fe<sup>III</sup> occurs, most likely because of the interaction of FeCl<sub>3</sub> or FeCl<sub>4</sub><sup>â</sup> with the dinuclear
Cu<sup>II</sup> macrocycle. Voltammetric measurements highlight a
modulation of both Cu<sup>II</sup> and Fe<sup>III</sup> redox potentials
in this adduct, but electron paramagnetic resonance spectroscopy indicates
that any CuâFe intermetallic interaction is weak. High ketone/alcohol
product ratios, a small reaction constant (Hammett analysis), and
small kinetic isotope effect for H-atom abstraction point toward a
free-radical reaction. However, the lack of reactivity with cyclohexane,
oxidation of 9,10-dihydroanthracene, oxygenation by the hydroperoxide
MPPH (radical mechanistic probe), and oxygenation in dinitrogen-purge
experiments indicate a metal-based reaction. Through detailed reaction
monitoring and associated kinetic modeling, a network of oxidation
pathways is proposed that includes âwell-disguisedâ
radical chemistry via the formation of metal-associated radical intermediates
Patient-reported outcome measures used in patients with primary sclerosing cholangitis: a systematic review
Abstract Background Primary Sclerosing Cholangitis (PSC) is a rare chronic, cholestatic liver condition in which patients can experience a range of debilitating symptoms. Patient reported outcome measures (PROMs) could provide a valuable insight into the impact of PSC on patient quality of life and symptoms. A previous review has been conducted on the quality of life instruments used in liver transplant recipients. However, there has been no comprehensive review evaluating PROM use or measurement properties in PSC patientsâ to-date. The aim of the systematic review was to: (a) To identify and categorise which PROMs are currently being used in research involving the PSC population (b) To investigate the measurement properties of PROMs used in PSC. Methods A systematic review of Medline, EMBASE and CINAHL, from inception to February 2018, was undertaken. The methodological quality of included studies was assessed using the Consensus-based Standards for selection of health Measurement Instruments (COSMIN) checklist. Results Thirty-seven studies were identified, which included 36 different PROMs. Seven PROMs were generic, 10 disease-specific, 17 symptom-specific measures and 2 measures on dietary intake. The most common PROMs were the Short form-36 (SF-36) (nâ=â15) and Chronic liver disease questionnaire (CLDQ) (nâ=â6). Only three studies evaluated measurement properties, two studies evaluated the National Institute of Diabetes Digestive and Kidney Diseases Liver Transplant (NIDDK-QA) and one study evaluated the PSC PRO; however, according to the COSMIN guidelines, methodological quality was poor for the NIDDK-QA studies and fair for the PSC PRO study. Conclusion A wide variety of PROMs have been used to assess health-related quality of life and symptom burden in patients with PSC; however only two measures (NIDDK-QA and PSC PRO) have been formally validated in this population. The newly developed PSC PRO requires further validation in PSC patients with diverse demographics, comorbidities and at different stages of disease; however this is a promising new measure with which to assess the impact of PSC on patient quality of life and symptoms
Getting into hot water:sick guppies frequent warmer thermal conditions
Ectotherms depend on the environmental temperature for thermoregulation and exploit thermal regimes that optimise physiological functioning. They may also frequent warmer conditions to up-regulate their immune response against parasite infection and/or impede parasite development. This adaptive response, known as âbehavioural feverâ, has been documented in various taxa including insects, reptiles and fish, but only in response to endoparasite infections. Here, a choice chamber experiment was used to investigate the thermal preferences of a tropical freshwater fish, the Trinidadian guppy (Poecilia reticulata), when infected with a common helminth ectoparasite Gyrodactylus turnbulli, in female-only and mixed-sex shoals. The temperature tolerance of G. turnbulli was also investigated by monitoring parasite population trajectories on guppies maintained at a continuous 18, 24 or 32 °C. Regardless of shoal composition, infected fish frequented the 32 °C choice chamber more often than when uninfected, significantly increasing their mean temperature preference. Parasites maintained continuously at 32 °C decreased to extinction within 3 days, whereas mean parasite abundance increased on hosts incubated at 18 and 24 °C. We show for the first time that gyrodactylid-infected fish have a preference for warmer waters and speculate that sick fish exploit the upper thermal tolerances of their parasites to self medicate
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