School of Chemistry and Chemical Biology

Quaternarized pdppz: synthesis, DNA-binding and biological studies of a novel dppz derivative that causes cellular death upon light irradiation.

2013-05-16T10:54:10Z

Quaternarized pdppz: synthesis, DNA-binding and biological studies of a novel dppz derivative that causes cellular death upon light irradiation. Elmes, Robert B.P.; Erby, Marialuisa; Cloonan, Suzanne M.; Quinn, Susan J.; Williams, D. Clive; Gunnlaugsson, Thorfinnur The quaternarized pdppz derivative 1 was shown to bind strongly to DNA with concomitant changes in its ground and excited state photophysical properties. Furthermore, the compound also showed rapid cellular uptake, and induced apoptosis upon light irradiation in various cancer cell lines after 24 hours of incubation.

Synthesis and photophysical evaluations of fluorescent quaternary bipyridyl-1,8-naphthalimide conjugates as nucleic acid targeting agents

2013-05-16T10:51:21Z

Synthesis and photophysical evaluations of fluorescent quaternary bipyridyl-1,8-naphthalimide conjugates as nucleic acid targeting agents Ryan, Gary J.; Elmes, Robert B.P.; Quinn, Susan J.; Gunnlaugsson, Thorfinnur A family of organic molecules containing the DNA intercalating chromophores, 4-nitro- and 4-amino 1,8-naphthalimide, conjugated to a diquat derivative by an ‘orthogonal’ phenyl spacer have been prepared and characterised. Their binding interactions with double-stranded DNA were studied by a variety of spectroscopic techniques. These charged organic compounds are found to exhibit excellent binding affinities to DNA with binding constants comparable to those exhibited by metal complexes.

A comparative picosecond transient infrared study of 1-methylcytosine and 5'-dCMP that sheds further light on the excited states of cytosine derivatives

2013-05-16T09:22:07Z

A comparative picosecond transient infrared study of 1-methylcytosine and 5'-dCMP that sheds further light on the excited states of cytosine derivatives Keane, Páraic M.; Wojdyla, Michal; Doorley, Gerard W.; Watson, Graeme W.; Clark, Ian P.; Greetham, Gregory M.; Parker, Anthony W.; Towrie, Michael; Kelly, John M.; Quinn, Susan J. The role of N1-substitution in controlling the deactivation processes in photoexcited cytosine derivatives has been explored using picosecond time-resolved IR spectroscopy. The simplest N1-substituted derivative, 1-methylcytosine, exhibits relaxation dynamics similar to the cytosine nucleobase and distinct from the biologically relevant nucleotide and nucleoside analogues, which have longer-lived excited-state intermediates. It is suggested that this is the case because the sugar group either facilitates access to the long-lived (1)n(O)Ï * state or retards its crossover to the ground state.

Ultrafast IR spectroscopy of polymeric cytosine nucleic acids reveal the long-lived species is due to a localised state

2013-05-16T09:17:19Z

Ultrafast IR spectroscopy of polymeric cytosine nucleic acids reveal the long-lived species is due to a localised state Keane, Páraic M.; Wojdyla, Michal; Doorley, Gerard W.; Kelly, John M.; Clark, Ian P.; Parker, Anthony W.; Greetham, Gregory M.; Towrie, Michael; Magno, Luís M.; Quinn, Susan J. The decay pathways of UV-excited cytosine polymers are investigated using picosecond time-resolved infrared spectroscopy. Similar yields of a non-emissive (1)nÏ * state are found in the single-stranded dC(30) polymer as in the dCMP monomer, but with a longer lifetime in the polymer (80 ps vs. 39 ps). A longer lifetime is also found in the d(CpC) dinucleotide. No evidence of excimer states is observed, suggesting that localised (1)nÏ * excited states are the most significant intermediates present on the picosecond timescale.

Excited state behaviour of substituted dipyridophenazine Cr(III) complexes in the presence of nucleic acids

2013-05-16T09:12:35Z

Excited state behaviour of substituted dipyridophenazine Cr(III) complexes in the presence of nucleic acids Wojdyla, Michal; Smith, Jayden A.; Vasudevan, Suni; Quinn, Susan J.; Kelly, John M. The photophysics and photochemistry of [Cr(phen)2(dppz)]3+ and its 11,12-substituted derivatives [Cr(phen)2(X2dppz)]3+ {X = Me or F} have been studied in the presence of purine nucleotides or DNA using steady state and time-resolved absorption and luminescence spectroscopy. 5'-Adenosine monophosphate (5'-AMP) shows only a weak interaction with the excited states of each complex. By contrast they are efficiently quenched by 5'-guanosine monophosphate (5'-GMP), consistent with photo-induced electron transfer. Laser flash photolysis spectroscopy in the presence of 5'--GMP suggests that both forward and back electron-transfers are rapid. All complexes also display a strong affinity for DNA and evidence for both static and dynamic quenching mechanisms is provided.

Incandescent Porous Carbon Microspheres to Light up Cells: Solution Phenomena and Cellular Uptake

2013-05-16T09:09:06Z

Incandescent Porous Carbon Microspheres to Light up Cells: Solution Phenomena and Cellular Uptake Duffy, Paul; Magno, Luís M.; Yadavc, Rahul B.; Roberts, Selene K.; Ward, Andrew D.; Botchway, Stanley W.; Colavita, Paula E.; Quinn, Susan J. Carbon based materials are attractive for biological applications because of their excellent biocompatibility profile. Porous carbons with high specific surface area are particularly interesting because it is possible in principle to leverage their properties to deliver high drug payloads. In this work, porous carbon microspheres with high specific surface area were prepared and studied in solution and in cells. Raman optical tweezer trapping of microspheres, excited by 532 nm, results in graphitization and incandescence in solvents that display poor heat conduction. Fluorescence confocal microscopy imaging was used to demonstrate the uptake of fluorescently labelled microspheres by cells and the ability to leverage their optical absorptivity in order to cause carbon graphitization and cell death.

Photophysical studies of CdTe quantum dots in the presence of a zinc cationic porphyrin

2013-05-16T08:30:41Z

Photophysical studies of CdTe quantum dots in the presence of a zinc cationic porphyrin Keane, Páraic M.; Gallagher, Shane A.; Magno, Luís M.; Leising, Miriam J.; Clark, Ian P.; Greetham, Gregory M.; Towrie, Michael; Gun'ko, Yurii; Kelly, John M.; Quinn, Susan J. The photophysical properties of 2.3 nm thioglycolic acid (TGA) coated CdTe quantum dots (QDs) prepared by a reflux method have been studied in the presence of cationic meso-tetrakis(4-N-methylpyridyl) zinc porphyrin (ZnTMPyP4). Addition of the CdTe QDs to the porphyrin in H2O results in a marked red-shift and hypochromism in the porphyrin absorption spectrum, indicative of a non-covalent binding interaction with the QD surface. Only low equivalents of the quantum dot were required for complete quenching of the porphyrin fluorescence revealing that one quantum dot may quench more than one porphyrin. Similarly addition of porphyrin to the quantum dot provided evidence for very efficient quenching of the CdTe photoluminescence, suggesting the formation of CdTe'porphyrin aggregates. Definitive evidence for such aggregates was gathered using small angle X-ray spectroscopy (SAXS). Ultrafast transient absorption data are consistent with very rapid photoinduced electron transfer (1.3 ps) and the resultant formation of a long-lived porphyrin species.

Synthesis and spectroscopic studies of chiral CdSe quantum dots

2013-04-30T11:16:22Z

Synthesis and spectroscopic studies of chiral CdSe quantum dots Gallagher, Shane A.; Moloney, Mícheál P.; Wojdyla, Michal; Quinn, Susan J.; Kelly, John M.; Gun'ko, Yurii K. Using microwave irradiation, water soluble, optically active, penicillamine (Pen) capped CdSe nanocrystals with broad spectral distribution (430-780 nm) of photoluminescence have been produced and studied by a range of instrumental techniques including absorption, circular dichroism and both steady state and time resolved photoluminescence spectroscopy. The photoluminescence of these nanocrystals is attributed to emission from surface defect states. The decay of the excited state in the nanosecond region, which can be analysed as a triple exponential, depends strongly on the emission wavelength selected, but only weakly on the excitation wavelength.

Towards selective catalytic oxidations using in-situ generated H2O2

2013-04-18T17:04:12Z

Towards selective catalytic oxidations using in-situ generated H2O2 Sullivan, James A.; O'Callaghan, Niamh A series of Ti-modified mesoporous SiO2 materials (SBA-15 and MCF) are prepared, characterised and used as catalysts in the selective epoxidation of a probe alkene (cyclohexene) using H2O2 as an oxidising agent. Similarly, a series of mesoporous SiO2-supported monometallic and bimetallic DMAP-stabilized Au and Pd nanoparticles were prepared, characterised and used as catalysts in the production of H2O2 from dilute H2(g) + O2(g) mixtures.The metallic nanoparticles were then supported on the Ti-modified mesoporous SiO2 and these hybrid materials were characterised and their activities in the selective epoxidation of alkenes in the presence of H2 (g) + O2 (g) mixtures (where H2O2 would be formed in situ) were studied. The bimetallic Au / Pd nanoparticles (which were most active in the production of H2O2 from H2 and O2) were not the most active or selective in the combined reaction. This was ascribed to the Pd component of the nanoparticles promoting hydrogenation of the probe alkene more rapidly than the formation of H2O2. The selectivity of Au nanoparticles in the presence of H2(g) + O2(g) was higher than that of the same catalysts in the presence of H2O2(aq).

Rare earth (La, Nd, Pr) doped ceria zirconia solid solutions for soot combustion

2013-04-18T11:10:19Z

Rare earth (La, Nd, Pr) doped ceria zirconia solid solutions for soot combustion Dulgheru, Petrica; Sullivan, James A. A series of rare earth (RE) (La, Nd, Pr) ceria zirconia materials were analysed for their soot combustion activity in air and in NO/O2. The materials were characterised using DRIFT spectroscopy. In general the presence of the RE dopant increases the activity of undoped CexZr1-xO2. The La and Pr doped catalysts showed increased low temperature activity in the presence of NO/O2 while the effect was less pronounced in case of Nd - doped samples. FTIR data has shown that the catalysts interact differently to NO/O2 mixtures in that they do not form significant quantities of adsorbed nitrite-type species. We postulate that this species is a precursor to NO2 formation which in turn increases soot combustion.

The nature of surface deposits following valeric acid interactions with Al2O3-supported Alkaline Earth Oxide catalysts: Towards cellulosic biofuels

2013-04-15T11:39:32Z

The nature of surface deposits following valeric acid interactions with Al2O3-supported Alkaline Earth Oxide catalysts: Towards cellulosic biofuels Sullivan, James A.; Sherry, Linda Two Al2O3-supported alkaline earth metal oxide catalysts (MgO and BaO) were contacted with valeric acid at 250 °C. Each formed amounts of 5-nonanone (BaO more than MgO). A significant deposition of hydrocarbonaceous material onto the catalyst surface is noted. This adsorbed material is characterised using TGA and FTIR and relates to a carboxylate species.

The Preparation of Group II Oxide Catalysts Through Acetate Calcination: The Influence of a Support on the Nature of the Final Catalyst.

2013-04-15T11:37:04Z

The Preparation of Group II Oxide Catalysts Through Acetate Calcination: The Influence of a Support on the Nature of the Final Catalyst. Sullivan, James A.; Sherry, Linda The preparation of supported and unsupported group II oxide catalysts through oxidation of analogous group II acetates (Mg, Ca and Ba) in the presence and absence of a mesoporous silica material (SBA-15) was analysed using TGA. In the absence of the mesoporous support the acetates oxidised at different temperatures with a stability trend whereby Mg

Synthesis of a conformationally constrained delta-amino acid building block

2013-03-20T12:15:19Z

Synthesis of a conformationally constrained delta-amino acid building block O'Reilly, Elaine; Pes, Lara; Ortin, Yannick; Mueller-Bunz, Helge; Paradisi, Francesca Conformationally restricted amino acids are important components in peptidomimetics and drug design. Herein, we describe the synthesis of a novel, non-proteinogenic constrained delta amino acid containing a cyclobutane ring, cis-3(aminomethyl)cyclobutane carboxylic acid (ACCA). The synthesis of the target amino acid was achieved in seven steps, with the key reaction being a base induced intramolecular nucleophilic substitution. A small library of dipeptides was prepared through the coupling of ACCA with proteinogenic amino acids.

Effect of organic solvents on the activity and stability of halophilic alcohol dehydrogenase (ADH2) from Haloferax volcanii

2013-03-01T10:01:10Z

Effect of organic solvents on the activity and stability of halophilic alcohol dehydrogenase (ADH2) from Haloferax volcanii Alsafadi, Diya; Paradisi, Francesca The effect of various organic solvents on the catalytic activity, stability and substrate specificity of alchohol dehydrogenase from Haloferax volcanii (HvADH2) was evaluated. The HvADH2 showed remarkable stability and catalysed the reaction in aqueous–organic medium containing dimethyl sulfoxide (DMSO) and methanol (MeOH). Tetrahydrofuran and acetonitrile were also investigated and adversely affected the stability of the enzyme. High concentration of salt, essential to maintain the enzymatic activity and structural integrity of the halophilic enzyme under standard conditions may be partially replaced by DMSO and MeOH. The presence of organic solvents did not induce gross changes in substrate specificity. DMSO offered a protective effect for the stability of the enzyme at nonoptimal pHs such as 6 and 10. Salt and solvent effects on the HvADH2 conformation and folding were examined through fluorescence spectroscopy. The fluorescence findings were consistent with the activity and stability results and corroborated the denaturing properties of some solvents. The intrinsic tolerance of this enzyme to organic solvent makes it highly attractive to industry.

A comparison of two novel alcohol dehydrogenase enzymes (ADH1 and ADH2) from the extreme halophile Haloferax volcanii

2013-02-28T11:10:16Z

A comparison of two novel alcohol dehydrogenase enzymes (ADH1 and ADH2) from the extreme halophile Haloferax volcanii Timpson, Leanne M.; Liliensiek, Ann-Kathrin; Alsafadi, Diya; Cassidy, Jennifer; Sharkey, Michael A.; Liddell, Susan; Allers, Thorsten; Paradisi, Francesca Haloarchaeal alcohol dehydrogenases are exciting biocatalysts with potential industrial applications. In this study, two alcohol dehydrogenase enzymes from the extremely halophilic archaeon Haloferax volcanii (HvADH1 and HvADH2) were homologously expressed and subsequently purified by immobilized metal-affinity chromatography. The proteins appeared to copurify with endogenous alcohol dehydrogenases, and a double Δadh2 Δadh1 gene deletion strain was constructed to prevent this occurrence. Purified HvADH1 and HvADH2 were compared in terms of stability and enzymatic activity over a range of pH values, salt concentrations, and temperatures. Both enzymes were haloalkaliphilic and thermoactive for the oxidative reaction and catalyzed the reductive reaction at a slightly acidic pH. While the NAD+-dependent HvADH1 showed a preference for short-chain alcohols and was inherently unstable, HvADH2 exhibited dual cofactor specificity, accepted a broad range of substrates, and, with respect to HvADH1, was remarkably stable. Furthermore, HvADH2 exhibited tolerance to organic solvents. HvADH2 therefore displays much greater potential as an industrially useful biocatalyst than HvADH1.

The influence of SO2-/4 on the catalytic combustion of soot using O-2 and NO/O-2 mixtures over Na-promoted Al2O3 catalysts

2013-01-11T14:54:04Z

The influence of SO2-/4 on the catalytic combustion of soot using O-2 and NO/O-2 mixtures over Na-promoted Al2O3 catalysts Sullivan, James A.; Keane, Orla; Maguire, Linda Na/Al2O3 catalysts are tested in the soot oxidation reaction using O-2 and NO/O-2 as oxidising agents. The activity of the catalysts . varies with oxidising agent and the presence of surface SO42-.. NO(g) increases conversion of soot to COx-. Surface SO42- has no effect on activity for the C-(s) + O-2 reaction but decreases activity in the Carbon + NO/O-2 oxidation. This is interpreted in terms of SO42- blocking NO adsorption sites, thus poisoning the 4 NO + O-2 --> NO2 reaction, which is essential for the NOx promoted oxidation.

Selective H-D exchange catalysed by aqueous phase and immobilised Pd nanoparticles

2013-01-09T17:33:12Z

Selective H-D exchange catalysed by aqueous phase and immobilised Pd nanoparticles Sullivan, James A.; Flanagan, Keith A.; Hain, Holger Pd nanoparticles (with a mean diameter of 3.4 ± 0.5 nm) prepared through BH4− reduction of Na2PdCl4 can catalyse selective H–D exchange (through reaction with D2O) at the carbon α to the N atom of a pyridine ring. Oxidised Pd(II) complex species also present as dissolved species in the nanoparticle dispersion play no part in the exchange. We have studied the effects of several different variables on the rate of the selective H–D exchange activity for a model pyridine, i.e. 4-dimethylaminopyridine (DMAP). These variables include temperature, nanoparticle aging, nanoparticle re-reduction with H2 and nanoparticle immobilisation onto multi-walled carbon nanotubes (MWCNTs). We have also extended the study to related pyridine containing molecules.

Attempts at an in-situ Raman study of Ceria / Zirconia catalysts in PM combustion

2013-01-09T17:26:14Z

Attempts at an in-situ Raman study of Ceria / Zirconia catalysts in PM combustion Sullivan, James A.; Dulgheru, Petrica; Atribak, Idriss; Bueno-López, Agustín; García-García, Avelina CexZr1−xO2 catalysts with various Ce/Zr contents were studied using Raman spectroscopy under different gaseous atmospheres, at different temperatures and in the presence of a model soot. Catalysts with high concentrations of Zr fluoresced at elevated temperatures making analysis of their spectra impossible. This effect became even more pronounced at higher temperatures. CeO2 and solid solutions with relatively low concentrations of Zr showed a red shift and a decrease in intensity of the characteristic F2g peak at high temperatures under different atmospheres. The magnitude of the latter effect was higher under reducing atmospheres. These changes are reversible upon cooling, showing that they relate to a lattice expansion effect rather than any major chemical change to the material. In the presence of the model soot the Raman spectra of all materials was much decreased due to the absorption of the incident and scattered radiation by the soot particles. The presence of soot does not change the relative intensities or positions of the peaks in the spectra of the solid solutions. Evidence is shown for the production of a Ce3+–CO species following interaction between the soot and the surface at high temperature in an inert atmosphere.

Beneficial and problematic interactions between NOx trapping materials and carbonaceous particulate matter

2013-01-09T17:02:06Z

Beneficial and problematic interactions between NOx trapping materials and carbonaceous particulate matter Sullivan, James A.; Keane, Orla; Cassidy, Andrew The presence of a NOx trapping BaO component with a Pt active phase does not per-se promote carbon combustion. However when coupled with trap regeneration, promotion of soot combustion is seen. Operating NOx trapping materials promote combustion due to the periodic localised increases in [NO2](g) generated during the regeneration rather than due to a co-generated exotherm. However, it has been noted that the presence of soot prevents NOx traps from employing their full trapping capacity.

Preparation and characterization of 4-dimethylaminopyridine-stabilized palladium nanoparticles

2013-01-09T16:40:18Z

Preparation and characterization of 4-dimethylaminopyridine-stabilized palladium nanoparticles Flanagan, Keith A.; Sullivan, James A.; Mueller-Bunz, Helge 4-Dimethylaminopyridine (DMAP)-stabilized palladium nanoparticles with a mean diameter of 3.4 +/- 0.5 mn are prepared from the aqueous phase reduction of Na2PdCl4 using NaBH4 in the presence of DMAP. TEM and UV-vis spectroscopy characterization of the nanoparticle dispersion shows no obvious change in the nanoparticles several months after preparation. H-1 NMR spectroscopy of the nanoparticles shows that the nanoparticle dispersion also contains a boron/DMAP complex and two palladium/DMAP complexes. One of the palladium complexes crystallizes out of the dispersion and is identified as Pd(DMAP)(4)(OH)(2) by X-ray crystallography. Following extensive analysis, it is believed that the palladium/DMAP complexes are formed following the oxidation of the palladium nanoparticles. The prepared nanoparticle dispersion promotes selective hydrogen/deuterium (H/D) exchange on the carbon atoms alpha to the endocyclic nitrogen atom on the DMAP-stabilizing ligands through reaction with D2O. This activity is attributed to the presence of the nanoparticles rather than to the presence of the oxidized palladium/DMAP complexes.

Suzuki coupling activity of an aqueous phase Pd nanoparticle dispersion and a carbon nanotube/Pd nanoparticle composite

2013-01-09T16:40:53Z

Suzuki coupling activity of an aqueous phase Pd nanoparticle dispersion and a carbon nanotube/Pd nanoparticle composite Sullivan, James A.; Flanagan, Keith A.; Hain, Holger An aqueous phase dispersion of Pd nanoparticles stabilised by 4-dimethylaminopyridine (DMAP) promotes model Suzuki coupling reactions. The dispersion contains Pd nanoparticles of 3.4 ± 0.5 nm and a Pd(II) species (Pd(DMAP)4(OH)2) which forms following aerobic oxidation of the nanoparticles. The activity of the nanoparticle dispersion in promoting the Suzuki reactions is directly proportional to the size of the halogen on the substrate (as is usual for these coupling reactions) and also to the age of the nanoparticle dispersion. The Pd(DMAP)4(OH)2 complex can be isolated from the dispersion and is found to be very active in promoting the reactions. Its formation following aerobic oxidation of the nanoparticles is proposed as the reason for the improved activity of the dispersion with age. The nanoparticles present in the dispersion can, through displacement of the stabilising ligand, be immobilised onto functionalised multi-walled carbon nanotubes (MWCNTs) and the composite formed is an active and recyclable catalyst. The MWCNT/Pd-DMAP NP composite acts as a reservoir of dissolved Pd species, which function as homogeneous catalysts under reaction conditions.

The combustion of carbon particulates using NO/O2 mixtures : The influence of SO42− and NOx trapping materials

2013-01-07T17:30:30Z

The combustion of carbon particulates using NO/O2 mixtures : The influence of SO42− and NOx trapping materials Sullivan, James A.; Keane, Orla The activity of several catalysts are studied in the soot combustion reaction using air and NO/air as oxidising agents. Over Al2O3-supported catalysts NO(g) is a promoter for the combustion reaction with the extent of promotion depending on the Na loading. Over these catalysts SO42− poisons this promotion by preventing NO oxidation through a site blocking mechanism. SiO2 is unable to adsorb NO or catalyse its oxidation and over SiO2-supported Na catalysts NO(g) inhibits the combustion reaction. This is ascribed to a competition between NO and O2. Over Fe-ZSM-5 catalysts the presence of a NOx trapping component does not increase the combustion of soot in the presence of NO(g) and it is proposed that this previously reported effect is only seen under continuous NOx trap operation as NO2 is periodically released during regeneration and thus available for soot combustion. Experiments during which the [NO](g) is varied show that CO, rather than an adsorbed carbonyl-like intermediate, is formed upon reaction between NO2 (the proposed oxygen carrier) and soot.

NH3 and urea in the selective catalytic reduction of NOx over oxide-supported copper catalysts

2013-01-07T17:24:05Z

NH3 and urea in the selective catalytic reduction of NOx over oxide-supported copper catalysts Sullivan, James A.; Doherty, Julie A. The temperature-programmed activity of a series of oxide-supported (TiO2, Al2O3 and SiO2) Cu catalysts formed from two different Cu precursors (Cu(NO3)(2) and CuSO4) for the selective catalytic reduction of NOx using solutions of urea as a reductant have been determined. These activities are compared to those found using NH3 as a reducing agent over the same catalysts in the presence of H2O and it is found that catalysts that are active for the selective reduction of NOx with NH3 are inactive for its reduction using solutions of urea. Poisoning of the surface by H2Oads is not responsible for all of this decrease in activity and it is postulated that the urea is not hydrolysing to form NH3 over the catalysts but rather is oxidising to form N-2 or forming passivated layers of polymeric melamine complexes on the surface. The catalysts were characterised by temperature-programmed reduction while temperature-programmed desorption and oxidation of NH3 and temperature programmed decomposition of urea are used to characterise the interaction of both reductants with the various catalysts.

The effect of support and copper precursor on the activity of supported CuO catalysts in the Selective Catalytic Reduction of NOx.

2013-01-07T17:12:24Z

The effect of support and copper precursor on the activity of supported CuO catalysts in the Selective Catalytic Reduction of NOx. Sullivan, James A. A series of Cu catalysts were studied as a function of support (Al2O3, TiO2 and SiO2) and Cu precursor (ex-SO4 and ex-NO3) for activity in the SCR-NH3 reaction. The catalysts were characterized using NOx TPD and SEM/EDAX analysis and the effects of residual sulphur interpreted in terms of site-blocking and NH3 activation mechanisms.

The effect of C(s) on the trapping of NOx onto Pt/Ba/Al2O3 catalysts

2013-01-07T17:11:54Z

The effect of C(s) on the trapping of NOx onto Pt/Ba/Al2O3 catalysts Sullivan, James A.; Dulgheru, Petrica Pt/BaO/Al2O3 catalysts react with NO/O2 mixtures to form barium nitrites and nitrates. In the presence of small amounts of C(s) admixed with the catalyst the concentrations of NOx adsorbed is considerably reduced and the stability of the stored NOx is decreased. These results suggest that there are important implications for any attempts to combine NSR and particulate combustion systems within a single catalytic system. Furthermore, it is clear that there are significant concentrations of mobile NOx species present in the environs of the NOx trap prior to fixation on BaO, i.e. the transfer between NO2 formed on the Pt component of the trap and the “fixed” NOx in Ba(NO3)2 involves significant concentrations of gaseous or surface mobile NO2.

Tethering of bi-nuclear complexes to SBA-15 and their application in CO2 hydrogenation

2013-01-07T16:47:22Z

Tethering of bi-nuclear complexes to SBA-15 and their application in CO2 hydrogenation Morgan, Grace; Fennell, Kevin; Kishore, M. Jhansi L.; Sullivan, James A. Two bimetallic cryptates (containing Cu and Co), which have previously been shown to react with and activate atmospheric CO2, have been tethered to modified mesoporous SiO2 and their activities in promoting the CO2 + H2 reaction has been analysed. The cryptates have been tethered to –C3H6Cl modified SBA-15 through a condensation reaction between surface alkyl chlorides and 2° amines of the ligands releasing HCl and forming a 3° amine. The materials have been characterised using BET, TGA, FTIR and elemental analysis and their activity in promoting the CO2 + H2 reaction has been tested under batch reactor conditions. Co ions appear to selectively populate the medal sites of the tethered ligands while Cu ions appear to deposit on the surface as Cu(BF4)2 salts. The composite materials generate CO and CH4 from the CO2 + H2 mixtures. Co-containing catalysts are more effective than the Cu analogues in promoting the reaction.

A combination of NOx trapping materials and Urea-SCR catalysts for use in the removal of NOx from mobile diesel engines

2013-01-07T16:35:01Z

A combination of NOx trapping materials and Urea-SCR catalysts for use in the removal of NOx from mobile diesel engines Sullivan, James A.; Keane, Orla Preliminary studies on a series of nanocomposite BaO–Fe ZSM-5 materials have been carried out to determine the feasibility of combining NOx trapping and SCR-NH3 reactions to develop a system that might be applicable to reducing NOx emissions from diesel-powered vehicles. The materials are analysed for SCR-NH3 and SCR-urea reactivity, their NOx trapping and NH3 trapping capacities are probed using temperature programmed desorption (TPD) and the activities of the catalysts for promoting the NH3 ads + NO/O2 → N2 and NOx ads + NH3 → N2 reactions are studied using temperature programmed surface reaction (TPSR).

The role of Bronstead acidity in poisoning the SCR-urea reaction over FeZSM-5 catalysts

2013-01-07T16:12:22Z

The role of Bronstead acidity in poisoning the SCR-urea reaction over FeZSM-5 catalysts Sullivan, James A.; Keane, Orla Two series of FeZSM-5 catalysts prepared from Na+ and NH4+ ZSM-5 precursors are studied in the selective reduction of NOx using NH3 and urea as reducing agents. All Fe-containing catalysts are active for NOx reduction in the SCR-NH3 reaction with ex-NH4+ catalysts being more active than ex-Na+ materials and the activity depending (to a minor extent within each series of catalysts) upon [Fe]. Catalysts with Bronstead acid sites also show a small transient deNO(x) activity at low temperatures. All catalysts are less active for the SCR-urea reaction but the ex-Na+ catalysts retain far more deNO(x) activity than the ex-NH4+ materials. NH3 TPD shows that strongly binding Bronstead acid sites are present on the ex-NH4+ materials and H+-treated parent zeolites while Urea TPD shows that the mode of decomposition of urea differs as a function of initial zeolite counter-ion. Urea TPSR shows that the reaction between adsorbed urea and gaseous NO/O-2 is related to [Fe]. It is proposed that the decreased activity of the ex-NH4(+) catalysts in the SCR-urea reaction is due to a less favourable mode of decomposition over these catalysts. Furthermore it is suggested that the Bronstead acidity plays some part in this less favoured decomposition.

Carbon-Doped TiO2 and Carbon, Tungsten-Codoped TiO2 through Sol-Gel Processes in the Presence of Melamine Borate: Reflections through Photocatalysis

2012-12-20T16:59:15Z

Carbon-Doped TiO2 and Carbon, Tungsten-Codoped TiO2 through Sol-Gel Processes in the Presence of Melamine Borate: Reflections through Photocatalysis Neville, Elaine M.; Mattle, Michael J.; Loughrey, David; Rajesh, Bashyam; Rahman, Mahfujur; MacElroy, J. M. Don; Sullivan, James A.; Thampi, Ravindranathan A series of C-doped, W-doped, and C,Wcodoped TiO2 samples have been prepared using modified sol-gel techniques. Reproducible inexpensive C-doping arises from the presence of melamine borate in a sol-gel mixture, whereas W-doping is from the addition of tungstic acid to the sol. The materials have been characterized using elemental analysis, N2 physisorption (BET), thermogravimetric analysis, X-ray diffraction, Raman, X-ray photoelectron, UV-vis spectroscopies, and photocatalytic activity measurements. Doping C and W independently results in an increased absorbance in the visible region of the spectrum with a synergistic effect in increased absorbance when both elements are codoped. The increased visible-light absorbance of the W-doped or codoped materials is not reflected in photocatalytic activity. Visiblelight- induced photocatalytic activity of C-doped material was superior to that of an undoped catalyst, paving the way for its application under only visible-light irradiation conditions. A significant fraction of the spectral red shift commonly observed with doped catalysts might be due to the formation of color centers as a result of defects associated with oxygen vacancies, and bandgap-related narrowing or intragap localization of dopant levels are not the only factors responsible for enhanced visible-light absorption in doped photocatalysts. Furthermore, bandgap narrowing through increases in the energy of the valence band may actually decrease photo-oxidation activity through a curtailment of one route of oxidation.

The reactivity of mesoporous silica modified with acidic sites in the production of biodiesel

2013-01-07T16:13:05Z

The reactivity of mesoporous silica modified with acidic sites in the production of biodiesel Sherry, Linda; Sullivan, James A. This work has studied a range of acidic catalysts which have been prepared through the modification of mesoporous silica materials. Specifically we have prepared –SO3H modified materials (in the presence and absence of a hydrophobic component) and–COOH and –CN materials. We have characterized these catalysts using TEM, TGA, TPD of probe molecules and IR and we have analysed their activity in the esterification of octanoic acid and the transesterification of triacetin. Activity in the esterification reaction (in which all catalysts are active) is not determined by the acid site concentration of the catalyst while the activity of active catalysts in the transesterification reaction (in which only –SO3H materials are active) appears related to acid site concentration.

Horse Liver Alcohol Dehydrogenase: new perspectives for an old enzyme

2012-11-29T15:24:42Z

Horse Liver Alcohol Dehydrogenase: new perspectives for an old enzyme Quaglia, Daniela; Irwin, Jane A.; Paradisi, Francesca The EE subunit of horse liver alcohol dehydrogenase (HLADH-EE) has been subcloned in pRSETb vector to generate a fusion His-tag protein. The migration from a multistep purification protocol for this well-known enzyme to a single-step has been successfully achieved. Several adjustments to the traditional purification procedure for Histag proteins have been made to retain protein activity. A full characterization of the fusion enzyme has been carried out and compared with the native one. The Km for EtOH, NAD and NADH in the His-tag version of HLADH are in line with the ones reported in literature for the native enzyme. A shift in optimal pH activity is also observed. The enzyme retains the same stability and quaternary structure as the wild type and can therefore be easily used instead of the native HLADH for biotechnological applications.

Pyridine-derived N-heterocyclic carbenes: An experimental and theoretical evaluation of the bonding in and reactivity of selected normal and abnormal complexes of Ni(II) and Pd(II)

2012-11-02T15:25:02Z

Pyridine-derived N-heterocyclic carbenes: An experimental and theoretical evaluation of the bonding in and reactivity of selected normal and abnormal complexes of Ni(II) and Pd(II) Stander-Gobler, Elzet; Schuster, Oliver; Cronje, Stephanie; Tosh, Evangeline; Albrecht, Martin; Frenking, Gernot; Raubenheimer, Helgard G.; Heydenrych, Greta We report a thorough investigation of a series of isomeric complexes with the general formula trans-(pyridylidene)M(PPh3)2Cl (M = Pd, Ni). For the first time, a systematic comparison of normal, abnormal and remote bonding modes is presented. X-ray structural and 13C NMR data indicate the importance of carbenoid mesomeric contributions in their compound class. The catalytic performance of the palladium complexes, trans-(pyridylidene)Pd(PPh3)2Cl, as precursors in Suzuki-Miyaura-type cross-coupling suggests a correlation of remote bonding with catalyst robustness and thus effectivity. When metal precursors M(PPh3)4 are reacted with 2,4-dichloro pyridinium salts, preferential formation of remote carbene complexes occurs and indications are that electronic parameters rather than steric influences are responsible for the observed selectivity. Calculations at the BP86/TZ2P level of theory support interpretation of the results.

Novel approach to the synthesis of aliphatic and aromatic alpha-keto acids

2012-08-24T15:44:19Z

Novel approach to the synthesis of aliphatic and aromatic alpha-keto acids Balducci, Daniele; Conway, Philip A.; Sapuppo, Giulia; Müller-Bunz, Helge; Paradisi, Francesca A new practical and efficient synthesis of alpha-keto acids was accomplished starting from the synthon 1,4-diacetylpiperazine-2,5-dione. The synthesis encompasses both aromatic and aliphatic substrates proving to be versatile and innovative with excellent carbon economy and recycling of the glycine by-product.

Towards Langmuir-Blodgett films of magnetically interesting materials: solution equilibria in amphiphilic iron(II) complexes of a triazole-containing ligand

2012-08-10T14:37:28Z

Towards Langmuir-Blodgett films of magnetically interesting materials: solution equilibria in amphiphilic iron(II) complexes of a triazole-containing ligand White, Nicholas G.; Feltham, Humphrey L. C.; Gandolfi, Claudio; Albrecht, Martin; Brooker, Sally As a first step towards ambiphilic SCO systems where the hydrophobic part of the system is introduced by a non-coordinating anion (i.e. where no modification of the ligands to introduce hydrophobic substituents is required), [FeII(OH2)2(C16SO3)2] and [CoII(OH2)2(C16SO3)2] have been reacted with the triazole-containing ligands adpt and pldpt (C16SO¬3 = hexadecanesulfonate anion, adpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole, pldpt = 4-pyrrolyl-3,5-bis(2-pyridyl)-1,2,4-triazole). In the solid state, HS complexes of the form [FeII(Rdpt)2(C16SO3)2] and [CoII(Rdpt)2(CH3OH)2](C16SO3)2 are observed, even when excess ligand is used (Rdpt = adpt or pldpt). In solution, the cobalt complexes remain in this form as evidenced by colour, Visible/NIR and IR spectroscopy. For the iron complexes, there is an equilibrium in solution between the neutral high-spin form of the complex [FeII(Rdpt)2(C16SO3)2] and the dicationic low-spin form [FeII(Rdpt)3](C16SO3)2. Polar solvents favour the dicationic form, while less polar solvents favour the neutral form (as evidenced by solution colour and solution IR spectroscopy). Visible/NIR spectroscopy and Evans’ method NMR spectroscopy show the equilibrium can be shifted towards the [FeII(Rdpt)3](C16SO3) form by adding additional ligand to the solution. The X-ray crystal structures of [FeII(adpt)2(C16SO3)2] and [CoII(adpt)2(CH3OH)2](C16SO3)2·1.33CH3OH are presented. [FeII(adpt)2(C16SO3)2] has a 2D bilayer structure with alternating layers of polar Fe(adpt)2 centres, and hydrophobic alkyl chains. The complex cations in [CoII(adpt)2(CH3OH)2](C16SO3)2·1.33CH3OH form 1-D columns in the solid state. The capacity of the amphiphilic complexes [FeII(pldpt)2(C16SO3)2] and [FeII(adpt)2(C16SO3)2] to self-assemble has been probed at the air-water interface using Langmuir techniques. The pertinent pressure-area isotherms reveal only a low tendency of the complexes to form films.

The Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active Materials

2012-08-10T14:31:17Z

The Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active Materials Schuster, Oliver; Mercs, Laszlo; Albrecht, Martin The application of N-heterocyclic carbene complexes as active sites in materials other than catalysis has been remarkably scarce. Inspired by the — often misleading — ‘carbene’ label, which implies a substantial degree of M=C π bonding, we have been interested in evaluating the potential of N-heterocylclic carbene complexes as building blocks for constructing electronically active materials. Electron mobility via the metal-carbon bond has been investigated in monometallic imidazol-2-ylidene complexes and subsequently expanded to polymetallic systems. In particular, ditopic benzobisimidazolium-derived ligands have been explored for the fabrication of bimetallic molecular switches and main-chain conjugated organometallic polymers. Electrochemical analyses have allowed for quantifying the degree of electronic coupling between the metal sites and for identifying the key parameters that govern the intermetallic communication.

Organization of spin- and redox-labile metal centers into Langmuir and Langmuir-Blodgett films

2012-08-10T14:25:48Z

Organization of spin- and redox-labile metal centers into Langmuir and Langmuir-Blodgett films Gandolfi, Claudio; Miyashita, Naoko; Kurth, Dirk G.; Martinho, Paulo N.; Morgan, Grace; Albrecht, Martin New sal2(trien) ligands that contain alkoxy substituents of various length in meta position of the salicyl entities were coordinated to electronically and magnetically active iron(III) and cobalt(III) centers. The electrochemical and spectroscopic properties of these amphiphilic complexes are virtually unaffected upon alteration of the alkoxy substituents, thus providing a system in which the physical behavior and the metal-centered chemical activity can be tailored independently. The amphiphilic character has been exploited for preparing Langmuir monolayers at the air-water interface and for constructing Langmuir-Blodgett films, hence allowing for hierarchical assembling of electronically and magnetically active systems. While Langmuir films were stable, transfer onto solid supports was limited, which restricted the magnetic analysis of the Langmuir-Blodgett assemblies.

Beyond catalysis: N-heterocyclic carbene complexes as components for medicinal, luminescent, and functional materials applications

2012-08-10T14:17:43Z

Beyond catalysis: N-heterocyclic carbene complexes as components for medicinal, luminescent, and functional materials applications Mercs, Laszlo; Albrecht, Martin This tutorial review compiles the advances that have been achieved in using transition metal complexes containing N-heterocyclic carbene ligands as components for materials. Applications of metal carbene complexes in fields different from catalysis are remarkably scarce. During the last few years, promising results have been accomplished in particular by utilizing such complexes as antimicrobial and cytotoxic agents, as photoactive sites in luminescent materials, for self-assembly into liquid crystalline materials and metallasupramolecular structures, and as synthons for molecular switches and conducting polymeric materials. These initial achievements clearly underline the great potential of N-heterocyclic carbene complexes in various fields of materials science.

Non-classical N-Heterocyclic carbene complexes

2012-08-10T14:01:42Z

Non-classical N-Heterocyclic carbene complexes Krüger, Anneke; Albrecht, Martin The expansion of the concept of N-heterocyclic carbenes as ligands for transition metals to mesoionic ligand systems has led to the discovery of a wide range of non-classical carbene-type ligands. These non-classical carbene-type ligands are characterised by a significantly lower heteroatom stabilisation of the (putative) free carbene, a situation that also affects the ligand donor properties and hence the reactivity of the coordinated metal centre. Based on the unique impact of non-classical carbene-type ligands, a number of attractive transition metal-catalysed processes have been disclosed in recent years, predominantly in the area of cross-coupling reactions, hydrogenations, and olefin metathesis

Expanding the family of mesoionic complexes: donor properties and catalytic impact of palladated isoxazolylidenes

2012-06-28T16:18:18Z

Expanding the family of mesoionic complexes: donor properties and catalytic impact of palladated isoxazolylidenes Iglesias, Manuel; Albrecht, Martin Abnormal isoxazolylidene complexes, a new subclass of mesoionic complexes containing an isoxazolium-derived carbene type ligand, have been synthesised via oxidative addition and compared to structurally related mesoionic complexes by using 31P NMR spectroscopy as a convenient probe for their donor ability and in catalytic cross-coupling reactions.

The coupled δ13C-radiocarbon systematics of three late Glacial/early Holocene speleothems; insights into soil and cave processes at climatic transitions

2012-06-26T16:01:25Z

The coupled δ13C-radiocarbon systematics of three late Glacial/early Holocene speleothems; insights into soil and cave processes at climatic transitions Rudzka, Dominika; McDermott, Frank; Baldini, Lisa M.; Fleitmann, Dominik; Moreno, Ana; Stoll, Heather The coupled δ13C-radiocarbon systematics of three European stalagmites deposited during the Late Glacial and early Holocene were investigated to understand better how the carbon isotope systematics of speleothems respond to climate transitions. The emphasis is on understanding how speleothems may record climate-driven changes in the proportions of biogenic (soil carbon) and limestone bedrock derived carbon. At two of the three sites, the combined δ13C and 14C data argue against greater inputs of limestone carbon as the sole cause of the observed shift to higher d13C during the cold Younger Dryas. In these stalagmites (GAR-01 from La Garma cave, N. Spain and So-1 from Sofular cave, Turkey), the combined changes in δ13C and initial 14C activities suggest enhanced decomposition of old stored, more recalcitrant, soil carbon at the onset of the warmer early Holocene. Alternative explanations involving gradual temporal changes between open- and closed-system behaviour during the Late Glacial are difficult to reconcile with observed changes in speleothem δ13C and the growth rates. In contrast, a stalagmite from Pindal cave (N. Spain) indicates an abrupt change in carbon inputs linked to local hydrological and disequilibrium isotope fractionation effects, rather than climate change. For the first time, it is shown that while the initial 14C activities of all three stalagmites broadly follow the contemporaneous atmospheric 14C trends (the Younger Dryas atmospheric 14C anomaly can be clearly discerned), subtle changes in speleothem initial 14C activities are linked to climate-driven changes in soil carbon turnover at a climate transition.

[Ru(bpy)3]2+ analogues containing a N-heterocyclic carbene ligand

2012-06-26T14:21:26Z

[Ru(bpy)3]2+ analogues containing a N-heterocyclic carbene ligand Ghattas, Wadih; Müller-Bunz, Helge; Albrecht, Martin A synthetic procedure is described that provides access to [Ru(bpy)3]2+ analogues in which one bpy ligand is replaced by a C,N-bidentate coordinating carbene-benzimidazole ligand (bpy = 2,2’-bipyridine). These new complexes were prepared by first installing the chelating carbene ligand onto a Ru(cymene) platform and subsequent ligand substitution using bpy or terpy (terpy = 2:2’,6’:2’’-terpyridine). The carbene ligand significantly affects the optical properties of the complex and lowers the ruthenium(II) oxidation potential substantially. Such modifications may be advantageous for the development of new classes of photosensitizer materials.

Cyclometalation using d-block transition metals: fundamental aspects and recent trends

2012-06-26T14:15:06Z

Cyclometalation using d-block transition metals: fundamental aspects and recent trends Albrecht, Martin

Role of cell cycle on the cellular uptake and dilution of nanoparticles in a cell population

2013-01-15T11:42:35Z

Role of cell cycle on the cellular uptake and dilution of nanoparticles in a cell population Kim, Jong Ah; Åberg, Christoffer; Salvati, Anna; Dawson, Kenneth A. Nanoparticles are considered a primary vehicle for targeted therapies because they can pass biological barriers, enter and distribute in cells by energy-dependent pathways1-3. Until now, most studies have shown that nanoparticle properties, such as size4-6 and surface7,8, can affect how cells internalise nanoparticles. Here we show that the different phases of cell growth, which constitute the cell cycle, can also influence nanoparticle uptake. Although cells in different cell cycle phases internalised nanoparticles with similar rates, after 24 hours of uptake the concentration of nanoparticles in the cells is ranked according to the different cell cycle phases: G2/M > S > G0/G1. Nanoparticles were not exported from cells but the internalised nanoparticle concentration is split when the cell divides. Our results suggest that future studies on nanoparticle uptake should consider the cell cycle because in a cell population, the internalised nanoparticle dose in each cell varies as the cell cycles.

A new, mild one-pot synthesis of iodinated heterocycles as suitable precursors for N-heterocyclic carbene complexes

2012-06-19T15:41:44Z

A new, mild one-pot synthesis of iodinated heterocycles as suitable precursors for N-heterocyclic carbene complexes Iglesias, Manuel; Schuster, Oliver; Albrecht, Martin The use of I2/AgOAc in dichloromethane constitutes a cheap, mild, and efficient method for the selective iodination of a variety of heterocycles. In a number of cases, this method provides superior yields than other literature methods and affords iodo-functionalized heterocycles that are suitable precursors for carbene complexes.

Rhodium-mediated activation of an alkane-type C–H bond

2012-06-14T14:22:51Z

Rhodium-mediated activation of an alkane-type C–H bond Krüger, Anneke; Neels, Antonia; Albrecht, Martin Abnormal C4-bonding of N-heterocyclic carbenes effectively modulates the electron density at rhodium and allows for the selective cleavage of an unactivated C(sp3)–H bond, whereas no such intramolecular C–H bond breaking is observed when the carbene binds normally through the C2 carbon.

Room-temperature spin crossover and Langmuir-Blodgett film formation of an iron(II) triazole complex featuring a long alkyl chain substituent : the tail that wags the dog

2012-06-14T14:14:20Z

Room-temperature spin crossover and Langmuir-Blodgett film formation of an iron(II) triazole complex featuring a long alkyl chain substituent : the tail that wags the dog Kitchen, Jonathan A.; White, Nicholas G.; Gandolfi, Claudio; Albrecht, Martin; Jameson, Guy N. L.; Tallon, Jeffery L.; Brooker, Sally [FeII(C16dpt)2(NCS)2]·⅔H2O displays temperature-mediated spin crossover (SCO) with T½ = 290 K and the long alkyl chain substituent on the dipyridyltriazole ligand facilitates the formation of a stable Langmuir-Blodgett film at an air water interface.

Main-chain organometallic polymers comprising redox-active iron(II) centers connected by ditopic N-heterocyclic carbenes

2012-06-14T14:04:12Z

Main-chain organometallic polymers comprising redox-active iron(II) centers connected by ditopic N-heterocyclic carbenes Mercs, Laszlo; Neels, Antonia; Stoeckli-Evans, Helen; Albrecht, Martin Main-chain organometallic polymers were synthesized from bimetallic iron(II)complexes containing a ditopic N-heterocyclic carbene (NHC) ligand [(cp)(CO)LFe(NHC~NHC)Fe (cp)(CO)L]X2 (where NHC~NHC represents a bridging dicarbene ligand, L = I– or CO). Addition of a diimine ligand such as pyrazine or 4,4’-bipyridine interconnected these bimetallic complexes and gave the corresponding co-polymers containing iron centers that are alternately linked by a dicarbene and a diimine ligand. Diimine coordination was depending on the wingtip groups at the carbene ligands and was accomplished either by photolytic activation of a carbonyl ligand from the cationic [Fe(cp)(NHC)(CO)2]+ precursor (alkyl wingtips) or by AgBF4-mediated halide abstraction from the neutral complex [FeI(cp)(NHC)(CO)] (mesityl wingtips). Remarkably, the polymeric materials were substantially more stable than the related bimetallic model complexes. Electrochemical analyses indicated metal-metal interactions in the pyrazine-containing polymers, whereas in 4,4’-bipyridine-linked systems the metal centers were electronically decoupled.

Palladium carbene complexes for selective alkene di- and oligomerization

2013-02-26T10:01:29Z

Palladium carbene complexes for selective alkene di- and oligomerization Khlebnikov, Vsevolod; Meduri, Angelo; Müller-Bunz, Helge; Montini, Tiziano; Fornasiero, Paolo; Zangrando, Ennio; Milani, Barbara; Albrecht, Martin A series of palladium complexes were synthesized that comprise three sterically different C,N-bidentate coordinating NHC-pyridine ligands (NHC = N-heterocyclic carbene). In one set, the pyridine and the carbene are linked by a flexible CH2 group (a), in the other two sets, the two ligand units are directly linked and feature a shielding mesityl substituent on the carbene and either an unsubstituted pyridine (b) or a xylyl-substituted pyridine unit (c). Investigation of the reactivity of cationic complexes [Pd(C^N)Me(NCMe)]+, 6, analogues to Brookhart’s α-diimine system, towards alkenes showed a strong correlation between the catalytic activity and selectivity and the ligand setting. While 6a was inactive in ethylene conversion, 6b afforded low-molecular weight olefins (oligomerization), and 6c produced exclusively butene (dimerization). With styrene as substrate, exclusive dimerization occurred with all three complexes. Steric and electronic factors were identified that govern the disparate activity and selectivity, and that allow for efficient tailoring of the catalytic performance.

Rhodium carbene complexes as versatile catalyst precursors for Si–H bond activation

2012-06-14T13:45:26Z

Rhodium carbene complexes as versatile catalyst precursors for Si–H bond activation Krüger, Anneke; Albrecht, Martin Rhodium(III) complexes comprising monoanionic C,C,C-tridentate dicarbene ligands activate Si–H bonds and catalyse the hydrolysis of hydrosilanes to form silanols and siloxanes with concomitant release of H2. In dry MeNO2, selective formation of siloxanes takes place, while changing conditions to wet THF produces silanols exclusively. Silylethers are formed when ROH is used as substrate, thus providing a mild route towards the protection of alcohols with H2 as the only by-product. With alkynes, comparably fast hydrosilylation takes place, while carbonyl groups are unaffected. Further expansion of the Si–H bond activation to dihydrosilanes afforded silicones and polysilylethers. Mechanistic investigations using deuterated silane revealed deuterium incorporation into the abnormal carbene ligand and hence suggests a ligand-assisted mechanism involving heterolytic Si–H bond cleavage.

Polyoxometalate-based N-heterocyclic carbene (NHC) complexes for palladium-mediated C-C coupling and chloroaryl dehalogenation catalysis

2012-06-14T13:38:38Z

Polyoxometalate-based N-heterocyclic carbene (NHC) complexes for palladium-mediated C-C coupling and chloroaryl dehalogenation catalysis Berardi, Serena; Carraro, Mauro; Iglesias, Manuel; Sartorel, Andrea; Scorrano, Gianfranco; Albrecht, Martin; Bonchio, Marcella