Preparació de nous catalitzadors basats en complexos de Ru i Pd, i el seu ancorament a polímers conductors per a la fabricació de materials multifuncionals

Aquesta tesi doctoral es basa en l'estudi de l'aplicació en catàlisi de dos tipus de complexos organometallics basats en dos metalls de tansició diferents. Concretament s'estudien complexos macrocíclics triolefínics de pal·ladi(0) com a catalitzadors per a les reaccions de Suzuki i Heck, i oxocomplexos carbènics de ruteni(II) com a espècies catalítiques en oxidacions de compostos orgànics. En el cas dels complexos de ruteni s'ha vist que en augmentar el nombre de lligands carbènics en l'esfera de coordinació del metall s'aconseguiex afavorir els processos bielectrònics, obtenint-se catalitzadors més actius i més selectius. En un segon pas, els dos tipus de catalitzadors homogenis s'han immobilitzat sobre la superfície d'un elèctrode mitjançant l'estratègia d'electropolimerització del grup pirrol. Els elèctodes modificats resultants s'han aplicat com a catalitzadors heterogenis. En ambdós casos els catalitzadors heterogenis han mostrat una activitat equiparable o superior a la del sistema homogeni corresponent. Finalment, s'ha assajat una reacció de catàlisi tàndem en què els dos catalitzadors (immobilitzats sobre el mateix elèctrode) actuen en cooperació. S'ha aconseguit realitzar dues transformacions consecutives d'un substat orgànic.

Rhodium(I) catalyzed [2+2+2] cycloaddition reactions: experimental and theoretical studies

The [2+2+2] cycloaddition reaction involves the formation of three carbon-carbon bonds in one single step using alkynes, alkenes, nitriles, carbonyls and other unsaturated reagents as reactants. This is one of the most elegant methods for the construction of polycyclic aromatic compounds and heteroaromatic, which have important academic and industrial uses. The thesis is divided into ten chapters including six related publications. The first study based on the Wilkinson’s catalyst, RhCl(PPh3)3, compares the reaction mechanism of the [2+2+2] cycloaddition process of acetylene with the cycloaddition obtained for the model of the complex, RhCl(PH3)3. In an attempt to reduce computational costs in DFT studies, this research project aimed to substitute PPh3 ligands for PH3, despite the electronic and steric effects produced by PPh3 ligands being significantly different to those created by PH3 ones. In this first study, detailed theoretical calculations were performed to determine the reaction mechanism of the two complexes. Despite some differences being detected, it was found that modelling PPh3 by PH3 in the catalyst helps to reduce the computational cost significantly while at the same time providing qualitatively acceptable results. Taking into account the results obtained in this earlier study, the model of the Wilkinson’s catalyst, RhCl(PH3)3, was applied to study different [2+2+2] cycloaddition reactions with unsaturated systems conducted in the laboratory. Our research group found that in the case of totally closed systems, specifically 15- and 25-membered azamacrocycles can afford benzenic compounds, except in the case of 20-membered azamacrocycle (20-MAA) which was inactive with the Wilkinson’s catalyst. In this study, theoretical calculations allowed to determine the origin of the different reactivity of the 20-MAA, where it was found that the activation barrier of the oxidative addition of two alkynes is higher than those obtained for the 15- and 25-membered macrocycles. This barrier was attributed primarily to the interaction energy, which corresponds to the energy that is released when the two deformed reagents interact in the transition state. The main factor that helped to provide an explanation to the different reactivity observed was that the 20-MAA had a more stable and delocalized HOMO orbital in the oxidative addition step. Moreover, we observed that the formation of a strained ten-membered ring during the cycloaddition of 20-MAA presents significant steric hindrance. Furthermore, in Chapter 5, an electrochemical study is presented in collaboration with Prof. Anny Jutand from Paris. This work allowed studying the main steps of the catalytic cycle of the [2+2+2] cycloaddition reaction between diynes with a monoalkyne. First kinetic data were obtained of the [2+2+2] cycloaddition process catalyzed by the Wilkinson’s catalyst, where it was observed that the rate-determining step of the reaction can change depending on the structure of the starting reagents. In the case of the [2+2+2] cycloaddition reaction involving two alkynes and one alkene in the same molecule (enediynes), it is well known that the oxidative coupling may occur between two alkynes giving the corresponding metallacyclopentadiene, or between one alkyne and the alkene affording the metallacyclopentene complex. Wilkinson’s model was used in DFT calculations to analyze the different factors that may influence in the reaction mechanism. Here it was observed that the cyclic enediynes always prefer the oxidative coupling between two alkynes moieties, while the acyclic cases have different preferences depending on the linker and the substituents used in the alkynes. Moreover, the Wilkinson’s model was used to explain the experimental results achieved in Chapter 7 where the [2+2+2] cycloaddition reaction of enediynes is studied varying the position of the double bond in the starting reagent. It was observed that enediynes type yne-ene-yne preferred the standard [2+2+2] cycloaddition reaction, while enediynes type yne-yne-ene suffered β-hydride elimination followed a reductive elimination of Wilkinson’s catalyst giving cyclohexadiene compounds, which are isomers from those that would be obtained through standard [2+2+2] cycloaddition reactions. Finally, the last chapter of this thesis is based on the use of DFT calculations to determine the reaction mechanism when the macrocycles are treated with transition metals that are inactive to the [2+2+2] cycloaddition reaction, but which are thermally active leading to new polycyclic compounds. Thus, a domino process was described combining an ene reaction and a Diels-Alder cycloaddition.

Síntesi i Caracterització de Nous Complexos de Ruteni contenint Lligands Quirals. Aplicacions en Catàlisi Asimètrica

La Tesis describe una serie de complejos de Rutenio conteniendo ligandos quirales y su aplicación en catálisis asimétrica. Por un lado se describe el complejo [RuCl(bpea)((S)-BINAP)](BF4), donde (S)-BINAP es una difosfina quiral y bpea un ligando N-tridentado. Su aplicación en catálisis de hidrogenación asimétrica de sustratos olefínicos y carbonílicos, tanto en fase homogénea como heterogénea (tras inmovilización sobre soportes alumino-fosfato), ha mostrado excelentes resultados de conversión y excesos enantioméricos. Aparte, se ha desarrollado una familia de complejos con fórmula [Ru(T)(B)X], donde T representa un ligando tri-N-dentado, B una di-oxazolina quiral y X es Cl o H2O. Se ha estudiado la actividad catalítica de los aquocomplejos con T = tpm en epoxidaciones, determinándose la influencia de los sustituyentes en B sobre el rendimiento y la quimioselectividad, favorecidos por interacciones de tipo π-stacking. El uso de otros ligandos tridentados ha llevado a su rotura, habiéndose caracterizado sin embargo los correspondientes complejos

Chemoselective catalytic hydrogenation of acrolein on Ag(111): effect of molecular orientation on reaction selectivity

The adsorption and hydrogenation of acrolein on the Ag(111) surface has been investigated by high resolution synchrotron XPS, NEXAFS, and temperature programmed reaction. The molecule adsorbs intact at all coverages and its adsorption geometry is critically important in determining chemoselectivity toward the formation of allyl alcohol, the desired but thermodynamically disfavored product. In the absence of hydrogen adatoms (H(a)), acrolein lies almost parallel to the metal surface; high coverages force the C=C bond to tilt markedly, likely rendering it less vulnerable toward reaction with hydrogen adatoms. Reaction with coadsorbed H(a) yields allyl alcohol, propionaldehyde, and propanol, consistent with the behavior of practical dispersed Ag catalysts operated at atmospheric pressure: formation of all three hydrogenation products is surface reaction rate limited. Overall chemoselectivity is strongly influenced by secondary reactions of allyl alcohol. At low H(a) coverages, the C=C bond in the newly formed allyl alcohol molecule is strongly tilted with respect to the surface, rendering it immune to attack by H(a) and leading to desorption of the unsaturated alcohol. In contrast with this, at high H(a) coverages, the C=C bond in allyl alcohol lies almost parallel to the surface, undergoes hydrogenation by H(a), and the saturated alcohol (propanol) desorbs.

Molecular guided catalytic hydrogenation by micelle-hosted Pd nanoparticle in supercritical CO2

Homogeneous dispersion of microemulsion containing palladium nanoparticles in scCO(2) is, for the first time, observed via sapphire window reactor and these particles show an unusual reluctance for double bond hydrogenation of citral aldehyde at hydrophobic end rather than hydrophilic end (high regioselectivity) owing to the unique micelle environment in supercritical carbon dioxide that guide a head-on attack of the molecule.

Nitrogen donor ligands bearing N-H groups: Effect on catalytic and cytotoxic activity of molybdenum eta(3)-allyldicarbonyl complexes

Reactions of [Mo(eta(3)-C3H5)Br(CO)(2)(NCMe)(2)] with the bidentate nitrogen ligands 2-(2'-pyridyl)imidazole (L1), 2-(2'-pyridyl)benzimidazole (L2), N,N'-bis(2'-pyridinecarboxamido)-1,2-ethane (L3), and 2,2'-bisimidazole (L4) led to the new complexes [Mo(eta(3)-C3H5)Br(CO)(2)(L)] (L = L1, 1; L2, 2; L4, 4) and [{Mo(eta(3)-C3H5) Br(CO)(2)}(2)(mu-L-3)] (3). The reaction of complexes 2 and 3 with Tl[CF3SO3] afforded [Mo(eta(3)-C3H5)(CF3SO3)(CO)(2)(L2)] (2T) and [{Mo(eta(3)-C3H5)(CF3SO3)(CO)(2)}(2)(mu-L-3)] (3T). Complexes 3 and 2T were structurally characterized by single crystal X-ray diffraction, showing the facial allyl/carbonyls arrangement and the formation of the axial isomer. In 2T, two molecules are assembled in a hydrogen bond dimer. The four complexes 1-4 were tested as precursors in the catalytic epoxidation of cyclooctene and styrene, in the presence of t-butylhydroperoxide (TBHP), with moderate conversions and turnover frequencies for complexes 1-3 and very low ones for 4. The increasing number of N-H groups in the complexes seems to be responsible for the loss of catalytic activity, compared with other related systems. The cytotoxic activities of all the complexes were evaluated against HeLa cells. The results showed that compounds 1,2,4, and 2T exhibited significant activity, complexes 2 and 2T being particularly promising. (C) 2008 Elsevier B.V. All rights reserved.

Synthesis and characterization of packed mesoporous tungsteno-silicates: application to the catalytic dehydrogenation of 2-propanol

The synthesis of highly ordered mesoporous tungsteno-silicas in which a high percentage of tungsten is introduced into a silica framework is reported hereafter. Powder XRD and TEM have been used to characterize the materials synthesized at room temperature. The materials are shown to be homogeneous as there is no evidence for any crystalline species other than the silica framework. The pore diameter and the surface area of the materials, evaluated from the nitrogen adsorption isotherms and unit cell parameter indicate a pore diameter of about 2 nm and a surface area of 1400 m(2) g(-1) for a content of 10% tungsten. Catalyzed dehydration of 2-propanol has been investigated and the activity of the materials synthesized is significant, even for low tungsten content W-MCM-41 materials. (C) 2003 Elsevier B.V. All rights reserved.

Heterogeneously catalyzed asymmetric hydrogenation of C = C bonds directed by surface-tethered chiral modifiers

Asymmetric hydrogenation of C=C bonds is of the highest importance in organic synthesis, and such reactions are currently carried out with organometallic homogeneous catalysts. Achieving heterogeneous metal-catalyzed hydrogenation, a highly desirable goal, necessitates forcing the crucial enantiodifferentiating step to take place at the metal surface. By synthesis and application of six chiral sulfide ligands that anchor robustly to Pd nanoparticles and resist displacement, we have for the first time accomplished heterogeneous enantioselective catalytic hydrogenation of isophorone. High resolution XPS data established that ligand adsorption from solution occurred exclusively on the Pd nanoparticles and not on the carbon support. All ligands contained a pyrrolidine nitrogen to enable their interaction with the isophorone substrate while the sulfide functionality provided the required interaction with the Pd surface. Enantioselective turnover numbers of up to similar to 100 product molecules per ligand molecule were found with a very large variation in asymmetric induction between ligands: observed enantiomeric excesses increased with increasing size of the alkyl group in the sulfide. This likely reflects varying degrees of ligand dispersion on the surface: bulky substituent groups hinder close approach of ligand molecules to each other, inhibiting close-packed island formation, favoring dispersion as separate molecules, and leading to effective asymmetric induction. Conversely, small substituents favor island formation leading to very low asymmetric induction. Enantioselective reaction most likely involves initial formation of an enamine or iminium species, confirmed by use of an analogous tertiary amine, which leads to racemic product. Ligand rigidity and resistance to self-assembled monolayer formation are important attributes that should be designed into improved chiral modifiers.

Synthesis of new phosphino amino alcohol ligands via ortho-alkyllithiation reactions. Versatile coordination behavior toward copper(I) and palladium(II)

2-[Methyl(2-methylphenyl)amino]ethanol undergoes an ortho-alkyllithiation reaction with n-butyllithium to lead to a new mixed benzyllithium−lithium alkoxide. This organolithium species reacts with PPh2Cl, with selective P−C bond formation, to afford the ligand 2-[methyl(2-((diphenylphosphino)methyl)phenyl)amino]ethanol L1. The coordination of the ligand L1 to copper(I) leads to the complex [Cu(L1)2](BF4), whose structure has been determined by an X-ray diffraction study. In the solid state, one of the ligands acts as a monodentate phosphine while the other adopts a tridentate P,N,O coordination mode. A variable-temperature 31P NMR study demonstrated the existence of an equilibrium between the two modes in solution, with a coalescence temperature of ca. 0 °C, indicating a double-hemilabile behavior for the nitrogen and the oxygen functions. L1 reacts with [Pd(Me)(Cl)(COD)] to give a dinuclear complex in which the ligand appears to behave as a bridging anionic P,O ligand. Such a complex could serve as a model for a key intermediate in the proposed mechanism for the homogeneous catalysis of the methoxycarbonylation of propyne by certain palladium(II) complexes containing P,N ligands. L1 can undergo a second ortho-alkylmetalation reaction with n-butyllithium which, after addition of PPh2Cl, provides the new ligand 2-{methyl[2-(bis(diphenylphosphino)methyl)phenyl]amino}ethanol (L2) in high yield.

Structures, reactivity, and catalytic activity of dithiolato-bridged heterobimetallic MRh (M = Pt, Pd) complexes

Heterobimetallic complexes [(P−P)Pt(μ-S−S)Rh(cod)]ClO4 (P−P = (PPh3)2, Ph2P(CH2)3PPh2 (dppp), and Ph2P(CH2)4PPh2 (dppb); S−S = -S(CH2)2S- (EDT), -S(CH2)3S- (PDT), -S(CH2)4S- (BDT), cod = 1,5-cyclooctadiene) reacted with CO to form the carbonyl complexes [(P−P)Pt(μ-S−S)Rh(CO)2]ClO4 and then with PR3 ligands to give [(P−P)Pt(μ-S−S)Rh(CO)(PR3)]ClO4. The binuclear framework of these cod complexes was maintained in the reactions reported. The cod complexes were tested as catalyst precursors in the hydroformylation of styrene. HPNMR in situ studies showed that mononuclear species formed under catalytic conditions.

Complexes of palladium using di-2-pyridyl ketone as ligand: synthesis, structure and, spectral and catalytic properties

During the reaction of di-2-pyridyl ketone (dpk) with Na(2)[PdCl(4)] in alcoholic media, the C=O fragment of dpk undergoes facile solvolysis and the transformed di-2-pyridyl ketone (dpk(ROH), R = Me or H) binds to palladium as NN-donor. When the reaction is carried out in refluxing methanol, a mono-complex of type [Pd(dpk(MeOH))Cl(2)] is obtained. A similar reaction in ethanol affords a bis-complex of type [Pd(dpk(ROH))(2)]Cl(2). Structure of both the complexes have been determined by X-ray crystallography. In acetonitrile solution the [pd(dpk(MeOH))Cl(2)] and [pd(dpk(ROH))(2)]Cl(2) complexes show intense absorptions in the visible and ultraviolet region, origin of which has been probed through uvr calculations. These two palladium complexes are found to be efficient catalysts for Suzuki cross-coupling reactions.

Time-resolved gas-phase kinetic, quantum chemical, and RRKM studies of reactions of silylene with alcohols

Time-resolved kinetic studies of silylene, SiH2, generated by laser flash photolysis of 1-silacyclopent-3-ene and phenylsilane, have been carried out to obtain rate constants for its bimolecular reactions with methanol, ethanol, 1-propanol, 1-butanol and 2-methyl-1-butanol. The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at room temperature. In the study with methanol several buffer gases were used. All five reactions showed pressure dependences characteristic of third body assisted association reactions. The rate constant pressure dependences were modelled using RRKM theory, based on Eo values of the association complexes obtained by ab initio calculation (G3 level). Transition state models were adjusted to fit experimental fall-off curves and extrapolated to obtain k∞ values in the range 1.9 to 4.5 × 10-10 cm3 molecule-1 s-1. These numbers, corresponding to the true bimolecular rate constants, indicate efficiencies of between 16 and 67% of the collision rates for these reactions. In the reaction of SiH2 + MeOH there is a small kinetic component to the rate which is second order in MeOH (at low total pressures). This suggests an additional catalysed reaction pathway, which is supported by the ab initio calculations. These calculations have been used to define specific MeOH-for-H2O substitution effects on this catalytic pathway. Where possible our experimental and theoretical results are compared with those of previous studies.

Structural and Biochemical Characterization of Peroxiredoxin Q beta from Xylella fastidiosa CATALYTIC MECHANISM AND HIGH REACTIVITY

The phytopathogenic bacterium Xylella fastidiosa is the etiological agent of various plant diseases. To survive under oxidative stress imposed by the host, microorganisms express antioxidant proteins, including cysteine-based peroxidases named peroxiredoxins. This work is a comprehensive analysis of the catalysis performed by PrxQ from X. fastidiosa (XfPrxQ) that belongs to a peroxiredoxin class still poorly characterized and previously considered as moderately reactive toward hydroperoxides. Contrary to these assumptions, our competitive kinetics studies have shown that the second-order rate constants of the peroxidase reactions of XfPrxQ with hydrogen peroxide and peroxynitrite are in the order of 107 and 106 M(-1) s(-1), respectively, which are as fast as the most efficient peroxidases. The XfPrxQ disulfides were only slightly reducible by dithiothreitol; therefore, the identification of a thioredoxin system as the probable biological reductant of XfPrxQ was a relevant finding. We also showed by site-specific mutagenesis and mass spectrometry that an intramolecular disulfide bond between Cys-47 and Cys-83 is generated during the catalytic cycle. Furthermore, we elucidated the crystal structure of XfPrxQ C47S in which Ser-47 and Cys-83 lie similar to 12.3 angstrom apart. Therefore, significant conformational changes are required for disulfide bond formation. In fact, circular dichroism data indicated that there was a significant redox-dependent unfolding of alpha-helices, which is probably triggered by the peroxidatic cysteine oxidation. Finally, we proposed a model that takes data from this work as well data as from the literature into account.

A single-step procedure for the preparation of palladium nanoparticles and a phosphine-functionalized support as catalyst for Suzuki cross-coupling reactions

We here report the preparation of supported palladium nanoparticles (NPs) stabilized by pendant phosphine groups by reacting a palladium complex containing the ligand 2-(diphenylphosphino)benzaldehyde with an amino-functionalized silica surface The Pd nanocatalyst is active for Suzuki cross-coupling reaction avoiding any addition of other sources of phosphine ligands The Pd intermediates and Pd NPs were characterized by solid-state nuclear magnetic resonance and transmission electron microscopy techniques The synthetic method was also applied to prepare magnetically recoverable Pd NPs leading to a catalyst that could be reused for up to 10 recycles In summary we gathered the advantages of heterogeneous catalysis magnetic separation and enhanced catalytic activity of palladium promoted by phosphine ligands to synthesize a new catalyst for Suzuki cross-coupling reactions The Pd NP catalyst prepared on the phosphine-functionalized support was more active and selective than a similar Pd NP catalyst prepared on an amino-functionalized support (C) 2010 Elsevier Inc All rights reserved

Catalytic hydrodechlorination of chlorobenzene over supported palladium catalyst in buffered medium

The catalytic hydrodechlorination (HDC) reaction, which is an attractive abatement process for chlorinated organic wastes, was studied over a magnetically recoverable supported Pd(0) catalyst. We investigated the most favorable reaction conditions under which to obtain the highest substrate conversion rates while preserving the catalyst properties and morphology. Sodium hydroxide, triethylamine and buffered solutions were used as proton scavengers in the HDC of chlorobenzene under mild conditions. It was observed that sodium hydroxide caused corrosion of the silica support, triethylamine in 2-propanol preserved the morphology of the catalyst which could be recycled for up to five successive H DC reactions, and aqueous buffer solutions preserved the catalyst morphology and the catalytic activity for up to four successive HDC reactions. The use of buffer solutions to neutralize the HCl formed during the HDC reaction is an interesting, less aggressive, alternative approach to HDC reactions. (C) 2010 Elsevier B.V. All rights reserved.