Molecular modelling of transition metal complexes with QM/MM methods : structure and reactivity /

Descripció del recurs: el 20-08-2008Consultable des del TDXTítol obtingut de la portada digitalitzadaEl trabajo presentado en esta tesis consiste principalmente en la aplicación del método IMOMM (un método que combina la mecánica cuántica y la mecánica molecular) al estudio de sistemas químicos donde se ven involucrados complejos de metales de transición. Por un lado se han realizado estudios de tipo estructural, en los cuales se analizan las causas de la interacciones agósticas que se encuentran en el complejo Ir(H)2(P(t-Bu)2Ph)2 y en la familia de complejos Mo(COR)S-S)(CO)(PR3)2, y también se analizan la causas de las distorsiones geométricas que presentan en una serie de complejos de Ir pentacoordinados. En todos ellos se comprueba que la introducción de los efectos estéricos juega un papel fundamental en la explicación de los hechos experimentales. Por otro lado se han realizado estudios de reactividad, concretamente sobre la reacción de dihidroxilación enantioselectiva de olefinas con OsO4. Para ello se ha llevado a cabo en primer lugar un estudio metodológico, en el cual se ha determinado cual es la base y el método cuántico óptimos para el estudio metodológico, en el cual se ha determinado cual es la base y el método cuántico óptimos para el estudio del sistema modelo, OsO4 NH3 + C2H4 y se comprobado también que el método IMOMM es un método válido para el estudio del sistema real, (DHQD)2PYDZ.OsO4 + CH2=CHPh. Una vez realizado esto, se ha procedido al estudio sobre el mecanismo de la reacción. El estudio sobre el sistema modelo ha permitido descartar uno de los dos mecanismos propuestos, concretamente el (2+2). El estudio sobre el sistema real ha pemitido por un lado la caracterización de un intermedio para el mecanismo (3+2) (que estaba propuesto experimentalmente), y por otro el análisis de cual se el origen de la enantioselectividad. En dichos análisis se demuestra que el origen de la enantioselectividad es debido a las interacciones atractivas entre los grupos aromáticos de la olefina y del del catalizador

Beyond Continuum Solvent Models in Computational Homogeneous Catalysis

Altres ajuts: Acord transformatiu CRUE-CSICIn homogeneous catalysis solvent is an inherent part of the catalytic system. As such, it must be considered in the computational modeling. The most common approach to include solvent effects in quantum mechanical calculations is by means of continuum solvent models. When they are properly used, average solvent effects are efficiently captured, mainly those related with solvent polarity. However, neglecting atomistic description of solvent molecules has its limitations, and continuum solvent models all alone cannot be applied to whatever situation. In many cases, inclusion of explicit solvent molecules in the quantum mechanical description of the system is mandatory. The purpose of this article is to highlight through selected examples what are the reasons that urge to go beyond the continuum models to the employment of micro-solvated (cluster-continuum) of fully explicit solvent models, in this way setting the limits of continuum solvent models in computational homogeneous catalysis. These examples showcase that inclusion of solvent molecules in the calculation not only can improve the description of already known mechanisms but can yield new mechanistic views of a reaction. With the aim of systematizing the use of explicit solvent models, after discussing the success and limitations of continuum solvent models, issues related with solvent coordination and solvent dynamics, solvent effects in reactions involving small, charged species, as well as reactions in protic solvents and the role of solvent as reagent itself are successively considered

Anti-Markovnikov Intermolecular Hydroamination of Alkenes and Alkynes : A Mechanistic View

Altres ajuts: acords transformatius de la UABHydroamination, the addition of an N-H bond across a C-C multiple bond, is a reaction with a great synthetic potential. Important advances have been made in the last decades concerning catalysis of these reactions. However, controlling the regioselectivity in the amine addition toward the formation of anti-Markovnikov products (addition to the less substituted carbon) still remains a challenge, particularly in intermolecular hydroaminations of alkenes and alkynes. The goal of this review is to collect the systems in which intermolecular hydroamination of terminal alkynes and alkenes with anti-Markovnikov regioselectivity has been achieved. The focus will be placed on the mechanistic aspects of such reactions, to discern the step at which regioselectivity is decided and to unravel the factors that favor the anti-Markovnikov regioselectivity. In addition to the processes entailing direct addition of the amine to the C-C multiple bond, alternative pathways, involving several reactions to accomplish anti-Markovnikov regioselectivity (formal hydroamination processes), will also be discussed in this review. The catalysts gathered embrace most of the metal groups of the Periodic Table. Finally, a section discussing radical-mediated and metal-free approaches, as well as heterogeneous catalyzed processes, is also included

Origin of the Rate Acceleration in the C−C Reductive Elimination from Pt(IV)-complex in a [Ga4L6]12− Supramolecular Metallocage

Altres ajuts: Acord transformatiu CRUE-CSICThe reductive elimination on [(MeP)Pt(MeOH)(CH)], 2P, complex performed in MeOH solution and inside a [GaL] metallocage are computationally analysed by mean of QM and MD simulations and compared with the mechanism of gold parent systems previously reported [EtPAu(MeOH)(CH)], 2Au. The comparative analysis between the encapsulated Au(III) and Pt(IV)-counterparts shows that there are no additional solvent MeOH molecules inside the cavity of the metallocage for both systems. The Gibbs energy barriers for the 2P reductive elimination calculated at DFT level are in good agreement with the experimental values for both environments. The effect of microsolvation and encapsulation on the rate acceleration are evaluated and shows that the latter is far more relevant, conversely to 2Au. Energy decomposition analysis indicates that the encapsulation is the main responsible for most of the energy barrier reduction. Microsolvation and encapsulation effects are not equally contributing for both metal systems and consequently, the reasons of the rate acceleration are not the same for both metallic systems despite the similarity between them

Challenges in modelling homogeneous catalysis : new answers from ab initio molecular dynamics to the controversy over the Wacker process

The controversial reaction mechanism considering experimental results and theoretical treatment from static to ab initio molecular dynamic simulations is reviewed.</p

Computational analysis on the Pd-catalyzed C-N coupling of ammonia with aryl bromides using a chelate phosphine ligand

The Buchwald-Hartwig amination of arylhalides with the Pd-Josiphos complex is a very useful process for the generation of primary amines using ammonia as a reactant. Density-functional theory (DFT) calculations are carried out to examine the reaction mechanism for this process. Although the general mechanism for the C-N cross-coupling reaction is known, there are still some open questions regarding the effect of a chelate phosphine ligand and the role of the base in the process. Reaction pathways involving the release of one of the arms of the phosphine ligand are compared with those where the chelate phosphine remains fully coordinated. Conformational analysis for the complex with the open chelate phosphine is required to properly evaluate the proposed pathways. The role played by the added base (t-BuO-) as a possible ligand or just as a base was also evaluated. The understanding of all of these aspects allowed us to propose a complete reaction mechanism for the Pd-catalyzed C-N coupling of arylhalides with ammonia using the chelate Josiphos ligand

Modeling Kinetics and Thermodynamics of Guest Encapsulation into the [ML] 12- Supramolecular Organometallic Cage

Altres ajuts: Acord transformatiu CRUE-CSICThe encapsulation of molecular guests into supramolecular hosts is a complex molecular recognition process in which the guest displaces the solvent from the host cavity, while the host deforms to let the guest in. An atomistic description of the association would provide valuable insights on the physicochemical properties that guide it. This understanding may be used to design novel host assemblies with improved properties (i.e., affinities) toward a given class of guests. Molecular simulations may be conveniently used to model the association processes. It is thus of interest to establish efficient protocols to trace the encapsulation process and to predict the associated magnitudes Δ G and Δ G ⧧. Here, we report the calculation of the Gibbs energy barrier and Gibbs binding energy by means of explicit solvent molecular simulations for the [GaL] 12- metallocage encapsulating a series of cationic molecules. The Δ G ⧧ for encapsulation was estimated by means of umbrella sampling simulations. The steps involved were identified, including ion-pair formation and naphthalene rotation (from L ligands of the metallocage) during the guest's entrance. The Δ G values were computed using the attach-pull-release method. The results reveal the sensitivity of the estimates on the force field parameters, in particular on atomic charges, showing that higher accuracy is obtained when charges are derived from implicit solvent quantum chemical calculations. Correlation analysis identified some indicators for the binding affinity trends. All computed magnitudes are in very good agreement with experimental observations. This work provides, on one side, a benchmarked way to computationally model a highly charged metallocage encapsulation process. This includes a nonstandard parameterization and charge derivation procedure. On the other hand, it gives specific mechanistic information on the binding processes of [GaL] 12- at the molecular level where key motions are depicted. Taken together, the study provides an interesting option for the future design of metal-organic cages

Catalysis by [Ga4L6]12− metallocage on the Nazarov cyclization : the basicity of complexed alcohol is key

The Nazarov cyclization is investigated in solution and within K[GaL] supramolecular organometallic cage by means of computational methods. The reaction needs acidic condition in solution but works at neutral pH in the presence of the metallocage. The reaction steps for the process are analogous in both media: (a) protonation of the alcohol group, (b) water loss and (c) cyclization. The relative Gibbs energies of all the steps are affected by changing the environment from solvent to the metallocage. The first step in the mechanism, the alcohol protonation, turns out to be the most critical one for the acceleration of the reaction inside the metallocage. In order to calculate the relative stability of protonated alcohol inside the cavity, we propose a computational scheme for the calculation of basicity for species inside cavities and can be of general use. These results are in excellent agreement with the experiments, identifying key steps of catalysis and providing an in-depth understanding of the impact of the metallocage on all the reaction steps

A Tetraferrocenyl-Resorcinarene Cavitand as a Redox-Switchable Host of Ammonium Salts

Tetraannulation of a resorcinarene-octaamino cavitand with ferrocenecarboxaldehyde allows the preparation of a tetrabenzimidazole-resorcinarene cavitand with four ferrocenyl moieties directly linked to the C2 atom of the imidazole units. Oxidation of the four ferrocenyl moieties produces important structural modifications of the molecule, as indicated by DFT calculations performed for the neutral and tetraoxidized forms of the cavitand. By means of 1H NMR spectroscopic analysis, the encapsulating properties of the new tetraferrocenyl-resorcinarene cavitand toward a series of ammonium salts were evaluated, and a clear cutoff point in binding affinity with respect to size was observed. Cyclic voltammetric studies allowed us to estimate the relative association constants for the neutral and oxidized forms of the cavitand, thus indicating that the guest was bound to the neutral (reduced) state of the cavitand and was released from the oxidized form. These redox-addressable conformational and binding properties of the resorcinarene-tetraferrocenyl cavitand constitute all the necessary features of a redox-switchable molecular gripper. By means of mass-spectrometric analysis, we could unambiguously confirm the molar stoichiometry of the host–guest complex (1:1) and assess the strong guest encapsulation, as indicated by triggering the covalent coupling between host and guest in the gas phase.MEC of Spain (CTQ2011-24055/BQU and CTQ2014-54071-P) and the Universitat Jaume I (P11B2014-02