Computational Catalysis of Homogenous and Heterogeneous Systems: New Insights into the Activation of Small Molecules

Esta Tesis Doctoral se ubica en el contexto del estudio teórico y computacional de catalizadores, tanto homogéneos como heterogéneos, para la fijación de moléculas pequeñas, en concreto N2, O2, CO, NH3, HCOOH e hidrosilanos, empleando DFT. Estos procesos presentan un gran interés en investigación química, ya que un conocimiento en profundidad de los mismos facilitaría el diseño racional de nuevos catalizadores más activos y respetuosos con el medio ambiente. A continuación se presenta un resumen de los procesos estudiados.En el campo de la catálisis homogénea, se estudiaron cuatro procesos químicos diferentes, catalizados por complejos organometálicos de Rh e Ir:i) Fijación de amoniaco mediante complejos de Ir con ligandos de tipo pincer. En particular, se estudiaron los factores cinéticos y termodinámicos que afectan a laactividad del catalizador. Para ello, se emplearon diferentes métodos para el estudio del enlace químico, como IQA y la ELF.ii) La sililación de enlaces C–H aromáticos mediante un complejo bien definido de Ir(III)– NHC. Se propuso un mecanismo de reacción completo mediante cálculoscomputacionales. Asimismo, se realizaron una serie de experimentos que apoyan dicho mecanismo. Los resultados obtenidos revelaron el papel clave de los grupos directorespresentes en el sustrato en el control de la selectividad del proceso.iii) El empleo de hidrógeno molecular como vector energético. En particular, se estudió el proceso de deshidrogenación de ácido fórmico para generar H2 catalizado por uncompuesto de Rh–NHO muy activo. Los estudios computacionales permitieron proponer un mecanismo de reacción plausible, que está de acuerdo con las barreras energéticas determinadas experimentalmente.iv) La alcoxicarbonilación de alcoholes y aminas con CO para preparar carbamatos. El proceso está catalizado por un complejo de Rh y requiere le acción de un oxidante(KHSO5). El estudio teórico realizado permitió proponer un mecanismo de reacción y determinar el papel clave del oxidante.Con respecto a la catálisis heterogénea, se abordó la propuesta de nuevos descriptores de la actividad catalítica basados en las propiedades magnéticas del catalizador y los reactivos. Los diferentes procesos estudiados se resumen a continuación:v) Las reacciones de reducción de oxígeno (ORR) y de evolución de oxígeno (OER) catalizadas por materiales basados en perovskitas. En concreto, es analizaron laspropiedades magnéticas de diferentes catalizadores derivados de LaMnO3. Los resultados mostraron la importancia de tener en cuenta la entropía electrónica y la fasemagnética del catalizador. Basándonos en estos resultados, se propusieron una serie dereglas para el diseño de nuevos catalizadores heterogéneos derivados de óxidosmetálicos magnéticos para ORR y OER. Finalmente, estas reglas se emplearon en eldiseño de un nuevo catalizador para OER basado en la perovskita LaFeO3.vi) Los principios previamente presentados se ampliaron al estudio de procesos de fijación de N2 catalizados por nitruros de Mo. Los resultados muestran la validez de la aplicación del momento magnético del molibdeno como descriptor de la actividad catalítica.<br /

Estudio computacional de sistemas basados en sub-nanoclústeres metálicos

Los sub-nanoclústeres metálicos presentan un gran potencial para su uso como catalizadores en reacciones de interés industrial, tales como el reformado seco de metano y la reacción de reducción de oxígeno. Tradicionalmente, se han empleado como catalizadores sistemas basados en metales nobles, que presentan, en general, una gran actividad catalítica. Sin embargo, tienen un coste económico muy elevado, por lo que se están buscando soluciones en otros metales de transición no nobles, los cuales son mucho más accesibles y, por tanto, más baratos. Entre ellos, se han seleccionado los sub-nano clústeres de Ni, que, depositados sobre soportes basados en nitruro de boro hexagonal (h-BN), son catalizadores prometedores para la reacción de reformado seco de metano. Con el objetivo de comprender mejor las características del sistema anterior, en este Trabajo de Fin de Grado se han aplicado técnicas de optimización global para explorar la superficie de energía potencial de sistemas formados por 4 y 5 átomos de Ni, tanto en fase gas como depositados sobre h-BN. Con la metodología anterior se espera poder identificar todos los mínimos de cada uno de los cuatros sistemas (Ni4 y Ni5 en fase gas y depositados sobre h-BN), y determinar cuáles de ellos son relevantes en función de la temperatura y en qué proporción. Esta búsqueda es relevante, ya que la distribución estadística de estructuras presentes en el sistema tiene una gran importancia en la actividad catalítica del mismo (que no ha sido estudiada explícitamente en este TFG). Además de la geometría, se ha analizado el magnetismo de los clústeres obtenidos, así como el cambio que experimenta el mismo cuando los clústeres pasan de estar en fase gas a depositarse sobre h-BN. Todos los cálculos del trabajo han sido realizados con el programa especializado en química (y física) cuántica Vienna ab Initio Simulation Package (VASP).<br /

Efficient Rhodium-catalyzed multicomponent reaction for the synthesis of novel propargylamines

[{Rh(μ-Cl)(H)2(IPr)}2] (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazole-2-ylidene) was found to be an efficient catalyst for the synthesis of novel propargylamines by a one-pot three-component reaction between primary arylamines, aliphatic aldehydes, and triisopropylsilylacetylene. This methodology offers an efficient synthetic pathway for the preparation of secondary propargylamines derived from aliphatic aldehydes. The reactivity of [{Rh(μ-Cl)(H)2(IPr)}2] with amines and aldehydes was studied, leading to the identification of complexes [RhCl(CO)IPr(MesNH2)] (MesNH2 = 2,4,6-trimethylaniline) and [RhCl(CO)2IPr]. The latter shows a very low catalytic activity while the former brought about reaction rates similar to those obtained with [{Rh(μ-Cl)(H)2(IPr)}2]. Besides, complex [RhCl(CO)IPr(MesNH2)] reacts with an excess of amine and aldehyde to give [RhCl(CO)IPr{MesN[DOUBLE BOND]CHCH2CH(CH3)2}], which was postulated as the active species. A mechanism that clarifies the scarcely studied catalytic cycle of A3-coupling reactions is proposed based on reactivity studies and DFT calculations.This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO CSD2009-0050, CTQ2011-27593, CTQ2012-35665 and CTQ2013-42532-P projects) and the DGA/FSE-E07. The support from KFUPM-University of Zaragoza research agreement and the Centre of Research Excellence in Petroleum Refining & KFUPM is gratefully acknowledged. V. P. thankfully acknowledges the resources from the supercomputer >Memento>, technical expertise and assistance provided by BIFI-ZCAM (Universidad de Zaragoza). L.R.-P thanks to CONACyT for a postdoctoral fellowship (204033).Peer Reviewe

An insight into transfer hydrogenation reactions catalysed by iridium(III) bis-N-heterocyclic carbenes

A variety of [M(L)2(L′)2{κC,C′-bis(NHC)}]BF4 complexes (M = Rh or Ir; L = CH3CN or wingtip group; L′ = I– or CF3COO–; NHC=N-heterocyclic carbene) have been tested as pre-catalysts for the transfer hydrogenation of ketones and imines. The conversions and TOF's obtained are closely related to the nature of the ligand system and metal centre, more strongly coordinating wingtip groups yielding more active and recyclable catalysts. Theoretical calculations at the DFT level support a classic stepwise metal-hydride pathway against the concerted Meerwein–Ponndorf–Verley (MPV) mechanism. The calculated catalytic cycle involves a series of ligand rearrangements due to the high trans effect of the carbene and hydrido ligands, which are more stable when situated in mutual cis positions. The reaction profiles obtained for the complexes featuring an iodide or a trifluoroacetate in one of the apical positions agree well with the relative activity observed for both catalysts.The authors would like to acknowledge the support by the Ministry of Higher Education, Saudi Arabia, in establishment of the Centre of Research Excellence in Petroleum Refining & Petrochemicals at KFUPM (KACST-funded project ART-32-68). The support under the KFUPM–University of Zaragoza research agreement is also highly appreciated. This work was further supported by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO CSD2009-0050, CTQ2011-27593 and CTQ2012-35665 projects) and the Diputación General de Aragón (DGA/FSE-E07).Peer Reviewe

Atoms in molecules in real space: a fertile field for chemical bonding

In this perspective, we review some recent advances in the concept of atoms-in-molecules from a real space perspective. We first introduce the general formalism of atomic weight factors that allows unifying the treatment of fuzzy and non-fuzzy decompositions under a common algebraic umbrella. We then show how the use of reduced density matrices and their cumulants allows partitioning any quantum mechanical observable into atomic or group contributions. This circumstance provides access to electron counting as well as energy partitioning, on the same footing. We focus on how the fluctuations of atomic populations, as measured by the statistical cumulants of the electron distribution functions, are related to general multi-center bonding descriptors. Then we turn our attention to the interacting quantum atom energy partitioning, which is briefly reviewed since several general accounts on it have already appeared in the literature. More attention is paid to recent applications to large systems. Finally, we consider how a common formalism to extract electron counts and energies can be used to establish an algebraic justification for the extensively used bond order-bond energy relationships. We also briefly review a path to recover one-electron functions from real space partitions. Although most of the applications considered will be restricted to real space atoms taken from the quantum theory of atoms in molecules, arguably the most successful of all the atomic partitions devised so far, all the take-home messages from this perspective are generalizable to any real space decompositionsWe acknowledge the spanish MICINN, grant PID2021-122763NB-I00 and the FICyT, grant IDI/2021/000054 for financial support. TRR gratefully acknowledges DGTIC/UNAM for computer time (LANCAD-UNAM-DGTIC 250

Novel homogeneous Ir-catalysts: Ligand design, applications and mechanisms

Resumen del trabajo presentado a la 2nd World Chemistry Conference and Exhibition (WCCE), celebrada en Valencia (España) del 9 al 11 de julio de 2018.This presentation will deal with two main subjects: (i) the preparation ofthe frrst PCP-type ligand based on an N-heterocyclic olefm (NHO) scaffold, accompanied by an evaluation of the impact of this type of ligand in the activity of iridium complexes in several relevant catalytic processes; and (ii) the development of well- defined Ir-NHC complexes as catalysts for the dehydrogenative silylation of aromatic C-H bonds. (i) A great variety of pincer complexes has been reported in the literature. In particular, transition metal complexes containing PCP pincer ligands have shown remarkable activities in relevant catalytic processes. Recent work by us on this subject has resulted in the preparation of an ewPCP-type ligand based on an N- heterocyclic olefin (NHO) scaffold. The flexibille coordination of this NHO-based PCP-ligand can be attributed to the dual nature (ylide-olefm) oftbe NHO. Iridium(I) complexes featuring this ligand show excellent activities in transfer hydrogenation reactions. The active species ([Ir(KP,C,P'-NHO-PPh2)(iPrO)]), formed via COD dissociation and subsequent isopropoxide coordination, features an NHO moiety that behaves as a hemilabile ligand, which allows the catalyst to adopt stabJe square planar geometries in the transition states, thus reducing the energetic barrier of the process. More recently, we have tested the activity ofthese complexes in the dehydrogenation of formic acid, showing outstanding activities in water and in neat formicacid. (ii) The preparation of fine chernicals by the catalytic functionalization of C-H bonds has seen an outstanding development in recent years, with borylation and silylation reactions being prominent examples of this chemistry. In this regard, the versatility of oganosilicon compounds can be attributed to the low cost and non-toxic nature of silicon reagents, together with their straight forward functionalization by various reactions. Moreover, conjugated organosilicon materials are attractive targets per se owing to their unique properties, which permit a widespread applicability in the field of organic electronics and photonics. Most of the catalysts employed so far for this reaction are generated >in situ> from commercial metal precursors and ligands. Hence, we have focused on the development of well-defined organometallic catalysts bearing appropriate ligands in order to improve the efficiency of current silylation catalysts. In particular, the use of NHC-Ir (III) complex [Ir(H)2(IPr)(py)3][BF4] (IPr = 1 ,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) as a catalyst has allowed for the preparation of a wide range of aryl- and heteroarylsilanes. The directed and non-directed functionalization of C-H bonds has been accomplisbed successfully using the areneas tbe limiting reagent and a variety of hydrosilanes, including Et3SiH, Ph2MeSiH, PhMe2SiH, Ph3SiH and(Et0)3SiH.Peer Reviewe

How electrons still guard the space: Electron number distribution functions based on QTAIMnELF intersections

Despite the importance of the one-particle picture provided by the orbital paradigm, a rigorous understanding of the spatial distribution of electrons in molecules is still of paramount importance to chemistry. Considerable progress has been made following the introduction of topological approaches, capable of partitioning space into chemically meaningful regions. They usually provide atomic partitions, for example, through the attraction basins of the electron density in the quantum theory of atoms in molecules (QTAIM) or electron-pair decompositions, as in the case of the electron localization function (ELF). In both cases, the so-called electron distribution functions (EDFs) provide a rich statistical description of the electron distribution in these spatial domains. Here, we take the EDF concept to a new fine-grained limit by calculating EDFs in the QTAIM n ELF intersection domains. As shown in AHn systems based on main group elements, as well as in the CO, NO, and BeO molecules, this approach provides an exquisitely detailed picture of the electron distribution in molecules, allowing for an insightful combination of the distribution of electrons between Lewis entities (such as bonds and lone pairs) and atoms at the same time. Besides mean-field calculations, we also explore the impact of electron correlation through Hartree–Fock (HF), density functional theory (DFT) (B3LYP), and CASSCF calculations

Understanding the reaction mechanism of the oxidative addition of ammonia by (PXP)Ir(i) complexes: The role of the X group

An analysis of the electronic rearrangements for the oxidative addition of ammonia to a set of five representative (PXP)Ir pincer complexes (X = B, CH, O, N, SiH) is performed. We aim to understand the factors controlling the activation and reaction energies of this process by combining different theoretical strategies based on DFT calculations. Interestingly, complexes featuring higher activation barriers yield more exothermic reactions. The analysis of the reaction path using the bonding evolution theory shows that the main chemical events, N-H bond cleavage and Ir-H bond formation, take place before the transition structure is reached. Metal oxidation implies an electron density transfer from non-shared Ir pairs to the Ir-N bond. This decrement in the atomic charge of the metal provokes different effects in the ionic contribution of the Ir-X bonding depending on the nature of the X atom as shown by the interacting quantum atoms methodology. © 2017 the Owner Societies

Exploring the potential energy surface of Pt6 sub-nano clusters deposited over graphene

Catalytic systems based on sub-nanoclusters deposited over different supports are promising for very relevant chemical transformations such as many electrocatalytic processes as the ORR. These systems have been demonstrated to be very fluxional, as they are able to change shape and interconvert between each other either alone or in the presence of adsorbates. In addition, an accurate representation of their catalytic activity requires the consideration of ensemble effects and not a single structure alone. In this sense, a reliable theoretical methodology should assure an accurate and extensive exploration of the potential energy surface to include all the relevant structures and with correct relative energies. In this context, we applied DFT in conjunction with global optimization techniques to obtain and analyze the characteristics of the many local minima of Pt6 sub-nanoclusters over a carbon-based support (graphene)—a system with electrocatalytic relevance. We also analyzed the magnetism and the charge transfer between the clusters and the support and paid special attention to the dependence of dispersion effects on the ensemble characteristics. We found that the ensembles computed with and without dispersion corrections are qualitatively similar, especially for the lowest-in-energy clusters, which we attribute to a (mainly) covalent binding to the surface. However, there are some significant variations in the relative stability of some clusters, which would significantly affect their population in the ensemble composition