Computational chemistry applied to hydrocarbon functionalization by iron catalysts, oxidation state characterization, and electrides design

Resumen

According to F. Jensen quote: \Chemistry is the science dealing with construction, transformation and properties of molecules." Introduction to Computational Chemistry, Second Edition. Frank Jensen. Page 1. In this thesis, the three constituent parts of chemistry (in agreement with Jensen's quotation) are addressed by means of theoretical and computational tools. In this sense, only the global knowledge given by the three di erent aspects of chemistry provides an enough detailed picture for the absolute understanding of any chemical system. It is necessary to know the molecular construction (atomic and electronic) to rationalize molecular properties, and, in turn, molecular properties govern the reactivity and molecular transformations. This thesis encompass different computational studies that together cover all the three parts of chemical science. For each part, we have selected a di erent research project, whose major feature is related with construction, transformation or properties of the chemical systems. The di erent research projects have been chosen taking into account their general chemical interest and the insights that computational chemistry can provide to the up-to-date knowledge of the topic. In that way, the study of the C-H bond functionalization of non-activated hydrocarbons by iron catalysts has been chosen as the research project for the molecular transformations section; the oxidation state is the property we focus on; and, regarding molecular construction, we describe the electronic structure of molecular electrides. Achieving direct and selective C-H bond functionalizations of non-activated hydrocarbons by organometallic catalysts is still a challenging work in synthetic chemistry. Nowadays it constitutes the main drawback to overcome in order to achieve ecient strategies in organic synthesis or to use alkanes as direct precursors for more important chemicals for the industry. Thus, currently, this research topic is focus of much activity and there is still plenty of room for development. The use of iron catalysts in such reactions has a special interest in the sense that iron is an economic, environmental friendly, and nontoxic metal. The main role of computational chemistry in this eld falls to the determination of the reaction mechanisms. In this thesis two C-H bond functionalization processes that share complementary interest are studied (Chapter 4). The rst one is the hydroxylation of alkanes by a nonheme FeV =O catalyst. Although this reaction has been widely studied computationally, the di erent studies report several reaction mechanisms that di er among them, the origin of their variability was not completely understood. Therefore, we investigate the e ects of the substrate and solvent in these processes in order to try to rationalize the di erences on the reaction mechanisms reported in literature. The di erent stability at gas and in solvent phase of the reaction intermediate is revealed as the key reason for the existence of di erent reaction mechanisms. The second studied process is the functionalization of C-H bonds of non-activated arenes by carbene-insertion reactions. Unlike hydroxylation reactions, this process has not been widely studied. Speci cally, a chemoselective carbene-insertion reaction into benzene by an iron-carbene catalyst was reported in 2016 by the experimental groups of Dr. Miquel Costas and Prof. Pedro J. Perez for the rst time (Angew. Chemie Int. Ed. 2016, 55, 6530-6534). In this sense, in the fourth chapter of this thesis we elucidate the reaction mechanism of this unprecedent reactivity and we also identify the features and reasons that favor this reactivity over other possible reactions such as the carbene addition into benzene. The oxidation state (OS) concept has utmost relevance in chemistry. In reaction mechanisms of organometallic catalysis, for example, it is an intrinsic descriptor and the ngerprint of the chemical reaction is not complete until the determination of the OSs of all the involved species. The OS concept is also an important concept in chemical nomenclature, or, in spectroscopic characterizations, where it is related to d-electron con gurations of TM. The role of computational chemistry in determining the OSs from rst principles has been little explored. However, the group of Dr. Pedro Salvador recently presented a new computational tool for the determination of OS, the so-called E ective Oxidation State (EOS) method (J. Chem. Theory Comput. 2015, 11, 1501-1508). In chapter 5, we present a benchmarking study about the applicability of the EOS method to determine OSs. Moreover, new key insights about the OS property based on data of the benchmarking study are provided. Electrides are a relatively new class of ionic compounds with an intriguing electronic structure: isolated electrons act as the stoichiometric anionic part of the crystal. This interesting feature, however, hinders the classical assignation of oxidation states in such compounds, for example. Despite just thirteen solid electrides have been synthesized up to date (only six stable at room temperature), their interesting properties have led to the development of a plethora of amazing applications for those compounds. Thus, electride are, without doubt, one of the promising research elds for the development of new materials which still has many room for development. Computational chemistry has always had an important role in the development of electrides and in the understanding of their electronic structure. In chapter 6 we present the rst computational study about the existence of electrides beyond the solid state. Despite many theoretical works have been addressed to the study of the electron con nement of electrides in the solid lattice, no other work before focused on the characterization of electrides in the gas phase. Herein, we investigate whether the electronic structure of molecular electrides truly constain isolated electrons or if it is actually just a formal picture to represent these molecules. Furthermore, we provide an unambiguous criterion to distinguish molecular electrides from other similar species. Overall, this thesis is divided into eight chapters. The rst one is an introduction where the reasons of the chemical interest of the di erent topics selected in this thesis are exposed. Furthermore, the rst chapter also summarizes the contributions that the quantum chemist community has provided to these topics up to date. Chapter 2 presents the theoretical methods, tools, and the di erent computational chemistry strategies used in the thesis to carry out the studies or to properly analyze the results. Chapter 3 lists the aims of the thesis. Chapters 4, 5, and 6 include the abovementioned publications presented in this thesis. Chapter 7 presents the discussion of the reported results and, nally, in chapter 8 the main conclusions drawn from this thesis are summarized.