DFT Studies on Terminal versus Bridged N2 Binding in Transition Metal Chemistry and Reactions of Metal-Nitride Complexes

Abstract

Structure determines function. Although this statement is simple in principle, understanding how structure can affect the function of a molecule, enzyme or catalyst can be a very complex process. Whether it is the coordination geometry, the arrangement of molecular orbitals, or the way intramolecular forces can cause a molecule to fold around itself, it is hard to determine exactly what causes a molecule or catalyst to react in a certain way. Fortunately, the development and advancement of quantum mechanical methods such as Density Functional Theory (DFT) has allowed chemists to take a deeper look at the structural properties of chemical systems. Herein, we utilize such computational methods to understand the behavior of molecular catalysts in the synthesis of ammonia from dinitrogen. The following work is divided into three main parts. After the introduction, the second chapter focuses on the equilibrium between terminal N2 metal complexes and bridging N2 complexes. The coordination of N2 to a metal catalyst is the first step in the ammonia synthesis process. While some molecular catalysts spontaneously form terminal N2 complexes in solution upon exposure to a flow of N2, others form bridged N2 complexes. In Chapter 2, we consider several organometallic complexes, all with previous experimental data, of different metal centers, coordination spheres and pi-electron counts. We split the equilibrium of interest into simpler fundamental steps and calculate the energies of each step. Based on the analysis of the computed data, we propose a pi-Bond order (pi-BO) model which explains why complexes prefer either mode of N2 coordination based on the number of pi-electrons in the system. In the third chapter, we consider the thermodynamics and kinetics of the formation and cleavage of bridged N2 complexes. We study the effect of the nature of the metal and the coordination sphere by considering two different organometallic systems with several metal centers. We utilize the proposed pi-BO model and MO-theory in attempt to determine the factors affecting the formation and cleavage of bridging N2 complexes. In Chapter 3, we also reproduce the the kinetics of the experimentally observed cleavage of Schneider's bridging N2 complex and see how changing the metal center affects these kinetic observations. The last chapter of this work considers reactions of metal nitride complexes, the products of the previously considered cleavage reactions. This has direct applications in molecular catalysis of NH3 synthesis from N2. We consider different transfer reactions to the metal nitride: a proton transfer, an electron transfer, a hydrogen atom transfer, a nitrogen atom transfer, and an oxygen atom transfer. We study the effect of the metal and coordination sphere on such reactions by considering two systems and alter the nature of the metal center. We hope that the studies included in this work contribute to the design and synthesis of more efficient catalysts for ammonia synthesis.