Theoretical Study Of Dinitrogen Activation By Iron, Ruthenium, And Osmium

Molecular nitrogen (N2) surrounds us as the major component of the atmosphere here on Earth. Only a few organisms are capable of utilizing this plentiful source of nitrogen. The process by which N2 is incorporated into biological systems is referred to as nitrogen fixation and involves the nitrogenase enzymes that contain a metal-sulfide cluster at the active site. The recent X-ray crystal structure of one such enzyme cofactor has inspired increased interest in the mechanism by which nitrogenase converts N2 to ammonia. Since the discovery of the first dinitrogen complex, [(H3N)5Ru(N2)]Cl2 in 1965, the coordination chemistry of this simple molecule has flourished, and dinitrogen compounds of almost every transition element have been prepared. Some of this early coordination chemistry of N2 was aimed at modeling what was believed to be the active site of nitrogenase. Additionally, there is continued interest in developing new kinds of reactivities for coordinated N2 in an attempt to achieve a different goal: the discovery of new catalytic processes for the fixation and functionalization of dinitrogen.At the turn of last century, the imminent need for a source of fixed nitrogen became apparent, as natural source of nitrogen compounds used largely for fertilizers, were being depleted. The Haber-Bosch process, which has proven the most successful commercially, reacts N2 gas with three equivalents of H2 gas over a transition metal iron catalyst to produce ammonia. While this reaction is exothermic and thermodynamically favored under ambient conditions, the feedstock gases must be compressed to several hundred atmospheres to favor ammonia production at the high temperatures currently required. A catalyst that could perform this reaction at lower temperatures, and therefore lower pressures, would be economically advantageous; advancements in this area have been made using a Barium-promoted oxide-supported ruthenium-based catalyst instead of an Fe-based catalyst..While the Haber-Bosch process differs substantially from the biological fixation of dinitrogen, both of these processes involve the activation of N2 by a transition metal-containing catalyst. Numerous efforts have been made to develop synthetic metal-based catalysts to functionalize molecular nitrogen under mild conditions. Although some progresses have been made in this area, many of the systems remain intriguing curiosities and are commercially impractical. The limited reactivity of dinitrogen and the harsh conditions required to convert N2 into useful nitrogen-containing compounds are in contrast with the reactivities of the other small molecules.. The catalytic activation of molecular nitrogen to produce nitrogen-containing compounds under mild conditions remains one of the loftier goals in chemistry. Theoretical and experimental study dinitrogen activation is one of the most interesting subjects.The present work presents a detailed theoretical study on the activation of dinitrogen by iron, ruthenium, and osmium atoms and diatoms. The contents of this thesis are described in three parts.Part I consists of two chapters. In the first chapter, we reviewed the structure and propertiesof N2. We summarized the most important achievements on the experimental and the theoretical studies of the activation and fixation of dinitrogen. Chapter 2 presented a brief introduction of density functional theory (DFT), which is the working-horse in the present study.In this work hybrid density functional method B3LYP, implemented in the Gaussian 98 suite of program, was employed to do full geometry optimizations. Harmonic vibrational frequencies were obtained at the same level to characterize the stationary points as local minima or first-order saddle points, and to obtain zero-point energy (ZPE) corrections for monoatomic and diatomic iron, ruthenium, and osmium containing-dinitrogen complexes and compounds with different spin multiplicities and electronic states. All electron basis sets were applied to Fe and N and effective core potential plus double-zet...