Fundamental Studies of Metal Centered Transformations Relevant to Catalysis
The overall objectives the research performed have been to advance the mission of the U. S. Department of Energy, with specific emphasis concerned with the conversion and use of fossil and renewable energy resources, and the synthesis of advanced chemicals. Achieving this objective requires improvements in rational catalyst design by establishing details of reaction mechanisms and their kinetics. Therefore, over the course of this award, emphasis has been directed towards obtaining detailed information concerned with fundamental transformations that occur at metal centers, with particular attention given towards reactions such as (i) hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), (ii) the use of carbon dioxide as a renewable C1 feedstock, (iii) the generation of hydrogen on demand, and (iv) transformations that involve the cleavage of H–H, C–H and C–C bonds. HDS and HDN are the essential processes for removing sulfur- and nitrogen-containing impurities from crude oil in order to obtain (i) cleaner fuels that minimize environmental pollution and (ii) cleaner chemical feedstocks that are less likely to poison the catalysts that are used for subsequent transformations. The catalysts employed for HDS and HDN are typically molybdenum and tungsten sulfides and, to gain insight into these transformations at the molecular level, we have focused on developing the coordination chemistry of heterocyclic sulfur and nitrogen compounds with molybdenum and tungsten. For example, we obtained the first examples of the cleavage of thiophene C–S bonds by employing molecular molybdenum compounds. The efficient utilization of carbon dioxide as a renewable C 1 source for the synthesis of value added organic chemicals and fuels has prompted much effort towards developing catalytic reactions to functionalize CO 2 . One such approach is hydrosilylation, which can proceed in a stepwise manner to afford a series of products with different levels of reduction, namely silyl formates, silyl acetals, methoxy silanes and methane. Despite the utility of these transformations, however, most catalysts employ precious metals, which operate at elevated temperatures, and control of the level of reduction is challenging. Therefore, it is significant that we have developed zinc and magnesium systems that can achieve hydrosilylation of CO 2 and which may be modified to control the level of reduction. The growing demand for energy requires the development of alternative energy sources that are more sustainable than fossil fuels and have a reduced impact on the environment. In this regard, an attractive energy storage medium is provided by hydrogen, which can, for example, be consumed in a fuel cell with only water as a byproduct. Unfortunately, a problem with using hydrogen in this manner is that present storage and transportation techniques are inadequate. For example, not only does storing liquid hydrogen present a safety risk, considerable energy is also required to liquefy the hydrogen and maintain it in this form. Therefore, we have focused on the application of chemical methods to provide hydrogen on demand. One such method utilizes formic acid as a storage medium and we reported the first examples of molybdenum compounds that serve as catalysts for releasing H 2 from formic acid. The nature of the interaction of a C–H bond with a metal center is of pivotal importance in many transformations of organic molecules, and we have examined several systems to probe how the interaction varies according to whether the metal is molybdenum or tungsten. These studies demonstrate that tungsten exhibits a greater propensity to cleave a C–H bond. While the oxidative addition of C–H bonds to a metal center is well precedented, transformations that feature oxidative addition of C–C bonds are uncommon and so it is noteworthy that we have observed the cleavage of a strong C–C bond that is a component of an unstrained 6-membered aromatic ring. The oxidative addition of dihydrogen to a metal center, and its microscopic reverse, reductive elimination, are two of the simplest yet most important transformations in organometallic chemistry. For example, oxidative addition of dihydrogen is a key step in many transition metal-catalyzed reactions involving H 2 . However, relatively few studies have established in detail the factors that influence the thermodynamics of this transformation. Therefore, it is significant that we demonstrated experimentally that that the energetics of the oxidative addition of H 2 to the six coordinate molybdenum and tungsten halide complexes depends very strongly on the nature of both the metal and the halogen. Deuterium kinetic isotope effects (KIEs) and equilibrium isotope effects (EIEs) serve as versatile tools to infer details about reaction mechanisms. In this regard, primary deuterium isotope effects are often interpreted by using two simple guidelines: (i) the KIE for an elementary reaction is normal (k H /k D > 1) and (ii) the EIE is dictated by deuterium preferring to be located in the site corresponding to the highest frequency oscillator. We have evaluated the applicability of these rules to the interactions of H–H and C–H bonds with a transition metal center and our results question the ability to predict primary EIEs in these systems based on the simple notion that deuterium prefers to occupy the highest frequency oscillator. The key findings are that the EIEs for (i) formation of σ–complexes by coordination of H–H and C–H bonds and (ii) oxidative addition of dihydrogen exhibit unusual temperature dependencies, such that the same system may exhibit both normal (i.e. K H /K D > 1) and inverse (i.e. K H /K D < 1) values.