Exploration of Second Sphere Reactivity
Chapter 1. Overview of Carbon Dioxide Hydrogenation for the Production of Formic Acid As the world’s energy demands increase, our resources dwindle and the need for a sustainable energy source is pertinent. Our current energy infrastructure is dominated by fossil fuel use. Hydrogen, on the other hand, is potentially an ideal energy carrier as it is emissions-free when burned and can be used in fuel cells. Significant advances are still needed to develop more efficient ways to produce and store H2. The hydrogenation of CO2 to formic acid and/or methanol provides an encouraging and reversible approach for a hydrogen storage material. The first example of homogeneously catalyzed hydrogenation of carbon dioxide was in 1976. Over the past 40 years, there has been excellent progress in the development of catalysts for CO2 hydrogenation. Typically, homogenous catalysts found to be effect are 2nd and 3rd row transition metals of groups 8-10. In recent years, base-metals (common and inexpensive metals) have demonstrated promising results. This chapter is designed to highlight important discoveries throughout the history of carbon dioxide hydrogenation. Chapter 2. Development of a Transition Metal / N-Heterocyclic Carbene Cooperative System for the Hydrogenation of Carbon Dioxide to Formic Acid Over the past few decades, the conversion of small molecules such as H2, N2, O2, CH4, C2H4, CO, and CO2 have attracted considerable attention. Many of these molecules are thermodynamically or kinetically stable and their usefulness depends on overcoming significant barriers. Frustrated Lewis pairs and N-heterocyclic carbenes have become common strategies to activate unreactive small molecule likes CO2 and H2. However, a hybrid approach utilizing both a transition metal and an activator has only recently been investigated for the transformation of small molecules to more useful and complex compounds. A novel method for these transformations is the use of a bifunctional catalyst system that incorporates a Lewis basic N-heterocyclic carbene and a Lewis acidic transition metal. This chapter highlights our serendipitous discovery that small quantities of bicarbonate and other inorganic salts enhanced the productivity of formic acid in CO2 hydrogenation reactions. The phenomenon was general for many noble-metal catalysts and for one of the most efficient base-metal hydrogenation catalysts. Additionally, the synthesis of a transition metal complex bearing a pendant dihydroimidazolium salt is described. Stoichiometric and catalytic applications of the newly designed complex were explored in investigate our Lewis base / transition metal approach to small molecule activation. Chapter 3. Chemistry of Iron N-Heterocyclic Carbene Complexes N-heterocyclic carbenes are one of the most versatile ligands in organometallic chemistry due to their unique properties as ancillary ligands. Although NHCs are typically potent σ-donors (a) with minor contributions from π*-backdonation (b), they also have the ability to accept electron density from the metal center as two-electron (c) or one-electron (d) interactions. Since the first examples of metal–NHC complexes were reported in the 1960’s, numerous studies have been devoted to the synthesis of new NHCs, to their characterization, and to their use as ligands in transition metal complexes. The coordination chemistry of NHCs with late transition metals has been studied extensively. However, the chemistry of iron–NHC complexes has not been developed to the same extent as other late transition metals. This chapter highlights important discoveries throughout the history of iron–NHC complexes, while emphasizing the nature of the metal–carbene bond. Chapter 4. Reactivity of Bis(amidinato)-N-Heterocyclic Carbene Iron Complexes Over the past few decades, the development of highly active and selective transition metal catalysts has attracted considerable attention. While the metal employed largely influences the expectations for catalytic activity, the importance of supporting ligands in tuning the reactivity of any given complex is vital. Our group recently synthesized a bis(amidinato)-N-heterocyclic carbene complex of iron as an analogy to the highly active bis(imino)pyridine iron complexes. We hypothesized that having an N-heterocyclic carbene as the central donor instead of pyridine could have significant impacts on the reactivity of such iron complexes. This chapter highlights the synthesis of iron bis(amidinato)-N-heterocyclic carbene complexes spanning multiple oxidation states previously described by our group. Through a combination characterization techniques, the bis(amidinato)-N-heterocyclic carbene was discovered to have unique interactions with the iron center, which change depending on the oxidation state of the metal. Additionally, we undertook investigations into the reactivity of these complexes with azides, hydrides, alkyl reagents, and ethylene. The results of which supported the capability of the bis(amidinato)-N-heterocyclic carbene ligand to act as a redox and chemical non-innocent ligand.