Abstract:
This dissertation recounts the synthesis and use of imidazol-2-ylidenes for their applications as ligands for main group elements and transition metals and as sterically hindered Lewis bases for hydrogen activation. Chapter 1 reviews the background of carbenes as ligands for main group compounds and transition metal complexes. The history and development of chelating Martin-type fluoroalkoxy ligands to form unique bonding architectures is covered. The synthetic strategies employed to functionalize carbene ligands with fluoroalkoxy substituents is reviewed leading to the development of stable fluoroalkoxy imidazolium zwitterions. A series of new ionic fluoroalkoxy imidazol-2ylidenes is described along with a comparison of the electronic properties to neutral imidazol-2-ylidenes. These fluoroalkoxy imidazol-2-ylidenes were used to prepare chelated hypervalent main group compounds. The structural characterization of the hypervalent adducts is described along with effects due to chelation. Additionally, the characterization of unique fused heterocyclic byproducts formed from unstable main group chelates is reviewed. The synthesis and characterization of transition metal complexes utilizing these ligands are described. Special emphasis is focused on catalytically active transition metals complexes and the influence of the tridentate ligand on catalyst activity. This ligand was also used to allow for the stabilization of iron benzylidene complexes which may have applications towards olefin metathesis. Chapter 2 describes work on hydrogen activation as a project with the U.S. Dept. of Energy's Alabama Chemical Hydrogen Storage Initiative. This project takes advantage of sterically hindered Lewis acid-base pairs to activate small molecules. A wide range of main group element Lewis acids were explored with a variety of mixed results. "Abnormal" coordination of Lewis acids to the carbene backbone results in structures with unique properties and reactivity. These systems were utilized with the goal of reversible heterolytic hydrogen cleavage followed by hydrogen elimination under another set of circumstances.
