|M.Sc Student||Sevilia Sagi|
|Subject||New Approaches for Water Oxidation|
|Department||Department of Chemistry||Supervisor||Professor Zeev Gross|
|Full Thesis text|
Tetrapyrrolic macrocycles receive large scientific attention because of their biology, medicine and many applications. One member of that family is the corrole, whose structure may be considered as a synthetic intermediate between two naturally occurring members: porphyrins (heme) and corrins (Vitamin B12). Corroles share an identical skeleton with corrins (C19) and differ from porphyrins (C20) by having one less carbon atom, while baring a fully conjugated system composed of 18 electrons like porphyrins. Two unique structural aspects of corroles relative to porphyrins are the presence of three rather than two NH protons in the coordination core and their lower symmetry.
The last decades have evidenced an extensive search for the ultimate water oxidation catalyst (WOC). Many different approaches were explored and a large number of factors were considered for achieving improved efficiency, by examining the type of metal, ligand, and number of metal sites within the complex.
This thesis describes the effort of developing corrole-based WOC's with improved activity, based on the conclusions from most recent publications. For fulfilling this task, a series of cobalt(III) corroles was prepared, which differ from each other by the number of ortho-pyridyl moieties present on the meso-carbon atoms of the corrole.
The four free-base corroles were metallated by cobalt, which serves as the catalytic center of the complex. Cobalt was chosen for its low cost, reactivity of high oxidation states and for its strong binding to the corrole. All four complexes were fully characterized by a variety of spectroscopic methods, and a X-ray structure was obtained for one of them: 10-(2-pyridyl)-5,15-bis(pentafluorophenyl)corrolato corrole(III) bispyridine.
Cyclic voltammetry was used for evaluation the effect of pyridyl vs C6F5 meso-substituents on the oxidation potentials. The first redox process was reversible and displayed a shift toward lower potential for each additional pyridyl group.
Analysis of the electrooxidation of water (3.2 M in acetonitrile) in the absence and presence of the catalysts disclosed 5,10,15-tris(pentafluorophenyl)corrolato cobalt(III) bispyridine as the most efficient catalyst: about 6 times more efficient than the non-catalyzed reaction, a onset potential of 0.95 V, kobs with a value of 0.14 sec-1 and maximal current at 1.1 V.
At higher concentration of water (6.1 M in acetonitrile), 5,10,15-tris(2-pyridyl)corrolato cobalt(III) bispyridine displayed an impressive value for kobs and a very low onset potential of 0.85 V. Its effectiveness was also quite large but maximal activity was achieved at the quite high potential of 1.5 V.