Transition Metal Complexes for Glycerol Dehydrogenation and Study of Water Oxidation Catalysi

This dissertation describes the study of transition metal complexes in relation to two types of oxidation catalysis, namely dehydrogenation and water oxidation.;Chapters 1 and 2 explore dehydrogenation catalysis as a means of glycerol valorization. Glycerol is the major byproduct of biodiesel production (~10%), and there is thus intense interest in developing methods to convert this waste glycerol to more valuable products. One such product is lactic acid, which is commonly used in the food and detergent industries, and is a platform chemical that is seeing increasing demand. All prior methods for convening glycerol to lactic acid employed heterogeneous catalysts, which often require high temperatures and give generally poor selectivity and catalytic activity. In this work, I describe our study of homogeneous catalysts for glycerol conversion to lactic acid. Our Ir bis-NHC (NHC = N-heterocyclic carbene) precatalysts are superior to the previous systems in terms of selectivity and activity, and function in neat glycerol without the need for a co-solvent. These complexes can convert samples of crude glycerol from the biodiesel industry without the need for prior purification, suggesting their possible industrial application. Additionally, hydrogen is produced as a valuable byproduct. Chapter 2, carried out in collaboration with Professor Nilay Hazari (Yale), describes the study of catalysts based on non-precious metals for this reaction. A family of Fe precatalysts with bifunctional PNP pincer ligands give excellent selectivity and activity, and represent the first examples of homogeneous base-metal catalysts for glycerol conversion to lactic acid. In studies of Ir species formed from our Ir bis-NHC precatalysts during glycerol dehydrogenation, we isolated a series of unusual NHC-rich Ir polyhydride clusters (Chapter 3). These compounds are unprecedented in terms of their high NHC content, and were fully characterized using a variety of methods.;Chapters 4 and 5, carried out in collaboration with Shashi Sinha and Dimitar Shopov, joint BrudvigCrabtree students, describe the study of model complexes related to resting states and high oxidation state intermediates in water oxidation catalysis. Water oxidation has garnered intense interest because of its potential application in the production of solar fuels, but effective catalysts are needed to carry out the reaction with low overpotentials. Our group previously found that upon oxidative activation, the Cp*Ir(pyalk)OH precatalyst (pyalk = 2-pyridyl-2-propanolate) generates one of the most active and robust water oxidation catalysts reported to date. Previous spectroscopic characterization and DFT studies revealed that the Cp* ligand is oxidatively degraded, and the catalyst resting state likely consists of a mixture of related species with a (pyalk)2IrIV-O-IrIV(pyalk) core. However, these species completely resisted purification and crystallization by standard methods. Therefore, we developed a protocol to more selectively prepare related CI(pyalk)2IrIV-O-IrIV(pyalk) 2CI complexes, which can be isolated and crystallographically characterized. These complexes are unusual examples of well-defined Ir(IV,IV) mono-mu-oxo dimers, and are stable under ambient conditions, in contrast to previous examples of Ir(IV,IV) mono-mu-oxo dimers containing organometallic ligands. Our study of these complexes sheds light on the resting state of our Ir water oxidation catalyst, and opens the door to future development of well-defined Ir-oxo dimers for water oxidation catalysis.;In a related study (Chapter 5), we use techniques and insights that build on our Ir oxo-dimer study to synthesize unprecedented Ir(V) coordination complexes with organic ligands. Study of such well-defined high oxidation state complexes is of interest in relation to oxidation catalysis, where Ir(V) species have been proposed as key intermediates. In order to access Ir(V), we developed the ligand dpyp, an N,O,Odonor analogue of pyalk. Importantly, dpyp forms coordination complexes with four coplanar alkoxogroups, an arrangement that favors attainment of high oxidation states based on our previous work. Indeed, oxidation of IrIV(dpyp)2gives IrV(dpyp) +2+, which was fully characterized including by X-ray crystallography and DFT methods.