Preparation and Characterization of Iridium Hydride and Dihydrogen Complexes Relevant to Biomass Deoxygenation

Abstract

This thesis describes the fundamental organometallic reactivity of iridium pincer complexes and their applications to glycerol deoxygenation catalysis. These investigations provide support for each step of a previously proposed glycerol deoxygenation mechanism. Chapter 1 outlines the motivations for this work, specifically the goal of using biomass as a chemical feedstock over more common petroleum-based sources. A discussion of the importance of transforming glycerol to higher value products, such as 1,3-propanediol, is discussed. Chapter 2 describes investigations into the importance of pincer ligand steric factors on the coordination chemistry of the iridium metal center. Full characterization of a five-coordinate iridium-hydride complex is presented; this species was previously proposed to be a catalyst resting state for glycerol deoxygenation. Chapter 3 investigates hydrogen addition to R4(POCOP)Ir(CO) [R4POCOP = κ3-C6H3-2,6-(OPR2)2 for R = tBu, iPr] and R4(PCP)Ir(CO) [R4(PCP) = κ3-C6H3-2,6-(CH2PR2)2 for R = tBu, iPr] to give cis- and/or trans-dihydride complexes. Two mechanisms of hydrogen addition are presented (concerted oxidative addition and proton-catalyzed addition); the mechanism of hydrogen addition is dependent on the steric environment at the metal center. Chapter 4 presents spectroscopic evidence for two new iridium-dihydrogen complexes only stable under high pressures of hydrogen (40-80 atm) and low temperatures. Furthermore, iridium-catalyzed isotope exchange between H2 and CD3OD is presented and its potential implications in supporting the glycerol deoxygenation mechanism. Chapter 5 outlines a fundamental reaction of oxidative addition of iodine to (tBu)4(POCOP)Ir(CO) complexes. Characterization of a cationic monoiodo iridium carbonyl complex as a potential oxidative addition intermediate is presented.