Synthesis and Reactivity of Late Transition Metal Pincer Complexes: Progress toward Alkane Functionalization

Abstract

Commercially viable methods to directly and selectively functionalize hydrocarbon C-H bonds would have a significant impact in the chemical and fuel industries. Two desirable transformations are the partial oxidation of alkanes to alcohols and the conversion of alkanes to alkenes via alkane dehydrogenation. For such functionalizations to be useful on large scale, the most ideal oxidant is molecular oxygen because it is abundant, cheap, and benign. Late transition metal complexes are promising candidates for accomplishing these transformations due to their ability to activate C-H bonds and selectively react with O2. As an introduction, chapter 1 surveys the literature for methane activation and functionalization and alkane dehydrogenation demonstrating what has been previously accomplished for these transformations and what improvements can be made. Chapter 2 focuses on the partial oxidation of alkanes to alcohols and explores how O2 reacts with late metal-carbon bonds. These findings could be applied to the design of new catalytic routes that combine C-H activation and functionalization using O2 as the oxidant. To this end, the reactions of tBuPNP, tBuPCP, and iPrPCP PdII-Me complexes with O2 are described and compared with the reported O2 reactivity of related PdII-Me complexes. [(tBuPNP)PdMe]Cl was found to react with O2 upon photolysis, resulting in oxidation of the pincer ligand backbone to produce a (tBuPNO)PdCl complex. In contrast, photolysis of (tBuPCP)PdMe with O2 resulted in oxidation of the Pd-Me moiety to form (tBuPCP)PdOCO2H. Additionally, photolysis of (iPrPCP)PdMe with O2 resulted in multiple products, suggesting that this complex is too active under photolytic conditions. Isotopic labeling, radical initiators, and solvent studies were employed to gain insight into the mechanisms of these unusual reactions of late metal alkyls with molecular oxygen. Chapters 3 and 4 discuss work towards accessing novel IrIII complexes for use in aerobic alkane dehydrogenation. Efforts have focused on the 1,3-bis(2’-pyridylimino)isoindoline (BPI- H) ligand framework. This ligand is ideal because it has been shown to be stable at high temperatures under oxidizing conditions and is easily modified to allow for variation of the electronic and steric effects. Chapter 3 discusses the synthesis and characterization of a series of both novel and known RBPI-H ligands. The percent volume buried has been calculated and compared for each ligand to determine how sterics affects the binding pocket or metalation of these ligand variations. OMeBPI-H, has the highest steric profile with a % volume buried of 71.9. Chapter 4 describes efforts to metalate these ligands with iridium and rhodium and explores the reactivities of these complexes. Novel IrIII complexes (BPI)IrEt(OAc) and (xylylBPI)IrEt(OAc) have been synthesized and fully characterized. The protonation, β-H elimination, and C-H activation reactivity for (BPI)IrEt(OAc) in the context of alkane dehydrogenation was investigated. β-H elimination of (BPI)IrEt(OAc) was found to be reversible, with the equilibrium favoring the Ir-Et. In addition, (BPI)IrEt(OAc) was found to activate C6D6 to form (BPI)Ir(CD2CD3)(OAc) and (BPI)Ir(C6D5)(OAc) via H-D exchange at 70 °C. With NaBArF24 present in the reaction of (BPI)IrEt(OAc) with C6D6, a dinuclear Ir complex [(BPI)Ir(CD2CD3)]2(μ-OAc)]BArF24 is formed. This difference in reactivity is attributed to the presence of NaBArF24 allowing a dinuclear complex to initially form in solution before C-H activation occurs. This differs in reactivity compared to the monomer with no NaBArF24 present. The β-H elimination and C-H activation reactivity of (xylylBPI)IrEt(OAc) was also explored and preliminary results are discussed. β-H elimination of (xylylBPI)IrEt(OAc) was found to be more facile than (BPI)IrEt(OAc) to produce a stable Ir-H species. Additionally, the C-H activation reactivity of (xylylBPI)IrEt(OAc) mirrored that of (BPI)IrEt(OAc). (xylylBPI)IrEt(OAc) was found to activate C6D6 to form (xylylBPI)Ir(CD2CD3)(OAc) via H-D exchange at 70 °C. No Ir-C6D5 product was formed in this reaction likely due to the increased steric profile of xylylBPI ligand. With NaBArF24 present, a dinuclear Ir complex [(xylylBPI)Ir(CD2CD3)]2(μ-OAc)]BArF24 is formed.