Mechanistic Insight Into The Partial Oxidation Of Methane To Methanol Using Platinum And Palladium Complexes

Hannah Elizabeth Zeitler, University of Pennsylvania

Abstract

The efficient conversion of methane into methanol is a priority in energy research as it converts methane, a greenhouse gas, to methanol, a transportable liquid that is ubiquitous in chemical industry. Detailed within are efforts towards the development of palladium and platinum homogeneous catalysts that will selectively oxidize methane to methanol using dioxygen as the oxidant. There are three proposed steps for this challenging transformation: 1) selective C–H activation 2) oxidation and 3) release of product. A potential mechanism for C–H activation is metal-ligand cooperation (MLC) in which the metal and ligand are involved in cleaving C–H bonds. As the C–H activation step can be thermodynamically unfavorable, Chapter 2 focuses on mechanistic studies of the microscopic reverse of C–H activation - the formation of methane from metal-methyl complexes with a protonated ligand backbone. For these studies, [H(BPI)M(CH3)][X] (BPI = 1,3-bis(2-pyridylimino)isoindole , M = Pd, Pt, X = OTf-, NTf2-, BF4-, BArF20-, IMP-CF-, Cl-), the proposed product of MLC C–H activation, were used. Thermolysis of these compounds resulted in the formation of methane. Kinetic and computational studies were carried out to investigate the mechanism of C–H bond formation. Chapter 3 evaluates different MLC mechanisms for the activation of C–H and H-X bonds. Three types of MLC were investigated: C–H bond activation, in which the BPI ligand is protonated, Concerted Metalation Deprotonation (CMD) and Internal Electrophilic Substitution (IES). C–H activation of benzene was observed to form a Pt-phenyl complex bearing a protonated imine BPI ligand when a non-coordinating anion was used. Attempts to promote activation of benzene by CMD or IES with (BPI)M complexes were unproductive. However, (BPI)Pt(OH) did activate H2 to afford (BPI)PtH. Chapter 4 investigates the reaction of oxygen with (BPI)M(CH3) (M = Pd, Pt). Oxygen was found to insert into the metal-methyl bond to form (BPI)M(OOCH3). (BPI)Pt(OOCH3), was the third reported crystallographically characterized Pt(OOCH3) complex. The results of kinetic and mechanistic studies are consistent with a radical chain pathway for the oxygen insertion reaction. Further reaction of (BPI)Pt(OOCH3) leads to the release of methanol in up to 55% yield.