Group 10 Methyl Transfer Reactions toward Catalyst Development for Oxidative Oligomerization of Methane.

Abstract

Abstract Group 10 Methyl Transfer Reactions toward Catalyst Development for Oxidative Oligomerization of Methane

by

Matthew Sean Remy

Chair: Melanie S. Sanford One of the challenges of developing a homogeneous catalyst for oxidative oligomerization of methane (OOM) is promoting a C–C coupling reaction from the product of alkane C–H activation. Heterolytic C–H activation of methane almost exclusively generates a monoalkyl-metal species. In order to make a new C–C bond, it is proposed that a polymethyl intermediate must be generated. Our studies showed that aryl disproportionation is far more favorable than methyl disproportionation. Careful tuning of ancillary ligands at monomethyl-palladium(II) complexes using density functional theory (DFT) calculations facilitated optimization of thermodynamics for methyl disproportionation. This allowed the first observed disproportionation reaction to form a dimethyl-palladium(II) complex. Heating dimethyl complex (tBu-bpy)PdII(CH3)2 (tBu-bpy = 4,4-ditertbutyl-2,2-bipyridine) to 100 °C produced a mixture of methane and ethane over 24 h. One-electron oxidants and 1,4-benzoquinone were found to be effective promoters of ethane formation from (tBu-bpy)PdII(CH3)2 at 25 °C. Mechanistic study of one-electron oxidation uncovered an oxidatively-induced methyl transfer reaction which produced ethane from [(tBu-bpy)PdIV(CH3)3(solvent)]+. Subsequently, one-electron oxidation of complexes of the general formula (tBu-bpy)PdII(CH3)X was developed as an effective method for generation of ethane from monomethyl-palladium(II) complexes. Platinum complexes, (N–N)PtII(CH3)2, similarly undergo one-electron to produce [(N–N)PtIV(CH3)3(solvent)]+. However, these platinum(IV) products are generally stable to reductive elimination of ethane. When the N–N ligand is designed to have steric interactions with axial ligands of platinum(IV), ethane reductive elimination becomes favorable, occurring cleanly over 8.5 hours in acetone and over 1 h in dichloromethane. This document describes experimental evidence for the generation of polymethyl palladium and platinum complexes from model palladium(II) and platinum(II) products of C–H activation subsequent ethane elimination from the polymethyl complexes. In an unrelated project, N-insertion into palladium-carbon bond was observed when palladium(II) complexes of bidentate C–N ligands were reacted with an iminoiodinane oxidant. The reaction was proposed to occur through an imido-palladium(IV) intermediate.