The quest for stable σ-methane complexes: computational and experimental studies

Abstract

A series of cationic late transition metal pincer complexes with tridentate, neutral pincer ligands and their corresponding metal methyl complexes have been investigated by density functional theory (DFT). The key calculated quantities of interest for each metal–ligand pair were the energy of the metal methyl hydride relative to the metal σ-methane complex and the methane dissociation enthalpy and free energy. A few promising pincer ligand frameworks emerged as candidates for the syntheses of σ-methane complexes with enhanced thermal stability. The calculational predictions have been tested experimentally, and new iridium and rhodium complexes of a tridentate pincer ligand, 2,6-bis(di-tert-butylphosphinito)-3,5-diphenylpyrazine (N-PONOP) have been prepared as well as a cationic palladium methyl complex with 2,6-bis(di-tert-butylphosphinito)pyridine (PONOP) and subjected to several protonation experiments. Protonation of the (N-PONOP)Ir methyl complex yielded the corresponding five-coordinate iridium(III) methyl hydride cation. Kinetic studies of the C–H bond coupling and reductive elimination have been carried out. Line broadening NMR spectroscopic techniques have been established a barrier of 7.9(1) kcal mol⁻¹ for H–C_alkyl bond coupling in the iridium(III) methyl hydride (−100 °C). A protonation of the iridium pincer complexes at the uncoordinated pyrazine-N atom was not achieved.