Mechanistic Studies of Ligand Substitution, Linkage Isomerism, and Insertion Reactions in Electron Rich Pd(II) Complexes of a Zwitterionic Diimine Ligand

We have prepared cationic palladium complexes possessing a new zwitterionic ligand bis-N,N’–1-(2,4,6-triphenylpyridyl) oxalamide [(N∧N)Pd(Me)(L)]⁺[BArF]⁻, (BArF = 3,5-(CF₃)₂C₆H₃, L = NCMe, CO). The structure of [(N∧N)Pd(Me)(CO)]⁺[BArF]⁻ was determined by X-ray diffraction analysis. Energy Decomposition Analysis (EDA) indicates this N∧N zwitterionic ligand is more electron-donating relative to bidentate diimine ligands. Low temperature NMR analysis shows the existence of linkage isomers with the N∧N isomer the most stable. Structures were assigned using NMR and DFT analysis. Barriers to interconversion of isomers are ΔG^‡ = 10–12 kcal/mol. Kinetics of acetonitrile displacement from [(N∧N)Pd(Me)(NCCH₃)]⁺[BArF]⁻ by CD₃CN, ethylene and ^tBu₃P were measured and mechanisms of exchange determined. The ethylene complex, [(N∧N)Pd(Me)(C₂H₄)]⁺ was generated at −45 °C, and the barrier of migratory insertion was determined at 0 °C (ΔG^‡ = 23.4 kcal/mol) and compared to related diimine complexes. The methyl carbonyl complex undergoes migratory insertion in the presence of CO at −70 to −55 °C (ΔG^‡ = ca. 15.7 kcal/mol) to yield the acyl carbonyl complex. The neutral bis-trimethylsilylmethyl complex, (N∧N)Pd(CH₂SiMe₃)₂ was prepared and characterized by X-ray diffraction analysis. It displays dynamic behavior at very low temperatures in the NMR spectrum (−90 °C, ΔG^‡ = 7.9 kcal/mol) which, supported by DFT analysis, is ascribed to rotation of the bulky −CH₂SiMe₃ groups.