Abstract

Dimethyl palladium(II) complexes with various bi- and tri-dentate ligands, (L2)PdII(CH3)2, react readily with the one-electron oxidant ferrocenium hexafluorophosphate (Fc+) in acetone-d6. These oxidations typically proceed by one of two pathways. Oxidatively-induced methyl transfer forms trimethyl palladium(IV) intermediates which usually undergo reductive C−C coupling and form ethane. Alternatively, oxidation can cause Pd–C bond homolysis, which yields predominantly methane and methylferrocene, a product of methyl radical trapping. The reaction selectivity is dependent on ligand identity and likely due to the ability of different classes of ligands to stabilize intermediates in the PdIV oxidation state. Complexes containing the bidentate diimine ligands 1,10-phenanthroline (phen) or 2,3-dimethyl-1,4-di-4-tolyl-1,4-diazabuta-1,3-diene (Tol2DAB) adhere to the ethane producing pathway previously reported for (tBu2bpy)Pd(CH3)2 (tBu2bpy=4,4′-bis(tert-butyl)-2,2′-bipyridine), as does the complex of the potentially tridentate 6-methyl-N,N′-bis-2-pyridinyl-2-pyridinamine ligand. In contrast, Fc+ oxidations of complexes with bis-carbene (1,1′-methylene-3,3′-tert-butyldiimidazole-2,2′-diylidene, tBuCCtBu), bis-phosphine (tert-butyl-bis(diphenylphosphinomethyl)amine (tBuN(CH2PPh2)2, PCNCP), or tris(pyrazolyl)methane ligands resulted in little or no ethane production. These reactions likely involve either methyl radicals or trimethyl PdIV complexes that are stable towards C−C reductive elimination. Oxidation of (TMEDA)Pd(CH3)2 (TMEDA=(N,N,N′,N′-tetramethylethylenediamine) yields both methane and ethane, suggesting that both Pd–C bond homolysis and oxidative methyl transfer are competitive in this case.

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