One-electron oxidation of L2PdII(CH3)2 complexes: Ligand effects on production of ethane vs. Pd-C bond homolysis

Dimethyl palladium(II) complexes with various bi- and tri-dentate ligands, (L-2)Pd-II(CH3)(2), react readily with the one-electron oxidant ferrocenium hexafluorophosphate (FC+) in acetone-d(6). 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 Pd-IV oxidation state. Complexes containing the bidentate diimine ligands 1,10-phenanthroline (phen) or 2,3-dimethy1-1,4di-4-toly1-1,4-diazabuta-1,3-diene (Tol(2)DAB) adhere to the ethane producing pathway previously reported for ((t)Bu(2)bpy)Pd(CH3)(2) ((t)Bu(2)bpy = 4,4'-bis(tert-butyl)-2,2'-bipyridine), as does the complex of the potentially tridentate 6-methyl-N,N'-bis-2-pyridiny1-2-pyridinamine ligand. In contrast, Fc(+) oxidations of complexes with bis-carbene (1,1'-methylene-3,3'-tert-butyldiimidazole-2,2'-diylidene, (COBtBU)-C-tBu), bis-phosphine (tert-butyl-bis(diphenylphosphinomethyl)amine ((BuN)-Bu-t(CH2PPh2)(2), PCNCP), or tris(pyrazolyl)methane ligands resulted in little or no ethane production. These reactions likely involve either methyl radicals or trimethyl Pd-IV 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. (C) 2014 Elsevier Ltd. All rights reserved.