1H NMR Detection of Cationic Organopalladium(IV) Intermediates in Oxidative-Addition Reactions and the Structure of fac-PdBrMe2(CH2COPh)(bpy)

Abstract

Phenacyl bromides react with dimethylpalladium(II) complexes PdMe2(L2) (L2 = bpy, phen) to form the palladium(IV) complexes fac-PdBrMe2(CH2COAr)(L2) (Ar = Ph, p-C6H4Br), which have both methyl groups trans to the bidentate nitrogen donor ligand. 1H NMR studies of the oxidative addition of phenacyl bromides to PdMe2(bpy) in (CD3)2C0 at -5 C indicate the formation of two similar cationic intermediates, prior to formation of the neutral product. As the cations have only one of the methyl groups trans to bpy, configurational changes at palladium occur on cation formation and on conversion of the cations to the neutral product. It is proposed that one of the cations may be stabilized via an intramolecularly coordinated phenacyl group, [PdMe2(CH2COAr-C,0)(L2)]+. A single cation, exhibiting NMR spectra similar to those of the cations of the bpy complexes, is detected as an intermediate in the reaction of PdMe2(phen) with ArCOCH2Br. A cation has also been detected for the reaction of PdMe2(bpy) with CD3I in (CD3)2C0 at -50 C, prior to formation of PdIMe2(CD3)(bpy). Both the cation, most likely [PdMe2(CD3)(bpy)((CD3)2CO)]+, and the neutral product display scrambled Me and CD3 groups, in contrast to the trans oxidative addition reported for the analogous reaction of PtMe2(bpy) with CD3I. Crystals of PdBrMe2- (CH2COAr)(bpy) are orthorhombic, space group Pbca, with a = 19.554 (3) A, b = 15.007 (9) A, c = 13.282(4) A, and Z = 8.