Synthesis and decomposition behavior of pallada(IV)cyclopentane complexes

The octahedral pallada(IV)cyclopentane complexes PdX(C4H8)R(bpy) [RX = MeI(1), EtI (2), PhCH2Br (3), CH2=CHCH2Br (4); bpy 2,2'-bipyridine] may be isolated on oxidative addition of organohalides to the palladium(II) complex Pd(C4H8)(bpy) in acetone at -20 degrees C. The complex PdBr(C4H8)(CF3)(bpy) (5) has been generated in solution but was too unstable to permit isolation as a solid. Complex 1 occurs as a mixture of isomers in a 4:1 ratio, where the dominant isomer has the methyl group trans to bpy and the minor isomer has 2-fold symmetry with the methyl group trans to the iodo ligand. Complexes 2-5 adopt only the configuration with R trans to X. The complexes exhibit low stability, allowing studies of decomposition in solution under mild conditions in CDCl3 or CD2Cl2. Formation of the major products from PdI(C4H8)Me(bpy) (1) and PdI(C4H8)Et(bpy) (2) is assumed to result from C4H8 ... R coupling at Pd(IV) to give undetected (PdCH2CH2CH2CH2R)-C-Pi species which undergo beta-elimination to form alkenes (48% of detected organic products), Pd(O), and HX, with subsequent protonation of (PdCH2CH2CH2CH2R)-C-Pi to give alkanes (39%). Complexes 1 and 2 also give iodopentane (similar to 12% from 1) and iodohexane (similar to 9% from 2). The complex PdBr(C4H8)(CH2Ph)(bpy) (3) gives phenylpentane (15%), phenylpentenes (66%), toluene (10%), and butenes (8%); PdBr(C4H8)(CH2CH=CH2)(bpy) (4) and PdI(C4H8)(CF3)(bpy) (5) give predominantly cyclobutane, together with minor amounts of butenes (4, 5) and propene (4). Deuteration studies implicate the occurrence of intermolecular hydrogen atom transfer for elimination of alkenes and alkanes, except for cyclobutane.