Carbon-Oxygen Bond Formation at Organopalladium Centers: The Reactions of PdMeR(L2) (R = Me, 4-tolyl); L2 = tmeda, bpy) with Diaroyl Peroxides and the Involvement of Organopalladium(IV) Species

The group 16 oxidants dibenzoyl- and bis(4-trifluoromethylbenzoyl)-peroxide react with dimethylpalladium(II) and methyl(4-tolyl)palladium(II) complexes of the bidentate nitrogen donor ligands 2,2′-bipyridine and N,N,N′,N′-tetramethylethylenediamine in discrete stepwise processes as the temperature is raised from -70°C. Carbon-oxygen bonds are formed during this reaction sequence but not from those Pd(IV) complexes detected spectroscopically. The initial reaction gives undetected "PdIV(O2CAr)2MeR(L2)" (Ar = Ph, ArF; R = Me, Tol; L2 = bpy, tmeda), which immediately undergo methyl aroate exchange with PdIIMeR(L2) to give PdII(O2CAr)R(L2) and PdIV(O2CAr)Me2R(L2), where all products except for PdIV(O2CAr)Me2-Tol(tmeda) were detected by 1H NMR spectroscopy. On raising the temperature, the PdIVMe3 complexes reductively eliminate Me-Me, and the PdIVMe2Tol complexes eliminate Me-Me and Tol-Me. The resultant Pd(II) complexes PdII(O2CAr)R(L2) react with (ArCO2)2 at higher temperatures to form PdII(O2CAr)2(L2) and R-O2CAr (R = Me, Tol), except for PdII(O2CAr)Tol(tmeda), which forms PdII(O2CAr)2(tmeda) and 4,4′-bitolyl. Each reaction step has been confirmed by the independent synthesis of intermediates PdII(O2CAr)2(L2) and PdII(O2CAr)R(L2) (Ar = Ph, ArF; R = Me, Tol; L2 = bpy, tmeda) and PdIV(O2CR)Me2R(L2) (R = Ph, ArF; R = Me, Tol; L2 = bpy) by metathesis reactions of halogeno complexes with Ag[O2CAr], followed by temperature-dependent studies of both the decomposition of Pd(IV) complexes and reactions of Pd(II) complexes with (ArCO2)2. Attempts to prepare "PdIV(O2CAr)2 MeR(bpy)" in a similar manner (and thus in the absence of PdMeR(bpy) with which they undergo exchange reactions) were unsuccessful, but the complexes PdIVI2MeR(bpy) (R = Me, Tol) that formed on reaction of diiodine with PdMeR(L2) were detected and found to reductively eliminate iodomethane. X-ray structural studies are reported for the square-planar palladium(II) complexes Pd(O2CPh)2(bpy), Pd(O2CAr)2(tmeda) (Ar = Ph, ArF), and Pd(O2CPh)(Tol) (bpy)•CH2Cl2.