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)Me2Tol(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)2MeR(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.