Light-Induced Palladium(IV)–Carbon Bond Homolysis

Palladium chemistry has been widely studied since the 1950s, particularly for cross-coupling reactions. It facilitates breaking C-X bonds through oxidative addition and forming C-C bonds through reductive elimination. These 2 electrons' elementary steps are the key features to construct highly elaborated molecules and explain their exceptional versatility. While Pd(0)/Pd(II) catalytic cycles are well understood, the behavior of Pd(IV) alkyl complexes is less studied, particularly due to their instability. Here, we report the synthesis and characterization by X-ray diffraction, solid-state magnetism, and ¹H NMR of several Pd(Alkyl)₄ fragments, which demonstrate unusual stability thanks to a Cp*₂Yb(bipym) fragment (Cp* is for pentamethylcyclopentadienyl and bipym for 2,2'bipyrimidine). As such, the Cp*₂Yb(bipym)Pd(Me)₃(R) (R = Me, 3^Me; Et, 3^Et) complexes have a room temperature half-life of more than 17 h, while the one-electron reduction of 3^Me leads to a Pd(Me)₄ fragment, 3^@crypt, which does not degrade over time. This unusual stability allowed us to study the original reactivities of these Pd(Alkyl)₄ fragments other than classical reductive elimination. Thus, we report the first light-induced Pd(IV)-C bond homolysis, which leads to the formation of alkyl radicals. The Cp*₂Yb(bipym)PdMe₄ complex, 3^Me, reacts under irradiation at 370 nm to form the Cp*₂Yb(4Me,4H-bipym)PdMe₄, 4, and the Cp*₂Yb(4Me,4H-bipym)PdMe₂, 5, in which the methyl radical couples with the bipym radical. The mechanism of this peculiar reaction has been determined by DFT. Similar reactivity with 3^@crypt leads to the formation of a free methyl radical, as shown by EPR reaction trapping.