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, 3Me; Et, 3Et) complexes have a room temperature half-life of more
than 17 h, while the one-electron reduction of 3Me 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, 3Me, 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.