DFT studies of isomerization in palladium(IV) chemistry and alkyl halide transfer from palladium(IV) to palladium(II)

A DFT study of alkyl halide exchange from Pd(IV) to Pd(II) centers supports proposals for an S_N2 process in which nucleophilic Pd(II) square-planar complexes attack the axial alkyl group in electrophilic squarepyramidal Pd(IV) centers, leading to transfer of R⁺ from Pd(IV) to Pd(II) involving a transition structure of the form [R₂(bpy)··R··PdR₂ (bpy)]⁺ (bpy = 2,20’ -bipyridine). Computation accounts for the selectivity in benzyl (Bn) over methyl transfer from PhMeBnPd^IV(bpy)Br to Me₂Pd^II(bpy), giving PhMePd^II(bpy) and BnMe₂Pd^IV(bpy). Transition structures are formed by the interaction of a d_z2-like HOMO at the Pd(II) nucleophile and the Pd-C σ* LUMO at the cationic Pd(IV) electrophile. Isomerization of octahedral palladium(IV) complexes via halide loss followed by involvement of trigonal-bipyramidal transition structures to facilitate isomerization is also examined. A lower barrier for Bn⁺ than Me⁺ transfer is attributed to stabilisation of the transition structure by delocalisation of positive charge within the benzyl group, reflected in a higher total charge density for the benzyl group than found for the methyl group.