Mechanistic Studies on the Stereoisomerization between Two Stereoisomeric, Isolable Five-Coordinate Borylpalladium(II) Complexes Bearing a Phenylene-Bridged PSiP-Pincer Type Ligand

The mechanism of the stereoisomerization of isolable five-coordinate borylpalladium(II) complexes bearing a phenylene-bridged PSiP-pincer ligand is investigated. The trigonal-bipyramidal borylpalladium(II) complexes undergo facile stereoisomerization under heating conditions from trans(B,Si) isomers having boryl and silyl ligands at apical positions to cis(B,Si) isomers in which boryl and silyl ligands are located in equatorial and apical positions, respectively. Kinetic studies and theoretical calculations clarified that the isomerization proceeds through a turnstile rotation mechanism via a five-coordinate transition state without dissociation of a monophosphine ligand. This is a quite rare example of stereoisomerization between two stereoisomeric five-coordinate group 10 metal complexes that are stable enough to be isolated.