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.