Formation and Structure of a Platinum(II) Complex Containing TwotransNonstabilized Phosphorus Ylide Ligands: Evidence for Reversible Ylide Dissociation

Abstract

The reaction of (nBu4P)2[PtBr4] with 2,2'-(C6H4Li)2 yields selectively (by 195 Pt and 31 P NMR) compound trans-PtBr2[CH(nPr)(PnBu3)]2 as a mixture of meso (R,S) and dl (R,R+S,S) diastereoisomers. The initially formed 1:1 mixture slowly evolves toward a solution richer in the meso compound, which then crystallizes selectively. In addition to the NMR characterization, the meso isomer has also been characterized by a single crystal X-ray diffraction analysis. DFT calculations in the gas phase and in solution (THF) confirm the slight thermodynamic preference for the meso isomer and reproduce the chemical shift difference of the 195 Pt NMR resonances. They also suggest the likelihood of a dissociative mechanism for the slow diastereoisomer interconversion.