η²-Palladium and Platinum(II) Complexes of a λ⁴-Phosphinine Anion: Syntheses, X-ray Crystal Structures, and DFT Calculations

A 1-methyl-2,6-bis(trimethylsilyl)-3,5-diphenylphosphinine anion (2) reacts with [MCl2(PPh3)2] (M = Pd, Pt) to afford the corresponding η2-palladium (3) and platinum (4) complexes in which coordination of the ligand to the [MCl(PPh3)] fragments occurs through the P−C bond. Complexes 3 and 4 were structurally characterized. Both complexes adopt the same stereochemistry, the Cl ligand lying cis to the carbon atom of the P−C bond. Variable-temperature NMR experiments demonstrate that in 3 and 4 the [MCl(PPh3)] fragment rapidly exchanges between the two P−C bonds in solution. DFT calculations on model complexes were performed to determine the structure of the transition state in the case of the palladium complex 3. The value of the calculated ΔG⧧ is discussed regarding the experimental values obtained by NMR studies. IMOMM (DFT-MM3, QM/MM hybrid method) calculations were also carried out on the real systems to rationalize the stereochemistry of both complexes.