Ligand Electronic Effect on Reductive Elimination of Biphenyl from cis-[Pt(Ph)2(diphosphine)] Complexes Bearing Electron-Poor Diphosphine: Correlation Study between Experimental and Theoretical Results

• Published in: Organometallics Vol. 29; no. 18; pp. 4025 - 4035
• Main Authors: Korenaga, Toshinobu, Abe, Kayoko, Ko, Aram, Maenishi, Ryota, Sakai, Takashi
• Format: Journal Article
• Language: English; Japanese
• Published: WASHINGTON American Chemical Society 27.09.2010; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Physical Sciences; Science & Technology; CARBON BOND FORMATION; PALLADIUM; ETHERS; PHOSPHANE LIGANDS; MECHANISM; COUPLING REACTIONS; AMINES; ARYL HALIDES; CHEMISTRY; ACRYLATE
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om100073j

The reductive elimination of biphenyl from cis-[Pt(Ph)2(diphosphine)] (3) was studied to clarify the electronic effects of diphosphine ligands on the reaction. Reaction kinetic data were evaluated in d 8-toluene within 80−110 °C using 1,2-bis(diphenylphosphino)ethane (dppe) and seven of its fluoroaromatic analogues as ancillary diphosphine ligands. The fastest reaction rate corresponded to 3, bearing the electron-poor 1,2-bis[bis(pentafluorophenyl)phosphino]ethane (dfppe) ligand, and was 1240 times faster than that for dppe-bearing 3, which has the slowest. The estimated rate constants k were highly correlated with Taft’s σ* values for phosphorus-bound aromatics in 3. However, their correlation was split between 2,6-fluorine aromatic and 2,6-hydrogen aromatic-bearing diphosphines, suggesting steric effects from the 2,6-fluorine atoms. The observed ΔH ⧧ values were correlated with theoretical values, which were calculated by the DFT method. The correlations revealed that electron-poor diphosphine ligands decrease the energy gaps between the HOMO−1 of 3 and the HOMO of transition state 3-TS, including the platinum d orbital, and reduce the destabilization of the platinum d orbital upon binding with the diphosphine in 3-TS as compared to the cases of an electron-donating ligand. This last mentioned effect is the true nature of electronic influence of the electron-poor diphosphine ligands in the reductive elimination from 3.