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

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 d8-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.