Aryl Fluoride Reductive Elimination from PdII Complexes: A Descriptor to Guide Ligand Selection

Abstract

The effect of ligand selection on the reductive elimination barrier of PdII aryl fluoride complexes was analyzed by using density functional theory (DFT) calculations. A separate Evans-Polanyi relationship between the activation barriers and the reaction energies is found for C, F reductive elimination from monodentate and bis(monodentate) PdII complexes. For comparable reaction energies, the reductive elimination barriers for monodentate complexes [LPd(Ar)F] (L=PR3, Ar=aryl) were calculated to be 80kJmol-1 lower than for bis(monodentate) complexes [L2Pd(Ar)F]. Natural population analysis demonstrated that the partial charges on the Pd center and on the aryl α-carbon can be used as a descriptor for the reductive elimination barriers. The presence of a trans phosphine ligand in bis(monodentate) complexes increases the Pd, C charge density and hence increases the C, F reductive elimination barrier. The importance of the Pd, C charge density can be understood by performing a natural bond orbital analysis. Indeed, C, F reductive elimination is best described as the nucleophilic attack of one of the fluorine lone pairs on the antibonding Pd, C orbital, and the energy difference between this fluorine lone pair and the antibonding Pd, C orbital determines the rate of this nucleophilic attack. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.