Mechanisms of Reductive Eliminations in Square Planar Pd(II) Complexes: Nature of Eliminated Bonds and Role of trans Influence

Abstract

The trans influence of various phosphine ligands (L) in direct as well as dissociative reductive elimination pathways yielding CH3CH3 from Pd(CH3)2L2 and CH3Cl from Pd(CH3)(Cl)L2 has been quantified in terms of isodesmic reaction energy, Etrans, using the MPWB1K level of density functional theory. In the absence of a large steric effect, Etrans correlated linearly with the activation barrier (Eact) of both direct and dissociation pathways. The minimum of molecular electrostatic potential (Vmin) at the lone pair region of phosphine ligands has been used to assess their electron donating power. Etrans increased linearly with an increase in the negative Vmin values. Further, the nature of bonds that are eliminated during reductive elimination have been analyzed in terms of AIM parameters, viz. electron density (ρ(r)), Laplacian of the electron density (∇2ρ(r)), total electron energy density (H(r)), and ratio of potential and kinetic electron energy densities (k(r)). Interestingly, Eact correlated inversely with the strength of the eliminated metal–ligand bonds measured in terms of the bond length or the ρ(r). Analysis of H(r) showed that elimination of the C–C/C–Cl bond becomes more facile when the covalent character of the Pd–C/Pd–Cl bond increases. Thus, AIM details clearly showed that the strength of the eliminated bond is not the deciding factor for the reductive elimination but the nature of the bond, covalent or ionic. Further, a unified picture showing the relationship between the nature of the eliminated chemical bond and the tendency of reductive elimination is obtained from the k(r) values: the Eact of both direct and dissociative mechanisms for the elimination of CH3CH3 and CH3Cl decreased linearly when the sum of k(r) at the cleaved bonds showed a more negative character. It means that the potential electron energy density dominates over the kinetic electron energy density when the bonds (Pd–C/Pd–Cl) become more covalent and the eliminated fragments attain more radical character leading to the easy formation of C–C/C–Cl bond.