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Keywords:

  • activation strain model;
  • density functional calculations;
  • homogeneous catalysis;
  • oxidative addition;
  • reaction mechanisms

Abstract

We investigate palladium-induced activation of the C[BOND]H, C[BOND]C, C[BOND]F, and C[BOND]Cl bonds in methane, ethane, cyclopropane, fluoromethane, and chloromethane, using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Our purpose is to arrive at a qualitative understanding, based on accurate calculations, of the trends in activation barriers and transition state (TS) geometries (e.g. early or late along the reaction coordinate) in terms of the reactants’ properties. To this end, we extend the activation strain model (in which the activation energy ΔE is decomposed into the activation strain ΔEstrain of the reactants and the stabilizing TS interaction ΔEint between the reactants) from a single-point analysis of the TS to an analysis along the reaction coordinate ζ, that is, ΔE(ζ)=ΔEstrain(ζ)+ΔEint(ζ). This extension enables us to understand qualitatively, trends in the position of the TS along ζ and, therefore, the values of the activation strain ΔEstrain=ΔEstrainTS) and TS interaction ΔEint=ΔEint(ζTS) and trends therein. An interesting insight that emerges is that the much higher barrier of metal-mediated C[BOND]C versus C[BOND]H activation originates from steric shielding of the C[BOND]C bond in ethane by C[BOND]H bonds. Thus, before a favorable stabilizing interaction with the C[BOND]C bond can occur, the C[BOND]H bonds must be bent away, which causes the metal–substrate interaction ΔEint(ζ) in C[BOND]C activation to lag behind. Such steric shielding is not present in the metal-mediated activation of the C[BOND]H bond, which is always accessible from the hydrogen side. Other phenomena that are addressed are anion assistance, competition between direct oxidative insertion (OxIn) versus the alternative SN2 pathway, and the effect of ring strain.