Oxidative Addition versus Dehydrogenation of Methane, Silane, and Heavier AH4Congeners Reacting with Palladium

We have computationally studied the oxidative addition of AH 4 to Pd (with A = C, Si, Ge, Sn, and Pb) using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Our purpose is threefold: (i) exploring the occurrence and competition between direct oxidative insertion (OxIn) into the A−H bond and an alternative S N 2-type mechanism that is known to occur for oxidative addition of carbon−halogen bonds; (ii) exploring the trends in activation and reaction enthalpies as the atom A in AH 4 descends in group 14 from carbon to lead; (iii) analyzing and understanding the emerging trends in terms of properties of the reactants using the activation strain model. We find that oxidative insertion of Pd proceeds via a reactant complex and a central barrier only for the C−H bond. For the heavier A−H bonds, the process is barrierless and significantly more exothermic. The higher barrier and smaller exothermicity in the case of Pd + CH 4 has two main reasons: (i) the higher strain associated with the stronger C−H bond and (ii) the weaker Pd−CH 4 interaction due to less attractive electrostatic interaction with the compact and less polar charge distribution of methane. Backside nucleophilic attack proceeds in none of the cases toward an S N 2-type mechanism for oxidative addition. Instead, the process evolves into a novel mechanism for α-elimination of molecular hydrogen.