Activation of C-H, C-C and C-I bonds by Pd and cis-Pd(CO)2I2. Catalyst-substrate adaptation

We have studied the oxidative addition reactions of methane and ethane C-H, ethane C-C and iodomethane C-I bonds to Pd and cis-Pd(CO)₂I₂ at the ZORA-BP86/TZ(2)P level of relativistic density functional theory (DFT). Our purpose, besides exploring these particular model reactions, is to understand how the mechanism of bond activation changes as the catalytically active species changes from a simple, uncoordinated metal atom to a metal-ligand coordination complex. For both Pd and cis-Pd(CO)₂I₂, direct oxidative insertion (OxIn) is the lowest-barrier pathway whereas nucleophilic substitution (S_N2) is highly endothermic, and therefore not competitive. Introducing the ligands, i.e., going from Pd to cis-Pd(CO)₂I₂, causes a significant increase of the activation and reaction enthalpies for oxidative insertion and takes away the intrinsic preference of Pd for C-I over C-H activation. Obviously, cis-Pd(CO)₂I₂ is a poor catalyst in terms of activity as well as selectivity for one of the three bonds studied. However, its exploration sheds light on features in the process of catalytic bond activation associated with the increased structural and mechanistic complexity that arises if one goes from a monoatomic model catalysts to a more realistic transition-metal complex. First, in the transition state (TS) for oxidative insertion, the C-X bond to be activated can have, in principle, various different orientations with respect to the square-planar cis-Pd(CO)₂I₂ complex, e.g., C-X or X-C along an I-Pd-CO axis, or in between two I-Pd-CO axes. Second, at variance to the uncoordinated metal atom, the metal complex may be deformed due to the interaction with the substrate. This leads to a process of mutual adjustment of catalyst and substrate that we designate catalyst-substrate adaptation. The latter can be monitored by the Activation Strain model in which activation energies ΔE^≠ are decomposed into the activation strain of and the stabilizing TS interaction between the reactants in the activated complex: .