Active anionic zero-valent palladium catalysts: Characterization by density functional calculations

This works uses DFT (B3LYP/LACVP^*+//B3LYP/LACVP* level) to ascertain the existence of the tricoordinate, anionic zero-valent palladium complexes that were postulated as the active species in the catalytic cycles of Pd-catalyzed Heck and cross-coupling reactions. The variety of complexes studied (1 and 2), include [Pd(PR₃)₂X] ^- species, in which R=H, Me, vinyl, and phenyl, and X=Cl, Br, I, AcO, and TFA, as well as bidentate complexes, [Pd{Ph₂P(CH ₂)_nPh₂P}X]^-, in which X = Cl, AcO and n = 3-6. The study shows that these complexes exist as distinct minima in the gas phase as well as in THF. In addition, it provides geometric features and Pd-X^- dissociation energies for all these complexes as well as some NMR and IR data, which show a clear distinction in these features between the tri- and dicoordinate Pd⁰ species. An orbital interaction model and perturbation theory arguments account for the bonding mechanism and rationalize all the trends in the stability of the Pd-X bond. These trends include the effects of variation of X, R, and the length of the linker in the bidentate ligands.