Mechanistic Studies of the Insertion of CO2 into Palladium(I) Bridging Allyl Dimers

In contrast to the chemistry of momomeric eta(1)-Pd allyls, which act as nucleophiles, and monomeric eta(3)-Pd allyls, which act as electrophiles, relatively little is known about the reactivity of Pd complexes with bridging allyl ligands. Recently we demonstrated that HI dimers containing two bridging allyl ligands react with one equivalent of CO2 to form species with one bridging allyl and one bridging carboxylate ligand. In this work we have prepared complexes from three different classes of Pd-I bridging allyl dimers: (i) dimers containing two bridging allyl ligands, (ii) dimers with one bridging allyl and one bridging chloride ligand, and (iii) dimers with one bridging allyl and one bridging carboxylate ligand. Complexes from all three groups have been characterized by X-ray crystallography, and their structures compared. Complexes with two bridging allyl ligands have the longest Pd bridging allyl bond lengths due to the high trans influence of the opposing bridging allyl ligand. For these species the HOMO is located almost entirely on the bridging 4,1 ligands, whereas for chloride- and carboxylate-bridged species the HOMO is primarily Pd based. A combined experimental and theoretical study has been performed to investigate the reactivity of the three different types of bridging allyl dimers with CO2. Complexes with one bridging allyl and one bridging chloride ligand and complexes with one bridging allyl and one bridging carboxylate ligand do not insert CO2 because the reaction is thermodynamically unfavorable. In contrast, in most cases the reaction of CO2 with species containing two bridging allyl ligands is facile and involves nucleophilic attack of the bridging allyl ligand on electrophilic CO2. An alternative pathway for CO2 insertion, which involves a monomer/dimer equilibrium, can occur in the presence of a weakly coordinating ligand. Overall, our results suggest that although the bridging allyl ligand is likely to be unreactive in carboxylate and chloride-bridged species, complexes with two bridging allyl ligands can act as nucleophiles like monomeric eta(1)-Pd allyls.