Synthesis, Electronic Structure, and Reactivity of Palladium(I) Dimers with Bridging Allyl, Cyclopentadienyl, and Indenyl Ligands

The synthesis of three new Pd(I) dimers, (mu-All)(mu-Cp){Pd(PEt3)}(2) (All = C3H5, Cp = C5H5), (mu-All)(mu-Ind){Pd(PEt3)}(2) (Ind = C7H9), and (mu-Cp)(mu-Ind){Pd-(PEt3)}(2), which contain a combination of bridging allyl, Cp, or indenyl ligands and are all supported by triethylphosphine as the ancillary ligand, is reported. The solid-state geometries, electronic structures, and reactivity of these new compounds have been compared with those of the dimers (mu-All)(2){Pd-(PEt3)}(2) and (mu-Cp)(2){Pd(PEt3)}(2), which have previously been reported. This work establishes that there are many similarities in the solid-state and electronic structures of complexes containing bridging allyl, Cp, or indenyl ligands. For example, in all cases the bridging ligands bind through three carbon atoms to the two Pd atoms, with only the central carbon atom of the bridging group bound to both metal centers. However, there are also important differences based on the identity of the bridging ligand. As a result of different overlap between the metal centers and the pi orbitals of the bridging allyl, Cp, or indenyl ligand, Cp ligands are more likely to result in an anti relationship between the two bridging ligands, while allyl and indenyl ligands are more likely to give a syn relationship. The solid-state structures indicate that bridging allyl ligands bind the most tightly to the metal center and bridging Cp ligands bind the least tightly. DFT calculations reveal that the nature of the bridging ligand alters the HOMO of the Pd(I) dimers. As a result, in some cases it is possible to selectively protonate one of the bridging ligands using the electrophile 2,6-lutidinium chloride.