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

The
synthesis of three new Pd­(I) dimers, (μ-All)­(μ-Cp)­{Pd­(PEt3)}2 (All = C3H5, Cp = C5H5), (μ-All)­(μ-Ind)­{Pd­(PEt3)}2 (Ind = C7H9), and (μ-Cp)­(μ-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 (μ-All)2{Pd­(PEt3)}2 and (μ-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 π 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.