Synthesis, Coordination Characteristics, Conformational Behavior, and Bond Reactivity Studies of a Novel Chiral Phosphapalladacycle Complex

A novel five-membered dimeric phosphapalladacycle was prepared using palladium(II) acetate as the ortho-palladation source. A successful optical resolution of the palladacycle was achieved through the separation of its (S)-prolinate derivatives by fractional crystallization using different solvent systems. Both the (R,R)- and (S,S)-di-μ-chloro dimeric palladium complexes can be obtained by treating the corresponding prolinato derivatives with 1 M hydrochloric acid, and the absolute configurations of both optically active dimers were concluded from the X-ray diffration studies. Two phosphine ligands, triphenylphosphine (PPh₃) and 3,4-dimethyl-1-phenylphosphole (dmpp), were able to coordinate with the dimeric palladium complex, and thus the palladacycle’s coordination characteristics were studied by both ³¹P NMR and X-ray diffraction. The phosphapalladacycle’s conformational behavior was further studied by the 2D ¹H−¹H ROESY NMR of its acetylacetonate derivative, which indicated that the α-methyl group was axially located and the palladacycle was conformationally rigid in solution. Moreover, the Pd−C bond in the palladacycle and the dmpp derivative was found to be ruptured immediately in the presence of concentrated HCl.