NHC Core Phosphonium Ylide-based Palladium(II) Pincer Complexes: The Second Ylide Extremity Makes the Difference.

The coordinating properties of N-heterocyclic carbene (NHC) (A), phenolate (B), and phosphonium ylide (C) moieties were investigated systematically through the preparation of a family of NHC, phosphonium ylide-based pincer ligands, where the third donor extremity can be either an NHC, a phenolate, or a phosphonium ylide. The overall donor character of such ligands [NHC(ABC)] ( + + = 2) was analyzed by comparison of the molecular orbitals (energy and shape), oxidation potentials (), and IR ν and ν stretching frequencies of their isostructural pincer Pd(II) complexes [NHC(ABC)PdL][OTf] (L = NCCH, CO, or CNBu). The three categories of pincer complexes based on phosphonium ylides were easily obtained by acidic treatment of their highly stable -metalated Pd(II) precursors prepared in a single step from readily available N-phosphonio-substituted imidazolium salts. Analysis of IR data indicated that NHC and phenolate ligands have a similar donor character but which remains lower than that of the phosphonium ylide. The impact on catalytic performance of the incorporation of a second strongly donating phosphonium ylide into the ligand architecture was illustrated in the Pd-catalyzed allylation of aldehydes.