Palladium(II) and Palladium(0) Complexes of Pyridylthioether-Based Metallodendrimers. Synthesis, Characterization, and Mechanistic Study of the Influence of Wedge Size on Allyl Amination

Abstract

Pd(II) allyl and Pd(0) fumaronitrile complexes bearing pyridyl-dithioether-based dendrimers as ancillary ligands have been synthesized and fully characterized by means of NMR spectrometry, elemental analysis, and MALDI-TOF technique. The fluxional behavior of the species was investigated by studying the 1 H NMR spectra recorded in CD 2 Cl 2 at variable temperature and interpreted on the basis of a windshield-wiper motion of the dendritic wings which alternatively coordinate at the metal core. The reactivity of piperidine on the Pd(II) allyl substrates to give the allylamine derivative and the corresponding Pd(0) fumaronitrile species was also studied. The second-order rate constants k 2 and the equilibrium constants K E relating to piperidine attack on the allyl fragment and to the concomitant displacement of the dendrimer ligand by piperidine, respectively, were determined and discussed taking into account the increasing dendritic size. Surprisingly, no macroscopic effects are observed on going from the model molecule to the second-generation dendritic substrates, and only with the third generation dendritic wedge complex are remarkable variations in the rate and equilibrium constants observed. We therefore advance the hypothesis that a sudden rearrangement at this stage occurs in solution. Such a rearrangement would induce an increase of steric hindrance at the allyl fragment and a concomitant distortion of the ligand in the environment of the metal core, thereby justifying the decrease of k 2 and the increase of K E values.