Apparent Allyl Rotation and Pd−N Bond Rupture in Allylpalladium Complexes with N-Donor Ligands − Evidence of an Associative Mechanism

The new didentate N‐donor ligands 2‐(4‐methyl‐1H‐pyrazol‐1‐yl)pyrimidine (4Me‐pzpm, 1) and 2‐(4‐bromo‐1H‐pyrazol‐1‐yl)pyrimidine (4Br‐pzpm, 2) have been synthesised and used to obtain the allylpalladium derivatives [Pd(η3‐2Me‐C3H4)(NN′)]X [X = BAr′4−, NN′ = 1 (3), NN′ = 2 (4); X = CF3SO3−, NN′ = 1 (5), NN′ = 2 (6)]. In complexes 3−6 two types of fluxional process have been found: apparent allyl rotation that is observed as Hsyn−Hsyn, Hanti−Hanti interconversions and H4−H6 interchange of the pyrimidine protons that must involve Pd−N(pm) bond rupture. The influence of different factors on both processes — such as the nature of the N‐donor ligand, counterion, solvent, complex concentration and addition of water — has been studied. It has been concluded that the apparent allyl rotation has a lower free energy of activation and in both cases the presence of a species with coordinating ability favours the process. Negative entropies of activation have been found. Associative mechanisms involving participation of five‐coordinate intermediates have been proposed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)