Apparent Allyl Rotation in New Allylpalladium(II) Complexes with Pyrazolyl N-Donor Ligands

The ligands 2,4,6‐tris(pyrazol‐1‐yl)‐1,3,5‐triazine (TPzT), 2,4,6‐tris(3,5‐dimethyl‐pyrazol‐1‐yl)‐1,3,5‐triazine (Me2‐TPzT), and 2,4,6‐tris(3,4,5‐trimethylpyrazol‐1‐yl)‐1,3,5‐triazine (Me3‐TPzT) have been synthesised and their reaction with [Pd(η3‐C4H7)(THF)2]X was explored. The reaction in a 1:3 molar ratio (ligand/solvate) leads to the new derivatives [{Pd(η3‐C4H7)}2(BPz′TO)]X (Pz′ = pyrazole, X = BF4, 2a; Pz′ = 3,5‐dimethylpyrazole, X = PF6, 2c; Pz′ = 3,4,5‐trimethylpyrazole, X = PF6, 2d) in which the N‐donor ligand has been partially hydrolysed. The complexes exist in the form of two isomers, a meso form and a d,l pair. An apparent allyl rotation process leads to a syn‐syn/anti‐anti interconversion and also to an interchange between the two isomers. The values of ΔGc‡ at the coalescence temperature have been calculated. The results, which have also been compared with those previously obtained for the complex with the 4‐methylpyrazole ligand, indicate that the activation barrier for the process is not affected by the change in the pyrazole group. Appreciable effects were not observed on changing the concentration or on the addition of water to the acetone solutions of 2c. However, the influence of the solvent (acetone versus chloroform) is worth noting. A negative activation entropy has been found and a mechanistic proposal for the process is included. The molecular structure of 2c has been determined by X‐ray diffraction. The meso isomer, in which the two C−Me axes of the allylic groups are oriented in the same direction, is found in the solid state. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)