New complexes with pyrazole-containing ligands and different metallic centres.: Comparative study of their fluxional behaviour involving M-N bond rupture

New complexes of palladium(II), silver(I), copper(I) and ruthenium(II) containing the ligands 2-(pyrazol-1-yl)pyridine (pzpy), 2-(pyrazol-1-yl)pyrimidine (pzpm), 2-(3,5-dimethylpyrazol-1-yl)pyrimidine (pz*pm) and 4,6-bis(pyrazol-1-yl)-pyrimidine (bpzpm) have been prepared and characterised. Only the isomer with the least steric hindrance was found for the derivatives PdClR(N-N'), R=Me or COMe, N-N'=pzpy or pzpm, while the two possible isomers were found for similar derivatives with pz*pm. In the reaction of [RuCl2(cod)](n) with pzpy, pzpm and bpzpm, the isomer trans-RuCl2(cod)(N-N') was always formed. Three different isomers were consecutively formed after the reaction of RuCl2(PPh3)(3) with bpzpm, with the thermodynamically stable isomer being cis,trans-RuCl2(PPh3)(2)- (bpzpm). A fluxional process involving interchange of the H-4/H-6 pyrimidine protons, which implies M-N(pm) bond rupture, was found in the silver, copper and some palladium derivatives. Such an interchange was not observed in the ruthenium complexes. The trans influence of the group situated trans to the pyrimidine ring was found to be extremely important for Pd-N bond rupture. The isomer interconversion for the derivatives PdClR(pz*pm), R=Me or COMe, was studied by variable temperature H-1 NMR studies. The process is of higher energy than the H-4/H-6 interchange and is facilitated by the presence of the acyl group.