New palladium and platinum polyfluorophenyl complexes with pyrazolyl N-donor ligands. Analysis of the restricted rotation of the polyfluorophenyl rings

Abstract

The new N-donor chelate ligands bis(pyrazol-1-yl)(anisol-2-yl)methane, bpzmArOMe, bis(3,5-dimethylpyrazol-1-yl)hydrocarbylmethane (hydrocarbyl = anisol-2-yl, bpz*mArOMe; cyclohexyl, bpz*mCy) and bis(3,5-dimethylpyrazol-1-yl)ferrocenylmethane, bpz*mFec, have been prepared and characterized. New palladium and platinum polyfluorophenyl complexes were prepared with these ligands and those previously described: bis(pyrazol-1-yl)(phenol-2-yl)methane, bpzmArOH; bis(3,5-dimethylpyrazol-1-yl)(phenol-2-yl)methane, bpz*mArOH; 2-(pyrazol-1-yl)-pyridine, pzpy and 2-(pyrazol-1-yl)-pyrimidine, pzpm. For the bpzmR or bpz*mR complexes only one isomer has been found with the R group in an axial orientation. The fluxional behaviour of the new complexes, mainly the polyfluorophenyl rotation, has been analyzed. For the complexes with the nonplanar ligands, derived from bis(pyrazol-1-yl)methane, rotation of the pentafluorophenyl groups has been found to occur with different energy barriers. The presence of methyl groups in position 3 of the pyrazole rings reduces this barrier. The process is not affected by the presence of coordinating solvents or anions (Cl⁻) or by a change in the complex concentration. For the derivative Pd(2,3,4,6-C₆HF₄)₂(pzpm), containing a planar ligand, two atropisomers are observed even at high temperature, which excludes the existence of polyfluorophenyl rotation. Processes of Pd–N bond rupture or cis,trans-isomerization have not been observed for this complex on the NMR time scale. The molecular structure of complex Pd(C₆F₅)₂(bpz*mFec) has been determined by an X-ray diffraction study.