Synthesis, spectroscopic properties and structural characterisation of Pd(II) and Pt(II) complexes with 1,3,5-pyrazole derived ligands. Rotation around the metal–N bond

Reactions of ligands 1-ethyl-5-methyl-3-phenyl-1H-pyrazole (L1) and 5-methyl-1-octyl-3-phenyl-1H-pyrazole (L2) with [PdCl2(CH3CN)2 and K2PtCl4 gave complexes trans-[MCl2(L)2] (L = L1, L2). The new complexes were characterised by elemental analyses, conductivity measurements, infrared, 1H and 13C{1H} NMR spectroscopies and X-ray diffraction. The NMR study of the complex [PdCl2(L1)2], in CDCl3 solution, is consistent with a very slow rotation of ligands around the Pd–N bond, so that two conformational isomers can be observed in solution (syn and anti). Different behaviour is observed for complexes [PdCl2(L2)2] and [PtCl2(L)2] (L = L1, L2), which present an isomer in solution at room temperature (anti). The crystal structure of [PdCl2(L1)2] complex is described, where the Pd(II) presents a square planar geometry with the ligands coordinated in a trans disposition.