Synthesis of 1,1-bis(pyrazol-1-ylmethyl)ethene and the structure of a trimethylplatinum(IV) derivative containing an eight-membered chelate ring

Abstract

The bidentate pyrazole-donor ligand 1,1-bis(pyrazol-1-ylmethyl)ethene, [(pz)CH₂]₂CCH₂, may be synthesized in high yield on reaction of the readily obtained potassium pyrazolide with commercially available (ClCH₂)₂CCH₂. Reaction of the ligand with [{PtMe₂(SEt₂)}₂], followed by addition of iodomethane, gives the platinum(IV) complex fac-[PtIMe₃{[(pz)CH₂]₂CCH₂-N,N′}]. Proton NMR and X-ray structural studies show that the eight-membered chelate ring PtNNC₃NN adopts a configuration in which the pyrazole rings are in different environments, forming angles of 26.0(3) and 38.2(3)° with the ‘PtC₂N₂’ mean plane. Thus, the four methylene protons are also in different environments, as are the PtMe groups trans to pyrazole groups. The eight-membered chelate ring is fluxional, undergoing inversion with ΔG^‡≈ 15 kcal mol^–1(≈ 63 kJ mol^–1) at 10 °C.