Synthesis of 1,1-Bis(pyrazol-1 -ylmethyl)ethene and the Structure of a Trimethylplatinum(1V) Derivative containing an Eight-membered Chelate Ring

The bidentate pyrazole-donor ligand 1,1-bis(pyrazo1-1-ylmethyl)ethene, [(pz)CH2]2C=CH2, may be synthesized in high yield on reaction of the readily obtained potassium pyrazolide with commercially available (CICH2)2C=CH2. Reaction of the ligand with [{PtMe2(SEt2)}2], followed by addition of iodomethane, gives the platinum(IV) complex fac-[PtlMe3{[(pz)CH2]2C=CH2-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 DGact ~ 15 kcal mol-1 (~63 kJ mol-l) at 10 C.