Reactions of bis(η-cyclopentadienyl)nickel with some pyrazoles. Unusual temperature dependence of the hydrogen-1 nuclear magnetic resonance spectrum of the cyclopentadienyl group

Abstract

Bis(η-cyclopentadienyl)nickel reacts with 3,5-dialkylpyrazoles in benzene to give dark red diamagnetic complexes [{Ni(η-C₅H₅)(N₂C₃HR₂)}₂](R = Me, Et, or Prⁱ). No reaction occurs when R = Bu^t. The position of the resonance of the η-C₅H₅ group in the ¹H n.m.r. spectrum is temperature dependent, moving ca. 1 p.p.m. upfield between 0 and 80 °C; this behaviour is consistent with an equilibrium (ΔHca. 45 kJ mol^–1) between the dimer and a paramagnetic monomer. The reaction of [Ni(η-C₅H₅)₂] with pyrazole and with 3-methylpyrazole results in the displacement of both η-C₅H₅ groups and the formation of polymeric bis(pyrazolates).