Carbon−Hydrogen Bond Activation in Hydridotris(pyrazolyl)borate Complexes of Iridium

Methylene chloride solutions of the tris(1-pyrazolyl)borate (Tp) complexes TpIr(PPh₃)(C₂H₄) or TpIr(C₂H₄)₂ react with excess PPh₃ via activation of a pyrazole C−H bond to form equilibrium mixtures of TpIr(PPh₃)(C₂H₄), (N,C⁵,N-Tp-H)Ir(PPh₃)₂H (1), and free ethylene upon standing at room temperature. Complete conversion to 1 was accomplished by removing the displaced ethylene. The hydride complex 1 was purified by protonation at the unbound nitrogen of the pyrazole ring to afford cationic complex 2 as the BF₄ salt. Deprotonation of 2 affords complex 1 in good yield and purity. The structures of 1 and 2 have been elucidated using ¹H and ¹³C NMR spectroscopy. The structures of TpIr(C₂H₄)₂ and complex 2 (CH₃CN solvate) been verified by X-ray crystallography. Reaction of isolated complex 1 with ethylene regenerates TpIr(PPh₃)(C₂H₄) in a reaction that is strongly inhibited by added PPh₃.