Synthesis of neutral and zwitterionic phosphinomethylpyrrolato complexes of nickel

Potassium salts of the new 2-phosphinomethyl-1H-pyrroles, K[R₂PCH₂C₄H₃N] (R = Ph, Cy) react with (η³-allyl)nickel bromide to give the chelate complexes (R₂PCH₂C₄H₃N)Ni(allyl), whereas the sterically hindered 2-diphenylphosphinomethyl-5-t-butyl-1H-pyrrole and (η³-allyl)nickel bromide afford a phosphine adduct (HNC₄H₂-5-Bu^t-2-CH₂PPh₂)Ni(allyl)Br which is stabilized by an intramolecular NHBr hydrogen bond. The addition of B(C₆F₅)₃ to (R₂PCH₂C₄H₃N)Ni(allyl) leads to an electrophilic attack in 5-position of the pyrrole ring, to give the thermally unstable zwitterions (η³-C₃H₅)Ni[NC₄H₃(2-CH₂PR₂)-5-B(C₆F₅)₃] which catalyse the isomerisation of 1-hexene. The addition of B(C₆F₅)₃ is reversible, and slow ligand rearrangement to Ni(N-P)₂ products appears to be the major catalyst deactivation pathway.