Strained Diphosphines Built upon a Calix[4]arene Skeleton. Synthesis of a Highly Active Norbornene Polymerization Catalyst†

Summary: The complexes cis-P,P′-(η5-cylopentadienyl)-{5,17-dibromo-11,23-bis(diphenylphosphino)-25,26,27,28-tetrapropoxy-calix[4]arene}nickel(II) tetrafluoroborate (1) and dibromo-{5,17-dibromo-11,23-bis(diphenylphosphino)-25,26,27,28-tetrapropoxycalix[4]arene}nickel(II) (2), both of which contain a constrained chelating diphosphine, were evaluated for the polymerization of norbornene. Combined with methylaluminoxane, they result in remarkably active systems for the production of high-molar-mass vinyl-type polynorbornene. Turnover frequencies of up to 7.5 × 105 mol(NBE) · mol(Ni)−1 · h−1 are observed. A plausible explanation for their high performances relies on a periodic P–Ni–P bite angle enlargement that temporarily increases the steric hindrance about the catalytic centre, which in turn favours the insertion steps. Molecular structure of 2.