Diphosphines with expandable bite angles:: Highly active ethylene dimerisation catalysts based on upper rim, distally diphosphinated calix[4]arenes

The binding properties of two large diphosphines, cone-5,17-dibromo-11,23-bis(diphenylphosphino)- 25,26,27,28-tetrapropoxycalix[4]arene (1) and cone-5,17-bis(diphenylphosphino)-25,26,27,28-tetrapropoxycalix[4]arene (2) toward Ni-II centres have been investigated. Whatever the starting complex, NiBr2 or [NiCp]BF4, quantitative formation of a chelate complex was observed, illustrating the preorganisation of the ligands. An X-ray structure determination was carried out for [NiCp.1]BF4 which revealed that the nickel atom is positioned to one side of the calixarene axis, the PNiP plane being roughly parallel to the calixarene reference plane. The molecule has C-1 symmetry in the solid state, a feature which is also observed in solution at low temperature. As shown by variable-temperature H-1 and P-31 NMR studies, the complex undergoes two distinct motions: 1) a fan-like swinging of the coordination plane which displaces the metal from one side of the calixarene axis to the other, a motion during which the PNiP angle is likely to undergo a significant enlargement; 2) a rapid oscillation of each PPh2 unit about the corresponding Ni-P bond. In the latter dynamics the two endo-oriented PPh rings alternately occupy the calixarene entry. The two flexible ligands were assessed in ethylene oligomerisation. Activation with methylaluminoxane of the paramagnetic complexes [NiBr2.(1 or 2)] afforded highly active ethylene dimerisation catalysts, with turnover frequencies up to 10(6)(mol C2H4)(mol Ni)(-1)h(-1). The selective formation of 1-butene can be rationally controlled by using low catalyst concentrations.