Diphosphines with Expandable Bite Angles: Highly Active Ethylene Dimerisation Catalysts Based on Upper Rim, Distally Diphosphinated Calix[4]arenes
Résumé
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 NiII 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 C1 symmetry in the solid state, a feature which is also observed in solution at low temperature. As shown by variable‐temperature 1H and 31P 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 NiP 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 106 (mol C2H4) (mol Ni)−1 h−1. The selective formation of 1‐butene can be rationally controlled by using low catalyst concentrations.