Diphosphines with Expandable Bite Angles: Highly Active Ethylene Dimerisation Catalysts Based on Upper Rim, Distally Diphosphinated Calix[4]arenes

Abstract

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 [NiCp1]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 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 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.

Efficient, Nickel‐Catalysed Kumada–Tamao–Corriu Cross‐ Coupling with a Calix[4]arene‐Diphosphine Ligand

“…The calixarene fragment adopts a pinched cone conformation, with interplanar angles between opposite phenoxy rings of 26.3 and 69.08, respectively. As already observed in cis-chelate complexes obtained from related "1,3-calixdiphosphines", [15,21] the metal unit is turned away from the calixarene axis (distance to the axis: ca. + (Cp = C 5 H 5 or Cp-derived ligand).…”

“…For convenience, these phosphines will be termed here "1,3-calixdiphosphines". These ligands, when associated with nickel(II), result in remarkably fast catalysts for C À C bond-forming reactions, notably ethylene [15] and propylene dimerisation [16] as well as norbornene polymerisation. [17] A major practical advantage of the 1,3-calixdiphosphines is their high stability towards oxidation, which makes them attractive with respect to the ligands usually employed for efficient coupling reactions based on Ni/phosphine systems, e.g., PCy 3 .…”

“…Similar findings have been made previously for an analogue of 2, containing four n-propyl substituents instead of benzyl ones. [15] Note that at À80 8C (CD 2 Cl 2 , 400 MHz) the second motion of the benzyl-substituted molecule 2 …”

Unusually Large Bite Angle of a Distally Diphosphanylated Calix[4]arene Chelator

“…These complexes were found to operate as remarkably efficient catalysts in carbon-carbon coupling reactions. [17][18][19][20][21][22] Their high reactivity was attributed to the relatively large ligand bite angle (100-110°), which may favour a reductive elimination step in the catalytic cycle through steric effects involving the phosphorus substituents. A factor likely to enhance this effect is the fanning motion of the P-M-P plane in solution.…”

Bioinspired Catalyst Design and Artificial Metalloenzymes

“…[7] In homogeneous catalysis sophisticated modular ligand systems have been designed by using bioinspired supramolecular interactions [8] or by introducing bulky organic scaffolds like cyclodextrins or calixarenes for substrate encapsulation. [9] In addition, ligand systems with specific hydrogen-bonding interactions have been designed to enforce steric interactions or to favourably orientate substrates towards key reaction intermediates. [10] A field that aims to combine the best features of natural and "man-made" catalysts is the design of transition-metal catalyst systems based on the building blocks of Nature.…”