Mechanism of Perfluoro-Nickelacyclopentane Formation from Tetrafluoroethylene: Effects of Ancillary Ligand Bite Angle

Metallacyclopentanes are important reaction intermediates for catalytic processes, such as selective ethylene oligomerization and diene cyclooligomerization. Reactions of fluoroalkenes with low-valent first row transition metal complexes tend to form stable fluorometallacyclopentane products. DFT studies conducted herein indicate that conversion of three-coordinate Ni­(η²-C₂F₄)­L₂ complexes (L₂ = 2 monodentate or one bidentate chelate) to perfluorometallacycle products Ni­[κ²-(CF₂)₄-]­L₂ occurs preferentially via a four-coordinate intermediate Ni­(η²-C₂F₄)₂L₂; an alternative three-coordinate pathway via Ni­(η²-C₂F₄)₂L occurs only when L is extremely bulky. The transition structure for cyclization requires a dramatic increase in the L–Ni–L angle, which is perpendicular to the C–C bond-forming plane and initially affords a kinetic metallacycle intermediate resembling a trigonal bipyramidal geometry with one equatorial vacant site. The donor solvent, or intramolecular O-donor association with this intermediate, provides a low-energy pathway to the formation of the more stable square-planar metallacycle product. For bidentate chelate ligands, differential bite angles affect both coordination of a second C₂F₄ ligand and the cyclization step. For small-bite-angle chelates, the cyclization transition state energies are higher relative to monodentate analogues. An experimental investigation of wide bite-angle bis­(phosphine) ligands such as DPEphos and Bisbi showed the formation of additional nickelacyclopentane products with the coordination of a single P center to each Ni [DPEphos = bis­(2-diphenylphosphino-phenyl) ether; Bisbi = 2,2′-bis­(diphenylphosphino-methyl)-1,1′-biphenyl].