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].