Rapid intermolecular carbon-fluorine bond activation of pentafluoropyridine at nickel(0): Comparative reactivity of fluorinated arene and fluorinated pyridine derivatives

Reaction of hexafluorobenzene with Ni(COD)(2) (COD = 1,5,-cyclooctadiene) in the presence of triethylphosphine or with Ni(PEt3)(4) at room temperature results in very slow formation of trans-Ni(PEt3)(2)(C6F5)F (1). The X-ray crystal structure reveals a molecular complex with approximately square-planar coordination at nickel, a Ni-F distance of 1.836(5) Angstrom and a Ni-C distance of 1.878(7) Angstrom. Analogous reactions with pentafluoropyridine and 2,3,5,6-tetrafluoropyridine proceed much faster. Both reactions yield C-F activation products analogous to 1 in which a single regioisomer is dominant. The crystal structure of trans-Ni(PEt3)(2)(C5HF3N)F (3) shows that the trifluoropyridyl ligand is metalated at the 2-position. The Ni-F and Ni-C distances are 1.856(2) and 1.869(4) Angstrom, respectively. In the structures of both 1 and 3, the plane of the aryl ring is perpendicular to the nickel coordination plane, These structures provide the first values of nickel-fluorine distances at square-planar Ni(II). The reaction of Ni(COD)(2) with PEt3 and 3,5-dichloro-2,4,6-trifluoropyridine yields exclusively trans-Ni(PEt3)(2)(C5ClF3N)Cl (4), the product of C-Cl activation with the metal at the 3-position of the ring. The corresponding reaction with 2,3,4,5,6,-pentafluorostyrene results in rapid formation of the alkene coordination product, Ni(PEt3)(2)(eta(2)-CH2=CHC6F5) (6). The crystal structure of 6 shows the typical trigonal Ni(0) geometry with the coordinated atoms almost perfectly coplanar with the nickel. The Ni-C distances average 1.960(6) Angstrom; the P-Ni-P angle is 115.0(1)degrees. Reaction with pentafluorobenzene and methoxypentafluorobenzene yield C-F activation products very slowly with little regioselectivity. The studies reported in this paper demonstrate that intermolecular C-F activation of a fluoroaromatic can take place rapidly and in good yield at a first row transition metal through the suitable choice of substrate.