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) Å and a
Ni−C distance of 1.878(7) Å. 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) Å, 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(η2-CH2CHC6F5)
(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) Å; the
P−Ni−P angle is 115.0(1)°. 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.
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