Experimental Study of the Reaction of a Ni(PEt3)(2) Synthon with Polyfluorinated Pyridines: Concerted, Phosphine-Assisted, or Radical C-F Bond Activation Mechanisms?
Hatnean, JA; Johnson, SA; ,
The mechanisms of the C F activation reactions of 2,3,5,6-tetrafluoropyridine and pentafluoropyridine with the phenanthrene adduct (PEt3)(2)Ni(eta(2)-C14H10) and Ni(PEt3)(4) as sources of (PEt3)(2)Ni were investigated. The activation of 2,3,5,6-tetrafluoropyridine proceeds via the mononuclear adduct (PEt3)(2)Ni(eta(2)-2,3,5,6-C5F4HN) (3) in equilibrium with the CH activation product, trans-(PEt3)(2)NiH(2,3,5,6-C5F4N) (2). The C-F activation reaction occurs by concerted oxidative addition, as judged by the observation of cis-(PEt3)(2)NiF(3,5,6-C5F3HN) (4), as an intermediate to trans-(PEt3)(2)NiF(3,5,6-C5F3HN) (1). The dinudear adduct [(PEt3)(2)Ni](2)(mu-eta(2):eta(2)-2,3,5,6-C5F4HN) (5) was not observed. Pentafluoropyridine reacted with the phenanthrene adduct (PEt3)(2)Ni(eta(2)-C14H10) to provide the mononuclear adduct (PEt3)(2)Ni(eta(2)-C5F5N) (6) and the dinuclear adduct [(PEt3)(2)Ni](2)-(mu-eta(2):eta(2)-C5F5N) (11). A (PEt3)(2)Ni adduct of the C-F activation product, (PEt3)(2)Ni[eta(2)-3,4,5,6-C5F4N-2-NiF(PEt3)(2)] (10), and the double C F activation product [trans-(PEt3)(2)NiF](2)(3,4,5-C5F3N) (9) were also assigned by NMR spectroscopy. Complex 10 was a precursor to both trans-(PEt3)(2)NiF(3,4,5,6-C5F4N) (7) and 9. The rate of formation of trans-(PEt3)(2)NiF(3,4,5,6-C5F4N) (7) is not first order with respect to the concentration of 6. The EPR spectrum of (PEt3)(2)Ni(eta(2)-C14H10) and pentafluoropyridine, in a toluene solution that was frozen in N-2(1) during reaction at -40 degrees C, revealed signals consistent with the speculative formulation (PEt3)(2)Ni(2-C5F4N). These results suggest that pathways other than concerted oxidative addition or phosphine-assisted pathways arising from 6 are required to account for the unusual selectivity of the C F activation of pentafluoropyridine by the (PEt3)(2)Ni fragment.