C-F Bond activation at Ni(0) and simple reactions of square planar Ni(II) fluoride complexes

The reaction of Ni(COD)(2) (COD = 1,5-cyclooctadiene) with triethylphosphine and pentafluoropyridine in hexane has been shown previously to yield trans-[NiF(2-C5NF4)(PEt3)(2)] (1a) with a preference for reaction at the 2-position of the heteroaromatic. The corresponding reaction with 2,3,5,6-tetrafluoropyridine was shown to yield trans-[NiF(2-C5NF3H)(PEt3)(2)] (1b). In this paper, we show that reaction of Ni(COD)(2) with triethylphosphine and pentafluoropyridine in THF yields a mixture of 1a and 1b. Competition reactions of Ni(COD)(2) with triethylphosphine in the presence of mixtures of heteroaromatics in hexane reveal a kinetic preference of k(pentafluoropyridine) : k(2,3,5,6-tetrafluoropyridine) = 5.4 : 1. Treatment of 1a and 1b with Me3SiN3 affords trans-[Ni(N-3)(2-C5NF4)(PEt3)(2)] (2a) and trans-[Ni(N-3)(2-C5NHF3)(PEt3)(2)] (2b), respectively. The complex trans-[Ni(NCO)(2-C5NHF3)(PEt3)(2)] (3b) is obtained on reaction of 1b with Me3SiNCO and by photolysis of 2b under CO, while trans-[Ni(eta(1)-C=CPh)(2-C5NF4)(PEt3)(2)] (4a) is obtained by reaction of phenylacetylene with 1a. Addition of KCN, KI and NaOAc to complex 1a affords trans-[Ni(X)(2-C5NF4)(PEt3)(2)] (5a X = CN, 6a X = I, 7a X = OAc), respectively. The PEt3 groups of complex 1a are readily replaced by addition of 1,2-bis(dicyclohexylphosphino)ethane (dcpe) to produce [NiF(2-C5F4N)(dcpe)] (8a). Addition of dcpe to trans-[Ni(OTf)(2-C5F4N)(PEt3)(2)] (10a), however, yields the salt [Ni(2-C5F4N)(dcpe)(PEt3)](OTf) (9a) by substitution of only one PEt3 and displacement of the triflate ligand. The structures of 2b, 4a, 7a and 8a were determined by X-ray crystallography. The influence of different ancillary ligands on the bond lengths and angles of square-planar nickel structures with polyfluoropyridyl ligands is analysed.