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

Abstract

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(N₃)(2-C₅NF₄)(PEt₃)₂] (2a) and trans-[Ni(N₃)(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(η¹-CCPh)(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.