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-C(5)NF(4))(PEt(3))(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-C(5)NF(3)H)(PEt(3))(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 Me(3)SiN(3) affords trans-[Ni(N(3))(2-C(5)NF(4))(PEt(3))(2)](2a) and trans-[Ni(N(3))(2-C(5)NHF(3))(PEt(3))(2)](2b), respectively. The complex trans-[Ni(NCO)(2-C(5)NHF(3))(PEt(3))(2)](3b) is obtained on reaction of with Me(3)SiNCO and by photolysis of under CO, while trans-[Ni(eta(1)-C [triple bond CPh)(2-C(5)NF(4))(PEt(3))(2)](4a) is obtained by reaction of phenylacetylene with 1a. Addition of KCN, KI and NaOAc to complex 1a affords trans-[Ni(X)(2-C(5)NF(4))(PEt(3))(2)](5a X = CN, 6a X = I, 7a X = OAc), respectively. The PEt(3) groups of complex are readily replaced by addition of 1,2-bis(dicyclohexylphosphino)ethane (dcpe) to produce [NiF(2-C(5)F(4)N)(dcpe)](8a). Addition of dcpe to trans-[Ni(OTf)(2-C(5)F(4)N)(PEt(3))(2)](10a), however, yields the salt [Ni(2-C(5)F(4)N)(dcpe)(PEt(3))](OTf)(9a) by substitution of only one PEt(3) 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.

Nickel fluoro complexes as intermediates in catalytic cross-coupling reactions

“…The angles about the nickel atom vary from 87.53(9)8 to 94.69(8)8. The Ni(1)-F(1) distance of 1.8383 (14) Å and the Ni(1)-C(11) distance to the tetrafluoropyridyl ligand of 1.892 (3) Å are in a similar range as the corresponding separations in [Ni(F)(2-C 5 NF 4 )(Cy 2 PCH 2 CH 2 PCy 2 )] (Ni-F = 1.8473(12) Å , Ni-C = 1.917(2) Å ) [26].…”

“…It exhibits a square-planar structure at Ni with the fluoro ligand coordinated in the trans position to the phosphorus atom. The angles about the nickel atom are distorted from an ideal square-planar geometry and vary from 89.48 (15) Comparable hydrogen-fluoro interactions have been observed before for other metal fluoro complexes [20,26,28,53,58]. …”

“…Comparable oxidative addition reactions at nickel bisphosphine moieties are known in the literature [6,26,27,[52][53][54]56]. Note that for the C-F activation step several mechanism have been discussed [3,6,26].…”

“…The use of THF instead of n-hexane as a solvent has no influence of the observed ratio of the products [26]. Treatment of [Ni(COD) 2 ] with one equivalent i Pr 2 PCH 2 CH 2 OMe and pentafluoropyridine gives nearly the same ratio of products, but in considerable lower yields.…”

Fluorine as a ligand substituent in organometallic chemistry: A second chance and a second research career

“…Much of this recent activity is driven by the need for synthetic procedures to incorporate fluorine or a CF 3 group strategically in the synthesis of pharmaceuticals or PET reagents, leading to the development of new methodology for formation of C-F and C-CF 3 bonds [110][111][112][113][114][115][116][117][118][119][120][121][122]. Organometallic chemistry involving the chemical functionalization of C-F bonds also continues to be an active field [123][124][125][126][127][128][129][130][131][132][133][134][135][136].…”