Silver(I) and Nickel(II) Complexes with Oxygen- or Nitrogen-Functionalized NHC Ditopic Ligands and Catalytic Ethylene Oligomerization

Potentially bidentate ditopic ligands containing a N-heterocyclic carbene (NHC) donor associated with an ether or an amine have been prepared and coordinated to Ni-II centers. The influence of the length of the alkyl chain, -(CH2)(2)- or -(CH2)(3)- connecting the ether or the amine group to the heterocycle was examined. In the analogous Ag-I complexes [Ag{Im(Dipp)(C3OMe)-kappa C-1(NHC)}(2)]Cl (8), 9 and 10, in the neutral Ni-II complexes with a C-3 spacer trans-[NiCl2{Im(Dipp)(C3OMe)-kappa C-1(NHC)}(2)] (5a), 6, and 7, and in the cationic cis-[Ni{Im(Dipp)(C3OMe)-kappa C-1(NHC)}(2)(NCMe)(2)](PF6)(2) (15) and cis-[Ni{Im(Mes)(C3OMe)-kappa C-1(NHC)}(2)(NCMe)(2)](PF6)(2) (16), the ligand is monodentate. [(ImH)(Dipp)(C3OMe)][NiX3{Im(Dipp)(C3OMe)-kappa C-1(NHC)}] (14a, X = Cl) and (14b, X = Br) are rare examples of complexes of the type [NiX3(NHC)](-). For comparison, [NiBr2{(Im)(Dipp)(C2NMe2)-kappa(2),C-NHC,N-amine}] (24) and [NiBr2{(Im)(Dipp)(C3NMe2)-kappa(2),C-NHC,N-amine}] (25) contain a six- or a seven-membered kappa C-2(NHC),N-amine chelate, respectively. Various Ni-II complexes were evaluated as precatalysts for ethylene oligomerization. The structures of the carbene (Dipp)(C3OMe)imidazole-ylidene (4) and of the complexes 5a, 7, 8, [(ImH)(Dipp)(C3OMe)](2)[NiCl4] (11a), [(ImH)(Dipp)(C3OMe)](2)[NiBr4] (11b), [(ImH)(Me)(C3OMe)](2)[NiCl4] (13), 14b, 16 center dot NCMe, [Im(H){C(Me)(=NDipp)}(C3OMe)](2) [NiCl4] (18), [AgCl{Im[C(Me)=NDipp](C3OMe)}-kappa C-1(NHC)] (19), [AgCl{Im(Dipp)(C3NMe2)-kappa C-1(NHC)}] (23), 24, 25 and trans-[NiCl2{Im(Dipp){CH2CH2C(O)OEt}}(2)] were analyzed by X-ray diffraction.