Cu(i), Ag(i), Ni(ii), Cr(iii) and Ir(i) complexes with tritopic NimineCNHCNamine pincer ligands and catalytic ethylene oligomerization

Abstract

With their N-amine and N′-imine substituents, the imidazolium chlorides [(ImH){C(Me)NDipp}(C₂NMe₂)]Cl (1) and [(ImH){C(Me)NDipp}(C₃NMe₂)]Cl (2) are suitable precursors to pincer-type tritopic N^imineC^NHCN^amine ligands for various transition metals and allow an investigation of the influence of (i) the functionalities associated with the NHC donor and (ii) the length of the spacer connecting the amine group to the heterocycle. The mononuclear Cu(I) complexes [CuCl{Im[C(Me)NDipp](C₂NMe₂)-κ¹C^NHC}] (3) and [CuCl{Im[C(Me)NDipp](C₃NMe₂)-κ¹C^NHC}] (4) were prepared by the reaction of 1 and 2 with mesitylcopper(I), respectively. The reactions of 1 and 2 with Ag₂O, in the presence of molecular sieves, afforded the Ag(I) complexes [AgCl{Im[C(Me)NDipp](C₂NMe₂)-κ¹C^NHC}] (5) and [AgCl{Im[C(Me)NDipp](C₃NMe₂)-κ¹C^NHC}] (6), respectively. A dinuclear Cu(I) complex containing a ladder-type skeleton formed by copper and iodine atoms, [Cu₄(μ₂-I)₂(μ₃-I)₂{μ-κ²-C^NHC,N^amine(Im){C(Me)NDipp}(C₃NMe₂)}₂] (7), was obtained by deprotonation of 2 using sodium trimethylsilylamide/CuI. A transmetallation reaction from 5 to [NiBr₂(dme)] afforded an unexpected mixed-metal tetranuclear, centrosymmetric complex [NiCl{Im[C(Me)NDipp](C₂NMe₂)-κ¹C^NHC}{μ₂-κ³-N^imine,C^NHC,N^amineIm[C(Me)NDipp](C₂NMe₂)}AgBr₂]₂ (8), which contains two mononuclear Ni(II) units, one with a κ¹C^NHC donor and the other with a κ²-N^imine,C^NHC chelating and μ₂-C^NHC,N^amine bridging ligand. These two monocationic complexes are connected by the dianonic, disilver unit [Ag₂Br₄]²⁻. Furthermore, the mononuclear tritopic Ni(II) pincer complexes [Ni(NCMe){Im[C(Me)NDipp](C₂NMe₂)-κ³N^imine,C^NHC,N^amine}][BF₄]₂ (9) and [Ni(NCMe){Im[C(Me)NDipp](C₃NMe₂)-κ³N^imine,C^NHC,N^amine}][BF₄]₂ (10) were obtained when [Ni(NCMe)₆](BF₄)₂ was used as a precursor in transmetallation reaction with 9 and 10, respectively. Transmetallation from the silver complexes 5 and 6 was also applied to the synthesis of chromium complexes and whether [CrCl₂(THF)₂] or [CrCl₃(THF)₃] was used as a precursor, the same chromium(III) complexes, mer-[CrCl₃{Im[C(Me)NDipp](C₂NMe₂)-κ³N^imine,C^NHC,N^amine}] (11) and mer-[CrCl₃{Im[C(Me)NDipp](C₃NMe₂)-κ³N^imine,C^NHC,N^amine}] (12), respectively, were isolated. Moreover, silver transmetalation reactions from 5 and 6 were successfully applied to the Ir(I) precursor [Ir(μ-Cl)(cod)]₂ (cod = 1,5-cycloocadiene) and the Ir(I) complexes [IrCl(cod){Im[C(Me)NDipp](C₂NMe₂)-κ¹C^NHC}] (13) and [IrCl(cod) {Im[C(Me)NDipp](C₃NMe₂)-κ¹C^NHC}] (14), respectively, containing a monodentate NHC ligand were isolated as yellow crystals in good yields. The cationic, N^imine,C^NHC-chelated Ir(I) complexes [Ir(cod){Im[C(Me)NDipp](C₂NMe₂)-κ²N^imine,C^NHC}]BF₄ (15) and [Ir(cod){Im[C(Me)NDipp](C₃NMe₂)-κ²N^imine,C^NHC}]BF₄ (16) were obtained by halide abstraction from 13 and 14, respectively. The following 10 complexes 3, 6, 7, 8·2Et₂O, 9–11, 12·2C₇H₈, 14 and 16·THF were characterized by X-ray diffraction analysis. In catalytic ethylene oligomerization, the Ni complex 10 showed a productivity of 14 600 gC₂H₄ per (gNi h), giving 65% butenes and 30% hexenes. With the Cr complex 12, the productivity was up to 9200 gC₂H₄ per (gCr h) and a very high selectivity for α-olefins was observed, from C₄ to C₈.