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)[double bond, length as m-dash]NDipp}(C2NMe2)]Cl (1) and [(ImH){C(Me)[double bond, length as m-dash]NDipp}(C3NMe2)]Cl (2) are suitable precursors to pincer-type tritopic NimineCNHCNamine 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)[double bond, length as m-dash]NDipp](C2NMe2)-κ1CNHC}] (3) and [CuCl{Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ1CNHC}] (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)[double bond, length as m-dash]NDipp](C2NMe2)-κ1CNHC}] (5) and [AgCl{Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ1CNHC}] (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-CNHC,Namine(Im){C(Me)[double bond, length as m-dash]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)[double bond, length as m-dash]NDipp](C2NMe2)-κ1CNHC}{μ2-κ3-Nimine,CNHC,NamineIm[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)}AgBr2]2 (8), which contains two mononuclear Ni(ii) units, one with a κ1CNHC donor and the other with a κ2-Nimine,CNHC chelating and μ2-CNHC,Namine 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)[double bond, length as m-dash]NDipp](C2NMe2)-κ3Nimine,CNHC,Namine}][BF4]2 (9) and [Ni(NCMe){Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ3Nimine,CNHC,Namine}][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)[double bond, length as m-dash]NDipp](C2NMe2)-κ3Nimine,CNHC,Namine}] (11) and mer-[CrCl3{Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ3Nimine,CNHC,Namine}] (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)[double bond, length as m-dash]NDipp](C2NMe2)-κ1CNHC}] (13) and [IrCl(cod) {Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ1CNHC}] (14), respectively, containing a monodentate NHC ligand were isolated as yellow crystals in good yields. The cationic, Nimine,CNHC-chelated Ir(i) complexes [Ir(cod){Im[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)-κ2Nimine,CNHC}]BF4 (15) and [Ir(cod){Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ2Nimine,CNHC}]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.