A new stable CNHC⁁CH⁁CNHCN-heterocyclic dicarbene ligand: its mono- and dinuclear Ir(i) and Ir(i)–Rh(i) complexes

Abstract

The ligand [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazol-2-ylidene) (2) (abbreviated as ^EtC_NHC⁁CH⁁C_NHC with CH = central aromatic ring, ⁁ = CH₂ spacer) has been synthesised, and isolated, by deprotonation of [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazolium) dichloride, ^Et(CH_imid.⁁CH⁁CH_imid.)Cl₂ (1) with LiN(SiMe₃)₂. This new, remarkably stable NHC dicarbene ligand is quantitatively protonated with HBr to form [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazolium) dibromide ^Et(CH_imid.⁁CH⁁CH_imid.)Br₂ (3) and reprotonated slowly enough in MeOH to allow observation of the monoprotonated intermediate 4. Dicarbene 2 reacts with S₈ to give the dithione compound [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazol-2-thione) ^Et[(SC)⁁CH⁁(CS)] (6). Reaction of 2 with [IrCl(CO)(PPh₃)₂] and [Ir(μ-Cl)(cod)]₂ yielded the mononuclear [Ir(CO)(PPh₃)₂^Et(CH_imid.⁁CH⁁C_NHC)(PF₆)](PF₆) (8) and [IrCl(cod)^Et(CH_imid.⁁CH⁁C_NHC)](PF₆) (10) complexes, respectively, and the dinuclear complexes [{Ir(CO)(PPh₃)₂}₂^Et(μ-C_NHC⁁CH⁁C_NHC)](PF₆)₂ (7) and [{IrCl(cod)}₂^Et(μ-C_NHC⁁CH⁁C_NHC)] (9), respectively, in which 2 acts as a bridging, non-pincer type ligand. Only one other Ir(I) complex has been reported before containing a C_NHC⁁CH⁁C_NHC ligand. The structures of 6 and 9 were determined by X-ray diffraction and depict different conformations of the (1,3-phenylene)bis(methylene) (or xylylene) moiety. The mononuclear Ir complex 9 reacted smoothly with [Rh(μ-Cl)(cod)]₂ and Cs₂CO₃ to generate a rare example of a heteronuclear NHC complex, [IrRhCl₂(cod)₂^Et(μ-C_NHC⁁CH⁁C_NHC)] (11).