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 EtCNHC--CH--CNHC 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(CHimid.--CH--CHimid.)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(CHimid.--CH--CHimid.)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(mu-Cl)(cod)]2 yielded the mononuclear [Ir(CO)(PPh3)2Et(CHimid.--CH--CNHC)(PF6)](PF6) ( 8) and [IrCl(cod)Et(CHimid.--CH--CNHC)](PF6) ( 10) complexes, respectively, and the dinuclear complexes [{Ir(CO)(PPh3)2}2Et(mu-CNHC--CH--CNHC)](PF6)2 ( 7) and [{IrCl(cod)}2Et(mu-CNHC--CH--CNHC)] ( 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 CNHC--CH--CNHC 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(mu-Cl)(cod)]2 and Cs2CO3 to generate a rare example of a heteronuclear NHC complex, [IrRhCl2(cod)2Et(mu-CNHC--CH--CNHC)] ( 11).