A new stable (CNHCCHCNHC)-C-boolean AND-C-boolean AND N-heterocyclic dicarbene ligand: its mono- and dinuclear Ir(I) and Ir(I)-Rh(I) complexes

The ligand [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazol-2-ylidene) (2) (abbreviated as (CNHCCHCNHC)-C-Et-C-boolean AND-C-boolean AND with CH = central aromatic ring, (boolean AND) = CH2 spacer) has been synthesised, and isolated, by deprotonation of [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazolium) dichloride, (Et)((CHimid.CHCHimid.)-C-boolean AND-C-boolean AND)Cl-2 (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.CHCHimid.)-C-boolean AND-C-boolean AND)Br-2 (3) and reprotonated slowly enough in MeOH to allow observation of the monoprotonated intermediate 4. Dicarbene 2 reacts with S-8 to give the dithione compound [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazol-2-thione) (Et)[(S=C)(CH boolean AND)-C-boolean AND(C=S)] (6). Reaction of 2 with [IrCl(CO)(PPh3)(2)] and [Ir(mu-Cl)(cod)](2) yielded the mononuclear [Ir(CO)(PPh3)(2)-(Et)((CHimid.CHCNHC)-C-boolean AND-C-boolean AND)(PF6)](PF6) (8) and [IrCl(cod)(Et)(CHimid.(CHCNHC)-C-boolean AND-C-boolean AND)](PF6) (10) complexes, respectively, and the dinuclear complexes [{Ir(CO)(PPh3)(2)}(2)(Et)(mu-(CNHCCHCNHC)-C-boolean AND-C-boolean AND)](PF6)(2) (7) and [{IrCl(cod)}(2)(Et)(mu-(CNHCCHCNHC)-C-boolean AND-C-boolean AND)] (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 (CNHCCHCNHC)-C-boolean AND-C-boolean AND 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)(2)(Et)(mu-(CNHCCHCNHC)-C-boolean AND-C-boolean AND)] (11).