Homodinuclear Rh I and Ir I Complexes Derived from Imidazolium‐Benzimidazolates

The reactions of [M(μ‐Cl)(COD)] 2 (M=Rh, Ir) with the conjugated mesomeric betaines 2‐(3‐methyl‐1‐imidazolium)‐benzimidazolate ( 1a H), 2‐(3‐isopropyl‐1‐imidazolium)‐benzimidazolate ( 1b H) and 2‐(3‐ tert ‐butyl‐1‐imidazolium)‐benzimidazolate ( 1c H) in equilibrium with their carbenic tautomers 1’a H, 1’b H and 1’c H were investigated. In the case of 1a H and 1b H, the homodinuclear complexes [MCl(COD){M(μ‐ 1’a )(COD)}] and [MCl(COD){M(μ‐ 1’b )(COD)}] were obtained, whose anionic NHC ligand 1’a − or 1’b − bridges the metal centres in a κ 1 N  : κ 1 N mode, while binding the M(COD) moiety in a C , N ‐chelating mode. For M=Ir, [IrCl(COD){Ir(μ‐ 1’’b )(COD)}] containing the “abnormal” NHC isomer 1’’b − was observed as a side product. In the case of 1’c H, [{RhCl(COD)} 2 (μ‐ 1c H)] containing a κ 1 N  : κ 1 N benzimidazolato ligand was obtained for M=Rh, while for M=Ir the reaction furnished [IrCl(COD){Ir(μ‐ 1’’c )(COD)}], which contains the anionic “abnormal” NHC ligand 1’’c − . This difference in behaviour is rationalised by the propensity of iridium(I) for oxidative addition, which in the present case involves a C−H bond of the dangling formamidium unit of [{IrCl(COD)} 2 (μ‐ 1c H)] to afford [Ir I Cl(COD){Ir III HCl(μ‐ 1’’c )(COD)}], followed by reductive elimination of HCl. The crystal structures of the new complexes were determined by single‐crystal X‐ray diffraction, except for [IrCl(COD){Ir(μ‐ 1’a )(COD)}], where by serendipity the closely related mononuclear complex [Ir( 1’a )(COD)] was structurally characterised instead.