Homodinuclear RhI and IrI Complexes Derived from Imidazolium-Benzimidazolates

The reactions of [M(mu-Cl)(COD)](2) (M=Rh, Ir) with the conjugated mesomeric betaines 2-(3-methyl-1-imidazolium)-benzimidazolate (1aH), 2-(3-isopropyl-1-imidazolium)-benzimidazolate (1bH) and 2-(3-tert-butyl-1-imidazolium)-benzimidazolate (1cH) in equilibrium with their carbenic tautomers 1'aH, 1'bH and 1'cH were investigated. In the case of 1aH and 1bH, the homodinuclear complexes [MCl(COD){M(mu-1'a)(COD)}] and [MCl(COD){M(mu-1'b)(COD)}] were obtained, whose anionic NHC ligand 1'a(-) or 1'b(-) bridges the metal centres in a kappa N-1 : kappa N-1 mode, while binding the M(COD) moiety in a C,N-chelating mode. For M=Ir, [IrCl(COD){Ir(mu-1''b)(COD)}] containing the "abnormal" NHC isomer 1''b(-) was observed as a side product. In the case of 1'cH, [{RhCl(COD)}(2)(mu-1cH)] containing a kappa N-1 : kappa N-1 benzimidazolato ligand was obtained for M=Rh, while for M=Ir the reaction furnished [IrCl(COD){Ir(mu-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)(mu-1cH)] to afford [(IrCl)-Cl-I(COD){(IrHCl)-H-III(mu-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(mu-1'a)(COD)}], where by serendipity the closely related mononuclear complex [Ir(1'a)(COD)] was structurally characterised instead.