Stepwise synthesis of a hydrido, N-heterocyclic dicarbene iridium(iii) pincer complex featuring mixed NHC/abnormal NHC ligands

Abstract

We describe a stepwise synthesis of the hydrido, N-heterocyclic dicarbene iridium(III) pincer complex [Ir(H)I(C_NHCCC_aNHC)(NCMe)] (3) which features a combination of normal and abnormal NHC ligands. The reaction of the bis(imidazolium) diiodide [(CH_imidCHCH_imid)]I₂ (1) with [Ir(μ-Cl)(cod)]₂ afforded first the mono-NHC Ir(I) complex [IrI(cod)(CH_imidCHC_NHC)]I (2), which was then reacted with 2 equiv. of Cs₂CO₃ in acetonitrile at 60 °C for 40 h to yield 3. These observations support our previously proposed mechanism for the formation of hydrido, N-heterocyclic dicarbene iridium(III) pincer complexes from the reaction of bis(imidazolium) salts with weak bases involving a mono-NHC Ir(I) intermediate. We describe the reactivity of the mono-NHC Ir(I) complex 2 under various conditions. By changing the reaction solvent from MeCN to toluene, we observed the cleavage of the imidazol-2-ylidene ring and the formation of an iminoformamide-containing mono-NHC Ir(I) complex [IrI(cod){[NHCHCHN(Ad)CHO]CHC_NHC}] (4). Complex 4 was also prepared in high yield from the reaction of 2 with strong bases (potassium tert-butoxide or potassium hexamethyldisilazane), via the initial formation of the complex [IrI(cod)(CH_NHCCHC_NHC)] (5), which contains a coordinated NHC moiety and a free carbene arm, followed by subsequent hydrolysis of the latter. The bis(imidazolium) salt 1 can be deprotonated by strong bases to form the bis(carbene) ligand C_NHCCHC_NHC (6), which readily reacts with [Ir(μ-Cl)(cod)]₂ to give the dinuclear complex [{IrI(cod)}₂(μ-C_NHCCHC_NHC)] (7), in which the N-heterocyclic bis(carbene) ligand bridges the two metals through the carbene carbon atoms.