Stepwise and one-pot syntheses of Ir(III) complexes with imidazolium-based carbene ligands

We report the preparation, crystal structure, electrochemistry, and emission properties of Ir(C⁁C:)3, where C⁁C: is an N-heterocyclic carbene ligand. Two synthetic approaches are introduced for generating Ir(III) complexes bearing imidazolium-based carbene ligands whose precursors are [pypiH2][Cl] (1a) (pyridyl[1,2-a]{2-phenylimidazol}-3-ylidene chloride) and [pympiH2][Cl] (1b) (pyridyl[1,2-a-{2-(p-methoxy)phenylimidazol}-3-ylidene chloride). The first method is a stepwise reaction: treatment of [Ir(µ-Cl)(COD)]2, where COD is 1,5-cyclooctadiene, with 4 equiv. of the corresponding carbene (C⁁C:) ligands in the presence of an excess amount of sodium methoxide affords Ir(III) dimers [Ir(µ-Cl)(C⁁C:)2]2 (2a, C⁁C: = pypi−; 2b, C⁁C: = pympi−). These chloro-bridged dimers 2a and 2b react with the corresponding carbene (C⁁C:) ligands to form the desired homoleptic compounds Ir(C⁁C:)3 (3a, C⁁C: = pypi−; 3b, C⁁C: = pympi−). The second method, using a one-pot reaction of [Ir(µ-Cl)(COD)]2 with 6 equiv. of the corresponding carbene (C⁁C:) ligands 1a and 1b in the presence of excess amounts of Ag2O, affords Ir(C⁁C:)3. The two methods are convenient and reproducible procedures for the synthesis of Ir(C⁁C:)3. Complexes 3a and 3b are obtained as mixtures of meridional and facial isomers, which can be separated by recrystallization or flash column chromatography.