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

Abstract

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.