Iridium-mediated N–H and methyl C–H bond activations in N-(2′,6′-dimethylphenyl)pyrrole-2-aldimine. Synthesis, characterization and catalytic applications

• N-(2′,6′-dimethylphenyl)pyrrole-2-aldimine reacts with Ir(PPh3)3Cl.
• N–H and methyl C–H bond activations of the imine ligand occur.
• Organoiridium complexes are formed.
• Only N–H bond activation occurs in N-(2′-methylphenyl)pyrrole-2-aldimine.
• The iridium complexes efficiently catalyze Oppenauer oxidation.

Reaction of N-(2′,6′-dimethylphenyl)pyrrole-2-aldimine (L-Me2) with Ir(PPh3)3Cl in refluxing toluene affords two organometallic complexes (1 and 2), where the imine-ligand (L-Me2) is coordinated to the metal center, via N–H and methyl C–H activations, as a di-anionic tridentate NNC-donor, along with two triphenylphosphines. In 1 the sixth coordination site is occupied by a hydride, while in 2 by a chloride. In both cases the hydride or chloride is trans to the coordinated imine-nitrogen, and the two triphenylphosphines are mutually trans. Similar reaction of N-(2′-methylphenyl)pyrrole-2-aldimine (L-Me) with Ir(PPh3)3Cl affords 3, where the imine-ligand is coordinated to the metal center as a mono-anionic bidentate NN-donor, along with two triphenylphosphines, a hydride and a chloride. Structures of 1, 2 and 3 have been determined by X-ray crystallography. DFT analyses have been carried out to understand the formation of the complexes. All the complexes show characteristic 1H NMR signals and, intense transitions in the visible and ultraviolet regions. Cyclic voltammetry on all three complexes shows two irreversible oxidations within 0.89–1.34 V vs. SCE and a reduction within −1.31 to −1.40 V vs. SCE. Complexes 1, 2 and 3 have been found to efficiently catalyze the Oppenauer oxidation of alcohols.

Reaction of Ir(PPh3)3Cl with N-(2′,6′-dimethylphenyl)pyrrole-2-aldimine (L-Me2) affords complexes 1 and 2, via N-H and methyl C-H activations of the imine-ligand. Similar reaction with N-(2′-methylphenyl)pyrrole-2-aldimine (L-Me) affords complex 3, via only N-H activation of the imine-ligand. All three complexes are found to efficiently catalyze Oppenauer oxidation.Figure optionsDownload as PowerPoint slide