Iridium mediated N–H and C–H bond activation of N-(aryl)pyrrole-2-aldimines. Synthesis, structure and, spectral and electrochemical properties

Reaction of N-(4′-R-phenyl)pyrrole-2-aldimines (L-R, R = OCH3, CH3, H, Cl and NO2) with Ir(PPh3)3Cl in refluxing ethanol affords a group of yellow complexes (1-R), in which an imine-ligand (L-R) is coordinated to the metal center as a mono-anionic bidentate NN-donor along with two triphenylphosphines, a chloride and a hydride. The hydride is trans to the coordinated pyrrole-nitrogen, while the chloride is trans to the imine-nitrogen, and the two triphenylphosphines are mutually trans. Structure of the 1-OCH3 complex has been determined by X-ray crystallography. Similar reaction of N-(naphthyl)pyrrole-2-aldimine (L-nap) with Ir(PPh3)3Cl affords an organometallic complex 2, where the imine-ligand is coordinated to the metal center, via C–H activation of the naphthyl ring at the 8-position, as a di-anionic tridentate NNC-donor, along with two triphenylphosphines and a hydride. Structure of this complex 2 has also been determined by X-ray crystallography. All the complexes are diamagnetic, and show characteristic 1H NMR signals and intense transitions in the visible region. Cyclic voltammetry on all the complexes shows two irreversible oxidations within 0.97–1.46 V vs. SCE and a reduction within −0.83 to −1.40 V vs. SCE.

Graphical abstractReaction of N-(4′-R-phenyl)pyrrole-2-aldimines (L-R) with Ir(PPh3)3Cl in refluxing ethanol affords a group of hydrido complexes of iridium (1-R), via N–H bond activation. Similar reaction of N-(naphthyl)pyrrole-2-aldimine (L-nap) with Ir(PPh3)3Cl affords an organometallic complex 2, via N–H bond activation followed by C–H bond activation of the naphthyl ring at the 8-position.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► N-(aryl)pyrrole-2-aldimines undergo N–H activation upon reaction with Ir(PPh3)3Cl. ► Hydrido complexes of iridium(III) are formed due to this N–H activation. ► In case of N-(naphthyl)pyrrole-2-aldimine C–H activation follows N–H activation. ► Iridium-bound chloride activates proximal C–H bond.