Efficient and versatile transfer hydrogenation catalysts: Iridium (III) and ruthenium (II) complexes with 4-acetylbenzyl-N-heterocyclic carbenes

•New 4-acetylbenzyl NHC Ir and Ru complexes were synthesized.•The synthesized complexes showed efficient and versatile catalytic activity.•The transfer hydrogenation of ketones and N-alkylation of amines were investigated.•Intramolecular CH activition was observed in Ir-catalyzed transfer hydrogenation.

New 4-acetylbenzyl-N-heterocyclic carbene ligands (1–4) have been used to synthesize iridium complexes 6–9 and ruthenium complex 10. All complexes were characterized by FT-IR, 1H and 13C NMR spectroscopy, elemental analysis, and in the case of 6, by X-ray diffraction studies. The catalytic performance of these iridium and ruthenium complexes for transfer hydrogenation of ketones and imines and N-alkylation of amines with primary alcohols were tested in a range of substrates, and showed high catalytic activity with 1 mol% catalytic loading. The neutral complex 8 with two acetyl groups also showed good catalytic efficiency under lower catalyst loading (0.01 mol%), with the maximum TON of 8000, while on the other hand, the cationic complex 9 with PF6− as counteranion showed good to excellent catalytic activity toward the N-alkylation of amines in a wide scope of substrates. We also found out that the Ir complex 6′ was formed through the intramolecular CH activition of 6 under the transfer hydrogenation conditions.

Graphical abstractA series of 4-acetylbenzyl N-heterocyclic carbene iridium and ruthenium complexes was developed and showed good to excellent catalytic activities toward both transfer hydrogenation of ketones and imines and N-alkylation of amines with alcohols. The iridium complex converted to an intramolecular CH activated one under the transfer hydrogenation conditions.Figure optionsDownload full-size imageDownload high-quality image (86 K)Download as PowerPoint slide