Arene-Ruthenium(II) and -Iridium(III) Complexes with Click-Based Pyridyl-triazoles, Bis-triazoles, and Chelating Abnormal Carbenes: Applications in Catalytic Transfer Hydrogenation of Nitrobenzene

The complexes [(Cym)Ru(L)Cl]PF6, 2-4, and [Cp*Ir(L)Cl]PF6, 6-8 (Cym = p-cymene, Cp* = pentamethylcyclopentadienyl), with L = click-derived pyridyl-triazol, bis-triazole, or bis-abnormal carbene, were synthesized and spectroscopically characterized. Structural elucidation of the complexes shows a half-sandwich, piano-stool type of coordination around the metal centers and a delocalized situation within the triazolylidene rings. All the complexes were tested for their catalytic efficiency in the transfer hydrogenation of nitrobenzenes, and the results were compared with their 2,2'-bipyridine (bpy) Ru counterpart 1 and Ir counterpart S. Remarkably, the nature of the final catalytic product is strongly dependent on the chosen metal center, with aniline being preferentially formed with the Ru complexes and azobenzenes with the Ir complexes. Judicious selection of catalyst and reaction conditions also facilitates the isolation of azoxybenzene. To the best of our knowledge, this is a rare example of a homogeneous catalytic synthesis of azobenzene from nitrobenzene. The influence of ligand substitution, metal substitution, and temperature variation on catalytic activity and selectivity has been investigated, whereby a systematic variation of the ligands from bpy, to pyridyl-triazole, to bis-triazole, to bis-abnormal carbene has been carried out. We also present a mechanistic investigation for this transformation with the aim of understanding reaction behavior.