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 5. 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.