Di- and Trinuclear Iridium(III) Complexes with Poly-Mesoionic Carbenes Synthesized through Selective Base-Dependent Metalation

Mutidentate carbene ligands based
on a rigid aromatic platform
are valuable synthons for generating carbene complexes with higher
nuclearity. We present here the selective, base-dependent synthesis
of a dinuclear or a trinuclear IrIII complex from the 1,3,5-substituted
benzene derived tris-triazolium salt. The dinuclear IrIII complex features an unreacted triazolium unit which enables us to
compare the metric parameters between the bonded 1,2,3-triazol-5-ylidene
to their parent triazolium salt present in the same molecule. Single
crystal X-ray diffraction studies confirm the di- and trinuclear nature
of the complexes and establish their configuration and conformation.
Both the di- and trinuclear IrIII complexes have been used
for catalytic transfer hydrogenation, and these complexes are potent
precatalysts delivering good to excellent yields for the reduction
of benzaldehyde, acetophenone, benzophenone, and cyclohexanone. Furthermore,
they show a preference for reducing nitrobenzene to either azoxybenzene
or azobenzene. Mercury poisoning tests conclusively prove the homogeneous
nature of the reported catalysis. The lack of orthometalation in these
complexes and the possible effect thereof on catalysis are discussed.