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.