Nonlinear Pathway for Remote Metal–Metal Communication in Heterobimetallic Complexes Enabled by Mixed NHC Ligands: Influence of Metal Coordination Modes

This work presents unique examples of two heterobimetallic complexes 4 and 5 that share the same overall molecular composition but display distinctly different electrochemical properties. These complexes were synthesized in high yields from an unsymmetrical bis-triazolium ligand precursor with a phenyl spacer (1) featuring differently acidic triazolium moieties via sequential regioselective metalation. Despite the potential for doubly orthometalated systems, these complexes demonstrate unusual binding modes on a single ligand platform: One is orthometalated with the normal NHC (1,2,4-TzNHC) moiety, while at the same time, the other metal is nonorthometalated at the unusual C4 position of the abnormal NHC (1,2,3-TzNHC) motif. This molecular design significantly enhances the electronic properties of the complexes, facilitating the intramolecular metal-to-metal electronic communication between the Ru^II and Rh^III centers through the bridging NHC ligand, although these metal ions are separated by ∼9 Å. Detailed spectroelectrochemical investigations reveal that out of the two constitutional isomers 4 and 5, complex 5, which features a cyclometalated Ru^II moiety, exhibits superior redox activity and enhanced UV–vis–NIR absorption compared to complex 4, also supported by DFT studies. This study provides a pioneering example of how the binding sites and coordination modes of metal ions influence the electrochemical behavior of NHC-coordinated heterobimetallic complexes.