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 <b>4</b> and <b>5</b> 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 (<b>1</b>) 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 <b>4</b> and <b>5</b>, complex <b>5</b>, which features a cyclometalated Ru^II moiety, exhibits superior redox activity and enhanced UV-vis-NIR absorption compared to complex <b>4</b>, 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.