Ruthenium Complexes of Tripodal Ligands with Pyridine and Triazole Arms: Subtle Tuning of Thermal,Electrochemical, and Photochemical Reactivity

Electrochemical and photochemical bond‐activation steps are important for a variety of chemical transformations. We present here four new complexes, Ru(Lⁿ)(dmso)(Cl)]PF₆ ( 1 – 4 ), where Lⁿ is a tripodal amine ligand with 4?n pyridylmethyl arms and n?1 triazolylmethyl arms. Structural comparisons show that the triazoles bind closer to the Ru center than the pyridines. For L², two isomers (with respect to the position of the triazole arm, equatorial or axial), trans‐ 2 _sym and trans‐ 2 _un, could be separated and compared. The increase in the number of the triazole arms in the ligand has almost no effect on the Ru^II/Ru^III oxidation potentials, but it increases the stability of the Ru?S_dmso bond. Hence, the oxidation waves become more reversible from trans‐ 1 to trans‐ 4 , and whereas the dmso ligand readily dissociates from trans‐ 1 upon heating or irradiation with UV light, the Ru?S bond of trans‐ 4 remains perfectly stable under the same conditions. The strength of the Ru?S bond is not only influenced by the number of triazole arms but also by their position, as evidenced by the difference in redox behavior and reactivity of the two isomers, trans‐ 2 _sym and trans‐ 2 _un. A mechanistic picture for the electrochemical, thermal, and photochemical bond activation is discussed with data from NMR spectroscopy, cyclic voltammetry, and spectroelectrochemistry.