Tuning ligand effects and probing the inner-workings of bond activation steps: generation of ruthenium complexes with tailor-made properties.

Activating chemical bonds through external triggers and understanding the underlying mechanism are at the heart of developing molecules with catalytic and switchable functions. Thermal, photochemical, and electrochemical bond activation pathways are useful for many chemical reactions. In this Article, a series of Ru(II) complexes containing a bidentate and a tripodal ligand were synthesized. Starting from all-pyridine complex 1(2+), the pyridines were stepwise substituted with "click" triazoles (2(2+)-7(2+)). Whereas the thermo- and photoreactivity of 1(2+) are due to steric repulsion within the equatorial plane of the complex, 3(2+)-6(2+) are reactive because of triazoles in axial positions, and 4(2+) shows unprecedented photoreactivity. Complexes that feature neither steric interactions nor axial triazoles (2(2+) and 7(2+)) do not show any reactivity. Furthermore, a redox-triggered conversion mechanism was discovered in 1(2+), 3(2+), and 4(2+). We show here ligand design principles required to convert a completely inert molecule to a reactive one and vice versa, and provide mechanistic insights into their functioning. The results presented here will likely have consequences for developing a future generation of catalysts, sensors, and molecular switches.