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 RuII complexes
containing a bidentate and a tripodal ligand were synthesized. Starting
from all-pyridine complex 12+, the pyridines
were stepwise substituted with “click” triazoles (22+–72+). Whereas
the thermo- and photoreactivity of 12+ are
due to steric repulsion within the equatorial plane of the complex, 32+–62+ are reactive
because of triazoles in axial positions, and 42+ shows unprecedented photoreactivity. Complexes that feature neither
steric interactions nor axial triazoles (22+ and 72+) do not show any reactivity. Furthermore,
a redox-triggered conversion mechanism was discovered in 12+, 32+, and 42+. 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.