Hydrazine Photorelease from Ru(II) Chugaev-Type Dicarbene Complexes

Ruthenium(II) polypyridyl complexes exhibit rich excited state reactivity from both photogenerated 3MLCT and higher energy thermally populated ligand-field (LF) excited states. In order to maximize the utility of the different reactivity of these two states, the energy gap between them must be tuned without sacrificing the favorable optical and photophysical properties of the complex. In probing design strategies aimed at enhancing the photostability of RuII complexes for nondissociative photochemical applications, we herein explore the use of Chugaev-type dicarbene ligands as strong σ-donors. Using a complementary combination of X-ray crystallography, electrochemistry, and optical and X-ray spectroscopies, we demonstrate how Chugaev-type dicarbene ligands destabilize LF states of photoactive RuII complexes without sacrificing the favorable optical and photophysical properties of the complex, and compare with π-accepting methylisocyanide ligands. Despite these design strategies aimed at suppressing dissociative excited-state ligand-field pathways, we further report an unexpected dechelative photoelimination of hydrazine from the RuII Chugaev-type dicarbene complexes that occurs directly from the 3MLCT excited state with quantum yields in the order of ca. 1-5%, and is sensitive to peripheral substitution of chromophoric bipyridine ligands. These findings contribute to growing understanding of molecular design principles for photoactive RuII complexes targeted at either dissociative/nondissociative photochemical applications and introduce a new potential scaffold and photorelease mechanism for photoactivated chemotherapy.