Tunable Luminescent Properties of Tricyanoosmium Nitrido Complexes Bearing a Chelating O^N Ligand

We have recently reported a strongly luminescent osmium(VI) nitrido complex [Os^VI(N)(NO₂-L)(CN)₃]^- [HNO₂-L = 2-(2-hydroxy-5-nitrophenyl)benzoxazole]. The excited state of this complex readily activates the strong C-H bonds of alkanes and arenes (Commun. Chem. 2019, 2, 40). In this work, we attempted to tune the excited-state properties of this complex by introducing various substituents on the bidentate L ligand. The series of nitrido complexes were characterized by IR, UV/vis, ¹H NMR, and electrospray ionization mass spectrometry. The molecular structures of five of the nitrido compounds have been determined by X-ray crystallography. The photophysical and electrochemical properties of these complexes have been investigated. The luminescence of these nitrido complexes in the solid state, in a CH₂Cl₂ solution, and in a CH₂Cl₂ solid matrix at 77 K glassy medium clearly shows that these emissions are due to ³LML′CT [L ligand to Os≡N] phosphorescence. The presence of strongly electron-withdrawing substituents in these complexes enhances the LML′CT emission. Our result demonstrates that the excited-state properties of this novel class of luminescent osmium(VI) nitrido complexes can be fine-tuned by introducing various substituents on the bidentate L ligand.