En Route to the Formation of High-Efficiency, Osmium(II)-Based Phosphorescent Materials

Triosmium cluster complexes [Os₃(CO)₈(fppz)₂] (2a) and [Os₃(CO)₈(fptz)₂] (2b) bearing two 2-pyridyl azolate ligands were synthesized in an attempt to establish the reaction mechanism that gives rise to the blue-emitting phosphorescent complexes [Os(CO)₂(fppz)₂] (1a) and [Os(CO)₂(fptz)₂] (1b) [(fppz)H = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole; (fptz)H = 3-(trifluoromethyl)-5-(2-pyridyl)triazole]. X-ray structural analysis of 2b showed an open triangular metal framework incorporating multisite-coordinated 2-pyridyltriazolate ligands. Treatment of 2 with the respective 2-pyridylazolate ligand led to the formation of blue-emitting complex 1b, confirming their intermediacy, while the reaction of 2b with phosphine ligand PPh₂Me afforded two hitherto novel hydride complexes 3 and 4, for which the reversible interconversion was clearly established at higher temperatures (>180 °C). The single-crystal X-ray diffraction analyses of 3 and 4 confirmed their monometallic and isomeric nature, together with the coordination of two phosphine ligands located in the trans-disposition and one CO and one hydride located opposite to the pyridyl triazolate chelate. Subtle differences in photophysical properties were examined for isomers 3 and 4 on the basis of steady state absorption and emission, the relaxation dynamics, and temperature-dependent luminescent studies. The results, in combination with time-dependent density function theory (TDDFT) calculations, provide fundamental insights into the future design and preparation of highly efficient phosphorescent emitters.