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.