Synthesis and Characterization of Metal Complexes Possessing the 5-(2-Pyridyl) Pyrazolate Ligands: The Observation of Remarkable Osmium-Induced Blue Phosphorescence in Solution at Room Temperature

A total of three distinctive main group and transition metal complexes containing the 2-pyridyl pyrazolate (pypz) ligand were prepared, namely, B(C 6 F 5 ) 2 (pypz) ( 1 ), Ru(CO) 2 (pypz) 2 ( 2 ), and Os(CO) 2 (pypz) 2 ( 3 ), where (pypz)H = 3-trifluoromethyl-5-(2-pyridyl)pyrazole. Single-crystal X-ray diffraction studies were carried out on complexes 2 and 3, revealing octahedral coordination geometry with two CO ligands located at cis dispositions. While the pypz ligand arrangement for complex 2 is in cis- (N py,N py ) and trans- (N pz,N pz ), complex 3 reveals a different configuration, cis- (N pz,N pz ) and trans- (N py,N py ) (N py for pyridine-N and N pz for pyrazolate donor sites). Similar to that of the in-situ-prepared pypz anion, the boron complex B(C 6 F 5 ) 2 (pypz) ( 1 ) exhibits a strong emission centered at 380 nm, which is unambiguously assigned to fluorescence derived from the S 1 (ππ*) → S 0 transition. In contrast to the nonluminescent behavior for Ru complex 2, the Os complex 3 displays unique, strong room-temperature phosphorescence, showing vibronic progressions at 430, 457, and 480 nm. The remarkable differences in photophysical properties were rationalized by a combination of π-electron accepting CO ligand, relative pypz orientation, and heavy-atom-enhanced spin−orbit coupling effects.